Proceedings of the International Conference on Colloid and Surface Science

Studies in Surface Science and Catalysis 132 PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON COLLOID AND SURFACE SCIENCE.TOKYO, JAPAN, NOVEMBER 5-8,2000

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Studies in Surfece Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 132

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON COLLOID AND SURFACE SCIENCE, Tokyo, Japan, November 5-8,2000 25th Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan Edited by Yasuhiro Iwasawa Department of Chemistry, Graduate School of ScienceJhe University of Tokyo, Hongo,BunkyO'lcu,Tol(yo 113-0033, Japan Noboru Oyama Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Nakacho, Koganei,Tokyo 184-8588, Japan Hironobu Kunieda Division ofArtificial Environment and Systems, Graduate School of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku,Yokohama 240-8501, Japan

2001 ELSEVIER Amsterdam —London - New York - Oxford —Paris — Shannon — Tokyo

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Preface The International Conference on Colloid and Surface Science was held from Sunday, November 5 to Wednesday, November 8, 2000 at Arcadia Ichigaya in Tokyo under the auspices of Division of Colloid and Surface Chemistry, the Chemical Society of Japan to commemorate the 25th anniversary of the foundation of the Division.

This special

volume of Studies in Surface Science and Catalysis contains articles submitted to the milestone Conference, financially supported by Grant-in-Aid for Publication of Scientific Research Result, the Ministry of Education, Science, Sports and Culture. The purpose of the Conference is to discuss the results of recent developments and the future prospect in science and technology of the field.

The field has been

growing andflourishing,while indicating many problems to be uncovered and solved. The Conference will be structured to encourage interactions and to stimulate the exchange of ideas to accomplish the above purpose. Key issues and materials related to the Conference were included as follows: (1) Molecular Assemblies in Solutions (micelles, surfactant solutions, emulsions and microemulsions, polymer solutions, gels, liquid crystals, etc.), (2) Fine Particles and Colloidal Dispersions (nano-micro fine particles, suspensions, polymer colloids, light scattering, electrokinetic phenomena, rheology, etc.), (3) Supramolecular Organized Films (insoluble monolayers, bilayers, Langmuir-Blodgett fibns, self-assembled monolayers, vesicles and liposomes, monoparticle films, biological aspects, etc.), (4) Nanostructural Solid Surfaces (adsorption in nanopores, catalysis, nanoparticle surfaces, functionalized surfaces, scanning probe microscopies, surface forces, nanotribology, microfabrication, colloids and interfaces in the environment, etc,), and (5) Industrial Applications and Products (energy and batteries: Li battery, Ni-hydrogen battery, fuel cell, gel, electrolyte, cosmetics and healthcare: skin-care, hair-care, drug delivery systems, foods: colloidal aspects of foods & foodstuff, processing, texture, nutrition, household: detergents, fabric-care, house-keeping goods, environmental applications, paint, etc.) The Conference comprised 2 plenary lectures, 42 invited lectures, 150 oral presentations and 266 poster presentations.

The camera-ready articles q^pearing in

this special issue were reviewed by expertsfroms^ropriate fields in accordance with standard guidelines of scientific journals in the field. The number of submitted papers

was much greater than we anticipated, reflecting the variety of scientific and technological activities in Colloid and Surface Science and the state of the Conference as an important event of the Division. We are grateful to the outstanding scientists in different fields of Colloid and Surface Science who accepted our invitation to overview vital research areas and to introduce the various topics of the sessions covered by the Conference.

We are also

grateful to Dr. Kostas I. Marinakis, Publisher, Dr. Matthias W.C. Wahls, Publishing Editor, and Drs. Huub Manten of Elsevier Science Publishers for the guidance and cooperation provided in getting this volume published in the book series of Studies in Surface Science and Catalysis. We believe that this Proceeding Book of the Conference may provide a valuable contribution towards a greater understanding of colloid and surface science, and also stimulate further developments and new ideas in future materials and processes.

Tokyo, November, 2000 The Editors

Table of Contents Preface

V

Plenary Session Structure of Liquid/liquid Interfaces Studied by Ellipsometry and Brewster Angle Microscopy G. H. Findenegg, J. Schulz and S. Uredat Assembly of Organic and Inorganic Molecular Layers by Adsorption from Solution T. Kunitake and I. Ichinose

1 15

Molecular Assemblies in Solutions Scattering Study of the Lyotropic Lamellar Phase in Aqueous Solutions of Nonionic Surfactants T. Kato, K. Minewaki, H. Yoshida, M Imai, and K. Ito

25

Microemulsions Composed of Metal Complex Surfactants, Bis(Octylethylenediamine (=0E)) Zn(II), Cd(II), and Pd(II) Chlorides, in Water / Chloroform and Water / Benzene Systems M. lida, H. Er, N. Hisamatsu, N. Asaoka and T. Imae 31 Kinetics of Lamellar to Gyroid Transition in a Nonionic Surfactant System M. Imai, A. Kawaguchi, A. Saeki, K. Nakaya, T. Kato and K. Ito

35

Efficiency Boosting by Amphiphilic Block Copolymers in Microemulsions: Dependence on Surfactant and Oil Chain Length R. Strey, M. Brandt, B. Jakobs and T. Sottmann

39

Thermotropic Phase Behavior of Binary Cationic Surfactant Mixtures in Water S. Kaneshina, H. Matsuki, R. Ichikawa and T. Kuwahara

45

Active Control of Surfactants N. L. Abbott

49

Droplet Microemulsion and Telechelic Polymer: Linear Rheology and Flow Instability at High Shear G. Porte, M. FilaH, E. Michel, J. Appell, S. Mora, E. Sunnyer and F. Molino

55

Synthesis and Micelle Formation of Fluorine-Containing Block Copolymers K. Matsumoto, T. Kitade, H. Mazaki, H. Matsuoka and H. Yamaoka

61

A Study of the Gelation of the Polysaccharide Curdlan H. Zhang, K. Nishinari, T. J. Foster, M. A. K. Wilhams and I. T. Norton

65

Formation of Highly Swollen L-Phases and Vesicle PhasesfromSingle Chain Surfactants by Chemical Reactions H. Hoffrnann, K. Horbaschek and J. Hao 69 Functions and Structures of Molecular Assemblies under High Magnetic Fields S. Ozeki

79

ESR Spectral Simulation Study of Oleic Acid/Oleate Solution by Using a Spin Probe H. Fukuda, A. Goto, H. Yoshioka and P. Walde

85

Controlled Association between Amphiphilic Polymers and Enzyme by Cyclodextrins in Heat Denatured Process: Artificial Molecular Chaperone K. Akiyoshi, M. Ikeda, Y. Sasaki and J. Sunamoto

89

Effect of Alcohols (Propanol, Propylene Glycol, and Glycerol) on Cloud Point and Micellar Structure in Long-Poly(oxyethylene)n Oleyl Ethers Systems K. Shigeta, U. Olsson and H. Kunieda 93 Critical Surface Charge Density for Counter-Ion Binding in Mixed Micelles of Ionic with Non-Ionic Surfactant M. Manabe, H. Kawamura, H. Katsu-ura and M. Shiomi

97

Dispersibility of Surfactant-Free OAV Emulsions and Their Stability Design: A Present Scope from Hydrocarbons to Some Oleate Esters K. Kamogawa, H. Akatsuka, M. Matsumoto, T. Sakai, T. Kobayashi, H. Sakai and M. Abe

101

Solidification of Liquid Hydrocarbons with the Aid of Carboxylate H. Sakaguchi

105

Methodology for Predicting Approximate Shape and Size Distribution of Micelles M. Kinoshita and Y. Sugai

109

Pre-Micelle and Micelle Formation of Local Anesthetic Dibucaine Hydrochloride H. Matsuki, T. Miyata, T. Yoshioka, H. Satake and S. Kaneshina

113

Ionic Partition to Zwitterionic Micelles K. Iso and T. Okada

117

Analysis of Local Structure of Ion Adsorbed on the Gas/Liquid Interface M. Harada, T. Okada and I. Watanabe Adsorption of Nonionic Surfactants, Triton X and Triton N, on Hydroxyapatite after Surface Modification with Sodium Dodecyl Sulfate in an Aqueous Phase S. Shimabayashi, M. Hoshino, T. Ohnishi and T. Hino

121 125

Miscibilty of Dodecylpyridinium Bromide and Dodecylquinolinium Bromide in Adsorbed Films and Micelle T. Fujii, K. Fujio and S. Ozeki 129 Interaction and Complex Formation of Pluronic Polymers with Ionic Surfactants S. Shimabayashi, A. Ichimori and T. Hino

133

Formation of Chiral Aggregates of Acylamino Acids in Organic Solvents H. Matsuzawa, H. Minami, T. Yano, T. Wakabayashi, M. Iwahashi, K. Sakamoto andD. Kaneko

137

Formation and Structure Control of Reverse Micelles by the Addition of Alkyl Amines and Their Applications for Extraction Processes of Proteins K. Shiomori, T. Honbu, Y. Kawano, R. Kuboi and I. Komasawa

141

Preparation and Surface-Active Properties of Cotelomer Type Surfactants of Alkyl Acrylate and Acrylic Acid T. Yoshimura, Y. Koide, H. Shosenji and K.Esumi

145

Iridescent and Coloured Colloidal Phases in Highly Dilute Systems Containing Decyl - and -D-glucopyranosides, Decanol or Octanol and Water B. Hoffmann and G. Platz

149

Complex Formation between Water-Soluble Calixarenes and Dodecylpyridinium Chloride K.Murakami 153 Influence of Oil Droplet Size on Flocculation/Coalescence in Surfactant-Free Emulsion T. Sakai, K.Kamogawa, F. Harusawa, N. Momozawa, H. Sakai and M. Abe

157

Morphology of Microemulsion Droplet Confining a Single Polymer Chain K. Nakaya, M. Imai, I. Miyata and M. Yonese

161

AOT Microemulsion Structure Depending on Both Apolar Solvent and Protein Concentration R. Kawai-Hirai, M. Hirai, H. Futatsugi, H. Iwase and T. Hayakawa

165

Phase Transition in Gibbs Monolayers of Mixed Surfactants M. M. Hossain, T. Okano and T. Kato

169

Mesoscopic Structures of J Aggregates of Organic Dyes at a Sohd/Liquid Interface and in Solution: Spectroscopic and Microscopic Studies H. Yao, S. Yamamoto, N. Kitamura and K. Kimura 173 Energy of Breaking of Aqueous GEMINI Surfactant Film T. Kidokoro and J. Igarashi

177

Molecular Aggregation States and Polymerizability of Potassium and Calcium 10-Undecenoates in Aqueous Systems Y. Shibasaki, H. Saitoh and A. Fujimori

181

Effects of Shear Flow on the Structure of the Lamellar Phase Formed in Nonionic Surfactant-Water System K. Minewaki, T.Kato and M. Imai

185

Solubility Behavior of Benzylhexadecyldimethylammonium Salts in Oils N.Ohtani

189

Deswelling Kinetics of Freeze-Dry-Treated Poly (A^-isopropylacrylamide) Gel in Sugar Solution N. Kato, S. Yamaguchi and F. Takahashi

193

NMR Study on the Effect of Added Salt on Alkylpyridinium Bromide Micelles S. Kobayashi, K. Fujio, Y. Uzu and S. Ozeki

197

Small-Angle Neutron Scattering Study of w/o AOT Microemulsion Entrapping Proteins M. Hirai, R. Kawai-Hirai, H. Iwase and T. Hayakawa

201

Neutron Spin Echo Investigations on Slow Dynamics in Complex Fluids Involving Amphiphiles T. Takeda, Y. Kawabata, H. Seto, S. Komura and M. Nagao

205

Neutron Spin Echo Studies on Effects of Temperature and Pressure in Dynamics of a Ternary Microemulsion Y. Kawabata, M. Nagao, H. Seto and T. Takeda

209

Dimerization of Penicillin V as Deduced by Frontal Derivative Chromatography S. Ishikawa, S. Neya and N. Funasaki

213

Two-Dimensional Clusters of Magnetic Fine Particles at the Surface of Magnetic Colloidal Suspension N. Tanaka, S. Doi and I. Takahashi

217

Colloid-chemical Properties of Chitosan S. Y. Bratskaya, M. V. Shamov, V. A. Avramenko and D. V. Chervonetskiy

221

Fine Particles and Colloidal Dispersions Science and Art of Fine Particles E. Matijevi

225

Hydrothermal Synthesis of Nano-Size ZrOa Powder, Its Characterization and Colloidal Processing O. Vasylkiv and Y. Sakka

233

Nanocrystal Self Assemblies: Fabrication and Collective Properties M. P. Pileni

237

Preparation, Characterization and Catalyses of Light-Transition-Metal/Noble-Metal Bimetallic Alloy Nanoclusters N. Toshima, P. Lu and Y. Wang

243

Novel Nanosize Borosilicate Colloid: Synthesis, Characterization, and Application J. M. Fu, A. Gerli, B. A. Keiser and A. Zelenev

247

Synthesis of Monodispersed Magnetic Particles by the Gel-Sol Method and Their Magnetic Properties H. Itoh and T. Sugimoto

251

Structural Change of Zinc Chloride Hydrate Melt Coexisting with Porous Solid Materials M. Mizuhata, Y, Sumihiro, A. Kajinami and S. Deki 255 Formation Conditions of Integrated Ordered Microstructure of Nano-size Silica Materials in Laurylamine/Tetraethoxysilane System M. Adachi, H. Taniguchi and M. Harada 259

Microscopic Morphology and SERS Activity of Ag Colloidal Particles M. Futamata, Y. Maruyama and M. Ishikawa

263

Effect of Polyvinylpyrrolidone on the Physical Properties of Titanium Dioxide Suspensions T. Sato and S. Kohnosu 267 Polymer Network Formation in the Pavement using SBR Latex Modified Asphalt Emulsions K. Takamura and W. Heckmann

271

Conformational Changes Induced by Competitive Adsorption in Mixed Interfacial Layers of Uncharged Polymers F. Csempesz and K. F.-Csaki 275 Electrostatic Potentials at Metal Oxide Aqueous Interface N. Kallay, D. Kova evi and I. Kobal

279

Microgravity Effects on the Properties of Colloidal Dispersions T. Okubo and A. Tsuchida

285

Colloidal Crystal Alloy Structure of Binary Dispersions of Polystyrene and Poly(Methylmethacrylate) Lattices by Ultra-Small-Angle X-ray Scattering T. Harada, H. Matsuoka and H. Yamaoka

289

The Role of Electrokinetic Properties on Adhesion of Nitrifying Bacteria to Solid Surfaces H. Hayashi, S. Tsuneda, A. Hirata and H. Sasaki 293 The Preparation of Porous Titania Films via Colloidal Crystallization between Electrodes Z.-Z. Gu, S. Hayami, Q.-B. Meng, A. Fujishima and O. Sato 297 Structure Formation of Poly(Furfuryl Alcohol)/Silica Hybrids S. Spange, H. Muller, D. Pleul, and F. Simon

301

Approach to a Unified Theory of Hydrophobic/Hydrophilic Surface Forces H.-J. Muller

307

Adsorption Behavior of Dispersing Agent to Pigment Surfaces N. Nakai, A. Hiwara and T. Fujitani

311

Measurement of Zeta Potentials in Concentrated Aqueous Suspensions of Ceramic Powders Using Electroacoustics R. Greenwood

315

Electrokinetic Phenomena in Concentrated Suspensions of Soft Particles H.Ohshima

319

Preparation of Nanosize Bimetallic Particles on Activated Carbon S. Hodoshima, T. Kubono, S. Asano, H. Arai and Y. Saito

323

Preparation of the PVA Film with Gold Fine Particles by a Counter Diffusion Method: Effect of Diffusion on the Distribution of Gold Fine Particles in the Film S. Sato, T. Shiono, H. Sato, M. Tsuji and M. Yonese

327

Optical Properties of ZnS: Mn Nanoparticles in Sol-Gel Glasses Y. Uchida and K. Matsui

331

Sonochemical Preparation of Noble Metal Nanoparticles in the Presence of Various Surfactants E. Takagi, Y. Mizukoshi, R. Oshima, Y. Nagata, H. Bandow and Y. Maeda

335

Liquid-Phase Synthesis of Y2O3: Eu Precursor Particles from Homogeneous Solution Y. Nishisu and M. Kobayashi

339

Processing of Zirconia and Alumina Fine Particles Through Electrophoretic Deposition Y. Sakka, B. D. Hatton and T. Uchikoshi

343

The Acceleration Behavior of Decomposition of Potassium Persulfate in the Dispersions of Polystyrene Particles Stabilized with Nonionic Emulsifier M. Okubo, T. Suzuki and S. Tasaki

347

Determination of the Size Distribution of Ultrafme Particles Based on a Measurement of Specific Surface Areas H. Yao, S. Yonemaru and K. Kimura

351

Dependence of Temperature-sensitivity of Poly (N-isopropylacrylamide-co-acrylic acid) Hydrogel Microspheres upon Their Sizes K. Makino, H. Agata and H. Ohshima

355

Deposition of Thiol-Passivated Gold Nanoparticles onto Glass Plates by Pulsed 532-nm Laser Irradiation: Effects of Thiol Y. Niidome, A. Hori, H. Takahashi and S. Yamada

359

Preparation of Silica-coated Magnetic Nanoparticles Y. Kobayashi and L. M. Liz-Marzan

363

Studies on the Preparation of Silica-Coated Carbon Particles by Sol-gel Method H. Shibuya, M. Shimada, N. Suzuki, H. Ito, K. limura, T. Kato and T. Kakihara

367

Synthesis and Catalysis of Polymer-Stabilized Ag and Ag/Pd Colloids Y. Shiraishi, K. Hirakawa, J. Yamaguchi and N. Toshima

371

Depletion Stabilization of Ceramic Suspensions with High Sohds Loading in the Presence of Zirconium Oxy-Salts O. Sakurada and M. Hashiba

375

Monte Carlo Study of Attractive Interaction between Charged Colloids T. Terao and T. Nakayama

379

Measurements of Elastic Constants of Colloidal Silica Crystals by Laser Diffraction T. Shinohara, T. Yoshiyama, I. S. Sogami, T. Konishi and N. Ise

383

Rheo-Optics of Colloidal Crystals T. Okubo, H. Kimura and T. Hatta

387

Relationship between the Electrorheological Effects and Electrical Properties in Barium Titanate Suspensions Y. Misono, N. Shigematsu, T. Yamaguchi and K. Negita

391

Solid-Liquid Separation and Size Classification of Ultra-fine Hematite Particles Using Bubbles K. Yamada, H. Hayashi, H. Sasaki and E. Matijevi

395

Rapid Separation of Oil Particles from Low Concentrated OAV Emulsion in the Presence of Surfactant using Surface Characteristics R. Sako, H. Ito, H. Hayashi and H. Sasaki 399 Fingering Pattern Dynamics in Magnetic Fluids Y. Enomoto

403

Coagulation of Negatively Charged Microspheres Dispersed in Cationic Surfactant Solution K. Fukada, K. Nakazato, T. Kato and M. Iwahashi 407 Nonlinear Electric Conduction in Zinc Oxide Suspension K. Negita, T. Yamaguchi, Y. Misono and N. Shigematsu

411

Heterocoagulation Behavior of PC Vesicles with Spherical Silica B. Yang, H. Matsumura, H. Kise and K. Furusawa

415

Electrokinetic Studies of Fullurene Dispersions in Aqueous Solutions of Surfactants H. Takahashi and M. Ozaki

419

Dynamic Mobility of Concentrated Suspensions in the Presence of Polyelectrolytes N. Tobori and T. Amari

423

Electrochemiluminescence Reactions of Metal Complexes Immobihzed on Surface of a Magnetic Microbead N. Oyama, K. Komori and O. Hatozaki

427

Fluorescence Spectra and Fluorescence Lifetime of Colloidal Solution of an Organic Dye, bis-MSB, and Third-order Optical Nonlinearities of Its Excitons K. Kasatani, H. Miyata, H. Okamoto and S. Takenaka 431 Supramolecular Organized Films Mechanistic Study of Model Monolayer Membranes and Their Interactions with Surfactants: Correlation to Effects on CHO Cell Cultures C. Yang, C. Ansong, L. Bockrath, J. J. Chalmers, Y.-S. Lee, M. O'Neil, J. F. Rathman and T.Sakamoto

435

Nanostructure and Dynamics of Polymers at the Interfaces by Neutron and X-ray Reflectometry E. Mouri, H. Matsuoka, K. Kago, R. Yoshitome, H. Yamaoka and S. Tasaki

439

Dynamic Cavity Array of Steroid Cyclophanes at Membrane Surface K. Ariga, Y. Terasaka, H. Tsuji, D. Sakai and J. Kikuchi

443

Mixed Langmuir Monolayer Properties of Sphingoglycolipids (Cerebrosides) and Lipids S. Nakamura, O. Shibata, K. Nakamura, M. Inagaki and R. Higuchi

447

Photoinduced Electron Transfer Processes in Polymer Langmuir-Blodgett Films T. Miyashita, S. Ugawa and A. Aoki

451

Monolayer Assemblies of Comb-Like Polymers Containing Fluorocarbons with Different Chain Length A. Fujimori, T. Araki, Y. Shibasaki and H. Nakahara 457 Self-organization of Amphiphilic Diacetylenes in Langmuir-Blodgett Films H. Tachibana, Y. Yamanaka, H. Sakai, M. Abe and M. Matsumoto

461

A Novel Understanding of Infrared Spectra of Langmuir-Blodgett Fihns by Factor Analysis T. Hasegawa, J. Nishijo and J. Umemura

465

Chemical Force Microscopies by Friction and Adhesion Using Chemically Modified Atomic Force Microscope (AFM) Tips M. Fujihira, Y. Tani, M. Furugori, Y. Okabe, U. Akiba, K. Yagi and S. Okamoto

469

In Situ Adsorption Investigation of Hexadecyltrimethylanmionium Chloride on Self-Assembled Monolayers by Surface Plasmon Resonance and Surface Enhanced Infrared Absorption Spectroscopy T. Imae, T. Takeshita and K. Yahagi

477

Molecular Assemblies Based on DNA-Mimetics:Effect of Monolayer Matrix on Photopolymerization of Diacetylene-Containing Nucleobase Monolayers K. Ijiro, J. Matsumoto and M. Shimomura

481

From Polymeric Films to Nanocapsules H. Mohwald, H. Lichtenfeld, S. Moya, A. Voigt, G. Sukhorukov, S. Leporatti, L. Dahne, A. Antipov, C. Y. Gao and E. Donath

485

The Effects of Substituents on the Aggregation of Bacteriochlorophylls CF and dp T. Ishii, H. Hirabayashi, F. Kamigakiuchi, M. Kimura, M. Kirihata, M. Kamikado, N. Tohge, Y. M. Jung, Y. Ozaki and K. Uehara

491

Dynamic Transformation of Liposomes Revealed by Dark-Field Microscopy F. Nomura, M. Honda, S. Takeda, K. Takiguchi and H. Hotani

495

Micelle-Vesicle Transition and Vesicle Size Determining Factor M. Ueno, H. Kashiwagi, N. Hirota and C. Sun

501

Active Control of Vesicle Formation with Photoelectrochemical Switching H. Sakai, A. Matsumura, T. Saji and M. Abe

505

Novel Cell Culture Substrates Based on Micro-Porous Films of Amphiphilic Polymers T. Nishikawa, J. Nishida, K. Nishikawa, R. Ookura, H. Ookubo, H. Kamachi, M. Matsushita, S. Todo and M. Shimomura

509

Nanoparticle Gold Preparation and Its Application in Biological Technology X. Y. Chen, L. Lin, Y. P. Deng, J. R. Li and L. Jiang

513

Strong Capillary Attraction between Spherical Inclusions in a Multilayered Lipid Membrane K. D. Danov, B. Pouligny, M. I. Angelova and P. A. Kralchevsky

519

Controlled Growth of Gold Nanoparticles in Organic Gels T. Yonezawa, M. Fukumaru and N. Kimizuka

525

A Model of Self-Assembling Nanoparticles due to Capillary Forces K. Yoshie, S. Maenosono and Y. Yamaguchi

529

Thin Films of Semiconductor Nanocrystals Self-Assembled by Wet Coating S. Maenosono, Y. Yamaguchi and K. Yoshie

533

Morphological Homogenization of Melamine Lipid Monolayer by Using Thermal Molecular Motion: Formation of Mesoscopic Pattem Based on Hydrogen Bonding Network T. Kasagi, M. Kuramori, K. Suehiro, Y. Oishi, K. Ariga and T. Kunitake

537

Formation and Structure of Organized Molecular Films of Fluorinated Amphiphiles with Vinyl Group A. Fujimori, T. Araki and H. Nakahara

541

Mixing Behavior of Binary Monolayer of Fatty Acid Based on -A Isotherm Measurement M. Kuramori, K. Suehiro and Y. Oishi

545

Fine Tuning of Chromophore Orientation Due to Hydrogen Bond Formation in Nucleobase-Terminated Azobenzene Monolayers M. Morisue, K. Ijiro and M. Shimomura

549

Monolayer and Bilayer Properties of Oligopeptide-Containing Lipids - Difference in Phase Transition Behavior S. Kawanami, T. Kosaka, T. Abe, K. Ariga and J. Kikuchi

553

Interactions of Sugars with Phosphatidylcholines H. Takahashi and I. Hatta

557

Study of J-Aggregate Formation of a Long-Chain Merocyanine in the Mixed LB Fihns and Their Optical Behavior M. Murata, T. Araki and H. Nakahara

561

Structure of H-Aggregate Formed in Merocyanine Dye LB Films Y. Hirano, T. M. Okada, Y. F. Miura, M. Sugi and T. Ishii

565

Impedance Analysis of Redox Polymer Langmuir-Blodgett Films A. Aoki and T. Miyashita

569

Molecular Orientation and Motion of Pyrene Molecules at the Interface of Polymer LB Films J. Matsui, M. Mitsuishi and T. Miyashita

573

Luminescence Properties of Europium Complexes in Polymer LB Films M. Mitsuishi, S. Kikuchi and T. Miyashita

577

Well-Defmed, Rigid Multiporphyrin Arrays: Interfacial Synthesis and Optical Properties in Monolayer Assemblies D.-J. Qian, C. Nakamura and J. Miyake

581

Surface Enhanced Infrared Absorption and UV-Vis Spectroscopic Study of a Monolayer Film of Protoporphyrin IX Zinc(II) on Gold Z. Zhang and T. Imae

585

Thermoreversible Vesicles with Semipolar Additives M. Gradzielski, H. Hoffmann, K. Horbaschek and F. Witte

589

Effect of L-Ascorbyl 2-Phosphates on Stability for Vesicles of Hydrogenated Soybean Lecithin S. Ban, K. Sasaki, S. Nakata and A. Kitahara

595

Novel Class of Organic-Inorganic Hybrid Vesicle "Cerasome" Derived from Various Amphiphiles with Alkoxysilyl Head K. Katagiri, K. Ariga and J. Kikuchi

599

Spin-Label Parameters of Detergent-Containing Liposomes and their Application to Micelle-Vesicle Transition H. Kashiwagi, S. Sagasaki, M. Tanaka, K. Aizawa, C. Sun and M. Ueno

603

Magneto fusion and Magnetodivision of Dipalmitoylphosphatidylcholine Liposomes H. Kurashima, H. Abe and S. Ozeki

607

Molecular Mechanism of Liposome Membrane Fusion Induced by Two Classes of Amphipathic Helical Peptides with Similar and Different Hydrophobic/Hydrophilic Balances T. Yoshimura, E. Sato, S. Lee and K. Kameyama

611

Stability of PA/PI Mixed Liposomes against Aggregation H. Minami, M. Iwahashi and T. Inoue

615

Preparation of Ultrathin Films Filled with Gold Nanoparticles through Layer-by-Layer Assembly with Polyions T. Yonezawa, H. Shimokawa, M.Sutoh, S. Onoue and T. Kunitake

619

Three Dimensional Assembly of Cationic Gold Nanoparticles and Anionic Organic Components: DNA and a Bilayer Membrane T. Yonezawa, S. Onoue and T. Kunitake

623

Preparation of Monolayers of Si02 and Ti02 Nano-Particles by Langmuir-Blodgett Technique M. Takahashi, K. Muramatsu, K. Tajima and K. Kobayashi

627

Size Controlled Mesoporous Silicate Thin Films using Block Copolymer as Template (I) T. Yamada, K. Asai, K. Ishigure, A. Endo, H. S. Zhou and I. Honma

631

Photocurrent of Purple Membrane Adsorbed onto a Thin Polymer Film: Effects of Monovalent and Divalent Ions A. Shibata, K. Yamada, H. Ikema, S. Ueno, E. Muneyuki and T. Higuti

635

Roles of Biomembranes - Effects of Surfactants Including Precursors of Endocrine Disrupters on the Interactions Between Acetylcholinesterase and Halothane I. Tsukamoto, H. Komatsu, T. Tsukamoto and N. Maekawa

639

The Effects of Alkyl Substituents and Formyl Group of Bacteriochlorophyll e on their Aggregation in Chlorosomes of Brown-Colored Photosynthetic Sulfur Bacteria H. Hirabayashi, T. Ishii and K. Uehara

643

Nanostructural Solid Surfaces Improved Molecular Models for Porous Carbons J. Pikunic, R. J.-M. Pellenq, K. T. Thomson, J.-N. Rouzaud, P. Levitz and K. E. Gubbins 647 Condensed Phase Property of Methanol in the Mesoporous Silica S. Kittaka, A. Serizawa, T. Iwashita, S. Takahara, T. Takenaka, Y. Kuroda and T. Mori

653

Structure and Relaxational Dynamics of Interfacial Water M.-C. Bellissent-Funel

657

Structural Analysis of Water Molecular Assembly in Hydrophobic Micropores Using in situ Small Angle X-Ray Scattering T. liyama, S. Ozeki and K. Kaneko

663

Synthesis, Characterisation and Chemistry of Transition Metals in Mesoporous Silica T. Campbell, J. M. Corker, A. J. Dent, S. A. El-Safty, J. Evans, S. G. Fiddy, M. A. Newton, C. P. Ship and S. Turin

667

Ethylene Hydrogenation on fee Ultra Thin Fe Films on a Rh(lOO) Surface - Effect of Co-Adsorbed CO and Growth Temperature C. Egawa, H. Iwai and S. Oki

673

Highly Isolated and Dispersed Transition Metal Ions and Oxides Studied by UV Resonance Raman Spectroscopy C. Li

677

Fluctuations in Nano-Scale Reaction Systems: Catalytic CO Oxidation on a Pt Field Emitter Tip R. Imbihl and Y. Suchorski

683

Why Copper Ion-Exchanged ZSM-5-type Zeolite Is So Active for CO Adsorption? - Comparison with Adsorption Properties of Silver Ion-Exchanged ZSM-5 Y. Kuroda, R. Kumashiro, H. Onishi, T. Mori, H. Kobayashi, Y. Yoshikawa and M. Nagao 689 Promotiong Effect of Zirconia Coated on Alumina on the Formation of Platinum Nanoparticles - Application on CO2 Reforming of Methane M. Schmal, M. M. V. M. Souza, D. A. G. Aranda and C. A. O. Perez

695

Structure Sensitivity in Reactive Carbon Dioxide Desorption on Palladium Surfaces M. G. Moula, S. Wako, M. U. Kislyuk, Y. Ohno and T. Matsushima

701

Conformational Order of Octadecanethiol(ODT) Monolayer at Gold/Solution Interface: Internal Reflection Sum Frequency Generation(SFG) Study S. Ye, S. Nihonyanagi, K. Fujishima and K. Uosaki

705

Doping Silver Nanoparticles in AOT Lyotropic Lamellar Phases X. Chen, S. Efrima, O. Regev, Z. M. Sui and K. Z. Yang

711

Modeling of the Kinetics of Metal Oxide Dissolution with Chelating Agents H. Tamura, M. Kitano, N. Ito, S. Takasaki and R. Furuichi

715

The Size-Induced Metal-Insulator Transition in Colloidal Gold P. P. Edwards, S. R. Johnson, M. O. Jones, A. Porch and R. L. Johnston

719

Design of Synthetic Glycolipids for Membrane Biotechnology M. Hato, J. B. Seguer and H. Minamikawa

725

Wetting of Ultrathin Layers of Polystyrene Studied by Atomic Force Microscopy S. Loi, M. Wind, M. Preuss, H.-J. Butt, H. W. Spiess and U. Jonas

729

Effect of Precursors on Structure of Rh Nanoparticles on Si02 support: in-situ EXAFS Observation during CO2 Hydrogenation K. K. Bando, H. Kusama, T. Saito, K. Sato, T. Tanaka, F. Dumeignil, M. Imamura, N. Matsubayashi and H. Shimada

737

Reduction of Photocurrents from Modified Electrodes with Cdi.xMnxS Nanoparticles in the Presence of Magnetic Fields H. Yonemura, M. Yoshida and S. Yamada

741

Ethylene Hydrogenation on fee Co Thin Fihns Grown on Ni(lOO) Surface C. Egawa, H. Iwai and S. Oki

745

Behavior of Pyridine on a Ti02(l 10) Surface Studied by Density Functional Theory T. Sasaki, K. Fukui and Y. Iwasawa

749

Observation of Indiviual Adsorbed Pyridine, Ammonia, and Water on TiO2(110) by Means of Scanning Tunneling Microscopy S. Suzuki, K. Fukui, H. Onishi, T. Sasaki and Y. Iwasawa

753

Three Dimensional Analysis of the Local Structure of Cu on Ti02(l 10) by in-situ Polarization-Dependent Total-Reflection Fluorescence XAFS Y. Tanizawa, W. J. Chun, T. Shido, K. Asakura and Y. Iwasawa

757

Insertion and Aggregation Behavior of PtCU between Graphite Layers M. Shirai, K. Igeta and M. Arai

761

Surface Properties of Silica-Titania and Silica-Zirconia Mixed Oxide Gels S. Ikoma, K. Nobuhara, M. Takami, T. Nishiyama, M. Nakamura and S. Kaneko

765

Characteristics of Supported Gold Catalysts Prepared by Spray Reaction Method L. Fan, N. Ichikuni, S. Shimazu and T. Uematsu

769

STM Observation of Oxygen Adsorption on Cu(l 11) T. Matsumoto, R. Bennett, P. Stone, T. Yamada, K. Domen and M. Bowker

773

Characterization of Si-O-C Ceramics Prepared by the Pyrolysis of Phenylsilicones Y. Tanaka, C. Mori, N. Suzuki, T. Kasai, K. limura and T. Kato

777

Structure and Growth Process of Niobium Carbide on Silica N. Ichikuni, F. Sato, S. Shimazu and T. Uematsu

781

In situ Energy-Dispersive XAFS Study of the Reduction Process of Cu-ZSM-5 Catalysts with 1 s Time-Resolution A. Yamaguchi, Y. Inada, T. Shido, K. Asakura, M. Nomura and Y. Iwasawa

785

Preparation and CO Hydrogenation Activities of Smectite-Type Catalysts Containing Cobalt Divalent Cations in Octahedral Sheets M. Shirai, K. Aoki, S.-L. Guo, K. Torii and M. Arai

789

Preparation of M0O3 by Spray Reaction Method and Photometathesis of C3H6 H. Murayama, N. Ichikuni, S. Shimazu and T. Uematsu

793

Molecular Mobility of Hydrogen-Bonded Acetonitrile on Surface Hydroxyls of MCM-41 with Kubo-Rothschild Analysis H. Tanaka, A. Matsumoto, K. K. linger and K. Kaneko 797 A Comprehensive Study of Surface State of MCM-41 having a Good Surface Crystallinity and Its Reactivity to Water Vapor T. Mori, Y. Kuroda, Y. Yoshikawa, M. Nagao and S. Kittaka 801 Vaporization and Oxidation of Poly- -Olefin on Metal Plates K. Hachiya

805

Molecular Geometry-Sensitive Filling in Micropores of Copper Complex-Assembled Microcrystals D. Li and K. Kaneko

809

Fluorescent Solubilizates in the Silica-Surfactant Composite Films K. Hayakawa, N. Fujiyama and I. Satake

813

Backbone Orientation of Adsorbed Polydimethylsiloxane I. Soga and S. Granick

817

Adsorption of Albumin on Organically-Modified Silicas (Ormosils) in Aqueous Solutions H. lyanagi, K. Yamane and S. Kaneko 821 Determination of the Acid-Base Properties of Surfaces by Contact Angle Titration with Buffered and Unbuffered Solutions H. Sakai

825

Sorption of Uranium(VI) on Na-Montmorillonite Colloids - Effect of Humic Acid and its Migration S. Nagasaki

829

Contribution of Preformed Monolayer to Micropore Filling T. Ohba, T. Suzuki and K. Kaneko

833

Magnetic-Field Control of Oxygen Adsorption H. Sato, Y. Matsubara and S. Ozeki

837

Viologen Monolayers: Dynamics on Electrode Surfaces T. Sagara, H. Tsuruta, Y. Fukuoka, S. Tanaka and N. Nakashima

841

Fluorescence Specific Micro Patterns in Two-Dimensional Ordered Arrays Composed of Polystyrene Fine Particles S. I. Matsushita, T. Miwa and A. Fujishima

845

Surface Force Measurement of Alumina Surfaces: Effect of Polyelectrolyte on the Dispersiveness of Aqueous Alumina Suspension R. Ishiguro, O. Sakurada, K. Kameyama, M. Hashiba, K. Hiramatsu and Y. Nurishi

849

Surface Properties and Photoactivity of Silica Prepared by Surface Modification M. Fuji, N. Maruzuka, T. Takei, T. Watanabe, M. Chikazawa, K. Tanabe and K. Mitsuhashi

853

Stability for Compressing Adsorbed Layers at Solid-Liquid Interface by the AFM Probe T. Kan-no, M. Fujii and T. Kato

857

Analysis of Bonding Nature being Operative in the M(Li, Na, K)Ion-Exchanged Zeolites CO Adsorption Systems R. Kumashiro, Y. Kuroda, H. Kobayashi and M. Nagao 861 Morphology of Octadecyltrimethylanmionium Halides Aggregates Adsorbed on Mica M. Fujii, T. Hasegawa and T. Kato

865

Photoinduced Long-Range Attraction between Spiropyran Monolayers Studied by Surface Forces Measurement Y. Nakai and K. Kurihara 869 Solid Monolayers of Simple Alkyl Molecules Adsorbedfromtheir Liquid to Graphite: the Influenece of Different Chemical Groups M. A. Castro, S. M. Clarke, A. Inaba, A. Perdigon, A. Prestidge and R. K. Thomas

873

Sorption Mechanism of lOs" onto Hydrotalcite T. Toraishi, S. Nagasaki and S. Tanaka

877

Thickness Dependence of Absorption of Molecular Thin Films Studied Using FECO Spectroscopy T. Haraszti, K. Kusakabe and K. Kurihara

881

Sorption of Neptunium on Surface of Magnetite K. Nakata, S. Nagasaki, S. Tanaka, Y. Sakamoto, T. Tanaka and H. Ogawa

885

Two Dimensional Auto-Organized Nanostructure Formation of Acid Polysaccharides on Bovine Serum Albumin Monolayer and Its Surface Tension S. Xu, T. Nonogaki, K. Tachi, S. Sato, I. Miyata, J. Yamanaka and M. Yonese

889

Trapping Behavior of Water on Metal Oxide and Active Desorption K. Chiba, T. Yoneoka and S. Tanaka

893

Effects of Adsorbed Water upon Friction at Layered K4Nb60i7-3H20 Surfaces Studied with FFM T. Sugai and H. Shindo

897

Sorption Behavior of Strontium onto C-S-H (Calcium Silicate Hydrated Phases) T. Iwaida, S. Nagasaki and S. Tanaka

901

Characterization and Direct Force Measurements of Fluorocarbon Monolayer Surfaces S. Ohnishi, V. V. Yaminsky and H. K. Christenson

905

Adsorption of Naphthalene Derivatives on Water-Soluble Polynuclear Aromatic Molecules Derived from Carbon Black K. Kamegawa, M. Kodama, K. Nishikubo and H. Yoshida

909

xxi

The Addition of Water and Alcohol to Alkenes by Alky-Immobilized Zeolite Catalysts in the Liquid Phase H. Ogawa, T. Hosoe, H. Xiuhua and T. Chihara

913

Magnetic Effects on Li' ExtractiodInsertion Reactions in Spinel-Trype Manganese Oxides Y. Kawachi, I. Mogi, H. Kanoh, K. Ooi, and S. Ozeki

917

Proton Conductivity and Water Adsorption Behavior of Complex Antimonic Acids K. Ozawa. Y. Sakka and M. Amano

92 1

Industrial Applications and Products High Performance Thin Lithium-Ion Battery Using an Aluminum-Plastic Laminated Film Bag T. Ohsaki, N. Takami, M. Kanda and M. Yamamoto

925

Surface Reactions of Carbon Negative Electrodes of Rechargeable Lithium Batteries Z. Ogumi, M. Inaba, T. Abe and S.-K. Jeong

929

Lithium Intercalation Mechanism of Iron Cyanocomplex N. Imanishi, T. Horiuchi, A. Hirano and Y. Takeda

935

New Lithium Insertion Alloy Electrode Materials for Rechargeable Lithium Batteries T. Sakai, Y. Xia, T. Fujieda and K. Tatsumi

939

Studies on the Interaction between Underpotentially Deposited Copper and 2,SDimercapto- 1,3,4-thiadiazole Adsorbed on Gold Electrode S. A. John, 0. Hatozaki and N. Oyama

943

Study of the Evolution of the LiElectrolyte Interface during Cycling of LiPolymer Batteries C. Brissot, M. Rosso, J.-N. Chazalviel and S. Lascaud

947

Analyses of the Preferential Oxidation of Carbon Monoxide in Hydrogen-Rich Gas over Noble Metal Catalysts Supported on Mordenite H. Igarashi, H. Uchida and M. Watanabe

953

Effects of Microstructure in Catalyst Layer on the Performance of PEFC J. Morita, E. Yasumoto, Y. Sugawara, M. Uchida and H. Gyoten

959

Ag-Etching Technique Based on Chemical Wet Process K.-S. Lee, J.-E. Park and S.-G. Park

963

Electrochemical Recognition of Ions with Self-Assembled Monolayers of Quinone Derivatized Calixarene Disulfide H. Kim, J. Kim, H. Lim, M.-J. Choi, S.-K. Chang and T. D. Chung

967

Milk Protein Adsorbed Layers and the Relationship to Emulsion Stability and Rheology E. Dickinson

973

Microscopic and Macroscopic Phase Transitions in Polyelectroyte-Micelle Systems P. L. Dubin

979

Self-Organization of Sucrose Fatty Acid Ester in Water K. Aramaki, H. Kunieda and M. Ishitobi

985

Development and Application of Microbial Transglutaminase Y. Kumazawa, T. Ohtsuka, K. Seguro and N. Nio

989

Fat Particle Structure and Stability of Food Dispersions D. T. Wasan, S. Uchil, A. D. Nikolov and T. Tagawa

995

The Investigation of Sodium N-Acyl-L-Glutamate and Cationic Cellulose Interaction N. Yamato, D. Kaneko and R. Y. Lochhead

1001

Properties of Aggregates of Amide Guanidine Type Cationic Surfactant with 1-Hexadecanol Adsorbed on Hair M. Arai, T. Suzuki, Y. Kaneko, M. Miyake and N. Nishikawa

1005

Preparation of 0/W/O Type Muhiple Emulsions and Its Application to Cosmetics T. Yanaki

1009

Visualization and Analysis of lontophoretic Transport in Hairless Mouse Skin B. D. Bath, J. B. Phipps, E. R. Scott, O. D. Uitto and H. S. White

1015

Development of New High Oil Contained Powder {Powder Gel) and Application to Powder Make-up H. Hotta, Y. Yago, R. Tsuchiya, M. Sasaki, H. Sugasawa, K. Minami, T. Minami and T. Suzuki

1021

Multiphase Emulsions by Liquid Crystal Emulsification and Their Application T. Suzuki, K. Yoda, H. Iwai, K. Fukuda and H. Hotta

1025

Rheology Studies to Investigate Sensorial Aspects of Emulsions K.-P. Wittem, R. Brummer and S. Godersky

1031

Effect of Chemical Structure on Aggregate Properties and Drag Reduction Behaviors of Quaternary Ammonium Salt Cationic Surfactant Solutions T. Horiuchi, T. Majima, T. Tamura, H. Sugawara and M. Yamauchi

1037

Interpretation of Foam Performance of Aerosol Type Glass Cleaner in Terms of Dynamic Surface and Interfacial Tensions M. Tanomura, Y. Takeuchi and Y. Kaneko

1041

NMR Specification of Lipid Bilayer Interfaces as Drug Delivery Sites E. Okamura, R. Kakitsubo and M. Nakahara

1045

Liquid/Liquid Extraction a New Alternative for Waste Water Remediation M. J. Schwuger, G. Subklew and N. Woller

1049

Characterization of Surfactants Used for Monodispersed Oil-in-Water Microspheres Production by MicroChannel Emulsification J. Tong, M. Nakajima, H. Nabetani and Y. Kikuchi

1055

Continuous Formation of Monodispersed Oil-in-Water Microspheres Using Vertically Mounted MicroChannel System I. Kobayashi, M. Nakajima and Y. Kikuchi

1061

X-ray Diffraction Study on Mouse Stratum Comeum N. Ohta, I. Hatta, S. Ban, H. Tanaka and S. Nakata

1067

Application of Self-Organizing Silicone Pol>iners for Long-Wearing Lipsticks M. Shibata, M. Shimizu, K. Nojima, K. Yoshino, H. Hosokawa and T. Suzuki

1071

Adsorption of Diols on Silica Gel Surface and Their Reactivities for Selective Monoacylation with Acetyl Chloride H. Ogawa, Y. Ide and T. Chihara

1075

Supported Liquid Film Catalyst and Biphasic Catalysis Using Water Soluble Metal Complexes in a Medium of Supercritical Carbon Dioxide B. M. Bhanage, Y. Ikushima, M. Shirai and M. Arai

1079

Effect of Oligosaccharide Alcohol Addition Concerning Translucent AI2O3 Produced by Slip Casting Using Gypsum Mold Y. Hotta, T. Banno, S. Sano, A. Tsuzuki and K. Oda

1083

Coal-Oil-Water Mixture Prepared by Disintegration of De-Ashed Coal Agglomerates H. Takase

1087

Author Index

1091

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Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) @ 2001 Elsevier Science B.V. All rights reserved.

Structure of Liquid/liquid Interfaces Studied by Ellipsometry and Brewster Angle Microscopy Gerfiard H. Findenegg, Jens Schulz and Steflfen Uredat Iwan-N.-Stranski Institut ftir Physikalische und Theoretische Chemie, Technische Universitat Berlin, StraBe des 17. Juni 112, D 10623 Berlin, Germany [email protected] Fax: +49 30 314 26602 The potential of ellipsometric measurements and the ellipsometric imaging technique for the study of structural properties of liquid/liquid interfaces is demonstrated. The ellipticity p of near critical interfaces in binary systems near an upper or lower critical solution point has been analyzed in terms of the Fisk-Widom profile n{z) of diffuse interfaces as well as theories of rough interfaces based on the coupling of thermal capillary waves. Phase transitions in Gibbs monolayers of simple amphiphiles (fatty alcohols and related substances) adsorbed at oil/water interfaces have been investigated by Brewster angle microscopy. The observation of domain patterns of the two coexisting phases provides direct evidence and unexpected aspects of the monolayer phase transition at this liquid/liquid interface. 1

INTRODUCTION

The interface between two liquids represents an inhomogeneous region in which local properties such as the number density or composition change in a smooth way fix)m the bulk value in phase a to that in phase (3. This feature becomes quite obvious in two-phase systems with high mutual solubility of the majority components. Such a situation arises when the constituent phases a and (3 are close to their consolute critical point. Near-critical interfaces have attained great interest since the days of van der Waals [1], and much progress towards a universal theory of simple fluid interfaces has been made in recent years [2, 3]. Apart from the structure normal to the interface, the interface may exhibit a lateral inhomogeneities on the length scale of micrometers or more. This case is met with adsorbed monolayers of amphiphiles at interfaces between chemically dissimilar liquids, such as oil and water. Within these monolayers the adsorbed molecules may be arranged in different ways, reminiscent of liquid, gaseous and liquid crystaline states of bulk phases. Like bulk-phases these interface phases undergo temperature dependent phase transitions. In two-phase coexistence regions the different monolayer phases cover patches of the interface with lateral extensions on a micrometer scale. Fluid interfaces are easily perturbed by mechanical devices as required in classical methods to study interfacial tensions and related properties of the interface. Noninvasive methods need to be used in the study the equilibrium structure of interfaces. In principle, reflectometiy of X-rays or neutons can yield detailed information about the interfacial composition profile on a molecular length scale [4, 5]. However, only recently

it has been possible to ^ply these methods to liquid/liquid interfaces [6]. Reflection ellipsometry of light represents a less demanding technique and is very useful for studying structural properties of interfaces in optically transparent liquid systems [7]. EUipsometry measures the state of polarization of light reflected at the interface (expressed by the coefficient of ellipticity), which is related to the profile of the refractive index in the direction perpendicular to the interface. Like X-ray and neutron reflecticity, ellipsometry is sensitive to the properties of the interfacial profile on a molecular length scale. In this article we will illustrate, how ellipsometric measurements can be used to elucidate structural features of different types of liquid/liquid interfaces: On the one hand, the thickness and composition profile of interfaces between coexistent nearcritical phases (critical interfaces) can be studied as a function of the distance from the critical solution temperature. Such systems are believed to constitute examples of weakly inhomogeneous interfaces. On the other hand, Brewster angle microscopy (BAM), a space-resolved imaging ellipsometric technique, was adapted to visualize lateral domain structures in monolayers of amphiphiles adsorbed at an oilAvater interface. Although BAM had been used successfully to study monolayer films of amphiphilic molecules at the free surface of water during the last decade [8], its application to the study of liquid/liquid interfaces is a relatively new technique developed in our laboratory. It was used to study monolayer phase transitions in Gibbs films of simple amphiphiles at the hexane/water interface. Some results of that study will also be discussed in this article. 2 STRUCTURE OF INTERFACES AND INTERACTION WITH LIGHT As explained in the Introduction, fluid interfaces do not represent sharp discontinuities at which the properties change abruptly in a step-like manner. Instead, real interfaces are best understood as inhomogeneous regions with non-vanishing gradients of the local number density and composition. These changes imply a characteristic profile of the refractive index and dielectric permittivity e in the direction normal to the interface, which in turn causes specific polarization properties of light reflected at the interface. In ellipsometric measurements a wave of polarized hght is reflected off the interface and the state of polarization of the reflected wave is analyzed. Generally, elliptically polarized light can be treated as the sum of two components, polarized in the plane of incidence (p-polarized) and perpendicular to this plane (s-polarized). Upon reflection, the amplitude and phase of the two components is altered. These changes in ampUtude and phase are represented by the complex reflection amplitudes, Vp = ppe*^^ and ^5 = PsC**' for p - and s-polarization, respectively, where p is the absolute amplitude and 5 is the phase of the wave. Ellipsometry measures the ratio of these two coefficients. It has its highest sensitivity at the Brewster angle OB^ where Ke(rp/rs) = 0, since at this angle of incidence tiiere is no background Fresnel contribution from the bulk phases. For sharp profiles (as compared with the wave length of light. A) the following relation was derived by Drude [9]:

= Im TT y/Cg -h 6f3 ^ ( g a - e{z)) (ejz)

X Ca - 60

J

- Ep) ^^

^^^

e(z)

— oo

where e = n^ represents the optical dielectric permittivity (square of the refractive index n) at the given wave length; Sa and cp denote the values in the two bulk phases (using the convention that the light wave is incident through phase /?), and £{z) is the dielectric permittivity along the normal coordinate z. The Dnide equation can be used in two different ways to attain information about the structure of interfaces: • Equation (1) allows to calculate p for parametric profile functions e(z) resulting from theoretical models of the interface structure, and to compare these theoretical predictions with experimental ellipticities. • In a more qualitative approach. Equation (1) can be used to translate trends of experimental p data (say, variations of p with temperature or composition) into changes of the interface profile 6(z), based on plausible qualitative models of the interfacial profile. Examples of the analysis of ellipsometric data for liquid/liquid interfaces in terms of the Drude equation are presented in the following section. 3 CRITICAL INTERFACES IN BINARY SYSTEMS Measurements of the absolute ellipticity were carried out in liquid/liquid systems close to the consolute critical point. Here the effective thickness of the inhomogeneous region between the bulk phases is proportional to the correlation length ^ of the critical fluctuations within the bulk phases. In this article we summarize some results for the interface between the constituent phases of phase-separated two-component systems close to the consolute critical point, at which the two phases become identical. For such interfaces, which are known as critical interfaces, one expects a marked increase of the thickness of the inhomogeneous region and/or an increasing roughness of the interface as the critical point is approached, and this increase of the effective interface thickness should cause a pronounced increase of p. Below it will be shown that this conjectured behavior is indeed observed. Critical interfaces in binary systems may exhibit either an upper critical solution temperature (UCST) or a lower critical solution temperature (LCST). Here we summarize briefly results for a nonaqueous system, hexane + perfluorohexane (CeH^ + CeF^), which exhibits an UCST, and an aqueous system of a short-chain amphiphilic compound, iso-butoxyethanol (i-C4Ei+water), wiiich exhibits a LCST, As the critical point

0.05 0.04 0.03

200

^\ _

160 El20

c 0.02

-/

0.01

f

^ ^

o

1

40 j_

)

h

9

t

0.01

0.02

0.03

^0

0.(

°

(T-TJ/Tc

0.01

0.02

0.(

(T-Tc)/Tc

Figure 2. 2-C4Ei + Water: Interfacial tension cr as a function of the temperature variable t = (T - Tc)/Tc and a fit of the data by cr = (TQI^ with ^=1.24.

Figure 1. i-C4Ei + Water: Refractive index increment An = np - Ua as a function of the temperature variable t = (T - Tc)/Tc and a fit of the data by An = Btf^ with ^=0.325.

is approached from the two-phase region, the mutual solubility of the two components increases and thus the compositions of the coexistent phases a and (3 become more and more alike, until at the critical point the two phases have become identical and the interfacial tension has disappeared. This behavior is exemplified in Figure 1 and Figure 2, where the temperature dependence of the refractive index increment An = rip — ria and of the interfacial tension a between the near-critical phases a and P of the system i-C4Ei+water is plotted as a function of the reduced temperature increment t = {T - Tc)/Tc, where Tc is the critical temperature. In the near-critical region for T -> T^ (t -> 0) the variation of An and a can be expressed by imiversal power laws, viz.. An

=

Tip —

UQ

= B t^

(2) (3)

where B and CTQ are material constants, while the critical exponents ^ and /i are expected to be universal constants for which modem theories of critical phenomena predict the values 0.325 and 1.26, respectively. In all cases studied by us these power laws are obeyed within experimental accuracy. Best-fit values for the parameters of the present systems are given in Table 1. Results for the ellipticity p of the critical interface a/0 of the two systems are presented in Figure 3. In both systems the magnitude of p increases progressively as the critical point is approached (t -^ 0). This behavior can be attributed to the growing thickness of the inhomogeneous interfacial region. Van der Waals theories [1, 2] relate the thickness of fluid interfaces to the correlation length ^ of critical composition fluctuations in the bulk phases. Following Fisk and Widom [10] the intrinsic composition

profile of the interface, expressed by the profile of the refractive index n, can be represented by a scaling function P{x) in the reduced length coordinate x = z/2^:

n{x) = where P(.)

=

-{ua-\-np)-\--{ria

- Ufs) P{x)

f^-M-) ^3 - tanh^(a:)

(4)

Inserting this profile function into the Drude equation (1) yields an expression for the ellipticity pi of the diffuse interface in terms of the correlation length ^ and refractive index increment Ua - np of the coexistent bulk phases,

where we have used the relation e = n^. Note that pi is proportional to the product of the refractive index increment An and the correlation length f of composition fluctuations in the bulk phases. These two quantities exhibit an antagonistic behavior in the critical region: while An tends to zero, the correlation length f increases and tends to infinity for t -^ 0 as The above power laws for An and f imply that the ellipticity resulting from the intrinsic profile of the interface will increase in magnitude and diverge as a power law PiCx{n^-n0)^

= B^ot-^''-^^

with 1/-/3 = 0.305. To test the above relation for p„ the correlation length ^ of the present systems was determined by turbidity measurements in the critical region. Best-fit values of the correlation length amplitude ^o are given in Table 1. Results for pi derived from the Fisk-Widom expression on the basis of our independent determinations of B and fo are shown as curve a in Figure 3. In these and other systems studied by us the Fisk-Widom theory yields a reasonable representation of the measured ellipticity data. Table 1. Parameters B^ CTQ and ^o in the power laws of the refractive index increment An, interfacial tension a, and correlation length ^, of binary systems near their critical solution temperature. The corresponding values of the critical exponents are /?=0.32; /i=1.26 and t/=0.63. _«.«««_ System B ao/mJm'^ ^o/^m z-C4Ei+Water CeHn+CeFn

0.116±0.005 0.218 ± 0.005

13.9±1.0 30.3 ± 1.0

0.35±0.02 0.20 ± 0.02

•

c

b

d

•

Te

5:^ Of

L

L.

0.01

0.02

0.03

-0.03

(T-Tc)/Te

-0.02

-0.01

(T-Tc)/Tc

Figure 3. Temperature dependence of the ellipticity p for two systems with critical interfaces near the critical temperature Tc. (left) 2-C4Ei+Water near the LCST; (right) CeH^+CeFH near the UCST. The curves represent calculations of p based on the Fisk-Widom theory of the intrinsic profile (eqn. 4; curve a), a combination of the Fisk-Widom theory and the capillary wave contribution (eqn. 6; curve d), and more sophisticated theories of critical interfaces). although close to the critical point pi determined in this way somewhat underestimates the experimental ellipticities. A different picture of fluid interfaces arises from the theory of thermally driven capillary wave fluctuations [11]. Unlike van der Waals theories, which treat interfaces as diffuse but flat, capillary wave theories consider the interface as sharp but rough. On the basis of the mode-coupling theory by Meunier [11] the ellipticity arising from the superposition of thermal capillaiy waves is 4.38 na-\-n0

(6)

Commonly the capillary wave expression p^w is of similar magnitude as p, (Equation 5) and, except for the weakly temperature-dependent prefactors, the two expressions exhibit the same temperature dependence. The effect of the capillary wave contribution to the ellipticity is shown in Figure 3 where the sum of the two contributions, pi + ^cw is shown as curve d. For most systems studied by us [12] it is found that pi +/Ocw somewhat overestimates the experimental ellipticities while pi, the ellipticity resulting from the intrinsic profile alone, falls somewhat below the experimental values. More sophisticated theories yield good agreement with the experimental ellipticities (curves 6 and c in Figure 3) without changing the underlying physical picture outlined above [12]. 4 PHASE TRANSITIONS IN GIBBS MONOLAYERS 4.1

Visualization of lateral structures

Soluble amphiphiles are preferentially adsorbed at oil/water interfaces. The decrease of the surface excess free energy due to this process can directly be monitored by measurement of the interfacial tension a. As the amphiphilic component is soluble

in one (or both) of the bulk phases, the adsorbed layer is always in thermodynamic equilibrium with the solution. The surface excess concentration F^^'^^ of the adsorbed amphiphile can be derived from concentration-dependent measurements of the interfacial tension using the Gibbs equation which, for constant temperature, yields r(i,2) "

L _ ^ RTdlnc

'

m ^ ^

where c denotes the equilibrium concentration of the amphiphile in one of the bulk phases. Adsorbed monolayers of amphiphiles are called Gibbs films. Gibbs films exhibit monolayer phase transitions in which the surface (excess) concentration shows discontinuous changes as a fimction of bulk concentration or temperature. Such phase transitions are causing a discontinuity of the slope of the interfacial tension as a fimction of temperature at the transition temperature T<: At temperatures above the phase transition the interfacial tension
A.<''^) = - ^

(8)

On the basis of this relation, a discontinuity of the slope of a vs. temperature can be ascribed to afirst-orderchange of the entropy of adsorption, which is a signature of phase transitions of the Gibbs film. At low temperatures the film is believed to be in a condensed state, where the adsorbed molecules are arranged as a dense monolayer of solid-like or liquid-like order, while at high temperatures the monolayer film is in a dilute gas-analogue state. However, so far there has been no direct proof of such monolayer phase transitions at liquid/liquid interfaces. In particular, nothing is known about the coexistence of gas-like and condensed monolayer phases, or about the morphology of the liquid-like domains at two-phase coexistence in the film. Monolayer phase transitions are well known from films of insoluble amphiphiles, so called Langmuir films, where the film density can be varied by compression with movable barriers. Such monolayers are usually examined at the free surface of water. Unlike the situation of Gibbs films, Langmuir films are not in thermodynamic equilibrium with the subphase. Langmuir films have been subject of intensive studies for several decades, and there is a wide variety of methods for their investigation, including imaging techniques, that give information on the film morphology at a lateral resolution of an optical Ught microscope (approx. 5 /im). The most favorable technique is Brewster angle microscopy (BAM) which, in principle, can be applied to Langmuir films as well as Gibbs films [13]. However, all commercial instruments so far are suitable only for investigations at fiee liquid surfaces, not for liquid/liquid interfaces. In order to overcome this limitation we have adapted the BAM technique to the requirements

met with adsorption layers at liquid/liquid interfaces. A detailed description of the instrumental setup has been published elsewhere [14, 15]. The instrument combines three features: (i) A BAM with a lateral resolution of ca. 5 ;xm; (ii) measurement of the integral reflectivity of p-polarized light; (iii) a capillary wave spectrometer to monitor changes of the interfacial tension in situ. In a typical experiment the temperature of a sample with constant composition is ramped up or down, while the interface is observed by the BAM. At the same time the capillary wave spectrometer monitors the power spectrum of light scattered by interface capillary waves. The resulting capillary wave spectra are fitted by a single Lorentzian function. Neglecting intrumental broadening effects, the center frequency of the Lorentzian function I/Q is related to the interfacial tension a by [16]

-o = = ;1-f ^/-^ /-^ 27rVpi4

,

(9)

where pi and p2 denote the mass densities of the two bulk phases. Monitoring i/max thus provides a means to trace the interfacial tension during the course of an experiment. The new device was used to study Gibbs films of several long-chain alcohols at the n-hexane/water-interface. 4.2

Systems

Monolayer phase transitions of Gibbs films at water/oil interfaces have been reported for several higher alcohols, such as octadecanol [17] (CisOH), 1,1,2,2 tetrahydroperfluorododecanol [18] (FC12OH), and cholesterol [19] at the n-hexane/waterinterface. These substances are soluble in n-hexane and virtually insoluble in water, allowing an easy interpretation of measurements according to equations (7) and (8), as the composition of the oil phase can easily be adjusted by the initial composition, while the aqueous phase can be regarded as pure water. Some results of a recent BAM study of these three systems are presented below. A full account of this work is given elsewhere [14]. Temperature dependent measurements of the interfacial tension of geraniol at the hexane/water interface revealed a behavior similar to that of the systems mentioned above. Therefore this system was studied with the new apparatus too (cf. Figure 4). 43

Results

The interfacial tension of the chosen alcohols at the hexane/water interface were measured as a function of temperature for several concentrations of the alcohol in hexane, using the pendant drop technique. Representative results for the chosen alcohols are shown in Figure 4. As can be seen in that figure, the graphs for FC12OH and CisOH exhibit a sharp kink, indicating the respective monolayer transition temperature Tt. Note that the graphs for geraniol and cholesterol do not exhibit such a sharp kink.

S

35-^

—, 20

1

r-

—I 35

/°C

Figure 4. Interfacial tension vs. temperature curves for A geraniol (x = 3.4-lO"'*), • cholesterol (x = 1.010-^), D FC12OH (x = 7.310-^) and • CigOH (x = 1.2M0-3). Presumed transition temperatures Tt are indicated by arrows. indicating that for these substances the state of the monolayer film changes in a more gradual manner than for the former substances. BAM investigations were made along a stepwise temperature scan in a temperature range above and below the expected transition temperature. Temperature steps of 1 K were chosen, and the system was allowed to equilibrate for 30 minutes after each temperature step. In each case measurement started with a descending temperature ramp, followed by an ascending scan after equilibration at the lowest temperature. 43.1

FC12OH

The average reflected intensity Ir for p-polarized light incident at the Brewster angle of the system is expected to be quite small at temperatures above Tt and higher below Tt. A typical measurement of this quantity is shown in Figure 5 for FC12OH at the hexane/water interface. For a first-order transition we would expect a stei>wise change of /^ at the transition temperature, perhaps with some hysteresis between descending and ascending temperature scans. However, such a step-wise change is not observed. Instead, the experiments yield a gradual increase of /^ with decreasing temperature, as shown in Figure 5. This increase commences at temperatures well below the nominal transition temperature as determined ex situ by interfacial tension measurements (Tt - T ^ 4 K). \^1thin experimental resolution, no hysteresis of Ir is observed. The capillary wave spectra taken in parallel with these measurements (cf Figure 5) show a change in the temperature behavior at 24 ""C, in good agreement with the transition temperature determined ex situ by the pendant drop measurements (cf Figure 4). BAM micrographs taken during the temperature scan reveal the appearance of circular domains of the condensed monolayer phase within a matrix of the gas-analogue phase (cf. Figure 6a). Domain formation starts approximately 4 K below the transition

10 908070605040-

•

1

1

*

\\

302010-

a /•> a -^5000 J

Q/i^ •

1

x^

kr

r ^

Xy'

* « « « « # «««iyarti 15

20

25

J 4000

30

T / 'C

Figure 5. Reflected intensity /^ (•,•) and peak frequency I/Q of capillary wave spectra (•), Gibbs film of FC12OH at the hexane/water interface vs. temperature T: full symbols, stepwise decrease of T; open symbols, stepwise increase of T.

Figure 6. BAM-images of a Gibbs film of FC12OH at different temperatures (see Fig. 5). a: 20^C, b: 12^C, c: 15°C. (a,b) stepwise decrease of T; (c) stepwise increase of T. temperature as determined by interfacial tension measurement and causes the observed increase of reflected intensity shown in Figure 5. On further decreasing temperature the area between these domains becomes smaller and finally the entire area of the interface is covered by a neat condensed film (Figure 6b). As the temperature is now increased, the domain pattem again occurs (Figure 6c), indicating that this domain morphology represents an equilibrium state of the system. 43.2

CisOH

Gibbs films of CigOH at the hexane/water interface exhibit a behavior similar to that of FC12OH layers. However, due to the favourable optical contrast, a higher resolution of the BAM micrographs can be obtained in this case as compared with

II

p#-^le Figure 7. BAM images of a Gibbs film of CigOH below the phase transition temperature. Contrast inversion resulting from a rotation of the analyzer (white bar represents a length of lOO^um) the former system. Here again, domains of the condensed phase can be observed at temperatures well below Tt. Unlike with FC12OH, circular domains of the condensed phase are found to coexist with extended (continuous) condensed regions (Figure 7). As shown in Figure 7, the contrast between the domains of condensed and dilute phase can be inverted by rotating the analyzer of the instrument. However, these changes of the analyzer setting do not lead to any sub-pattems within regions appearing in homogeneous brightness in Figure 7. This fact indicates that the observed domains are uniform in thickness and density, supporting the conjecture that they indeed represent monolayer domains of the alcohol film. (Otherwise, if the film would exhibit a non-imiform thickness profile, the local angle of incidence within a domain would be non-uniform and thus no sharp contrast inversion would occur on rotating the analyzer.) 43.3

Cholesterol and geraniol

Gibbs fihns of cholesterol and geraniol were found to show a similar behavior at the hexane/water interface. Results obtained for these two substances will be summarized here only briefly. As seen from Figure 4 the changes in the temperature dependence of the interfacial tension are much less pronounced for these substances than for FC12OH and CigOH. The BAM micrographs do not give any evidence of a true phase transition in the Gibbs fihns of these substances. Although a stepwise decrease of temperature leads to the formation of domain patterns below Tt (Figure 8), these domains are not stable and disappear a few minutes after the temperature step. It was found that the transient domains are more pronounced for greater temperature steps, but in any case they are unstable and disappear with time. No transient domains were observed for ascending temperature steps.

:>! Figure 8. Brewster angle micrographs of cholesterol (x = 7.110"^, after a rapid decrease of temperature from 35 to 20X) and geraniol (x = 3.4-10-^, after a rapid decrease of temperature from 25 to 22.5°C) 4.4 Discussion The observation of domain patterns in the BAM micrographs of Gibbs monolayers of the two simple alcohols at the hexane/water interface provides direct evidence of a phase transition and two-phase coexistence within Gibbs monolayers. Nontheless our findings provoce several questions as to the nature of these transitions: • If the observed transition represents a true first-order phase transition, two-phase coexistence should occur only at a singular transition temperature. So why does two-phase coexistence extend over a rather wide temperature range? • Why do the domain patterns appear only at a temperature some Kelvins below the transition temperature Tt as obtained by interfacial tension measurements? • Why are the patterns observed in films of cholesterol and geraniol unstable? A two-phase coexistence region extending over a finite range of temperature can be explained by assuming the presence of insoluble amphiphilic impurities. Assume, for simplicity, that the alcohol (A) contains a single surface active impurity (B) which is insoluble in the condensed surface phase of A, while A and B form an ideal mixture in the dilute (gas-like or expanded) surface phase. In this case the mole fraction of the impurity in the dilute surface phase (x%) is given approximately by [14] AH (Tt - T) (10) RTtT where AH is the molar enthalpy of the surface phase transition of pure component A and Tt is the transition temperature in the absence of the impurity. According to this simple relation the mole fi^action of the impurity in the dilute phase must increase as T is decreased. Now, when temperature is lowered both components will be adsorbed fix)m the subphase but while the majority component (A) can be accommodated in the condensed phase, the impurity (B) remains in the dilute phase. Accordingly, as the temperature is lowered the fi-action of surface covered by the condensed phase will increase while in the remainmg part of the surface the mole fraction of the impurity will increase in accordance with equation (10). In(l-x^):

13

The above conjecture does not explain why interfacial tension measurements indicate a phase transition temperature greater than the temperature at which domains of the condensed phase can be detected by BAM. Furthermore, we do not understand why condensed phase patterns of cholesterol and geraniol are unstable. These open questions indicate that phase transitions in Gibbs monolayers are more complicated than thought up to now, and our findings show the need for further experimental and theoretical work in this field. On the other hand, the results outlined above also show that BAM is indeed a useful tool for the study of Gibbs films at liquid/liquid interfaces. Future studies will take advantage of this new technique. Acknowledgement. The authors wish to thank Dr. P. Marczuk and B. Paeplow for help with the interfacial tension measurements. Discussions with Prof. S. Dietrich (Wuppertal) and Prof. B. Law (Manhattan, Kansas, USA) are gratefully acknowledged. This work was supported by the Deutsche Forschimgsgemeinschaft (DFG) under grant FI 235/11 as part of the Priority Program SPP 728 'Transportmechanismen " er fluide Phasengrenzen' . References 1. J.D. van der Waals, Verhandel Konink Akad Weten. Amsterdam (Sect. 1), Vol. 1, No. 8 (1893); Engl, translation (J.S. Rowlinson) in J. Statist. Phys. 20 (1979) 197 2. J.S. Rowlinson, B. Widom, Molecular Theory of Capillarity, Clarendon Press, Oxford, 1982 3. H. Ted Davis, Statistical Mechanics of Phases, Interfaces and Thin Films, VCH Publishers, New York, 1996 4. A. Braslau, M. Deutsch, P.S. Pershan, A.H. Weiss, J. Als-Nielsen, J. Bohr, Phys. Rev. Lett. 54 (1985) 114; J. Als-Nielsen, Physica B 126 (1984) 145 5. P. Lang, in Modern Characterization Methods of Surfactant Systems, Surfactant Science Series, Vol. 83 (B.R Binks, ed.). Marcel Dekker, 1999, chap. 10; R.K. Thomas, ibid, chap. 11 6. D.M. Mitrinovic, Z. Zhang, S.M. Williams, Z. Huang, M.L. Schlossman, J. Phys. Chem. B 103 (1999) 1779; Z. Zhang, D.M. Mitrinovic, S.M. Williams, Z. Huang, M.L. Schlossman, J. Chem. Phys. 110 (1999) 7421 7. D. Beaglehole, in Fluid Interfacial Phenomena (C. A. Croxton, ed.), Chichester, 1986, chap. 11 8. S. Henon, J. Meunier, in Modern Characterization Methods of Surfactant Systems (ref 5), chap. 4 9. R Drude, Ann. Phys. Chem. (Leipzig) 43 (1891) 91

14

10. S. Fisk, B. Widom, J. Chem. Phys. 50 (1969) 3219 11. J. Meunier, in Light Scattering by Liquid Surfaces and Complementary Techniques, Surfactant Science Series, Vol. 41 (D. Langevin, ed.), Marcel Dekker, New York, 1992, p. 333 12. J. Schulz, A. Hirtz, G.H. Findenegg, Physica A 244 (1997) 334 13. D. Vollhardt, Adv. Colloid Interf, Scl 64 (1996)143; V. Melzer, D. Vollhardt, Phys. Rev. Lett. 76 (1996) 3770; D. Vollhardt, V. Melzer, J. Phys. Chem. B 101 (1997) 3370 14. S. Uredat, G.H. Findenegg, Langmuir 15 (1999) 1108 15. S. Uredat, G.H. Findenegg, Colloids and Surfaces A 142 (1998) 323 16. D. Langevin, in Light Scattering by Liquid Surfaces and Complementary Techniques, Surfactant Science Series, Vol. 41 (D. Langevin, ed.). Marcel Dekker, New York, 1992, chap. 2 17. N. Matubayasi, K. Motomura, M. Aratono, R. Matuura, Bull. Chem. Soc. Japan 51 (1978) 2800; T. Ikenaga, N. Matubayasi, M. Aratono, K. Motomura, R. Matuura, Bull. Chem. Soc. Japan 53 (1980) 653; H. lyota, M. Aratono, K. Motomura, R. Matuura, Bull. Chem. Soc. Japan 65 (1983) 2402 18. Y. Hayami, A. Uemura, N. Ikeda, M. Aratono, K. Motomura, J. Colloid Interf. Sci. 172 (1995) 142; T. Takiue, A. Yanata, N. Ikeda, Y. Hayami, K. Motomura, M. Aratono, J Phys. Chem. 100 (1996) 20122 19. M. Matsuguchi, M. Aratono, K. Motomura, Bull. Chem. Soc. Japan 63 (1990) 17

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. Ail rights reserved.

15

Assembly of organic and inorganic molecular layers by adsorption from solution T. Kunitake and L Ichinose The iDStitute of Riysical and Chemical Research (RIKEN), Frontier Research System 2-1, Hirosawa, Wako, Saitama 351-0198 JAPAN Fax: +81-48-464-9361 E-mail: [email protected] 1. Introduction Fabrication of ultrathin molecular films can be classified as either dry or wet. Diy processes involve depositions of precursors and monomers in vapor phase onto solid sur&ces. Activation of reactants by plasma is one of the most commonly used methods for chemical vapor deposition. It, however, often results in structures that do not allow detailed characterization. The Langmuir-Blodgett technique has been ahnost the sole wet method to prepare ordered, moleculariy-thin films in the past several decades. This situation changed recently, and newer techniques based on physi-sorption and chemi-sorption of small molecules and polymers became widely used. Simple adsorption processes can produce regular molecular layers, if proper preparative conditions are employed. 2. Polymerizatioii-Induced Adsorption ,S.

NH, H In-situ deposition of primarily insoluble conductive polymers has been studied by several groups. Gregory et al. pyrrole aniline 3-hexylthiophene obtained uniform thin films of polypyrrole (1) and polyaniline (2) on textile substrates (2) (3) during polymerization via adsorption of an intermediate from solution. Paley et al. succeeded in photodeposition of HoN-, ^ - o - • Q ^ N H , polydiacetylene films from the monomer N=C=0 solution and used it for the surface tolyiene-2,4-diisocyanate 4,4'-oxydianiline patterning. Combination of in-situ (5) (4) deposition and electrostatic altemate adsorption was recently explored by Rubner and his co-woricers. ' p-Doped conjugate polymers, such as polypyrrole, polyaniline and poly(3-hexylthiophene) (3), were directly adsorbedfix)mdiluted polymerization solutions, and the substrate was subsequently inunersed in poly(styrenesulfonate) solution. Polypyrrole is uniformly adsorbed on a negatively-chaiged precursor surface. A typical example of the polymerization-induced adsorption is given in Figure la. A gold-coated QCM resonator is inunersed into a fiiesh polymerization solution. After recording the fi:equency change, the electrode is immersed again into a newly prepared

6

6

16

polymerization solution, hisoluble polymer occasionally precipitates finom the reaction mixture, but they are readily removed by washing. Frequency shifts during repeated adsorption of polypynole (1) are shown in Figure lb. The oxidative polymerization was started by adding a small amount of copper ( E ) chloride into pyrrole in propanol. The frequency changes observed here point to regular growth of an ultrathin fihn of polypyrrole with thickness increase of 12 ± 5 nm. Apparently, more than ten molecular layen of polypyrrole are adsorbed during a single adsorption process. The XPS and UV-VIS-NIR spectra mdicate that this film contains a small amount of copper atoms used as oxidation catalyst. Figure 2 shows a scaiming electron micrograph of the cross section of a polypyrrole fihn of 90± 10 nm thickness deposited onto a gold-coated QCM resonator.

b) ^5000: X ^

4000Polypyrrole

1

Polymenzation solution

/C^K J

o c Measurement of frequency

1 ^/

E 3000 •

«

Drying

£ "^ Washing

v\ ^A

Polyiirea i

oi 0

1

2

4

i i

1 1

6

1

8

Cycles of polymerization

Figure 1. (a) Schematic illustration of polymerization-induced adsorption, (b) QCM frequency shifts with cycles of polymerization. Another case of the polymerization-induced adsorption is a polyurea thin film. Repeated inunersion in a 1:1 mixture of 4,4'-oxydianiline (4) and tolylene-2,4-diisocyanate (5) gives regular adsorption of 8 ± 2 nm in each cycle. Reflection IR spectroscopy of this fihn indicates formation of the urea linkage via polyaddition of amine and isocyanate groups along with 17 % of uiueacted isocyanate groups. The extent of the adsorption was fairiy constant among different polyureas, although film morphologies are widely varied.^ 3. Polymerization-Induced Epitaxy

Figure 2. Scanning electron micrographs of polypyrrole thin film.

When a flat, crystalline soUd is immersed in a polymerization solution, an ultrathin film of ordered polymer chains may be foimd on the sohd surface. Stmctural analyses indicate regular aUgrunent of almost all train chains with an epitaxial orientation. ' The polymerization process itself is indispensable for film growth, and we refer to this epitaxial adsorption as polymerization-induced epitaxy. Freshly

17 cleaved, highly oriented pyiolytic graphite is convenient as substrate. After polymerization, the substrate is taken out of thereactionmixture and is rinsed exhaustively with solvents of polymers. The first polymer layer on the surface is insoluble in common solvents, probably due to difficulty in solvating epitaxially adsorbed highly packed chains. Ring-opening polymerization by cationic and anionic catalysts, radical polymerization, polycondensation and polyaddition are found to yield oriented films on graphite. Thus any one-phase solution polymerization appears to induce film growth as long as the substrate surface remains chemically intact. The film morphology and the surface coverage depend heavily on polymerization conditions such as concentration, temperature, andreactiontime. Figure 3 is a probable aUgmnent of PTHF chain based on scanning tunneling microscopy (STM) images. Planar zigzag chains in all-trans conformation lie so that every ethylene unit is commensurate with the graphite hexagonal lattice. This film was polycrystalline, as other orientations as well as different chain packings were observed on the same sample surface. Polycrystalline structures were also observed for aliphatic and aromatic polyesters.

kb^b^'f :^-. 4.3 A'""(

ly^}/?'^^'^ \, }'-/'"(

'Xy^'^

.>--«'''""*<

Figure 3. An epitaxial model that is consistent with the STM image. Insolubility of the epitaxial film is advantageous when performing substitution reaction directly on the film. A highly aligned film of polyurethane containing a bisbromomethyl group was grown first. Treatment of theresultingfiilmwith sodium azide in solution caused substitution of the bromide for azide. It was not known, however, whether the chains remained epitaxially attached to the substrate surface after the substitution reaction. The reactedfilmstayed insoluble in common solvents. However, it is possible that only parts of the chains are detached by the reaction, leaving the whole chain still tethered to the substrate surface. This may lead to controlled disordering of organized films at a molecular level. It is also possible to take advantage of segmental epitaxial growth by designing the order of polymerization. Epitaxy of diblock copolymer was used for fixation of an amorphous polymer byfirstpolymerizing a crystalline polymer (PTHF), followed by polymerization of a non-crystalline chain (a random copolymer of THF and BCMO) at the terminal of the crystalline polymer. The resulting copolymer PTHF-block-P(THF-BCMO) on the graphite

surface was insoluble even in good polymer solvents. The copolymer was tethered to the substrate surface by PTHF block epitaxy, and the non-ciystalline P(THF-BCMO) block remained free of surface attachment. Thus chlorine atoms of BCMO could be substituted by azide, using a conventional polymer reaction. 4. Alternate Electrostatic Adsorption Alternate adsorption of charged polyions rapidly expanded for assembling organized thin films, because of its general applicability. The cracial feature of this method is excessive adsorption of polyions at every stage of the layering process that results in excess electric charge on the outermost surface. The principle of altemate adsorption for charged colloidal particles was proposed in 1966 by Her.^ Mallouk and coworkers developed altemate adsorption of Zr'^^ions and diphosphonic acid.^ Fromhertz proposed an idea of assembling proteins by the adsorption method. Decher et al. ignited this field by showing successftd film assembly by means of altemate adsorption of linear polyions and bipolar amphiphiles. Stoichiometric and excessive adsorptions of linear polyions have been investigated by the surface force measurements by Bemdt et al. S]s

1© overcompensation

compensation

1^ ^ Adsorption! Eg Poiyanion

Adsorption Polycation

P f?iiSSfctMnii?S!tfni£/?iSiy!!S!^ sub - compensation

Figure 4. Altemate adsorption of oppositely-charged polyions. Figure 4 schematically describes the altemate assembly. A solid substrate with a positively charged planar surface is immersed in a solution containing an anionic polyelectrolyte, and a layer of poiyanion is adsorbed. When the adsorption is carried out at a relatively high concentration of polyelectrolyte, an excessive number of anionic groups is present on the surface, and the surface charges becomes reversed. After rising, the substrate is inunersed in a solution containing a cationic polyelectrolyte, and an excess amount of polycations is adsorbed. By repeating both steps, an alternating multiplayer assembly is obtained. Simplicity of the procedure suggests that, in principle, there is no restriction on the choice of polyelectrolytes. Presently, the technique has been applied successfully to a large variety of water-soluble polyions, including conductive polymers, DNA, and polypeptides, at a number of laboratories. It is also applicable to charged nano-particles: ceramics and a-ZrP plates, virases, and proteins. ' The driving force of altemate adsorption is not restricted to electrostatic force. Metal coordination has been used for this purpose by Keller et al and by Watanabe & Regen.^^ In the case of biomolecules, specific interactions such as the strong binding between concanavalin A and glucose can be used. Akashi and coworkers

19

a)5000

.

.

.

.

J

.

.

.

1

1

.

.

.

,

.

.

.

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.

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Step of growth for Mh/PSS bilayer is -AF=494Hz: 300 for Mb and 194 forPSS ~ '^'^•*"~'*.

e.ead 0

5

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10 15 Cycles of adsorption

zee

- i . -

^''~*- '

H

lee Wavelength (nn)

Figure 5. (a) QCM frequency shifts and cycles of alternate Mb/PSS adsorption Changes of UV absorption with Mb/PSS assembly.

(b)

starch '^

PEI (PEI/GA)2

Figure 6.

Sequential enzymatic process based on GA and GOD. 17

took advantage of stereo-complex formation of poly(methacrylic acids). Multilayer fihns of layered proteins were assembled by means of alternate adsorption using positively-charged PEI and PDDA or negatively-charged PSS and poly(aniline propanesulfonic acid) (PANPSA).^^ The pH values of the protein solutions were set far enough from the isoelectric point for the proteins to be sufficiently charged. Proteins studied so far include cytochrome C, lysozyme (Lys), histone type YIE-S, myoglobin (Mb), peroxidase (POD), hemoglobin, glucoamylase (GAM), concanavalin A (Con A), glucose oxidase (GOD), catalase, invertase, and diaphorase. The structure of the solid support surface was found to affect the stability of the proteins. A substrate covered with alternate PEl/PSS is often used as the standard surface. Time needed for saturated adsorption was measured to be 15 to 20 min. Linear growth of fihn thickness during adsoq)tion cycles was observed. As an example. Figure 5a indicates the dependence of QCM frequency shift on

20

myoglobin/PSS adsorption cycles and shows protein monolayer fonnation at every step of adsorption. Figure 5b shows UV spectra for a Mb/PSS film at consecutive steps of the assembly. The Soret band absorbance increased linearly with the number of Nfb layers. Enzymatic activities are examined for GOD/GAM protein films prepared on an ultrafiltration membrane. The whole set up is shown in Figure 6. Filtration of an aqueous solution of "water-soluble starch" at the flow rate of 1 - 3 mL/h is conducted by applying pressure. Hydrolysis of the glycoside bond in starch by GA produces glucose. Glucose is converted to gluconolactone by GOD with H2O2 as co-product. Sequential conversion of starch is made possible by the ordered protein layers. 5. Surface Sol-Gel Process Fabrication and nanostmctural control of metal oxide thin films have been playing important roles in many areas of materials science, since surface oxide layers give rise to unique properties. High-vacuum dry-processes, such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE), have made it possible to precisely control the thickness of metal oxide layers. The preparative conditions like pressure and substrate temperature can be widely varied, and the elemental composition in individual atomic layers is controllable by sequential supply of precursor gases. The dense, defect-free oxide films thus prepared arefi:equentlyused as underlayers of microelectronics devices. On the other hand, preparation of metal oxide thin films by means of adsorption of metal alkoxide has been carried out in the past for the activation of heterogeneous catalysts. For example, Asakura et al. prepared one-atomic layers of niobium oxide by repeating chemisorption of Nb(0Et)5 on silica beads. Ferguson et al. prepared an ultrathin Ti02 film by repeated adsorption of Ti(0^u)4 onto silicon wafers with oxidized surface. We have developed the surface sol-gel process based on stepwise adsorption of metal alkoxides. It can be widely employed for the fabrication of novel ultrathin films. In this technique, metal alkoxide is chemisorbed on a solid substrate possessing surface hydroxyl groups. The oxide gel films with molecular thickness are formed after hydrolysis of the chemisorbed alkoxides. Active hydroxyl groups reproduced on the surface are used for further adsorption of alkoxides to grow metal oxide multilayers. The films thus obtained usually contain some unhydrolyzed alkoxide groups. For example, ultrathin Ti02-gel fihn formed from Ti(0°Bu)4 has 2 to 5 times larger carbon contents against titanium atom, and gives a density of 1.6 g/cm^. This film is composed of a partially cross-linked chain structure (-Ti(OR)2-TiO(OR)-0-). The structural flexibility of the gel gives useful properties for forming precise nano-composites with organic compounds and for its use in molecular imprinting. The active surface hydroxyl group need not be restricted to hydrolyzed metal oxide layers. We found that polyhydroxyl compounds, polymers and small organic molecules alike, are adsorbed readily onto the surface of the oxide layer. The adsorbed polyhydroxyl compounds provide fi:ee hydroxyl groups on the surface, and metal alkoxides are adsorbed subsequently. Thus, it is possible to extend the surface sol-gel process to alternate adsorption of polyhydroxyl organic compounds and metal alkoxides, as follows. A solid substrate is inmiersed into metal alkoxide solutions for given periods of time. The chemisorbed alkoxides are hydrolyzed, and the substrate is immersed in a solution of a polyhydroxyl compound for adsorption. Repetition of these procedures gives an alternately layered film of organic and inorganic components. Figure 7 shows QCM ftequency changes due to altemate

21

adsorption of Ti(OT3u)4 and poly(acrylic acid) (PAA). The linear finequency decrement indicates regular growth of Ti02-gel/PAA multilayers, and thicknesses of Ti02-gel and PAA layers calculated from QCM frequency changes are 14±4 A and 13 ± 7 A, respectively. b)

3

4

5

6

7

Cycles of adsorption

8

9

10 O-nsu

Figure 7. (a) QCM frequency changes due to ri02-gel/PAA and TiC^-gel/starch assemblies, (b) Schematic illustration of Ti02-gel/PAA fihn. Further examples of the alternate organic/inorganic adsorption include combinations with cyclodextrin, polyrotaxane, and dendrimer. Very recently, two-dimensional aligrunent of metal nanoparticles was successfully conducted by using the Ti02-gel surface. A long-alkyl disulfide with two hydroxyl groups at both molecular termini (HO(CH2)iiSS(CH2)iiOH) was 800 used as a protective agent for gold 400 500 600 nano-particles with average diameter of Wavelength / nm 4.7 nm. When a soUd substrate overlaid with a TiC^-gel film was Figure 8. UV-vis spectra of layer-by-layer inmiersed in an aqueous dispersion of a assembly of gold nanoparticles and Ti02 layer. gold nanoparticle, a densely packed monolayer of the nanoparticles was obtained. Saturation of the adsorption requires as long as 10 h, in contrast to adsorption of polymeric PVA and starch that requires only 10 min. Au nanoparticle /Ti02-gel multilayers were obtainable by alternating formation of a TiCh-gel layer for 15 cycles and adsorption of Au nanoparticles (Figure 8). Unique catalytic and optical properties are expected for metal nanoparticles organized in oxide gel matrices. 6. Molecular Imprintiiig Molecular imprinting is an equally important development of the surface sol-gel process.^^ Recently, the unprinting of biologically active substances such as amino acid

22

derivatives and dipeptides was successfully achieved. A 4:1 mixture of Ti(0'*Bu)4 and carbobenzyloxy-L-alanine (Cbz-L-Ala) in toluene/ethanol was subjected to the surface sol-gel process. The template molecule, Cbz-L-Ala, incorporated into Ti02-gel film was removed by treatment with 1 wt % aqueous ammonia, as confirmed fix)m dis^pearance of characteristic peaks of titanium-carboxylate complex and carbamate inreflectionFT-IR spectra. In-situ QCM measurements were carried out forre-bindingof the original template and other guests in CH3CN. Figure 9a shows selected examples of the guest binding experiment. The binding is rapid and saturated in 30 to 60 s. Among the peptide derivatives, Cbz-Gly showed the largest binding (53 Hz) and the extent of binding became suppressed with increasing sizes of the side chain. It is noteworthy that the original template, Cbz-L-Ala (43 ± 3 Hz), is less efficiently bound than smaller Cbz-Gly. However, the binding efficiency is not determined solely by the size of guest molecules. For example, adamantane-1-carboxylic acid, l-AdC02H, gave much smaller binding than all the amino acid derivatives, in spite of its smaller molecular weight. Organic carboxylic acids such as benzoic acid and cirmamic acid also gaverelativelylow binding efficiencies. These binding data strongly suggest that the imprinting has produced a specific receptor site in the Ti02-gel fihn. As illustrated in Figure 9b, this receptor cavity is probably composed of carboxylate site, hydrogen bonding site and hydrophobic site(s). The carboxylate-binding site is common to all the guest molecules. The Ti02-gel fihn contains hydroxyl group (Ti-OH) as hydrogen-bond site and hydrophobic site (Ti-(y*Bu in addition to the network of Ti-O-Ti) to accommodate non-polar moiety. These varied interaction sites act cooperatively to realize selective recognition of amino acid derivatives.

b) Hydrogen bonding site

30

60

Immersion

Carboxylate binding site

90

time/sec

Figure 9. (a) In-situ QCM frequency decrements due to binding of a series of guest molecules, (b) A schematic illustration of an imprinted film Imprinted Ti02-gel films were prepared in a similar way by using L- or D-amino acid derivatives (Cbz-Ala, Cbz-Leu, Cbz-Phe) as template. The rcbinding data for L- and

23

D-enantiomers in these imprinted films are shown in Table 1. Enantioselectivity factor ( a ) in each imprinted film is a ratio of bound template molecule and its enantiomer (M-template/M-enantiomer, mol/mol). Figure 10 shows selected examples of the relative binding efficiency. Table 1.

QCM frequency changes due to binding of guest molecules and -U-D enantioselectivity factor.

imprinted film

amount of bound molecule^

^L

mo

a^

173/43^ (65) Hz

153/38(60) Hz

1.13

Cbz-D-Ala

165/41(67) Hz

177/44(70) Hz

1.08

Cbz-L-Leu

129/38(74) Hz

75/22(58) Hz

1.73

Cbz-D-Leu

81/24(62) Hz

139/41(79) Hz

1.71

Cbz-L-Phe

120/4(X69)Hz

60/20(49) Hz

2.00

Cbz-D-Phe

96/32(44) Hz

162/54(66) Hz

1.68

Cbz-L-Ala

* amount of adsorbed molecule (pmol) calculated from QCM frequency change, ^ QCM frequency change after collection for the adsorption on outermost layer

In the Cbz-Ala imprinted film, the fiiequency change between the template and its enantiomer was 5 or less, and the enantioselectivity factor was about 1.1 in either of the L- and D-imprinted films. In contrast, the template molecule was bound much better than its enantiomer in the Cbz-Leu- or Cbz-Phe-imprinted Cbz-o-Lau Imprinted flm film. The difieience of the frequency Cbz<-Lau change between these optical isomers Imprinted film was 16-22 Hz, and the Cbz-o-Leu enantioselectivity factor amounted to 1.7-2.0. The enantioselectivity is attained only when all three substituents Figure 10. Relative binding efficiency for Cbz-D(or-L)-Phe imprinted fihns. (-COOH, Cbz-NH, and side chain) on the a carbon are recognized. When a D- enantiomer is adsorbed in an L-imprinted fihn, the configuration of side-chain and carboxyl group is reversed. Water-soluble peptides could be imprinted in Ti02-gel fihns by the altemate adsorption approach with Ti(OT3u)4 and glycyl-L-tyrosine (Gly-L-TVr). The template molecule was removed by treating with 10 mM aqueous sodium hydroxide. Similar 'n02-gel films could be prepared for glycyl-L-glutamic acid (Gly-L-Glu), glycylglycine (Gly-Gly), glycylglycylglycine (Gly-Gly-Gly). These peptide-imprinted Ti02-gel films are able to recognize peptides at low concentrations less than 10 /x M in water.

24

We can conclude from these results that imprinted Ti02-gel j&lms are capable of precisely recognizing the size of template molecules, the nature and position of functional groups, and stereochemistry. It is particularly noteworthy that monosaccharide isomers in which only the configuration of hydroxyl groups differs are discriminated. Current investigations by other group aim at construction of highly discriminating molecular recognition systems by 3D arrangement of specific guest fimctional groups. However, the synthesis of such host compounds is not readily achieved. For example, Wulff et al. attempted enhancement of substrate selectivity by fixing (or imprinting) fimctional groups which specifically interact with the template molecule. Appropriate spatial organization of the hydrogen-bonding unit in cross-linked polymers of divinyl monomers has been studied in great numbers. Divinyl monomers are not necessarily suitable for producing precisely-structured cavities complementary to small guest molecules, hi contrast, Ti02-gels have flexible netwoik stmctures that provide metal coordination site, hydrogen bonding site, charged site and hydrophobic domain. Such structural flexibility and fimctional diversity are greatest advantages of the TiC^-gel for molecular imprinting. Additional advantage is that the Ti02 nano-film is readily prepared on surfaces of different shapes; particle surface, inner-wall of capillary and inside of a porous support. REFERENCES 1. R. G. Gregpry. W. C. Kimbrell, H. H. Kuhn, Synth. Met, 23 (1989) 823. 2. M. S. Paley. D. O. Frazier, R Abdeldeyem, S. Armstrong, S. P. McManus, J. Am. Chem Soc, 117 (1995) 4775. 3. J. R Cheung, A F Fou, M. F Rubner. Thin SoUd Films, 244 (1994) 985. 4. A F Fou, M. F Rubner, Macromoiecules, 28 (1995) 7115. 5.1. Ichinose, R Miyauchi, M. Tanaka, T. Kunitake, Chem U t t , (1998) 19. 6.1. Ichinose, T. Kunitake, Adv. Mater., 11 (1999) 413. 7. M. Sano, D. Y. Sasaki, T. Kunitake, Science, 258 (1992) 441. 8. M. Sano, Y. Lvov, T. Kunitake, Anna Rev. Phys. Chem, 26 (1996) 153. 9. R. K. Her, J. CoUoid Interface Set, 21 (1966) 569. 10. R Lee, L. Kepley, R -G. Hong. T. E. MaUouk, J. Am Chem Soc., 110 (1988) 618. 11. R Fromhertz, **Electron Microscopy at Molecular Dimension" 1989. ed by W. Baumeister, W. VogeU, Springer-Verlag (Berlin) 338-349. 12. G. Decher, J.-D. Hong, MakromoL Chem MacromoL Symp., 46 (1991) 321. 13. R Bemdt, K. Kurihaia, T. Kunitake, Langmuir, 8 (1992) 2486. 14. S. Keller, R-N. Kim, T. E. Mallouk, J. Am Chem Soc, 116 (1994) 8817. 15. Y. Lvov, K. Ariga, L Ichinose, T. Kunitake, J. Am Chem Soc., 117 (1995) 6117. 16. S. Watanabc, S. Regen, J. Am Chem Soc, 116 (1994) 8855. 17. T. Serizawa, K. Hamada, T. Kitayama, N. Fujimoto, K. Hatada, M. Akashi, J. Am Chem Soc, 122 (2000) 1891. 18. K. Asakura, and Y Iwasawa, Chem Lett, (1988) 633. 19. E. R. Kleinfeld, G. S. Ferguson, Mater. Res. Soc Symp. Proc, 351 (1994) 419. 20.1. Ichinose, R Senzu, T. Kunitake, Chem Mater., 9 (1997) 12%. 21. L Ichinose, T. Kawakami, T. Kunitake, Adv. Mater., 10 (1998) 535. 22. T. Yonezawa, S. Onoue, T. Kunitake, Adv. Mater., 10 (1998) 414. 23. S.-W. Lee, L Ichinose, T. Kunitake, Langmuir, 14 (1998) 2857. 24. G. Wulff, Angew. Chem, Im. Ed EngL, 34 (1995) 1812.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyania and H. Kunieda (Editors) 'C! 2001 Elsevier Science B.V. All rights reserved.

25

Scattering Study of the Lyotropic Lamellar Phase in Aqueous Solutions of Nonionic Surfactants Tadashi Kato^, Koji Minewakia, Hirohisa Yoshida*', Masayuki Imai^, and Kazuki Ito^ » Department of Chemistry, Tokyo Metropolitan University, Minamiohsawa, Hachioji, Tokyo 192-0397, Japan ^ Department of Applied Chemistry, Tokyo Minamiohsawa, Hachioji, Tokyo 192-0397, Japan

Metropolitan

University,

c Department of Physics, Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo 112-0012, J a p a n ^ Institute of Applied Biochemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8572, J a p a n Small angle x-ray scattering (SAXS) has been measured on the lamellar phase and concentrated micellar solutions for heptaethylene glycol-n-hexadecyl ether (Ci6E7)-water system. The repeat distance d follows the swelling law d oc^c'^ where^c is the volume fraction of hydrophobic layers. In the higher temperature range, the exponent s is unity as expected. However, reduction of temperature induces gradual transition of s from unity to 2/3 which corresponds to the upper limit of theoretical values for the mesh phase. Based on these results and line shape analyses of SAXS, it has been shown that the deviation from the classical lamellar structure is enhanced as the temperature and concentration are reduced and the most pronounced near the boundary with bicontinuous cubic and micellar phases. Relations with phase behaviors and the structure of the micellar phase are also discussed. 1. INTRODUCTION Aqueous solutions of nonionic surfactant exhibit a variety of phase behaviors merely by varying temperature or surfactant concentration. In the transition between these phases, not only the arrangement but also the shape of building block itself is changed, which makes it difficult but fascinating to clarify the mechanism of the phase transition in surfactant systems. Variation in the shape of building block occurs even far from the transition point. Investigation of such

26

a structural change in the single phase region is considered to give a clue for clarifying mechanism of the phase transition. In our previous studies [1-4], we have measured static and dynamic light scattering and pulsed-gradient spin-echo on semidilute micellar solutions of CieE?, C14E6, and C14E7 (CnEm represents a chemical formula CnH2n+i (OC2H4)mOH). It has been shown that entanglement of wormlike micelles occurs and that there exist transient connections with life time of lO'^^^lO'^ s above about Tc -30K (Tc. the lower critical consolution temperature). Above about TJ 5K (about 45°C for the CieE? system), stable connections are formed and the number of these connections increases with increasing temperature and concentration. As the temperature and concentration increases further (above about 55°C for the CieE? system) so that the solution approaches the boundary with the lamellar and the bicontinuous cubic (Vi) phases, the microstructure becomes similar to that of the Vi phase except for the lack of regularity. These results led us to investigate the structure of the lamellar phase because the building block of the micellar phase (one-dimensional aggregates) and the lamellar phase (two-dimensional aggregates) are quite different. Recent studies on the systems of homologues [6-8] suggest that the lyotropic phases exhibit much more wide variety of morphologies than considered before. In the present study, we have performed measurements of small angle x-ray scattering (SAXS) on the micellar and lamellar phases of the CieE? system to compare the structures of these phases in the wide concentration and temperature range. 2. EXPERIMENTAL Procedure for the preparation of sample has been reported elsewhere [9]. SAXS measurements were performed using an apparatus (MAC Science) constructed from an x-ray generator (SUA, MXP18, ISkW, X =0.154nm), incident monochromator (W/Si multilayer crystal), Kratky slit, and imaging plate (DIP 200). For line shape analyses, we used the synchrotron radiation SAXS spectrometer installed at the BL-IOC and BL-15A instruments at the photon factory (PF) of the High Energy Accelerator Research Organization (KEK), Tsukuba. Details of measurements are reported elsewhere [9]. 3. RESULTS AND DISCUSSION 3.1 Peak position analysis Figure 1 shows temperature dependence of the repeat distance (d) obtained from the position of the first-order reflection of SAXS at different concentrations. In the lower concentration range, the repeat distance rapidly decreases with decreasing temperature in spite that surfactant concentration is kept constant. In Figure 2, plots of log d vs. log (/> he are shown where ^ he is the volume fraction of the hydrophobic layers. All the plots can be fitted to a straight Une, indicating the swelling law d oc ^c*^. Temperature dependence of the exponent s

27

is shown in Figure 3. Above about 70^:, s is close to unity as expected from the relation for the stacked bilayer sheets of constant thickness d" 2Ac^hc

(1)

where 2<Sic is the thickness of the hydrophobic layer. However, reduction of temperature induces gradual transition of s from unity to 2/3 which corresponds to the upper limit of theoretical values for the mesh phase [10,11]. In the region denoted as "La" in the phase diagram shown in Figure 4, only first-order and second-order reflections were observed in the positional ratio 1:2 (see Figure 5). Also, the ^H NMR spectra observed in this region consists of only one doublet, 11.0 10.5 10.0 9.5 9.0 8.5 8.0

32.0% 40.0% 48.0% 51.0% 54.0% 57.0% 60.0% 64.0% 67.0% 70.0% 75.0%

.-D—

D

• A

•

O •

0 •

o A

V

1 80.0%•

7.5 7.0 6.5 6.0 5.5

o-o-^

5.0

* *~^ • • > #

4.5

30

40

50 60 //'C

70

80

Fig. 1. Temperature dependence of the repeat distance d at different concentrations. The dashed lines indicate the micellar/lamellar coexistence region. The d values at the temperatures lower than 60°C at 32 and 40 wt% are obtained for the micellar phase.

Fig. 2. Double logarithmic plot of the repeat distance rfvs. volimie fraction of the hydrophobic layer ^c. Ordinate is arbitrarily shifted. The solid Unes are the least-square fits to the relation d oc ^c~^. The region between the two vertical bars indicate the micellar/lamellar coexistence region. The symbols located at the lower concentrations than these bars indicate the lvalues for the micellar phase.

28

indicating that the possibiUty of coexistence with other phases can be excluded. The above discussion is based on the assumption that Ac is constant. If (Sic depends on the concentration, 5 may deviate from unity even if Eq.(l) holds true. So we have analyzed line shape of SAXS to determine Ac directly. 3.2 Line shape analysis In the line shape analysis, we assume that layer displacement fluctuations are independent of the transverse position. Then the scattering intensity can be written in terms of the form factor of a membrane B^q) and the structure factor S{q) as [12] I(q) = (2 7z/d)I\q)S(q)/q^

(2)

In the calculation of I\q), the membrane is assumed to be composed of three layers; one hydrophobic layer and two hydrophihc layers. Then I(q) can be expressed as a function of Ac, the thickness of the hydrophilic layer, and a AA A A A A

^ 0.8

30

40

50

60

t/'C

Fig. 3. Temperature dependence of the exponent s obtained from the least-square fitting of the data in Figure 2. The solid line is an aid to the eye.

10 20

30 40 50 60 wt%

70

80

Fig. 4. Phase diagram of C16E7-D2O system, redrawn from figure 1 in Ref.[5]. La, lamellar phase; Vi, cubic phase; Hi, hexagonal phase; Nc, nematic phase; Li, isotropic micellar phase; and W + Li, coexisting liquid phases. Filled and open triangles indicate the presence and absence of the broad component, respectively. The dotted lines are constant contours of A, (see Eq.(3)). The dotted hnes in the Li phase indicate constant contours of the activation energy for self-diffusion processes.

29 parameter correlated with the layer displacement fluctuations. We have determined these parameters by the least-square fitting of g^/(g) because /(^) is inversely proportional to ^ . Figure 5 (left) shows examples of observed SAXS data and least-squares fits. It has been found that the Ac value thus obtained depends on concentration and temperature only slightly [9], suggesting that the deviation of-5 from unity is not due to the variation of Ac but due to the change in the structure itself. 3.3 Relations with phase behaviors and structures of micellar phase For the CieEe system, existence of water-filled defects has been proposed by Holmes's group [6-8]. In this case, ^ c can be expressed as (3)

^ c = (1 - fv,)(2St,c)/d

where £,v is the volume fraction of the defects. We have tried to calculate £N at each concentration and temperature by utilizing Ac values which satisfy the relation ^c^= 2dticld^ (d^: lower limit of the repeat distance) corresponding to the assumption that defects disappear at ^c^. Based on the results in Figure 1, we set d^ = 4.63 nm. Although the absolute value of i?v depends on d^, this does not affect the discussion below. Figxire 4 contains the lines where fw is constant. As

q /nm-

q I nm-^

Fig. 5. Examples of SAXS patterns, left: Least-squares fits of g*/at 75°C (40, 51, and 55 wt% fi-om the bottom), right: Logarithm of the SAXS intensity at 48 wt% (55, 60, 65, 70, 75°C firom the bottom).

30

can be seen from the SAXS patterns in Figure 5 (right), a broad component is superimposed on the first diffraction peak at 55 and 60°C. The filled and open triangles in Figure 4 indicate the presence and absence of the broad component, respectively. It can be seen from the figure that the broad component is observed in the region where i?v is relatively large. The observation of such a broad component has been already reported for the CieEe system [6-8] where it has been assigned to the reflection from the water-filled defects. This assignment is consistent with the strong correlation between £s and the appearance of the broad component in the present system. As described in the introduction, our previous studies [1-5] suggests that in the Li phase three-dimensional networlc is formed in the vicinity of the "La" and Vi phases. The dotted lines in the Li phase indicate constant contours of the activation energy for self-diffusion processes (40, 60, 80, 100, 120, 140, and 160 kJ mol-i firom the top) which can be regarded as the measure of the fraction of such a network. On the other hand, bilayer sheets with water-filled holes can be regarded as two-dimensional network. In the lower concentration range in the "La" phase, gradual transition into such a structure occurs by the reduction of temperature. Then the structures of the "La" and Li phases become similar as the temperature decreases. It should be noted that £N takes a maximum near the boundary with Li and Vi phases. As can be seen from Figure 2, there is only a small discontinuity in the repeat distance at the "La" to Li transition in the lower temperature range. On the contrary, the repeat distance is not continuously varied at the "La" to Li transition in the higher temperature range (see Figure 1). All these results are consistent with the variation in the structure of the "La" phase described above. REFERENCES 1. T. Kato, T. Terao, and T. Seimiya, Langmuir, 10 (1994) 4468. 2. T. Kato, N. Taguchi, T. Terao, and T. Seimiya, Langmuir, 11 (1995) 4661. 3. T. Kato, Prog. Colloid Polym. Sci., 100 (1996) 15. 4. T. Kato, N. Taguchi, and D. Nozu, Prog. Colloid Polym. Sci., 106 (1997) 57. 5. T. Kato and D. Nozu, J. Mol. Liquid, in press. 6. M.C. Holmes, M.S; Leaver, A.M. Smith, Langmuir, 11 (1995) 356. 7. S.S. Funari, M.C. Holmes, G.J.T. Tiddy, J. Phys. Chem., 96 (1992) 11029, 98 (1994) 3015. 8. C.E. Fairhurst, M.C. Holmes, M.S. Leaver, Langmuir, 12 (1996) 6336, 13 (1997) 4964. 9. K. Minewaki, T. Kato, H. Yoshida, M. Imai, K. Ito, submitted to Langmuir. 10. S.T. Hyde, Colloque de Physique, 51 (1990) C7-209. 11. S.T. Hyde, CoUoids and Surfaces A, 103 (1995), 227. 12. F. NaUet, R. Laversanne, D. Roux, J. Phys. II France, 3 (1993) 487. 13. M. Imai, A. Kawaguchi, A. Saeki, K. Nakaya, T. Kato, K. Ito, Y. Amemiya, Phys. Rev. E, in press.

Studies in Surface Science and Catalysis 132 Y. iwasawa, N. Oyama and H. Kunieda (Editors) ^c 2001 Elsevier Science B. V. All rights reserved.

31

Microemulsions composed of metal complex surfactants, bis(octylethyienediamine (= OE)) Zn(II), Cd(II), and Pd(II) chlorides, in water/ chloroform and water/benzene systems Masay asu lida,* Hua Er,^ Naoko Hisamatsu,* None Asaoka,^ and Toy oko Imae ^ ^Department of Chemistry, Nara Women's University, Nara 630-8506, Japan ^ Research Center for Materials Science, Nagoy a University, Nagoy a 464-8602, Japan Bis(A^-octylethylenediamine (= OE)) zinc(II), cadniiiun(II), and palladium(II) complexes (Zn(OE)2Cl2, Cd(OE)2Cl2, and Pd(OE)2Cl2) were prepared, and the characteristic structures of the aggregates were investigated in such mixed solvents as water/chlorofomi, water/benzene, and water/methanol using ^H NMR pulsed-gradient spin echo (PGSE) and TEM (transmission electron microscopy) methods. These complexes form reverse micelles or w/o microemulsions in water/chloroform or water/benzene depending on the hydrophilicity of the headgroup. 1. INTRODUCTION Double-chained surfactants of metal complexes have shown unique aggregation behavior in orgsnic solvents or water,^"^ since their HLB is intermediate and they sometimes display characteristic stereochemistry."* We have prepared such kinds of metal complex surfactants as Zn(OE)2Cl2, Cd(OE)2Cl2, and Pd(OE)2Cl2. They are readily to form singje crystals and not hygroscopic. They are thus desirable for the X-ray crystallographic analysis as doublechained surfactants. We have previously clarified the structures of the Zn(II) and Pd(II) complexes in crystals as follows.^'^ In the former complex, the geometry is octahedral, the aniono groups are tram, and the octyl chains are transoid; on the other hand, in the Pd(II) complex the geometry is square planar and the chloride ions are not coordinated. The molecular structure of rra/i5-dichloro-/ra«5oa/-bis(A^-octylethylenediamine)zinc(II) complex is given in Fig. 1. In the present paper, we report the solubilities and aggregation behavior of M(OE)2Cl2 (M= Zn, Cd, Pd) in water/methanol, water/chloroform and water/benzene systems in comparison of the metal-chloride interactions between the three metals. 2. EXPERIMENTAL The M(OE)2Cl2 (M= Zn, Cd, and Pd) complexes were prepared according to the previous

32

methods. ^'^ The purities were confirmed by elemental analysis and ^^C NMR spectra. The selective solubilities of the complexes in water/chloroform or water/benzene system were visually determined and the ternary phase diagrams were drawn. The aggreg3tion behavior was studied by the measurements of diffusion coefficients of the complexes and water on a JEOL FX 90 NMR spectrometer. The diffusion coefficients were reproducible within a precision of 5% or better. TEM was observed on a Hitachi H-800 electron microscope at an accelerating voltage of 100 kV. Freeze-fracture repUcas were prepared by using a Balzers cryofract (BAF-400). Details of the procedure were described elsewhere. "^» ^ 3. RESULTS AND DISCUSSION All the complexes are poorly soluble in water (around 1% for the palladium complex and below 0.1% for the zinc and cadmium complexes). The Zn(II) and Cd(II) complexes are significantly soluble in chloroform while the Pd(II) complex is poorly soluble. The solubilities in chloroform increased with the addition of water; for the palladium complex, the magnitude was especially large in spite of the low solubility in neat water. Partial ternary phase diagrams for the three complexes in water/chloroform mixed solvents are given in Fig 2. It is remarkable that the palladium complex has a large L2 region, which reflects its hi^er hydrophilicity (or polarity) of the headgroup caused by the ionic character of the palladiumchloride bond. In the Zn(II) and Cd(II) complexes, on the other hand, the chloride ion coordinates to the metal center in crystal. The strength of the metal-chloride bond in the complexes would be in the order, Cd(OE)2Cl2 > Zn(OE)2Cl2 » Pd(OE)2Cl2, whereas in the simple chloride salts the covalency of the metal-chloride ion bond is in the order, PdCl2 > CdCl2 > ZnCl2.

(a) H2O-X 40 Cd(OE)2 V60

. 80

^ CIF Pd(OE)2 \60

CIF H2O-

H2O 40

Fig 1. A molecular structure of Zn(OE)2Cl2. (In Figures, M(OE)2Cl2 is abbreviated as M(0E)2.)

^ 60

60

80

CIF 40

60

80

Fig 2. Partial mass (wt %) ternary phase diagram for the three complexes in water/chloroform (CIF) mixed solvents. L2 is water in oil phase.

33

Figure 3 shows the diffusion coefficients for water and the Zn(II) complex in the chloroform and methanol systems as functions of the complex concentrations. Appreciable decrease in the water diffusion coefficients compared to that for the neat water (2.32x10"^ m^s~^) means the motional restriction of water molecules by the Zn(II) complex We furthermore found that the addition of water slightly (20-30%) retards the diffusion of the Zn(II) complex in the concentration ranges of 0.5-1.0 mol kg"^ in the chloroform system. This result suggests the formation of aggregates incorporating water. The trend for the diffusion coefficients in the Cd(II) complex system was similar to that in the Zn(II) complex system, as seen for the solubility behavior. (Fig. 2) The difference in the diffusion coefficients between the Zn(II) complex and water is significantly smaller in the chloroform system (Fig 3) compared to the methanol (Fig 3) and benzene (Fig 4) systems. The diffusion coefficients and phase diagrams suggest that the reverse micelles are formed for the Zn(II) and Cd(II) complexes in water/chloroform medium, while the microemulsions are formed for the Zn(II) and Cd(II) complexes in water/benzene and for the Pd(II) complex in water/chloroform. (Fig. 5) It is characteristic that the diffusion coefficients of water molecules and metal complexes increase with an increase in water content in the Pd(II) microemulsion system (Fig 5) while they decrease in the Zn(II) (Fig 4) and Cd(II) systems. In the Pd(II) complex system, the degree of the dissociation of the chloride ion would be larger compared to the other two metal systems since the Pd-Cl bond is more ionic; therefore, water molecules may more easily move from one water pool to the other ones together with the chloride ions. 10*

^ :

HDD in MeOD Zn(0E)2 in MeOD

• •

HDO in GIF Zn(0E)2 in GIF iWo'

1.5,

1 W

-

1 1 1 1 1 1 1 11 [ 1 1 1 1 1 1 1 1 1 1 1 1

: • •

ZrC) 7

10'

10-10

j

IZn(0E)2]s

A

E

\ J ^ .

Bz HDD Zn(0E)2

2.26 mol kg-M

:

Q

•

•

• 10'

. . . I . . . . I . . . . t.

0.4 0.2 m I mol kg"

0.6

Fig 3. Diffusion coefficients for the Zn(II) complex and water in chloroform and methanol systems depending on the complex concentrations.

• •

in-ii

0

10

1

20

30

..y..... 40

50

60

Fig 4. Diffusion coefficients for the Zn(II) complex and water in the water/benzene systems depending on the water content. {W^ = [H20]/[Zn(II) complex])

34

The structures of the microemulsions were directly observed by TEM. Figure 6 shows typical cases of TEM photographs for the Zn(OE)2Cl2/H20/benzene system where the wei^t ratio of the Zn(OE)2Cl2: benzene is 1:1. In thisfigure,at 15 wt % water content (1), spherical particles with diameters of 25-30 nm were uniformly dispersed. Those particles must be water-in-oil (w/o) microemulsions having water pools in the cores. The TEM texture changed at 20 wt % water content (2) to sponge phase with water channels. For a solution with 40 wt % water content (3), the TEM photograph displayed typical texture of bicontinuous phase which has benzene and water domains. A solution with 30 wt % water content shows a mixture of sponge and bicontinuous phases. Bicontinuous phase was observed even for a 1.5 : 1 solution of the Zn(OE)2Cl2: benzene ratio with 50 wt % water content.

S! io-«

[

A

^

j

A

1

A Jk.

m / moi kg''

E

10-^Oh

HDO Pd(0E)2 HDO Pd(0E)2

• •

0.15 J 1.04

]

Z«(OE)i

Q

r

11

•

•

•

1

V60

.\<2«

10-^

u^u., M

20

1 t l..^..,Ul...4,.,i«JL-i,„>-A.„>..,A...

30

40

50

6 0 H:0

Fig 5. Diffusion coefficients for the Pd(II) complex and water in water/chloroform system depending on the water content (WQ),

Fig 6. Ternary solubility diagram for the Zn(II) complex/water/ benzene system and the TEM photogr^hs at the respective (1-3) compositions on the phase diagram.

REFERENCES 1. M. lida, A. Yonezawa, and J. Tanaka, Chem. Lett., (1997) 663. 2. M. lida, T. Tanase, N. Asaoka, and A. Nakanishi, Chem. Lett., (1998) 1275. 3. M. lida, H. Er, N. Hisamatsu, T. Tanase, Chem. Lett., (2000) 518. 4. D. A. Jaeger, V. B. Reddy, N. Arulsamy, D. S. Bohle, D. W. Grainger, and B. Berggren, La/igmw/r, 14(1998)2589. 5. X. Lu, Z. Zhang, and Y. Liang, Langmuir, 12 (1996) 5501; ibid., Langmuir, 13 (1997) 533. 6. Y. Ikeda, T. Imae, J. C. Hao, M. lida, T. Kitano, andN. Hisamatsu,Langmuir, 16 (2000) 7618.

Studies in Surface Science and Catalysls 132 Y Iwasawa. N Oyama and H Kunleda (Edltors) (c1 2001 Elsevier Science B.V All rights reserved

35

Kinetics of Lamellar to Gyroid Transition in a Nonionic Surfactant System M. Imai,

A. Kawaguchi, A. Saekia, K. Nakaya, T. Katob, and K. ItoC

Faculty of Science, Ochanomizu University, Bunkyo, Tokyo 112-0012, Japan aDepartment of Physics, Keio University, Yokohama 223-8522, Japan bGraduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan %stitUte of Applied Biochemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan Kinetics of Lamellar to Gyroid Transition in a Nonionic Surfactant System has been investigated by means of a small angle x-ray scattering (SAXS) technique. For large AT ( AT=T-TL,, T: temperature, TLG:lamellar to gyroid transition temperature) in the lamellar phase, the SAXS profiles can be described by a structure factor for undulation fluctuation lamellae. Approaching the temperature to the TLG,an excess diffuse scattering grows at lower Q (Q: magnitude of scattering vector) side of the first lamellar peak. This diffuse scattering arises from the modulation fluctuation of lamellar layer. At the T,, the PFL transformed to the gyroid phase through a transient ordered structure having a rhombohedra1 symmetry. 1. INTRODUCTION

One of the most fascinating properties of surfactantfwater systems and block copolymers is their ability to form a variety of ordered mesophases, such as hexagonally ordered cylinders (C), lamellae (L), and a gyroid (G) structure having a bicontinuous cubic network with Is3,, symmetry. The similarity of phase behaviors for surfactant/water systems and block copolymers suggests universal nature of ordered mesophases originated from their incompatibility effects. Hence the behaviors of ordered mesophases have been the subject of the extensive experimental and theoretical investigations. An important target of the order-order transition (OOT) studies is to reveal the kinetic pathways between the ordered mesophases. From experimental point of view, OOT’s proceed by a nucleation and growth mechanism [l]. Matsen [2] showed the nucleation and growth process for C to G transition on the basis of self consistent mean field theory. However some pre-transition structures are observed in OOT’s, such as a hexagonally perforated layer (HPL)phase [3] in L to G transition of block copolymers, an intermediate phase [4] having Rhombohedra1 symmetry between L and G phases,

36 and a characteristic fluctuation mode predicted by Laradji et al. [5], in C to bodycentered-cubic spheres phases of a tribiock copolymer [6]. Then it is quite meaningful to elucidate the stability of the fluctuations around the equilibrium ordered mesophases prior to the OOT's. In this report we reveal fluctuations of the L phase prior to the L to G transition in the surfactant/water system using a small angle x-ray diffraction (SAXS) technique with synchrotron radiation source and newly developed large area charged coupled device (CCD) detector. 2. EXPERIMENTS We examined the L to G transition of a Cj^Ev/DjO system [7]. The samples containing 46, 50, 52, 55, and 60 wt% of C,6E7 were sealed in a glass vial. For homogenization we annealed the sample for 3 hours at about 55°C and then held it at room temperature for 21 hours. This annealing procedure was repeated about one week and then the samples were transferred to a temperature control cell for SAXS measurements. Standard sample dimensions were diameter of 3.0 mm and thickness of 1.0 mm, which brings polycrystalline L phase. In order to obtain highly oriented L phase, we used thin sample cells with a thickness of 50 |im. The orientation of the L phase was checked by polarized optical microscope observations. SAXS measurements were performed using BL-15A instrument [8] at the photon factory (PF) in the high energy accelerator research organization (KEK). The samples were heated from room temperature (C phase) to 67 "C (L phase) and then annealed isothermally (±0.1 X ) for 100 min to reduce the sample history. After the isothermal annealing, the samples were cooled from 67 X to the TLG with stepwise manner and these processes were followed by SAXS measurements. 3. RESULTS AND DISCUSSIONS First we show brief features of the L to G transition of 55 wt% C,6E7/D20 system obtained from the polycrystalline samples. We plot development of one dimensional scattering profiles as a function of temperature in Fig. 1. At 66.1 °C, two Bragg peaks can be observed at g = 0.098 A*' and 0.19 A'', indicating stacked lamellar structure. The first lamellar peak is composed of a sharp Bragg peak and diffuse asymmetric tail spread around the Bragg peak. The diffuse tail originates from the undulation of lamellae and can be described by the Caille correlation function [9]. From fitting of observed profile by the Caille function, we obtained a bending modulus K=0.7 kBT. Decreasing the temperature to the T^Q, peak positions shift to higher Q side and diffuse shoulder appears at |g=0.09 A'. This new diffuse shoulder can not be explained by the Caille correlation function, indicating the existence of another type of fluctuations. Approaching the temperature to the JLG, the intensity of the excess diffuse scattering increases with keeping the peak maximum position and is reversible against the temperature. At 47.5 °C, in addition to the lamellar peaks the scattering profile shows apparent new peaks at g = 0.093, 0.107, 0.15, 0.16, and 0.19 A*'. Holmes et al. investigated phase behavior of CiEj/D20 systems and reported intermediate phases between C and L

37 phases or G and L phases. They attributed the intermediate phases to rhombohedral (R) structures. The intermediate phase having space group of R 3m provides the most likely structure, which explains the diffraction pattern of the transient structure observed in this study. It should be noted that the R structure is not an equilibrium structure but a transient structure which transforms to the G phase spontaneously during isothermal annealing. When we decreased the temperature to 43.8 °C, the scattering profile changed to another pattern. The new diffraction profile agrees well with that of the G structure (Ia3d). Thus the L to G transition proceeds by lamellae-»fluctuating lamellae-^R structure-»G structure. Hereafter we focus our attention to fluctuations in the lamellar structure prior to the L to G transition. Recently Saeki et al. [10] developed a new computational simulation scheme to obtain the equilibrium structure of ordered mesophases. Using this scheme, they found perforated lamellae close to the T^ as shown in Fig. 2. The channels in the lamellae fluctuate with time and do not show characteristic spatial symmetries such as hexagonal symmetry observed in block copolymers (HPL phase [3]). Thus, this structure is not the HPL phase but perforation fluctuation layer (PFL) structure. They showed that the PFL gives a diffuse peak appears in SJ^Q) (scattering function in lamellar plane) having its peak position at slightly lower Q side of the first lamellar peak. In order to confirm the validity of PFL model we performed scattering experiments for oriented lamellae, which reveals behavior of in-plane and out-of-plane fluctuations. Fig. 3 shows the evolution of the 2D scattering patterns, Si.{Q^,Qy), during the cooling process from the L phase to the G phase when the x-ray beam irradiates perpendicular to the lamellar plane. The scattering pattern at 65.1°C (Fig. 3 (a)) is monotonic without Bragg peaks confirming that the x-ray beam irradiates perpendicular to the lamellar plane. Decreasing the temperature to the TLQ, a diffuse isotropic scattering ring appears (Fig 3 (b)) and grows, indicating the development of the in-plane density fluctuations. At 47.6 X hexagonal diffraction spots (corresponding to diffraction spots from {300} crystal planes) of the R structure appear with hexagonally symmetry diffuse fragments (Fig. 3 (c)-(d)). Further anneahng at 47.6 X the scattering pattern transforms to the typical G phase pattern (Fig. 3 (e)-(f)). Here it should be noted that diffraction spots from {211} crystal planes are appears at the diffuse peak position of the R structure. In conclusion. At high temperature region in the L phase the amplitude-modulation fluctuated lamellar structure is observed. Modulation fluctuations increase their amplitude as the temperature approaches T^ and finally develop to the stable PFL. At the TLG, first the transient R structure appears and then transforms to the G phase. It is quite interesting to note that (110) peak of the R structure appears at the diffuse scattering peak position of the PFL structure and (211) peak of the G phase grows at the diffuse scattering peak position of the R structure. This indicates that the fluctuations around the equilibrium ordered mesophases play an important role in the OOT's.

38

(b)49.4'C Qy (A-l) 0.2

• \

\

Sit .;

^W^.J 0.2

(d)47.6'C

0.24

Qy (A-^) 0.2

Fig. 1. Evolution of SAXS profiles from L to G phases as a function of temperature.

-0.2]

- 0:

0.0 0.2 Qx (A-J)

(e)47-6°C lOmin

- 0.2

0.0 0.2 Qx (A-l)

(f)47.6'C13min o.v (A-i) 0.2 I

Fig. 2. Perforating lamellar structure obtained in Saeki's simulation.

Fig.3. Evolution of 2D scattering patterns 5l(Gx,Cy) from the L phase to the G (a) 65.1 X , (b) 49.4 "C, (c) 48.3 X , and (d) ~ (f) time evolution of scattering patterns at 47.6 'C.

REFERENCES 1. M. Clerc, P. Laggner, A.M. Levelut, and G. Rapp, J. Phys. II5, 901, (1995). 2. M.W. Matsen, Phys. Rev. Lett. 80, 4470 (1998). 3. D.A. Hajduk, et al., Macromolecules 30, 3788 (1997). 4. J. Burgoyne, M.C. Holmes, and G.T.T. Tiddy, J. Phys. Chem. 99, 6054 (1995). 5. M. Laradji, A.-C. Shi, R.C. Desai, and J. Noolandi, Phys. Rev. Lett. 78, 2577 (1997). 6. QY. Ryu, M.E. Vigild, and TP. Lodge, Phys. Rev. Lett. 81,5354 (1998). 7. T. Kato, N. Taguchi, T. Terao, and T. Seimiya, Langmuir 11, 4661 (1995). 8. Y. Amemiya, et al., Nucl. Instr. and Meth. 208, 471 (1983). 9. A. Caille, C.R, Acad. Sci. Ser. B 274, 891 (1972). 10. A. Saeki, and F. Yonezawa, Prog. Theor. Phys. Suppl. in press.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyamaand H. Kunieda (Editors) P 2001 Elsevier Science B.V. All rights reserved.

39

Efficiency boosting by amphiphilic block copolymers in microemulsions: Dependence on surfactant and oil chain length R. Strey, M. Brandt, B. Jakobs and T. Sottmann Universitat zu Koln, Institut fur Physikalische Chemie, Luxemburger Str. 116, D-50939 Koln, Germany We examine the enhancement of the solubilization capacity of medium-chain surfactants in microemulsions of ternary base systems water - n-alkane - CjEj by block copolymers of the poly-(ethylenepropylene)-co-poly(ethyleneoxide) (PEP-PEO) type. The effect is an enormous increase of the swelling of the middle phase microemulsion by water and oil. The magnitude of the effect depends slightly but systematically on the chain length of the surfactant forming the base system and on the chain length of the alkane. Interestingly, the less efficient the base system the larger the boosting effect. Furthermore, the lamellar phase, which usually develops as surfactants become more efficient, is suppressed, the more the less efficient the surfactant of the base system. 1. INTRODUCTION In this paper we further investigate the efficiency boosting effect of block copolymers in microemulsions^'^. In order to appreciate the magnitude of the effect let us recall that in microemulsions, which are thermodynamically stable and macroscopically isotropic mixtures of at least three components water, oil and surfactant, the surfactant forms an extended interfacial film separating water and oil on a local scale. We previously determined the elastic properties of the amphiphilic film for suitable systems^"^ There the correlation between the state of highest efficiency and the bending constants for a variety of nonionic surfactants was determined^. It is well-known that increasing the hydrophobic chain length of the surfactant the amount of surfactant needed to form a one-phase microemulsion is systematically lowered, i.e. the efficiency increases. Also, one generally observes that concomitantly the lamellar phase is stabilized^. We have discovered and reported recently*'^ how to increase the efficiency of surfactants by adding block copolymers while reducing the range of stability- of the lamellar phase. Even more recently an explanation of the effect in terms of an enhancement of the saddle-splay modulus of the film was given^. While mixtures of two surfactants of comparable chain length show small synergistic effects in microemulsions^, adding an amphiphilic block copolymer to a conventional microemulsion system leads to a large efficiency increase already by traces of polymer. The oil and water excess phases are progressively incorporated in the surfactant-rich middle phase which thereby increases in volume. Interestingly, the efficiency boosting experiments can be performed at constant temperature because the hydrophilic-lipophilic balance of the base system is not or only little affected^. Here we report how the observed

40

efficiency boosting effect depends on the surfactant chain length and the oil chain length while keeping the block size of the amphiphilic block copolymer constant. 2. EXPERIMENT 2.1. Phase Diagrams The phase diagrams are determined using thermostated water baths with temperature control up to 0.02 K. The sample composition is given by the oil in (water plus oil) mass fraction a = me / (mA + me), the overall mass fraction of the surfactant (or surfactant plus polymer mixture) y = (mc ^mo) / (mA + me + mc +mD) and the mass fraction of the polymer in the surfactant/polymer mixture 5 = mp / (mc + mp). In this study all samples were prepared at an oil / (water + oil) - volume fraction of (|) = 0.5. 2.2. Materials The n-alkanes were either from Sigma Aldrich (Steinheim, Germany) or Merck (Darmstadt, Germany) with a purity > 99%. The alkylpolyglycol ether surfactants (CjEj) were obtained from Fluka (Neu-Ulm, Germany) and Bachem (Bubendorf, Switzerland) with a purity > 98 %. All substances were used without further purification. The amphiphilic block copolymer is poly-(ethylenepropylene)-co-poly(ethyleneoxide), abbreviated PEP5-PE05, where the approximate molar masses of the blocks are 5 kg/mol. The polymers were synthesized by a two-step process as described elsewhere^"''. A narrow M^/Mp ratio of 1.02 was obtained. 3. RESULTS AND DISCUSSION 3.1. The ternary base systems The phase behavior of the ternary water - n-alkane - CjEj systems has been described in various connections^^"^^. A useful way to characterize these systems are vertical sections through the phase prism at constant water/oil-ratio. In these sections the coexistence curves show the well-knovm "fish" diagram. At low temperatures a microemulsion of o/w type coexists with excess oil (denoted by 2). At high temperatures a microemulsion of w/o type coexists with excess water (2). At lower surfactant concentrations the three phase body occurs (3), at higher surfactant concentrations the one phase region appears (1). Such a "fish" for the water - n-octane - CgEa system is depicted by the hollow circles in Fig. 1. The minimum surfactant concentration for complete solubilization of water and oil, that is where the three-phase and one-phase region meet, is denoted by y at temperature f ^^. This point is referred to as X-point and is a useful measure for the efficiency of a surfactant. The lower Y the more efficient a surfactant is.

41

30

25 [

U o H

" '^' - ^^^-^*^^^-^^v .^

20

15

10

0.00

0.05

0.10

0.15

0.20

0.25

Y

Fig. 1 Section through the phase prism at equal volumes of water and n-octane ((t)=0.5). The well-known "fish" is shown for water-n-octane-CgEa as hollow circles. The effect of adding the amphiphilic polymer PEP5-PE05 leads to an efficiency increase, a shift to smaller y (fiill circles) at unchanged hydrophilic-lipophilic temperature. 3.2. Efficiency boosting by the amphiphilic block copolymer The striking phenomenon of adding the polymer PEP5-PE05 is demonstrated by the full circles in Fig. 1. These mark the fish-tails for increasing 5. Addition of polymer leads to a reduction of y from 0.19 at 6=0 to 0.069 at 6=0.10. Proceeding to 6=0.15 the minimum amount of surfactant plus polymer to form a one-phase microemulsion drops to y =0.019. Remarkably the addition of PEP5-PE05 does not lead to the formation of the lamellar phase in the fish tails presented in Fig. 1. Even for the most efficient fish tail (6=0.15) the lamellar phase does not appear up to y=0.08, the highest surfactant concentration we checked. Only strong streaming birefringence is observed. In spite of the small quantities of polymer added a striking efficiency enhancement is achieved. Extending the measurements to y<0.02 becomes increasingly difficult because of the extremely strong light scattering of the microemulsion phase. As mentioned above and as is evident from Fig. 1, the efficiency enhancement by polymer addition is obtained without significantly shifting the hydrophilic-lipophilic balance temperature. Only for the largest 6 a slight increase is observed due to the slightly more hydrophilic nature of PEP5-PE05 compared to CgEs. 3.3. Effect of surfactant hydrophilicity and hydrophobicity Without added polymer increasing the hydrophilic head group size of the surfactant by one oxyethylene (O-CH2-CH2) unit, that is proceeding to C8E4, the "fish" is shifted to f =41.7 °C, and y increases to y =0.239. Proceeding to CgEs the fish is systematically shifted ftuther up on the temperature scale, i.e. f =61.5 °C, and y increases to y =0.283. These X-points of the base systems are given in Fig. 2 as hollow points.

42

C12E7

:Ci2E6/

ClOE6

"'

/ 5E5"

F

//

/

/jCsEA

/ [Cl2E5\ L

CIOEX

05 = 0 • S = 0.05

CgE^^ 0.1

0.2

03

Fig. 2 Matrix of characteristic X-points for water - n-octane - CjEj systems (hollow points)^ Note that both the surfactant hydrocarbon chain length i and the number of ethyleneoxide j are varied. The effect of adding 5=0.05 of amphiphilic block copolymer PEP5-PE05 is to systematically lower the amount of surfactant at the X-point. hicreasing the hydrophobic tail by two CH2 groups at constant oxyethylene number, i.e. proceeding e.g. from CgEs to C10E5 and on to C12E5, a significant efficiency increase is observed. The corresponding X-points are also included in Fig. 2. The mean temperature in general drops while y decreases. In numbers, for C10E5 we find f =44.6 °C and y =0.141 and for C12E5 f =32.6 °C and y =0.048. When searching for more and more efficient systems, one might wonder why not proceed to C14E5? As it turns out, for C14E5 the efficiency increase leads to the collision of the lamellar mesophase with the three-phase body, so that the onephase microemulsion does no longer exist. Adding now polymer an efficiency increase is observed for all surfactans (see the full points in Fig. 2). It is obvious that the less efficient the base system the stronger the efficiency boosting effect. It is an important feature of the efficiency boosting by polymers that higher efficiencies, as those by C14-surfactants or longer, may be achieved while not developing the lamellar phase to the extent one usually encounters for such efficient surfactants. The lamellar mesophase, which is in general formed at higher surfactant concentrations^^, is to found to start in binary H2O - CiEj systems for surfactant with chain length larger than 10 carbon atoms *^ and usually increases with increasing efficiency of the surfactant. In our experiments we find that the lamellar phase is most destabilized, if one compares equally efficient systems, the shorter the surfactant chain length. To give an example, a C12E5 microemulsion reaches y=0.05 with 5=0, displaying the lowest appearance of a lamellar phase in form of a tip at Y=0.07. A C10E4 microemulsion reaches Y=0.05 with 5=0.09, displaying the lamellar tip at Y=0.12. A CgEa microemulsion reaches y=0.05 with 5=0.11, displaying no lamellar phase up to surfactant concentrations of y=0.25.

43

According to most recent theoretical developments the efficiency increase is associated with an increase in the saddle-splay rigidity constant - ic ^^•^^. Increasing the rigidity K on the other hand has the effect of stabilizing the lamellar phase in these systems '

3.4. The effect of oil chain length The quality of oils as solvent for hydrophobic polymers decreases with oil chain length. In Fig. 3 we examined the effect of the oil chain length on the efficiency boosting. The hollow points mark the y of the base systems^ The ftill points demonstrate the shift by adding the block polymer PEP5-PE05 at 6=0.05. Again the less efficient the base system the stronger the efficiency boosting effect.

Fig. 3. Dependence of the characteristic X-points for water-n-alkane-CioE4 systems (hollow points)^ on oil chain length. The effect of adding 5=0.05 of the block copolymer PEP5-PE05 is to lower the amount of surfactant needed for all oil chain lengths k (full points). 3.5. Discussion The individual observations of the efficiency boosting by the block copolymers in the C10E4 (in ref 2) and CgEs (in this paper) systems have been generalized by studying the effect for systems with various surfactant and oil chain lengths. Here we found the efficiency boosting by the polymer to occur for all these systems (see Fig. 2 and 3). Also, the shorter the chain length of the surfactant of the base system the stronger the boosting effect, and the more the lamellar phase is suppressed. We have recently reported^ that the efficiency boosting effect is accompanied by a comparatively large length scale increase in the microemulsion. Also a decrease of the ultralow interfacial tension between water- and oil-rich phases was seen. These features were concluded to be related to the location of the amphiphilic block copolymer in the film while extending the individual blocks into the adjacent sub-phases. The effect of polymer adsorption on the curvature and rigidity of monolayers and bilayers has been discussed in a number of papers.^^"^^ Recent theoretical considerations by Lipowsky and coworkers^^'"^^ describing the effects of anchored polymers were found to provide a basis

44

of discussion^ for the block copolymers considered here. These authors discuss mushroomlike conformations of the individual blocks of the polymers. However, in our system we have to consider polymeric ends on both sides of the monolayer which oppositely affect curvature and jointly add to the bending elastic properties. The exact architecture of the amphiphilic film with block copolymers we presently investigate by SANS experiments. Acknowledgment. We thank Dr. Allgaier, Prof Dr. Richter and Prof Dr. Gompper of the FZ Julich for pleasant cooperation.

REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28)

Jakobs, B.; Sottmann, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter, D. German Patent Application No. 19839054.8,1998. Jakobs, B.; Sottmann, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter, D. Langmuir, 1999, 75, 6707 Strey, R. ColloidPolym. Sci. 1994, 272, 1005. Sottmann, T.; Strey, R. J. Chem. Phys. 1997,106, 8606. Sottmann, T.; Strey, R.; Chen, S.H. J. Chem. Phys. 1997,106, 6483. see also: Burauer S.; Sachert T.; Sottmann T.; Strey R., Phys. Chem. Chem. Phys. 1999, 7, 4299. Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233. Endo, H., Allgaier, J., Gompper, G., Jakobs, B., Monkenbusch, M., Richter D., Sottmann T., Strey R. Phys. Rev. Lett. 2000, 85,102 Kunieda, H.; Shinoda, K. J. Coll. Interface Sci. 1985,107, 107. Allgaier, J.; Poppe, A.; Willner, L.; Richter, D. Macromolecules 1997, 30, 1582. Allgaier, J.; Willner, L.; Richter, D. German Patent Application No. P 19634477.8, 1996. Poppe, A.; Willner, L.; Allgaier, J.; Stellbriick, J.; Richter, D. Macromolecules 1997, 30, 7462. Shinoda, K.; Friberg, S. Adv. Coll. Interface Sci. 1975, ^, 281. Kunieda, H.; Friberg, S.E. Bull. Chem. Soc. Jpn. 1981, 54, 1010. Kahlweit, M.; Strey, R. Angew. Chem. Int. Ed 1985, 24,654. Kahlweit, M.;Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. Kahlweit, M.;Strey, R.; Busse, G. Phys. Rev. E 1993, 47,4197. Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. Strey, R.; SchomScker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc. Faraday Trans. 1990, 86, 2253. Golubovic, L. Phys. Rev. E 1994, 50, R2419 Morse, D.C. Phys. Rev. E 1994, 50, R2423 Morse, D. C. Current Opinion in Colloid & Interface Sci. 1997,2, 365 Gompper, G.; KroU, D.M. Phys. Rev. Lett. 1998,81, 2284 Ji, H., Hone, D. Macromolecules, 1988,21, 2600 deGennes, P.G. J. Phys. Chem. 1990, 94, 8407 Brooks, J.T., Marques, CM., Gates, M.E., J. Phys. II, 1991,1,673 Hiergeist, C ; Indrani, V.A.; Lipowsky, R. Europhys. Lett. 1996, 36,491. Lipowsky, R. Colloids and Surfaces A 1997,128, 255. Breidenbach, M.; Netz, R.R.; Lipowsky, R. Europhys. Lett. 2000, 49, 431.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

^

Thermotropic phase behavior of binary cationic surfactant mixtures in water Shoji Kaneshina, Hitoshi Matsuki, Ryoichi Ichikawa and Toshiharu Kuwahara Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506, Japan Thermotropic phase behavior of four systems of aqueous binary cationic surfactant mixtures, that is, hexadecyltrimethylanmionium bromide (C16TAB)-octadecyltrimethylammonium bromide (C18TAB), CI6TAB-tetracaine hydrobromide (TCHBr), C18TAB-TCHBr, and C18TAB-dibucaine hydrobromide (DCHBr) systems, has been examined by means of a differential scanning calorimetry (DSC). Tetracaine and dibucaine are micelle-forming local anesthetics. In all the systems, the transition temperature from the coagel phase (so-called hydrated solid) to the micellar solution, which can be regarded as the Krafft temperature in the binary surfactant solutions, showed a minimum at a certain composition, that is, an eutectic point. In the binary systems including local anesthetics, two components are miscible with each other in the micellar solution but are immiscible in the coagel phase. Therefore, the transition temperature vs. composition curves are reproducible by a simple eutectic model assuming ideal mixing in the micellar phase. In the system of C18TAB-DCHBr, the eutectic point was indeterminable because of the Krafft temperature of DCHBr to be below 0 T . In the binary surfactant system of C16TAB-C18TAB, the transition temperature from the coagel phase to the micellar phase had a minimum at an eutectic point, and the phase diagram showed partial miscibility in the coagel state like a solid solution. The difference in surfactant miscibility in between coagel and micelle was elucidated by the DSC of aqueous binary surfactant mixtures. 1. INTRODUCTION The properties of the mixed micelles and mixed adsorbed films of surfactants have been widely studied from the theoretical and practical interest. The miscibility of binary surfactants has been discussed thermodynamically for the micellar and adsorbed states of various surfactant mixtures [1]. On the other hand, few studies [2-4] have been reported on the miscibility in the hydrated crystal. In our previous study [5] differential scanning calorimetry (DSC) has been used to elucidate the phase behavior of aqueous solutions of a homologous series of cationic surfactants, alkyltrimethylanmionium bromide. Three kinds of endothermic peaks were observed by the heating scans of DSC. The endothermic peak observed at the highest temperature is assigned to the transition from the coagel phase (or the so-called hydrated crystal) to the micellar solution, which corresponds to the Krafft point. In the present study, the methods of DSC and surfactant solubility are employed to examine the phase behavior of aqueous binary surfactant solutions. The surfactant miscibility in the coagel phase as well as that in the micellar solution is discussed from the composition-dependent manner for transition temperature.

46 2. EXPERIMENTAL Four cationic surfactants, hexadecyltrimethylammonium bromide (C16TAB), octadecyltrimethylammonium bromide (CISTAB), tetracaine hydrobromide (TCHBr) and dibucaine hydrobromide (DCHBr) were used. Tetracaine, 2-(dimethylamino)ethyl-4(butylamino) benzoate, and dibucaine, 2-butoxy-N-[2-(diethylamino)ethyl]-4quinolinecarboxamide, are tertiary amine local anesthetics, which have the ability tofrommicelle by themselves. TCHBr and DCHBr were synthesized from their free base with hydrobromic acid in an ethanol solution. Thermotropic phase behavior of four systems of aqueous binary surfactant mixtures, that is, C16TAB-C18TAB, C 16TAB-TCHBr, C18TAB-TCHBr and C18TABDCHBr, has been examined. DSC of aqueous binary surfactant solutions was performed with a Seiko SSC-560U calorimeter. Weighed amounts of the binary surfactant solutions of 0.06 ml were sealed in DSC cells made of silver. Prior to the calorimetric scans, the temperature of the DSC cell was kept at 5 °C for a suitable period of time (2-14 days). The scanning rate was 0.8 K min-^ in an ascending mode. Since the solubility of surfactants in water increases abruptly above the Krafft temperature, the Krafft point is approximately equal to the temperature at which the hydrated solid surfactants are completely dissolved above the critical micelle concentration (CMC). The Krafft point was determined by the previous method [6] using a spectrophotometer (Hitachi 100-60 model) with a jacketed cuvette. The hydrated solid surfactants were precipitated by cooling from the micellar solution. Then the solution was heated slowly (0.5 K minO with stirring. The Krafft point was determined by observing the disappearance of turbidity during the course of heating. The cuvette temperature was monitored by a digital thermometer (chino DI) with a C-C thermocouple probe inserted into the cuvette. 3. RESULTS AND DISCUSSION The CMC, Krafft temperature and enthalpy changes of transition from coagel to micelle for four surfactants are summarized in Table 1, which were taken from our previous studies [5,7-9]. The aqueous binary surfactant solutions with various compositions were prepared at the total surfactant concentration of 0.03 mol kg-^ for C16TAB-C18TAB system and 0.3 mol kg"^ for other systems taking into account of the CMC for these surfactants. DSC thermograms for binary surfactant system showed one or two endothermic peaks, which are corresponding to the Krafft temperature and the eutectic horizontal temperature. As is seen from Fig. 1, the U-ansition Table 1 The CMC, Krafft temperature and enthalpy changes of coagel-micelle transition {AH) for four surfactants Surfactant

CMC(molkg-0

C16TAB C18TAB TCHBr DCHBr

0.00092 0.00034 0.109 0.062

Krafft temp. (K) 299.1 311.9 313.9 < 273.15

A//(kJmol-0 55.4(10.2) 62.5 (± 0.2) 36.6 (±0.2)

47

temperatures from the coagel phase to the micellar solution in the binary surfactant solutions showed a minimum at a certain composition, that is, an eutectic point. The temperature of coagel-micelle transition can be regarded as the melting temperature of the coagel or the freezing temperature of the micelle. With respect to the coagel-micelle equilibrium, we assume the pseudo two component system of surfactants 1 and 2, which is based upon the idea of organized solution [10]. Excess water is assumed present, so that the water phase can be neglected. The temperature of coagel-micelle transition in the simple eutectic system is give by

(1)

1/7 = 1/r, - (/?/A//,)ln(l - X2) for component 1 and 320

330 (b)

290 0 0.2 (C16TAB)

0.4

0.6

0.8

Xj

1 (01 STAB)

290 (C16TAB)

0.2

0.4

0.6 Xj

0.8

1 (TCHBr)

330 (c)

320

-

Micelle

/ O

310

\^

/

Micelle^ Coagel D

ID ^ ^

"°

0

^

Coagel D

n

300 Coagel

290 (C18TAB)

1

0.4

0.6

0.8

1 (TCHBr)

(C18TAB)

Fig. 1. Phase diagrams of binary surfactant mixtures in water. Transition temperatures from coagel to micelle determined by the DSC (O) and solubility (•) methods are shown as a function of mole fraction, X2. The eutectic horizontal temperatures (D) were observed by DSC. Solid lines show the transition temperature calculated from a simple eutectic model (Eqs. 1 and 2). (a) C16TAB-C18TAB, (b) C16TAB-TCHBr, (c) C18TAB-TCHBr and (d) C18TAB-DCHBr systems.

48

\IT = 1/72 - (/?/A//2)lnX2

(2)

for component 2, where 7, and T2 are the Krafft temperature of surfactant 1 and 2, respectively, A//j and A//2 are the enthalpy changes of transition from coagel to micelle for respective surfactants, and X2 is the mole fraction of component 2. 3.1. C16TAB-C18TAB system The phase diagram of C16TAB-C18TAB system is shown in Fig. 1(a). The Krafft temperature of binary mixtures has a minimum. Solid line shows the Krafft temperature calculated from Eqs. 1 and 2 using thermodynamic data shown in Table 1. The eutectic composition was calculated as 0.26, which is in good agreement with the experimental result, but the eutectic temperature differs slightly in the calculated and experimental values. The eutectic horizontal temperatures were observed only in the mole fraction of 0.1 - 0.4 on the phase diagram. Judging from the phase diagram, it may be concluded that both surfactants of C16TAB and CI STAB are completely miscible with each other in the micelle, but in the coagel phase both surfactants are partially miscible with each other. 3.2. C16TAB-TCHBrand ClSTAB-TCHBr systems The phase diagrams of both systems are shown in Figs. 1(b) and 1(c). Higher temperature of transition was observed for pure surfactants, which is responsible for the high concentration of surfactants. As is seen from phase diagrams, the transition temperature vs. composition curves for both systems can be reproduced by a simple eutectic model assuming ideal mixing in the micellar solution. The eutectic point was calculated as X2 = 0.319 and T = 295.8 K for the C16TAB-TCHBr system, X2 = 0.487 and r = 304.5 K for the C18TAB-TCHBr system. Since the eutectic horizontal temperatures were observed throughout the whole composition ranges, two components in these systems are miscible with each other in the micellar solution, but are inmiiscible in the coagel phase. 3.3. ClSTAB-DCHBr systems The phase diagram of this system (Fig. 1(d)) does not show the eutectic point because the Krafft temperature of DCHBr lays under 0 °C. The depression of Krafft temperature of C18TAB by the addition of DCHBr can be well reproduced by a simple eutectic model. REFERENCES 1. K. Ogino and M. Abe (eds.) Mixed Surfactant Systems, Marcel Dekker, New York, 1992. 2. M. Hato and K. Shinoda, J. Phys. Chem., 77 (1973) 378. 3. K. Tsujii, N. Saito and T. Takeuchi, J. Phys. Chem., 84 (1980) 2287. 4. Y. Moroi, T. Oyama and R. Matuura, J. Colloid Interface Sci., 60 (1977) 103. 5. S. Kaneshina and M. Yamanaka, J. Colloid Interface Sci., 131 (1989) 493. 6. S. Kaneshina, H. Kamaya and I. Ueda, J. Colloid Interface Sci., 83 (1981) 589. 7. H. Matsuki, R. Ichikawa, S. Kaneshina, H. Kamaya and I. Ueda, J. Colloid Interface Sci., 181(1996)362. 8. H. Matsuki, S. Hashimoto, S. Kaneshina and M. Yamanaka, Langmuir, 10 (1994) 1882. 9. H. Matsuki, M. Yamanaka and S. Kaneshina, Bull. Chem. Soc. Jpn., 68 (1995) 1833. 10. K. Shinoda, Langmuir, 7 (1991) 2877.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyamaand H. Kunieda (Editors) c 2001 Elsevier Science B. V. All rights reserved.

49

Active Control of Surfactants Nicholas L. Abbott Department of Chemical Engineering, University of Wisconsin, Madison WI 53706, USA. E-mail: [email protected] This review describes some recent work at University of Wisconsin that is aimed at the development of strategies for active control of interfadal properties of aqueous solutions of water-soluble surfactants [1-7]. These strategies, which indude the use of redox-active surfactants in combination with electrochemical methods, make possible spatial and temporal control of the adsorption of surfactant at interfaces. Here we focus on a portion of our work that is based on the oxidation and reduction of ferrocene-containing surfactants, such that changes in the oxidation state of ferrocene substantially perturb the equilibrium partitioning of surfactant between an interface and bulk solution. Because the behavior of liquids on millimeter and smaller scales are often dominated by interfadal stresses, we illustrate how these surfactants can provide new materials and methods for microfluidics. We report use of these surfactants to achieve vectorial transport of droplets of liquid through a network of fluidic channels and to pattern the dewetting of liquids on energetically homogeneous surfaces. The work described below is based on the use of ferrocenyl surfactants that have the structure Fc(CH2)nN*(CH3)3Br', where Fc is the redox-active ferrocene group. A surfactant witih this structure (n=ll) was first synthesized by Saji and coworkers and subsequently used in studies of the deposition of thin films of organic dyes on electrodes [8,9]. The ferrocene group consists of an Fe^* ion sandwiched between two cydopentyldienyl anions and it is, therefore, an electrically neutral complex. The complex has a volume of --150 A^ (roughly three times the volume of a methyl group) and a low solubility in water (5 x 10'^ M). Ferrocene, when hosted in surfactants with the structure given above, can undergo a reversible one electron oxidation in aqueous solution to form the ferrocenium cation. The redox potential for the oxidation is typically around 0.15 V (versus SCE). Oxidation of the ferrocene to the ferrocenium cation transforms the ferrocenyl surfactant with a single ionic headgroup into a surfactant with two ionic headgroups (one at each end of the molecule). We have synthesized ferrocenyl surfactants with n = 8 (I*), 11 (ir), 15 (Iir) as well as a dimeric ferrocenyl surfactant (DI*^).

50

Figure 1 shows equilibrium surface tensions of aqueous solutions (0.1 M 02804, pH 2) of IV and If* that were measured using the Wilhelmy plate method (and confirmed using the pendant drop and du Nouy methods). The reduced surfactant (IV) has a critical micellar concentration (CMC) of 0.1 mM and a limiting surface tension of 49 mN/m (measured at concentrations above the CMC). The oxidized surfactant (IP*) is not measurably surface active below concentrations of 0.1 mM. Oxidation of 11* to IP* at a concentration of 0.1 mM does, therefore, change the equilibrium surface tension of the aqueous solution from 49 mN/m to 72 mN/m. The oxidation of 11* to iV* is reversible over many cycles, and we have measured the reduction of IP* to 11* to result in a return of the equilibrium surface tension to 49 mN/m (see below). The measurements shown in Figure 1 reveal the largest changes in equilibrium surface tension (-23 mN/m) upon oxidation of 11* to IP* to occur at a concentration of 0.1 mM. At concentrations of surfactant above 0.1 mM, the change in surface tension upon oxidation of 11* to IP* is smaller than the maximum value (22 mN/m at 0.1 75 HI 1 ' ' ""'^ mM) because the H • • • 70 L B surface tensions • 1 of the solutions i • 65 Lf of IP* are lower 0 • than that of the 60 L[ aqueous solution i 55 L 0 of electrolyte h • (not containing 0 50 L 0 0 0 surfactant). We • 0 0 measured the • 45 r H surface tension of a • aqueous ..u. 40 "' 102 solutions of IP* IOO 10-3 10-2 IOI 10-1 to decrease with Concentration of Surfactant (mM) increasing concentration of Figure 1: Equilibrium surface tension as a function of IP* without any concentration for aqueous solutions of II in oxidized sign of a CMC. (closed drcle) and reduced (open circle) states. Measurements performed with 0.1 M Li2S04, pH 2,25 °C. Surprisingly, above concentrations of 10 mM, oxidation of 11* to IP* no longer leads to an increase in surface tension but rather a decrease in the surface tension. This decrease in surface tension can be reversed by reduction of IP* back to 11*. We also point out that the changes in surface tension shown in Figure 1 have been observed when using both chemical (Fe2(S04)3 as oxidizer) and electrochemical oxidation of 11* to IP*. By using the Gibbs adsorption equation, we estimate the limiting surface areas 1 1 1 iin|

rn

1

! • !11 i i i i |

T — 1 - 1 T T T "*T~

1

1

IIIWj

C

J

P

T

p

Lu

1

1 111,11 i

1 1 mill

1

a..

Ull

1

1 1kXkJ . . 1

«

• • • Miui

occupied by molecules of IV and 11^* to be 85±4 A^ and 65±4 A^ respectively. This result is also somewhat surprising because it suggests that oxidation of IV to 11^* at concentrations greater than -ImM leads to an increase in the excess surface concentration of these surfactants even though the charge carried by each surfactant is increased upon oxidation. In order to understand the origin of the above described interfadal behavior of 11* and 11^*, we developed a molecular thermodynamic model for Gibbs monolayers of these surfactants. The model, which is described in detail elsewhere [6], assumes equilibrium to exist between surfactants dissolved in the bulk solution and at the interface. The results of the model are in good agreement with experimental measurements of surface tension and offer several insights into the balance of forces that controls the surface activity of ferrocenyl surfactants. First, the model reveals that the limiting surface area occupied by a molecule of IV hosted within a Gibbs monolayers is large because IV adopts a looped conformation at the surface of the aqueous solution. This looped configuration is a result of an effective attraction between the ferrocene group of the surfactant and the aqueous subphase. Second, the model indicates that the desorption of surfactant from the surface of the solution which accompanies oxidation of a 0.1 mM solution of IV to 11^* is caused by a reduction in both the hydrophobic driving force for adsorption and the change in electrostatic contributions to the standard free energy of formation of the Gibbs monolayer. In particular, the configurational (chain packing) contribution to the standard free energy of formation of the monolayer does not drive the oxidation-induced desorption of the surfactant from the surface of the solution because the surfactant adopts a looped conformation in both oxidized and reduced states. Third, the model also offers an explanation of the oxidation-induced adsorption of the ferrocenyl surfactant to the surface of a solution containing high (>1 mM) concentrations of the surfactant. As noted above, aqueous solutions of IV possess a CMC of 0.1 mM, and thus at concentrations greater than 0.1 mM for ir, the chemical potential of the surfactant changes little with concentration. In contrast to aqueous solutions of 11^"^, no experimental evidence for the existence of a CMC at concentrations up to 30 mM was determined for solutions of 11^*. That is, oxidation of IV in aqueous solutions with concentrations between 0.1 mM to 30 mM leads to the dissolution of micelles of 11* to singly dispersed molecules of 11^*. Accompanying the disruption of the micelles is an increase in the cratic contribution to the chemical potential of the surfactant in the bulk aqueous solution. The increase chemical potential in the bulk drives the adsorption of surfactant onto the interface and leads to the decrease in the limiting area per molecule at the interface. The increase in the excess surface concentration of molecules at the interface, in addition to the increase in the electrostatic contribution to the surface pressure, results in a decrease in surface tension of the solution.

52

The final section of this review illustrates the manner in which the above described principles for active control of the interfaciaJ properties of liquids can be used to form the basis of new ways to drive liquids into motion and to position liquids on surfaces in periodic arrays. First, we comment that a simple arrangement of electrodes can be used to dispense or consume IV at localized regions, thereby creating gradients in the surface excess concentration of ferrocenyl surfactant. The resulting gradients in surface tension cause surface flows (Marangoni phenomena) directed away from the cathode. Sulfur dust sprinkled on the surface can be used to visualize the flow. Application of a reducing potential (-0.3 V) at one electrode and an oxidizing potential (0.3 V) at a second electrode causes the surface fluid to flow away from the cathode. Reversing the oxidizing and reducing potentials causes the surface to flow in the opposite direction. We have used the electrochemical oxidation and reduction of ferrocenyl surfactant to direct the flow of liquid through a network of channels. Figure 2 shows a network of four intersecting channels. Three of the four channels end at Pt electrodes while the channel at the top ends at a reference electrode and a coimter electrode. The fluidic network was filled with 0.3 mM 11^* to a depth of -1 mm. Small drops of a nematic liquid crystal (LC), 4-n-pentyl-4'-cyanobiphenyl, were placed on the surface so that the fluid flow could be visualized easily through crossed polarizers. Application of oxidizing (0.3 V) and reducing (-0.3 V) potentials to any

Figure 2: Time lapse images of pumping of LC droplets across the surface of an aqueous solution of 0.3 mM II (0.01 M Li2S04). Platinum electrodes protrude through the surface of the solution at the ends of the left,right,and lower channels. The end of the top channel contains an SCE and a counter electrode. A) Droplet of LC is dispensed at the bottom channel. The LC droplet is pumped by application of -0.3 V to the bottom electrode and 0.3 V to therightelectrode. B) Droplet of LC is pumped to the left electrode by application of -0.3 V to the right electrode and 0.3 V to the left electrode. C) E>roplet of LC dispensed at the bottom is pumped by application of -0.3 V to the bottom electrode and 0.3 V to the left electrode.

53

pair of electrodes caused the LC droplets to be pumped between the two electrodes. The velocity of the droplets was controlled by the magnitude of the potential applied to the electrodes. The oxidation and reduction of IV was also used to demonstrate flow of LC droplets across an unconfined surface.

t-Os

mmmm t = 10s

Because local changes in the surface tension of aqueous solutions can give rise to localized imbalances of force at the threephase contact line of liquid supported on a surface and thereby place a liquid into motion, we have explored the use of ferrocenyl surfactants in strategies for patterned wetting and dewetting of aqueous solutions on Figure 3: top view of dewetting of an surfaces. We illustrate this aqueous film of 0.3 mM II (0.01 M capability here by using ferrocenyl Li2S04, pH 1.3) supported on surfactant to form a 2-dimensional microscope slide patterned with a mesh array of droplets on the surface of of electrodes (dark region). Etewetting glass (Figure 3). The glass was was induced at the top left comer of the patterned by evaporating 50 A of microscope by application of 0.5 V (vs. titanium and 500 A of gold SCE) to the electrode. The receding film through a micromachined leaves droplets on the regions of glass. aluminum mask. The surfaces The image was illununated from Sie were made energetically lower right side causing shadows to homogeneous by immersing into a form diagonal lines across the droplets. solution of 0.9 mM 11mercaptoundecanoic add and 0.1 mM hexadecyl mercaptan for 30 minutes. After application of a thin film of aqueous solution of i r , an oxidizing potential (0.5 V) was applied to the array of electrodes. The aqueous solution dewets the gold electrodes leaving droplets on the glass. Without any applied potential, no patterned dewetting was observed. The patterned dewetting can be repeated on the same surface by reapplying a fresh film of aqueous surfactant solution. Whereas gradients in surface tension caused by changes in the oxidation state of i r drive the phenomena shown in Figure 1, the patterned dewetting of solutions of W is caused by electrochenucally induced changes in the contact angle of the solution on the surface. Surface plasmon reflectometry measurements suggest that

54

electrochemically induced desorption of surfactant from the solid-liquid interface plays a central role in the phenomena. An increase in the contact angle leads to a net force on the contact line causing the solution to dewet the electrode. In other work, we have shown that spreading can be induced by electrochemical control of gradients in surfactant concentration near the contact line. In conclusion, the results summarized above demonstrate the use of ferrocene-based surfactants in combination with electrochenucal methods to achieve both spatial and temporal control of the adsorption of surfactant at interfaces. This capability provides new ways to control interfadal phenomena in surfactant systems, including Marangoni effects and the spreading and wetting of liquids on surfaces. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

B.S. Gallardo, M.J. Hwa, N.L. Abbott, Langmuir, 11 (1995) 4209. D.E. Bennett, B.S. Gallardo, N.L. Abbott, J. Am. Chem. Soc. 118 (1996) 6499. B.S. Gallardo, K.L. Metcalfe, N.L. Abbott, Langmuir, 12 (1996) 4116. B.S. Gallardo, N.L. Abbott, Langmuir, 13 (199^ 203. B.S. Gallardo, V.K. Gupta, F.D. Eagerton, L.I. Jong, V.S. Craig, R.R. Shah, N.L. Abbott, Science, 283 (1999) 57. N. Aydogan, B.S. Gallardo, N.L. Abbott, Langmuir, 15 (1999) 722. J.Y. Shin, N.L. Abbott, Langmuir, 15 (1999) 4404. T. Saji, K. Hoshino, I. Yoshiyuki, G.J. Masayuki, J. Am. Chem. Soc. 113 (1991) 450. T. Saji, K. Hoshino, S. Aoyagui, J. Am. Chem. Soc. 107 (1985) 6865.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) cc) 2001 Elsevier Science B.V. All rights reserved.

55

Droplet microemulsion and telechelic polymer: linear rheology and flow instability at high shear. G. Porte, M. Filali, E. Michel, J. Appell, S. Mora, E. Sunnyer and F. Molino Groupe de Dynamique des Phases Condensees, Case 026, Universite MontpelUer II 34095 MontpelUer Cedex 05, France Our purpose is to examine in detail the flow behaviour of model transient networks. Such a network is obtained from an o/w droplet microemulsion into which we incorporate a hydrophobically end capped hydrosoluble polymer. At high enough droplet and polymer concentrations, the system exhibits viscoelastic behaviour with quasi maxwellian stress relaxation. The variations of the elastic modulus and terminal time at the percolation threshold are discussed in the light of the Tanaka-Edwards theory: droplets rearrangements after the escape of a sticker are essential. Under steady shear, a sharp flow instability is observed. We show that it arises from a fracture like formation of a lubricating layer at the surface of the shear cell. INTRODUCTION Temporary networks are intuitivelly appealing examples of viscoelastic materials and a lot of theoretical effort have been spent to account quantitatively for their rheological behaviour both in the linear regime and under high deformation [1,2]. The finite instantaneous elastic modulus G(0) arises from the existence of junctions capable to sustain the stress while the terminal time T/^ is related to the finite residence time of the extremities of the strands into the junctions [3]. The most refined theories to date rely however on two simplifying assumptions [2]: i) the deformation of the network is homogeneous at all scales, (affine to the macroscopic deformation applied); ii) once one end of an active strand snaps off a junction, it definitely forgets the strained state of the network. These theories predict quasi maxwellian stress relaxation after step strains of moderate amplitude (linear regime) and shear thinning behaviour at high rates under steady shear (due to the shorter residence time of strands submitted to high tensions). These predictions agree qualitatively with the observations. However, although reasonnable for densely linked networks, the above mean field assumption becomes questionnable for tenuous network close to the percolation threshold. Moreover, the flow curve in steady shear often show up an unexpectedly sharp drop of the stress. To examine these points, we study transient networks in which the average number density and ftinctionnality of the junctions can be controlled separately. They are obtained from an o/w droplet microemulsion into which we incorporate a hydrophobically end capped water soluble polymer. The microemulsion involves TXlOO and TX35 as non-ionic surfactant, the oil is decane and the proportions are chosen so to fix the size of the droplets (82A radius checked by neutron scattering) at all concentrations. The telechelic polymer is a 10k POE end grafted

56 with C18H37. The endcaps stick to the oil droplets with a residence time (of the order of Is) controlled by their degree of hydrophobicity. The droplet concentration determines the number density of the junctions while the concentration of added polymer (number r of stickers per droplet) determines their average fimctionnality. In section 1 we report on the percolation behaviour from step strain experiments. Comparisions with simple numerical simulations show that droplets reorganization due to the balance of strand tensions play a crucial part in the stress relaxation. In section II we characterize the flow instability under steady shear and show that its origin is located near the walls of the shear cell. 1. LINEAR RHEOLOGY: In all samples, the droplet volume fraction is 10%. Their average distance is thus of the order of the end to end distance of a free 10k POE coil: so bridging is easy. Rheological measurements are performed with a strain controlled rheometer (Rheometrics RFSII). Figure 1 shows typical stress relaxation after a moderate step strain. The behaviour is close to Maxwell; the fit with a slightly stretched exponential is very good:

l U

^ 1 I I I I I I I I I I I I I I ! I I I j I I I I i I 1 ^

lOOOi £

o

100 10

0.1 0

0.49

0.97 time (s)

1.5

Figure 1: Stress relaxation for the 10% droplet sample with r=18 stickers per droplet

0.85 X G{t) = a(t)/Yo = G(0)exp(-(r/T^)^°^)

(1)

allowing an accurate determination of G(0) (=2300Pa) and of T/? (= 0.137s). The stress relaxation as function of r is fitted for all sample with expression (1). Both G(0) and T/^ decrease upon decreasing r and vanishes below a finite threshold value rp.

57 I I I 11 I I I I I I I I I I I I I I I I •

I I

2000 CO

6

1000

0

'

0

'

•

'

' ^ ^ • ^ '

5

I

I

I

•

I

10

I

I

I

15

I

I

'

I

I

20

I

I

I

25

r Figure 2: G(0) as function of r at 10% fraction of droplets

Figure 2) illustrates the percolation behaviour of G(0) at r [4]. The continuous line is a fit with the power law: G(0,r) = Go(r-rJ^

(2)

which gives rp= 4.23 for the threshold and ^ = 1.42 for the exponent. Of course a percolation pattern is not unexpected: afiniteminimum connectivity density must be exceeded so that an infinite connected cluster builds up capable of transmitting the torque all through the gap between the mobile and the fixed wall of the shear cell. We note however that these values do not coincide with those reported in the litterature [5] for bond percolation calculated on simple cubic lattice (threshold about 1.5 and exponent 1.7). But in our situation, some polymers loop on the same droplet and therefore do not contribute to the stress. Moreover, neighbouring droplet may be linked by two or even more polymers strands. These specific features may account for the discrepancies. A mean field simplification is usually postulated at the starting point of the current interpretation of the stress relaxation: macroscopic deformations applied to the sample are assumed to propagate homogeneously at all scales into the material. Immediately after the application of a step strain, the initially isotropic distribution of the nodes is affinely convected so that, depending on their initial orientation with respect to the shear direction, threads are stretched or compressed compared to their initial length. The resulting anisotropic distribution of tension is at the origin of the intantaneous elastic stress a(t = 0). As time goes on, stickers randomly escape from the droplets with a characteristic time (the residence time: T^.^^). Each time a sticker escapes, the tension or compression of its thread vanishes immediately and so does its contribution to the total stress. Of course, the escaped sticker will soon recombine with another droplet, but it will do so in an isotropic manner and therefore no longer participate to the stress history: this is the second simplifying assumption in the current interpretation. In this primitive

58

description, ait) is simply proportional to the number r(t) of stickers per droplet that have not yet escaped at time t and therefore are still under tension or compression. Since the escape is a random process, we expect: G(r)/G(0) = r(t)/r = exp(-r/r,,,)

(3)

that is a Maxwell relaxation with r^^^ as characteristic time. The experiments confirm a quasi-Maxwellian stress relaxation. But, according to the above mean field description, we would expect G(0,r)ocr in contradiction with the singular behaviour of G(0,r) at r^. Clearly, not all the threads deform affinely to the macroscopic strain tensor: threads which belong to finite size clusters as well as dangling threads decorating the infinite cluster are indeed not elastically active. Only the threads which belongs to the skeleton of the infinite cluster are active. In order to remove the assumption of homogeneous affine deformation, one could start from the r dependence of G(0) as fitted from the experiment (expression (2)) and write: Git) = G(0,r(r)) = Go(r(r) - r^)^

(4)

the modulus Git) at time / is so taken identical to the instantaneous modulus of a sample having a degree of connectivity equal to r(r). Assuming again the stickers to escape at random: r(r) = rexp(-r/r^^J

(5)

one gets: Git) = Go(rexp(-r/T,,,) - r^)^

(6)

Proceeding so, we extend the effect of the nonhomogeneous connectivity of the network evidenced in the intantaneous elastic response to all the later stages of the stress relaxation. However expression (6) is in obvious contradiction with the quasi-Maxwellian relaxation found experimentally: all stress in (6) is totally removed after a finite time t = T^^^ln(r/rp) and it cannot coincide with the observed stretched exponential decay. Therefore, correcting for the non-homogeneous, non-affine deformation alone is clearly not sufficient. We must consider the second explicit assumption of the current theories -namely that escaped threads no longer participate to stress history. A given droplet, has a mechanical equilibrium position where all tensions of the attached threads compensate. Each time a sticker escapes, the tension balance is broken and the droplet moves to a new equilibrium position within a typical time certainly much shorter than the residence time of the stickers. Therefore, at all stage of the stress relaxation, the position distribution of the droplets rearranges so insure instantaneously mechanical equilibrium. As mentioned above, a sticker escaped at time t inmiediately recombines with another droplet but after having relaxed its initial tension: the new junction formed bears no tension. However as time proceeds, the relative positions of the droplets changes so that a tenseless thread at

59 time t may well find itself stretched or compressed (with respect to the equilibrium length distribution) at time t' >t. Therefore, due to the rearrangement of the droplets positions, a sticker having escaped early from the initial droplet, will nevertheless participate to the later stages of the stress relaxation. We believe that tensions arising after recombinations and equilibrations explain the regular character of the stress relaxation at long time. Numerical simulations on a 2D distribution of beads and springs are performed and support our interpretation.

2. FLOW INSTABILITY AT HIGH STEADY SHEAR RATES

600

CO CO

300 h

(D

rate (s'"*)

Figure 3: Stress versus rate for the 10%r=21 sample The flow curve of the sample with 10% droplet fraction and r= 21 under steady shear rate is shown in figure 3). It is obtained with a titanium cone and plate; the cone angle is 0.02rad. Special attention is paid to make sure that all data points correspond to true steady state (slow transients are observed close to the discontinuity). There is two regimes with a sharp drop in stress at the cross over. The low shear rate regime is close to Newtonian with a viscosity % = 270 Pa.s. The discontinuity occurs at a rate Yinst"^ ^-^^^ ^^^ ^ stress 0^^^^^= 400 Pa. Between 1.6s"^ and 3s"\ we find a range where the stress is a decreasing function of the rate. At still higher rates, we recover a regime where the stress increases linearly with the rate but according to the unexpected form: (7 = 0"y

+ n/^r

(7)

The positive apparent "yield stress" Oy (80 Pa) is reminiscent of a Bingham behaviour and the effective viscosity 7]/^^= 23 Pa.s is much lower than % .

60 Homogeneous flows are unstable in the range where the stress decreases as the rate increases. The drop in stress then suggests the onset of a fracture-like non-homogeneous flow pattern: a lubricating layer of low viscosity forms above Yinst- ^^ such fracture were to appear at any position in the gap, additionnal fractures would occur each time the stress exceeds Oif^st- This is not the case on the upper stable branch in figure 4. So, the fracture occurs at a special position in the gap: that is at the wall of the shear cell To check this point, we established the flow curve (figure 4) using different aluminium tools: sand blasted cone with 1.5° angle, polished cone with 1.5° again, polished cone with 3° angle. Considerable differences are actually observed for the flow instability (see figure 4 for an illustration). 300

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I M [ I I I I

i sand blasted 1 OH

150

Figure 4: different surface roughnesses To sum up: -the slope of the upper stable branch {7]^^) only depends on the cone angle (roughly proportional) but not on the surface roughness (figure 4). -on the other hand the onset of the instability (cr,^^^; and Yinst)^ ^^ ^^^^ ^^ ^^^ y'\Q\d stress Gy, decrease strongly when the surface is smoother. -of course the viscosity in the Newtonian low rate regime is invariant. So the stress drop discontinuity involves a shear induced transformation of the material in the vicity of the shear cell surface. Note that the stress pattern is different from that observed in case of simple sliding at the interfaces. Specially intriguing is the yield-like aspect of the high shear regime (cr^). We currently study the influence of the chemical nature of the surface (more or less hydrophobic) on the onset of the instability in order to elucidate the adhesion of the network onto the shear cell wall. REFERENCES [1] M. S. Green and A. V. Tobolsky, J. Chem. Phys.,U, (1946), 80. [2] F. Tanaka and S. F. Edwards, Macromolecules, 25, (1992), 1516. [3] T. Annable et al, J. Rheology, 37, (1993), 695. [4] H. Bagger-Jorgensen et al, Langmuir,U, (1997), 4204. [5] D. Stauffer, Introduction to percolation theory, Taylor & Francis Pub, London, (1985)

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) •c 2001 Elsevier Science B. V. All rights reserved.

61

Synthesis and micelle formation of fluorine-containing block copolymers Kozo Matsumoto*, Taku Kitade, Hiroaki Mazaki, Hideki Matsuoka, and Hitoshi Yamaoka^ Department of Polymer Chemistry, Kyoto University, Kyoto 606-8501, Japan ABSTRACT: Fluorine-containing block copolymers composed of poly{2-(2,2,3,3,3pentafluoropropoxy)ethyl vinyl ether)} (polyPFPOVE) and poly(2-hydroxyethyl vinyl ether) (polyHOVE) were synthesized and their micelle formation in water was investigated. The block copolymers were prepared by sequential polymerization of PFPOVE and 2-acetoxyethyl vinyl ether (AcOVE), followed by hydrolysis of the acetyl-protecting group. The surface tension of the block copolymer solution decreased to ca.32 mN/m. The experimental data of the small-angle X-ray scattering (SAXS) measurement of 1 wt% aqueous polymer solution were well-reproduced by the calculated scattering curves for core-shell micelle models. 1. INTRODUCTION Fluorine-containing block copolymers are a new series of polymer materials.'" They have attracted much attention because of the unique properties of fluorinated segments such as low surface energy, high contact angle, reduced coefficient of friction, high biocompatibility, and lipoand hydrophobicity. We have recently reported the synthesis and the micelle formation of fluorinecontaining amphiphilic block copolymer, poly(2-hydroxyethyl vinyl ether)-/7/d?c/:-poly[2(2,2,2trifluoroethoxy)ethyl vinyl ether] (poiy(HOVE-6/(9c/:-TFEOVE)).' In this study, we synthesized a new amphiphilic block copolymer with a higher fluorine content, that is polyiHOVE-blockPFPOVE), and examined its surface activity and micelle formation. 2. EXPERIMENTAL SECTION Measurements. Gel permeation chromatography was carried out in chloroform on a JASCO GPC-900 equipped with four polystyrene gel columns (Shodex K-802, K-803, K-804, and K-805). 'H NMR spectra were recorded on a JEOL GSX 270 spectrometer in CDCI3. Surface tension was measured on a CBVP-Z (Kyowa Interface Science Co., Ltd.) using a Pt plate in full automatic mode. Small-angle X-ray scattering (SAXS) of polymer solution was measured using a Kratky-type camera manufactured by Rigaku Corporation installed on a rotating anode X-ray generator and a position-sensitive proportional counter.

^Present address: Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500, Hassaka, Hikone, Shiga, 522-8533, Japan

62 Synthesis of Poly(HOVE-^/oc/c-PFPOVE). Polymerization was earned out under nitrogen by addition of /z-butyl vinyl ether-HCl adduct and ZnCK into a mixture of PFPOVE and in methylene chloride at -40 ""C. After PFPOVE had been converted, AcOVE was added, and the mixture was allowed to warm at -20 °C. The polymerization was quenched with methanol containing ammonia The mixture was washed with water, extracted with diethyl ether, and concentrated to give poly(AcOVE-Z?/oc/:-PFPOVE). The acetoxy groups of the polymer were hydrolyzed by treatment with aqueous sodium hydroxide in 1,4-dioxane. Removal of impurities by dialysis in water followed by freeze drying gave poly(HOVE-6-PFPOVE). MJM„ values were determined by GPC with a polystyrene standard calibration for poly(AcOVE-/7/(9ci(:-PFPOVE). The number-average degrees of polymerization of AcOVE segment (m) and PFPOVE segment (n) were determined by 'HNMR. Data Analysis of SAXS Measurements. model were calculated as follows:

The theoretical profiles for a spherical core-shell

I(q) =/np[47c/3 • R^^p.-p^WR^q) + 4n/3 • R,^(p,-i)o)<^(Rsq)]% 0(x) = 3[(sinx-xcosx)/x']

(1)

where np is the number density of the micelle, R^ and R, are the radii of the core and of the whole micelle, p^, p^, and Po are the electron densities of the core, shell, and solvent, respectively, q is 4nsinB/X with X-ray wavelength X and scattering angle 26, and/is the shift factor. It was assumed that the electron density distributions inside the core and the shell were homogeneous. Using the electron density of the monomer unit PPFPOVE ^<^ PHOVE» PC ^^^ P$ ^ ^ given by Pc = PPFPOVE

(2)

Ps = solPo + (l-
(3)

where (])«,, is the volume fraction of the solvent in the shell, which can be calculated by the following equation with the degree of polymerization of HOVE (m), the volume of HOVE repeated units(VHovE)» ^he volume of the core (V^), and the volume of the overall micelle (VJ: sol=l-^aggmVHovE/(Vs-V,)

(4)

Nagg denotes the aggregation number of the micelles, which is calculated from V^, the degree of polymerization of PFPOVE (n), and the volume of PFPOVE repeated units (VPFPOVE)^agg = Ve/(nVpppovE)

(5)

Assuming that all polymers contribute to the micelle formation, the number density np of the micelles is then calculated "p =
(6)

where 0 is the volume fraction of copolymer in solution. The values of PPFPOVE = 0.435, PHQVE = 0.382, and Po = 0.334 [A"] were calculated from bulk densities of polyPFPOVE (1.50 [g/cm^]), polyHOVE (1.17 [g/cm']), and H,0. 3. RESULTS AND DISCUSSION Synthesis of Poly(HOVE-^/ocA:-PFPOVE). AcOVE was added as the second monomer to a cationic living polyPFPOVE. Four block copolymers having different compositions of AcOVE and PFPOVE were synthesized. The GPC charts for the obtained polymers shifted toward higher

63 moleculai- weight regions by the addition of AcOVE, keeping the monomodal shapes {MJM^ =1.09). This indicated the clean formation of the block polymers. The obtained copolymers were easily converted to poly(HOVE-^/t7C)^PFPOVE)s by a basic hydrolysis of the acetyl protecting group in 1,4-dioxane. All block polymei*s prepared in this study (m:n = 61:21, 52:21, 39:20, 31:21) were soluble in water, CHCI3, and CH.Cl,.

OCHo

9

0^

n-C4H9

o HO

CH2 CF2CF3

Fig. 1. Chemical structure of poly(HOVE-^/oc^-PFPOVE)

Surface Activity of the Block Copolymer. 80 • 61:21 \ 1 To examine the surface activities of poly(HOVE70 • 52:21 1I t hlock'VYVOVE) in aqueous media, the surface tension 60 A 39:20 of the aqueous solution was measured. Fig. 2 shows so • 31:21 1 the surface tension of aqueous block copolymers. - • * 40 Ambiguous breakpoints in the surface tension curves 30 were observed around IxlO"* mol/L, indicating the 20 existence of a critical micelle concentration (CMC). ; 10 All polymer solutions showed a decrease in the surface \ 0 tension to less than 35 mN/m with increasing polymer concentration, indicating the high surface activities of Concentration (mol/L) tliese polymers. Small Angle X-ray Scattering (SAXS) Fig. 2. Surface tension of poly(HOVE Measurement. SAXS measurements of 1.0 wt% -block-?¥VOVE) aqueous solutions. aqueous solutions (above CMC) were carried out. Fig. 3 shows the SAXS profiles for poly(HOVE-^- Table 1. Stmctural Parameters PFPOVE)s (m:n = 61:21, 52:21, 39:20, 31:21). In all Poly(HOVE-^/oc^PFPOVE) Micelles in 1.0 wt% Aqueous Solutions. cases, strong scattering was observed in the small-angle Copolymer R; K ^J
'*t* ^

64 1000 F

1000 r

100 t-

100 t-

0.01

0.1

0.2

1000

0.1

0.01

0.2

qU'^l

qU'M

1000

100 t-

100

qU-'l

Fig. 3. SAXS profiles of poIy(HOVE-Z7/oc^PFPOVE) 1.0 wt% aqueous solutions. m:n = 61:21 (a), 52:21 (b), 39:20 (c), 31:21 (d). Solid lines are theoretical curves of core-shell spheres. 4. CONCLUSIONS The high surface activity of a new fluorine-containing block copolymer, poIy(HOVE-/?PFPOVE), was confirmed by surface tension measurements and its micelle formation in water was also confirmed by a SAXS analysis. A theoretical calculation of the SAXS profile revealed that the micelle has a core-shell spherical morphology. Further investigations on the properties of the fluorine-containing polymer micelle are now in progress. Acknowledginents: The present work was supported by a Grant-in-aid for Encouragement of Young Scientists (No. 11750764) from Japan Society for the Promotion of Science. REFERENCES 1. (a) J. Sheirs, (ed.). Modem Fluoropolymers, Johy Wiley & Sons, New York, 1997. (b) E. Kissa, (ed.), Fluorinated Surfactants; Surfactant Science Series, vol. 50, Marcel Dekker, New York, 1994. 2. (a) M. Miyamoto, K. Aoi, and T. Saegusa, Macromolecules, 22, 3540 (1989). (b) T. Ishizone, K. Sugiyama, Y. Sakano, H. Mori, A. Hirao, and S. Nakahama, Polym. J. 31,983, (1999). 3. K. Matsumoto, M. Kubota, H. Matsuoka, and H. Yamaoka, Macromolecules, 32, 122 (1999).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B.V. All rights reserved.

65

A Study of the Gelation of the Polysaccharide Curdlan Hongbin Zhang*, Katsuyoshi Nishina^i^ Tim J. Foster**, Martin A. K. Williams*" and Ian T. Norton*' ^Department of Food & Nutrition, Faculty of Human Life Science, Osaka City University, Sumiyoshi, Osaka, 558-8585, Japan *TJnilever Research, Colworth House, Shambrook, Bedford, MK44 ILQ United Kingdom Gelation mechanism of curdlan was studied by confocal microscopy, pulsed *H NMR relaxation, DSC and rheological measurements. 1. INTRODUCTION Curdlan is a bacterial polysaccharide formed by fermentation of Alcaligenes faecalis and its linear structure (Fig.l) is composed entirely of 1,3-P glucosidic linkages that occur widely in nature. Since it has unique gelling properties in the ability to form either a thermo-reversible or a thermo-irreversible gel* and shows strongly physiological functions such as anti-tumor and anti-HIV activities, recently, curdlan has attracted much attention, and may become one of the industrially important biopolymers.

Fig.l Chemical structure of curdlan In contrast to the majority of gelling synthetic polymers, where the gelation is commonly attributed to formation of a covalently crosslinked network, the gelation of biopolymers such as polysaccharides, is attributed to physical interactions such as hydrogen bonds, for example, in the cases of agarose, gelatin, carrageenan, and gellan gum etc., and hydrophobic interactions in cases such as hydroxypropylmethylcellulose and methylcellulose. Generally, polysaccharide gels formed exclusively by hydrogen bonding or by hydrophobic interactions have the opposite temperature dependence of the elastic modulus. The former melts on heating whereas the latter melts on cooling.

66 Curdlan is insoluble in water but capable of forming two distinct types of gel according to the temperature-time history of the sample. In the present work the gelling characteristics of curdlan aqueous suspensions has been studied by pulsed ^H NMR, DSC, microscopy and rheological measurements. 2 MATERIAL AND METHODS Curdlan was supplied by Takeda Chemical Industries Ltd., Japan, with a molecular weight of 1.92X10^ and polydispersy index (Mw/Mn) of about 1.3 as determined by static light scattermg in an alkaline solution. Its intrinsic viscosity is 5.19dl.g'^ in a 0.2N NaOH aqueous solution at 25°C. Curdlan aqueous suspensions were prepared by using a homogenizer and then stirred at room temperature before measurement. The development of morphology of the sample, labeled with a fluorescence dye of aqueous rhodamine solution of 0.05% w/w, was monitored by a confocal laser scanning microscope system (Biorad MRC 600) using a x 60 objective and video recorder (Video Mag. x 2000) and laser excitation wavelength of 488nm at a scan rate of 2°C/min from 25 to 90°C. Rheological measurements were carried out using a strain-controlled Fluids Spectrometer RFSII with parallel plate geometry of 25mm in diameter and 1.5mm in gap. DSC measurements were performed in a Setaram micro DSC-III calorimeter. Pulsed ^H NMR measurements were carried out at 23 MHz using a Resonance Instruments MARAN spectrometer. Spin-spin relaxation times, Ti, were recorded using the CPMG pulse sequence. 3. RESULTS AND DISCUSSION Curdlan in the solid state may exist in a triple helical structure^ or partial triple helical chains that bind with single helical chains^. It is poorly crystalline"* and is manufactured as a granule^ in which extensive hydrogen bonding strongly binds the helices to form microfibrils that are themselves associated. Despite the lack of water solubility, curdlan is readily dispersed in water and in the present work a paste-like suspension has been prepared, at room temperature, by using a homogenizer. The variation of the storage modulus G' exhibited by a 2% curdlan sample as a function of temperature is shown in figure 2. Initially, in the temperature range 30-55°C, the rheological results show a slight increase followed by a sight decrease in elastic modulus. If a sample is subsequently cooled following this thermal history then a low temperature set gel is formed (results not shown). Such gels are believed to be associated with space-filling by swollen hydrated granules, that nevertheless retain much of the integrity of internal triple helical organisation. Video microscopy obtained during this temperature regime does indeed clearly evidence the swelling of the granules and the NMR data shown in figure 3 show a reduction

67 in Ti that is consistent with increased hydration facilitating increased accessibility to exchangeable biopolymer hydroxyl protons. On further heating it is observed that, at ca. 55°C, the suspension becomes clear. Indeed, it is clear from all the techniques employed in this study that there is a significant organisational change occurring at around this temperature. The sample becomes more optically transparent, DSC reveals the presence of an endothermic peak at 40-63 °C (as shown in figure 2), and the NMR relaxometry evidences a significant increase in Tj that is consistent with an increase in biopolymer chain mobility (as shown in figure 3). Furthermore, figure 4 shows confocal images of 2% curdlan at 50 and 60°C respectively (The snapshots were reproduced from a videotape). The initial microstructures below -SO'^C are seen to contain fme long fibrous strands which are very closely packed. However, above ~60°C, such fibrous strands are not observed and the brightness waned significantly. It has been suggested that the appearance of the endothermic peak in DSC on heating is attributable to the breakup of intra- and intermolecular hydrogen bonds existing in microfibrils of curdlan^ and that above this temperature, curdlan triple helices dissociate^. Indeed the data presented here from a number of diverse techniques is consistent with this view. With continued heating the storage modulus increases and a high temperature set gel is formed. The NMR data is again consistent with this picture, showing a decrease in Ti that would be consistent with increasingly restricted chain motion. On subsequent cooling of this sample it can be seen in figure 2 that the resultant gel becomes more resilient especially below 40°C, consistent with the appearance of exothermic peaks in the DSC that are considered as reformation of hydrogen bonds and/or molecular ordering. However, the details of the gel properties at such lower temperatures depend critically the temperature-time sample history. Indeed the NMR results clearly show that on holding at 80°C, in preference to subsequent cooling, the gel still forms and Ti continues to decrease, clearly demonstrating the kinetic aspect of the gel formation, once initiated by the pre-requisite molecular rearrangement. On subsequent cooling, although a slight perturbation in the relaxation behaviour can be seen (consistent with the DSC exotherm and storage modulus increase seen in figure 2), this is substantially reduced compared to that found if the sample is subject to immediate cooling. This clearly establishes a connection between the "completeness" of high temperature set gel formation and the ability of the system to reform hydrogen bonded or molecularly ordered entities at lower temperatures. The relative importance of hydrophobic chain interactions and severe topological entanglement in controlling the high temperature set gel formation and determining its properties is, at present, unclear and this area is being addressed in ongoing work.

68

Cooling If-

itf-

—o—G"

Heating

DSC Up-

«

n

80

90

T/«C

Fig. 2 Temperature dependence of G' for a 2% curdlan. The temperature course of thermal change in DSC is shown for direct comparison. Scan rate: TC/min.

Fig. 3 NMR spin-spin relaxation time T2 plot of 2% curdlan as a function of temperature (heldat80°C forSh). Scan rate: TC/min.

Fig. 4 The confocal images of 2% curdlan sample (Left 50°C, Right 60°C). ACKNOWLEDGEMENTS The authors would like to thank D.R. Martin and R Knight for carrying out NMR measurement and microscopy, respectively.

REFERENCES 1. T. Harada, M. Terasaki and A. Harada, in Industrial Gums, 3rd Ed. R. L. Whistler and J. N. BeMiller (eds.), Academic Press, New York, 1993. 2. Y. Deslandes, R. H. Marchessault and A. Sarko, Macromolecules, 13 (1980) 1466. 3. H. Saito, in Viscoelasticity of Biomaterials. W. Glasser and H. Hatakeyama (eds.), American Chemical Society, Washington, DC, USA, 1992. R. H. Marchessault, Y. Deslandes, K. Ogawa and R R. Sundararajan, Can. J. Chem., 55 (1977)300. 5. Y. Kanzawa, T. Harada, A. Koreeda and A. Harada, Agric. Biol. Chem., 51 (1987) 1839 6. A. Konno, K. Okuyama, A. Koreeda, A. Harada, ' Kanzawa and T. Harada, in Food Hydrocolloids: Structures, Properties and Function K. Nishinari and E. Doi (eds.). Plenum Press, New York, 1994.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. Ail rights reserved.

69

Formation of highly swollen L^-phases and vesicle phases from single chain surfactants by chemical reactions H. Hoffmann, K. Horbaschek, J. Hao* Department of Physical Chemistry I, University of Bayreuth, D-95440 Bayreuth, Germany Several general methods for the preparation of highly swollen lyotropic La-phases fiom single chain surfactants are described. It is shown that these La-phases can be prepared by mixing of surfactants with cosurfactants, by mixing of anionic and cationic surfactants, by combining ionic surfactants with hydrophobic counterions and surprisingly by mixing of Ca-salts of ionic surfactants with zwitterionic surfactants. Independently of their chemical composition the La-phases have several features in common: If the bilayers of the La-phases are ionically charged, and the phases are prepared by mixing of the components whereby the phases are exposed to shear forces, La-phases are obtained that have highly viscoelastic properties and consist of densely packed uniand multilamellar vesicles. If, on the other hand, shear forces are avoided in the preparation, low viscous La-phases with stacked bilayers are obtained. Shear can be avoided when the phases are prepared with the help of a chemical reaction that produces one necessary component of the final phase in the stagnant fluid. Several reactions are described which can be used for the preparation of La-phases. It is shown that phase transformations to La-phases can be accomplished by the production of protons, of cosurfactants, or of ionic surfactants by chemical reactions. When the La-phases are produced by one of these reactions classic stacked bilayer phases are always obtained. These phases can easily be transformed to vesicle phases by exposing the La-phases to shear forces.

1. Introduction In recent years many investigations in surfactant science have been concerned with vesicles. The first vesicles have been observed in biological systems and they consisted of phospholipids like lecithin. Vesicles from these compounds can be prepared by sonication or extrusion of phospholipid dispersions [l]. Later on it has been observed that double chain surfactants in general can also form vesicles [2]. More recently vesicles have also been observed in solutions of single chain surfactants [3, 4, 5]. Home address: Department of Chemistry, Shandong Normal University Jinan 250014, PR China

70

Several general methods are known today with which vesicles can be prepared from single chain surfactants or mixtures of single chain surfactants and cosurfactants. It has been claimed in some of these situations that the vesicles form spontaneously when the components are mixed. However, it is also known that shearing forces can have a large influence on micellar structures. Actually, several groups have shown that swollen Laphases are transformed to vesicle phases when the La-phases are exposed to shear [6, 7, 8]. When two micellar solutions are mixed the micellar phases and hence the micellar structures in these phases are exposed to shear. The question therefore arises whether the vesicles that have been reported in some mixed systems and where it was claimed that the vesicles had formed spontaneously, were not the result of the shearing forces in the mixing process. In this paper we answer the question related to this problem. In the first part several general methods will be shown which are known today with which vesicles or vesicle phases can be prepared from single chain surfactants. In this part the vesicles are prepared by mixing two solutions. The vesicles will be characterised by FFTEM micrographs and by their rheological properties. In the second part the same phases are prepared in a different route in which the fmal micellar structures are not exposed to shear forces. We would like to emphasize that in this investigation situations are discussed in which La-phases are produced in the single phase region. The results may be different when two phase regions are formed by the mixing process.

2. Results and discussion 2.1. Preparation of vesicle phases 2.1,1. Vesicles from surfactants and cosurfactants Many groups have shown that micellar solutions of zwitterionic or nonionic surfactants are transformed with increasing cosurfactant concentrations into a vesicle phase, a classic La-phase and a sponge phase. If the total surfactant concentration is above 100 mM, one usually obtains single phase situations. These sequences of phases have been observed for the cosurfactants from pentanol to octanol. The L3-phase is usually stable only in a very narrow cosurfactant/surfactant ratio but over a large concentration region [9, 10]. When this uncharged Ls-phase is mixed with a few mole-percent of an ionic surfactant, either cationic or anionic, it is spontaneously transformed into a viscoelastic phase with multilamellar vesicles [ l l ] . Such vesicle phases have been studied in great detail. They are stable for long times. Their viscoelastic properties and their yield-stress are due to the electrostatic repulsion between the ionically charged bilayers. A FF-TEM micrograph of a vesicle phase that has been prepared from a Ls-phase and is charged with a cationic surfactant is shown in fig. 1. A rheogram of the phase is shown in fig. 2. When salt is added to this phase the shear modulus of the phase decreases and the yield stress disappears.

71

10'^

10°

frequency/Hz Fig. 1. FF-TEM micrograph of a La-phase Fig. 2. Rheogram of a viscoelastic ionically with multilamellar vesicles. Composition of charged La-phase with multilamellar vesicles. the phase: 90 mM C H D M A O , 10 mM Composition of the phase: 100 mM C M D M A O , 220 mM CeOH, 10 mM HCl. CnTMABr, 220 mM CeOH, T = 25 ° C. Note that the storage modulus G' (o) is independent of frequency and the loss modulus G" is about 10 times lower than G\ 2,1.2. Vesicles form cationic and anionic surfactants

"1 81

7-

L,

6-

•

L„'L, •

\J

^ s'.

o

^

2.7.2.7. The combination TTABr + SL

L

• 4-

L^

•

- 3 ^ iitf

'

•

210- 1 0

'

1

10

'

1

20

'

1

30

'

1

40

•

1•

50

,

,

60

,

1

70

,

,

80

,

•

•

1

•

•

1.90 1 7100 • no1

c (lauric acid) [mM] Fig. 3 a. Phase diagram of a 100 mM solution of TTAOH when titrated with lauric acid. Note that the conductivity (K) in the vesicle phase Lv is very low and the minimum of K is at the equimolar composition.

A very popular way of preparmg vesicles consists of mixing cationic and anionic surfactant solutions. Several systems have been studied by the group of E. Kaler [3, 12]. Usually the triangular phase diagram of such systems is symmetric to the equimolar mixing ratio. Very often one observes for the equimolar mixing ratio a precipitate while to the right and left of the equimolar situation a vesicle phase is observed. In contrast to the vesicle phase in chapter 2.1.1. this vesicle phase is a low viscous phase, because the vesicles have a small charge density and because the remaining charge is shielded by the salt that is formed from the two counterions of the anionic and cationic surfactants. Vesicle phases without excess salt from cationic-

72

COTABr) [mM]

Fig. 3 b. Phase diagram for the situation when a 100 mM solution of sodium-laurate (SL) is mixed with a 100 mM solution of tetradecyltrimethylammonium-bromide (TTABr). Note the precipitation at the equimolar position and the vesicle phase L* to the left and to the right of the precipitation region.

anionic surfactants can easily be prepared when the hydroxide of the cationic surfactant is titrated with an alkylcarbonic acid. Such a cut in a phase diagram is shown in fig. 3 a. For comparison the phase diagram for the systems that is prepared from TTAB and sodium laurate is given in fig. 3 b. Both systems contain the same phases. In the shielded system the La-phases to the left and to the right of the precipitate region are wider [13, 14]. 2.7.2.2.

Ci2(EO)25S03H

CMDMAO

Cationic-anionic surfactant systems can also be prepared by adding the acids of alkylsulfonates or alkylsulfates to zwitterionic surfactants [15]. The acids protonate the zwitterionic surfactants what results in the formation of a cationic surfactant that combines with the anionic surfactant to the cat-anionic CJDMAO ^ l""'' f"^***! surfactant systems. In fig. 4 the Viscoutl phase diagram for 100 mM mixtures of tetradecyldimethylamineoxide and dodecylethoxysulfuric acid 1.0 X [mole fraction] C.,DMAO (DES) is shown. With increasing mole-fraction of DES one finds f rst a Li-phase, then a two phase Li/LaFig. 4. Phase diagram of a 100 mM solution of CuDMAO with a 100 mM solution of the region, a single La-phase, a acid of dodecylethoxysulfate (DES). Note the precipitate phase, a single La-phase, two La-phases to the left and right of the a two phase Li/La-region and finally again a Li-phase. The precipitate precipitation zone. dissolves for T = 50° C and forms a La-phase. The two La-phases to the left and to the right of the precipitate region show very weak birefringence. The two phases differ however in their rheological properties. The La-phase on the left hand side is an uncharged phase and for this reason has low viscoelastic properties while the Laphase on the right hand side is ionically charged and has strong viscoelastic properties. Theoretically it is possible to use the dodecylsulfuric acid in combination with C14DMAO for the preparation of the cat-anionic surfactant system. Preliminary experiments have shown however that this combination forms a precipitate over a much

73

Fig. 5. FF-TEM micrograph of a La-phase with multilamellar vesicles in the system Ci4DMAO/Ci2(EO)xS03H with the composition 100 mM C14DMAO / 30 mM C,2(EO)xS03H.

wider mixing ratio. It is thus more favourable to use the DES for the preparation of the system. The acid form of the DES was prepared from sodium dodecylethoxysulfate (SDES) by ion exchange process. As already indicated by its extremely weak birefringence, the La-phase that is prepared by mixing the two surfactants consists of vesicles as is shown in a micrograph in fig. 5. In contrast to the vesicles in fig. 1 the vesicles in fig. 5 are smaller in size and do not contain large multilamellar vesicles. ionic 2.1,3, Vesicles from hydrophobic surfactants and counterions

The surface activity and the phase behavior of an ionic surfactant depend strongly on the • • l(X)-j hydrophobicity of the counterions. 1 Early measurements on homologues • • • 10^ • series like tetradecyltrimethylanmionium-alkylsulfonates have •j h globular micdles entangled vesicles j shown that both the CMC, the 2* 0,h rodlike miceiles •j surface tension at the CMC and the 0.01 ^ • • phase behavior change continuously • with the chainlength in the alkylrlE-3—1 • r ' r group. For short alkyl-groups like c (HNQ [mM] methyl and ethyl, the systems Fig. 6. Phase diagram of a 100 mM solution of behave like hydrophilic surfactants CTAOH when titrated with 3-hydroxy-2- with univalent counter-ions while for longer R-groups the systems napthocarboxylic acid. behave more like double chain surfactants. The main switch in the phase behavior occurs around the R-group with seven CH2-groups. While the systems with short alkyl-groups in the counterion have the typical phase sequence Li, Hi, La, the systems with the longer alkyl-groups show the sequence Li/La- Hydrophobic counterions with a large hydrophobicity are counter-ions based on benzene or naphthene. A particular interesting counter-ion is the 3-hydroxy-2-naphthalene-carboxylate [16, 17]. If this counter-ion is combined with a typical cationic surfactant the result is a bilayer phase. In fig. 6 the sequence of phases is shown when a solution of 100 mM CTAOH is titrated with 3-hydroxy-2-naphthoic acid. With increasing counter-ion concentration one 1 un)-i

«

1

'-

1

'

1

•

1

1

«

j

.- . 1 •I

•1

1

1

r-

—1

'

74

observes first a low viscous Li-phase, then a highly viscoelastic Li-phase and finally across an extremely narrow Li/La two phase region, a single region La-phase. The Laphase shows similar rheological properties as the La-phases prepared in section one and two and is formed from multilamellar vesicles. 2.1.4. Vesicles from Ca^salts of ionic surfactants and zwitterionic surfactants 0,7

It is generally known in surfactant science that there is a large isotropic. synergism between anionic and zwitterionic surfactants [18]. Mixtures of the two surfactants h03 i> usually have lower CMC's and have a lower surface tension than either yoa^ of the two components. In many 1-0.1 mixtures of the two surfactants, the viscosity shows a maximum with 0.6 the mixing ratio. The reason for the large synergism between the two Fig. 7. Partial phase diagram of a solution of components is likely the fact that the 100 mM CuDMAO and 50 mM Ca(DS)2. positive charge from the zwitterionic surfactant can attract the negative charge of the anionic surfactant and in this way the packing of the micellar interface becomes more dense than that in the case of the two components. In spite of their large synergism, mixtures of the two components are completely miscible and form a single Li-phase for moderately concentrated solutions of a few percent. Recently, it was shown that the synergism between zwitterionic and anionic surfactants is even larger when anionic surfactants with bivalent counter-ions are used for the combination [19]. For the mixtures between the C14DMAO and Ca(DS)2 the synergism becomes strong enough in order to form a La-phase. This is shown in fig. 7. As is obvious from the comparison of fig. 7 and fig. 3 the combination of Ca(DS)2 and C14DMAO acts practically as a mixture between a cationic and an anionic surfactant. 2.2. Preparations of swollen La-phases with the use of chemical reactions in the solutions at rest 2.2.1. A lamellar phase from a Ls-phasefrom C14DMAO and methylformiate In the discussed preparations of the vesicle phases two solutions were mixed under stirring. The final phases and the micellar structures in the phases have therefore been exposed to shearing forces. In this chapter, we show how the same phases can be prepared by different routes in which the same final compositions are reached but in which the final micellar structures have not been exposed to shear. In chapter 2.1.1., we had prepared a La-phase by mixing an ionic surfactant or a strong acid like formic acid with a L3-phase. The result of the procedure was a viscoelastic transparent phase that

75

Fig. 8. Birefringence pattern with time in a Ls-phase of 100 mM C M D M A O and 250 mM CeOH when mixed with 10 mM methylformiate; (a) = 18 min; (b) = 30 min; (c) = 1 hour; (d) = 1 day, (e) = after completion of the hydrolysis, (f) and (g) = after briefly shaking, sample (f) has been centrifuged. contained densely packed multilamellar vesicles. Now, we mix the La-phase with a precursor of the formic acid, namely methylformiate [20]. Obviously, during the mixing of the components we also shear the La-phase. However, the hydrolysis reaction is so slow that the La-phase is at rest again before any substantial amount of formic acid has been produced. Even though the La-phase is rather sensitive to different additives, the La-phase is stable towards the addition of small amounts of methylformiate. The transformation of the La-phase to the La-phase can be detected simply by viewing the sample. The La-phase is an isotropic phase and the phase looks therefore dark between crossed polarizors. The La-phase is a birefringent phase. If one observes therefore the sample with time between crossed polarizors, one can clearly see the developing birefringent speckles with time, at first very weak and then becoming more bright with time. A sequence of the pattern with time is shown in fig. 8. For comparison, we also show the birefringence pattern that is obtained after one day and after the sample had briefly been shaken. Now, the pattern looks very different. It is thus possible just by visual inspection of the samples to conclude that the structure in the La-phases that are prepared with and without shear must be different even though their compositions are the same. In order to demonstrate the difference in the structures even more clearly we prepared a FF-TEM micrograph from the samples that had not been sheared. In order to avoid the shear that is normally associated with the filling of ihe copper disks during the preparation of the sample for the rapid freezing, the La-phase with the Fig. 9. A micrograph of a La-phase (composition methylformiate was filled between 100 mM CuDMAO + 220 mM CeOH + 10 mM the copper sandwich and the MF) that has been prepared without shear.

76 chemical reaction in the sample occurred while the sample was in the sandwich. After the reaction was completed the sample was plunged into liquid propane and was then prepared for the TEM micrograph. The result of the whole procedure is seen in fig. 9. The micrograph shows a perfectly stacked classic La-phase and no vesicles. The micrograph unambiguously demonstrates that the chemical reaction in the sample at rest leads to a La-phase and not to vesicles. The vesicles as shown in fig. 1 were thus not formed spontaneously but were the result of shear during the mixing process. The vesicles can now easily be prepared from the stacked La-phase simply by shaking of the samples. The macroscopic properties of samples that contain vesicles and samples that contain the classic La-phase are very different. The vesicle phase is a viscoelastic phase with a yield stress value and a high storage modulus while the La-phase is a low viscous phase with a storage modulus that is about 2 orders of magnitude lower than that in the vesicle phase. These features are typical for the two phases and can be used to differentiate the two phases. 2.2.2. The La-phase form SDES, CiJDMAO and the hydrolysis of methylformiate In section 2.1.2.2. we had discussed the phase diagram and the preparation of vesicles from C14DMAO and DES and the use of an acid, for the protonation of C14DMAO [15]. Now we add methylformiate, the precursor of the acid, to the low viscous Li-phase of SDES and C14DMAO. With time the samples develop from an isotropic Li-phase over a macroscopically separated Li/La situation into the final birefringent La-phase in the single phase region. The final state that is prepared in this way looks very different from the state that is obtained when the Li-phase from SDES and C14DMAO is mixed with formic acid. In particular the unsheared phase is transparent and birefringent while the sheared phase is turbid and more or less isotropic. Furthermore, the sheared phase is viscoelastic while the unsheared phase is a low viscous phase. 2.2.3. The La-phase form tetradecyUrimethylammonium laurate

0 5 u*n

Fig. 10. FF-TEM micrograph of a La-phase Fig. 11. FF-TEM micrograph of the same (composition 100 mM TTAOH + 25 mM system as in fig. 10 when the phase was lauric acid + 73 mM ML) that has been prepared with shear, prepared without shear.

77 In section 2.1.2.1, it has been shown that the La-phases that are prepared from TTAOH and lauric acid consist of vesicles and that the phases with 100 mM surfactant are highly viscoelastic. Now, we would like to produce the same phases by a process in which the final phases are not exposed to shear. In principle we could think of solubilizing methyllaurate (ML) directly into TTAOH and then wait for the hydrolysis to occur. The solubilisation capacity of TTAOH for ML would probably be large enough in order to reach the La-phase. The problem lies with the solubilisation time. The alkaline hydrolysis is rather fast and we would have to solubilize the ML in a time that is short in comparison to the hydrolysis. Even though we did not try, we doubt that this will be possible because it is known from other investigations on solubilisation that this process is rather slow. For this reason we preferred another strategy [13]. We solubilized ML in a vesicular phase of TTAL. We had expected that the vesicles would be transformed to microemulsion droplets by the ML. The experiment showed however that at a ML/TTAL ratio of about one we still have vesicles. When this phase is mixed however with TTAOH we obtain rather quickly an isotropic Li-phase which can now be left at rest for the hydrolysis to proceed. Even though the samples change their appearance with time from turbid to clear to birefringent, the samples never phase separate macroscopically. The final phase is a birefringent stacked La-phase. The structure of the La-phase is demonstrated in a FF-TEM micrograph in fig. 10. When the phase is sheared its properties change from low viscous to highly viscoelastic fig. 11. All the investigated systems demonstrate unambiguously that vesicles are not spontaneously formed when two surfactant solutions are mixed in such a way that bilayer structures are formed which have zero curvature. In the systems in which vesicles are produced the vesicles are the result of the shearing forces during the mixing process and not due to the adjustment of the hydrophilicity of the system. If the shearing is avoided and the hydrophilicity of the system is adjusted by the use of a chemical reaction in a stagnant phase a classic bilayer phase is obtained. It is without consequence whether the La-phase is approached from the more hydrophilic Li-phase or from the more lipophilic Ls-phase.

3. Conclusions Highly swollen aqueous La-phases can be prepared from combinations of single chain surfactants or from surfactants and cosurifactants. We describe several mixtures of surfactants and cosurfactants from which highly swollen La-phases can be prepared. Independently of their particular chemistry these phases have several features in common: The structures and the properties of the phases depend on the way the phases are prepared. When the phases are prepared from the compounds with the help of a mixing aid or simply by stirring the mixed systems, the final phases will consist of multilamellar and unilamellar vesicles. The size of the multilamellar vesicles (onion) depends on the strength and the duration of the shear stress to which the system has been exposed to. When phases with the same compositions are prepared in the absence of shear they consist of classic La-phases with stacked bilayers. Both phases with

78

identical composition but different morphology are stable for long times (months). Several routes are described for the preparation of La-phases in which the phase is not exposed to shear. In these routes one component of the final phase is formed by a chemical reaction in the stagnant solution. When the vesicle phases carry ionic charges which are not shielded by excess salt the phases are highly viscoelastic fluids with a yield stress value. The phases can be optically isotropic and can show little or no birefringence between crossed polarizers. The stacked La-phases on the contrary have much weaker viscoelastic properties and show domainlike birefringence. The different state of the phases can thus be easily recognised by visual inspection of the samples. The La-phases are easily transformed to the vesicle phases by modestly shaking the samples.

References 1. R. G. Laughlin, The aqueous phase behavior of surfactants, Academic Press, London, 1994. 2. D. D. Miller, J. R. Bellare, T. Kaneko, D. F. Evans, Langmuir 4 (1988) 1363. 3. E. W. Kaler, A. K. Murthy, B. E. Rodriguez, J. A. N. Zasadzinski, Science 245 (1989) 1371. 4. H. Hoffmann, C. Thunig, P. Schmiedel, U. Munkert, Langmuir 10 (1994) 3972. 5. J. Oberdisse, C. Couve, J. Appell, J. F. Berret, C. Ligouve, G. Porte, Langmuir 12 (1996) 1212. 6. D. Roux, F. Nallet, O. Diat, Euro Phys. Letter 24 (1993) 53. 7. J. Berghausen, J. Zipfel, P. Lindner and W. Richtering, Europhys. Lett. 43 (1998) 683. 8. M. Bergmeier, M. Gradzielski, H. Hoffmann, K. Mortensen, J. Phys. Chem. B 103 (1999) 1605. 9. C. A. Miller, M. Gradzielski, H. Hoffmann, U. Kramer, C. Thunig, Progr. Colloid Polym.Sci. 84(1991)243. 10. G. Porte, J. Marignan, P. Bassereau, R. May, J. Phys. 49 (1988) 511. 11. H. Hoffmann, C. Thunig, P. Schmiedel, U. Munkert, II Nuovo Cimento Vol. 16 D, N. 9, 1994. 12. E. W. Kaler, L. Kathleen, A. Herrington, A. K. Murthy, J. A. N. Zasadzinski, J. Phys. Chem. 96 (1992) 6698. 13. K. Horbaschek, H. Hoffmann, J. Hao, J. Phys. Chem. B 104 (2000) 2781. 14. Y. Chevalier, T. Zemb, Rep. Prog. Phys. 53 (1990) 279. 15. J. Hao, H. Hoffmann, K. Horbaschek, J. Phys. Chem. B, in press. 16. K. Horbaschek, H. Hoffmann, C. Thunig, J. Colloid and Interf. Sci. 206 (1998) 439 . 17. P. A. Hassan, B. S. Valaulikar, C. Manohar, F. Kern, L. Bourdieu and S. J. Candau, Langmuir 12 (1996), 435. 18. A. Shiloach, D. Blankschtein, Langmuir 14 (1998) 1618. 19. H. Hoffmann, D. Grabner, U. Homfeck, G. Platz, J. Phys. Chem. B 103 (1999) 611. 20. M. Bergmeier, H. Hoffmann, C. Thunig, J. Phys. Chem. B 101 (1997) 5767.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) cc 2001 Elsevier Science B. V. All rights reserved.

Functions and structures of

79

molecular assemblies under high

magnetic fields Sumio Ozeki Etepartment of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan Deformation in membranes of dipalmitoylphosphatidylcholine (DPPC) led to the fusion and division of its liposome and large changes in the membrane potential of its black membrane (BLM) under magnetic fields of up to 30 T. The magnetofusion and magnetodivision of DPPC liposomes significantly depended on their initial particle size and aromatic compounds doped. There seem to be discrete liposome sizes stabilized at a given magnetic field. The changes in liposome size due to magnetofusion give an estimation of the local curvature of the membrane. Undulation of a membrane due to high magnetic fields may relax any orientational defects in a bimolecular Upid membrane, which may cause the magnetoresponse in the membrane potential. The membrane potential and resistance of BLMs markedly in the region of 0.2 T. High magnetic fields induced much larger responses in their electrical properties, corresponding to magnetofusion. The magnetic-field effects on the electrical properties seem to occur not via the Lorentz force on the ion flux, but via the cooperative orientation of lipid molecules. The hexadecyltrimethylammonium/silicate hybrids were prepared in acidic and basic conditions under magnetic fields (< 30 T). With increase in magnetic field, (1) hexagonal structure developed in one-phase systems, but depressed in two-phase sysems, (2) lamellar structure was generally depressed, but promoted in certain basic conditions, (3) in two-phase systems their composition changed. Mesoporous silicas obtained by calcination of the hybrids showed specific pore structures and adsorptivity depending on magneticfieldintensity. These examples demonstrate potentiality of steady magnetic fields in control of structures and functions of molecular assemblies. 1. INTRODUCTION There are several methods for structural control of organized molecular assemblies, such as use of a flow and an electric field. A magnetic field is one of potential methods to align and orient molecules and domains, because it has an advantage that even diamagnetic materials can be

80

w
Figure 1. A typical example of the time course of changes in membrane potential ^ of a BLM of didcxtecyl phosphite with the application of various magnetic fields perpendicular to the membrane.

Figure 2 Apparent fixed charge density ^A^of the BLM estimated from the V'values as a function of the magnetic field (/f).

aligned by magnetic fields if they have the magnetic anisotropy. It is well established that diamagnetic assemblies having magnetic anisotropy will become oriented and rotate in a magnetic field so as to obtain the minimum-energy state. The magnetic orientational energy (E^) of a diamagnetic lipid domain containing A^ molecules (volume A^v), whose long molecular axis is at an angle ^ to H

and which have the diamagnetic anisotropy.

Ax, and the magnetic susceptibihty perpendicular to //, X^. is given by the following equation[l]: E, = -(/^/2)( A: -L-+ A A: COS' 0 )A^V

(1)

When H and/or N is large enough, the long molecular axis of lipid molecules in a domain can be cooperatively aligned in the direction of averaged (i>. We present here two examples for structural control and function regulation of lipid membranes [2-4] and surfactant/silicate assemblies [5]. Both topics are based on molecular orientation of diamagnetic organic molecules having magnetic anisotropy, which may lead to the magnetic deformation of lipid membranes and surfactant mesophases. 2. MAGNETIC REGULATION OF MOLECULAR ASSEMBLIES 2.1. Magnetic-field effects on liposomes and black membranes of phospholipids In a previous paper, it was reported that the membrane potential of black lipid membranes, comprising didodecyl phosphite (DP), changed remarkably under low, steady magnetic fields less than 0.5T[2]. The magnetic-field effects on the electrical properties seem to occur not via the Lorentz force on the ion flux, but via a cooperative orientation of Upid molecules. Another magnetic-field effect on a bilayer membrane comes from the elastic properties of the membrane. Helfrich[6] predicted theoretically the magnetic deformation of phospholipid liposomes from a sphere to an ellipsoid.

81 Fig. 1 shows a typical example of changes in membrane potential '? of a DP membrane with the application of magnetic fields {H) perpendicular to the membrane[2].

The BLM of DP or

DPPC in aqueous NaCl solutions was formed in the hole of a thin Teflon sheet which divided the Teflon cell into two compartments.

"^ arising across a BLM was measured by maintaining a

10-fold difference in the NaCl concentration (1x10'^ mol/dm^ and 1x10'^ mol/dm^).

The general

features in the magnetic response are its reversibility, the maximum response at around 0.2 T (H^) and the reverse response at H >0.3 T.

When a magnetic field was parallel to the lipid membrane,

no magnetic effect was observed, suggesting that no ion flow would be directly affected by the Lxjrentz force. We assume that magnetic fields should modify the ^parent fixed charge density X of the membrane.

The features in the experimental magnetic responses of "^ and R seem to be

consistent with each other via <j)X within the theory[7].

Fig.2 shows that the estimated X

changed with magnetic field through a minimum {^X^J at H^.

These trends in Fig. 2 may be

brought about by the molecular orientation in a domain, which leads to changes in the molecular density at the membrane surface.

When a lipid molecule tilts under a magnetic field, the

occupied molecular area increases monotonically with tilt angle and thus charge density would also decrease monotonically with magnetic field. On the other hand, with increasing a tilt angle, the hydrocarbon/water interface at the membrane surface should increase and destabilize the tilted structure.

Thus, the critical tilt angle must exist.

Then, one possible way to increase charge

density at higher magnetic fields would be to introduce membrane deformation in a plain

o

noneOh) A 10 inol% benzene (3h) A 20 niol% benzene (3h) D lOmoHb atthnceae (3h) 20mol% anUutcene (3b) nooedOOh) V 20 niol% benzene (lOOh) T 20mol% uUncene (lOOfa) O 10 nK>l% pyrene • 20 ntol% pyicne

|

•

•

MagnetoAision

• ••^.y o 12 16 Hll Figure 3 Variations of radiiB r ( # ) of IFPC liposomes and changes in membrane potential AV'(O) of a black lipid membrane at 318K with a steady magnetic field intensity.

i^

f

Magnetodivision i» 4 .

800 /nm

Rgur« 4 Comparison between experimental and n values. Theoretical for p = 20 nm: gray region. Experimental: all symbols.

82 surface. Under higher magnetic fields, membrane deformation out of a plain surface may be expected so as to relax the orientational defects among domains having different orientation at a tilt angle. Thus, we may expect great changes in membrane potential. In fact, the membrane potential changed markedly with magnetic field more than 12 T (Fig. 3). Also, when DPPC liposomes were exposed to higher magnetic fields than 12 T, they markedly grew. These large changes should result in an out-of-plain orientation, i.e., undulation of a membrane. The undulation structure of a membrane may be similar to the ripple structure, because when DPPC liposomes were cooled from 45 °C down to 30 °C under 12 T, an H-NMR pattern of the undulation phase was very similar to its ripple phase.^ The total energy of a liposome comprising the curvature-elastic and magnetic energies should determine the liposome growth from radius r^to r with the association of n hposomes under a magnetic field. Using the Helfrich theory for the magnetodeformation of a spherical bilayer liposome of radius r^, in which the domains in the bilayer have a local radius of curvature Po, we can get the condition for the magnetofusion and magnetodivision may occur[4]: 6{l-n)^r,(n"'-n)/p,^0

(2)

This condition is illustrated for a given Po in Fig. 4. The shading region shows magnetofusion (n >1) and magnetodivision (w <1) for Po = 20nm. When n = 1, a liposome is stable against a certain magnetic field. Small DPPC hposomes less than 100 nm and more than 200 nm were unchanged in size by a 10-T magnetic field, but the addition of benzene and anthracene to DPPC liposomes induced the magnetofusion. But, in medium size liposomes it was slightly different, e.g., in the benzene systems the liposome size was unchanged. Moreover, pyrene induced the magnetodivision as well as magnetofusion, depending on the initial size and pyrene content Fig. 4 summarizes the experimental ratio of r to TQ or square root n as a function of TQ. A comparison of the theoretical to the experimental liposome size under magnetic fields shows that almost all experimental points drop in the theoretical region for pg = 20 nm. The discrete stabihzation (n = 1) of liposomes might be based on fundamental vibrational modes of a membrane. 2.2. Structural control of mesoporous materials due to magnetic fields The mesoporous silicas comprising a honeycomb structure have homogeneous, straight pores with a narrow size distribution and non-interconnecting-networks. Therefore, the potential materials have been paid attention to from the point of view of both fundamentals and application. It is, however, desirable that the oriented domain of a honeycomb structure is much larger than

83 10000

600

500

8000

: :

28

400

6000

]

' " p

^

«

T* • •/• ' •

0 H I T

300

4000

^

200

2000 I.I 1 J1 1 1

0

0.2

1

1

0.4

1 1

_ A _

0.6

i _ _ i — . — . _ X. J

0.8

Relative }nxssure. p I p^

le I Figure 5. XRD patterns of mesoporous silicas prepared at a zcro-fidd and a 22-T magnetic fidd Composition: TEOS: H2O: NH3: EtOH: CTAB = 1: 40: 2: 1.8: 0.2

Figure 6. Nj adsorption isotherms of mcsoporous silicas prepared at various magnetic field intensities.

that of conventionally synthethized mesoporous silicas.

Since the mesoporous silicas are

prepared by calcination of surfactant molecules in surfactant/silicate hybrids having hexagonal structure, it must be possible to align a silica skeleton through magnetic orientation of a surfactant mesophase of hexadecyltrimethylammonium bromide, CTAB, as a template.

We adopted the

sol-gel method to fix the memory under a magnetic field. The mesoporous silicas were obtained by calcination (at 823K in air for 5h) of the organosilicates (hybrids) prepared from ethanol-water mixtures containing tetraethyl orthosilicate (TEOS) and CTAB at 298 K under magnetic fields (<30T). comprised the one-phase region and two-phase region. magnetic field in a one-phase system. hybrid from a basic mixed solution.

The phase diagrams for the systems

Hexagonal structure developed with a

Fig. 6 shows an example for a magnetic field effect on a An amorphous hybrid at a zero-field was transformed to a

typical hexagonal phase by a 22-T magnetic field. When hexagonal and lamellar phases coexisted, a 28 T-magnetic field depressed almost perfectly only the growth of a lamellar phase to lead to a pure hexagonal phase. The silicas prepared from the hybrids in Fig. 5 changed from microporous at a zero-field to mesoporous ones with increasing a magnetic field, and the mesopore became smaller and more homogeneous (Fig. 6).

The increase in adsorption amount of Nj and a step-like increase

suggest that more homogeneous mesopores in the hexagonal structure. On the contrary, in two-phase systems the hexagonal structure was depressed with increasing magnetic field, as shown in Fig. 7.

The hexagonal peak intensities in X-ray diffraction (XRD)

decreased with increasing magnetic field. Corresponding to these results, the mesopores of silicas from the hybrids decreased with magnetic

fields.

In the two-phase systems,

displacement of a liquid/liquid equilibrium between lamellar and hexagonal phases may occur, as suggested by an increase in surfactant content in the bottom hexagonal phase.

84

Figure 7. XRD patterns of mcsoporous silicas prepared at various magnetic field intensities.

The great dependence of the magnetic-field-induced structural changes on the compositions seems to arise from stability and domain size of surfactant mesophases. Especially, basicity may affect hybrid structure via polymerization of silicate ion, because an acid and a base are a hydrolysis and polymerization catalyst, respectively. 3. CONCLUSION Deformation in lipid membranes led to the magnetofusion and magnetodivision of its liposome and large changes in the electrical functions of its black membrane under magnetic fields of up to 30 T. Also, surfactant/silicate molecular assemblies were effectively regulated by steady magnetic fields via orientation of surfactant molecules as a template, which may led to specific pore structures and adsorptivity of mesoporous silicas. These results demonstrate the potentiality of steady magnetic fields that can regulate functions, structures, and sizes of artificial organized molecular assemblies. REFERENCES 1. L.F. Bragonza, B.H.Blott, T.J.Coe and D.Melville, Biochim. Biophys. Acta, 801(1984)66. 2. H.R. Khan and S. Ozeki, J. Colloid Interface Sci., 177(1996)628. 3. S. Ozeki, H. Kurashima, M. Miyanaga and C. Nozawa, Langmuir, 16(2000)1478. 4. S. Ozeki, H. Kurashima and H. Abe, J. Phys. Chem. B, 104(2000)5657. 5. S. Ozeki, M. Yamamoto, K. Nobuhara, Characterization of Porous Solids IV, Eds. B. MaEnamey, et al.. The Royal Society of Chemistry, London, 1997, p.648. 6. W. Helfrich, Phys. Lett., 43A( 1973)409; Zeit. Naturforsch., 28C( 1973)693. 7. T. Ueda, N. Kamo, N. Ishida and Y. Kobatake, J. Phys. Chem., 76(1972)2447.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (0 2001 Elsevier Science B.V. All rights reserved.

85

ESR Spectral Simulation Study of Oleic Acid/Oleate Solution by Using a Spin Probe Hiroshi Fukuda''\ Ayako Goto''\ Hisashi Yoshioka'^'Und Peter Walde'^ *^School of Informatics, "^Graduate School of Nutritional and Environmental Sciences, University of Shizuoka, Yada52-1, Shizuoka-shi, 422-8526, Japan ''Institut fur Polymere, ETH-Zunch, Universitatstrasse 6, CH-8092 Zurich, Switzeriand ESR spectra of fatty acid spin probe(16-doxylstearic acid, 16-DS) incorporated into an aqueous oleic acid/oleate system were analyzed by spectral simulation based upon a modified Bloch equation and the overlapping spectra in the range of pH7-pH10 were separated into a spectrum of the vesicle and that of the micellar solution to get insight into equilibrium vesicle system of oleic acid/oleate. As a result, it was shown that transformation of micelle to vesicle occurred continuously with decrease of pH and micelles coexisted with vesicles in the range of pH7-pH10. Vesicles were partly destructed into micelles and monomers by dilution. 1. Introduction The understanding of the process of transition from micelle to vesicle is important for practical application as well as for fundamental aspects of vesicle formation. It is of interest to study the dynamic aspect of the tiansformation of micelle into vesicle in aqueous dispertions of fatty acid/soaps with change of pH[l-2]. In previous reports[3-4], ESR spectra of a spin probe(16-doxylstearic acid, 16-DS) in oleic acid/oleate solutions were measured at various pHs. Micelles were formed above pHlO.2, but with decreasing pH vesicles came to be formed. The formed vesicles seemed to coexist with micelles in the range of pH7-pH10 because the high-field lines of the ESR spectra were asymmetric as a result of the overiapping two lines. In this study, fine analysis of the high-field lines was carried out with the aid of

86 computer simulation based upon a modified Bloch equation and the overlapping spectra were separated into a spectrum of the vesicle and that of the micellar solution. Peak-to-peak line width of vesicle which is related with property of the vesicle and the fraction of the vesicle were estimated in the equilibrium system of oleic acid/oleate. 2.Experimental Sodium oleate was purchased from Sigma. 6-Doxylstearic acid(16-DS), which is stearic acid spin labeled at carbon 16, was used as a spin probe and was obtained from Aldrich. ESR spectra were recorded on a JEOL JESRE3X spectrometer between 10 *C and 50*C using a flat quartz cell. The ESR spectra of 16-DS were recorded in the presence of various amounts of sodium oleate(0-50mM); Mn^"^ in MgO was used as the external reference to get the corresponding peak positions precisely. 3.Results and Discussion Based upon the ESR results[3-4], we propose a following model for the equilibrium system of oleic acid/oleate solution in the range of pH7-pH10, as shown in Fig. 1. Vesicles, micelles and monomers always coexist at equilibrium, and 16-DS probe molecules are distributed into vesicles, micelles, and aqueous domain containing monomer. While the probe molecule quickly exchanges their position between the micelles and aqueous domain, it exchanges very slowly between the bilayer and micelles. It is reasonable to consider that the latter exchange rate is nearly close to zero in the frequency of ESR, but the former exchange rate is very rapid. Therefore, when the probe is distributed between the micelles and aqueous phase, the ESR line is apparently observed as one peak, but when vesicles coexist with micelles, the absorption line is a overlapping of two lines. Since a modified Bloch equation can simulate the line shape of such a chemical exchange system[5], the overlapping ESR lines on high field in the intermediate pH were simulated based upon the modified Bloch equation. The solution of this equation is shown inEq.(l). F(a))= fX oi' oim) -^ U <^- <^v)+ 7(P [ ( C u - a j

+yPvJ[(cO-

a„)+7P„v]+PvmP-nv

(D

87 Here, the ESR spectra is a function of angular frequency, o). f^ and /^ are the fraction of the probe located in the vesicle( J and in the micellar solution(^). j is the imaginary unit, o)^ and co^ are correlated with the positions of the original lines, co^ and 00^, and the spin-spin relaxation timeCTj ), which is related to the peak-to-peak width (A o)) of the original lines of the doublet in the case that the line shape is Lorentzian. P^^ and P^ mean the transition rate from the vesicle to the micellar solution and that from the micellar solution to the vesicle, respectively, but in the frequency of ESR, P^^ , and P„^, are apparently nearly close to zero. The imaginary part of the function F (co) in Eq.(l) gives the line shape of the absorption spectrum, but ESR spectrum was recorded as the first derivatives of the absorption line. Therefore, the imaginary part was differentiated with coand was calculated using six parameters. As a result, the overlapping two lines could be separated, as shown in Fig. 2. One corresponds to the line of the probe in the vesicles and another to the line of the probe distributed in the micellar solution. These results mean that the probe behaviour follows the Ijorentzian and the Scheme of Fig. 1 was supported. In these cases the parameters were obtained by the revised Marquadt method and their asymptotic 95% confidence intervals were estimated according to the nonlinear regression analysis^'^^. However, the line of the vesicles could not be separated in the pH>10, and pH<7.4. Here, based upon the estimated parameters, A co^, A co^, and /^, we could show three results, i) The line widths of the vesicles could be estimated in the intermediate pH. In this case, we could take out the line corresponding to the vesicles to estimate the the width of vesicles which is related with the rigidity of the vesicular bilayer. As a result, it was shown that the stable vesicles are formed around pH8.5. ii)It was assumed that the micelles coexisted with the vesicles. iii)In the region of pH8.5-10, the vesicles increased

little by little with decrease of pH, indicating that the

transformation of the micelles to the vesicles occurred continuously with decrease of pH. iii)From the effect of temperature upon/, , it was suggested that the entropical effect contributed to the transformation of the micelles to the vesicles. iv)lt was shown that the fraction of the vesicles increased with the concentration of oleic acid/oleate at higher pH. This means that by dilution of the oleic acid/oleate solution, the vesicles are partly destructed into the micelles and monomer. This must be related with

88 formation of giant vesicles by dilution[3]. In conclusion, the oleic acid/oleate solution in the intermediate of pH 10-7 is a dynamic system, where vesicles, micelles and monomer coexist. This dynamic system is significantly different from that of phospholipid system. References 1) Walde, P.; Wick, R.; Fresta, M.; Mangone, A.; Luisi, P.L. J. Am. Chem. Soc. 1994,116, 11649.2)Wick,R.; Walde,?.;Luisi,P.L. J. Am. Chem.Soc. 1995, 117, 1435.3)Goto,A.;Suzuki,A.; Yoshioka,H.; Goto,R.; Imae, T.;Yamazaki, K.; Walde,P. "Giant Vesicles", edited by Luisi,P.L. and walde,P. John & Sons Ltd., 1999.4)Goto,A.; Yoshioka,H.;Goto, R.; WaIde,P. submitted.5)Yoshioka, H.; Kazama,S. J. Colloid Interface Sci., 1983,95, 240.6)Marquardt,D.W. SIAMJ.Appl. Math. 1963,11, 431.7)Osborne,M.R. J. Australian Mathmatical Soc. 1976, 19, 343.

••••

328.6 328.8 329.0 329.2 329 4 329.6 329.8

V

^V

mT

'

'

1 '

'

' T '

'

'

'

A

4U

/ 1/

20

WT

*""

/

Aw.

60 r 1 1 1 11 1

1 1 1 1 1 1 1 i .J

_ ,__

.

. : \ : -

/

00 -20

•

, ,j

328.6 328.8 329.0 329.2 329.4 329.6 329.8 mT

Fig. 1 Scheme of the equilibrium of the oleic acid/oleate system in the intermediate pH region

Fig.2 Separation of the ESR spectra of oleic acid/oleate solution of pH9.4 into the vesicles and the micellar solution at20t:. The concentration of oleic add/oleate was 25mM.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ic 2001 Elsevier Science B.V. All rights reserved.

89

Controlled association between amphiphilic polymers and enzyme by cyclodextrins in heat denatured process : artificial molecular chaperone K. Akiyoshi* , M. Ikeda, Y. Sasaki, and J. Sunamoto Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Yoshida-Hommachi, Sakyo-ku, Kyoto 606-8501, Japan"^ The irreversible aggregation of carbonic anhydrase B (CAB) upon heating was completely prevented by complexation between the heat-denatured enzyme and hydrogel nanoparticles of hexadecyl groups-bearing pullulan (C16P). The complexed CAB was released by dis-aggregation upon the addition of cyclodextrins. The released CAB refolded to the native form, depending on the concentrations and kinds of cyclodextrins. 1. INTRODUCTION Most enzymes irreversibly lose enzyme activity upon heating because denatured protein exposes hydrophobic residues of polypeptide and this induces irreversible aggregation. In living system, molecular chaperones selectively trap heat-denatured proteins [1] or their intermediates in refolding [2] to prevent their irreversible aggregation. The host chaperone releases the protein in a refolded form through a change in the conformation of the complex with the aid of ATP and another co-chaperone. For example, GroEL, which are a wellinvestigated family of molecular chaperones, acts as a host in macromolecular self-assembly by enclosing a folding intermediate protein as a guest in its central hydrophobic cavity [3]. In artificial system, it is difficult to control association between macromolecules. We describe here well-controlled associations between hydrophobized polysaccharides and proteins by cyclodextrin. This is similar to the two-step mechanism of a molecular chaperone; i.e., capture of a heat-denatured protein and release of the refolded protein. 2. DESIGN OF ARTIFICIAL MOLECULAR CHAPERONE The amphiphilic nature of proteins, especially their hydrophobicity, is important in the complexation between a chaperone and denatured protein. We used an amphiphilic hydrogel nanoparticle that behaves as a host for a guest protein to simulate the heat shock protein activity of molecular chaperones. Hydrophobized polysaccharides or hydrophobized poly (amino acids) spontaneously form hydrogel nanoparticles in water by their selfassociation [4] The self-association is controlled by cyclodextrins [4c, 5]. * To whom correspondence should be addressed. "*" This research was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.

90 The hydrogel nanoparticles spontaneously complex with various soluble proteins in water [6]. We previously reported that carbonic anhydrase B (CAB) was complexed by the nanoparticles of cholesterol group-bearing pullulan (CHP) in a heat-denatured form above its denaturation temperature (55 °C) [7]. The complexed CAB was effectively released in its refolded native form in the presence of b-cyclodextrin. In addition, this system provided effective thermal stabilization of CAB. These results suggest that structure of hydrophobized pullulans and kinds of cyclodextrins should be important factor for chaperon like activity. The pullulans with long alkyl chains such as hexadecyl group-bearing pullulan (C16P) also form monodispersive nanoparticles by self-aggregation in dilute aqueous solution. The characteristics of the nanogel were much different. The density of the polysaccharide in one aggregate of CHP and C16P was -0.13 g/mL and -0.04 g/mL. C16P formed less densely packed nanoparticles than CHP. One hydrophobic domain of C16P self-aggregate consists of 12-17 alkyl groups, while that of CHP consists of 4-5 cholesterol groups. The number of cross-linking point (3-4) of the ALP hydrogel nanoparticles is significantly smaller than that of CHP (10-15). In this paper, we describe size heat shock protein activity of molecular chaperones using hexadecyl groups-bearing pullulan (C16P) and various cyclodextrins.

zOH

CH20CONHC6Hi2NHC(0)0-R

CHP HO

OCH? OH

OH

CH2OH

CH2OH

OH

^0^, OH

. ^ . . ^ ^ L - n -

'(CH2)15CH3 C 1 6 P

OH

Fig. 1 Structures of hydrophobized pullulan 3. HEAT-SHOCK ACTIVITY OF ARTIFICIAL MOLECULAR CHAPERONE The preparation and characterization of C16P nanoparticles have been reported elsewhere [4]. Alkyl group-bearing pullulan (C16P-108-1.2, where the molecular weight of the parent pullulan was 108,000 and the degree of substitution of the cholesteryl moiety per 100 glucose units was 1.2) was used in this study. The aggregation number of the selfaggregate was approx. 4 determined by the static light scattering method. The root meansquare radius of gyration was 18.2 nm. High-performance size-exclusion chromatography (HPSEC) was carried out to study complex formation between C16P nanoparticles and CAB. The samples of the C16P nanoparticles gave a single peak. The apparent polydispersity of the aggregates was below 1.2, calculated using a calibration curve of standard pullulans. No precipitation was observed even after heating at 90 °C for Ih. The self-aggregates are coUoidally stable. When free CAB (2 \JM) was heated at 70 ""C for 10 min and then cooled to room temperature, white precipitates were produced. After the suspension was passed through a Millipore fiher (pore size 0.45 mm), the filtrate was subjected to HPSEC. No peak of CAB was observed on the chromatogram; all of the CAB aggregated and precipitated under these conditions

91 a)

E c o f 00

b)

C16P self-aggregate elf-aggregate

I

KlJCL heating complex)

M

6P-CAB ?C?6I

CM

^•^ (d

CO OQ <

c)

yv

C16P-CD complex

d)

1

0.0

—r

15.

1^ 0

RT{min)

Fig. 2 Chromatograms of interaction between C16P, CAB and cyclodextrin a) After mixing C16P self-aggregate and CAB for Ih at 25°C, b) heating at 70 °C for 10 min and then cooled to 25 °C, c) Mixture of C16P and a-cyclodextrin (3mM), eluent contains acyclodextrin (3mM), d) After addition of a-cyclodextrin (3mM) to C16P-CAB complex, eluent contains a-cyclodextrin (3mM). When a mixture of the C16P nanoparticle (4 \xM) and CAB (2 jxM) was incubated at 70 °C for 10 min and then cooled to 25 °C, the solution was still transparent. Fig. 2b shows the quantitative complexation of CAB with the nanoparticles. The size of the nanoparticles did not change so much even after complexation because of the complete incorporation of CAB in the hydrogel matrix of the nanoparticles. No dissociation of CAB was observed even after the solution was maintained for a week at room temperature. The C16P nanoparticles (4 ^M) did not bind native CAB (2 iiM) even after incubation for 24 h at 25°C. C16P nanoparticles may capture the heat-denatured intermediate of CAB. C16P self-aggregates undergo dissociation with the addition of cyclodextrin. The cross-linking domains provided by the hydrophobic alkyl groups are destructed upon encapsulation of the alkyl moiety by the cyclodextrin cavity. The destruction of the nanoparticles subsequently induces the release of complexed CAB. To the C16P (4 jiM)CAB (2 ^xM) complex was added a-cyclodextrin (3 mM), and the mixture was kept for 24 h at 25 °C. HPSEC analysis of the reaction mixture showed a complete release of free CAB from the complex under these conditions (Fig.2d). The recovery of the enzyme activity of CAB was estimated by the hydrolysis of p-nitrophenylacetate. Complexed CAB (2 mM) did not show any enzyme activity in the absence of cyclodextrin. However, with the addition of a-cyclodextrin (3 mM) to the complex (CHP, 2 (iM), almost 100 % of the enzyme activity recovered within 3 h. Fig. 3 shows the recovery of enzyme activity after addition of various cyclodextrins. A a-cyclodextrin is known to most strongly bind alkyl groups

92 among the cyclodextrins. Effective dis-aggregation of nanoparticle by cyclodextrin results in high efficiency of refolding of trapped CAB. A Htatino C A B l t a t l v t Z;;;!^ CABDcnaturwJ •

Agg

C16P-CD ComDiu

10

12

Cycl.
jj^^„„j,

Cie-CABconiDltx

CD (mM)

Fig.3 Recovery of enzyme activity as a function of various cyclodextrins at pH 7.5 at at 25 T .

Fig.4 Scheme of artificial chaperone

In conclusion, in this study, we mimicked the heat shock activity of a molecular chaperone using a simple system consisting of self-aggregate nanoparticles and cyclodextrin (Fig.4). This is a novel example of the well-controlled association between amphiphilic polymers and proteins by cyclodextrin. Protein refolding in urea or GuHCl-induced denaturation system is under investigation using a similar hydrophobized polysaccharide system. REFERENCES 1. For example, H. Taguchi, and M. Yoshida, J. Biol. Chem. 268, (1993) 5371. b)Y. Kawata, K. Nosaka, K.Hongo, T. Mizobata, and J. Nagai, FEES Letters, 345, (1994) 229. 2. For example, M. Mayhew, A. C. da Silva, J. Martin, H. Erdjument Bromage, P. Tempst, and F. U. Hartl, Nature 379, (1996) 420. 3. For example, a) J. L. Cleland, and D. I. Wang, In Biocatalist design for stability and specificity, ACS Symposium Series, 151 (1993) b)D. Rozema, and S. H. Gellman, J. Am. Chem. Soc. 117, (1995)2373. 4. a)K. Akiyoshi, and J. Sunamoto, Supramolecular Science i, (1996) 157. b)K. Akiyoshi, S. Deguchi, H. Tajima, T. Nishikawa, and J. Sunamoto, Macromolecules 30, (1997) 857. c) K. Akiyoshi, E-C. Kang, S. Kurumada, J. Sunamoto, T. Principi, and F. M. Winnik, Macromolecules 33 (2000) 3244. d) K. Akiyoshi, A. Ueminami, S. Kurumada, and Y. ]
Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

93

Effect of Alcohols (Propanol, Propylene Glycol, and Glycerol) on Cloud Point and Micellar Structure in Long-Poly(oxyethylene)„ Oleyl Ethers Systems Kazuki Shigeta', Ulf 01sson^ Hironobu Kunieda'' ^ Division of Artificial Environment Systems, Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan ^ Division of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Effects of propanol, propylene glycol, and glycerol on cloud points and micellar structures in aqueous poly(oxyethylene)„ oleyl ether (C,g ,EOJ solutions were investigated. Upon addition of propanol or propylene glycol, the cloud point increases whereas it almost remains constant or decreases upon addition of glycerol. The self-diffusion data of surfactants measured by NMR suggest that the micelles tend to dissociate upon addition of propanol or propylene glycol. The change in cmc measured by the fluorescence-probe method also indicates the dissociation of micelles in propanol and propylene glycol systems. As a result, the cloud points increase. However, the micellar structure may be maintained even at a high glycerol concentration in C,8 jEO^ (n=19.2, 30.1, and 50.8) systems, and the molecular weight of micelle largely increases with increasing the glycerol content in C,g ,EO,o7 system. The cloud point is, therefore, almost does not change with glycerol content in C,8 jEO^ («=19.2, 30.1, and 50.8) systems, and it largely decreases in C,8 jEO,o7 system. 1. INTRODUCTION Polyols, inorganic salts, and other additives influence phase behavior of poly(oxyethylene)„-type nonionic surfactant aqueous solutions, and especially, the effect of these additives on the cloud point has been extensively investigated. However, rather short poly(oxyethylene)„-chain (EO-chain) surfactants were used for the previous studies, and as a result, the cloud point is limited in the range of 0-100 °C. When the EO-chain length of surfactant increases, it approaches the cloud point of poly(oxyethylene)„ with practically infinite molecular weight. In the case of poly(oxyethylene)„ oleyl ether (C,8 ,E0„) aqueous solutions, the cloud point increases up to 118 °C at M=30.1, and then slightly decreases with the further increase in n. Namely, the cloud point in long-EO-chain surfactant aqueous solution approaches to that of water-soluble polymer, poly(oxyethylene)„ [1]. In our previous study, the effects of three alcohols with the same carbon number (propanol, propylene glycol, and glycerol) on the phase behavior of aqueous CjjEOg solutions were investigated [2]. It was found that propanol and propylene glycol tend to dissociate micelles, whereas the aggregation number increases upon addition of glycerol. These differences influence the cloud point. However, the influence of alcohol on the cloud point in a very long-EO-chain surfactant has not been widely studied. In the present study, to clarify what mainly causes the change of cloud point, the effect of propanol, propylene glycol, and glycerol on the cloud point in C,g,EO„ (n=10.7, 19.2, 30.1, and 50.8) systems were investigated by fluorescence and NMR self-diffusion measurements.

94

2. EXPERIMENTAL SECTION 2.1. Materials Poly(oxyethyIene)„ oleyl ethers (C,8,E0„, n=10.7, 19.2, 30.1, and 50.8) were obtained from NOP Co. These surfactants were synthesized from pure oleyl alcohol (>99.7%) and oleyl group of these surfactants is pure. Propanol (Junsei), propylene glycol(Wako), and glycerol(Wako) were used as water-soluble short-chain alcohols. 2.2. NMR Self-Diffusion measurement The self-diffusion coefficients of surfactants were measured with the FT-PFG technique, monitoring the 'H spectra [3]. The measurements were performed on a Bruker DMX 200 spectrometer. Heavy water (D2O, 99.8% isotopic purity obtained from Sigma) was used in the NMR experiments. 2.3. Fluorescence measurement Steady state fluorescence spectra of pyrene were recorded on a spectrofluorophotometer (SHIMADZU, RF-5300PC). Micropolarity of pyrene was measured from the relative intensities of first and third bands (///j), and cmc is obtained from the change in the micropolarity [4]. 3. RESULTS AND DISCUSSION 3.1. Cloud-point behavior Figure 1 shows the change of cloud point in 5wt% surfactant aqueous solution with alcohol content. In propanol systems, the cloud points in the C,g,EO„ (n=19.2, 30.1, and 50.8) systems are almost constant or slightly decrease up to 15wt% propanol, and then rapidly increase. The cloud point in the Cjg,£0,0^ system monotonically increases. In propylene glycol systems, the cloud points increase in all systems. In glycerol systems, the cloud point is almost constant in the Cjg,EO„ (n=19.2, 30.1, and 50.8) systems, but it decreases in the C,8.,EO,o7 system.

(c)glycerol 1

0.4 ^propanol

^propylene

glycol

0.2

0.4

0.6

^glycerol

Fig. 1 Change of cloud points in C,g ,EO,o ,(0), C,g,E0,,.2(A), C,g,E03o,(V), and Cjg jEOsogCD) systems with weight fraction of (a) propanol, (b) propylene glycol, and (c) glycerol. The cloud point in binary water-surfactant system is related to the micellar molecular weight [1]. When micellar molecular weight is low, the cloud temperature is high and vice versa. Therefore, it is considered that the change in cloud point may be related to the change in micellar molecular weight upon addition of alcohols.

