Fluorinated Surfactants and Repellents, Second Edition, (Surfactant Science)

FLUORINATED SURFACTANTS AND REPELLENTS

SURFACTANT SCIENCE SERIES

FOUNDING EDITOR

MARTIN J. SCHICK 1918-1998 SERIES EDITOR

ARTHUR T. HUBBARD Santa Barbara Science Project Santa Barbara, California

ADVISORY BOARD

DANIEL BLANKSCHTEIN

ERIC W. KALER

Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts

Department of Chemical Engineering University of Delaware Newark, Delaware

S. KARABORNI

CLARENCE MILLER

Shell International Petroleum Company Limited London, England

Departntent of Chemical Engineering Rice University Houston, Texas

LISA B. QUENCER

DON RUBINGH The Procter& Gamble Company

The DOWChemical Compaq) Midland, Michigan JOHN F. SCAMEHORN

Institute for Applied Surfactant Research University of Oklahoma Norman, Oklahoma P. SOMASUNDARAN Henry KruntbSchool of Mines Columbia University New, York, New York

Cincinnati, Ohio BEREND SMIT Shell International Oil Products B.V. Amsterdam, The Netherlands JOHN TEXTER

Strider Research Corporation Rochester, New York

1. Nonionic Surfactants, edited by Martin J. Schick (see also Volumes 19, 23, and 60) 2.Solvent Properties of SurfactantSolutions, edited by Kozo Shinoda (see Volume 55) 3. Surfactant Biodegradation, R. D. Swisher (see Volume 18) 4. Cationic Surfactants, edited by Eric Jungermann (see also Volumes 34, 37, and 53) 5. Detergency:Theory and Test Methods(inthree parts), edited by W. G. Cutler and R. C. Davis (see also Volume 20) 6. Emulsions and Emulsion Technology (in three parts), edited by Kenneth J. Lissant 7. Anionic Surfactants (in two parts), edited by Warner M. Linfield (see Volume 56) 8. Anionic Surfactants: Chemical Analysis, edited by John Cross 9. Stabilization of Colloidal DispersionsbyPolymerAdsorption, Tatsuo Sat0 and Richard Ruch I O . Anionic Surfactants: Biochemistry, Toxicology, Dermatology, edited by Christian Gloxhuber (see Volume 43) 11. Anionic Surfactants: Physical Chemistry of Surfactant Action, edited by E. H. Lucassen-Reynders 12. Amphoteric Surfactants, edited by B. R. Bluestein and Clifford L. Hilton (see Volume 59) 13. Demulsification: Industrial Applications, Kenneth J. Lissant 14. Surfactants in Textile Processing, Awed Datyner 15. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, edited by Ayao Kitahara and Akira Watanabe 16. Surfactants in Cosmetics, edited by Martin M. Rieger (see Volume 68) 17. Interfacial Phenomena: Equilibrium and Dynamic Effects, Clarence A. Miller and P. Neogi 18.SurfactantBiodegradation:SecondEdition,Revised and Expanded, R. D. Swisher 19. Nonionic Surfactants: Chemical Analysis, edited by John Cross 20. Detergency:Theory and Technology, edited by W. Gale Cutler and Erik Kissa 21. Interfacial Phenomena in Apolar Media, edited by Hans-Friedrich Eicke and Geoffrey D.Parfitt 22. Surfactant Solutions: New Methods of Investigation, edited by Raoul Zana 23. Nonionic Surfactants: Physical Chemistry, edited by Martin J. Schick 24. Microemulsion Systems, edited by Henri L. Rosano and Marc Clausse 25. Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C. C. Gray 26. Surfactants in Emerging Technologies, edited by Milton J. Rosen 27.Reagents in MineralTechnology, edited by P. Somasundaran and Brij M. Moudgil 28.Surfactants in ChemicaVProcessEngineering, edited by Darsh T. Wasan, Martin E. Ginn, and Dinesh 0. Shah 29. Thin Liquid Films, editedby I. €3. lvanov 30. Microemulsions and Related Systems: Formulation, Solvency, and Physical Properties, edited by Maurice Bourrel and Robert S. Schechter 31. Crystallization and Polymorphism of Fats and Fatty Acids, edited by Nissim Garti andKiyotaka Sat0

32. Interfacial Phenomena in Coal Technology, edited by Gregory D. Botsaris and Yuli M. Glazman 33. Surfactant-Based Separation Processes, edited by John F. Scamehorn and Jeffrey H. Hatwell 34. Cationic Surfactants: Organic Chemistry, edited by James M. Richmond 35. Alkylene Oxides and Their Polymers, F. E. Bailey, Jr., and Joseph V. Koleske 36. Interfacial Phenomena in Petroleum Recovery, edited by Norman R. Morrow 37. Cationic Surfactants: Physical Chemistry, edited by Donn N. Rubingh and Paul M. Holland 38. Kinetics and Catalysis in Microheterogeneous Systems, edited by M. Gratzel and K. Kalyanasundaram 39. Interfacial Phenomena in Biological Systems, edited by Max Bender 40. Analysis of Surfactants, Thomas M. Schmitt (see Volume 96) 41. Light Scattering by Liquid Surfaces and Complementary Techniques, edited by Dominique Langevin 42. Polymeric Surfactants, lrja Piirma 43. Anionic Surfactants: Biochemistry, Toxicology, Dermatology. Second Edition, Revised and Expanded, edited by Christian Gloxhuber and Klaus Kunstler 44. Organized Solutions: Surfactants in Science and Technology, edited by Stig E. Friberg and Bjorn Lindman 45. Defoaming: Theory and Industrial Applications, edited by P. R. Garrett 46. Mixed Surfactant Systems, edited by Keizo Ogino and Masahiko Abe 47. Coagulation and Flocculation: Theory and Applications. edited by Bohuslav DobiaS 48. Biosurfactants: Production Properties Applications, edited by Naim KOsaric 49. Wettability, edited by John C. Berg 50. Fluorinated Surfactants: Synthesis Properties Applications, Erik Kissa 51. Surface and Colloid Chemistry in Advanced Ceramics Processing, edited by Robert J. Pugh and Lennart Bergstrom 52. Technological Applications of Dispersions, edited by Robert B. McKsy 53. Cationic Surfactants: Analytical and Biological Evaluation, edited by John Cross and Edward J. Singer 54. Surfactants in Agrochemicals, Thatwat F. Tadros 55. Solubilization in Surfactant Aggregates, edited bySherrilD. Christian and John F. Scamehorn 56. Anionic Surfactants: Organic Chemistry, edited by Helmut W. Stache 57. Foams: Theory, Measurements, and Applications, edited by Robert K. Prud'homme and Saad A. Khan 58. The Preparation of Dispersions in Liquids, H. N. Stein 59. Amphoteric Surfactants: Second Edition, edited by Eric G. Lomax 60. Nonionic Surfactants: Polyoxyalkylene Block Copolymers, edited by Vaughn M. Nace 61. Emulsions and Emulsion Stability, edited by Johan Sjoblom 62. Vesicles, edited by Morton Rosoff 63. Applied Surface Thermodynamics, edited by A. W. Neumann and Jan K. Spelt 64. Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal 65. Detergents in the Environment, edited by Milan Johann Schwuger

66. Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu Kunieda 67. Liquid Detergents, edited by Kuo-Yann Lai 68. Surfactants in Cosmetics: Second Edition, Revised and Expanded, edited by Martin M. Rieger and Linda D. Rhein 69. Enzymes in Detergency, edited by Jan H. van €e, Onno Misset, and Erik J. Baas 70. Structure-Performance Relationships in Surfactants, edited by Kunio Esumi and Minoru Ueno 71. Powdered Detergents, edited by Michael S. Showell 72. Nonionic surfactants: Organic Chemistry, edited by Nico M. van Os 73. AnionicSurfactants:AnalyticalChemistry, Second Edition, Revised and Expanded, edited by John Cross 74. Novel Surfactants: Preparation, Applications, and Biodegradability, edited by Krister Holmberg 75. Biopolymers at Interfaces, edited by Martin Malmsten 76. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, Second Edition, Revised and Expanded, edited by Hiroyuki Ohshima and Kunio Furusawa 77. Polymer-SurfactantSystems, edited by Jan C. T. Kwak 78. Surfaces of Nanoparticles and Porous Materials, edited by James A. Schwatz and Cristian 1. Contescu 79. Surface Chemistry and Electrochemistry of Membranes, edited by Torben Smith Serrensen 80. Interfacial Phenomena in Chromatography, edited by €mile Pefferkorn 81. Solid-Liquid Dispersions, Bohuslav DobiaS, Xueping Qiu, and Wolfgang von Rybinski 82. Handbook of Detergents, editor in chief: Uri Zoller Part A: Properties, edited by Guy Broze 83. Modern Characterization Methods of Surfactant Systems, edited by Bernard P. Binks 84. Dispersions: Characterization, Testing, and Measurement, Erik Kissa 85. Interfacial Forces and Fields: Theory and Applications, edited by Jyh-Ping Hsu 86. Silicone Surfactants, edited by Randal M. Hill 87. Surface Characterization Methods: Principles, Techniques, and Applications, edited by Andrew J. Milling 88. Interfacial Dynamics, edited by Nikola Kallay 89. Computational Methods in Surface and Colloid Science, edited by Malgorzata Borowko 90. Adsorption on Silica Surfaces, edited by Eugene Papirer 91. Nonionic Surfactants: Alkyl Polyglucosides, edited by Dieter Balzer and Harald Liiders 92.FineParticles:Synthesis,Characterization, and Mechanisms of Growth, edited by Tadao Sugimoto 93. Thermal Behavior of Dispersed Systems, edited by Nissim Garti 94.SurfaceCharacteristics of Fibers and Textiles, edited by Christopher M. Pastore and Paul Kiekens 95.LiquidInterfaces in Chemical,Biological, and Pharmaceutical Applications, edited by Alexander G. Volkov

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96. Analysis of Surfactants: Second Edition, Revised and Expanded, Thomas M. Schmitt 97. Fluorinated Surfactants and Repellents: Second Edition, Revised and Expanded, Erik Kissa 98. Detergency of Specialty Surfactants, edited by Floyd E. f-riedli 99. Physical Chemistry of Polyelectrolytes, edited by Tsetska Radeva

ADDITIONAL VOLUMESIN PREPARATION

Reactions and Synthesis in Surfactant Systems,edited by John Texter Chemical Properties of Material Surfaces, Marek Kosmulski Protein-Based Surfactants: Synthesis, Physicochemical Properties, and Applications, edited by lfendu A. Nnanna and Jiding Xia Oxide Surfaces,edited by James A. Wingrave

SURFACTANTS AND REPELLENTS Second Edition Revised and Expanded

Erik Kissa Consultant Wilmington, Delaware

MARCEL

MARCELDEKKER, INC. D E K K E R

NEWYORK BASEL

First edition published as Fluorinated Stufactnrzts. by E. Kissa, Marcel Dekker, Inc., NY, 1994. Antron. Atsurf, Fluorad, Fluosol, Fluowet, Forafac. Lodyne, Monflor. Oxydent, Pluronic. Quilon. Scotchban, Stainmaster, Surflon, Teflon, and Zonyl are registered trademarks.

ISBN: 0-8247-0472-X This book is printed onacid-free paper.

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PRINTED IN THE UNITED STATES OF AMERICA

Preface to the Second Edition

The revised edition of Fluorinated Surfnctants has been expanded by adding four chapters on repellency and the protection against soiling and staining. Chapter 11 is devoted to the theory of repellency. Chapter 12 describes repellents, including hydrocarbon-type, silicone-based, and fluorinated repellents. Chapter 13 reviews the mechanisms of soiling and soil retardance, as well as soil retardants. Chapter 14 discusses the intricacies of making stain-resistant carpets by using fluorinated polymers and nonfluorinated stain-resist agents. The organization of the first ten chapters has been maintained from the first edition. All the chapters have been brought up to date and a substantial amount of new material has been added. I wish to thank my former employer, the Du Pont Company, for continued access to the library facilities at the Experimental Station. Wilmington, Delaware. Finally, my thanks go to Joseph Stubenrauch and Anita Lekhwani of Marcel Dekker, Inc., for their proficient assistance in preparing this volume. Erik Kissa

iii

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Preface to the First Edition

Fluorinated surfactants are truly the super surfactants. Fluorinated surfactants can decrease the surface tension of water below the lower limit reached by hydrocarbon-type surfactants. The perfluorinated hydrophobe is extremely resistant to chemical attack, and fluorinated surfactants can be used in media where conventional surfactants do not survive. Since a perfluoroalkyl chain is not only hydrophobic but oleophobic, fluorinated surfactants can serve as oil and fat repellents. Compounds consisting of a fluorinated chain and a hydrocarbon group can function as surfactants in hydrocarbon media. Because of their unique properties, fluorinated surfactants are indispensable in certain practical applications and of great theoretical interest for the study of surfactants and micellar systems. Chapter 1 presents an overview of fluorinated surfxtants. The synthesis of fluorinated surfactants is discussed in Chapter 2. Since the space limitations precluded a detailed description of processes, patent citations are augmented by references to Chemical Abstracts. Physical and chemical properties are reviewed in Chapter 3. Chapters 4-7 are devoted to the theory of fluorinated surfactants: liquid-vapor and liquid-liquid interface (Chapter 4), solid-liquid interface (Chapter 3 , solutions of fluorinated surfactants (Chapter 6), and the structure of micelles and mesophases, including mixed surfactant systems, in Chapter 7. The practical application of fluorinated surfactants is the subject of Chapter 8. Various applications are listed in alphabetical order for easy access to information. Chapfor theinvestigation ter 9 reviewstheanalyticalandphysicalmethods of fluorinated surfactants. Chapter 10 examines the environmental and toxicologicalaspects,includingtheuse of fluorinatedsurfactants in biological systems. V

vi

Preface to the First Edition

Because of my intention to write a stand-alone book, material dealt with in other monographs has been included. Related theories and principles are presented along with references to the literature for those who wish to study the fundamental theories in depth. Some discussion of hydrocarbon-type surfactants is given so they can be compared with fluorinated surfactants. Several computer-aided literature searches were conducted. Ongoing research on fluorinated surfactants is very active, and while the book was being written new material had to be constantly reviewed. An effort was made to keep all chapters up to date. Since the book was written at home after regular working hours, I am immensely grateful to my wife, Selma, whose support and patience made this book possible. I am indebted for valuable comments and suggestions to Du Pont chemists who read the chapters in which they have expertise: Drs. J. E. Dowd, T. A. Liss, and J. F. Neumer (synthesis), K. S. Prowse (applications), M. W. Duch (ESCA). J. T. Cronin (IR), A. Foris (NMR). J. R. Valentine (MS), B. E. Baker (toxicology), R. C. Bergman and S. Raynolds (intravascular oxygen carriers). I am also grateful to my son Erik H. Kissa, M.D., for reviewing the chapter on blood substitutes. Last but not least, I am indebted to Joseph Stubenrauch, Marilyn Ludzki, and Sandra Beberman of Marcel Dekker, Inc., for their assistance in preparing this volume.

Erik Kissn

Contents

PREFACE TO THE SECOND EDITION PREFACE TO THE FIRSTEDITION

...

111

V

PARTA.FLUORINATEDSURFACTANTS

1. STRUCTURE OF FLUORINATED SURFACTANTS 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

2.

Introduction Structural Features of Fluorinated Surfactants. The Hydrophobe Anionic Fluorinated Surfactants Cationic Surfactants Amphoteric Surfactants Nonionic Surfactants Silicon-Containing Fluorinated Surfactants Fluorinated Surfactants Without a Hydrophile Polymeric Fluorinated Surfactants References

SYNTHESIS 2.1 2.2 2.3 2.4

Starting Materials Electrochemical Fluorination Telomerization Oligomerization of Tetrafluoroethylene

1 1

3 4 7 8 10 11 14 15 21 29 29 31 36 40 vi i

Contents

viii

2.5 2.6 2.7 2.8 2.9

Miscellaneous Reactions Anionic Fluorinated Surfactants Cationic Surfactants Amphoteric Surfactants Nonionic Surfactants References

3. PHYSICAL AND CHEMICAL PROPERTIES 3.1 3.2 3.3 3.4 3.5

4.

Chemical Properties Melting Points Boiling Points Density Refractive Index References

LIQUID-VAPOR AND LIQUID-LIQUID BOUNDARIES. SURFACE TENSION 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9

Theory of Surface Tension Adsorption at Liquid-Vapor Boundary Surface Tension in Water. Surfactant Structure Kinetics of Adsorption Surface Tension in Acids and Alkali Surface Tension in Organic Liquids Liquid-Liquid Interface Emulsions Foams References

5. SOLID-LIQUID INTERFACE 5.1 5.2

Adsorption from Solution Adsorption of Mixed Surfactants References

6. FLUORINATED SURFACTANTS IN SOLUTION 6.1 6.2 6.3 6.4 6.5 6.6

Solubility Micelle Formation Krafft Point Cloud Point Thermodynamics of Micellization Critical Micelle Concentration

43 44 56 59 64 70

80 80 90 94 99 100 101

103 103 108 124 133 139 145 155 160 166 169

175 175 190 194 198 198 202 210 217 220 228

ix

Contents

6.7 6.8

7.

Solubilization Association with Cyclodextrins References

STRUCTURE OF MICELLES AND MESOPHASES 7.1 7.2 7.3 7.4 7.5

Structure of Micelles Theory for Mixed Micelles Surfactant Mixtures Mesophases and Liquid Crystals Hybrid Surfactants References

8. APPLICATIONS 8.1 8.2 8.3

Performance Characteristics of Fluorinated Surfactants Selection of Fluorinated Surfactants Specific Applications References

9. ANALYSIS OF FLUORINATED SURFACTANTS 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.1 1 9.12 9.13 9.14 9.15 9.16 9.17 9.18

Determination and Characterization of Fluorinated Surfactants Elemental Analysis Volumetric Methods and Ion-Pair Spectroscopy Chromatography Ultraviolet and Infrared Spectroscopy Mass Spectrometry Nuclear Magnetic Resonance Electron Spin Resonance Chemical Relaxation Methods Small-Angle Scattering Methods Light Scattering Luminescence Probing Methods X-Ray Photoelectron Spectroscopy Electrochemical Methods Ultrafiltration Surface Tension Fluorinated Surfactants in Biological Systems Fluorinated Surfactants in the Environment References

256 269 269 277 277 288 299 330 340 342 349 349 350 352 379 390 390 390 393 394 396 399 405 409 410 413 415 41 6 419 422 426 427 434 436 437

Contents

X

10. TOXICOLOGY AND ENVIRONMENTAL ASPECTS 10.1 10.2 10.3 10.4 10.5

PART B.

Toxicology Environmental Aspects Physiology: Sorption, Metabolism, and Excretion Fluorochemical Emulsions for Biomedical Oxygen Transport Drug Delivery and Other Pharmaceutical Applications References

45 1 456 46 1 467 486 487

FLUORINATED REPELLENTS AND SOIL RETARDANTS

11. THEORY OF REPELLENCY

11.1 Definitions 11.2 Wetting 11.3The Equilibrium Contact Angle 11.4 Contact Angles in Real Systems 11.5 Critical Surface Tension and Surface Energy 11.6 Kinetics of Wetting 1 1.7 Repellency of Fabrics References 12. FLUORINATED REPELLENTS 12.1 12.2 12.3 12.4 12.5 12.6

45 1

Repellents with Hydrocarbon Hydrophobes Silicones (Polysiloxanes) Fluorochemical Repellents Repellent Finishing with Fluoropolymers Repellency Tests Future References

494 494 495 498 499 501 506 510 513 516 516 525 530 54 1 543

551 551

13. FLUORINATED SOIL RETARDANTS

557

Soils Soiling Mechanisms Theory of Soil Retardation Fluorinated Soil Retardants Soil-Resistance Tests Fluorinated Soil-Release Agents References

557 558 560 565 568 574 579

13.1 13.2 13.3 13.4 13.5 13.6

i

Contents 14. STAIN-RESISTANT CARPETS 14.1 Soiling and Staining 14.2 Carpet Fibers 14.3 Stains 14.4CoffeeStains on Nylon 14.5 Theoriesfor Stain Resistance 14.6 Stain-Resist Agents 14.7 Stain-Resistance Tests References INDEX

xi

582 582 582 584 5 87 5 89 592 598 602 607

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FLUORINATED SURFACTANTS AND REPELLENTS

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Structure of Fluorinated Surfactants

1.I

INTRODUCTION

Surfactants have a very important role in our everyday life. Surfactants are essential in biological systems and industrial processes. Our food, cosmetics, medicine, and household items, such as soap and detergents, contain surfactants. The wide variety of surfactant applications has required different types of surfactants and a large number of surfactant structures is available for the specific need. The literature on surfactants is voluminous. The word suffactunt is an abbreviation of the more descriptive term suffacenctitv agent. A surfactant is a substance which, even at low concentrations, effectively lowers the surface tension of its medium by selective adsorption on the interface. A surfactant can be a pure chemical compound or a mixture of homologs or different chemical compounds. The characteristic feature of surfactants is their efficiency in lowering surface.tension. The surface tension of a liquid can be lowered by mixing it with another liquid of lower surface tension. For example, one part of ethanol added to four parts of water decreases the surface tension of water from 73 mN/m to below 40 mN/m. However, only 0.1% of a typical surfactant is needed for the same surface tension reduction. The efficiency of surfactants in lowering surface tension is related to selective adsorption of the surfactant at the interface. The adsorption, in turn, is a result of the amphiphilic nature of the surfactant. The term a~nphiphilicor amphiputhic, as it is sometimes called, implies attraction to two different kinds of media. The surfactant structure can be described as consisting of two parts with vastly different solution characteristics: a “solventsoluble” lyophilic part and a “solvent-insoluble” lyophobic part. Conventional 1

2

Chapter 1

surfactants consist of a water-soluble hydrophilic part and a water-insoluble hydrophobic part which is lipophilic, compatible with fats and hydrocarbons. The hydrophobe is usually a hydrocarbon group, but surfactants containing oxygen, nitrogen, sulfur, silicon, and/or halogens are also used. In fluorinated surfactants, the hydrophobic part of the surfactant molecule contains fluorine. At least one hydrogen atom in the hydrophobic segment of a surfactant has been replaced by fluorine. Both the extent of fluorination and the position of iluorine atoms in the surfactant molecule affect the characteristics of the surfactant. Hence, fluorinated surfactants can be classified as perfluorinated surfactants or partially fluorinated surfactants. In perfluorinated surfactants, all hydrogens in the hydrophobic segment have been replaced by fluorine. In partially fluorinated surfactants, the hydrophobic part of the surfactant molecule contains both fluorine and hydrogen atoms. The location and the number of fluorine atoms in the partially fluorinated hydrophobe are important. Partially fluorinated surfactants with a terminal CF3 group differ in their characteristics from partially fluorinated surfactants with a hydrogen-containing terminal group. Substitution of fluorine for hydrogen changes the properties of a surfactant drastically [l-121. The hydrophobic part of the fluorinated surfactant not only repels water but repels oil and fat as well. Hence, fluorinated surfactants exhibit both water and oil repellency when adsorbed on substrates such as textiles or paper. Fluorinated surfactants are much more surface active than their hydrocarbon counterparts. Fluorinated surfactants can lower the surface tension of aqueous systems to below 20 mN/m and are effective at a very low concentration. Only 10 ppm of a fluorinated surfactant may be needed to lower the surface tension of water to 40 mN/m. Fluorinated surfactants exhibit surface activity in organic systems and are stable to heat, acids, and bases, as well as reducing and oxidizing agents. On the negative side is the higher price of fluorinated surfactants, but this is at least partially offset by the small quantities usually needed. Because of their unique properties, fluorinated surfactants are irreplaceable in many applications. The term fluorinated sulfactarzt, although widely used, can be misleading, as it implies that the hydrocarbon segment of a surfactant has been fluorinated. This, of course, is not the real synthetic route to surfactants with a fluorine-containing hydrophobe. The author therefore prefers the shorter term fluorosz~~factmt, in analogy to the frequently used terms flfluoroc-hemicaland fluorocarbon. However, the term.fluorinated srufactcuzts is conventional and, consequently, the title of this book. Some surfactants have counterions which contain fluorine but do not have fluorine in their hydrophobic part. Although such surfactants do not really belong to the class of fluorinated surfactants proper, the presence of fluorine in the counterion affects the behavior of the surfactant. Such surfactants have therefore been included in this book.

I

c

Structure of Fluorinated Surfactants

3

1.2 STRUCTURALFEATURES OF FLUORINATED SURFACTANTS. THE HYDROPHOBE

To understand how surfactants function and to select a surfactant for a specific purpose, it is necessary to classify surfactants according to their structural features. Like all surfactants, fluorinated surfactants are either ionic or nonionic. Ionic surfactants can, unlike nonionic surfactants, dissociate into ions in an aqueous medium. The hydrophobic part can belong to a negative or positive ion. Some surfactants have negatively and positively charged functional groups on the same backbone. The surfactants can therefore be classified into four types: Anionic surfactants-the hydrophobicpartisananion, for example, R1-COO-Na+,where Rf is a fluorine-containing hydrophobe. Cntiorzicswfnctnrzts-the hydrophobicpartis a cation, for example, C7FI&ONH(CH2)3N+ (CH3)3I-. Amphoteric surjuctants-have at least one anionic and one cationic group at their isoelectric point. Nonionic surfactants-do not dissociate into ions, for example, C7F1 sCH2CH20(CH2CH20),H.A special class of nonionic fluorinated surfactants are compounds which do not have a hydrophile but consist of an oleophobic (fluorinated) segment and a oleophilic segment (see Section 1.8). The structure of the hydrophobe of an anionic fluorinated surfactant can be varied more extensively than the structure of the hydrophile. The hydrophobe can be a fully or partially fluorinated alkyl group having a linear or a branched alkyl chain. The hydrophobe can have an aromatic group or contain other elements (0, N, C1, S, and Si) as well, as shown with the following examples:

C, H, F CIF2,Z+ 1CfIF2fz+ 1CHKH2C, H, F. 0 C,IF2,,+ 1 OCF2CF2CnF212+ I %H4C, H, F, 0, N C,lF212+1 CONH(CHdsN< c, H, F, 0, s CnFZr,+I SO,NH(CH2)3N< C, F, C1 CF3CClZ(CFZCFCljll- 1 CFZC, F. Si C8F17CH2CH2Si(CH&-

4

Chapter 1

The hydrophobes of partially fluorinated surfactants contain both fluorine and hydrogen atoms. Unlike the hydrophobe of hydrocarbon surfactants, the partially fluorinated hydrophobe consists of two mutually phobic parts which are not compatible. Partially fluorinated surfactants therefore exhibit anomalies in macroscopic characteristics, such as the critical micelle concentration (cmc), and in microscopic phenomena as well. Partially fluorinated surfactants have several advantages over perfluorinated surfactants. The hydrocarbon segment provides solubility in more commonly used solvents, lowers the melting point of the surfactant, reduces volatility, and decreases the acid strength of fluorinated acids [ 131. Hydrocarbon-type surfactants with fluorinated counterions are not truly fluorinated surfactants, because the surface-active ion is not fluorinated. However, fluorination of the counterion affects the solution characteristics of the surfactant and has been the subject of several investigations. Moss and co-workers [14,15] used the CF3S03- anion as a counterion for a sulfonium methylating agent and a hydroperoxy surfactant. Hoffmann et al. [16,17] investigated surfactant association in solutions of dodecylammonium and tetradecylammonium trifluoroacetates and tetradecylpyridinium perfluorobutyrate. Sugihara et al. [ 181 studied the solubility and cmc of dodecylam~noniumperfluorocarboxylates in water. The effect of the counterion (trifluoroacetate, pentafluoropropionate, and heptafluorobutyrate)hydrophobicity on solubility,cmc, and theKrafftpointwere determined. 1.3 ANIONIC FLUORINATED SURFACTANTS Ionic surfactants dissociate in water and form a surface-active ion with an oppositely charged counterion. The surface-active ions of anionic surfactants bear a negative charge. Anionic fluorinated surfactants can form water-insoluble ion pairs with cationic species and are usually not compatible with most cationic surfactants, Anionic surfactants are the most important class of fluorinated surfactants. Based on the hydrophile structure, anionic fluorinated surfactants can be divided into four main categories:

RfCOO-M+ Carboxylates Sulfonates R~SO:M+ Sulfates R~OSO:M+ R,OP(O)O;-M; Phosphates where Rf is a fluorine-containing hydrophobe and M" an inorganic or an organic cation. Some anionic surfactants contain nonionic hydrophilic polyoxyethylene segments, which increase their solubility and compatibility with cationic or amphoteric surfactants. The fluorinated hydrophobe can be a perfluoroalkyl group or apartially fluorinated alkyl group. Some fluorinated surfactants have hydrophobes containing

rences

Structure Surfactants of Fluorinated

5

aryl groups as well. The hydrophobe may be attached via a sulfide, carbonamido, or sulfonamido linkage to the rest of the surfactant molecule. Examples of anionic fluorinated surfactants with a carboxylic acid or carboxylate hydrophile are given in Table 1.1. Although fluorinated carboxylic acids are stronger acids than alkanoic acids, fluorinated carboxylates have some disadTABLE 1.1 Carboxylic Acids and Salts ~~

Structural features Perfluoroalkanoic acid RfCOOHor RfCOOM Fluorinated alkanoic acid Rf(CH&F2),CH&OOH Alkanoic acids with terminal Rf group Rf(CH2),COONa Alkenoic acids with terminal Rf group RfCH = CH(CH2),COONa Perfluoroalkoxyalkanoicacids RfO(CH2),COOH Perfluoroalkylethyleneoxyalkanoicacids C8Fl7CH2CH2OCH2CH2COOH Perfluoroalkoxybenzoicacid RfOCsH4COOH(Na) Sulfide linkage RfCH2CH2SCH2CH2COOLi Bis(polyfluoroalkoxyalkylcarboxylic acid)sulfide or -S(CH2),Slinkage ( r n = 2 or 6) ~

~

~

~

3

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1,2,10, 19 20 21 21 22 23 6,7 24

2

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~

~

Perfluoroacylaminoalkanoicacids (Na or NH4salts) C8F,7CONH(CH2)5COONa Perfluoroalkanesulfonamidogroup RfS02NH(CH2)3N(CH2COONa)2 RfS02N(CH&H(OH)CH2COOH)(CH2)3N(CH3)CONHC2H5 Hydroxy and amine function CgF1gCH2CH(OH)CH2N(CH3)CH&OOK Perfluoroalkylatedaminocarboxylateswith oxy and hydroxy groups RfCH2CH(OH)CH2NH(CH2)~O[(CH2)40]8(CH2)3NHCH2COONa Perfluoropolyether carboxylic acids CF3[OCF2CF(CF3)],0CF&OOH ( n = 1, 2, 3) HOOCCF2(0CF2CF2)~(OCF2)mOCF2COOH (n, rn = 1,2,3) Oxyethylated segment F3C(CF2),S02N(C2H5)(CH2CH20)n(CH2)3COOK(or Na, Li) ( r n = 3-25,n = 2-50) CgFlgCH2CH(OH)CH2NH(CH2)3O(CH2CH20)8(CH2)3NHCH2COONa RfCH2CH2SCH(COOH)CH2COO(CH2CH20)22H RfCH2CH2SCH(CH&OOH)COO(CH&H20)22H Hexafluoropropene oxide asthe hydrophobe C3F70[CF(CF3)CF20],- 2CF(CF3)COONa( n = 2-6)

25 ~ 26 27 28 29

30

31

32 30 33 34

~

~

2

~

~

~

Chapter 1

6

TABLE 1.2

Fluorinated Sulfonic Acids and Salts

Structural features

i

References

Perfluoroalkanesulfonicacids C,F2, + 1SO3H (or salts) Tetraethylammoniumperfluorooctanesulfonate C~FI~SO~N(C~H~)~ Ammonium salts of fluorinated sulfonic acids RfS0gR2N+(CH2CH20H)2,R = alkyl Perfluoroalkylethanesulfonates CnF.2, + I C H ~ C H ~ S O ~ N H ~ Perfluoropropylalkanesulfonates C3F7(CH2),S03Na ( n = 5, 7, 9) Perfluoroalkylbenzenesulfonates C"F2, + 1C6H4SO3H (Or Salts) Perfluoroalkoxybenzenesulfonic acid RfOC6H4SO3H (Or Salts) Perfluoroacylbenzenesulfonates RfCOC6H4SO3H or Salts HC,F~,COCGH~SO~H or Salts Tri(perfluoroalkyl)methoxyalkanesulfonates (C2F5)3CO(CH2)3S03K Neos "Ftergent NF' (CF3)ZC = C(CF3)0C6H4S03Na C3nF6" - I C ~ H ~ S O ~ H , Fluorinated hydrophobes with sulfide and carbonamide linkages CF3(CF2),CH2CH2SCH2CH2CONHC(CH3)2CH2S03H (or salts) Perfluoroalkanesulfonamidoand carbonamido functions RfS02NH(CH2)3NMeCONH(CH2)2S03Na Perfluoroacylcarbonamidogroup R&ONR(CH2)3S03Na Oligo(hexaf1uoropropene oxide) hydrophobe CF3CF2CF20-(CFCF20)n-Ar-S03H, where Ar is an arylene group

I1 CF3 Oxafluoroalkanesulfonates CF3CF2[CF2CF(CF3)0],CF2CF2S03M, M = K, Na Oxyethylenesulfonates CF3C6H12CH20(C2H40)5S03Na Perfluoroalkylether amides F[CF(CF3)CF20],CF(CF3)CONHCH2CH2S03Na Fluorinated sulfosuccinates RfC2H400CCH(S03Na)CH2COOC2H4Rf H(CF2),C2H400CCH(S03Na)CH2COOC2H4(CF2)nH Perfluoroalkylethersulfonate CF3CF2(CF2CF2),0CF2CF2S03M, M = alkali metal Perfluoroalkylsulfopropionatesand sulfobutyrates CF3(CF2),CH200CCH(CH3)CH2S03Na

8,35-37 8, 38

39,40 41 42 43 6, 7

44 45

46,47 48,49 50 51

52 53 54,55 56

10,57 50 59

Structure of Fluorinated Surfactants

7

TABLE 1.3 Fluorinated Alkyl Sulfates Structure Perfluoroalkylated methyl sulfate C7F1&H20S03Na Fluorinated sulfatopoly (oxyethylene)

References 10

H(CF2CF2),CH2(0CH2CH2),0SOSO3NH4 CF3(CF&F2),CH2(0CH2CH2),0S03NH4 Perfluoropropoxylated sulfate (F3C)2CFO(CH2)60S03Na Fluorinated aminosulfate C8F17S02NH(CH2)3NH(CH2)3NHCH2CH20S03Na

60 61

62

vantages characteristic of their hydrocarbon-type counterparts, such as insolubility in strong acids and in water containing divalent or trivalent metal ions. Examples of fluorinated surfactants with a sulfonic acid group (or its salt) as the hydrophile are shown in Table 1.2. Fluorinated sulfonic acids are less sensitive to low pH, electrolytes, and calcium ions than their carboxylic acid analogues. Some examples of fluorinated surfactants with a sulfate group are given in Table 1.3. A sulfate group is a stronger hydrophile than the sulfonate group. Sulfated fluorinated surfactants are readily available from fluorinated alcohols, but their lower hydrolytic stability limits their use. Table 1.4 shows examples of fluorinated surfactants featuring a phosphate hydrophile. In general, the phosphates are less prone to cause foaming than other anionic fluorinated surfactants and some phosphate ester salts function as antifoaming agents. 1.4

CATIONICSURFACTANTS

In cationic fluorinated surfactants the fluorinated hydrophobe is attached directly or indirectly to a quaternary ammonium group, a protonated amino group, or aheterocyclic base. Cationic surfactants dissociate in water, forming a surface-active positively charged ion and a negatively charged counterion. Like anionic surfactants, cationic surfactants are sensitive to electrolytes and the pH of the medium. Cationic surfactants are usually incompatible with anionic surfactants. Cationic fluorinated surfactants adsorb on negatively charged surfaces. Because most surfaces and particles are negatively charged, the adsorption can be advantageous or disadvantageous, depending on the intended use of the surfactant. For example, adsorption on clay and sludge in wastewater cleaning systems simplifies the removal of cationic fluorinated surfactants from effluents.

Chapter 1

8

TABLE 1.4

Fluorinated Phosphates ~~~

~

~

Structure

References

63,64 Perfluoroalkylethyl phosphates CF3(CF2)"CH2CH20P(O)(OH)2 [CF3(CF2),CH2CH2o12P(O)(OH) [CF3(CF2)"CH2CH2012P(O)(ONH4) 65 Mono- and bis(fluoroalky1)phosphateamine salts (C~FI~CH~CH~~)~~PO[(OH)NH(C~H~~H)~~~.~ 66 Perfluoroalkyl phosphates (CF3)2CF(CF2)6FCH2CH2op(o)(OH)2 [ ( ~ ~ 3 ) 2 ~ ~ ( ~ ~ 2 ) 6 ~ ~ ~ 2 ~ ~ 2 ~ 1 2 ~ ( ~ ) ( ~ ~ )

[(CF3)2CF(CF2)6FCH2CH20]3P(o) Oligomer phosphate CeFl&H=C(CF3)OPO(OH)2 (Perfluoroalkyl)glycol monophosphates F(CF2),CH(OH)CH,OP(O)(OH)2 F(CF2),CH(CH2OH)OP(O)(OH)2 C6F,3CH2CH2S(CH2)3P(0)(0C2H5)2 Perfluoroalkanesulfonamidederivatives CF~C(CF~)~SO~N(C~HS)CH~CH~OP(O)(OH)~ CF3C(CF2)7S02N(C2H5)CH2CH20P(0)(ONa)2 [CF3C(CF2)7S02N(C2H5)CH2CH20]2P(0)ONa Polymerizable phosphate esters CH2=CRCOOCH2CH(OH)CH20P(0)(OR')ONa, where R = H or CH3, R' = alkyl, fluoroalkyl

6, 7

67 68 69

70

Some cationic fluorinated surfactants contain both quaternary and secondary amino groups and carbonamido or sulfonamido linkages. Examples of cationic fluorinated surfactants are given in Table 1.5. Amine oxides, or more correctly amine oxides of tertiary amines, are nonionic in an alkaline or neutral solution but cationic in acid solutions. The synthetic routes to amine oxides and cationic surfactants are similar. Although amine oxides are intrinsically nonionic, they may be considered to belong to the class of cationic surfactants. 1.5 AMPHOTERICSURFACTANTS

Amphoteric fluorinated surfactants are bifunctional compounds which can function both as anionic and as cationic surfactants, depending on the pH of the medium [87]. Amphoteric surfactants have at least one cationic group and at least

I

!

i

Structure of Fluorinated Surfactants

9

one anionic group and are electrically neutral around their isoelectric points. Outside the isoelectric range, amphoteric surfactants function as anionic or cationic surfactants, depending on the pH of their medium. Ideally, typical amphoteric fluorinated surfactants function in an alkaline medium as anionic surfactants, whereas in an acid medium, they assume a cationic character: OH-

RfNfH2CHaCH2COOH RfNfH2CH2CH2COO-

OH-

H+

RfNHCH2CH2COO-M+

In reality, surfactants having cationic and anionic functional groups are not always truly amphoteric. The dissociation constants of anionic and cationic functional groups are rarely equal. As a consequence, either the cationic or the anionic character may dominate the behavior of the surfactant in solution. TABLE 1.5 Cationic Fluorinated Surfactants Structural feature (Perfluoroalkyl)alkyIC,F2, + ICH2CH2N+(CH3)2C2H51-, n = 6 or 8 C7Fl5CH2NH(CH2)2Nf(CH3)3ClH(CF2),-,CH2N+R(CH3)21-, n = 2, 4, 6; R = CH3, C2H5, C3H7, C4H9 (CF3)2CF(CF2)6CH&H(OH)CH2N+(CH3)31PerfluoroalkaneamidoC7F1&ONH(CH2)3Nf(CH3)1F[C(CF,)CF20],CF(CF3)CONH(CH2)3N+(C2H5)2CH31RfO(C3F6O)CF(CF3)CONH(CH2)3N+(CH3)3ClCF3CF2CF&ONHCH2CH2CH2N+(OH)(CH3)2HOCH2COOPerfluorooctanesulfonamidoC8F17S02NH(CH2)3Nf(CH3)31Sulfide linkage Rf(CH2)nS(CH2),Nf(CH3)2CH2COOH CIC6F13CH2CH2SCH2N+(CH3)2CH2CH20H Br(CF3)2CF(CF2)4(CH2)2SCH2CH(CH3)2COOCH2CH2N+(CH3)31RfCH(OH)CH2SCH2CH2N+(CH3)2CH2C6H5Br-, Rf = CSFl3 or c8F17 Ester linkage C6F13S020(CH2)3N+(CH3)31Amine salts [F(CF2)8CH(OH)CH2]2NCH2CH2NH2*H2SO4 Heterocyclic nitrogen N-(PerfluorooctanesuIfony1)piperazinederivatives Terminal perfluoroalkyl (CF3-) CF3(CH2)nN+(CH3)3Br-

References

71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

Chapter 1

10

Amphoteric fluorinated surfactants usually have a ( 1) carboxybetaine (I), (2) sulfobetaine (11),or (3) sulfatobetaine (111) structure: RfXYN+(CH3)2(CH2),,COO(1) R~XYN+(CH~)~(CH~),,SOS (11) RfXYN+(CH3)2(CH2)nOS020- (111) where n = 1, 2. or 3, (4) an amino acid function. or ( 5 ) a dialkylated heterocyclic nitrogen, for example, n-dialkylpiperazine (88) or rz-dialkyl- 1. 4-oxazine ~91. Amphoteric fluorinated surfactants are compatible with other types of surfactants and adsorb on either negatively or positively charged surfaces. Amphoteric fluorinated surfactants are used in fire-extinguishing agents, foam stabilizers, wetting agents, spreading agents on hydrocarbon surfaces. emulsifiers for manufacturing tluoropolymers, repellents for paper and textiles, cleaning agents for degreasing metal surfaces, and body or hair shampoos. Their main disadvantage is a higher price. Examples of amphoteric fluorinated surfactants are given in Table 1.6. 1.6

NONIONIC SURFACTANTS

Nonionic surfactants do not dissociate into ions in water. As a consequence, nonionics are less sensitive to electrolytes and pH changes. Nonionic fluorinated surfactants are soluble in anacid or an alkaline medium and are compatible with ionic and amphoteric species. Unlike ionic surfactants, nonionic fluorinated surfactants are not preferentially adsorbed on charged surfaces. The hydrophile of nonionic surfactants is usually a polyoxyethylene chain or consists of polyoxyethylene and polyoxypropylene segments. The length of the hydrophilic chain can be conveniently varied to modify the hydrophile-lipophile balance (HLB) of the surfactant, a property which affects interfacial behavior and the stabilization of emulsions. The solubility of nonionic surfactants decreases with increasing temperature, and at the cloud point (see Section 6.4), the aqueous solution becomes turbid. In general, nonionic fluorinated surfactants are more soluble in organic solvents than ionic fluorinated surfactants. The polyoxyethylene hydrophile is less stable chemically than carboxylate or sulfonate hydrophiles. Hence, nonionic fluorinated surfactants cannot be used in strongly oxidizing media. Examples of nonionic fluorinated surfactants are given in Table 1.7. Typical nonionic fluorinated surfactants are oxyethylated alcohols, amines, or thiols (mercaptans).

Structure Surfactants of Fluorinated

11

TABLE1.6 Amphoteric Fluorinated Surfactants Structure Carboxybetaines RfCH2CH(OOCCH3)CH2Nf(CH3)&H&0ORfCH&H(OH)CH2N+(CH3)2CH2COORfCH2CH(OCOCH3)CH2Nt(CH3)2CH2C0OCgF1gCONH(CH2)30(CH2)2Nt(CH3)2CH2COOCF3(CF2),CH&H2CONH(CH2)mN+(CH3)2CH2CH&OO( n = 7 or 5, m = 3 or 2) RfCH2CH2SCH2CH2N '(CH3)2CH2COORrCH2CH2SCH2CH(OH)CH2NCH(CH3)CH2COOCF3(CF2),CH&H2SO2NHCH&H2N+(CH3)2CH2CH&OO( n = 5, 7 , or 9) p-CgF170C6H4S02NH(CH2)3Nf(CH3)2CH&OOPerfluoroalkyletheramidoalkylbetaines C3F70CF(CF3)CF20CF(CF3)CONH(CH2)3N+(CH3)2CH2COOSulfobetaines C8F17CH2CH2CONH(CH2)3N+(CH3)2CH2CH2CH2SOg C~F~~OCGH~CONH(CH~)~N+(CH~)~CH~CH~CH~SO~ Sulfatobetaines CF3(CF2)6CF = CHCH2Nf(CH3)2CH2CH20SOi RfCH2CH2SCH2CH(OSOi)CH2N'(CH3)3 Trianion-type amphoteric fluorinated surfactants C8F1&ONH(CH2)3N+(C2H4OH)(CH2COOH)2CH2COOC8F,7S02NHCH2CH(OH)CH2N+CH3[CH2CH(OH)CH2SO3Na]2ClC6F13S02NHCH2CH2CH2N+CH3(CH2CH2CH2S03Na)21~C6F13S02NH(CH2)3N+(CH~COONa)[CH2CH(OH)CH2S03Na]2Cl4-WC8F17C6H4S02NH(CH2)6Nf(CH2CH20S03Na)3Clr+C8F17S02NH(CH2)3N+(CH2COONa)3CI-

References

90 89 24 91

92 93 94 95 96 97 98 99 100 101

102 103 103 104 104 105

1.7 SILICON-CONTAININGFLUORINATEDSURFACTANTS The surface tensions of silicone surfactants [ 126-1 3 11 are, in general, below those of hydrocarbon surfactants. For example, the surface tension of dimethicones (polydimethylsiloxanes) in water is about 20-2 1 mN/m. Because perfluoroalkyl substituents usually decrease the surface tension of the parent compound, fluorination of silicone surfactants [ 131-1581 can lower the surface tension of silicone surfactants as well. The reduction of this concept to practice is not straightforward, however. Fluorinated substituents in the (x and f3 position to silicon weaken the Si-C bond and make it vulnerable especially to a nucleophilic attack [126]. Fluorination of a silane such as CH3SiC13or (CH3)2SiC12yields compounds of considerable thermal stability, butthe exceptionally high electronegativity of fluorine and the consequential strong inductive effect make the Si-C bond hydrolytically unstable [132,133]. Hydrolytic cleavage of the Si-F bond in

12

Chapter 1

TABLE 1.7 Nonionic Fluorinated Surfactants Structure Oxyethylated alcohols CF3(CF2)nCH20(CH2CH2O),H CF3CF2(CF2CF2),CH2CH2O(CH2CH2O)nH F(CF2),(CH2),(0CH2CH,),OH(q = 6, 7, m 1, 3, n = 4, 5, 6, 8) H(CF2CF2)&H20(CH2CH2O),H (CFB)~CFO(CH~)GO(CH~CH~O)~H Propylene oxide segments CF3CHFCF2CH20[CH(CH3)CH2O]mH CF3CHFCF2CH20[CH(CH3)CH20],(CH&H2O)nH Hydroxyl hydrophile CgFIgCH2CH(OH)CH20C2H5 Fluorinated polyhydric alcohols C8F17C2H40[CH2CH(CH20H)O],H ( n = 1.7 average) C8F,7C2H4S02NH[CH2CH(CH20H)O]nH ( n = 1.7 average) Oxyethylated perfluorophenol C6F5(0CH2CH2)lOOH Perfluoroalkyl-2-ethanethiolderivatives CsF13CH2CH2S(CH2CH20)3H R&H2CH2SCH2CH(OH)CH2O(CH2CHO)CH3 (avg. n = 7, Rf = c4-12) R&H4S(CH2CH20),C2H4S(CH2CH2O)nH Carbonylamidofunction CnF2, + 1CONH(CH2CH20),H ( n = 6-9, m = 2-4) C7F15CONH(CH2)3N(CH2CH2OH)2 CnFn + ICH~CON[(CH~CH~O),CH~]~ Sulfonamido function C~FI~CH~CH~SO~N(CH~)CH~CH~OH F&(CF2)7CHFCF2S02N[(CH&H20)nH]2 R$02N(CH3)CO(OCH2CH2),OC4Hg ClOF190C6H4S02N(C2H5)(CH2CH2o)nH

References 10,106-1 09 24 110 106,111-1 13 114 114 114 115 116 116 117 118,119 120 121 122 10 123 124 125 8 6

compounds CF3SiC13 or (CF3)?SiC12 liberates the fluorinatedhydrocarbon CF3H. To reduce the inductive effect on the Si-C bond, the perfluorinated group has to be isolated from the silicon atom. Fluorination of an alkylsilane even in the p position yields hydrolytically unstable compounds which undergo thermal rearrangements and liberate CF2=CH2. Fluorination of an alkyl group in the y position, CF3CH2CH2-, does not affect the Si-F bond significantly and the hydrolyticstabilityisadequateforpracticaluse [ 1321. Thedichlorosilane CF3CHzCH2Si(CH3)C12 hydrolyzes to form a polysiloxane. However, the ethylene link increases the bulkiness of the side chains, increases the hydrocarbon/fluorocarbon ratio, and causes fluorocarbon-hydrocarbon and fluorine-silicone interactions and orientation effects which may reduce the surface activity of the fluorinated compound.

L

Structure 13 Surfactants of Fluorinated

The surface tension of fluorosiloxanes in the liquid state is measured directly and usually the values obtained are not affected by the measurement technique. The equilibrium surface tension in water is related to the liquid surface tension value. Siloxanes with trifluoropropyl groups are less effective in lowering the surface tension of water than the nonfluorinated poly(dimethy1) siloxanes. The surface tension of poly(3,3,3, trifluoropropylnlethylsiloxane) is higher than that of the lower-cost dimethicones [poly(dimethylsiloxane)] [134,1351. Because of the high affinity of fluorine to silicon [ 1361, fluorine atoms may be inclined to coordinate with silicone atoms. The distorted orientation may partially expose the hydrocarbon link of the pendant side chain. To increase the surface activity, Owen and Groh [ 1351 increased the fluorocarbon content of the side chains while maintaining the ethylene link between the fluorocarbon group and the silicon atom. Nonafluorohexyl (3,3,4,4,5,5,6,6,6nonafluorohexyl) disiloxane and trisiloxane surfactants have equilibrium surface tensions in water at the cmc of 20 mN/m [ 1421. However, the response of their surface tension to dynamic changes is impeded by bulkiness of the flouroalkyl group [142]. The surface tension of the fluorosilicones is shown in Table 1.8. The solid-surface tension, indicated by a contact angle of a liquid drop (e.g., 17-hexadecane,water, and methylene iodide) on the fluorosiloxane film on glass is different from the liquid-surface tensions. Siloxanes containing trifluoropropyl groupshavelower a surfacetension than thenonfluorinatedsiloxane [poly(dimethylsiloxane)] [ 1521. The cmc values of nonionic fluorosilicone surfactants having two trifluoropropyl groups are similar to those of methylsiloxane surfactants with a branched trisiloxane hydrophobe. However, surfactants containing three trifluoropropyl groups and those containing nonafluorohexyl radicals have higher cmc values, attributed to the bulkiness of the hydrophobe [142].

TABLE 1.8 Surface Tension of Fluorosilicones

Structure Surface tension of liquid (Me3CO)2[CF3(CF2)6CH20]2Sia CF3(CF2)6(CH2)2Si(OMe)3 CF3(CF2)3(CH2)2Me2SiOSiMe2H Aqueous solution (1.O% conc.) [CF3(CF2)6CH202CCH2CH(SOiNa+)C02(CH2)3Me2Si]20 CF3(CF2)6CH202CCH2CH(SOiNa+)C02(CH2)3Me2SiOMe2Si(CH2)3OH Me = CH3-. Source: Ref. 135.

a

Surface tension (nM/m)

18.4 18.1 16.4 12.5 13.0

Chapter 1

14

Siloxanes have been designed by connecting the perfluoroalkyl group to silicon with a link other than ethylene. Polysiloxanes featuring perfluoroalkylcarbonamide groups have been described in patent literature [ 137-1 391: CH3

I I (CH2)3 I N"CH3 I c=o I

(CH,),Si-O-[Si-O],,,Si(CH3)3

CnF211+1 Co-oligomers of fluorinated silicon containing sulfo groups [ 135.1591 or carboxyl groups [ 15I] have been prepared. Co-oligomerization of fluoroalkanoyl peroxideswithacrylicacidandtrimethylvinylsilaneyieldedamphiphilic oligomers soluble and surface active both in water and aromatic solvents, such as benzene, toluene, and rn-xylene:

RF-(CH~CH),,,"(CH~CH),,"R~

I

Si(CH3j,

I

COOH

RF = C3F7, CF(CF3) [OCF$2F(CF3)Ip0C3F7: y

I

0,1,2

Amphiphilic oligomers surface active in both water and aromatic solvents were obtained as well by reacting fluoroalkanoyl peroxides with acrylic acid and dimethylsilcones containing one vinyl or one methacryloxypropyl end group [153]. Owen and Groh [ 1351 prepared fluorosilicone copolymers, composed of CF3(CF2),,CH2CH2Si(CH3)0, (CH3),SiO, and (CH3)3Si01/7.where rz = 3 , 5 , and 7. These fluorosilicone copolymers were found to be more effective antifoam agents than conventional silicone antifoams at defoaming silicone surfactants but ineffective against foaming of fluorinated surfactants having a very low surface tension (see also Foams and Defoamings, Chapter 8).

I .8 FLUORINATED SURFACTANTS WITHOUT A HYDROPHILE Surfactants are used most frequently in aqueous systems. Their amphiphilic character arises from the presence of a hydrophilic group and a hydrophobic group in the same molecule. A perfluoroalkyl group of fluorinated surfactants is not only hydrophobic but oleophobic as well. In a hydrocarbon medium, a perfluoroalkyl

15

Structure of Fluorinated Surfactants

group is a lyophobe (oleophobe), whereas an unfluorinated alkyl group is a lyophile (oleophile). In a fluorocarbon medium, a perfluoroalkyl group is a lyophile, whereas a unfluorinated alkyl group is a lyophobe. Hence, a compound featuring a perfluoroalkane segment and a hydrocarbon segment is amphiphilic in a hydrocarbon or fluorocarbon medium. Semifluorinated alkanes are low-molecular-weight block copolymers of normal perfluorocarbons and hydrocarbons [ 160-17 11. Their structure formula F(CF&(CH2),1H is usually abbreviated to F,,,H,, [ 1641 or F,,H,, [ 1631. The semifluorinated alkanes so far reported have 6 to 32 carbon atoms ( m 1 1 ) . The compounds 171 = 12 and 11 = 8, 14, or 18 lower the surface tension of rzdodecane [ 1641. The adsorption of the semifluorinated alkane at hydrocarbon-air surfaces increases with increasing length of its fluorocarbon chain [ 1731, increasing chain length of the alkane solvent and decreasing temperature [169]. Static and dynamic light scattering, small-angle neutron scattering, nuclear magnetic resonance (NMR), viscocity, vapor-phase osmometry, and fluorescence techniques have shown that semifluorinated alkanes form micelles in hydrocarbon and fluorocarbon solvents: F8H1-,in perfluorotributylan~ine [ 1651, F8HI6 in toluene [165], perfluorooctane [165,168], and perfluorodecalin [170]. F10H16in toluene [165], and FlzH16in perfluorooctane [168]. The estimated aggregation numbers of the micelles are small, about four to six. The compound FI2Hl0forms a gel in tluorocarbon solvents (perfluorodecalin) [166]. FloHloforms a liquid-crystal phase [167]. Semifluorinated alkanes FloH1-,, F13H14, and F12H18form a gel with n-dodecane at higher concentrations (10”M) when allowed to cool to ambient temperature from above their melting points [ 161,1641.When dissolved in a hydrocarbon solvent and placed on a water surface, semifluorinated alkanes (e.g., F12H8,F10H12.and F12H18)spread and form monolayers [ 1641. The observed area occupied by a molecule suggests that the perfluoroalkyl segment extends out from water while the hydrocarbon segment is immersed in water [ 1641. The polymeric surfactants (HFPO),,Ar, where (HFPO),, is a hexafluoropropylene oligomer and Ar an aryl group [52,173], lower the surface tension of hydrocarbons such as m-xylene and belong, therefore, to the class of hydrophilefree surfactants (see Section 1.9). The semifluorinated alkanes, such as F6Hlo. stabilize aqueous perfluoro emulsions by forming an interface between the fluorocarbon phase and the phospholipid emulsifier [174]. The stabilizing effect is attributed to a more favorable structure of the interfacial film [ 1751.

+

1.9 POLYMERIC FLUORINATEDSURFACTANTS

Sfrucfureof Fluorinated Polymeric Surfacfanfs Polymeric and oligomeric surfactants [ 176-1 781 are used in various technical and biological applications, most importantly as stabilizers or flocculants of disper-

Chapter 1

16

sions [ 1791. Polymeric surfactants are more strongly adsorbed than monomeric surfactants and are less affected by electrolytes and temperature changes. Silicone-containing polymeric surfactants have been described in Section 1.7 and Chapter 12. Polymeric fluorinated surfactants with a carbon containing backbone are discussed below. Polymeric fluorinated surfactants (1) have a fluorinated backbone or (2) contain perfluroalkyl groups, as pendant perfluoroalkyl groups attached directly or indirectly to the polymer backbone, or as one or two fluorinated end groups of the polymer. A perfluoroalkyl tail can be attached to poly(tris(hydroxymethy1)acrylamidomethane or to a natural product, such as a sugar, polyol. and so forth [180]. A fluorinatedgroup may be attachedtoa poly(oxyethy1ene) chain -CH2CH20-, to an ethylene oxide-propylene oxide block polymer, or to an acrylic comonomer. A fluorinated backbone may contain -CF2CF2or polyether units. The pol yether chain may be perfluropoly(oxypropy1ene) -OCF2CF-

I

or -CF2CFZCF20-

CF3 or a perfluorinated polymer containing -[(CF2CF20),, (CF20)J-

repeat units.

lonomers Perfluoroalkane ionomers consist of a perfluorinated backbone and pendant chains terminated with an anionic group. for example with sulfonic acid groups (Nafion H) [181-1851: -(CFZCF&CFZCF-

I

OCF~CFOCF2CF~OS03H

I

CF3 Ionomers are used mainly as ion-selective and ion-separation resins.

Petfluoropolyethers Perfluoropolyethers [ 185-1 931 are thermally and chemically stable materials of great commercial importance. Perfluoropolyethers CF3[CF(CF3)CF20),,CF2CF3, where rz = 27 average, being chemically inert, form stable films on concentrated acids (e.g., sulfuric acid, nitric acid, and phosphoric acid) [194]. The fluorinated polyether is insoluble in the acids and lowers their surface tension. Steady-state spreading pressures, calculated from the nearly constant surface tension values, correlate with the Hammett acidity of the acid. This relationship suggests that the

Structure of 'FluorinatedSurfactants

17

driving force for spreading is hydrogen-bonding between the acid and the oxygen or fluorine atoms of the polyether. Perfluorocarbon chains, especially linear ones, are less flexible than hydrocarbon chains [52.132] and the melting points of fluorinated surfactants are usually higher than those of their hydrocarbon analogs. Fluorinated polyethers, such as oligomers of hexafluoropropene oxide (HFPO), are more flexible and have a lower melting point than perfluoroalkanes. Fluorinated polyethers are therefore more suitable as long-chain hydrophobes or oleophobes for polymeric surfactants [ 195-1 991. Some examples of fluorinated surfactants with an oligohexafluoropropene chain (HFPO),,+] [52,200] are shown below (see also Chapter 4, Figs. 4.2 1 and 4.22):

CF3CF2CF20(CFCF20j,,CFCOONa CF3

I

CF3

I

and

CF3CF2CF20(CFCF20),,-ArSO3Na

I

CF3 where Ar is an aryl group. Perfluoropoly(oxypropy1ene) F(CF2CF2CF20 j,zCFzCF2R and perfluoropoly(oxyethy1ene-co-oxymethylene), R-CF~0[(CF2CF20)o.4(CF20)o.6],l-R, where R = CF3, OH, or COOH, perfluoropolyethers are not surface active in water when R = CF3 [201.202]. By contrast, the polyethers with hydrophilic head groups " O H or "COOH are surface active in water. Surface-pressure measurements by the Langmuir-Blodgett technique suggested that all hydrophilic head groups are attached to the water surface, whereas the polymer chains are located above water. The nature of the polar end groups affects the tribological response of linear perfluoropolyalkyl ethers (PFPAE). The introduction of carboxyl, hydroxyl, or piperonyl end groups increases the ability of the PFPAE polymers to protect the substrate against wear [203]. The results suggest considerable interactions of polar end groups in an otherwise perfluorinated system. Nonionic fluorinated poly(propy1eneoxide) surfactants (HFPO),,+l-Ar are soluble in aromatic solvents (e.g. toluene or xylene) and lower the surface tension of these solvents [52] (see Chapter 4, Table 4.17 and Section 1.8 of this chapter). Ishikawa and Sasabe [52] found the effectiveness of these fluoroalkylated benzenes to increase with the increasing HFPO chain length. This is in apparent discord with the results obtained by Sawada et al. [153] for low-molecular-weight fluoroalkylated benzenes with a short HFPO chain. Sawaka et al. found that surfactants with only one or two HFPO linkages lowered the surface

18

Chapter 1

tension ofnz-xylene further than thesurface tension decrease reported by Ishikawa and Sasabe for the long-chain analogs. Although different techniques were used for surface tension measurement, an incongruity is apparent. Stevenson etal. [204] investigated the reduction in surface tension of xylene by phenylcarbonylpoly(hexafluoropropy1ene oxide) and phenylpoly(hexafluoropropylene oxide) surfactants. They suggested that the surfactants used in Refs. 52 and 173 may have contained a component which adsorbed on the Wilhelmy plate used for surface tension measurements and prevented the complete wetting of the plate. Monduzzi et al. [205] studied three- or four-component water-oil microemulsions containing perfluoropolyether (PFPE) oils and a anionic surfactant, MW 723, with a PFPEoleophobic chain

where R is a perfluoroalkyl group. The surfactant formed microemulsions even without a cosurfactant, behaving in some ways similar to a double-chained ionic surfactant, such as didodecyldimethylammonium bromide. Miyamoto et al. [206,207] prepared block polymers of poly[(acylimino)alkylene] imino ethers featuring alkyl and perfluoroalkyl hydrophobes. Short perfluoroalkyl groups (C2FSor C3F7) of the copolymers were sufficiently hydrophobic to lower the surface tension of water below 20 mN/m.

Poly(oxyethy1ene) Ethers withFluorinated End Groups Polymeric fluorinated surfactants RfO(CH?CH20),,Rf (12 = 7, 13, 23, or 90) featuring a long-chain hydrophile are usually prepared from polyethylene glycols. Houghton[208]reactedthetetrafluoroethylene, CF?=CF?, pentamerwith polyethylene glycol [molecular weight (MW) 10001 to produce the surfactant CloF190(CH2CH~O)2~Cl~F~9. The sulfonyl chloride C10F190C6H~S01C1 yielded CloF190C6H4S020(CH~CH20)2302SC6H40C1~F19. The polymer C7FIsC00(CH3CH10)230CC7F15wasprepared by reactingpolyethyleneglycol with C7FI5COC1.These surfactants improve the gloss and leveling of coatings, polishes, and paints. 1H, lH,2H,2H,3H-Perfluoro- 1,2-nonylene oxide and t-butyl glycidyl ether gaveasurfactant of the statistical composition C10F271CH?CH1_0{ [CH2CH(CH1C~FI~)0]5[CH~CH(CH20H)O]18}H (209), used as surfactant a in shampoos. Mueller [210] prepared perfluoroalkyl-substituted half-esters and amides featuring a Rf-R-Sfunction, a carboxylic group, and a polyoxyethylene chain. As an example, maleic anhydride was reacted in methylethyl ketone with

i I

i

i

!

Structure of FluorinatedSurfactants

19

polyoxyethylene of MW 1000. The reaction product was reacted with perfluoroalkylethylmercaptan (Rf consisted of Cg, cg, and Clo isomers) in the presence of triethylamine as a catalyst to form the fluorinated surfactant RJH2CHZSCHCOOH

I

Fluoroalkyl-terminated ethylene oxide-propylene oxide block polymers have been described by Gross et al. [2 111.

Fluorinated Vinyl Polymers Random acrylic copolymers (2 12) containing -(CH?CH,)

I

COOCH2CHzCgF17 and “(CH2C(CH3)COOR, R

= “CH3,

-CISH37,

repeat units have been used as emulsification agents. Polymeric fluorinated surfactants containing ionic groups may or may not be surface active, depending on their structure [213]. The choline methacrylate-l,l,2,2-tetrahydroperfl~~orooctylmethacrylate copolymers [214] exhibit a considerable surface activity in water but without a break point expected for a critical micelle concentration. The reduced viscocities are low, like those of corresponding polymers without hydrophobic groups. The surface activity, aggregation, aqd solubilization resemble “polysoaps” not containing fluorine but with significant differences. The solubilization capacity of fluorinated “polysoaps” is much smaller than that of the hydrocarbon analogs. This is not surprising because the fluorinated groups are both hydrophobicd a i oleophobic. 4-Vinylpyridinium chloride oligomers with one or two fluorinated end groups, C3F7- or CF(CF3)0[CF2CF(CF3)0],,C3F7,tn = 0, 1,2, have been prepared by Sawada et al. [215]. These oligomers reduce the surface tension of water to about 10 mN/m and exhibit a break point resembling a critical micelle concentration. Perfluoropropylated and perfluoro-oxaalkylated 4-vinylpyridinium chloride oligomers have been prepared by reacting fluoroalkanoyl peroxides with 4vinylpyridinium chloride [2161. Sawada and co-workers prepared also fluorinated 2-acryloxyethyltrimethylammonium chloride oligomers [217,218] and fluoroalkylated allylammonium chloride and diallylammonium chloride oligomers [2191.

Chapter 1

20

Fluoroalkylated sulfonic acid oligomers have been prepared by reacting various fluoroalkanoyl peroxides with 2-(methacryloxy) ethanesulfonic acid under mild conditions [218,220]:

0

II

0

II

RFCOOCRF + I Z " C H ~ " C C H+ ~ RF(CHZCCH~),~RF

I

I

C02CH2CH2S03H C02CH2CHZS03H

Rf= C3F7 and CF(CF3) [OCF$F(CF3)],,,0C3F7,

N Z = 0, 1,2. Anionic 2-(methacryloxy)ethanesulfonic acid oligomers containing perfluorooxaalkylene end groups reduce the surface tension of water and are soluble in aqueous polar organic solvents as well. Acrylic acid oligomers Rf(CH2CHCOOH),,Rf containing two fluorinated end groups Rf = C3F7, or C6F13,CF(CF3)[OCF2CF(CF3)],120C3F7, nz = 0, 1, 2 [221], and Rf(CH2CHCOOH),,CH2CHRfCOOH[222] have been prepared by oligomerization of acrylic acid with fluoroalkanoyl peroxides. Oligomericsurfactants, RfCH2CH200CCH2S[CH2C(CONH2)H] loH. have been prepared from perfluoroalkylethanol, mercaptoacetic acid, and acrylamide [223]. The surfactant (0.01% w/w) lowered the surface tension of a protein-containing fire-extinguishing foam composition to 23.4 mN/m.

Polymerizable Surfactants Polymerizable surfactants (surfmers) [224-23 11 are of current interest because of their potential application in drug delivery [232], in paints, in polymerization processes, for surface modifications, as well as for studies of polymerizable micelles [224,225,230]. Stahler and co-workers [224,225,230] prepared polymerizable surfactants with the structures Bro CH3

01

CH~=CH"C"N"CH~"CH~"N"CH~-C"O"R~

II

0

I

R1

I

CH3

II

0

R1 = H, C2H5and R3 = C2H4CsF17, C14HZ9 and investigated their micellar aggregation by viscosity, surface tension, and electrical conductivity measurements. The results indicated that the mixing and demixing of the fluorinated surfmer with the corresponding hydrocarbon analog is not determined solely by the lengths of the fluorocarbon and hydrocarbon tails. An interaction related to H bonds has a considerable effect on the demixing behavior. Solubilization of hydrocarbon and fluorocarbon dyes in pure or mixed micellar solutions revealed a highly selective

Structure of Fluorinated Surfactants

21

preference of the dyes for a micelle, providing a similar environment. The study concluded that micellar polymerization is a suitable method for synthesizing multicompartment polymeric micelles. In certain applications, polymerizable surfactants have several advantages over nonreactive surfactants. For example, a surfactant, used in a paint as a dispersant or emulsifier, can soften the paint film. Polymerization of the surfactant in paint during curing hinders migration of the surfactant to the surface and reduces softening of the paint film (see Chapter 8). In emulsion polymerization, polymerizable surfactants improve the shear stability of latices and reduce foaming. Polymerizable surfactants can be used to modify surfaces by polymerization of the adsorbed surfactant by ultraviolet (UV) irradiation. A surface of a paint film or lacquer can be fluorinated by cross linking a polymerizable fluorinated surfactant which has migrated to the surface and formed a monolayer [231].

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22

Chapter 1

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Structure of Fluorinated Surfactants

23

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Chapter 1

24 90. 91. 92. 93. 94. 95. 96. 97. 98.

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Structure Surfactants ofFluorinated

25

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26

Chapter 1

159. H. Sawada, A. Ohashi. M. Yoshimizu, J. Kyokane. T. Kawase, and K. Yoshino. Zairyo Gijutsu 15, 25 (1997); CA 126, 187646 (1997). 160. R. J. Twieg and J. F. Rabolt. J. Polym. Sci., Polym. Lett. Ed. 21,901 (1983). 161. J. F. Rabolt, T. P. Russell, and R. J. Twieg. Macromolecules 17, 2786 (1 984). 162. R. J. Twieg. T. P. Russell. R. L. Siemens, and J. F. Rabolt, Macromolecules 18, 1361 (1985). 163. T. P. Russell, J. F. Rabolt, R. J. Twieg, R. L. Siemens. and B. L. Farmer, Macromolecules 19, 1135 (1 986). 164. G. L. Gaines, Langmuir 7,3054 (1991). 165. M. P. Truborg and J. E. Brady. J. Am. Chem. SOC.110,7797 ( 1988). 166. J. Hopken, C. Pugh, W. Richtering, and M. Moller, Makromol. Chem. 189. 911 (1988). 167. W. Mahler, D. Guillon. and A. Skoulios, Mol. Cryst. Liq. Cryst.. Lett. Sect. 2. 111 (1 985). 168. P. Lo Nostro and S-H. Chen. J. Phys. Chem. 97,6535 (1993). 169. B. P. Binks, P. D. I. Fetcher, W. F. C. Sager, and R. L. Thompson, Langmuir 1I , 977 (1995). 170. B. P. Binks, P. D. I. Fetcher, and R. L. Thompson. Ber. Bunsen Ges. 100, 232 ( 1 996). 171. B. P. Binks. P. D. I. Fletcher, W. F. C. Sager, and R. L. Thompson, J. Mol. Liq. 72, 177 (1 997). 172. M. Napoli, C. Fraccaro. A. Scipioni. and P. Alessi, J. Fluorine Chem. 51, 103 (1991). 173. M. Abe. K. Morikawa, K. Ogino. H. Sawada. T. Matsumoto. and M. Nakayama, Langmuir 8,763 (1992). 174. C.Cornelius, M.-P. Krafft. and J. G. Riess, J. Colloid Interf. Sci. 163, 391 (1994). 175. L. Trevino. M. Postel. and J. G. Riess, J. Colloid Interf. Sci. 166.414 (1994). 176. I. Piirma, “Polymeric Surfactants,’’ Marcel Dekker. New York (1992). 177. P. Anton, P. Koberle, and A. Laschewsky, Makromol. Chem. 194, 1 (1993). 178. B. Jonsson B. Lindman, K. Holmberg, and B. Kronberg, “Surfactants and Polymers in Aqueous Solution.” John Wiley & Sons. Chichester (1998). 179. E. Kissa. “Dispersions. Characterization, Tests, and Measurements,” Marcel Dekker, New York (1999). 180. E. Myrtil, L. Zarif, J. Greiner, J. G. Riess, B. Pucci. and A. A. Pavia, J. Fluorine Chern. 71, 101 (1995). 181. A. Eisenberg and H. D. Yeager (eds.), “Perfluorinated Ionomer Membranes,’’ ACS Symposium Series No. 180, American Chemical Society, Washington. DC (1980). 182. S. A. Lossia, S. G. Flore, S. Nimmala, H. Lei, and S. Schlick, J. Phys. Chem. 96,6071 ( 1 992). 183. E. Szajdzinska-Pietek, S. Schlick, and A. Plonka. Langmuir 10, 1101 (1994). 184. E. Szajdzinska-Pietek, S. Schlick, and A. Plonka. Langmuir 10,2188 (1994). 185. S. Ristori, G. Martini, and S. Schlick, Adv. Colloid Interf. Sci. 57,65 (1995). 186. M. K. Bemett and W. A. Zisman. J. Phys. Chem. 77,2324 (1973). 187. E. N. Squire. U.S. Patent 4409393 (1983). 188. D. D. Saperstein and L. J. Lin, Langrnuir 6, 1522 (1990). 189. S. Ristori, M. F. Ottaviani. D. Lenti, and G. Martini, Langmuir 7, 1958 (1991).

Surfactants Fluorinated

of

Structure

27

190. R. J. Lagow, T. R. Bierschenk, T. J. Juhlke, and H. Kawa, in “Synthetic Fluorine Chemistry.” G. A. Olah, R. D. Chambers, and G. K. Surya Prakash, eds., John Wiley & Sons, New York (1992). 191. B. E. Smart and D. A. Dixon, J. Fluorine Chem. 57,25 1 (1992). 192. M. Hung, W. B. Farnham. A.E. Feiring, and S. Rozen. J. Am. Chem. SOC.115,8954 ( 1993). 193. B. B. Sauer and G. Y. Dee. J. Colloid Tnterf. Sci. 162, 25 (1994). 194. J. K. Klassen, M. B. Mitchell, S. D. T.Govoni, and G. M. Nathanson. J. Phys. Chem. 97, 10166 (1993). 195. H. S. Eleuterio, A. S. Milian, Jr., and E. P. Moore, Jr. (DuPont), U.S. Patent 3250808 ( 1966). 196. E. P. Moore (Du Pont). U.S. Patent 3322826 (1967). 197. F. L. Arbogast (Du Pont) U.S. Patent 3412148 (1968). 198. H. S. Eleuterio, J. Macromol. Sci.. Chem., A6(6), 1072 (1972). 199. L. A. Shits, L. V.Dikhtievskaya, S. P. Krukovskii, L. V.Cherendnichenko,and V. A. Pomarenko, Kolloidn. Zh. 38, 1130 ( 1976). 200. K. Ogino, H. Murakami, N. Ishikawa, and M. Sasabe, Yulcagaku 32(2). 95 (1983); CA 98, 145452. 201. W. A. Goedel, C. Xu, and C. W. Frank, Langmuir 9, 1184 (1993). 202. W. A. Goedel. H. Wu. M. C. Friedenberg. G. G. Fuller. M. Foster, and C. W. Frank, Langmuir 10,4209 (1993). 203. M. Ruths and S. Granick. J. Phys. Chem. B 103, 871 1 (1999). 204. P. A. Stevenson, D. A. R. Jones, J. Lin. and L. A. M. Rupert, Langmuir 11, 4167 ( 1995). 205. M. Monduzzi, A. Chittofrati. and M. Visca, Langmuir 8. 1278 (1992). 206. M. Myamoto, K. Ago, and T. Saegusa, Macromolecules 21, 1880 (1988). 207. M. Myamoto, K. Ago. and T. Saegusa. Macromolecules 22,3540 (1989). 208, L. E. Houghton (ICI). Ger. Offen. 2215388 (1972). 209. G. Vanierberghe and H. Sebag, Ger. Offen. 3021447 (1980). 210. K. F. Mueller, (Ciba-Geigy) U.S. Patent 4171282 (1979). 21 1. U. Gross, M. Herbst. and T. Szekrenyesy, Tenside Surfact. Deterg. 28,250 (1991). 212. M. Morita. M. Kubo, and M. Matsumoto. Colloids Surfaces A109, 183 (1996). 213. A. Laschewski. Adv. Polym. Sci. 124, 1 (1995). 214. D.Cochin,P.Hendlinger.andA.Laschewsky,ColloidPolym. Sci. 273, 1138 ( 1995). 215. H. Sawada, A. Wake, M. Oue, T. Kawase, Y. Hayakawa, Y. Minoshima, and M. Mitani. J. Colloid Interf. Sci. 178, 379 (1996). 216. H. Sawada, A. Wake,T.Maekawa, T. Kawase, Y. Hayakawa, T. Tomio, and M. Baba, J. Fluorine Chem. 83, 125 (1997). 2 17. H. Sawada. S. Katayama, M. Oue, T. Kawase.Y. Hayakawa, M. Baba,T. Tomita. and M. Mitani, Nihon Yukagakkaishi 45, 161 (1996); CA 124.317989. 21 8. H. Sawada, E. Sumino,Y. Hayakawa, T. Tomita, and M. Baba, Zairyo Gijutsu 15(3), 79 (1 997); CA 127. 18032. 219. H. Sawada, K. Tanba. T. Kawase, M. Baba. and Y. Hayakawa. Nihon Yukagakkaishi 46(2), 191 (1997); CA 126. 187645.).

28

Chapter 1

220. H. Sawada, A. Ohashi, M. Baba, T. Kawase, and Y. Hayakawa. J. Fluorine Chem. 79, 149 ( 1996). 221. H. Sawada, Y. F. Gong, Y. Minoshima, T. Matsumoto, M. Nakayama, M. Kosugi, and T. Migita, J. Chem. SOC.,Chem. Commun. 537 (1992). 222. H. Sawada, Y. Minoshima, and H. Nakajima, J. Fluorine Chem. 65, 169 (1993). 223. Asahi Glass, JpnKokai Tokkyo Koho, JP 601 18228 (1985); CA 104,132007. 224. K. Stahler, J. Selb, and F. Candau, Colloid Polym. Sci. 276, 860 (1998). 225. K. Stahler. J. Selb, P. Barthelemy, B. Pucci, and F. Candau, Langmuir 14, 4765 (1998). 226. R. Elbert, T. Folda, and H. Ringsdorf, J. Am. Chem. SOC.106,7687 (1984). 227. F. Szonyi, L. Conte, and A. Cambon, Tenside Surfact. Deterg. 31,257 (1994). 228. A. Hedhli. M. M. Chaabouni, A. Baklouti, S. Szonyi, and A. Cambon, J. Dispers. Sci. Technol. 15,639 (1994). 229. A. Guyot, Curr. Opin. Colloid Interf. Sci. 1,580 (1996). 230. K. Stahler, J. Selb, and F. Candau, Langmuir 15,7565 (1999). 231. M. Torstensson, B. Rinby, and A. Hult, Macromolecules 23. 126 (1990). 232. M. Yokoyama. G. S. Kwan. T. Okano, Y. Sakurai, and K. Kataoka, in “Polymeric Drugs and Drug Administration,’’ ACSSymposium Series No. 545. R. M. Ottenbrite (ed.). p. 126. American Chemical Society, Washington, DC (1994).

Synthesis

2.1

STARTINGMATERIALS

Direct fluorination with elemental fluorine is not practical for commercial synthesis of fluorinated surfactants. Elemental fluorine is extremely reactive and difficult to handle. The heat of formation of the C-F bond (about 460 kJ/mol or 1 10 kcal/mol) and the H-F bond (566 kJ/mol or 135 kcal/mol) exceeds that of the C-C bond (about 348 W/mol or 83 kcal/mol) [l]. Hence, the fluorination with elemental fluorine leads to a violent fragmentation of the substrate unless the reaction is carefully controlled and the reaction heat effectively dissipated [2,3]. Commercially important pathways to fluorinated surfactants are electrochemical fluorination, telomerization, and oligomerization of tetrafluoroethylene [4-61. Electrochernicnl fluorination utilizes anhydrous hydrofluoric acid as the fluorine source. Hydrofluoric acid is produced by a reaction of calcium fluoride with sulfuric acid [7]: CaF2

+ H2SO4 ”+ 2HF + Cas04

Industrially produced hydrofluoric acid contains impurities, mainly fluorosulfonic acid, silicon tetrafluoride, sulfur dioxide, sulfuric acid, and water. Purification of hydrogen fluoride by simple physical means is difficult and various chemical methods have been developed for the removal of impurities. Anhydrous hydrogen fluoride has been prepared by thermal decomposition of anhydrous sodium hydrogen fluoride or potassium hydrogen fluoride. The salt, KHF2, is difficult to dry and electrolysis of the salt has been employed to remove traces of water and other impurities [8-lo]. Highly pure hydrogen fluoride can be manufactured by electrolytically oxidizing impurities remaining in hydrogen fluoride [ 111. 29

Chapter 2

30

The starting materials of the telonzerization process are tetrafluoroethylene and a perfluoroalkyl iodide. Of the various perfluoroalkyl iodides used intelomerization processes, pentafluoroethyl iodide is the most important (see Section 2.3). The one-step process for manufacturing pentafluoroethyl iodide is based on theaddition of iodine fluoride to tetrafluoroethylene. Iodine fluoride, IF, is too unstable to be isolated and is therefore formed from iodine pentafluoride and iodine ill situ. Iodine pentafluoride is synthesized from iodine and elemental fluorine [ 121: I?

+ 5F2 + 2IFS

Iodine pentafluoride and iodine react with tetrafluoroethylene in the presence of a catalyst, such as antimony trifluoride. under pressure [ 13,141: 5CF?=CF;!

SbF, + IF5 + 212 + 5CF3CF2I

The reaction of hexafluoropropylene with IF, formed i n situ from IFs and I?, gives heptafluoroisopropyl iodide [ 15,161: SCF3CFrCF2

+ IF5 + 212 + 5(CF3)2CFI

Heptafluoroisopropyl iodide as a telomerization agent (telogen) produces telomers featuring a branched fluorocarbon chain end. The disadvantage of the IF addition process for making perfluoroalkyl iodides is that it requires elemental fluorine. Fluorine in the form of fluorides is abundant in nature. However, liberation of fluorine from fluorides requires vigorous conditions to break the strong bond between fluorine and alkali and alkaline earth metals. Oxidation of hydrogen fluoride or fluorides to elemental fluorine is difficult, although such reactions have been reported in the literature [17]. Elemental fluorine is produced by electrolysis of anhydrous hydrogen fluoride containing dissolved potassium hydrogen fluoride [ 17-19]. Handling of elemental fluorine, because of its high reactivity and corrosiveness, aggravates the difficulty. Hence, numerous attempts have been made to synthesize perfluoroalkyl iodides without using elemental fluorine [20-261. Scherer and Futterer [20] prepared pentafluoroethyl iodide by passing pentafluoroethyl bromide through a layer of KI at 500°C. Millauer [2 1-23] prepared pentafluoroethyl iodide and heptafluoropropyl iodide by a reaction of 1,2-diiodotetrafluoroethane,CF21CF21.with HF or of CF?CF=CF:! with iodine and HF, respectively. 1,2-Diiodotetrafluoroethane,obtained by the addition of iodine to tetrafluoroethylene, is heated in the presence of HF and a catalyst or an oxidant to form pentafluoroethyl iodide [21-241: CF2ICF2I + HF + CF3CFZI

+ HI

Chlorine, sulfonyl chloride, and antimony pentafluoride have been used as catalysts with HF to facilitate the conversion of the diiodide to the monoiodide [21,221.

Synthesis

31

A Montecatini Edison patent [ X ] describes a reaction of 1,2-diiodotetrafluoroethane with HF and PbO? to form pentafluoroethyl iodide. However, oxidizing acids, such as HN03, HC103, HI03, and H5IO6, form only volatile by-products and therefore have an advantage over metal oxides, such as HgO and Pb02 [23]. The reaction with HsI06 iscarried out in an excess of HF at 150°C: 7CF21CF21+ 7HF

+ H5106 -+ 7CF3CF21+ 412 + 6H20

Electrochemical synthesis of pentafluoroethyl iodide from CF21CF21 inthe presence of anhydrous hydrogen fluoride has been claimed in Hoechst patents [261. 2.2

ELECTROCHEMICALFLUORINATION

Electrochemical fluorination of organic compounds in anhydrous hydrogen fluoride [27-341 was invented by Simons [27-301. The organic substance to be fluorinated is dissolved or dispersed in liquid hydrogen fluoride. A direct electric current of a voltage below 8 V, usually between 5 and 7 V, is passed through hydrogen fluoride containing the substance. Hydrogen is evolved at the cathode and the organic substance is fluorinated. The voltage applied is insufficient €or fluorine evolution but adequate for fluorination at the anode. All hydrogen atoms in the molecule are replaced by fluorine, but some functional groups such as carboxylic halides and sulfonyl halides are retained. A diagram of a electrochemical fluorination plant is shown in Fig. 2.1. The electrolytic cell is built of a metal, resistant to corrosion by hydrogen fluoride, like copper, nickel, Monel, or iron. The anode plates are made of nickel and the cathode plates are usually made of iron or nickel. The cell is equipped with a reflux condenser, cooled to - 10°C to -4O"C, to retain hydrogen fluoride. Alternatively, solid sodium fluoride is used to trap hydrogen fluoride escaping through the condenser. The temperature of the cell is usually kept at OOC, although a temperature range of -80°C to +75"C has been claimed in the patent literature [30,36]. The cell temperature is selected as a compromise between two opposing factors. On one hand, the conductivity of solutions in hydrogen fluoride increases with increasing temperature. On the other hand. anhydrous hydrogen fluoride boils at 19°C and lower fluorination temperatures are therefore more convenient. The process can be operated batchwise or continuously. The conductivity of pure anhydrous hydrogen fluoride is very low. If the substrate dissolved in hydrogen fluoride does not adequately increase the conductivity, organic or inorganic additives are used to increase conductivity. Some additives are consumed in the process; some are inert. Water increases conductivity of HF. However, the amount of water in hydrogen fluoride has to be kept below 1%. Water in amounts above 1% lowers the yield [30,31,371, and above 10%w/w. water produces dangerously large amounts of explosive oxygen difluoride. Alkali

Chapter 2

32

FIG.2.1 Schematic outline of an electrochemical fluorination plant. (From Ref. 35. Reproduced by permission of Chapman & Hall Ltd.)

or alkaline earth metal fluorides used as conductivity additives do not react during electrolysis. They do not need to be replenished during fluorination but can corrode the anode. The mechanism of electrochemical fluorination is still incompletely understood [6,31,38-41]. Fluorination is believed to occur by fluorine adsorbed on the nickel fluoride layer formed on the anode surface while hydrogen is liberated on the cathode: Cathode: Anode:

+ 2e- + H2 2F- "+ 2F + 2e2H+

A free-radical mechanism assumes the following fluorination reaction sequence:

F-

> Fgds

RH + Fgd, + Rid, Rids + Fids + RF

e-

+ HF

According to an ionic fluorination mechanism, adsorption of organic molecules on the NiF2 layer is the first step, followed by their anodic oxidation to

Synthesis

33

carbonium ions, which react with fluoride ions [42]: CH4

-

--e-, - H +

CH.&d,

F-

&CH2F,+dS-%

CH3Fads CH,Fgd, CH2F3ads

-eCHTads

-2e-, -H+,F-

CHF3

-2e-, -H+,F-

> CF4

Electrochemical fluorination of an alkanoic acid yields a perfluoroalkanoic acid fluoride [43-47]: C,lH2,1+1 COOH

+ (212 + 2)HF -+CnF2,*+COF + by-products

Electrochemical fluorination of carboxylic acids produces perfluoroalkanoic acid fluorides in a low yield (.about 10-20%) [311 and water as a by-product. Water forms explosive oxygen difluoride and causes oxidative degradation of the carboxylic acid. For acids with six or more carbon atoms, cyclic perfluoroethers are among the by-products formed. The anhydrides of carboxylic acids give perfluorocarboxylic acid fluorides in a higher yield than the parent carboxylic acids. The electrochemical fluorination of carboxylic acids or their anhydrides is now obsolete. The yield of a perfluorocarboxylic acid fluoride is higher when a carboxylic acid chloride or fluoride is fluorinated instead of the carboxylic acid or its anhydride [48,49]. For example, acetic acid, acetic acid anhydride, and acetyl fluoride give trifluoroacetyl fluoride in 17%,32%, and 76% yields, respectively [32,49]: C,2H3,t+~ COCl

+ (212 + 2)HF -+ C,,Fa,+

COF

+ HCl + by-products

The carboxylic acid fluorides are soluble in HF, and the solutions of alkanoic acid fluorides with more than four carbon atoms are conductive. The yield of the perfluoroalkanoic acid fluoride decreases with increasing chain length of the carboxylic acid fluoride fluorinated, from as much as about 80% for acetyl fluoride to 10% for perfluorooctanoyl fluoride [34]. The yield is lower when an acyl chloride is fluorinated instead of an acyl fluoride, but acyl chlorides are more readily available than acyl fluorides [48]. Electrochemical fluorination of dicarboxylic acid fluorides also gives a higher yield than fluorination of the parent acids. The lower yield given by the parent carboxylic acids has been attributed to decarboxylation, similar to the Kolbe reaction [3 11. The perfluorocarboxylic acid fluorides can be converted to perfluorocarboxylic acids, esters, amides, or other intermediates for surfactants (Fig. 2.2). Hydrolysis of the acid fluoride yields the carboxylic acid (I) or its salt (11). Alcohols form esters (111), which can be hydrogenated to an alcohol (IV), an intermediate for nonionic surfactants. The reaction of the acid fluoride with an amine [e.g., NH2C3H6N(CH3)?]yields an intermediate (V) for an amine salt (VI), a cationic

Chapter 2

34 C,F,,COOH

(I)

C,F,,COOM

(11)

C,F,,COf C,F,,COOR

(111)

L

-C,F,CH,OH

(IV)

C,F,,CONHC,H,N(CH,),

C,F,,CONHC,H,N(CH,),

(V)

E

HX (VI)

[C,F,,CONHC,H,N(CH,),R]+X-(Vll)

C,F,,CONHC,H,~(CH,),C,H,COO-

(VIII)

FIG.2.2 Fluorinated surfactants derived from perfluorocarbonyl fluoride. (From Ref. 50.)

surfactant (VII), or an amphoteric surfactant (VIII). The surface activity of surfactants derived from pe~rfluorocarbonylfluoride is shown in Fig. 2.3. Electrochemical fluorination of alkanesulfonyl chlorides or fluorides yields the corresponding perfluorosulfonyl chloride or fluoride in a 12-79% yield, depending on the alkane chain length [51]:

+ (212 + 2)HF -+ CrlF3n+S02F + HCl + by-products

CIIH3r,+I S02Cl

Fluorination of ethanesulfonyl chloride yields pentafluoroethanesulfonyl fluoride in 79% yield [52]. Fluorination of octanesulfonyl chloride yields perfluorooctanesulfonyl fluoride in a 25% yield [52], but perfluorodecanesulfonyl fluoride is obtained in only 12% yield [53]. The by-products formed are shorter-chain sulfonyl fluorides or chlorides and fluorocarbons [51-541. The structures of the by-products suggest a cleavage of carbon-sulfur and carbon-carbon bonds and some oxidative degradation as well. Fluorination of alkanesulfonyl fluorides or chlorides gives higher yields of perfluoroalkanesulfonic acids than fluorination of alkanesulfonic acids [49,5 132,541. Another disadvantage associated with the fluorination of free alkanesulfonic acids is the potential explosion hazard caused by excessive amounts of oxygen difluoride and hypofluorites formed as by-products [55]. The perfluorosulfonyl fluorides obtained by the electrochemical fluorination process can be readily converted into the corresponding acids and salts used as surfactants (Fig. 2.4). Hydrolysis yields the perfluoroalkane sulfonic acid (I) or its salt (11). Primary or secondary amines yield sulfonamides (111), which can be converted to a carboxylic acid (IV) or an alcohol (V). The alcohol is an internmediate for nonionic surfactants (VI), phosphates (VII), or sulfates (VIII). The tertiary aminosulfonamide (IX) can form acationic surfactant (X), a amphoteric surfactant (XI), or an amine salt (XII).

Synthesis

35

I

70

C,F,,CONHC,H,h(CH,)2C2H4COO-

t I 60

/

[C7F15CO&HC,H,N(CH,)JI-

50 40

30 20 I

I

0.00010.001

I

I

I

0.01

0.1

1.o

10

Wt % solids FIG.2.3 Surface activity of aqueous solutions of fluorinated surfactants derived from perfluorocarbonyl fluoride. (From Ref. 50.)

c8F17s02F

FIG. 2.4 Fluorinated surfactants derived from perfluorosulfonyl fluoride. (From Ref. 50.)

Chapter 2

36

FIG.2.5 Surface activity of aqueous solutions of fluorinated surfactants derived from perfluorosulfonyl fluoride. (From Ref. 50.)

The surface activity of fluorinated surfactants derived from pedluorosulfonyl fluoride is shown in Fig. 2.5.

2.3 TELOMERIZATION Telomerization was initially developed by the Du Pont Company for free-radical polymerization of ethylene [56-611 and defined as a process of reacting a molecule, called telogen, with two or more ethylenically unsaturated molecules, called taxogens:

YZ Telogen

+

IZA Taxogen

+

Y-(A),,-Z Telomes

Haszeldine [62,63] allowed trifluoromethyl iodide to react with ethylene and obtained oligomers of the type CF3[CH2CH2],I ( n = 1, 2, and 3). The reaction of trifluoromethyl iodide with tetrafluoroethylene, catalyzed by ultraviolet (UV) irradiation,yieldedshort-chainpolymers of thegeneralformula CF3[CF?CF2],,1, where 12 = 1-10. Some of the members of the telomeric series were isolated. i

Synthesis

37

The photochemically catalyzed reaction of trifluoromethyl iodide with tetrafluoroethylene involves a radical chain [64]:

+

CF3I + hv + CF3. 1. CF3. + CF2 = CF:, -+ CF3CF2CFa. CF3CFZCF2. + CF? = CF2 -+ CF3CF2CFZCF2CF2.

(initiation) (chain propagation)

CF3CF2CF2. + CF3I -+CF3CFZCFJ + CF3. (transfer) CF3(CF:,CF?),,. + CF31-+ CF3(CF?CF?),,I+ CF3. 2CF3. -+ CF3CF3

(chain termination)

Thepolymerizationmechanismshavebeenextendedfromtheoriginal free-radical reaction to anionic, cationic, and transition-metal-catalyzed telomerization. The value of IZ in CF3(CF2CF2),,Idepends on the relative concentration of the chain transfer agent (the telogen), the monomer (tetrafluoroethylene). and the length of the radical chain. Telomerization in the vapor phase favors the formation of longer-chain perfluoroalkyl iodides with 17 > 2 because the relative concentration of the telogen is lower. The probability is therefore higher that heptafluoropropyl iodide undergoes a subsequent homolytic fission and generates a C3F7.radical, which can react with tetrafluoroethylene. Pentafluoroethyl iodide (1 -iodopentafluoroethane) reacts with tetrafluoroethylene under conditions similar to those for iodotrifluoromethane and yields perfluoroalkyl iodides, CF3CF2[CF2CF2],,I , with an even number of carbon atoms. Commercial telomerization of tetrafluoroethylene with pentafluoroethyl iodide was developed by the DU Pont Company [65,66]. The telogen, pentafluoroethyl iodide, is prepared by reacting iodine pentafluoride and iodine with tetrafluoroethylene [ 13,141 (see Section 2.1): 5CF2 = CF2

+ IF5 + 2

1 2 3 5CF3CF21

Telomerization of tetrafluoroethylene with pentafluoroethyl iodide produces a mixture of even-carbon-numbered telomers differing in their overall carbon chain length: C2F5I

+ I I C ~ F3 J C,F5(C2FLF),,I

Numerous modifications of the original Du Pont telomerization process have been patented [67-791. Fluorides, such as HF/SF+ SbF3/SbFS, or IFs/AlC13, used inolder telomerization processes as catalysts [4], are corrosive and have been replaced by selected metal salts dissolved in amines or solid metal salt-amine complexes. Hauptschein [67] has prepared telomers by reacting tetrafluoroethylene with a secondary iodide. having two same or different perfluoroalkyl groups, at 150-220°C under pressure. A Ciba patent [68] discloses telomerization of per-

38

Chapter 2

fluoroalkyl iodides with perfluoroethylene or perfluoropropylene, catalyzed by a system consisting of an amine and a salt of a metal belonging to group IB or IIB [68] or group IIIA, IIIB, or VIIIB [69]. The preferred compound is ZrCI4 [69]. Kali-Chemie patents [70,7 11 disclose metal salt-amine telomerization catalysts, prepared from ethanolamine or butylamine and chlorides of Zn, Cu, Ti, or Sb. Pentafluoroethyl iodide was telomerized with tetrafluoroethylene at 180°C in the presence of a catalyst, on silica or alumina, prepared from CuCl, TiC14, SbC13,and ethanolamine [7 11. Organic peroxides are used as telornerization catalysts, in spite their potential instability and handling hazards. Telomerization processes patented by Hoechst use bis(trichloroacryloy1)peroxide [72], (C,1F2,1+COO)? [73], or bis(4t-butylcyclohexy1)peroxydicarbonate [74] as a telomerization catalyst. Peroxyacids of the formula CnX2n+lC(O)OOH, where X = H, F, or C1 andz i = 1-12, have been claimed in Kali-Chemie patents [75]. Both primary and secondary alkyl iodides are useful as telogens in telomerization processes. The telomerization of 1,2-diiodotetrafluoroethane is complicated by its thermal instability. The formation of ethylene iodide has been suppressed with tetrafluoroethylene to telomerize 1,2-diiodotetrafluoroethanewith ethylene [76]. The perfluoroalkyl iodides do not react with nucleophiles, such as OH- or NH3, and cannot be converted directly to common intermediates for fluorinated surfactants. Perfluoroalkyl iodides are therefore reacted with ethylene:

A Ciba patent [77] describes a reaction of olefins with a perfluoroalkyl iodide of the formula CnF2,1+ 11,wherez I = 4-14. The olefins were bubbled through the liquid perfluoroalkyl iodides at 50-220°C in the presence of a free-radical catalyst. Perfluoroalkyl iodides with 6-24 carbon atoms were reacted with an olefin in the presence of a metal-amine complex [78]. The perfluoroalkylethyl iodides can be readily converted to the corresponding alcohols, thiols, and sulfonyl chlorides used as intermediates for fluorinated surfactants (Fig. 2.6). Perfirz~oronl~~Z-2-ethanols areobtained by hydrolysis of 2-perfluoroalkylethyl iodides. A variety of processes have been described in the patent literature. A Du Pont patent describes a process of treating 2-perfluoroalkylethyl iodide with oleum [80] at 25"C, pouring the reaction mixture into water containing NaHS03 and distilling the organic layer. As analternative to the Du Pont process, 2-perfluoroalkylethyl iodides have been hydrolyzed with nonoxidizing acids, like sulfuric acid or an arylsulfonic acid [811. AHoechstprocesshydrolyzes2-perfluoroalkylethyliodidesin Nmethylpyrrolidone and water by heating at 150°C for 13 h [82]. An Asahi process

i

i

i Y

Synthesis

X

LJ X

1 X

I

c",

Y a

0"

I 0

U-

0 1" 0

0 I" X"

0

U-

0

0 0 I"

0

U-

0

I"

U-

0

I

39

Chapter 2

40

I

I

I

uses aqueous dimethylformamide (DMF) for hydrolysis at 140°C for 6 h, followed by a treatment with aqueous 10% KOH for 3 hat 120°C [83]. A two-stage process [84] for preparing 1,1,2,2-tetrahydroperfluoroalka1101s converts perfluoroalkylethyl iodides, RfCH2CH,I, nitrates, to RfCH2CH20N02.by a reaction with 70-98% HN03. In the second stage, the nitrates are hydrogenated in the presence of a hydrogenation catalyst to the corresponding alcohols. The hydrolysis of the perfluoroalkylethyl iodides is catalyzed by certain metals. Copper salts (e.g., cupric sulfate) catalyze the hydrolysis of perfluoroalkylethyl iodides in aqueous acetonitrile heated at 160°C for 12 h, followed by heating in 10% NaOH at 70°C for 2 h [85]. A Hoechst patent [86] describes a process for preparing 2-perfluoroalkylethanol by a reaction of the iodide in water containing a metal and a phase transfer agent in an acid medium (pH 1-6). Another Hoechst patent [87] discloses a reaction of 2-perfluoroalkylethyl iodide with a peracid to yield 2-perfluoroalkylethanol. patents claim the preparation of fluorinated alcohols, Du Pont H(CF2CF2),,CH20H,by telomerization of methyl alcohol with tetrafluoroethylene in the presence of a peroxy or azo catalyst [88,89]. Pe~~uo~-onl~l-2-etl?crl2et~~iols are obtained by reacting a perfluoroalkyl-2ethyl iodide with thiourea and hydrolyzing the thiouronium salt formed [90-941. Perfluoroalkyl-2-ethanethiols can be converted to perfluoroalkylethylsulfonyl chlorides by a reaction with chlorine and water [92,93]: RfCH2CHzSH

CI?/H?O

RfCH2CH2S02Cl

The perfluoroalkylethanesulfonyl chlorides are useful intermediates for producing cationic and amphoteric fluorinated surfactants. 2.4

OLIGOMERIZATION OF TETRAFLUOROETHYLENE

A process developed by IC1 [95-971 is based on anionic polymerization of tetrafluoroethylene. Unlike high-molecular-weight poly(tetrafluoroethy1ene) produced by free-radical polymerization, anionic polymerization catalyzed by a fluoride (e.g., cesium, potassium, or tetraalkylammonium fluoride) produces highly branched oligomers. The main products are a tetramer, a pentamer, and hexamers (Fig. 2.7). The pentamer, the most abundant of the oligomers. is an unsaturated perfluorocarbon susceptible to a nucleophilic attack. The pentamer can react with a phenol. The product can be sulfonated to form a sulfonic acid, which after neutralization, functions as an anionic surfactant: so3 CloF20 + CsH50H %_* C~OF~~O -_ C* ~ HC10F190C6H~S03H ~

Synthesis

41

Tetramer

CF

5 \ c = c / ‘ZF5 \ CF, CF, C*FI6

and trans isomer in approximatelyequal amount

Pentamer c2F5\

\ c = c/ c2F57 / \ cF3

CF,

F

C F,

Hexamers (only two most abundant isomers shown) 2‘

F5\

C F-

c;,”/

C-C-CF / tF2

CF3

\ c2F5

c2F5\

/F \ CF, / c = c \ CF,

cF37c

c4Fg

FIG.2.7 Structures of tetrafluoroethylene oligomers. (From Ref. 96.)