95 3.2. Micellar structure In order to figure out the change of micellar structure with alcohol, FT-PFG 'H-NMR self-diffusion measurements were performed. The self-diffusion coefficients of surfactants (D^) as a function of alcohol concentration in water are shown in Figure 2, where the surfactant concentration is the same as for the cloud point data. The self-diffusion behavior in the water/C,g,EO„ systems is well explained by the colloidal hard-sphere model [1]. In the colloidal hard-sphere model, the self-diffiision coefficient is given by following equation [5,6]. kj

A = 67vr]R^ 1 -

(1)

0.

where /?^ is the hydrodynamic radius, r] is the solvent viscosity, and k^T is the thermal energy. 0„ is the critical random packing volume fraction of hard spheres and it is equal to 0.63. (f>„s is the volume fraction of hard spheres. In the water/C,g iEO„ binary systems, 0^^^ is proportional to 0^and we can write „s=(^(l>s^ where a=2.07 for C,g.iEO,o7, 2.54 for C,8,EOj9 2» 3.25 for C,8.jEO30,, and 4.22 for C^^.fiO^Qg [1]. Assuming that no structural change in micelles takes place upon addition of alcohol, D^ depends only on solvent viscosity because R„ and a do not change. Hence, the theoretical D^ can be calculated by equation (1). The theoretical D^ is shown by solid lines in Figure 2, where R^^ is used 4.26nm for C,8 ,EO,o7, 4.44nm for C,8 ,EO,92, 5.09nm for Cjg jEOjo j, and 5.82nm for €,« .EOsog Ul

0.4 Tpropanol

Tpropylene g tyco I

0.4

0.6

0.8

r glycerol

Fig. 2 Change of self-diffusion coefficients (D,) of C, .,EO,o.7(0), C,g,EO,, ,(A), C,g,EO30,(V), and C,8 jEOjogCD) in the micellar phase upon addition of (a) propanol, (b) propylene glycol, and (c) glycerol at 25°C. Solid lines show the calculated values for D^ by using equation (1) for (i) C,g ,EO,o7, (ii) C,g ,EO,92, (iii) C,g.EOjop and (iv) C,^,EO^, systems. In propanol systems, D^ slightly decreases, and turns to increase with further addition of propanol. Finally, the measured D^ becomes markedly larger than solid lines. In propylene glycol systems, D^ changes along solid lines, but it begins to increase at a high alcohol content. In glycerol systems, D^ monotonically decreases along solid lines, but in C,g.,EOio7 system D^ rapidly decreases. In surfactant systems, D^ is the average self-diffusion between monomers and micelles. To confirm the contribution of monomer to D5, cmc was measured by the fluorescence-probe method. The results are shown in Table 1. In propanol and propylene glycol systems, cmc increases with increasing alcohol content. As a result, D^ increases due to the increase of cmc

96 in propanol and propylene glycol systems. Namely, the increase of cmc means that the miscibility between water and surfactant increases, and micelles are dissociated. Hence, the cloud points increase upon addition of propanol and propylene glycol. In glycerol systems, cmc does not change in the CigjEOjo^ system, and slightly increases in the Ci8,EO„ (n=19.2, 30.1, and 50.8) systems. However, D^ in C,8,EO,07 system extremely decreases, and the decrease indicates the micellar growth. As a result, the cloud point decreases with increasing glycerol content in the C,8 ,EO,o7 systems. On the other hand, the miscibihty between water and surfactant is almost unchanged and micellar structure does not change in the C,8 ,E0„ (n=19.2, 30.1, and 50.8) systems. Therefore, the cloud points keep almost constant with glycerol content in the Cjg ,E0„ (n=19.2, 30.1, and 50.8) systems. Table. 1 Change of cmc (mol kg"') of Cjg,EO„ (n=10.7, 19.2, 30.1, and 50.8) with weight ratio of propanol, propylene glycol, and glycerol in water + alcohol at 25°C. C EO C EO Weight ratio ^18:1^^10.7 ^18:1^^50.8 ^18.1^^30,1 ^18.1^^19.2 Alcohol-free 3.0x10-° 4.0x10-" 6.3x10-" 7.2x10-° Propanol 0.1 2x10' 1x10-' 6x10-' 3x10-' 0.2 1x10-' 6x10-' 9x10-' 9x10-' 0.4 >2.0xl0-' >6.8xlO-' >3.1xlO-^ >4.5xl0-^ propylene glycol 4x10-' 0.6 5x10-' 2x10-' 9x10-' glycerol 0.6 6x10' 3x10-' 9x10-' 4x10-' 4. CONCLUDING REMARKS In the Cjg ,EOn systems having long EO chain, micelles are dissociated upon addition of propanol and propylene glycol, as is observed in relatively short-chain CjjEOg system. As a result, the cloud points increase. On the other hand, micelles grow upon addition of glycerol in the C,g iEOio7 system similarly to C,2E0g system, while micelles do not change in C,g.iEO„ (n=19.2, 30.1, and 50.8) systems. Therefore, the cloud point decreases only in C,g ,EOjo7 system because of the micellar growth, but the cloud points in C,8,E0„ (/2=19.2, 30.1, and 50.8) systems are unchanged with glycerol content. Namely, the cloud points increase because of the dissociation of micelles upon addition of propanol and propylene glycol, and this fact is independent on the EO-chain length. On the other hand, the cloud point decreases upon addition of glycerol because of the micellar growth in relatively short EO-chain C,g.iEO,o7 system, but the cloud point does not change in C,g ,EO„ systems having a very long EO chain (>19.2) because of no structural change in micelles. Concerning the correlation between cloud point and micellar size, therefore, we can conclude that the cloud point increases when micelles are dissociated and the cloud point decreases when micellar size increases. REFERENCES 1. K. Shigeta, U. Olsson, H. Kunieda, submitted 2. K. Aramaki, U. Olsson, Y. Yamaguchi, H. Kunieda, Langmuir, 15 (1999) 6226 3. P. Stilbs, Prog. Nucl. Magn. Reson. Spectrosc. 19 (1987) 1 4. A. Nakajima, Bull. Chem. Soc. Jpn.,50 (1977) 2473 5. D. Quemada, Rhol Acta, 16 (1977) 82 6. A. van Blanderen, J. Peetermans, G. Maret, J. K. G. Dhont, J. Chem. Phys., 96 (1992) 4591

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B. V. All rights reserved.

97

Critical surface charge density for counter-ion binding in mixed micelles of ionic with non-ionic surfactant M.Manabe. H.Kawamura, H Katsu-ura, and M.Shiomi Niihama National College of Technology, Niihama, 792-8580, Japan It was found that in the mixed micelles of ionic with non-ionic surfactant, there exists a critical mole fraction Xim* (=0.103) of ionic surfactant below which the counter-ion of ionic surfactant is completely released, without any counter-ion binding. The fact implies that in the phase diagram of mixed micellar solution, the micellar region can be separated into the traditional region and a novel region in which the ionization degree of mixed micelles is unity. As for the mixed micelle at Xim*, the amount of charge (hexavalent aggregate) and the aggregation number were evaluated based on the conductivity of micellar solution. INTRODUCTION Colloid electrolytes in water release some amount of counter-ions. The degree of ionization has been considered to be a function of surface charge density The value of degree lies around 0.3 for the micelles of most ionic surfactants [1], and when a non-ionic amphiphile is solubilized in the ionic micelles, the more counter-ions are dissociated with increasing amount of the additive [2] Another evidence is that for polyelectrolytes in a simple linear form, they behave as a strong electrolytes when the distance between nearest neighbor ionizable groups is farther than a critical distance, as developed by Manning [3]. These facts suggests that in general, colloid electrolytes must have a critical surface charge density below which the counter-ions are dissociated at all. Then we have attempted to confirm the existence of such critical surface charge density in the mixed micelles of ionic with non-ionic surfactant by conductometry For the analysis of conductivity, a distinctive quantity, differential conductivity, was applied [4]. EXPERIMENTAL Respective ionic and non-ionic surfactants are used: synthesized sodium dodecylsulfate (SDS) [5] and purchased hexaoxyethylene glycol mono-dodecyl ether (RE) (Nikko Chemicals Co.,Ltd.). The aqueous solution of RE (concentration: CJ was prepared as a solvent which was used for preparing a stock solution of SDS. Small amounts of the SDS solution was consecutively added in a given amount of the non-ionic surfactant solution

98 which was kept in a conductivity cell (cell constantO 5474 cm') immersed in a water bath controlled at 25.0-1-1/100 X . After each addition, the conductivity measurements were made on a conductivity meter (HP: 4284A). RESULTS AND DISCUSSION At all C^'s studied, the specific conductivity (K) increases monotonically with an increase in the SDS concentration (CJ , where the increasing tendency in the presence of RE was more complex compared to in water. For the detail analysis of the increasing tendency, a differential conductivity was adopted, defined as the increment of K between a pair of nearest neighbor data : dic/dC, = (K.-K,)/(C,.-C,,) The dependence of dic/dC, on the square root (SQR) of C, is illustrated in Figs. 1 and 2. In water, dic/dC, decreases linearly to a break point at which the value of C is taken to be the critical micelle concentration, CMC, and then it drops in a narrow C, region to a low constant dK/dC, value. When C, is lower than CMC,,. (CMC of RE in water, 0 09mM [6]), dK/dC, decreases in the similar manner as in water, where the curve starts deviating from the straight line at lower C, than CMC,., (CMC of SDS in water), and break becomes moderate The curve reflects the CMC decrease of SDS in the presence of RE. The same tendency of differential conductivity curve was observed in the SDS solution containing a small amount of long chain alkanols[4]. Just above CMC^,,, a maximum appears at a low C, (Fig.l). When SDS is added in a micellar solution of RE, a definite maximum is observed as seen in Fig.2 which has some characteristics. The value of dK/dC, at maximum denoted by (dic/dCJ* is the highest just above CMC^., and then decreases with C„. It is noticed that (dK/dCJ* is higher than dK/dC, of SDS solution below C M C , The fact suggests the formation of 90

ou

i

I 80

^60 c o

^

^

"

!

X

:

^

^40 c/)

Cj60

u -a

13

50 0

2

4

6

8

10

12

14

0

1 2

3

4

5

6

100SQR(Q)/S(y(m)l/1^ H g l I^lationbetv^endK/dGsandGs. fig 1. Relation betv^eendK/dCs and Q . -CrpOmmol/l^ D 00995 A 0 6

0 1.5 • 5.07 A 19.999 ': 9.997 - CrF=()niid/kg

99 polyvalent aggregates. In addition, it should be emphasized that extrapolated value of dK/dC, to C,->0 at each C^ in Fig.2 is close to each other and the value is regarded to be 50 which corresponds to the equivalent conductivity of Na ion at limiting dilution. The coincidence at 50 and the increase for dK/dC, gives a following model of mixed micelle. On the addition of a small amount of SDS in a micellar solution of RE, the complete amount of surfactant ion (DS) is accommodated in the non-ionic micelles in which the electric charge becomes greater with increasing C„ whereas Na ion is freely dissolved in the bulk water. The complete accommodation was confirmed from the result that the dic/dC, values for homologous sodium alkylsulfates is coincident with each other at given C^ and C, below the maximum point, no data being shown here. On the basis of the model, the decrease of dK/dC, above the maximum can be attributed to the counter-ion binding. Namely, for the mixed micelles, the highest charge density is accomplished at the maximum which gives the critical composition i.e., the critical surface charge density below which the counter-ion is completely ionized. Then, the SDS concentration at the maximum, denoted by C,*, is plotted against C^, in Fig 3 h is obvious that the relation is linear: the least mean squares method analysis provides the slope denoted by q (= 0.1154 ) and the intercept (0.0089). The very small intercept can be taken to be zero and represents that C,* is proportional to C„. By ignoring the concentrations of both SDS and RE dissolved in monomerical state, the critical mole fraction of DS in the mixed micelle, X,^*, can be approximated as X,^* = C,*/(C,*+CJ. As a result, X,^* can be calculated as X,^*=q/(l+q)=0.103 since C,* = q C^ This value of X.^,* is significant. If the effective cross sectional area of each head group, DS and RE, can be assigned the critical surface charge density can be estimated. In addition, when the values of X,^* in some other systems will be collected in future, it can be confirmed whether the critical surface charge density is common or not in general colloid electrolytes The amount of charge, Z, of the critical ionic aggregates is estimated in the following manner. (dic/dCJ* is plotted against SQR(C,*) and SQR(C,) as illustrated in Fig.4. Each curve has a maximum. In the concentration region above the maximum in Fig.4, each curve 2.5

0

5

10

15

Cn/(inniol/kg) Fig. 3. Relation between Cs* unci Cn.

20

0

5 10 15 IOOSQR(Cii,Cs*) / SQR(mol/kg)

Mg. 4. Dependence or(dK/dCs)* on Cn and Cs*. • 'SQRcCn) DSQR(Cs*)

100 can be regarded to be linear and the linear regressive analysis provides the relations. (dk/dCJ* = -1.274 x 100SQR(CJ + 85.59, (dk/dCJ* = -3.736 x 100SQR(CJ + 85.63 (1) It is apparent that the intercepts are in good agreement with each other Both Hnear relations can be considered to have the same physical meaning: the dependence of differential conductivity on the concentration of the critical ionic aggregates Along this explanation, the intercept denoted by (dK/dCJ*" indicates the equivalent conductivity of the critical ionic aggregates at limiting dilution. If the Stokes' law is applied to the critical ionic aggregates with the charge amount (Z e), the radius R, in the aqueous solvent with its viscosity r| , the ionic mobility U, is expressed and also the ionic mobility U, of SD ion with the radius r is done as U, = Z e / (67rnR) ; U, = e/(67rrir) (2) After all, the ratio is obtained. U/U, = Z/(R/r). (3) Provided that DS ion and the critical ionic aggregate are spherical and the volumes of respective species, monomerical DS ion, and micellized DS ion and RE are identical with each other, the relation can be derived. (R/r)'-^ = Z+N (4), where N stands for the aggregation number of RE in the aggregates From Eqs.(3) and (4) and taking X,^*^ =Z/(Z+N) into account, Z can be calculated as

z = (uyu,)^^Vx,,*'^

(5)

On numerical calculation for Z, Uc/U, can be evaluated form the equivalent conductivity of respective ionic species, DS and the critical ionic aggregate, where the conductivity is evaluated by subtracting the conductivity of Na+ ion (50 1) from respective intercepts obtained in Fig.4 for the aggregate, and in Fig. 1 for SDS in water UyU, = (85.6-50.1)/(73.2-50.1)= 1.54 (6) Finally Z is calculated by Eq.(5) as 5.94 yielding N as 51 7 which seems to be reasonable. REFERENCES 1. Y. Moroi, Micelles, Prenum, New York, 1992,p.62. 2. M.Manabe, H.Kawamura, S.Kondo, M.Kojioma, and S.Tokunaga, Langmuir 6 (1990) 1596. 3. G.S.Manning, J.Phys.Chem., 79(1975)262. 4. M.Manabe, H.Kawamura, A.Yamashita, and S Tokunaga, J Colloid Interface Sci, 115 (1987)147. 5 M.Manabe, S.Kikuchi, YNakano, YKikuchi, S Tokunaga, M Koda, Memoirs of the Niihama Technical College (Science and Engeneering), 19(1983)50. 6. PBehcer, Nonionic Surfactants, Ed by M.J.Schickic, Dekker, New York, 1967, p 483

Studies in Surface Science and Catalysis 132 Y. Ivvasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

101

Dispersibility of Surfactant-free OAV Emulsions and Tlieir Stability Design: A Present Scope from Hydrocarbons to Some Oleate Esters. K. Kamogawa^•^ H. Akatsuka^ M. Matsumoto', T. Sakai^ T. Kobayashi^ H. Sakai^''^ and M. Abe^''^ ^Elem.&Sec. Ed. Bureau, Ministry of Education, Science, Sports and Culture, Kasumigaseki3-2-2, Chiyoda, Tokyo, 100-0013, Japan ''Institute of Colloid and Interface Sci., Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo, 162, Japan. ^Science University of Tokyo, 2641, Yamazaki, Noda, Chiba, 278-8510, Japan This paper presents recent development in surfactant-free emulsion (SFE) chemistry on the dispersibility and the stability for growth. Appropriate selection of the hydrophobic oil and the mixing control with the second oils realized fine dispersibility and stability for long period up to 1 year. The SFE droplets reveal prototype behavior of emulsion, which has been shielded by the amphiphilic groups. 1. INTRODUCTION Emulsion technology has been developed on the basis of surfactants and other amphiphilic substances. While they are quite useful, another needs arise today at organic-recycling or energetic consumption in the recovery.

Surfactant-free 0/W

emulsion (SFE) is, if possible, one promising solution for these needs, allowing variety of hydrophobicity design of oil phase. The oils concerned here are completely hydrophobicor slightly polar oils with HLB<1. However, their dispersibility and growth nature have not well been understood, except for phase separation at visible order. Our earlier works on C6-molecular SFE found that the dispersibility has some distinctive characteristics, not much popular for surfactant-added emulsions [1,2]. The discrete characteristics can be explained with the critical aspect in the coalescence as shown in Fig. 1. In ordinary scheme (a), coalescence is collision-controlled, stepwise

102

CXD-O Figure. 1 (a) stepwise coalescence

(b) criticalflocculation/coalescenceto

to give continuous drift in size.

give discrete, bimodal distribution.

growth. When SFE droplets are resistive enou^, critical degree offlocculationwould be required before the ftision (b). We found electron-microscopic evidences for (b) [3]. (1) Discrete, multimodal size distribution at ^ l O ' , -10^ and ^10^ nm scale classified here as S,M and L-class. (2) The peak size increased discretely as a population transfer from lower to higher class, not continuously drifted as ordinary emulsions. (3) These profiles vary sensitively to the types of oils, e.g. benzene generated S-class droplet (^30 nm) while n-hexane and cyclohexane did not. Further study on n-hexadecane SFE realized [2], (4) S-class droplet is generated also for n-hexadecane but unimodal distribution with higher stability than benzene. (5) Mixing of n-hexadecane with benzene stabilizes the dispersion for 6 month. 2. RESULTS AND DISCUSSION 2.1 Initial Dispersibility of Hydrocarbon-and Oieyl Ester SFE In contrast to current view, SFE droplets at sub- fi m order are substantially metastable, as shown in Fig.2. S-class droplets of n-decane generated at 30*C are observable for

Sonicationifor 8 min.'

180 (3hour^

^

200|-

120 (2hours) (Ihour) 10

10

to

10

Time / min

Particlesize/nm

Figure 2. Size distribution of n-decane.

Figure 3. Principal peak drift of

SFE at 301: for 10,30 min, 1,2,3 hrs.

n-decane SFE at 10-501;.

103

400,

Figure 4. < d 0 > vs. y / 97 for n-alkanes n= 8,10,12,14 and 16fromrightto left.

Figures. < d o > vs. y Irj for alkyl oleate. CHjCB), decyKD), oleyl(0),glycerol(A) and acid(#).

30min, before discrete growth to M-class. At other temperatures, the peak positions unchanged except for a transition as shown in Fig.3. The initial dispersibility can be evaluated with the average diameter < d o>. As often referred for emulsions, the droplet size was analyzed as aftinctionof the interfacial tension(y) normalized to viscosity ( 7 ), y I rj in mPas-s/mN-m' [4]. Figure 4 reveals high correlation between < d 0> and y / rj for n-alkane oils. kXy I rj = 20-100. < d o> decreases with y I rjso that the droplet evolution above M-class in n-hexane and cyclohexane SFE is ascribable to the low viscosity. The linearity suggests that nm-scale droplets may be generated at / / 7 ~* 1. Expected oils are alkyl oleate or glycerol trioleate giving y / 7 = 1 ^ 5 . lOxlO'r CO

E CO _3 "O

2

% a

I "5 3

o

20 40 60 80 100120140 160180

Time /min

Figure 6. vs. time. Oleic acid(#), CH3(H) and glycerol(A) esters.

2

3

4

5

6

7

Time / day

Figure?. vs. time. CH3(•), decyl(n), oleyl(O) ,gIyceroI(A) esters.

104 Figure 8. log[ A < d(t) > / A t] plot against the oil

100

viscosity. CH3 ( • X d e c y U D ) , oleyl(O) and glycerol(A) esters.

As shown in Fig.5, however, their < d o > reached only at 60nm. Althou^ it seems to be a disruption 10

20 ~30 40 50 ©0 ^i^t ^^^ viscous droplets, < d o >

Viscosity /mPa • s

is certainly

regulated by the viscosity.

2.2 Two Growth Modes and Their Control The growth of SFE can be analyzed with the rate, A < d ( t ) > / At.

In the case of

oleic esters (or n-hexadecane-added tetralin), < d ( t ) > showed bipbasic increase, at the scale of few hrs and several days respectively, as seen Fig3.(6) and (7). Therefore, two modes can be distinguished in growth, as fast and slow ones. Fast mode was often found for oils soluble in water slightly. The growth presented linear rise of < d(t) ^ > with time in accord with the LSW theory as shown in Fig.6. This mode is assignable to the Ostwald ripening. Shrinkage of smaller droplet with the growth of larger droplets, an indication of the Ostwald ripening, was found for oleic acid. Stabilization of benzene- and tetralin droplets by n-nexadecane mixing [2] allows us to protect the Ostwald ripening with 2"^ oil. Slow mode rate was analyzed for oleic acid esters, in which [ A < d ( t ) > / A t

]

significantly decreased with an increase in oil viscosity in a semi-log3rithmic manner, as shown in Fig.?. This relationship does not arise from the collision-controlled ftision but from an activation energy factor, because diffusional motion is almost invariant for these dilute SFE dispersions. This indicates that the slow coalescence can be controlled dynamically with the oil phase viscosity. REFERENCES 1 .K.Kamogawa, T.Sakai, N.Momozawa, M.Shimazaki, M.Enomura, H.Sakai and M.Abe,

J. Jpn. Oil. Chem. Soc.,47( 1998 ), 159.

2.K.Kamogawa, M.Matsumoto, T.Kobayashi, T.Sakai, H.Sakai and M.Abe, .,Langmuir, 15(1999), 1913. 3.T.Sakai,.K.Kamogawa, N,Momozawa, H.Sakai and M.Abe, under submission. 4..K.Kamoga\va and M. Abe, in Encyclopedia in Emulsion Technology, Mercei Dekker Inc.NY in press.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c^ 2001 Elsevier Science B.V. All rights reserved.

105

Solidification of Liquid Hydrocarbons with the Aid of Carboxylate Hiroshi Sakaguchi National Institute of Materials and Chemical Research, Higashil-I, Tsukuba 305-8565, Japan It was shown that, with the aid of water and very small amounts of alkyl chains of surfactants, almost all liquid hydrocarbons were easily solidified at room temperature. l.INTRODUCTION 1.1. Importance of the solidification of liquid hydrocarbons 20th century has been the period of petrochemistry. Because liquid or gaseous hydrocarbons are used as reactants, chemical reactions can be controlled easily and much amounts of high quality products have been produced. This is the essentially different point of petrochemistry compared with coal chemistry. However, at the same time, very large scale explosion or leakage accidents have increased greatly. Much care should be paid during transport, handling and storage[l]. If all hydrocarbons can be kept in solid state at room temperature, and can be returned back to the original liquid or gas state at any time we want, it will be very useful, not only for practical chemical industry or for the benefit of environment, but also on the pure scientific point of view. 1.2.Importance of van der Waals forces among alkyl chains in water Depended on intermolecular van der Waals forces[2], all hydrocarbons (and all kinds of molecules) originally have properties to gather together to make themselves solididified state. However, depended on thermal energy, which is the function of temperature, hydrocarbon molecules have properties to move around freely and randomly. As the consequence, actual pure hydrocarbons exist in a state of solid, liquid or gas, depended on temperature. 2.EXPERIMENTALS 99-100% pure sodium carboxylate, 99-100% pure hydrocarbon, and ultra-pure water(standard molar ratio 1:10-1000:1000) were put into a 20ml test tube, and heated (to 60-95°C) until sodium carboxylate was melted and

106 dissolved completely in water or in hydrocarbon. At this point, two liquid phases were coexisting. Then the liquids were mixed completely for several minutes by using vortex-mixer, and the tube was stood still at room temperature. After some time, from a few minutes to several days depended on sample to sample, self assembling occurred and a white gigantic solidified aggregate, practically including all the existing surfactant and hydrocarbon molecules, was separated from pure water. Self-assembled aggregate was analyzed by DSC, TGA, photo-microscopy, FT-IR, elemental analysis, ICP emission spectroscopy, and so on. 3.RESULTS Figure 1 shows a typical example of self-assembled solidified aggregates of sodium tetradecanoate and n-paraffins(molar ratio of 1:50) in pure water(molar ratio 1000). These aggregates were very hard and stable at room temperature, and could be picked up by fingers.

Fig. 1. Self-assembled solidified aggregates of sodium tetradecanoate and n-paraffins (molar ratio 1:50) in pure water (molar ratio 1000). From left to right; n-pentane, n-hexane and n-heptane. As sodium carboxylates, n-alkyl chain length of 12(dodecanoate) to 22 (docosanoate) were used. And as hydrocarbons, all liquid n-paraffins (from n-pentane to n-octadecane), branched parafrins(ex. 2,2,4-trimethylpentane), olefins(ex. 1-decene), and aromatic hydrocarbons (benzene, toluene, xylenes, ethylbenzene, cumene) were tested. In all the combinations of these carboxylates and hydrocarbons, and in wide ranges of molar ratios(l:l to

107

1:400-800), self-assembling always occurred, solidified aggregates were obtained, and separated from pure water. And so, in maximum cases, more than 99 weight per cent of one aggregate was composed of pure hydrocarbon, and the remaining less than 1% was composed of carboxylate. However, when molar ratio of hydrocarbon became over the limit, solidified aggregate was not produced, and two liquid phases, hydrocarbon and water, remained. Of course, in the absence of water, such self-assembling and solidification phenomenon did not occur at all. Mixture of hydrocarbon and carboxylate separated easily, mixture of caboxylate and water became stable emulsified liquid, and mixture of water and hydrocarbon separated immediately. DSC analysis showed that, in the aggregate, hydrocarbon was kept liquid state at room temperature. In case of n-heptane and sodium pentadecanoate aggregate, for example, during cooling process of DSC measurement, quantitative phase transition peak from liquid to crystal of pure n-heptane was obtained at -93°C. And so, solidified aggregate was only an apparent one. Intrinsically, the aggregate was liquid, but it could be treated like solid. By DSC measurement, it became clear that water content in each aggregate was very small and practically negligible. In water, separated from self-assembled aggregate, ICP analysis revealed that only SOppm of Na was contained. This meant that almost all carboxylate was in the aggregate. By TGA analysis, each white aggregate was usually stable at room temperature, and was decomposed around at melting point of the sodium carboxylate included. And so, at higher than the decomposition temperature, pure hydrocarbon could be easily recovered. The aggregate was really liquid, only apparently solid. However, after some adequate heat and cool treatment, the aggregate became real co-crystal of carboxylate and hydrocarbon [3]. 4.DISCUSSION This self-assembling and solidification phenomenon can be explained by intermolecular interaction, van der Waals attraction force, among alkyl chains of carboxylate and hydrocarbons. Water plays decisive role for alkyl chains attract each other, gather together and finally construct themselves to solidlike aggregate. Without water, as thermal movement of each short alkyl chain hydrocarbon is much stronger than mutual van der Waals attraction, each molecule easily go away from surrounding alkyl chains. Beside, carboxylate plays also decisive role. Carboxylate plays like anchor to catch and fasten hydrocarbons tightly in water. However, once hydrocarbons are caught and gather together, as hydrocarbons intrinsically have ability to attract each other, they can attract each other and stabilize themselves. And

108

so, only small amounts of carboxylates are necessary compared hydrocarbons. Figure 2 shows a schematic model to explain this phenomenon. H20 H20 H20 H20 H20 H20 H20 H20

H20 H20 H2O

H2O

H20 H20 H20 H20 H20 H20 H20 H20

H20

H20

^•S^.^^VX-V/

L

H20 H20 H20 H20 H20 H20 H20 H20

^

with

H20 H20 H20 H20 H20 H20 H20 H20

H20 H2O

H2O

H2O

H20 H20

H20

H20 (

H20 H20

H2O H2O H2O H2O H2O H2O H2O

Fig. 2. Self-assembling and solidification scheme. REFERENCES 1. Eberhard Weise(Volume editor), Ullmann's Encyclopedia of Industrial Chemistry, Volume B8, (1995) 497. 2. Paul C. Hiemenz and Raj Rajagopalan(eds.), Principles of Colloid and Surface Chemistry, 3rd ed.. Marcel Dekker(1997) 462. 3. H. Sakaguchi, R. Tzoneva, T. Yoshimura, and K Itoh, Proceeding on the IWCPB-HMF'99, (2000)379.

Studies in Surface Science and Catalysis 132 Y. iwasawa, N. Oyama and H. Kunieda (Editors) o 2001 Elsevier Science B.V. All rights reserved.

109

Methodology for predicting approximate shape and size distribution of micelles M. Kinoshita and Y. Sugai Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, JAPAN We propose a methodology for predicting the ^jproximate shape and size distribution of micelles with all-atom potentials. A themiodynamic theory is combined with the Monte Carlo simulated annealing technique and the reference interaction site model theory. Though the methodology can be applied to realistic models of surfectant and solvent molecules with current computational capabilities, it is illustrated for simplified models as a preliminary step. 1. INTRODUCTION Theoretical prediction of the shape and size distribution of micelles formed by aggregation of surfactant molecules in solvent is one of the most challenging problems. Molecular Dynamics (MD) and Monte Carlo (MC) simulations treating surfactant molecules and surrounding solvent molecules simultaneously have been performed with all-atom potentials. Some of them use realistic models of surfactants and solvents [1-3], but they suffer fix)m extremely heavy computational load: only a single, pre-assembled aggregate can be simulated for a very short period ( ~ 0 . Ins), and the final structure relies heavily on the initial structure chosen. Even with simplified models for surfactants and solvents [4-7], analysis on the equilibrium px)perties of the system is not an easy task, mainly because the number of solvent molecules to be treated is extremely large. A problem of the simulations is that they are able to sample only a limited region of the configurational phase space. The number of degrees of fieedom involved in the simulations is substantially reduced if the solvent molecules are omitted. This can be justified by accounting for solvent effects using the reference interaction site model (RISM) theory, an elaborate statistical-mechanical theory for molecular fluids. We propose a metiiodology in \\iiich a thermodynamic theory [8] is combined witii the MC simulated annealing technique [9-10] and tiie RISM tiieory. The RISM equations are solved using a robust and very efficient algorithm [11]. As a preliminary step of our research, the methodology is illustrated for simplified models of surfactant and solvent molecules. 2. MODEL The solvent molecule is a spherical particle V, and the surfactant molecule is a chain comprising two

110 types of atoms: a solvophilic atom A and a solvophobic atom B. The V-V, V-A, and B-B interactions are Lennard-Jones (U) potentials, and the V-B, A-A, and A-B interactions are repulsive parts of the LJ potentials. Rigid, linear surfactant molecules, AB, AABB, and ABBB, are treated with L/^L^^=Lf^= 0.20nm {LpQ is the distance between the centers of P and Q\ £'w=^vA^^vB'^^AA'=0-6kcal/mol, ^BB^^AB'^^kcal/mol, and crw=crvA=crvB=cTAA~<^BB^<^AB^'28nm.

The temperature of the system T is

298.15K and the reduced solvent density PvCTw^ is 0.7317. More details are described in [12]. 3. METHODOLOGY We employ the themiodynamic theory by Ben-Shaul and Gelbart [8]. It was shown by Displat and Care [13] and by von Gottberg et al. [14] that the theory gives good results for the size distribution of micelles. However, the key quantity, v„=(/i„^-//,^(JfeB^ iMn is the chemical potential of a surfactant molecule in an aggregate of size n that is present in solvent at infinite dilution and k^ is the Boltzmann constant), must be calculated via another route, which is a drawback of the theory. In our methodology [12] v„ can be estimated solely from the solvent-solvent, solvent-surfactant, and surfactant-surfactant interactions and the approximate shape of the aggregate is also obtained as a function of«. Consider a supennolecule of n surfactant molecules in solvent. The siq^ermolecule is present at infinite dilution. Each surfactant molecule consists of m atoms, and the si^rmolecule has mn atoms. A conformation of the supermolecule is specified by positions of n surfactant molecules. For the supermolecule in a particular conformation, we define the total enei^gy Ej by Ef^Eo^A/u^,

(1)

where E^ is the conformational energy and Afd^ the solvation fiee energy. EQ is defined as the sum of all the intermolecular atom-atom potentials for the surfactant molecules. Aju^ is calculated using the RISM theory. A number of conformations are sampled in accordance with the MC simulated annealing technique, and the conformation that gives £V ^ ^ lowest value, i.e., the most stable conformation in the solvent, is chosen. The conformation witii the lowest value of £V is a compact aggregate, and we refer to the aggregate as the micelle of size n. Note that tiie MC technique is ^plied only to the surfactant molecules. The ensemble-averaged structure of the solvent, which is in equilibrium with the supermolecule in a particular conformation, is theoretically and rapidly calculated. [The long-range Coulomb potentials can be handled without problems.] Determination of the supermolecule conformation with the lowest value ofE^ is performed for many different values of«. The values of £V, Ea an^ ^ / ^ ^ ^ determined for the supermolecule comprising n surfactant molecules are denoted by Ej^(n), Ec^Qi), and A^^(n% respectively. The quantity v„ can be interpreted as the difference between a surfactant molecule in an aggregate of size A? and a fi^ surfactant molecule in terms of the stability in the solvent, and the stability is dependent on the supermolecule confomiation. The most stable conformation of the supermolecule is chosen for calculating v„:

Ill ^^ri)rEc^Jn)l{nk^n

(2)

^{ny={A^^Jji)ln-A^l\)}l{kJ),

(3)

v;=^«)=£J:(«>f£^(«)={J^^(«)/«.^/i3(l)}/(^7).

(4)

In the above equations, Aju^iX) is the solvation free energy of a single surfactant molecule, and we have found that zl/is(l)(AB)4). This characterization is performed by analysis on the principal moments of inertia of the micelle. The eigenvalues (A„ A2, and /I3; ?i{>X^>X^ of the moment of inertia matrix are calculated, and a characteristic length is obtained as l=X,' (/=1, 2, 3). The asphericity of the micelle, wiiich is dependent on n, can be measured by monitoring /,//2 and / / / j . 4. RESULTS AND DISCUSSION We have verified that the results are independent of the initial conformation of the supermolecule. Figure 1 illustrates ^ ( « ) , E^{n\ and E^n) for AABB. E^{n) predominates over Ec{n), and this is also tme for AB and ABBB. The surfactant molecules are forced to assemble into aggregates mainly by the solvent. Since the solvent efifects are thus substantially large, the average shape of micelles of size n should be similar to the most stable shape in the solvent As for AB, the micelles of large sizes (??>10) are not spherical (they are more disc-like), and the asphericity increases as the size becomes larger. On the other hand, the micelles for AABB are nearly spherical in the size range tested. The micelles for ABBB of smaller sizes («<10) are more disc-like and those of larger sizes («>10) are more cylindrical than the micelles for AABB. We have found that the micelles with the lowest value of ^c^ which are determined by the MC simulated annealing technique without the solvent, are almost completely spherical for all the three model surfactant molecules. Those micelles are destabilized in the solvent due to relatively high values of the solvation free energy. E^{n) possesses a minimum wiiile ^^{n) is a decreasing function of n. For AABB the minimum in E^(ri) is the most distinct with the result that E^n) also possesses a minimum as observed in Fig. 1. Thus, large micelles can be destabilized by the solvent effects as well as by the translation entropy effects. For ABBB a minimum is not present in Ej{n\ and a larger aggregate can be more stabilized by packing of the long solvophobic chains. Figure 2 shows the size distribution of micelles f„ {f„^nXJLnX„, X~NJNy\ N„ is the number of micelles of size n) at some different concentrations of surfactant molecules for AABB (A^ is the number of the surfactant molecules and A^v is that of the solvent molecules). The critical micelle concentration (cmc) is at N/Ny/-\0'\ We have found that cmc(ABBB)«cmc(AABB)«cmc(AB). The micelle size is the smallest for AABB because of the apparent minimum present in Sj(n). Slightly larger micelles can be formed for AB and much larger micelles for ABBB.

112 r

• r—r

AABB

AABB

o AA

AA

/

-^

I' 1'

0.4 ,

^ A A A A A A A

o

apaDO

DD

DD

N/Nv=5.0X10-*

/ \

N/Nv=1.0X10-' -

/ /\ \ /

D D D ° °

0.2

II]

-8

A

I1'.

II]

6

N/Nv=1.0X10-«

\-

-4

[I]

•

i \1

o

i

\\

ooooooo^ 10

20

4

8

12

nH Fig. 1. S^nX ^(n), and Sj(n) for AABB.

Fig. 2. Size distribution for AABB.

5. CONCLUDING REMARKS Although the model systems for wdiich our methodology is illustrated are simple, the results provide some useful information. First, the solvent effects are substantially large and must fully be incorporated. The micelle shape is variable depending on the surfactant molecule and the micelle size. The cmc decreases largely with increasing solvophobicity of the surfactant molecule. The average size of the micelles does not always become larger as the solvophobicity increases. We are extending our study to more realistic models of the surfactant and solvent molecules. REFERENCES 1. D. Brown and J. H. R. Clarice, J. Phys. Chem. 92 (1988) 2881. 2. J. C. SheUey, M. Sprik, and M. Klein, Langmuir 9 (1993) 916. 3. J. Bocker, J. Brickmann, and P. Bopp, J. Phys. Chem. 98 (1994) 712. 4. B. Smit, et al., Langmuir 9 (1993) 9. 5. S. Karabomi, et al.. Science 266 (1994) 254. 6. D. R. Rector, R van Swol, and J. R. Hendereon, Molec. Phys. 82 (1994) 1009. 7. B. J. Palmer and J. Liu, Langmuir 12 (19%) 6015. 8. A. Ben-Shaul and W. M. Gelbart, J. Phys. Chem. 86 (1982) 316. 9. S. Kiriq)atrick, C. D. Gelatt, Jr., and M. P Vecchi, Science 220 (1983) 671. 10. M. Kinoshita, Y. Okamoto, and R Hirata, J. Am. Chem. Soc. 120 (1998) 1855. 11. M. Kinoshita and R Hirata, J. Chem. Phys. 104 (1996) 8807. 12. M. Kinoshita and Y. Sugai, Chem. Phys. Lett., 313 (1999) 685. 13. J.<:. Desplat and C. M. Care, Molec. Phys. 87 (1996) 441. 14. R K. von Gottbeig, K. A. Smith, and T A Hatton, J. Chem. Phys. 106 (1997) 9850.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyamaand H. Kunieda (Editors) 'c:> 2001 Elsevier Science B.V. All rights reserved.

113

Pre-micelle and micelle formation of local anesthetic dibucaine hydrochloride Hitoshi Matsukia, Takahiro Miyata^, Toshiharu Yoshioka^, Hiromu Satake*> and Shoji Kaneshinaa ^Department of Biological Science and Technology, Faculty of Engineering, and ^'Center for Cooperative Research, The University of Tokushima, Minamijosanjima, Tokushima 770-8506, Japan The molecular-aggregate formation of local anesthetic dibucaine hydrochloride (E)C»HC1) was investigated from the electrode potential and light scattering measurements of the aqueous solutions. The electrode potential of E)C»HC1 solutions showed the deviation from Nemstian response at low concentrations below the critical micelle concentration (CMC). We considered the possibility of pre-micelle formation of DC»HC1 in the solution and found that dibucaine cation forms a trimer with two chloride ions in the concentration range. On the other hand, the aggregation number of 15 were obtained for DC^HCl micelles in water from the light scattering measurements. Five dibucaine trimers associate cooperatively and form the micelle at concentrations above the CMC. Further, we observed the E)C»HC1 micelle grow one-dimensional direction like small rod-hke micelle in solutions of high ionic strength. The pre-micelle, micelle formation and micellar growth of dibucaine may be attributable to the stacking of dibucaine molecules due to the ;r electron interaction of an aromatic ring in the molecule. 1. INTRODUCTION Local anesthetics have tertiary amine structures with an aromatic ring. Some with large hydrophobicities form molecular aggregates such as micelles in aqueous solutions. The potency of local anesthetics to block nerve conduction is based on their ability to associate with membranes. There are numerous reports demonstrating that the clinical potency of local anesthetics correlates extremely well with their ability to adsorb to lipid monolayers and bilayers. The formation of molecular aggregates and the partitioning into hydrophobic environments of cell membranes, which is an important process for local anesthesia, are both based on the hydrophobicity. The micelle formation of local anesthetics in aqueous solutions has beenreportedby several researchers [1-4]. They showed that the aggregation of local anesthetics is different from that of surfactants with a straight hydrophobic chain. Recently, we have also shown the existence of micellar aggregates for local anesthetics in the aqueous solution by ionic activity, differential scanning calorimetry (DSC), density, and surface tension measurements [5], etc. In the present study, we examined the formation of pre-micellar and micellar aggregates for dibucaine hydrochlorides (DC»HC1) from the electrode potenrial and light scattering measurements of the aqueous solutions. The aggregation behavior of DC^HCl in the aqueous solution at concentrations below and above the critical micelle concentration (CMC) and that in solutions of high ionic strength was characterized.

114 2. EXPERIMENTAL Dibucaine hydrochloride (2-butoxy-N-[2-(diethylamino)ethyl]-4-quinolinecarboxamide hydrochloride: C2oH29N302»HCl) was purchased from Sigma Chemicals (St. Louis, MO) and purified by the method described previously [6]. Water was distilled twice after deionization. The coated-wire electrode selectively sensitive to dibucaine cation was prepared by the same method described previously [7]. The electromotive force (EMF) of the dibucaine cation and chloride ion (Toko Chemicals' electrodes 7020) was measured by a digital multi-ion monitor (Yamashita Giken Co. Ltd.,Tokushima, Japan) at 298.15 K under atmospheric pressure [7]. Static and dynamic light scattering measurements were performed by light-scattering spectrophotometer DLS-7000 (Otsuka Electronics, Osaka, Japan) with Ar and He-Ne lasers. The scattered light of aqueous dibucaine solutions in the absence and presence of sodium chloride was measured under the same condition as the EMF measurements. The increment of refractive index for dibucaine solutions was measured by differential refractometer DRM-1021 (Otsuka Electronics) to calculate the weight average of molecular weight for dibucaine. 3. RESULTS AND DISCUSSION 3.1. Pre-micelle formation of DC-HCI at concentrations below the CMC The electrode responses of dibucaine cation (DC»H+) and chloride ion (CI) in the aqueous solution were presented as a function of their concentrations in Fig. 1. The DC*H+ and Clelectrodes showed a linear response with a Nemstian slope at concentrations below 20.0 mmol kg-^ Both electrode responses deviated from the Nemstian slope at concentrations above 20.0 mmol kg-^: the depression of activities for both ions was observed. At concentrations above 80.0 mmol kg->, the activity of DC»H+ had a constant value and the slope of activity of CIbecame about one-third of Nemstian response. Since the CMC of DC^HCl has a value of 78.7 mmol kg-' [6] and no hydrolysis of DC»H+ occurred in this study [5], we examined the activity depressions taking account of premicellar formation due to self-association of E>C»HC1 in the aqueous solution. Because the activities of DC»H+ and CI" were depressed each other, it is expected that premicelles including both ions are formed in the solution. We consider that other monovalent cationic species except DC»H+ are able to response the DC^H+ electrode as follows -3.0

-2.5

-2.0

-1.5

-1.0

log [DC-HCI],. log [NaCI] / mol dm^

mDC-H"*" + (m - l)Cr

Fig. 1. Relationship between electrode potential and E)C»HC1 or NaCl concentration: The equilibrium constant {K) of the above reaction (1) dibucaine cation, (2) chloride ion is expressed as (DC*HC1), (3) chloride ion (NaCl). -(DC-H)^Cl(,.,/

(1)

115 K = [(DC*H)^Cl(^.,)^]/[DC.H*r[Cr]^'"-^^

(2)

obtained from the electrode is given by The apparent concentration of DC • H+ ([DC • H"^\^^) iapp>' [DC-H^],pp = [DC-H*] + A [ ( D C - H ) , a ( , . , ) n

(3)

where X is the response coefficient of the electrode for (DC»H)^C1(^. ^"^ ion. Total concentration of dibucaine ([IX: • HClJt) can be written using the concentration of DC-H+ [DC*HC11^ = [DC*H^] + m[(DC*H)^Cl(^.,)^]

(4)

From Eqs. (3) and (4), we obtain [DC-HCl], - [DC-H^],pp = (m -A)[(DC-H)^C1(,. ,)^]

(5)

Alternatively, using CI" ion concentration, [DC* HClj^ is written as [DC*HCl]t - [CI] = (m .1)[(DC*H)^C1(^.,)^]

(6)

By combining Eqs. (5) and (6), ([DC*HCl]t - [DCH^],pp)/([DC•HCIJj - [CI]) = (m - ^)/(m - 1)

(7)

The A value can be determined experimentally if we can know the m value. Considering ionic equilibrium given by Eq. (1), the following two equations can be obtained. ([DC*HCl]t - [DC•H-'])/[DC•H^]'" = mA:[Cr]^'" ''^

(8)

and ([DC*HC1], - [Cr])/[Cl-]^'"-^> = (m -1)A:[DC«H^]'"

(9)

where [DC • H"*"] is not measurable directly and obtainable from the equation [DC*H^] = [DC-H^],pp - A/(m -l)([DC-HCI]t - [Cl'l)

(10)

Right hand side of Eq. (10) can be determined with the m value by the EMF measurements. Both activity depressions were analyzed by the above equations with several m values. We found that the m value of 3 held on the Eqs. (8) and (9). The results are shown in Fig. 2,

10

20

(Crf X I0*/mol^dm*

20

40

60

[DC«H*f X I0*/mol^clm'"

Fig. 2. Validity of pre-micelle formation with m = 3: (A) plot of Eq. (8), (B) plot of Eq. (9).

116 respectively: good linear relationship in both figures with m = 3 was obtained. The values of ^ and A were found to be 6.21 (± 0.18) x 10* (moH kg^) and 0.50 ± 0.14 at 298.15 K, respectively. This fact indicates that at the concentration below the CMC, DC^HCl forms a trimer with two chloride ions. The pre-micelle formation of dibucaine may be attributable to the n electron interaction of an aromatic ring with a butyl chain in the molecule. 3.2. Formation and growth of DC*HC1 micelle at concentrations above the CMC We next performed the static and dynamic light scattering (SLS and DLS) measurements on various concentrations of aqueous E>C»HC1 solutions in the absence and presence of added sodium chloride (NaCl). The static scattered light intensity greatly increased at concentrations above the CMC. The aggregation numbers of the anesthetic micelle were evaluated from Debye plot of the intensity data. The value of 15 was obtained for DC»HC1 micelles in water and it is consistent with literature ones [3,4]. Since E)C»HC1 form trimers at concentrations below the CMC, five dibucaine trimers associate cooperatively and form the micelle at concentrations above the CMC. Furthermore, the CMC decreased and the aggregation number increased by the addition of NaCl. Micellar properties of E)C»HC1 in water and NaCl solutions were sununarized in Table 1 together with the average diameters of DC^HCl micelle, which were obtained by DLS measurements. Although the average diameter increased with increasing NaCl concentration, the variation of the aggregation number with NaCl concentration seemed not to be threedimensional as seen in surfactants with a straight hydrophobic chain. This fact suggests that the E>C»HC1 micelle grows one-dimensional direction like small rod-like micelle in the aqueous solution by addition of NaCl. The micellar growth of DC*HC1 in solutions of high ionic strength may result from the stacking of an aromatic ring in the molecule. Table 1 Micellar properties of DC*HC1 in water and NaCl solutions NaCl cone. (molkg-»)

0

0.10

0.15 a)

0.30

0.40

0.50

0.60

79.4

51.0

40.9

28.2

22.5

20.8

18.3

Aggregation number

15

29

34

40

45

53

59

Average diameter (nm)

1.9

2.8

3.3

4.1

4.8

5.2

6.1

CMC(mmolkg-0

*) physiological saline concentration. REFERENCES 1. T. Eckert, E. Kilb and H. Hoffman, Arch. Pharm., 297 (1964) 31. 2. R. Jaenicke, Kolloid. Z., 212 (1966) 36. 3. E. H. Johnson and D. B. Ludlum, Biochem. Pharmacol., 18 (1969) 2675. 4. D.Attwood and P Fletcher, J. Pharm. Pharmacol., 38 (1986) 494. 5. H. Matsuki and S. Kaneshina, Hyomen (in Japanese), 37 (1999) 20. 6. H. Matsuki, M. Yamanaka, S. Kaneshina, H. Kamaya and I. Ueda, Colloids Surfaces B: Biointerfaces, 11 (1998) 87. 7. H.Satake, T. Miyata and S. Kaneshina, Bull. Chem. Soc. Jpn., 64 (1991) 3029.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyamaand H. Kunieda (Editors) c: 2001 Elsevier Science B.V. All rights reserved.

^1/

Ionic partition to zwitterionic micelles Kenji Iso and Tetsuo Okada Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan Capillary electrophoresis has been used to measure the ^-potential of zwitterionic micelles in various electrolytes. Even for intrinsically neutral zwitterionic micelles, detectable ^-potential is induced by the imbalance between anionic and cationic partition. Its magnitude and sign are determined by the natures of ions (dominantly anionic ones) and the polarity of surfactant molecules. However, the former is a principal factor governing the <^-potential of zwitterionic micelles in aqueous solutions. 1. Introduction The natures of electrolytes are important factors to affect various interfacial phenomena. Electrolytes play significant roles particularly in the solution and interfacial chemistry of electrically neutral micelles (e.g. zwitterionic and nonionic micelles) [l], and often cause drastic changes in micellar characteristics, such as cloud points, critical micelle concentration etc. Studies of electrolyte effects are thus expected to provide essential clues to solve puzzles involved in micellar chemistry. The ^-potential of micelles is an efficient measure to characterize phenomena taking place at solution/micelle interfaces. Electrophoresis and its related methods are useful for evaluating (^-potential, because even rather small In the present paper, the ionic potential can be determined thereby [2,3]. partition to zwitterionic micelles has been studied by measuring their <^-potential with capillary electrophoresis. In previous papers, we indicated that the partition of ions into A^-dodecyl-A^-dimetyl-ammonio-propanesulfonate (DDAPS) micelles are governed by anionic nature, and that partition selectivity can be explained by solvation changes or ion-association of anions [3,4]. DDAPS micelles have inner positive and outer negative charges; anions are attracted by the inner cationic groups while cations are accumulated in the vicinity of the outer surface. It is the main purpose of the present paper whether the anion-dominated partition to DDAPS micelles originates from the molecular

118

polarity of DDAPS or from the natures of the hydration of ions. The ionic partition into zwitterionic micelles having different molecular polarities and charge densities is studied to elucidate this aspect, and results are discussed on the basis of (^-potentials evaluated by capillary electrophoresis. 2. Experimental Capillary electrophoretic apparatus was basically the same as used in the previous research [3]. The length of a fused-silica capillary (0.05 mm i.d., 0.375 mm o.d.) was 600mm, and the effective length of the capillary was 400 mm. The applied voltage was ±12kV under the current lower than 80|iA, which depends on the nature and concentrations of electrolytes. A sample solution was introduced basically from the anodic end by siphoning; sample introduction was done from the cathodic end in case of the reversed electroosmotic flow. The detection wavelength was set to 280 nm. The entire system was set in an incubator thermostated at 25°C. DDAPS was recry stalled from acetone twice. Octaethyleneglycol monododecyl ether(OGME) and hexadecyl phosphocholine(HPC) were treated with an ion-exchange resin, Amberlite EG-4, to remove ionic impurities. The structures of these surfactants are shown below.

OGME

3. Results and Discussion 3.1 Zeta potential determination from capillary electrophoretic data Solution containing pjnrene and acetone was introduced as a sample into the capillary filled with a micellar electrolyte. The former, which is completely partitioned into the micelle, migrates together with micelles, while the latter, which is present in a bulk solution, acts as an electroosmotic flow marker. Thus, two peaks of spiked pyrene and the electroosmotic flow marker appear on an electropherogram. The mobility of a micelle under a given condition (//) is represented by

119

/^ = -

(1) V app

^eoJ

where ^ i s the magnitude of the applied electric field, L is the effective length of the capillary (the length between the injection end and the detection window), and tapp and teo are the migration times of the micelle and the electroosmotic flow. When the ^'•potential is not very high, the Henry's equation is applicable to the determination of f-potential based on electrophoretic mobility. ^o<

K^B)

where rj is the viscosity of a medium andy(;d?5) is a Henry's coeflacient, thus determine the ^-potential of the micelle from //. 3.2

(2) We can

DDAPS micelle Table 1 summarizes the ^'-potential of the DDAPS micelles measured in various electrolytes. These values indicate that the (^-potential of the DDAPS micelles is dominantly determined by anionic nature, while cationic nature plays a minor role. There is a clear correlation between the (^-potential and the hydration energy of an anion, suggesting that the partition mechanism be related to the hydration of anions; the poorer the hydration of an anion the larger the partition. We indicated that there are two possible origins in ionic partition selectivity, and that large and polarizable anions (C104- and I ) are likely to form ion-pairs with DDAPS molecules while small and well-hydrated anions (e.g. CI) undergo hydration changes upon going fi-om bulk to the palisade portion of the DDAPS miceUes [3]. These selectivity terms can be involved in Table 1 <^ -potential of DDAPS micelles the Poisson-Boltzmann equation for induced by anion'dominated partition spheres, which provides the spatial ^-potential distributions of electrostatic cone salt /mV /mM potential and ionic concentrations. 50 -57.2 NaC104 The (^-potential of the DDAPS Nal 50 -39.6 micelles was successfully NaBr 50 -19.0 interpreted on the basis of this -10.4 50 NaCl model. -5.8 20 The charge density of the -6.1 20 LiCl DDAPS micelles can be lowered by -8.8 50 BU4NC1 adding OGME, which forms micelles with almost the same size

120

and the same aggregation number as DDAPS [5]. The partition of ions to the mixed micelles is also governed by anionic nature, and the ^'-potential decreases with decreasing the relative concentrations of DDAPS. Interestingly, the ^'•potential of nonionic OGME micelle is also negative by predominant partition of an anion; e.g. -llmV for 60mM NaC104, and the negative (^-potential was detected even for NaCl though quantitative evaluation was not possible because of a very small difference in migration between the micelle and an electroosmotic flow. 3.3

HPS micelles As stated above, the anion dominant partition occurs even in nonionic micelles. This strongly suggests that the partition of ions be related to differences in the hydration nature between anions and cations rather than to the polarity of zwitterionic micelles. In order to elucidate this intrinsic aspect in the ionic partition into zwitterionic micelles, the ^-potential of HPS micelles was studied in various electrolytes. Results are summarized in Table 2. Negative <^-potential occurs for perchlorate salts, which is almost the same as induced in OGME micelles. If the polarity of the zwitterionic micelles were a principal factor to govern ionic partition, the potential should be more positive. In contrast, positive potentials are confirmed for tetrabutylammonium chloride, which induces negative potential for DDAPS. Thus, the polarity of surfactant molecules is a factor to determine the ionic partition into zwitterionic micelles, but less important than the hydration nature of ions. The Gibbs free energy of transfer for individual ions should be determined for more detailed discussions. Table 2 <^-potential of HFC micelles in various electrolytes ^ , . ,^ cone C-potential surfactant salt , T,, , \T /mM /mV HFC 160 -15.8 LiC104 -13.9 160 NaC104 28.4 80 Bu4NCl HFC(5mM) + 1.9 80 Bu4NCl OGME(30mM) OGME

BU4NC1

80

References 1. 2. 3. 4. 5.

Y.ChevaUer, N.Kamenka, M.Chorro, R.Zana, Langmuirl2il996)3225. S.Terabe, KOtsuka, TAndo, Anal Chem. 57(1985)834 K.Iso, T.Okada, Langmuirheing submitted. T.Masudo, T.Okada, Phys. Chem. Chem.Phys. 1(1999)3577. M.Zulauf, KWeckstrom, J.B.Hayter. V.Degiorgio, M.Corti, J.Phys.Chem. 89(1985) 3411.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^c 2001 Elsevier Science B.V. All rights reserved.

121

Analysis of local structure of ion adsorbed on the gas/liquid interface Makoto Harada,^ Tetsuo Okada^ and Iwao Watanabe^ ^Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, 152-8551, J a p a n ^'Department of Chemistry, G r a d u a t e School of Science, O s a k a University, Machikaneyama, Toyonaka, Osaka, 560-0043, J a p a n

The local structure of Br" in the vicinity of a surface film of a zwitterionic surfactant, iV-dodecyl-iV,iV-dimecyl-ammonio-butane-sulfonic acid (DDABS), was analyzed by X-ray absorption fine structure (XAFS) spectroscopy.

It was

suggested t h a t DDABS molecules are cross-linked by Br" attracting a few water molecules d u e to the strong interaction between DDABS and Br". 1. Introduction Surfactant molecules are adsorbed, and regularly aligned on the aqueous solution surface (gas/liquid interface). On the solution site, the charged groups of s u r f a c t a n t s attract water molecules a s well a s ions to constitute specific surface structures.

DDABS is a zwitterionic s u r f a c t a n t w i t h a cationic

q u a t e r n a r y a m m o n i u m g r o u p a n d a n anionic sulfoic group.

The surface

s t r u c t u r e s of a t t r a c t e d ions depend on the concentration a n d n a t u r e of a n electrolyte a d d e d to t h e solution.

In this work, t h e surface s t r u c t u r e s of

bromide ions adsorbed on the surface film of DDABS were analyzed by XAFS technique, which can be an effective tool to probe s u c h local atomic structures. The total-refiection total-electron-yield (TRTEY) XAFS methodfl] allows u s to distinguish the information on the local structure of Br" at the solution surface from that in bulk.

122

He Gas

electron

X-ray

JL. He Gas Electrode TlBias \r)lt:age

incident angle

t \\ypJJi ca.l()()A\i,, ' -y

X-ray A^indow

/

/ evanescent wave , / ,. ahsorbmg atom Fig. 1. Principle of TRTEY-XAFS.

V

Sample Water BathJ_ 'smml^^^ Cel Sample Cell ^=^ odinpit, V^A^U Fig.2. TRTEY-XAFS Cell.

2. Experimental 2 . 1 . TRTEY-XAFS Method The principle of the TRTEY method is shown in Figure 1.

X-ray was

i n t r o d u c e d a t a grazing incident angle to the solution surface, which w a s smaller t h a n the critical angle of total reflection for Br K-edge X-ray at water s u r f a c e (ca. 2 m r a d for t h e e n e r g y of Br K-edge), Auger e l e c t r o n s a n d p h o t o e l e c t r o n s a r e i n d u c e d from atoms absorbing evanescent wave(atoms existing writhin ca. 100A of the solution surface).

Since the n u m b e r of the

emitted electrons is proportional to X-ray absorption intensity, XAFS spectra can be estimated from current intensity. 2 . 2 . XAFS m e a s u r e m e n t Figure 2 shows a schematic diagram of the TRTEY-XAFS a p p a r a t u s . The cell w a s filled o u t w^ith helium gas. grazing incidence angle.

X-ray was introduced at a b o u t 1 m r a d

Ejected electrons successively ionize helium atoms.

Helium ions were collected by the cathode, to which an appropriate bias field (ca. 1.5 kV-m'^) w a s applied; t h u s the amphfied c u r r e n t was detected.

The

XAFS spectra were estimated from / / 4 , where 4 is the incident X-ray intensity detected by 4 c m ion-chamber filled with Nj gas and / is the X-ray absorption intensity of bromine measured by the cell shown in Figure 2. All XAFS spectra were recorded a t BL-7C of Photon Factory, High Energy Accelerator Research Organization in T s u k u b a .

123

Br iC-edge

/\

1

Kji^/f\w^y^ t/(b?v \j/y/ F-~UcL-

\Yv / ^

rNidj^^-r-'T''^

,

0

3

2

^^'^

r/A Fig.3. Fourier transforms for samples with Smmol-dm'^ DDABS a n d (a)5, (b) 10,(c)20,(d)40 mmol-dm-^ CuBr2 aq.

Fig.4. The model for Br" and DDABS molecules on the solution surface.

3 . Results and Discussion Figure 3 shows the Fourier transforms of obtained XAFS spectra.

A

p a r a m e t e r , r, c o r r e s p o n d s to t h e distance between Br' a n d its coordinating atoms.

XAFS p a r a m e t e r s for DDABS a n d CuBr2 aqueous solutions at Br K-

edge were determined by curve-fitting.

Results are listed in Table 1. Nis the

n u m b e r of neighboring a t o m s a n d o is the Debye-Waller like factor, which s t a n d s for thermal fractuation.

The parameters for 3mol-dm"^ KBr aqueous

s o l u t i o n d e t e r m i n e d by t h e t r a n s m i s s i o n m e t h o d are also listed.

These

parameters represent the local structure for Br' in water. It was reported that Br' in water a d o p t s the octahedral configuration, and t h u s the coordination n u m b e r is six[2].

TV values for Br' adsorbed on DDABS surface films were

d e t e r m i n e d r e l a t i v e to t h i s s o l v a t i o n n u m b e r in b u l k w a t e r .

As t h e

concentration of CuBr2 decreases, the coordination distance becomes longer. The bromide ions electrostatically interact with hydrogen atoms of the methyl groups bonded to the quaternary nitrogen atom, a s schematically illustrated in Figure 4. The distance between Br' and the carbon atom of a methyl group can be assigned the r v a l u e , 3.41 A. This interaction distance and the coordination number suggest t h a t DDABS molecules adsorbed on the solution surface form the network structure bridged by bromide ions. Although Br" seems to connect

124

Table 1 Curve-fitting parameters for 5mmol-dm'^ DDABS a n d CuBr2 aq. Concentration of CuBrj aq.

r/k

N*

a/A

Smmol-dm'^

3.41

3.47

0.152

lOmmol-dm"^

3.42

3.88

0.152

20mmol-dni'^

3.20

3.03

0.107

40mmol-dm'^

3.18

5.24

0.096

3mol-dm-^ KBr aq.

3.20

6.00

0.144

* N for KBr aq. is a s s u m e d to be six.

four DDABS molecules b e c a u s e t h e coordination n u m b e r within the plane parallel to the solution surface is four, this should be u n s t a b l e due to steric hindrance.

XAFS a n a l y s e s c a n n o t completely distinguish the coordination

distance of water molecule for B r (3.20A) from t h a t of DDABS (3.41 A).

The

hydrogen a t o m s of water molecules a p p r o a c h Br" a n d stabilize the network structure illustrated in Figure 4; two methyl groups a n d two water molecules coordinate Br'.

Otherwise, Br' may be sandwiched by two DDABS molecules,

and surrounded by four methyl groups. In this case, it is expected that Br has no coordinated water molecules.

The result for 40 mmoldm"^ CuBra and 5

mmoldm"^ DDABS aq. is similar to t h a t for KBr aq. because usually solvated bromide ions (not interacting with DDABS) exist in t h e observable region. T h u s , the surface a n d bulk s t r u c t u r e s of bromide ions are superimposed for s u c h high concentrations.

REFERENCES 1. I. Watanabe, H. Tanida, S. Kawauchi, M. Harada and M. Nomura, Rev. Sci. Instrum., 68 (1997) 3307. 2. H. Ohtaki, N. Fukushima, T. Yamaguchi, in: H. Ohtaki, H. Yamatera (eds.), S t r u c t u r e a n d Dynamics of Solution. S t u d i e s in Physical a n d Theoretical Chemistry 79, Elsevier, Amsterdam, 1992.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) •c 2001 Elsevier Science B.V. All rights reserved.

125

Adsorption of Nonionic Surfactants, Triton X and Triton N, on Hydroxyapatite after Surface Modification with Sodium Dodecyl Sulfate in an Aqueous Phase *) Saburo Shimabayashi, Masashi Hoshino, Takahisa Ohnishi, and Tomoaki Hino The University of Tokushima, Faculty of Pharmaceutical Sciences, Sho-machi 178-1, Tokushima 770-8505, Japan. E-mail: [email protected] ABSTRACT Although Triton X-100 and Triton N, which are water-soluble nonionic surfactants, were scarcely adsorbed by HAP without SDS, these were adsorbed in the presence of SDS. On the other hand, methyl yellow, a water-insoluble dye, was also captured on the surface of HAP when HAP was treated with SDS. It was concluded that surface modification of HAP by SDS and hydrophobic interaction with SDS on the surface of HAP play important roles in the adsorption.

1. INTRODUCTION Adsorption mechanism of a surfactant by a solid surface has been discussed so far[l, 2]. It has been concluded that the adsorbed surfactant ions were arrayed and oriented on the adsorbent surface after the adsorption, that is, the hydrophobic tails or head groups were protruded into an aqueous phase from the surface. Whether hydrophobic or hydrophilic group was protruded depends on the combination of an adsorbent and a surfactant. When the adsorption amount is high enough, hemimicelle/admicelle is formed on the surface. Hydroxyapatite(Cai0(PO4)6(OH)2, HAP) easily adsorbs sodium dodecyl sulfate (Ci2H25S04Na, SDS) mainly through 2 mechanisms: (1) electrostatic attractive force between Ca2+ on the surface and dodecylsulfate anion(DS") of the added SDS, and (2) isomorphous substitution of the surface phosphate ion with a sulfate group of DS" [3, 4]. The surface of HAP becomes hydrophobic to some extent after the adsorption, because the Ci2 hydrocarbon tails of the adsorbed SDS were protruded to an aqueous phase from the surface. These tails interact with each other on the surface (i.e., lateral interaction and formation of hemimicelle), resulting in formation of the hydrophobic domain. It is expected that another hydrophobic compound should be captured by this modified surface through hydrophobic interaction[3, 4]. In the present paper, the interaction between the adsorbed SDS and hydrophobic or amphiphiUc compounds are discussed. *T This study was supportedby Grant-in-Aid for Scientific Research (C)-(2) #11672143(1999-2000) of The Ministry of Education, Science, Sports and Culture, Japan.

126

2. EXPERIMENTAL SDS, methyl yellow(MY), polyethylene glycol mono-p-isooctylphenyl ether (Triton X-100,TX-100,n = ca. 10), polyethylene glycol mono-p-nonylphenyl ether (Triton N, TN, n = ca.lO), and matured HAP were purchased from Nacalai Tesque Inc.(Kyoto). The concentrations were determined by an Epton method for SDS, a UV absorptiometry at 274 nm for TX-lOO and TN, and a colorimetry at 418 nm for MY. The experimental conditions are shown in a respective figure caption. 3. RESULTS AND DISCUSSION 3.1. Adsorption of Triton X-100 and Triton N Triton X-IOO(TX-IOO) and Triton N(TN) are scarcely adsorbed to the surface of HAP in the absence of SDS. This is because their affinity for a raw surface of HAP is weak. However, it was found that these nonionic surfactants were adsorbed to the HAP in the presence of SDS. This fact suggests that the SDS adsorbed on HAP offers the adsorption site for TX-lOO and TN, as expected above. Figure 1 shows the adsorption amounts of TX-lOO and TN in the presence of 0.50 m m o l / d m 3 SDS. The adsorption isotherms are sigmoidal, where the adsorption amount of TN was higher than that of TX-lOO and this is due to the fact that the hydrocarbon chain of TN is longer than that of TX-lOO. The adsorption amount of TX-lOO slightly decreased after attaining a maximum. The results for TX-lOO will be mainly discussed hereafter, because those for TN were qualitatively almost the same as those for TX-lOO. An initial slope in the adsorption isotherm of TX-lOO became steeper while 30

^

25 20 h

§

15

Io

10

o

-^

h r

y

5 h

200

400

600

[Nonionic surfiictant]free/(^inioI/dm ) Fig. 1. Adsorption Amounts of Triton X-lOO(diamond) and Triton N(square). [SDS] = 0.50 mmol/dm3, [HAP] = 25.0 g/dm3,Temp = 30 OC. NaCl was not added.

127

the adsorption amount of TX-lOO at a plateau region decreased after attaining a maximum with a concentration of SDS added (data not shown). These facts suggest that the affinity of TX-lOO for HAP increases with a concentration of SDS when that of TX-lOO is low. On the other hand, when both concentrations becomes high enough, some molecules of TX-lOO tend to interact with SDS to form a mixed micelles in a mother solution but not on the surface. This results in an increase in the concentration of TX-lOO unbound to HAP and, therefore, in a decrease in the adsorption amount of TX-lOO. The effect of NaCl on the adsorption amount of TX-lOO was also studied. Some results are shown in Fig. 2, where NaCl was added by 500 mmol/dm^. An initial slope of the adsorption isotherm increases in the presence of NaCl. This is the salting-out effect to increase in the adsorption of TX-lOO. On the other hand, the adsorption amount significantly decreases after attaining a maximum in the presence of 500 mmol/dm3 NaCl with a concentration of TX-lOO. This phenomenon might be explained as follows. Formation of the mixed micelle of SDS with TX-lOO in an aqueous solution of 500 mmol/dm^ NaCl is more preferable than formation of the mixed adsorption layer on the surface of HAP. 3. 2. Adsorption of MY on HAP in the Presence of SDS [5] In the above section, it was shown that formation of the adsorption layer of ^ o B

30

2t

§ ^

0

200

400

600

800

1000

[TX-100]fr^(/imol/dm3) Fig. 2. Adsorption Amounts of Triton X-100 in the Presence of NaCl and SDS. [HAP] = 25.0 g/dm3. Temp. = 30 OQ (diamond): [SDS] = 0.50 mmol/dm3, (square): [SDS] = 0.50 mmol/ dm^ together with [NaCl] = 500 mmol/ dm^, (cross): [SDS] = 1.00 mmol/dm^, (triangle): [SDS] = 1.00 mmol/ dm^ together with [NaCl] = 500 mmol/dm3.

128

SDS on the surface of HAP is responsible for the adsorption of TX-lOO and TN. In order to confirm and expand this idea, the adsorption and/or capturing of MY to the surface of HAP was studied. It was found that MY, insoluble in distilled water and nonadsorbable to PiAP, was easily adsorbed to the surface of HAP in the presence of SDS. Some of the results are shown in Fig. 3. The adsorption amount of MY is decreased after attaining a maximum. These tendencies are quite similar to those mentioned above for the adsorption of TX-lOO and TN, and shows that the formation of hemimicelle/adsorption layer of SDS plays a significant role in the adsorption of MY to the HAP surface. Details are mentioned elsewhere[ 5 ]. lb

(B)

O

1-1

oT 10 .

<

Di:

%_ «~.& „

3a

bO

a^--a-

^>»

"o £ £

5 " ^

^^ >S X

I J

1

10

fSDSl^otal/(iniiiol/din3)

»

15

[SDSl^ot^/(iiimol/din3)

Fig. 3. Adsorption Amount of MY on the Surface of HAP in the Presence of SDS andNaCl. [HAP]=25g/dm3, [NaCl]= 0(A) and 5 mmol/dm3(B). Data shown in (A) and (B) are for 2 runs. MY was adsorbed from its saturated solution at 30 ^C. Therefore, chemical potential of MY in the solution is equal to that of MY of the solid phase over the range of the SDS concentration studied [ 5 ]. REFERENCES I. K. Shinoda, T. Nakagawa, B. Tamamushi, and T. Isemura(eds.), "Colloidal Surfactants", Academic Press, New York, 1963, pp.216-247 2.G. D. Parfitt and C. H. Rochester(eds.), "Adsorption from Solution at the Solid/ Liquid Interface", Academic Press, London, 1983, p. 116, p. 254, p.288, and p. 294. 3. S.Shimabayashi, S.Nishine, and T.Uno, "Adsorption of Hydroxypropylcellulose on Hydroxyapatite via Formation of Surface Complex with Sodium Dodecyl Sulfate", in Zahid Amjad(ed.), "Water Soluble Polymers: Solution Properties and Apphcations", Plenum Press, New York, 1998, Chap.9, pp. 105-116. 4. S. Shimabayashi and T. Uno, "Hydroxyapatite-Polymer Interactions", in J. C. Salamone (ed.), "Polymeric Materials Encyclopedia," vol.5(H-L), CRC Press, Boca Raton, 1996, pp. 3142-3147. 5. S. Shimabayashi, T. Hino, and T. Ohnishi, Phosphorus Research Bulletin, Vol. II, in press(2000).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c 2001 Elsevier Science B.V. All rights reserved.

129

Miscibility of Dodecylpyridinium Bromide and Dodecylquinolinium Bromide in Adsorbed Films and Micelle Takayuki Fujii, Katsuhiko Fujio, and Sumio Ozeki Department of Chemistry, Faculty of Science, Shinshu Asahi,Matsumoto,Nagano 390-8621, Japan

University

3-1-1

Surface tension was measured for the dodecylpyridinium bromide (DPB) / dodecylquinolinium bromide (DQB) / water system. Using the regular solution approximation for nonideal mixing, the molecular interaction parameter, 0, and composition were estimated in adsorbed films and micelle. The 0 values obtained are slightly positive, which indicate that a weak repulsive force acts between DPB and DQB in adsorbed films and micelle. It is also shown that adsorbed films and micelle are richer in DQB than bulk solutions. 1.Introduction In most applications surfactant mixtures are used rather than pure species. Mixtures of different surfactant types often exhibit synergism in their effects on various properties of a system. There is increasing interest in understanding the structure and properties of mixed micelle and monolayer. The behavior of single surfactant system has been widely investigated, but that of surfactant mixtures has been investigated only to a limited extent. In this work the surface tension of aqueous solutions of a mixture of dodecylpyridinium bromide (DPB) and dodecylquinoUnium bromide (DQB) was measured. On the base of regidar solution theory the miscibihty of these surfactants in adsorbed films and micelle was investigated. 2.Experimental DPB was synthesized by several recrystallizations of dodecylpyridinium

130

chloride in concentrated NaBr solution. DQB was synthesized from 1bromododecane and quinoline. Surface tension was measured as a function of the total molality and fixed composition of surfactant mixture at 298K by the drop-weight method. Water doubly distilled from alkaUne permanganate was used. S.Results and Discussion Total molality of mixed surfactants is defined by

where mppg and m^Qg are molalities of DPB and DQB, respectively, and the mole fraction of DPB in total mixed solute is given by nir

Fig.l shows the concentration dependence of surface tension for the DPB/DQB system at constant a jrjp^ . The surface activity of DQB is stronger than that of DPB. CMC increases with increasing (2r ^^^^. The molecular interaction parameter and composition can be estimated in

S

-5

-4 5

-4

-3 5

-3

-2 5

-2

-15

log m Fig. 1. Surface tension vs log total molality for aqueous solutions of DPB and DQB mixture at constant compositon.

131 adsorbed films and micelle by using the regular solution theory approximation for nonideal mixing. In adsorbed films the molecular interaction parameter ,0\ is given by

p"

ln(a

,)

(1)

(i-^D.«r

where ml^„ is the molality of pure DPB at a certain surface tension [1] . XQPQ is the mole fraction of DPB in mixed adsorbed films which give that particular surface tension, estimated by

^ DFB

*^V^DPB

mix,,

yyinPH)

(2)

= 1. (1 - ^DPB f ln[(l - a J,,, )m /(I - X ^,^ )m]^^ J

In Fig.2 the total molality of mixed surfactant in aqueous solution with the surface tension of 45mN m"^ is plotted versus a ^p^ . The X^p^ obtained by solving Eq.(2) indicates that adsorbed films are richer in DQB than bulk solutions. The average oi J^"" values calculated by Eq.(l) is 0.24, which means that in adsorbed films the interaction between DPB and DQB is sUghtly repulsive.

1 1

nonide*! Mdt 1(0=0 23)

nonideal a o d t U f i ^ 24) O

exper mental

O

cxperMtntal

OOIS

be be

0 01

'o

s

^^^^^

^
1^—-o--"*'^"'*'^

Fig. 2. Total molality vs mole fraction of DPB in the total solute at the surface tension of 45mN m'^

0 005

^^^^^^..^^x**^

Fig. 3. CMC of mixtures vs mole fraction of DPB in the total solute

132

Similarly, the molecular interaction parameter in mixed micelles, 0^, expressed as

is

(^ ~ XopB )

where w^^^ is the CMC of pure DPB and mf^ is the mixed CMC [2,3] . X'llpg represents the mole fraction of DPB in mixed micelle, determined by

i^DFB)

^^(Q^DPB ^

I ^DPB ^DPB)

_ |

(4)

(1 - Xl^.p, y ln[(l - a,,p, )m'^ /(I - X^,, )m^, ]

Fig. 3 shows the dependence of mixed CMC on a^PB • From the X^^p^ obtained by solving Eq.(4), it is found that mixed micelles are richer in DQB than bulk solutions. The average j3^ value of 0.23 estimated by Eq.(3) indicates the weak repulsion between two surfactants in mixed micelles. Reference 1. M.J.Rosen and X.Y.Hua., J. Colloid Interface SCL, 86,164(1982) ^.D.N.Rubingh iz? "Solution Chemistry of Surfactants", Vol.1, KLMittalEd, Plenum Press, New York, 1979, p337. 3. P.M.Holland and D.N.Rubingh, J. Phys. Chem,. 87,1983(1984)

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. A l l rights reserved.

133

interaction and Complex Formation of Pluronic Polymers with Ionic Surfactants Saburo Shimabayashi, Akihito Ichimori, and Tomoaki Hino The University of Tokushima, Faculty of Pliarmaceutical Sciences, Sho-machi 1-78-1, Tokushima 770-8505, Japan E-mail: [email protected] ABSTRACT: Interaction of a pluronic po(ymer(Plu) with an ionic surfactant was studied. Specific behaviors of a solution of Plu mixed with an ionic surfactant, such as cloud point, relative viscosity, and solubilization of methyl yellow, were discussed, taking the binding isotherm into considerstion.

1. INTRODUCTION Pluronic polymer(Plu) is a block coplymer of polyethylene oxide(PEO) and polypropylene oxide(PPO), (EO)A-(PO)B-(EO)A, where the PPO group is more hydrophobic than the PEO group. It is, therefore, known that the Plu Interacts more strongly with hydrophobic and amphiphilic compounds than the simple PEO [1]. In the present paper, the interaction and complex formation of Plu with an ionic surfactant were discussed, taking the published informations into consideration[2,3]. 2. EXPERIMENTAL Plu F68(A=76, B=29) and F127(A=100, B=65) were obtaind from Sigma Ltd.(USA). Sodium dodecylsulfate(SDS) and cetylpyridinium chloride (CPC) were products of BDH Ltd.(England) and Tokyo Kasei Ltd.(Japan). Other chemicals used were of the reagent grade from Nacalai Tesque Ltd.(Japan). Binding ratio of a surfactant to a Plu was determined by a dialysis method with a cellulose tubing at 30 00. Viscosity was measured with an Ubbelohde-type capillary viscometer also at 30 00. Oloud point of the solution was observed with the naked eye. 3. RESULTS AND

DISCUSSION

Binding ratio(XsDS) of SDS to a Plu was determined at 30 oo over the concentration ranges of 0.025-0.2 g/dl Plu and 0-0.1 mol/dm^ Na2S04. The XSDS sigmoidally increased and then decreased after attaining a maximum, as shown in Fig. 1. The sigmoidal increase means that the binding of SDS to Plu is cooperative, while the decrease suggests that micelle formation of SDS on the polymer chain is competitive with that in the bulk solution[2-5]. The X S D S also decreased with a concentration of Plu, as shown from the top to the bottom of Fig. 1. On the other hand, it increased with a concentration of Na2S04 added, as shown from the left-hand side to the right-hand side in Fig. 1. These facts are showing that the space between the polymer chains, into which SDS could penetrate and bind to the polymer segments, becomes narrower with an increase in the polymer concentration due mainly to the steric hindrance of the polymer chain itself and its

30,

30

A

O X -

0

2

4

8

10 12 ISOSlfI mM

8

-

14

18

18

:

0

IPluronlcy(gld1) 0.m ~F+OJ/M = 0

2

10 12 14 ISDSlf/ mM [PlumnkV(gldl) 0.025 p+S04]/M 4

8

8

-

I8 I

18

0

2

0.01

4

8

10 12 lSOSh I mM

8

IPluronlcy(g/dl)

-

0.025

18

14

18

:

[ N + O J / M = 0.1

30,

0

2

4

6

8

10

12

14

18

18

0

2

ISOSlf / mM IPluronkV(gld1)= 0.1

Il?E t l 0

4

8

8

-

10

12

I8

14

18

:

18

,

ISDSlf I mM IPluronkU(gld9 0.1 IN@O4l/M -0.1

IN@O4l/M = 0.01

30

2

4

020 25

I

8

10 12 14 ISOSlr / mM [PluronkV(g/dl)= 0.2 [&SOJ/M 8

18

-

0

18

t 0

2

4

8

8 10 12 [SOSlf I mM

[PluronicV(g/dl)I0.2

14

18

I N ~ S 0 4 1 / M= 0.01

18

0

2

4

8

8

10

IsoslfI [PluronkV[gldl) 0.2

12 mM

14

IN+OJ/H

Fig. 1. Binding isotherm of SDS to Plu F127 and F68 at 30 OC. (triangle):F127, (circle): F68. The concentration of Plu increases from the top to the bottom, while that of Na2S04 from the left-hand side to the right-hand side.

-

18

0.1

135

hydration sheath[6]. When the salt, Na2S04' is added, the hydration layer of the polymer becomes thinner and the salting-out effect of the salt on SDS and Plu accelerates the binding of SDS to the polymer. The binding ratio of the surfactants to Plu F127 was almost the same as or slightly larger than that to Plu F68(Fig. 1). Similar tendencies were observed in the binding ratio of CPC to Plu. However, the binding ratio of CPC was remarkably smaller than that of SDS(data not shown). Cloud point[4] of an aqueous solution of 0.5 g/dl Plu F127 was higher than 100 oc. It decreased with a concentration of Na2S04, while it increased again after attaining a minimum with a concentration of SDS in the presence of 0.15-0.30 mol/ dm^ Na2S04. On the other hand, it increased monotonously with a concentration of CPC(0-8 mmol/dm3) in the presence of Na2S04. Some of the results are shown in Fig. 2. Relative viscosity of a Plu solution decreased after attaining a maximum with a concentration of SDS, while it monotonously increased with that of CPC over iicr locr

::: 90 ^x>

o oo

80

o ooo

A A A A A A A ^ ^ 70

A D

L

601 50^

2.5

5.0 7.5 [SDS] / mM

10.0

5.0 7.5 [CPC]/mM

12.5

12.5

Fig. 2. Cloud point of an aqueous solution of 0.5 g/dl Plu F127 + an ionic surfactant -hNa2S04. [Na2S04] /(mmol/dm^) = 0.15(circle), 0.25(triangle), and 0.30(square) 1.125

o o

o

o

^ o

o

1.125h

1.100

1.075

1.100

f

22

1.075h

A 1.050'

1.050' 2.5

5.0 7.5 [SDS]/mM

10.0

12.5

5.0 7.5 [CPC]/mM

12.5

Fig. 3. Relative viscosity of 0.5 g/dl Plu F127 in an aqueous solution of an ionic surfactant+Na2S04 at 30 OC. [Na2S04] / (mmol/dm3) = O(diamond), 0.15(circle), 0.25 (triangle), and 0.30(square)

136

the concentration range studled(Fig. 3). Thus, SDS and CPC are considerably different in their effects on the cloud point and relative viscosity as well as their binding ratio. Data for Plu F68 are not shown In the present paper. These results may be explained as follows, by taking the binding isotherms into consideration. At a low binding ratio of SDS, the SDS forms hydrophobic moiety on the polymer chain, which bridges between the adjacent polymer segments with each other.This fact corresponds to the decrease in the affinity of the polymer coil for water(i.e., lowering a cloud point) and to the increase in the volume of the flow unit of the polymer coil after the aggregation(l.e., an increase In relative viscosity). With an increase in the binding ratio of SDS. the intermolecular bridging collapses due to the increase in the intermolecular repulsion, resulting in the redlspersion of the polymer[2]. This fact appears as a high cloud point and a low relative viscosity. These effects were not observed in the system of CPC, probably because of its low affinity for the polymer or low binding ratio, as mentioned before. Solubilization of methyl yellow(MY) by the polymer solution was studied. Results are shown in Fig. 4. MY was scarcely solubilized by a solution of 2 g/dl F68 in the absence of a surfactant. The solubilized amount, however, increased with the addition of SDS or CPC, which was higher than that solubilized by SDS or CPC alone. This effect is owing to the complex formation. On the other hand, 2 g/dl F127 showed a high solubillzing ability even in the absence of a surfactant. It increased with a concentration of a surfactant added. Thus, the solubillzing ability of F127 was higher than that of F68 owing mainly to the fact that the content and length of the PPO chain Is larger in F127 than in F68 and, therefore, the mixed micelles[3, 5, 6] of the surfactant and Plu F127 easily capture the MY molecules into them. 0.5, 2 0.4 E ^AA

o I E CD

"D , © 0.2/f n o 0.1

5

10 15 [SDS]/mM

20

0.5

1.0 1.5 [CPC]/mM

2.0

Fig. 4. Solubilization of MY(p-dimethylaminoazobenzene) in the system of Plu and an ionic surfactant at 30 oc. (circle): an ionic surfactant only, (triangle): an ionic surfactant + 2 g/dl F127, (square): an ionic surfactant + 2 g/dl F68

REFERENCES 1. P.AIexandridis and T.A.Hatton, Colloids and Surfaces A, 96(1995), 1. 2. S.Shimabayashi and T.Uno, Prog. Colloid Polym. Sci., 106(1997), 136. 3. M.AImgren, J.van Stam, and P.Bahadur, J.Phys.Chem., 95(1991), 5677. 4. R.Cardoso da Silva and W.Loh, J.Colloid Interface Sci., 202(1998). 385. 5. E.Hecht, K.Mortensen, and H.Hoffmann, J.Phys.Che.. 99(1995). 4866. 6. A.Caragheorgheopol and S.Schlick, Macromolecules, 31(1998), 7736.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (Q 2001 Elsevier Science B.V. All rights reserved.

137

Formation of Chiral Aggregates of Acylamino Acids in Organic Solvents H. Matsuzawa *, H. Minami', T. Yano *, T. Wakabayashi *, M. Iwahashi *, K. Sakamoto ** andD. Kaneko** * School of Science, Kitasalo University, Sagamihara 228-8555, Japan ^ Ajinomoto Co. Kawasaki 210-8681, Japan Through measurement of circular dichroism (CD), NMR and vapor pressure osmometry for optically active acylamino acids in CH3CN, these acids were found to be present as monomers in a polar solvent

The decrease in CD intensity and amide proton NMR

chemical shifts with a rise in temperature were considered to arise from conformational change in the acid molecule. 1. Introduction Acylamino acids, abbreviation for amino and fatty acids, are mild to living organisms and easily decomposed through the action of microorganisms.

The salts of these acids are

thus often used as mild surfactants,^ whose properties in water, including chiral and racemic modifications, have been studied in detail.^ The properties of less water-soluble acylamino acids in organic solvents have been also studied. Optically active acylamino acids in organic solvents show strong capacity for hydrogen-bonding and form three-dimensional gelstructures able to trap numerous organic solvents.^ Sakamoto et al found that optically active acylamino acids form lyotropic liquid crystals in aromatic solvents when suspended in solvents that do not dissolve the acids and these crystals undergo swelling. During swelling, optically active acid molecules assume an orientation that results in the formation of a helix. "* Sakamoto et al found optically active N-lauroylglutamic acid (L- or D-LGA) and Nlauroylvaline (L- or D-LVA) to completely dissolve in polar solvents such as methanol and ethanol and show circular dichroism (CD) bands at 212 nm. They proposed the formation of helix aggregates of acylamino acids.^ However, the size and structures of the aggregates have not been determined. For clarification of these parameters for aggregates of L-LGA, L-LVA, or methyl ester of

138 L-LVA (L-LVMe) in polar solvent, CD and NMR measurements were conducted in the present study.

Apparent mean aggregation numbers of L-LGA, L-LVA, and L-LVMe were

also obtained by vapor pressure osmometry (VPO) method. CH3CN was used, mstead of alcohol, to prevent proton exchange that would make difficult IR and NMR measurements for solvent-acylamino acid systems. 2. Experimental L-LGA, L-LVA and L-LVMe (Ajinomoto Co. Ltd.) were used after being dried in vacuum. CH3CN (spectroscopic grade, Dojin Chem. Co.) and CH3CN-flf3 (98 %, Aldrich Chem. Co.) were used without further purification. VOP was conducted using molecular weight apparatus (model 117, Corona Electric Co. Ltd.); diphenylethanedione (Junsei Co. Ltd.) was used for apparatus calibration.^ NMR was carried out with a JEOL EX-400NMR spectrometer. 20'C

3. Results and Discussion Figs.

1-A

and

B

indicate

representative temperature dependency of CD and UV absorption spectra for 1.0 X 10-^ mol dm-^ L-LVA in CH3CN. The CD band intensity (Ae, difference in molar absorptivity for left and right

245

250

circularly polarized light) was found to decrease with temperature, as also noted when using methanol as solvent.^

The

Steep portion of the absorption band i n

E 1200

Fig. 1-B at shorter wavelength is the

1

hem of the strong band due to C=0 n-n* transition, the peak of whose band noted to appear at 150 nm.^ The hem has a

205

210

215

220

225

230

235

240

245

250

Wavelength /nm

shoulder attributable to the weak band produced as a result of the C=0 n-K* transition*^ at 215-220 nm. The CD spectra thus is ultimately given by the C=0 n-TT* transition.

Fig. 1 (A) CD and (B) absorption spectra for 1.0 X 10^ mol dm ^ LVA in CH3CN at 20, 30,40, 50, and 60 °C.

139 The CD spectra are generally influenced by the intensity change of the absorption band

0.07

itself. Thus, for detailed comparison of Ae,

0.06

normalization of this parameter is necessary. Accordingly, we divided Ae by the molar absorptivity, e, at the same wavelength. Fig. 2 shows the temperature dependence

0.05 u> 0.04

_ -g-,. •i^. '

< 0.03

of Ae /e for L-LVA and L-LVMe at 215 nm.

0.02 h

Ae /e obviously decreased with temperature

0.01

but did not depend on concentration of L-LVA

0

t^ •

1

! -

LVMe

, . _ j ..«.„

i ...,

10

or L-LVMe within experimental error. The

LVA

•-e-

20

30

40

50

60

70

T /x:

decrease in Ae /e may possibly result from dissociation of helix aggregates.^ To confirm

Fig. 2

this point, apparent molar weights of L-LVA

symbols) and LVMe (closed symbols):

and L-LGA in CH3CN were determined at

Circles indicate 1.0x10^ mol dm ^ triangles,

various temperatures by VPO and then the

5.0 x 10*' mol dm ^ squares, 1.0 x lO"' mol

apparent mean aggregation

dm'^ diamonds, 5.0 x 10*^ mol dm''

number was

AE / e VS. temperature for LVA (open

calculated. Surprisingly, the mean aggregation number in nearly all cases was about 1.0. Acylamino acids exist not as aggregates but as monomers in a polar solvent Namely, the decrease in CD may possibly arise from the conformational change of the acid molecule and such change would derive from diminished intramolecular hydrogen bonding (H-bonding) or acid-solvent H-bonding (solvent H-bonding). To confirm H-bonding, NMR was carried out on L-LVA. N-H proton signals of L-LVA shifted to a higher magnetic field with a rise in temperature, with no dependence on L-LVA concentration, as neither did CD spectra.

Conformational

change would thus appear due to decrease in intramolecular H-bonding or solvent H-bonding. However, the intramolecular H-bonding between NH and COOH moieties is structurally difficult; L-LVMe possessing no COOH moiety is similar in CD decrease to L-LVA, as evident from Fig. 2 H, N, C and O atoms within the amidic group have resonance structures and are on a same flat surface.

Intramolecular rotation about the C-N bond in the group is somewhat restricted

since the bond has a nature of double bond.

Solvent H-bonding between CH3CN and NH of

an acylamino acid enhances electron delocalization of the amidic bond and thus also

140 resonance with consequent restriction of rotation.

Restricted rotation may serve to maintain

the chirality of the acylamino acid molecule. At a higher temperature, the solvent H-bonding weakens, causing electrons to become localized.

With the disappearance of the solvent H-

bonding the rotation should occur more easily about the C-N bond.

Acylamino acid may

undergo conformational change with consequent reduction in chirality. NMR data for Nmethylacetamide and acylamino acid have shown electron localization with a rise in temperature.^'*^ Based on the present results, a polar solvent may be concluded to influence rotation about the C-N bond, resulting in conformational change of acylamino acid *° and lower CD intensity. References 1. K. Sakamoto, J. Soc. Cosmet. Chem., 35 (1987) 353; J. Oleochem., 44 (1996) 256. 2. H. Staudinger, M. V. Bechker, Ber., 70 (1937) 889; M. Naudet, Bull. Soc. Chim. France, (1950) 358; P. Heit-mann, European J. Biochem., 3 (1968) 346; M. Takehara et al. J. Am. Oil Chem. Soc., 49 (1972)143; 50 (1973) 227; 51 (1974) 419; 49 (1972) 157; 51 (1974) 419; M. Shinitzky and R.Haimovitz, J. Am. Chem. Soc. 115 (1993) 12545; T. Imae, Y. Takahashi and H. Muramatsu, J. Am. Chem. Soc. 114 (1992) 3414. 3. K. Hanabusa, K. Okui K. Karaki, T. Koyama, and H. Shirai, J. Chem. Soc., Chem. Commun., (1992) 1371, K. Hanabusa, J. Tange, Y. Taniguchi, T. Koyama and H. Shirai, /7?i^., (1993) 390. 4. K. Sakamoto, R. Yoshida, M. Hatano and T. Tachibana, J. Am. Chem. Soc. 100 (1978) 6998. 5. K. Sakamoto and M. Hatano, Bull. Chem. Soc. Jpn., 53 (1980) 339. 6. R. M. Silverstein, G. C. Bassler and T. C. Morrill, "Spectroscopic Identification of Organic Compounds" John Wiley &Son, Inc. (1991) 7. K. Imabori "Introduction to the experimental biophysical chemistry 11" Baifukan (1972). 8. S.H. Gellman, G. P. Dado, G-B. Liang and B. R. Adams, J. Am. Chem. Soc. 113 (1991) 1164. 9. M. Akiyama and T. Ohtani, Spectrochimica Acta 50A (1994) 317. lO.J. Manzur and G. Gonzale, S. Naturfousch B36 (1981) 763.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) P 2001 Elsevier Science B.V. All rights reserved.

141

Formation and Structure Control of Reverse Micelles by the Addition of Alkyl Amines and their Applications for Extraction Processes of Proteins K. Shiomori^*, T. Honbu^, Y. Kawano^, R. Kuboi^ and I. Komasawa^ ^Department of Applied Chemistry, Miyazaki University, 1-1 Gakuenkibanadai-nishi, Miyazaki 889-2192 Japan ^Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Osaka 560-8531 Japan Extraction of water and lysozyme in mixed micellar systems of sodium bis(2-ethylhexyl) sulfosuccinate and various long chain alkyl amines were investigated. Extraction of water in the mixed micellar systems decreased rapidly with decreasing pH in the aqueous phase. The pH dependence on water extraction is affected by the structure of the amines. Extraction of lysozyme is also controlled by the formation of the reverse micelles, and does not completely occurr in the pH range in which no water is extracted into the organic phase. Lysozyme extracted into the mixed micellar systems can be successfully back-extracted with high activity yield by destruction of the micelles at acidic pH. By increasing the amine concentration, the pH values initiated by the back-extraction of lysozyme are raised, and the activity of the back-extracted lysozyme is decreased. 1. INTRODUCTION Reverse micelles, which are self-organization molecular aggregates of surfactants in apolar media, have been extensively investigated as media for the extraction of proteins [2, 5]. One intensively studied surfactant is sodium bis(2-ethylhexyl) sulfosuccinate (AOT). The extraction of proteins occurrs by interaction between the protein surface and the reverse micelles. Extraction control of the protein in the usual reverse micellar system is carried out using changes of the surface characteristics of proteins and the size of reverse micelles effected by the pH and salt concentration. However, proteins are known to suffer from denaturation, and the efficiency of protein separation using reverse micelles decreases significantly when the interaction between proteins and micelles is too strong. If it is possible to greatly change the characteristics and structure of reverse micelles by means of external stimulation, it will be possible to construct a more effective separation process of proteins. Some studies have reported on the control of the formation of reverse micelles by pH [3,4], pressure [6] and temperature [1]. Previously, the addition of either tri-«-octylamine or di-2ethylhexylamine to the AOT reverse micellar system was found effective to control the formation of reverse micelles by pH change [7]. In this paper, various long chain alkyl amines are used for the additives to the AOT reverse micelles. The effect of the amine structure on the formation of reverse micelles and the extraction characteristics of lysozyme by the mixed micellar systems are investigated.

142

2. EXPERIMENTAL AOT was purchased from Nacalai Tesque Co. The long chain alkyl amines used are shown in Fig. 1. AOT was dissolved in isooctane, then the amine was solubilized in the solution. A buffer solution containing NaCl (0.1 - 1.0 M) was used as the aqueous phase [7]. Egg white lysozyme was purchased from Wako Pure Chemical Co. Extractions of water and proteins were carried out by the phase transfer method [7]. The water concentration in the micellar organic phase was determined by Karl-Fisher titration. The concentration of lysozyme in the aqueous and organic phases, respectively, was measured by the adsorption at 280 nm. Lysozyme activity was measured by the lysis rate of Micrococcus lysodekticus.

• H

0139^2 ,H CH3CHiCH2CH2CH CH2-r<^ 2-Cthylhexylamine

n-OctyUiinlne (OA)

(EHA) CH39H2 CHaCHaCHzCHaCHCHav CHgCHaCHzCHaCHCHa^

Oi-ff-Octylamlne (DOA) /CH3 .^ ^CH3 Dimethyl-n-Octyiamlne (OMOA)

CH3CH2 Di(2-Ethylhexyl) amine (DEHA)

CsH..

CeHi7^

CeH,7-N CeH,7 Trl-n-OctyWimlne

aOA)

Fig.l

3. RESULTS AND DISCUSSION 3.1 Extraction behavior of water

C8Hi7^ .N-CH3 C8H,7^ D(-/M>ctyl methylamlne (OOMA) /

CH3 CH3 \ CH3CCH2CHCH24N

V

CH3

A

Trl-/!$o-Octyl«mlne CnOA)

Structures of long chain alkyl amines used.

The effect of pH on the water extraction in the mixed micellar systems is shown in Fig. 2. Extraction of water in the AOT system is almost constant regardless of pH. In the mixed micellar systems, the water extracted into the organic phase decreased with pH lower than a specific pH value, and was negligibly small at lower pH. This change of the water extraction effected by pH corresponds to the formation of the reverse micelle. The pH values at which the water extraction begins to decrease are dependent on the kind of amine used, and they are raised in the following order: tertiary amines < secondary amines < primary amines, branched-alkyl chain < straight alkyl chain, trialkylamines < dialkyl methylamines < alkyl dimethylamines. These tendencies agree well with the tendency demonstrated by the p/Ca value of amines, determined in the research of extraction behavior of acids by alkyl amines, to change in association with the alkyl group [8]. The control mechanism of the micellar formation in the mixed micellar systems is illustrated in Fig. 3. The amines, B, undergo protonation and form a cationic ammonium ion in the acidic pH range (Eq. (1)). This reaction 2.5 Key

cr2.o E E ^-^ ?1.0 o

• O • A A n ^ O

(AOTlo^g = 50 (molAn3l

Amines OA EHA DOA DEHA DMOA DOMA TIOA TOA

(Aminel^g =50[mol/m3] AOT system

(NaCI]^ = 0 1 [kmolAn3]

pH>pKa

P'^^^P^^

Long chain alkyl amine (B) O OT(ion form of AOT) CM A B + H+ ;=rBH+ (1) s«L I 0.5 An°^°A AOT :^ OT(2) OT-+BH+;::OT-BH (3) _S2a^ 0.0 OT-BH + (n-1 )OT- 71 (AOT)n-BH (4) 3 5 7 9 11 13 pH H Fig.3 Schematic illustration of formation control of reverse micelles in the mixed Fig.2 Effect of pH on water extraction in the long micellar system chain alkylamine-AOT system

vvl

143

will be affected by the basicity of the amine added. The ammonium ion reacts with the ion form of AOT (OT") to form the intermolecular ion complex by electrostatic interaction (Eq.(3)). It is considered that the complex has lower surface activities and no ability to form reverse micelles. Further, the higher-order complexation of the ammonium ion with 0T~ expressed by Eq. (4) has been implied when the amine concentration is lower than that of AOT [7]. The decrease in the water extraction is considered to be due to the consumption of AOT, which forms reverse micelles and extracts water, induced by these complexation reactions. 3.2 Forward extraction of lysozyme Effects of pH on the extraction of lysozyme in the mixed and the AOT single systems, respectively, are shown in Fig. 4. In the AOT system, lysozyme extraction effectively occurred at a wide pH range around neutral pH. In the pH range lower than 5, the extracted fraction of lysozyme into the organic phase, £f, was decreased, whereas the removed fraction of lysozyme from the aqueous phase, Rf, is very high and a large number of aggregates was observed at the interface and in the aqueous phase. Lysozyme is considered to be denatured by strong electrostatic interaction with AOT. In the DEHA-AOT and DOA-AOT mixed systems, £f was also decreased with pH values lower than a certain pH, a pH higher than that in the AOT system. Under this condition, /?f decreased with pH and the aggregates were not formed. The decrease in both £f and /?f almost corresponded to the decrease in the water extraction. Given that the reverse micelles do not form at acidic pH by the interaction with the alkyl ammonium ion, it is considerd that lysozyme was not extracted, and that the interaction between lysozyme and AOT was supressed.

8 10^12 PHH pi Fig.4 Effect of pH on extraction of lysozyme in the mixed and the AOT systems 1.0

3.3 Back-extraction of lysozyme from the micellar phase Back-extraction of lysozyme extracted into the micellar phase at pH 9 and 0.1 M NaCl was carried out by contact with a new aqueous phase. The effects of pH on the backextracted fraction of lysozyme into the aqueous phase, E\y, the removed fraction of lysozyme from the organic phase, R\^, the concentration of water in the organic phase, and the residual activity of the back-extracted lysozyme, RSA, are shown in Fig. 5. In the AOT system, lysozyme could not be back-extracted using the aqueous phase containing 0.1 M

e

e

10 i

pHH P» Fig.5 Effect of pH on backextraction of lysozyme in the mixed and the AOT systems

144

NaCl, regardless of the pH values. By increasing NaCl concentration up to 1.5 M, lysozyme could be backextracted in the pH range higher than the pi. Because lysozyme has a very high pi (pl=ll.l), back-extraction of lysozyme in the AOT system requires that the aqueous phase have a at very high pH and high salt concentration. In the mixed micellar system, lysozyme was effectively backextracted at a more acidic pH range than that of the forward extraction, in which the water extraction is very low and the reverse micelles are not formed. RSA was high and decreased moderately with pH. Back-extraction of lysozyme in the mixed system is considered to be carried out by the destruction of the reverse micelles that are caused by the complex formation between AOT and the cationic ammonium ion formed in the acidic pH range. By increasing the DOA concentration at a fixed AOT concentration, the pH values, at which the back-extraction of lysozyme began, were raised, and the activity of the backextracted lysozyme decreased (Fig. 6). However, the concentration change of both AOT and DOA, sustaining an equimolar ratio, was unaffected by the back-extraction behavior (Fig. 7). These results suggest that a quantitative relation among the concentrations of the amine, AOT, and lysozyme is present; this would explain the back-extraction mechanisms of proteins in the mixed micellar system. It is also obvious that lysozyme is denatured by addition of the amine to excess.

1.0 _ 0.8 — 0.6 ( A O T l ^ = 5 0 moJAn3

0.2

Fig.6 Effect of pH on backextraction of lysozyme at various concentrations of DOA.

REFERENCES 1. M. Dekker, K. Van't Riet, J. J. Van Der Pol, J. W. A. Baltussen, R. Hilhorst and B. H. Bijsterbosch, Chem. Eng. J.,46(I991)B69. 2. M. P. Pileni (ed.). Structure and Reactivity in Reverse Micelles, Elsevier Co., Amsterdam, 1989. 3. M. Goto, K. Kondo and F. Nakashio, J. Chem. Eng.,Japan, 23 (1990) 513. 4. T. Kinugasa, A. Hisamatsu, K. Watanabe and H. Takeuchi, J. Chem. Eng.Japan, 27 (1994) 557. 5. M. E. Leser and P. L. Luisi, Chimia, 44 (1990) 270. 6. J. B. Philips, H Nguyen and V. T. John, Biotechnol ?rog.,7(1991)43. 7. K. Shiomori, Y. Kawano, R. Kuboi and I Komasawa, J. Chem Eng, Japan, 32 (1999) 177. 8. A. M. Eyal, B. Hazan and R. Bloch, Solv. Extr. Ion Exch.,9 (\99\)2\\.

PEDTCXBXKXBO

80

£ 60 i

40 20

• ^ L L r

i^^lAOTlorg [DGAlofg '^[molAn3lImolMi3l O ^ n o

50 30 20 10

50 30 20 10

V-

0 100

80

6

8

10

12

PHH Fig. 7 Effect of pH on bac^ extraction of lysozyme at various concentrations of DOA and AOT.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (.o 2001 Elsevier Science B.V. All rights reserved.

145

Preparation and surface-active properties of cotelomer type surfactants of alkyl acrylate and acrylic acid T. Yoshimuraa Y. Koide^, H. Shosenjia and K. Esumi^ aDepartment of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kumamoto 860-8555, Japan bDepartment of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Tokyo 162-8601, Japan Cotelomer type surfactants of w-hexyl acrylate, «-octyl acrylate, 2-ethylhexyl acrylate or «-dodecyl acrylate and acrylic acid having hydropholic groups of 2-3 were prepared by the cotelomerization and examined for surface activity. Surface tensions of aqueous solutions of cotelomers with alkyl chain length of 6-8 were 27-32 mN m'\ The addition of 300 ppm of Ca^^ to aqueous solutions of cotelomers reduced the surface tension. Cotelomers with shorter alkyl chains had high foam stability, while those with branched alkyl chains showed poor stability. Highly stable oil-in-water type emulsions, which were brought about by shaking toluene with aqueous solutions of cotelomers, were formed by using cotelomers having alkyl chains of 2-3 and carboxylate functions of 2-3. 1. INTRODUCTION Dimeric (gemini) surfactants possessing two alkyl chains and two or three hydrophilic groups and trimeric surfactants possessing three alkyl chains and two or three hydrophilic groups have been developed [1-4]. Dimeric and trimeric surfactants are known to exhibit enhanced surface-active properties such as better water solubility, lower critical micelle concentration (cmc) and more efficient in lowering surface tension of water than conventional monomeric surfactants. These excellent properties may be derived from the enhancement of the interfacial density of surfactants having multi-alkyl chains and multihydrophilic groups. However, relations between structures of multi-alkylated surfactants and their properties have not been elucidated in detail. Telomers (oligomers), a type of polymer with polymerization degree (Pn) of 5-20, are obtained by polymerizing monomers in solvents with large chain-transfer constants. Telomers are considered to be suitable for introducing the multi-alkyl chains of the determined number. Recently we have synthesized vinylpyridine telomer type surfactants having multi-alkyl chains and multi-pyridinium functions [5-7]. Many of them showed abilities to lower surface tension and properties of emulsification superior to the corresponding monomeric telomers. In this study, we prepared multi-alkylated surfactants of cotelomers of «-hexyl acrylate (R^A), «-octyl acrylate (R8(No)A), 2-ethylhexyl acrylate (R8(Eh)A) or w-dodecyl acrylate (R12A) and acrylic acid (AA) with Pn of 3-6 and investigated their surface activities such as surface tension, foaming property and emulsification.

146

2. EXPERIMENTAL 2.1. Preparation Cotelomer type surfactants having alkyl chains of 2-3 (xR^A-yAA, x, y and m mean number of alkyl chains, number of hydrophilic functions and alkyl chain length, respectively) were prepared by the radical cotelomerization of AA with R^A, RgACNo), RgACEh) and RjA respectively, using 2-aminoethanethiol hydrochloride as a chain-transfer agent in the presence of a,a'-azobisisobutyronitrile as an initiator at 60 °C for 6 h under nitrogen atmosphere. After cooling to room temperature, mixture solutions of sodium hydroxide and methanol were added to the NHoCoH.S^CHg-CHV(cHo-CH4f^ solution and the precipitates were collected by ^ c-0 6=cy filtration. The precipitates were dissolved in water, 6lNa+
147 y+1 for xR„A-yAA since ionic surfactants are dissociated into positive and negative ions. The A was calculated from A=l / Nr, where N is Avogadro's number. Cotelomer molecules are considered as bundles of sodium alkanoate of conventional surfactants. Hence surface activities of cotelomers were compared with those of sodium alkanoate. The cmcs of cotelomers were smaller by one to two orders of magnitude than those of monomeric surfactants v^th the same alkyl chain length. 2.1R8(No)A-1.2AA and 2.8R8(Eh)A-2.5AA gave lower cmc than sodium «-dodecanoate [9], which has longer alkyl chain. Cotelomers having shorter alkyl chains gave lower surface tension than the monomeric surfactants, while that having dodecyl chains gave somewhat higher surface tension than sodium w-dodecanoate. The increase of the number of alkyl chains in cotelomers reduced the surface tension and the increase of the hydrophobic chain length rendered the cotelomers less surface active. The values of A of cotelomers were greater than that of sodium Ai-dodecanoate (92A). Cotelomers seem to be adsorbed at surface between water and air by orienting their alkyl chains along with skeletal hydrocarbon chains to air. The cross-sectional molecular areas per one alkyl chain of 2.1R8(No)A-1.2AA and 2.7Ri2A-2.9AA were smaller than that of sodium «-dodecanoate. Cotelomers having 2-3 alkyl chains seem to orient themselves so as to cause effective surface activities due to good balance between hydrophobic functions and hydrophilic functions. Cotelomers dissolved in hard water containing 300 ppm of Ca^". Cotelomers in the presence of Ca^* gave lower cmc, lower surface tension and smaller cross-sectional molecular area than those in the absence of Ca^^ The addition of Ca^^ ion to aqueous solutions of cotelomers improved their surface activities by reducing the static repulsion among carboxylates which prevented the dense packing of cotelomers at surface of water. 3.2. Foaming property The foaming abilities of 2.9R^A-2.3AA, 2.1R8(No)A-1.2AA, 2.8R8(Eh)A-2.5AA and 2.7Ri2-2.9AA in water were as high as that of sodium «-dodecanoate. The foam stabilities of cotelomers were significandy influenced by alkyl chain length and nature of hydrophobic functions. Fig.2 shows a relation between HLB (hydrophile-lipophile balance) and foam stabilities after 60 min of standing. The values of HLB of cotelomers were calculated by Oda's equation [10, 11]. The foam stabilities of cotelomers were correlated to their HLB. Cotelomers with HLB of about 20 gave high foam stabilities in homologous series. xR^A-yAA and xR8(No)A-yAA, which have shorter alkyl chains, had high foam stabilities. xR8(Eh)A-yAA, which has branched alkyl chains, showed poor foam stabilities. Conventional surfactants having branched alkyl chain are known to reduce the foam stabilities [12]. Cotelomers having branched alkyl chains seem to orient themselves at interface less easily than those having straight alkyl chains. It is important to control the balance of hydrophobic functions and hydrophilic functions of cotelomers in order to form 2 0 30 40 60 the high foam stability. HLB In the presence of 300 ppm of Ca^*, the foaming Fig 2 Relation between HLB and properties of cotelomers were also influenced by alkyl foam volume after 60 min. • : R6, • : RgCNo), A: RgCEh), Q: R ^

148

chain length. xRjjA-yAA in the presence of Ca^* showed higher foam stabilities than those in the absence of Ca^"^. In the presence of Ca^"^, cotelomers were packed closely at surface between water and air due to interaction between carboxylates and Ca^^ 3.3. Emulsiflcation Emulsifications of organic solvents were formed by shaking vigorously with aqueous solutions of cotelomers. The degrees of emulsifications were in the order : toluene > hexane > kerosene > ligroin > chloroform. Fig.3 shows the percentage of emulsion phases prepared with aqueous solutions of xR^A-yAA and toluene. Number of hexyl chains and carboxylate functions of cotelomers affected the stabilization of emulsions. 2.9R6A-2.3AA showed good stability, with more than 60% emulsion phases after 24h. Emulsions that were produced with xR^A30 40 yAA dissolved in water led to oil-in-water (o/w) type Time /min emulsions. 2.1R8(No)A-1.2AA, 2.8R8(Eh)A-2.5AA Fig 3 Relation between elapse of and 2.7Rj2A-2.9AA as well formed highly stable o/w time and volume of emulsion layer. type emulsions. In the present cotelomer molecules 0:1.1R6A-3.9AA, A: 2.3R^-3.0AA, having alkyl chains of 2-3 and hydrophilic functions • : 2.9R6A-2.3AA, • : 3.2R5A-6.5AA. of 2-3, carboxylates as well as alkyl chains are combined by chemical bonds so tightly that they easily orient themselves at the interface to give rise to efficient in stabilizing emulsions. REFERENCES 1. M. J. Rosen, CHEMTECH, (1993) 30. 2. M. J. Rosen, D. J. Tracy, J. Surfact. Deterg., 1 (1998) 547. 3. K. Esumi, K. Taguma, Y. Koide, Langmuir, 12 (1996) 4039. 4. E. Onitsuka, J. Beppu, T. Yoshimura, Y. Koide, H. Shosenji, K. Esumi, J. Jpn. Oil Chem. Soc, 49 (2000) 929. 5. Y. Koide, T. Yoshimura, H. Shosenji, K. Esumi, J. Jpn. Oil Chem. Soc, 48 (1999) 123 6. T. Yoshimura, Y. Koide, H. Shosenji, K. Esumi, J. Jpn. Oil Chem. Soc, 48 (1999) 1297. 7. K. Esumi, H. Mizutani. K. Shoji, M. Miyazaki, K. Torigoe, T. Yoshimura, Y. Koide, H. Shosenji, J. Colloid Interface. Sci., 220 (1999) 170. 8. M. J. Rosen, "Surfactants and Interfacial Phenomena", 2nd ed, John Wily & Sons, New York (1989). 9. Nihon-Yukagaku-Kyokai, "Yushi-Kagaku-Binran", Maruzen, Tokyo (1990) 480. 10. A. Fujita, Kagaku No Ryoiki, 12 (1957) 719. 11. R. Oda, K. Teramura, "Kaimen-Kasseizai No Gosei To Sonooyo", Maki Shoten, Tokyo (1962)501. 12. T. Yoshida, S. Shindo, T. Ogaki, K. Yamanaka, "Kaimen-Kasseizai-Handobukku", Kougakutosyo, Tokyo (1987) 156.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

149

Iridescent and coloured colloidal phases in highly dilute systems containing decyl a and |3-D-glucopyranosides, decanol or octanol and water B. Hoffmann and G. Platz Department of Physical Chemistry I, University of Bayreuth, D-95440 Bayreuth, Germany The phase behaviour of decyl a-D-glucopyranoside / decanol or octanol / water is extremely influenced by traces of sodium decylsulfate. No swollen lyotropic phases are present in the pure ternary systems. Milky emulsions which belong to an extended liquid liquid miscibility gap are obtained above the solution temperature of the surfactant. The addition of traces of sodium decylsulfate to the ternary systems of decyl a-D-glucopyranoside or decyl a/p-D-glucopyranoside / octanol or decanol / water is necessary to induce the formation of highly swollen phases. Iridescence can be obtained with extraordinary bright colours reaching from blue to red. These phases remain far stable below the Kraffi boundaries of the binary systems. 1. INTRODUCTION Alkyl a/p-glucopyranosides are obtained from Fischer syntheses in an 7:3 ratio together with higher glucosides [1,2]. The alkyl a-D-glucopyranosides are characterized by substantially higher Krqfft points and higher crystallisation energies than those of the Panomers [2]. Therefore there are various applications for the alkyl p-D-glucopyranosides but until now none for the alkyl a-D-glucosides. Highly diluted lyotropic single phase regions are of special interest for surfactant applications. It is well known that dilute micellar solutions of surfactants in water can be transformed to vesicular dispersions, lamellar and sponge phases when increasing amounts of alcohols with medium chain lengths are added [3]. Such a behaviour is known for polyglucopyranosides [4] but not for alkyl a- or P-D-glucopyranosides. For example octyl p-D-glucopyranoside / octanol / water forms an extended L2phase but no swollen lyotropic phase [5]. Further more the high Krqfft points of the a anomers hinder the phase formation in the room temperature region. We show that highly dilute lamellar and brilliant iridescent phases can be obtained with decyl a-D-glucopyranoside (a-CioGi) or decyl a/p-D-glucopyranoside / decanol or octanol and water when traces of ionic surfactant like sodium decylsulfate (SDeS) are present. 2. MATERIALS AND METHODS Decyl a-D-glucopyranoside was obtained by Fischer synthesis and crystallisation from water [2]. Sodium decylsulfate was synthesised according to Dreger et al. [6]. Decanol and octanol were purchased from Fluka Chemical. Water was HPLC quality. - 2 ml samples were

150 prepared in 10ml test tubes with screw caps. After heating and homogenisation the samples were stored at 30 "^C. The iridescent phases develop within several hours or days. 3. RESULTS AND DISCUSSION Fig. 1 explains the phase behaviour of a-CioGi and water in the dilute region (cf. [7]). The solution temperature of the surfactant remains constant at 45 °C (Krqfft boundary). The crystalline surfactant is found at the bottom of the test tube. On heating above the solution temperature of the surfactant clear isotropic solutions or, at locations within the miscibility gap, turbid emulsions were obtained. The two phase region transforms to clear and optical isotropic solutions above the upper solution temperature. When the turbid emulsions are cooled dovm, clear and isotropic solutions are found from all points within the gap. The crystallisation of the undercooled surfactant solution takes place more than 10 °C below the lowest solution temperature of the two phase region. Thus a closed boundary loop exists between 40 and 93 °C which separates the coazervate from the isotropic liquid single phase region.

100

0

2

4

6

8

10

12

14

aClOGl wt% 0.002

Fig 1: phase diagram of a-CioGi • solution temperature on heating o recrystallisation on cooling A 1(|) to liquid-liquid transition

0.004 0.006 decanol wt%

0.008

0.01

Fig 2a Influence of traces of SDeS on the phase diagram of a-CioGi (2.6 wt%) 2b Influence of decanol on the phase behaviour of 0.93 wt% a-CioGi which contains 0.07 % SDeS, Symbols see fig 1

151 The extension of this miscibility gap decreases strongly when small amounts of sodium decylsulfate are present (fig 2a, cf decyl p-D-glucopyranoside [8]) and increases when decanol (fig 2b) or octanol are added. Only milky dispersions are obtained when the ternary systems of a-CioGi / octanol or decanol / water are heated above the solution temperature of the surfactant. No single phase regions are formed in the investigated temperature region up to 100 °C. On the other side a-CioGi and SDeS form low viscous isotropic micellar solutions above the solution temperature of the surfactant mixtures. The phase behaviour becomes much more interesting when traces of SDeS are present in the a-CioGi / decanol / water system. At 30 °C 0.9 wt% a-CioGi and 10'^ wt% SDeS (that means 1.1 % in a-CioGi) form a crystalline dispersion. The phase volume intersection (fig 3a) shows that the crystals are transformed to coloured phases when decanol is added. With 0.15 - 0.25 % decanol an iridescent phase grows from the bottom of the tube below a turbid isotropic phase. This region disappears by about 0.30 wt% decanol and a blue iridescent single phase region is obtained. At 0.73 wt% the system becomes turbid and colourless. Higher amounts of decanol remain undissolved and form a decanol rich L2 phase which separates as concentrated upper phase above the iridescent region. Blue iridescence is found above green, red and colourless which indicates sedimentation effects. The phase diagram intersection fig. 3b elucidates the transformation from ternary system with decanol to the swollen lamellar phase in dependence of the SDeS concentration. Below 0.6 % SDeS only milky dispersions are found. With concentrations between 0.6 ~ 0.9 % SDeS in a-CjoGi a turbid phase which display blue to green colours is observed. Above 1% a blue iridescent single phase region appears . The iridescent disappears when the amount of SDeS is increased above 1.7 %, however a bluish scattering remains. It should be emphasized that the weight ratio of ionic surfactant to decyl a-D-glucopyranoside which is necessary for the swollen lamellar phase is in the order of 0.010 - 0.017. This means that it is sufficient that only about one of 100 surfactant molecules in the lamellar structure is an ionic one. Increasing the fraction of the sulfate decreases the optical birefringence. This is an indication for a continuous transformation from a planar lamellar structure to a system with charged vesicles.

0,5 Ti

0.5 4-

1

0.2

0.4 0.6 0.8 decanol wt%

1.2

b

B

C

^

• • • • < !

0.5

1

1.5

2

2.5

(mass ratio SDeS / a^^^G^riOO

Fig 3) phase volume intersection, T= 30 °C a) 0.92 wt% a-CioGi, 0.0092 wt% SDeS b) 0.92 wt% a-CjoGi, 0.48 wt% decanol t = slightly turbid, R,G,B = red, green or blue iridescence, m = milky dispersion, c = coloured milky dispersion , b = colourless region with flow birefringence and bluish scattering.

152 Strongly iridescent phases can be obtained with pure decyl a-D-glucopyranoside or with mixtures of decyl a/p-D-glucopyranosides and octanol or decanol. The system aCioGi/decanol/SDeS in a weight ratio of 97/66/0.97 displays linear swelling on dilution for a total volume fraction > 0.009. This corresponds to a maximum interlamellar distance of 248 nm with a Bragg Peak at 660 nm. The bilayer thickness is 2.30 nm. Some more examples are presented in table 1. The appearance of two 5r
wt% 0,93 0,78 0^64

decanol (wt%) 0,64 0,53 OM

SDeS ^^ 1,01 1,01 1,01

decyl a/p-D-glucopyranoside (a:p = 85:15) wt% octanol (wt%) SDeS ^^ 0,92 0,58 0,66 0,77 0,48 0,66 0,70 0,44 0,66