Reaction of the pentamer with p-cresol yields an ether which can be oxidized to yield a fluorinated carboxylic acid. The acid can be neutralized to give an anionic fluorinated surfactant:

Chapter 2

42

The same fluorinated carboxylic acid can be prepared by reacting the pentamer with p-hydroxymethylbenzoate and hydrolyzing the ester formed:

hydrolysis

C 1 OF,~ O C ~ H A C O O C H ~

Alkali causes a stepwise degradation of the pentamer to a ketone which, like the pentamer itself, can react with strong alkali and yield an anionic fluorinated surfactant:

Cationic fluorinated surfactants can be prepared by reacting the pentamer with phenol and, subsequently, with chlorosulfonic acid. The sulfonyl chloride reacts with N,N-dimethylpropanediamine to form a tertiary amine sulfonamide, which can be quaternized to form a cationic surfactant:

.L

CloF19OC6H4SO?NHCH2CH?N(CH3)2

I

CH,I

C,oF190C6H,S02NHCH~CH~N+(cH~)~IReaction of the amine with P-propiolactone instead of methyl iodide yields the corresponding amphoteric fluorinated surfactant, of the structure C~~F~~OC~H~SO~NHCH~CH~N+(CH~)~CH?CH?_COO-. Alternatively, the pentamer can be reacted with 3-hydroxypyridine. The pyridyl ether formed can be quaternized to give a cationic surfactant: CloF7,0+ HOCSH4N

base

>

(CH3 )?SO3

CloF190CSH4N+CH3CH3SOY Nonionic surfactants can be prepared by a reaction of the pentamer with alcohols or with phenol. The subsequent sulfonation of the product gives a sulfonyl chloride which with N-ethylethanolamine yields an alcohol. An addition of ethy-

i

I

Synthesis

43

The base-catalyzed reaction of the oligomers with oxyethylated alcohols gives ethers which can also be converted to nonionic surfactants:

+

C10F20 HO(CHzCH20),,R -+ CloF190(CH2CH20),,R, R = H, alkyl, aryl, etc. If R is H, the reaction yields a nonionic fluorinated surfactant with a fluorocarbon group at both ends of the molecule: C I 8 2 0 + HO(CH2CH20),1H + C I o F ~ ~ O ( C H ~ C H I oF19 ~O),C or CSF]6

+ HO(CH?CH?O),,H-+ CsFl SO(CH2CH?O),,CsF15

Unlike the phenyl ethers, the alkyl ethers thus formed react readily with nucleophilic reagents. The reaction with another oxyethylated alcohol molecule gives a nonionic surfactant with two oxyethylene chains, C F

\"

O(CH?CH?O),R 13//

/"=Y

R(OCH,CH?),,,OC"O CF3 Another example of nucleophilic attack on the alkyl ethers is the reaction with an aqueous base which replaces a CF3-group with hydrogen and yields a stable nonionic fluorinated surfactant C6F13CH=C(CF3)0(CH2CH20),,R. 2.5

MISCELLANEOUSREACTIONS

Anionic fluorinated surfactants containing an oligo(hexafluoropropene oxide) chain have been prepared by anionic oligomerization of hexafluoropropene oxide (HFPO) [98,99] and hydrolysis of the oligomer, an acid fluoride [ 1001: rzCF3CF-CF2 '0'

-

> CF3CF2CF20(CFCF20),,-2CFCOF CF3

I

CF3

I

I

CF3CF2CF20(CFCF20),,-2CFCOONa (or NH,) CF3

I

CF3

I

Chapter 2

44

The HFPO oligomers are acid fluorides and acylate benzene or toluene in the presence of a Friedel-Crafts catalyst such as AlC13 [101,102]. The resulting oligomers with an aryl end group are soluble and exhibit surface activity in toluene and m-xylene [ 1021: C6Hh

CF3CF3CF2O(CFCF20),,-2CFCOF CF3

A,CI,

>

CF3

CF3CF?CF?O(CFCF20),,-2CFCOC6H, CF,

CFCOC6H4SO3H

I

CF3 The acylated benzenes and toluenes were sulfonated with 30% oleum at 50-60°C. The sulfonic acid group was found to be in the meta position relative to the acyl (HFPO),, group. The sodium salts of the sulfonic acids, (HFPO),,-Ar-S03Na (where Ar is a phenyl or tolyl group), lowered the surface tension of water (see Section 4.3). Pittman et al. [ 103,1041 treated hexafluoroacetone with nucleophilic agents such as an alkoxide, cyanide, and an alkali metal fluoride to prepare fluoroalkyl acrylates and methacrylates with a perfluoroisopropyl group: (CF3)2C=O (CF3)zCFO-K+

+ KF + (CF3)2CFO-K+

+ CHz=CHCOCl+

CHZ=CHCOOCF(CF3)2

+ KC1

These fluoroacrylate monomers can be copolymerized with other acrylates to form polymeric surfactants. The reaction of potassium fluoride with hexafluoroacetone has been employed to prepare nonionic fluorinated surfactants (see Section 2.9). Because hexafluoroacetone has been found to be highly toxic and a teratogen, it is no longer used for industrial preparation of fluorinated surfactants. Kokelenberg and Pollet [ 1051 attempted to introduce a few short perfluorinated groups into compounds featuring one or more hydrophilic functions. The addition of chlorotrifluoroethylene and hexafluoropropene to aliphatic or aromatic polyhydroxy compounds in the presence of a base and a dipolar aprotic solvent gave mixtures of products resulting from incomplete addition and partial dehydrofluorination. 2.6

ANIONIC FLUORINATED SURFACTANTS

The synthetic method selected for the preparation of an anionic fluorinated surfactant depends on the hydrophile of the surfactant: a carboxylate. sulfonate, sul-

I

-

.

...

.

.

Synthesis

45

fate, sulfite, or phosphate group. Within each group, the choice of the preparative method is determined by the availability of starting materials.

Carboxylates From Carbolzvl or Sulfor~ylFluorides or Chlorides

Perfluoroalkanecarbonyl fluorides produced by the electrochemical fluorination process are hydrolyzed to produce a perfluoroalkanoic acid or its salt: C7FIsCOF+ 2NaOH -+C7FIsCOONa + NaF

+ H20

Perfluoroalkanesulfonyl fluorides, prepared by the electrochemical fluorination process, yield fluorinated carboxylic acids via the sulfonamide as the intermediate. The N-substituted perfluorohexanesulfonamide is condensed with sodium monochloroacetate in aqueous isopropyl alcohol [ 1061: C~FI~SO + ~NHz(CH?)_?NH?+ F C6F13SO2NH(CH&NH,

I

CICH,COONa

C6F,,S02NH(CH2)3N(CH2COONa)2 A fluorinated sulfonyl chloride can be reacted with an amino acid to give a fluorinated anionic surfactant with a carboxylic group and a sulfonamido linkage [107]: CH3NHCHZCOOH

CgF17OC6H4SO2Cl CgF17OC6Hs

CISO3H NaOH

>

Alkanoic acids with a terminal perfluoroalkyl segment have been prepared by the addition of perfluoroalkanesulfonyl chlorides to unsaturated acids [ 1081. From Telonner Iodides

Perfluoroalkyl iodides produced by the telomerization process can be oxidized to carboxylic acids, but drastic conditions are needed for the reaction to occur. Perfluoroalkyl iodide is oxidized with oleum (1545% SO3) at 100-180°C under pressure. The upper layer is treated with water and the perfluoroalkanoic acid formed is separated by distillation [109]: C F 3 ( C F 2 C F 2 ) , I aCF3(CF2CF2),,-ICF2COOH+ I3 + by-products

3

The liberated iodine is recovered by filtration. Oxidation of perfluoroalkyl iodides with fuming sulfuric acid in the presence of P205 andrectification of thereactionmixturegivesthefluoride C,2FZ,l+ ICOF, which ishydrolyzed with water to produce the acid CIzF2,1+ I COOH. Perfluoroalkanoic acids have also been obtained by carbonation of fluo-

Chapter 2

46

roalkyl iodides in the presence of a copper-zinc catalyst [ 1101 or group VI11 transition metal complexes and subsequent hydrolysis [ 1 111:

The Grignard reaction has been employed to prepare fluorinated carboxylic acids available hitherto only by electrochemical fluorination [ 1 121: CF3(CF2)s1+ C6HsMgBr"+ C6HsI + CF3(CF2)SMgBr

1

( 1 ) (C,H,O),CO

(2) hydrolysls

CF3(CF2)5COOH The conversion of telorner iodides to chlorosulfates and the subsequent hydrolysis of the chlorosulfate yield perfluorinated carboxylic acids [ 113.1 141: CIS0,H

CnzF?nt + 1(CF?_CF?_),zI

C,,,FZ,,,+ 1(CF2CFz),,OSO:Cl

+ HI

C,,,F2,,,+ (CF2CF2),,0S0,C1 + 6NaOH+ Cn2F2r,z + (CF2CF2)12-,CF,COONa

+ 2NaF + Na,SO, + NaCl + 3H20 Long-chain alkanoic acids with terminal perfluoroalkyl segments can be prepared by a two-step synthesis [ 115,1161: R,-I + CH,=CH(CH2),,,-2COOR

+

RfCHzCHI(CH2),,,-2COOR

Zn cllcoho,

> Rf (CHZ),,COOR

Hydrolysis of the ester gives a alkanoic acid with a terminal perfluoroalkyl segment. Perfluoroalkyl iodides are converted with ethylene to perfluoroalkylethyl iodides, which can be oxidized more readily than the parent perfluoroalkyl iodides. Oxidation with chromic acid yields a perfluoroalkanoic acid and a fluorinated alkanoic acid with an a-methylene group [93,117]: CF3(CF2CFZ),zCH2CH?I

e K2Cr207

CF3(CF?CF?),,CH2COOH+ CF3( CF2CF?),,COOH

From Perflrror-oalk?,let~l~?lol Perfluoroalkylethanols, readily available by hydrolysis of perfluoroalkyl iodides, can be oxidized to fluorinated carboxylic acids. As an example, oxidation of H(CF2CF?),,CH?OH with nitrogen oxides gave whydroperfluorocarboxylic acids [ 1181:

Synthesis

47

Photochemical chlorination of perfluoroalkylethanols gives perfluoroalkylacetyl chlorides [ 119,1201, which are hydrolyzed to the carboxylic acid: C1JUV

CF3(CF2),,CH?CH?OH CF3(CFz),,CH?COCl+ CF3(CF2),,CH?COOH The reaction of perfluoroalkylethanols with acrylonitrile and hydrolysis of the nitrile formed yields a fluorinated carboxylic acid [ 1101:

I

conc HCl at reflux

C8F&H2CH20CH3CH2COOH The polyphilic surfactant F(CF2)6(CH2)I 10C6H4C6H4COOH[ 1211 is synthesized from the polyfluorinated alcohol prepared by 1-2 addition of perfluorohexyl iodide on o-undecenol, followed by reduction. The polyfluorinated alcohol is reacted with HBr/H2S04,and the resulting bromide is etherified with 4-cyano4’-hydroxybiphenyl to give 4-(polyfluoroalkoxy)-4’-cyanobiphenyl.Hydrolysis yields the polyphilic carboxylic acid.

From Pe~fluoroalh?~lthiols The reaction of perfluoroalkylthiols (see Section 2.3) with an alkyl acrylate in the presence of a free-radical catalyst yields esters, which are hydrolyzed to obtain perfluoroalkylcarboxylates [90]: RfCH2CHZSH + CH,=CHCOOCH,+ RfCH?CH2SCH?CH2COOCH3 KOH RfCH2CH2SCH2CH2COOK + CH30H

From Telorner Chlorides Telomerization of ClCF=CF2 with CF3CC13 gives the telomer CF3CCL(CF2CFCl),,Cl, which can be converted to carboxylic acids. Halogen interchange with aluminum chloride and subsequent hydrolysis of the trichloromethyl end group yields a carboxylic acid containing fluorine and chlorine [ 1221: CF3CCI?(CF2CFCl),Cl

AlCI,

CF3CC12(CF2CFC1),,-1CF2CC1_?

1

hydrolysis

CF3CC12(CF2CFCl),,-1CF2COOH From Esters Esters of fluorinated acids can be hydrolyzed to obtain the parent acid [ I 16,1231. Thus. hydrolysis of hydrocarbon-segmented fluorinated carboxylic

Chapter 2

48

acid esters with KOH in 90% aqueous alcohol gives the potassium salt of the parent carboxylic acid [ 1161: (CF3)2CF(CHz)1oCOOC2H5 + KOH + (CF3)2CF(CHz)1 oCOOK + CzH5OH Condensation of an aminocarboxylic acid with the isopropyl ester of a perfluorocarboxylic acid yields a fluorinated surfactant with a modified hydrophobic chain [ 1241: HzN(CH2)5COONa+ CsF I 7COOCH(CH3)2-+ CsF17CONH(CH2)5COONa HOCH(CH3)2

+

From Oligomers A tetrafluoroethylene pentamer forms an ester with p-hydroxymethylbenzoate which can be hydrolyzed to prepare a salt of a fluorinated carboxylic acid [96,97]: hydrolysis

C ~ O F ~ ~ O C ~ H ~ C O O C H ~ CloF190C6H4COONa Oxidation of a methyl group on a cresyl ether, obtained by reacting p-cresol with the tetrafluoroethylene pentamer, yields a fluorinated carboxylic acid. The acid is neutralized to give a fluorinated surfactant [96,97]: I

-

c1OF190C6H-lCH3

oxidation

NaOH

Cl~F1~0C~H4COOH"---+ CloF190C6H4COONa

From Fluorinated Epoxides Fluorinated epoxides produce fluorinated surfactants with a hydroxyl in the hydrophobic chain [ 125,1261: C,zF2,1+ ICH-CH2 \ / 0

+ H,NCH(CH,),-+ CrlF?_?Z

+ 1CH(0H)CH2HNCH(CH3)2

CICH,COONa C,H,ONa/C,H,OH

>

CnF211+ ICH(OH)CH2N[CH(CH3)z]CH2COONa Fluorinated epoxides condensed with potassium sarcosine in aqueous isopropyl alcohol at 50°C yield a fluorinated aminocarboxylate [ 1271: C9F1&H2"CH"CH2 0 ''

+ CH3NHCHZCOOK + CgF19CH?CH(OH)CHzN(CH3)CH?COOK

By Photooxidation Perfluoropolyether surfactants [ 128-1 3 11 have been prepared using perfluoroalkeneoxide as the hydrophobic group. Monocarboxylic acids were obtained

Synthesis

49

starting from intermediates produced by photooxidation of hexafluoropropylene [131]: CF3(0CF?CF),,OCF2COOH (n = 1,2.3)

I

CF3 Dicarboxylicacidswereprepared by reductivecleavage of perfluoropolyperoxide obtained by photooxidation of tetrafluoroethylene [ 1311:

HOOCCF2[(0CF2CF~),,(OCF3_),,,1,0CF2COOH ( n = m = l , p = 1,2, 3) By Ozonizztiorz Perfluoroalkyl ethylene, CF3(CF2CF2),,CH=CH2, obtained by dehalogenation of perfluoroalkylethyl iodide, is treated with ozone to give ozonides. Oxidative cleavage of the ozonides with hydrogen peroxide yields perfluoroalkanoic acids [ 1321. From Oxoalkurzoic Acids

6,6- and 10,lO-Difluorooctadecanoic acids have been prepared from the corresponding oxooctadecanoic acids by esterification with diazomethane, fluorination with diethylaminosulfurtrifluoride in carbon tetrachloride, followed by hydrolysis of the ester to yield the carboxylic acids [133].

Sulfonates By Electr-oclzernicalFluorination Perfluoroalkanesulfonyl fluorides or chlorides obtained by electrochemical fluorination are hydrolyzed to give the corresponding sulfonic acid or its salt [51-53]: CF3(CF2),,S02F+ 2NaOH -+ CF3(CF2),1S03Na+ NaF

+ H20

The industrial process removes the fluoride liberated with calcium oxide [ 1341:

CF3(CF2),,S02F+ KOH

+

CaO -+ CF3(CF2),S03K + f CaF2 + H20

Quaternary ammonium salts of perfluoroalkanesulfonic acids can be prepared by neutralizing perfluoroalkanesulfonic acid with tetraalkylammonium hydroxide. This procedure is not economical, however, for industrial use. A discovery [ 134-1 361 that quaternary ammonium salts of perfluoroalkane sulfonic acids can be obtained directly by reacting the perfluoroalkanesulfofluoride with a tertiaryamineand an alkoxysilaneisthebasisforacommercial

Chapter 2

50

process: C,,F2,?+I S 0 8

+ N(C,,,H?,,,+1)3 + Cll?H2112+lOSiR3 -+ C12F?,t+ 1SOdN(C,,,H~,,,+ ,)A]+

+ FOSiR3

The fluorinated surfactant Fluortensid FT 248 was made by reacting perfluorooctanesulfonyl fluoride with triethylamine and ethoxysilane in a anhydrous polar solvent (e.g., monochlorobenzene, diethyl ether, chloroform, acetonitrile, or tetrahydrofuran). The product precipitated and was then separated by filtration or centrifugation. The reaction mechanism was elucidated by reacting perfluorobutanesulfonyl fluoride with triethylamine and an amine: CAF9SO2F + H*N(CH2)4Si(CH3)?OCH3+ N(C2H5)3+ C4F9SOL7[N(C2H5)3CH3]++ H2N(CH2)4Si(CH3)2F Contrary to the expected reaction path, the methoxy group reacted instead of the primary amine with the sulfonyl fluoride. The reaction mechanism is supported by the observation that in the absence of triethylamine, the reaction of the amine with the perfluorobutanesulfonyl fluoride produces the compound

CH3

H

Perfluoroacylbenzenesulfonic acids, RtCOC6H4S03H, or their salts [ 1371 have been prepared by acylating benzene with a perfluoroacyl halide in the presence of a Lewis acid and sulfonating the reaction product. Sodium (perfluorooctanoy1)benzenesulfonate was prepared by reacting perfluorooctanoyl chloride with benzene in the presence of AlC13: C7FlSCOCl + C6H6

-

aC ~ F I ~ C O C ~ H S

oleum

C7F15COC6H~S03H

The sulfonic acid was neutralized with NaOH to give the sodium sulfonate C7F15COC6HAS03Na. Perfluoroalkylether amides featuring a sodium sulfonate or sodium carboxylate group have been prepared by reacting an acid fluoride with an aromatic or aliphatic amino acid. For example. F[CF(CF3)CF20]3CF(CF3)COF has been reacted with sulfanilic acid and triethylamine, followed by NaOH, to give F[CF(CF3)CF20I3CF(CF3)CONHC6H$3O3Na [ 1381. By Telornerization Perfluoroalkylethyl iodides can be converted to sulfonic acids utilizing a thiol as an intermediate. The thiol formed with thiourea [92-941 is dispersed in acidified water and oxidized with chlorine:

Synthesis

51

,NH, CF,(CF?),,CH2CH21 + (NH?)2C=S -+CF3(CF,),,CH?CH2SC * HI

base

\\NH

CF3(CF?),,CH2CH2SH

i 1

cl,m20

CF3(CF2),,CH?CH?S02Cl hydrolysis

CF3(CF2),,CH2CH2S03Na Alternatively, a perfluoroalkylethanethiol, C6F13CH2CH2SH, can be reacted in the presence of a free-radical catalyst with l-allyloxy-2,3-epoxypropane to form epoxides. A reaction of the epoxides with CH3NHCH3CH2SO3Nagives the fluorinated surfactants [ 1391

and

A fluorinated surfactant CF3(CF2),,CH2CH2S0~NH~ (NH: or H+) is obtained by a reaction of the perfluoroalkylethyl iodide with KSCN. The thiocyanate formed is oxidized with a peroxycarboxylic acid (e.g., peracetic acid) [ 1401:

CF3(CF,),,CH?CH21 + KSCN +CF,(CF,),CH2CH2SCN

1

CH,C002H

CF3(CF2),,CH?CH2S03NH~ Potassium perfluoro-3-oxaalkanesulfonate,

can be prepared using sulfur trioxide and tetrafluoroethylene as the starting materials to form the tetrafluoroethane sulfone as an intermediate [ 1411: CF2-CF2 CF,-COF

I

I

4

SO?-0 SOZF [FS02C2F,0]-K+ + C,F,

KF

[CF2-CF?O]-Kf

> I

502f

+ I,+

2F2F4

+

FSO C F OC2F41 KI

FSO~C?F,0(C2F4),lCZF~I

Chapter 2

52

The resulting iodide is converted to an anionic surfactant by substituting iodine with fluorine. A fluorine-nitrogen mixture (1 : 3) is bubbled into a solution of the telomers in 1,1.2-trichlorotrifluoroethanecontaining NaF and MgF2. The solvent is removed by distillation and the residue is treated with KOH. The surfactant, CF3CF2(CF2CF2),,OCF2CF1SO3K, is isolated by neutralization and extraction with methanol. BY Oligornerizntion The tetrafluoroethylene pentamer (Fig. 2.7) is allowed to react with phenol. The phenylether formed is treated with oleum to yield a sulfonic acid, which is neutralized with NaOH [96,97]: NaOH

f SO3 + C I O F 1 9 0 C ~ H ~ S 0 ~ H ~ C I ~ F 1 ~ 0 C ~ H ~ S 0 ~ N a CI~F190C6H5

Bv Sulfonation with a Sultone Sulfopropylated N-alkylperfluorooctanamides [ 1421 have been prepared by reacting ethyl perfluorooctanoate with an amine. The amide formed was reacted with metallic sodium and 1,3-propanesultone to give the fluorinated surfactant RtCONR(CH2)3S03Na, where R = H or an alkyl in the C1 to C12 range. The nbutyl group generally gave the best surfactant properties. From Hexurfluoropropene Oligomers Fluorinatedsurfactants with an oligo(hexafluoropropeneoxide)hydrophobe have been prepared by sulfonating (HFPO),,Ar, where (HFPO),, is an oligo(hexafluoropropene oxide) group and Ar is an aryl group [ 1021.

A perfluoroalkene is reacted with the disodium salt of y-hydroxybenzenesulfonic acid in a aprotic solvent (DMF) [143]: [(CF3)2CFI2C=CFCF3

+ NaOC6H4So3Na + [(CF3)2CF],C=CCF30C6H~S03Na

+ NaF

Michael Addition

The Michael addition of a perfluoroalkyl group containing amine to a potassium acryloamidoalkanesulfonate gives a sulfonate with a perfluoroalkyl end group [ 1441: C ~ F ~ ~ S O ~ N H C H Z C H+~ CH2CHCONC NHZ (CH3)2CH2S03K + CsF17SO?_NHCH?CH?NHCH2 CH2CONHC(CH3)2CH2SOjK

c

-

”

-.”. ””.””&

.“””

II_ , . I ”

Synthesis

53

Sulfosuccinates Fluorinated alkyl-2-sulfosuccinates are prepared by a reaction of a fluorinated alcohol with maleic anhydride. The reaction yields monoesters or diesters, which are treated with Na2S03 or Na&OS. Two moles of the fluorinated alcohol CF3CF2CHzCH20Hand 1 mol of maleic acid are heated at 140°C to give the diester, which is heated at reflux with Na2S205in aqueous isopropyl alcohol to obtain a fluorinated sulfosuccinate [ 1451: 2CF,CF,CH2CH20H

+ HOOCCH=CHCOOH

+

CF3CFzCH2CH200CCH=CHCOOCH2CHzCFCF3

I

Na,SO,

CF3CF2CH,CH200CCHCH?COOCH2CH2CF?CF3

I

S03Na Yoshino et al. [ 146,1471 esterified a fluorinated alkanol. such as lH, lH, 9H-hexadecafluorononanol-l , with maleic anhydride in the presence of y-tolue-

nesulfonic acid monohydrate. The resulting bis-maleate ester reacted with sodium hydrogen sulfite to yield the sodium salt of the fluoroalkyl-2-sulfosuccinate. Oxyethylene groups were introduced by reacting the corresponding fluorinated alcohols with ethylene carbonate prior to esterification with maleic anhydride [148].

Hybrid Surfactants Hybrid anionic surfactants contain a fluorocarbon chain, a hydrocarbon chain, and a hydrophilic head group in the same molecule. Guo et al, [149] synthesized hybrid surfactants by the reaction scheme shown in Fig. 2.8. The hydrophilic group

FIG.2.8 Synthesis of hybrid surfactants. (Reproduced with permission from Ref. 149. Copyright 0 1992 by the American Chemical Society.)

Chapter 2

54

SO,:l,?-Dioxme CHICICH2CI

-

R

f

o CO-CH(SO,H)R

FIG. 2.9 Synthesis of hybridsurfactantscontaininganaromatic group. [Reproduced with permission from Ref. 150. Copyright 0 1995 by the American Chemical Society.]

is attached to the surfactant molecule through an ester bond -COS03Na sensitive to hydrolysis. The surfactants hydrolyze slowly in humid air and in an aqueous solution, which limits their use. The hybrid surfactants synthesized by Yoshino et al. [150] contain an aromatic ring. The synthesis involves the following sequence of reactions shown in Fig. 2.9. The hydrophile is a sulfonate attached to the molecule through a -CS03Na bond, which is stable to hydrolysis.

Sulfates Perfluoroalkylmethyl sulfates, X(CF2),,CH2OSO2Na,where X is H or F and 17 = 5-1 2, are prepared by a reaction of the corresponding alkanols with concentrated H2S04or C1SO3H and neutralization [ 15I]. Greiner al.et [ 1521 prepared fluorinated ether sulfates, H-(CF2CF*),,CH* [OCH2CH2],,,0S03NH4, 17 = 2, 3, 4, rn = average 3. The telomer alcohols were oxyethylated using BF3 as the catalyst, sulfonated with chlorosulfonic acid, and neutralized with ammonia. The fluorinated surfactants (CF3)2CFO(CH~)60S03Na (Na+ or NHI) have been prepared by sulfation of (CF3)2CFO(CH2)60H,which is obtained by a reaction of hexafluoroacetone, CF3COCF3,with KF and Cl(CH2)60H [ 1531.The toxicity of hexafluoroacetone limits the usefulness of this process. Dipropylene glycol 5.5,6,6,6-pentafluorohexylether sodium sulfate has been prepared by reacting dipropylene glycol 5,5,6,6,6-pentafluorohexylether with chlorosulfonic acid and neutralizing with NaOH [ 1541.

Bunte Salts Fluorinated Bunte salts, featuring an -S203M group, are prepared by reacting sodium thiosulfate with perfluoroalkylethyl iodide [ 155,1561:

Synthesis

55

C,,F2,,+ICH~CHJ+ Na2S203 -+ C,lFZ,l+CHzCH3S203Na Fluorinated Bunte salts are effective dispersants and emulsifiers for fluorinated polymers [ 1561.

Phosphates Thealkylphosphatesaremixtures of monoestersanddiestersformulated ammonium as salts [e.g., (R+-CH,CH2O)P(O)(O-NHz)2 and (RfCH2CH20)2P(0)(0-NH:)] or as free acids [e.g., (RfCH2CH,0)P(0)(OH)2 and (RfCH2CH20)2P(O)(OH)]. By Telomerization

Bis-(fluoroalkyl)phosphate esters are prepared by reacting POC13 with a perfluoroalkylethanol [ 1571:

+

2CF3(CF3),lCH2CH20H POC13 +

(CF3(CF?_),CH?CHz0)2P(O)Cl+ 2HC1 /H20

(CF3(CF2),,CH2CH20)?PoOH Reaction conditions can be selected to minimize the formation of the monoester and the triester. Perfluorooctylethanol has been reacted with P203C14and diethanolamine to obtain a product with the triester content of 3.0% and the average composition (CF~(CF~)~CH?_CHZO) 1 .5PO[OH*NH(C2H40H)2] 1.5 [ 1581, A reaction of perfluoroalkyl iodides with yellow phosphorus produces fluorinated surfactants [93]: 6RfI + P4 RfZPI + RfPI2

2Rf2PI + 2RfPIZ H ~ O>

Rf2POzH + RfP03H2

By Electrochemical Fluorination Perfluoroalkane sulfonyl fluorides produced by electrochemical fluorination are reacted with ethylaminoethanol. The fluorinated alcohol formed is converted with POC13 to a fluorinated phosphate ester [159]: 2CsF17SO?N(C?HS) CHzCH2OH-

POC13

[C8F17SO~N(C2H5)CH2CH20]3P(O)Cl

[C~F17S03N(C2H5)CH2CH20]2P(0)ONa

Chapter 2

56

By Oligomerization Reaction of the tetrafluoroethylene tetramer [96,97] with dilute aqueous sodium hydroxide and acidification of the reaction product give the ketone C6F13CH2COCF3.Bases convert the ketone to its enol form, which reacts with phosphorus oxychloride. Hydrolysis of the reaction product gives the fluorinated surfactant C6F13CH=C(CF3)OPO(OH)3. 2.7

CATIONIC SURFACTANTS

Carbonyl or sulfonyl fluorides obtained by electrochemical fluorination yield cationic fluorinated surfactants by a reaction with N,N-dimethyl- 173-diaminopropane and quaternization with methyl iodide or dimethyl sulfate [5,160]:

CSFI~SO~NH(CH~) N(CH3W In a similar fashion, cationic fluorinated surfactants are prepared from telonler iodides by converting perfluoroalkylethyl iodides to sulfonyl chlorides and reacting these with N,N-dimethyl- 173-diaminopropane[ 1611. Likewise, cationic surfactants are prepared by the oligomerization process. The tetrafluoroethylene tetramer or pentamer is allowed to react with phenol and the phenyl ether obtained is chlorosulfonated [96,97]. The sulfonyl chloride is reacted with N,N-dimethyl- 1,3-diaminopropane and quaternized: CIS03H

+C ~ O F ~ ~ O C ~ H ~ F C10F20 f C~HSOH C I oF190C6H~SO?Cl

1 1

(CH3)2N(CH2)3NH2

C~OFI~OC~H~SO~NH(CH~)~N(CH,)~ CH3I

C,OF~~OC~H~SO'NH(CH~)~N+(CH~) 31Perfluoroalkyl-2-ethanethiols are prepared by reaction of a perfluoroalkyl2-ethyl iodide with thiourea and subsequent alkaline hydrolysis of the thiouronium saltformed[90,94]. A reaction of the perfluoroalkyl-2-ethanethiol

Synthesis

57

with CICHZCH(OH)CH~N+(CH~)~C~gives cationic the surfactant RCH?CH?SCHZ CH(OH)CH2N+(CH3)3Cl- [ 1621. In some cationic fluorinated surfactants, the positively charged nitrogen atom is part of a heterocyclic ring [ 1631. Katritzky et al. [ 1641 prepared cationic fluorinated surfactants derived from N-(perfluorooctanesulfony1)piperazine: C,F,,SO,F

n

HN

-

wNH

n

2RI ___)

UNH

C8F17S02N

/-/ C8F17S02N

WNk2r

where R = C,lH3,2+1, 11 = 1,2,4,6, or 8. lH, l H , 2H, 2H-Perfluoroalkylpyridinium chloride is synthesized from lH, lH, 2H. 2H-perfluoroalkyl iodide and pyridine. The alkylpyridinium iodide obtained is passed through an ion-exchange column to yield the desired lH, lH,2H, 2H-perfluoroalkylpyridinium chloride [ 1651. Cationic fluorinated surfactants have been prepared from perfluoroalkyl esters, obtained by converting an acid fluoride into an ester. Reaction of the ester with a diamine and alkylation with a halide or sulfonate gives a cationic surfactant, for example [ 1661:

CF3CF?[CF20CF(CF3)]2COOCH3+ HzN(CH2)3N(CH3)2+

CFr(

CH31

CF2[CF2OCF(CF3)]~COHN(CH2)3N(CH3)~~

CF~CFZ[CFZOCF(CF~)]~COHN(CH~)~N+(CH~) 31Matsui et al. [ 1671 reacted the fluorinated ester C7F15COOC1H5with H2N(CH1)2N(CH3)2and reduced the reaction product with LiAlH4. Subsequent quaternization with CH31 and ion exchange to replace iodide with chloride produced the cationic surfactant C7F15CHZNH(CH2)2N+(CH3)3C1-. Using 3-(perfluoro- 1,l-dimethyl-butyl)-1-propene as the starting material, cationicfluorinatedsurfactantshave been prepared via thecorresponding epoxypropane reacted with a secondary amine [168-1701: CF3

I

R'

/

C6F3CH2CH-CHzN'

I

OH

\

R2

The alkylation with methyl iodide gave quaternary ammonium salts [ 1701.

Chapter 2

58

A reaction of fluorinated alkylethoxy-, alkylethylthio-, or alkylethylamino1,2-epoxypropanes with dimethylamine. followed by quaternization with methyl iodide, yields cationic surfactants [ 171-1751: HN(CH,)2

RfC2H4QCH2CHCH?N+(CH3)31RfCZH4QCH2CHCH2

I

V

OH

A glycidyl ether intermediate can react cationic surfactant directly [6]: RfC2H,SC2H,OCH?CHCH?

with tertiary amines to give a

N(CH,),.HCI

\ / 0

R&2H4SC,H10CH2CHCH,N+(CH3)31-

I

OH

A reaction of the fluorinated epoxypropane with morpholine gave a cationic surfactant with a heterocyclic nitrogen:

Amine oxides [ 1761 or, more correctly, N-oxides of tertiary amines, are electrically neutral and belong formally to the class of nonionic surfactants. However, the chemistry involved in the production of amine oxides is related to that of cationic surfactants and is therefore described here. The synthesis of amine oxides involves oxidation of a tertiary amine with hydrogen peroxide [ 160,170.177- 1791 or peracetic acid [ 1801: CH3C(O)OOH

CSF~~CH~CH(OOCCH~)CH~N(CH~)~A CSF~~CH&H(OOCCH~)CH~N(CH~)?

\1

0 Fluorinated amine oxides with a heterocyclic nitrogen have been prepared [ 1701, for example:

A Amine oxides are effective foaming agents.

Synthesis

59

2.8 AMPHOTERICSURFACTANTS Amphoteric fluorinated surfactants have at least one anionic and at least one cationic group at about their isoelectric point [ 1811. The cationic group is usually a quaternary amine or a protonated tertiary or secondary amine. The anionic functionality isa carboxylate, sulfonate, sulfate, or phosphate group. Most widely used fluorinated surfactants are carboxybetaine-type amphoteric surfactants which have. like betaines, a cationic amine functionality and an anionic carboxylate group. Instead of acarboxylate group, the anionic component of sulfatobetaines isa sulfate group and that of sulfobetaines is a sulfonate group. Cnrboxybetaines have been prepared by reacting a fluorinated tertiary amine with P-propiolactone [ 182-1 861, chloroacetic acid or its sodium salt [ 185-1 941 oracrylic acid [ 182,185,1861.P-Propiolactoneisasuspected carcinogen and is probably no longer used commercially asa quaternizing agent. The synthetic methods utilized for the preparation of amphoteric fluorinated surfactants are similar to those used for cationic surfactants, except for the alkylation step. A tertiary amine can be prepared by reaction of a fluorinated ester or acid halide with a diamine which contains both a primary or a secondary amine and a tertiary amine [195]. The resulting amine is then treated with chloroacetic acid:

Fluorinated tertiary amines with a sulfonamido linkage are obtained by a reaction with a fluorinated alkanesulfonyl chloride and a primary or a secondary amine. For example, 1-amin0-3-dimethylaminopropane reacts with perfluoroalkylethanesulfonyl chloride to give a fluorinated tertiary amine which is quaternized to yield a betaine [ 1961: C6FI3CH2CH2S02C1+ H?N(CH2)3N(CH3)2

1. base

CH,C,

A reaction of perfluoroalkoxybenzenesulfonyl chloride with 1 -amino-3-

Chapter 2

60

dimethylaminopropane yields a betaine with an aromatic sulfonamido linkage [ 1941: base

C ~ F ~ ~ O C ~ H ~+SH?N(CH2)3N(CH3)? OZC~ d

C9F190C6HiFS02NH(CH2)3N(cH3)3

I

ClCHzCOOH

C~F~~OC SO?NH(CH&N+(CH3) ~HA 2CH2COOOligomerization of tetrafluoroethylene provides intermediates for amphoteric fluorinated surfactants. The tetrafluoroethylene pentamer reacts with phenol to form aphenyl ether [96]. Chlorosulfonation of the phenyl ether with chlorosulfonic acid yields a sulfonyl chloride, which is allowed to react with N,N-dimethylpropanediamine: base

C I O F ~ ~ O C ~ H+ ~S NH2CH?CHzCHzN(CH3)2 O~C~ +

A reaction of the resulting tertiary amine with P-propiolactone gives an amphoteric fluorinated surfactant:

Because P-propiolactone is a suspected carcinogen, sodium chloroacetate (or chloroacetic acid) has become the preferred alkylation agent:

Amino-acid-based amphoteric surfactants containing a hydroxyl group have been obtained by reacting an amino acid with a perfluoroalkylated epoxide [ 1971: Rf"CH-"CH2 O '/

+

+ NHZCHCOOH + RfCHCH2NCH2CHCOOR

OH

RI

The reaction is catalyzed by triethylamine in 65% aqueous ethanol. Carboxybetaines with a sulfide linkage have been prepared by a reaction of

Synthesis

61

perfluoroethyl iodide with (2-mercaptoethy1)dimethylammonium chloride and subsequent quaternization with chloroacetic acid [ 198.1991: NaOH

RCH2CH2I + HSCH2CH2N(CH3)2*HClA RfCH2CH2SCH2CH2N(CH3)2

I

CICHzCOOH

R~CH~CHZSCH~CH~N+(CH~)~CH~COOCarboxybetaines with a sulfide link and a hydroxyl group are obtained by reaction of perfluoroalkylethanethiol with epichlorohydrin. The resulting epoxide is allowed to react with sarcosine to give a betaine [200]: RfCH2CHzSH + CICH2CH-CH2 \ /

-+ RfCH2CHZSCH2CH-CH2 \ /

0

0

I

H,CNHCH,COOH

RfCH2CH2SCH2CH(OH)CH2N+H(CH3)CH,COOAmphoteric fluorinated surfactants with heterocyclic nitrogen have been prepared by treating a fluorinated ester with N-(2-hydroxyethyl)piperazine and subsequently with chloroacetic acid [ 1951: C9F,9COOC,Hy

ANCH2CH2OH W

+ HN

/

NCHzCH2OH CgF19CON

\

ClCH,COOH

W

n

C9F19CON

\

Nf(CH2CH20H)CH2COO-

/

Perfluoroalkyl esters react with cyclic N-aminoquaternary salts to give an amphoteric fluorinated surfactant with a heterocyclic nitrogen [201]:

A 0

W

NNH,

+ CHJ

A

+0

/CHs 10

Chapter 2

62

A

/CH3

0

-

0

I

+ CH30Na+ F (CF2)7C02C2H3

Amphoteric fluorinated surfactants with heterocyclic nitrogen have also been prepared by the Michael addition of 2-perfluoroalkylethanethiol to either a maleic monoarnide or monoester [ 163,202-2041 :

e

0

H

0

I

U

Sulfobetnines [ 187,205-2081 were usually prepared by a reaction of a tertiary amine with propane sultone [205,206,209]. However, propane sultone has been found to be a carcinogen and is no longer used for this purpose. Instead of propane sultone, chlorosulfonic acid or its salt [210] and sodium 3-chloropropanesulfonate [207,208] are used as quaternizing agents to obtain fluorinated sulfobetaines. Sulfktobetairles contain a sulfuric acid ester group, introduced by sulfating a hydroxyl group or an olefinic bond [210-2131. Sulfur trioxide, with dimethyl sulfate or as a trimethylamine complex, has been used as a sulfating agent to preparesulfatobetaines.BillensteinandEhrl[212]treatedthecompound CF3(CF?)6CF=CHCH2N(CH3)CH2CH20Hwith a S03-air mixture and with dimethyl sulfate to give the betaine

Sulfatobetaines RCH2CH(OS0y)CH2N+(CH3)3, where R = C6F13-,C6F13CH2CHzS-, C6F13CH2CH2S(CH?)30-,Cp,Hl7CH?CH?S--, or C8HI7-, have been prepared by reacting an epoxide RCH2CH-CH~0 with a (CH3)3N-SO3complex [214]. '0' Plzosylzatobetnines have been preparedfromafluorinatedalkanol, Rf(CHz),,OH,where Rf = C6FI3or C8FI7and n = 2 or 11. Riess et al. [215,216] phosphorylated the fluorinated alcohol with phosphorus oxychloride in the presence of triethylamine:

Synthesis

63

The reaction product, Rf(CH2),,0P(0)CL, was subsequently reacted with bromoethanol. Hydrolysis of the phosphoryl chloride group and a reaction of the bromoethyl group with trimethylamine and silver carbonate yielded the perfluoroalkylated phosphotidylcholine:

0

II I

Rf(CH2),,0POCH~CH~N+(CH3)3 0 Alternatively, the phosphorylated alcohol, RXCH2),20P(0)C12, wastreated with choline tosylate in pyridine. Subsequent hydrolysis gave the same perfluoroalkylated phosphotidylcholine:

Chlorination of the fluorinated alcohol by phosphorus oxychloride was suppressed by using anhydrous diethyl ether as the solvent [216]. Amine(polyfluoroalkoxyacy1)imides have been prepared by the reaction of esters containing polyfluoroalkoxy groups with tertiary alkylaminimides and hydroxyalkylaminimides [2 171:

R = RtCX20CH2, X = F or H, R” = CH3, CH2CH20H, or CH2CHCH20H OH

Chapter 2

64

2.9

NONIONIC SURFACTANTS

The method selected for the preparation of a nonionic fluorinated surfactant depends on the hydrophile, which can be either an polyalkyl ether chain or a polyhydroxy group. Because oxyethylation results in a mixture of oligomers, special methods have been devised for the preparation of monodisperse surfactants. A unique group of nonionic surfactants are amphiphiles without a hydrophile. Semifluorinated alkanes with an oleophilic and an oleophobic segment function as nonionic surfactants in oleophilic solvents (see Section 1.8).

Nonionic Fluorinated Surfactants witha Polyoxyethylene Chain Fronz Fluorinated Alcohols

The preparation of nonionic fluorinated surfactants from fluorinated alcohols is straightforward [218-2301. Perfluoroalkylethanol (see Section 2.3), obtained from telonler perfluoroalkylethyl iodides, is oxyethylated in the presence of a catalyst, for example, BF3: /"\

R+-CHZCHZI-+ RfCHZCH20H

CH?"CH?

R+-CH2CH20(CH?CH20),,H

The oxyethylation conditions affect the formation of dioxane, a toxic byproduct. Yang et al. [221] have found that a catalyst, consisting of an aluminum alkoxide (average C l o alkoxide) and hydrogen fluoride, reduces the dioxane concentration to 9000 ppm, compared to 45.000 ppm for the surfactant prepared with BF3 as the catalyst. Recent improvements of the oxyethylation process have reduced the dioxane content further. Ethylene carbonate has been used as the alkylating agent for O-hydroxyethylation of l, l-dihydroperfluorinated alcohols, catalyzed by tetraalkylammonium iodides and trialkylamines [2311. Telomer perfluoroalkylmethanols have been oxyethylated to form nonionic surfactants having the structure RfCH20(CH2CH20),,H[222,223]. Oxyethylation of the telomeric alcohol H(CF2),,CH20H, where 171 = 6, 8, 10. in the presence of BF3 yields the surfactant H(CF2),,,CH20(CH2CH20) ),H [224,225]. The surfactant with the hydrophile length n = 10 was found to have the best surfactant properties. The nonionic fluorinated surfactants CF3(CFZ),,,CH20(CH2CH20),,Hand H(CF2),CH20(CH2CH20) IIH have been prepared from the corresponding fluorinated alcohols in the presence of a base catalyst [226]. Perfluorooctylethanol reacted with epichlorohydrin gives an ester which, on hydrolysis with HC1, yields surfactant the of structure the CgF17CH2CH?O [CH2CH(CH?OH)O],,H(average n = 1.7) [227]. Leempoel et al. [228] and Selve and Castro [229] condensed H(OCHZCH~)~OH and C7FIsCH20Hto form a nonionic fluorinated surfactant:

Synthesis

65

From Acids or Acid Halides

Nonionic fluorinated surfactants are prepared from acid fluorides, obtained by electrochemical fluorination, usually with an amide as the intermediate. Sulfonyl fluorides are converted with an amine to the amide, which is reacted with ethylene chlorohydrin and oxyethylated:

1

/O\ CH,-CH,

C8FI7S02N(C2HS)CH,CH,O(CH,CH,O),,H The perfluorinated carbonyl fluorides, prepared by electrochemical fluorination, are converted to nonionic fluorinated surfactants via the amide by a similar process:

1

/O\ CH,-CH,

.L

C7FI~CON(C2H~)CH~CH~O(CH1CH,O),,H Meussdoerffer and co-workers [ 134,2321have developed the following process for preparing nonionic fluorinated surfactants: CSF17SOZF + 2H?NCH,+ C8FI7SO2NHCH3 + HZNCH3.HF C,H,OH COC12

/O\ + /CHZ"CHz

+ H(OCH?CH?),,OC4H9

+ H(OCH2CH2),,0CJH,+ ClC(OCH?CH?),,OCJH9+ HC1 II

0

0

C~FI~SO~N(CH~)C(OCH~CH?),,OC~H~ + HCl

II

0

Chapter 2

66

Alkoxylated perfluoroalkane sulfonamides have been prepared by reacting aliphatic polyamines with less than the stoichiometric amount of a perfluoroalkanesulfonyl fluoride and alkoxylating the reaction product with ethylene oxide or propylene oxide [233]: (C~HS)~N

CsF17S02F + H~N(CH~CHZNH)~CH~CH?NH~ (CH?CHz)0

C ~ F I ~ S O ~ N H ( C H ? C H ~3CHZCH2NH2 NH) A CsF17S02NH(CH2CH2NH) 3CH2CH2NH(OCH?CH?),,H Perfluoroalkanoic acids can be converted to nonionic fluorinated surfactants by a reaction with oxyethylated propylenediamine [93]: C7FlsCOOH + HzN(CH2)3N[CH2CH20),,H]2+

Surfactants RfCONH(CH2CH20)24H have been prepared by condensing a perfluorocarboxylic acid derivative with an oxyethylated amine. A perfluorinatedcarbonylhalideisreactedwithbromoethylamineandtheproduct, RfCONHCH2CH2Br,is condensed with HO(CH2CH20)24H. The same surfactant canbe prepared by condensing the ester, RI-COOCH3, with the amine H2N(CH?CH20)2H [234]. An esterification reaction has been utilized to introduce perfluoroalkyl groups into commercial nonionic hydrocarbon-type surfactants, Pluronic F68 and Butronic R1. Gangoda et al. [235] esterified Pluronic F68 (block copolymer of ethylene oxide and propylene oxide) with perfluorocarboxylic groups to increase the stability of fluorochemical (perfluorodecalin and perfluorotripropylamine) microemulsions. Nonionic fluorinated surfactants having the structures

can be obtained by alkoxylation of the corresponding carboxylic acids [236].

From Tet~.u~uoroetlzvlelze Oligomers For the oligomerization of tetrafluoroethylene, see Section 2.4, Fig. 2.7. Nonionic surfactants can be prepared by a reaction of the tetrafluoroethylene pentamer with alcohols or with a phenol [96,97]. The reaction product obtained with phenol is sulfonated to produce a sulfonyl chloride. A reaction of the sulfonyl chloride with with N-ethylethanolamine yields an alcohol, which is oxyethylated to produce a nonionic surfactant:

Synthesis

67

Nonionic fluorinated surfactants with an aliphatic hydrophobe are produced by a base-catalyzed reaction of the oligomers with oxyethylated alcohols. The ethers formed are converted to nonionic surfactants: C10F20+ HO(CH2CH20),,R + CloF190(CH2CH?0),,R7 R = H, alkyl. aryl, etc. If R is H, both ends of the molecule react with the fluorocarbon [237]: cIOF20

+ HO(CH2CH2o)nH

ClOFl90(CH2CH20)rrCIOF19

or CgF16

+ HO(CH?CH20),,H + CgF,SO(CH2CHzO),,CgF,5

Unlike the phenyl ethers, the alkyl ethers fornled react readily with nucleophilic reagents. The reaction with oxyethylated alcohol yields a nonionic surfactant with two oxyethylene chains, Cfh3 ,O(CH2CH11),R /

\

A stable nonionic fluorinated surfactant, of structure the C6F13CH=C(CF3)0(CH?CH?0),,R7 isobtained by a reaction of the alkyl ether with an aqueous base which replaces a CF3- group with hydrogen. From Fluorinated Thiols

A fluorinated thiol is reacted with monochlorohydrin and the resulting alcohol is oxyethylated to yield a nonionic surfactant [238]. Perfluoroalkylmercaptan, RfCH2CHzSH,epichlorohydrin, and Carbowax 350 were reacted to prepare the surfactant RfCH2CH2SCH2CH(OH)CH20(CHzCHO)CH3[239]. From Hexafluoroacetone Hexafluoroacetone has been converted with potassium fluoride to the alkoxide which is reacted with 1-chloro-6-hexanol: (CF,)ZC=O (CF,),CFOK

+ KF

(CF3)ZCFOK

+ CI(CH2)6OH+(CF3)2FCO(CH2)60H + KC1

Chapter 2

68

where 17 is from 1 to about 20 [240]. Hexafluoroacetone is highly toxic and a teratogen. It is therefore unlikely that hexafluoroacetone is used for the industrial production of fluorinated surfactants. Nonionic surfactants with perfluorohexyl a hydrophobe and a polyethyleneglycol methylether hydrophile, with a prolongator unit between them, have been synthesized from perfluorohexyliodide, perfluorohexylbutyliodide, or sodium l H , lH, 2H, 2H-perfluorooctoxide [241]:

F,,C&,H,ONa

-t ClC3H6C(O)O(C2H4O),,CH3

"NaCI

F,3C6C2H,OC3H6OC(O)O(C2H,O),,CH3 F,3C61+ CH2=CHCH20(C2H40)n,CH3-+F13C6CH2CHICH30(C3H30),,,CH3

i

+NaOC2H, - NaI

F13C6CH2CH(OC?HS)CH,O(C2H~O),,,CH, for 12 = 12 and 17 and IIZ = 7, 12, 17.

Monodisperse Nonionic Fluorinated Surfactants

i

Oxyethylation results in oligomeric mixtures, RdCHZ)lor 30(CH3CH20j,,H, differing in the length, M , of their hydrophilic chain. Monodisperse nonionic surfactants can be prepared by grafting a perfluoroalkyl group to oligo(ethy1ene glycol) of defined molecular weight [242-2441. One end of the oligomer is protected by alkylation and the other end is activated by (1) chlorination using thionylchloride [242.243] or (2) tosylation [244]. The compound formed is condensed with a fluorinated alkanol and the end cap is removed by hydrogenation in the presence of a catalyst. Because the length of the polyoxyethylene hydrophile could contribute to membrane toxicity of microemulsions considered for biomedical oxygen transport (see Section 10.4), Guittard et al.[245] synthesized monodisperse fluorinated surfactants containing two hydrophilic oligo(oxyethy1ene) groups linked to the fluorinated chain through an amine bond :C2H40)pH F(CF:),,CH?CH?N

\

(C2H40jp where rz

= 4,6,

and 8 andp

=

2 and 3.

Synthesis

69

Monodisperse bipodal nonionic fluorinated surfactants, perfluoroalkyl N,Npolyethoxylated amides, have been synthesized as dimethyl ether derivatives, RfCH2C(O)N[(CH2CH?O),,CH312.to prevent equilibration with aminoesters [2461. A different method for preparing monodisperse nonionic fluorinated surfactants has been described by Szony and Cambon [247]. The thiol RfCH2CH2SH is reacted with Cl(CH2CH20),,H. where IZ = 1-4, a known value. Thiolation of the compound, IZ = 1-3, and second a substitution reaction with C1(CH2CH?0),,H monodisperse yields the nonionic surfactant RtCH2CHZS (CH2CH20),,CH2CH2S(CHZCH2O),,,H (12 = 0-2,112 = 1-4). Later, Szonyi and Cambon [248,249] developed a three-step synthesis with an oxirane as the intermediate:

1

CICH,CHCH

\o/

where Nu is a nucleophile: diethylamine, (trishydroxymethyl)aminomethane, or N-methylglucamine. Polymerizable nonionic surfactants can be obtained by reacting the polyoxyethylated oxirane with N,N’-diallylamine, acrylic acid, or methacrylic acid as the nucleophile [250].

Nonionic Fluorinated Surfactants with a Polyhydroxy Hydrophile Nonionic fluorinated surfactants with a tris(hydroxymethy1) hydrophilic group have been synthesized by Pavia et al. [25 I]. A perfluoroalkylthioethanol reacted with tris(hydroxymethy1)amidonlethane in the presence of a radical initiator in boiling methanol: CH?=CHCONHC(CH2OH)3

+ HSCH2CH2CnIF2,,,+1 + C,nF3m+ ICH2CH2S(CH?_CH),,H

I

CONHC(CH?OH)3 Riess and co-workers [252-263] have prepared nonionic fluorinated surfactants with a polyhydroxy hydrophile by perfluoroalkylating natural products such as monosaccharides and disaccharides, pentitols, hexitols, and so forth. Per-Oacetylglycopyranosyl bromides were reacted with 1 1-(F-alkyl)- 10-undecenols,

Chapter 2

70

RfCH=CH(CH2)90H, Rf=C6FI or C8FI7 , prepared by reacting the perfluoroalkyl iodide with undecenol using copper(1) chloride and ethanolamine catalysis [254]. The reaction product was deacetylated in a methanol-triethylamine-water mixture to give the fluorinated nonionic surfactant (see Section 10.4). Perfluoroalkylated fatty acid 6-esters of sucrose and cy. a-trehalose were prepared by a reaction of sucrose or cy,&-trehalosewith a perfluoroalkylated acid, Rf(CH2),,COOH (Rf = CJFg, C6FI3,or C8FI7,rz = 2, 4, 10) in the presence of triphenylphosphine and diisopropyl azodicarboxylate in N,N-dimethylformamide [262]. The surfactants were used to prepare fluorochemical emulsions intended as intravascular oxygen carriers (see Section 10.4). Acetalation of pyranosides with perfluoroaldehydes in the presence of dicyclohexylcarbodiimide (DCC) yields perfluoroalkydine sugars with amphiphilic properties [264]. Wagner and Prescher (265) reacted the condensation product of 2-piperazinylethylamine and D-gluocono-&lactone with an epoxide CF3(CF2)7CH2CH"CHZ O '/ to obtain a fluorinated polyhydroxycarbohydrate.

Fluorinated Sun'actants Without a Hydrophile Oleophilic/oleophobic fluorinated surfactants without a hydrophile, designed for use in hydrocarbon systems, are in a structural sense also nonionic fluorinated surfactants (see Section 1.8); for example, the semifluorinated alkanes [266-2701, block polyethylene-polypropylene glycol ethers prepared with perfluoroalkene trimers [27 11, surfactants featuring an oligo(hexafluoropropene oxide) chain [272], and carboxamides and sulfonamides derived from N-(perfluorooctanesulfony1)piperazine [273]. Semifluorinated alkanes are prepared by a free-radical-initiated addition of perfluoroalkyl iodide to a terminal olefin [ 141,274,2751. Oil-soluble surfactants, CF3CF2CF20[CF(CF3)CF20In-2CF(CF3)COR(R = phenyl or y-tolyl), are obtained by arylating hexafluoropropylene oxide oligomeric acid halides [272]. Carboxamides and sulfonamides are prepared by conventional reactions using N-(perfluorooctanesulfony1)piperazine as the intemlediate [273]. i

REFERENCES 1. G, Siegemund, W. Schwertfeger, A. Feiring. B. Smart, F. Behr. and H. Vogel. in "Ullmann's Encyclopedia of Industrial Chemistry." Vol. A1 1. p. 349. VCH Verlagsgesellschaft, Weinheim (1988). 2. F. R. Feher. P. W. Foerst. P. H. Liu, D. J. Kalota. and J. S. McConaghy (Monsanto), Eur. Patent EP 332601 (1989).

L

. . .

Synthesis

71

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230. H.-N. Hunag and R. A. Halling (Du Pont), PCT Int. Appl. WO 95 35,272 (1995). 231. S. M. Heilmann, G. J. Drtina. E. P. Janulis, L. R. Krepski, J. K. Rasmussen. S. A. Babirad, D. M. Doren. D. S. Bonham, S. V. Pathre. and G. W. Greening. J. Fluorine Chem. 59.387 (1 992). 232. J. N. Meussdoerffer, H. Niederprum, and M. Dahm. Ger Offen. 2238740 (1 974). 233. G. Reitz and G. Boehmke (Bayer), Ger Offen. DE 2639473 (1978). 234. J. Afzai. B. M. Fung, and E. A. O’Rear. J. Fluorine Chem. 34, 385 (1987). 235. M. Gangoda. B. M. Fung, and E. A. O’Rear. J. Colloid Interf. Sci. 116,230 (1987). 236. N. 0. Brace (Du Pont), U.S. Patent 3,231.604 (1966); CA 64, 9963a. 237. L. E. Houghton (ICI), Ger. Offen. DE 2215388 (1972). 238. J. P. Lampin, A. Cambon, F. Szony. J. J. Delpuech, G. Serratrice, G. Thiollet, and L. Lafosse. Eur. Patent Appl. EP 165853 (1985); CA 105. 155095. 239. T. W. Cooke (Ciba-Geigy), Eur. Patent Appl. EP 10523 (1980); CA 93. 97302. 240. A. J. Szur (Diamond Shamrock). U.S. Patent 3,980,715 (1976): CA 85, 194514. 241. H. Meinert. P. Reuter. J. Mader, L. Haidmann, and N. Northoff, Biomater. Artificial Cells Immob. Biotech. 20( l), 115 (1992). 242. G. Mathis and J. J. Delpuech, Fr. Patent 8022875 (1980). 243. T. Gartiser, C. Selve, L. Mansuy, A. Robert, and J. J. Delpuech, J. Chem. Res. (S) 292 (1984). 244. S. Achilefu, C. Selve. M.-J. StCbe, J.-C. Ravey, and J. J. Delpuech, Langmuir 10, 2131 (1994). 245. F. Guittard. E. Taffin de Givenchy. F. Szonyi, and A. Cambon. Tetrahedron Lett. 36, 7863 (1 995). 246. C. Selve, E. M. Moumni, and J. J. Delpuech, J. Chem. SOC.,Chem. Commun. 1437 (1987). 247. F. Szony and A. Cambon, J. Fluorine Chem. 36. 195 (1987). 248. F. Szonyi and A. Cambon, MC Patent 02132 (1990). 249. F. Szonyi and A. Cambon, Tenside Surfact. Deterg. 31, 124 (1994). 250. F. Szonyi and A. Cambon. Tenside Surfact. Deterg. 31,257 ( 1994). 251. A. A. Pavia, B. Pucci, J. G. Riess, and L. Zarif, Bioorg. Med. Chem. Lett. l(2). 103 (1991). 252. L. Zarif, A. Manfredi, C. Varescon, M. Le Blanc, and J. G. Riess, J. Am. Oil Chem. SOC.66, 1515 (1989). 253. L. Zarif, J. Greiner, S. Pace. and J. G. Riess, J. Med. Chem. 33. 1262 (1990). 254. A. Milius, J. Greiner, and J. G. Riess, New J. Chem. 15. 337 (1991). 255. A. Milius, J. Greiner, and J. G. Riess, Colloids Surfaces 63,281 (1992). 256. J. G. Riess, C. Arlen, J. Greiner. M. Le Blanc, A. Manfredi, S. Pace, C. Varescon, and L. Zarif, Biomater. Artif. Cells Artif. Organs. 16,421 (1988). 257. J. Greiner, A. Manfredi, and J. G. Riess. New J. Chem. 13,247 (1989). 258. A. Manfredi, S. Abouhilale, J. Greiner, and J. G. Riess, Bull. SOC.Chim. Fr. 872 (1989). 259. L. Zarif, J. Greiner. and J. G. Riess, J. Fluorine Chem. 44, 73 (1989). 260. C. Varescon, A. Manfredi, M. Le Blanc, and J. G. Riess, J. Colloid Interf. Sci. 137, 373 (1990). 261. S. J. Abouhilale, J. Greiner, and J. G. Riess, Carbohydr. Res. 212, 55 (1991).

Synthesis

79

262. S. Abouhilale, J. Greiner, and J. G. Riess. J. Am. Oil Chem. SOC.69, 1 (1992). 263. J. Greiner. J. G. Riess, and P. Vierling. in “Organofluorine Compounds in Medicinal Chemistry and Biomedical Applications,” R. Filler. Y. Kobayashi. and L. M. Yagupolski, eds.. Elsevier, Amsterdam (1993). 264. C. Zur, A. 0. Miller. and R. Mietchen, J. Fluorine Chem. 90, 67 (1998). 265. R. Wagner and D. Prescher. Ger. Offen. DE 19,541,788 (1997). 266. R. J. Twieg and J. F. Rabolt. J. Polym. Sci., Polym. Lett. Ed. 21, 901 (1983). 267. J. F. Rabolt, T. P. Russell, and R. J. Twieg, Macromolecules 17. 2786 (1984). 268. R. J. Twieg, T. P. Russell, R.L. Siemens. andJ. F. Rabolt. Macromolecules 18,1361 (1985). 269. T. P. Russell, J. F. Rabolt, R. J. Twieg, R. L. Siemens. and B. L. Farmer, Macromolecules 19, 1135 (1986). 270. G. L. Gaines. Langmuir 7, 3054 (199 1). 271. Neos Co., Jpn. Kokai TokkyoKoho JP 5952520 (1984); CA 101.40225. 272. N. Tshikawa and M. Sasabe, J. Fluorine Chem. 25,241 (1984). 273. A. R. Katrizky, T. L. Davis, G. W. Rewcastle, G. 0. Rubel, and M. T. Pike, Langmuir 4,732 ( I 988). 274. N. 0. Brace. J. Org. Chem. 38, 3167 (1973). 275. N. 0. Brace, J. Org. Chem. 44,2 12 (1979).

1

Physical and Chemical Properties

3.1

CHEMICAL PROPERTIES

Perfluorinated surfactants are remarkably stable. Their outstanding thermal and chemical stability permits applications under conditions which would be too severe for conventional hydrocarbon-based surfactants. The very strong C-F bond is stable to acids, alkali, oxidation, and reduction, even at relatively high temperatures. The unusual properties of fluorosurfactants arise from the unique properties of elemental fluorine [ 11: High oxidation potential F2 + 2e- -+ 2F-, E0-2.65 V (Table 3.1) High ionization energy F -+ F+ + e- (Table 3.2) High electron affinity F + e- + F- (Table 3.2) High electronegativity of covalently bonded fluorine (Table 3.2). Fluorine is the most electronegative element. Fluorine is very difficult to polarize (1). The low dissociation energy of fluorine (F2 4 2F) (Table 3.3) provides a sufficient number of fluorine atoms for a reaction to occur. This is probably the main reason for thehigh reactivity of elemental fluorine [l]. The unusual chemical properties of fluorine as asubstituent in organic compounds have been attributed to (1) the high electronegativity of fluorine, (2) the three nonbonding electron pairs on fluorine, and (3) the excellent match between the 2s and 2p orbitals of fluorine and the corresponding orbitals of other secondperiod elements [3]. Fluorine can therefore form very strong covalent bonds with carbon and hydrogen. The carbon-fluorine bond is among the strongest of known 80

Physical and Chemical Properties TABLE 3.1

81

Standard Electrode Potentials Eo (V)

Electrode reaction Li+ + e- = Li Na+ + e- = Na 2H+ + 2e- = H2 l2+ 2e- = 21Br2 2e- = 2BrC12 2e- = 2CIO2 + 2H+ + 2e- = 2H20 F2 + 2e- = 2F-

-3.024 -2.71 4 0.000 0.536 1.065 1.358 2.07 2.65

+ +

Source: Ref. 33.

TABLE 3.2 Selected Ionization Energies, Electron Affinities, and Electronegativities Element

Ionization energya

Electron affinityb

ElectronegativityC

F CI Br I

401.5 300 272.9 242.2 313.8 315.0 125.8

83.5 87.3 82.0 75.7

4.0 3.0 2.8 2.5 3.5 2.1 1.o

0 H Li

-

0 0

in kcal/mol forthe first ionization energy of the reaction X + X+ Values in kcal for X + e- + X-. Pauling scale (Ref. 2, p. 88). Source: Ref. 2. Reproduced by permission of Cornell University Press.

a Values

TABLE 3.3 Dissociation Energies of Elements (X2 + 2x1 Element

Dissociation energy (kcal) 37 58 46 225 118

Source: Refs. 1 and 9.

+ e-

(Ref. 2,p. 57).

Chapter 3

82

n

FIG.3.1

Steric effects of fluorocarbons. (From Ref. 5.)

covalent bonds [4]. The bond strength increases further with increasing substitution at a carbon. The heat of formation of the carbon-fluorine bond increases in the order CH3F 448 kJ/mol. CH2F2 459 kJ/mol, CHF3 480 kJ/mol. and CF4 486 kJ/mol. The stability of fluorinated surfactants results from a strong C-F bond and effective shielding of carbon by fluorine atoms. The atomic radius of covalently bonded fluorine is only 0.72 A. Because of their small size, fluorine atoms can shield a perfluorinated carbon atom without steric stress (Fig. 3.1) [5]. Although the unique chemistry of fluorinated organic compounds is not completely understood,the stability of the C-F bond is very important in industrial applications. The tlzennd stclbilif?,of fluorinated surfactants is based on the outstanding stability of the fluorocarbon hydrophobe. Fluorocarbons are usually more stable than the corresponding hydrocarbons [6]. Other functional groups of the surfactant are usually less stable than the fluorocarbon group and may lower the thermal stability of the fluorinated surfactant. Hence, not all fluorinated surfactants are thermally stable. Perfluoroalkanecarboxylic acids and perfluoroalkanesulfonic acids are the most stable fluorinated surfactants. Their salts decompose more readily, however, than the parent free acids. Pe~fluo~oalli~rzeccr~bo~~ylic acids can be heated to 400°C in borosilicate glass without significant decomposition [7]. At higher temperatures (550°C), perfluoroalkanecarboxylic acids decompose, yielding an olefin, HF, and C 0 2 [8] (Table 3.4). Salts of per-uoroalkanecarboqlic acids are less stable and are decarboxylated at a lower temperature than the parent acid, depending on the chemical nature of the cation [ 1,10,1I]: RtCF2CF2COOM + RfCF=CFZ

+ CO2 + MF

The yields of perfluoroolefin and the decomposition temperature depend on the cation of the salt. The sodium salt of normal-chain perfluoroalkanoic acids gives the terminally unsaturated perfluoroolefin in yields ranging from 65010 to

Chemical Physical and

83

Properties

TABLE 3.4 Thermal Stability of Perfluoroalkanoic Acids and Alkanoic Acids Conditions

Reactions

Acid heated Alkanoic acid

RCOOH -+ various decomposition reactions

Perfluoroalkanoic acid RrCH2COOH=Rf Salt heated Alkanoic acid

RCOONa

=

CF2

+ HF + C02

-

3RH + C02 (poor yield)

Perfluoroalkanoic acid CF3(CF2)3COOK CF3(CF2)3COOK

165-200°C

CF3CF2CF=CF2

HOCH,CH,OH 2000~

2CF3CF2CF2COOAg

260-270°C

)

+ CF3CF=CFCF3

CF3(CF2)2CHF2 CF3(CF2)4CF3

Source: Ref. 1.

loo%, depending on the chain length of the parent acid [101. Lines and Sutcliffe (1 1) observed that the thermal stability of perfluorooctanoates decreases with increasing ionic radii of the cations within a group of the periodic table: Li+ > Ba+ = Ca+ > Na+ > Pb'+ > Agf > K+ > Cs+ > NH: (Table 3.5). Barium appeared to be an exception in the correlation between thermal stability and ionic radii. However. Glockner et al. [12,13] found that the thermal stability of perfluoTABLE 3.5 Decomposition of Perfluoroalkanoates

YO Residue of perfluorooctanoates

20% Decomposition temperature ("C) Salt Li Ba Ca Na Pb Ag K cs NH4 a

Perfluorooctanoates 341 320 35 320 298 290 287 244 237 1 67

Heptafluorobutyratesa

Ionic radii

Found

Theoretical

-

0.060

4 20 10 9 15 19 14 30

6 18 9 10 24 20 13 28

0.1

275

-

235

-

0.1

295 200 -

85

1

21

69

0.099 0.095 0.1 26 0.135 0.1 0.148

Data from Ref. 10.

Source: Ref. 11. Reproduced by permission of Elsevier Sequoia.

Chapter 3

84

roalkanecarboxylates and o-H-perfluoroalkanecarboxylates decrease in the order Ca'+ > Li" > Na+ > K+ > NH; (Figs. 3.2 and 3.3). Because this relationship is in accord with the ionization energies of the metals, the authors suggested that the primary step of the thermal decomposition reaction is dissociation of the salt and heterolysis of the carboxylate anion to a carbanion and C02. The w-H-perfluoroalkanecarboxylates are somewhat less stable than the perfluorocarboxylates, but the effect of the terminal hydrogen on thermal decomposition is minimal. Anhydrous yer~uurunllca~zesulfor~ic acids are stable at 400°C in the absence of air but form hydrogen fluoride when moisture is present. Gramstad and Haszeli

400

I 2a4 27

U W

n I-

200 LI

I

c7

c4

I

LI Na K NH6 Ca Ca C9 PERFLUORALKANOATES LI Na Ca

Li Na K NH6Ca

400

t !? v

W

5 300 tU W

n

3I-

200

c11

c9

c7

W-H-PERFLUORALKANOATES

FIG. 3.2 Salts of perfluoroalkanoicacidsanda-H-perfluoroalkanoic acids. Decomposition temperatures corresponding to a 50% mass loss (IO"C/min; N2 atmosphere). (From Ref. 13. Reproduced by permission of Carl Hanser Verlag.)

Physical and Chemical Properties

85

0-

-

40 20

tn I-

3

60-

80 100

I

I

I

150

200

250

300

TEMPERATURE ("C)

I

I

350

400

+

FIG.3.3 Thermal decomposition of salts of perfluorooctanoic acid (1O°C/min; N2 atmosphere). (From Ref. 13. Reproduced by permission of Carl Hanser Verlag.)

dine [14] found that anhydrous perfluorooctanesulfonic acid, heated at 400°C for 3 h, liberates only a trace of hydrogen fluoride. Pyrolysis of anhydrous perfluoropropanesulfonic acid at 500°C gave perfluorohexane, pentafluoropropionyl fluoride, sulfur dioxide, carbonyl fluoride, sulfuryl fluoride, and hydrogen fluoride. The pyrolysis products indicate that (1) the C-S bond is the weakest link of the molecule and (2) the C3F7. radical is formed, which degrades further via the alkoxy radical during pyrolysis. The saZts ofye~3cluorosulfonicacids are more thermally stable than the corresponding perfluoroalkanecarboxylates. Lithium, sodium, potassium, or barium salts of perfluorooctanesulfonic acid can be heated to 350400°C without decomposition [ 141. Fluoride is liberated at temperatures above 420°C. Gramstad and Haszeldine [ 141 found that anhydrous potassium perfluorooctanesulfonate was unaffected when heated i n vacuo at 400°C for 5 h. Fluoride was liberated above 430°C. The thermal stability of tetraethylammonium perfluorooctane sulfonate is remarkable for a quaternary ammonium salt, although the tetramethylammonium salt is less stable than the corresponding alkali metal salt. Tetraethylammonium perfluorosulfonate can be heated to 350°C before decomposition begins (Fig. 3.4) [5,15,16]. Decomposition temperatures corresponding to a 50% weight loss have been reported for perfluoroalkane sulfonates by Glockner and co-workers [13] (Fig. 3.4).

Chapter 3

86

0

100

200 400 300 TEMPERATURE ("C)

FIG. 3.4 Thermogravimetric analysis C ~ F I ~ S O ~ N ( C (From ~ H ~ )Ref. ~ . 5.)

500

(TGA) curves of C8FI7SO3K and

The pyrolysis of perfluoroalkanesulfonates proceeds by a different reaction mechanism than the pyrolysis of perfluoroalkanoates. Like its parent acid, potassium perfluorooctanesulfonate does not produce an olefin when heated but yields perfluoroalkanes of various chain lengths. SO?, K2S04, and soot [13]. The thermal stability of perfluoroalkanesulfonates decreases in the order of their cations: Ca2+ > K+ > Naf > Li+ > NH: (Fig. 3.5) [ 131. Calcium perfluorooctanesulfonate is, like calcium perfluorooctanecarboxylate, more stable than the analogous alkali metal salts. However, the thermal stabilities of alkali metal perfluoroalkanesulfonates decrease with increasing ionic radii in an order opposite thau the stabilities of perfluoroalkanecarboxylates. The excellent clzerniccd stubilizy of perfluorinated alkanoic and alkanesulfonic acids to acids, oxidants. and alkali [5,13,17] allows their application under conditions too severe for hydrocarbon-based surfactants. Gramstad and Haszeldine [14] did not detect fluoride when they heated potassium perfluorooctanesulfonate (0.9 g) in water (5 mL) at 300°C for 8 h. However, when potassium perfluorooctanesulfonate was heated in 10% aqueous potassium hydroxide for 8 h at 27OoC, 8% of the theoretical amount of fluoride was liberated. Perfluorooctane-

Physical and Chemical Properties

87

sulfonic acid, heated in water for 3 h at 400"C, produced 17% of the theoretical amount of hydrogen fluoride. Perfluorooctanesulfonic acid is remarkably stable to oxidants: Heating in concentrated nitric acid for 12 h at 160°C did not liberate hydrogen fluoride. Potassium perfluorooctanesulfonate is also unusually stable to oxidants (nitric acid, hydrogen peroxide) and reducing agents (hydrazine) [18]. Glockner and co-workers [ 131 showed that anionic perfluorinated surfactants are stable even when stored for a long time in 60% HN03 or 98% sulfuric acid containing 10 g/L chromic oxide. After 15 days at ambient temperature, followed by 28 days at 90°C, the surface tension of the solutions did not change significantly (see also Section 4.4). However, nonionic surfactants having the structure C,,F2,,-1 (OCH2CH2),,0CH3 were not stable in chromic-sulfuric acid. The solution turned green immediately. The chemical properties of anionic fluorinated surfactants also differ from those of their hydrocarbon analogs by the increased acid strength of the parent acids. Replacement of hydrogen by fluorine increases the dissociation constant of carboxylic acids (Table 3.6) [ 191. Perfluorinated alkanecarboxylic acids are strong

!

ll lL FIG. 3.5 Decomposition temperaturescorresponding to a 50% mass loss of perfluoroalkane sulfonates. (From Ref. 13. Reproduced by permission of Carl Hanser Verlag.)

Chapter 3

88 TABLE 3.6 Acidity of Fluorinated Acids

Acid

CF3COOH CHFZCOOH CHZFCOOH CH3COOH

Dissociation constant, K

I .8 X 1o

-~

2.2

5.7

X

x

5.5

x IO"

Source: Refs. 19 and 20.

acids, comparable to mineral acids [20]. Equivalent conductance measurements 171have shown that perfluoroalkanecarboxylic acids are completely dissociated in water. The strength of perfluoroalkanoic acids has also been established by potentiometric titration of rz-perfluorodecanoic acid, rz-perfluoroheptanoic acid, and n-perfluoropropionic acid with NaOH. The similarity of the titration curves to the titration curve of nitric acid (Fig. 3.6) [21] indicated complete dissociation of the perfluoroalkanoic acids in water. Perfluoroether surfactants featuring one or two carboxylic acid groups [22] are also strong acids. The pKa values for the monocarboxylic acids are about 1.7; the pKUl-+ pK,2 values for the dicarboxylic acids about 2-2.5.

200

acid

:

IO-*mol dm"

(100 ~ m ' ~ )

100

> €

G

0

- 100 -200t-

0

I 0.5

1

1.5

2

v(NaOH)/cm' FIG.3.6 Potentiometric titration curves of perfluorodecanoic acid (CgFlgC0OH) (0)and nitric acid (HN03) (0).Glass electrode and Ag/AgCI reference electrode at 25°C. (From Ref.21. Reproduced by permission of Steinkopff.)

Physical and Chemical Properties

89

TABLE3.7 Effect of Distance Between Perfluoroalkyl and Carboxyl Groups on Acidity of Fluorinated Acids Fluorinated acid CF3COOH CF3CH2COOH CF3CH2CH2COOH CF3CH2CH2CH2COOH CH3CH2CH2CH2COOH

Dissociation constant, K 5.5 x 1X 7X 3.2 X 1.56 X

10" 10-3 10-5 10-5 1OP5

Source: Refs. 19, 20, 23, and 24. The Macmillan Co.

Henne and Fox [20] observed that the shielding effect of one methylene group between the perfluorinated segment and the carboxylic acid group is considerable. Perfluoroacetic acid and perfluorobutyric acid are strong acids but CF3CH2COOH and CF3CH2CH2COOH are weak acids, although stronger than the corresponding nonfluorinated carboxylic acids. Hence, even two methylene groups cannot completely shield the electronegative induction effect of the CF3 group (Table 3.7). Brace [25] determined dissociation constants of long-chain alkanoic and alkenoic acids featuring a terminal CF3 group in 50% aqueous ethanol, because of limited solubility of the acids in water. Dissociation constants of perfluorooctanoic acid and the corresponding unfluorinated alkanoic acids were also measured (Table 3.8). The data revealed that the electron-withdrawing effect of the TABLE3.8 Apparent Dissociation Constants for Perfluorooctanoic Acid, Perfluoroalkyl Segmented Alkanoic and Alkenoic Acids, and Alkanoic Acids in 50% Aqueous Ethanol Concentration Compound

(W

PKa

K X IOp6

0.005 0.002 0.002 0.002 0.002 0.002 0.004 0.003 0.002, 0.005 0.002, 0.005

2.80 ? 0.03 5.12 k 0.06 5.63 ? 0.03 5.81 t 0.06 5.95 5 0.005 6.26 t 0.08 5.40 5 0.05 5.83 -t- 0.01 6.17 -t- 0.01 6.13 5 0.01

159 0.76 0.22 0.16 0.1 1 0.06 0.38 0.1 5 0.068 0.074

Source: Ref. 25. Reproduced by permission of the American Chemical Society.

Chapter 3

90

perfluoroalkyl group is transmitted through alkylene segments of two to five carbon atoms with decreasing force.

3.2 MELTINGPOINTS The melting point of a compound is defined as the temperature at which the solid and liquid phases are in equilibrium at the pressure of 1 atm. The heat of fusion depends on the lattice energy of the solid. the type of the crystal lattice, and the entropy of fusion. Although the intermolecular attractive forces affect the heat of fusion and the melting point, a general correlation between the crystal energy and the melting point does not exist. Melting can be viewed as a transition from a highly ordered solid state to a liquid state of a low degree of order. Consequently, the entropy increases during melting. The numerical values of the melting points depend therefore not only on the heat of fusion but also on the entropy of fusion [26]. Perfluoroalkanes have higher melting points than the corresponding hydrocarbons, except for the first member, CF4, of the homologous series (Fig. 3.7). However, the melting points of the perfluoroalkanes with less than five carbon atoms increase not continuously but alternately. In both series, the homolog with three carbon atoms has the lowest melting point. In analogy to perfluoroalkanes, the melting points of fluorinated surfactants are higher than those of the corresponding hydrocarbon-type surfactants. The

1

2

3

4

5

6

P

6

9

1

0

Number of Carbon Atoms

FIG.3.7 Melting points of alkanes (a) and perfluoroalkanes (b). (From Ref. 19. Reproduced by permission of The Macmillan Co.)

Physical and ChemicalProperties

91

1201 -

IO0I -

80I -

c

0

/c /z

60

I -

L

E“

40I -

p’

20 0 0

I

1

I/

I

I

I

I

I

4 6 8 10 12 14 16 TOTAL NUMBER OF CARBON ATOMS IN THE n-ALKYL CHAIN

2

3

FIG.3.8 Melting points of alkanoic and fluorinated alkanoic acids. (From Ref. 28. Reproduced by permission of the American Chemical Society.)

melting points of fluorinated surfactants are affected by the stiffness of the fluorinated hydrophobe. Perfluorinated carbon chains of surfactant molecules, particularly the linear chains, are hard and inflexible. Perfluorinated surfactants therefore have a high melting point and a high Krafft point, and their solubility in solvents is low. Surfactants with a polyether segment are more flexible, have lower melting points, and are more soluble [27]. Branching of the hydrophobe lowers the melting point of a fluorinated surfactant. Branched-chain perfluoroalkanecarboxylic acids have lower melting points than the normal-chain perfluoroalkanecarboxylic acids with the same number of carbon atoms (Fig. 3.8). The melting point differences among curves A, B, and C for equal-length fluorinated surfactants are larger than the difference between curves D and E for unfluorinated surfactants. Branching of a fluorinated chain has a larger disrupting effect on the melting point of a surfactant than a unfluorinated alkyl group. Fluoroalkyl groups are larger than unfluorinated alkyl groups and induce more steric hindrance than unfluorinated alkyl groups [28]. Brace [25] synthesized long-chain alkanoic and alkenoic acids with a terminal perfluorinated segment Rf (Table 3.9). The chain length and branching of the terminal Rf have a marked effect on the melting point of the segmented acids (Fig. 3.9). The shape of the curves depends on the interaction of the Rf and alkylene segments. As the hydrocarbon segment in the CF3(CF2)6(CH2),,,COOHseries

Chapter 3

92 TABLE3.9 Melting Points Segmented Alkanoic Acids

Compound

of Terminal Perfluoroalkyl Melting point ("C) 48.5-50 70-7 1 49-50 36-37 73-74.5 41 -42 55 58-59.8 43 77-79 67-69 86-88 73 79-80 64-65 79.5-80 82-83 63 9 1.5-92 89-90 88 111-112

Source: Ref. 25.

I

(curve A) is increased, the melting point increases first and then drops to a minimum. Further extension of the hydrocarbon segment gives a slope approaching roughly that of the curves for unfluorinated alkanoic acids. The transition of the slope corresponds to a maximum interaction of the Rf and alkylene segments. Long-chain carboxylic acids with a branched Rf segment have a lower melting point than the straight-chain analogs. The unusual melting behavior of the acids is caused by (1) the greater stiffness of the (CF,), chain than the (CH2)Iz chain and (2) the larger cross-sectional area, 29 1$ for (CF2),2versus 20 A for (CH2)IZ. Melting points of perfluorooctanoates with inorganic cations are shown in Table 3.10 [ 111. Their melting points do not increase regularly with increasing ionic radii, as may have been expected. The melting points of the salts are probably affected by the thermal stability of the salts, which decreases with increasing ionic radii.

Physical and Chemical Properties

93

130r"-I20

-

(?

100

b

I

110-

I I

-

I

J

I 90

-

4

C 0

.d

a

bL,

c

80-

.d

4

c .

70-

50

401

3

0

8

1

" " IO

1 ~ ' ' 12 14

1

I6

'

1

I8

20

1

1

22

1

1

24

Total Number of Carbon Atoms FIG. 3.9 Melting points of perfluoroalkanoic, segmented perfluoroalkylalkanoic, and alkanoic acids: (A) CF3(CF2)n-2COOH, CF3(CF2)6(CH2),COOH; (B) CF3(CF2)n(CH2)10COOH,n = 0-10; (C) CF3CF(CF3)(CF2)n(CH2)loCOOH, n = 0-6; (D) CF3(CF2)n(CH2),6COOH,n = 0-6. The melting points of alkanoic acid are shown with dashed curves: even-carbon-number alkanoic acids (upper curve), odd-carbon-number alkanoic acids (lower curve). (From Ref. 25. Reproduced by permission of the American Chemical Society.)

Chapter 3

94 TABLE3.10 Melting Points Perfluorooctanoic Acid Salts

I

of

Salt

Melting point ("C)

Li Na K cs NH4 Ca Ba Ag Pb

2 19-224 273-275 222-235 230da 157-1 65 145-1 55d 230-240 220-225 265-270

~~~

a d = decomposition. Source: Ref. 11. Reproduced by permission of Elsevier Sequoia.

Commercial fluorinated surfactants are usually mixtures and the relationship between structure and melting point is less clearly defined. For the tetraethylammonium salt of perfluorooctanesulfonic acid a broad melting range, about 170-1 9OoC,has been reported [15]. The parent acid of the salt was a mixture of isomers, formed by the electrofluorination process. The mixture consisted of the normal-chain isomer (70%) and branched isomers (30%).

3.3 BOILING POINTS The transition of a liquid phase to its vapor phase involves the separation of molecules in the liquid and the removal of molecules from the surface of the liquid into the vapor phase. The energy absorbed when a definite quantity of a liquid is vaporized (the latent heat of vaporization) therefore depends on the intermolecular attractive forces which have to be overcome in order to separate molecules. According to Trouton's rule, the boiling points of nonassociated liquids, on the absolute-temperature scale, are approximately proportional to their latent heats of vaporization. Hence, the boiling point of a liquid depends on the relative strength of cohesive intermolecular forces. The low polarizability of fluorine and the relatively weak attractive forces between fluorine atoms suggest that the boiling points of fluorinated compounds should be lower than those of other halocarbons. Indeed, the boiling points of alkyl halides increase in the order H < F < C1 < Br < I (Fig. 3.10) [ 191. The unusual effect of fluorination on volatility can be illustrated with halomethanes. The boiling points of chlorinated and brominated methanes in-

I

I

L

I

Physical and Chemical Properties

Number

95

of Carbon Atoms

FIG. 3.10 Boilingpoints of alkanes(a),monofluoroalkanes(b),monochloroalkanes (c), monobromoalkanes (d), and monoiodoalkanes (e). (From Ref. 19. Reproduced by permission of The Macmillan Co.)

crease with increasing halogen substitution. The fluorinated methanes exhibit. however, a maximum boiling point for difluoromethane (Fig. 3.1 1) [ 13. Perfluoroalkanes with more than four carbon atoms have lower boiling points than the corresponding hydrocarbons. However, the boiling points of perfluoroalkanes with less than four carbons are higher than those of the corresponding hydrocarbons (Fig. 3.12). Fluorination has a more pronounced effect on the boiling points of carboxylic acids than on the boiling points of hydrocarbons [30]. The boiling points of pefluoroalkanoic acids are about 45°C lower than those of the corresponding alkanoic acids (Fig. 3.13) [19,31]. The boiling points of perfluoroalkanoic acids increase with increasing chain length, like the boiling points of unfluorinated alkanoic acids. Branching of the carbon chain decreases the boiling point of a perfluoroalkanoic acid. A terminal chlorine atom increases the boiling points of branched perfluoroalkanoic acids CF2Cl(CF3)CF(CF2),,COOH, (Table 3.1 1) [28].

Chapter 3

96

FIG.3.1 1 Boiling points of halomethanes. (From Ref. 1.)

TABLE3.11 MeltingPointsandBoilingPoints CF2X(CF,)CF(CF2)nCOOH

of BranchedFluorinated Acids

x = CI

X=F Melting point ("C) (mm)

n

-

1 3 5 7

9 11

-

-64

33-36 61 88-89 100-1 07

Boiling point ("C)

Melting point ("C)

Boiling point ("C)(mm)

141 (760) 130 (150) 115 (20) 125 (IO) 117 (0.5)

-

100 (50) 115 134 (24) 141 (8) 117 (0.3)

-

44-50 71-74.5

(28)

Source: Refs. 28 and 29. Reproduced by permission of the American Chemical Society.

Number qf Carbon Atoms

FIG.3.12 Boiling points of alkanes (a), perfluoroalkanes (b), and the difference between the boiling points (boiling point of the alkane minus that of the perfluoroalkane) (c). (From Ref. 19. Reproduced by permission of The Macmillan Co.)

FIG. 3.13 Boiling points of alkanoic and perfluoroalkanoic acids. (From Ref. 7. Reproduced by permission of the American Chemical Society.)

Chapter 3

98 TABLE3.12 Boiling Points of Perfluoroalkanesulfonic Acids Compound

(760)

Boiling point, ("C)(mm)

178 C2F5S03H 87 (25) (760)1 96 0-21 21 2 (760) 105 (22) 224, (760) 226 97 (4) (760) 238-239 110 (5) (760) 247-249 122 (5)

C3F7S03H C4FgS03H c5F11 S03H

c6F13S03H c7F15S03H (760) 258-260 c8F17S03H

145

(IO)

Source: Ref. 14.

In analogy to the effect of fluorination on the boiling points of perfluoroalkanoic acids, fluorination also lowers the boiling points of alkanesulfonic acids. Unlike alkanesulfonic acids, ye~uo~oalkanesuIfor~ic acids are relatively volatile. The boiling points of perfluoroalkanesulfonic acids, like those of perfluorocarboxylic acids, increase with increasing carbon chain length (Table 3.12) TABLE 3.13 Physical Properties of Monofunctional Perfluoropolyether Surfactants ____

~~~~

~

~~~

~

n

Structure

CF3(0CF2-CF),-OCF2COOH

I CF3 CF3(0CF2-CF),-OCF2-CH20H

I

CF3 CF3(OCF2-CF),-OCF2CH2NH2

I CF3 CF3(0CF2--CF),-OCF2CN

I CF3

Boiling point ("C/760mm Hg)

1 158 2 193 3 220 1 1.6835 126 163 2 3 1.791 197 1 134 2 152 3 188 1 115 2 124 3 1 60

Source: Ref. 22. Reproduced by permission of Academic Press.

~~~

Density (d;')

Refractive index (&O)

1.7237 1.7794 1.8032

1.2989 1.2960 1.2994

1.7515 0

1.2950 1 2972

Physical and Chemical Properties

99

[ 141. Perfluorooctanesulfonic acid boils at 258-260°C at atmospheric pressure. However, the boiling points of perfluoroalkanesulfonic acids cannot be determined accurately at atmospheric pressure, because perfluoroalkanesulfonic acids form an anhydride when heated to the boiling point at atmospheric pressure. Like alkanesulfonic acids, perfluoroalkanesulfonic acids are hygroscopic and form a solid monohydrate. Boiling points of per-uoropolyether su$actarzts (as well as their densities and refractive indexes) [22] increase with increasing length of the fluorocarbon chain and decrease in the order of their functional groups: carboxylic acid > alcohol > amine > nitrile (Table 3.13).

3.4

DENSITY

The density of organic compounds RX increases in the order X = H < F < C1 < I (Fig. 3.14). Substitution of fluorine for hydrogen increases the density of a hydrocarbon. The density increase resulting from substituting one fluorine for hydrogen at the same carbon atom decreases in the order of first > second > third > fourth [ 191. In accord with the densities of fluorinated organic chemicals in general, the densities of fluorinated surfactants are higher than those of their hydrocarbon-type analogs. Perfluoroalkanoic acids have higher densities than the corresponding unfluorinated alkanoic acids (Fig. 3.15) [7,33]. The densities of per-

70

90

170 150 130 110

190

210

MOLECULAR WT

FIG.3.14

Density of halobenzenes C6H5X.(Data from Ref. 32.)

Chapter 3

100

I .7

t

si 1.2 z W 0 I I -

I

> 1.3

IO.9 -

I 2 3 4 5 6 7 8 9 1 0 NUMBER OF CARBON ATOMS FIG.3.15 Liquid densities of alkanoic and perfluoroalkanoic acids. (From Ref. 7. Reproduced by permission of the American Chemical Society.)

fluoroalkanoic acids increase with increasing number of carbon atoms, whereas for the series of nonfluorinated alkanoic acids the opposite is true [7] (Fig. 3.15). Apparently, the difluoromethylene segment -CF2contributes more to the density of the acid than a carboxyl group. In contrast, the incremental effect of the methylene segment -CH2on density is smaller than the effect of a carboxyl group. The density of carboxylic acids with a perfluoroether hydrophobe, derived from hexafluoropropylene, increases with increasing chain length (Table 3.13) [221. 3.5

REFRACTIVE INDEX

Fluoro compounds have a lower refractive index than their hydrocarbon and halocarbon analogs. The refractive indexes of monosubstituted halobenzenes increase in the order F < H < C1 < Br < I (Table 3.14). The low refractive index of fluorobenzene is related to the low polarizability of the fluorine atom. The polarizability, PE,can be calculated from atomic contributions (Table 3.14), of which fluorine has the lowest value.

Physical and Chemical Properties TABLE3.14 C6H5X

101

Refractive Indexes and Surface Tensions of Benzene Derivatives

Atomic refractive constant of Xa

Refractive indexa

X

(do)

Surface tension (7) 20°C (vapor)

F H CI Br I

0.95 1. I O 5.97 8.87 13.90

1.47 1.50 1.52 1.56 1.57

27.3 28.9 33.6 35.8 39.3

a

Data from Ref. 32.

The refractive index of perfluorinated conlpounds is exceedingly low [ 191. Perfluoropentane has probably the lowest refractive index 1.333) ever recorded. Refractive indexes of some fluorinated surfactants are listed in Table 3.13.

(n?

REFERENCES 1. W. A. Sheppard and C. M. Sharts, “Organic Fluorine Chemistry,” W. A. Benjamin, New York (1 969). 2. L. Pading. “The Nature of the Chemical Bond,” 3rd ed.. p. 95. Cornell University Press. Ithaca, NY( I 960). 3. B. E. Smart. in “Molecular Structure and Energetics.” J. F. Liebman and A. Greenberg. eds., Vol. 3, pp. 141-191, VCH Publishers, Deerfield Beach. FL (1986). 4. N.V. Sidgwick, “The Chemical Elements and Their Compounds.’’ Vol. 11, p. 1099, Oxford University Press. Oxford(195 1). 5. H. G. Klein. J. N. Meussdoerffer, and H. Niederpriim. Metalloberflache 29, 559 (1975). 6. T. J. Brice, in “Fluorine Chemistry,” J. H. Simons, ed.. Vol. I. Academic Press. New York (1950). 7. E. A. Kauck and A. R. Diesslin. Ind. Eng. Chem. 43, 2332 (1 95 1). 1). 8. L. Hals, T. S. Reid, and G. H. Smith. J. Am. Chem. SOC. 73.3054 (195 9. T. L. Cottrell. “The Strength of Chemical Bonds,’‘ 2nd ed.. Butterworths Scientific Publications, London (1958). 10. J. D. LaZerte, L. J. Hals, T. S. Reid, and G. H. Smith, J. Am. Chem. SOC.75, 4525 (1953). 11. D. Lines and H. Sutcliffe. J. Fluorine Chem. 25, 505 (1984). 12. V. Glockner. K. Lunkwitz, and D. Prescher, Abh. Akad. Wiss. DDR, Abt. Math. Naturwiss., Tech. 1986, 539 (1987). 13. V. Glockner, K. Lunkwitz. and D. Prescher, Tenside 26.376 ( 1989). 14. T. Gramstad and R. N. Haszeldine.J. Chem. SOC.2640 ( I 957).

102

Chapter 3

15. H. G. Klein. J. N. Meussdoerffer. H. Niederpriim, and M. Wechsberg. Tenside 15,2 (1978). 16. J. N. Meussdoerffer, and H. Niederprum, Chem. Z. 104.45 (1980). 17. J. Burdon, I. Farzmand, M. Stacey. and J. C. Tatlow. J. Chem. SOC.2574 (1957). 18. H. G. Bryce, in “Fluorine Chemistry,” J. H. Simons. ed., Vol. V, Academic Press, New York (1964). 19. M. Hudlicky. “Chemistry of Organic Fluorine Compounds,” Macmillan, New York ( 1962). 20. A. L. Henne an$ C. J. Fox. J. Am. Chem. SOC.73.2323 (1951). 21. X.-J. Fan, M. Colii, N.Kallay,and E. Matijevii, ColloidPolym. Sci. 266. 380 ( 1 988). 22. G. Caporiccio, F. Burzio. G. Carniselli. and V. Biancardi, J. Colloid Interf. Sci. 98, 202 ( I 984). 23. A. L. Henne and C. J. Fox. J. Am. Chem. SOC. 75,5750 (1953). 24. E. T. MacBee. 0. R. Pierce, and D. D. Smith, J. Am. Chem. SOC. 76,3722 (1954). 25. N. 0. Brace, J. Org. Chem. 27,4491 (1 962). 26. A. R. Ubbelohde, Quart. Rev. (Chem. SOC.London) 4.356 (1954). 27. N. Ishikawa and M. Sasabe, J. Fluorine Chem. 25,241 (1984). 28. M. K. Bernett and W. A. Zisman. J. Phys. Chem. 71,2075 ( 1967). 29. M. Hauptschein and M. E. Miville, Canad. Patent 735.493 (1966). 30. A. V. Grosse and G. H. Cady, Ind. Eng. Chem. 39.367( 1 947). 31. J. 0. Hendrichs, Ind. Eng. Chem. 45, 99 (1953). 32. J. A. Dean. ed., “Lange‘s Handbook of Chemistry.” 13th ed., McGraw-Hill, New York (1986). 33. C. H. Arrington. Jr. and G. D. Patterson. J. Phys. Chem. 57. 247 (1953).

Liquid-Vapor and Liquid-Liquid Boundaries. Surface Tension

4.1

THEORYOFSURFACE TENSION

An interface is the region at the boundary between two immiscible phases. Ideally, the interfacial boundary is considered to be the plane dividing the phases. In real systems, the interface is not a plane but a region which has a composition different from that of the bulk phase (Fig. 4.1). The boundary may be solid-solid, solid-liquid, solid-vapor, liquid-liquid, or liquid-vapor immiscible phases. If one of the two phases in contact with each other is a gas or vapor, the term surface is used to denote the boundary. Energetics predict that the free energy of interface formation has to be positive. If the free energy is negative, the interface is unstable or metastable. If the free energy is zero, the two phases are miscible and a boundary cannot exist. Because the free energy of interface formation is positive, work has to be done in order to expand the interface. The work. Wlnln,required to expand the surface of a liquid reversibly and isothermally by a unit area is called the surface tension, y,

where AA denotes the area created and y denotes the surface tension (in ergkm’, dyne/cm, or mN/m). The methods for the determination of surface tension are reviewed in Chapter 9. As a consequence of surface tension, a liquid tends to contract its surface area. In the absence of other forces, a drop assumes a spherical shape. The cause of surface tension is the difference in attractive forces acting on molecules at the surface. From the energetics point of view, molecules at or near 103

Ideal interface(a) and real interface(b).

the surface are in a different condition than the molecules in the bulk of the liquid. The molecules in the bulk phase are surrounded by other molecules. The force field around a molecule is uniform and the resulting net force acting on the molecule is zero. Unlike the molecules in the bulk, molecules at the surface are not surrounded by other molecules, The attractive forces on theinner side of the interface are not balanced because the interaction with the vapor phase is weak. The resulting net attractive force tends to pull the molecules out of the surface into the interior of the liquid. This migration of molecules out of the surface into the interior is in dynamicequilib~umwith the diffusio~of molecules in the opposite direction, from the interior of the liquid to the surface [11. The exchange rate, p, has been calculated from theequation [2]

p =f f ( 2 ~ k ~ ) I f 2 P ~

(2)

where a is a coefficient, po is the equilibrium vapor pressure of the liquid, m the mass of the molecule, T is the absolute temperature, and k is Boltzmann,~ constant. Theaverage residence time of a water molecule at the water surface at25°C has been estimated to be 3 ps or less. When a surface of a liquid is expanded, more molecules are brought from the interior of the liquid to the surface. The quantity of work required to bring more molecules to the surface depends on the strength of the attractive forces between the molecules and, consequently, on the energy of cohesion, Hence, a correlation, albeit with many exceptions, exists between the boiling point and surface tension [3].

Liquid-Liquid Boundaries Liquid-Vapor and

105

A surfactant molecule dissolved in water is surrounded by water molecules. The energy needed to transport the surfactant molecule to the surface depends on the effect the molecule has on water structure [4]. Water is believed to consist of regions of free unbound water and regions of hydrogen-bonded water with a structure similar to ice [5].A surfactant molecule dissolved in water causes a distortion or disorientation of the water structure. Water molecules near the hydrophobic group(s) of the surfactant molecule are restructured into an even more extensively ordered structure [6].The internal torsional vibrations of the hydrophobic chains are restricted by the highly structured water [7].The surfactant molecule orients itself at the surface with the hydrophobic part directed away from water but the hydrophilic part in water. The resulting increase in entropy and decrease in free energy of the system decrease the work needed to transport the surfactant molecule to the surface of water. Because less work is needed to transport a surfactant molecule to the water surface than a water molecule, the surfactant reduces surface tension. Because removal of the hydrophobic segment of the surfactant from water is entropically and energetically favorable, the surfactant molecules accumulate in the surface region of water. The equilibrium between the surfactant molecules in the interface or at the surface is given by the Gibbs equation

where d y is the change in the surface or interfacial tension, I?,, is the surface excess concentration of the t2th component, and is the change inthe chemical potential of the nth component. The surface excess concentration is the difference between the surfactant concentration in the surface region and the surfactant concentration in the bulk phase at equilibrium with the surface region. Surface excess concentration has been defined in general terms as the excess, per unit area of interface, of the amount of the component actually present in the system over the amount present in an ideal system of the same volume in which the bulk concentration remains constant up to the hypothetical dividing plane separating the two phases. The term surjuce excess concentrutionis, unfortunately, not very descriptive, especially when used simply as the surjkce excess. Attempts have been made to replace it, but the term sz4pe1ficialdensity, originally suggested by Gibbs, has not been a better choice. At equilibrium between the interfacial and bulk phase concentrations, d p = - RTdln url

where a, is the activity of the nth component in the bulk phase, R is the gas constant, and Tis the absolute temperature. Hence,

Chapter 4

106

For a dilute solution ( e d 0.01M) containing only one nondissociating component, the activity coefficients of the solute and solvent can be considered to be constant:

where c is the molar concentration of the surfactant (or solute in general). For a nondissociating surfactant in dilute solution, the surface excess concentration can becalculated from the slope of a curve obtained by plotting the surface tension against log c:

For an ionic surfactant completely dissociated in a dilute solution into ions A and B,

+ rBd I n LIB)

(8)

d y = -RT(rA cl In c ~ A

In the absence of any other solute or electrolyte. assuming r A

=r B

and cA

= CB to maintain electroneutrality,

d y = -2RTT d In N = -4.606RT dl11

(9)

For dilute solutions of a surfactant behaving like a strong electrolyte [8], the activity coefficient can be calculated using the Debye-Hiickel relation and Eq. (9) becomes d y = -2RTT( 1 - 0 . 5 8 G ) d In c

(10)

If the activity coefficient of a completely dissociated ionic surfactant in a dilute solution can be assumed to be unity, d y = - 4 . 6 0 6 ~d ~log ~

(11)

The surface excess concentration r i n a dilute solution of a completely dissociated surfactant in the absence of other electrolytes can be calculated from the slope of a surface tension plot against the logarithm of surfactant concentration:

A typical plot of surface tension against the logarithm of concentration is shown in Fig. 4.2. The plot provides valuable practical information as well as theoretical insight. Initially, at low surfactant concentrations, the surface tension decrease is gradual (section A-B). With increasing surfactant concentration, the surfacetensiondecreasesmoresteeply and thecurvebecomeslinear, as the liquid-vapor boundary becomes saturated with the adsorbed surfactant at the point B. The plot is linear (section B-C) until an inflection point C isreached. The inflection point usually corresponds to the critical micelle concentration (cmc)

Liquid-Vapor and Liquid-Liquid Boundaries

107

P YS

Surface Tension

YCMC

I I

I I

I

I

CS

CMC

Log Concentration FIG.4.2 Typical surface tension-log surfactant concentration curve.