^^ (mass ratio SDeS / a-CioGO* 100

~~~"

ACKNOWLEDGEMENTS The authors thank Deutschen Forschungsgemeinschaft (DFG) for financial support. REFERENCES 1. A.J.J. Straathof, H. van Bekkum, A.P.G. Kieboom, starch/Starke, 40 (1988) 229. 2. V. Adasch, B. Hoffmann, W. Milius, G. Platz, G. Voss, Carbohydr. Res., 314 (1998) 177. 3a) H. Hoffmann, C. Thunig, U. Munkert, H.W. Meyer, W. Richter; Langmuir, 8 (1992) 2629; b) U. Munkert, H. Hoffmann, C. Thunig, H.W. Meyer, W. Richter, Prog. Colloid Polym. Sci.,93(1993)137. 4. G. Platz, J. Policke, W. Kirchhoff, D. Nickel, Colloids Surfaces A, 88 (1994) 113. 5a) S. Kunugi, Y. Hayashi, A. Koyasu, N. Tanaka and M. Shiraishi, Bull. Chem. Soc. Jpn., 68 (1995) 1012; b) J. Chopineau, M. Ollivon, D. Thomas, M.-D. Legoy, Pure&Appl. Chem., 64 (1992) 1757; c) J. Chopineau, D. Thomas and M.-D. Legoy, Eur. J. Biochem., 183 (1989)459. 6. E.E. Dreger, G.I. Keim, G.D. Sheldovsky, J. Ross, Ind Eng. Chem., 36 (1944) 610. 7a) L.D. Ryan, E.W.Kaler, J. Colloid Interface Sci.,201 (1999) 251 ;b)L.D. Ryan, E.W. Kaler, Langmuir, 13 (1997) 5222; c) M. Kahlweit, G. Busse, B. Faulhaber, Langmuir, 11 (1995) 3382; d) H. Kahl, K. Quitzsch, E. H. Stenby, Fluid Phase Equilibria, 139 (1997) 295. 8) L.D.Ryan, E.W. Kaler, J. Phys. Chem. B, 102 (1998) 7549.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

153

Complex formation between water-soluble calixarenes and dodecylpyridinium chloride K. Murakami Faculty of Education, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8513, Japan The complex formation between water-soluble calixarenes (p-sulfonatocalix[n]arenes, n = 4, 6, 8, CALXSn) and dodecylpyridinium chloride (DPC) has been studied by the potentiometric titration method using a surfactant-selective electrode, at [CALXSn] = 1 X lO""^ mol dm•^ 1=0.01, pH=2.0, 4.0, and 6.9, and 25°C. The observed binding isotherms showed that the binding of DPC to CALXSn occurs in two stages. The first stage is the strong binding to one site, and the second stage is the cooperative binding to the residual sites. White precipitates have been observed at the beginingof the second stage. The values of the binding constant and the coop erativity parameter have been found to change with an increase in pH, depending on CALXSn species. These results were discussed from the view point of small cooperative binding system compared to large systems. 1. INTRODUCTION The binding of amphiphilic substances such as surfactants and dyes to macromolecules such as Unear polymers and proteins occurs frequently in concerted manners and sometimes leads to the reduction of their solubility and the conformational changes of the macromolecules including protein denaturation.^'"* This cooperative binding phenomenon has been of strong interest for many scientists. Although several theories of statistical mechanical analysis have been formulated^"^ and successfully appHed to linear polymer and globular protein systems, the detailed nature of the interactions involved seems not to be fully recognized in molecular level, since the systems studied are too large and too complex to be discussed in detail. It is therefore desirable to study small cooperative binding systems which are composed of small number of binding sites, as the models of local structures of the macromolecules. This paper describes the complex formation between /7-sulfonatocalix[n]arenes (n = 4, 6, 8, CALXSn) and dodecylpyridinium chloride (DPC) to discuss the nature of the small cooperative binding system as the model of the binding of amphiphilic substances to protein local structures.

154 2. EXPERIMENTAL Sodium/?-sulfonatocalix[n]arenes (n = 4, 6, 8) gifted from S u ^ Kagaku Co. were purified by two reciystallizations from an aqueous methanol solution. DPC purchased from Tokyo Chemical Industry Co., Ltd. was purified from three recrystallizations from acetone solution. These were dried in a vacuum at 110 ""C for 24 h. AH the other chemicals used were of reagent grade. The sample solutions were prepared in the three buffer solutions of 1=0.01: pH=2 (KCl- HCl buffer), pH=4 (acetate buffer), and pH=6.9 (phosphate buffer). The extent of DPC binding was measured by the potentiometric titration^ using a surfactant-ionselective electrode at 25.0±0.1 **C. The potentiometric measurements were made with the electrochemical cell: Ag^AgCl, KCl | salt bridge | reference solution | PVC manbrane | sample solution | salt bridge i A^AgCl, KCl. The slope of the plot of emf. vs. log(surfactant concentration) showed a good Nernstian slope, i.e., 58.9 mV / decade. 3. RESULTS AND DISCUSSION Figure 1 shows the Scatchard plot^^ for the binding of DPC to CALXSn at pH=2.

Q

15

s S

10

a 1^ ^

5 -A. D

A

o^'-o-Oo

-^^ooX

0

1

2

3

4

5

6

'^•Bj 7

8

V

Fig. 1. The Scatchard plots for the binding of DPC to CALXS4(0), CALXS6(A), and CALXS8(n) at pH=2, I-O.Ol, and 25^C.

155 This figure shows that the binding of DPC to CALXSn occurs in two stages; the first stage is the strong binding to one site and the second stage is the cooperative binding to the residual sites. Assuming that these binding stages are independent each other, the number of binding sites (wi) and the binding constant (Ki) for the first stage were at first evaluated from the data in the low concentration region where the second stage does not appear. Next the binding isotherms for the second stage have been calculated by subtracting the contribution of the first stage from the overall binding number. The number of binding sites (W2) and the cooperativity parameter (w) for the second stage were evaluated from the binding isotherms. Here, u is defined, at the half saturation point, by^'^^ u={4(d6/d\nLf)y^,,,

(1)

where 6 is the degree of binding calculated as the binding number devided by the number of binding sites. The values of these parameters are listed in Table 1. Table 1 Binding parameters and cooperativity parameters for the binding of DPC to CALXSn at 1=0.01 and 25°C. CALXS4 pH wi 10%/morMm^W2 2 0.88 4 0.78 6.9 0.84

4.3 7.6 13.

CALXS6

CALXS8

w Wi 10%/morMm^ «2 w

3 9.9 1 3 71. 1 4 18. 1

0.68 1.88 18.8

4 5 5

105 69 68

Wi 10%/mor'dm^ «2 w 1 1 1

5.3 14. 80.

6 106 6 237 7 144

The values of Ki are comparable to or larger than the values of the binding constants reported for the stilbene dyes and alkylammonium ions binding to CALXSn.^^ The large values ofKi suggest that the hydrophobic interaction between the alkyl chain of DPC and the cavity of CALXSn as well as the electrostatic interaction between the charged groups are incorporated in this binding stage. Ki for each CALSXn has the tendency to increase with an increase in pH, i.e., the deprotonation of the hydroxyl groups of CALXSn. The values of the intrinsic binding constant of the second stage, K2, were estimated to be of the order of 100 mol'Mm^ this suggests that the main force of this step is electrostatic in nature. «2 tends to increase with an increase in pH. The fact that the values of/72 are smaller than the numbers of the residual ne^tively-charged groups, which are estimated from the pKa values^^'^"* of CALXSn, shows that not all of these groups serve as the cooperative binding sites. The values of A'2 and u were found to be comparable to those for the other

156 polymer-surfactant systems in the similar conditions.^'^^ The pH dependences of u for CALXS4 and CALXS8 systems have the similar tendency of having maximum values at pH=4, but u for CALXS6 tends to deaease with an increase in pH. These results suggest that the differences in the binding bahavior arise from the differences in the conformation of CALXSn and the manner of its change with pH. For all of the present systems, white precipitates have been observed at the begining of the second stage, i.e., around v - 1 . This means the presence of neutral complexes to form precipitates at the low level of binding and suggests that there are two kinds of complex species; one is the species which bound one DPC molecule due to the first stage and the other is the species in which almost all of the binding sites are occupied by DPC molecules. The latter neutral complexes tend to aggregate. This all-or-none type binding behavior which is typical of the strong cooperative binding may be attained by the small system size. This is in contrast to the case of the large sy stems such as sodium dextransulfate-surfactant systems, in which the precipitates appear only at the high binding level where almost all of the sulfate groups are bound by the surfactant molecules.^^ This difference in the binding behavior between the small and the large systems may be due to the difference in the magnitude of the cluster size of bound surfactants relative to the system size.^^ REFERENCES 1. J. Steinhardt and J. A. Reynolds, Multiple Equilibria in Proteins, Academic Press, New York, 1969. 2. S. Lapanje, Physicochemical Aspects of Protein Denaturation, John Wiley & Sons, New York, 1978. 3. K. Murakami, Bull. Chem. Soc. Jpn., 71 (1998) 2293. 4. K. Murakami and K. Tsurufuji, Bull. Chem. Soc. Jpn., 72 (1999) 653. 5. G. Schwarz, Eur. J. Biochem., 12 (1970) 442. 6.1. Satake and J.T. Yang, Biopolymers, 15 (1976) 2263. 7. J.D. McGhee and P.H. Von Hippel, J. Mol. Biol., 86 (1974) 469. 8. K. Murakami, Langmuir, 15 (1999) 4270. 9. K. Shirahama, Y. Nishiyama, and N. Takisawa, J. Phys. Chem., 91 (1987) 5928. 10. G. Scatchard, Ann. N. Y Acad. Sci., 51 (1949) 660. 11. T.L. Hill, Cooperative Theory in Biochemistry, Springer, New York, 1985. 12. M. Nishida, D. Ishii, I. Yoshida, and S. Shinkai, Bull. Chem. Soc. Jpn., 70 (1997) 2131. 13.1. Yoshida, N. Yamamoto, F. Sagara, D. Ishii, K. Ueno, and S. Shinkai, Bull. Chem. Soc. Jpn., 65 (1992) 1012. 14. M. Sonoda, K. Hayashi, M. Nishida, D. Ishii, and I. Yoshida, Anal. Sci., 14 (1998) 493. 15. Y. Moriyama, K. Takeda, and K. Murakami, Langmuir, in press

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) © 2001 Elsevier Science B.V. All rights reserved.

157

Influence of Oil Droplet Size on Flocculation/Coalescence in Surfactant-Free Emulsion Toshio Sakai\ Keiji Kamogawa'''^, Fuminori HarusawaS Nobuyuki Momozawa*^ Hideki Sakai"'^ and Masahiko Abe'^ Taculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278-8510, Japan **Ele. & Sec Ed. Bureau, the Ministry of Education, Sports, Culture and Science, Kasumigaseki, Chiyoda, Tokyo 100-0013, Japan ""Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan We investigated the influence of oil droplet size on the growth processes (flocculation and/or coalescence) of benzene and fluorobenzene droplets ultrasonically dispersed in water by freeze-fracture electron microscopy (FFEM). Oil droplets with the size of 100-1000 nm (M class) and those with the size of >1000 nm (L class) were formed through flocculation of small droplets ( 100 nm, S class) and coalescence of M class droplets, respectively. Furthermore, FFEM observation verified the presence of benzene and fluorobenzene droplets with diameters below lOOnm and appearance of discrete distributions of S and M class droplets. 1. INTRODUCTION Emulsions are mixtures of two immiscible liquids, such as oil and water, stabilized by an emulsifier. Emulsions are thermodynamically unstable systems that generally break down over a short time through a variety of physicochemical destabilizing processes, e.g., gravitational separation, flocculation, coalescence, and the Ostwald ripening [1-4]. These destabilizing processes cause changes in the spatial distribution and/or size of droplets. We have investigated surfactant-free O/W emulsions (SFEs), which can be prepared without addition of surfactant with ultrasonicator at extremely low oil concentrations (-0.1 vol%) slightly above oil solubility [5-7], to elucidate the mechanism of stability and growth processes of emulsions. SFE is the simplest system of emulsions capable of helping us clarify the evolution and growth processes, and stabilizing factors of emulsion systems. In our recent investigation, discontinuous growth of oil droplets and discrete droplet size distributions were observed for SFEs by dynamic light scattering (DLS) measurements. For example, SFE using benzene as the oil phase had three size distributions appearing one by one with the lapse of time; 20-100 nm (S class), 200-1000 nm (M class) and 3000-4000 nm

158 (L class)[5]. But DLS couldn't easily distinguish growth processes, for example, flocculation and coalescence. In this paper, we focus on the growth processes of benzene and fluorobenzene droplets in SFEs and examine the processes by a combination of dynamic light scattering (DLS) and freeze fracture electron microscopy (FFEM). FT^EM is very useful to discuss the growth processes because it can yield direct imaging of size, aggregates, and shape of oil droplets dispersed in water. 2. EXPERIMENTAL SECTION Materials and Preparation of dispersions Benzene, and fluorobenzene (Tokyo Kasei Co., Ltd.) were GR-grade and used as received. Distilled and deionized water of injection grade (Ohtsuka Pharmacy Co., Ltd.) was used without further purification. Oil (weighed by volume with a syringe) was mixed with water in a flask and the mixture was kept at 25 "C. The concentrations of oils (benzene and fluorobenzene) added to water were 30mM and 17mM, respectively, which were determined from the solubility of the oils in water. The mixture was then sonicated for 2 min in a cleaning bath (Bransonic 220, 125 W, Smith Kline Company). Freeze Fracture Electron Microscopy Immediately after the dispersion treatment, samples for electron microscopy were prepared by freeze-fracture replication. A small volume (-10 ^L) of each sample was placed on a small holder plate (0=3 mm; Hitachi) and the samples were frozen in liquid nitrogen at -190 T . The specimens were transferred to a freeze fracture device (Hitachi, FR-7000A), fractured at -120 "C and < 10^ Torr. The fractured surfaces were immediately replicated by evaporating platinum-carbon mixture from an electrode at an angle of 45"* onto the fractured surface, followed by carbon film at normal incidence, to increase the mechanical stability of replica. The replicas were washed in acetone (Tokyo Kasei Co., Ltd.), rinsed with distilled and deionized water of injection grade (Ohtsuka Pharmacy Co., Ltd.), and collected on 300 mesh copper electron microscope grids (OKENSHOJI Co., Ltd.). The replicas thus prepared were examined in a transmission electron microscope (JEM1200EX, JEOL) in conventional transmission mode using 80-kV electrons. Images were recorded on an electron-microscopic film (FUJI PHOTO FILM Co., Ltd.). Shadows (absence of platinum) appear light in the prints. 3. RESULTS AND DISCUSSION Figures 1 and 2 show the oil droplet size distribution obtained by DLS and FFEM images of benzene and fluorobenzene droplets, respectively, immediately after sonication. For benzene SFE, the number based-droplet size distributions (diameter) at 20^-200 nm (S class) was observed by DLS [5]. Furthermore, spherical droplets with diameters at

159 30-100 nm (S class) were also observed by FFEM as shown in Fig. 1. For fluorobenzene SFE, the droplet size distribution at 60-900 nm (M class), which is different >«.. from that of benzene SFE, was monitored by DLS immediately after preparation as shown in Fig. 2(a). Figure 2(b) is FFEM of fluorobenzene droplets, in which #^ droplets at 100-1000 nm are observed. These sizes are Figure 1 FFEM image of benzene droplets in agreement with those obtained by DLS. Furthermore, (30mM) dispersed in water immediately after sonication. droplets at 20-60 nm (S class) could be observed by FFEM in another spot of the replica as shown in Fig. 2(c). The amount of droplets with these size, however, was less than that of M class droplets, which could be hardly detected by DLS. The size of S class droplets observed for both benzene and fluorobenzene is one order of

f'

magnitude smaller than those of M class droplets.

10 Dropiet size / nm

(b) Figure 2 (a) Size distribution measured by dynamic light scattering (Sub-miaon particle analyzer system 4700 Malvern Instrument Co.), (b) FFEM image at M class droplets, and (c) at S class droplets of fluorobenzEne (l7mM) dispersed in water immediately after sonication.

Figures 3 and 4 show FFEM images of benzene and fluorobenzene droplets 60min after sonication, respectively. We were able to observe those images which show benzene droplets at S class size flocculated keeping their shape and size as individual droplets in Fig. 3, and two or more fluorobenzene droplets at M class size merged into a bigger one in Fig. 4. We also obtained similar results, the coalescence process, for benzene droplets at M class. Thus, these processes (flocculation and/or coalescence) are independent of the kinds of oil, but depend on droplet size (S and/or M class droplets). These results proved that M and L class droplets are formed through flocculation of S class droplets and coalescence of M class droplets, respectively. These findings are explained in terms of the classical DerjaguinLandau-Verwey-Overbeek (DLVO) theory [8, 9]. When the surface-to-surface distance between two droplets

%

{

>

V * SL

;•>

Figure 3 FFEM image of aggregaes of benzene droplets (S class)60min after sonication.

Figure 4 FFEM image of coalescence of fluorobenzene droplets (M class) 60min after sonication.

becomes less than twice the thickness of the electrical double layer (K"') around the droplets there arises a strong electrostatic repulsion between the droplets [10-12]. The electrostatic repulsion

160 between droplets increases with decrease in droplet size because of the overlapping of the electrical double layers surrounding the droplets. In fact, benzene droplets in surfactantfree condition had a ^-potential around -35 mV, because hydroxyl ions in water adsorb preferentially on the droplet surface due to the difference in dielectric constant between the oil and aqueous phases [13], and the value decreased with increase in droplet size [7, 14]. Weiss and McClements reported that 25 wt% n-octadecane oil-in-water emulsions stabilized with sodium dodecyl sulfate (SDS) underwent a liquid -to- solid change when the oil droplet size decreased below 85 nm (radius) because of the overlapping of the electrical double layers surrounding the droplets [10]. As has been mentioned so far, FFEM can provide valuable information on fine liquid droplets and their growth. The technique verified the presence of benzene and fluorobenzene droplets with diameters below lOOnm and appearance of discrete distributions of S and M class droplets. REFERENCES 1. D. F. Evans, H. Wennersrrom, THE COLLOIDAL DOMAIN SECOND EDITION, Wiley-VCH, Inc.: New York, 1999. 2. D. J. McClements, Food Emulsions: Practice and Techniques; CRC Press: Boca Raton, FL, 1998. 3. R. J. Hunter, Foundations of Colloid Science; Oxford University Press: Oxford, 1986; Vol. 1. 4. P. C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York. 5. K. Kamogawa, T. Sakai, N. Momozawa, M. Shimazaki, M. Enomura, H. Sakai, M. Abe, J. Jpn. Oil Chem. Soc. 47 (1998) 159. 6. K. Kamogawa, M. Matsumoto, T. Kobayashi, T. Sakai, H. Sakai, M. Abe, Langmuir 15 (1999) 1913. 7. K. Kamogawa, H. Akatsuka, M. Matsumoto, S. Yokoyama, T. Sakai, H. Sakai, M. Abe, to be submitted. 8. B. V. Derjaguin, L. D. Landau, Acta Physicochim. URSS 14 (1941) 633. 9. E. J. W. Verwey, J. Th. G. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. 10. J. Weiss, D. J. McClements, Langmuir 16 (2000) 2145. 11. J. W. Goodwin, A. M. Rhider, Colloid and Interface Science, Kerker, M., Ed.; Academic Press: New York, 1976; Vol. 4, p 529. 12. R. Buscall, J. W. Goodwin, M. W. Hawkins, R. H. Ottewill, J. Chem. Soc., Faraday Trans. 78 (1982) 2889. 13. R. S. Schechter, A. Garcia, J. Lachaise, J. Colloid Interface Sci. 204 (1998) 398. 14. K. Kamogawa, N. Kuwayama, T. Sakai, H. Sakai, M. Abe, to be submitted.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) o 2001 Elsevier Science B.V. All rights reserved.

161

Morphology of Microemuision Droplet Confining a Single Polymer Chain K. Nakaya", M. Imai", I. Miyata^ and M. Yonese*' 'Department of Physics, Faculty of Science, Ochanomizu University, 2-1-1 Bunkyo, Tokyo 112-0012, Japan

Otsuka,

''Faculty of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe-dori, Mizuho, Nagoya 467-8603 Japan We have investigated morphological change of a microemuision droplet induced by confinement of a polymer chain using a small angle neutron scattering (SANS) technique. The confinement induces two morphological changes, 1) increase of the mean droplet size and 2) increase of the droplet size polydispersity. The increase of mean droplet size can be described by a simple scaling concept on the basis of membrane rigidity and entropy loss of polymer chain by the confinement. 1. INTRODUCTION Microemulsions are thermodynamically stable structured fluids consisting of water, oil, and surfactants. By changing compositions or external fields, morphologies of the surfactant membrane shows a variety of meso-structures having order of hundreds of angstroms, such as droplet (sphere), lamellar, and bicontinuous structures. Among these morphologies, water-in-oil droplets provide isolated meso-scale water region separated by continuous oil phase. Recently behaviors of surfactant membrane confining polymer chains have received much attention. For example, the confinement of polymers between lyotropic lamellar structure brings change of elastic nature of the membrane [1]. If we introduced water soluble polymers into the water-in-oil microemuision droplet system, polymer chains can be confined in the spherical meso-space surrounded by the surfactant membrane. Lai and Auvray [2] investigated perturbations of microemuision droplets by confining poly(ethylene glycol) chains and observed increase of the droplet size distribution. Quellet et al. [3] and Hearing et al. [4] introduced concentrated associating polymer (gelatin) into dense microemuision droplet system (isooctane/Aerosol OT/water) and found that the droplet system transforms to three dimensional network structure (formation of transparent stable gels). Thus the confined polymers affect behavior of surrounding membrane deeply. In this study we introduced a single associating polymer chain into a microemuision droplet, where the size of the droplet is comparable to the radius of gyration of the confined polymer chain and investigated morphology change of the surfactant membrane using the SANS method.

162 2. EXPERIMENTS In this experiment, we used water-in-oil microemulsion droplets consisting of isooctane, Aerosol-OT, and water. For the neutron scattering experiments, deuterated isooctane and deuterated water were mixed with corresponding hydrogenated components in order to adjust the scattering length densities. As confined polymers we used gelatin (type A, Bloom 300) having molecular weight of 9.6x 10\ The radius of gyration (Rg) of this gelatin is 81 A, which is obtained by the SANS measurement. We varied the droplet radius by changing the water to surfactant ratio (COQ) in the vicinity of the Rg of the gelatin. The volume fraction of droplets (water+AOT) was fixed to 7%. The microemulsion droplets confining polymer chains were made as follows: gelatin was allowed to swell in water at 60 °C and then mixed with solution of AOT in isooctane at 60 °C. After well stirring, the samples were transferred to SANS cells and hold for 1 hour at 30 °C before the measurements. The SANS measurements were performed using a SANS-U instrument of Institute for Solid state Physics, the University of Tokyo at JRR-3M reactor of Japan Atomic Energy Research Institute at Tokai [5]. All measurements were carried out at 30 "^C. 3. RESULTS AND DISCUSSIONS First we show behavior of microemulsion droplets without polymers. In Fig. 1 we plot the scattering profile for coo(=[H20]/[AOT])=57.6. The scattering profile shows a broad peak at ^=0.03 A ' and q'^ decay in ^>0.1 A"' region, which is typical behavior for microemulsion droplets under film contrast condition. The q'^ decay indicates sharp and smooth membrane interface of sphere droplets. The obtained scattering fimction can be described by a scattering fimction for spherical shell model [6] taking account of size polydispersity given by I(q)-Np(r)P{q,r)dr

(1)

P(q)'l6n'{p, -pJ{Rlf,{qR,)-AI^fo{qRr)f with /o= (sinx- Jccos;c)/x^

(2)

^'{Ps-p^er)/{ps''Pou)

where ^N is the number density of droplets, P(q) is form factor, and p^, Po,,, and p^,„ are the scattering length density of surfactants, oil, and water, respectively. For the polydispersity function we adopted a Schultz distribution /(r). Here we ignore the influence of the structure factor because we use the dilute system. The shell model without the structure factor well describes the observed profiles and as an example, the fitted curve is given as solid line in Fig. 1. From the fitting we extracted two parameters, mean radius of the droplet (R) and the polydispersity parameter p {p^=/^'\). We obtained good linear relationship between R and CDQ, and the samples with a)o=57.6, 65.4, and 74.0 had /?= 78.0, 86.0, and 98.0 A, respectively. Next we examined the microemulsion droplets confining a polymer chain system. The average number of polymer chain in a droplet (/ip) is calculated from the number of

163

0.01 n ni

^ q [A-»] Fig.I. SANS profiles for the system of AOT-isooctane-water microemulsion droplets of a)Q=57.6 without(circle) and with(trianglc) gelatin at 30*C. The solid lines arc curves fitted with the shell model.

Fig.2. Changes of droplet radius (open) and polydispersity (closed) of microemulsion without (circle) and with (triangle) gelatin.

droplet and the number of gelatin chains in the system. In this study we prepared three samples a)o=57.6 (/Zp=0.82), 65.4 (0.88), and 74.0(0.88). We plot the SANS profile for the droplet ((Oo=57.6 and /jp=0.82) with corresponding SANS profile without polymer in Fig. 1. The features of scattering profiles for the droplets confining a polymer chain are smearing of the characteristic peak of the droplet and asymptotic q^ behavior in high q region. The latter indicates that the droplets are not deformed significantly by the contmement. It should be noted that increase of scattering intensity in low q region is due to unmatching of the scattering length densities between inner water pool containing hydrogenated gelatin chain and outer oil matrix. We fitted the experimental profiles with the shell model described above. The obtained R and/? without gelatin and with gelatin are plotted as a ftmction of COQ in Fig. 2. The R and p are increased by the confinement of a polymer chain. Taking account that the gelatin chain has i^g=81 A, the smaller droplets confine a polymer chain, the larger increase of the droplet size and polydispersity are observed. Unfortunately at present, we can not make clear origin of observed increase of the polydispersity. Here we focus our attention on the change of the mean droplet size. In order to explain observed increase of the mean droplet size, we consider a simple scaling law describing free energy of droplet membrane confining a polymer chain. The droplet membrane with the spontaneous curvature HQ (or sphere radius R^ confines a polymer chain having Flory radius of chain R^, resulting in the formation of sphere droplet having the radius of R, In this treatment we assume that the polymer adsorption at the interface and electrostatic interaction are negligible. The total free energy of the system may be composed of polymer confinement term [7] and membrane elasticity term (Helfrich expression) [8] as

F ^F ^tot

+F

•• conf ^

^mt

~T(^y^f[2K(H-H,y^KK]dS

(3)

where T is temperature, K is the bending modulus, K is the saddle-splay modulus, H is mean curvature, and K is the Gaussian curvature. For simplicity we assume that the

164

• o

? 1

s

•

• 0.45

0.9 08 07

09 oT" Rg/R

1

Fig.3 A log-log plot of R/RQ versus R^/R. The slop represents the power law behabior (R>T^h(Rg/R)0*5.

droplet radius without polymer is RQ, parameter, R^, R, and R^ as follows Fu. ~ n j p ^

Then we can express F,o, using three observable

+4;I[2K - 4 K ( - | ) + 2 K ( - | ) ^ + K ] .

(4)

Minimizing the free energy against R gives a simple scaling law (5) In Fig. 3 we compared the experimental data with the scaling law, where we replaced Rj: by ^g of the gelatin. Roughly speaking, the experimental data obeys this simple scaling law, although the data is limited in the narrow region. This is because that if we confine the polymer chains in a smaller droplet, large deformation of droplet is observed. This issue will be discussed forthcoming paper. ACKNOWLEDGEMENTS This work was supposed by Sasakawa Scientific Research Grant from the Japan Science Society, Grant of ihe Sumitomo Foimdation (No. 990575), and Grant-in-Aid for Exploratory Research (No. 12874047) from the Ministry of Education, Science, and Culture of Japan. This work was done under the approval of the Neutron Scattering Program Advisory Committee, Japan (No. 9564).

REFERENCES 1. C. Ligoure, G. Bouglet, G. Porte, and O. Diat, J. Phys. II (France), 7 (1997) 473. 2. J. Lai, and L. Auvray, J. Phys.II (France), 4 (1994) 2119. 3. C. Quellet, and H.-F. Eicke, CHIML\, 40 (1986) 233. 4. G. Haering, and P.L. Luisi, J. Phys. Chem., 90 (1986) 5892 . 5. Y. Ito, M. Imai, and S. Takahashi, Physica B, 213&214 (1995) 889. 6. M.Gradzielski, D.Langevin, and BFarago, Phys. Rev., E 53 (1996) 3900. 7. P.G. de Gennes, Scaling Concepts in Polymer Physics, Cornell I niversity Press, New York, 1979. 8. W.Helfrich: Z.Naturforsch, 28 (1973) 693.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

165

AOT microemulsion structure depending on both apolar solvent and protein concentration Rika Kawai-Hirai,t Mitsuhiro Hirai,^* Hisae Futatsugi,^ Hiroki Iwase,§ and Tomohiro Hayakawa.S tMeiwa Gakuen Junior College, Maebashi, Gunma 371-0034, Japan. ^Department of Physics, Gunma University, Maebashi 371-8510, Japan. By using synchrotron radiation small-angle X-ray scattering (SR-SAXS) method we studied w/o AOT (sodium bis(2-ethylhexyl)sulfosuccinate) microemulsion systems (water/AOT/n-hexane, /i-heptane, and n-octane) entrapping proteins. We foimd that the structure of the w/o AOT microemulsion clearly depends on the protein concentration and the length of hydrocarbon chain of apolar solvent. 1, INTRODUCTION Enhancement of catalytic activity, so-called super-activity, of enz3nnes entrapped in w/o microemulsions has attracted significant interest concerning not only with future practical applications such as microreactors but also with biophysical catal3rtic mechanisms of enzymes at a limiting condition such as a water pool of w/o microemulsion [1, 2]. By using SR-SAXS and enzymatic activity measurements we clarified that the catalytic activity of a-chymotrypsin entrapped in the water/AOT /isooctane microemulsion is enhanced at low WQ (= [H201/[A0T]) range of 8-16 and that the three different phases (oligomeric phase, transient phase and monomeric phase) appear successively with increasing WQ value [3]. Our neutron spin echo (NSE) study of water/AOT/heptane system showed that the effective diffusion coefficient relating to the bending fluctuation of the microemulsion is significantly enhanced at the transient phase [4]. As shown in other SR-SAXS study [5], there exists the penetration limit of apolar solvent depending on the linear hydrocarbon chain length, which results in the shift of the WQ value of the above phase boundaries. These previous studies suggest that the presence of the transient phase and the enhancement of the bending fluctuation of the microemulsion would induce the increase of an effective surface area of enzymes for the contact with substrates, which would result in the acceleration of the metabolic turnover. Then, we have carried out further SR-SAXS experiments to examine more precisely the structural features of the various AOT microemulsions entrapping enzymes. Corresponding author.

166

2. EXPERIMENTAL AOT was purchased from Nacalai Tesque Inc. Apolar solvents used were 96.5+ % n-hexane, 99.9+ % n-heptane and 97+ % n-octane, which were purchasedfromWako Pure Chemical Industries Ltd. The enzyme used was a-chymotrypsin from bovine pancreas, type 11 produced by Sigma Chemical Co. Water was purified by a Millipore system. The AOT microemulsions were prepared by using an ii^jection method. The wo values of the samples were varied from 0 to 50. The AOT molar concentrations were 0.1 M for all samples. a-Chjrmotrypsin was solubilized in 10 mM Hepes buffer adjusted at pH 8.0. The molar concentrations of o-chymotrypsin in the samples were varied from 2.4x10*5 M to 2.8x10"^ M depending on WQ. S R - S A X S experiments were carried out by using a small-angle X-ray scattering equipment installed at the synchrotron radiation source (PF) at the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. The measurement condition was the same as the previous experiments [5]. We carried out the following standard analyses for the obtained scattering data as follows. To estimate structural changes of the AOT microemulsion depending on both a-chymotrypsin concentration and apolar solvent, we calculated the distance distribution functions p(r) by the following Fourier transform of the scattering intensity Kq). p(r) = ^-2-J''^/(^)sin(r<7)^

(1)

0

where q is the magnitude of the scattering vector defined as q- (4;r/A)sin(^/2), 6 and Xy are the scattering angle and the X-ray wavelength. The radius of gyration -Rg of the solute particle was obtained from the p(r) function as follows.

R^^kjll

(2)

3. RESULTS AND DISCUSSION Fig. 1 shows the wo dependence of the scattering curves Kq) of the AOT microemulsions with three different apolar solvents (n-hexane, n-heptane and /ioctane) imder the constant protein concentration of 2.4xl0"5 M. The decrease of the scattering intensity below q = 0.02 A'^ is attributed to the beam stopper. With increasing the wo value, the scattering curve shifts to a small-^ region with the rise of the scattering intensity below q = -0.04 A*^ for all cases of the three different AOT microemulsion systems, suggesting the enlargements of the microemulsion structure. The p(r) functions in Fig. 2 using Eq. (1) show this process more clearly. Namely, the position of the maximimi value p(r)niax of the p(jr) function shifts to a long distance side with the increase of the maximum diameter Dmax estimatedfromp(r) = 0 for r

167

0.06

(a) I 0.04

|10* -6

IAS

1, gio*

^N, > ?"

(a)1 1

0

>\-|

0.06

'^S^^v^

1—i_i

11J

rk-

CL 0 . 0 2

n-hex

~" ^

10^

n-hexane 1

r—r-^

rT)

it

1

1

'

•

(b)

j

= 0.04 n-heptane

(0

rr

'S 0.02

0 0.06

0

Fig. 1. wo dependence of the scattering curves of w/o AOT microemulsions (water/AOT//i-hexane, (a); n-heptane. (b); n-octane, (c)) occluding achymotiypsin (2.4x10-5 M).

50

100 150 r(A)

200

^^' 2. Distance distribution functions p(r) of the scattering curves of AOT microemulsions shown ^" ^ - 1./i-hexane, (a);/i-heptane, (b); n-octane, ^«>- V«"°^^ ^^"^« ^^^ ^ ^" ^^- ^•

>^max. Whereupon, the change of the p(r) occurs in the different manner for each system, especially at low water contents. The tailing of the p{r) function to a long distance direction can be seen at wo = 4 for the n-hexane system, at wo = 4 - 8 for the n-heptane system, and wo = 4 - 12 for the n-octane system, respectively. As explained previously [3], the tailing of the p(r) function suggests the presence of a certain amount of oUgomeric AOT microemulsion particles at tiie low water contents. Thus, Pig. 2 shows that the lengthening of the hydrocarbon chain of the apolar solvent extends the transient region from the oligomeric phase to the monomeric phase. Figs. 3(a) and

168

10 20 30 40 [HgOMAOT] (M/M)

50

Pig. 3. wo dependence of the J7g (a) and p{r)max (Jb) of w/o AOT microemulsions occluding achymotiypsin (2.4xl0-^ M).

0.1

0.2 [El (mM)

Fig. 4. Protein concentration dependence ofp(r) function (a) andp(r)inax (b). In (a), water/AOT/nhexane at wo = 20.

3flb) show the WQ dependence of the Rg andp(r)max, respectively. The relation between the WQ andp(r)niax values shows a good linearity for all systems. Whereas, with the lengthening of the hydrocarbon chain the wo vs. Rg relation becomes to deviate from a simple linearity and to separate into three different regions with different slopes. The slope above wo = 16 becomes smaller with the lengthening of the hydrocarbon chain. These results depending on the hydrocarbon chain length are essentially the same as those observed in the w/o AOT microemvdsion systems without proteins [5]. In Fig. 4 the protein concentration dependence of the pir) function and p(r)max shows that the occlusion of the proteins tends to decrease the microemulsion radius, which is more clearly seen with increasing water content or with shortening the hydrocarbon chain length. This would result from the attractive electrostatic interaction between the polar head of AOT and the basic residues of the protein surface.

REFERENCES 1. R. Hilhorst, In Structure and Reactivity in Reversed Micelles; Pileni, M. P. (ed.), Elsevier, Amsterdam, (1989) 323. 2. R. H. Pain (ed.). Mechanisms of Protein Folding, IRL Press, New York, 1994. 3. M. Hirai, et al., J. Chem. Soc. Faraday Trans., 91 (1995) 1081; J. Phys. Chem., 99 (1995) 6652. 4. M. Hirai, et al., J. Phys. Chem. Solids, 60 (1999) 1297. 5. M. Hirai, et al., J. Phys. Chem. B, 103 (1999) 9658.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B.V. All rights reserved.

169

Phase transition in Gibbs monolayers of mixed surfactants Md. Mufazzal Hossain,* Tomomichi Okano^ and Teiji Kato** * Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan ^ Lion corporation, Tokyo, Japan Abstract Phase transitions in mixed monolayers of 2-hydroxyethyl laurate (2-HEL) and Na-salt of 3,6,9,12tetra oxa octacosanoic acid (TOOCNa) formed by co-adsorption fiom their mixed bulk solutions have been studied. The presence of cusp points followed by plateau regions in tiie 7c-t curves, vs4iich is accompanied by two phase coexistent state indicates afirst-orderphase transition. The domains have circular shape with internal segments whereas those of pure 2-HEL at the same temperature are of fingering pattem with uniform brightness all over the domains. With increasing thefi:actionof TOOCNa, the domains show more and more expanded behavior which favor easy fusion of tiiem. 1. INTRODUCTION In recent years, we as well as several other research groups have demonstrated the existence of first-order phase transition fiom gas or liquid expanded to condensed phase in Gibbs monolayers of highly purified amphiphiles.[l,2] Extensive research has been performed on mixed surfactant systems, since they can show superior performance as compared to single surfactant alone. When a trace of dodecanol is added to sodium dodecyl sulfate solution die interfacial properties of the aqueous solutions are markedly altered. This is attributed to an increase in packing and ordering of the monolayers at the air-water interfece due to the co-adsorption of dodecanol.[3] Shah et aL [4] proposed that when two surfactants are mixed with a molar ratio of 1:3, the properties of the surfactant systems are changed strikingly due to 2D hexagonal packing oftiiemolecules. Recently, Bain et aL [5] reportedtiiephase transition in the mixed monolayers of cationic surfactants and dodecanol at the airwater interface by sumfiiequencyspectroscopy. In this paper, we evidently report that two water soluble surfactants show a first-order phase transition and form liquid condensed (LC) domains in an appropriate mixture. We have chosen 2hydroxyethyl laurate (2-HEL) and Na-sak of 3,6,9,12-tetraoxa octacosanoic acid (TOOCNa) as amphiphiles (Fig.l).

170 2-HEL:

CH3(CH2),OCCXX:H2CH20H

/ COONa TOOCNa:

CH3(CH2)i50"

^^

^^

^O'

Fig. 1. Chemical structures of the surfactants used in this study 2. EXPERIMENTAL The material, 2-HEL was synthesized with a purity of > 99.5% and TOOCNa was obtained fix)m Lion corporation, Japan with a purity of > 99%. The solutions were prepared separately in ultra pure water of resistivity 18 M Q -cm and then mixed in appropriate volume ratio to obtain the desired molar ratio. To specify the ratio of a given mixture in the latter part of this paper, we always follow the order 2-HEL: TOOCNa. The surfece pressure-time (u-t) curves were measured in a home buik Langmuir trough of very shallow type. The experimental procedure [6] was detailed elsewhere. The surface pressure was measured by the Wilhehny metfiod and the domain morphology was characterized simultaneously by Brewster angle microscopy (BAM) [7] using 20 mW He-Ne laser as a light source. 3. RESULTS AND DISCUSSION Fig. 2 shows 7i-t adsorption kinetics of 2.0 X10'^ M aqueous solutions of both the surfactants separately and in mixed solutions v^th different concentration ratios at 15°C. This concentration is suflBcient to form condensed domains by only 2-HEL but not sufficient to do that by TOOCNa. Condensed domain

formation

in

pure

TOOCNa

monolayer is not possible even with more OHicentration solutions because these are

Fig. 2.7c-t adsorption kinetics at 15°C of 2.0 X10'^ M

above its cmc. Under the present conditions,

aqueous solution of 2-HEL (I), TOOCNa (V), and

the rate of adsorption for pure TOOCNa is

their mixtures with diffoent molar ratios of 2-

higher than that of 2-HEL but in the mixed

HELrTOOCNa; 3:1 (II), 1:1 (ffl), 2:3 (iV). The

system this rate is in between the pure

vertical arrows indicate the position of the cusp points

systems. WrAi increase in the fraction of

in the reflective curves.

TOOCNa in the mixture, the overall rate of adsorption increases indicating the co-adsorption of the surfactants. However, the 7c-t curves in the figure show the cusp points followed by plateau regions up to die ratio 2:3. For spread monolayers of some otiier amphiphiles, a true cusp point in the surface

171 pressure-area (TC-A) isothenns indicates a discontinuity in 5G/57C (G, Gibbs fiee eneigy) because 5G/57C =A. This is the characteristic of afirst-orderphase transition. Since, A decreases with time in Gibbs monolayers, a cusp point in the n-t curve also indicates afirst-orderphase transition.[l,2] hi the 7c-t curve of ratio 3:1 (curve II), the concentration of 2-HEL is only 1.5 X10'^ M which is not suflScient to form condensed domains at 1 5 t when it is used alone.[8] However, phase transition is possible in the mixed system of ratio 2:3 where the concentration of 2-HEL is only 0.8 X10'^ M. These results demonstratetiiatthe existence of phase transition in these systems are due to both of the surfactants. The critical surfece pressures {n^ necessary for the phase transition are almost Ae same except in one case where the fiction of 2-HEL isrelativelylow (curve IV). hi the latter case, tiie TC, is higher than those of the other systems. This should be due to the rapid adsorption of mainly TOOCNa molecules which cover most of the surfaces before a considerable amount of 2-HEL can accumulate to initiate condensation. However, once the concentration of 2-HEL becomes sufficient LC domain formation starts, but before this can happen the surfece pressure becomes almost close to the equilibrium value of TOOCNa. With further decrease in thefiactionof 2-HEL beyond 2:3, phase transition vanish. The in situ BAM observation for all the curves with cusp points shows condensed domains which become larger with time and finally solution surface is covered with them. Fig. 3 presents the typical shape and texture of the domains of pure 2-HEL and the mixed surfactants at 15°C. For pure 2-HEL, the domains are of fingering pattern with uniform brightness all over the domains (image A) at this temperature. The shape of tiie domains for mixed monolayers is circular with inner segments (images B-D) at and above this

Fig. 3. Shapes and textures of the domains formed

temperature. For the mbced surfactant system of

in the monolayers of 2-HEL (A) and the mixed

ratio 3:1, the most of the domains have stripe

systems of ratio 2:3 (B-D). Size: 400 X 300 pml

pattem. With increase in thefiactionof TOOCNa, the texture of the domains becomes irregular with a variety of pattem. Few examples of different defects are given in the Fig. 3 (images B-D). The fusion of tiie domains is rarely observed during the formation process of the domains in pure 2-HEL and even in mixed surfactants containing higher fiaction of 2-HEL (ratio 3:1). However, fusion becomes more and more favorable witii increase in the flection of TOOCNa. Fig. 3D presents an elliptical domain which is formed by the fusion of two circular domains. This type of fusion is rather a common phenomenon during the monolayer formation in the mixed system of ratio 2:3.

172

All theseresultscan be explained considering formation of the mixed monolayers. Withtfieratio 3:1 of the surfactants, the monolayers is dominated by tiie 2-HEL, but considerable extent of TCXXnSIa molecules is included intotiiedomains. For other cases, the extent of 2-HEL molecules is comparatively small. Since, TOOCNa contains sixteen carbon chain, it is expected to have higher line tension if it is introduced into the domains. This high line tension causes circular shape domains although the pure 2-HEL show fingering pattem.[8,9] At 15°C, the 2-HEL molecules remain almost normal to tiie surface, wiiich causes uniform brightness in these monolayers.[8] Nevertheless, the TOOCNa molecules containing longer carbon chain as well as larger hydrophilic group should be tilted. Thus, overall balance among these molecules should cause such complex pattern in the monolayers. With decrease in thefractionof 2-HEL, the monolayers have a tendency to show more irregular textured domains. This is a clear evidence in favor of the formation of domains by both of the surfactants because for pure surfactantsregularpattern is ahvays observed. The irregular pattern most probably due totfieuneven distribution of the components in the domains. We must take consideration of the repulsive forces e.g. electrostatic, dipolar, hydration etc. of the long hydrophilic head groups of TOOCNa molecules. W^ith decrease in the fraction of 2-HEL, tiie tendency of incorporation of TOOCNa molecules into the domains increases. When the concentration of 2-HEL is sufficiently low to initiate condensed domain formation, phase transition does not occur. 4. CONCLUSIONS We provide evidence for the first-order phase transition in mixed monolayers of two water soluble amphiphiles, 2-HEL and TOOCNa. It is clearfix)mthe BAM images that the formed domains contain both of the component, although the exact composition is still unknown. The circular domain formation in these monolayers is a direct evidence for the effect of line tension on domain shape. The irregular textured domain formation may be attributed to an uneven distribution of the surfactants at the diflFerent part of the domains. REFERENCES l.D. Vollhardt, V. Melzer, J. Phys. Chem. B 101 (1997) 3370. 2.M. M. Hossain, M. Yoshida, T Kato, Langmuir 16 (2000) 3345. 3. B. D. Casson, C. D. Bain, J. Phys. Chem. B102 (1998) 7434. 4. A. Patist, S. Devi, D. O. Shah, Langmuir 15 (1999) 7403. 5. B. D. Casson, C. D. Bain, J. Phys. Chem. B 103 (1999) 4678. 6. M. M. Hossain, M. Yoshida, K. Iimura,N. Suzuki, T Kato, ColloidSmf.A 171 (2000) 105 7. S. Henon, J. Meunier, Rev. Sci. Imtnrn. 62 (1991) 936. 8. M. M. Hossain, T. Kato, Langmuir 2000 (in press) 9. S. Siegel, D. Vollharxit, Thin Solid Films 284/285 (19%) 424.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

173

Mesoscopic structures of J aggregates of organic dyes at a solid/liquid interface and in solution: spectroscopic and microscopic studies Hiroshi Yao,* Sadaaki Yamamoto,** Noboni Kitamura*' and Keisaku Kimura" ^ Faculty of Science, Himeji Institute of Technology, Hyogo 678-1297, Japan ^ Material Science Laboratory, Mitsui Chemicals, Inc., Chiba 299-0265, Japan "^ Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Supramolecular structures in J aggregate systems are characterized. J aggregates of a pseudoisocyanine dye (PIC) at a mica/solution interface were in situ observed by tapping-mode atomic force microscopy (AFM). The single aggregates possessed a three-dimensional disk-like island structure in a mesoscopic scale. The island size ranged -400-600 nm long, -100 nm wide, and -3-6 nm high. Morphological differences can be observed between the J aggregates at a solid/liquid interface and those in bulk solution. Mesoscopic string structures of 5,5'-dichloro-3,3'-disulfopropyl thiacyanine (TC) J aggregates were detected in an aqueous solution for the first time by both fluorescence microscopy and microspectroscopy. The length of the J string was several tens of \im while the width was very narrow. 1. INTRODUCTION Extensive research has been directed toward a better understanding of supramolecular aggregate systems and their interesting optical and electronic properties (!]. 7 aggregates are specific dye supramolecular assemblies characterized by a narrow and intense absorption band that shows a bathochromic shift compared to the relevant monomer band. Since the aggregate structure reflects highly on its spectroscopic properties, detailed investigations of the structures and/or morphologies of the single aggregates are of primary importance. Thus, we examined morphologies and optical properties of different types of mesoscopic J aggregate systems: J aggregates at a solid/liquid interface and in a solution phase. 2. EXPERIMENTAL l,r-Diethyl-2,2'-cyanine chloride (pseudoisocyanine; abbreviated as PIC) and 5,5'-dichloro-3,3'-disulfopropyl thiacyanine (abbreviated as TC) were purchased from Nippon Kankoh-Shikiso Kenkyusho Co., and used as received. Conventional absorption and fluorescence measurements were carried out on a Hitachi U-3300 spectrophotometer and an F-4500 spectrofluorometer, respectively.

174

In order to investigate J aggregate formation at a mica/water interface, cationic PIC dye was used. A sample for spectroscopic measurements was prepared by placing an aliquot of an aqueous PIC solution between mica and hydrophobic glass plate. A TC dye was used for examining the structure of J aggregates produced in a solution phase. A solution sample was prepared by dissolving TC sodium salt in an aqueous NaCl solution (5.0 mM). AFM images were recorded on a Nanoscope Ilia (Digital Instruments) operating at a tapping-mode in a liquid phase. Triangular Si3N4 microcantilevers (Nanoprobe; NP-S, Digital Instruments) possessing a spring constant of 0.58 Nm*' were used. Fluorescence microscope images were obtained by using a CCD camera (Hitachi, Remote Eye) set on an optical microscope (Nikon, Optiphoto-2). Fluorescence microspectroscopy was conducted by using a polychromator-multichannel photodetector set (Hamamatsu Photonics, PMA-11) equipped on the microscope. A monochromatic beam (454.5 nm) was used as the excitation sources. A sharp-cut filter (Y-47) was mounted in front of the CCD camera and photodetector set. 3. RESULTS AND DISCUSSION 3.1.

Mesoscopic island structures of PIC single J aggregates at a mica/water interface Figure 1 shows an optical path length dependence of the absorption spectrum of an aqueous PIC solution (2.0 mM). The spectrum showed a sharp and intense J band (580 nm). It is worth noting that no J band can be observed when using a hydrophobic glass cell. Moreover, the figure indicates clearly that the J band is independent of the path length while the bulk monomer (525 nm) or dimer band (480 nm) increases with increasing the path length. The results indicate that J aggregate formation is concluded to be confined to the vicinity of the mica/solution interface.

400

500 600 Wavelength / nm

Fig. 1. Optical path length dependence of the absorption spectrum measured for an aqueous PIC solution (2.0 mM) between mica and hydrophobic glass plate.

2 Mm

Fig. 2. AFM top-view image of the PIC J aggregates at a mica/solution interface (|PIC| = 0.2 mM). Arrows show the periodic orientation of negative holes on mica surface.

175

Thus, atomic force microscopy (AFM) was conducted to examine the microstructures of the J aggregates at the interface. Figure 2 shows AFM image at [PIC] = 0.2 mM. Since the mica surface was unchanged and atomically flat until the J band appeared (< 0.1 mM), the observed mesoscopic leaf-like islands were considered to be the J aggregates. The size of these islands ranged -400-600 nm long, -100 nm wide, and -3-6 nm high. Interestingly, our AFM image revealed that the J aggregates have a three-dimensional disk-like structure but not a two-dimensional monolayer structure. In addition, morphological changes of the islands were observed with changing the PIC concentration: The number density of the J islands increased with increasing PIC concentration, and then, they coalesced into larger domains. In contrast, the height of the islands was independent of the PIC concentration [3]. The constant height of the islands would be determined by the balance between the adsorption/aggregation and dissolution energies. Furthermore, the long axis of the islands are anisotropically oriented relative to the alignment of the negative holes on mica surface formed by dissociating K"^ ions, which was shown as white arrows (three directions) in Figure 2. The results suggest the existence of epitaxial interaction between PIC molecules and the lattice of a mica substrate. The highly probable epitaxial interaction is the one that the positively charged N atoms of the dye are placed at the negative holes on mica [4,5]. jisland _ _ - --_ According to this epitaxial interaction, there are two possible alignments of / \ ^ the dye molecules in the islands: the L Q Cl C Q O C . long axes of the dye are either parallel ^ ' ^/ r^crc" o :, s^ o o or 60** relative to the long axes of the islands. In terms of energy, however, the dye molecules may grow so that

O

the long axis of dye molecules is parallel to the long axis of the islands as shown in Figure 3, which shows an energetically stable brick-stonework alignment of dye molecules.

cziizj

negative holes on the mica surlace

PIC molecules

Fig. 3. Schematic model of the alignment of PIC molecules in a J aggregate island.

3.2. Mesoscopic string structures of TC single J aggregates in solution On the other hand, structural and/or morphological differences are expected between the J aggregates at a solid/liquid interface and those produced in bulk solution. We demonstrate that fluorescence microscopy and microspectroscopy enable a direct observation of single J aggregates in solution. Here, we examined the microstructures of 5,5'-dichloro-3,3'-disulfopropyl thiacyanine (TC) J aggregates in an aqueous solution. Figure 4 shows a fluorescence microscope image at [TC] = 0.05 mM, above which the J band appears, and mesoscopic string structures were clearly observed. Since a characteristic fluorescence image was not detected below this concentration.

176

the strings distributed in solution were considered to be 7 aggregates of TC. To the best of our knowledge, this is the first observation of mesoscopic J aggregates in a solution phase. The length of the string was several tens of \km while the width was very narrow; sub-^m. This string structure is probably due to anisotropic interactions between TC molecules in solution (i.e., quasi-one-dimensional stacking interactions), different from that of PIC J aggregates observed at a mica/water interface. It is noteworthy that a single string is likely to bend in an arc form, suggesting that the mesoscopic J aggregate is flexible and polycrystalline-like. Figure 5 shows fluorescence spectra observed for a single string of J aggregates and at the periphery of the string. The excitation beam diameter was -10 jim, and the measurements were conducted at various position in the string. However, the spectral shape did not change with the observation position in the string. Since the spectrum was quite similar to that measured in bulk solution, the strings detected in Figure 4 were concluded to be TC 7 aggregates. It is worth noting that fluorescence was scarcely observed at the outer periphery of the string, indicating that TC in an aqueous solution produces exclusively mesoscopic-size J aggregates.

i 200 >,0 c «

periphery of the string

S 100

30f,im

Fig. 4. Fluorescence microscope image of the TC J aggregates in an aqueous solution ([TC] = 0.05 mM). The strings correspond to the TC J aggregates.

u

^m^^^

400

450

600 500 550 Wavelength / nm

650

Fig. 5. Fluorescence spectra observed for a single string of TC J aggregates and at the periphery of the string.

REFERENCES 1. P. W. Bohn, Annu. Rev. Phys. Chem., 44 (1993) 37. 2. H. Schmidt, J. Vac. Sci. Technol. A, 8 (1990) 388. 3. H. Yao, S. Sugiyama, R. Kawabata, H. Ikeda, O. Matsuoka, S. Yamamoto and N. Kitamura, J. Phys. Chem. B, 103 (1999) 4452. 4. V. Czikkely, H. D. Forsertling and H. Kuhn, Chem. Phys. Lett., 6 (1970) 11. 5. S. S. Ono, H. Yao, O. Matsuoka, R. Kawabata, N. Kitamura and S. Yamamoto, J. Phys. Chem. B, 103 (1999) 6909.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) i£' 2001 Elsevier Science B.V. All rights reserved.

177

Energy of Breaking of Aqueous GEMINI Surfactant Film Tadahiko Kidokoro and Junichi Igarashi Department of Chemistry, Faculty of Science, Tokai University 1117 Kita-Kaname, Hiratsuka-shi, Kanagawa-ken 259-1292, Japan l.Introduction A number of methods have been proposed for measuring the surface tension of a liquid. Single liquid film is considered to be a very simple colloid system, and is useful for elucidating various colloidal and interfacial phenomena. Interest has been shown by many investigators, particularly upon the effect of aqueous ionic surfactant for its drainage of the film. However, there are few such reports on ionic Gemini surfactant films. Surface active substance usually forms a stable adsorption film in the surface or interface of liquids. As the typical methods for studying the mechanical behavior of such a surface film, equilibrium method of measuring surface free energy( Op), namely the surface tension measurement and dynamic film detachment method of measuring surface film detachment followed by its breaking( a p) are listed. In the case of pure liquid, a p= a p is assumed to hold, since pure liquid rarely produces a stable foam. However, in the case of aqueous surfactant solution, excess energy corresponding to a F ~ cr p = a pp is confirmed to detach the aqueous surfactant thin film from its aqueous surface. Thus, the value of o pp in such a case is considered as the measure of the film stability. In the present study, a reliable measurement of this energy of surface film detachment is attempted. For this purpose, we adopted the frame method that enabled the process of dynamic film detachment to occur in a well-defined condition. The measurement of 0 pp is carried out for the aqueous solutions of Gemini type surfactant, and the stability of aqueous thin film is discussed by taking into account of the structure of Gemini molecule. For this purpose, frame method is original and especially suited for the measurement of surface tension in the present case. Here, we attempt to establish an empirical equation available for the calculation of a p, since there are no empirical equations of Harkins and Brown, and Harkins and Jordan in drop weight method and ring method, respectively. The difference of the interfacial tension of several mNm' is obtained in ionic surfactant and nonionic surfactant surface, which exists qualitative values.

178 Here, the Gemini style surfactants that were synthesized in recent years were investigated sufficiently the surface properties especially for the properties mentioned above. 2.£xperimental Z.l.Materials The sample of Gemini surfactant used here is disodium N-N - bis [(2-carboxyethyl) lauroylamide] ethylene diamine corresponding to monomer, made by KANEBO, Ltd., Cosmetics Lab. Product. The water used was distilled from a solution prepared by dissolving potassium permanganate and sodium hydroxide in ion-exchanged water, using borosilicate glass equipment immediately before each measurement. Pure liquids (water, cyclohexane, toluene, and chlorobenzene) were obtained by twice simple distillation. These solutions were freshly prepared just before the use. 2.2.Apparatus The cell was submerged in a thermostated water bath of 303.15±0.2K, and the whole apparatus was set in a chamber of 303.15 ±0.2K for about 15 hours before the measurement. Air room (gas phase) is filled with nitrogen gas. 2.3.Method In the present measurement, the glass frame used for frame method was shown in Fig.l- i . A microscopic cover glass with a perimeter of 21=8.030cm was used as a plate, to which a thin glass rod carrying the hook was attached as shown Fig.lii. The frame used was made of glass rod of 0.51mm in diameter and 2L=8.856cm in width as shown in Fig.li. = 0 . 5 1 Kill! Pure water and the aqueous solution, whose surface tension is considered to 21-8.()3()cni be constant at a constant temperature is put in vessel, which is slowly raised Fig.l-ii Fig.1- i until the surface just touches the low end Glass f r a n i e s ( i ) and P l a t e ( i i ) used in t h i s study. of the plate. Gemini solution used was put in the vessel, o p and o p were measured after about 15 hours aging cither by Wilhelmy method or frame method. The solutions were gradually ra sed. le frame was vertically withdrawn at the rate 0.75~2.5mmsec"^ from the solution by electric motor. The vessel was

179 contained about 300ml of the solution. The surface tension of both the plate and frame are converted to the electric signal by strain gauge UR-2GR of MINEVEA Co., Ltd. and is put into the recorder connected. The plate and frame were cleaned before each measurement by immersing it in chromic acid mixture for about 8 hours , followed by rinsing with water. 3.Result and Discussion The frame method of measuring downward pull of liquid is considered as the more idealized method of measuring the force of film detachment. Since we have no equations applicable for the frame method, as it exists in the case of drop weight JQQ method of Harkins and Brown, and ring 79J [• 79.6 ^ method of Harkins and Jordan, we 79.4 attempted first to establish an empirical ' e 79J r z equation applicable for the calculation I 79.0 •' 3 of a p, by using pure liquids such as water, cyclohexane, chlorobenzene and 78.6 ^ 78.4 [ toluene. 782 j^ In the case of film extension rate of 78.0 — 0.75"^2.5mmsec'^ range, a straight line 0.4 0.6 0.0 V7(n»n-»ec")'' relation was obtained as shown in Fig.2 between film tension (P/2L) and inverse Fig.2 Surface tension as a function of V'^ of film extension rate V"^ for pure liquid. From the extrapolated value of (P/2L) to V^=0 as expressed by (P/2L)oo, and a p, A a is obtained as shown equation(l). (P/2L)oo-ap=Aa (1) Figure 3 shows plots of A a in each pure liquid against (P/2L)c», showing a straight line of equation (2). A a =0.0899(P/2L)«,-0.6018 (2) In the present study, surface detachment energy a p measured by the frame method is defined as Eq.(3), aF=(P/2L)oo-A a (3) From Eqs. (2) and (3), Eq. (4) is obtained, a F=0.9101(P/2L)oo+0.6018 (4) Here mNm'^ is used as the unit of surface tension. As examples, the detachment process of the film in the frame method at regular rate of film expansion is shown in Fig.4. The pattern

180 showed maximum B, and shows a linear rise in according to the change in expansion rating of the film in the case of Gemini systems A.B.C and D. increased with downward speed.

As shown in Fig.4 surface cut energy

Similar variation was also observed with an increase in

concentration.

D

B

b 30

<

aoo

4Q0

eao

8Q0

(P/aj./fTWm'

Fig.3 Aa-(P/2Lio curves for pure liquids.

Fig.4 Chan of dynamic surface tension by frame meihod.

Energy of detaching and breaking thin aqueous Gemini film o p according to Eq.4 are shown

in

Fig.5.

breaking surface film,

This energy of

50.0 45.0

which is considered

as an indication of the stability for an

40.0

adsorbed film,

35.0 -

is seen to change with

concentration of surfactant and shows a maximum

in the range of the

surface

saturation concentration of the solute.

The

difference of detachment energies between

30.0 25.0 -115

-10.5

-9.5

-8.5

ln(C/mol-dn"')

the monomer and dimer Gemini compound is considered to be explained by the difference in

the

hydrophobic

interaction

respective compounds. characteristic

connection

of

the

Also, the special effect

Gemini should be taken into account.

that

of

Fig.5 The static and dynamic surface tension of Gemini surfactant solution plotted against inc.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

181

Molecular Aggregation States and Polymerizability of Potassium and Calcium 10-Undecenoates in Aqueous Systems Yoshio Shibasaki, Hideki Saitoh, and Atsuhiro Fujimori Faculty of Science, Saitama University, Urawa, 338-8570, Japan Tel:81 - ^ 8 - 8 5 8 - 3 3 8 1 , Fax:81 -^8-858-3700, Email:saitoh(achem.saitama-u.ac.ip Molecular aggregation states of potassium and calcium 10-undecenoates in aqueous systems with different water contents were studied by differential scanning calorimetry (DSC) and a temperature controlled X-ray diffraction measurement in relation to the polymerizability of these amphiphilic monomers by a r -ray-initiated polymerization in different aggregation states. 1. INTRODUCTION Amphiphilic compounds in aqueous systems exhibit various states of molecular aggregation, such as coagel, gel, liquid crystal, and disk or rod-like micelle, depending on the water content and temperature, as reported previously [1,2]. In the case of amphiphilic monomers, their polymerizability is expected to be influenced by the aggregation states through differences in the separation, orientation and mobility of the functional groups [2-4]. Polymerization in lamellar structure, thin films, micelle, etc. is an important problem in the field of colloid and surface science. In this work, we studied the molecular aggregation states in relation to the polymerizability of potassium and calcium 10-undecenoates in aqueous systems with different water contents. The aggregation states were investigated by thermal analysis using a differential scanning calorimeter and a controlled temperature X-ray diffractometer. Polymerizabilities in various aggregation states were examined by the low temperature r -ray irradiation post-polymerization.

2. EXPERIMENTAL 2.1 Materials Potassium 10-undecenoate [CH^ =^ CH(CH^)8C(= 0)0K:Abbr.KUD] was synthesized by the reaction ol 10-undecenoic acid (mp. 2 3 . 5 — 2 4 . 1 0 with potassium ethoxide in ethanol, and was purified by recrystallization from methanol solution. Calcium 10-undecenoate [ ( C H , = CH(CH J 8 C ( = 0)0)2 Ca:Abbr.Ca(UD)2] was synthesized by the dropwise addition of 10-undecenoic acid in a cone, solution of calcium hydroxide and used after purification with n-hexane. Carefully dried undecenoates were mixed with given amounts of water in a mortar and were homogenized by storing in a refrigerator for 1 week at ca. 5 X). 2.2 Apparatus and Procedures Thermal analysis were carried out by a Seiko Instruments model DSC 6200. Aggregation mode of molecules was examined by a Mac Science MXP18VA dif-

182

fractmeter (CaKa radiation) equipped with a refrigerator using liquid Ng. Polymerization in various states were carried out at various temperatures ( - 2 0 ' - 5 0 t ; ) . The monomer samples in aqueous systems of about Ig were sealed in Pyrex tubes (id. 10mm) in nitrogen atmosphere, and irradiated with ®°Co r -rays (30 kGy at desired temperatures in a Dewer flask). The r -ray irradiation was carried out at the Japan Atomic Energy Institute at Takasaki. The polymerization process was followed by a Perkin Elmer system 2000R. 3. RESULTS AND DISCUSSION 3.1 Aggregation States of Monomer Molecules DSC curves for the samples of KUD and Ca(UD)2 with different water contents of 10-90wt% are shown in Figure 1. In the case of KUD-water system, an endothermic peak at O'X: appeared from the water content of 20wt% and increased with water content, while a peak at \5V decreased. In addition, three small peaks were observed at ^0, 85 and 120t:. In the case of Ca(UD)2water system, a clear endothermic peak appeared at 30'€, except for a peak at OV, and two small peaks were observed. Moreover, in a range of water content of 10-50wt%, two broad peaks were observed at 140 and 180t:. To estimate aggregation states of monomers in these systems. X-ray diffraction patterns of the samples at various temperatures were observed. In the case of KUD (water content 30wt%, Fig.2A), the coagel state at - 30t: was changed to a gel-1 state at 5V, in which some free water penetrate into the headto-head layer of KUD molecules. It can be expected that the aggregation state of KUD changed progressively to a gel-B (at 20t:), a lamellar liquid crystal (at 30t:), a disk-like micelle (at 80T:). In the case of Ca(UD)2water system (water content 10wt%, Fig.2B), the regular arrangement of long alkyl chains are almost destroied between 30T: and 50T:, because a peak at 20 =r diminished at 50't;, and above 50X: their aggregation state changed (B) Ca(UD)2-water system

(A) KUD-water system

I 1

-20 0 20 40 60 80 100 120 140 t /Op

-30 0

30 60 90 120 150 180

Fig.l DSC curves of KUD and Ca(UD)^-water systems.

183

(B) Ca(UD)2

(A) KUD

(water content

10 wt%)

30 "C

30 "C

10 X,

-30 r

- 3 0 "C

tf. r. ^^^.•^r7r^>^rAJ:

20 ' ' 26 Fig.2 X-Ray d i f f r a c t i o n patterns of KUD and Ca(UD)g-water systems at various temperature.

to a disk-like micelle. The exothermic peaks at 1^0 and ISOt: in DSC curve of Ca(UD)2 (water content 10%) were examined by FTIR spectral change of the samples, those were heated up to ]30V and 190T:. Band intensities at 3090 cm" ( ] ^ C - M ) , 16^0 c m - ' ( y c - c ) , and 990 and 910 c m - ' ( 6 c - i i ) corresponding to - C H = C H 2 were clearly decreased indicating the thermal polymerization. Phase diagrams of KUD-water and Ca(UD)2-water systems constructed from thermal analysis data are shown in Figure 3. In the case of KUD-water system (B) Ca(UD)2-water system

(A) KUD-water system

180

©xl

160 140

0)

x

-

% 80

I 60 40 20

0

Disk-like ^ micelle \

r

160

Micellar solution

120 - " N

£ 100

180

iiiiiiiin

\

iiiiiiiHi

120

1 Gel-n 1 •GCI-I" Coagel

•

Pioo

(Micellar solution)

0)

r Lamellar j liquid crystal i

m i

(Thermal Dolymerization)

140

ro 80 0)

^ Liquid crystal

i

(hexagonal)

|

1 •

•

•

•

.

40 20 '

.

mvmjtt www

0

-20 20 40 60 80 Water Content (%)

60

100

Disk-like micelle - . "Lamellar" " .^ Liquid crystal fjd^. Gel S ^ Coagel

•

^ WA XM

-20 20 40 60 80 Water Content (%)

Fig.3 Phase diagrams of KUD and Ca(UD)^-water systems.

100

184 (B) Ca(UD) ..-water system

(A) KUD-water syste H20

70X

60 ^ 80% 50%

w

50 L 30% 20%

--15

^40 h

90%

3 30 h

10%

L 0%

20 h

i.

Coagel

V ^ ; yeo* "A

K\'

Gel- 1;

Gel- n

-40 -20 0 20 40 Polymerization Temperature (tT)

-40 -20 0 20 40 60 Polymtrization Temperature (°C )

F i g . 4 Saturated conversion-temperature relationships of KUD and Ca(UD)2water systems at various temperatures. (Fig.3A), in a range of water content (10-30wt%), aggregation state changes as follows coagel-* gel-I -> gel-H^ lamellar L.C.-* disk like micelle, while in a region of ^0-70wt% the state changes at 15'X:gel-I-^ hexagonal L.C., and above 70% the hexagonal L.C. changes to a micellar solution at 100-130X:. In the case of Ca(UD)2-water system (Fig.3B), aggregation state changes as coagel^ gel-* lamellar L.C.-* disk-like micelle, and a region of 50-80wt% a transition expected to be a disk-like micelle-* micellar solution transition was observed. Above ]50V thermal polymerization occurred gradually. 3.2 7 -Ray-Irradiation Polymerization in Various States In the 7 -ray-irradiation polymerization of KUD-water and Ca(UD)z-water systems, polymerization proceeded rapidly and saturated after 3-^ hours. The polymerizability of KUD was highest in the coagel state at water contents of 70'-80wt% and the maximum conversion reached to 60 % as shown in Figure ^A. On the contrary, the polymerizability of Ca(UD)2 was highest in the coagel at water contents of ]0---20wt%, and the maximum conversion was ca. 23 % (Fig. ^B). In both cases, the polymerizability decreased in the order of coagel, gel-1, gel-B and lamellar liquid crystal. It can be concluded that the regular arrangement of the monomer molecules together with mobility of terminal vinyl groups is an important factor for the polymerization, although details of the results obtained in this work are not consistent with the results of sodium and zinc 10-undecenoates in aqueous systems[2]. REFERENCES 1. M.Kodama and S.Seki, J.Colloid Interface Sci., 117(1987)^85. 2. Y.Shibasaki and K.Fukuda, Colloids and Surfaces, 67(1992)195. 3. A.Fujimori, H.Saitoh and Y.Shibasaki, J.Therm.Anal.Calori.,57(1999)631. ^. A.Fujimori, H.Saitoh and Y.Shibasaki, J.Polym.Sci.,A:Polym.Chem.,37( 1999)3845.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

185

Effects of Shear Flow on the Structure of the Lamellar Phase Formed in Nonionic Surfactant-Water System K. Minewaki^, T. Kato^ and M. Imai^ ^Department of Chemistry, Tokyo Metropolitan University, Minamiohsawa, Hachioji, Tokyo, 192-0397, Japan ^Department of Physics, Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo 112-0012, Japan Small-angle neutron scattering has been measured on the lamellar phase formed in a nonionic surfactant (Ci6E7)-water system under shear flow at shear rates in the range 10~'^-10^ s""^ From the dependence of the peak position and the peak intensity on the shear rate it has been suggested that the lamellar domain is disrupted into small fragments at shear rates about 0.1^1 s~^ and that the original microstructure is reconstructed at higher shear rates. 1. INTRODUCTION In recent years, the effects of shear flow on the structure of the lamellar phase formed by nonionic surfactants have been investigated by using microscopy[1], viscometry[l], small-angle neutron scattering (SANS) [1-3], light scattering[l], and NMR[1,4] and attention has been paid to the formation of multilayered vesicles by shear flow and the orientation of the sample for structural analyses. In a previous paper, we studied the effects of shear flow on the structure of the lamellar phase in Ci6H33(OCH2CH2)70H (abbreviated as Ci6E7)-water system by using SANS in the range of shear rate 7 = 10"^10^ s~^ which is much lower than that of other studies reported so far. It has been shown that significant changes in the peak position and the peak intensity were observed at 7 c^ 0.1 s~^[5]. In these measurements, however, we used the neutron beam only along the gradient direction by which the perpendicular and transverse orientations are observed (orientation of the layer normal along the vorticity, flow, and velocity gradient directions are referred to as perpendicular, transverse, and parallel, respectively). In this study, measurements have been made by using a neutron beam along both gradient (radial) and flow (tangential) directions alternatively at each shear rate. In addition, effects of the

186

0.5

1.0

1.5

2.0 0.5

q/nm-

1.0

1.5

2.0

q / nm~

Fig. 1. Scattering intensities for the vorticity direction at 328 K (a) and 343 K (b) integrated over a sector of ±10° at different shear rates. The results at 343 K have been already reported [5].

0.5

1.0

1.5

q I nm-^

2.0 0.5

1.0

1.5

2.0

q I nvar^

Fig. 2. Scattering intensities for the flow (a) and vorticity (b) directions at 328 K integrated over a sector of ±10° at different shear rates (different run from that of Figure 1(a)).

shear history have been examined. 2. E X P E R I M E N T S Measurements of SANS were carried out at the instrument SANS-U of Institute for SoHd State Physics of University of Tokyo in JRR-3M at Tokai with a Couette shear cell[6]. All the measurements were made for the sample containing 55 wt% of CieEy at 328 K and 343 K. 3. R E S U L T S A N D D I S C U S S I O N Figures l a and l b show the scattering intensities for the vorticity direction at 328 K

187

«

vorticity direction —o— gradient direction - -o—

1.4 1.3 1.2

p^ 500 IS 400 [

0 0.0010.01 0.1

1

10 100

Fig. 3. Shear rate dependence of the peak position (a) and the peak intensity (b) of the first reflection for the vorticity direction at 328 K an 343 K.

vorticity direction —o— gradient direction -o--0-'-<^

0 0.0010.01 0.1 Fig. 4. Shear rate dependence of the peak position (a) and the peak intensity (b) in the vorticity and gradient directions at 328 K.

and 343 K, respectively. SANS patterns were recorded twice (10 min each) at each shear rate. The peak positions and the peak intensities are plotted against the shear rate in Figure 3. As reported before[5], for 7 < 0.1 s"•^ the peak intensities for the vorticity and flow directions decrease with increasing shear rate without the change in the peak position. At the shear rates around 0.1 s~\ a new peak appears at the higher scattering angles. As the shear rate increases further, the peak position shifts to the lower scattering angle while the peak intensity increases. Taking into account the results of small-angle x-ray scattering measurements[5,7], we proposed the following scenario[5]. At lower shear rates, the lamellar domain contracts into smaller domains and some of them become disordered states. At the shear rates around 0.1 s~^, a new domain is formed composed of bilayer sheets with water-filled defects. As the shear rate increases further, the fraction of the water-filled defects decreases and the original microstructure is reconstructed. In the present study, we performed five independent runs which gave different shear histories to the sample. Results for one of them are shown in Figures 2 and 4. Below

188 ^-N

3

^ Ul

^ ^ w* c«

a

c

350 300 250 200 150 100 50 01

r

-I

1

— 1

328 K j

r

^ 1 go ....1

1

0.9

1.0

^

1

.-,...

1.1 1.2 1.3 /nm-1 Fig. 5. Peak intensity vs. peak position for the vorticity direction at 328 K and 343 K. Different symbols indicate different runs. ^max

1 s~\ the scattering intensity for the gradient direction increases while that for the vorticity direction decreases, which may result from the fact that a reorientation from perpendicular to parallel alignment is enhanced. Above about 1 s~\ on the other hand, the intensities for both the vorticity and gradient directions reach a minimum and then increase with increasing shear rate. The specific shear rate where the peak intensity reaches a minimum depends on the shear history (0.1 ~ 1 s~^). However, a systematic relation can be found between the peak position and the peak intensity regardless the shear history as can be seen from Figure 5 where the results of different runs are shown. These results in the present study confirm the scenario reported before [5]. REFERENCES 1. S Miiller, C. Borschig, W. Gronski, C. Schmidt, D. Roux, Langmuir, 15 (1999) 7558. 2. C. E. Fairhurst, M. C. Holmes, M. S. Leaver, Langmuir, 12 (1996) 6336. 3. J. Penfold, E. Staples, A. K. Lodhi, I. Tucker, G. J. T. Tiddy, J. Phys. Chem. B, 101 (1997) 66. 4. M. Lukaschek, S. Miiller, A. Hansenhindl, D. A. Grabowski, C. Schmidt, Colloid Polym. Sci., 264 (1996) 1. 5. K. Minewaki, T. Kato, H. Yoshida and M. Imai, J. Thermal Analysis Calorimetry, 57 (1999) 753. 6. Y. Takahashi, M. Noda, M. Naruse, H. Watanabe, T. Kanaya, T. Kato, M. Imai, submitted to J. Rheol. . 7. K. Minewaki, T. Kato, H. Yoshida, M. Imai and K. Ito, submitted to Langmuir.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) G 2001 Elsevier Science B.V. All rights reserved.

189

Solubility behavior of benzylhexadecyidimethylammoniuin salts in oils. Noritaka Ohtani Department of Materials-process Engineering and Applied Chemistry for Environments, Faculty of Engineering and Resource Science, Akita University, Akita, 010-8502 Japan The solubility of benzylhexadecyldimethylammonium salt (BHDAX) in aromatic hydrocarbons was examined as a function of temperature. The solubility curve afforded characteristic features, which corresponded to Krafft point and critical micelle concentration of ionic surfactants in water. When the molecular sizes of oil and counter ion were small, the solubility was high because of the low critical temperature. Above the critical concentration, BHDAX was assumed to aggregate in benzene based on NMR analyses. 1. INTRODUCTION Benzylalkyldimethylammonium salts have been used as the catalysts of phase-transfer catalysis (PTC)' or as the catalytic moieties of the corresponding polymer-supported phasetransfer catalysis." Our early studies on polymer-supported phase-transfer catalysts suggested the presence of a specific microstructure like reversed micelles within the insoluble catalyst polymer matrix based on the analyses of oil and water, which were imbibed under the reaction conditions."^ The solution behavior and reactivity of the corresponding linear polymer attached by quaternary salts (cationic ionomers) have shown that the quaternary salts aggregate in nonpolar oils.^ The recent development of studies on both theoretical and experimental aspects of microemulsions has provided a better understanding of the parameters controlling the phase behavior of microemulsions, though the conditions dealt with have been far from those of phase-transfer catalytic conditions.^ In this article, we examine the phase behavior of BHDAX (X = CI, Br, I, OAc, OMs). We will show that simple solubility measurements afford valuable information about how the parameters, such as temperature, organic solvent, or counter-anion, influence the formation of reversed micelles or w/o microemulsions. 2. EXPERIMENTAL 2.1. Materials and Equipments BHDACl was obtained from Tokyo-Kasei and used without further purification. BHDAX with other counter ions was prepared through the usual ion-exchange method from BHDACl and dried in vacuo. The water content was determined by 'H analysis. The degree of the ion-exchange was ascertained by the GLC analyses of decyl derivatives that were formed by the reaction of decyl methanesulfonate with the BHDAX in benzene.^ The purities of all the prepared BHDAX were over 98%. 'H(270.05MHz) NMR spectra were recorded at probe temperature of 60 °C on a Varian Mercury 300. Chemical shifts are referenced to proton impurities of C^D^ (8 7.15) and are reported downfield of TMS.

190 60

hmm p50i

I

I

I

60| 1 < gHbACl ' O 50

!

I « BHDAI

2 40

: mem

2 30,

S 2of

i 20 10 0

0.2

1

_,_-•—-"•"^

1

1

'

1

0.5

0.6

"

0-Q 0.1

1

1

1

1

0.3

0.4

0.5

0.6

0

0.2

0.3

0.4

weight fraction of BHDAX

weight fraction of BHDAX (a) Solubility of BHDAX in benzene

0.1

(b) Solubility of BHDAX in water

Fig. 1 Solubility of BHDAX in benzene and water 2.2. SolubUity To a 10 mL screw-capped test tube with Teflon-coated stirring bar, given amounts of BHDAX and oil were added. The tube was transferred to a temperature-variable oil or water bath, and the temperature of the system was raised with stirring at a rate of TC / min '. The temperature was read when the BHDAX crystalline solid disappeared. The uncertainty of the temperature measurement was ±0.2 °C. Then, the solution was progressively diluted with the oil, and the measurement of the solubility temperature was repeated in the same way. When the BHDAX concentration was low, the system tended to form a stable supersaturated solution. In this case, the mixture in a tube was cooled to a dry ice-methanol temperature. The tube was then transferred to a water bath maintained at 6 °C, and the BHDAX crystalline was separated out. 3. RERULTS AND DISCUSSION The solubility curve of BHDAX in benzene is shown in Fig. la. The curves bend at certain BHDAX concentrations. At low temperatures, solid BHDAX (Q phase) coexists with a liquid solution (O phase). The concentration of BHDAX in the O phase is very low. Within a very limited range of temperatures, however, the amount of the solid BHDAX is sharply decreased as if the quaternary salt melts at the temperatures. The solubility suddenly increased, and BHDAX was soluble freely in benzene up to a considerably high concentration above the temperature. This type of solubility behavior resembles to those observed for a number of ionic surfactants in water. In fact, the solubility of BHDAX in water affords unique features characterized by Krafft point and cmc as shown in Fig. lb. At low temperatures, most of BHDAX (X = CI, Br, or OMs) is present as a solid phase (Q phase) that coexists with an aqueous solution (W phase). The concentration of BHDAX in the W phase is similarly very low. BHDAOAc was very soluble in water. Its Krafft point was not observed because BHDAOAc gave a homogeneous solution irrespective of its concentration even at 5 °C. On the other hand, BHDAI was hardly soluble in water. The solid BHDAI merely melt at 56.5 °C in the presence of water to give a liquid-liquid two-phase separation at higher temperatures. The bottom phase was an aqueous phase that contained few amounts

191

4.0 «o 3.0 *^2.0 <1.0

H7 6 -o-J 5 (f(PhC (UPhCH^)(5(NCH3) 13 2 1 0 1.0 1.5 2.0 2.5 3.0

o benzene D toluene 0 p-xylene • ethyl benzene

h^ ^^"^""V:^ 0

0.5

1 / [BHDACI] (mmo|-^dm^) Fig. 2. Influence of BHDAQ concentration on the chemical-shift difference between benzyl methylene protons and A^-methyl protons in benzene-4,. Dashed lines indicate predicted set of two straight lines when the micelle aggregation number is constant.

0.1

0.2

1

I

I

0.3

0.4

0.5

0.6

weight fraction of BHDACI Fig. 3. Solubility of BHDACI in aromatic oils.

of BHDAl. The upper phase was a surfactant-rich liquid phase. It is said that surfactants are able to dissolve as an aggregate above Krafft point and their solubility increases sharply at a temperature a few degrees higher than the Krafft point. At the temperatures higher than Krafft point, surfactants form a micelle (Wm phase) if the concentration is higher than cmc. Below cmc (W phase), they are present as monomer or small aggregate. In the same way, the results in Fig. la suggest that it is possible to define a critical solubility temperature and a critical solubility concentration, which correspond to the Krafft point and cmc of ionic surfactants in water, respectively. In benzene, therefore, it is assumed that BHDAX forms an aggregate like a reverse micelle instead of a micelle above the critical concentration. We represent the phase as Om, where BHDAX is assumed to form aggregates. The Om phase is able to imbibe a large amount of water to form microemulsions (M phase) in the same way as the Wm phase can imbibe benzene to form microemulsions. Fig. la also shows that, with an increasing volume of halide counter-anion, Krafft point and cmc of BHDAX increase. The sharp bending of solubility curve diminished for the BHDAX whose counter-ion was large, particularly for BHDAOMs. These indicate BHDAX with small counter-anions is rather soluble in benzene probably due to the feasible formation of compact reverse micelles. BHDAOAc is the most soluble in benzene among the BHDAX used, suggesting that acetate ions assist to form aggregates. The aggregation of BHDACI is proved by the measurement of 'H NMR in benzene-J^ at 60 °C. As shown in Fig. 2, the chemical-shift difference of two singlet peaks, benzyl methylene proton and N-methyl proton, is plotted against the inverse BHDACI concentration in a similar way as the method of Lindman.*" At low concentrations, the chemical-shift difference was independent of BHDACI concentration but roughly at I mM it started to decrease because of downfield shift of A^-methyl proton and upfield shift of benzyl methylene proton. The two straight lines intersect around 10 mM, which may be taken as cmc of BHDACI in benzene. The transient concentration region from 1 mM to 10 mM probably indicates the presence of pre-reversed micelle aggregates and/or the polydispersity of reversed micelles.^ The assumed cmc from this NMR measurement is coincident well to the bending

192 point of the solubility curve in Fig. la. The concentration range from the pre-reversed micelles to reversed micelles is also in good agreement with the results obtained by fluorescence quenching method.*^ In Fig. 2 is also shown the chemical shift of the residual water proton; water to surfactant molar ratio was adjusted to 2.0. Water peak shifted with BHDACl concentration in concert with the peaks of BHDACl. Water peak was very close to that of free water in benzene (0.41 ppm) at low BHDACl concentrations and moved downfield with an increase in BHDACl concentration. This indicates that, at low BHDACl concentrations in the absence of reversed micelles, most of the water is freely dissolved in bulk benzene and that, at high BHDACl concentrations, water resides in the BHDACl aggregates.^ In fact, an incremental addition of water to a benzene solution of BHDACl, of which concentration is higher than the assumed cmc, induces further downfield shifting of water peak, indicating an increase in the unbound water in the core of microemulsions. Fig. 3 shows the influence of oil on the solubility of BHDACl. The solubility curves resemble each other except in ethylbenzene. The phase transition from an 0-Q to a Wm was observed for all oils. Introducing a methyl group to benzene ring heightened the Krafft point of BHDACl: p-xylene > toluene > benzene. This suggests that the oil with a small volume easily penetrates into the surfactant tail, leading to the formation of more compact aggregates. The solubility temperature in ethylbenzene approached the value in p-xylene when the BHDACl concentration was high. However, the bending of the solubility curve was rather gentle compared with other oils, suggesting a slow increase in the aggregation number of reversed micelles and/or the wide polydispersity of the aggregation number. REFERENCES 1. C M . Starks and C. Liotta, 'Thase Transfer Catalysis Principles and Techniques." 1978, New York: Academic Press; E. V. Dehmlow and S. S. Dehmlow, 'Thase Transfer Catalysis. 2nd Ed. ed." 1983, Weinheim, Veriag Chemie. 2. S. L. Regen, Angew. Chem.. 91, 464(1979); F. Montanari, D. Landini, and F. Rolla, Top. Curr. Chem., 101, 147(1982). 3. N. Ohtani, C. A. Wilkie, A. Nigam, and S. L. Regen, Macromolecules, 14, 516(1981); N. Ohtani and S. L. Regen, Macromolecules. 14, 1594(1981). 4. N. Ohtani, Y. Inoue, H. Mizuoka, and K. Itoh, J. Polym. Sci., Polym. Chem. Ed. 32, 2589(1994); N. Ohtani, Y. Inoue, Y. Kaneko, and S. Okumura, J. Polym.Sci., Polym. Chem. Ed., 33, 2449(1995); N. Ohtani, Y. Inoue, Y. Kaneko, A. Sakakida, I. Takeishi, and H. Furutani, Polymer J. 28, 11(1996). 5. A. Jada, J. Lang, and R. Zana, J. Phvs. Chem. 94, 381(1990); A. Jada, J. Lang, and R. Zana, R. Makhloufi, E. Hirsch, and S. J. Candau, J. Phys. Chem. 94,387( 1990). 6. B.-O. Persson, T. Drankenberg, and B. Lindman, J. Phys. Chem., 83,3011( 1979). 7. Y. Moroi and R. Matuura, Bull. Chem. Soc. Jpn., 61,333( 1988). 8. C. D. Borsarelli, C. M. Previtali, and J. J. Cosa, J. Colloid Interface ScL 179, 34( 1996). 9. F. Heatley, J. Chem. Soc. Parody Trans. 1, 84, 343( 1988); M. Seno, K. Swada, K. Araki, K. Iwamoto, and H. Kise, J. Colloid Interface Sci., 78, 57( 1980).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyamaand H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

193

Deswelling kinetics of freeze-dry-treated poly(A^-isopropylacrylainide) gel in sugar solution Norihiro Kato, Shinichiro Yamaguchi, Fujio Takahashi Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University 7-1-2 Yoto, Utsunomiya, 321-8585 JAPAN The kinetic parameters of deswelling on a freeze-dry-treated polyiNisopropylacrylamide) gel were investigated using glucose and maltose as hydi-ophilic probes. The activation entropy change at 313K (ASs^y) shifted from negative (-190 J K^mol'O in water to positive (+410 J K'^mol"^) in 0.2 M glucose. This is explained by the structural similarity between /3 -D-glucopyranose and water clusters. jS-D-Glucopyranose retains the normal CI conformation (chair form). On the other hand, a water cluster possesses a tridymite-structure. Consequently, the equatorial OH of glucose can easily form hydrogen bonds with the surrounding water in the gel. The apparent activation energy (EJ and AS^i.^ are higher in 0.2 M glucose than in water, which may reflect the presence of larger clusters of the iceberg around isopropyl groups in the presence of hydrophihc glucose possessing many hydroxyl groups in equatorial positions. 1. INTRODUCTION Our previous report described that poly(A^-isopropylacrylamide) (NlPAAm) gel turned fast-responsive concerning thermal deswelling after freeze-drying (FD) and hydration in water [1]. The deswelling rate accelerated 10' times as compared with the conventional gel (without the FD-treatment). The hydrophobic interaction between isopropyl groups on the side chains of polymers was considered to play an important role during the deswelling process. The kinetic parameters for deswelling were discussed in the viewpoint of the dehydration process from gel in connection with the bond-formation between polymer chains in the gel [2]. The structured water called "iceberg" (ice-like clusters formed around the hydrophobic isopropyl gi'oups in the gel) was particularly important concerning a lower critical solution temperature (LCST) for the poly(NIPAAm) gel.

194

The structure of sugars retaining the chair conformation was reported to resemble stereospecifically that of the ice (tridymite structure) [3]. It remains, however, to explain how the structure of the hydrated water changes during the deswelUng process. In this report, we present the kinetic parameters of desweUing of the FD-treated poly(NIPAAm) gel in mono- or disaccharide solution as hydrophihc probes. The kinetic parameters obtained are discussed in connection with the stereospecific structure of sugars. 2. METHODS 2.1 Preparation of poly(NIPAAm) gel Cylindiical poly(NIPAAm) gel ( 0 2mm) crosslinked by 4 mol% A';A^ 'methylenebisacrylamide was prepared in water and then freeze-dried in the same way as described in our previous reports [1,2]. The dried gel rods were re-swoUen in 0-1.0 M glucose or maltose solutions at 22 °C. 2.2 Kinetic parameters The initial desweUing rate is equal to the desweUing rate constant (k) when the decrease of the volume for the FD-treated gel can be regarded as a zero-order reaction. The apparent activation energy (EJ and the activation entropy change at 313 K (AS313) were calculated according to the Arrhenius equation. 3. RESULTS The representative deswellingcurves in glucose solutions are shown in Fig. 1. The gel rods equilibrated in 0.25 M glucose solution at 22°C (T^) were transferred to freshly prepared glucose solution (C=0.25 M) at T2 (above the LOST). The desweUing of the FD-treated gel was accelerated with increasing T2. The initial desweUing rate, -d(L/LJ^/dt, was determined from the slope of the straight Une in Fig. 1. The desweUing rate constants (k's), obtained from the desweUing profiles in glucose solutions, were plotted against C (Fig.

1.0 0.9 -J -J

0.8 0.7 0.6 400 Fig. 1. DesweUing profiles of FDtreated poly(NIPAAm) gel.

195

2). In addition to glucose, Fig. 2 showed the plots of desweUing rate constants against the maltose concentration. The minima of deswelling rates appeared at around 0.15 M glucose and around 0.01 M maltose, respectively. Arrhenius plots with respect to various concentrations of glucose solutions fell on straight lines (data not shown). The E,'s and ASgis's were calculated from the slopes and intercepts of these straight Unes. Similarly, the de-swelling kinetics of the FD-treated gel was investigated using maltose solutions. The E^'s and ASsis's calculated were plotted against the concentrations of glucose and maltose (Fig. 3). The maxima of E, and AS313 appeared at around 0.2 M glucose and 0.02 M maltose, respectively. The cuives of E,—C and ^Ssio—C showed a similar tendency for each sugar solution. 4. DISCUSSION Figure 3a showed that E^'s reversed to decrease when glucose concentration exceeds 0.2 M. This is because the desweUing rate increases. The maximum E^ was 230 kJ mol^ in 0.2 M glucose. This is almost twice larger as compared with around 130 kJ mol"^ obtained by using alkaUmetal halides or the NIPAAm monomer as additives [2]. It was considered that collapse of the iceberg during the

0.08 Glucose # ' 0.06 1

^ 0.04 ^

Maltose

0.024

4

)

^ ^

1

0.2 C/M

1

1

0.4

Fig. 2. Dependence of the desweUing rate constant (k) with sugar concentration.

300 Glucose

<-200

Fig. 3. (a) Dependence of the apparent activation energy (EJ with sugar concentration, (b) Dependence of the activation entropy change at 313 K (AS313) with sugar concentration.

196 dehydration process was caused by the strong interaction between glucose molecules and the iceberg around the isopropyl groups. Excess glucose might destroy the water clusters from the utmost surface of the iceberg at concentrations higher than 0.2 M. This may be deduced from the fact that E^ and AS313 decrease with increasing glucose concentration. The kinetic parameters obtained should reflect the rate-determining steps of the deswelling process. The sign of AS313 is considered to be important forjudging the rate-determining steps of deswelling. There should be two rate-determining steps in 0 and 0.15 - 0.20 M glucose: One for a bond formation process between polymer chains (AS;:.i3 < 0) and another for a dehydration process from hydrated polymers in the gel (AS313 ^ 0)- It is plausible that the additive glucose stabilizes the water clusters around polymer chains. It means that the collapse process of the water clusters may become the rate-determining step. Maltose concentration for the maximum E^ or AS313 (around 0.02 M) was smaller than that for glucose solution (C = 0.2 M). These facts are explained by the fact that twice numbers of hydroxyl groups in a molecule of maltose are able to interact with the iceberg. The oxygen atom of each water molecule is linked tetrahedi-ally by two hydi-ogen bonds and two covalent bonds in clusters. The conformation of water clusters is similar to that of tridymite SiOo [3]. The distance between two adjacent oxygen atoms bound to the D-glucopyranose hydroxyl groups is known to be 0.286 nm. This is consistent with the distance between two adjacent oxygen atoms of water molecules in the tridymite water (0.290 nm) [3]. Equatorial hydi'oxyl gi'oups on/3-D-glucopyranose retaining the normal CI conformation (chair form) are capable of forming hydrogen bonds with the adjacent water molecules in the tridymite cluster. The molecule of glucose might substitute the adjacent water molecules in the tridymite cluster of the gel. ACKNOWLEDGEMENTS This work was partly supported by Utsunomiya University SateUite Venture Business Laboratory (SVBL). REFERENCES 1. N. Kato, F. Takahashi, BuU. Chem. Soc. Jpn. 70 (1997) 1289. 2. N. Kato, H. Hasegawa, R Takahashi, Bull. Chem. Soc. Jpn. 73 (2000) 1089. 3. M. A. Kabayama, D. Patterson, Can. J. Chem. 36 (1958) 563.

Studies in Surface Science and Catalysis 132 Y. Iwasawa. N. Oyama and H. Kunieda (Editors) >c 2001 Elsevier Science B.V. All rights reserved.

197

NMR Study on the Effect of Added Salt on Aikylpyridinium Bromide Micelles Shinya Kobayashi, Katsuhiko Fujio, Yuhei Uzu, and Sumio Ozeki Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan The self-diffusion coefficients, D^^^, of dodecylpyridinium bromide (DPB), l-dodecyl-2propylpyridinium bromide (2ProDPB) and tetradecylpyridinium bromide (TPB) were measured in aqueous NaBr solutions by the pulse-gradient spin echo NMR technique. The self-diffusion coefficients of micelles of these surfactants, D^, determined from their D^ 's increase with increasing NaBr concentration. These increments of D^ would be attributed to the decrease in an electrostatic drag due to the interaction between a micelle and an electrical double layer (the relaxation effect). 1. INTRODUCTION Added salts change the size and shape of ionic surfactant micelles. For example, the aggregation number of dodecylpyridinium bromide (DPB) micelle increases from 46.0 in water to 71.0 in 6 mol dm'^ NaBr solution without a significant shape change[l ]. Adding NaBr makes tetradecylpyridinium bromide (TPB) micelle grow from a spherical micelle with the aggregation number of 77 at 0 mol dm'^ NaBr to a rod-like micelle with the aggregation number of 499 at 0.5 mol dm'^ NaBr [2]. In this study, we measured the self-diffusion coefficients of DPB, 1-dodecylpyridinium bomide (2ProDPB) and TPB in solutions of NaBr in DjO by the pulse-gradient spin echo (PGSE) NMR technique in order to investigate the effect of added salt on the self-diffusion coefficient or hydrodynamic radius of micelles of these surfactants. 2. EXPERIMENTAL DPB, 2ProDPB and TPB were synthesized from the corresponding pyridine and 1bromoalkane.

198

The PGSE NMR was measured on protons of methyl group of hydrophobic alkyl chain of surfactants at 399.6 MHz by a JEOL JNM-LA400 spectrometer at 25±0.11:. The absolute magnitude of the field-gradient was calibrated against the self-diffusion coefficient of pure water [3]. Deuterium oxide purchased from Merk (99.8%) was used as a solvent. 3. RESULTS AND DISCUSSION Fig. 1 shows the concentration dependence of the self-diffusion coefficient, D^, of DPB in aqueous NaBr solutions. Due to the fast exchange between the monomeric and micellized surfactants, the observed self-diffusion coefficient, D^^^, is a weighted average of selfdiffusion coefficients of the monomer, Df, and the micelle, D^, which is expressed as _{m-m,)D^-^m,D,

1

on the assumption that above the critical micelle concentration (CMC) the monomer

E o O JS o

0.02

0.04

0.06

0.08

0.1

-1

m /mol kg '

Fig. 1. Concentration dependence of the self-diffusion coefficient of DPB at NaBr concentrations: 0(0), 0.10(#), 0.30(A) mol dm'\

100

200

m' /kg mol*

Fig. 2. Self-diffusion coefficient vs reciprocal of concentration of DPB at NaBr concentrations: 0(0), 0.10(©), 0.30(A) mol d m \

199 concentration is equal to the CMC, TTIQ [4]. m represents the total surfactant concentration. Because of low surfactant concentrations measured, we assume that Df and D^ are almost independent of m. This assumption is supported by the fact that the D^^ vs /w"' plot is linear above the CMC as shown in Fig. 2. Therefore, the micellar self-diffusion coefficient can be obtained from the intercept of the D^^ vs m~^ plot. Fig. 3 shows D^ of DPB, 2ProDPB and TPB micelles as a function of NaBr concentration, Wg. Hydrodynamic radii, /?„, of these micelles calculated from D^ by Stokes-Einstein equation are listed in Table 1, together with values of D^ . It is found that for all surfactant micelles studied D^ increases and /?„ decreases with increasing NaBr concentration. But the increment of D^ can not be attributed to the decrease in micellar size because of the static light-scattering result that the aggregation numbers of DPB and TPB micelles increase with NaBr concentration [1,2], and /?„ values without NaBr are too large compared with the lengths of the surfactant molecules with a completely stretched alkyl chain, which are 2.18, 2.17 and 2.43 nm for DPB, 2ProDPB and TPB. For charged colloidal particles, in addition to

Q

0.6

Ws / mol kg" Fig. 3. Dependence of micellar self-diffusion coefficient on NaBr concentration for DPB(O), 2ProDPB(#) and TPB( A ) . Dashed curves represent calculated values.

200

Table 1 Self-diffusion coefficients and hydrodynamic radii of DPB, 2ProDPB and TPB micelles TPB DPB 2ProDPB m^ /Ws m^ D^ RH ^. ^n, mol kg"' 1 0 ' V s ' 0 0.530 0.10 0.877 0.931 0.30

nm 3.74 2.26 2.13

mol kg*' 10'Ws' nm 0 0.696 2.85 0.953 2.08 0.10 0.980 2.02 0.30

mol kg"' 0 0.02 0.10

10'Ws' 0.497 0.834 0.840

nm 3.98 2.37 2.36

the hydrodynamic drag (the contribution of particle size), the electrostatic drag due to the interaction between a charged particle and small ions in the electrical double layer surrounding it (the relaxation effect) are predicted by some theories[5, 6]. According to Ohshima et al.[5] and Tominaga and Nishinaka[6], the diffusion coefficient, D, of a charged particle is expressed as

D==D,{\-cQl^) where D^ is the diffusion coefficient of an uncharged particle, Q^^ is the particle charge, and c is a complicated function of particle radius and ionic strength. Dashed curves in Fig. 3 show the values calculated from the above equation taking the maximum D^ values as D^ and using Q^^ values estimated from aggregation number [1,2] and degree of counterion binding[7, 8]. Although calculated D^ values without NaBr are larger than observed ones, the electrostatic drag can qualitatively explain the dependence of /),„ on NaBr concentration. To quantitative discussion further studies are necessary.

REFERENCES 1. K. Fujio and S. Ikeda, Langmuir, 7 (1991) 2899 2. K. Fujio, Bull. Chem. Soc. Jpn., 71 (1998) 83 3. R. Mills, J. Phys. Chem., 77 (1973) 685 4. P. Stilbs, J. Colloid Interface Sci., 87 (1982) 385 5. H. Ohshima, T. W. Healy and L. R. White, J. Chem. Soc. Faraday Trans. 2, 80 (1984) 1299 6. T. Tominaga and M. Nishinaka, J. Chem. Soc. Faraday Trans., 89 (1993) 3459 7. W. A. Wan-Badhi, R. Palepu, D. M. Bloor, D. G. Hall and E. Wyn-Jones, J. Phys. Chem., 95(1991)6642 8. M. BeSan, M. MalavaSi^ and G. Vesnaver, J. Chem. Soc. Faraday Trans., 89 (1993) 2445

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) 'c 2001 Elsevier Science B.V. All rights reserved.

201

Small-angle neutron scattering study of w/o AOT microemulsion entrapping proteins Mitsuhiro Hirai,t* Rika Kawai-Hirai,§ Hiroki Iwase,t and Tomohiro Hayakawa.t tDepartment of Physics, Gunma University, Maebashi 371-8510, Japan. §Meiwa Gakuen Junior College, Maebashi, Gunma 371-0034, Japan. We have studied the structure of the water/AOT/heptane microemulsion containing a-chymotrypsin by using the solvent contrast variation method in smallangle neutron scattering. The protein entrapped in the microemulsion is clearly indicated to locate at water pool by the scattering data analyses. The present results agree well with the previous ones obtained using X-ray scattering. 1. E^RODUCTION A solvent contrast variation method in small-angle neutron scattering is known to have an advantage for determining an internal scattering density distribution of a solute particle (contrast meaning an effective excess average scattering density of a solute particle in comparison with an average scattering density of a solvent). A w/o microemulsion occluding proteins consists of different components (surfactant, water, and proteins) which have distinct contrasts. In neutron solution scattering method we can easily change the scattering density of the solvent by using a mixture of protonated and deuterated apolar solvents. Hence, we can measure scattering data in a variety of contrast-profiles of solute particles to determine an internal structure of a w/o microemulsion. On the other hand, native structures of water-soluble proteins are ordinarily considered to be maintained under biological solvent conditions. Studies of unfolding-and-folding of proteins show that the addition of apolar solvent or surfactant greatly affects native structures of proteins and induces unfolding of proteins in many cases. Thus an appearance of super-catalytic activity of proteins entrapped in w/o microemulsions is a very attractive phenomenon not only from the viewpoint of a possible practical application of w/o microemulsions such as microreactors but also from that of biophysical mechanisms of protein structure stability. By using synchrotron radiation small-angle X-ray scattering (SR-SAXS), smallangle neutron scattering (SANS), and neutron spin-echo (NSE), we have been studying

202

the relation between w/o microemulsion structure and enzymatic activity of proteins, where we found the following evidences. 1) Hydrolysis of some esters catalyzed by achymotrypsin entrapped in the water/sodium bis(2-ethylhexyl)sulfosuccinate (AOTVisooctane microemulsion is enhanced only at a low wo range (WQ S [water]/[AOT]) [3]. 2) At this low wo range the microemulsion is in a transient phase between the oligomeric and monomeric phases [4]. 3) A bending fluctuation of the microemulsions is significantly enhanced at this low wo range [5]. 4) Structural properties and phase boimdaries between different phases (oUgomeric, transient and monomeric phases) of w/o AOT microemulsions are defined by the penetration limits of apolar solvents depending on the linear hydrocarbon chain lengths of those solvents [6]. In the present study, to clarify the location of proteins in w/o AOT microemulsions we have carried out SANS experiments using the solvent contrast variation method. 2. EXPERIMENTAL AOT was purchased from Nacalai Tesque Inc. Solvents used were 99.9+ % nheptane from Wako Chem. Co., die-n-heptane (99+ atom % D) and deuterium oxide (100 atom % D) from Aldrich Chem. Co. Normal water purified by a Millipore system was used. Type-II o-Chymotrypsin firom bovine pancreas produced by Sigma Chemical Co. was used without further purification. The a-ch)rmotrypsin was solubilized in D2O/H2O mixed water (98 % D2O v/v) whose scattering density is matched to that of di6-n-heptane. The microemulsions were obtained by using an injection method. The fractions of di^-n-heptane in the apolar solvent were 100 % v/v, 85 % v/v, and 70 % v/v. The [waterMAOT] molar ratios ( s wo) were selected to be 12 and 20. The AOT concentrations of all samples were 0.1 M. The concentrations of a-chymotrypsin [E] were 0 M and 1.32 x 10-4 M for wo = 12, and 0 M and 2.21 x 10-4 M for WQ = 20. SANS experiments were carried out by using a SANS instrument of JRR-3M of JAERI, Tokai, Japan. The sample solutions were contained in a quartz cell controlled at 25 ""C. The following standard analyses of SANS data were executed. By using the Guinier plot (iTiIiq) versus q^) on the data sets in a defined small q range (0.03-0.05 A^), we determined the values of the zero-angle scattering intensity 1(0) and the radius of gyration Rg of the solute particle by using the following equation /(^) = /(0)exp(V/?//3)

(1)

where q = {4n/X)sin{d/2), 6 and X are the scattering angle and the neutron wavelength. The distance distribution function p(r) was obtained by the following Fourier transform of the scattering intensity Hq). P(r) = '^]rql(q)smirq)dq

(2)

203

Thep(r) function depends both on the geometrical shape and on the internal scattering density distribution of the solute particle. 3. RESULTS AND DISCUSSION Fig. 1 shows the change of the scattering curve I(q) depending on the ratio of [di6-n-heptane]/[n-heptane] and on the protein concentration,. The significant decrease of the scattering intensity below q = 0.03 A'l is attributed to the presence of a beam stopper. In Fig. 1(a), the scattering curve Kq) varies with decreasing the die-nheptane concentration, which results from the change of the contrast ofthe AOT microemulsion. The protein concentration dependence of the scattering curve is shown in Fig. 1(b). Due to the occlusion of the proteins in the microemulsion, the internal scattering density distribution greatly varies, which results in the change ofthe scattering curve. As is well known in the solvent contrast variation method, a heterogeneity in the internal scattering density distribution of a solute particle is more clearly reflected with approaching the solvent scattering density to the solute scattering density. J I I L In the present case, the effect of the 10' 1 1 1 1 1 presence of the proteins on the microemulsion structure can be seen in 70 % d-heptane (b) most evidently at the low contrast solvent condition, namely, at 70 % v/v [E1 = 0 di6-n-heptane since the contrast matching points of the AOT and protein [E]«2.21x10-^M molecules are lower than 70 % v/v die-nheptane. The distance distribution function pir) using Eq. 2 in Fig. 2 directly shows the change ofthe internal structure ofthe j _ microemulsion in real space by the 0 0.05 0.1 0.15 0.2 0.25 0.3 presence of the proteins. The peak position of thep(r) function shifts from 55 A to 49 A with holding the bell-shaped pir) profile, indicating that the occluded Fig. 1. Change of the scattering curve of AOT microemulsion (wo = 20) depending on the d- proteins are located at the center ofthe heptane fraction in the solvent (a) and on the microemulsion. protein concentration (b). The zero-angle scattering intensity

204

1

1 1 1 in 70 % d-heptanej

1.0

/

€ CO

T

1

/ A

0.5

/ y

[E]«2.2x10-^M*\ \ 1 1 '\ 1 25

50

100

75

(A) Fig. 2. Difference between the distance distribution functions p(r) obtained from the scattering curves in Fig. 1(b).

- e — w=20, [EJsO

7(0) and the radius of gyration Rg of the solute particle were determined by the Guinier plot using Eq. 1. The contrast matching point of the solute particle was obtained from the plot of [7(0)] 1^2 against d-heptane concentration, as shown in Fig. 3(a). As we know the contrast matching point and can calculate the average scattering density of the heptane solvent at each d-heptane fraction, we can plot the Rg^ against 1/contrast. This plot, socalled the Stuhrmann plot, well reflects a heterogeneity of internal scattering density distribution of a solute particle [7]. The negative slope in Fig. 3(b) indicates that the solute particle consists of a high density core surroimded by a low density shell. This agrees with the present microemulsion structure since its water pool contains 98 % D2O v/v. The decrease of the absolute value of the slope by the occlusion of the proteins also shows that the proteins locate at the center of the microemulsion to descend the scattering density of the water pool.

REFERENCES 0

20 40 60 80 d-heptane fraction (%v/v) T

r

1

1500

'^0)1250 cc 1000

(r

-./ •

1 i 1 -0.02 -0.015 -0.01 -0.005 1/contrast (% d-heptane)

Fig. 3. (a). Square root of the zero-angle scattering intensity plotted against d-heptane fraction, (b), Change of Stuhrmann plot due to the occlusion of the proteins. Marks in (b) are as same as in (a).

1. R. Hilhorst, In Structure and Reactivity in Reversed Micelles; Pileni, M. P. (ed.), Elsevier, Amsterdam, (1989) 323. 2. R. H. Pain (ed.), Mechanisms of Protein Folding, IRL Press, New York, 1994. 3. M. Hirai, et al., J. Chem. Soc. Faraday Trans., 91 (1995) 1081. 4. M. Hirai, et al., J. Phys. Chem., 99 (1995) 6652. 5. M. Hirai, et al., J. Phys. Chem. Solids, 60 (1999) 1297. 6. M. Hirai, et al., J. Phys. Chem. B, 103 (1999) 9658. 7. H. B. Stuhrmann, and A. Miller, J. Appl. Cryst., 11 (1978) 325.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) cc 2001 Elsevier Science B.V. All rights reserved.

205

Neutron Spin Echo Investigations on Slow Dynamics in Complex Fluids Involving Amphiphiles Takayoshi Takedaa, Youhei Kawabatab, Hideki Setoa, Shigehiro Komurac and Michihiro Nagao^ aFaculty of Integrated Arts and Sciences and ^Graduate School of Bio-Sphere Science, Hiroshima University, Higashi-Hiroshima 739-8521, Japan cDepartment of Physics, ChuoUniversity, Bunkyo-ku, Tokyo 112-8551, Japan ^Institute for Solid State Riysics, University of Tokyo, Tokai, Naka Ibaraki 319-1106, Japan In order to elucidate the self-assembling mechanisms in complex fluids involving amphiphiles, we have studied slow dynamics in complex fluid systems such as the nonionic surfactant C12E5 / n-octane / D2O system and lipid DPPC / E)20 / CaCla system using neutron spin echo spectroscopy (NSE). The intermediate functions I(Q, t) obtained from the NSE experiments were well fitted to a stretched exponential function in time l(Q,t) = / (Q,0)exp[- (17 )2/3]. The relaxation rate F increased as (?3. The NSE results support the theory presented by Zilman and Granek [Phys. Rev. Letters 77 (1996) 4788.]. We estimated the bending modulus x of the interfacial membrane using their theory. 1. INTRODUCTION In order to elucidate the self-assembling mechanisms in complex fluids involving amphiphiles, microscopic parameters like the bending modulus x on the local scale of the interfacial membrane deduced from dynamical experiments using neutron spin echo spectroscopy(NSE) are important. The wave vector Q and time t dependent intermediate functions /((?,/) were obtained from neutron spin echo (NSE) experiments on the following two systems; (a) film and bulk contrast samples in the non-ionic surfactant H(CH2)i2(OCH2CH2)50H(=Ci2E5)/n-octane / D2O ternary system at equal volume fraction of octane and water with 0.2 volume fraction of C12E5, which shows a sequence of low temperature micro-emulsion(LTM)/lamellar(MTL)/high temperature microemulsion (HTM)phase with increasing temperature[l, 2], (b) the lipid dipalmitoylphosphatidyl-choline (= DPPC)/D20/CaCl2) system. In the C12E5 /n-octane/D20 system, the LTM and HTM phase have a bicontinuous structure. The lamellar repeat distance ^1 in the liquid crystalline phase La varies greatly upon addition of salt in the DPPC/water system[3]. In order to study the dynamics in undulation of lipid bilayers, NSE experiments were carried out on the dilute lamellar phase of the DPPC / water / CaCl2 system with d\ longer than 300 A in order to avoid interactions between neighbouring sheets of membranes.

206 2. EXPERIMENTAL For sample preparation, 99.7%pure C12E5 was purchased from Tokyo Chemical Co., 99% n-octane from Aldrich Chemical Co., 99 % deutrated n-octane from Isotec Inc., 99.9 % D2O from Isotec Inc., 99% a-L-DPPC from Sigma Chemical Co. and 99.5% CaCl2-2H20 from Wako Pure Chem. Ltd. These materials were used without further purification. In the ternary system C12E5/ n-octane / D2O at equal volume fraction of octane and D2O with volume fraction 0.2 of C12E5, protonated n-octane was used for the bulk contrast samples and deutrated n-octane for the film contrast samples. To prepare the lipid samples, 7.7wt% DPPC was dispersed in D2O solutions with 6.8mM CaCk for DSl sample and 6.3wt% DPPC in D2O solutions with 6.6mM CaCk for DS2 sample. In small angle neutron scattering patterns, the peak corresponding to d\ = 430 A was observed for DSl sample. The NSE experiments were performed using ISSP- NSE at C2-2 port of JRR-3M, JAERI[4-6]. The experiments were also carried out as the performance test of the spectrometer. Silica gel and Grafoil were used to measure the resolution function of the spectrometer. Neutron beams with wavelength X = 5.9 A (FWHM of its resolution AX/X = 15%) and X = 7.14 A (AX/X = 18%) were used. 3. RESULTS AND DISCUSSION The intermediate correlation functions I(Qj) obtained from the NSE experiments were well fitted to the following equation, /((?,O=/((?,0)exp[-(rO2/3 ]

(1)

in both the bicontinuous microemulsion and the lamellar phases of the Ci2E5/n-octane/D20 system and also in the lamellar phase of the DPPC/D20/CaCl2 system. The relaxation rates r obtained from the fitting to Eq.(l) increased as (? 3 over the range of Q from 0.08 A-l to 0.17 A-l for the C12E5 / n-octane/D2O system and Q from 0.05 A-l to 0.14 A-l for the DPPC / D2O / CaCl2 system as shown in Fig. 1. In our previous papers concerning mainly the Ci2E5/n-octane/D20 system[7-9], we reported that the NSE results supported the theory presented by Zilman and Granek[10]. They predicted a stretched exponential relaxation of / (Q/) as follows Eq.(l) where the relaxation rate F is given by r = 0.025y (/CBT/X )l/2(ter/rj )Q 3 .

(2)

Here, x is the bending modulus of the membrane and rj the viscosity of the surrounding medium. The factor y originates from averaging over the angle between Q and the plaquette surface normal in the calculation of / {Qj). Figure 2(a) shows x estimated in the bicontinuous microemulsion and the lamellar phases of the C12E5 / n-octane / D2O system using Eq.(2) where we put y = l a n d used 3 times the value of average solvent (n-octane and D2O) viscosities for rj (rj = 3rjsoivent) taking the local dissipation at the membrane into consideration[ll]. The values of x are nearly same as that estimated from dynamical light scattering in the C12E5 / hexanol / water system at room temperature[12]. x in Fig.2(a) decreases monotonically with increasing temperature independently of the mesoscopic structure and the scattering contrast. The rate F deviated from Eq. (2) at Q higher than

207

3.5

! - ' • ' . ' ' • •

tbicontinuous C12E5/D20/n-octane 1

-^-T

1

11

1 1 1 1

1

1

1 O/

c/^/ ^

0.1

Qm/

'in

0.01

295

/Jk//^

-1

14

n /v/

1

^

12

z-y i

3

^

10

-d

L '

1 — • — »

0.04

•

1

—

0.06 0.080.1

-

—

,

1

0.2

Q (A-^)

Fig. 1. The dependence of the relaxation rate F on Q obtained using the fitting to Eq.(l) in the LTM(full circles) and HTM phase (open circles) of the film contrast sample of the CiaEs/n-octane/water system, and DSl sample at 46*C ( full triangles) and at 52oC ( open triangles). The lines indicate Q3 proportionality.

310

315

'

DPPC/DjO/CaClj

i V.

:

•^-..

r(b)

4 1.J-J

315

-^

'

8 6

•

305

T(K)

>i

0.001 r =

300

1

d i ^ ^A

u

1 \

Rale

•b T— >-^

bicontinuousi

J

1 1

J 1 1

320

• 0 *—.. • ^... _1 .. .—._ . 1 . _. . . 1 325 330 335

T(K)

Fig. 2. The dependence of bending modulus X of the membrane on the temperature T obtained from NSE experiments for the two systems; (a) the bulk( open circles) and film sample(full circles) in the C12E5 / n-octane / water system , and (b) DSl sample(full circles) and DS2 sample(open circles) in the DPPC/DaO/CaCb system. The lines which are a guide for the eye are fitting curves to an exponential function x - a exp(- bIT ), where a and A are constants.

0.18 A-l and at Q lower than 0.08 A-l as shown in Fig. 1. At Q lower than 0.08 A-l, the dependence of F on Q depended on the mesoscopic structure and the scattering contrast. The collective motions of the membranes that are not considered in the Zilman and Granek's theory may play an important role in the dynamics at Q lower than 2x1^ where ^ is a typical size of the mesoscopic structure, while the Zilman and Granek's model is considered to be applicable to dynamics of a single membrane at higher Q, The deviation at Q higher than 0.18 A-l suggests that the effect of the thickness of the membrane which is neglected in the the Zilman's model plays a significant role in the dynamics at higher Q. In the case of the DPPC /DaO/CaCb system, the Zilman and Granek's model is considered to be applicable, since
208

same order as that estimated from the microscopic observation of giant flaccid vesicles[13]. The values of x seem to depend strongly on d\ though d\ is longer than 300A. The estimated values of x decrease with increasing temperature and seem to be reasonable over the range oi x from 0.86 ArBr to 13 iteT'in the present study. This result indicates that F depends on « in the manner predicted by Zilman and Granek which shows an anomalous dependence of T on p^, F'^x -i/2, in contrast to other theories of membrane undulations and that their theory describes well membrane undulations in these complex fluids involving amphiphiles. ACKNOWLEDGEMEl^TS This experiments at JRR-3 were done under the approval of the Neutron Scattering Program Advisory Committee. One of the authers(T. T.) was financially supported by the Grant-in-Aid for Scientific Research (No. 06640505, No. 07236103, No. 08044089, No. 09640466) from the Japanese Ministry of Education, Science, Sports and Culture. REFERENCES 1. M. Kahlweit, R. Strey, D. Haase, H. Kunieda, T. Schmeing, B. Faulhaber, M. Borkovec, H. F. Eicke, G. Busse, F. Eggers, T. Funck, H. Richmann, L. Magid, O. Soederman, P. Stibs, J. Winkler, A. Dittrich, W. Jahn, J. Colloid & Interface Sci. 118 (1987) 436. 2. S. K. Ghosh, S. Komura, J. Matsuba, H. Seto, T. Takeda, M. Hikosaka: Progr. Colloid Polym.Sci. 106(1997)91. 3. T. Takeda, S. Ueno, H. Kobayashi, S. Komura. H. Seto, Y. Toyoshima : Physica B 213&214 (1995) 763. 4. T. Takeda, S. Komura, S. Seto, M. Nagai, H. Kobayashi, E. Yokoi, C.M.E Zeyen, T. Ebisawa, S. Tasaki, Y. Ito, S. Takahashi and H. Yoshizawa : Nucl. Instr. and Methods in Phys. Research, A364 (1995) 186. 5. T. Takeda, H. Seto, S. Komura, S. K. Ghosh, M. Nagao, J. Matsuba, H. Kobayashi, T. Ebisawa, S. Tasaki, C. M. E. Zeyen, Y. Ito, S. Takahashi, H. Yoshizawa; J. Phys. Soc. Jpn. 65 SuppLA (1996) 189. 6. T. Takeda, H. Seto, Y. Kawabata, D. Okuhara, T. Krist, C. M.E. Zeyen, I.S.Anderson, P. Hfghfj. M. Nagao. H. Yoshizawa, S. Komura. T. Ebisawa, S. Tasaki and M. Monkenbusch, J. Phys. Chem. Solids 60 (1999) 1599. 7. T. Takeda, Y. Kawabata, H. Seto, S. Komura, S. K. Ghosh and M. Nagao, AIP CP-469 (1999) 148. 8. T. Takeda, Y. Kawabata, H. Seto. S. Komura, S. K. Ghosh and M. Nagao, D. Okuhara, J. Riys. Chem. SoUds 60 (1999) 1375. 9. T. Takeda, Y. Kawabata, H. Seto, S. K. Ghosh. S. Komura and M. Nagao, AIP CP-514 (2000) 190. 10. A. G. Zihnan and R. Granek, Phys. Rev. Letters 77 (1996) 4788.4. 11. B. Farago, M. Monkenbusch, K. D. Goecking, D. Richter, J. S. Huang: Physica B 213&214 (1995) 712. 12. F. Nallet, D. Roux, J. Prost: J. Phys. France 50 (1989) 3147. E. Freyssingeas, D. Roux, F. Nallet: J. Phys. H France 7 (1997) 913. 13. H. Engelhardt, H. P. Duwe, E. Sackmann: J. Phys. (Paris) Lett. 46 (1985) L-395.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (O 2001 Elsevier Science B.V. All rights reserved.

209

NEUTRON SPIN ECHO STUDIES ON EFFECTS OF TEMPERATURE AND PRESSURE IN DYNAMICS OF A TERNARY MICROEMULSION Y. Kawabata^ M. Nagao^ , H. Seto" and T. Takeda" ^Graduate School of Bio-Sphere Science, ^Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan ^Institute for Sohd State Physics, The University of Tokyo, Tokai 319-1106. Japan In order to understand dynamical features of amphiphile membranes, we performed neutron spin echo (NSE) experiments in a dilute droplet system consisting of AOT(dioctyl sulfosuccinate sodium salt), D2O and C10D22 at room temperature, at high temperature, and at high pressure. The bending modulus of membranes was obtained from the model fitting to a theory proposed by Milner and Safran[9] describing diffusive dynamics of droplets. We found that membranes at high temperature was more flexible than those at room temperature, while, those at high pressure was more rigid. This result was consistent with our previous study in a dense droplet microemulsion system. 1. Introduction Ternary complex fluids, consisting of water, oil, and amphiphile, form various structures depending on its composition, temperature, and/or pressure. We have investigated static and dynamical features of the A0T/D20/n-decane system by means of small angle X-ray(SAXS) and neutron scattering(SANS), and neutron spin echo(NSE) so far[l]-[5], and showed that temperature and pressure induced a phase transition from one-phase dense droplet to two-phase coexistence with an ordered lamellar phase and a disordered bicontinuous phase. Our results indicated that the static features of the high-temperature and the high-pressure phases were similar, however, the dynamical features were different. We concluded that amphiphihc membranes between oil and water became flexible with increasing temperature, while rigid with increasing pressure. However, the correlation between droplets cannot be ignored in the dense droplet system, and it was difficult to understand the dynamical features of the membranes only with the dense droplet system. In this study, we focused on the dynamical features of amphiphilic membranes in a dilute droplet system (0 ~ 0.1, where 0 is the droplet density). In this system, observed dynamics was successfully explained in terms of the shape and size fluctuation and the translational diffusion of droplets, as shown by Huang et al[6]. We also calculated the ratio w = ro/vs of a radius of droplet ro to a spontaneous radius of droplet rg. We found that the behavior of the n was the same with the previous results[4,5].

210 2. E x p e r i m e n t The NSE experiments were performed at the ISSP-owned NSE spectrometer at JRR3M in JAERI, Tokai[7]. The measured momentum transfer q ranged over 0.04 < ^ < 0.14 [A~^] and Fourier time t over 0.15
= exp[-D,ff{q)qH],

(1)

where Deff{q) is the effective diffusion coefficient and q is the scattering vector. Taking the shape deformation of droplets into account, the Deff(q) is a sum of two terms, DefM)

= Dtr^DdeM).

(2)

where A r is the translational diffusion coefficient and Ddef{q) the shape deformation diffusion coeflficient. The Ddef{q) can be written as, D

(n) = 5A2/2(gro)(|a2p) '^^^"^^ q^inryoiqro) + bf2{qro){\a2\')]

.3.

with the second mode(n = 2) of the shape deformation is taken into consideration. The n = 2 mode can be mainly observed using the ISSP-NSE spectrometer, because the damping frequency of the deformation of the other modes except the second mode is out of the dynamic range of the spectrometer. Here, A2 is the damping frequency of the second mode, ro the radius of droplet, w the ratio of ro to a spontaneous radius of droplet r^, and fn the n - t h weighting factor including the spherical bessel function. The mean square of the second mode fluctuation amphtude, (|a2p), and the A2 can be expressed as,

(|a,P) = A^^^^rg,

(4)

9 6

X..

K

A2 = —^ir^w,

rjr^ 55

(5)

21]

t [nsl

Figure 1. Observed intermediate correlation function from NSE experiments at HP. Lines are fitting results to Eq.(l).

Figure 2. The effective diffusion coefficient Deff obtained from the fitting to Eq.(l). Full circles correspond to the RTF phase, open triangles the HT phase and full squares the HP phase. The lines are fitting curves to Eq.(2).

where T] is the average of the viscosity inside and outside the droplet, n the bending modulus of amphiphilic membranes. A2 was obtained from the fitting to the observed Deff{q). In order to calculate TS and u^ we used the following expression[9], r{qro = 7r)

y ^ Q.3A2 To J

W

7r/2

4-

0.15

(6)

This equation is reduced by taking polydispersity of droplet due to the averaging numerator and denominator over the different radii in the sample into account[6,9]. 4. R e s u l t s and Discussion The observed I{q,t)/I{q,0) at HP and De/f are shown in Figure 1 and Figure 2 with the fitting curves to Eq.(l) and Eq.(2) respectively. In Figure 2, a peak position and peak height are corresponding to the characteristic wave number of the droplet radius {Qpeak = 7r/ro) and shape fluctuation diffusion coefficient Ddef. respectively. The qpeak shifts to larger q and the Deff at
212

Table 1 The obtained fit parameters for each condition from the NSE experiments. RTP HT HP K[kBT] 1.4 1.7 2.0 ll£2[!l 0.10 0.13 0.08 ^0 Ar[xlO-^cmVs] 3.9 4.7 5.6 -o[A] 36 31 32

parameters from the fitting resuhs to Eq.(2) and (5) at each condition are summarized in Table 1. The fractional displacement against droplet radius. (|a2p)/'"o' decreased with increasing pressure and increased with increasing temperature. This indicates that the shape fluctuation is suppressed with increasing pressure and vice versa with increasing temperature. The K at HP was larger, and that at HT was smaller than that at RTP. This result shows that the membranes become rigid with applying pressure, and flexible with increasing temperature. The value of K is consistent with the result obtained by Farago et al[8]. These evidences are consistent with our previous results in the dense droplet system[2.3]. and we concluded that the dynamical features at HT are different from those at HP. This difference between the effects of temperature and pressure is due to the interaction around surfactant molecules as discussed in our previous articles[l]-[3]. The tail-tail attractive interaction increases with increasing pressure, while the head-head repulsive interaction increases with increasing temperature because of the dissociation of counter-ion from head group. REFERENCES 1. M. Nagao and H. Seto, Phys. Rev. E 59 (1999) 3169. 2. M. Nagao H. Seto Y. Kawabata and T. Takeda, J. Appl. Cryst 33 (.2000) 653. 3. H. Seto, D. Okuhara, Y. Kawabata, T. Takeda, M. Nagao, J. Suzuki, H. Kamikubo and Y. Amemiya, J. Chem. Phys. 112 (2000) 10608. 4. M. Nagao, Y. Kawabata, H. Seto and T. Takeda MP Conference Proceedings 469 (1999) 154. 5. Y. Kawabata, M. Nagao, H. Seto and T. Takeda AIP Conference Proceedings in press. 6. J. S. Huang, S. T. Milner, B. Farago and D. Richter, Phys. Rev. Lett. 59 (1987) 2600. 7. T. Takeda, H. Seto, S. Komura, S. K. Ghosh, M. Nagao, J. Matsuba, H. Kobayashi, M. Nagai, H. Kobayashi, T. Ebisawa, S. Tasaki, C. M. E. Zeyen, Y. Ito, S. Takahashi and H. Yoshizawa, J. Phys. Soc. Jpn. 65, Suppl. A (1996) 189. 8. B. Farago, D. Richter, J. S. Huang, S. A. Safran, and S. T. Milner, Phys. Rev. Lett. 65, (1990) 3348. 9. Milner, S. T., and Safran, S. A., Phys. Rev. A 36, (1987) 4371.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

213

Dimerization of penicillin V as deduced by frontal derivative chromatography Seiji Ishikawa, Saburo Neva, and Noriaki Funasaki Kyoto Pharmaceutical University, Misasagi, Yamashina-ku, Kyoto 607-8414, Japan The aggregation pattern of penicillin V (PCV) is investigated by frontal derivative chromatography on Sephadex G-10 columns. The height and position of the trailing peak, computer-simulated on the basis of the stepwise aggregation model, are in excellent agreement with the observed data, whereas those simulated on the basis of the trimer dodecamer model remarkably differ from the observed data, particularly in dilute solutions. Thus, PCV forms dimers and self-associates stepwise. 1. INTRODUCTION Many drugs form aggregates in water by hydrophobic interactions. This causes significant changes in bioactivity, chemical stability, and osmotic pressure of the drugs in aqueous solutions [1]. Dimerization must take place in all the self-associating systems being considered. However, the formation of higher multimers may often overshadow it and may lead to difficulties involved in even detecting it. Frontal derivative chromatography is an excellent method for detecting dimerization and determining the dimerization constant, because the frontal derivative chromatogram of the dimerizing system exhibits a characteristic pattern [1 ]. This characteristic pattern of dimerization can be used to detect dimer even in the presence of other multimers. Generally, dimerization does not cause any inflection, corresponding to a cmc, in the concentration dependence of physical properties. This often leads to the misinterpretation that dimmers are absent in micellar systems. In 1994 we showed by frontal gel filtration chromatography (GFC) that penicillin V (PCV) self-associates stepwise in a 150 mM potassium chloride solution at 298.2 K [1]. In particular, we determined the dimerization constant of PCV on the basis of GFC data, such as the centroid volume and the position and height of the trailing peak. Very recently, however, it was reported that PCV forms only two species, trimer and dodecamer, in remarkable contrast with our model [2]. In the present work we provide further chromatographic evidence for our stepwise aggregation model of PCV to resolve the above disagreement on the aggregation pattern of PCV. 2. EXPERIMENTAL SECTION A specimen of PCV (potassium phenoxymethylpenicillinate) was received from Sigma. Sephadex G-10 (Pharmacia) columns were treated as suggested by the manufacturer. The

214

double-distilled water was degassed just before each GFC experiment. All GFC experiments were carried out in a 150 mM potassium chloride solution under a flow rate of ca. 0.36 cm^ min'\ Two columns, B and C, were used for PCV. The columns were jacketed in order to maintain them at a constant temperature of 298.2 ± 0.2 K. A large volume of sample was applied so that the plateau region appeared on the elution curve. The concentration of PCV in eluate was monitored continuously with a differential refractometer. The GFC experiments were carried out at a number of concentrations. For the case of two concentrations, Co = 0.2002 and 0.2600 M, on column C, 2.00 cm^ of eluate was taken in a test tube with afi-actioncollector, because the refractometer did not respond to such high concentrations. Simulations of chromatograms were carried out by a plate theory. The number of the plate (AO and the void volume (Vo) of the column are the same as those already reported [1 ]. 3. RESULTS AND DISCUSSION 3.1. Frontal chromatogram of PCV A large volume of a dilute PCV solution was applied on the Sephadex G-10 column, so that the plateau region appeared on the chromatogram (Fig. la). This chromatogram is termed the frontal chromatogram. Because the concentration C of PCV in the plateau is the same Co as that applied, this chromatogram affords us quantitative information on the self-association of PCV. We may assume that the equivalent sharp boundary for the leading or trailing edge of the solute zone is approximately the initiation or termination of the plateau region (centroid) of the elution profile. In Fig. 1, S denotes the applied volume of the sample. Because the centroid volumes, Fc' and Fc, at the leading and trailing boundaries were equal to each other within experimental errors, we took the average of them as Fc. Frontal derivative 5 chromatograms (Fig. lb) reflect o aggregation patterns. As we have shown [3], we can determine the monomer concentration C] from Cy=(Vc-Vrn)C/iV,-Vm)

(1)

Here V\ and F^ denote the centroid volumes of the monomer and aggregates of PCV, respectively: Fi = 24.00 cm^ and Vm = 7.03 cm^ on column B and Fi = 21.84 cm^ and Vm = 6.35 cm"^ on column C. From Eqn. 1 the monomer concentration of PCV was determined as a function of the total PCV concentration. According to multiple equilibrium theory for self-association, the micellar weight average aggregation


0.04

0.00 80 40 V{cr[9) Fig. 1. (a) Frontal chromatogram of PCV at CQ = 0.2002 M and 5 = 42.50 cm^ and (b) its derivative with definitions of chromatographic parameters.

215 number w^ is calculated from «w = dlog(C-C,)/dlogC,

(2)

Our data showed that PCV forms dimer at low concentrations and larger multimers at higher concentrations [1]. Figure 2a shows the observed derivative chromatograms of C'o ^ 0.04177 mM at the leading and trailing boundaries on column B. The volume coordinate for the trailing boundary is shown as F - S\ the volume coordinate is assigned a zero value when the trailing boundary of the applied sample leaves the column bed. The shape of the derivative chromatogram at the trailing boundary reflects the aggregation pattern. The peak volume decreases with increasing concentration. According to asymptotic theory, the volume, Fp, of the trailing peak for the dimerization system approximately obeys the following equation [3]: {{V,;-F2p°^)/(Kp - K2p*)}^ = 1 + 9.6/:2C^x

(3)

Here Cmax denotes the concentration at the peak volume, Fp, at the trailing boundary (Fig. 1) and Ki stands for the dimerization constant. The Fip"" value may be estimated from extrapolation of the Fp values to zero concentration, and the F2p'' value may be set as the Fp value of blue dextran; Fip"" = 23.50 cm^ and Vi^ = 6.35 cm^ for column B. As Fig. 3 shows, Eqn. 3 holds true at dilute concentrations. The dimerization constant of PCV evaluated from the slope of the linear portion in Fig. 3 is 4.7 M* and is close to 4.38 M"^ obtained from the centroid volume data [1]. 3.2. Two self-association models of PCV We have proposed a stepwise aggregation model for PCV. According to this model, the total PCV concentration can be written as oo

C = Ci + IK iC^ + I /CVexp(a/ - ^/'^' - ci^^')

(4)

3

Here A^i stands for the overall aggregation constant for the formation of i-mer from / monomers. The term a corresponds to the driving force of micellization due to the transfer of the hydrophobic group from water to the micelle. The term h expresses the reduction of hydrophobic surface area caused by spherical micelle formation. The term c denotes the electrostatic repulsion between the hydrophilic groups at the micellar surface. These aggregation parameters were determined to fit the calculated Fc value to the observed one for a given concentration. The best fit parameters were determined to be ^^2 = 4.38 M\ a = 78.2, h = 75.8, and c = 19.3. On the other hand, Valera et al. proposed that PCV forms trimers and dodecamers alone [2]. According to their model, the total concentration of PCV is written as

c = c, + 3A:3Cr'+i2^i2cV^

(5)

216 0.010 0.008

L

c3

1

f

•9. 0.002 L h

(b)

'

5

/\

i

•• •• • • • •

I 0.006 h r ^0.004

(a)

— f

^

•

rf'"^o«

o ° ; °4

0.000 82__« 10

1 20

/' \ '• /•' \' /' \' /' \' /'

\'i

/lp\J,

-/1\:

••^n 10

20

30

V (cml \/ (cm') Fig. 2. (a) Observed derivative chromatograms of PCV of Co = 0.04177 M at the leading (closed circles) and trailing (open circles) boundaries and (b) simulated on the basis of the stepwise aggregation model (solid lines) and the trimer - dodecamer model (dashed lines).

0.00

Fig. 3. Plots of trailing peak positions according to Eqn. 3: O; observed, solid line; simulated on the basis of the stepwise aggregation model, dashed line; simulated on the basis of the trimer dodecamer model.

Using our observed Vc data, we determined the best fit trimerization and dodecamerization constants of A'3 = 71.93 M"' and AT^^ 19.53x10^ Wr^\ respectively. 3.3. Simulations of derivative chromatograms We simulated the derivative chromatograms at Co = 0.04177 M on the basis of the two aggregation models for PCV. As Fig. 2b shows, our stepwise model is clearly better fit to the observed chromatogram (Fig. 2a) at Co = 0.04177 M than the trimer - dodecamer model. In particular, the peak positions of the derivative chromatogram simulated on the basis of the trimer - dodecamer model are distant from the observed ones. This disagreement is mainly ascribed to the neglect of dimer in the trimer - dodecamer model. As Fig. 2 shows, the peak heights simulated on the basis of the stepwise aggregation model are in an excellent agreement with the observed ones, but those simulated on the basis of the trimer - dodecamer model are not so. Figure 3 depicts that the peak volume data simulated on the basis of the stepwise aggregation model are in an excellent agreement with the observed ones, but those simulated on the basis of the trimer dodecamer model are far from the observed ones. This is a very useful plot to show dimerization. In conclusion, our stepwise aggregation model for the self-association of PCV is better than the trimer - dodecamer model. REFERENCES I.N. Funasaki, S. Hada, and S. Neya, Chem. Pharm. Bull., 42 (1994) 779. 2. L. M. Varela, C. Rega, M. J. Suarez-Filloy, J. M. Ruso, G. Prieto, D. Attwood, F. Sarmiento, and V. Mosquera, Langmuir, 15 (1999) 6285. 3. N. Funasaki, Adv. Colloid Interface Sci., 43 (1993) 87.

Studies in Surface Science and Catalysis 132 Y. Ivvasawa, N. Oyama and H. Kunieda (Editors) 'o 2001 Elsevier Science B.V. All rights reserved.

217

Two-dimensional Clusters of Magnetic Fine Particles at the Surface of Magnetic Colloidal Suspension N. Tanaka, S. Doi and I. Takahashi School of Science^ Advanced Research Center of Science, Kwansei Gakuin University (ARCS-KGU), 2-1 Gakuen, Sanda 669-1337, Japan We report the in-plane structure of clusters of magnetic nanoparticles condensed at the free surface of a magnetic fluid (ferrofluid). Strong diffuse X-ray reflectivity which could not be explained without assuming the clusters was observed. Fractal dimension of the cluster and the surface tension of the magnetic fluid were evaluated. The fractal dimension strongly suggests a chain-like cluster of the colloidal spheres lying beneath the surface which isfluctuatingdue to capillary waves. The average direction of the magnetic dipole moment is also suggested to be parallel to the surface. They are consistent with the stability of such a surface-induced clusters observed by specular X-ray reflectivity at various temperatures and under AC magnetic fields. l.INTRODUCnON Specular The aggregation process of particles has RetJecitvitv attracted a great deal of interest in recent years. pifjuse Rcfiecfiviir Not a few systems are known to have strong tendency to spontaneously form highly concentrated regions or clusters. From X-ray '1:, Sample Surface reflectivity (XR), we found a stable, concentrated layer of the magnetic ^7> nanoparticles at the free surface of a magnetic Fig.l Schematic drawing of X-ray surface fluid (the coverage of the particles was scattering. Notations are explained in the text. estimated as about 70-80% [1]). Although the Dotted curve represents a transverse scan. magnetic fluid can be regarded simply as an ensemble of disordered magnetic dipole moments, the mechanism of such an aggregation phenomenon has not yet been understood. In this paper, we report diffuse XR from the surface of the magnetic fluid. After having the structure, we discuss the magnetic structure of the aggregates.

218 2. EXPERIMENTAL In order to measure the XR from the free surface of liquids, several difficulties should be overcome. At least, the sample surface must be kept horizontal, and the vibration from the rotating anode X-ray generator should be sufficiently insulated. The details on our diffractometer and experimental setup were described elsewhere [1,2]. The sample was a conunercially available magnetic fluid (LS-40, Taiho Industry Inc.). The superparamagnetic nanoparticles, about 100 A in diameter, were dispersed in alkyl naphthalene. Due to appropriate surfactant molecules which coat the particles, the bulk colloid remains stable until the volumetric packing fraction of

5

LOOE'28

LOOE-31 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.6

a -sin '^ (q A /4 Ji) (deg,) Fig.2

Transverse scans for different q values. For clarity all

the curves are displaced by three order of magnitude.

the nanoparticles exceeds O.I. ]

\ D

q = 0.1138

-TV

•^ so

' A"

5

\

^^at—.

T|-l= -0.1084\

I.OE-06

\ LOB-^

l.OE-04

j

B % l.OE-03

l.OB-02

qs(A-') Fig.3

Typical Log-Log plot of diffuse XR after a correction of the

illuminating area of the incident X-ray beam. The slope of the diffuse scattering at ^=0.1138[A"^] was fitted to b e - 0 . 1 0 8 4 (the solid line). The calculated XR with Y=0.026[N/m] is also plotted.

3. RESULTS Figure 1 shows the scattering geometry of specular and diffuse XR; whole the measurements were carried out by a transverse scan, namely the sum of the incident and outgoing angles (denoted as a and x in the figure) remains constant throughout the scan. Figure 2 indicates the transverse scans for different q (= \ q \ ) values (the momentum transfer q is defined as ko-ki, where ki, ko are the incident and scattered wavevectors of X-ray (\ki\ = \ko\ =1/X)).hithe

219 middle, a narrow specular XR is seen. The broad diffuse XR component is also visible for all the scans. 0.00

4. ANALYSIS Figure 3 is a Log-Log plot of the typical scattered intensity for fixed ^=0.1138[A-^]. A power-law region is clearly seen, leading us to free capillary waves are present on the surface [3]. However, the surfacefluctuationsof the magnetic fluid could not be explained only by the capillary waves of uniform liquids: Neither intensity nor slope of the diffuse XR for kerosene

•0.04 Y I -0.08

h

'0.12 •0.16 0.00

Fig.4

0.02

0.04

0.06

0.08

0.10

Ti-1 vs. q^. The line is a fit to the data.

agreed with observation, although the surface tension of kerosene was thought to be ahnost the same as that of the magnetic fluid (notice the theoretical diffuse XR from kerosene plotted in Fig. 3). Therefore, an alternative stmctural model is necessary to understand the diffuse XR from the magnetic fluid. To analyze the diffuse XR, we considerfractalclusters of colloidal particles lying beneath the surface which isfluctuatingdue to capillary waves. When the amplitude of the capillary waves is much smaller than the radius of the nanoparticles rnp, the electron density may be written as ^{x, y, z)

^ H[z - h(x, y)] { Apo g(x, y) + p(alkyl naphthalene) },

(1)

where H is the step function ( / / ( z ) = 0 for z<0, and / / ( z ) = 1 for z > l ) , h represents the height of the surface at {x, y), Apo is the density contrast (= p(nanoparticle) - p(alkyl naphthalene) ), and g represents a function on occupancy of the nanoparticles ( g = 0 if there is not the nanoparticlc at (x; y, h(x, y) + rnp), and g = 1 if there is the nanoparticle at (x; y, h(Xy y) + r„p) ). Forfractalaggregation withfractaldimension D, the pair correlation ftinction is given by < g(0, 0) g(x, y) > ^ ( x'^y')

(^-^,

(2)

where d is the space dimension set to be 2 in the present study. Since the instrumental resolution in the direction qy out of the scattering plane was rather loose, qy must be integrated over. Thus, the diffuse scattering 7^,^ g ) can be given by «

q ) - / J J P( O

P{ r' ) exp{ iq'(r-r')}6r

dr'd^y.

Inserting Eq. (1) into Eq. (3), and assuming that there is no correlation betweenH{ r)H(r')

(3) and

g( ^ )g(''') statistically, we obtain ^diffi q .^nJ^-' )-q:

(4)

220

Ti= d-D + ^ 9 ^

(5)

where ks, T, y are the Boltzmann constant, temperature[K] and surface tension coefficient, respectively. The third term of Eq. (5) represents the contribution of capillary waves [3]. Figure 4 represents the fitted T^ as a function ofq^'. The r\ revealed to be well reproduced by Eq. (5); the solid line in Fig. 4 is the fitted line. From the slope and the intercept at the ordinate, D and y were readily evaluated as 1.13 and 0.05 [N/m], respectively. 5. DISCUSSION In the present study we have obtained the reasonable value of surface tension (y at IS'C = 0.073(water), 0.026(kerosene) [N/m]). It might be one reason that our structural model is a plausible one. The fractal dimension of diffusion limited aggregation (DLA) in two dimensional system is known to be D^l.6. Since obtained fractal dimension of D ^ l . l for the magnetic fluid is rather small compared to 1.6, the surface induced clusters must not originate solely from the accepted processes, like DLA. Furthermore, fractal clusters with D ^ l . l can be regarded roughly as one-dimensional rather than two-dimensional. Therefore, the clusters would not have a net-like or two-dimensional crystalline (or porous) stmcture. Consequently, we can conclude that a chain-like structure with few branches is the most plausible. In Ref.[l], two types of arrangements of dipole moments, i.e. surface normal and surface parallel arrangements in which the average direction of the dipole moment is surface noraial and surface parallel respectively, were pointed out, although we could not decide which arrangement was reasonable (Fig.6 in [1]). The chain-like in-plane particle structure obtained in the present work clearly allows us to support the surface parallel arrangement. By using X-ray diffraction, the in-plane particle structure of the clusters of the magnetic fine particles was discussed on the standpoint of fractal aggregation. Computer simulation studies of which results can be compared with those of the present study are required so as to understand such a unique aggregation at the free surface of magnetic fluids. Acknowledgments We are greatly indebted to Prof. H. Terauchi and Prof. J. Harada for fruitful discussions and criticisms. Part of this study was supported by a special research grant from Kwansei Gakuin University, and by grant-in-aid for scientific research 09740252,11874055 from the Ministry of Education, Science and Culture in Japan. REFERENCES [1] I. Takahashi, K. Ueda, Y. Tsukahara, A. Ichimiya, J. Harada, J. Phys.: Condens. Matter 10 (1998) 4489-4497. [2] S. Doi and I. Takahashi, Philos. Mag. A 80 (2000) 1889-1899. [3] M. K. Sanyal, S. K. Sinha, K. G. Huang, B. M. Ocko, Phys. Rev. Lett. 66 (1991) 628-631.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c; 2001 Elsevier Science B.V. All rights reserved.

221

Colloid-chemical properties of chitosan Bratskaya S.Yu., Shamov M.V., Avramenko V.A., Chervonetskiy D.V. Institute of Chemistry, Far East Department of the Russian Academy of Sciences, 159, Prosp. 100-letya Vladivostoka, Vladivostok 690022, Russia Flocculation properties of chitosan in solutions of humic substances were investigated at various pH and ratios chitosan to humic acid. Effect of additions of Fe^"^ on the flocculation effectiveness was also studied. Experimental results on chitosan interaction with carbonic acids of low molecular weight have shown that hydrophobic internal domains of chitosan helices along with chitosan amine groups play an important role in chitosan interaction with organic substances. INTRODUCTION In the recent years, chitosan has been attracting the researches interest as a very promising material with widely ranging applications in water treatment, food and pharmaceutical industries, cosmetology, and biotechnology. Chitosan is a basic polymer of helix structure having reactive amine groups that gives a lot of possibilities of modification and ionic interactions. Combination of chitosan reactive amine groups and helix structure with internal hydrophobic domains determines such colloid-chemical properties of chitosan as organic substances sorption, flocculation, and stabilization of colloid systems that are of great interest both in theoretical terms and in industrial application possibilities. In this paper we discuss use of chitosan for humic substances flocculation. Some results on carbonic acids sorption on chitosan are also presented to illustrate the mechanism of chitosan interaction with organic substances. EXPERIMENTAL Chitosan flakes (dacetylation degree=75%) obtained from crab shells were used in all experiments. Humic acids were isolated from peat [1], amide of humic acids was obtained by heating humic acid solution with NH3 at 140 ° C and high pressure for 5h. For testing chitosan flocculation properties, 1 g of chitosan flakes were dissolved in 100 ml of O.IN HCl solution. Fixed amount of prepared chitosan solution was added to solutions of humic substances with concentration of 50-500 g/L and pH was adjusted to 7. After 24h solutions were filtered and optical density was measured at 413nm. Effectiveness of flocculation was calculated as a percentage of color removal. Interaction of chitosan with carbonic acids was studied as follows: 40mg of dry chitosan flakes and 20 ml of acidic solutions with concentrations from 1 up to 100 ^mol/L were shaken for 24h at controlled temperature 20''C. When equilibrium was established, the equilibrium acid concentration was determined by potentiometric titration with "Radelkis 0P211/1" pH-meter and "ORION 96-21" combination pH-electrode as described in [2].

222 The same method of titration was used to determine pK-distribution in humic acids and their amide. RESULTS AND DISCUSSION Flocculation of humic substances is of great interest for decontamination of natural and waste water from humic substances and metals bound to them as well as for recovery of valuable metals from technological solutions. Conventional method of humic acids removal from solutions is flocculation at pH=l-2 but it does not allow obtaining color removal higher than 80-90%. Our experimental results have shown that flocculation effectiveness of 96-100% can be achieved at pH=6.5-7.5 when chitosan is used as a flocculant in humic acid solutions Fig.l. This figure also illustrates that too high content of chitosan is resulted in the flocculation effectiveness reduction because of the stabilization of humic colloids by chitosan. Effect of chitosan concentration on effectiveness of humic acids and their amides flocculation is shovm in Fig.2.

30

40

50

60

Cchitosan'lO^.^

Fig. 1. Effectiveness of humic acids flocculation at ratios chitosan to humic: • -1:5, •-1:2, A- 1:1, T.2:1,•-4:1

Fig.2. Effect of chitosan concentration on flocculation of humic acids: •-lOOmg/1,. •-500mg/l; amide of humic acids: A-lOOmg/1, T-500mg/l.

It is obvious that chitosan concentration required for effective flocculation depends on humic acid concentration in solution and the nature of their functional groups. As a result of modification, amide of humic acids does not contain functional groups with pK less than 6.8 while original humic acids have carboxylic groups with average pK equal to 4 and 5.5 - Fig.3. Thus, amide of humic acids has a lower charge density in comparison vsdth original humic acids, and, therefore, less amount of chitosan is required to satisfy "cationic demand" of negatively charged colloid particles of humic acid amide. Fig.4 illustrates flocculation of humic acids by chitosan when Fe3+ is added. It is seen that in this case effective flocculation was obtained at significantly lower concentrations of chitosan. It should also be mentioned that even very high concentrations of Fe3-»- used as a coagulant without chitosan addition do not provide effective flocculation of humic acids. Thus, charge neutralization is not the only reason of chitosan effective work as a flocculant.

223 100

0.020

I-

0.015 i

humic acids humic acids amide

0.010

0.005 i

0.000 0

1

2

3

4

5

6

Cchitosan-103.%

Fig.3. pK-distribution o f functional groups of humic acids and their amide

Fig.4. Effect of Fe^* on humic acids (HA) and their amide (AHA) flocculation: • -HA(100mg/l)+ Fe^*(50mg/1), • - H A ( 5 0 0 m g / l ) +Fe^*n00mg/1), A- AHA(100mg/l)+ Fe^*(50mg/I),

W e assume that chitosan internal hydrophobic domains aside from its amine groups play a n important role i n interaction o f chitosan with organic substances including humic acids. T o confirm this assumption w e have studied interaction o f chitosan with homologous series o f carbonic acids - acetic, propionic, butyric, n-valeric and isovaleric acids in aqueous and water-ethanol solutions. Strong correlation w a s found b e t w e e n m a x i m u m sorption o f acid on chitosan and its p K value in aqueous solution with the only exception - isovaleric acid Fig.5. 2.2

2.2 y^valericadd

2.0

2.0

aceticacid

propionic add butyric add

I 1.8 \ acetic aa

r

J

V""^* \

•

1

•

c 1.6 o

butyric a d d ^ \ ^

isQK/afericacid

\

]

vatericadd \ ^

/ \\

1.2 propionic add

1.0 4.74

\ \

CO

1.2

isovaleric add •

butyric add

\

I 1.4

I

//

\

\ valeric add

propionic add 1.0

4.76

4.78

4.80

4.82

4.84

4.86

pK Fig. 5. Correlation between maximum sorption on chitosan and pK of carbonic acids: • - in aqueous solution

4.8

i

2

3

4

5

the number of carbon atonns Fig.6. Correlation between hydrocarbon radical length of carbonic acids and their maximum sorption on chitosan: • - in water/edianol (2:1) solution

6

224

Taking into account that this acid has a branched hydrocarbon radical we suggested that structure and length of the radical can also effect carbonic acids sorption on chitosan. This effect should be more explicit in solvent of polarity less than polarity of water, where hydrophobic interactions play more important role, for instance, in water/ethanol solutions. Our experimental results have shown that there is no strong correlation between pK and maximum sorption of carbonic acids on chitosan in water/ethanol solutions. But if we plot maximum sorption versus hydrocarbon radical length, we observe increase of sorption with increase of radical length (Fig.6) that was not observed for aqueous solutions. In water/ethanol solutions internal hydrophobic domains of chitosan helices become more accessible for interaction with hydrocarbon radical of carbonic acids that can result in acid trapping inside chitosan helices. It is obvious that hydrophobic interaction between chitosan and carbonic acids will be stronger with increase of the acid hydrocarbon radical length. In our experiments valeric acid has shown the highest values of sorption in water/ethanol solutions. Nevertheless, isovaleric acid with the same number of carbon atoms has the smallest value of sorption. This effect can be explained by space hindrances during penetration of a branched hydrocarbon radical inside chitosan helices. CONCLUSION We conclude that chitosan can be used as a very effective and biodegradable [3] flocculant of humic substances. Amount of chitosan required for effective flocculation depends on humic acid origin and concentration as well as on content of metal ions, especially Fe"'"*". Chitosan shows the best flocculation properties in pH range from 6.5 up to 7.5, where the highest values (up to 100%) of color removal were obtained. Summarizing the results obtained on sorption of low molecular weight carbonic acids on chitosan, we can conclude that the structure of hydrocarbon radical of the acid effect its sorption on chitosan due to possibility of hydrophobic interaction with internal domains of macromolecular helices. This effect becomes stronger with increase of the hydrocarbon radical length. Thus, we can suggest that for high molecular weight humic acids containing fragments with aromatic and long aliphatic radicals [4] hydrophobic interaction with chitosan should play a very important role. Most likely this type of interaction, aside from ionic interaction with chitosan amine groups, determines high effectiveness of chitosan as a flocculant in humic acids solutions. REFERENCES 1. 2. 3. 4.

Kemdorff H., Schnitzer M., Geochim. et Cosmochim. Acta, Xe 11 (1980) 1701. Bratskaya S.Yu., Golikov A.P., J.Anal.Chem., XeS (1998) 234. Boryniec S., Ratajska M., Fibers and Text. East.Eur. Xo4 (1995) 60. Orlov D.S. Soil Chemistry, Moscow State University, Moscow, 1992.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c 2001 Elsevier Science B.V. All rights reserved.

225

Science and art of fine particles Egon Matijevic Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5814, USA Without science we should have no notion of equality, without art no notion of liberty. W.HAuden INTRODUCTION This presentation deals with an area of materials science, which is causing a great deal of excitement for academic and practical reasons, i.e. with the so called monodispersed colloids. While the subject is an old one, dating back to Faraday's gold sols described in 1857, it has become a topic of wide-spread interest only relatively recently. There are some intriguing aspects of thisfieldof science and technology which justify some comments. First, it is obviousfi*omthe title, that the size of the "fine particles" to be discussed is missing. Presently we have two groups of scientists interested in fmely dispersed matter, some dealing in the nanometer and the other in the micrometer range. In many cases these systems are treated as separate worlds, yet it will be shown that they are closely connected, especially when these dispersions are prepared by chemical reactions in solutions. Another interesting aspect is that the uniformity of particles has seldom been encountered in natural environments. Indeed, there are only a few examples of indigenous monodispersed systems, opal being probably the best known. Yet scientists have produced a large number of such solids over a broad range of modal sizes, of simple or mixed (internally or externally) compositions, of different structures, and in a variety of shapes. The reasons for the discrepancy between the natural occurrences and research achievements will be addressed in the presentation. It will also be shown that, despite this progress, many questions remain to be resolved, which represent major challenges to the workers in the field. One of the problems is the actual mechanism (or mechanisms) by which uniform particles are formed. A recent development on the subject will be described. Another aspect is the predictability of shapes and structures of monodispersed colloids, to which there is still no answer. Finally, the title of the lecture requires further explanation. The word "art" has two meanings

226 in the English language: it is used to describe skilled craftsmanship or something of beauty. In dealing with well defined particles there is room for both of these interpretations. EXAMPLES OF UNIFORM PARTICLES There are many different techniques, which may yield particles of uniform size and shape, but none can be used to produce every kind of dispersions of desired properties. Due to the simplicity, versatility, and practicality, the precipitationfi-omsolutions is the method of choice. In the past this procedure yielded a number of monodispersed systems, although mostly by serendipity. About a quarter of a century ago systematic studies were initiated, which resulted in a multitude of well defined dispersions as reviewed in several articles (1-3). In most instances the processes have involved either mixing reactants or decomposing complexes, normally under mild temperatures and in moderate concentrations. A few examples of imiform particles of simple chemical compounds are illustrated by the electron micrographs in Figure 1. Using the same technique, it is also possible to precipitate composite particulates. The latter can be homogeneous of exact stoichiometry, as exemplified by pure or doped barium titanates. To achieve these conditions rapid mixing is required, such as by using the controlled double jet precipitation process. In contrast, slow precipitation results, as a rule, in internal inhomogeneity, i.e., the composition changesfi'omthe center to the periphery, although the particles may still be perfectly spherical, as observed with mixed alumina/silica, or copper/lanthanum, or copper/yttrium oxides. Another major area of interest are coated particles. Again, it is possible to produce uniform surface layers of varying thickness on inorganic cores with either inorganic or organic coatings or organic cores with inorganic coatings. It is interesting that the shell of the same material can be deposited on different cores. For example, yttrium basic carbonate coatings were produced on silica, hematite, and latex! It is essential to note that conditions needed to obtain a given material as a uniform dispersion are sensitive to a great degree to the experimental parameters, such as the temperature, concentration of reactants, pH, ionic strength, solvent composition, etc. In some cases even a small change in these conditions can not only affect the particle uniformity, but it may yield solids of different chemical composition, structure, or morphology. This sensitivity explains the paucity of naturally occurring monodispersed particles. It should also be noted that the laboratory preparations require minutes or hours, while the processes in nature extend over geological times. A final comment in this section refers to scaling up the production of well defined powders. Interestingly, the two materials first obtained in large quantities are the polymer latex and silica. The first is not a subject of this presentation, while silica will be exemplified in connection with some applications

227

One important aspect of scaling up precipitation processes, which yield uniform particles, is the necessity that any engineering design must consider the optimum conditions established in small batches. Much success has been achieved by using a plug-flow type of a reactor for continuous precipitation of a variety of uniform colloid dispersions, such as yttria, silica, aluminum hydroxide, and barium titanate (4). The schematic presentation of this equipment is given in Figure 2.

0.5/cm

0.5/cm

r 2\ITX\

S^m

Figure 1. Transmission electron micrographs (TEM) of (a) zinc sulfide; (b) manganese(II) phosphate, (c) and (d) hematite (Fe203) particles.

228

RESERVOIR 1

RESERVOIR 2

' PRODUCT

PERISTALTICI PUMP

TEMP

com.

SCR

U

Figure 2. Schematic presentation of the apparatus for continuous flow precipitation. MECHANISM OF THE FORMATION OF MONODISPERSED PARTICLES As one would expect, the understanding of the mechanism of the formation of monodispersed colloids by precipitation has been of major concern to workers in the field. For a long time the concept developed by LaMer was generally accepted; i.e., such dispersions should be generated, if a short lived burst of nuclei in a supersaturated solution is followed by controlled diffusion of constituent solutes onto these nuclei, resulting in the final uniform particles. This mechanism is indeed operational in some, albeit limited cases, and more often only at the initial stage of the precipitation process. For a long time the writer of this article has been puzzled by some experimental observations, one of which is illustrated by the electron micrograph of zinc sulfide in Figure la. These perfectly spherical particles, obtained by precipitation in ionic solutions, exhibit X-ray characteristics of a known mineral (sphalerite). Obviously, it is not easily understood why would such homogeneously precipitated perfect spheres have crystalline characteristics. Importantly, low angle X-ray measurements showed these particles to be made up of essentiality identical nanosized subunits. Electron microscopy and other methods of evaluation demonstrated on numerous other dispersions, that particles of different morphologies and chemical compositions clearly exhibited particulate substructures.

229 Based on the illustrated samples and many others, it is now firmly established that the prevailing mechanism in the formation of monodispersed particles proceeds in several stages: (7) nucleation, (2) growth to nanosize particles, and (3) aggregation of nanosize particles to uniform fmal colloids. Extensive studies have indeed documented the existence of the aggregation stage (5). For example, the electron micrographs, X-ray analysis, and independently prepared precursor particles all yielded the average diameter of crystallite subunits of 3 5 ^ nm of spherical gold particles displayed in Figure 3(6).

Figure 3. Colloidal gold particles It was then necessary to derive a mechanism, which would account for the aggregation of a huge number of nanosized particles into identical larger colloids. Recently a kinetic model has been developed that explains this size selection (7). The latter is based on the assumption that the nucleation is followed by a rapid formation of singlets, i.e. primary (nanosized) particles, which are sufficiently sparsely populated, and once formed are not further generated. Thus, their concentration decays by aggregation, when the conditions in the system eliminate repulsion between them. The latter can be due either to an increase in the ionic strength or to a change in the pH in course of the process. It is also assumed that the diffusion constant of singlets is larger than that of aggregates. The dominance of the irreversible singlet capture in the growth process can, under certain set of conditions, result in the size selection (i.e. uniformity) of the fmal particles.

230

f=0.1 sec a = 0.57 HJm 60

oc

"O

40

1 sec

[ 0.1

I 0.2

10sec

0.3

Figure 4. Distribution of secondary particles by their sizes at 0.1, 1, and 10 sec, calculated for the precipitation of spherical gold particles, using the model described in (7). The calculated size distributions of the secondary (final) spheres for three reaction times, using the parameters for the precipitation of the displayed gold sol, are given in Figure 4. The model describes reasonably well the experimental observations, considering the simplification used in the calculations. An interesting consequence of the newly established mechanism, is the fact that precipitation in solutions yields, as a rule, nanosized particles. If the process is arrested at this stage by additives, such as surfactants or microemulsions, one obtains stable nanosystems. However, in the absence of stabilizers, these particles aggregate (rather than to grow) into larger final products, which under appropriate conditions consist of monodispersed colloids. Thus, here we have found the *'bridge" between the two ''worlds" mentioned earlier! NANOSIZED PARTICLES The understanding of the mechanism of the formation of colloids throws new light on the possible preparation procedures of nanosized particles. Normally, wet routes involve precipitation in the presence of large amounts of surfactants (e.g. microemulsions, vesicles, etc), which affect the nucleation stage, but also stabilize the resulting finely dispersed matter. These processes make it difficult to separate solids from the additives. Obviously it is of interest to produce monodispersions in quantities with a minimum amount of stabilizers. One such process is based on the recognition that larger particles are aggregates of much smaller precursors. Should it be possible to peptize such colloids, one would have a new avenue of approach in the generation of nanoparticles. This method was indeed proven possible in some cases. Thus, monodispersed colloidal indium hydroxide was prepared in ethylene glycol. On

231 addition of water the original organic solvent was leached out and the precipitated solids fell apart into constituent subunits of nanometers in size. Another useful technique proves to be the controlled double jet precipitation (CDJP), which can yield in larger quantities nanparticles in the presence of moderate amounts of surfactants, as demonstrated on a variety of systems, including ZnO, PbS, BaTiOa, etc. "ART" AND SCIENCE The new developments in the understanding of the mechanisms of formation of monodispersed colloids have greatly advanced the scientific aspects of this area of materials. Yet in actual preparations the art, i.e. skills, still play an essential role, especially since we do not know how to predict and control some properties, such as the shape or even the composition of the resulting particles. The finely dispersed matter offers much in terms of the other aspect of art, i.e. the beauty. The latter can be affected by shape or color or both. Examples of such artistic impressions will be offered using electron micrographs of monodispersed particles and their surfaces. Even more importantly, pigments, marbles, metals, etc. are made of fine particles, without which we would have no paintings, sculptures, and other works of art, which so much embellish our lives. REFERENCES 1.

2. 3. 4. 5.

6. 7.

E. Matijevic: Formation of Monodisperse Inorganic Particulates. In Controlled Particle, Droplet and Bubble Formation (D.J. Wedlock, Ed.), Butterworth-Heinemann, London, 1994, pp. 39-59. E. Matijevic: Uniform Colloid Dispersions - Achievements and Challenges. Langmuir, 10, 8-16(1994). E. Matijevic: Preparation and Properties of Uniform Size Colloids. Chem. Mater., 5,412426(1993). Y.-S. Her, S.-H. Lee, and E. Matijevic: Continuous Precipitation of Monodispersed Colloidal Particles. II. SiOz, A1(0H)3, and BaTiOs- J. Mater. Res., 11, 156-161 (1996). S.H. Lee, Y.-S. Her and E. Matijevic: Preparation and Growth Mechanism of Uniform Colloidal Copper Compounds by the Controlled Double-Jet Precipitation. J. Colloid Interface Sci., 186, 193-202 (1997). D.V. Goia and E. Matijevic:Colloids Surf, 146, 139-152 (1999). V. Privman, D.V. Goia, J. Park and E. Matijevic: Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors. J. Colloid Interface Sci., 213,36-45(1999).

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Studies in Surface Science and Catalysis 132 Y. lwasawa, N. Oyama and H. Kunieda (Editors) c¢~2001 Elsevier Science B.V. All rights reserved.

233

Hydrothermal synthesis of nano-size ZrO2 powder, its characterization and colloidal processing O. Vasylkiv and Y. Sakka National Research Institute for Metals, 1-2-1, Sengen, Tsukuba, Ibaraki Pref., 305-0047, Japan Nano-size 3Y-TZP powder with controlled secondary particles size was synthesized by hydrolytic coprecipitation. Hydrous-zirconia gel produced by urea precipitation results in a nano-powder with a primary particles size of 5 - 8 rim, and secondary aggregates size of 45 - 65 rim. After calcination powders exhibit a single tetragonal phase. Determination of the optimal formation parameters, postsynthesis treatment for fabrication of agglomerate-free zirconia powder with finest primary crystallites uniformly packed into the secondary aggregates, as far as determination of the best suspensions properties allowed preparation of the uniformly densified green body with high packing density of 58 % after slip casting and CIP. 1. INTRODUCTION Fine ceramic particles, uniformly agglomerated are generally desirable for producing dense ceramic, because of the close packing and uniform densification [ 1-12]. Powders produced by wet chemical methods are usually polydispersed and consist of: primary crystallites, aggregates of primary crystallites, which are formed during reaction time, and powders agglomerates [2-11]. The crystallites size is dependent of the nature of powder processing i.e. technique or methodology of synthesis. The rate of crystallites growth is negligible low up to the temperature of 900 °C [5]. Tetragonal zirconia polycrystal (TZP) based ceramics have attracted special attention because of its excellent mechanical properties and attractive possibility of obtaining the nanograined ceramic with controllable microstructure [2-10]. Nano-grained 3Y-TZP ceramics are expected to show the excellent mechanical properties. Nano-scale zirconia-based ceramic is a good start for post-sintering heat treatment, the grains in the final microstructure can be grown to any desired size, provided the grain growth can be controlled [6]. The purpose of our study is to obtain a finest possible powder with narrowest particle size distribution in order to prepare the uniform slurry and slip cast for obtaining the uniform green microstructure, with high reactivity during sintering. 2. EXPERIMENTAL Z r O C I a * 8 H 2 0 , Y203, urea (High Purity Chemicals, J a p a n ) , and HCI (Kosochemical, J a p a n ) , were used for this investigation. The mixed sols of composition ZrO2 + 3 mol% Y203 at different cations concentration with urea, were hydrothermally treated at 155 °C. The urea decomposed into NH3 and CO2 through reaction with H20 and pH changed to 9. The homogeneous precipitate formed was hydrous ZrO2 with Y203 and it is crystallized under hydrothermal conditions. Several techniques were used for powder washing and drying in order to minimize the agglomeration of the powder [5]. Powders were washed with deionized water to remove CI-

234 ions and ammonia, and with ethanol. Subsequently ethanol was evaporated (T= 65 °C). After pH overreached 8 the micro tip ultrasonication for 10 min, using 20 kHz and 160 W (Shimazu, USP-600) was used to destroy the powder agglomerates in suspension [5, 11]. Phase identification was determined from X-ray diffractometry data (JEOL JDX-3500). The primary crystallite size was determined through an X-ray diffraction line-broadening method, from surface area data (Coulter SA 3100) and from TEM observation (JEM-100-CX, Japan) data. The aggregates and agglomerates size have been analyzed using a Laser Particle Size Analyzer (LSPZ-100, Otsuka Electronics). Aqueous suspensions were prepared from the powders with different morphology and from the same powder by changing the mount of additional dispersant (ammonium polycarboxylate, Toaghosei Co., ALON A-6114). The suspensions were ultrasonically dispersed for 10 rain and stirred under same conditions of mixing with a magnetic stirrer [11, 12]. The rheological behavior of the suspensions was studied with a viscometer (Toki-Sangyo Co., RE500L). Consolidation of the suspensions by slip casting and subsequent CIP at 400 MPa were applied. The densities of the green bodies were measured by the Archimedes method using kerosene. Relative density was based on a 6.02 g/cm3. 3. RESULTS AND DISCUSSION

3.1. Powder preparation To prepare powders with homogeneous composition and uniform morphology metalchlorides and urea-contained sol thermal hydrolysis have been used. Each of the produced particles of hydrous zirconia powder was an aggregate of many small primary particles with calculated diameter of 5 - 8 nm. The initial solution concentration influences the powders properties. With an increasing solution concentration, the size of the resulting secondary aggregates and degree of agglomeration became larger. The powder's surface area decreases only from 96 to 87 m/g 2 for calcination at 600 °C, or from 53 to 47 m2/g for calcinations at 800 °C (for 1 h holding) if the concentration of the stock solution increases from 0.1 to 0.5 mol/l. At the same time the size of the primary particles obtained from TEM observation was nearly the same for all concentrations at the same conditions of calcination. We clarified the decreasing of the surface area to the primary particles bonding and appearance and thickening of the solid bridges between the crystallites packed into the agglomerated secondary aggregates. The size and morphology of the aggregates of primary crystallites is strongly depends on the solution concentration, temperature-time conditions of hydrolysis, conditions of washing and drying, and timetemperature conditions of calcination. During hydrolytic precipitation, dense aggregates of Fig. 1. TEM photograph of hydrous zirconia primary particles forms when the hydro-thermally derived and capillary forces overcome the interparticle repulsive forces ealcinated 3YTZP powder, and bring as formed oxide particles closely to each other. The free particles are bound together into the aggregates by van der Waals forces and such bonds forms necks between the particles, the initial reagents react into the solution and then precipitate at the torroidal region between the particles. Figure 1 shows the TEM photograph of the 3Y-TZP aggregates synthesized from urea-chlorides aqueous solution with initial concentration of 0.2 mol/l and calcinated at 800 °C for 1 h.

235 No tendency appears for bridging between neighboring aggregates for the powder washed with ethanol. However, after drying of washed powders the surface of free powder can absorb up to -- 3 wt.% of water. Such water amount is enough for particles bounding. In addition, the particles can be densely packed during initial stage of calcination, and solid necks between primary crystallites and inter-aggregate bridges forms during subsequent calcinations. Stabilizing of oxide powders against coagulation in an aqueous solution required a low initial solution concentration (0.05 - 0 . 2 mol/l) and pH value - several units above isoelectric point (IEP). Micro-tip ultrasonic treatment for breaking up the appeared agglomerates has been used at the final stages of washing [5] and after calcination. The average size of secondary aggregates could be reduced close to the value of 45-65 nm. The bonds between the crystallites into the aggregates with such average size has to be rather strong because ultrasonication was unable to break up them and further reduction in size was found to be impossible. It is clear that the powders produced with different wet chemical techniques are in fact agglomerates of a small, strongly bond aggregates of the crystallites [2-9]. Zirconia nano-powder (Fig. 1) with primary crystallites size of 5 - 10 nm, with average secondary particles size of 50 nm, and without tertiary agglomerates has been prepared. The optimum inintial sol's concentration was found to be 0.05 -0.2 mol/l.

3.2. Colloidal processing Aqueous suspensions were prepared by changing the solid content and the amount of additional dispersant. The suspensions with the minimum viscosity i.e. best flowability [1012] were prepared from the powders with different degree of aggregation-agglomeration. The appropriate solid content of the suspension was 25 so A' ' ' ~~ ' ' '.v~r.0~o.~/ vol% for the finest agglomerate-free 3Y-TZP powder. a g g r e g a t e size, n n ~ . I ~xx \ --O-s0 % 1 For coarser, agglomerated powder the solid content of --4- ~0 30 vol% was applied. Figure 2 shows the changes of = the suspensions viscosities (at the share rate of 100 s"l) over the amount of dispersant for suspensions with different average secondary particle size and degree of .~ agglomeration. The viscosity of the suspensions could 20 be reduced by the addition of water. However, subsequent slip casting of the suspensions with solid 10 content lower than 25 vol% leads to non-uniform densification of green body and cracking. The amounts I ~ i ~ I ~ of dispersant 4.5 wt% for 50 nm powder, 3.5 wt% for 0 , I ~ J I 0 1 2 3 4 5 6 l l 0 nm powder, and 2.5 wt% for coarser powder were Amount of dispersant, wt% found to be appropriate for preparing well-dispersed suspensions. Suspension's stability as far as good Fig. 2. Suspensions viscosity dispersion is the main trend of the uniform green versus theamountofdispersant, microstructure. The high viscosity of suspensions implies the rapid flocculation of the particles in nonstirred suspension. Viscosity of the suspension prepared with 25 vol% of solid and 4.5 wt% of dispersant changed from 8 mPa-s to 12 mPa.s during first 2 hours (without stirring) and atter 7 hours reached the value of only 27. Such time is enough for preparing of uniform green body. At the lower or higher amount of dispersant the suspension became to stiff for slip casting immediately or during the first 2 hours. 4.5 wt% of dispersant and 25 vol% of solid content have been chosen as the best conditions for preparation of the uniformly densified green body. Schematic of influence of agglomeration on the densification and green microstructure is depicted in Fig. 3. The agglomerates in zirconia powder prepared from the sols with concentration of more than 0.2 mol/l, water-washed, dehydrated with ethanol, dried and

236 concentration of more than 0.2 mol/l, water-washed, dehydrated with ethanol, dried and calcinated cannot be completely breaking up during subsequent processing. It can be indicated (Fig. 4) from lower green density of as-slip casted compacts (Relative density D = 35 - 4 3 % in comparison with density of compacts from the powder with narrow size distribution of secondary particles (D= 45 - 52 %). Non-uniform packing from agglomerates lead to localized, inhomogeneous densification during subsequent sintering. The highest green density was obtained for the sample prepared from the powder with average aggregate size of 110 nm. The green densities for the f'mest uniformly aggregated powder atter slip casting and slip casting with CIP were 46 % and 52 % respectively. Uniformly aggregated primary particles

uniform green rnicrostructure from slip casting

CIP best packing density

~9o high pecking density

The size range of intraaggregate and interaggregate pores is same

Circuits (agglomerates) of the secondary aggregates ~ i ~ nonuniform microstructure low packing density

CIP poor packing density

80 ~60 eu

--0- slip casting ~ slip casting + CIP

50 4o t

Large interaggtornerate pores

Fig. 3. Schematic of influence of agglomeration on the densification and green microstructure.

' 0

I 100

'

i 200

'

i 300

~

1 400

'

I 500

'

I

T

600

Meanaggregate(agglomerate)size,nm Fig. 4. Effect of aggregate (agglomerate)

I 700

size on compacts density.

IV. CONCLUSION The main potential of nano-powders - increasing reactivity and reducing of sintering temperatures can't be realized unless powder agglomeration will be minimized during preparation or agglomerates will be eliminated, and uniform densification of green body will be achieved. This study showed that determination of the optimal powder formation parameters, and post-synthesis treatment for fabrication of non-agglomerated zirconia powder with f'mest primary crystallites uniformly packed into the secondary aggregates with narrow size distribution, and determination of the best suspension's properties allowed preparation of the uniformly densified green body with density high for nano-size powders. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

W.H. Rhodes, J. Am. Ceram. Soc. 64, (1981) 19. W. Luan, L. Gao, and J. Guo, NanoStructured Materials 10, (1998) 1119. S. Theunissen, A. Winnubst and A. Burggraaf, J. Eur. Ceram. Soc. 11, (1993) 315. O. Vasylkiv and Y. Sakka, J. Am. Ceram. Soc. submitted (2000). P. Duran, M. Villegas, F. Capel, C. Moure, J. Mater. Sci. 15, (1996) 741. S. Lawson, J. Eur. Ceram. Soc. 15, (1995) 485. N. Enomoto, S. Maruyama, and Z. Nakagawa, J. Mater. Res. 12, (1997) 1410. O. Vasylkiv and Y. Sakka, J. Am. Ceram. Soc. 83 [9] (2000). O. Vasylkiv H. Borodians'ka and Y. Sakka, J. Am. Ceram. Soc. accepted (2000). F.F. Lange, J. Am. Ceram. Soc., 72, (1989) 3. T. Suzuki, Y. Sakka, K. Nakano and K. Hiraga, Materials Transactions, JIM. 39, (1998) 689.

Studies in Surface Science and Catalysis 132 Y. lwasawa, N. Oyama and H. Kunieda (Editors) ~c; 2001 Elsevier Science B.V. All rights reserved.

237

Nanocrystal Self Assemblies: Fabrication and collective properties M.P.Pileni Laboratoire S.R.S.I, URA CNRS 1662, Universit~ P. et M. Curie (Paris VI), BP 52, 4 place Jussieu, 75252 Paris cedex 05, France. I. Fabrication Nanocrystal are made by using reverse micelles (1-3). At the end of the synthesis, the nanocrystals are coated with dodecanethiol . To get 2D and 3D superlattices, the deposition procedure and the nanocrystal concentration play a key roles. A drop of the solution is deposited on HOPG substrate with a filter paper underneath. The solution migrate from the substrate to the filter paper and after few seconds the solvent is totally evaporated. At rather low concentration (particle volume fraction, ~=0.01%), monolayer made of nanocrystals are observed on a very large domain (50~m) (4). The nanocrystals are organized in compact hexagonal network. The average distance between particles is 1.8nm. Immediately after the solution containing the nanocrystals is deposited on the substrate, the solvent begins to evaporate and droplets form. The nanocrystals themselves are fully solvated (or "dressed") by the heptane, which prevents their assembly into dense structures (5). As the droplets grow and begin to merge, some of the particles (which are still mobile because of the thin solvent layer present on the HOPG surface) are expelled away from the merge center. These dressed particles form compact monolayer islands, whose density increases after all of the solvent evaporates and interdigitation of the alkyl chains on the nanocrystal occurs. Other particles are caught in the center of the droplet merge point. The pressure exerted on these particles by the droplet menisci is large, and while a monolayer initially form, continued

238 droplet coalescence engenders the formation of a 3D structure. The three dimensional structure of dressed particles dries out as the solvent evaporates, and thus interdigitation of the particles' alkyl chain coating occurs in 3D instead of 2D. The sizes of the 3D aggregates are similar, which implies that the merge regions between growing solvent droplets are also similar in size. Enhancement of the TEM pattern shows that the nanocrystals self assemble In four-fold symmetry (6,7). This can be attributed to the [001] plane of an FCC lattice can easily be seen. The center-to-center distance between

two

nanocrystals along the [010] plane is ca. 11 nm. The average particle diameter is 5.8 nm, and the shortest center-to-center distance is 7.8 nm, leaving a 2 nm edge-to-edge separation that is consistent with alkyl chain interdigitation. By deposition of a drop of solution containing nanocrystal on HOPG substrate with an anticapillary tweezer formation of rings instead of monolayer self organized in compact hexagonal network (8). Similar behavior is observed with spherical silver, cobalt, ferrite and with flat tringular CdS nanocrystals. When nanocrystals dispersed in hexane are deposited on a TEM grid under a "quasi" saturated atmosphere, they are then randomly dispersed without any ring formation. Such a change In the nanocrystal organization from rings to a random dispersion is due to the evaporation rate. As matter of fact, the evaporation time (3 minutes) Increases compared to that under air

(30

seconds). Hence formation of rings made of nanocrystals are related to the evaporation rate. This is strongly confirmed by the calculated values of temperature gradient (AT) and Marangoni number (Mg). The estimated values of temperature gradient and Marangoni number are 29 and 10^ under air and 4.8 and 1.8.10"^ under saturated hexane respectively. Hence, the decrease in the evaporation rate induces a decrease in the AT and M^. This induces a decrease in the instabilities. By reducing the evaporation time, the system equilibrates faster than the heat loss by the evaporation process. Under such conditions, instabilities disappear and nanocrystals are randomly distributed

239

on the carbon film. This means that formation of rings is related to the instabilities, Induced by a fast evaporation process. The physical properties of the nanomaterials used (semiconductors, metals, oxides) and their shape (spheres or triangle) are not related to ring formation.

II.

COLLECTIVE

OPTICAL

AND

ELECTRONIC

TRANSPORT

PROPERTIES Both the experimental

and simulated

absorption

spectra

of silver

nanocrystals show a decrease in the plasmon resonance band intensity and increase in bandwidth with decreasing particle size. When silver nanocrystals are organized into a 2D lattice, the plasmon resonance peak Is shifted to energies lower than what is obtained for dilute solutions of isolated particles. UV/vis polarization spectroscopy can reveal information about interparticle electromagnetic interactions.

In s-polarizatlon, the electric field vector is

oriented parallel to the plane of the substrate at all incidence angles 9. Plasmon resonance modes with components polarized perpendicular to the plane of the substrate are not seen when the incident light Is s-polarized. On the other hand, p-polarized light, whose electric field is parallel to the plane of incidence can probe plasmon resonance excitations whose components are either parallel or perpendicular to the substrate. In s-polarlzation, the absorption spectra are virtually independent of incidence angle and show a plasmon resonance band centered at 2.9 eV, which is similar to that seen in isolated silver nanocrystals. In p-polarization, a second band appears at higher energy as the incidence angle is increased. At large angle (60°), the two peaks are well-defined: the first is close in energy (2.8 eV) to the absorption maximum for isolated particles (2.9 eV), but the second is centered at ca. 3.8 eV. The high energy band at 3.8 eV is attributed to the self-organization of the

240 silver nanocrystals into a hexagonal network (10). The position of the peak can be explained in terms of local field effects. When a single silver nanoparticle is deposited on a gold 111 substrate, the scanning tunneling spectroscopy

measurement indicates a double tunnel

junction. Upon increasing the applied bias voltage V, the capacitor elements (defined by the tip-particle interface and particle-substrate charged up,

interface) are

and the detected current I is inititally close to zero. Above a

certain threshold voltage, electrons can tunnel through the interfaces and the current increases with the applied voltage. A plot of dl/dV versus V clearly shows that the derivative reaches zero at zero V. The non-linear profile of the l(V) curve and zero dl/dV at zero bias voltage are characteristic of the wellknown Coulomb blockade effect. For silver nanoparticles self-organized in a 2D superlattice on an Au(111) substrate, the current is an order of magnitude lower than that observed for Isolated particles. The Coulomb gap Is small (ca. 0.45 V, compared to the 2V seen in the isolated particles), and the overall l(V) curve is more linear. This indicates an increase in the ohmic contribution to the current.

In other words, the tunneling contribution to the total current

decreases, and more conductive pathways between particles are established. The derivative curve indicates a metallic conduction behavior with dl/dV ^^ 0 at zero bias voltage.

From the l(V) and dl/dV curves, it can be concluded that

when the particles are arranged in a 2D lattice, the tunneling current exhibits both metallic and Coulomb contributions. This indicates that lateral tunneling between adjacent particles is very important and contributes to the total electron transport process. When silver nanocrystals are assembled in a 3D FCC structure, the l(V) curve shows a linear ohmic behavior. The dl/dV curve is essentially flat, indicating metallic behavior without Coulomb staircases. The ohmic behavior cannot be attributed to the coalescence of the particles. Thus we conclude that the FCC structure of the superlattice induces an increase in the tunneling rate via a decrease in resistance between the particles.

The

241

electron tunneling between adjacent particles becomes a major contribution to conduction, and the Coulomb blockade effect in the l(V) curves is inhibited (11).

The mechanism may involve an enhanced dipole-dipole interaction

along the vertical (z) axis. When subjected to a voltage bias, the Fermi level of the individual nanocrystals is also perturbed.

The details remain to be

uncovered, but it is clear that a supercrystal of coated metal nanoparticles can behave as a metal.

III. COLLECTIVE MAGNETIC PROPERTIES A comparison of the magnetic properties of isolated magnetic nanocrystals and 2D hexagonal assemblies reveals cooperative effects in the latter system (12,13). The magnetization curves for cobalt nanocrystals deposited on an HOPG substrate, for magnetic fields applied parallel and perpendicular to the substrate are compared to that observed when nanocrystals are isolated in a matrix. When the magnetic field is parallel to the substrate, the Mr/Ms ratio is 0.60, and the hysteresis loop is squarer than that obtained for particles In solution. When the field is perpendicular to the substrate, the loop is less square, and the Mr/Ms ratio decreases to 0.40. These results show that for a given saturation magnetization, the remanence magnetization markedly varies with the orientation of the magnetic field. The observed changes cannot be attributed to coalescence of the nanocrystals, since TEM images taken over large areas of the sample show no evidence of this. Among the possible explanations for the change in magnetic properties (isolated particles versus 2D assemblies) is magnetic dipolar coupling between particles. This lead to enhanced magnetization. The scenario for dipole coupling enhancements is an enhancement due to the long-range order of the 2D lattice and collective "flips" of the magnetic dipoles. The dependence of the magnetization cun/es on field orientation is calculated. The simulated cun/es resemble the experimental

242

curves showing variations when the magnetic field is applied parallel and perpendicular

to the

substrate

(14).

From these

theory-experiment

comparisons, it seems reasonable to conclude that the collective magnetic properties observed when the cobalt particles are arranged in a 2D lattice are due to an increase in magnetic dipole-dipole interactions. REFERENCES 1. M.P. Pileni, ed. Reactivity in Reverse Micelles. Amsterdam, Elsevier, 1989. 2. M.P. Pileni, J Phys Chem 97 (1993) 6961. 3.. M.P. Pileni,. Langmuir 13 (1997) 3266. 4. L Motte, F Billoudet, MP Pileni. J Phys Chem 99 (1995) 16425. 5. L Motte, E Lacaze, M Maillard, MP Pileni. Langmuir 16, (2000) 3803. 6.L Motte, F Billoudet, E Lacaze, MP Pileni. Adv Mater. (1996) 8:1018- , 1996. 7. L Motte, F Billoudet, E Lacaze, J Douin, MP Pileni. J.Phys.Chem, B 101 (1997),138. 8. M Maillard, L Motte, T Ngo, MP Pileni J.Phys.Chem. (2000) in press 9.A Taleb, C Petit, MP Pileni. J.Phys.Chem, B 102 (1998) 2214 10. A Taleb. V Russier, A Courty, MP Pileni. Phys Rev B 59 (1999) 13350. 11. A.Taleb, F.Silly, O.Gusev, F.Charra, M.P.Pileni, Adv. Mat.12, (2000), 119. 12. C. Petit, A Taleb, . M.P. Pileni,. Adv.Mater 10 (1998) 259.13. C Petit, A Taleb, MP Pileni. J. Phys; Chem. B 103 (1999) 1805. 14. V. Russier, C. Petit, J. Legrand,. M.P. Pileni Phys. Rev. B 62 (2000) 3910.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

243

Preparation, characterization and catalyses of lighttransition-metal/noble-metal bimetallic alloy nanoclusters Naoki Toshima, Ping Lu, and Yuan Wang Department of Materials Science and Engineering, Science University of Tokyo in Yamaguchi, Onoda-shi, Yamaguchi 756-0884, Japan Colloidal dispersions of bimetallic alloy nanoclusters of light transition metals like Cu, Ni, Fe and noble metals like Pd, Pt, and Rh were prepared by using ethylen glycol as a reductive solvent and metal hydroxide colloids as metal precursors. Even though the nanoclusters of light transition metals are easily oxidized by oxygen and thus difficult to be handled under practical conditions, the bimetallic alloy nanoclusters containing these light transition metals at high concentrations are enough stable imder air at room temperature and thus enough easily handled to be used as catalysts for hydration of nitrile to amide (Cu/Pd), reduction of nitro to amino group (Ni/Pd), etc. 1. INTRODUCTION Various metal nanoclusters have recently received increasing attention especially as building blocks for nanotechnology as well as selective and active catalysts for industries [1]. The nanoclusters of light transition metals like Cu, Ni and Fe are easily oxidized by oxygen in air, although those of noble metals are usually stable even under air. Tlius, bimetalization to form alloy with noble metals is considered as a good method to stabilize nanoclusters of light transition metals. Here we report a novel chemical method to prepare colloidal dispersions of bimetallic alloy nanoclusters of light transition metals and noble metals especially at high concentrations of light transition metals. Since the alcohol reduction of noble metal ions in the presence of poly(Ar-vinyl-2-pyrrolidone) (PVP) is an excellent method to prepare the colloidal dispersions of noble metal nanoclusters with good monodispersity, we have used ethylene glycol as a reductive solvent having high boiling point and metal hydroxide colloids as metal precursors [2]. The bimetallic alloy nanoclusters thus prepared have been charactarized by transission electron microscopy (TEM), X-ray diffl-action (XRD) pattern, X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS) technique, etc., and used as catalysts for hydration of nitrile to amide (Cu/Pd), reduction of nitro to amino group (Ni/Pd), hydrogenation of olefins, dehalogenation of halogenated compounds, and so on. The magnetic properties of Fe, Co, and Ni ultrafine particles have been of interest for many years, but complicating features of surface oxide effects on magnetism are still not well understood. Thus, the bimetallic alloy nanoclusters containing these elements are of great interest from the viewpoint of stabilization of the light transition metals as well as increase of coercivity [3].

244

2. EXPERIMENTAL For preparation of bimetallic alloy nanoclusters the following salts were used for the source of the corresponding elements: CuS04-5H20, NiS04-7H20, FeS04* 7H2O, Pd(OCOCH3)2, H2PtCl6-6H20, RhCl3-3H20. The solvent mainly used was ethylene glycol. When the starting salts were not soluble in ethylene glycol, rather small amount of dioxane or water were used to solubilize these paticular salts. After adjusting the pH of the mixtures at about 10 by addition of an aqueous solution of NaOH, the mixed solution of the coresponding salts and PVP were heated to refluxing at 190°C for hours with a nitrogen flows passing through the reaction system to take away water added and formed, and byproducts produced. The resulting colloidal dispersions have a transparent dark-brown color. For separation of the PVP-protected nanoclusters from solvents two techniques, vacuum evaporatiion of solvents and filtration under argon with an ultrafilter, were uesd. The following equipments were used for characterization of bimetallic nanoclusters: Hitach H-7000 and H-9000 NAR electron microscopes for TEM, a Rigaku Rint 2400 diffractometer for XRD, JEOL JIS-90 SX and Kratos AXIS-HS photoelectron spectrometers for XPS, and BL-TC and BL-lOB beamlines of Photon Factory, the National Laboratory for High Energy Physics (KEK-PF) for EXAFS. 3. RESULTS AND DISCUSSION 3.1. Preparation and characterization of bimetallic aUoy nanoclusters Colloidal dispersions of bimetallic alloy nanoclusters of light transition metal and noble metal were prepared by simultaneous reduction of both metal ions by refluxing the alkaline ethylene glycol solutions at 198°C. Since the redox potential of light transition metal is more negative then that of noble metal, the reduction of light transition metal ions may not simultaneously occur at the same time with that of noble metal ions, but may by catalyzed be the preformed nanoclusters of noble metals.

-

Pd Pd/Cu9:l Pd/Cu6:4 Pd/Cu5:5 Pd/Cu2:8

0 3 •0 C

/I

a> n S

h

1

.1

.'t 1

-

I

> * ^ ^ 1 « ^

n %^42^s^ 4

Distance/10'^ nm

•

•

• .V •/ *

'

"

.

-

-

.

•

>

•

.

'

/

Pd coUoid Pd/Ni(4/l)coUoid Pd/N 1(3/2) coUoid - - Pd/Ni(|/ncoUoid Pd/NKl/4)coUoid

A ;,1

1

'\ V"•v-J^ •••'

:r.; —.^'~-

TrVrni mtmttnt

0 Distance/10 nm

Fig. 1. Fourier-transformed EXAFS spectra of nanoclusters at Pd K-edge.

245

TEM micrographs and the size distributions, cahbrated from the corresponding micrographs, have indicated that sizes of the PVP-protected bimetaUic alloy nanoclusters are enough small (ca. 1-3 nm) and monodispersed. The alloy structure has been confirmed by XRD, XPS and EXAFS. Fouriertransformed EXAFS spectra at Pd K-edge of Pd/Cu and Pd/Ni bimetallic nanoclusters are shown in Fig. 1. These data indicate the existence of bonds between the corresponding two elements to form an alloy. Recent precise analyses suggest that the bimetallic nanoclusters have a tendency to have the structures of intermetallic compounds [4]. 3.2. Catalyses of bimetallic alloy nanoclusters In general the catalyses of bimetallic alloy nanoclusters are expected to be varied linearly with the content of active element because the amount of surface atoms of the particular element increases linearly with increasing content of the element in the bimetallic nanoclusters. In practice, however, the adjacent element can give electronic and steric effects on surface atoms of the active element. In the case of PVP-protected Cu/Pd bimetallic nanocluster [2a] for example, the rate of selective partial hydrogenation of 1,3-cyclooctadiene to cyclooctene (eq. 1) under mild conditions can increase rapidly when the Pd content is higher than 25 mole %, and reaches that of pure Pd nanoclusters at the Cu/Pd ratio of 1. In hydration of acrylonitrile to acrylamide (eq. 2), for which Cu is known as an active catalyst, the activity of Cu/Pd bimetallic nanoclusters increases rapidly from the beginning and that at high concentration of Cu is at least 50 times higher than that of pure Cu colloids [2a]. This is probably due to the ensemble (steric) effect of adjacent Pd on surface Cu atoms.

Cu/Pd

(1)

-^ Cu/Pd

CH2 = C H - C = N + H20

•

CH2 = C H - C - N H 2

<^>

O R - < ( O j > - N O , -h 3H,

•

R - < 2 /

NH2 + H2O

(3)

The colloidal dispersion of Ni/Pd bimetallic alloy nanoclusters were applied to catalysis of reduction of nitrobenzene deriratives to aniline deviratives (eq. 3) by hydrogenation under mild conditions [5]. Although both Ni and Pd are known as t h e c a t a l y s t s for t h e r e d u c t i o n s h o w n in eq. 3, t h e Ni/Pd bimetallic nanoclusters have higher catalytic activity t h a n both of the corresponding monometallic nanoclusters. The increasing Pd content can increase the hidride

246

(Pd-H) content, and the increasing Ni content can promote the interaction between the nanocluster catalyst surface and the substrate. Both effects can result in the maximum catalytic activity at a certain intermediate composition ratio, for example at Ni/Pd = 2/3 for nitrobenzene. For reduction of para-substituted nitrobenzenes, a linear relationship is observed between the reduction rate and the LUMO energy level of the substrate, and not with the HOMO level, as shown in Fig. 2, indicating that the reaction proceeds by the nucleophilic attach of hydride to the LUMO orbital. The Ni/Pd bimetallic nanoclusters are also used as the catalysts for dehalogenation of halogenated benzenes by molecular hydrogen and hydrogenation of benzonitrile to benzylamine.

'hoiMO / CV

-031

- 0 •?

- 0 ?•>

-O ?R

n.?T

-O.M

-0.13

-0.12

-on

-0.1

(V7fi

-0.09

'•|iiino/eV

Fig. 2. Relationship between LUNG energy Ei^^^ ( • ) or HOMO energy £homo (O) and the catalytic activity r of nitrobenzene derivatives catalyzed by Ni/Pd(l/4) nanoclusters.

4. CONCLUSION Although the ions of light transition metals like Cu, Ni, Fe are more difficult to be reduced to form metal nanoclusters, bimetallic alloy nanoclusters of light transition metals and noble metals have been successfully prepared by using ethylene glycol as alcohol with high boiling point and metal hydroxide colloids as metal precursors. The resulting bimetallic alloy nanoclusters are enough stable under practical conditions and work as more active catalyst than the corresponding monometallic nanoclusters. They are also interesting from the viewpoint of magnetic properties, which will be published elsewhere. REFERENCES 1. (a) V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, Nature, 370 (1994) 354. (b) R. P. Andres, et al., Chem. Eng. News,. Nov. 18 (1992). (c) N. Toshima and T. Yonezawa, New J. Chem., 22 (1998) 1179. 2. (a) N. Toshima and Y. Wang, Langmuir, 10 (1994) 4574. (b) Y. Wang, H. Lin, and N. Toshima, J. Phys. Chem., 100 (1996) 19533. 3. (a) S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science, 287 (2000) 1989. (b) M. Jacoby, Chem. Eng. News. June 12 (2000) 37. 4. K. Asakura, C.-R. Biam, P. Lu, and N. Toshima, 78"^ National Spring Meeting of Chem. Soc. Jpn., March 2000, Funabashi, 3H308. 5. (a) P. Lu, T. Teranishi, K. Asakura, M. Miyake, and N. Toshima, J. Phys. Chem. B, 103 (1998) 9673. (b) P. Lu and N. Toshima, Bull. Chem. Soc. Jpn., 73 (2000) 751.

Studies 111 Surface Science and Catalysis 132 Y Iwasawa, N. Oyama and H. Kunieda (Editors) ^c: 2001 Elsevier Science B.V. All rights reserved.

247

Novel Nanosize Borosilicate Colloid: Synthesis, Characterization, and Application Josepha M. Fu^, Alessandra Gerli*', Bruce A. Keiser^, and Andrei Zelenev^ 'Nalco Chemical Co., One Nalco Center, Naperville, IL 60563-1198 USA Valco Europe, P.O. Box 627, 2300 AP Leiden, The Netherlands The synthesis of stable amorphous nanosize borosilicate particles dispersed in water is reported. The formation of B-O-Si bonds is established by ^^B NMR spectroscopy and examined by dialysis and chelation. 1. INTRODUCTION Borosilicates are commonly found as commercial glasses. Crystalline borosilicates also have important industrial application as catalysts and adsorbents [1]. Typically, crystalline borosilicates and glasses are prepared by heating mixtures of suitable silicon and boron compounds above 150°C. Her [2] pointed out that the B-O-Si bonds are hydrolytically unstable and can form only upon dehydration at high temperature. Recently, investigators have turned to sol-gel techniques [3,4] or concentrated aqueous tetrapropylammonium hydroxide solutions to study B-O-Si bond formation [5]. Borosilicate formation was observed only in the beginning of the sol-gel process and disappeared within hours [3,4]. Mortlock, et al [5] observed borosilicate only at temperatures below 10°C. In the present work we report the aqueous synthesis of stable amorphous nanosize borosilicate (ANBS) above 10°C and without high temperature treatment. 2. EXPERIMENTAL 2.1. Sample Preparation The amorphous nanosize borosilicates, ANBS, were prepared as outlined in reference [6]. 50mL of a 0.025M solution of sodium tetraborate decahydrate was combined with 18.3mL of O.IM NaOH. A "silicic acid" solution having a density of 1.032g/mL at 25°C was made via standard methods [6]. This solution was added to the borax/NaOH solution at room temperature. The resulting mixture was post-treated at 40°C for an additional hour. The boron to silicon molar ratio was 0.04 (0.04 ANBS) and confirmed by elemental analysis using Inductively Coupled Argon Plasma Spectroscopy (ICP). A sample of colloidal silica, Nalco 8671® was obtained from Nalco Chemical Company, Naperville, IL, 60563, USA. 2.2. Separation of dissolved boron species from ANBS A 0.04 ANBS sample was repeatedly dialyzed against distilled water using a Centriprep" concentrator with a 3000 Dalton cut-off membrane. In this manner, the soluble borates (i.e., <3000 Daltons) were separated from the solid phase. Dialysis was continued until no soluble borate was detected by '^B NMR in the low or high molecular weight (MW) fractions. 2.3. Characterization of colloidal borosilicate A Coulter N4 Plus instrument was used to measure the hydrodynamic diameter of ANBS samples at 25°C by Dynamic Light Scattering (DLS) using a 5 minute data collection time at a scattering angle of 90°.

248

20

El6 + a IE

4+ ^ o4 3 I I I I I I I I I I I I I I I I I I I I I I I I [ I I t I I I I I I I

15

10

5

0

-5

ppm

Fig. 1. " B N M R spectrum of 0.04ANBS.

Peak A, Fig. 1 Group B, Fig. 1 Borate Solution How,etal[10] -h 5

QO O • I ' • I I I I I

7 9 Solution pH

11

Fig. 2. The ^^B NMR chemical shifts as a ftinction of solution pH.

A sample of 0.04 ANBS was air-dried. The resulting powder was examined by X-ray diffraction (XRD) using a Phillips APD 3720 instrument. The ^^B NMR (59 MHz) NMR spectra were obtained with a Varian INOVA spectrometer equipped with a modified 10mm broadband probe. PTFE-FEP tubes were used. ^ B chemical shifts are reported relative to boron trifluoride diethyl etherate. 2.4. Boron chelation with catechol All samples were prepared in a glove box under a continuous flow of nitrogen. Catechol (Alfa Aesar) was dissolved in DI water. Catechol solution was added to ANBS to yield a 3:1 molar ratio with respect to total boron. The resulting mixture was adjusted to pH 9.5 with NaOH as needed. 2.5. ANBS Performance in papermaking A synthetic acid paper furnish was prepared from hardwood (HWK) and softwood (SWK) kraft drylap pulps as described previously [7]. Additionally, cationic starch, HI-CAT® 1164 (Roquette) was used at 5 kg/t along with 0.5 kg/t of cationic flocculant, a 10 mol % copolymer of acrylamide and trimethyl-(3-methacrylamido-propyl)-ammonium chloride. All retention chemicals were added on the basis of furnish solids. The performance of ANBS as a retention aid was evaluated by Focused Beam Reflectance Measurement (FBRM) technique as described previously [7-9] using a Lasentec® Model M500 instrument. 3. RESULTS AND DISCUSSION 3.1. Characterization and properties of amorphous nanosize borosilicate (ANBS) The 0.04 ANBS had an average hydrodynamic diameter by DLS was 25 nm, confirming its colloidal nature. The analysis of 0.04 ANBS powder by XRD revealed only a broad peak at 20 «20^, characteristic of an amorphous material.

249

12.733

15

0

ppm

10 0 -10

Fig. 3. The " B N M R spectrum of "high"(left) and "low" (right) MW fractions of 0.04 ANBS.

I IIIIIIIIIIIII IIIIIIIIIII IIIIIIIIIII

15 10 5

0

-5

ppm

Fig. 4. ^^B NMR spectrum of 0.04 ANBS in the absence (a) and presence (b) of catechol atpH9.5.

The colloidal borosilicate was further characterized by ^^B NMR spectroscopy. The ^^B NMR spectrum of ANBS, Fig. 1, contains two features: a large peak at ~5 ppm, and a group of two smaller, overlapping peaks in the region from -1 to -4 ppm. The ^'B NMR spectra of ANBS were recorded at several pH values (Fig. 2). The chemical shift of "Peak A" in Fig. 1, exhibited a solution pH dependence (Fig.2) similar to that of aqueous borates [10], leading to its assignment as soluble borate. The chemical shift of the peaks around -2 ppm, did not vary with pH. Their chemical shifts are consistent with B(OSi) and B(0Si)2 bonds [3-4,11]. In the ' ' B NMR spectrum of the low MW solutes obtained in the first cycle of dialysis (in Fig. 3, b), only soluble borate is observed. In the spectrum of the high MW fraction obtained during the same cycle, both soluble borate and borosilicate species are observed. However, the amount of soluble borate relative to borosilicate decreased as compared to ANBS, labeled "a" in Fig. 3. Each successive cycle of dialysis continued to remove soluble borate from the high MW fraction until none was detected in either the high or low molecular weight fractions. At this point, the B-O-Si peak, associated with the high molecular weight fraction, can still be observed. Therefore, stable B-O-Si bonds were formed in our synthetic process. Had the B-O-Si bond in ANBS been hydrolytically unstable, as suggested by Her [2] and others [3-5], the dialysis would have led to dissociation, resulting in soluble borate formation. This soluble borate would then have been removed by dialysis, leading to the disappearance of all borosilicate. Additionally, the high MW fraction was lyophilized and analyzed by Secondary Ion Mass Spectroscopy (SIMS). Both boron and silicon were found. These findings indicate that the solid phase remaining after dialysis is borosiHcate. Catechol is known to form strong complexes with soluble borates [11-13]. The spectrum of a mixture of catechol and 0.04 ANBS at a 3:1 molar ratio of catechol to boron is shown in Fig. 4. The soluble borate peak at ~5 ppm in the spectrum of ANBS has disappeared, and

250

peaks at 7.29 and 12.73 ppm, characteristic of catechol-B and (catechol)2-B complexes appeared instead [13]. Importantly, the peak at around -2.6 ppm, i.e., the borosilicate peak, remains. This confirms that the B-O-Si 60 bonds are not in equilibrium with D 0.04 ANBS soluble borate as suggested by Mortlock 50 o Colloidal Silica et al. [5], but rather are an integral part R' = 0.91 40 D of the solid phase of ANBS. U

30

3.2. Performance of borosilicate as a retention aid in papermaking

20

R =1.00

10 0 0.4

0.8 1.2 1.6 2.0 Particle Dose (kg/t)

Fig. 5. Papermaking performance of 0.04 ANBS and colloidal silica in synthetic acid furnish.

It is well known that the efficiency of colloidal silica as a papermaking retention aid decreases significantly under acidic conditions [14]. An example of the unique performance of colloidal borosilicate in "acid" furnish is presented in Fig. 5. Clearly, 0.04 ANBS is more effective at promoting retention than colloidal silica [6-9].

4. CONCLUSION An aqueous, low temperature synthesis of amorphous nanosize borosilicate has been presented. The analysis by ^'B N M R has demonstrated that the colloidal dispersed phase contains B-O-Si bonds. Further, colloidal borosilicates prepared by the described process have an increased activity in papermaking. REFERENCES 1. M.R. Klotz, S.R. Ely, Method of Preparing a Metal-Cation-Deficient Crystalline Borosilicate, US Patent No. 4 285 919 (1981). 2. R.K. Her, The Chemistry of Silica, John Wiley & Sons, New York, 1979, pi 90, 410. 3. A.D. Irwin, J.S. Holmgren, T.W. Zerda, and J. Jonas, J.Non-Cryst. Solids, 89 (1987) 191. 4. A.D. Irwin, J.S. Holmgren, and J. Jonas, J.Non-Cryst. Solids, 101 (1988) 249. 5. R.F. Mortlock, A.T. Bell, and C.J. Radke, J. Phys. Chem., 95 (1991) 372. 6. B.A. Keiser, and J.E. Whitten, Colloidal Borosilicates and Their Use in the Production of Paper, WO 9916708 (1998). 7. I. Clemencon and A. Gerh, Nordic Pulp and Paper Res., 14 (1999) 23. 8. J.C. Alfano, P.W. Carter, and A Gerli, Nord. Pulp Pap. Res. J., 13 (1998) 159. 9. J.C. Alfano, P.W. Carter, and J.E. Whitten, Nord. Pulp Pap. Res. J., 25 (1999) 189. 10. M.J. How, G.R. Kennedy, and E.F. Mooney, Chem. Commun., No. 6 (1969) 267. 11. R. Pizer and L. Bobcock, Inorg. Chem., 16 (1977) 1677. 12. C. Uncuta, I. Bally, C. Draghici, F. Chiraleu, and A.T. Balaban, Rev. Roum. Chim., 29 (1984)121. 13. K. Yoshino, M. Kotaka, M. Okamoto, and H. Kakihana, Bull. Chem. Soc. Jpn., 52 (1979) 3005. 14. K. Andersson, P. Barla, and J. Yrjans, Papermaking Process, EP 0 218 674 (1985).

Studies In Surface Science and Catalysis 132 Y. Ivvasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

251

S y n t h e s i s of m o n o d i s p e r s e d magnetic particles by the gel-sol method a n d t h e i r magnetic properties H. Itoh and T. Sugimoto Institute for Advanced Materials Processing, Tohoku University Katahira 2-1-1, Aobaku, Sendai 980-8577, Japan As an application of the gel-sol method, developed by one of the authors for the synthesis of general monodisperse particles, monodispersed magnetite (Fefi^ and maghemite (y-FejOa) particles were prepared, and the magnetic properties of these particles were studied systematically, especially for their dependence on their size, shape and structure. First, monodisperse spindle-t5rpe single-crystal particles of hematite (a-Fe^Og) were prepared by aging a highly condensed suspension of P-FeOOH particles, containing a prescribed amount of P O / ions in their interiors, in the presence of hematite seed particles in an acidic medium at 140**C. The resulting hematite particles were converted into magnetite by reduction in a H2 stream, and then into maghemite by reoxidation of the magnetite in an air stream. The coercivity of the monodispersed maghemite particles thus prepared increased drastically with the increasing aspect ratio, and the maximum coercivity was obtained with particles of 4 x 10"*^- 10^ ^an^ in volume, regardless of their aspect ratio. But the coercivity at a given particle volume increased with increasing aspect ratio. It was essential for the achievement of high magnetic properties to s t a r t from single-crystal hematite and not from polycrystal ones. Magnetite particles, as the intermediate, were very close to the maghemite particles in magnetic properties and their size- and shape-dependence. 1. DJTRODUCTION It is known that the acicular maghemite particles have superb magnetic properties, so t h a t they have widely been used as a magnetic recording material in industry. For attaining the ultimate functions of this magnetic material, the particles should be as uniform as possible and perfectly controlled in size, shape and structure. While Ozaki et al. have prepared monodisperse spindle-t)rpe single-crystal hematite particles from a dilute homogeneous solution system in the presence of phosphate ions [1], and studied the magnetic properties of maghemite particles prepared from the hematite particles of some sizes and aspect ratios [2], the systematic control of these factors of monodispersed magnetic particles has never been achieved yet. In addition, many trials to prepare ideally luiiform maghemite particles have been imsuccessfvd in condensed systems, as is necessary for industrial production. Hence, the precise assessment of the magnetic properties has never been performed with monodispersed particles systematically controlled in these factors. In the meantime, one of the authors has developed a unique method, named the "gel-sol method", for the synthesis of general monodisperse particles in a highly condensed system, in which the final product is S3mthesized by a phase transformation of a highly condensed precursor gel as a reservoir of the metal ions and hydroxide ions [3,4]. The present study pertains to the synthesis of monodispersed spindle-type magnetite and maghemite particles systematically controlled in shape, size, and

252

structure, as an application of the gel-sol method, and to study the magnetic properties of those particles. Special emphasis is placed on the effects of particle shape, size and internal structure on their magnetic properties. 2. EXPERIMENTAL First, needle-like p-FeOOH particles containing phosphate ions (P04^) in each interior were prepared as follows: 90 cm^ of 6.0 mol dm*^ NaOH solution was added dropwise to 100 cm^ of well stirred 2.0 mol dm*^ F e d , solution at room temperatxu-e, and then 10 cm^ of prescribed concentration of NaEl2P04 solution was additionally added. The final concentration of phosphate ions in the gel was varied firom 0 to 3 x 10^ mol dm^ The resulting Fe(OH), gel was completely converted into needle-like B-FeOOH particles by aging at 100 **C for 6 h. Then, thoroughly washed P-FeOOH particles were used as a starting material for the preparation of hematite particles: Monodisperse spindle-type single-crystal particles of hematite were synthesized by aging a highly condensed suspension (0.5 mol dm'^) of the p-FeOOH particles, containing a prescribed amount of phosphate ions, with hematite seed particles [4] in an aqueous medium, consisting of 0.06 mol dm-^ in HCl and 0.5 mol dm^ in NaNO,, at 140*^C for 1 day. Systematic control of the aspect ratio and mean size was conducted by regulating the content of phosphate ions in the interiors of the B-FeOOH particles and the amoimt of the hematite seeds added, respectively. The resulting hematite particles were transformed into magnetite by reduction in a BL^ stream for 6 h at 330**C, and then into maghemite by reoxidation of the magnetite in an air stream for 2 h at 240**C. The original shape of the hematite particles was retained even after the transformation into magnemite as shown in Fig. 1. 3. RESULTS AND DISCUSSION Figures 2 and 3 illustrate coercivity and squareness (ratio of residual magnetization to saturation magnetization) as a function of particle volume of the maghemite particles with different aspect ratios. The magnetic properties of maghemite strongly depend on both the aspect ratio and the size of the particles. Figure 4 simunarizes the effect of aspect ratio of the maghemite particles on tneir maximum coercivity and squareness at each aspect ratio. The coercivity and the squareness increase drastically with the increasing aspect ratio, but their increase above aspect ratio 7 is rather limited. The dependence of the coercivity on the particle volume is also clearly revealed in Fig. 2. Interestingly, the maximum coercivity is located in a narrow particle volume range from 4 x 10^ to 10^^ ^m^, regardless of aspect ratio. This optimum )article volume corresponds to a particle ength, 0.36 M^m, when the aspect ratio is 5. The reason for the presence of the optimum particle volume to give the highest coercivity has been explained in terms of the increasing coercivity by the 0.25 ^m transition of ferromagnetic partides firom mametic multidomain to single-domain with decreasing particle size and the Fig. 1. TEM of a typical maghemite powder.

f

253 450

1 Aspect ratio = 7 Aspect ratio = 6 Aspect ratio = 5

400 350

0.9 0.8

Aspect ratio s 7

Aspect ratio = 4

^300^ A^)ectratio = 3

O

0.71- Aspect ratio = 6 , Aspect ratio - 5
."tS

-Aspect ratio = 4 "Aspect ratio = 3

0

^250

£ 0.5

(0

{£ 200

Aspect ratio = 2

150 h

0.3[Aspect ratio

100

0.21-

50 1x10-6

0.1

1x10-5

1x10-^

1x10*3

1x10-2

1x10"^

Particle volume (^m^)

Fig. 2. Effect of mean particle volume of the maghemite particles on their coercivity.

1x10-^

1x10-5

1x10-*

1x10-3

1x10'2

1x10*^

Particle volume (^m^)

Fig. 3. Effect of mean particle volume of the maghemite particles on their squareness, for the same samples used in Fig. 2.

lowering coercivity of single-domain particles by the increasing contribution of the superparamagnetism, as elucidated by Morrish and Yu [5]. According to their tneoretical prediction, the threshold of single domain maghemite particles has been expected ca. 0.9 ^im in length when the aspect ratio is 5. This value is considerably larger than om* corresponding result (0.36 ^m). Although there is so far no other systematic study using a variety of well-defined magnetic particles to assess the theoretical prediction, our result is much closer to those of Ozaki and Matijevic, - 0 . 4 um long at aspect ratio 5 - 6 [2]. TTie nighest coercivity obtained in this study, 412 Oe, is comparable to the value reported by Ozaki and Matijevic [2] on the monodispersed spindletype maghemite particles of - 0.4 ^un in length and aspect ratio of 5 - 6, which had been transformed from monodispersed hematite particles S3nithesized in a dilute homogeneous FeClg solution system (0.02 mol dm^). The particles of maghemite and the intermediate, magnetite, were foimd to be both polycrystalline with randomly oriented subcrystals, 3 4 5 after the redox treatments of even Aspect ratio the single-crystal hematite particles, as revealed by the OPML-XRD Fig. 4. Effect of aspect ratio of the maghemite technique [6]. Nevertheless, it was particles on their maximum coercivity and essential for the achievement of high squareness at each aspect ratio.

254

400

1 0.9

350 Prepared from singleciystal hematite

0.81-

Prepared from singleciystal hematite

300 0.7 ® 250

80.6Ioc

"1*200

Prepared from polyciystal hematite

2 0.5 (S

Prepared from polycrystal hematite

5150 0.3 100 0.2 50

1x10*

0.1

1x10"^

1x10"^

IxlO*^

1x10'^

Particle volume (^m^)

Fig. 5. Comparison of coercivity between maghemite particles prepared from singlecrystal and polycrystal hematite particles. Aspect ratio of each sample is ca. 4.

1x10*5

1x10^

Ixia^

1x10-2

1x10-'

Particle volume (^m^)

Fig. 6. Comparison of squareness between maghemite particles prepared from singlecrystal and polycrystal hematite particles, for the same samples as in Fig. 5.

magnetic properties to start from single-crystal hematite and not from polycrystal ones, as shown in Figs. 5 and 6, which illustrate the coercivities and squarenesses of the spindle-type maghemite particles prepared from the polycrystal hematite particles and from the single-crystal hematite particles. Prom the line broadening analysis of the XRD profiles of representative face indices, it was foxmd that the average size of the subcrystals of the former particles ( - 1 5 nm) were smaller t h a n those of the latter (- 25 nm), and t h a t the aspect ratio of the subcrystals of the former was somewhat smaller. Although the correlation between the size and aspect ratio of t h e subcrystals and the magnetic properties is not well luiderstood, the present results may suggest that the size and the aspect ratio of subcrystals have significant effects on the magnetic domain structure and the magnetic anisotropy, respectively. Magnetite particles, as the intermediate, were very close to the maghemite particles in magnetic properties and their size- and shape-dependence. A more detailed study will be published elsewhere. REFERENCES 1. M. Ozaki, S. Kratohvil and E. Matijevic, E., J . Colloid Interface Sci. 102 (1984) 146. 2. M. Ozaki and E. Matijevic, J. Colloid Interface Sci., 107 (1985) 199. 3. T. Sugimoto and K. Sakata, J. Colloid Interface Sci., 152 (1992) 587. 4. T. Sugimoto, Y. Wang, H. Itoh and A. Muramatsu, (I!olloids Surf. A, 134 (1998)265. 5. A. H. Morrishand S. P. Yu, J. Appl. Phys., 26 (1955) 1049. 6. T. Sugimoto, A. Muramatsu, K. Sakata and D. Shindo, J. Colloid Interface Sci., 158(1993)420.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) P 2001 Elsevier Science B.V. All rights reserved.

255

Structural change of zinc chloride hydrate melt coexisting with porous solid materials Minoru Mizuhata, Yasushi Sumihiro, Akihiko Kajinami, and Shigehito Deki Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University, 1-1 Rokko-dai, Nada, Kobe 657-8501, Japan EXAFS and Raman spectra of ZnCh nHiO melt (n = V3 - 6) coexisting with the a-AhOa fine powder were measured in order to discuss the influence of the solid phase to the ionic species in the hydrates. For the system containing ZnCh' 73H2O (n = 2.5 - 6), dehydration was promoted the coexistence of a A b O s powder by an intensification of the ionic interaction between Zn(II) and CI ions. For the system containing ZnCU ^/3H20, the structure of the highly aggregated complex was broken. These structural changes bring about the variation of the conduction of the zinc chloride hydrate melts near the porous solid materials. 1, INTRODUCTION Physicochemical properties and behavior of the hydrate melt are influenced by the solid phase in the interfacial region. In the coexisting systems containing electrolyte solution and porous solid materials, macroscopic properties of the liquid phase vary with an increase in the solid content, since the area of interface extremely increases. We have been studying that changes of the structure and properties of the electrolyte solution coexisting with solid phase [l, 2]. In this study, the structure of zinc chloride hydrates (ZnCh /3H2O; n-^U' 6) with the inorganic powder was observed by the extended X-ray absorption fine structure (EXAFS) and Raman spectroscopy. The hetero-phase effect caused by the coexisting solid phase is discussed with the results of the electrical conductivity measured by AC impedance method and thermal analysis [l]. 2. EXPERIMENTAL 2.1. Samples The ZnCl2 hydrates were prepared from anhydrous ZnCh guaranteed reagent (Nacalai Tesque Inc.) and double distillated water. The n values; i.e., ratio number of water/ZnCh was determined by chelatometric titration method. As a solid phase, a A b O a powder (Showa Denko K. K., specific surface area: 3.0-32.8 m2/g) was used. Prior to mixing with hydrates, the powder was dried at 400**C for 48 h in order to avoid a change of the hydration number. The hydrate was mixed thoroughly with the powder sample in alumina mortar. The hydrate content ranged from 10-70 vol%.

256 25, 20 15 10!

"~ 9600

9800 lObOb 10200 Photon energy/eV

Fig.l. Zn K-XAFS spectrum for a-AbOa powder (23.8m2/g) / ZnCb' I.5H2O melts coexisting systems. Melt content: 30 vol%.

Fig.2. EXAFS spectra for a-AI2O3pow der (23.8m2/g) / ZnCh I.5H2O melts coexisting systems. Solid lines; nor* malized spectra, and dashed lines; nonlinear lease squares fit spectra.

2.2. EXAFS measurement and analysis The EXAFS of Zn element was measured with the Rigaku R-EXAFS Super operating with 20 kV and 750 mA output of Mo target, using curved Ge(400) single crystal for monochromator and solid semiconductor detector (SSD) for detector, in order to find the change of the coordination structure with the coexisting of the solid phase. Measured EXAFS spectra, as shown in Fig.l, were analyzed with Rigaku EXAFS Software, "REX2". Zn-K EXAFS spectra were subtracted by extrapolating pre-edge absorbance through the energy region above the edge. The EXAFS oscillations were isolated by cubic spline-fitting of the subtracted data and normalized using a Victoreen polynomial. The Eo values were detected at 9660eV. Spectra were weighted by J^ to compensate for damping of oscillations at high k, as shown in in Fig.2. Normalized EXAFS were filtered over A-ranges from 3.6 - 1 0 A1, and fourier-transformed to produce radial structure functions (RSFs)[3]. Nonlinear least-squares methods were used to fit unknown spectra to reference EXAFS phase-shift and amplitude function. For ZnCh V3H2O crystal was used as reference sample to evaluate the phase shift. The harmonic Debye-Waller factor, CT, and the photo electron mean free path, A, among adjustable parameters were fixed at values estimated from the reference sample; 00= 0.062, cTa= 0.087, /k)= 2.8, ^ 1 = 2.8. The other adjustable parameters, coordination number, N, and bond length, /?, were alternately varied and fixed until the least-squares function was minimized. 2.3. Raman spectra Raman spectra were obtained using 100 mW Ar^ laser (514.5 nm) and monochromator U-1000 (Jobin-Yvon). We mainly measured the variation of several

257

bands [4]; Vi of [ZnClJ^ vibration mode at ca. 292 cm ^ and the band assigned to aggregated Zn species [5] at ca. 237 cm 1, and the variations of their peak positions and their integrated intensity ratio are discussed. 3. RESULTS AND DISCUSSION 3.1. Coordination number around Zn(ll) ionic species In the bulk systems of the hydrate melts, the tetrahedral structure of [ZnClJ^ complex predominantly exists. However, as shown in Fig.3, 40 60 too The coordination number ratio of O Liquid content /vol% to Cl; NolNci obtained from EXAFS measurement decreased as the liq- Fig.3. Variations of the coordination uid phase decreased for the systems number ratio NolNc\ obtained by EXAFS containing a-AhOa powder and measurement with the hquid content for ZnCl2 6H2O or ZnCl2-2H20. On the other hand, the parameter of bond a-AI2O3 powder (23.8m2/g) / ZnCh hydrate lengths, ifeno and ifenci, were melts coexisting systems. nearly constant in the overall range of the melt content. It is suggested that the dehydration was promoted by an increase of the solid content. In the system containing a-Al203 powder and ZnCl2V3H20, the coordination number increased as the liquid phase decreased. It is well-known that the coordination structure with high aggregation was stabilized in the bulk system [5]. It is suggested that the coordination structure was broken by the coexistence of the solid phase. 3.2. Structural change of the dissolved species in ZnCh hydrate. Raman spectra for a-Al203powder (3.0 m2/g) / ZnCl2V3H20 melts coexisting systems are shown in Fig.4. According to Irish's studies [4], the band at 237cm ^ is assigned to the stretching mode of the polynuclear aggregated complex Vi([ZnCl2]/7). In the Raman spectra of the systems containing ZnCh V3H2O, the intensity ratio of the band at 237cm 1 to the band at 292cm*^ decreased as the liquid content decreased. These results show that the aggregated dissolved species are no longer stable under the condition coexisting with the a-AhOs powder. According to the previous study [l], the activation energy of the electrical conductivity decreased as the liquid content decreased in the coexisting system of a-Al203 and ZnCl2V3H20. In such condition, the ionic species transfer with ease, and consequently, the activation energy of the conductivity decreased for the system containing ZnCl2 V3H2O melts. These results support the variation of the ratio of the coordination numbers obtained for the system containing ZnCl2^/3H20 by the EXAFS measurement.

258 v^azncg^) v^([ZnCy') v/ZnCI^, [ZnCyi

Wavenumber/ cm Fig.4. Raman spectra for a-AhOa powder (3.0 m2/g) / ZnCl2V3H20 melts coexisting systems. Each assignment of the Raman bands was referred from Ref. [4].

40 60 Liquid content /vo\%

Fig.5. Variations of intensity ratio of the band at 237cm^ to the band at 292cm 1 for a-AhOa powder/ ZnCh hydrate melts coexisting systems.

4. CONCLUSION EXAFS and Raman spectra of a-Al203 powder / ZnCb 72H2O melt (n = ^k - 6) coexisting systems were measured. For the system containing ZnCb 73H2O (72 = 2.5 " 6), dehydration was promoted the coexistence of aAl203 powder by an intensification of the ionic interaction between Zn(II) and CI ions. For the system containing ZnCb' V3H2O, the highly aggregated complex was diminished. The structural changes bring about the variation of the conduction of the zinc chloride hydrate melts near the porous solid materials. This study was supported by the Proposal-Based New Industry Creative Type Technology R&D Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

REFERENCES 1. S. Deki, M. Mizuhata, and A. Kajinami, Proc. of 11^^ Symp. of Molten Salt, The Electrochem. Soc, PV98-11, 1998, 513. 2. M. Mizuhata, H. Ikeda, A. Kajinami, and S. Deki, J. Mol. Liq., 83, 1999, 179. 3. E. A. Stern, "Theory of EXAFS" in "X-Ray Absorption : Principles, Applications, Techniques of Exafs, Sexafs and Xanes, Chemical Analysis, Vol 92", D. C. Koningsberger and R. Prins eds., pp. 3*51, John Wiley & Sons, 1988. 4. E. Irish, B. McCarroU, and T. F Young, J. Chem. Phys.. 39, 1963, 3436. 5. T. Yamaguchi, S. Hayashi, H. Ohtaki, J. Phys. Chem., 93, 1989, 2620.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

259

Formation conditions of integrated ordered microstructure of nano-size silica materials in laurylamine/tetraethoxysilane system Motonari Adachi, Hiroflimi Taniguchi and Makoto Harada Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan Variation in morphologies of silica microstructure was investigated under various pH conditions in laurylamine/tetraethoxysilane system. Integrated ordered structure such as hexagonal array or bicontinuous cubic phases was formed only in the narrow pH regions from 10 to 11. Liquid-liquid interface between the organic and the aqueous phases played an important role for the formation of the integrated ordered structure. 1. INTRODUCTION Formation processes of silica nanotubes at pH 4.5 were elucidated by measuring the evolution of the shape and size of the produced materials in laurylamine hydrochloride (LAHC) /tetraethoxysilane (TEGS) system [1,2]. Although we succeeded in the formation of silica-nanotubes, integrated ordered silica materials such as hexagonal array or bicontinuous cubic phase could not be produced at pH 4.5 in the above system. PH-values affect strongly the hydrolysis and condensation reaction rates of TEOS [3,4] and the dissociation equilibrium of LAHC [5] as well, because laurylamine (LA) is primary amine and has different properties in comparison with quaternary ammonium compounds such as CTAB [6]. Thus, pH-value is expected as an important factor to control the microstructure of produced materials in LAHC/TEOS system. There are some reports concerning with the formation of integrated ordered structure using air-liquid [7] or liquid-liquid interface [8,9]. Interface is also expected as an important factor. Only Schacht et al. [8] reported the mechanistic consideration about the formation of MCM41 at liquid-liquid interface. However, there is still no sufficient mechanistic understanding, which can explain the reason why MCM41 was produced. In this paper we present that various morphologies of microstructure were synthesized by pH variation in LAHC/TEOS system, and liquid-liquid interface between the organic and the aqueous phases plays an essential role for the formation of the integrated ordered structure. 2. EXPERIMENTAL We used LA/TEOS systems. Mostly, TEOS was contacted with a 0.1 M LAHC aqueous solution with various pH-values either in the stirring cell, or quietly through the still liquid-liquid interface (LAHC aq. soln./TEOS system). In some experiments, TEOS solution in LA was contacted with aqueous solutions of desired pH (TEOS in LA/aq. soln. system). Reaction temperature was kept at 313 K. The produced materials were characterized by

260

measuring SAXS, TEM, SEM, isotherm of nitrogen adsorption and ^^Si-NMR. The interfacial tension between cyclohexane and the aqueous phase containing LA was measured by Wilhelmy plate method. 3. RESULTS

AND

DISCUSSION

Variation in the ratio of the concentration of non-charged LAnon to that of cationic LA , [LAnoJ/LLA"^] at dilute conditions is given as Equation (1), [LAnon]/[LAl=10"'°^V[H"]

(1)

because pKa of LA is reported as 10.63 [5]. Partition coefficient D of LA between cyclohexane and the aqueous phases was measured under various pH conditions and expressed as Equation (2). D=200/(l+[H^]/10-^°^^)

(2)

Partition coefficient D decreased logarithmically with decreasing pH in the range pH<10, because LA^ does not dissolve into the organic phase, and LAnon decreases logarithmically with decreasing pH. On the other hand, D became constant value at pH>11.5, because almost all LA became LAnon at pH>11.5, and D was determined by the equilibrium partition of LAnon between cyclohexane and the aqueous phase. Morphologies of microstructure changed with pH-values as follows in the experiments using the stirring cell in LAHC aq. soln./TEOS system. Silica nanotubes or bundles of a few of them were formed in acidic conditions (l
r t _ witlji interface wilthout interface

tie

[dct)

Scattering a ^ k 20 IdcitJ

Figure 1 SAXS spectra of the precipitates obtained by contacting O.IM LA aqueous solution of pH 10.5 with TEOS (a) or TEOS solution of cyclohexane (b) through the still liquid-liquid interface.

Scattering angle 2 0

[deg]

Figure 2 SAXS spectra of the precipitates obtained with interface and without interface.

261

no silica materials were formed. Formation of ordered structure was tried by contacting pure TEOS or TEOS solutions of cyclohexane with the LA aqueous phase of pH 10.5 through the still liquid-liquid interface. Precipitates were formed at the interface in both cases after about 3 min from adding TEOS. The SAXS results showed hexagonal array structure for pure TEOS and gyroid-like structure for TEOS solution of cyclohexane as shown in Figure 1. These results strongly suggest that the liquid-liquid interface played an important role for producing the integrated ordered structure. Next, we examined the role of interface by checking the difference in the structure of produced materials with interface and without interface. The experiments without interface were performed as follows: LA was extracted from 0.1 M aqueous LA solution of pH 10.5 to cyclohexane. TEOS was added to the LA solution of cyclohexane. The mole ratio of water to LA in the organic phase WQ was 2.5. The mole ratio of TEOS to LA was 4. The experiments with interface were carried out by contacting pure TEOS with 0.1 M LA aqueous solution of pH 10.5 through the still liquid-liquid interface. Both SAXS spectra of the obtained precipitates are shown in Figure 2. The spectrum with interface had a peak in the meso-scale region, indicating the existence of meso-scale periodical structure. On the other hand, the spectrum without interface had no peak in the meso-scale region. This result clearly shows that liquid-liquid interface played the important role for the formation of the integrated ordered structure. When integrated ordered structure is formed at the liquid-liquid interface, similar ordered structure has to be obtained by contacting the organic phase containing both LA and TEOS with pure water or the aqueous NaOH solution of pH 10.5. Also, the contacting method, i.e., by stirring or quiet contact through the still liquid-liquid interface, does not affect the produced structure significantly. Figure 3 shows the SAXS spectra obtained by the three different conditions, a) The aqueous LA solution of pH 10.5 was contacted with pure TEOS through the still liquid-liquid interface, b) The organic phase containing both LA and TEOS was contacted with pure water by the stirring condition, c) The organic phase containing both LA and TEOS was contacted with pure water through the still liquid-liquid interface. These three cases gave the almost same spectra. Further, we checked that the integrated ordered structure was not formed in the aqueous bulk phase by contacting 5 ml of TEOS solution in LA with tiny amount of water, i.e., 0.64 ml. Precipitates were formed at the liquid-liquid interface and showed similar SAXS spectrum with that shown in Figure 3. This confirmed that the amount of the aqueous bulk phase does not affect the integrated ordered structure. We measured the variation in interfacial tension with time between the cyclohexane solution of LA and the aqueous solution of various pH when a constant amount of TEOS was added to the measuring system. Figure 4 presents the results. Interfacial tension decreased with time at pH 10.5, indicating that surface active substances gathered at the interface with proceed of the reactions between LA and TEOS. On the other hand, interfacial tension increased at pHs below 7, indicating that the surface active substances left the interface. This strongly suggests that the restriction of the reaction products to the two dimensional interface is essentially important for the formation of integrated ordered structure. Lastly, we tried to find out the necessary conditions for formation of integrated ordered materials. When the 0.1 M LA aqueous solution of pH<7 was contacted with pure TEOS (mole ratio Si/LA =4) through the still liquid-liquid interface, no reaction was observed for one hour half from the contact. When 0.1 M CTAB aqueous solution of pH 10.5 was contacted with pure TEOS (mole ratio Si/CTAB=4) through the still liquid-liquid interface, no

262

reaction was observed over one day. In both cases, both aqueous and organic phases and the liquid-liquid interface were transparent, and integrated ordered structure was not formed. These findings suggest that integrated ordered structure is not formed regardless the existence of the liquid-liquid interface, so far as the reaction products are not captured at the interface.

45 20000 r

40

isooo

35

.1

10000

25

g

sooo

£

30

to

20

O OCDO

OO

»o y

15 10 0

1

2

3

4

5

Scattering angle (2 0) [deg]

5 0 20

Figure 3 Comparison of SAXS spectra obtained follovsdng three conditions: a) D Aqueous LA solution was contacted v^th pure TEOS through the still liquid-liquid interface. b ) # Organic phase containing both LA and TEOS was contacted with pure water by the stirring condition, c) O Organic phase containing both LA and TEOS was contacted with pure water through the still liauid-liauid interface.

40

60

Time [min]

Figure 4 Variation m interfacial tension with time. A :pH 10.5, BipHT, 0:pH4.5, • :pH2

REFERENCES 1. M. Adachi, T. Harada and M. Harada, Langmuir, 15 (1999) 7097. 2. M. Adachi, T. Harada and M. Harada, Langmuir, 16 (2000) 2376. (posted on Web site on January 27) 3. D.W. Schaefer, Science, 243 (1989) 1023. 4. C.J. Brinker and G.W. Scherer, Sol-gel Science. The physics and chemistry of sol-gel processing. Academic Press, San-diego, 1990. 5. Kagaku Binran (Handbook of Chemistry), Maruzen, Tokyo, 1966. 6. J.S. Beck, J.C. Vartuli, W.j. Roth, M.E. Leonowicz, C.T Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J.Am.Chem.Soc., 114(1992)10834. H.Yang, N.Coobs, I.Sokolov and G.A.Ozin, Nature, (1996) 381, 589. S.Schacht, Q.Huo, LG.Voigt-Martin, G.D.Stuckey, and F.Schuth, Science, 273 (1996) 768. A.Imhof and D.J.Pine, Nature, 389 (1997) 948.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

263

Microscopic morphology and SERS activity of Ag colloidal particles M. Futamata, Y. Maruyama* and M. Ishikawa* Joint Research Center for Atom Technology (JRCAT)-National Institute for Advanced Interdisciplinary Research (NAIR), JRCAT-Angstrom Technology Partnership (ATP) * 1-1-4 Higashi, Tsukuba, 305-8562 japan 1. INTRODUCTION Extremely high sensitivity in Surface Enhanced Raman Scattering (SERS) has been reported with respect to the single molecule identification [1-4]. However, Signal processing system there are substantial uncertainties in the enhancement mechanism, although APDor Polychro.+ extensive efforts were devoted in the last CCD two decades. Two distinct mechanism were suggested by various experimental Raman Signal and theoretical evidences: (1) Electromagnetic enhancement based on ]• the Localized Surface Plasmon Polariton Diode laser Position sensor X=670 nm Probe ; Sample (LSPP) of metal, and (2) Charge Transfer (CT) mechanism, which arises from the electronic interaction between metal and Light source adsorbates at the specific (active) site. Ar"*" laser Accordingly, both of these depend on a Xe lamp microscopic morphology (size, shape or Scanner tube ^'^ggr^g^tio" state) of a metal surface. Fig. 1 ATR-SNOM-Raman set up However, most of the research adopted macroscopic samples, and thereby obtained the population-averaged enhancement. Therefore, we have studied to elucidate the relation between surface morphology of Ag colloidal particles and SERS enhancement factors using AFM, absorption spectroscopy and Raman microscopy |5]. Moreover, w^e have built an ATR (Attenuated Total Reflection) device (Fig. 1) for SNOM (Scanning Near-field Optical Microscopy) - Raman spectroscopy to establish an analytical tool to characterize a single molecule at an active catalysis |6-7|.

I

0-

264

2. RESULTS AND DISCUSSION 2.1 Absorption spectra of the Ag colloid The size and aggregation state substantially depend on the preparation method. AgNO, was reduced by NaBH4 to yield homogeneous isolated Ag particles with a diameter of ca. 20 nm (see Fig. 2a), while sodium citrate yields various sizes and shapes (isolated sphere or ellipsoid, dimer, rod or aggregates). These are probably due to electrostatic repulsion between the Ag particles and relaxation by citrate. In fact, the addition of NaCl (1 mM) and cationic dye (Crystal Violet, CV) to isolated Ag colloid in the dispersed solution causes an aggregation (ca. 1 - 5 pim in scale, see Fig. 2b) similar to that in the citrate system. Accordingly, quite sharp absorption at 393 nm (50 nm in width) originates from the LSPP of isolated Ag particles shifts to longer wavelength and becomes extremely broad (almost continuous in visible region, see Fig. 3). In contrast, the broad absorption band centered at 420 nm (ca. 300 nm in width) for an inhomogeneous Ag colloid prepared by citrate is not significantly changed by the addition of dye and NaCl. U\) ^ iiiTi X r> am

(b) lOum \ lOum

Fig. 2 AFM image of Ag colloid prepared by NaBH^ method: (a) Isolated particles and (b) Aggregates 2.2 Raman spectra on Ag colloid by NaBH4 The Ag colloidal particles containing CV molecules were fixed onto a glass slide with a silane-coupler. Raman spectra of CV were obtained only from Ag aggregates with a size of ca. 1-5 ^m (Fig. 3). The aggregated colloid consists of larger Ag particles than the isolated one, which is rationahzed by the dissolution-precipitation mechanism during the incubation with NaCl and CV: (1) AgCl," "* complex ions (n= 2, 3,...) are formed by the adsorption of CI onto Ag surfaces, (2) cationic CV interacts with this negative charge to suppress the repulsion between each Ag particle, and thus (3) the Ag colloid

265

aggregates. The enhancement factor was estimated by comparing Raman spectra from a single molecule on Ag colloid with that in bulk solution. It was found to be 10-10"^ for CV. Similar enhancement factor of 10' was obtained for adenine and c\'tosine, which are DNA bases and have no electronic absorption in visible wavelength region. Thus, the enhancement factor from the Ag colloid does not depend on molecular structures nor if the molecule is under- or off-resonance with the excitation wavelength.

400

600

Wavelength (nm) 3 Extinction spectra (left) and Raman spectra (right) from Ag colloid 2.3 Raman spectra on Ag colloid by sodium citrate Similar enhancement was obtained for the aggregated particles in the citrate system. Moreover, significantly larger enhancement was obtained for lower surface coverage of adsorbates (1.5 x 10' for 10' M CV and 6.4 x 10' for 1 0 " M CV). It suggests the adsorption sites with significantly different enhancement factors. The Raman signal was detected for lower surface coverage, only when CV adsorbs at the extremely active site. Therefore, the enhancement factor is apparently much larger for lower coverage than higher one. Competitive adsorption of CV and citrate was observed for extremely diluted dye concentration <10 "^ M. Only the Raman signal from CV was obtained by addition of much concentrated NaCl (10 mM) which eliminates the residual citrate from the Ag surfaces. Worth noting is that the Raman signal of CV abruptly decreases after the first scan (0.3 ^tW x 10 sec) or increases during sequential measurements only for < 10 '" M. The 'turn-on and -off (blinking) of the Raman signal suggest the existence of the extremely active site on Ag and/or photochemical instability of the adsorbates, although we must have detail experimental data to reach the conclusion. 2.4. ATR-SNOM-Raman spectra from CuPc Our ATR-SNOM-Raman system possesses various virtues such as (1) high-sensitivit>^ utilizing LSPP, (2) fluorescence and Raman signal from the optical fiber are negligible under ATR condition, (3) elimination of Rayleigh light is feasible, and (4) much stronger

266

excitation source is relevant because the metal coating is unnecessary. These are in contrast to the conventional SNOM with an illumination mode, in which the excitation light is incident through the optical fiber coated by thin metal film [8]. We have confirmed that the topography and corresponding SNOM image of the Au films prepared by a sputtering method are simultaneously obtained using the ATR-SNOMRaman system. The spatial resolution is ca. 20 nm, which corresponds to the diameter of the optical fiber prepared by the chemical etching |8|. With using the white light source, the extinction spectra of CuPc on the prism substrate were obtained, which agree well with the spectra obtained with a conventional to ID spectrometer. We have 0.9 1900 1500 1100 obtained SNOM-Raman Raman shift ( cm ' ' ) spectra from CuPc (copper Fig. 4 ATR-SNOM-Raman spectra from CuPc on phthalocyanine) on the ATR the prism prism without any resonance effects (Fig. 4). Moreover, the Raman scattering intensity was enhanced by a factor of ca. 100 with using the LSPP on Ag island films [9]. 3, REFERENCES 1. K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari and M. S. Feld, Chem. Rev., 99 (1999) 2957. 2. I. T. Krug, G. D. Wang, S. R. Emory and S. Nie, J. Am. Chem. Soc, 121 (1999) 9208. 3. H. Xu, E. ]. Bjerneld, M. Kail and L. Borjesson, Phys. Rev. Lett., 83 (1999) 4357. 4. A. M. Michaels, et al. ]. Phys. Chem., 121 (1999) 9932. 5. Y. Maruyama, M. Ishikawa, M. Futamata, in preparation. 6. S. Takahashi, M. Futamata and I. Kojima, ]. Microsc, 194 (1999) 519. 7. M. Futamata, Chem. Phys. Lett. 317 (2000) 304, and Appl. Opt., 36 (1997) 364. 8. M. Ohtsu (Ed.), Near-Field Nano/Atom Optics and Technology (Springer, 1998), Chap. 1. 9. M. Futamata and A. Bruckbauer, in preparation.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

267

E£Eect of polyvinylpyrrolidone on the physical properties of titanium dioxide suspensions Tatsuo Sato*, Shigeru Kohnosu Monsanto Agricultural Research Station for Asia Pacific 1-5-1 Arakawaoki-ffigashi Tsuchiura, Ibaraki 300-1171, JAPAN The effect of polyvinylpyrrolidone (PVP) on the physical properties of aqueous and non-aqueous (methanol-based) TiOg suspension was studied by measuring PVP adsorption and the rheological properties and dispersion stabiUty of the suspensions. The amount of PVP adsorbed onto TiOg from the methanol solution is much larger than that firom the aqueous solution and the effect on the dispersion StabiUty of the suspension is much greater in the methanol-based suspension than in the aqueous suspension. The amount of adsorption of PVP is independent of the molecular weight, while the thickness of the adsorbed layer increases as the molecular weight increases. As the PVP concentration increases, the viscosity of the PVP solution increases, while the relative viscosity and the dynamic rheological parameters(G', G") of the suspension decrease, indicating that the network structure formed by dispersed TiOg particles in the absence of PVP diminishes with the increase in the PVP concentration. The dispersion stabiUty of the methanol-based suspension increases with the increase in the concentration of PVP of high molecular weight, while the dispersion stabiUty of the aqueous suspension is not significantly improved by PVP. It appears firom the conformation of the adsorbed PVP molecules, determined by the measurement of the amount adsorbed and the thickness of the adsorbed layer, that the stabiUzation of the methanol-based suspension by PVP is due to entropic repulsion caused by the interaction of protruded tails of the adsorbed PVP molecules. It also appears firom the measurement of the adsorption isotherm and the rheological properties that the increase in the dispersion stabiUty with PVP at high PVP concentration is due to the increase in the concentration of non-adsorbingfireePVP molecules.

Corresponding author.

268

1. EXPERIMENTAL 1.1.

Materials

1.1.1. PVP Table 1 Agrimer® supplied by ISP AjL5 MW

8 XIO^

A30

A60

A 90

58 XIO^

400 XIO^

1300 XIO^

1.1.2. TLO2 Rutile type TiOg supplied by Ishihara Industry Co., Tokyo. Surface area : 8 m / g Average diameter : 0.21 // m Specific gravity : 4.2 1.2.

Procedures

1.2.1. Determination of adsorption of PVP onto TiOs. The amount of adsorption was calculated from measurements of the differences in concentrations of the polymer solutions before and after preparation of the suspension. The PVP concentration was determined by measuring the intensity of the adsorption band at 212 nm by UV spectrophotometer. 1.2.2. Determination of the adsorbed layer thickness. The hydrod5niamic thickness of the adsorbed layer was determined by measuring the viscosity of the suspensions by an Ubbelobde viscometer. The thickness was calculated from the Guth-Gould equation. 1.2.3. Kheological measurement. Rheological parameters were measured by using Rheometrics Spectrophotometer RFS 11 equipped with Couette geometry.

Fluid

269

2. RESULTS AND DISCUSSIONS 2.1. Adflorptioii of PVP onto IIO2 and the thicknesa ci adsorbed layers. The following results were obtained. i ) The amount of adsorption is independent of the molecular weight of PVP as shown in Fig. 1. ii) The adsorption of PVP from the methanol solution is much greeter than that from the aqueous solution as shown in Fig. 1.

E C 0 •H 4-1

Ck M 0 CO V

< >

iao

0

Afi

1

AQ

I

2

J-

_L

3

4

E q u i l i b r i u m PVP C o n c e n t r a t i o n ,

5 %wt

Fig. 1. Adsorption isotherms of PVP of various molecular weights from aqueous and methanol solutions at 25 °C: • , A 15-methanol; A, A 30-methanol; • , A 60-methanol; • , A 90-methanol; O, A 15-water; A, A 30-water.

2.2. Rheological properties As the PVP concentration increases, the viscosity of the methanol and the aqueous solutions increases as shown in Fig. 2, while the viscosity (or relative viscosity) of TiOg suspensions decreases markedly as shown in Fig. 3.

270

0

5

10

15

20

PVP Concentration, %wt Fig.2. Change in Newtonian viscosity of Aqueous and methanol solution with PVP concentration at25*C: 9 ^ 15-methanol; A^SO-methanol; 0 ^ 1 5 - w a t e r ; A^30-water.

2

4

6

8

10

PVP Concentration, %wt Fig.3. Change in relative viscosity of TiOj suspensions with PVP concentration at 25*^ # ^ 15-methanol; • ^ 30-methanol; 0 , A 15-water, A.A 30-water.

2.3. Dispersion stability The stability of the methanol-based suspensions increases markedly as the PVP concentration increases, while the stabiUty of the aqueous suspensions in not significantly influenced by PVP. It appears that the stabilization of the methanol-based suspensions by PVP is due to entropic repulsion caused by the interaction of protruded tails of the adsorbed PVP molecules that the increases in the dispersion stabiUty at high PVP concentration is due to the increase in the concentration of non-adsorbing fi'ee PVP molecules.

REFERENCES 1. T. Sato, S. Kohnosu, CoUoids Surf A, 88 (1994) 197. 2. T. Sato, A. Sato, T. Arai, Colloids Surf. A, 142 (1998) 117. 3. T. Sato and R. Ruch, StabiUzation of Colloidal Dispersions by Polymer Adsorption, Surfactant Science Series Vol.9, Marcel Dekker, N.Y., 1980.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

271

Polymer network formation in the pavement using SBR latex modifled asphalt emulsions Koichi Takamura^ and Walter Heckmann^ 'BASF Corporation, 11501 Steele Creek Road, Charlotte, NC, 28273, USA BASF AG, Polymer Physics Department, D-67056, Ludwigshafen, Germany SBR latex particles in the polymer modified emulsion remain in the aqueous phase and spontaneously transform into a continuous microscopic polymer film surrounding asphalt particles upon curing. In the microsurfacing mix, honeycomb structures of flexible Portland cement-polymer complex are present among aggregates and within entire pavement. The latex film formation provides early strength development and rut filling capability. The honeycombs made only with Portland cement would be very brittle and this is the case when the polymer is present in the asphalt phase. Only 3% SBR latex in the asphalt is needed to provide excellent improvement in the rutting resistance, demonstrating advantages of fine polymer networks in the asphalt. 1. INTRODUCTION Asphalt pavements become too soft under strong sunlight on hot summer days but very brittle during cold nights in winter. Heavy traffic on the soft asphalt paving causes permanent deformation to the pavement, known as '^rutting". Cracking of the pavement develops during winter months when the asphalt binder becomes too brittle. This means the asphalt binder works well only within its application window, within which it is visco-elastic enough to dissipate stress exerted by the traffic. Polymer modification extends this application window, mostly increasing visco-elasticity at high temperature. The modified asphalt also has better fatigue resistance and extends pavement lifetime, e.g. 10 years vs. 15 years with and without modification, respectively. There are two major technologies of constructing the asphalt pavement: '*Hot Mix'* and "Cold Paving". For the hot mix process, aggregates are heated to above 200°C to remove residual water and mixed with molten asphalt at >165°C. Heating of the aggregate accounts for nearly 90% of the total energy usage for this hot mix process. The cold paving uses the asphalt emulsion, which contains 65-75% asphalt dispersed in water. Cold aggregates can be used, thus significantly reducing the energy consumption. Optical and scanning electron microscope techniques were applied to understand the curing mechanism of the SBR latex modified, cationic asphalt emulsion. The study demonstrates that the modified asphalt emulsion with the SBR latex is not just an emulsion of the polymer-modified asphalt, but rather the emulsion containing dispersed latex particles.

272

Latex particles remain in the aqueous phase and spontaneously transform into a continuous microscopic polymer film surrounding asphalt particles upon curing. This explains the excellent rut filling capability of the pavement constructed with the SBR latex modified asphalt emulsion. 2 MICROSURFACING FORMULATION The microsurfacing for the preventive maintenance is gaining popularity because of its versatility and performance. The same formulation can be used for the rut filling and thin surface dressing of
273

honeycombs made only with Portland cement would be very brittle and this would be the case when the polymer is present in the asphalt phase. 2.2. Mechanism of the Polymer Honeycomb Formation SEM photographs shown in Fig. 1 indicate the modified asphalt with the SBR latex is not just an emulsion of the polymer modified asphalt, but rather the asphalt emulsion containing dispersed latex particles in the aqueous phase, as schematically shown in Fig. 2. Menisci of water containing latex and Ijitrx MiHUfted Kmuhittn C'urwJ A,sphi*lt K«nubii«n cement particles form among asphalt particles when water starts to evaporate from the Figure 2, Ixfr. Schematic illusCiaiiun of the iaiex iTKxJincd emulsion showing latex particles, which emulsion. The latex particles are partially remuiti in the aqueous phase. Right. I^tex panicles destabilized during this process due to the transform to a continuous jx^jyrner lllm surrounditig increased electrolyte concentration in the asphalt pttfticles, which cures to torm the aqueous phase by evaporation and reduced honeycomb iiiructure surfactant concentration by adsorption to the aggregate surface. The latex particles spontaneously transform to a continuous polymer film surrounding asphalt particles. The SBR latex for the asphalt modification is designed to create the polymer film without coagulum formation; , ; promoting early strength development. Majorities of latex particles migrate together with water and present in the I menisci, thus act as "spot welding" of asphalt particles to | ensure the maximum binding power*^' as shown in the right | side of Fig. 2. It is important to realize that the asphalt ! emulsion should not break (coalesce) during this process to ' i form the finest honeycomb structure. The polymer honeycomb formation in the cured asphalt ...^•.,,^ binder was successfully confirmed by optical microscopy. 'lOMIIi; Here, the asphalt emulsion modified with BUTONAL® NS198 was mixed with Portland cement to create an Fi^. 3 A conforcal Laser scanning asphalt binder for the microsurfacing. The emulsion does photomicrograph o( i\\c dried not break, but rather becomes viscous after 1-2 minutes of nucrosurtacing binder (asphah mixing, resembling the mixing (open) time of the entire emulsion-cement mixture) demonstrating the polymer microsurfacing formulation. networks sunounding asphalt The cured specimen was fractured and the freshly particles. exposed surface imaged with the fluorescence contrast method using a Confocal Laser Scan Microscope (Leica TCS4D). Fig. 3 is a composite image of 32 thin sections illustrating approximately 50|im deep. Polymer networks surrounding asphalt particles are clearly seen. The presence of a few 20-50|im diameter asphalt particles indicates some coalescence caused by the cement addition.

274

2.3. Residue Characterization (a> lb) The SuperPave dynamic shear [ «-—^ rheometry'"^' was applied to determine the rheological properties of the emulsion ^ : ; 1 residue. Although the drying rate is slower f .<* ^with cement addition, the mechanical * ^.^^ . . 4-' -..-c strength develops quickly enough for the DSR measurement within a few hours. The rutting resistance temperature, Tr (temperature at G*/sin(5)=lkPa, where G* is the complex modulus and 8 is the phase Fig. 4 Ihe rutting resistance tein|xjrarure oi^ the angle) was determined as a function of the microsurtacing emulsion, emulsion plus cement and curing time for the emulsion only, the emulsion, cement and }^c SBR L^tex. Two PG emulsion plus cement, and emulsion, grades improvement can be observed with the polymercement system, which maintains elasticity of the cement and 3% SBR latex to the asphalt. residue as seen with the low measured phase angle. A typical microsurfacing formulation consists of lOOg aggregates, 12g of 65% asphalt emulsion containing 3% latex polymer, lOg water and Ig Portland cement. The formulation used for this study is the same but without the aggregate and lOg water. As seen in Fig. 4a, Tr of the unmodified emulsion residue increased slighdy from 66°C to 68°C after one month. The sample with Portland cement shows gradual increase in Tr to 7l°C within three weeks. This increase in Tr is mostly due to stiffening of the asphalt as the phase angle of the residue increased from 82° to 88° at Tr. The value of Tr showed a rapid increase to 76°C within the first 3 days of curing when 3% of the SBR latex is also present in the mix. The phase angle at Tr remained nearly constant value of 77-78° throughout the curing, confirming the SBR modified asphalt binder maintains the elasticity. Differences in the phase angle of these three samples are summarized in Fig. 4b. \

•:<->.fc:of!.>-:?rti<-.-^:.>?;r>»»

—L—^j

'

• * ' '

^,.i*-T • "

••

'

i

•

••

8mu>«lonpnJv

I

SniutwoR * C«m«oi *3% t«Mx

Curing TJm«. day

3. CONCLUSION Bi-continuous phases of fine SBR polymer and asphalt exist in the polymer modified asphalt. More pronounced improvement in the asphalt rheology can be obtained with the finer polymer structure. Honeycomb structures of the Portland cement-SBR polymer complex surround asphalt particles and fill among aggregates cavities, providing excellent rutting resistance in the microsurfacing. 4. REFERENCES 1. D.L. Wolfe, D. Armentrout, C.B. Arends, H.M. Baker, H. Plancher and J.C. Petersen, Transportation Research Record 1096, National Research Council, Washington, DC. 2. J.G. Sheehan, K. Takamura, H.T. Davis and L.E. Scriven, TAPPl Journal, 76, 93-101, 1993. 3. ASTM Method D 244 and AASHTO Test Method T 59, A Basic Asphalt Emulsion Manual, Asphalt Institute Manual Series No. 19. 4. Background of SUPERPAVE ASPHALT BINDER TEST METHODS, FHWA-SA-94069, U.S. Department of Transportation, Federal Highway Administration, July 199.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors)
275

Conformational changes induced by competitive adsorption in mixed interfacial layers of uncharged polymers Ferenc Csempesz^ and Katalin F.-Csaki Department of Colloid Chemistry, Eotvos Lorand University, Budapest* H-1518 Budapest 112, P.O. Box: 32, Hungary ABSTRACT The interfacial behaviour of nonionic polymers on negatively charged polystyrene latex and silver iodide sol, respectively, and the structure of mixed polymer layers formed in simultaneous competitive adsorption from ternary polymer solutions at particle/solution interfaces were investigated. Preferential adsorption parameters for the polymers in pairs have been determined and used as a measure of the affinity for surface sites of the competing macromolecules. The hydrodynamic and the electrophoretic thickness of the polymer layers formed on the particle surfaces have been measured by photon correlation spectroscopy (PCS) and laser Doppler-electrophoresis (LDE), respectively. Correlation was found between the spatial properties of the composite polymer layers and the adsorption preference of the competing polymers. At partial surface coverages, irregularly extended interfacial layers formed in which the molecules of the less preferred polymer adopted a non-equilibrium conformation over wide time. At high surface coverages, the preferentially adsorbed species slowly displaced the molecules of the weakly attached polymer from the interfaces. 1. INTRODUCTION For controlling the stability/flocculation behaviour of colloidal dispersions by uncharged polymers, the extent of adsorption and the state of chains at the interfaces have alike a marked effect on how the macromolecular substances act. The preferred polymer chains should well anchored to the particle surface and should also give a thick adsorption layer extending into the dispersion medium [1,2]. To describe the state of chemically different chains in a composite layer, the adsorbed amount and the chain conformations should be certainly established for each macromolecular adsorbate. Information is required on the preferential affinity of the macromolecules competing for particle surfaces and also, on the displacement processes that may take place between different polymer molecules [3,4]. In this paper we are going to reveal typical alterations caused in the structure of composite polymer layers by the competitive adsorption from ternary solutions of chemically different polymers at particle/solution interfaces. The hydrodynamic and the electrophoretic thickness of the polymer layers formed from methylcellulose (MC), poly(vinyl alcohol) (PVA) and from 1:1 (w/w) MC-PVA mixtures in individual adsorption and in simultaneous competitive adsorption, respectively, on negatively charged polystyrene (PS) and silver iodide (Agl) particles have been measured. In addition, the preferential adsorption parameters for the polymers in pairs have been determined. Keywords: polymer mixture, competitive adsorption, conformational changes, adsorbed layer thicknesses. * Corresponding author, e-mail: csf(@.ludens.elte.hu * This work was supported by the Hungarian Science Foundation and the Research and Development Division of the Ministry of Education under grants OTKA T 022923 and FKFP-0174/2000.

276

2. EXPERIMENTAL 2.1. Materials Silver iodide sol (Agl) with negatively charged particles was prepared. The samples were fairly monodisperse with a 34±2-nm volume-average mean particle size. The particle size and the polydispersity of the fresh sol were checked by dynamic light scattering measurements. The sol concentration in the size measurements was 1.26x10'^ gdm"^ [5]. Monodisperse polystyrene latex (PS) with 67±lnm particles was prepared according to the Vanderhoff method. The particles are negatively charged with a 3.2 jiCcm'^ surface charge density. The concentration of the PS in the size determinations was 1.0x10"^gdm"^ [5]. Fractionated polymer samples, prepared from commercial Tylose MH 50 methylcellulose (Hoechst A.G., Germany) and Powal 420 hydrolyzed poly(vinyl alcohol) (Kuraray Ltd., Japan), and 1:1 (w/w) MC-PVA binary mixtures were used. The degree of polymerization of the MC and the PVA is 490 and 2450, respectively. 2.2. Methods Photon correlation spectroscopy (PCS) Mean size, size distribution, and polydispersity of the particles with and without adsorbed polymer were measured at 25**C by an advanced technique of photon correlation spectroscopy (PCS) using a Malvern Zetasizer 4 apparatus (Malvern Instruments, UK) with autosizing mode and auto sample time. Determination of the hydrodynamic layer thickness (5*^) at particle/solution interfaces by dynamic light scattering is based on the fact that the adsorbed polymer layer limits the diffusion of coated particles. Adequate determination of 5^ required, therefore, the use of monodisperse and kinetically stable dispersions [5,6]. Laser Doppler-electrophoresis (LDE) For electrically charged particles moving in response to an applied electric field with and without adsorbed polymer, respectively, a correlation ftmction of laser Doppler-shift was measured with Zetasizer 4 apparatus at 25**C and the resulting frequency spectrum was translated to electrophoretic mobility and zeta potential {Q. The electrophoretic thickness (6^) of the adsorbed polymer has been evaluated at low ionic strengths, as proposed by Cohen Stuart et al. [7] using the formula: tanh (ze(74kT) = tanh (zev|/d/4kT)exp[-K {b^-d)]

(1)

where e is the unit charge, z is the charge of the counter-ions, v|/d is the Stem potential, d is the thickness of the Stem layer (about 0.4 nm), K is the Debye-Huckel parameter, k is the Boltzmann constant and Tis the absolute temperature. Preferential adsorption In simultaneous competitive adsorption from binary polymer mixtures, independent adsorption isotherms for each polymer were determined by measuring the concentration difference (before and after adsorption) in the continuous phase. To characterize the affinity for solids of chemically different macromolecules in competitive adsorption, preferential adsorption parameters (f^ for the polymers in pairs have been established as described elsewhere in detail [3,5]. For polymer / in competition with polymery, f ^ is given by: fp = ri/(ri+rj)

(2)

where Ti and Tj are the adsorbed amounts per unit surface area, corresponding to "zero" concentration in the equilibrium solution of both / andy polymers.

277

3. RESULTS AND DISCUSSION 3.1. Adsorbed layer thicknesses The effective thickness of an interfacial polymer layer may be defined and measured in different ways. Measurements of the hydrodynamic thickness by PCS of polymer-coated dispersions might be expected to reflect also to the influence of tail segments protruding far from the particles [6]. As characteristic results, the hydrodynamic thicknesses of adsorbed layers obtained for individual MC and PVA (dashed lines) and for their 1:1 (w/w) binary mixtures (solid lines) on polystyrene latex and silver iodide sol particles at various initial polymer concentrations (Cj) are shown in Figures la,b, respectively.

20-| 1

--X--MC; o Py/A

'•- MMVA(lh): - « - M W \ A ( 2 « D |

^1 I

- X - - MC; <-- P^K

\tOPsl^ (1h); - • - MOPVA (Sh)

|

-X

.--*

15-

1

"l.---—-^

X-'''

10-

5-

\J

0- f 4 - n

-

_J:

,

,

1

•

1

60

V

1

\

1

1

1

80 c, (mjxlnni^

Figs. la,b. Hydrodynamic thicknesses (5^) of individual and mixed adsorption layers of methylcellulose and poly(vinyl alcohol) on PS latex (a) and Agl sol (b) particles. The dependence of layer thicknesses on polymer concentration shows the expected trend, but the results for the MC-PVA mixture reveal some unusual effects. At low surface coverages, 5^ values obtained for the binary mixture on silver iodide sol demonstrate the existence of irregularly extended mixed layers with nearly the same thickness (Fig. lb) or somewhat thicker than that for either single polymer layers. This unusual behaviour suggest that, as a result of competition between chemically different chains for particle surfaces, certain macromolecules in the mixed layers are in an „elongated state" compared to the conformation they adopt in the individual adsorbed layers. After longer contact times, opposite trends for the spatial characteristics of the MC-PVA layers can be observed at the different interfaces. The mixed layer developed on PS particles becomes thicker, but on Agl particles the average thickness of the mixed layer decreases in time and approaches that of the individual PVA layer. The latter effects can presumably be ascribed to a slow exchange of the polymer molecules adsorbed at the particle/solution interfaces [2,4]. Similar interfacial behaviour for these polymers can be revealed from the results of electrokinetic investigations. For comparision purposes, the data of the thicker individual layer (MC) and that of the mixed layer are shown together in Table 1. Definite differences in the spatial properties between the mixed layers formed on PS and Agl particles, respectively, are demonstrated by these results as well. Moreover, the higher 8^ values obtained for the mixture at low polymer concentrations on Agl sol confirm the idea that the macromolecules in the composite adsorption layer may be constrained to adopt an extended conformation, at least over some time at partial surface coverages.

278 Table 1. Electrokinetic thicknesses (8*) of adsorbed MC and MC-PVA l:l(w/w) layers at various initial polymer concentrations on polystyrene and silver iodide particle surfaces ci PS latex^ AgfsoP 5*^(nm) 8*^(nm) (mgdm"^) MC MC-PVA MC MC-PVA 5 3 2 2 2 10 10 7 9 5 14 20 12 9 8 50 17 16 12 11 12 100 18 16 16 ^contact time:lh, K (m"^): 33 10 ^ Ka: 1,2 ^contact timeilh, K (m''): 83 10 ^ Ka: 1,7 3.2. Adsorption preference To characterize the affinity for solids of chemically different macromolecules in competitive adsorption, preferential adsorption parameters for the polymers in pairs have been established. T h e / ' ' values determined for the MC and PVA on the two dispersions with different surface properties are collected in Table 2. The higher/'^ is for a given polymer (from 0 to 1.0), the more pronounced is its adsorption preference in competitive adsorption at the solid/solution interfaces. Table 2. Preferential adsorption parameters {f^ for MC and PVA in simultaneous adsorption from binary mixture on polystyrene latex and silver iodide sol * fP Polymer Agl sol PS latex 0,41 MC 0,81 0,59 PVA 0,19 *Data are taken from Ref. 5. The / ^ values demonstrate that on PS particles the adsorption of methylcellulose molecules is preferred. On silver iodide sol, however, the PVA molecules are adsorbed preferentially with respect to the MC molecules. These results confirm the idea of displacement from the particle/solution interfaces of the weakly adsorbed polymers by the preferentially adsorbed ones and are in line with that of the hydrodynamic and the electrokinetic measurements, as well. Formation of irregularly extended mixed layers reveals that the interfacial polymer chains of different segment affinity for particle siufaces change their conformation and at low surface coverages may take on an "elongated" form. Considering the preferential affinity of the polymers, the weakly adsorbed macromolecules are presumably constrained to adopt non-equilibrium conformations in the extended mixed layers, at least over some time. REFERENCES l.Th.F. Tadros, Polymers in Colloid Systems, Th.F. Tadros (ed.), Elsevier, Amsterdam, 1986. 2.G.J. Fleer, M.A. Cohen Stuart, J.M.H.M. Scheutjens, T. Cosgrove and B. Vincent, Polymers at hiterfaces. Chapman and Hall, London, 1993. 3. F. Csempesz and S. Rohrsetzer, Colloids Surfaces, 11 (1984) 173. 4. M. Kawaguchi, Adv. Colloid Interface Sci., 32 (1990) 1. 5. F. Csempesz and K.-F. Csaki, Langmuir, 16 (2000) 5917. 6. V.S. Stenkamp and J.C. Berg, Langmuir 13 (1997) 3827. 7. G.B. Van der Beek, M.A. Cohen Stuart and G.J. Fleer, Langmuir, 5 (1989) 1180.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

279

Electrostatic potentials at metal oxide aqueous interface N. Kallay", D. Kovacevic^ and I. Kobal^ ^Laboratory of Physical Chemistry, Faculty of Science, University of Zagreb, Marulicev trg 19, 10001 Zagreb, P. O. Box 163, Croatia ''Department of Physical and Organic Chemistry, J. Stefan Institute, 1001 Ljubljana, P. O. Box 3000, Slovenia Equilibrium at metal oxide aqueous interface is discussed in terms of Surface Complexation Model. It is shown that measurements of the surface potential ^ , in the plane where interactions of potential determining ions take place, help in evaluation of interfacial equilibrium parameters. The nonlinearity (inflection) of ^(pH) function in the p.z.c. region may appear at high ionic strengths, low densities of active surface sites, and low values of protonation and deprotonation surface equilibrium constants.

1. INTRODUCTION Interaction of ionic species at a solid liquid interface results in charging of the surface and development of the electrostatic surface potential. Characterisation of the interfacial equilibria involves measurements of the surface charge density, electrokinetic properties and adsorption of counterions or specifically adsorbable species. The evaluation of parameters that determine surface equilibria is not accurate due to application of different theoretical models associated with numerous adjustable parameters. One solution to these problems is the introduction of additional experimental methods. This article shows that measurements of the ^ surface potential may help in the interpretation of the interfacial equilibria. The potential ^ could be measured by constructing special electrodes, as shown for ice [1], or by using the field effect transistors [2]. The analysis will be performed on the basis of the Surface Complexation Model [3]. 2. THEORY The concept of surface complexation takes into account interaction of ions from the bulk of the solution with specified active surface groups. Classical 2-pK model [4] assumes amphoteric surface MOH groups that undergo protonation (p) and deprotonation (d), resulting in positive and negative charged surface groups, respectively

280

MOH + H^ -^ M0H2^ ;

K^ = cxp{(P(yF/RT) ^^^^"^ ^

(1)

<3(H^)7"(MOH)

MOH -> MO" + H^

;

K^^- exp(-^//?7)^^^ ^^^^^ ^

(2)

7"(M0H)

where Kp and K^ are the corresponding thermodynamic equilibrium constants, /^denotes surface concentration, and ^ is the electrostatic potential at 0-plane affecting the state of M0H2^ and MO" surface species. Charged surface groups may bind counterions, i.e. ions of the opposite charge

M0H2^ + A"-> M0H2^A" ;

K^ = Qxp(-(p^F / RT) ^^'^^"^ '^ ^^

(3)

a(A")/"(MOH/)

MO"+C'^->MO"-C^

;

Kc = expiipoF / RT) ^^^^ "^ ^

(4)

a(C*)AMO )

A^A and Kc are equilibrium constants of association of anions A" and cations C^, the state of which is affected by electrostatic potential of P-plane ( ^ ) . Total surface concentration of active surface sites in the interfacial layer (/Jot) is the sum of all contributions. In absence of specific adsorption Tiot = /IMOH) + T^MOHz^) + /TMO") + IJUO-C) + /iMOHz^A")

(5)

Surface charge densities in the 0-plane (ao) and in the P-plane (<7p) are given by CTo = F(/(MOH20 + r(M0U2'A-) - JJMO) - r(MO~C))

(6)

Op = F{IlMO-'C) - TlMOHz'A-))

(7)

Surface charge density in the diffuse layer (od) is equal in magnitude, but different in sign, to the net density of charges bound to the surface ((JS) CTs = -Od = Ob + Op = FiflMOHzl

- r(MO-))

(8)

The potential drop between different planes in the interfacial layer can be calculated on the basis of the constant capacitance assumption C,=cTj{(f>,-(/>,)

;

C,=cTj{,-(/>,)

(9)

281 Surface charge density in the diffuse layer (oa) and potential at the onset of diffuse layer (^d) are, according to the Gouy-Chapman theory, related by CT, = -yjMT£l^sinh((p^F/2RT)

;

^, = 2RTF-' arsinh(-o-j /^MTsIJ

(10)

where Ic is the ionic strength, and € is the medium permittivity. The point of zero charge (p.z.c.) is defined through the consumption of potential determining ions from the bulk of the solution. In the case of metal oxides p.z.c. corresponds to pH at which HMOUJ") + /(M0H2'-A") = T^MO") + riMQ-C)

(11)

In the absence of specific adsorption p.z.c. corresponds to CTQ = 0. In such a case, according to Eqs. (1,2), the point of zero charge is related to protonation and deprotonation equilibrium constants by pHpzc = 0.5 1og(V^d)

(12)