(see Chapter 6), although a limit of surfactant solubility, if reached below the cmc, can also cause an inflection. Above the cmc, the surface tension is essentially constant, in the absence of complicating factors. The slope dyld log c is related to the surface excess concentration r [Eq. (12)]. Hence, the constant slope of the linear portion of the curve indicates that the surface excess concentration is constant and the surface is saturated. However, the surface tension continues to decrease with increasing surfactant concentration, in spite of dyld log c being constant. This apparent contradiction has been explained by surfactant concentration and activity changes in the bulk phase [9,lo]. The information needed for practical application of the surfactant is (1) the maximum surface tension decrease achievable with the surfactant, ( 3 )the amount of surfactant needed to lower surface tension, and (3) the time needed to achieve the desired surface tension decrease (see Section 4.3). Rosen [ 1 11 has introduced two terms to describe surfactant performance: efficiency and effectiveness. Efficiency is defined as the bulk liquid-phase concen-

Chapter 4

108

tration of the surfactant at equilibrium required to depress the surface tension of the solvent by 20 dynkm. The definition of efficiency is a compromise because is not achieved for all surfactants above the arbitrary the surface saturation (I?,,?) surface tension value of 20 dynkm. EfSectiveness is defined as the maximum surface tension reduction that can beobtained, regardless of bulk phase concentration of the surfactant. Instead of the term effectiveness the term surface tension at cmc, ycmc,is also being used. Below the Krafft point, the maximum surface tension decrease achievable is limited by the solubility of the surfactant. However, practical surfactant applications are usually above the Krafft point. Above the Krafft point (see Section 6.3), the minimum surface tension, ycmc,is indicated by the inflection point (C), corresponding to the cmc of the surfactant. Consequently, the surfactant concentration needed to achieve the minimum surface tension is the critical micelle concentration. 4.2

ADSORPTIONATLIQUID-VAPORBOUNDARY

Surface tension curves, such as the one shown in Fig. 4.2, yield useful information, in addition to defining the minimum surface tension at cmc. ycmc.The surfactant concentration needed to attain the surface excess concentration, c, corresponds to surface saturation, I?,,,. The slope dyld log c of the linear portion of the curve is related to the surface excess concentration, I?,,,. The excess surface concentration r,,,can be determined from the linear portion of the surface tension-log surfactant concentration curve. Although direct determination of r is experimentally more difficult, several satisfactory methods have been developed [ 12-30]. McBain and co-workers [ 13,141analyzed a slice of the surface removed with a microtome. Radioisotope tracer methods [15-211 are simpler and the agreement with calculated values has been satisfactory. Recently, the structure and composition of the adsorbed surface layer has been determined by neutron reflection [22-301. Neutrons traveling through the air-liquid interface are refracted at locations where the refractive index changes. Neutrons are reflected if the neutrons are traveling from amedium of high refractive index to a medium of low refractive index and the incidence angle is smaller than a critical angle. Because neutron reflection is a function of the composition in the surface region. the arrangement of molecules and the concentration of a surfactant in the monolayer at the surface of a dilute surfactant solution can be measured [27]. The contrast of the reflectivity profile may be varied by isotopic substitution (e.g., by using deuteriated water or surfactant). The neutron reflection technique assumes that (1) homogeneous single layer has been formed, (2) equilibrium has been established, and (3) the surface coverage is sufficiently high to preclude a phase change from a liquid expanded phase to a gaseous phase.

Liquid-Vapor and Liquid-Liquid Boundaries

109

Downer et al. [29,30] used neutron reflection and surface tension measurements to investigate the adsorption of four fluorinated surfactants at the air-water interface. The surfactants used were two single-chain carboxylates, sodium perfluorononanoate (NaPFN) and sodium 9H-perfluorononanoate7 and two doublechain sulfosuccinates, sodium bis( I H , 1H-perfluoropentyl)-2-sulfosuccinate (DCF4) and sodium bis( 1H , lH, 5 H , 5H-octafluoropentyl)-2-sulfosuccinate (DHCF4). The replacement of a terminal fluorine by hydrogen creates a permanent dipole in the hydrophobic chain and consequently, increases the cmc and the limiting surface tension as well. The adsorption of surfactant at the surface and, consequently, I?,,, depends on the surfactant structure. The r,,,values listed by Rosen for nonfluorinated anionic, cationic, nonionic, and amphoteric surfactants [ 111 indicate that the hydrophilic group has a considerable effect. For ionic surfactants, r,,,decreases with increasing cross-sectional area of the hydrated hydrophile. Carboxylates generally have higher T,, values than sulfonates or sulfates [lo]. The values for cationic surfactants decrease with increasing size of the quaternary hydrophile. The rill values for nonionic surfactants decrease with increasing length of the oxyethylene chain. The length of the hydrophobic chain of ionic or nonionic surfactants beyond 10 carbon atoms, however, has very little effect below a chain length of 16 carbon atoms. A decrease in r,,,for achain length exceeding 18 carbon atoms has been explained by coiling of the carbon chain. Branching of the hydrocarbon chain has very little effect. Similarly, a central placement of the hydrophile in the middle of the hydrocarbon chain, instead of a terminal position, has no significant However, two terminal hydrophiles on opposite ends of the hyeffect on drophobe reduce r,,,considerably. The hydrophobe is looped between both of its hydrophiles located in water (Fig. 4.3) [31]. In contrast, r,, values for nonionic

r,,,

r,,,.

I

?42

+N

R’ I ‘R R

y42

+A

R’ I

R

‘R

Air Water

FIG.4.3 Orientation of a surfactant with two hydrophilic groups on the air-water interface. (From Ref. 31. Reproduced by permission of the American Chemical Society.)

Chapter 4

110

surfactants with two oxyethylene chains attached to the same end of the hydrophobe are similar to the r,,,values for surfactants with one oxyethylene chain and the same number of oxyethylene units [32]. Rosen [ 1 I ] has suggested that fluorination of the hydrophobe has apparently only a small effect on Tn1,in contrast to the large effect fluorination has on surface tension. If fluorination does not affect the surface excess concentration significantly the large effect of fluorination on surface tension must be related to the chemical potential of the fluorinated surfactant [Eq. (3)]. The chemical potential depends on the nature of the hydrophobe, as well as the hydrophile. The main factors in reducing surface tension are the decrease in the work needed to transport a surfactant molecule to the surface and the low surface energy of the monomolecular film formed by adsorbed surfactant molecules. Under conditions of surface saturation, the Gibbs equation can be used to determine the maximum extent of surfactant adsorption on the surface. The area occupied by a molecule, A, (expressed in nm2j can be calculated from the surface excess concentration I?,,, of the saturated surface (mol/cm2j:

r,,,.

A,

=

10I4/NT,,,

where N is Avogadro’s number. The area occupied by a surfactant molecule provides valuable information on the orientation of the surfactant molecule at the surface. The adsorbed surfactant can be visualized to form a monomolecular film which exerts surface pressure, T:

where yo is the surface tension of the pure solvent and ys is the surface tension of the solution. Shinoda and co-workers [33-351 varied the counterion concentration and found that usually only about half of the expected amount of the counterion was adsorbed at the surface. Recently, An et al. [27] examined the surface excess of perfluorooctanoate counterions with neutron reflection and surface tension measurements. The authors found that a prefactor less than 2 in the Gibbs equation [Eq. (9)] is an artifact caused by the presence of a divalent cation impurity. Once the impurity, usually calcium. is removed, neutron reflection results are in agreement with surface tension results using a Gibbs prefactor of 2. Downer et al. [29] attempted to remove divalent metal ions by an addition of EDTA in amounts sufficient to chelate the contaminants, but below the concentration at which EDTA affects the surface tension. However, adsorption isotherms derived from surface tension with a Gibbs prefactor of 2 did not agree with those obtained from neutron reflection data. A better agreement was found when using aprefactor of 1.7,consistent with about 30% dissociation of counterions.

Liquid-Liquid Boundaries Liquid-Vapor and

111

The cross-sectional areas of the salts of perfluorooctanesulfonic acid decrease in the order Li == Na > K > NH4 (Table 4. l). The sparse data available do not show a similar trend for perfluorocarboxylic acid salts. The effect of added ammonium chloride and sodium chloride on the area occupied by an adsorbed perfluorooctanoate molecule has been estimated using the Gibbs equation [Eq. (13)] and by neutron reflection [26]. The results indicated that sodium and ammonium ions have a different specific effect on the formation of perfluorooctanoate micelles and on the surface tension above the cmc (see Section

6.6). Adsorption of ammonium perfluorooctanoate (APFO) and ammonium decanoate (AmDec) at the air-liquid interface was studied by Simister et al. [24] using surface tension and neutron reflection measurements. At the cmc, the areas ocTABLE 4.1

Surface Tensions of Fluorinated Surfactants in Water at 25°C Ymm

Surfactant

(mN/m)

27.8 15.2 24.6 20.6 24.6 21.5 20.6 14.8 15.9 21.9 20.5 13.8 16.9 =I 5.5 20.2 19.5 1 9.4a 37.3 29.8 40.5 34.5 27.8 21.5 22.0 In 0.02N KOH. Source: Refs. 33, 37,and 54.

a

rrn

(mol/cm2 X 1 0'')

4.0 3.9

3.8 3.1 3.0 3.1 3.7 4.1 3.9

A (A2)

pC2,

41.5 42.0 2.50 43.0 2.57

48.0 43.5 2.57 47.5 52.5 2.76 55.2 3.20 52.5 3.23 3.56 45.1 41 .O 42.5 3.44 45.2

3.40

Chapter 4

112

cupied by the adsorbed surfactant anion were found to be 41 ? 2.5 A2 for APFO and 35 2 3 A* for AmDec. The structure of the adsorbed layer was examined using the neutron reflection technique with three different isotopic combinations of water and surfactant. The adsorbed layer was found to be thicker than the fully extended surfactant anion. The APFO layer was about 30% larger and the AmDec layer 15 % larger than the expected thickness, suggesting a partial immersion of the surfactant anion in the aqueous phase. Silver cations interact with adsorbed ammonium perfluorooctanoate at the air-water interface [36]. The adsorption of silver and the surfactant is not competitive, however, but cooperative. At a low ammonium perfluorooctanoate bulk concentration (0.0059%).the surfactant surface excess increases with increasing silver concentrations. At medium ammonium perfluorooctanoate bulk concentration (0.03%), silver has no significant effect. At a higher bulk concentration (0.07%), the surfactant surface excess decreases with increasing silver concentration, whereas the surface tension decreases. The silver surface excess increases with increasing surfactant concentration, as well as with increasing silver concentration in water. The effect of silver on the adsorbed layer has been interpreted [36] by a hexagonal arrangement of surfactant molecules at the surface, with a silver ion at the center of each hexagon. Tadros [37] determined the area per molecule for Monflor surfactants from surface tension measurements. The area per molecule in the adsorbed monolayer was calculated from the surface excess concentration, r, determined from the linear portion of the y-log C curve above cmc (Fig. 4.4). Equation (7) was used for nonionic and Eq. (12) for ionic surfactants (Table 4.2). The area covered by the fluorinated surfactant Monflor 71 molecule, 0.26 nm2, is much smaller than the area, 0.47 nm', covered by a similar hydrocarbon surfactant, cetyltrimethylammonium bromide (CTABr), molecule. The small area covered by the adsorbed fluorinated surfactant reveals very tight packing at the interface. The amine(polyfluoroalkoxyacy1)imide surfactants

a.X =F b. X = H C.X = H d. X = H

R =-CH3 R ="CH3 R ="CH'CH20H R = "CHZ"CH"CH20H

I

OH form aqueous solutions with extremely low surface tensions (16.7, 15, 16.2, and

Liquid-Vapor and Liquid-Liquid Boundaries

113

10-6 10-5 10-4 10-3 SURFACTANT CONCENTRATION FIG.4.4 Surface tension curves for Monflor 51 at various temperatures. (From Ref. 37. Reproduced by permission of Academic Press, Inc.)

15.8 mN/m" for a, b, c, and d, respectively) [38]. These low surface tension Values are probably related to the very tight packing of the adsorbed surfactant. The surface areas occupied by the individual surfactant molecules, determined by using the Gibbs equation [Eq. (3)], range from 32 to 39 For surface tension measurements. the surfactant has to be adequately soluble in water. In contrast, surface pressure measurement using the Langmuir film balance requires that the surfactant be water immiscible and spread on the water surface.

w2.

TABLE4.2 Area Covered by a Molecule in Adsorbed Monolayers of Monflor Surfactants

Trade name ~~~~~~

~

Chemical formula

Area/molecule at 25°C (nm2)

CloF190(CH2CH20),CloFlg, average n = 23 CloFl 90C6H4S03Na CloFlgOC6H4N(CH3);CH3SOT

0.53 0.36 0.26

~

Monflor 51 Monflor 31 Monflor 71

Source: Ref. 37

Chapter 4

114

Area per molecule, A' FIG.4.5 Force-area curve for H(CF2),&H20H. (From Ref. 39. Reproduced by permission of the American Chemical Society.)

Arrington and Patterson [39] spread the fluoroalcohol H(CFz),&HzOH on water and determined the area occupied per molecule from the force-area curve of the spread monolayers (Fig. 4.5). By extrapolating the upper part of the curve to zero pressure. a close-packed area of 29 A' was obtained. The fluorocarboxylic acid H(CFz)lzCOOHwas spread on water and on 0.01 N hydrochloric acid (Fig. 4.6). The force-area curve obtained for hydrochloric acid agrees with the curve shown for the fluoroalcohol in Fig. 4.5. When the fluorocarboxylic acid film on hydrochloric acid was recompressed, the curve was duplicated. The compression of the fluorocarboxylic acid film on water gave a smaller area per molecule than obtained on acid. A second compression gave even a smaller area (1 9 A'). The instability of the fluorocarboxylic acid film on water may have been caused by the significant, albeit very low, solubility of the acid in water. The fact that the fluorocarbon chain has a larger cross-sectional area than the carboxyl group may also have contributed to film instability. Caporiccio et al. [40] prepared two series of surfactants: Series A:

CF~(O"CF~"CF),-O"CF~COOH

(17 =

1,2,3)

Series B:

HOOCCF~-(O-CF~CF~),,-(OCF~),,l-OCF~COOH(11 = 1,2,3; 171

1,2,3) The area occupied by a surfactant molecule was calculated using Eq. (10)

Liquid-Vapor and Liquid-Liquid Boundaries

115

I FIRST COMPRLSSION

SECOND COYPRESSION

a IO

20

1

40

30

FIG.4.6 Force-area curves for CI2F2&OOH at (a) the water-air interface and (b) at a 0.01N hydrochloric acid-air interface. (From Ref. 39. Reproduced by permission of the American Chemical Society.)

from linear plots of surface tension versus log surfactant concentration (Figs. 4.7 and 4.8). The area covered by a molecule in a monolayer was also determined from surface pressure-area curves (Figs. 4.9 and 4.10) obtained with the Langmuir film balance. Silver and zinc salts of Series A and silver salts of Series B were used for the measurement. Two sets of cross-sectional areas were calculated.

Y CF3(0C3F6)nOCF2COONH4 A1 CF3i0C3F6i0,0CF2COOH A1

.

H

NH4

N H 4 Ao2 n = l

A2H

0

n=2

A3H

0

n=3

A3-NH4

L

n-1

T

n=2

b

n=3

dynes cm

,70

\

110

10.6

1o

.~

1o

.~

IO

10 2

10

'

mol/\

FIG. 4.7 Surface tension curves for monocarboxylic perfluoroether surfactants of the general formula CF3(0C3F6),0CF2COOH orits ammonium salt. (From Ref. 40. Reproduced by permission of Academic Press, Inc.)

Chapter 4

116

-"

o

B1 p=' B2-H p=2

0

63-H p=3

HOOCCF2[COC,F4)!OCF2)] pOCF2COOH

Y lyr1cs I -

ern

70

60

z

0 50 W

I" W

40

y n: 3 v)

30

20

10 3

IO 4

10 2

10

t 10 10

rnol/l

FIG.4.8 Surface tension curves for dicarboxylic perfluoroether surfactants of the B1-H, p = 1; B2-H, p = general formula HOOCCF2[(0C2F4)(0CF2)]pOCF2COOH; 2; BH-3, p = 3. (From Ref. 40. Reproduced by permission of Academic Press, Inc.)

One set represents molecules close together in a complete monolayer without strain (uncompressed monolayer). The other set is related to the limiting area per molecule below which the molecules in the monolayer begin to overlap (compressed monolayer). The values of the first set were obtained from surface pressure-area curves as the area per molecule below which the surface pressure begins to increase rapidly. The limiting area values of the second set were calculated from the two-dimensional van der Waals equation of state [41): ( F ) ( A

-

Ao) = kT

where A is the area per molecule at a surface pressure T,A0 is the limiting area per molecule, n is a constant, k is the Boltzmann constant, and T is the absolute temperature. The cross-sectional areas are listed in Table 4.3. together with areas estimated from Stuart molecular models. The values calculated from surface excess concentration and values for uncompressed areas are in fairly good agreement. The areas of Series A surfactants for the compressed monomolecular film agree

Liquid-Vapor and Liquid-Liquid Boundaries

50

MOLECULE

100

117

150

FIG.4.9 Surface pressure-area per molecule curves for the silver and zinc salts of the monocarboxylic acid surfactants A2-Ag, n = 2; A2-Zn, n = 2; A3-Ag, n = 3; A3-Zn, n = 3. (From Ref. 40. Reproduced by permission of Academic Press, Inc.)

reasonably well with the areas estimated using Stuart molecular models. However, the compressed monolayer areas of Series B surfactants are twice as large as the Stuart areas. This suggests a folded structure of the polyether chain on the surface with both carboxylic acid groups oriented toward water 1401. Matos et al. 1421 synthesized nonionic fluorinated surfactants which contained both oxyethylene and thioethylene groups:

CF3(CF?),,,C7_H~(SC?H~)(OC2H~),,OH

(Series I)

CF_1(CF2),,C’H4(SC2H~~(OC?-H~)p(SC1H4)(OC2H~),IOH (Series 11) The surface excess concentration and the area covered by the surfactant molecule adsorbed in the water-vapor boundary were calculated from the slopes of the surface tension curves. The areas occupied by the adsorbed Series I surfactant molecules with a short hydrophile (12 = 2 or 3) were small 135 and 37

Chapter 4

118

30

20

10

0

I

100

1

I

200

300

FIG.4.10 Surface pressure-area per molecule curves for the silver salt of the dicarboxylic acid surfactant B-3-Ag, p = 3. (From Ref. 40. Reproduced by permission of Academic Press, Inc.)

A2), indicating a dense packing and perpendicular orientation of the fluorocarbon chains. For the less hydrophilic surfactants in Series 11, the areas were only a little larger, also revealing dense packing of the fluorocarbon chains (Fig. 4.1 1). The areas occupied by the more hydrophilic surfactants of Series I or I1 increased to values comparable to A = 47 A2 in the lamellar mesophases of C~FI~CH~(OCZHJ)JOH. Selve et al. [32] synthesized fluorinated nonionic surfactants with a twochain polyoxyethylene hydrophilic head linked to the hydrophobe via an amide bond. F(CF2)1(CH2),71C(0)N[(C2H~O),,CH3]2. They calculated the area A per surfactant molecule adsorbed on the air-water interface from the slope of the surface tension curve using the Gibbs equation [Eq. (13)]. The area A increases with increasing number of oxyethylene units for both fluorinated and hydrocarbon sur-

TABLE4.3 Limiting Area per Molecule Salts and for a Difunctional Silver Salt

Code" AI A2 A3 B1 B2 B3

81

Area calculated from dy/dT

(A2)

for Monofunctional Silver and Zinc

Area determined with the Langmuir film balance Uncompressed

Stuart model area

Compressed

63

42 76b-1 01'

95 136 136 128

112c

51b-53c

42 28 28 28

56'

Surfactant structure code:see Figs. 4.7 and 4.8. Zn salt. Ag salt. Source: Ref. 40.

a

50

40

30

" (0) P+9 2

4

(e)

6

NUMBER OF OXYETHYLENE UNITS FIG.4.1 1 Area A occupied by the adsorbed surfactant plotted against the number of oxyethylene units n (surfactant Series I) and p + g (surfactant Series 11). (From Ref. 42. Reproduced by permission of Academic Press, Inc.)

119

120

Chapter 4

factants. For nonionic surfactants with six oxyethylene units, the area A occupied by linear fluorinated surfactants is smaller than those for the corresponding hydrocarbon-based nonionic surfactants. For nonionic surfactants with a larger number of oxyethylene units, the A values are similar for fluorinated or nonfluorinated surfactants. The adsorbed layers at the air-liquid interface have been studied mainly by surface tension measurements, and the surface excess as well as the surface area covered by the adsorbed surfactant ion or molecule have been determined indirectly. Recent methods of surface analysis have permitted the determination of the conformation of the adsorbed layer, the orientation and alignment of molecules at the air-liquid interface, the composition and structure of the adsorbed layer, and intermolecular forces in the adsorbed layer directly. Monolayers of adsorbed molecules at the air-water interface display phase transitions. Monolayers of octadecylamine salts of perfluoroalkanoates on 4.4M NaCl solution exhibit two or three types of phase transitions in their T-A (surface pressure-area) isotherms [43]. The phases were classified and phase diagrams drawn from the phase-transition pressures and surface potentials. A x-ray diffraction study by Barton et a1.[44] of a C10F31 CHICOOH monolayer spread on water revealed a coexistence region between ordered condensed islands and a dilute disordered phase. The molecular tilt, 2" ? 3". of the fluorinated surfactant differs from that of hydrocarbon monolayers in whichthe ordered phase is compressible with a continuously variable tilt ranging from 30" to 0" at closest packing. The difference was attributed to the stiffness of the fluorinated chain. The large tilt angle of C7F1s(CH3)4COOHlends qualitative support to this interpretation. Qualitative aspects of the molecular dynamics simulations by Shin et al. [45] are consistent with the conclusions by Barton et al. [44] for CIOF21CH1COOH monolayers. A discrepancy between experimental and theoretical values of the tilt angles was attributed to the inadequacy of the atom-atom potentials used in the simulations. The collective tilt angle predicted was found to be a sensitive function of the area per molecule.

Mixtures of Surfactants In ideal systems. the surface area covered by mixtures of surfactants adsorbed at the liquid-vapor boundary is the sum of the areas covered by the components:

where XI and X 2 are the mole fractions and A and A? are the molecular areas of pure components 1 and 2, respectively. When the mixed surfactants have similar molecular structures, the surface pressure-area isotherms obey the additivity rule.

Liquid-Vapor and Liquid-Liquid Boundaries

121

Thus, the FA curves of two cationic fluorinated surfactants,

C3F70CF(CF3)CF30CF(CF3)CONH(CH2)3N+(C3H~)3CH~I-(FCN3) and C3F70CF(CF3)CF?OCF(CF3) X

CF?OCF(CF3)CONH(CH2)3N+(C2H5)2CH31-(FCN4)

coincide with curves calculated by Zhang and co-workers using the additivity rule (Fig. 4.12) [46]. Surprisingly, the T-A curves of mixtures of perfluorononanoic acid and sodium octadecanesulfonate also obey the additivity rule. The experimental curves coincide with the calculated curves (Fig. 4.13). although the interaction between the hydrocarbon and fluorocarbon chains is mutually phobic. Mixtures of perfluorooctanoic acid and sodium octadecylsulfate behave similarly. Hence, the additivity rule is obeyed by two entirely different systems (1 2,46): (1) the mixed surfactants are so similar that they can form a two-dimensional ideal solution or (2) the surfactants are so different that they are incompatible and form separate monolayers. Similar results were obtained by Zhang et al. [46] using benzene-acetone-ethanol as the spreading solvent to form a monolayer. A mixed monolayer can be considered an ideal monolayer when the surface pressure n is near zero. Zhang et al. [46] suggested that at near-zero surface pressure, mixtures of mutually phobic surfactants indeed form ideal monolayers. As

40

60

80

100

120

140

160

ACA2/decul#3) FIG.4.12 Surface pressure-area per molecule isotherms for FCN3-FCN4 mixtures (20°C). The mole fractions of FCN3 are (1) 1.00, (2) 0.506, and (3) 0.00. (From Ref. 46. Reproduced by permission of Academic Press, Inc.)

Chapter 4

122

20

40

60

80

100

A(i2/moleaJe)

FIG. 4.13 Surface pressure-area per molecule isotherms for CgFHC18H37S03Namixtures (20°C). The mole fractions of CgFH (perfluorononanoic acid) are (1) 1.00, (2) 0.832, (3) 0.665, (4) 0.512, (5) 0.335,(6) 0.167, and (7)0.00. (From Ref. 46. Reproduced by permission of Academic Press, Inc.)

the film is compressed, the distance between molecules in the monolayer decreases and the mutual phobic interaction increases. Consequently, the mixture exhibits a positive deviation from the additivity rule. When the film is compressed further, the phobic interaction becomes so strong that the surfactants become incompatible and form separate monolayers. However, when the phobic interaction is very strong, the phase separation may occur at a low pressure where the deviations are experimentally insignificant and escape observation. Mixtures of CF3C(C2Fs)2C(CF3)=C(CF3)OC6H4S03Na (6201) and sodium octadecylsulfate deviated positively from the additivity rule at low pressures, but at higher pressures. the n-A curves coincided with the calculated curves (Fig. 4.14). The apparent ideality at higher pressures is probably caused by the separation of the phobic components, in accordance with the postulated mechanism. When the phobic interactions are reduced by a hydrocarbon segment in the fluorocarbon chain or branching of the hydrophobe chain, a positive deviation from the additivity rule may also be evidenced at higher pressures. As an example, the interaction between the FCN4 surfactant and octadecyltrimethylammonium bromide, both cationic surfactants, is not strong enough to form separate monolayers and the n-A curves deviate positively from the calculated curves (Fig. 4.15). When the hydrocarbon character of the hydrophobe is increased further, the deviation from ideal additivity decreases and may even become negative.

Liquid-Vapor and Liquid-Liquid Boundaries

123

50

40 n

r

I

E 30

z

E

v

t=

2o 10

0

FIG.4.14 Surface pressure-area per molecule isotherms for 6201-C16H33S04Na mixtures (10°C). The mole fractions of 6201 are (1) 1.OO, (2) 0.833, (3) 0.501, and (4) 0.00. (From Ref. 46. Reproduced by permission of Academic Press, Inc.)

k

FIG. 4.15 Surface pressure-area permoleculeisotherms for FCN4-C18H37 N(CH&Br mixtures (13°C). The mole fractions of FCN4 are (1) 1.OO, (2) 0.665, (3) 0.495, and (4) 0.00. (From Ref. 46. Reproduced by permission of Academic Press, Inc.)

Chapter 4

124

The competitive adsorption of perfluorocarbon and hydrocarbon surfactants at the air-water interface was studied below and above the cmc [47]. The solutions of mixed surfactants, lithium perfluorooctanesulfonate (LiF0S)-lithium tetradecy1 sulfate (LiTS) and ammonium perfluorononanoate (LiPFN)-ammonium dodecyl sulfate (ADS), exhibited two break points. corresponding to cmcl and cmc?, in their surface tension-concentration curves. The surface excess of hydrocarbon surfactants was measured by the radioisotope tracer technique using 3H labeling. LiTS was adsorbed preferentially below cmcl but replaced by LiFOS with increasing total concentration. Above cmc2, the surface was covered mainly by LiFOS. In contrast, the fluorinated surfactant LiPFN was adsorbed in larger amounts than the hydrocarbon surfactant ADS and exhibited a maximum adsorption value below cmc

4.3

SURFACE TENSION IN WATER. SURFACTANT STRUCTURE

The surface tensions of Zonyl fluorinated surfactants [48] are shown i n Table 4.4. When selecting a surfactant to lower the surface tension of an aqueous solution it is useful to know the following: 1. The concentration needed to lower surface tension to a given value 2. The maximum surface tension lowering, or the minimum surface tension achievable at any surfactant concentration

TABLE4.4

Surface Tension of Zonyl Fluorinated Surfactants, Rf = F(CF2CF2)3-8 Surface tension in water at 25°C (mN/m)

Zonyl

Structure

0.001%

0.01%

0.1%

FSA FSP FSE UR

48 42 45 46

22 24 27 40

18 21 20 28

FSN FSO FSC FSK TBS

29 22 58 40 58

24 19 21 21 38

23 18 19 19 24

Note: All surfactant concentrations are given in percent solids. Zonyl UR was neutralized with sodium hydroxide to pH 7-8.

Liquid-Vapor and Liquid-Liquid Boundaries

125

The minimum surface tension achievable with a surfactant in solution at equilibrium depends on the following:

1. The number of surfactant molecules accumulated per unit surface area, indicated by the surface excess concentration r,,,.The number of molecules adsorbed at the surface is inversely proportional to the area occupied by each surfactant molecule [Eq. (1 3)]. 2. The surface energy of the monomolecular film of adsorbed surfactant molecules 3. The free energy of adsorption of a surfactant molecule from the bulk solution phase to the surface. As already discussed in Section 4.1, the surface tension of a surfactant has its minimum value at cmc, yCmC. Variations of surface tension values above cmc are usually small. Hence, cmc represents fairly accurately the surfactant concentration needed to achieve the minimum surface tension. The effect of surfactant structure on cmc is discussed in Section 6.6. The complex relationship between surface tension and cmc depends on the hydrophobe and the hydrophile, including the counterion, of the surfactant. An increase in the chain length of the hydrophobe decreases cmc; branching of the carbon chainincreasescmc.Fluorination of thehydrophobelowerscmc considerably. In addition to the chemical structure of the surfactant, cmc depends on external factors, including electrolyte effects, temperature, and other dissolved or solubilized organic components. The concentration needed to achieve a given surface tension below cmc depends qualitatively but not solely on the same structural factors as cmc. The concentration needed to lower surface tension decreases with increasing hydrophobicity of the surfactant. The efficiency of a fluorinated surfactant in lowering surface tension is determined mainly by the structure of the hydrophobic group. Hydrophilic groups have a smaller effect.

The Hydrophobe Fluorination of the hydrophobe decreases markedly the surfactant concentration needed for asubstantial surface tension reduction. At 1 g/L concentration, sodium octanesulfonate, CsHI7SO3Na,lowers the surface tension of water at 20°C to 65 nM/m, but sodium perfluorooctanoate, C8H17S03Na,at the same concentration lowers surface tension to 32 mN/m [49]. Conventional surfactants with a hydrocarbon-type hydrophobe can lower surface tension to 30-35 mN/m at 0.1% concentration. With fluorinated surfactants, surface tensions below 30 mN/m have been achieved at concentrations as low as 10-1 00 ppm. With some fluorinated surfactants, only 100-200 ppm of the surfactant is needed to lower surface tension below 20 mN/m [48,50].

Chapter 4

126

The minimum surface tension achievable is also much lower for fluorinated surfactants than for nonfluorinated surfactants. The minima of the surface tensions of surfactants with a hydrocarbon hydrophobe are in the range 25-35 mN/m [51-531, whereas those of fluorinated surfactants are as low as 15-20 nM/m or even lower [54]. The surface tension of aqueous solutions above cmc varies only slightly with surfactant concentration. Surface tension above cmc decreases with increasing fluorocarbon chain length and depends on the counterion (Table 4.1). Increasing the carbon chain length increases the efficiency of the surfactant in surface tension reduction [50,51,55] (Fig. 4.16). Surface tension of perfluoroalkanoic acid solutions plotted against their log molar concentration is shown in Fig. 4.17 [56].The surface tension of perfluoroalkanoic acids obeys Traube's rule: log c,, = c - 171 log K T

(17)

where C,,, is the concentration of a homolog surfactant needed for agiven surface tension, 112 is the number of carbon atoms in the hydrophobe C,,IF2,,l+ I , KT is the Traube constant, and C is a constant. The logarithm of the concentration needed to achieve a given surface tension value plotted against the number of carbon atoms in the fluorocarbon hydrophobe yields a straight line (Fig. 4.18).

W

I

O

U

0.001

FIG.4.16 Surface tensions of perfluoroalkanoic acids in aqueous solutions versus log(weight percent). (From Ref. 51. Reproduced by permission of the American Chemical Society.)

Liquid-Vapor and Liquid-Liquid Boundaries

127

70

10 10-4

10-3 10-2 10" SURFACTANT CONCENTRATION (mol/L)

0

FIG.4.17 Surface tensions of perfluoroalkanoic acids in aqueous solutions versus log(molar concentration). (From Ref. 56. Reproduced by permission of Carl Hanser Verlag.)

Traube's rule states that each CH2 group added to the hydrocarbon chain reduces the concentration required to give a certain surface tension value by approximately a factor of 3. Accordingly, the addition of two CH2 groups lowers the required concentration by a factor of 9. Figure 4.18 shows that two CF2 groups decrease the required concentration by a factor of about 10, in good agreement with Traube's rule. Although the surface tensions of members of a homologous series exhibit certain regularities, Traube's rule seems to hold because of a fortuitous mutual compensation of various factors [ 121. The effect of the chain length of some perfluoropolyether surfactants [40] is shown in Figs. 4.7 and 4.8. Branching of the fluorocarbon chain decreases the efficiency of a fluorinated surfactant in surface tension reduction [57,58]. In analogy, a condensed (spread) monolayer of a perfluorinated n-alkanoic acid has a lower critical surface tension than its terminally branched isomer. Bernett and Zisman [59] attributed the effect of branching to different molecular packing and carbon chain adlineation. The Hydrophile The effect of the hydrophilic group on surface tension depends on the structure of the hydrophile and, for ionic surfactants, also on the counterion. For a constant chain length of C7F15 , the surface tension of 0.1 % solutions of fluorinated surfactants varies between 17 and 47 mN/m, depending on the nature of the hydrophile [56] (Table 4.5). Nonionic fluorinated surfactants usually have lower surface tensionsthantheirioniccounterparts.Nonionicsurfactantsderivedfrom

Chapter 4

128

10"

30"

-

IO-'

40"

1o-z

10-:

1

3

5

7

9

NUMBER OF C ATOMS IN HYDROPHOBE FIG. 4.18 Traube plot for fluorinated surfactants C,F,+lCOOH. The log of the concentration required for attaining equal surface tensions is plotted against the number of carbon atoms n. (From Ref. 56. Reproduced by permission of Carl Hanser Verlag.)

TABLE4.5 Surface Tensions of Various Fluorinated Surfactants with a C7FI5 Perfluoroalkyl Group Surfactant

Surface tension for 0.1% aqueous solutions (mN/m) 47 40 28 25 17

Source: Ref. 56. (Reproduced by permission of Carl Hanser Verlag.)

Liquid-Vapor and Liquid-Liquid Boundaries

z G

CT)

60

5

20

c/)

129

I 0-3 10-* 10-4 SURFACTANT CONCENTRATION(mol/L)

FIG.4.19 Surface tension of ammonium o-H-perfluorononyl sulfate (- - - -) and ammonium o-H-perfluorononyl sulfate with an oxyethylene segment (-). (From Ref. 60. Reproduced by permission of Dr. A. Huttig Verlag.)

N-methylperfluorosulfonamide lower surface tension to 20 mN/m, whereas the surface tension of perfluorosulfonic acid salt solutions averages 22.8 mN/m [49]. However, the carbamide linkage is chemically not resistant to hydrolysis. which limits the use of these surfactants under severe conditions. The surface tension of an anionic fluorinated surfactant can be lowered by inserting an oxyethylene group between the fluorinated hydrophobe and the sulfate hydrophile. Greiner and co-workers [60] prepared surfactants having the structure H[CF2CF2],,CH2[OCH2CH2],,0S03NH+ The oxyethylene group has lowered the surface tension by about 5 dyn/cm (Fig. 4.19). According to Klein and co-workers [61], the minimum surface tension of perfluorooctanesulfonic acid salts depends very little on the nature of the counterion. The minimum surface tension depends mainly on the hydrophobe. However, the concentration needed to achieve the minimum surface tension, presumably ycmc,varies considerably (Table 4.6). In contrast, Shinoda and co-workers [33] reTABLE 4.6 Surface Tension of Perfluorooctanesulfonates

Y: Surfactant c8F17S03K c8F17S03NH4 1.2 1.2 C8F17S03N(CH3)4 0.25 C8F17S03N(C2H5)4 a

1 (mN/m) at(g/L) 23.0 22.4 23.1 22.6

cs 4.0

tb No. (min)

g/L 600 1050 200 100

ys = minimum surface tension at concentration C.,

t = time required to attain ys Solid phase present; limited solubility. Source: Ref. 61. (Reproducedby permission of Carl Hanser Verlag.)

of phases at 10 g/L 2c 2c 2 1

Chapter 4

130

ported large differences between the surface tension minima for n-perfluorooctanesulfonic acid salts (Table 4.1). Thediscrepancy between the results in Tables 4.1 and 4.6 may have been caused by different purities of the surfactants used.

Partially Fluorinated Surfactants Partially fluorinated surfactants are not as surface active as perfluorinated surfactants [5 11. Ammonium a-[perfluorooctyl]-w-sulfato(oxyethylene),CF2(CF2)6CH2[0CH~CH2),,0S03NH~. lowers the surface tension of water to 19.5 dyn/cm, but its w-hydrogen analog to only 25 mN/m [60]. The effect of a hydrocarbon segment on the surface activity of a partially fluorinated surfactant depends on the position of the hydrocarbon segment in the molecule. Terminal hydrogen or hydrocarbonsegmentsdecreasesurfaceactivityinwatermorethan an internal hydrocarbon segment. Very low surface tensions have been achieved with fluorinated surfactants featuring internal hydrocarbon segments. Brace [62] prepared perfluoroalkyl-segmented alkanoic and alkenoic acids and determined their surface tensions as a function of their concentration in water (Fig. 4.20). An increase

CONCENTRATION WT% IN WATER

FIG.4.20 Surface tensions of aqueous solutions of perfluoroalkyl segmented (0),CF3(CF2)6CH=CH(CH2)8 alkanoic acids CF3(CF2)6CF(CF3)(CH2)lo~~~Na COONa (H),tetradecanoic acid (-), perfluorooctanoic acid (O), and its ammonium salt (a).(From Ref. 62. Reproduced by permission of the American Chemical Society.)

Liquid-Vapor and Liquid-Liquid Boundaries

131

TABLE4.7 Surface Tension of Aqueous Solution of FluorinatedSurfactants C7F15CON(R)CH2CH2CH2 CH2S03Na at 30°C Surface tension (mN/m) at each concentration (g/lOOmL)

0.5 0.4 0.3 0.2 R 0.10.05 0.02 0.01 H 59.6 54.4 CH3 C2H5 21.7 22.2 22.7 23.5 29.6 38.2 44.9 54.8 53.7 C3H7 51.2 C4H9 28.0 C6H13 22.0 C8H17 28.2 C10H21 53.2 C12H25

54.0 50.0

32.8 41.0

29.8 31.4

23.2 22.4

16.4 23.8

-

-

23.2

22.8

47.0 44.2 21.5 20.3 26.8 48.2

36.8 32.0 18.4 19.6 24.6 33.6

29.2 23.8 18.0 19.6 24.6 33.6

21.8 21.0 17.7 19.4 23.0 26.6

20.9 20.0 17.5 19.1 21.8 25.6

20.0 19.6 17.4 18.6 20.6 23.8

19.1 19.1 17.3 18.6 19.6 22.2

Source: Ref. 63.

in the length of the Rf segment in R+(CH2),,,COONato Rf = CF3(CF& decreased the minimum surface tension obtained from 27 to 14.8 mN/m. The longer the hydrocarbon chain (higher value of m), the lower the concentration needed to lower surface tension to a given value. Kimura and co-workers [63] prepared sulfopropylated N-alkylperfluorooctanamides, C7FIsCON(R)CH2CH2CH2S03Na, where R is H or an alkyl group. The surface activity of the compounds depend on the chain length of the alkyl group. A short-chain (<6 carbons) or a long-chain (12 carbons) alkyl substituent reduces the efficiency of the surfactant, although the effect on effectiveness is small. Alkyl groups of a length in the range of 6-10 carbon atoms give the highest surface activity (Table 4.7). Tshikawa and Sasabe [64] prepared fluorinated surfactants of the structure (HFPO),,ArS03Na, where (HFPO),, is an oligo(hexafluoropropene oxide) group and Ar an aryl group. The effect of 12 on surface tension in water is shown in Figs. 4.21 and 4.22. In both series featuring an (HFP0)4 group, surfactants derived from benzenesulfonic acid or toluenesulfonic acid exhibited the highest surface activity, lowering the surface tension of water down to 16 dyn/cm. This observation suggests that oxygen atoms in the perfluorinated oligoether chains are not hydrophilic. Nonionic fluorinated surfactants which contain both oxyethylene and thioethylenegroups, CF3(CF2),,,C2H~(SC2H4)(0C2H4),,OH (Series I) and CF3(CF2),,C~H4(SC2H~)(0C2H&(SC2H~)(OC~H~)yOH (Series 11) [42], have an exceptionally high surface activity (18 mN/m). Selve et al. [32] prepared fluorinated nonionic surfactants with a two-chain polyoxyethylene hydrophilic head linked to the hydrophobe via an amide bond,

Chapter 4

132

70 60 50

30

20

10

I

0

1o

- ~

1o

I

-~

I

1o

- ~

mol / 1 FIG.4.21 Surface tensionversus log concentration for (HFPO),C6H4S03Na aqueous solutions (20°C). (From Ref. 64. Reproduced by permission of Elsevier Sequoia.)

F(CF2)1(CH3),,lC(0)[N(C2H40),,CH3]2. The surface tension, ycmc,increased with increasing number of oxyethylene groups, regardless of the hydrophobe structure (1 6 and 24; 18 and 24; 27 and 30 mN/m for structures 1 = 6, rn = 1, rz = 2 + 2 and4+4;1=8,m= l,rz=2+2and4+4;Z=O,rn= 117n=2+2and4 + 4, respectively). Klevens and Raison [65] studied micellization of sodium dodecyl sulfate and perfluorooctanoic acid. Their data have been reviewed by Mukerjee and Mysels [66].who concluded that partially fluorinated surfactants behave like mixtures of fluorinated and hydrocarbon-based surfactants. The surface tension data for the mixtures containing sodium dodecyl sulfate and 1.0, 0.75, and 0.20 mole fractions of perfluorooctanoic acid are plotted as a function of perfluorooctanoic acid concentration in Fig. 4.23. All three systems attain the same low and constant surface tension value after a sharp change in the slope corresponding to the formation of perfluorooctanoic acid micelles. The cmc values for the 1.O and 0.75 molar systems are indistinguishable, indicating that the solubility of sodium dodecyl sulfate is insignificant in those micelles. The curve

Liquid-Vapor and Liquid-Liquid Boundaries

133

70

60 50

5

40

\

c

2,

U

30

\ \

\

\

20 10 I

0

1o

I

- ~

1o

1

-~

10-3

mol /1 FIG.4.22 Surface tension versus log concentration for (HFPO),C6H3(CH3)S03Na aqueous solutions (20°C). (From Ref. 64. Reproduced by permission of Elsevier Sequoia.)

for the 0.2 mole fraction shows a change of slope and the cmc value is higher than for the other two systems. The differences in the curve shapes suggested that some perfluorooctanoic acid is dissolved in sodium docecyl sulfate micelles while the rest continues to increase its monomer activity until cmc is reached. 4.4

KINETICS OF ADSORPTION

For practical purposes, it is not only important to know how much of a fluorinated surfactant is needed to reduce surface tension to a desired value. The time required to decrease surface tension is also highly significant. Many industrial processes do not allow sufficient time for a surfactant to attain equilibrium and depend on the kinetics of surfactant adsorption. The time required for surface tension reduction depends on diffusion processes involved in surfactant adsorption. Kinetic models for surfactant adsorption divide the adsorption process into two steps [67]. The first step is the transport of the surfactant to the subsurface, driven by a concentration gradient or hydrody-

"". ,

" "

L

" " " _ " " I

-

134

Chapter 4

per F-C8 OOH. mM

FIG.4.23 Surface tension of perfluorooctanoic acid and its mixtures with sodium dodecyl sulfate. Mole fractions of perfluorooctanoic acid: 1.OO, 0.75, 0.20. (From Ref. 66. Reproduced by permission of the American Chemical Society.)

namic forces. The second step is the transfer of the surfactant molecule from the subsurface to the adsorbed state. Usually, the second step is rapid and the first step determines the adsorption rate, especially when the transport of the surfactant molecule is a diffusion process. Because the bulk aqueous phase contains surfactant micelles, the influence of micelles on adsorption kinetics has to be considered. A kinetic model proposed by Miller [68] takes the formation and dissociation of surfactant micelles into account. Impurities can alter adsorption kinetics markedly and simulate adsorption or desorption barriers [69-7 11. Pure surfactants adsorb by a diffusion-controlled adsorption mechanism which excludes barriers to sorption processes [72]. Different mathematical solutions to the problem of diffusion-controlled adsorption kinetics have been reviewed and compared by Miller and Ziller [73]. The kinetics of surfactant adsorption depend on the surfactant structure. Fluorination of the hydrophobe increases the rate (dyldt) of surface tension decrease, but the time needed to attain equilibrium may notbe affected considerably. The surface tensions of sodium perfluorooctanesulfonate and its hydrocarbon-

"

Liquid-Liquid Liquid-Vapor and

Boundaries

135

type analog are shown in Fig. 4.24 [74]. Both surfactants reach their equilibrium surface tension in about the same time. However, the fluorinated surfactant has rapidly decreased the surface tension in less than a half a minute by about 30 dyn/cm. Dikhtievskaya and Makarevich [75] examined perpendicular and lateral interactions of fluorinated surfactants, IZ-C~F~O[CF(CF~)CF~O],,,-CF= C(CF3)COONH4,at the air-water boundary. The time required to attain equilibrium surface tension increased with increasing fluoroalkyl chain length or decreasing surfactant concentration. Equilibration is more rapid above cmc than below cmc [61]. The time required for attaining surface tension equilibrium decreases with increasing temperature. Selve et al. [33] have suggested that extremely hydropho-

70

C8H, 7S03Na

t I

6o

50

SURFACE TENSION (mN.m-')

.

40

30

C8F,7S03Na

20

0

1

10

10000 100

1000

LOG TIME (min) FIG. 4.24 Surface tension of 1-g/L solutions of sodium octanesulfonate and sodium perfluorooctanesulfonateversus log time. (From Ref. 74. Reproduced by permission of Dr. A. Huttig Verlag.)

Chapter 4

136

h

7

S

2

E

v

?-

201 0

50

100

150

200

t (mn) FIG. 4.25 Time dependence of surface tension of two surfactants at various concentrations at 5°C. From top to bottom: F10.1.33, 1.O X 106, 1.3 X 1OP6, 3.5 X mol/L; F8.1.33, 3.9 x IO6, 8.0 x IOP6, 1.2 x IOe5 mol/L. (From Ref. 32. Reproduced by permission of Pergamon Press.)

------191 171

FIG. 4.26 Surface tension of the surfactant solutions shown in Fig. 24 but at 45°C. In contrast to Fig. 24, adsorption is rapid at 45°C and the time dependence of surface tension is not detectable by the method used for measurement. (From Ref. 32. Reproduced by permission of Pergamon Press.)

Liquid-Liquid Boundaries Liquid-Vapor and

137

bic surfactants at low temperatures and low bulk concentrations have such a strong tendency to escape the aqueous environment that they form a rather disordered superficial layer. Minutes or even hours may berequired for the surface film to reorganize. Increasing temperature accelerates the diffusion processes and the equilibrium surface tension is attained in a shorter time. The temperature and concentration effects of adsorption kinetics are illustrated in Figs. 4.25 and 4.26. The adsorption of the fluorinated surfactants F8.1.33 and F10.1.33 [structures are given in Section 4.3 [32)] is slow at 5°C but essentially time independent at 45°C. The rate at which a surfactant reaches equilibrium surface tension also depends on its counterion. Klein et al. found that the tetraethylammonium salt of perfluorooctanesulfonic acid attains surface tension equilibrium more rapidly than the potassium salt (Fig. 4.27) [49], the tetramethylammonium, or the ammonium salt (Table 4.6) [61]. The adsorption rate of a surfactant on the liquid-air interface is affected by the presence of impurities and isomers with different adsorption characteristics. The diffusion rate of the surfactant to the liquid-air interface depends not only on the effective cross section of the isomers but on their solubility as well. A frac-

70 0.1 gIL

60

SURFACE TENSION 50 (dynkm)

0.1 gIL

40

30

1 g/L

c

c8F1

1 gIL

LOG TIME FIG. 4.27 Surface tension versus log time curves for C8F17S03K and C8F17S03N(C2H5)4. (From Ref. 49. Reproduced by permission of Carl Hanser Verlag-)

Chapter 4

138

tionation of the surfactant may occur when the solubility limit is exceeded. If the fluorinated surfactant is a mixture of normal-chain and branched-chain isomers, the normal-chain surfactant components precipitate, and the branched-chain isomers remain in solution [49]. Surfactant adsorption kinetics has been studied by static and dynamic methods. Although static methods can reveal kinetic effects of surface tension, dynamic methods can measure surface tension in a few milliseconds and are eminently suitable for investigating the kinetics of surfactant adsorption (see Chapter 9). Thedynamic surface tensions of fluorinated surfactants consisting of telomeric mixtures (Zonyl FSA, Zonyl FSN, and Zonyl FSC) were studied at the Textile Research Institute [76] by the maximum-bubble-pressure method. The surface tension can be measured by varying the flow rate of the gas used to generate a bubble in the liquid. When the bubble grows slowly, surfactant molecules have time to diffuse to the expanding gas-liquid interface. However, when the bubble generation is rapid. the surfactant does not have sufficient time for adsorption and the measured surface tension is higher than the equilibrium surface tension. Although the equilibrium surface tension of 0.30 n M Zonyl FSA, an anionic fluorinated surfactant, was found to be 17 mN/m. the dynamic surface tension at a bubble frequency of 0.1 bubble/s remained above 60 mN/m (Fig. 4.28).

0 030 mM 70 60

,.-"_I:_

50

3.0mM

40

30

30 mM c

20

CMC

0

1

1

1

I

2

4

6

8

- 0.1 mM 10

Bubble Frequency(11s) FIG. 4.28 Effect of surfactant concentration on the dynamic surface tension of aqueous solutions of an anionic fluorinated surfactant Zonyl FSA. (From Ref. 76. Reproduced by permission of Elsevier Science Publishers.)

Liquid-Vapor and Liquid-Liquid Boundaries

139

NP-10 AND FSNNATER AT 30 mM

20

I 2 0

4

6

8

10

Bubble Frequency (l/s) FIG.4.29 Synergistic effect on the dynamic surface tension of aqueous solutions of Tergitol NP-10 and Zonyl FSN at a total concentration of 30 mM. (From Ref. 76. Reproduced by permission of Elsevier Science Publishers.)

A mixture of Zonyl FSN, a nonionic fluorinated surfxtant, and Tergitol NP10. a nonionic hydrocarbon-type surfactant (nonylphenol with 10 oxyethylene units: Union Carbide Corp.), exhibited surface tension synergism (Fig. 4.29). The dynamic surface tension of the 1 : 1 mixture is lower than the dynamic surface tension of the individual components.