In the case of negligible (/(MOH2* A') = 0, flMO'C) = 0) or symmetric (/^MOHj^A") = /|MO"C^)) counterion association at p.z.c. all interfacial potentials (^, ^ , ^d) are equal to zero so that p.z.c. corresponds to the electrokinetic isoelectric point (i.e.p.) and to the point of zero potential (p.z.p.) defined by ^ = 0. The electrokinetic (^-potential could be used as a starting point in calculation of other electrostatic potentials. The Gouy-Chapman theory provides the relationship between the potential (pd and the ^-potential (/>,=2RTF-'\n\

exp(- X^K)+ ianh{F<;/4RT) Qxp{- x^K)-i3nh{FC/4RT)

(13)

where Xe is the electrokinetic slipping plane separation [5] (- 15 A) and fc is the DebyeHiickel reciprocal length (fc= {2F^Ic/eRTf'\ 3.

RESULTS AND DISCUSSION

The effect of ionic strength and values of surface equilibrium parameters in the <Jo(pH), ^(pH) and ^d(pH) functions is examined by calculations based on the Surface Complexation Model (Eqs 1-13). (^d(pH) function is denoted by dashed line). The parameters used in calculations correspond to typical metal oxide aqueous system with low affinity of counterions towards surface association. For this reference system: ^p = 5 1 0 \ Kd = 510-'\ Ci = 1.88 F m*\ C2 --* 00, ATc = 0.01, ^d = 0.01, /, = 0.01 mol dm•^ Ttot = 1.5-10'^ mol m"^, / = 25 °C, £r = 78.54. To examine the effect of parameters on behaviour of the systems some of them were varied, while other remained the same as for the reference system. Those parameters that are changed are denoted in the figures.

282

200

Figure 1. Effect of ionic strength. Figure 1. shows the effect of ionic strength on the surface charge density (oo) and on the potentials ^ (full line) and ^a (dashed line). At the ionic strength h = 0.001 mol dm'-^ ^ was found to be practically a linear function of the pH with the slope of-42 mV being somehow lower than the Nemstian (-59 mV). However, at high ionic strength the predicted ^ function is no more linear showing the inflection in the p.z.c. region. As expected, the Ppotential function is lower than the ^, function which is more pronounced at higher ionic strength. The inflection in ^ function appears also at low ionic strength (0.01 mol dm'^) if total density of surface sites decreases (Fig. 2). 200

U. 1

V

10V,ot/molm"^:

\

0.1

loV ot/mol m'

i

100

^

N

\

0.0

^

> E

R"\

-0.1 '

1

0

\N.

0.1 x*'.

-100

1.5

1.5^

n1

-200 2

4

6

8

10

12

pH

Figure 2. Effect of density of active surface sites.

2

4

6

8

pH

10

12

283

0.2

300 Nv

logKpKd:

logKpKd:

200

0.1 100 0.0

%

1

> E

i^^^^^l

1 "^^^^

0

-100 -0.1

-0.2

1 I

' I

I

'^ ^

-200 -300

'

6

8

10

6

12

PH

8

10

12

pH

Figure 3. Effect of the Kp and A^d values at their constant ratio (constant pHpzc)The similar effect appears if the magnitudes of the protonation and deprotonation equilibrium constant are decreased, but in such a manner that their ratio, and consequently the pHpzc, remains constant (Fig. 3). In all of the above calculations the association equilibrium constants for cations and anions were kept low and equal. It may happen that cations are chemically bound to the negative surface sites, so that their binding equilibrium constants become high. According to Fig. 4 the calculated functions are significantly shifted. Three different conditions of zero charge do not coincide any more; i.e.p. is shifted to higher pH values, while p.z.c. and p.z.p. are moved to the lower pH values. These shifts may be used to deduce the specific adsorption of ionic species.

6

8

pH Figure 4. Effect of specific adsorption of counterions C^.

284

0.2

300

Su Nu N 200

0.1

100

> E

I

0

-100

-0.1

I

1

] \;/\^A

1

-200

A

-0.2

\B

-300

6

8

PH

10

12

6

8

10

12

pH

Figure 5. Different sets of equilibrium parameters leading to approximately the same cro(pH) function, but different ^(pH) and ^d(pH) functions. A: K^ = S'\0\ Kd = 510*^\ /c = 1.0 mol dm'^ B: Kp = 5•10^ K^ = 5•10'^ 4 = 0.01 mol dm"^ Other parameters are the same as for the reference system. Interesting results were obtained by interpreting the adsorption isotherm of lead species on hematite [6]. Two possibilities regarding location of adsorbed lead species were examined. The resulting ^(pH) functions were markedly different when binding of Pb species was assumed to take place in the 0-, instead of the p-plane. The aim of this article is to show how the measurements of the ^-potential may be useful in the interpretation of the surface equilibria. This conclusion is supported by the results presented in Fig. 5. The cro(pH) functions for two different sets of equilibrium parameters are practically the same; they cannot be distinguished by classical potentiometric titration. However, the difference in ^ potentials is so pronounced that these measurements can be used to distinguish between the correct and false sets of parameters obtained by fitting the cro(pH) function. REFERENCES 1. 2. 3. 4. 5. 6.

N. Kallay and D. Cakara, J. Colloid Interface Sci., in press S. Ardizzone and S. Trasatti, Adv. Colloid Interface Sci., 64 (1996) 173. N. Kallay (ed.), Interfacial Dynamics, Marcel Dekker Inc., New York, 1999. J. Lyklema, Fundamentals of Interface and Colloid Science, Academic Press, London, 1995. D. Kovacevic, N. Kallay, I. Antol, A. Pohlmeier, H. Lewandovski and H. D. Nanres, Colloids Surf 140 (1998) 261. J. A. Davis and J. O. Leckie, J. Colloid Interface Sci., 67 (1978) 90.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

285

Microgravity effects on the properties of colloidal dispersions Tsuneo Okubo and Akira Tsuchida Department of Applied Chemistry, Gifti University, Gifti 501-1193, Japan Effect of the microgravity on the physico-chemical properties of most colloidal dispersion systems has not been discussed so in detail hitherto. However, we should note that the gravitational effect is significant for the translational diffusion of colloidal particles larger than 0.1 |im and larger than 0.1 in the density difference of the particles against solvent (Ap). Peclet number, P^, which is the ratio of the time of sedimentation of a sphere in gravity against that of translational Brownian diffusion, is one of the convenient parameters to know the gravitational effect compared with the Brownian diffusion. P, = rS/D,

= (4m-'Apg) I (Sk^T)

(1)

where r is the sphere radius, S and D, the sedimentation- and translational diffusioncoefficients, g the gravitational acceleration, ^B the Boltzmann constant and T the absolute temperature, respectively. When Ap-values are taken to be 0.1 and 1, for example, P^values are evaluated to be 10*^ (r = 10 nm), 10"* (100 nm), 1 (1 ^m) and 10^ (10 ^m), and 10'^ (10 nm), 10-^ (100 nm), 10 (1 jam) and 10^ (10 ^m), respectively. We have made several microgravity experiments hitherto on the colloidal dispersion systems such as colloidal crystallization kinetics of single- and two-component spheres in aqueous suspensions [1-3], polymerization kinetics of colloidal silica spheres formed [4,5] and the rotational relaxation times of anisotropic-shaped particles of tungstic acid colloids [6,7]. 1. COLLOIDAL CRYSTALLIZATION IN MICROGRAVITY Colloidal crystallization takes place for the monodispersed colloidal particles in suspension. Many researchers have clarified that the colloidal crystals are formed by Brownian movement of colloidal size of particles resulted in the inter-particle repulsion by the principle of minimizing dead space where is not occupied by the particles. In other words, each particles form crystal-like distribution automatically with the help of Brownian movement of each particles in order to maximizing packing density [8]. When the extra repulsive interaction like electrostatic repulsion is effective among colloidal particles in addition to the repulsion forces from their Brownian movement, colloidal crystallization takes place easily even at very low particle concentrations. Most colloidal particles get negative charges on their surfaces in polar solvent like water. The ionic groups leave their counterions, and these excess charges accumulate near the surface forming an electrical double layer. The counterions in the diffuse region are distributed according to a balance between the thermal diffusion forces and the forces of electrical attraction with colloidal particles. The lattice spacing of colloidal crystal changes by the particle concentration exclusively, which supports strongly the importance of the intersphere repulsion induced by Brownian movement of particles themselves and also the electrostatic interparticle repulsive forces. Furthermore, nearest-neighbored intersphere

286 distance of the colloidal crystal observed, I^ was equal to or a bit shorter than the effective diameter of spheres including the electrical double layers, (i^^ = JQ + 2 x D, [9]. Here, D, is the thickness of the electrical double layers given by eq. 2. D, = {4;re^n/

sk^T)"'

(2)

where e is the electronic charge and ^ is the dielectric constant of the solvent, n is the concentration of "diffusible" or "free-state" cations and anions in suspension. Colloidal crystals are so soft (10*^ to 10^ Pa) and packed densely that the crystals are compressed easily even by the gravitational force. From the microscopic and reflection spectroscopic measurements of the lattice constants in the crystals of colloidal spheres and rods, static elastic moduli have been evaluated in the state of sedimentation equilibrium in normal gravity. In microgravity no compression should take place and the strictly homogeneous crystal structure should be formed, though the wall effect, /. e., most dense crystal planes orient always parallel to the cell wall, still remains. It should be mentioned here that the diffusion equilibrium measurements have been made in normal gravity for determining the elastic moduli of colloidal crystals [10]. Centrifugal compression has been discussed in order to obtain elastic information and is deeply correlated to the gravitational compression [11]. Recently we have made microgravity experiments produced by the parabolic flights of an aircraft on colloidal crystallization kinetics of colloidal silica spheres in highly deionized and diluted aqueous suspensions [1-3]. Sphere concentrations ranged from 0.0016 to 0.0045 in volume fraction. Nucleation rates decreased in microgravity. Crystal growth rates of the fee lattices decreased in microgravity (0 G) by about 25 % compared with those in normal gravity (1 G). One of the main causes for the retardation in microgravity was suggested to be elimination of the downward diffusion of spheres, which may enhance the inter-sphere collisions. No convection of the suspensions in microgravity was also or much more important than the elimination effect of the downward diffusion. Crystal growth rates of the colloidal alloys of binary mixtures of monodispersed polystyrene and/or silica spheres have been studied in microgravity using aircraft. The rates increased substantially up to ca. 1.7 folds in o ° microgravity compared with I 9 o 1 those in normal gravity as is shown in Fig. 1. We should note that the segregation effect is familiar for binary mixtures of colloid and powder science, /. e., large spheres are segregated upward and small ones downward in normal gravity. 0.5 In microgravity such segregation should disappear and the homogeneous mixing should Fig. 1. Growth rate ratios in microgravity and normal gravity as take place favorably, which a function of sphere volume fraction in [D1B76],., + [CS-91]x leads fast alloy crystallization. mixtures at 25 °C, ^,o,ai = 0.005. Large circles show the mean The substantial acceleration of each run shown by small circles. effect of microgravity in the

i

287 alloy crystallization supports again strongly that the colloidal crystallization takes place by the packing model described above, /. e., alloy structure is determined by the condition whether the maximum packing density is achieved or not for a given ratio of the sphere sizes including surrounding electrical double layers. Quite recently, we have made aircraft experiments to know the microgravity effect on the kinetics of colloidal crystallization in the presence of sodium chloride [3]. Crystal growth rates decreased especially at the low salt concentrations in microgravity, which is similar to those observed for the deionized systems. 2. SYNTHESES OF COLLOIDAL SPHERES IN A MICROGRAVITY Recently, keen attention has been paid for the chemical reactions, especially polymerization reactions in microgravity as a new technology in space. Polymerization rates of colloidal silica spheres from tetraethylorthosilicate, ammonia and water in ethanol were studied in microgravity using an aircraft [4,5]. Specific gravity of colloidal silica spheres is ca. 2.2, high enough compared with that of solvent ethanol of 0.82. Thus, the spheres formed are sedimented in normal gravity and the reaction mixtures must be agitated in most cases throughout the polymerization processes. In microgravity, on the other hand, the products do not sediment and the reaction proceeds homogeneously without stirring. Another advantage of the microgravity experiments is the substantial decrease in the convection of reaction mixtures. For example, it is observed clearly by CCD camera observation that the movement of the very small air bubbles formed by chance in the reaction mixtures stop completely in the microgravity within ±0.01 G produced by the parabolic flight of an aircraft. Surprisingly, the polymerization rates were lowered greatly when the reaction mixtures experienced the microgravity in the beginning stages in the reaction processes. It is highly plausible that decrease in the formation rate of the primary particles and also decrease in their number in microgravity influenced the polymerization (coalescence) rates. Increase in the induction times, where the primary particles are formed were also observed in microgravity. One of the main reasons for their observations is the fact that the translational Brownian movement of product spheres is free from the downward translational movement in microgravity. No convection of the reaction suspensions in microgravity will be another important factor. Microgravity effect on the formation reactions of colloidal silica spheres was much more substantial than our expectation. 3. PHYSICOCHEMICAL MICROGRAVITY

PROPERTIES

OF

COLLOIDAL

PARTICLES

IN

As was described in introduction, experimental clarification of the microgravity effect on the translational diffusion coefficient of colloidal particles is important. Recently, we have made free-fall and aircraft experiments to determine the rotational diffusion coefficients of anisotropic-shaped colloid, i.e., tungstic acid colloids in microgravity [6,7]. Peclet number of the colloids is estimated roughly to be 230, which shows the contribution of sedimentation being much larger than Brownian translational diffusion. Clearly, sedimentation of non-spherical particles is complicated by the coupling of the translational and rotational motions. The particles sediment in a direction that depends on their orientation. Brownian motion changes the orientation of the particles and hence their sedimentation speeds and directins. However, on average, the particles will move in the direction of gravity only. The rotational relaxation time (r) were evaluated from the relaxation traces of the optical transmittance of the suspension using the stopped-flow

288 technique. For four different flow directions of the sample suspension in the observation cell, rvalues were observed at 0 G and 1 G. The probing light always passed through the cell perpendicular to the largest planes of the cell. The observed rvalues at I G scattered rather significantly, which is explained by the convection of the suspension. It should be noted further that r values at 1 G were about 6 % larger than those evaluated theoretically using Perrin's equations. These rvalues agreed with those calculated at the Debye-length as 21 nm. Conductivity measurements of the colloidal suspension gave 36.7 |iS/cm (23.3 °C) at (?J= 0.20 wt%, which corresponds to 2.5 x 10"^ mol/L KCl and the Debye-length of 27 nm. Thus, agreement between the observed and calculated values of the electrical double layers is satisfactory. Clearly, the r values in microgravity were not scattered so much compared with those in normal gravity, and there was no difference between the r values observed at 1 G and 0 G. Again, the observed r values were larger than the theoretical ones due to the significant contribution of the electrical double layers. The experimental errors in r values at 0 G were smaller than those at 1 G. Furthermore, the r values in microgravity were quite insensitive to the flow directions in the cell. Very reliable values of r were observed in microgravity, which supports the fact that the lack of sedimentation of the colloidal particles and no convection of the suspension raise reliability of the experimental data significantly. In near future, studies on the microgravity effects upon the other kinds of thermodynamic properties should be exciting, since the fundamental relation, eq. 3 holds. dU=TdS-PdV^

adA^

g(h - h^)dM - {rW/2)dM + A ^ « .

(3)

where U is the internal energy. T, 5, P, V, a. A, g, h, h^, M, r, w, //, and n, are the temperature, entropy, pressure, volume, surface tension, area, acceleration of gravity, height, reference height, molecular weight, radius, angular velocity, chemical potential of /component and number of moles of /-component, respectively. The authors further notes that the experimental studies between 0 G and 1 G and above 1 G are also important to understand the microgravity effects. REFERENCES 1. T. Okubo, A. Tsuchida, T. Okuda, K. Fujitsuna, M. Ishikawa, T. Morita and T. Tada, Coll. Surf. A153 (1999) 515. 2. T. Okubo, A. Tsuchida, S. Takahashi, K. Taguchi and M. Ishikawa, Coll. Polym. Sci., 278 (2000) 202. 3. A. Tsuchida, K. Taguchi, E. Takyo, H. Yoshimi, S. Kiriyama, T Okubo and M. Ishikawa, Coll. Polym. Sci., in press. 4. T. Okubo, A. Tsuchida, H. Yoshimi and H. Maeda, 277 (1999) 474. 5. T. Okubo, K. Kobayashi, A. Kuno and A. Tsuchida, Coll. Polym. Sci., 277 (1999) 483. 6. T. Okubo, A. Tsuchida, H. Yoshimi and H. Maeda, Coll. Polym. Sci., 277 (1999) 601. 7. A. Tsuchida, H. Yoshimi, K. Ohiwa and T. Okubo, Coll. Polym. Sci., in press. 8. T. Okubo, Prog. Polym. Sci., 18 (1993) 481. 9. T. Okubo, Ace. Chem. Red., 21 (1988) 281. 10. T. Okubo, J. Chem. Phys., 102 (1995) 7721. 11. T. Okubo, J. Chem. Phys., 112 (1990) 5420.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) o 2001 Elsevier Science B.V. All rights reserved.

289

Colloidal crystal alloy structure of binary dispersions of polystyrene and poly(methylmethacrylate) latices by ultra-small-angle X-ray scattering Tamotsu Harada, Hideki Matsuoka,* Hitoshi Yamaoka^ Department of Polymer Chemistry, Kyoto University, Kyoto 606-8501, JAPAN We have performed a systematic investigation of the colloidal crystal formation with the ultra-small-angle X-ray scattering (USAXS) technique. We estimated the distribution of poly(methylmethacrylate) (PMMA) latex particles in the binary colloidal dispersion of PMMA latex and polystyrene (PS) latex dispersions by USAXS. A clear shift of the Bragg peak position was observed by changing the mixing ratio of the dispersions, which reflected the change of the colloidal alloy structure. 1. INTRODUCTION The recent investigations of the colloidal crystal structure using highly advanced apparatus have revealed novel and interesting aspects of the colloidal crystal formation. Especially, the USAXS technique has been shown to be useful [1, 2]. One of the most important merits of the USAXS technique is that the investigation of bulk structure inside the dispersion is possible, while it is hard with the microscopic technique. The other merit is that it is possible to see structures on a larger scale. USAXS has revealed the behavior of colloidal crystals, which could not be observed by the other techniques [3, 4]. For example, in a USAXS study, we observed peculiar behavior of the colloidal crystal with regard to the salt concentration [3]; The interparticle distance in colloidal crystals has a maximum value as a function of the salt concentration. This implied that novel factors might exist in the interparticle Coulomb interaction in such low ionic strength systems. However, the behavior of colloidal crystals in the binary colloidal system has not been revealed clearly, while the behavior in a one-component colloidal system has been investigated eagerly. Here, we describe the results of a USAXS investigation using a binary colloidal system. 2. EXPERIMENTAL SECTION 2.1 Sample Properties Latex dispersions used were prepared in our laboratory by the emulsion polymerization without an emulsifier. After the polymerization, ultra-filtration was performed to reduce the impurities in the dispersions. Then, ion-exchange resins (mix-bed, H* form and To whom corresponds should be addressed. ^Tel: +81-75-753-5631, Fax; +81-75-753-5609, e-mail: [email protected] Present address Department of Materials Science, University of Shiga Prefecture, Hikone City, Shiga 522-8533, JAPAN

290 OH" form) were added to reduce ionic impurities. To estimate the radii and the distributions of latex particles, we examined the dried samples using the USAXS technique were performed. We fitted the USAXS profile to the theoretical scattering profile for an isolated particle, in which the particle size distribution was taken into account. The surface charge number of the particle was estimated by the titration with NaOH. The properties of the samples are shown in Table 1. 2.2 USAXS measurement and data analysis Our USAXS system is based on the Bonse-Hart principle. This optical system has two silicon channel-cut crystals. The Bragg reflection in these crystals makes it possible to detect the scattering at very small scattering angles. The optical system is described in detail elsewhere [2]. Table 2 shows the experimental conditions for USAXS. For all scattering angles, a smoothing treatment was performed with a width of 5 points. Then, the desmearing treatment was performed. In this analysis, we assumed an infinitely thin width and infinitely long beam geometry [3,4]. Table I Properties of samples Sample; Name Sample Type MS32 PMMA latex PS 169 PS latex Table 2 USAXS experimental conditions Run No. [MS32] [vol%] 2DjAl 4.95 1 4360 4.79 2 4410 3 4.55 4490 4 4.37 4550 5 4.14 4630 6 3.25 5020 7 0

Diameter 2810 1210

Surface charge density 3.1 3.1

[SS169] [vol%] Number ratio MS32:SS 169 0 100:0 0.10 80:20 0.25 60:40 0.36 50:50 0.50 40:60 1.05 20:80 3.07 0:100

2Do: the average interparticle distance calculated from the MS32 concentration with the assumption of fee symmetry.

3. RESULTS AND DISCUSSION Figure 1 shows the USAXS profiles obtained for the mixture of PMMA latex dispersion (MS32) and PS latex dispersion (SS169) at various mixing ratios. The electron density difference between the PMMA latex and water is so large that the scattering from PMMA particles in dispersion is detectable by USAXS. On the other hand, since the electron density of PS is almost the same as that of water, the scattering from PS latex particles cannot be detected by USAXS. Clear Bragg peaks were observed in the profiles. The arrows show the position of the first Bragg peak for each measurement. The presence of Bragg peaks on

291 the scattering profiles means the existence of colloidal crystals, formed by PMMA latex particles. Under the condition of MS32:SS169 = 100:0, i.e. one-component colloidal dispersion, the relative peak position of Bragg peaks was compatible with that for the facecentered cubic (fee) lattice structure. On the other hand, the scattering profile was the same as the background scattering profile under the condition of MS32:SS169 = 0:100. When the mixing condition was changed, the Bragg peaks was clearly shifted. This indicated that the structure of the colloidal crystals formed by P M M A particles changed drastically with the change in the mixing ratio. For the estimation of the precise structure of colloidal crystals, it is necessary to estimate each interparticle structure factor S(q) separately. Here, q is the scattering vector (q = 47Csin6/A, 2^, the scattering angle, A = 1.5406A; the wavelength of X-rays). The observed intensity I(q) is the multiple of the form factor for an isolated particle Piq) and S(q) [3]. Siq) reflects the spatial distribution of the center of mass of the particles. Hence, the exact information about the colloidal crystal structure can be obtained from the analysis of S(q). Figure 2 shows S(q) for each mixing ratio. With the decrease of the fraction of MS32, Bragg peaks became broad. At the mixing ratio MS32:SS169 = 20:80, there was no clear Bragg peak. Arrows in Figure 2 show the positions of the first and the second peaks in Siq) profiles. At the mixing conditions of MS32:SS169 = 100:0, 80:20 and 60:40, the S(q) profile could be reproduced with the assumption of fee symmetry. However, it should be noted that the height ratio of the first peak and the second peak was changed at the mixing conditions of MS32:SS169 = 50:50. Since the second peaks were relatively high, these profiles were not fitted to the theoretical curves calculated with the assumption of fee symmetry. This observation indicates the possibility of the structural change of colloidal crystals in the colloidal mixture. Since this peak height cannot be explained by simple cubic and body centered cubic symmetry, a peculiar lattice structure for the alloy crystal might be formed. 25 Run No. 30 Run No. 6 (20:80) (IV1S32:SS169) <*^.«.,-*.*«*'-^^ 25 L ° \ 7(0:100) 5 (40:60) 6 (20:80) 5 (40:60) 4 (50:50)

20

[

*

^ « 14 (50:50)

^ 1 5

3 (60:40)

3 (60:40) 10

F

OtahBl/'

1

oiBDB.ap'

%«.««0?>**^°

°0

"cPo^o <«-«'>o<M 2 (80:20) -^^^ ^-^ 1 (100:0)

2 (80:20) 1

° %

0

0.001

^1«%
''OtroJ

°

\

1 (100:0)

0

0

0.001 0.002 0.003 0.004

Fig. 1. U S A A S profiles obtained for the mixture of PMMA and PS latex dispersions at various mixing ratios. To avoid the superposition, all curves were shifted upward.

0.002

0.003

Fig. 2. Interpartifcie structure factor S(q)'s at various mixing ratios. All data were shifted vertically.

292 0.0015

0

20 40 60 80 fraction of PMMA latex Fig. 3. The first peak position of S(q), Qraax^ plotted against the fraction of PMMA latex

20 40 60 80 100 fraction of PMMA latex Fig. 4. The height of the first peak position, S(q)„„, plotted against the fraction of PMMA latex

Figure 3 shows the behavior of q^^, which is the first peak position of S{q), against the fraction of PMMA latex. The maximum value was obtained at the mixing ratio MS32:SS169 = 60:40. The behavior of ^„^ also suggests the transition of the colloidal crystal structure. Figure 4 shows the behavior of the height of the first peak, S{q),^, as a function of the fraction of PMMA latex. S(q),^ decreased monotonically with the decrease of the mixing ratio of PMMA. This means that the colloidal crystals formed by PMMA latex particles were distorted by the addition of the other components. We have already started an ultra-small-angle neutron scattering (USANS) investigation of the colloidal mixture [5]. Using the USANS technique, we can obtain information about the distribution of each component in the colloidal mixture. Further USANS and USAXS investigations are needed to elucidate the behavior of colloidal alloy crystals. Acknowledgement This work was financially supported by Grants-in-Aid for Scientific Research (C10650887 and CI2450386) by the Ministry of Education, Science, Sports and Culture of Japan to whom our sincere gratitude is due.

REFERENCES 1. U. Bonse and M. Hart, Appl. Phys. Lett., 7 (1965) 238. 2. (a) H. Matsuoka, K. Kakigami, N. Ise, Y. Kobayashi, Y. Machitani, T. Kikuchi, and T. Kato, Proc. Natl. Acad. Sci. USA, 88 (1991), 6618. 3. (a) H. Matsuoka, T. Harada, and H. Yamaoka, Langmuir, 12 (1996) 5588. (b) T. Harada, H. Matsuoka, and H. Yamaoka, Colloids Surfaces A, 174 (2000) 79. 4. (a) T. Harada, T. Ikeda, H. Matsuoka, and H. Yamaoka, Langmuir, 16 (2000) 1612. (b) H. Matsuoka, T. Harada, T. Ikeda, and H. Yamaoka, J. Appl. Cryst., 33 (2000) 855. 5. H. Matsuoka, T. Ikeda, H. Yamaoka, M. Hashimoto, T. Takahashi, M. M. Agamalian, and G. D. Wignall, Langmuir, 15 (1999) 293.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

293

The role of electrokinetic properties on adhesion of nitrifying bacteria to solid surfaces Hiroshi Hayashi", Satoshi Tsuneda'*, Akira Hirata' and Hiroshi Sasaki'' "•Department of Chemical Engineering, Waseda University ^Department of Environment and Resources Engineering, Waseda University Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan. Electrokinetic properties of nitrifying bacteria of Nitrosomonas europaea and Nitrobacter winogradskyi were investigated by electrophoretic mobility measurement and analyzed by soft particle electrophoresis theory. Also bacterial adhesion onto glass beads surface was examined by packed bed method. A^. europaea had more negative and rigid surface character compared with A^. winogradskyi. Cell adhesion properties were significantly dependent on pH and critical increase of cell attachment was observed below pH 4.3 for A^. europaea, and below pH 5.0 for N. winogradskyi, respectively.

1. INTRODUCTION Much attention has been paid to control of attachment/detachment of specific bacterial cell due to increasing demand for utilizing specific microbial cell to mediate biological reactions for technological applications. A typical example in wastewater treatment technology is immobilizing nitrifying bacteria, which is difficult to retain in bioreactor because of its low growth rate and lack of production of exopolymeric saccharides to facilitate biofilm formation. In order to develop immobilizing technique of specific bacteria onto support material, it is essential to elucidate interfacial characteristics of the strain and their influence on adhesion to solid substratum. Although many works has been made to investigate bacterial adhesion in terms of DLVO theory [1], knowledge of surface properties of nitrifying bacteria has been still limited due to complexity of environments where they usually exist. The purpose of this study is to investigate electrokinetic properties of typical nitrifying bacteria of Nitrosomonas europaea and Nitrobacter winogradskyi by electrophoretic mobility measurement and reveal its relevance to adhesion onto glass beads by packed column bed method. For the interpretation of cell electrokinetics, soft particle electrophoresis theory [2] was applied to analyze surface structure peculiar to biological cell. DLVO-type interaction energy between cell - glass was calculated and compared with collision efficiency parameter OQ.

2. MATERIALS AND METHODS 2.1 Bacterial strains and electrophoretic mobility measurement Ammonia-oxidizing bacterium of Nitrosomonas europaea IFO-14298 (A^. europaea). * Corresponding author. Tel:+81-3-5286-3210; Fax:+81-3-3209-3680, E-mail:[email protected]

294 nitrite-oxidizing bacterium of Nitrobacter winogradskyi IFO-14297 {N. winogradskyi) were used in this study. A^. europaea and A^. winogradskyi were aerobically cultured at 30 ^Q in an inorganic medium containing 1.0 gdm'^of (NH4)2S04 and NaNOj. Cells at their exponential growth phase were harvested by centrifligation (10000 g, 10 min) and washed five times by 10 mM KNO3 solution (pH 6.0) and then given to the experiments. Measurement of electrophoretic mobility was carried out by electrophoretic light scattering spectrophotometer (ELS-800, Otsuka Electronics, Japan). All of the measurement was carried out in 12 hours after harvesting from culture medium for to avoid changing cell surface properties. Mechanically ground glass particulates were used for measurement of glass beads. Supporting electrolyte used in all experiments was KNO3 and droplets of HNO3 or KOH that had the same ionic strength as the prepared suspensions were added to adjust pH. Cell suspension was dispersed in ultrasonic bath for 3 min and quickly supplied to the apparatus. For soft particle analysis, plots of mobility data were fitted with the electrophoresis formula given by Ohshima and Kondo [2]: £oS, T Q / K , + y p o ^ / A ^ ez^ 11

1/K,+1/A

(1)

TiA^

where \i is electrophoretic mobility, CQ and e^ are the permittivitiy of vacuum and relative permittivity of medium, respectively, r\ is the viscosity, K^, is Debye-Huckel parameter of surface region, e is elementary charge, ZN is special charge density in the surface region, k is a parameter characterizing the resistance to liquid flow in the surface region and T^ON is Donnan potential. ^ 0 is surface potential and written as follows. -\V2^

Izn)

Izn)

(2)

Here, k is Boltzmann constant, T is absolute temperature, z and n are the valence and concentration of bulk ions, respectively. This theory assumes particles with a charged layer of finite thickness at its outer region and best fitted combination oiZN and MX is experimentally given from eq(l). 2.2 Packed column experiments and analytical procedure Ten grams of glass beads (GB) were packed at the bottom of glass-made column (inner diameter: 15 mm, height: 40mm, porosity: 0.52). Cell suspension (concentration: 5 X 10* cm'^) containing 10 mM KNO3 was introduced into the column at flow rate of 9.0 mLmin'^ (0.08 mm-sec'). Cell concentration of effluent was examined by 00^60 measurement. Cell deposition was analyzed by clean bed collision efficiency followed by Rijinaarts et al [3]: CyCo = e x p [ - 3 / 4 { ( l - e ) / a > a o ( l - B 0 > ]

(3)

where C, and CQ are cell concentrations of feed and effluent at time t, respectively, e is bed porosity, a, is collector radius, 0 is collector mass transfer efficiency, OQ is clean bed collision efficiency, B is the blocking factor influenced by attached cells and 6 is the fraction of surface covered. DLVO-type interaction energy Vj between cell and GB were calculated by

295 following equation [4]: K, =So£.a,[(%. +^o2)'log(l + e-"*)+(vFo, - 4 ^ 0 2 ) ' M l - e - * ) ] -

h

• + log

h + 2a,

(4) h-\-2a

where a^^ is cell radius, ^01 ^ ^ ^ 2 ^ ^ surface potential of cell and solid, respectively, h is separation distance, A is Hamaker constant and referred to as [5].

3. RESULTS AND DISCUSSION 3.1 Electrokinetic behavior of bacterial cell Figure 1 shows electrophoretic mobility of A^. europaea and A^. winogradskyi as a function of pH in 10 mM KNO3. Mobility of both strains as well as GB showed negative value while absolute value of A^. europaea (-2.75 ^im/sA^/cm) was larger than A^. winogradskyi (-1.48 \mdsNlQm) at neutral pH. Mobility significantly increased below pH 5. Figure 2 shows electrophoretic mobility of A^. europaea and A^. winogradskyi as a function of KNO3 concentration and the solid lines were theoretical curves fitted by eq (1). Both mobility data converged to nonzero values as the KNO3 concentration got higher, which was a typical property of soft particle and indicated that application of this model was reasonable. 0

i

-0.5

0 i-1.5 « > -2 |i-2.5 2 =i -3

1

w

10 11

Fig.l Electrophoretic mobility of bacterial cells and GB as a function of pH.

--; / ^ \j/ >

tt/ yf -3-5 I B_ .4

0

U-*-'

•

A^. w

nogri idsky\

0.05 0.1 0.15 KNO3 concentration, M

Fig.2 Electrophoretic mobility of bacterial cells as a function of KNO3 concentration.

Best fitted combinations of ZN and 1/A (pH 7.0) that represents cell surface character are listed in Table 1. It was found that A^. europaea had larger ZN and smaller MX values than A^. winogradskyi, indicating surface layer of A^. europaea had more negative charge density and rigid structure as compared with A^. winogradskyi based on soft electrophoresis analysis. 3.2 Adhesion assay and DLVO interaction energy Figure 3 shows breakthrough curves of A^. europaea (a) and A^. winogradskyi (b) and solid lines were theoretical curves fitted by equation (3). In the case of A^. europaea, little attachment was occurred in the pH range from 7.0 to 5.0, where both A^. europaea and GB had relatively high negative charge as observed in Fig.l. On the other hand, cell collection drastically increased below pH 4.3, probably due to suppression of electrostatic repulsive forces between cell and GB. Similar phenomenon was observed in attachment of A^. winogradskyi to GB, in which increment of cell collection took place below pH 5.0. Figure 4 shows relationship between cell - GB collision efficiency OQ and DLVO interaction maximum V^^ calculated by eq (4). Electric potential of glass was approximated as zeta potential by converting mobility with Smoluchowski formula and cell surface potential at pH 7.0 could be given from eq (3) using ZN in Table 1. In other pH conditions, lA. values

296 are assumed to be constant at pH ranging from 4.0 to 7.0 because pH has great influence on surface dissociation group, i.e., ZN, compared with lA., even though MX values are more or less affected by solution pH [6]. ZN values at any pH were derived from mobility data from Fig. 1 and converted to surface potentials. From Fig. 4, OQ got lower in proportion as the V ^ increased when OQ was less than 4 X 10'^ On the other hand, Oo substantially got higher with a slight decrease of V^^^ when OQ was above 4X lOl This critical change of OQ was observed when V^3^ is around 35 kT for both strains, probably indicating rapid heterocoagulation of cell - GB came to take place. (a) A^. europaea I 0.8 o

rn

53

• ^

pH4.2 pH4.3

•

pH5.0

•

pH7.0

-•— -d •

pH4.0

^

pH4.5

•

pH5.0

•

pH6.0

EI50 ^ 100

•

pH7.0

50 0

^"0.4

i 0

Table 1 Best fitted values of ZA^and \/X for A^. europaea and N. winogradskyi at pH 7.0 l//l(nm) ZN(M) Strain 0.90 -0.0531 N. europaea -0.0227 1.23 N. winogradskyi

•

5^0.6 0.2 0

pH4.0

200 400 600 800 1000

Time, s (b) N. winogradskyi 1 0.8

\tn t t | ft" • - • C3Z TJL.

y*o.6 iisM ^^0.4

• • • WTT r •

-T—

:M

•

• "A

0.2 T

0

0

200 400 600 800 1000

Time, s Fig. 3 Breakthrough curves of A^. europaea (a) and N. winogradskyi (b) at various pH in 10 mM KNO3 as a function of time.

350 300 ^250 , ;200 I-

101-2

N. europaea N. winogradsky.

Collision efficiency tto

Fig.4 Relationship between collision efficiency Oo and potential energy maximum V„^.

4. CONCLUSION Both A^. europaea and A^. winogradskyi revealed soft particle behavior by electrophoretic mobility measurement. A^. europaea showed more negative and rigid surface layer compared with A^. winogradskyi based on soft particle analysis. Electrokinetics and adhesion of cells were influenced by pH of the suspension and significant increase of cell - GB attachment was observed around V ^ , o f 35 kT for both strains.

Reference 1. M Hemiansson, Collokis Surf. B: Biointei&es, 14 (1999) 105. 2. R OishimaandT. Kondo, J. CoUokilnterfeceSci., 130(1989)281. 3. R R Nl Rijinarrts, W. Noide, E. J. Bouvv«; J. Lyklema, A. J. ZdindCT, Enviioa Sc^ 4. R. J. Hunter, Foundatiais of Colloid Science, Vol.1 (1987) Oxford, New York 5. R R NL Rijinants, W. Node, E. J. L)4dema, A. J. B. Zdmdei; Colbkls Surf B: Biointe^^ 6. R ScMxtora, N. Muramatsu, R CAishimaandT. Kondo, Bioph^^

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c; 2001 Elsevier Science B.V. All rights reserved.

297

The preparation of porous titania films via colloidal crystallization between electrodes Z.-Z. Gu,* S. Hayami," Q.-B. Meng," A. Fujishima,*' and O. Sato" *Kanagawa Academy of Science and Technology, KSP Bldg. East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan **Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8565, Japan As described in the paper, a new method for the fabrication of sandwich-like inverse opal devices was developed. Mono-dispersed particles are firstly assembled between two parallel substrates by taking advantage of capillary forces and Theological forces. Then a liquid precursor is infiltrated into the voids between the substrates. The following hydrolysis and calcination give rise to the inverse opal with interconnected voids. 1. INTRODUCTION In the past few years, interconnected porous films with three-dimensional ordered nano-meter scale features have garnered great attention, not only for their abilities to control light propagation, but also for their applications in coatings, catalysis and other emerging nanotechnologies[l-3]. A number of methods have been developed to fabricate the porous films, such as selective etching, self-assembly of block copolymers and replica molding against templates. Among these methods, replication provides an easy and efficient method for the fabrication of three-dimensional interconnected porous structures[4]. The work of our group is aimed at producing electrically tunable photonic films, in which liquid crystals and electro-chromic dyes are infiltrated into three-dimensionally ordered void regions[5]. In order to fabricate such tunable photonic crystal devices, it is necessary to make high quality inverse opal films with interconnected voids and predetermined film thickness between two parallel transparent electrodes, such as ITO-coated glass plates. In this report, we describe a new method to prepare such sandwich-like inverse opal devices. Mono-dispersed particles are firstly assembled between two parallel substrates by taking advantage of capillary forces and rheological forces. Then a liquid precursor is infiltrated into the voids between the substrates. The following hydrolysis and calcination give rise to the inverse opal with interconnected voids. This method is both simple and efficient when compared with other known methods[6]. Using this method, we have derived high quality inverse opal with a predetermined thickness. 2. FABWCATION OF TEMPLATES FROM MONO-DISPOSED SPHERES Several methods including natural sedimentation[7-9], vertical deposition[10], and

298 spacer

Infiltration

ma

'*

-4

h-jo- « V. .-T_-

Calcination

«| '0 1

.^

J

(a) (b) Fig. 1. (a) Outline of the procedure for the self-assembly of mono-dispersed spheres between substrates, (b) Procedure for the fabrication of titania porous film using an opal template. electrophoretic deposition[ll] of mono-disposed spheres have been developed to fabricate high quality opal film on substrates. However, it is still a challenge to obtain opal films with a smooth surface over a large area and pre-determined thickness[8]. It has been reported that such a sandwich-like opal device can be fabricated by using the external pressure induced by a gas flow[6]. However, a complex photolithography technique is required for the making of a porous frame and for the control of the opal film thickness. This difficulty makes it necessary to develop a much simpler method for the fabrication of the sandwich-like opal. Here, we show that high quality sandwich opal film can be fabricated using the self-assembly of the mono-dispersed particles between two parallel substrates by taking advantage of capillary forces and rheological forces. The mono-dispersed polystyrene particles used to prepare the templates were commercial products of Sekisui Chemical Company, Osaka, Japan. These samples were aqueous suspensions. All of the samples were carefully purified and the solutions were exchanged fix)m water to ethanol using an ultrafiltration cell (model 8400, membrane: XM300, Amicon Co.). Then the samples were treated with a mixed bed of cation- and ainon-exchange resions (Bio-Rad, Ag501-X8(D)) for several weeks before use. The procedure for the film fabrication is shown in Figure la. A cell for the particle assembly was constructed from two substrates (glass, Sn02-coated glass or ITO-coated glass) and silica ball spacers with diameters of 2 (im 10 ^m. The two substrates were tightened together with binder clips. When the bottom of the cell was inmiersed into the alcohol suspension, the gap between the substrates wasfilledby the particle-dispersed solution due to the capillary force. The opal film formed on the substrate in the course of the evaporation of the solution from the gap. As the evaporation from the gap was always followed by the introduction of the particle-dispersed solution from the vial due to the rheological force, the whole space was ultimately covered by the particles. The film formation was initiated at the edge of the cell and then expanded gradually to the inner space. The quality of the opal film depended on the concentration of the particles. A dilute solution was appropriate for the fabrication of a high quality film, although the growth rate decreased with a decrease in the concentration. It was found that a dense solution, with a volume fraction of over 5%, produced a disordered arrangement. In this experiment, the density was set at 0.5% by considering both the qualities of the opal film and the growth rate. Afilmwith an area of 15 nun X 20 nmi was derived after one week. A sample made of spheres with a diameter of 540 nm on ITO-coated glass is shown in Fig. 2. Removing one of the substrates derives the samples used for SEM observation. The hexagonal arrangement of the spheres may be assigned to the (111) surface of cubic-close-packed (CCP) structure. It has been reported that samples prepared by the conventional gravity-sedimentation

299

Fig. 2. Hexagonal arrangement of the polystyrene spheres.

Fig. 3. Inverse opal fabricated using a template with a sphere size of 540 nm.

method have both hexagonal and tetragonal arrangements, In contrast, the samples fabricated using this method always have a hexagonal arrangement. 3. FABRICATION OF 3D ORDERED POROUS FILMS Fig. lb depicts the procedure for the fabrication of inverse opal. The dried opal film sandwiched between two substrates was immersed into pure titanium tetra-ethoxide (TTED) or its ethanol solution. The liquid precursors fill the void spaces between the substrates due to the capillary force. Experiment shows that the viscosity of pure TTED is high and it takes along time for the precursor to infiltrate the void. The infiltration speed can be increased using TTED diluted with ethanol. As the TTED is very sensitive to the moisture in the air, all of the operations were performed in a glove box with a nitrogen atmosphere. After the voids were completely filled by the precursor, the film was taken out and left in the air for several days for hydrolysis. This procedure was repeated several times. Then, the film was calcinated at 5(X)**C for the purpose of removal of the polymer beads and the formation of titanium dioxide. The inverse opal fabricated using a template composed of spheres with a diameter of 540 nm is shown in Fig. 3. Pure TTED solution is used as the precursor. The images show that the crystalline grains in the film are highly oriented with the close-packed planes parallel to the substrate. Hexagonal arrangement of the air spheres can be observed over a large area indicating that the template was well replicated. The high-magnification image clearly shows that the second layer also has the same hexagonal arrangement as the top layer and the voids between the different layers are interconnected. Cracks can be observed in wide-view image. These cracks originate from the shrinkage of the liquid precursor during the solidification. According SEM image, the distance between the centers of two nearest pores is 424nm. This distance is 79% of the diameter of the spheres used in the template. Such shrinkages were observed m all of the samples. Center-to-center distances of 221 nm, 293 nm and 323 nm were obtained using templates composed of spheres with diameters of 285 nm, 373 nm and 408 nm, respectively. These shrinkages are around 21% with respect to the initial size of the polystyrene spheres. The surfaces of the samples prepared by this method are very smooth. The normal reflection spectra of the samples are shown in Fig. 4, which were measured with a multi-channel photodetector (Otsuka Elec.). The light reflections are due to the Bragg diffraction. The

300

diffraction

wavelength,

as: k~h63d(nl'sm^dy'\

X,

is

given

where d and 0

represent the center-to-center distance between neighboring pores and incident angle of the light, respectively. The average refractive index, na, is expressed as: nl '^nlf-^n^^^^(1 - / ) , where f is the volume fraction of the air spheres. Hence, the diffraction wavelength should linearly change with the center-to-center distance between neighboring pores. An appropriate choice of the diameter of the latex spheres in the templates allows one to change the center-to-center Fig. 4. Normal transmission spectra of distance between neighboring voids and thus tune titania inverse opals. the rejection wavelength (X), In our experiment, the center-to-center distance between pores was varied from 221 nm to 424 nm. All the samples exhibited strong reflection peaks at wavelengths which varied from 417 nm to 825 nm (Fig. 4). The wavelength of the reflection linearly increased with the center-to-center distance. From the slop of the line, the average refractive index was estimated to be 2.01. Since the refractive indices of air and titanium dioxide are 1 and 2.5 respectively, the volume fraction of the air spheres was calculated as 90%. This value is larger than the volume fraction occupied by the polystyrene beads in the template, which is 76%. 4. CONCLUSION In conclusion, a new method has been developed to fabricate inverse opal frlms, with controllable thickness, between electrodes. This method is both simple and efficient. Hence, this technique can be used to fabricate a variety of photonic crystals. This method has been demonstrated for the preparation of electrically tunable opals infiltrated with liquid crystal[5]. REFERENCES I.E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059 . 2. C. M. Soukoulis, NATO ASI Ser. 3, Vol. 315, Kluwer Academic Publishers, Dordrecht 1996. 3.N. Normile, Science 286 (1999) 1500. 4.0. D. Velev and E. W. Kaler, Adv. Mater. 12 (2000) 5 3 1 . 5. M.-Q. Bo, Z.-Z. Gu, F. Fujishima and O. Sato, to be published . 6.S. H. Park and Y. Xia, Langmuir 15 (1999) 266 . 7.R. Mayoral, J. Requena, J. S. Moya, C. Lopez, A. Cintas, H. Miguez, F. Meseguer, L Vazquez, M. Holgado and A. Blanco, Adv. Mater. 9 (1997) 257 . 8.H. Miguez, F. Meseguer, C. Lopez, A. Mifsud, J. S. Moya and L Vazquez, Langmuir 13 (1997) 6009. 9.H. Miguez, C. Lopez, F. Meseguer, A. Blanco, L Vazquez, R. Mayoral, M. Ocana, V. Foraes and A. Mifsud, Appl Phys Lett 71 (1997) 1148. 10. R Jiang, J. F. Bertone and K. S. Hwang, V. L Colvin, Chem. Matter. 11 (1999) 2132 . 11. M. Trau, D. A. Saville and I. A. Aksay, Science (Washington, D. C.) 272 (1996) 706.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) 2001 Elsevier Science B.V.

301

Structure formation of poly(furfuryl alcohol)/silica hybrids S. Spange*, H. Muller", D. Pleul\ and F. Simon*' * Department of Polymer Chemistry, Institute of Chemistry, Chenmitz University of Technology, StraBe der Nationen 62, D-09111 Chemnitz, Germany ^ Institute of Polymer Research, Hohe StraBe 6, D-01069 Dresden, Germany The interfacial step growth polymerization of furfliryl alcohol on silica particles yielded a hybrid of the core-shell polymer type (silica/cross-linked poly(furfuryl alcohol)). The structures of the core-shell polymers of poly(furfuryl alcohol) (PFA) on silica were examined by X-ray photoelectron spectroscopy (XPS). Labelling reactions of the hybrids with trifluoroacetic anhydride (TFAA) were used to characterize the functional end groups. The existence of carbonyl groups found by XPS in the polymer layer is also evidenced by FT-IR spectroscopy, and derivatization with primary amine yields the corresponding azomethine groups. The model reaction of furane derivatives with carbenium ion pair intermediates on silica shows that the attack of a carbenium ion occurs at the five position of the fiiranering. 1. INTRODUCTION Previously, we have been able to show that furfliryl alcohol can be polymerized cationically on a surface of silica which results in efficient and controlled grafting of the silica particle [1,2]. The polymer obtained at the surface of silica which is dark brown or black, consists of different structure units as head to tail sequences (I), conjugated sequences (fl) and cross-linked units (HI) (Fig. 1), as evidenced by FT-IR-spectroscopy, UVA^is-spectroscopy and solid state ^^CNMR-spectroscopy. The controlled formation of different structures in a polymer layer is a very important feature for the development of advanced materials in the wide field of hybrids. The objectives of this paper are to investigate model reactions of fiiranederivatives on silica gel and to examine the polymer formation on the silica surface by XPS. 2. EXPERIMENTAL PART 2.1. Hybrid materials The syntheses of the PF A/silica hybrids were carried out using the procedure described in [1]. To produce 2-methyl-5-triphenylmethylfurane (MPMF)/silica hybrids a suspension of 200 mg triphenylmethyl chloride (PMCl) and 1 g silica (Kieselgel 60 from Merck, dried in vacuum at 200 °C for 4 hours) in 20 ml 1,2-dichloroethane (DCE) was stirred for 10 min at 20 °C. Then, 1 ml of 2-methylfurane was added rapidly. After having stirred the suspension for 24 hours the silica was filtered off and washed with DCE. The organic solutions were combined

302

and the solvent was evaporated under reduced pressure. The oily raw product was purified by column chromatography (Kieselgel 60, eluent toluene). The yield of pure product was 81 %. ^H-NMR (CDCI3): 5 = 2.30 (s, 3H, Me), 5.87-5.95 (m, 2H, C-H), 7.09-7.35 (m, 15H, Ph); ^^C-NMR (CDCI3): 5 = 13.7 (--CHsl 60.7 (-CPha), 105.7 (=CH~, furane), 112.3 (=CH-, furane), 126.5 (Ph), 127.6 (Ph), 130.4 (Ph), 145.5 (Ph), 152.1 (>C= furane), 157.3 (>C=, furane) 2-(triphenylmethyl)furane (TPMF) and 2,5-bis(triphenylmethyl)furane (BPMF) were synthesized according to the given procedure. 2.2. Spectroscopy A KRATOS ES 300 spectrometer was used to acquire photoelectron spectra of pure and modified silica powder surfaces. Unmonochromized Mg Kai,2 radiation was used as the excitation source. The qualitative determinations of the elemental surface composition have been carried out taking into account the instrumentally determined sensitivity factors. Fitted parameters of the Gaussian component peaks were the peak maximum position, the fiill width half maximum and the peak area. The model reaction of methylflirane with PMCl on silica was monitored by UVA^is spectroscopy using a diving cuvette. The spectra were obtained on a Carl Zeiss MCS 400.

CH2-<^3^CH2-\^3^CH2A^>—^

(D

m (ffl)

Fig. 1: Different structures of PFA on silica surfaces. Tail sequences (I), conjugated sequences (H), and cross-linked units (IID. 3. Results and Discussion The initiation step, the reaction of furfiiryl alcohol with the initiator CF3COOH or CCI3COOH,

303 respectively, involves the formation of the 2-furfurylium ion pair on silica surface as shown in eq. 1. The high reactivity of furfuryl alcohol towards an electrophilic attack of a carbenium ion

O-H

//-\\ 0 © ^X-H + H0-CH2-\ 7

0

© /r-\

X

CH9-<

O-H

O

}

+ HoO (1)

o

o

o

o

o

(X = CF3COO ; CCI3COO ; Br^

•\

-I

Jv^-CHj-X-H-OO

o

°o

J_ o X0 °

o

o

H O -HY

ru«,r^<

-CH=<

.>=CH-

^:^c„,^

i.j:^o«j:^c«^c«,j:}

@ o

-CH

-H"

0 - y—CU=k^ C H = 0 =^^=cU—\ C H ^ : X )C^ CHH== ^0 =>=CHCHX> -X o o o o o

(2)

304

pair is shown by the model reaction of 2-methylfurane with the complex of triphenylmethylium chloride and silica. It can be demonstrated that only the Friedel-Crafts alkylation does occur and that hydride ion transfer reaction do not takes place. Thus, the formation of a red coloured species observed in the first stage of interfacial reaction is attributed to a strong a-complex of the triphenylmethylium ion and the 2-methylfurane. The shape of the absorption band (494 nm) is very similar to that of the absorption of protonated methylfiirane (488 nm). The reaction of fiirane with triphenylmethylium chloride/silica leads to TPMF or BPMF, depending on the stoichiometric ratio of fiirane and PMCI of 1:1 or 1.2. In contrast to the interfacial reaction, an ester is formed in the bulk phase of the solution. The ester can react with the fiirfiiryl alcohol forming both the diflirfuryl ether and the FriedelCrafls product [3]. The formation of difurfuryl ether is suppressed during the interfacial polycondenzation in comparison to the solution process. However, the preferred formation of the desired head-to-tail connected monomers offers the material as an efficient hydride donor. The driving force of the easy hydride abstraction is the formation of the bis-(2-fiirfuryl)methylium ion which is more stable than the furfiirylium ion. The hydride transfer may be followed by a proton transfer to the counter ion due to conjugated double bonds. This new structure has a higher 7i-reactivity towards a carbenium ion than the furfliryl alcohol monomer or a bis-(2-furfuryl)methane unit in the polymer chain. That means, the conjugation process is followed by the reaction of a growing cation upon the bridged sp^-hybridized carbon atom. This explains the effective coverage of silica particles by a cross-linked PFA obtained via interfacial polycondenzation. The specific mechanism of interfacial ion pair propagation is due to both the formation of the polymer and the occurrence of the cationically active species in an two dimensional layer (eq. 2). In contrast the extractable polymer obtained fi-om the suspending liquid using an excess of fiirfiiryl alcohol is not cross-linked, colourless (grew white) and soluble. In the presence of solved monomers a possibly primarily formed trifluoroacetic acid ester undergoes a following condensation reaction according to eq. 3. IJ ^^CH2 + CF3COOH ^F=:^ II >-CH2 O OH (a)

(

>-CH2

O

O + H2O >-CF3 O

O^CH2-^l^CH2

o H2 OH

CF3COOH (3)

Fig. 2 shows a typical high-resolved spectrum of the C Is peak of a silica powder surface modified by fiirfiiryl alcohol. The C Is peak was divided into three component peaks. The largest component peak A involved the CxHy bonds. It has been used as the binding energy reference and set on 284.70 eV. Different oxygen containing fimctional groups induces shifts

305

to higher binding energies. A chemical shift of the binding energy by approximately 1.6 eV indicates alcoholic groups (C-OH) and ether groups (C-O-C) and the one of about 3.4 eV indicates carbonyl groups (C=0), respectively. According to the two possible reaction mechanism polymerization and polycondenzation the yielded polymers must differ in the [0]:[C] ratio. An elemental ratio of the organic polymer component of [0]:[C] = 2:5 is expected in case of a polymerization mechanism. In contrast, an elemental ratio of the synthesized PFA of [0]:[C] =1:5 shows a polymer build-up reaction by polycondenzation mechanism. As the influences of the oxygen in silica substrate materials on the O Is peak cannot be discriminated it is not possible to discuss the elemental ratio gotfromsurvey spectrum. However, the peak analysis of the C Is spectrum (Fig. 2) gives the number of oxygen containing functional carbon groups (B ^ C) and the number of oxygen-free carbon-carbon and carbon-hydrogen bonds (A). The found ratio [^]/([^]-*-[C] =1.37 indicates a preference of a polycondenzation mechanism of fiirfuryl alcohol on silica surfaces according to eq. 3 (theoretical ratio =1.5).

satellites

285 290 binding energy [eV]

280

285

290 295 binding energy [eV]

Fig. 2: Typical C Is photoelectron spectrum of Fig. 3: C Is spectrum of a PFA/silica hybrid a of PF A/silica hybrid labelled with TFAA In the following the possible reasons of the small excess of oxygen containing carbon groups and the kind of function groups in the polymer chains will be discussed. The component peak B of the spectrum in Fig. 2 may represent alcohol and ether groups. Ether groups are undisputed in the grafted polymer chains. The existence of alcohol groups can be proved by a labelling reaction according to Briggs et al. [4]. This has been done by the specific gas phase reaction of TFAA with alcohol groups. With the formation of the corresponding trifluoroacetic acid ester (eq. 3, structur a),fluorinehas been introduced in the polymer chains. In contrast to the unlabelled samples the photoelectron spectra of such labelled PF A/silica hybrids show a F Is peak and a modified C Is peak (Fig. 3). The Cls peak of the labelled hybrid shows two additional component peaks shifted to higher binding energies. The two additional component peaks resultfromthe introducedfluorine(-CF3) and carboxyl (0-C=0) species. According to eq. 3, the number of the introduced -CF^ and ester groups, respectively, must equal the number of alcoholic OH groups on the unlabelled hybrid surface. That means, the peak area of one

306 of each additional component peaks D or E represents the number of OH groups on the unlabelled hybrid surface. The knowledge of the number of OH groups has allowed to separate B for alcoholic and ether species, consequently the number of ether groups on the surface of the PFA/silica hybrid can be determined. According to the above method a ratio of [ether]:[alcohol] = 6.3 has been found. The ratio shows that only a small part of the component peak B represents alcoholic groups. The main part of this component peak results from the fiirfuryl rings containing ether groups. The results of the investigation of PFA hybrids labelled by TFAA support the assumption of a preferred polycondenzation reaction during the polymer build-up reaction of furfiiryl alcohol on silica surfaces. The C Is peak of the PF A/silica hybrids shows a component peak C (Fig. 2). The amount of the shift in the binding energy allows to interpret this component peak with C=0 groups. The drift-FT-IR spectrum of the hybrid material shows a carbonyl band at 1703 cm'* [1]. After a reaction of the carbonyl groups with ethylendiamine the carbonyl band has disappeared and the new band of azomethine groups can be observed near thefiirfiirylband [1]. The results demonstrate that the PFA chains of the hybrids contain carbonyl groups. In a previous paper the existence of covalent bonds between the inorganic silica surface and the organic buih-up polymer chains has been discussed [5]. The Si 2p photoelectron spectrum of PF A/silica hybrids shows the same feature discussed for of poly(cyclopentadiene)/silica hybrids. Therefore, it can be concluded that Si~0-C bonds are formed during the polymer buildup reaction of furfiiryl alkohol onto silica particle surfaces. Drift-FTIR studies show corresponding results. After the polymer build-up reaction on the silica surface the adsorption band of the Si-OH valency vibration is removed, and a shoulder at 964 cm"* indicates Si-O-C bonds. 4. CONCLUSIONS Structure determination of the hybrids of PFA and silica is not easy. The examination of the hybrids by XPS and the comparison of the results with NMR and IR experiments show that CC-connecting reactions between the fiirfiiryl alcohol molecules are the dominating reactions. This is one of the differences between the homogeneous reaction in solution and the reaction at the silica surface. The carbocation intermediates are stabilized by the silica surfaces. The model reaction of fliraneand PMCl onto silica in DCE shows the possibility of an attack of a carbocation at the five position of the furanering. The deep brown colour of the hybrid is the result of conjugated structures formed by a sequence of hydride transfers with subsequent proton transfers.

REFERENCES 1 S. Spange, B. Heublein, A. Schramm, and R. Martihez, Makromol. Chem., Rapid Commun. 13(1992)511. 2 S. Spange, H. Schutz, and R. Martinez, Macromol. Chem. 194 (1993) 1537. 3 R. Gonzalez, R. Martinez, and P. Opitz, Makromol. Chem., Rapid Commun. 13 (1992) 517. 4 A.P. Ameen, R.J. Ward, R.D. Short, G. Beamson, D. Briggs, Polymer 43 (1993) 1795. 5 I. Voigt, Simon, H. Komber, H.J.; Jacobasch, and S. Spange, Coll. Polymer Sci 278 (2000) 48.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2(10! Elsevier Science B.V. Ail rights reserved.

307

Approach to a Unified Theory of Hydrophobic/Hydrophilic Surface Forces H.-J. Muller Max-Planck-Institut fiir KoUoid- und Grenzflachenforschung D-14476 Golm, Germany In the last two decades, a multitude of forces between hydrophilic and hydrophobic surfaces in water has been observed which do not fit in the classical scheme of colloidal interactions. Until now, numerous different physical mechanisms have been discussed for these phenomena. Here, a mechanism is proposed, in which the nonDLVO forces result from the influence of the classical forces on an excess of hydrogen bonds at water interfaces. 1. INTRODUCTION In the last decades a host of forces between two approaching interfaces have been observed, which will not expected from the DLVO theory. This was especially the case in the interaction of hydrophobic or hydrophilic interfaces in water or aqueous solutions. Examples are the very strong attraction between hydrophobic surfaces, the repulsive hydration forces measured between hydrophilic solid surfaces or between lipid bilayers, and the strong non-DLVO repulsion in wetting films of water on very hydrophilic substrates. Until now the underlying physical mechanisms of these phenomena are a matter of discussion. Ideas about structurized layers of water near interfaces are not able to explain the forces of a longer range, since molecular dynamics simulations show that only the structure of two or three water layers near the interface deviate from the bulk of water. Numerous theoretical models have been proposed. However, until now, neither the theoretical understanding of these forces is satisfying nor does any generalized picture for all these phenomena exist. Here, a mechanism is proposed which is able to describe interactions in very different systems. It will be shown that an excess of H-bonds, positive or negative, exists in water layers near hydrophobic and hydrophilic surfaces, which is varied by the effect of the classical long-range forces. If the ratio between closed and broken H-bonds in the interfacial layer changes on decreasing distance between the interfaces, additional contributions to the interaction forces appear.

308

2. EXCESS OF H-BONDS AT INTERFACES OF WATER If we create a new surface of water by cutting all hydrogen bonds crossing a mathematical plane introduced in a bulk phase of water, each water molecule in the surface water layer should have two broken H-bonds. However, molecular dynamics simulations [1] and surface sensitive sum-frequency generation vibrational spectroscopy of water interfaces [2] delivers evidence that only one of the H-bonds of water molecules near a hydrophobic interface is broken. Water minimizes its surface excess energy at hydrophobic interfaces by the formation of a half additional H-bond per water molecule. The H-bond excess has been used for the explanation of additional forces in the presence of hydrophobic interfaces in [3]. On the other hand, it has been shown that the H-bond density is diminished near hydrophilic walls [4]. The results given in [1-4] show that a positive or negative excess of H-bonds is a general feature of interfaces of water. 3. ENHANCED COLLOIDAL INTERACTION We now assume that the water molecules in the interfacial layer are distributed between the two possible states of one additional closed or broken H-bond in agreement with the Boltzmann theorem. The distribution is varied, if the free energy of the interfacial molecules is increased or decreased by the effect of the classical long-range interaction forces on approach of the interfaces. The change of composition or structure of interfacial layers due to the approach of two interacting interfaces has been investigated in [5] in relation to the variation of the adsorption density of surfactant molecules depending on the thickness of a foam films. In [3] the influence of disjoining pressure in a water wetting film on the H-bond excess at the water has been discussed. In every case it results an "enhanced colloidal interaction (ECI)", because of the energy costs of changing the composition of the interfacial layer. This idea will be generalized here. The interaction between the interfaces delivers an increment

Ag^{h)=j^n{h'}ih' + nS

(1)

to the density of Gibbs energy in the interfacial water layer ( n : disjoining pressure in the gap between the interfaces; h: distance between the interacting interfaces; S: effective thickness of the interfacial water layer). In agreement with the Boltzmann theorem the increment Ag^ changes the ratio between broken and closed H-bonds in the interfacial water layer. This ratio can be given [3] by

309

(r^^: number density per unit of area of the excess hydrogen bonds at an interface interacting with a second one across a distance h; r;^: density of hydrogen bonds at the same interface, however at a large distance to the second interface; r": number density of water molecules belonging to the surface excess phase of the water interface without interaction; r / : number density of water molecules belonging to the surface excess phase of an interacting interface; g": Gibbs surface excess energy of the non-interacting interface). Choosing a value for the quantity r ; defines the surface excess phase. The quantity r/can be obtained numerically from the following implicit equation

-T:H = exp

RT

r;

r/(A)

(3)

(g° is the surface excess energy of a surface of water created in the above mentioned way by cutting all H-bonds which cross a mathematical plane). We can describe the mechanism as follows. The structurized water layer near an interface "feels" the approach of a second interface by the effect of the weak classical forces. An unavoidable consequence then is a change in the number density of H-bonds. This causes additional contributions to the interaction energy and to the force between the interfaces. The final equilibrium between the "new" strength of interaction and the "new" density of H-bonds is given by the equations (2) and (3). 4. APPLICATION OF THE THEORY 4.1 Strong hydrophobic attraction Among the numerous effects between hydrophobic surfaces we restrict our discussion to the interaction between surfaces coated with a polymerized hydrophobic layer attached covalently to the underlying substrate. The range of the force in such systems does not greatly exceed the range of the van der Waals force, however the hydrophobic force is much stronger [6,7]. Here, it is proposed that the weak van der Waals attraction diminishes the surface excess energy as described by eq. (1). This is followed by an increase in H-bond density in agreement with eq. (2) and (3). Since the H-bond density now increases with a decreasing distance of the interfaces, an additional attractive force appears. 4.2 Hydration forces between lipid bilayers It has been shown that the hydration force between lipid bilayers is proportional to the square of the component of the dipole moment of the lipid molecules in a bilayer normal to the bilayer plane [8]. The repulsive force, however, resulting from this dipole potential is much too small to explain the hydration force in this system. In the picture of the ECI-theory, the interaction between the dipole moments of the two approaching bilayers causes a decrease in the H-bond density in the water layers

310

nearest the lipid head groups. This has been confirmed by means of molecular dynamics simulations [9]. There it is shown that the H-bond density near the lipid head groups decreases significantly if the distance between the opposing bilayers is diminished (Fig. 5a). The derivative of the increasing surface excess energy on decreasing H-bond density in respect to the thickness of the water layer between the lipid bilayers is the hydration force. 4.3 Wetting layers of water on very hydrophilic substrates In the investigation of multilayer adsorption of water on hydrophilic substrates [10,11], a repulsive disjoining pressure has been observed much stronger than expected from the DLVO theory. In wetting films of water on hydrophilic substrates, the van der Waals interaction delivers a positive contribution to the disjoining pressure. However, measurements of the thickness of the wetting fihn depending on the water vapor pressure show that the repulsion in the wetting film is much stronger than the van der Waals pressure. From the application of eq. (1), (2) and (3) results that H-bonds are broken at the water/vapor interface, if the thickness of the water film is diminished by reduction of the vapor pressure. The cost of energy for breaking these H-bonds delivers the additional repulsion in the wetting layer. 5. CONCLUDING REMARKS The mechanism discussed above offers a unified picture of the interactions between hydrophobic and hydrophilic surfaces in water. Consideration of the influence of the surface forces on the ratio between closed and broken hydrogen bonds delivers an explanation of the sign and the strength of different non-DLVO forces. For wetting films on very hydrophilic solids it has been demonstrated in [3] that the theory describes the interaction force in quantitative agreement with the experiment.

REFERENCES 1. C.Y. Lee, J.A. McCammon, P.J. Rossky, J. Chem. Phys. 80 (1984) 4448 2. Q. Du, E. Freysz, Y.R. Shen, Science 264 (1994) 826 3. H.-J. Muller, Langmuir 14 (1998) 6789 4. R. Kjellander, S. Marcelja, Chem. Phys. Lett. 120 (1985) 393 5. R. Krustev, H.-J. Muller, Langmuir 15 (1999) 2134 6. J. Wood, R. Sharma, Langmuir 11 (1995) 4797 7. N. Ishida, M. Sakamoto, M. Miyahara, K. Higashitani, Langmuir 16 (2000) 5681 8. S.A. Simon, T.J. Mcintosh, A.D. Magid, D. Needham, Biophys. J. 61 (1992) 786 9. S.-J. Marrink, M. Berkowitz, H.J.C. Berendsen, Langmuir 9 (1993) 3122 10. M.L. Gee, T.W. Healy, L.R.White, J. Colloid Interface Sci. 140 (1990) 450 11.V. Panella, R. Chiarello, J. Krim, Phys. Rev. Lett. 76 (1996) 3606

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) 2001 Elsevier Science B.V.

311

Adsorption behavior of dispersing agent to pigment surfaces Noboru Nakai, Atsunao Hiwara, Toshihide Fujitani Technical Research Laboratory Kansai Paint Co., ltd. 17-1 Higashi-Yawata 4-Chome, Hiratuka-shi, 254-8562 Japan Important factors for pigment dispersion process are wettability and adsorption of polymers to the pigment surfaces. Interactions of pigments and dispersing agents was studied. It found that dispersing agents with a similar function to group which exhibit more heats of irreversible adsorption on pigment surfaces could disperse the pigment better. 1. Introduction There are two main factors to contribute pigment dispersion process. One is wettability on pigment surfaces with polymer solutions and the other is adsorption of polymers to the pigment surfaces. In order to improve pigment dispersion, effective dispersing agents are very often used to promote the wettability and the polymer adsorption in these processes. Up to the present, dispersing agents were selected from the standpoint of acid-base theory. In order to clarify the operation of dispersing agents, [1 ] influence of compound with similar functional groups to several dispersing agents on the wettabilify and [2] relationship between structure of the dispersing agents and adsorbed amount of them were studied. 2. Experimental 2.1 Wettabilify between the pigments and solutions Examined compounds were Acetic acid, Triethylamine(TEA), Diethyamine(DEA), Dimethylfon7iamlde(DMF),Dimethylacetamide(DMA),Tolylenediisocyanate{TDI), Ethanol and Methyl benzoate as following Table 1. Examined pigments were Titanium dioxide(Ti02), Carbon, Bismuth vanadate, Iron oxide, Diketopyrrolopyrrole(DPPred), Anthraquinone, Quinacridone, Perylene, Cu-phthalocyanine blue. Table 1 List of compounds with functional groups Compound Acetic Acid Triethylamine (TEA) Diethylamine (DEA) Dimethylfonnamide (DMF) Dimethylacetamide (DMA) Tolylenediisocyanate (TDI) Ethanol Methyl benzoate

Functional Group -COOH -NR^ -NR, -NHCO -NHCO -NCO -OH -COOR

312

Liquid Input

Flow Rate: 3niVlir Carrier Adaorptive Solution 0.04moleyL

Pigment'

Carrier

.JL Heat of [adaorption (Hi)

i ^

Heat of neaorption (Hd)|

Heat of irreveraible adaorption (Ha-m-Hd) Fig.1 Structure of Flow Micro -Calorimeter (PMC) 3Vi

Fig.2 Diagram of heat of adsorption and heat of desorption

Wettability was estimated by heats of adsorption(immersion) and desorption. The amount of heat of adsorption and desorption to pigments were measured by FMC3Vi (MICROSCAL Co., ltd. made). A earner solvent was xylene. First, the carrier solvent only was flowed through a cell stuffed with a pigment. After thermal equilibrium was reached, the carrier was changed to xylene solution contained each compound in concentration of 4X10 ^mol/L. The heats of adsorption (HI) was estimated the area of exothemiic region at this time . Then, after thermal equilibrium was reached, the carrier was changed to xylene only. The heats of desorption (Hd) was estimated the area of endothermic region at this time. The heats of irreversible adsorption (Ha) were calculated by subtraction the heats of desorption from the heats of adsorption. 2.2 Adsorbed amount on the pigment surfaces of various dispersing agents having different acid-base characteristics. Dispersing agents with following functional groups were examined. Functional groups were COOH, COOR, COONR3, NR3, SO3M, POsH, TDI-Block . A mixture of a pigment, solvent and dispersing agent (4-10wt.% per pigment) was ground by paint-conditioner. After this paste manufactured was centrifuged, a difference(Co-C) of concentration of supematant (C) and initial concentration (Co) was estimated as an amount of adsorption by weight. Besides, dispersibility of pigment in this paste was estimated by particle diameter of pigment and yield value of viscosity of the paste. 3. Results and Discussion As examples for relationships between heats of adsorption and heats of desorption, TiOz-solution system were shown in Fig.3 and DPPred-solution system were shown in Fig.4. The farther a plot lay away a broken line, the more one is irreversible for adsorption and desorption behavior. Namely, the farther the compound lay on the right of a broken line, the stronger one adsorbs on the pigment surface. In consequence, a series of this measurement enables us to infomn compounds exhibit irreversible heats for each pigments and to evaluate adsorption strength between the pigment surfaces and each functional group quantitatively. In case of T1O2, tolylenediisocyanate(TDI) was the most adsorptive compound. In case of DPPred, acetic acid was the most adsorptive compound.

313 M 5

50.5 I 0.4

•

-

EtOH

-

3 h

I'

' '

= 0

I

1

1

•

• ™

• TEA

-1—1—,1

—1—1—1

2

3

|o.i

1 ri02 1 .1

4

1

1

1

1

1

X

- •

*-DMF

0

0.2

Fig.4

-

0.4

i2

-1 1

1 "^

•r

|1

IV 1

flffl 1

CQ

1

K

1

20

I aa

OQ

1

u

1<

—0-

<1

1 0 1

e

<

ji

Dispersing agent

Fig. 5

The adsorbed amount of dispersing agent for TiOa

I

- | 20 r

ns r

1

1 0-4 1 •§ ^

0.3 • 0.2 -

1 b

Heat of adsorption of solvent's fuDCtiooal Group in DPP .«

0

0

< ^^ f 0

c

I- u

-»,

3

0 c E

<

0

<

n L

u u

u 0 CQ

e

•8 1

U—1 0 CQ

e

< E

<

0) U]

Dispersin g a j ;enl

Fig. 6

1—

1

0.6

1

i_

0.8

Heat of Adsorption and Desorption for DPP red

^catof adsoqjtic)n( rf solvent's functional Group in TiOj

S 4 f

| 3 !

E

40

1

Heat of adsorption(J/g)

tioo

- H

1 DPP 1 1

5

Heat of Adsorption and Desorption for T1O2

60 "5

• Acteic acid

0

Heat of adsorption(J/g)

Fig. 3

DEA

#'

S 0.2

Acteic A d d

DEA

•'

,•' EtOH

The adsorbed amount of dispersing agent for DPP red

__ J

314 l.«KfM

The Amoant of AdMrptioii(iii^g)

Fig.7

The relationship between yield value, particle diameter of paste and the amount of desorption for TiOg hVRtm

I

LW*«1 L9&40 IMM. 5

M

IS

30

25

The Amount of AdsorptiooCmg^g)

Fig. 8

The relationship between yield value, particle diameter of paste and the amount of desorption for DPP

Contrasting the adsorbed amount of the dispersing agents and the heats of irreversible adsorption Ti02-solutlon system was shown in Fig. 5 and DPP redsolution system was shown in Fig. 6. As obviously in these figures, there are similarities in the tendency, that is, dispersing agent having more adsorptive functional group adsorbed more on the pigment surfaces. Fig. 7 and 8 show the relationships between yield value, particle diameter of pigment in pastes and the amount of adsorption of dispersing agent on the pigment surfaces respectively. These figures indicate the dispersing agent which have more absorbed amount more improved in dispersability.

4. Conclusion The most suitable functional group could be selected for each pigment by measuring the heat of adsorption and desorption. The dispersing agent with the most suitable functional group can disperse the pigment very well. More detailed criterion for selection of dispersing agents for pigments could be found.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) C 2001 Elsevier Science B.V. All rights reserved.

315

Measurement of zeta potentials in concentrated aqueous suspensions of ceramic powders using electroacoustics. R. Greenwood School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT. U.K. Electroacoustics is a powerful tool for measuring the zeta potentials of concentrated aqueous suspensions of ceramic powders, revealing an insight into the nature of the interparticle forces. This paper reviews the potential of the Acoustosizer ( Colloidal Dynamics, U.S.A. ) to investigate various effects important in the colloidal processing of ceramic powders. 1. INTRODUCTION Unless the interparticle forces between ceramic particles dispersed in aqueous media are repulsive in nature then the particles will flocculate together. These floes may persist through the processing stage to form flaws in the fmal sintered product. These flaws are then the source of defects and reduce the mechanical strength of the body. By understanding and controlling the nature of the interparticle forces it is simple to minimise the floes and reduce their presence in the fmal body. Electroacoustics is a relatively new technique that allows concentrated aqueous ceramic suspensions to be investigated. Previously the only other methods of measuring zeta potentials were in dilute systems, e.g. electrophoresis. However the very act of dilution itself alters the very property to be measured. Hence electroacoustics readily allows the investigation of industrially relevant concentrated suspensions. 2. THEORY An alternating electric field of known frequency is applied to 400 ml of the suspension, which then causes the particles to oscillate at the same frequency. The liquid within the shear plane around the particles also moves with the particles, but due to differences in permitivity and density ( thus inertia ) the movements are slightly out of phase with the input signal. This results in a cyclic longitudinal pressure variation in the suspending liquid, i.e. a, sound wave. The acoustic wave is then detected by the transducers and its an^litude and phase difference recorded in the dynamic mobility spectrum ( frequency range 300 kHz - 11.5 MHz ). From this spectrum the zeta potential and particle size distribution is calculated automatically.

316

3. ELECTROSTATICALLY STABILISED SYSTEMS 3.1 Dissolution of material The effect of time on the suspension stability can be investigated. If the zeta potential of a suspension varies with time then this may cause some potential processing problems. In Figure 1 the increase in zeta potential with time is shown for two yttria doped zirconia powders. This increase was attributed to yttrium dissolution / precipitation [1,2]. A similar increase has been noted for spinel powders, which was attributed to dissolution of sodium ions [3].

Dissolution of yttria from zirconia powders -~ * ' " > 30 -25

1 1 o a 1

20 15 10 51

"

00

• 3YZiraxiia • SYZroonia

^

60

1

1

1

120 180 240 300 360 420 time (nrins)

Fig. 1. Increase in zeta potential of two yttria doped zirconia suspensions. 3.2 Iso electric point In designing a stable ceramic suspension in order to impart the greatest repulsive force between the particles it is necessary to know where the point at which zero zeta potential occurs. This is known as the iso electric point and identifies the pH value where there are no repulsive forces present in the system and the particles are flocculated. Once the i.e.p. of a system is known, then the pH is adjusted so that a large repulsive force is generated between the particles. Generally -- 2 pH units away from the i.e.p. is sufficient. The Acoustosizer allows the pH of the system to be carefully controlled via automatic titration of acid and base. By varying the nature of these chemicals the effect of different ions on the i.e.p. can also be investigated [1]. 4. ELECTROSTERICALLY STABILISED SYSTEM Traditionally in the ceramic processing industry polyelectrolytes have been utilised to stabilise. It is important to add the correct dosage of polyelectrolyte to the suspension. Too little and floes will still persist and too much may cause destabilisation and is also uneconomic.

317

4.1 Measurement of the optimum amount of dispersant Electroacoustics is an excellent technique for studying the adsorption of ionic polyelectrolytes as a small amount may alter the zeta potential significantly. Figure 2 shows the adsorption of a cationic and an anionic polyelectrolyte onto an alumina powder. Both show the same trend in that a small amount of polyelectrolyte alters the surface charge on the particles. Adsorption of more dispersant occurs until the particle is con^letely covered in dispersant. If more polyelectrolyte is added at this stage it remains unadsorbed in the medium, hence the zeta potential does not increase further but plateaus out. The start point of this plateau is taken to be the optimum dispersant concentration. This optimum amount agrees with results from adsorption isotherms and rheology [4]. The technique also allows numerous dispersants to be screened quickly [5].

Fig. 2. Adsorption of a cationic dispesant on an alumina powder ( squares ) and adsorption of an anionic dispersant on alumina (diamonds). No added salt. 4.2 Effect of ionic strength The conformation of a polyelectrolyte chain in solution is determined by the ionic strength of the suspension. With no salt present the polyelectrolyte is a large extended coil, but as the salt concentration increases the coil collapses in on itself to form a tight coil. This then affects the way the polyelectrolyte adsorbs onto the particle surfiice hence the optimum amount. At low salt concentrations and high pH the polyelectrolyte adsorbs as a train, so the adsorbed amount is low, but at higher salt concentrations more loops and tails are formed by the polyelectrolyte hence the adsorbed amount is higher. This obviously this has implications for the layer thickness and stability [6].

318 4.3 Effect of molecular weight of the dispersant. The molecular weight of a polyelectrolyte and how it adsorbs at a particle interface is vital for particle stability. At low salt concentrations and high pH the optimum amount of a polyacrylic acid was independent of its molecular weight as the dispersant adsorbs as trains, see Figxire 3. 4.4 Surface area of powders Another area where electroacoustics has been applied is studying the optimum amount of one polyacrylic acid based dispersant adsorbed onto an alumina powder of different surface areas. Over the range of surface areas studied the optimum amount was independent of the area [7]. Again this was carried out at high pH values and low salt concentration.

•? 0.6 f 0.5 -

'^ ^ i

,'

I

1 0.4 -

i °^ ' " 0.2 -

l i 0.1 ^ 2

^

n

10 00

1

10000

!

100000

1000000

molecular weight ( Daltons ) Fig. 3. Optimum amount of poly (acrylic acid) required to cover SDK160 alumma powder at a background electrolyte strength of ImM KCl. 5. CONCLUSIONS Electroacoustics is a powerful technique for studying concentrated suspensions of ceramic particles. This then allows the interparticle forces to be manipulated to ensure better properties in the final product be they electrical, optical or mechanical. REFERENCES 1. R.Greenwood and K.Kendall, J. European Ceramic Soc. 19 (1998) 479. 2. R.Greenwood and K.Kendall, J. European Ceramic Soc. 20 (2000) 77. 3. R.Greenwood and K.Kendall, Brit. Ceramic Trans. 97 (1998) 174. 4. L. Bergstrom and R.Greenwood, J. European Ceramic Soc. 17 (1997) 537. 5. RGreenwood and K.Kendall, World Congress on Particle Technology, Brighton U.K. (1998) Paper 5, CD ROM, I.Chem.E. 6. R.Greenwood and K.Kendall accepted for Powder Technology. 7. M.Burke, R.Greenwood and K.Kendall, J.Mat.Sci. 33 (1998) 5149.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c 2001 Elsevier Science B.V. All rights reserved.

319

Electrokinetic phenomena in concentrated suspensions of soft particles Hiroyuki Ohshima Facility of Pharmaceutical Sciences and Institute of Colloid and Interface Science, Science University of Tokyo, 12 Ichigaya Funagawara-machi, Shinjukuku, Tokyo 162-0826, J a p a n A theory for electrokinetic phenomena in concentrated suspensions of spherical soft particles (i.e., spherical hard colloidal particles coated with a layer of poly electrolytes) is reviewed. Simple analytic expressions are given for the electrophoretic mobility, electrical conductivity, and sedimentation potential of concentrated soft particles. 1. INTRODUCTION Electrokinetics of soft particles (polyelectrolyte-coated particles) (Fig. 1) is quite different form that of hard particles because there is a liquid flow inside the poly electrolyte layer. In previous papers [1-6], the author presented a general theory for the electrophoresis of soft particles on the basis of Debye-BuecheHermans-Fujita model [7, 8] for spherical poly electrolytes. Since a soft particle becomes a hard particle in the absence of the surface layer and tends to a spherical polyelectrolyte in the absence of the particle core, the electrophoresis theory of soft particles imites the theories of the electrophoresis of hard colloidal particles and of poly electrolytes. This theory has shown that the concept of zeta potential loses it meaning for soft particles and the mobility of soft particles is determined by two parameters, the charge density distributed in the polyelectrolyte layer and the softness parameter. Quite recently, on the basis of Kuwabara's cell model [9] (Fig. 2), the author has extended the electrokinetic theory of soft particles [1-61, which is applicable for dilute suspensions, to cover the case of concentrated suspensions [10-12]. In the present paper we review this electrokinetic theory of concentrated suspensions of soft particles. ."".

*''""*'*^

/ ^ ^ V^^

Fig. 1. Soft particle

'~'*~'*^

\'K4SI

Fig. 2. Cell model

\

320

2. ELECTROKINETIC EQUATIONS Consider a concentrated suspension of charged spherical soft particles (polyelectrolyte-coated particles) moving with a velocity U in a liquid containing a general electrolyte in an applied electric field E. We assume that the particle core of radius a is coated with an ion-penetrable layer of polyelectrolytes with a thickness d. The polyelectrolyte-coated particle has thus an inner radius a and an outer radius b^a-nl. We employ a cell model [9] in which each particle is surrounded by a concentric spherical shell of an electrolyte solution, having an outer radius of c such that the particle/cell volume ratio in the imit cell is equal to the particle volimae fraction 0 throughout the entire dispersion, viz., 0 = (b/cf. We adopt the model of Debye-Bueche [7] that the polymer segments are regarded as resistance centers distributed in the poly electrolyte layer, exerting fiictional forces on the Uquidflowingin the polyelectrolyte layer. The fimdamental equations for the flow velocity of the liquid u(r) at position r and that of the i th ionic species v,(r) are ;7VxVxu + V/7 + p^,V^ + }ti = 0, 77VxVxu + Vp + p^,Vv^ = 0,

a
b
Vu=:0,

(1) (2) (3)

v,=u-lv/i„

(4)

A,-

V(n,v,) = 0,

(5)

where r] is the viscosity,/}(r) is the pressure, p^iir) is the charge density resulting from the mobile ionic species, v
321

^ r/ with

llK^ + m

Ha''^j^„A2'

1.

- I ^^-y 2(l+d/a)^Jl_.

.

(7)

with A = (yry)^^ and 0^ = (a/c)^ where 1/A is a parameter that characterizes the "softness'' of the soft particle, xjf^^ is the Donnan potential in the polyelectrolyte layer, y/; is the potential at the boundary between the polyelectrolyte layer and the surrounding solution, which we call the surface potential of the soft particle, K^ is the Debye-Hiickel parameter of the polyelectrolyte layer and (p^ is the volume fraction of the particle core. Equation (6) is applicable for the simple but important case where the double layer potential remains spherically symmetrical in the presence of the applied electric field (the relaxation effect is neglected), and Afe »1, Kb »1 and Ad = A(6 - a) »1, Kd= idb - a) » 1. This condition is satisfied for most practical cases The limiting forms of the mobility for the two cases dJa «1 and d/a »1 are given by

^^M^y^JL^llmilA^Pn^^ l/KT^ + l / A

7]

3r7

d.a,

(8)

riA'

1/K:^4-1/A

rjA"

Note that the 0 dependence of the mobility disappears for d«a. It is to be noted that the mobility fi has been defined as /x = U/E (where f/= IUI and 5 = IEI). There is another way of defiining the electrophoretic mobility in the concentrated case, where the mobility /z* is defined as ju* = U/<E >, <E> being the magnitude of the average electric field <E> within the suspensfion [ISIS] . It follows from the continuity condition of electric current that K * (E) = K'^E, where K^ andIC are, respectively, the electric conductivity of the suspension and that of the electrolyte solution in the absence of the particles. Thus jj and //* are related to each other by 1/* = - ^ / / .

(10)

4. ELECTRICAL CONDUCTIVITY In a previous paper [11] the author derived an expression for the electrical conductivity -K* of a concentrated suspension of soft particles, which reduces to, for low potentials.

322

^ =±:i^ K^

(11)

l+0e/2

It is interesting to note that Eq. (11) is Maxwell's relation with respect to the volume fraction of the particle core, implying that the polyelectrolyte layer contributes little to the conductivity for the low potential case. 5. SEDIMENTATION POTENTIAL It is well known t h a t an Onsager relation holds between electrophoretic mobility and sedimentation potential ESED- For a concentrated suspension of soft particles with low potentials, this relation is given by [12] E . ^ ^ = _ (l~0c) (0cAPe^<^sAPs) (12) ^™ (l+(^c/2) K'^ with 0g = Vg/(4;tcV3), where V^ is the volimie of the polymer segments, 0^ is the volume fraction of the polyelectrolyte segments, Ap^ = p, - p^, Ap^ = Ps - Po (Pc* Ps and Po are, respectively, the mass densities of the particle core, of the polyelectrolyte segments and of the electrolyte solution) and g is the gravity. It follows from Eqs. (10) and (11) that Eq. (12) may be generalized to EsEP=-.:^(^cAPc^J^sAPs)^g,,(0cAPe^j>sAP^

(13)

REFERENCES 1. H. Ohshima, J. Colloid Interface Sci., 163 (1994) 474. 2. H. Ohshima, H., Adv. Colloid Interface Sci., 62 (1995) 189. 3. H. Ohshima, Colloids and Surfaces A: Physicochemical and Engineering Aspects., 103 (1995) 249. 4. H. Ohshima, Colloid Polym Sci., 275 (1997) 480. 5. H. Ohshima, and K Furusawa (Eds.), Electrical Phenomena at Interfaces, 2nd Edition, Chap. 2, Dekker, New York, 1998. 6. H. Ohshima, J. Colloid Interface Sci., 228 (2000) 190. 7. P. Debye and A. Bueche, J. Chem. Phys., 16 (1948) 573. 8. J.J. Hermans andH. Fujita, Koninkl. Ned Akad. Wetenschap., Proc. Ser. B 58, (1955) 182. 9. S. Kuwabara, J. Phys. Soc. Japan, 14 (1959) 527. 10. H. Ohshima, J. CoUoid Interface Sci., 225 (2000) 233. 11. H. Ohshima, J. CoUoid Interface Sci., 229 (2000) 140. 12. H. Ohshima, J. CoUoid Interface Sci., 229 (2000) 307. 13. V.N. Shilov, N.I. Zharkih and Yu.B. Borkovskaya, CoUoid J., 43 (1981) 434. 14. A.S. Dukhin, H. Ohshima, V.N. Shilov and P.J. Goetz, Langmuir, 15 (1999) 3445. 15. A S . Dukhin, V.N. Shilov, and Yu.B. Borkovskaya, Langmmr, 15 (1999) 3452. 16. A.S. Dukhin, V.N. Shilov, H. Ohshima and P.J. Goetz, Langmuir, 15 (1999) 6692.

Studies in Surface Science and Catalysis 132 Y. iwasawa, N. Oyamaand H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

323

Preparation of nanosize bimetallic particles on activated carbon Shinya Hodoshima*, Takashi Kubono, Shintaro Asano, Hiroshi Aral and Yasukazu Saito *Department of Industrial Chemistry, Faculty of Engineering, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Carbon-supported nanosize platinum-iridium bimetallic particles prepared by impregnation gave uniform compositions, being almost equal to the given mixed ratio of Pt / Ir, at the particle level irrespective of its size, whereas narrow size-distribution together with uniform particle compositions were obtained for platinum-ruthenium nanoparticles by a dry-migration method in spite of heavy metal loading. 1. INTRODUCTION Nanosize bimetallic particles have attracted wide attention in the studies of bimetallic catalysts [1,2], especially on those exhibiting narrow distributions of both size and composition [3]. It was revealed recently that nanosize platinum-iridium bimetallic particles on a carbon support play an important role in hydrogen mobile storage by use of decalin dehydrogenation / naphthalene hydrogenation pair [4], where the dehydrogenation catalyst should be active at the temperature range of waste heats. As well known, nanosize platinum-ruthenium bimetallic particles are widely adopted as the anode catalyst of fuel cell, when reformed hydrogen is used [5]. Moreover, this bimetallic catalyst was found to be an excellent endothermic catalyst of 2-propanol/acetone/hydrogen chemical heat pump [6]. In both cases, heavy metal loading on carbon supports are required for Pt-Ru composite catalysts. In spite of the fact that Pt-Ru / C bimetallic particles within a nanosize range were prepared at heavy metal loading successfully by a dry-migration method [7], where powder mixtures of the Pt / C particles and the H4Ru4(CO)i2 complex were heated and stirred under Nj, followed by H2, atmosphere, conventional preparation methods of coimpregnation from the aqueous solution of mixed chlorides for Pt-Ru / C catalysts gave wide distribution of both size and composition [8]. In the present study, preparation of Pt-Ir and Pt-Ru bimetallic particles has been attempted with impregnation and dry-migration methods in order to control size and composition. 2. EXPERIMENTAL 2.1. Preparation of carbon-supported metal particles The granule powders of microporous carbon [9] ( KOH-activated, BET specific surface area : 3100 m^ / g, pore volume : 1.78 cm^ / g, average pore size : 2.0 nm, Kansai

324

Netsukagaku Co. Ltd. ) were immersed with a NaOH aqueous solution ( pH = 12 or 14 ) for 24 h, followed by washing with distilled water, until base was removed from the bulk solution and the surface except for the micropore of carbon granule. Before metal loading, this basepretreated carbon was evacuated at 70°C for 10 h. Carbon-supported platinum particles ( Pt / C, 14 wt-metal% ) and platinum-iridium bimetallic particles ( Pt-Ir / C, 5 wt-metal%, Pt / Ir mixed ratio = 1 ) were prepared from the powders of base pretreated carbon ( Pt / C : pH = 14, Pt-Ir / C : pH = 12 ) by impregnating with the K2PtCl4 or K2PtCl4 • HzIrCl^j aqueous solutions for 48 h, followed by dropwise addition of the NaBH4 aqueous solution at 90*^0 for 30 min. After standing for another 30 min, they were washed with a large amount of water to remove the residual ions through filtration and then evacuated at 70°C for 10 h. Carbon-supported platinum-ruthenium bimetallic particles ( Pt-Ru / C, 14 wt-Pt%, Pt / Ru mixed ratio = 1 ) were prepared by stirring the mixture of Pt / C and ruthenocene complex under N2 atomosphere at 120°C for 1 h and then H2 at 180°C for 1 h. 2.2. Characterization of carbon-supported metal particles TEM measurements were made on a JEOL JEM-2010 spectrometer at the accelerating voltage of 200 kV. Particle size-distributions and average particle sizes were determined from randomly picked-up 200 particles, using 700,000 x ( Pt-Ir / C ) and 1,500,000 x images ( Pt / C, Pt-Ru / C ). Bimetallic components at the particle level were analyzed by an EDX spectrometer equipped inside the TEM system. UV spectra ( Hitachi U-3300 ) of the residual metallic species ( PtCl4^' : 216 nm, IrCl^^^" : 233 nm ) in the supernatant solutions were recorded separately but time-sequentially during the impregnation procedure. 3. RESULTS AND DISCUSSION 3.1. Characteristics of carbon-supported platinum-iridium nanosize bimetallic particles According to TEM-EDX analyses at the particle level ( Fig. 1(A) ), the carbon-supported platinum-iridium bimetallic particles ( Pt-Ir / C, 5 wt-metal%, Pt / Ir = 1 ) gave uniform compositions almost equal to the given mixed ratio of Pt / Ir irrespective of its size. As revealed from UV measurements of the supernatant solution during impregnation of Pt (II) and Ir (IV) species toward the carbon support ( Fig. 1(B)), their adsorption rates were almost equal. Simultaneous adsorption of these species on carbon seems to be essentially important for keeping the particle composition to be uniform. 3.2. Effect of base pretreatment for carbon and characteristics of carbon-supported platinum-ruthenium nanosize bimetallic particles TEM images and particle size-distributions of Pt / C and Pt-Ru / C particles are shown in Fig. 2(A) and (B), respectively. A base pretreatment ( pH = 14 ) on carbon was effective for preparing homogeneously-spaced and sharply-dispersed nanosize platinum particles in spite of heavy loading (14 wt-metal% ). Strong adsorption of PtCl4^" species by exchanging its CI'

325

^

100 80 |_ Pt-Ir / C ( 5 wt-metal% ) Pt / Ir mixed ratio 1 60

"oo- -6'-^-

40 20 0

Av. particle size : 3.2 nm I

I

I

5 6 7 Particle size / nm TEM image : Direct mag. 400,000 x Ace. vol. 200 kV EDX analyses : Electron beam dia. 2.0 nm Fig. 1(A) TEM-EDX analysis on size dependence of Ir molar fraction in Pt-Ir / C particles at the particle level

X)

5 Impregnation time / h UV measurements of residual ions in supernatant solutions PtCl4^": 216 nm, IrClg^": 233 nm Fig. 1(B) Time course of either Pt (II) or Ir (IV) species adsorbed on carbon support

liga'^d with OH" of base only inside the micropore would make it possible to form a lot of met; lie nuclei at the reduction step with NaBH4 and to surpress the crystal growth of platmum as the result of limited migration. Preparation of sharply distributed and uniformly composed, with the mixed ratio of Pt / Ru reflected, Pt-Ru / C nanosize particles was successfully accomplished by the dry-migration method ( Fig. 2(B) and Table 1 ). After heating ruthenocene under N2 atmosphere over the carbon support for its uniform migration, hydrogen reduction at higher temperatures was attempted to occur only on the surface of platinum particles. 4. CONCLUSION Nanosize Pt-Ir / C bimetallic particles with uniform compositions irrespective of their sizes were prepared by a conventional impregnation method, when the carbon support was pretreated with base. Furthermore, combination of this base-pretreated carbon and the drymigrated ruthenocene toward Hz-adsorbed Pt particles made it possible to prepare Pt-Ru / C with narrow distributions of both size at the nano-range and composition at the particle level. Table 1 TEM-EDX analysis on particle-level composition of carbon-supported platinumruthenium particles prepared by the dry-migration method Ru / mol% B / mol% 48.7 51.3 47.3 52.7 43.6 56.4 Particles : The sample as illustrated as in Fig. 2(B), Electron beam diameter : 0.5 nm Particle no.

326

f

^

* ^ . ^ *

'^Mm-' TEM image : Direct mag. 150,000 x. Ace. vol. 200 kV

13 34 njn

100 ^

a £ 0 12 3 4 5 6 7 Particle size / nm (A)Pt/C(14wt%)

80 _ Av. particle size 2.7 nm 60 40 20 I

1 —

X

1

1

0 12 3 4 5 6 7 Particle size / mn (B) Pt-Ru / C (14 wt-Pt%, Pt /Ru = 1)

Fig. 2 TEM images and particle size distributions of Pt / C, with the carbon-support basepretreated, and Pt-Ru / C, with ruthenocene dry-migrated to the hydrogen-treated surface of platinum.

REFERENCES [1] J. H. Sinfelt, "Bimetallic Catalysis : Discoveries, Concepts and Applicataions", Willey, New York, ( 1983 ). [2] B. C. Gates, L. Guczi and H. Knozinger, "Metal Clusters in Catalysis", Elsevier, Amsterdam, ( 1986 ). [3] M. S. Nasher, A. I. Frenkel, D. L. Adler, J. R. Shapley and R. G. Nuzzo, J. Am. Chem. Soc, 119 ( 1997) 7760. [4] S. Hodoshima and Y Saito, J. Hydrogen Energy Systems, 24 (1999) 13. [5] H. Takenaka, Handbook of Electrochemistry, ed., *The Electrochemical Society of Japan", Maruzen, Tokyo ( 2000 ), pp. 537-539 ; K. Tsurumi, Catalysts and Catalysis, 41 (1999 ) 547. [6] Y Saito, H. Kameyama and K. Yoshida, Int. J. Energy Res., 11 (1987) 549. [7] M. Nakabayashi, M. Yamashita and Y Saito, Chem., Utt., (1994) 1275. [8] Y Saito, H. Ogino and T. Fukushima, Kagaku Kogaku Ronbunsyu, 21 ( 1995 ) 984. [9] T Otowa, Hyoumen ( Surface), 34 ( 1996) 62.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) lO 2001 Elsevier Science B.V. All rights reserved.

327

Preparation of the PVA film with gold fine particles by a counter diffusion method: Effect of diffusion on the distribution of gold fine particles in the film Shizuko Sato, Tsuyoshi Shiono, Hirotoshi Sato, Masaya Tsuji, Masakatsu Yonese Faculty of Pharmaceutical Sciences, Nagoya City University, Tanabe-dori 3-1, Mizuho-ku, Nagoya 467-8603, Japan 1. Introduction The fabrication of ordered superlattices from nanocrystals is one of the subjects that recently occupied considerable attention [1,2]. If nanocrystal is formed in the media with the order, we can easily obtain the ordered lattices of nanocrystals. We have studied on the formation of colloidal gold in the presence of an acetylenic glycol nonionic surfactant, Surfynol 465 [3-6]. This method had marked advantages in the simplicity (formed by mixing two aqueous solutions) and the temperature (in the wide range from 5 °C to 95 °C). In this work, we formed a colloidal gold in a polymer film matrix to have a polymer film with gold particles. As a polymer polyvinyl alcohol (PVA) was used. 2. Experimental 2.1. Materials Tetrachloroaurate, HAUCI4 • 3H2O, was obtained from Aldrich Chem. Surfynol 465 (number-averaged molar mass: 600 g mol"') was a gift from Air Product and Chemicals, U.S.A. PVA (average degree of polymerization : 200) was from Wako Pure Chemical Industries, Ltd., Japan. Deionized and distilled water was used. 2.2. Apparatus The UV-VIS absorption spectrum of film was measured using a spectrometer (type U-3000, Hitachi, Japan). Gold particles in film were observed by a transmission electron microscope (type H-7500, Hitachi, Japan). Gold particles on the surface of film were observed by a scanning electron microscope (type S-4300, Hitachi, Japan). The energy dispersive spectrum of film was measured using a spectrometer (type Emax-7000, Horiba, Japan). 2.3. Procedure A PVA film was prepared casting PVA aqueous solution (1 wt%) in a stainless ring on polyethylene sheet. The PVA film was treated with formalin to be insoluble in water. The thickness of PVA film was 10-13 jim. To form a colloidal gold in the inside of PVA we used the twin-cell device with a diaphragm holder in between. After the PVA film was set in the holder as diaphragm, aqueous solutions of HAuCl4and Surfynol 465 were separately put into

328

Au-PVA

PVA

S465

HAUCI4

Surfynol 465 / PVA film / HAuCI^ Fig.l. Experimental system of the preparation of Au-PVA film each cell. We proceeded the experiment on a Surfynol 465 / PVA film / HAUCI4 system (Fig.l). The concentration of solution in the cell was regulated by two methods. Method A and Method B. In Method A, two solutions in twin-cell were renewed at one day intervals, and the concentrations changed periodically. In Method B, two cells were continuously supplied with fresh solutions by pumps to keep the concentrations in constant. 3. Results According to the experimental system in Fig.l, each cell was filled with HAUCI4aqueous solution (0.2 mM) or Surfynol 465 aqueous solution (6 mM). Several hours after the PVA film was lightly tinted pink. In the UV-VIS spectrum of PVA film an absorption peak at 530 nm was observed. It was suggested that HAUCI4 and Surfynol 465 entered from two opposite surfaces of PVA film into the inside and formed a colloidal gold in the film matrix. The absorbance of peak increased with time. Thus, we could have a PVA film in which the gold fine particles were embedded. The film was called a PVA film with gold fine particles or an Au-PVA film. Two days after the Au-PVA film was wine red in color and could not have a significance difference in appearance between the films prepared by two methods. However, in the Au-PVA film which was provided with HAUCI4 and Surfynol 465 for a long time we observed a distinct difference between two methods. Now, we studied on the Au-PVA films prepared four days, i.e., 96 hours, in duration through an electron microscopy. 3.1. Transmission electron micrograph (TEM) Figure 2 showed TEM of the cross section of the Au-PVA film prepared by Method A. A lot of number of gold fine particles was observed in the film. In the inside of film the particles without aggregation were distributed exclusively in Surfynol 465 site than in HAUCI4 site. It was indicated clearly that a gold particle was formed in the PVA film matrix. 3.2. Scanning electron micrograph (SEM) In TEM in Fig.2 we observed many particles around the surface of the Au-PVA film. To visually examine the location of particles the surface of film was directly observed by SEM. Figure 3 showed SEM on the surface of the Au-PVA film prepared by Method A. On two surfaces we observed many particles and aggregates. The number of particle was extremely larger in Surfynol 465 site than in HAUCI4 site, showing the agreement with the results of the inside of film. On the other hand, in the Au-PVA film prepared by Method B we observed a little particle or aggregate on the bare plain surface (Fig. 4).

329

j i

S465

" —y

'

-..- ' •:.

HAUCI4 2 ^A

Fig. 2. TEM of the cross section of the Au-PVA film prepared by the method A.

(a)

(b)

2jia

Fig.3. SEM of the surface of the Au-PVA film prepared by the method A. (a) Surfynol 465 site, (b) HAuCU site

(a)

(b)

Fig.4. SEM of the surface of the Au-PVA fihn prepared by the method B. (a) Surfynol 465 site, (b) HAUCI4 site

2jia

330

Au-M„,

H

5

U

V*>v«,*>*\jifW*\^y
2

Energy /

3

keV

Fig. 5. Energy dispersive spectrum of the surface of the Au-PVA film 3.3. Energy dispersive spectrum (EDS) The EDS on the surface of the Au-PVA film in Fig. 5 showed the peak due to Au-Mal [7]. It was exactly confirmed that the particle or the aggregate observed in TEM and SEM was made of gold. 4. Discussion In the inside of Au-PVA film the gold fine particles were distributed exclusively in Surfyno 465 site. From the fact that the diffusion rate of HAUCI4 in the PVA film was remarkably larger than the rate of Surfyno 465, it was reasonably expected that the particles finally located near Surfynol 465 site. Also, we observed many particles and aggregate on the surfaces of film prepared by Method A. The number of aggregate on the surface was greater in Surfynol 465 site than in HAUCI4 site. The depth of x-ray penetration of gold is about 150 nm at 15 keV [8]. The aggregates were located in the outer region of film, and not the interior of film. Thus, it might be say that through the PVA film HAUCI4 passed to another cell contained Surfynol 465, and formed a colloidal gold. Some gold particles in the cell adsorbed on the film surface. With time the particle adsorbed steadily on the surface to completely cover the surface with particle, and finally formed an aggregate.

REFERENCES 1. C.B. Murray, C.R. Kagan and M.G. Bawendi, Sciences, 270 (1995) 1335. 2. C.R Collier, R.J. Saykally, J.J. Shiang, S.E. Henrichs and J.R. Heath, Sciences, 277 (1997) 1978. 3. S. Sato and H. Kishimoto, J. Surface Sci. TechnoL, 8 (1992) 209. 4. S. Sato, H. Sezaki and H. Kishimoto, Prog. Colloid Polym. Sci., 93 (1993) 277. 5. S. Sato, Colloid Polym. Sci., 274 (1996) 1161. 6. S. Sato, K. Toda and S. Oniki, J. Colloid Interface Sci., 218 (1999) 504. 7. D.R.Lide (eds) Handbook of Chemistry and Physics, CRC Press, Boston, 1999. 8. L.S. Birk, J. Appl. Phys., 33 (1962) 233.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

331

Optical properties of ZnS:Mn nanoparticles in sol-gel glasses Yoshikuni Uchidaand Kazunori Matsui* Department of Industrial Chemistry, College of Engineering, Kanto Gakuin University, Mutsuura-cho 4834, Kanazawa-ku, Yokohama 236-8501, Japan. Mn^^-doped ZnS nanoparticles dispersed in silica were prepared by the sol-gel process. The particles were grown under various stages of sol-gel processing. The luminescence spectra mainly show a blue band (~ 430 nm) and an orange band (- 600 nm). The relative intensity of the bands changes according to the preparation conditions and aging time. Explanation for these results is done based on the adsorption of Zn^^ and Mn^^ on siloxane polymers and the structural relaxation in the nanoparticles. 1. INTRODUCTION Semiconductor nanoparticles have attracted much attention in the past decade because of their unique physical properties such as quantum size effects, nonlinear optical properties and luminescence ll-4|. Dispersions of semiconductor nanoparticles in an optically transparent matrix will have various applications including the above properties. There is considerable interest in semiconductor nanoparticles trapped in sol-gel silica from the viewpoints |5-7|. In this work, we have prepared sol-gel glasses doped with ZnSiMn nanoparticles synthesized from aqueous solutions of Zn(N03)2, ^"(^03)2. and NSLJS. In order to study the effect of the sol-gel reaction on the growth of ZnS:Mn particles, the precursor solutions were added at various stages of the sol-gel process such as the initial solution and the gel. 2. EXPERIMENTAL Zn(N03)2-6H20 (Aldrich), Mn(N03)2-6H20 (Aldrich), Na.S-QH.O (Wako), tetramethoxysilane (TMOS, Tokyo Kasei), and methanol (MeOH, Wako) were used as received. Water was deionized and distilled. Synthesis of ZnS:Mn-doped sol-gel silica was carried out at room temperature as shown in Fig. I: (1) 5 mL of TMOS, 5 mL of water, and 5 mL of MeOH

332

were mixed by stirring for 0.5 h in the presence TMOS + H2O + MeOH with catalysis of HCI

of HCI to adjust the pH to be 4; (2) 2.0x 10' M aqueous solutions of the following compounds were added to the sol-gel solutions in the order

lt3 = 0-^24h

of Zn(N03)2-6H20 (I mL), Mn(N03)2-6Hp

Zn(N03)2 aqueous solution

1

before adding these solutions to the sol-gel

^ ~ | Mn(N03)2 aqueous solution

1

solutions ( t j was changed at 0 - 24 h.

^ ~ | Na2S aqueous solution

(0.1 mL) and Na2S-9H20 (1 mL).

^

The time It

V

should

be noted here that the gel

was

1

.—_—M

• ZnSMn doped sol

impregnated with the colloidal solutions at t^ = 24 h because gelation had occurred already; (3)

1

sol-gel transition drying

the mixtures were stirred further for 0.5 h and •

allowed to gel and dry in polystyrene beakers

ZnS.Mn doped xerogel 1

sealed with plastic film with several pinholes. Luminescence (X^^= 300 nm) and excitation

Fig. 1. Scheme of the process used for the preparation of ZnS.Mn nanoparticle-doped silica xerogel.

(>^,n = 600 nm) spectra were recorded on a JASCO FP-777 spectro-fluorometer.

3. RESULTS AND DISCUSSION Fig. 2 shows the luminescence and excitation spectra of ZnSiMn particles in a sol-gel system just prepared (a) and after 4 weeks (b).

These spectra are essentially similar to those

observed in aqueous colloidal ZnS:Mn nanoparticles.

In general, the luminescence spectra

can be divided into three broad bands ~ 380 nm (ultraviolet band), - 430 nm (blue band), and - 600 nm (orange band).

The orange luminescence band is due to the "^i -t^A, transition of

Mn^^ located at Zn^^ lattice sites on the inside of the nanoparticles |2|.

The ultraviolet band

is attributed to Mn^^-activated ZnS nanoparticles (viz., Mn^* introduced on the outside of the ZnS) |2|.

As for the blue band, the origin is ascribed to defect sites such as vacancy and/or

interstitial sites of Zn^^ and/or S^, although the details are still uncertain |2,4|. The luminescence spectra at tg = 0 and 3 h was similar for particles just prepared as shown in Fig. 2(a), whereas there was a slight difference in the ultraviolet and blue band regions. significant decrease in intensity of the orange band was observed for ta = 24 h.

A The

luminescence intensity of the blue band against the orange band (Ibi^/'orangc) at aging time of 0 d (just prepared) increased with an increase in t^ as shown in Fig. 3.

The particle diameters

estimated by the excitation peaks are - 2.5 nm for t^ = 0 and 3 h, and - 2.0 nm for t^ = 24 h |31. These results show that the particle diameter decreases and the number of defect sites

333 increases at initial growth of ZnSiMn particles as the sol-gel reactions such as hydrolysis and polymerization of TMOS proceed.

It is thus suggested that a polymerized siloxane network

acts as a confinement matrix for controlling the growth of ZnS:Mn particles. The luminescence and excitation spectra aged 4 weeks are shown in Fig. 2(b).

Different

changes are seen for the luminescence; the orange band decreases for t^ = 0 and 3 h, but increases for i^ = 24 h.

The detailed results of aging are shown in Fig. 3.

Ibiue/lorange for ta = 0

~ 3 h gradually increase, while Ibiue/'oiange for t. = 24 h decrease considerably within 1 day and then gradually decrease. Murase et al. reported for aqueous solution that the orange luminescence increases by about I order of magnitude within I day, while blue luminescence is rather insensitive to time after the preparation |4|.

The results were explained as follows; the orange luminescence

efficiency of Mn^^ inside the nanoparticles is low at the early stage of preparation because the excitation energies are transferred into the randomly distributed defect centers in the vicinity of Mn^^ ions.

These defect centers will move to the surface, and its number in the crystal

will decrease with time in the colloidal solution, leading to the increase in the orange luminescence.

2000 r 0.7 F "I O t,= 0 h O t,= 3 h 0.6 ^J D t,=0.5h • t.=24h H 1 A t,= 1 h

0.b[300

400 500 600 Wavelength/hm

700

c

0.4 h

A

T

^

0.3 h 0.2 [ 0.1 h

a

0.0 b- 1 0 300

400 500 600 700 Wavelength/hm Fig. 2. Luminescence and excitation spectra of ZnSiMn particles in the sol-gel system: just prepared (a) and after 4 weeks (b) for different adding time {i^: Oh, ;3 h, ;24h, - - -.

• . _L- . _l 10 20 Aging time/d

j_-d

Fig. 3. Relative intensity of the blue band against orange band for various ta as a function of time.

334

According to Weller et al., the blue luminescence increases by adding SiOj particles during the precipitation of ZnS nanoparticles |8|. That is attributed to the formation of vacancies due to adsorption of Zn^* ions by SiOj particles. Considering these findings and our results together, we can explain the present results as follows. For ZnS:Mn nanoparticles prepared during the early stage of sol-gel processing such as ta = 0 ~ 3 h, the growth reaction may proceed in a similar manner as in aqueous solution with a slight restriction. The structural relaxation in ZnSrMn nanoparticles and polymerization of TMOS simultaneously occur after the preparation. At pH 4, the net charge of the siloxane polymer is negative due to Si-O groups. Zn^* vacancies thus would increase during the structural relaxation in ZnS:Mn nanoparticles by the adsorption of Zn^* on the polymer surface, resulting in an increase in Iwuc/IorangcAt ta = 24 h, where the polymerization of TMOS has proceeded considerably and gelation has occurred, ZnS:Mn nanoparticles grow in relatively confined environment of siloxane polymers, as indicated by the smaller particle diameter - 2.0 nm in comparison with those 2.5 nm for t, = 0 and 3 h. In addition, the negative siloxane polymer surface can adsorb Zn^* and Mn'^, resulting in an increase in the blue band due to Zn^^ vacancies and a decrease in the orange band due to Mn^^ at Zn^^ lattice sites. Subsequently, the structural relaxation in ZnSiMn nanoparticles gradually proceeds, increasing the orange luminescence as explained by Murase at al |4|. It seems probable that the interaction between the surface of ZnS:Mn nanoparticles and silica matrix is different for ZnSiMn nanoparticles incorporated into the silica network which was beforehand completed and ZnSiMn nanoparticles grown during the polymerization of siloxane network, inducing the different relaxation behaviors. REFERENCES 1. R.N. Bhargava, D. Gallagher, X. Hong and A. Nurmikko, Phys. Rev. Lett., 72 (1994) 416. 2. K. Sooklal, B.S. Cullum, S.M. Angel and C.J. Murphy, J. Phys. Chem., 100 (1996) 4551. 3. Y. Nakaoka and Y. Nosaka, Langmuir, 13 (1997) 708. 4. N. Murase, R. Jagannathan, Y. Kanematsu, M. Watanabe, A. Kurita, K. Hirata, T. Yazawa and T. Kushida, J. Phys. Chem. B, 103 (1999) 754. 5. Y. Zhang, N. Raman, J.K. Bailey, C.J. Brinker and R.M. Crooks, J. Phys. Chem., 96 (1992) 9098. 6. K.M. Choi and K.J. Shea, J. Phys. Chem., 98 (1994) 3207. 7. G. Counio, S. Esnouf, T. Gacoin and J.-P. Boilot, J. Phys. Chem., 100 (19%) 20021. 8. H. Weller, U. Koch, M. Gutierrez and A. Henglein, Ber. Bunsenges. Phys. Chem., 88 (1984)649.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

335

Sonochemical preparation of noble metal nanopartides in the presence of various surfactants E. Takagi» Y. Mizukoshi^ R. Oshima*> Y. Nagata^

H. Bandowa and Y Maeda«

^Faculty of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, 599-8531, Osaka, Japan **Research Institute for Advanced Science and Technology, Osaka Prefectxire University, 1*2, Gakuen-cho, 599-8570, Sakai, Osaka, Japan Noble metal particles were prepared by the sonochemical reduction of noble metal ions, NaAuCU (Au (III)), PdCk (Pd (II)), H2PtCl6 (Pt (IV)) and K2PtCl4(Pt (H)) in the aqueous solution containing surfactants, such as sodium dodecylsidfate (SDS), sodium dodecylbenzensulfonate (DBS) and polyethylene glycol monostearate (PEG-MS). The sonochemical reduction rates of Au (lU) to Au (O), Pd (II) to Pd (O), Pt (IV) to Pt ai) and Pt (II) to Pt (0) in SDS solution (CMC; 8 mM) are 83,140, 97 and 24 jiM/min, respectively. The size of metal nanopartides were measured by TEM, in the case of SDS solution, the particles of Au, Pt and Pd are 13 nm, 3 nm and 7 nm in diameter with narrow distribution. The reduction rates in PEG-MS (Au (III) to Au (0); 380 nM/min, Pd (E) to Pd (O); 330 pM/min, Pt (IV) to Pt (II);150 MM/min) were faster than those in SDS and the size of nanopartides of Pd (4 nm) in PEG:MS was smaller than that in SDS. The size of the particles in the growing process became feasible to be measured by removing coexisting ions at each desired time by means of anion-exchange resin. 1. INTRODUCTION MetaUic nanopartides have been widely studied in various fields because of their characteristic and attractive properties such as catalytic activity, optical and magnetic property til. And many methods of preparation of metal particles have been reported such as the reduction by chemical reductants^^^sJ, photochemical or radiation chemical reductionists! and gas evaporation^^'^l In this paper, we report

336

that noble metal ions in aqueous phase are sonochemically reduced to form stable nanoparticles in the presence of various surfactants. 2. EXPERIMENTAL Ultrasonic irradiation was carried out by using a multiwave ultrasound generator with a barium titanate oscillator (Kaijo 4021, 65 mm diameter, 200kHz, 6 W/cm2). The argon-purged aqueous solution of noble metal ions containing surfactants in the glass vessel was sonicated in a water bath. During the sonication, the vessel was closed to prevent the solution exposing to air. All chemicals piuxihased from Wako Chemicals were reagent grade and used without further purification. 3. RESULTS AND DISCUSSION 3.1. Reduction of noble metal ions The aqueous solution of Au (III) (l mM) containing SDS or PEG-MS was completely reduced to form nanoparticles by 15 minutes sonication. During the sonication, the color of the solution turns pale-yellow to reddish-violet. In the case of Pd (II), the reduction finished in 15 minutes sonication (pale-yellow to dark-brown). The reduction of Pt ions was completely reduced in about 90 minutes sonication (pale-yellow to dark-brown). Table 1 Reduction rates of noble metal ions in the presence of various surfactants

Metal ions Pt(IV)^Pt(n)

PtaO^Ptto)

Auan)-^Au(o) PdOD-PcKo)

Surfactants SDS(8mM) DBS (3 mM) PEG-MS (0.4 mM) SDS (8 mM) DBSOmM) PEG-MS (0.4 mM) SDS(8mM) PEG-MS (0.4 mM) SDS (8 mM) PEG-MS (0.4 mM)

Rate (jiM/min) 97 75 149 24 22 23 83 383 140 328

337

Table 1 shows the reduction rates of noble metal ions with sonication. The reduction rates of Au (lU) to Au (O), Pd ttl) to Pd (0) and Pt (IV) to Pt (ID in PEG-MS were faster than those in SDS. In the case of Pt (ID to Pt (0), however, the reduction rate in PEG-MS was equal to that in SDS. The differences of the reduction rates may be caused by the reactivity of each ion to each radical produced from surfactants^^^ by sonochemical reaction in aqueous solution. 3.2. The size of noble metal nanoparticles TEM images of Pd nanoparticles produced by the sonochemical method are shown in Fig.l. The size of Pd nanoparticles in SDS was 7 nm, in the case of Au and Pt, the size of nanoparticles were 13 nm and 3 nm respectively. The size of Pd particles in SDS was larger than that of 4 nm in PEG-MS. The same tendency was observed in the case of Au and Pt. The size of nanoparticles seems to be correlated with the reduction rates, the kinds of siufactants and the stabilizing abihties of the surfactants. 3.3. Change of the size of nanoparticles in reduction process It was revealed that coexistent ions in the sonicated solution can be removed by means of anion-exchange resin. This makes possible to investigate the morphology of the particles in the sonochemical growing process by TEM observation. It was found that characteristic UV-VIS absorption due to Pd (II) disappeared by the treatment of sonicated solution by anion-exchange resin (Fig.2), and the TEM images of the particles in the initial stage of the growing process could be investigated (Fig.3).

(a) S

(b)

' 4 *

•*

* »

,-•

20 nm

^' .^^;j20nm

Fig.l. TEM images of Pd nanoparticles (a) in 8 mM SDS after the completion of the reduction (Pd; 1 mM, 15 min sonication). (b) in 0.4 mM PEG-MS after the completion of the reduction (Pd; 1 mM, 15min sonication) .

338

20 nm 250 300 350 400 450 500 Wavelength (nm) Fig.2. UV-VIS spectra of Pd /SDS solution after 90s sonication. (a) The spectrum of the solution. (b) The spectrum of the solution treated by anion-exchange resin.

Fig.3. TEM images of Pd particles in SDS in the initial stage of the growing process (90 s sonication, Pd (0) / Pd (II) = 0.24 mM / 0.76 mM)

In this stage, though the size of particles was 4 nm and smaller than that observed in Fig. 1. (a) (7 nm), the number of particles in this stage was 6.8 x lO^^ and it was almost same as that in Fig 1. (a) (7.5 x lO^^). From the result, the nucleation rate of Pd (0) may be roughly equal to the coagulation rate of Pd particles. In the case of Au (III) solution, coexistent Au (III) ions were also found to be removed by this method, and the size of Au particles in the growing process will be able to be measured in similar manner. 4 REFERENCES 1. G. Schmid, Ed. CoUoids and Clusters, VHC Press, New York (1995). 2. L. Zeiri and S. Efirima, J. Phys. Chem. 96 (1992) 5908. 3. R. Seshadri et al., J. Phys. Chem. 99 (1995) 5639. 4. D. Lawless, S. Kapoor, P. Kennepohl, D. Meisel and N. Serpone, J. Phys. Chem. 98 (1994) 9619. 5. N. L. Pocard, D.C. Alsmeyer, R. L. McCreery, T. X. Neeman and M.R. Callstorm, J.Am. Chem. Soc., 114 (1992) 769. 6. N. Satoh and K Kimura, BuU. Chem. Soc. Jpn., 62 (1989) 1758. Z R. P. Andres et al., J. Mater. Res., 4 (1989) 704. 8. Y. Mizukoshi, R. Oshima, Y. Nagata and Y Maeda, Langmuir 15 (1999) 2733.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

339

Liquid-phase synthesis of Y203:Eu precursor particles from homogeneous solution Yoshihiro Nishisu* and Mikio Kobayashi* ^Materials Processing Department, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba-Shi, Ibaraki, Japan Liquid-phase synthesis of precursor fine particles of Y203:Eu from homogeneous aqueous solution using urea as a precipitant generator with and without addition of colloidal nucleus-particles was investigated. Spherical monodispersed fine particles of the submicron order were formed fi-om the homogeneous solution of 0.01 mol/dm^ YCI3 and EuCb- By using nucleus-particles, the particle size changed fi-om about SOOnm to lOOOnm. Calcined particles showed fluorescence property of 61 Inm, which was peculiar for Y203:Eu phosphor. 1. Introduction Europium (Eu^"") -doped yttrium oxide phosphor (Y203:Eu) particles are very important as a red phosphor for application of displays and fluorescent tubes [1]. These phosphor particles are generally required to be monodispersed and to have homogenous shape. The size of phosphor particles is very important for high resolution of display and high efficiency of luminescence [2]. The conventional combustion method which most of all phosphors for displays made by is simple and economical but not easy to control shape and size of phosphors. In this work, liquid-phase synthesis of precursor fine particles of Y203:Eu fi-om homogeneous aqueous solution using urea as a precipitant generator with and without addition of colloidal nucleus-particles was investigated. 2. Experimental Method YCI3-61120, EuCl3-6H20, Y2O3 099.9% purity) and urea (reagent grade) were used as starting materials and a precipitant generator, respectively. Homogeneous stock solutions were prepared by the dissolution of the reagents in pure water. The particles were synthesized through the precipitation reaction of the stock solution with and without addition of the particles as nuclei. Precipitation was carried out for 60 min at 97 °C in the reactor with a reflux condenser. The nucleus-particles were synthesized using the part of stock solution in the same way. Precipitates were washed and then the solvent, water, was

340

replaced with 2-propanol to prevent granulation of the precipitates during drying. The total metal ion concentration was 0.01 mol/dm^ and the concentration ratio of Y to Eu in the initial solution was changed. The average particle size of precipitate and size distribution was observed with transmission electron microscope, TEM. The yields of Y and Eu as the particles were obtained from the ion concentration in the supernatant. The part of the synthesized particles was dissolved with the acid solution, and the ratio of Y to Eu in the particles was calculated from the concentrations of Y and Eu in this solution. For measuring of metal ion concentration in the solution, ICP spectrometer was used. The fluorescence spectra of fine particles were measured by the spectrophotofluorometer with excitation wavelength of 254nm of Xe lamp. 3. Results and Discussion

Table 1. Concentration condition.

3.1. Precipitate witliout Nucleus-Particles The concentration condition of the synthesis without the nucleus-particles is shown at Table 1. Both metal ion of Y^^ and Eu^"*^ precipitated by aging the stock solution at 97 "C for 1 hour, at the yield over 99%. The TEM photograph of the precipitate is shown in Fig. 1. The shape of the precipitated particles was spherical and monodispersed. Figure 2 shows the size distribution of the particles. Average particle size was about 300nm on the system only including Y as a metal composition. And, it was about lOOnm on the system only including Eu. On the system including both, particle size changed by the Y/Eu ratio in the nonlinearity. It

Concentration (lO'^mol/dm^) YCb EuCb Urea

10 9.5 9.0 8.5 8.0 6.0 4.0 2.0 0

0 0.5 1.0 1.5 2.0 4.0 6.0 8.0 10

500 500 500 500 500 500 500 500 500

200 400 600 Particle size (nm) Fig. 1. TEM photograph of the particles obtained from 0.5 mol/dm^ urea solution of 9.5 X 10-^ mol/dm^ YCI3 and 0.5 x 10'^ mol/dm^ EuCb.

Fig. 2. The volume frequency distribution of the particles obtained from 0.5 mol/dm^ urea solution of 9.5 x 10"^ mol/dm^ YCI3 and 0.5 xlO-^mol/dm^EuCl3.

341

increased from 100 to 300nm (Fig. 3) as the ratio of concentration of Eu in the stock solutions were decreased. In spite of the change of particle size, there was no the remarkable extent for the particle size distribution, and there was no difference of the shape. These results mean that the synthesis of the YzOsiEu precursor fine particles of uniform shape and size is possible by this coprecipitation reaction, even if the activation quantity of Eu changes.

350 ^ 3001

I 250 B

200

E 150 > 100

0 0.2 0.4 0.6 0.8 1 3.2. Precipitate with Nucleus-Particles [Eu]/[Y+Eu] in initial solution The experimental condition of the synthesis with the nucleus-particles is shown at Table 2. The Fig. 3. The relationship between the nucleus-particles were synthesized using solution A mean diameter of the particles and of various volumes by the procedure equal to the [Eu]/[Y+Eu] in the initial solution. synthesis without nucleus-particles. The number of the nucleus-particle is proportional to the volume of the solution A, as number of generation of the particle per unit volume is equal. The shape of the precipitated particles was spherical and monodispersed as well as the system without the nucleus-particles. The particle size was bigger than the system without the nucleus-particles under the equal concentration condition. In addition, the particle size is different according to the number of nucleus-particle, and it increased with the decrease in the nucleus-particles. For example, it changed from about 500nm to lOOOnm in the experimental condition shown in Table 2. These results show that the final number of generated particle was limited by using the nucleus-particle. The number of nucleus-particles was easily adjusted by the volume of solution A, and it could widely control the size of finally composed particles. Table 2. Experimental conditions. Initial Volume of Concentration (10"Wl/ciiT1^) solution A PH YCh EuCb Urea (dm') 0.7 50 9.5 0.5 6.2 9.5 0.5 50 0.5 6.3 9.5 0.5 50 6.0 0.3 9.5 0.5 50 0.1 6.5

Volume of solution B (dm') 0.2 0.4 0.6 0.8

Final Mean pH diameter (nm) 8.6 566 8.6 634 8.4 780 968 8.8

3.3. Characterization of the Precipitate The ratio of Y to Eu in the coprecipitated spherical monodispersed particles synthesized from the homogeneous solution including Y, Eu and urea, could be controlled by adjusting the concentration of the starting materials in the stock solution. Composition of the synthesized fine particles was guessed with basic carbonate including COs^' and OH", which derive from the hydrolysis of the urea [3,4]. XRD patterns of the spherical monodispersed

342

fine particle around the calcination are shown in (b) after calcination Fig. 4. The synthesized precursor fine particles were amorphous (Fig. 4(a)), and they were inverted to (Yp,Euq)203 (p+q=l) of the single phase by ^ calcination over about 600 t (Fig. 4(b)) [3]. \ \ kk,k i «A« i.iAi Though the size of the precursor fine particles : I befor calcination decreased about 20% by the calcination, the effect i (a) was not remarkable for spherical sh^>e and j characteristics of the monodispersion. The fluorescence spectra of the fine particles around the calcination are shown in Fig. 5. The fine particle 10 20 30 40 50 60 70 80 before the calcination hardly showed fluorescence 2 0 (degrees) property. The fine particles that were inverted to Fig. 4. X-ray diffraction patterns of the oxide by the calcination showed remarkable and the spherical monodispersed fine red luminescence property. They showed the particles around calcinations at 850 peculiar emission spectrum with sharp luminescence peak of the 611nm wavelength. This luminescence property agrees with characteristics of the Y203:Eu After phosphor. There was no effect by the difference in 1 calcination present synthesis condition for the form of the emission /'^^ spectrum. On the other hand, the emission intensity ti^ became a slightly high value, as particle size increased. •e < From these results, spherical monodispersed fine ^^ particle synthesized by this liquid-phase method is G concluded with that the utilization is possible as a a> jd precursor of the Y203:Eu phosphor. And, the use of 1 Before the nucleus-particles can be ^plied as an effective calcination technique of the particle size control.

K

AMJ

REFERENCES

560 580 600 620 640 660 Wavelength (nm)

1. J. Koike, T. Kojima, R. Toyonaga, A. Kagami, T. Hase Fig. 5. Emission spectra of the and S. Inaho, J. Electrochem. Soc, 123 (1979) 1008. spherical monodispersed fine 2. R. E. Siever, P. D. Milewski, C. Y. Xu and B. A. particles around calcination. Watkins, Proceedings of the 3rd International Conference (Excitation: 254 nm). on the Science and Technology of Display Phosphors, Huntington Beach, CA, 303 (1997). 3. Y. Nishisu and M. Kobayashi, Proceedings of the 1st Int. Conference on Processing Materials for Properties, 549 (1993). 4. B. Aiken, W. P. Hsu and E. Matijevic, J. Am. Ceram. Soc, 71 (1988) 845.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

343

Processing of zirconia and alumina fine particles through electrophoretic deposition Y. Sakka^ B. D. Hatton^ and T. Uchikoshi^ ^National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, 305-0047, Japan ^ University of Toronto, #140, 184 College St., Toronto, Ontario, Canada M5S 3E4 The effect of washing on the stability, electrophoretic deposition (EPD), and density of consolidated body for aqueous and nonaqueous suspensions of asreceived zirconia (TZ3Y, Tosoh Corp.) and alumina fine particles are presented. An increase in the zeta potential and a decrease in the particle size of the zirconia-ethanol system were observed due to washing. As a result, the EPD rate increased dramatically from the as-received condition. There were no significant beneficial effects of washing on the aqueous system, indicating a much greater sensitivity to ionic contaminants in an ethanol suspension, especially for zirconia. Green densities prepared by EPD using aqueous suspensions were higher than those prepared by EPD using ethanol suspensions and similar to those obtained by shp casting using aqueous suspensions. 1. INTRODUCTION Fine particles will inevitably aggregate due to van der Waals attraction and other forces, to an extent that depends on their processing history. Colloidal processing has been developed for consohdating fine particles to avoid heterogeneous agglomerates by using electrostatic repulsive forces and/or steric stabilization, since they greatly eliminate large pores and reduce the sintering temperature, resulting in a dense and fine microstructure [1]. Electrophoretic deposition (EPD) is a useful colloidal processing technique for consolidating fine particles and fabricating films, multilayered composites, etc. [2]. The process of EPD usually requires the use of nonaqueous colloidal suspensions, to avoid the problems of void formation associated with an aqueous solvent by the electrolysis of H2O. Recently, we found that no voids were formed in a deposit prepared by aqueous suspension using a palladium substrate, since palladium electrodes absorb hydrogen caused by electrolysis of water [3]. The preparation and characteristics of suspensions are important for factors such as the deposition rate, zeta potential, and particle size distribution. Experimental results showing the effect of washing on the stabihty and EPD for aqueous and nonaqueous suspensions of as-received zirconia and alumina fine particles are presented here. A comparison is also made between the consolidated bodies obtained by EPD and those obtained by slip casting.

344

2. EXPERIMENTAL Particles used were 3 mol% Y2O3 doped tetragonal ZrOg (TZ3Y; average particle size: 60 nm; Tosoh Corp.) and three types of AlgOgi a-AlgOg (TMDAR; 0.15 |Lim; Taimei Chem.), a-AlgOg (AKP50; 0.20 jun; Sumitomo Chem.) and Y-AI2O3 (Nanotek; 33 nm; CI Chem.). Suspensions of fine particles (5 vol% solids concentration) were prepared in ethanol (EtOH) or distilled water. Washing of the powder using EtOH or water was performed by first separating the powder from the supernatant by centrifuging (5,000-10,000 rpm) until the supernatant was clear. After washing, the as-received and the washed powders were redispersed in suspension using an ultrasonic horn for 10 minutes at 20 kHz and 120 kW, and HNO3 was used to adjust the pH to 4.0, the point at which the zeta potentials of ZrOg and AI2O3 have been reported to be large enough to stabilize the suspensions both in diluted EtOH [4] and aqueous systems [5]. Measurements of particle size distribution and zeta potential were made by electroacoustic spectroscopy directly on the electrostatically stabilized suspensions without dilution. The details are described elsewhere [6]. Calibration of the equipment was performed using standard monodispersed silica suspensions. EPD deposition rates of the colloidal suspensions were measured from the weight deposited on stainless steel or palladium electrodes using a galvanostat operated at a fixed current. The samples were dried after deposition and separated from the substrate. Bulk density was measured by Archimedes' method using kerosene. Inductively coupled plasma spectroscopy (ICP) was used to analyze the Na* concentration of the TZ3Y powder after washing. 3. RESULTS AND DISCUSSION The amount of deposition of TZ3Y powders without and with 3 washes was examined and a dramatic increase in the amount of deposition with washes was observed [7]. Deposition of the as-received powder was insignificant and did not increase with time; i.e., the deposit mass was similar to that obtained by simply dipping the electrode with no appUed potential. However, no such significant phenomenon was observed for the AI2O3 powders. Therefore, the effect of washing on the particle size and zeta potential was examined for the TZ3Y powders. The mean particle size measurements for the EtOH suspension as a function of the number of washes are shown in Fig. 1. The measured particle size in EtOH without washing is quite large, suggesting that the primary (particle size 20 nm) and secondary (mean particle size 60-70 nm) particles agglomerate. The particle sizes decrease steadily with washing. The zeta potential of the EtOH suspension increases with washes, and appears to reach a maximum at 3 washes. These results suggest an increase in suspension stabiUty due to the increase in zeta potential which is related to some surface contamination in the EtOH system. EPD deposition rates were measured from the weight deposited on stainless steel electrodes using a galvanostat operated at 0.5 mA/cm^ for 60 s, as shown in Fig. 2. The deposition rate increased with 1 and 2 washes, but did not continue to increase, which appears to match the change in zeta potential.

345

-••• O

particle size Zeta potential

> 40

E

^

35

1(S''

1

2 3 4 Number of washes

5

Fig. 1 Measurements of zeta potential and mean particle size for TZ3Y.

0

1

2 3 4 Number of washes

5

Fig. 2 EPD deposition mass and relative green density of EPD sample of TZ3Y.

As a t5^ical example of ionic impurity, Na^ concentration was measured. The Na* concentration of the as-received and 1-, 2-, and 3-washed TZ3Y powders in EtOH was 0.007, 0.002, > 0.001, and 0.002 wt%, respectively. It is noted that colloidal suspensions in EtOH have a high sensitivity to ionic surface contamination. A similar situation has been reported in which the zeta potential decreases as the electrolyte concentration increases [8]. The measured relative green densities of the EPD samples are also shown in Fig. 2. The low packing density of the as-received powder is improved by washing. The most important point of the consolidation technique is to control the pore volume and pore size distribution that is originated by the network of the secondary and/or tertiary agglomerated particles. Therefore, the washing treatment is necessary not only to increase the deposition rate of EPD but also to obtain a homogeneous packing body for the EtOH TZ3Y suspension [9]. The particle size and zeta potential were also measured for the aqueous TZ3Y suspension at pH = 4, but there was no obvious effect of washing and a constant value was maintained. This situation appUed in aqueous suspensions examined. An aqueous system has advantages of high stabiUty, low processing cost, low electrical potential requirement, and low environmental cost in comparison with nonaqueous systems. Fig. 3 shows optical micrographs of the surfaces of AI2O3 (AKP50) deposits onto various substrates after 30 min deposition at 0.75 mA/cm^ The voltages were considerable higher than the theoretical voltage for the decomposition of water (1.25 V), as seen in Fig. 3(b). Many micropores are observed on the surfaces of the deposits formed on the nickel, platinum, and stainless steel substrates, but not on the palladium substrate. Table 1 compares the relative green densities by EPD and slip casting using aqueous or ethanol suspensions. Green densities prepared by EPD using aqueous suspensions are larger than those prepared by EPD using ethanol suspensions after washing and similar to those obtained by slip casting using aqueous suspensions.

346

10

15 20 time (min)

Fig. 3 (a) Micrographs of AKP50 deposit surfaces onto various substrate and (b) voltage variation as a function of time at a constant current of 0.75 mA/cm^. Table 1 Comparison of relative green densities (%) prepared by slip casting (SC) and EPD using aqueous and ethanol suspensions. (NM:not measured) TZ3Y Nanotek AKP50 TMDAR Aqueous (SC) 49.9 61.3 61.0 60.0 Ethanol (SC) 38.0 44.8 56.4 NM Aqueous (EPD) NM NM 61.1 60.5 Ethanol (EPD) 40.4 43.5 54.7 59.0 The appUcation of EPD to bulk, monohthic materials is relatively inefficient and expensive, and appears to offer Uttle advantage over slip casting. An exception is for the deposition of very small particles. The time required for colloidal filtration can be extensive for very small particles, due to the narrowness of the pore channels in the consolidated layer [9]. EPD is likely to be best suited to the fabrication of film coatings, multilayered composites, or functionally gradient materials consisting of fine particles. REFERENCES 1. F. F. Lange: J.Am. Ceram. Soc, 72 (1989) 3. 2. P. Sarkar and P. S. Nicholson: J. Am. Ceram. Soc, 79 (1996) 1987. 3. T. Uchikoshi, K. Ozawa, B. D. Hatton and Y. Sakka: Trans. Mater. Res. Soc. Jpn., 25 (2000) 107. 4. G. Wang, P Sarker and P S. Nicholson: J. Am. Ceram. Soc, 80 (1997) 965. 5. Y. Sakka and K. Hiraga: Nippon Kagaku Kaishi, [8] (1999) 497. 6. Y Sakka, T. Uchikoshi and B. D. Hatton: J. Jpn. Soc. Powder Powder Metal. 47 (2000) in press and references are cited. 7. B. D. Hatton and Y Sakka: J. Am. Ceram. Soc (submitted). 8. G. Wang, P Sarkar and P S. Nicholson: J. Am. Ceram. Soc, 82 (1999) 849. 9. B. D. Hatton, T S. Suzuki and Y Sakka: J. Jpn. Soc. Powder Powder Metal. 46 (1999) 1284.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) 'C' 2001 Elsevier Science B.V. All rights reserved.

347

The acceleration behavior of decomposition of potassium persulfate in the dispersions of polystyrene particles stabilized with nonionic emulsifier Masayoshi OKUBO, Toyoko SUZUKI and Satoshi TASAKI Dept. of Chem. Sci. & Eng., Faculty of Engineering, Kobe University, Japan The acceleration behavior of potassium persulfate (KPS) decomposition was examined in dispersions of polystyrene (PS) particles stabilized with polyoxyethylene solbitan monooleate nonionic emulsifier (Tween 80) molecules. Tween 80 molecules forming micelle and being adsorbed onto PS particle accelerated KPS decomposition, but it was much weaker than free Tween 80 molecule which dissolves in an aqueous medium in a monomolecular state. 1. INTRODUCTION Brooks et al. reported that the decomposition of potassium persulfate (KPS)

12

was greatly accelerated in the presence of

10

sodium dodecyl sulfate (SDS) and/or polymer

A

o

^

8

particles [1], which is shown as closed circles in Fig. 1. KPS and SDS are, respectively, a

£, ^

CMC: 2.7 mM

conventional initiator and emulsifier for emulsion polymerization. Their result raised some doubts on emulsion polymerization mechanisms including the representative Smith-Ewart theory [2], in which the initiator and emulsifier are treated as independent of

0

2

4

6

8

Sodium dodecyl sulfate (mM)

Fig.1 Effect of concentration of sodium dodecyl sulfate on kd values of potassium persulfate (7 mM) at 60°C; O, our work; • . Brooks' work

each other. We re-examined the acceleration behavior of KPS with isotachophoresis method [3-5]. The open circles in Fig. 1 show our result. It indicates that "free" SDS molecules dissolved in a monomolecular state accelerated markedly the KPS decomposition, whereas SDS

348 molecules forming micelle did not accelerate it. In addition, SDS molecules being adsorbed onto polystyrene (PS) particles also did not accelerate it, though in Brooks' result, they accelerated five times faster than that for SDS-free sysytem. Our results support the traditional concept of the emulsion polymerization mechanism. In this article, in order to develop the above knowledge, the acceleration behavior of KPS decomposition with polyoxyethylene solbitan monooleate nonionic emulsifier (Tween 80) molecules in place of SDS will be examined in the absence/presence of PS particles.

2. EXPERIMENTAL 2.1 Thermal decomposition of KPS The KPS decompositions in Tween 80 aqueous solutions and/or PS particles produced by emulsifier-free emulsion polymerization were carried out at 60°C in a glass reactionflask while being stirred under nitrogen atmosphere. The initial concentration of KPS was 7.0 mM, which corresponds to a conventional initiator concentration in emulsion polymerization. The pH value of each emulsion was maintained in the range from 8 to 10 with potassium hydroxide to prevent any influence of pH decrease in the decomposition, which is caused from by-production of sulfuric acid in the decomposition of KPS. The emulsion of 1 mL was periodically pipetted out. 2.2 Analytical methods Isotachophoretic analysis was applied

to d e t e r m i n e

the

decomposition rate of KPS as described in the previous articles[35]. A Shimadzu model IP-2A capillary-type isotachophoretic analyzer with a potential gradient detector was used. The above pipetted solutions of 3 \iL, from which the polymer particles had been removed by ultracentrifuging at about 10°C, were injected into the column. The analytical conditions

Fig. 2 A typical isotachophoretic chart of potassium persulfate in the thennal decomposition system. L1, persulfate ion; L2, sulfate ion. a) Potential gradient curve, b) Differential curve.

349 were the same as those reported in the previous paper[5]. Figure 2 shows a typical isotachophoretic chart of KPS in the thermal decomposition system. The amount of persulfate was obtained from the length LI by using a previously determined calibration curve. In addition, the amount of sulfate ion which was derived from sulfuric acid as a decomposition by-product of KPS was simultaneously determined from L2. 3. RESULTS AND DISCUSSION Figure 3 shows time-decomposition curves at various concentrations of Tween 80 aqueous solution at 60°C, and their first order plots. The good linear relationship in the first-order plots shows that the kinetics of the decomposition in the presence of Tween 80 is first order with respect to the KPS concentration Figure 4 shows the relationship between the decomposition rate coefficient (kd) of KPS and the Tween 80 concentration. The kd value markedly increased linearly with an increase in the Tween 80 concentration. However, the slope became gentle above 30 [iM. The Tween 80 concentration of 30 |iM at the bending point agreed with the critical micelle concentration of Tween 80 at 60°C, which was obtained from surface tension measurement. These results suggest that "free" Tween 80 molecules which dissolved in the monomolecular state accelerate markedly the KPS decomposition as well as the SOS system [3]. On the other hand, Tween 80 molecules forming a micelle still had the decomposition acceleration ability, though it was weak. This behavior is different from the SDS system in which SOS

4 6 8 Time (h)

10

'

0

2

4 6 Time (h)

8

Fig. 3 Decomposition (a) and first-order plot (b) of potassium persulfate (7 mM) in various concentrations of Tween 80 aqueous solutions at 60°C: Tween 80 (^M);o , OjO , 20;o , 30;e , 40;Q,60; • ,

80

350

molecules forming micelle did not accelerate it. Figure 5 shows the relationship between o

the kd and the total surface area (At) of PS

X

particles dispersed in Tween 80 aqueous solution. The kd value decreased linearly with an increase in the At value, and then decreased gradually. The At value at the

20

bending point agreed with the concentration at which "free" Tween 80 molecules just disappear in water phase, which was

40

60

80

100

TweenSO (^M) Fig. 4 Effect of concentration of Tween 80 on kd values of potassium persulfate (7 mM) at 60°C

estimated from the soap titration method. Furthermore, kd value of 5.5 h ' at the bending point was larger than that of 4.5 h ' observed for the SDS system. This suggests that Tween 80 molecules being adsorbed on the PS particles still had the decomposition acceleration ability, though it was much weaker than that of "free" ones. 0

From these results, it is concluded that

molecules, and Tween 80 molecules forming

40

60 • 80

100

Total surface area of PS particles (nf/L)

"free" Tween 80 molecules accelerated to decomposition of KPS as well as "free" SDS

20

Fig. 5 Effect of the concentration of PS particles on the kd values of KPS (7 mM) in Tween 80 aqueous solution (30 ^iM) at 60°C

micelle and being adsorbed onto PS particles still had weak acceleration ability. The latter point, which is different from the case of SDS, seems to be based on that there are no electrostatic repulsion between Tween 80 molecule and sulfate radicals and persulfate ions. REFERENCES 1. B. W. Brooks, B. O. Makanjuola, Makromol. Chem. Rapid Commun, 2 (1981) 698 2. W. V. Smith, R. H. Ewart, J. Chem. Phys., 16 (1948) 592 3. M. Okubo, T. Mori, Colloid Polym. Sci., 266 (1988) 333-336 4. M. Okubo, T. Mori, Makromol. Chem., Macromol. Symp., 31 (1990) 143-156 5. M. Okubo, M. Fujimura, T. Mori, Colloid Polym. Sci., 269 (1991) 121-123

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

351

Determination of the size distribution of nltrafine particles based on a measurement of specific surface areas Hiroshi Yao, Shuichi Yonemaru and Keisaku Kimura Faculty of Science, Himeji Institute of Technology, 3-2-1 Koto, Kamigori, Ako-gun, Hyogo 678-1297, Japan We propose a new method for the determination of a size distribution of crystalline ultrafme particles based on a measurement of surface areas. By assuming a log-normal function (mean diameter: D^, geometrical standard deviation: a^) for the size distribution of spherical particles, specific surface area (5) is expressed as a function of D^ and a^. Since D^ and 5 can be estimated experimentally from the width of XRD profiles and BET analyses of N2 adsorption, respectively, o^ is then evaluated. Size distributions determined on the basis of these procedures were compared to those obtained by TEM observations for various ultrafine particles. 1. INTRODUCTION Physical and/or mechanical properties of powders constituting of ultrafine particles depend highly on the particle sizes and their distribution [1]. Therefore, determination of the size distribution is of primary importance for various applications of ultrafine particles. A particle size distribution is often determined by transmission electron microscopic (TEM) measurements through long-time elaboration with counting many particles. Thus, facilitated methods for estimating particle size distributions are desired. Since ultrafine particles are characterized by large specific surface areas, we propose a novel method for determining their size distribution based on a measurement of surface areas. 2. THEORETICAL CONSIDERATION We assume that spherical ultrafine particles are distributed in a log-normal function (/LN(0)) expressed as eq.(l),

where D^ or o^ is a mean diameter or geometrical standard deviation of the size distribution, respectively. It is noteworthy that the fraction of number of particles per

352

diameter interval can be written as bjiln - fuJiOyM^ID [1]. Therefore, the specific surface area (5) can be calculated given as eq.(2), S - • .^

, ' ,3— - - • — r r -

exp ---In ^«

^'

where p is a density of the material. Thus, we can estimate o^ by knowing the parameters p, 5 and D^. 3. EXPERIMENTAL Surface areas of ultrafme particles were determined by BET (Brunauer-EnunettTeller) adsorption experiments of Nj gas operated at 77K. The monolayer volume was obtained firstly from the BET plot in the relative pressure range between 0.05 and 0.3. Then, a specific surface area (S) was calculated by using values of the cross section of N2 molecule (= 0.162 nm^), mass of powders introduced in the cell, and the monolayer volume obtained [2]. A representative diameter of crystalline ultrafine particles (DXRD) can be estimated from the width of X-ray diffraction (XRD) profiles through the Scherrer formula [3] 0.9A )8.cos(^o) where A is the wavelength of the X-ray radiation, 0o is the angle of reflection, and /8 is the full width at half-maximum (FWHM). In the present case, DXRD is assumed to be AI2O3 (Nihon Aerosil, Aluminum Oxide C), ZnO, and Fe-Co alloy are used as samples of ultrafine particle powders. Nanocrystalline ZnO powders were prepared according to the literature [4]. All samples were pre-heated at 150-200 **C for -10 hours under reduced pressure. Adsorption experiments were conducted by using a volumetric adsorption apparatus constructed in our laboratory. This apparatus can provide successive measurements of equilibrium pressures by using a volume-changeable bellows under constant amounts of a gas. Powder XRD measurements were performed by using a Rigaku RINT-2000 with CuKa tube (A =1.541 A) attaching graphite monochromator. A Hitachi-8100 transmission electron microscope (TEM) operated at 200 kV was used to analyze particle size distributions. 4. RESULTS AND DISCUSSION 4.1. Estimation of the particle size distribution of AI2O3 powders Figure 1 shows XRD powder spectrum for AI2O3 (Aluminum Oxide C) sample. The observed peaks, which were assigned to those of 6-AI2O3, were broadened

353

slightly due to a small crystallite size of the sample. By using Scherrer formula for prominent peaks, an average DXRD was obtained to be 14.5 nm (= DJ. Figure 2 shows the BET plot for the 8-AI2O3 powder sample (38.2 mg). The monolayer volume of N2 was 0.763 STP cm^, and the specific surface area S = 87.0 mVg was then obtained. Since the density of 8-AI2O3 is 2.9 g/cm^ [5], the standard deviation (OgCcalc)) was estimated to be 1.56 by eq.(2).

40

50

20 / degree

Fig. 2. BET plot for N 2 adsorption onto 6-AI2O3 at 77K. x: relative pressure (= equilibrium pressure/saturation pressure), v: total volume of N 2 adsoprtion.

Fig. 1. XRD powder spectrum for AI2O3 sample (Aluminum Oxide C). The pattern shows that the peaks were assigned to those for 6-AI2O3. In order to compare a particle size distribution determined by the present procedures with that obtained by microscopic observations, TEM measurements were performed. The inset in Figure 3 shows a typical TEM image of 6-AI2O3 sample. Figure 3 also shows a size histogram by the TEM image and the calculated log-normal distribution curve. The figure indicates that the calculated distribution was broader, and the peak diameter was slightly shifted to a smaller-sized region as compared to that obtained by the TEM observation. Since DXRD represents a mean crystallite length in the grains, it is demonstrated that DXRD is often

-TTT 1 T

ou 40 M

« !«, &

1

i

/

1 LL \

0

J!'"

'

£

f 10 / .

. ^ , \

• . .

10

1L 20

30

40

Diameter

Fig. 3. Size histogram of 5-AI2O3 ultrafme particles obtaied by a TEM image. The inset shows the TEM image of the sample. The solid curve also shows a calculated size distribution.

354

smaller than the mean diameter obtained by TEM images (DJEM). Therefore, the facts would bring about the observed peak shift in particle size distributions. Detailed discussion will be described in the next section. 4.2. Comparison of size distributions obtained by the present method with that obtained by TEM observations Size distributions for other ultrafme particle powders were examined according to the present method. The results are summarized in Table 1 together with that of 6AI2O3. The BET diameter (DBET) is also listed. This value means that the sample powder is assumed to be composed of spherical monodisperse particles alone with DBET in diameter. For tiny nanocrystalline ZnO powders, the peak diameter in the simulation was similar to DJEM* indicating that the crystallite length was almost equal to the observed particle diameter. However, the estimated size distribution was quite broad compared to that obtained by TEM observations. The size distribution of ZnO was resultantly narrow. Therefore, the discrepancy is probably due to underestimation of the specific surface area (5), since the obtained DBET was larger than Z>rEM- When 5 is underestimated, ag(calc) becomes large compared to that expected according to eq.(2). Thus, determination of the broad size distribution for 6-AI2O3 described above would be also due to underestimation of the S value. In this case, moreover, differences between D^ and DTEM also Table 1 contribute to broad size distributions. For Fe-Co Obtained parameters for various ultrafme particles alloy sample, differences 6-AI2O3 Fe-Co Sample ZnO in Qg became smaller. 87.0 98.1 Since DBET is similar to 32.9 Specific surface area (m^/g) ^TEM. suggesting a XRD diameter (DXRD=0„ : nm) 14.5 14.5 7.0 reasonable estimation of S, 21.7 23.8 10.7 BET diameter (DBET : nm) a slight discrepancy in a^ 22.4 17.3 4.9 TEM diameter (DTEM : nm) is ascribed to that between ^XRD (i-^M crystallite og (calc) 1.56 1.50 1.50 length) and Dj^ for 1.14 1.20 1.10 ag(TEM) ultrafme particles of tens of nanometer in size. The BET diameter (D^^j) was calculated by 6/(p5). REFERENCES 1. K. Kimura, Bull. Chem. See. Jpn., 60 (1987) 3093. 2. A. L. McClellan and H. F. Hamsberger, J. Colloid Interface Sci., 23 (1%7) 577. 3. M. G. Bawendi, A. R. Kortan, M. L. Steigerwald and L. E. Brus, J. Chem. Phys., 91 (1989) 7282. 4. P. Hoyer, R. Eichberger and H. Weller, Ber. Bunsenges. Phys. Chem., 97 (1993) 630. 5. Production catalogue of Aluminum Oxide C (Nihon Aerosil).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^c 2001 Elsevier Science B.V. All rights reserved.

355

Dependence of temperature-sensitivity of poly (N-isopropylacrylamidec£i-acrylic acid) hydrogel microspheres upon their sizes Kimiko Makino^' ^, Hideki Agata^ and Hiroyuki Ohshima*' ^ * Faculty of Pharmaceutical Sciences, Science University of Tokyo, 12 Ichigaya Funagawara-machi, Shinjuku-ku Tokyo 162-0826, Japan. ^ Institute of Colloid and Interface Science, Science University of Tokyo, Shinjuku-ku Tokyo 162-8601, Japan. The surface properties of monodisperse poly (N-isopropylacrylamide-co-acrylic acid) hydrogel microspheres with different sizes were studied by measuring the electrophoretic mobility of the microspheres in the solutions at pH 7.4 with the ionic strengths between 0.005 and 0.154 M at 25, 30, 33, 35, 40, and 45 °C. Poly (N-IPAAm) microspheres show positive mobility at all ionic strengths and temperatures, while poly (N-IPAAm-co-AAc) microspheres have negative mobility. Higher absolute values of electrophoretic mobility were obtained with smaller microspheres than the larger ones at each temperature. By analyzing the data with an electrokinetic theory for "soft" surfaces, it was shown that smaller microspheres have hi^er surface charge density than the larger ones, althou^ the microspheres were prepared from monomer solutions with the same monomer composition. The observed size dependence of the electrophoretic mobility suggests that charged acrylic acid monomers have a tendency to be localized in the microsphere core region, whereby the surface region of microspheres becomes poor in charges, reducing the mobility of larger microspheres. On the other hand, poly (N-IPAAm) is a thermosensitive hydrogel with a phase transition temperature around 33 °C, under which it is in a swollen state and above which in shrunken state. Therefore, the surface charge density of poly (N-IPAAm-co-AAc) microspheres increased above their phase transition temperatures. Also their surfaces became harder by the shrinkage of the polymer chains at the surfaces. It was found that the smaller microspheres show hi^er temperature-dependent changes in their surface charge density than the larger ones. 1. INTRODUCTION Recently, many attention has been focused on thermos ens it ive-hydrogels and their properties [1, 2]. Most of the work on thermosensitive hydrogels involves gels based on poly (N-IPAAm). Poly (N-IPAAm) hydrogel has a phase transition temperature at 33°C, above which it is in a shrunken state and under which it is in a swollen state. In a previous

356 paper [3], we have shown that poly (L-lysineisopropylamide-terephthalic acid) microcapsules containing water with thin membranes, the chemical structure of which has NIPAAm moiety are hi^ly thermosensitive and have shown that smaller device having thinner membrane is required to obtain hi^er stimuli-sensitive drug release. The aim of the present work is to show that the hi^er stimuli-sensitivity is obtained in smaller devices, and to show the reason from the comparison of the surface properties of microspheres with various sizes, which will be obtained from the electrokinetic study of the microspheres. 2. METHODS 2.1. Preparation of poly (N-IPAAm-co-AAc) hydrogel microspheres Monodisperse poly (N-IPAAm-c^-AAc) hydrogel microspheres were prepared using a membrane emulsification technique equipped with Sirasu porous ^ s membranes of pore diameters of 0.32,0.73, 1.15, and 1.70 ^im [4] (samples 1-4, respectively). The composition of the microspheres is shown in Table 1. Ammonium persulfate was used as a polymerization initiator. 2.2 Measurements of the electrophoretic mobility of poly (N-IPAAm-co-AAc) microspheres The surface properties were studied by measuring the electrophoretic mobility of the microspheres in the solutions at pH 7.4 with the ionic strengths between 0.005 and 0.154 M at 25, 30, 33, 35,40, and 45 °C. 3. RESULTS AND DISCUSSION Poly (N-IPAAm) microspheres show positive mobility at all ionic strengths and temperatures, while poly (N-IPAAm-co-AAc) microspheres have negative mobility, as shown in Figs. 1 and 2. Hi^er absolute values of electrophoretic mobility were obtained with smaller microspheres than the larger ones at each temperature. These observations suggest that the structure of the microspheres are dependent on their sizes, as have been reported about poly (aery lamide-co-acry lie acid) microspheres by us [4]. On the other hand, the electrophoretic mobility of the microspheres was also affected by the temperature, since poly (N-IPAAm) hydrogel is a thermosensitive hydrogel, changing its volume drastically around 33 °C. The obtained electrophoretic mobility data were analyzed by Ohshima s theory for soft particles, which provides us information on charge density [zN] and softness parameter [\/X] of the soft particle surfaces layers [5].The electrophoretic mobility ^ is then expressed as Eq. (1) [5]. Table 1. Composition of microspheres Poly (N-IPAAm) MS Poly (N-lPAAm-co-5 mol % AAc) MS Poly (N-IPAAm-co-10 mol % AAc) MS

N-lPAAm [mol/L] 4.0 3.8 3.6

AAc [mol/L] 0.0 0.2 0.4

357

0.08 0.12 Ionic strength [M]

0.16

0.16 Ionic strength [M]

Fig. 1. Electrophoretic mobility of poly (N-IPAAm) microspheres with various sizes. Symbols are experimental data measured as a function of the ionic strength in the suspending medium at pH 7.4 and 35 °C: A,sample 1; • , sample 2; D , sample 3; and # , sample 4. Solid curves are theoretical ones calculated with z / / = 0.018 M, and 1/ X =1.59 nm (curve 1), zA^= 0.016 M, and 1/ X =1.69 nm (curve 2), zN = 0.011 M, and 1/ A. =1.70 nm (curve 3) and zN - 0.009 M, and 1/ A. =1.83 nm (curve 4).

Fig. 2. Electrophoretic mobility of poly (N-IPAAmco-5 mol % AAc) microspheres with various sizes. Symbols are experimental data measured as a function of the ionic strength in the suspending medium at pH 7.4 and 35 °C: A,sample 1; • , sample 2; D , sample 3; and # , sample 4. Solid curves are theoretical ones calculated with zN - - 0.018 M, and 1/ X =2.64 nm (curve 1), zN - - 0.011 M, and 1/ A. =3.00 nm (curve 2), zN = - 0.010 M, and 1/ X =3.19 nm (curve 3) and zN ~ - 0.007 M, and MX =3.40 nm (curve 4).

(1)

with 1 +

2(1 + 4)

(2)

MD / ON = ^ l n [ ; f ^ + { ( ^ ) 2 + l } ' ^ 2 j

"^

2v«

2vn

A. = (7/T1)"', Kn, = K [ l + ( ^ ) 2 ] " ^ 2vrt , v (2ne}y}\\i2

(3)

TN

Ivn

(4) (5) (6) (7)

358 Here, a is the radius of the particle core, d is the thickness of the ion-penetrable surface layer. r\ is the viscosity, y is the frictional coefficient of the surface layer, Cr is the relative permittivity of the solution, EQ is the permittivity of a vacuum, \|/DON is the Donnan potential of the surface layer, \|/o is the potential at the boundary between the surface layer and the surrounding solution, and K is the Debye-Hiickel parameter. We term \|/o the surface potential of a soft particle and Km can be interpreted as the Debye-Hiickel parameter in the surface layer. The parameter X characterizes the degree of friction exerted on the liquid flow in the surface layer and zN represents the number density of the fixed charges in the surface layer. The reciprocal of X, i.e., 1/ X has the dimension of length and can be considered to be a "softness" parameter, since in the limit 1/ X -•O, the surface layer becomes rigid. Figure 3 shows the thermosensitivity of microspheres depending on their sizes. It was found that the smaller microspheres show higher temperature dependent changes in their surface charge density than larger ones. That is, the surface charge density of poly (N-lPAAm-co-5 mol % AAc) microspheres prepared using a membrane with a pore size of 0.33 ^im (sample 1) changes more than those prepared using a membrane with a pore size of 1.73 ^im (sample 4) between 25 and 45 °C, while the temperature-dependent changes are less clear than poly (N-IPAAm-co-10 mol % AAc) microspheres [6]. Also, the surface softness of the microspheres prepared using 30 35 40 45 50 ^ membrane with a smaller pore size Temperature [°C ] decreased more than those prepared using a 1, ^ nu Ki r 1 nwi IDA A < 10/ Fig. 3. Changes in zN of poly (N-IPAAm-co-5 mol% AAc) microspheres depending on their sizes and temperature. Symbols show:0, sample l;D, sample 2;0,sample 3; A,sample 4.

membrane with a larger pore sizes between ^ ^ 25 and 45 C.

Acknowledgments This work was partly supported by a Grant-in-Aid (10680805) from the Ministry of Education, Science, and Culture, Japan.

REFERENCES 1. 2. 3. 4. 5. 6.

H. G. Schild, Prog. Polym. ScL, 17 (1992) 163. R. Pelton, Ad Coll. Inter. Sci.. 85 (2000) 1. K. Makino, et al. Colloids and Surfaces, B:Biointerfaces, in press. S. Nagashima, et al. J. Colloid Interface Sci., 197 (1998) 377. H. Ohshima, J. Colloid Interface Sci.. 163 (1994) 474. K. Makino, et al. / Colloid Interface. Sci., in press.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c^ 2001 Elsevier Science B.V. All rights reserved.

359

Deposition of thiol-passivated gold nanoparticles onto glass plates by pulsed 532-nm laser irradiation: effects of thiol Yasuro Niidome, Ayako Hori, Hironobu Takahashi, and Sunao Yamada Department of Materials Physics and Chemistry, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581

Irradiation of pulsed 532-nm laser light on dodecanethiol-passivated gold nanoparticles in cyclohexane resulted in the deposition of larger gold nanoparticles onto glass plates. The degree of deposition depended on the laser pulse energy and the concentration of dodecanethiol in the cyclohexane solution. 1. INTRODUCTION Metal nanopaticulate films have been attractive because of their potentials in technological applications such as catalysts and surface enhanced Raman scattering sensors [1-4]. Thus, the development of convenient methods to elaborately deposit the metal nanoparticles on substrates is very important. Quite recently, we found that the gold nanoparticles were easily deposited on a glass plate which was immersed into a cyclohexane solution of dodecanethiol (DT)-passivated gold nanoparticles, by pulsed 532-nm laser irradiation [5]. In this study, we have investigated the effects of DT on the deposition process. 2. EXPERIMENTAL Cyclohexane solutions of DT-passivated gold nanoparticles were prepared according to the previous procedure [6]. Pulsed 532-nm laser light of a Nd-YAG laser ( ~ 8 ns, 33 mJ/pulse, 10 Hz) irradiated a sample glass cell (1 x 2 x 3 cm) containing the glass plate and the cyclohexane solution of DT-passivated gold nanoparticles [7]. The deposited gold nanoparticles were characterized by absorption and FT-IR spectra, and scanning electron micrograph (SEM) observations. 3. RESULTS AND DISCUSSION After laser irradiation, the glass plate was rinsed with cyclohexane and dried in air. A reddish spot coinciding with the laser irradiation area could be seen with the naked eye. Figure 1 shows absorption spectra of the spots formed on the glass plates after different irradiation times. Absorption peaks at around 540 nm are characteristic of the surface plasmon bands of the deposited gold nanoparticles. The surface plasmon band increased with the irradiation time up to 60 seconds. However, further irradiation (> 60 seconds) resulted

360

400

500 600 Wavelength / nm

700

Figure 1. Absorption spectra of gold particles deposited on glass plates at different irradiation times of the 532-nm laser light: a, 10; b, 30; c, 60; d, 180 seconds, (gold particles: 3.2 ± 0.95 nm, 8 mg/ml).

Figure 2. SEM photographs of the glass plate after 180 seconds of laser irradiation. Conditions are identical to those of Fig. 1.

in considerable degradation and broadening of the plasmon band in longer wavelength regions than 700 nm (Fig. 1 (d)). Typical SEM photographs of the deposited gold particles after 180 seconds of laser irradiation are shown in Fig. 2. Most of the deposited particles are spherical, and their size distribution (8.5 ± 5.2 nm) is larger than that of the initial particles before irradiation (3.2 ± 0.95 nm). The SEM observation also shows the existence of some huge agglomerates as shown in the inset of Fig. 2. It has been known that aggregated colloidal particles exhibit broad peaks in longer wavelength regions as compared with typical plasmon bands of the spherical nanoparticles [3]. Therefore, it is most likely that the broadening of the absorption band observed in longer irradiation times (> 60 seconds) is due to the formation of agglomerates. The growth of the particle is certainly caused by proceeding repeated coagulation (or agglomeration)-fusion cycles [7]. At this stage, there are two plausible pathways for the deposition of larger particles: the size growth at the plate surface, and the deposition of larger particles formed in the solution. In order to verify the deposition mechanism, the effect of DT in the colloidal solution (5.8 ± 2.0 nm, 1 mg/ml) was investigated. Figure 3(a) shows absorption spectra of the irradiated regions of the glass plates which were immersed in the colloidal solutions with different concentrations of DT (0, 1, 10 mM); in all cases the irradiation time for deposition was 30 seconds. In the absence of DT, a clear plasmon band was seen as in Fig. 1. However, no appreciable bands due to gold particles were observed in the presence of DT (1, lOmM). This clearly indicates that DT molecules hinder deposition of the particles from the cyclohexane solution. As a next step, a new glass plate was immersed into the cyclohexane solution that had been used for the first deposition experiment (Fig. 3(a)), and then the laser deposition was again carried out for 30 seconds (second deposition), and then the absorption spectrum was recorded (Fig. 3(b)). In a similar manner, the third (Fig. 3(c)) and the fourth (Fig. 3(d)) deposition experiments were carried out by replacing the new glass plate, while the cyclohexane solution

361

(a)

0.2

L (b)

0.2

o c

[DT]= 0 mM

-A.

c

nCO o O.H <

CO

U^^-v-"

o0.1 <

[DT]= 1 mM

400

500 600 Wavelength / nm

(C)

400

••-..-... [DT]= 0 mM

0.2 U

py^^.^-

CO

o 01 < 400

1

700

..^..->..,. [DT]= 0 mM

(d) ..•V

Mmrf I 0.1

hT."

[DT]=

[DT]= 1 mM (DT]=10r

[DT]=10mM

500 600 Wavelength / nm

_J

.-V

c

S

.

500 600 Wavelength / nm

O

O C

[DT]= 1 mM

(DT]= 10 mM 1

I

700

••••^''

p-*-—-c

[DT]=10mM

0.2

[DT]=OmM

CD O

700

400

500 600 Wavelength / nm

700

Figure 3. Absorption spectra of the irradiated regions of the glass plate in air, after 30 seconds of laser irradiation in the cyclohexane solution with (1 or 10 mM) or without (0 mM) DT. In every step, new glass plates were immersed into the cyclohexane solutions for deposition (30 seconds), while the solutions were unchanged. Thus, the cyclohexane solution was pre-irradiated for 0 (a), 30 (b), 60 (c), or 90 (d) seconds before each deposition step, (gold particles: 5.8 ± 2.0 nm, 1 mg/ml) was unchanged; thus, the cyclohexane solution was pre-irradiated for 0 (a), 30 (b), 60 (c), or 90 (d) seconds before each deposition step. In absence of DT, the plasmon band became more intense with proceeding the deposition step. Situation was more prominent in the case of the cyclohexane solution containing 0.1 mM DT. In Fig. 3(d), the plasmon band was clearly seen even in the case of [DT] = 10 mM. These observations suggest that some larger particles are formed in the sample solution and they increase with proceeding the exposed time. The larger particles are favorable for the laser-induced deposition because of their instability in solution. However, the plasmon band is substantially smaller in the presence of high concentration of DT (10 mM), especially in Figs. 3(c) and (d). This is quite consistent with the results of Fig. 3(a). Thus, the DT molecules hinder aggregation (and/or fusion) of the particles in the solution. In order to investigate whether the passivated DT molecules are retained on the deposited gold particles or not, FT-IR measurements were carried out using a CaF2 substrate instead of the glass plate. Figure 4(a) shows the FT-IR spectrum of the gold particles deposited on the

362


ti

(a)

o o c CO

^W""^^

E "^ 03 C

2 H

$:: E

M

2954 1 1 1 2850 Gold Particles '2922

3500

Wavenumber/cm -1

en c CO

h-

2500

3500

2500 Wavenumber/cm 1

Figure 4. FT-IR spectra of the irradiated regions of the CaF2 plate (a) and DT casted on the CaF2 plate (b). Laser irradiation condition: 532 nm, 40 mJ/pulse, 30 sec.. Gold particles: 3.8 ± 1.6 nm, 1 mg/ml.

CaFj plate. Three C-H stretching bands based on methyl and methylene groups can be clearly seen at 2954, 2922, and 2850 cm'. They are quite similar to those of DT molecules casted on the CaFj plate (Figure 4(b)). Thus, some of the immobilized DT molecules are still retained on the gold particles even after the deposition on the CaF, plate. Accordingly, it is suggested that the removal of some immobilized DT molecules is one of the key processes for the present laser-induced deposition. In the presence of excess DT molecules, re-adsorption of DT-molecules to the particles can occur quickly, even after removal of the immobilized DT molecules by laser irradiation; this must retard aggregation and deposition.

REFERENCES 1. 2.

See for example: M. A. Hayat (ed.). Colloidal Gold, Academic Press, 1989. See for example: a) A. Henglein, J. Phys. Chem., 97, 5457 (1997). b) G. Schmidt, Chem. /?^v., 92, (1992) 1709. 3. a) R. G. Freeman, K. C. Grabar, K. J. Allison, R. M. Bright, J. A. Davis, Andrea P. Guthrie, M. B. Hommer, M. A. Jackson, P. C. Smith, D. G. Walter, M. J. Natan, Science, 267, (1992) 1629. b) K. C. Grabar, R. G. Freeman, M. B. Hommer, M. J. Natan, Anal. Chem., 67,(1995)735. 4. K. Murakoshi and Y. Nakato, Adv. Mater,, 12, (2000) 791. 5. Y. Niidome, A. Hori, Y. Gotoh, S. Yamada, to be submitted. 6. D. V. Leff, P. C. Ohara, J. R. Heath, W. M. Gelbart, J. Phys. Chem., 99, (1995) 7036. 7. Y. Niidome, A. Hori, T. Sato, S. Yamada, Chem. Lett., 2000, 310.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^c) 20()l Elsevier Science B.V. All rights reserved.

363

Preparation of silica-coated magnetic nanoparticles Y Kobayashi and L. M Liz-Marzan Departamento de Quimica Fisica, Universidade de Vigo, E-36200, Vigo, Spain The synthesis of monodispersed, amorphous cobalt fine particles in aqueous solution, and their subsequent coating with a silica shell of controlled thickness is described The silicacovered cobalt particles can be transferred into cyclohexane using n-decylamine as a stabilizer Furthermore, the transformation of the cobalt cores into crystalline, metallic cobalt upon heating in air is demonstrated through X-ray diffraction data Both the initial, amorphous fine particles, and their crystalline counterpart are magnetic, which promises important applications for ferrofluid preparation and for magnetic recording media 1. INTRODUCTION Nanometer sized colloidal particles have special interest because they are in an intermediate state between atoms or molecules and bulk material and thereby they can be expected to exhibit excellent properties different from those of bulk material [1-3] In the case of magnetic materials, when the nanometer size range is achieved, the particles can show superparamagneticity It is well known that when the particles have the superparamagnetic properties their magnetic dipole can be oriented by means of an external magnetic field This enables the magnetic particles to be applied very interestingly In general, nanoparticles show a tendency to aggregate forming larger entities Therefore, the magnetic particles must be separated by some means, so that the individual properties are preserved and each particle acts as a single magnetic dipole. Traditionally the stabilization of colloids is performed through surface modification processes, such as the adsorption of macromolecules forming a physical barrier against other approaching particles The coating of the particles with inert silica shells has previously been used as a stabilizing technique [4-7], due to the anomalously high stability in aqueous media of colloidal silica, even at high salt concentration This technique offers the additional possibility of varying the thickness of the silica shell, which would allow for a control of the interactions among individual particles An example of silica-coated megnetic particles, silica-coated Fe304 (magnetite) particles were produced, which exhibited excellent magnetic properties when they were deposited on Si wafers by means of LBL [8, 9] However, no ilirther studies on magnetic properties have been reported to our knowledge Metallic cobalt is one of the most interesting magnetic materials However, it is unstable with respect to oxidation in air Therefore, it seemed interesting to prepare silica-coated cobah particles (Co@Si02) In this study, we report on the preparation of Co@Si02 sub-micron particles which are stable in aqueous dispersion, but can also be transferred into organic solvents.

364 2. EXPERIMENTAL 2.1. Synthesis The preparation of the starting cobah particles was performed by the reduction of 4x10"^ M CoCb with 4x10'^ M NaBfij in an aqueous solution that had been deaerated by purging with N2, as manifested by a gray coloration of the dispersion Nitrogen bubbling was maintained during the reactions. In order to coat the cobalt particles with silica, 400 mL of ethanol containing 6 |iL of 3-aminopropyl-trimethoxysilane (APS) and 67 5 ^L of tetraethylorthosilicate (TEOS) ([APS] [TEOS] = 1 9 ) was added to 100 mL of the obtained gray-colored colloid, 1 min after mixing the aqueous solutions APS was used as a stabilizer, and also served as a surface primer to promote the adsorption of silica onto the surface TEOS was used as a silica precursor for shell formation The mixture was allowed to stand for one day The produced colloid was centrifuged at 3000 rpm for 1 hour and redispersed in ethanol To make the silica shell thicker, TEOS and aq ammonia were added to the colloid To transfer the Co@Si02 colloid into an organic solvent, cyclohexane and n-decylamine were added to the colloid and the mixture was shaken vigorously After a short time, the particles were dispersed in the upper, organic layer, while the lower phase was colorless Powder-like samples were obtained by simple solvent evaporation Crystallization experiments were carried out by heating the powder at 500 °C for several hours in air with a standard oven. 2.2. Techniques The samples obtained by the above procedures were characterized with transmission electron microscopy (TEM) using a Philips CM-20 microscope operating at 200 kV X-ray diffraction (XRD) was measured with a Siemens D5000 operated at 40 kV and 30 mA with Cu Ka radiation and graphite monochromator Vibrating sample magnetometry (VSM) was performed with a magnetometer from Digital Instruments 3. RESULTS AND DISCUSSION 3.1. Co@Si02 colloid As exemplified in Figure 1, TEM micrographs clearly show the core-shell nature of the prepared particles, due to the much higher electron absorption by the metallic Co core. The presence of Co, Si and O was confirmed by EDS The average size of the core particles is 280 nm in the sample shown, and was observed to be quite uniform all over the grid. The shell formation is assumed to occur by initial attachment of APS on the surface of the cobalt particles and fiirther growth through hydrolysis among OH groups derived fi-om APS and TEOS

lOdimi

Figure 1 Transmission electron micrograph sho^^ing the core-shell structure of the Cal^SiO: particle

365 3.2. Extraction into non-polar solvent Solvent exchange is an important procedure, from both the point of view of properties and fundamental studies of the magnetic colloid in non-polar solvents The Co@Si02 particles could be extracted into cyclohexane through surface modification with n-decylamine (Figure 2) Since the amino group of n-decylamine and the silica surface of Co@Si02 are oppositely charged, the former adsorbs onto the particles, so that the hydrocarbon tail of n-decylamine would provide the colloid with steric stabilization 3.3. Co@Si02 powder The X-ray diffractogram of the powder obtained after drying the colloid is shown in Figure 3 Its featureless nature clearly points toward an amorphous structure, although it is also possible that the size of the crystal domains is too small to detect a peak in XRD. The transition between the amorphous and the crystalline states was promoted by heating at 500 X for 2 h in a standard oven, in the presence of air The XRD resuhs were undoubtedly assigned to metallic cubic Co, as indicated by the vertical lines in Figure 3 Longer heating processes were carried out (up to 6 h), but the crystallinity of the powder was not improved In addition, no peaks were observed which could be due to cobalt oxide such as C03O4 This indicates that the silica shell protects the metallic cobalt core against oxidation and during the heating the shell is likely to be compressed, which would provide basically full protection against external agents after heating Moreover, any peaks of boron compound such as C02B were not observed either It has been reported that compounds of Co and B are produced besides metallic Co when cobalt ions are reduced in water by borohydride [10] However, the environment around Co particles coated with silica must be different from that involved in the reduction of cobalt ions in water without silica coating TEM

Figure 2. Immiscible layers (a) Co(S)SiO: colloid in water (at the bottom) and the clean cyclohexane (on top) (b) n-Dec>laminedenvatized Co/«)SiO: colloid in cyclohexane (on top) and the clean aqueous solution (at the bottom)

20 30 40 50 60 70 80 90100

2 e (degree) CuKa Figure 3 X-ray diffractograms of Co^io^SiO: before and after heatmg at 500 °C in air for 2 h Vertical dashed lines show the diffraction lines for metallic cubic cobalt

366

observation of the particles after heating showed that the Co cores were still uniform in size, with spherical shape and undamaged silica shell. 3.4. Magnetic properties Magnetic characterization of the amorphous and crystalline products is shown in Figure 4 The measurements were performed at room temperature It is remarkable that the amorphous sample was also magnetic, even though in XRD the existence of metallic cobalt was not confirmed It should be noted that the magnetization values of the crystalline sample are ca 10 times higher than those of the amorphous sample. The crystalline sample is therefore very promising as a magnetic material. Further investigation is necessary and currently under progress

20H

E 10 c a

amorphous H OH

CD N

/

-20 i -i

.

1

-10000 -5000

1

1

1

0

1

5000

.

r

10000

H(Oe) Figure 4 Room temperature magnetization vs field data for Cof/SiOz nanoparticles

ACKNOLEDGEMENTS This work has been supported by the Spanish Xunta de Galicia (Project No PGIDT99PXI30104B), and by the Ministerio de Educacion y Cultura (Project No PB981088) Y.K. acknowledges the Spanish Ministerio de Educacion y Cultura for a personal grant The authurs express their thanks to J B Rodriguez in the C A C T I from Universidade de Vigo for his assistance during XRD measurements and TEM observations REFERENCES 1 L Brus, J. Phys Chem , 90 (1986) 2555 2 A. I Ekimov, A L Efros and A A Onushchenko, Solid State Commun , 88 (1993) 947 3 K Kimura, Z Phys D, 11 (1989) 327 4 L. M. Liz-Marzan, M Giersig and P Mulvaney, Chem Commun., (1996) 731 5 L M Liz-Marzan, M Giersig and P Mulvaney, Langmuir, 12 (1996) 4329 6 M A Correa-Duarte, M Giersig and L M Liz-Marzan, Chem Phys Lett, 286 (1998) 497. 7 T Ung, L M Liz-Marzan and P Mulvaney, Langmuir, 14 (1998) 3740 8 M A. Correa-Duarte, M Giersig, N A Kotov and L M Liz-Marzan, Langmuir, 14 (1998)6430 9 F G Aliev, M A Correa-Duarte, A Mamedov, J W Ostrander, M Giersig, L M LizMarzan and N A Kotov, Adv Mater 11 (1999) 1006 10 G N. Glavee, K. J Klabunde, C M Sorensen and G C Hadjipanayis, Langmuir, 9 (1993) 162

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) 'c) 2001 Elsevier Science B.V. All rights reserved.

367

Studies on the Preparation of Silica-Coated Carbon Particles by Sol-gel Method Hajime Shibuya*, Mika Shimada*, Noboru Suzuki**, Hirotomo Ito*, Ken-ichi limura", Teiji Kato* and Toshiaki Kakihara** ^Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan *lshikawajima-Harima Heavy Industries Co., Ltd., 3-1-15 Toyosu, Koto-ku, Tokyo 135-8732, Japan Silica-coated phenol resin particles have been prepared by sol-gel method using tetraethoxysilane solution. Silica-coated carbon particles have been prepared by the controlled pyrolysis of silica-coated particles in argon atmosphere. A bell like structure consisting of carbon core and silica shell has also been prepared by the partial oxidation of the silica-coated carbon particles. By examine conditions of sol-gel coating for the thicker silica film formation, it is better to decrease or remove ethanol as the solvent of coating solution. 1. INTRODUCTION Excellent functional materials such as complex glasses or ceramics have been prepared by physical or chemical vapor deposition (PVD and CVD) or sol-gel method. By sol-gel method, uniform coating of sample surfaces can be achieved easily, and the chemical or mechanical properties of the surfaces can be improved. Although the sol-gel method has been generally applied to the coating of metals, glasses and ceramics, the technique is also applied to the organic film coating. Azuta et al. have prepared silica thin films containing organic polymers on poly(ethylene terephtalate) substrate by sol-gel method to suppress gas permeability [1]. Also, many of encapsulated particles are prepared by this method. Silica and/or titania have coated on barium sulfate and magnetite [2] and on polystyrene particles [3]. We have tried to prepare ceramic-coated resin particles by sol-gel method. In this study, phenolic resin particles are used for seed particles as carbon precursor and tetraethoxysilane (TEGS) is for metal alkoxide. The coated particles are heat-treated in argon atmosphere to prepare silica-coated carbon particles.

368

2. EXPERIMENTAL 2.1 Silica-coating of phenolic resin particle A mixture of ethanol-H20-HCl was added to a TEOS-ethanol solution. The solution was stirred for Ih at room temperature to hydrolyze TEOS. Then the phenolic resin particles were added into the solution. The solution was stirred for 6h for coating. After the particles were filtrated and washed by ethanol, they were dried in desiccator. To examine the most suitable conditions of silica coating, we repeated the experiments with changing the cycle number of muhi-coating and the molar ratio of chemicals. 2.2 Carbonization of silica-coated phenolic resin particles The coated resin particles were heated at 400, 800 and 1000°C in argon atmosphere to prepare silica-coated carbon particles. Some of the particles prepared were heated at 550°C in air for the partial oxidation. These products were characterized mainly by scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). 3. RESULTS AND DISCUSSION 3.1 Carbonization and silica coating of phenolic resin particles Figure 1 shows SEM photographs of phenolic resin particles and silica-coated phenolic resin particles where TE0S:H20:C2H50H:HC1 molar ratio was 1:11:7:0.05. Surface of particles is very smooth and there is no significant change before and after the coating. Figure2 shows XPS survey spectra of silica-coated carbon particles heated up to 1000°C. Because Si2p peak was observed, it is confirmed that silica-coating fihn was prepared in this method. Then, the heat-treated particles were etched by Ar ion. In the spectra obtained after etching, it was observed that Cls peak increases with a decrease of Si2p peak. Thus, the silica film should be very thin. The thickness of silica film was calculated fi'om etching speed and was about 15 nm. FigureS shows SEM photographs of silica-coated carbon particles obtained by heatmg up to 1000°C. Although the origmal silica-coated phenolic resin surface was very smooth, two types of surface ^ ^ ^ ^ ^ s ^ ^ ^ ^ ' ^ i i ISMHMV'.^^^^N.^ structures were observed for the ^ ^ P ^ ^ ^ H ^ ^ B f e l |j^^n|(^^^HI^fe| samples same lot. One has many p ' ^ ^ ^ S ^ B H B H ^^^H^^^^v^^P irregular wrinkles and some circles ^ f i j ^ K M p ' V B B ^ B ^ l f l H I ^ ^ ^ ^ I ^ K whose internal surface very smooth ^ ^ H j j j ^ ^ B ^ ^ ^ H 1 ^ ^ I j ^ B ^ a H ^ ^ H (Fig.3 a), and another has many radial H J J H R l H R R R I V i l9HbM>4iflll^^ mm wrinkles around some points (Fig.3 b). (a) (b) The circles are considered to be made Figl SEM photographs of phenolic resin by the break of the silica bridging particles, (a), and silica coated phenolic resin portion formed during silica coating particles, (b).

369

or drying process. It is thought that the radial wrinkles of the latter structure might be formed by shrink of the phenolic resin during the heat treatment. FigureS (c) shows the particles heat-treated at SSO^'C in air. Carbon core exists in silica shell, and a bell like structure is observed. This is named as a 'micro-beir structure.

Fig.2 XPS survey spectra of silica-coated phenolic resin particles before (a) and after (b) Ar ion etching.

3.2 Repeated coating on phenolic resin To obtain thicker coating film, we tried repeated coating on phenolic resin. Coating of the particles was carried out with the following two procedures. In the first method, phenolic resin particles were coated with silica and dried, and then the procedure was repeated three times. The second method was that phenolic resin particles were coated, dried and heated up to 400°C in one cycle, and this was repeated for three cycles. No practical change was observed by SEM for one, two or three times coating particles by the former method. By the latter, partially thicker film was observed for the particles after two or three times coating. Figure4 (a) shows SEM photgraph of the particles coated two times. From this result, it seems that coating film was peeled off during the second or third coating process. 3.3 Effect of ethanol concentration on the thickness of silica film Even if a two-phase mixture consisting of water and TEOS was used as the starting material, homogenization of the solution will occur during in reaction because ethanol is formed as a product of TEOS hydrolyzation [4]. Thus the coating was carried out with lower concentration of and without ethanol. The latter case is the two-phase mixture system. By both cases (molar ratios of 1:3.5 and 1:0 for TEOS:C2H50H), a relatively thicker coating fihn has been formed compared with the above condition in the section 3.1. Figure4 (b) shows SEM

pj^ 3 ^^^ photographs of silica coated phenolic resin heated up to 1000°C, (a) and (b), and that of carbon-silica 'micro-bell', (c).

370

photograph of the coated particle without ethanol. After heat-treatment at 400°C, the surface of particles was smooth and there was no wrinkle on the surface. It can be thought from this resuh that the coating film has not been pulled into the inside of the silica shell by the shrink of phenolic resin particle, because the coated film is comparatively thick. 4. CONCLUSION Phenolic resin particle has been successftilly coated with silica by sol-gel method using TEGS, and silica-coated carbon particle has been also prepared by heat treatment. Wrinkles and pleats are formed on the surface of heat-treated particles. Partial oxidation (combustion) of the silica-coated Fig.4 SEM photographs of carbon particle leads to form a bell like structure named the particles coated two 'micro-beir. With the TE0S:C2H50H:H20:HC1 molar ratio times, (a), and without of 1:7:11:0.05, the thickness of silica coating film is very thin. However, by examining coating conditions, it was found that ethanol (b). decreasing ethanol concentration is better for the formation of thicker silica films Acknowledgement This study was partially supported by Grant-in-Aid for Scientific Research on Priority Areas (Carbon Alloys) of the Ministry of Education, Science, Sports and Culture, Nos. 10137206 and 11124204, and the Aid of Satellite Venture Business Laboratory of Utsunomiya University. REFERENCES 1. K. Azuta, K. Tadanaga and T. Minami, J. Ceram. Soc. Jap., 107(1999)293. 2. Kimata, M. Mitsuhiro and M. Hasegawa, J. Soc. Powder Technol., Jap., 34(1997)206. 3. M. Akazawa, K. Minamihashi, M. Iwasaki, H. Tada and S. Ito, Shikizai, 71(1998)100. 4. D. Avnir and V. R. Kaufman, J. Non-Crystal. Solids, No. 192 (1987)180.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c^ 2001 Elsevier Science B.V. All rights reserved.

371

Synthesis and catalysis of polymer-stabilized Ag and Ag/Pd colloids Yukihide Shiraishi*, Kazutaka Hirakawa**', Jun-ichi Yamaguchi', and Naoki Toshima* * Department of Materials Science and Engineering, Science University of Tokyo in Yamaguchi, Onoda-shi, Yamaguchi 756-0884, Japan. ^ Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Silver monometallic and silver/palladium bimetallic colloids stabilized by poly(sodium acrylate) (PSA) were prepared by UV irradiation of an ethanol/water solution of silver (I) perchlorate with and without palladium (11) acetate in the presence of PSA. Oxidation of ethylene, catalyzed by PSA-stabihzed Ag (PSA-Ag) and Ag/Pd (PSA-Ag/Pd) colloids, was performed in glycol under 1 atm of ethylene/oxygen (2/1, v/v). The PSA-Ag/Pd bimetallic colloids showed the higher catalytic activity than PSA-Ag monometallic colloids. 1. INTRODUCTION Colloidal dispersions of metal nanoclusters have various interesting properties based on high surface area and quantum size effect, being one of the promising advanced materials for application to electronic devices, to optical materials and especially to catalysts [1]. MetaUic colloids are often applied to reductive catalysts, but seldom to oxidative catalysts. Silver has been investigated and utilized as an oxidative catalyst, especially in the production of ethylene oxide from ethylene. In the previous paper [2] we preliminarily reported the oxidation of ethylene, catalyzed by poly(N-vinyl-2-pyrrolidone)(PVP)-stabilized silver colloids. However, the catalytic activity was not so high. Multimetallic catalysts have widel y been studied with a view to improve the quality of catalysts. Torigoe et al. [3] have prepared

2 C2H4 +

O2

^'^°"°'^'»

2 CH2-CH2

V This work was partially supported by a Grant-in-Aid for Sdentific Research on Priority Areas "Molecular Synchronization" (No. 11167281, to NT.) and a Grant-in-Aid for Encouragement of Young Sdentisls (No. 11750738, to Y.S.) from the Ministry of Education, Science, Sports, and Culture, Japan. *Present address: Radioisotope Institute, School of Medicine, Mie University, Tsu-shi, Mie 514-8506 Japan.

372

Ag/Pt colloids by reduction of the silver oxalate precursor in the presence of PVP. However, the colloids have not been applied to a catalyst. In the present paper we report on the synthesis of PSA-Ag and PSA-Ag/Pd colloids, which are applied to oxidative catalysis. 2. EXPERIMENTAL PSA-stabilized Ag nfionometallic and Ag/Pd(l/1) bimetallic colloids were prepared by photoirradiation of a 50 cm^ ethanol/water (1/1, v/v) solution of silver (I) perchlorate without and with palladium (II) acetate, respectively, (total metallic ion = 0.066 mmol) in the presence of PSA (2.64 mmol in monomeric units) as a protecting polymer. The solutions in a quartz vessel were degassed by three freeze-thaw cycles, filled with pure nitrogen and then exposed to the light of a 500 W super-high-pressure mercury lamp for 1 h. Ultraviolet and visible (UV-Vis) spectra were obtained using a Shimadzu 2500PC recording spectrophotometer equipped with a 10 mm quartz cell. PSA-Ag monometallic and PSAAg/Pd bimetallic colloids employed here were characterized by transmission electron microscopy (TEM) at 75 kV on a Hitachi H-7000 electron microscope. Oxidation of ethylene, catalyzed by PSA-Ag and PSA-Ag/Pd colloids, was performed in glycol under 1 atm of ethylene/oxygen (2/1, v/v). Glycol (30 cm^) and PSA-Ag or PSAAg/Pd colloids (0.015 mmol) were stirred under ethylene/oxygen at 170 "C. The oxidation product was analyzed with a Shimadzu GC-14B gas chromatograph using a 3.1 mX3.2 mm column of TSG-1 at 60 °C, being identified to be ethylene oxide. 3. RESULTS AND DISCUSSION 3.1. Preparation and characterization of Ag monometallic and Ag/Pd bimetallic colloids PSA-stabilized Ag monometallic or Ag/Pd bimetallic colloids were prepared by reduction of the ethanol/water solution of silver (I) perchlorate and PSA, without or with palladium (II) acetate with UV light irradiation, having 1 Ag 1 2Ag>Pd(9/1) transparent yellow or dark brown color and 3 AgyPcl(4A1) 4 Ag>Pd(1/1) being stable for months at room temperature. 5 Ag/Pd(1/4) 6Pd © 2 o FFig. 1 shows UV-Vis absorption spectra of c ca \ \ the colloids obtained by varying the Ag/Pd o 0) molar ratio. An absorption p>eak at 414 nm of < 1 h ^ ' V-6. PSA-Ag monometallic colloids is due to \ . 4 - -^Vplasmon oscillation characteristic of silver, 1 1 . ^^""""^^ L suggesting reduction of silver (I) ions. With 300 600 400 500 increasing molar ratio of Pd, the absorption Wavelength / nm Fig. 1. UV-Vis absorption spectra of peak at ca. 400 nm became board and shifted the PSA-stabilized Ag/Pd colloids.

N^

-A

373

PSA-Ag/Pd(9/1) d =3.5 nm

\L 0

2

4

6

8

10

12 14

Diameter / nm

A

PSA-Ag/Pd(4/1) d =2.8 nm

IB

'I 0

5 =1-1 nm

^ ^ ¥ T

2

4

I T r I I I I I I—' 6 8 10 12 14

Dameter/nm

PSA-Ag/Pcl(1/4) d =2.1 nm

0

2

4

6

8

10 12 14

Diameter/nm

IL 0

2

4

r r I 6

6

d

PSA-Pd =3.0 nm

=0.70 nm

I I I I I I I I 8 10 12 14

Diameter/nm

ifWlPif ^^l I 0 2 4 6 8

[ I I I I I I' 10 12 14 I

Diameter / nm

1

so nm

Fig.2. Transmission electron micrographs and particle size distribution histograms of PSA-Ag/Pd colloids: dav = average diameter, a = standard deviation.

to shorter wavelengths. The plasmon absorption of silver disappears at the molar ratio Ag/Pd=l/1. This strongly suggests that PSA-Ag/Pd (1/1) bimetallic colloids are mainly covered by Pd atoms, and the Ag atoms are located near the center of the bimetallic colloids, forming an Ag-core/Pd-shell structure. The same tendency was observed on PVP-Au/Pt bimetallic colloids [4]. Figure 2 depicts transmission electron micrographs and the corresponding histograms indicating the particle size distributions of colloids. The particles of all the colloids especially bimetallic colloids have very narrow size distribution with average diameters in the range of 2.1-3.5 nm. The Ag/Pd(l/4) bimetallic colloids have the smallest average diameter of 2.1 nm. In PSA-stabilized Ag and Pd monometaUic colloids, the particles have a larger average diameter i.e. 3.7 and 3.0 nm, respectively, than those of the bimetallic cases, although no aggregation is observed. These facts suggest that PSA-Ag/Pd bimetallic colloids are not mixtures of monometallic Ag and Pd colloids but consist of single particles containing both elements. 3.2. Catalysis for oxidation of ethylene

Silver colloid acts as an effective catalyst for oxidation of ethylene to ethylene oxide (EO) in solution under an atmospheric pressure of ethylene and oxygen (2/1, v/v) [2,5]. The catalytic activity increases remarkably with increasing temperature from 130 to 170 "C, and reaches the maximum (2,700 mmol-EO mol-Ag' h"') at 170 T as shown in Fig. 3. This catalytic activity of PSA-Ag colloids is much higher than that of commercial silver powders at 170 T (56 mmol-EO mol-Ag* h'^). The low activity of the commercial silver catalyst may

374 10000

6000

O)

Q.

< •5 1000 h E

S4OOO

o 111

100

E 2000 E

10 Ag powder PSA-Ag PSA-Ag PSA-Ag

(1701:) (1301) {^5Qy:) (i7or) Fig. 3. Relationship reaction temperature and the catalytic activity of PSA-Ag colloids for oxidation of ethylene.

^ -L. 75 100 50 Pd/mol% Fig. 4. Dependence of the catalytic activity of PSA-Ag/Pd bimetallic colloids on the metal composition for oxidation of ethylene. 25

be due to the aggregation of silver particles, since commercial silver powders are composed of relatively lager particles and their aggregates, as we have reported in the previous paper [2]. PSA-stabiHzed Ag/Pd bimetaUic colloids were applied to the catalyst for oxidation of ethylene as well. Figure 4 exhibits the relationship between catalytic activity and the metal composition of PSA-Ag/Pd bimetalUc colloids. Since palladium have low activity and silver have high activity for the oxidation as described above, PSA-Ag/Pd bimetallic colloids were thought to exhibit activities somewhere between the corresponding monometallic colloids. However, PSA-Ag/Pd bimetallic colloids show higher catalytic activity than the monometallic colloids; catalytic activity at a molar ratio of 20 mol% of Pd is the highest among the colloids. The high catalytic activity can be explained by an electronic effect of neighboring Ag on the surface Pd. 4. CONCLUSIONS PSA-Ag monometallic and PSA-Ag/Pd bimetallic colloids were prepared by UV irradiation of an alcohol/water solution of the coiresponding salts in the presence of PSA. These colloids were well monodispersed and very stable at room temperature for months. Catalytic properties of PSA-Ag monometallic and PSA-Ag/Pd bimetallic colloids were studied for oxidation of ethylene. PSA-Ag/Pd bimetallic colloids are more active than PSAAg monometaUic colloids. The higher activity of the PSA-Ag/Pd bimetalhc colloids can be understood by the electronic effect of the core upon the surface atoms. REFERENCES 1. N. Toshima and T. Yonezawa, New J. Chem., 22 (1998) 1179. 2. Y. Shiraishi and N. Toshima, J. Mol. Catal. A, 141 (1999) 187. 3. K. Torigoe and K. Esumi, J. Phys. Chem., 97 (1993) 8304. 4. T. Yonezawa and N. Toshima, J. Mol. Catal., 83 (1993) 167. 5. Y. Shiraishi and N. Toshima, Colloids Surf. A, 169 (2000) 59.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) CO 2001 Elsevier Science B.V. AH rights reserved.

375

Depletion stabilization of ceramic suspensions with high solids loading in the presence of zirconium oxy-salts Osamu Sakurada and Minoru Hashiba Department of Chemistry, Faculty of Engineering, Gifii University 1-1, Yanagido, Gifu, 501-1193, Japan It was found that the addition of zirconium acetate enhanced the fluidity of ceramic suspensions in the acidic pH range comparing the addition of poly(acrylic acid) to adjust at around pH 10. It could thicken the suspension up to 60 vol% alumina keeping fluidity. Effects of zirconium acetate on the dispersion and the fluidization of the suspensions were discussed. As a result, it would be explained by the depletion stabilization attributing to the positive surface charges of ceramic particles and zirconium acetate species. 1. INTRODUCTION Dispersion of submicron-sized ceramic powders is one of the key factors in explaining the property of fine particles in ceramic forming process such as slip casting, tape casting, etc. [1]. The fluidity is especially important for concentrated suspensions that are used to fabricate structural ceramic parts. It has been already reported that the fluidity of alumina suspension was enhanced by the addition of anionic polyelectrolyte such as poly(acrylic acid) and poly(methacrylic acid) in the vicinity of pH 10 [2, 3]. Lewis et al. have recently used highly repulsive, hydrous zirconia nanoparticles as a depletant to stabilize uniform silica microspheres suspension [4]. We found that the addition of some oxy-zirconium salts instead of zirconia nanoparticles also enhanced the fluidity of silica, alumina and silicon carbide suspensions. In this paper, we show that zirconium oxy-salts are effective dispersants on ceramic powder suspensions in the acidic pH range. We investigate the effect of concentration of zirconium oxy-salts and of pH on the shear-rate versus shear-stress curves (flow curves) of suspensions and measure the iso-electric point (iep) of ceramic powders and the adsorption isotherms of zirconium oxy-salts onto the particle surface. 2. EXPERIMENTAL 2.1. Powders and chemicals We have used three different ceramic powders: commercial mono-dispersed silica (Gel-Tech, USA), alumina (AKP-30, Sumitomo Chemical Co., Japan) and silicon carbide (Betarundum, Ibiden Co., Japan). Table 1 summarizes the characteristics of the powders that were supplied by the manufacturers. Zirconium oxy-salts used were ZrOCb'81120 (Zr-Cl) (Nacalai Tesque, Japan) and zirconium acetate (Zr-Ac) (Aldrich Chemical Co., USA). Nanometric zirconia particles (Nyacol Products Inc., USA) and poly(acrylic acid) (PAA) (average molecular weight ca. 5000, Aldrich Chemical Co., USA) were also used.

376

Table 1 Physical characteristics of powders Silica Particle size (\im) 0.57 Specific surface area (mVg) 4^9

Alumina 0.33 6.8

Silicon carbide 0.29 19.9

2.2. Suspension preparation The suspensions were prepared at volume fractions of ceramic powders in the range of 2 - 60 vol%. A fixed amount of PTFE balls, water, fluidizing agent (zirconium species or PAA) and ceramic powders were put together in a polypropylene bottle and milled for 24h at room temperature. The pH of the suspensions was adjusted with HNO3 and NaOH. Water used was from a commercial Milli-Q system (Millipore, USA). 23. Measurements The flow behavior of the suspensions was determined from shear rate versus shear stress curves, obtained at 25*'C by a controlled stress rheometer (RS-50 and RS-150, Haake, Germany) equipped with either the parallel plate geometry (20,35, and 60 mm in diameter) or the double gap cylinder geometry (DG41). The electrokinetic behaviors of suspensions were characterized by using an acoustophoretic method (Acoustophor 8000, Pen Kem, Inc., USA). The ceramic powder suspensions containing Zr-Ac were centrifuged at 15000 g for 60 min to determine the adsorption of dispersant on the particle surfaces. Residual Zr-Ac amount in the supernatant was determined by ICP-AES (PS-IOOOUV, Leeman Labs Inc., USA). 3. RESULTS AND DISCUSSION 3.1. Effect of pH and added amount of Zr-Ac on the fluidity The desirable prepared conditions for dispersion and concentration of suspensions were clearly determined by the flow curves, i.e., selection of zirconium oxy-salts, effective added -y/"

I— Shear stress: 0.0057 Pa

—»•—pHO.5 —•—pHl.O —0—pHl.5 - A — pH 2.0

\

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lO""

ul

10'

• •^•7

V ^

I I I mill

10-^ [Zr] / M

10

Fig. 1. Apparent viscosity of 10 vol% silica suspensions as a function of Zr-Ac concentration at various pH.

377

Shear rate / s"' ••—2 • 1 0 •*• 100 — • - 6 0 0

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— 1 —

1

10'

— 1 —

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/•—-

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•

•

" "•..

""• A

i 10' ^ 10'

;' A

J" T-,

•-"

'•-. "'•

• 10 A 100 —•—600

10^ 4 PH

Fig. 2. Apparent viscx)sity of 40 vol% alumina suspensions contained with lO'^M [Zr]zr-Ac as a function of pH.

1

Shear rate / s

A:

3

A

fr

T

I 10-'

A

- •

3

: 1

4 pH

Fig. 3. Apparent viscosity of 32 vol% silicon carbide suspensions contained with 7xlO"^M [Zr]zr-Ac as a function of pH.

amount and optimum pH range. Zirconium acetate was most effective for whole of ceramic suspensions in this study. Figure 1 shows the apparent viscosity, Tiapp of the silica suspensions as a function of Zr-Ac concentration at various pH. At 2 < pH <3, riapp decreases to a low value at 2xlO"^M [Zr] and keeps a level up to 0.2 M [Zr] and then increases. At pH > 3, it is hardly to prepare for suspension. Figures 2 and 3 also show Tiapp of the alumia and silicon carbide suspensions as a function of pH. The optimum pH ranges for silica, alumina and silicon carbide were pH 2 - 3, around 4.3 and 3.5, respectively. Figures 4 and 5 show riapp of the alumia and silicon carbide suspensions as a function of added amounts of Zr-Ac at pH 4.3 and 3.5, respectively. For alumina, the values of T^app decrease to a low value at lO'^M [Zr] and keep a level up to lO'^M [Zr] and then increase. For silicon carbide, r^app decreases to a low value at 0.06 M [Zr] and then increases. The optimum concentration ranges of Zr-Ac for silica, alumina and silicon carbide suspensions were 2.5x10'^ - lO'^M, 10"^- 10"^M and 6x10"^ - 8xlO"^M, respectively. It was seen that Zr-Ac promoted the fluidization of alumina suspension on very wide concentration range. Especially for alumina, it could thicken the suspensions up to 60 vol% alumina keeping fluidity. 3.2. Dispersion mechanism of this study ICP-AES measurements showed that the amounts of adsorbed zirconium were negligible small (< lO'^mol Zr m'^ of the particle surface). It was reported that zirconium oxy-salts hydrolyzed in acidic solutions to form polymer like networks [5]. Zr-Cl and Zr-Ac formed the precipitation by the addition of anionic dye, such as methyl orange and fluorescein at pH 2 - 4.3 and did not form the precipitation with o-toluidine blue as cationic dye. Therefore, it was found that Zr-Cl and Zr-Ac in aqueous solution gave a species having positive charge at pH 2 - 4.3. The electrokinetic measurements showed that the iep values of silica, alumina and silicon carbide were 3, 9.3 and 4, respectively. Under these pH, silica, alumina and silicon carbide particles also have to charge positively. As

378 —r-y/m

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<

•

-

*

•^ "

•

•-.^-*—-s• •

t

*

A * *

'

'

'.*

V

10

"• •

•

A

*•

8

:

lA

•

-A

T

>

«,

^

-

•

0

lO''

10'

]

•

J 1

-••

1

•*

10^ lO' [Zr]/M

•*

lO'

• 1

A

lO'

1

Fig. 4. Apparent viscosity of 40vol% alumina suspensions as a function of Zr-Ac concentration at pH 4.3.

•

B

10^

10°

A

Shear rate / s' 1


•

3

• A

A

•

^2-

•

•

• ••

it

-

AAA

•^

10^ • /j*"*

•

m

V ^ ^ •

T

«

-«^ ^

,,-^

8 , 210* • • c n a. a.

.• "^—m.

/ • • ;

•••)>

T

-y/T

10'

Shear rate / s"' ——2 - . . - • 10

LJL_

V/'

• ••

«

J

2

•

10

A

100

\

•

600

1

«

1

1

10' [Zr] / M

Fig. 5. Apparent viscosity of 32 vol% silicon carbide suspensions as a function of Zr-Ac concentration at pH 3.5.

mentioned above, the optimum pH of silica, alumina and silicon carbide were pH 2 - 3, 4.3 and 3.5, respectively. The adsorption measurements indicated that Zr-Cl and Zr-Ac did not play as adsorbents. As a result, the mechanism of dispersion in this study should be explained by the depletion stabilization [6, 7] attributing to the positive surface charges of ceramic particles and zirconium species. 4. CONCLUSIONS We found that the addition of zirconium acetate with the optimum concentration much more enhanced the fluidity of silica, alumina and silicon carbide suspensions at the acidic pH range. Dispersion mechanism would be explained by the depletion stabilization attributing to the positive surface charges of ceramic particles and zirconium species. ACKNOWLEDGMENT We are grateful to Mr. Seizou Obata of Gifii Prefectural Institute for Ceramic Research and Technology, Japan for his helpful in the electrokinetic measurements. REFERENCES 1. J.S. Reed, Principles of Ceramics Processing, 2nd ed., John Wiley & Sons, New York, 1995. 2. J. Cesarano III and LA. Aksay, J. Am. Ceram. Soc., 71 (1988), 1062. 3. M. Itoh, O. Sakurada, M. Hashiba, K. Hiramatsu and Y. Nurishi, J. Mater. Sci., 31 (1996), 3321. 4. J.A. Lewis, V. Tohver, O. Sakurada, P.V. Braun and P. Wiltzius, submitted. 5. W.B. Blumenthal, The Chemical Behavior of Zirconium, D. Van Nostrand, Princeton, 1958. 6. D.H. Napper, "The Effect of Free Polymer on Colloid Stability"; pp. 332-352 in Polymeric Stabilization of Colloidal Dispersions, Academic Press, New York, 1983. 7. A.L Ogden and J.A. Lewis, Langmuir, 12 (1996), 3413.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C' 2001 Elsevier Science B.V. All rights reserved.

379

Monte Carlo study of attractive interaction between charged colloids T. Terao and T. Nakayama Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan We have numerically investigated the properties of like-charged colloidal particles confined between two parallel plates. The interparticle force between a pair of colloidal particles has been clarified, with monovalent as well as multivalent microions. The calculated results demonstrate that the effective interaction between highly-charged colloids strongly depends on the strength of the electrostatic coupling in an aqueous solution.

1. INTRODUCTION It is of fundamental interest to understand the effective interaction between colloids because a vast number of industrial processes depend on controlling the interaction between colloidal particles[l, 2]. While the bare Coulomb interaction between charged colloidal particles is purely repulsive, the problem is nontrivial by the presence of the microscopic counterions dispersed in an aqueous solution, which screens the direct Coulomb repulsion. In the framework of continuum theory, effective potential between charged colloidal particles have been derived from the Poisson-Boltzmann equation. For weak Coulomb interaction or high dilution of macroions, the linearized screening theory of Debye and Hiickel always leads to an eflfective pure-repulsive interaction. This phenomenon is described by the Derjaguin-Landau-Verwey-Overbeek(DLVO) theory, which predicts the screened Coulomb repulsion between charged colloidal particles in an aqueous solution. The properties of quasi-two-dimensional colloidal system are very interesting, because the phase behaviors of colloidal suspensions under geometrical confinement are expected to be drastically different from those in three-dimensional system[3, 4]. In this paper, we perform Monte Carlo simulation of charged colloidal particles confined between two parallel plates. The eflPective force F{r) between a pair of colloidal particles is clarified with different valence of microions in an aqueous solution.

380 2. NUMERICAL RESULTS We adopt the primitive model of strongly asymmetric electrolytes involving the excluded volume and the Coulomb interaction of charged particles under geometrical confinement[5]. The solvent enters into this model by its dielectric constant €, which reduces the Coulomb interaction. We consider two spherical colloidal particles with the surface charge —Ze {Z > 0), where e is the elementaly charge of an electron. In addition, Nc counterions are disposed carrying an opposite charge qe {q > 0). The interactions between particles are given by \/ { \ ^ 1 i 00 °° Z^y^wer Vmm[r)-<^\ ZV/47r6r T/ / N _ f

0 0oo Zqe^/Aner

1/ r^^ - /i ^oc ''^^^ " \1 q^e'^/iirer (?2eV47r6r

for r ^< ad lor for r > d , for for r r<<(i/2 (i/24-4-r,T-c for r > d/2 -f r^ for r < 2rc < 2re 2TC ,, 2r,

for T r >>

... ^'^ /^x .^x ^^

where KnmiT'), Vmci'r) and K:c(7') represent the pair potential on macroions(m) and counterions(c), and d and TC are the diameter of macroions and the radius of counterions, respectively. The linear system size in x- and y- directions are taken to be L, and the distance between two parallel plates is defined as L^ ( - ^ / 2 < x,y < L/2, -Lz/2 < z < L2/2). In the following, the two macroions are placed on the positions Ri = ( - r / 2 v ^ , -r/2y/2,0) and R2 = {r/2y/2, r/2%/2,0). The effective force between like-charged colloidal particles confined between parallel plates are studied in the (N,V,T) ensemble. We start with an arbitrary counterion configuration which does not penetrate into the two colloidal particles. It takes 2 x 10"* Monte Carlo steps(MCS) to get the system into equilibrium, and 1 x 10^ MCS to take the canonical average after the equilibrium. We also take sample average over 60 samples for each run. In the following, the temperature T and the relative dielectric constant of water €r are taken to be T = 300 K and e^ = 78, respectively. The diameter of macroions d and the radius of microions r^ are taken as c? = 20 nm and re = 2.8 A, respectively. These are realistic values for actual colloidal particles which are experimentally studied. We impose the hard-wall condition in x-, y-directions, and the periodic-boundary condition in z-direction. Figure 1 is a typical snapshot of colloidal particles and counterions. The distance between two parallel plates L^ and the interparticle distance r between two colloidal spheres are taken to be Lz = 2.bd and r = 4d, respectively. The magnitude of the charge on colloidal particles Z and surrounding counterions q are taken to be Z = 600 and 9 = 1 , respectively. We can see that counterions are surrounding around a pair of colloidal particles and screen the Coulomb repulsion. Figure 2 shows the effective force F{r)/Fo between two colloidal particles confined between parallel plates. Here FQ is defined to be FQ = ksT/XB (AB = e'^/AirekBT is

381 the Bjerrum length), and F{r) is defined as F{r) = Fi(r) • (Ri - R2)/|Ri - R2I , where Ri and R2 are the positional vectors of the first and the second colloidal particles, respectively. Hence a positive value of F{r) impUes repulsion, and a negative value indicates attraction. On the charge of counterions q, we consider three different cases such as monovalent ions {q = 1) and multivalent ones {q = 2,3) to clarify the eflPect of the strength on electrostatic coupling. Solid squares, solid circles and solid triangles denotes the results with ^ = 1, 2, and 3, respectively. We can observe attractive interaction between highly-charged colloidal particles for strong electrostatic couplings(q' = 2,3). These results give a consistent explanation with regard to the recent experiments, which show that the interaction between charged colloidal particles cannot be described by the conventional DLVO theory in low-salt content[6]. The interaction between colloidal particles strongly depends on the strength of electrostatic couplings in an aqueous solution. Our study has shed light on the microscopic origin of attractive force between negatively charged colloidal particles. For strong electrostatic coupling, the attraction becomes dominant due to nonlinear screening eflfect produced by surrounding counterions. This screening effect induces strong cationic atmosphere around a pair of negatively charged colloidal particles. The comparison between our work and previous studies suggests that the observed attractive interaction between like-charged colloids in an aqueous solution is essentially fluctuation-based phenomenon, where the eflPect of fluctuations is neglected in the traditional Poisson-Boltzmann equation[7, 8]. In the theoretical treatment, the continuum mean-field approximation is not appropriate to describe this problem, but the primitive-model approach including the spatial and temporal density fluctuation of counterions is required, which is a characteristic feature of fluctuation-induced attract ions [9].

3. CONCLUSION In conclusion, we have investigated the eflfective interaction between highly charged colloidal particles numerically, using the primitive model of strongly asymmetric electrolytes. By Monte Carlo simulation, the effective force between colloidal particles under geometrical confinement has been clarified. With smaller electrostatic coupling, the eflfective forces between colloidal particles are pure repulsive over the range of distances explored. With larger coupling, on the contrary, the attractive interaction dominates. We have shown that the eflfective interaction between highly-charged colloidal particles in a low-salt content contradicts with the theoretical prediction by the conventional DLVO theory. These results provide a deep insight into recent experiments, where the strength of the electrostatic coupling in an aqueous solution is an essential parameter to determine the effective potential between like-charged colloidal particles. These non-DLVO behavior between like-charged colloids will become impor-

382 tant to understand the physical nature of colloidal crystal[3, 10].

Fig. 1. Snapshot on the equilibrium state of counterions surrounding two colloidal particles, which are confined between parallel plates.

Fig. 2. Dimensionless effective force F{r)/Fo between two colloidal particles under geometrical confinement.

ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid from the Japan Ministry of Education, Science, and Culture for Scientific Research. The authors thank the Supercomputer Center, Institute of Solid State Physics, University of Tokyo for the use of the facilities.

REFERENCES 1. S. Alexander, P. M. Chaikin, P. Grant, G. J. Morales, P. Pincus and D. Hone, J. Chem. Phys., 80 (1984) 5776. 2. I. Rouzina and V. A. Bloomfield, J. Phys. Chem., 100 (1996) 9977. 3. T. Terao and T. Nakayama, Phys. Rev., E 60 (1999) 7157. 4. K. J. Strandburg, Rev. Mod. Phys., 60 (1988) 161. 5. T. Terao and T. Nakayama, J. Phys.: Condens. Matter, (2000), in press. 6. G. M. Kepler and S. Fraden, Phys. Rev. Lett., 73 (1994) 356. 7. J. C. Neu, Phys. Rev. Lett., 82 (1999) 1072. 8. J. E. Sader and D. Y. C. Chan, J. Colloid Interface Sci., 213 (1999) 268. 9. M. Kardar and R. Golestanian, Rev. Mod. Phys., 71 (1999) 1233. 10. J. Yamanaka, H. Yoshida, T, Koga, N. Ise and T. Hashimoto, Phys. Rev. Lett., 80 (1998) 5806.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

383

Measurements of elastic constants of colloidal silica crystals by laser diflFraction* Tadatomi Shinohara*'^, Tsuyoshi Yoshiyama^, Ikuo S. Sogami^ Toshiki Konishi*^ and Norio Ise^ ^Department of Physics, Kyoto Sangyo University Motoyama, Kita-ku, Kyoto 603-8555, Japan ^Central Laboratory, Rengo Co., Ltd., 186-1-4, Ohhiraki, Fukushima, Osaka 553-0007, Japan We estimated the lattice constants and the elastic moduli of the colloidal sihca crystals using the laser diffraction method. The cube of the lattice constant increased in proportion to the height of the dispersion and the elastic moduh were obtained. 1. INTRODUCTION Investigation on ordering formation processes in colloidal dispersions is helpful for understanding the fundamental properties of colloidal crystals. We observed sedimentation effects on the colloidal silica crystals using the laser diffraction method. Due to the density difference between silica particles and water, there exist small but still detectable gravitational effects. In the laser diffraction experiment [1-3], we used parallel Ar laser beams and observed intrinsic Kossel images of colloidal crystals. Kossel pattern analysis makes the highly-precision determination of the lattice constant of colloidal crystal possible. We photographed the Kossel images of colloidal crystals at various heights. From these Kossel images, we estimated the lattice constants and the elastic moduli of the colloidal crystals. 2. EXPERIMENTAL The colloidal silica dispersion used in this work was Cataloid SI-80P produced by Catalyst & Chemicals Co., Ltd., Tokyo, Japan. The average radius of the spherical colloidal particles was determined to be 500 A by ultra-small-angle X-ray scattering [4]. After purification and deionization by ion-exchange resin particles, the effective surface charge density of the silica particles was determined as 0.2 /iC/cm^ [5,6]. By dilution with pure water, we obtained several semi-dilute specimens of volume fraction 0.01 ~0.05. These specimens were introduced into quartz cuvettes (height 45 mm; width 10 mm; inner thickness 1 mm) together with ion-exchange resin particles. The upper and lower parts of the *This study is funded by a part of "Ground Research for Space Utilization" promoted by NASDA and Japan Space Forum.

384

containers were filled with the resin paxticles. The dispersion began to show iridescence in several minutes and became iridescent throughout the container in a few hours. The cuvettes were kept standing vertically. The temperature was held at room temperature. The laser diffraction experiments were done as follows [1-3]. The specimen cuvette was mounted upright on a goniometer head and the incident Ar laser beam (wavelength 4880 A) was applied normally to the wide surface of the cuvette. Backward Kossel images were photographed on Fuji FG films with a modified rotating camera consisting of a cyhndrical film holder with diameter 2i?cam = 57.3 mm, a colhmation slit of 1 mm diameter and the goniometer head. Time of exposure was about three seconds. To correct the refractive eflPect by using Snell's law, we adopted the formula for the index of refraction of colloidal dispersions n{(/>o) proposed by Hiltner and Krieger [7], n((t>o) = n^aterCl " 0 o ) + ^particle^O,

W

where nwater and riparticie are indices of refraction of water and silica particle (riwater = 1-33 and riparticie = 1-47 [8]). ^o is the initial mean volume fraction of the dispersion. On the supposition that the cuvette's quartz plate is infinitely thin, we considered refraction at the boundary between the colloidal crystal surface and an air. 3. SEDIMENTATION EFFECT OF COLLOIDAL CRYSTAL Spatial coordinate system of this analysis consists of an x-axis being normal to the wide surface of the cuvette, a ?/-axis being horizontal and parallel to the cuvette surface and a z-axis with vertically upward direction. Crandall and Williams [9] derived first a simple expression relating the height dependence of the lattice parameter in a column of colloidal crystals to its elastic modulus. The sedimentation effect leads to inhomogeneous distribution of particles and the volume fraction of the colloidal particles becomes of a function of the vertical coordinate z. In the case where the whole dispersion is considered to be filled with a single crystal, at any given height z from the bottom of the dispersion part, the lattice constant ao{z) of the colloidal crystal and the volume fraction (p{z) are related by ao{z) =

STT

,1/3

[3(/)(^)J

for bcc structure, where R is the colloidal particle radius. In the present experiment, after the colloidal silica dispersion was introduced into the container, the dispersion began to show iridescence in several minutes, while the time taken by a dispersion to reach gravitational equihbrium is the order of a few hours. We observed the crystals with cubic structure throughout the container and did not detect anisotropy of their lattice constants in both horizontal and vertical directions in almost all cases. The lattice constants of the colloidal crystals were determined as a function in height of the dispersion (Fig. 1). The volume of unit cell at the height z is represented as a^{z) using the lattice constant a{z). Equation of balance between the gravitational force and the elastic restoring force is given as [10,11] Qeffg(t){z)dz = —^

-/-

^B,

(3)

385 where ^eff = ^particle — ^water IS the cfFective density of the particle and g is the acceleration due to gravity. B is the bulk modulus of "static" elasticity of the dispersion and the height dependence of B has not been explicitly put in. Equation (3) can be rewritten as d(a3(.)) =

'.^a\z)6z.

(4)

The initial mean volume fraction 0o of the dispersion is given as

/i0o= l%{z)dz,

(5)

where h is the total height of the dispersion part in container. Peculiar Kossel images which are different from those of cubic structure were also observed. Analysis of such a rare observation will be done in a near future. 4. RESULTS AND DISCUSSION The crystal structure, the lattice constant and the orientation of crystals in the salt-free dispersions were determined by the laser diffraction. The results showed that the bodycentered-cubic (bcc) crystalhtes grew with the (110) plane being parallel to the cuvette surface. The lattice constants had a tendency to decrease with the lapse of time. As shown from Figure 2, the observed lattice constants were found to be smaller than the values calculated by Eq. (2) under the assumption of the uniform space fiUing distribution, but the differences were small. Assuming ^(2:) is inversely proportional to the third power of the lattice constant a{z), we get a^{z) = a^(0) + bz

(6)

from Eq. (4). Here, b = Q^fig(t)[z)a^{z)/B. The detailed results of the laser diffraction experiments showed that the lattice constants of the colloidal crystals decreased from the upper to the lower parts of the cuvette as is expected by Eq. (6) and that the orientations of the crystals in the lower part tended to deviate from those in the upper part. These features which can be interpreted as effects of gravity appeared prominent for the dilute dispersions. Figure 1 shows variation of the cube of the lattice constant (in A) of the colloidal crystal as a function of the height z (in mm) from the bottom of the dispersion. The height z ranges from 0 mm to 22 mm. Specimen has the volume fraction 0o of 0.0109. We observed in 121 days after crystallization. From Figure 1, the cube of the lattice constant a{z) follows Eq. (6). From Eqs. (5) and (6) we get ,f

X ^

"^^^^

bhcpO

1

loge[l 4- 6/i/a3(0)l a^O) -^ bz'

(J^

^^

Concentration difference between the upper part and the bottom part of the dispersion lies in the range of the shift of at most ±5 % from (t)o. Thus B are obtained as log,[l + 6/./a3(0)]'

^ ^

386

o o

c

ILO

C O

O 'X;

3000

e6 6

10

16

20

Height z [mm] Fig. 1. The cube of the lattice constant a{z) (in A) of the colloidal crystal as a function of the height z (in mm) from the bottom of the dispersion. The height z ranges from 0 mm to 22 mm.

^

0.01

0.02

0.03

0.04

0.06

Volume Fraction 0 Fig. 2. The averaged lattice constants (in A) of the sample with the volume fractions 0.0109, 0.0294, 0.0360, 0.0446 and 0.0474 measured at 121, 85, 66, 77 and 85 days after, respectively. We drew curve of Eq. (2) as a function of the volume fraction 0.

where ^eff^ = 118 x 10^ N/m^. As a result, we get B = 36 Pa for the specimen of (^0 = 0.0109 and of 121 days after the preparation. The value of h of Eq. (6) was obtained from the slope of the best fit in Figure 1. It turned out that the elastic constant of colloidal crystals is very small compared with that of metals ranging in 10^^-10^^ Pa. In colloidal crystals the particle concentration is approximately 10^^-10^"* cm~^, while the atomic concentration is approximately 10^^ cm"^ in metals. This means the ratio of the elastic constant to the particle concentration of the colloidal crystal is of the same order as that of the metal. It is worthwhile to emphasize that we determined the elastic constant of colloidal crystal by using the quite different principle (diffraction images inside dispersion) from the methods of usual measurements. REFERENCES 1. T. Yoshiyama, I. Sogami and N. Ise, Phys. Rev. Lett., 53 (1984) 2153. 2. T. Yoshiyama and I. S. Sogami, Langmuir, 3 (1987) 851. 3. T. Yoshiyama and I. S. Sogami, in: A. K. Arora and B. V. R. Tata (eds.). Ordering and Phase Transitions in Charged Colloids, VCH Publisher, New York, 1996, p.41. 4. T. Konishi and N. Ise, Phys. Rev. B, 57 (1998) 2655. 5. J. Yamanaka, H. Yoshida, T. Koga, N. Ise and T. Hashimoto, Phys. Rev. Lett., 80 (1998) 5806. 6. H. Yoshida, J. Yamanaka, T. Koga, T. Koga, N. Ise and T. Hashimoto, Langmuir, 15 (1999) 2684. 7. P. A. Hiltner and I. M. Krieger, J. Phys. Chem., 73 (1969) 2386. 8. R. K. Iler, The Chemistry of Silica, John Wiley h Sons, New York, 1979, p. 19. 9. R. S. Crandall and R. Williams, Science, 198 (1977) 293. 10. R. Kesavamoorthy and A. K. Arora, J. Phys., A18 (1985) 3389. 11. R. Kesavamoorthy and A. K. Arora, J. Phys., C19 (1986) 2833.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) io 2001 Elsevier Science B.V. All rights reserved.

387

Rheo-optics of colloidal crystals T. Okubo, H. Kimura and T. Hatta Department of Applied Chemistry, Gifu University, 1-1 Yanagido, Gifti 501-1193, JAPAN Colloidal crystals, i.e., crystal-like distribution of colloidal particles in suspension, is so soft that the crystal structures are distorted and even broken by the shear stress quite easily. In this study the relationship between rheological (macroscopic) and optical (microscopic) properties have been measured simultaneously for the colloidal crystals of silica spheres in the exhaustively deionized aqueous suspensions. 1. INTRODUCTION Small particles ranging from ca.lO nm to 0.1 mm in diameter are called as colloidal particles. Generally speaking, most colloidal particles in aqueous suspension get the negative charges on their surfaces by two mechanisms; one is the dissociation of ionizable groups and the other is the preferential adsorption of ions from suspension. These ionic groups leave their counterions, and the excess charges accumulate near the surface forming an electrical double layer. The counterions in the diffuse region are distributed according to a balance between the thermal diffusive force and the forces of electrical attraction with colloidal particles. When the suspension is exhaustively deionized with the mixed beds of the ionexchange resins, the electrical double layer expand and the particles arrange regularly [1-9]. In other words, phase of the colloidal suspension changes from "liquid" to "crystal" in the process of the deionization. The elastic modulus of the colloidal crystals is extremely low compared with that of other common crystals such as metals. Thus, the colloidal crystals are distorted quite easily by the external fields such as shearing forces, gravity, electric field, centrifugal force, high pressure, and even suspension temperature and ionic species. Several reports have appeared so far for colloidal crystals in the shearing forces; (i) microscopic change in the lattice constant in a flow cell [10], and (ii) the macroscopic visco-elastic properties [11]. Purpose of this work is to clarify the relationship between the visco-elastic and optical properties from the measurements of the rheological parameters and reflection spectra simultaneously.

388

2. EXPERIMENTAL Aqueous suspensions of colloidal silica spheres (CS-91, 110±4.5 nm in diameter, Catalyst & Chemicals Ind. Co. (Tokyo)) were used. It took more than eight years for the exhaustive deionization with ion-exchange resins (Bio-Rad, Richmond, Calif., USA, AG501-X8(D), 20-50 mesh). Sphere concentration of the stock suspension was 0.13 in volume fraction (0). Four sample suspensions from 0.02 to 0.13 were prepared. Water used for the purification and for suspension preparation was purified by a Milli-Q reagent grade system (Milli-R05 plus and Milli-Q plus, Millipore, Co., Bedford, MA). A coaxial type rheometer (Rheosol-G2000W-GF, UBM Co.(Kyoto)) was used. An outer cup was made of pyrex glass for the simultaneous optical measurements. The shear rates increased linearly up to 0.09 s' in the shear stress-strain measurements. The shear rates changed from 0.001 to 10 s* in the steady shear flow. The reflection spectrum at incident angle of 90' were recorded on a Photonic Multi-channel Analyzer (PMA-50, Hamamatsu Photonics Co.(Tokyo)). The measuring temperature is 25*C 3. RESULTS AND DISCUSSION 3.1. Strain dependence Shear stress, a of colloidal crystals was studied as a function of strain, y increasing up to 5.0 from zero. At 0 of 0.129 CTincreased as 0 increased. Especially, in the case of y< 0.3 c increased linearly with increasing 7, which supports, of course, the elastic nature of colloidal crystals. The elastic modulus estimated from the slope of stress-strain curve was 7.0 Pa. At 7= 0.3, the suspension showed yielding though the graph showing this was omitted. When 7 is larger than 0.3, akept constant at 1.1 Pa irrespective of strain. At low sphere concentrations, the yielding point disappeared since the crystal structures are broken in part by the shearing forces. The reflection spectra were obtained at 0 = 0.043. A single sharp peak was observed. The lattice structure is deduced to be fee. The peaks became broad as strain increased. It is highly plausible that fee and bcc lattices coexist in shear stress. The peak wavelength, X^ shifted to the longer when 7 increased. Peak intensity, /^ decreased as 7 increased. These observations mean that the lattice constant increases and crystal size decreases as 7increases. Fig. 1 shows that the 7^ values decrease as 0 increases. At low 0, 0.022, \ decreased slightly when 7 increased. When 0 is 0.043, ^ increased especially at high y> 1.0. At high 0 values of 0.086 and 0.129, A^ values were insensitive to 7 smaller than 1.0, while they turned to decrease as 7 increased. The closest intersphere distance / observed [12] and /^ calculated [13] are given, respectively, /= 0.612 Ay« /, = 0.904 c/,0-'^^

(1) (2)

389

where n is the refractive index of the suspension, and taken to be that of water, 1.33 (251C). df^ is the diameter of silica spheres. The / values at 0 = 0.022, 0.043, 0.086 and 0.129 were 364.9, 293.5, 238.7 and 211.7 nm, while theoretical values /, are 354.9, 283.8, 225.3 and 196.8, respectively. The agreement between the observation and the calculation is satisfactory. These changes in ^ under shear are ascribed to the distortion of the electrical double layers by the shearing forces [13]. Decrease in the lattice spacing by the shearing forces is due to 794 the distortion of the shape of the 792
3.2. Shear rate dependence (7 as a function of shear rate, y were examined. At high 0 values of 0.086 and 0.129, crkept constant irrespective of ^. On the other hand, at low 0 values of 0.022 and 0.043, cr increased as y increased. Clearly, phase transition from "crystal" to "liquid" occurred as 0 increased. Fig. 1 Peak wavelength, ^'-z? of colloidal crystals of Optical measurements on /^ and CS-91 spheres as a function of strain, 7 at 25 "C. ?ip were made as a function of y fmax = 0.093 s'K O: 0 = 0.022, X: 0.043, for the sample, 0 = 0.129. / A:0.086, 0:0.129. decreased sharply around 0.1 s' of y, while Xp kept constant. These results support the fact that colloidal crystals melted in part and the crystal size decreased at the shear rates larger than 0.1 s"'. 3.3. Dynamic properties Storage modulus, G' was studied as a function of angular frequency, co. When 0 is high, G'was insensitive to co. At low sphere concentration of 0.022 and 0.043 in volume fraction, the phase transition from "crystal" to "liquid" was observed. At / < 0.2, G' decreased

390 drastically when yincreased, whereas at larger /than 1.0, G'approached to a constant value. The elastic modulus G obtained by stress-strain relationship was 7.0 Pa at 0= 0.129, which is slightly larger than the storage modulus G' = 2 Pa (y = 0.3), observed in the dynamic measurements. This may be due to the contamination of the ionic impurities into the colloidal samples during the measurements. 4. CONCLUSION It is clear that distortion of the electrical double layers play an important role for the micro- and macro- properties of colloidal crystals. At small strains, colloidal crystals are distorted, but its structure does not change so much. On the other hand, at large strains, structural changes such as sliding of the lattice planes, change in shape of the electrical double layers from spherical to flame-like, and also decrease in crystal size occurs. Acknowledgments The rheometer was purchased by the Grants-in-Aid for Scientific Research on Priority Areas (A) (11167241) from Japanese Ministry of Education, Science and Culture, to whom the authors thank deeply. Sample of colloidal silica spheres was a gift from Catalyst & Chemicals Ind. Co. (Tokyo).

REFERENCES 1) Kose, A., M. Ozaki, K.Takano, Y. Kobayashi and S. Hachisu : J. Coloid Interface ScL, 44, 330(1973) 2) Hachisu, S., A. Kose, Y. Kobayashi and K. Takano : J. Coloid Interface ScL, 55, 499 (1976) 3) Lindsay, H. M. and P. M. Chaikin : J. Chem. Phys., 76, 3774 (1982) 4) Pieranski, P.: Contemp. Phys., 24, 25 (1983) 5) Ottewill, R. H.: Ber. Bunsenges. Phys. Chem., 89, 281 (1985) 6) Aastuen, D. J. W., N. A. Clark, L. A. Cotter and B. J. Ackerson : Phys. Rev. Lett., 57, 1733(1986) 7) Okubo, T. :Acc. Chem. Res., 21, 281 (1988) 8) Lowen, H., T. Palberg and R. Simon : Phys. Rev. Lett., 70,1557 (1993) 9) Stevens, M. J., M. L. Falk and M. O. Robbins : J. Chem. Phys., 104, 5209 (1996) 10) Okubo, T.: J. Coloid Interface Sci., 117, 165 (1987) 11) Okubo, T.: Ber. Bunsenges. Phys. Chem., 92, 504 (1988) 12) Okubo, T.: J. Coloid Interface ScL, 135, 259 (1990) 13) Okubo, T.: J. Chem. Soc. Faraday Trans. 1 : 84, 1171 (1988)

Studies in Surface Science and Catalysis 132 Y. iwasawa, N. Oyama and H. Kunieda (Editors) c; 2001 Elsevier Science B.V. All rights reserved.

391

Relationship between the electrorheological effects and electrical properties in barium titanate suspension Yasuhito Misono, Nammi Shigematsu, Takashi Yamaguchi and Keishi Negita Department of Chemistry, Faculty of Science, Fukuoka University Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan Effects of the electric field on the rheology, electrorheological (ER) effects, and electrical properties in a suspension composed of barium titanate and silicone oil are investigated. From the ER measurements, it is newly found that with increase in the frequency of the electric field the ER effect decreases at lower electric fields while it increases at higher electric fields. On the other hand, the electrical measurements show that a change from a nonlinear to a linear conduction occurs with increasing the frequency of the electric field. On the basis of these results, the ER mechanism in the barium titanate suspension is discussed. 1. INTRODUCTION Rheological properties of suspensions are modified by applying an electric field of a few kV mm'^ Such an electrorheological (ER) effect has been extensively investigated in recent years from technological and scientific interests. From a number of researches it has been clarified that the ER effect in suspensions is caused by a chain-like structure formed across the electrodes. The application of the electric field induces an interfacial polarization on the particle and the resultant dipole-dipole interaction is responsible for the formation of the structure. The ER effect in suspensions, thus, is closely associated with the interfacial polarization, which is caused by a dielectric mismatch between the particle and the liquid; P oc a^e|(ep-£,)/(2ep+£i)£, where P is the polarization, E is the applied electric field, a is the radius of the particle, and e and 6, are dielectric constants of the particle and the liquid, respectively [1]. According to this mechanism if the frequency of the electric field increases, the polarization cannot follow the alternating electric field, which results in a decrease in the polarization effective for the ER effect and hence the smaller ER effect. Such a behavior has been observed in various ER suspensions, e. g. carbon and zeolite suspensions [2]. In our recent study on the electrical properties of the carbon suspension, an interesting result is obtained; at lower frequencies the current increases nonlinearly with the electric field

392 strength, accompanied by a distortion of the current waveform from a sinusoidal one, while at higher frequencies the current increases Hnearly with the electric field strength without a waveform distortion. This result indicates that the electric field localizes the mobile charge near the interface between the particle and the liquid, which leads to the induction of the polarization responsible for the ER effect as well as the nonlinear conduction. In our previous study on the barium titanate (BT) suspension, it is found that the ER effect increases monotonously with the electric field frequency [3]. This result, however, cannot be understood if we consider the same mechanism as that for the carbon suspension. In the present study, to explore the ER behavior of this suspension, details of the frequency dependence of the ER effect are studied together with the electrical and the dielectric properties. 2. EXPERIMENTAL The BT suspension was prepared by dispersing 10 vol% barium titanate particles having the particle size of ca. 0.3 ^m (BT-01, Sakai Chem. Engineering Co., Ltd.) into 50 cS silicone oil (TSF-451-50, Toshiba Silicone Co., Ltd.). The ER effect was measured using a viscometer of a double cylinder type by applying a high voltage across the 1 mm gap between the inner and outer cylinders made of stainless steel 304 [4]. The high voltage was generated using a high voltage amplifier (664 Trek, Inc.); a small AC voltage from a multifunction synthesizer (1946, NF electric Instruments, Co., Ltd.) was amplified up to 2 kV. The current waveform was recorded with a digital oscilloscope (DL708E, Yokogawa Electric Co.). The electric flux density (D) - electric field (E) hysteresis was measured by Sawyer and Tower method [5]. All the measurements were made at 300.0 K, by controlling the temperature of the suspension to within ± 0.1 K (KPIOOO, Chino Co.). Throughout this paper, the amplitude of the AC electric field and the current is expressed in rms. ,Q3 lOkVmm'

3. RESULTS 3.L Rheological Properties Varying the electric field strength, an increment of the shear stress due to the electric

k 1.4 kV mm '

^

1.0kVmm'

10'*

*

y''tif^'^'^i')M^fMf
0.6 kV mm'

Q)^ Q OJkVn

field, yield stress Ar, is measured as a function of lo*' electric field frequency. As is obvious from Fig. '®' '®' ^^f/\lf ^^' ^^ 1, the yield stress decreases with the frequency at ^-^ ^ yield stress (^r) vs. electric field 0.2 kV mm•^ while it increases with the frequency (/) at some electric field frequency above 1.0 kV m m ' ^ At 2.0 kV m m ' ^ a amplitudes. Shear rate = 329.5 s''.

393 remarkable increase in the yield stress is observed; the yield stress at 4.1 kHz becomes about 50 times as large as that at 1 Hz. In order to examine the ER effect in detail, keeping the electric field strength at 1.0 kV mm•^ the frequency dependence of the shear stress is measured at some shear rates. Figure 2 shows that the shear stress begins to increase steeply above a frequency Z^. As indicated by the solid line,^ shifts to a higher value in proportion to the shear rate with a relation of/^ = >/47C.

/ / Hi F^g- 2- Shear stress (r) vs. electric field frequency (/) at some shear rates.

3.2. Electrical and Dielectric Properties Measurements of the electric field frequency dependence of the current show that a distortion of the current waveform from a sine wave of the electric field is recognized at 10 Hz, which is reduced at higher frequencies. Using these waveforms, the current vs. the electric field strength is obtained (Fig. 3), showing that a change from a nonlinear to a linear conduction occurs with increase in the frequency. These behaviors are similar to those in the carbon suspension. In addition to the current measurements, D - E hysteresis, which gives us information about the dielectric constant £* = DIE, is observed. Figure 4 shows that at 10 Hz there appears a hysteresis indicative of a larger dielectric constant at higher electric field strength. At higher frequencies, the hysteresis becomes smaller; at 1 kHz an almost linear relationship is obtained between D and £, suggesting that ^ is not dependent on the electric field strength.

10" E/kVmm*

10'

Fig. 3. Current (/) vs. electric field strength {E) at some frequencies. Shear rate = 329.5 s-'.

-2.0

-1.0

0.0

1.0

2.0

3.0

ElVMrnm^ Fig. 4. Electric flux density (Z)) electric field (£) hysteresis at 10 Hz. Shear rate = 329.5 s"'.

394 4. DISCUSSION At lower electric field strength, the ER effect of the BT suspension decreases with the electric field frequency, which is consistent with the result that the nonlinear conduction changes to the linear one with the frequency. It is, therefore, plausible to consider that at lower electric fields the ER effect in the BT suspension is caused by the same mechanism as that in the carbon suspension; the electric field induces a polarization on the particle owing to the localization of the mobile charge near the interface, which is responsible for the formation of a chain structure causing the ER effect. On the other hand, at higher electric field strength, the ER effect of the suspension increases with the frequency. Our result that the nonlinearity in the conduction and the D - £ hysteresis is reduced with increasing the frequency indicates that the ER effect at higher electric field strength cannot be understood in terms of the mobile charge localized at the interface. For understanding the mechanism, the relation obtained from Fig. 2, /^ = yf4n, is suggestive. It is theoretically clarified that if a steady shear deformation is applied to the suspension at a shear rate y, a rotation of the particle occurs with an angular velocityfi)^= yfl [6]. The relationy^ = y/4K suggests that the ER effect largely increases if the electric field fi-equency becomes higher than the rotational frequency of the particle, f^ = (OJ2K = y/4K. Although the effect of the particle rotation on the ER effect is not well understood, a theoretical study shows that a suppression of the particle rotation due to the electric field is responsible for an increase in the ER effect [7]. Furthermore, experimentally it has been clarified in the TiOj suspension that if the frequency of the electric field matches the rotational frequency, an increase in the ER effect occurs [8]. These results may suggest that the suppression of the particle rotation would be associated with the ER effect at higher electric field strength.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

R. W. Sillars, J. Instr. Elect. Eng. (London), 80 (1937) 378. K. Negita, Y. Misono, and T. Yamaguchi, Trans. Mater. Res. Soc. Jpn., 25 (2000) 159. Y. Misono, T. Yamaguchi, and K. Negita, J. Mol. Liquids, in press. K. Negita, Netsu Sokutei, 22 (1995) 137. C. B. Sawyer and C. H. Tower, Phys. Rev., 35 (1930) 269. G. B. Jeffery, Proc. R. Soc. London A, 102 (1923) 161. J. Hemp, Proc. R. Soc. London A, 434 (1991) 297. K. Negita and Y. Osawa, Phys. Rev. E, 52 (1995) 1934.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B.V. All rights reserved.

395

Solid-liquid separation and size classification of ultra-fine hematite particles using bubbles K. Yamada', RHayashi', HSasaki*, and E.Matijevic*' "School of Science and Engineering, Waseda University, Tokyo 169-8555, Japan *Tnst Colloid and Interface Sience, Claricson University, Potsdam, NY 13676, U.S.A.

Solid-liquid separation and size classification of ultra-fine hematite particles (smaller than 1 n m) were examined by using column flotation method Ultra-fine hematite particles prepared by hydro-thermal aging method were suspended in water, followed by introducing argon bubbles into die colunmflotationcell. Since no sur&ce active agent as a collector was added, particle surfaces were maintained in hydrophihc conditioa Recovery of hematite particles dianged significantly by regulating chemical condition of suspension such as pH, and electrolyte concentration. In addition, size classification of poly-dispersed hematite particles was achieved in acid condition by the same method of introducing bubbles. Small sized hematite particles were floated selectively by bubbles, while larger particles remained in the suspensiwi. It was found that separation teclmiques using bubbles in colunm flotator were very usefiil for solid-liquid separation and size classification of ultra-fine particles firom low concentrated suspensions. Driving forces of those processes are suggested to be interfecial chemical interactions such as hetero-coagulation between bubble and hematite particle, or homocoagulation of hematite particles. 1. INTRODUCTION When ultra-fine particles smaller than 1 iz m are suspended in water, they disperse stably due to their low coagulation and low sedimaitation velocity [1]. In those suspensions, colloidal effects such as Brownian motion, or repulsive force between particle surfeces dominate the characteristic of the suspension [2]. In general, separation of ultra-fine particles fi-om low concentrated suspension is diiSicult Separating method aj^licable to ultra-fine particles has yet to be estabUshed, so new alternative separating methods are now desired [3]. Two of the authors in this paper, have already reported new methods for rapid separation of ultra-fine particles from low concentrated suspension using glass beads or fibrous slag wool as particle collecting materials [4-7]. In these papers, it is described that interfacial chemical interactions between particle and those collecting materials enabled excellent separations. In this study, argon gas bubbles inflotationcolumn were used as the collecting material. Bubble

396 as the collecting material can concentrate particles when it ruptures one after another at liquid surface. Through all experiments, flotation collector for making hydrofAobicity to particle surface wasn't added, so particle surfeces were kept in hydro[Ailic ccmditioa On that point, this method makes exact distinction from mineral flotation [8,9]. This paper describes the results of sohd-liquid separation and size classification of artificial ultra-fine hematite particles prepared by hydro-thermal aging method, and discusses the influences of chemical conditions of the suspensions such as pH which would affect inter&cial chemical interactions between hematite particles and gas bubbles. 2. EXPERIMENTAL 2-1. Materials More than 99.999% purity of argon gas was used to generate bubbles in the experiments. Flow rate of argon gas was regulated by afloatflowmeter and a soapflowmeter. Ultra-fine hematite particles were piepared by the hydro-thermal aging method as described by Matijevic and Scheiner [10]. A solution c(mtaining 0.018 mol/L ¥eCl^ and 0.001 mol/L HCl was aged at lOOt: for 24hours. After aging, the suspension was centrifiiged, and the supernatant solution was decanted, followed by resuspension with doubly distilled water in an ultrasonic bath Centrifugation and resuspension were repeated until the conductivity of the supernatant solution remained constant (1 n S/cm). By these procedures, mono-dispersed spherical hematite particles sized 130 nm were obtained. By the same aging method, poly-dispersed hematite particles ranging from 100 to 500 nm were prepared by changing the concentration of FeCla to 0.025mol/L, HCl to 0.008 mol/L, and aging time to 7days. Here, the median diameter and the size distribution of hematite particles were determined by dynamic hght scattering method with an apparatus ELS800 (Ohtsuka Electronics). All chemicals used in the e^qjeriments were of analytical reagent grade, and the water used for the \s4iole experiments was doiily distilled. 2-2. Methods 2.4 mg^L of hematite suspension was charged into a cylindrical pylex glass column flotator of 36mm inner diameter, aiKl 850mm h e i ^ . Argon gas was introduced into the suspension with flow rate lOOmL/min through the glass filter equipped at the bottom of the columnflotator.10~^ mol/L of l-butarK)l, asfirotherwas added into the suspension, to help generating small bubbles of median diameter 0.40 mm [8,9]. After introducing bubbles for 15 minutes, concentration of hematite particles in column flotator was determined by measuring the intensity of He-Ne laser scattered in the suspension. The recovery, in other words, the proportion of hematite particles floated by bubbles to the initial amount of them was determined by comparing the concentration of suspension before and after gas bubbling. Size classification of poly-dispersed hematite particles was also examined by introducing gas bubbles into the suspension containing 2.8 mg/L of hematite. The concentration of polydispersed hematite particles was determined by dissolving them into hydrochloric acid, followed by introducing to ICP quantmeter. The results of size classification were evaluated by comparing size distribution and recovery of hematite particles before and after gas bubbling.

397 3. RESULTS AND DISCUSSIONS 3-1. Solid-Liquid separation Fig.1 shows the recovery of hematite particles for various pH conditions obtained by introducing bubbles into the suspension containing 10~^ mol/L of KNO3 and mono-dispersed hematite particles of 130 ran. Nearly 90% of hematite particles were recovered at the condition of pH7.5 in ISnunutes. In acid conditions, about 50% of hematite particles were recovered, while recovery declined below 40% in alkaline conditions. To explain these results, surface characteristics of bubble and hematite particle seem to have great importance. Fig.2 shows the zeta-potential of hematite, and bubble. The zeta-potential of hematite was determined by electrophoresis method using ELS-800, and zeta-potential of bubble was determined by sqjplying the sedimentation potential method [11]. As shown in Fig.2, bubble surface has negative charge with insignificant dependence of pR Contrary, hematite has its isoelectric point at pH7.8, and changes its surface charge in accordance with pH. It has positive charge in acid conditions, and negative charge in alkaline conditions respectively > 60 .^-40

r

» > €

Jt^ i

-600 > -400 a

£-20

-200

£

0

0

o

a>

1

200

D Hematite 1J 1 20 • Bubble |1 400 B 1 o-Crf^ Ia 40 1 ___] L 1 1 i 600 a 2

4

6 „8 10 12 pH Fig.l. Recovery of hematite within ISminutes for various values of pH

60

8 10 12 pH Fig.2. Zeta-potential of hematite and bubble 6

In acid conditions, bubble and hematite have opposite surfece charge, so they attract each other, namely such hetero-coagulation promoted separating hematite particles, but in alkaline conditions, both bubble and hematite have negatively charged surface, so they repel each other, which accounts for the lowest recovery. At pH7.5, near the iso-electric point of hematite, homocoagulation is likely responsible for the maximum recovery. Hematite particles attracted each other due to the absence of electric repulsive force between particle surfeces with rupturing of bubbles at the liquid sur&ce. 3-2. Size classification From the results of above session, it seems that there is a possibility of achieving size classification in acid and neutral pH conditions v^ere hematite particles were recovered eflFectively. Size classification was exanuned in those pH conditions. By introducing gas bubbles into the suspension of poly-disperesed hematite particles at the condition of pH4.0, small sized hematite particles rangingfirom110 to 200 nm were recovered selectively, while larger particles ranging fi-om 170 to 460 nm remained in the suspension as

398 shown in Fig.3. This means hematite particles were divided into two classes according to their sizes. On the other hand, in neutral condition of pH7.5, transition of size distribution was negligible before and after gas bubbling, so size classification was not achieved In this case, coagulation of hematite paiticles seems to have prevented selective separation by size.

100 200 300 400500 100 200 300 400 500 Diameter n m Diameter nm Fig. 3.Size distributions of hematite particles (a)pH 4.0, (b)pH7.5; • : before gas bubbling, O : after gas bubbling, broken lines: recovered hematite caluculated by mass balance U

0

4. CONCLUSIONS Solid-liquid separation and size classification of ultra-fine hematite particles were examined by introducing bubbles into the low concentrated (2.4-2.8 mg/L) suspensioa The following results were obtained (l)Recovery of hematite particles as a float changed remaiteibly depending on pH of the suspension. Especially, 90% of hematite particles wererecoveredin 15minutes at pH7.5. (2)Smaller hematite particles were collected selectively by bubbles, while larger particles remained in the suspension at pH4.0. These results were explained in terms of interfecial chemical interaction of gas bubbles and hematite particles.

REFERENCES 1. RJ.Hunter: Foundations of Colloid Science Vohime.l, Oxford university press (1986), p49 2. R J.Hunter: Intrcxhiction to Modem Colloid Science, Oxford university press (1993), p32 3. A.Rushton, A.S.Ward, R.G.Holdich,: Solid-Liquid Filtration and separation Technology, VCH(1993),pl 4. RSasaki, Grace.E.Drige, T.Sugimoto: Powder Tedinology ,78 (1994), pl37 5. HH^'Hshi, H.Sasaki: Global Metals Environmeiit*99, IntcmaticMial Academic Publishers (1999), p332 6. XRIbanez, RSasaki: IntJ. of Mat Eng. For Resources Vol.3 No. 1(1995), pl67 7. J.P.Ibanez, Y.Umetsu, H.Sasaki: Hydromctalluigy, 47(1998), p353 8. DriscolJD.K: Thesis of Bach Sci. Clarkson CoU. Tech. (1979) 9. Jiang, Meiling: Thesis of Ms. Engineering, Tohoku Univ (1992) (In Japanese) 10. E.Matijevic, D.Scheiner,: J.CoUoid Interfece Sci. Vol.63, No.3(1978), p509 11. S.Usui, RSasaki, HMatsukawa,: J.CoUoid Interfece Sd. Vol.81, No. 1(1981), p80

Siudies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

399

Rapid separation of oil particles from low concentrated OAV emulsion in the presence of surfactant using surface characteristics Remi SAKO^ Hitoshi ITO', Hiroshi HAYASHF and Hiroshi SASAKF' ''Department of Resources and Environmental Engineering, Waseda University, Okubo 3-4-1, Shinjuku-ku, Tokyo 169-8555 Japan ^Advanced material lab, Ebara Research co, Honfujisawa 4-2-1, Fujisawa-shi, 251-8502, Japan "" Department of chemical engineering, Waseda University A novel technique of rapid separation of low concentrated fine n-decane droplets (mean diameter 400 nm) in the presence of anionic (SDS) or cationic (CTAB) surfactant was examined using column bed packed with fibrous slag, which was solid waste from Fe-Ni smelting process. Efficient recovery of oil droplets was attained below 3 X 10"^ M for SDS and 1X10'^ for CTAB, where coagulation of oil - collector or oil - oil droplet easily occurred. On the other hand, at relatively higher surfactant concentration of 1 X 10*^ M for SDS and 3X10'^ M or above for CTAB, collection efficiency monotonically decreased, suggesting coalescence of oil droplets were hindered. It was found that our separation method, using interfacial characteristics, is effective for dilute and stable dispersed oil droplets due to its low cost and simplicity.

1. INTRODUCTION It has been generally accepted that separation of fine oil particles from low concentrated and stable dispersed 0/W emulsion without coalescence by chemical or mechanical treatment is very difficuh because physical property of particle is undominant in colloidal range, failing to remove particles by skimming, straining or interception. Hence, rapid separation of low concentrated emulsified oil is one of the hardest techniques and effective method has yet to be established so far. On the other hand, our method using surface characteristics, application of heterocoagulation phenomena, was a good alternative for separation of colloid particles from stable dispersed system [1]. Particle separation was carried out while passing the solution * Corresponding Author: Tel:++81-3-5286-3327 E-mail :[email protected]

400

through the column packed with fibrous collector at relatively fast superficial velocity. Whether spontaneous attachment of particles takes place or not is largely dependent on interfacial physicochemical interaction between particle and collector surface. In this study, liquid/liquid separation of low concentrated emulsified n-decane (100 v/v ppm) in the presence of sodium dodecyl sulfate (SDS) or hexadecyl trimethyl ammonium bromide (CTAB) was examined by packed column method. Material employed for particle collector was fibrous ferro-nickel slag, which was by-productfi-omferro-nickel electrosmelting process, attempting to utilize industrial waste as ecomaterial for wastewater treatment. Also interfacial tension between water/n-decane and zeta potential measurement of oil droplets were carried out to examine surface character of emulsions.

2. MATERIALS AND METHODS 2.1 Materials OAV emulsified oil (100 v/v ppm) was prepared by dispersing n-decane mechanically using ultrasonic homogenizer (BRANSON SONIFIER 250) into the lOmM NaNOj solution in the presence of desired concentration of surfactant. Sodium dodecyl sulfate (SDS) and hexadecyl trimethyl ammonium bromide (CTAB) were used as anionic and cationic surfactants in the experiment, respectively. It was confirmed that emulsified oil droplets had mean diameter of 400nm by DLS. Fibrous Fe-Ni slag was purchased fi-om Pacific Metals Co. (Japan). The chemical compositions of the slag used in the experiment were SiOj (52.9%), MgO (22.2%), AI2O3 (18.6%), Fe203 (1.13%), and CaO (1.66%). The individual fiber had a mean diameter of 8 - 10 ^un and a completely smooth surface. The dissolution of slag was quite negligible in the pH range 4 - 10 at 25^0 [2]. All chemicals used were of analytical grade and purchased fi-om Kanto Chemical Co., Japan. 2.2 Interfacial tension between water/n-decane and zeta potential of n-decane droplets Interfacial tension between water/n-decane was measured by Wilhelmy method using automatic surface tensiometer (CBVP-Z, Kyowa Interface Science, Japan) with platinum plate. Zeta potential measurements were carried out by electrophoretic light scattering spectrophotometer (ELS-800, Otsuka Electronics, Japan). Supporting of electrolyte for water was 1 X 10"^ M NaNOj for both experiment. 2.3 Separation of n-decane emulsion by packed bed method Fibrous slag (13.5 g) was homogenously packed at the bottom of the glass-made column (cross-sectional area: 19.6 cm^ height 30cm) at void volume fraction of 0.95. Prepared emulsion was introduced from the top of the column into the bed at constant flow rate of 250 cm^-min"' (superficial velocity: 0.85 mms'^). Time course of n-decane concentration in effluent was measured by gas chromatograph with FI detector after n-hexane extraction.

401

3. RESULTS AND DISCUSSION Fig.l

shows

interfacial

tensions

between

water/n-decane

as functions

of

surfactant

concentration. Interfacial tension gradually decreased with the addition o f surfactant in both cases and finally leveled off at concentration o f 3 X 10 ^ M and 3 X 10 ^ M for S D S and CTAB systems, respectively. 125 100 >75| . 50

•7 50

e c30

I 25

S20 B -50 1 -100 -125 '

a 0

-3

10-^ W 10^ 10'" 10"^ 10'^^ 10r\-2 Surfactant concentration, mol* dm* • SDS • CTAB

10-^ 10-^ 10^ 10-^ 10^ 10-^ IQ-^ Surfactant concentration, mol* dm""^ • SDS • CTAB Fig. 1. Interfacial tensions between water/n-decane systems as a function of surfactant concentration

Fig.2. Zeta potential curves of n-decane droplet as a function of surfactant concentration

Figure 2 shows zeta potential curves of n-decane as a function of surfactant concentration. Adsorption of SDS onto the n-decane droplet surface, which was originally negatively charged (-55.3mV in the absence of surfactant), increased the magnitude of negative potential to reach - 9 2 mV eventually at 1X10*^ M. In the case of CTAB, significant increment of zeta potential was observed at lower concentration (<1 X 10'^ M) and then turned to slight increase. Isoelectric point was estimated to be at 2 X 10 "^ M CTAB. 1 0.9 0.8 ^0.7 XO.6

wsJ-^^ -fi

-S"^O- C ^ >-A-<^

p^^ -^<^%
V

-J ffX~ > X

y-(M

V\

V0.5 J 0.4 ""0.3 0.2 0.1 0

^-a 2

4

6 8 10 12 14 16 18 20 Dimensionless Time

-<>-3X10''M

9 0.7 ^0.6

c-ro.5

"N

0

1 0.9 0.8

1 X lO"^ M

Fig.3. Result of n-decane collection as a function of time in the presence of SDS

r A ] r-An

¥1\

^-^ -
U

A-n A

/\ L.

o-

r

Ff^

H \

\

1

^-0

1 hi

SI' T^ s

(J 0.4 ^0.3 0.2 0.1 0

n

0

2

4

6

8

10 12 14 16 18 20

Dimensionless Time -O-OM , 3 X 10'^ M

3X10'^M

Fig.4. Result of n-decane collection as a function of time in the presence of CTAB

402

Figure 3 and 4 show the resuh of n-decane collection as a function of time in the presence of SDS and CTAB, respectively. In both figures, collection efficiency was defined as the ratio (Co'Cf)/Co, where Q and C, represents the initial and outlet n-decane concentration. The abscissa, dimensionless time, was defined as the time divided by the time required for the solution to pass through the bed. Dimensionless time 1 corresponded to approximately 1 min in the experimental condition, hi the case of SDS, efficient collection was achieved below 3 X 10"^ M. Gradual increment of collection efficiency indicated that coalescence between previously attached and approaching oil particles (homocoagulation) promoted n-decane recovery as the passage of time. On the other hand, at 1 X 10"^ M of SDS, n-decane collection curve monotonically decreased, suggesting that homocoagulation and coalescence of oil particles was hindered. In Fig. 4, excellent collection was observed at 1 XIO"^ M CTAB, where CTAB adsorption on n-decane changed zeta potential from -15mV to +80mV, resulting in promoting n-decane attachment onto negatively charged collector surface. But recovery monotonically decreased above 3 X 10'^ M CTAB, mainly due to the interference of mutual approach between n-decane particles.

4. Conclusion Our novel method, using surface characteristics of dispersed oil droplet, is very effective, especially for low concentrated ultrafine emulsified oil. Efficiency of oil droplet recovery was significantly dependent on their interfacial characteristics, which was peculiar to our method compared with conventional oil recovery technique. This system generally applies to ultrafine particulate matter dispersed in liquid phase such as metal oxides and sulfides, clay minerals and microbial cells and so on [3-4].

Reference 1. J. P. Ibanez, M. Murakami and H. Sasaki, Int. J. of The Soc. Mat. Eng. for Resources, Vol.5 (1997)No.l,p.41-57. 2. J. P. Ibanez and H. Sasaki, Int. J. of The Soc. Mat. Eng. for Resources, Vol.3 (1995) No.l, p.167-177. 3. J. R Ibanez, Y. Umetsu and H. Sasaki, Hydrometallurgy, Vol. 47 (2000) 353. 4. H. Hayashi and H. Sasaki, Proc. of Global Metals Environment, (1999) Beijing, China, p.332-339.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c) 2001 Elsevier Science B.V. All rights reserved.

403

Fingering pattern dynamics in magnetic fluids Y. Enomoto Department of Environmental Technology, Nagoya Institute of Technology, Gokiso, Nagoya 466-8555, Japan We analyze the Saffman-Taylor problem in a Hele-Shaw cell, when a magnetic fluid is injected into a nonmagnetic immiscible viscous fluid in the presence of a perpendicular magnetic field. From the hydrodynamics treatment of the problem, an interface separating two fluids is shown to exhibit complex patterns due to the competition between surface tension, injection effect, and long-range dipolar repulsive force. 1. INTRODUCTION Magnetic fluids (ferrofluids) are oil- or water- based colloidal suspensions of ferromagnetic nanoparticles, such as Co-ferrite (CoFe204) and magnetite (Fe304) [1]. Because they are fluids, these suspensions can flow in response to apphed forces such as a magnetic field [2,3]. Due to this hydrodynamic property of ferrofluids, various interesting instabilities have been observed. Recent experiment has examined the interfacial fingering dynamics, when ferrofluids are injected into nonmagnetic immiscible viscous fluids in the presence of a perpendicular magnetic field [4], confined in a narrow gap between the parallel plates (the Hele-Shaw cell [2,3]). To study the mechanism generating complex fingering patterns (known as the Saffman-Taylor problem [2]), we here formulate and analyze this problem hydrodynamically. 2. DARCY'S LAW FOR MAGNETIC FLUIDS We consider two inuniscible, viscous fluids, separated by an interface with a. surface tension a, in the Hele-Shaw cell with a thickness b (see Fig.l). Here, a ferrofluid (fluid 1) having viscosity rji is injected into nonmagnetic fluid 2 having viscosity 7]2 at a constant injection rate Q. The flows are assumed to be irrotational, except at the interface. In order to include magnetic forces, we apply a magnetic field perpendicular to the cell. Following the standard approach in Hele-Shaw problems with a constant ferrofluid magnetization magnitude M and a magnetic scalar potential
2r/v = - 6 ^ V ( p - 2M6-V)

(^)

Eq.(l) describes the nonmagnetic fluid 2 by simply setting M = 0. Considering the incompressibihty constraint, V • v = 0, we obtain from eq.(l) a jump

404

of an effective pressure, Ui = pi - 2M6" Vt (z = 1,2), across the interface by n2 - Hi = aK{s) 4- 2M%-'l{s)

(2)

where s is a parametric coordinate along the interface, and K is the curvature of the interface. The I{s) in eq.(2) is proportional to the magnetic potential, given by [5,6] I{s) = JdA

[\T{S) - r r ' - {\r{s) - r f + 62)-i/2]

(3)

where the integration is carried out on the top of the cell within the interior of ferrofluid region.

Fig.l. Schematic configuration considered here.

3. INTERFACE DYNAMICS Within the assumption of Darcy's law, one can obtain the interface equation of motion between the ferrofluid and nonmagnetic fluid. To do so, it is convenient to rewrite eq.(l) in terms of the velocity potential ij; for the irrotational velocity by v = -Vip as I2rjiilji = b'^Ui

(4)

after dropping an arbitrary constant. Substituting eq.(4) into eq.(2), the interface equation of motion reads A{'tp2 + t/;i) -f ^2 - V'l = 2a[aK{s) -h 2M^b-^I{s)]

(5)

with the viscosity contrast A = (r;2-Tyi)/(r/2+r/i) and a = 6^/[12(7?2+'?i)]- To close eq.(5), we should consider the kinematic boundary condition such that the normal components of each fluid's velocity at the interface (represented by p = p(9, t) in polar coordinates) equals the normal velocity of the interface itself: [1]

~di~ r'^de~de~"d^

(6)

405

3.1. Linear stability During the ferrofluid injection, the interface has a perturbed shape described by p = R(t) + C(^,*), as is shown in Fig.l, where R(t) is the time-dependent unperturbed radius given by R{t) = y/R{Oy + Qt/n. To determine the stabihty of the initial shape, we hnearize eqs.(5) and (6) in the amphtude of small disturbances about a circle. Setting C(^, t) = En Cn{t) exp(m^), and also solving to lowest order in Cn{t)/R{t)y we obtain

dUt) _ dt

Q aa {A\n\-l)-^^\n\{{n'-l)-NsDr.{x)} [27rR{t)

Cn(t) = a;(n)CnW

(7)

with the linear growth rate a;(n), geometric aspect ratio x = 2R{t)/b, magnetic Bond number A^^ = 2M%/a, giving the relative strength of dipolar to surface forces, and sm'z)''^' Dn{x) = y x'^{2k - 1)-' + 2-'x^ r ' dz{cos{2nz) - cos(2z))(l + x' si

(8)

For details of Dn{x), except that Drx{x) > 0 for any n and x, see Ref.[6]. From these results we can find that (1) the perturbation is destabilized by both injection effect (Qterm in eq.(7)) and dipolar forces {NB), but is stabilized by surface tension (n^ - 1), and (2) due to the comphcated n and t dependence of a;(n), the competition between many unstable modes can occur, probably leading to complex interface patterns, as is shown in Fig.2.

Fig.2. Time evolution of the interface in properly rescaled time unit with NB — 2 and X = 50, given by the hnear theory (7) starting from a circle with small randomness. A thick curve is a snapshot given by nonhnear theory (11)-(12) at the same simulation time as the final linear result.

406 3.2. Boundary element method simulations The numerical method to simulate the interface evolution beyond the linear theory is based on a boundary integral equation formulation for the Darcy equation eq.(l) with V^Ui = 0. First, we find from Darcy's law that the tangential components of the fluid velocity are related to the effective pressure boundary condition through 62t.(Vn2-Vni) = -12t.(772V2-7;iVi) .

(9)

Then, we rewrite eq.(9) in terms of an average velocity V = (vi -f V2)/2 as 2at-V(n2-ni) = 2 a ^ ( n 2 - n i ) = -[t.(v2-vi)-2At.V]

(10)

where f{s) = t • (v2 - Vi) gives the strength of the tangential velocity discontinuity. In addition, the mean velocity V is given by the Birkhoflf-Rott integral [6] 27rV(5) = Pfds'z

X (r(s) - r(5'))|r(5) - r(s')rV(5')

W

where P means a principal-value integral. Hence, the discontinuity strength /(s) can be obtained to solve the integral equation /(.)-2>lt(.).V(.) = 2 a [ a ^ + ? ^ ^ ] .

(12)

The above equations (11) and (12) can be solved numerically by using the usual boundary element method [6]. In Fig.2 we show a simulation result, compared with linear theory (7), starting from the same initial condition. This result shows the possibihty that more complex pattern beyond the linear theory can be obtained in nonlinear simulations. 4. CONCLUSION Patterns dynamics in Saffman-Taylor experiments with injected ferrofluids in applied magnetic fields has been formulated and analyzed hydrodynamically. Both linear stability and simulations have revealed that complex fingering interface patterns occur due to the competition among sinrface tension, injection rate effect, and dipolar force. Detailed discussion of the present analyses, as well as comparison with experiments, is beyond the present scope, and it will be done in near future. REFERENCES 1. 2. 3. 4. 5. 6.

R.E.Rosensweig, Ferrohydrodynamics, Cambridge University Press, Cambridge, 1985. P.G.SaflPman and G.I.Taylor, Proc.R.Soc.London A245 (1958) 312. D.Bensimon et al, Rev.Mod.Phys., 58 (1986) 977. C.Flament it et. a l , Phys.Rev.E 53 (1996) 4801. J.A.Miranda and M.Widom, Physica D 120 (1998) 315. D.P.Jackson et al, Phys.Rev.E 50 (1994) 298.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) P 2001 Elsevier Science B.V. All rights reserved.

407

Coagulation of negatively chained microspheres dispersed in cationic surfactant solution Kazuhiro Fukada*, Kyoko Nakazato\ Tadashi Kato*, and Makio IwahasW ^Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Minamioosawa 1-1, Hachiohji, Tokyo 192-0397, Japan ^Department of Chemistry, Faculty of Science, Kitasato University, Sagamihara, Kanagawa 228-0829, Japan Turbidity and ^-potential of negatively charged polystyrene latex (PSL) dispersions in alkyltrimethylammonium bromide solutions were measured as a function of surfactant concentration. By the adsorption of a small amount of surfactant, coagulation of PSL occurred though the net surface charge of PSL particles was still negative. 1. INTRODUCION Polystyrene microspheres prepared by emulsion polymerization have been utilized as model dispersions for the study of colloidal stability because of the well-defined shape and the high degree of monodispersity. The DLVO theory, which considers the interaction potential between two spheres as the sum of Van der Waals attraction and electrostatic repulsion, is generally accepted to explain the stability of colloidal systems. Several discrepancies, however, between theory and experiment have been reported for polymer particles [1]. In some cases, disagreement was attributed to the adsorbed molecules or ions on the surfaces of particles. Adsorption of ionic surfactants on charged colloids can modify the surface properties, e.g. surface charge density [2, 3], and influence the stability of dispersed state of colloids. In this study, we focused the attention to the coagulation behavior of negatively charged polystyrene latex (PSL) particles dispersed in cationic surfactant solutions. It will be shown that rapid coagulation occurred only in a narrow concentration range of surfactants where PSL particles were still negatively charged. 2. EXPERIMENTAL Uniform polystyrene latex particles with sulfate groups on the surface were obtained from Dow Chemicals (Indianapolis, IN) as the 10-wt% solid content dispersion in water. The PSL particles were ultra-clean uniform microspheres with a number averaged diameter of 0.22 [im (std. deviation = 0.0065 |im), and were extensively dialyzed by the manufacturer to remove impurities. Guaranteed grade reagents of dodecyl-, tetradecyl-.

408 and hexadecyltrimethyammonium bromide (C12TAB, Ci^TAB, Cj^TAB; Tokyo Kasei Inc.) were purchased and used after recrystallization from an acetone/ethyl ether mixture. Water passed through a Milli-Q Lab purification system after distillation was used. The PSL dispersion was diluted with various concentrations of alkyltrimethylammonium bromide (QTAB) + 10 mM NaCl aqueous solution and equilibrated overnight at room temperature, and ^-potential of the particles was measured at 25 °C by an electrophoretic light-scattering photometer (ELS-800, Otsuka Electronics, Hirakata). This apparatus uses laser-Doppler velocimetry to measure the electrophoretic mobility, M. The ^-potential was calculated by Smoluchowski's equation; t, = urj/ e,€o

(1)

where rj and e^ is coefficient of viscosity and relative dielectric constant of dispersion medium, respectively, and CQ is the dielectric constant of vacuum. Surfactant solutions containing 10 mM NaCl were employed as the dispersion medium since PSL exhibit anomalous electrokinetic behavior in low ionic strength solutions [4]. Coagulation behavior of the PSL particles in surfactant solutions was monitored measuring the transmission coefficient of 480-nm light with a spectrophotometer (UV160A, Shimadzu). The turbidity x was calculated by the relation;

QLi//"*^

0

10-1

.J

-I

^

^

-.

100

101

102

103

104

IC„TAB] / nM Fig. 1. ^-potential of polystyrene latex (PSL) particles dispersed in CJAB + 10 mM NaCl as a function of surfactant concentration at 25 °C. The used surfactants are C^jTAB ( O ) , C,JAB ( A ) , and C^JAB ( D ) , respectively. Diameter of PSL is 0.22 ^m.

Fig. 2. Time course of the turbidity of PSL dispersions after the mixing with C14TAB. Different symbols indicate different CjJAB concentration after the mixing. Concentration of PSL in dispersion mediums (C14TAB + 10 mM NaQ) is 0.014 wt%.