4.5

SURFACE TENSION IN ACIDS AND ALKALI

The outstanding chemical stability of fluorinated surfactants to strong acids and alkali permits their use under conditions in which conventional surfactants would decompose. Certain fluorinated surfactants can be used in the presence of strong oxidants, acids, or alkalies even at elevated temperatures. The surface tension of Zonyl fluorinated surfactants, featuring the hydrophobe F(CF3CF&-&H3CH2-, in acids and alkali is shown in Table 4.8. Anionic, cationic, nonionic, or amphoteric surfactants listed in Table 4.8 can lower the surface tension of 10% potassium hydroxide. 25% sulfuric acid, 37% hy-

TABLE 4.8

Surface Tension (mN/m) of Zonyl Fluorinated Surfactants at 25°C (77°F) 10% Potassium hydroxide

25% Sulfuric acid

Zonyl

0.001%

0.01%

0.1%

0.001%

FSA FSP FSE UR** FSJ FSN FSO FSC FSK TBS

33 30 34 35 38 30 25 27 54

15 17 19 20 24 25 24 18 32 36b

16 17 16 21 22 23 24 18 23 35b

1 1 1 1 29 25 28 26 30

a

5Ib

Insoluble. Partially insoluble. For 10% HCI.

la

0.01%

1 1 1 1

1 25 20 19 20 20

37% Hydrochloric acid

0.1%

0.001%

0.01%

0.1%

1 1 1 1 1 23 19 19 18 16

1 1 1 1 1 28 32 31 29 29'

1 1

1 1 1 1 1 23 22 18 17 16'

1 1

1 24 22 20 19 22'

70% Nitric Acid 0.01%

0.1%

1 1 1

1 1 1

1

1 1 25 31a 25 22 28

1 1 1 1 1 23 21 20 18 21

0.001%

1 30 41 36 34 50

Liquid-Vapor and 141 Liquid-Liquid Boundaries

drochloric acid, or 70% nitric acid below 25 mN/m, unless the surfactant is insoluble in acid.

Anionic Surfactants Fluorinated surfactants with a carboxylate or phosphate hydrophile precipitate in strongly acid media, whereas surfactants with a sulfonate hydrophile are more soluble and also effective in strongly acid media [77,78]. Talbot [77] measured surface tensions of perfluorinated sulfonic acid potassium salts in acids. Addition of a strong inorganic acid to a 10-3M solution of I I - C ~ F ~ ~ SinO water ~K lowered the surface tension to a value of 17 mN/m. The surface tension decreased with decreasing pH of the acid solution. Talbot explained the effect of acids by the formation of free surfactant acid molecules which are more strongly adsorbed on the surface and lower surface tension. Acetic acid had a lesser effect on surface tension of n-CSFI7SO3K,but its effect was proportional to its acidity and ionization. The surface tensions of some perfluorinated anionic surfactants with a C8Fl7- hydrophobe in strong acids are given in Table 4.9 [56]. The surfactant CsFl7SO3Kis very effective in concentrated acids, although its solubility in 37% hydrochloric acid is limited to 0.2 g L . The surface tension of the tetraethylammonium salt of the perfluorinated surfactant CSF17S03N(C2H5)d in acids is shown in Figs. 4.30 and 4.31 [49]. Glockner et al. [79] tested the stability of perfluorinated surfactants in acids or alkali under severe conditions. The 0.1 % solutions of anionic surfactants in 60% nitric acid or 98% sulfuric acid containing 10 g/L chromic oxide were stored for a time span of 43 days, of which 28 days were storage at 90°C. The surface tensions of the anionic surfactants changed very little during the test (Table 4.10) [791. TABLE 4.9

Surface Tensions of Fluorinated Surfactants (0.01Yo)in Acids Surface tension (mN/m) Hydrochloric acid

Surfactant

2.5%) (I

Sulfuric acid (50%)

Source: Ref. 56. (Reproduced by permission of Carl Hanser Verlag.)

Chromic oxidesulfuric acid

Chapter 4

142

70

60

SURFACE 50 TENSION (dynlcm)

40

30

20 LOG CONCENTRATION (9/L) FIG.4.30 Surface tension curve of C8F17S03N(C2H5)4 in 10% nitric acid. (From Ref. 49. Reproduced by permission of Carl Hanser Verlag.)

The stability of perfluorinzted anionic surfactants is also remarkable in alkali. Glockner et al. [79] found the surface tensions of perfluorinated anionic surfactants in saturated aqueous sodium hydroxide to be constant for at least 100 days (Table 4.11). Salts of perfluoroalkanoic. perfluoroalkanesulfonic, or perfluoroalkenesulfonic acids survived an alkaline fusion [5 g KN03/LiN03 (35-65 mol%), 0.5 g NaOH, 0.05 g surfactant] for 2 h at 200°C, as indicated by the surface tensions of the aqueous solutions of the fusion mass.

Nonionic Surfactants Perfluorinated nonionic surfactants of the structure ClzF3,2- (OCH2CH2),,0CH3 [79] and nonionics with an o-H-perfluoro hydrophobe [80] are effective surfactants in acids and in alkali, if stable. Greiner and Herbst [80] prepared nonionic surfactants with the general formula H(CF2CFZ),,CH2O(CH2CH2O),,,H.The oxyethylated 9-hydroperfluorononanol, at a concentration of 0.3-0.5%, lowered

70 0

60

SURFACE TENSION

\

0

-

\

50

(dynlcm) 40

30

20 1 0 - 6 10-5

10-3

10-4

10-2

1 0 ~ ~, - . = O L . L 10-1 I I O loo 1000

LOG CONCENTRATION (W) FIG.4.31 Surface tension curve of C8F17S03N(C2H5)4 in 10% phosphoric acid. (From Ref. 49. Reproduced by permission of Carl Hanser Verlag.) TABLE 4.10 Surface Tension of 0.1% Solutions of Anionic Fluorinated Surfactants in Concentrated Mineral Acids at 22°C

Surface tension (mN/m) ~~~~~

~

~

~~

~

90% H2S04 + 10 g/L Cr03

60% HN03

C7F15COOH H(CF2)8COONa c8F17S03K CBF17S03N(CZH5)4 C7F13S03Na 41.8 42.6 C,F2, - S03Na ( n = 10-11) No surfactant 63.3

1 da

43db 15d

33.8

-

-

19.8 23.2 42.6 20.3

1d 33.7

-

18.6 22.8

20.9 22.7

19.4

20.1

41.4 50.8 39.7 34.6 43.0 26.2

43db 15d -

42.6

50.5' 38.8 32.5 43.3 27.0

40.1 32.4 48.7 27.8

-

62.0

d = time in days. Including 28 days at 90°C. 218d. Source: Ref. 79. (Reproduced by permission of Carl Hanser Verlag.)

a

143

42.2

Chapter 4

144 TABLE4.11 Surface Tension of Anionic Fluorinated Surfactants After an NaOH/KOH Fusion at 300°C Surface tension (mN/m) Surfactant 5h

2h

c6F13S03K c8F17S03K C8F17SO3(C2H5)4 No surfactant

39.5 43.0 43.7 51.3

46.3 57.0 55.5 53.4

Source: Ref. 79. (Reproduced by permission of Carl Hanser Verlag.)

the surface tension of 50% sulfuric acid from 76 to 28 mN/m. Heating of the solution for 6 h at reflux did not change its surface tension. Thin-layer chromatography indicated, however, partial degradation of the oxyethylene chain. Although not significantly soluble in alkali, the oxyethylated 9-hydroperfluorononanol, at a concentration of 0.05%, lowers the surface tension of 20% NaOH to 27 mN/m. Highly oxyethylated (55 EO) 11-hydroperfluoroundecanol has a similar effect on surface tension. The nonionic fluorinated surfactants are more stable in alkali than in acid. Glockner et al. [79] found that, unlike perfluorinated anionic surfactants, perfluorinatednonionicsurfactants C,2F2,1-l(OCH2CH2),,,0CH3are not stablein chromic-sulfuric acid. The solution immediately turns green, indicating decomposition. In spite of the degradation by chromic-sulfuric acid, nonionic surfactants lower the surface tension of the acid substantially. The surface tension does not change significantly even after more than 80 days of storage (Table 4.12). ApTABLE 4.1 2 Surface Tension of Nonionic Fluorinated Surfactants in Concentrated Chromic-Sulfuric Acid

42.9

10-1 1 8-1 1 5-8 5-8 Monflor 52 No surfactant

57.9

17 7.6 17 8.7

42.3 36.7 56.4 60.7

45.5 41.2 55.7

64.2

Source: Ref. 79. (Reproducedby permission of Carl Hanser Verlag.)

96 96 86 90 112

Liquid-Vapor and Liquid-Liquid Boundaries

145

parently, the degradation of the nonionic surfactant produces surface-active species, perhaps perfluoroalkenyl sulfates or perfluoroalkanoic acids [79]. When selecting a fluorinated surfactant, the stability of the surfactant in the particular working medium has to be considered. Although the C-F bond is stable, the stability of fluorinated surfactants in strong acids and alkali also depends on the stability of other functional groups in the molecule. For example, nonionic surfactants derived from perfluorooctanesulfonamide can lower the surface tension of water more than the corresponding sulfonic acid salts. However, their use in alkali is limited by the hydrolytic instability of the sulfonamide linkage [49]. 4.6

SURFACE TENSION IN ORGANIC LIQUIDS

Many fluorochemicals are remarkably surface active in organic liquids [81-851 Scholberg et al. [50] found that perfluorocarboxylic acids lower the surface tensions of organic solvents more than any other chemical ever reported. Ellison and Zisman screened various compounds for surface activity in organic liquids by using a Langmuir film balance. The compound was spread on an organic liquid and the surface film pressure measured as a function of surface area [85]. The results indicated that the surface activity of fluorinated compounds in organic liquids can be predicted approximately from the Harkins equation for the spreading coefficient: Sb/a = y a -

(y b

+ yab)

(18)

where &/a is the spreading coefficient of liquid b spreading on liquid a and ya and y b are the surface tensions of liquids a and b, respectively. ?ab is the interfacial tension between liquids a and b. If y a b is small compared with the surface tension of the organic liquids, yab can be neglected. Hence, spreading behavior is approximately indicated by the difference y a - y b . The greater the surface tension of the solvent and the lower the surface tension of the solute, the greater the surface activity of the solute in the solvent. Because the surface tensions of fluorinated compounds are considerably lower than those of their hydrocarbon analogs, the surface activity of fluorinated compounds in organic solvents is not surprising. However. spreadability is anecessary but not a sufficient condition for surface activity. Even a positive S h does not always indicate the extent of surface tension lowering [86]. The effect of surfactants on surface tension is much smaller in solvents than in water. Surfactants can lower the surface tension of water by more than 50 mN/m, to below one-quarter of its original value. In aromatic hydrocarbons, only a few fluorinated surfactants can decrease the surface tension to half of the initial value. The surface tension depression is smaller, mainly for two reasons. First, the initial surface tension of solvents, 19-30 mN/m for hydrocarbons, is lower to start. Second, surfactants are not as effective in organic media as in water because ad-

Chapter 4

146

sorption of the surfactant at the surface of an organic liquid has less effect on the nature of the interface. Fluorinated surfactants are the most effective surfactants in organic media as well as in water. However, even for fluorinated surfactants, the lowest surface tension achieved in water is lower than the lowest surface tension in organic liquids. The structural requirements for a surfactant in an organic liquid are quite different from the structures of typical fluorinated surfactants used in water. In order to function as a surface-active agent, a compound has to be amphiphilic and consist of lyophobic (solvent insoluble) and lyophilic (solvent soluble) groups in the molecule [86]. In water, the hydrophobic group of the surfactant causes a re-

C 00

0 08

004 I

I

1

1

0.12 1

I

25.8

256

5

\

In

254 U

z

0

m

252

W

I-

202 0

2

E 3

Ln

200

"-"

19.8

04

1 .O

14

18

2.2

26

C,2 H25NH2 HCI, M FIG.4.32 Surface tension of n-dodecylammonium chloride in water (curve I), a 50% water-ethanol mixture (curve Il), and absolute ethanol (curve 111) versus surfactant concentration at 50°C. (From Ref. 87. Reproduced by permission of Wiley Interscience.)

Liquid-Vapor and Liquid-Liquid Boundaries

solute

147

M

FIG.4.33 Surface tension of toluene solutions of n-dodecylammonium chloride ( 0 )and tri-n-dodecylammonium bromide (A)as a function of surfactant concentration at 50°C. (From Ref. 88. Reproduced by permission of Academic Press.)

structuring of hydrogen-bonded water. The surfactant molecule is transported to the surface, where the molecule orients with the hydrophobic part directed away from water. In organic solvents, the situation is different. An alkyl tail, hydrophobic in water, may be lyophilic in a solvent and the hydrophilic head group in water may be lyophobic in a solvent. Conventional surfactants may orient with the hydrophilic lyophobic group away from the solvent surface, resulting in higher surface energy and an increase in surface tension (Fig. 4.32) [87] and (Fig. 4.33) [88]. A fluorinated hydrophobe functions in solvents, as in water, as the lyophobe. A lyophilic group provides solubility in the solvent. As an example, fluorinated carboxylic esters [86] can function as surfactants in organic solvents where the ester group is lyophilic and soluble. The fluorinated chain of the molecule is the lyophobic part. In aqueous solutions of nonfluorinated surfactants, the lowest surface tensions are attained by covering the surface with a close-packed monolayer of vertically oriented hydrocarbon chains forming a continuous layer of “CH3 groups exposed to air [89]. By analogy, the surface tension of a solution of a fluorinated

Chapter 4

148

TABLE 4.13 20°C)

Surface Tensions of Saturated Solutions in Various Organic Liquids (at Surface tension of

Surface tension (mN/m)

Propylene carbonate ( y = 41.1)

Tricresyl phosphate ( y = 40.2)

+’-Octyl alcohol +’-Octyl ethanesulfonate

Solid 19.1

Bis-(4’-octoxy)-bis-(t-butoxy)silane Tris-(4’-octoxy)-t-butoxysilane Bis-(4’-octyl)-tetrachlorophthalate Bis-4’-octyl)-a-n-dodecenylsuccinate Bis-(~’-octyl)-2,4-toluenedicarbamate Tris-(4’-butyl)-tricarballylate

18.4 18.5 Solid 19.4 Solid 18.5

24.ga 23.8a (22.3 at 0.16M) 28.3 28.3 30.7 25.3 32.5 24.5a

27.6a 22.1a (20.9 at 0.1 4M) 25.9 38.0

Tris-(4’-octyl)-tricarballylate

Solid

28.2

Surface-active solute

0.1 mol/L. Source: Ref. 89.

a At

surfactant is at its minimum when the surface is covered with a closely packed monolayer of fluorocarbon groups with their “CF3 groups oriented toward air. The orientation has a considerable effect on surface tension. Vertical orientation exposes the terminal -CF3 groups to air, whereas tilting of fluorocarbon chains exposes the -CF2groups. A surface film consisting of -CF2groups has a higher surface tension than a surface film of “CF3 groups [90.91]. Zisman and co-workers found that at the surface of an organic liquid, even the lowest values of area per molecule, calculated for compounds containing only one fluorinated chain, are much larger than the cross-sectional area of 24.5 A determined from the Stuart-Briegleb molecular models [89]. This suggests incomplete surface coverage by a fluorocarbon film, even though the fluorinated chain, at least in some solvents, is oriented away from the surface. Compounds containing two fluorinated chains [e.g., bis-(cp’octyl)tetrachlorophthalate] have lower values of area per molecule, approaching the cross-sectional value of 50 A indicated by the Stuart-Briegleb models [the symbol cp‘ is an abbreviation for an F(CF2)J2H2- group]. However, the minimum surface tensions found were higher than expected for a surface covered with vertically oriented fluorocarbon chains (Table 4.13). Compounds with three fluorinated chains behaved similarly. Zisman and co-workers concluded [89] that factors contributing to close packing are (1) structure and solubility of the organophilic group, (2) type and position of the polar group connecting the organophile with the fluorinated

Liquid-Liquid Boundaries Liquid-Vapor and

149

saturated solution (mN/m) Alkazene 42 ( y = 38.2)

Nitromethane ( y = 36.4)

31.2 23.4

26.8a 26.7a

25.4 23.8

24.2 27.4

25.7 36.0 31.2 23.1 Too insoluble 27.1a (22.2 at 0.68811.1) 23.6

21.2 24.0

Dioxane

Ethylbenzene ( y = 28.6)

( y = 32.9)

30.4 21 .ga 24.7 27.1 a (23.6 at 0.559M) 22.1

Hexadecane ( y = 27.6)

20.7 21 .ga Too insoluble 24.9

Too insoluble

organophobe, and (3) fluorination, length, and number of organophobic chains. A terminal hydrogen in an otherwise fluorinated chain increases solubility in polar solvents and decreases adsorption. Longer chains decrease solubility and lead to closer packing at the surface. Too many chains attached to the molecule interfere sterically with eachother and increasetheareaoccupied by the molecule. The solubility of the fluorinated compound has a significant effect on surface activity. Jarvis and Zisman [92] found that the maximum surface tension lowering of some fluorinated compounds is limited by their insufficient solubility. whereas others were too soluble to be adsorbed at the surface of the solution. Fluorinated chemicals must be designed to have a proper balance between soluble and insoluble groups of the molecule. An increase in the organophobic part of the molecule increases the initial slope of the surface tension-concentration curve (Fig. 4.34) but decreases the solubility of the molecule. Eventually. inadequate solubility of the molecule limits the minimum surface tension (maximum surface tension lowering) that can be achieved by increasing the concentration of the surfactant. Katrizki et al. [93] prepared a series of N-(perfluorooctanesulfony1)piperazine derivatives (Table 4.14). The most effective member of the series lowered the surface tension of diesel fuel from 27.2 to 21.7 mN/m at a concentration of 1% (w/v). Interestingly, the surface tension decrease, 5.5 mN/m, was smaller

"".._

" " "

I

_"

."_

. ." " " ".7

~ . - " "

_-

Chapter 4

150

I

0

I I

I 2

3

I

I

5

4 MOL/L

x

I 6

I

7

J8

IO*

FIG. 4.34 Surface tension-concentration isotherms of partially fluorinated carboxylicesters in organicliquids: bis-(cp'-butyl)3-methyIglutarate (@); bis-(cp'hexyl)3-methylglutarate (A);bis-(cp'-octyl)3-methylglutarate(U); bis-(cp'-heptyl)3methylglutarate ( 0 )hexylq-butyrate ; (A);1,2,3,-trimethylolpropanetris-(cp-butyrate) (m). (From Ref. 92. Reproduced by permission of the American Chemical Society.)

than the surface tension depression observed for a Zonyl surfactant (Table 4.15) [94]. Zonyl FSJ, a mixture of an amphoteric fluorinated surfactant and a hydrocarbon-based surfactant, was found to be more effective in lowering surface tension than a fluorinated surfactant alone. The synergism between fluorinated and nonfluorinated surfactants is important for lowering interfacial tensions (see Section 7.2). Initially, the surface activity in organic solverlts was studied with surfactants featuring a fluoroalkyl lyophobe. Recently, fluorinated surfactants with an oligo(hexafluoropropene oxide) chain have been developed. Oil-sol\lble surfactants with the structure (HFPO),,-Ar, where Ar is an aryl group and (RFPO),,is an oligo(hexafluoropropene oxide) group, 12 = 2-5, decrease the surface tension of toluene or rn-xylene down to 12-14 mN/m at 20°C (Fig. 4.35) [95]. The lowest surface tensions, ymin,the corresponding concentrations for the surfactants (HFP0)2-6C6H5,and their cmc values are given in Table LC.16. The surface tension of xylene (28.6 mN/m) was lowered considerably even at small surfactant concentrations. The surface tension decrease depends on the length of the perfluorinated chain, exhibiting a maximum decrease at 11 = 5 . However, Abe et

Liquid-Vapor and Liquid-Liquid Boundaries

151

42r 40HEXADECANE

38-

34 3236

A

A

I

"0""- 4322

I

1 0

2

1

I 3

1

I

I

I

4 MOL/L

5

6

?

x

I

lo2

PROPYLENE CAR6ONATE

0

FIG.4.34

I

continued

2

3

4 MOL/L

5

x to2

6

7

8

Chapter 4

152

TABLE4.14 24°C

Equilibrium Surface Tensions of 1 wt% Solutions in Diesel Fuel at

27.2 26.2 27.2 24.6 27.2 25.1 26.5 26.6 27.0 21.7 22.8

Blank 1 2

3 4 5 6 7 8 9 10 a

22.8 26.7 25.8 26.0 26.2 26.2 26.9 24.3 23.9 27.0 26.8

11 12 13 14 15 16 17 18 19 20 21

Compound structures:

Compound

Y

Compound

~

9 10 11 12 13 14 15 16

1 2 3 4 5 6 7 8

Compound

Source: Ref. 93.

Y

Y

Liquid-Liquid Liquid-Vapor and

CH&H20)2P(O)O"NH4+

TABLE4.15 Fuel

Boundaries

Surface Tension Depression by Fluorinated Surfactants in Diesel

Zonyl

Structure (Rf = C,F2,

FSK FSC FSN FSA FSE FSP FSJ

153

+ 1;

n

=

A y (mN/m)

3-8

RfCH2CH2CH(OCOCH3)Nf(CH3)&H2COO[RfCH2CH2SCH2CH2Nf(CH3)312SOzRrCH2CH20(CH2CH20),H RfCH2CH2SCH2CH2COOLi

4.6 6.4

(RfCH,CH,o)P(o)(o-)2(NH4+)2 FSP + nonfluorinated surfactant

0.7 9.6

0.4 0.5

Source: Refs. 93 and 94. (Reproduced by permission of the American Chemical Society.)

k

I

I 0-4

I

I

I 0-3 10-2 Concentration (mol/L)

FIG.4.35 Surface tension-log concentration curves for (HFPO),-Ph (From Ref. 95. Reproduced by permission of Elsevier Sequoia.)

in rn-xylene.

154

Chapter4

TABLE4.16 Surface Tensions of(HFPO),"C6H5 rn-Xylene (20°C) Yrnm

N/m) n

~~~

(WtYO)

cmc (mol/L)

0.58 0.21 0.23 0.23 0.29

1.48 X 1OP2 3.75 X 10-3 3.19 X 10-3 2.58 X 2.18 X lop3

Crnm

(m

2 3 4 5 6

Solutions in

17.8 12.8 12.1 9.9 11.3 ~

~

~~~~

Source: Ref. 95. (Reproduced by permission of Elsevier Sequoia.)

al. [96] found the surfactant containing only one ether linkage to be especially effective in reducing the surface tension of rn-xylene. In the (HFPO),,C6H4CH3series, the surfactant (HFP0)&,H4CH3was the most effective i n lowering the surface tension of toluene and xylene (Table 17). Presumably, the maximum surface tension lowering corresponds to an optimum TABLE4.17 Surface Tension Mimima (Ymln) and Corresponding Concentrations (Cmln) for Ar-(HFPO), in Toluene or Xylene at 20°C Ar-(HFPO), Ar Toluene C6H5

kCH3C6H4

m-Xylene p"CH3C6H4

n

Yrnm (mN/m)

Cmln (WtYO)

2 3 4 5 6 2 3 4 5 6

21 .o 12.8 13.5 12.2 12.7 21.4 13.7 14.8 12.7 12.0

2.3 2.3 2.3 0.46 0.46 2.3 2.3 0.46 0.46 0.46

2 3 4 5 6

21.5 14.3 13.8 12.8 11.9

2.3 2.3 2.3 0.46 0.46

Source: Ref. 95. (Reproduced by permission of Elsevier Sequoia.)

Liquid-Vapor and Liquid-Liquid Boundaries

155

balance between the oleophilic y-tolyl group and the oleophobic (HFP0)6 group in the molecule. The development of fluorinated surfactants for organic solvents is still ongoing. The structural design is limited by (1) the dependence of surface activity on the solubility of the surfactant in the solvent and (2) the initial surface tension of the solvent. Hence, a fluorinated surfactant cannot have the same optimum structure for all organic solvents [92]. For semifluorinated alkanes see Section 1.8. 4.7

LIQUID-LIQUID INTERFACE

The interface between two immiscible liquids is ideally a dividing plane between the two phases (Fig. 4.1). In reality, the interface is a region of finite thickness formed by the adsorbed surfactant and the subsurface. Adsorption of surfactants at a liquid-liquid interface is amore complex process than adsorption at a liquid-gas interface. The simplest case is a system consisting of water, an oil phase, and a surfactant. When the surfactant is soluble only in water, then factors, such as structural effects of the sulfactant. which govern the oil-water interfacial tension are similar to those affecting the surface tension of water. However, if the surfactant is soluble in both phases, the situation is much more complex. Various interactions and solubilization of oil in water or water in oil have to be considered. The theory of interfacial tension has been reviewed in depth by several authors [2,3,97-991. Interfacial tension is an important factor in chemical and physical processes involving a large interface, such as formation of emulsions, spreading of one liquid on another, wetting, foaming, enhanced oil recovery, and other processes of technical interest. Emulsification requires a low, near-zero, interfacial tension because a large interface has to be created. A low interfacial tension is a result of positive adsorption of the surfactant at the interface. The effect of a surfactant on interfacial tension depends on the extent of adsorption and the nature of the adsorbed film. Scholberg et al. [SO] observed that fluorinated surfactants exhibit a low interfacial energy between water and a fluorocarbon, but the emulsions of these materials were not stable. However, Greiner and co-workers [60,80] found fluorinated surfactants of and their sulfates to lower interfacial the type H(CF2CF2),,CH20(CH2CH3_o),,H tension between hydrocarbons and water and function as effective emulsifiers. The sulfates lowered water-nonane interfacial tension between water and nonane to 5 mN/m. Motomura et al. [loo] measured the interfacial tension of aqueous sodium perfluorooctanoate against hexane as a function of temperature around cmc. The entropy and energy of adsorption were found to be higher for sodium perfluorooctanoate than for dodecylammonium chloride or sodium dodecyl sulfate. The

156

Chapter 4

results indicated a weak mutual interaction between the fluorocarbon chain and hexane molecule as well as the strong hydrophobicity of the fluorinated surfactant. Thoay [ 1011 studied interfacial activity of partially fluorinated surfactants in an oil-water medium. The fluorocarbon group was located either at the end of the hydrocarbon tail, inserted in the middle of the hydrocarbon chain, or attached as a branch to a conventional hydrocarbon-type cationic surfactant. Three types of surfactants were used: 1. C1gH37NCH2CH2N(C2H&, 2C2HSBr

In the first type, a fluorocarbon branch was attached to a cationic surfactant. At low concentrations, these fluorinated surfactants lower the interfacial tension, y2, more than their hydrocarbon analogs, but the limiting value of y2is much higher for the fluorinated surfactants than for the hydrocarbon analogs (Fig. 4.36). The second surfactant type had a terminal fluorocarbon group. Fluorination increased interfacial efficiency but also increased the limiting value of y2 (Fig. 4.37). Thoay concluded that the main effect of a terminal perfluoro group istodecreasethesolubility of thesurfactantandincreaseitsinterfacial efficiency. In the third type, a fluorocarbon segment was inserted between a hydrocarbon chain and the hydrophilic group. The interfacial efficiency is similar to that of type TI surfactants: greater interfacial efficiency and approximately the same yl limit (Fig. 4.38). None of the fluorinated surfactants was suitable for preparing oil-water microemulsions.Thoayconcludedthatfluorinatedsurfactants weaken the CH--CH interchain interaction essential for the formation of stable oil-water microemulsions. Mukerjee and Handa [ 102-1041 measured surface and interfacial tensions of dilute aqueous solutions of sodium perfluorobutyrate, sodium perfluorooctanoate, sodium perfluorodecanoate, sodium octyl sulfate, and sodium decyl sulfate at the air-water, hexane-water, and perfluorohexane-water interfaces. The surface and interfacial tension values, y. were converted to interfacial pressures, n, where I3 = 'yo - y. The data obtained for dilute solutions indicated the affinity of the surfactant for the interface.

Liquid-Vapor and Liquid-Liquid Boundaries

vi

157

dyne/cm

FIG.4.36 Water-heptane interfacial tension as a function of the concentration of C18H37N(CH2R)CH2CH2N(C2H5)2, 2C2H5Br.(From Ref. 101. Reproduced by permission of Academic Press.)

FIG. 4.37 Water-heptane interfacial tension as a function of surfactant concentration. Effect of the fluorinated chain length. (From Ref. 101. Reproduced by permission of Academic Press.)

Chapter 4

158

11 35

1

dynelcm

A

o

C,, H,,OSO, CHI),,

Na

OS4Na

X C7YsOCH2(CF2 l2 CFH CH2OSO, Na

15 10

10-4

FIG.4.38 Water-heptane interfacial tension as a function of surfactant concentration. Effect of hydrophobe structure. (From Ref. 101. Reproduced by permission of Academic Press.)

Interfacial pressures below 2 mN/m increased linearly with surfactant concentration (activity) (Fig. 4.39). The linear relationship is in agreement with a thermodynamic model of adsorption developed by Lucassen-Reynders [ 1051 and others [ 106-1081. For surfactants which are strong 1 : 1 electrolytes, the model suggests the limiting form of the equation of state:

rIA

=

2kT

(19)

where A is the surface per surfactant ion, k is the Boltzmann constant, and Tis the absolute temperature. For low surfactant concentrations and low II values, the mutual interactions of adsorbed surfactant molecules at the interface are very weak. The initial dII/dc- values therefore indicate the relative affinity of the surfactants for the interface. A comparison of the sodium pwfluorooctmoate and sodium decyl sulfate interfacial pressures at various interfaces showed striking differences between the relative affinities of the surfactant for various interfaces. Although both surfactants have nearly identical cmc values, their adsorption at low surfactant concentrations differ markedly. The perfluorinated surfactant had a higher affinity for the air-water and perfluorohexane-water interfaces than the nonfluorinated surfactant. However, the latter has a higher affinity for the hexane-water interface (Fig. 4.39). The higher affinity of the nonfluorinated surfactant for the hydrocarbon-water interface has been attributed to a more favorable interaction of the hydrocarbon chains at the hydrocarbon-water interface. Mukerjee and Handa [lo21 estimated the free energies of the surfactants at the air-water, heptane-water, and perfluorooctane-heptane interfaces and calcu-

Liquid-Vapor and Liquid-Liquid Boundaries

159

lated the incremental changes in free-energy values for the addition of a -CF2or a -CF2group. The affinity of a surfactant with a hydrocarbon tail for the hexane-water interface was found to be much higher than that for the air-water interface. In contrast, the difference between the affinities of a surfactant with a fluorocarbon tail for the perfluorohexane-water interface and for the air-water interface is not very large. This smaller difference is in accord with the weak interactions among fluorocarbons. The results obtained by Mukerjee and Handa substantiate the fundamental difference between liquid-air and liquid-liquid interfaces. The surface tension of a surfactant solution depends mainly on interactions with the solvent and adsorption at the liquid-air interface. Interactions with air or vapor are weak. In contrast, a surfactant at a liquid-liquid interface interacts with two liquid phases. The lyophobic part of a surfactant oriented away from one of the liquid phases interacts with the second liquid phase in contact. The interaction between the lyophobic film and the second liquid phase is much more significant than the interaction with air or a vapor. Zhao and Zhu (1091 studied the interfacial tension and spreading properties of mixed solutions of anionic (cationic) and cationic (anionic) fluorocarbon and hydrocarbon surfactants. The anionic-cationic system exhibited much higher surAir / Water Hexane

/ Water

0

P- f - hexane/ Water

0

1

1

1

3

6

9

FIG.4.39 Interfacial pressures, plotted against the surfactant concentration in the aqueous phase, C, at 25°C; (0)SDeS; ( 0 )SPFO. (From Ref. 102. Reproduced by permission of the American Chemical Society.)

Chapter 4

160

face activity than the single components. The aqueous solution of the surfactant mixture had a lower surface tension and a lower oil-water interfacial tension and spread readily on n-heptane and light kerosene. The increase in surface activity was attributed to a strong interaction, possibly ion-pair formation, between the surfactant ions with opposite charges. 4.8

EMULSIONS

An emulsion is a reasonably stable mixture of one liquid (the dispersed phase) dispersed in an immiscible liquid (the continuous phase). Aqueous emulsions are oilin-water (O/W) or water-in-oil (W/O). Here. the term oil designates a water-immiscible liquid. The basic theory of emulsions has been discussed in several books and articles [ 12,110-1 121. The formation of emulsions increases immensely the interfacial area, A,, between the two phases and therefore requires a low interfacial tension, y,. It can be readily seen that the work, W, required to create the new interface,

w = YlAl is very large unless yl is small. However, a low, near-zero, interfacial tension is not a sufficient prerequisite for-the formation of a stable emulsion. The stability of an emulsion also depends on the nature of the interfacial boundary. The stabilization mechanisms of emulsions are complex and not completely understood. The main mechanisms are steric stabilization, ionic stabilization, and the less effective stabilization by hydrosols. Droplets in the emulsion have to be prevented from approaching each other to a distance where coalescence results. Also, a droplet has to resist deformation during a droplet-droplet collision and a disturbance of the protective film. For the selection of a suitable surfactant structure, the hydrophile-lipophile balance (HLB) has been a useful index [ 112-1 161. The HLB values have played an important part in selecting fluorinated surfactants for ernulsification. A distinction has to be made between kinetically and thermodynamically stable emulsions. Mixtures of water. an oil, a surfactant, and a cosurfactant in proper proportions can spontaneously form transparent, thermodynamically stable emulsions, called microemulsions. The definition is not quite unequivocal. Thermodynamically unstable but kinetically stable systems which form spontaneously have also been called microemulsions [ 1 171. Micellar solutions have been included in the definition of microemulsions and the debate on microemulsion structure is ongoing. Fluorinated surfactants are unique. The stability of fluorinated surfactants to heat or chemical attack is advantageous when preparing emulsions containing corrosive chemicals. The fluorocarbon part of the molecule is not only hydrophobic but also oleophobic and contributes to a very low free-surface energy of the ad-

Liquid-Liquid Boundaries Liquid-Vapor and

161

sorbed surfactant. Fluorinated surfactants are effective emulsifiers for fluorinated monomers (e.g., tetrafluoroethylene) in the polymerization process [118]. Fluorinated surfactants are also used for emulsifying other low-surface-energy liquids, such as silicones and fluorocarbons. However, perfluorinated surfactants are not as effective as hydrocarbon-type surfactants in emulsifying hydrocarbon oils. Synergistic effects of conventional hydrocarbon [ 11 91surfactants have been employed to increase the effectiveness of either surfactant alone and to lower the total surfactant concentration needed. The low efficiency of fluorinated surfactants in emulsifying hydrocarbon oils is beneficial in some applications. Selected fluorinated surfactants can provide wetting without emulsification. To generate nzicroernulsions. a surfactant must lower the oil-water interfacial tension to a small value (near zero) [120]. A strong association between the hydrocarbon tails of oil and the hydrophobe of conventional hydrocarbon-based surfactants is essential for forming oil-water microemulsions. Substituting hydrogen with fluorine reduces this hydrocarbon-hydrocarbon interaction and perfluorinated surfactants lack lipophilic groups needed for emulsifying hydrocarbons. Thoay [ 1011 attempted to compensate for the lack of lipophily by preparing partially fluorinated surfactants (see also Section 4.6) which have three parts: a lipophilic group, a hydrophilic group, and a fluorocarbon group. However, none of the fluorinated surfactants was found to be effective in generating a microemulsion of hydrocarbons in water. An attempt to prepare a hydrocarbon-water microemulsion produced a gel. Large amounts of the fluorinated surfactants were needed to prevent gel formation. A keen interest in microemulsioru offluorocarbonsin water was kindled by the need for synthetic oxygen carriers in blood (see Section 10.4). Gerbacia and Rosano [121] prepared a stable fluorocarbon emulsion using a mixture of fluorinated and hydrocarbon-type nonionic surfactants. The droplet size was not determined, however, and the emulsions were not characterized. Oliveros et al. [ 1221 prepared perfluorinated microemulsions consisting of four components: (1) sodium perfluorooctanoate, an anionic fluorinated surfactant; (2) 2,2.3,3,4,4,4heptafluoro- 1-butanol; (3) perfluorohexane; (4) water. The pseudoternary-phase diagram (Fig. 4.40) shows two regions, M I and M1. of respective W/O and O N microemulsions [ 1221. These optically transparent microemulsions were characterized by nuclear magnetic resonance spectroscopy. Anionic perfluorinated and almost completely fluorinated surfactants were used by Ceschin et al. [ 1231 to prepare microemulsions of fluorocarbons. The particle size in monophasic regions is related to surfactant and cosurfactant nature, the weight ratio of surfactant to the oil phase, and the nature of the oil phase. A French team [ 1241prepared nonionic microemulsions from aternaxy mixture of water, a fluorocarbon (e.g., perfluorodecalin), and a nonionic perfluoroalkylpoly(oxyethylene) surfactant. The microemulsions formed spontaneously when the three components were mixed and shaken by hand. The concentration of

Chapter 4

162

/ FIG. 4.40 Pseudoternary-phasediagram of the system C7F15COONa (I)-& F7CH20H(2)-C6F14 (3) for a constant surfactantkosurfactant weight ratio of 1 : 1 (molar ratio 1 : 2.2) at 23°C. M1and M2 are the regions of respective W/O and ONV microemulsions. (From Ref. 122. Reproduced by permission of Verlag Helvetica Chimica Acta.)

the fluorocarbon was high [up to 50% (w/w)], resulting in high solubility of gases in the emulsion. The microemulsions remained stable when diluted with water. Small-angle neutron scattering (SANS) measurements gave an average droplet size of about 100 A and the apparent aggregation number of 2100. The size and shape of droplets remained about the same when the microemulsions were diluted with water.The droplets were visualized, comprisinga fluorocarbon core surrounded by the fluorinated surfactant. The surfactant is oriented with its fluorocarbon end toward the fluorocarbon core and its hydrophilic group directed toward water. According to Shinoda and Kunieda [ 1111, aqueous microemulsions can form from ternary mixtures of a hydrocarbon, a nonionic surfactant, and water when the temperature of the system is kept between the solubilization temperature (lower limit) and the cloud point (higher limit). The area between the solubiliza-

Liquid-Vapor and Liquid-Liquid Boundaries

163

tion and cloud-point curves [ 1241 obtained by the French team represents aqueous microemulsions (Winsor IV solutions [ 1251). The solubilization temperature and the cloud point increase with increasing fluorocarbon content of the system. Both curves ascend until they join at the phase-inversion temperature (Fig. 4.41). To maximize solubilization, it is desirable to have the phase-inversion temperature fall into the temperature range where the system is used. Both phase-inversion temperature and the cloud point are related to the structure and the HLB of the surfactant (see Section 6.6).

(C

1

1

S

.

1

I

1

10 1s 20 25 YoFLUOROCARBON

FIG. 4.41 Partial phase diagrams of systems containing 10 wt% C6F13C2H4(OC2H4)140H, water, and (a) C6F13CH=CHC6F13, (b) C8F17C2H5,(c) C6F13CH=CH2 as a function of temperature. Upper curves are cloud-point curves; lower curves are solubilization curves. (From Ref. 124. Reproduced by permission of the American Chemical Society.)

I

Chapter 4

164

Mathisetal. [ 1241 preparedsurfactants of thegeneralstructure C,,,F,,,+I(CH2),(OC2H4),,0H.Surfactants with the subscripts HZ = 6, p = 1, IZ = 5 , and y71 = 7, y = 1, IZ = 5 or 6 were selected for their appropriate HLB. The phase diagrams, like the one shown in Fig. 4.42, indicated high sensitivity to small variations of the HLB. This HLB dependence and an increase in solubilization effectiveness [ 1261 mandated the use of monodisperse surfactants. Robert and Tondre [ 1271 studied reverse microemulsions consisting of water emulsified in a binary mixture of a fluorocarbon and a nonionic fluorinated surfactant. Surfactants of the structure C6F13CH2(OC2H4),,0H. with n = 4, 5 , or 6 , emulsified large amounts of water (up to 20 wt% water in perfluorodecalin) in fluorocarbons. The solubilization of water by nonionic fluorinated surfactants in fluorocarbons follows the same trends exhibited by hydrocarbon-type systems. Schubert and Kaler [128] have shown that microemulsions of a variety of fluorinated oils in water can be made by using a mixture of a fluorinated surfactants and a nonionic hydrogenated surfactant. The relative amount and chain

PERFLUORODECALIN

FIG. 4.42 Phase diagrams of the systems water-perfluorodecalinC7FI5CH2(OC2H&0H and w a t e r - p e r f l u ~ r o d e c a l i n - C ~ F ~ ~ C H ~ ( O C ~at H~)~OH 37°C (only the microemulsion region is shown, hatch area). (From Ref. 124. Reproduced by permission of the American Chemical Society.)

Liquid-Vapor and Liquid-Liquid Boundaries

165

length are parameters which can be used to increase the efficiency of the amphiphile mixture or to vary the temperature of microemulsion formation. The fluorinated surfactants, F(CF?),CH2CH20),H, used were Zonyl FSO-100 (i = 7.5, j = 8) and Zonyl FSN-LOO (i = 8.2, j = 10) from Du Pont. The hydrogenated surfactants were mostly rz-alkyl polyglycol ethers. The fluorinated oils were 1-bromoperfluoroctane,perfluorodecalin, and perfluoro(tetradecahydrophenanthrene). Chittofrati et al. [ 1291prepared microemulsions of perfluoropolyethers (PFPES) having the general structure

Ammonium salts of PFPE carboxylates are very effective surfactants for the preparation of monophasic, transparent, isotropic PFPE emulsions that behave as true microemulsions. With increasing hydrophobic chain length. the solubility of the surfactant in water decreases and the solubility in oil increases. Short-chain surfactants form O/W microemulsions. In contrast, oil-soluble long-chain surfactants form W/O ternary microemulsions. The formation of the microemulsions was investigated by electric conductivity and static light scattering [ 1301. Conductivity measurements across the isotropic oil-rich region were used to monitor the progressive transition from a network of hydrated surfactant to a microemulsion in oil while adding water. Conductivity measurements were also used to study the solubilization of water in PFPE oil by a PFPE carboxyIic acid in the presence of alkanols [13 11. The conductivities of these four-component systems were explained by hydratiodpartial dissociation equilibria of surfactants. The dominant conduction mechanism, either by ionic charge carriers or statistically charged droplets, depends on the type of alkanol. Amine(polyfluoroalkoxyacy1)imides [38) (see Section 4.2) formed stable perfluorodecalin emulsions when the surface activity of the surfactant was sufficient to lower the surface tension of water below 18 mN/m. Aminacylimides capable of lowering the surface tension of water only to 18-25 mN/m formed unstable emulsions that broke after a few hours within a few weeks. Aminacylimides with a higher surface tension in water formed emulsions that were even less stable. Selve at al. [32] synthesized fluorinated nonionic surfactants with a twochain polyoxyethylene hydrophilic head linked to the hydrophobe via an amide In this surfactant series, the surfacbond, F(CF2),(CH2),,,C(0)N[(C2H40),,CH3]2. tant with the subscripts I = 10, wz = 1, and 12 = 33 has an HLB value phase suitable for forming microemulsions with inversion temperatures near the ambient temperature. Hence, the surfactant emulsified larger amounts (1 8 wt%) of a perfluorocarbon than other surfactants prepared in this series.

Chapter 4

166

The application of fluorinated surfactants in emulsions and dispersions is covered in Chapter 8. Fluorocarbon emulsions used as oxygen carriers in blood are discussed in Chapter 10. 4.9

FOAMS

Foams are coarse dispersions of gas in a relatively small amount of liquid. In solid foams, the continuous phase is a solid which, at one point in time, has been a liquid. Pure liquids cannot form foams and nearly all liquid foams are thermodynamically unstable. Only liquids containing a surface-active component can form foams. The surface-active component may be a solute dissolved in the liquid or insoluble matter at the interface, such as a solid in the form of insoluble particles. a liquid-crystal phase, or an insoluble monomolecular film. Surfactants can stabilize but also destabilize foams and cause their collapse. Foams are complex systems and many factors affect foaming and foam stability [3,11.132-1361. Surface tension trying to pull the gas bubble together is opposed by pressure inside the bubble and the forces within the film between the bubbles, perpendicular to the interface (the disjoining pressure). Foams are subject to mechanical disturbances and environmental stresses that affect the persistence of foam. A foam collapses when (1) the gas from the interior of the bubble diffuses into the gas phase outside the foarn or into a larger bubble because of the pressure differences caused by the larger curvature of the smaller bubble, (2) the lamellae are ruptured. or (3) the liquid drains from the lamellae and the lamellae are thinned to below a minimum thickness (about 50-100 A). Drainage is caused by gravity or by pressure differences across the interfxe. According to Laplace’s equation. the pressure difference across a curved interface, AP, depends on the curvature of the interface R:

In the foam bubble, the curvature of the interface is largest (smallest radius) where bubbles join (referred to as the plateau border region or the Gibbs triangle). The pressure in the plateau region is therefore smaller than in the central areas of the lamellae. The pressure difference causes liquid to flow into the plateau border regions and depletes the central parts of the lamellae. An important factor determining foam stability is the ability of foam to resist excessive localized thinning of the lamellae and to repair thinning of the lamellar film caused by drainage or by expansion of the bubble. Gibbs defined this property as foam elasticity, E,:

4

t

”

t

Liquid-Vapor and Liquid-Liquid Boundaries

167

An incremental increase of the surface area, A, is counterbalanced by an increase in surface tension. A surface tension gradient formed when the surface area is expanded causes a flow of the liquid toward the thinner areas and hinders stretching of the surface. Ewers and Sutherland [ 1371 proposed a surface transport mechanism in which the surface tension gradient causes the surface monolayer and the subsurface layer to flow from a low to a high surface tension region. The surface elasticity effect arising from a surface tension gradient as proposed by Gibbs is complemented by the Marangoni effect, which results from the time dependence of surface tension. The Gibbs effect deals with foam in the state of equilibrium, whereas the Marangoni effect results from a kinetic time lag of surfactant diffusion which causes a surfactant concentration gradient and, consequently, a surface tension gradient. Because the diffusion of surfactant from the bulk solution to the interface requires time, the concentration of surfactant in the interface decreases when the surface is expanded. The surface tension increase counteracts the forces stretching the surface. The Marangoni effect is operative only in dilute solutions. Other important factors governing foam persistence are surface viscosity, bulk viscosity, and the electrostatic repulsion between the two sides of the film. Foams can be desirable or undesirable. It is therefore useful that fluorinated surfactants differ widely in their foaming propensity, as shown for Zonyl surfactants in Table 4.18. (For structure, see Table 4.4). Some fluorinated surfactants are effective foaming agents, whereas others prevent foaming. Most effective foaming agents can be found among amphoteric, cationic, and anionic fluorinated surfactants. Their foaming characteristics depend on the structures of the hydrophobe and the hydrophilic group. In general, nonionic fluorinated surfactants, like hydrocarbon-type nonionic surfactants, produce less foam and less persistent foam than ionic fluorinated surfactants. Surface tension lowering is aprerequisite for foam formation but not a sufficient condition for foam stability. For example, the fluorinated surfactants Zonyl FSA, Zonyl FSP, and Zonyl FSK lower the surface tension of water to about 20 dyn/cm (Fig. 4.43). However, Zonyl FSK is an outstanding foaming agent, whereas Zonyl FSP can function under some conditions as an antifoam agent. The third fluorinated surfactant represented in Fig. 4.43, Zonyl FSA, is neither an effective foaming agent nor an antifoam agent. The antifoam activity of Zonyl FSP, a mixture of (RKH2CH20)P(0)(ONH& and (RCH,CH20)2P(O)(ONH,), is related to its phosphate ester hydrophile. Other fluorinated surfactants featuring a phosphate-ester-type hydrophile function as antifoam agents. For example, Surflon S-112, a fluorinated surfactant with a phosphate ester hydrophile, depressed foaming of a solution containing a nonionic surfactant from 80 to 38 mm of foam height measured by the Ross-Miles test. After 5 min, the foam height was 58 mm in the absence of Surflon S- 1 12 but only 6 mm in the presence of 0.004% Surflon S-112 [ 1381. Similar results were obtained with Atsurf S-112, a phosphate-ester-

!

168

Chapter 4

TABLE4.18 Foaming Action in Water: 0.1 % Solids in Water, Ross-Miles Foam Test [ASTM-D1173-53] Foam height (mm) Deionized water ppm 300 Zonyl

FSA FSP FSE UR + sodium hydroxidea FSJ FSN FSN-100 FSO FSO-100 FSC FSK

TBS

Temperature ("CP F)

Initial

3 min

41I105 681155 41I105 681155 41I105 681155 41I105 681155 41I105 681155 41I105 681155 41I105 6811 15 41I105 681155 41I105 681155 41I105 681155 41I105 681155 41I105 681155

100 125 35 35 120 50 15 15 130 140 145 130 135 130 74 134 90 105 220 220 220 220 228 246

95 120 30 30 110 40 15 15 120 130 140 130 130 100 65 95 85 47 220 220 220 220 235 264

min 10 9'5 50 25 10 90 10 10 10 100 60 135 40 130 40 63 62 80 33 220 70 220 80 235 208

hard water Initial

3 min

min 10

1

Surfactant precipitates

65 60 135 130 130 120 35 95 78 115 225 200 220 220

30 20 20 10 135 130 120 20 130 130 95 30 30 24 58 35 75 73 55 35 225 225 200 80 220 220 200 80 Surfactant precipitates ~