409

/ = /oexp(-T/)

(2)

where IQ and / are the intensities of the incident and transmitted radiation, respectively, and / is the optical path length of the cell. 3. RESULTS AND DISCUSSION Figure 1 indicates the results of electrophoretic light-scattering measurements as a function of surfactant concentration. The measured ^-potential for the dispersion without surfactant ([QTAB] = 0) was -25 mV. This means that the "bare" latex particles have negative surface charge probably due to the ionization of the sulfate groups. With increasing concentration of surfactant the negative ^-potential of the particles increased and eventually became zero. 100 101 102 The surfactant concentration where ^[C„TAB] / JAM potential became zero (isoelectric point) was 7 ^iM, 60 ^iM, and 80 jiM for Fig. 3. The surfactant concentration CiJAB, CiJAB, and C12TAB, dependence of turbidity for 0.22^m PSL respectively. Further increase in dispersion (0.014 wt%) 1 day (O) or 2 concentration of the surfactant caused days (A) after the preparation in QTAB the ^-potential to become increasingly + 10 mM NaCl. The used surfactants are positive, until in the region on the Cj^TAB (a), C,,TAB (b), and C^J A B (c), critical micelle concentration of the respectively. Thick (thin) arrows indicate surfactant (cmc), it became nearly the isoelectric point (cmc). constant (ca. 70 mV). We confirmed the similar dependence of ^-potential on the cationic surfactant concentration for 1.09-M.m diameter PSL particles having sulfate groups (data not shown here). These results suggest that the net surface charge of PSL particles became positive from negative above the isoelectric point by the adsorption of cationic surfactants. Figure 2 shows the time course of turbidity of PSL dispersions after the immediate mixing with CiJAB solution. When CiJAB concentration was 2 or 7 |iM, turbidity increased with time. We confirmed by optical microscopy that PSL particles dispersed in 2 - 7 jiM CiJAB solutions flocculated to form aggregates. So, the observed increase of turbidity could be attributed to the light scattering by the large aggregates of PSL particles. About 3 min after the mixing, the turbidity of PSL in 2 \M CiJAB ^adually decreased because the large aggregates begun to sediment. The sedimentation was completed within 1 day to give the transparent supernatant. Figure 2 cleariy indicates

410

that the rapid coagulation of PSL occurred in C14TAB solutions with the concentration of one order below the isoelectric point, and the dispersion in the isoelectric condition (60 \iM C14TAB) was stable. Figure 3 shows the surfactant concentration dependence of turbidity for 0.22-jim PSL dispersions in C^jTAB, C14TAB, and CiJAB measured 1 or 2 days after the preparation. In the concentration range where zero turbidity was observed, the PSL particles completely sedimented giving the transparent supernatant. At the lower or higher surfactant concentrations, on the other hand, the turbidity did not change so much for 1 - 2 days; the particles were stably dispersed. One can see from Figure 3 that 1) the coagulation occurred in surfactant solutions with lower QTAB concentration than the isoelectric points where the net surface charge of PSL particles were negative, 2) the dispersions at the isoelectric condition were stable at least 1-2 days, and 3) the unstable region where the coagulation was observed became lower and narrower when the alkyl chain length of surfactant was longer. It is not possible to explain these results in terms of DLVO theory that predicts the unstable region to be around the isoelectric point. We here propose a possible mechanism explaining the aggregation behavior of PSL particles by the adsorption of cationic surfactants as follows. In dilute surfactant solutions below the isoelectric point, small number of C„TA* ions adsorb on the particle surfaces mainly by the electrostatic interaction with the ionized sulfate groups, and the hydrophobic tail of absorbed QTA* ions is supposed to be toward the aqueous environment. Then, the attractive hydrophobic interaction between the adsorbed surfactant tails becomes operative and leads the coagulation of particles. When the surfactant concentration is increased, adsorption of QTA* ions proceeds to form bilayer structure on the surfaces. In this situation, the hydrophilic head of C„TA* ions is toward the aqueous environment, and the particles are stably dispersed. This mechanism is proposed as a tentative one, and we now are trying to obtain supporting data concerning the amount of adsorbed QTA* ions and surface structure of PSL particles. It should be noted that the ^-potential and the colloidal stability of negatively charged silver iodide sols in the presence of cationic surfactants had been reported 40 years ago [5]. There was a marked displacement of the minimum stability point form the zero point of charge. With increasing chain length of surfactant, the stability minimum was displaced towards \nghcT positive ^-potential values contrary to the present study. It is obvious that further studies are necessary to establish the theory explaining the stability of charged colloids dispersed in surfactant solutions with opposite charge.

REFERENCES 1. A.M. Puertas and FJ. de las Nieves, J. Colloid Interface Sci., 216 (1999) 221. 2. R. Xu and G. Smart, Langmuir, 12 (1996) 4125. 3. R.G. Alargova, I.Y. Vakarelsky, V.N. Paunov, S.D. Stoyanov, P.A. Kralchevsky, A. Mehreteab, and G. Broze, Langmuir, 14 (1998) 1996. 4. R. Folkerama, AJ.G. van Diemen, and H.N. Stein, Langmuir, 14 (1998) 5973. 5. R.H. Ottewill and M.C. Rastogi, Trans. Faraday Soc., 56 (1960) 880.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) yo 2001 Elsevier Science B.V. All rights reserved.

411

Nonlinear electric conduction in zinc oxide suspension K. Negita, T. Yamaguchi, Y. Misono, and N. Shigematsu Departmentof Chemistry, Faculty of Science, Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka 814-0180, Japan

Electrical properties of a suspension composed of zinc oxide (ZnO) particles and silicone oil are measured under a study shear flow. At lower electric fields, the current increases linearly with the electric field strength without a distortion of the current waveform from the sinusoidal one of the electric field. But when the electric field strength exceeds a certain value, the current waveform begins to distort and the current abruptly increases by some orders of magnitude with a small increase of the electric field strength. Experimental results of such a newly found "fluid varistor" are given, and are discussed on the basis of well known solid varistor. 1. INTRODUCTION In suspensions composed of small particles and liquids, there appear some interesting phenomena. One of these is an electrorheological (ER) effect; when an electric field of a few kV/mm is applied to the suspension, a large increase of the viscosity is observed [1.2]. From our recent studies on the carbon suspension it has been clarified that the electrorheological (ER) effect is closely related to the non-linear electric conduction of the suspension [3]. To ascertain our result we have also studied ZnO suspension, and in our course to measure the electrical properties of this suspension, we find that the current passing through this suspension increases abruptly above a certain electric field strength. Such a behavior has been well known as a varistor in ZnO ceramics, but, to authors' knowledge, is not known in fluids. For preparing ZnO varistor, ZnO was sintered with some additives such as Bi203„ MnO, CoO and Cr203 to form a grain boundary enriched with the doped metal ions [4]. The grain boundary thus formed has been suggested to be responsible for the appearance of the nonlinear conduction of the varistor. Some theoretical studies have shown that the double Shottkey barrier is formed around the grain boundaries, and the application of the electric field modifies the electron transfer through the barrier, making a large amount of electrons

412 pass through the grain boundary above a certain filed [5,6]. In the present work, the details of the abrupt increase of the current in the ZnO suspension are studied to make clear the nonlinear electric conduction of this suspension. Measurements of the current waveform, the current vs. electric field strength, and its frequency dependence are made. On the basis of these results, the behavior of the varistor of this suspension is discussed in comparison with the solid state varistor. 2. EXPERIMENTAL To prepare anhydrous suspension composed of ZnO particles and silicone oil, ZnO particles of 11.9 jLim (PZ, HakusuiTech) was dehydrated by keeping the particles at 150 'C for more than five hours, and 20 vol% of the particles were dispersed in the silicone oil (TSF451-50, Toshiba Silicone) dehydrated by molecular sieves 4A. The resistivity of the ZnO particles is reported to be 7.12 x 10^ Q cm [7]. The measurements of the electrical property were made under a steady shear flow at 300 K using a viscometer of a double cylinder type; a high voltage up to a few kV is applied to the gap (1mm) between the inner and outer cylinders, and the current passing through the specimen was monitored by a digital oscilloscope (DL708E, YEW). The electrodes, thus, are these cylinders made of stainless steel 304. For generation of the high voltage a small voltage from an oscillator (1946, NF) was amplified by a high-voltage amplifier (664, Trek). Throughout this article, the amplitude of the electric field and the current is expressed in rms. 3. RESULTS The change of the current waveform was monitored at a shear rate y of 329.5 s"* by varying the amplitude of the electric field of 100 Hz (Fig. 1). As this figure shows, at lower fields the phase of the current shifts from that of the electric field E = E^^xxnii (Fig. 1-a), indicating that the current is composed of a conduction and a displacement currents; the equivalent circuit of the measurement cell is given by a parallel connection of the resistor R and a capacitor C, leading to the conduction and the displacement currents proportional to VR and i£yC, respectively. The current increases regularly with the electric field strength when the electric field is not so large, but above 1.4 kV/mm, the current waveform begins to distort (Fig. 1-b), and if the amplitude of the electric field is increased to 1.6 kV/mm an abrupt increase of the current is observed (Fig. 1-c). Above this break-down field E^, the current was so large that we could not measure it owing to current limiter of the high voltage amplifier. Comparing the waveform of the electric filed (Fig. 1-a) with that of the current (Fig. 1-c), we find that the current at 1.6 kV/mm increases largely when the electric field strength approaches to its maximum and that the phase of the electric field is almost coincide with that of the current. This result indicates that the large increase of the current is due to the conduction current, but not to the displacement one. When varying the frequency of the

413 electric field to 10 Hz and 1 kHz, the distortion of the current waveform is somewhat different from that at 100 Hz, but the break-down field E^ is not so influenced. Even under the DC electric field, the abrupt increase of the current is observed with E^ similar to that under the AC fields. In Fig. 2, the current / vs. the electric field strength E is given. Here the solid line exhibits the linear relationship between / and E. At lower fields, the current increases linearly with the electric field strength, showing that the conduction mechanism is governed by the Ohm's law. On the other hand, at higher fields the current increases nonlinearly with the electric field strength, leading to the abrupt increase of the current above ca. 1.6 kV/mm. Such a change of the / - £ relationship has been known as a characteristic behavior of the varistor in the solid state ZnO ceramics [3]. To make clear the effect of the shear rate, measurements were also made under other shear rates of 69.5 and 695 s', leading to a result that the changes of the waveform and the / - £ relationships are similar to those under 329.5 s ^ Furthermore, electrical properties of the suspension composed of ZnO particles of high resistivity (> 10^^ Q ^m) were measured. In such a suspension, any reproducible data could not be obtained, but if the electric field of larger than 5 kV/mm was applied, an abrupt large increase of the current was also recognized.

0.005

0.01

/ / s Fig. 1. Waveforms of the electric field (a) and the current at lower (b) and higher fields (c). For the electric field, normalized waveform is given (£7 EQ). ,0-2 g

, r • DC O 10 H7

1 0 ' t-

^ '<><•"' A

10-'* t-

I kHz

A

,«^:»

10-^

io-« 10'

10"

E I kV mm*

Fig. 2. Current / vs. electric field strength E. The straight line exhibits / «= E

414 4. DISCUSSION To understand the appearance of the non-linear conductivity of the ZnO suspension, the behavior of the solid ZnO varistor is suggestive. The solid ZnO varistor is fabricated by sintering ZnO powder with some additives such as Bi203, CoO, MnO, Cr203, and Sb203, which results in the formation of the ZnO grain and the grain boundary composed of ZnO and respective metal oxides [4]. The grain boundary thus formed has high resistivity and contains defects and dopants, which are responsible for the formation of the double Shottkey barrier at the grain boundary. Owing to the presence of this barrier, the number of the carrier (electron) passing through the boundary is varied nonlinearly by the electric field strength, and the varistor action of the ZnO ceramics occurs [5,6]. The grain boundary, thus, plays an essential role for the appearance of the non-linear conduction of the solid varistor. In the present ZnO suspension, the grain boundary is not present. However, the concept of the solid ZnO varistor leads us to consider that the current flow is accomplished by the electron transfer from a particle to a particle and for the electron transfer there is some energy barrier as a result of the presence of the highly resistive silicone oil. Although the details of the mechanism for the nonlinear conduction of this suspension should be clarified in future, a recent result for the ZnO single crystals gives us a clue for understanding the mechanism; it is reported that without the grain boundary the varistor behavior is observed if the single crystals of ZnO are properly contacted [8]. Referring to these results, exploring the mechanism for the varistor action of the ZnO suspension is underway. ACKNOWLEDGEMENTS This study was supported by Special Coordination Funds for Promoting Science and Technology from the Japanese Science and Technology Agency, to whom we are deeply indebted. REFERENCES 1. H. Block and J. P. Kelley, J. Phys. D, 21 (1988) 1661. 2. K. Negita and Y. Ohsawa, J. Phys. II France, 5 (1995) 883. 3. T. Yamaguchi, N. Shigematsu, Y. Misono, and K. Negita, 78th CSJ National Meeting (2000). 4. M. Matsuoka, Jpn. J. Appl. Phys., 10 (1971) 736. 5. G. D. Mahan, L. M. Levinson, and H. R. Philipp, J. Appl. Phys., 50 (1979) 2799. 6. D. R. Clarke, J. Am. Ceram. Soc., 82 (1999) 485. 7. Data sheet of HakusuiTech. 8. Y. Nakamura, T. Harada, H. Kuribara, A. Kishimoto, N. Motohira, and H. Yanagida, J. Am. Ceram. Soc., 82 (1999) 3069.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) vc 2001 Elsevier Science B.V. All rights reserved.

415

Heterocoagulation behavior of PC vesicles with spherical silica B.Yang*, H. Matsumu^a^ H. Kise\ and K. Furusawa*^* * Institute of Material Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan " Electrotechnical Laboratory, AIST, MITI, Tsukuba, Ibaraki 305, Japan "" Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305, Japan The coagulation behavior of spherical silica after adding thephosphatidylcholine (PC) vesicles was investigated by determining the enlarging size ratio (P) of silica particles in 10"* M LaClj aqueous solution. Furthermore, the formation mechanism of PC vesicle/silica or silica/PC vesicle/silica composite was discussed by using the heterocoagulation concept. 1. INTRODUCTION Recently, composite particles comprised of organic/inorganic materials have been widely investigated, because they have important applications in the practical fields such as biology, medicine, catalysis, electronics, and separation [1]. These composite particles are usually prepared by the heterocoagulation technique. The technique depends strongly on the relative charge and the size ratio of core and shell particles [2]. Most of the studies on the formation of heterocoagulation have been conducted for organic/inorganic material systems such as latex/silica, or latex/magnetic particle. However, the behavior including soft particles as vesicles has not been reported. Such a system is important to the application in a bio-medical field such as diagnostic andreatment medicine. In this study, the coagulation behavior of PC vesicle^silica particle system has been investigated by using some colloid chemical techniques. 2. EXPERIMENTAL 2.1. Materials Egg yolk phosphatidylcholine was purchased from Sigma Chemical Co. Ltd, (USA). The silica particles were offered by Nippon Catalysis Co., Ltd, (Japan). 2.2. Preparation of vesicles The PC vesicles were prepared by the extrusion method [3]. The mean diameter of the vesicles was determined by a light scattering apparatus ELS-8000 (Otsuka Elect. Japan). 2 J. Analysis of coagulation behavior The coagulation of silica particle was evaluated by the enlarging size ratio of the particle (P) using the ELS-8000 after adding PC vesicle to the silica dispersion. The P is given by P = (D^ - Do) / Do, where, D, is the mean diameter of silica at time t after mixing PC vesicles * To whom correspondence should be addressed.

Tel and Fax: +81-298-534426.

416

with silica, DQ is the mean diameter of silica particles at a starting time (t = 0). 2.4. Measurement of electrophoretic mobility Electrophoretic mobilities of the vesicles and of silica particles were measured by an apparatus of Zeecom (Microtec Co., Ltd., Japan), ^-potentials was calculated by O'BrienWhite equation [4]. 2.5. Adsorbed amount of PC vesicles on large silica particles ImL silica suspension was mixed with ImL PC vesicle dispersion, and the mixture was stirred for 4h at 25*'C. To separate the coagulated particles from the single vesicle particles, it was centrifuged by 4000 rpm for 30 min at 15°C. The phospholipid concentration in the supematant and original vesicle dispersion was determined by the Bartlett method [5]. The adsorption amount of PC on the silica particles was calculated from the differences of them. 3. RESULTS AND DISCUSSION The coagulation behavior of silica particles with vesicles was investigated under 10^ M LaCls aqueous solution. In this salt condition, the ^-potentials of PC vesicles and silica particles (d =L0 |Lim) were +32 mV and -30 mV, respectively. Therefore, it is expected that a strong electrostatic attraction will be operated between them. In Fig.l, the enlarging size ratio (?) of silica particle at 15 min after adding PC vesicle (d = 0.2 ^im) is plotted against the PC vesicle concentration. The increase of P depends on the amounts of PC vesicle, and there is an optimum PC concentration on the flock size. The PC concentration conesponding to give the maximum P is called "maximum coagulation concentration (MCC)". In order to make clear the coagulation mechanism, we carried out the electrophoresis measurement, ^-potential of silica particles after adding PC vesicles with various concentrations is also shown in Fig.l. The ^-potential of silica particles decreased quickly with increasing PC concentration and reversed to a positive value at a very low PC concentration. This indicates

^—

'*'

0.6

|J

PC vesicle

W\

>

o q^/

Interparticle bridging

B 0.4

(a)

l\

(b)

0.2

i-i._»_l j - .j

1 1 1

i..,i_J.

Stabilization by adsorbed vesicles

PC concentration (mM) Fig. 1. The ?(%) or ^-potential (A)of composite particle vs. PC concentration curves.

Fig. 2. Schematic picture to show the coagulation mechanism of silica particles after adding PC vesicles: (a) at a low PC concentration, (b) at a high PC concentration.

417

that the positively charged PC vesicles by binding of l£* to the head group of PC adhered strongly on the negatively charged silica surface. It is found that the ^-potential at MCC is +15 mV. This behavior of ^-potential cannot be explained by the simple charge neutralization mechanism. The "interparticle bridging" of silica by PC vesicles must be taken into account. Because the PC vesicles and the silica surface have opposite charges, the attractive force will arise between the adsorbed PC vesicle on one silica particle and the bare surface of other silica particles. The schematic image of "interparticle bridging" is shown in Fig. 2. When the PC concentration increased, the number of bridging site increases, which causes a substantial increase in the coagulation rate. However, when the negative surface charges on the silica surface are neutralized completely and finally reverse to positive sign by the adsorbed PC vesicles, the electrostatic attraction becomes weak. When adsorption of PC vesicles proceeded over a certain value, the silica surface is covered completely with the vesicle particles. The electric repulsion between the PC vesicles on the silica surface becomes predominant. So, the coagulation is prevented by the electrostatic repulsion. The fact that PC vesicles adsorb on the silica surface as a vesicle particle is proved by the adsorbed amount experiment. In Fig. 3, the adsorbed amount of PC on the silica surface as a function of the PC concentration is shown. The adsorption amount is expressed by the number of phospholipid molecules adsorbed per square meter of silica surface. The solid line represents a theoretical curve for the adsorption amount of a single bilayer membrane model assuming that the area per PC molecule equals 0.7 nm^. The dotted line shows a theoretical curve for adsorption of the single layer model of vesicle spherical particles. The saturated amounts of adsorption in the experimental data are located near the value for the latter. So, we expect that PC vesicles would be adsorbed on the silica surface as a single vesicle layer, i.e., the PC vesicle/silica composite was produced under a high PC concentration region. S 300

-a 200 i

100

0.2 0.4 0.6 0.8 1.0 PC concentration (mM)

Fig. 3. Adsorption amount of PC vesicles on silica particles at 10"^ M LaCls

Fig. 4. Optical micrograph of silica /PC vesicle/silica composite.

418 The same concept was applied to form multilayer composite particles. After preparing the PC vesicle/silica composite particles, we removed the free PC vesicles from the system by the ultrafiltration method, and mixed the composite particles with a small silica dispersion (d = 0.5 \im) under the same ICT* M LaClg concentration. The ^-potentials of the products reversed from a positive (+28.4 mV) to a negative (-30.0 mV), indicating that the small silica particles (with negative charges) were adhered on the surface of the positively charged composite particle. The image for the small silica adsorbed on PC vesicle/large silica composite surface was observed by a special microscope [6]. In Fig. 4, the photo clearly shows that the small silica particles are adsorbed on the core PC vesicle/silica composite particle. 4. CONCLUSION The coagulation mechanism of silica particles with PC vesicles is found as follows: At a low PC concentration, the silica particles coagulate by "the interparticle bridging" of vesicle particles. On the other hand, at a high PC concentration, the silica surfaces were covered with a lot of PC vesicles and the surface charge of silica particles shifted to a positive side. Hence, the coagulation of silica particles was prevented by their electrostatic repulsion. Furthermore, formations of silica particle layer on the PC vesicle/silica composite have been confirmed. 5. ACKNOWLEDGE Authors thank Dr. K. Katoh (ETL, AIST) for his cooperation in optical microscopic studies. 6. REFERENCES 1. R. Davies, G.A. Schurr, P.D. Meenan, H.E.Bergna, C.A.S. Brevett, R.H. Goldbaum, Adv. Mater,10 (1998) 1264. 2. R.D. Harding, J. Colloid Interf. Sci., 40 (1972) 57. 3. M. J. Hope, M. B. Bally, G. Webb, R K. Cullis, Biochim. Biophys. Acta., 812 (1985) 55. 4. R. W. O'Brien, L. R. J. White, Chem. Soc, Faraday Trans., 74 (1978) 1607. 5. G. R. Bartlett, J. Biol. Chem., 234 (1959) 466. 6. B.Yang, H. Matsumura, H. Kise, K. Furusawa, Langmuir, in submission.

Studies in Surface Science and Catalysis 132 Y. IwQsawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

419

Electrokinetic studies of fuUerene dispersions in aqueous solutions of surfactants Hanae Takahashi and Masataka Ozaki Graduate School of Integrated Science, Yokohama City University, Yokohama, 236-0027 Japan Fullerene dispersions were prepared and electrokinetic properties of fullerene particles dispersed in ionic surfactants were investigated. The interaction energy between hydrocarbon chains were estimated to 0.95/cTor 570 cal/mole from the variation in the chain length of the surfactant concentration at zero zeta-potential, which is in good agreement with the value reported in the literature. This result indicates the validity of the hemimicelle hypothesis of surfactants at the fullerene-water interface. 1. INTRODUCTION Fullerenes have been widely investigated for a variety of purposes after the finding of the compound by Kroto et al. [1]. However, only a few studies on the colloidal dispersions of fullerenes have been reported. Among them the preparation of fullerene dispersion and the stability of the dispersion in water are investigated by Mchedolov-Petrossyan et al. [2,3]. They found that the stability of the fullerene dispersion could be explained by the well known DLVO theory which showed a typical hydrophobic colloidal property. The adsorptions of surfactants on colloidal particles are very important in many fields from the point of the stability of the dispersions. A number of studies on the adsorption of surfactants on colloidal particles have been reported [4-9]. Electrokinetic measurements of colloidal particles on which ionic surfactants are adsorbed are useful for the study of the interaction of a solid with surfactants. Watanabe et al. investigated the electrokinetic properties of the positive silver iodide-surfactant system in detail and they gave a quantitative interpretation for the adsorption of the surfactant(8). Somasundaran et al. suggested the hemimicelle formation at the watersolid interface from the study of the electrokinetic measurements of silica in water-surfactant systems [9]. Mchedolov-Petrossyan et al. observed that the stability of the fullerene dispersion is enhanced by the adsorption of alkyltrimethylammonium ions. In this study, preparative methods for the fullerene dispersions and the electrokinetic properties of the fullerene particles dispersed in ionic surfactants are investigated.

420

2. EXPERIMENTAL Preparation of fullereneparticles: FuUerene (purity 99.9 %) was purchased from Wako Corporation (Tokyo, Japan). Ionic surfactants were obtained from Tokyo Kasei Co. Ltd. (Tokyo, Japan) and used as received. Fullerene particles were produced by adding a large amount of solvent in which fullerene does not dissolve or dissolve slightly into a fullerene solution of a proper organic liquid with vigorous stirring. Benzene, toluene, monobromobenzene, and cyclohexane were tested for the solvent of fullerene. Alcohols, water and alcohol-water mixtures were chosen for the diluent. The dilutions were made at room temperature and at 5, 30 and 60 °C with different ratios of fullerene solution:solvent. The fullerene particles so produced were examined by X-ray powder diffraction and by scanning electron microscopy (SEM). Electrokinetic measurements: The fullerene particles were then dispersed in aqueous solutions using an ultrasonic water bath at the desired concentrations of surfactants and electrolytes for use in the electrokinetic measurements after washing several times with distilled water. The electrophoretic measurements were made at a constant ionic strength of potassium chloride of 10^ mol/l at pHs of about 5.8 without using pH controlling reagent. The zeta-potential was calculated from the mobility measured by a Pen-Kem System 3000 using the Smoluchovski equation. 3. RESULTS AND DISCUSSION The X-ray powder diffraction of the fullerene particles showed the typical reflection pattern of fullerene powder. The widths of the peaks of X-ray reflections were wider than the reflection peaks of the powder as received from the market. This result means that the fullerene particles produced are smaller in crystal size than that of commercial one. Fig. 1 shows the typical SEM micrographs of the fullerene particles produced by adding ethanol into benzene solutions of fullerene at room temperature with

Fig. 1. SEM micrographs of fullerene particles; a, benzene : ethanol = 1:5 ; b, benzene : ethanol = 1:2.

421

dilution ratios of 2 and 5. Fullerene particles are in good uniformity although the sizes are coarse in the range of 500 nm~ 1000 nm. The particle became small as the ratio of the benzeneralcohol increased. The size of the particle decreased with the increase in temperature. The best uniformity was obtained by adding 2-5 times the volume of ethanol into a benzene solution of fullerene slightly lower than the solution saturated with fullerene. Good uniformity was also obtained with toluene-ethanol system. Fullerene particles having less uniformity deposited when nonaqueous solutions containing fullerene were mixed with water. The zeta-potential of the fullerene particles was negative in the pH range where measurements were made. The mechanism of the generation of the fullerene particles is not clear as yet, Mchedolov-Petrossyan et al. suggested that the potential develops as a result of the formation of the first hydration shell due to the donor-acceptor interaction of the n electrons of fullerene with water. Fig.2 shows the change in the zeta-potential of the fullerene particles dispersed in n-alkyltrimethylammonium bromide solutions having a variety of hydrocarbon chain lengths as a function of the concentration of the surfactant. The zeta-potential changed its sign from negative to positive at a peculiar concentration for each surfactant. The sign of the zetapotential changed at lower concentrations for the surfactants having longer chains. This is the direct result of the higher adsorptivity of the cationic surfactant for a longer alkyl chain since the interaction of the surfactant with fullerene increases in the increase of the chain length. The critical

>

a o LUlia

10-'

10-^

10'^

1 I IIIIM

10-^

C / moldm "^

L-LLU

io-»

10"

6

8

10

12

14

16

18

Number of carbon atoms in alkyl chain

Fig. 2. Zeta-potential vs concentration of Fig. 3. In Q vs number of carbon n-alkyltrimethylammonium bromide. atoms n in surfactants.

422

micelle concentrations for n-alkyltrimethylammonium bromide are Cg, 9.8 X 10-2 M; Q^^^ 6.8 X 10-2 M; C12, 1.2x lO-^M and C,e> 4.8 x 10'^ M respectively [4]. It must be noticed that the change occurred at far below the respective c.m.c values. According to Somasundaran et al. (9), the relation between In Q and 0 can be expressed by the following relation, where Q is the surfactant concentration at zero zeta-potential and 0 , the interaction energy of adsorption per alkyl chain due to van der Waals interaction: In Co = - n 0 / / c T - l n ( r 5 ) + Ln2r,

(1)

in which n is the number of alkyl chains, k is the Boltzmann constant, T is temperature, T^ is the amount of surfactant adsorbed on surface, r is radius of the surfactant. (t> can be evaluated from the slope of the linear relation of InQ versus n, the number of chain of surfactants. Fig.3 shows the relation between InQ and n. From the slope of the linear function, the interaction energy per alkyl chain is estimated to 0,95kT. This is very close to 0.97/cr obtained by Somasundaran et al. in the silica-alkylammonium system in an aqueous solution. They suggested possibility of the formation of the self association of surfactants called hemimicelle at the silica-water interface due to the interaction between the alkyl chains since the value obtained was very close to the hydrocarbon-hydrocarbon interaction energy obtained by other methods. Mchedolov-Petrossyan et al. states that the fullerene dispersion behaves as a typical hydrophobic colloid. The hydrophobic nature of the fullerene surface will promote the selfassembling of surfactants at the fullerene-water interface thus enhancing the hemimicelle formation.

REFERENCES 1. H. W. Kroto, J. R. Heath, S. C. OBrien, R. F. Curl and R. E. Smalley, Nature, 318 (1985) 162. 2. N. O. Mchedolv-Petrossyan, V. K. Klochkov and G. V. Andrievsky, J. Chem. Sci., Faraday Trans., 93 (1997) 4343. 3. G. V. Andrievsky, M. V. Kosevich, O. M. Vovk, V. S. Shelkovsky and L. A. Vashchenko, J. Chem. Soc, Chem. Commun., (1995) 1281. 4. P. Comer and R. H. Ottewull, J. Colloid Interface Sci., 37 (1971) 642. 5. I. Piirma and Shih-R. Chen, J. Colloid Interface Sci., 74 (1980) 90. 6. H. Schott and I. J. Kazella, J. Oil Chemist's Sci., 44 (1967) 416. 7. K. Meguro and S. Tomioka, N. Kawashima and K. Esumi, Progr. Colloid and Polymer Sci., 68 (1983) 97. 8. A. Watanabe, Denkikagaku (J. Electrochem. Soci. Japan), 29 (1961) 777. 9. P. Somasundaran, T. W. Healy and D. Fuerstenau, J. Phys. Chem., 68 (1964)3562.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) •c 2001 Elsevier Science B.V. Ail rights reserved.

423

Dynamic Mobility of Concentrated Suspensions in the Presence of Polyelectrolytes N. Tobori and T. Amari Graduate School of Science and Technology, Chiba University 1-33 Yayoi-cho, Chiba 263-8522, Japan The dynamic mobility of CaC03 suspensions in the presence of polyelectrolyte as a dispersing agent was measured with varying volume fraction of particles, and it was converted to zeta potential and size distribution by theoretical treatment of dynamic mobility spectra. The results of the suspension that the particles have cationic, anionic and nearly neutral charge by adsorption of polyelectrolytes fitted to the theoretical calculation derived by O'Brien. For the suspension stabilized by steric hindrance, the zeta potential and size distribution calculated by dynamic mobility spectra can be obtained up to 0.6 of volume fraction. 1. INTRODUCTION Dynamic mobility can be determined by measuring the magnitude and phase angle of the sound wave generated by electrophoretic oscillations of particles (electrokinetic sonic amplitude; ESA effect), if an ahemating electric field applied in aqueous suspension. The theoretical treatment of dynamic mobility was developed by O'Brien [1], and it was verified for various systems. Since the frequency dependence of magnitude and phase lag corresponds to the effect of particle inertia, we can obtain the particle size distribution of particles in concentrated suspensions without any dilution [1,2]. This technique is very useful for the suspensions in many industrial situations, especially for investigating the effect of dispersing agents that is used to attain high solid concentration with adequate fluidity [3]. In this study, we attempted to apply the electroacoustic measurement for the suspensions with high solid concentration. To do this, the dynamic mobility of CaCO^ suspensions including a polyelectrolyte as a dispersing agent were measured with varying volume fraction of particles, and it was converted to zeta potential and size distribution by theoretical treatment of dynamic mobility spectra. Since we can obtain highly concentrated suspensions with volume fraction up to 0.6 by adding the polyelectrolytes, the results provide some useful information about electroacoustic characterization in highly concentrated suspensions. 2. EXPERIMENTAL 2.1. Materials In this study, a dry powder of heavy calcium carbonate was used. The average size of CaCOs particles(median diameter) was 1.8/z m and its size distribution was relatively broad that ranged from 0.3 to 10 M m. The specific surface area was 3.2mVg determined by BET

424

technique, and the density was 2.71g/ml. Two types of polyelectrolyte; sodium polyacrylate (PAA) and comblike graft copolymer based on methacrylate (PG) were used as dispersing agents. The PG copolymer consists of sodium methacrylate and methacrylate ester of methoxy-polyethylene glycol (polymerization degree: 22) with molar ratio of 0.75/0.25. These polymers were obtained by radical polymerization in our laboratory. The molecular weight of PAA and PG polymer were 9000 and 40000, respectively. Aqueous concentrated suspensions of CaCOj particles were prepared by slowly adding the required amount of dry powder of CaCOs to an aqueous solution of the polymer employed under a controlled stirring. Air bubbles in the suspension were removed by evacuation. The obtained suspensions were allowed to stand for 1 day and were re-stirred using a homogenizer before loading for each measurement. 2.2. Measurements The dynamic mobility was determined using an Acoustosizer instrument (Colloidal Dynamics Ltd.). This instrument applies a highfrequencyalternating voltage to a suspension, causing the particles to oscillate at a velocity that is dependent on their size and charge. The resulting pressure force that arise at the suspension boundaries produce pulses of sound waves in a phenomenon known as the electrokinetic sonic amplitude (ESA) effect. The ESA signal is related to the particle averaged dynamic mobility, < ju d>, by the equation: ESA( a))=A( CO,«)<<)A p/

p

(1)

where co istheangularfrequencyof applied field, <> / is volumefractionof particle, A is an instrument constant depending on co and * , A /o is the density difference between the particle and the medium density, io . For a spherical particle with a thin double layer, n ^ can be related to the C -potential by the relationship: /id=G(coa^/v) £ C / T ]

(2)

where f and TJ are the permittivity and viscosity of the medium respectively. G term of the equation is a measure of the inertia of particles, having the radius a and kinetic viscosity of medium v at the frequency co. In a polydisperse system, = J M(co,a)/7(a)da

(3)

where /7(a)da is the mass fraction of particles with radius between a+da/2 and a-da/2. Therefore, the zeta potential C and size distribution /7(a) can be determined by fitting measured dynamic mobility spectrum to theoretical one.

8 10 12 Frequency (MHz) Fig. 1. Dynamic mobility spectra of the CaCOs suspensions containing various polyelectrolytes. Filled symbols; magnitude, open symbols; phase lag and curves; theoretical fits to experimental data.

425

(a) magnitude

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1

1

1

1

1

• 0.039 o 0.084 A 0.136 A 0.197 • 0.269 o 0.355 • 0.462 L... 1

10 12 4 6 8 10 12 • 4 6 8 0 Frequency (MHz) Frequency (MHz) Fig. 2. Dynamic mobility spectra (a; magnitude, b; phase lag) for PAA suspensions at different volume fraction. Symbols indicate experimental results and curves indicate theoretical fits to experimental data. 0

>

3.RESULTS AND DISCUSSION The magnitude and phase lag of the dynamic mobility measured as a function of frequency for CaCOj suspensions at (t> =0.289 including various polyelectrolytes are shown in Fig. 1. In the case of additive free suspension, the sign of particle charge was positive. With addition of PAA or PG in the suspension, the surface of particles became high negative charge for PAA and little charge for PG. From the rheological measurements, the viscosity of the suspension decreased drastically when the adequate amount of PAA or PG was added into the suspension. Thus, it can be considered that the particles in suspension were stabilized by electrostatic force for PAA suspension and by steric force for PG suspension. In addition, it was found that the resuUs for three suspensions with different surface structure agreed with the theoretical calculation assuming a log-normal distribution using Eq. (l)-(3). The volume fraction dependence of dynamic mobility was also examined. Fig. 2 shows the dynamic mobility spectra for PAA suspensions at various volume fractions as indicated. The samples being measured are diluted to adequate volume fractions from original suspension of 0 =0.462. It was also found that the results of all suspension fitted to 60 [-0 o additive free theoretical calculation. The frequency 40 1 0 o 0 0 A PAA: 0.4% dependence was gradually decreased with 20 D PG: 0.4% increasing volume fraction, because of 0 preventing oscillation of particles and absorbing the generated sonic wave at high -20 volume fraction. The zeta potentials 1-40 calculated from dynamic mobility spectra ^ - 6 0 [A A are shown in Fig. 3. The zeta potential . ^ A, A , A , , 1 -80 1 seemed to be constant irrespective of kinds of polyelectrolyte and volume fraction. In 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 particular, in the case of PG suspension, it is Volume fraction interesting that the zeta potential can be Fig. 3. Zeta potential calculated from dynamic measured and indicate constant value up to mobility spectra for various suspensions as a 0=0.6. function of volume fraction.

1

426 Figure 4 shows the particle size distributions (PSD) calculated by the dynamic mobility for these suspensions. In this figure, D15, D50 and D85 denote the particle size showing 15%, 50% and 85% of cumulative mass fraction assuming a log-normal distribution, respectively. For additive free suspension, the particle size was large as volume fraction decreased. This can be attributed to the change of pH in suspension because of dilution. At constant volume fraction, the PSD of PG suspension was smaller than that of PAA suspension at same polymer concentration. This means that the particles in PG suspension is more dispersed by steric hindrance. In addition, an interesting behavior was observed that the PSD becomes narrower with maintaining the median diameter as increasing volume fraction. This tendency was remarkable for weakly flocculated systems such as additive-free and PAA suspensions at high volume fraction. On the contrary, the PSD was kept constant as increasing volume fraction for PG suspension. From the viscoelastic measurements, it was found that the storage modulus, G' of additive free and PAA suspension was larger than that of PG suspension. Therefore, It can be deduced that the flocculated network structure in suspension affects the response of dynamic mobility, especially high frequency region. 100 a)additive free

• Disl OD50

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•

100

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• D15J b)PAA

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.

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0 0.2 0.4 0.6 0 0.2 0.4 0.6 0.2 0.4 0.6 Volume fraction Volume fraction Volume fraction Fig. 4. Particle size distributions calculated from the dynamic mobility for various suspensions as a fiinction of volume fraction. a)additive free suspension, b)suspension with 0.4% PAA, c^susnension with 0 4% PG 0

4. CONCLUSIONS Dynamic mobility for highly concentrated CaCO^ suspensions can be measured. Since the zeta potential and size distribution calculated from dynamic mobility was mostly reasonable, the measurement of dynamic mobility is usefiil technique to analyze the dispersing state of particle in concentrated suspensions. The difference in the size distribution obtained by dynamic mobility can be explained by the difference of dispersing state between PAA suspension (electrostatically stabilized) and PG suspension (sterically stabilized). REFERENCES 1. R. W. O'Brien, D. W. Cannon and W. N. Rowlands, J. Colloid Interface Sci., 173 (1995) 406 2. R. J. Hunter, Colloids and Surfaces A, 141(1998) 37 3. H. Ohshima, and K. Furusawa (eds), Electrokinetic phenomena at interfaces. Marcel Dekker, 1998

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^g 2001 Elsevier Science B.V. All rights reserved.

427

Electrochemiluminescence reactions of metal complexes immobilized on surface of a magnetic microbead N. Oyama, K. Komori and O. Hatozaki Dq)artment of implied Chemistry, Faculty of Technology, Tokyo UnivCTsity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan In this study, electrochemiluminescence (ECL) reactions of Ru(bpy)3^* complex immobilized on magnetic microbeads (himinomicrobeads) were examined to develop highly sensitive ECL detection systems. It was found that intensity of ECL response of the luminomicrobeads was dependent on a coreactant (tripropylamine) concentration and solution pH. It is observed that the luminomicrobeads highly aggregated when collected on the electrode surface using a magnet, and the ECL intensity was strongly dependent on the degree of the aggregation of luminomicrobeads. l.INTRODUCTION In the last two decades, a luminescence technique has found widespread use in immunoassays, especially whwe sensitivity is of primary concern [1, 2]. Compared to chemiluminescence and bioluminescence immunoassays, electrochemiluminescence(ECL)based immunoassay offers some advantages. For example, since an ECL reaction is trigg^ed by potential or current application to electrodes to oxidize or reduce ECL labels [3], measurements can be readily controlled with electrochemical instruments. To develop highly sensitive ECL detection systems for immunoassay, we employ a magnetic microbead whk:h has been recentiy applied to immunoassay [4-6]. The surface of the microbead is coated with polystyrene layer and thus serves as an immunoassay support with immobilized antibodies. In a detection scheme, an analyte antigen, for example, is sandwiched between the complimentary antibody immobilized on microbead surface and antibody modified with an ECL label. The magnetic microbeads carrying the conjugate are then collected on an electrode surface with a magnet to oxidize or reduce the ECL label to induce an ECL reaction. The use of microbeads facilitates renewal of electrode surfaces in consecutive assays. In this study, we carried out experiments to assess factors affecting ECL reactions. In situ observation of surface aggregation of microbeads was also carried out to correlate ECL intensity with aggregation of microbeads. 2.EXPERIMENTAL Magnetic microbeads used in this study were superparamagnetic, polystyrene microsphere (Dynabeads* M-450, diameter:4.5^m). A derivative of ruthenium tris(bipyridyl) con?)lex (Ru(bpy)3^*) was covalentiy attached to immunoglobulin G as an ECL label, and the Ru-labeled IgG was then immobilized to the surface of microbeads to yield luminomiaobeads. To measure

428

ECL responses of luminomicrobeads and Ru(bpy)3^* dissolved in solutions, a photon detection system was locally built by fixing PMT onto a black box in which an electrolysis cell was contained. Aggregation of luminomicrobeads on electrode surface was in situ observed using a CCD video camera. 3.RESULTS AND DISCUSSION It has been observed that Ru(bpy)3^* comphx and its derivatives give stable ECL responses in aqueous solutions [7, 8]. Figure 1 shows ECL response of luminomicrobeads (7x10^ beads/cm^) in the presence of 0. IM tripropylamine (TPA) as a coreactant in a phosphate buffo* (pH=7.5). Surfactants (Tween 20 and Triton X-lOO) and sodium azide were also added to the buffer solution (vide infra). When tiie electrode potential was scanned in the positive direction from OV vs. Ag/AgCl, intense light emission from the limiinomicrobead was observed in a potential region above 0.8V, where both the Ru complex and TPA ware oxidized. ECL response was not observed in the absence of the Ru complex and/or TPA. Scheme 1 shows a proposed mechanism for the ECL reaction of the luminomiaobead [5, 9]. At Pt electrode surface, the Ru(II) labels on luminomicrobead and TPA are concurrently oxidized to Ru(III) and TPA radical cation, respectively. It is also possible that TPA radical cation is formed from an electron-transfer reaction between TPA and Ru(bpy)3^* [9]. Highly reducing TPA radical (TPA) was subsequently formed from the deprotonation reaction of the TPA radical cation and reduces Ru(ni) complex to Ru(II) in an excited state, Ru(bpy)3^**. As a result of the deexcitation of the Ru(n)* back to a ground state, a photon is emitted. In the ECL cycle of Ru(bpy)3^, ruthenium complex is rq)eatedly oxidized at electrode surface and produces multiple photons. The use of luminomicrobeads would promise high efficiency of an ECL cycle and thus high light emission, since ECL labels do not diffuse away from electrode surface during ECL reactions. (A) 1mA

(B)

L 0

l400mV

±

0.4 0.8 1.2 E/Vvs.Ag/AgCl Figure 1 (A) Cyclic voltanunograms and (B) ECL responses of luminomicrobeads on a Pt electrode (I) in the presence and (II) absence of O.IM TPA.

electrodel

Scheme 1 A reaction scheme for the ECL reaction of Ru(bpy)32+ complex immobilized on magnetic microbeads in the presence of TPA.

429

Note that in Figure 1, however, the ECL intensity goes through a maximum at ca.0.95V vs. Ag/AgCl and afterward decreases in the positive scan, even though the oxidation current for TPA was still increasing. Furthemwre, in the reverse scan, oxidation of TPA failed to produce strong light emission. These results suggested that the ECL reaction scheme shown in Scheme 1 was hindo'ed at some stage(s) by unknown reasons. HowevCT, since we also obtained similar ECL response with a maximum for Ru(bpy)3^* complex dissolved in solutions of the same con^sition, the deactivation of the ECL reaction was not patinent to the use of luminomicrobeads. Oxidation products of TPA, sodium azide and the surfactants could be a possible origin for the obso^^ed decrease in the ECL intensity. The effects of the surfactants and sodium azide are currentiy studied in detail. In this study, ECL intensity of the luminomicrobead is measured with changing TPA concentration and solution pH. Figure 2 shows dependence of the peak height of ECL response on concentration of TPA ECL intensity linearly increased with increasing TPA concentration up to 0.2M TPA. Figure 3 shows pH dependence of ECL intensity in tiie presence of 0. IM TPA. The pH dependence shows a maximum at pH=7.5 as observed for Ru(bpy)3^* complex dissolved in solutions containing TPA [9]. The increase in ECL intensity below the maximum may be asaibable to the fact that the deprotonation equilibrium of TPA**;^ TPA+H* lies to the right at a higho- pH. One of possible origins for the decrease in the ECL intensity above pH=7.5 is decomposition of Ru(III)(bpy)3^* in alkaline solutions [10]. Lower solubility of TPA at higher pH could also decrease ECL intensity.

"0

0.05 0.10 0.15 0.20 0.25 [TPA]/M Figure 2 Dependence of the ECL response on the concentration of TPA (pH=7.5). Surface population of the luminomicrobead : 7x10^ beads/cm-2.

Figures pH-dependence of the ECL response. TPA concentration :0.1M. Surface population of the luminomicrobead: (a) 2.2x10^ beads/cm^.

In situ observation of the luminomicrobeads on electrode surface was carried out using a CCD video camera. CCD images displayed that luminonucrobeads highly aggregated when collected on electrode surface with a magnet The degree of aggregation increased with increasing surface population of luminomicrobeads. Figure 4 shows dependence of ECL intensity on surface population of luminomicrobeads. Although a linearly increasing region was found at a lower population than l.lxlO' beads/cm^ at a higher population the intensity deviated greatly from the linearity. We ascribed this saturation behavior of the ECL intensity to surface aggregation of luminomicrobeads. Although luminomicrobeads gave intense ECL

430

response as shown above, all of the Ru labels immobilized on the microbeads should not be ECL-active, because 4.5|im-diameter is too large to electrochemically oxidize the whole surface of the microbeads at least within our expCTimental time scale. Additionally, highly reactive TPA radical could not diffuse over such long distance from electrode surface. As a result, only Ru labels close to electrode surface are capable of emitting photons according to the ECL reaction scheme shown in Sdieme 1. Tha-efore, since a PMT was placed above the Pt working electrode in our photon measurem^t system, surface aggregation of luminomicrobeads will shield photon emission from a PNTT and thus diminish ECL intensity.

5 10 15 20 10^ Population of luminomicorbead / beads cm-2 Figure 4 Dependence of the ECL response on surface population of the luminomicrobead. [TPA]:0.2M.

4.CONCLUSIONS Luminomicrobeads prepared in this study showed intense ECL response when the immobilized ruthenium labels were oxidized in tiie presence of a strong reducing agent, TPA radical. It was observed that ECL intensity was strongly affected by surface aggregation of luminomicrobeads at a high surface population. Therefore, control of surface aggregation of luminomicrobeads was essential for quantitative measurements. ACKNOWLEDGMENT This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (No. 11167221).

REFERENCES 1. D. S. Age, Anal. Chem. 71 (1999) 294R, and references therein. 2. C. P. Price and D. J. Newman (eds.), Principles and Practice of Immunoassay, 2nd. ed., Stockton Press, New York, 1997. 3. J. G. Velasco, Electroanalysis, 3 (1991) 261. 4. C. H. PoUema, J. Ruzicka, G. D. Christian and A. Lermark, Anal. ChenL, 64 (1992) 1356. 5. D.R.Deaver, Nature 377 (1995) 758. 6. A. G. Gehring, J. D. Brewster, P. L. Irwin, S. I. Tu and L. J. Van Houten, J. Electroanal. Chem., 469 (1999) 27. 7. L Rubinstein and A. J. Bard, J. Am. Chem. Soc., 103 (1981) 512. 8. A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and V. Zelewsky, Coord. Chem. Rev., 84(1988)85. 9. J. K. Leland andM. J. Powell, J. Electrochem. Soc., 137 (1990) 3127. 10. P. K. Gosh, B. S. Brunshwing, M. Chou, C. Creutz and N. Sutin, J. Am. Chem. Soc., 106(1984)4772.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (P 2001 Elsevier Science B.V. All rights reserved.

431

Fluorescence spectra andfluorescencelifetime of colloidal solution of an organic dye, bis-MSB, and third-order optical nonlinearities of its excitons K. Kasatani*, H. Miyata^ H. Okamoto^ and S. Takenaka*' ^Institute for Fundamental Research of Organic Chemistry, Kyushu University, Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai, Ube, 755-8611 Japan. We made microparticles of an organic dye, bis-MSB, and measured fluorescence spectrum, fluorescence lifetime, and the third-order optical nonlinearities of exciton of the microparticles. We evaluated that an effective x^^^ of excitons of bis-MSB in colloidal solution was about 1/23 of that of the excited dye molecule from the observed DFWM signal intensity. Taking the concentration of excitons of the dye in colloidal solution into the consideration, we concluded that the value of molecular hyperpolarizability for an exciton in microparticle should be about one third of that for an excited state of the dye in solution. 1. INTRODUCTION Organic compounds have been attracting much attention for their applications in all-optical devices, because of their large optical nonlinearities and fast response. However, third-order optical nonlinearities of organic materials reported so far have been several orders smaller than the criterion for a practical use. In 1991, Garito and coworkers [1] predicted the possibility of obtaining larger optical nonlinearities on populating the excited states of organic molecules. They demonstrated experimentally that two organic molecules in excited states had very large optical nonlinearities [1,2]. We also found that the molecular hyperpolarizability of an organic dye, bis-MSB (p-bis(o-methylstyryl)benzene) in an excited state is about 10* times larger than that in the ground state [3]. Recently, Nakanishi and coworkers [4,5] demonstrated that microparticles of organic molecules show different optical properties compared with single molecules (molecules in solutions). In the present study, we made microparticles of an organic dye, bis-MSB, and measured fluorescence spectra, / ^ fluorescence lifetimes, and the third-order L \ ^ ^—^ H^ optical nonlinearities of exciton of the microparticles. We think that this is the first pig. 1. Structural formula of bis-MSB.

432

paper reporting UV excitation enhancement of third-order optical nonlinearities of solid state organic compound. 2. EXPERIMENTAL Bis-MSB (dye for liquid scintillation) was purchased from Ishizu-seiyaku, Co., and used without further purification. Figure 1 shows structural formula of bis150 200 MSB. Colloidal solution of bis-MSB was Diameter / nm prepared by reprecipitation method [4], i.e., typically, 2 ml of acetone solution of bisFig. 2. Size distribution of colloidal MSB (concentration: 6 x 10^ mol dm'^), solution of bis-MSB. was dispersed dropwise into 10 ml of vigorously stirred pure water at 40 °C. The colloidal solution obtained was concentrated to one-third in volume, and YAG laser was filtered by a Millipore filter with pore Probe beam diameter of 0.5 |xm. The colloidal solution Pump (1064nm) of this dye was very stable, and there was beam no difference in the UV-visible absorption (355nm) spectra after one day. Particle size distribution of colloidal solution was determined by the dynamic light scattering method, using Otsuka electronics DLS7000DL. The incident beam for DLS was an argon ion laser (488.0 nm) and the scattering angle was fixed at 90°. The Sample DFWM distribution determined is shown in Fig. (1064mii) 2. The average diameter of the particles Photowasca. 180 nm. transistor Third-order optical nonlinearities were measured at 1.06 ^m by the degenerate Fig. 3. Experimental setup for four-wave mixing (DFWM) method using DFWM measurement. a YAG laser (Spectra-Physics, GCR-190) (see Fig. 3). Samples were simultaneously irradiated by the THG output (355 nm) of the laser in order to produce suitable amount of excitons in colloidal solution. Fluorescence lifetime was measured using the combination of a femtosecond TiiSapphire laser (Spectra-Physics, Tsunami) and a streak camera (Hamamatsu, Streak Scope C4334).

433

3. RESULTS AND DISCUSSION First, we measured second molecular hyperpolarizability y for tetrahydrofuran (THF) solutions of several organic dyes in excited states by the DFWM method. We found that BBO (2,5-(4-biphenyl)oxazole), POPOP (2,2-p-phenylene-bis(5-phenyloxazole), and bisMSB have very large hyperpolarizability in excited states. Values of y determined at 1.06 urn are 1.5 x 10'' A s m ' V for BBO, 1.3 x 10'' A s mV^ for POPOP, and 0.8 x 1 0 " A s mV'^ for bis-MSB. These values were determined by comparison with that of pyrene in an excited state (1.5 x 10'^ A s m^V"^ [6]). These dyes have no transient absorption at 1.06 |Lim. Then we tried to make colloidal solution of these dyes, but only bis-MSB gave stable colloidal solutions at suitable concentrations for DFWM measurements. Figure 4 shows absorption spectra of bis-MSB in THF solution and colloidal solution. The spectrum of colloidal solution has a shorter and broader band than that of THF solution. On the other hand, fluorescence spectrum of colloidal solution shows large red shift, and 500 350 400 its intensity is only one-tenth of that Wavelength / nm of THF solution (Fig. 5). Purification of the dye by Fig 4. Absorption spectra of colloidal solution recrystallization gave no difference (solid line) and THF solution (broken line) of to fluorescence. bis-MSB. Fluorescence decay curve of colloidal solution can be fitted well -j—7T: —rI — with a single exponential function •a with time constant of about 2.4 ns, 0.8 -^ ^ which is much longer than the fluorescence lifetime of this dye in 0.6 H ' '\ THF solution, 1.4 ns. Very small G 4> dependence of lifetime on particle 0.4 -% -J ''\ size was observed; clear size 1> dependence of lifetime has been C 0.2 H reported for microcrystals of J _Zfcr= 0 perylene [5]. 3 400 550 450 500 Figure 6 shows an example of E Wavelength / nm DFWM signal from bis-MSB Fig 5. Fluorescence spectra of colloidal solution colloidal solution. When IR laser (solid line) and THF solution (broken line) of beam was blocked, there was almost bis-MSB. no signal, which means that there

I

434

was no detectable scattering laser 1.5 T" T light. When UV laser beam was 1 1 blocked, signal intensity decreased ^ • to about two thirds. DFWM signal 1.06^1 m ON 1.06mnON without UV irradiation comes from 355nmON 355iim OFF solvent (water in this case). 1 •a We evaluated that an effective X^^^ of excitons of bis-MSB in colloidal solution was about 1/23 of that of the excited dye molecule rO.5 L [V-A/V based on the observed DFWM signal intensity. Taking the concentration of excitons of the dye in colloidal solution into the consideration, we concluded that the value of molecular _L 0 0 40 50 hyperpolarizability for an exciton in 10 20 30 microparticle should be about one Time / s third of that for an excited state of Fig. 6. DFWM signal for bis-MSB colloidal the dye in solution. The absolute solution. The signal intensity when UV laser value of the second beam was blocked is attributed to the solvent hyperpolarizability y for an exciton (water). in the bis-MSB colloidal solution should be ca. 0.3 x 10"^^ A s m^V"^ at 1.06 ^im. We measured transient absorption spectra of THF solution and colloidal solution of bis-MSB. The former has strong transient absorption at 733 nm (e = 2.8x10^ mol"' dm^ cm*), while we could obtain no distinct spectra of the latter. We think one of the reasons why hyperpolarizability y is small in colloidal solution is weak transient absorption in the solid state. We are very grateful to Professor Kobayashi, Kyushu University, for the use of the YAG laser and the apparatus for picosecond transient absorption measurements.

REFERENCES 1. Q. L. Thou, J. R. Heflin, K. Y. Wong, O. Zamani-Khamari and A. F. Garito, Phys. Rev. A43 (1991) 1673-1676. 2. D. C. Rodenberger, J. R.Heflin and A. F. Garito, Nature, 359 (1992) 309-311. 3. K. Kasatani, J. Luminescence, 87-89 (2000) 889-891. 4. H. Kasai, H. S. Nalwa, H. Oikawa, S. Okada, H. Matsuda, N. Minami, A. Kakuta, K. Ono, A. Mukoh and H. Nakanishi, Jpn. J. Appl. Phys., 31 (1992) LI 132-Ll 134. 5. H. Kasai,Y. Yoshikawa, T. Seko, S. Okada, H. Oikawa, H. Matsuda, A. Watanabe, O. Ito, H. Toyotama and H. Nakanishi, Mol. Cryst. Liq. Cryst., 249 (1997) 173-176. 6. M. Assel, T. Hofer, A. Laubereau and W. Kaiser, J. Phys. Chem., 100, (1996) 11836-11842.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

435

Mechanistic study of model monolayer membranes and their interactions with surfactants: correlation to effects on CHO cell cultures Chihae Yang^ Charles Ansong\ Lena Bockrath\ Jeffrey J. Chalme^s^ Yoon-Seob Lee\ Marianne CNeir, James F. Rathman^ Takahiro Sakamoto* 'Chemistry Department, Otterbein College, Westerville, OH 43081, USA ^'Chemical Engineering Department, The Ohio State University, Columbus, OH 43210, USA A quantitative comparison of the penetration of dipalmitoyl-5A2-glycero-3phosphatidylcholine (DPPC) and palmitoyloleoyl-5«-glycero-3-phosphatidylcholine (POPC) monolayers by various types of surfactants and polymers is presented. Surface pressure (11)area compression isotherms and Brewster angle microscopy (BAM) were used to investigate the incorporation of amphiphiles from a subphase solution into lipid monolayers. Parallel experiments were performed to study the effects of these same additives on cell growth in Chinese hamster ovary (CHO) cell cultures. Results show a strong correlation between additive effects on model lipid monolayers and their impact on cell growth and viability. 1. MOTIVATION Surface active agents are commonly used in cell culture to prevent cells from adhering to gas-liquid interfaces, to solubilize hydrophobic nutrients, to minimize foaming in sparged cultures, and to inhibit cell clumping.^' Currently, the use of these additives for use in industrial processes is highly empirical. To develop a more fundamental understanding of additive effects on cell membranes, the penetration of model membranes by surfactants and polymers were systematically correlated with their effects on actual animal cell membranes. 2. EXPERIMENTAL METHODS 2.1.1. Materials Surfactant additives studied were sodium octyl- and dodecylsulfate (SOS and SDS, respectively), dimethyldodecylamine oxide (DDAO), dodecyl- and hexadecyltrimethylammonium bromide (DTAB and CTAB), octyl- and decyUp-glucoside (OG and DG), and octyl and decyl-p-maltoside (OM and DM). Polymeric additives used in this study were two triblock (polyethylene oxide/polypropylene oxide) copolymers, Pluronic F-68 and Pluronic P-103, and a polyethylene glycol with average molecular weight of-8000 (PEG 8000). 2.1. Penetration of model lipid membranes by subphase components A customized 7x7x0.4 cm two-compartment, double barrier Langmuir trough (Nima Technology, Coventry, UK), was used throughout this study (Fig. 1). A known volume of

436

solution containing lipid — i l i g i i i I fflfiCi—I dissolved in chloroformyliexane i H i U U | l l l U l J . A i A was spread on the buffer r solution in the first I compartment and then compressed at 10 cmVmin to a target surface pressure of I 25 mN/m. The subphase I solution in the second compartment contained the Fig. 1. Monolayer penetration experiment using twosurfactant or polymer additive of compartment Langmuir trough, interest dissolved in the same buffer. All surfactant concentrations were orders of magnitude lower than the critical micelle concentration (cmc) to insure that no micelles were present (at least initially) in the subphase. The stabilized monolayer film in the first compartment was then transferred to the second compartment while holding the area constant; the two barriers were moved at a constant speed of 10 cmVmin during the transfer step. Once the transfer of the monolayer onto the subphase in the second compartment was completed, the surface pressure was held constant at 25 mN/m by allowing the area to vary as needed. The measured area values were recorded as a function of time until no further change was observed, or in some cases until the trough area was exceeded. Similar methods have been used previously to study rates of surfactant adsorption onto monolayers.^

EUiiuauiuiiQ

2.2.

Additive effects on CHO cell growth and viability Cells were cultured in a T-75 flask until confluence was observed, at which point the flask contained approximately 1x10^ total cells. The cells were then resuspended and used to inoculate a 24 well plate, which had been pretreated with 10% fetal bovine serum. The initial cell density in each well was 2x10^ cells/well. All media solutions were made fresh prior to each experiment by adding the additive into serum-free Ham F12 media and then passed through a 0.2 |im filter for sterilization. 250 jiL media aliquots were then added to the appropriate wells so that the ratio of cells to the media volume was the same for all wells. In each experiment, 6 wells were used as controls (no additive). The experiments were monitored for 3 days until the controls reached confluence. Cells were detached and resuspended for counting at the end of the experiment. The number of viable cells were determined by the Trypan Blue exclusion technique. 3. RESULTS AND DISCUSSION The area increase of a monolayer at constant surface pressure resulting from partitioning of a surfactant or polymeric additive from the subphase into the monolayer was used to determine the amount of additive in the monolayer. The mole fraction in the monolayer (X^) of a component initially present only in the subphase is calculated from: X^ =

— , where «^.

a„N,,

437

where n^ is the moles of lipid, ns is the moles of surfactant or polymer in the monolayer after partitioning has occurred, AA is the resulting increase in the area on the Langmuir trough, a^ is the effective area of the surfactant headgroup, and N^v is Avogadro's number. Values for a^ were calculated by application of the Gibb's equation to surface tension data of aqueous solutions. Results are summarized in Tables 1 and 2 for additive penetration from cell buffer solution at 25°C and surface pressure 25 mN/m. Table 1 Surfactant penetration from cell buffer subphase into lipid monolayers Xs in lipid monolayer atc/c/wc = 0.01 additive SDS SOS DDAO DTAB CTAB OG OM DG DM

DPPC 0.48 0.12 0.60 0.06 0.57 0 0 0 0

POPC 0.26 0.02 0.08

Table 2 Polymer penetration from cell buffer subphase into POPC monolayers X,in polymer cone (wt%) POPC Pluronic F-68 0.0125 0.15 a 0.34 0.025 0.37 0.035 " 0.37 0.040 (0.16) 0.050 0.0005 0.08 Pluronic P-103 " 0.27 0.001 a 0.63 0.002 0 0.05 PEG 8000 a 0 0.25 tt

ii,

0.01 0.01 0.03 0.03

Despite having identical head groups, the lipids DPPC and POPC exhibit much different susceptibility to penetration by components in the subphase. At the temperature used in these experiments (25°C), DPPC in aqueous solution forms highly structured solid-like Lc mesophases, while the more fluid liquid crystalline phase (L„) dominates the POPC/water phase diagram. Relatively small, linear surfactants penetrate both DPPC and POPC effectively, while those with bulkier head groups (glucosides and maltosides) penetrate only POPC. None of the polymers partition into DPPC to any significant degree, but the Pluronic polymers partition strongly into POPC monolayers (Table 3). For F-68, an apparent maximum in X^ is observed with increasing concentration; however, this result is most likely due to formation of mixed polymer/lipid aggregates in the subphase, depleting the -/^'^^ lipid at the interface so that the method of calculating X^ is no longer valid. Brewster angle microscopy (BAM) reveals changes in monolayer structure that occur during the penetration experiments. An example of an extreme case are the local defects caused by the penetration of DDAO into DPPC monolayers, as shown in Fig 2.

Fig. 2. BAM images of DPPC monolayers at 25 mN/m before (left) and after (center) penetration by dimethyldodecylamine oxide (DDAO). Image on right is observed upon ftirther compression to higher Fl.

438 The effects of various additives on CHO cell growth are summarized in Fig 3. Based on these results, media solutions containing amphiphilic additives can be broadly classified into three groups based on their effect on CHO cell growth: 1) stimulatory; 2) neutral or moderately detrimental; 3) toxic. For a given additive, the effect observed in cell growth experiments generally correlates very well with effects observed in the monolayer penetration measurements, indicating that additive/lipid interactions are of primary importance. Surfactants that partition rapidly into the cell membrane also cause rapid cell death. Surfactants such as DDAO that induce monolayer defects are highly toxic to cells. The results for Pluronic F-68 are especially relevant, given this polymer's widespread use in cell culture bioprocesses. It is often assumed to be an inert component in bioprocesses, but these data clearly PEG 8000 (0.1) ^/Z7777s W/y^/7/ ^Z^cZZZZ/i ^ indicate that F-68 does indeed interact I'-68(0.]) Yyyy^^j'/^/y>'///yy^y//yy/^y^yA i SDS (0.0002) ^'TTZZT/ZTZi with cell membranes and affect cell DM (0.002) r/TTTT? growth. yz/jf^ DM (0.02) X^/jfyZ/

OM (0.02) OM (0.2) OG (0.02)

•;.._._

-i

...^ Ilf—i ^K

OG(0.2) Zl-H \ p. 103 (0.01) P-103 (0.1) control CTAB(O.Ol) DTAB(O.l) DD.AO(O.Ol) ~ M B H DDAO (0.03) -1 SDS (0.02) SDS (0.1) •i

F 1

•1

50000

1

WT7^ stimuhiiorv r \ moderate • H t^>'i^^

l(K)(M)0 150000 201)000

250(K)0

II density (ceJls/weU)

Fig. 3. Effect of surfactants and polymers on CHO cell growth. Concentration is given in mM for surfactants, wt% for polymers.

A given additive may be stimulatory at one concentration and highly toxic at another. SDS is but one example of a surfactant that is generally thought of as being toxic - SDS is in fact conmionly used to promote cell lysis to facilitate the extraction of cellular components; however, at very low concentrations SDS is actually stimulatory. Understanding the influence of amphiphilic components on cell membranes demands a better understanding of concentration effects as well as other properties of the aqueous media such as pH and ionic strength.'*

ACKNOWLEDGEMENT Funding for this research was provided by U.S. NSF-POWRE Grant 9806185.

REFERENCES 1. Michaels, J., Nowak, J., Millik, A., Koczo, K., Wasan, D., Papoutsakis, E., Biotechnol. Bioeng.(1995),47,407. 2. Chattopadhyay, D., Rathman, J., Chalmers, J., Biotechnol. Bioeng (1995), 45,473. 3. Sundaram, S.; Stebe, K.. Langmuir (1997), 13, 1729. 4. Yang, C , Bockrath, L., O'Neil, M., Chalmers, J., Rathman, J., Younger, A., Biotechnol. Bioeng, submitted.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) cc) 2001 Elsevier Science B.V. All rights reserved.

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Nanostructure and dynamics of polymers at the interfaces by neutron and X-ray reflectometry Emiko Mouria, Hideki Matsuoka^*, Keitaro Kago^l, Ryuji Yoshitome^, Hitoshi Yamaoka3§ and Seiji Tasaki*' department of Polymer Chemistry, Kyoto University, Kyoto 606-8501, Japan ^Research Reactor Institute, Kyoto University, Kumatori 590-0494, Japan 1. INTRODUCTION X-ray and neutron reflectivity (XR, NR) are highly powerful techniques for the study of surface, interface, thin films and adsorbed layers at the interfaces. We have been investigating the nanostructure of amphiphilic polymer monolayer on water by XR[1]. Here, we applied NR to the study of the adsorbed protein layers at the solid/liquid interface since it can not be studied by XR. In addition, we have tried to extract information on the dynamic properties of the water surface system by applying the time-resolved XR measurement and correlation analysis. 2. EXPERIMENTAL 2.1. Materials Bovine serum albumin (BSA) for the NR study was purchased from Sigma (IXcrystallized, >97%). BSA was dissolved into D2O which was a product of Aldrich(99.9%). The pH of the sample solution was adjusted by addition of 0.0IN HCl or NaOHaq. The lipid, distealoylphosphatidylcholine(DSPC) was purchased from Sigma and used without further purification. 2.2. Neutron reflectivity measurement for polymers at solid/liquid interface quartz block The NR experiment was (Optically polished) Neutrons performed by MINE of the Institute for Solid State Physics, the University of Tokyo at JRR- I>20 solution 3M in Tokai, Japan. The details of the apparatus were fully ^. , ^ • ,, ^ t vir* « r described elsewhere[2]. For the ^'^' » The specially designed NR cell for liquid/solid interface study by solid/liquid interface. NR, we designed and made a special cell. The principle is shown in Fig.l. The sample H Present address: Venture Business Laboratory, Kyoto University § Present address: Department of Materials Science, University of Shiga Prefecture

440

polymer solution was put into a thin gap of a Teflon block and sandwiched over with a quartz block, which was optically polished. The neutron beam entered the side wall of the quartz block and was reflected by the quartz/solution interface. The reflected neutron exited from the other side wall of the quartz block and then reached the detector. The NR measurements were performed under a specular condition. The typical accumulation time was 100 sec for the smaller angles (6=0°-1.2**), 400sec for the middle angles( 1.0-2.2°), and lOOOsec for the larger angles(1.8°-5.0°). The wave-length of the neutron(X) was 12.6A. Model calculations were performed by mlayer. 2.3. X-ray reflectivity measurements for dynamics of polymer layers on water The XR experiments were performed with an instrument in our laboratory, whose details were described elsewhere[l]. The dynamic XR measurements were performed under specular condition, and at the fixed angle with a time interval of 0.006l.Osec. Since our XR apparatus has an LB trough at the sample position, the monolayer on water surface can be studied as a function of the surface pressure. 3. RESULTS AND DISCUSSION 3.1. Nanostrucuture of BSA adsorbed at quartz/heavy water interface Figure 2 shows the specular NR profiles of _ the quartz/BSA solution interface at three different f pH conditions. The data for quartz/pure D2O 1 .4 [ interface was also shown, indicating a | ; monotonical decrease after passing through the-^ critical point. The solid line is the best fit by model calculation and the interface roughness was evaluated to be 39A. The profiles for pH=6.0 (close to the isoelectric point (pl)[3]) and pH=9.6 0.02 0.04 (well above pi) also showed a monotonical 0.06 Q(A-i) decrease and the interface roughness was 2 NR profiles for evaluated to be about 6OA for both cases. Figure/DCA 1 /• n n^ Although the very small increase of the interface ^^^^/ .x^^^ ^rc \ u 1^ A . ^ . ^. ^ ^ mterface at three different pHs' and roughness may mdicate an absorption of a very pure D2O. (Q=47csin0/A.) small amount of BSA molecules, it is safe to say [BSA]=l.lmg/ml. that almost no adsorbed layer was formed at these conditions. Since the quartz surface is negatively charged slightly, this observation is duly acceptable. However, at pH=3.8, i.e. at which the BSA molecules are positively charged, the NR profile shows a clear fringe, which indicates a layer formation at the interface. By the fitting shown by the solid line in the figure, the thickness of the adsorbed BSA layer was estimated to be 280±30A. The interface roughness was evaluated to be 120±20A both for the quartz/BSA layer and BSA layer/D20 interfaces. Since the BSA molecule is a prolate ellipsoid with the major axis of 140A and the minor axis of 40A, the layer thickness obtained might indicate a formation of multi-

441

layer adsorption (almost double layers). The large interface roughness values for both interfaces are quite understandable when the shape of the BSA molecule is taken into account. Although the data shown here preliminary, they clearly indicate the high performance and importance of the NR technique for interface study, especially the solid/liquid interface. Since the X-ray can not pass through the quartz block due to absorption, XR can not be applied to a solid/liquid interface: NR can only be used for these systems. 3.2. Dynamics of water surface systems Although the nanostructure studies of surfaces and interfaces are performed eagerly by XR and NR recently, the dynamics of the structure has been received little attention. The surface dynamics might be a new field of science since many kinds of techniques for surface studies, such as AFM, surface plasmon, and evanescent wave[4] have been recently established. We have examined the dynamics of the water surface and monolayer on water surface by using the XR technique. Figure 3 shows the time fluctuation of the X-ray intensity ^dir^ct b e a ^ reflected by the glass surface, water surface, and DSPC monolayer on *gi«ss water. For comparison, the intensity fluctuation of the direct beam is also shown. The intensities of the direct beam and reflection by glass surface are quite stable, while those for Xrays reflected by the water surface and DSPC monolayer show large fluctuation. Obviously, the Figure 3 Timefluctuationsof X-ray intensity nanostructure of water surface reflected by air/water interface at the critical systems is not frozen but quite angle and of the direct beam. " I ' 1 • 1' T ' l ' dynamic. This nature is more clearly Gias s M • • yVater ; and qualitatively understood by Fig.4 iceL: _^iirtac:)K... which is the result of histogram -i-f-f f 4analysis of time fluctuation data like i1 Fig.3. One more interesting observation shown in Fig.3 might be the larger _..;..;.. fluctuation for DSPC than water in spite of stronger intensity of reflection. -i i To obtain more quantitative l^ i . i .1 L i > information and to examine whether 20 -20 0 20 20 - 2 0 0 -20 0 20 - 2 0 0 the fluctuation shows periodicity, we Amnied(%) have performed a time-correlation Figure 4 Histograms of the mtensity

[,-J:mp$^

-fffff-:

-iij

i.A

11m

analysis of the time-fluctuation data, fluctuation of the direct beam and reflected XThe time correlation function C(r) of ray be glass surface, water surface and monolayer on water.

442

quantity x can be represented by C(T) = (jc(r)jc(r + r)) (1) where / is the time, i is the delay time, and o denotes time average. This function can be easily calculated from the data in Fig.3. 512 data points were used for this analysis. If there is a machine noise in the data, the measured correlation function Cmeasi^) should be the convolution of the real function Q^r/T^ and that of noise C„oise(t). This relation can be represented by a simple mathematical form in the reciprocal space; 3[c„^(T)l = 3[C_(T)]/3[C^,,(r)] ( 3 : Fourier transform) (2) Here we regarded the data from solid surface as a noise and evaluated C„oise(T) . It was found to be constant, which means that Csur/r) is proportional to C,„eas(T), Figure 5 shows the power spectrum P(coX=^( Cmeas(T) ),fiXfrequency).P((o) 0.1 0.2 0.3 for the water surface shows a specific peak a>(hz) at co=0.06Hz, which means that some Figure 5 Power spectrum of the time periodic dynamics exists in the system. fluctuation of reflected X-ray by silicon This peak was reproducible. wafer, water surface, and DSPC monolayer on water, (correlation analysis) 4. CONCLUSIONS NR measurement by MINE was useful to determine the nanostructure of protein layer adsorbed at the liquid (water) /solid(quartz) surface. The pH dependence of adsorption behavior was also clearly shown. Multi-layered adsorption was predicted at a low pH condition. It was confirmed that the interface dynamics can be studied using the XR technique. Slow fluctuation of water surface (ca.lSsec) was detected by a timeresolved XR measurement with the time-correlation analysis. In future, the XR and NR technique will be applied to surface/interface systems, including liquid/solid, liquid/air, to study not only nanostructure but also dynamics. These studies are expected to bring about further developments in surface and interface science. Acknowledgment The neutron reflectivity measurements by MINE were technically supported by Drs.Tohru Ebisawa, Masahiro Hino (Research Reactor Institute, Kyoto University) and Dr. Naoya Torikai (KENS, High Energy Accelerator Organization) to whom our sincere thanks are due. This work is financially supported by Grant-in-aid from the Ministry of Education, Science, Sports and Culture (No.A09305063 and B12555266)

REFERENCES 1. H.Matsuoka et al. Langmuir, 15(1999), 4298; Langmuir, 15(1999), 5193; Langmuir, 15(1999), 2237; Supramol. Science, 5(1998), 349;Chem. Phys. Lett., 295(1998), 245. 2. T.Ebisawa, S.Tasaki, Y.Otake, H.Funahashi, K.Soyama, N.Torikai, Y.Matsushita, Physica B 213-214 (1995) 901-903. 3. The pl=4.8 was reported for BSA, but pl=6.0 was found for our sample by titration. 4. H.Matsuoka, Macromol.Chem.Phys.(2000) in press ; D.C.Prieve, Adv.ColLInt.Sci., 82(1999), 93.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^c: 2001 Elsevier Science B.V. All rights reserved.

443

Dynamic cavity array of steroid cyclophanes at membrane surface K. Ariga, Y. Terasaka, H. Tsuji, D. Sakai and J. Kikuchi Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan In this Study, we dynamically controlled the cavity structures of steroid cyclophanes placed at the air-water interface and demonstrated repeatable piezoluminescence based on molecular recognition. The repeated compression and expansion of the monolayer of a colic-type steroid cyclophane induced a periodic change in the fluorescence intensity increase upon binding of a fluorescent guest. We also investigated binding of the fluorescent guest in aqueous mixtures of the steroid cyclophane and an artificial lipid. Emission wavelength of the fluorescent guest in the mixtures was significantly altered by controlling the mixing ratios. 1. INTRODUCTION Information conversion based on specific molecular recognition is an attractive research target, because it is indispensable for the development of molecular devices. Inclusion of a guest molecule into a host cavity sometimes induces changes in the spectral characteristics [1, 2]. Such a system provides information conversion from "chemical structure" to "photosignal". So far, static host cavities such as cyclodextrin have been mainly used for molecular recognition on membrane surfaces [3-5]. However, more sophisticated conversion systems would be developed if the host cavity is dynamically controlled. Recently, we developed steroid cyclophanes as a dynamic cavity which are unique hosts with a cyclic core of a l,6,20,25-tetraaza[6.1.6.1]paracyclophane connected to four steroid moieties through a flexible L-lysine spacer (Fig. 1). Polarity of the cavity interior can be controlled by several factors, resulting in a change in fluorescence of the bound probe (6-(^toluidino)naphthalene-2-sulfonate, TNS). In this paper, we present preliminary results of the information conversions in the monolayer on water and in aqueous mixed assemblies with an artificial lipid. 2. EXPERIMENTAL Details of the syntheses of 1 [6], 2 [7], and 3 [8] were described elsewhere. The JC-A isotherms were measured at 20.0 ± 0.2 °C and at a compression rate of 0.2 mm^ s'* with an FSD-300 computer-controlled film balance (USI System). The subphase pH was adjusted to 11 with aqueous KOH. Surface-reflective fluorescence spectra were measured using a photodiode array-equipped spectrometer (Otsuka Electronics, Model MCPD-7000) with an excitation wavelength of 323 nm. The fluorescence spectra of the aqueous mixture were similarly measured at 30 °C (Shimadzu, RF-5300PC).

444

O

CH3

H

^(CH2)i5CH3

O 3

K^ "O3S TNS 1 : X = OH 2:X = H

Fig. 1. Structures of compounds used in this study.

3. RESULTS AND DISCUSSION 3.1. Piezoluminescence of dynamic cavity array at the air-water interface Cholic acid with three hydroxyl groups forms a rigid plane with hydrophobic and hydrophilic faces, and is expected to lie on water with its hydrophilic side facing the bulk water. The 7C-A isotherms of the steroid cyclophane 1 showed a transition behavior at ca. 20 mN m"* and ca. 4 nm^ on 0.1 mM aqueous TNS (Fig. 2A). A limiting area of ca. 2 nm^ is between the cross-sectional area of 1 with a standing-up conformation of the cholic acid moieties (2.3 nm^) and area for the close packing of the four cholic acids (1.6 nm^). Therefore, the three-dimensional cavity is probably formed at the second rise in the isotherm. The area estimated for the open confomiation of 1 with flattening of the four cholic acid moieties on water (ca. 7 nm^) is close to the molecular area where the surface pressure starts increasing. These n-A characteristics strongly suggest that the cavity conversion occurs upon compressing the monolayer of 1. The fluorescence increase upon the binding of TNS to these monolayers was investigated using surface-reflective fluorescence spectroscopy. Since the fluorescence intensity of TNS is significantly suppressed in a polar medium, TNS has a large fluorescence only through its insertion into the hydrophobic core. The repeated compression and expansion induced a periodic change in the fluorescence intensity increase upon the binding of TNS [9]. Changes in the fluorescence intensity and the surface pressure are plotted in Fig. 2B. Both the parameters show periodical changes in unison. This result confirms that the dynamic pressure application is converted to luminescence based on the molecular recognition by the monolayer of 1. A similar experiment was carried out using the monolayer of the colanic-type steroid cyclophane (2). Both the phase transition in the n-A isotherm and the change in the

445

fluorescence intensity were hardly detected. This control experiment confirms that the dynamic characteristic of the cavity is crucial for the pressure-induced luminescence.

p

£ 0

5

0

10

50 100 150 200 250 300

Time / ruin

Molecular Area / nm^

Fig. 2. (A) 7C-A Isotherms of the steroid cyclophanes on pure water at 20 °C. (B) Changes of fluorescence intensity at 435 nm and surface pressure upon reversible guest binding induced by repeated compression-expansion of monolayer of 1.

(A) [3]/[I] =0.1

Hydrophilic Cavity 3

H ^ 433 nm

TNS (B)[3]/[l] =5

zHydrophobic Cavity 409 nm

(C)[3]/[l] =80

It

Hydrophilic Cavity 426 nm

Fig. 3. Plausible model of structural changes in mixtures of 1 and 3.

446

3.2. Modulation of emission wavelength by cavities embedded in lipid bilayer An aqueous dispersion of 3 was mixed with aqueous TNS (0.1 |iM) and an ethanolic solution of 1 (0.01 mM), and the fluorescence spectra over a wide range of mixing ratios of 1 and 3 were measured. When the content of 1 is dominant ([3]/[l] = 0.1), the fluorescent maximum of TNS is 433 nm. This value is close to that observed for the methanolic solution of TNS. Under such circumstances, the hydrophobic cavity would be imperfectly formed and TNS is bound to 1 mainly through an electrostatic interaction with the lysine residue at the hydrophilic part of the host (Fig. 3 A). At the [3]/[l] ratio of 5, the fluorescence maximum is ca. 409 nm, indicating that the host cavity provided a more hydrophobic environment like THF (Fig. 3B). Interestingly, a large excess of 3 reversed the fluorescence maximum up to 426 nm, i.e., the host cavity became hydrophilic again. The steroid cyclophane can disperse in the lipid bilayer of 3 by contacting the hydrophobic face of the steroidal moieties to the hydrophobic region of the completed bilayer, leading to the formation of hydrophilic cavity (Fig. 3C). The fluorescence maxima are almost independent of the lipid content when 2 was used instead of 1. The lack of the bifacial nature of the side wall of 2 leads to an unchanged cavity. This result reveals that the bifacial nature of the wall in the steroid cyclophane is indispensable for the hydrophobic-hydrophilic cavity conversion. 4. CONCLUSION In this study, we prepared the dynamic cavity array of the steroid cyclophanes at the airwater interface and in lipid mixtures, and two kinds of cavity conversions were demonstrated. The flexible and amphiphilic nature of the steroid cyclophane results in pressure-induced and lipid-dependent photo-signal modulation. These systems would lead to a novel design of molecular photonic devices based on controllable molecular recognition.

REFERENCES 1. A. Ueno, T. Kuwabara, A. Nakamura and F. Toda, Nature, 356 (1992) 136. 2. Y. Kubo, S. Maeda, S. Tokita and M. Kubo, Nature, 382 (1996) 522. 3. H. Nakahara, H. Tanaka, K. Fukuda, M. Matsumoto and W. Tagaki, Thin Solid Films, 284285(1996)687. 4. K. Kurihara, K. Ohto, Y. Tanaka, Y. Aoyama and T. Kunitake, J. Am. Chem. Soc, 113 (1991)444. 5. Y. Ishikawa, T. Kunitake, T. Matsuda, T. Otsuka and S. Shinkai, J. Chem. Soc, Chem. Commun., (1989) 736. 6. J. Kikuchi, T. Inada, H. Miura, K. Suehiro, O. Hayashida and Y. Murakami, Reel. Trav. Chim. Pay-Bas, 113 (1994) 216. 7. J. Kikuchi, T. Ogata, M. Inada, and Y. Murakami Chem. Lett., (1996) 771. 8. Y. Murakami, J. Kikuchi, T. Takaki, K. Uchimura and A. Nakano, J. Am. Chem. Soc, 107 (1995)2161. 9. K. Ariga, Y. Terasaka, D. Sakai, H. Tsuji and J. Kikuchi, J. Am. Chem. Soc, 122 (2000) 7835.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c) 2001 Elsevier Science B.V. All rights reserved.

447

Mixed Langmuir monolayer properties of sphingoglycoiipids (cerebrosides) and lipids S. Nakamura, O. Shibata*, K. Nakamura,^ M.Inagaki^, and R. Higuchi^ Department of Molecular Bioformatics and ^^partment of Medicinal Chemistry, Graduate School of Pharmaceutical Sciences, Kyushu University, Maidashi, 3-1-1 Higashi-ku, Fukuoka 812-8582, Japan Surface pressure (ji) - , surface potential (AVO - , and dipole moment (]U^) - area (A) isotherms were obtained for Langmuir monolayers made from sphingolipids(cerebrosides) (LMC-2), cholesteryl sodium sulfate (Ch-S) and cholesterol and their combinations. 1. INTRODUCTION The interactions of animal cells with surfaces regulate such fundamental biological processes as growth, differentiation and motility. Although the nature of the interactions is not understood at the molecular level it is thought that the complex glycolipid and glycoprotein molecules which lie at the outer surface of the cells are involved. Taking into account the above phenomena, as the first attempt, the surface pressure (Jt) and the surface potential(A V)- area (A) isotherms were obtained for mixed monolayers of different glucocerebrosides with lipids on substrate solution of 0.5M NaCl as a function of compositions in the mixture by employing the Langmuir method and the ionizing electrode method. 2 . EXPERIMENTAL PART Sphingoglycolipid was isolated from starfish Luidia maculataXMC and purified as reported previously 111. 1 -0-0-D-glucopyranosylH 1 S,3S,4R)-2-|(2R)-2-hydroxycarbonylamino|-alkane-l,3,4-triol (LMC-2) is chemical species whose chain lengths are 22, 23, and 24on carbonyl group and 17, 18, and 19 chain lengths on alkane, respectively. Cholesteryl sodium sulfate (Ch-S) is the by-product from LMC. Its purity was checked by TLC and showed single spot. Cholesterol (Ch) was purchased from Nu-Chek-Prep, Inc., and also checked by TLC. The pure compounds or their mixtures were spread from a nhexane/ethanol mixture (7/3) at the air /aqueous solution interface. While Ch-S was spread from a chloroform / methanol mixture (2/1) which are both Merck (Uvasol).

448 Other experimental conditions were the same as described in the previous papers |21. 3.RESULT AND DISCUSSION 3.1.Surface pressure (K) -, surface potential (AV) -, and dipole moment (/Xj^) - area (A) isotherms The K-A, AV-A, and ^^-A isotherms of monolayers made from pure LMC-2 and Ch-S spread on a 0.5 M NaCI substrate (at 298.2 K) are shown on Figure la-b. LMC-2 was stable up to 47.2mN m"' with a liquid-expanded (LE) phase (Fig. la). The extrapolated area in the condensed state was 0.65 nm^ and the collapse area 0.44 nm^. Cholesteryl sodium sulfate (Ch-S) isotherm was condensed, indicating that the monolayer was in a liquid condensed (LC) phase. It collapsed at 52.2 mN m* (0.30 nm^) and the extrapolated area was 0.36 nm^. These values indicate that the steroid skeletons are at close contact at high pressure. The surface potentials {AV) of LMC-2 and Cholesteryl sodium sulfate (Ch-S) show positive change (Fig. lb). The Cholesteryl sodium sulfate (Ch-S) monolayer showed an increase of AV under compression. AV became gradually jump and reached the value of about 280 mV (starting from 40 mV, an absolute difference of 240 mV) at the collapse area (0.30 nm^). The LMC-2 monolayer displayed a much smaller variation of AV (from 0 mV to 100 mV) at the collapse area (0.44 nm^). The small variation of Z\V( LMC-2) reflects the orientation change during compression. The LMC-2 head area is much larger than the

500

I I I I I I

X(CH.S)= «-^-

X = 0.7 X = 0.5 X = 0.3 X = 0.1 LMC.2

400

I I I I I I I I I I I I I I I I I I I I I I

b

Mixed monolayer LMC-2 / Ch-S

300 200 100 0

0.2

0.4 0.6 0.8 Area / nm^

1

1.2

• 100

I I I I I I I I I I I I I I I I

0.2 0.4 0.6 0.8 Area / nm^

1

1.2

Fig. 1. Surface pressure (it) - area (A) isotherms (a), and surface potential (AV) - area (A) isotherms (b) on 0.5 M NaCI at 298.2 K.

449 choiesteryl sodium sulfate (Ch-S) head area, which results in a loose packing of LMC-2 chains. The larger increase of AV observed in the case of Ch-S can be explained by the higher organization of the monolayer, as compared to that of LMC-2, due to increased mutual polarization. The variations of the vertical component of the surface dipole moment, jUj^, of LMC-2 and Ch-S monolayers under compression are obtained. jUj^ strongly depends on the polar head group's nature. Upon compression, jUj^ (Ch-S) increased from about 100 mD to 215 mD, while jU^(LMC-2) only increased from 40 mD to 110 mD. 3.2.Ideality of the mixture The two-component mixed monolayer system composed of LMC-2 and Ch-S was studied in order to assess the impact of the molecular structure of the amphiphiles on their miscibility in the monolayer, and on the state of the monolayer. For the above purpose, the JT - A , AV-A and jUj^- A isotherms of the LMC-2 and Ch-S mixed monolayers were measured for various Ch-S molar fractions (X^^,^.^) (298.2K, 0.5M NaCI substrate). Results are shown in Figure I. All the curves of the mixed system exist between those of the respective pure components, and they successively change with increasing mole fraction. An understanding of the interactions between LMC-2 and Ch-S is provided by examining whether the variation of the mean molecular area as a function of XQ^_^ does satisfy the additivity mie or not. For all surface pressures (5, 15, 25, and 35 mN m ' ) , it clearly shows a negative deviation between the theoretical and experimental curves, indicating some interactions between LMC-2 and Ch-S. These interactions may likely result from attractive interactions between LMC-2 and Ch-S polar heads. We have also examined the influence of X(-j^_5 on the AV-A and jUj^ - A curves. Both AV and jUj^ reflected the Jt-A behavior, the higher the collapse pressure, the larger the ^V and jUj^ values. An analysis of the surface potential (AV) and of the surface dipole moment ( jUj^ ) of the monolayer was also made in terms of the additivity rule. For LMC-2/Ch-S mixed system, assuming the additivity rule, variations of ^V and jUj^ at various surface pressures (15, 25 and 35 mN m ' ) showed a significant negative deviation.

450

3.3.Two-Dimensional Phase Diagram Two-dimensional phase diagrams of LMC-2 and Ch-S monolayers were constructed by plotting the values of collapse pressures as a function of

Ch-S mole fractions. The

coexistence phase boundary between the expanded phase of the LMC-2 and Ch-S mixture and the bulk phase can be theoretically simulated by the Joos equation | 3 | . It is noteworthy that the LMC-2 /Ch-S system produced a positive interaction parameter ( | = 1). Such a positive parameter implies that the interaction energy between LMC-2 and Ch-S (which was calculated to be 413 J mol'^) is lower than the mean energy between similar molecules. That means that the two components are completely miscible in the expanded state but are not miscible in the condensed state. However their mutual interaction is weaker than the mean of interactions between pure component molecules themselves |4|.

The above

procedures were performed for LMC-2/ Ch system. To ascertain the above phenomena, the miscibility of LMC-2 / Ch-S and LMC-2 / Ch systems have to be further studied by other techniques such as Brewster angle microscopy, and fluorescence microscopy which will be reported in a separate paper. In conclusion^ the new fmding of this study is that cerebrosides (LMC-2) and Cholesteryl sodium sulfate (Ch-S) can be spread as a stable monolayer at 298.2 K on a 0.5M NaCI subphase. Judging from the collapse pressure, our phase diagrams might be classified into two types. The first might be an eutectic type, which the two combinations of cerebroside (LMC-2) with

cholesteryl sodium sulfate (Ch-S) is assigned; they are

miscible in the expanded state but immiscible in the condensed state.

The second is a

completely immiscible type in the expanded state and in the condensed state; the combination of cerebroside (LMC-2) with cholesterol (Ch). REFERENCES 1. R. Higuchi, M. Inagaki, K. Togawa, T. Miyamoto, and T. Komori, Liebigs Ann. Chem. (1994)653. 2. O. Shibata , Y. Moroi, M. Saito, and R. Matuura,Thin Solid Films, 327, (1998) 123. 3. Joos, P. and Demel, R.A., Biochim. Biophys. Acta. 183 (1%9)447. 4. Matuo, H., Motomura, K., and Matuura, R., Chem. Phys. Lipids 30 (1982) 353.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

451

P h o t o i n d u c e d E l e c t r o n Transfer P r o c e s s e s in Polymer Langmuir-Blodgett Films Tokuji Miyashita, Shinsaku Ugawa and Atsushi Aoki Institute for Chemical Reaction Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan The photo-electric conversion system using the hetero-deposited redox polymer Langmuir-Blodgett (LB) films consisting of poly(N-dodecylacrylamide-coferrocenylmethylacrylate) (Fc copolymer) and poly(iV-dodecylacrylamideco-(4-(acryloylmethyl)-4'-methyl-2,2'-bipyridine)-bis(2,2'-bipyridine) ruthenium diperchlorate)) (Ru copolymer) were investigated. On irradiation on the Ru/Fc polymer LB film, efficient photocurrent generation was observed. Various factors influencing the photocurrent generation were examined. Especially, the deposition surface pressure and the sacrificial electron donor effects on photocurrent are investigated at the hetero-deposited redox polymer LB film electrode containing tris(bipyridine)ruthenium complex as a sensitizer and ferrocene as an electron donor. The maximum photocurrent is observed at the deposition surface pressure of the inner Ru monolayers of 35 mN/m because of the relationship between the formation of the well-defined hetero-interface between Ru and Fc and the interlayer electron transfer rate. The photocurrent in the sacrificial electron donor, iodide solution is higher than that in the triethanolamine solution due to the effective electron transfer and the ion complex formation between the ferrocenium cation in the outer layer and the iodide anion in the solution. 1.INTRODUCTION Langmuir-Blodgett (LB) technique is one of the methods to fabricate highly ordered molecular assembUes and ultrathin films on sohd substrates. The organized molecular assemblies similar to biomembrane where each component is spatially arranged for functional performance can be fabricated by the LB technique. We have attempted to incorporate various functional groups into polymer LB films for the fabrication of nano-organic devices. Functional groups with photo-activity, molecular recognition, redox activity and chiral have been successfiilly incorporated. In the previous study, we have reported that Ruthenium complex and ferrocene

452

derivatives are safely incorporated into PDDA polymer LB films, and the heterodeposited LB films composed of Ruthenium layers and ferrocene layers show an efficient photocurrent generation. In this work, we investigated various factors influencing the photocurrent generation to fabricate an effective photoenergy conversion soft device. 2. EXPERIMENTAL 2.1. Fabrication of redox polymer LB films PolyCAT-dodecylacrylamide-co-ferrocenylmethylacrylate) (Fc copolymer) and poly(iV-dodecylacrylamide-co-(4-(acryloylmethyl)-4'-methyl-2,2*-bipyridine)bis-(2,2'-bipyridine)ruthenium diperchlorate) (Ru copolymer) were prepared as described previously [1,2] (Figure 1). The mole fractions of the redox species in the copolymers were determined from the UV-vis absorption spectra to be 0.54 and 0.11 for Fc and Ru copolymers, respectively. Molar absorption coefficients of FcA homopolymer and Ru(bpy)32* used as a standard compound for determination of copolymer composition are e = 110 M^ cm^ at 438 nm and 1.4 x 10^ M^ cm^ at 454 nm, respectively. All other chemicals were of reagent grade and used without further purification. The measurement of surface pressure (n) - area (A) isotherms and deposition of the monolayers were carried out with a computer-controlled Langmuir trough FSD-11 (USD at 20 *'C. The monolayers of the Fc and Ru copolymers were transferred onto an ITO electrode as a substrate by vertical dipping method at a dipping speed of 10 mm min^ under various surface pressures at 20 °C. The typical structure of the hetero-deposited redox polymer LB film consists of Ru copolymer monolayers (two layers) as an inner layer and Fc copolymer monolayers (three layers) as an outer layer on an ITO electrode (Figure 1). 2.2.Photoelectrochemical Measurements Photocurrent measurement was carried out at a constant applied potential on light irradiation using a potentiostat (HA-501, Hokuto). A 500 W xenon lamp equipped with IR-cut off filter (IRA-2S, Toshiba) and UV-cut off filter (VY43, Toshiba) was used as a light source. The light intensity at the irradiating substrate surface was measured with a thermopile MIR-IOOC (Daiya Instrument). Photocurrent was measured on a y-t recorder. The electrochemical cell is equipped with a window for mounting an ITO electrode. The ITO electrode is mounted at the cell window using a silicon rubber 0-ring (14 mmf). An electrode area of 1.54 cm2 is exposed to the electrolyte solutions. A Pt wire is used as an auxiliary electrode and the potential is referenced to a saturated calomel electrode (SCE). 1.0 M NaC104 solution is employed as an electrolyte solution. Solutions were initially purged with N2 for 30 min and then maintained under a flow of N2.

453

-(CH2-XH ) - ( C H 2 - C H ) OssC 0.89

^lH CHo

0 = C 0.11

6

0 - C 0.46 IJH

dH

"

0-C 054 6 CK

Fe

Homo-deposited Ru LB film electrode

Hetero-deposited redox polymer LB film electrode Fig. 1 Copolymers with Ru complex and ferrocene, and the strucutre of hetero- and homodeposited LB films

3. RESULTS AND DISCUSSION RuCbpy^a^"^ is well known to be a redox-active photosensitizer with strong absorption in the visible light. The photoexcited ruthenium complex (Ru(bpy)3^'^*) can perform both oxidative and reductive electron transfer reactions by means of acceptors and donors, resulting in the oxidized form, RuCbpy)^^"^, and the reduced form, Ru(bpy)3*, respectively. Ru(bpy)3^* and Ru(bpy)3* can be detected as a photocurrent at the electrode. The ferrocene derivative was chosen as a donor for the excited Ru(bpy)3^*. The polymer LB films with different deposition structure are constructed by var3dng the deposition order of these redox polymer monolayers by the LB technique. By light irradiation on the LB films, the anodic photocurrent was observed at the hetero-deposited redox polymer LB films consisting of Ru copolymer LB film as an inner layer and Fc copolymer LB film as an outer layer on the ITO (Fc/Ru/ITO) electrodes, whereas the cathodic one is observed at the reverse layered structure (Ru/Fc/ITO) electrodes. The direction of photocurrent flow depends on the deposition order of the redox polymer LB films on the ITO electrode. The effective photocurrent generation was investigated under various conditions. 3.1. Deposition surface pressure e£feet on photocurrent The electrochemical properties of the hetero-deposited redox polymer LB films containing ferrocene and tris(bipyridine)ruthenium derivatives were strongly dependent on the deposition surface pressure as shown in our previous work [4].

454

The photocurrent generated at the LB film electrodes is also expected to depend on the deposition surface pressure. Figure 2 shows the dependence of photocurrent on the deposition surface pressure of the Ru copolymer monolayers obtained at the homo-deposited Ru LB film and the hetero-deposited redox polymer LB film. The photocurrent at the hetero-deposited LB film structure is about three times higher than that of the homo-deposited LB film structure because the effective photoinduced electron transfer between the ruthenium complex and ferrocene takes place [3]. On the other hand, the dependence of the photocurrent on the deposition surface pressure shows the same trend at both of homo-deposited and hetero-deposited LB films. The both photocurrents increase with the deposition surface pressure below 35 mN/m whereas decrease above 35 mN/m. At the deposition surface pressure below 35 mN/m, the alkyl chains of the LB film is loosely packed so the redox species is not fixed at the hydrophilic interface of the LB film and the electron produced by the photoinduced electron transfer is consumed by the recombination before becoming photocurrent. Meanwhile, the photocurrent reduction above 35 mN/m is caused by the following two reasons: One is the increase of the interlayer electron transfer distance and the other is the packing density of the alkyl side chain at the deposition surface pressure above 35 mN/m. Therefore, the optimum deposition surface pressure is 35 mN/m and the photocurrent of 580 nA/cm^ is three times higher than the photocurrent of the hetero-deposited redox polymer LB film at any deposition surface pressure. Because the well-defined hetero-interface between the Ru and Fc copolymer monolayers is formed and the interlayer electron transfer rate is enough fast at the deposition surface pressure of 35 mN/m.

20 25 30 35 40 45 Deposition surface pressure (mN/m)

Fig. 2. The dependence of photocurrent on deposition surface pressure of the Ru monolayers at the hetero-deposited LB film(A) and the homo-deposited LB film.

0

0.05

0.1

0.15

0.2

0.25

Potential (V vs. SCE)

Fig. 3. The depdendence of photocurrent on potential at the hetero-deposited LB film inl.O M Nal (A) and 0.5 M TEOA (B) solution.

455

The results are supported by the cycUc voltammograms at the hetero-deposited redox polymer LB film [4]. They indicated that the inner Ru monolayers do not behave as an insulator for the outer Fc monolayers below 35 mN/m. The well-defined hetero-interface between Ru and Fc has been fi)rmed above 35mN/m. Above 35 mN/m, the interlayer electron transfer rate becomes smaller due to the increase of the electron transfer distance and low mobility of counterion in the LB films. 3.2. Sacrificial electron donor effect on photocurrent The sacrificial donor effect on photocurrent was investigated to improve the photocurrent generation. Figure 3 shows photocurrent dependence on applied potential at the hetero-deposited LB film electrode in two different sacrificial donors, 0.5 M triethanolamine (TEOA) and 1.0 M Nal solution, respectively. These sacrificial electron donors are electroinactive in the experimental potential region. Both photocurrents shows the same trend against the electrode potential whereas the magnitude of the photocurrent in Nal solution is higher than that in TEOA solution. The both sacrificial electron donor concentrations are chosen for the condition obtaining the saturated photocurrent. The difference of photocurrent would be caused by the difference of the electron transfer rate between the ferrocenium in the outer layer and the sacrificial electron donor and by the ion complex formation between the ferrocenium cation and iodide anion. The magnitudes of photocurrent increase with the concentration of the sacrificial electron donor, I at the homo-deposited and hetero-deposited redox polymer LB film electrode (Figure 4). The photocurrent is almost linearly dependent on the r concentration below 0.2 M. In this sacrificial electron donor concentration, the magnitude of photocurrent is controlled by the rate of the electron transfer reaction between the ferrocenium in the outer layer and T in the solution. The magnitude of photocurrent was saturated at the I" concentration above 0.2 M. Therefore, the photoinduced electron transfer fi-om ferrocene to photoexcited Ru complex is the rate-determining step in the overall photocurrent process at the I concentration above 0.2 M. The photocurrents depend linearly upon the intensity of the light irradiation on the homo-deposited and the hetero-deposited redox polymer LB film electrodes as shown in Figure 5. The result indicates that the excitation of the ruthenium complex is the rate-determining process in the photocurrent generation at the sacrificial electron donor concentration of 1 M. The maximum photocurrent of 4.27 mA/cm^ was obtained in the hetero-deposited redox polymer LB film at 0.3 V vs SCE, 80 mW/cm^ and 1 M NaL This value was four times higher than that of the homogenous Ru redox polymer LB film. The pseudo-photocatalytic rate constant (k^t) was determined to be 0.65 s"^ according to the following equation.

456

where 1^ is the saturated photocurrent, n is the number of electron, F is Faraday constant, and F is surface concentration of Ru complex.

rT 1000

(A)^

o

:$ 800 c >—• S 600

g 8 o

400

1

200

o-^s^—H

0. [ 0

1

1

200

400

. 600

, 800

1000

Nal concentration (mM) Fig. 4. The dependence of photocurrent on the Nal concentration at the heterodepsoited LB film (A) and homodepsoited LB film (B) at 0.3 V vs. SCE.

0

20

40

60

80

light intensity (mW/cm^) Fig. 5. The dependence of photocurrent on light intensity at the hetero-deposited LB film (A) and homo-deposited LB film (B) at 0.3 V vs. SCE.

In summary, the photocurrent is dependent on the deposition surface pressure of the inner Ru copolymer monolayers. The maximum photocurrent is obtained at a deposition surface pressure of 35 mN/m because of the relationship between the well-defined hetero-interface and the interlayer electron transfer rate. The iodide ion is superior to the triethanolamine as a sacrificial electron donor in the present photoinduced electron transfer system using the heterodeposited redox polymer LB film due to the effective electron transfer and the ion complex formation between the ferrocenium cation and iodide anion.

REFERENCES 1. AAoki and T.Miyashita, Macromolecules, 29(1996)4662. 2. AAoki and T.Miyashita, Chem.Lett., (1996)563. 3. AAoki, Y.Abe and T.Miyashita, Langmuir, 15(1999)1463. 4. AAoki and T.Miyashita, J.Electroanal.Chem., 473(1999)125.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (o 20()1 Elsevier Science B.V. All rights reserved.

457

Monolayer Assemblies of Comb-Like Polymers Containing Fluorocarbons with Different Chain Length Atsuhiro Fujimori, Tohru Araki, Yoshio Shibasaki and Hiroo Nakahara Department of Chemistry, Faculty of Science, Saitama University, 255 Shimo-okubo, Urawa, 338-8570, Japan The monolayer behavior and the structure of transferred films of the comb-polymers with different chain length obtained by solid-state polymerization with y-ray irradiation, from the corresponding esters of acrylic and methacrylic acids containing fluorocarbon chains, were investigated by surface pressure-area (7t-A) isotherm, and scanning electron microscopy (SEM), scanning probe microscopy (SPM), respectively. It was found that adifference of the side-chain length for the polymers induced a drastic change in the isotherms was found, suggesting the relationships between crystallinity in the bulk state and collapsed surface pressure of the monolayer on the water surface. In addition, the monolayers were deposited by the horizontal lifting, the Langmuir-Blodgett, and the surface-lowering methods to give the X-, Y-, and Z-type films, respectively, which were characterized by scanning probe microscopy, to clarify the mesoscopic surface structures related to the friction properties ascribed to composition of the outermost surface for the organized molecular films. 1. INTRODUCTION The fluorocarbon chain is thicker and less flexible than hydrocarbon chain, since the van der Waals radius of fluorine atom is too large to allow of making precise trans zig-zag planar conformation of -(CF2-CF2)n- chain, as the poly(tetrafluoroethylene) [1-4]. Previously, for long-chain vinyl compounds containing the fluorocarbon or hydrocarbon with different lengths the thermal behavior related to the molecular packing in bulk states were investigated by measurements of differential scanning calorimetry (DSC) and X-ray diffraction [5-6]. And further, the formation and structure for the organized molecular films for fluorinated amphiphiles with vinyl groups were investigated by TT-A isotherm, and SEM, AFM, X-ray diffraction, respectively [7]. In the present work, for the fluorinated comb-polymers with different side-chain length, the monolayer behavior at the air/water interface and the structures of build-up films have been studied by 7t-A isotherms, and SEM together with a SPM, respectively, to obtain the two-dimensional molecular orientation in the films depending on both the length and the atom at the a-position of the side-chain and also the ester groups of the polymers as well as the effect of spreading solution. 2. EXPERIMENTAL 2.1. Materials As shown in Table 1, fluorinated comb-polymers used in this work, the acrylate and methacrylate derivatives containing fluorocarbon chains, Poly-2-(perfluoroalkyl)ethyl acrylate and methacrylate, F(CF2)nCH2CH20[COC(X)=CH2]m, [abbreviated as Poly-FFnEA

458

(n=6,8,10) for X=H and Poly-FFnEMA (n=6,8,10) for X=CH3] and also Poly-1H, 1 H,(n+1 )H-(partialfluoroalkyl) acrylate and methacrylate, H(CF2)nCH20[COC(X)=CH2]m, [Poly-FnA and Poly-FnMA (n=4,6,8,10) for X=H and CH3, respectively] were obtained by solid-state polymerization with ^Co y-ray irradiation. The tacticity of these polymers was estimated to be almost syndiotactic (Diad : 58 %) by ^H-NMR analysis according to the reference 8 [8]. The molecular weights seemed to be above tens of from the intrinsic viscosity of [T]] = 0.12 - 0.54 for these trifluoroacetic acid solutions at 30 °C. While, TOF-Mass measurements using trifluoroacetic acid as matrix, are in progress. Table 1 The comb-poiymers containing fluorocarbon chain used in this work Sample name Abbrev. F(CF2)nCH2CH20[COC(X)=CH2]m ; X=H, CH3, n=6,8,10 Poly-2-(perfuluoroalkyl)ethyl acrylate and methacrylate

Poly-FFnEA and Poly-FFnEMA

H(CF2)„CH20[COC(X)=CH2]ni; X=H, CH3, n=4,6,8,10 Poly-lH,lH,(n+l)H-(partialfluoroalkyl) acrylate and methacrylate

Poly-F„A and Poly-FJ^A

(n=6,8,10) (n=4,6,8,10)

2.2. Procedures The monolayers were spread from trifluoroacetic acid solution to the distilled water [9]. A complete spreading was to allow the spreading solution to flow down a small glass rod projecting from the water surface in the trough. The TI-A isotherms for the monolayers on water surface were measured by a Lauda film balance. The X-, Y-, and Z-type films were deposited by a horizontal lifting, a Langmuir-Blodgett, and a surface lowering methods, respectively. Morphologies of the surface for the deposited films were observed by a scanning electron microscope (Hitachi, model 4100) and a scanning probe 5°C 1 microscope (Seiko Instruments, SPA 70 ry f ^ 4 0 °C n=ia^ (^F2)„ (^F2)„ 300 with SPI3800 probe station). r n=8^ 3. RESULT AND DISCUSSION 3.1 The dependence of side-chain length of fluorinated comb-polymers for the isotherms The monolayers of Poly-FFioEA and Poly-FFgEA on the water surface, formed extremely stable condensed monolayers in the temperature range of 5 to 40 °C (Fig. 1). However, Poly-FF6EA monolayers on the water surface exhibited relatively low collapsed surface pressure. In the previous report, Poly-FFioEA and Poly-FFgEA formed crystalline polymers in the bulk state, while Poly-FF6EA couldn't form a layer structure and gave amorphous polymer [5]. These results indicated the relationships between crystallinity in the bulk state and collapsed surface pressure of the

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70 80 30 40 50 60 Area/A^repeating unit'* Figure 1 The 7t-A isotherms of monolayers on water surface for Poly-FFJEA at 5 and 40 °C.

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Area/A^repeating unit"* Area/A^repeating unit* Figure 2 The TC-A isotherms of monolayers on water surface for Poly-F„A (a) and poly-F,MA (b) at 5 °C.

459 monolayers on water surface for the fluorinated comb-polymers. Figure 2 also indicates the 7i-A isotherms of another comb-polymers, the depending on the side-chain length. The Poly-FnAs with the short side-chain length, where n = 4, pi^ur^a The AFM images of X-type monolavcrs on 6, didn't form a layer structure and mica for poly-F6MA at 5 (left) and 15 mNm (right), was amorphous polymer in the bulk before and after the phase transition respectively (5 °C). state. These monolayers exhibited low collapsed surface pressure. This tendency was similar to the results of Poly-FFnEA monolayers. On the other hand, the Poly-FnMAs exhibited the phase transition and the higher collapsed surface pressure together with a very small molecular area. In the AFM observation of Poly-F6A monolayers before and after the phase transition at about 10 mNm''(Fig. 3), the difference of height and roughness for films was confirmed. At the lower surface pressure region, Poly-F6A monolayers on mica formed relatively flat surface, while at the higher surface pressure region, the large domain structure was formed. This result indicated the formation of a multilayer on the water surface, that is, collapsed monolayer which piled up the polymer monolayer after the phase transition. These fluorinated comb-polymers were hard to dissolve the any organic solvents, except of trifluoroacetic acid. However, it is important to discuss the difference of molecular weights for these polymers in order to consider the any effect caused by these differences. 3.2. Effect of intermolecular hydrogen bonds for polymers substituted hydrogen atom at the (D-position of fluorocarbons ^ The 7C-A isotherms for the monolayers of Poly-FioA, which has a hydrogen atom in the * co-position of fluorocarbon chains, were dependent on t z the spreading solution of trifluoroacetic acid | containing different hexane contents (Fig. 4). The^* trifluoroacetic acid as spread solvent is known having «g« ability of scission for the intermolecular hydrogen ^ bonds. If the volume of the poor solvent in the spreading solvent increased, the promotion of the • intermolecular hydrogen bonds would be remarkable. The results of 7i-A isotherms of poly-FioA, increase of Area/A^repeating unit'^ n-hexane content in spreading solvent, that is, Figure 4 The ic-A isotherms of promotion of the intermolecular hydrogen bonds, monolayers on water surface for brought to decrease the molecular area and collapsed Poly-FioA spread from trifluoroacetic surface pressure. On the other hand, the p-A acid solution containing different isotherms for the other compounds were independent n-hexane contents at 5 °C. on the hexane contents. The AFM images of the built-up film for Poly-FioA the indicated increase of the domain size together with hexane contents in the spreading solvent (Fig. 5). It was confirmed by the SEM images for the monolayers on silicon substrate. These effects of intermolecular hydrogen bonds were also supported by the shift of carbonyl band caused by formation of intermolecular hydrogen bonds in IR spectra for the multilayers built-up the

460

poly-FioA monolayers spread from trifluoroacetic acid solution with several n-hexane contents. 3.3. The friction Figure 5 The AFM images of Z-type monolayers for Poiy-FioA properties for Z-type spreading from trifluoroacetic acid solution containing different films for fluorinated hexane contents (CF3COOH : hexane = left; 1:0, center; 1:1, right,l:4). comb-polymers 1 • PolyFFioEA For the outermost I • PolyFFioEMA surface with the fluorocarbon chains, the friction I A F.oA A [ A PoIyF.oA ^' A 1 properties of these Z-type films could be also 1 • PolyF.oMA/ examined using atomic force microscopy by ,-' 1 M measuring frictional forces as a function of the ^••* 'A 1 'A applied load for each system (Fig. 6) [10]. In this measurements, the applied load to the Z-type 11 A*" films were increased every a few nN unit in 1.0 [im X 1.0 (jm scan scale, and the friction forces L 'A were measured up to the limit of the movement >"" for Z piezo in an atomic force microscopy 1 .^^ system. The difference of voltage, that is, the r^^ __.*•*• difference of FFM signals which mean the force iMUBKBES^mm^^mmi^ in order to modify the distortion or twisting of 0 5 10 1^ 20 probe during the scan for go and back in scan p.^^^ ^ ^^ p,^j ^ friction force versus area, was detected as a measure of friction force, applied load of the Z-type films for The inclination and intercept on vertical axis fluorinated comb-oolvmers on mica. could be considered a friction and attractive force between tip and film. A significant difference between the friction forces of poly-FFioEA and poly-FioA has been found, which can be ascribed to the friction properties of only the outermost surface of the Z-type films. The poly-FioA and poly-FioMA films have hydrogen in fluorocarbons, showing relatively large friction, and also having any interaction with the tip even at zero load. In comparison with the result of FioA monomer films, it was found that the effect of outer atoms on the friction properties was expected to be enhanced by the polymerization.

A

I ^ ^ ^ A M I ^ ^

REFERENCE 1. N. A. Plate and V. P. Shibaev, J. Polym. ScL Macromol. Rev,, 8(1974) 117. 2. J. Schneider, C. Erdelen, H. Ringsdorf and H. Rabolt, Macromolecules, 22(1989) 3475. 3. J. M. Rodriguez-Parada, M. kaku, and D. Y. Sogah, Madomolecules, 27(1994) 1571. 4. X. -d. Li, A. Aoki, and T. Miyashita, Langmuir, 12 (1996) 5444. 5. A. Fujimori, H. Saitoh and Y. Shibasaki, J. Therm. Anal and Calo., 57 (1999) 631. 6. A. Fujimori, H. Saitoh and Y Shibasaki, J. Polym. Sci. Polym. Chem.Ed., 37 (1999) 3845. 7. A. Fujimori, T. Araki and H. Nakahara, Langmuir, (2000) to be submmited. 8. K. Yamada, T. Nakano, and Y Okamoto, Polymer J., 30 (1998) 641. 9. H. J. Trunit, / Colloid Sci., 15 (1960) 1. 10. A. Fujimori, T Araki and H. Nakahara, Chem. Lett., N0.8 (2000) 898.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

461

Self-organization of amphiphilic diacetylenes in Langmuir-Blodgett films H. Tachibana", Y. Yamanaka^ H. Sakai^ M. Abe^ and M. Matsumoto* ''National Institute of Materials and Chemical Research, Tsukuba 305-8565, Japan ''Science University of Tokyo, Noda 278-8510, Japan In 1:1 mixed Langmuir-Blodgett films of an amphiphilic diacetylene (10,12pentacosadiynoic acid: DA) complexed with polyallylamine (PAA) and neutral form of DA without PAA, spontaneous morphological changes were observed after kept for several days at room temperature. We have demonstrated that the changes are induced by self-organization of DA molecules of the neutral form in the mixed LB film. 1. INTRODUCTION Microscopic structures and macroscopic morphologies of crystals, thin films, bilayers, and self-assembly have been observed using various technologies such as scanning probe microscopies (SPMs) [1,2]. Recently, studies of the structure-property relationship have received considerable attention from the view point of achieving a fine-tuning of electronic and optical properties. Interesting photoelectronic properties have been developed by introducing butadiyne groups into the molecules [3-5]. It is well known that thin films with structures defined at molecular level can be fabricated using Langmuir-Blodgett (LB) techniques [6]. Polyion complex method coupled with the LB technique is promising candidates as a method of simple modifications of molecular structures in LB films, because the functional LB films can be designed easily by varying the combination of amphiphilic molecules and water-soluble polymers such as polycations and polyanions [7]. We have reported that chromic reaction [8], morphology [9], and orientation [10] can be controlled by cis-trans photoisomerization of azobenzene in the polyion complex LB films. In this paper, we report a spontaneous self-organization of amphiphilic diacetylene molecules in LB films fabricated by the polyion complex method. 2. EXPERIMENTS 10,12-Pentacosadiynoic acid (DA) and polyallylamine (PAA; Mw=lxlO^) were used as an amphiphilic diacetylene and a water-soluble polymer, respectively. Surface pressure-area isotherms were measured using a Lauda films balance at 17 °C. Chloroform solution of DA was spread onto the subphase containing 0.01 mM of PAA. The monolayers were transferred at 25 mNm'^ onto solid substrates using vertical dipping method. A Seiko SPA300 atomic force microscope, operating a noncontact

462

mode, was employed to image the morphology of the single-layer LB film on mica. The AFM image was obtained at line scan rates of 1 Hz using silicon cantilevers with a resonance frequency of 28 kHz and a spring constant of 1.9 Nm '. 3. RESULTS AND DISCUSSION We investigated the effect of water-soluble polymer, PAA, on the structures in LB films by measurements ^as(COO-) 1 of FT-IR spectra. Figure 1 shows the D(C=0) transmission IR spectra in the 10-layer LB films. Vibrational modes of the carbonyl group are sensitive to the local bonding geometry of COOH and COO o c groups [11,12]. The IR spectra show an \\ absorption peak due to the asymmetric O (vjCOO)) stretching vibrations bands (A at 1537 cm'. In addition, the C=0 < stretching vibration (v(C=0)) due to the carboxylic acid was observed at 1693 cm'. The COO stretching vibration 1 1 bands are caused by the salt formation 2000 1800 1600 1400 1200 1000 correlated with an acid-base interaction Wavenumber / cm'^ between the carboxylic acid of DA and Fig. 11. Transmission FT-IR spectra of 1 1 the amine of PAA. The protonation 10-layer DA/PAA complex LB films. from the carboxylic acid to the amine was confirmed by measurements of the XPS spectra. The result is shown in Figure 2. The XPS spectra show only an intense peak at 401.1 eV, which is assigned to the +1 state of N Is emission originating from PAA [13,14]. No peaks due to the neutral state of N Is are observed in the lower energy region. From the analysis of the integrated area of two peaks due to the N* Is and C Is, the component ratio of DA to PAA was 2. These results indicate that a half of DA molecules form the salt complex with PAA (DA/PAA' complex) and the other is the neutral form without PAA (DA^ in the LB films. This is consistent 396 402 400 with the result of IR spectra. Energy / eV We measured the morphology of Fig. 2. XPS spectra of lO-Iayer single-layer 1:1 mixed LB films of DA DA/PAA complex LB films. /PAA' complex and DA° using atomic

X

CO

J

VXz

463 force microscopy (AFM). The surface of the as-deposited LB films was flat though a few irregular structures were observed on the surface. The morphology changed spontaneously while kept the LB films in the dark for several days at room temperature. Figure 3 shows the AFM image in wide rage (100 ^im x 100 \im) after kept in the dark for 7 days. Large three-dimensional structures with few ten microns in both width and length, and with 20-30 nm in height were observed in the AFM image. When following the morphological changes in the same regions, small threedimensional structures appear on the surface, grow up with increasing the Fig. 3. AFM images (100 ^m x 100 [im) after length and the width while kept in the kept single-layer DA/PAA complex LB films dark, and finally become a constant in the dark for 7 days at room temperature. size. Such morphological changes were not observed in the 1:1 mixed LB films of arachidic acid (C20) complexed with PAA and C20 of neutral state without PAA. This result indicates that the butadiyne groups play an important role in the formation of the three-dimensional structures. The structural change accompanied with the morphological change was investigated by measurements of FT-IR reflection-absorption (RA) spectra. Figure 4 shows the RA spectra in single-layer DA/PAA complex LB films after as-deposition and after kept in the dark for 15 days. The RA spectra show drastical as-deposition a(CH2l changes accompanied with the morphological changes. One is that v(C=0) the intensity of the C=0 stretching band becomes strong remarkably, which is caused by the crystallization of DA° molecules in the 1:1 mixed LB films of DA/PAA' complex and DA®. The other is that the position of two bands assigned to CH. antisymmetric (V3(CH2)) and symmetric (V5(CH2)) stretching band shifts to lower wavenumber with the -L -SSr1600 2800 1800 3000 increase in the intensity, indicating Wavenumber / cm"' gauche-to-trans conformational changes of alkyl chains [15]. The Fig. 4. FT-IR reflectance-absorption spectra morphological change is coupled after as-deposition and after 15 days in single-layer DA/PAA complex LB films. with the disorder-order transition of

464

alkyl chains. From these results, we consider that the three-dimensional structures are formed by rearrangement of the DA^ molecules in the 1:1 mixed LB films of DA7PAA* complex and DA^. This should be responsible both for disorder-order transition of the alkyl chain and the K-K interaction between butadiyne groups of DA° molecules, resulting in the morphological changes. We investigated the morphological changes in LB films fabricated on varying the PAA concentration into the subphase. No spontaneous morphological changes are observed in DA7PAA* complex LB films without DA° of neutral state, but similar three-dimensional structures are observed in as-deposited LB films of only DA° fabricated on pure water. These results support above mechanism that the three-dimensional structures consist of self-organization of the DA° molecules in the 1:1 mixed LB films of DA /PAA* complex and DA^ 4. CONCLUSION Spontaneous morphological changes were observed after kept 1:1 mixed LB films of DA PAA* complex and DA° in the dark for several days at the room temperature, which is caused by the rearrangement of the DA° molecules. The DA° molecules are forced to mix with DA7PAA* complex in the as-deposited LB film are driven to selforganize spontaneously. REFERENCES 1. J. Frommer, Angew. Chem. Ind. Ed. Engl., 31 (1992) 1298. 2. H. -J. Guntherodt and R. Wiesendanger (eds.), Scanning Tunneling Microscopy I, Spring-Verlag, Germany, 1994. 3. M. Masuda, T. Hanada, K. Yase, and T. Shimizu, Macromlecules, 31 (1998) 9403. 4. J. Y. Chang, J. H. Baik, C. B. Lee, M. J. Han, and S. -K. Hong, J. Am. Chem. Soc, 119(1997)3197. 5. N. Tamaoki, G. Kruk, and H. Matsuda, J. Mater. Chem., 9 (1999) 2381. 6. M. C. Petty (ed), Langmuir-Bldogett Films: An Introduction, Cambridge University, New York, 1996. 7. M. Shimomura, Prog. Polym. Sci., 18 (1993) 295. 8. H. Tachibana, R. Azumi, M. Tanaka, M. Matsumoto, S. Sako, H. Sakai, M. Abe, Y. Kondo, and N. Yoshino, Thin Solid Films, 284 (1996) 73. 9. M. Matsumoto, D. Miyazaki, M. Tanaka, R. Azumi, E. Manda, Y. Kondo, N. Yoshino, and H. Tachibana, J. Am. Chem. Soc., 120 (1998) 1479. 10. H. Tachibana, N. Yoshino, and M. Matsumoto, Chem. Lett., (2000) 240. 11. J. F. Rabolt, F. C. Bums, N. E. Schlotter, and J. D. Swalen, J. Chem. Phys., 78 (1983) 946. 12. F. Kimura, J. Umemura, and T. Takenaka, Langmuir, 2 (1986) 96. 13. M. Shimomura and T. Kunitake, Thin Solid Films, 132 (1985) 243. 14. B. Zhao, K. G. Neoh, F. T. Liu, E. T. Kang, and K. L. Tan, Ungmuir, 15 (1999) 8259. 15. C. Naselli, J. F. Rabolt, and J. D. Swalen, J. Chem. Phys. 82 (1985) 2136.

Studies in Surface Science and Catalysis 132 Y. hvasawa, N. Oyama and H. Kunieda (Editors) c 2{)()1 Elsevier Science B.V. All rights reserved.

465

A Novel Understanding of Infrared Spectra of Langmuir-Blodgett Films by Factor Analysis T. Hasegawa^, J. Nishijo* and J. Umemura*' *Kobe Pharmaceutical University, Motoyama-kita, Higashinada-ku, Kobe 658-8558, Japan ''Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto-fu 611-0011, Japan A minute spectrum hidden in infrared spectra of a Langmuir-Blodgett (LB) film was readily resolved by use of principal component analysis that is a factor-analytical technique. The resolved spectrum suggested that minute amount of water remained even after the drying of the LB film, and they combined to a part of the head group of the phospholipid molecule. 1. INTRODUCTION Infrared spectrometry is a highly sensitive tool to investigate molecular packing, ordering, and orientation in organic ultrathin films represented by Langmuir-Blodgett (LB) film. Nevertheless, infrared spectra comprise a number of bands that are arisen from many group vibrations in the molecular aggregates, which often makes the discussion of the spectra very difficult. When some bands are overlaid on each other, in particular, it is almost impossible to draw fine information from the spectra. In this case, thus far, some analytical techniques [1] have been employed to the complicated spectra such as, second derivative, Fourier self-deconvolution, subtraction and two-dimensional correlation analyses. Regardless, it is still difficult to resolve a spectrum of minute chemical species from that of other dominant components. In the present study, an intrinsic characteristic of principal component analysis (PCA) is demonstrated to have a powerful property, which resolves a spectrum of a minute chemical species from a collection of infrared reflection-absorption (IRRA) spectra [2] of an LB film deposited on a gold-evaporated glass slide. The new characteristic of the spectral resolution was found to be more powerful than the asynchronous map of two-dimensional correlation analysis [3], since it is impervious to spectral noise. In the present study, the mechanism of molecular interaction of L-a-dipalmitoyl-

466

phosphatidylcholine (DPPC) monolayer with sucrose was investigated by the new characteristic of PCA. Minute water molecules that play a key role in the interaction were readily detected as an infrared spectrum. 2. MATERIAL AND METHODS An extrapure-grade reagent of alkyl-deuterated DPPC (DPPC-^62, >99%) was purchased from Avanti Polar-Lipids (Alabaster, AL), and it was used without further purification. Other materials are described in detail in a previous report [4]. The preparation of Langmuir and LB films of thermally-pretreated DPPC-d62 films associated with sucrose was performed as described previously [4]. PCA calculations of simulated and observed spectra were performed with the MathWorks (Natick, MA) MATLAB software ver. 4.0 by use of Chemometric Toolbox purchased from Applied Chemometrics (Sharon, MA). 3. Results and Discussion 3.1 Theoretical simulations When a spectrum consists of A^ wavenumber points, the spectrum can be expressed as a vector, a = (AI, ai, ..., UN), which can be considered a point in A^-dimensional space. When this spectrum increases in intensity without changing its shape, the point would move in the space along a line that connects the origin and point, a, since every vector component would change proportionally. In this manner, we can consider that the direction from the origin to a point in space corresponds to spectral shape, and the length (norm) between the origin and the point corresponds to the intensity of the spectrum. This is a key concept to understand the multivariate analysis of spectra. On the other hand, what would happen if the spectrum changes its shape? When the spectrum changes its shape, it means that the chemical species is not a single component, but multi-components species. In this case, the point move in the space becomes different from that for the single-component spectrum. The points plotted in the space scatter along a line that does not go through the origin. This implies a very important characteristic of spectral change expressed in multi-dimensional space: the least-squares (LS) line to the plot does not go through the origin, while the first eigenvector connects the origin and the average point of the scattered plot. The first eigenvector corresponds to the first principal component spectrum yielded by PCA. In other words, when the observed spectra are arisen from a single component, the first PCA solution would be identical to the LS solution, but it would greatly differ when the spectra are derived from multi-component

467 chemical species.

Since the direction from the origin corresponds to the spectral shape, a

LS solution would give a chemically understandable spectrum.

In a multi-component

system, however, PCA solution would not give a chemically understandable spectrum, since its direction is different from the LS solution.

As a result, it is considered that PCA

itself cannot be directly used for the spectral separation for the mixture spectra. Nevertheless, the situation would change greatly if the concentration ratio becomes very high.

For example, let us consider that one of the two chemical constituents has very

minute concentration in comparison to the other constituent.

Therefore, the minute

component would not affect the spectral change, even if both constituents change their concentrations.

In this case, the first PCA solution is expected to give an almost identical

solution to the LS solution to the same plot.

This means that PCA would resolve a

spectrum of the minute chemical component from the mixture spectra. To confirm this mechanism, simulation spectra were generated, and they were subjected to PCA.

Two spectra that individually comprise two bands were prepared, and

one of the spectra was made a very small spectrum. the smaller one was about 100.

The ratio of the larger spectrum to

The band locations of the two bands in the larger

spectrum were 80 and 120, and those in the smaller one were 90 and 130 channels, respectively.

Random noise was also added to the synthesized spectra, in which the

smaller spectrum and the noise were invisible due to the small intensities. The first three abstract spectra by PCA

that are representations

of

the

eigenvectors are overlaid in Figure 1. The first two abstracts (thick solid and dotted lines) correspond well to the two source spectra used for the simulation. This proves that even the minute-intensity spectrum

is

readily

resolved

characteristic of PCA as expected.

by the Since

the third abstract spectrum presents noise only, PCA is found to economically draw the information of the minute spectrum, although comparative noise was added to the spectra.

In this manner, the spectral

resolution by PCA has a powerful property that it is impervious to noise.

100 Channel Fig.l Resolved spectra by PCA.

468 3.2 Experimental Application DPPC-J62 monolayers prepared on various concentrations of sucrose aqueous solution were transferred onto a gold-evaporated glass slide by the vertical-dipping method. Significant changes in the IRRA spectra (left panel of Figure 2) of them were governed by

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Fig. 2 IRRA spectra and third loading, the concentration changes of sucrose, and minute information about the molecular interaction between phospholipids and sucrose was not observed in the raw spectra. The spectra were subjected to PCA. The first abstract spectrum presented a typical spectrum of sucrose, as expected. The second one gave a spectrum of DPPC-^62 and incorporated sucrose molecules in the monolayer. This means that the incorporated sucrose is readily discriminated from bulk sucrose without using probe molecules. The third abstract spectrum (right panel of Figure 2) provided, on the other hand, a spectrum of minute water molecules remained in the dried LB film. It was also found that it strongly bound the PO2" group in the phospholipid via strong hydrogen bonding. It is the first time to observe the spectrum of minute water molecules, which is hidden in the raw spectra. REFERENCES 1. Peter R. Griffiths and James A. de Haseth, "Fourier Transform Infrared Spectrometry" (Wiley Interscience, New York, 1986). 2. T. Takenaka and J. Umemura, "Vibrational Spectra and Structure(J. R. During ed.)" (Elsevier Science, Amsterdam, 1991). 3. I. Noda: Appl Spectrosc, 47,1329 (1993). 4. T. Hasegawa, H. Kawato, M. Toudou and J. Nishijo, J. Phys. Chem. B, 101 (1997) 6701.

Studies in Surface Science and Catalysis 132 Y. Iwasavva, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

469

Chemical force microscopies by friction and adhesion using chemically modified atomic force microscope (AFM) tips M. Fujihira,* Y. Tani, M. Furugori, Y. Okabe, U. Akiba, K. Yagi, and S. Okamoto Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan Dependence of friction and adhesion on surface chemical species on samples has been studied by friction force microscopy and pulsed-force-mode (PFM) AFM, respectively, using chemically modified atomic force microscope (AFM) tips. We concentrated on i) how to modify gold coated AFM tips reproducibly with thiol compounds, ii) examination of chemical recognition of Si(lOO) surfaces partially covered with hydrocarbon monolayer domains using the modified tips, and iii) dependence of friction and adhesion on composition of phaseseparated domains in mixed hydrocarbon and fluorocarijon Langmuir-Blodgett films. These films were also studied with scanning surface potential microscopy, which provides information on permanent dipole moments of the terminal groups. Chemical force microscopy of self-assembeld monolayers prepared by microcontact printing was also studied by PFM-AFM. 1. INTRODUCTION Recently, friction at a sliding interface between organic monolayers such as LangmuirBlodgett (LB) films and self-assembled monolayers (SAMs) has been studied experimentally by friction force microscopes (FFM) [1-3] and surface force apparatuses (SFA) [4]. The difference in friction between hydrocarbon (HC) and fluorocarbon (FC) [5,6] and among monolayers of alkyl chains having different terminal groups [7-9] against atomic force microscope (AFM) tips with or without monolayers having different terminal groups has been observed. The chemical modification by silanization of oxide surfaces [10] or SAM formation on gold surfaces with thiol or disulfide compounds [11] has been utilized for the tip functionalization. The first systematic chemical derivatization of the tips was carried out with trichlorosilane [12,13]. Today, chemical differentiation of the terminal groups by FFM [5-9] or adhesive force measurements [12-14] is called chemical force microscopy (CFM) [7]. Adhesive and frictional forces can be mapped in x-y planes [15,16] as CFM images. The adhesive force mapping was performed by pulsed-force-mode AFM (PFM-AFM) [17-20]. Recently, we attempted to interpret the origin of the friction differences among the organic monolayers by comparing the MD simulation of the sliding friction between these monolayers with a corresponding simplified phenomenological simulation [21-25]. The dependence of adhesive forces between the sample and the AFM tip surfaces on the terminal functional groups was also studied using the simplified phenomenological simulation [23] and using PFM-AFM with chemically modified tips [15,16]. In the present paper, first, we will describe how we can modify gold-coated AFM tips reproducibly with thiol compounds. Then, dependence of friction on surface chemical species * To whom correspondence should be addressed (Email: [email protected]).

470

of oxidized Si(lOO) wafer surfaces partially covered with HC monolayer domains and dependence of friction on composition of phase-separated domains in mixed HC and FC LB films will be described. Finally, the observed frictional behavior will be compared with other properties of these organic monolayers. The methods used for the other properties were scanning surface potential microscopy (SSPM) and PFM-AFM. The former provides information on permanent dipole moments of the terminal groups, while the latter affords adhesive force maps measured between the terminal groups on the tip and sample surfaces. In particular, contrast changes in FFM images and adhesive force maps by mixing of two components in phase-separated LB films and patterned SAMs will be discussed. 2. EXPERIMENTAL Chemical modification of AFM tips was performed by the previous method [15] with a little modification. The chemical modification of AFM tips just after vapor deposition of gold in an Olympus factory and the modification of the commercially available gold-coated tips after cleaning in our laboratory gave almost the same results for chemical differentiation of surface chemical species [15]. Therefore, chemically modified AFM tips were prepared from Olympus Si3N4 rectangular cantilevers OMCL-RC800PB-1 (0.73 N m' or 0.38 N m"^) with a sharpened pyramidal tip according to the following procedure. The commercially available tips were already coated with Cr/Au films when we received them. The tips were first cleaned using an ozone cleaner (Nippon Laser & Electronics Lab.) for 30 min and, immediately after the oxidation step, the gold-coated tips were immersed in pure hot ethanol at ca. 65 °C for 30 min for complete reduction of surface gold oxides. Further chemical modification was performed by immersing the cleaned gold tips into a 1 mM 1-decanethiol (CH3(CH2)9SH) or 11-mercaptoundecanoic acid (HOOC(CH2)ioSH) ethanol solution for 24 h at room temperature immediately after the two-step cleaning procedure. Finally, the modified tips were rinsed thoroughly by pure ethanol and dried under a stream of nitrogen. The chemically modified tips with CH3(CH2)9SH and HOOC(CH2)ioSH will be called hereafter CHj-tips and COOH-tips, respectively. The chemically modified sample surface with pattern was prepared by the following procedure. Firstly, a phase-separated mixed monolayer was formed by spreading a mixed chloroform solution of a carboxylic acid with a partially fluorinated carbon (C9F19C2H4OC2H4COOH, PFECA) and octadecyltrichlorosilane (C,gH37SiCl3, OTS) and by compressing a mixed monolayer at an air-water interface in a Langmuir trough. Secondly, the resulting phase-separated monolayer was deposited on an oxidized Si(lll) wafer by the LB method followed by heating at 80 °C overnight and then ultrasonication in ethanol. The bare oxidized Si surface was exposed by removing the physically adsorbed PFECA domains, but the chemically bound OTS monolayer domains remained on the Si surface even after ultrasonication. In this way, an oxidized Si wafer surface partially covered with HC monolayer domains was prepared. The sample thus prepared was used for comparison between friction force and adhesive force mapping by FFM and PFM-AFM, respectively. A stamp for microcontact printing (ji-CP) was fabricated by the previous method [16]. The height of the convexities of the stamp was determined to be ca. 0.4 |jim from a cross section of the AFM image. The gold-coated substrates were prepared by the following method. Micro cover glass plates obtained from Matsunami Glass Industry were cleaned by the same ozone cleaning for 30 min as that used for the AFM tips described above. Gold films

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Fig. 1 Preparation procedures of patterned samples with (a) the wet-inking and (b) the contact-inking method, followed by further chemical modification of the unprinted area with a 1 mM HOOC(CH2),oSH ethanol solution for ca. 5 min. (ca. 11 nm) were deposited on the cleaned cover glass plates with a Hitachi E-1030 ion sputter using an argon plasma at about 6 Pa, 15 mA with a deposition rate of about 11 nm min'\ The patterned SAM samples were prepared by the procedures illustrated in Fig. 1. As the first step, two types of (bi-CP methods [26] were used to make stamped patterns on the goldcoated substtates. One is called the wet-inking method, in which inking of the stamp was done by placing a thiol ethanol solution for 30 s and then removing the excess solution under a stream of nitrogen. The ethanol solution v^th a thiol concentration of 0.1 mM was examined as the inking solution. 1-Alkane thiols CH3(CH2)nSH with different chainlengths of n = 11, 15, and 19 were examined as the inking thiol compounds. The other is called the contact-inking method, in which an inker pad (ca. 3x15x15 mm^) made of polydi methyl si loxane (PDMS) was dipped overnight in a 0.1 mM thiol ethanol solution, then the excess solution was removed by a stream of nitrogen, and finally the stamp was placed for 30 s on the inker pad impregnated with the thiol solution. The second step for pattern formation was the same for the two methods and the gold surfaces patterned with alkane thiols by ^-CP were further reacted with HOOC(CH2)ioSH in a 1 mM ethanol solution for ca. 5 min. The resulting patterns with CH3- and COOH- terminated regions were analyzed by imaging the adhesive forces in an aqueous solution with the chemically modified gold coated AFM tips with a SAM of CH3 terminal functional groups. All measurements of AFM, FFM, and the adhesive force mapping with PFM-AFM were performed by a commercial AFM (a Seiko Instruments SPA 300 AFM unit with an SPI-3700 AFM controller) and a PFM box [19,20]. The cantilever bending forces [27] for FFM and PFM-AFM was ca. 10 nN and 3 nN, respectively. The frequency and the amplitude for PFMAFM were 0.5-1 kHz and 30-70 nm, respectively. Relative humidity of measuring air atmospheres was 50-60 %. The adhesive force mappings in water were performed in a 0.1 mM NaHCOj aqueous solution at room temperature. This slightly basic aqueous solution was used to confirm dissociation of the surface COOH terminal groups [28]. Brighter contrast in friction and adhesive force maps corresponds to higher friction and adhesive forces. SSPM was carried out as previously [29].

472

3. RESULTS AND DISCUSSION 3.1. CFM of patterned surface prepared by the LB method In Fig. 2 are shown 5x5 jun^ (256x128) images of (a) a topography and (b) a friction force map of an oxidized Si wafer partially covered with polymerized OTS monolayer domains prepared by the LB method described above. These two images were taken simultaneously by contact-mode AFM using a CH3 tip. The height of the HC domains in Fig. 2(a) was determined to be ca. 2.2 nm by AFM. The AFM result indicated that the HC layer was a polymerized OTS monolayer and the lower surface was the bare oxidized Si surface. In Fig. 2(b), the much higher friction was observed on the bare oxidized Si surface than on the HC domains. As discussed previously [27], the contrast in the friction map can be attributed to the change in the effective normal loads rather than difference in the intrinsic friction forces between these two domains with different chemical species. In other words, a hydrophilic surface on the oxidized Si substrate was covered with a water film in the ambient atmosphere and water capillary forces on this region would increase the effective normal load [27].

Fig. 2 5x5 \im^ (256x128) images of (a) a topography and (b) a friction force mapping, simultaneously obtained by contact-mode AFM using a CH3 tip in air for a patterned sample prepared with the LB method.

(c) Fig. 3 5x5 [im^ (256x128) images of (a) a topography and (b) an adhesive force map, simultaneously obtained by PFM-AFM, and (c) a histogram of the adhesive forces measured in Fig. 3(b), using the same CH3 tip on the same patterned area as that observed in Fig. 2.

473

To do adhesive force mapping, we also performed PFM-AFM on the same area using the same CH3 AFM tip. Figure 3 shows 5x5 ^m^ (256x128) images of (a) a topography and (b) an adhesive force map taken simultaneously by PFM-AFM. Figure 3(c) is a histogram of the adhesive forces measured in Fig. 3(b). Here, the histogram shows the adhesive force versus the number of times, with which this force was observed in 32,768 (256x128) times adhesive force measurements during the adhesive force mapping. In PFM-AFM, the z-position of the sample is controlled so as to keep the peak signal constant using sample and hold circuits and a feed back loop in AFM, which is used to produce a topographic image. The cantilever bending forces for the peak signal of the PFM images shown in Figs. 3(a) and 3(b) was ca. 3 nN and the frequency and the amplitude for the images were 1 kHz and 70 nm, respectively. The height of the HC domains in Fig. 3(a) was found to be the same as that observed in the contact mode AFM topography in Fig. 2(a) with a cantilever bending force of ca. 10 nN. The adhesive force map shown in Fig. 3(b) gave also a clear contrast between the HC and the bare oxidized Si domain. As we would expect, the higher adhesive forces were observed on the hydrophilic oxidized Si surface than on the hydrophobic HC domain. From the histogram shown in Fig. 3(c), average values of the adhesive forces on the Si and the HC surface were found to be ca. 3.9 and 1.6 nN, respectively. As described above, it was found that CFM by mapping the friction and the adhesive force with FFM and PFM-AFM, respectively, is useful for the patterned sample of the oxidized Si wafer partially covered with polymerized OTS monolayer domains. The similar measurements were also carried out using a COOH tip. In both images of FFM and PFM-AFM, the much higher contrast was observed between these two domains using this hydrophilic tip. The water capillary force is again considered as the main origin of the contrast obtained with this tip. 3.2. Friction force map in phase separated HC-FC mixed LB films with Ca^^ counter ions In our first report of FFM of phase separated HC and FC mixed monolayers, we used cationic polymers as the counter ions of Langmuir mixed monolayers of carboxylates with HC and FC long chains [5,6]. The phase separated mixed monolayers was, however, found to be "on-top" structure rather than conventional "side-by-side" structure [29]. One of the factors

(a)

(b)

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Fig. 4 8x8 jim^ (256x128) images of (a) an AFM topography and (b) an FFM friction, observed simultaneously by contact-mode AFM, and (c) a surface potential mapping by SSPM of the same area of a mixed monolayer of behenic acid and perfluorotetradecanoic acid with a molar ratio of 2:1 using a gold-coated tip under nitrogen.

474

giving unusual two story structure was considered to be due to an amphiphilic property of cationic polymers. Therefore, we attempted to form phase-separated HC and FC mixed monolayers with "side-by-side" structure using hydrophilic counter ions such as Ca^^ ions. Figures 4(a) and 4(b) show 8x8 jim^ images of a topography and a fiction force map of a phase-separated mixed monolayer with a molar ratio of behenic acid (C21H43COOH) to perfluorotetradecanoic acid (C,3F27COOH) of 2:1. The subphase was water from a Milli-Q system containing 3 mM CaClj and 0.1 mM NaOH (pH 9-10). The resulting mixed monolayers exhibited "side-by-side" structure, as we would expect. From comparison of the FFM image shown in Fig. 4(b) with previous HC and FC patterned samples using silanization reagents [30], we could conclude safely that the frictional contrast of the present FFM images of the "side-by-side" monolayer was surprisingly low. The possible explanation of the low contrast is that compositions of the separated two phases can be similar each other. To confirm the idea, we also observed a surface potential map of the same area with SSPM [31]. The image of the surface potentials is shown in Fig. 4(c). It is known from the surface potential measurements of single component films [29] that difference in the surface potentials between the pure FC and the pure HC domains is expected to be ca. 1.3 V. The potential difference observed in Fig. 4(c) was found to be only 0.2 V. The result clearly supports the idea that the low contrast in FFM image reflects similar compositions of the separated two phases. In this way, simultaneous observation of FFM and SSPM images of the same samples is useful to chemical recognition of the phase-separated domains. As the other useful combinations of two different methods, we already demonstrated that combination of scanning near-field optical microscopy (SNOM) with FFM [32] or SSPM [33] is a powerful tool for chemical recognition with lateral resolutions of submicron scales. 3.3. CFM of patterned surfaces prepared by ji-CP To develop a new method, by which we can study mixing of two components in each domain in the patterned surfaces, we studied CFM of patterned surfaces prepared by fi-CP methods. Figure 5 shows comparison between the patterned samples prepared by two different (i-CP methods, i.e. the wet-inking and the contact-inking method [26]. All adhesive force maps in Fig. 5 were observed in a 0.1 mM NaHC03 aqueous solution using a CHj-tip on three samples. The z-piezo was modulated sinusoidally at a frequency of 0.5 kHz with amplitudes of ca. 30-70 nm. Figures 5(a) and 5(b) are 10x10 juun' adhesive force maps of the samples prepared by the contact-inking method using inker pads impregnated with 0.1 mM ethanol solutions of 1-hexadecanethiol and 1-octanethiol, respectively. For comparison, in Fig. 5(c) is shown an image of adhesive forces observed on a sample prepared by the wet-inking method with 0.1 mM 1-hexadecanethiol ethanol solution. The latter sample was prepared with 30 s for both the inking and the stamping time. Although the conditions, such as the solution concentration for impregnation and the times for inking and stamping, have not been optimized yet, the contrast of the adhesive forces was observed on the patterned samples prepared by the contact-inking method as shown in Figs. 5(a) and 5(b). We are now under investigation for the optimized conditions. It is the most interesting to note that widths observed on the samples prepared by the contact-inking method were 1.0 |jim for both cases using inker pads impregnated with 1-hexadecanethiol and 1-octanethiol as shown in Figs. 5(a) and 5(b), respectively, while the width for the sample prepared by the wet-inking method was 1.25 jxm as shown in Fig. 5(c). In terms of the spatial resolution of the printed pattern, it was found clearly that the contact-inking method was much better than the wet-inking method. As

475

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Fig. 5 10x10 |xm^ (256x128) CFM images of patterned samples prepared by the contactinking method using a 0.1 mM ethanol solution of 1-alkanethiol CH3(CH2)nSH with different chainlength (a) n = 15 and (b) n = 7, respectively, and by the wet-inking method with (c) n = 15, followed by the further reaction with HOOC(CH2),oSH. The inking only on faces of the convexities is possible by the contact-inking method, which prevents any noticeable spread of thiols from the points of contact. to the defect density on the printed area, a further study will be necessary for the optimization of the contact-inking method, because the defect density seemed to be higher for the samples prepared by the contact-inking method than those by the wet-inking method when the same thiol concentration was used for impregnation and for wet-inking. This was concluded from less clear peak separation in their histograms of the adhesive force mappings on the patterned samples prepared with the contact-inking method than the wet-inking method. In other words, mixed SAMs were formed over the printed area through the further chemical modification with HOOC(CH2)ioSH due to the much higher defect concentration in the printed area with the contact-inking method. In the case of the wet-inking method, the contamination via vapor phase transfer can be expected to increase with the decrease in the chainlength of alkane thiols due to the increase in their vapor pressures [34]. Therefore, in addition to the sample used for Fig. 5(c), other samples were prepared using 1-alkanethiols, CH3(CH2)„SH with different chainlengths of n = 11 and n = 19 as inks in the wet-inking method. It is clear from these adhesive force maps that the contrast of the adhesive force map becomes better as the chain length increases. The improvement of the contrast can be interpreted by the less contamination of the unprinted area by vapor phase transfer. In other words, the adhesive force maps can be used to study the degree of contamination, which has been hard to investigate by other methods. To express the degree of contamination more quantitatively, we made such histograms from the adhesive force maps as shown in Fig. 3(c). It was found from the histograms (not shown) that average adhesive forces in the CH3 and the COOH terminated regions could be obtained readily and that the values were always higher in the CH3 regions than in the COOH regions. In addition, it was also found that the two peaks corresponding to the high and the low adhesive forces on the CH3 and the COOH region, respectively, separate more clearly with the increase in the chainlength. In other words, the unprinted regions were covered with a mixed SAM of HOOC(CH2)ioSH and CH3(CH2)„SH and the concentration of 1-alkanethiol in the mixed SAMs increased v^th the decrease in the chainlength.

476 REFERENCES 1. CM. Mate, G.M. McClelland, R. Erlandsson, S. Chiang, Phys. Rev. Lett. 59 (1987) 1942. 2. G. Meyer and N.M. Amer, Appl. Phys. Lett. 57 (1990) 2089. 3. O. Marti, J. Colchero, J. Mlynek, Nanotechnology 1 (1990) 141. 4. J.N. Israelachvili, P.M. McGuiggan, A.M. Homola, Science 240 (1988) 189. 5. E. Meyer, R. Ovemey, R. Luthi, D. Brodbeck, L. Howald, J. Frommer, H.-J. Guntherodt, M. Fujihira, H. Takano, Y. Gotoh, Thin Solid Films 220 (1992) 132. 6. R. Ovemey, E. Meyer, J. Frommer, D. Brodweck, L. Howald, H.-J. Guntherodt, M. Fujihira, H. Takano, Y. Gotoh, Nature 359 (1992) 133. 7. CD. Frisbie, LF. Rozsnyai, A. Noy, M.S. Wrighton, CM. Lieber, Science 265 (1994) 2071. 8. J.-B.D. Green, M.T. McDermott, M.D. Porter, L.M. Siperko, J. Phys. Chem. 99 (1995) 10960. 9. J.L. Wilbur, H A Biebuyck, J.C MacDonald, G.M. Whitesides, Langmuir 11 (1995) 825. 10. R. Murry, Molecular Design of Electrode Surfaces, in: A Weissberger (Ed), Techniques of Chemistry, Vol. 22,Wiley, New York, 1992. 11. J. Tien, Y. Xia, and G.M. Whitesides, in: A. Ulman (Ed), Self-Assembled Monolayers of Thiols, Thin Films, Vol.24, Academic Press, San Diego, 1998, p. 227. 12. T. Nakagawa, K. Ogawa, T. Kurumizawa, S. Ozaki, Jpn. J. Appl. Phys. 32 (1993) L294. 13. T. Nakagawa, K. Ogawa, and T. Kurumizawa, J. Vac. Sci. Technol. B12 (1994) 2215. 14. A. Noy, CD. Frisbie, LF. Ronzsnyai, M.S. Wrighton, C M. Lieber, J. Am. Chem. Soc. 117(1995)7943. 15. M. Fujihira, Y. Okabe, Y. Tani, M. Furugori, U. Akiba, Ultramicroscopy 82 (2000) 181. 16. Y. Okabe, M. Furugori, Y. Tani, U. Akiba, M. Fujihira, Ultramicroscopy 82 (2000) 203. 17. A. Rosa, E. Weilandt, S. Hild, O. Marti, Meas. Sci. Technol. 8 (1997) 1. 18. Th. Stifter, E. Weilandt, O. Marti, S. Hild, Appl. Phys. A66 (1998) S597. 19. T. Miyatani, M. Horii, A. Rosa, M. Fujihira, O. Marti, Appl. Phys. Lett. 71 (1997) 2632. 20. T. Miyatani, S. Okamoto, A. Rosa, O. Marti, M. Fujihira, Appl. Phys. A66 (1998) S349. 21. T. Ohzono, J. N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 37 (1998) 6335. 22. T. Ohzono, J. N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 38 (1999) L675. 23. M. Fujihira and T. Ohzono, Jpn. J. Appl. Phys. 38 (1999) 3918. 24. T. Ohzono and M. Fujihira, Jpn. J. Appl. Phys. 39 (2000) 6029. 25. T. Ohzono and M. Fujihira, Phys. Rev. B, in press. 26. L. Libioulle, A. Bietsch, H. Schmid, B. Michel, E. Delamarche, Langmuir 15 (1999) 300. 27. M. Fujihira, D. Aoki, Y. Okabe, H. Takano, H. Hokari, J. Frommer, Y. Nagatani, F. Sakai, Chem. Lett. (1996)499. 28. D.V Vezenov, A. Noy, LF. Rozsnyai, CM. Lieber, J. Am. Chem. Soc. 119 (1997) 2006. 29. M. Fujihira, Annu. Rev. Mater. Sci. 29 (1999) 353. 30. M. Fujihira and Y. Morita, J. Vac. Sci. Tech. B 12 (1994) 1609. 31. K. Yagi and M. Fujihira, unpublished results. 32. H. Muramatsu, N. Chiba, M. Fujihira, Appl. Phys. Lett. 71 (1997) 2061. 33. Y. Horiuchi, K. Yagi, T. Hosokawa, N. Yamamoto, H. Muramatsu, M. Fujihira, J. Microscopy 194 (1999) 467. 34. E. Delamarche, H. Schmid, A. Bietsch, N.B. Larsen, H. Rothuizen, B. Michel, H. Biebuyck, J. Pys. Chem. B 102 (1998) 3324.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyamaand H. Kunieda(Editors) >c) 2001 Elsevier Science B.V. All rights reserved.

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In Situ Adsorption Investigation of Hexadecyltrimethylammonium Chloride on Self-assembled Monolayers by Surface Plasmon Resonance and Surface Enhanced Infrared Absorption Spectroscopy Toyoko Imae,^'^* Tomohiro Takeshita,^ and Koichi Yahagi*' ^Research Center for Materials Science and ^Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan In situ adsorption kinetics of hexadecyltrimethylammonium chloride (C16TAC) on 1dodecanethiol and 3-mercapto-l-propanol self-assembled monolayers (SAM) was investigated by surface plasmon resonance and surface enhanced infrared absorption spectroscopy. Adsorption from a dilute solution on 1-dodecanethiol SAM proceeded slow after initial fast increase. Adsorption from solutions at medium concentrations converged to a constant value through a maximum at the initial stage. Adsorption at a high concentration above critical micelle concentration became constant without through a maximum. These behaviors indicate the existence of two types of adsorption; one is faster bulk adsorption (physisorption), where excess accumulation sometimes occurs, and another is slower adsorption (chemisorption) being accompanied with rearrangement Adsorption on 3-mercapto-l-propanol SAM was always slow over the wide CieTAC concentration regions. This implies that the adsorption mechanism of C16TAC depends on chemical structure of thiol. 1. INTRODUCTION Adsorption and desorption kinetics of molecules on solid substrates is important concerns in many applications and has been explored experimentally and theoretically [1,2]. High attension was for kinetic control of the preparation of self-assembled monolayer (SAM) with high quality, which is one of typical thin films with high ordered array and prepared by the spontaneous chemisorption of molecules onto the metal surface. The kinetics investigations of SAM were carried out by a quartz crystal microbalance (QCM) monitor method, surface plasmon resonance (SPR) spectroscopy, atomic force microscopy, ellipsometry, and attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRA) [13,4]. These techniques are applicable to in situ investigation of not only general adsorption and desorption kinetics of molecules on solid substrates but also chemical reaction kinetics of molecules with SAM or of antigen-antibody on substrate [1,2,5,6]. One of authors (T. 1.) and her collaborator [1] performed the adsorption investigation of 3-mercaptopropionic acid (MPA) on Au surface and of hexadecyltrimethylammonium chloride (C16TAC) on MPA SAM by using ATR-SEIRA. In the present work, in situ adsorption kinetics of C16TAC on 1-dodecanethiol and 3-mercapto-l-propanol SAMs are investigated by SPR and ATR-SEIRA. The results are compared with the previous system. *To whom coTFespondence should be addressed.

478 2. EXPERIMENTAL SECTION 1-Dodecanethiol (CH3(CH2)iiSH), 3-mercapto-l-propanol (HS(CH2)30H), ethanol, and chloroform-d are commercial products. CieTAC (CH3(CH2)i5N^(CH3)3Cl-) was recr}'stallized from ethanol-acetone mixture. SPR spectroscopic examination was performed at 30 "C on a Biosensor Analytical System (Nippon Laser & Hectronics Lab.) [2]. A SAM-coated Au substrate used was prepared from a Au-evaporated glass substrate by treating with ethanol solutions (1 mM) of thiols. Infrared spectra were recorded at room temperature (~25"C) on a Bio-Rad FTS 575C FT-IR Spectrometer. SAM was prepared from a chloroform-d solution (7 mM) of 1dodecanethiol, and infrared reflection absorption spectrum (IRAS), time-resolved ATRSHRAS, and ATR-SEIRAS were measured according to the procedure reported before [1]. Time resolved ATR-SEIRAS were also measured for an aqueous solution (0.1 wt %, 3 mM) of C16TAC on 1-dodecanethiol SAM and Au surface. Interferograms were accumulated 256 or 512 times (except time resolution experiment) at 4 cm*^ resolution. 3. RESULTS SPR reflectance angle shifts as a function of adsorption time on 1-dodecanethiol and 3mercapto-1-propanol SAMs are shown in Fig. 1 for aqueous solutions with different CigTAC concentrations. The adsorption for a 10"^ wt% solution proceeded with initial fast and successive slow increases. The equilibrium reached after -5000 sec. On the other hand, the angle shift values for 0.5 x 10"^ and 5 x 10'^ wt% solutions converged to a constant value through a maximum. The maximum was reached at shorter time with larger shift value with increasing the concentration up to 10"^ wt%. The angle shift for a solution of 10'^ wt%, which is larger than critical micelle concentration (cmc), reached to a constant value (-0.06 degree) without through a maxiumum after the initial increase. The angle shift values at the adsorption equilibrium are consistent with each other for solutions above 0.5 x 10"^ wt%, indicating the saturated adsorption or the Langmuir monolayer adsorption. As seen in Fig. 1(b), angle shifts for aqueous solutions with (5.0 - 100) x 10"^ wt% C16TAC concentrations on 3-mercapto-lpropanol SAM continued to increase even after -5000 sec, although the shift value was highest for a 10"^ wt% solution. This indicates that the adsorption equilibrium is not accomplished even after -5000 sec. 0.1

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Fig. 1. SPR reflectance angle shifts as a function of time for adsorption from aqueous solutions with various C16TAC concentrations on 1-dodecanethiol (a) and 3-mercapto-l-propanol (b) SAMs.

479

IRAS was measured for a 1-dodecanethiol SAM on Au surface. Since a SH stretching band around 2580 cm"^ disappeared, all observed bands are ascribed to SAM. Four bands were found around 3000 - 2800 cm'^ and their wavenumbers and assignments are listed in Table 1. It is suggested from positions of the CH2 stretching vibration bands that alkyl chains of 1dodecanethiol in SAM take trans-zigzag configuration. A time-resolved ATR-SHRA was measured for 1-dodecanethiol adsorption from a chloroform-d solution on Au surface. Absorbance rapidly increased in the initial adsorption and reached to the equilibrium value. Band positons in spectra in equilibrium with solution and of dried SAM were consistent with those of IRAS, as seen in Table 1, indicating no configurational change of molecules in SAM. It should be noted that the absorbance of ATR-SEIRA was 100 time stronger than that of IRAS, suggesting the advantage of ATR-SEIRA for the detection of tiny amount of adsorbed molecules. Figure 2(a) shows a time-resolved ATR-SEIRAS for Ci^TAC adsorption from an aqueous 10"^ wt% solution on 1-dodecanethiol SAM. The equilibrium was reached at early stage of adsorption. It was estimated from the comparison between spectra before and after adsorption of CieTAC that the contribution of SAM on absorption bands of CH3 and CH2 stretching vibration modes is about 1/4 - 1/3 of total absorbance, indicating the effective adsorption of C16TAC on SAM. Measurement was carried out even on Au surface without SAM. Although both time-resolved profiles were similar to each other, absorbances of CH3 and CH2 stretching vibration bands on SAM were 3 times larger than those on Au surface. This indicates the efficiency of the SAM on adsorption of C16TAC. As seen in Table 1, alkyl chains of adsorbed Ci^TAC have trans-zigzag configuration as well as those of 1-dodecanethiol. Table 1 Observed IR band positions (cm'^)^^ and their assignments. 1-dodecanethiol in chloroform-d

Ci^TAC in H2O

IRAS ATR-SEIRAS ATR-SEIRAS, with solu. without solu.^^ with solu.^^ without solu.^^ on SAM ^^ on Au^^ 2965m 2920s 2879m 2850W

2963m 2914s 2871m 2846VW

2961W 2920s 2873W 2852m

2955w 2915s 287 Iw 2847m

2954W 2917s sh 2847m

assignment

CH3 as}Tnmetric stretching CH2 antisymetric stretching CH3 symmetric stretching CH2 symmetric stretching

a) vs, ver>' strong; s, strong; m, medium; w, weak; vw, ver\' weak; sh, shoulder. b) Background is Au substrate, c) Background is chloroform-d. 4.

DISCUSSION The adsorption of Ci^TAC on thiol SAM depends on molecular species of SAM and C16TAC concentration. It is noticed from SPR reflectance angle shift values that the adsorption of C16TAC proceeds more abundant for 1-dodecanethiol SAM than for 3-mercapto-l-propanol SAM. Since 3-mercapto-l-propanol SAM does not have any specific interaction site with C16TAC, C16TAC on the SAM must be physisorbed. Then the adsorption is less and slow.

480

(a)

r^N»CM»>

r^heCfU*!

g^^T

C.sTAC

•ft?

dodecanethtoi

Fig. 2. C16TAC adsorption from an aqueous solution (0.1 \\l%) on 1-dodecanethiol SAM. (a) A time-resolved ATR-SEIRAS (125 sets of 4 times accumuration). (b) Schematic presentation of adsorption model at equilibrium. Ci^TAC adsorption on 1-dodecanethiol SAM obeys Langmuir monolayer isotherm but not the kinetics. The adsorption mechanism through maximum for the CieTAC adsorption from aqueous 0.5 x 10"^ and 5.0 x 10"^ vvt% solutions on 1-dodecanethiol SAM implys the coexistence of chemisorption and physisorption. Physisorption happens at short term (within -12(X) second). Then excess physisorbed molecules are desorbed with rearrangement to chemisorption. The schematic adsorption model at equilibrium is shown in Fig. 2(b). 1Dodecanethiol SAM interacts hydrophobically with C16TAC. The equilibrium is accomplished when the monolayer of C16TAC covered hydrophobic SAM surface. C16TAC molecules in the monolayer on SAM have trans-zigzag configuration but do not orient normal to the surface, as estimated from the comparison of CH2 and CH3 stretching \'ibration band intensities. For C16TAC adsorption on MPA SAM, the fast adsorption at early stage was followed by the slow adsorption [1]. This adsorption accompanied with the transition of MPA SAM to the carboxylate SAM and the ion-pairing of carboxylate SAM and C16TAC by the electrostatic interaction, different from the case of CieTAC adsorption on 1-dodecanethiol SAM. Kinetics and mechanism of adsorption of hexadecyltrimethylammonium bromide at the interface of negatively charged silica and water were investigated by Pagac et al. [7]. They demonstrated that monometic surfactants adsorb on silica with a defective bilayer sUoicture at concentrations below cmc, while a close-packed monolayer of micelles is formed on the surface at concentrations above cmc. This is not the present case, because the surface of 1dodecanethiol SAM is not hydrophilic, and no bilayers and no micelles of C16TAC adsorb.

REFERENCES 1. T. Imae and H. Torii, J. Phys. Chem. B, 104 (2000) 9218 and references cited therein. 2. T. Imae, M. Ito, K. Aoi, K.Tsutsumiuchi, H. Noda, and M. Okada, Colloids Surfaces, APhys. Eng. Asp., 175 (2000) 225. 3. S. Xu, S. J. N. Cruchon-Dupeyrat, J. C. Gamo, and G.-Y. Liu, J. Chem. Phys., 108 (1998)5002. 4. N. Gang, J. M. Friedman, and T. R. Lee, Langmuir, 16 (2000) 4266. 5. K. Matsuura, A. Tsuchida, Y. Okahata, T. Akaike, and K. Kobayashi, Bull. Chem. Soc. Jpn., 71 (1998) 2973. 6. T. Hasegawa, K. Matsuura, K. Ariga, and K. Kobayashi, Macromolecules, 33 (2000) 2772. 7. E. S. Pagac, D. C. Prieve, and R. D. Tilton, Langmuir, 14 (1998) 2333.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

481

Molecular Assemblies Based on DNA-Mimetics: Effect of Monolayer Matrix on Photopoljnnerization of Diacetylenecontaining Nucleobase Monolayers Kuniharu Ijiro**^, Jin Matsumoto^ and Masatsugu Shimomura^^* ^Research Institute for Electronic Science, Hokkaido University, N12W6, Sapporo, 060-0812, Japan ^Core Research for Evolutional Science and Tfechnology (CREST), Japan Science and TBchnology Corporation (JST), N12W6, Sapporo, 060-0812, Japan Pressure-area isotherm of a ternary component monolayer of nucleobase amphiphiles, diacetylene-containing adenine and thymine amphiphiles (DA-Ade and DA-Thy), and octadecylcjrtosine (Cis-Cyt), on the aqueous dTso subphase was changed by addition of poljKG) into the subphase. Photopolymerization of the ternary monolayers on the aqueous dTao solution was almost identical with that of a binary monolayer (DA-Ade/DA-Thy) on the same subphase. Addition of poly(G) into the subphase suppressed photopolymerization of the ternary monolayer. Since the DA-Ade/DA-Thy pairs complexed with dTso were clustered in the Ci8-Cyt matrix, both intra- and inter-complex polymerization were occiured on the dTao subphase. The inter-complex polymerization was restrained by mixing with Ci8-Cyt / poly(G) matrix. 1. INTRODUCTION Double-hehcal DNA is a supramolecular architecture composed of complementary base pairs of adenine-thjmaine and cytosine-guanine based on specific hydrogen bonding. We have already succeeded to prepare two-dimensional DNA mimetics by using a simple nucleobase monolayer, octadecylc3^sine (Cis-Cj^), on the water subphase containing guanine nucleoside^^*^]^ Another t)npe of DNA-mimetic was mixed monolayer of octadecyladenine (Cis-Ade) and octadecylthjmiine (Cis-Thy)^^]^ Triplex was formed between the Cis-Ade/Cis-Thy monolayer and thymidine (uridine or poly(U)) dissolved in the subphases^. We propose the DNA-mimetics as novel functional molecular materials based on the molecular recognition-directed self-assembly. AmphiphiUc nucleobase derivatives having a diacetylene moiety, DA-Ade and DA-Thy are newly sjmthesized to materiahze the DNA-mimetics by polymerization. Diacetylenes can be photopoljnaoierized topotactically in monolayers, and thus the ordered structure of the two-dimensional assembly can

482

be preserved. The polymerization can be followed by absorption spectral changes. If the polymerization of the DA-Ade/DA-Thy monolayer is achieved on the oUgonucleotides subphase of thymine base, the molecular weight of the pol3mier can be controlled by the template effect of the oUgonucleotide. Isolation of the DA-Ade/DA-Thy pairs complexed with a single oligonucleotide chain is required to achieve the template pol3nnerization. Inter-complex polymerization must be excluded for the molecular weight control. Ternary mixtures of DA-Ade, DA-Thy and Ci8-Cyt were spread on aqueous subphases containing oUgonucleotides, deoxythymidyhc acid 30 mer (dTao) with and without poly(G). The nonpolymerizable Cis-Cjrt molecule is expected to be a space-filling matrix isolating DA-Ade/DA-Thy/dTso complex in the monolayers. 2. Experiment The syntheses of nucleobase amphiphiles were described elsewhere. The 1:1:0, 1:1:1 or 1:1:2 mixture of DA-Ade, DA-Thy and Cis-Cjrt (Scheme) was spread from the chloroform/ ethanol (9/1, v/v) solution on the surface of lOmM Tris-HCl buffer solution (pH7.8) containing 25nM of dTso. The buffer solution containing dTao and poly(G) (1:2 in monomer unit), whose total concentration was equivalent to 25nM of 30-mer oUgonucleotide, was also used as the water subphase. The mixed monolayer compressed at 15mN/m was irradiated with UV Ught (254nm) on the water subphase and spectral changes due to the photopolymerization were foUowed by a fiber-optics reflection spectrophotometer (LB-100, JASCO).

NH2

DA-Ade ^N

N

;c-0-(CH2)9C»CC-C(CH2)iiCH3

CZH/

c

Cr.

HN O

J

N

DA-Thy

;c-0-(CH2)9C»CC-C(CH2)iiCH3

CJH/

^_^o

Cs-Cyt

H2N-^N-(CH2)i7CH3

Scheme Chemical structure of nucleobase amphiphiles.

3. RESULTS AND DISCUSSION Figure 1 shows the pressure-area isotherms of the 1:1:2 mixture of the DA-Ade/DA-Thy/Ci8-Cyt monolayer on the buffer solution with and without oUgonucleotides. When the buffer solution without oUgonucleotide is used as the water subphase, only the DA-Ade/DA-Thy pair contributes to the pressure-area isotherm because Cis-Cjrt can not form stable monolayer without complementary guanine bases in a water subphase^i'2], fhe isotherm was changed when dTzo was added to the buffer subphase. Hoogsteen type base triplex of A-T and T was formed between the DA-Ade/DA-Thy monolayer and dTao at the air-water interfacel^J. Moreover, the isotherm was most expanded when poly(G) was added in the dTao subphase. StabiUzation of the Cis-Cyt monolayer requires the complementary nucleotide, poljKG), in the water subphase. Difference between

483 50 I 1:1 (DA-Ade/DA-Thy) sndTao

1:1:1 mixture ondTaol 2 mixture ondTaq

9n dTao on dT3o+poly(G)

400 Area (nm /molecule)

Fig. 1. Pressure-area isotherms of 1:1:2 mixed monolayers on various subphases at 20**C.

500 600 Wavelength ( n m )

700

Fig. 2. Reflection spectra of the mixed monolayer on various oligonucleotide subphases after UV irradiation for 20 min.

two isotherms on dTao with and without poly(G) suggests that the DA-Ade/DA-Thy molecules and Cis-Cyt molecules are recognized by dTso and poljKG), respectively. Figure 2 shows the reflection spectra of the mixed monolayers on various oligonucleotide subphases after 20 min photoirradiation. These spectra are normahzed by the contents of the diacetylene amphiphiles at the molecular area of 15 mN/m. Absorption spectra of photopolymerized 1:1 (DA-Ade/DA-Thy) and 1:1:1 (DA-Ade/DA-Thy/Ci8-Cyt) mixed monolayers formed on the dTso subphases are almost identical. They show **blue form" polydiacetylenes with absorption Top view of the monolayers

(^) wOOOOOmOO nn ^ intra-complex UU jJU

(b) intra-complex polymerization

polymerization,

Xmax

00

Xmax

638nm

612nm poly{G)

DA-Ade disc

Tny^

Ci8-Cyt

""^"^maamoa Fig. 3. Plausible models of molecular arrangement in the 1:1:2 mixed monolayers on (a) the dTso subphase and (b) dTao+polyCG) subphase.

Ci8-Cyt

484

inaximums at 637 nm and 638 nm, respectively. This indicates that the DA-Ade/DA-Thy pairs complexed with dTao are separated from non-polymerized matrix of Ci8-C3rt in the ternary monolayer. The photopolymerization of the 1:1:2 mixed monolayer on the dTao subphase was sUghtly suppressed to compare with that of the 1:1:1 mixed monolayer on the same subphase. Dilution with Cis-Cyt restrains the polymerization of the DA-Ade/DA-Thy/dTao complexes. However the absorption maximima of the polymerized 1:1:2 mixture monolayer (638 nm) was almost identical with that of the 1:1:1 mixed monolayer. This means the DA-Ade/DA-Thy/dTao complexes are still aggregated in the 1:1:2 mixed monolayer on the dTao subphase(Figure 3a). On the other hand, addition of poly(G) into the dTso subphase reduced further the absorption intensity of the pol3nnerized monolayer. This indicates that Ci8-Cyt/poly(G) complex can mix with the DA-Ade/DA-Thy/dTao complexes to suppress the inter-complex polymerization (Figure 3b). 4. CONCLUSION Photopolymerization of the ternary monolayers (DA-Ade/DA-Thy/Cis-Cj^) on the dTao subphase with and without poly(G) was characterized by the absorption spectra of polydiacetylenes. The DA-Ade/DA-Thy pairs complexed with dTao were separated from the Cis-Cyt matrix in the ternary monolayer on the dTao subphase without P0I3KG). Polymeric matrix of Ci8-C3rt/poly(G) complex was efBciently mixed with the DA-Ade/DA-Thy/dTao complex and restrained the inter-complex polymerization of the diacetylene monolayer.

REFERENCES 1. M. Shimomura, F. Nakamura, K Ijiro, H. Taketsima, M. Tanaka, H. Nakamura, and K Hasebe, Thin Solid Fihns, 284-285 (1996) 691. 2. M. Shimomura, F. Nakamura, K Ijiro, H. Taketsuna. M. Tanaka, H. Nakamura, and K Hasebe, J. Am. Chem. Soc., 119 (1997) 2341. 3. F. Nakamura, K Ijiro, M. Shimomura, Thin SoUd Fihns, 327-329 (1998) 603. 4. M. Shimomura, J. Matsimioto, F. Makamura, T. Ikeda, T. Fukasawa, K. Hasebe, T. Sawadaishi, O. Karthaus, and K Ijiro, Polym. J., 31 (1999) 1115.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

485

From polymeric films to nanocapsules H. Mohwald, H. Lichtenfeld, S. Moya, A. Voigt, G. Sukhorukov, S. Leporatti, L. Dahne, A. Antipov, C. Y. Gao and E. Donath Max-Planck-Institute of Colloids and Interfaces, Am Muhlenberg 2, D-14476 Golm, Germany I. INTRODUCTION In recent years much has been learnt about the structure of organic films and interfaces, and techniques to prepare these films in a controlled way have been developed. These have concerned mostly planar systems, but there is no obvious reason why this knowledge could not be transferred to curved interfaces. This is most desirable in order to coat colloids for various reasons: One would be able to obtain systems with much specific surface area. This would allow not only many applications requiring controlled surfaces (e.g. chromatography, enzyme technology, separation technology), but also other techniques to study interfaces could be employed. Our motivation resulted fi-om the application of methods typically used for bulk samples: NMR, differential scanning calorimetry and flash spectroscopy. Coating colloids in a defined way is also a prerequisite to understanding colloidal solutions, because the interparticle interactions are determined by their interfaces. Having succeeded in coating colloids there is an obvious next step, to template colloids by dissolving the core and thus obtaining hollow capsules. This route will be proceeded in the following, demonstrating also the interesting properties and possible applications of these capsules. n. Preparation of coated Colloids and Capsules A technique to prepare polymeric films with nm precision has been introduced by Decher, and is called layer-by-layer adsorption /I/. With this technique a charged surface can be coated by dipping it into a solution of an oppositely charged polyelectrolyte. The latter is adsorbed reversing the surface charge under suitable conditions, and thus a polyelectrolyte with again the opposite charge can be adsorbed. Repeating the process leads to polymeric films of low roughness (< Inm), thickness controllable with nm accuracy by the number of dipping cycles and ionic conditions. Functional molecules can be integrated into the films with a position along the surface normal, controlled with nm precision. Depending essentially on electrostatic interactions the technique is applicable to many multiply charged systems: synthetic or natural polymers, proteins, colloidal particles, and dyes. It also does not require planar and flat surfaces. Instead the surface can be rough, porous or strongly curved. Consequently we have developed various protocols to coat colloidal particles by this technique /2/ (Fig. 1). These protocols either exist in incubating the colloidal particles in a

486

solution containing more polyelectrolyte than : ^ 1 '^ needed for adsorption and then removing ® "^*J \ ^ ^ excess polyelectrolyte by centrifugation or filtering before adding polyelectrolyte with the pig. 1 Scheme of the layer-by-layer opposite charge, or by adding just sufficient adsorption of polyelectrolytes on colloidal polyelectrolyte as needed for saturation coating, particles. The excess polyelectrolyte in the The conjecture that each adsorption step leads supernatant has to be removed prior to to reversal of the surface charge is verified by adsorption of the next polyion species measurements of the electrophoretic mobility. The coating can be followed quantitatively by single particle light scattering. Fig. 2 shows that the distribution of the scattering intensity per particle shifts to higher values depending on the number of coating processes. This shift can be quantified applying Rayleigh-Debye- 2000 ^ Gans or Mie theory knowing the refractive index of the polymer /3/. One thus derives 15001thickness increases of about 1 nm per step which is close to the value determined by X-ray 10001reflectivity for planar surfaces. One also realizes that there is little broadening of the distribution 500 \ with coating proving that the deposit thickness is the same for different particles. This does not ^, ^ ^, .• • -r TT T 200 300 400 500 600 700 prove that the coating is umform. However, if .,. ^ ^, ij t Scattering Intensity (arb. units) this were not the case one would observe particle aggregation, because there would be 7 Q- i n' 1 1' ht sc tterine Coulomb attraction between oppositely charged . *^* . .^^^f ^ rnccmAo *3 j-^* 11 J -ru- • / u J intensity distnbutions of PSS/PAH coated areas on different colloids. This is not observed , , ^ ir . x 1 * ^'^ r ^Ar\ uru* ^ ' A Auu * polyfstyrene sulfate) latex particles of 640 by light scattenng. A dimer or higher aggregate }• . would appear at higher intensities, in contrast to '^"^ ^^^^ ^^ our findings. Because the basic force for layer build up is electrostatic attraction the wall material can exist of many different, but multiply charged entities. We have demonstrated this versatility using synthetic and natural charged polymers, proteins, inorganic colloids, DNA, charged dyes and trivalent metal ions /4/. Also the colloidal core to be coated can be of many different nature, and this advantage will be apparent below. We have coated successfully latices, biological cells, inorganic particles, precipitates of enzyme, DNA or dyes, and even hydrophobic particles and oils could be coated after rendering their surface charged via adding low or high molecular weight amphiphiles /5/. These cores have become very important since they define conditions under which they can be removed. This process then yields hollow capsules and is possible since generally the polyelectrolyte films are permeable to small molecules but impermeable for macromolecules with molecular weight of some thousands. In the case of weakly cross-linked melamine formaldehyde the core could be removed by going to pH 1 /6/, biological templates were removed via deproteinizer solution, inorganic particles could be removed by milder pH changes, and organic cores were dissolved adding proper solvent /5/. Altogether these different procedures lead to hollow capsules with the following main features - Sizes and size distribution, depending on the template,,, between 70 nm and 10 jim

487 -

Wall thickness tunable in the nm range Wall composition tunable along the surface normal Inner and outer surface variable Variable wall material.

This unique variability should lead to a wide tunability of properties and examples of this will be given in the next section. III. Properties of Hollow Capsules III. 1 Encapsulation and Release Fig. 3 shows a confocal fluorescence micrograph of a hollow capsule after adding a fluorescently labe^._ :,, led polyelectrolyte to the outside aqueous medium /7/. The left image was taken at a pH close to that during Apsjilatid preparation. The daric interior demonstrates that the wall is impermeable for the polyelectrolyte, and this holds over times of at least Fig. 3: Top: Scheme of pH reversible opening and closing of ^ However, polyelectrolyte capsules: Bottom: Confocal fluorescence micrograph j-g^jucing the pH to obtained with fluorescently labeled dextran ^ (middle) the wall becomes permeable as evidenced by the fluorescence fi-om the interior. Increasing the pH again to 8 and removing the outside polyelectrolyte one observes that the polyelectrolyte could be entrapped, i. e. the wall has become impermeable again. This intriguing finding was encountered also for proteins and labeled dextrans with molecular weights fi"om 70 000 to 2.000 000 and can be explained as follows /If: The multilayer is composed of a strong polyelectrolyte, PSS i. e. all dissociable groups are charged, and a weak polyelectrolyte, PAH, where depending on the pH only part of the side groups are charged. At the pH of preparation the negatively and the positively charged groups compensate and the fihn interior is nearly neutral. This is the case for preparation at a pH between 7 and 8. Now reducing the pH more amine groups of PAH become charged and the interior of the fihn is charged up. The intemal Coulombic repulsion will be partly compensated by the counter ions fi-om solution. This yields an intemal osmotic pressure which overall gives a force destabilizing the fihn. This destabilizafion may resuh in disrupture of the fihn or in pore formation which is obviously the case here. The finding of a reversible pore formation is accidental and we would expect that this reversibility cannot be repeated too many times. This is, however, not needed for many practical applications.

488

The above arguments indicate that this release threshold can be varied via preparation parameters and type of polyelectrolyte as well as that ionic strength yields another control parameter. This has indeed been found. The above experiment yields parameters for controlled enc^qpsulation and release of macromolecules including proteins. For small molecules (Mw <10^), however, films prepared up to now were largely permeable as measured by confocal microscopy with dye probes in similar experiments as in Fig. 3. Then one may encapsulate by controlled precipitation in an experiment as described with Fig. 4 /8/. In this case the negatively charged dye carboxyfluorescein was used as model drug. The capsule was prepared with the inner surface positively charged, the outer one negatively. The dye in solution is distributed nearly homogeneous inside and outside the capsule but with some enrichment near the inner surface because of Colombic Fig. 4: Electron micrograph of carboxyfluorescein forces. Lowering the pH causes dye precipitates in templated erythrocytes precipitation and this occiu^ predominantly near this inner surface. From there crystal growth proceeds towards the center, more dye is sucked into the inside until the capsule is filled with precipitate. The image in Fig. 4 shows that crystallization occurs exclusively in the inside for weak supersaturation and if the outside is positively charged. In this case even more complicated shapes like the discoidic erythrocytes can be templated (Fig. 4 right). III.2 Mechanical Properties and Stability Fig. 5 shows a sequence of confocal micrographs that enable measurement of the elastic modulus \i. The wall was stained by a fluorescent dye, the polyelectrolyte PSS was added to

Fig. 5 Fluorescence micrograph of hollow shells increasing the concentration of PSS outside fi-om left to right the outside and the sequence corresponds to increasing PSS concentration /9/. Since PSS does not penetrate the capsule it creates an osmotic pressure fi-om outside and this causes deformation. Coimting thefi-actionof deformed capsules one can define a critical pressure Pc where the instability sets in. In a simple model assuming a hollow sphere with radius R

489 and homogeneous wall thickness d one expects a relation P^

•-=-(!)"

(1).

The square dependence on R and 5 could be verified and one thus obtains \i = 500 MPa. This value is somewhat less than that for a glassy polymer like PMMA and we thus may consider the wall like a glassy polymer. Although the above experiment is elegant and meaningful one should not consider \i as typical for the shell material. We have mentioned above that the shell could be composed of many different materials including inorganic particles, and thus \i may vary drastically. However, even for the same material \i may vary, as expected from the above pH variation but also as a function of temperature as revealed from Fig. 6 /lO/. In this case one observes a

Fig. 6: CLSM images (a), (b), and SFM (Contact Mode) topview images (c), (d) of 10-layer (PSS/PAH)5 polyelectrolyte capsules prepared on melamine formaldehyde resin latex solution, (a), (c) before and (b), (d) after annealing at 70°C for 2 h capsule shrinking on increasing the temperature to 70 °C. Detailed analysis of the height profiles from the AFM measurements reveals that this shrinkage is accompanied by a wall thickness increase. Hence the individual layers, initially rather stratified, would like to coil for entropic reasons. A thickness increase at constant density then has to cause a surface area decrease and thus lateral shrinking. The remaining question now is. How is it possible, in view of the many electrostatic bonds in the film, to have a molecular reorientation? Indeed if one assumes that reorientation requires simultaneous breaking of many bonds (say 15), and each bond is screened partly by water (E « 4 0 ) one may estimate with Eyring's theory reorientation times between hours and months and a strong temperature dependence /lO/. On the other hand these estimates show that varying molecular parameters like charge-charge distance or water content in the film one may obtain stability over years or instantaneous instabilities.

490 Another aspect of stability, the so-called colloidal one, that against precipitation can trivially be tuned. The preparation process yields charge stabilized colloids and these are stable in solution at least over months. On the other hand reducing the surface charge, e. g. by adsorbing oppositely charged polyelectrol)^es without reversing the charge eventually causes flocculation, as known in many applications of polyelectrolytes as flocculants. Acknowledgement: We acknowledge the skillful assistance of H. Zastrow and I. Bartsch. The work was supported by the Bundesministerium fiir Forschung imd Technologic, the Senate of Berlin, BMBF grant 03C0293A1 REFERENCES 1. G. Decher, Science, 277 (1997) 1232 2. G. B. Sukhorukov, E. Donath, H. Lichtenfeld, E. Knippel, M. Knippel, H. M6hw3ild,ColLSurf. A, 137(1998)253 F. Caruso, E. Donath, H. Mohwald, J. Phys. Chem. 102 (1998) 2011 3. H. Lichtenfeld, L. Knapschinsky, C. Diirr, H. Zastrow, Progr. Coll. Polym. Sci, 104 (1997) 148 4. G. B. Sukhorukov, E. Donath, S. A. Davis, H. Lichtenfeld, F. Caruso, V. L. Popov, H. Mohwald, Polym. Advan.. Technol 9 (1998) 759 L L. Radtchenko, G. B. Suhorukov, S. Leporatti, G. B. Khomutov, E. Donath, H. Mohwald, J. Col & Int. Sci. 230 (2000), 272-280 A. Rogach, A. Susha, F. Caruso, G. B. Sukhorukov, A. Komowski, S. Kershaw, H. Mohwald, A. Eychmuller, H. Weller, Adv. Mat., 12 (2000) 333 F. Caruso, H. Mohwald, Langmuir, 15 (1999) 8276-8281 F. Caruso, R. A. Caruso, H. Mohwald, Science 282 (1998) 1111-1114 S. Moya, E. Donath, G. B. Sukhorukov, M. Auch, H. Baumler, H. Mohwald, Macromolecules, 33 (20000) 4538-4544 5. G. B. Sukhorukov, N. A. Moroz, N. L Larionova, E. Donath, H. Mohwald, Appl. Biochem. Biotech., (2000),: submitted F. Caruso, D. Trau, H. Mohwald, R. Renneberg, Langmuir, 16 (2000) 1485-1488 F. Caruso, W. Yang, D. Trau, R. Renneberg, Langmuir 2000, in press E. Donath, G. B. Sukhorukov, H. Mohwald, Nach. Chem. Tech. Lab., 47 (1999) 400 A. A. Antipov, G. B. Sukhorukov, S. Leporatti, I. L. Radtchenko, E. Donath, H. Mohwald, Coll. Surf. A. 2000, submitted 6. E. Donath, G. B. Sukhorukov, F. Caruso, S. Davis, H. Mohwald, Angew. Chem. 1998, 2323, Angew. Chemie, Inter. Ed. Eng, 37 (1998) 2201 7. G. B. Sukhorukov, A. A. Antipov, A. Voigt, E. Donath, H. Mohwald, Macromol. Rap. Comm., 2000, in press 8. G. B. Sukhorukov, L. Dahne, J. Hartmann, E. Donath, H. Mohwald, Adv. Mat. 12 (2000) 112-115 9. C. Y. Gao, E. Donath, S. Moya, V. Dudnik, H. Mohwald, Eur. Phys. J. E, 2000, in press. 10. S. Leporatti, C. Y. Gao, A. Voigt, E. Donath, H. Mohwald, submitted to Eur. Phys. J. E., 2000

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) vc 2001 Elsevier Science B.V. All rights reserved.

491

The Effects of Substituents on the A^regation of Bacteriochlorophylls c^ and^fp Takasada Ishii\ Hiroki Hirabayashii', Fuminori Kamigakiuchi\ Mikiko Kimura^, Mitsunori Kirihata^, Miki Kamikado^, Noboni Tohge"*, Young Mee Jung"^, Yukihiro Ozaki^ & Kaku Uehara' ^Research Institute for Advanced Science and Technology, Osaka Prefecture University, Gakuencho, Sakai, Osaka 599-8570, Japan.

^Department of Applied Biochemistry,

Osaka Prefecture University, Gakuencho, Sakai, Osaka 599-8531, Japan. ^Faculty of Science and Engineering, Kinki University, Kowakae, Higashiosaka, Osaka 577-0818, Japan. ^Department of Chemistry, School of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662-8501, Japan, We observed spectroscopically different aggregation and disaggregation behavior in bacteriochlorophyll (BChl) c and BChl d. The final aggregates of /?-[£>!]- and /?-[E,E]BChl dp showed resistance against acid and heat. These BChl molecules appear to form hydrophobic aggregates which exclude water and organic solvent molecules. 1. INTRODUCTION Chlorosomes are light-harvesting systems of green photosynthetic bacteria. It is believed that each chlorosome possesses a large amount of BChl c,d or e molecules in a rod-like self-aggregate without any significant interaction between protein. The chlorosomes absorb light energy and transfer it to the photochemical reaction center (1). Four major homologs of BChl c and BChl d were isolated from Chlorohium (Cb.) limicola which are /?-[E,M]-, /?-[E,E]-, /?-[P,E]- and 5-[I,E]-BChl c^ and BChl d^ where E, P, I, and M stand for ethyl, propyl, iso-butyl and methyl groups, respectively, R- and 5-, epimer at position 3', and F, famesyl group (Fig. 1) (2). In this study, we investigated the difference between BChl c and BChl d homologs on artificial aggregation and disaggregation

492

behavior in aqueous dimethylsulfoxide (DMSO) solution. We found that aggregation of R[E,M]- and /?-[E,E]-BChl d^ proceeded via a two-stage process to form extremely stable aggregates compared with those formed from the BChl c^ homologs. 2. MATERIALS AND METHODS Each individual homologs of BChl c^ and BChl Jp were extracted from Cb. limicola, and separated on a reverse phase (ODS) HPLC column (3). Aggregates of these BChls were formed in a DMSO-water (50:50) which maintained the pH at 7.5 without buffer salts (Fig. 2), so that the experiments might be carried out in the absence of the effect of ionic strength. The solution can maintain the composition ratio because the boiling point of DMSO (183 XI) is higher than that of water and because there are specific hydrogenbond interactions between DMSO and water molecules. The formation and the decomposition behavior of the BChl aggregates were measured by absorption spectra recorded at 5 min intervals from 5 to 120 min at 30 t!;. The thermal and acid decomposition were carried out by the heat treatment at 60 *C and the addition of 2N HCl (pH 3.0), respectively. The 1064 nm excited Fourier transform Raman (FT-Raman) spectra were measured for the aggregates of the BChls formed in the DMSO-water mixture (4).

Xa.

u

e d

farnesyt

R8 R12 R20 Et/n-F¥//-Bu Me/B Me EX/nPr/l-Ba Me/Et H

Fig. 1. Chemical structures of homologs of BChl c and d.

0

20 40 60 80 100 DMSO content (%)

Fig. 2. Buffer action of EMSO-water mixture solution: Relationship between pH and composition of DMSO-water mixture solution.

493

60 120 180 240 300 360 420 Time/min Fig. 3. The kinetics of the appearance for Qy absorption maxima of the aggregate of each individual homologs of /?-BChl dp 3. RESULTS and DISCUSSION Aggregates of R-[EM]- and /?-[E,E]-BChl d^ were found to form via a two-stage process (Fig. 3). In the first stage, the Qy absorption maxima changed from 660 nm (monomer) to 705 nm (first aggregate). In the second stage, the 705-nm peak shifted further to 760 nm (final aggregate). This two stages behavior was not observed for /?[P,E]BChl Jp and each homologs of BChl c^ (Fig. 3). The final aggregates of ^-[E,M]and /?-[E,E]-BChl d^ showed resistance against acid and heat. The single-stage aggregates might be induced to decompose easily by the thermal molecular vibration, but the final aggregates of /?-[E,M]- and /?-[E,E]-BChl d^: were not dissociated easily, suggesting that the BChl molecules are strongly coordinated and hydrogen-bonded to each other. Usually, the central magnesium atom of the BChl is replaced by two protons (pheophytinization) at lower pH. Fig. 4 shows the kinetics of the disappearance of BChl aggregates upon acid treatment evaluated by fitting Qy absorption. The final aggregates of /?-[E,M]- and R[E,E]-BChl dj: showed high resistance to decomposition. On the other hand, it took only 20 min for the decomposition of the first-stage aggregates of them. In the case of the single-stage aggregates, they were as weak as that of the first-stage aggregates of /?-[E,M]and /?-[E,E]-BChl Jp. Fig. 5 shows FT-Raman spectra of the two-stage aggregates of R[E,E]BChl df and single-stage aggregate of /?-[E,E]BChl Cp were compared. The signals of the five-coordination number of the Mg atom (1606 cm^) and Mg- • •0H(3*)- • •0=C(13*) (1645 cm') were found in the final-aggregate of /?-[E,E]BChl d^. On the contrary, the signals of the six-coordination number of the Mg atom (1593 to 1597 cm') and Mg- • •

494

0=C(13') (1648 to 1653 cm"^) were observed in the first aggregate of /?-[E,E]BChl Jp and the aggregate of /?-[E,E]BChl Cp These results suggest that the first aggregate of R[E,E]BChl Jp and the aggregate of /?-[E,E]BChl Cp have a longer distance between BChl molecules coordinated via water or organic solvent (DMSO) molecules and cannot make a hydrophobic environment arround the porphyrin rings of the BChls, whereas the final aggregates of /?-[E,E]BChl
lEAfjBChl J740nm

40 60 Time/min Fig. 4. The kinetics of the disappearance for Qy absorption maxima of the aggregate of/?-[E,M] and /?-[E.E] of BChl Cp BChl Jp adjusted at pH3.0. Fig. 5. The 1064 nm excited FT-Raman ^ c t r a of the aggregate of /?-[E,E]BChl cp(a), the first (b) and the final (c) aggregate of /?-[E,E]BChl dp.

1800

1500 Raman Shift (cm'1)

REFERENCES 1) L. A. Staehelin, J. R. Golecki, and G. Drews, Biochim. Biophys. Acta, 589,30-45 (1980). 2) K. M. Smith, Photosynth.Res., 41,23-26 (1994). 3) J. M. Olson and J. P. Pedersen, Photosynth. Res., 25,25-37 (1990). 4) K. Okada, E. Nishizawa, Y. Fujimoto, Y. Koyama, S. Muraishi and Y. Ozaki, Applied Spectroscopy, 46,518-523 (1992).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) *c) 2001 Elsevier Science B.V. All rights reserved.

Dynamic transformation of microscopy

495

liposomes revealed by

dark-field

F. Nomura, M. Honda, S. Takeda, K. Takiguchi and H. Hotani Department of Molecular Biology, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan Liposomes are micro compartments made of lipid bilayer membrane of which characteristics are quite similar to those of biological membrane. Direct observation of Uposomes by optical dark-field microscopy revealed that membrane vesicles possess the ability to transform into a variety of cell organelle-like shape in response to alterations in the surface-to-volume ratio caused by osmotic pressure. Topological changes such as fusion and division of membrane vesicles also play an essential role in cellular activities. We investigated the mechanism of these topological transformations by visualizing their real-time processes. A variety of novel topological transformations were found, including the opening-up of liposomes, the direct expulsion of inner vesicles and the inside-out inversion. 1. SEQUENTIAL TRANSFORMATION OF LIPOSOMES Many years ago, we made the first systematic study on the liposomes undergoing transformation [1]. Liposomes are found to transform sequentially in a well-defined manner through one of several transformation pathways. A circular biconcave form was an initial shape m all the pathways and it transformed into a stable thin flexible filament or small spheres via a variety of regularly shaped vesicles which possessed geometrical symmetry (Fig. 1). The driving force for the transformations is found to be osmotic pressure that decrease the liposome volume thereby altering the surface to volume ratio. Using the mechanical principle of the least bending energy of membranes, we investigate the stabiUty and shape transformation of the liposomes. By computer simulations and theoretical analyses, we show that there are energetically stable liposome shapes possessing intrinsic geometrical symmetry [2]. We find that by reducing the volume, the stable shape changes from a circular biconcave shape to elliptical, triangular, square or other polygonal shapes. 2. TOPOLOGICAL TRANSFORMATION OF LIPOSOMES 2 . 1 . Direct observation of membrane fusion Membrane fusion plays an essential role in cellular activity such as exocytosis, vesicle transport among cell organelles and release of neurotransmitters at synapses. To investigate the mechanism of membrane fusion, we used liposomes as a model of biological vesicles.

496 monitoring the fusion process in real time by using optical dark-field microscopy. To induce the membrane fusion, we used the following peptides, i) HA (influenza hemagglutinin) peptide

Fig. 1. Transformation pathways of liposomes. Biconcave form transformed via one of the possible pathways shown by arrows. : 20 residues peptide derived from amino-terminal end of HA2 subunit, ii) synthesized analogue peptides of HA; negative charged analogue (E5) or positive charged analogue (K5), and iii) bee venom peptide: melittin. We succeeded to visuaUze the membrane fusion process induced by mehttin or the mixture of E5 and K5 at neutral pH. In addition to the fusion phenomenon, various behaviors of membrane vesicles including aggregation, shrinkage and opening-up of liposomes were observed, depending on the buffer conditions. However, HA or E5 alone did not induce the membrane fusion at acidic pH, where previously reported as the optimal conditions for the membrane fusion. The fusion occurred only at the narrow range of the conditions with the peptide concentrations, ionic strength and lipid composition of the membrane. When unilamellar liposomes composed of phosphatidylcholine and phosphatidyl glycerol were mixed with E5 and K5 peptides, they transformed into multilamellar ones prior to the fusion (Fig. 2). Their average diameters became smaller than half and the light scattering intensity of each liposome increased remarkably. Only the multilamellar liposomes fused with each other (Fig.3).

0

15

25

35 Time (sec) Fig. 2. Transition into a multilamellar liposome.

45

55

6S

Bar: 5\im

2.2. Opening-up of liposomes Recently, we found that the band 4.1 family proteins make large stable holes on liposome

497 • i ••

m-

•«••

illiri--•••'

Fig. 3. Fusion process of liposomes caused by amphiphilic peptide. membrane. When talin, one of the protein in this family, was added to a liposome solution, liposomes opened stable holes and were subsequently transformed into cup-shaped liposomes [3]. The holes became larger with increasing talin concentration, and finally the cup-shaped Uposomes were transformed into a lipid bilayer sheet These morphological changes could be reversed by protein dilution, i.e. the sheets could be transformed back into closed spherical Hposomes (Fig.4). We demonstrated that talin is locahzed mainly along the membrane verges, presumably avoiding exposure of its hydrophobic portion at the edge of the lipid bilayer. This was the first demonstration that a Upid bilayer can stably maintain a free verge in aqueous solution, and refutes the established dogma that all hpid bilayer membranes inevitably form closed vesicles. We found that some proteins derived from sea urchin eggs also possessed the similar opening-up activity. It further showed that taUn is a useful tool for manipulating liposome topology.

Fig.4. Opening-up (A-H) and closing (I-L) of a liposome. Bar: 5jim.

498 2 . 3 . Cyclic shrinkage of liposomes and direct expulsion of inner vesicles Membrane fusion and its lysis seem to be almost opposite phenomena to each other. Actually, however, they are closely related behaviors, because the membranes must be lysed once in the vicinity of the fusion point to undergo the membrane fusion. Hence, we studied the effect of the membrane-^iestabilibilizing reagents such as surfactants on liposome topology. When Triton X-100, electrically neutral detergent, was added to a liposome solution, spherical hposomes showed intermittent quakes of liposomes coupled with their continuous-stepwise shrinkage. Liposomal quakes are transitions from the tense state to the transient state which are characterized by vigorous fluctuations in their spherical shape. Transition from the tense to the quaking state was a rapid process accomphshed within the time resolution limit (1/30 sec), while recovery from the quaking to the tense state was a slow process on the second scale, liposomes quaked intermittently and decreased their size step by step (Fig. 5, 6). The interval between quakes decreased as the liposomal size decreased.

Time (sec) • Phase T Fig. 5. Intermittent quakes of a liposome.

1 cycle:

^134 E

|l2t^ E m O 11

r I RucUMMion phiMS

ioo^

1—I—r-r

H—I ' I 'I—I I I—I—r

5 10 Time (sec)

15

Fig. 6. Stepwise shrinkage of a liposome. Our preparations of liposomes contained giant liposomes encapsulating baby liposomes (a few micrometers in size), and those giant hposomes never released their babies externally as long as they remained tense. Surprisingly, however, giant hposomes direcdy released their babies during their quaking periods (Fig. 7). These observations indicate that Uposomal membranes that are in contact with surfactant temporally open holes as large as a few micrometers in size, while they maintain their overall shape and continuity. Small hposomes encapsulated by giant ones never shrank; however, once exposed to the external solution, they

499 began to shrink similarly to the giant liposomes. The release of the babies from giant hposomes may be a prototype for exocytosis, a process whereby lipid membrane vesicles are released from the cell surface. Continuous-stepwise shrinkage with intermittent quakes is the pathway of liposomal solubihzation when hposomes are exposed to a non-ionic surfactant However, when an ionic surfactant replaced a non-ionic one, Hposomes decreased their size smoothly. In this case, surface areas calculated from the diameters of hposomes decreased exponentially with time. We assume that hposomes quaked at various magnitudes depending on the nature of the surfactant, and that continuous-smooth shrinkage was brought about by invisible magnitudes of membrane quakes. In the case of continuous-stepwise shrinkage, which accompanies the intermittent quaking, hpids may also be excluded continuously from a hposomal membrane. The hposome could, however, discharge water molecules from its interior, as well as encapsulated vesicles, only during the intermittent quaking by perforation of its hpid bilayer. Liposomes vigorously fluctuate their membranes within each quaking state due to membrane perforation and to the sudden decrease in internal pressure which results from the discharge of the water within.

11. 0

8.9

1X0

12.3

13.9

Time (sec) Fig. 7. Direct expulsion of inner vesicles. 2 . 4 . Inside-out inyersion of liposomes. A remarkable behavior of hposomes (Fig. 8) was found when the plus charged hposomes were mixed with cationic surfactants. In this case, hposomal membranes were opened, transformed into hpid bilayer sheets, turned themselves inside out and then closed again. The hposomal behavior of this inside-out topological inversion is demonstrated for the first time in this study.

1^

1.7

Time (sec) Fig. 8. Inside out transition of a liposome. Liposomes repeated this topological inversion several times, decreasing their size step by step (Fig. 9). After repeating such inside-out inversions several times, the lipid bilayer sheets often became unable to close any more and were then solubihzed over their entire surface areas

500

equally. In this case, it seems likely that the surfactant molecules penetrate into and exclude Upids from only one side of the lipid bilayer (the outer leaflet) which is exposed to the solution containing the surfactant (Fig. 10). Although lipids may be continuously excluded from the outer leaflet of a liposomal membrane, the Uposome can not decrease its size and remains tense because the rapid escape of interior water molecules is prevented by the intact inner half layer of its membrane. If a flip-flop translocation of lipid molecules does not take place in the membrane, the surface tensions working in each of the two leaflets of the Uposomal membrane will lose balance, resulting in the inside-out inversion that is eventually induced in the Uposome (Fig. 10). It has been suggested that the molecular shape of Upids in a bilayer membrane can determine the membrane curvature and shape. The inside-out topological inversion of Uposomes described in this study could be the first direct demonstration showing that a Upid bilayer which has different numbers of Upid molecules in two leaflets can occasionaUy generate forces strong enough to change the membrane curvature and/or topology. T : Tensed phase I : Inside-out

one cycle

0

10

20

30 40 Time(jsec)

Fig. 9. Stepwise shrinkage of a ]iposome.

Out In ^&^^ TOS

- ^ / ^ ^ ^ N ^

/

Out

V Perforating ife'S ^s> •v Inside-out ^ ^ ' " ^ ^ Ms? N^ XSZ^*"^

" ^

Tensed phase

•MM^SSSi

Inside-out phase

Fig. 10. A model for the inside-out inversion process of a liposome. REFERENCES 1. H. Hotani, J. Mol. Biol., 178(1984) 113. 2. T.Sekimura and H.Hotani, J. Theor. Biol., 149 (1990) 325. 3. A.Saitoh, K.Takiguchi, Y.Tanaka and H.Hotani, Proc NaU Acad Sci USA, 95 (1998) 1026.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) '•€) 2001 Elsevier Science B.V. All rights reserved.

501

Micelle-Vesicle Transition and Vesicle size Determining Factor Masaharu Ueno, Hiroshi Kashiwagi, Noriko Hirota, and Changqui Sun Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, Sugitani, Toyama 930-0194 Japan The process of vesicle solubilization and size growth by detergents was examined in order to clarify the size-determining factor of vesicles prepared by removing detergent from phospholipid-detergent mixed micelles. When the detergent concentration in the vesicle membrane reached a critical value, breakdown of LUV into SUV was observed. This SUV was named SUV*. The SUV* is fusible and resulted in LUV when octylglucoside or CHAPS was used as a detergent but not when sodium cholate or SDS was used. The fusion and size growth of SUV* were also time-dependent. On the basis of these findings, we concluded that the fusion of SUV* appeared in the micelle-vesicle transition is the main factor determining vesicle size. 1. INTRODUCTION Micelle-vesicle transition has been studied in close relationship to the functional reconstitution of membrane proteins after they have been purified in detergent solutions. A four-stage model, instead of the three-stage model, to explain the process of vesicle destruction by a detergent has become widely accepted [ 1 ]. Although the four-stage model is useful for understanding many observations in the vesicle-to-micelle transition, it cannot answer the following fundamental and simple questions [2]. Why are octylglucoside- or CHAPS-removed vesicles large (200-300 nm in diameter) but cholate- or SDS-removed vesicles small (30-50 nm)? Why are vesicles prepared by detergent removal by dialysis larger than those prepared by rapid detergent removal using hydrophobic porous beads? Furthermore, the model caimot explain the discrepancy in partition behaviors of a detergent for LUV and SUV. The apparent partition coefficient of octylglucoside for LUV is greatly dependent on the detergent concentration, being 70-90 M"' in a very low concentration range and decrease to about 30 M"' as the concentration is increased [3,4]. On the other hand, the partition coefificient for SUV remains almost constant (33 M'') over a wide concentration range [5]. In Tables 1 and 2, the sizes of vesicles prepared using different types of detergents and different methods of detergent removal are summarized. 2. METHODS 2.1. Vesicle preparation Phospholipid vesicles were prepared by detergent removal by dialysis or stepwise- or one step- dilution of mixed-micellar solution, and size of the resultant vesicles was evaluated by quasi-elastic light scattering.

502

2.2. Vesicle destruction The process of the vesicle destruction by adding detergents was examined by pursuing turbidity and apparent particle size. The changes of size and shape of the vesicles were observed by electron microscope with freeze fracture technique. Table 1 Vesicle size prepared by detergent removal Detergent octylglucoside sodium cholate s CHAPS SDS

Average size (nm) 210 100 50 60 380 40

Method dialysis XAD.2* dialysis SM-2* dialysis dialysis

Table 2 Vesicle size prepared by mixed micelle dilution

Dialysis stepwise dilution one-step dilution

CHAPS

Choi Size (nm)

380 335 40

50 45 38

Chol/Ca

90 60

•Detergent was removed by hydrophobic porous beads, Amberlite XAD-2 or Biobeads SM-2.

3. RESULTS 3.1. Breakdown Of LUV to SUV* During the process of destruction of LUV (large unilamellar vesicles) by detergent, complicated phenomena were observed. As the detergent concentration was increased, the optical density (OD) of the suspension and apparent particle size initially increased, then decreased, subsequently increased and decreased again. These observations suggest that the collapse of LUV occurs via at least two stages. The first decrease in OD and in size corresponds to the breakdovm of LUV to small vesicles containmg large amount of detergent. Electron microscopy shows the appearance of small vesicles in this stage [6]. We tentatively named these small vesicles SUV*.

3.2. Size growth of SUV* containing octylglucoside and sodium cholate SUV* can be reproduced by adding an appropriate amount of detergent to SUV prepared by ultrasonication. When octylglucoside was added to SUV to make the detergent concentration of 1.0-1.1 in molar ratio (detergent/phospholipid) in the membrane phase [7], the vesicles grew time-dependently, as shown in Fig. 1, and a figure of fusion was observed under an electron microscope (Fig. 2). On the other hand, there was little size growth of SUV* containing sodium cholate. Assuming that the limited size growth of the latter was due to the electrostatic repulsion of SUV* containing an anionic surfactant, sodium cholate, the effect of CHAPS, which has the same steroid structure as that of sodium cholate but no net charge, was examined. The effect of Ca ions on the size growth of sodium cholate-containing SUV* was also examined.

503

3

4

Time/ d

Fig. 1 Time course of vesicle size after addition of detergent. R is molar ratio of detergent to phospholipid.

Fig. 2 Electron micrograph of A and B in Fig.l.

3.3. Sizes of vesicles prepared using sodium cholate, CHAPS and sodium cholate with Ca ions Table 2 shows the sizes of vesicles prepared using different types of detergent and diflferent methods of detergent removal. When a detergent was removed by rapid one-step dilution, the size of the prepared vesicles was small regardless of the type of detergent used. With slow stepwise dilution, large vesicles were prepared in all cases except for when sodium cholate without Ca ions was used. These observations suggest that the vesicle size is controlled by both a kinetic factor and the nature of detergents. 3.4. Size growth of SUV* containing CHAPS and sodium cholete with Ca ions SUV* was reproduced by adding 3 mM of CHAPS. Rapid size growth was observed. The size growth was completed within 10 minutes, as shown in Fig. 3. Electron microscopy showed large vesicles (C in Fig. 5). With addition of 4 mM of CHAPS, the shape of the vesicles changed to a worm-like intermediate structure (D in Fig. 5). When sodium cholate with Ca ions was used, size growth of SUV* was observed with the addition of 4 mM of detergent (F in Fig. 5), but the size growth was rather slow compared to the case of CHAPS and was rather small. 4. CONCLUSION The size growth of vesicles containing a large amount of detergent (SUV*) determines the ultimate vesicle size when the vesicles are prepared by detergent removal from detergent/phospholipid mixed micelles. The size growth occurs by fusion between SUV*, as shown in Figs. 2 and 5. The fusion is affected both by the nature of detergents and by a kinetic factor in detergent removal. The discrepancy in partition behaviors of a detergent for LUV and SUV can also be explained by the breakdown of LUV into SUV*.

504

400

CHAPS/mM Fig. 3 Apparent particle size vs CHAPS concentration

Sodium cholateAnM Fig. 4 Apparent particle size vs. sodium chorate concentration in the presence of Ca ion.

Fig. 5 Freeze fracture electron micrograph of C, D in Fig. 3 and E, F in Fig. 4.

REFERENCES 1) 2) 3) 4) 5)

J. Lasch, Biochim. Biophys. Acta, 1241,268 (1995) M. Ueno, Membrane, 18,96 (1993) M. Ueno, Biochemistry, 28, 5631 (1989) M. Patemostre, M. Rourex, and J. L. Rigaud, Biochemistry, 27, 2668 (1988) S. Almog, B. J. Litman, W. Wimley, J. Cohen, E. J. Barenholtz, A. Ben-Shaul, and D. Lichtenberg, Biochemistry, 29,4582 (1990) 6) M. Ueno and Y. Akechi, Chem. Lett., 1991, 1801 7) M. Ueno, H. Kashiwagi, and N. Hirota, Chem. Lett., 1997,217

Studies in Surface Science and Catalysis 132 Y. iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B.V. All rights reserved.

505

Active Control of Vesicle Formation with Photoelectrochemical Switching Hideki Sakai*'^''\ Atsutoshi Matsumura'\ Tetsuo SSL]I\ Masahiko Abe'*^ a) Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki,Noda, Chiba 278-8510, Japan b) Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku-ku,Tokyo 162-0825, Japan c) Department of Applied Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro-ku,Tokyo 152-8552,Japan. Abstract Vesicle formation could be photochemically controlled in aqueous mixtures of a "photo-switchable" azobenzene-nKxlified cationic surfactant (4-butylazobenzene-4 -(oxyethyl)trimethylammonium bromide; AZTMA) and an anionic surfactant (sodium dodecylbenzenesulfonate; SDBS). Vesicles were formed spontaneously in a wide composition in aqueous trans -AZTMA / SDBS mixtures. Transmission electron microscopic observations via freeze replica technique demonstrated the disruption of the vesicles into larger aggregates (precipitate) with UV light irradiation (cis - AZTMA formation) and the following visible light irradiation (trans formation) resulted in vesicle reformation. The release rate of aqueous compounds encapsulated in vesicles was shown to be photochemically controllable in the present AZTMA/SDBS mixed system. 1. INTRODUCTION Reversible control of formation and disruption of surfactant molecular assemblies like vesicles by external stimuli''^^; e.g., thermal, electrical, and optical ones, has been a subject of significant attention with the view to apply it to the controlled release of drugs and perfumes, and to the removal of organic impurities dissolved in water. Actually, we have recently reported a reversible control of vesicle formation with an aqueous mixture of a "redox switchable" ferrocene-modified cationic surfactant and a simple anionic surfactant.^^ Among various ways of control, optical control is promising since it requires no addition of a third component to the system. In the present study, spontaneously forming vesicles are prepared by mixing a cationic surfactant modified with "photo switchable" azobenzene moiety and a simple anionic surfactant. The effect of trans-cis photoisomerization of the azobenzene-modified surfactant on the aggregation state

506 of vesicles is also studied by means of measurements.

microscopic observations and spectroscopic

2. EXPERIMENTAL 4-Butylazobenzene-4'(oxyethyl)trimethyl- ammonium bromide (AZTMA, Scheme 1) was used as the cationic surfactant. AZTMA was synthesized and purified as described previously . Anionic sodium dodecylbenzenesulfonate (a)calionic surfactant (SDBS) was used as received from Tokyo pH3 Kasei. Sample preparations were done by first ^*—V ^I^ \ _ / ^ ^ ' " * " ~ ^ ^ " ' Br rcK making stock solutions of AZTMA and SDBS CH3 4-Butylazobenzene-4'-(oxyethyl) at desired concentrations in deionized water. trimethylammoniumbromide (AZTMA) Samples were then prepared by mixing stock (b)anionic surfactant solutions of AZTMA and SDBS gently at desired ratios for three seconds. All samples Sodiumdodecylbenzenesulfonate (SDBS) were equilibrated in a thermostated bath at 25°C. Scheme 1 Molecular structure of AZTMA and SDBS

3. RESULTS AND DISCUSSION Figure 1 shows the temary phase diagram of aqueous mixtures of trans - AZTMA and SDBS at 25°C. Mixed micellar phases (M) existed on the binary surfactant-water axes in Fig. 1. Precipitates (P) were formed along the equimolar line. Spontaneously formed H20(wt%) vesicles (V) were observed in a relatively wide range of mixing compositions on both cation-rich and anion-rich sides (V+L and V+L+P regions). Freeze replica TEM observation and glucose dialysis experiments also confirmed vesicle formation. 98.0 The effect of light irradiation with Hg-Xe lamp (San-ei Supercure-203S) on the aggregation state of cation-rich aqueous mixtures of trans - AZTMA and SDBS (total AZTMA(wt%) SDBS(wt%) 1.00 ^+^<.p 2.00 concentration; 0.05 %, AZTMA: SDBS = 6 : 4 wt%, point A in Fig.l) was studied. Fig. 1 Ternary phase diagram for AZTMA/ SDBS / H20 at 25°C. Micelle region is M. UV/vis absorption spectra showed that the Precipitate is P. Two-phase region is spectroscopic characteristics of A2TMA in vesicles and lameller phase (V+L). Three-phase region is vesicles, lameller aqueous mixtures with SDBS were almost the phase and precipitate (V+L+P). same as those of AZTlMA alone in its aqueous

507

solutions

as

reported^^

and

AZTMA

constituting vesicles underwent reversible

1^

trans - cis isomerization upon alternate UV and visible light irradiation. The effect of light irradiation on the aggregation state was then directly observed using a transmission electron microscope via freeze replica technique. The samples were frozen in liquid nitrogen and fractured at -120°C using a freeze fracture device (Hitachi,

FR-7000A).

The

Fig. 2

fractured

surfaces were immediately replicated by evaporating platinum at an angle of 45°,

Freeze-replica TEM micrographs of AZTMA/SDBS (= 6/4 wt%) mixed aqueous solutions (a) as prepared, (b) after UV irradiation

followed by carbon film at normal incidence, to increase the mechanical stability of replica. The replicas thus prepared were examined on an electron microscope vJEOL TEM1200EX). In a micrograph of the as-prepared solution {trans form. Fig. 2(a)), spherical vesicles with an average size of 50-100 nm were observed. After 2h UV-light irradiation (Fig. 2(b)), spherical vesicles disappeared and large elongated molecular aggregates were observed, though the nature of these large molecular aggregates has not yet been identified except that this solution is highly turbid. Furthermore, subsequent visible light irradiation resulted in reformation of vesicles with an average size of ca. 50nm. These results clearly demonstrate that vesicle formation and disruption can be reversibly controlled with photoirradiation in the present catanionic AZTMA/SDBS system. The

effect

photoirradiation trapping

of

on

the

efficiency

of

concentration of SDBS / wt% 1J)

AZTMA/SDBS vesicles was investigated with the glucose

c

\

dialysis technique^^

CO

B

Figure

3 represents the trapping

3 E C CO

efficiency

8 Q

of

AZTMA

/

SDBS mixed solutions (total surfactant


O^^S^ 1

1

—#- nonirradiated —•- UV-irradiated "Q- Vis-irradiated

V

0.2

-T-

1

0.0

-

l\

0»

2h

0 ^

0.6

1

11 /

1

v\

<

concentration;

lwt%) as a function of their composition. efficiency

of

The trapping as-prepared

(trans form) solutions was high (2-4% against

total

'0.0

0.2

0.4

— 1

0.6

0.8

concentration of AZTMA / wt% Fig. 3 Glucose amount uptaken by AZTMA/SDBS mixture (Total surfactant concentration 1.00wt%)

508 glucose amount) at the compositions where vesicles are formed (AZTMA compositions are 0.30-0.50 and 0.60 0.75), whereas it was almost zero at the equimolar composition (the AZTMA ratio is 0.547). After cis form formation induced by UV-light irradiation for 12 hours, the trapping efficiency decreased drastically. For instance, it decreased from 3.1 % to 0.3 % by UV-light irradiation for the AZTMA/SDBS = 6/4 wt% solution. Furthermore, the following visible light irradiation for 12h (in advance of dialysis) caused re-increase in the trapping efficiency to 3.2 %. These results confirm the disruption and reformation of vesicles induced by the UV and visible light irradiation, and also suggest that the release of aqueous compounds encapsulated in the vesicles can be controlled by the photochemical reaction of AZTMA. The allowed packing of surfactant molecules is governed by the "surfactant number^^" ^/^o^c where v is the volume of the hydrophobic portion of the surfactant, 1^ is the length of the hydrophobic group ,and a^ is the head group area of the surfactant molecule.

In terms of the

surfactant mixtures of interest here, the dynamic ion pairing of ionic single-tailed surfactants apparently brings about a pseudo double-tailed zwitterionic surfactant, which results in a smaller head group and a larger hydrophobic region than those in the individual surfactants. This ion pairing was confirmed by Kaler et. al^^ with surface tension and conductivity measurements. The dynamic pairing jf trans-AZTMA and SDBS should roughly double the surfactant number, leading to a transition from spherical micelles in the pure component systems to vesicles in the mixed surfactant systems. On the other hand, the formation of cw-form by UV-light irradiation causes an increase in the critical micelle concentration (cmc)^\ Thus, the cmc of trans-AZTMA corresponds to that of alkyltrimethylammonium bromide with carbon chain length of 16, while the cmc of cw-AZTMA corresponds to that of alkyltrimethylammonium bromide with carbon chain length of 14. Furthermore, the volume of the hydrophobic portion (v) increases and the length of the hydrophobic group (1^) decreases through cw-form formation . This would produce an increase in the surfactant number of AZTMA, thereby causing transformation from vesicles to a planer lamellar structure.

REFERENCES 1) T. Saji,, K Hoshino, Y. Ishii, M. Goto, 7. Am. Chem. Soc. 113, (1991) 450. 2) L. Yang, N. Takisawa, T. Hayashita, K. Shirahama, 7. Phys. Chem., 99, (1995) 8799. 3) H. Sakai, H. Imamura, Y.Kakizawa, Y. Kondo, N. Yoshino, J. H. Harwell, Denki Kagaku 65,(1997)669. 4) T. Saji, K. Ebata, K. Sugawara, S. Liu, K. Kobayashi, J.Am.Chem.Soc. 116, (1994) 6053. 5) W. R. Erode, J. H. Gould, G. M. Wyman, J.Am.Chem.Soc, 74, (1952)4641. 6) J. N. Israelachvili, D. J. Mitchell, B. W. Ninham, J. Chem. Soc, Faraday Trans. 72, (1976) 1525. 7) M. T. Yatcilla, K. L. Herrington, L. L. Brasher, E. W. Kaler, S. Chiruvolu, J. Zasadzinski, J. Phys.Chem., 100, (1996) 5874.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) o 2001 Elsevier Science B.V. All rights reserved.

509

Novel Cell Culture Substrates based on Micro-Porous Films of Amphiphilic Polymers T. Nishikawa", J. Nishida^ K. Nishikawa^ R. Ookura^ H. Ookubo', H. Kamachi', M. Matsushita^ S. Todo", and M. Shimomura*'^ ^Spatio-Temporal Function Materials Research Group, Frontier Research System, RIKEN, Hirosawa, 2-1, Wako, Saitama, 351-0198, Japan ^'Research Institute for Electronic Science, Hokkaido University, N12W6, Sapporo, 060-0812, Japan ""School of Medicine, Hokkaido University, N15W7, Sapporo, 060-8638 Honeycomb-like micro-porous films (honeycomb films) can be fabricated by casting dilute solutions of amphiphilic polymers on solid supports in humid atmosphere. We have been studying how the honeycomb films can be used as artificial extracellular matrices. Here we introduce two fabrication methods for the honeycomb films which provide different surface coverage to solid substrates.

The honeycomb films which can make the solid

substrates appear from their micro pores were utilized for the micro-patterning of cell culture substrates. Hepatocytes recognized the difference in the surface coverage of solid supports. 1. INTRODUCTION It is noteworthy that surface morphology of materials, especially in a sub-cellular scale will significantly influence the behavior of cells. For example, microgrooves etched on

Part of this work was financially supported by The Kao Foundation for Arts and Science.

510 metal surface can guide cell movement [1].

Size of the cell adhesive sites and their

geometrical distribution on material surfaces determine the destiny of cells; live and death [2]. Normally these microstructures are fabricated by photolithography and the related techniques such as micro-contact printing. On the other hand, we have recently found that thin polymer films possessing regular array of micro pores (honeycomb film) can be fabricated by casting dilute solution of amphiphilic polymers in a humid atmosphere [3]. We have been studying the honeycomb films with respect to cell culture substrates [4]. In this article, we describe the fabrication of honeycomb films which can control the surface coverage of solid supports and the application of the honeycomb films to micro-patterning of cell culture substrates. We will show how mammalian cells will interact with the exposed surfaces of solid support through honeycomb micro pores. 2. EXPERIMENTS 2.1. Fabrication of honeycomb films Tow types of honeycomb films were fabricated by casting dilute solution of amphiphilic copolymer (fig. 1: copolymer 1) (solvent: benzene, concentration: 1 mg/mL) on either glass substrates (film-1) or water surface (film-2), and followed by applying humidified HO I ^^^ on ^ 3 4i^o^^^o'^V**N'"^-^"*^-^'Y^

air (80 % r.h. at 20 °C) to the cast solution. The

"SP^^"^^^ o ^ n

surface morphology of these honeycomb films was

^N"'^^

studied by contact mode of atomic force microscope

film-2 was transferred onto a bare glass plate. The

m

1

(m:n=4:i)

(AFM). The flat films of copolymer 1 were also fabricated by casting the same dilute solution on

Figure 1 Amphphilic copolymer 1 applied to the fabrication of honeycomb-like glass plates in dry atmosphere, and they were micro-porous films. ,. . „ , . ^ . applied to cell culture substrates. 2.2. Cell culture experiment Hepatocytes which were isolated from rat livers were cultured on each cast films of copolymer i (honeycomb films (film I and film 2) and flat films) and glass plates. The cell

morphology of hepatocytes was observed by phase contrast microscope at 24 hrs after seeding of cells.

511 3. RESULTS AND DISCUSSION 3.1. Surface morphology of honeycomb films The honeycomb films (film-1 and film-2) have resulted in almost identical surface morphology in an average size of micro pores (3 ^m) (figure 2 (a) and (b): top) and in a film thickness (approximately 0.7 ^m which was measured by cross sectioning of AFM images). However a significant difference in surfaces of glass plates which can be accessed through the micro pores was found between these honeycomb films.

The portion of a micro pore on the glass surface was scratched by an

(a)

AFM tip loaded with a constant force of 100 nN. 5 ixm

5 Jim

The AFM

measurement revealed that there was a 7 nm thick film which covered the surface of the glass

h

Scratched area

plate (figure 2 (a): bottom) in addition

to

the

honeycomb

structure of the film-1. , 1

1000

2000

3000 nm

0

1000

J

1

2000

3000 nm

Figure 2 Optical microscope images (top) of honeycomb films ((a) film-1 and (b) film-2) and cross sectional profiles (bottom) of atomic force microscope images which were scanned at the scratched surfaces of glass plates.

On the

other hand, the thin films were not detected on the glass plate

which was overlaid with the film-2 (figure 2 (b): bottom).

3.2. Recognition of micro-patterned surfaces by hepatocytes On the cell adhesive surfaces such as a glass plate, the shape of hepatocytes is fully stretched in all directions (figure 3 (a)). The hepatocytes which were cultured on flat films of copolymer i maintained their round shape and formed cellular aggregates (spheroids) (figure 3 (b)). This indicated that the flat films of copolymer 1 were less adhesive to hepatocytes. The hepatocytes were cultured on a micro-patterned substrate whose entire surface was covered with the film-1 (substrate-1). By utilizing the film-2, we fabricated a new micro-patterned surface which consisted of low adhesive areas (a honeycomb film of copolymer!) and high adhesive areas (exposed surface of a glass plate) (substrate-2). The hepatocytes adhered weakly to the substrate-1 and exhibited round shapes which

512

are similar to those of hepatocytes observed on the flat films of copolymer 1 (figure 3 (c)). On the other hand, hepatocytes were strongly adhered to the substrate-2 and spread extensively (figure 3 (d)). This suggests that glass surface which appear from the micro pores

of the

film-2

considerably

influence the adhesion of hepatocytes. Thus

hepatocytes

recognize

the

difference in the surface coverage of Figure 3 Phase contrast microscope images of hepatocytes which were cultured on various substrates, (a) glass plates, these two substrates, (b) flat films of copolymer 1, (c) substrate-1, and (d) substrate-2. Bar: 20 mm.

4. CONCLUSION We succeeded in the fabrication of two types of honeycomb films which can control the surface coverage of solid substrates. The surface of solid substrate was appeared from micro pores of honeycomb film which was fabricated on water surface.

Hepatocytes recognized

the difference in the surface coverage of glass plates where the honeycomb films were deposited.

It is our belief that this micro-patterned surface can be utilized for regulating

various cell behaviors such as adhesion, tissue formation, and expression of functions. REFERENCES 1. E. den Braber, J. de Reijiter, L. Ginsel, A. von Recum, J. Jansen, J. Biomed. Mater. Res., 40, 291,(1998). 2. C. Chen, M. Mrkisch, S. Huang, G. Whitesides, D. Ingber, Science, 276, 1425, (1997). 3. N. Maruyama, T. Koito, J. Nishida, T. Sawadaishi, X. Cieren, K. Ijiro, O. Karthaus, M. Shimomura, Thin Solid Films. 327-329, 854, (1998). 4. T. Nishikawa, J. Nishida, R. Ookura, S. Nishimura, S. Wada, T Karino, M. Shimomura, Mater. Sci. & Eng. C, 10, 141, (1999).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

513

NANOPARTICLE GOLD PREPARATION AND ITS APPLICATION IN BIOLOGICAL TECHNOLOGY X.Y.Chen, L. Lin, Y. P. Deng, J.R.Li, L. Jiang* Lab. Colloid and Interface, Institute of Chemistry, Chinese Academy of Sciences, 1. INTRODUCTION The preparation of nanometer sized gold particles has been proved to be a promising regime in information and biological science since it has been revealed that besides enormous surface area effect the physical and chemical properties of nanoparticles are dominated by the spatial confinement of electronic and vibrational excitations[l-6]. Nanoparticle gold less than lOnm in our laboratory was produced by nucleation method in aqueous solution or in surfactant system such as reversal micelle, vesicle and monolayer to obtain hydrophilic as v^ell as hydrophobic particles.[7-l 1] A general rule has been found that the concentration of surfactants in the interface, by means of increasing the surfactant concentration in the bulk solution or prolonging their chain length, will cause the smaller particle formation. This fact could be interpreted in terms of providing more nucleation sites at the interface. To make a homogeneous two dimensional arrangement of gold particles, monodispersity is critical. After careftilly size control process, we could get a 2D gold nanosized particle arrangement ranging from lOnm down to the atomic size. Fig.1-2., [9,10]. Few examples in this paper have been taken to show the role of gold nanoparticle in the biological system such as sensitivity enhancement of glucose biosensor, DNA detection and AIDS virus detection.

* Corresponding address: 3A Da Tun Rd. Beijing 100101, RR.China. Fax: 86-1064888171; E-mail: [email protected]

This work was supported by NSFC(No.29733110)

514

•.•••it?^wj^JSZ5

Fig.l. Electronmicrogram of 2D arrangement of gold

Fig.2. AFM image of Au atom grown on the poly-pentacosadiynoic acid

2. MONODISPERSED PARTICLE PREPARATION. In order to achieve a quantitative understanding of the nanoparticle phenomena, one needs to have monodisperse particles with well-defined surface. It is not able to touch all methods in a paper and we have to restrict only in describing some results, which we think, are important in the monosize nanoparticle preparation. Thefirstthing we would like to emphasize is the effect of chainlengths of surfactants in monolayer or in micelles or reversed micelles on the formed particle size and stability. Gibbs gave a famous formula for nucleation in his classical work. [12]. With Wg denoting the work required to form the surfaces (i. e. I(as)) of a new phase and Wy the work gained in forming the mass as distinguishedfi-omthe surfaces due to the change of chemical potential fi-om a metastable state to a stable state. The work required for nucleation may be denoted by Ws-Wv. Ws-Wv= -W^^^ya*S

(1)

It means the nucleation energy is uniquely determined by the surface (interface energy). On the other hand, from Traube rule, we know that the surface (interface) energy will decrease as the surfactant chain length became longer by one CHj, the surface tension will decrease by 3 times. Langmuir gave a theoretical interpretation of this rule.[13] , clearly indicating the nature of surface energy decrease is the surface enrichment of the surfactants . From this interpretation, hydrophobic long chain in the molecular could make the surfactant concentrate at the surface more easy, or to make a more compact monolayer at the surface. These surface molecules provide more sites for the nucleation, resulting in smaller nuclei. To prove the validity of above assumption in the preparation of nanometer particles, particle size and stability of the amines functionalized gold particles in chloroform has been investigated with narrow size distribution. C,g, C,2 and C,o functionalized gold particles purified with ethanol. As shown In Fig.3, The gold particles formed under long chain surfactant C,8 were smaller than that under C,2.

515

Fig.3. Electronmicrograph of gold particles capped by (a) C,8H37NH2 and (b)C,2H25NH2. Fig 4. shows the stability. The Cig, C,2 and C,o ftinctionalized particles have absorption at 500nm, 506nm and 507nm respectively (exactly after preparation), indicating the particles capped by longer chain amine have the smaller size. Then part of these samples were separated and placed under room temperature for 24 hours, after which, UV-vis spectra was again recorded. As evidenced by which, X^^^ of the C,2 and C,o ftinctionalized particles has shifted obviously to longer wavelength (511nm and 517nm respectively), while the absorption curve of the C,8 functionalized particles did not change at all. This is indicative of the variance of the size distribution of the C,2 and C,o functionalized particles to larger size while gold capped by Cjg remained the same size.

Ci8 capped partides Ci2 capped partides Cio capped partides

300

400

500 Wavetength(nm)

600

700

300

400

500 600 Wavelength(nm)

Fig.4. UV spectroscopy curves of gold particles capped by aliphatic anime for less than 20 min(a) and 20 hrs(b) after preparation. Although great achievement have been made in recent year in the preparation of monodisperse particles, to make a monodisperse nanometer particles still is not a easy job. We found that gold nanoparticle could be also separated by the method suggested by M.G.Bawendi et al.[14] and think it is a common methods for all nanoparticle preparation.

516 Fig. 5a comes from particles without size separation, pic.5b comes from particles after size separation. From these pictures we could understand the importance of monodisperse particles in 2D arrangement.

Fig. 5 Two-dimentional array of gold particles before(a) and after(b) purification by replacement 3. THE EFFECT OF NANOMETER PARTICLES IN BIOLOGICAL SYSTEM. Four examples of nanogold particles have been introduced here to show the nanogold effect used in biological system. That is, enzymatic and photoizomerization properties in immunology and gene diagnose and gene therapy. 3.1. Glucose oxidase (GOD) biosensor Glucose oxidase is an enzyme that could produce H2O2 when contact with glucose and could be used as the sensitive element in the glucose biosensor. It has been found that when glucose oxidase mixed with hydrophobic SiOj particles its enzymatic activity will increased dramatically, while mixed with hydrophilic particles its enzymatic activity remained the same as it without addition of silica particles. It is surprising that hydrophobic oil droplets could also cause the same effect [15], indicating the nature of this effect — surface effect. Electronmicroscope on hydrophobic Si02 and hydrophilic Gold showed the enzyme will change their orientation after adsorption, protecting the hydrophobic particles and cause coagulation of hydrophilic particles[16 ] We attributed this effect to the surface orientation of glucose oxidase. When we worked on nanoparticle effect, a strange phenomenon caused our special attention. That is when the particle become smaller, Gold particles has much larger effect than the SiOj . This phenomenon could not be explained by the surface orientation effect. When we ftirther investigated the nanoparticle effects quantitatively, it has been found that this enzyme activity enhancement effect origin not only from the larger specific area of nanoparticles, but also from the catalyst nature of the particle [17] 3.2. Bacteriorhodopsin (bR) photoresponse effect. Becteriorhodopsin is a biomolecule found in patches in the membranes of Halobacterium,

517

is composed of the retinylidene chromophore retinal linked through a Schiff base to the protein bacteriorhodopsin, which could be used to make a film in mimecking the vision process. The nature of its photoresponse moiety — protonated shift base of retinal. Many papers reported its artificial membrane could be used as an photoelectric electrode [18-20] Koyama etc. has proved that antibody-mediated bacteriorhodopsin orientation could increase the photoresponse of bR apparently,[21] In our experiment it has been found that the nanoparticle gold could increase the life time and photoresponse intensity of retinal[22] LB film. We attributed this effect to be the orientation of bR at the gold solid interface. Photoelectric response, UV adsorption and cyclovaltalic diagram showed there has very strong interaction between the gold particles and the biomolecules, 3.3. Immunology for AIDS detection Fig.6. shows the principle of nanogold particles with diameter of 10-30nm could be used as an carrier of antibody of Aids virus. When the antibody-gold particles meet the antigen of AIDS in a filter paper, the combination of antigen and antibody will be displayed by the red color of gold which remains in the filter paper[23]. Otherwise, there will be no color appeared in the filter paper at all. The difficult point is that the test solution must remove the excess antibody and gold particles so that then can have high reactivity.

^^ ^

i.

^

^-J3o\d

particles

-—HIV antigen

HIV antibody-^ A

A

•^—Filter paper

Negative samples

Positive samples

Fig.6. Configuration for HIV antibody test by IgG-gold conjugate 3.4. DNA detection Gold particles could be used in the recognition of target DNA by improvement of the combination of the probe single strand DNA probe on the substrate as shown as follows . Colloidal Au

^ JS\/\/vgS\/vyN,/^r)/Ni--ATG GGC CTC AGO TTC AT

i

0" - A T G GGC CTC AGG TTC AT

^

0"

518 By using Quartz Crystal Microbalance(QCM) immobilization of'-12nm-ciiameter colloidal Au on to an Au-coated QCM resulted in an easier attachment of oligonucleotide, with a mercaptohexyl group at the 5'-phoaphate end and an increased capacity for nucleic acid detection, the frequency change on 50ng colloidal system is about 4 times than that observed on unmodified system for target DNA at the same concentration. [24] REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Y. Wang, and N. Herron., J. Phys. Chem., 95 (1991) 525-532. A.Henglein, RMulvaney., and T.T. Linnert, Farday Discuss, 92 (1991) 31-44. J.H.W.Cramp, and P.J.Hillson, The jounal of Photographic Science, 24 (1976) 25-299. A.Henglein, RMulvaney, and T.T. Linnert, Farday Discuss, 92 (1991) 31 -44. RMulvaney, T Linnert,.and A.Henglein, J.Phys.Chem., 95 (1991) 7843-7846. R.Tausch-Treml, A.Henglein, and J.Lilie, Ber.Bunsenges, Phys.Chem.,82 (1978) 13351343. C. Y. Fan, L. Jiang, Langmuir, 13 (1997) 3059-3062 X.M. Ou, J.R. Li, X.C. Li, L. Jiang, Chinese Science Bulletin, 43 (1998) 790. X.Y Chen, J.R. Li, S.J. Pang, A.L. Shen, L. Jiang, Surface Science, 441 (1999) L891896 X.Y. Chen, J.R. Li, L. Jiang, Nanotechnology, 11 (2000) 108-111. S.X. Ji, C.Y. Fan, RY. Ma, L. Jiang, Thin Solid Film, 242 (1994) 16-20. J. Willard Gibbs, The collected v^orks of J. Willard Gibbs, Ph. D., LL. D., Longmans, Green and Co., New York, 1931. L J. Langmuir, Am. Chem. Soc, 39 (1917) 1848. C.B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc, 115 (1993) 8706-8715. RQ. Tang, J.R. Li, L. Zhang, L. Jiang, Biosensor and Bioelectronics, 7 (1992) 503 Z.J. Chen, X.M. Ou, RQ. Tang, L. Jiang, Colloid & Surfaces B: Biointerface, Colloids and Surfaces B: Biointerfaces, 7 (1996) 173-179. X.Y.Chen,J.R.Li, X.C.Li, L.Jiang, Biochemical and Biophysical Reserch Communication, 245(1998)352-354. T.Miyasaka, K.Koyama, I.Itoh, Science, 255 (1992) 342-344. RT.Hong, Progress in Surface Science, 62 (1999) 1-237 R.R.Berge, L.A.Findsen, B.M.Pierce, J.Am.Chem.Soc, 109 (1987) 5041-5043. K.Koyama, N. Yamaguchi, T. Miyasaka, Science, 265 (1994).762-765. Y.H. Sun, J.R. Li, B.R Li, L. Jiang, Langmuir, 13 (1997) 5799-5801. Gerald Schochetman and J.Richard George (eds.), AIDS Testing, T^ Edi., SpringerVerlag, New York, 1994 L. Lin, H.Q. Zhao, J.R.Li, J.A.Tang, M.X. Duan and L. Jiang, Biochemical and Biophysical Reserch Communication, 274 (2000) 817-820.

Sludies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) t^C' 2001 Elsevier Science B.V. Ail rights reserved.

519

Strong capillary attraction between spherical inclusions in a multilayered lipid membrane K. D. Danov", B. Pouligny^ M. I. Angelova'''" and P. A. Kralchevsky' ^Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, University of Sofia, 1 J. Bourchier Ave., Sofia 1164, Bulgaria ^'Centre de recherche Paul-Pascal, CNRS, Avenue Schweitzer, Pessac 33600, France "^Institute of Biophysics, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Strong attraction has been experimentally observed between two spherical latex particles, which are included into the membrane of a giant spherical phospholipid vesicle. We interpret this attraction as a capillary force resulting from the overlap of the menisci formed around each of the two encapsulated particles. A theory is proposed to describe films containing large inclusions, i.e. when the particles are much greater than the natural film thickness. First theoretical results for the capillary forces are in line with the experimentally observed trends.* 1. INTRODUCTION The attachment of colloid particles to a fluid interface, or their confinement in a liquid film, is often accompanied with interfacial deformations, i.e. appearance of menisci around the separate particles. The overlap of the menisci around such two particles gives rise to a lateral capillary force between them which is attractive for similar particles, see e.g. Ref [1]. Depending on the physical origin of the interfacial deformations, two types of lateral capillary forces can be distinguished. In the case of flotation force the meniscus is due to the weight of floating particles, including the buoyancy force [1-5]. This force causes two-dimensional aggregation of floating particles [6,7]; its magnitude is proportional to R^ (R = particle radius). For that reason the flotation capillary force strongly decreases with the decrease of R and becomes immaterial for R smaller than c.a. 5 |im. The other type of capillary force, the immersion force, appears between particles that are confined in a liquid film with one or two deformable surfaces [8], see Figure la,b. In this case the deformation is related to the wetting properties of the particle surfaces (contact angles, edges), rather than to gravity. It turns out that the immersion force can be significant between particles of radius larger than a few nanometers [1,4]. It has been found to promote the growth of two dimensional crystals from colloid particles, viruses and globular proteins, see Ref [9]. The theoretical description of the lateral capillary forces is based on the Laplace equation which determines the shape on the liquid meniscus, z = C(^j). In general, it is a second order *This study was performed in the framework of the Franco-Bulgarian Laboratory "Vesicles and Membranes" supported by CNRS, France.

520

CCv,v)

Fig. 1. The capillary "immersion" force is due to the overlap of the menisci formed in a vicinity of colloidal particles confined in liquid films, (a) Two particles in a wetting film on a solid substrate, (b) Two particles in a free film; z - Cj<x,y) describes the meniscus shape (deviation from planarity). nonlinear partial differential equation. However, when the particles only slightly deform the interfaces, i.e. when slopes are small (iVn^^«: 1), the Laplace equation simplifies into the linear form [1,8,9]:

v?.f = ^^C,

^

d

d

(1)

Vii is the two-dimensional gradient operator in the x,y plane. When gravity drives the deformation, q^ is the usual capillary length. In the case of a thin film of thickness /?, Eq.(l) holds when ^//i «: 1. Then the characteristic length, cf^ , is given by: (2) where n is the disjoining pressure and a is the surface (membrane) tension, /zo is the thickness of the non-perturbed film. Using cylindrical coordinates (r,(jo) in Eq. (1) one can determine the shape of the meniscus around a single particle in the form ^(r) = A Ko(^r), where Ko is the modified Bessel function of the second kind and zeroth order and A is a constant of integration. Next, one can apply the superposition approximation [2], i.e. assume that the interfacial deformation caused by two particles (Figure 1) is equal to the sum of the deformations caused by the separate particles in isolation. Then, in view of the expression for ^(r), the energy of lateral capillary interaction between the two particles can be expressed in the form [1,3,9]

b^N^-lncQxQ.Y^iqL)

(3)

where L denotes the distance between the two particles, Q, = r, sini//; (/ = 1,2) are the so called "capillary charges", r, and \\fi are the radius of the contact line and the slope angle at the contact line of the respective particle. AW represents a variation in the gravitational energy or in the energy of wetting, for floatation or immersion forces, respectively, see Ref. [9]. It is worthwhile noting that Eq. (1) is not valid for all physically possible cases. Indeed, the relationship C,lh^« 1 corresponds to comparable magnitudes of the particle diameter and the film thickness. However, it is possible the particle size to be much greater than the film thickness, ^//2o» U see Figure 2. Then, the meniscus profile obeys the equation

521

Fig. 2. Two colloidal spheres entrapped in a free liquid film, whose thickness ho is much smaller than the particle diameter. "Plateau borders" of outer radius r^ and peripheral contact angle a are formed around each sphere; 6c is a solid-liquid-fluid contact angle. aV^if = AP = const

(^//ZQ »

1)

(4)

Here AP is the pressure jump across the meniscus; AP is constant if the effect of the gravitational hydrostatic pressure is negligible. Note, that mathematically Eq. (1) is a Helmholtz-type equation, whereas Eq. (4) is a Laplace-type equation. Consequently, unlike Eq. (1), equation (4) has no solutions which are finite at infinity. The latter fact implies that the meniscus around each particle must end at a peripheral contact line (of radius rp), out of which the film is plane-parallel (^= 0), see Figure 2. In this case the overlap of the menisci, and the interaction between the particles, begins when they come at a distance L < 2r^ from each other; such type of interaction is obviously different from that described by Eq. (3). For menisci oi finite extent, whose shape is described by Eq. (4), the problem about the lateral capillary force has not yet been addressed theoretically. This problem deserves to be investigated insofar as the experiment provides evidence about the existence of capillary interaction between particles surrounded by such menisci. For example, Velikov et al. [10] observed a strong attraction between latex particles of diameter 2R ~ 1 \xm entrapped in a foam film of thickness ho ~ 0.07 ^im. In the next section, we further illustrate this point by similar observations with micron-sized latex spheres encapsulated within the multi-lamellar membrane of a giant lipid vesicle. As we will see, these experiments neatly reveal the film deformation caused by the particles and the related attraction. In Section 3, we briefly report some results of the theory, essentially in relation to the experiments of interest in Section 2. A complete presentation of the theory will be the matter of a forthcoming article [II]. 2. EXPERIMENTS WITH LATEX PARTICLES ATTACHED TO LIPID VESICLES Phospholipids in water self assemble into bilayers about 4 nm in thickness. Different preparation procedures allow to produce vesicles, whose membranes are constituted of one or a few such lipid bilayers [12]. In our experiments, we use so-called "giant vesicles", grown by electroformation [13]. The method produces a cluster of vesicles on the surface of a platinum electrode. The vesicles at the outer boundary of the cluster are approximately spherical, with diameters in the 10-100 micrometers range. On their "rear" sides (towards the electrode), they are connected to neighboring vesicles by a few contact points, sometimes by a small adhesion area. Their outer sides are free of contacts.

522

(a)

(b)

Fig. 3. Photos of a couple of latex beads trapped between two detached lipid membranes composing a multilamellar spherical vesicle. Each photo is an equatorial cut of the vesicle, (a) The two beads are side by side in the same equatorial plane, (b) Section perpendicular to the line that goes through the centers of the beads: the two beads are still present, but one is just on top of the other. Electroformation is a nice method to produce unilamellar giant vesicles; nevertheless, multilamellar specimens can be found in the cluster, in proportions that subtly depend on the used lipid and on the parameters of the procedure. The experiment of interest here is one of many which were carried out [13,14] to study the interactions of polystyrene latex particles with the membranes of giant lipid vesicles. We use an optical trap to manipulate single particles and bring them in contact with vesicles. It is generally observed that simple latex spheres (whose surfaces bear sulfate groups) spontaneously adhere to neutral lipid vesicles [13]. The membranes in the photographs below (Figure 3) were in fact positively charged, but similar behaviors, in terms of particle encapsulation and capillary interactions, were observed with neutral vesicles. When a particle gets in contact to a lipid membrane, different scenarios may happen, leading to a partial wetting of the particle surface by the lipids or to a complete encapsulation [13]. The photos in Figure 3 reveal that the vesicle was bi-lamellar. Each particle was incorporated between two lipid lamellae and pushed them apart, creating a water gap in between. Below we will call "Plateau border" this narrow region filled with water, making analogy with the similar formations in foams. At the present stage, the dynamics of particle incorporation and membrane delamination is poorly understood [13], but this is not the important point here, in as much as we are interested only in the final configuration. Indeed, the experiment is a physical realization of the configuration sketched in Figure 2, i.e. a film with 2 solid inclusions whose diameters, about 4.3 micrometers, are considerably greater that the equilibrium film thickness (a few nanometers). The optical setup can be operated in a double trap configuration (2 pairs of laser beams, see Ref. 15), which allowed us to catch each particle and to vary the interparticle separation. For instance, starting with the configuration of Figure 3a, i.e. with both particles in the vesicle equatorial plane, we pulled them apart. When the particles were separated, we switched off the beams and noticed that the particles would move back towards contact, proving the existence of a long range (micrometers) attraction. The observation could be repeated at will. Interestingly, we found that the capillary attraction would reach a maximum not when the particles were in contact but when they were separated by a distance comparable to their diameter (about 4 microns). We estimated the maximum force to be of the order of 10 pico-N.

523

As we mentioned, the observation was repeatable. This does not mean that the experiment can be systematically reproduced with any vesicle and any couple of particles. In fact, we observed capillary attractions only sporadically among hundreds of different systems. This is not surprising, because the simultaneous occurrence of a bi- or multi-lamellar membrane and of a proper encapsulation of both particles is a rare event. We observed the attraction first with small particles, about 2 microns in diameter [16]. In this case, we estimated the interaction energy at contact on the order of ksT (the thermal energy). Conversely, the attraction energy of large (15 microns) particles turned out enormous. By "enormous", we mean a value well beyond 10^ ICBT, which is about the maximum optical trapping energy in these experiments. Apparently, the attraction drastically increases with the particle size. 3. THEORETICAL CALCULATION OF THE CAPILLARY FORCE To calculate the capillary force for the configurations in Figures 2 and 3 we solved the Laplace equation (4), which governs the shape of the capillary menisci (of the Plateau borders). We applied bipolar coordinates (w,v) in the plane xy\ see Ref [17]: x = gsinhu, ^

+^

y = gsinv, = g^^

g =a/(coshu-cosv)

(5) (6)

where a is a parameter related to the radius of the contact line and the distance between the two particles. To determine AP, which is equivalent to determining the pressure of the inner liquid (captured between the two detached menisci. Figure 2), we used the assumption that the total volume of the captured liquid is constant: V =4\\^{u,v)g'

dudv = const

(7)

Equation (6) was solved by numerical integration, using Eq. (7) and a combination of boundary conditions at the peripheral line (fixed peripheral line or fixed peripheral angle a) and at the contact line on the particle surface (fixed contact line or fixed contact angle 6c), see Figure 2 for the notation. Thus the meniscus shape ^(w,v) was determined. Next, from the calculated shape we computed the capillary force F exerted on each particle. The latter is a sum of contributions due to the pressure, integrated over the particle surface, and the surface tension, integrated along the contact line, see Ref [9] for details. Due to the symmetry of the system, F is directed along the jc-axis, passing through the centers of the two particles. Figure 4 shows a plot of the dimensionless force, Fx/{(Tr,), vs. the surface-to-surface distance between the two particles, scaled with the radius of the contact line, n (see Figure 2); here Fr = ^x^ For typical experimental values (r, > 2 ^m, CT > 0.1 mN/m), the calculated capillary force is greater than the lateral force of the optical trap and can explain the observed spontaneous "escape" of the particles from the trap and their sticking together. In consonance with the experimental observations, the calculated attractive force exhibits a local maximum (maximum negative Fx) at a certain intermediate value of the interparticle separation. It should be noted that Eq. (7) and the boundary condition at the peripheral contact line lead to a nonlinear boundary^ problem, despite the fact that the differential equation (6) is linear. For that reason we had to use a numerical solution. Moreover, the nonlinearity of the boundary

524

problem forbids the application of the superposition approximation [2]. A detailed description of our experiments and theoretical considerations can be found in Ref. [11].

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Distance between particles Fig 4. Calculated plot of the dimensionless force FJ{Grc) vs. the surface-to-surface distance between two identical particles, scaled with the contact line radius, r^. The contact line at the particle surface is fixed at hc/rc = 1, whereas the peripheral contact line is movable at peripheral contact angle a = 0; the total volume of the liquid in the two Plateau borders is VliTtrJ") = 2, see Figure 2 for the notation. REFERENCES 1. 2. 3. 4.

P.A. Kralchevsky and K. Nagayama, Langmuir, 10 (1994) 23. M.M. Nicolson, Proc. Cambridge Philos. Soc., 45 (1949), 288. D.Y.C. Chan, J.D. Henry and L.R. White, J. Colloid Interface Sci., 79 (1981) 410. V.N. Paunov, P.A. Kralchevsky, N.D. Denkov and K. Nagayama, J. Colloid Interface Sci., 157(1993) 100. 5. C. Allain and M. Cloitre, J. Colloid Interface Sci., 157 (1993) 261; ibid. p. 269. 6. P. Somasundaran, R. Varbanov and S. Tchaliovska, Colloids Surf., 64 (1992) 35. 7. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1992. 8. P.A. Kralchevsky, V.N. Paunov, LB. Ivanov and K. Nagayama, J. Colloid Interface Sci., 151 (1992)79. 9. P.A. Kralchevsky and K. Nagayama, Adv. Colloid Interface Sci., 85 (2000) 145. 10. K.P. Velikov, F. Durst and O.D. Velev, Langmuir, 14 (1998) 1148. 11. K.D. Danov, B. Pouligny and P.A. Kralchevsky, Langmuir (2000) - submitted. 12. R. Dimova, C. Dietrich and B. Pouligny, in "Giant Vesicles", P. Luisi and P. Walde (eds.), John Wiley & Sons, New York, 1999; p. 222. 13. C. Dietrich, M. Angelova and B. Pouligny, J. Phys. II France, 7 (1997) 1651. 14. R. Dimova, C. Dietrich, A. Hadjiisky, K. Danov and B. Pouligny, Eur. Phys. J. B, 12 (1999)589. 15. M.I. Angelova and B. Pouligny, Pure Appl. Optics, 2 (1993) 261. 16. M.I. Angelova, B. Pouligny, G. Martinot-Lagarde, G. Grehan and G. Goulesbet, Progr. Colloid Polym. Sci., 97 (1994) 293. 17. G.A. Kom and T.M. Korn, Mathematical Handbook, McGraw-Hill, New York, 1968.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors)
525

Controlled Growth of Gold Nanoparticles in Organic Gels Tetsu Yonezawa,* Masaya Fukumaru, and Nobuo Kimizuka* Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan Behavior of dodecanethiol-passivated gold nanoparticles immobilized into organic gels was investigated. Although these nanoparticles are quite stable and no significant changes are observed for months in solutions, growth of nanoparticle in the gel matrix was found by UV-Vis and TEM observation in a few days after the preparation.

1. INTRODUCTION Control on the dimension of metal nanoparticles has been a topic of gathering much interests, since they often display novel properties arising from "quantum size effect", [l] Especially, immobiUzation and assembling of nanoparticles is an important issues for their practical applications. [2] As the immobihzation procedure, adsorption of nanoparticles onto solid substrates has been extensively investigated. On the other hand, soft molecular assembUes as exemplified by organogels has never been employed as a matrix for immobilization of nanoparticles. Organogels formed by low molecular mass gelators contain a large volume of liquid. [3] As uniform metal nanoparticles are usually prepared in liquid phase, gelation of the liquid should provide a simple way for the homogeneous immobiUzation of nanoparticles. In this study, we report on the specific growth of gold particles in organogels, which provides a novel means to control nanoaprticle structures. 2. EXPERIMENTAL Dodecanethiol-passivated gold nanoparticles were prepared by the well-established two-phase reduction procedure of AuCU by NaBH4 in the presence of dodecanethiol. [4] The average diameter of the nanoparticles obtained by TEM investigation was 2.7 nm. For an immobilizing matrix, we have chosen a physical gel formed by 12-hydroxysteaUc acid (CH3(CH2)5CH(OH)(CH2)ioCOOH, Wako) in toluene. [5] Into the toluene dispersion of gold nanoparticles, toluene solution of 12-hydroxysteaUc acid (60 ""O was introduced. Then, the mixture was kept at 50 °C for 5 min to obtain a homogeneous red dispersion. The obtained

526

dispersion was kept at r. t. under saturated toluene vapor, and homogeneous toluene gels containing gold nanoparticles were obtained. UV-Vis spectra were observed at 20 °C by Jasco V-550 spectrophotometer equipped with a 1-mm quartz cell. Transmission electron microscopy (TEM) was conducted by the use of JEOL JEM-200CX (HVEM Lab., Kyushu Univ.) at an acceleration voltage of 200 kV. 3. RESULTS AND DISCUSSION 3.1.

Stability of Gold Nanoparticles

In order to prepare organic gels by using self-assembly of 2.0 small organic molecules, their 1.5 heated homogeneous solution • should be added to the gold oc m 1.0 nanoparticles dispersed in -e toluene. Therefore, we firstly o 0.5 investigated thermal stability of gold nanoparticles in toluene. 0.0 In Figure 1, UV-Vis spectra 300 400 700 800 500 600 of fireshly prepared gold wavelength (nm) nanoparticle in toluene and that after keeping for 5 min. at 60 °C Fig. 1. UV-Vis spectra of the toluene dispersions of were collected. As can be seen Ci2SH-8tabili2ed gold nanoparticles. Narrower lineas prepared, thicker line: after heated up to 60 **C. in this figure, the two spectra [Au] = 3.6 X 10 3 mol dm 3. 7= 1 mm, 20 *»C are completely identical, and it clearly indicates that no aggregation or flocculation of gold nanoparticles was occurred in toluene dispersions even at 60 **C. 3.2.

UV-Vis spectral change of toluene gel incorporating gold nanoparticles

2.5 The freshly prepared toluene gels containing 2.7-nm gold 2.0 nanoparticles initially showed a 1.5 red color with ^max at ca. 500 nm (Figure 2), which is 1.0 analogous to that of the 0.5 disperse state (Figure 1). However, after one day, the gold 0.0 500 600 nanoparticle containing gel 300 400 700 800 turned purple and the spectrum Wavelength / nm was broadened considerably with a red-shift (Xmax at ca. 540 Fig. 2. UV-Vis spectra of the toluene gel containing Ci2SH-8tabilized gold nanoparticles. [Au] = 7.2 x nm), although such a color 10 3 mol dm 3; [gelator] = 6.0 x 10 2 g cm 3.

527

5.8 nm

•i.-ifT: ; • %

••

•?

50 45

"

• « • ;

'•

40 ^ *•,

•*., '%:::• •#*!'*

^^-t

35

-^30 2 25 3

•e 20

1

.2 15 "O

10

•'• 0 50 nm

0

Jxl ' iW^'P--

1 2 3 4 5 6 7 8 9 diameter (nm)

10 11

Fig. 3. TEM image and the size distribution of Ci2SH-8tabilized gold nanoparticles immobilized in a toluene gel for 4 days. [Au] = 1.8 x lo 3 mol dm'^; [gelator] = 6.0 x 10 2 g cm 3. Average particle size = 5.8 nm. The image was taken with a JEOL JEM-200CX, acceleration voltage = 200 kV.

change was not observed for the toluene solution without 12"hydroxystearic acid. After 4 to 10 days, the red shifted absorption became narrower, and the 540-nm absorption was well preserved. The observed red shift and broadening of the plasmon band of gold nanoparticles should be ascribed to the formation of larger particles or aggregates. [6] In many cases, the spectral change of organic molecule stabilized gold nanoparticles was ascribed to the formation of aggregates. However, narrower peaks observed for the gels after 4 to 10 days cannot be explained simply by aggregation of the particles. Thus, TEM observations of the nanoparticles were carried out. 3.3

TEM observations of gold nanoparticles in toluene gels

To investigate the structural changes of the gold nanoparticle, TEM observations were conducted for samples prepared by pressing carbon-coated copper grids onto freshly cut gel surface. We have reported that gel-surface nanostructures are transferable onto soUd supports and are amenable to AFM observations (Gel-assisted transfer technique). [7] In Figure 3, the TEM image and the size distribution of incorporated nanoparticles in toluene gel (after 4 days of preparation, [Au] = 1.8 x 10"3 mol dm"^, [gelation reagent] = 6.0 x 10 2 g cm^) were collected. Gold nanoparticles have close-packed structures in small domains. Surprisingly, the particle size became much larger (average 5.8 nm) than that of the particles in dispersion (average 2.7 nm). The particles observed in the image are quite uniform and all spherical. Moreover, no aggregated structure was found in the image. The edge-to-edge inter-particle distance is ca. 1.5 nm, which is analogous to the reported values of dodecanthiol-stabilized nanoparticles. [8] This indicates that the larger particles shown here should be covered by

528

dodecanethiol monolayer. High resolution TEM observation was also applied to the nanoparticles in gels (not shown here). Clear fringes were observed in each particle. These fringes suggest that the each particle has one crystal structure. On a solid substrate, 4.5-nm gold nanoparticles stabiUzed by dodecanethiol were connected to form a semi-continuous network after keeping under air for several months. Smaller cubooctahedra particles exposing both {111} and {100} surface facets, and the latter facet is not stable. [9] In our case, in the toluene gel, the particle size of gold nanoparticles grew much faster (4 ~ 10 days) than on solid surfaces, probably due to the relatively higher mobility of nanoparticles in gel matrix. Interestingly, however, prolonged immobiUzation did not give larger particles. Such restricted particle growth in the matrix has been reported for those surrounded by the {ill} facets. [9] These observations clearly indicate that organogels formed from molecular assembly provide unique opportunity to control structures of gold nanoparticles. 4. CONCLUSION Immobilization of gold nanoparticles by using a low molecular mass gelator was proposed. The particle dispersion could be immobiUzed homogeneously. The obtained gel was stable but Xm&x of the specific plasmon absorption shifted to longer wavelengths. Uniform growth of nanoparticles was observed by TEM investigation, and this provides a novel means to control dimensions of nanoparticles by the use of soft molecular systems. REFERENCES 1. N. Tbshima and T. Yonezawa, New J. Chem., (1998) 1179; T. Yonezawa, S. Onoue, and N. Kimizuka, Langmuir. 16 (2000) 5218; G. Schmid, M. Baiunle, M. Geerkens, I. Heim, C. Osemann, and T. Sawitowski, Chem. Soc. Rev., 28 (1999) 179; J. H. Fendler, Chem. Mater., 8 (1996) 1616; A. Henglein, J. Phys. Chem. B, 104 (2000) 6683. 2. T. Yonezawa, S. Onoue, and T. Kunitake, Adv. Mater., 10 (1998) 414; T. Yonezawa, S. Onoue, and T. Kunitake, Chem. Lett., (1998) 689; T. Yonezawa, H. Matsune, and T. Kunitake, Chem. Mater., 11 (1999) 33; J. P. Spatz, S. Mossmer, M. MoUer, Chem. Eur. J., 2 (1996) 1552. 3. T. Pierre and R. G. Weiss, Chem. Rev., 87 (1997) 3133; K. Hanabusa, H. Kobayashi, M. Suzuki, M. PCimura, and H. Shirai, Colloid Polym. Sci., 276 (1998) 252. 4. M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, and R. Whyman, Chem. Commun., (1994) 801. 5. T. Tachibana, T. Mori, and K. Hori, Bull. Chem. Soc. Jpn., 53 (1980) 1714. 6. J. S. Bradley, in Clusters and CoUoids, G. Schmid Ed., VCH, Weinheim (1994) pp. 506-508. 7. N. Kimizuka, M. Shimizu, S. Fujikawa, K. Fujimura, M. Sano, and T. Kunitake, Chem. Lett., (1998) 967. 8. B. A. Korgel, S. Fullam, S. Connolly, and D. Fitzmaurice, J. Phys. Chem. B, 102 (1998) 8379. 9. C. J. Kiely, J. Fink, M. Brust, D. BetheU, and D. J. Schiffrin, Nature, 396 (1998) 444. Acknowledgement: This work is partly supported by Grant-in-Aids for COE Research (08CE2005), for Basic Research (C) (11640585) and for Encouragement of Young Scientists (for TY, 12740383) from Ministry of Education, Science, Sports, and Culture, Japan.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) fc) 2001 Elsevier Science B.V. All rights reserved.

529

A model of self-assemling nanoparticles due to capillary forces Ken-ichi Yoshie^ Shinya Maenosono'', Yukio Yamaguchi*' a Non-equilibrium Lab. Yokohama Research Center, Mitsubishi Chemical Corporation. 1000 Kamoshida Aoba-ku, Yokohama 227-8502, Japan b Dept. Chem. System Eng. Univ. of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan We studied self-assembling pattern formation during the evaporating alkane suspension of nano particles. We found that these patterns show self-similarity and their fractal dimension increases as the particle concentration of suspension increases or as the molecular weight of the solvent alkane decreases.

We discussed the mechanism of the pattern formation

regarding the inhomogeneous convective motion of particles, which is likely resulted from the fact that the contact angle varied with the particle concentration.

The simulation results of

particle accumulation by two-dimensional capillary force were compared with the experiments. 1.Introduction The ultimate goal of our study is to fabricate nano particle based devices like LED, sensors, and laser. Coating of nano particles suspension is easy and cheap, however there are many problems on controlling the process in order to obtain desired structure of thin particles layer. To understand the fundamental of the wet coating of nano particles, we studied the relation between the pattern formation and process parameters during drying the droplet of nano particle suspension on a glass substrate. 2.£xperiniental CdSe nano particles that mean diameter is ca. 3.5nm and its standard deviation is ca. 10%

530

were synthesized by so-called hot soap method. The synthetic procedure is described elsewhere'^'^\

The particles adsorb TOPO (Tri-Octyle-Phosphine-Oxide) that weight ratio is

ca 50 % of the particle. Nano particles were dispersed in Pentane, Hexane, Heptane, and Octane at .0.05, 0.5, and 5 wt %. The suspension of 3^1 was dropped on the glass plate that was cleaned by sulfuric acid and was dried prior to the experiment, then we observed the particle layer pattern, which was formed during the drying, through optical microscope. 3.$imulation We simulated pattern formation of particles regarding two-dimensional capillary force^^'*^ We assumed that the liquid depth is half of the particle diameter and the liquid is not vaporized during the coagulation.. 4.Results As shown in Fig.l, nano particles formed different patterns according to the particle concentration and the kind of solvent. The images of the patterns were converted to binary data and the relations between the number of boxes which contain particles and the box size were obtained by box counting method^^ as shown in Fig.2. The patterns exhibit selfsimilarity then the fractal dimensions were calculated. The fractal dimension is increased as the molecular weight of the solvent alkane decreases or the particle concentration decreases (Fig.3). We also found that the contact angle between the suspension and glass plate decreases as the particle concentration increases (Fig.4). Simulation results showed that the dot like aggregate structure appeared as shown in Fig.5. S.Discussion The pattern formation of the particles is possibly determined by following factors. l)Convective motion of particles and two-dimensional capillary force among the particles^^'^)'^^'«>. 2)Dependence of contact angle of the suspension on the particle concentration. 3)Deposition of particles on the substrate.

531 4)Separation of the suspension liquid from the deposited particles during the drying. The ring formation

and

Particle concentration [\\t %

the separation of liquid from the

ring

domain

were

repeated then the multi-ring IPentane morphology were observed. The flux of particles towards the

droplet

periphery

is

increased, as the contact jHexane angle of the suspension is decreased"*\ In the case of the nano particle suspension, the lower the concentration the larger the contact angle. Thus Heptane it is possible that if there are fluctuations of the particle concentration beginning

at

the

of the droplet Octane

evaporation on a substrate, particles

tend

to

be Ftg.l The patterns of nano particles after

transported more to higher

drying the suspension droplet.

concentration region. This inhomogeneous

convective

motion

of

10 -S- 0.05wt% -O-0.5wt% -0-5wt%

particles is likely the cause of the petal-like pattern formation, which was observed in the case of, for instance. Heptane 0.5%. At low particle concentration, dots array were formed. According to the simulation

00

o

results, the aggregates form one large dot if the particles were isolated in a certain area. Fig.2 The relation between box size s and the number of boxes N(s), which contain particles. The solvent is Pentane.

-4

-3

-2 log(l/s)

532

1.8 -B- 0.05wt% -O-0.5wt% -O 5wt%

c B

1.0

0

50

100

150

200

250 300

1

vapor pressure fmmHgl

2

3

4

5

concentration fwt%l

Flg3 Thefractaldimension of the pattern

Fig.4 The nomializied contact angle of

Vapor pressure atroomtonperature of

the nanoparticle suspension in Hexane.

the solvent is used as the variable.

Bs is the contact angle of Hexane

5fxm

%

Fig.5 DEM simulation results. 200 nano particles of 5nm are assembled by 2D capillary force References l)AP.Alivisatos, Science 221933 (1996). 2) C.B.Murray,D.Jnorris and M. Bawendi, J. Am. Chem. Soc.Jii, 8706 (1993) 3)S. Maenosono, C.D.Dushkin Y. Yamaguchi, K. Nagayama and Y. Tsuji , Colloid Polymer Science 2ZZn52

(1999)

4)S.Maenosono, C.Dushkin , S. Saita and Y. Yamaguchi,

Langmuir,, 15, 957-965 (1999)

5)P. Meakin, "Fractals,scaling and growth far from equilibrium", Cambridge University Press, (1998) 6) Nagayama Supramolecular Science, 1,(1-3) 111 (1996) 7)M. Yamaki, J. Higo and K. Nagayama, Langmuir l i , (8) 2975 (1995) 8)A.S. Dimitrov, C.D.Dushkin, H. Yoshimuraand K. Nagayama, UngmuirlD, 432 (1994)

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

533

Thin films of semiconductor nanocrystals self-assembled by wet coating S. Maenosono*, Y. Yamaguchi*, and K. Yoshie*" 'Department of Chemical System Engineering, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan **Non-equilibrium Laboratory, Yokohama Research Center, Mitsubishi Chemical Corp., 1000 Kamoshida-cho, Aoba-ku, Yokohama, Kanagawa 227-8502, Japan Thin films of colloid chemically synthesized CdSe nanocrystals are self-assembled by wet coating. The CdSe nanocrystal films exhibit an intriguing phenomenon called excitation-time dependent luminescence (ETDL).^ Photoluminescence (PL) intensity of the film increases dramatically under continuous excitation. ETDL strongly depends on coating processes and conditions, namely structures of nanocrystals array in the film. Here we report ETDL of thin films obtained by spin coating and drying a suspension droplet. 1. INTRODUCTION The synthesis and self-assembly of nanocrystals and the physical properties of an isolated nanocrystal have been intensively studied. However there are few studies on a correlation between self-assembled nanostructure and its physical properties'"^ which can be different from properties of an isolated nanocrystal."* It comes to be more important to study physical properties of a nanocrystal array because new properties emerge from the array.^'^ There are many kinds of method to assemble nanocrystal arrays, e.g. aggregation in a wettingfilm,^'^spin coating,^ electric field deposition.^^ Recently, a much simpler process has been utilized to make rings of latex particles and semiconductor nanocrystals*' by drying a suspension droplet on solid substrate. Here we show ETDL of spin-coated film of CdSe nanocrystals and demonstrate the process dependence by measuring ETDL of a CdSe nanocrystal array obtained by drying a suspension droplet. 2. EXPERIMENTAL 2.1. Synthesis CdSe nanocrystals were synthesized by a colloid chemical method.* They were grown in tri-/i-octylphosphine oxide (TOPO) maintained at 350**C by injection of a mixture of Cd(CH3)2 and Se dissolved in tri-butylphosphine (TBP). Two batches of CdSe nanocrystals with average diameters d = 3.7 nm (absorption peak at X = 579 nm) and d = 2.2 nm (X = 500 nm). After the synthesis, they were separated by precipitation by adding poor solvent and drying in N2 atmosphere. The nanocrystals of diameter 3.7 nm and 2.2 nm dissolved in toluene and pentane, respectively. The solid concentration was 5wt% for both suspensions.

534

carbon •-^"-'^'A nanocrystal \:v. TOPO '^>1^^^£ Si02 Si wafer Fig. 1. Microstnicture of the CdSc nanocrystal monolayer obtained by spin coating: a) High-resolution SEM micrograph, b) Cross-sectional TEM micrograph. The nanocrystal suspension contains excess TOPO which can not be separated by precipitation processes. The nanocrystal monolayer is formed on an adsorption layer of excess TOPO molecules. 2^. Assembly Thin films of CdSe nanocrystals were formed by following two different methods. One of them was prepared by spin coating of a 100 \i\ of toluene suspension on Si wafer at 5000 rpm. The Si wafer was washed by sulfuric acid before use. As shown in Fig. 1, a densely packed monolayer of CdSe nanocrystals is obtained The other was prepared by drying a 3 jil of pentane suspension droplet on a glass substrate washed by sulfuric acid. The droplet was dried up within 10 seconds under ambient conditions (temperature ZS^'C, humidity 50%). As shown in Fig. 2, several concentric circles of nanocrystal rings are obtained. In the case of a CdS/pyridine suspension, only one thick ring is formed at the initial position of contact line.^^ The increase of the number of rings in case of CdSe/pentane is mainly due to the difference of drying rate of solvent. When the drying rate is too fast, a contact line rapidly recedes and a stick-slip is occurred. 2 J . Optical measurement The PL spectra from a spin-coated film were recorded using a Hitachi F-3000 fluorescence spectrophotometer. To measure the time dependence of PL intensity, the peak intensity was recorded under continuous excitation. The monochromatic light of wavelength 400 nm was used for excitation. The total luminescence intensity from a ring formed by drying of the suspension droplet is measured by taking fluorescent micrographs using a Nikon Fluophoto and analyzing the images by a computer. The weak monochromatic UV light was used for excitation. To observe the time dependence of luminescence intensity, the fluorescent images were taken at regular time interval (5 min) under continuous excitation.

Fig. 2. SEM micrograph of the CdSe nanocrystal rings which are obtained by drying of CdSe/pentane suspension droplet. The many white points of size about 1 \im seen in the image are aggregates of CdSe nanocrystals formed on the film.

535

3. RESULTS AND CONCLUSION As shown in Fig. 3, the PL spectrum from a thin film formed by spin coating clearly exhibits ETDL (PL intensity is dramatically enhanced as a function of excitation time). ETDL can be explained as a slow kinetics of electron transfer between CdSe nanocrystals and matrix (TOPO).^ Electrons which leave the nanocrystal core can be localized at trap sites in TOPO matrix and they gradually fill trap sites during continuous excitation. The accumulation of localized electrons increases the potential barrier for ionization and it leads to an increase of emission efficiency. The fluorescent micrograph of the CdSe nanocrystal ring is shown in Fig. 4. The intensity profile between point A and B in Fig. 4 is measured by analyzing the image (see Fig. 5). As shown in Fig. 5, one can see ETDL at the ring region, however it vanishes at outer region of the ring. In the inset in Fig. 5, the average intensities at each region are plotted versus time. This difference of ETDL between the ring and outer region of the ring can be explained by the difference of packing density of nanocrystals in the film. During the formation of ring region, the contact line is pinned and the nanocrystal array is epitaxially grown.^^'^^ On the other hand, the nanocrystals are rapidly aggregated when the contact line recedes forming outer region of the ring. It can be assumed that the ring region has much higher packing density of nanocrystals than outer region of the ring and a non-radiative recombination rate of the localized electron in TOPO matrix is accelerated by adsorption of ions from the air in the case of low packing density and the accumulation of localized electrons is declined. An intriguing phenomenon called excitation-time dependent luminescence (ETDL) observed in a CdSe nanocrystals thin film obtained by wet coating is reported. The PL intensity increases dramatically as a function of time under continuous excitation. A coating process dependence on ETDL is demonstrated by measuring ETDL of a CdSe nanocrystals ring obtained by drying a suspension droplet. ETDL is observed at ring region and not observed at outer region of the ring. It can be due to low packing density of nanocrystals at outer region of the ring.

0>

. . . . I . . . i i . . ..I

100

200

300

400

500

t (min) Fig. 3. ETDL effect: a) Time evolution of the PL spectrum from a spin-coated film at repeated excitation, b) PL intensity at the peak wavelength recorded under continuous excitation. The intensity / is normalized by the initial intensity /Q.

536 200

outer region of ring

c«

150

1 ^—'100 2:^

a c

ring

o Omin n 5min c lOmin A i5min

jsf ^-^ ^»11

30

1 0

A Fig. 4. Fluorescent micrograph of CdSe nanocrystal ring fabricated by drying a suspension droplet of CdSe/pentane on a glass substrate. The length between point A and B is 270 |im.

5 10 15 time (tilin)

B

Fig. 5. Time evolution of the PL intensity profile measured between point A and B in Fig. 4. The inset shows the normalized average intensities at each region (•: ring region, • : outer region of ring)

REFERENCES 1. S. Maenosono, CD. Dushkin, S. Saita, and Y. Yamaguchi, Jpn. J. Appl. Phys., 39 (2000) 4006 2. C.B. Murray, C.R. Kagan, and M.G Bawendi, Science, 270 (1995) 1335 3. C.R. Kagan, C.B. Murray, M. Nirmal, and M.G Bawendi, Phys. Rev. Lett., 76 (1996) 1517 4. M. Nirmal, B.O. Dabbousi, M.G Bawendi, JJ. Macklin, J.K. Trautman, T.D. Harris, and L.E. Brus, Nature, 383 (1996) 802 5. J.J. Shiang, J.R. Heath, C.R Collier, and R.J. Saykally, J. Phys. Chem. B, 102 (1998) 3425 6. G Markovich, C.R Collier, and J.R. Heath, Phys. Rev. Lett., 80 (1998) 3807 7. N.D. Denkov, O.D. Velev, P.A. Kralchevsky, LB. Ivanov, H. Yoshimura, and K. Nagayama, Langmuir, 8 (1992) 3183 8. A.S. Dimitrov and K. Nagayama, Langmuir, 12 (1996) 1303 9. A. Chevreau, B. Phillips, B.G Higgins, and S.H. Risbud, J. Mater. Chem., 6 (1996) 1643 10. M. Trau, D.A. Saville, and I.A. Aksay, Science, 272 (1996) 706 11. R.D. Deegan, O. Bakajin, T.F. Dupont, G Huber, S.R. Nagel, and T.A. Witten, Nature, 389 (1997) 827 12. S. Maenosono, CD. Dushkin, S. Saita, and Y. Yamaguchi, Langmuir, 15 (1999) 957 13. X. Peng, M.C Schlamp, A.V. Kadavanich, and A.R Alivisatos, J. Am. Chem. Soc., 119 (1997) 7019

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ip 2001 Elsevier Science B.V. All rights reserved.

537

Morphological homogenization of melamine lipid monolayer by using thermal molecular motion: formation of mesoscopic pattern based on hydrogen bonding network T. Kasagi,^ M. Kuramori," K. Suehiro," Y. Oishi," K. Ariga" and T. Kunitake' "" Faculty of Science and Engineering, Saga University, 1 Honjo, Saga 840-8502, Japan ^ Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan *" Graduate School of Engineering, Kyushu University, Fukuoka 812-8581, Japan The morphology and mesoscopic patterns of dialkylmelamine monolayer bound to thiobarbituric acid were investigated by atomic force microscopy. The surface morphology was effectively controlled through adjustment of the thiobarbituric acid concentration and thermal treatment. The thermal treatment of the monolayer provided uniform films comprised of striped mesoscopic patterns with molecular thicknesses. 1. INTRODUCTION We have been developed two-dimensional molecular patterning through specific molecular recognition at the air-water interface. For example, we demonstrated that a regular arrangement of alkyl chains can be artificially created in dialkylmelamine monolayers upon the binding of barbituric acid [1]. Melamines form an alternate hydrogen bonding pair with barbituric acid derivatives resulting in a linear hydrogen bonding network [1-5]. This structure forces the dialkylmelamine amphiphiles to align in a specific direction. Although the molecular arrangement can be defined, morphological control at the mesoscopic level is still unknown. Therefore, in this study we investigated the morphology of dialkylmelamine monolayers on aqueous thiobarbituric acid with respect to the thiobarbituric acid concentration and the thermal treatment by atomic force microscopy (AFM). 2. EXPERIMENTAL The benzene/ethanol solution (10:1 v/v) of 2C,4mela [6] (see Fig. 1) with a concentration of 7.5 X 10"* M was spread on aqueous thiobarbituric acid at a subphase temperature of 293 K. The monolayer was compressed to a surface pressure of 15 mN m ' and maintained for one hour to prepare mechanically stable monolayers. Each monolayer was transferred onto a freshly cleaved mica by the horizontal drawing-up method [7] for AFM observation. LB films for the other measurements were prepared on a gold-deposited glass plate by the vertical dipping method. The transfer ratio in both cases is unity, indicating that the substrate is completely covered with the monolayers. The AFM images of the monolayers were obtained

538

with SFA300 (Seiko Instruments) in air at 293 K. A 20 x 20 ^im' scan head and a sihcon nitride tip on a cantilever with a spring constant of 0.02 N m ' were used.

Regular Molecular Pattern in Two-Dimensional Plane

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"'O^S^O^'

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Thiobarbituric Acid

Fig. 1. Hydrogen bonding network ofdialkylmelamine and thiobarbituric acid.

3. RESULTS AND DISCUSSION The binding behavior of aqueous thiobarbituric acid to the melamine derivative monolayers was first investigated. Preliminary XPS analysis on the transferred LB films revealed that a dialkylmelamine binds aqueous thiobarbituric acid with a large binding constant (> lO'* M~') in an equimolar stoichiometry [8]. Hydrogen bonding between the melamine core and the thiobarbituric acid was confirmed by peak shifts in the FT-IR spectra of the LB films [9]. These results indicate formation of a hydrogen bonding network between the 2C,4mela monolayer and aqueous thiobarbituric acid. Morphology of the monolayer transferred on a mica plate was investigated by AFM. The 2C,4mela monolayer transferred from 5 mM aqueous thiobarbituric acid showed a uniform surface, but large platy aggregates with a height of 200—300 nm were detected (Fig. 2A). Tape-like patterns with a width of ca. 50 nm were observed in the uniform region (Fig. 2B). Assembly of the linear hydrogen bonding network probably results in formation of the tapelike mesoscopic pattern. The decrease in the concentration of the aqueous thibarbituric acid changes the morphology of the 2C,4mela monolayer. In the AFM images of the 2C,4mela monolayer transferred from 1 mM of aqueous thiobarbituric acid (Fig. 2C), the large platy aggregates disappear, whereas the small aggregates still remain. The magnified image in Fig. 2D indicates that the surface structure was comprised of domains with a diameter of 100 nm. Interestingly, each domain is an assembly of the short linear structures. Since the binding

539

Fig. 2. AFM images of 2Ci4mela monolaters on mica: (A), (B), monolayer transferred from 5 mM aqueous thiobarbituric acid; (C), (D), monolayer transferred from 1 mM aqueous thiobarbituric acid; (E), (F), monolayer transferred from 1 mM aqueous thiobarbituric acid with thermal treatment.

540 constant of the thobarbituric acid to the 2C,4niela is larger than 1 X 10^ M ' , most of the 2C,4mela molecules bind thiobarbituric acid even at 1 mM. Therefore, only small amount of deficiency in the hydrogen bonding network probably results in the shorter structure. Flexibility in the shorter structure would be advantageous for removing the platy aggregates. Thermal treatment was applied to this monolayer system. The 2C,4mela monolayer was first spread on 1 mM aqueous thiobarbituric acid at 330 K, and then the monolayer was cooled to 293 K at a rate of 0.6 K min"'. The obtained monolayer was compressed to 15 mN m ' at a rate of 0.8 mm s"'. After a waiting time of 1 h, the monolayer was transferred on freshly cleaved mica. The obtained monolayer shows a flat morphology without visible domains (Fig. 2E). Aligned long and wide striped structures are seen in the magnified image (Fig. 2F). A hole was detected at the mismatched boundary of the striped structures. Since the depth of the hole (1.4—1.8 nm) is comparable to the length of the 2C,4mela molecules, the striped structure has a thickness equal to that of the network. The thermal treatment (heating and gradual cooling) would lead to extension of individual hydrogen bonding networks and the alignment of the networks. 4. CONCLUSION Molecular arrangement in two-dimensional plane is controllable by the hydrogen bonding network, but it provides only a local structure. This study demonstrates that physical modification such as thermal treatment is indispensable for regulating structures at the mesoscopic level. The appropriate combination of molecular recognition and physical treatment would create regular molecular and mesoscopic patterns in two-dimensional plane. REFERENCES 1. H. Koyano, K. Yoshihara, K. Ariga, T. Kunitake, Y. Gishi, O. Kawano, M. Kuramori, and K. Suehiro, Chem. Common., 1996, 1769. 2. H. Koyano, P. Bissel, K. Yoshihara, K. Ariga, and T. Kunitake, Chem. Eur. J., 3 (1997) 1077. 3. Q. Huo, K. C. Russell, and R. M. Leblanc, Langmuir, 15 (1999) 3972. 4. C. M. Drain, K. C. Russell, and J.-M. Lehn, Chem. Commun., 19%, 337. 5. J. C. MacDonald and G. M. Whitesides, Chem. Rev., 94 (1994) 2383. 6. 2C,4mela was synthesized according to the procedure described in ref 2. A colorless powder, mp, 97.0—97.9 X ; TLC RfOA4 (CH2CI2-CH3OH (10:1 v/v)); 'H NMR (CDCI3, 300 MHz) 6 0.88 (t, 6, J = 6.7 Hz, 2 CH3), 1.0—1.3 (m, 44, 22 CH2), 1.3—1.5 (m, 4, 2 CH2CH2N), 3.1—3.3 (m, 4, 2, CH2N), 5.5—6.4 (br, 4, NH2 and 2 NH). Anal. Calcd for C3,H62N6:C, 71.76; H, 12.04; N, 16.20%. Found: C, 71.51; H, 11.95; N, 16.12%. 7. Y. Gishi, T. Kuri, Y. Takashima, and T. Kajiyama, Chem. Lett. 1994, 1445. 8. Guest/host ratio determined by XPS analysis on dialkylmelamine LB fibns: 0.626, 0.866, 0.974, L007, 1.043, L037 at 0.1, 0.5, 1.0, 5.0, 10.0, and 20.0 mM of thiobarbituric acid, respectively. These data provide binding constant of 1.45 xlO"* M"' with saturated guest/host ratio of 1.03 (20 °C). 9. FT-IR peaks (cm"^) of dialkylmelamine LB fihns: (C=N)„„g = 1545 and v(C=0) ^ 1719 for film transferred from 10 mM thiobarbituric acid; (C=N)„ng = 1606 for film transferred from pure water (v(C=0) peak was absent).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ic' 2001 Elsevier Science B.V. All rights reserved.

541

Formation and Structure of Organized Molecular Films of Fluorinated Amphiphiles with Vinyl Group Atsuhiro Fujimori, Tohru Araki and Hiroo Nakahara Department of Chemistry, Faculty of Science, Saitama University, 255 Shimo-okubo, Urawa, 338-8570, Japan The monolayer behavior at the air/water interface and the structure of deposited multilayers on solid substrates for esters of acrylic and methacrylic acids containing fluorocarbon chains, were investigated by surface pressure-area (TC-A) isotherms, Brewster angle microscopy (BAM), and scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction, respectively. It has been found that a minor change of the chemical structures have significant effects on the molecular packing and the surface structure of the organized molecular fihns. A introduction of a methyl substituent at the a-position of the vinyl ester groups influenced the molecular orientation in the monolayers and transferred films by the vertical dipping method, and also the atoms, fluorine or hydrogen, at the ©-position of the fluorocarbons of fluorinated amphiphiles have a considerable effect on the friction force for the Z-type films with the fluorocarbon chains exposed to air. 1. INTRODUCTION Organized molecular fihns [1] shall be developed to be candidates for biomimetic models [2] and molecular electronic devices [3], which are widely interested in the fundamental sciences as well as their potential applications [4]. In comparison with hydrocarbons, fluorinated amphiphiles are expected to provide a low energy surface as well as a characteristic surface potential by use of the monolayer assembling methods [5-6]. Recently, comb-polymers with fluorocarbons as the side-chams were used for their lubricant properties with low friction in addition to the high thermal and chemical stability as well as the extreme toughness and flexibility at very low temperatures [7-8]. Previously, for long-chain vinyl compounds containing the fluorocarbons with different lengths the thermal behavior related to the molecular packing in the solid states were investigated by differential scanning calorimetry (DSC) and X-ray diffraction [9]. In this study, for the fluorinated amphiphiles with a vinyl group the monolayer behavior on the water surface and the structures of the build-up fihns on substrates have been studied by TC-A isotherms, BAM, and X-ray diffraction, SEM together with a scanning probe microscopy, respectively, to obtain the two-dimensional molecular packing and orientation in the films depending on both the surface pressure and the temperature of the aqueous subphase as well as the chemical structure of the amphiphiles. 2. EXPERIMENTAL 2.1. Materials

542

As shown in Table 1, fluorinated amphiphiles used in this work, the aery late and methacrylate derivatives containing fluorocarbon chains 2-(perfluorodecyl)ethyl acrylate and methacrylate, F(CF2)ioCH2CH20CCX:(X)=CH2, [abbreviated as FFioEA for X=H and FFioEMA for X==CH3] and also IH, IH, 11 H-icosafluoroundecyl acrylate and methacrylate, H(CF2)ioCH20COC(X)=CH2, [F,oA and FioMA for X=H and CH3, respectively] were purchased from Daikin Fine Chemicals Co. Ltd. and purified by recrystallization repeated nme times from n-hexane solutions. Table 1 The long-chain vinyl compounds containing fluorocarbon chain used in this work Abbrev. Sample name FFioEA and FFioEMA 2-(perfuluorodecyl)ethyl acrylate and methacrylate F(CF2)ioCH2CH20CCX:(X)=CH2; X=H,CH3 FioA and FioMA IH, IH, llH-icosofluoroundecyl acrylate and methacrylate H(CF2)ioCH20COC(X)=CH2; X=H,CH3 2.2. Procedures The 7t-A isotherms for the monolayers on the water surface were measured by a Lauda film balance. The X-, Y-, and Z-type multilayers were deposited by a horizontal lifting, a Langmuir-Blodgett, and a surface lowering method, respectively. Morphologies of the outermost surface of the deposited films were observed by scanning electron microscope (Hitachi, model 4100) and a scanning probe microscope (Seiko Instruments, SPA 300 with SPI 3800 probe station). Values of the long spacing for the layer structures of the films were measured by a X-ray diflfractometer (Rigaku, Rad-B, CuKa radiation, 40kV, 30mA) equipped with a graphite monochromator. 3. RESULTS and DISCUSSION The 7C-A isotherms for the FFioEA monolayers exhibited characteristic phase transition about 10 to 15 mN/m in the temperature range of 5 to 20 °C and the molecular areas tend to decrease with an increase of the temperature of the aqueous subphase (Fig. 1). At 10 and 15 **C the limiting areas obtained 30 40 5(» 6« 20 i« Area / A'moleciile' from the second condensed state were 32 and 27 A^/molecule, Figure 1 The ic-A isotherms of monolayers on water respectively. These values are surface and corresponding AFM images of X-type well corresponding to the monolayers on mica for FFioEA cross-sectional area reported previously for the other perfluorinated amphiphiles, which is large enough to accommodate its helical segment. In the AFM images, a relatively flat surface was confumed at the lower surface pressure region, while in a little higher region, the defect of film appeared and the domain with higher hill was observed. After the phase transition, this defects disappeared gradually, and the structure with the hill direction developed remarkably, more and more.

543

(a) (b) 3.0 fim (^) ^-^M"^ Figure2 (a)BAM images of monolayer on the water surface at 25 mNm* and SEM images of X-type monolayers on silicon substrate for FFioEA at 25 mNm" (b) and 7 mNm* (c), before and after the phase transition respectively (5°C). Figure 2(a) is shown BAM images of monolayer on the water surface for the FFioEA at 25 mNm'^ after the phase transition. At 25 niNm'\ nearly hexagonal structure formed on the 15 mNm^ water surface, in spite of 35 mNm* observation for the formation of liquid monolayer at lower 70 B surface pressure region. The SEM image of FFioEA monolayers on silicon substrate 40 1 ^i'A \ \ \ showed dendrite structure at 25 30 7mNm" j L \ \ \ \ jtk-' mNm"* after the transition (Fig. 20 2(b)). Whereas, at 7 mNm'* 10 .^ i ?^c ^c hc ».c r,c ct. cr, cr, cr, cr. for FFioEA (Fig. 3). However, hc hc r,c ".c hc Ch I cr. cr. c^ ct. V f.cy \''^J \ r.cy V lic J \hc the tendency of decrease for the molecular area was remarkable at the second condensed state, / C M (C9A (ch\ r^M 'cr, ^ r,c '>c r,c r,c r,c in spite of appearance for the 18A Cf. cr. cp; \ch cr. r,c r,c k c ^c \»,c cr. Cfi cr. |cr, cr, larger molecular area about 90 r,c r,c \hc r,c r,c Cf, cr. cr. cr, cr. r,c r,c '>c 'fC A^molecule* at the fu*st 1 ''^ cr, ci^ \c9u cr, cr. 1 1 r.c r.c r.c r,c condensed state in comparison \\ J\ i \ jy x\ >/ 1 MMrate { with FFioEA monolayers. •)FFMEA b)FFwEMA While the FioA and FioMA Figure 4 Molecular structure for FFioEA and could form stable monolayers FFioEMA multilayers deposited by LB method. on the water surface without any plateau at lower temperature 5 °C, however these began to be partially collapsed at 10 °C. The result of X-ray diffraction showed that the molecules in the multilayers for FFioEA

^Jkif j | 1

v^c

^

•

^

544 1 / S . 'C'C "C* A ^^Mi^B tilted for 33 degrees to the surface normal, while FFioEA the FFioEMA molecules having relatively strong D FFioEMA A 'C - A aggregation force were ahnost perpendicular to the 1 A r" loA O FioMA surface (Fig. 4). In addition the FFioEA r multilayers formed the double layer structure as an a AA /A alternating Y-type fikn, while the latter fihn gave a 0.30 \ 'A O a o the first-order reflection only and formed A' CD QOD 1 non-alternating X-type fihn. This result / ^ g 0.20 corresponded to the previously report for the orientation of these compounds in the bulk state tS ois o estimated by X-ray powder diffraction, that is, 0 10 r FFioEA formed double layer structure in the bulk state, while the stable structure of FFioEMA was single layer structure. It has been found that a 10 15 minor change of the chemical structures influence Load/nN the aggregation of the fluorocarbons and the molecular arrangement. Figure 5. Plot of friction force versus applied load for the 2>type films of The fluorocarbons are known having characteristic low friction coefficient. Hence, we fluorinated amphiDhiles on mica. carried out the measurement of friction property for Z-type films with the fluorocarbon chains exposed to the air by friction force microscopy (Fig. 5) [10]. For the outermost surface with the fluorocarbon chains, the friction properties of these Z-type films could be also examined using atomic force microscopy by measuring frictional forces as a function of the applied load for each system. The difference of voltage was detected as a measure of friction force. The inclination and intercept on the vertical axis could be considered a friction and a attractive force between the tip and the film, repectively. A significant difference between the friction forces of FFioEA and FioA has been found for applied loads up to 20 nN, which can be ascribed to the friction properties of only the outermost surface of the Z-type films. For the Z-type fihns of FFioEMA a low friction was observed to be similar to that of FFioEA for the loads up to 15 nN. While the FioMA fihns took a relatively high friction behavior similar to the FioA for the loads only up to 10 nN.

i

REFERENCE 1. Gaines, G.L.Jr. Insoluble Monolayers at Liquid Gas Interfaces, Wiley: New York, 1966. 2. Kuhn, H., Mobius, D. and Bucher, H. Spectroscopy of Monolayer Assemblies, in Physical Methods of Chemistry, (eds. Weissberger, A. and Rossiter, B.W.), Vol.1, Part IIIB, pp.577 702, Wiley, New York, 1972. 3. Uhnan, A. Ultrathin Organic Fihns, Academic Press: London, 1991. 4. Thin Solid Films, 1(1980) 68 ; ibid,, (1983) 99 ; ibid,, (1985) 133 ; ibid., (1988),159, 160 ; ibid., (1989) 178 ; ibid., (1992) 210 ; ibid., (1994) 242 ; ibid,, (1996) 284 ; ibid., (1998), 327. 5. Kunitake, T., Okahata, Y. and Yasunami, S, J. Am. Chem. Soc, 104 (1982) 5547. 6. Elbert, R., Folda, T. and Ringsdorf, H., J. Am. Chem. Soc, 106 (1984) 7687. 7. Laschewsky, A., Ringsdorf, H. and Schmidt, G., Thin Solid Films, 134 (1985) 153. 8. Li, X.-d., Aoki, A. and Miyashita, T., Langmuir, 12 (1996) 5444. 9. A. Fujimori, H. Saitoh and Y. Shibasaki, J. Therm. Anal, and Calo., 57 (1999) 631. 10. A. Fujimori, T. Araki and H. Nakahara, Langmuir, (2000) to be submitted.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors)

545 e 2001 Elsevier Science B.V. All rights reserved.

Mixing beiiayior of binary monolayer of fatty acid based on Jc-A isotherm measurement M. Kuramori, K. Suehiro, and Y. Oishi Faculty of Science and Engineering, Saga University, 1 Honjo, Saga 840-8502, Japan The mixing behavior of binary monolayer in various combinations of palmitic (Cj^), arachidic (C2o)» behenic (C22) and lignoceric (C24) ^^'^ ^ ^ investigated on the basis of a JiA isotherm measurement The (C16/C20) and (C16/C22) mixed monolayers were in a misciWe state, whereas the (C16/C24) mixed monolayer was an inmiiscible one at 293 K. This mixing character is probably due to an enthalpic contribution based on the difference in cohesive energy between the monolayer components. 1. INTRODUCTION The miscibility and molecular interaction for mixed monolayers of various amphiphilic molecules have been examined by n-A isotherm analyses based on the additive rule with respect to the molecular area [1-5], mechanical properties [6], evaporation behavior [7]. However, litde has been done concerning a systematic understanding of phase separation mechanism in a monolayer. The phase separation in a monolayer depends on many factors. Hence, to obtain a general concept of phase separation in a monolayer, it is necessary to investigate a simple experimental system of fatty acid monolayer as a first step. In this study, the mixing behavior of binary monolayer of fatty acids was investigated on the basis of a n-A isotherm measurement with respect to the difference in cohesive energy of each alkyl chain. 2. EXPERIMENTAL Benzene solution of each binary mixture of Cj^, C20. C22 and C24 was prepared with molar fractions of 100/0, 75/25, 50/50, 25/75 and 0/100. The sample solution was spread on the water surface at a subphase temperature, Tsp of 293 K. After standing for 60 min, n-A isotherm of each monolayer was measured under compression by two barriers at an area change rate of 8.6 x 10-^ nm^molecule-i-s-i with a microprocessor-controlled film balance system (FSD-300, USI System). 3. RESULTS AND DISCUSSION Fig. 1(a) shows n-A isotherms of pure Cj^, C20, and of (Ci^C2o) mixed monolayers with different molar fractions on the water suri'ace at a Tsp of 293 K. This figure was illustrated by sliding the abscissae corresponding to the five molar fractions for clarity. The n-A isotherms of the pure palmitic and arachidic acid monolayers (Ci6/C2o- 100/0, 0/100) exhibited a sharp rise in surface pressure at a surface area of ca. 0.25 nm^-moleculel This increase in surface pressure results from contact between two-dimensional domains grown just after spreading a solution by surface compression. Fig. 1(b) shows the composition dependence of surface pressure at the collapse and transition points from L2to LS of monolayer phase [8].

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Collapse pressure linearly changed with the molar fraction. Surface pressure at the transition point decreased with an increase in C20 content to be a minimal value of 16.7 mN m-^ at the molar fraction of around 0.5. These indicate that die monolayer is composed of a single phase, and also that the aggregation state of monolayer changes with the monolayer composition. Rg. 1(c) shows the composition dependence of the measured molecular areas for the mixed monolayer at constant siuface pressures. Those plotsrevealeda negative deviation from the additive line. It is, therefore, reasonable to considertiiatdie (Ci^C2o) mixed monolayer with the difference in the cohesive energy corresponding to four CH2 groups (24 - 30 kJ molO is in a miscible state at 293 K. Fig. 2(a) shows n-A isotherms of pure C15, C22, and of (Ci^C22) ™^^ monolayers with different molar fractions at 293 K. The n-A isotiierms of the pure pdmitic and behenic acid monolayers {CxJC^i- 100/0,0/100) exhibited a sharp rise in surface pressure at a surface area of ca. 0.25 nm^molecule-i. Unlike the (Cj^/Cjo) mixed system, die collapse pressure abruptiy lessened by an addition of the small amount of C^^ molecules and was almost constant except the pure C^2 monolayer, as shown in Rg. 2(b). This indicates the existence of segregated regions with different mechanical properties in monolayer. On the other hand, the transition point decreased with an increase in C22 content to be a minimal value of 13.5 mN m-^ at the molar fraction of around 0.5. Furthermore, as shown in Rg. 2(c), plots of the measured

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1.0

548

molecular areas for the mixed monolayer at constant surface pressures versus the monolayer composition revealed a negative deviation from the additive line. The composition dependence of transition pressure and molecular area suggests the miscibility of the mixed monolayer. Putting the above-mentioned results together, it is reasonable to consider that the (C15/C22) mixed monolayer with the difference in the cohesive energy corresponding to six CH2 groups (36 - 45 kJ molO is not in a complete miscible state at 293 K. Fig. 3(a) shows n-A isotherms of pure C,6, C24, and of (C16/C24) mixed monolayers with different molar fractions at 293 K. The n-A isotherm of the pure palmitic acid monolayer (Ci^C24: 100/0) exhibited a sharp rise in surface pressure at a surface area of ca. 0.25 nm^molecule-i. For a pure lignoceric acid monolayer (C16/C24: 0/100), a plateau region was observed in an area range of 0.21 - 0.23 nm^molecule-^ on the n-A isotherm. The plateau region appears owing to the consumption of compression energy for the transition from a hexagonal system (Lj) to a rectangular one (CS) [9]. Collapse, transition and plateau pressures for the mixed monolayers were almost constant, irrespective of monolayer composition, as shown in Fig. 3(b). This constancy of characteristic pressures indicates that the aggregated regions of C^^ and C24 molecules separately exist in the mixed monolayers. Moreover, the molecular areas which were plotted against the monolayer composition lay on the additive line (Fig. 3(c)). This additivity suggests that the components are ideal mixing or complete segregation. Hence, it is reasonable to consider that the (C16/C24) mixed monolayer with the difference in the cohesive energy corresponding to eight CH2 groups (48 - 60 kJ molO is in an inuniscible state at 293 K. The subphase was in the neutral pH range in this study, which corresponds to an unionized state of the carboxylic group of fatty acids [10]. Accordingly, the difference in cohesive energy of 48 - 60 kJ mol-i reduces the interaction potential overcoming the entropic contribution,resultingin the inuniscibility of the (C16/C24) mixed monolayer. 4. CONCLUSIONS The present study suggested that the difference in the cohesive energy between the hydrophobic groups in monolayer components was an important factor for determination of the mixing behavior. The mixing behavior depends on the intermolecular interaction, the thermal molecular motion and so on. A further systematic investigation is required to understand the aggregation mechanism in the multicomponent monolayer.

REFERENCES 1. W. D. Harldns and R. T. Florence, J, Chem. Phys., 6 (1938) 847. 2. T. Isemura and K. Hamaguchi, Mem, Inst. ScL Ind. Res. Osaka Univ., 8 (1951) 131. 3. R. A. Demel, L. L. M. Van Deenen, and B. A. Pethica, Biochim. Biophys. Acta, 135 (1967) 11. 4. D. A. Cadenhead and R. J. Demchak, 7. Colloid Interf. Sci., 30 (1969) 76. 5.1. S. Costin and G. T. Barnes, J. Colloid Interf. Sci., 51 (1975) 106. 6. G. E. Boyd and F. Vaslow, J. Colloid Sci., 13 (1958) 275. 7. V. K. LaMer, L. A. G. Aylmore, and T. W. Healy, J. Phys. Chem., 67 (1%3) 2793. 8.1. R. Peterson, V. Brzezinski, R. M. Kenn, and R. Steitz, Langmuir, 8 (1992) 2995. 9. Y. Oishi, H. Kozuru, K. Shuto, and T. Kajiyama, Trans. Mat. Res. Soc. Jpn., 15A (1994) 563. 10. Y. Oishi, Y. Takashima, K. Suehiro, and T. Kajiyama, Langmuir, 13 (1997) 2527.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) vc' 2001 Elsevier Science B.V. All rights reserved.

549

Fine Timing of Chromophore Orientation Due to Hydrogen Formation in Nucleobase-Terminated Azobenzene Monolayers

Bond

Mitsuhiko Morisue^ Kuniharu Ijiro' ^ and Masatsugu Shimomura''^* Molecular Device Laboratory, Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan. ^Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation (JST), Sapporo 060-0812, Japan. The structures of two-dimensional azobenzene chromophore assemblies were finely tuned by molecular recognition in thy mine-terminated azobenzene monolayers at the air-water interface. A large spectral shift of the azobenzene chromophore was found when the adenosine or nucleobase analogue was dissolved in the water subphase. Versatile combination of hydrogen bonding at the air-water interface can make fine tuning of chromophore orientation in the twodimensional molecular assemblies. 1. Introduction DNA is a typical supramolecular architecture of the nucleobases. Spontaneous assemblages of nucleobase or nucleobase-analogue derivatives through hydrogen bonding have received great interest to tailor well-defined molecular assembUes. For nucleobase-bearing chromophores, specific interchromohore behavior was controlled by molecular recognition in organic media [1]. Amphiphilic nucleobases or nucleobase analogues were reported to generate monolayer assemblies throu^ hydrogen bonding at the air-water interface [2,3]. Azobenzene-containing cytosine amphiphiles provided base-specific monolayer assemblies in two-dimension [4]. We show herein organization of chromophores in the monolayer assemblies of azobenzenecontaining thymine amphiphiles at the air-water interface. Orientation of azobenzene chromophore was regulated by hydrogen bonding of the thymine head group with the various guest molecules, adenine derivatives or nucleobase analogues, dissolved in the subphase. Spectral shift of the azobenzene chromophore due to hydrogen bond formation is ascribable to the versatile chromophore orientation in the monolayer [5]. 2. Experimental Materials. Synthetic procedures of the single-chain thymine amphiphiles containing an azobenzene chromophore, C„AzoC;„Thy, are described elsewhere [6]. The nucleobase analogues, 2,6-diaminopyridine (DAP) and 2,4,6-triaminopyrimidine (TAP),

^

j^cny-o-Cy-H "* ^ ^ 'N--HQKO-^CHJ^,CH, o CH ' C^oC„-Thy 1"' '"i == i^^' ^1' 1^' ^®I

H-^)

550 were purchased from Wako Pure Chemical Ltd. Adenosine was purchased from TCI. Nucleobases and nucleobase analogues were dissolved in the subphase without further purification. Monolayer experiments. A dilute chloroform/ethanol (9/1 v/v) solution of the thymine amphiphile was spread on water surface. Water was purified to 18 MQcm by a Milli-Q SP reagent water system, (Millipore Co.). Surface monolayer e?q)eriments were carried out by a computer controlled Langmuir film balance (USI System, FSD-50) with a Wilhelmy pressure sensor at 20 °C in a dark room. The monolayers were transferred onto quartz plates at a constant surface pressure as Z-type single layer by Langmuir-Blodgett technique and their absorption spectra were measured by a UVA^is spectrophotometer (JASCO, V-530). 3. Results and Discussion Based on Kasha's molecular exciton theory and systematic X-ray crystallogr^hy [5], chromophore orientation in two-dimensional molecular assemblies of singje-chain ammonium amphiphiles containing an azobenzene chromophore can be described as a function of alkylchain length connecting the chromophore and hy drophilic head [7]. According to this concept, Ci2AzoC5-Thy and CgAzoCio-Thy monolayers were expected to be assembled in a tilted and in a parallel orientation of azobenzene chromophore, respectively. UV-visible absorption spectra oftheCi2AzoC5-Thy monolayer, however, showed hyp sochromic shift indicating the parallel orientation of the azobenzene chromophore. Absorption maximum of the isolated azobenzene chromophore is located at 360 nm. In the Ci2AzoC5-Thy monolayer, absorption maximum is located at 344 nm at 8 mN/m. Phase transition induced spectral shift to 325 nm at 20 mN/m. These spectral changes indicate that the azobenzene chromophores stack parallel in the monolayer. Thymine bases are known to form hydrogen bonded dimer in crystalline state [8].

at 20 mN/m

at 8 mN/m solution

0.1

0.2

0.3

0.4 0.5 0.6 0.7 Area [nm^/molecule]

0.8

300 350 400 Wavelength [nm]

500

Figure 1. (a) Surface pressure - area isotherm of the C^AzoCs-Thy monolayer on pure water at 20 °C. (b) UV-visible absorption spectra of C|2AzoC5-Thy monolayer transferred on quartz substrate at 8 mN/m and 20 mN/m.

551

Nucleobase analogues, TAP and DAP, form 1:2 and 1:1 con:^)lexes with thymine base throu^ multiple hydrogen bonds, respectively. TAP molecule sandwiched between thymine bases can draw apart two azobenzene chromophores. In fact, small red shift of C^AzoCs-Thy to 332 nm indicates that distance of azobenzene chromophores sli^tly increases with keeping closed parallel orientation. On the other hand, DAP fractured T T hydrogen bonding interaction to form heterogeneous 1:1 complex. The Ci2AzoC5-Thy/DAP complex can not form hydrogen bond mutually. Therefore, a large red shift to 360 nm, indicating azobenzene chromophore in a tilted orientation, was observed (Figure 2). On the other hand, absorption maximum of CgAzoCio-Thy monolayer was located at 326 nm on 10 mM of DAP subphase, as e^qsected from the above concept of chromophore orientation controlled by alkyl-chain length [7]. Adenine base is a complement of thymine base and expected to form A T base-pair or A T T base-trimer at the air-water interface. On the adenosine subphase, hypsochromic shift remained (326 nm), indicating azobenzene chromophore in the parallel orientation similar to the T T combination (Figure 2). Then binding mode of the adenine base to the thymine head is similar to that of TAP. Spectral change was not found even at a high adenine concentration (3 mM). Thus A T T base-trimer by multiple hydrogen bonding is more stable than AT pair with double hydrogen bonding.

CH, 1 J. i H H H I

H

i

> % .

o

I

H

2,4,6-triammo pyrimidine (TAP)

2,4-diamino pyridine (DAP)

HO ° " adenosine

(a)

' 50

£

\X \

2 30

DAP

3

120

^

R

JX^TAP

.adenosine

.g 10 3 CO

pure water 0 = 0.1

1

0.2

1

\ L

0.3 0.4 0.5 0.6 Area [nm'/molecule]

0.7

0.8

300 350 400 Wavelength [nm]

Figure 2. (a) Surface pressure - area isotherms of the Ci2AzoC5-Thy monolayers on pure water, 1.0 mM of 2,4,6-triaminopyrimidine (TAP), 10 mM of 2,6-diaminopyridine (DAP), and 2.0 mM of adenosine at 20 °C. (b) UV-visible absorption spectra of C^AzoCs-Thy monolayers transferred on quartz substrates at 20 mN/m.

500

552

4. Conclusion Chromophore orientation of azobenzene was controlled by host-guest interaction through hydrogen bonds. Molecular-recognition-directed fine tuning of chromophore orientation and spectral shift can be applicable to a novel photonic device based on DNA-mimetic twodimensional molecular assemblies. References [1] (a) C. M. Drain, R. Fischer, E. G. Nolen, J.-M. Lehn, J Chem. Soc, Chem. Commun. (1993) 243. (b) J. L. Sesslar, B. Wang, A. Harriman, J. Am. Chem. Soc. 115 (1993) 10418. (c) N. Armaroli, F. Barigelletti, G. Calogero, L. Flamigni, C. M. White, M. D. Ward, Chem. Commun. (1997)2181. [2] (a) H. Kitano, H. Ringsdorf, Bull. Chem. Soc. Jpn. 58 (1985) 2826. (b) K. Kurihara, K. Ohto, Y. Honda, T. Kunitake, J. Am. Chem. Soc. 113 (1991) 5077. (c) K. Arig?, T. Kunitake, Ace. Chem. Res. 3\(\99^) 371. [3] (a) M. Shimomura, F. Nakamura, K. Ijiro, H. Taketsuna, M. Tanaka, H. Nakamura, K. Hasebe, Thin Solid Films 284/285 (1996) 691.; J. Am. Chem. Soc. 118 (1997) 2341. (b) F. Nakamura, K. Ijiro, M. Shimomura, Thin Solid Films 327/329 (1998) 603. (c) M. Morisue, K. Ijiro, M. Shimomura, to be published in a special issue of Mol. Cryst. Liq. Cryst. for the 3rd Asian Symposium on Organized Molecular Films for Electronics and Photonics. [4] M. Morisue, H. Nakamura, K. Ijiro, M. Shimomura, Mol. Cryst. Liq. Cryst. 337 (1999) 457. [5] (a) M. Kasha, Radiat. Res. 20 (1963) 55. (b) M. Shimomura, R. Ando, T. Kunitake, Ber. Bunsenges. Phys. Chem. 87 (1993) 1134. [6] M. Morisue, K. Ijiro, M. Shimomura, to be published. [7] (a) M. Shimomura, S. Aiba, N. Tajima, N. Inoue, K. Okuyama, Langmuir 11 (1995) 969. (b) M. Shimomura, S. Aiba, S. Oguma, M. Oguchi, M. Matsute, H. Shimada, R. Kajiwara, H. Emori, K. Yoshiwara, K. Okuyama, T. Miyashita, A. Watanabe, M. Matsuda, Supramol Sci. 1 (1994) 33. [8] N. Tohnai, Y. Inaki, M. Miyata, N. Yasui, E. Mochizuki, Y. Kai, Bull. Chem. Soc. Jpn., 72 (1999)851.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c 2001 Elsevier Science B.V. All rights reserved.

553

Monolayer and bilayer properties of oligopeptide-containing lipids - Difference in pliase transition behavior S. Kawanami, T. Kosaka, T. Abe, K. Ariga and J. Kikuchi Graduate SchcxDl of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan Lipids containing peptides between a polar head and a dialkyl tail were synthesized, and their bilayer and monolayer properties were investigated by DSC measurements, FT-IR spectroscopy, and n-A isotherm measurements. The FT-IR spectra of aqueous bilayers of these lipids directly proved that hydrogen bonding was formed both in the gel and liquid crystalline states. Phase transition behavior was significantly different between a monolayer and a bilayer. This would originate from the difference in dielectric environment between the two interfaces. 1. INTRODUCTION Hydrogen bonding plays important roles in lipid assemblies. For example, Brockerhoff postulated that a "hydrogen (bonding) belt" strengthens naturally-occurring lipid bilayers through hydrogen bonding formation [1]. Murakami et al. developed peptide-containing lipids (peptide lipids) which have hydrogen bonding sites between a polar head and hydrophobic tails. The peptide lipids form a stable bilayer vesicle due to the presence of the hydrogen bonding belt. Interestingly, phase transition temperatures from the gel to the liquid crystalline state are virtually independent of amino acid residues introduced in the peptide lipid [2]. The hydrogen bonding belt may separate the molecular motion of alkyl chains from the remaining parts, leading to an unchanged transition temperature. Although there are several experimental indications for the hydrogen bonding belt [2], it has not been fully characterized. Therefore, we synthesized oligopeptide-containing lipids (see below), and their assembling properties were systematically investigated both in bilayer and monolayer states.

1 (R = CH3, n = 1); 2 (R = H, n = 1); 3 (R = H. n = 2); 4 (R = H, n = 3)

554

2. EXPERIMENTAL Syntheses of the compounds used in this study have been described elsewhere [3] and in future publications. An aqueous solution of the lipids (10 mM) was vortex-mixed for 5 min at room temperature, and the resulting suspension was subjected to three freeze-thaw cycles to obtain clear and stable vesicles. Differential scanning calorimetry (DSC) measurements were performed at a scanning rate of 1 °C min'^ with a DSC-61(X) calorimeter (Seiko Instruments, Inc.). The peak in the excess heat capacity versus temperature plot was taken as the main transition temperature Tn,. The IR spectra were recorded on a Nicolet 560 Fourier transform infrared spectrometer with an MCT detector. The sample suspension was sandwiched between the CaF2 windows with a 12-|xm Teflon spacer. The CaF2 windows were mounted in a temperature-controlled flow-through cell (Harrick Scientific Co.,TFC-M19). Three hundred interferograms were co-added and Fourier transformed with one level of zero filling to yield spectra with a high S/N ratio at a resolution of 1 cm ^ Surface pressure (7C)-area(A) isotherms were obtained at various temperatures using a computer-controlled film balance system (USI-System Co., Ltd, FSD-300). Benzene-ethanol (7:3 v/v) was prepared as a spreading solvent. The accuracy of the temperature was ±0.2 °C, and the area change rate was 60 nmi min . 3. RESULTS AND DISCUSSION 3.1. Direct evidence for hydrogen bonding belt in the bilayer vesicle of lipids containing single amino acid residue Phase transition temperatures determined by DSC measurements were 25.3 °C (enthalpy change, .4^/ = 34.1 kJ mol"') and 29.1 °C (AH = 32.7 kJ mol'^) for aqueous vesicles of 1 and 2, respectively. Only a slight difference is observed. The FT-IR spectra of both

2924

1680

"(B) •

1 1660 1

1

1640

3

1 1620 ^

10

^

1

20 30 40 50 Temperature / °C

10

20

30

40

50

Temperature / °C

Fig. 1. Temperature dependencies on absorption frequencies of 1 (O) and 2 ( • ) of lipid assemblies in water: (A) Vas(CH2); (B) v(C=0). Plot A represents the peak position of v(C=0) of 2 measured in ethanol.

555 vesicles show drastic frequency shifts of Vas(CH2) band at their phase transition temperature (Fig. 1(A)). These shifts correspond to alkyl chain melting. In contrast, the peak position of v(C=0) was not affected by the phase transition (Fig. 1(B)). The observed frequencies are apparently lower than those observed in ethanol (1661 cm"' for 1 and 1659 cm"' for 2). Therefore, hydrogen bonding at the amino acid residue was maintained in both the gel and liquid crystalline states. The maintenance of the hydrogen bonding belt would lead to similar phase behavior of 1 and 2 in the bilayer state. 3.2. Difference in phase transition behavior between monolayer and bilayer systems 7C-A Isotherms of 1 and 2 were measured at various temperatures (Fig. 2(A)). Compressibilities (-(3A/37C)T/A) were calculated at each point and were plotted as a function of molecular area. One example is shown in Fig. 2(B) where two minimums of the compressibility can be seen. From these minimums, we determined the phase transition pressure (Tip) and pressure at the most condensed state (TCc). The obtained values are plotted against the subphase temperature (Fig. 3). Both TCp and Tic were detectable at lower temperature, but the former pressure disappeared at a certain temperature. In addition, a discontinuous change in Tic was also observed at the same temperature. This temperature was defined as the phase transition temperature of the monolayers. Kajiyama et al. conducted an analogous analysis of the phase transition of lipid monolayers [4]. The obtained phase transition temperatures of the monolayers of 1 and 2 were observed at 14.5 and 27.0 °C, respectively. The difference in the transition temperature (12.5 °C) is significantly larger than the corresponding difference in the bilayer system (3.8 °C). This finding provides interesting implication on difference between the two types of assemblies, although the meaning of the phase transition in both the systems cannot be regarded as identical. It suggests that the assembling properties of these lipids cannot be treated equally in the monolayer and bilayer states. Hydrogen bonding efficiency is probably different between the two types of interfaces. Similar differences between the interfaces were

0.15

0

0.5

1.0

Molecular Area / nm^

1.5 Molecular Area / nm

Fig. 2. (A) Ti-A isotherms of 2: a, 10 ''C; b, 20 T ; c, 30 °C; d, 40 ^'C. (B) Ti-A and compressibility-A isotherm of monolayer composed by 2 at 10 °C.

556 reported for acid dissociation behavior [5] and molecular recognition efficiency [6]. The phase transition temperatures were similarly determined for monolayers of 3 and 4. The obtained temperatures (33.0 °C and 32.0 °C for 3 and 4, respectively) are higher than that of 1, but the transition temperature does not increase in proportion to the number of hydrogen bonding sites. The efficiency of the hydrogen bonding would be affected by the disposition of the sites at the interface, resulting in the unusual behavior. Detailed analyses based on FT-IR measurements are currently under investigation.

20 40 '0 20 40 Temperature / °C Temperature / °C Fig. 3. Temperature dependencies on Kp (O) and TCC ( • ) for monolayer composed by peptide lipids: (A), 1; (B), 2. 0

4. CONCLUSION The formation of hydrogen bonding in aqueous bilayers of peptide lipids was confirmed both in the gel and liquid crystalline states. The phase transition behavior is somewhat different between the monolayer and bilayer states. This suggests that hydrogen bonding properties are significantly affected by the type of the interface. Further systematic studies would reveal the specific behavior of the hydrogen bonding at interfaces, which must be a key to design novel supramolecular structures at interfacial media. REFERENCES 1. H. Brockerhoff, Biooi^ganic Chemistry, Vol. 3, E. E. van Tamelen, ed.. Academic Press, New Yoric, N.Y., 1977,1. 2. Y. Murakami, J. Kikuchi, Biooig. Chem. Frontiers, H. Dugas, ed., vol. 2, Springer-Verlag, Berlin, 1991,73. 3. Y. Murakami, A. Nakano, A. Yoshimatsu, K Uchitomi, Y Matsuda, J. Am. Chem. Soc., 106 (1984) 3613. 4. T. Kajiyama, H. Kozum, Y. Takashima, Y. Gishi, K. Suehiro, Supramol. Sci., 2 (1995) 107. 5. K. Ariga, T. Abe, J. Kikuchi, Chem. Lett., (2000) 82. 6. M. Onda, K. Yoshihara, H. Koyano, K. Ariga, T Kunitake, J. Am. Chem. Soc., 118 (1996) 8524.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

557

Interactions of Sugars with Phosphatidylcholines Hiroshi Takahashi* and Ichiro Hatta" *Department of Physics, Gunma University, Aramaki 4-2, Maebashi, 371-8510, Japan ^'Department of Applied Physics, Nagoya University, Chikusa-hi, Nagoya, 464-8603, Japan 1. ABSTRACT Sugars and sugar alcohols are capable of protecting organisms against low temperatures. In order to clarify the cryoprotective mechanism of these sugars, by means of various methods, we have investigated the interactions of these sugars with phosphatidylcholines which are one of major lipid components of biomembranes. In this study, calorimetry and X-ray diffraction were used to study the effects of trehalose on dihexadecylphosphatidylcholine and neutron scattering was used to study the effects of sorbitol on dihexanoylphsohatidylcholine. The results obtained in this study suggest that both trehalose and sorbitol reduce the interfacial area between the hpid and aqueous phases. On the basis of this conclusion, we consider the role of these sugars for the ability of organisms to survive in low temperatures. 2. INTRODUCTION Certain sugars and sugar alcohols act as neutral cryoprotectants [1]. For example, it have been found that the concentration of trehalose (sugar) or sorbitol (sugar alcohol) (see Fig.l) is increased in insects acclimated to low temperatures [2]. In order to reveal the mechanism of cryoprotective function of sugars and sugar alcohols, the interaction between these sugars and model biomembrane systems has been extensively studied [3]. In such studies, phosphohpid-water systems have been widely used as model systems for biomembranes Based upon the studies of effects of sugars on the phase transition temperatures of phospholipid membrane systems, Koynova et al. [4] have proposed a following hypothesis: Sugars or sugar alcohols act as a kosmotropic regent stabilizing the water structure, and as a result, the presence of these substances reduces the interfacial area between the lipid and the aqueous phases. In order to examine above hypothesis, we studied the phase behavior and structure of phospholipid assemblies in sugar solutions. We used phosphatidylcholines (PCs), because PC is one of major phospholipid components of biomembranes. In this study, the effects of trehalose on dihexadecylphosphatidylcholine membranes were studied by calorimetry and X-ray diffraction and the effects of sorbitol on dihexanoylphsohatidylcholine micelles were studied by neutron scattering.

558

(b)

OH

OH

HOHC

CH.OH OH

OH

Fig.1 Chemical structures of (a) trehalose and (b) sorbitol.

3. RESULTS 3.1 EflBect of trehftlose on dialkylphosphatidyldioliDe Dihexadecylphosphatidylcholine (DHPC) is one of dialkylphosphatidylcholines. In the lipid, hydrocarbon chains are linked to a glycerol backbone by ether-bond. DHPC forms an interdigitated gel phase below a ripple phase [5]. In the interdigitated gel phase, the interfacial area per lipid molecule is about twice as compared with that of a normal gel phase bilayer.

60 Liquid Crystalline Phase

50 40

Ripple ex^-^v^- Phase

30 20

Normal H Gel Phase

Interdigitated Gel Phase

10

•

0

•

.

I

•

0.4

•

.

•

i

0.8

1.2

.

.

i

1.6

Trehalose (M) Fig.2 Trehalose concentration dependence of the phase behavior of DHPC. The closed symbols show the peak temperatures of calorimetric thermograms. The open symbols show the temperature at which x-ray diffraction pattems change.

559

We constructed a phase diagram of DHPC as a function of trehalose concentration based on the phase transition temperatures obtained by calorimetry and X-ray structural analysis data for DHPC in trehalose solution (Fig.2) [6]. It can been seen from the figure that trehalose destabilizes the interdigitated gel phase of DHPC bilayers, i.e., the presence of above 1.0 M trehalose induces a normal gel phase bilayer. This indicates that the interfacial area between the water layer and the hpid bilayer drastically decreases in the presence of trehalose. 3.2 Effect of SQibitol on short-chain phosphatidyldioline A short-chain PC, dihexanoyl-PC (dC(6)PC), forms a micellar structure in water [7]. We investigated the effect of sorbitol on the dC(6)PC micelles [8]. In such a study, neutron scattering is useful. According to a standard theory for a globular micellar system, the neutron scattering intensity at
S=1.38nm^ 67 nm'

V ^'O-

n=19

OT ¥

S=1.13nm V=OMnm

k^^^W/^

n = 32

Watpr

2M Sorbitol solution Fig.3 Schematic representation for dC(6)PC micelles in water and sorbitol solution S : interfacial area, V : volume of the lipid, n : mean aggregation number.

560 4. DISCUSSION The present study provides clear evidence that the interfacial area between the water and PC is decreased by the presence of trehalose or sorbitol. Let us consider the relation of this result and the role of these sugars in a low temperature tolerance for some organisms. It has been reported that these sugars hardly bind to phospholipid monolayers on aqueous phases [9] and that the change of the gel-to-liquid crystalline phase transition temperatures of phospholipid membranes induced by these sugars does not depends on the difference of polar headgroups [10]. These facts suggest the effect of these sugars is not specific, i.e., the effect might be common for amphiphilic macromolecules or assembhes of amphiphilic molecules. Thus, it would be expected that sugars or sugar alcohols also reduce the interfacial area of proteins in aqueous phases. Proteins undergo a cold-induced denaturation, i.e., proteins do not adopt a native state at some subzero temperatures 11,11]. Denaturated proteins do not fulfill their function. In the native state of proteins, the hydrophobic amino residues of proteins are located in a inside region and only hydrophilic amino residues face with surrounding water molecules. In the denatured state, however, the hydrophobic residues of proteins are exposed to water and the interfacial area contacting with water increases in comparison with the native state. Taking accimts of this fact, these sugars would be expected to stabilize the native state of proteins at some subzero temperatures by reducing the interfacial area. This may be correlated with the low temperature resistance by producing certain sugars or sugar alcohols in some organisms. REFERENCES 1. F. Franks, Biophysics and Biochemistry at Low Temperatures, Cambridge University Press, Cambridge, 1985. 2. K.B. Storey, Phil. Trans. R. Soc. Lond. B., 326 (1990) 635. 3. J. H. Crowe, L.M. Crowe, J.F. Carpenter, and C.A. Wistrom, BiochemJ, 242 (1987) 1. 4. R.D. Koynova, B. G. Tenchov, and P. J. Quinn, Biochim. Biophys. Acta, 980 (1989) 377. 5. P. Laggner, K. Lohner, G. Degovics, K. Muller, and A. Schuster, Chem. Phys. Lipids, 44(1985)31. 6. H. Takahashi, H. Ohmae, and I. Hatta, Biophys. 7., 73 (1997) 3030. 7. T. L. Lin, S. H. Chen, N. E. Gabriel, M. F. Roberts, / Am. Chem. Soc. 108 (1986) 3499. 8. H. Takahashi, M. Imai, Y. Matsushita, and I. Hatta, Progr. Colloid. Polym. Sci., 106 (1997)223. 9. E. A. Amett, N. Harvey, E. A. Johnson, D. S. Johnston, and D. Chapman, Biochemistry, 25 (1986) 5239. 10. R. Koynova, J Brankov, and B. Tenchov, Eur. Biophys. J., 25 (1997) 261. 11. P. Privalov and S. J. Gill, Adv. Protein Chem., 39 (1988) 191.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B.V. All rights reserved.

561

Study on J-aggregate formation of a long-chain merocyanine in the mixed LB films and their optical behavior Michio Murata, Tohru Araki and Hiroo Nakahara Department of Chemistry, Faculty of Science, Saitama University, 255 Shimo-okubo, Urawa, 338-8570, Japan We have studied the J-aggregates in binary mixed LB films of the amphiphilic merocyanine dye (abbr. 5-ClMc) with cadmium arachidate (AA) (5-ClMc:AA=l:2) and in ternary system with octadecane (OD) (5-ClMc:AA:0D=l:l:l). The absorption spectra of the mixed dye LB films were fitted by the Lorentzian. As a result, the ternary and the binary systems were consisted of five and seven components, respectively. By using the extended dipole model, the energy shift of each component was in good agreement with the corresponding aggregation number N = 4-14 at the slip angle a = 26°. Furthermore, the J-aggregate in the mixed dye LB films was observed by using the scanning near-field optical microscopy (SNOM). Round domains with diameter of 250nm were observed in the binary system, while in the ternary system smaller bright parts appeared. 1. INTRODUCTION Cyanine dyes form a J-aggregate showing an intense narrow absorption band shifted to longer wavelength relative to the monomer band and an almost Stokes-shiftfi-eefluorescence band. The J-aggregate can be regarded as two-dimensional system with a brickstone arrangement of the chromophores. The red-shifted absorption band due to the J-aggregates could be explained by the extended dipole model [1]. The J-aggregates of amphiphilic cyanines and unsymmetrical merocyanines are formed by the LB technique which can organize molecular films to control two-dimensional arrangements. The ternary mixed monolayer containing a merocyanine exhibited the spectral changefi^omthe monomer and the card-liked dimmer or oligomer to the J-aggregate with an apparent isosbestic point under the compression [2]. The aggregates are of practical and theoretical interest because of their particular photophysical behavior [3-4]. Previously, for the binary LB system containing an amphiphilic merocyanine [Mc], the extended dipole model was applied only for the lowest energy band of the electronic absorption spectrum [5].And also for the binary dye Mc mixed films with the more content of fatty acids the optical density of the J-band linearly decreased. In this investigation, for the ternary mixed dye LB films as well as the binary system containing the J- and other aggregates the absorption spectra were analyzed with fitting by the Lorentzian and the number of classes of aggregates as well as their contents in the films were estimated. And fiirther, for the corresponding aggregates in the spectrum, the two-dimensional arrangements of the dye in the films were obtained by applying the extended dipole model. In

562 addition, the J-aggregates formed in the mixed dye LB films were observed by using SNOM and then were studied in correlation to the aggregation bands and the molecular environments. 2. EXPERIMENTAL 2.1 Materials The long-chain merocyanine dye substituted with chlorine (abbr. 5-ClMc) used in this work as shown in Figure. 1 (A) was purchased from Japanese Research Institute for photosensitizing Dye, Co. (Okayama, Japan). As the matrix lipids which were stabilized by mixing with the dye monolayer, octadecane (OD) and cadmium arachidate (AA) were used. The molar ratios of 5-ClMc:AA=l:2 and 5-ClMc:AA:0D=l:l:l chloroform solutions were spread onto the aqueous subphase vnth 3x10"^ M CdCb and 5x10'^ M KHCO3 (pH 6.8). The monolayers were transferred at 25 mN/m and 12.5 °C by the Langmuir-Blodgett method onto a solid substrate. A quartz and a glass substrates precoated with five monolayers of cadmium stearate (SA) for the binary system and four monolayers for the ternary system were used, i.e., the SA monolayer was transferred as an outermost layer for the binary system. 2.2 Procedures The mixed dye LB films were made by using a Lauda film balance (Lauda Filmlift FL-1). Absorption spectra of the mixed dye LB films were measured by absorption spectrophotometer (HITACHI, U-3210). SNOM images of the mixed dye LB films on slide-glass were obtained by scanning near-field optical microscopy (SNOAM; Seiko Instruments Inc.) with transmission mode using bent probe with aperture of 125^m in diameter. The J-aggregates in the mixed dye LB films were excited by using diode laser at 532nm and fluorescence was observed by a photon counting system at 572.5~647.5nm. 2 3 Deconvolution of the absorption spectra The absorption spectra of the mixed dye LB films containing the J- and other aggregates were fitted Lorentzian (fitting spectrum) which can be ascribed to only the lifetime of excitation state related to absorption of light and emission. From longer wavelength area, the spectrum was adjusted with a peak position and a ftill width at half maximum (FWHM). From the area ratio the fitting spectrum to the absorption one at each wavelength, the content was estimated. The deconvolution was repeated until excess 90% in total. 2.4 Calculation of the aggregation number and the slip angle For the J- and other classes of aggregates, the aggregation number N and the slip angle a of the chromophores were calculated by using the extended dipole model, as shown in Figure 1 (B), in which r, is the separation between the point charges e of two interacting dipoles and the dielectric constant of medium is assumed to be Z) = 2.5 for hydrocarbons. The excitation energy AE' of the dye aggregate is represented with that of the dye monomer, A£, and the interaction integral Jj2 approximated by A£'-A£ = ±2Sy,2 The values of J/2 can be calculated by assuming / = 0.9 nm and also estimating the distance between the adjacent transition moment at about d = s = 0.5 nm, which corresponds to the diameter of the cross-section area of hydrocarbon chain,fi-omthe result of the pressure-area isotherms and the transition moment // = / e = 7.52 Debye fi-om the absorption spectrum of chloroform solution for 5-ClMc and also estimating the molecular length M/ = 1.45 nm fi-om the molecular model.

563

3. R E S U L T S AND DISCUSSION Figure 1 shows the result of the deconvolution of the absorption spectra of the mixed dye LB films. In both the ternary and the binary systems, the absorption spectra have the highest content at the Al band of which were 54.7% and 45.8% for the ternary and the binary systems, respectively. Therefore, the Al was the main component in these spectra. For the J- and other classes of aggregates, the ternary and the binary systems were consisted of five and seven components, respectively. The peak positions between the ternary and the binary systems had a little difference of 21cm"' - 184cm'' and an unsymmetrical moiety at the high-energy region tended to increase in the binary system. The homogeneous J-aggregate was formed in the ternary system as compared with the binary system in spectral properties.

0.08

Source spectrum l ' ^ l = - 2 . 8 0 xlO'^^J (45.8%)

0.06

A2 = -2.62 X J (8.8%)

§ 0.04

A3

o

^

2.38 x*^°J (6.9%) A4 = -2.08 x*^°J (5.2%) . A

0.02

pxq pxq Al = l x l 4 A5= 1 x 9 , 3 x 3 A2=lxl3 A6= 1 x 6 , 2 x 3 A3=l X 12.2x6 A7=l x 4 , 2 x 2 A4 = I X 11

A5 =-1.63 x"^°J (7.5% A6 = -1.05x' J(8.9%) 20 A7.0.44X J(7.7%)

000 14

16

18

20

22

Wavenumber / cm xl0 Figure 1. The deconvolution of absorption spectra; (A) the ternary and (B) the binary systems. The inset shows two-dimensional arrangement of the dye molecules with the long axis horizontally oriented and parallel to each other.

564

Figure 2. The SNOM images; (A) and (B), the AFM images; (A') and (B') in the same area for the ternary (A) and the binary (B) systems. For each fitting spectrum, varying the numbers p and q of the column and the row for the arrangement of the transition moments, respectively, and the angle a between the transition moments and these centers, the obtained values of 7/2 are indicated in the insert of Figure 1. In both the ternary and the binary systems, the aggregation numbers N agreed with the corresponding energy shifts and the slip angle a (a is almost around 26 °). The Al component was N = p x q = l x l 4 both the ternary and the binary systems. In the binary system, both p x q =1 X 6 and 2 x 3 were probable for the A6 and p x q =1 x 4 and 2 x 2 for the A7 components. These were very small aggregates. As film characteristics, Figure.2 shows SNOM and AFM images in the same area. The SNOM fluorescence images were not correspondent to the AFM topographic images. It is suggested that the fluorescence image of the J-aggregate in the LB films could not be observed in relation to the topographic image at this microscopic range. The contrast image of the SNOM was considered to be due to the fluorescence of the J-aggregate because the monomer of 5-ClMc did not have emission and could not be observed by the SNOM. The round domains with diameter of 250nm were observed in the binary system, while in the ternary system, smaller bright parts appeared. Therefore, the J-aggregate distributed extensively in the ternary system while the domain seemed to be composed of above hundred of the J-aggregates in the binary system. From these results, it is probable to be considered that the ternary system comparatively consists of a large amount of the J-aggregate. And the content of the most numerous aggregates at the A1 component in particular exceeds 50%. And fiirther, it is imagined that the J-aggregate of the ternary system formed domains at disadvantage, while in the binary system included the small aggregates the A6 and A7 with the minor components shall assist the domains formation of the J-aggregate. REFERENCES 1. V. Czikkely, H.D. Forsterling and H. Kuhn, Chem. Phys. Letters, 6 (1970) 207. 2. H. Nakahara and D. Mobius, J. Colloid Interface Sci., 114 (1986) 363. 3. R. Steiger, R. Kitzing and P. Jund, Photographic Sensitivity, (eds. R.J.Cox), pp 221-240, Academic press, London and New York, 1973. 4. David M. Sturmer, Special Topics in Heterocyclic Chemistry, (eds. Arnold Wessberger and Edward C. Taylor), chapter VIII, An Interscience Publication, New York, 1977. 5. H. Nakahara, K. Fukuda, D. Mobius and H. Kuhn, J. Phys. Chem., 90 (1986) 6144.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

565

Structure of H-aggregate formed in merocyanine dye LB films Yoshiaki Hirano, Takuya M. Okada, Yasuhiro F. Miura, Michio Sugi and Toshio Ishii** Toin University of Yokohama, Aoba-ku Yokohama 225-8502, Japan ^ Tsurumi University, Tsurumi-ku, Yokohama 230-8501, Japan

We have studied the layered structure of the mixed LB films of the merocyanine (MS)-arachidic acid (C20) binary and the MS-C20-«-octadecane (ALis) ternary systems with the molar mixing ratio [MS]:[C2o]:[ALi8]=l:10:x (0^x^5.0) by means of the X-ray dif&action measurement. In the binary and the ternary systems with the C20 matrix content, the X-ray diffraction pattems give only one-regime for 2* <20<2O* . The d-spacings in all the systems are in good agreement with the value of the pure C20-C20 bilayer unit cells in the LB films reported in the earlier works. The results indicate that the observed X-ray scatterings are mainly due to C20-C20 bilayer unit cells and that the accumulation of AL18 at the hydrophobic interfaces is discarded. INTRODUCTION The Langmuir-Blodgett (LB) method is one of the most versatile techniques for fabricating organic thin films with well controlled structures, compositions and thicknesses at the molecular level.[l, 2] Due to the features, the LB method has been utilized to construct prototypes of the molecular devices based on the dye molecules.[l, 2] The study of the dye aggregates in the LB system has been the subject of keen interest because of their potentiality for organic solar cells and optical memory media. [3] In most cases, to control the aggregation state and the orientation of the dye molecules in a desired manner is one of the most important key issues, since the microscopic structure of the dye molecules in the organic thin films is expected to be reflected to the optical, the electrical and the magnetic properties in the system. [3] As is well known, LB films of the merocyanine dye (MS, Fig. l)-arachidic acid (C20) binary system show a sharp absorption peak at 590 nm, when they are prepared under the conventional subphase condition containing Cd2"*^.[4-7] The absorption band shows the characteristics of the J-band, with its absorption maximum remarkably red-shiftedfi*omthat of the MS monomer peak. The optical characteristics are ascribed to a specific alignment of the MS molecules which is called J-aggregates. The sharp and narrow absorption is due to the excitons delocalized over the aggregate. We have found that a pronounced blue-shifted band at 505 nm is induced in the mixed LB films of the merocyanine dye (MS)-arachidic acid (C2o)-«-octadecane ternary

566 system. The blue-shifted band at 505 nm is assigned to an H-aggregate referring to the energy shift, the sharpness and the in-plane anisotropy of the band.[8-l 1] In previous papers [10], we have examined the associated layered structure in the mixed LB films with the molar mixing ratios of both [MS]:[C2o]:[ALi8]=l :2'JC and 1:5.x ( 0 ^ ^5.0) by means of the surface pressure (n) - area (A) isotherm and the X-ray diffraction measurements. In these systems, two different regimes have been recognized in the X-ray diffraction patterns: 20<9' and 9' <20, although three different bilayer unit cell species are considered to exist in the MS-C20 phase-separated system: Type (1) (CdC2o-C2oCd), Type (2) (CdC20-MSCd)/(CdMS-C2oCd) and Type (3) (CdMS-MSCd). The bilayer unit cell species mainly corresponding to two different regimes have been identified in each molar mixing ratio. The X-ray scatterings of the mixing ratios with l:2:jc and l:5'jc are mainly due to Type (3) (CdMS-MSCd) and Type (2) (CdC2o-MSCd)/(CdMS-C2oCd) for 2G<9' , respectively. On the other hand, the X-ray scatterings of both mixing ratios are mainly related with Type (1) (CdC20-C2oCd) for 9' <29. The results indicate that the bilayer unit cell species governing the X-ray diffraction patterns 20<9' changes with increasing the C20 content. There arises a question whether two different regimes are recognized in the X-ray diffraction patterns for 2* <28<20' by further increase of the C20 matrix or not. In this paper, we discuss the layered structure of the mixed LB films of the MS-C20 binary and the MS-C20-AL18 ternary systems with high C20 content ([MS]:[C2O]:[ALI8]=1:10:JC) based on the results of the X-ray diffraction measurement. EXPERIMENTAL

O--^' r^^^N^'

s

The surface active merocyanine dye (MS), ° CH2C00H arachidic acid (C20) and n-octadecane (ALis) were FIGURE 1 The chemical structure of used in this study. MS, C20 and AL18 were merocyanine dye (MS), dissolved in the freshly-distilled chloroform with the molar mixing ratio [MS]:[C2O]:[ALI8]=1:10JC (0;$JC;$5.0). The mixed monolayers on the aqueous subphase containing Cd^^ ions were transferred onto the glass substrate hydrophobized with 1,1,1,3,3,3-hexamethyldisilazane using the standard vertical dipping method. The aqueous subphase and the deposition condition were the same as reported previously.[4-11] All the LB films were of Y-type with a transfer ratio close to unity. The X-ray diffraction measurement was carried out by the ordinary 0-29 scan method with a CuKa source (X.=1.5418A) operated at 30 kV and 30 mA using a Shimadzu X-ray dififractometer, immediately after the sample preparation. Fifty-layered LB films were used. The 29 angles were calibrated using the LB fihns of the cadmium arachidate (CdC2o) as a reference. RESULTS AND DISCUSSION hi the Y-type LB films of the Cd salt of the fatty acid, it is well known that Cd^^ ions

567 dominantly contribute to the X-ray scatterings among all the constituents involved and that the diffraction peaks of the odd order is stronger in intensity than those of the even order. [12] The laycjred structure can be easily characterized using the Cd-Cd spacing as a marker. Figures 2(a) and (b) show the X-ray diffraction profiles of the mixed LB !4.0h x2 films of the binary and the ternary systems w 2.01i the molar mixing ratio with 0) .^,^vJV^^J^/^^v.^.^^>^-^ [MS]:[C2o]:[ALi8]=l:10:x, where (a) x=0 6 8 10 12 14 16 18 0 2 2 e angle (degree) and (b) jf=2.0, respectively. In each figure, (a) the peaks at 29w3* is assumed to be assigned to the 2nd-order peak, leading to a reasonable estimate of the Cd-Cd spacing. ^ 4.0 x2 of up to the The diffraction peaks 4 w 2.0 llth-order are seen in the binary and the ini I A T I 0 2 ternary systems for 2* <20<2O* , for 6 8 10 12 14 16 18 2 e angle (degree) which the peaks are generally higher in (b) intensity than corresponding ones with low C20 contents ([MS]:[C2O]:[ALI8]=1:2JC and

1:5:JC)[11], suggesting the highly-ordered stacking of the layered structure of the LB films due to the increase of the C20 matrix content. In Figs. 2(a) and (b), the X-ray diffraction profiles are almost the same regardless of the increase of the AL18 content. It is also noted that no significant change in the X-ray diffraction profiles is seen in the other ternary systems, although a slight increase in the peak intensity is observed in the ternary system for jc=5.0 for 20<9* . The results indicate that the X-ray diffraction patterns give only one-regime in the system of the C20 matrix

2 The X-ray diffraction FIGURE profiles of the mixed LB films of the MS-C20 binary and the MS-C20-AL18 ternary sytems with the molar mixing ratio [MS]:[C20]:[ALi8]=l:10jc, where (a) ;c=0 and(b)x=2.0.

content ([MS]:[C2O]:[ALI8]=1:10:JC) for T

<29<20* and that the similar results can be FIGURE 3 The Cd-Cd spacing of the mixed also obtained in the ternary systems with LB films of the MS-C20 binary system plotted higher C20 contents. Therefore, the Cd-Cd against the reciprocal of X-ray diffraction order 1/n. The molar mixing ratio is estimated using the spacing [MS]:[C20]=1:10. extrapolation method from the X-ray diffraction peaks, assuming a linear dependence. [12] Figure 3 shows the Cd-Cd spacing of the mixed LB films of the MS-C20 binary system ([MS]:[C2o]=l:10) plotted against the reciprocal of X-ray diffraction order 1/n. The line is fitted by the least-squares method, assuming that Cd-Cd spacing=a(l/n)+b with a=2.50

568 and b=55.5 (A) of which the estimated value corresponds to the Cd-Cd spacing. Figure 4 shows the Cd-Cd spacings of the mixed LB fihns of the MS-C20 binary and the MS-C20-AL18 ternary systems plotted against the molar mixing ratio x of AL18 ([MS]:[C2o]:[ALi8]=l:10:x). In all the binary and the ternary systems, the estimated Cd-Cd spacings remain almost the same.(55.1—55.7 A) 1— ^ 7 0 3 — 1 —1 ^ We can interpret the results of the ? 65 X-ray diffraction measurements as follows. 0 The MS-C20 binary system is known to form 5 60 heterogeneous monolayers where MS 0 0 0 0 d £ 55k> molecules are phase-separated from C20 0 molecules due to the formation of S. 50 ^ CO J-aggregates.[4-7] If MS and C20 are also ^ S 45 assumed to be phase-separated from each , S 40 D other in the MS-C20-AL18 ternary systems, 1 1 _j d 4 5 1 2 3 0 the mixed LB films of the binary and the temary systems should be composed of three Molar mixing ratio x GfALis types of the Y-type bilayer unit cells: (1) (CdC2o-C2oCd) (2) (CdC2o-MSCd) and FIGURE 4 The Cd-Cd spacing estimated (CdMS-C2oCd) (3) (CdMS-MSCd). The from the mixed LB films of the MS-C20 estimated Cd-Cd spacings are consistent with binary and the MS-C20-AL18 temary systems plotted against the molar mixing ratio x of the value of the pure (CdC2o-C2oCd) bilayer AL18. The molar mixing ratio unit cells in the LB films reported previously rMSl:[C20l:fALi8l=l:10:jc. (55.2A).[12] The result indicates that the X-ray scatterings for 2 ' < 20 < 20 * are mainly due to (CdC2o-C2oCd) bilayer unit cells. Therefore, we can discard the possibility that AL18 molecules are accumulated between CH3 end groups. The study of the orientation of AL18 added as a third component is now in progress. REFERENCES 1. H. Kuhn, Thin Solid Films, 178 (1989) 1. 2. M. Sugi, J. Mol. Electron., 2 (1985) 3. 3. See, for example, T. Kobayashi, J-Aggregates, World Scientific, Singapore, 1996. 4. M. Sugi, T. Fukui, S. lizima and K. Iriyama, Mol. Cryst. Liq. Cryst., 62 (1980) 165. 5. S. Kuroda, M. Sugi and S. lizima. Thin Solidfilms,99 (1983) 21. 6. H. Nakahara and D. Mobius, J. Colloid and Interface Sci., 114 (1986) 363. 7. H. Nakahara, K. Fukuda, D. Mobius and H. Kuhn, J. Phys. Chem., 90 (1986) 6144. 8. Y. Hirano, H. Sano, J. Shimada, H. Chiba, J. Kawata, Y.F. Miura, M. Sugi and T. Ishii, Mol. Cryst. Liq. Cryst., 294 (1997) 161. 9. Y. Hirano, J. Kawata, Y.F. Miura, M. Sugi and T. Ishii, Thin Solid Films, 327-329 (1998) 345. 10. Y. Hirano, K.N. Kamata, Y.S. Inadzuki, J. Kawata, Y.F. Miura, M. Sugi and T. Ishii, Jpn. J. Ar n|. Phys., 38 (1999) 6024. 11. Y. Hirano, T.M. Okada, Y.F. Miura, M. Sugi and T. Ishii, Trans. MRS-J, 25 [2] (2000) 417. 12. A. Matsuda, M. Sugi, T. Fukui, S. lizima, M. Miyahara, Y. Otsubo, J. Appl. Phys., 48 (1977) 771.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) f^c^ 2001 Elsevier Science B.V. All rights reserved.

569

Impedance Analysis of Redox Polymer Langmuir-Blodgett Films Atsushi Aoki and Tokuji Miyashita Institute for Chemical Reaction Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan The capacitance property of the redox polymer Langmuir-Blodgett (LB) films containing tris(bipyridine)ruthenium complex was investigated as a function of the number of monolayers and the deposition surface pressure using an impedance method. It was found that the membrane charging of the LB film took place in a short time and then the doublelayer charging at the electrode/LB film followed according to charging current restricted by the membrane resistance. The membrane capacitance was linearly proportional to the inverse of the number of monolayers whereas the double-layer capacitance was almost constant. The membrane capacitance was deviated from the theoretical line at the low deposition surface pressures because of immersion of electrolytes into the LB films. 1. INTRODUCTION Redox modified elecu-odes with a highly ordered structure have been greatly investigated in order to understand the electron transfer process in ultrathin films and to fabricate novel redox molecular devices [1]. Langmuir-Blodgett (LB) technique has an attractive feature to fabricate a molecular assembly with a controlled thickness at a molecular dimension and well-organized molecular orientation. Recently, polymer LB films have received much attention in terms of their mechanical and thermal stability. We have succeeded in incorporating A^-dodecylacrylamide polymer with tris(bipyridine)ruthenium and ferrocene moieties by free radical polymerization [2,3]. In a previous study, we demonstrated that the hetero-deposited redox polymer LB films with ruthenium and ferrocene derivatives showed the electrochemical molecular rectifying property [4]. Furthermore, we reported the photoinduced electron transfer between the photoexcited ruthenium and ferrocene derivatives in the hetero-deposited LB films [5]. In order to improve the rectifying property and the photoinduced electron transfer efficiency, it is relevant to understand the capacitance property in the LB film structure and the interfacial and interlayer electron transfer processes in the redox polymer LB film. An impedance technique has offered several advantages such as the small potential perturbation and the wide range of applicable frequencies, which allows for a convenient determination of kinetic.

570

mass-transport as well as membrane capacitance parameters [6,7]. In paiiicular, the impedance technique is a useful tool for the film modified electrodes because ol the small potential perturbation. In this study, the capacitance property of the redox polymer LB film is investigated as a function of the number of monolayers and the deposition surface pressure using electrochemical impedance method. 2,EXPERIMENTAL 2.1.Fabrication of redox polymer LB films Poly(A^-dodecylacrylamide-co-(4-(acryloylmethyl)-4'-methyl-2,2'-bipyridine)-bis(2,2'-bipyridine)ruthenium diperchlorate) (Ru copolymer) were prepared as described previously [1], The mole fraction of the redox species in the copolymer was determined from the UV-vis absorption spectrum to be 0.11 for Ru copolymer. All other chemicals were of reagent grade and used without further purification. The measurement of surface pressure (n) - area (A) isotherms and deposition of the monolayers were carried out with a computer-controlled Langmuir trough FSD-11 (USI) at 20 °C. A highly oriented pyrolytic graphite (HOPG) electrode, which was freshly peeled off by an adhesive tape before the deposition was used as a substrate. The monolayer of the Ru copolymer was transferred onto a HOPG by vertical dipping method at a dipping speed of 10 mm min ' under various surface pressures at 20 °C. 2.2.Electrochemical Impedance Measurement Impedance measurements were carried out in the frequency range 20 kHz to 0.1 Hz with an amplitude of 10 mV using an electrochemical measurement unit (Solartron SI1280B). The measurements were performed at 0.2 V vs SCE, which means that no faradaic process dominates the electrode process. An electrode area of 0.785 cm^ was exposed to 0.5 M NaC104 electrolyte solutions. A Pt wire was used as an auxiliary electrode and the potential is referenced to a saturated calomel electrode (SCE). 3.RESULTS AND DISCUSSION 3.1.Behavior of complex impedance and capacitance of the redox polymer LB films The plots for the complex impedance and the complex capacitance of the LB films with various numbers of monolayers are shown in Fig. 1. The straight capacitive line is only observed at the bare HOPG electrode in the complex impedance plane. A simple approximation of the dielectric properties of the interface is suggested as a serial RC circuit. Here, C is the double-layer capacitance (Cj,) and R is the solution resistance (Rs), which is easily obtained graphically from the extrapolated intercept with the real axis. Two straight capacitive lines in the high- and low-frequency regions are observed at all redox polymer LB

571 films on ihe electrodes. The high Irequency resistances are ahnosi the same value with thai of the bare HOPG electrode, indicating the solution reMsiance. The low-frequency resistances increase with the number of monolayers, meaning ihc membrane resistance (Rv,). On the other hand, a transformation of the impedance data mio the complex capacitance plane shows that two superimposed semicircles are observed in complex capacitance plots of all redox polymer LB films on the electrodes. The diameter of the high-frequency semicircle is observed to decrease with the number of monolayers on the electrode surface. However, the total diameters of the high- and low-frequency semicircles are almost constant. From the results, we propose the equivalent circuit model as shown in Fig. 2, assuming the two charging processes which are charging of the LB film with the dielectric property and charging of the double-layer capacitance through the membrane. The membrane charging occurs at a smaller time constant than charging of double-layer capacitance through the relatively high membrane resistance which increases with the number of monolayers. 3.2. Dependence of the membrane capacitance on the number of monolayers The membrane capacitance can be regarded as the serial circuit of monolayer capacitance (CLB) in the following equation.

where n is the number of monolayers. The membrane capacitance is plotted by the inverse of the number of monolayers (Fig. 3). The membrane capacitance is almost linearly proportional to the inverse of the number of monolayers. The monolayer capacitance is determined to be 2.37 p.F cm'^ from the slope. As the alkylchain length of DDA was determined to be 1.8 nm, the relative dielectric constant of the monolayer is determined to be 4.80. This value is much higher than the value (2.64) of stearic acid reported in the literature because of the existence of tris(bipyridine)ruthenium dication and perchlorate anion in the monolayer. The CM of the LB film deposited at 20 mN/m is deviated from the linear slope and is larger than those deposited at 30 and 40 mN/m. The result indicates that the film structure of the LB film deposited at 20 mN/m is not closely packed and the electrolyte can be immersed into the LB film whereas the LB film structure deposited at 30 and 40 mN/m is closely packed and the electrolyte can not be immersed into the LB film. Therefore, the monolayer capacitance can be determined from the membrane capacitance of the LB film at the high deposition surface pressure. In summary, the electrochemical impedance method is a useful technique for the investigation of the LB film structure. It has been found that the membrane charging of the LB film takes place in a short time and then the double-layer charging at the electrode/LB film interface follows with restriction of charging current by the membrane resistance. The relative dielectric constant of the redox polymer LB film containing tris(bipyridine)ruthenium

572

dication is higher than that of the stearic acid LB film. The excess electrolyte can not be immersed into the LB film deposited at high surface pressures. Acknowledgement This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of * Electrochemistry of Ordered Interface' (No.l 1118211) and for Encouragement of Young Scientists (No.l 1750756) from the Ministry of Education, Science, Sports and Culture, Japan. REFERENCES 1. R.Murray, Molecular Design of Electrode Surface, John-Wiley&Sons, New York, 1992. 2. A.Aoki and T.Miyashita, Macromolecules, 29( 1996)4662. 3. A.Aoki and T.Miyashita, Chem.Lett., (1996)563. 4. A.Aoki, Y.Abe and T.Miyashita, Langmuir, 15(1999)1463. 5. A.Aoki and T.Miyashita, J.Electroanal.Chem., 473(1999)125. 6. B.Lindholm-Sethson, Langmuir, 12(1996)3305. 7. J.R.Macdonald, Impedance Spectroscopy, John-Wilely&Sons, New York, 1987. CM

Rs

J-AAr-j H

I

RM

CI

Fig.2. Equivalent circuit of the redox polymer LB film on the electrode.

1*^

o A • A V

Z

Bare HOPG 2 layers 4 layers 6 layers 8 layers

X 10*. ohm cm'

1^

• 1.0

o A

a

0.51-

•V

A

Bare HOPG | 2 layers 1 4 layers 1 6 layers 1 8 layers |

£«:;^i«^*'4-.> 0.5

1.0

Xo

•

0.8

06

1

20mN/m 30mN/m

o 40mN/m

y

o/

• •

04

0.6

0.1

02

0.3

0.4

0.5

0,6

1.5

C - • u F cm"'

Fig.l. Complex impedance and capacitance plots of the redox polymer LB films on the electrodes at 0.2 V vs SCE.

Fig.3. Plots of the membrane capacitance vs the inverse of the number of the monolayers at different deposition surface pressures.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 20()l Elsevier Science B.V. All rights reserved.

573

Molecular Orientation and Motion of Pyrene Molecules at the Interface of Polymer LB Films Jun Matsui, Masaya Mitsuishi and Tokuji Miyashita Institute for Chemical Reaction Science (ICRS), Tohoku University, Katahira 2-1-1, Aoba, Sendai 980-8577, Japan The molecular orientation and the molecular motion of pyrene chromophore which is incorporated into polymer LB films were studied by UV-vis spectroscopy and fluorescence anisotropy measurement. The pyrene orientation was largely afifected by the side alkyl chains of the polymers. The pyrene chromophore orients perpendicular to its layer plane in A^-dodecyl acrylamide polymer (p(DDA)) LB films, while liie pyrene orients parallel to its layer plane in r^rr-pentyl acrylamide polymer (p(/PA)) LB films. Both polymer LB films show high anisotropy values, indicating that the molecular motion of pyrene chromophore is restricted in the polymer LB films. 1. INTRODUCTION Langmuir-Blodgett (LB) technique is one of methods to fabricate highly ordered structures and ultrathinfilmson solid substrates. We have reported that poly(/er/-pentylacrylamide) (p(/PA)), a short-branched type polymer, as well as poly(A^-dodecylacrylamide) (p(DDA)) can form a stable polymer LB film [1,2]. Recently, variousfiinctionalpolymer LB films can be fabricated from the copolymers of these acrylamide polymers. In this study, pyrene chromophore were incorporated into the polymer LB monolayers as a comonomer of p(DDA) or p(/PA), and the orientation of the pyrene molecule and its molecular motion are ^ Q ^ —CH) fCHa—CH)— discussed. C=0 ^ C=o" NH

2.EXPERIMENTAL SECTION

I

R

The copolymers of 1-pyrenylmethyl acrylate (PyMA) with DDA and with /PA were prepared as described previously [3]. The molecular structure of the copolymer R= was shown in Figure 1. The mole fraction of the pyrene chromophore in the copolymers was determined with UVvis spectroscopy, using molar extinction coefficient for 1-pyrenylmethyl acrylate(e=4.6xlO*M"^ cm^at345 nm p{t PA/PyM A) in chloroform. The molecular weights were determined p(DDA/PyMA) by gel permeation chromatography using a polystyrene Fig. 1. Molecular structrure of standard. They are summarized in Table 1. The p(DDA/PyMA)s and p(rPA/PyMA)s

574

Table 1. Characterization of p(DDA/PyMA)s and p(/PA/PyMA)s Copolymers PyMA (mol p(DDA/PyMA3)

3

p(DDA/PyMA9) p(/PA/PyMA3) p(/PA/PyMA10)

%}_ Mwx 10^

Mw/Mn

1.90

1.55

9

5.69

1.63

3

7.08

1.40

10

5.00

1.61

measurement of 3i-A isotherms and the deposition of monolayers were carried out with an automatic Langmuir trough (USI, LB lift Controller FSD-51 using a Wilhelmy-type film balance). Fluorescence depolarization spectra and UV-vis absorption spectra were measured with a Hitachi F-4500 spectrofluorophotometer and a Hitachi UV-vis spectrophotometer, respectively 3. RESULTS AND DISCUSSION 3.1 Molecular orientation of pyrene in polymer LB films The stable condensed monolayer formation of these polymers was confirmed by the JC-A isotherms as described in the previous study [3]. The surface area for the pyrene chromophore estimatedfromthe isotherms indicates that the pyrene chromophores in p(/PA/PyMA) monolayers take a parallel orientation to the monolayer plane at the air/water interface, while they orient vertically to the water surface in the case of p(DDA/PyMA) monolayers. In this study, the orientation of pyrene chromophore in the LB films deposited on quartz was investigated from the UV absorption spectra of pyrene chromophore. The absorbance around 277 nm is assigned to the transition moment which is perpendicular to the molecular long axis (A^) of pyrene, and the band around 345 nm is related to the transition moment which is parallel to it (A^^) [3-5]. Figure 2 shows the UV absorption spectra for 40 layers of (a) P ( D D A A ^ M A 9 ) and (b) p(rPA/ PyMAlO) LB films. The ratio of A^A^j for p(/PA/PyMA10) is 1.2 and almost the same as the (a)

260 280 300 320 340 360 380 Wavelength (nm)

260 280 300 320 340 360 380 Wavlength (nm)

Fig. 2. Normalized absorption spectra for 40 layers LB films (a) p(DDA/PyMA9) and(b)p(/PA/PyMA10)

575 value measured in the chloroform solution (AJ\=\. 1). On the other hand the AJ\ value of p(DDA/PyMA9) is 1.7. This is 1.5 times larger than the data measured in chloroform. These findings indicate that the pyrene molecules orient perpendicular to the layer plane in p(DDA/ PyMA)s LB films, whereas they orient parallel to it in p(/PA/PyMA)s LB multilayers. In other words, the orientation of the pyrene molecules in the monolayers at the air/water interface is kept in the LB films after the transfer of the monolayer onto solid substrate. 3.2 Molecular movement of the pyrene molecules in LB films The molecular motion of the pyrene in LB films was characterized by fluorescence depolarization technique. Fluorescence depolarization reflects the molecular motion when other factors, e.g., energy transfer, are negligible in the observed system. To achieve this condition, the polymers with low pyrene content (about 3 mole%) were employed. Assuming that the pyrene is randomly distributed in the LB monolayer because the copolymers are prepared by a random copolymerization, the average distance between neighboring pyrene residues in p(DDA/ PyMA3) and p(/PA/PyMA3) can be estimated to be 3.5 and 3.4 nm respectively, indicating the effect of energy transfer could be negligible since the Forester radius for pyrene is ca. 1.0 nm. Figure 3 shows the polarizedfluorescencespectra of (a) p(DDA/PyMA3) and (b) p(/PA/PyMA3) LB monolayer As shown in thefluorescencespectra, there is no excimer emission, which support the above assumption. The anisotropy r is calculated as r=(Ijj'IJ I (Ijj-^-lIJ, where /^ and /^ are the intensities of emission parallel and perpendicular to the polarization vector of excitation light, respectively. The r-value for each polymer was obtainedfi-omFigure 3 and listed in Table 2. The r-values of both LB films were higher than the values for the cast films. In general the cast film is an inflexible matrix, which contains a great deal of free volume. On the other hand, the pyrene would be tightiy constrained between alkyl side chains in the polymer LB films, resulting in the higher r-values. In a comparison between p(DDA/PyMA3) LB film and p(/PA/ PyMA3) LB film, the r-value is higher in the former one. In previous study, it is suggested that the pyrene chromophore is exposed onto the surface of p(/PA/PyMA)s , whereas the pyrene is surrounded by the dodecyl side chains in p(DDA/PyMA)s. Therefore it could be considered that (b)

(a) 25

I 360 400 440 480 520 560 600

360 400 440 480 520 560 600 Wavelength (nm) Fig. 3. Fluorescence deporaization spectra for (a) p(DDA/PyMA3) and (b) p(rPA/PyMA3) h, L

576 Tabic 2. The r-value for p(DDA/PyMA3) and p(/PA/PyMA3) Copolymers cast p(DDA/PyMA3) p(/PA/PyMA3)

0.27 0.26

fiIm

1 l^er LBfilm 0.34 0.29

1 layer LB film overcoated by p(DDA) 41ayers 0.33 0.29

the pyrene motion is more restricted in p(DDA/PyMA)s than in p(/PA/PyMA)s. Next we investigated the efFect of capping layers (4 layer-p(DDA) LB film) on the r-value. The r-value, however did not change with the capping (Table 2). This is quite differentfi-omthe result of the cadmium arachidate/10-(l-pyrene)decanoic acid mixed LB films which was reported by Wu et al. [7]. In the mixed fatty acid LB films, the capping with other layers restricted the motion of the pyrene molecule. No effect of capping layer observed in the present polymer LBfilmsmeans that the motion of the pyrene chromophore is already restricted in p(DDA/PyMA3) and p(/PA/ PyMA3) monolayer films due to not only the packing of the side chain, but also the polymer backbone. As the other factor, the difference in r-value between p(DDA/PyMA)s and p(/PA/PyMA3) is also explained by the difference of the orientation of the pyrene in those LB films because the distribution of molecular orientation also depolarized the fluorescence. As mentioned in the previous section, the pyrene oriented parallel to the substrate and takes uniaxial orientation in p(DDA/PyMA)s, although it take no axial orientation in p(/PA/PyMA)s. Therefore in p(/PA/ PyMA3) LBfilmsthefluorescenceis more depolarized than in p(DDA/PyMA3) LB films because of the randomness of the pyrene ring orientation. Consequently the r-value of p(DDA/PyMA3) LB films become higher than the value in p(rPA/PyMA3) LB films. ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for the "Research for the future" Program (JSPS-RFTF97P00302) from the Japan Society for the Promotion of Science. J. M. would like to thank to Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. REFERENCE L T Miyashita, Prog. Polym. Sci., 18 (1993) 263. 2. T Taniguchi, Y. Yokoyama and T Miyashita, Macromolecules, 30 (1997) 3646. 3. J. Matsui, M. Mitsuishi and T Miyashita, Macromolecules, 32 (1999) 381. 4. R M. Winnik, Chem. Rev, 93 (1993) 587. 5. S. AJdmoto, A.Ohomori and L Yamazaki, J. Phys. Chem. B 101 (1997) 3753. 6. B. L Berlman, Energy Transfer Parameters of Aromatic Compounds, Academic Press, New York, 1973. 7. H. Wu, M. D. Foster, S. A. Ross, W. L. Mattice and M. A. Mattice, Langmuir, 12 (1996) 3015.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

577

Luminesceiice Properties of Europium Complexes in Polymer LB Films Masaya Mitsuishi, Shinji Kikuchi and Tokuji Miyashita Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-1, Aoba, Sendai 9808577, Japan Polymer LB films containing europium complexes (p(DDA/Eu(TTA)3Phen)) were prepared by spreading a mixed solution of pDDA (= poly(Ar-dodecylacrylamide)) and Eu(TTA)3Phen (= 4,4,4-trifluoro-l-(2-thienyl)-l,3-butanediono)-l,10-phenanthroline europium(III)) onto a water subphase in which the complexes and their ligand groups were saturated. From the surface pressure - area isotherms, it was found that p(DDA/Eu(TrA)3Phen) takes a condensed monolayer formation. The monolayer could be transferred on solid substrates as Y-type LB films. The spectroscopic properties of Eu(TTA)3Phen in the polymer LB films were investigated by fluorescence spectroscopy and time-resolved luminescence decay measurement. 1. INTRODUCTION The utilization of lanthanide ions, in particular, Eu^ and Tb^, have been paid much attention of scientists because of their outstanding luminescence properties [1]. Although the lanthanide ions themselves have very weak absorption band, they show fascinating luminescence features, if they are bound to the suitable ligands such as p-diketone derivatives [2]. For example, europium complexes emit a strong red light with fairly long luminescence lifetime (submillisecond) and its bandwidth is very narrow. They are also very stable in terms of photochemistry. Actually, the lanthanide ion compounds are expected to be a promising material for the application of laser and/or biochemistry fields. Langmuir-Blodgett (LB) technique enables us to fabricate well-ordered nanostructure. Recently, the preparation of LB films containing europium or terbium complexes have been studied by using fatty acids as an LB matrix [3]. Using 7V-alkylacrylamide as a comonomer, a wide variety of functional groups could be incorporated into highly condensed polymer monolayer within 1.8 nm thick [4]. In the current article, we describe the fabrication of composite LB films containing europium complexes. The monolayer formation and the luminescence properties of the europium complexes in polymer LB films have been investigated. 2. EXPERIMENTAL Europium complexes, Eu(TTA)3Phen (4,4,4-trifluoro-l-(2-thienyl)-l,3-butanediono)-l,10phenanthroline europium(III)) were synthesized according to a reference [3]. TTA and Phen were commercially available, and used without further purification. The synthesis of Ndodecylacrylamide (DDA) and its polymerization were described elsewhere [5]. The chemical structures of Eu(TTA)3Phen and pDDA are shown in Figure 1. A Langmuir trough with Wlhelmy type film balance (HBM, Kyowa Interface Science CO

578 Ltd.) was used for the measurement of surface pressure (jc) - area (A) isotherms and the deposition of the monolayer onto quartz — ( C H 2 - C H V ~ substrates. A mixed chloroform solution of c=o pDDAand Eu(TTA)3Phen (1:1 molar ratio) was spread onto the water subphase in which NH Eu(TTA)3Phen, TTA, and Phen were saturated. The monolayer was compressed at a CH3 speed of 14 cmVmin. UV-Vis absorption and luminescence spectra were measured with a HITACHI UEu(TTA)3Phen pDDA 3000 spectrophotometer and an F-4500 fluorescence spectrophotometer, respectively. Fig.l. Chemical structure Time-resolved luminescence decay measurement was carried out using single photon counting system. The europium complexes were excited by a nitrogen laser (fwhm 700 ps) and the emission decay was monitored by streak camera system (Hamamatsu Photonics). All measurements were carried out at room temperature unless otherwise indicated. 3. RESULTS AND DISCUSSION 3.1. Monolayer formation 70The ji - A isotherms for p(DDA/ B 6 0 - 15'C Eu(TTA)3Phen) are shown in Figure 2. They Z 20'C show steep rise in surface pressure and high B 5 0 - 25'C collapse pressure. As the temperature in^^^ « ^ 40creases, however, the collapse pressure desvi creases drastically. This is a similar phenom30enon to that of pDDAhomopolymer [5]. We scd 2 0 observed no plateau region in the Ji-A isoVtherms. This is totally cHfferent from the re3 10W sult reported in the composite system consist0 ing of Eu(TTA)3Phen complexes and low 0.0 0.1 0.2 0.3 0.4 0.5 molecular weight compounds [3]. In the above Surface Area (nmVmonomer unit) report, the Eu(TTA)3Phen complexes form microcrystals onto the water subphase. This imFig. 2. ji - A isotherms of p(DDA/ plies that Eu(TTA)3Phen complexes are uniEu(TrA)3Phen) formly distributed in the polymer monolayer without the microcrystal formation. The average surface limiting area, which is determined from the extrapolation of the linear portion to zero pressure, is fairly larger than that of pDDA(0.28nmVunit)[5]. <ji

i

32. Deposition of p(DDA/Eu(TIA)3Phen) LB films The deposition of p(DDA/Eu(TTA)3Phen) was carried out at 15 °C, keenng the surface pressure at 30mN/m. The p(DDA/Eu(TTA)3Phen) monolayer was successfully deposited with the

579

(a) 3

/\

3

>% C

C

-

c

"

•' 1 • • ' ' 1

"

4J

• •^' n • • •

450 500 550 600 650 700 Wavelength (nm)

2

4 6 8 10 12 Number of Layers

Fig. 3. (a) Luminescence spectra of p(DDA/Eu(TTA) Phen) LB films as a function of deposited layers (from bottom 2,4, 6, 8 and 10 layers); (b) plots of luminescence intensity at 613 nm vs, number of deposited layers.

transfer ratio of almost unity. Therefore, Y-type LB films could be obtained by the vertical deposition method. Although it is not clearly understood at present why Eu(TTA)3Phen compounds can be incorporated into pDDA LB monolayer during the deposition, it should be mentioned that the europium complexes could not be deposited onto solid supports when the "pure" pDDA was spread on the water subphase which contained the Eu(TTA)3Phen. Anyway, as Yang et al. noted, the use of the "composite" subphase is necessitate to transfer the p(DDA/ Eu(TTA)3Phen) monolayer onto solid supports [3]. 33. Laminescence properties of p(DDA/Ea(TIA)3Phen) LB films Figure 3(a) shows the luminescence spectra of p(DDA/Eu(TTA)3Phen) LB tllms as a function of deposited layers (excitation wavelength = 280 nm). As shown in Figure 3(a), the luminescence peak observed at 613 nm is assigned to ^D^ - • ^F^ emission which is the strongest emission band [1]. The linear relationship between the intensity (at 613 nm) and number of layers, shown in Figure 3(b), clearly proves the regular deposition of c p(DDA/Eu(TTA)3Phen) monolayer. The similar proportionality was also observed in UV-Vis absorption spectra. In addition, the X-ray diffraction measurements showed the 0.5 1.0 1.5 0.0 layer structure of p(DDA/Eu(TTA)3Phen) LB Time (ms) films with 2.1 nm thickness per monolayer. Fig. 4. Luminescence decay of p(DDA/ It can be concluded that the Eu(TTA)3Phen Eu(TTA)3Phen) LB films with 10 layers. is located in the polymer LB assemblies so that each monolayer contains Eu(TTA)3Phen

580

keeping at the constant contents. Table 1 Luminescene lifetime of p(DDA/ In general, europium complexes have a Eu(TTA)3Phen). long lifetime in the range of 0.1 to 1 ms [1]. Figure 4 shows the luminescence decay of T (mS) • y2 p(DDA/Eu(TTA)3Phen) LB films with 10 layers. The decay curve could be well fitted LB film 0.714 1.17 by a single exponential decay function. As shown in Table 1, the luminescence lifetime 0.335 cast film 1.19 of the europium complexes drastically changed; they show the longest lifetime (x) solution (AT) 0.488 1.14 in the LB films (0.714 ms) but is the shortest one in cast film (0.335 ms). The 0.238 1.15 Eu(TTA)3Phen complexes were homoge- solution (no bubbling) neously distributed in the polymer LB films, while they form aggregates in the cast films ' Chi-square of the weighted residuals. [7]. In other words, the regulation of the structure at nanoscale results in the stabilization in the excited state. In conclusion, composite polymer LB films, p(DDA/Eu(TTA)3Phen) LB films were prepared by using pDDA homopolymer and Eu(TTA)3Phen complexes. From spectroscopic measurements, it was confirmed that the p(DDA/Eu(TTA)3Phen) LB films show the uniform and well-ordered structure. Furthermore, the well-ordered stacking results in the uniform distribution of the chromophore, leading to the stabilization of its excited state. The present polymer LB films containing the Eu complexes are expected as a potential material applicable to new emitting devices in nanotechnology. This work was partially supported by "Molecular Sensors for Aero-Thermodynamic Research (MOSAIC)", the Special Coordination Funds of Science and Technology Agency.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

F. S. Richardson, Chem. Rev., 82 (1982) 541. L. R. Melby, N. J. Rose, E. Abramson, and J. C. Caris, J. Am. Chem. Soc., 86 (1964) 5117. G. -L. Zhong and K. -Z. Yang, Langmuir, 14 (1998) 5502. T. Miyashita, Prog. Polym. Sci., 18 (1993) 263. T. Miyashita, Y. Mizuta and M. Matsuda, Br. Polym. J., 22 (1990) 327. R R. Selvin, T. M. Rana and J. E .Hearst, J. Am. Chem. Soc, 116 (1994) 6029. S. Kikuchi, M. Mitsuishi and T. Miyashita, to be published.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c: 2001 Elsevier Science B.V. All rights reserved.

581

Well-defined, rigid multiporphyrin arrays: interfacial synthesis and optical properties in monolayer assemblies Dong-Jin Qian, Chikashi Nakamura and Jiin Miyake National Institute for Advanced Interdisciplinary Research, Agency of Industrial Science and Technology, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan We report a readily available strategy to prepare multiporphyrin arrays by using Cd^*mediated assembly of tetrapyridylporphyrin (TPyP) on Cd^^ subphase surface. By using Langmuir-Blodgett (LB) technique, monolayers were transferred onto solid supports as multilayers intercalated by dipalmitoylphosphatidylthioethanol. Only 4-nm red shift of porphyrin Soret band was measured in the LB film of Cd^^-TPyP multiporphyrin array whilst 25-nm was measured in the TPyP LB film. Similar orientation angle was estimated for porphyrin rings in LB films of either TPyP or Cd^*-TPyP multiporphyrin array. 1. INTRODUCTION Well-defined supramolecular arrays of functional molecules are of interest for the design of molecular electronic and opto-electronic devices and for the modeling of biological energy or charge transfer. Porphyrins are particular attractive species to incorporate into supramolecular arrays as functional materials including the transport of energy, charge, molecules and ions, and as model of photosynthetic reaction center [1]. Architecture of large porphyrin (multiporphyrin) arrays is generally achieved by using linkers for porphyrins or the metal centers of metal porphyrins in solutions [2] and at interface [3]. Furthermore, by introduction of an interconnetcted unit or a guest molecule to support the multiporphyrin layers, threedimensional (3D) organized metalated multiporphyrin arrays (or 2D-polymer sheet) could be achieved [4]. We report here an easily available strategy to prepare 2D/3D muhiporphyrin arrays by using Cd^^-mediated assembly of tetrapyridylporphyrin at the air-water interface and by using Langmuir-Blodgett (LB) technique. These LB films were characterized by using UV-vis and absorption dichroism spectra. The molecular arrangement for porphyrins in the LB films was also discussed. 2. EXPERIMENTAL DETAILS 2.1. Materials Tetrapyridylporphyrin (TPyP, Fig. 1) was purchased from Aldrich Chem. Co.; dipahnitoylphosphatidylthioethanol (DPPTE, Fig. 1) from Avanti Polar-Lipids INC; cadmium chloride from Wako Pure Chemical Industries Ltd; and HPLC-grade chloroform used as spreading solvent was from Nacalai Tesque, INC. AU reagents were used as received and without further purification. Ultrapure water (18.3 MQ) for the subphase was prepared with a Milli-Q filtration unit from Millipore Corp.

582

1 2 3 Area (nm molecule )

TPyP

4

DPFIE

Fig. 1. Molecular structure of porphyrin and phospholipid.

Fig. 2. Ji-A isotherms for the monolayers of (a) TPyP, (c) DPPTE-TPyP on water and (b) TPyP, (d) DPPTE-TPyP on 0.1 M Cd^^ subphase surfaces.

2.2 Methods Cd^+.XPyP multiporphyrin array was prepared by spreading a dilute solution of TPyP or DPPTE-TPyP onto Cd^* subphase surface. The surface pressure-area (JI-A) isotherm measurements and LB film deposition were done with a KSV 5000 minitrough (Finland) using a continuous speed of 10 cmVmin at 20 °C. Transfer of monolayers of TPyP, DPPTETPyP or (DPPTE)Cd^''-TPyP multiporphyrin arrays onto solid plates was done by vertical dipping method. The dipping speed was 2 mm/min. Absorption and dichroism spectra of transferred LB films were measured with a Shimadzu UV-1601 UV-vis spectrophotometer. 3. RESUTLS AND DISCUSSION 3.1. Surface pressure-area isotherms Fig. 2 shows the JI-A isotherms of TPyP and DPPTE-TPyP monolayers on water and 0.1 mol/1 Cd^^ subphase surfaces. All curves indicate that a condensed phase is formed when the monolayers are compressed. By extrapolating the tangent on the curves at a fixed point to zero pressure, we find that the mean TPyP molecular area is 0.65 nm^ for TPyP monolayer on water surface and 2.5 - 2.6 nm^ on 0.1 mol/1 Cd^* subphase surface, respectively. It is expected that mean molecular area of tetraarylporphyin be 0.9 nm^ if the porphyrin rings He vertical to the water surface and 2.25 nm^ if the porphyrin rings lie parallel [5]. Hence, the mean occupied area per TPyP molecule on water surface is even smaller than that of porphyrin rings lie vertical to the water surface, the phenomenon which has been recently attributed to the formation of a tilted and overlapped arrangement or the formation of aggregates [6]. The mean TPyP molecular area on 0.1 mol/1 Cd^* subphase surface is close to that of porphyrin rings lie parallel to water surface, which is ascribed to the formation of Cd^^-TPyP multiporphyrin arrays by Cd^^ ions coordination to pyridyl groups of TPyP [4]. For the monolayers of DPPTE-TPyP (molar ratio, 2 : 1) on water and 0.1 mol/1 Cd^* subphase surfaces, the measured JI-A isotherms showed a little smaller mean molecular area but a close compressibility compared to the ideal mixing curve (Fig. 2c' and d'), indicating a condensed effect in the mixed monolayers. A large occupied mean TPyP molecular area (2.2 - 2.4 nm^) could be calculated from the mixed Ji-A isotherms, suggesting that the Cd *-TPyP multiporphyrin array also formed in the DPPTE-TPyP mixed monolayers.

583 3.2. Absorption spectra of the transferred LB films Absorption characters of chromophores are usually used to evaluate their aggregation and orientation in monolayer assemblies. Fig. 3 shows the absorption spectra of one-layer LB films of TPyP, DPPTE-TPyP, Cd^^-TPyP and (DPPTE)Cd^'^-TPyP multiporphyrin arrays in comparison with TPyP in solution. As can be seen from Fig. 3 that absorption spectrum of TPyP solution consists of a strong Soret band centered at 417 nm and several Q-bands between 500 and 650 nm. This Soret band produced a 25-nm red shift for the LB films of TPyP and DPPTE-TPyP and a 4-nm red shift for the LB films of Cd^^-TPyP and (DPPTE)Cd^"'-TPyP multiporphyrin arrays, respectively. Kasha predicted that a red shift of chromophores corresponds to a titled "deck of cards" (J^^ggregate) aggregation of chromophores [7], which results from a JC-JC interaction between porphyrin rings. The extent of the shift depends on several factors including the degree of aggregation and the interplanar separation distance. As we have measured that mean TPyP molecular area in TPyP monolayer is rather small (0.65 nm^), that is, large aggregates are formed on water surface, so it is reasonable that there is a large red shift for its Soret band. On the other hand, the mean TPyP area in Cd^^-TPyP multiporphyrin array is close to that of the rings parallel to substrate surface, so that JI-JI interaction between porphyrin rings is largely weakened, leading to the Soret band only slightly shifted. In addition to the peak shift, absorption spectra of the LB films reveal another three features. The first one is a small peak at 429 nm for the DPPTE-TPyP LB film, which could be attributed to the reduction of porphyrin-porphyrin interaction after the introduction of DPPTE. The second feature is that all absorption band in LB films are broadened as compared to that in solution due to an increase of the electronic interaction among porphyrin moieties in LB films. The third feature is that the relative intensity (RI) of Q-band to the Soret band is rather larger for TPyP in the TPyP LB film (RI > 0.1) than that in solution (RI ~ 0.05) and multiporphyrin array LB films (RI ~ 0.01). This means that absorbance of Qband increases with the formation of J-aggregate in TPyP LB film, the observation of which agrees with spectroscopic studies on the monomer and J-aggregate of several water-soluble porphyrins by Periasamy and his coworkers [8]. For a two-layer LB film of (DPPTE)Cd^*-TPyP multiporphyrin array on hydrophobic glass surface, the absorption spectra show a shoulder peak at 440 nm and another two Q-bands between 500-650 nm. The results could be ascribed to a face-to-face interaction of porphyrin rings between two layers of Cd^*-TPyP multiporphyrin arrays in Y-type LB films.

0.00 350

400

450

500

550

600

650

Wavelength (nm)

Fig. 3. Absorption spectra for (a) TPyP solution and LB films of (b) TPyP, (c) DPPTE-TPyP, (d) Cd^^-TPyP, and (e) (DPPTE)Cd^*-TPyP.

. = DPPTE;0=Py; a = C^ 9=Cd2+

Scheme 1. Schematic models for LB films of (a) DPPTE-TPyP and (b) (DPPTE)Cd^^-TPyP multiporphyrin.

584 3 J . Arrangement of TPyP in monolayer and mulUporphyiin arrays Porphyrin orientation was investigated by measurement of polarized UV-vis spectra at incidence angles of O'' and 45**. The titled angle between the mean porphyrin plane and substrate surface, 6, was calculated according to Yoneyama et al's method [9]. Our calculation indicated that 6 was in the range of Id" -31° for LB films of TPyP and Cd^^-TPyP multiporphyrin array, and of 35° -^ 38° for DPPTE-TPyP and (DPPTE)Cd^*-TPyP multiporphyrin array, respectively. Considering the uncertainty of ± 5° by using the polarized UV-vis method [8], little difference was found for the orientation of the porphyrin rings in the LB films of TPyP or Cd^"'-TPyP muhiporphyrin array. However, we have revealed that the mean molecular areas per TPyP unit in these two systems were largely different; 0.65 nm^ for TPyP monolayer and 2.5 nm^ for Cd^^-TPyP multiporphyrin arrays, the similarity of porphyrin orientation should relate to their different arrangement. A crystal structure analysis of Cd^^-TPyP coordination polymer synthesized by TPyP layered with metal sahs in methanol solution indicated that TPyP molecules are nonplanar and adopt a butterfly configuration [6]. Only 5° - 10° increase of 6 was measured by immersion of DPPTE molecules into the TPyP or Cd^*-TPyP multiporphyrin arrays, indicating no significant effect of DPPTE on the arrangement of TPyP in organized films. Based on the crystal analysis and our results from Ji-A isotherms, UV-vis spectra and orientation angle, we propose an organization scheme shown in Scheme 1 for the LB films of (DPPTE-)TPyP and (DPPTE)Cd^"'-TPyP multiporphyrin array. In Scheme la where TPyP rings are titled and overlapped, smaller mean occupied TPyP area and larger red-shift of the Soret band should be measured; while in Scheme lb where Py-Cd^*-Py coordination networks separate TPyP rings, larger mean occupied TPyP area and smaller red-shift of the Soret band should be measured. 4. CONCLUSIONS Interfacial metal-mediated assembly at the air-water interface pro'/ides a readily available route for the preparation of well-defined, 2D muhiporphyrin arrays. Moreover, 3D organized multiporphyrin arrays could be obtained by using LB technique.

REFERENCES 1. V. S.-Y. Lin, S. G. Dimagno, M. J. Therien, Science 264 (1994) 1105. 2. J. Fan, J. A Whiteford, B. Olenyuk, M. D. Levin, P. J. Stang, E. B. Fleischer, J. Am. Chem. Soc. 121 (1999) 2741. 3. L. Wen, M. Li, J. B. Schlenoff, J. Am. Chem. Soc. 119 (1997) 7726. 4. C. V. K. Sharma, G. A Broker, J. G. Huddleston, J. W. Baldwin, R. M. Metzger, R. D. Rogers, J. Am. Chem. Soc. 121 (1999) 1137. 5. A Ruaudel-Teixier, A Barraud, B. Belbeoch, M. Roulliay, Thin Solid Films 99 (1983) 33. 6. H. Chou, C. -T. Chen, K. F. Stock, P. W. Bohn, K. S. Suslick, J. Phys. Chem. 98 (1994) 383. 7. M. Kasha, Radiat. Res. 20 (1963) 55, 8. N. C. Maiti, S. Mazumdar, N. Periasamy, J. Phys. Chem. B, 102 (1998) 1528. 9. M. Yoneyama, M. Sugi, M. Saito, K. Ikegami, S. Kuroda, Jpn. Appli. Phys., 25 (1986) 961.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

585

Surface Enhanced Infrared Absorption and UV-Vis Spectroscopic Study of a Monolayer Film of Protoporphyrin IX Zinc (II) on Gold Zhijun Zhang^ and Toyoko Imae* Research Center for Materials Science, Nagoya Univ., Nagoya 464-8602, Japan This paper describes preparation and structural characterization of a selfassembled monolayer film of protoporphyrin IX Zinc (II) by surface enhanced infrared absorption, normal infrared reflection absorption, and ultravioletvisible spectroscopies.

1. INTRODUCTION Axial ligation of metalloporphyrins to a ligand terminated self-assembled monolayer (SAM) formed on solid substrates offers a simple and effective way to prepare porphyrin monolayers [1]. It is, however, not always easy to employ surface infrared spectroscopy, due to poor S/N ratio, to probe these monolayers [la]. Surface enhanced infrared absorption spectroscopy (SEIRAS) has recently proven to be a powerful tool in study of surface monolayer films [2]. We report herein on characterization of a SAM of protoporphyrin IX Zinc (II) (ZnPP) on an Au island film by SEIRA and ultraviolet-visible (UV-vis) spectroscopies. Two step strategy was employed to prepare the SAM of ZnPP on Au surface: formation of a SAM of 4-pyridinethiol (PySH) as a coupling layer on Au surface and then axial ligation of ZnPP to the pyridine SAM.

2. EXPERIMENTAL ZnPP (Figure 1) and PySH were purchased from Aldrich and used as received. A 200 nm thick Au evaporated glass substrate was used for infrared reflection absorption spectra (IRAS) and UV-vis RAS measurements. It was 1. Japan Society for the Promotion of Science (JSPS) postdoctoral fellow. * Corresponding author.

586 treated with piranha solution (H2SO4: 30% H^O. 3:1 v/v) prior to SAM formation. A 15 nm Au island film that was used for SEIRAS measurements was prepared via thermal evaporation onto a flat glass substrate in a vacuum chamber at a pressure of 10^ Torr. Formation of a SAM of PySH onto Au substrate was described elsewhere [3]. The SAM was then immediately immersed into an ethanol/DMF (4:1 in v/v) solution of ZnPP (2 x 10"^ M) and kept for 72 hours at room temperature. After withdrawal, the sample was sonicated in the ethanol/DMF mixed solvent for 1 min. (the SAM on the Au island film) or 5 min. (the SAM on the thick Au substrate). Finally, it was rinsed again with a copious amount of the mixed solvent. UV-vis RAS were taken on a Shimadzu UV-2200 Spectrophotometer with a reflection attachment. Measurement of IR spectra in reflection mode was performed on a Bio-Rad FTS 575C FT-IR Spectrometer with a Harrick reflectance attachment at an incidence angle of 75°.

3. RESULTS AND DISCUSSION 3.1. UV-vis spectra Figure 2 presents UV-vis (a) RAS of the porphyrin SAM on a thick Au substrate and (b) transmission spectrum (TRS) of the porphyrin SAM on the Au island film. For comparison, a UV-vis spectrum of ZnPP in ethanol/DMF (4:1 in v/v) is added as (c). The solution shows a strong band (Soret band) at 417 nm and two weak bands (Q bands) at 545 and 583 nm. For the porphyrin SAM on the Au island film, the spectrum shows the Soret band at 422 nm, suggesting formation of a SAM of ZnPP. In addition, small Soret band shift reveals weak effect of axial ligation on electronic structure of the porphyrins [3]. However, for the thick Au film, the spectrum gives rise to a strong band at 433 nm and another unresolved broad absorption band at about 480 nm occurring as a shoulder on the former band. This phenomenon was also previously observed by Zhang, one of the present authors and his colleagues [4] in a study of LB film of a long alkyl chain appended porphyrin on Au. It was confirmed now that the spectral differences between the porphyrin SAMs formed on Au island film and on thick Au surface are mainly due to spectral distortion caused by the UV-vis RAS measurement. The UV-vis results indicate successful formation of the ZnPP SAM on both kinds of Au films precoated with a SAM of PySH.

587

;H=CH,

.08-f

CH,

ZiiPP SAM (a) on thick Au

;H=CH. \

(b) on Au island film \.

ZnPP Xc) in solution

\

COOH

I

COOH 400

500

600

W a v e l e n g t h (nm) Fig. 1. Structure of ZnPP

Fig. 2. UV-vis (a) RAS, (b) TRS of ZnPP SAM, and (c) TRS of ZnPP solution (a.u.)

3.2. IR spectra Figure 3 exhibits (a) SEIRAS in reflection mode and (b) normal IRAS in 1700-1400 c m \ of a SAM of PySH on the Au island film and on the thick Au surface, respectively. The bands at 1612, 1564, and 1474 cm"' can be assigned to the stretching modes 8a, 8b, and 19a of the pyridyl group, respectively [3]. Compared to the normal IRAS, the three IR absorption bands in the SEIRAS show drastic enhancement of 10, 26, and 17, respectively. Figure 4 presents (a) SEIRAS and (b) IRAS of the porphyrin SAM on the 15 nm Au island film and on the thick Au surface, respectively, precoated with the SAM of PySH, and (c) IR spectrum of ZnPP solid in KBr pellet. Two bands at 1725 c m ' and 1658 cm"' are due to C=0 stretching mode of the carboxylic acid group, and the skeleton of the porphyrin molecule, respectively [5]. The band at 1576 c m ' is due to 8b mode of the pyridyl group. These bands are significantly enhanced in the SEIRAS, being 19, 35, and 25 times, respectively, stronger than the corresponding bands in the normal IRAS. Moreover, a weak peak at 1476 cm"', assignable to the 19a stretching mode of the pyridyl group, and a medium band at 1385 cm"', due to the C-H deformation of the alkyl part of the porphyrin molecule, either of which was not detected in the normal IRAS, are clearly seen in the SEIRAS. These facts strongly demonstrate that SEIRAS is more suitable for study of surface structures, compared with normal IRAS. It is also interesting to mention that, the band at 1576 cm"' in the SEIRAS of the SAM of PySH before and after the formation of the porphyrin SAM shows no noticeable alteration in band intensity, but a frequency shift by 12 cm"', suggesting the effect of axial ligation on the vibration mode of the pyridine. Another feature is that, in the KBr spectrum, the band at 1725 cm"' is

588 predominant, and the band at 1658 cm'' appears just as an unresolved shoulder. In the SEIRAS, however, the two bands are well resolved and with comparable intensities, indicative of different environment and interactions for ZnPP molecules in solid and the SAM.

.03-

x5

b Tf vO wo

i02-

A

JD u. O

CN

JO

vO

.01- a 1700

^^^

/

'^ r^

/\

''T

/ \

/

A

-A.

W

1

1

1600 1500 Wavenumber (cm"')

1400

1800 1600 1400 Wavenumber (cm')

Fig. 3. (a) SEIRAS and (b) IRAS

Fig. 4. (a) SEIRAS, (b) IRAS of ZnPP SAM

of a SAM of PySH on Au surface.

and (c) IR TRS of ZnPP solid in KBr (a. u.).

4. CONCLUSIONS Conclusions reached from this work include: (1) UV-vis spectra suggest formation of ZnPP SAM and weak interactions between ZnPP molecules in the SAM. In addition, formation of the SAM on Au island film allows us to measure UV-vis spectra in transmission mode that avoids spectral distortion occurring in the UV-vis RAS. (2) SEIRAS offers much more detailed structural information than normal IRAS in study of the porphyrin SAM.

REFERENCES 1. For examples, see: (a) D. A. Offord, S. B. Sachs, M. S. Ennis, T. A. Eberspacher, J. H. Griffin, C. E. D. Chidsey, and J. P Collman, J. Am. Chem. S o c , 120 (1998) 4478. (b) Z. -J. Zhang, S. F. Hou, Z. H. Zhu, and Z. F. Liu, Langmuir, 16 ( 2000) 537. 2. M. Osawa, Bull. Chem. Soc. Jpn. 70 (1997) 2861. 3. Z. -J. Zhang and T. Imae, J Colloid and Interface Sci., in press. 4. Z. -J. Zhang, A. L. Verma, M. Yoneyama, K. Nakashima, K. Iriyama, and Y. Ozaki, Langmuir, 13 (1997) 4422. 5. L. J. Boucher and J. J. Katz, J. Am. Chem. Soc. 89 (1967) 1340.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c 2001 Elsevier Science B.V. All rights reserved.

589

Thermoreversible vesicles with semipolar additives M. Gradzielski, H. Hoffmann, K. Horbaschek, F. Witte Lehrstuhl fur Physikalische Chemie I, Universitat Bayreuth, D - 95440 Bayreuth, Germany The influence of the semipolar additive hexylacetate (HA) on micellar structure and phase behavior of surfactant solutions of tetradecyldimethylamine oxide (TDMAO) has been studied. It was found that the semipolar additive behaves at low concentration as a cosurfactant and at higher concentration as an apolar additive. 1. INTRODUCTION Additives to surfactant systems can be divided into two groups, polar and apolar additives. Polar additives (cosurfactants) such as medium or long chain alcohols are located in the micellar interface. Their head group area is considerably smaller than that of a surfactant molecule. In this way the micellar curvature is reduced by the addition of a cosurfactant and structural transformations can be induced. It is thus possible to form hexagonal, lamellar or bicontinuous lyotropic mesophases from Lj-phases by addition of cosurfactants [1-4]. In contrast apolar additives such as alkanes are incorporated into the hydrophobic interior of the micelles. Between these 2 extreme situations a variety of molecules exist with intermediate polarity. One class of such semipolar additives are asymmetrical esters. E. g. Hexylacetate (HA) has both a relatively polar acetate and a hydrophobic hexyl group. For this reason the molecule shows an amphiphilicity which is of course much lower than that of the corresponding alcohol - 1-hexanol. In this pa|:»er the aggregation behavior of hexylacetate in the ternary system 200 mM tetradecyldimethylamine oxide (TDMAO)/ hexylacetate (HA)/water is discussed. 2. RESULTS AND DISCUSSION The phase behavior was observed by adding hexylacetate to a constant concentration of 200 mM of the surfactant TDMAO (fig. 1). At 20 X the phase behavior is very simple. Over the whole concentration range no lyotropic mesophase is formed. Up to the solubilization limit of 500 mM hexylacetate all solutions are totally clear and show no birefringence. Only at low hexylacetate concentrations there is a viscosity increase which is due to the growth of the rodlike micelles. For temperatures above 25 °C an interesting phenomenon is observed. At intermediate HA concentrations the solutions are turbid and possess distinctly different rheological properties. The viscosity is much higher. The samples are viscoelastic, show a yield stress and are birefringent between crossed polarizers. This properties indicates that this phase is an La-phase. These features are typical for La-phases in the vesicular state. If the temperature is lowered again to the initial value of 20 °C the new phase vanishes and the solution becomes again clear, isotropic, and shows Newtonian flow behavior.

590

to (0 Q.

E

a 100 [hexylacetate] / mM

Fig. 1: The phase diagram of a 200 mM TDMAO solution as a function of temperature and hexyl-acetate concentration.

200

•

D

D

300

D

D

O

400

[hexylacetate] [mM]

Fig. 2: The zero shear viscosity T|O of a solution of 200 mM TDMAO/hexylacetate as a function of the hexylacetate concentration at 20 °C.

In fig. 2 the zero-shear viscosity is shown at different hexylacetate concentrations at 20 °C, where one is below the temperature of the phase transition to the vesicle phase. When hexylacetate is added the viscosity increases steeply and reaches a maximum value of 80 mPas for 60 mM HA. Beyond that concentration the viscosity decreases again. Beyond 250 mM HA the viscosity remains constant at 1.6 mPas which is significantly lower than the initial value of the pure surfactant solution. The viscosity increase can be attributed to a growth of micellar rods, i. e. at low HA concentration it behaves like a cosurfactant [5-7]. At high ester concentrations (300 mM HA) the samples are less viscous and show Newtonian flow behavior. This is typical for solutions which contain small spherical aggregates, i. e. here the long rodlike micelles have been transformed to spheres. This can be explained by solubilization of the HA into the micellar core thereby forming microemulsion droplets [8-10]. Only at temperatures below the critical temperature the micellar phase exists over the whole concentration range from 0 to 500 mM HA. At higher temperatures the Lj-phase is interrupted by the L(x-phase. The minimum of this phase is at 25°C and 230 mM HA. The higher the temperature the more stable is the La-phase, i.e. its concentration range increases. But the temperature dependence is significantly different at the lower and upper phase boundary. The lower phase boundary of the two phase region micelles / vesicles shifts from 170 mM HA at 25 °C to 140 mM HA at 60 °C. The phase border is therefore only little temperature dependent. The situation at the upper phase border is totally different. By a temperature increase from 25 to 60 °C it shifts from 270 to 460 mM HA. The upper phase border shows a similarly strong temperature dependence. The macroscopic behavior of the samples indicates also that the birefringent phases are indeed vesicle phases since they show the typical schlieren texture. Rlieological investigations with an oscillating experiment, show the typical rheogram of a solution consisting of densely packed multilamellar vesicles (df fig. 4). In the next step we try to elucidate the phase transition from the micellar to the vesicle phase. If one increases the cosurfactant concentration in an usual hydrocarbon system the cosurfactant goes into the micellar interface. Because of its small head group the curvature of the micelles is decreased and

591 spherical micelles are transformed into rodlike and/or discoid micelles. Further increase of the cosurfactant leads to the formation of bilayers, i. e. vesicles or stacked bilayers are formed. Beyond the lamellar phase usually a sponge phase or an reverse micellar phase is observed [11]. In the system TDMAO / HA the phase transition from micelles to vesicles should have the same origin. The ester in the micellar interface reduces the aggregates curvature and vesicles are formed. But why does this phase transition only take place at higher temperatures? T>pically temperature dependent phase transitions occur for nonionic amphiphilic systems. TTiis phenomenon is explained by a temperature dependent change of the packing parameter which is due to the reversible dehydration of the nonionic head group with increasing temperature [3,12]. For our system it appears that below 23 °C not enough HA molecules are located in the interface. Therefore only elongated aggregates are formed and with increasing HA concentration swollen spherical micelles are formed. But these swollen micelles in the TDMAO / HA system have a fundamental difference in comparison to usual swollen micelles such as ones that contain decane. An alkane is totally hydrophobic. There is no reason for the molecule to leave the micellar core. The HA which is solubilized at lower tempeiatures in the TDMAO micelles is not apolar like an alkane but amphiphilic. From the energetic view it can be better for the HA molecules to move from the micellar core to the micellar interface where the polar head group is in contact with water and the hydrophobic tail remains in the inner part of the aggregate. This decreases the curvature and lead to a transition from micelles to vesicles takes place. Such a mechanism can explain the phase transition from micellar aggregates to vesicles that takes place upon increasing the temperature. This model can also explain why the lower phase boundary of the two phase vesicle regime never extends below 140 mM. Vesicles cannot be formed until a certain concentration of HA in the interface is surpassed. Evidently the limit for this process is 140 mM and therefore further increase of the temperature cannot induce a phase transition. The upper phase boundary can be explained in similar terms. Here at low temperature a certain amount of the HA is located in the micellar core. With increasing temperature its tendency to become incorporated into the amphiphilic layer increases. However, the larger the total amount of HA present the more difficult it becomes to transform the micellar aggregates into bilayers. This explains the phase boundary that increases towards higher temperatures with increasing total content of HA. It should be noted that such a phase sequence: L] - vesicle - Lj upon increasing concentration of additive is observed in no other classical surfactant system. In surfactant/cosurfactant systems one can observe the transition L] - vesicle, in microemulsion systems the phase transition La - Lj is observed. The ambivalent character of the hexylacetate becomes very clear at this point. Low ester concentration makes the HA behave like a cosurfactant, high ester concentration results in an oil-like behavior. As shown above one way to transform micelles into vesicles is to increase the HA concentration. Another possibility which is given in this special case is simply to increase the temperature of a TDMAO /HA solution. The sample 200 mM TDMAO / 220 mM HA changes into a single vesicle phase at 25 ^C. The vesicle formation is totally reversible and can be repeated as often as one likes to increase and decrease the temperature. This is an indication that the vesicle phase in this hydrocarbon system might really be a thermodynamically stable state. As mentioned above the existence of a vesicle phase in an all hydrocarbon system as the only lamellar phase is unusual. In nonionic perfluoro or mixed systems of both hydrocarbon and perfluoro surfactant this phenomenon is more common [13,14]. These systems usually tend to form vesicle phases instead of classical lamellar phases.

592 The transition from micelles to vesicles goes along with the change of various microscopic and macroscopic parameters. One way to determine the transition from a L]- to a vesicle phase is via the conductivity of the samples. L]-phases show relatively high conductivities because the solution is water continuous and the aggregates are small. In contrast to this vesicles entrap ions in between their shells which are no longer able to contribute to the conductivity. Therefore the conductivity decreases steeply in the two phase regime Li/vesicles due to the increase of the vesicle volume fraction. In fig. 3 conductivity is shown as a function of temperature for different samples df the system 200 mM TDMAO/HA. Conductivity was measured while stirring. At 120 mM HA it depends linearly on the temperature due to an increased mobility of the ions. The sample docs not undergo a phase transformation as there is no decrease in conductivity. At 220 mM HA the situation is totally different. Until 22.5 °C the conductivity increases but then drops to a value which is about 10 times lower than the initial value. This temperature corresponds exactly to the macroscopically observed phase boundary between isotropic phase and 2-phase regime L\/ vesicles. For solutions with higher HA concentration the conductivity curves look similar. Only the decrease shifts to higher temperatures. The macroscopic rheological behavior is affected by the phase transition, too. For this reason a sample consisting of 200 mM TDMAO/220 mM HA was observed with a rheological double gap oscillation experiment at different temperatures. At low temperatures the system is in the micellar state and the solution shows a Newtonian flow behavior. The viscosity is frequency independent at 7 mPas. Both moduli are rather low, between 10"^ and 10"^ Pa (not shown here). The loss modulus is higher than the storage modulus which reveals the mainly viscous behavior of the solution.

o ooo

A A

T [*»C]

Fig. 3: Conductivity of samples of 200 mM TDMAO and different concentrations of HA as a function of the temperature.

•

n

o

G'

A

G-

A AAAJJ^

Frequenz [Hz]

Fig. 4: Frequency dependent oscillating rheogram of 200 mM TDMAO / 220 mM HA measured with a Couette geometry at 30 °C.

At 30 °C the solution is transformed well into the vesicle phase. Fig. 4 shows the rheogram which is typical for a sample consisting of densely packed multilamellar vesicles. Both moduli are frequency independent and the storage modulus is by a factor 10 higher than the loss modulus. Now the elastic properties dominate the rheological behavior. The viscosity shows no plateau any more, it decreases with increasing frequency in the whole investigated frequency range. At this temperature the sample has a yield stress which can be seen by entrapped air bubbles in the solution which do not rise. The phase transition has also been investigated by means of DSC and a first order phase transition has been observed at 23° C (for 200 mM TDMAO / 220 mM HA) The peak is relatively narrow which is

593 due to the small temperature range of the two phase area at this HA - concentration. From the peak area the phase transition enthalpy can be calculated with 0.15 mJ/mg. 3. Conclusions The phase behavior of the zwitterionic surfactant TDMAO with a new class of semipolar additives has been studied. This additive, in our case hexylacetate, shows rather interesting phenomena. With a polarity between cosurfactants and hydrocarbons exhibits both cosurfactant and oil like behavior. The character of the additive is mainly determined by the additive/surfactant ratio. For increasing concentration of HA one finds for a 200 mM TDMAO solution above 25° C an Li-phase, a two phase region Lj/La, a single La-phase of multilamellar vesicles and then a reentry into the Lj-phase of swollen micelles again similar to the solubilization of oils. Below 25° C the La-phase vanishes. The L a-phase is parabolic in shape with the minimum at an additive/surfactant ratio of about 1.1 In contrast to normal uncharged surfactant/cosurfactant systems only one bilayer phase, the vesicle phase, is found. This phase is also the thermodynamically stable state of the lamellar phase in this system. At about a molar ratio of 1:1 the L]-phase can reversibly be transformed into the vesicle phase just by raising the temperature. The driving force for this phase transition is the changing amphiphilicity of the additive. It is incorporated in the hydrophobic core of the micelles at lou temperatures. For higher tem-perature it becomes energetically more favorable for the system to locate the amphiphilic ester mole-cules at the interface where the polar head group is in contact to water. The considerably lower hydro-philicity compared to common cosurfactants and the much higher amphiphilicity compared to oils is the reason for the special behavior of the investigated surfactant systems. With the solubilized HA, the system is already very close to the phase boundary and small energetic changes are sufficient to induce the phase transition. Therefore the character of the semipolar additive can be influenced by only small variation of various physico-chemical parameters, such as temperature or concentration. Acknowledgements We would like to thank the DFG (Deutsche Forschungsgemeinschaft) for financial support. This work has been sponsored by the project Ho 457/22-1 and Ho 457/22-2. REFERENCES 1. R. G. Laughlin, The aqueous phase beha\'ior of surfactants. Academic press, London, 1994. 2. C. Tanford, J. Phys. Chem. 76 (1972) 3020. 3. J. N. Israelachvili, D. J. Mitchell, B. W. Ninham, J. Chem. Soc. Faraday Trans. 2, 72 (1976) 1525. 4. O. Bayer, H. Hoffmann, H. Thum, W. Ulbricht, Adv. Coll. Interface Sci. 26 (1986) 177. 5. V. K. Bansal, D. O. Shah, J. Colloid Interface Sci. 65 (1978) 451. 6. H. Hoffmann, Organized solutions. Surfactant Sci. Series 144 (1992) 169. 7. B. Jonsson, H. Wennerstrom, J. Phys. Chem. 91 (1987) 378. 8. J. C. Russel, D. G. Whitten, J. Am. Chem. Soc. 104 (1982) 5937. 9. P. Lianos, J. Lang, R. Zana, J. Phys. Chem. 86 (1982) 4809. 10. O. Bayer, H. Hoffmann, W. Ulbricht in surfactants in solution. Vol. 4, Plenum, New York, 1987. 11. H. Hoffmann, U. Munkert, C. Thunig, M. Valiente, J. Coll. Interface Sci. 163 (1994) 217. 12. M. Corti, C. Minero, J. Phys. Chem. 88 (1984) 309. 13. H. Hoffmann, F. Witte, J. Wurtz, Proceedings Int. School Phys. Enirco Fermi, 134 (1997) 391. 14. M. Bergmeier, H. Hoffmann, F. Witte, S. Zourab, J. Coll. Interface Sci. 203 (1998) 1.

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Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. Ail rights reserved.

595

Effect of L-Ascorbyl 2-phosphates on Stability for Vesicles of Hydrogenated Soybean Lecithin Sadanori Ban*, Kahori Sasaki", Satoru Nakata* and Ayao Kitahara^ "Nippon Menard Cosmetic Co., Torimi-cho, Nishi-ku, Nagoya 451-0071, Japan ^'Science University of Tokyo, (Prof. Emeritus) The effect of bivalent ions (Mg^* and Ca^^) and monovalent ions (Na* and K*) of an organic salt on the stability of the vesicles prepared by a new method was investigated. For comparison, the effect of inorganic salts was studied. The addition of the bivalent ions increased the stability of vesicles and that of the monovalent ions lowered the stability in both organic and inorganic salts. The addition of the bivalent ions shifted C-potential of vesicles to the positive side. This shows the adsorption of the ions on the vesicles. The increase of C,potential elucidates the increase of the stability of the vesicles. The increase of Tc by DSC was also observed by the addition of the bivalent ions. The addition of the monovalent ions did not change C, and Tc. The stabilization of vesicles by the bivalent ions is related to the adsorption of the ions and the destabilization of vesicles by the monovalent ions seems to be related to dehydration. 1.

INTRODUCTION

In the cosmetic field, hydrogenated soybean lecithin (HSL) is widely used as liposome products and emulsifiers etc. L-Ascorbyl 2-Phosphates (AP) is used as effective whitening, anti-aging and anti-oxidant products. The effectiveness of the liposome for cosmetics seems to increase by mixing HSL and AP. However, there seems to be no references on the stability of vesicles by the addition of various AP sahs. The interaction between vesicle and inorganic salts has been studied [1]. The adsorption of bivalent ions (Mg^^ and Ca^*) on vesicles has been indicated [2]. We report the effect of the bivalent ions and the monovalent ions of the organic salts on the stability of vesicles prepared by a new method. The effect is compared with that of inorganic salts. 2. EXPERIMENTAL 2.1 Material Hydrogenated soybean lecithin (HSL, PC purity:98%) was purchased from NOP Corporation. It was used without further purification. L-Ascorbyl 2-Pshosphate salts AP-Mg, AP-Ca, AP-Na and AP-K were supplied from Showa Denko Co. and used without further purification. Chlorides of Mg, Ca, Na, K were purchased from Katayama Chemical Co. and used without further purification.

596

2.2 Preparation of vesicles Vesicles used were prepared from HSL with a high-pressure homogenizer of a new type (DeBEE equipment; Mini-DeBEE)[3]. Aqueous dispersions of HSL were added in aqueous solutions of various salts. The concentration of HSL was 1.0% throughout the experiment. The mixture was passed five times in the high-pressure homogenizer under 40,000 psi pressure. The formation of vesicles was confirmed by an electron microscope. 2.3 Measurements The transmittance of vesicle dispersions was measured at 25°C using UV-160A (Shimazu Manufacturing Co.). The wavelength used was 600 nm. The particle size of vesicles was measured at 25°C using DLS-7000 (Otsuka Electronics Co.). The light source was He-Ne laser of wavelength 632.8 nm. The time-dependent correlation function of the light scattering intensity was measured at scattering angle of 90°. The C-potential of vesicles was measured at 25°C using LEZA-600 (Otsuka Electronics Co. which is a laser doppler electrophoresis apparatus equipping with He-Ne laser (632.8 nm). The transition temperature (Tc) of vesicles was measured using DSC 120(Seiko Co.). The vesicles (40-50 mg) were put into a silver DSC cell (70pil). The sample was then heated from 0 to 70X at rC/min. 3. RESULTS AND DISCUSSION The result of the transmittance measurement on the stability of vesicles was shown in Figure 1. Change of particle size with time was quite similar to Figure 1. Transmittance after 48 hrs, T(48), was assumed to be T(oo)

100

W^

b

50

10

20

30

Time/hr

40

50

10

20

30

40

50

Time/hr

Figure 1 The time dependence of transmittance (T) of vesicle dispersions. Added sahs: (a) organic, (b) inroganic. ( • ) AP-Mg l.OmM; (A) AP-Ca l.OmM; ( • ) AP-Na 1.0mM;(^)AP.K 1 OmM; (O) MgCl2 1.5mM; (A) CaCl2 1.5mM; (n)NaCl 3.0mM; (O) KCl 3.OmM; (X) Control Since the value of T(0) is uncertain, T(48) was used for the estimation of the stability of vesicles. From Figure 1, it was observed that the addition of the bivalent ions increases the stability of vesicles and that of the monovalent ions lowers the stability. Effect of the bivalent ions of inorganic saks was a little larger than that of organic sahs. The sah concentration dependence on C and Tc was depicted in Figures 2 and 3.

597

0 0.01

1

100

Concentration of salts / mM

100

0 0.01 Concentrarion of salts / mM

Figure 2 Relationship between the concentration of salts and C of vesicles. Added salts: (a) organic, (b) inorganic. ( • ) AP-Mg; (A) AP-Ca; ( • ) AP-Na, ( • ) AP-K; (O) MgCl2; (A) CaCl^; (D) NaCl; (O) KCl 53.5 O

a

53.0 52.5

O o

P"

u H

52.0 51.5

1

0 0.01 0.1

1

-.1,

1

10

L

Concentration of salts / mM

0 0.01 0.1

1

10

100 1000

Concentration of salts / mM

Figure 3 Relationship between the concentration of sahs and Tc of vesicles. Added salts: (a) organic, (b) inorganic. ( • ) AP-Mg; (A) AP-Ca; ( • ) AP-Na; ( • ) AP-K; (O) MgCl2; (A) CaCl^; (D) NaCl; (O) KCl AP-Ca did not dissolve in water above lOmM. The addition of the bivalent ions increased Cpotential to the positive side as seen in Figure 2. It is known that the bivalent ions are adsorbed to the membranes of vesicles [2]. The repulsive interaction between electric double layers of charged vesicles which was generated as a resuh of the adsorption elucidates the increase of the stabilization [4,5]. On the other hand, the monovalent ions do not adsorb on the vesicle. As the result, Cpotential is nearly invariable as seen in Figure 2. The tendency of the change of Tc seen in Figure 3 corresponds to Figure 2. Tc increases by the addition of the bivalent ions as well as C-potential, though the increase in the case of the organic salts is poor The increase of Tc is related to the adsorption of the ions. It is considered that the rise of Tc shows the decrease of the membrane fluidity which is the result of the interaction between the bivalent ions adsorbed and the hydrophilic group of the membrane. The little difference in the stability between the organic and the inorganic salts described above reflects the differences seen in C, and Tc in Figures 2 and 3. The difference seems to be

598 induced from the degree of approach to the charged membrane of the organic ion (AP ion). Effect of Ca^^ on the stability of vesicles was larger than Mg^^ as seen in Figure 1. This difference is seen in ^-potential and Tc in Figures 2 and 3. This phenomenon is seen in other cases and elucidated from the difference in the adsorption force [4,5]. The addition of the monovalent ions destabilized the vesicles, though C and Tc were invariable by the addition. The reason of the destabilization is considered to be the dehydration by the ions. The effect of Na* was larger than K\ The order corresponds with the lyotropic series. 4. CONCLUSION The stabilization and destabilization of vesicles by the addition of the bivalent and the monovalent ions were elucidated by the adsorption and the dehydration from the measurement of ^-potential and Tc of DSC. ACKNOWLEDGE The authors thank Mr. E. Ogata (Showa Denko Co.) for supplying L-Ascorbyl 2-phoshates. REFERENCES 1) MNakagaki, T Handa, S. Shakutsui, M Nakayama, Yakugaku Zasshi, 102, 17, (1982) 2) J. Marra, J. Israelachvili, Biochemistry, 24, 4608 (1985) 3)US patent No: 5,720,551 4) H. Oshima, T Mitsui, J. Colloid Interface Sci., 63, 525 (1978) 5) H. Oshima, Y. Inoko, T Mitsui, J. Colloid Interface ScL, 86, 57 (1982)

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

599

Novel class of organic-inorganic hybrid vesicle "Cerasome" derived from various amphiphiles with alkoxysilyl head K. Katagiri, K. Ariga, and J. Kikuchi Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan A novel class of organic-inorganic hybrids, "Cerasome," which has a bilayer vesicle structure and a ceramic surface, has been developed by a sol-gel reaction and self-assembly of amphiphiles bearing a triethoxysilyl head and a dialkyl tail. When an amphiphile with a triethoxysilyl head was used, the Cerasomes were obtained most efficiently at pH 3 where hydrolysis of the triethoxysilyl head proceeded without disturbing vesicle formation. In contrast, stable dispersion of the Cerasomes was obtained under various pH conditions in the case of the amphiphiles bearing an additional quaternary ammonium group. Hydrolysis behavior of triethoxysilyl groups was monitored using ' H - N M R spectroscopy. Hydrolysis of 1 proceeded slowly at first, but was accelerated after a certain induction period. On the other hand, the hydrolysis reaction was so rapid and did not have an induction period in the case of 2. These results indicate that the structural difference of the hydrophilic group has a significant effect on the formation process of the Cerasome. 1. INTRODUCTION Organic-inorganic hybrid materials are attracting a great deal of attention at the moment because they possibly possess the advantageous characteristics of both organic and inorganic materials [1]. The sol-gel method is one of the most promising techniques for producing organic-inorganic hybrids. It is a low-temperature solution process for preparing ceramics [2-4], and thus decomposition of the organic component can be minimized. However, most of the organic-inorganic hybrid materials prepared via the sol-gel method, e.g., ORMOSILs [5] are ceramics simply modified with an organic polymer or other functional groups and are not materials controlled on a meso-, or nano-scopic scale. Our major efforts have been devoted to the creation of a novel class of hybrid materials, which have a highly organized structure. Recently, we have prepared a novel class of hybrid materials, "Cerasome," which form lipid-bilayer vesicles with a silicate framework on the surface from amphiphiles bearing a triethoxysilyl head and a dialkyl tail (Fig. 1) [6]. The key point of the formation process of the Cerasome is the hydrolysis behavior of the triethoxysilyl moiety. In the present study, we have synthesized two types of amphiphiles with a triethoxysilyl head. We investigated the effect of the structure of the hydrophilic group on the hydrolysis process to optimize the catalytic conditions for the preparation process of Cerasome. This research was supported by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), 12450364, 2000

600

Hydrolysis

\-0^

Condensation Assembly Amphiphite

Si S^i S(i OH 0~ OH

Vesicular Structure

L

Silicate Surface Bilayer Structure

Fig. 1. Structures of Cerasome-forming compounds 1, 2 and schematic drawing of Cerasome forming process.

2. EXPERIMENTAL The synthesis of N,A^-dihexadecyl-^-(3-triethoxysilyl)propylsuccinamide (1) was described elsewhere [5] and the synthesis of A^,A^-dihexadecyl-A^-[6-[{3-triethoxysilyl) propyldimethylammonio]hexanoyl]glycineamide (2) will be shown in future publications [7]. Water used for the Cerasome preparation was distilled and deionized using an Autostill WS33 (Yamato Scientific, Japan) and Milli-Q Labo (Nihon Millipore, Japan), respectively. Standard solutions of diluted hydrochloric acid (HCl) and diluted ammonium hydroxide (NH4OH) were used for pH control. The Cerasomes were prepared by mixing an aqueous dispersion of 1 or 2 (1 nmiol dm'^) using a vortex mixer (VORTEX-GENIE 2, Scientific Industries) at a temperature of 25 ± 5 °C. The ^H-NMR of the samples dispersed in a deuterated solvent was measured by a JEOL JNM-LA400 NMR specu-ometer. 3. RESULTS AND DISCUSSION The ethoxysilyl head groups in 1 can be hydrolyzed under acidic or basic condition, whereupon 1 could become amphiphilic. Condensation of the resulting silanol groups leads to the formation of silica-like inorganic frameworks, and the Cerasomes were then prepared. Therefore, control of the hydrolysis speed is important because the sol-gel process and the bilayer forming process have to co-proceed at the same time in this system. We investigated the effect of pH conditions for Cerasome preparation. Table 1 shows the dispersion state of 1 in aqueous media after vortex mixing for 6 h under various pH conditions. At neutral pH, 1 could not be changed to amphiphilic and remained as oil droplets probably due to extremely slow hydrolysis of the head groups. On the other hand, a translucent solution characteristic

601 of a vesicular dispersion was obtained under moderate acidic Table 1 conditions (pH 3 with HCl) Dispersion state of 1 in aqueous media under various pH where hydrolysis of the conditions: 1 mmol dm ^ 25 °C, 6 h. ethoxysilyl head groups was gently accelerated by acidic Dispersion state pH catalysis. Under a stronger Insoluble aggregates 1 acidic condition (pH 1), however, a stable dispersion was not 3 Translucent solution obtained and precipitation 7 inmiediately occurred. In this Oil droplets case, the hydrolysis and 12 Translucent solution + Oil droplets subsequent condensation reaction would be too fast to maintain the vesicular structure. In the case of pH 12, quite a different result was obtained. That is to say, the resultant solution was translucent but oil droplets remained. The hydrolysis seems to proceed inhomogeneously under this condition. The observed behavior can be explained by a mechanism of acidic and basic catalysis in the sol-gel process [8]. Under acidic conditions, it is likely that an alkoxide group is protonated in an initial step. An electron is withdrawn from a silicon atom, making it more electrophilic. As a result, the alkoxysilyl groups are more susceptible to attack by water. Once substitution of OH for OR occurred, the electron density on silicon decreases. Consequently, the hydrolysis is suppressed with each subsequent hydrolysis step. Therefore, the hydrolysis of the triethoxysilyl group proceeds equally for all the molecules in a one-by-one manner. This process prevents inhomogeneous hydrolysis and uncontrolled condensation, and thus this condition is suitable for preparation of the Cerasome. The hydrolysis process of 1 was monitored from the estimation of the resulting ethanol by ' H - N M R measurements. From the H-NMR spectra of the aqueous dispersion of 1 at pD 3 for various mixing times, it was proved that hydrolysis was accelerated after a certain induction period and was completed after 8 h. Oil droplets of 1 were gradually dispersed as a vesicle by hydrolysis of the first ethoxy groups in the initial stage, and subsequent hydrolysis steps were accelerated on the surface of the formed vesicles due to increased contact with the bulk water. On the other hand, the hydroxyl anion directly attacks the silicon atom under basic conditions. Because the resultant hydroxyl groups withdraw a negative charge from a silicon atom, substitution of an Table 2 alkoxy group promotes further Dispersion state of 2 in aqueous media under various pH hydrolysis of the same molecule. conditions: 1 mmol dm'^, 25 °C, 6 h. Therefore, the particular Dispersion state molecules are hydrolyzed pH preferentially and other Translucent solution 3 molecules remain unreacted. This process leads to Translucent solution 7 inhomogeneous hydrolysis, and Translucent solution 12 thus this condition is not suited to preparation of Cerasomes.

602 Table 2 shows the dispersion state of 2 in aqueous media after vortex mixing for 6 h under various pH conditions. Stable dispersion of the Cerasomes was obtained at all pH conditions examined. This behavior is different from that of 1, and the difference can be explained by the nature of the hydrophilic head group. Hydrolysis of the alkoxysilyl groups can proceed efficiently when the amphiphiles are well dispersed in water. In the case of 1, silanol groups formed by the hydrolysis act as a polar head. Therefore, the initial hydrolysis through acid catalysis was indispensable for vesicle formation, and the hydrolysis and condensation can be accelerated only after the hydrolysis. In contrast, 2 originally has a quaternary ammonium group as a polar head; thus it can be spontaneously dispersed in water without regard to hydrolysis. From ^H-NMR spectra of the aqueous dispersion of 2 at pD 3 for various mixing times, it was proved that the hydrolysis reaction was quite rapid and that the reaction was completed within 3 hours. Interestingly, no induction period was observed. In this case, pre-Cerasome vesicles were rapidly formed, and the hydrolysis and condensation would occur efficiently at the vesicle surfaces. 4. CONCLUSIONS Preparation of a novel organic-inorganic hybrid, "Cerasome," having a vesicle structure and a ceramic surface was investigated under various pH conditions. In the case of 1, a stable dispersion of Cerasome was obtained only under mild acidic conditions. In contrast, 2 gave an aqueous Cerasome dispersion over a wide pH range. These results indicate that the structural difference in the hydrophilic group has a significant effect on the formation process of the Cerasome, especially on hydrolysis. We are currently optimizing the design of the amphiphile structure and the preparation conditions to obtain a superstable Cerasome with a well-developed silicate surface. In addition, modification and fuctionalization of the Cerasome are also in progress. REFERENCES AND NOTE 1. C. Sanchez and F. Ribot, New J. Chem., 18 (1994) 1007. 2. B. Wang and G L. Wilkes, J. Macromol. Sci. Pure Appl. Chem., A31 (1994) 249. 3. A. B. Wojcik and L. C. Klein, J. Sol-Gel Sci. Technol., 5 (1995) 77. 4. H. Schmidt, J. Sol-Gel Sci. Technol., 1 (1994) 217. 5. J. D. Mackenzie, J. Sol-Gel Sci. Technol., 2 (1994) 81. 6. K. Katagiri, K. Ariga, and J. Kikuchi, Chem. Lett., (1999) 661. 7. A colorless oil. E.A.; Anal Found: C, 63.98; H, 11.29; N, 4.40%. Calcd for C5iH,o6BrN305Si • I/2H2O: C, 63.91; H, 11.25; N, 4.38%. 8. C. J. Brinker and G W. Scherer, Sol-Gel Science; The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

603

Spin-label parameters of detergent-containing liposomes and their application to micelle-vesicle transition Hiroshi Kashiwagi, Shinji Sagasaki, Manabu Tanaka, Kazutoshi Aizawa, Changqi Sun, and Masaharu Ueno Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, Sugitani, Toyama 930-0194, Japan Order parameters of 5-doxylstearic acid, and hyperfine splitting constants, half widths, and molar fractions of membrane components of 2,2,6,6-tetrametiiylpiperidine-N-oxyl were applied for examining properties of detergent-containing egg yolk phosphatidylcholine liposomes. Fundamentally, regardless of kinds of detergents, 3ie destruction of liposomes by detergents possessed three transition points. However, there were striking diflferences in the membrane properties if different detergents were used. Especially, an ESR spectrum of 12-doxylstearic acid incorporated into sodium cholate-containing liposomes suggested the coexistence of at least two components in the hydrophobic region of membranes. 1. INTRODUCTION The mechanism of micelle-vesicle transition has been studied in connection with the reconstitution of membrane proteins after having been purified in detergent solutions. Many researchers have proposed different transition models using different lipid-detergent systems. Among them, the three-stage model by Lichtenberg et. al. [1] and its modification by OUivon et. al. [2] are widely accepted. We have studied on the micelle-vesicle transition mechanism in order to find out the size-determining factors of liposomes [3,4]. Consequently, in the course of the destruction of egg yolk phosphatidylcholine (EPC) large unilamellar vesicles (LUV) by detergents (octyl P-D-glucoside (OG), sodium cholate (Na-chol), or octaethyleneglycol monon-dodecyl ether {C^^X following four stages were found to occur with an increase in detergent concentration: I) distribution of detergent molecdes into vesicles in the first stage, II) transition to small vesicles (SUV*) containing high amount of detergents in the second stage. III) transformation to intermediate structures in the third stage, and IV) formation of mixed micelles in the fourth stage [3,4]. The phenomena observed in the process of vesicle formation from mixed micelles by removing detergents were found to be opposite symmetry. The most remarkable characteristic of our model is the presence of SUV* in the second stage [3,4]. On the other hand, spin labeling isfrequentlyemployed in order to obtain membrane properties, such as fluidity, diffusion, flip-flop, partitioning of a spin probe between water and lipid, and so on. We have obtained several spin-label parameters, using 5- and 12-doxylstearic acid (5- and 12-DS) and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as functions of dete^ent concentrations. The obtained results will be discussed in relation to micelle-vesicle transition mechanism. 2. METHODS EPC and dimyristoylphosphatidylcholine (DMPC) LUV (diameter: 200 nm) suspensions were prepared by extrusion. Small unilamellar vesicles (SUV) possessing diameters of about 30 nm were obtained by ultrasonication. Detergent (OG, Na-chol, or C^^E^) solution was added to these suspensions, left for 1 day, and then ESR measurement was performed with a JES-FE3X type spectrometer (JEOL, Japan) at 25°C. 5- or 12-DS was incorporated into the membranes in

604

the course of the vesicle preparation, while TEMPO solution was added to the suspensions simultaneously with the detergent solution. 3. RESULTS AND DISCUSSION 3.1. Order parameters of 5-DS incorporated into EPC LUV Fig. 1 depicts the dependence of order parameters of 5-DS incorporated into EPC LUV (lipid concentration: 5 mM) on detergent (OG, Na-chol, or CyEg) concentrations. A large order parameter means a small fluidity in hydrocarbon chains. The concentration dependence curves of order parameters do not vary monotonously. Inflection points appearing in Fig. 1 agree with the phase transition points (indicated by broken lines) [4]. Regardless of the kinds of detergents, the micelle-vesicle transition mechanism seems to possess three transition points, as described above. However, there are differences in the membrane properties when different detergents are added. For example, the membrane fluidity is small in Na-chol-containing LUV probably due to the aggregation of membrane induced by a steroid, while it is large in C.jEg-containing LUV probably because membrane destruction occurs mainly in the hydrocarron chains of LUV. A decreased fluidity of Na-chol-containing EPC LUV was also observed with rotational correlation times of 12-DS. 3.2. Analyses of hyperfme splitting patterns of TEMPO dissolved in EPC LUV As described previously [4], one (high-field component) of three hyperfine splitting bands of TEMPO dissolved in detergent-containing LUV was the superposition of the spectra due to TEMPO dissolved in water (abbreviated as w), in lamellar EPC (previously reported component (C)

> • 4k •

Injiil T x l 40 0 10 20 0 '0 Total detergent concentration / mM Fig. 1. Dependence of order parameters on OG (A), Na-chol (B), or Cj^E, (C) concentrations. Broken lines indicate phase transition points. 1.7

%

^'^0

100

200

0 100 200 OG c o n c e n t r a t i o n / mM Fig. 2. Dependence of A (A), 5H (B), and molar fractions of total membrane components (C) on OG concentrations. Broken lines indicate phase transition points.

605 [5]; abbreviated as m ), and in nonlamellar EPC (new component; abbreviated as m^). The g values (g) and hypertine splitting constants (A) of the w, m^, and m were determined to be g=2.0065 andA=1.75, g=2.0069 and A=1.64, and g=2.0065 and A=1.63, respectively. As far as membrane components (m, and m,) were concerned, only mj existed in the first and second stages of the vesicle destruction. The amount of m^ increased, while that of m^ decreased with increasing detergent concentration in the third stage. The amount of m, was small in the fourth stage [4]. On the basis of these findings, following analyses of ESR spectra of TEMPO were applied to obtain membrane properties of polar regions of detergent-containing LUV systems. Fig. 2 shows the dependence of hyperfine splitting constant (A), half widths (5H), and molar fi-actions of total membrane components of TEMPO dissolved in LUV (lipid concentration: 40 mM) on OG concentrations. Phase transition points are indicated by broken lines in Fig. 2. In the first and second stages of the LUV destruction by OG, the values of A decrease, indicating that TEMPO is transferred to less polar part near the surface of EPC membranes. The 8H values increase with increasing OG concentrations due to increase in the suppression of movement of TEMPO (Fig. 2B). In the third stage, the A values increase at increased OG concentrations. Apparent half widths of total membrane components increase in the first half of the third stage because of the coexistence of two membrane components possessing different g values. In the latter half of the third stage, an ESR spectrum become gradually sharp with an increase in the amount of n v Inflection points appearing in the curves for molar flections of total membrane components (Fig. 2C) agree with the phase transition points of the LUV. When Na-chol or C^£^ was used for destructing EPC LUV, tiie behavior of A and 5H was strongly dependent on the kinds of detergents. Instead of the TEMPO parameter [5], A, 5H, and molar flections of total membrane components are thus proved to be useful parameters for examining the behavior of detergents in hposomes in the course of the micelle-vesicle transition. 33. Behaviors of 12-DS in the Na-chol-containing SUV Fig. 3 illustrates ESR spectra of 12-DS incorporated into EPC SUV (lipid concentration: 5 mM) as a function of Na-chol concentrationfi-om0 to 3.0 mM (the first stage of the vesicle destruction). Isotropic triplet bands shown in Fig. 3A are well known to be assigned to 12-DS incorporated into the hydrophobic region of EPC. As indicated by a solid arrow in Fig. 3B, a weak additional band appeau^ at Na-chol concentration =1.0 mM. Its intensity increases with

Fig. 3. ESR spectra of 12-DS in EPC SUV at Na-chol concentration = 0 (A), 1.0 (B), 2.0 (C), 2.5 (D), and 3.0 (E) mM. Arrows indicate the additional bands.

Fig. 4. ESR spectra of 12-DS in DMPC SUV containing 0 (A), 13 (B), 15 (C), 17 (D), and 25 (E) mol% cholesterol. Arrows indicate the additional bands.

606 an increase in Na-chol concentration (Fig. 3C-E). As was not shown in Fig. 3, the intensity of this additional band decreased at a further increased Na-chol concentration, and became unobservable at a concentration > 6.5 mM (the third stage of vesicle disruption). In the presence of appropriate amounts of Na-chol, additional bands similar to those shown in Fig. 3 appeared in spectra of 12-DS/DMPC SUV, but did not in those of 5-DS/EPC, 5-DS/DMPC, or 16-DS/EPC SUV. It can be seen from Fig. 3 that the additional band is scarcely influenced by the presence of the main band. Moreover, by using 12-DS, Kawamura et. al. reported the coexistence of two (strongly and weakly immobilized) immiscible components in dihydroxy and trihydroxy bile salt micelles [6]. Then, we presumed that two (or more) inuniscible sites for 12-DS might be separated by the presence of an appropriate amount of Na-chol. In die case of Na-chol/EPC systems, total spectral width in the presence of the additional band (Fig. 3B-E) is slightly narrower than that in the absence of it ^ i g . 3 A), and the location of the additional band is slightly lower in magnetic field than that oftfiestrong band. Therefore, we presumed that this additional band is caused by a more weakly immobili;^ component induced in the hydrocarbon chains of EPC. As will be described elsewhere, we also observed the presence of two or more phases in Na-chol-containing SUV by means of differential scanning calorimetry. Fig. 4 depicts ESR spectra of 12-DS dissolved in DMPC SUV containing 0-25 mol% cholesterol. Similar to the results of Na-chol-containing systems (Fig. 3), the spectrum obtained when 15 mol% cholesterol is added (Fig. 4B) is explained as the superposition of strong triplet bands and a weak additional signal. The intensity of the additional band increases with an increase in cholesterol concentration. Eventually, the additional band might be originated from a steroid structure in Na-chol or cholesterol. The fact that the vesicles are disrupted by sodium cholate, but are not by cholesterol might arise from the presence of carboxyl group in Na-chol. As will be described in the near future, the occurrence of a phase possessing high electronic charges prior to vesicle destruction was demonstratai by the use of electrophoretic light scattering. 4. CONCLUSION In the present work, dependence of order parameters of 5-DS dissolved in EPC LUV on detergent concentrations was examined as a measure of fluidity of hydrophobic region of membranes. Similarly, A, 8H, and molar fractions of total membrane components of TEMPO in LUV was obtain^ as parameters of physical properties of hydroi^iUc parts of liposomes. These results were discussed in connection witii the micelle-vesicle transformation mechanism. Eventually, regardless of the kinds of detergents, the transition mechanism fundamentally possessed three transition points. However, membrane properties were strikingly dependent on the kinds of detergents. Furthermore, ESR spectra of 12-DS demonstrated another function of Na-chol in the vesicle destruction: the formation of an additional band, which suggested the coexistence of two or more immiscible sites in hydrocarbon chains of EPC at the first stage of the vesicle destruction. REFERENCES 1. D. Lichtenberg, R. J. Robson, and E. A. Dennis, Biochim. Biophys. Acta, 737,285 (1983). 2. M. Ollivon, O. Eidelman, R. Blumenthal, and A. Walter, Biochemistry, 27,1695 (1988). 3. M. Ueno and Y. Akechi, Chem. Lett., 1991,1801; M. Ueno, Membrane, 18,96 (1993); M. Ueno, H. Kashiwagi, and N. Hirota, Chem. Lett., 1997,217; M. Ueno, H. Kashiwagi, N. Hirota, and C. Sun, Proceedings of the International Conference on Colloid and Surface Sciences, Tokyo, 2000, Ca.311. 4. H. Kashiwagi, K. Aizawa, and M. Ueno, Chem. Lett., 2000,134. 5. H. M. McConnell, K. L. Wright, and B. G. McFarland, Biochem. Biophys. Res. Commun., 47,273 (1973); E. J. Shimshick and H. M. McConnell, Biochem. Biophys. Res. Conmiun., 53, 446 (1973); S. W. Wu and H. M. McConnell, Biochemistry, 14 , 847 (1975). 6. H. Kawamura, Y. Murata, T Yamaguchi, H. Igimi, M. Tanaka, G. Sugihara, and J. P Kratohvil, J. Phys. Chem., 93,3321 (1989).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

607

Magnetofusion and magnetodivision of dipalmitoylphosphatidylcholine liposomes H. Kurashima,' H. Abe*" and S. Ozeki* • Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan ^ National Research Institute for Metals, 3-13 Sakura, Tsukuba, Ibaraki 305-8565, Japan The fusion and division of liposomes of dipalmitoylphosphatidylcholine were induced by high magnetic fields of up to 30T. A stable liposome size was found for each magnetic field intensity, and changed by the addition of aromatic compounds. These suggest that structures and thus functions of membranes should be controlled by steady magnetic fields, to which the sensitivity of membranes will be subject to their magnetic anisotropy. !• INTRODUCTION The magnetic effects on chemical processes in biosystems have been reported [1-3]. However, since biosystems include many chemical compounds and complicating processes, it is difficult to detect a magnetic response itself and much more so to elucidate a mechanism for magnetic responses in biosystems. Therefore, we have used an artificial lipid membrane as a very simple model for a biosystem. We previously reported great magnetoresponses in membrane potential and resistance of a black lipid membrane [4, 5] and magnetofusion of dipalmitoylphosphatidylcholine (DPPC) liposomes [6]. DPPC liposomes grew up under high magnetic fields over 12T and the addition of aromatic additives having large magnetic anisotropy led to magnetofusion even at a few T. According to a theory [6], magnetodivision as well as magnetofusion should occur by a balance between a magnetic energy and a curvature-elastic energy of a liposome.

608 In this paper, we have reported the magnetodivision and a possibility of the existence of a stable size at each magnetic field intensity. 2. EXPERIMENTAL SECTION DPPC liposomes in Tris buffer solutions (pH=7.5) were prepared by the Bangham method [7]. 20 mol% anthracene and pyrene were added to DPPC liposomes. A portion of a liposome solution was filtered through a cellulose nitrate membrane filter (0.2,0.45 or 0.65 fim).

Liposome solutions were exposed to magnetic fields of up to 30T for 5 sec to 3 h at

318 K and cooled down to 298 K under magnetic fields. The average liposome size was determined as a sphere from the Einstein-Stokes equation using diffusion constants measured by dynamic light scattering (an Otsuka ELS800 spectrophotometer) at 298 K. 3. RESULTS AND DISCUSSION Fig. 1 shows an example of changes in average-size distribution of DPPC liposomes by lOT and 30T magnetic fields; the former induced magnetofusion, the latter magnetodivision. According to Helflich et al. [8], the magnetic deformation of spherical bilayer (liposome)

I

1—1—1

1 II

M

'—'—r—n

J

8

A

/ rv^

* 6

2 0 100

^4

I

ff ^ \ / 9 *

t /^ / 64

^fi-o-i—•

• • ••

1000 V

log d Ij-Ljk nm

\

-tb-^"

i

It]

'—^

Fig. 1 An example for changes in size distribution of DPPC liposomes containing 20 mol% pyrene due to magnetic fields. Symbols: O , OT; • . lOT (magnetofusion); • . SOT (magnetodivision)

10

15

20

25

30

Magnetic field, / / / T Fig. 2 Association number of liposomes as a function of magnetic field Figures mean r^ values (nm). Symbols: O.DPPC; A , anthracene-DPPC; D.pyrene-DPPC

609 occurs by a balance between a magnetic energy and a curvature-elastic energy of a liposome. When liposome size changes under a magnetic field, the total energy of a liposome under a magnetic field must be smaller than that at a zero field. This condition was given by eq.l [6]. 6(l^n)-c^o(n'^'n)^0

(1)

Where, n is the association number of liposomes, r^ and CQ are the initial radius and the curvature of liposome. The association number of liposomes, n, is given by (r/ro)^, where r is the radius of liposomes after magnetic field exposure. Eq. 1 shows that liposomes can both fuse and divide under magnetic fields for a given set of TQ and CQ, in accordance with the results (Fig. 1 and also Fig. 2). Rg. 2 illustrates size dependence of doped liposomes having different initial size, 4) (= 2ro) on magnetic field intensity; 474, 376 and 656 nm for DPPC, anthracene-DPPC and pyrene-DPPC liposomes, respectively.

The magnetic dependence of liposome size was

subject to the kind of additives, i.e., the magnetic anisotropy. Fig. 3 demonstrates that a stable liposome size may stand for each magnetic field. In addition, magnetic fields may activate membrane deformation, judging from the fact that the size at zero-field, after a magnetic field was removed, disagreed with the initial size.

100

1000

884 nm

1000

933 nm

1000

478 nm

Rg. 3 Changes in liposome size (2ro/nm) of pyrene-DPPC due to magnetic field treatments. The allows indicate the time course.

610 4. CONCLUSION Liposome sizes changed through undulation of a membrane caused by steady magnetic fields.

The magnetofusion and magnetodivision depended strongly on the initial liposome

size, and led to a stable size at each magnetic field intensity. Thus, it seems to be possible that a steady magnetic field should be used as a new technique for the size control of liposomes. REFERENCES L W. Haberditzl and K. Z. Muller, Naturforsch, 20b (1965) 517 2. W. Haberditzl, Nature, 213 (1967) 72 3. S. Ueno and M. Iwasawa, J. Appl. Phys., 79 (19%) 4705 4. H. R. Khan and S. Ozeki, J. Colloid Interface Sci., 177 (19%) 628 5. S. Ozeki, H. Kurashima, M. Miyanaga and C. Nozawa, Langmuir, 16,4 (2000) 1478 6. S. Ozeki, H. Kurashima, and H. Abe, J. Phys. Chem. B, 104,24 (2000) 5657 7. A. D. Bangham, M. M. Standish, and J. C. Watkins, J. Mol. Biol., 13 (1%5) 238; S. M. Johnson, A. D. Bangham, M. W. Hill, and E. D. Kom, Biochim. Biophys. Acta., 223(1971)820 8. W. Helfrich, Phys. Lett. 43A (1973) 409; Zeit. Naturforsch. 28C (1973) 693

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

611

Molecular mechanism of liposome membrane fusion induced by two classes of amphipathic helical peptides with similar and different hydrophobic/hydrophilic balances* T. Yoshimura", E. Sato', S. Lee\ and K. Kameyama' Taculty of Engineering, Mie University, Mie 514-8507, Japan "Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan Taculty of Engineering, Gifu University, Gifu 501-1112, Japan Membrane fusion reactions consist of two steps: close apposition and disruption and reformation of bilayer membranes. In this paper, we have studied the molecular mechansim of liposome membrane fusion induced by two classes of amphipathic helical peptides with similar and different hydrophobic/hydrophilic balances, and proposed a possible mechanism that close apposition of the liposome membranes occurs through "cross-bridge formation" of the membrane surfece-bound peptides and disruption and reformation of the membranes occur through "transbilayer movement" of the peptides. 1. INTRODUCTION An amphipathic helix is defined as a helix in which the distribution of amino acid residues forms opposing polar and nonpolar faces. It is an important structural unit included in proteins and peptides and is responsible for interaction with biological membranes to elicit their biological functions such as membrane fusion [1]. Membrane fusion reactions consist of two steps: close apposition of bilayer membranes and their disruption and reformation. To clarify the role of amphipathic peptides in membrane fusion reactions, we synthesized one class of amphipathic peptides with similar hyrophobic/hydrophilic balances containing different types of amino acid residues and another class of amphipathic peptides with different hydrophobic/hydrophilic balances containing similar types of amino acid residues and examined their helix formation, self-aggregation, transbilayer localization, membrane binding, and membrane fusion properties using liposome membrane systems [2-4]. In the present paper, we briefly review these studies and discuss a possible mechanism for amphipathic helical peptideinduced close apposition and disruption and reformation of the bilayer membranes. 2. MATERIALS AND METHODS Amphipathic peptides were synthesized as described previously [2,5,6]. Liposomes were prepared by the reverse-phase evaporation method, followed byfiltrationthrough polycarbonate membranes of 0.1-/i m pore size [2]. Membrane fusion induced by the peptides, their binding to liposome membranes, and their self-aggregation were measured 25 °C as described previously [7] with a Hitachi F-4010 fluorescence

612

Spectrophotometer. The helix contents of the peptides were estimated from CD spectra measured with a Jasco spectropolarimeter model J-720M as described previously [2]. Molecular weights and partial specilSc volumes of the peptides were determined at 20 °C by the sedimentation equilibrium method in a Beckman analytical ultracentrifuge model XL-A [8]. The transbilayer localization of the peptides was assessed by the quenching of fluorescence of the peptides by doxyl stearic acids (DS) in the membranes [9], and the rate of fluorescence quenching was measured in Unisoku RS-501 stopped-flow spectrometer. The size of fused liposomes was evaluated by their diffusion coefficients measured at 25 "C in an Otsuka photon-counting laser light scattering photometer model DLS-700 as described previously [10]. Measurements were carried out in 10 mM Tris-HCl (pH 7.5) with or without 10 mM NaCl. 3. RESULTS AND DISCUSSION 3.1 Liposome membrane fusion induced by amphipathic helical peptides with similar hydrophobic/hydrophilic balances The amphipathic peptides with similar hyrophobic/hydrophilic balances containing different kinds of amino acid residues were peptides with 51, 47, and 22 amino acid residues (KL-51 [2], KL-47, and KL-22, respectively), all of which consist of tandem repeats of a KKLL sequence, a model peptide of leucine zipper with 26 amino acid residues (LZ-26), and a cationic peptide as an analog of the influenza hemagglutinin fusion peptide with 20 amino acid residues (HK-20) [5]. In solution, KL-51, LZ-26 and HK-20 were in random and monomeric forms, whereas KL-47 and KL-22 had helical and oligomeric (dimeric and tetrameric, respectively) structures, but they all formed helical structures on association with liposome membranes containing phosphatidylserine (PS). The monomeric peptides LZ-26 and HK-20 always induced fusion of PS membranes, but intramolecularly interacting KL-51 and oligomeric KL-47 and KL-22 could not trigger fusion of the membranes in the presence of excess peptides and induced fusion only in the presence of excess liposomes, suggesting that the monomeric or dissociated form of amphipathic helical peptides is fusion-active [2-4]. Moreover, KL-47 and KL-22 were shown to localize and form assemblies or clusters at the surface of the membranes. The amphipathic peptides had little or no activity of fusion of liposome membranes containing phosphatidylcholine (PC) alone. However, they adopted helical conformations in the presence of ATP and became self-aggregative above an ATP concentration at which the positive charges of the peptides were neutralized by the negative charges of ATP, and above an ATP concentration at which they became self-aggregative, the peptides, except oligomeric KL-47 and KL-22, interacted with the PC membranes and triggered fusion, suggesting that the hydrophobic helical peptides are fusion-active [2-4]. 3.2 Liposome membrane fusion induced by amphipathic helical peptides with different hydrophobic/hydrophilic balances The amphipathic peptides with different hydrophobic/hydrophilic balances containing similar types of amino acid residues were five types of peptides with 18 amino acid residues composed of hydrophobic amino acids, Leu and Trp, and a hydrophilic amino acid, Lys, the respective ratios of the hydrophobic to hydrophilic amino acids being 5:13, 7:11, 9:9, 11:7 and 13:5 (denoted as Hel 5-13, Hel 7-11 and so on) [6]. As shown in Table 1, all the peptides bound to the PS liposome membranes, formed helical structures, and induced fusion of the membranes. In contrast, the peptides

613

Table 1 Properties of amphipathic peptides bound to liposome membranes Addition Liposome Peptide Helix content* Membrane binding* Fusion rate (%) (%) (%/s) None PS 0.02 Hel 5-13 90.2 37.8

PC

ATP

PS

PC

Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5 Hel 5-13 Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5 Hel 5-13 Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5 Hel 5-13 Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5

56.9 65.6 59.8 82.9 1.0 3.3 12.1 40.5 72.0

+ +

-1-1-h 1.2 6.2 29.8 58.4 80.9

94.0 99.6 100 100 0.4 6.5 2.3 18.1 80.1 96.4 100 -14-f 1.6 6.6 -1-1-

+

0.31 5.2 17.6 31.5 0.0 0.0 0.002 0.025 0.037 3.8 7.8 9.9 30.6 37.9 0.001 0.005 0.012 0.25 0.93

* -f- means that the values could be estimated qualitatively but not quantitatively. showed little activity for fusion of PC liposome membranes, and in particular, although Hel 13-5 associated with the membranes and adopted helical conformation, low fusion activity was observed. Interestingly, they were localized both at the surface and in the interior of the PS membranes, but only buried in the interior of the PC membranes (data not shown). When the positive charges of the peptides were neutralized by addition of ATP, the peptide became helical and self-aggregative, bound to the PS and PC liposome membranes, and induced fusion of the membranes (Table 1 and 2). These results obtained by using two types of the amphipathic peptides suggest that close apposition of the negatively charged membranes occurs through the interaction between the exposed nonpolar feces of amphipathic helical peptides that are formed by Table 2 Properties of amphipathic peptides in solution Addition None

ATP

Peptide Hel 5-13 Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5 Hel 5-13 Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5

Helix content (%) 1.1 4.6 9.1 133 38.5 6.4 7.6 30.9 41.1 81.3

Self-aggregation (%) 1.7 0.7 4.8 2.1 4.7 46.6 48.3 84.8 97.0 98.8

614

Table 3 Rate of fluorescence quenching for amphipathic peptides by 5- and 16-DS PS 5-DS

16-DS

Peptide Hel 5-13 Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5 Hel 5-13 Hel 7-11 Hel 9-9 Hel 11-7 Hel 13-5

Rate constant (S^ x lOT 0.66 1.03 1.03 1.10 1.38 0.13 0.64 0.83 1.35 1.52

their electrostatic binding to the surface of the membranes, and that upon neutralization of the positive charges of the amphipathic peptides by negatively charged compounds such as ATP, close apposition of the membranes occurs through the membrane-binding and self-aggregative properties of amphipathic helical peptides. In other words, close apposition of liposome membranes occurs through "cross-bridge formation" by the self-aggregation (hydrophobic interaction) of the membrane surface-bound amphipathic helical peptides. When the peptides were fusogenic in the absence or presence of ATP, they always existed both at the surface and in the interior of the membanes, and both the rate of fusion of liposome membranes and the probability of the peptides in the interior of the membranes increased with increase in their hydrophobic/hydrophilic balance (data not shown). Moreover, the rate of penetration of the peptides into the interior of the membranes (the rate of fluorescence quenching by 16-DS) increased with increase in the hydrophobic/hydrophilic balance and correlated well with the rate of fusion (Table 3). These results suggest that disruption and reformation of the membranes occurs through "transbilayer movement" of the bound peptides between the interior and exterior of the membranes resulting in destabilization of the lipid bilayer structure.

REFERENCES 1. E.L. Pecheur, J. Sainte-Marie, A. Bienvene and D. Hoekstra, J. Membr. Biol., 167 (1999) 1. 2. T. Yoshimura, Y. Goto and S. Aimoto, Biochemistry, 31 (1992) 6119. 3. T. Yoshimura, K. Kameyama, S. Aimoto, T. Takagi, Y. Goto and S. Takahashi, Progr. Colloid Polym. Sci., 106 (1997) 219. 4. T. Yoshimura, E. Sato, S. Lee, K. Kameyama, S. Aimoto, Y. Goto and S. Takahashi, Peptide ScL, (N. Fujii, ed.) 1999 (2000)455. 5. S. Takahashi, Biochemistry, 29 (1990) 6257. 6. T. Kiyota, S. Lee and G. Sugihara, Biochemistry, 35 (1996) 13196. 7. S. Maezawa, T. Yoshimura, K. Hong, N. Duzgunes and D. Papahadjopoulos, Biochemistry, 28 (1989) 1422. 8. S.J. Edelstein and H.K. Schachman, Methods EnzymoL, 27 (1973) 82. 9. F.S. Abrams and E. London, Biochemistry, 31 (1992) 5312. 10. T. Yoshimura, S. Maezawa, K. Kameyama and T. Takagi, J. Biochem. (Tokyo), 115 (1994) 715.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) CO 2001 Elsevier Science B.V. All rights reserved.

615

Stability of PA/PI Mixed Liposomes against Aggregation H. Minami**, M. IwahasW and T. Inoue'' ^ Department of Chemistry, School of Science, Kitasato University, Kitasato, Sagamihara 228-8555, Japan Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka, 814-0180, Japan Liposome aggregation induced by Mg~^ ion was kinetically investigated for phosphatidic acid (PA)/phosphatidylinositol (PI) mixed system. Initial velocity of the aggregation was suppressed above the gel-to-liquid-crystalline phase transition temperature of PAPI mixed bilayers. Moreover, PA/PI liposomes aggregated more slowly than the acidic phospholipid liposomes; the aggregation for the PI liposomes was irreversible with respect to salt concentration. These results suggest that a repulsive hydration force acts between the liposomes containing PI. However, the repulsive effect is not so strong as to suppress the irreversible aggregation. 1. INTRODUCTION It is generally known that unilamellar liposomes of phosphatidylcholines (PCs) do not aggregate in the presence of multivalent cations and that the stability of PC liposomes is attributed to a so-called hydration force instead of an electrostatic repulsive interaction. However, the mechanism, in a molecular scale, of the repulsive hydration force acting between PC liposomes has not yet been fully elucidated: The repulsive hydration force would be ascribed to steric origins such as bilayer undulation, bilayer thickness fluctuations and the motion of hydrated Hpid hcadgroups [1], or to the weak electrostatic origin resulting from the polarization charges induced by zvvitterionic headgroup of PC [2]. In any case, the water molecules hydrated around PC headgroups would participate in this repulsive interaction. On the other hand, the liposomes composed of acidic phospholipids such as phosphatidic acid (PA), phosphatidylglycerol (PG) and phosphatidylscrinc (PS) readily and irreversibly aggregate in the presence of multivalent cations, because these liposomes having negative charges are stabilized mainly by electrostatic repulsion [3,4].

616 The head group of phosphatidylinositol (PI) possesses also a negative charge and, furthermore, five hydroxyl groups having strong affinity to water molecules. Therefore, it is interesting to know what interaction acts betv^xen the PI liposomes. In the present study, in order to know the interaction, we studied the stability of PA / PI mixed liposomes against the aggregation induced by magnesium ion. 2. EXPERIMENTAL 2.1. Materials. Soybean PI (>99%, sodium salt), dilauroylphosphatidic acid, DLPA (>98%, sodium salt), dimyristoylphosphatidic acid, DMPA (>98%, sodium salt), dipalmitoylphosphatidic acid, DPPA (>99%, sodium salt) from Sigma Chemical Co. and MgCl, (analytical grade, Wako Pure Chemicals Co.) were used without further purification. Water was purified by deionization followed by distillation t\\'ice. Unilamellar vesicles with a controlled size distribution, about 90 nm in diameter, were obtaind by applying the extrusion method. In a PA/PI mixed system, the mole fraction of PI was kept at 0.5 throughout the experiments. 2.2. Turbidity Measurements. Time course of the vesicle aggregation at constant temperature was followed by monitoring the change in optical density (turbidity) at 400 nm after mixing of a vesicle preparation with a salt solution of various concentration, by 1:1 in volume, using a stopped-flow spectrophotometer (Otsuka Denshi Model RA-401). The slope of turbidity increase just after the mixing was used to evaluate the initial velocity of liposome aggregation. 2.3. Dynamic Light Scattering Measurements. The reversibihty of the aggregation was examined by monitoring the change in size of aggregates associated with the change in salt concentration. The size of the aggregates was evaluated in terms of a mean hydrodynamic diameter, Dh, obtained by dynamic light scattering measurements using a NICOMP Submicron Particle Sizer Model 370 with an argon-ion laser (>^=488.0 nm) of a maximum power of 75 mW. 2.4. Differential Scanning Calorimetry (DSC). DSC measurements were performed using a Seiko Denshi Model SSC5200. PA/PI mixture of about 5 mg was weighted in a sealable sample pan made from aluminium, to which 20 \i\ of water was added, and then the pan was sealed. Alumina was used as a reference material. The sample was held at 90 °C for about 90 min in the oven of the DSC apparatus in order to assure homogeneous mixing of the lipid and water. Then, a cooling/heating cycle was repeated for several times at the rate of 4 "C/min in the temperature range from -40 to 90 °C. A

617

good reproducibility was obtained for the DSC thermograms recorded by the repeated scan. 3. RESULTS AND DISCUSSION

0'

10

20

30

Concentration / mM Fig.l Plot of Ko against MgCh concentration for DLPA/PI mixed liposome aggregation at 30 "C

Figure 1 shows the plots of initial velocity, F^, of DLPA/PI mixed liposome aggregation induced by mixing with MgCU solution as a function of the salt concentration. V^ increases with the salt concentration and approaches a saturation value, Vg'^''- The initial velocities of other PA/PI-liposome aggregations also approached a F^"*"" by the addition of Mg"^ ion above 26 mM. These results suggest that electrostatic repulsion betw^een PA/PI- liposomes is completely disappeared by the addition of Mg^* ion above 26 mM If some extra

repulsive force(s) such as hydration repulsion does not act, the aggregation of PA/PI-liposomes, occurring at high salt concentration, would be due to a diffusion-controlled process, i.e., the rapid aggregation. In such a case, F^"""" exponentially increases with temperature [3,4]. The temperature dependence of VQ"""" is depicted in Fig. 2. Arrows show gel-to-liquid-crystalline phase transition temperatures (Tm) measured with DSC for various bilayers (see Table 1). The F^'^'of pure DLPA-liposome aggregation exponentially increases with temperature and is not affected by the state of the lipid bilayer, /. e., gel or liquid-crystalline state. On the other hand, DLPA for PA/PI-liposomes, K^"""" increases rather linearly with temperature up to Tm^ and then the increase of F^*™"" is suppressed. These results suggest that some repulsive force(s) is acting bet\\'een PA/PI-liposomes and the repulsive force acts more strongly above Tm. It has been reported that the hydration force acting bet\\'een PC bilayers enlarges with temperature and becomes stronger especially above the gel-to-liquid-crystalline phase transition temparaturc [1,5,6]. Thus, the repulsive force which acts bet\\'een PA/PI liposomes would be also hydration force. 10

20

30

40

50

Table 1

60

Temperature (X) Fig. 2 Variation of Fornax ,^ix\\ temparature for DLPA and several PA/PI mixed liposomes. Arrows indicate Tm measured with DSC

Lipid

DLPA

DLPA/PI

DMPA/PI

DPPA/PI

Tm

33 °C

23 °C

40 °C

56 "C

(jel-to-liquid-cr) stalline pliase transition temperature, Tm, measured with DSC for several pure or mixed phospholipid bila\ ers.

618

2.0h 8mM

>3* h W)inM LOh

Tdilution 0.1

J

0

L—

2

4

6

Time (min) Fig. 3 Effect of dilution of MgCl2 concentration on the particle size, Z>h, of PI liposome at 55 °C. MgCl2 concentrations before and after the dilution are indicated in the figure. Tlie initial PI concentration is 0.17mM Tlie threshold MgCh concentration is 10 mM for PI liposome aggregation and Vo of PI liposome aggregation reaches Vd^^ by addition of MgCb solution over 60mM.

It is well known that the macroscopic aggregates are often formed with the progress of the aggregation in most colloidal dispersion system. Especially, in the rapid aggregation region, macroscopic aggregates are rapidly formed. Actually, in the rapid aggregation region (V^ = Vo""'') for PA, PG and PS, their liposomes rapidly form visible, macroscopic aggregates. However, macroscopic aggregates for PI or PAPI-liposomes have not been observed in such a short period in the rapid aggregation region. This fact also means that the repulsion, except the electrostatic repulsion, is acting bet>^'een PI or PA/PI-liposomes with a sufficient strength. Then, the reversibility of the aggregation was examined for pure PI

liposome. This is because, if the hydration force superiorly acts betv^xen liposomes, the aggregation behavior would show the reversibility [7,8]. However, as shown in Fig. 3, the growth of the aggregate was abnost stoped by diluting the liposome solution, but the particle size of the aggregate did not return to original value. Namely, the aggregation of PI liposomes induced by Mg^* ion is irreversible. In conclusion, the hydration force would be surely acting bet>^'een the liposomes containing PI, and the repulsive effect owing to the hydration force remarkably appears above Tm. However, the hydration force is not so strong as to suppress the irreversible aggregation. ACKNOWLEDGMENT This work was supported in part by funds from Kitasato University Research Grant for Young Researchers. REFERENCES 1. J. Marra, and J. N. Israelachvili. Biochemistrv' 24 (1985) 4608 2. B. Jonsson, and H. Wennerstrom. J. Chem. Soc. Faraday Trans. 2. 79 (1983) 19 3. H. Minami, T. Inoue, and R. Shimozawa. J. Colloid Interface Sci. 158 (1993) 460 4. H. Minami, T. Inoue, and R. Shimozawa. J. Colloid Interface Sci. 164 (1994) 9 5. L. J. Lis, M. McAlister, N. Fuller, and R. P. Rand. Biophys. J. 37 (1982) 657 6. J. N. Israelachvili. Chemica Scripta 25 (1985) 7 7. H. Minami, T. Inoue, and R. Shimozawa. Langmuir 12 (1996) 3574 8. H. Minami, and T. Inoue. Langmuir 15 (1999) 6643

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

619

Preparation of ultrathin films filled with gold nanoparticles through layer-by-layer assembly with polyions Tetsu Yonezawa,* Hiroto Shimokawa, Mizuki Sutoh, Shin-ya Onoue, and Toyoki Kunitake^ * Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan Gold nanoparticles stabilized by a small mercapto-ligand were assembled by the layer-by-layer alternate adsorption. Due to high chemical stability of these nanoparticles, sodium chloride could be added to the dispersion to reduce the electrostatic repulsion between the particles. Obtained layers of gold nanoparticles showed golden metallic luster and electro-conductivity. 1. INTRODUCTION Researches on metal nanoparticles have been conducted extensively from expectation to obtain novel properties arising from "quantum size effect", [l] Especially, immobilization and assembUng of nanoparticles are important technology of great practical and fundamental interest. Preparation of densely-packed monolayers of inorganic nanoparticles is not easy. Careful drying or dip-coating of organosols of nanoparticles is required to give their two-dimensional arrays on sohd substrates. [2] Polyelectrolytes and self-assembled monolayers (SAM) are often used as underlying substrates for organization of inorganic particles. [3] Metal nanoparticles are usually stabiUzed by organic molecules which are attached to the particle surface. Recently, thiol derivatives are often used as the stabilizer molecule. In this study, we propose to use a small anionic mercapto ligand, mercaptopropionic acid (MPA) as the stabilizer molecule. With this small ligand, the close-packed assembly of the nanoparticles was electroconductive because the insulating organic layer between the particles becomes very thin. It can be a useful module of nano-devices. 2. EXPERIMENTAL Poly(diallyldimethylammonium chloride) (PDDA, Aldrich, medium MW) and sodium poly(4-styrenesulfonate) (PSS, American Polymer Standards, MW = 88000) were used as the polymer layers for layer-by-layer assembly. tPresent address: RIKEN, Hirosawa, Wako-shi, Saitama 351-0198, Japan.

620

Mercaptopropionic acid (MPA)-stabilized gold nanoparticles were prepared by citrate reduction of AuCLt in the presence of MPANa (sodium mercaptopropionate). [4] Fig. 1 Schematic illustration of layer-by-layer assembly procedure to from The average gold nanoparticle multilayers on QCM resonators. diameter of the nanoparticles obtained by TEM investigation was 11.8 nm. Dispersions of the MPA-stabilized gold nanoparticle were concentrated by repeated ultrafiltration under N2 pressure (Advantec Toyo, Ultrafilter). The concentration of the obtained dispersion was determined by inductively coupled plasma spectrometry (TCP; Shimadzu, ICPS-5000). Silver-coated quartz crystal microbalance (QCM, USI Systems Co., 9 MHz, AT-cut) resonators were used as the substrate. The resonators were immersed for given periods of time in aqueous solutions of polyions, and gold nanoparticles, dried under N2 flow, followed by firequency measurements to determine the adsorbed mass. [5] A well-defined precursor fikn of PSS and PDDA was assembled onto resonators with a thickness of ca. 10 nm. The precursor film contained eight polyion layers beginning with PDDA in the alternate mode PDDA/PSS and the terminal layer was negative PSS. The resonator was then immersed for 20 min in an aqueous solution of PDDA, 50001 washed with water, and immersed in a dispersion of gold 40001 nanoparticles. 30001-

3. RESULTS AND DISCUSSION 3.1. Adsorption of MPAstabilized gold nanoparticles onto polyelectrolyte layers MPA-stabihzed gold nanoparticles were very stable, and no precipitation was found for months. Figure 2 shows the time course of adsorption of MPA-stabilized gold nanoparticles

«

2000h 1000!

Time / min

Fig. 2 QCM frequency decrease (Ai^ upon adsorption of the MPA-stabilized gold nanoparticles on poljnner layers. LA.U atom] = 2.0 X 10^ mol dm 3 in water at r t. without NaCl addition.

621

onto PDDA (the average value of repeated experiments for more than 5 times). If the ideal hexagonal packing is attained for a monolayer of MPA-stabilized gold nanoparticles, a flat QCM resonator will give a frequency decrease (AP) of ca. 4000 Hz (100 % coverage). At 60 min immersion, AF was 3300 Hz, which 4 6 8 NaCl / ia3 mol dm-^ indicates that 83 % of the QCM surface was covered by Fig. 3 Effect of NaCl concentration of the aq. MPA-stabilized gold nanoparticles. MPA-stabilized gold nanoparticle dispersion in The adsorbed mass increased the adsorption on PDDA. [Au atom] = 2.0 x 10'3 mol dm^, at r. t, immersion time = (O) 5 upon longer immersion, but SEM min, ( • ) 60 min. investigation suggested that multilayer stacking began at 60 min. To increase the surface coverage without stacking of the particles, NaCl was added to the dispersion, in order to reduce electrostatic repulsion among the particles. [5b] Gold nanoparticles prepared by citrate reduction of AuCU without MPA immediately precipitated by addition of very small amount of NaCl. In contrast, NaCl could be added up to 1.0 x 10 2 mol dm ^ into MPA-stabilized ones without precipitation. The highest NaCl concentration we can use was 1.0 X 10*2 mol dm 3, and the particle precipitated above this concentration. Figure 3 shows the frequency decrease after the immersion of 5 min and 60 min. The adsorbed mass linearly increases with the concentration of NaCl. Obviously, more than one layer of gold nanoparticles was adsorbed at higher ionic strengths. 3.2. Multilayer formation of MPA-stabilized gold nanoparticles by layer-by-layer assembly

25000 S

20000 h

15000 Multilayer of MPA-stabilized gold nanoparticles was prepared by 10000 alternate layer-by-layer assembly. 5000 h Precursor coated QCM resonators were immersed into the gold nanoparticle dispersion containing NaCl at 5.0 x 10*3 mol dm-3 for 5 min, then immersed into solutions of Adsorption Cycle PDDA, PSS, and again PDDA. Fig. 4. QCMfrequencydecreases (DF) due to Figure 4 shows QCM frequency the alternate gold/PDDA/PSS/PDDA changes in the assembly of gold adsorption at r. t. [Au atom] = 2.0 x lO'^ mol nanoparticles and PDDA/PSS/PDDA dm-3, [NaCl] = 5.0 x lO^ mol dm-3, immersion polymer layers. A linear increase in time = 5 min.

622

film mass is observed. Each layer adsorbed constant amounts of the particle. The dispersion of MPA-stabilized gold nanoparticles showed red color due to the specific plasmon absorption. The color of the plasmon absorption becomes purple to blue in aggregated forms. A multilayer of citrate-stabilized gold nanoparticles by layer-by-layer assembly showed blue color. [6] In contrast, the multilayer showed golden metallic luster in our case. The gold nanoparticles were adsorbed on the substrate quite effectively and surface coverage of the particle layer was very high. This is consistent with the fact that the obtained multilayer is electroconductive even the particle surface was covered by small organic Ugands. 4. CONCLUSION Ultrathin films of mercaptopropionic acid-stabilized gold nanoparticles were successfully obtained by layer-by-layer assembly with polycations. According to the sufficient stabilization by mercaptopropionic acid, the particle dispersion remained stable even after addition of NaCl to reduce the electrostatic repulsion among particles. The particles were adsorbed effectively so that the obtained thin films showed metaUic luster, indicating that the film was electroconducitve. Acknowledgement* This work is partly supported by Grant-in-Aids for COE Research (08CE2005) and for Encouragement of Young Scientists (for TY, 09740527) from Ministry of Education, Science, Sports, and Culture, Japan. TY also thanks the financial support from Shiseido Science Research Fund. REFERENCES 1. a) G. Schmid, Chem. Rev., 92 (1992) 1709; b) P. Buffat and J. P. Borel, Phys. Rev A, 13 (1976) 2287; c) N. Toshima and T. Yonezawa, New J. Chem., (1998) 1179; d) N. Toshima, T. Yonezawa, and K. Kushihashi, J. Chem. Soc, Faraday Trans. 89 (1993) 2537; e) A. Henglein, Ber. Bunsenges. Phys. Chem., 99 (1995) 903. 2. a) K. Vijaya Sarathy, G. U. Kuklarni, and C. N. R. Rao, Chem. Commun. (1997) 537; b) T. Yonezawa, S. Onoue, and N. Kimizuka, submitted. 3. a) D. Bethell, M. Brust, D. J. Schffirin, and C. Kiely J. Electroanal. Chem. 409 (1996) 137; b) K C. Grabar, K J. AUison, B. E. Baker, R. M. Bright, K. R. Brown, R. G. Freeman, A. P. Fox, C. D. Keating, M. D. Musick, and M. J. Natan, Langmuir, 12 (1996), 2353. 4. T. Yonezawa, M. Sutoh, and T. Kunitake, Chem. Lett., (1997) 619. 5. a) K Ariga, Y Lvov, M. Onda, I. Ichinose, and T. Kunitake, Chem. Lett. (1997) 125; b) Y Lvov, K. Ariga, M. Onda, I. Ichinose, and T. Kunitake, Langmuir 13 (1997) 6195; c) T. Yonezawa, S. Onoue, and T. Kunitake, Chem. Lett. (1998) 689. 6. J. Schmitt, G. Decher, W. J. Dressick, S. L. Brandow, R. E. Geer, R. Shashidhar, and J. M. Calvert, Adv. Mater. 9 (1997) 61.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) p 2001 Elsevier Science B.V. All rights reserved.

623

Three-dimensional assembly of cationic gold nanoparticles and anionic organic components' DNA and a bilayer membrane Tetsu Yonezawa,* Shin-ya Onoue, and Tbyoki Kunitake^* Department of Applied Chemistry, Facidty of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan Cationic gold nanoparticles and anionic components, DNA and anionic bilayer were assembled layer-by-layer to form controlled three-dimensional composite thin film. The QCM frequency changes revealed formation of ordered sandwich tjrpe layers. Densely charged positive particles formed close-packed layers on these anionic components. For formation of stable composites, rigid organic components were indispensable. 1. INTRODUCTION Layer-by-layer assembly procedure is interested as a simple method to prepare ultrathin films with nanometer thickness. Until now, various materials have been used as components of ultrathin films by using this method, [l] On the other hand, metal nanoparticles were investigated extensively as new materials with novel properties based on quantum size effect (QSE), and are expected as an important components for nanomaterials. We have recently created highly stable, positively charged metal nanoparticles that are totally covered by quaternary ammonium bromide groups. [2] Close-packed two-dimensional regular arrays could be successfidly obtained by using electrostatic interaction, thanks to the strongly charged surface of these particles. [3] In this study, we report that an ordered sandwich type three-dimensional structure was prepared by the layer-by-layer assembly

I

Br® Fig. 1. Schematic iUustration of 2.2 nm-cationic gold nanoparticle used in this study.

tPresent address: RIKEN, Hirosawa, Wakoshi, Saitama 351-0198, Japan.

624

Anionic DNA or

Rinse in water

bilayermolecule »rmolecule

procedure with highly charged positive nanop articles and anionic organic components, such as DNA and an anionic bilayer. 2. EXPERIMENTAL

IT

43.

QCM Measurement

<J= Rinse in water

Au Nanoparticle

Preparation of cationic gold nanop articles was already reported elsewhere. [3] The particles were Fig. 2. Schematic totally covered by trimethylammonium bromide illustration of formation of group and their average diameter was 2.2 nm (Fig. ultrathin films of gold nanoparticles by layer-by1). layer assembly procedure. Poly(diallyldimethylammonium chloride) (PDDA, Aldrich, medium MW) and sodium poly(4-styrenesulfonate) (PSS, American Polymer Standards, MW = 88000) were used as the precursor layer. X.-DNA (Nippon Gene, 48502 bps, MW = 3.15 X 107) and a bilayer molecule 1 [4] were used as anionic components for the composite. Quartz crystal microbalance (QCM) can detect mass increase on an electrolde as its frequency decrease. [5,6] We used silver-coated QCM resonators (9 MHz, AT-cut) as the substrate. First of all, a precursor film of PDDA and PSS was assembled onto resonators in the alternative mode (PDDA/PSS)4 and the terminal layer was positive PDDA. The resonator was then immersed for 20 min in 10 15 20 a buffer (pH = 7.5) solution of DNA Adsorption Cycle ([VDNA] = 72 ^ig cm 3) or 1 (lO mM) and Fig. 3. Frequency decrease (A/) of immersed in the dispersion of cationic gold QCM upon layer-by-layer assembly of nanoparticles ([Au] = 5.0 x 10 3 mol dm 3). cationic gold nanoparticles ( • ) and This alternate layer-bylayer adsorption anionic components (a DNA (O) b 1 was repeated 10 times. Every time after (D)). Precursor film PSS (O), PDDA the immersion in the nanoparticle (•). dispersion, the resonator was dried under N2 flow and then the frequency change was

625

measured. (Fig. 2) 3. RESULTS AND DISCUSSION Alternation with anionic organic Table 1. Frequency decrease and surface coverage of cationic gold components (DNA, nanoparticles on DNA and bilayerl surface bilayer) is essential for s u c c e s s f u l Average of First multiple layer 10 Cycle Monolayer assembly of cationic Multilayer gold nanoparticles. Frequency Decrease / Hz 365 361 Figure 3 shows QCM Au / DNA 111 Surface coverage / % frequency changes in no the assembly of the 366 364 Frequency Decrease / Hz Au/1 cationic gold 111 110 Surface coverage / % nanoparticles and the anionic components (a^ DNA, b* l). One can see a linear increase of film thickness (-AFis proportional to mass Ad. This should be the first example of the regularly ordered layer-bylayer stacking of nanoparticles and self-assembled molecules (in our case, bilayer). We have already reported that the cationic gold nanoparticle form a densely-packed two-dimensional structure on anionic surface, with a imiform inter-particle distance of 3.8 nm. [2a] When the cationic nanoparticles fiiUy cover the QCM surface with this inter-particle distance, the frequency decrease can be estimated as ca. 330 Hz. Table 1 gives the firequency decreases and surface coverage of the cationic nanoparticle on the first layer on A,-DNA and 1, and the average A/'of the 10-cycle layer-by-layer assemblies. It can be clearly seen in this Table that the frequency decrease upon adsorption of cationic gold nanoparticles onto X-DNA and bilayer 1 were ca. 360 Hz. The surface coverage could be calcidated as 110 ~ HI %. These data strongly indicate that gold nanoparticles form close-packed monolayers in each step. In this multilayer film, close-packed monolayers of gold nanoparticles are sandwiched between the organic components. This is the first example of sandwich t5rpe three-dimensional assembly of inorganic nanoparticles and organic counter layer. It was reported previously that inorganic nanoparticles could not form dose-packed monolayers onto organic counter layer due to electrostatic repulsion between the particles, [lb, le] The flatness and rigidity of the organic layer can help formation of the close-packing, which induced the formation of regular multilayers. This result is an important for preparing novel homogeneous materials containing inorganic nanoparticles.

626

4. CONCLUSION Regular sandwich type thin fibns of cationic nanoparticles and organic components such as DNA or bilayer membranes could be successfully obtained by the layer-by-layer assembly procedure. Thanks to the strongly charged surface, the particles could be assembled onto DNA or bilayer with close-packed structures. Acknowledgement' This work is partly supported by Grant"in-Aids for COE Research (08CE2005) and for Encouragement of Young Scientists (for TY, 09740527) from Ministry of Education, Science, Sports, and Culture, Japan. REFERENCES 1. a) G. Dehcer, Science, 277 (1997) 1232; b) Y. Lvov, K. Ariga, M. Onda, I. Ichinose, and T. Kunitake, Langmuir, 13 (1997) 6195; c) Y. Lvov, K. Ariga, I. Ichinose, and T. Kunitake, J. Am. Chem. Soc., 117 (1995) 6117; d) T. Yonezawa, H. Matsune, and T. Kunitake, Chem. Mater., 11 (1999) 33; e) J. Schmitt, G. Decher, W. J. Dressick, S. L. Brandow, R. E. Geer, R. Shashidhar, and J. M. Calvert, Adv Mater., 9 (1997) 61. 2. a) T. Yonezawa, S. Onoue, and T. Kunitake, Chem. Lett.,(l999) 1061; b) T. Yonezawa, S. Onoue, and N. Kimizuka, Langmuir, 16 (2000) 5218. 3. T. Yonezawa, S. Onoue, and T. Kunitake, Kobunshi Ronbunshu, 56 (1999) 855. 4. J.-M. Kim, Doctor Thesis, Kyushu University, (1980) pp. 41-90. 5. G. Z. Sauerbrey, Z. Phys., 155 (1959) 206. 6. Y. Okahata, K. Ariga, and K. Tanaka, Thin SoUd Films, 210/211 (1992) 702. 7. The relationship between adsorbed mass M (g) and frequency decrease ^F (Hz) in this system is AF= 1.83 x 10^ x MA, [5] where A is the apparent area of the QCM. By using the coefi&dent of the two dimensional packing, 0.9069, the number of particle adsorbed on both sides of the resonator is A^ = (0.9069 x 2A) I TCA^, where n is the average total radius of the gold particles containing the stabilizer shell (= 3.0 nm). The adsorbed mass is Af = W{, x A^a, where W^ is the weight of a particle. From elemental analysis, one particle contains 70.9 wt% of Au. [2a] Therefore, W^ = p(4/3 (TCTC^)) / 0.709, where TC is the average radius of the metal core (= 1.1 nm).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

627

Preparation of monolayers of SiOi and TiOi nano-particles by LangmuirBlodgett technique Masashi Takahashi*, Kunihiko Muramatsu*, Kazuo Tajima** and Koichi Kobayashi** ^Department of Energy Science and Engineering, Musashi Institute of Technology, Setagaya, Tol^o 158-8557, Japan ^Department of Chemistry, Kanagawa University, Yokohama 221-8686, Japan The preparation of two-dimensional arrays of nano-size Si02 and Ti02 particles on a solid substrate was attempted by the Langmuir-Blodgett (LB) technique. The most favorable conditions for yielding a uniform and dense packing of particles were examined for both Si02 and Ti02. From scanning electron microscopy (SEM) and atomic force microscopy (AFM) images it was found that the quantity of deposited particles is dependent on the electrostatic interaction between the spreading monolayer and the particles. The typical resuh for Si02 was a monoparticle-layer with a hexagonal packing arrangement. The distribution of particle sizes was found to have a marked influence on the arrangement of spherical particles. 1. INTRODUCTION It is well known that thin layers of nano-size particles have practical uses such as photocatalysts for decomposing organic environmental contaminants and coating materials for improving certain surface properties. The arrangement of particles has become increasingly important for improving the functionality of the particle-assemblies. In order to produce a two-dimensional packing of nano-particles, several methods have been proposed, for example, the casting of a colloidal solution [1] and the self-assembly of particles [2] including more sophisticated alternative layer-by-layer assembly technique [3,4]. Based on the electrostatic immobilization of particles at an air-water interface, a method g^jplying the versatile LB technique has also been reported [5-7]. Mayya et al. presented a series of studies demonstrating the immobilization of gold, silver and CdS nano-particles using a Langmuir monolayer of fatty amine. The studies showed that the degree of incorporation of particles can be controlled electrostatically by changing the pH of the colloidal subphase [5]. Furthermore, as the LB technique has the advantage of producing multilayers of nanoparticles with a hi^ly ordered layer structure, it could be widely extended to the formation of varied heterogeneous or alternative nano-assemblies. In our recent studies, two different approaches have been adopted; the self-assembly method using a positively-charged LB monolayer of long-chain alky lammonium salt [8], and the LB technique of depositing a cationic Langmuir monolayer on a colloidal subphase.

628 In this paper we describe the latter attempt; fabricating an LB monolayer such that the Si02 or Ti02 nano-particles are arranged two-dimensionally on the sohd substrate. In order to determine the optimum conditions that give uniform and close-packed layers, we primarily examined the influence of particle size and the pH of the colloidal subphase in the immobilizing processes of both Si02 and Ti02 particles. 2. EXPERIMENTAL Colloidal Si02 particles (mean diameter: d=90, 110, 300 and 560 nm) were sourced from Nissan Chemical Industries, Ltd. and Nippon Shokubai Co., Ltd. The Si02 particles were confirmed to be almost entirely monodispersed by SEM and AFM observations, except for the particles of d=90 nm. Dispersed solutions of Ti02 particles (d=6, 13 and 45 nm) were kindly supplied by Taki Chemical Co., Ltd. According to Taki Chemical Co., Ltd., the Ti02 particles were anatase form and were treated with org9nic acids. Cationic fihn material octadecylamine (ODA, Aldrich Chemical Company, Inc.) was employed as an adsorbent matrix for both Si02 and Ti02 particles, and was used without further purification. All other chemicals used were of the highest grade available. A moving-wall type film balance (NL-LB150S-MW, Nippon Laser & Electrics Lab.) was used for measuring surface pressure-area (jt-^) isotherms and monolayer depositions. Each of the dispersed solutions of Si02 and Ti02 was diluted in order to provide the colloidal subphase at a concentration of 0.40 mgcm'^ The subphase pH was systematically varied by adding hydrochloric acid or sodium hydroxide. The ODA monolayer was spread from a 1.0 x 10"^ mol dm"^ chloroform solution on the colloidal subphases at 20 "C. The monolayer was transferred onto a glass substrate by vertical dipping method at dipping and withdrawal speeds of 7 mm min"^ at a surface pressure of 10-30 mN m'^ The morphological features of the deposited layer were observed using a Hitachi S-4100 scanning electron microscope and a Digital Instruments Nanoscope III atomic force microscope. To obtain the AFM images, we used a commercially available 125 jim etched silicon probe cantilever (Digital Instruments, Inc., type: TESP). All AFM images were obtained in tapping mode under atmospheric conditions. 3. RESULTS AND DISCUSSION In the literature [9], the point of zero charge (pHo) of the Si02 (sol) was reported to be about 1.8, and hence the colloidal Si02 particles are considered to carry ne^ive charges at over pHo. On the other hand, pHo of synthesized Ti02 (anatase) was reported to be ^^proximately 6.0 [9]. Nevertheless, the Ti02 particles used in this study were confirmed to be anionic even at pH 3.0, probably due to the treatment with organic acid, and thus are sufficiently stable in a neutral solution. As the pH of the colloidal subphase affects the ionic state of ODA simultaneously, it is necessary to measure the K-A isotherms of the ODA Langmuir monolayer at various pH values before deposition. The results for both colloidal subphases; Si02 (d=110 nm) and Ti02 (d=45 nm), implied the presence of interaction between the monolayer and the particles.

629

Subsequently, SEM and AFM images of the deposited LB monolayers on the gjass substrate were obtained, and the morphological features and quantity of deposited particles were evaluated. Fig 1 is a typical SEM image of the ODA-SiOj (d=110 nm) LB monolayer fabricated at pH 7.0, clearly showing the deposition of a monolayer of particles. In this assembly, most particles are packed relatively close together in a hexagonal packing arrangement. However, the arrangement appears to be partially disordered due to several particles with dimensions that deviatedfromthe mean diameter. Furthermore, the assembly of Si02 particles with a broad size distribution (d=90 nm) exhibited only randomly arranged packing Therefor, it is concluded that monodispersion of particle size is required in order to realize hi^ly ordered arrangements of spherical particles. From the SEM images, we calculated the surface coverages of the SiOj particles with various diameters, plotted in Fig 2 as a function of pH. The incomplete curves at acidic pH are attributable to the lower stability of the floating ODA monolayer in this region, preventing deposition. The d=l 10 nm particle assemblies e?diibit hi^ surface coverage of 81% in the pH 5.5-8.7 range. However, the coverage decreases abruptly with increasing pH, with the result that at pH greater than 11, virtually no particles are seen in the SEM images, ionic ofca. amines, particle surface Si02 d=110 nm, and becomes much lower when the particles of d=560 nm are used. In addition, the images with low surface 1 SEM image of the ODA-SiOi LB monolayer coverage revealed that most particles Fig.deposited at pH 7.0. The mean diameter of the gathered to form domains. This impHes that particles is UOnm. the surface charges of the particles are 100 neutralized after adsorption on the ODA monolayer, and hence the particles tend to cluster together. Thin-layers of Ti02 particles were similarly assembled, however it was difficuh to obtain SEM images clearly because of charge accumulation on the Ti02 surface. Instead, we employed AFM as the primary means of observing morphological features. Fig 3 shows the AFM image of an ODATi02 monolayer (d=45 nm) deposited at pH Fig 2 Surface coverage of SiOz particles as a function of pH of the colloidal subphase. The dotted line at a 6.6. In the image, the surface of the substrate surface coverage of 91% is for the perfect hexagonal is densely covered with Ti02 nano-particles. packing of spherical particles, d: O, 110 nm; A, Another 0DA-Ti02 monolayer, prepared at

VHifmy'HQ

300 nm; D, 560 nm.

630 a different pH (pH 5.5-10.7), also ejdiibited a layer of densely packed particles, similar to that in Fig 3. However, the mean particle sizes in these images were obviously smaller than 45 nm. Also, the diameter of the TiOa particles, as evaluated by l i ^ t scattering seemed to vary according to the pH of the colloidal solution. These results indicate that the colloidal Fig. 3 AFM image of the 0DA-Ti02 LB monolayer on a particles in the solution are made up of scan area of 1 ^m x 1 fim. Left is height profile and smaller particles, and consequently, the right is phase profile. The monolayer was deposited with the particles of d=45 nm at pH 6.6. Ti02 particles are considered to adsorb in the form of aggregates on the ODA monolayer. At the same time, the AFM images also show that the Ti02 particles are somewhat distorted in shape from a sphere, and their sizes are not homogeneous, instead being distributed, which probably accounts for the less ordered arrangements compared to the Si02 assemblies. Unlike the above ODA results, when we used anionic film materials, e g , long-chain fatty acids and long-chain sodium sulfonate derivatives, as adsorbent matrixes, neither Si02 nor Ti02 were found to be immobilized on the LB monolayer deposited under conditions corresponding to those for ODA. Taking into account the pH-dependence seen in Fig 2, it is clear that the formation of the particle layer is primarily gDvemed by ionic interactions between the charged particles and the floating monolayer. Accordin^y, the charge density of the Langmuir monolayer, as well as the pH of the subphase, will be a key factor in controlling the adsorbancy of particles.

REFERENCES 1. A. S. Dimitrov and K. Nagayama, Langmuir, 12 (1996) 1303. 2. V. L. Colvin, A. N. Goldstein, A. P. Alivisatos, J. Am. Chem. See., 114 (1992) 5221. 3. Y. Lvov, K. Ariga, M. Onda, I. Ichinose andT. Kunitake, Langmuir, 13 (1997) 6195. 4. Y. Sun, E. Hao, X. Zhang, B. Yang, J. Shen, L. Chi and H. Fuchs, Langmuir, 13 (1997) 5168. 5. K. S. Mayya, V. Patil and M. Sastry, J. Chem. Soc., Faraday Trans., 93 (1997) 3377; K. S. Mayya and M. Sastry, J. Phys. Chem. B, 101 (1997) 9790; K. S. Mayya, V. Patil, P. M. Kumar and M. Sastry, Thin Solid Fihns, 312 (1998) 300; K. S. Mayya and M. Sastry, Langmuir, 14 (1998) 74. 6. L. S. Li, J. Zhang, L. J. Wang, Y. Chen, Z. Hui, T. J. Li, L. F. Chi and H. Fuchs, J. Vac. Sci. Technol. B, 15(1997) 1618. 7. H. Du, Y. B. Bai, Z. Hui, L. S. Li, Y. M. Chen, X. Y. Tang and T. J. Li, Langmuir, 13 (1997) 2538. 8. K. Tajima, Y. Imai, U. Tamaki, T. Kawagoe, M. Takahashi and K. Kobayashi, The Ninth International Conference on Organized Molecular Films, Potsdam, Germany, 2000. 9. G. A. Parks, Chem. Rev., 15 (1965) 177; R. H. Yoon, T. Sahnam and G. Donnay, J. Colloid Interface Sci., 70 (1979) 483.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) *P 2001 Elsevier Science B.V. All rights reserved.

631

Size Controlled Mesoporous Silicate Thin Films using Block Copolymer as Template (I) T. Yamada^ K. Asai", K. Ishigu^e^ A. Endo^ H. S. Zhou*' and I. Honma' "Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan **Department of Chemical Syxstems, National Institute of Materials and Chemicals Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ''Energy Fundamentals Division, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan The hexagonal type mesoporous silicate templated by triblock copolymer was synthesized in powder and film states. Their pore size can be successfully controlled by the synthesis condition. 1. Introduction According to the definition of lUPAC [I], the mesoporous materials have pore diameter between 2nm to 50nm. The novel mesoporous materials MCM-41 were reported in 1992 [2]. MCM-41 is silicate / aluminosilicate mesoporous material produced using ionic surfactant template by sol-gel method, which has uniform hexagonal arrangement. Since 1992, the mesoporous materials have attracted considerable interest because of applications in molecular sieve, catalyst, and adsorbent [3]. Efforts have been devoted to create sensor, electronic and photonic device [4, 5], on the bases of these self-assembled mesoporous materials. Further more, if functional molecules could be incorporated into the pore of mesoporous materials [6], new novel functional device might be made. However ordinary mesoporous materials included MCM-41 have too small pore size to incorporate some functional molecules. It is necessary to synthesize size controlled new large mesoporous materials for the further applications. Recently, a new large mesoporous materials which were made from poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPOPEO) type triblock copolymer as a structure directing agent using sol-gel method, was reported [7, 8], in which the pore size was larger than the pore size of MCM-41. However, tfiis mesoporous material has not been sufficiently systematically investigated in size controlling process. Generally, there are two methods to control the size of mesoporous material in sol-gel synthesis process. One is the condensation control of silicate, which control the condensation and shrinkage of pore during polymerization and removing the template materials by silica framework condensation. The other is direct control of template * To whom all correspondence should be addressed

632 polymer size during assembling process in synthesis. The size control of mesoporous materials, which used block copolymer as structure directing agent, was reported only using former process in powder state [8]. Furthermore, for using new device applications, it is important to change the state of the mesoporous material from powder to film [9-13]. Here we report the size controlling of block copolymer EO20-PO70-EO20 (BASF: Pluronic PI23; A/^,= 5750) template mesoporous silicate in both powder and film state using polymer control process and condensation control process, respectively. 2. Synthesis The mesoporous silicate powder and fihn were synthesized in different way. In powder synthesis, the triblock copolymer PI23 was dissolved in diluted HCl solution with stirring in a series of different temperature. Which is called the synthesis temperature Ts in this paper. Then tetraethoxysilane (TEOS) was added into the above solution. And a precipitated product was appeared. After that, the mixture solution was aged at SOT without stirring. Then, the precipitated product wasfiltered,washed by water, and air-dried at room temperature. Finally, calcination was carried out at 500°C. We changed Ts in synthesis condition to obtain different template polymer size and result in different mesoporous silicate size. In film synthesis, mesoporous silicate film was prepared by spin coating method on glass substrate. The coating solution was made from two solutions. One is the ethanol solution of PI23 with stirring at room temperature. The other is the silicate sol-gel solution mixture of TEOS, ethanol, water and diluted HCl with stirring at a range from room temperature to 75°C. Here, we call silicate sol-gel solution's stirring temperature as polymerization temperature Tp, because these parameters controlled the polymerization of the silicate [14]. After mixing these solutions, the mixed solution was stirred at room temperature. Then, the coating solution was used for film deposition on glass substrates by spin coating. Finally calcination was carried out at 450°C. We can get homogeneous and transparent mesoporous silicate thin films. The characteristics were investigated by X-ray diffraction (XRD) pattern and nitrogen adsorption desorption isotherm. 3. Results and Discussion Fig. 1 shows a typical XRD pattern of mesoporous silicate powder using PI 23 as template. Six well resolved peaks are observed in this figure at low angle 28 of 0.86°, 1.51°, 1.74°, 2.28°, 2.63° and 2.95° with d spacing of 10.2, 5.85, 5.07, 3.87, 3.36 and 2.99nm, respectively. These peaks have d spacing ratio of 1: 1/^3: 1/2: 1/V7: 1/3: 1/Vl2 and can be indexed as (100), (110), (200), (210), (300) and (220) reflections of hexagonal mesostructure, respectively [8]. The unit cell size is 11.8nm (a = d,ooX2/V3 [2]). Fig. 2 shows the pore and the unit cell size of this material dependence on Ts. The pore size from D-H method of nitrogen desorption isotherm [15] is increased with increasing Ts. The pore size of this material is influenced by the core of the block copolymer PI 23 micelle. The block copolymer PI23 micelle has two regions. One is the core that is constructed by the Pl23's hydrophobic part. The other is the mantle that is constructed by the P123's hydrophilic part. The core of micelle doesn't react and connect with framework silicate of this material due to hydrophobicity. Therefore, it only influences pore volume of this material. Hydrophobic part of PI23 is constructed PO block and a part of EO block. So, the

633

E c

2

3

4

Fig.l. The XRD Pattern of

^25 30 35 40 45 50 55 60 65 Ts [T] Fig.2. The size dependence on Ts

mesoporous silicate powder.

(unit cell size • , pore size • )

26 r i

hydrophobicity of the EO block is increased [16] with Ts. Consequently, the core volume of micelle and the pore size of this material depend on Ts. The unit cell size from powder XRD pattern is increased with increasing Ts under 45°C. After over 45°C, the unit cell is gradually decreased with increasing Ts in spit of the pore size is increased. Because the silicate is condensed with increasing temperature [6, 14] therefore the silicate framework is condensed and decreased. However, under 45°C, the unit cell is increased and the pore size is gradually increased with increasing Ts. There is no sufficiently space in this paper to discuss this phenomenon. Please see details in next coming paper [17]. The size of mesoporous silicate thin films only using PI23 as template is controlled by synthesis conditions. A typical XRD pattern of calcined mesoporous silicate thin films (Fig. 3) shows three well resolved peaks with d spacing of 4.81, 2.50 and 1.65nm, which index as (100), (200) and (300) reflections of one dimension hexagonal structure, respectively [13]. Comparing to Fig. 1, the (110), (210) and (220) reflections disappeared, which suggests that

3 ed

4>

c

50 60 Tp [OC]

70

Fig.3. The XRD pattern of

Fig.4. The unit cell size dependence

mesoporous silicate thin film

onTp

80

634 the films have a highly oriented hexagonal structure and the pore channels are parallel to the substrate surface [13, 18]. The (100) peak reflects a d spacing of 4.81nm, corresponding to a large unit cell parameter (a = d,ooX2/V3= 5.55nm [2]). Fig. 4 shows the unit cell size dependence on Tp. This figure shows the d spacing increased with increasing Tp. Tp should be related wiA condensation of silicate [14]. And the condensed silicate should prevent pore shrinkage during calcination [6, 14, 19]. Therefore the pore size might be controlled by Tp because of the unit cell size could be controlled [6, 14, 19]. 4. Conclusions Both powder and thin fihn mesoporous silicate was synthesized by using PI23 type triblock copolymer as a template. These powders and thin films formed hexagonal and one dimensional hexagonal mesostructure according to XRD pattern. We succeeded in controlling the size of mesoporous silicate both in powders and thin fihns. References [I] K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure & Appl. Chem. 57 (1985) 603. [2] J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Lenowicz, C. T. Kreage, K. D. Schmitt, C. T-W. Chu, D. H. Olson, E. W. Sheppard, S. B. NcCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] X. S. Zhao, G. Q. (Max) Lu and G. J. Millar, Ind. Eng. Chem. Res. 35 (1996) 2075. [4] H. S. Zhou and I. Honma, Chem. Lett. (1998) 973. [5] H. S. Zhou, H. Sasabe and I. Honma, J. Mater. Chem. 8 (1998) 515. [6] M. Ogawa, H. Ishikawa and T. Kikuchi, J. Mater. Chem. 8 (1998) 1783. [7] D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredirckson, B. F. Chmelka and G. D. Stucky Science, 279 (1998) 548. [8] D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky; J. Am. Chem. Soc. 120 (1998) 6024. [9] M. Ogawa, J. Am. Chem. Soc. 116 (1994) 7941. [10] Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Huang and J. I. Zink, Nature 389 (1997) 364. [II] D. Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka and G. D. Stucky, Adv. Mater. 10 (1998) 1380. [12] H. S. Zhou and I. Honma, Jpn. J. Appl. Phys. 38 (1999) L958. [13] H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara and G. A. Ozin, Nature, 379 (1996) 703. [14] Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Sciith and G. D. Stucky, Chem. Mater. 6 (1994) 1176. [15] D. Dollimore and G. R. Heal, J. Appl. Chem. 14 (1964) 109. [16] R. Zana, Colloids Surf. A123-124 (1997) 27. [17] T. Yamada, K. Asai, A. Endo, H. S. Zhou and 1. Honma, manuscript in preparation. [18] D. Kundo, H. S. Zhou and I. Honma, J. Mater. Sci. Lett. 17 (1998) 2089. [19] T. Yamada, K. Asai, A. Endo, H. S. Zhou and I. Honma, J. Mater. Sci. Lett, in print.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

635

Photocurrent of purple membrane adsorbed onto a thin polymer film: effects of monovalent and divalent ions A. Shibataa*, K. Yamadaa, H. Ikemaa, S. Uenoa, E. Muneyukib and T. Higutia apaculty of Pharmaceutical Sciences, The University of Tokushima, Shomachi, Tokushima 7708505. ^Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuka, Midoriku, Yokohama 226-8503, Japan. Bacteriorhodopsin (bR) in purple membrane (PM) binds divalent cations (Ca2+ and Mg2+) and these cations are essential for the proton transport function of bR. The capacitive current in bR in the absence and presence of electrolyte at concentrations in excess of'- 0.1 mol dm-3 has been examined in the membrane adsorbed system, in which the PM fragments were adsorbed onto a thin polymer film. The photoelectric current in bR was reduced concentration-dependently and reversibly in different manner by each electrolyte. We found that adding monovalent and divalent ions to the PM caused supression of its proton pumping activity. The sequences of the EC50 (a 50% suppression parameter) for monovalent anions, monovalent cations, and divalent cations were as follows: F- > CI- > Br- > I- > SCN- > CIO4Li+ > Na+ > K^^ > Rb+ > Cs+ Mg2+ > Co2+ > Mn2+ > Ca2+ » Hg2+ The sequences follow the Hofmeister series and are explained on the basis of changes in the hydration of the PM. The absorption and fluorescence spectra showed that the binding site of Ca2+ to bR in the PM differs from that of Mg2+. The suppression of the bR pump is attributed to surface phenomena and not to ion binding at specific sites in the PM. 1. INTRODUCTION Bacteriorhodopsin (bR) is the photoreceptor protein in the purple membrane (PM) of Halobacterium salinarium. BR in the PM is organized in a hexagonal crystalline structure in the membrane and functions as a light-driven proton pump, allowing the transformation of light into chemical energy. The chromophore is a retinal in a protonated Schiff base linkage with Lys2i6 residue. BR in the PM binds 4mol of Ca2+ and Imol Mg2+ per mol of bR [1]. Removal of these cations from the PM or lowering the pH of the medium changes its color from purple (^ax=568 nm) to blue (A^ax=605 nm) and the blue membrane does not pump protons [2]. The divalent cations in the PM can be replaced by monovalent cations, Na+ and K+, or trivalent cation, La3+, and the replacement with La3+ suppresses the signal amplitude of flash photolysis[3]. Two different proposal have been made to explain the requirement of divalent cations for the purple color of PM which is closely associated with the pumping activity of bR: specific binding sites, probably to carboxyl groups of protein side chains [3] and a nonspecific binding within the Gouy-Chapman double layer [4]. Water also plays an important role in the molecular process of the proton pump in bR. Water in

636 the binding site plays a major role in controlling pKa of the protonated Schiff base and its counterions [5]. The photoelectric signals of bR are affected by the level of the purple membrane hydration [6]. A defined geometrical structure of the active site of bR that includes the protonated Schiff base linkage, bound water and the primary proton acceptor Asp-85 was suggested to stabilize the ion pair in the binding site [7]. Our purpose of this study is to elucidate the effect of electrolyte on the proton pumping activity and the properties of the ion binding sites of the PM. The photoelectric current in bR in the presence of electrolytes was suppressed concentrationdependently and reversibly in different manner by each electrolyte. Fluorescence spectra showed that Mg2+ modified the tertiary structure around the tryptophan residue in bR in the PM. 2. EXPERIMENTAL PM fragments (d=ca. 500 nm) were isolated from Halobacterium salinarium S9. Electrolytes used were NaF, NaCl, NaBr, Nal, NaNOs, NaC104, NaSCN, LiCl, NaCl, KCl, CsCl as uniunivalent electrolytes and CaCh, MgCh, MnCh, C0CI2, HgCh as di-univalent electrolytes. The apparatus and techniques for the electrical measurements were basically the same as described by Muneyuki et al [8]:The PM adsorbed onto a thin polymer film (0.9 ^im thickness; Lumirror, TORAY CO.) was equilibrated with the buffer solution (10 mM Tris-HCl/0.1 M KCl (pH 7.4)) for 10 min, followed by 5 sec continuous illumination (100 W halogen lamp). Capacitive currents in bR were recorded before and after expose to the electrolyte. Intrinsicfluorescencewas obtained by a Hitachi fluorescence spectrometer operating in photon counting mode, excitation at 285 nm, and emission between 300 and 550 nm. The absorption spectra were obtained by a JASCO V500 spectrophotometer. All measurements were carried out at 23 ± 1 <>€. 3. RESULTS and DISCUSSION Fig. 1 shows the photocurrent of bR in the PM in the absence and presence of the various monovalent anions (NaF, NaCl, NaBr, Nal, NaNOa, NaC104, and NaSCN). As the electrolyte concentration was raised, the photocurrents generated by proton pump of bR were cooperatively decreased. All of the monovalent anions tested suppressed the pumping activity and their effectivenesses varied greatly with different ions. Concentrations (EC50) required for a 50% suppression of bR proton pumping activity were estimated from the action curves in Fig. 1 and given in Table 1. The monovalent cations (LiCl, NaCl, KCl, RbCl and CsCl) also decreased concentration-dependently and in different manner the photocurrent of bR. The effective concentrations of divalent cations (MgCl2, MnCl2, C0CI2, CaCl2 and HgCl2) to suppress the pumping activity were similar in the order of magnitude to those of monovalent cations. We found that the divalent cations at the concentration in excess of - 100 mmol dm-3 suppressed bR pumping activity although bR pumps proton at the concentration below 1 mmol dm-3 [2]. The obtained sequences for monovalent anions , monovalent cations, and divalent cations were as follows: F- > CI- > Br- > I- > SCN > CIO4(1) Li+ > Na+ > K+ > Rb+ > Cs+ (2) Mg2+ > Co2+ > Mn2+ > Ca2+ » Hg2+ (3) The effects of the electrolytes can be devided into two major classes [9]; the first is due to 'direct interactions* of the ions with specific charged groups on the PM. The second class of effects arises primarily from an indirect process whereby the ions affect the PM through the modification

637

of hydration around the PM. The manner in modification is markedly dependent upon the structure and constituents of the ions. The sequences in Eqns. (1), (2) and (3), indicating ability of ions to suppress the bR function, follow the Hofmeister series [10]. The sequences are strikingly similar to those observed for hydration changes of globular protein caused by electrolytes [11] and for salting-in and salting-out capabilities of these electrolytes for the peptide groups and nonpolar side chains of polypeptides [12]. In order to account for the suppression of the proton pumping activity of bR in the presence of electrolyte, the values of EC50 were plotted against the reciprocal of the ionic radii for monovalent anions (CI-, B r and I- ), monovalent cations (Li+, Na+ , K+ , Rb+ and Cs+) and divalent cations (Mg2+, Co2+. Mn2+ and Ca2+) (data not shown). The results obtained were markedly similar to the plots of hydration number of ion vs. the reciprocal of the ionic radii [13]. Caotropic ions (C1-, Br-, I-, SCN-), which interact less favorably with surrounding water, have higher affinity for positively charged residue of protein because of their smaller effective size. On the other hand, the monovalent cosmotropic anion, Fand SO42- (Fig. 1), which increase the water structure by interacting with nearby water molecules, have lower affinity of protein because of their larger size. The sulfate anion is heavily hydrated (Table 1). Table 1. 1.2

1 ] iiilii|

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The concentrations (EC50) of monovalent anions and cations and divalent cations required for a 50% suppression of bR photocurrent (EC50; mol din-3). Anions Cations Cations Salt EC50 Salt EC50 Salt EC50 3.5 LiCl MgCl2 3.9 NaF >2.5 2.4 NaQ coa2 2.6 2.4 NaCl 0.5 Mna2 1.2 KQ 1.0 NaBr 0.2 CsCl CaCl2 0.6 0.3 Nal

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1

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Fig.l. Effect of monovalent anions on the photocurrent of bR in the purple membrane.

NaNOs

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1. Control 2. 0.05M MgCl2 3. 0.5M 4. l.OM 5. 3.0M 6. 4.0M 7. 5.0M

1. Control 2. 0.05MCaCl2 \

1

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400

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,

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.

450

Wavelength /nm

.

.

.

I

.

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. ,

.

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Fig.2. Fluorescence spectra of bR in the P M in the presence of C a Q 2 (A) and M g a 2 (B).

0.06

638 The binding of the monovalent cations to bR in the PM was highly sensitive as shown in Eqn. (2). Native PM contains 75% proteins and 25% lipids by weight. Protein, BR, has 19 aspartic acid and glutamic acid residues, 14 lysine and arginine residues. Most of them are presumably near the membrane surface and at least some of them must ionized. 80% of lipid are strong acids and hence render the membrane surface very acidic [11]. Thus the PM will be expected to have a significant surface potential. In fact, we found that the zeta potential of the PM dispersed in 10 mM Tris-HCl buffer solution (pH 7.4) at 22 oC was -43 mV. When the monovalent cation was added, the zeta potential of the PM increased with increasing the concentration. The cations ( Li+ and Cs+) change the surface potential by acting as free Gouy-Chapman ions and by lowering the surface charge density of the membrane through specific binding. The extended investigation of negatively charged DNA indicated that T^ (a destabilizing parameter of the helical conformation) decreases linearly with increasing ionic radius of the counterions in the series Li+ < Na+ < K+ < Rb+ < Cs+ [ 14]. Electrophoretic data showed that binding of the alkali metal ions to DNA is in the reverse order, Li+ > Na+ > K+ > Rb+ > Cs+ [15]. The data are compatible if the decrease in Tm is associated with the decreasing degree of ionic hydration in the series Li-*- > Na+ > K+ > Rb-*- > Cs+, particularly since the Li+ ion is markedly more hydrated than the other ions in the series. Figure 2 shows the fluorescence spectra of bR in the PM in the presence of CaCl2 and MgCl2. The fluorescence maximum of the control bR was 315 nm, indicating that the location of the tryptophan residues in bR was mainly interior of the molecule, away from water in the bulk phase [16]. As the concentrations of Mg2+ are raised, the tryptophan band was shifted up to the 330 nm band, but not shifted in the presence of Ca2+, suggesting perturbation of the tertiary structure around the tryptophan residues in bR by Mg2+. The absorption and fluorescence spectra showed that the binding site of Ca2+ to bR in the PM differs from that of Mg2+. The suppression of the bR pump are attributed to surface phenomena and not to ion binding at specific sites in the PM. REFERENCES 1. C. -H. Chang, J. -G. Chen, R. Govindjee, and T. Ebrey, Proc. Natl. Acad. Sci. USA. 82 (1985)396. 2. J. A. Griffiths, J. King, R. Browner and M. A. El-Sayed, J. Phys. Chem., 100 (1996) 929. 3. R. Jonas and T.G. Ebrey, Proc. Natl. Acad. Sci. USA. 88 (1991) 149. 4.1. Szundi and W. Stoeckenius, Biophys. J., 56 (1989) 369. 5.1. Rousso, N. Friedman, A. Lewis and M. Sheves, Biophys. J., 73 (1997) 2081. 6. G.Varo and L. Keszthelyi, Biophys. J., 43 (1983) 47. 7.1. Rousso, N. Friedman, M. Sheves and M. Ottolenghi, Biochemistry, 34 (1995) 10259. 8. E. Muneyuki, D. Okuno, M. Yoshida, A. Ikai and H. Arakawa, FEBS Lett., 427 (1998) 109. 9. F. Franks (ed.), Water, Vol. 4 : Aqueous solutions of amphiphiles and macromolecules. Chap. 5, Plenum Press, New York, 1975. 10. Hoffmeister K. D. Collins and M. W. Washabaugh, Q. Rev. Biophys., 18 (1985) 323. 11. H.B. Bull and K. Greese, Arch. Biochem. Biophys. 137 (1970) 299. 12. P.K. Nandi and D.R. Robinnson, J. Amer. Chem. Soc., 94 (1972) 1308. 13. J. O'M. Bockris and P.P.S. Saluja, J. Phys. Chem. 76 (1972) 2140. 14. C. Zinmier and H. Venner, Naturewiss, 49 (1962) 86. 15. P.D. Ross and R.L. Scruggs, Biopolymers, 2 (1964) 231. 16. E.A. Permyakov, Luminescent Spectroscopy of Proteins, Chapt. 4, CRC Press, Inc., 1993.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

639

Roles of Biomembranes -Effects of Surfactants Including Precursors of Endocrine Disrupters on the Interactions Between Acetylcholinesterase and HalothaneI. Tsukamoto*, H. Komalsu*, T. Tsukamoto'', N.Maekawa' ^Department of Anesthesiology and Emergency Medicine, Kagawa Medical University Miki Kagawa 761-0793, Japan ''Department of Phannacy, Kagawa Medical University Hospital Miki Kagawa 761-0793, Japan Effects of surfactants on the interactions between halothane and acetylcholinesterase (E.C.3.1.1.7. abbreviated hereafter as AChE) were examined. Halothane inhibits the activity of AChE. Three kinds of Triton X (polyoxyethylenegrycol/?-r- octylphenyl ether); X-114, X-100 and X-102, strengthened the inhibition effect of halothane at below their c.m.c. These are different in polymerization numbers. Since the effects of Triton X were not affected by the size of the hydrophilic portions, the effects were considered to be caused by the hydrophobic portion, i.e., octylphenol. With respect to the cationic, anionic and other nonionic surfactants examined in this work, no similar effect was observed. As octylphenol is one of the chemical compounds listed as endocrine disrupters, the present data indicated the possibility of the interactions between endocrine disrupters and medicines. This effect was observed only when AChE existed in the membrane. The lipid bilayer structure controlled the interactions between halothane, AChE and surfactants. 1. Introduction Nowadays, many surfactants are spreading in the environment, and some of them are listed as precursors of endocrine disrupters. The endocrine disrupters alter the balance of biological functions. On the other hand, many medicines are usually administered to regulate the functions. In case the organs have been already polluted with endocrine disrupters, unexpected effects of medicines may occur. Though the reports dealing with this problem have not published yet, it will be a serious problem if the process how the endocrine disrupters interact

640

with the medicines in vivo becomes clear. Triton X is a widely used surfactant It includes the structure of octylphenol that is listed as one of the endocrine disrupters. As we reported previously, a general anesthetic, halothane (CF3CHClBr) inhibits the activity of human acetylcholinesterase in vitro(I). Recently, reports about the interactions between anesthetic molecules and functional proteins are increasing, and those about the interactions between anesthetics and membrane lipids are decreasing. Functional proteins are of course very important to interpret the anesthetic actions. We think however the membrane lipids are also important Membrane structure is not simply a supporting matrix for the proteins, but should be a kind of controller of their functions. In this study, the effect of surfactants on the interactions between halothane and the enzyme in the membrane was discussed. 2. Material and Methods Blood samples were taken from healthy volunteers. Erythrocyte was washed with saline, and prepared at Ht=50%. Acetylcholinesterase (E.C.3.1.1.7.: abbreviated hereafter as AChE) exists in the erythrocyte membrane. The activity of AChE was measured by Ellman's method using HITACHI U-3200 spectrophotometer (2). Halothane and surfactants were solved in buffer solution (20mM phosphate buffer pH=8.0) and appropriate amount of the solution was added into the enzyme solution. The concentration of halothane in the buffer was determined by head space gas chromatography. An HS-GC system (Perkin Elmer Autosystem Gas chromatograph equipped with HS-40 Headspace sampler) was used. For surfactants, a cationic surfactant, cethyltrimethylammonium bromide (CTAB), an anionic surfactant, sodium dodecyl sulfate (SDS), and nonionic surfactants, sucrose myristate (SE), Triton X-114, Triton X-100 and Triton X-102 were used. All surfactants except SE were commercially available reagent grade, and used without further purification. SE was kindly supplied from Mitsubishi-Kagaku Foods Co.. Sucrose includes three R-CHj-OH groups where esterification can be performed. More than 80% of SE used were the mixture of 3 kinds of monoester. Another 20% included di- and tri- esters. Using the molecular weight of a monoester, the concentration of SE was estimated. 3. Results and Discussion Increasing the concentration of Triton X-100, the activity of the enzyme in erythrocyte membrane once decreased and then recovered (Fig.lB O). During the process, the enzyme is transferred from erythrocyte membrane into the micelle (from membrane-AChE to micelle-AChE), and the bilayer structure disappears. The minimum of the activity was observed at [Triton X-100] = 0.004%. In case the micelle solution is diluted below c.m.c, the micelle

641

Fig.l Effects of surfactants on the activity of AChE with ( • ) and without (O) halothane (15mM).

A :

Triton X-114

D:

SDS

1.2 1.0 - 0.8 4^ , , '.> 0.6

S 0.4 ^

0.2

0.0 0 0.00

0.01 0.02 0.03 [Triton X-114] (%)

500 1000 1500 [SDS]

IJLM

CTAB

Triton X-100

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o <

0.00

0.01

0.02

[Triton X-100]

0

0.03

[CTAB] AdM

(%)

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Triton X-102

0.00

0.01

0.02

[Triton X-102]

50 100 150 200

0.03 (%)

0

SE

200 400 [SE] ULM

600

642 structure is broken, and the enzyme exists as a free form (free-AChE). However, during the process from micelle-AChE to free-AChE, the activity of the enzyme is constant even at [Triton X-100] = 0.004%. The decrease in the activity can occur only with membrane-AChE. So, the decrease is not caused by the interaction between Triton X-100 and enzyme protein, but by the interaction between Triton X-100 and membrane lipids. The change of the properties of the membrane alters the activity of the enzyme in the membrane. 15mM halothane decreases the activity of membrane-AChE to 70-80% of the control ([halothane]=0, [Triton]=0). As for micelle-AChE and free-AChE, the activity decreases to 80-90 %, which is not affected by the concentration of Triton X-100. Two reasons can be considered for why the effect of halothane is stronger with respect to membrane-AChE compared with micelle- or free-AChE.

One is the accumulation effect

As halothane is

hydrophobic and tend to accumulate in the membrane, the local concentration of halothane around membrane-AChE might be much higher than that in the bulk concentration. Another is that the indirect effect of halothane on AChE via membrane lipid can be considered. As we can see in Fig.lB,

0.0007% Triton X-100 does not affect the activity of

membrane-AChE by itself. However, when both 15mM halothane and 0.0007% Triton X-100 exist, the activity decreases from 70 to 40-50%. Strengthened inhibition can be observed only with membrane-AChE, and not with free-AChE nor micelle-AChE. The membrane structure is essential for this case, too. Similar phenomena were observed with respect to all Triton X (Fig. 1 A-C).

The

polymerization numbers of Triton X-114, X-100 and X-102 are 7-8, 9-10, and 12-13, respectively.

As the sizes of hydrophilic portion did not affect, the phenomena should be

caused by the hydrophobic portion, i.e., octylphenol. SDS (Fig. ID) and CTAB (Fig. IE) inhibited the AChE activity dose dependently. SE (Fig. IF) did not affect the AChE activity both in the presence and in the absence of halothane. In the present study, we examined 6 kinds of surfactants. Present data shows that the structure of octylphenol and lipid bilayer are essential to strengthen the inhibition by halothane. Erythrocyte membrane was really one of controllers of the activity of AChE, and the posibility that endocrine disrupters affect the actions of medicines was revealed.

REFERENCES 1.

I. Tsukamoto, S. Yokono, H. Komatsu, K.Ogli Toxicol. Let.100-101, (1998)447-450

2.

G. L. Ellman, K. D. Courtney, V. Andres Jr., R.M. Featherstone Biochem. Riarmacol. 7, (1961) 88-95

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) vc 2001 Elsevier Science B.V. All rights reserved.

643

The Effects of Alkyl Substhuents and Formyl group of Bacteriochlorophyll e on their Aggregation in Chlorosomes of Brown-colored Photosynthetic Sulfur Bacteria Hiroki Hiiabayashi, Takasada Ishii & Kaku Uehara Research Institute for Advanced Science and Technology, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599-8570, Japaa Fax: 0722-51-7139, E-mail: [email protected] We found the spectral difference in the aggregates of bacteriochlorophyll (BChl) c and e, which depended on the effect of a methyl and a formyl group at position 7 of the BChls, respectively The Qx absorption maximum appeared for tiie artificial aggregates of BChl e but did not clearly for those of BChl c. The formyl group at position 7 of BChl e might adapt to the optical absorption range for the brown-colored photosyhthetic bacteria which live in deeper sea under lower light intensity. INTRODUCTION The green sulfur bacteria have a characteristic light harvesting system called chlorosome. The chlorosome contains a large amount of BChl c, BChl d and/or BChl e which might form rod-like self-aggregates without interaction of protein. These chlorosomes are surroimded by a monolayer envelope of galactolipid (1). These BChls consist of homologs possessing different alkyl groups at positions 8 and 12. Three major homologs of BChl e have been isolatedfix)mbrown-colored [rfiotosynthetic sulfur bacteria Chlorobium {Cb.) phaeobacteroides contains [E,E]-, [P£]-, and [I3]BChl ef where E, P and I stand for ethyl, propyl and wo-butyl groups, respectively, and F, famesyl group. BChl ep differs from BChl CF in having a formyl group at position 7 such as chlorojAyll b which plants contain (Figl) (2). We investigated the spectroscopical difference in the in vitro aggregation behavior in aqueous dimethyl sulfoxide (DMSO) solution (DMSO-water 50:50) for the individual homologs of BChl ep and BChl CF based on the effects of alkyl substituents and a formyl group (3). MATERIALS AND METHODS Cb.phaeobacteroides was grown in batch culture (30 nmi diameter of 100 ml Pyrex tubes) at 30 and irradiated at 15 // E/m^/sec from a fluorescent lamp. To investigate the effect of Qx absorption band, a tube was covered with a green cellojAane film which shieldsfi-om300 nm to

644

500 nm and from 600ranto 800ran,respectively. Each individual homologs of BChl ep and BChl cp were extracted from Cb.phaeobacteroides and Cb.tepidum, respectively and separated on a reverse-phase (ODS) HPLC column (4). Artificial aggregates of these BChls were formed in DMSOwater (50:50) which is automatically maintained at pH 7.5 without buffer salts. The aggregation behavior of the BChls were measured by absorption spectra recorded at 5 min intervals at 30. After complete aggregate formation, the orientation of the BChl molecules was investigated by CD spectra.

o famesyt chain(Ci5) R^

Fig.2 Absorption spectra of Chlorobium lepidum containing BChl c (solid line) and Cb.phaeobacteriodes containing BChl e (dashed line).

R? Kn Rl2 BCMe Me Et/n-Pr/i-Bu Et BCU« CHO Et/D-Pr/i-Bu Et Fig. 1 Chemical structures of chlorosomal bactcriochlorophyll.

RESULTS AND DISCUSSION Fig.2 shows the absorption spectra of the living bacteria, Cb.tepidum containing BCbl cp and Cb.phaeobacteroides containing BChl ep. Quite dififerent aggregated forms were observed between BChl cp and BChl epin vivo. The Qy absorption maximum of BChl ep aggregates was found in the shorter wavelength region (720 nm) compared with that of BChl Cf(753 nm). Further, the (Jx absorption band 24)peared in the BChl ep aggregates but did not in BChl cp The molecules of chlorophylls and bacteriochlorophylls have two dipole moments, Qx and Qy. Generally, the Qx absorption band was observed only in the spectra of bacteriochloropyll a, b and g, which they possess bacteriochlorin ring. The bacteriochlorin ring \s converted from chlorin ring by reduction between position 7 and 8. In the case of chlorophylls and chlorosomal bacteriochlorophylls, their absorption spectra do not have Qx absorption band because they possess chlorin ring. However, the absorption spectrum of BChl ep aggregate indicated the Qx

645 absorption band although they possess chlorin ring. We observed that Cb.phaeobacteroides could grow in a culture tube covered with a green cellophane fibn, but Cb.tepidum could not The wavelength area of it is the same as the absorption spectrum of Cb.tepidum, therefore it can not grow From the in vitro study, time-depended absorption spectra of 5-[I,E]BChl c/rand ep are

w

shovm in Fig.3 (a) and (b) and Fig.4.

nr

At 1

1

•

1

400

11

1

•

•

•

•

„, 600 ^ ^ Wavelength (nun)

1 ,J

800

"

400

\K 1

1

•

•

om

— 1

I

•

•

ij

• 1

^

200

0 1

•

400

•

•

•

I

I

I

600 Wavelength (mm)

!

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800

600 WavdeQglh(nim)

Fig.3 Absorption spccUra of the aggregate of S-[I,E]BChl c (a) and S-[IJE]BChl c (b) formed in DMSO-watcr mixture (50:50) recorded at 5 min intervals at 30*'C. CD spectra of the aggregated S.[I,E]BChl c (c) and S-[I^]BChl e (d) in DMSO-watcr mixture (50:50). It was observed that transformation from monomer ((Jy peak: 670 nm) to aggregates ((Jy peak: 740 nm) was given for BChl cp, and that from monomer (Qy peak: 660 nm) to aggregates (Qy peak: 700 nm) was given for

BChl ep. The aggregates of BChl ep showd the Qx absorpticm at

516 nm but those of BChl c^did not. We found the increase of the Qx absorption and a red shift of the Qy peak of BChl ep aggregates were attended with the alkylation at the position 8 and 12 of the BChl (data not shown). Fig.3 (c) and (d) showed the CD spectra of the final aggregates of 5^[lJE]BChl c/r and eF. The CD spectrum of the 5-[l£]BChl ep aggregates indicated a rather strong exciton interaction among the BChl molecules at the Qx and the (Jy absorption band than

646

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• 120

11 140

Time/min Fig,4 The kinetics of the appearance for Qy absorption maxima of the aggregate of S-[I£]BChl c and S-[I,E]BChl e. that of the S-[I3]BChl Cf aggregates. This interaction can prepare the eflFective energy transfer between BChl molecules. The formyl group at position 7 of the BChl CF might convert the aggregates to make up the interaction. It was reported that the initial stage of aggregation behavior of BChl Cf is the coordination of 3^ hydroxyl group of one BChl Cf molecule to the central magnesium atom of another BChl cp molecule, after that, 13^ keto C=0 group of the other BChl CF molecule coodinated to OH- • • Mg. The BChl ep molecule has the formyl group at position 7 for the coordination site. The alkaylation at position 8 and 12 of the BChl e showed increase when Cb.phaeobacteroides was cultured under the lower light intensity and a red shift of the Qy absorption maxima of BChl e increased concomitant with the alkylation (5). Anyone has not reported the effect of the Qx absorption band of the BChl e but they believed that Cb. phaeobacteroides containing BChl ep can live under the lower light condition than CbJepidum containing BChl cp. In this study, we investigated the effect of the formyl groi^) at position 7 of BChl e compared with BChl c. These results suggested that the efficiency of the light harvesting and energy transfer of aggregated BChl ep is superior to that of aggregated BChl cp REFFRENCES 1) J.M Olson, Photochemistry and Photobiology, 67(1), 61-75 (1998). 2) KMSmith, Photosynth.Res.,41,23-26 (1991). 3) K.Okada, E. Nishizawa, Y.Fujimoto, Y.Koyama, S. Muraishi and Y.Ozaki, Applied Spectroscopy, 46,518-523 (1992). 4) JMOlson and J.PPedersen, Photosynth.Res.,25,25-37 (1990). 5) Bobe, F. W., N. Pfennig, K. L. Swanson and K. M. Smith, Biochemistry, 29,4340-4348 (1990).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) 'C' 2001 Elsevier Science B.V. All rights reserved.

647

Improved Molecular Models for Porous Carbons J. Pikunic', R. J.-M. Pellenq^ K.T. Thomson''" J.-N. Rouzaud\ P. Levitz^ and K.E. Gubbins' ' Department of Chemical Engineering, North Carolina State University, 113 Riddick Labs, Raleigh, NC 27695-7905, USA ^ Centre de Recherche sur la Matiere Divisee (UMR 131 CNRS), IB rue de la Ferollerie, 45071 Orleans Cedex 02, France We describe two approaches based on Reverse Monte Carlo to model porous carbons. We use these approaches to model two different porous carbons and highlight the features of the resulting structures. In the first approach, RMC moves are applied to a configuration of basic carbon units that resemble the structure of graphene plates. In the second approach, RMC moves are applied at the atomic level, allowing the creation of defects in the carbon plates. 1. INTRODUCTION The widespread interest in porous carbons stems from their high surface activity and consequent adsorption capacity, and from the fact that they are relatively cheap to produce. Activated carbons are the most widely used of all general purpose adsorbents in industry [1]. Industrial uses of activated carbons include sugar refining, purification of drinking water, solvent recovery, deodorization, and purification of air and other gas streams. They have also been used extensively as catalyst supports in chemical processing applications. Porous carbons are disordered materials, and as yet cannot be fully characterized from experiment. Techniques such as X-ray and neutron scattering, and high resolution transmission electron microscopy, give useful partial information about the molecular structure, but are not yet able to provide a complete picture at the atomic level. In order to interpret experimental data on the carbons themselves, and on the behavior of adsorbates in carbons, we must resort to structural models of the pore morphology and topology, in addition to models or the intermolecular forces involved. There are two general approaches to the problem of constructing a molecular model of a porous material. The first, which we term mimetic simulation, involves the development of a simulation strategy that mimics the synthetic process used to fabricate the material in the laboratory (see for example ref 2). The second, termed the reconstruction method, seeks to build a molecular model whose structure matches the available experimental structure data. In

^ Present address: Purdue University, School of Chemical Engineering, West Lafayette, IN 47907-1283, USA

648 the case of most porous carbons, the synthesis process is so poorly understood that mimetic simulation is not feasible. 2. REVERSE MONTE CARLO METHOD The Reverse Monte Carlo (RMC) method was originally proposed by McGreevy and Putszai [3]. The goal of this method is to produce an atomic configuration that is consistent with a set of experimental data. The method consists of changing the atomic positions of some initial atomic configuration through a stochastic procedure. The initial configuration may have either some random structure or a structure generated with some previous knowledge about the material that is being modeled. Using the Metropolis algorithm [4], changes in the atomic configuration, or moves, are accepted or rejected based on the agreement between some simulated structural property and a corresponding target. Throughout the simulation, the differences between the simulated and the target functions are minimized. The most commonly used structural properties in RMC methods are the structure factor, S(q) and the radial distribution function, g(r). If the experimental g(r) is used as the target function, then the quantity to be minimized is: ««P,

'

(1)

/=i

where riexp is the number of experimental points, gsim&i) is the simulated g(r) and gexpfrj is the experimental g(r) evaluated at n. Similarly, if the S(q) is used as the target function, then the quantity to be minimized is:

1=1

where Ssim(qi) is the simulated S(q) and Sexp(qi) is the experimental S(q) evaluated at qi. After each move, the quantity ^ is calculated. The move is accepted with a probability Pace (eq. 3). In eq. 3, P^ is a weighting parameter. The number of accepted moves will be a function of this parameter. Note that when P^ is set to infinity, the moves are only accepted if ^ ^ < ^ / j . P^, = min[l,exp{-/>^ [zL - ^ L ) } ]

^^^

It has been pointed out that different structures can be found by RMC with the same set of structural data [5,6]. Moreover, some of the models generated by RMC may have unphysical features. In order to overcome this problem, one may include some chemical and physical constraints. RMC methods have been used before to study different types of carbons [7-14]. We now present two different approaches to model porous carbons by RMC methods. 2.1. Basic Carbon Unit Approach The details of this approach have been presented elsewhere [14]. We used the experimental g(r) as the target function combined with the following constraints: (1) any atom can only have two or three neighbors, (2) all the interatomic distances are 1.42 A, (3) all the bond

649 angles are 120*". When these three constraints are applied together, we can define some basic carbon units. These units are rigid aromatic sheets of sp^ bonded carbon which resemble the structure of graphene plates. The carbon atoms are, thus, disposed in hexagonal rings. We begin by generating an initial carbon structure within a cubic simulation cell with periodic boundary conditions with a target density p. We construct graphene plates by randomly adding rings to the exposed edges of a starting ring. We then perform the RMC simulation by allowing three types of moves: (1) plate translation/rotation, (2) ring creation/annihilation, and (3) plate creation/annihilation. After each move, we use eq. 1 to calculate ^ and accept/reject the move based on the acceptance criterion shown in eq. 3 with the parameter P^ set to infinity. 2.2. Atomistic Approach In this approach, we use the experimental S(q) as the target function and the following constraints: (1) the bond angle distribution is centered at 120°, (2) the distribution of the number of neighbors is centered at 3. Any two atoms are considered neighbors if the distance between their centers is within a minimum and a maximum bond length. These limits are arbitrarily set. This approach is different than the basic carbon unit approach in the sense that the interatomic distances and the bond angles are not restricted to specific values. The advantage of this method is that it allows modeling the local defects observed in real porous materials, such as curvature and roughness. However, the definition of a basic carbon unit is not trivial in this case, and thus each atom must be moved independently. One can think of this approach as a simultaneous minimization of three quantities. One is the usual ^ (eq. 2) and the other two are: 0,--

2;.r

<^)

where Oi are the different bond angles in radians, w^ is the total number of bond angles, N^ is the number of atoms with three neighbors and A^' is the total number of atoms in the simulation box. We begin by generating an initial configuration with a random structure with a target density /7 to ensure that the resuUing model is not biased by the choice of initial configuration. As in the previous approach, we use a cubic simulation cell with periodic boundary conditions. We then perform only one type of RMC move, similar to the Metropolis algorithm for MC simulations in the NVT ensemble. After each move, we calculate the three quantities to be minimized, ^, y?, ^. If the move causes any two atoms to be at a distance less than the minimum bond length, the move is rejected. Otherwise, we accept/reject the move based on the following acceptance criterion:

650

Pacc-rmn[uxp{-[p^{zL-zL)^PMe^-^^^^^^^

(6)

The acceptance criterion now includes the change in the three quantities that are being minimized. This introduces two additional weighting parameters, P^ and P^. The ratio between any two weighting parameters determines the "relative importance" of the structural properties. When the three weighting parameters are set to infinity and the maximum and minimum bond lengths are set to 1.42 A, this method is exactly equivalent to the basic carbon unit approach. 3. RESULTS 3.1. Basic Carbon Unit Approach We used the basic carbon unit approach to model an activated carbon made fi'om coconut shell [15]. We used a simulation cell of 11 nm length with a target density of 1.90 g/ml based on the actual carbon density estimated by helium pycnometry. The simulated and the target g(r) are shown in Fig. 1. A structural representation of this model is shown in Fig. 2.

Fig. 1. Radial distribution function of activated carbon from Fig. 2. Structural representation of coconut shell. Experimental (line) and RMC model the activated carbon from coconut (diamonds). shell. The experimental and the simulated g(r) are in good agreement. However, we observe deviations for the first four peaks (r less than ~ 6 A). In this range of r, the structure of the model is driven by the constraints imposed in the simulation. The three constraints applied in this approach do not allow the appearance of any defects, such as rings of 5 or 7 carbon atoms, which would cause curvature in the plates. The structural representation of this model (Fig. 2.) reveals that the carbon plates are nearly parallel. The structure is very similar to that of a graphite crystal with defects. 3.2. Atomistic Approach In order to exemplify this model, we modeled the glassy carbon formed at a heat treatment temperature of 2500°C presented in the work by O'Malley, Snook and McCulloch [11]. We

651 performed the RMC simulations in a box of 3 nm with a density of 1.57 g/ml. The simulated and the experimental S(q)s are presented in Fig. 3. The structural representation is shown in Fig. 4.

Fig. 3. Structure factor of glassy carbon. Experimental (dots) and RMC model (stars).

Fig. 4. Structural representation of glassy carbon.

The simulated and the target S(q) are in very good agreement. The structural representation reveals that the plates in this model contain a significant number of rings of more than and less than six carbon atoms. These type of defects create the curvature that we observe in all the carbon plates. The bond angle distribution and the neighbor distribution are presented in Fig. 5 and 6, respectively. 0.035 1 ,

1-

0.03 -

^

ll

0.025 0.02 ^0.015 a, 0.01 -

0.8 -

o c o

0.60.4-

1 1

V

\

0.005 0-1 ()

1

2

//

,—-/ 50

,

100 0 (degrees)

Fig. 5. Bond angle distribution.

V T^

150

x> E 3

0.2 -

^C 0^

- ••,l 0

1 2

* 1

3

1

4

1

5

:

r

6

7

8

number of neighbors

Fig. 6. Neighbor distribution.

The bond angle distribution is centered at 120°. O'Malley and co-workers found a small peak in the bond angle distribution at 60° [11] which revealed the presence of rings of three carbon atoms. We observe that this p e ^ does not appear when we include the angle constraint. The addition of this angle constraint also causes a better agreement between the simulated and the experimental S(q) in the q region corresponding to the first and second

652 coordination shell. There is a very low fraction of atoms with four and no neighbors. This feature is unphysical. However, note that the minimum and maximum bond lengths are arbitrary in this model. Any two atoms separated by a distance less than the maximum bond length will be considered as neighbors. 4. CONCLUSIONS We describe two different approaches for modeling porous carbons based on Reverse Monte Carlo. The basic carbon unit approach, presented elsewhere, produces structures that are similar to those of graphite crystals. The graphene plates observed in the models constructed with this method have no curvature. The atomistic approach is useful to consruct models for porous carbons with curved plates. In this approach, we simultaneously minimize the deviation of the simulated S(q) with respect to experimental results, the sum of the differences between the bond angles and the equilibrium angle for sp^ carbon and the fraction of atoms with a number of neighbors different than 3. We are currently working on the generation of TEM pictures for the simulated structures in order to compare with experimental TEM pictures to test these structural models. REFERENCES 1. 2. 3. 4.

M. Smisek and R.S. Cemy, Active Carbon, Elsevier, Amsterdam, 1970. L.D. Gelb, K.E. Gubbins, Langmuir 14 (1998), 2097. R.L. McGreevy and L. Pusztai, Molecular Simulation 1 (1988), 369. M.P. Allen and D.T. Tildesley, Computer Simulation of Liquids, Clarendon Press, Oxford, 1987. 5. R. Evans, Molecular Simulation 4 (1990), 409. 6. L. Pusztai, J. of Non-Crystalline Sol. 227-230 (1998), 88. 7. V.A. Bakaev, J. Chem. Phys. 102 (1995), 1398 8. J.K. Walters, J.S. Ridgen and R.J. Newport, Physica Scripta T57 (1995), 137. 9. J.S. Ridgen and R.J. Newport, J. Electrochem. Soc. 143 (1996), 292. 10. J.K. Walters, K.W.R. Gilkes, J.D. Wicks and R.J. Newport, J. Phys.: Condens. Matter 9 (1997), L457. 11. B. O'Malley, I. Snook and D. McCulloch, Phys. Rev. B 57 (1998), 14148. 12. V. Rosato, J.C. Lascovich, A. Santoni and L. Colombo, J. of Modem Physics 9 (1998), 917. 13. J.K. Walters, K.W.R. Gilkes, J.D. Wicks and R.J. Newport, J. of Non-Crystalline Sol. 232-234 (1998), 694. 14. K.T. Thomson and K.E. Gubbins, Langmuir 16 (2000), 5761. 15. K. Kaneko. Chiba University. Personal communications. ACKNOLEDGEMENTS We thank Prof K. Kaneko, Chiba University, for providing X-ray data for the activated carbon from coconut shell. We thank the Department of Energy for support of this research under grant no. DE-FG02-98ER14847. Supercomputer time was provided under a NSF/NRAC grant (no. MCA 93 SOI 1).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

653

Condensed Phase Property of Methanol in the Mesoporous Silica S.Kittaka^ A.Serizawa^ T. Iwashita^ S. Takahara^ T. Takenaka^ Y. Kuroda*' and T. Mori'' * Department of Chemistry, Faculty of Science, Okayama University of Science, Okayama 700-0005, Japan ^ Department of Chemistry, Faculty of Science, Okayama University, Okayama 700-8530, Japan Adsorbed state of methanol condensed in MCM-41 was studied by analyzing the dynamic properties using quasielastic neutron scattering. Dynamic properties of methanol above monolayer range were markedly suppressed loosing translational motions and overall rotations. 1. INTRODUCnON After development of the mesoporous silica samples having uniform pore sizes like FSM-16, MCM series and SBA series, studies on the molecular processes in a confined space have been acceleratingly developed.^^ Dynamics of liquid molecules in a confined space is an important property related to the various chemical processes in biocells, soils, catalysis etc. Present authors have been working with dynamics of water molecules in the low dimentional spaces, i.e., on the metal oxide surface, in the layered compound, and in mesopores of MCM-41. In these systems hydrogen bonding is a main force to determine the structure and dynamic property of the adsorbed phases. Methanol has been chosen as a next sample to study the role of hydrogen bonding property of molecules in low demensional phases, since it is a simple molecule with a single hydroxyl and a methyl group. Alcohols on the Cr203 was studied by adsorption measurements, FT-IR , and thermal analyses.^^ The exchange of alkyl groupe with one hydroxyl of water looses several properties of adsorbed phases. A two dimensional structuring on the Cr203 surface, which is driven by lateral hydrogen bonding, was lost. Instead a new kind of condensed phase was observed after monolayer adsorption. Present work aims to study the adsorbed state of methanol confined in MCM-41 saniples having various pore sizes by using quasielastic neutron scattering measurements in combination with adsorption and FT-IR measurements. 2. EXPEMMENTAL MCM-41 samples were prepared by the method developed by Beck et al.^^ using tetra-alkyl

654

ammonium halides. The pore sizes were controlled by changing the methylene numbers of the longest alkyl group and determined by N2 adsorption; CIO (2.1mn), C14 (2.8nm), and C18 (3.7nm). Adsorption isotherm of methanol was measured gravimetrically UL5 QA%J^^^^^^ 298K with RUBOTHERM. The changes in a6 hydrogen bond was studied by FT-IR m -v_-^ measurement at decreasing temperatures with JIR-100 (JEOL). Dynamic property was studied by measuring quasielastic neutron scattering (QENS) with the spectrometer AGNES in J AERI.

014^.,^k#^^^

^04

010^ ^"^^

02 A 1^

1

1

02

3. RESULTS AND DISCUSSION Figure 1 shows the adsorption

1

1

adsorpdcxKopoi^inbGl desorption: dosed symbol 1

04

1

1

1

1

08

06

PlPo

Fig. 1. Adsorption isotherms of methanol on MCM-41 with varying pore sizes.

isotherms of methanol on the samples CIO, C14, and C18, which are of typical type IV with two steps. In the monolayer range of adsorption, interaction of methanol molecules with surface hydroxyls (2.66-2.70 OHsnm-^) was similar to that for typical silica surface having 14 surface hydroxyls."^^ In the former two 12 samples, adsorption was reversible, while in the latter slightly irreversible around at the 10 h second step, which is due to cappillary condensation in a larger cylindrical pore. The isosteric heat of adsorption, ^st, determined for the second step were larger than the heat 4 h of liquefaction, L, due to pore size effect. The surface energy hs of adsorbed phase was estimated to be 68 mJm'^by the equation [1] 4000 3600 3200 2800 2400 from ^st. W avenumber/cm'* 2v/i, Fig. 2. FT-IR spectra of methanol in MCM-41 q,-L' [1]

I

V is a molar volume. The calculated value

(C-10) determined temperatures.

at

decreasing

655 was much smaller than 45.1 mJm'^forthe flat surface determined from the surface free energy measurements. The hs value is fairly larger than the surface energy, indicating that loss the monolayer surface upon adsorption of the 2nd layer or condensed phase brings in the more interacting packing of adsorbed molecules than that of the liquid phase. Figure 2 shows the FT-IR spectra of methanol filled in the sample CIO observed at decreasing temperatures. As the temperature is decreased , 3270 cm'^ band grows while 3420 cm* band remains. This change is in contrast to that of bulk methanol in which higher side of the peak decreases markedly when cooled. This supports the idea that methanol phase is heterogenized by the pore surface. Figure 3 shows the neutron scattering spectra of the methanol filled in the sample CIO at decreasing temperatures. Quasielastic neutron scattering probes dynamic properties of molecules as translational and rotational motions. In the present system, observed spectra gave small quasielastic component and was analyzed by fitting with single Lorentzian function L((o,r), as expressed by the relation [2]. ^^^ jtco^ + r^ where S(Q, (o) is a scattering law, d((o) a delta function of energy transfer co. F is a half width at half maximum of the peak, r values are almost constant against momentum transfer, Q. The EISF

(elastic

factor, A)

incoherent

structure

plots of the data gave

decreasing curve with increase in Q, ^^ These facts indicate that dynamic motion is not composed of translational diffusion

but

rotational

one.

The

relaxation time r for the rotational motion of methanol was estimated by

1 T = —

r

oj /me\

[3]

the relation [3]. Here, T is a mean

Fig. 3 Quasi-elastic neutron scattering spectra for MCM-41 (CIO) at (2=150 A'. Solid line is a fitted line and broken line for the quasi-elastic part.

656 value for Q ranges observed. Figure 4 shows the Arrhenius plots of the relaxation time, in which r values for pore filled methanol increase slightly with decrease in temperature. And decrease in pore size gave larger T values. The rotational motion is divided into two parts, Le,, around the C3 axis of the methyl group and around the axis vertical to the main molecular axis (overall rotation). The relaxation times for these two rotations of CD3OH are shown in Fig. 4 as TBI and TB2,

3.5

4

4.5

Fig. 4. Relaxation time of rotational motions of methanol in the MCM-41. C14: Tci4mi. monolayer; values for CH3OH will be smaller than T<:\Ap{y pore filled and CIO: Xci4pf pore filled. Bulk methanol deuterated: TBI, CsroutionOf CD3; TB2. overall these. In the case of monolayer rotation, methanol, rotational motion should be in the CH3 group since hydroxyls of methanol is

respectively.^^ Naturally, corresponding

anchored to the surface hydroxyls. Important is that xtHmi value for monolayer methanol is much higher than TBI and rather close to TB2. This signifies that the rotational motion of CH3 was strongly decelerated by adsorption. It is also noted that rvalues for pore filled methanol are also high and become closer to that of monolayer methanol with decrease in pore size. This sequence of dynamics of inner methanol is ascribed to the deceleration of C3 rotation of CH3 by confinement in smaller pores, since overall rotation must be strongly limited already by their structuring inside the pore as suggested by heat of adsorption. REFERENCES 1. (a) P.L.Uewellyn et aL, Surface Sci., 352 (1996) 468. (b) S. Takahara et al., J. Phys. Chem., 107 (1999) 6965. (c)N. Floquel et al., Proc. Int. Zeolite Conf, 12(1999) 65. 2. S. Kittaka er fl/., Langmuir, 14 (1998)832. 3. H.Yamauchi and S.Kondoh, Colloid Polym. Sci., 266 (1988) 855. 4. J. S. Beck et al , J. Amer. Chem. Soc., 114 (1992) 10834. 5. M. Bee, Quasielastic Neutron Scattering, Adam Hilger, Bristol, 1988. 6. F. J. Bermejo et al, J. Phys. Condens. Matter 2 (1990) 1301.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) cc) 2001 Elsevier Science B.V. All rights reserved.

657

Structure and relaxational dynamics of interfacial water M.-C. Bellissent-Funel Laboratoire L6on Brillouin (CEA-CNRS), CEA-Saclay, 91191 Gif-sur-Yvette, France This paper gives the more recent up to date account of the structure and dynamics of interfacial water as compared with that of bulk water. In particular, neutron diffraction studies and high resolution quasi-elastic neutron scattering studies of water molecules confined in the pores of a hydrophilic model system are presented. The retardation of the translational motions of water molecules is discussed in light of some model of alpha relaxation familiar in the theory of kinetic glass transition in dense supercooled liquids. 1. INTRODUCTION The structural and dynamic properties of bulk water are now mostly well understood in some ranges of temperatures and pressures. Many investigations using different techniques such as X-ray diffraction, neutron scattering, nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), molecular dynamics (MD), monte carlo (MC), simulations have been performed in the deeply supercooled regime [1-4] and in a situation where the effects due to the hydrogen bonding are dominant. In many common and relevant situations, water is not in its bulk form but instead attached to some substrates or filling small cavities. Common examples are: water in porous media, such as rock or sand stones and water in biological material as in the interior of cells or bound at the surface of biological macromolecules and membranes. This is what we define here as the "confined" or "interfacial water". Water in confined space has attracted a considerable interest in the recent years. Understanding of the modification from bulk liquid water behaviour when water is introduced into pores of porous media or confined in the vicinity of metallic surfaces is important to technological problems. In particular, the assessment of perturbation of liquid water structure and dynamics by hydrophilic and hydrophobic molecular surfaces is fundamental to the quantitative understanding of the stability and enzymatic activity of globular proteins and functions of membranes. Water in porous materials such as Vycor glass, silica gel, polymeric membranes, and zeolites have been actively under investigation because of their relevance in catalytic and separation processes [5-10]. A microscopically detailed method for exploring the change in hydrogen-bonding pattems as well as the translational and rotational diffusion constants and residence times of water molecules, when they are near the surfaces is computer molecular dynamics (CMD) [11-13]. Traditionally, the dynamics of interfacial water has been studied by nuclear magnetic relaxation techniques [14]. Results of CMD simulations generally agreed that the dynamics of water molecules on protein and silica surfaces suffer only a mild slowing down compared to bulk water. From the above comparison it seems clear that there are considerable discrepancies in the degrees of slowing down between NMR experiments and CMD. This is

658

especially true for the translational diffusion constant. We therefore had a strong motivation of performing neutron scattering experiments to resolve these discrepancies [15-17]. We studied water in a porous silica glass, Vycor, which is a model hydrophilic system, as functions of level of hydration and temperature. We report here on the more recent results about microscopic structure and single particle dynamics of interfacial water. The high-resolution quasi-elastic neutron scattering data have been analysed using the relaxing cage model.

2. MODE COUPLING THEORY AND RELAXING CAGE MODEL This paper focuses on the relaxing cage model that appears now the more appropriate one to describe the diffiisional dynamics in supercooled and interfacial water. In supercooled or interfacial water due to the reduced thermal energy of the molecule and for water due to the formation of a stabler, hydrogen-bonded cage around each molecule, a molecule can translate a substantial distance only by rearranging positions of a large number of molecules around it. Thus the diffusion is strongly coupled to the local structural rearrangements or the structural relaxation. The usual, Markovian, Brownian-like diffusion is no longer valid in the case of supercooled water. The relaxing cage model uses an idea borrowed from mode-coupling theory (MCT) of supercooled liquids. Mode-coupling theory focuses its attention on the "cage effect" in the liquid state which can be pictured as a transient trapping of molecules by their neighbours as a result of lowering of the temperature [18]. A detailed discussion of the intermediate scattering function F^ ( 2 , 0 for bulk water and its extension to the case of confined water is given in references [4,17,19]. One shows that in the time interval 10"^
(2)

in which the initial decorrelation associated with the motion in the cage (first term) is followed by a stretched exponential decay (second term). The slow relaxation is characterised by a correlation time x, related to the lifetime of the cage and a stretched exponent p. A (Q) is the so-called Debye-Waller factor (DWF) which has a Gaussian shape. This implies that the short time dynamics of interfacial or supercooled water is, to a good approximation, harmonic. A{Q) = txp(~Q'a')

(3)

where a is the root mean square vibrational amplitude of water molecules in the cage in which the particle is constrained during its short time movements. From the results of the MD simulation of the SPC/E water at supercooled temperature, a is found to be about 0.5 A. Thus for a Q-range between 0 and 1.0 A" 1, applicable to high resolution QENS spectra, the DWF in Eq.3 is very nearly unity. Thus in Eq. 2 the contribution of the first term on the right hand side is negligible. We can thus write with a good approximation the final intermediate scattering function as:

F,(G.r)=A(Q)exp|-fiT

(4)

659 3. SAMPLES Vycor brand porous glass n° 7930 is a product of Coming Glass Works [20]. The average pore size in Vycor 7930 is 50 A, as stated by the manufacturer. The procedure for the preparation of the Vycor samples is described in ref. [15]. At full hydration, a Vycor glass absorbs water up to 25% of its dry weight. A partially hydrated sample is then obtained by absorption of water in the vapour phase until the desired level of hydration is reached. 4. RESULTS AND CONCLUSION 4.1. Structure of water in Vycor The structure of D2O in Vycor has been studied by neutron diffraction as a function of level of hydration of Vycor. The experiments have been performed on the 7C2 spectrometer of the Orphee reactor of the Laboratoire Ldon Brillouin at Saclay (France) for a Q ranging between 0.6 and 16 A'* (Q=47C/X, sin (6/2), X^.705 A and 9 Max=128 degrees). The sample consists of a D20-hydrated rod of 8 nmi diameter and 60 nun height. The structure of water in the fully hydrated Vycor is almost identical to that of bulk water and that of water in the partially hydrated Vycor is of little difference [15] (Fig. 1). It is evident that the three site-site radial correlation functions are indeed required for a sensible study of the orientational correlations between neighbouring molecules.

Fig.l. dL( r) for (a) confined D2O from fully hydrated Vycor (27°C); (b) confined D2O from partially hydrated Vycor (35°C) compared with (c) bulk water (27°C) [2].

Fig.2. Spectrum of cubic ice (dotted line) [2] and confined D2O from partially hydrated Vycor (full hne) at -45°C; there is -36% liquid water still present below the Bragg peaks of cubic ice.

However the evolution of the total functions as a function of temperature is of big interest. In effect, we have shown that he temperature of nucleation of water in Vycor decreases with the level of hydration. For 50% hydrated sample, the deepest supercooling temperature is -27 °C, while for the fully hydrated sample it is -18 °C. As the temperature goes below the limit of

660

supercooling, there is appearance of Bragg peaks of ice. In Vycor, it is worth noting that water always nucleates into cubic ice which is in sharp contrast to bulk water that always nucleates into hexagonal ice (Fig. 2). The nucleation of cubic ice can be explained in terms of distortion of the microscopic structure of water confmed in a hydrophilic substrate [21]. 4.2 Translational mottons of water in Vycor The quasi-elastic neutron scattering experiments have been performed at the High Flux Reactor of the Institut Laue Langevin in Grenoble using the D^5 time of flight spectrometer. The energy resolution (FWHM) at the elastic position was 10 peV and the Q range covered was from 0.15 k'^ to 0. 99 A'^ (using 10 A neutrons). The sample consists of H20-hydrated Vycor rectangular plates of thickness 1.9 nwn and surface area of 32x36 nmi^. Vycor plates are thin enough to ensure that neutron transmission with water inside is 90%. According to the relaxing cage model, the self dynamic structure factor, SsiQ.Q)), which is a time Fourier transform of the stretched exponential function in Eq. 4, shows a sharp line near co=0 with an extended slowly decaying side wing [17]. The procedure of fitting the spectra is described into details in ref. 17. In particular, the structural relaxation rate and the stretch exponent P are obtained. Fig. 3 shows a plot of the structural relaxation rate as a function of Q in log-log scale for 100% hydrated sample at five temperatures, three of the lower ones are supercooled. The values of exponent y are given in the inset for each temperature. One observes that values of y are larger than but not far from 2. In principle, for a sufficiently small values of Q, y should reduces to 2 so that a correct hydrodynamic behaviour is recovered. The Q-dependence of the stretch exponent p for the same sample shows that values of P are significantly below unity for large Q, but approaching unity for Q less than 0.1 A" 1. One can infer from these results that even at the highest temperature measured, the hydrodynamic limit is probably reached for Q only below 0.1 A"!. 1000.0

100.0 h

i i

TOO 0 293K o 278K \^ 268K o 263K [7_258K

Y 2.77 2.40 2.35 2.29 2.03

10.0

OJ

0.8 1i>

Q(A"^

Fig. 3. Evidence of a power-law dependence of the structural relaxation rate on Q, measured with 10 |ieV resolution, in the 100% hydrated sample. This graph shows a log-log plot of l/x vs Q. Within a Q range of 0. 1 -1 A'^ the exportent y, which is the slope of the solid lines joining the data points, seems to be a constant. The corresponding data from MD simulation [19] show that the exponent y is Qdependent, approaching 2 as Q goes below 0.1 k'\

661

Since both xand (3 control the quasi-elastic line shape and have Q dependences, it is convenient to try to combine the two to obtain a single parameter which characterise the structural relaxation. In the case of an exponential relaxation as a result of a continuous diffusion, one has F3(Q,t)=exp(-DQ^).

(5)

For this case, one can characterise the relaxation by its first cumulant (initial slope) which is DQ^ or by its area, which is 1 / DQ^. The two are equivalent. In the case of a stretched exponential relaxation, the first cumulant diverges for p < 1, but the area under the curve is [4] (6)

x=j;dtexp[.(iy] = i r ( l ]

Fig. 4 is a log-log plot of the average relaxation time x as a function of Q. One sees that the average relaxation time has, within this Q range, a power law dependence, x « Q"^ , with an exponent Y approximately equal to 2 at room temperature, similar to the sijnple diffusion case. One may then define an average diffusion constant by the relation x = 1 / DQ and use it to estimate the average diffusion coefficient D. From Fig. 4, for 100% hydrated sample at 293 K, we get Y= -1.95, x = 944 ps at Q = 0.1 A"!. Substituting these two numbers into the above relation, we get D = l.ljclO"^ cw} /sec, compared to the measured self diffusion constant for bulk water at this temperature which is D = 2.0JC10"^ cm} /sec [3], We thus arrive at a ratio D / D = 1.8, agreeing with estimate of Lee and Rossky [11]. We may also say that as far as the single-particle dynamics is concerned, water in Vycor at 293 K behaves as that in a bulk water at 273 K (20 degrees below). As the temperature goes below the freezing point, the exponent y becomes less than 2, indicating a deviation from simple diffusion. /—1^t0.13

0.1

(U

04

OSILBIi)

0.1

•J

I

OJ

^04

•T

»

I

.

I

OJOSIU)

I

I

I

I' I

y*—1.73t0J0

0.1

OJ

04

06 0814)

01

02

04

O6O81.0

Fig.4. Average relaxation time x [defined in Eq.6] for the 100% hydrated sample plotted against Q in log-log scales. It can be seen that the slope is approximately -2 at room temperature.

662

The recent results from neutron scattering experiments [17,22-23] and molecular dynamics simulations [19,24] establish clearly the existence of alpha relaxation in supercooled or interfacial water suggesting that the dynamics of supercooled water can be described in the general frame of the MCT scheme of glass-forming liquids. Acknowledgements The author is pleased to thank S.H. Chen for his active collaboration in the field of confined water. REFERENCES 1. C.A. Angell, Water : A comprehensive treatise, Franks F. Editor, vol.7 (Plenum Press, New-York, London) Chap.l (1981). 2. M.-C. Bellissent-Funel, in ''Hydrogen Bonded Liquids'' (J.C. Dore and J. Teixeira eds.) C329, p. 117, Kluwer Academic Dordrecht, 1991. 3. M.-C. Bellissent-Funel and J. Teixeira, J. Mol. Structure, 250 (1991) 213. 4. S.H. Chen. "'Hydration Processes in Biology : Theoretical and Experimental Approaches", (Ed. M.-C. Bellissent-Funel) lOS Press Publishers. Vol. 305,1999 and references therein. 5. P.M. Wiggins, Prog. Polym. Sci., 13 (1988)1. 6. J.C. Dore, F. Coveney and M.-C. Bellissent-Funel, Recent developments in the Physics of Fluids, (W.S. Howells and K. Soper, eds.), Adam Hilger Pubs., p.299,1992. 7. M.J. Benham, J.C. Cook, J.C. Li, AD.K. Ross, P.L. Hall and B. Sarkissian, Phys. Rev,. B39 (1989) 633 8. L. Bosio, G.P. Johari, M. Oumezzine and J. Teixeira, Chem. Physics Lett., 188 (1992) 113. 9. J.D.F. Ramsay and C. Poinsignon, Langmuir, 3 (1987) 320. 10. V. Venuti, V. Crupi, S. Magazu, D. Majolino, P. Migliardo and M-.C. Bellissent-Funel, J. Phys. IV France, 10 (2000) Pr7-211. 11. S.H. Lee and P.J. Rossky,J.Chem.Phys.,100 (1994) 3334. 12. C.F. Wong and J.A. McCammon, Isr. J. Chem Phys.. 27 (1986) 211. 13. P. Ahlstrom, O. Teleman and B. Jonnson, J. Am. Chem. Soc., 110 (1988) 4198. 14. B. Halle '"Hydration Processes in Biology: Theoretical and Experimental Approaches", (Ed. M.-C. Bellissent-Funel) NATO ASI : Life Sciences Series, lOS Press Publishers. Vol. 305,1999 and references therein. 15. M-.C. Bellissent-Funel, L. Bosio and J. Lai, J. Chem. Phys., 98 (1993) 4246. 16. M.-C. Bellissent-Funel, J.-M. Zanotti and S.H. Chen, Far. Disc. 103 (1996) 281. 17. J.-M. Zanotti, M-.C. Bellissent-Funel and S. H. Chen, Phys. Rev. E 59 (1999) 3084. 18. W. Gotze and L. Sjogren, Rep. Prog. Phys., 55 (1992) 241. 19. S.H Chen,.C. Liao, F. Sciortino, P. Gallo and P. Tartaglia, Phys. Rev., E59 (1999) 6708. 20. General information on Vycor Brand Porous "thirsty glass, n^ 7930, Coming Glass Works, is available from OEM Sales Service, Box 5000, Coming, NY 14830, USA . 21. F. Brani, M.A. Ricci and A. K. Soper, J. Chem. Phys., 109 (1998) 1478. 22. S. Dellerue and M.-C. Bellissent-Funel, Chem. Phys.,258 (2000) 315 23. M.-C. Bellissent-Funel, S. Longeville, J.-M. Zanotti and S. H. Chen, PRL, (2000), to appear. 24. M.A. Ricci, F. Bruni, P. Gallo, M. Rovere and A. K. Soper, J. Phys.: Condens. Matter, 12 (2000) A345.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B.V. All rights reserved.

663

Structural Analysis of Water Molecular Assembly in Hydrophobic Micropores Using with in situ Small Angle X-ray Scattering T. Iiyama«, S. Ozeki^ and K. Kaneko*' ^Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan ''Material Science, Graduate School of Science and Technology, Chiba University, 1-33 Yayoi, Chiba 263-8522, Japan The in situ small-angle X-ray scatterings (SAXS) of water adsorbed on pitch-based activated carbon fibers (ACF) were measured at 303K. The SAXS spectrum of ACF of w = l.lnm in the direction of desorption did not overlap that in the adsorption direction, showing hysteresis. The density fluctuation data from Ornstein-Zemike analysis for the SAXS profiles indicated that water molecules form great clusters in the course of adsorption, but desorption proceeds through uniform molecular evaporation. 1

Introduction

The behaviors of water molecules confined in a small space have attracted much attention from biology, geology and chemistry. The surface tension of water depends strongly on the curvature of the gas-liquid interface, leading to the depression of thefireezingpoint of water in a confined space [1]. The macroscopic properties of water confined in the small space are not sufliciently understood fi-om the molecular level yet. Although the adsorption of water by microporous carbon has been studied for many years, the adsorption mechanism is not clearly elucidated. The water-carbon micropore system has a contradictory nature, because the carbon surface is hydrophobic. A predominant water adsorption begins at the middle range of relative pressure. This steep adsorption uptake has been believed to be associated with the cluster formation through Dubinin-Serpinsky mechanism [2]. Recently, liyama et aJ. gave an obvious evidence for the formation of organized molecular assembly of water with in situ X-ray diffraction [3]. In the preceding paper these authors showed the effectiveness of the X-ray diffraction (XRD) method for determining the intermolecular structure of water in graphitic micropores due to the good transparency of carbon against X-ray [3]. An electron radial distribution function (ERDF) analysis for XRD data showed the presence of a well-ordered structure of water in the graphitic micropores at 303K; the adsorbed water has a more

664

ordered structure than liquid, but less than ice and the structure of water molecules in the 0.8nm-pore is more ordered compared with that of l.lnm-pore case at 303K [4]. The authors showed also the effectiveness of in situ XRD method for determination of the intermolecular structure of CCI4 [5] and alcohols [6] in the graphitic micropore . However, still we have no information on the shape and size of adsorbed molecular assemblies. The in situ small angle X-ray scattering (SAXS) can provide these information. It is well known that activated carbon (AC) strongly scatters X-ray at small angles due to the porosity of carbon sample itself [7,8]. The scattering profile of AC at smaller angles changed dramatically with water humidity and the scattering entity water-adsorbed AC in air were discussed [8,9]. We need to measure a more exact SAXS data on water-adsorbed AC in order to discuss the adsorption mechanism of water. We tried to elucidate the adsorption mechanism of water on activated carbon by in situ SAXS measurement [10]. 2

Experimental

Two kinds of pitch-based activated carbon fibers(ACF), P5 and P20 (ADD'ALL Co., Ltd.) were used. ACF have the great micropore volume and considerably uniform slitshaped micropores, compared with conventional AC [11]. The micropore structures were determined by N2 adsorption isotherms at 77K. The water adsorption isotherms were gravimetricaUy determined at 303K after preheating at 383K and ImPa for 2h. We constructed a new in situ SAXS apparatus. The sample chamber was connected to the vacuum-adsorption system. The adsorbed amount can be measured by a volumemetric method simultaneously on the SAXS measurement. We measured SAXS spectra by use of a two-axial three-slit system (Mac Science Model No.3310) with an X-ray tube operated at 35kV and 15mA. A scattering parameter s {s = 47rsin^/A) ranging from 0.035 to I.2A-1 was covered. 3

Result and Discussion

3.1 Pore structures and water adsorption N2 adsorption isotherms at 77K were of Type I. The pore structures were determined by the subtracting pore effect method using the high resolution a^ plots 0.2 0.4 0.6 0.8 1 [11]. The external surface area is negliRelative pressure, P/PQ gibly small compared with total surface area. The micropore widths (w) of P5 and Figure 1: Water adsorption isotherms of ACFs at 303K. D , • ; P20, O, • ; P5. The soUd and P20 were 0.8 and l.lnm, respectively. dotted curves denote adsorption and desorption. The water adsorption isotherms at 303K are shown in Figure 1, that were of Type V. The satiu-ated amounts of water adsorption of P5 and P20 were 290 and 790mg/g, respectively. The densities of water

665 adsorbed on P5 and P20 are 0.86 and 0.81g/cm^, respectively, on the assumption that water completely filled for pore volume that were determined by N2 adsorption. The adsorbed water density smaller than bulk liquid density should be caused by an ordered sparse structure even at room temperature [4]. It is noteworthy that the adsorption hysteresis depends on the pore width. In case of narrow pore system (P5; w = O.Snm), the desorption-course almost overlaps the adsorption one, which has a steep rising part near P/FQ = 0.4. On the contrary, the wide pore system (P20; w = l.lnm) has a remarkable adsorption hysteresis. 3.2

Ornstein-Zernike analysis of SAXS behaviors

We applied the Ornstein-Zernike (OZ) plot to the SAXS data. According to the OZ theory for the samples, the scattering intensity is given by

/(s) = /(0)/(l + ^ V )

2.5

T 2

1 1\

1.5

(1) 1 C

—i""*

where f is the OZ correlation length and 7(0) is the zero-angle scattering intensity. 11 0.5 The plots of the water adsorbed ACF are _ _i_ . ^ 1 — . — — linear over the whole fractional fiUing (6) 0 0.2 0.4 0.6 o.s 1 rr.i_ 1 to v^/ Fractional filling,^ region. I n e zero-angle scattermg mtensity, 7(0), is directly associated with the I'ig^e2: The zeroangle X-ray scattering intensi1 -4. £1 X X//A Ar\2\ /AT c .^ ties 7(5 = 0) of water adsorbed ACFs against the density fluctuation, ({AA^)^)/iV, of the fractional fiLg0 of water at 303K. D • ; P20 system. We can descnbe the change of (yj = i.inm), O, • ; P5 (u; = O.Snm). The solid electron density fluctuation of the system and dotted curves denote adsorption and desorpwith water adsorption using the 7(0). ^i^n, respectively. Figure 2 shows the 7(0) vs. 0 relationships for water-adsorbed ACFs at 303K. The ordinate of Figure 2 is normalized by use of the 7(0) values of carbon samples without adsorbed water in vacuo. The 7(0) of P5 decreases gradually as increases . The 7(0) vs. (j) plots for adsorption and desorption courses are overlapped each other over the whole (j) region. On the other hand, the 7(0) of the wider pore system (P20) increases with (f> until 0 = 0.7 and then drops above <^ = 0.7 for adsorption. The 7(0) vs. (p plot for desorption course is situated at a lower position than that for the adsorption course above <^ = 0.1 and it coincides with that for adsorption course below 0 = 0.1. Accordingly, the 7(0) vs. (f) plots for adsorption and desorption form an explicit loop whose closing 0 agrees with that of the adsorption hysteresis. 3.3 Structural adsorption mechanism The increase of 7(0) with (f) for the adsorption course of P20 indicates the formation of the cluster in micropores. The steep drop of 7(0) above (p = 0.7 should indicate that the vacant pore spgice decreases owing to formation of larger clusters and their merging.

666

On desorption, the 7(0) vs. <j) plot does not change until 0 = 0.1 . This suggests the uniform evaporation of water molecules. Hence, water molecules form the clusters on adsorption, but desorption proceeds through uniform molecular evaporation in case of P20 {w = l.lnm). On the contrary, the 7(0) vs. (f> plot for P5 indicates the uniform adlayer mechanism. The micropore walls of ACF are hydrophobic. Accordingly, water molecules cannot form the uniform adlayer on the pore walls. Then, we presume that water molecules form small unit clusters whose size is assumed to be about 0.5nm. Those unit clusters behave as if they were a single molecule. The unit clusters form uniform adlayer in small micropore of P5. Hence, formation of the adlayer are continuous and 7(0) does not increase with adsorption. Here, water clusters can form a imiform layer structure in the hydrophobic space, as suggested by the recent molecular simulation [12]. The adsorption isotherm of P5 does not steeply rise, which indicates that small clusters have the size distribution such as dimers to pentamers. If water molecules desorb in the form of small clusters having the size distribution, the desorption isotherm can be close to the adsorption one, as observed. As the serious geometrical restriction for formation of the ordered structure in micropores of P5 produces a defective solid-like structure and the intercluster binding is not strong, the smsill cluster can be detached from the adsorbed phase to dissociate into molecules in the gas phase. These results indicate that the formation of water molecular clusters in the hydrophobic spaces. Thus, in situ SAXS examination can elucidate the dependence of adsorption mechanism of water in hydrophobic micropores with the pore width. 4

Acknowledgment The authors thank Professor K. Nishikawa Chiba University for her comments on the SAXS analysis. We acknowledge the Ministry of Education for the Grant-in-Aid for Scientific Research on Priority Areas (Carbon Alloys) of Japanese Government. References 1. R. Defay, L. Pigogine, A. Bellemans and D. H. Everret, Surface Tension and Adsorption (Longman, London, 1966) p.251 2. M. M. Dubinin and V. V. Serpinsky, Carbon 19 (1981) 402. 3. T. liyama, K. Nishikawa, T. Otowa and K. Kaneko, J. Phys. Chem. 99 (1995) 10075. 4. T. liyama, K. Nishikawa, T. Suzuki and K. Kaneko, Chem. Phys. Lett. 274 (1997) 152. 5. T. liyama, K. Nishikawa, T. Suzuki, T. Otowa, M. Hijiriyama, Y. Nojima and K. Kaneko, J. Phys. Chem. B 101 (1997) 3037. 6. T. Ohkubo, T. liyama, K. Nishikawa, T. Suzuki and K. Kaneko, J. Phys. Chem. B 103 (1999) 1859. 7. M.Ruike, T.Kasu, N.Setoyama, T.Suzuki and K.Kaneko, J.Phys.Chem. 98 (1995) 9594. 8. K. Kaneko, Y. Fujiwara and K. Nishikawa, J. Colloid Interface Sci. 127 (1989) 298. 9. Y. Fujiwara, K. Nishikawa, T. lijima and K. Kaneko, J. Chem. Soc. Faraday Trans. 87 (1991) 2763. 10. T. liyama, M. Ruike and K. Kaneko, Chem. Phys. Lett, in press. 11. K. Kaneko, C. Ishii, M. Ruike and H. Kuwabara, Carbon 30 (1992) 1075. 12. K. Koga, X. C. Zeng and H. Tanaka, Phys. Rev. Lett. 79 (1997) 5013.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

667

Synthesis, Characterisation and Chemistry of Transition Metals in Mesoporous Silica Tom Campbell,* Judith M. Corker,**^ Andrew J. Dent,*' S A El-Safty,' John Evans,* Steven G. Fiddy,* Mark A. Newton,* Chee Peng Ship * and Sandra Turin* *Department of Chemistry, University of Southampton, Southampton, SOI 7 IBJ, U.K., ^'CLRC Daresbury Laboratory, Warrington, WA4 4AD, U.K. ^Department of Chemistry, Tanta University, Tanta, Egypt The hexagonal phase mesoporous silica Hi Si02 has been investigated as a support to form a series of transition metal and acid catalysts. Incorporation of Pt wires, uranium oxide and heteropolyacids into the channels was observed. Binding of transition metal ion oxidants was effected using an aminated version of this support. 1. INTRODUCTION Mesoporous silicas offer many attractions as supports for metal catalysts. This study has used the liquid crystal templated silicas of Attard et al [1] to provide high pore order and internal/external surface area ratios. These materials can provide: (i) a spaceefficient array of active sites, (ii) sufficient space for supporting metal complexes, (iii) some size selectivity and (iv) thermal stability for a wide range of working temperatures. However, there are some potential problems: the pores may become blocked during the metal impregnation stage, migration of metals from the pores may occur and the presence of long-range order pore order does not define the distribution of reaction sites on the pore walls. As for amorphous silicas, there is a distribution of isolated silanols, vicinal and geminal silanediols and strained siloxane bridges. The sites may differ within these concave surfaces from being on gentle convex surfaces of sils. The introduction of metal sites onto such supports can be effected in a variety of ways, including adsorption from solution, precipitation from solution, vapour deposition, and tethering onto ligand-ftmctionalised surfaces. Metallic or oxide catalysts may then be formed by post-adsorption pretreatment. In all of these cases, establishing the nature of the sites and their distribution is an important aspect of understanding their reactivity. To achieve this we have combined in situ monitoring techniques {DRIFTS, Energy Dispersive EXAFS (EDE) and mass spectrometry} with a wider array of ex situ characterisation methods (Gas adsorption, powder X-ray diffraction, TEM, EXAFS, NMR, ESR, and UV-vis-NIR reflectance). * This paper is dedicated to the memory of Judith Corker who initiated this research programme.

668 2. INCORPORATION AND CHEMISTRY OF Pt(acac)2 The hexagonal- phase mesoporous silica Hi-Si02 was impregnated with Pt(acac)2 from a dry toluene solution. Ft Lm edge EXAFS and IR spectroscopy indicates that the complex remains intact on this material, as it does on silica and alumina. However, in the HiSi02 alone, there is evidence for a close approach of oxygen to the metal {2 Pt...O at 3.12(2)A}. The thermal degradation and hydrogenation of these samples has been monitored in situ within a microreactor by energy-dispersive X-ray absorption spectroscopy [2]. For all three supports, exposure to hydrogen while operating a temperature ramp, causes conversion from the complex to metallic platinum over a narrow temperature range{110-120°C for HiSi02; 100-120°C for silica; 90-105°C for alumina}, with only a slight increase in stabilisation in the mesopores. More significant differences are evident for thermolyses under nitrogen atmosphere. In mesoporous silica a process begins to be evident by XAFS at 192° and analysis indicates a coordination of -3 Ft at 2.68A and ~ 4 C at 1.97 A. There is a continual reduction in the carbon coordination as the temperature is ramped. A model of a single shell of Ft atoms (--5.3

I Fig. 1 Transmission electron micrograph of Ft(acac)2 on HiSi02 after H2 reduction. at 2.70 A) is observed. There is also evidence from DRIFTS measurements of terminal carbonyl groups being formed as intermediates during the decomposition. A band at 2087 cm"* is apparent by 149^ which is lost by 210°, while a second v(CO) feature is -K evident at 2045cm'* at 178°, and there are shifts to lower frequency (down to 1990cm') with increasing temperature. On amorphous silica, the v(CO) IR band is apparent by 105°C, followed by a terminal band at about 130°C. At 150°C, the EXAFS analysis indicates a mean Ft environment of 2-4 Ft atoms (2.66 A) and --3 C atoms (2.02 A). Somewhat larger clusters become apparent at 250°C (-6 Ft at 2.70 A) than in the case of the mesoporous silica. So the mesoporous structure does influence the decomposition process and the morphology of the metal particles formed.

669 In Figure 1, particles of platinum can be observed within the track of the mesporous channels. Other, larger particles, however, grow outside the channels and reside on the exterior surface of the silica particle. So it appears that, though there is a specific interaction of the precursor, Pt(acac)2, with the channels of the mesoporous silica, during the reduction by hydrogen, migration of the metal atoms can occur.

11200 11300 Ptioton Eneigy (eN/)

11400

11500

11700 11800 11900 Photon Energy (eV)

Fig. 2. Ge K- (top) and Pt Lm (bottom) edge EDE of Pt(acac)2+GeBu4/Hi Si02 silica during heating under 10%H2/N2 under various temperatures (increasing down the plots). The effect of alkylgermaniums on the decomposition pathway could be monitored readily by energy-dispersive X-ray absorption spectroscopy, due to the proximity of their absorption edges: Ge K at 11108 eV and Pt Lm at 11572 eV. Stack plots of the changes in their spectra for a 1:1 Pt(acac)2/GeBu4 on HiSi02 sample during a temperature ramped reaction under 10% H2/N2 is shown in Figure 2. The addition of GeBu4 has an affect on the on platinum formation, increasing the onset to 185®C. At higher temperatures (> 250°C) the EXAFS features of both edges change simultaneously indicating some form of alloying. 3. INCORPORATION OF METAL CATIONS INTO MESOPOROUS SILICAS An alternative method of a metal complex to a support is via a tethering ligand. In this study, HiSi02 was functionalised using H2N(CH2)3Si(OEt)3. IR studies show that the

670

isolated silanol groups are lost by this reaction. ^^Si NMR shows that two main types of binding occur to two and to three neighbouring siloxanes. The first transition row dications for Mn, Co, Cu and Zn were incorporated into the functionalised mesoporous material [3]. During these steps, the intensity of the (100) X-ray diffraction peak is sequentially reduced consistent with a reduction in long range order caused by their disordered inclusion. Spectroscopic studies indicate that the majority of the copper is coordinated as a species of the type shown in Figure 3. Part of the manganese appear to have oxidised to Mn(IV). O— Si(CH2)3NH2

CuiOHjhf"

— ^O-^ \

/ Si(CH2)3NH2

Fig. 3. Main species formed from the incorporation of Cuaq into aminated HiSi02. The ligand-bonded cations were tested for the ability to oxidise aniline derivatives and displayed a reactivity order: Mn^^^^>Cu">Co">Zn". Interestingly, after the amine oxidation reaction, the intensity of the (100) reflections of the mesoporous array increase back to those of the original silica, indicating that there has been a relief of the disorder imposed by the incorporation of the metal. A supported ruthenium(III) material was also synthesised fi-om RuCb, and was found to have intermediate reactivity between the manganese and copper materials. Since Ru(in) has a t2g^ electronic configurarion, it is substitutionally inert, but can readily undergo an outer sphere electron transfer reaction, indicating that this is the likely mechanism of these redox reactions. 4. ACIDIFICATION OF MESOPOROUS SILICA Two of the reagents that have been used to create supported solid acids have been the sulphonic and heteropolyacids (HPAs). Both of these may in principle by applied to the acidification of HiSi02 since the pore sizes are close to twice Uie diameter of the Keggin ion structures. One of the most acidic HP As is H3Wi2P04o- Incorporation into the pores by impregnationfi*omsolution has resulted in very low levels of uptake, as compared to Aerosil 200. Under comparable conditions, the loading was 0.14 molecules/nm^ on Aerosil 200, but only 0.044 molecules/nm^ on HiSi02. Pyridine adsorption was monitored by DRIFTS (Figure 4a) and this showed adsorption as hydrogen bonded sites on silanols and also as the pyridinium ion at the acid sites. Little of the latter was observed using the mesoporous silica, presumably due to the low loading. P NMR studies of P0(0Et)3 adsorption do suggest enhanced acidity within the pores of the few HPA molecules residing there. In contrast, adsorption pyridine onto materials pretreated with MeSOaH onto both silicas only showed protonation to the pyridinium ion, showing

671 4A) «t room tcmpcniture

%T

HPA deposited on Aerosil 200 Pyridiniuin ion

MeSOjH supporteiLoD Aerosil 20(

TTK ' 1486

162M6

%T

^

.

'

•

•

k^v,.i 1

"•

"

/

V,

1 ••"'

MeSOaH sfipported on HiSiOi

\^'r\V,yAM^-^^'

',

t

U'

!

y'

Fig. 4 a) Sections of DRIFTS spectra showing the pyridine adsorption sites on silicas before and after treatment with H3W12PO40. b) DRIFTS spectra of pyridine adsorption on MeSOsH treated Aerosil 200 and HiSiOzessentially saturation of the surfaces with acid sites (Figure 4b). The sulphonic acid sites also show higher temperature stability and thus appear to provide the better method of synthesising acid sites within mesoporous silicas. 5. CATALYTIC ACTIVITY OF MESOPOROUS SILICA SUPPORTED URANIUM OXIDES The study of supported uranium oxide, which is a catalyst for the selective reduction of NO, offered an opportunity to investigate the effect of using mesoporous silica as a catalyst support over amorphous alternatives. The wet-impregnated catalyst precursor, uranyl nitrate, was shown by in situ X-ray powder diffraction to convert to the uranium oxide U3O8 on calcination in air. This conversion occurred at 590**C for the amorphous alumina supported material, at 600**C for the amorphous silica analogue, but at a

672

reduced temperature of 490**C when mesoporous silica was used as the support. This suggests that mesoporous silica destabilises the uranyl nitrate phase with respect to U3O8, and under standard catalyst preparation conditions, 800°C in air, this leads to a greater extent of sintering of this phase. This results in much larger uranium oxide particles when compared to those formed on the amorphous analogues. The effect of this reduction in the surface area of the active uranium phase, is that the mesoporous silica supported material pre-treated at 800°C is a less active catalyst for the selective reduction of NO with CO than the amorphous support counterparts. This is despite a threefold increase in support surface area for the mesoporous materials. However, if the supported catalyst precursor is not calcined prior to reaction, but instead, the active uranium oxide phase is allowed to form under an atmosphere of the reactants CO and NO, then the catalytic performance of the mesoporous silica based material is superior to that of the amorphous analogues, regardless of their preparation conditions. It is suggested that this is due to the uranium oxide formation conditions, where the catalytic reaction itself prevents the sintering of the active phase, and allows the higher surface area of the mesoporous silica to become the dominating factor. This dependence on pre-treatment conditions is presented in Figure 5.

400 Temperature/C

Fig.5 %N0 conversion as a function of reaction temperature for the mesoporous silica supported uranyl nitrate system; a) with no pre-treatment; b) pre-treated at 450*^0; c) pre-treated at 800**C ACKNOWLEDGEMENTS We wish to thank the EPSRC, University of Southampton, the Egyptian Government, the British ORS scheme, BP Chemicals, and BNFL for support. We wish also to thank the Director and staff of the CCLRC Daresbury Laboratory for access to their facilities and technical support. REFERENCES 1. G. S. Attard, J. C. Glyde, and C. G. Goltner, Nature, 378 (1995) 366. 2. S. G. Fiddy, M. A. Newton, A. J. Dent, G. Salvini, J. M. Corker, S. Turin, T. Campbell, and J. Evans, Chem. Commun., (1999), 851. 3. J. Evans, A. B. Zaki, M. Y. El-Sheikh, and S. A. El-Safty, J. Phys. Chem, in press.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) C) 2001 Elsevier Science B.V. All rights reserved.

673

Ethylene hydrogenation on fee ultra thin Fe films on a Rh(lOO) surfaee - Effeet of eo-adsorbed CO and growth temperature C. Egawa *, H. Iwai and S. Oki Dept. of Energy and Environ. Sci., Utsunomiya University, Mine 350, Utsunomiya 321-8505, Japan The effect of co-adsorbed CO on the bonding of hydrogen atoms and the hydrogenation of ethylene on 1 ML Fe thin fihn on Rh(lOO) has been investigated by AES, LEED and TPD methods. Co-adsorption of CO caused a desorption peak of hydrogen at 150 K at the expense of the state at 210 K. This weakening of surface-hydrogen bond promoted the formation of ethane from the reaction v^th ethylene at 140 K. The ethane formation at low temperature was also observed in the absence of co-adsorbed CO from the Fe thin fihns grown above 500 K, which produced hydrogen desorption at 150 K in addition to the state around 300 K. Accordingly, it is concluded that the activation of hydrogen atom inducing the formation of ethane at low temperature is attributed to the decrease in the bonding energy of hydrogen atom principally due to the electronic effect. 1. Introduction Recently, it has been shown that the reaction of ethylene with pre-saturated hydrogen atoms on bcc Fe(lOO) surface produces surface ethyl species which are released as second desorption peak of ethylene upon further heating [1]. However, the presence of co-adsorbed CO destabilizes adsorbed hydrogen and enhances the formation of ethane [2]. Similar two desorption states of ethylene have been observed from the adsorption of ethylene on H atom pre-saturated 1 ML Fe thin films grown on Rh(lOO) surface [3]. This 1 ML Fe thin fihn greatly reduced the bonding energy of hydrogen atom due to the reduced electronic density of states just below the Fermi energy and evolved two states of ethane even in the absence of CO as similarly as in the desorption of ethylene [4]. We have shown H-D exchange reaction was mainly observed in the higher temperature state of ethylene, indicating the hydrogenation and dehydrogenation through an ethyl intermediate. In this study, the activation of adsorbed hydrogen atom for ethylene hydrogenation has been investigated by changing the coverage of co-adsorbed CO or the growth temperature of Fe thin fihn.

Financial support by Toray Science Foundation and the Ministry of Education, Science and Culture is gratefully acknowledged.

674

2. Experimental All the experiments were carried out in an UHV system with facilities of LEED, AES and TPD as described elsewhere [4]. TPD was taken by a QMS by monitoring several masses simultaneously in a heating rate of 3 K/s. Fe films were grown by evaporation of a Fe plate. For each TPD experiment, a fresh Fe fihn was prepared on the Rh(lOO) surface at 300 K after the sample was cleaned by repeated Ar ion bombardment and subsequent annealing procedures. The 1 ML Fe film showed a (1x1) structure dictated by the Rh(lOO) substrate. After evaporation of 1 ML Fe film, surface impurities (mainly CO) level was close to the coverage of 0.03. 3. Results and discussion Fig. 1 shows CO TPD spectra from the adsorption at 100 K on 1 ML Fe film as a function of CO coverage. There are three molecular desorption peaks at 200 K, 360 K and 450 K sequentially filled in addition to a recombination state at 710 K. The coverage of these four peaks is obtained as 0.2, 0.25, 0.05 and 0.08, respectively, based on the saturation coverage of CO on clean Rh(lOO) as 0.5 [5]. They are well corresponding to those reported on bcc Fe(lOO) surface[6], where three 100 200 300 400 500 600 700 800 molecular adsorption states and one Temperature / K dissociative state are observed at 200 K, 300 K, 420 K and 800 K. It indicates Fig.l CO TPD spectra after adsorption that these states can be assigned in a of CO at 100 K on 1 ML Fe thinfilmsas similar way as on bcc (100) surface; afimctionof increasing CO coverage. fi-om the low temperature side CO desorption occurs fi-om on-top and bridged sites, whereas the latter two desorption states are arising from molecular CO lying dovm in four-fold hollow sites [7]. The weakened bonding energy of CO at four-fold hollow site, however, leads to a decrease of its saturation coveragefi^om0.25 on bcc (100) surface to 0.13 on Fe 1 ML fihn and a shift of the desorption temperature by 90 K. Adsorption of hydrogen at 100 K on 1 ML Fe film grown on the Rh(lOO) gave a desorption peak at 210 K as shown in the bottom curve of Fig. 2. The desorption temperature is greatly lowered firom 290 K on bcc Fe(lOO) and/or 320 K on Rh(lOO) as similar to adsorption of CO mentioned above. This weakened Fe-H bonding is induced by the fomfiation of an interfacial electronic state between Fe overiayer and the Rh substrate [4]. Fig. 2 displays H2 TPD spectra obtainedfix^mpost-adsorption of CO on H pre-saturated 1 ML Fe

675 film. The post-dosed CO coverage was determined simultaneously in the TPD measurement. The post-dosed CO induced a new desorption peak at 150 K together with reduction of the peak at 210 K. The total coverage of hydrogen stays almost constant, which indicates no displacement of H-atoms by post-dosed CO. By the comparison with Rh(lOO), saturation coverage of H-atom is obtained as 0.8 on 1 ML Fe film. Thus, the strength of Fe-H bond is further reduced by 20 kJ/mol in the presence of co-adsorbed CO. The effect of post-dosed CO on the hydrogenation of ethylene is then examined by 100 150 200 250 300 350 400 450 TPD after the adsorption of ethylene at 100 K Temperature / K on H-preadsorbed 1 ML Fe followed by CO Fig. 2 H2 TPD spectra after adsorption dose. Fig. 3 demonstrates two peaks for both of of hydrogen on 1 ML Fe thin films. ethylene and ethane (small dots) evolutions. With increasing post-dosed CO coverage, ethane formation at 140 K grows at the expense of that at 180 K in a similar manner as the change in the hydrogen desorption. The ethane formation at 140 K is thus enhanced by the destabilized H-atom probably due to the lowering of an activation barrier. On the other hand, the post-dosed CO decreased the populations of ethylene at 130 K and 200 K by displacement. With the decrease in the concentration of the ethylene peak at 200 K, a sharp H2 desorption peak at 220 K disappeared in the simultaneous TPD. It supports that these are conmionly derived from P-H elimination of surface ethyl species and ethane formation at 180 K occurs through fiirther hydrogenation of this ethyl intermediate as proposed previously [3]. Similar two desorption states of ethylene were reported at 160 K and 220 K in TPD from adsorption of 100 150 200 250 300 350 ethylene on the H-saturated bcc Fe (100) Temperature / K [1]. However, the formation of ethane is only induced by co-adsorbed CO on the Fig.3 TPD spectra of ethylene and ethane Fe(lOO) surface[2], which lowers the from adsorption of ethylene at 100 K on activation energy barrier of ethane H-preadsorbed 1 ML Fe thin films.

676

formation to 20 - 25 kJ/mol in a similar manner as in this study. The effect of co-adsorbed CO on adsorption state of H-atom has been investigated by the combination of HREELS and TPD methods on bcc Fe(lOO) surface[8]. It has been shown that the post-dosed CO is forced to adsorb molecularly at on-top and/or bridged sites, because every four-fold hollow site, as the most stable site for adsorbed CO, is blocked by Hatoms. As a result, each CO molecule destabilizes the nearest H-atoms by a short-range CO-H repulsive interaction, inducing the movement of H-atoms from four-fold hollow site to asymmetric three-fold position corresponding to the increase in the Fe-H stretching frequency in HREEL spectra. However, it is not evident that the short-range CO-H interaction is due to geometrical direct repulsion or electronic effects mediated by the substrate. In order to study the origin of the destabilization of surface H-atoms, we have performed TPD experiments on Fe thin films as a function of growth temperature. Upon the growth temperature above 500 K, significant change was observed on CO and hydrogen desorption spectra as shown by the symbols of open circles in Figs. 1 and 2. The development of CO desorption peak at 450 K and hydrogen desorption state around 300 K is quite resemble to the desorption feature from clean Rh(lOO) surface. However, since surface Fe coverage measured by AES stayed constant, it is reasonable that these desorption states occur from the modification of the surface electronic structure of Fe thin film. In consistent with the existence of the hydrogen desorption peak at 150 K, significant amount of ethane evolution was only observed at 150 K and ethylene desorption at low temperature increased due to the absence of CO. Accordingly, it is concluded from these results that the activation of hydrogen atom inducing the formation of ethane at low temperature is attributed to the decrease in the bonding energy of hydrogen atom mainly induced by the electronic effect of Fe thin film.

References 1. M.L.Burke and R.J.Madix, J. Amer. Chem. Soc. 113 (1991) 3675. 2. M.L.Burke and R.J.Madix, J. Amer. Chem. Soc. 113 (1991) 1475. 3. C.Egawa, S.Oki and Y.Murata, Surf. Sci. 304 (1994) L488. 4. C.Egawa, Y.Tezuka, S.Oki and Y.Murata, Surf. Sci. 283 (1993) 338. 5. B.A.Gumey, L.J.Richter, J.S.Vilarrubia and W.Ho, J. Chem. Phys. 87 (1987) 6710. 6. M.L.Burke and R.J.Madix, Surf. Sci., 237 (1990) 20. 7. D.W.Moon, S.L.Bemasek, J.P.Lu, J.L.Gland and D.J.Dwyer, Surf. Sci., 184 (1987) 90. 8. P.B.Merrill and R.J.Madix, Surf. Sci., 347 (1996) 249.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors)

677

Highly isolated and dispersed transition metal ions and oxides studied by UV resonance Raman spectroscopy Can Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics Chinese Academy of Sciences, Dalian 116023, China Email: [email protected] Fax: 86-411-4694447

Highly isolated (atomic level) and highly dispersed (nano scale) transition metal ion and oxide species are the active sites or the active phases of catalysts for many kinds of reactions. It is a key issue to characterise the active sites in the highly isolated and dispersed forms in order to gain an insight into the catalytic nature. This presentation focuses on the identification and characterisation of the highly isolated transition metal ion and oxide species in the framework of molecular sieves and on supports by UV resonance Raman spectroscopy. Two kinds of typical examples are shown: Ti, Fe, V ions (oxide) highly isolated in molecular sieves, e.g., TS-1, V-MCM-41, and Fe-ZSM-5, and molybdate species highly dispersed on alumina.

1. INTRODUCTION 1.1. Characterisation of highly isolated and dispersed species It is a challenging task to obtain the information on the incorporation and the coordination of the transition metal ions and oxides in molecular sieves [1] and on supports with high surface area since their concentration is usually very low and various surface species may coexist. This is not only an interesting topic of catalysis, but also an important subject for material science because the highly isolated or dispersed species may play an important role in the modification of the material properties. The transition metal ion substituted in molecular sieves, for example, a crucial issue of synthesis and catalysis of molecular sieves [2], has not been well characterised although a number of techniques have been used to tackle the problem. Raman spectroscopy has been considered to be a potentially powerful tool to supply the structural information on the transition metal ion and oxide species highly isolated or dispersed. 1.2. Conventional Raman spectroscopy applied in catalysis Raman spectroscopy is an important spectroscopic technique for characterising the molecular structures. It has been extensively applied to the study on the issues of chemistry, physics, biology and material science [3]. The visible laser lines are usually used as the

678 excitation sources for conventional Raman spectroscopy. Unfortunately the fluorescence frequently occurs in the visible and near-UV regions. The intensity of the fluorescence is higher than that of Raman signal by several orders of magnitude. As a result, the visible Raman spectra are often obscured by the strong fluorescence interference. Another shortcoming of the conventional Raman spectroscopy is the inherently low intensity of Raman scattering. The fluorescence interference is extremely severe for catalysts because the fluorescence impurity is frequently present on the catalyst surfaces. In particular, the hydrocarbon species, which have the strong fluorescence, inevitably derive on the catalyst surfaces under working conditions since most catalytic reactions are involving the carbon-containing molecules. This fluorescence interference often occurs in the Raman spectra of molecular sieves because of the organic template left behind the synthesis. It is hard to obtain the visible Raman spectra once the surface fluorescence is present. Therefore, avoiding or eliminating the fluorescence interference and increasing the sensitivity are the urgent requirements for the effective application of Raman spectroscopy in catalysis, materials science as well as in many other fields. 1.3. UV Raman spectroscopy The conventional Raman spectroscopy has not been widely used in catalysis mainly because of the inherently low sensitivity and strong fluorescence interference often arising from catalyst surfaces. A recent advance in the Raman spectroscopic studies is the successfiil application of UV Raman spectroscopy in catalysis [4] as well as in other fields [5]. It has been demonstrated that the UV Raman spectroscopy has several merits. 1) The fluorescence is avoided successfully by moving the laser line from the visible region to the UV region, because the most fluorescence appears in the visible region (as schematically shown in Figure 1). 2) The sensitivity will be increased significantly since the Raman scattering intensity is inversely proportional to X"^, where X is the wavelength of the Raman scattering. 3) UV resonance Raman spectra (UVRRS) can be obtained by exciting the electronic states with ultraviolet laser lines since the electronic transition of chemical compounds mostly occurs in the UV region. In UVRRS the Raman intensity of some bands can be increased by several orders of magnitude, so that the local structure of a complex system, for example, heterogeneous catalysts, can be selectively identified. The advantages of the UV Raman spectroscopy over conventional Raman spectroscopy are confirmed by a number catalyst examples [6]: zeolites(ZSM-5, USY) [4], coked zeolites [7], superacid catalyst (sulfated zirconia) [8], transition metal substituted zeolites [9, 10] and supported oxides [11, 12]. This paper reviews the applications of UV Raman spectroscopy in catalysis and materials science. The emphasis is placed on the study of highly isolated transition metal ions incorporated in the framework of molecular sieves, and extremely low loading of transition metal oxides supported on alumina. The UV Raman spectroscopy is demonstrated to be a very useftil technique for the characterisations of catalysts and other materials thanks to avoiding the fluorescence interference and enhancing the sensitivity.

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2. RESULTS AND DISCUSSION 2.1. Isolated transition metal ions incorporated in molecular sieves Zeolites and mesoporous materials substituted with transition metal ions are a new class of catalysts showing potential applications in the selective oxidation of hydrocarbons. The extensively studied are the titanium-substituted silicalites (e.g., TS-1) which are active and selective for a number of selective oxidation reactions, such as, hydroxylation of phenol and epoxidation of olefins using dilute H2O2 as the oxidant. It is commonly believed that the framework titanium ions are responsible for the excellent catalysis. However, it is hard to tell how and whether the titanium ions are incorporated in the framework of the zeolites although many techniques, e.g., FT-IR, XRD, UV-visible, and NMR, have been used to characterise the framework transition metal ions in TS-1 as well as in other zeolites. UV resonance Raman spectroscopy opens up the possibility to definitely identify the framework transition metal ions based on the resonance Raman phenomenon. The characteristic Raman bands of framework titanium species in TS-1 [9], framework iron species in Fe-ZSM-5 [13], and framework vanadium species in V-MCM-41 [10, 14] can be all selectively enhanced by taking advantage of the resonance Raman effect. Compared with visible Raman spectrum, there are three new bands at 490, 530 and 1125 cm"^ appeared in the UV Raman spectrum of TS-1 (Figure 2) and these bands are totally different from those of Ti02 (Figure 3). The 244.0-nm laser line excites the charge transfer transition between the framework metal ions and framework oxygen ions therefore the Raman bands associated with the framework transition metal atoms are selectively enhanced

680

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Raman Shift / cm''* Fig. 2. Raman spectra of TS-1 excited by different laser lines.

1^ , , , ^ , ^ r 300 500 TOO 9 0 0 1100

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Fig. 3. Raman spectra of TiOi excited by different laser lines.

by resonance Raman effect. This is the direct evidence for the framework Ti atoms in TS-1, based on the resonance Raman effect. The band at 960 cm'^ is not enhanced by resonance Raman effect, indicating that it is not associated with the Ti-O-Si bond directly, but can be assigned to Si-O-Si bonds next to the Ti-O-Si bonds or defects. Similarly, the new bands at 930 and 1070 cm"^ are observed in the UV resonance Raman spectrum of V-MCM-41. These bands are assigned to the vanadium species in the extraframework and framework, respectively, because the Raman bands of vanadium species in the framework and extra-framework are enhanced by resonance Raman effect simultaneously. The framework vanadium species in MCM-41 is in the distorted tetrahedral form, and that in the extra-framework is in the polymerized octahedral form. The concentration of framework vanadium atoms in the zeolite is limited to a certain amount. As the concentration of vanadium species is beyond the limit, the vanadium oxides in the extra-framework begin to appear and their amount increases with increasing the concentration of vanadium ions. UV resonance Raman bands at 516, 580, 1026 and 1185 cm'^ are observed for the iron species in the framework of Fe-ZSM-5. These bands can be detected even if the content of the framework iron is below 0.1 %wt. suggesting that the presence of Fe impurities in the framework of ZSM-5 will greatly enhance the Raman bands due to the resonance Raman effect. This further demonstrates that the UV resonance Raman spectroscopy is a very sensitive technique.

681 2.2. Highly dispersed metal oxide supported on high surface area oxide The supported molybdenum oxides have draw much attention since they are the important catalyst precursors used in chemical industry processes. However, past Raman spectroscopic studies on the supported molybdate were mainly focused on high loading catalysts. For the low loading ones, it is impossible to obtain the Raman spectrum because of the fluorescence interference and the low sensitivity of conventional Raman spectroscopy. A study on low loading molybdate, i.e., highly dispersed molybdate species, will provide more definite information about the interaction between the surface species and the supports. Supported molybdate on y-alumina with extremely low loading is characterised by UV resonance Raman spectroscopy. The resonance Raman effect not only increases the sensitivity by several orders of magnitude, but also gives the information for the coordination structures of surface molybdate species. The surface octahedral molybdate species are detected as well as the tetrahedral molybdate by UV resonance Raman spectroscopy even with the molybdate loading dovm to 0.1 wt.%. This suggests that the octahedral species be formed at all loading together with tetrahedral species on y-A^Os while it was assumed that only tetrahedral species were formed as long as the loading is very low. The genesis of the surface metal oxides prepared by the chemical equilibrium adsorption method is investigated by UV resonance Raman spectroscopy. It is shown that the species adsorbed on the support in the wet state are not always in agreement with that in the

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500

1000

1500

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500 1000 1500 2000 2500 3000

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Fig. 5. UV Raman spectra of MoOa/y-AI2O3 prepared by adsorption in NaMo04 solution.

682 impregnation solution. The surface molybdate species in the wet state is determined by both the pH value of the impregnating solution and the pH value at PZC of the support. During calcination some tetrahedral molybdate species aggregate into the octahedral species for the catalysts prepared by (NH4)6Mo7024 (Figure 4). This is due to the decrease in the surface pH value when NH3 run away from the surface. The tetrahedral molybdate species do not polymerize when the catalyst is prepared with the solution of NaMo04 because the surface pH value hardly changes during calcination (Figure 5). The surface pH value, which is influenced by the pH value of the impregnating solution, the pH value at PZC of the support, molybdenum loading and compensatory cations, is a crucial factor for controlling the coordination structure of the surface molybdate species. 3. SUMMARY In summary, UV Raman spectroscopy broadens the applications of Raman spectroscopy in catalysis owing to avoiding the fluorescence and increasing the sensitivity. The highly isolated transition metal ions in the framework of molecular sieves, such as TS-1, V-MCM41, Fe-ZSM-5, can be definitely identified by UV resonance Raman spectroscopy. The highly dispersed vanadate species in molecular sieves and molybdate species on alumina are also characterised based on the UV resonance Raman effect. The author would like to acknowledge his colleagues who have also contributed to this study, among them are Guang Xiong, Zhaochi Feng, Yi Yu, Pinliang Ying and Qin Xin. This work was financially supported by the National Natural Science Foundation of China (NSFC) for Distinguished Young Scholars (Grant No. 29625305) REFERENCES 1. M. Hartmann and L. Kevan, Chemical Reviews, 99 (1999) 636. 2. G. Bellessi and M.S. Rigutto, Studies in Surface Science and Catalysis, 85 (1994) 177. 3. L.A. Lyon, CD. Keating, A.P. Fox, B.E. Baker, L. He, S.R. Nicewamer, S.P. Mulvaney and M.J. Natan, Anal. Chem., 70 (1998) 34IR. 4. Can Li and Peter C. Stair, Studies in Surface Science and Catalysis, 110 (1996) 881. 5. S.A. Asher, Anal. Chem., 65 (1993) 59A; 65 (1993) 201 A. 6. Peter C. Stair and Can Li, J. Vac. Technol. A, 15 (1997 ) 1679. 7. Can Li and Peter C. Stair, Catalysis Today, 33 (1997) 353. 8. Can Li and Peter C. Stair, Catal. Lett., 36 (1996) 119. 9. Can Li, G. Xiong, Q. Xin, J. Liu, P. Ying, Z. Feng, J. Li, W. Yang, Y. Wang, G. Wang, X. Liu, M. Lin, X. Wang and E. Min, Angew. Chem. Int. Engl. Ed., 38(1999)2220. 10. G. Xiong, Can Li, Z. Feng, J. Li, P. Ying, H. Li and Q. Xin. Studies in Surface Science and Catalysis, 130 (2000) 341. 11. G. Xiong, Can Li, Z. Feng, P. Ying, Q. Xin and J. Liu, J. Catal., 186 (1999) 234. 12. G. Xiong, Z. Feng, J. Li, Q. Yang, P. Ying, Q. Xin and Can Li, J. Phys. Chem., 104(2000)3581. 13. Y. Yu, G. Xiong, Can Li, F.S. Xiao, J. Catal., 194 (2000) 487. 14. G. Xiong, Can Li, H. Y. Li, Q. Xin and Z. C. Feng, Chem. Commun., (2000) 677.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c* 2001 Elsevier Science B.V. All rights reserved.

683

Fluctuations in nano-scale reaction systems: catalytic CO oxidation on a Pt field emitter tip Ronald Imbihl and Yuri Suchorski Institut fiir Physikalische Chemie und Elektrochemie, Universitat Hannover, Callinstr. 3 - 3a, D-30167 Hannover, Germany Fluctuations which arise in catalytic CO oxidation on a Pt field emitter tip have been studied with field electron microscopy. Fluctuation-driven transitions between the active and the inactive branch of the reaction occur close to the bifurcation point terminating the bistable range. Very close to this point one finds critical behavior similar to an equilibrium phase transition. Spatial correlations of fluctuations on different orientations are studied. 1. Introduction Due to the statistical nature of the elementary processes of reaction and diffusion fluctuations are always present in chemical reaction systems [1-3]. As outlined in standard textbooks their relative amplitude varies with (1/N)^^ with N being the number of molecules so that in a macroscopic system with, say 1/100 moles, fluctuations in the particle density are on the order of 10'^^ whereas for a microscopic system with N=100 the amplitude reaches 0.1. In heterogeneous catalysis systems of that size are realized by metal particles of a supported catalyst [4]. In particular, such small numbers of adsorbed particles (range 10^ - 10^) are present on the fecets of a field emitter tip. In recent years we have employed field electron microscopy (FEM) to study the reactioninduced fluctuations which occur in catalytic CO oxidation on a Pt field emitter tip [5-9]. This system was chosen because it has been comprehensively investigated, the reaction mechanism is well known and because it has been demonstrated that effects of the applied electric field are negligible for the conditions under which we investigate the reaction [10]. Mathematical models of various types have been proposed for this reaction system. Commonly mean field models represented by a set of coupled differential equations are used. These mean field models, however, do not exhibit any fluctuations at all due to the deterministic nature of the equations. Monte Carlo (MC) simulations are a technique suitable for studying fluctuations and we therefore compare the experimental results with the results of MC modeling. For small size systems deviations from the results of mean field equations have been predicted [1-3]. For example, in a bistable system of macroscopic size represented by mean field equations the reaction would reside for an infinite period of time on either of the two stable states whereas in a microscopic system fluctuation-induced transitions between the two states can occur. The system Pt/CO+Oi has been intensively investigated in recent years because of interesting spatiotemporal dynamics including rate oscillations, chemical wave patterns and reactioninduced roughening/facetting [11]. All these phenomena have also been observed on a field emitter tip. In order to reduce complexity we studied the fluctuations under conditions where the system is bistable, i. e., we worked at low temperature where the Pt surface structure is to a certain extent frozen in so that adsorbate-induced surface phase transitions are suppressed.

684 Coupling via a mobile adsorbate (CO) leads to spatial correlations between fluctuations on different points of a surface. Synchronized behavior, propagating reaction fronts or more complex forms of spatiotemporal patterns can result. On the other hand, a field emitter tip is structurally very heterogeneous because it exhibits different facet orientations and these orientations will have different reactivities. We thus have competing influences and the net resuh is difficult to predict. In addition to the correlation analysis of local time series we also employ Karhunen-Loeve decomposition (also known as proper orthogonal decomposition = POD) to detect coherent modes in the video-sequences [8]. Here we attempt to present a short overview of the experimental studies of fluctuations in catalytic CO oxidation with FEM. 2. Experimental and methods of analysis Figure 1 displays a FIM (field ion microscopy)- image and a FEM-image of the [100]oriented Pt tip. The magnification of both methods, FEM and FIM, is given by a pointprojection of the tip onto the screen and therefore practically identical. FIM with its high resolution of ca. 3 - 4 A FIM can therefore be used to identify the precise surface crystallography of the surface region which is probed under reaction conditions with FEM with a much lower resolution of ca. 20 A. As in situ method for studying fluctuations we employ FEM because of the lower field strength of « 0.4 V/A as compared to the 1.2 -1.5V/A of FIM (with O2 as imaging gas), so that field-induced effects in catalytic CO oxidation are negligible in FEM as was demonstrated in measurements with on-off duty cycles [10],

Fig.l: Catalytic CO oxidation on a Pt field emitter tip: (a) FIM image showing the crystallography of the Pt tip (T=78 K, imaging gas Ne,fieldstrength F=3.6 V/A). The rectangular window indicates the area in whichfluctuationswere studied, (b) FEM image of the same area (same scale) shown in (a) under reaction conditions (T=310 K, po2=4.0xl0'^ Torr, pco=4xl0'^ Torr, F= 0.4 V A^). (From.ref 7). The FEM images are recorded with a video-camera (40 ms/frame) and digitized with 8 bit resolution. Local time series can be constructed by integrating the intensity in small 20x20 A^ "windows" corresponding to the resolution of FEM. From the time series we obtain the autocorrelation, the cross-correlation and the spatial (two-point) correlation functions. With POD we take advantage of the full spatiotemporal information available with the videosequences [8]. In POD the spatiotemporal signal w(x,t) is represented as a superposition of eigenimages or modes, v^n(x) where the dynamics are contained in the time-dependent coefficients An(t): w(x,t) = ZAn(t) ^„(x), [12]. Primarily, POD is a method of data reduction and noise filtering but applied to complex spatiotemporal dynamics the resulting modes can be viewed as effective degrees of freedom. Thus, in cases where the spatiotemporal dynamics are governed by only a few degrees of freedom, the first few modes akeady capture most of the signal content, i. e., they contain most of the "energy" which is in the system.

685

3. Local fluctuations and critical behavior in experiment Catalytic CO oxidation on a structurally stable Pt surface exhibits two branches of the kinetics: an active branch in which the surface is predominantly oxygen covered so that CO can still adsorb and react, and an inactive branch on which a high CO coverage inhibits O2 adsorption and hence poisons the reaction [11]. In the bistable regime the two stable branches coexist leading to a hysteresis upon variation of the bifurcation parameter. The bifurcation diagram showing the range in which the reaction is bistable and exhibits a hysteresis is displayed in Fig. 2 [7]. Remarkably, although the various orientations on the tip differ quite strongly in their reactivity (due to different oxygen sticking coefficients) [11], fast CO diffusion apparently ties the different facets together so that the tip behaves as one dynamical system during such transitions. At the left and right boundary of the bistability range the whole tip becomes CO covered and oxygen covered, respectively. 10-^

Fig. 2: Bifurcation diagram for catalytic CO oxidation on a [100]-oriented Pt tip at p02= 4.0x10^ Torr. The inset shows the hysteresis in local FEM brightness (20x20A^) for the area marked in Fig. la upon cyclic variation of T: filled triangles - heating, empty triangles - cooling. (From ref 7).

s I

8 3.1

3.2 3.3 1/T[1000/K]

3.4

3.5

The brightness in FEM varies with the local work function (WF) and the CO covered and bare surface (low WF) are therefore imaged as bright areas whereas the oxygen covered surface (high WF) appears dark. The different time series displayed in Fig. 3a correspond to a COcovered (inactive) and an oxygen covered (active) surface in the monostable range (a,b in Fig.2) and to states in the bistable range (c,d in Fig. 2) of the reaction. From the time series, probability distributions of the intensity fluctuations have been constructed (Fig. 3b). In the monostable ranges (a,b) relatively narrow distributions of roughly Gaussian shape are found, but in the bistable range the distributions become rather broad. On the active branch, the peak just broadens and becomes slightly asymmetric (c), but on the inactive branch (d) the distribution actually becomes bimodal. This bimodal distribution is evidence for fluctuation (noise)-induced transitions between the two "stable" states, where the system is typically in one of these states, and spends comparatively little time in transition between them. Of decisive influence for the amplitude of the fluctuations is the proximity to the cusp point C in Fig. 2 which terminates the bistable range of the reaction. As demonstrated by Fig. 4 by data taken from the Pt(112) facet upon approaching C the amplitude of the fluctuations increases drastically [9]. As supported by simulations the cusp point C plays the role of a critical point in an equilibrium phase transition, i. e. one observes an increase in correlation length and critical slowing down [9].

686

200 w 0 c © g §

200 100

E -100 S -200 c 100

(a)

0.05

~5

0 W*i*>>m^i<m i»j -100 -200 100 0

t/v/*yW^^^^^ I

-

t

kt'''^0^Mr*^^

1 -100 a -200 •g 20

25

30 time [s]

35

40

50

100

amplitude [a.u]

Fig. 3: Fluctuations in catalytic CO oxidation on Pt(110) under dififerent reaction conditions. (From ref. 7). (a) Time series of the local (20x20 A^) FEM brightness in the area marked in Fig. la The data were recorded at different points marked on the hysteresis loop in the inset of Fig. 2. (b) Probability distributions corresponding to the time series shown in (a). (From ref 7).

Fig. 4: Critical behavior offluctuations.Shown is the increase in the average amplitude of the local FEM brightnessfluctuationson a Pt(l 12) facet with decreasing distance to the critical point C in the bistability range. (From ref 9).

4. Spatial correlations The Pt tip is structurally a very heterogeneous system because it exhibits facets of different orientation and hence different reactivity which are coupled together via fast CO diffusion. Atomic steps terminating low-index planes will, however, slow down CO diffusion and it is therefore a priori not clear to what extent and when the facets will act as independent entities and when they will be coupled together so strongly so that they behave as one single dynamic system. From local time series recorded at different positions we can construct spatial (two-point) correlation functions. We find that over a single facet the fluctuations are typically well correlated but that no correlations exist between fluctuations on different facets unless we are in close proximity to the critical point [5-9,13]. The spatial correlation on the Pt(lOO) and Pt(l 11) facets are displayed in Fig. 5 for different regimes of the reaction [6].

687

K

PKlll)

1

\ ^=^^^:^:r-~-
" ' • • ' • • • • - - — ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

) is

\'

0-5 i

0

"^"^^^"' 1

b

V ^ >

'i

^H

^

20

40 Distance (A)

60

80

Fig. 5: Comparison of the spatial correlation (two-point correlation function) for fluctuations on a P^lOO) facet and on a Pt(lll) facet. The different curves represent different reaction conditions (curves a and c corresponds to me points a and c, curve b to the point d and curve d to the point b of the inset of Fig.2). The distance zero refers to the left edge of the facet. (From ref 6). We first consider Pt(l 11). On the CO-poisoned surface in the monostable range we find only very short-ranged correlations limited to < 20 A (curve a). The correlation is practically identical to that for pure CO, i. e., the reaction plays no role. On the active branch in the monostable range (curve d) the correlation is practically the same as on the inactive branch despite the fact that the surface is oxygen covered under these conditions. Far reaching correlations we observe in the bistable range of the reaction for both, the active (curve c) and for the inactive branch of the reaction (curve b). In all cases the correlation decays very rapidly when we enter a neighboring facet (not shov^ in Fig. 5). The spatial correlation depends also on the proximity to the critical point C and upon approaching C we find, in general, an increase in correlation length. When we study under identical p,T-parameters the fluctuations on different orientations we see that their fluctuation behavior is quite different: the amplitude distribution functions for varying reaction conditions are different for different orientations [6]. The differences are also reflected in the spatial correlation functions displayed in Fig. 5. On Pt(100), e.g., the correlation of the active monostable branch (curve d) is over a large part of the region much higher than on Pt(lll). The similarity between the correlation functions for the active and inactive branch in the bistable range (curves b and c) is by far less pronounced on Pt(100) thanonPt(lll). The differences in the fluctuation behavior of different facets can be rationalized if we take into account that despite identical p,T-parameters different facets can be in different dynamic regimes due to their differences in reactivity. Coupling via CO diffusion is apparentiy not strong enough to synchronize the fluctuations on different facets which retain their local character. Quite in contrast, upon crossing one of the global bifurcation lines in the existence diagram in Fig. 2, the whole tip undergoes simultaneously the transition to a CO covered and oxygen covered surface, respectively. During fluctuation-induced transitions tiie changes in coverages occur well correlated over the size over a facet [8]. In POD this synchronized behavior is reflected by tiie dominance of one mode which typically captures already 70 - 90% of the total energy alone. Different interpretations are possible to explain this observation. One possibility is of course a true spatially homogeneous transition but it may also well be that the long-reaching correlation is just caused by reaction fronts that sweep across the surface in a time too short for the time resolution of a standard video camera (40 ms). So far no reaction fronts at all have been

688 observed in our studies of fluctuations. If we take a typical front velocity of 1 ^m/s observed on macroscopic single crystal planes then this front would move across a 100 A wide facet in 1/100 s [11]. This is below the time resolution of a standard video equipment but it is well within the capabilities of digital cameras which can take several hundred images per second. 5. Simulations Atomistic lattice-gas models are well suited for modeling fluctuations [14,15]. Numerous Monte Carlo simulations of catalytic CO oxidation have been carried in the literature most of them based upon the ZGB (=Ziff-Gulari-Barshed) model [14]. The ZGB model, however, is unrealistic because it allows for oxygen poisoning in contradiction to experimental results and it neglects essential processes like CO diffusion. Moreover, the interactions between adparticles are not considered. In order to obtain a more realistic model which accounts for the high mobility of adsorbed CO a hybrid model was constructed by Evans et. al [7,9]. Adsorbed CO is represented by a single (mean field) variable whereas the distribution of adsorbed oxygen is described by a full lattice gas model, i. e. it is assumed that CO diffuses infinitely fast. The simulations reproduce very well the different amplitude distributions for varying reaction conditions demonstrated in Fig. 3. In particular, the bimodal distribution which was considered as experimental evidence for noise-induced transitions is obtained in the simulations as well [7,9]. They also demonstrated that close to the cusp point the behaviour is similar to that of an equilibriimi system near a critical point. 6. Outlook We have demonstrated that FEM is a suitable method for studying reaction-induced fluctuations. Fluctuation-induced transitions and critical behavior have been found. In future studies the time resolution will be improved so that more of the microscopic details become visible. The results are important for understanding the reaction behavior of small sized (nanoscale) systems vAiich can be found in many different areas in chemistry and biochemistry. Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft. References 1. N. G.vanKampen, Stochastic processes in physics and chemistry, (North Holland, Amsterdam 1987) 2. W. Horstfiemke, R. Lefever, Noise-induced transitions (Springer, Berlin, 1984); A.S. Mikhailov and A. Yu. Loskutov, Foundations of synergetics II, T^ Ed. (Springer, Berlin, 1996); 3. M.O. Vlad and J. Ross, J. Chem. Phys. 100 (1994) 7268, 7279, 7295; A. S. Mikhailov, Z. Phys. B 41 (1981) 277. 4. C. N. Satterfield, Heterogeneous Catalysis in Practice (McGraw - Hill, New York, 1980); V. P. Zhdanov and B. Kasemo, Surf. Sci. 405, 27 (1998). 5. Yu. Suchorski, J. Bebcn and R. hnbihl. Surf. Sci. 405 (1998) L477. 6. Yu. Suchorski, J. Beben and R. hnbihl. Prog. Surf Sci. 59 (1998) 343. 7. Yu. Suchorski, J. Bebcn, E.W. James, J.W. Evans and R. hnbihl, Phys. Rev. Lett. 82 (1999) 1907. 8. Yu. Suchorski, J. Beben, and R. hnbihl. Surf. Sci. 454-456 (2000) 331. 9. Yu. Suchorski, J. Beben, E.W. James, D.-J. Liu, J.W. Evans, and R. hnbihl, Phys.Rev.B, submitted. 10. Yu. Suchorski, R. hnbihl, and V. K. Medvedev, Surf. Sci. 401 (1998) 392 (1998). 11. R. hnbihl and G. Ertl, Chem Rev. 95 (1995) 697. 12. J.L. Lumley, Stochastic Tools in Turbulence (Academic Press, New Yoiic, 1969). 13.Yu. Suchorski et al., ECOSS-19, Madrid, Spain, September 5 - 8, 2000, to be pubhshed. 14. R. M. Ziff; E. Gulari, and Y. Barshad, Phys. Rev. Lett. 56 (1986) 2553. 15. K. Fichtiiom, E. Gulari, and R.M. Zif5 Phys. Rev. Lett. 63 (1989) 1527.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C' 2001 Elsevier Science B.V. All rights reserved.

689

Why copper ion-exchanged ZSM-5-type zeolite is so active for CO adsorption ? - Comparison with adsorption properties of silver ion-exchanged ZSM-5 Yasushige Kuroda,** Ryotaro Kumashiro,^ Hideo Onishi/ Toshinori Mori,* Hisayoshi Kobayashi,''Yuzo Yoshikawa,* and Mahiko Nagao** ""Department of Chemistry and ^Research Laboratory for Surface Science, Faculty of Science, Okayama University, Okayama 700-8530, Japan ""College of Science and Industrial Technology, Kurashiki University of Science and the Arts, Kurashiki, Okayama 712-8505, Japan We have examined the properties of copper or silver ion-exchanged zeolites to picture the bonding nature, as well as to clarify the specificity of the electronic state of the copper ion exchanged in ZSM-type zeolite. There are a few kinds of the adsorption sites giving the IR band at around 2155 or 2190 cm'* for CO molecule adsorbed on copper or silver ion exchanged in ZSM-5, respectively. It is clearly indicated that the property of metal-ionexchanged ZSM-5 samples for CO adsorption is well correlated with the fact whether the parent Bronsted acid site exists or not, by comparing with the adsorption property of CO on metal-ion-supported silica-alumina or Si02 sample. By applying the relationship between the differential adsorption heats of CO, ^difr, and the stretching vibration of adsorbed CO, Vco, the interaction between CO molecule and the copper ion exchanged in the ZSM-5 sample is well explained in terms of a cF-bonding nature, as is different from the case of Ag^-CO ZSM-5 system where the electrostatic attraction is operative. As for Ag"^, the energy separation between 5s and 4d levels is expected to be large, as suggested from the data that the larger absorption energy between the electronic configuration 4d*^ and the first excitation 4d^5s* as well as the larger second ionization energy of silver metal, in comparison with these values for copper ion (between 4s and 3d). As a result, we come to the conclusion that the mixing of orbital of 4s and 3d levels plays an important role in the adsorption property of copper-ion exchanged on the Bronsted acid site. 1. INTRODUCTION Copper ion-exchanged ZSM-5 zeolite (CuZSM-5) exhibits high and sustained activities for NOx decomposition [1,2] and specific N2 adsorption [3,4]. A large number of studies have been made on the catalytic properties of this sample, intending to develop new

690

practical catalysts, and have reached the conclusion that Cu^ is in the center of the active sites in CuZSM-5. However, little is known about the electronic and structural environment of the copper-ion exchanged and such information could also help to rationalize the catalytic behavior exhibited by the monovalent copper ion. Considering the outer shell orbitals, Ag"^ takes the isoelectronic staicture with Cu"*". To examine the state of Ag^ is helpful for getting the information on the specificity of Cu^ in zeolite. In the present paper, a thorough study of ZSM-5, silica-alumina, and Si02 that were modified by copper or silver ions is presented to clarify the specificity of the electronic state of Cu"^ in CuZSM-5. In particular, the IR technique in combination with microcalorimetry was used to picture the bonding nature of metal ion supported on the solid materials by utilizing CO as a probe molecule. 2. EXPERIMENTAL Materials, A sodium type of ZSM-5 zeolite (NaZSM-5: Si/Al=l 1.9) and silica sample (aerogel 200) were kindly provided by Tosoh and Japan aerogel co., respectively. A silicaalumina [hereafter abbreviated as Si02'AI2O3] sample was prepared in our laboratory. Copper or Silver ion-exchanged ZSM-5-type zeolites and Si02-Al203 [Cu/Si02-Al203, Ag/Si02-Al203] samples were prepared by a conventional ion-exchange method using an aqueous solution of copper or silver nitrate. The metal ion-exchanged samples thus obtained are abbreviated as CuZSM-5-X or AgZSM-5-A^, where X is the exchange level of copper or silver ion in %. In this article, ion exchange capacity is expressed by the molar quantity of metal ions exchanged. Isotherm and adsorption heat measurements. The measurements of adsorption isotherm and adsorption heat of CO were performed for the samples that were evacuated at 873 K under a reduced pressure of 1 mPa. The measurement of adsorption-heats was carried out by using an adiabatic-type calorimeter made in our laboratory. The equilibrium pressure of CO gas was measured with a capacitance pressure sensor, MKS 310 BH IR spectra, IR spectra were measured by a Mattoson 3020 FT-IR spectrophotometer with a TGS detector at a nominal resolution of 2 cm'^ The sample was pressed into the self-supporting disk and was placed in an IR cell that can be treated at higher temperatures in vacuo or in various gases in situ condition. Calculation method The gradient corrected density functional (DF) as well as the hybrid functional methods were employed in the present calculation. The Slater and Becke exchange and Lee-Yang-Parr correlation functionals (BLYP) were used for the pure DF method. The Gaussian 94/98 programs were used throughout this work. 3. RESULTS AND DISCUSSION Figures 1 and 2 show the IR spectra for the CuZSM-5 and AgZSM-5 samples that were evacuated at 873 K, followed by CO adsorption at 300 K. The observed bands can be assigned to the CO molecule adsorbed on the Cu"^ or Ag^ species existing on the

691 exchangeable sites in ZSM-5 [5,6]. In the case of copper ion-exchanged system, a broad IR band (VQO) due to Cu^-CO species was observed at around 2155 cm'^ with a taihng toward the lower wavenumber side. This band was resolved into three components, 2159, 2151, and 2135 cm"^ bands (Fig. 1, e.g. spectrum 7) They have been assigned to the CO molecules adsorbed on two-coordinated and three-coordinated copper ion and on Cu20-like species deposited on silicious part of zeolite [5]. On the other hand, the spectrum of the AgZSM-5 sample was resolved into one major component, the 2193 cm"^ band, with a minor component, the 2183 cm'^ band (Figure 2).

13

U e«

I

10

-JfCliizri] 12 -^K^umi^ -4^iiiizid U S - -#^^=1^ L i -JKuzzzd 1 3 -J^^cnznl -^^HIZIIU LEI- 4 \ K ^ Z Z : ^

<

1 2 1 •

2300 2200 2100 2000 Wavenumber / cm'^ Fig. 1. IR spectra for CuZSM-5-74: (1) evacuated at 873 K;(2>(6) equilibrated with CO gas of increasingly but nearly zero pressure at 300 K; (7) 13 Pa, (8) 80 Pa, (9) 800 Pa, and (10) 3.51 kPa.

4U

2300

^

J

1.... 1

2200 2100 2000 Wavenumber / cm-'

Fig. 2 IR spectra for AgZSM-5-83: (1) evacuated at 873 K; (2H5) equilibrated with CO gas of nearly zero pressure; (6) 11 Pa, (7) 19 Pa,(8) 30 Pa, (9) 65 Pa, (10) 115 Pa, (11) 442 Pa, (12) 1.1. kPa, (13) 3.17 kPa.

In order to know the important factor determining the IR bands for metal-ion-exchanged ZSM-5 samples, FTIR spectra were measured by sequential dosing of CO onto Cu/Si02-Al203 or Ag/Si02-Al203 at 300 K. Increasing the coverage from nearly zero to monolayer causes a shift of the atop band for all the spectra as plotted in Figure 3. The respective main peaks at 2158 cm"^ and 2191 cm'^ are associated with CO adsorbed on the copper and silver ions exchanged in Si02-Al203 sample, and CO adsorption causes a shift of each atop band to 2148 cm'^ and to 2183 cm'^ with increasing coverage for the respective samples. The respective bands having a weak intensity at around 2132 cm"^ and 2177 cm'^ for the Cu/Si02 and Ag/Si02 samples are observed, being compared with the 2159 cm'^ band for the CuZSM-5 and Cu/Si02-Al203 samples and also 2190 cm"^ for the AgZSM-5 and Ag/Si02*Al203 samples. The positions of the bands due to CO molecule bonded to atop sites are different depending on the bonding nature between the metal ion deposited on the respective sites and a CO molecule; the state of metal-ion exchanged is expected to be correlated with the existence of the parent Bronsted acid site prior to the ion-exchange.

692 In order to get the information on the states of the adsorption sites for CO, the measurements of adsorption 2200 isotherm and heat of adsorption of CO on the 873 K-treated CuZSM-574 and AgZSM-5-83 samples were -r 219C performed at 301 K (Fig. 4). The rAg/SiOjAljOj monolayer capacities were estimated 2180l-Ag/SiO2 to be 28.3 and 16.1 cm^(S.T.P.)/g for the first isotherms on the respective samples. The ratio CO/M (M=Cu or Ag) can be evaluated to be 1.5 and 0.80 for the respective samples by considering both values of the ionexchanging capacity and the adsorbed amount. The ^diff of CO on CuZSM-5-74gave 120 kJ/mol in the initial stage of irreversible adsorp0.4 0.6 0.8 1.2 tion, and then it decreased to 100 Coverage kJ/mol to give a plateau (Fig. 4). Beyond this region, the heat of Fig. 3. Variation of atop bands with coverage in the various systems. adsorption drastically decreased to 45 kJ/mol in the reversible adsorption region and then exhibited a gradual decrease. It is quite obvious that such variation of the heat of adsorption with the adsorbed amount is characteristic of the interaction between the copper ion species in CuZSM-5 and the CO molecules. These adsorption data support the conception of a strong bonding (chemisorption) between the Cu^ species and CO molecules. Moreover, the existence of three adsorption ranges in the chemisorption region, corresponding to the heats of adsorption of 120 (plateau), 100 (plateau), and 100-60 kJ/mol (decrease), suggests a presence of at least three kinds of Cu"^ species interactmg with CO molecules. As for the AgZSM-5-83 sample, the initial heat of CO adsorption is about 105 kJ/mol. The value of ^diff decreases to 90 kJ/mol in the adsorption range from 1 to 9 cmVg. As increasing the amount adsorbed to the monolayer capacity of adsorption, ^aifr decreases to about 40 kJ/mol. The adsorption heat and isotherm data also support the following tendency; the existence of metal ion exchanged on the Bronsted acid site is responsible for liberation of the higher adsorption heat during the CO adsorption. We can discuss the various properties of these samples from the standpoints of acidity of solids and of electronic feature of the respective ions exchanged. As described above for the CuZSM-5-CO system, a presence of three kinds of the adsorption sites is suggested, corresponding to ^dift of 120, 100, and 100-60 kJ/mol, respec-

693 lively. The relationship between <7diff and vco for the respective sites gives a linear relationship as shown in Fig. 5. An analogous relationship can be found in the AgZSM-5 system (Fig. 5), though the plots are differently situated in the figure from the case of CuZSM-5 system. These facts lead us to conclude that the interaction through the obonding is operative in the copper ionexchanged systems [7] and that the electrostatic interaction is dominant in the silver ionexchanged systems. The relation between ^diff and Vco for the metal 5 10 15 20 25 ions deposited on Si02*Al203 and Volume adsorbed / cm g"^ Si02 was also examined. As a Fig. 4 Adsorption heats (a) and isotherms (b) of CO result, it became clear that the on CiiZSM-5-74 and AgZSM-5-83 samples. parent Bronsted acid site plays an important role in the CO adsorption [8]. On the basis of the quantum 140 chemical calculation, the difference in the bonding nature AgZSM.5.83 between copper and silver ions 100 was elucidated. In this approach, ^3 we have assumed two types of 60 model for the metal ions CuZSM-5-74 / exchanged in ZSM-5: two I I I I I I I I 20 I coordinated and three coordinated 2170 2190 2130 2150 sites relating to Bronsted acid center. According to the DF Fig. 5 Relationship between the differential heat of theoretical calculation method, it adsorption and the wavenumber of stretching was indicated that the former site vibration of adsorbed CO. corresponds to the site giving larger heat-values and higher IR vibrational band and the latter species has lower energy and lower frequency of IR band. The discussion relating to the bonding nature between metals or metal ions and CO molecules is interesting from the viewpoints of classical or nonclassical classification of M -

694 CO bond-type. Hence it is meaningful to discuss the bonding nature between the copper ion and CO molecule, comparing with the case of Ag*-CO, to clarify the specificity of copper ion as a catalytic center. Since n back-donation involves a transfer of electron density from thefilledmetal d-orbital to the empty CO orbitals, it is related to a removal of electron density from the metal, i.e., ionization potentials. In this stage, it is worthwhile to describe the two points. First, Rack and Strauss reported that the 4d^5s^(^D) state of Ag^ lies above 548 kJ/mol from the ground state 4d^°5s°(^A), compared with 314 kJ/mol for the corresponding state in Cu* [9], and their datum was also substantiated by the photoemission spectra which give the larger energy separation between 5s and 4d levels of the silver ion, compared with that between 4s and 3d levels for the copper ion [10]. Second, the second ionization potential of silver metal is larger than that of copper or gold metals [11]. Taking account of the data described above, the contribution of n back-donation is ruled out in the copper ionCO systems, and also in the silver ion-CO systems. In the case of the latter systems, another covalent nature, cr-bonding, is considered in the similar manner to the former systems. It has demonstrated that the extent of cx-donation from the CO 5cr MO, i.e. the lone pair of carbon atom, to empty levels of the monovalent silver ion is not such to allow the classification of the Ag*-CO bonding as a a-dative surface complex in the present system. The interaction of CO molecules with silver ions can be almost entirely described in terms of electrostatic interaction without involving any cr-donation; the large energy difference between 5s and 4d levels for the silver ion reflects in the larger d-population and smaller s-population of AgCO* compared with the case of CuCO^, indicating less 5s4d ahybridization and more a-repulsion. These tendencies were well explained by the theoretical points of view. On the other hand, Strauss et al proposed that the ^-bonding interaction is operative in the copper carbonyl complexes and a-boning in the silver ion carbonyl complexes [11], being different from our cases. The existence of difference between these carbonyl complexes and zeolite systems may be explained by considering that Wall effect (Pauli's repulsion) is operative in the zeolite systems. From these data, the difference in bonding nature between Cu^-CO and Ag*-CO systems is become apparent: the specificity of the copper ion in zeolite. REFERENCES 1. M. Iwamoto et al., J. Chem. Soc, Chem. Commun., (1986) 1272. 2. M. Shelef, Chem. Rev., 95 (1995) 209. 3. Y. Kuroda et al., J. Chem. Soc. Chem. Commun., (1993) 18. 4. G. Spoto et al., J. Chem. Soc, Faraday Trans., 91 (1995) 3285. 5. R. Kumashiro et al., J. Phys. Chem. B, 103 (1999) 89. 6. A. Zecchina et al., J. Phys. Chem. B, 103 (1999) 3833. 7. Y. Kuroda et al., J. Phys. Chem. B, 101 (1997) 6497. 8. Y. Kuroda et al., Phys. Chem. Chem. Phys., 1 (1999) 3807. 9. J. J. Rack and S. H. Strauss, Catal. Today, 36 (1997) 99. 10. S. M. Kanan et al., J. Phys. Chem. B, 104 (2000) 3507. 11. S. H. Strauss, J. Chem. Soc, Dalton Trans., (2000), 1.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

695

Promoting Effect of Zirconia Coated on Alumina on the Formation of Platinum Nanoparticles - Application on CO2 Reforming of Methane M. Schmal, M. M. V. M. Souza, D. A. G. Aranda and C. A. C. Perez NUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502,21945-970, Rio de Janeiro, Brazil Pt/Zr02/Al203 samples have been prepared by impregnation of the alumina with a nitric acid solution of zirconium hydroxide. A monolayer of zirconia on alumina was observed around 10 wt% as shown by XPS and ISS; beyond this value crystallites were formed. FTIR spectra of CO adsorbed on Pt/Zr02 showed the formation of Pt-ZrOx interfacial sites and there was a decrease in Pt-CO bond strength. The presence of surface cations of zirconia was confirmed by ISS with CO adsorption in situ. The Pt/10%ZrO2/Al2O3 catalyst exhibits high activity and stability during CO2/CH4 reforming at 1073 K. 1. INTRODUCTION There has been a general interest in recent years in the more efficient use of natural gas (methane) and in the reduction of CO2 in the atmosphere. The production of syngas (CO and H2) by CO2 reforming of CH4 over heterogeneous catalyst is one of the most attractive routes for the utilization of methane and CO2 resources [1]. Prior investigations of CO2 reforming of CH4 over supported platinum catalysts have indicated that metal-support interactions can affect both catalyst activity and activity maintenance [2]. It has been shown that Pt/Zr02 is a suitable catalyst for reforming of methane with CO2 [3], which results in a very interesting surface system, where zirconia participates as oxygen donor, creating active sites at the Pt-ZrOx interface. Our work was focused on the surface properties of the Pt/Zr02/Al203 systems in order to investigate the contribution of Zr02 to the CO2/CH4 reforming and the elementary reaction steps that occur. The model system with zirconia covered by platinum layer was also characterized. Con^aring the results of the model system with supported Pt catalysts, we tried to elucidate some details of the mechanism of the CH4 reforming by CO2, regarding that zirconia has reducible and oxygen storage properties. 2. EXPEWMENTAL 2.1. Catalyst Preparation Zr02/Al203 samples were prepared by impregnation over alumina powder (Harshaw A13996) with a nitric acid solution (50%) of zirconium hydroxide (MEL Products), The mixture was stirred for 2 h at 363 K. The resulting solid was dried at 393 K for 16 h and calcined at 823 K for 2 h under flowing air. The zirconia loading was 1,

696 5, 10 and 20 (%w/w). Zirconia contents were determined by X-ray fluorescence. Supports will be referred to as xZrAl for x^oZrOi/AbOa. The catalysts were prepared by incipient wetness technique, using an aqueous solution of H2PtCl6 (Aldrich), followed by drying at 393 K for 16 h and calcination in air at 823 K for 2 h. For all catalysts, the platinum content was around 1% (w/w), which was measured by atomic absorption spectrometry. Prepared catalysts will be referred to as PtAl for Pt/AfeOa, PtZr for Pt/Zr02 and PtxZr for Pt/x%Zr02/Al203. 2.2. Characterization Textural properties were obtained in ASAP 2000 apparatus (Micromeritics) after pretreatment at 573K. The BET areas were determined from nitrogen isotherms at 77K. The XPS measurements were obtained in a PHI model 1257 spectrometer using a Al Ka radiation source (1486.6 eV). The supports were analyzed without pretreatment and PtZr and PtAl catalyst were reduced in situ at 573 and 773 K. After reduction, PtZr and PtAl were transferred back to the reaction chamber and exposed to flowing 10%CO /He for 10 min, evacuated and analyzed again. The outmost layer of PtAl and PtZr were probed by scattering of low energy (1 KeV) He^ ions. The FTIR analysis was carried out with a Perkin-Elmer model 2000 at a resolution of 2 cm"*. The catalysts were dried at 423 K, reduced at 573 K or 773 K for 1 h and evacuation for 30 min. After cooling down to room temperature, CO was admitted at 30 Torr for 15 min and after evacuation the IR spectrum was taken. This procedure was repeated for desorption temperatures at 373, 473 and 573 K. TPD was performed in a dynamic mode apparatus. First the catalyst was dried at 423 K for 30 min and reduced at 573 or 773 K for 1 h. After reduction the sample was purged with He during cooling down to room ten:^)erature. CO was admitted by injecting pulses until complete saturation of the catalyst surfece was obtained. Desorption was performed by heating the catalysts at 20 K/min from 298 to 873 K in flowing helium. The effluent gas composition was monitored by a quadrupole mass spectrometer (Dycor MAIOOM- Ametek). 23. Catalyst testing The CO2 reforming of CH4 was performed in a flow tubular quartz reactor at atmospheric pressure and 1073 K. The catalysts (20mg) were dried in situ with flowing nitrogen at 423 K before reduction with 10%H2/N2 for Ih at 773 K. After reduction the catalyst was purged with nitrogen for 30 min at the same temperature. The reaction mixture consisted of C02/CH4/He (molarl/1/18) and a total feed flow rate of 200 cmVmin (WHSV= 160h"*). The reaction products were analyzed by on-line gas chromatogr^h (CHROMPACK CP9001), equipped with a Hayesep D column and a TCD detector. 3. RESULTS AND DISCUSSION 3.1. Dispersion of Zirconia on Alumina Platinum and zirconia loading and BET surfru:e areas for all catalysts are presented in Table 1. The experimental XPS values of IZr 3d/IAl 2p and ISS intensities as afimctionof the atomic ratio between Zr and Al in bulk are plotted in Fig. 1.

697 Table 1- Chemical composition and surface area Catalyst Pt (wt%) Zr02 (wt%) PtAl 0.86 . PtZr 0.83 PtlZr 0.64 0.80 PtSZr 0.87 4.00 PtlOZr 0.95 8.28 Pt20Zr 19.10 0.85

BET area (mVg) 199 62 183 190 180 147

The 1-10% zirconia containing samples show specific area values close to the alumina. The loss in BET surface area was approximately 26% for the sanple containing 20% of Zr02, which can be associated to blockage of the alumina pores by zirconia crystallites.

0 04

0 06

(Zr/AJ)bulk

0.O8

Damyanova et al. [4] showed by XPS through the BE of Zr 3d5/2 that there was a slight increase with increasing Zr02 loading up to 17% con^ared to pure zirconium, indicating changes in the coordination number, which could be explained by an interaction of Zr with alumina atoms, due to the formation of Zr-O-Al bond. This phase should be amorphous.

Fig. 1. XPS and ISS mtensities for Zr02/Al203 samples. We observed similar behavior in Figure 1, ahhough the maximum surfece concentration of Zr was obtained for 10%wt Zr and above this concentration, crystallites of Zr02 nucleated, decreasing the intensity ratio of Zr 3d and Al 2p photoelectrons. The surface coverage of zirconia on alumina, as probed by ISS which is more surface sensitive than XPS, also increased up to 10%wt Zr. 3.2. FTIR of Carbon Monoxide Figure 2 presents the FTIR spectra of CO adsorbed on the PtAl, PtZr, PtSZr and Pt20Zr catalysts, after evacuation at 298K- The peaks between 2070-2082 cm"' are attributed to the linearly bonded CO on platinum at room temperature. Negligible amounts of bridge-bonded CO (1780-1860 cm'^) were observed on alumina catalysts. Noteworthy is the peak around 1540-1650 cm* on zirconia contained catalysts, which can be attributed to carbonate species on the support [5]. The spectra of CO adsorption on alumina-supported zirconia catalysts are very similar to the PtAl catalyst spectra. On the other hand, the IR spectra of PtZr catalyst

698 reduced at 773 K provides valuable information about the nature of the metal-support interaction.

2200 2100

2000

1900

1800

.700

1600

1500

IR spectra of PtZr displayed a small band at 2178 cm"^ at 298 K, v^hich was attributed to the linear adsorbed CO species on cationic sites (either Zr"*^ or Zr^^). This adsorption involves the a-donation type to coordenatively unsatured surface cationic sites (cus), acting as Lewis acid centers, according to prcvious work

Wavenurnber(cm )

[6]. Fig. 2. Infrared spectra of CO for different catalysts reduced at 773K, after evacuation at 298K. The PtZr catalyst reduced at 773 K showed also an intense band at 2130 cm ^ which can be associated to CO adsorption on Pt-Zr02 interfece, that means, on Pt sites interacting strongly with zirconia. The increasing vibration frequency of CO can be explained by an electron transfer phenomenon from platinum particles to partially reduced zirconia, which decreases the Pt-CO bond strength. The PtZr catalyst reduced at 573 K presented linearly adsorbed CO band close to the PtAl catalyst band, but displayed lower intensity and con^)lete desorption at 573 K, indicating weaker Pt-CO bond on the zirconia support The frequency of linearly adsorbed CO bands, obtained by thermal desorption experiments, was extrapolated for zero surfece coverage for all catalysts. It was observed around 2040-2047 cm"' for all supported alumina catalysts, while on PtZr catalyst there was a large increase of X (0=0), mainly after reduction at 773 K (X=2086 cm"'). It confirms less Pt-CO bond strength on PtZr catalyst due to the Pt-Zr"*^ interface. 3.3. XPS and ISS Measurements The surface state of PrZr catalyst was also studied by XPS and ISS using CO as a probe molecule. Conparing the XPS intensity band of the oxide and after reduction at 773 K the same value was obtained (BE 182.2 eV for Zr3d), however, after adsorption of CO this band was shifted to 181.4 eV, corresponding to AE= 0.8 eV. This shift was not observed on the sanq)le reduced at 573 K. A possible explanation is that H2 during the reduction step splits in H-H, occurring a spillover through Pt^ settling over the O linked to Zr-0 leading to Zr^^ cation in the first layer of the surfece. Noteworthy are the ISS data, a refined method for the identification of atoms in the first layer. These results indicated an enhancing intensity of the Zr+Pt ISS peak after reduction at 773 K, that can

699 be attributed to the increasing surface population of Zr"*^. The adsorption of CO decreased this peak, suggesting the presence of surface cations of zirconia, which are also able to catch the probe molecule . This is not the case when the reduction is 573 K, remaining the Pt^ metallic form without affecting the Zr02 state. There are two important consequences from Zr^ presence. First, the surfece acidity is affected. According to Damyanova et al. [4] the increase in positive charge on zirconium corresponds to the formation of Lewis acid sites, due to the high electronegativity of the Zr"*^ cation in a oxide matrix. The other possibility is related to the CO adsorption form. In the presence of Zr"*^ the adsorption is different, since the interaction between Zr^ and Pt® decreases the electron density of the noble metal, which affects the CO adsorptive properties. 3.4. TPDofCO The TPD profiles of CO on PtAl and on alumina-supported zirconia catalysts, reduced at 773 K, were very similar, exhibiting similar shapes and position of the desorption peaks. It showed that all CO chemisorption was released as CO2 and H2. The CO2 formation started up at low temperature (around 323 K) exhibiting two distinctly regions: in the first one, it extends up to 500 K releasing only CO2, while in the second region a maximum CO2 desorption occurred at 550 K with simultaneous and symmetric formation of H2. The CO desorption profiles on PtZr catalyst TPDofCO are shown in Figure 3 for RZr different reduction temperatures at 573 and 773 K. After reduction at 773 K no H2 was observed and the CO2 profile showed two peaks at 373 K and 513 K. However, the TPD profiles after reduction at 573 K were very similar to the alumina supported catalysts reduced at 773 K. 400 800

y^-^V

Temperature (K)

Fig. 3. Desorption profiles, after CO adsorption on PtZr catalyst reduced at 773 K: (a) CO2, (b) H2, (c) CO; reduced at 573 K: (d) CO2, (e) H2, (0 CO. The first region can be related to the Bourdouard reaction on metallic Pt^ (2C0 -^ CO2 + C) and in the case of zirconia containing catalysts, to the reaction of CO with the oxygen lattice of Zr02 in the neighboring of Pt-Zr02 interface: CO(Pt*) + [0](ZrOx) -^ CO2, where Pt* is the Pt surface active site and ZrOx is the reduced surface site.

700

After reaching 500K, there is a simultaneous formation of CO2 and H2 with the exception of the PtZr catalyst reduced at 773K. According to Jackson at al. [7], the synmietric profiles of CO2 and H2 are ascribed to the surfece reaction between carbon monoxide adsorbed on metallic surface and hydroxyls at the metal-support interfece. 3.5. Catatyst Testing A comparison of the time-dependent activity is shown in Fig. 4 for all supported catalysts. The PtAl and PtlZr catalysts exhibited high deactivation rates, presumably due to the rapid deposition of inactive carbon. On the other hand, PtlOZr catalyst operated for 60 h without significant deactivation. These results would explain the C H , ^ Rcfcrning (CH,/C0,-1) general reaction mechanism of the 1073 K, 200 n#min methane reforming with CO2. O'Connor [8] suggested that methane is activated on metallic Pt^, splitting in C and H, that combine with the O from the zirconia lattice . The oxygen is supplied by CO2. These combination would favor the H2 and CO formation. However, our results would suggest that the methyl radical and CO may be -. adsorbed on the PtZr^ interface, 30 40 favoring the electron deficiency of the » on stream (h) Pt-Zr interface. The presence of Zr"*^ Fig. 4. Deactivation tests at 1073 K. and Pt° were observ^ from ISS and XPS data that suggest the interfacial site, confirmed by IR measurements, explaming the electromc phenomena tor the reaction path, due to the electron transfer from Pt to Zr, justifying the CH4 splitting at this interface besides the metallic platinum. Moreover, our resuhs point out to decrease in the adsorption energy of CO on PtZr02 system, in other words, zirconia inhibits the C-0 bond breaking on platinum surface. Thus, the intensity of back donation is lower in CO/electron deficient platinum model. The main consequence is a lower amount of carbon on the surface and even a larger stability during the reaction. REFERENCES [1] J.T. Richardson, S.A. Paripatyadar, Appl. CataL, 61 (1990), 293. [2] M.C.J. Bradford, M.A. Vannice, J. CataL, 173 (1998) 157-171. [3] J.H. Bitter, K. Seshan, and J.A. Lercher, J. Catal. 171 (1997) 279. [4] S. Damyanova, P. Grange, B. Dehnon, J. Catal. 168 (1997) 421. [5] W. Hertl, Langmuir 5 (1989) 96. [6] C. Morterra, G. Cerrato, F. Pinna, Spect. Acta Part A 55 (1999) 95. [7] S.D. Jackson, B.M. Glanville, J. Willis, G.D. McLellan, G. Webb, R.B. Moyes, S. Sinu)son, P.B. Wells, R. Whyman, J. Catal. 139 (1993) 207. [8] AM. O'Connor; Ph.D. Thesis, University of Limerick, Ireland, 1998.

Studies in Surface Science and Catalysis 132 Y. IwasawQ, N. Oyama and H. Kunieda (Editors) P 2001 Elsevier Science B.V. All rights reserved.

701

Structure Sensitivity in Reactive Carbon Dioxide Desorption on Palladium Surfaces Md. Golam MOULA/ Sugio WAKO," Mikhail U. KISLYUK,*-^ Yuichi OHNO* and Tatsuo MATSUSHIMA* " Graduate School of Environmental Earth Science and Catalysis Research Center, Hokkaido University, Sapporo 060-0810, Japan ^ Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia The velocity distribution of desorbing product CO2 was determined in steady-state CO oxidation on Pd(ll 1), Pd(l 10) andPd(100) by cross-correlation time-of-flight techniques. The distribution curve commonly involved two components. One is thermalized to the surface temperature, and the other holds an excess translational energy, depending on the surface structure. 1 Introduction CO oxidation on platinum metals is an important catalytic process that has been frequently used for examining new concepts of surface reactions. This paper first reports remarkable differences in CO2 desorption dynamics between Pd( 110), Pd( 100) and Pd( 111), irrespective of very similar chemical kinetics. In steady-state CO oxidation on palladium, CO2 formation proceeds via CO(a)+0(a)—•C02(g). The product CO2 carries high translational and internal energy [1,2]. The spatial and velocity distributions of desorbing CO2 provide the dynamics of this reactive desorption event, even if the event is not rate-determining. Desorbing CO2 involves two velocity components, whose separation becomes clear by inducing a site switching for CO2 formation. This switching takes place around a critical CO pressure where the rate-determining step shifts from the CO adsorption to O2 dissociation [3-5]. 2 Experimental The apparatus was described elsewhere[3]. Briefly, it consisted of a reaction chamber with LEED and XPS optics, a chopper house and an analyzer with a mass spectrometer. The chopper house had a slit on each end and a cross-correlation random chopper. The signal due to CO2 at the spectrometer was registered on a multi-channel scaler synchronized with the chopper rotation, and then velocity distributions were obtained with the standard deconvolution techniques[4]. The distance between the chopper blade and the ionizer of the mass spectrometer was 377 mm and the time resolution of 20 /is was used. A Pd( 111), Pd( 110) or Pd( 100) single crystal in a disk-shape slice was mounted on a rotatory manipulator. The partial pressures of CO (Pco) and O2 {P02) were kept constant by dosing gases, continuously. The surface temperature (Ts) was regulated at fixed values. The crystal was rotated around the axis of the [ 110] direction for Pd( 111), or the [ 110] direction for Pd( 110), or the [010] direction for Pd(lOO), to change the desorption angle (0 ; polar angle). No crystal azimutii dependence of CO2 desorption was confirmed on Pd(l 11) and Pd(100)[l]

702

3 Results

9 = 0*

3.1 Steady-state kinetics Similar kinetics were found on Pd(lll), Pd(110) and Pd(100). The steady-state CO2 formation rate was monitored in AR (angle-resolved) form in the range of 7V=400 -700 K. The Pco dependence of the rate was characterized by a transition at a critical Pco value. Below tiiis value, the rate increased linearly with Pco, ^^ereas above it, the rate decreased, showing negative orders in CO. Hereafter, the former is named the "active region" and the latter the "inhibited region." Aroimd the critical value, no sharp drop was found in the CO2 formation rate on Pd(lll), whereas on Pd(llO) and Pd(lOO), large jumps were found [5,6]. With increasing 7i, or P02 value, the critical Pco shifted to higher values. 3.2 Velocity distribution The velocity distribution curve of CO2 1 2 consisted of two components, a fast one and a Velocity/km 8 '^ slow one. Their contribution largely changed in the inhibited region. Typical velocity Fig. 1 Velocity distributions on Pd(lll) at distribution curves on Pd(l 11) at various Pco Po2=3xlO'^ Torr and Ts=500 K. The dashed values and at e^(f , Ts = 500 K and P02 curves show typical deconvolutions and the =3x10-^ Torr are shown in Fig. 1. The curve in solid lines show the summation of both the active region showed a single peak, components. The reaction regions are whereas a bi-modal form of the distribution became clear in the inhibited region. The slow indicated on the right-hand side. component was described by a Maxwellian distribution at Ts. The remaining signal after subtraction of the slow component was fitted to a modified Maxwellian form. Deconvolutions are shown by broken curves in the figure. The resultant energy is indicated (in the temperature units) as r<£i>=<E>/2k, where <E> is the mean translational energy and k is the Boltzmann constant. The fast component showed a nearly constant temperature of 1750 ± 100 K. This component was suppressed above the critical CO pressure. On the other hand, the slow one still increased with Pco at 3x10'^ Torr of O2, although it decreased at lower O2 pressures (Fig. 2a). Both components increased linearly with Pco in the active region. On Pd(110), the translational temperature of the fast component decreased from 3000 K to 2300 K with increasing Pco under the conditions in Fig. 2. The value at lower Pco increased vnihPo2 and reached 4000 K at 3x10"^ Torr of O2 [5]. The fiction of the fast component in the normal direction suddenly decreased from about 0.9 at the critical point to about 0.2 in the inhibited region. The latter value increased at higher pressures [5]. On Pd(100), only a single peak was found in the velocity curves. The slow component was found only at O2 pressures above 1x10'^ Torr and below 7V=520 K. The fast component showed a translational temperature of 1500 ± 100 AT throughout the kinetic transition region [6].

703

3.3 Angular distribution In order to examine the angular distribution of the two components separately, the velocity distribution curve was analyzed at various desorption angles. On Pd(lll), the fast component showed a cos^^-^^e form angular distribution in both regions. On Pd(110), it showed a cos^^^-^^ 0 form in the (001) plane, whereas it obeyed a cos^^^ -^^6 form on Pd(lOO) [5,6]. The slow component commonly showed a cosine distribution.

«6 c 3

(a)

Pd(111) 8=0*^ Kinetic transition

Ts-SOOK

4 Discussion 4.1 Component Change The contribution of the two components changed in the inhibited region. Their fraction in the total CO2 formation was estimated from the signal at 0 =0 " and the angular distribution of each component by considering the relation between the observed AR signal and the total flux [3]. The fraction of the fast component is shown in Fig. 2b. The - 7 - 6 - ^ - 4 - 3 fast component on Pd(lll) was kept around log(P CO f Tonr) 50 % in the active region and decreased below Fig. 2 (a) Flux variation of the fast and slow 10 % at high Pco in the inhibited region. The slow change in the component and the components on Pd( 111) with Pco atfixedP02 significant amount of the slow component values of ( • , < » 3x10"^ Torr, (•,0)3x10^ Torr and ( • , O )3xlO'^ Torr. (b) Pco should be noted. The results on Pd(llO) and Pd(lOO) under dependence of the fraction of the fast similar conditions are also shown in the figure. component at fixed P02 of 3x10"^ Torr. The On Pd(110), the fast component was sharply vertical dashed lines show kinetic transition suppressed at the critical point above which positions. the slow component became major [5]. On the other hand, on Pd(100), only the fast component was found throughout the transition region below 1x10'^ Torr of O2 pressure. The slow component was observed only above this O2 pressure and below 7>=520 K [6]. 4.2 Site switching The composition change is commonly explained by the site-switching model, in which each component is formed on different sites [3,5,6]. The fast component is formed on non-reconstructedflatparts and the slow one on structural defects providing dissociation sites of O2 because the latter becomes major in the inhibited region. On the three surfaces, the rate-determining step shifts from CO adsorption to O2 dissociation at critical Pco- Around this critical value, the surface species switches from 0(a)>CO(a) to 0(a) Pd(llO) > Pd(lll)[5,6]. This sequence is mostly caused by the order of the adsorption heat of CO At the critical point, the amount of CO(a) on Pd(110) and Pd(lOO) is high enough to clean off 0(a), whereas it remains small on Pd(lll). Hence, the oxygen distribution on the latter is not largely shifted to the dissociation site in the

704

inhibited region. This may cause slow changes in both kinetics and adsorbed species around the critical point. This becomes possible when CO and oxygen form separate domains [7]. Table 1. Parameters describing product CO2 states Pd(100) Ref. Surface Pddll) Pd(110) 1800-2000 Present work 3500-4000 Fast comp. T<E> * K 1900 - 2300 2 Vibrational temp. • • K 1530 1600 1300 2 1150 Rotational temp. ** K 1130 960 • Ts= 500-700KandP02 =3x10-^ Torr,and** 7i-650KandP02 =-1x10' Ton. 4.3 CO2 Energy Typical desorption parameters describing product CO2 are summarized in Table 1. The translational temperature may increase a little more at around 10'^ Torr. The structure sensitivity is remarkable at the translational temperature. Infrared emission work is based on irradiation emitted from vibrationally excited molecules [2]. CO2 molecules in the fast component do not necessarily emit irradiation. Without simultaneous measurements of both the energies, it is difficult to discuss the correlation between them. The repulsive force is enhanced as nascent CO2 is closer to the surface plane because the site exerts Pauli's repulsion towards the product [1]. By considering that CO2 is formed on adsorption sites for 0(a), the distance of this reactant against the metal plane must be a principal factor to affect the translational energy. On Pd(100), 0(a) is located on four-fold hollow sites at 0.95-90 A above the metal plane [8]. On the other hand, it is close to the metal atom plane on Pd(llO) by considering that CO2 is formed in the surface troughs [5]. 0(a) on Pd(lll) may be located on three-fold hollow sites at 0.85 A above the metal plane [9]. High excess translational energy is expected on Pd(l 10), whereas similar lower values are predicted on Pd(l 11) and Pd(lOO). These were actually observed. Acknowledgments Md. Golam Moula and Mikhail U. Kislyuk are indebted to the Ministry of Education, Science, Sports and Culture of Japan for a scholarship and support throughout the foreign-researcher (COE) invitation program. The authors thank Ms. Atsuka Hiratsuka for her technical assistance. References 1. 2. 3. 4. 5.

T. Matsushima, Heterogen. Chem. Rev, 2 (1995) 51. H. Uetsuka, K. Watanabe, H. Ohnuma and K. Kunimori, Surf. Rev and Lett. 4(1997) 1359. G. Cao, M. G. Moula, Y. Ohno and T Matsushima, J. Phys. Chem. B, 103 (1999) 3235. G. Comsa, R. David and B. J. Schumacher, J. Rev Sci. Instrum. 52 (1981) 789. S. Wako, Md. G. Moula, G. Cao, K. Kimura, I. Kobal, Y. Ohno and T Matsushima, Langmuir, 16(2000)2689. Md. G. Moula, S. Wako, Y Ohno, M.U. Kislyuk, I. Kobal and T Matsushima, Phys. Chem. Chem. Phys. 2 (2000) 2773. T Matsushima and H. Asada, J. Chem. Phys. 85 (1986) 1658. K.H. Rieder and W Stocker, Surf. Sci. Lett. 150 (1985) L-66. K. Mortensen, C. Klink, F. Jensen, F. Besenbacher and I. Stensgaard, Surf. Sci. 220 (1989) L-701.

Studies m Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) P 2001 Elsevier Science B.V. All rights reserved.

705

Confonnational Order of Octadecanethiol (ODT) Monolayer at Golc^ Solution Interface: Internal Reflection Sum Frequency Generation (SFG) Study Shen Ye, Satoshi Nihonyanagi, Ken Fujishima and Kohei Uosaki* Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Sum frequency generation (SFG) spectroscopy using internal reflection geometry was employed to investigate the conformational order of octadecanethiol (ODT) monolayer on gold thin films in em electrolyte solution. This approach is convinced to be useful to study the molecular structure on the electrode/solution interface imder electrochemical condition. 1. Introduction Sum frequency generation (SFG) spectroscopy is becoming a powerful tool in the research on surface science because of its high siuface/interface selectivity and versatile applicability [1-6]. SFG is a second-order nonlinear optical process in which two photons at frequencies (Oi 2ind 0)2 generate one photon of sum-frequency at 0)3=0)1 + 0)2. The SFG is forbidden in the bulk of a cetrosymmetric mediimi and is only active on the surface or interface where the inversion symmetry is necessarily broken. For IR-visible SFG, a pulsed visible laser at a fixed frequency 0)i and a pulsed infrared laser with a tunable frequency 0)2 are used. SFG signal is resonantly enhanced when 0)2 matches vibrational modes (o)n) on the interface as: |2

^SFG '•

I. V^j-fi^J+'T.

^Ax'^ir'

(1)

where IsFG is the intensity of SFG signal, 0)n An and Tn are the resonant frequency, strength and damping constant of the vibration modes, respectively, and z^im and e are the non-resonant contribution and its phase angle, respectively. Therefore, SFG spectroscopy can be regarded as an interface specific vibrational spectroscopy. When SFG spectroscopy is applied to the solid/aqueous solution interface, the

706 input energy of infrared laser pxilse is significantly reduced before reaching the electrode surface by the aqueous solution layer between the electrode and the optical window even its thickness is minimized by pressing the electrode against the window [6,7]. Recently, Williams et al proposed to obtain SFG spectra of molecules adsorbed on the gold ultra-thin films in solution using total internal reflection geometry [8]. The attenuation effect of input infrared by the aqueous solution layer can be avoided and a large enhancement of surface electric field is expected in this geometry. The films used in their study were, however, too thin (<10 nm) to be used in electrochemical application because of their low electric conductivity [8]. In the present work, we obtained SFG spectra of octadecanethiol (ODT: CH3(CH2)i7SH) monolayers adsorbed on gold thin films with various diickness, which were prepared by sputtering of gold on a fused quartz surface, using an internal reflection geometry. It was foimd that the optimtim thickness of the gold film to obtain a high-quality SFG spectrum of ODT monolayer both in air and in electrolyte solution was 40-60 nm. The gold film of this tiiickness has a good electric conductivity and can be used as an electrode for in situ SFG measurement imder potential control. 2. Experimental The SFG system employed in this study is schematically shown in Figure 1. A picosecond NdiYAG laser (PL2143B, EKSPLA) was used to pump an optical parametric generation and amplification (OPG/OPA) system. The output from the OPG/OPA was mixed with 1064 nm laser output in a nonlinear infrared crystal, Ag2GaS2, to generate a tunable infrared output between 2.3 and 8.5 ^m. The second harmonic generation output (532 nm) from the laser was used as visible light [9,10].

532nm P^ Mode-locked NdiYAG Laser 355nm

i

PL2143B

^

1 Quartz Prism 2 Gold Film 3 Solution 4 Monochromator

^

' OPG/OPA/DFG I

Sr PMT

Figure 1. The SFG system and an experimental arrangement for SFG measurement with internal reflection geometry.

707

An IR-grade fused quartz circular plate (d=31.75 mm, 1=3.18 mm, Esco Products) was used as a substrate. The quartz plate was thoroughly cleaned by chromic acid solution and then rinsed with Milli-Q water several times. Gold thin films of various thickness were prepared on the fused quartz substrate by gold sputtering using a fine coater (JFC-1200, JEOL) with a deposition rate of 10 nm/min. A gold single crystal Au(lll) surface prepared by Qavilier's method [11] was also used a substrate. ODT was used as received from Wako Pure Chemicals. ODT monolayers were constructed by immersing the fresh gold films or annealed Au(lll) electrode into a ImM ODT ethanol solution overnight. An experimental arrangement for the SPG measurement with internal reflection geometry is also shown in Fig. 1. Electrolyte solution was exposed to the face of gold film in a homemade cell and the infrared and visible beams were incident from quartz side with incidence angle of 50 and 70 degrees, respectively. The two beams were overlapped at the interface between gold and quartz. The SFG signal was collected by a photomultiplier (Hamamatsu R630-10) after passing through a holographic SuperNotch filter (HSPF-532-1.0, Kaiser Optical System) and a monochromator. Polarizations of SFG, visible and infrared lights were all p. Each data point was obtained by averaging the SFG signals corresponding to 100 pulses aind was normalized against the intensities of the infrared and visible inputs. 3. Results and Discussions Figure 2 shows an SFG spectrum (circles) of ODT monolayer on Au(lll) surface in air by using normal external reflection geometry, i.e., both visible and infrared beams were incident from the gold side. The SFG spectrum was fitted to Eq. 1 (top, solid line) with five resonance components (bottom, solid lines). The SFG spectrum is in agreement with that previously reported [3,12,13]. Three dominant peaks attributed to CHa group of ODT monolayer were observed. The peak at 2886 and 2972 cm-i were assigned to the symmetric and asymmetric C-H stretching of CHb group. 2950 3000 The peak at 2948 cm-^ W2is attributed to the Wavenumber/ ari^ Fermi resonance between symmetric C-H 2. SFG Spectrum of ODT adsorbed on stretching of CH3 and CH3 bending Figure Au(lll). Top: data andfittedcurve; Bottom: overtone. Compared to the relatively deconvoluted resonance bands.

708

strong C-H bands corresponding to the CH3 group, bands of symmetric (2848 cm-^) and asymmetric (2924 cm-i) C-H stretching of CH2 group were very weak. This result confirmed that the ODT monolayer was densely packed on the gold surface with very small ntunber of gauche defects, which results in the CH2 peak in the SFG spectnim. Figure 3 shows SFG spectra (circles) of the ODT monolayer adsorbed on gold films of various thickness in contact with 0.1 M H2SO4 solution obtained by using internal reflection geometry. Solid lines correspond to the results fitted to Eq. (1). Figure 3 shows that the SFG spectra of ODT monolayer essentially depend on the thickness of the gold films. When the gold film was thin (Figs. 3(a)~l(b), 2(H30 nm), three small peaks attributed to CH3 groups of ODT monolayer were observed and peaks attributed to CH2 groups were hardly observed. These features are similar to that in Fig. 2 where external reflection geometry was used, although the intensities of these peaks were much less than those in Fig. 2. Although Williams et al. reported that total internal reflected SFG spectra of ODT monolayer was only observed on the gold thin films with thickness less than 10 nm by using their nanosecond laser system [8], the high quality SFG spectra were not obtained on such thin films in the present work. Fxirthermore, the surface of the gold thin layer was easily damaged by laser illimunation during the SFG measiu-ement.

3 CO

O CD

2800

2850

2^00

2^50

3000

Wavenumber / cm-1 Figure 3. Internal reflection SFG spectra of ODT adsorbed on Au films with various thickness in contact with 0.1 M H^SO^.

709

As the thickness of the gold fibns increased to 40-60 nm (Figs. 3(c)-l(d)), the signal/noise (S/N) ratio of the SFG spectra improved and the three peaks attributed to CH3 group were observed clearly. The gold films were more tolerant to the laser illimiination. These SFG spectra were almost the same as that in Fig. 2. It should be mentioned here that the gold films with the thickness of 40'-60nm show a good electric conductivity and can be used in electrochemical measiu-ement. In fact, this gold film is using as the substrate for in situ SFG measurement imder electrochemical condition. When the gold film was thicker than 80 nm, no peak was observed (Fig. 3(e)). The intensity of light decreases exponentially with thickness of the gold films as: E = Eoe-'^=Eoe^~^

(2)

where a is the absorption coefficient, K is the imaginary part of refractive index, k is wavelength and d is the thickness of the film. The penetration depths, where light intensity is decreased to 1/e (36.8%), for visible (532 nm) and infrared (3.4 ^un) were estimated to be 17 nm and 11 nm, respectively [14]. Since the penetration depths were much smaller than the optimiun thickness for internal reflection SFG measurement shown in Fig. 3, the intensities of both visible and infrared lasers are expected to be attenuated significantly by the gold thin films. A strong SFG signal from ODT monolayer on gold surface was, however, observed clearly as that in normal external reflection SFG spectra (Figs. 2 and 3). Thus, another enhancement factors should be considered here. It is well known that the intensity of the Raman scattering signal for a surface adsorbed species is enhanced by lO^-lO^ when a grained metal thin layer is used [15]. The strong electromagnetic (EM) field associated with the surface plasmon polariton and the collective electron resonance are considered as major reasons for the surface enhanced Raman scattering (SERS). Similar surface enhanced phenomenon associated with surface roughness of die evaporated metal thin layer was reported for IR absorption [16]. Surface structure and morphology of the gold thin film should play an important role also in the thickness dependent SFG spectra shown in Fig. 3. One should be able to reduce the light loss within the gold films by illuminating the visible and observing SFG output both from the water side because die absorption of visible and SFG lights by water are negligible. This study is now in progress.

710

In conclusion, we have demonstrated that internal reflection SFG spectroscopy is useful in determining conformational order of ODT monolayer on gold films in electrolyte solution. The internal reflection SFG spectroscopy can be used as an efficient tool to study the electrochemical interface. Acknowledgment This work was partially supported by Grant-in-Aids for Scientific Research on Priority Area of "Electrochemistry of Ordered Interfaces" (No. 09237101) and for Encouragement of Yoimg Scientists (No. 10740314) from the Ministry of Education, Science, Sports and Culture, Japan. REFERENCES 1. Y. R. Shen, Nature 337,519 (1989). 2. Y. R. Shen, Proc, Natl Acad. Sci. USA 93,12104 (19%). 3. C. D. Bain, /. Chem. Soc. Faraday Trans. 91,1281 (1995). 4. P. B. Miranda and Y. R. Shen, /. Phys. Chem. B 103,3292 (1999). 5. D. E. Gragson and G. L. Richmond, /. Phys. Chem. B 102,3847 (1998). 6. A. Tadjeddine and A. Peremans, in Spectroscopy for Surface Science (R. J. H. Qark and R. E. Hester, eds.), Wiley & Sons Ltd , Chichester, UK, 1998, p. 159. 7. W. Daimi, K. A. Friedrich, C. Klimker, D. Knabben, U. Stimming, and H. Ibach, Appl. Phys. A. 59,553 (1994). 8. C. T. Williams, Y. Yang, and C. D. Bain, Lagmuir 16,2343 (2000). 9. S. Ye, S. Nihonyanagi, and K. Uosaki, Chem. hett.,72A (2000). 10. S. Ye, T. Saito, S. Nihonyanagi, K. Uosaki, M. Paulo, D. Kim, and Y. R. Shen, submitted. 11. J. Clavilier, R. Faure, G. Guinet, and R. Durand, /. Electroanal. Chem. 107,205 (1980). 12. M. A. Hines, J. A. Todd, and P. Guyot-Sionnest, Langmuir 11,493 (1995). 13. S. Ye, S. Nihonyanagi, and K. Uosaki, Nonlinear Optics, submitted. 14. D. W. Lynch and W. R. Hunter, in Handbook ofOptiad Constants of Solids (E. D. Palik, ed.). Academic Press, Inc., Orlando, 1985, p. 275. 15. W. Suetaka, Surface infrared and Raman spectroscopy, methods and applications, Plenimi Press, New York, 1995. 16. M. Osawa, K. Ataka, K. Yoshi, and T. Yotsuyanagi, /. Electron Spectroscopy and Related Phenomena 64/65,371 (1993).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c 2001 Elsevier Science B.V. All rights reserved.

711

Doping silver nanoparticles in AOT lyotropic lamellar phases* X. Chen^ * , S. Efrima', O. Regev ^ Z. M. Sui* and K. Z. Yang* *Key Lab for Colloid and Interface Chemistry of Education Ministry, Shandong University Jinan, 250100, P. R.China ^Department of Chemistry, T)epartment of Chemical Engineering, Ben-Gurion University R 0. BOX 653, Beer-Sheva, Israel A new approach for assembling a inorganic/organic hybrid by dual-doping Ag nanoparticles to a lamellar template of AOT (sodium bis(2-ethylhexyl) sulfosuccinate) lyotropic liquid crystal is reported. Doping of both hydrophilic and hydrophobic particles simultaneously is realized for the first time. The origin of stability in the dual-doped AOT system suggests a delicate balance between different interaction forces. 1. INTRODUCTION Assembly of ordered systems of inorganic matter in organic matrices is of recent practical and fundamental significance. There is also a continuous interest in organized hybrid assemblies combining nanoparticles and lyotropic lamellar phases (LLPs)[l-5]. Such hybrid materials may yield novel physical effects and are also very promising in view of possible technological applications. The first single-phase system combining a LLP and magnetic particles has been described by Fabre, et al.[l]. Since their work, most studies, with only few exceptions, concentrated on the ferrofluid and SDS (sodium dodecyl sulfate)[6-9]. To further understand how a metal colloidal dispersion can incorporate into a liquid structured phase and to elucidate the way it affects the stability and the order in this complex system, we have chosen the LLPs of AOTwater-isooctane ternary system as the matrix for assembling the organized inorganic/organic hybrid. Silver colloids were chosen as particles to be doped. Simultaneous doping of the lamellar phase with both hydrophilic and hydrophobic particles is achieved for the first time. 2. EXPERIMENTAL AOT, AgNOj, NaBH4 and oleic acid are A. R. grade and purchased fi-om Aldrich. HPLC isooctane is from Fluka. All reagents are used as received. The water is treated in a Barnstead * Financial support from the Key Teacher Fund and CSC of Education Ministry, State Major Basic Research Project of China, Israel Science Foundation and K. C. Wong Education Foundation. Hong Kong are acknowledged.

712

E-pure water purifier (resistivity > 18 MQ cm). Oleate capped silver colloidal hydrosols and organosols are prepared as described elsewhere [10]. Their UV-visible spectra are taken with a HP 8453 diode array spectrophotometer. Small-angle X-ray scattering (SAXS) experiments are performed using Ni-filtered Cu Ka radiation (0.154 nm) from a Seifert ID 3000 sealed tube X-ray generator operating at 40 kV and 40 mA. 3. RESULTS AND DISCUSSION 3.1. Lyotropic lamellar phases AOT is widely used and has been extensively studied. The boundaries we measured for the lamellar phase, in the two-component, water/AOT system, are about 8 and 75 wt % AOT, in general agreement with Fontell [11]. However, except near 10-20 % AOT, the system allows only very limited amounts of isooctane in the lamellar region. It is also known that some lamellar regions of the AOT system are riddled with structural defects (< 20 wt % AOT). How these defects affect the colloid-matrix interactions is of interest to learn 3.2. Silver sols The UV-visible spectra of the sols are given in Figure 1. The silver particles exhibit welldefined extinction maxima at 414 ± 2 and 416 ± 2 nm, in water and isooctane respectively, due to a localized plasmon resonance. The average diameters of the silver particles, measured by SAXS measurements, are 4.4 ±1.2 and 5.0 ±1.9 nm in isooctane and water, respectively [12]. The sizes are also in good agreement with TEM results [10]. Electrophoresis shows that the hydrophilic Ag particles are negatively charged. i

o

\

3 CD

>»

/!\ \ / i\ i

vu >N.5

C

$

5

-doptd with 1 hydrosol i ' • doptd with 1 both hydrosol | and organosol undoptd i pur* phast |

c

300 400 500 600 700 Wavelength (nm)

Fig. 1. UV-visible spectra of colloidal silver particles dispersed in (a) water and (b) isooctane.

0.2

0.4

0.6 0.8 q (nm 1)

1.0

Fig. 2 SAXS spectra for pure and doped lamellar phases. W=W'=\40.

3.3. Doped lyotropic lamellar phases The dual-doped lamellar systems are formed simply. By using a silver hydrosol instead of pure water and a silver organosol instead of the pure organic solvent simultaneously, a phase doped with both hydrophilic and hydrophobic silver particles is constructed, each confined to its own environment. Ag hydrosol and organosol with [Ag] = 0.0025 M are incorporated into

713 three systems with W (W is the molar ratio of Ag-hydrosol/AOT) of 140, 99 and 74 The isooaane organosol is added to a level of 2.5 wt %. Though all systems appear uniform just after preparation, only the system with W'= 140 exhibits long range stability (over 3 months). In the lower W systems, the particles are excluded out gradually within several days. 3.4. Structural analysis from SAXS measurements Figure 2 shows SAXS curves for pure and doped AOT systems. The system with W (molar ratio of HoO/AOT) =140, corresponding to 15 wt % AOT, is reported to be deformed lamellae, in which bridges between two neighboring membranes are formed with polar heads facing inward (Figure 3a) [13, 14]. This is corroborated by the single peak observed in the SAXS curves of the systems non-doped or doped by hydrophilic particles, and by a distinct shoulder for the dual-particle system, indicating one with only very weak ordering. Yet, the systems are all birefringent. Basically this deformed structure seems to be retained after doping with a hydrosol. However, dual-particle doping strongly perturbs the structure. The absence of high order Bragg peaks beyond the first order and the broadening of the single apparent diffraction peak indicate the absence of long-range order, consistent with highly deformed lamellae. The short-ranged lamellar structure seems still kept from the similar texture to non-doped system, as observed by polarized optical microscopy. The little change of q-value of the shoulder compared to the system doped only by hydrophilic particles may be caused by the small doping amount of the organosol and the deformed lamellar structure as shown in Figure 3a

(a)

(b)

Fig. 3. (a) A schematic representation of AOT bridges in a lamellar phase with the possible insertion position of the hydrophobic particles, (b) A possible intermediate structure between lamellar and sponge L3 phase for our dual-doped system. It is interesting and unexpected that the presence of a hydrosol stabilizes the organosol in the AOT system. In the present AOT/water/isooctane system, there is a size mismatch between the oil layer (the layer thickness for 2.5 % isooctane is ~ 0.6 nm) and the particle (diameter of the bare hydrophobic particle is 4 nm). However, unlike in the case of SDS [12], AOT can retain the particles in spite of their large (relative) size. We believe this is made possible by the many defects existing in the AOT lamellae and less rigidity of the membranes. The large cross-section of the AOT surfactant tails relative to the head-group, a giving low surfactant number (N3 =V/la), causes adjacent surfactant molecules to pack so that the film

714 develops preferred radius of curvature. The tendency for the surfactant layer to bend is much greater in the AOT system than in the SDS system. The ethyl side of AOT tends to favor a natural packing curvature with the polar heads facing inward. It is plausible then to imagine a lamellar structure in which some of the amphiphile membranes curve across the water layers to form a bridge, on a local scale, (see Figure 3a) The existence of an attractive interaction between the surfactant bilayers and the hydrophobic particles makes the latter tend to concentrate in the bridge area, wrapped up in the molecular film. In those regions, there is more space for the particles, than in the plate, merely 1.9 nm thick, parts of the membrane. The hydrophilic particles might also cluster near these bridges and stabilize the hydrophobic ones via van der Waal's interaction (Figure 3a). This perhaps explains the origin of the stability of the AOT system doped with silver, both hydrophilic and hydrophobic particles, which fails to form in SDS systems. The bridged deformed lamellar phase can be considered as an intermediate structure between lamellae and a sponge phase L3, as shown in Figure 3b. 4. CONCLUSIONS We have demonstrated that the LLP of AOT/water/isoortane can be doped successfully with both hydrophilic and hydrophobic silver nanoparticles at the same time. It is confirmed that merely size matching between the particles and the doping space is at times not always an absolute requirement, especially for hydrophobic ones. The presence of defects or their inducement by the particles can be much more important for the stability of doped hybrid. REFERENCES 1. P. Fabre, C. Casagrande, M. Veyssie, V. Cabuil and R. Massart, Phys. Rev Lett., 64 (1990) 539. 2. V. Ponsinet and P Fabre, J. Phys. Chem., 100 (1996) 5035. 3. L. Ramos, P Fabre and R. Ober, Eur. Phys. J. B, 1 (1998) 319. 4. V. Berejnov, Yu. Raikert, V. Cabuil, J.-C. Bacri and R. Perzynski, J. Colloid Interface Sci., 199(1998)215. 5. C Menager, L. Belloni, V. Cabuil, M. Dubois, T Gulik-rzywixki and Th Zemb, Langmuir, 12(1996)3516. 6. J. Arrault, C Grand, W C K. Poon and M. E. Cates, Europhys. Lett., 38 (1997) 625 7. P Poulin, A. Raghunathan, P Richetti and D. Roux, J. Phys, II, France, 4 (1994) 1557 8. A. Raghunathan, P Richetti and D. Roux, Langmuir, 12 (1996) 3789. 9. I. Grillo, P Levitz and Th. Zemb, Langmuir, 16 (2000) 4830. 10. W Wang, S. Efi-ima and O. Regev, Langmuir, 14 (1998) 602. 11. K. Fontell, J. Colloid Interface Sci., 44 (1973) 318. 12. W. Wang, S. Efi-ima and O. Regev, J. Phys. Chem. B, 103 (1999) 5613. 13. G. Chidichimo, C La Mesa, G. A. Ranieri and M. Terenzi, Mol. Cryst. Liq. Cryst., 150b (1987)221. 14. E. van der Linden and C J. Buytenhek, Physica A, 245 (1997) 1.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

715

Modeling of the kinetics of metal oxide dissolution with chelating agents Hiroki Tamu^a^ Masahiko Kitano", Naotsugu Ito*, Shinichi Takasaki**, and Ryusaburo Fuiuichi" 'Laboratory of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628 Japan ''Kurita Water Industries Ltd., 7-1, Wakamiya, Morinosato, Atsugi, 243-0124 Japan The kinetics of the dissolution of metal oxides with chelating agents were modeled, and the model rate equations were able to explain and reproduce the time changes in dissolved metal concentrations and the pH peak dissolution rates. 1. INTRODUCTION Weak acid chelating agents dissolve metal oxides at higher pH than strong acids, and are used to remove radioactive contamination of corrosion products in cooling systems of nuclear power plants. In technological application of dissolution, it is important to be able to explain and predict the extent of dissolution quantitatively, and modeling of the dissolution kinetics was made by considering the coupling of metal and oxide ion transfer. 2. EXPERIMENTAL 2-1 Preparation of metal oxide specimens Powder metal oxide samples (CuO, Fe304, Fe203, PbO) were sintered, formed into disks, embedded in resin, and the disk cross-sections were exposed by grinding. These specimens have constant surface areas throughout dissolution experiments due to the "steady-state surface morphologies" [1]. 2-2 Measurement of dissolution rate The dissolving solutions were mainly 0.475 dm^ of 3.0x10*^ mol dm"^ EDTA - 0.1 mol dm'^ NaClQ, mixtures. The dissolving solution was kept at 80°C and stirred with a magnetic stirrer. The solution pH was adjusted by adding NaOH or HCIO^ solution, and maintained constant with a pH Stat. Dissolution experiments were started by dipping the specimen in solution, and the concentration of metal ions was measured by atomic absorption spectrometry. 3. RESULTS AND DISCUSSION Figures 1 and 2 show the time changes in concentration of Cu(II) dissolved from CuO with

716 EDTA. The dissolution behavior follows a linear rate law at pH^7, and is nonlinear at pH>8. The amount of Cu(II) dissolved showed a maximum at pHT-^S (Fig. 3). The other oxide samples also showed pH peak behaviors (Fig. 4), as other investigations have reported for Fcfi, [2,3], Fe203 [4], FeOOH [5], and Fe(III) (hydr)oxides [6]. 1.6 1.4

•o

"O

^ ^

•X 1 0

o E

0

1

—

pH 8.0 8.6 9.0 9.5 10.0

•

• ^ 1.2 E

—

-I

A T

•

j / ^

""^ -^

E

t 0.8 ^ ^ 0.6

3

H •/x

^OA

o

-1

02

3

1 2

4

O.Cj! 0

5

• 1

2

3

4

5

timeAi

Fig. 1 Plot of [Cu(IDl vs time at different pH. • T

1 — — 1 — —1 0

1.4

1

Fig. 2 Plot of (Cu(II)l vs time at different pH.

1

r -

V^8

.7 1.2 E

0

i/ 0 J

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nn

n

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_x

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1

1

7 8 pH

L.

9

10 11

Fig. 3 Plot of (Cu(II)] vs pH at 5 h.

Fig. 4 Plot of (MJ vs pH at 5 h for different oxides.

The following pathways are proposed to describe the dissolution behavior of oxides of divalent metal ions M(II). -MO + H„Y^^">

- • MY'+nir + -0'

0)

-tf+2H'-^H20 (2) It is assumed that lattice M(II) and oxide ion sites on the oxide behave as an -MO pair. An n protonated EDTA species H^Y^^""^ transfers an M(II) ion to the solution as an EDTA chelate MY^" by liberating n protons and by leaving a "lone oxide ion" -O^" on the oxide, this proceeds with rate constant ^,. This forward step in (1) breaks the electric neutrality in the solid and

717

solution phases and a backward reaction occurs with rate constant ^,. In the successive reaction (2) the "lone oxide ion" is protonated and transferred to the solution as water with rate constant k^ to recover electric neutrality. The "lone oxide ion" -O^" may be regarded as a very minor component, because the anions and cations being transferred to the solution must be coupled tightly. The rate equation was derived by applying the Principle of Stationary States to the "lone oxide ion" as follows (electric charges on species except for H* are omitted hereafter): d[MYl/d,-''*''-^°"^''-f''-(5/n «3) it_.[MY]+it2[H^f-'' where S is the surface area, Fthe solution volume, and a the fraction of H„V'*'"^ Rate equation (3) can be simplified by considering the proton concentration. At low pH where A:., [MY] < k^[Kf-" (n may be regarded to be less than 2 [7]), d[MY]/dr = ^,[-M0][Y]T«4. (SIV) (4) The right-hand side of Eq. (4) is constant at a pH, and this equation is integrated to: [UY]-k\-UO][Y],a,,„{SIV)t = k't (5) Here k' [=^,[-M0][Y]ta4.„ {SIV)] is the composite rate constant. Equation (5) explains the linear kinetics at pH<7 in Fig. 1, as the dissolved Cu(II) is the CuY chelate ([Cu(II)]=[CuY]). The values ofk are obtained from the slope of the linear [Cu(II)] vs. time relations in Fig. 1. At high pH where it., [MY] > k^[Wt\ d[MY]/d/= ^^[.MO][Y],[H1^-V„(5/m^.,[MY]) (6) Integration of Eq. (9) leads to: [MYf = 2^^[-MO][Y],[Hl^^a,, Sl(yk_,) t = 2Jt"/ (7) Here A:" [='Kk^[-lAO]\Y]-^\[Vf''a^ SI{Vk,^] is the composite rate constant, and Eq. (7) expresses parabolic kinetics. The data which showed nonlinear kinetics in Fig. 2 at pH>8 were replotted according to Eq. (7) in Fig. 5. The relations are linear, and it is clear that the experimental nonlinear data at pH>8 obey the parabolic rate law Eq. (7). The values of ^' are determined from the slope of the linear [Cu(II)]^ vs. time relations in Fig. 5. The values of it' obtained fi-om the slope of the linear [Cu(II)] vs. time relations (pH 4-7 curves in Fig. 1) are plotted against pH in Fig. 6. The log k vs. pH plot is linear and gives the following expression for the composite rate constant k' at pH 4-7: k=\Q'''\lttf (8) The values of it" obtained from the slope of the [Cu(II)]^ vs. time relations (Fig. 5) are also plotted against pH in Fig. 6. The linear log it" vs. pH plot results in the following expression for the composite rate constant A:" at pH 8.5-10: it" = ia''[H']'*' (9) Rate equation (3) for the entire pH range can be rewritten with k and k' as d[CuY]/dr = ifit"/(if [CuY] -f it") (10) This is integrated to: k [CuY]72 + it"[CuY] = kk'U (11) The observed it' and it" in Eqs. (8) and (9) (Fig. 6) are constant at one pH and EDTA concentration with respect to time, and were introduced into Eq. (11) with the assumption that

718

2

3 time/h

Fig. 5 Plot of [Cu(II)f vs time at differemt pH.

4

5

6

7 pH

8

9

10

Fig. 6 Plot of A:' and r vs pH

these relations can be extended outside the pHrangeswhere they were established. Then the dissolved Cu(II) concentration [CuY] at/=5 h was calculatedfromEq. (11) as a function of pH. The results are shown as the curve in Fig. 3, which reproduces the peaking dissolution with respect to pH. The pH peak corresponds to the boundary of the pH regions for the linear and parabolicrateequations obtained by simplifyingrateequation (3). The model rate equation (3) was able to explain and reproduce the time changes in dissolved Cu(II) concentration and the pH peak dissolution rate for CuO. It was also possible to explain and reproduce the dissolution behavior of Fe^O^ with EDTA and citrate by modifying Eq. (3) with the relevant values of « (maximum number of protons possessed by chelating agents) and the valence of lattice metal ions [8-10]. REFERENCES 1. B. Wehrli, J. Colloid Interface Sci., 132 (1989) 230. 2. I. G. Gorichev, V. S. Dukhanin, and N. A. Kipriyanov, Zhumal Fizichenskoi Khimii, 54 (1980)1341. 3. E. B. Borghi, A. E. Regazzoni, A. J. G. Maroto, and M. A. Blesa, J. Colloid Interface Sci., 130(1989)299. 4. H.-C. Chang and E. Matijevi'c, J. Colloid Interface Sci., 92 (1983) 479. 5. J. S. Lakind and A. T. Stone, Geochim. Cosmochim. Acta, 53 (1989) 961. 6. A. Amirbahman, L. Sigg, and U. von Gunten, J. Colloid Interface Sci., 194 (1997) 194. 7. H. Tamura, N. Ito, S. Takasaki, and R. Furuichi, Zairyo-to-Kankyo (Corrosion Engineering), 49 (2000) 22. 8. S. Takasaki, K. Ogura, H. Tamura, and M. Nagayama, Zairyo-to-Kankyo (Corrosion Engineering), 44 (1995) 86. 9. S. Takasaki, K. Ogura, H. Tamura, and M. Nagayama, Zairyc ^o-Kankvo (Corrosion Engineering), 45 (1996) 67. 10. H. Tamura, S. Takasaki, and R Furuichi, Bunseki Kagaku (J jpn. Sr... Anal. Chem.), 47 (1998)397.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) tc 2001 Elsevier Science B.V. All rights reserved.

719

The Size - Induced Metal - Insulator Transition in Colloidal Gold P. P. Edwards^^ S. R. Johnson', M. O. Jones'^ A. Porch' and R.L. Johnston^ ' School of Chemistry, ^ School of Metallurgy and Materials, ' School of Electrical and Electronic Engineering, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. 1. INTRODUCTION There are countless examples in nature where stubbornly resistive insulators or nonmetals can be spectacularly transformed into highly conducting metals, and viceversa. Such Metal - Insulator Transitions (MIT) are always accompanied by gross changes in the d.c. electrical conductivity, routinely over ten - to fourteen - orders of magnitude, over relatively small changes in temperature, density and composition. We are interested here in the related but, as yet, not intensively studied phenomenon of the Size - Induced Metal - Insulator Transition (SIMIT) occurring entirely within a single, isolated cluster or small particle of a metal.^'^ Here one imagines that the inevitable consequence of the successive fragmentation, or division, of a single grain of, for example, bulk gold must be the ultimate cessation of metallic conduction within that particle. An artists impression of the problem of the SIMIT in ""Divided Metals'\ first identified in 1857 by Faraday^ for colloidal particles of silver and gold, is given in Figure 1; this attempts to identify the key features Macroscopic Metal

Mesoscopic

Microscopic

Size-Induced Metal-Insulator Transition Insulator J J ^ -J J .J ^

Bulk Metal

Metal Colloids ^ and Nanoparticles

Atoms and Molecules

Figure 1 . A representation of the division of a single grain of metal.

720

of the macroscopic, mesoscopic and microscopic regimes derived from the successive fragmentation or division of a metal such as gold. The accepted definition^ of the colloidal regime is "... between a nanometer and a micron..'\ and it is now clear that the modem synthesis of colloidal metals extends down to this lower limit, where the distinction between, for example, colloidal metal sols and so called ''megaclusters'' becomes only one of semantics. The colloidal regime, therefore, occupies the enviable position as the natural intermediary between the atomic, molecular and the (bulk) metallic states of matter;fi-omthis perspective one can now appreciate the current resurgence of interest in the chemistry and physics of metal colloids - within the modem terminology, identified as ''nanoparticles'\ Our research centres on the electronic phase transition ft-om metal - to - insulator which inevitably must occur upon the successive fragmentation or division of a single grain of (bulk) metallic matter. Equivalently, one is interested in the precise manner in which the key defining properties of a metal {e.g. high electrical conductivity) evolve or develop fi'om the progressive agglomeration of its atomic constituents. We have attempted to develop not only reproducible synthetic approaches to the large - scale preparation of monodisperse, ultrafine metallic colloids (suitably engineered down to particle diameters less than 1 nm), but also physical measurements to directly probe the SIMIT over wide ranges of particle dimensions, and temperatures. The aim of this lecture, therefore, is to highlight some of our recent work which attempts to probe the Size - Induced Metal - Insulator Transition in small colloidal particles of gold, at room temperature, within a sol. 2. ELECTRONIC STRUCTURE ACROSS THE SIMIT At a temperature of absolute zero (T = OK) the highest occupied electronic energy state of a metal is the so - called Fermi Energy, Ep. Above EF, there are an infinite number of infinitesimally separated, and empty, electronic energy levels which can easily be populated by conduction electrons, even at low temperatures. In contrast, when a metal is sufficiently divided to mesoscopic or microscopic dimensions, the assumption of an electronic energy continuum breaks down and this continuum of electronic energy levels gives way to a manifold of discrete levels with an average separation, 5 « EF / N, where N is the total number of atoms contained within the colloidal particle. When this energy level separation, taken here as the so - called Kubo gap becomes comparable with the (ambient) thermal energy, kT, the electronic energy levels now become discrete, rather than continuous. This process will ultimately determine the metallic or insulating status of the colloidal particle. A schematic representation of the emerging discreteness of electronic energy levels with decreasing metal particle size is shown overpage in Figure 2. In Figure 3 we show the size (particle diameter) dependence of the (average) energy level spacing of gold. Here the relevant values of the Kubo gap are given in degrees Kelvin. Of course, the finite size of the particle will also lead to a significant number of gold atoms being located on the surface of the cluster or particle. We also show in

721 The Size-Induced Metal-Insulator Transition Macroscopic

6=0

Bulk Metal

WKKKKKm Mesoscopic

••••Mi

5
Metallic Clusters and Particles increasing

Microscopic

5>kT

Insulating Clusters and Particles Particle Diameter

5»kT

Atoms and Molecules

decreasing •

Figure 2 . The effect on electronic structure of the division of a metal. Figure 3 the effective percentage, Ps, of surface atoms for gold particles as a function of particle diameter. It is readily apparent that colloidal particles of as many as 10,000 atoms still have almost 20 % of their constituent atoms on the surface. In fact Ps only drops below 1% for a system with N > 6.4 x lO'^ atoms (corresponding to a diameter of approximately 0.16 jam for gold clusters). The onset of such electronic ""quantum size - effects"' in a finite system may well signal the occurrence of a genuine SIMIT, but this remains to be rigorously justified. One opinion argues that genuine 'metallic' properties can only be sustained in particles at finite temperatures when 5 < kT, a situation enabling the facile creation of electronic charge carriers via thermal excitation. From the viewpoint, the following simple criteria; 5kT: Insulator

(1) (2)

would presumably define the experimental parameters for a SIMIT in a finite, colloidal gold particle at temperatures above T = OK. Of course, as we move closer and closer to the microscopic regime, we should become increasingly concerned at the validity of any approaches derived from what one might term the continuum physics of macroscopic metals. Conversely, any theoretical treatment originating from the quantum - chemistry viewpoint ('Atoms and Molecules' regime in Figure 1) becomes increasingly unrealistic as we enter the mesoscopic and macroscopic regimes.

722

Particle diameter (D / A)

Figure 3. Particle diameter dependence of the electronic energy level structure of particulate gold.

For gold, with EF = 5.5 eV, at room temperature (kT « 0.025 eV), the coarse - grained conclusion 8 < kT (Equation 1) and presumed metallic conduction would then be realised in a metal colloid or particle containing more than 200 atoms, or a diameter of ca. 20 A. Equally, this leads to the inevitable SIMIT for gold particles with a diameter below ca. 20 A, and measured at room temperature.

We pose the question, therefore, whether it is possible to observe the SIMIT within individual colloidal gold particles in a sol, at room temperature? 3. ELECTRICAL CONDUCTIVITY WITHIN COLLOIDAL PARTICLES FROM MICROWAVE LOSS STUDIES

GOLD

The electrical conductivity mthin colloidal particles of gold has been measured via a new approach developed recently in Birmingham based on microwave resonance techniques. Microwave techniques are ideally suited for this task since they allow for contactless electrical conductivity measurements of individual particles in solution. This is achieved by the careful measurement of the dissipated and stored energy within the colloid particle itself; effectively, the real and imaginary parts of the dielectric constant. For microwave measurements, the dissipation energy is proportional to aF, where a is the electrical conductivity of the sample and V is the total sample volume. A direct measurement of the electrical conductivity of individual nanoparticles can therefore be obtained. In microwave loss experiments there are two important particle size regimes related to the physical dimensions of the particle as compared to the characteristic

723 microwave skin depth, ^; in the large particle limit, r » ^, (where r is the radius of the particle), the microwave loss cc \ / G '^\ In the small particle limit, where r « ^, the microwave loss oc a. The skin depth of gold at 3 GHz, the operating frequency of the resonator, is 1.3 fim and for gold colloids below this size we are therefore operating well within the small particle regime. Importantly, the small particle regime encompasses two further categories; the macroscopic particle limit, r « ^, where a takes the value of the bulk metal, and the microscopic particle limit, r < To « ^, where a is greatly reduced due to the SIMIT and ro is the radius of the gold colloid for which criterion (2) is satisfied. Recall, the present experiments are on colloidal gold solutions at room temperature. The microwave resonator technique involves inserting the sample in a region of high microwave electric field within a copper stripline resonator, shown in Figure 4. The microwave dissipation is determined directly from the measured bandwidth of the resonator, A / B , and if the sample volume V is known, then the electrical conductivity can be deduced, vzz;

SIDE VIEW O

Electric field maximum

O

Magnetic field maximum

20mm

Coupling loops

a ^ (7i8o VAU / V R ) A / B ,

where VAU is the volume of gold and VR is the volume of PLAN VIEW the resonator. Figure 5 shows the effect of inserting various —> Electric Field colloidal - sized gold particles —> Magnetic field into the microwave resonator. As can be seen, the resonant frequency increases when large «^Radiation Sample in quartz tube particles are added (r » ^), and shield the resonant bandwidth increases significantly when Figure 4 . The Copper Stripline Resonator particles in the macroscopic particle limit are added (r « ^). Remarkably, the response for ultrafine colloidal gold particles below ca 50 A is extremely small, even though the sample contains the same amount of gold. The dispersion, or division, of the gold into the nm - scale regime (r < ro « ^), therefore, produces a qualitative change in the microwave response from the colloidal solution. Quarter Wave Stripline Resonator

724

In Table 1 we show a comparison between the electrical conductivities of bulk gold and gold colloids in hexane, determined via the microwave resonator technique. The conductivity value for the colloidal gold particle of ca. 20 A diameter is some ten orders of magnitude lower than that of bulk gold. For this size regime, colloidal gold is behaving not as a metallic conductor, but rather it exists in a nonmetallic state.

Cavity

- 3>im 1 -2nm

^\

^^\

o Q_
\/

X

/

[-

1

\ /

(A

C (D

\-

- ^ - J

-50 -10nm 1—_

- .1

300

310

Frequency (GHz)

Figure 5 . Microwave loss of three colloidal solutions of gold of different particle sizes. Table 1 Electrical conductivities within colloidal particles of gold. Size regime of Gold Conductivity ( Q'^ m'*) 4.26x10' Bulk 1.43x10' i 2 nm

4. CONCLUSIONS We hope to have illustrated here the concept, and the experimental realisation, of the Size - Induced Metal - Insulator Transition in individual particles of colloidal gold in a sol. It is indeed remarkable that the SIMIT is observed at room temperature; a direct manifestation of quantum size - effects within such Divided Metals.

REFERENCES 1. 2. 3. 4.

P. P. Edwards et al., Sol. State Phys., 52 (1999), 229. C. N. Rao et al., Chem. Soc. Rev., 29 (2000), 27. M. Faraday, Phil. Trans., 147 (1857), 145. H. B. Weiser, A Textbook of Colloid Chemistry, 2"^ Edition, John Wiley and Sons, Inc., New York. Acknowledgements: We wish to thank Mr M. J. Edmondson for the schematic of the Copper Stripline Resonator

Studies in Surface Science and Catalysis 132 Y. Iwasawa. N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

725

Design of synthetic glycolipids for membrane biotechnology Masakatsu Hato, Joan B. Seguer,t and Hiroyuki Minamikawa Surface Engineering Laboratory, Department of Polymer Physics, National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki, 305-8565, Japan. 1. ABSTRACT Correlation between chemical structures of glycolipids and their gel to liquid crystalline phase transition temperature, Tm, was examined. This indicates combination of highly branched alkyl chains such as a phytanyl chain and maltooligosaccharide (MalN) headgroups can give a novel class of glycolipids, providing us with versatile freedom to control their physical properties and aqueous structures to meet a range of applications. 2. INTRODUCTION Glycolipids are important in both scientific and technical fields [1,2]. From the ecological view points, alkyl polyglycosides are becoming increasingly important as environmentally friendly surfactants [3]. Moreover, biological functions involved in sugar-headgroups make them attractive as a new material for various aspects of biotechnology [4]. For example, owing to their stabilizing activity of structures and functions of proteins, synthetic glycolipids have proved useful in solubilization, crystallization, and reconstitution of membrane proteins [5,6]. It is well known that a lipid/water system has its specific temperature Tm above which a hydrated solid transforms into a liquid crystalline phase. Tm is important in the sense that lipids are usable only at temperatures above Tm [7]. Tm's of glycolipids are however generally higher than room temperatures, making them less amenable to applications that would otherwise be possible [8,9]. In this communication, we examine factors that affect Tm of glycolipids and propose chemically stable glycolipids whose values of Tm are well below 0 **C . 3. EXPERIMENTAL Mal5(Ci4)2 and MalN(Ci6)2 (N = 2, 5) and Cel5(Phyt)2 were synthesized by the same method reported previously [11]. Chemical structures of the typical lipids examined are shown in Fig.l and Fig. 3. We determined Tm by a Seiko SSC/560U differential scanning calorimeter (DSC). 4. RESULTS AND DISCUSSION 4.1. Effects of headgroup stereochemistry We have previously shown that stereochemistry of sugar-headgroup has significant effects on Tm- The cellooligosaccharide (CelN) and MalN headgroups exhibit "opposite" effects on the physical properties of the glycolipidAvater systems. For MalN(Ci2)2, increasing N decreases the Tm and enhances the "hydrophilicity" of the lipids, making lipids increasingly more water soluble as N increases. Mal7(Ci2)2 is soluble in water [12]. On the other hand, the Tm of CelN(Ci2)2 tends to increase with t Present address: Laboratorios Miret S.A. C/Geminis, 4 Poligono Industrial Can Parellada, Barcelona, Spain.

726

Mali^(Cn)2

HO

p

HO

p

HO-

Y

N = l - 5 CelN(Ci2)2

Y

11=12

Fig. 1. Chemical structure of lipids. In ccllooligosaccharides (CelN), all the glucose residues are linked through P-l,4-0-glycosidic bonds. In the maltooligosaccharides (MalN), all the glucose residues are linked via a-l,4-0-glycosidic bonds. N: The number of glucose residue in the headgroup. increasing N. In particular, Tm jumps from 59 T (N = 4) to above 160 'C (N = 5) and Cel5(Ci2)2 is totally insoluble m water [10]. These headgroup effects arise from different conformations of the headgroups: the "helical" conformation for MalN and the "extended" conformation for CelN [10]- Ehie to the "helical" conformation, the cross section areas of MalN headgroup increase as N increases, while those of CelN headgroup remain nearly constant. Thus, MalN headgroups can serve as a useful headgroup both to depress Tm and control the structures of aqueous glycolipids. However, as shown in Fig. 2, Tm-depressing effects considerably diminish as alkyl chain length, n, increases. That is to say, an average Tm depression per glucose unit.

Fig. 2. Effects of alkyl chain length (n) on Tm of MalN(Cn)2- ° ' Glc(Cn)2 '• Mall(Cn)2; ^ : Mal2(Cn)2; • : Mal5(Cn)2; O: Mal3(Ci2)2 and Mal4(Ci2)2

727

dTjdN, 1).

is only 4.5 "C for MalN(Ci6)2 as compared to 14 X for MalN(Ci2)2 (Tab.

Table 1. Thennodynaniic parameters for glycolipids. Lipid

AH (kJmol-1)

CQ Mal5(Ci4)2 Glc(Ci6)2 Mal2(Ci6)2 Mal5(Ci6)2 Cel2(Ci8)2 Cel2(Ci8:l)2 MalN(Phyt)2 (N=1.5) Cel5(Phyt)2

AS (JK-l-moI-l)

18 ± la (30b ) 60 ± 1 53 ± 1 42 ± 1 76 ± 1 26 ± 1 <0

78 43 30 67 51 -

234 132 95 192 170 -

135 ± 1

41

99

ref.

[13]

a: A main endothermic peak, b: A sub-endothermic peak. 4.2. Highly branched phytanyl chain as a novel alkyi chain unit. The previous results indicated that a choice of headgroup alone is not sufficient to depress Tm- We have to find suitable hydrophobic part to achieve our goal. We here propose a phytanyl chain, glycerol di-ether of highly branched 3,7,11,15tetramethylhexadecyl group as a useful alkyl chain unit. An example of chemical structures of phytanyl-chained glycolipids, Nfel3(Phyt)2, and Cel5(Phyt)2 are shown in Fig. 3. Ability for a phytanyl chain to depress Tm was remarkable. Neither endothermic nor exothermic peak associated with phase transition of MalN(Phyt)2 appeared on DSC thermograms over a temperature range, -120 'C to 100 "C. This indicates that the values of Tm of MalN(Phyt)2 are well below 0 'C [13], in marked contrast to straightchained glycolipids such as MalN(Ci6)2 (Tab. 1). For straight-chained lipids, increased van der Waals attractions between the alkyl chains can increasingly more suppress headgroup hydration, depressing ^I^/^iV as n increases. For the highly

Mal3(Phyt)2

Fig. 3. Chemical structures of l,3-di-0-phytanyl-2-0-(P-glycosyl)glycerols bearing Mal3 and Cels headgroups. A phytanyl chain contains 16 carbon atoms in the main chain and 4 branched methyl groups.

728 branched phytanyl chain, such suppression can not occur effectively due to steric hindrance between the highly branched alkyl chains. One interesting exception is Cel5(Phyt)2 whose Tm is 135 "C. Since CelN possess the same repeating structure as cellulose, strong inter-headgroup attractions that rapidly increase with increasing N, can significantly suppress Cels headgroup hydration [8,10,11]. It is noted that cellotetraose (Cel4) already exhibits X-ray diffractions characteristic of cellulose and the aqueous solubility of CelN rapidly decreases particularly when N ^ 5. Tm may then approach a melting point of (dry) Cel5(Phyt)2, giving the value as high as 135 'C. Apart from their low Tm, one salient feature of phytanyl chain is its chemical stability. Though an introduction of double bonds into alkyl chains can be an alternative approach to depress Tm (see Tab. 1), advantages of the phytanyl chain are obvious in view of its better chemical stability. Moreover, compared with conventional ester-type lipids, the ether linkages are more stable to hydrolysis at high temperatures and low values of pH. 5. CONCLUDING REMARKS The combination of highly branched phytanyl chain and MalN headgroups gives a novel class of glycolipids that possess both good chemical stability and low values of Tm (< 0 ' Q - MalN(Phyt)2 provides us with versatile freedom to control their physical properties and aqueous structures to meet a range of applications [8,13,14]. For example, we have recently confirmed that Mal3(Phyt)2 vesicles are useful in functional reconstitution of cyanobacterial photosystem II complex (PS II) [6], representing a first evidence that a synthetic glycolipid is useful for the reconstitution of complex and labile membrane proteins, such as PS II. Glycolipids with highly branched alkyl chains of varying chain length are also of interest and will be a subject of our forthcoming publications. Acknowledgments J. S. is grateful to the STA-fellowship. We thank T. Baba for providing us with DSC data of Mal2(Ci6)2- This work was performed as part of R &D Projeas of Industrial Science and Technology Frontier Program (Physical properties of membrane protein/lipid assemblies) supported by AIST, Japan. REFERENCES 1. S. Hakomori, Pure Appl. Chem. 63 (1991) 473. 2. O. Lockhoff, Angew. Chem., Int. Ed. En^. 30 (1991) 1611. 3. K. Hill, W. von Rybinski, G. Stoll, Alkyl Polyglycosides, VCH, Weinheim, 1997. 4. T. Irimura (ed.). Sugar Chains in Biological Cell Functions, Nikkei Science Publ., Tokyo, 1994. 5. R.M. Garavito, U. Hinz, J.M Neuhaus, J. Biol. Chem., 259 (1984) 4254. 6. T. Baba, H. Minamiakwa, M. Hato, A. Motoki, M. Hirano, D. Zhou and K. Kawasaki, Biochim. Biophys. Res. Commun., 265 (1999) 734. 7. K. Shinoda, Solution and Solubility, Maruzenn, Tokyo, 1991. 8. M. Hato, H. Minamikawa, K. Tamada, Baba and Y. Tanabe, Adv. Colloid and Interface Sci., 80 (1999) 233. 9. R. Koynova and M. Caffrey, Chem. Phys. Lipids, 69 (1994) 181. 10. M. Hato, and H. Minamikawa, Langmuir, 12 (1996) 1658. 11. H. Minamikawa, T. Murakami and M. Hato, Chem. Phys. Lipids, 72 (1994) 111. 12. M. Hato, J. B. Seguer and H. Minamikawa, J. Phys. Chem. B., 102 (1998) 11035. 13. H. Minamikawa and M. Hato, Langmuir, 13 (1997) 2564. 14. H. Minamikawa and M. Hato, Langmuir, 14 (1998) 4503.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2(M)I Elsevier Science B.V. All rights reserved.

729

Wetting of ultrathin layers of polystyrene studied by atomic force microscopy Simona Loi*, Michael Wind'', Markus Preuss*, Hans-Jurgen Butt**, Hans W. Spiess^ and Ullrich Jonas^ * Institut fur Physikalische Chemie II, Universitat Siegen, 57068 Siegen, Germany ^ Max-Planck-Institut fur Polymerforschung, 55099 Mainz, Germany The wetting of ultrathin films of polystyrene on the high energy surface of mica and the low energy surface of graphite was studied by atomic force microscopy. The average thickness of polystyrene films was between 0.2 and 2 nm. Experiments showed that polystyrene spreads on graphite. On mica it forms a continuous layer only after annealing. On top of this continuous layer of 1-2 nm thickness droplets were observed. When preparing films under argon atmosphere (instead of air) drastic changes of the wetting behaviour were observed on graphite. 1. INTRODUCTION The wetting of thin polymer films on solid substrates plays an important role in many practical applications like coating, lubrication, and adhesion. Much work has been devoted to the mesoscopic regime, where the film can still be considered as a continuous medium governed by van der Waals interactions or other simple, general laws and can be described by parameters like the interfacial tensions. In particular the wetting behavior of polydimethylsiloxane (PDMS) on differently treated silicon has extensively been studied (e.g. U'^]) but also other materials were investigated by different methods (e.g. [^» ^1). In contrast, films of molecular thickness are often controlled by short-range interactions and the discrete molecular nature of the materials involved has to be taken into account. The aim of the project described in this paper was to study the wetting behavior of ultrathin layers of a polymer on high and low energy substrates. The average thickness of polymer films was between 0.2 and 2 nm. Polystyrene was chosen because of its great practical importance and its simple structure. Since we are interested in the structure on the molecular level the substrates had to be atomically smooth. Experiments were done with mica and on the basal plane of graphite. The surface energy of mica is yM=0.13-0.17 J W [^"^^1 under ambient conditions. For graphite we estimate a surface energy of yG=0.04 J/m^. Wetting and dewetting structures were imaged with an atomic force microscope (AFM).

• To whom correspondence should be addressed.

730

2. MATERIALS AND METHODS Polystyrene (PS) was synthesised by anionic polymerisation. The molecular weight, characterised by gel permeation chromatography and calibrated with PS standards, was Mw=11120 g/mol and Mw/Mn= 1.04 (Mn is Uie number average molar mass). The glass transition temperature measured by differential scanning calorimetry was Tg=107*'C. PS was dissolved in dichloromethane at a concentration of O.Olmg/ml. Samples were prepared by spin casting a drop of solution on mica or on the basal plane of graphite (HOPG, highly oriented pyrolytic graphite. Piano, Wetzlar, Germany). Within 5 s after putting the drop on a freshly cleaved mica or HOPG, the samples were rotated for 10 sec at 810 rpm. Samples were imaged at room temperature with a commercial AFM (Nanoscope III, Digital Instruments, Santa Barbara, California) in tapping mode. We used rectangular silicon cantilevers (Nanosensors, 125 ^im long, 30 ^m wide, 4 ^m thick) with an integrated tip (nominal tip radius 10 nm), a nominal spring constant of 42 N/m, and a resonance frequency of 330 kHz. Images were filtered by flattening. To get information about the interfacial energy between polystyrene and mica, YMPS, and between polystyrene and graphite, yops, adhesion measurements were done. Therefore, polystyrene particles (Bangs Labs Inc., Carmel, USA) ofR = 4.38 jim radius were glued using a small amount of epoxy resin (Epikote 1004, Shell) to the end of tipless cantilevers (Digital Instr., California, V-shaped, 100 ^m long, 0.6 ^m thick, spring constant «0.3 N/m). Spring constants of each individual spring used were determined by moving them against a reference cantilever as described before [^^l. The reference cantilever was calibrated with a method described by Cleveland et al. by measuring its resonance frequency with attached spheres [12]. Adhesion measurements were done with the particle interaction apparatus which was described in detail before [^ ^1. With this apparatus the adhesion force (pull-off force) between a polystyrene particle and a planar mica (or graphite) surface was measured. To obtain the force acting between the particle and mica (or graphite), the cantilever deflection was measured and multiplied with the spring constant. The deflection of the cantilever was determined by the change m position of a laser spot reflected off the free end onto a position sensitive device (United Detectors, UK, active area 30x5 mm^). Cantilevers were fixed to a movable cantilever holder. The experimental procedure was started by positioning the particle a few fim above the freshly cleaved sample surface. Positioning was done with a micrometer stage under optical control by use of two microscopes with long-distance lenses which were mounted mutimlly perpendicular. Then, the mica (or graphite) surface was moved towards the PS particle using a 15 |xm range piezoelectric translator (Queensgate, DPT-CS, England) until contact was established. The translator stage was equipped with integrated capacitance position sensors with an accuracy of 1 nm. At contact the maximal applied load was roughly 150 nN. After contact had been established the mica (or graphite) surface was moved away from the particle. At the point of separation, the pull-off force was determined. Complete force curves were usually taken in 20 sec time intervals. This leads to typical relative velocities of the particle of 0.5 |im/s which we regard as quasi-static. The position of the sample and the deflection of the cantilever were

731

recorded with a digital oscilloscope (12 bit effective resolution). All experiments were done at room temperature. The value of the macroscopic contact angle of PS on mica was determined by putting a heap of powder of PS on a mica surface, heating it to 150°C, monitoring its melting until it formed a liquid drop and measure the contact angle with a goniometer of a commercial contact angle measuring device (Kriiss, GIO, Hamburg, Germany). 3. RESULTS AND DISCUSSION Wetting on graphite. On graphite PS formed a layer of roughly 0.5 nm height (fig. 1). The layer did not cover the whole surface but besides the steps, which are typical for HOPG, holes with a sharply defined rim and roughly circular shape (diameters ranging from 0.5-4 |Lim) were visible. No significant changes were observed when annealing the samples at 95°C for 30 min and imaging it at room temperature. Wetting on mica. Directly after spin casting PS onto mica we usually observed drops with a height of h=14±3 nm (the error is always the standard deviation), a diameter of w=226±37 nm (fig. 2) and a typical density (number of drops per surface area) of 10-20 drops per jim^. The shape of drops was that of a spherical cap. Contact angles of drops were 14°±2°. The values of the contact angle for the drops did not deviate significantly from the macroscopic contact angle of 13°±2°. On a certain sample the drops were quite homogeneous in size. For different samples we observed a higher variation of widths, heights, and lateral densities. The detailed shape seemed to depend sensitively on the rotation time during the spin coating of the solution on the mica substrate. In order to check the effect of the solvent, we diluted the PS in toluene. Drop formation on mica did not change compared with the samples obtained from solution of PS in dichloromethane.

Fig. 1: Polystyrene spin coated on a graphite substrate. Image size: 5x5nm^, vertical scale 50 nm.

Significant changes were observed when annealing the samples. After annealing ultrathin layers of PS on mica at 95 °C for 30 min a continuous "foot" of defined thickness formed around each drop (fig. 3). The mean thickness of the foot was 1.3 nm (fig. 4). The height of drops did not change significantly (h=13±2 nm) but the diameter of the spherical cap in the centre decreased (w=128±16 nm). Accordingly, the contact angle slightly increased to 22°±2°. The perimeter of the foot was irregular and curved but not strictly circular.

732

Fig. 2: Height image in tapping mode of polystyrene on mica. Image size: 5x5^m , vertical scale 50 nm. The detailed image from which the cross-section is shown is 518x518 nm^ in size.

Fig. 3: Drop of PS on mica directly after annealing for 30 min at 95°C (top). After waiting 1 h at room temperature the bottom image was taken. Scan size: 500 x 500 nm^.

733

(O

Fig. 4.: Histogram of thickness values measured on different premelting "foot" like structures.

c O

O

In addition, each drop was surrounded by a more diffuse PS layer that extended over an area of roughly 500 nm diameter. Hence, the premelting layer consists of two regions: a foot of 1.3 nm thickness with clearly defined rim plus an extended diffuse region.

Imaging the samples after 1 h we observed that the foot increased in extension (no change in thickness). Therefore the diffuse layer around the drops started to aggregate into small islands. Thickness (nm) The height of these islands was similar to the thickness of the foot. That this aggregation into small islands was not induced by the tip of the AFM could be shown by imaging the sample after different time intervals without scanning in between. This observation shows that individual polystyrene molecules are still mobile on a mica surface although they are 80° below their transition temperature.

JL

Which factors determine the thickness of the foot? One factor is the van der Waals attraction between the substrate and polystyrene. The van der Waals force tends to pull the polymer molecules as close to the surface as possible. The van der Waals attraction is probably responsible for the flattening of the polymer molecules. Opposite to the van der Waals force acts the configuration entropy which is reduced when the polymers are forced into a thin layer. This entropic contribution tends to thicken polymer films. A thickness of 1.3 nm probably represents a thickness at which both components are equally strong. When annealing at 140°C for longer periods (t> 2 h, cooling slowly overnight) PS forms a continuous layer of defined thickness on mica (fig. 5). On top of the continuous PS layer small droplets of roughly 2 nm height and 30 to 50 nm diameter were observed. They appeared with a density of 50-60 drops per jim^ which was significantly higher than the density of drops observed directly after spin casting PS onto mica. To verify the existence of the continuous layer we scratched holes into it. Therefore we switched to contact mode scanning. When scanning at forces below w40 nN the layer remained intact and only the droplets are pushed to the side. However, directly after switching to contact mode and before being able to adjust the force to a minimal value the tip penetrates into the continuous layer and rectangular holes are formed. From the depth of these holes the layer thickness was estimated. It is roughly similar to the thickness of the foot observed at lower annealing temperatures.

734

Fig. 5: Height image of a PS sample amiealed at 140°C for 2 h. In the right image a hole of 500x500 mn^ was scraped into the PS layer.

Dewetting of PS films on high energy surfaces like silicon has been observed before i^^' ^^]. In these cases, however, PS films thicker than 5 nm were investigated and dewetting was only observed after annealing. In our case dewetting structures were observed right after spin casting while after annealing at 140°C complete wetting was observed. At first sight it is surprising to fmd more complete and faster wetting on graphite rather than on mica because the surface energy of mica is higher than the surface energy of graphite. Consequently the spreading coefficient of PS on mica, S ^/j^ -/p^ -YMPS »is expected to be higher than the spreading coefficient for PS on graphite, S ^y^-yps - /GPS • Here, yps is the surface energy of polystyrene (yps=0.042 J/m^ [^^]), YM and yo are the surface energies of mica and graphite, respectively. However, the higher surface energy of mica might be overcompensated by the higher interfacial energy of PS on mica (compared to PS on graphite). That YMPS is indeed higher than yops was confirmed by adhesion experiments. To release a PS particle from a mica surface a mean adhesion force of (152±45) nN was required. On graphite the adhesion force was only (98±28) nN. The error is due to the uncertainty in determining the spring constant. Since the adhesion force is proportional to the interfacial energy we conclude that YGPS < YMPSAt this point we would like to point out one problem concerning the quantitative interpretation of adhesion measurements. Usually the adhesion force is related by F^ = ^Tryj^p^R to the radius R of the particle and the interfacial energy [1^1. Alternatively, the relation ^adh = ^^MPs^ ^s applied for small and hard particles [^^1. Using these two equations the interfacial energies are 0.0028-0.0037 J/m^ for polystyrene/mica and 0.0018-0.0024 J/m^ for polystyrene/graphite. These values are much too low. A possible reason is the surface roughness of typically 1-2 nm of polystyrene particles (as revealed by AFM images of

735

particles). Due to the roughness the actual surface in contact with the substrate is smaller than expected for a perfectly spherical particle. Wetting in argon atmosphere. To test if water adsorbed to the mica surface influenced wetting behaviour of polystyrene AFM measurements were performed in a glove box filled with argon. Also the sample preparation was done under argon atmosphere. On mica we did not notice significant changes. Hence, adsorbed water does not seem to influence the wetting behaviour PS on mica drastically. We expect, however, that even under argon monolayer of water remains on the mica surface like as is known for silica. In contrast, drastic changes were observed on graphite. On graphite and under argon PS formed drops of «12 nm height and a width of «220 nm (fig. 6, Left). The whole graphite surface was covered by a diffuse layer. In addition, needle-like structure which were oriented parallel, 60°, or 120° with respect to each other appeared on the surface. The length of the needles variedfi-om500 to 100 nm and the height was roughly 1 nm. In order to check if the presence of the needles was due to contamination inside the glove box or to the solvent itself, we imaged the graphite surface spin casting only the dichloromethane. The images showed the bare graphite surface. Taking images of the PS samples on graphite 2 h after the preparation, the diffuse PS layer under the drops seemed to have disappeared and the needles became longer (fig. 6, Right). When we took one of this sample out from the glove box and annealed it in a oven at 120° for 30 min, the images taken in air showed that these structures were stable. These results were surprising for two reasons. First, at room temperature neither argon, nor nitrogen or oxygen adsorbs to graphite [^^1. Hence, wetting of polystyrene under argon should be similar to wetting in air. Second, the needles observed are probably polystyrene molecules which show a certain preferred orientation with respect to the underlying graphite. The graphite induces a specific lateral structure of the polymer.

Fig. 6: Polystyrene on graphite in argon. Left: Directly after spin casting. Right: 2 h after spin casting.

736 In conclusion, our results suggest that there is no simple rule to extrapolate the wetting behaviour of ultrathin films (average thickness below 2nm) from that of thin films (average thickness above 5 nm). For example, thin films of PS on a high energy surface like a bare silicon surface dewet after annealing [15]. In contrast, ultrathin films on the high energy surface of mica completely wet the surface after annealing. Hence, an ultrathin premelting layer of 1-2 nm thickness, which was previously undetected, is probably present in all cases. Drastic changes in the wetting behavior were observed when doing the experiments in an argon atmosphere rather than ambient conditions.

4. ACKNOWLEDGEMENTS We would like to acknowledge financial support the Deutsche Forschungsgemeinschaft program "wetting and structure formation on surfaces" (grant Bu 701/11). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

D. Ausserre, A. M. Picard, L. Leger, Phys. Rev. Lett. 57 (1986) 2671. J. Daillant, J. J. Benattar, L. Bosio, L. Leger, Europhys. Lett. 6 (1988) 431. F. Heslot, A. M. Cazabat, P. Levinson, N. Fraysse, Phys. Rev. Lett. 65 (1990) 599. J. De Conick, N. Fraysee, M. P. Valignat, A. M. Cazabat, Langmuir 9 (1993) 1906. C. Redon, F. Brochard-Wyart, F. Rondelez, Phys. Rev Lett. d(5 (1991) 715. U. Albrecht, A. Otto, P. Leiderer, Phys. Rev. Lett. 21 (1992) 3192. J. W. Obreimoff, Proc. Royal Soc. London 127 A (1930) 290. J. N. Israelachvili, E. Perez, R. K. Tandon, J. Colloid Interface Sci. 78 (1980) 260. H. K. Christenson, J. Phys. Chem. P7 (1993) 12034. A. Bailey, S. M. Kay, Proc. Roy. Soc. A 301 (1967) 47. M. Preuss, H.-J. Butt, Langmuir 14 (1998) 3164. J. P. Cleveland, S. Manne, D. Bocek, P. K. Hansma, Rev. Sci. Instrum. 64 (1993) 403. G. Henn, D. G. Bucknall, M. Stamm, P. Vanhoome, R. Jerome, Macromolecules 29 (1996)4305. G. Reiter, Phys. Rev. Lett. 68 (1992) 75. G. Reiter, Langmuir 9 (1993) 1344. D. K. Owens, R. C. Wendt, J. Appl. Polymer Sci. 13 (1969) 1741. K. L. Johnson, K. Kendall, A. D. Roberts, Proc. Royal Soc. London ^ 52^ (1971) 301. B. V. Derjaguin, V. M. Muller, Y. P. Toporov, J. Colloid Interface Sci. 5i (1975) 314. G. B. Hess, Phase Transitions in Surface Films 2 1991, p. 357.

Studies m Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

737

Effect of precursors on structure of Rh nanoparticles on Si02 support: iii'Situ EXAFS observation (luring CO2 hydrogenation K. K. Bando, H. Kusama, T. Saito^, K. Sato, T. Tanaka, F. Dumeignil, M. Imamura, N. Matsubayashi, and H. Shimada National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305-8565, Japan To elucidate the effect of Rh precursors on the catalytic selectivity of Si02 supported Rh catalysts in CO2 hydrogenation, the structures of Rh catalysts prepared from an acetate precursor (Rh/Si02 (A)) and a chloride precursor (Rh/Si02 (CI)) were investigated by means of in-situ EXAFS technique. The results elucidated the differences in the morphology and properties of Rh nanoparticles; the Rh particles in Rh/Si02 (CI) were found to be spherical, whereas those in Rh/Si02 (A) were suggested to be with lower-dimension and to be more easily subject to oxidation upon exposure to air than Rh/Si02 (CI). 1. INTRODUCTION Catalytic activity or selectivity of supported metal catalysts sometimes depends on the metal precursors used in the catalyst preparation. For instance, in the CO2 hydrogenation reactions over Si02 supported Rh catalysts with metal loading lower than 5 wt%, the product distribution greatly depended on the precursor used in the impregnation process; Rh/Si02 prepared from a chloride precursor dominantly produced CH4, whereas CO was the major product over a Rh/Si02 catalyst derived from an acetate precursor [1]. For precise understanding of the structure-activity relationship of catalysts, it is necessary to analyze the active catalytic sites under reaction conditions. In the present study, we conducted in-situ EXAFS analysis of the Rh/Si02 supported catalysts using a newly developed high-pressure and high-temperature EXAFS cell [2] and discussed the structure and properties of the Rh nanoparticles in the two catalysts.

"^Present Address: Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan

738

Quartz Tube

2. EXPERIMENTAL

Two supported Rh catalysts were prepared by conventional impregnation using Si02 (Davison X-Ray #57) as a support. The Si02 supports dried under vacuum at Water 473 K for 2 h were immersed into O-ring aqueous solutions of precursors, Window RhCh 3H2O (Wako) and Rh(CH3C00)s 5H2O (Soekawa), Fig.l. A schematic of an in-situ XAFS cell designed for observation under high pressure and high temperature followed by drying under vacuum conditions. The volume of Ae cell is 18 ml. at 473 K. The prepared catalysts with 1 wt% of Rh were denoted as Rh/Si02 (CI) and Rh/Si02 (A), respectively. Prior to the reaction, the catalysts were reduced at 723 K in a flow of 20% H2/Ar for 1 h. The average size of Rh particles in both catalysts was estimated at 2.3 nm by H2 chemisorption measurement. The CO2 hydrogenation reaction was carried out using a fixed bed reactor A premixed gas (25% CO2 and 75 % H2) was pressurized to 3.8 MPa and flowed over 0.3 g of each catalyst at a rate of 100 ml/min. The reaction temperature was set at 523 K.

Gas

\

\ Thermocouple ^

EXAFS measurements were carried out at BLIOB of the Photon Factory of the Institute of Materials Structure Science, High Energy Accelerator Research Organization. About 0.3 g of each catalyst was pressed into a disk with a diameter of 10 mm and set in an in-situ EXAFS cell designed for measurements under higher pressure and high temperature conditions (Fig.l) [2]. EXAFS spectra in the transmission mode were obtained in the following order; after reduction in H2, during CO2 hydrogenation and after exposure to dry air Curve-fitting analysis of EXAFS spectra was carried out using a commercially available code (REX, Rigaku Co.). Parameters such as back scattering amplitudes and phase shifts were extracted from the EXAFS spectra obtained for Rh foil at various temperatures in a flow of Ar. 3. RESULTS AND DISCUSSION Figure 2 shows Fourier transformed EXAFS spectra of the Rh/Si02 catalysts after reduction in a flow of 20%H2/Ar. The main peak at 0.24 nm in both spectra was assigned to Rh-Rh scattering. The coordination numbers (CN) determined by curve-fitting analysis were 8.1 for Rh/Si02 (A) and 9.4 for Rh/Si02 (CI) (Table 1). The particle size of Rh on

739

15 B 3

10

C

o

% c o o U

1 2 3 4 5 6 Distance / (0.1 nm) Fig. 2. Fourier Transform of EXAFS oscillations (k^X(k)) for reduced Rh/SiO^CA) (soild line) and Rh/SiO,(Cl) (dashed line). Rh/Si02 (CI) estimated from the CN of 9.4

1

2 3 4 Coordination Shell Fig.3 Coordinaiton numbers for higher shells determined by curve-fitting analysis of reduced catalysts. Circles represent Rh/SiO^CCl) and squares represent Rh/SiO (A).

was about 2.3 mn, which was consistent with that estimated by H2 chemisorption. In contrast, the Rh particle size of about 1.5 nm for Rh/Si02 (A) estimated from the CN of 8.1 was much smaller than that estimated by H2 chemisorption (2.3 nm). Note that the curve fitting analysis gave small errors (Rf < 1%) and reasonable Debye-Waller factors (about 0.007 nm). The discrepancy of the Rh particle sizes was not due to the analytical error of EXAFS or the structural distortion/disorder of Rh particles. The above particle sizes by H2 chemisorption and EXAFS methods were obtained assuming spherical particles. Thus, the discrepancy suggests that the Rh particles in Rh/Si02 (A) were not spherical. Table 1 Curve-fitting results for Rh/Si02(A) and Rh/Si02(Cl) under various conditions. Rh/Si02(A)

Rh/Si02(Cl)

Conditions Scattering Reduction Rh-Rh Reaction Rh-Rh Air rRh-Rh Rh-0 Reduction Reaction Air

Rh-Rh Rh-Rh Rh-Rh

-0.8 0.5 -1.7

a/nm** 0.0073 0.0078 0.0094 0.0070

Rfy%"' 0.14 0.54 1.87 2.31

1.48 -0.9 0.01

0.0067 0.0065 0.0071

0.08 0.5 0.4

CN 8.1 7.7 6.0 2.4

R/nm 0.269 0.270 0.269 0.202

AEo' -0.04

9.4 8.1 7.3

0.270 0.270 0.0269

* Difference in photoelectron kinetic energy between a reference and an observed specti ** Debye-Waller like factor ~ Xcaiy I z\Xobs

»where x represents a normalized EXAFS oscillation

740

35 30 25 20 15 10

5 0 1 2 3 4 5 6 Distance/(0.1 nm) Fig.4 Fourier Transform of EXFAS oscillations (k'x(k)) for air-exposed Rh/SiO^CA) (solid line) and Rh/SiO (CI) (dashed line).

Figure 3 shows the CN*s for higher shells determined by curve-fitting analysis. The third-shell CN for Rh/Si02(Cl) was about 14, which was in good agreement with the calculated CN assuming spherical particles with a diameter of about 2 nm. In contrast, the third-shell CN for Rh/Si02 (A) was 7.8, which was much smaller than that of the spherical model. The Rh particles in Rh/Si02 (CD were presumably spherical, whereas the small third-shell CN for Rh/Si02 (A) suggested that the Rh particles were not spherical but with a lower dimensional structure such as a disk-like

structure. Table 1 compares the first-shell CN of the catalysts before and during the CO2 hydrogenation reaction. The higher-shell CN could not be analyzed, because the effect of thermal oscillation was large. For both of the catalysts the first-shell CNs were slightly decreased, but no other peak was observed in the Fourier transformed spectra. Thus, the structures of Rh nanoparticles were preserved under the reaction conditions. Figure 4 shows the Fourier transformed EXAFS of the catalysts after exposure to air. The exposure slightly decreased the first-shell CN for Rh/Si02 (CI) with little change in the Debye-Waller factor (Table 1). In contrast, the decrease of that for Rh/Si02 (A) was significant together with large increase in the Debye-Waller factor. In addition, a new peak assigned to Rh-0 scattering appeared at 0.16 nm. These results indicate that the Rh nanoparticles in Rh/Si02(A) was deformed and easily subject to oxidation upon exposure to air. This may be related to the lower-dimensional structure of Rh particles with large surface area. Thus, the Rh particles in Rh/Si02 (A) had different morphology and different affinity with oxygen from Rh/SiO: (CI). The high affinity of Rh/Si02 (A) toward oxygen likely results in high affinity toward CO2 and is thus related to the characteristic catalytic selectivity in the hydrogenation of CO2 [1]. References 1. H. Kusama, K. Okabe, K. Sayama, and H. Arakawa, Stu. Suf Sci. Catal., 114 (1998) 431. 2. K. K. Bando, T. Saito, K. Sato, T. Tanaka, F. Dumeignil, M. Imamura, N. Matubayashi, and H. Shimada, J. Sync. Rad., accepted.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

'^^

Reduction of Photocurrents from Modified Electrodes with Cdj.xMnxS Nanoparticles in the Presence of Magnetic Fields H. Yonemura*, M. Yoshida, and S. Yamada Department of Materials Physics and Chemistry, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Modified electrodes with diluted magnetic semiconductor nanoparticles (Q-Cdi.xMnxS) were fabricated by immobilizing them on a self-assembled monolayer of hexanedithiol prepared on the Au electrode. Photoirradiation of the Q-Cdi.xMnxS-modified electrode in the presence of triethanolamine afforded stable anodic photocurrents. In the presence of external magnetic fields, larger reduction (-8%) of photocurrent from the Q-Cdi.xMnxS-modified electrode was observed as compared with that from a cadmium sulfide nanoparticles (Q-CdS). This magnetic field effect is most likely ascribed to the quantum size effect (quantum confinement effect) and exchange effects in the exciton states due to Mn^^ ions in Q-Cdi.xMnxS. 1. Introduction Recently, semiconductor nanoparticles have intensively been studied because of their properties as quantum size effects [1]. We have reported, for the first time, magnetic field effects (MFEs) on photoelectrochemical reactions of modified electrodes with Langmuir-Blodgett films of porphyrin-viologen linked compounds [2,3]. We have also observed MFEs on photocurrents of modified electrodes with cadmium sulfide nanoparticles (Q-CdS) ascribing to their quantum size effects [4], but they were smaller than those on the modified electrodes with the porphyrin-viologen linked compounds. In diluted magnetic semiconductor and its nanopartciles, variety of unusual magnetic and magneto-optical properties due to exchange interaction between the band electrons and the magnetic ions have been reported [5,6]. In this study, we have found that the MFEs on the photocurrent responses from modified electrodes with diluted magnetic semiconductor nanoparticles (Q-Cdj.xMnxS) •Corresponding author. Tel.: +81-92-642-3580; fax: +81-92-642-3611. E-mail address: [email protected] (H. Yonemura)

742

were substantially enhanced by the presence of Mn^^ ions. 2. Experimental Q-Cdi-xMnxS and Q-CdS were prepared by the use of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reversed micelles following the method as reported by Steigerwald et.al. [7]. Co-precipitation took place by mixing two kinds of AOT reversed micellar hepatane solutions with water-to-surfactant ratio (W-value =[H20]/[A0T]); one is made of a reversed micelle containing NaiS, and the other is made of a reversed micelle containing Cd(C104)2 and Mn(C104)2 or Cd(C104)2. The relative amount of Mn^* ion compared to Cd^"" ion in the semiconductor is defined as Q-Cdi.xMnxS or Cdi.xMnxS, where X=[Mn^'^]/([Cd^^]+[Mn^^]), by assuming that all of Cd^"", Mn^^, and S^" in the mixed solution formed the foregoing particles. The size of Cdi.xMnxS particle was controlled by the W-value of the heptane solution; larger Cdi-xMnxS particles were obtained for a large W-value (W=54). A self-assembled monolayer (SAM) of 1,6-hexanedithiol was prepared by immersing on a Au electrode in an ethanol solution of 1,6-hexanedithiol (0.1 mmol dm"^). The modified electrode with Q-Cdi-xMnxS, Q-CdS, or Cdj.xMnxS was fabricated by immersing the SAM electrode in the AOT heptane solution containing Q- Cdi.xMnxS, Q-CdS, or Cd^xMnxS particles. Measurements of photocurrents and their MFEs were carried out using a three electrode cell as described in the previous procedure [4]. Spectroscopic studies were carried out by electronic absorption (Shimadzu UV-2200) and fluorescence (Hitachi F-3010) spectrometers. 3. Results and discussion Absorption spectra of Q-Cdi.x MnxS with various W-values and X-values were observed in hepetane solutions. In the case of the identical X-value, both the absorption onset and the absorption peak showed blue-shift as the W-value decreased. The emission peak of Q-Cdi.xMnxS in the hepetane solution was also blue-shifted with decreasing the W-value. These resuhs are ascribed to quantum size effects. In the case of the identical W-value (W=3.0), the presence of

1.5 1 h JO

<

0.5 h

300

350

400

450

500

X /nm Fig.l Absorption spectra of Q-CdS (a) and Q-Cdi.xMnxS (X=0.2) (b) particles in heptane solution containing AOT reversed micelles (W=3.0).

743

Mn ^ ions (X=0.2) resulted in blue-shifts of both the absorption onset and the absorption peak (Fig. 1). The emission peak was also blue-shifted slightly in the presence of Mn^^ ions. These results are in good agreement with the previous report [8]. Photoirradiation of Q-CduxMnxS 3 (X=0.2,W=3.0)-modified electrodes OT ^JK76X_^ afforded stable anodic photocurrents (Fig. off off ^ ^ >^ y 2). In the presence of the magnetic field n nnn (0.76T), the photocurrents clearly i 1.5 a decreased (left side of Fig. 2). The magnitude of MFEs is evaluated by the 0.5 VJ ' ^ >J v j VJ VJ U t t 4 ton ^ ^ ^ following equation; A=(I(0)-I(H))/I(0) X 0 0 5 10 15 100, where the photocurrents in the Time (min.) absence and presence of the magnetic Fig.2 MFEs on the photocurrents of the field are denoted by 1(0) and 1(H), Q.Cd,-xMnxS (X=0.2,W=3.0)- modified respectively. In Fig. 2, the photocurrent electrode (E=0.4 V vs Ag/AgCl). was reduced for -8% (A=~8 %). The modified electrode with Q-CdS (W=3.0) was examined as a reference system [4]. Stable anodic photocurrents and the MFEs were also observed. However, the magnitude of MFEs at 0.72 T (A=3 %) was considerably smaller than that of the Q-Cdi-xMnxS-modified electrode (Fig. 2). In addition, the magnitude of MFEs in the Q-CdS-modified electrodes (A='-3 %) did not change appreciably in the range of W=3.0-5.4, in spite of different particle size. These results strongly suggest that the magnitude of MFEs on the photocurrent of the Q-Cdi.xMnxS-modified electrode is enhanced by the presence of Mn^"^ ions. MFEs on exciton emission have been reported in diluted magnetic semiconductor microcrystallites in a Si02 glass support; the red-shift and the enhancement of the emission band have been observed in the presence of magnetic field [9]. In those cases, the MFEs are caused by the exchange interaction of the exciton with incorporated Mn^^ ions in the semiconductor. The modified electrode with large Cdi.xMnxS (X=0.2) particles was also examined. Although stable photocurrents were generated in the anodic direction, no MFEs on the photocurrents were observed. The result is fairly well consistent with the previous result in the Q-CdS system [4] showing the presence of the quantum size effect (quantum confinement effect) on the reduction of photocurrent induced by the magnetic field (Fig. 2).

I .

On the basis of these observations, the large MFEs observed in the Q-Cdi-xMnxS-modified electrode are probably explained by both the quantum size effect (quantum confinement effect) and the exchange effects in the exciton states, which are caused by the incorporated Mn^"" ions. Recently, magnetic interactions of Q-Cdi.xMnxS have been

744

found to increase with decreasing particle sizes, have been reported [10]. Further investigations using Q-Cdi.xMnxS-modified electrode with different X- and/or W-values as well as other diluted magnetic semiconductor nanoparticles are in progress to elucidate the role of magnetic ions. Acknowledgments The authors wish to thank Mr. H. Horiuchi, in preparing the three-electrode photoelectrochemical cells. The present study was financially supported by KAWASAKI STEEL 21st Century, Shinkagakuhatten, and the Murata Science Foundations and by the following Grant-in-Aid for Scientific Research: Priority Area of "Electrochemistry of Ordered Interfaces" (No.l 1118257), and Encouragement of Young Scientists (No. 12750737). References 1. S. Ogawa, F. -R. F. Fan, A. J. Bard, J. Phys. Chem., 99, 11182 (1995) and references cited therein. 2. H. Yonemura, K. Ohishi, and T. Matsuo, Chem. Lett., 661 (1996). 3. H. Yonemura, K. Ohishi, and T. Matsuo, Mol. Cryst. Liq. Cryst., 294, 221 (1997). 4. H.Yonemura, M. Yoshida, S. Miyake, and S. Yamada, Electrochemistry, 67,1209 (1999). 5. J. K. Furdya, J. Kossut, Semiconductors and semimetals; Academic: New York (1988), Vol. 25. 6. N. Feltin, L. Levy, D. Ingert, M. R Pileni, J. Phys. Chem.B, 103, 4 (1999). 7. M. L. Steigerwald, A. R Alivisatos, J. M. Gibson, T. D. Harris, R. Kortan, A. J. Muller, A. M. Thayer, T. M. Duncan, D. C. Dougalss, L. E. Brus, J.Am.Chem.Soc, 110, 3046 (1988). 8. L. Levy, J. R Hochepied, M. R Pileni, J. Phys. Chem., 100, 18322 (1996). 9. Y. Yanata, K. Suzuki, and Y. Oka, J. Appl. Phys., 73,4595 (1993). 10. N. Feltin, L. Levy, D. Ingert, M. R Pileni, Adv. Mater., 11, 398 (1999).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) © 2001 Elsevier Science B.V. All rights reserved.

745

Ethylene hydrogenation on fee Co thin films grown on Ni(lOO) surfaee C. Egawa, H. Iwai and S. Oki Dept. of Energy and Environ. Sci.,Utsunomiya Univ., Mine 350,Utsunomiya 321-8505, Japan Epitaxial layer-by-layer growth of fee Co thin films was estimated up to 6 ML on Ni(lOO) at 300 K by a sharp (1x1) LEED pattern and the breaks in the uptake curves of Ni and Co 2p XP signals. The Co thin fihns of 1 - 3 ML display the dispersion of the valence band along T-K direction in the Brillouin zone in a similar way as the Ni(lOO) substrate. At 6 ML, the energy band moved upwards in consistent with the shift of the unoccupied states. In closely corresponding to the electronic states of Co thin films, ethane evolution from the reaction of ethylene with pre-saturated hydrogen atoms increased considerably for 1 - 3 ML fihns and decreased at the coverage of 6 ML. 1.

Introduction It is well established that the materials with unstable crystal phases under ordinary thermodynamic conditions can be fabricated by thin fihn growth employing suitable substrates. Although Co undergoes an hep to fee phase transition at 690 K, an epitaxial growth of Co on Ni(lOO), as an example, is demonstrated to form fee films up to 30 ML at room temperature with no tetragonal expansion of the Co lattice in the direction normal to the interface [1]. In contrast, very thin Co fihns on Cu(lOO) are known to show a tetragonal distortion (about 4-5%) in the cubic overlayer to accommodate the compressive stress at the interface [2]. Since it is shown that small changes in the films' structural parameters show up clearly in the electronic structure [3], the effects of tetragonal distortion on the electronic states will be of great importance to understand the reactivity of the overlayers. The aim of the present study is to investigate the surface electronic properties of fee Co fihns and correlate them with the reactivity for the hydrogenation reaction of ethylene. 2. Experimental The experiments were performed in an UHV chamber with facilities for XPS, UPS, LEED and TPD as described previously [4]. TPD measurements were made by a QMS using a linear heating rate of 3 K/s. The Ni(lOO) crystal can be cooled to 100 K using liquid nitrogen and resistively heated to 1200 K. The temperature was measured by a chromel-

746 alumel thermocouple spot-welded to the side edge of the crystal. The Ni(lOO) crystal was cleaned by repeated Ar ion sputtering and annealing at 1200 K procedures and Co fihns were newly prepared by evaporating a Co wire on the clean surface kept at room temperature, because of the carbon contamination resulting fh)m TPD. Research grade reactant gases were used without further purification. 3. Results and discussion The growth mode of Co thin fihns on Ni(lOO) was studied by the intensities of Co and Ni 2p core level photoelectrons taken at the collection angle of 36° from sample normal direction as a function of deposition time. The curves exhibit the exponential form for the Co overlayer as well as the Ni substrate and can be approximated by a series of straight-line segments expected for layer-by-layer growth at least up to 6 ML. The attenuation coefficient of the Ni signal by a monolayer of Co is found to be 0.64, which leads to a mean free path for 400 eV electron of about 2.6 ML taking the corrected escape depth into consideration. It is a reasonable value compared with those reported in the literature [5]. The layer-by-layer growth is also consistent with appearance of a sharp (1x1) LEED pattern with a low back ground up to 6 ML thick. The change of the electronic structure of the Co fihns over 1 - 6 ML thick has been followed by angle-resolved He I photoemission in the plane of (011) taken at 30° off-normal direction as presented in Fig. 1. The Ni(lOO) substrate spectrum displayed a main peak at 1.6 eV with a shoulder at 0.5 eV below the Fermi level. Both of two peaks slightly shifted to -1.7 eV and -0.7 eV binding energies on Co 1 ML film. On the other hand, these peaks showed a shift towards the Fermi level at 3 ML in consistent with the increase by 0.2 V in the work function of Co fihns. However, a main peak remained at - 1.1 eV binding energy at 6 ML. ; ; / ;\ : It is confirmed by ARUPS measurements :/ : : : • f : •• : along the [Oil] direction of the surface that 6ML| the Co overlayer-induced structure clearly shows the similar dispersion of two1 3M]LJ^_^ dimensional state as the Ni substrate along ill ^LI surface Brillouin zone of A line. It indicates that the shoulder state present up to 3 ML I IMLi H - - ^ A r v \ i overlayer around 0.5 eV below EF moves upwards and intercept the Fermi energy. ] OML' Consistently, in inverse photoemission the As

-yjm-

^ M \N

unoccupied state is observed at 0.55 eV above the Fermi level for Co overlayers more than 5 ML thick on Ni(lOO) [6]. In order to correlate the electronic

1

.7 ^

i

1

.5 ^

i

i

i i — - ^

.3 - 2 - 1 0 1 EF ®*"^*"*S ^"^"^ ^ ^^ ^'^'^ ARUPS spectra for Co thin films on Ni(lOO) surface taken at 30^ off-normal.

747

states of the Co films with the chemical reactivity, we have studied adsorption of hydrogen as a function of fibn thickness as shown in Fig. 2. Hydrogen desorption appeared as a broad peak at 300 K with a shoulder around 240 K in TPD from the clean Ni(lOO) surface. Deposition of the first Co overlayer produced a desorption peak at 200 K together with a shift of the main peak to 270 K. In contrast, TPD spectra from the Co films above 2 ML thick gave rise to one broad peak. The weakening of hydrogensurface bonding on the Co 1 ML is characteristic 150 200 250 300 350 400 450 of the chemisorptive properties derivedfix)mthe Temperature / K formation of an interface state. Fig. 2 H2 TPD spectra after adsorption of hydrogen at 100 K on Co thin fihns. The hydrogenation of ethylene to form ethane under UHV was examined after saturation of the hydrogen pre-adsorbed Co overlayers with ethylene at 100 K. Fig. 3 provides TPD spectra of ethylene and ethane (small dots) evolved simultaneously as a function of Co fihn thickness. H2 desorption features in the corresponding TPD are plotted as small dots with those from adsorption of hydrogen only in Fig. 2. The direct comparison clearly demonstrates that H2 desorption state at lower temperature lost its population but instead the intensity at a maximum desorption temperature increased. It indicates that this low temperature hydrogen state is involved in the hydrogenation of ethylene. It is in good agreement with ethane evolution at the same temperature shown in Fig. 3. On the other hand, there are at least three states observed in the ethylene desorption from the Ni(lOO) surface. Ethylene desorption at the lowest temperature is considered to be very weakly adsorbed ethylene as similar to physisorption. The concomitant appearance of the shoulder ethylene peak and ethane evolution around 240 K suggests that ethylene and ethane are produced from a common intermediate, i.e., surface ethyl species. Since the hydrogenation of 150 200 250 300 ethyl species occurs at a low Temperature / K temperature below 180 K on Ni(lOO) [7], the hydrogenation of ethylene is Fig. 3 TPD spectra following the adsorption of ethylene at 100 K on H-preadsorbed Co thin films.

748 limited by the formation of surface ethyl species. Accordingly, a main peak at 170 K could be assigned as desorption of chemisorbed ethylene. Addition of Co 1 ML on the Ni(lOO) surface considerably shifted the desorption of this chemisorbed ethylene to a lower temperature in a similar manner as H2 desorption, giving a broad feature by merging into the physisorption state. The weakened ethylene bonding results in a considerable increase in ethane formation as indicated by a larger peak at 190 K in the TPD spectra. Upon increasing Co thickness up to 3 ML the low temperature peak of ethylene was clearly separated by the shift of chemisorbed ethylene to a higher temperature. It is interpreted that stabilization of chemisorbed ethylene is almost compensated by the increased stability of surface ethyl species. Hence this gives no effect on the activation energy for the initial C-H bond formation involved in the hydrogenation of ethylene and does not affect on the amount of ethane production. On the contrary, 6 ML Co film caused a reduced ethane formation with the marked increase of the desorption temperature by 20 K. The reduction in the ethane TPD area due to the increased surface-ethylene bonding energy is closely related with the upward shift of the d-band states of Co fihns. Thus, the rate of ethylene hydrogenation critically depends on small changes in the electronic structure of fee Co thin fihns. 4. Summary XPS and LEED studies indicate the epitaxial growth of Co films on Ni(lOO) at least up to 6 ML coverage. At 1 ML Co fibn the bonding energies of hydrogen atom as well as ethylene molecule are weakened, thus giving a large evolution of ethane around 190 K. The ethane formation critically depends on the Co film thickness, showing a close relation with the upward shift of the d-band states of Co fihns. Financial support by Toray Science Foundation and the Ministry of Education, Science and Culture is grateftilly acknowledged. References 1. S.A.Chambers, S.B.Anderson, H.-W.Chen and J.H.Weaver, Phys. Rev. B35, 2592 (1987). 2. A.Clarke, G.Jennings, R.F.Willis, P.J.Rous and J.B.Pendry, Surf. Sci., 187, 327 (1987). 3. C.M.Schneider, P.Schuster, M.Mammond, H.Ebert, J.Noffke and J.Kirschner, J. Phys. C 3,4349(1991). 4. C.Egawa, S.Katayama and S.Oki, Chem. Phys. Lett., 266 (1997) 169. 5. C.D.Wagner, W.M.Riggs, L.E.Davis, J.F.Moulder and G.E.Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Division, Perkin-Elmer Corporation, Eden Prairie, Minnesota, 1979. 6. C.Hwang and F.J.Himpsel, Phys. Rev. B52,15368 (1995). 7. C.Egawa, D.Shimizu, H.Iwai and S.Oki, in preparation.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

749

Behavior of pyridine on a TiO 2(110) surface studied by Density Functional Theory Takehiko Sasaki ^ Ken-ichi Fukui ^ and Yasuhiro Iwasawa ^ a: £>epaitment of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan b: Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Density Functional calculations were performed to explain the behavior of pyridine adsorbed on a TiO2(110) surface observed by STM and to gain a detailed picture for the interaction between pyridine and the surface. Adsorption energies for various conformations of pyridine interacting with defect-free TiO2(110) surface were calculated. The most stable adsorption state for pyridine is the upright conformation with the 2, 6-hydrogen atoms interacting with bridging oxygen atoms via a hydrogen-bond like interaction. The flat-lying pyridine was found with a low surface diffusion barrier along the [001] direction, that corresponds to a movable species observed by STM. The effect of defect sites were also examined and the 4-fold coordinated Ti atom obtained by the removal of two adjacent bridging oxygen atoms was found to be a stronger Lewis acid site than the 5-fold coordinatedTi atom. 1. Introduction Metal oxides are quite versatile materials used as components for catalysts, ceramics, electronic devices and so on. Roles of metal and oxygen ions are associated with various properties of metal oxides. It is important to understand and control the acidic property of oxide surfaces. We have been studying pyridine adsorption on Ti02(l 10) using STM to gain insight on active sites of metal oxide surfaces as well as interaction between the basic molecule and surface defect sites. Fig. 1 shows a schematic model for TiC>2(l 10) surface and an STM image for pyridine adsorption on the surface [1]. TiO2(110) surface consists of alternating rows of the 5-fold coordinated Ti atoms and rows of the bridging oxygen atoms along the [001] direction. The protrusion of the bridging oxygen from the layer containing Ti atoms and in-plane oxygen atoms is 0.124 nm for the bulk terminated structure and 0.093 nm measured by the surface X ray diffraction [2]. Pyridine molecules on Ti02( 110) were imaged as bright spots along the Ti rows (Fig. 1 right) and most of them are mobile at room temperature, indicating that the interaction with the substrate is rather weak. In the STM observation of 4-methyl pyridine on TiC>2(110) [3], immobUe flat-lying and upright species were found in addition to the mobile flat-lying species from the height analysis of the STM images. Furthermore, pyridine was found to strongly adsorb on step sites with several specific ledge angles, indicating that the 4-fold coordinated Ti atoms exposed to the surface are responsible for the strong interaction [4]. The aim of this work is to obtain the pictures for the interaction between pyridine and TiO2(110) by means of a conq)Utational method, which would be helpful for the understanding of acid-base interaction on solid surfaces including the effect of surface defects.

750 lUd]

H-adatom a

b

c

mn

Fig. 1 (Left) A schematic model of TiO2(110Hlxl) surface (top and side views) including defect sites (a^). Filled dide, open circle and shaded dide ooirespond to Ti atom, the bnc|ging oxygen, and the in-plane oxygen, respectively. Hydogen adatom and defect models arc included Defect sites correspond to (a) vacancy of the bridjging oxygen, (b) vacancy of the 5-fold coordinated Ti atom, and (c) vacancy of the two adjacent bridging oxygen atoms. (Right) A constant current STM image of pyridne adsorbed on Ti02(l 10) at room temperature. Sample bias voltage = +2.5 V, tunneling current = 0.30 nA, 8 x 8 nm^.

2. Method The program used in this study was CASTEP working with Cerius2 interface released from Molecular Simulations Inc. Single point calculations were done with plain wave basis sets and the Perdew-Wang 1991 generalized gradient approximation (GGA). In this study TiO2(110) surface with the bulk terminated structure was treated as a slab model with 1 nm crystal thickness comprised of three layers (3 layers of 2 x 2 superstructure) and 2 nm vacuum spadng. The actual unit cell size was 1.271 nm x 0.579 nm x 3.000 nm. The structure of pyridine molecule was also kept at the gas phase structure. 3. Results and Discassion As to the interaction between pyridine molecule and the TiO2(110) surface without defects, a flat-lying conformation (Fig. 2a) and two upright conformations (Fig. 2b and c) on the Ti row were examined, since STM images of pyridine molecules on Ti02( 110) were found along the 5-fold coordinated Ti rows [1,4]. For the flat-lying configuration the nitrogen atom of pyridine was positioned above the 5-fold coordinated Ti atom. Later, the lateral displacement was considered. For the upright pyridine the nitrogen atom interacts with the 5fold coordinated Ti atom with the C2v axis perpendicular to the surface. The distance between the molecule and the surface was varied in the range 0.2 nm to 1.2 nm. The distance of 1.2 nm was taken for the case without the interaction between the substrate and die molecule. Adsorption energy as a function of die Ti-N distance for the flat-lying and upright conformations is shown in Fig. 3. The flat-lying pyridine takes the most stable adsorption state (11.3 kcal/mol) with the distance from the first layer of 0.325 nm. The upright conformation with die molecular plane parallel to [110] (perpendicular to die bridging oxygen rows. Fig. 2b) was found to be much more stable than the one with the molecular plane parallel to [001] (along die bridgmg oxygen rows. Fig. 2c). The maximum adsorption energy of 37.76 kcal/mol was found at Ti-N distance of 0.25 nm. The larger slab model (2x3x3 layer) was adopted for die calculation of this conformation, yielding an adsorption energy of 38.23 kcal/mol. This diff^ence of 0.47 kcal/mol is indicative of die accuracy of the present calculation. The stabilization due to hydrogen bonding between 2- and 6hydrogen atoms of pyridine and bridging oxygen atoms seems to be operative. The distance

751

(a)

Fig. 2 Adsoiption modds of pyridne on TiO2(110). [lIO], (c) upright pyridine parallel to [001].

(a) Flat-lying pyridne, (b) upright pyridne parallel to

between the hydrogen atom and the bridging oxygen is 0.173 nm and the stabilization due to a pair of O and H is 9.71 kcal/mol. The mobility of adsorbed pyridine was examined by changing the position of adsorbed pyridine. When the most stabilized upright pyridine (Fig. 2b) was moved 0.1447 nm along [001] corresponding to the midpoint between two adjacent Ti atoms at the fixed Ti-N distance, the adsorption energy decreased by 0.56 eV (12.9 k ^ m o l ) still in the attractive region. 0.56 eV can be roughly taken as an activation energy for diffusion along [(X)l]. Judging from this value, the upright pyridine is immobile at room temperature. In the case of displacement along [lIO] the destabilization was 3.1 eV, indicating that the diffusion along [110] is quite difficult Fig. 4 shows the change of adsorption energy as a function of azimuthal angle for the flat-lying pyridine that shows the nitrogen-end of pyridine is aligned along the [001] direction. The change of adsorption energy of the flat-lying pyridine was examined with varying displacement along the [001] direction at the fixed Ti-N distance of 0.325 nm. The activation energy of the diffusion along the [001] direction for the flat-lying pyridine was found to be 3 kcal/mol, indicating that the movable pyridine at room temperature in the STM observation corresponds to this flat-lying pyridine. Interconversion from the flat-lying pyridine to the upright one was examined as shown in Fig. 5. This figure was obtained by rotating the flat-lying pyridine at 0.325 nm from the Ti atom with respect to the axis that is running through the center of the molecule and parallel with [110] direction. i V

1

1

1

T—11

0.5 0.0

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|-0.2

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|-0.3

5-0.4

1

^

_ y^

-| H

/

4H J

&-0.5

'

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30

40

1-0.6 -1.5 UL

,1

,1.

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1

\\

-0.7 t 0

l _ J' 10 20

-0.5* s ; — 1

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-0.7

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-0.8 -10

1 0

11 10

1 20

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4 6 8 10 12 Rotation angle / degree Rotation Angle / degree n-N distance/O.I nm Fig. 3 (Left) Adsorption energy for pyridine on Ti02( 110) as a function of Ti-N distance, Square, circle and triangle symbols correspondto Fig. 2 (a), (b) and (c), respectively. Fig. 4 (Middle)Adsorption energy of the flat-lying pyridine as a function of the azimuthal angle. Fig. 5 (Right) Adsorption energy of the flat-lying pyridine versus a rotation angle with respect to the axis that is running through the center of the pyridine and parallel to [iTO]. 2

752

Transition from the flat-lying cx>nformation to the upright conformation was found to be an easy process without a potential barrier. However, most of the observed particles were mobile in the STM observation at room temperature [1]. There seems to be some mechanism to exclude the immobile upright species. The presence of the surface hydrogen was reported to be 0.13 ML for the regularly prepared clean surface [5]. These surface hydrogen atoms might block the upright pyridine. And/or a different type of adsorbed pyridine might appear via a hydrogen bonding. Preliminary calculations indicated that hydrogen adatom located above the bridging oxygen corresponding to a hydroxyl group, interacts with the flat-lying pyridine azimuthally rotated by the right angle with an adsorption energy as pyridine by 2.0 kcal/mol larger than the normal flat-lying pyridine on a clean surface. Flat-lying pyridine interactingwith hydroxyl group is likely to be associated with the immobileflat-lyingspecies. The influence of defect sites on TiO2(110) surfaces was also investigated because defect sites play important roles for surface processes [6]. As to the possible defect models, (a) vacancy of the bridging oxygen, (b) vacancy of the 5-fold coordinated Ti atom, and (c) vacancy of the two adjacent bridging oxygen atoms as depicted in Fig. 1 were considered for die stabilized pyridine adsorption states. Here, the structural relaxation around defect sites was not considered and the effect of the formation of defect sites on the adsorption energy was estimated. Single atom vacancy formation resulted in a small destabilization for adsorbed species by 0.5 - 5 kcal/mol, indicating that single atom vacancy is not intportBui for pyridine adsorption. As to the case (c), the 4-fold coordinated Ti atom obtained by die removad of two adjacent bridging oxygen atoms was examined using the 2x3x3 slab model. The upright pyridine adsorbed on this site with the Ti-N distance of 0.25 imi gave the adsorption energy of 1.214 eV (28.0 kcal/mol), indicating that die Lewis acidity of diis site is stronger tiian that of die 5-fold coordinated Ti atom without a hydrogen-bond Uke interaction (Fig. 2c). This is in line with the importance of 4-fold coordinated Ti atom as evidenced by the strongly adsorbed pyridine at step edges with specific directions, though the geometry is different in detail [4]. The present computational study indicates that the hydrogen bonding between die 2, and 6- hydrogen atoms of pyridine and the bridging oxygen atoms is important in addition to the traditional acid-base interaction between the pyridine nitrogen atom and the coordinative unsaturated T i ^ ion. Three kinds of pyridme adspecies on the Ti-row corresponding to the STM observation were found in the order of adsorption strength as follows, upright pyridine with hydrogen bonding to the bridging oxygen atoms > flat-lying pyridme with hydrogen bonding widi hydrogen adatom on the bridging oxygen > mobile flat-lying pyridine. This work has been supported by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST). References [1] S. Suzuki, Y. Yamaguchi,H. Onishi, T. Sasaki, K. Fukui and Y. Iwasawa, J. Chem. Soc. Farad. Trans., 94(1998) 161. [2]G. Charlton, P.B. Howes, C.L. Nicklin,P. Steadman, J.S.G. Taylor, C.A. Muryn, S.P. Harte, J. Mereer, R. McGrath, D. Norman, T.S. Turner and G. Thornton, Phys. Rev. Lett. 78(1997)495. [3] S. Suzuki, H. Onishi, T. Sasaki, K. Fukui and Y. Iwasawa, Catal. Lett. 54 (1998) 177. [4] S. Suzuki, Y. Yamaguchi.H. Onishi, K. Fukui, T. Sasaki and Y. Iwasawa, Catal. Lett., 50(1998)117. [5] S. Suzuki, K. Fukui, H. Onishi, and Y. Iwasawa, Phys. Rev. Lett, 84(2000) 2156. [6] MA.Barteau, Chem. Rev., 96 (1996) 1413.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ic> 2001 Elsevier Science B.V. All rights reserved.

753

Observation of individual adsorbed pyridine, ammonia, and water on TiO2(110) by means of scanning tunneling microscopy Shushi Suzuki*^ Ken-ichi Fukui'.Hiroshi Onishi*^, Takehiko Sasaki\ and Yasuhiro Iwasawa* 'Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ^Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Adsorption states of pyridine, ammonia (NH3) and water (HjO) molecules on TiO2(110)(1x1) were studied by scanning tunneling microscopy (STM). Water molecules may be dissociated at step sites and gave OH species at the step edges. Different activity for ammonia adsorption depends on the orientation of the step in azimuth, which is similar to pyridine adsorption. 1.

Introduction The acidic property of oxide surfaces is one of the key factors to understand and control the catalytic reactions on oxide catalysts [1]. Scanning probe microscopy has great advantages on structural studies of the inherently heterogeneous oxide surfaces, discriminating and visualizing surface atoms and molecules at each site with different properties [2]. In the present study, we have studied adsorption of ammonia and water molecules on a typical oxide surface of TiO2(110)-(lxl) by scanning tunneling microscopy (STM). Ammonia is a typical Lewis-base molecule which is often used to probe acidity of oxide surfaces. Adsorption behavior of ammonia on Ti02(l 10) will be discussed comparing with that of pyridine imaged by STM, which is also a typical Lewis-base compound [3, 4]. Site-specific adsorption similar to that of pyridine was visualized. Adsorption of water molecules on Ti02(l 10) has been a target of many theoretical studies [5-7] and experimental studies with various macroscopic methods [8-12], however, some discrepancies exist between these results. STM observations revealed that the coverage at room temperature was very small, and the minor species, which may be dissociation products, were observed at step edges. 2.

Experimental The experiments were performed with an ultrahigh vacuum (UHV) STM (JEOL JSTM4500VT). A polished rutile TiO2(110) sample (Earth Chemicals) was cleaned by cycles of Ar^ sputtering followed by UHV annealing at 900 K. The sample was exposed to ammonia or water at room temperature (RT) through a doser by 3.0 L (1 L=133xl0^ Pa s). All the STM images were recorded at RT at V,=+1.0 V and 1^=0.05 nA. * Present address: Catalysis Research Center, Hokkaido University, Sapporo, Hokkaido 060-0811, Japan "Present address: Kanagawa Academy of Science and Technology. Sakado, Takatsu-ku, Kawasaki. Kanagawa 213-0012. Japan

754

3. Results and Discussion 3.1 STM observation of ammonia adsorption on TiOjCl 10) Figure 1 shows a typical STM image of an ammonia-exposed TiOjCllO) surface. The TiO2(110)-(lxl) surface is composed of the alternating rows of the five-fold coordinated Ti atoms and the bridging oxygen ridges. It is now established that STM observes the Ti rows as bright contrast at positive sample bias voltages [13]. Bright round protrusions observed on the bright Ti rows (A in Fig. 1) are derived from ammonia and other protrusions with vague contrast between the bright rows (B in Fig. 1) are hydrogen adatoms on the bridging oxygen rows (hydroxyl hydrogen). The species B was formed on the surface during the cleaning procedure and was recently identified by electron-stimulated desorption (ESD) and STM [14]. The species A had an average height of 0.09±0.02 nm and the coverage between 0.01 and 0.03 ML, where 1 ML is defined as the density of the (1x1) units, 5.2x10'^ m l The species A showed a round protrusion in most cases, but it changed their positions frame by frame during successive STM observations. The mobility of the species was smaller than that of adsorbed pyridine, which migrated rapidly on the surface and sometimes gave a fragmental image [3,4]. Previous works suggested that ammonia adsorbs on TiO2(110)-(lxl) in a molecular form. No N-containing species were detected by X-ray photoelectron spectroscopy (XPS) after heating an NH3-covered TiOjCllO) surface to 395 K [15], suggesting no thermally induced dissociation of NH3. An ultraviolet photoelectron spectroscopic (UPS) study [16] also suggested the molecular form at low coverages at 300 K, with some residual amount of NHj and OH radicals. A periodic Hartree-Fock calculation showed that molecular adsorption was energetically favored than dissociative adsorption [17]. The present TiO2(110) surface possessed the hydrogen adatoms of 0.14 ML, which would prevent NH3 from dissociating to NHj and H. Therefore, the species A is probably assigned to molecular NH3. Tightly bound species were found at step edges. As shown in Rg. 2, some adsorbed species were found at the step sites which running along the [112] and the [114] direction, but nothing was found on the step along the [111] direction. Thus, the activity on adsorption of ammonia depended on the orientation of the step in azimuth. The dependency is quite similar to that observed on pyridine adsorption, which was discussed in a previous paper in terms on the different local environment [4]. However, some differences were found

Fig. 1. STM topograph (174x17.4 nm^) of a wide terrace of an NH3-exposed TiO2(n0) surface. The labels are explained in the text

Fig. 2. STM topograph (14.1x14.1 nm^) of an NH3-exposed TiO^l 10) surface at a region

with steps along different azimuths.

755 Ti rows

Fig. 3. (A) STM topograph (16.1x93 nm^) of step regions of an NH3-exposedTi02(l 10) surface. Squares with a label of 'R' and 'L' indicate step regions where asymmetric bright protrusions were observed relative to therightand left side of the bright rows of five-fold Ti atoms at the upper teirace, respectively. (B) A zoomed-in image of the bottomrightsquare in (A). (C) The positions of the asymmetric bright protrusions in (B) were shown on the model of the substrate.

between step-bound species of ammonia and pyridine. Although the center of the observed step-bound pyridine was on the four-fold coordinated Ti"^ site, which is the end of the bright row at the upper terrace, that of the step-bound ammonia was shifted from the site away from the edge depending on the step orientation as shown in Fig. 3. Besides, the height of the step-bound ammonia was 0.05±0.02 nm, which was much smaller than that of terraceadsorbed anmionia. It is not possible to determine exact adsorption geometry of the stepbound anmionia only from these STM images, however, the species inclined from the surface normal to the direction away from the step edge may be a possible explanation of the image. Partial dissociation of NH3 to NHj and H on the bridging oxygen row may occur at the step edges depending on the step orientation. Theoretical calculation at the step sites is necessary to confirm the origin. STM observation of water adsorption on TiOjCl 10) It has been demonstrated that water on TiO2(110)-(lxl) mainly adsorbs in a molecular form below 200 K and a residual amount of dissociation species (OH) is left on the surface by heating to RT by UPS, low energy ion scattering spectroscopy (LHS), and high-resolution electron energy loss spectroscopy (HREELS) [8-10]. A modulated molecular beam study

3.2

Fig. 4. (A) STM topograph (11.5x7.0 nm^) of step regions of an H2O-exposedTiO2(n0) surface. The labels are explained in the text. (B) A zoomed-in image of the middle right square in (A). (C) The positions of the bright protrusions in (B) were shown on the model of the substrate.

756 also suggested very few water dissociation at RT [12]. Water adsorption on TiOjCl lO)-(lxl) have also been a subject of theoretical calculations. Even if we limit the references to recent ab initio calculations, the conclusion is different: preference for dissociative adsorption [6], dissociative adsorption at low coverage followed by molecular adsorption [5], and molecular adsorption [7]. Figure 4 shows a typical STM image of water-exposed TiOzCllOHlxl) surface. Other than the hydrogen adatoms between the bright rows, there were few species found on the terrace sites. The species Q and R were observed with its center on the dark bridging oxygen row and the bright Ti row, respectively. The species P is a part of a TijOj added row formed on the (1x1) surface [13]. The density of adsorbed species was larger at step sites (labeled S with arrows in Fig. 4). As shown in Figs. 4B and 4C, the center of the step-bound species was located on the four-fold coordinated Ti'** site in contrast to ammonia adsorption in Fig. 3. Successive STM observations showed that the step-bound species was not moved from their position. Although previous works showed that the density oxygen defects did not affect the water adsorption [8-10], the step-bound species may be explained to be the water dissociation products (OH). Preferential dissociation of water at step or kink of TiO2(110) was suggested by temperature-programmed desorption (TPD) from surfaces of different annealing temperature after Ar* sputtering [11]. This work has been supported by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST). References [I] Y. Iwasawa, Stud. Surf. Sci. Catal. 101 (1996) 21. [2] Y. Iwasawa, Surf. Sci. 402-404 (1998) 8. [3] S. Suzuki, Y. Yamaguchi, H. Onishi, K. Fukui, T. Sasaki and Y. Iwasawa, J. Chem. Soc., Faraday Trans. 94 (1998) 161. [4] S. Suzuki, Y. Yamaguchi, H. Onishi, K. Fukui, T. Sasaki and Y. Iwasawa, Catal. Lett. 50 (1998)117. [5] P.J.D. Lindan, N.M. Harrison and MJ. Gillan, Phys. Rev. Lett. 80 (1998) 762. [6] K.F. Ferris and L.Q. Wang, J. Vac. Sci. Technol. A 16 (1998) 956. [7] E.V. Stefanovich and T.N. Truong, Chem. Phys. Lett. 299 (1999) 623. [8] R.L. Kurtz, R. Stockbauer, T.E. Madey, E. Roman and J.L. De Segovia, Surf. Sci. 218 (1989) 178. [9] J.M. Pan, B.L. Maschhoff, U. Diebold and T.E. Madey, J. Vac. Sci. Technol. A 10 (1992) 2470. [10] M.A. Henderson, Surf. Sci. 355 (1996) 151. [II] M.A. Henderson, Langmuir 12 (19%) 5093. [12] D. Brinkley, M. Dietrich, T. Engel, P. Farrall, G. Gantner, A. Schafer and A. Szuchmacher, Surf. Sci. 395 (1998) 292. [13] H. Onishi, K. Fukui and Y. Iwasawa, Bull. Chem. Soc. Jpn. 68 (1995) 2447. [14] S. Suzuki, K. Fukui, H. Onishi and Y. Iwasawa, Phys. Rev. Lett. (2000) 2156. [15] U. Diebold and T.E. Madey, J. Vac. Sci. Technol. A 10 (1992) 2327. [16] E.L. Roman, J.L. Desegovia, R.L. Kurtz, R. Stockbauer and T.E. Madey, Surf. Sci. 273 (1992)40. [17] A. Markovits, J. Ahdjoudj and C. Minot, Surf. Sci. 365 (1996) 649.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c^ 2001 Elsevier Science B.V. All rights reserved.

757

Three dimensional analysis of the local structure of Cu on TiOsCllO) by in-situ polarization-dependent total-reflection fluorescence XAFS Y. Tanizawa/ W. J. C h u n / T. Shido,* K. Asakura' and Y. Iwasawa '* "Graduate School of Science, The University of Tokyo, Kongo, Bunkyo-ku, Tokyo 113-0033, Japan ''Tsukuba Laboratory, CACs Inc., Ami 300-0332, Japan ^Catalysis Research Center, Hokkaido University, Kita-ku, Sapporo 060-0811, Japan. Cu K-edge XAFS of Cu/TiOjCllO) was measured by polarization-dependent totalreflection fluorescence XAFS technique. XAFS of [001], [iTO], and [110] directions were measured to elucidate the three dimensional structure of Cu species on the TiOzCl 10) surface prepared by the deposition of Cu(DPM)2 followed by reduction by Hj. Simulation of the EXAFS functions as well as conventional curve fitting analysis revealed that plane Cu3^ small clusters with similar structure to Cu(l 11) plane were formed by the reduction at 363 K. The small clusters converted into spherical metallic Cu particles by the reduction at 473 K. 1. Introduction In supported metal catalysts, the support not only disperses the metal on its surface but also affects the selectivity and activity of catalysts through the metal-support interaction, which changes the electronic structure, orientation and morphology on the surface. In general, metal-support interactions and structure of surface species are anisotropic and asymmetrical. Thus it is difficult to clarify the metal-support interaction by conventional XAFS technique. The EXAFS oscillation xfj^) depends on the angle 0, between the ith bond direction and the polarized electric-field vector of the incident X-rays as shown in Eq. (1);

X{k) = J^Xi{^)^os'e,,

(1)

where XxiH) is an EXAFS oscillation accompanying the rth bond. When a single crystal oxide is used as a support, structural information on the bonds both parallel and normal to the sur&ce can be independently obtained. However, there are two problems in applying the polarization dependent XAFS to species on a flat substrate. First, concentration of the surfece species is very low, and hence it is impossible to measure XAFS spectra in a transmission mode. Fluorescence yield detection is preferable for such dilute samples. Second, in a hard Xray region, incident X-rays penetrate deeply into the bulk, yielding a large amount of scattering X-rays, which hinder detecting the fluorescence X-rays from the surfece species. In order to reduce the scattering X-rays from the bulk substrates, we adopted the measurement under a total-reflection condition. When the incident X-rays hit a flat substrate below a critical angle (SJ, the X-rays are totally reflected and can penetrate only few nm into the bulk resulting in a decrease in

758

scattering X-rays from the substrate. The fluorescence yield technique under a total reflection condition make it possible to measure the polarization-dependent XAFS spectra for metal surface species on a flat surface. The technique is called as polarization-dependent totalreflection fluorescence XAFS (PTRF-XAFS) method hereinafter. We have determined the asymmetric structure of supported metals and metal oxides structure such as Pt on aAl2O3(0001) and Mo oxide dimeric structure on TiOjCl 10)^' 2. Rutile TiOjCllO) has an anisotropic structure with an akemative alignment of bridging oxygen ridges and five-fold coordinated Ti*^ rows along the [001] axis^. Therefore, formation of an anisotropic structure of Cu on the surface is expected. TiOz-supported Cu is important as catalysts for the reduction of NO^' ^ i^ this study, we have applied PTRF-XAFS method to probe the morphology change of the Cu/TiOjCl 10) before and after the reduction by Hj. 2. Experimental The sample was prepared by depositing a diethyl ether solution of Cu(DPM)2 (DPM = 2,2,6,6-tetramethyl-3,6-heptadionate) on a TiOzCHO) surfece, which was pre-calcined at 673 K in air. The loading of Cu was 5x10'"* atoms/cm^, which was estimated by XPS. The obtained sample was moimted in a vacuum chamber equipped with a 6-axis goniometer, and a heating device. The sample was reduced at given temperatures by Hj (2 Torr) in the chamber and PTRF-XAFS spectra were measured. The measurement was done at BL-12C at KEK-PF. A Si(l 11) double crystal was used to monochromatize the X-rays. The intensity of the incident X-rays (IQ) and that of fluorescence X-rays from the sample {Ij) were monitored by an ion chamber filled with N2 gas and a scintillation counter, respectively. The sample was rotated by the goniometer to measure Xray absorption spectra in three different directions where the electric vectors of the incident X-rays were adjusted parallel to the [001], [1TO], and [110] axes of TiOzCl 10). The spectra were analysed by the UWXAFS package ^. After background subtraction, k'weighted EXAFS fimctions were Fourier transformed into R-space and the curve fittings were done in the R-space. The k range of the Fourier transformation was 30-90 nm"'. The Fourier transformed EXAFS fimctions were fitted by Cu-0, Cu-Cu and/or Cu-Ti contributions. The backscattering amplitude and phase shift of these shells were calculated by the FEFF8 code 7. The fitting parameters were interatomic distances, effective coordination numbers and DebyeWaller &ctors for each shells and the correction of edge energy for all shells. 3. Results and discussion Figure 1 (a)-(c) shows k' weighted EXAFS fimctions in the [001], [llO], and [110] directions of Cu(DPM)2/Ti02(l 10) before and after reduction at 363 and 473 K. In the sample reduced at 363 K, strong polarization dependence was observed and the EXAFS functions in the three directions were substantially different from each other. In the [001] direction, EXAFS oscillation in the higher k range was stronger than those in the other directions, which suggests that the absorber was surrounded by heavier atoms in the [001] direction. Another characteristic feature of this direction is that a large amplitude oscillation appear at 30-40 nm"', which also appeared in the EXAFS of Cu foil. From these results, Cu-Cu bondings were

759

preferably formed along the [001] axis. On the other hand, the EXAFS oscillation in the [110] direction damped quicker than the other directions with k values. Probably in the [110] direction, the absorber was coordinated by the surface atoms of the TiOjCl 10) surface. From these features of the EXAFS functions measured in the three directions, the formation of Cu cluster along the [001] direction is suggested. In the san^le before the reduction, Cu-0 bonds were observed at 0.192 nm in all three directions. There was no resemblance in XANES spectra between Cu(DPM)2 complex and the Cu(DPM)2/Ti02. These results indicates that the framework of Cu(DPM)2 complex was broken. Thus, Cu atoms may be highly dispersed, and occupy the three-fold hollow sites of surface oxygen atoms because a Cu-0 contribution appeared in all three directions. In the sample reduced at 473 K, the N* values of Cu-Cu bond were similar in all the directions. Thus, spherical Cu metal particles were formed at this temperature. In the sample reduced at 363 K, Cu-Ti bond was observed in the [110] direction perpendicular to the surface, while Cu-Cu bond instead of Cu-Ti was observed in the directions parallel to the surfece. This suggests that plane Cu clusters were formed. To investigate the structure of the reduced Cu species in more detail, we simulated the EXAFS spectra of the san:^)le reduced at 363 K by the FEFF8 code. As the Cu clusters may have a similar structure to that of closed-packed Cu(l 11) plane, which is suggested from the curve fitting results in [001] and [iTO] orientations, we put the clusters in different positions on the TiOjCl 10) surfece and simulated EXAFS functions to know which structure reproduces well the observed spectra. Figure 2 (a)-(c) shows the calculated EXAFS functions for a Cuj model shown in Fig. 3. As shovm in Fig. 2 (a)-(c), the calculated spectra for the Cuj model reproduced the observed spectra in all the directions. Bigger Cu clusters than Cuj with similar structure could not reproduce the experimental data. The results of the simulation are summarized as below: 1) The Cu clusters consist of CU3 close-packed structure with the Cu-Cu distance at 0.25 nm. 2) The Cu clusters were tilted by 15 degree as shown in Fig. 3. 3) The Cu atoms in the cluster are nearly located on the three-hold hollow sites of surface oxygen atoms. It has been demonstrated that precise local structural information can be obtained from

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Figure 1.

k^ weighted Cu K-edge EXAFS functions of Cu(DPM)2/riO2(110) before (a) and after reduction at 363 K (b) and 473 K (c). Solid, broken, and dotted lines represent EXAFS for the [001], [iTO] and [110] directions, respectively.

760

PTRF-XAFS technique. Especially, the PTRF-XAFS technique has an advantage in investigating the chemical interaction between the deposited species and the substrate. This work has been supported by CREST (Core Research for Evolutionary Science and Technology) of Japan Science and Technology Corporation (JST). The XAFS measurements have been done in the approval of Photon Factory advisory committee (proposal No. 98G305). (a) . :

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reduced at 363 K (solid lines) with calculated results (broken line) for a nrxxlel structure proposed in Figure 3 : (a) [001], (b) [1TO] and (c)

[IK ']•

REFERENCES 1. W. J. Chun, K. Asakura, and Y. Iwasawa, J. Phys. Chem. B, 102 (1998) 9006. 2. K. Asakura, W. J. Chun, and Y. Iwasawa, Topics in Catal, 10 (2000) 209. 3. H. Onishi, K. Fukui, and Y. Iwasawa, Bull. Chem. Soc. Jpn., 68 (1995) 2477. 4. H. Aritani, et al, J. Chem Soc. Faraday Trans., 92 (1996) 2625. 5. H. Aritani, et aU J. Catal, 168 (1997) 412. 6. E. A. Stem, et aU Physica B, 208 (1995) 117. 7. A. L. Ankudinov, B. Ravel, J. J. Rehr, and S. D. Conradson, Phys. Rev. B, 58 (1998) 7565.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

761

Insertion and aggregation behavior of PtCl4 between graphite layers Masayuki Shirai *, Koichi Igeta", and Masahiko Aral ^ * Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba, Sendai, 980-8577, JAPAN ** Division of N4aterials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, JAPAN

Platinum chloride (PtCk) was intercalated between graphite layers under pressurized chlorine atmosphere. The analyses of XRD and EXAFS showed that platinum chloridegraphite intercalation compounds (PtCU-GICs) with a stage three structure were formed and PtCU molecules aggregated in interlayer space from low platinum loading. TEM and XMA analyses showed that platinum nanosheets with thickness 2-3 nm containing a number of hexagonal holes were formed between graphite layers by hydrogen reduction of the PtCU-GIC samples.

1. INTRODUCTION Platinum nanoparticles, nanorods [1] and nanowires [2] are formed in the nanotubes and nanochannels with template techniques. Graphite has a layered structure and each layer is a regular hexagonal net of carbon atoms. The graphite layer interacts with each other via van der Waals force. Several kinds of transition metal chlorides can be inserted into the interlayer space to produce graphite intercalated compounds (MClx-GICs). Transition metal particles intercalated in graphite layers (M-GICs) are formed by reduction of MClx-GICs. Graphimet (commercially available M-GICs) is obtained by treating MClx-GICs with lithium biphaiyl at 223 K under a helium atmosphere [3]. Small platinum metal particles (1-10 nm) were observed in Pt-Graphimet by HRTEM analysis [4]. We have reported that platinum nanosheets can be obtained by hydrogen reduction of PtC^-GIC samples [5]. In this paper we report the structure and reduction behavior of platinum chloride molecules between graphite layers.

762 2. EXPERIMENTAL Details of sample preparation were described previously [6]. Platinum (IV) chloride and gn^hite (KS6, Lonza) were mixed in a thick walled glass reactor under nitrogen atmosphere and dried in vacuum at 423 K for 2 h. The intercalation reaction was carried out in the reactor at 723 K for 2 weeks under 0.3 MPa of chlorine (Takachiho, 99.999 %), to obtain the platinum chloride intercalated compounds (PtCU-GIC). The PtCk-GIC samples were reduced at 573 K for 1 h under 40 kPa hydrogen to produce the platinum metal intercalated compounds (PtGIC).

3. RESULTS AND DISCUSSION 3 . 1 . Structure of PtCU-GIC Figure 1 shows the XRD pattern of 5 wt% PtCU-GIC (platinum loading: 5 wt%). The peaks ascribed to platinum chloride disappeared and the (002) diffraction of graphite at 26 = 26.57" weakened after the reaction under chlorine atmosphere. Peaks ascribed to platinum chloride and graphite were observed on XRD patterns of a reference mixture of platinum chloride and graphite. Peaks at 26 = 10.0, 14.6, and 20.3* in Figure 1 can be ascribed to PtCU-GIC structure. The diffraction peak positions calculated for (002), (003) and (004) reflections for PtCU-GIC of the repeat distance along the c axis (Ic = 1.76 nm) are 10.0, 15.1, and 20.2% respectively, being in good agreement with the experimental ones. The distance of 1.76 nm corresponds to the sum of three graphite layers and one intercalated layer (0.75 nm). This result shows that platinum chloride is intercalated between every three graphite layers (stage three structure) for 5 wt% PtCl^-GIC sample. Peaks of platinum chloride and stage structures were not observed and the peak intensities of graphite decreased in the XRD pattern of the 1 wt% PtCl^-GIC sample. With increasing the amount of platinum chloride loaded, the diffraction for (002) of gr^hite decreased (Figure 2). We cannot conclude from the XRD result that the 1 wt% PtCU-GIC sample has stage structures, but it is probable that platinum chloride molecules are inserted into graphite layers. The maximum amount of platinum chloride that can be intercalated is ca. 38 wt% and thus platinum chloride intercalated compounds are mixed with the graphite matrix at low platinum chloride loadings. Figure 3 shows the Fourier transform for Pt Uii-edge EXAFS spectra of the 1 wt% PtCU-GIC sample. A peak at 0.2 nm is assigned to the Pt-Cl bond and a peak at 0.34 nm to Pt-Cl-Pt bond (phase shift is not corrected). A similar EXAFS Fourier transform was obtained for the 5 wt% PtCU-GIC sample. The observation of Pt-Cl-Pt bond indicates that platinum chloride molecules aggregated with each other at low platinum loadings. XRD and EXAFS results indicates that platinum chloride is intercalated between graphite layers and several platinum chloride molecules aggregated with each other in the interlayer space of PtCU-GIC samples for low platinum chloride loadings under our experimental conditions.

763

15

20

30

25

35

2e/deg(CuKa)

Fig. 1. XRD pattern of 5 wt% PtC14-GIC ( • : PtCl^-GIC, T : Graphite).

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20

30

R loading / wt%

Fig. 2. The intensity of the graphite (002) diffraction.

0.2

0.3

0.4

0.5

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Fig. 3.

The Fourier transforms of Pt Ull-edge EXAFS spectra of 1 wt% of PtCU-GIC sample (phase shift is not corrected).

3.2. Structure of Pt-GIC Figure 4(a) and 4(b) shows TEM images of the 5 wt% Pt-GIC sample reduced at 573 K. Figure 4(a) shows a number of dark images in a rod-like arrangement in parallel. The

764 thickness of dark images is 2-3 nm. Figure 4(b) shows the large sheets with a number of hexagonal holes and that the edge angles of the large sheets are constant at 120'. X-ray microanalysis results confirmed the dark images to be platinum metal. The image of a number of sheets in parallel presumably indicates that platinum sheets exist between graphite layers. Platinum chloride can aggregate only two dimensionally between graphite layers during hydrogen reduction because of the steric hindrance of graphite layers, so that the resulting platinum metal partides of Pt-GIC have two-dimensional structures. The movement during the reduction would determine the morphology of the resulting platinum nanosheets having hexagonal holes. For comparison, the mixture of platinum chloride and graphite upon reduction at 573 K (Pt/Gmix) showed spherical platinum particles according to TEM For Pt/Gmix samples platinum chloride molecules on the graphite surface can aggregate in a three dimensional manner and there is no interfering factor for the growth of metal particles, and so large platinum particles are formed.

Fig. 4. TEM images of 5 wt% Pt-GIC ((a) Side view and (b) Top view). 4. ACKNOWLEDGEMENT We thank Dr. E Aoyagi and Dr. Y. Hayasaka (High Voltage Electron Microscope Laboratory of Tohoku University) for TEM and XMA analysis. This work was carried out under the approval of PF advisory committee (No.95G211 and 98G108). This research was partially supported by The Japan Securities Scholarship Foundation

REFERENCES L B. K. Pradhan, T. Toba, T. Kyotani, and A. Tomita, Chem. Mater. 10 (1998) 2510. 2. M. Sasaki, M. M. Osada, N. Higashimoto, T. Yamamoto, A. Fukuoka, and M. Ichikawa, J. Mol. Catal. A 141 (1999) 223. 3. J. M. Lalancette, US Patent No. 3, 874,963 (1979). 4. D. J. Smith, R. M. Fisher, and L. A. Freeman, J. Catal, ,72 (1981) 51. 5. M. Shirai, K. Igeta, and M. Arai, Chem. Commun., (2000) 623. 6. M. Shirai, K. Igeta, and M. Arai, Mol. Cryst. Liq. Cryst., 340 (2000) 127.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) © 2001 Elsevier Science B.V. All rights reserved.

765

Surface Properties of Silica-Htania and Silica-Zirconia Mixed Oxide Gels Shi^i Ikoma. Kazunori Nohuhara,* Masayoshi Takami, Takuo Nishiyama, Motoshi Nakamura, and Shoji Kaneko Department of Materials Science and Technology, Shizuoka University, Johoku, Hamamatsu 432-8561, J^>an *Fuji-Silysia Chemical, Ltd., Kozoji, Kasugai 487-0013, Japan The preparation of mixed oxide gels composed of silica and titania, and of silica and zirconia was tried by a coating method in order to examine the surface pHToperties of the gels. Metal oxide was dispersed homogeneously over the silica surface. Both the surface silanol groups and the acid strength changed depending on the preparation conditions. The characteristics of tiie silica gel as solid acid was enhanced by the introduction of the amphoteric metal oxide. The zirconia-coated gel prepared by the hydrolysis of alkoxide exhibited the greatest acid strength. 1. INTRODUCTION The availability of the surface modification of silica gel with other metal oxide has appeared in its wide applications such as catalysts, fillers and adsorijents [1-6]. Their properties depend considerably not only on the nature of the second oxide phase such as the coordination number of metal and oxygen but also on its dispersion in the silica matrix [7]. The excellent separation ability for basic pyridine using silica-magnesia prepared by a coprecipitation method [1]. Schrijnemakers et al. [5] tried to create a chemically bound anatase layer on silica by a grafting method. It has been shown that a very small amount of metal oxide (even less than 1 wt%) introduced to silica gel changes drastically the surface properties such as solid acidity and adsorptive ability [1,8]. Tbe present p^jer discussed on the relationship between the preparation and the surface properties of silica gel coated with titania and zirconia 2. EXPERIMENTAL 2.1 Preparation oftitania-coatedand ztrconianroated silica gels Spherically shaped silica gel beads (Super Microbead lOOA-10, average pore diameter 100 A and average particle size 10/L/m; Fuji-Silysia Chemical, Japan) was used as matrix. Other chemicals used here were of analytical-reagent grade.

766 The following three procedures were adopted for surface coating: (1) Pore-filling metiiod (pf-w gel) TitaniumOTV) chloride and zirconyl chloride solutions were added dropwise to silica gel beads, and the resultants were aged over 24 h at room temperature, washed with methanol (without washing, pf gel) and dried at 180°C. The aqueous suspension of each gel was adjusted to pH 7 by the addition of ammonia, and filtered. The gel was washed once more with water and methanol, and finally dried at 180°C. (2) Chloride method (CI gel) In a sealed flask dried sihca gel beads were immersed in 2-propanol and mixed with the above chloride, and the suspension was refluxed under an inert atmosphere for half an hour. The gels obtained were washed with methanol and dried at 180°C. (3) Alkoxide method (al gel) In sealed flasks tiie mixtures of titanium(IV) isopropoxide and zirconium(IV) n-propoxide dissolved in 2-pix)panol, and dried silica gel beads were refluxed in an atmosphere of nitrogen for half an hour. The resulting gels were washed with methanol and dried at 180°C. 22 Characterisation of gel The contents of titanium and zirconium in these mixed oxide gels were determined by atomic absorption spectrometer. The specific surface areas and pore volumes of the gels were detennined by using the nitrogen adsorption isotherms measured at 77K. The surface acid strengths of gels were examined visually by using Hammett indicators [8]. The solid acidities of the gels were also determined fix)m the temperature-programmed desorption of ammonia supported. 3. RESULTS AND DISCUSSION 3.1 Preparative conditions and physical properties of the surface of gels Table 1 shows that the specific surfece area and the pore volume of the mixed oxide gels have a tendency to decrease with an increase in the amount of metal oxide coated. Therefore silica surfaces are coated with metal oxide clogging in part micropores on the surface. Titania and zirconia seem to be deposited on the silica surface not as hydrous metal oxides but as some titanium and zirconium species such as Ti^ and Zr^^ ions. In comparison with the pf-w gel it can be seen that the titania content of the pf Table 1 Composition and surface characteristics of silica gels (M = Ti and Zr) Gel Amount of MO; (w1%) Surface area (mVg) Pore volume (cmVg) titania zirconia titania zirconia zirconia titania 1.14(untreated) SiOj 344(untreated) 1.00 0.91 10.4 17.2 297 316 pf pf-w 1.10 1.02 3.7 324 274 11.9 2.9 1.18 1.09 CI 6.4 338 338 0.99 1.10 18.4 341 al 8.5 318

767

gel wasreducedto one third its initial value (10.4 wt%) by washing with methanol. The titanium species contained in the pf-w gel are adsorbed chemically by the fonnation of Si-O-Ti bond on the silica surfece, whereas the ones eliminated easily by washing are attached physically to the silica matrix. The amount of titania coated of the al gel is the largest (18.4 wt%) in the present work. It is weU known that there 8 silanol groiqjs per 1 nm^ of the of silica surface. In the al gel about a half of these silanols reacted with titania, and then about four 11-0 bonds per 1 nm^ existed on the silica surface. The amount of zirconia loaded was always more than that of titania, although these amounts varied possibly with the preparation conditions of the mixed oxide gels, as shown in Table 1. The FT-IR spectra of both untreated and titania-coated silicas gave a strong and sharp absorption at 3750 cm"^ assigned to stretching vibration of isolated silanol group [9]. However, the hydrogenbonded silanol groups, wiiich showed the broad absorption band centred around 3400 cm\ wwe diminished by the titania coating. The ^Si-NMR alsorevealedthat the hydrogen-bonded silanols as the water-attracting groiq? react easily with metal species in aqueous media and the isolated silanols with metal species in 2-propanol. On the other hand, metal alkoxide forms nonequivalent bridged Si-O-M (M = Ti or Zr) bond [10] through the use of both silanols in 2-propanol. 32 Surface acid strength and solid acidity of gels The solid acid strength (HQ) of the silica gel was raised by the introduction of the amphoteric metal oxide (titania and zirconia), as shown in Table 2. In contrast, the magnesia (basic oxide) coating had a tendency to weaken the surface acid strength of the silica gel [1]. The acid strength of titaniacoated silica was weaker than that of zirconia-coated one. Furthermore, the acid strength of the gels increased in order of the silica gel, the pf-w gel, the CI gel, and the al gel. The acid strength of metal oxide-containing silica gels prepsaed by a coprecipitation method (initial mole ratio [metal]: [Si] = 1/3 ~ 3/1) depends considerably on the nature of the metal oxide phase [11]. The high acid strength of the pf gel, in v^ch metal oxide particles are attached physically to the silica gel, should be ascribed in part to the acidic nature of titania and zirconia. When the gel was prepared in alcoholic solution (the CI gel and the al gel), its acid strength was raised. The CI gel has stronger acid sites than the pf-w gel, despite the fact that the metal oxide loading is the lowest in the gels examined, and the al gel the strongest ones. Metal alkoxide is labile and can form Si-O-M linkages uniformly through the condensation with two types of silanol groups (hydrogen-bonded and isolated), and thus the total amount of acid site is the Table 2 Acid strength of silica gels Gel titania-coated zirconia-coated SiOj +1.5
768 Tables NH3-TPDresultsof silica gels Gel NH3desoibed(jL/mol/g) Peak temp.CQ titania zirconia titania 2drconia 116(imtreated) 197(imtreated) Si02 pf-w 71.8 288 197 222

CI al MO2

116 190 447

219 440 313

197 198 230

219 211 225

Highest temp.CC) titania zirconia 420(imtreated) 380 480 390 420 400 460 500 500

largest intibeal gel (Table 3). The zirconia-coated gel prepared by the hydrolysis of alkoxide exhibited the greatest strength (HQ <—82) of all the gels. The solid acidities of original silica and coated gels detenninedfiiomthe temperature-programmed desorption of ammonia si^ported the discussion of the surfece properties mentioned above. It is assumed that the Si-O-M bond creates more Lewis and/or Broensted acid sites by the shift of electron fiom silica to titania and zirconia due to the difference of electronegativities between silicon and metals. Since zirconium is more electropositive and has a larger coordination number than titanium, silica-zirconia q^pears to be more acidic than silica-titania REFERENCES 1. K. Nobuhara, M. Kato, M. Nakamura, M. Takami, and S. Kaneko, J. Chromatogr. A, 704 (1995) 45. 2. B. G. Anderson, Z. Dang, and B. A. Mom)w, J. Phys. Chem., 99 (1995) 14444. 3. JL Kunat, J. E. Prenosil, and J. J. Ramsden, J. CoUoid Interface Sci., 185 (1997) 1. 4. L. J. Alemany, M. A. Banares, E. Pardo, F. Martin, M. Galan-Fereres, and J. M. Blasco, Appl. Catal. BEnviroa,13(1997)289. 5. K. Schrijnemakers, N. R. E. N. Impens, and E. E Vansant, Langmuir, 15 (1999) 5807. 6. S. Dcoma, K. Nobuhara, M. Takami, T. Nishiyama, M. Nakamura, and S. Kaneko, "Prq>aration and evaluation of titania-coated silica gel as column packing materials for high-performance liquid chromatography", (in preparation). 7. K. Tanabe, Solid Acids and Bases, Academic Press, London, 1970. 8. V. A. Dzisko, in Proceedings of the 3rd International Congress on Catalysis, Amsterdam, 1964. 9. S. G. Bush, J. W. Jorgenson, M. L. Miller, and R.W. Linton, J. Chromatogr., 260 (1983) 1. 10. C. U. L Odenbrand, S. T Lundin, and L. A. H. Andersson, Appl. Catal., 18 (1985) 353. U.S. Kaneko, T. Mitsuzawa, S. Ohmori, M. Nakamura, K. Nobuhara, and M. Masatani, J. Chromatogr. A, 669 (1994)1.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

769

Characteristics of supported gold catalysts prepared by spray reaction method Lin Fan^, Nobuyuki Ichikuni*', Shogo Shimazu*' and Takayoshi Uematsu'' ^Center for Frontier Electronics and Photonics, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan ** Faculty of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan In order to clarify the unique catalysis of gold nanoparticles, supported gold catalysts, Le,, different Au/Ti02 samples in the form of fine composite, prepared by spray reaction method (SPR) were investigated. The structure of spr-Au/Ti02 catalysts was studied by SEM, TEM and XRD. The effects of preparation conditions for the spray reaction, such as the solution concentration and the calcination temperature, on the surface characteristics are discussed. The catalytic behavior of spr-Au/Ti02 catalysts for CO oxidation was studied. The spr-Au/Ti02 catalysts calcined at high temperature, i.e., sprO.1/1073 and sprO.2/1073, gave the relative high turnover frequency (TOF) irrespective of the limited active sites. The high catalytic activity of sprO.1/1073 and sprO.2/1073 was attributed to the extensive perimeter interface between Au and Ti02 support particles, which enhanced a synergistic effect. 1. INTRODUCTION Since gold has been proven to be active as a catalyst when it is dispersed on metal oxides as nanoparticles, many studies have been made on finding an effective method for preparing gold catalysts with ultrafine particles and high dispersion by coprecipitation, deposition-precipitation, impregnation, vapor-phase deposition and grafting methods [1-6]. In our research, spray reaction method (SPR) was applied for the preparation of gold catalysts. We have previously demonstrated that SPR is an excellent preparation method for various nano-composites including multicomponent catalysts, e.g., Pd/Zr02, Ni/Zr02, RU/AI2O3 and Ru/Ti02 [7-9]. The novel characteristics of sprayed catalyst, the homogeneous mixing of the components and the improvement of the interaction between them, are probably caused by the components formed simultaneously from a homogeneous solution in a quick-heating and quick-quenching preparation process. In the present study, the effects of preparation conditions for spray reaction, i.e., the solution concentration and the calcination temperature, on the surface characteristics of spr-Au/Ti02 catalysts are discussed. The catalytic properties for CO oxidation over spr-Au/Ti02 catalysts were also investigated. 2. EXPERIMENTAL The preparation of spr-Au/Ti02 catalysts was carried out as follows: the spray solution, a mixed aqueous solution of HAuCU and TiCU in a certain concentration, was atomized by an ultrasonic device, followed by calcination in an air flow through a heated quartz tube reactor

770 under the suction of an aspirator. The formed fine particles were collected on a glass filter. The spray solution concentrations were changed from 0.1 to 0.2 mol/1 and the calcination temperatures were controlled at 673, 873 and 1073 K, respectively. The catalysts with 1 mol-% loading of gold supported on Ti02 were obtained. The spr-Au/TiOa catalysts are hereafter symbolized as spr-solution concentration / calcination temperature (e.g., sprO. 1/673). The characterization of spr-Au/Ti02 catalysts was investigated by SEM (JSM-6340F FE-SEM), TEM (JEOL JEM-4000FXII) and XRD (Science MXP3 X-ray Diffractometer). The crystallite size of gold was determined by the calculation of Au (200) reflection of XRD. The CO chemisorption was measured in a static system at 373 K. CO oxidation was carried out with a closed circulation reaction system. A reaction mixture of CO (20 Torr) and O2 (10 Torr) was introduced into the circulatory line and the reaction products were analyzed by a gas chromatograph (SHIMAZU GC-8A) with a TCD detector. The initial rate of CO2 formation was used to stand for the catalytic activity. All of the catalysts used for CO chemisorption and CO oxidation were pretreated with H2 (300 Torr) for 1 hour at 573 K followed by evacuation for 30 minutes at the same temperature. 3. RESULTS AND DISCUSSION The SEM photographs show that spr-Au/Ti02 catalysts are sub-micron particles with spherical shape in the range of 0.2 to 0.6 pm. According to the backscattering electron images of SEM, it is also found that the gold particles are homogeneously dispersed on the support surface. The XRD patterns of spr-Au/Ti02 catalysts are shown in Figure 1. In all the XRD patterns of spr-Au/Ti02 catalysts, the peaks corresponding to anatase phase of Ti02 and metallic Au are clearly observed. However, in the XRD pattern of sprO.2/1073, the peak due to rutile phase of Ti02 is also detected. Moreover, the Ti02 peaks show more sharply as the calcination temperature is increased. It is indicated that the crystal growth was happened as the spr-Au/Ti02 catalyst 26 I degree calcined at high temperature, even the Fig. 1 XRD patterns of spr-Au/Ti02 catalysts, rutile phase was formed for the (a) sprO.1/673, (b) sprO.1/873, (c) sprO.1/1073, catalyst prepared from concentrated (d) spK).2/673, (e) sprO.2/873, (0 sprO.2/1073 solution and high calcination temperature, Le., sprO.2/1073. The results of crystallite size of Au calculated from Au (200) peaks of XRD are shown in Table 1. The crystal growth of Au, which is due to the coagulation of gold particles, is also found in the case of the catalysts calcined at high temperature. However, the crystallite sizes of Au for sprO.1/673 and sprO.1/873 show no differences (ca. 9 nm), being smaller than those found on the catalysts prepared from concentrated solution (0.2 mol/1). It is suggested that the coagulation and the crystal growth of gold particles could be prevented by preparing the gold catalysts from a diluted solution and calcination at low temperature.

771

Table 1 Crystallite sizes of Au (200) and characteristics of spr-Au/Ti02 catalysts Catalysts sprO.1/673 sprO.1/873 sprO.1/1073 sprO.2/673 sprO.2/873 sprO.2/1073

Crystallite size of Au (nm) 9.1 8.4 24.3 14.9 22.5 21.7

CO/Au (10') 15.6 18.0 7.6 16.3 5.9 4.6

TOP (10-" s') 2.1 2.5 27.7 1.4 10.8 34.4

'TOFat373K

The results of CO chemisorption measurement indicate that the amount of CO chemisorption for 0.2 mol/1 samples decreased when a relatively high calcination temperature was applied. Only 0.46% of gold active sites are exposed at the surface for sprO.2/1073. On the other hand, for the catalysts prepared from the diluted solution (0.1 mol/1), spr0.1/1073 gives a low CO/Au Fig. 2 TEM photographs of (a) sprO.2/673 and (b) sprO.2/873 value while sprO. 1/673 and sprO. 1/873 show no significant change. The reason why only limited gold active sites are exposed on the surface, especially for the catalysts calcined at high temperature, can be explained by the result of TEM photographs shown in Figure 2. As comparing the TEM photographs of sprO.2/673 and sprO.2/873, it is found that most of the gold particles in the latter are partially buried in the TiOa support particles. We tentatively explain this result by the different decomposition temperatures of HAuCU (ca. 473 K) and TiCU {ca. 673 K). We suppose that as the catalyst calcined at a relatively high temperature, the gold particles 2.4 2.6 2.8 are formed instantaneously. The gold 1/TX103(K-1) particles are then covered in the surrounding Fig. 3 Arrhenius plots for CO oxidation TiCU droplets, which leads to most of the over spr-Au/Ti02 catalysts. AsprO. 1/673, gold particles becoming partially buried in •sprO.1/873, •sprO.l/1073, AsprO.2/673, OsprO.2/873, nspK).2/1073 Ti02 support particles formed later in the reaction process.

772

When the catalytic activity of spr-Aii/Ti02 catalysts for CO oxidation was examined, the spr-Au/Ti02 catalysts calcined at a high temperature, i.e., sprO.1/1073 and sprO.2/1073, gave the relative high turnover frequency (TOF) irrespective of the limited active sites. The apparent activation energies of spr-Au/Ti02 catalysts are also deduced from the Arrhenius plots for the initial rates of CO2 formation shown in Figure 3. The activation energies of the catalysts prepared at 673 or 873 K are close to 50-60 kJ/mol, with no significant differences. However, the activation energies of sprO.1/1073 and sprO.2/1073 give much lower values, being ca. 18 and 19 kJ/mol, respectively. The high catalytic activity of sprO.1/1073 and sprO.2/1073 is attributed to the formation of extensive perimeter interface between Au and Ti02 support particles because most of the gold particles are partially buried in the Ti02 support particles, which enhanced a synergistic effect. In an attempt at preparation of high dispersion catalysts by spray techniques, suspended spray reaction method (SSP), a modification of spray reaction techniques, has been applied successfully for the preparation of RU/AI2O3 with high dispersion by our research group [10]. For comparison, the Au/Ti02 catalysts were also prepared by SSP method, where the catalysts were prepared from a suspension of HAuCU and Ti02 (Nippon Aerosil, P-25, specific surface area; 50 m /g, mainly composed of anatase) and calcined with a same procedure as described above for SPR catalysts. Detailed results for ssp-Au/Ti02 catalysts will be reported later. 4. CONCLUSIONS Based on our experimental results, the following conclusions may be drawn: (1) The Au/Ti02 catalysts prepared by spray reaction method are sub-micron particles with a spherical shape and the gold particles are homogeneously dispersed on the support surface. (2) The surface structure of spr-Au/Ti02 catalysts is affected by the preparation conditions, where most of the gold particles are partially buried in the support particles as a relative high calcination temperature is applied. (3) The high catalytic activity of sprO.1/1073 and sprO.2/1073 for CO oxidation is attributed to the formation of extensive perimeter interface between Au and Ti02 support particles, which enhanced a synergistic effect. REFERENCES 1. M. Haruta, N. Yamada, T. Kobayashi, and S. lijima, J. Catal., 115 (1989) 301. 2. S. Tubota, D. A. H. Cunningham, Y. Bando, and M. Haruta, in: Preparation of Catalysis VI, G. Poncelet et al (eds.), Elsevier, Amsterdam, 1995, P. 227. 3. M. Haruta, B. S. Uphade, S. Tubota, and A. Miyamoto, Res. Chem. Intermed., 24 (1998) 329. 4. J.-D. Grunwaldt, C. Kiener, C. Wogerbauer, and A. Baiker, J. Catal., 181 (1999) 223. 5. G. C. Bond, and D. T. Thompson, Catal. Rev.-Sci. Eng., 41 (1999) 319. 6. A. I. Kozlov, A. P. Kozlova, H. Liu, and Y. Iwasawa, Appl. Catal. A: General, 182 (1999) 9. 7. T. Uematsu, S. Shimazu, Shokubai (Catalysts & Catalysis, Japan), 36 (1994) 252. 8. D. Li, N. Ichikuni, S.Shimazu, and T. Uematsu, Appl. Catal. A: General, 172 (1998) 351. 9. D. Li, N. Ichikuni, S.Shimazu, and T. Uematsu, Appl. Catal. A: General, 180 (1999) 227. 10. T. Tsuchiya, N. Ichikuni, S.Shimazu, and T. Uematsu, Chem. Lett., (2000) 652.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) 'C' 2001 Elsevier Science B.V. All rights reserved.

773

STM observation of oxygen adsorption on Cu(lll). T. Matsumoto% R.A. Bennett»>, P. Stone»>, T. Yamada^ K. Domena* and M. Bowkei* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan KlcnteT for Surface Science and Catalysis, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK ^Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishi-Waseda Shmjuku-ku, Tokyo, 169-0051, Japan The adsorption of O2 on Cu(lll) in a pressure of O2 at 10-5-10-3 Pa was studied by scanning tunneling microscopy (STM) at 300-773 K. Three processes were observed in the oxidation of Cu(l 11). The well-ordered '44'- and '29'-structures were observed by STM at atomic resolution. The selectivity to produce those structures is also discussed. 1. Introduction Metallic Cu is used as an industrially important catalyst for such chemical reactions as the water-gas shift reaction and methanol synthesis, and O atoms are suggested to adsorb on the Cu surface during these catalytic reactions. On the Cu(l 11) surface, many ordered structures have been reported dependent on oxidation temperatures, and only disordered structures were reported for oxidation at room temperature [1]. This is the first research about the processes of oxide formation on Cu(l 11), which has been complicated by the structural diversity of resulting the oxidized Cu(l 11) surfaces. 2. Experimental The experiments were carried out in an UHV chamber equipped with a W.A. Technology variable temperature STM operating at a base pressure of 10-* Pa, which is described in detail elsewhere [2]. The Cu(l 11) surface was cleaned by cycles of ion bombardment at RT and annealing at 773 K. All STM images were acquired in the constant-current mode. 3. Processes of Cu(lll) oxidation at 300 and 573 K The processes of Cu(l 11) oxidation at 300 K was investigated by STM. Fig.l shows the STM images of Cu(lll) m a pressure of O2 at 7X10"^ Pa. The bare Cu(lll) surface is shown in Fig. 1 (a). A terrace contains of vacancy islands (arrows). The step edges became The authors gratefully acknowledge University of Reading for the use of the STM, the EPSRC(RB) and JSPS Research Fellowships for the Young Scientists for financial support (TM), and Oxford Instruments for the provision of a CASE award for one of us (PS, RB, and MB).

774

angular, and dark area was found along step edges after 10 minutes exposure in Fig. 1 (b). The vacancy islands (arrows) in Fig. 1 (a) grow to form dark triangles. Light patches appeared on terraces after 33 minutes in Fig. 1 (c). The domain boundaries of dark areas along step edges were almost the same as the original step edges on the Cu(lll) surface before oxidation. New dark areas (arrows) were produced in terraces between light patches on terraces in Fig. 1 (d). The dark area along step edges was observed in UHV (Fig. 2) after O2 exposure at 12.4 X 10"^ Pa for 15 minutes at 300 K. Dark dots array hexagonally in some parts as marked with white circles. The distances between adjacent dark dots were 0.55-0.7 nm. In the top of Fig. 2, the model of the Cu20(lll) surface, the O atoms in the first layer (filled circles) array hexagonally with the 0.6 nm distance between adjacent O atoms in the first layer. O atoms are generally observed dark on metal surfaces [3]. The dark area along the step edge is considered to be oxide whose structure is similar to Cu20(l 11). Similar STM images were obtained of the structure in the dark triangles and the light patches on the terraces indicating that they were similar oxides. The density of Cu atoms of Cu(l 11) is about 5.5 times lager than that in a layer of Cu20(l 11) so Cu atoms should be released during oxidation at step edges. We consider that these released Cu atoms combine with O atoms on the terraces to form the light patches. The processes of Cu(l 11) oxidation were observed at 573 K by STM in a pressure of oxygen at 1-2.4 X 10-5 Pa. Fig. 3 (a) shows the clean Cu(l 11) surface at 573 K. Step edges are streaky because of rapid diffusion. Fig. 3 (b) and (c) show the Cu(l 11) surfaces oxidized

Fig. 1 STM images of Cu(l 11) in a pressure of O2 at 7 X 10-5 Pa at 300 K. (a) Clean Cu(l 11) before oxygen exposure (100 X100 nm2, V=500 mV, 1=0.03 nA). (b) After 10 min (120X120 nm2, V=700 mV, 1=0.07 nA). (c) After 33 min (120X120 nm2, V=700 mV, 1=0.07 nA). (d) After 39 min (100 X100 nm2, V=700 mV, 1=0.03 nA).

Fig. 2 Model of Cu20(l 11) and STM image of Cu(lll) after oxidation after O2 exposure at 1-2.4X10'^ Pa for 15 min (20 X 20 nm2, V=500 mV, 1=0.1 nA).

775

for 4 and 14 minutes after starting O2 introduction, respectively. Dark oxide domains were formed along the step edges but light patches and dark triangles were not observed on terraces and atomic resolution was not obtained in Fig. 3 (b) suggesting that the Cu and O atoms of oxide presumably diffused rapidly. The STM image of the •44'-structure [4] was acquired at atomic resolution after the surface was covered by oxide completely (Fig. 3 (c)). This indicates that the close packing of the atoms inhibited the diffusion of the surface species. Thus, the processes of Cu(l 11) oxidation at 300 and 573 K were revealed (Scheme 1). At 300 K oxidation starts by growth into the step edge which releases Cu atoms, these Cu atoms and O atoms combine on Cu(l 11) terraces. O atoms also form pits of oxide in the terraces. In contrast, at 573 K oxide grows only from step edges.

Fig. 3 STM images of Cu(lll) at 573 K in a pressure of oxygen at 1-2.4 X 10-5 Pa. (a) Cu(l 11) surface before oxidation (50 X 50 nm2, V=500 mV, 1=1 nA). (b) After 4 min (50 X 50 nm2, V=500 mV, 1=0.1 nA). (c) After 15 min (15 X 15 nm^, V=500 mV, 1=0.1 nA)

QxidatiQnat300K 1. oxidation at step edge with releasing Cu atom

oxidation in terrace

Oxidation at 573 K step e d g e v / ^ ^ ^ ^ ^ ^ v ^ oxidation at step edge with releasing Cu atom Scheme 1 Model for Cu(l 11) oxidation. 4. Ordered *29*-stnicture and *44*-stnicture STM images of the '44'-structure (Fig. 4 (a)) were observed after oxidation at 300-573 K and annealing at 423-623 K. The '29'-structure (Fig. 4 (b)) was formed by oxidation at 423-723 K and annealing at 573-723 K. Annealing the '29'-surface at 773 K could also produce the '44'-structure. White parallelogram lines indicate unit cells. The dark spots basically have hexagonal structures with a periodicity of ~ 6 A which are similar to the array of the O atoms of the CmO (111) in the both STM images. O atoms generally appear dark due to a low density of states available for tunneling. These structures are related to the Cu20(l 11) surface as depicted in Fig. 4. These models are proposed on the basis of the STM images and LEED patterns, and corroborative techniques are necessary to determine their definitive structures.

776

Fig. 4 STM images of the ordered structures and their structure models. Parallelogram lines indicate a unit cell. Open and filled circles indicate Cu and O atoms, respectively, (a) '44'-structure at 300 K after exposure of Cu(l 11) to 02 at 0.2-1 X 10-5 Pa for 40 min at 300 K and annealed at 473 K for 5 min (5X5 nm2, V=700 mV, 1=0.2 nA). (b) '29'structure at 300 K after exposure of Cu(l 11) to O2 at 5 X10-5 Pa for 30 min at 673 K and annealed at 723 K for 5 min. (4.3 X 4. 3 nm2, V=700 mV, 1=0.03 nA).

Fig. 5 STM images of oxidized C u ( l l l ) at various temperatures, (a) Cu( H I ) surface at 493 K after oxidation in a pressure of O2 at 5 X 10-5 Pafor30 min at 493 K(10X10nm2, V=500mV, 1=1 nA). (b) Surface in Fig. (a) at 573 K(10X10nm2, V=1000mV, 1=0.1 nA).

The '44'-structure produced at 423-623 K (Fig. 5 (a)) was transformed to the '29'structure at 573-673 K (Fig 5 (b)). This surface appeared uncorrugated at 773 K presumably because of rapid diffusion of surface species. The Cu(l 11) surface oxidized at 300 K was annealed at 623 K where the '29'-structure is expected to be observed as seen in Fig. 6 (b) but could not be observed at atomic resolution. These uncorrugated surfaces were found to have the '44'-strucrture at 300 K indicating that rapid diffusion stopped below the temperature for production of the '29*-structure to form the '44'-structure. References [1] J. Haase, and H. -J. Kuhr, Surf. Sci. 203 (1988) L695. [2] M. Bowker, S. Poulston, R. A. Bennett, P. Stone, A. H. Jones, S. Haq and P. Hollins, J. Mol.Catal.A131 (1998) 185. [3] D. J. Coulman, J. Wintterlin, R. J. Behm and G. Ertl, Phys. Rev. Lett. 64 (1990) 1761. [4] F. Jensen, F. Besenbacher, E. Uegsgaard. and I. Stensgaard, Surf. Sci. 259 (1991) L781.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) vC 2001 Elsevier Science B.V. All rights reserved.

777

Characterization of Si-O-C Ceramics Prepared by the Pyrolysis of Phenylsiiicones Yoshito Tanaka, Chisato Mori, Noboru Suzuki*, Takehiko Kasai, Ken-ichi limura and Teiji Kato Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan We have characterized Si-O-C ceramics prepared by pyrolysis of phenylsilicone polymer derived from hydrolytic polymerization of phenyltrichlorosilane. The samples heat-treated at 600 to 900°C have much free carbon with about 60% of carbon atoms of original polymer. The samples heat-treated at 900 to 1400°C are recognized to have Si-O-C network of silicon oxycarbide and free carbon in the bulk phase, and the surface covered with silica. The electric conductivity of these samples increases with the treatment temperature, and the values arerather than the reported one of silicon oxycarbide glasses pyrolyzed from methylsilicone polymers. 1. INTRODUCTION The preparation and the characterization of Si-O-C ceramics that contain only silicon, oxygen and carbon have been studied by many researchers [1-7]. This material that has Si-O-C network (siliconoxycarbide) is attractive for its high temperature strength and chemical stability and for the application to the high performance composite materials. Although mainly used starting materials were methyl, ethyl or dimethyl silicone polymers in many reports, phenylsilicone polymers from phenylchlorosilanes such as phenytrichlorosilane(PTCS) and diphenyldichlorosilane(DPE)CS) will be good candidates for precursors carbon rich Si-O-C materials having not only a high mechanical strength but also a moderate surface area and an electric conductivity. In this paper, we have reported preparation of Si-O-C ceramics by the pyrolysis of phenylsilicone polymer from PTCS and characterization of the produsts by means of scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD) and electric conductivity. 2. EXPERIMENTAL PTCS used as a starting material was purchased from Kanto Chemicals and was used as

778 received. PTCS was poured into distilled water with stirring and hydrolyzed, and then a solid material, phenyl silicone polymer(PSP), was formed by the condensation polymerization. After leaving the suspension one night, the polymer sample was filtrated, washed with water and dried in vacuum for Ih. Then the sample was heated at 200°C for Ih in argon to complete the reaction of cross linking. After pulverizing the polymer, tablets 10mm diameter x 2mm thickness, were formedfi*omthe powder of 300mg by pressing at 3380kg/cm^. Finally, these were heat-treated at several temperatures for Ih. The samples used for the treatment above lOOO^C were preheated at 900°C. The apparatus used for XPS measurements was Physical Electronics ESCA 5600 with the conventional MgKa x-ray source. Binding energies of XPS spectra were calibrated to set the main CIs peak at 284.5 eV as pyrolysed carbon or graphite. X-ray powder diffraction patterns were taken with a Rigaku Rint 2100 diffractometer, using Cu Ka radiation. 3. RESULTS AND DISCUSSION 3.1. Weight and color changes during pyrolysis Firstly, we observed color change during pyrolysis. The color was white under 500°C, brown at 500 to 600°C and black above 600°C. The total weight loss of the sample heat-treated up to 400°C was less than 3%. On the other hand, those of at 500 and above 550°C were about 18 and 25%, respectively. From 600 to 900°C, there is no significant change in weight. Thus the decomposition of PSP is considered to proceed at around 500°C. If all phenyl groups are removed by the pyrolysis, the weight loss showed be about 60%. From this difference between 25 and 60%, about 60% of the initial carbon atoms contained in the PSP have remained after the pyrolysis. This indicates that a lot of free carbon remains by pyrolysis of PSP than methylsilicone polymers. 3.2.SEM Observation SEM photographs showed many large cracks on the sample surfaces heat-treated at 1000 and 1200°C, which may be caused by the shrinkage during the cooling process. However, the sample heat-treated at 1400°C has only micro cracks, which suggests progress of sintering and/or solid reactions. 3.3.XPS result Before measuring XPS, samples prepared at various temperatures were evacuated at 120°C to remove adsorbed gasses. Atomic concentration (AC) and binding energies (BE) are obtained before and after mild argon ion etching. Those values of bulk were obtained after mild etching of the cross section of cut samples. From the XPS spectra, we obtained the AC and BE of every peaks. Results are summarized in Table 1. For the sample heat-treated at above 900°C, silica is formed at the topmost surface(before etching) because O/Si atomic ratios are around 1.9 and BEs of Si2p are around 103.6 eV. The

779 carbons ACs before and after argon ion etching of a 900°C treated sample are 39 and 34%, respectively. Therefore, this sample surface consists of silica and free carbon. On the other hand, those of a sample heat-treated above 1000°C are only a few percent or less. Thus, the treatment at higher treatment may progress the silica formation. Although the reason and the mechanism are unknown yet, oxidation of surface of the 900°C treated sample may have proceeded during exposure to ambient atmosphere before the successive heat treatment at higher temperatures. On the cross section analysis, O/Si and C/Si ratios are 1.04 to 1.27 and 1.97 to 2.47, respectively, and the BEs of Si2p are about 102.0 eV. The BE is smaller than that of silica (103.2-103.8eV), larger than that of siliconcarbide (99.8-100.8eV) and similar to that of silicone polymer (101.8-102.8eV) and silicate (101.0-102.0eV). So, the samples heat-treated above 900°C have Si-O-C network and free carbon in the bulk phase and the silica layer on the surface. Table. 1 Atomic concentration (AC) of heat-treated samples by XPS Surface Before etching 900°C 1000°C 1400°C After etching 900°C 1000°C 1400°C Bulk After etching 900°C 1000°C 1400°C

C(%)

0(%)

Si (%)

O/Si

C/Si

BEsi2p (eV)

39.12 5.08 6.06

40.76 61.77 61.35

20.12 33.15 32.59

2.03 1.86 1.88

1.94 0.15 0.19

103.6 103.4 103.8

33.58 1.67 0.52

38.89 62.64 61.31

27.53 35.69 38.18

1.42 1.76 1.61

1.22 0.05 0.01

102.9 104.0 103.8"^

54.82 48.61 46.58

23.06 27.47 29.89

22.12 23.92 23.53

1.04 1.15 1.27

2.47 2.03 1.97

102.0 102.1 102.0

a) BE was referred to the CIs of 284.8eV as contamination carbon 3.4. Crystallinity On the XRD patterns, there is no sharp peak for all samples. This indicates that the crystallization of silica, graphite, silicon oxycarbide or silicon carbide does not proceed under 1400°C treatment. 3.5.£le€tric conductivity The electric conductivities of heat-treated samples were measured by the four-terminal methods. Results are summarized in Table 2. The conductivity can not be measured for the samples treated under 600'*C. However, the conductivity of black samples and increase with heating

Table.2 Electric conductivity. Temp. (°C) 200 550 900

1000 1200 1400

Color White Blown Black Black Black Black

Conductivity (Scm'^) 6.7 X 10-^ 1.0 X 10'^ 2.9X10'

2.6

780

temperature, and the values much higher than those of corresponding siHcon oxycarbides pyrolyzed from methylsilicone polymers[2]. 4. CONCLUSION Si-O-C ceramics have been prepared by the pyrolysis of phenylsilicone polymers derived from the hydrolytic polymerization of phenyltrichloro silane. From the change in color and the weight loss, the pyrolysis proceeds at around 500°C and about 60% of carbon atoms of the original polymer have remained as free carbon in the samples heat-treated at 600 to 900°C. XPS were measured for samples heat-treated above 900°C to obtain the structural information. The atomic concentration ratio of 0/Si for the surface is around 1.9 and the BE of Si2p is around 103.6, which indicate that the surface is covered by silica. For the cross section of cut samples, however, the 0/Si ratio is 1.04 to 1.27 and the BE of Si2p is around 102.0 eV, indicating that the bulk materials have the Si-O-C network of silicon oxycarbide. The electric conductivities the samples of heat-treated at 900 to 1400°C increase with the treatment temperature, and the value much higher than that of reported silicon oxycarbide glasses. Consequently, we have prepared the carbon rich Si-O-C ceramics having a high mechanical strength, a high electric conductivity and a moderate surface area. The high strength will be due to Si-O-C network. These properties will be controllable by using other precursor materials such as diphenylsilicone polymer and the composite with methylsilicones, which are now in progress. Acknowledgement This research was partially supported by Grant-in-Aid for Scientific Research on Priority Areas (Carbon Alloys) of the Ministry of Education, Science, Sports and Culture, Nos.10137206 and 11124204, and the Aid of Satellite Venture Business Laboratory of Utsunomiya University. REFERENCE 1. G. M. Renlund, S. Prochazka and R. H. Doremus, J. Mater. Res., 6(1991)2716. 2. G. M. Renlund, S. Prochazka and R. H. Doremus, J. Mater. Res., 6(1991)2723. 3. E.Breval, M.Hammond and C. G. Pantano, J. Am. Ceram. Soc, 77(1994)3012. 4. S.Manocha, D.Vashistha and L.M.Manocha, Key Eng. Mater., 164-165(1999)81. 5. D. M. Wolfe, F. Wang, B. J. Hinds and G. Lucovsky, Mat. Res. Soc. Symp. Proc, 483(1998)203. 6. D. R. Bujalski, S. Grigoras, W. L. Lee, G. M. Wieber and G. A. Zank, J. Mater. Chem., 8(1998)1427. 7. L. Bois, J. Maquet, F. Babonneau and D. Bahloul, Chem. Mater., 7(1995)975. 8. M. Inagaki, T. Ibuki and T Takeichi, J. Appl. Polym. Sci., 44( 1992)521. 9. T. Takeichi, Y. Eguchi, Y. Kaburagi, T. Hishiyama and M. Inagaki, Carbon, 37(1999)569.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^ 2001 Elsevier Science B.V. All rights reserved.

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Structure and growth process of niobium carbide on silica Nobuyuki Ichikuni, Fumio Sato, Shogo Shimazu and Takayoshi Uematsu Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522, Japan Nb205/Si02 was carburized by TPR process in a stream of 20% CH4/H2 gas mixture to produce NbC/SiOj. The XAFS analysis confirmed that NbjOj was reduced to NbOj at the first stage, and converted into NbC at the second stage. The growth of Nb particle was not observed in the former process, but in the latter process. Although the NbC particle size of 4wt% catalyst (3.15 nm) was larger than those for the conventional 2wt% catalyst (2.29 nm), the successive carburized catalyst (4wt%MC) had a similar size (2.51 nm) to the 2wt%. The design of highly dispersed NbC particle on Si02 surface was achieved by the MC method. The effect of carbon on the structure has also been investigated. 1. INTRODUCTION Most of works on the catalytic application of eariy transition-metal carbides have been focused on MOjC and WC [1], and very few have been reported on the other compounds such as NbC. The carburization of Nb205 into NbC requires significantly high temperatures {ca. 1370 K) [2] which may lead to a decrease in surface area, and, therefore, is limited in use as catalysts. In this study, we prepared Nb205 supported on high surface area SiOj and transformed it into NbC/SiOj, expecting that the support effect would prevent the aggregation of Nb species during carburization to keep the high surface area. The growing process of NbC particle, the effect of carbon on the structure and the catalysis were also investigated. 2. EXPERIMENTAL The precursors (Nb205/Si02) were prepared by an impregnation method of Si02 (Aerosil #200) with a NbClj/methanol solution. The calcined precursors were carburized in a 20% CH4/H2 mixed gas stream to produce NbC/SiOj by temperature programmed reaction (TPR) process as follows: The reaction temperature was increased at a linear rate of 10 Kmin^ to the final temperature (T^arb)* which was maintained for t^ minutes until no CO was detected by TCD. The successive impregnation (Nb 2wt%) - carburization method and therepeatedimpregnation (Nb 2wt% X 2) - carburization method were applied, which were designated as MC and SC, respectively. Characterization was carried out by TEM (JEOL JEM-4000FXII, 400 kV), XAFS (KEKPF, Proposal No. 98G317, BL-lOB with Si(311)), Raman spectroscopy (JASCO NRS-2100, Ar^ 514.5 nm excitation) and XRD (MAC Science, MXP3).

782

CO oxidation reaction at 573 K was studied in a closed circulating system with the initial pressures of 2.6 and 1.3 kPa for CO and O2, respectively. 3. RESULTS AND DISCUSSION 3 . 1 . Particle size of surface carbide The XAFS analysis for the 2wt% Nb/SiOj catalysts revealed that Nb component was reduced to NbOj in the course of TPR, just elevated to 1073 K. In the remaining carburization stage, at 1073 K for t^ min, NbOj was gradually reduced and carburized to become NbC. Table 1 showed the average particle size determined from the TEM measurement for each carburization process. Although the particle size between Nb205 (1.92 nm) and NbOj (1.96 nm) was almost unchanged, the particle size growth was observed from NbOj (1.96 nm) to NbC (2.47 nm). It was suggested that the aggregation of Nb particles occurred at the step from NbOj to NbC. Considering the densities of NbjO^ and NbC, the NbC particle on NbC/SiOj (7;a,^=1073 K) may be converted from ca. five Nb205 particles on the original NbjOj/SiOj catalyst. Figure 1 shows the XRD patterns for 2wt% NbC/SiOj carburized at 1273 K. The XAFS analysis confirmed that the tj^ = 30 min (T^^ = 1273 K) was long enough for 2wt% catalyst to be converted from NbjOs to NbC. The observed peaks were attributed to NbC and amorphous SiOj for the catalyst of t^ = 30 min. On the other hand, the graphite peaks besides NbC peaks were appeared for lengthening the carburization time (r^ = 180 min). The excessive carburization seemed to promote the growth of graphite on the catalyst surface. Table 1 Average particle size of 2wt% Nb/SiO, catalysts Obtained Nb compound condition Vnm calcined Nb205 1.92 Nb02 7,^,^=1073 K,/^=0 min 1.96 NbC 2.47 r_,=1073K, r„=120min ' Average particle size determined from TEM measurement. n

1

\

\—

cartx)n (amorphous) (0

graphite (crystal)

(A C 0)

c

(0

E (0 DC

60 80 20 /degree

Fig. 1. XRD patterns for 2wt% NbC/SiO^ carburized at 1273 K.

1800

1600

1400

1200

1000

Raman shift / cm'^ Fig. 2. Raman spectra for 2wt% NbC/SiOj carburized at 1273 K.

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Raman spectra for these two catalysts showed two peaks as shown in Fig. 2. These peaks at around 1585 and 1360 cm^ were attributed to the highly crystalUzed graphite (/^) and the amorphous carbon (4«,), respectively. The increasing 7^/,^ratioby the increase of r^ means that the growth of crystal graphite preferably took place during the cari)urization process. The average particle sizes were determined from the TEM measurements. The sizes of 2wt% NbC/SiOj catalysts {T,^^ = 1273 K) were 2.29 and 2.27 nm for the t^, of 30 min and 180 min, respectively, indicating that the carburization time did not affect the particle growth as listed in Table 2. On the other hand, the drastic deactivation of CO oxidation reaction was found by lengthening r^ (75.8x10"^ molmin-^gj^^,^ and 42.7x10"^ molmin^g^b^ for r^ = 30 min and 180 min, respectively). The TEM observations and XRD analysis revealed that the graphite grew on the catalyst. The Raman measurements suggested that there were both the crystal and the amorphous carbon on the catalyst. It is concluded that the excessive carburization may cause the deactivation by covering the NbC surface with the crystal graphite. Table 2 Particle size, dispersion of NbC/SiO, and initialratefor CO oxidation d *^ /nm dispersion / % r^ V 10^ mol-min'-g. Catalyst* Jiiu aw 2wt% (/^ = 30 min) 2.29 58.9 75.8 2.27 2wt% (/^ = 180 min) 42.7 59.0 4wt% 3.15 42.5 41.7 4wt%SC 3.42 39.2 39.7 4wt%MC 2.51 53.4 11.9 • Carburized at 1273 K. ^ Average particle size determined from the TEM measurement. ' Initialratefor CO oxidation, P(CO) = 2.6 kPa, PiO^) = 1.3 kPa, T = 573 K.

"5

t 2

3 4 r/10'^nm Fig. 3. Fourier transforms of A:^-weighted Nb K-edge EXAFS oscillations for NbOSiO^ catalysts carburized at 1273 K.

100 60 80 26 /degree XRD patterns for 4wt% NbC/Si02

40

Fig. 4. catalysts.

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3.2. Successive carburization method To prepare the highly dispersed 4wt% NbC/SiOj comparable to the 2wt% NbC/SiOj, the successive carburization methods were applied. Figure 3 demonstrates the Fourier transform of /:^-weighted EXAFS oscillations. All catalysts showed the same FT profile like bulk NbC except for the intensity. The FT intensities for MC catalyst and SC catalyst resembled to each for 2wt% and 4wt%, respectively. Particle size analysis by using TEM were carried out and summarized as shown in Table 2. Although the NbC particle size of 4wt%SC catalyst (3.42 nm) was larger than those for the conventional 2wt% catalyst (2.29 nm), 4wt%MC catalyst was nearly the same (2.51 nm) in size as shown in Table 2. XRD patterns for these catalysts were exhibited in Fig. 4. The distinct graphite peaks were observed for MC catalyst, suggesting that the graphite was present MC surface more than on the SC surface. Y. Zhang et al, reported that carbon nanotubes and solid metal reacted to produce metal carbide at above 1073 K under UHV or inert condition [3]. Taking into account the fact that the Nb particle aggregation occurred mainly in NbOj to NbC step, the surface excess graphite seemed to react with NbOj at the second carburization process and prevent the aggregation. 4. CONCLUSION SiOj supported NbC catalysts were prepared by carburizing the Nb205/Si02. The pardcle size growth was occurred in NbOj to NbC step and the excessive carburization did not affect the paiticle size growth. The design of highly dispersed NbC particle on SiOj surface was achieved by use of the successive impregnation-carburization method. The high dispersion may be due to the presence of surface graphite. This work is paitially supported by Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan (No. 12740311). REFERENCES 1. S. T. Oyama ed.. The Chemistry of Transition Metal Carbides and Nitrides, Blackie Academic and Professional, London, 1996. 2. V. L. S. Teixeira da Silva, E. I. Ko, M. Schmal and S. T. Oyama, Chem. Mater., 7 (1995) 179. 3. Y. Zhang, T. Ichihashi, E. Landree, F. Nihey and S. lijima. Science, 285 (1999) 1719.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (& 2001 Elsevier Science B.V. All rights reserved.

785

In situ energy-dispersive XAFS study of the reduction process of CuZSM-5 catalysts with 1 s time-resolution A. YaInaguchi^ Y. Inada^ T. Shido", K Asakura', M. Nomura^ and Y. Iwasawa"* ' Department of Chemistry, Graduate School of Science, the University of Tokyo, Kongo, Bunnkyo-ku, Tokyo 113-0033, Japan •^ Research Center for Materials Science, Nagoya University, Nagoya 464-8602, Japan "^ Catalysis Research Center, Hokkaido University, Kita-ku, Sapporo 060-0811, Japan ^ Institute of Materials Structure Science, Photon Factory, KEK, Ibaraki 305-0801, Japan The reduction processes of Cu-ZSM-5 were investigated by a time-resolved dispersive XAFS (DXAFS) technique. Two Cu-ZSM-5 samples with different Cu loadings were prepared by an ion-exchange method. In the low loading sample (84 % ion exchanged), Cu^* cations were highly dispersed in the channel of ZSM-5, while CuO particles were formed in the high loading sample (104 % ion exchanged). The isolated Cu^* cations were reduced to isolated Cu* cations at 400-450 K and the Cu* ions were reduced to Cu° at 550-650 K The curve fitting analysis of the DXAFS data revealed that CU4 clusters were initially formed and then grew up to big particles. On the contrary, the CuO particles were reduced directly to Cu° big metallic particles aroimd 450 K 1. Introduction Elucidating the dynamic structure change of catalyst precxursors is a key issue to understand the chemistry of catalyst preparation and also to develop new catalytic materials in a controllable manner. Reduction of catalyst precursors to prepare active catalysts and the chemistry during reduction has been characterized by temperatin^programmed experiments. X-ray absorption fine structure (XAFS) is often used to investigate the structure of noncrystalline materials such as catalysts [1,2]. The dviration of data acquisition in a conventional XAFS, however, is more than 10 min.

786

and the applications to dynamic processes have been hmited. Recently, energydispersive XAFS (DXAFS) capable of data acquisition in several seconds has been developed [3]. DXAFS is a powerful technique to elucidate the structural change of metal sites during catalyst preparation. We have studied the reduction processes of Cu-ZSM-5 during temperature programmed reduction (TPR) by DXAFS in this paper. Cu-ZSM-5 shows unique catalytic performances for NO decomposition [4,5]. The redox property of Cu in CuZSM-5 is assiuned to be essential for the NO decomposition. However, structure change of the Cu species that occurs during the reduction with Hg is still unknown. 2. Experimental The Cu-ZSM-5 samples were prepared by ion exchange using an aqueous Cu(N03)2 solution. The pH during the ion exchange was 5.5. The sample with 84 % ion exchange is denoted as Cu-ZSM-5-A hereinafter. The sample with 104 % ion exchange (denoted as Cu-ZSM-5-B) was prepared imder the condition where the pH of the suspension during ion exchange was increased to 7 by addition of diluted ammonia. DXAFS measurements were carried out using synchrotron radiation at BLr9C at KEK-PF. A Si(lll) bent crystal was used to focus polychromatic X-ray beam. The sample was placed at the focus. The diverging X-rays were detected by position sensitive detectors. Absorption spectra were calculated by hi(Io(E)/I(E)), where 1(E) and Io(E) are X-ray intensity with and without sample as a function of X-ray energy (E). The spectra were acquired at an interval of 1 s dxuing the TPR and analyzed by the UWXAFS package [6]. 3. Results and discussion Figures 1 and 2 show a series of k^-weighted Fourier transformed EXAFS functions at Cu K-edge for Cu-ZSM-5-A and Cu-ZSM-5-B, respectively, during TPR in Hs (5.3 kPa) at a heating rate of 5 K min' in the range jfrom 300 K to 700 K. The data acquisition time for each spectrum was 1 s. Figure 3 shows the temperature dependence of the coordination numbers (CN) of Cu-0 and Cu-Cu determined by the curve fitting analysis for figure 1. The bond distances of Cu-0 and Cu-Cu did not change during TPR. The CN of Cu-0 was 2.8 at the initial Cu state and did not change till 410 K It began to decrease after this temperature and became 1.5 (nearly half of the initial value) at 450 K Then the CN was maintained up to 550 K and began to decrease above 550 K and became 0.1 at 700 K. The second change was accompanied by appearance of a metaUic Cu-Cu peak at a distance of 0.25 nm, which is similar to

787

the Cu-Cu distance of Cu metal. It indicates that all the Cu species were reduced to form Cu metallic particles at 700 K We assume that the two-stage change in the CN of Cu-0 corresponds to the reduction steps, Cu^* —• Cu* ~* Cu°. We can estimate the size of Cu metallic particles by using the CN determined by the EXAFS analysis which is an averaged value over all Cu atoms in the sample and by using the proportion of Cu° species determined by the XANES analysis. The ClSTs of Cu metaUic particles are obtained by the following equation; CN of Cu metaUic particle = (observed CN of metaUic Cu-Cu) / (proportion of Cu° species). The observed XANES spectra (8950-9050 eV) were deconvoluted using the three independent XANES spectra for Cu^\ Cu* and Cu°, so we could estimate the population of Cu^*, Cu* and Cu° species. As shown in figure 4, the CN of Cu metallic particles was 3 at 550 K It is likely fix)m CN = 3 that C\x\ clusters are formed at the initial stage of Cu* reduction. The CN increased gradually during the TPR and reached 7.8 at 670 K This suggests that a part of the Cu^ clusters migrate to the outer surface of ZSM-5 on which they merge to form Cu° particles. The reduction process of Cu-ZSM-5-B which contained big CuO particles was completely different irom that of the isolated Cu'* species in the channels of ZSM-5 in Cu-ZSM-5-A. The DXAFS study demonstrated that the direct reduction, Cu'* -^ Cu°, occurred with the Cu-ZSM-5-B. The CN of metaUic Cu-Cu bond showed that the Cu particles were big irom the beginning and any particle growing process was not observed, as shown in figure 4.

2

3 4 5 6 R/0.1 nm Figure 1 Fourier transformed k'^-weighted DXAFS functions for Cu-ZSM-5(84 %) obtained by background subtraction from the spectra during the TPR under 5.3 kPa of Hj from 300 K to 700 K.

0

1

2

3 4 R/0.1 nm Figure 2 Fourier transformed kWeighted DXAFS functions for Cu-ZSM.5(104 %) obtained by background subtraction from the spectra during the TPR under 5.3 kPa of K, from 300 K to 700 K.

788

z o

Figure 3 The coordination numbers (CN) of Cu-0 (O) and Cu-Cu (A) as a function of reduction temperatures during the TPR of CuZSM.5(84 %).

Figure 4 The CN of Cu-Cu bond in Cu metallic particles against reduction temperature; A: Cu.ZSM-5(84 %); # : Cu-ZSM-5(104 %).

4. Conclusion Isolated Cu^ species in the channels of ZSM-5 in Cu-ZSM-5-A were reduced stepwise, Cu^* -^ Cu* -* Cu°. At the initial stage of the reduction of Cu\ small clusters CU4 were formed and a part of them went out to the outer surfaces of the ZSM-5 to form Cu particles. On the contrary, CuO particles on the outer surfaces in Cu-ZSM-5-B were reduced at 450 K to Cu° big particles. The CN of metaUic Cu-Cu bond showed that the Cu particles on the outer surfaces were big from the beginning and the particle growing process was not observed. This work has been supported by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST). References 1. D.C. Koningsberger and R. Prins (eds.), X-ray Absorption. Principles, AppUcations, Techniques of EXAFS, SEXAES and XANES, Wiley, New York, 1988. 2. Y. Iwasawa (eds.), X-ray Absorption Fine Structure for Catalysts and Surfaces, World Scientific, Singapore, 1996. 3. T. Matsushita and R.P. Phizackerley, Jpn. J. Appl Phys., 20 (1981) 2223. 4. M. Iwamoto, H. Yahiro, N. Mizuno, W.-X. Zhang, Y. Mine, H. Furukawa and S. Kagawa, J. Phys. Chem., 96 (1992) 9360. 5. M. Shelef, Chem. Rev., 95 (1995) 209. 6. E.A. Stern, M. NewviUe, B. Ravel, Y. Yacoby and D. Haskel,Physica B, 208&209 (1995) 117.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ••o 2001 Elsevier Science B.V. All rights reserved.

789

Preparation and CO hydrogenation activities of smectite-type catalysts containing cobalt divalent cations in octahedral sheets Masayuki Shirai *, Kuriko Aoki *, Shi-Lin Guo *, Kazuo Torii ^ and Masahiko Arai' * Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba, Sendai, 980-8577, JAPAN ** Tohoku National Industrial Research Institute, Nigatake, Miyagino, Sendai, 983-8551, JAPAN *" Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, JAPAN Smectite-type materials containing cobalt cations in octahedral sheets (MST (Co)) were synthesized with a hydrothermal method. The MST(Co) catalyst prepared at pH 6.6 during hydrothermal treatment, having larger surface area and pore volume than that prepared at pH 8.8, showed higher CO hydrogenation activity than a conventional impregnated silica supported cobalt catalyst.

1. INTRODUCTION Smectite-type materials have layered structures in which each layer is composed of one octahedral sheet sandwiched by two tetrahedral sheets. The octahedral sheets contain divalent or trivalent cations which are surrounded by six oxygen atoms in octahedral structure and the tetrahedral sheets contain Si"** ions which are surrounded by four oxygen atoms in tetrahedral structure. We reported that porous smectite-type materials (MST) were synthesized from water glass and several metal chlorides with a hydrothermal reaction [1,2]. Because of their relativdy high surface area and easy introduction of various transition metal cations in the lattice, MST can be potentially useful catalysts. Nickel substituted smectite-materials (MST(Ni)) showed high activities for the isomerization of 1-butene [3], the oligomerization of ethylene [4], and hydrogenation of acetonitrile [5]. Cobalt substituted smectite-type materials (MST(Co)) showed high activity for the hydrodesulfurization of thiophene [6]. We also reported that the pore structure (surface area, pore volume, and pore size distribution) of MST(Mg) can be controlled by changing pH [7] and adding alkyl ammonium chloride [8-10] during hydrothermal treatment In the present paper, we report the effect of slurry pH during hydrothermal treatment for the pore structures and the CO hydrogenation activities of MST(Co) catalysts.

790 2. SAMPLE PREPARATION Si-Co hydrous oxide was prepared from water glass (SiO^ 29.04%, Na209.4 %, NIHON KAGAKU KOGYO) and cobalt chloride. A Si-Co hydrous precipitate was obtained by adding an aqueous solution of cobalt chloride to an aqueous water glass solution conu-olled in pH with an aqueous solution of sodium hydroxide. The Si/Co ratio was fixed at 8/6. After filtration and washing the precipitate with distilled water, a slurry was prepared from the Si-Co hydrous oxide precipitate and an aqueous solution of sodium hydroxide. The pH of the slurry was controlled by the aqueous solution of sodium hydroxide. Following a hydrothermal reaction, the slurry was filtrated, dried at 353 K and calcined at 573 K, and then the final product (MST(Co)) was obtained. 3. RESULTS AND DISCUSSION 3 . 1 . Characterization of MST(Co) Fig. 1 shows XRD patterns of MST(Co) samples and cobalt oxides. Peaks at ca. 10', 24*, 44% and 79' are assigned to (001), (020, 110), (130, 200), and (060,330) diffraction peaks of smectite structures, respectively [11]. The peaks ascribed to cobalt oxides were not observed in the diffraction patterns of MST(Co) samples after calcination at temperature up to 873 K.

3 cd

^-

---...^^PH 6.6

^J

c 0)

^^^^^COO

. i.

^^^^^COA. !

1

0

.

.

.

1

20

1

..1 .

.

.

1

40

.

.

. .

.

.

. A 1

60

.

.

.

1

J

80

2 e / degree (Fe Ka) Fig. 1. The XRD patterns of MST(Co) Fig. 2 shows Fourier transforms of EXAFS spectra of the MST(Co) samples after calcination at 573 K. Both Fourier transforms showed two peaks at similar distances (phase uncorrected); the peaks between 0.1 and 0.2 nm are ascribed to Co-O bond and the peaks between 0.2 and 0.3 nm are ascribed to the Co-aCo and Co-O-Si bonds. The radial distribution functions in Fig. 2 are similar to those of EXAFS Fourier transforms for nickel divalent cations in octahedral sheets of smectite-type materials [4]. No other bonds derived from cobalt oxides were observed in the EXAFS Fourier transforms of the MST(Co) samples calcined at 873 K, which suggests that the divalent cations are incorporated in the octahedral lattice. The intensity of peak assigned to Co-O-Co and Co-O-Si bonds in Fig. 2(b) is larger than that in Fig. 2(a), indicating that the size of smectite fragments in the MST(Co) sample prepared at pH 8.8 is larger than that prepared at pH 6.6. XRD and EXAFS analysis indicates that both MST(Co) samples prepared at pH 6.6 and 8.8 have smectite structure.

791

(a) pH 6.6

0.1

0.2

0.3

0.4

0.5

0.6

R/nm

Fig. 2.

Fourier transforms for Co K-edge EAFXS of MST(Co).

Pore size distribution of two MST(Co) samples evaluated from desorption isotherms with the BJH method are shown in Fig. 3. Both MST(Co) samples have high surface areas (SA), large pore volumes (PV) and micro and mesopores. The MST(Co) samples prepared at pH 6.6 has more micro pores than the MST(Co) sample prepared at pH 8.8. We reported that small fragments of smectite exist as pillars between silicate layers in MST(Mg) samples and that the size of small smectite fragments related to the pore size of MST(Mg). Small smectite fragments would also exist between silicate in MST(Co) samples. Smaller smectite fragments would exist in the MST(Co) sample prepared at pH 6.6 because the distribution of micropores in the MST(Co) sample prepared at pH 6.6 was larger than that of pH 8.8. 0.05, (a) pH 6.6 SA 451 m^g' PV 0.51 cmV'

0.04 0.03 [ 0.02 ^

E u

0.01

0 (b) pH 8.8 SA 239 m^g' PV 0.26 cm^g'

I 0.04 5 0.03

100

Pore diameter /A

Fig. 3.

1000

Pore size distribution of MST(Co).

792 3.2. CO Hydrogenation with MST(Co) catalysts The CO hydrogenation reaction was carried out in slurry phase over MST(Co) catalysts with an autoclave (Table 1). The activity of MST(Co) synthesized at pH 6.6 was active and higher than that of the sihca supported catalyst The activity of MST(Co) synthesized at pH 8.8 was less active. The ratio of H2 to CO reacted on MST(Co) synthesized at pH 6.6 is lower than that on the sihca supported catalyst, suggesting that the higher hydrocarbons were much produced on the MST(Co) catalyst than that over the silica supported catalyst. Table 1. Activities for CO Hydrogenation Conversion ^ H2/CO reacted Catalysts * CO(%) H2(%) MST(Co) (pH 6.6) 16 19 0.6 MST(Co) (pH 8.8) 3 0 35wt%Co/Si02 7 31 2.2 a: Reduction: H2, 673 K, 3 hours. b: CO/H2/Ar = 60/30/10,4 atm, 543 K, 6 hours.

4. ACKNOWLEDGEMENT This work was carried out under the approval of PF advisory committee (No.97G022). This work was partially supported by JSPS (RFTF-98P01001).

REFERENCES 1. M.Arai, S.-L.Guo, M.Shirai, Y.NishiyamaandK.Torii, J. Catal., 161 (1996) 704. 2. M.Shirai, K.Torii, and M.Arai, Jpn. J. Appl. Phys., 38 (1999) 69. 3. M.Shirai, K.Aoki, K.Torii and M.Arai, Appl. Catal. A, 187 (1999) 141. 4. M.Shirai, K.Aoki, Y.Minato, K.Torii, and M.Arai, Stud. Surf. Sci. Catal. 129 (2000) 435. 5. M.Arai, M.Kanno, Y.Nishiyama, K.Torii, and M.Shirai, J. Catal, 182 (1999) 507. 6. M.Arai, Y.Minato, K.Torii, and M.Shirai, Catal. Lett. 61 (1999) 83. 7. K.Torii, Y.Onodera, T.Iwasaki, M.Shirai, M.Arai, and Y.Nishiyama, J. Porous Mater., 4(1997)261. 8. M.Shirai, N.Suzuki, Y.Nishiyama, K.Torii, and M.Arai, Appl. Catal., 177 (1999) 219. 9. M.Shirai, K.Torii, and M.Arai, Stud. Surf. Sci. Catal., 130 (2000) 2105. 10. M.Shirai, KTorii and M.Arai, Mol. Cryst. Liq. Cryst., 341 (1999) 321. 11. M.Shirai, K.Aoki, T.Miura, K.Torii, and M.Arai, Chem. Lett. (2000) 36.

Studies m Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c) 2001 Elsevier Science B.V. All rights reserved.

793

Preparation of M0O3 by spray reaction method and photometathesis of C3H, H. Murayama,N. Ichikuni, S. Shimazu and T. Uematsu Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522, Japan Molybdenum oxide catalysts prepared by the spray reaction method had anisotropy in their crystal structures. In the present paper, the surface structure and the photoactivity for propen metathesis of the spray catalyst compared with the commercial M0O3 were investigated. In the XPS, the main peak of the spray catalysts was similar to that of the commercial M0O3 with the other characteristic peaks of Mo 3d^,-,^ and 3d^^f^y The activity of spray catalysts showed higher than that of the commercial M0O3, and increased with increasing the T^^y suggesting the formation of the characteristic active species due to the preparation method, Le. rapid heating and quenching steps, for the spray reaction method. 1. INTRODUCTION Most of photocatalystshave been successfully prepared by tailoring the active species on the supports with high surface area These surface species dispersed as atomic or molecular unit showed high photoactivity. For example, the supported molybdenum oxide species on SiOj show photocatalyticactivity in the oxidativedehydrogenation of alcohols [1], the oxidation of hydrocarbons [2] and the metathesis of alkenes [3]. On the other hand, the bulk molybdenum oxide catalysthas no photocatalysisfor the QH^ metathesis reaction [4]. Sprayed particles, which were prepared by rapid heating and quenching process, can provide the characteristic properties in various catalyses [5-7] due to nano structures in the fine composites with metastable surface states [8]. The unsupported bulk oxides prepared from the spray reaction method were expected to show high photocatalysis as well as the supported catalysts. In this study, molybdenum oxide photocatalysts were prepared by the spray reaction method, and their characterization of the active site structure and the catalytic behavior in the photometathesis of C^H^ were investigated. 2. EXPERIMENTAL 2.1. Catalysts preparation The spray molybdenum oxide catalysts were prepared as follows: The 0.15 mol/1 molybdenum aqueous solution was prepared by dissolving M0O3 (Junsei Chemical Co., Ltd.) in dilute NH3 solution, and used as a starting solution. Atomized droplets from the starting solution were dried and calcined in a quartz tube reactor heated at the spray reaction temperature (T^pr) between 673-973 K. The catalyst was designated with T^^ as spr 673. 2.2. Characterization The catalyst was pretreated under O2 at 673 K for 120 min, followed by evacuation for 15

794 min prior to use for each of measurements and reactions. Characterization was carried out by methods of BET surface area measurement, XRD, XPS and Raman spectroscopy. The Raman spectra were taken with the 514.5 nm line of an Ar* laser (JASCO NRS 2100). The XPS was measured with using Mg-K^^. The pretreated catalyst was sealed in a pyrex glass tube, it was opened in a Ar glove box before setting it on a holder. The number of active sites was quantified by the CO photooxidation reaction performed under the same condition of the C3Hg photometathesisreaction. 23.

Reaction The photocatalytic metathesis reaction was performed at 275 K with 3.3 kPa of C^H^ in a quartz cell connected with a closed circulating system. UV irradiation was carried out by using a 75 W high pressure Hg lamp with a water filter. The reaction products (CjH^ and C4H8) were analyzed by a GC. The TON was calculated from the amount of formed C2H4 for 180 min divided by the number of photoactive sites. 3. RESULTS AND DISCUSSION 3.1. XRD As shown in Fig. 1, the interesting differences in the relative intensity of XRD peaks of a-MoOg {20 = 23.4, 25.8, 27.4, 33.8 and 39.0 degree) between the spray catalyst and the commercial M0O3 were observed. The peaks at 25.8 and 39.0 degree were diffracted from (040) and (060) plane, respectively. The ratio of these two peaks {26 = 25.8 and 39.0 degree) to the peaks at 26 = 23.4 and 27.4 degree was much smaller for the spray catalysts than that for the conunercial M0O3 w^^ich has the ratio over unity. These observations indicate that the anisotropic growth of crystals [9] was promoted by the spray reaction method. With increasing T,^, the anisotropic profile of the spray catalysts was more distinguished. The XRD peaks of spr 673, spr 773 and the commercial M0O3 were assigned to only aM0O3. As T^p^ raised to 873 K, two phases, a-MoOj and P-M0O3, were coexistent The impregnation catalyst (Mo/SiOj) showed no P-M0O3. Since P-M0O3 is stable below 623 K [10], it is unusual that the spray catalysts prepared at 7,^^ above 873 K contained P-M0O3. The rapid heating and quenching steps for the spray reaction may form metastable sites on the surface. Thus, the surface of the spray molybdenum oxides could be partly converted to the different stable phase as |J-MoQ. 3.2. Raman spectra A band due to the Mo-O-Mo bridged bond of a-Mo03 was observed at 820 cm' in the Raman spectra of spr 973 as shown in Fig. 2. Moreover, a band observed at 1000 cm' was assigned to the terminal Mo=0 stretching vibration of a-MoOj. It suggests that the pseudoisolated molybdenum oxide species existed on the spray catalyst The p- M0O3 phase was also observed at 775 and 850 cm *, which was in good agreement with the XRD results. The intensity of peak at 1000 cm^ reduced after the reaction as shown in Fig. 2 (b). However, in the case of spr 773, there is little difference in intensity between before and after the reaction. The different photocatalytic activity between spr973 and spr773 was expected.

795 S.

^MoO^

I

\ i \

20

i J L L »^^ 1 25

30 35 26 I degree

(a)

40

Figure 1. XRD patterns for Mo oxide catalysts; (a) the commercial MoQ, (b) spr 673 ,(c) spr 773, (d) spr 873 and (e)spr973.

45

'i

M W\

(a) ••

A\

(b) 11

J

1000

^'•---'''^^

.'--"^

Vwx/N.^^^^-^ 800

600

400

Raman shift / cm'

Figure 2. Raman spectra for spr 973; (a) pretreatedand (b) after the photometathesis.

3.3. XPS The surface species of the spray catalysts were characterized by the XPS. As for spr 973, binding energy (BE) of Mo 3d(3/2) and 3d(5/2) were higher by 0.15 eV and that of 01s was lower by 0.2 eV, respectively, than those of the commercial M0O3. Since the FWHM was broadened over the spr 973 compared to the commercial M0O3, it was deconvoluted into two peaks by gauss functions, as shown in Fig. 3. The main peaks at 235.6 and 232.5 eV 230 236 234 232 238 corresponded to those of the commercial binding energy / eV M0O3. The other peaks (BE = 234.4 and Figure 3. XPS spectra of Mo (3d3;2 ^^^ 231.3 eV) were shifted to lower BE by 1.2 3d5,2) level for (a)the commercial M0O3 and eV, and observed solely for the spray (b)spr973. catalysts. The energy gap between Mo 3d(5/2) and 3d(3/2) and the peak ratio of Mo 3d(5/2) and'^^^,2)were almost constant for the deconvoluted peaks. These new lower BE peaks could be contributed metastable surface structures of the spray molybdenum oxide. It can be said that these new BE peaks were formed by the rearrangement of the surface structure during the spray reaction.

796 Table 1 Activity for QH^ photometathesis reaction catalyst TON (C,H^) number of active sites /10 ^ mol g,^' spr673 27 1.7 spr 773 29 3.0 spr 873 36 2.2 48 2.7 spr 973 MoQ* 13 OT * Purchased from Junsei Chemical Co., Ltd. 3.4. Photometathesis reaction of CJl^ Although all samples had almost equal BET surface areas (ca 20 m-g'), the spray catalysts had much more active sites than the commercial M0O3 as shown in Table 1. The number of active sites were in order of spr 773 > spr 973 > spr 873 > spr 673 » the commercial M0O3. ^^ decrease in active sites for spr 973 and spr 873 was caused by the content of P-M0O3 which had lower amount of active sites. From the results of the XRD and the Raman spectra, it is suggested that the anisotropic growth of a-Mo03 on the spray catalysts resulted in the production of molybdenyl double bonds associated with the active site. The TON became higher with increasing T^^^. Considering die fact that the higher T^^^ could produce much more unstable surface, these unstable states may have high efficiency of the charge transfer process. It can be speculated that molybdenyl double bonds on the individual surface of the unstable a - MoC^ would show the higher activity. REFERENCES 1. T. Ono, M. Anpo and Y. Kubokawa, J. Phys. Chem., 90 (1986) 4780. 2. K. Marcinkowska, S. Kaliaguineand P. C. Roberge, J. Catal., 90 (1984) 49. 3. M. Anpo, M. Kondo and Y. Kubokawa, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 2771. 4. Y. Kubokawa and M. Anpo, in "Adsorption and Catalysis on Oxide Surface", Eds. M. Che and G. C. Bond, Elsevier, Amsterdam, (1985) 127. 5. D. Li, N. Ichikuni, S. Shimazu and T. Uematsu, Appl. Catal. ArGeneral, 172 (1998) 352. 6. D. Li, N. Ichikuni, S. Shimazu and T. Uematsu, Appl. Catal. A:General, 180 (1999) 227. 7. N. Ichikuni, D. Murata, S. Shimazu and T. Uematsu, Catal. Lett., 69 (2000) 33. 8. T. Tsuchiya, N. Ichikuni, S. Shimazu and T. Uematsu, Chem. Lett., (2000) 652. 9. S. Li and J. S. U e , J. Catal., 162 (19%) 76. 10. E. McCarron, J. Chem. Soc, Chem. Commun., (1986) 336.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

797

Molecular mobility of hydrogen-bonded acetonitrile on surfece hydroxyls of MCM-41 with Kubo-Rothschild analysis H. Tanaka, A. Matsumoto \ K. K. linger', K. Kaneko Physical Chemistry, Material Science, Graduate School of Natural Science and Technology, Chiba University, 1-33 Yayoi, Inage, Chiba, Japan ^Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohasi 441, Japan ^Institute for Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University, D55099, Mainz, Germany The infrared-band shapes of the Vj (C=N stretching) mode of hydrogen-bonded acetonitrile on the surface hydroxyls of MCM-41 were analyzed with the Kubo-Rothschild theory in order to understand the vibrational dephasing dynamics. The rms magnitude of the Bohr frequency modulation A and its correlation time r^ were determined for the vibrational dephasing of the V2 mode.

1. INTRODUCTION Mesoporous molecular sieves with a hexagonal structure such as MCM-41 [1] and FSM-16 [2] have received an extensive attention concerning adsorption and catalysis. The concentration, reactivity and accessibility of internal surface hydroxyl groups (SiOH) are responsible for ion exchange and postmodification, such as silylation and chemical deposition. It is, therefore, important to explore a basis for the molecule - SiOH group interactions. It is well known that acetonitrile (CH3CN) forms a coordination complex with an electron acceptor molecule [3]. Then C=N bond is expected to form a hydrogen bond of the -N •• HO type with SiOH groups of MCM-41. The hydrogen bonding affects the C = N stretching mode significantly. The infrared-band width of the C=N stretching v^ mode of the hydrogen-bonded CH3CN is larger than that of the unassociated molecule by a factor of about 2. It indicates that the vibrational dephasing rate of the Vj mode is strongly influenced by hydrogen-bond formation. Vibrational phase relaxation in condensed phases has received considerable attention in recent decades, because it is recognized as a powerful probe of intermolecular interaction dynamics [4]. The simple dephasing model of Kubo-Rothschild theory can be extended to describe an adsorbed molecule, which interacts with the solid surface [5]. In the preceding works authors examined the relaxation of CH3CN molecules in the multi adlayers on MCM-41 using the infrared-band shape analysis [6, 7]. In this study, we measured infrared adsorption bands of hydrogen-bonded CH3CN

798

on SiOH groups of MCM-41 as a function of temperature (283 ~ 333 K) and F/P^ for the infraredband shape analysis. We determined the rms magnitude of the Bohr frequency modulation A (the factor hA can be looked upon as a rms intermolecular energy, which reflects the static distribution of interaction sites) and its correlation time r^.

2. THEORETICAL METHODOLOGY It is quite difficuh that hydrogen-bonded CH3CN on SiOH groups of MCM-41 undergoes a free reorientational motion of the main Cjy symmetry axis because of the hydrogen bonding. The resonant vibrational energy transfer process can be also negligible because hydrogen-bonded CH3CN may be isolated each other. We expect that the vibrational relaxation process of hydrogenbonded CH3CN mainly show aspects of pure vibrational dephasing. Hence, we neglect all other relaxation except for the vibrational phase relaxation and assume that the infrared-band shape I( v) can be represented by I(v) = Cvll-exp(-hcv/A:T)]£°cos2m:r(v-vJ
(1)

where c is the light velocity, C is associated with the integrated absorbance of the band, VQ is the band center and Op^ is the following Kubo relaxation function [5]: ^(0 = e x p { - A ^ [ r , H e x p ( - / / r J - l K r , / ] }

(2)

The parameters, A and r^ were determined for the vibrational dephasing of the V2 mode of hydrogen-bonded CH3CN by using the Damped Least-Squares (DLS) method so that the calculated band profile (eq. (1)) has the best fit with the observed values. Fig. 1 displays the Vj band resolved into the fundamental (Vj*), hot bands (v2'*'=V2+v'8-V8 v^^v^^lv^-lv^ and combination band (V3+V4). The relative intensities of the fundamental and hot bands were determined by the Boltzmann population analysis [8].

3. EXPERIMENTAL MCM-41 samples were prepared by Johannes Gutenberg University. The pore structure of MCM41 was determined by Nj adsorption at 77 K. The surface area, pore volume, and pore width were 940 m^ g', 0.74 ml g*', and 3.2 nm, respectively. Acetonitrile of spectroscopic grade reagent from Wako Pure Chemicals was used. The infrared specu^ of CH3CN adsorbed on MCM-41 were measured using an in situ IR cell with KRS-5 windows for FT-IR spectrometer (Jasco FT/IR-550). The spectra were measured with a resolution of 1 cm"' (256 scans each background and sample). The MCM-41 sample was uniformly coated on the KJBr disk under the pressure of 5MPa for the IR measurement, which does not induce the destruction of pore structures [9].

799

4. RESULTS AND DISCUSSION The 0-H stretching band of free surface hydroxyl of MCM-41 at 3738 cm' is shifted downward by about 300 cm', when the hydroxy! group forms a hydrogen bond with CH3CN. Then, the difference spectra between the CH3CN adsorbed sample and the outgassed one (background spectra) gave a negative band at 3738 cm'. The integrated absorbance of the negative 0-H band at 3738 cm' had a linear correlation with that of the V2 band of hydrogen-bonded CH3CN (2264 cm') in the range of P/PQ = 0.001 - 0.004 (surface coverage 9 = 0.15 - 0.32). When temperature was lowered (333 ~ 283 K), the surface coverage increased and the linear correlation was also observed. This confirms the formation of the 1:1 hydrogen-bonded complex. The VQ, A and r^ values obtained by fitting the observed spectra to eq. (1) are listed in Table 1 together with the value of r^A. The r^A-value denotes the modulation regime of the system. If r^A « 1, the system is in a fast modulation limit; the perturbation to a vibrating molecule during a ver> short time does not intensively vary the phase, so its phase is retained for a long times. In this case I(v) (eq. (1)) is a Lorentzian curve. If r^A » 1, it is in a slow modulation limit; the phase is rapidly lost and I(v) is a Gaussian form [4, 5]. As calculated r^A-values are approximately 1.3, the system is in an intermediate modulation regime. The correlation times r^ for the V2 band of hydrogenbonded CH3CN is about 0.8 ps, being independent of the surface coverage. The r,-value determines the average time between perturbative events (such as, collisions and librations). The independence of r^ on the surface coverage suggests that adsorbed molecules are highly isolated and the correlation time, r^ should be associated with the interaction with SiOH group. The observed r^ agrees with typical lifetimes of hydrogen bonds in liquid systems including solvent water [10]. Furthermore, in the case of pyridine (QHjN) - water system [11], 988 cm' band of C^HjN is sensitive to hydrogen bonding of the N •• HO type, the r^-value of the mode is about 1.2 ps and r^A is approximately 0.8 over the whole concentration range. Accordingly the reason for the observed vibrational dephasing must be ascribed to the hydrogen bonding with the surface SiOH group. Both of Tc and VQ decrease as 0 14 1 increasing temperature, as shown in Table 0 12 A .V,' 1. According to Purcell [3] the C = N 0.1 bond force constant of CH3CN is strengthened and the stretching frequency 008 V 3 + V4 always increases, when CH3CN forms a 0.06 coordination complex with an electron f\\ 0.04 acceptor molecule. That is, in the case of hydrogen-bonded CH3CN on the surface 002 hydroxyl of MCM-41, the upward shit of 0 the C = N stretching frequency depends 2280 2260 on the electron transfer from the nitrogen Wave number / cm atom of the CH3CN due to hydrogen Fig. 1 Resolved infrared absorption spectrum of bonding. The Vj band of hydrogen-bonded hydrogen-bonded acetonitrile on MCM-41 in the C=N CH3CN shifts upward by 12 cm*' at 283 K stretching Vj region at 283 K.

/ W\ Y "v,'""^

f

800 and 10 cm*' at 332 K from the bulk liquid phase value. Therefore, the downward frequency shift shows that the hydrogen bond becomes weaker with increasing temperature. Then the lifetime of the hydrogen bond should become smaller as observed. The observed temperature dependence of A can be interpreted by the temperature dependence of the hydrogen bond. We can conclude that analysis of the infrared-band shape of CH3CN on the MCM-41 using the Kubo-Rothschild theory provides the microscopic dynamics of an adsorbed molecule on the surface SiOH group. Table 1 Kubo parameters for the V2 mode of hydrogen-bonded CH3CN Vo (cm •) A (cm') TA ^c(PS) 00 ^

0.15

2263.1 ±0.1

0.77 ±0.06

9.1 ±0.1

1.3 ±0.2

2 S > 0 ^ ti

0.23

2263.5 ±0.1

0.77 ±0.06

9.1 ±0.1

1.3 ±0.2

0.31

2263.5 ±0.1

0.78 ± 0.06

9.1 ±0.1

1.3 ±0.2

^

283

2265.4 ±0.1

0.86 ±0.07

8.1 ±0.1

1.3 ±0.2

303

2264.7 ±0.1

0.75 ± 0.07

8.6 ±0.1

1.2 ±0.2

323

2263.9 ±0.1

0.70 ±0.05

8.9 ±0.1

1.2 ±0.1

332

2263.5 ±0.1

0.66 ±0.04

9.1 ±0.1

1.1 ±0.1

2253.3

0.13

8.2

0.20

^

Bulk liquid [12]

REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nture (London), 359 (1992)710. 2. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 681. 3. K. F. Purcell and R. S. Drago, J. Am. Chem. Soc., 88 (1966) 919. 4. A. Morresi, L. Mariani, M. R. Distefano and M. G. Giorgini, J. Raman Spectrosc, 26 (1995) 179. 5. W. G. Rothschild, Dynamics of Molecular Liquids, Wiley, New York, 1984. 6. H. Tanaka, T. liyama, N. Uekawa, T. Suzuki, A. Matsumoto, M. Grim, K. K. linger and K. Kaneko, Chem. Phys. Lett., 292 (1998) 541. 7. H. Tanaka, A. Matsumoto K. K. Unger and K. Kaneko, in Characterization of Porous Solids V, K. K. Unger, G. Kreysa and J. R Baselt (eds.), Elsevier, (2000), p 251. 8. Hashimoto, T. Ohba, S. Ikawa, Chem. Phys., 138 (1989) 63. 9. Y. G. Vladimir, F. Xiaobing, B. Zimei, L. H. Gary, A. O. James, J. Phys. Chem., 100 (1996) 1985. 10. J. Lascombe and M. Perrot, Faraday Disc. Chem. Soc., 66 (1978) 216. 11. W. Shindler and H. A. Posch, Chem. Phys., 43 (1979) 9. 12. A. Morresi, R Sassi, M. Paolantoni, S. Santini and R. S. Cataliotti, Chem. Phys., 254 (2000) 337.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (C) 2001 Elsevier Science B.V. All rights reserved.

801

A Comprehensive Study of Surface State of MCM-41 having Good Surface Crystallinity and Its Reactivity to Water Vapor Toshinori Mori,^ Yasushige Kuroda,^ Yuzo Yoshikawa,^ Mahiko Nagao,^ and Shigeharu Kittaka"" ^Department of Chemistry and '^Research Laboratory for Surface Science, Faculty of Science, Okayama University, Okayama 700-8530, Japan '^Department of Chemistry, Faculty of Science, Okayama University of Science, Okayama 700-005, Japan. We succeeded in preparing the MCM-41 sample, which possesses durability against water, through longer aging of a mixed solution of silica and surfactant in the temperature range from 308 to 413 K; the MCM-41 sample thus prepared was survived even after treating it in boiling water for one week. The properties of this sample were examined by XRD, ^^Si CP MAS NMR, and FT-IR measurements to know the factor governing such prominent features. As a result, it was concluded that the surface crystallinity of the sample plays a pivotal role in protecting the sample against the attack of water. 1. INTRODUCTION Mesoporous molecular sieve composed of silica, MCM-41, possesses the hexagonally-arranged uniform-pore structure [1,2]. The important characters of this material are its large specific surface area and controllable and narrowly-distributed pore sizes that manifest itself as a promising candidate for catalyst or catalyst support. Furthermore, the MCM-41 sample is also interesting from the viewpoints of elucidating the properties of molecules confined in its uniform pore. These characteristic features are expected to be different depending on the surface state of the sample. However, this MCM-41 sample has a weak point; this sample is readily collapsed and easily lose its characteristic hexagonally arranged structure when it was merely exposed to saturated water vapor for a long time [3,4]. So far, only few attempts have been made elucidating the effect of the surface state of such materials on retaining the lattice structure. In the present study, we intend to prepare the sample which has water-resisting property and to explore the surface properties of this sample as prepared, as well as the samples treated in boiling water, by means of ^^Si CP MAS NMR and also of IR using CO as a probe molecule.

802

2. EXPERIMENT Samples The MCM-41 sample was prepared according to the similar method reported in the several literatures [1,2,5,6]. A fumed silica sample (Aerosil 200) and hexadecyltrimethylammonium bromide (CTABr) were used as the silicon source and as the template organic reagent, respectively. The silica-surfactant gel was aged at 308 K for two days and then treated at 413 K for additional two days to improve surface crystallinity. The sample thus prepared was repeatedly washed with distilled water. The sample dried at ambient temperature was finally calcined at 873 K for 6 h in air to remove an organic template. This as-prepared sample. Sample A, was also treated in boiling water for one week(abbreviated as Sample B). IR measurement IR spectra were measured on a JEOL JT-100 spectrophotometer. The sample was pressed under a pressure of 40 kg cm*^ to a self supporting disk and was evacuated at 298 K for 4 h under a reduced pressure of 1 mPa. The spectra were recorded under various pressures of CO at 123 K by using an in situ cell, which was capable to treat in vacuo at 873 K and to adsorb various gases at 100 K in situ condition. X-ray measurement X-ray powder diffraction (XRD) measurements were performed on a Rigaku RAD-IC diffractometer using Cu Ka radiation. NMR measurement The solid state ^^Si CP MAS NMR spectra of several samples were recorded on a VARIAN UNITY INOVA300 spectrometer equipped with a magic angle spinning (MAS). Measurement conditions were as follows: resonance frequency, 59.59 MHz; pulse width, 4.2 jis; pulse delay, 10 s; spinning rate, 3.5 kHz. The values of chemical shift were seterminded relative to the value of tetramethylsilane as an external standard. Isotherm and water-content measurement The measurement of adsorption isotherm of Ar was carried out volumetrically. Before the measurement, samples were evacuated at 278 K under a pressure of 1 mPa. The water content of the same samples was determined by the successive ignition loss method. l(A) 3. RESULTS AND DISCUSSIONS The XRD pattern of Sample A and Sample B exhibits characteristic diffraction pattern of the hexagonal lattice, though that for the latter sample gives slightly broadening nature of each band (Figure 1 (A)). It is obvious from XRD and Ar adsorption data (Figure 1(B)) that the pore size becomes small by the water treatment, if examined the data in detail. TEM images of respective samples reveal the typical hexagonal honeycomb structure of the MCM-41 sample, whether treating with boiling water or not (Figure 2). The most striking aspect of the sample used in this experiment is

Sample A

l\L 2

Sample B I I I 4 6 8 1( 2theta / deg rny. M^^ Sample B

Ijjfj^'^^

es

Sample A

"O

1

D

1

1

1

0.4 0.8 p/Po Figure 1. XRD patterns and Ar isotherms.

1

803 that this sample keeps its original structure even after treating in boiling water, though the sample ordinarily utilized easily loses its structure with this procedure. Relating to the feature of Sample A, its water content was evaluated to be 0.48 OH/nm^ being far smaller than that reported so far for MCM-41 sample, 2.5-3.0 OH/nm^ [7]. Si CP-MAS NMR measurements were performed to know detailed information on the state in the vicinity of the surface. As shown in Figure 3, the spectra give two bands at - 108 and - 98 ppm for Sample A and three broad signals at - 108, - 98, and - 90 ppm for Sample B; the former two bands were assignable to Q"^ and Q^ Figure 2. TEM images. silicon species, and the latter one at - 90 ppm to a Q^ species. A large portion of the band is composed of Q^ species for Sample A. In addition, it is shown that the relative intensity of Q^ species to Q^ species for Sample B becomes strong, compared with that for Q^ species. These data substantiate that the condensation reaction in the surface region proceeds to form the stable siloxane bond for Sample A. As for Sample B, the hydrolysis of the siloxane bond takes place on the surface, resulting in the formation of the geminal-type silanol group, as well as the increase in quantity of the -130 single-type silanol group. -90 -110 5/ppm FT-IR spectra of the MCM-41 samples are Figure. 3. ^^Si CP MAS NMR shown in Figure 4. Sample A brings about a sharp spectra. and strong band centered at 3745 cm'^ due to the free silanol group with slightly tailing toward the lower wavenumber side; the band being assignable to hydrogen-bonded silanol (at around 3500 cm"') is scarcely found. Needless to say, both bands were clearly recognized for Sample B and the original silica (Aerosil 200) sample. It is worth noting that a distinct band appears at 3745 cm'', though its shape become broader and its intensity lower compared with those for Sample A. The most striking characteristic for Sample B is the appearance of the distinct shoulder band at around 3725 cm''. This band is stronger and clearly found for Sample B, and slightly observable for Sample A. CO is known as a probe that is suitable for analyzing the surface acidic properties [8]. IR spectra are measured at 123 K by using CO to evaluate the types of silanol group existing on surfaces of both samples, and these data are shown in Figure 4. At the initial stage of adsorption of CO on Sample B, the intensity of the 3725 cm*' band decreases, accompanying the appearance of a new band at around 3600 cm"'. This tendency can be

804

A

Sample B

i

1

^

1

1

1

1

1

1

3400 3400 3600 3800 3600 Wavenumber / cm' Wavenumber / cm' Figure 4. Infrared spectra of Sample A and B. Sample A and B were contacted with CO gas under increasing pressures from 0 Pa to 13.3 kPa.

3800

explained quite naturally as due to the formation of hydrogen bonding between the geminal-type silanol group with the adsorbed CO molecule. Additional increase in the pressure of CO gas, the band at 3650 cm"' increases its intensity and its intensity is finally saturated under the pressure of 13.3 kPa accompanying disappearance of the band at 3745 cm"'. It should be concluded that the last type of band is resulting from the interaction between the single-type silanol and a CO molecule. Though the water content of Sample B (1.50 OH/nm^) was almost three times larger than that of Sample A, its amount is still one-half that of the MCM-41 sample reported by other groups, indicating that the water content of Sample A is extremely small. The value for the silica sample was evaluated to be 3.3 OH/nm^. These data clearly indicate that the condensation reaction in the surface region proceeds to form the stable siloxane bond for Sample A; the structural break does not occur in the case of the present MCM-41 sample. In the sense that the highly condensed surface that retains the sufficient resistance for the hydration reaction and protects from the break of lattice structure exists, it is considered that our MCM-41 sample has a good surface crystallinity.

REFERENCES 1. C. T. Kresge et al.. Nature, 359 (1992) 710. 2. J. S. Beck et al., J. Am. Chem. Soc, 114 (1992) 10834. 3. J. M. Kim and R. Ryoo, Bull. Korean Chem. Soc, 17 (1996) 66. 4. R. Ryoo et al., J. Phys. Chem., 100 (1996) 17718. 5. C.-F. Cheng et al., J. Chem. Soc, Faraday Trans., 93 (1997) 193. 6. C.-F. Cheng et al., J. Chem. Soc, Faraday Trans., 93 (1997) 359. 7. X. S. Zhao et al., J. Phys. Chem. B, 101 (1997) 6525. 8. T.P. Beebe et al.. Surf. Sci., 148 (1984) 526.

Studies in Surface Science and Catalysis 132 Y. Ivvasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.

805

Vaporization and Oxidation of Poly- a -olefin on Metal Plates Kazuaki Hachiya Department of Mechanical Engineering, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan The amounts of a lubricant, poly- a -olefin, vaporized on aluminium, iron, stainless Steel, and copper plates at 150, 170, and 190 °C in the air were measured to a precision of 10"^ g with a balance. The oxidation of the lubricant, caused by the heating at the same temperature as the vaporization experiment, was observed by the absorbance at 1716 cm"^ with a FT-IR spectrophotometer. The changes in the vaporization and oxidation with time were remarkably different between an aluminium plate and a group of iron and stainless steel plates. Taking into account these differences, the three types of the lubricant films (i.e., the films of a fresh lubricant, of a partially oxidized lubricant, and of a polymerized lubricant) appear to exist on the metal plates at 150-190 ° C. 1. INTRODUCTION The seizure occurs between metal surfaces of a machine, which contact with each other. When the temperature on the contact surfaces rises by the seizure, the lubricant is vaporized and oxidized. The conventional experimental methods for the oxidation were to measure the amount of oxygen uptake in lubricant and to analyze the oxidation products of the lubricant [1]. These methods do not seem to be the direct measurements of the seizure occurring on metal surfaces of a machine. In the present experiment, the oil film, whose thickness was the same order as the one of fluid film lubrication, was formed on a metal plate surface with the lubricant of poly- a -olefin (PAO). After the lubricant film was oxidized in the air around the temperature where the seizure occurred on the contact metal surfaces, the vaporization and oxidation of the lubricant were followed by the measurements of the weight and of the absorbance with a FT-IR spectrophotometer, respectively. 2. EXPERIMENTAL The lubricant was poly- a -olefin, LUCANT HC-20 (Mitsui Chemicals, Inc.) with an average molecular weight of 800 and a density of 0.833 gizrv? at 15 Q. Metal plates tested were commercial products of rolled plates, and were aluminium (JIS A105OP, 99.5 % Al), iron (JIS SS400, 99.90 % Fe), stainless steel (JIS SUS430, 82 % Fe and 18 % Cr), and copper (JIS CI 100, 99.90 % Cu). They were polished with alumina of average diameter of 10 - 20 /i m, and were washed with a detergent. After the metal plates were washed with distilled water, they are dried in hot air [2]. The arithmetic average roughnesses, Rg, and the maximum height roughnesses, R^^^, were 0.73 and 12 /x m for an aluminium pate, 0.84 and 7.0 M ni for an iron plate, 0.41 and 10 /z m for a stainless steel plate, and 0.48 and 5.8 /z m for a copper plate, respectively. The plate was 5 cm in length, 6 cm in width, and from 0.1 to 0.5 mm in thickness. After 10 /il of 10 wt% benzene solution of lubricant was spread on a metal plate and the

806 solvent was evaporated, a thin oil film was formed. The lubricant film was heated on a hot plate at 150, 170, or 190 "C in the air. In the present experiment, the heating temperature of a metal plate was adjusted within ±V C, At high temperature, the lubricant was evaporated and oxidized. The amount of lubricant remaining on the metal plate was measured to a precision of 10"^ g with a Sartorius balance BP211D, which was grounded in order to prevent the electrification of the metal plate. The area of the lubricant, spread on a metal plate, was measured by a computer analysis of a picture which was taken with a digital camera. As a result, the film thickness was calculated from the ratio of the lubricant weight to the film area and the density. The change in the infrared absorbance caused by the oxidation of the lubricant was measured with a reflection type of a JASCO spectrophotometer FT/IR-410. After this absorbance was divided by the film thickness, the absorbance per 1 cm could be finally obtained. 3. RESULTS AND DISCUSSION The amounts of PAO vaporized on various metal plates were measured at 150, 170, and 190 ° C. Figure 1 shows the lubricant weight remaining on an aluminium plate as a function of time. The lubricant weight decreased monotonously with time at any temperature. As seen from Figure 1, most of the lubricant was vaporized, but only a small amount of lubricant remained on the plate for a long time at 150, 170, and 190 " C, because several interference fringes, formed by a thin oil film, were observed on the plate even at the end of the experiment. Figure 2 shows the amount of PAO remaining on aluminium, iron, stainless steel, or copper plate at 170 ° C. The weight of the lubricant on a iron plate at 170 *C decreased initially with time, but the vaporization of the lubricant almost stopped at about 20 min. At this time, about 50 % of the lubricant remained on the plate surface. The times, when the vaporization stopped at 150 and 190 ' C, were 100 and 10 min, respectively. The time dependence of the lubricant weight on a stainless steel plate changed similarly to that on the iron plate at 150, 170, or 190° C. On a copper plate, 20 to 30 % of the total weight decreased during 20 min, and then the amount of the remaining lubricant was kept almost constant. When PAO on the metal plate was heated under the same temperature condition as

100 150 time/min Fig. 1. Weights of poly- a -olefin remaining on an alminium plate at 150 (D), 170 ( • ) , and 190 (O) " C as functions of time.

40 60 time / min

80

00

Fig. 2. Weights of poly- a -olefin remaining at 170 " C on aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.

807

40 60 time/min

40 60 80 time / min Fig. 3. Absorbanccap at 170 ° C for aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.

Fig. 4. Area S (A), film thickness d (D), and weight W (O) of poly- a -olefin on an alminium plate at 170 ° C as functions of time.

the above vaporization experiment, an infrared spectrum showed a new peak at 1716 cm''. The appearance of the peak at this wave number would indicate an oxidation of the lubricant (i.e., a stretching vibration of C=0). The apparent absorbance, AbsorbancCap, of the lubricant on the metal plate at 1716 cm'' and 170 *C is shown in Figure 3. After the apparent absorbances for aluminium, iron, and stainless steel plates increased first with time, they decreased a little, and then were kept almost constant. The decrease in the apparent absorbance would be caused by the decrease in the oil film thickness, which was obtained by the ratio of the lubricant weight to the density and the film area, as shown in Figure 4. In order to obtain the absorbance for a constant film thickness, the ordinary absorbance per 1 cm, Absorbance, was calculated from the ratio of the apparent absorbance to the film thickness. Figure 5 shows the time dependence of the Absorbance of the lubricant on an aluminium plate. The Absorbance at any temperature increased with time, and became constant for some time. However, it increased again with time. The Absorbances for aluminium, iron, stainless steel, and copper plates at 170 *C are shown in Figure 6. The

lU

"o

8

~a5

6

c cs Jd

1

1

T —

I

/

1

J

^ 4 < 2 jfljarir? • • ^ - ^ — • "T *'

50

100 150 200 time / min

250

Fig. 5. Absorbances of poly-a-olefin on an alminium plate at 150 (D), 170 ( • ) , and 190 (0)°C as functions of time.

0

20

40

1

60 80 time / min

—1

100 120

Fig. 6. Absorbances of poly- a -olefin at 170 ° C on aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.

Absorbances of the lubricant on iron and stainless steal plates at 170 * C increased with time, but became almost constant after about 20 min, respectively. The time, when a constant Absorbance was reached, became short with increasing temperature. The Absorbance for a copper plate increased monotonously with time, and was relatively smaller than the Absorbances for iron, stainless steal, and aluminium plates. The Absorbance of the lubricant at 1716 cm"^ increased with the progress of the oxidation in Figures 5 and 6. The assignment of this peak was done by the following way. When stearic acid was spread on a fresh surface of an aluminium plate at room temperature, the infrared spectrum showed the peaks at 1698 (a stretching vibration of C=0) and 940 (a bending vibration of 0-H) cm"^ However, when the same acid was spread on an aluminium surface that was formed by heating at 190 ° C for 5 min and then was cooled to room temperature, the peak at 940 cm'^ disappeared, but the one at 1698 cm'^ remained. This would indicate that an oxide film was formed on a fresh metal surface, when a metal plate was heated at 190 °C for more than 5 min. Furthermore, the spectrum of the stearic acid, observed on the oxidized aluminium surface, was similar to the spectrum of the oxidized PAO on the metal plate that was heated at 150, 170, or 190 ° C. This would mean that the oxidized PAO is a carboxylic acid and reacts with the oxidized metal surface [2]. Because such a thin oil film is formed on the oxidized aluminium surface, a small amount of lubricant would remain on the plate for a long time as shown in Figure 1. Bowden and Tabor classify the metal plates into two types: (a) in which chemical attack is absent or slight (iron, aluminium, nickel, and chromium), and (b) in which chemical attack is marked (copper) [3]. Then, the difference in the Absorbance between the copper and the other metals in Figure 6 is considered to arise from the difference in the reactivity between the oxidized PAO and the metal plates. The amounts of the remaining lubricant for iron and stainless steel plates are different from that for an aluminium plate in Figure 2. This would indicate that a different substance with a higher boiling point (i.e., a larger molecular weight) than PAO is newly produced on the iron or stainless steel plate. In other words, there seem to exist at least three types of the lubricant films on the metal plates: the one is the lubricant film which is not oxidized, and the other are two types of oxidized films of the lubricants, whose molecules are partially oxidized and are polymerized to form larger molecules (i.e., sludge [4]), respectively. On an aluminium plate, the polymerized molecule would be hardly produced in comparison with the partially oxidized one. Although the partially oxidized molecule reacts strongly with the oxidized aluminium plate, the PAO, not oxidized, vaporizes from the plate. Since the concentration of the partially oxidized lubricant increases on the metal plate because of the PAO vaporization, the absorbance for aluminium would increase again as shown in Figure 5. Because the increase in the molecular weight causes the increase in viscosity or in coefficient of friction, the formation of sludge might be one reason why the seizure occurs between two metal surfaces of a machine. REFERENCES 1. E.L. Lederer, Petroleum, 31 (1935) 44. 2. J.V. Sanders and D. Tabor, Proc. Roy. Soc. A, 204 (1951) 525. 3. F.P. Bowden and D. Tabor, The Friction and Lubricant of Solids, Clarendon Press, Oxford, 1954, pp. 200-227. 4. M. Rasberger, Chemistry and Technology of Lubricants (R. M. Mortier and S. T. Orszulik, ed.) , Blackie A & P, London, 1997, pp. 104-106.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) Q 2001 Elsevier Science B.V. All rights reserved.

809

Molecular Geometry-Sensitive Filling in Micropores of Copper Complex-Assembled Microcrystals Di Li and Katsumi Kaneko Dept. of Chem., Faulty of Sci., Chiba Univ., 1-33 Yayoi, Inage, Chiba 263-8522, Japan. A series of [Cu2(pzdc)2(PL)]„« xHjO (pzdc = 2,3-pyrazinedicarboxylate) complexes with permanent micropores was synthesized. The PL is a pillar ligand (pyrazine, 4,4'-bipyridine or trans-l,2-bis(4-pyridyMethylene). The microporosity of the Cu complexes was confirmed by XRD analysis and Nj, Ar, and COj adsorption. DR plots of the adsorption isotherms provided that the micropore volume for each complex changes with the different adsorbate, indicating the mechanism of the molecular geometry-sensitive micropore filling.

1. INTRODUCTION Micropore filling has been mainly studied on activated carbon and zeolite which have basically slit and cylindrical shaped micropores, respectively. Less attention was paid on the micropores with other geometry. Steele and Bojan [1] showed that comers can work as strong sites for micropore filling in rectangular micropores with the aid of GCMC simulation. However, we cannot confirm their prediction experimentally because there is no good microporous solid whose micropore shape is rectangular. Recently substantial advances have been made toward supramolecular chemistry field; a typical example is the successful construction of some metal complex frameworks with stable microporosity [2-4]. Especially, Kitagawa et al observed that crystalline Cu complexes can adsorb abundantly supercritical methane [2]. Although XRD had shown that these compounds have rectangular micropores, the microporous structure has not been hitherto characterized by gas adsorption. Additionally, a comparative study on micropore filling of different gases in the rectangular micropores is quite helpful to design a high performance adsorbent. In this work, we synthesized a series of Cu complex [Cu2(pzdc)2(PL)]„-xH20 (pzdc = 2,3 -pyrazinedicarboxylate, PL = pillar ligand) with stable semi-rectangular microporous structure and measured the adsorption isotherms of N2, Ar, and CO2 on these complexes. Pyrazine(pyz), 4,4*-bipyridine (bpy) or trans-l,2-bis(4-pyridyl)ethylene (bpe) was selected as a P L The relationship between the molecular geometry and adsorption behavior was elucidated.

2. EXPERIMENTAL Synthesis of Cu2(pzdc)2(pyz) • 2H2O (Cu-4x6). A 0.02 M Na2(pzdc) aqueous solution was added dropwise to a refluxing aqueous solution containing 0.02 M Cu(C104)2 and 0.05M pyz. The blue microcrystals were filtrated and dried under vacuum for 4 h. Cu2(pzdc)2(bpy) • 4H2O (Cu-9x6) and Cu2(pzdc)2(bpe) • SHjO (Cu-llx6) were prepared in a similar way. The gas adsorption isotherms were carried out on Autosorb-1, Quantachrom. Prior to the adsorption measurements, the samples were outgassed under vacuum at 373 K for 2 h.

810

3. RESULTS AND DISCUSSION 3.1 Crystal structure of Cu complexes The XRD analysis indicates that the Cu complex crystals possess a 3D structure with ID semi-rectangular channels, as shown in Fig. 1. Cu-4x6 miaocrystals belong to monoclinic space group ?2,lc with a=4.693, b=19.849, c=11.096A, /3 =96.90°, V = 1026. l A \ Z=2, and p=1.862 g/cm\ Cu-9x6 and Cu-llx6 have similar networks with Cu-4x6, but they have an elongated b parameter to 29.0A for Cu-9x6 and 32.9A for Cu-llx6. By the XRD results and the van der Waals radii of constituent atoms, the cross-sectional dimensions (A) of the channels of Cu complexes are estimated and are included in the nomenclature of each complex. 250

»

1

1

1

1

00

-M

O

f^

ooo oo

ooooooooooc^^-^

1 150 \ lioo

b

c

P3 50 < n

•Cu

i)N

©O

Fig. 1. Space-filling view of Cu-4 X 6 structure

r-

J CU-4X6 ^ CU-9X6 o H CU-11X6 •

e t

t.

0.0

.

0.2

-1

L.

1

A

0.4

0.6 0.8 1.0 P/Po Fig. 2. N2 adsorption isotherms at 77 K.

3.2 Microporosity of Cu complex-assembled microcrystals The N2 adsorption isotherms measured at 77 K on Cu complexes are given in Fig. 2. All samples show typical isotherms of type I, confirming the presence of micropores. The Nj adsorption isotherms on a logarithmic relative pressure are shown in Fig. 3. A pore blocking effect was observed on the isotherm of Cu-4x6. The SPE method, a extended a,-analysis method proposed by Kaneko [5], was used to analyze the Nj adsorption isotherms of Cu complex-assembled solids. The surface area of Cu complexes from the a^-analysis are summarized in Table 1. It is noteworthy that the surface area of Cu-llx6 goes over 1000 mVg. The micropore volume V^ by a^-plot has a good coincidence with that by DR analysis for all samples. Here, only the result of DR analysis was listed in Table 1 The Ar adsorption isotherms at 77 K on Cu complexes are very similar to that of Nj adsorption except the adsorption amount. The pore blocking effect also occurs in Ar adsorption on Cu-4x6. The V^ values by DR analysis of Ar adsorption isotherms are given in Table 1. CO2 adsorption at 273 K can provide the correct filling state of narrow micropores without blocking effect [6]. Figure 4 gives the COj adsorption isotherms on the Cu complexes. Cu4x6 and Cu-9x6 with smaller pores have an enhanced adsorption comparing with Cu-llx6 with larger pores at the low relative pressures. DR plots are shown in Fig. 5. It exhibits that each DR plot is linear at low or high PIP^ region and shows a steep increase in adsorption

811 amount at medium PIPQ range. This suggests that the adsorption behavior of each complex is qualitatively different at low and high relative pressures. Steele and Bojan [1] suggested that an adsorbate is strongly adsorbed on the comer site at first, followed by multilayer adsorption or pore filling on the walls for rectangular micropores. At low relative pressures, the COj molecules occupy the energetically most favorable positions. There is a Cu atom at the comer of the rectangular pore of these Cu complex crystals (Fig. 1). The great quadmpole moment of a COj molecule can interact with the Cu atom and thereby the comer site should be the strongest for the COj molecule. At high relative pressures, COj molecules fill the pores. Moreover, the steep increase in adsorption amount at medium PIPQ range is contributed to the rearrangement of adsorbed COj molecules. 250 F

1

•"

1

41

140

I

r—— 1

1

1

1—^ 0 1 0 J 0 tf 1 A^ 1 * 1 0

'00

120 E •^ 1(K) 80

g»200 o

1 150 r

i

•

O

0

A

r

<

CU-4X6 Cu-9X6

^

•

fi

5/5

"i 100

I 50 L

°

0 .- ' .• 0^^ •

A

^ 1 o J

CU-11X6 • 0 I ^^'^, 1 1 -4 -3 -2 -1 1

3 0

<

il 0

60 40 70 0

•

CU-4X6 CU-9X6

0

* •• -

0

1 J

•

•

s

* J 0 J1

•

H^ 0 5

log(P/Po) Fig. 3. The logarithmic relative pressure expression of N2 adsorption isotherms.

A

CU-11X6 • 1 i

1

1

10 15 20 P/PoXlO^

1

il

25

30

Fig. 4. CO2 adsorption isotherms at 273 K.

3 . 3 Molecular geometry-sensitive fliling in Cu complex-assembled micropores DR plot analysis of the Nj, Ar and COj adsorption isotherms indicates that the micropore volume for each compound changes with the different adsorbate (Table 1), suggesting that the filled effectiveness in rectangular micropore of Cu complexes depends on adsorbate molecular geometry. The observed space filling ratio (SFR), listed in Table 1, were obtained from the ratio of the void volume calculated from the XRD crystallographic data to the micropore volume measured by gas adsorption. These results indicate that COj molecules of an ellipse shape can fill the rectangular micropores more effectively than spherical Ar molecules. Table 1: The micropore parameters of Cu complexes and space filling ratios (SFR) of micropores with different molecules. Complex CU-4X6 CU-9X6 CU-11X6

571 846 1013

Observed SFR (Calculated SFR)

Vm(ml/g)

a (m2/g) N2

Ar

CO2

N2

Ar

CO2

0.15 0.22 0.28

0.12 0.19 0.28

0.18 0.34 0.39

0.39 (0.44) 0.42 (0.50) 0.50 (0.55)

0.31 (0.38) 0.37 (0.42) 0.50 (0.52)

0.46 (0.48) 0.65 (0.68) 0.70 (0.66)

812

1

2.0 rp %' 1.5 ^

1.0

O

r% \*

TLA°

•s

O

\ i rN°I * o

p*

X^ A 1»

0.0

0

,

,

@

H

^^ °o

"" 0.5

[

o \

CU-11X6 • 1 J

*\

r

-0.5

11

CU-9X6

[1 \>°

[

1

CU-4X6 ^ 1

1

O

° A

1 >a

10 15 20 [log(Po/P)]2

11 J 1

25

Fig. 5. DR plots of CO2 adsorption isotherms,

/""iBt^^^ f"^> -^ ^

/\

(^.Q .^j

^

f ^ V.^ O' r

^'^ x^

VJitL/ <:.!

r^C^t^r '-"' 1 ^s^y V:J>^

Fig. 6. Schematic of filling state of N2, Ar, and CO2 in the micropores of Cu-9 X 6.

Fig. 6 shows the schematic of the filling states of the rectangular micropores with Nj, Ar, and CO2 molecules on Cu-9x6. The 12-6 U size parameter was used to describe the effective diameter of an adsorbate molecule, O^N is 3.32A for Nj [7] and the N-N distance is 1.09A. Ooo = 3.03A and a^c = 2.82A for COj [8]; the O-O separation is 2.32A and the C-0 separation is 1.16A. a^^^, = 3.40A for Ar [1]. If adsorbate molecules are considered as a rigid body and fill the rectangular micropores with the largest density, we can calculate the SFR of the rectangular micropore with an arbitrary length by geometric principles. We also assumed that the linear Nj or COj molecules fill the rectangular micropores along the channel direction [9]. The observed and calculated SFRs are given in Table 1. The calculated SFRs are close to the observed SFRs for COj, but, larger than observed ones for Nj and Ar. This fact indicates that the adsorption conditions of Nj and Ar at 77 K do not necessarily allow a complete adsorption due to the insufficient intrapore diffusion. Nevertheless, both SFR values are influenced by pore size and adsorbate molecules, supporting the presence of the molecular geometry-sensitive micropore filling. REFERENCES 1. M. Bojan and W.A. Steele, Carbon, 36 (1998) 1417. 2. M. Kondo, T. Okubo, A. Asami, S. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka and K. Seki, J. Angew. Chem. Int. Ed., 38 (1999) 140. 3. D. U and K. Kaneko, J. Phys. Chem., 104 (2000) 8940. 4. H. Li, M. Eddaoudi, M. O'Keeffe and O.M. Yaghi, Nature, 402 (1999) 276. 5. K. Kaneko and C. Ishii, Colloids Surf., 67 (1992) 203. 6. D. Cazorla-Amoros, J. Alcaniz-Monge, M.A. de la Casa-Lillo and A. Linares-Solano, Langmuir, 14 (1998) 4589. 7. K. Kaneko, R.F. Cracknell and D. Nicholson, Langmuir, 10 (1994) 4606. 8. M. Heuchel, G.M. Davies, E. Buss and N.A.Seaton, Langmuir, 15 (1999) 8695. 9. A. Vishnyakov, P.I. Ravikovitch and A.V. Neimark, Langmuir, 15 (1999) 8736.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

813

Fluorescent Solubilizates in the Silica-Surfactant Composite Films Katumitu Hayakawa, Natsuki Fujiyama, and Iwao Satake Department of Chemistry & BioScience, Faculty of Science, Kagoshima University, Korimoto, Kagoshima 890-0065, Japan Layered siUca-surfactant films were synthesized through the hydrolysis and polymerization of tetramethylorthosiHcate in an acidic micellar solution using a sol-gel technique on glass plates. Some fluorescent probes were adsolubihzed in the composite films and the emission anisotropy was measured. The anisotropy results indicate the restricted mobihty of anthracene and pyrene and the coordination of the acridine orange derivative with dodecyl group in the composite film. Inorganic-organic composite materials with a well-defined ordered structure of nanometer size have been extensively investigated due to their potential utiUties. This type of research was accelerated with the development of low temperature sol-gel synthesis techniques. The technique allows for the incorporation of organic compounds in the inorganic crystal frame without decomposition. Fluorescent probes, for example, incorporated in an inorganic glass have been found to be resistant to photo-degradation and reactive chemicals [1-5]. On the other hand, inorganic materials of well-defined nanometer pores have been synthesized using surfactant micelles as a template, controlling the pore diameter (MCM series and FSM) [6-12]. We may expect to control the microstructure of layered siUca films using surfactant micelles as the template. Ogawa et al. prepared transparent sihca-surfactant composite fihns on glass using micellar solutions [12, 13]. A hydrophobic fluorescent probe may be adsolubihzed in the surfactant aggregates in the film to monitor the microstructure of the sihca-surfactant nanocomposite films on the glass surface. In this paper, we report the anisotropy of fluorescence probes incorporated into the composite films. 1. Experimental Materials. Tetramethyl orthosihcate (TMOS, Sin-Etsu Chemical, Tokyo) was used without further purification. Hexadecyl- (CieTAB), tetradecyl- ( C H T A B ) , dodecyl- (C12TAB) and decyltrimethylammonium bromide (CioTAB) were all GR grade of TCI, Tokyo and used as received. Pyrene (Wako, Osaka) was recrystaUized firom ethanol. Rhodamine B (99%, Aldrich), anthracene (Wako, Osaka) and 3,6-bis(dimethylamino)-10-dodecylacridinium bromide (C12AO,

814

Dojindo, Kumamoto) were used without further purification. Preparation and measurements. TMOS was hydrolyzed in hydrochloric acid solution for 60 min followed by addition of 0.5 mol dm-^ micellar solution (1/4 mole ratio of TMOS) including the fluorescent probes, and further hydrolyzed and polymerized for 30 min at a room temperature. The solution was used for spin coating and dip coating on the glass plates. The fluorescent spectra were recorded using a Shimadzu RF-5000 spectrofluorometer with a polarizer attachment. The anisotropy of fluorescence (r) is defined by Eq. 1 when the excitation Hght beam is polarized in the zdirection, and is calculated from the degree of polarization (P) of fluorescence,

r=AzjL.,J^

(1)

I,+21, 3-P where P is defined as (h • Ix)/(Iz + L) and was calculated by Eq. 2 by taking into account the correction of apparatus polarization characteristics. ' ! + (/«//«)x(/zx//.z)

The first and second characters in subscript correspond to the polarized direction of the incident and emitted Ught beams respectively. Since the z-axis is perpendicular to the plane consisting of incident and emitting beam, the /zz corresponds the component parallel to the polarized incident beam and the others correspond to the perpendicular components. The basal spacing was determined by means of an X-ray diffiractometer Rigaku D-8C. 2. Results and Discussion XRD measurements result in a basal spacing of 3.0 nm for the CieTAB-siUca films including the fluorescence probes. The extended length of CieTA"^ ion is estimated as 2.54 nm as proposed by Tanford [14]. Assuming a 1.0 nm thickness of the sihca layer [15] and stacked bilayer of surfactant, the maximum basal spacing is estimated as 3.5 nm for the monolayer arrangement and 6.1 nm for the bilayer arrangement. The observed basal spacing of 3.0 nm leads to an unreaUstic slanting angle of 23 degrees for the bilayer arrangement. Based on this estimate we propose a hexagonally packed cylindrical aggregate of surfactant surrounded by sihca layer. The fluorescence anisotropy was measured to investigate the mobiHty of the probes solubiUzed into the surfactant aggregates of nanometer size. The intensity difference between /zz and /zx appeared in the fluorescence spectra of pyrene indicates a non-zero anisotropy of the emission of pyrene incorporated into the sihca-CieTAB composite film. The calculated anisotropy of some fluorescence probes is Usted in Table 1. Depolarization can be induced by the following molecular mechanisms [16]: 1) Distribution of absorption and emission moment. An ordered orientation results in a higher anisotropy (r>0.4). 2) Rotational Brownian motion. r=0 for the completely firee motion. 3) Excitation energy transfer between fluorescence probes. It can lead to depolarization. The anisotropy was nearly zero in solvents like water, ethanol and hexane for all

815 Table 1 Anisotropy of the fluorescent probes incorporated in the silica-surfactant composite films. Probe Micelle Surfactant Film Dip-coating Spin-coating Anthracene

CieTAB

-

0.12

0.02

Pyrene (374 nm)

CioTAB CuTAB

0.20 -

0.00 0.01

C16TAB Non

0.13

0.11 0.05 0.06

0.03

0.18 0.04 0.03

0.12 0.15

Non

-

0.27

-

CloTAB

-

0.50

0.18

0.61 0.34

0.17 0.17

0.48

0.18

Rhodamine B

C12AO

CuTAB CieTAB

C12TAB C14TAB CieTAB

0.55 0.58

0.04

* The solutions in water, ethanol and hexane indicate minor anisotropy (0 to 0.04) as expected. probes, suggesting their free motion. In micellar solutions, the neutral probes of pjnrene and anthracene result in a low anisotropy, but the cationic probes of rhodamine B and C12AO show a clear anisotropy of the fluorescence, indicating their restricted motion. This finding suggests the effect of the cationic head groups of the micelles. Rhodamine B and C12AO reside in the palisade layer of the micelle, and the intense electric field may influence their mobiUty. In the silica-surfactant hybrid films, the anisotropy also depends on the molecular structure of fluorescent probes. Rhodamine B results in a negUgible anisotropy in the silica-surfactant composite films, but a clear anisotropy (r=0.27) in the siUca fiilm, suggesting the high mobihty only in the composite films. The anisotropy of anthracene and pyrene fluorescence suggests a restricted mobiUty in the composite films. Pyrene shows a stronger restriction in the siHca film than in the composite films, and also in the composite film with short chain surfactant (CioTAB) compared to the composite films with the long chain surfactant (C14TAB and CieTAB). The anisotropy of C12AO is larger than 0.4, suggesting an ordered orientation in the films. The long chain in C12AO contributes to the orientation. The micropolarity of the surfactant aggregates in the film can be estimated from the intensity ratio (/1//3) of the first and second vibronic bands of pyrene emission spectrum. Higher I\lh values indicate higher polarity of the probe environment. The values in Table 2 indicate a similar polarity of the film and

816

the micellar aggregates in aqueous solution. However the value in micellar solution decreases as the surfactant chain length increases, while the film polarity is constant. The film without surfactant shows a higher polarity. These findings suggest the inclusion of water in the composite film as well as in the silica film without surfactant. This work was supported by a Grant-in-Aid for Scientific Research No. 09640693 firom the Ministry of Education, Science, Sports and Culture. Table 1 Micropolarity probed by pjnrene fluorescence Film Surfactant Spin Coating Dip Coating CioTAB 1.29 1.35 CuTAB 1.35 CieTAB 1.35 1.30 None 1.52

Micelle 1.47 1.42 1.25

REFERENCES 1. D. Avnir, D. Levy, and R. Reisfeld, J. Phys. Chem., 88 (1984) 5956. 2. D. Avnir, V.R. Kaufman, and R. Reisfeld, J. Non-CrystaHne SoUds, 74 (1985) 395. 3. T. Tani, H. Namikawa, K. Arai, and A. Makishima, J. Appl. Phys., 58 (1985) 3559. 4. K. Matsui and N. Usuki, Bull. Chem. Soc. Jpn., 63 (1990) 3516. 5. N. Negishi, M. Fujino, H. Yamashita, M.A. Fox, and M. Anpo, Langmuir, 10 (1994) 1772. 6. J.S. Beck, J.C. VartuU, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, and J.L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. 7. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. VartuU, and J.S. Beck, Nature, 359 (1992) 710. 8. C.-Y. Chen, S.L. Burkett, H.-X. Li, and M.E. Davis, Microporous Mater., 2 (1993) 27 9. S. Inagaki, Y. Fukushima, and K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 680. 10. G.S. Attard, J.C. Clyde, and C.G. Goltner, Nature, 378 (1995) 366 11. Y. Fukushima, S. Inagaki, and K. Kuroda, Nippon Kagakukaishi, (1995) 327. 12. M. Ogawa and N. Yamamoto, Langmuir, 15 (1999) 2227. 13. M. Ogawa, Hyoumen, 35 (1997) 563. 14. C. Tanford, J. Phys. Chem., 76 (1972) 3020. 15. M. Ogawa, T. Igarashi, and K. Kuroda, BuU. Chem. Soc. Jpn., 70 (1997) 2833. 16. K. Mihashi, in Keiko Sokutei: AppUcation for Biological Science, L. Kinoshita and K. Mihashi, Editors. Gakkai Shuppan Center, Tokyo, 1983, p. 1.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^^ 2001 Elsevier Science B.V. All rights reserved.

817

Backbone Orientation of Adsorbed Polydimethylsiloxane Iwao Soga* and Steve Granick** ^Yokohama Research Center, Mitsubishi Chemical Corporation, Yokohama, Kanagawa 227-8502 Japan **Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801 USA The backbone orientation of an adsorbed polymer with respect to a flat solid surface was measured, using infrared spectroscopy in attenuated total reflection (FTIR-ATR), for polydimethylsiloxane (PDMS) adsorbed onto oxidized germanium from dilute solution in cyclohexane at 25.0 ""C, From a siloxane stretch, which originates from the main chain of PDMS, the backbone orientation of the adsorbed PDMS was evaluated. Orientational information was the effective parameter to evaluate the adsorbed polymer conformation and the internal structure change of the adsorbed layer. 1. INTRODUCTION The normal approach to the polymer adsorption is handling scalar amount. Though various properties, such as adsorbed mass, bound fraction, density profile and layer thickness, have been examined experimentally and theoretically [1], they did not focus on the vector information of the adsorbed polymers. Because the random coil conformation of the polymer chain in the solution averages and eliminates the orientational direction of the segments, the scalar approaches provide precise answers in most cases. But, so long as the polymers are consisted of the strings of the atoms, the orientation of the chain is worth to be concerned. Considering the importance of the adsorbed polymer layer for a colloid stability and various products qualities, the vector approaches will also be meaningful in many cases. When polymer chains adsorb onto the solid surface, the chain conformation becomes asymmetric against the surface. Hence, the isotropic random coil is not true for the adsorbed polymer. The distortion from the isotropic state is usually evaluated by the combination of the adsorbed mass and the layer thickness. The information from the orientation of the adsorbed polymers could directly answer to such a problem. It also provides the insight about the internal structure of the adsorbed layer, which may be deleted from the results of the scalar experiments. Especially, if there is an in-plane anisotropy, it can only be detected by the orientational analysis. To examine the segmental orientation of the adsorbed polymer chain, we adapted a model adsorption system of PDMS/cyclohexane. Because PDMS has a siloxane backbone, which is infrared active, the backbone orientation of the adsorbed polymer can be evaluated by the polarization spectroscopy. The segmental orientation was measured in both static and kinetic condition and analyzed with the adsorbed mass and the bound fraction.

818 2. EXPERIMENTAL Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR), the ATR liquid cell, experimental procedures and analysis methods were reported previously [2]. The ATR crystal was a trapezoidal germanium element in this experiment. The sample of polydimethylsiloxane (PDMS) was purchased from Polymer Laboratories (Mw=118,000 and Mw/Mn=L08). The spectrum grade cyclohexane was used as a solvent. The temperature was controlled at 25.0 °C. Experiments started with introducing dilute PDMS solution (0.02-3.0 mg-mL^) into the liquid cell. In some experiments, it was followed by replacing the solution by the pure solvent. FT-IR spectra were collected in p polarization (parallel to the incident plane) and s polarization (perpendicular to the incident plane) altemately. Polarization spectroscopy of FTIR-ATR was analyzed based on the FloumoyShaffers relations [3]. Measured infrared absorbance in p direction and s direction were analyzed to deduce the infrared absorbance in the Cartesian direction parallel to the solid surface (Ax = Ay) and perpendicular to the solid surface (Az). The dichroic ratio (D) was defined as the ratio of the absorbance in these two orthogonal directions. DsAz/AxsAz/Ay

(1)

3. RESULTS AND DISCUSSION 3.1. FTIR-ATR spectra The IR spectrum of PDMS in the cyclohexane solution is shown in fig.l. The sharp peak at 1261 cm"^ is assigned to Si-C stretch, and complex peaks around 1000-1150 cm"^ are assigned to Si-0 stretches, which are separated by a curve fitting. Si-0 peak of higher wavelength (1098 cm"^) is assigned to a symmetric stretch and lower one (1015 cm'^) is to an asymmetric stretch [4]. From the PDMS configuration, the dipole moment of Si-C stretch is perpendicular to the PDMS backbone. Si-0 symmetric stretch is also perpendicular to the backbone and Si-0 asymmetric is parallel to the one. Figure 1 also shows the spectra of the adsorbed PDMS on the Ge ATR crystal. Absorbance of Si-0 asymmetric stretch is clearly different for z and x direction. Larger Ax means the PDMS backbone tends to orient " r ' — T 1 1 parallel to the surface. A new shoulder at 984 c m \ which is not appeared in the 4 0001 spectrum in the solution, is stronger for the parallel direction. Because the decrease of ' / \ the total absorbance is observed for Si-0 -Sokjtbn flll c asymmetric stretch, it is reasonable to assign -e the peak to modulated Si-0 asymmetric / / W Adsoited/l o stretch. From the analogy of a polyethylene -< oxide adsorption study [5], this is supposed L_ . 1 , ,. J ; to originate from the bound oxygen atoms. — 1 Under the assumption of these assignments, 1400 1300 1200 1100 1000 900 the orientation of bound and fi-ee Si-0 IKavenumbers fcm"^) segments can be determined in addition to Fig.l. PDMS spectra in cyclohexane. the bound fraction and the adsorbed mass. •"•

CO

819 3.2. PDMS orientation in the static condition Segmental orientation of the adsorbed PDMS was determined from several IR bands. Figure 2 shows the dichroic ratio and the bound fraction against the adsorbed mass. When the adsorbed mass is low (-0.3 mg/m^), the surface is starved and the chain can be flatten on the surface. Higher bound fraction proves the flattened conformation. Lower D values of bound (984 cm"^) and free (1015 cm'^) Si-0 asymmetric stretch, which means parallel orientation of the PDMS backbone, are coincident with that image. D of Si-C stretch should be higher than 1.0 with this orientation. But bonding of the oxygen atoms to the surface may break the radial isotropy around the backbone and distort the relation. Hence, perpendicular dipoles against the backbone are not suitable to estimate the orientation, when the ratio of the bound segments is higher. At higher adsorbed mass, the bound fraction is lower. 1.2 T 1 r— 1 r- — ! 0.6 The adsorbed layer is crowded and the 0 , ^ J 0.5 o 1.0 V ratio of tails and loops increases. The °y^ [ ^^s^f^ •^ increase of the free segments adds to more ^ H 0.4 Q: 0.8 \ ^ v * o isotropic conformation chains, which is L D r n ^ V ^ ^•^ _* • ^n 0.3 0.6 'o V. confirmed by higher D (close to one) of • ° o 0.4 r o 1 0.2 free Si-0 asymmetric stretch. O 0.2

3 3 . Adsorption kinetics PDMS adsorption kinetics was examined from the segmental orientation. Figure 3 shows the time dependence of the adsorbed mass, the bound fraction and the dichroic ratio in the solution (0.5 mg/mL). Basically it is a continuous process from the initial starved condition, to the saturated dense layer. While the adsorbed mass increases with time, the bound fraction decreases. It is supposed that the later arrived chains cannot find enough sites to adsorb, so that the ratio of the bound segments in the chain becomes smaller. By this image, the later arrived chains will absorb weakly and the conformation will be more isotropic. This is confirmed by the increase of D values of Si-0 asymmetric stretch. Overlapping adsorption of the later arrived chains on the initial flattened conformation adds the vertically oriented segments. But even at the saturated condition, the total backbone orientation still keeps parallel preferential. So the conformation of the adsorbed PDMS is a little spread one along the surface at this condition. D of bound Si-0 asymmetric stretch is almost constant

0.1

L

0.0

1

1..

..L

1

1 ..

1

0.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Adsorbed Mass (mg/m^)

Fig.2. Segmental Orientation of Adsorbed PDMS. Open diamonds; 1261cm'\ Open squares; 1015cm'\ Open circles; 984cm\

E 0.3

-r0.6 o o - O.b 0.4 a D - 0.3

1 0.2

0.2

c o

8 0.1 <

0.1

CD

E 0.6 ^

1

r—1

1

1

0.4

o 1.0

c o o

•43

I •o

b) - < - I s o t r o p i c j:p^^^„^l

1 0.8

_, D D

^

.2 0.6

"n

"° °

1o 0.4

-a D

0 -©-

Q 0.2 1

0.0

0

50

1

'

100 150 200 250 Time (min) Fig. 3. Adsorption kinetics in the solution, b) Open squares; 1015cm"\ Open circles; 984cm''

820 (0.2) for the whole process. The bound E 0.6 0.6 segments is orientated parallel to the .a) 0.5 1 0.5 surface and is considered to be stable. 04 ^ 0.4 Though the kinetic changes could " ^ D D D D a 0.3 be explained by the arrival of the new E 0.3 chains, it may still include the internal 0.2 1 0.2 change of the adsorbed layer. This kind of 0.1 8 0.1 change will be observed more clearly in < — the solvent, where there are no later 0 1.0 -<•-Isotropic 1 ^' ' . • . • • arrived chains. Figure 4 shows the PDMS tParalleia D 1 0-8 l~l adsorption kinetics in the solvent. After .« 0.6 the replacement of the solution by the solvent, the adsorbed mass keeps constant "°" "oo . within the experimental error. So the 5 0.2 .^S> initially attached chains remain on the 0.0 surface without desorbing. In this 0 50 100 150 200 250 condition all changes are attributed to the Time (min) internal rearrangement of the adsorbed Fig. 4. Adsorption kinetics in the solvent. chains. In fig.4, a slight decrease is b) Open squares; 1015cm \ Open circles; 984cm observed in the bound fraction. Dichroic ratio shows a small increase at the first stage. These results mean some of the chains became rather vertical from the parallel orientation. Hence, it is suggested a relaxation process is going on inside the adsorbed layer. The relaxation will be the transition from the initial flattened conformation formed by a random deposition on a lot of unoccupied sites, to the equilibrium state matched to the adsorbed amount on the surface. For a strong adsorption system, ^frozen state was reported [2]. For PDMS at the non-saturated condition, the conformation was not frozen and the rearrangement of the chains in the adsorbed layer proceeded in the experimental time scale. n r-»_ l l

»-kJ

1 ^'^ Q

•

.

•

•

4. CONCLUSION In this study we have examined the backbone orientation of the adsorbed polymer. It provided the useful information about the adsorbed chain conformation. At the starved condition, the orientation tended to be parallel to the surface, which was coincident with the flattened conformation. In the kinetics studies, the addition of the later arrived chains increased the vertically oriented Si-0 segments. The internal change in the adsorbed layer was also observed, which will be the relaxation process from the initial random adsorption. REFERENCES 1. G. J. Fleer; M. A. Cohen Stuart; J. M. H. M. Scheutjens; T. Cosgrove; B. Vincent, Polymers at Interfaces', Chapman and Hall, New York, 1993. 2. P. Frantz and S. Granick, Macromolecules 28 (1995) 6915. 3. P. A. Floumoy and W. J. Schaffers, Spectrochim. Acta. 22 (1966) 15. 4. Tsao, M. -W.; Pfeifer, K.-H.; Rabolt, J. F.; Castner, D. G.; Hassling, L.; Ringsdorf, H., Macromolecules 30 (1997) 5913. 5. E. P. Enriquez and S. Granick, Colloids Surfaces A 11 (1996) 113.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c^ 2001 Elsevier Science B.V. All rights reserved.

821

Adsorption of Albumin on Organically-Modified Silicas (Ormosils) in Aqueous Solutions Hirokazu lyanagi, Kuriko Yamane,* and Shoji Kaneko* Pola Corp., Kashio-cho, Totsuka-ku, Yokohama 244-0812, Japan *Dept. Mater. Sci. Tech., Shizuoka University, Johoku, Hamamatsu 432-8561, Japan Organically modified silicas (Ormosils), which have an organic group covalently bonded into the silica network, were prepared from tetramethoxysilane (TMOS) and trimethoxysilane RSi(0Me)3 in ethanol. The adsorption behavior of albumin on Ormosils in aqueous solutions was studied. It has been proved that Ormosil including HS group (TP) was most effective to the albumin adsorption, depending on the solution pH. The ratio of HS group to SiO, in TP ( r HS) showed the optimum value for the albumin adsorption. 1. INTRODUCTION As biochemical production develops in the field of medicine and cosmetic, the separation of protein impurities from the products becomes an important manufacturing process. The removal of impurities in liquid phase with adsorbent is very useful from the viewpoint of purification and handling. Silica gel is one of promising inorganic adsorbents. The surface modification of silica gel leads to the enhancement in the adsorption ability for organic substances [1-3]. Thus we have prepared organically modified silicas (Ormosils) which have organic groups covalently bonded into the silica network. The adsorption behavior of a typical protein, albumin onto Ormosils in aqueous solutions was studied. 2. EXPERIMENTAL 2.1. Preparation of Ormosils Ormosils were prepared from tetramethoxysilane (TMOS) and trimethoxysilane RSi(0Me)3 in ethanol by hydrolysis and condensation with hydrochloric acid and triethanolamine, respectively, where R is H2C=C(CH3)COOC3H6 (MP), ClCjHg (CP), or HSC3H6 (TP). After filtration, the Ormosils were washed with deionized water, dried in

822 oven at SO'C for 24 h, and then at 80t^ for 24 h. The dried Ormosils were shaped to the grains of 20-42 mesh size (average diameter ca. 0.5 mm) without further heat-treatment as adsorbents. Also, silica gel was prepared similarly from TMOS. FT-IR and TG-DTA measurements were carried out to confirm the introduction of organic group into silica. Specific surface area, isoelectric point, and acid strength were measured. The amount of reactive HS group in TP was determined by a colorimetry using Ellman's reagent [4]. 2.2. Adsorption test of albumin on Ormosils The adsorption test was carried out by shaking the mixture of 0.1 g adsorbent and 25 ml of 50 ppm albumin solution in the pH range between 4 and 9 at 25*0 for 24 h. The initial pH of an albumin solution was adjusted by adding hydrochloric acid or sodium hydroxide solution. The residual amount of albumin in solution was determined by a colorimetry using bromocresol green. Adsorption percentages was calculated by equation (1),

Adsorption percentage =

[Initial amount] - [Residual amount] [Initial amount]

X100

^^^

3. RESULTS AND DISCUSSION 3.1 Effect of organic group on albumin adsorption As shown in Fig. 1, the adsorption percentages of albumin on MP and CP were low. In addition, Table 1 shows that the specific surface areas of MP and CP are small compared with that of the original silica, suggesting that the low adsorption percentages were due to the small specific surface areas. The adsorption percentage for TP was relatively high, particularly in the pH range from 4 to 6, and the highest adsorption percentage was found at pH4, despite its specific surface area was small. This finding suggests the occurrence of an interaction between HS group and albumin. The isoelectric point (ie.p.) of TP and albumin shown in Table 1 could explain this pH dependence of the albumin adsorption as follows In the pH range from 4 to 6, albumin molecule approaches closely to TP surface, because both of the electoric charges of TP and albumin are very small or almost zero. Therefore, TP and albumin electrostatically attract with each other to give high adsorption percentages. At pH < 4 and pH>6, however, TP and albumin electrostatically repel each other, because of the appearance of the same kind of the electric charge. Thus the adsorption percentages were low in these pH ranges. FT-IR spectra of albumin and albumin adsorbed on TP are shown in Fig.2 The

823

6 Initial pH Fig.l Adsorption of albumin on ormosils TP(A), MP (•), CP ( • ) and silica gel (D)-

3000

2000 1500 1000 650 Wavenumber (cm') Fig.2 FT-IR spectra of albumin (a) and albumin adsorbed on TP (b).

Table I The specific surface area and isoelectric point of varios adsorbents and albumin. specific surface area (mVg) i.e.p.(pH)

SiO,

MP

CP

TP

360

N.D.

140

120

3.74

4.05

6.95

3.80

albumin

3.8-4.6

absorption peaks at 1660 cm*' and 1550 cm'* which are assigned to C=0 stretch vibration and NH in-plane deformation vibration, respectively, became broad and shifted toward higher wavenumbers. It is, therefore, assumed that the HS group on TP attracts strongly NH and C=0 groups of albumin. 3.2 Effect of amount of HS group in the Ormosil on albumin adsorption Since the novel Ormosil, TP including HS group was proved to be an good albumin adsorbent, the optimum ratio of HS group to SiO, in TP (THS) was investigated for the enhancement of the adsorption abilities for albumin (Fig. 3). Below T HS^O.7, albumin was not adsorbed on TP at all, but the adsorption percentage increased abruptly between r HS"0.7 and 1.0. Furthermore, the adsorption percentage decreased gradually over THS"! 0. As shown in Fig.4, the slight weight loss until 200*0 was found in TP below rHs'^0.85 presumably due to the loss of OH group by dehydration, but it was not recognized in one at THS^IO- It is, therefore, supposed that the activity of HS group for albumin adsorption became higher as OH group was lost step by step between 7^3=0.85 and 1.0. This high activity of TP could be obser/ed between r HS"0S5 and 1.0 where the acid strength changed (Table2). The specific surface area of TP decreased with increasing r HS^ ^S shown in Fig. 3. It seems that dense HS groups on TP with decreased specific surface area have an interaction

824

400 600 800 TOOO Temperature(*C) Fig.4 TG-DTA cureves of TP. 200

Fig.3 Adsorption percentage of albumin at pH4 (i) and specific surface area of TP ((j as a function of r H5r

Table 2 The relationship between r HS ^^^ acid strength of TP. ^ ^ ^ T H S

Ho

0-0.85

1.0

1.15-2.0

+4.8

+3.3

+ 1.5

Fig.5 Amount of HS group reacted with Ellman's agent as a function of r HS

such as steric hindrance.

It prevents HS group from the attraction of albumin.

This is

supported by the fact that the Ellman's agent could not react with HS group on TP above r Hs~ 1 0, as shown in Fig. 5. 4. CONCLUSION We have prepared organically modified silicas (Ormosils) from TMOS and different RSi(0Me)3 and their adsorption behavior for albumin in aqueous solutions was studied. including HS group is most effective to albumin adsorption. of TP depended on pH and it was the highest at pH4.

TP

Also, the adsorption percentage

The ratio of HS group to SiO. in TP

( r HS) showed the optimum value of 1.0. REFERENCES 1. L. T. Kubota, A. Gambero, A. S. Santos and J. M. Granjeiro, J. Colloid Interf. Sci., 183 (1996)453. 2. W. Frenzel, S. Krekler, Anal. Chim. Acta., 310 (1995) 437. 3. L. T. Kubota, Y. Gushiken, A. Mansanares and H. Vargas, J. Colloid Interf. Sci., 173 (1995)372. 4. A. M. Glazer, H. kamimura, A. J. Grant, Y. Natsume, M. Scheriber, A. D. Yoffe, J Phys. C , 9 (1976) 291

825

Determination of the Acid-Base Properties of Surfaces by Contact Angle Titration with Buffered and Unbuffered Solutions RSakai Faculty of Human Life Science, Osaka City University Sugimoto 3, Sumiyoshi-ku, Osaka 558-8585 JAPAN A method of measuring the surface density of ionizable functional groups on solids by comparing the contact angle of buffered solution with that of unbuffered solution is studied. 1. INTRODUCTION The acid-base reactions (i.e., proton transfer reactions) at the solid-liquid interface are important and the establishment of measurement techniques of these properties of solid surfaces is desired in the colloid and interface science[l,2]. For this purpose, one should observe the ionizable functional groups on a solid that can be directly contact with liquid and cause acid-base reactions: The spectroscopy such as XPS and FT-IR observes functional groups that do not influence wettability because these methods detect groups in a thick layer of surfaces (about 5 nm for XPS and 100 nm for FT-IR)[3]. On the contrary, the surface probe microcopy only detects the outermost layer and misses groups in a small crack or hollow[4]. Thus, to investigate the acid-base properties, a liquid itself is used as a probe and the analysis of the interfacial free energy based on the contact angle measurement is extensively performed. (The presence of the acid-base interactions leads to a decrease in the contact angle.) By measuring the contact angle of aqueous solutions as a function of the solution pH, acid-base properties of various polymer surfaces[2,5,6] and oxide films on metals[7-9] were investigated. However, it is difficult to determine the effective surface density of functional groups from the contact angle. The contact angle 6 (i.e., the angle between a solid and a liquid surface at point of contact) relates to the solid surface tension r s, the liquid surface tension r L and the solid-liquid interfacial tension r SL by Young's equation:

Here, Young's equation is of the thermodynamic origin and 6 must be the equilibrium contact angle, which is difficuh to measure because of the contact angle hysteresis. The advancing contact angle is usually used as the alternative. Thus, it is said that the analysis based on the contact angle measurement is qualitative not quantitative. A breakthrough to circumvent this difficulty was achieved by Holmes-Farley et al.[5]. They showed that the effective surface density of ionizable functional groups can be measured by comparing the contact angle of buffered solution with that of unbuffered solution. The basic idea is as follows. When a drop of aqueous solution comes into contact with a solid

826 having ionizable functional groups on it, functional groups will ionize or neutralize according to the solution pH. Then, the hydrogen ions are transferred between the solid and the solution. For a small (e.g., a few u L) drop of unbuffered solution, it causes pH variation because of the change in the hydrogen ion concentration. The magnitude of the pH variation depends on the number of ionized functional groups. Therefore, the pH variation will be used to count the ionizable functional groups on the solid surface. The question is how to know the pH value of a small drop deposited on the solid. A few u L-drop is too small to measure its pH directly by the pH meter. The answer is to utilize the dependence of the contact angle on pH. By measuring the contact angle as a function of pH with buffered solutions, whose pH values remain constant in contact with solids, the calibration curve between the solution pH and the contact angle is obtained beforehand. Then, referring to this calibration curve, pH of a small drop of unbuffered solutions deposited on solids can be estimated from its contact angle. Here, the advancing contact angle will serve this purpose because it is used just as an index and its absolute value is not important. The aim of this paper is to investigate how accurately the effective surface density is determined by the contact angle measurement with small drops of unbuffered solutions. 2. CALCULATION As a model, we pick the simple solid surface on which the acidic groups are the only ionizable functional group. (We have high density of functional groups, such as carboxyl and hydroxyl groups, which cause the strong acid-base interactions with liquid in mind. The surface density of ionizable functional groups a is in the order of nm-2[7] and one group gives up a proton.) The typical calibration curve 6 (pH) [2,5-9] is modeled by e,.i(e,-e,)exp[p(pH-pHj]

,pH
e(pH)

(2)

i ( e , - e , ) e x p [ - p ( p H - p H j ] + e, ,pH>pH„

As mentioned above, the value of contact angle itself is meaningless and the pH variation is essential. Thus, 9 i=40° and 6 2=20° with 3 =10 are used irrespective of the value of a .

40

—p——1

1

r-

'^"

•

^

1

T

'^'

1

11

j

\

\

\ \

\

-g30 ""^^

^ ^ 1 * *

\

I

\

\

\

\

\

J

\ \

1

\

kri.

\

20

1.0 (nm" —1

1

1

1

L

1

1

1

1

1

_L _-J

1

1—

11 13 7 P^init Fig. 1. Titration curves of contact angle of buffered (solid lines) and unbuffered (dashed lines) solutions on the acidic surface with pHm=4, 7 and 10. a =1.0 nm-2. I

5

827 The solid lines in Fig.l are the calibration curve calculated by eq.(2) with pHni=4, 7 and 10; pHm indicates the solution pH at which the contact angle is the midpoint (6 =30° ). The degree of ionization of fiinctional groups at solid surfaces ot is related to 6 (pH) by a (pH)-(cose (pH)-cose,)/(cose2-cose,).

(3)

This relationship is derived under the assumption that r s and r L are independent of pH[2] so that the change in r SL with changes in pH depends linearly on the degree of ionization a: Y (pH) = ( l - a ) Y * SL ^ '^

'

^

+aY

^ ' SLl

, ' SL2 '

(4) ^ ^

where T SLI is the value of r SL when all the groups on the surface do not ionize {6 = 6 \)\ r SL2 is the value when all of them ionize ( 0 = 0 2)- By introducing eq.(l) into eq.(4), eq.(3) is obtained[5,10]. Next, let us explain how to calculate the pH variation of small drops of unbuffered solutions in contact with solids, a is assumed to be 0 before a drop is deposited. When the drop of a given pH=pHinit is deposited on the solid, the surface groups partly ionize according to the solution pH, i.e., Q!= Q:(pH). Then, a(pH) • a • ASL hydrogen ions are released into the drop. Here, ASL is the contact area of drop with solid. It is a function of the contact angle 6 and the drop volume V: - M 2YAsL-Jt'v'sin'e - c o s ' e - c o s e + - j ' .

(5)

The released hydrogen ions move the equilibrium of the reaction formula, H20;i^H+ + 0H-, in order to maintain the value of the ionic product of water Kw=[H+][OH-]=1.0X 10-14 (mol/L)2 at 25^0. Then, the hydrogen ion concentration in the drop changes. Therefore, the drop pH varies. By this pH variation, the values of 6 (pH) (thus, ASL( 0)) and a (pH) change and thus the number of released hydrogen ions also changes. Then, pH varies again. After these successive calculations, the equilibrium pH value, pHeq, is obtained at which the number of transferred ions and the pH variation are equilibrated. As is pointed out in the Introduction, the solution pH of a small drop is difficult to measure. Thus, to compare the calculated results with experimental ones, referring to the calibration curve 6 (pH), the calculated pHeq should be translated into the contact angle, which is readily compared with the experiment. 3. RESULTS AND DISCUSSION To obtain the effective surface density a, the contact angle difference due to the pH variation must exceed the experimental error in the contact angle measurement ("^1 ° ). The dashed lines in Fig.l show the contact angle of an 1 /x L-drop of unbuffered solutions on solids with a =1.0 nm-2 when pHjnit ranges from pH 1 to 13. When pHm=4 and 7, the difference between the buffered (solid line) and the unbuffered (dashed line) solutions is distinguishable. However, when pHm=10, it is negligible. This is because that 6 X lO^ ^ hydroxide ions in the 1 /x L-drop at pH 10 are too much to be buffered by about 10^3 hydrogen ions released from the a =1.0 nm-2 functional groups. To investigate the acidic component of a solid surface.

828

Fig. 2. Titration curves on the acidic surface with (a) pHm=4 and (b) pHm=7.

the transferred hydrogen ions must outnumber the hydroxide ions in the drop. For pHni=4 and 7, the titration curves of an \ u L-drop of unbuffered solutions are calculated for several surface densities of ionizable functional groups in Fig.2. As can be seen, the 2-fold difference of the value of a is necessary to make the measurable difference in 6 . This shows the limit of the precision by this method. In experiment, the parameter a can be determined by fitting the calculated ^ (pHcq) to the measured 6 as a function of pHjnit-Note that, for inorganic surfaces such as a metal oxide, to control the initial ionization state may be difficuh because the composition is unstable. For organic surfaces of some polymers, o may change due to the reorganization of a surface structure[3,10]. In summary, we have demonstrated the utility of the contact angle measurement with small drops of unbuffered solutions and showed its limit of sensitivity to the surface density of ionizable fimctional groups on solids. REFERENCES 1. J.C.Berg, "Wettability (Surfactant Science Series Vol. 49)" , ed. by J.C.Berg (Marcel, New York, 1993) Chap.2, p.75. 2. KJ.Huttinger, S.Hohmann-Wien and G.Krekel, J. Adhesion Sci. Technol., 6 (1992) 317. 3. T.Kawase, M.Uchita, T.Fujii and M.Minagawa, Textile Res. J., 61 (1991) 146. 4. C.D.Frisbie, L.F.Rozsnyai, A.Noy, M.S.Wrighton and C.M.Lieber, Science 265 (1994) 2071. 5. S.R.Holmes-Farley, R.H.Reamey, T.J.McCarthy, J.Deutch and G.M.Whitesides, Langmuir 1(1985) 725. 6. G.M.Whitesides, H.A.Biebuyck, J.P.Folkers and K.L.Prime, J. Adhesion Sci. Technol., 5 (1991)57. 7. L.K.Chau and M.D.Porter, J. Colloid Interface Sci., 145 (1991) 283. 8. E.McCafferty and J.P.Wightman, J. Adhesion Sci. Technol., 13 (1999) 1415. 9. C.Vittoz, P.Bossis, F.Lefebvre and J.C.Joud, J. Adhesion Sci. Technol., 13 (1999) 1045. 10. S.R.Holmes-Farley and G.M.Whitesides, Langmuir 3 (1987) 62.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

829

Sorption of uranium(VI) on Na-montmorillonite colloids — Effect of humic acid and its migration -^ S. Nagasaki Institute of Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Sorption kinetics of U(VI) ions and U(VI)-humic acid (HA) complexes on dispersed Na-montmorillonite colloids was investigated at pH 6.0 by kinetic spectra. For the sorption of U ions, we observed the fast and the slow processes. The fast one was considered to be assigned to the sorption on the outer surface of montmorillonite. The slow one was assigned to the sorption in the interlayer. On the other hand, assuming that U-HA complexes were sorbed on montmorillonite by pseudo-fast-order reaction, we observed only the fast process, indicating that U-HA complexes were sorbed on the outer surface of montmorillonite. We studied the migration of U ions and U sorbed on montmorillonite colloids in the absence and in the presence of humic acid through quartz-packed column. It was found that U ions strongly retained in the column, but that more than 60 % of U sorbed on montmorillonite colloids was eluted from the column. 1. INTRODUCTION Predicting the environmental behavior of uranium (U) in aquifer systems is important for the safety assessment of a given high level nuclear waste repository and groundwaters contaminated with anthropogenic uranium sources. A critical component of the groundwater constituents is humic acids (HA) and the extensive investigations on the complexation of U02^^ with humic acids have been carried out [1 and the references cited in 1]. It is pointed out that the generation of pseudocolloids may enhance the migration possibility of actinide elements such as U in an aquifer system. Pseudocolloids are formed by sorption and association of actinides on/with groundwater colloids. Montmorillonite is one of the typical inorganic groundwater colloids and the role of the montmorillonite particles in the migration of actinides has also been studied [2]. However, few studies have been performed on the generation of U-montmorillonite pseudocolloids in the presence of humic acids. Pohlmeier [3] has proposed the kinetic spectrum method for understanding the interaction of ions with a great number of different binding sites located on the heterogeneous surface of solid (ex. illite and montmorillonite). In our previous studies [4,5], we found that the kinetic spectrum method is useful to study the generation of pseudocolloids of which surface is heterogeneous. In the present work, we studied the sorption of U(VI) ions and U(VI)-HA complexes on Na-montmorillonite by using the kinetic spectrum method. Because the pH range in the groundwater is 6 to 9 and U(VI) can dominantly form U(VI)-HA complexes at pH 4 to pH 7, we studied the sorption at pH 6 in this work. Furthermore, we studied the migration of U ions, U-montmorillonite colloids and U-HA-montmorillonite colloids through quartz packed column.

830 2. EXPERIMENTAL 2.1. Chemicals All chemicals except natural U were reagent grade and used without further purification (Wako Pure Chemical Ind., Japan). Water was prepared from doubly distilled water by further purification with Milli-Q system (Millipore) and ultrafiltered by use of a 2 nm pore size ultrafilter (UFPl, Millipore) immediately before use. Natural uranium nitrate solution was from stock on hand at the University of Tokyo. Impurities and the daughter nuclides of uranium decay chains were removed by repeated extraction and back-extraction with TBP and 0.01 M HNO3. After that, the pH of U stock solution was adjusted to pH 6 with NaOH. The oxidation state of U was spectroscopically confirmed to be hexavalent. Highly pure Na-montmorillonite produced in Tsukinuno, Japan was purchased from Nichika, Japan. Dispersed Na-montmorillonite colloid solution was prepared at pH 6, according to the same procedure in our previous study [5]. The Aldrich humic acid is used. This is originally the Na-salt form as available from Aldrich Co. We adjusted the Aldrich humic acid to H-form according to the procedure by Kim et al [6]. The humic acid solution was prepared by dissolving the humic acids into the distilled water and the pH was adjusted to pH 6. The ionic strength of all solutions was adjusted to 0.1 M by addition of NaNOs. 2.2. Sorption experiment All experiments were performed in a chamber filled with highly pure N2 gas (> 99.999 %). First of all, we prepared the U-HA complexes by adding U stock solution into humic acid solution. The concentrations of U and HA were 2 x 10'^ mol/dm^ and 20 mg/dm^, respectively. From the ultrafiltration (UFPl; pore size = 2 nm), we found that 100 % U (ionic size = about 1 nm) were associated with humic acids. Based on the complexation constants [1], U was considered to exist as UO2OH-HA. Uranium(VI) stock solution and UO2OH-HA solution had absorption peaks at 414 nm and 419 nm at pH 6, respectively. When U and UO2OH-HA are sorbed on dispersed Na-montmorillonite colloids, the absorbance at 414 nm and 419 nm is expected to decrease. After the addition of U stock solution or UO2OH-HA solution into the dispersed Na-montmorillonite colloid solution, the variation of absorbance at 414 nm or 419 nm was measured spectroscopically (Shimadzu PC-9100UV). The concentrations of U in U ion system and UO2OH-HA system were 1 x 10"^ mol/dm^, which is less than solubility limit at pH 6. In this work, we studied kinetic spectra at 12°C to 28 °C. Experimental procedure of kinetic spectra was described in detail in the literature [4,5]. 23. Column experiment A column used was an inner diameter of 5 mm and filled with quartz powder (diameter = 8 ^im) up to a height of 300 mm. The porosity of the column was found to be 0.4 from the breakthrough phenomenon of non-sorbing tracer, HTO (tritiated water; T = H). Temperature of the column was controlled at 25 ± 1 °C by a water jacket. The quartz powder was supplied from APPIE (Association of Powder Process Industry and Engineering, Japan) and immersed in 0.1 M NaNOs solution (pH 6) for more than one year. The pre-conditioning solution (pH 6, 0.1 M NaNOs) was continuously pumped through the column at the flow velocity of 3 x 10'^ ml/h, before U feed solution was injected. After pre-equilibrium in the column was achieved, the 2 ml of U feed solution (i.e. U ion solution, U ion —bearing montmorillonite colloid solution, or UO2OH-HA —bearing montmorillonite colloid solution) were introduced into the column. The concentrations of U in eluate

831 fractions were determined by Ge detector (y-ray counter). All experiments were performed in a chamber filled with highly pure N2 gas (> 99.999 %). The detailed experimental procedure was mentioned in elsewhere [2]. 3. RESULTS AND DISCUSSION 3.1. Sorption experiment It was found that U ions (mainly UO2OH*) and UO2OH-HA were completely sorbed on Na-montmorillonite colloids at pH 6, because U was not detected in the ultrafiltrates after ultrafiltration (UFPl) of experimental solutions. In the present study, we assumed that not only U ions but also UO2OH-HA were sorbed on Na-montmorillonite by a pseudo-first-order reaction. Then, the time dependence of change in total U concentration may be described as An(t)« F^A{k)txp(-kit)dk where A/i(r) = nP-ni(t), /i/° the total concentration of site Z, sorbed by U at equilibrium, /i/(r) the total concentration of site Z, at time r, ki the apparent rate constant at Z„ F the proportionality factor. A(k) represents the distribution of sites, sorbed by U, as a function of k, specific for each site. The/I(it) plotted vs. log k is termed the kinetic spectrum. The A(k) was calculated by an inverse Laplace transform on AAi(r) which could be experimentally obtained from the change in the absorbance at 414 nm and 419 nm with time. Figure 1 shows the kinetic spectra for the sorption of U ions on the surface of Na-montmorillonite as a function of temperature. Two processes could be observed: a fast process with mean rate constants between 3 s"^ and 20 s\ and a slow process with mean rate constants between 0.1 s'^ and 3 s'\ The mean rate constants of both fast and slow processes are nearly in the same order of magnitude as the sorption of Np02* on the outer surface and in the interlayer of Na-montmorillonite [5]. Hence, we considered that U ions were sorbed on the outer surface of Na-montmorillonite by a fast process and consequently sorbed on the interlayer surface by a slow process. The half widths at half maximum (HWHM) of the spectra of fast process were not broad compared with those of slow process, suggesting that the heterogeneity of the interlayer surface is stronger than that of the outer surface. It was also found that the mean rate constants of both processes increased with temperature. Figure 2 shows the kinetic spectra for the sorption of UO2OH-HA on the surface of Na-

Fig.l Kinetic spectra for sorption of U ions onto Na-montmorillonite as a function of temperature

Fig.2 Kinetic spectra for sorption of UO2OH-HA onto Na-montmorillonite as a function of temperature

832 montmorillonite as a function of temperature. Only a fast process could be observed. This might suggest that U ions associated with humic acids were sorbed on the outer surface of Na-montmorillonite, but could not approach to the interlayer surface because of their geometrical structures. The mean rate constants increased with temperature, but the values of the constants were relatively smaller than those for U ions. In future, we will study the difference in sorption behavior between U ions and UO2OH-HA in detail, considering the structure, stability of complexes and diffusivity/mobility and so on.

PH = 6 1 = 0.1 M

U(VI) sorbed on montmorillonite in the presence of humic acid ^ ^ 0

80

60

f

* 0

U(VI) sorbed on montmorillonili in the absence of humic acid

40 20

•

-

0

J

^

U(VI) ions

0

0.5

1.5

2.5

V(i)/V,

Fig.3 Recovery curves of U from quartzpacked column.

3.2. Column experiment Figure 3 shows the ratio of the accumulation amounts of U recovered from the column to the injected amount of U as a function of V(i)/Veff, where V(i) is the eluted volume and Vgff the effective pore volume of the column. In the preliminary experiment, it was found that the recovery curve of montmorillonite colloids was similar to that of U ion —bearing montmorillonite colloids. Our results indicated that U associated with montmorillonite colloids migrated through the column at a similar velocity of HTO in the absence and in the presence of humic acids, although U ions which were not associated with montmorillonite colloids retained strongly in the column. In the presence of humic acids, U continued to elute, while the U recovery seemed to stop in the absence of humic acids at V(i)/Veff > 2. In this region, elution of humic acids was also detected in the experiment of UO2OH-HA—bearing montmorillonite colloid. Therefore, it may be considered that UO2OH-HA was desorbed from montmorillonite colloids which were retained in the column and eluted as UO2OH-HA. 4. CONCLUSIONS U ions were sorbed not only on the outer surface but also on the interlayer surface of Na-montmorillonite colloids, but UO2OH-HA were sorbed only on the outer surface of Na-montmorillonite colloids. In the presence and in the absence of humic acids, a substantial portion of U migrated through the column when U was associated with montmorillonite colloids. REFERENCES 1. P. Zeh, K.R. Czerwinski and J.I. Kim, Radiochim Acta 76 (1997) 37. 2. S. Nagasaki, S. Tanaka and A. Suzuki, Colloid Surfaces A, 155 (1999) 137. 3. A. Pohlmeier, Progr. Colloid & Polym. Sci., 95 (1994) 113. 4. S. Nagasaki, S. Tanaka, M. Todoriki and A. Suzuki, Radiochim Acta 82 (1997) 263. 5. S. Nagasaki and S. Tanaka, Radiochim Acta (in press). 6. J.I. Kim, G. Buckau, G.H. Li, H. Duschner and N. Psarros, Fresenius J. Anal. Chem., 338 (1990) 245.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) €> 2001 Elsevier Science B.V. All rights reserved.

833

Contribution of preformed monolayer to micropore filling T. Ohba \ T. Suzuki" and K. Kaneko ' * Department of Chemistry, Faculty of Science, Chiba University, 1-33, Yayoi, Inage, Chiba 263-8522, Japan

The effect of the preformed monolayer to secondary filling in a graphite slit pore was studied using GCMC simulation and potential calculation. The potential depth of a second layer molecule induced from monolayer molecules having hexagonal closed packing structure is 70 % of the total potential. Secondary filling induced by the interaction of preformed monolayer molecules shows an uptake after monolayer adsorption step in adsorption isotherm.

1. Introduction A graphite slit pore system has gathered much attention from technological interests. Molecular simulation study on the graphite slit pore has elucidated the reason why activated carbon can adsorb a plenty of vapor molecules even from an extremely low pressure range. Sing et al. have studied extensively adsorption of vapor molecules on activated carbon [1-3]. They proposed Os-plot for elucidation of the micropore filling process, suggesting the presence of two different processes of primary and secondary (or cooperative) filling. The detailed mechanism of molecular processes in micorpores was not sufficiently understood. Recently Kaneko et al. showed that molecules adsorbed on micropores form an ordered structure by use of several techniques [4, 5]. Then it was suggested that the interaction of the monolayer molecule with the second-layer molecule plays an important role in the ordered structure formation. Consequently we studied the contribution of the preformed monolayer to further adsorption in the case of the nitrogen-graphite slit pore system using the potential calculation and GCMC simulation.

834 2. GCMC simulation We used the hexagonal close-packing model (HCP-model) for preformed monolayer (Fig. 1(a)). The potential calculation was carried out using the 12-6 Lennard-Jones potential for the fluid-fluid interaction using one center approximation.

(1)

Mn^'^^ir

Here Cff, ou and r^ are the fluid-fluid potential depth, the effective diameter and intermolecular distance, respectively. Their values are Eff / kB=104.2 K and Off = 0.3632 nm. The following Steele's 10-4-3 potential function [6] for the N2 molecule-a single graphite slab interaction was used.

. Ifta^^e^pH

i'f

(2) 3^(z + 0.61Ay

Where z is the vertical distance of a molecule from the surface, p is the carbon atomic

1000

(b)

500

I -500 -1000

-1500

Wx -0.4

-0.2

0

0.2

0.4

z/nm

Fig. 1 Hexagonal close packing monolayer (HCP) model (a), the potential profiles of an N2 molecule in the monolayer-coated slit pore (b). The solid curve is the total potential. The dotted curve shows the interaction potential profile of the molecule with the slit pore.

835 number density, A is the interlayer distance of the graphite. Csf and Osf are fitted parameters of the N2 molecule-carbon potential well depth and effective diameter, respectively. They were obtained with the use of the Lorentz-Berthelot rules. The pore width w is associated with the physical width H using eq. 3 [7]. H is defined as the distance between opposite graphite surface.

wH-{IZQ-O^)

,

(3)

zo = 0.856asf

Here zo is the distance of the closest approach. The adsorption isotherm was calculated using the established grand canonical Monte Carlo (GCMC) simulation method [8].

0 10

10"'

0.0001 0.001 p/p„

0.01

0.1

>v= 1.0 nm w- 1.1 nm

H'= 1.4 nm

ax)M:ffrriiQimZ2n]aigi) ^ Fig. 2 Simulated adsorption isotherms of N2 at 77 K in a graphite slit pore of different w values and snap shots at P/Po = 3X lO"^ and 4 x 10'^, respectively : O, w = 1.0 nm; A, w = 1.1 nm; D , w= 1.4 nm

836 3. Results and discussion Fig. 1(b) shows the potential profiles of a second-layer molecule in the monolayer coated pore of w = 1.1 nm using the HCP model. For the potential profiles of the monolayer molecule with the graphite pore are shown for comparison. The potential depth of the molecule in the monolayer-coated pore can be briefly comparable to that of the molecule with pore wall. The potential depth of the molecule with the monolayer molecules is 70 % of the total potential depth from the interaction of the monolayer molecules and pore wall. Therefore, this strong intermolecular interaction gives a great effect on further adsorption [9]. Fig. 2 shows simulated adsorption isotherms of an N2 at 77 K and snap shots. As the micropore of w =1.4 nm has enough room for the second layer adsorption in the void space between the monolayer-coated pore-walls, a steep uptake at P/Po = 2 x 10"^ by the second layer adsorption is observed. The strong interaction in the second layer leads to the perfect filling near P/Po = 2 x 10'^. The void space of the monolayer-coated micropore of w = 1.0 nm is not sufficient and very narrow. Hence filling in the void space begins from much lower pressure than 2X 10'^ of P/Po, given no sharp jump in the adsorption isotherm. The adsorption in the micropore of iv = 1.1 nm has an intermediate nature between pores of iv = 1.0 and 1.4 nm. These results explicitly showed the acceleration effect of the monolayer on the pore walls for the filling by molecules.

References 1. K. S. W. Sing, Carbon 27 (1989) 25. 2. K. S. W. Sing, Carbon 32 (1994) 1311. 3. A. Vishnyakov, P. I. Ravikovitch and A. V. Neimark, Langmuir 15 (1999) 8736. 4. 5. 6. 7. 8. 9.

K. Kaneko, Carbon 38 (2000) 167. K. Kaneko, Supramolecular Sci. 5 (1998) 267. W.A. Steele, Surface Sci. 36 (1973) 317. K. Kaneko, R. F. Cracknell and D. Nicholson, Langmuir 10 (1994) 4606. T. Suzuki, K. Kaneko and K.E. Gubbins, Langmuir 13(1997)2545. T. Ohba, T. Suzuki and K. Kaneko, Chem. Phys. Lett, in press.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

837

Magnetic-field control of oxygen adsorption H. Sato/ Y Matsubara^ and S. Ozeki" "Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan Apartment of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan The influence of magnetic fields for adsorption of paramagnetic O2 and diamagnetic N2 on various activated carbon fibers (ACFs) was examined at 77K. The adsorption amount of O2 increased by applying magnetic fields less than 8 T, and decreased over it, which are referred to magnetoadsorption and magnetodesorption, respectively. The magnetoadsorption became maximum at around a few T and the magnetodesorption reached to no saturation even at 10 T, depending on pore size and pressure. Amounts of N2 adsorbed on ACFs decreased monotonically with increasing magnetic fields, different from the magnetoadsorption of O2. From these results, we conclude that adsorption of O2 and N2 on solids was controlled by steady magnetic fields, and inferred that air separation due to magnetic fields should be possible. L INTRODUCTION It is not easy for adsorption equilibrium to be affected by external magnetic fields, since the energy of adsorption is much larger than that of magnetic fields. It was found, however, that external, steady magnetic fields modified adsorption of paramagnetic NO and diamagnetic H2O and organics at 303K [1,2]. Magnetic-field-promotion and -depression in adsorption were referred to magnetoadsorption and magnetodesorption, which depend on the kind of solids and adsorptives, magnetic field intensity, and adsorption state of adsorptives [3]. The magnetic effects may be expected when the magnetization of the adsorption system changes during adsorption [3]. In this paper, the effects of magnetic fields to adsorption of paramagnetic O2 and diamagnetic N2 at 77K were investigated with the activated carbon fibers having different

838 pore width. 2. EXPERIMENTAL Three kinds of pitch-based activated carbon fibers (ACF; AlO, A20, and Y; Ad'all Co. Ltd.) were used (Table 1). O2 adsorption isotherms of ACFs were measured gravimetrically at 77K. The samples were pretreated at 383K and 10"^ torr for 2 h prior to adsorption measurements. ACFs were characterized by the Os-analysis of O2 adsorption isotherms. An ACF sample (10 mg) was packed into an ESR tube (adsorption cell), pretreated at 383K and 10'^ torr for 2 h, and then kept at 77K by dipping in liquid nitrogen whose level was kept constant using an automatic supplier of liquid nitrogen. O2 or N2 of different pressures was introduced over the sample at 77K. Magnetic fields were applied to the cell with a liquid helium-free superconducting magnet (Sumitomo Heavy Industries, Ltd.; ^ lOT) after adsorption equilibrium. O2 and N2 gases (Nippon Sanso Co., Ltd.; Okaya Sanso Co, Ltd.) were used after removing water at 77K. Table L Pore data of ACFs from Q2 adsorption adsorbent AlO A20 Y

pore volume* (mLg') 0.33 0.81 0.59

surface area* (m'g"') 1100 1900 1350

pore width* (ran) 0.7 0.9 1.5

• The values were determined by the Os-method.

3. RESULTS AND DISCUSSION Fig. 1 shows magnetic-field dependence of O2 pressure changes, AP, due to application of steady magnetic fields to AIO/O2 equilibrium systems at their different relative pressures, 0.01, 0.1, and 0.4. The pressure change was reversible at all examined relative pressures. Amounts of O2 adsorbed on AlO increased with increasing magnetic field intensity below around 8T, but decreased over 8T. The critical magnetic field (//c = 8T), at which no magnetic effect was observed, shifted to a lower field with increasing relative pressure. The magnetoresponses, AP, seems to be independent of O2 pressure. However, when changes in O2 adsorption amount, Av, estimated from the AP values are plotted as a fimction of magnetic

839

2 4 6 Magnetic F i e l d , / / / !

2 4 6 Magnetic Field,///T

10

Fig. 11^.

1. Magnetic-field dependence of O2 Fig. 2. Magnetic-field dependence adsorption-amount 1. ivio^^^uv-uciu ucpcui pressure change due to applied magnetic field change of O2 due to applied magnetic field on A10 at pressure change due to applied i Relative pressures P/PQ: O ; 0.01, D ; 0.1, on AlO AlO at at 77K. 77K. Relativ^ Relative pressures PlPo- 71K. on O;0.01, n;0.1. A; 0.4. field intensity, Av varies according to P.

The lower relative pressure, the larger Av is

obtained below He, although no corelation between Av and P is over He. shifts with O2 pressure.

Also, //© itself

The magnetoresponses in the pore saturation system (P/Po = 0.4)

were small at the examined H region. From thermodynamical aspects, the observed A/^ should be attributed to magnetization changes during adsorption, which also must depend on magnetic field.

Since O2 molecules

in low density adsorption phase could behave as a paramagnetic gas, 02-surface interaction must be dominant rather than O2-O2 interaction.

The magnetoadsorption at low fields

suggests that the interaction should bring about more paramagnetic character for adsorbed O2. The micropore-filling O2 at higher O2 pressure forms a diamagnetic condensed phase like a 0.15

2

4

6

8

Magnetic Field, HIT

10

Fig. 3. Magnetic-field dependence of O2 pressure change due to apphed magnetic field on various ACFs at 77K and P/Po = 0.01. Adsorbents: O;A10, V;A20, 0 ; Y .

840

0.15 h

0.05

<3

-0.05 0

2

4

6

8

10

Magnetic Field, HIT Fig. 4. Magnetic-field dependence of O2 (O) and N2 ( • ) adsorption on AlO at 77K and P//>o = 0.01.

2 4 6 8 Magnetic Field,///!

10

Fig. 5. Magnetic-field dependence adsorption-amount changes of O2 (O) and N2 ( • ) due to applied magneticfieldon AlO at 77K and PIP^ = 0.01.

liquid O2, which may depress the magnetoadsorption. The magnetodesorption at higher magnetic field suggests more diamagnetic adsorption phase. It is suggestive that the largest magnetodesorption occurred at PIPQ = 0.1, wiiere micropores was not perfectlyfilledwith O2. Fig. 3 demonstrates magnetic field dependence of AP for ACFs having different pore structures (Table 1). O2 adsorption for AlO and A20 having pores less than 1 nm was promoted below 8T, but for Y (>1 nm) was depressed with increasing magnetic field. From these results, there seems to be a suitable cluster size for magnetoadsorption, as suggested by a suitable pressure (PIPQ = 0.1 in Fig. 1) and pore size (A20 in Fig. 2). O2 adsorbed on ACF exist in different O2 states, depending on O2 pressure and pore size [4]. Therefore, the magnetoadsorption and magnetodesorption should arise from magnetic stability associated with magnetization changes due to adsorption. N2 was magnetically desorbed under the examined H region, as shown in Fig. 4. The magnetodesorption of N2 seems to be seasonable, because N2 is diamagnetic. The difference between these gases in Av was not very large (Fig. 5), but it suggests a prospect for separation of O2 and N2 in air by magnetic fields. REFERENCES 1. S. Ozeki, H. Uchiyama and K. Kaneko, J. Phys. Chem., 95 (1991) 7805. 2. S. Ozeki, J. Miyamoto, S. Ono, C. Wakai and T Watanabe, J. Phys. Chem., 100 (1996) 4250. 3. S. Ozeki, J. Miyamoto, Y. Matusbara, H. Kurashima and T. Tazaki, J. Japan Inst. Metals, 61 (1997) 1300. 4. H. Kanoh and K. Kaneko, J. Phys. Chem., 100 (1996) 755.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) <.c) 2001 Elsevier Science B.V. All rights reserved.

841

Viologen monolayers: Dynamics on electrode surfaces* Takamasa Sagara'^*', Hiroaki Tsuruta*", Yumika Fukuoka**, Saori Tanaka^, and Naotoshi Nakashima^ ^^Organization and Function", PRESTO, JST (E-mail [email protected]) ^Department of Applied Chemistry, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan Dynamic processes of viologen monolayers at electrified interfaces were described for self-assembled monolayers (SAMs) of viologen-thiols on a Au electrode and the spike vohammetric response of heptylviologen on a HOPG electrode using the results of electrochemical and electroreflectance measurements. We found that the viologen-thiol SAM memorizes anion present while forming the SAM. The spike response of heptylviologen at a HOPG electrode was tentatively assigned as Gibbs monolayer-Langmuir monolayer phase transition. 1. INTRODUCTION Viologen is among well-suited electroactive moieties to investigate dynamic behavior of redox center-tethered amphiphiles bearing long alkyl chains. Various types of thiolfunctionalized alkyl viologens (viologen-thiols) have been immobilized on Au electrodes and characterized [1,2]. Of interest is the ingress/egress of anions into/from the SAM and its dynamics. Dynamic processes of viologen with long alkyl chains at electrified interfaces are also of our great interest. Especially, well-known spike responses in cyclic voltammogram [3] are the target to be explored in the aspects of both molecular level mechanism and kinetics. We describe herein the behavior of SAMs of viologen-thiols on a Au electrode and the nature and kinetics of the spike response of heptylviologen on a HOPG electrode using the results of electrochemical and electroreflectance (ER) measurements. *This work was made as the research project of "Organization and Function", PRESTO, JST but was additionally supported financially in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Culture and Sports of Japan (to TS) and Asahi Glass Foundation (to TS).

We thank Mr. Yi Yuan for his technical assistance.

842 2. EXPERIMENTAL A viologen-thiols (CsV^'^CnSH 2PF6": V^^ stands for viologen dication) was synthesized. The polycrystalline Au electrode modified with a viologen-thiol SAM prepared by dipping a mirror-polished Au disk in 1 mM CsV^^CuSH 2PF6* /acetonitrile (AN) for 24 h was designated as "Eid-N" and that prepared in 1.0 mM CsV^'^CnSH 2PF6" + 0.1 M tetraethylammoniumbromide (Et4Nl3r')/AN as "Eld-Br". A freshly peeled-ofF basal plane HOPG electrode was set to a hanging-meniscus (H-M) configuration at a Ar gas/aqueous solution (1 mM heptylviologen + 0.3 M KBr) interface. All the electrochemical measurements were carried out in an Ar atmosphere at 23±2°C using a Ag/AgCl sat'd KCl reference electrode. 3. RESULTS AND DISCUSSION 3.L Viologen-thiol SAM memorizes coexisting anion while forming the monolayer The formal potential of V^^^*" couple of a viologen-thiol SAM is more negative when the softness of the electrolyte anion is higher [1,4]. For a softer anion, the ratio of the anion binding equilibrium constant to V^^ against that to V^ is greater, and this is the origin of the electrolyte anion dependence of the formal potential [4]. The behavior of "Eld-N" and "Eld-Br" was compared using cyclic voltammogram (CV) and ER measurements in various aqueous electrolyte solutions. The CV of "Eld-Br" in 0.1 M KBr solution showed ca. 100 mV more positive formal potential (£^') than that of "Eld-N" with smaller peak separation and width (Figure 1). This difference was observed also in 0.1 M KF but was not observed in 0.1 M KPF6 (Table 1). These results indicate that Eld Br EldN

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Table 1 Results of film transfer experiments: shift of £^' values in mV

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-100 0 100 / mV

Fig. 1. CV of Au/viologen-thiol SAM in 0.1 M KBr solution.

Electrode Initial electrolyte Second electrolyte 0.1 M KBr - ^ . 0.1MKPF6 -418 -479 Eld-N -317 -493 Eld-Br 0.1 MKF — ^ 0.1 M KBr -360 -400 Eld-N -192 -347 Eld-Br O.lMKPFfi —^.O.lMKBr -501 -366 Eld-N -503 -368 Eld-Br

843

the viologen-thiol SAM memorizes anion (Br), which is present while forming the SAM in AN, in KF and KBr electrolyte solutions, though the memory is lost in KPFe solution. When both "Eld-N" and "Eld-Br" tested initially in 0.1 M KBr solution were transferred into 0.1 M KPFs, the difference of £^' between them disappeared (Table 1). On the other hand, when "Eld-N" and "Eld-Br" tested in 0.1 M KF solution were transferred in 0.1 M KBr, the difference remained (Table 1). However, when "Eld->r' and "Eld-Br" tested in 0.1 M KPF6 solution were transferred in 0.1 M KBr, no considerable difference in CV was observed (Table 1). Therefore, PFs" in the electrolyte solution acts as an eraser of the memory of anion present while forming the SAM. Almost complete exchange of an anion in the monolayer for another anion in the solution was confirmed by the electrolyte injection experiment in the case of the exchange of Br' for PF6" and by the CV measurements negative to the second reduction wave of V*"*^^° in the case of the exchange of PF6' for Br'. To sum up, the memory of the anion present while forming the monolayer by the viologen-thiol SAM is retained in the electrolyte solution containing anions of lower or same softness. Electroreflectance measurements revealed that the structural differences of the SAM due to the counter anion of viologen in regards to monomer-dimer equilibrium of V*^ and molecular orientation are minimal. 3.2. Spike response of heptylviologen at a HOPG electrode in hanging-meniscus configuration The spike response observed in the present work (Figure 2) at a HOPG electrode in the H-M configuration on heptylviologen (HV) aqueous solution was in accord with the previous 2.4

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844 report by Tokuda et al [3]. In the present work, we found that the sharp spike response is observable in very restricted conditions so that we can see it only at the first horizontal touch of the HOPG basal plane to the surface of heptylviologen solution to set up the H-M configuration. When the electrode is dipped in the solution using an electrode holder with an 0-ring, the spike response was never observable. This fact indicates that the molecular assembling structure created at the initial touch is critical to the dynamic behavior of the adsorption layer. CV curves showed a sweep rate (v) dependence that the peak current is proportional to v'" with m = 0.6, suggesting that nucleation-growth-collision (NGC) process takes place as in the case of Hg electrode [5]. The current transient as a response of potential step over the spike peak potential decayed within a shorter period than 15 ms, and NGC component of the current transient synchronized with the transient of simultaneously measured reflectance of visible light. The ER spectrum at a normal incidence measured at the midpoint potential of the anodic and cathodic spike peaks (Figure 3) appeared different from absorption spectrum of V*"". The experimental ER spectral profile could be reproduced by the simulation with the assumptions that HV*^ moieties assume flat-lying or edge-on orientation and are in direct contact with the HOPG surface (Figure 3). Detailed potential step chronocoulometry measurements revealed that the total charge associated with the spike response exhibits a jump which amounts to more than half of the total surface excess of heptylviologen monolayer. We propose that the spike response is the redox-triggered first-order phase transition between a Gibbs monolayer and a Langmuir monolayer and the kinetics is controlled by NGC mechanism. 4. CONCLUSION Dynamic ingress/egress of anions into/from the viologen-thiol SAM governs the electrochemistry of the SAM. The spike response of heptylviologen at a HOPG electrode can be tentatively assigned as a phase transition phenomenon.

REFERENCES 1. X.-Y. Tang, T W Schneider, J. W Walker, D. A. Buttry, Langmuir, 12, 5921 (1996). 2. T Sagara, N. Kaba, M Komatsu, M. Uchida, N. Nakashima, Electrochim. Acta, 43, 2183 (1998). 3. K. Arihara, F. Kitamura, K. Nukanobu, T. Ohsaka, K. Tokuda, J. Electroanal. Chem., 473, 138(1999). 4. T Sagara, H. Maeda, Y. Yuan, N. Nakashima, Langmuir, 15, 3823 (1999). 5. J. I. Millan, R. Rodriguez-Amaro, J. J. Ruiz, L. Camacho, Langmuir, 15, 618 (1999).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ^o 2001 Elsevier Science B.V. All rights reserved.

845

Fluorescence Specific Micro Patterns in Two-Dimensional Ordered Arrays Composed of Polystyrene Fine Particles S. I. Matsushita*, T. Miwa^ and A. Fujishima* "Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Frontier Research Program for Deep-Sea Extremophiles, Japan Marine Science & Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan Light propagation in composite two-dimensional particle arrays, in which fluorescent particles are involved as light sources, were examined. Detail discussion in tetragonal packing and observation of stacking-faults in tetra layers support the data in hexagonal close packing and face-coitered-cubic packing domains, which were discussed the straight -line propagation and diffraction patterns. 1.

Introduction Two-dimensionally (2D) ordered arrays, in which fine particles or protein molecules are densely packed in a hi^ly oriented fashion [1], have been extensively studied from the viewpoint of high-density optical storage media [2], adheskxi with glue that of the orientation of protein molecules [3], and that glass spacer strip of the preparation of other microstructures [4, 5]. Recently, the 2D array as optical devices, especially as a photonic crystal [6], is gathering much attention both from theoretical [7] and experimental [8] fields. When we first considered the 2D array for the study of photonic devices, we were interested in precisely how the photons propagate in the array. We had already reported the preparation of two-dimensional array, in which fluorescent particles are involved as light sources [9], and the examination how light propagates in the single layer and the triple layer, which reveled Fig. 1. Schematic diagram of that the Ught propagation makes us determine directly Dimitrov cell used to fabricate the stacking mode, e.g., face-centered-cubic packing the arrays. The thickenss of the (fee) or hexagonal close packing (hep), in up to three array was controlled by adjusting layers [10]. In this paper, we describe the expanding the distance between the cell and information of the staking mode: the light the substrate using the z-axis propagation in tetragonal packing and observation of stage. During preparation of the array, the substrate is translated stacking-faults in tetra layers. horizontally using the jc-axis 2. Experimental We prepared composite arrays from two types stage.

846 of fluorescent particles, i.e., red (XexA^m 541/612 nm; 1.01 ± 0.05 M^m; Polymer Microspheres, Red Fluorescing, Duke Scientific Corp.) and green (X«/X«n 458/540 nm; 0.973 ± 0.028 fim; Fluoresbrite carboxylate microspheres, Polysciences Inc.) with nonfluorescent particles (1.034 ± 0.020 Jim; Particle-Size Standards, NIST Traceable, Duke Scientific Corp.) such that the fluorescent particles were present at a relatively low fraction, i.e., 1:1:400 (red-green-nonfluorescent). Approximately 0.5 mL of the mixed suspension was used for the array fabrication. The technique for the preparation of 2D arrays originally proposed by Nagayama et al. makes use of water evaporation and lateral capillary forces to control the number of layers (Figure 1) [11]. The substrates were nonfluorescent glass slides ("Micro-Slide" glass, Matsunami Co., Japan). Well-ordered 2D arrays with typical dimensions of 18 x 25 mm were prepared. The 2D arrays were observed by use of optical microscopy and fluorescence microscopy (BX60-34-FLBD1, Olympus, Japan). The microscope light source was a Hg-Xe lamp (Olympus). For thefluorescencemicroscopy, an excitation filter (BP430-450, Olympus), which allows light of wavelengths from 420 to 450 nm to pass, was used. Also, an absorption filter (BA515F, Olympus), which allows light of wavelengths greater than 515 nm to pass, was used, and thus the fluorescence emission from both red- and green-fluorescing particles was detected. 3. Results and Discussions Even with different types of particles, the composite 2D arrays exhibited high density, highly oriented hexagonal packing. Although these arrays are composed of many polycrystalline domains, the sizes of the domains were large enough to observe the light propagation in each domain, as discussed below. 3.1. Light Propagation of Tetragonal Packed Structure. Generally, 2D array consists of closest packing structures. However, in the transition zone between two multilayers, one can always observe a tetragonal lattice (Fig. 2d) [12]. Figure 2 shows a series of detailed through-focus images for tetragonal packing domain. As the focal plane is raised, the observed patterns exhibit the following changes: in the bottom layer, the fluorescing particles show up as fuzzy circles (Fig. 2a). When the focal plane is raised to the second layer, the fuzzy circles changed to fuzzy squares composed of four bright spots, 5fim Top view which are in direct contact with the Straight liglit fluorescent particles in the first layer, propagation and are thus illuminated efficiently Focal plane , Fluorescent by it. The pattern observed when particle the focal plane is at the bottom edge The light propagation patterns of of the top layer is a square, each Fig. 2. vertex being composed of four tetragonal packed structure in a triple layer for smaller squares (Fig. 2b). By particle diameters of 1 |im. The fluorescent careful observations, one can images are changed with the focal plane (fp) from observe these smaller squares the bottom layer (a) to the top layer (c). The composed of three parts. The large optical microscopic image is also shown (d). part of each square arises due to the Corresponding schematic models are shown to the straight-line propagation, i.e., the bottom of thefluorescenceimages.

847

emitted light propagates within the bulk of the particle and between particles at the point of contact. Two small oval-shapes are caused by the diffraction of the light, which emitted from the fluorescent particle propagates through the surrounding air. As the focal plane is raised to the middle of the third layer, each small square divides into four parts, one large dot at each of the four vertexes of the big square, due to particles that are on a direct straight line from the fluorescent particles, and three small oval-shaped dots on the sides and center of the big square. Gradually the latter merges as the focal plane is raised. Finally, at the top of the layer, the small ovals from each small square have completely merged at the midpoint of each side and center of the big square, and we can observe a big square pattern of nine illuminated particles in the tetragonal domains (Fig. 2c). These light propagaticm phenomena in tetragonal packing support the data in hep and fee domains, which were also discussed the straight -line propagation and diffraction patterns [10]. 3.2. Stacking Fault in the Tetra Layer. L

twinning-plane C

Fig. 3. Detail depictmg uie light propagation patterns of face-centered closest packed structure in a tetra layer for particle diameters 1 |im, with (I, ABCB) and without (II, ABCA) twining plane. The fluorescence images show the focal plane (fp) shifting from the bottom layer (a) to the top layer. Corresponding schematic models are shown to the bottom of the flucx-escence images. The points of fp were shown as black arrows, and straight line propagations were shown as skew lines.

848 As usual as other periodic materials, two-dimensional arrays have also many kinds of defects; point defects, dislocations, and so on [12]. When the number of the layers becomes over tetra, stacking faults should not be ignored, hi tetra layers, four stacking mode can be concerned; ABAB (hep), ABAC (hep with a stacking fault), ABCA (fee), and ABCB (fee with a stacking fault). By using this through-focusing technique, all those four stacking structures were observed. Here we discuss the Hght propagaticm pattern in fee packing structure with and without a stacking fault, which is more attractive than hep packingfromthe photonic crystal's point of view [13, 14]. While the focal plane is raised from the bottom layer to the third layer, the observed patterns are same with (Fig. 3-Ia, lb) and without (Fig. 3-IIa, lib) stacking fault. The illuminated patterns differ with and without twining-plane when the focal plane is raised to the top (fourth) layer. Let us explain the light propagation with the stacking fault using the red pattern in Fig. 3-1. In this case, there is no particle on the straight-line propagation in the fourth layer. Thus, the origin of the observed patterns is mainly diffraction. When the focal plane is at the bottom of the fourth layer, the light diffracted at six spaces that positioned on the illuminate particles in the third layer and observed as "Y" shapes (Fig. 3-Ic). As the focal plane is raised to the top of the top (fourth) layer, the diffracted lights from each space were divided into three, respectively, and merged on 12 illuminated dots (Fig. 3-Id). The green pattern that illuminatedfroma green fluorescing particle on the second layer shows this domain includes hep structure (BCB). On the other hand, without the stacking fault, there are three particles on the straight-light propagation. The observed phenomena is almost same in tetragonal packing (discussed above). Finally, we can observe a big triangle composed of eight illuminated points at the top view (Fig. 3-IIe). Even there are many reports related with the multi layers of two-dimensional array, depending on our short knowledge, this might be the first report that shows the stacking faults in multi layers directly. 1 N.D. Denkov, O.D. Velev, P.A. Kralchevsky, LB. Ivanov, H. Yoshimura, and K. Nagayama, Nature, 361 (1993) 26. 2 R. Micheletto, H. Fukuda, M. Ohtsu, Langmuir 11 (1995) 3333. 3 K. Nagayama, S. Takeda, S. Endo, H. Yoshimura, Jpn. J. Appl. Phys., 34 (1995) 3947. 4 F. Burmeister, C. Shafle, T. Matthes, M. Bohmish, J. Boneberg, and P. Leiderer, Langmuir 13(1997)2983. 5 S. Matsushita, T. Miwa, and A. Fujishima, Chem. Lett., (1997) 925. 6 J.D. Joannopoulos, R.D. Meade, and J.N. Winn (eds.) Photonic Crystals, Princeton University, Press, New Jersey, 1995. 7 H. Miyazaki and K. Ohtaka, Phys. Rev. B, 58 (1998) 6920. 8 T. Fujimura, T. Itoh, A. Imada, R. Shimada, T. Koda, N. Chiba, H. Muramatsu, H. Miyzaki, and K. Ohtaka, J. of Luminescence, 87-89 (2000) 954. 9 S. Matsushita, T. Miwa, and A. Fujishima, Langmuir, 13 (1997) 2582. 10 S.I. Matsushita, Y. Yagi, T. Miwa, D.A. Tryk, T. Koda, and A. Fujishima, Langmuir, 16 (2000) 636. 11 A.S. Dimitrov, T. Miwa, and K. Nagayama, Langmuir, 15 (1999) 5257. 12 CD. Dushkin, G.S. Lazarov, S.N. Kotsev, H. Yoshimura, and K. Nagayama, Colloid Polym.Sci., 277 (1999) 914. 13 H. Miguez, A. Blanco, F. Meseguer, C. L6pez, Phys. Rev. B, 59 (1999) 1563. 14 i.i. Tarhan and G.H. Watson, Phys. Rev. Utt., 76 (1996) 315.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c) 2001 Elsevier Science B.V. All rights reserved.

849

Surface Force Measurement of Alumina Surfaces: Effect of Polyelectrolyte on the Dispersiveness of Aqueous Alumina Suspension Ryo Ishiguro, Osamu Sakurada, Keiichi Kameyama, Minoni Hashiba, Koichi Hiramatsu, and Yukio Nurishi Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifii 501-1193, JAPAN Surface forces between a-alumina surfaces were measured in aqueous solutions by colloidal probe atomic force microscopy, to elucidate the effects of presence of three sodium polyaciylate (PANa) preparations with different molecular weights (1200,6500, and 30000) on the electrostatic forces and steric forces. PANa30000 and 6500 formed the steric layer at the concentration higher than 10~^M, and their layertiiicknesseswere independent of the molecular weight, while PANal200 could not form such effective steric layer. On the other hand, electric double layer force decayed efficiently as increasing the PANa molecular weight. 1. INTRODUCTION The fluidity and dispersion of ceramic slurries are important factors in slip casting, a critical process of ceramic manufacturing. In order to develop these characteristics, we have employed various kinds of organic polymers as additives or dispersants. The effect of polyelectrolyte on fluidity and viscoelasticity of slurries has been systematically studied, and it has been discussed that electrostatic and steric repulsion between the organic polymer layers adsorbed on the ceramic particle surfaces should contribute to the stable dispersion of the slurries [1-3]. Recently, direct measurements of the interparticle repulsion andfrictionforces by the use of surface force apparatus or atomic force microscopy were applied to account for macroscopic properties of the suspensions [4-6]. In the present study, we measured directly the dispersion force between a-alumina surfaces in an aqueous solution of sodium polyacrylate (PANa) using colloidal probe atomic force microscopy, and characterized the electrostatic and steric forcesfromthe results, which depend on the concentration and the molecular weight of the polyelectrolyte.

850

2. EXPERIMENTAL Sodium polyacrylates (PANa) with weight-average molecular weights of 1200, 6500, and 30000 were purchasedfromAldrich. Alumina spheres of about 5^m in radius, were purchased from Admatechs, and sintered at ISOO^C. a-Alumina plates were prepared by shaping aqueous suspension of alumina particle (AKP-30, purchasedfromSumitomo Chemical, 0.3 jim in diameter) followed by sintering at ISOO'C. Atomic force microscope (SPA400; SEIKO instruments) was used to measure die forces between a-alumina surfaces, following the procedures by Ducker et al. [7] (Fig. 1). An a-alumina sphere is attached to the top of a rectangular cantilever probe of lOO^m length with the spring constant of 0.75N m-* (RC800PSA, Olympus) by epoxy resin (Epikotel004, Shell), and loaded on the a-alumina plate settled on the sample table to record the force-distance profiles. The origin of surface distance was chosen as the point where the cantilever deflection was linear with respect to the plate displacement at high force O^^i* ^ ^ lOmN/m), and any deformation of steric layer was not expected. Surface force measurements were carried out in aqueous lO'^M NaCl solutions at various pH's, adjusted by HCl and NaOH, or in PANa solutions of various concentrations at pH9 without additional salts, where PANa*s were expected to be fully ionized.

laser beam 4-partitioned photodetector cantilever a-alumina sphere (5^im in radius, R> suiface force, F distance. D_ C

M^~~~~~-^- a-alumina plate scanner table (piezoelectric device)

Fig. 1. Schematic drawing of the surface force measurement system employing the atomic force microscope. The alumina plate is driven by the piezoelectric device. Surface force, F, is given as the product of spring constant of cantilever and cantilever deflection, which is detected from the direction of the laser light reflected on the back of the cantilever. The laser light is detected by the 4-partitioned photo diode.

3. RESULTS AND DISCUSSION Surface forces measured between a-alumina surfaces in aqueous lO'^M NaCl solution exhibited electric double layer repulsion at pH3-4, but not at pH5-9 (data not shown). These are consistent with pH dependency of zeta potential of alumina suspensions. Addition of PANa to the bulk solution at a concentration 10-^M at pH9 caused repulsion (Fig. 2), which can be the electric

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10 20 30 40 0 10 20 30 40 Distance, D/ nm Fig. 2. The force-distance profiles between a-alumina surfaces in aqueous solutions of PANal200 (a), 6500 (b), and 30000 (c) at the concentrations, OM (solid circles), I^IO"^ M (open circles), 1 x 10'^M (open squares), 1 x 10-^M (open triangles), 1 x 1 O^'M (crosses), at pH9. The forces between the surface, F, are normalized by sphere radius to give F/R which is proportional to the interaction energy between flat surfaces per unit area, and plotted with surface distance, D. ^

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double layer and steric forces due to the charged polyaciylate adsorption to the alumina surfaces. The magnitude of the repulsive force observed in a very short range became stronger with an further increase of PANa concentration. This indicates the steric and charged layer growth by PANa adsorption. On the other hand, the distance range of the repulsive force became shorter with increase of the concentration, and this tendency was more pronounced with higher molecular weight PANa. These can be ascribed to the electrostatic screening offreePANa molecules. Fig. 3a shows the vertical stress, P as d(F/2nR)/dD, during surface approach in lO'^M PANa6500. In thisfigure,the distance dependency of the stress is observed to change sharply at 4.3nm. At this distance the facing two steric layers are expected to contact each other. Hence the half of this distance can be regarded as the steric layer thickness. Thus in the distance beyond this transition point, the electric double layer force is dominated, in which the decay length corresponding to the Debye length is determined to be 3.4nm (Fig. 3b). Fig. 4 shows the steric layer thickness and the decay length depending on PANa molar concentration. For PANa6500 and 30000, the steric layer was observed to grow with increase of the PANa concentration higher than 10-^M in a similar manner but not evidently for PANal200 of which molecular dimension is presumably not large enough to form appreciable steric layer even after adsorption. On the other hand, the electric double layer force decayed more effectively for higher molecular weight PANa, because the ionic atmosphere around the adsorbed PANa layer is primarily consisted of the aery late residues infreePANa and their counterions.

852

5 10 Distance, D/nm

15

Fig. 3. The stress-distance (a) and the forcedistance (b) profile between a-alumina surfaces in aqueous solution of I^IO'^M PANa6500, at pH9. The stress changes sharply at 4.3nni separation, beyond which the force decayed exponentially with 3.4nm decay length.

10'2 10-* 10*^ PANa Concentration, C/ M Fig. 4. PANa concentration dependencies of the steric layer thickness (a) and the decay length of electric double layer force (b) with respect to three molecular weights; circles, 1200; squares, 6500; triangles, 30000.

These results indicate clearly that the efficiency of PANa as a dispersant depends upon its molecular weight. Higher molecular weight PANa enhances short range repulsion, and suppresses the long range one simultaneously. The presence of appropriate molecular weight should, therefore, be expected, which totally leads the most effective interparticle repulsion. The results of the present study are qualitatively in agreement with thosefix>mthe studies of viscoelastic measurements of alumina dispersions with various molecular weights of PANa's. REFERENCES 1. H. Okamoto et al., J. Mater. Sci., 26 (1991) 383 2. M. Hashiba et al., J. Mater. Sci., 28 (1993) 4456 3. M. Itoh et al., J. Mater. Sci., 31 (1996) 3321 4. W.A. Ducker et al., J. Am. Ceram. Soc., 80 (1997) 575 5. Z. Xu et al, Langmuir, 12 (1996) 2263 6. H.G. Pedersen and L.Beigstrom, 82 (1999) 1137 7. W.A. Ducker et al., Langmuir, 8 (1992) 1831.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (0 2001 Elsevier Science B.V. All rights reserved.

853

Surface properties and photoactivity of silica prepared by surface modification M. Fuji^ N. Ma^uzuka^ T. Takei^ T. Watanabe^ M. Chikazawa^ K. Tanabe*' and K. Mitsuhashi** ^Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397 JAPAN* ^Research and Development Center, Nittetsu Mining Co. Ltd., 8-1 Hirai, Hinodechou, Nishitama-gun, Tokyo 190-0182 JAPAN Properties of sites introduced onto silica surface using surface modification with a monofunctional titanate-based coupling agent were investigated. The surface density of sites introduced onto the surface was quantitatively increased with the repeat times of modification. The photocatalysis appeared before formation of crystalline TiOi. The ability as photocatalyst was increase with the amount of sites. It was recognized that the appearance of photoactivity was related to the existence of Si-O-Ti on surface. 1. INTRODUCTION Solid-state photocatalysts such as titania has received attention recently. Two methods to obtain the thin layer of titania have been reported mainly. In the sol-gel method [1], titanium based alkoxide such as titanium iso-propoxide (Ti[(CH3)2CHO]4) is used. The other method involves reacting titanium tetrachloride (TiCU) with on the solid surface [2]. Photoactivity in either method, titania layer is attributed to a layer composed of small particles. In these cases, titania layer acts as not only the photocatalyst but also a shielding material. In other words, the ability as a photocatalyst might be lower than theoretically expected. Recently, a new method to obtain the photocatalyst by surface modification was proposed [3]. The surface layer made by this method can be controlled at an atomic scale. In this paper, the properties of silica surface prepared by surface modification with the monofunctional titanate-based coupling agent are investigated. The relationship between the state of introduced sites and the appearance of photoactivity are discussed.

854 2. EXPERIMENTAL 2.1. Surface modification The silica powder (Aerosil OX50) was obtained from Nippon Aerosil Co. Surface modification reagent (CH3)2CHOTi(OCOCi7H35)3 (Ken-react; KR-TTS) was supplied by Kenrich Petrochemicals Co. The surface modification is carried out by chemical reaction between surface hydroxyl and monofunctional titanate-based coupling reagent using reflux method for 1 hour in n-hexane solvent. The solution containing the sample powder was filtered out and washed with n-hexane, and then dried up under reduced pressure. Subsequently, The organic part of surfate modifier was oxidized at 400'C in an electric furnace under oxygen atmosphere. Finally, the sample was exposed to humidity of 85%rh for lOhr to hydoxylat the surface. 2.2. Characterization of sample Surface density of modifier was determined from specific surface area and weight loss of the samples with combustion of the modifier. The specific surface area was obtained by nitrogen adsorption measurement, and the weight loss was calculated from TG-DTA curves. Surface density of Ti was regarded to be the same as surface density of modifier. To investigate chemical bonding state of the introduced site. X-ray photoelectron spectroscopy was measured. 2.3. Evaluation of photoactivity Photoactivity was evaluated by degradation of Methylene Blue molecules in water. Ultraviolet irradiation to the suspension of 40mg powder in 25 ml Methylene Blue solution of 0.02mM was carried out using black light (6Wx2). The change in absorbance was measured by i il.8 UV-VIS spectroscopy. •^ pl.6 X ^^1.4 y 3. RESULTS AND DISCUSSION / .^1.2 7A y"^ 1 = 1 _ 0) » 3.1. Surface modification T3 y< From the results of surface <2 0.6 / modification using variety of b. So.4 yi concentrations, it was recognized that ^ l 0 2 the surface density of Ti saturated at -/ ' 1 ^ CM /^ about 0.35 nm"^ at a concentration of 0 1 2 3 4 above 0.1 mmol g'^ Therefore, the Repeat times surface density of about 0.35 nm'^ is Fig. I. The relationship between obtsiined for a modification procedure repeat times of modification at content with a reagent concentration of above of 0.3 mmol g'and total density of Ti 0.1 mmol g"*. By repeating surface introduced onto silica surface. modification at above 0.1 mmol g'^ \

855 surface density of Ti would be controlled quantitatively. Fig. 1 shows the surface density of Ti on the samples which were modified repeatedly at a concentration of 0.3 mmolg" . The surface density of Ti increased in proportion to the number of repetitions. It is noted that Ti sites introduced by monofunctional reagent do not chemically bond to each other. In other ward, they are independent at the first modification at least. This result indicate not only the surface density of Ti increase quantitatively by the repetition of surface modification but also the sites introduced after second modifications exists independently according to the same manner of the first modification. 3.2. Properties of modified surface /'\ Fig. 2 shows the results of XPS. The peak of about 537eV was assigned to Si-O-Si [4,5], new peak was observed at about 534eV as shoulder of 537eV. The peak at 537eV was shifted to lower energy around 536eV by generating new peak with the repetition of modification step. The shoulder peak around 534eV increased in proportion to the repetition of modification. This peak was assigned to Si-O-Ti, which is knovm to form by the modification. Moreover, it is confirmed that the amount of Si-O-Ti increased with the repetition of modification step.

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3.3. Photoactivity of modified silica 540 536 532 530 To investigate the photocatalytic Binding Energy /eV Fig. 2. X-ray photoelectron spectra ability of the modified silica, the of 01s of the modified samples. The decomposition of Methylene Blue was number is repeat times. measured by UV spectroscopy. Fig. 3 shows the change in absorbance of Methylene Blue against the irradiation time of ultraviolet ray. In this measurement, there are two reasons for the decrease of absorbance except for the decomposition caused by photocatalysis. The first one is the direct decomposition of Methylene Blue molecule by ultraviolet ray. To estimate the amount, the blank experiment without silica powder was measured. However, the amount was negligible small. The other reason is the decrease for adsorption of Methylene Blue molecules on the sample surface. The blank experiment was carried out in the dark to all samples. The decrease of absorbance for each sample was almost the same as the result for original silica sample. These results indicate that the decrease absorbance for

856 original silica in Fig. 3 is due to the adsorption on the silica surface. Further, this indicates that modified samples possess photocatalytic ability. Moreover, it is important that photocatalytic ability was clearly confirmed at the surface density of Ti 0.38 nm"^ by first modification. These states of Ti on the surface are not crystalline titania but individual sites because of using monofunctional reagent to modify 2 4 6 surface. This result may provide Irradiation time / hr. some clues to the mechanism of photocatalysis. From the modified Fig. 3. Degradation of Methylene Blue solution surface characteristics, the on the modified sample. photocatalytic ability can be O: Original, • : Repeat 1 (0.3 8nm'^), correlated with the generation of Si-O-Ti on the surface. It is A: Repeat 2 (0.82nm'^), • : Repeat 3 (1.33nm'^) assumed that the conversion of independent Si-O-Ti sites on silica surface to Si-0-Ti(OH)n groups may play an important role in photocatalysis. Overall, the appearance of the photoactivity appears to be related to the formation of independent Si-O-Ti sites on silica surface and the ability as photocatalyst is decided by the amount of Si-O-Ti sites. REFERENCES 1. H. Tamon, T. Sone, M. Mikami and M. Okazaki, J. Colloid Interface Sci., 188 (1977)493. 2. V. M. Gun'ko, V. I. Zarko, V. V. Turov, R. Leboda, E. Chibowski, L. Holysz, E .M. Pakhlov, E. F. Voronin, V. V. Dudnik and Y. I. Gomikov, J. Colloid Interface ScL, 198(1998).141. 3. M. Fuji, N. Maruzuka, T. Takei, T, Watanabe, M. Chikazawa, K. Tanabe and K. Mitsuhashi, J. Mater. Jpn., in press 4. J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Minnesota, 1995 5. Y. Hayashi and K. Matsumoto, J. Ceram. Soc. Jpn 100 (1992) 1038. ACKNOWLEDGEMENTS Authors thank Ajinomoto Co. Ltd. for providing the modification reagent and Nippon Aerosil Co. Ltd for supplying the silica powder.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (^c) 2001 Elsevier Science B.V. All rights reserved.

857

Stability for compressing adsorbed layers at solid-liquid interface by the AFM probe. Kan-no, Tsutomu; Fujii, Masatoshi; Kato, Tadashi Graduate school of Science, Tokyo Metropolitan University 1-1 Minamioosawa, Hachioji, Tokyo 192-0397, Japan The stability of an adsorbed layer for compressing at silica / alkyltrimethylammonium bromide aqueous solution is investigated by surface force measurement using atomic force microscope (AFM). Applied pressure that is needed to break the adsorbed layers and squeeze them out from the gap increases as carbon chain lengths of amphipathic molecules. The spherical silica probe needs more compressing force to squeeze the layers out than the pyramidal silicon nitride tip does. 1. Introduction Adsorption behavior of amphipathic molecules at solid-liquid interface has been investigated by surface force measurement using surface force apparatus (SEA). These studies show that a bilayer is formed at the interface over critical micelle concentration (cmc). SPA is also used to characterize an adsorbed layer: the surface potential, the thickness and others. Cationic surfactant is widely used as surface modification reagent for metal oxide. Atomic force microscope (AFM) is used to study adsorption behavior to metal oxide by imaging and employing the force curve measurement. AFM can apply higher pressure to the adsorbed layer than SFA does because of having smaller contact area. It is suitable to study the stability of an adsorbed layer for compressing by different shapes of AFM probes. The investigation for the stability of an adsorbed layer gives the properties of an intermolecular interaction between hydrocarbon chains. In this study, force curves for different carbon chain lengths of amphipathic molecules are measured. Applied force onto the flat plate by the AFM probes and adhesion force between surfaces are focused. Dependence of the stability of the film for carbon chain lengths is discussed.

858 2. Experimental Section 2-1. Sample Preparation Alkyltrimethylammonium bromide homologue, C„TAB (n= 12, 14, 16, 18) were obtained from TOKYO KASEI Inc. The cmcs of the aqueous solutions of C^TABs were 16mM, 3.5mM, 0.9mM, 0.3mM, respectively. The C„TABs were purified by recrystallization from distilled acetone. Silica surfaces were prepared by oxidizing the silicon wafer ((1 1 1) Shin-etsu Handotai). Water used here is passed through a MilliQ Labo system. The purified water had a conductivity of 18.3M Q cm'\ 2-2. Force Measurement Force measurements were performed using Nanoscope II AFM (Digital Instruments Inc., CA) with fluid cell. Two types of AFM probes were used: one was spherical silica (SiOj) probe (radius of curvature: 2.5^m) and the other was pyramidal silicon nitride (Si3N4) tip (radius of curvature: O.OT^im). Silica sphere was adhered to the tip of V-shaped silicon nitride cantilever with epoxy resin (Epikote 1004, Shell). The AFM probes were irradiated by oxygen plasma (9kV, 20mA) for 5 sec at 30 Pa atmosphere. SiOj planar surfaces were cleaned in SCI (NH3: HjOj: H20= 1: 1: 5, volume fraction) and SC2 (HCl: H2O2: HjOs 1:1:6, volume fraction) reagents just before measurement. All the reagents were EL grade without further purification. Oxide silicon plate was placed on the AFM stage and the fluid cell was set on it. At first pure water was injected into the fluid cell to clean it, then aqueous solution was injected. The concentrations of the aqueous solutions of QTABs for the measurement were selected over cmc: 19mM (C.^TAB), 3.5mM (C^JAB), 0.96mM (Ci^TAB), and 0.30mM (CigTAB). 3. Results & Discussion Force measurement by AFM can take two force curves; one is approaching force curve that is recorded decreasing the distances between the probe and the plate and the other is separating one. Fig. 1 (a) shows separating force curves, which are force separation curves between AFM probes and plates while the surfaces separate, in 3.5mM C14TAB aqueous solution for two different probe sizes and different applied forces to the surfaces. The applied force corresponds to a maximum force applied to

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Si02 probe (2.5nm)

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the surface by AFM probe as the separation between surfaces is zero. Adhesion force between two surfaces corresponds to a maximum attraction force when contacting surfaces separate each other. When applied force is 14nN for sphere probe (bold solid line), and 5nN for pyramidal tip (bold dotted line), separating force curves have same profiles as those of approaching and there is no adhesion force under the applied force region. This indicates that there is no fusion that caused the capillary force between surfaces. When applied force is llOnN for sphere probe (open circle), and 22nN for pyramidal tip (open inverse triangle), separating force curves show adhesion force is 210nN and 13nN, respectively. Fig. 1 (b) is zoomed out Fig. 1 (a). The adhesion force is the capillary force caused by the adsorbed layers on the surfaces fusing each other. Applying high pressure to the surface, the probe breaks and squeezes the adsorbed layers out from the gap. Then the two surfaces jump into contact and the adsorbed layers fuse together. We adopt the adhesion force as an indication of fusion. To decide a threshold applied force, F„„, which can be determined

^^^'

as the value of applied force when the

200 h

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adhesion force suddenly increase, we measure the adhesion force, F^, as a

150

function of applied force, F ^ , in 3.5mM C14TAB aqueous

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1000 100 Fapp / n N Fig.2 Adhesion force. Fad, as a function of applied force, Fapp, in C14TAB aqueous solution (3.5 mM). (O) Si02 probe (2.5nm), ( T ) Si3N4 tip (O.OTum).

1

10

860 15 Table 1 Fmax and Fad in CnTAB (n= 12,14,16,18) using Si02(2.5nm) and Si3N4(0.07^mi) a. 10 I AFM probes. Si02 (2.5 Mm)

Si3N4 (0.07 ^lIn)]

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Carbon chain lengths Fig.3 Compressing pressure, Pmax, and adhesion energy, y, for sphere probe as a function of carbon chain length.

Table 1 shows F^^x and F^ values as a function of carbon chain lengths of amphipathic molecules for different probe sizes. F^,, increases as carbon chain lengths. The spherical SiOz probe needs more compressing force, F„„, to squeeze out the layers than the pyramidal Si3N4 tip does because of the larger contact area between surfaces. We can estimate a mean pressure, P,pp, between the surfaces dividing applied force, Fapp, by contact area. The contact area calculated from the applied force and elastic modulus of fused quartz IXlO^Nm"^ according to the Hertz theory [1-2]. The adhesion energy, y, was estimated from adhesion force, F^ [3]. Fig.3 shows Pn,ax ^^^ Y as a function of carbon chain length for the sphere probe. Both P„„ and y show larger values at carbon chain length 16 than these of 12 and 14. This shows that amphipathic molecules having carbon chain length 16 has large cohesive interaction compared with that having 12 or 14 has. The difference of the cohesive interaction between carbon chain lengths was observed by surface force measurement by AFM. References 1. Helm, C. A.; Israelachvili, J. N.; McGuiggan, P. M. Biochemistry, 31 (1992) 1794 2. Parker, J. L. and Attard, P. /. Phys. Chem., 96 (1992) 10398 3. Rutland, M. W. and Senden, T. J. Langmuir, 9 (1993) 412

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

861

Analysis of Bonding Nature being Operative in the Af(Li, Na, K) Ionexchanged Zeolites-CO Adsorption Systems Ryotaro Kumashiro,^ Yasushige Kuroda,** Hisayoshi Kobayashi,^ and Mahiko Nagao* ^Research Laboratory for Surface Science and ^Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700-8530, Japan ^College of Science and Industrial Technology, Kurashiki University of Science and the Arts, Tsurajima, Kurashiki 712-8505, Japan The nature of the interaction of alkali-metal ion-exchanged ZSM-5 zeolite (A/ZSM-5; M= Li'',Na"^, and K"") with carbon monoxide (CO) molecules was investigated by the adsorption calorimetry and IR spectroscopy, in comparison with the results obtained by the density functional (DF) calculation. For the A/ZSM-5-C0 systems, a major IR band due to the stretching vibration of CO molecules adsorbed could be resolved into two components. Corresponding to them, two types of adsorbed species with different heats of adsorption were suggested to exist. A linear relationship between the heat of adsorption and the wavenumber of IR band can be established for the A/ZSM-5-C0 systems, as for the case of copper ion-exchanged ZSM-5 (CuZSM-5)-C0 system, though its slope and position are different from the latter case. Taking into consideration that the o-bonding is operative in the latter system, it is assumed that in the A^SM-5-CO systems the interaction is of predominantly electrostatic. On the basis of DF theory, the optimized structures for the CO species adsorbed on the two types of alkali-metal ion-exchanged sites were obtained. From these results, it was found that the vibrational frequency and the bonding energy for the CO species adsorbed on the two-coordinated site are higher than those on the three-coordinated site. Such models explained the experimental data satisfactorily. 1. INTRODUCTION Metal cations exchanged in zeolite are known to play as catalytic center for various reactions in gas phase. Therefore, it is important that the elucidation of the bonding nature between the metal cation and the gas molecule. We have already reported that the combination of adsorption calorimetry and IR spectroscopy provides important information on the bonding nature of Cu^-CO [1,2]. This approach is very useful for the analysis of the bonding nature between the metal cation and the gas molecule in zeolite, and is applicable to various systems. In the present study, we intend to confirm the usefulness of such an approach in the analysis of the bonding nature between the alkali-metal ions and adsorbed CO molecules in A^SM-5, in comparison with the results obtained by the DF calculation. 2. EXPERIMENTAL A sodium type of ZSM-5 zeolite (Si/Al ratio = 11.9) supplied from Tosoh Co. was used as the starting material. Ion-exchange by using NH4CI solution was carried out to obtain the proton exchanged ZSM-5 sample (HZSM-5). HZSM-5 was ion-exchanged in an aqueous solution (0.3 M) of the corresponding alkali-metal nitrates at 363 K. The alkali-metal ion-

862 exchanged ZSM-5 samples, hereafter, are designated as MZSM-S-X, where A/and A'denote the kind of alkah-metal ion and the exchanging capacity in percentage, respectively. For the measurement of IR spectra, a self-supporting disk prepared by compressing iWZSM-5 was placed in an in situ cell. The sample was first treated at 873 K under a reduced pressure of 1 mPa and then treated with CO gas of given pressures at 301 K. The spectra were recorded at 301 K on a Mattson 3020 spectrophotometer. Simultaneous measurements of the adsorption heat and the adsorption isotherm of CO were performed at 301 K using an adiabatic-type adsorption calorimeter. The sample was treated at 873 K under a reduced pressure of 1 mPa. The gradient corrected density functional (DF) as well as the hybrid functional method were employed in the present calculation. 3. RESULTS AND DISCUSSION Figure 1 shows the IR spectra for A^SM-A' in equilibrium with CO gas of different pressures. Their curve-fitted spectra are also involved. For all MZSM-X, a major band attributable to the stretching vibration of adsorbed CO molecules is observed at the higher wavenumber compared with that for the free CO gas molecule (2143 cm''). It is well-known that such a major band can be separated into two bands, which indicates the presence of two types of alkali-metal ions in AfZSM-5 that have different interaction energies with CO molecule. The major band observed here can be separated into two bands: 2195 and 2185 cm"' for LiZSM-5-74, 2177 and 2162 cm"' for NaZSM-5-66, and 2162 and 2147 cm*' for KZSM5-68, respectively.

2300 2200 2100 2300 2200 2100 2300 2200 2100 2000 Wavenumber / cm' Figure 1. IR spectra for MZSM-X pretreated in the following manner: 1, evacuated at 873 K; 2, equilibrated with CO at a pressure of 1.42 kPa; 3, 3.96 kPa; 4, 13.7 kPa; 5, evacuated at 873 K; 6, equilibrated with CO at a pressure of 1.32 kPa; 7, 3.96 kPa; 8, 14.6 kPa; 9, evacuated at 873 K; 10, equilibrated with CO at a pressure of 1.38 kPa; 11, 3.94 kPa; 12, 13.9 kPa. A, LiZSM-5-74; B, NaZSM-5-66; C, KZSM-5-68.

863 Figure 2 shows the adsorption isotherms and the differential heats of adsorption (^diff) of CO at 301 K for MZSM-X. The ^diff for LiZSM-5-74 gives ca 80 kJ mol'^ at the initial stage of adsorption and decreases to ca. 55 kJ mo^^ and then with increasing adsorbed amount it shows a monotonous decrease to ca. 35 kJ mol''. Taking account of the IR data, it can be considered that the region of 55-35 kJ mol*^ corresponds to CO adsorption on the lithium ions exchanged in ZSM-5 zeolite. Here, by the detailed observation for all A^SM-5 we tried to combine the adsorption heats with the IR band for CO adsorption; for LiZSM-5-74 the respective bands at 2195 and 2185 cm"' correspond to the heat values of 49 and 39 kJ mor\ and for NaZSM-5-66 the respective bands at 2177 and 2162 cm'^ correspond to the heat values of 33 and 29 kJ mol'', and for KZSM-5-68 the bands at 2162 and 2147 cm** correspond to the heat values of 28 and 21 kJ mol'^ respectively. Figure 3 shows the relationship between the ^diff and the wavenumber of IR band (Vco) for the A^SM-5-CO systems, together with the previous resuh for the CuZSM-5-CO system [1]. As can be seen from this figure a linear relationship is established in the A/ZSM-5-C0 systems. If the same type of interaction takes place in the CO adsorption systems, the change of the energy of cationCO bond formation gives an influence on the strength of C - 0 bond in a similar manner, and hence the ^diff~v^co relationship becomes linear. From these considerations, it is suggested that the same type of interaction takes place in the A^SM-5~C0 systems. It is also apparent that the slope and position of straight line are different between the CuZSM-5-CO and A^SM-5-CO systems; in particular the plots for the latter system are situated in the lower ^diff range than those for the former system. These arise from the difference in the nature of bonding, and it is also indicated that the strength of interaction is weaker in the A^SM-5-CO systems than in the CuZSM-5-CO system. Zecchina et al. have proposed that the CO molecules interact electrostatically with the alkali-metal ions

0

2 4 6 8 10 12 V,,, I cm3(S.T.P.)g-»

Figure 2. Adsorption isotherms and differential heats of adsorption of CO at 301 K for A^SM-A'pretreated at 873 K. (•) LiZSM-5-74; (O) NaZSM-5-66; (A) KZSM-5-68.

2100

2150 2200 V I cm-^

2250

Figure 3. Relationship between ^diff and Vco for the A^SM-5-CO systems: •, LiZSM-5-74; O, NaZSM-5-66; A, KZSM-5-68. The result for the CuZSM-5-CO system (A) [1] is also given in the figure.

864 exchanged in ZSM-5 zeolite [3]. In our previous study, we have concluded that the bonding between the Cu"" species in CuZSM-5 and the CO molecule is a-bonding in nature [1]. Taking account of these results, it can be considered that in the A/ZSM-5-C0 systems the interaction is of predominantly electrostatic with a negligible a-character. We have already proposed the existence of two dominant types of exchangeable sites for copper ions in the CuZSM-5 zeolite; one is assigned to the bridged-type in which the copper ion makes the bonding to two lattice oxygen atoms, and the other assigned to the pseudo trigonal (distorted from plane) type with three lattice oxygen atoms [4, 5]. On the basis of DF theory, the structures for the adsorbed CO Figure 4. Results of DF calculation for species on the two types of sodium ion- (a) (OH)2Al(OH)2NaCO and (b) exchanged sites were obtained. Figure 4 shows (0H)Al(0H)3NaC0. the resultant optimized structures. The vibrational frequency of C-0 and the bonding energy of Na-C were estimated to be 2258 cm'' and 32.5 kJ mol'^ for (OH)2Al(OH)2NaCO, and 2254 cm"^ and 30.6 kJ mol*' for (0H)Al(0H)3NaC0, respectively. From these results, it is found that both vibrational frequency and bonding energy for adsorbed CO species are higher on the two-coordinated sites than on the three-coordinated sites. The experimental data, which indicate the presence of two types of alkah-metal ions differing in the ^diff and the Vco for the adsorbed CO species, can be explained satisfactorily by these models. 4. CONCLUSIONS The usefulness of combination of adsorption calorimetry and IR spectroscopy was confirmed in the analysis of the bonding nature between the alkali-metal ions in A/ZSM-5 and the CO molecules. By the detailed analysis of the heats of adsorption and the IR spectra, the linear relationship between ^diff and Vco, which are derived from the adsorbed CO species on the two types of alkali-metal ions, was obtained. This fact confirmed that the bonding nature in the A/ZSM-5-C0 systems is of electrostatic. Such an interpretation is also supported by the DF calculation. REFERENCES 1. Y. Kuroda, Y. Yoshikawa, R. Kumashiro, and M. Nagao, J. Phys. Chem. B, 101 (1997) 6497. 2. Y. Kuroda, T. Mori, Y. Yoshikawa, S. Kittaka, R. Kumashiro, and M. Nagao, Phys. Chem. Chem. Phys., 1 (1999)3807. 3. A. Zecchina, S. Bordiga, C. Lamberti, G. Spoto, L. Camelli, and C. Otero Arean, J. Phys. Chem., 98 (1994) 9577. 4. R. Kumashiro, Y. Kuroda, and M. Nagao, J. Phys. Chem. B, 103 (1999) 89. 5. Y. Kuroda, Y. Yoshikawa, S. Emura, R. Kumashiro, and M. Nagao, J. Phys. Chem. B, 103 (1999)2155.

865

Morphology of Octadecyltrimethylammonium Halides Aggregates Adsorbed on Mica Masatoshi FUJII, Takenobu HASEGAWA, Tadashi KATO Graduate School of Science, Tokyo Metropolitan University, 1-1 Minamioosawa, Hachioji, JAPAN, 192-0397 Fax: +81-426-77-2525, E-mail: [email protected] The morphology of the aggregates, which formed on cleaved mica dipped in an aqueous solution of octadecyltrimethylammonium halides, C18TAX, was observed by AFM. The surface covered with nonuniform precipitate of projections and constant height of islands. The height of the island, 1 nm, was smaller than the length of the molecule. The islands consist of regularly arrayed molecules tilted on the surface.

1. Introduction Amphipathic molecules form various shapes of aggregates at a solid/Uquid interface similar to that in a bulk solution. Potassium ions on a cleaved mica surface are exchanged for solute cations in an aqueous solution of ionic amphipathic molecules [1]. Thus, the aggregates are anchored on the mica surface by ionic bonds and change the form by the strong intermolecular interaction. In this study, we investigate the morphology of adsorbed assembles of octadecyltrimetylammoniimi hahdes on cleaved mica after prolonged period of dipped in the aqueous solution below the Kraffl; point observed by AFM; the phase t r a n s i t i o n t e m p e r a t u r e of the gel to m i c e l l a r s o l u t i o n of octadecyltrimetylammonium bromide and that of octadecyltrimetylammonium chloride is 37.7 °C and 16.0 °C, respectively [2]. The surface morphology reflects the shapes not only for the surface aggregates, hemimicelle and admicelle, but also for a gel phase structure. Variations of the morphology under the conditions of rinse process were also studied.

866

2. Experimental section Octadecyltrimethylammonium halides (C18TAX; X= CI, Br, Tokyo Kasei Kogyo Co., Ltd., Japan) were used as received. Waterfroma Milli-Q Lab was used in the preparation of the aqueous solution of C18TAX. Mica was dipped and cleaved in the solution to prevent the siu^ce contamination. After specific times, the mica was retracted and rinsed with warm MiUipore water (50 °C) for 5,10, or 20 min and dried with a dry nitrogen stream. The samples were imaged with NanoScope-II AFM (Digital Instruments, USA) in atomospheric condition. All images were acquired in contact mode using a silicon nitride tip with 0.58 N/m cantilever and either a wide range, 14000 nm, or a narrow range, 700 nm, scanner. The height scales of the scanners were calibrated by a step height of cleaved mica. To clarify the reproducibility of the surface morphology, we observed AFM images at least two macroscopically separated regions on one sample and repeated that for several different samples. The wettability of the surface was estimated by a measuring contact angle of 2 - 5 ^1 water droplets placed on the surface in a humidified environment at 25 °C.

3. Results and discussion Typical images of surfaces dipped in 1x10*^ M aqueous solution of C18TAB at 5°C for 30 h are shown in Fig. 1. The image size is 5000 nm x 5000 nm in width and 3.75 nm in height, except Fig. la: z=10 nm. The sectional profiles at the middle of the images are also hsted. The image of the surface without rinse is shown in Fig. la. The surface is covered with many projections about 200 nm in width and from 1 nm to 5 nm in height; the heights of the projections are randomly distributed. Figure lb shows the surface rinsed with warm Millipore water for 5 min. The projections shown in Fig. la are diminished but islands having regular height appear on a flat base plane. The shape and the width of the islands are randomly distributed but the height is almost 1 nm. Each island has flat terrace whose edge is steep. After rinsed for 10 min. Fig. Ic, the surface became smooth but small numbers of particles remained. The gentle imdulation of the image is caused by bend of the basal mica attached to the sample stage of AFM. Finally, the 20 min rinsed surface shows fine structures. Fig. Id. The surface consists of many tiny particular structure about 100 nm in diameter. A hole exists at the center of left; the depth and the width are 1.5 nm and 250 nm, respectively.

867

1

2 3 w/^m

2 ,

3

Fig. 1. Series contact mode AFM images showing the surface topography of mica dipped in 1 x 10* M aqueous solution of C18TAB at 5 °C for 70 h as a duration of rinsing with water at 50 *'C. a) without rinse, b) rinse for 5 min, c) rinse for 10 min d) rinse for 20 min. The image size for XY: 5000 nm x 5000 nm, and Z: 10 nm for a) and 3.75 nm for others. Corresponded sectional profiles at the center of the images are listed blows.

The projections on the siirface without rinse have various sizes and shapes. After 5 min rinse, they are completed washed out. The mica without rinse is covered with not only the adsorbate but also the precipitate of C18TAB. Thus, the projections, which desorbed easily, are the precipitates of C18TAB attached on the surface and are not formed by regularly assembled molecules on the mica surface. After washed out the projections, the islands appeared on the surface. The formation of the island structure has already been observed for the mica and C18TAB system by both in-situ and ex-situ AFM [3 - 5]. In both cases, small islands appear at an initial stage, then they grow only lateral direction. The islands also have flat terrace on the top, and the height observed by ex-situ AFM is 1.7 ± 0.3 nm [4]. The height of the islands in this study is lower than that for reported. Comparing with the molecular length of C18TAB, 2.5 nm, the height of the island is less than the half of the molecular length. The molecules in the islands attached to the surface with considerably tilted. The thickness of the flat layer under the island is estimated to 1.5 nm, because the depth of the hole in the images of the surface rinsed for 20 min, Fig. Id, shows 1.5 nm. Since the base layer has the same thickness as that of the island reported [4], the layer consists of arrayed structure of molecular assembly as that of the islands.

868

An enlarged image, 700 nm, of the surface rinsed for 20 min shows flat a surface with tiny holes randomly distributed (Fig. 2a). The particular structure in Fig. Id reflects the tiny holes that cannot be distinguished in the wide image size. Figure 2b is an atomic-scale image, 10 nm, at the flat area of the same surface. Although the quality of the image is much poor, the regularly arrayed spots can be observed. The base layer, thus, consists of regularly packing molecules with some defective areas. The hydrophobisity of the surface was tested by a contact angle measurement of water. The time dependence of the contact angle is shown in Fig. 3. The contact angle for the surface without rinse shows a constant value of 80°. While for the surfaces rinsed with water, the contact angle decreases with the elapsed time of the observation. Since the surface without rinse has been experienced in the solution for long duration at low temperature, the molecules assembled to a stable structure in the aqueous solution, ie, hydrated soUd. Therefore, the adsorbate does not easily desorb to the water droplet, and the angle shows stable value. The decreasing of the contact angle for the rinsed surface shows that the surface is not washed completely with water. The remaining molecules, which do not bind to the ion-exchangeable sites, gradually desorbed from the surface make the angle decreases. 0

References

Fig. 2. AFM images of the surface rinsed for 20 min, a) 700nm x 700 nm, b) 10 nm X IQ nm scale.

20

40 60 t/min Fig. 3. Time dependence of contact angle of water. O without rinse, Arinsefor 5 min, # 10 min, and • 20 min.

1. Y. L. Chen, S. Chen, C. Frank, J. IsraelachiviU, J. Colloid Interface Sci., 153 (1992) 244. 2. M. Kodama, K. Tsujii, S. Seki, J. Phys. Chem., 94 (1990) 815. 3. M. Fujii, B. Li, K. Fukada, T. Kato, T. Seimiya, Langmuir, 15 (1999) 3689. 4. W. A. Hayes, D. K. Schwartz, Langmuir, 14 (1998) 5913. 5. T. Hasegawa, M. Fujii, J. Surface Sci. Soc. Jpn„ 21 (2000) 468.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) CO 2001 Elsevier Science B.V. All rights reserved.

869

Photoinduced long-range attraction between spiropyran monolayers studied by surface forces measurement Yasuhiro Nakai and Kazue Kurihara'*' Institute for Cbemcai Reaction Science, Tohdcu University, Katahira, Aoba-ku, Sendai 980-8577, Japan Photoinduced changes in surfece interactions were studied using surface forces measurement. Surfeces were modified by the Langmuir-Blodgett (LB) deposition of a mixed monolaya: of a ^iropyran derivative (SP1822) and n-octadecane (C18). The LB film exhibited a reversible change in its structure between the SP form (neutral closed fonn) and the PMC form (zwitterionic open form) upon irradiation by ligjit at specific wavelengths, whidi were confirmed by UV-VIS spectroscopy. A long-range attraction extending to ca 100 nm was observed for the surfaces containing the PMC forms, wiiile an interaction between the SP forms was rqxilsive in the long-range and attractive at distances less than 20 nm. 1. INTOODUCnON Development of a sur&ce force apparatus (SFA) and an atomic force microscq)e (AFM) has lJ^7y=^'^^^^^^^ ^ CH2OCOC21H43 enabled us to directly study CH2OCOC21H43 interactions between the ^. ,^^f°™ , ^o^.«^^ _^ , . Fig. LPhotochromismofSP1822. surfeces bearing vanous fimctional groiq)s [1,2]. The measurement has been successfiilly utilized to probe the van da: Waals, DLVO and stoic forces, etc., leading to a more ccxicrete understanding for controlling surface interactions and {^operties. An attractive way of controlling the interaction should be photoswitdiing, whidi is recently used to reversibly change surface prq)erties such as the morphology [2] and the contact angle [3] of LB films. In this study, we directly demonstrated, for the first time, the photoinduced switching of the surface interactions using the surfeces modified with ^iropyran monolayers whidi could exhibit the {rfiotochromism as shown in Figure 1, and found a long-range attraction between the PMC foratis. Importance of this s^jproadi in elucidating origins of surface interactions is also discussed. * To whom correspondence should be addressed.

870 2. EXPERIMENTAL SECTION 2.1. Preparation of SP1822-C18 mixed monolayers An amphiphile bearing the spiropyrm grcxq) as a hydrophilic head (SP1822) was purchased fixxn Nif^xxi Kanko Shikiso and used as delivered as a mixture with n-octadecane (C18) [4]. The surface pressure-area (7C-A) isotherm measurement and the LB dqx)sition were performed using a computer controlled film balance (FSD 50) and a Ufter(FSD 21) system (USI system) at 20.0 ± 0.1 t : in the dark The solvent used for spreading the SP1822-C18 mixed monolaya* (1 : 2 molar ratio) was a mixture of dichloromethane and benzene ( 1 : 1 volume ratio). The monolayer was spreod over the pure water subphase and compressed at a rate of 0.3 cmVs after incubati(Mi for 10 miiL The monolayer formed a stable liquid-condensed phase as previously rqxwted [4]. It was transfored at a dipping speed of 3 mm/min in the down-stroke mode onto solid substrates, mica (for SFA) and quartz (for UVVIS) plates, which were rendered hydrophobic by the LB deposition of dioctadecyldimethylanmionium bromide [5]. The deposition pressures of the mixed monolayer were 30 mN/m and 23 mN/m, and the transfer ratios were 0.95 ± 0.02 and 1.00 ± 0.03 for quartz slides and mica sheets, re^)ectively. 2.2. Photoreaction of SP1822-C18 mixed monolayers The SP1822-C18 mixed monolayers deposited on quartz plates wCTe irradiated in water with a 500 W xenon lamp (SX-UI500XQ, Ushio) through aipx)priate filters, i. e., BP-36 (Kenko, 350--370 nm (UV) to form PMC) or G-56 (Toshiba, 540--600 nm (VIS) to form SP). Formati(Mi of the SP and Xenon lamp PMC form was monitored by a UVVIS spectrophotometer (U-3300, Hitadii). 2 3 . Surface forces measurement

Distance |F=

k AD (AD : Deflection of the spring) Motor FECX) fringes Water

Double cantilever spring I Filter Halogen lamp ' (FECO observation light souroe)

Forces measuronoits were carried out using a sur&ce forces Fig. 2. A schematic drawing of surface forces apjpardim (NL-SFOOl, Nippon Laser measurement under photoirradiation. & Electronics Lab.). The jM'ocedure was similar to the one previously employed for LB surfaces [5]. The surface sqiaration D was measured using the fringes of equal diromatic order (FECO). The force F was determined frwn deflection of a double cantilever spring. The measured fcMce normalized by the mean radius R of the cyliiKirical surfeces is proportiorial to thefreeenergy (Gf) of interaction betweoi flat surfaces: F/R=2 7C Of [1]. The system to paform the measurement under jrfiotoillumination is shown in Figure 2. The monitoring light (200 W halogai LS-150F, Sumita) was filtered by 25)propriate filters to prevent overl^jping with the reacticm light; the light longer than 470 nm was used to study the SP form, and 430 nm'^470 nm was used for the PMC form.

871

3. RESULTS AND DISCUSSION 3.L Photochromic behavior of SPI822-C18 mixed monolayers First, we examined the changes in the UV-VIS spoctnm of a SP1822-C18 mixed monolayer under I*otoillumination to establish the conditions for the surface forces measurement (Figure 3). Before the measurement, the monolayer was irradiated with VIS li^t for 30 min to convert any existing PMC ^jeciestotheSPform. This pretreated monolayer exhibited a peak at 340 nm ascribed to the SPfomi. Upon inadiation by UV l i ^ a new peak aj^jeared at 580 nm and increased in intensity, vMdti was accompanied by a red-shift of the 340 nm peak (Figure 3a). This result was in good agreement with the characteristics rqx)rted for the PMC formation [4]. The peak intensity at 580 nm reached saturation, ca. 3 0 % of thefiillconversion, after 180 min and did not change up to 3 60 min. Here, the li^t intensity was chosen to maintain the reversibility of the photodiromic reaction This intensity was also applied to the SFA measurement. The 580 nm peak disappeared when the monolayer was inadiated by VIS ligjit for 120 min (Figure 3b). Further illumination did not alter the spectrum, which was idaitical with the initial one. Therefore , we carried out SFA measurements during the UV irradiaticm of 180^360 min for the PMC fcxm, and the VIS irradiation of 120'-'180 min for the SP form. 32. Surface forces measurement 0.008 A force profile measured b^ween the SP a) : PMC form (UV irradiation) b) : SP form (VIS inadiation) 0.006 monolayers (SP form) in pure water (pH-5.6) is shown m Figure 4. A weak repulsion appeared at ca 100 nm, e^qjonentially increased with the decreasing distance (decay length = 26 ± 1 nm), and suddenly changed to attraction at ca. 20 nm where the surfecejumped into contact Thepull-oflF -0.002 500 600 700 800 300 400 (adhesive) force was 229.7 ± 24.6 mN/m, which Wavelength (nm) was in the ejq)ected range for hydrophobic surfeces Fig. 3. Absorption spectrum of a in pure water [6]; the contact angle of water on the SP1822 : CIS mixed monolayer in water SP monolayer was 90.3 ± 2.9 ** . The addition after photoirradiation : a) by UV for 180 of a salt (KBr, 0.1 and 1.0 mM) did not change the min to 360 min ; b) by VIS for more than decay length of the repulsion, thus the rqjulsion is 120 min. This change was reversible. not a pure electric double layer force [1][7]. UV irradiation of the SP form producestfiePMC form and dramatically dianged the interaction. A long-raige attraction appeared at 100 nm, ejqxMientially increased with a decay length of 37 ± 2 nm, and jumped into contact at 20 nm. The pull-oflf force was 174.2 ± 24.0 mN/m, a value lower than that oftfieSPfoim due to the zwitterionic nature which exhibited the contact angle of water of 85.7 ± 0.5 ** . The interacticMi retumed to the initial one vvtei the PMC monolayer was subsequently illuminated by visible light, confirming that the diange was reversible and reproducible. These observaticMis must berelatedto the photocromic changefi-omthe neutral, hydrophobic SP fonn to the zwitterionic, less hydrc^Mic PMC form. It is reasonable that the surface of lower hydrojAiobicity exhibits a smaller pull-oflf force. We think that the long-range attraction that operated

872

between the PMC surfaces could be 0.6 Jump in A • SP form attributed to the interaction between the X • PMC form dipoles of the zwitterions on the surfaces. A o Reproduced SP The long-range nature of the attraction may be brought about by the macroscq)ic size of the interaction units (size of surfaces and/or domain-like distribution of dipoles). The latter medianism was suggested by Womerstrom et al. [8] in order to interpret the long-range attraction y Jump out found between hydrophobic surfaces. r Jump out For the rq)ulsion between the SP surfaces, -240' 200 we can not identity the origin at this 160 0 40 80 120 moment The double layer rq)ulsion due Distance (nm) to adsorbed ions is one possibility, Fig. 4. Surface interactions between SP1822-C18 mixed monolayers under photoirradiation. however, the decay length is mudi shcoter than the Debye length. Further investigation is necessary to examine other possibilities as well as to clarify whether the double layer force overlapping witii a long-range (-^100 run) attraction, often found betweai hydrophobic surfaces [6], can exhibit the observed shorter decay length. 4. CONCLUSION This woik directly demonstrated the photo-induced switdiing of surface interactions. We found tiie long-range attraction extending to 100 nm between surfaces bearing the zwitterionic groiq>s, wiiich could serve as dipoles. Elucidation of the origins of the long-range attractions observed between hydnc^obic and/or electrically neutral surfaces have become an important issue in recent years, but cxie of the diflBculties involved in this study is identification of flie diflFerences in the surface diemical structures and morphologies. Our approach should provide a useful and novel means to study such interactions in liquids, because we can diange the diemical structures of the surface spedcs in a welldefined manner. REFERENCES 1) J. N. Israeladivili, Intermoleadar and Surface Forces (2 nd). 2) S. Terrettaz, R Tadiibana, M. Matsumoto, Langmuir, 14,7511(1998). 3) K- Idiimura, S. K- Oh, M. Nakagawa, Science, 288,1624(2000). 4) E. Ando, J. Miyazaki, K. Mwimoto, Thin Solid Films, 133,21(1985). 5) T. Abe, N. Higashi, M Niwa, K. Kurihara, Langmuir, 22,7725(1999). 6)K.Kurihara,T.Kunitake,^.4m. ChemSoa, 114,10927(1992). 7) J. N. Israeladivili, G. E. Adams, J. Chem. Soa Faraday Trans. 1,74,975(1978). 8) Y. H. Tsao, D. F. Evans, H. Wenneretrdm, Science, 262,547(1993).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) © 2001 Elsevier Science B.V. All rights reserved.

873

Solid Monolayers of Simple Alkyl Molecules Adsorbed from their Liquid to Graphite: the Influence of Different Chemical Groups. Miguel A. Cast^o^ Stuart M. Clarke*''*, Akira Inaba^ Ana Perdigon^ Amber Prestidge^ and Robert K. Thomas^ ^Institute de Ciencia de Materiales de Sevilla, Avda. Americo Vespucio, Sevilla, Spain. *^P Institute, Department of Chemistry, University of Cambridge, UK. ^Department of Chemistry and Research Centre for Molecular Thermodynamics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan. "^Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford. •Address correspondence to this author

Sensitive calorimetry has been used to demonstrate the formation of solid monolayers adsorbed from liquid alk-1-enes, phenyl-alkanes and fluoro-alkanes on to graphite as a means of investigating the influence of different chemical groups on solid monolayer formation. We highlight that unsaturation of alk-1-enes and phenyl-alkanes destabilises solid monolayers. Linear fluoro-alkanes also have limited solid monolayer formation despite being similar to linear hydrocarbons in terms of molecular architecture. 1. Introduction Solid monolayers adsorbed from liquids and solutions are central to many areas of academic and industrial interest, including wetting, detergency, photovoltaic devices, lubrication, and colloidal stabilisation. We have already investigated the adsorption behaviour of pure and binary mixtures of the pure linear alkanes where solid monolayers are adsorbed from the liquid and solutions to graphite( 1-4). Here we address the adsorption of alkyl molecules with a variety of chemical functionality. 2.0 Experimental The sensitive calorimetry and neutron scattering techniques used here have been described previously(2, 5, 6). These methods can identify the formation of solid monolayers adsorbed from their liquids and solutions and provide other detailed information including the monolayer melting temperature and enthalpy and molecular orientation on the surface. In this work calorimetry measurements were performed on a Perkin-Elmer DSC7 at the University of Cambridge and a Pyris 1 system at the University of Oxford. Transitions appear in the thermograms as peaks. The heating rate was 10 C/min. Measurements were made on the bare substrate, pure adsorbate as well as mixtures of adsorbate and substrate, which allows assignment of features from the pure bulk materials and the adsorbed monolayer. 2.1 Samples The materials investigated were the homologous series of alk-1-enes, and phenylalkanes and fluoroalkanes adsorbed on graphite, Papyex. The specific surface of each sample of

874 graphite was characterised by nitrogen adsorption. Additionally we used calorimetry to characterise the substrate with the adsorption of dodecane, an adsorbate for which the monolayer peaks are well established. This data indicated that variations in the monolayer melting enthalpy are due to variations in the specific surface area of the graphites. This correlation also indicates that the melting enthalpy of dodecane is an ideal indicator of the specific surface area of the graphite. The graphite used in the studies presented in this paper had a specific surface area of 25 m^/g. We give enthalpy data in units of Joules per gram of graphite (J/gg) as discussed previously(4). The protonated adsorbates were obtained from Aldrich and the fluorinated alkanes from Flurochem and were used without further purification. The amounts adsorbed are given in units of the amount of material required to cover a monolayer completely, an 'equivalent' monolayer, based on the specific surface areas of the graphite and the adsorbed molecules. For the DSC study 40-50 equivalent monolayers have been added. 3. Results and discussion 3.1 Alk-1-enes Figure 1 presents the thermogram from dec-1-ene adsorbed on graphite. Table 1, Alkenes: monolayer melting temperature and enthalpy Alkene

T''' (K)

T^'^ (K)

Hexene (c6) Heptene (c7) Octene (c8) Nonene (c9) Decene(clO) Hexadecene(cI6) Octadecene(cl8) Eicosene (c20) Docqsene (c22)

Not Observed Not Observed 188.7 K 215K 227.7 K 310K 334 K 341 K 366.9 K

172 K 192 K 207 K 274 K 288 K 300 K 311K

AH (J/gg) Bi BulkMpt (Lit, C) -140 -119 -101 0.26 0.48 -66 0.28 0.27 4.1 0.29 17 0.24 28 0.24

J^^W^

1.097 1.12 1.10 1.13 1.19 1.14 1.18

Endothermic transitions correspond to peaks in this Figure. The bulk melting transition is at 66.7 C (206 K), (lit. -66 C or 207 K). The peak at -44.9 C (228.1 K) does not occur in the thermograms of graphite substrate nor the pure adsorbate and is attributed to the melting of the adsorbed monolayer. The relative melting temperatures of the monolayer and bulk, T^°/T^^, is 1.10 and the enthalpy of the monolayer transition is 0.278 J/gg, in reasonable agreement with previous work on alkanes(l, 2, 4) on graphite. Table 1 summaries DSC data for all the alk-1-enes investigated. Monolayer peaks are evident for alkyl chain lengths of 8 or greater with similar T^^/T^^ values and melting enthalpies to dec-l-ene. We conclude that longer chain alkenes form solid monolayers when adsorbed from their liquid to graphite. However, it is significant that the peak from the melting of the adsorbed layer could not be observed for chain lengths less than 8.

875 The fact that we are able to observe solid monolayers for the higher homologues does provides some comfort that this is not simply an experimental artifact from poor equilibration. These results indicate that the unsaturation of the double bond inhibits the formation of the solid monolayer, possibly due to unfavourable n interactions between the double bonds and the graphite. The solid monolayer forms as the influence of the double bond is reduced with increasing saturated chain length. As octene is the shortest alkene to form a solid monolayer, 6 CH2 units win over a single double bond. Figure 1. DSC Thermogram of 40 equivalent monolayers of decene on graphite. Peaks in this Figure correspond to endothermic transitions. The bulk melting transition is at -66 C and the much weaker monolayer melting transition is at -45C Tshown in the inset).

f

i

1

J

3.2 Phenyl-Alkanes Table 2 presents DSC data for phenyl alkanes adsorbed on graphite. This data confirms that phenyl alkanes adsorbed from their liquids onto graphite form solid monolayers for alkyl chain lengths of 8 CH2 units or longer. For chain lengths shorter than 8 we do not find evidence for solid monolayer formation again suggesting that the unsaturation of an aromatic ring inhibits solid monolayer formation. The ratios T^^/T^^ are typically 1.05, significantly smaller than that found for alkanes and alkenes. The non-occurrence of a solid monolayer by phenyl alkanes shorter than c8 is also supported by incoherent elastic neutron scattering measurements on IN 10 at the ILL, France on toluene (cl) and ethyl benzene (c2). Additionally, previous neutron diffraction work has reported that benzene forms bulk crystallites once the coverage exceeds a monolayer (7). Table 2. Phenylalkanes: monolayer melting temperature and enthalpy. AH (J/gg) T'^'/T'" Phenylaikane T^) W(K) Phenylhexane (c6) Phenylheptane (c7) Phenyloctane (c8) Phenylnonane (c9) Phenyldecane (c 10) Phenylundecane (clO) Phenyldodecane (cl2) Phenyltetradecane (c 14)

Not Observed Not observed 240 K 251 K 272 K 270 K 294 K 305 K

237 K 240 K 261 K 260 K 280 K 287 K

0.445 0.423 0.433 0.318 0.579 0.396

1.02 1.05 1.04 1.04 1.05 1.06

876 3.3 Fluoroalkanes DSC data for the fluoroalkanes is given in Table 3 showing that solid monolayers for members of the series with alkyl chains of 12 carbon atoms and longer. The enthalpies are similar to those of the alkenes and phenyl alkanes but the values of T^^/T^^ are significantly smaller. No evidence of solid monolayers of fluoroalkanes could be found for members shorter than 12 carbon atoms although the distinction of the monolayer melting transition from the bulk melting peak is difficult. Table 3, Fluoroalkanes, monolayer melting temperatures and enthalpies Fluoroalkane Fluorohexane (c6) Fluoroheptane (c7) Fluorooctane (c8) Fluorodecane(clO) Fluorododecane (cl2) Fluorotetradecane (c 14) Fiuorohexadecane (c 16)

T'" (K) Not observed Not observed Not observed Not observed 351 K 383 K 411K

T'" (K)

AH (J/gg)

T'"/TF

343 K 373 K 398 K

0.360 0.406 0.299

1.02 1.03 1.03

4. Discussion This work has demonstrated that alkenes, phenyl alkanes and fluoroalkanes all form solid monolayers when adsorbed form the liquid onto graphite. It is evident that unsaturation in the molecules destabilises the solid monolayers. Fluorinated alkanes also form solid monolayers but the layers are only stable for a small temperature interval above the bulk melting point. These preliminary results highlight that solid monolayers are formed by an increasingly wide variety of molecular species with very different functionality. Within that differing functionality there are groups that seem to favour solid monolayer formation and those that inhibit it. References 1. Castro, M. A., et. al. 1998. Journal of Physical Chemistry, B102: 10528 2. Castro, M. A., et. al, 1997. Journal of Physical Chemistry. BlOl: 8878 3. Castro, M. A., et. al. 1999. Physical Chemistry Chemical Physics. 1: 5203 4. Castro, M. A., et. al. 1999. Physical Chemistry Chemical Physics. 1: 5017 5. Castro, M. A., et al. 1998. Physica. B241-243: 1086 6. Castro, M. A., et al 1998. Journal of Physical Chemistry. B102: 777 7. Meehan, P., et. al. 1980. J. Chem. Soc. Faraday Trans. I. 76: 2011

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) cc 2001 Elsevier Science B.V. All rights reserved.

877

Sorption mechanism of IO3" onto hydrotalcite Takashi TORAISHf, Shinya NAGASAKI*' and Satoru TANAKA' ^ Department of Quantum Engineering and Systems Science, Graduate school of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPAN ^ Institute of Environmental Studies, Graduate School of Frontier Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, JAPAN Sorption mechanism of IO3" onto anionic clay hydrotalcite (HT) was investigated. Sorption isotherm was measured by batch experiment and the structural change in sorbing IO3' were studied by IR spectroscopy. In adsorption isotherm, two curves were observed, suggesting the two mechanisms of sorption. We also found the change in IR spectra around [I03']=5X 10"^ M in liquid at equilibrium. From those two experiments, it was suggested that two mechanisms are included in the sorption of IO3 onto HT system. We characterized that IO3* has C3V symmetry at [IO3 ] < 1 X 10'^ M and C2vOr Cs symmetry at[I03"] > SXIO'^M. 1. INTRODUCTION For the safety assessment of high-level radioactive waste disposal, IO3" is considered to one of key radionuclides that dominate the long-term radiation hazard. However, in the present high revel radioactive waste disposal system, we cannot expect high retention of anionic species. In the present work, we investigated sorption of IO3 onto anionic clay hydrotalcite (HT) that has high sorption ability to anionic species and that is a potential material in the advanced disposal system. Formula of HT is Mg6Al2(OH)i64C03V7H20 and it has brucite-like double layered structure. In general, sorption mechanism of HT is mainly anionic exchange by the interlayer C03" [ 1 ]. This paper describes the work carried out to ascertain the sorption mechanism of IO3

878 onto HT. In the batch experiment, we obtained sorption isotherm. We also measured IO3 vibrational spectrum by Fourier Transform Infrared Spectroscopy (FT-IR) before and after IO3' was sorbed onto HT. 2.EXPERIMENTAL 2.1. Batch Experiment In order to understand how long it takes to reach sorption equilibrium, we measured the dependence of sorption on time. Under the N2 atmosphere (N2 purity > 99%), 0.30 g HT powder (Kyowa Chem. Industry Co.Ltd. , average size=1.0p.m) were mixed with pure water that was bubbled by N2gas in polyethylene tube. After one day swelling, we added 100 ^il 0.1 M KIO3 solution to the mixture and gently shook at room temperature. After pre-decided periods, the sample was taken from supernatant, and the IO3 concentration was measured by ICP-AES. It was found that sorption equilibrium was attained within 3 hours. In order to obtain sorption isotherm, we carried out the batch experiment. Under the N2 atmosphere, 0.10 g HT powder was mixed with KIO3 solutions of different concentrations. The mixture was shaken for 1 week at room temperature and centrifuged to separate the liquid phase from the solid phase. We measured the concentration of IO3" in the liquid phase. However, the difference in IO3 concentration before and after sorption was very small. Hence, we determined the amount of IO3' sorbed onto HT by desorption of 103' and by dissolving the solid phase after desorption. In the recent work, it was reported that C03^' is sorbed on HT more preferentially than CI" [2]. Hence, the desorption of 103'from HT were carried out by using 1.0 M KCl (3 times) and 0.1 M (NH4)2C03(1 time), and 103' concentrations that were desorbed were measured. Since there was a possibility that IO3' that could not be desorbed by these desorption experiments, the solid phase after desorption was dissolved in IM HN03and IO3 concentration was measured. The sum of desorbed and dissolved IO3' was considered to be amount of IO3" sorbed on HT. 2.2.

Foulier Transform Infrared Spectroscopy (FT-IR)

0.10 g HT were mixed with several concentrations of KIO3 solutions under N2 atmosphere. After I week, mixture were filtered with Millipore filters (pore size 0.10 |Lim). The solid phase was dried in vacuum for 24 hours. 0.01 g dried solid was mixed with

879 highly pure 1.0 g KBr powder and pressed to circular pellet. Infrared spectra were measured by Shimadzu FT-IR 8200PC spectrometer. The resolving power was settled in 1.0 cm'. 3. RESULT AND DISCUSSION 3.1. Batch Experiment Sorption isotherm of IO3' - HT system was shown in Fig.l. In the desorption experiment, we found that IO3" sorbed onto HT was completely desorbed by 3 times KCl treatment. It was found that the sorption isotherm consisted of two curves. These two curves crossed around [IO3'] = 5X 10'^ M. In general, precipitation is appeared in the high concentration. When precipitation is appeared, it is known that equilibrium concentration in liquid phase becomes plateau [3]. In this study, since such plateau was not observed, we considered that the precipitation was not taken place. Therefore, this result suggested that I03"that sorbed onto HT has two sorption mechanism around [IO3'] = 5X10' M ,one dominated at [lOi] < SXIO'M and the other at [IO3] > 5X10^M. Since IO3" was completely desorbed by CI, we considered that these two mechanisms of IO3" sorption were ion exchange reaction. ecMiiibtiuM concent rat ion fu)

1.2

/ •e o

A

0.6

M

/ ^ 0.4 0.2

A

oL.-^^ 0 0.05

0.1

0. 15

0.2

equiIibriurn concentration in Iiquid phase (M)

Fig. 1.

Sorption isotherm of IO3 onto HT

1000

900

800

700

600

500

Wave nuntjer (cnfO

Fig.2 FT-IR spectra as a function of IO3 concentration

880 3.2.

Fourier lyansform Infrared Spectroscopy (FT-IR)

Fig.2 shows IR spectra of IO3" sorbed onto HT as a function of IO3' concentration. It was shown that IO3" has C3V structural symmetry and two peaks appear at 805 cm"'(Vi) and 775 cm''(V3) in the aqueous solution and at 796 cm"' (V|) and 745 cm"' (V3) in the KIO3 solid [4,5]. In general, V3 vibrational mode sprits into two modes, V2 and V5, when C3v structural symmetry of IO3' is reduced into C2v or Cs. In this work, four peaks were found at 740 cm"', 760 cm"', 780 cm' and 800 cm*' in the sample that was swelled at [IO3'] > 4.70X 10"^M and only one broad peak appeared at 780 cm' at [IO3"] = 8.70X 10" M. According to literature [6], a broad peak around 780 cm"' was assigned to the summation of V| and V3 mode peaks. From those, it was suggested that IO3" sorbed onto HT has C3v symmetrical structure in the [IO3 ] = 8.70X 10"^ M. On the other hand, other three modes of 103" indicated the reduction of IO3" symmetrical structure from C3V to Czv or Cs, suggesting that IO3" adsorbed onto HT has two different structures C3V and C2V or Cs. It was also observed that the structural strain of IO3* became harder in higher concentration. This structural change around [IO3"] =5X10"''M seems to be consistent with the sorption mechanisms discussed in sorption isotherm (Fig.l.)4. CONCLUSIONS In this work, it was suggested that IO3 sorbed onto HT has the two different sorp tion structures. From infrared spectroscopy, it was found that IO3" sorbed on HT at [IO3 ] < 1 X 10"^ M has C3v symmetrical structure and that at [IO3"] > 5X 10"^ M has C2v or Cs symmetrical structure. REFERENCES 1. Vicente Rives et.al., Coodination Chemistry Reviews 181 (1999) 61 -120. 2. Rajib Lochan Goswamee et.al.. Applied Clay Science 13 (1998) 21-34. 3. Werner Stumm: CHEMISTRY OF THE SOLID-WATER INTERFACE, John Wiley & Sons, Inc. (1992). 4. D.J.Gardiner, R.B.Girling, and R.E.Hester, J. Mol. Structure, 13 (1972). 105. 5. W.E.Dasent and T.C. Waddington, J. Chem. Soc, (1960) 2429,3350. 6. K. Nakamoto: Infrared and Raman Spectra of Inorganic and Coodination Compounds, John Wiley & Sons, Inc. (1986).

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) fc> 2001 Elsevier Science B.V. All rights reserved.

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Thickness dependence of absorption of molecular thinfilmsstudied using FECO spectroscopy^ Tamas Haraszti, Kenichi Kusakabe and Kazue Kurihara* Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan Multiple beam interferometry, the fringes of equal chromatic order (FECO), was used to investigate the absorption properties of aqueous dye (Rhodamine B) films confined between mica surfaces of crossed cylindrical geometry. A new normalization method for the data analysis, using the transmitted intensities of both odd and even order FECO, was developed to obtain the thickness dependence of the absorption by the film. We found that the Rhodamine B molecules were trapped between the mica surfaces below 15 nm separation distance, and only water was expelled from the confined space upon compression. l.INTRODUCnON The structuring of liquids is well known in a confined space such as nanometer size pores or thin films, and plays important roles in many industrial processes, like catalysis or filtration. To study confined liquid films, a common and effective tool is the surface forces apparatus (SFA) and the different shear force devices designed for in-situ application with the SFA [1-4]. Atomically smooth mica surfaces are brought close at nanometer distances with a liquid confined between them, and the interaction force and the shear properties are investigated between the surfaces at different separations which are determined using the fringes of equal chromatic order (FECO). Two silver coated (ca. 45-50 nm) mica sheets confine a liquid or liquid crystal film forming a five layer, synmietric Fabry-Perot interferometer [5-8]. Recently, Helm et al. proposed applymg FECO for studying the adsorption properties of confined liquids [7,8]. They found anomalous behavior of the transmitted intensity of the odd order FECO, which limited their quantitative analysis of an aqueous Rhodamine B film to very small thicknesses around a few nanometers. In this study we interpreted the behavior of the odd fringes by changes in the overall optical properties, especially deformation of the surface geometry. Analysis of both the odd and even fiinges enabled us to do a full analysis of the thickness dependent change in the absorptive properties of confined liquids, which was applied to aqueous Rhodamine B solutions. tThis work was supported in part by Grants-in-aid (No. 9554044,11167204) from the Ministry of Education, Sciences, Sports and Culture of Japan. *To whom correspondence should be addressed.

882

III Fringes of eq«al chromatic order (FECO)

2. E X P E R I M E N T A L IV FECO tBtensHy profUe (specrrum)

Rhodamine B (TCI, Tokyo, ACE grade) was used as received. An aqueous Rhodamine B solution at 8.6 mM (molar ratio: xi = 1.55x10"*) was prepared with pure water (NANOpure n., Bamstead; 18 MQ/cm resistance). A 30-50 \il solution was injected into the space between the mica surfaces after characterizing the properties of the mica substrate. Silver The measurements were performed Figure 1 The experimental system: I, light source; II, using a surface forces apparatus (Mark silica disks with mica sheets and the confined 4, ANUTECH) equipped with a homesample; HI, FECO image observed in the built shear force attachment [4,9] spectrograph; IV, an intensity profile for mica (Figure 1). The FECO images on a surfaces in air contact (top) and with confined spectrograph (Nippon Jarrell Co., 3401 Rhodamine B (bottom). The dashed line is drawn to HR), were recorded through an ICCD provide a visual guide for the even order peaks to camera (C2400-89H, Hamamatsu), show the effect of dye absorption. and a video gauge (Image Tech VGIIP) on a personal computer as digital image files. The light intensity spectrum was taken at the center of the FECO and used for the calculations. The images were 640 x 480 pixels with a horizontal resolution of 15.4 pixel/nm; the intensity was converted by a 10 bit A/D converter. Each image was the average of 32 standard video frames, and the spectrum (light intensity versus wavelength) profiles were extracted by the program (LEPASll, Hamamatsu). The experiments were performed at optical wavelengths between 520 and 590 nm. The green and/or yellow lines of a high pressure Cd/Hg lamp were used for calibration (at 546.1 and 577 nm respectively). 3. RESULTS AND DISCUSSION The odd order FECO for a confined dye solution showed transmission lower than the theoretically expected value for surfaces with a flat contact geometry, as reported by Helm et al. [7,8]. Therefore, in order to elucidate this behavior, we first studied the thickness dependence of the light intensity transmitted through nonabsorbing pure water films in the ransmission < of the N order fiinge (TN) is calculated using equation 1 [7,8]: SFA. The transmission _ /. (1) *iV,0

where IN,O and IN are the intensity for the mica surfaces in air contact and with confined material between them. For a nonabsorbing system the theory predicts unit transmission (TN = 1) [7,8], however, the experimental results were quite different as presented in Figure 2. The transmitted intensity was around TN - 0.5 upon approach until the surfaces started to deform into a flat contact, then TN rose rapidly to unity (TN -* 1 as D - • 0). We obtained identical behavior for both the odd and even order fringes. These results indicated that the

883

anomalous behavior of the odd FECO is not related to the absorption of the liquid fibn, push into contact 0.8| but should be related to the experimental < setup, possibly the deformation of the surface geometry from the crossed cylinder to flat contact. Because the mica sheets were glued to cylindrical silica using an elastic epoxy resin, and the surfaces could deform upon high compression [5,10]. We can take into 10 20 30 account this apparatus effect by normalizing thickness, nm the even order FECO intensity (shown in Figure l/TV), which contained the absorptive Figure 2 The transmission of odd order FECO information, by that of the odd. for the water sample. The transmitted intensity Subsequently, we characterized the rapidly increases as the surfaces are pushed absorption properties of the confined into flat contact as in the case of the air Rhodamine B solutions. The extinction contact. The sketched figures show that the coefficient (k), which is the imaginary part of surface geometry can flatten as the surfaces the complex refi-active index (equation 2), are pressed together. was calculated using equation 3 [7]. The value of k is proportional to the concentration of the dye molecules (k • kRh x xi, where kRh and xi are the extinction coefficient and the molar ratio of the dye, Rhodamine B, respectively). (2)

n, - /I + ik ^^nucaO^-r)

2njTD{l + r)

t->)

(3)

where r is the reflectivity of the silver coating, k is the wavelength of the peak of the even order FECO and n and Umica are the refractive indices of the confined material and the mica, respectively. The obtained k value was plotted versus the fihn thickness (i.e., the separation distance, D) as shown in Figure 3. The wavelength (X) is in the range of 552-561 nm for the data marked with X (N = 46) and 592-597 nm for the other data (N = 18). We can clearly distinguish three regions in the figure. The k value was nearly constant at D>80 nm, raised about four times of its initial value with decreasing Dfi^om80 nm to 30 nm, then sharply increased at D<30 nm. Based on these data we could gain insights into the properties of the dye solution films. The constant extinction coefficient for thick films showed that, the concentration and the orientation of Rhodamine B dye molecules remained the same at D > 80 nm as those in the bulk. The slow increase in k at 80 nm > D > 30 nm indicated that the confined dye film was no longer the same as the bulk. There are three possibilities to account for this change. The first is a change in the dye spectrum due to close packing or aggregation of the dye molecules, and to the different chemical and physical conditions of the medium (water) in the confined space. Actually, the broadening and the red-shift of the spectral peak of the Rhodamine B has been previously reported, which does not explain well the observed increase in k [7]. The second is a change in the orientation of the flat Rhodamine B molecules. If they orient parallel to the mica surfaces, the extinction coefficient should increase to 3/2 times of that for

884

a random oriented dye in the bulk [7-9]. Though this effect is X -i probable, the increase in k is so ^ 1.2 1 large that we can not explain the .2 J *3 only by these 1 observation 56 mechanisms. Third is an increase in 8 0.8 1 the dye concentration which should 1 ^ -S -1 significantly contribute to the o 1 change. We found this effect •B 0.4 becomes even more dominant at \ smaller thicknesses (see the following). - > , ^^ K , ^ ": i The sharp increase at D < 30 nm 0 20 40 60 80 100 120 140 indicates that the concentration of thickness (D), nm the dye significantly increased. In Figure 3 The extinction coefficient as a function of the Figure 3, the solid line is a 1/D thickness of the confined film. Different experiments curve which well fits the data at are plotted together with different symbols. The solid smaller (D < 20 nm) thickness values. This shows that, the curve has the equation k=2/fD-l) number of dye molecules in the confined space is approximately constant in this range. To maintain a constant dye concentration, only water molecules have to be expelled from the space upon compression. Shear force resonance measurements indicate that the resonance peak shifted from the free to the contact resonance state at 6-10 nm thickness, where the number of molecules was constant [9]. There should be strong attractive interactions between the dye molecules and the dye and solid surfaces which leads exclusion of the water molecules between the space. 1

2"

T

1 T ^ 4

Summary: In this work, we developed a full analysis of the FECO intensity to study the absorptive properties of Rhodamine B aqueous solutions confined between two mica surfaces. This method, we may call it FECO spectroscopy, is found useful to obtain novel insights into the behavior of confined liquids which is regulated by liquid-liquid, liquid-surface interactions and the packing of the liquid molecules. REFERENCES 1. 2. 3. 4.

J.N. Israelachvili, G.E. Adams, J.Chem.Soc., Faraday Trans. 74(1978), 975. J.N. Israelachvili, P. McGuiggan, A. Homola, Science 240(1988), 189. A. Dinojwala, S. Granick, J.Chem.Soc.,Faraday Trans. 4(1996), 619. CD. Dushkin, K. Kurihara, Coll. & Surf. A. 129-130(1997), 131-139. CD. Dushkin, K. Kurihara, Rev. Sci. Instrum. 69(1998), 2095-2104 5. J.N. Israelachvili, J. Coll. & Int. Sci. 44(1973), 259. 6. P. Yeh, Optical Waves in Layered Media, John Wiley and Sons: New York, 1989. 7. P. Machte, C Muler, CA. Helm, J.Phys.H France 4(1994), 481-500. 8. C Miller, P. Machte, CA. Helm, J.Phys.Chem. 98(1994), 11119-11125. 9. T. Haraszti, K. Kusakabe, K. Kurihara, Paper in preparation 10. M. Heuberger, G. Luengo, J. Israelachvili, Langmuir 13(1997), 3839-3848.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.

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Sorption of neptunium on surface of magnetite K. Nakata^ S. Nagasaki\ S. Tanaka', Y. Sakamoto^ T. Tanaka' and H. Ogawa' ^Department of Quantum Engineering and System Science, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan ^'Graduate School of Frontier Sciences, University of Tokyo Bunkyo-ku, Tokyo 113-0033, Japan ^'Departments of Environmental Safety Research, Tokai Research Establishment Japan Atomic Energy Research Institute, Tokaimura, Naka-gun, Ibaraki 319-1195,Japan Sorption and desorption experiments of Np on magnetite under aerobic and anaerobic conditions were carried out to investigate the possibility of reduction of Np(V) to Np(IV) at pH=6. The amount of sorbed Np on magnetite under anaerobic condition was about 3 times greater than that under aerobic condition. Furthermore, the results of desorption experiment with 1 M KCl solution indicated that the dominant sorption mechanism on magnetite under anaerobic condition was quite different from that under aerobic condition. Oxidation-reduction reaction of Np(V) and Fe(II) in liquid phase was also investigated. It was found out that only 6% of Np(V) was reducted by Fe(II) at pH=6. According to this result, it was conjectured that the reduction of Np(V) to Np(IV) took place not in the liquid phase by Fe(II) ion but on the surface of magnetite. 1. INTRODUCTION Because of its long half-life and its mobile nature under aerobic conditions due to the high chemical stability of its pentavalent state, NpOz^, Np-237 from high-level radioactive wastes is considered as a possible long-term pollutant of the ecosystem [1]. In rock systems, iron oxides are expected to play an important role in regulating the migration of radionuclides because of their widespread existence, and high sorption capacity. Therefore, understanding the sorption mechanism of Np on iron oxides are required to describe the mobility of Np in natural waters. Furthermore, there is a possibility of reduction of Np(V) to Np(IV) at the surface of minerals containing Fe(II) [2]. It is necessary to investigate such a reaction to understand the sorption mechanism. However, few works have been performed to investigate the reduction of Np(V) to Np(IV) and the sorption/precipitation of Np(IV). In this work, sorption and desorption experiments of Np on magnetite(Fe304) under aerobic and anaerobic conditions were carried out to investigate the possibility of reduction of Np(V)

886 to Np(IV). The sequential desorption method by using 1 M KCl was applied to this magnetiteNp system to obtain the infomiation on sorption mechanism. 2. EXPEWMENTAL 2.1. Chemicals Magnetite powder was obtained from Rare Metallic Co., Ltd. (Tokyo, Japan). Magnetite powder was sieved to a particle size of less than 250 pm before the sorption experiment. 9.2x10"^ M Np in 2.2 M HNO3 was obtained from LMRI, France and diluted with deionized water to obtain a l.SxlO'^M Np stock solution. The oxidation state of Np was confirmed to be pentavalent by a solvent extraction technique using 0.5 M thenoyltrifluoroacetone (TTA) in xylene [3]. 2.2. Procedures Batch experiments under the aerobic and anaerobic condition were conducted at 25 °C. In both conditions ionic strength was adjusted to 0.1 M with NaNOa solution and pH value was adjusted to about 6 for anaerobic condition and between 4 to 8 for aerobic condition with HNO3 and NaOH solutions. The start concentration of Np was 6x10"^ M in both cases. The ratio of liquid and solid phase was 120 ml / 3 g for anaerobic condition and 40 ml / 1 g for aerobic condition, respectively. Blank tests with 0.1 M NaNOj solutions were carried out in parallel with the sorption experiments in order to measure the Np sorption onto the vessel walls and to investigate the possibility of precipitation in the solution phase under both aerobic and anaerobic conditions. The desorption of Np from the individual samples was investigated after the end of the sorption experiments using extraction technique utilizing 40 ml of 1 M KCl [4] in order to investigate the sorption mechanism of each conditions. Anaerobic condition was achieved by using Ar gas to prevent the oxidation of Np(IV) to Np(V) by the oxygen in the solution. All experiments under anaerobic condition were carried out in an atmospheric control chamber and all the solutions used under anaerobic condition were deaerated by bubbling with Ar gas. The detailed procedures of sorption and desorption experiments are described elsewhere[5]. 3. RESULTS 3.1. Sorption The results of sorption experiments are summarized in Fig. 1. The sorption ratio of Np, K (%), was calculated by using the following equation, C -C A' = ~^ ^xlOO, Q where Cf, and Cf represent the final concentrations of Np in the absence and presence of magnetite, respectively. It was found out by blank test that the sorption on the vessel walls

887

Anaerobic' 75 _Eh=24mV # Eh=40 mV • Aerobic

5^

25

J 1 D

Eh=395 mV D Eh=412 mV D

-

\

1

0 Ig

LD__J

6 8 pH Fig.l The sorption ratio of Np on magnetite under aerobic and anaerobic conditions

was negligible and that precipitation did not take place in the solution. The amount of sorbed Np under anaerobic condition was about 3 times greater than that under aerobic condition. After the sorption experiment the presence of Np(IV) in the aqueous phase was investigated by solvent extraction technique with 0.5 M TTA in xylene. However, the evidence of presence of Np(IV) was not obtained by this extraction experiment.

3.2. Desorption In the desorption method used in this study, Np loosely or ion-exchangeably sorbed was considered to be desorbed by a KCl solution. The remaining Np which was not desorbed by KCl solution was considered to be sorbed on magnetite non-exchangeabley. Therefore, in this paper, we defined the Np desorbed by the KCl solution as "Np sorbed exchangeably" and the remaining Np as "Np sorbed non-exchangeably". The results of the desorption experiment on magnetite under anaerobic and aerobic conditions are summarized in Tables 2 and 3, respectively. The desorption ratio K' (%) in Tables 2 and 3 was calculated using the following equation, ir = ^ x l O O , A, where A, and A^ represent the amount of sorbed Np before the desorption experiment and that of desorbed Np, respectively. As shown in Tables 2 and 3, exchangeable sorption is one of the dominant sorption mechanism under aerobic condition around pH=6 because approximately 55% of sorbed Np was desorbed by KCl solution. On the other hand, exchangeable sorption is not a dominant sorsorption mechanism because only less than 1 % of sorbed Np was desorbed by KCl solution under anaerobic condition. Table 2. Desorption ratio of Np on magnetite under anaerobic condition Sorbed non-exchageably(%) Sorbed exchangeably (%) at pH=6.0 0.2 99.8 atpH=6.5 0.4 99.6 Table 3 . Desorption ratio of Np on magnetite under aerobic condition Sorbed non - exchageably(%) Ion-exchangeably (%) /^'(%)atpH=6.1 54.2 45.8 A:'(%)atpH=7.0 57.4 42.6

888

4. DISCUSSIONS It was found out by using XPS that the surface structure of magnetite after Iweek contact with 0.1 M NaNOj solution at pH=6 under aerobic condition was not different from that under anaerobic condition. Therefore, the difference of sorption and desorption behavior between imder anaerobic and aerobic condition was not caused by the change of surface structure of magnetite. Some previous studies [6] clarified that Np(IV) has the higher stability constant of complexation than Np(V). Therefore, it is considered that the amount of sorbed Np increase and the complexation, i.e. non-exchangeable sorption, becomes the do-minant sorption mechanism if the reduction of Np(V) to Np(IV) took place. Therefore, results of sorption and desorption experiments are considered to indicate the possibility of reduction of Np(V) to Np(IV). It is found out that reduction of Np(V) do not take place in the solution in absence of magnetite from the blank test. Therefore, magnetite, especially Fe(II) in magnetite, is considered to cause the reduction of Np(V). However, the evidence of presence of Np(IV) in the liquid phase was not obtained by extraction experiment with TTA. Furthermore, oxidation-reduction reaction of Np(V) and Fe(II) in liquid phase was investigated under the same condition as sorption experiment under anaerobic condition. It was found out that only 6% of Np(V) was reduced by Fe(II) to Np(IV) under this condition after 1 week since Np(V) and Fe(II) was contacted. Thus, the sorption enhancement of Np on magnetite under anaerobic condition was not able to be explained only by the reduction in the solution. Therefore, these results are considered to indicate that the reduction take place not in the liquid phase by Fe(II) ion but on the surface of magnetite. 5. CONCLUSIONS 1. The amount of sorbed Np on magnetite under anaerobic condition was about 3 times greater than that under aerobic condition. 2. The results of desorption experiment indicated that the dominant sorption mechanism on magnetite under anaerobic condition was quite different from that under aerobic condition. 3. The possibility of reduction of Np(V) to Np(IV) by Fe(II) on the surface of magnetite was indicated. REFERENCES 1. R.C. Thompson, Radiation Research, 90 (1982) 1. 2. R.E. Meyer, W.D. Amold and F.I. Case, US-DOE Report NUREG/CR-3389 (1984). 3. F. L. Moore and J. E. Hudgens Jr, Analytical Chemistry, 29 (1957) 1767. 4. K. Idemitu, K. Obata and Y. Inagaki, Mat. Res. Soc. Symp. Proc, 353 (1995) 981. 5. K. Nakata, T. Fukuda, S. Nagasaki, S. Tanaka, A. Suzuki, T. Tanaka and S. Muraoka, Czechoslovak JPhys., 49 (1999) 159. 6. C. Keller, The chemistry of the Transuranium Elements, Verlag Chemie, Weinheim, 1971.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c; 2001 Elsevier Science B.V. All rights reserved.

889

Two dimensional auto-organized nanostructure formation of acid polysaccharides on bovine serum albumin monolayer and its surface tension Shouhong Xu, Tsuyoshi Nonogaki, Keita Tachi, Shizuko Sato, Isamu Miyata, Junpei Yamanaka and Masakatsu Yonese'*' Faculty of Pharaiaceutical Sciences, Nagoya City University, Tanabe-dori Mizuho-ku Nagoya 467, Japan The layer-by-layer interactions between bovine serum albumin BSA and acid polysaccharides such as sodium hyaluronate NaHA and sodium chondroitin sulfate Na2ChS were studied by a quartz crystal microbalance QCM method. The surface structures of BSA layer and acid polysaccharide layers, and their surface tensions y were investigated using an atomic force microscopy AFM. The surface structures of NaHA and Na2ChS adsorbed on the BSA monolayer were found to form hexagonal-like networks and their y increased with increasing the absolute values of the charge densities. INTRODUCTION Layer-by-layer interactions between polyelectrolytes have attracted special interests pursuing to elucidate the interfacial functions. Proteins and acid polysaccharides interact electrostatically, and form soluble and/or insoluble complexes depending on pH and the ionic strength of solutions^l The kinds of the charged groups of acid polysaccharides and their combination affect their characteristics^^ The layer-by-layer interactions between proteins and acid polysaccharides, and their surface properties are very important for basic and applicable points of views. In this report, the layer-by-layer interactions between bovine serum albumin BSA and acid polysaccharides such as sodium hyaluronate NaHA and chondroitin sulfate NazChS were studied by a quartz crystal microbalance QCM method. The surface structures of BSA layer and acid polysaccharide layers, and their surface tensions were investigated using an atomic force microscopy AFM. EXPERIMENTALS Materials. Bovine Serum Albumin BSA was purified by delipidizing a BSA Fraction V (Seikagaku kogyou). The average molecular weight was determined to be A/w=70,000 g mol"^ using a static light scattering. Sodium hyaluronate NaHA were offered from Seikagaku kogyou Co.(Tokyo). Their A/w were 850,000 g mol'* (NaHA85), and 210,000 g mol"^ (NaHA21). Chondroitin sulfate A (A/w =23,000 g mol"^) (Na2ChS) was of a commercial origin (Seikagaku kogyou Co. (Tokyo)). Poly ( y-methyl-L- glutamate) (Mr=307,000 g mol-1) (PMLG) was also of commercial origin (SIGMA Co.).

890

Methods Adsorption of BSA on PMLG cast film by using QCM PMLG cast film was prepared by spreading of PMLG casting solution (2 ii 1) dissolved in 1,2 dichloroethane on the quartz crystal microbalance QCM (USI system Co.) disk tip on which Au is covered by evaporatioa The tip is denoted by PMLG tip. The adsorbed amounts of BSA on the PMLG cast film were obtained by inmiersing the PMLG tip into various concentrations of BSA solutions and by measuring the decrease of the resonance frequency of QCM. Adsorption of acid polyBaccharides on BSA monolayer by using QCM The adsorbed amounts of NaHA and Na2ChS on the saturated BSA film were measured by inmiersing the BSA tip into their solutions of various concentrations and the time courses of the adsorption were also determined by QCM. Analysis of adsorbed amounts by QCM Adsorbed amounts were obtained using QCM. According to Sauerbrey's equation, the frequency decrease of IHz corresponded to a mass increase of 1 ng on the QCM tip. Observation of surface structures using AFM Surface structures of the adsorption layer of BSA, NaHA and Na2ChS were observed using AFM (NanoscopeDI, Digital Instruments Co.). After drying the samples under the condition of the room temperature and the atmospheric pressure, the surface structures were observed by a tapping and a phase-difference mode. Estimation of surface tension using force mode of AFM Adhesion force Fad between the AFM tip and sample surface can be given directly from force-distance curves using force mode AFM. The value of Fa/ can be related to the surface tension of AFM tip ( v t) and sample surface ( 7 s) by the Derjaguin-Muller-Toporov (DMT) theory taking Lennard-Jones interactions into account. When the tip-sample system is sketched as a sphere on a flat surface. Fad is given by equation (1).

F^^-AnR^jTrTs

(1)

where R is tip radius. RESULTS AND DISCUSSION As shown in the previous paper^, the adsorptions of BSA on the PMLG film were Langmuir type and the immobilized BSA molecules were confirmed to adsorb on the PMLG surface in the monolayer state. The adsorptions of NaHA85, NaHA21 and NaiChS on the BSA tip were also the Langmuir type. The equilibrium adsorption masses of NaHA /HA were found to depend on the molar masses as shown in Fig.l. The adsorption constants K and the saturation adsorption mass Too obtained from the reciprocal plots were summarized in Table 1 with the results of BSA adsorbed on the PMLG film. The values of the molecular numbers and the repeating units of NaHA and Na2ChS adsorbed on a BSA molecule in the saturated monolayer state are also shown in Table 1. The results show 1/10 of NaHA85, 7/10 of NAHA21 and 5 of Na2ChS molecules adsorb on one BSA molecule. The numbers of dissociation groups of NaHA85, NaHA21 and Na2ChS on one BSA

891

molecule were not constant. From these results, NaHA molecules do not interact electrostatically with the BSA monolayer primarily, but hydrophobically and/or sterically. 7, • :NaHA85 0:NaKA21

2

4

6

Cj^/lO-^kgdm-^ Fig.1 Adsorption behavior ofNaHA to BSA moEoiayers; C j ^ : conccntratioii of NaHA, r Yu^' adsorption mass ofNaHA.

Tablc 1 Adsorption cbaractczistics of BSA on PMLG cast film, NaHA on BSA monolayer and Na2ChS on BSA ZDOOOUQ^.

Adsorption consL(IC) /10* dm' kg'' Satadaoiption mass ( F «) / lO'^kg m'^ Acid poiysaccfaande / BSA Repeating unit / BSA Electrolytic dissodatioQ group / BSA

BSA \2 2.7

Langmuir adsoqition tsodienn

NaHA85 3J 2.9 1/10 232 232

NaHA21 1.7 5.1 7/10 380 380

NajCbS 8.4 3.6 5A 265 530

P = F^C I (I+ATC)

Surface structures of the saturated adsorption layer of NaHA85, NaHA21 and Na2ChS on the saturated BSA adsorption monolayer are shown in Fig.2. Network structures were found to spread over the BSA monolayer and almost a hexagonal in shapes different in sizes. Their average mesh sizes of NaHA85, NaHA21 and Na2ChS were 153.4,113,2 and 37.7 nm. Their average dimensions of the strands were 29.4,34.3 and 20.1 nm in width. The mesh sizes and the widths of the strands depended on the molar masses and the charge densities. The strands of NaHA should be formed by clusters which be composed of 15-18 double helixes of NaHA and the strands of Na2ChS should be formed by 8 double helixes of Na2ChS. The surface tension y of the PMLG tip increased with increasing the adsorption of BSA. The values of y of the saturated BSA monolayer decreased with increasing the adsorption of NaHA85. However, in the cases of NaHA21 and Na2ChS, r increased as shown in Fig.3. The values of y of the saturated acid polysaccharides were found to increase with increasing the absolute values of charge densities. The AFM images of frictional forces were observed also, and the frictional force

892

of the network strands of acid polysaccharides were found to be more than the surface of BSA monolayer. NaHA85

NaHA21

NaAS

Fig.2 The AFM images

•

6 a

0

• NaHAfS p

NiHA21

• Na2^3iS

LA ^

•

Fig.3

0

A

•

•

ff

Adsoiption effects ofacki polysaccharides on the surface t e n ^ BSA monolayer. C: concentration of acid polysaccharides.

y of

CONCLUSION 1. The adsorptions of NaHA and NaiChS on the saturated BSA adsorption monolayer were found to be the Langmuir adsorption isotherm. Their auto-organized structures were networks and their mesh sizes decreased with decreasing their molar masses. 2. The surface tensions of the saturated adsorption layers of NaHA and Na2ChS increased with their charge densities. 3. The surface frictional forces of the adsorption layers of NaHA and Na2ChS were more than that of the saturated BSA adsorption layer. REFERRNCES 1. Xu S.H., Yamanaka J., Sato S., Miyata I. and Yonese M. Chem, Pharm. Bull, 48(6), 779-783, (2000) 2. Nonogaki T, Xu S.H., Kugimiya S., Sato S., Miyata I. and Yonese M. Langmuir, 16(9): 4272-4278, (2000)

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c; 2001 Elsevier Science B.V. All rights reserved.

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Trapping behavior of water on metal oxide and active desorption Kunihiko Chiba, Toshiaki Yoneoka and Satoru Tanaka Department of Quantum Engineering and Systems Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Adsorption and desorptbn of HP or D p on the surface of iron oxkJe were studied by TD, XPS and SEM. Adsort)ed states of water on iron oxide were multiple and adsort)ed water was described around 400 K and 600 K. Desorption around 400 K was mainly attributable to the dehydration of FeOOD or FeOOH. Repetition of adsorption and thermal desorptkxi of water on the surface resulted in extension of the surface area. 1. Introduction It is necessary to understand the interaction of tritium with various materials in tritium handling system including fusion reactor. The developnnent of adequate process for decontaminating tritiated water is also strongly desired. Hence It is Important to elucidate the mechanism of adsorption and desorption of tritiated water on the material surface and to control actively the tritiated water. Surface of metallic materials used in actual condition is usually covered with oxkte or organk: matter It is easily expected that the adsorptbn and desorption mechanism on such surface are different from those on ideal metal surface with cleaned or highly evacuated condition. In the present study, we used H p or D p as simulating material of HTO and the pure iron as one of the metallk) materials on which thin films of iron oxkie grew. Adsorptton and desorptton of H p or D p on the surface of iron oxide were studied by thermal desorption (TD), X-ray photoemisskxi spectroscopy (XPS), and scanning electron mrcroscopy (SEM). 2. Experimental Pure iron plate (length: 40 mm, width: 5 mm, thickness: 0.5 mm) was used as a sample. Electrolytic polishing was conducted using phosphoric acid for 10 minutes and heated at 673 K in O2 (20 Pa) for 1 hour to oxkiize the sample surface. Thickness of the iron oxkie was about several tens of nanometer From XRD analysis it was found that the iron oxkie was mainly FePg. TD experiments were made in a chamber equipped with QMS and a heating system of

894

the sample. The sample was exposed to D p In air for 30 minutes and Introduced to the chamber. Base pressure of the chamber was about 5X10^ Pa. The sample was heated in vacuum under a constant heating rate (5 K/min) in the temperature range of 298-773 K. Desorbed species were detected using QMS. The TD experiments, D p exposure followed by heating were repeated several timesforthe same sample. XPS obsen/atbn was made in the measurement chamber with JPS-9800 (JOEL). Base pressure of the chamber was about 10' Pa. MgKa (1253.6 eV) X-ray was used to generate the spectra. After the sample was exposed to H^O in air for 30 minutes, it was introduced to XPS measurement chamber and XPS spectra were obtained. After that the sample was gradually heated to 473 K and 773 K, and XPS spectra were measured at each temperature. These spectra measurements after Hfi exposure were repeated several timesforthe same sample. The sample surface was observed using SEM, before and after D p exposure. Furthemiore, the samples used in TD and XPS experinrjents were also obsen/ed using SEM. 3. Results and Discussion 3.1.TD Fig.1 shows themrial desorption spectra of M/e=1,2,3,4,17,18 (H2O), 19 (HDO) and 20 (DP) from the sample surface. There were at least two distinct adsorption states of water corresponding to the peaks around 400 K and 600 K. Since the sample surface was exposed to D p in air, isotopic exchange took place tjetween D p and Hfl in air and water vapor in air was adsorlDOd on the sample surface. Accordingly H p (M/e=18) and HDO (M/e=19) were desorbed from sample surface. FeOOH is reported to dehydrate at 409 K [3]. The peak around 400 K of TD was considered to be mainly attributable to the dehydratfon of FeOOD, which was generated on the sample surface by reacting with water. The peak around 600 K could not be assigned from only TD experiments. As shown in Flg.2, when the set of D p exposure foltowed by thermal

300

300

400 500 600 700 Sample teinparature(K)

Fig. 1 Thermal desorptk>n spectra

400 500 600 700 Sample temparature(K)

Fig2 Thermal desorptk>n spectra of Mass19 forrepeatedD p exposure and heating

895

desorption wasrepeatedforthe same sample, the peak around 400 K was increased but there was little change in the peak around 600 K. The intensity increase of the peak around 400 K was considered to result from the increase of adsort)ed Dp. This indicated that the surface area was extended by themial desorption of D^O. 3.2. XPS Fig.3 shows XPS spectra of 01s after exposing the sample to HjD and after heating at 473 K and 773 K in the second time measurement. XPS spectra for each conditfon were nomnalized to the biggest peak around 530 eV, which was assigned to oxygen of iron oxkte [1]. After exposing the sample to H p the intensity between 531 and 534 eV was increased, and after heating in vacuum that was decreased. The spectrum at 773 K was different from the spectrum before H p exposure and this indicated that the adsort)ed water was not completelyremovedby heating up to 773 K in vacuum. Afewpeaks were conskiered to exist between 531 and 534 eV, but further infomnatfon was necessary to assign the peaks ctearly 01 s from hydroxyl of FeCK)Hreportedto appear at 531.2 eV and that from Hfl at 533.1 eV [1]. Furthemnore on metal oxide surfaces OH appears -1 eV higher than O of metal oxide in binding energy (BE), at -531 eV, and molecularly adsorbed water appears -1.5-3.5 eV higher than O of metal oxide in BE [2,4). Accordingly the peak -531 eV was considered to be attributabte to FeOOH, and the peak at -532-533eV to surface OH and molecularly adsorbed H^O [4]. Fig.4 shows XPS spectra afterrepeatedH2O exposure. Intensity between 531 and 534 eV was Increased with increasing times of exposure to water exceptfourthtimes (not shown). This indicated that amount of the adsort>ed KO was increased. 3.3. SEM The sample surface after oxidation is shown in Fig.5 and that after exposure to HgO in Fig.6. After exposure to Hp, needle-like structure was generated on the part of sampte -

' ' • 1 ' ' ' • 1 « •» » T ' ' 1' ''

' ' ' 1 ' ' ' • 1 • ' ' ' 1 '

'

Before H O 2

' S

-

Exposure - ^ H O-lst - ^ H 0-2nd

//

2

1

1

- ^ H 0-3rd

^0.4 I E eO.2

\ r.T'.

534 533 532 531 530 529 528 Binding Energy(eV) Fig. 3 XPS spectra after exposure to H^O and heating

2

f T ^ i . . . . 1 . . . . 1^

\

\ 1..

535 534 533 532 531 530 529 528 Binding Encrgy(eV) Fig. 4 XPS spectra after multiple H,0 exposure (R.T.)

896

surface, and it was also generated on the sample surfaces used in TD and XPS experiments. This needle like structure was considered to be the reactbn products between iron oxide and D p or HgO. This needle-like structure was considered to be FeOOD because crystal structure like a needle of FeOOH existed. This result was in agreement with that of TD and XPS. 3.4. Discussion The peak around 400 K of TD was Rg^SEM image rig.6 SEM image after after oxklatkm attributable to the dehydration of FeOOD. This exposure to HJO attribution was supported by the results of XPS (peak around 531 eV) and SEM (needle-like structure). But as shown in results of XPS (between 531 and 534 eV), the adsorbed states of water on the sample were not unifbmri. Accordingly it was considered that the peak around 400 K partly included desorption of surface OD and molecularly adsorbed Dp. However, from the results of XPS the adsort)ed water was not completely removed by heating up to 773 K. From results of TD and XPS, the amount of adsortjed H p or D p was increased with increasing the times of cycle of adsorptkxi and thermal desorption of water. This indicated that the cycle roughened the sample surface and extended the sample surface area. We couW not assign the peak around 600 K clearly Further experiments are necessary to assign the peak around 600 K. 4. Conclusions The nature of adsort)ed water on the thin films of iron oxide fbnned on the iron surface was not unifbmri and the adsorbed water was desorbed around 400 K and 600 K. However, complete removal of adsorbed water was impossible by heating up to 773 K. Desorption around 400 K was mainly attributable to dehydration of FeOOD. The cycle of adsorptton and desorption of water extended the surface area of the iron oxide. REFERENCES 1. Handbook of X-ray photoemission spectroscopy, JOEL (1991) in Japanese. 2. Mari<us B. Hugenschmidt, Lara Gamble, Charies T Campbell, The interaction of H^O with aTiOg (110) surface. Surface Science 302 (1994) 329-340. 3. The Chemical Society of Japan, Kagaku-binran Kiso-hen 4th ed., Maaizen, Tokyo 1993, p. 1-725 in Japanese. 4. K. Chiba, R. Ohmori, H. Tanigawa, T Yoneoka, S. Tanaka, H P trapping on various materials studied by AFM and XPS, in press.

Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) ic; 2001 Elsevier Science B.V. All rights reserved.

897

Effects of Adsorbed Water upon Friction at Layered ICjNbeOn •3H20 Surfaces Studied with FFM T. Sugai^ and H. Shindo^ ^Department of Applied Chemistry, Chuo University, Kasuga 1-13-27, Bunkyo-ku, Tokyo 112-8551,Japan ^The Institute of Science and Engineering, Chuo University, Kasuga 1-13-27, Bunkyo-ku, Tokyo 112-8551, Japan Frictional properties of two cleaved surfaces of K4Nb60i7-3H20 with different structures were compared with frictional force microscopy (FFM) at various relative humidity (RH) levels. Surface(I) is rough and facing the crystal water layer in the bulk. The other surface(n) is smooth and not hydrated in the bulk. In more humid (RH>30%) conditions, lower friction was observed with the more easily hydrated surface I. When RH level was increased, frictional coefficients at the two surfaces both decreased. Adsorbed water is lubricating the surface. In drier conditions (RH<30%), however, the RH dependence of the friction is reversed. Lower friction was observed at drier conditions at both surfaces and the difference between the two was small. The adsorbed water works as a glue at smaller surface coverage, while it acts as a lubricant at larger coverage. 1. INTRODUCTION Adsorbed water affects frictional properties of solid surfaces. While some workers report on lubrication effect of water on mica[l], some others claim that friction is enhanced by water on the same material[2]. We have observed lubrication effect of water at alkali halides[3] and CaSO4(100) [4] surfaces, but 'glue' effect on CaSO4(001) surface. The effect of humidity upon friction is highly dependent on the materials. In this work we have compared frictional properties of two cleaved surfaces of K4Nb60i7 •3H2O having different levels of affinity with water. The work was supported by The Ministry of Education, Science, Sports and Culture (Grantin-Aid for Scientific Research #12440188), The Promotion and Mutual Aid Corporation for Private Schools of Japan, and The Salt Science Research Foundation (grant #9918, #0011). The authors are grateful to Prof K. Domen of Tokyo Institute of Technology for the gift of K4Nb60i7-3H20 crystals.

898 The crystal structure of K4Nb60i7 -31120, a layered compound, is shown in Fig.l. Here, the structure of Rb salt is used instead of K salt[5]. The space group is orthorhombic, Pmnb , and the unit cell dimensions are ao=7.83A, ^o=39.06A , co=6.57A and 2=4. Thickness of a niobate sheet is about Inm. Depending upon the method of cleavage, the crystal can be separated either at the rugged and hydrated interlayer I, or at the smooth and unhydrated interlayer H, and the the resulting surfaces I and H, respectively, are distinguished by observing atom-resolved AFM images[6,7]. Judging from the images, we consider that the water molecules in the interlayer I is lost upon cleavage. We can expect, however, that the two surfaces have different levels of affinity with water, and that certain amount of water is collected from the air beneath the tip due to capillary condensation. By measuring frictional coefficients at the two surfaces with frictional force microscopy (FFM), we can study the effect of adsorbed water in more detail. 2. EXPEMMENTAL The niobate crystal was cleaved by quickly peeling off the upper part using a sticking tape. This way, the two surfaces I and II can be observed with AFM/FFM at the same time[6]. NanoScope IE AFM/LFM of Digital Instruments was used with commercially available V-shaped cantilevers with an oxide-sharpened silicon nitride tip. In order to control RH,

Interlayer I Interlayer II

I I

m E

IK 0 » i b l ^ 1

cv

^ ^ ^ i i ^ m • - ^ i p •H'iNp n ip •

•

V2bo

Fig. 1. Crystal structure of K4Nb60i7 •3H20[5]. Only the rugged interlayer I is hydrated. *W' denotes the oxygen atoms of the water molecules. The water molecules combine niobate layers by hydrogen-bonding with 0 atoms in the niobate layers and by coordinating with K ions. Only K-0 ionic bonds combine the niobate layers in the smooth interlayer H.

899 the AFM/FFM apparatus was placed inside a plastic bag, and the air with controlled RH was introduced into it. The frictional coefficients are calculated by analyzing the friction loop, geometry of the optical detection system and the structure parameters of the cantilever[3]. 3. RESULTS AND DISCUSSION In Fig. 2 are shown the AFM and FFM images observed at 49% RH. Both surface I and surface II are imaged at the same time. The two surfaces were distinguished by observing the atom-resolved AFM images. In Fig. 2(b), the difference in the friction level between the two surfaces is clearly demonstrated. Lower friction was observed at the surface I, which has adsorption sites for water molecules. The comparison between the two surfaces was continued by changing RH. The results are shown in Fig.3. Both surfaces show less friction toward higher RH. Adsorbed water clearly shows lubrication effect. The effect is more pronounced with surface I. At much drier conditions (RH<30%), however, a different trend is observed. In this range, no clear difference was observed between the two surfaces. The difference in the affinity with water does not affect the friction in this range. It is likely that the stabilization of water at surface I become significant only when the water coverage exceeds certain level. Another important point here is that the friction becomes stronger with the increase in RH. The adsorbed water works as a glue rather than a lubricant. We have observed similar phenomenon with CaS04 (001), a corrugated surface, but never with atom-flat alkali halide (001) surfaces or (100) and (010) surfaces of CaS04[3,4]. At the corrugated surface with ridge-and-valley structure, we speculate