~

L

I

~~~~~

a Aqueous solution prepared by heating 10.0 g Zonyl UR to 50-55°C (122-131°F) in water containing 0.53 g sodium hydroxide and stirring. Source: Ref. 48.

I

type fluorinated surfactant [ 1391. Alkylphosphate esters are known to function as foam inhibitors. Their foam-inhibition mechanism is probably related to a large spreading coefficient and drainage promotion in the foam lamellae. A mixture of an anionic (Fluorad F-95) and a cationic fluorinated surfactant (Fluorad FC-134) has been recommended for foamsuppression in organic liquids [ 1401. Kozhanov et al. [141] studied stability of foams as a function of the concentration of a fluorinated surfactant in the aqueous phase. Foams were made by shaking equal volumes of toluene and water. The stability of foams depended on

Liquid-Vapor and Liquid-Liquid Boundaries

70 I-

169

PURE WATER

60 50

c I

FSA

6

40

30 20

0.001

0.01

0.1

Surfactant Concentration(% W N ) FIG.4.43 Surface tensions of three Zonyl fluorinated surfactants in water at 25°C. (From Ref. 48.)

the final volume of the spreading coefficient of the surfactant solution with respect to toluene. Foams were formed when the spreading coefficient values were positive. Fluorinated surfactants are used in fire-extinguishing foams which can spread on hydrocarbons and organic solvents (see Chapter 8).

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170

Chapter 4

6. F. Franks. J. Ravenhill, P. A. Egelstaff, and D. I. Page, Proc. Roy. SOC.(London) A3 19. 189 ( I 970). 7. I. Danielssson. B. Lindman, and L. Odberg. Suomen Kemistilehti 42.209 (1969). 8. B. Tamamushi, in “Colloidal Surfactants,’‘ K. Shinoda, T. Nakagawa, B. Tamamushi, and T. Isemura, eds., p. 179, Academic Press. New York (1963). 9. H. Schott, J. Pharm. Sci. 69. 852 (1980). 10. F. Van Voorst Vader. Trans. Faraday SOC.56, 1067 (1960). 11. M. J. Rosen, “Surfactants and Interfacial Phenomena.’’ 2nd ed., Wiley, New York ( 1989). 12. A. W. Adamson. “Physical Chemistry of Surfaces,” 5th ed., Wiley. New York (1990). 13. J. W. McBain and C. W. Humphreys. J. Phys. Chem. 36. 300 (1932). I 4. J. W. McBain and R. C. Swain. Proc. Roy. SOC.(London). A154.608 (1936). 15. D. J. Salley. A. J. Weith, Jr.. A. A. Argyle, and J. K. Dixon, Proc. Roy. SOC.(London) A203,42 (1950). 16. J. K. Dixon. C. M. Judson, and D.J. Salley, “Monomolecular Layers,” p. 63. AAAS, Washington, DC ( 1954). 17. K. Tajima, M. Muramatsu. and T. Sasaki. Bull. Chem. SOC.Japan 43. 1991 (1970). 18. K. Tajima. Bull. Chem. SOC.Japan 43, 3063 (1 970). 19. N. H. Steiger and G. Aniansson, J. Phys. Chem. 5 8 , 228 (1954). 20. K. Shinoda and K. Ito. J. Phys. Chem. 65, 1499 ( I 96 1 ). 21. S. J. Rehfeld, J. Colloid Interf. Sci. 3 1,46 ( 1969). 33. T. C. Crowley. E. M. Lee, E. A. Simister, R. K. Thomas, J. Penfold, and A. R. Rennie. Colloids Surf. A 52. 85 ( I 990). 23. J. Penfold and R. K. Thomas, J. Phys. Condens. Matter, 2, 1369 (1990). 24. E. A. Simister, E. M. Lee, J. R. Lu. R. K. Thomas, R. H. Ottewill. A. R. Rennie, and J. Penfold, J. Chem. SOC.Faraday Trans. 88. 3033 ( 1992). 25. J. R. Lu, E. M. Lee, R. K. Thomas, J. Penfold. and S. Flitsch, Lamgmuir 9, 1352 (1993). 26. N. Downes, G. A. Ottewill. and R. H. Ottewill, Colloids Surf. A, 103, 203 (1995). 27. S. W. An, J. R. Lu. R. K. Thomas. and J. Penfold. Langmuir 122. 2446 (1996). 28. Z. X. Li, J. R. Lu, and R. K. Thomas, Langmuir 13,3681 (1997). 29. A. Downer. J. Eastoe, A. R. Pitt, J. Penfold, and R. K. Heenan, Colloids Surf. A 156, 33 (1999). 30. A. Downer. J. Eastoe. A. R. Pitt, E. Simister, and J. Penfold. Langmuir 15, 7591 (1999). 31. F. M. Wrenger and S. Wrenn, J. Phys. Chem. 78, 1387 (1974). 32. C. Selve, J. C. Ravey, M. J. Stebe, C. El Moudjahid. E. M. Moumni, and J. J. Delpuech. Tetrahedron 47,411 (1991). 33. K. Shinoda, M. Hatd, and T. Hayasdhi, J. Phys. Chenl. 76, 909 (1972). 34. K. Shinoda and H. Nakayama, J. Colloid Sci. 18,705 (1963). 35. K. Shinoda and K. Katsura, J. Phys. Chem. 68, 1568 (1964). 36. E. Gorodinsky and S. Efrima. Langmuir IO. 2 152 (1994). 37. Th. F. Tadros, J. Colloid Interf. Sci. 74, 196 (1980). 38. L. Haywood. S. McKee, and W. J. Middleton, J. Fluorine Chem. 51,419 (1991).

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39. C. H. Arrington and G. D. Patterson, J. Phys. Chem. 57,247 (1953). 40. G. Caporiccio, F. Burzio. G. Carniselli, and V. Biancardi. J. Colloid Interf. Sci. 98, 202 ( 1984). 41. I. H. Smith and R. H. Ottewill, “Symposium on Surface Active Agents,” p. 77. Society of Chemical Industry, London( 1979). 42. L. Matos, J. C. Ravey, and G. Serratrice, J. Colloid Interf. Sci. 128. 341 (1989). 43. 0.Shibata, Y. Moroi. M. Saito, and R. Matuura, Langmuir 8, 1806 (1992). 44. S. W. Barton, A. Goudot. 0. Bouloussa, F. Rondelez. B. Lin. F. Novak, A. Acero. and S. A. Rice, J . Chem. Phys. 96, 1343 ( 1992). 45. S. Shin, N. Collazo, and S. A. Rice, J. Chem. Phys. 96, 1352 (1992). 46. C. H. Zhang. B. Y. Zhu, and G. X. Zhao. J. Colloid Interf. Sci. 144. 483 (1 991 ); C. H. Zhang, G. X. Zhao, and B. Y. Zhu. J. Colloid Interf. 144,491 (1991j. 47. M. Koshinuma.A. Nakamura, andK. Tajima, J. Surf. Sci. Techno]. 8(2). 173 (1 992). 48. Du Pont Performance Products, “Zonyl Fluorosurfactants,” DU Pont, Wilmington, DE (1987). 49. H. G. Klein, J. N. Meussdoerffer, and H. Niederprum. Metalloberflache 29, 559 (1975). 50. H. M. Scholberg, R. A. Guenthner. and R. I. Coon, J. Phys. Chem. 57,923 (1953). 51. M. K. Bernett and W. A. Zisman, J. Phys. Chem. 63. 1911 (1959). 52. A. Lottermoser and B. Baumguertel, Trans. FaradaySOC. 31,200 (1935). 53. J. Powney, Trans. Faraday SOC. 31, 151 (1935). 54. H. Kunieda and K. Shinoda. J. Phys. Chem. 80,2468 ( 1976). 55. J. 0. Hendrichs, Ind. Eng. Chem. 45, 99 (1953). 56. E. Schuierer, Tenside 13. 1 (1976). 57. H. C. Fielding. in “Organofluorine Chemicals and Their Industrial Applications,” R. E. Banks, ed.. p. 2 14, Society of Chemical Industry, LondonEllis Horwood, Chichester ( 1979). 58. E. G. Shafrin and W. A. Zisman. J. Phys. Chem. 66,740 (1962). 59. M. K. Bernett and W. A. Zisman.J. Phys. Chem. 7 I , 2075 (1967). 60. A. Greiner. B. Kruger, and M. Herbst, Z. Chem. 18,383 (1 978). 61. H. G. Klein, J. N. Meussdoerffer. H. Niederprm, and M. Wechsberg. Tenside 15, 2 (1978). 62. N. 0. Brace, J. Org. Chem. 27,4491 (1 962). 63. C. Kimura, K. Kashiwaya,M. Kobayashi, and T. Nishiyama. J. Am. Oil Chem.SOC. 61, 105 (1984). 64. N. Ishikawa and M. Sasabe. J. Fluorine Chem. 25,241 (1984). 65. H. B. Klevens and M. Raison, J. Chim. 51, 1 (1954). 66. P. Mukerjee and K. J. Mysels, in “Colloidal Dispersions and Micellar Behavior,” ACS Symposium Series 9, American Chemical Society, Washington. DC. ( 1975), p. 239. 67. A. F. H. Ward and L. Torday. J. Chem. Phys. 14,453 (1946). 68. R. Miller. Colloid Polym. Sci. 259, 1124 (1981). 69. K. J. Mysels and A. T. Florence, Colloid Interf. Sci. 43. 577 (1973). 70. K. Lunkenheimer and R. Miller, Tenside 16, 312 (1979).

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71. 72. 73. 74. 75.

R. Miller and K. Lunkenheimer, Tenside 17,288 (1980). R. Miller and K. Lunkenheimer, Colloid Polym. Sci. 264, 367 ( 1986). R. Miller and M. Ziller, Colloid Polym. Sci. 266,532 (1988). J. N. Meussdoerffer and H. Niederpriim, Chem. Z. 104,45 (1980). L. V. Dikhtievskaya and N. A. Makarevich, Vestsi Akad. Navuk BSSR, Ser. Khim. Navuk (l), 24 1991; CA 114,254357. D. E. Hirt, R. K. Prud’homme. B. Miller, and L. Rebenfeld, Colloids Surf. 44, 101 (1990). E. L. Talbot, J. Phys. Chem. 63, 1666 (1959). H. G. Bryce, in “Fluorine Chemistry.” Vol. V, p. 370, J. H. Simons, ed., Academic Press, New York (1964). V. Glockner, K. Lunkwitz, and D. Prescher, Tenside 26, 376 ( 1989). A. Greiner and M. Herbst. Z. Chem. 16,229 (1976). G. B. Blake, A. H. Albrecht, and H. G. Bryce, Am. Chem. SOC.Div. Petrol. Chem. Gen. Pap., No. 32, 131 (1954). J. G. O’Rear and W. A. Zisman, U.S. Patent 2,824,141 (1958). P. D. Faurote. C. M. Murphy, C. M. Henderson, J. G. O’Rear, and H. Ravner, Ind. Eng. Chem. 48,445 (1954). A. H. Ellison and W. A. Zisman, J. Phys. Chem. 59, 1233 (1955). A. H. Ellison and W. A. Zisman, J. Phys. Chem. 60, 416 (1956). N. L. Jarvis and W. A. Zisman, J. Phys. Chem. 63.727 (1959). A. S. Kertes and H. Gutmann, in “Surface and Colloid Science,” E. Matijevic. ed.. Vol. 8, p. 193, Wiley-Interscience, New York (1975). H. Gutmann and A. S. Kertes, J. Colloid Tnterf. Sci. 5 1,406 (1975). M. K. Bernett. N. L. Jarvis, and W. A. Zisman, J. Phys. Chem. 66, 328 (1962). H. W. Fox and W. A. Zisman, J. Colloid Sci. 5.514 (1950). E. F. Hare. E. G. Shafrin. and W. A. Zisman, J. Phys. Chem. 58,236 (1954). N. L. Jarvis and W. A. Zisman, J. Phys. Chem. 64, 150 (1960). A. R. Katrizki. T. L. Davis, and G. W. Rewcastle, Langmuir 4,732 (1988). R. Holzleiter, Report to First Diesel Fuel Conference, Aberdeen Proving Grounds, MD (January 1986). N. Ishikawa and M. Sasabe J. Fluorine Chem. 25,241 (1984). M. Abe. K. Morikawa, K. Ogino, H. Sawada, T. Matsumoto. and M. Nakayama, Langmuir 8.763 (1992). C. A. Miller and P. Neogi, eds.. “Interfacial Phenomena,” Surfactant Science Series Vol. 17, Marcel Dekker. New York (1985). F. Tadros. ed., “Surfactants,” Academic Press, London ( 1 984). M. Bourrel and R. S. Schechter, “Microemulsions and Related Systems: Formulation, Solvency and Physical Properties,” Surfactant Science Series Vol. 30, Marcel Dekker, New York (1988). K. Motomura. I. Kajiwara, N. Ikeda. and M. Aratono, Colloids Surf. 38,61 (1989). N. Thoay, J. Colloid Sci. 62, 222 (1977). P. Mukerjee and T. Handa, J. Phys. Chem. 85,2298 (1981). P. Mukerjee. J. Am. Oil Chem. SOC.59, 573 ( 1 982). T. Handa and P. Mukerjee, J. Phys. Chem. 85,3916 (198 I).

76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99.

100. 101. 102. 103. 104.

Liquid-Vapor and Liquid-Liquid Boundaries 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133.

134.

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E. H. Lucassen-Reynders,J. Phys. Chem. 70,1977 (1966). G. L. Gaines,J. Chem. Phys. 69, 2627 (1978). I. D. Robb and A. E. Alexander, J. Colloid Interf. Sci. 28. 1 (1966). L. Ter-Minassian-Saraga. Proc.Int. Cong. Surface Activity 2nd. London, vol. I, p. 36 (1 957). G-X. Zhao and B-Y. Zhu, Colloid Polym. Sci. 261, 89 (1983). K. J. Lissant, ed., “Emulsions and Emulsion Technology.” Marcel Dekker, New York (1974). K. Shinoda and H. Kunieda, in “Microemulsions. Theory and Practice,” L. M. Prince, ed., p. 57, Academic Press, New York (1977). P. Becher, ed., “Encyclopedia of Emulsion Technology,” Vol. I. Marcel Dekker, New York (1983). W. C. Griffin, J. SOC. Cosmet. Chem. 1, 311 ( I 949). 5, (1949). W. C. Griffin, J. SOC. Cosmet. Chem. 249 J. T. Davies, Proc.Tnt. Cong. Surface Activity,2nd. London, Vol. I, 426 (1957). P. Becher, J. SOC. Cosmet. Chem. 1 1, 325 ( 1960). S. Friberg. Colloids Surf. 4,201 (1982). H. G. Bryce, in “Fluorine Chemistry.’’ J. H. Simmons. ed., Vol. 5 , p. 380, Academic Press. New York (1 964). M. K. Bernett and W. A. Zisman, J. Phys. Chem. 65,448 (1961). L. M. Prince, J. Colloid Interf. Sci. 23, 165 (1967). W. Gerbacia and H. L. Rosano, U.S. Patent 3,778,381 (1973). E.Oliveros, M. T.Maurette, andA. M. Braun,Helv.Chim.Acta66,1183 (1 983). C. Ceschin,J. Roques, M.C. Malet-Martino,and A. Lattes,J. Chem. Tech. Biotechnol. 35A, 73 ( 1 985). G. Mathis, P. Leempoel. J. C. Ravey, C. Selve, and J. J. Delpuech, J. Am. Chem. SOC. 106,6162(1984). P. A. Winsor. Trans. Faraday SOC. 44,376 (1948). K. Shinoda and H. Kuneida. J. Colloid Tnterf. Sci 42, 381 (1973). A. Robert and C. Tondre.J. Colloid Interf. Sci. 98, 515 (1984). K.-V. Schubertand E. W. Kaler, Colloids Surf. A 84.97 (1994). A. Chittofrati. D. Lenti, A. Sanguineti. M. Visca, C. Gambi, D. Senatra, and Z. Zhou, Colloids Surf. 41.45 (1989). A. Chittofrati. A. Sanguineti, M. Visca, and N. Kallay, Colloids Surf. 63, 219 (1992). A. Chittofrati. M. Visca. and N. Kallay. Colloids Surf. A 74. 25 I (1993). J. J. Bikerman, “Foams,” Springer-Verlag.New York ( 1973). E. H. Lucassen-Reynders,in “Anionic Surfactants:Physical Chemistry of Surfactant Action,” E. H. Lucassen-Reynders,ed., Surfactant Science SeriesVol 11, p. 1, Marcel Dekker, New York(198 1). M. J. Schick andT. R. Schmolka, in “Nonionic Surfactants: Physical Chemistry,” M. J. Schick, ed., Surfactant Science SeriesVol. 23, p. 835. Marcel Dekker,New York (1987). P. Walstra, in “Foams: Physics, Chemistry and Structure.” A. J. Wilson, ed., Springer-VerlagBerlin (1989).

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5 Solid-Liquid Interface

5.1 ADSORPTIONFROMSOLUTION The general theory of adsorption at a solid-liquid interface has been reviewed in several monographs and journal articles [ 1-20]. Adsorption at the solid-liquid interface is associated with a decrease in freesurface energy. The adsorbed surfactant is held at the solid surface by physical, ionic, or chemical forces. Physical adsorption is caused primarily by van der Waals forces. The adsorption mechanisms may involve dispersion forces, hydrophobic bonding. charge transfer, or hydrogen-bonding. Zonic adsorption results from an ionic bond formed between ionic sites at the surface and oppositely charged surfactant ions. Chemisorption involves the formation of acovalentchemicalbond. Chemisorption provides the strongest bond, van der Waals forces the weakest [17].

Adsorption at the solid-liquid interface depends on several factors:

1. The structure of the surfactant. Strength of the adsorptive bond formed by the adsorbed surfactant depends on the structure of the hydrophobic and hydrophilic groups of the surfactant and decreases usually in theorder: cationic > anionic > nonionic surfactant. 2. The nature of the substrate. Substrates are nonpolar, polar, or metallic (high surface energy). The surface characteristics of the substrate surface, such as the polarity, chemical composition, and surface geometry (porosity), are major factors affecting adsorption. 175

176

Chapter 5

3. The composition of the liquid phase. The nature of the solvent and the dissolved electrolytes or additives influences adsorption. 4. The physical conditions: temperature, pressure, and agitation. Adsorption can be characterized by the amount of the surfactant adsorbed as a function of equilibrium concentration. The quantitative amount of the adsorbed surfactant is usually determined by analyzing the liquid phase by spectroscopic, calorimetric, and electrophoretic methods [5,6]. The physical and chemical state of the adsorbed layer can be examined by spectroscopy, electron diffraction, x-ray diffraction,electronspectroscopyforchemicalanalysis (ESCA), interferometry, and electron microscopy. The orientation of the adsorbed surfactant molecules depends on the nature of the adsorbent and the surfactant, the interaction between the adsorbent and the surfactant, the lateral interaction between the adsorbed surfactant molecules, and the concentration of the surfactant in solution. The adsorbed layer may be homogeneous or consist of coadsorbed surfactants of different composition. Solvent molecules may also be adsorbed at the solid interface. The amount of a surfactant adsorbed at a constant temperature and pressure increases with the surfactant concentration to an adsorption limit or exhibits maxima and minima. Adsorption isotherms have been classified by Giles et al. [21,22] using letters and numbers to indicate subgroups (Fig. 5.1). However, two shapes of isotherms, 2L and 3L, are most common [14]. Zisman and his co-workers [23-331 characterized the wettability of low-energy solids, adsorbed monolayers, and Langmuir-Blodgett films of fluorinated surfactants by measuring the contact angle 8 and plotting cos 8 against the surface tension (see Chapter 11). Extrapolation to cos 8 = 1 gave a surface tension of the liquid (yLv)value called the critical surface tension yc. Zisman proposed that only liquids having surface tensions below this value can spread on the solid or film. Condensed monolayers were prepared on smooth surfaces of polished platinum, brass, copper, or glass by adsorption from solution or by the retraction method [25-291. The wetting characteristics of adsorbed films indicate that the adsorption of perfluorinated alkanoic acids and the wettability of their monolayers depend on the length of the perfluorinated carbon chain. Hare et al. [26] prepared monomolecular films of perfluorinated butyric, valeric, caproic, caprylic, capric, and lauric acids by adsorption on platinum from their solutions in sz-decane. The short-chain perfluoroalkanoic acids were adsorbed rapidly but the two longest-chain acids required a longer time to form a complete monolayer. The wetting properties of the monomolecular films were characterized by contact angle measurements for a number of liquids. The cosine of the contact angle OE for 12-alkanes plotted against the number of carbon atoms of the perfluorinated acids (Fig. 5.2) fell along parallel straight lines. The remarkable repellency of the perfluoroalkanoic acid monolayers was attributed to an exposed plane rich in perfluoromethyl groups, shown for perfluorododecanoic acid by electron diffraction.

Solid-Liquid Interface

177

€OUILIBRIUM

SOLUTION

CONCENTRATION

FIG.5.1 Classification of isotherm shapes. (From Ref. 12.)

The wettability of the monolayer-coated surfaces is determined mainly by the nature and packing of the outermost atoms in the exposed surface. The dominant effects of fluorinated terminal groups on the wettability of the adsorbed monolayer has been demonstrated with partially fluorinated fatty acids. Shafrin and Zisman [30] studied wetting characteristics of heptadecanoic acids which featured perfluoroethyl, perfluoropropyl, perfluoropentyl. and perfluoroheptyl groups in the terminal 17-position. Monomolecular films were prepared by adsorption from the melt on chromium. 17-(perfluorohepty1)heptadecanoic acid was also adsorbed onto platinum, nickel, quartz, and soda-lime glass. The films adsorbed on siliceous surfaces were more wettable than films on metals. A cornparison of partially fluorinated acids featuring a terminal perfluoroheptyl chain

178

Chapter 5

FIG.5.2 Contact angles observed for n-alkanes on monolayers of perfluorinated alkanoic acid homologs. (From Ref. 26. Reproduced by permission of the American Chemical Society.)

with perfluorinated acids revealed that the wettability of the monolayer is determined by the fluorinated outermost segment in the exposed surface plane. The adjacent alkyl segment, if adequately shielded by the fluorocarbon segment, does not affect the wetting characteristics of partially fluorinated surfactants with a terminal fluorinated segment. The effect of the fluorinated segment on wettability is illustrated in Fig. 5.3 with a plot of yc versus the number of fluorine-substituted carbon atoms in the hydrophobe. Straight lines represent the ycvalues for surfactants featuring more than three fluorine-substituted carbon atoms. The line for partially fluorinated surfactants intercepts the line for perfluorinated surfactants at seven carbon atoms. The data suggest that the perfluoroheptyl segment is sufficiently long to shield the hydrocarbon segment from the external boundary, and the wettability of the monolayer is determined mainly by the packing of fluorocarbon chains. In accord with the dominant effect of the terminal group on wettability, terminal branching and terminal chlorine substitution increase the wettability of the adsorbed monolayers formed by perfluoroalkanoic acids [29]. Condensed monolayers of twohomologousseries of acids,(CF3)2CF(CF3),,COOHand CF2C1(CF3)(CF2),,COOH,were adsorbed on chromium or platinum by the melt retraction method [31]. This technique is based on the observation that a drop of nonspreading acid rolled over the metal surface leaves an adsorbed monomolecular film behind. The increased wettability resulting from terminal branching or chlorine substitution was explained by changes in adsorption-site spacing. molecular packing, and chain adlineation of the monolayers. The packing and configuration of the adsorbed monolayer depend mainly on the relative sizes of the fluorocarbon and hydrocarbon segments and the cohe-

Solid-Liquid Interface

179

24

12

0 HfCH2),,COOH

X

-

( f r o m solution)

N U M B E R OF F L U O R I N A T E D C A R B O N ATOMS P E R M O L E C U L E

FIG.5.3 Effect of fluorination of the adsorbed alkanoic acid monolayer on the critical surface tension. (From Ref. 30. Reproduced by permission of the American Chemical Society.)

sive forces between adjacent segments. Shafrin and Zisnlan [30]considered three models for the acids with a perfluorinated segment of three carbon atoms or longer (Fig. 5.4). Models A and B assume that the perfluoro segments are in a closely packed extended position. The larger diameter of the fluorinated segments separates the hydrocarbon segments and reduces the otherwise strong cohesive forces between them. Model A postulates a short-range order for the hydrocarbon segments, a condition similar to the liquid state. Model B assumes a somewhat greater order for the hydrocarbon chains. Model C describes a high degree of order and close packing of both the hydrocarbon and fluorocarbon segments. To reduce steric hindrance, hydrocarbon segments have to be inclined. The main objection to model C is the unlikely parallel alignment required for two sets of planes representing the hydrocarbon and fluorocarbon segments. The wetting of a solid surface by a surfactant solution depends on the adsorption of the surfactant on the liquid-vapor and liquid-solid interfaces. If a surfactant is equally adsorbed on liquid-vapor and liquid-solid interfaces and the solid-vapor surface tension, ySv, is not affected by the liquid, the equilibrium

180

- -

-a-2.-

I

i-

r

yg

I I I

I

c

Chapter 5

Solid-Liquid Interface

181

contact angle 8 should correspond to the surface tension of the liquid, yLv,in accordance with Young’s equation: cos 8 =

Ysv

- YSL

YLV

If this assumption is correct, the wettability of a solid should depend solely on the surface tension, yLv, of a pure liquid or a surfactant solution, regardless of the structure of the surfactant used to lower the surface tension of water. However, this assumption was found to be incorrect. When Bernett and Zisman [32] used surfactant solutions instead of pure liquids in their wetting studies, fluorinated surfactants behaved differently than conventional surfactants with a hydrocarbon chain. Furthermore, the wetting characteristics of a fluorinated surfactant solution depend on the nature of the solid surface. According to the critical surface tension concept, the equilibrium contact angle 8 and the ability of the solution to spread on polyethylene and polytetrafluoroethylene (Teflon) are related to the critical surface tension yc of the solid and the surface tension yLv of the solution. Surfactants with a hydrocarbon-based hydrophobe cannot lower the surface tension of water below 18 mN/m, the critical surface tension yc of Teflon. Hence, aqueous solutions of hydrocarbon surfactants cannot wet Teflon. Fluorinated surfactants can decrease the surface tension of water below the critical surface tension of Teflon [18 mN/m) and wet it completely [33] (Fig. 5.5). The critical surface tension value (19 mN/m) obtained for Teflon by plotting cos 8 versus yLv is in good agreement with the anticipated value of 18 mN/m. However, a similar plot for fluorosurfactant solutions on polyethylene gave a critical surface tension (yc) value for polyethylene of 20 mN/m, instead of the anticipated value of 31 mN/m obtained with pure liquids (Fig. 5.6). Bernett and Zisman explained the apparent shift of yc by adsorption of fluorinated surfactants on polyethylene and a decrease of the solid-vapor interfacial tension ysv. Contact-angle data for hexadecane were used to propose a more tenacious adsorption of fluorinated surfactants on polyethylene than on Teflon (Fig. 5.7). According to the hypothesis, fluorinated surfactants adsorbed on polyethylene are oriented with their fluorinated hydrophobe toward water and render polyethylene more hydrophobic. The rationale of this explanation, disputed later by others [34] was based on the assumption that the change in free energy is larger when a surface consisting of -CF2-- or -CF3groups replaces a surface consisting of
Chapter 5

182

FIG.5.5 Wettability of Teflon by aqueous solutions of highly fluorinated compounds. (From Ref. 33. Reproduced by permission of the American Chemical Society.)

Using the Young and Gibbs equation [Eq. (l)] and Fowkes and Harkins' (351 assumption that dysv -0 dYLV

"

Johnson and Dettre [7] derived the equation COS

e = 2% - YLV YLV

The equation states that the adsorption at both the solid-liquid and liquid-vapor interfaces is similar if it is governed by interactions between water molecules and the surfactant hydrophobic tails and not by attractive forces at the interface. If this assumption is valid and Eq. (2) holds, then the yc values obtained from solvent

Solid-Liquid Interface

183 0 30

45

60

c

75

w"

0

0

90

05

-0.5IO

20 20

30

40

50

70

60

SURFACE TENSION (DYNES / CM AT 25O

00

C)

FIG. 5.6 Wettability of polyethylene by aqueous solutions of highly fluorinated compounds. (From Ref. 33. Reproduced by permission of the American Chemical Society.)

contact-angle data should be equal to the yc values obtained using surfactant solutions. A comparison of yc values (Table 5.1) shows that the agreement is reasonably good for hydrocarbon surfactants on nonpolar surfaces and for a fluorinated surfactant on Teflon. In other cases, large discrepancies of yc.values indicate the adsorption to the solid--liquid and liquid-vapor interfaces is not equal. The unequal adsorption to the liquid-vapor and solid-liquid interfaces has been the subject of several studies [34,36,38]. In contrast to Bernett and Zisman [33], Pyter et al. [34] explained the different wetting characteristics of hydrocarbon surfactants and fluorinated surfactants by low adsorption of fluorinated surfactants on nonpolar solids. The higher contact angles exhibited by solutions of fluorinated surfactants on polyethylene were explained by poor adsorption of fluorinated surfactants instead of the strong adsorption proposed by Bernett and Zisman [33].

Chapter 5

184 70

I

0

01

I

I

02

0.3

1

04

1

0.5

I

0.6

0.7

SOLUTE CONCENTRATION (Wr %)

FIG.5.7 Hexadecane contact angles on polymer surfaces with perfluorooctanoic acid adsorbed from aqueous solutions. (From Ref. 33. Reproduced by permission of the American Chemical Society.)

Pure liquids and aqueous surfactant solutions having the same surface tension, yLv,do not always exhibit the same contact angle. Pure liquids are often better wetting agents than surfactant solutions. The differences in wetting have been attributed to differences in ysL.The value of ysL is higher for the surfactant solution than for the pure liquid. This suggests lower adsorption of surfactants to the solid-liquid interface than to the liquid-vapor interface. For hydrocarbon surfactants, these differences are larger for semipolar solids such as poly(methy1 methacrylate) than for nonpolar solids, such as polyethylene. For a perfluorinated surfactant, such as perfluorooctanoic acid, the effect is significant for semipolar and nonpolar solids as well (e.g., paraffin and polyethylene). Fluorinated surfactant solutions exhibit higher contact angles on nonpolar solids, such as polyethylene and paraffin. than solutions of hydrocarbon-derived surfactants having the same surface tension (Fig. 5.8). The higher contact angles of fluorinated surfac-

Solid-Liquid Interface TABLE 5.1

185

Critical Surface Tension Values (mN/m) Solid

Liquid system

Paraffin

Polyethylene

Teflon

PMMAa

Nylon

Pure liquids Hydrocarbon surfactant in water Fluorinated 20 surfactant in water Predicted by Eq. (3) and solvent 8 data

25.5 [7] 23

31 [32] 30 [32]

18 [32] 22, 18 [32]

40 [38] 26

43 [37] 26

[33]

13, 18 [33]

11

17

47

15 28

33

48

~~

Note: Numbers in brackets are reference numbers. a

PMMA-poly(methy1 methacrylate).

Source: Values withoutreference numbers are from Ref.34. (Reproduced bypermissionof Academic Press, Inc.)

I .o 0.8 0.6

8

0.4

*)

0" 0.2 0.o

- 0.2 1

I

IO

20

I

I

1

40 50 yLV ( dynes / cm 1

30

1

I

60

70

FIG.5.8 Cos 8 versus surface tension of aqueous surfactant solutions on paraffin at 25°C: ( 0 )Aerosol OT in 0.1 MNaCI; (0) perfluorooctanoic acid. (From Ref. 34. Reproduced by permission of Academic Press.)

186

Chapter 5

tants are in accord with the lower adsorption of fluorinated hydrocarbon surfactants on Graphon [34], used as a hydrocarbon-type model substrate for comparing the adsorption of fluorinated and hydrocarbon surfactants. The orientation, thickness, and surface coverage of adsorbed fluorinated surfactants have been studied by infrared dichroism [39], scanning electron microscopy, atomic force microscopy [40], x-ray reflectivity [39], and x-ray photoelectron spectroscopy (XPS, also known as ESCA, see Section 9.12). Gerenser et al. (411 determined the orientation of a cationic fluorosurfactant, Zonyl FSC, on Si02 and poly(ethy1ene terephthalate) surfaces. XPS indicated that at low surfactant concentrations, Si02 is covered by a uniform monolayer. The surfactant orientation changed as the packing density was increased. On poly(ethy1ene terephthalate). the surface coverage was incomplete and the surfactant orientation did not change when the surfactant coverage was increased. Oxidation of the surface via corona discharge resulted in a uniform and continuous distribution of the surfactant over the surface [42]. These results suggested chemisorption of the fluorinated surfactant on the oxidized surface by ion exchange. X-ray photoelectron spectroscopy was used also by Mitsuya [43] to examine chemisorption of 11-H-eicosafluoroundecanoic acid from a liquid phase to a single crystal of silicon. The n-type silicon wafers were first oxidized in boiling HC1/H202/2H20solution, dipped into 1% HF to remove the amorphous oxide on the surface. The oxidized wafer was immersed i n 50% HF to produce a surfixe with Si-F bonds. An immersion in a hexane solution of 11-H-eicosafluoroundecanoic acid generated a XPS peak attributed to a C-F bond and another assigned to carboxylic carbon. The XPS spectra indicate the growth of a monomolecular film in which each molecule is adsorbed at two sites. Batts[44]studiedtheadsorption of thefluorosurfactantFC134 at a gelatin-air interface by x-ray photoelectron spectroscopy (XPS). Their data indicated that less fluorinated surfactant is needed for decreasing the critical surface tension. 'yc, of dried gelatin layers to its minimum value than for lowering the surface tension of water, yLv,to the minimum values of the aqueous surfactant solution. The time-of-flight secondary ion mass spectrometry (ToF-SIMS) results [45] were in agreement with XPS data. No change occurred in the outermost surface once monolayer coverage had been completed. The adsorption of perfluorocarbonsurfactants C,,F2,,+ COONaand C,7F2,1+ 1 COOH ( 1 1 = 6,7, and 8) on aluminum oxide has been measured in 0.1M NaCl and in 1.1,2-tri~hl0r0trifluoroethane(TCFE) [46]. In aqueous solution, maximum adsorption decreased with increasing carbon chain length and the pH of the medium. At an initial pH of 4, the maximum adsorption corresponded to bilevel coverage; at higher pH values, an adsorption plateau was attained before complete bilayer formation. Contact angles and friction coefficients for adsorbed films were measured as well. In TCFE [47], the adsorption of the perfluoro acids onto aluminum oxide was cooperative. involving attractive lateral interactions between

I"

1

Solid-Liquid Interface

187

the head groups of the acids within the adsorbed layer. The plateau values of adsorption isotherms corresponded to monolayer coverage. Clayfield et al. [48] attempted to prepare monolayers of fluorinated surfactants FC134 and FX161 on glass by adsorption from acetone solutions. Stable and reproducible contact angles were obtained with 12-alkanes, but polar liquids such as water caused desorption of the surfactant from the solid-liquid interface. Langmuir-Blodgett films of a polyphilic surfactant on float glass or calcium fluoride have been investigated by infrared dichroism, scanning electron microscopy, and x-ray reflectivity [39]. The polyphilic surfactant, F(CF2)6(CH2)Il O-C6H&6H4-COOH, consisted of a perfluorinated chain, a hydrocarbon spacer, a rigid biphenyl core, and a carboxylic hydrophile. At the transfer pressure used, the fluorocarbon chains are practically perpendicular to the plane of preparation. In multilayer films, however, the molecular axes are tilted toward the dipping direction. Virtually uncharged fluorocarbon surfaces on smooth sheets of muscovite mica have been prepared using the double-chain cationic surfactant N-(a-trimethylammoniumacetyl)O,O'-bis(lH, 1H.2H,2H-perfluorodecyl)-L-glutamate chloride [49]. Surfaces were coated with the fluorinated surfactant either by Langmuir-Blodgett deposition or by adsorption from solution [50]. Slow adsorption from solution gave an initially very thin surface layer of surfactant molecules lying flat on the mica surface. The surface neutralization was essentially complete, and it took 2-5 days for a reasonably well-packed uncharged monolayer to form. The initially uncharged monolayer is atransient state. With time, a surface charge builds up, caused by additional adsorption or loose association of surfactant molecules. This adsorption mechanism casts some doubt on the stepwise mechanism of surfactant adsorption on solid surfaces. In contrast, Langmuir-Blodgett films gave deposited monolayers which did not change within the time period of the study. Because the surfactant did not adsorb to Langmuir-Blodgett monolayers, their charge did not vary with time during the adsorption process. Chen and Israelachvili [51] have shown that physisorbed monolayers are not rigid structures but are affected by the environmental conditions of temperature, vapor pressure, humidity, a contact with another surface, and any applied pressure. These variables may alter the composition, thickness, compressibility, and structure of themonolayers. Humidity and organic vapors affect fluorocarbon monolayers less than hydrocarbon monolayers. The adsorption of fluorinated surfactants on coal was investigated by Lai and Gray [52] by measuring the contact angles, the spreading coefficient, and the effect of the fluorinated surfactant on flotation. The fluorinated surfactants used were Zonyl FSN (nonionic), Zonyl FSC (cationic), Zonyl FSK (amphoteric), and Zonyl FSA (anionic). The contact angle of water on coal is about 38'42". The anionic fluorinated surfactant increased the contact angle to 80"-90". The other fluorinated surfactants were less effective. The adsorption of the fluorinated surfac-

188

Chapter 5

tants on coal increased in the order nonionic < cationic < amphoteric < anionic. Lai and Gray proposed an adsorption mechanism involving acid-to-acid dimerization. According to the mechanism, the carboxylic group of the anionic fluorinated surfactant Zonyl FSA combines with carboxylic acid groups on the surface of coal. Lai and Gray considered the acid-to-acid dimerization adsorption to be stronger than the ionic or hydrogen-bonding type of adsorption. As a result of strong adsorption, the anionic fluorinated surfactant is most effective in transferring coal fromthe aqueous phase into the air phase for froth flotation. Thestructure andconformation of lithium perfluorooctanesulfonate (LiFOS) adsorbed at a graphite-solution interface has been examined by atomic force microscopy [40]. (For a review of atomic force microscopy, see Ref. 53.) At a low surface coverage, the surfactant is adsorbed as individual molecules. At higher surfactant concentrations, the adsorbed surfactant forms aggregates, admicelles, or hemimicelles. The spacing of the aggregates is determined by a balance between repulsive electrostatic forces and attractive forces such as interfacial energy terms and van der Waals forces. For sodium dodecyl sulfate (SDS), an increase in SDS or salt concentration decreases the distance between the aggregates. For LiFOS, an increase in salt concentration also reduces the separation of the adsorbed aggregates. However, an increase in LiFOS concentration increases the size of the aggregates as well. The adsorption of fluorinated surfactants from aqueous solutions of dye particles depends on the structure of the fluorinated surfactant and the polarity of the dye surface. Hartmann [54] investigated the adsorption of the anionic surfactant C8F17S03N(C2H5)4 and nonionic surfactants C,F2,+ ,0(CH3CH20),,,H(171 = 6,9, 13; n = 7, 10) as single solutes by contact-angle and surface tension measurements. The anionic fluorinated surfactant was found to be a more effective dispersant than the nonionic ones. Hartmann observed that, in contrast to hydrocarbon surfactants, fluorinated surfactants are adsorbed more strongly on polar surfaces than on nonpolar surfaces and are, therefore, better dispersants than hydrocarbon-type surfactants for certain dyes. The interesting question of how fluorinated surfactants stabilize the dye dispersion remains to be answered. The adsorbed fluorinated surfactants increase the contact angle of water on dye particles. This suggests that the surfactant was adsorbed with its hydrophile segment oriented toward the dye surface and the fluorocarbon tail toward water. Hence, the adsorption of the fluorinated surfactant reduced the surface energy and wettability of the dye particles by water, in analogy to autophobic wetting. The phenomena observed by Hartmann et al. [54,55] differ from stabilization mechanisms of dye dispersions with hydrocarbon-type surfactants. Kissa [56,57,58] used a water-immiscible solvent, in which the dye but not the surfactant is soluble, as a surface probe for dye dispersions. Dispersed dye particles favor the water phase in a solvent-water two-phase system because the adsorbed

Solid-Liquid Interface

189

dispersant renders the dye particles more hydrophilic. Fluorinated surfactants function as dispersants for fluorocarbon polymers by adsorbing on the particles with the hydrophile oriented toward water. Clearly, the adsorption phenomena observed by Hartmann et al. for dyedispersions need to be elucidated and the mechanisms of stabilizing dispersions with fluorinated surfactants investigated. The simultaneous adsorption of poly(vinylpyrro1idone) (PVP) and an anionic hydrocarbon or fluorocarbon surfactant from their binary mixtures on polystyrene latex was studied by Otsuka et a1 [59]. The bare particles and the particles coated with PVP/surfactant were sized by sedimentation field flow fractionation (SFFF). The adsorption of PVP was enhanced by LiDS or LiFOS at low surfactant concentrations, but decreased at high surfactant concentrations. The conformation of adsorbed PVP changed from loops and trails to trains with the increasing surfactant concentration. However, in the PVP-LiFOS system, the fraction of train segments increased steeply at a lower surfactant concentration and was greater than that in the PVP-LIDS system. The thickness of the adsorbed layer was also determined by photon correlation spectroscopy after prefractionation by SFFF. The thickness of the adsorbed layer decreased with increasing surfactant concentration. (For the characterization of adsorbed surfactant layers, see Ref. 56, pp. 205-216). The adsorption of fluorinated surfactants at the electrode-solution boundary is of considerable practical interest for the application in electrochemical systems [60-64) (see Chapter 8). The electrochemical behavior of Zonyl FSN (nonionic), Zonyl FSD (cationic), Zonyl FSA (anionic), Fluorad FC-99 (anionic), and Fluorad FC- 135 (cationic) at Hg and Pt electrodes has been investigated by using cyclic voltammetry and interfacial differential capacitance measurements. When the electrode is relatively hydrophobic, such as Hg, and the surface charge density is relatively low, the fluorinated surfactants, as well as hydrocarbon surfactants, are adsorbed with their hydrophobic segments oriented toward the electrode. The interaction of fluorinated surfactants with the Hg electrode is weaker and the adsorbed layer is less compact than those of hydrocarbon surfactants. When the electrode is more hydrophilic, such as Pt, or the surface charge density is high, the surfactants adsorb with their hydrophilic end group toward the electrode surface. Although much progess has been made, more fundamental information on the adsorption of fluorinated surfactants on solid surfaces is needed. Because most commercial applications involve surfactant interactions at the solid-liquid boundary, the adsorption phenomena are of great practical interest. Fluorinated surfactants are usually mixtures of homologs and are frequently employed as mixtures with hydrocarbon surfactants. Therefore, a thorough understanding of adsorption mechanisms for surfactant mixtures on solid substrates is also very important (see Section 5.2).

Chapter 5

190

ADSORPTION OF MIXED SURFACTANTS

5.2

Adsorption of binary surfactant mixtures at a solid-solution interface has been exploited commercially in many applications. In spite of the widespread use of hydrocarbon surfactant-fluorinated surfactant mixtures, very few articles have been published on their adsorption at a solid-solution interface. The interaction of hydrocarbon and fluorocarbon surfactants on the surface of dispersed particles has been studied through a flocculation and redispersion process [65-671. Dispersions of positively charged particles can be flocculated with an anionic surfactant. An excess of the anionic surfactant forms a bilayer on the particle surface and causes redispersion of the flocculated sol. This flocculation reversal was used to study the interaction between mixed surfactants on a solid surface. A dispersion ofiron(TI1) oxide hydrate particles was flocculated with an anionic hydrocarbon or fluorocarbon surfactant at pH 3.5, where the sols had a positive zeta potential. Subsequently, a second fluorocarbon or hydrocarbon surfactant was added to the flocculated sol. The extent of redispersion depended on the interaction between the two surfactants on the solid particle surface. Changes in zeta potential and turbidity of iron(TI1) oxide hydrate sols flocculated by sodium dodecyl sulfate (SDS) are shown in Fig. 5.9. When SDS was

+20-

..

-0Q -A-

> E

SDS SDS+NF~OO SDS*NF7

0-

\

l

d

.-b CI

C Q,

CI

-20-

0. O

0 CI

Q,

N

-40t n1o - ~

1o-2

10"

1

Conc of surfactant/ mM FIG.5.9 The zeta potential and turbidity of a-Fe203sols flocculated by SDS as a function of surfactant concentration; closed symbols, zeta potential; open symbols, turbidity; arrow, turbidity of sols in the absence of surfactant. (From Ref. 65. Reproduced by permission of Elsevier Science Publishers.)

Solid-Liquid Interface

191

Conc of surfactant1 m M FIG.5.10 The zeta potential and turbidity of a-Fe20s sols flocculated by NFIOO as a function of surfactant concentration; closed symbols, zeta potential; open symbols, turbidity. (From Ref. 65. Reproduced by permission of Elsevier Science Publishers.)

added to the sols, the positive zeta potential of the sol became negative. The turbidity increased sharply when more than 8 X 10” mM SDS was added. The turbidity increase indicated that the sol flocculated by SDS was redispersed by SDS in excess of the optimum flocculation concentration of 8 X mM. When the iron(II1) oxide hydrate sols were flocculated with 8 X mA4 SDS and sodium 4-[Z”eptafluoroisopropyl- I ,3-bis(trifluoromethyl)butenoxy]benzenesulfonate (NF100) or 3-heptafluoroisopropyl-~,4-bis(trifluoromethyl)-5,8,11,14,17,20,23, 26-octoxa-heptaicos-3-ene (NF7) was added, the sols flocculated by SDS were redispersed. The turbidity of the sols increased. The zeta potential of the sols decreased when NFlOO was added. Addition of NF7. a nonionic surfactant, had very little effect on the zeta potential. Figure 5.10 shows changes in zeta potential and turbidity of iron(II1) oxide hydrate sols flocculated with NF100. The optimum flocculation concentration was about 3 X lop3 mM NFlOO. The sols were redispersed byNF7 or NP7.5, a hydrocarbon-type nonionic surfactant (polyoxyethylene nonylphenyl ether with a polyoxyethylene chain of average 7.5 EO). The turbidity increased sharply. The zeta potential changed only a little, as expected for a nonionic surfactant. Sols flocculated by NFlOO were not redispersed by SDS. The inability of SDS, an anionic hydrocarbon surfactant, to redisperse the sols was attributed

Chapter 5

192

to very low adsorption of SDS on the sols flocculated by NFl00, a fluorinated surfactant. Changes in zeta potential and turbidity of iron(II1) oxide hydrate sols flocculated with lithium perfluorooctanesulfonate (LiFOS) are shown in Fig. 5.11. The nonionic surfactants NF7 and NP7.5 redispersed the sols. However, the anionic hydrocarbon surfactant LiDS (lithium dodecyl sulfate) had no significant effect. Accordingly, sols flocculated by LiDS were redispersed by a nonionic surfactant, NF7, but not by the anionic surfactant LiFOS (Fig. 5.12). Esumi et al. [65-671 explained the flocculation and redispersion mechanisms by adsorption processes. Low concentrations of anionic surfactants neutralize the positive charge of iron(II1) oxide hydrate sols and cause flocculation. The adsorbed anionic surfactant is oriented with its hydrophilic groups toward the particle surface and the hydrophobic groups toward water. If the second addition of a surfactant results in adsorption on the flocculated sols, the sols redisperse. The adsorption is caused by hydrophobic interactions and the surfactant is oriented with its hydrophilic groups toward water.

0 LIFOS*NF 7 LlFOS* NP7 0 LIFOS*LiDS

*

= e3

)r

2 I-

Conc of surfactant/mM FIG.5.11 The zeta potential and turbidity of a-Fe203 sols flocculated by LiFOS as a function of surfactant concentration; closed symbols, zeta potential; open symbols, turbidity. (From Ref. 65. Reproduced by permission of Elsevier Science Publishers.)

Solid-Liquid Interface

+40t

193

-0 LIDS

I

Cone of surfactant/ mM FIG.5.12 The zeta potential and turbidity of a-Fe20s sols flocculated by LiDS as a function of surfactant concentration; closed symbols, zeta potential; open symbols, turbidity. (From Ref. 65. Reproduced by permission of Elsevier Science Publishers.)

The amounts of the surfactants adsorbed [67] indicate that the formation of mixed bilayers between anionic (hydrocarbon or fluorocarbon) surfactants and nonionic (hydrocarbon or fluorocarbon) surfactants is more favorable than the formation of mixed bilayers between anionic fluorocarbon and anionic hydrocarbon surfactants on ferric hydrosols. Esumi et a1 [68] used dispersions of a-alumina as well to study the interaction between anionic fluorocarbon and hydrocarbon surfactants. The anionic fluorocarbon surfactants used were LiFOS and NF100, the anionic hydrocarbon surfactants were SDS and LiDS, and the nonionic surfactant was NP7.5. Like the flocculation behavior of iron hydroxide, a low concentration of an anionic surfactant precipitates alumina. A further addition of a surfactant, different from the first one, forms mixed bilayers and redisperses alumina. Measurements of zeta potentials, the size of adlayers, and the amounts of adsorbed surfactants indicated that mixed bilayers consisting of anionic hydrocarbon-nonionic hydrocarbon surfactants or anionic fluorocarbon-nonionic hydrocarbon surfactants are formed preferentially to hydrocarbon-fluorocarbon surfactant bilayers. The adsorption of LiDS and LiFOS on alumina depends on the feed concentration and the LiFOSLiDS mole ratio [69]. Below a feed concentration of 5

194

Chapter 5

mmol/L, the total amounts of the surfactant adsorbed from the binary solution are larger than those from a single surfactant solution. Above a feed concentration of 6 mmol/L, the amounts adsorbed from the binary solution are smaller than the amounts adsorbed from single surfactant solutions. Esumi et al. [69] related the adsorptiondecreaseathighconcentrationstotheformation of amixed LiFOS-LIDS micelle in the bulk phase. In contrast to the LiDS-LiFOS system, the total adsorption of SDS and 4[2-heptafluoroisopropyl-1,3-bis(trifluoromethyl)-butenoxy]benzenesulfonate on alumina was lower than that of the individual surfactants adsorbed separately [70]. The decrease in adsorption suggests an unfavorable interaction between these surfactants in the adsorbed layer. Esumi et al. [71] arrived at a similar conclusion when studying the adsorption ofbinary mixtures of cationic surfactants, bis(2-hydroxyethyl) (2-hydroxy-3perfluorooctylpropy1)methylammonium chloride and hexadecylpyridinium chloride from their solution in water to negatively charged silica. The total adsorption of the mixture was found to be lower than that of single surfactants adsorbed separately, indicating a mutual phobicity between hydrocarbon and fluorocarbon chains. The simultaneous adsorption of LiDS, LiFOS and poly(N-vinyl-2-pyrrolidone (PVP) from PVD-LIDS and PVP-LiFOS mixed aqueous solutions has been investigated on hydrophilic and hydrophobic silica [72] and on alumina [73]. The adsorption of PVP on silica increases with increasing LiDS concentrations at low LiDS concentrations and decreases at higher LiDS concentrations after passing a maximum. The PVP-LiFOS system behaves similarly. In both systems, the conformation of the adsorbed PVP-surfactant layer is changed from loops and tails to trains with the increasing surfactant concentration. A similar trend was observed for the adsorption on alumina [73]. It is noteworthy and perhaps puzzling that very few fundamental articles have been published on the adsorption of mixed fluorocarbon and hydrocarbon surfactants at the liquid-solid boundary, in spite of its great practical importance. One reason for this apparent dormancy may be the proprietary nature of knowledge. The results of research conducted by industrial laboratories may be found in the patent literature (Chapter 8).

REFERENCES 1. J. A. Finch and G. W. Smith, Contact Angles and Wetting. in “Anionic Surfactants. Physical Chemistryof Surfactant Action.” E. H. Lucassen-Reynders,ed., Surfactant Science Series, Vol. 11, p. 317. Marcel Dekker, New York (1981). 2. A. Cahn and J. L. Lynn, Jr., in “Kirk-Othmer, Encyclopediaof Chemical Technology,” 3rd ed.. Vol. 22,p. 332 Wiley,New York (1983).

Solid-Liquid Interface

195

3. B. Tamamushi, in “Colloidal Surfactants,” by K. Shinoda, T. Nakagawa, B. Tamamushi, and T. Isemura, Physical Chemistry Vol. XII. p. 179, Academic Press, New York, 1963. 4. F. M. Fowkes, ed.. “Contact Angle, Wettability, and Adhesion,’‘ Advancesin Chemistry Series 43, American Chemical Society. Washington. DC(1963). 5. J. J. Kipling. “Adsorption from Solution of Nonelectrolytes,” Academic Press. London ( 1965). 6. E. Kissa. “Dispersions,” pp. 197-20 1 . Marcel Dekker. New York, 1999. 7. R. E. Johnson, Jr. and R. H. Dettre, in “Surface and Colloid Science.”E. Matijevic’, ed.. Vol. 2. p. 81, Wiley-Interscience, New York (1969). 8. D. H. Everett, in “Colloid Science.’’ D. H. Everett, ed.. Vol. 1, p. 49, The Chemical Society, London (1973). 2. p. 52. The Chemical So9. C. E. Brown and D. H. Everett, in “Colloid Science,’‘ Vol. ciety, London (1975). 10. A. W. Adamson, “Physical Chemistry of Surfaces,”ed.. 5thWiley, New York( 1 990). 11. K. I. Mittal, “Adsorption at Interfaces,” ACS Symposiunl Series No. 8, American Chemical Society, Washington, DC (1975). 12. C. H. Giles. Adsorptionat Solid/Liquid Interfaces, in “Anionic Surfactants. Physical Chemistry of Surfactant Action,” E. H. Lucassen-Reynders, ed.. Surfactant Science Series, Vol. 11, p. 143, Marcel Dekker, New York(1981). 13. D. Attwood and A.T. Florence. “Surfactant Systems.’’p. 20, Chapman & Hall, London (1 983). 14. G. D. Parfitt and C. H. Rochester, in “Adsorption from Solution at the SolidLiquid Interface.’’ G. D. Parfitt, ed.. Academic Press, New York (1983). 15. R. Aveyard. in “Surfactants,” Th. F. Tadros, ed., p. 153, Academic Press, London ( 1984). 16. D. K. Chattoray andK. S. Birdi. “Adsorption and the Gibbs Surface Excess,’’ Plenum, New York ( 1 984). 17. W. von Rybinski and M. J. Schwuger, in “Nonionic Surfactants. Physical Chemistry,” M. Schick, ed.. Surfactant Science Series Vol. 23,p. 45, Marcel Dekker. New York. 1987. 18. D. Myers, “Surfactant Science and Technology,” VCH,New York ( 1 988). 19. M. J. Rosen. “Surfactants and Interfacial Phenomena,” 2nd ed., Wiley. New York ( 1989). D. N. 20. B. T. Ingram andR. H. Ottewill, in “Cationic Surfactants. Physical Chemistry.” Rubingh and P. M. Holland. eds.. Surfactant Science Series Vol. 37, p. 87. Marcel Dekker. New York (1991j. 21. G. H. Giles, T. H. Mac Ewan. S. N. Nakhwa, and D. Smith. J. Chem. SOC.3973 (1 960). 22. G. H. Giles, D. Smith, and A. Huitson.J. Colloid Interf. Sci. 47, 755 (1974). 23. W. A. Zisman, in “Contact Angle, Wettability and Adhesion”, R. F. Gould, ed.. Advances in Chemistry Series Vol. 43, p. 1, American Chemical Society, Washington, DC (1964). 24. H. W. Fox and W. A. Zisman,J. Colloid Sci. 7, 109 (1956). 25. F. Schulman and W. A. Zisman, J. Colloid Sci. 7,465 (1952). 26. E. F. Hare, E. G. Shafrin, and W. A. Zisman, J. Phys. Chem. 58,236 (1954).

196 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64.

Chapter 5 E. G. Shafrin and W. A. Zisman. J. Phys. Chem. 64,519 (1960). C. 0. Timmons and W. A. Zisman, J. Phys. Chem. 59, 984 (1965). M. K. Bernett and W. A. Zisman, J. Phys. Chem. 7 1, 2075 (1967). E. G. Shafrin and W. A. Zisman, J. Phys. Chem. 66,740 (1962). R. L. Cottington. E. G. Shafrin. and W. A. Zisman. J. Phys. Chem. 62,5 13 (1958). M. K. Bernett and W. A. Zisman. J. Phys. Chem. 63, 1241 (1959). M. K. Bernett and W. A. Zisman. J. Phys. Chem. 63, 1911 (1959). R. A. Pyter, G. Zografi, and P. Mukerjee, J. Colloid Interf. Sci. 89, 144 (1982). F. M. Fowkes and W. D. Harkins, J. Am. Chem. SOC. 62. 3377 (1940). E. H. Lucassen-Reynders. J. Phys. Chem. 67,969 (1963). A. El-Shimi and E. D. Goddard. J. Colloid Interf. Sci. 48.242 (1974). W. J. Murphy, M. W. Roberts, and J. R. H. Ross, J. Chetn. SOC.Faraday Trans. I 68. 1190 (1972). F. G. Tournilhac, L. Bosio, J. P. Bourgoin, and M. Vandevyver, J. Phys. Chem. 98, 4870 ( 1994). R. Lamont and W. Ducker,J. Colloid Interf. Sci. 191, 303 (1997). L. J.Gerenser, J. M. Pochan. M. G. Mason, and J. J.Elman, Langmuir 1,305 (1985). L. J.Gerenser, J. M.Pochan, J. F. Elman, and M.G. Mason. Langmuir 2,765 (1986). M. Mitsuya, Langmuir 10, 1635 (1994). G. N. Batts, Colloids Surf. 22, 133 (1987). G. N. Batts and A. J. Paul. Langmuir 10, 218 (1994). C.-L. Lai, J. H. Harwell, E. A. O’Rear. S. Komatsuzaki, J. Arai, T. Nakakawaji, and Y. Ito, Colloids Surf. A 104, 231 (1995). C.-L. Lai, E.A. O’Rear, and J. H. Harwell, J. Colloid Interf. Sci. 183. 166 (1996). E. J. Clayfield, A. G. Dixon, A. W. Foulds. and R. J. L. Miller. J. Colloid Interf. Sci. 104.500 (1985). M. Claesson and H. K. Christenson. J. Phys. Chem. 92, 1650 (1988). H. K. Christenson, P. M. Claesson. J. Berg, andP. C. Herder, J. Phys. Chem. 93, 1472 (1989). Y.-L. Chen and J. N. Tsraelachvili. J. Phys. Chem. 96,7752 (1992). R. W.Lai and H. L. Gray, in “Interfacial Phenomena in Biotechnology and Materials Processing,” Y. A. Attia, B. M. Moudgil, and S. Chander, eds.. p. 293, Elsevier Science, Amsterdam (1 988). E. Kissa, “Dispersions,” pp. 41 9-426, Marcel Dekker, New York ( I 999). G. Hartmann, Colloids Surf. 57.205 (1991). G. Hartmann. H. J. Konig. and D. Prescher, Textiltechnik 36, 541( 1 986). E. Kissa, “Dispersions,” pp. 262-274, Marcel Dekker. New York (1999). E. Kissa. Langmuir 6, 478 (1990). E. Kissa. Langmuir 6, 12 17 ( 1990). H. Otsuka, T. A. Ring, J.-T. Li. K. D. Caldwell. and K. Esumi, J. Phys. Chem. 103, 7665 (1999). G. Juhel. B. Boden, C. Lamy. and J. M. Leger, Electrochim. Acta 35,479 (1990). C. Cachet. Z. Chami. and R. Wiart. Electrochim. Acta32,465 (1987). C. Cachet. B. Saidani. and R. Wiart, J. Electrochem. SOC.138, 678 (1991). C. S. Cha and Y. B. Zu, Russ. J. Electrochem. 31,796 (1995). C. S. Cha and Y. B. Zu, Langmuir 14.6280 (1998).

ace

Solid-Liquid

197

65. K. Esumi, Y. Ono, M. Ishizuka, and K. Meguro. Colloids Surf. 32, 139 (1988). 66. K. Esumi, Y. Sakamoto, K. Yoshikawa, and K. Meguro, Bull. Chem. SOC.Japan 61. 1475 (1988). 67. K. Esumi, Y. Sakamoto, K. Yoshikawa. and K. Meguro, Colloids Surf. 36. 1 (1989). 68. K. Esumi, Y. Sakamoto, and K. Meguro, Colloid Polym. Sci. 267, 525 (1989). 69. K. Esumi, H. Otsuka. and K. Meguro, Langmuir 7.23 13 (1991). 70. K. Esumi. H. Otsuka, and K. Meguro, J. Colloid Interf. Sci. 142.582 (1991). 71. K. Esumi, Y. Tokui, T. Nagahama. and K. Meguro. J. Colloid Tnterf. Sci. 146, 3 13 (1991 ). 72. H. Otsuka. K. Esumi, T. A., Ring, J.-T. Li. and K. D. Caldwell, Colloids Surf. A 1 16, 161 (1996). 73. H. Otsuka and K. Esumi. Langmuir 10,45 (1994).

6 Fluorinated Surfactants in Solution

6.1

SOLUBILITY

The solubilities of fluorinated surfactants are related to the unusual properties of the fluorine atom and the C-F bond. Fluorine is the most electronegative element and is very difficult to polarize. Fluorine can form avery stable bond with hydrogen or carbon (see Section 3.1). The rigidity of the C-F bond causes stiffening of the perfluoroalkane chain and limits interactions with other molecules. Because of their small size, fluorine atoms can shield the perfluorinated carbon atom without steric stresses. Perfluoromethyl or perfluoromethylene groups therefore form compounds with very weak intermolecular forces. As a consequence of weak interactions, perfluoroalkanes are insoluble in common organic solvents. Perfluoroalkanes are more hydrophobic than hydrocarbons, evidenced by solubility data: CF4 is seven times less soluble in water than CH4 [ 1,2]. Water is almost 7 times less soluble in perfluoroheptane than in heptane on a equal weight basis [3] and 25 times more on a molar basis. The solubility of fluorinated surfactants depends on the hydrophile of the surfactant, in addition to the structure of the fluorinated group. The solubility of yerfluoronlknnoic acids in water, like the solubility of nonfluorinated alkanoic acids, decreases with increasing chain length. At 25"C, perfluorohexanoic acid and shorter-chain perfluoroalkanoic acids are miscible with water in all proportions. However, perfluorooctanoic acid and perfluorodecanoic acid are only slightly soluble in water [4]. The solubility of alkali metal salts of per-uorirznted alkanoic acids in water decreases with increasing chain length. Some metal salts of perfluoroacetic and perfluorobutyric acids are soluble in water and in organic solvents as well 198

Fluorinated Surfactants in Solution

199

(Table 6.1) [ 5 ] .Alkali metal salts of perfluorocarboxylic acids having a chain length of four or more carbon atoms form micellar solutions. Their solubility in water can be characterized by their Krafft point (see Section 6.3) and critical micelle concentration (see Sections 6.2 and 6.6). acids having one to six carbon atoms are colThe yel~'uoi-ocrlkalzesuIfor2ic orless oils which fume in moist air and form monohydrates [6]. The C2 and C3 acids are miscible with water in all proportions. The solubility of perfluoroalkanesulfonic acids in water decreases with incre,asingchain length and the C7 and Cs acids are only moderately soluble in water. Their surface activity increases with decreasing solubility in water. The solubilities of the salts of yeI~uoroaZkclnesulfonicacids depend on the nature of the counterion. The sodium and potassium salts of C l , C2, and C3 perfluoroalkanesulfonic acids are soluble in water, but their solubilities decrease with increasing chain length [6]. The tetraethylammonium salt of perfluorooctanesulfonic acid is more soluble in water than the potassium salt and is soluble in organic solvents such as alcohols, dimethyl formamide (DMF), or dimethyl sulfoxide (DMSO) as well [7]. The solution behavior of pentaerythritol tetraperfluorobutyrate reveals an extremely weak attraction between fluorocarbon and hydrocarbon groups [8,9]. Critical compositions are unsymmetrical and in agreement with values calculated for regular solutions using ideal entropy instead of Flory-Huggins entropy. The solubilities of some commercial fluorinated surfactants are shown in Table 6.2 [ 101. Nonionic fluorinated surfactants of the structures shown in Table

TABLE 6.1 Solubilities of Perfluoroalkanoates Solubility (g/lOO mL) ~~~~

~~

____

Petroleum Salt Perfluoroacetates Sodium Mercury Aluminum Barium Nickel Perfluorobutyrates Sodium Lithium Calcium Lead Silver

Source: Ref 5.

Water

>IO >IO >IO >IO 4-1 0 >10 >lo >IO >IO >IO

Methanol Acetone ether Benzene Ether

4-1 0

<2

>10

>IO

>10 >IO >IO

<2 <2

4-1 0

>10

<2

>IO >IO >IO >lo

4-1 0 <2

4-1 0 >10

<2

4-1 0 <2 <2 <2 <2 <2 <2 <2

>10

4-1 0 4-1 0 4-1 0 >IO 4-1 0

N

0 0

TABLE6.2 Solubility of Zonyl Fluorinated Surfactants Zonyl

Stability, 25°C (77°F) (“S’= stable, “I” = insoluble) 10% Potassium hydroxide 25% Sulfuric acid 37% Hydrochloric acid 70% Nitric acid Solubility, 25°C (77°F) (g solids/l00 g solution) Water lsopropyl alcohol 1 : 1 Water isopropyl alcohol Methyl alcohol Acetone Ethyl acetate Toluene n- Heptane

FSA

FSP

FSE

URa

FSJ

FSN

FSN-100

FSO

FSO-100

FSC

FSK

TBS

S

S I I I

S

S I I

S I I I

S S S S

5 5 5 5

5 5 5 5

5 5 5 5

5 5 5 5

s

I I I

5 5 5

PSb 5

>2

>2

>2 0 >2

>2 >2 >2 >2 >2 >2

>2

0.1

0.1

>2 >2

0.7

0 0

>2 >2 0.1 0 0

0

0

0

I I I

0.5 0 0 0 0

I 2 2 2 >2 2 2 0 0

>2 0.4

>2 >2 -0.1 -0.1 0 0

0

>2 >2 >2 >2 >2 >2

0

0

0.1

>2 >2

>2 >2 >2 0 0

>2

>2

>2 >2 >2

>2 >2

>2

>2

>2

>2

1 0 0 0

>2 >2 0

>2

1 >2 0 0

IC 5

0.1 0.1 0.1 0.1 0 0 0 0

201

Chapter 6

202

6.2 are soluble in several organic solvents, including acetone, ethyl acetate, toluene, tetrahydrofuran, and methyl chloroform. The amphoteric surfactant Zonyl FSK is soluble in acetone and ethyl acetate but only very slightly soluble in tetrahydrofuran. The surfactant Zonyl TBS, featuring a sulfonate hydrophile, is insoluble in organic solvents other than alcohols, in which it is slightly soluble. All other surfactants listed in Table 6.2 are soluble in isopropyl alcohol and in methanol. None of the fluorinated surfactants listed in Table 6.2 are soluble in aliphatic hydrocarbons. 6.2

MICELLEFORMATION

The solution behavior of surfactants is a result of their amphiphilic nature. Surfactants have lyophobic and lyophilic groups (the term lyophilic denotes a solvent compatible species; the term lyophobic indicates the opposite). Surfactants used in water have one or several hydrophilic head groups compatible with water and one or several hydrophobic tails, which repel water. The surfactant therefore orients itself at the water-air interface with its hydrophilic part directed toward water and the hydrophobic tail pointing toward air. In water, surfactant molecules associate to form micelles or aggregates. The hydrophobic tails of the surfactant molecules form the interior of the micelle and the hydrophilic head groups are exposed to water (Fig. 6.1 ). In organic solvents, the orientation of surfactant molecules is reversed. The hydrophilic groups are lyophobic and form the interior of the aggregate, whereas the lyophilic segments are oriented toward the solvent. The term rnicelle describes an equilibrium aggregate of colloidal dimensions [ 1 1-1 31. Franses et al. [14] have suggested that the term micelle stands for stable, disjoint, cooperative, closed, equilibrium colloidal aggregates. The term stable means constancy of the overall macroscopic properties of the aggregate population, although individual aggregates are not permanent. By disjoint is meant that the extent of aggregates is limited in all three dimensions and the aggregates remain clearly identifiable even when closely packed together. The term coopemtive refers to the association pattern, the dependence of the free energy of aggregation on the aggregate size. The term eqrdibriurn states that the aggregates form spontaneously and reversibly and are limited in size. The definition requires that a closed surface can be visualized to divide the hydrophilic and hydrophobic moieties. This condition is not always met and should be dropped from the otherwise precise definition. Micelle formation or micellization occurs at a narrow surfactant concentration range, called the critical micelle concentration (cmc). Below the cmc, the surfactant dissolves in the molecular state. At cmc, monomeric surfactant molecules associate to form micelles and the solubility of the surfactant increases abruptly. The physical properties of the surfactant solution, such as surface tension, electric

Fluorinated Surfactantsin Solution

SPHERICALMICELLES

203

CYLINDRICALMICELLES

FIG.6.1 Idealized structures that may exist in solutions of surfactants. (From Ref. 223. Reproduced by permission of Academic Press.)

conductivity, osmotic pressure, and light scattering, also exhibit a sharp discontinuity at the cmc (Fig. 6.3). Micelles are in a dynamic state [ 15,161. Surfactants in a micelle are mobile. Above the cmc, the molecularly dissolved surfactant molecules are in a dynamic equilibrium with the associated surfactant molecules in a micelle. A surfactant molecule may leave one micelle and adjoin another micelle. Micelle formation has been explained by several theories which regard the micelle as either a chemical species or a separate phase. The simplest to understand and probably the most adequate is the mass action model [17-291, which regards the micelle as a chemical species. The mass action model is based on association of monomeric surfactant molecules in dynamic equilibrium with the micelle: rzA

A,,

where IZ is the number of surfactant molecules in the micelle and K,,, is the association constant. The stepwise association theory, developed by Aniansson and co-workers [30-361, describes micellization as a stepwise process involving the association

"."-."7"""-C"----rr

"

l

"

"

"

.

Chapter 6

204

cmc

I I

Turbidity

~

~~

Surfactant conc FIG.6.2 tration.

Changes of physical properties occurring at the critical micelle concen-

and dissociation of a surfactant monomer with a micelle. In a stepwise process, one monomer at a time enters or leaves the micelles,

A

+ A,-I

A,,l n

=

2,3,4,. . .

(2)

where A,, is a micelle with the aggregation number 12. Because the association of the monomeric surfactant molecules occurs stepwise, the mass action model requires an association constant, K,,,, for every association step. Because of experimental limitations, numerical values for each association constant cannot be determined and have to be assumed [26,37,38]. Usually, as an approximation, a micellar solution is described with one K,,, value as if the solution were monodisperse. Burchfield and Woolley [39,40] have described a one-step mass action model that includes only one micellar species, with the assumption that the surfactant is a strong 1 : 1 electrolyte at infinite dilution. A surfactant solution having a concentration above cmc is considered a mixed electrolyte. The mass action equilibrium is given by

(nP)M+ + HAe M,,pA,'*'"p)

(3)

Solution

in

Surfactants Fluorinated

205

where rz is the surfactant aggregation number and p is the fraction of surfactant counterions associated with the micelle. The surfactant is represented as [M+,A-] and the micellar aggregate as [M,,pA~ncl-p)]. Equation (3) is applicable to cationic surfactants by reversing the charge signs of the surfactant and its counterion. Other parameters of the model are the screening factor 6 for the micellar charge and the Guggenheim ion interaction parameters for counterion-monomer and counterion-micelle interactions, B 1 and B2. The two-phase (phase separation) model [24,4143] regards the micelle as a separate phase, albeit a small entity of microscopic dimensions. The cmc is considered to correspond to the maximum solubility of the monomeric surfactant. If the saturation concentration is exceeded, a new phase, the micelle, appears. The micelle is thermodynamically stable and reversible. The phase-separation model assumes that the activity of the surfactant molecule [44-511 and/or the surface tension [50,52] of the surfactant solution remains constant above its cmc. This assumption is not correct, however [29]. Furthermore, the phase-separation model is not consistent with the number of degrees of freedom given by the Gibbs phase rule [29]. In spite of these difficulties, the two-phase model has explained solubilization and mixed-micelle formation reasonably well and is therefore widely used. Other theories have emerged, such as the charged phase-separation model for ionic surfactants [50] and the theory based on thermodynamics of small systems [53-571. The debate on tnicellization theories is still ongoing. Solutions of fluorinated surfactants have been investigated and their micellar nature has been confirmed [58,59]. The substitution of the larger and highly electronegative fluorine atom for the smaller hydrogen increases the amphiphilic nature of the surfactant and lowers the surface tension and cmc. The alkali and ammonium salts of perfluoroalkanoic acids exhibit surfactant properties and form micelles for a chain length of four carbon atoms, whereas eight carbon atoms are needed for the nonfluorinated alkanoates. The size and the structure of micelles formed by fluorinated surfactants are discussed in Chapter 7. The interior of typical micelles appears to have a liquidlike structure. Therefore, micellar solutions have solvent characteristics not exhibited by molecular solutions. Aqueous micellar solutions can dissolve water-insoluble substances by incorporating their molecules in or on the micelle (see Section 6.6). The formation and dissociation of micelles is a very rapid process, usually occurring in a fraction (10-2-10-9) of a second. Because the rate constants of such extremely rapid processes cannot be measured by conventional techniques, kinetic studies of micellization began when relaxation methods had been developed [36]. Kinetics of fluorinated surfactant micellization has been investigated by nuclear magnetic resonance (NMR) relaxation, chemical relaxation, electric birefringence, and luminescence probing methods (see Chapter 9) as well as by stopped-flow small-angle x-ray scattering [60].

Chapter 6

206

Sams et al. [61] proposed a two-state kinetic model which assumed a monomeric state and an associated state consisting of aggregates in various sizes larger than the monomer. The model describes only the fast process and assumes that the rate constants for association and dissociation are independent of the micelle size. A revised version of the two-state model [62,63] assunled micelle formation to be an adsorption phenomenon, with micelles at equilibrium with monomers adsorbing and escaping from the surface of micelles. A more detailed kinetic model utilized in studies of micellization kinetics is based on the theory by Aniansson and Wall [30-321 and modified by Kahlweit and co-workers [64-661. The theoretical model developed by Aniansson and Wall [30,31] assumes a stepwise aggregation of surfactant monomers to form micelles [36]. When a micellar solution at equilibrium is perturbed by an instant change of temperature or pressure, the size distribution of micellar aggregates is shifted. The reestablishment of the equilibrium is characterized by two relaxation times, representing a fast process and a slow process. The fast process involves an exchange of monomeric surfactants between the micelles and the intermicellar solution. A monomer or several monomers dissociatefromorassociate to existingmicelles. As aresult,themicellar distribution curve shifts without a change in the number of micelles [61.62,67, 681. In the slow process, the number of micelles changes as a result of micelle formation from monomers or a complete breakdown of micelles to monomers. The concentration of micelles relaxes to a new equilibrium value. The aggregation number does not change, however. The theory formulated by Aniansson and Wall and its modifications by Kahlweit and co-workers [34,64-661 have been the basis for most interpretations of chemical relaxation times and provided valuable information on kinetics of micellization. The relaxation time constant, q for the rapid relaxation process is given by the expression [30,31) -I 71

-

-

o2 +

k-a( 1 + CO) n

where k- is the rate constant for the dissociation of the surfactant monomer from the micelle, &? is the variance of the Gaussian distribution curve of micellar sizes, and cOis the average deviation from equilibrium (usually less than 1%). The factor CI is given by a = - A,,,

A1

and A,,,

= A,,, - A ,

where A,,, is total surfactant concentration and A I is the monomer concentration.

Solution

in

Surfactants Fluorinated

207

As an approximation, A I can be equated with cnlc and co can be neglected:

Alternatively, the Aniansson and Wall equation can be written as

where k+ is the association constant. The relaxation time of the slow process r2 is given by

7-2' =

k-c,n' A + (a'l/r~)(A,,,- cmc)

where ciis the micelle concentration at the micellar distribution curve minimum. Rassing et al. [62] studied kinetics of sodium perfluorooctanoate micellar systems using the ultrasonic relaxation method. They observed a fast relaxation process attributed to a micelle formation, The ultrasonic relaxation times revealed that periodic fluctuations in temperature and pressure caused by the acoustic wave are several magnitudes less than the temperature or pressure perturbations of jump techniques. Rassing et al. [62] suggested that the ultrasonic and jump methods measure different modes of micelle formation whose relaxation times differ by several orders of magnitude. Ultrasonic absorption techniques [69-711 have also been used to measure relaxation spectra of sodium perfluorooctanoate and cesium perfluorooctanoate [72,73]. The kinetics of micellization of perfluorinated surfactants has been investigated by Hoffmann and co-workers [74-801 by pressure jump and a shock wave method with conductivity detection [74-801. Hoffmann and Ulbricht [75] also used a temperature jump relaxation technique [81] with optical detection, utilizing a pH indicator (thymol blue) to observe relaxation processes of a 1 : 1 mixture of perfluorooctanoic acid and its sodium salt. For micellar systems in which fast relaxation times could be measured, the parameters k - h , k - l a ? , a2/n,and k+ln were calculated. The exchange of surfactantmonomersbetweenthemicellar and the monomeric state is diffusion controlled for monomers with hydrophilic counteri011s. The exchange rate is slower for monomers with hydrophobic counterions. For lithium and substituted ammonium salts of perfluorooctanesulfonic acid [76], the values of the micellar distribution curve, a'ln, increased with increasing hydrophobicity of the counterion or with a decrease of temperature. Both the association rate constant, k+ln, and dissociation rate constant, k-ln, decreased with increasing counterion hydrophobicity. The increase in k-ln values was larger than the decrease in cmc values, suggesting that hydrophobic counterions form a barrier to the monomer exchange process.

Chapter 6

208

A log-log plot of x"/n versus cmc is linear, with the exception of the point representing the surfactant with a C3H7NHTcounterion (Fig. 6.3) [76]. The system with a C3H7NHl counterion also has a different temperature dependence, shown in Fig. 6.4 [76] with an Arrhenius plot. The temperature dependence of the rate constant is given by

II-I1 =

(t) -+) enp(

where D is the diffusion coefficient of the monomer, 1 is the length of a CH2group, and E is the energy of monomer transfer from the micelle interior to the outside of the micelle. The energy of momomer transfer calculated from Eq. (9) includes the heat of activation of the diffusion process plus the heat oftransfer of the monomer from the micellar interior to the transition state. Both values appear to be lower than the corresponding values for hydrocarbon surfactants. The reciprocal relaxation time of the fast process increases linearly with increasing surfactant concentration (Fig. 6.5) [75,77], in accord with Eq. (6).

5.0

1

-

1.5

0

UI U c5

3 -*9 40 3.5 -

- 3.5

I . "

-

-3.0 -2.5 log CMC(mMol/dm')

-2.0

FIG.6.3 Plot of k-ln versus log cmc for perfluorooctanesulfonates at 35°C. (From Ref. 76. Reproduced by permission of Akademische Verlagsgesellshaft.)

. .

"

I

I

I

FIG.6.4 Plot of k-lnversus l/Tfor perfluorooctanesulfonates.(From Ref. 76. Reproduced by permission of Akademische Verlagsgesellshaft.)

20.1 0 4

C,F,,COO-

+N(CH,),

15

c 0

a,

0

:10 v

z r

5 0 4

7

10

13.10 3

C, (mol/L) FIG.6.5 Reciprocal values for the short relaxation time T~ plotted against the total concentration co for C8F~7COON(CH3)4 at different temperatures. (From Ref. 77. Reproduced by permission of Akademische Verlagsgesellshaft.) 209

Chapter 6

210

Relaxation measurements yield information on the size and shape of micelles. Hoffnlann et al. [77] observed that below a certain temperature, the amplitudes of the two relaxation processes decreased rapidly and amplitudes of new relaxation processes appeared. The new processes were attributed to the relaxation effects of another type of micelle, which appeared to be emulsion dropletlike giant molecules. The residence time of the surfactant molecules in the new micelle was unusually long, explained by the incorporation of ion pairs, formed by the surfactant and its counterion. in the micelle. The existence of giant micelles has been disputed by Fontell and Lindman [82]. Subsequent studies by Hoffnlann et al. have indicated that the giant aggregates are probably dispersions of liquid crystalline mesophases (see Section 7.1). Hydrocarbon-derived surfactants with a fluorine-containing counterion do not belong strictly to the fluorinated surfactant class but, nevertheless, have some unusual properties. Hoffmann et al. [78,80] investigated micelle formation kinetics in solutions of cationic surfactants having short-chain perfluoroalkanoate counterions. The surfactants consisted of a docedylammonium, tetradecylammonium, dodecylpyridinium, or tetradecylpyridinium cation and a perfluoroacetate, perfluoropropionate, or perfluorobutyrate counterion. Pressure-jump and shock-wave techniques revealed two relaxation processes in accord with the Aniansson-Wall theory. The residence time of a monomer in the micelle. the association and dissociation rate constants, the mean aggregation number of the micelles, the width of the micelle size distribution curve, and the concentration of the micellar nuclei were determined. The kinetic data suggested that the short-chain perfluorinated anions, CF3COO- and C2F5COO-, are incorporated into the micelle containing dodecylpyridinium or tetradecylpyridinium cations but the longer-chain perfluorinated anions, C3F7COO-, are located at the micelle surface [78]. The perfluoroacetate and perfluoropropionate ions facilitate the exchange of surfactant monomers between the micelle and the intermicellar solution. In contrast, the perfluorobutyrate counterions retard the surfactant monomer exchange. When the chain length of the fluorinated counterion is increased further, the counterion assumes surfactant characteristics in its own right. The cationic surfactant then becomes a mixed surfactant consisting of a fluorinated surfactant anion and a hydrocarbon surfactant cation (see Section 7.2). 6.3

KRAFFTPOINT

The solubility of surfactants in water increases gradually with increasing temperature until, at the Krafft point, the solubility increases abruptly [83,84].The Krafft point is the temperature at which the solubility of monomeric surfactant molecules is equal to the cmc at that temperature. The Krafft point can also be defined as the temperature at which the solubility versus temperature curve intersects the cmc

211

Fluorinated Surfactants in Solution

versus temperature curve [29,85]. Below the Krafft point, the surfactant solution is molecular and the solubility of the surfactant is low. In a saturated solution, the monomeric surfactant molecules are in equilibrium with a crystalline solid phase. Above the Krafft point, the surfactant forms micelles, and the crystalline solid phase, if present, is in equilibrium with the liquid phase containing micelles and monomeric molecules of the dissolved surfactant (Fig. 6.6) [86]. Shinoda and Hutchinson treated the micelle as a separate phase [41,45.87] and proposed that the Krafft point is the temperature above which the solid hydrated surfactant melts and dissolves as micelles in water [88,89]. In a phase diagram ofan aqueous surfactant, the Krafft point is the “triple point” at which monomolecular surfactant coexists with micelles and the solid surfactant [42]. At the Krafft point, micelles are in equilibrium with monomeric surfactant molecules

30

Solublll t y curve

-

20

Molecular solutlon +

cn 0 cn

I

crystals

u

Micellar

10

solutlon

i” 0

10

20

FIG. 6.6 Phase diagram of the sodium dodecyl sulfate-water system near the Krafft point TK.(From Ref. 86. Reproduced by permission of Pergamon Press.)

Chapter 6

212

[88,90-921. If micelles are treated as a separate phase, the equilibrium is univariant because the phase rule allows only one degree of freedom and the equilibrium Hydrated solid e Monomers

Micelles

(10)

is fixed at a given pressure. Moroi at al. [39] and La Mesa and Coppola [93] have argued that this assumption is incorrect because three phases coexist all along the Krafft line (Fig. 6.7) [93]. Experimental evidence that the cmc changes with pressure [94,95] also contradicts the assumption that the Krafft point is invariable. The mass action model [Eqs. (1) and (2)] gives two degrees of freedom at the Krafft point, because a solution phase is in equilibrium with a solid [93]. The number of components is three: water, molecularly dissolved surfactant, and micelles:

f=c-y+2-r

(1 1)

where f is the degrees of freedom, c is the number of conlponents (water, monomeric surfactant, and micelle), y is the number of phases (surfactant solution phase and surfactant solid phase), and I- is the number of equilibrium reactions (one) [39]. The two degrees of freedom are in accord with the evidence that solubility of a surfactant in water is defined by temperature and pressure. However, this conclusion based on the mass action model cannot be strictly correct either because an ionic micelle is surrounded by counterions. The condition of electroneutrality would reduce the degrees of freedom to one. Moroi has suggested that the electroneutrality problem can be overcome if the solubility trend around the Krafft point is known. The overall solubility above the Krafft point is then given by

where [ST] is the analytical concentration of micelles at temperature T, [SI is the solubility of the monomer, [SJ and [SIk are the analytical concentrations of micelles and monomeric concentrations at the Krafft point, respectively, and 112 is the aggregation number. Equation (12) states that the overall solubility above the Krafft point is related to the formation of micelles and depends on micelle aggregation numbers. In spite of some complications, the mass action model serves as a useful approximation for describing micellization processes. The Krafft points of perfluoroalkanoic and perfluoroalkanesulfonic acids are given in Table 6.3 [59,96]. The Krafft points of carboxylates are generally lower than those of sulfonates. In a homologous series, Krafft points increase approximately with increasing order of the melting points. Increasing the carbon chain length elevates the Krafft point of a fluorinated surfactant, in analogy to that of a hydrocarbon-derived surfactant. The incremental increase of Krafft points for

Fluorinated Surfactants in Solution

213

0

6C -

e

0

c 0

e

A

20

FIG.6.7 Partial phase diagram of the water-NaPFN system, obtained by using electrical conductivity (A),turbidity (m), and density data (0).Critical micelle concentrations at different temperatures ( 0 )were obtained from electrical conductance or surface tension data. (*) Krafft point. (From Ref. 93. Reproduced by permission of Elsevier Science Publishers.)

an increase of the chain length by one carbon atom is much larger than that for the corresponding hydrocarbon-derived surfactant. Branching of the carbon chain lowers the Krafft point [59]. A moderately branched surfactant has a lower Krafft point and melting point than a straightchain surfactant (Table 6.4).

TABLE 6.3

Krafft Points of Fluorinated Surfactants

Compound

Krafft point ("C)


Source: Refs. 59 and 96. 214

215

Fluorinated Surfactants in Solution

TABLE 6.4 Melting Points and Krafft Points of Normal-Chain and Branched-Chain Perfluorooctanoic Acids ~

Surfactant II-C~F~~COOH (CF3)2CF(CF2)4COOH

Melting point ("C)

Krafft point ("C)

56.4-57.9 13-1 4


20

Source: Ref. 59. (Reproduced by permission of the American Chemical Society.)

Krafft points depend on the counterion of perfluorosulfonates (Fig. 6.8) and perfluoroalkanoates (Fig. 6.9) [59]. Krafft points are affected by several factors, including the nature of the bond between the surfactant anion and the counterion, the structure of the hydrated solid, and the degree of hydration. Shinoda and coworkers [59] have shown that the incremental increase of the Krafft point corresponding to an increase of the carbon chain by one carbon atom depends on the hydration of the counterion. The more hydrated the counterion is, the steeper the slope. For univalent counterions, the slope decreases in the order H > Li > Na > K == NHJ Rb. La Mesa [97] measured the temperature dependence of cmc for ionic surfactants including Li perfluorononanoate. Minima of cmc values indicated that metastable micellization processes function below the Krafft point. The Krafft point depends on pressure and increases with increasing critical solution temperature (see Section 6.6). The Krafft point is affected by other ingredients present in the surfactant solution. Another surfactant added to the solution of a surfactant depresses the Krafft point [59]. Shinoda et al. determined the Krafft points of binary mixtures of perfluorosulfonates (Fig. 6.10) and perfluoroalkanoates (Fig. 6.1 1) from solubility data. They observed that depression of the Krafft point increases with decreasing Krafft point of the added surfactant. Shinoda and co-workers found the change in the Krafft point to be analogous to the melting-point depression in accord with Raoult's law. The Krafft point is also depressed by alcohols. Small amounts of hexanol, heptanol, and octan01 increase the solubility of dodecylsulfate in water and depress the Krafft point. Nakayama et al. [92] have proposed that the melting point of hydrated solid is depressed by the formation of a mixed micelle consisting of surfactant and alcohol. However, the melting-point model of a hydrated solid for explaining the Krafft point is not valid [24]. The Krafft point can be lowered by (1) increasing the solubility of the monomer or (2) decreasing the cmc value of the surfactant. Krafft points for fluorinated nonionic surfactants have not been reported. However, the widespread view that nonionic surfactants do not have a Krafft point is incorrect. Although only a few nonionic surfactants exhibit a Krafft point, an implication that the Krafft point is related to the ionic charge of a surfactant is mis-

Chapter 6

00

85

65

\ '

45

25

1

1

\

\ \

-

\

I

1

27

29

I / T x I@

31

I

I

33

FIG. 6.8 Solubilities of perfluoroalkanesulfonates, CnFan+lS03M, in water as a function of temperature. CgFI9SO3K is less pure than the other surfactants. (From Ref. 59. Reproduced by permission of the American Chemical Society.)

leading.Nonionicsurfactantshave low cmcvalues and thesolubility of monomeric nonionic surfactants does not increase considerably with increasing temperature [98]. Krafft points are not observed when the (fictitious) Krafft point is below the freezing point of water. Krafft points have been reported for C16E7 [99], Brij 56, polyoxyethylated (10) cetyl alcohol [100,101], and Brij 76, polyoxyethylated (10) stearyl alcohol [ 1011. Kuwamura [ 1021 found that Krafft points of nonionic surfactants having a glycine residue steadily decrease with increasing number of ethylene oxide units. L-Alanine raises the Krafft point somewhat, whereas sarcosine has only a slight effect. This suggests that a hydrogen-bondforming secondary amino linkage in a nonionic surfactant molecule increases crystallinity and hydrophilicity of a nonionic surfactant.

Fluorinated Surfactantsin Solution

217

r-”l

C

1

I

I

32

I

34

I/T x

1

1

36

IO’

FIG.6.9 Solubilities of C7FI5COOM in water as a function of temperature. (From Ref. 59. Reproduced by permission of the American Chemical Society.)

6.4

CLOUD POINT

Aqueous solutions of nonionic surfactants become suddenly turbid when the temperature is raised. The temperature at which the sudden onset of turbidity occurs is called the cloud point. The cmc decreases with increasing temperature and the micellar weight increases [ 103-1061. At the cloud point, the aggregates have become so large that turbidity becomes perceptible even to the naked eye [98,106]. The surfactant solution separates into two coexistent phases: a surfactant-rich phase and a phase in which the surfactant is depleted. The cloud point depends on the surfactant concentration, but the concentration effect is usually weak. The cloud point and the surfactant-water systems have been extensively studied for nonfluorinated nonionic surfactants. The phase separation at the cloud point has been explained by dehydration of the polyoxyethylene chain of the nonionic surfactant with increasing temperature [107]. The dehydration theory has

-IO'

1

2

0

mole

1

1

1

4

6

8

fraction

of

1

1 0

CeF,,S03K

FIG. 6.10 Krafftpoint of binarysurfactantmixtures: (I) C8Fl7so3NH4 + ZH~OH C8F17SO3K; (II) C8F17SO3H + C8F17SO3K: (III) C ~ F ~ ~ S O ~ N H ~ C + C8F17S03K.(From Ref. 59. Reproduced by permission of the American Chemical Society.)

30

y

v

1

I

I

I

20

?3!

c

2

al

a. IO E

al

I-

O

1 6 1

I

0

2

4

mole

fraction

I

I

8 1 0

of

FIG. 6.11 Krafft point of binarysurfactantmixturen-C7F15COONa + nC7F15COOK.(From Ref. 59. Reproduced by permission of the American Chemical Society.) 218

21

Solution

in Surfactants Fluorinated

9

lost some of its appeal in view of light-scattering, viscosity, and vapor-pressure data [ 1061. Although it is true that hydration of oxyethylene ether oxygens decreases with increasing temperature, the amount of water entrapped by the polyoxyethylene chains may increase [ 1061. Because the cloud point is related to solubility of the nonionic surfactant in water, structural features of the surfactant molecule affect the cloud point. Nonionic surfactants having a longer polyoxyethylene chain have a higher cloud point. At constant oxyethylene content, the cloud point is lowered by (1) decreasing molecular weight of the surfactant (at a constant oxyethylene content, decreasing the size of the hydrophobe), (2) broader distribution of polyoxyethylene chain lengths, (3) branching of the hydrophobic group, (4) a central position of the hydrophilic group in the surfactant molecule, (5) replacement of the terminal hydroxyl by a methoxy group, or (6) replacement of the ether bond between the hydrophilic and hydrophobic groups by an ester bond [98,108]. A cloud-point curve determined by Mathis et al. [ 1091 for afluorinated surfactant is shown in Fig. 6.12. Aliquots (1-2 mL) of surfactant solutions were heated in capped vials and the temperature was gradually raised until the solution became turbid. The curve exhibits a sharp decline and a flat minimum, followed by a gradual increase when the mass fraction of the surfactant is increased from 0% to 1%. The lowest point on the curve is the cloud point of the surfactant in the absence of any additives. Critical micelle concentrations of fluorinated surfactantsare approximately equal to those of their hydrocarbon analogs with a 1.5-1.8 times longer

20

1

I

1

1

1

1

1

1

I

1

3

n

u

W

W

U

n

a

I

0

2

o

r

1

4 6 WT% SURFACTANT

1

8

10

FIG.6.12 Cloud-point curve for the system C6C13CH2(0C2H4)60H/H20 system. (From Ref. 109. Reproduced by permission of the American Chemical Society.)

220

Chapter 6

hydrocarbon chain (see Section 6.6). Thus, the cmc of the fluorinated surfactant C6F13C2H4(OC2H4)140H (4.5 X 1OP4M) is of the same order of magnitude as that of surfactant C 12H?5(OC2H4)120H (1.4 X This rule does not appear to hold for cloud points of fluorinated surfactants. Mathis et al. [lo91 found a cloud point of 75°C for the fluorinated surfactant C6F13C3H3(OC2H4)140H. The estimated cloud point is about 120°C for surfactant Cl2H?5(OC2H4)120Hand about 140°C for surfactant C12H?15(OC2H4)140H. Although cloud points vary with the homolog distribution of nonionic surfactants, the observed discrepancy is significant. Hence, the cloud points of fluorinated surfactants are lower than anticipated on the basis of cloud points for hydrocarbon-type surfactants with comparable cmc values. The cloud point is affected by the presence of other solutes. Salts may increase or decrease the cloud point, depending on the nature of the anion and cation. Alcohols, fatty acids, and phenols depress the cloud point [ 1061. Several nonfluorinated ionic surfactants have been found to exhibit a cloud point [ 110-1 131. Recently, Yu and Neuman [1131 have observed cloud phenomena in an aqueous system containing sodium perfluorooctanoate with anexcess of tetrapropylammonium bromide. 6.5

THERMODYNAMICS OF MlCELLlZATlON

The thermodynamic description of micelle formation is complicated by the dynamic nature of micellar solutions and ambiguities of models defining the contents of micelles and the micellization process (see Section 6.2). The mass action model, which regards micellization as a reversible aggregation, has problems with the definition of standard states, polydispersity of micellar solutions, and the dependence of the aggregation number on temperature and surfactant concentration. The phase-separation model, on the other hand, has difficulties with the degrees of freedom given by the Gibbs phase rule and the definition of the heat of micellization [24,54,114,115]. According to the original mass action model, micellization is a reversible association of micelles [20,116): nA f A"

(13)

where n is the number of surfactant molecules. The association constant, KM,is given by

where aM is the activity of micelles and aA is the activity of surfactant molecules. At low surfactant concentrations, the activities of the surfactant and its mi-

Fluorinated Surfactants in Solution

221

celles can be replaced with their concentrations [27):

where cMand cA are the micelle and surfactant concentrations, respectively. Assuming ideal behavior, the chemical potentials, p A and ,x,,, of the surfactant monomers and micelles respectively are

At equilibrium,

The freeenergy of micellization, AGO, is then given by

)::( +)::(

AGO = RT In cA = RT

In CA

-

In cng

-

In n -

(Y) -

In ch1

When n is large, AGO = RT In cA = RT ln(cmc)

(20)

If the aggregation number 12 varies with temperature or, as in some cases, with surfactant concentration, Eq. (20) is not valid. If the variation of the aggregation number n with temperature is assumed to be small and can be neglected [27,45,47,54.105,115,117-1221, an equation of the Clausius-Clapeyron type can be written

Equation (21) is not strictly valid for calculating the heat of micellization because certain assumptions made in its derivation do not hold here. The equation implies that the micelle is at equilibrium near cmc in a standard state [27,54]. However, micelles are not definite stoichiometric entities but aggregates of different sizes that are in dynamic equilibrium with themselves and surfactant monomers. The aggregation number may vary with temperature. An extended mass action model describes micellization as a multiple equilibrium characterized by a series of equilibrium constants (see Section 6.2). Because these equilibrium constants cannot be determined, the micellar equilibrium is usually described by

Chapter 6

222

one equilibrium constant and an average aggregation number is used as an approximation. Equation (2 1 ) describes a highly idealized model for the association of nonionic surfactant molecules or ions. The simple mass action model assumes monodispersity of micelles. Counterions of surfactant ions are not included. In reality, large numbers of counterions are associated with micelles of anionic or cationic surfactants. Hence, if the surfactant dissociates into ions, the counterion and the degree of dissociation have to be considered [34): , 1 ~ -+

Iy2x+

~ ( t 7 - j ~ ~ ) -

(22)

or

and the equilibrium constant, Khf,is given for the equilibrium by

The activity coefficients are assumed to be unityand 111< n, because the micelles have a net charge. For cationic surfactants [Eq. (23)], the same equation [Eq. (24)] applies after reversing the charge signs. The freeenergy of micellization is given for ionic surfactants by

The use of Eq. (35)to calculate AGO for ionic systems is complicated by the uncertain value of m / ~ z ,especially in the presence of electrolytes. The ratio d n , the number of counterions. rn, per number of surfactant ions, 11, in the micelle, is the degree of counterion binding p. The degree of dissociation of micellar counterions, a, has been calculated from electrical conductivity data:

dK = dC

CY(^

+ Fp)

where K is the specific conductivity, c is the surfactant concentration, h is the equivalent conductance of the counterion, F is the Faraday constant, and p is the electrophoretic mobility of the micelles [123]. The value of CY for sodium perfluorooctanoate was estimated to be 0.45 at 30°C at 1 atm. This is significantly higher than the value of CY,0.28, for sodium dodecyl sulfate. The degree of binding of counterions. p, is obtained as the absolute slope of the plot of ln(cmc) versus In c.,, where c., is the total counterion concentration

Fluorinated Surfactants in Solution

223

[24,25,42,123,124]. The experimental value for sodium perfluorooctanoate, 0.56, is fairly close to the value 0.55, obtained by assuming that p = 1 - a = 1 - 0.45 = 0.55. Some p values and aggregation numbers for perfluorinated surfactants are given in Table 6.5 [ 1241. According to the mass action model of nlicellization of ionic surfactants [24], the equilibrium between surfactant monomers and micelles is given by [25) (37)

I$-]

+ pn[Na+]

where S- denotes the anionic surfactant monomer, M denotes the micelle, and 11 is the aggregation number of the micelle, and the concentrations are expressed in mole fraction units. In accord with this model, the standard free energy of micelle formation per monomer unit, AGZ,, is given by AGp,i - In[S-] RT

”

+ /3 ln[Na+]

-

Mukerjee et al. [125] have emphasized that calculations based on the mass action model or the pseudophase separation model give significantly different enthalphy values. However, the discrepancies between both models diminish if the aggregation number increases to infinity [ 107,1271. Data published on thermodynamic properties of fluorinated surfactants are scarce. Shinoda and Hutchinson [42] treated the micelle as a separate pseudophase of very small dimensions. The phase-separation model describes the micelle as a separate phase which begins to form at crnc. If micelle formation is analogous to phase separation, the heat and entropy of micellization can be calculated from the temperature dependence of the activities of micelle-forming species. Shinoda and Katsura [ 1261 calculated the entropy of micellization from the temperature dependence of cmc for potassium perfluorooctanoate under the con-

TABLE6.5 Values of Aggregation Number, n, and Degree of Binding, p, for Salts of Perfluorooctanoic Acid (PFO) and Perfluorononanoic Acid (PW

n

P

Ref

-

0.56 0.54 0.52 0.60 ? 0.002

123 125 126 85

Surfactant NaPFO NaPFO KPFO LiPFN

20 15

20 2 2

Source: Ref. 124. (Reproduced by permission of Elsevier Science Publishers.)

Chapter 6

224

dition of a definite counterion concentration. If micelle is considered a phase, the entropy, SIL1.and enthaply, Hhf, of micellization are given by [42,107,126]

where Tis the absolute temperature, R is the gas constant, X. and X 3 are the mole fractions of the surfactant and counterions, respectively, and Kg is a constant. If X 3 is kept constant, the second term in Eq. (29) is zero and the activity of micelle-forming species is dependent only on the surfactant ion concentration. In the absence of added salts, X. = X 3 , and

The phase-separation model has been criticized by proponents of the mass action model [24,128] (see Section 6.3). Tadros [ 1291 determined some physical parameters for the Monflor industrial fluorinated surfactants. The free energy, enthalpy, and entropy of micellization for Monflor fluorinated surfactants were obtained from the y versus log c (surface tension plotted against log of the concentration) curves at various temperatures. The nonionic, anionic, and cationic surfactants used in the study were Monflor 5 1, CloF190(CH2CH20),C,OFl9. where sz = 23;Monflor3 1, C 1 OF90C6H4S03Na; and Monflor 7 1, C I 190C6H4N+Me3MeSOT. Tadros calculated the standard free energy of micellization, AGG, by the equation

AGG

=

zRT In -rcn,c

(3 1)

where ,rcnlcis the mole fraction of the surfactant at cmc and z is the number of ions formed by the surfactant molecule (for a nonionic surfactant z = 1). The standard enthalpy of micellization was calculated using

The standard free entropy of micellization, S&, was calculated from

AGG

=

A H i f - TAS&

(33)

Equations (32) and (33) suggest that AH& and S& can becalculated from the variation of log xCmc with 1IT. However, plots of log xcnlc versus 1/T were not linear (Fig. 6.13),indicating a decrease in enthalpy of micellization with increasing temperature. AHgl was therefore calculated at each temperature from the tangent to the curve. The results are given in Tables 6.6 and 6.7 for Monflor 51, a nonionic surfactant, and Monflor 3 1, an anionic surfactant, respectively.

i

Fluorinated Surfactants in Solution

225

-

Monflor 51

3.2

3.1

(1IT) x lo3

FIG.6.13 The temperature dependence of cmc for Monflor 51 and 31. (From Ref. 129. Reproduced by permission of Academic Press, Inc.)

TABLE 6.6 Area/Molecule, cmc Values, and Thermodynamic Parameters of Micellization for Monflor 51 Temp. ("C)

r (mol/m2 x lo6)

Aredmolecule (nm2)

cmc

(x

lo6 M )

-AG (kcal/mol)

~~

0.56

2.97 3.1 19.3 0.52 3.19 0.49 3.3 (0.52) 7) (3.1 45 (0.58) (2.87)

20 2510.8 30 35 40 45

8.42

3.47

0.53

Source: Ref. 129.(Reproduced by permission of Academic Press.)

AH (kcal/mol)

TAS (kcal/mol)

226

Chapter

TABLE6.7 Area/Molecule, cmc Values, and Thermodynamic Parameters of Micellization for Monflor 31 Temp. ("C)

4.71 4.56 4.37 4.32 4.17 4.07

20 25 30 35 40 45

r (mol/m2 x

lo6)

Aredmolecule (nm2)

cmc

-AG

( x 1 O6 M)

(kcal/mol)

AH (kcal/mol)

TAS (kcal/mol)

0.35 0.36 0.38 0.39 0.40 0.41

9.12 6.61 5.63 5.01 5.25 4.68

12.8 13.4 13.8 14.2 14.3 14.7

17.8 8.4 8.7 8.1 3.9 3.1

21.3 21.8 22.5 22.3 18.3 17.8

Source: Ref. 129.(Reproduced by permission of Academic Press.)

The free energy of transfer, AGOM,for a -CF2- group from an aqueous environment to a micelle is similar to that of corresponding hydrocarbon surfactants [80,130] (Table 6.8). However, the free energy of adsorption for a-CF2- group at the air-aqueous solution interface is much higher than the free energy of adsorption for acorresponding hydrocarbon surfactant, in accord with the greater surface activity of fluorinated surfactants. The behavior of perfluorinated surfactants has been explained [85,131] by chain-chain and chain-solvent interactions. Micellization processes involve weaker chain-chain interactions. Transfer from an aqueous environment to the air-aqueous interface is driven by the chain-solvent interaction, which is stronger for perfluorinated surfactants than for hydrocarbon surfactants. La Mesa and Sesta [85] have suggested that the thermodynamic bulk

TABLE 6.8 Free Energy of Transfer (A\&) for a -CF2Group from Aqueous to Micellar Environment and Free Energy of Adsorption for a -CF2Group (AG&) at the Air-Aqueous Solution Interface ~~

~

AGL = -762 ? 25 cal/mol, at 25°C AGL = -825 caVmol, at 30"C, for KPFOa AGL = -706 cal/mol, at 30"C, for KDS AG&-Js= -1.3 kcal/mol, at 25°C AG& = -1.2 kcal/mol, at 30"C, for KPFO AGgd, = -620 cal/mol, at 30"C, for KDS KPFO, potassium perfluorooctanoate (131); KDS, potassium decyl sulfate. Source: Ref. 85. (Reproduced by permission of the American Chemical Society.) a

Surfactants Fluorinated

in Solution

227

and surface properties of perfluorinated surfactants are related to the hydrophobic effect of perfluoromethylene chains. Fisicaro et al. [ 1241 determined the enthalpy of micellization for sodium perfluorooctanoate from heats of dilution using an LKB flow microcalorimeter. The enthalpy value, 8.70 kJ/mol at 25OC, calculated by using the pseudophase transition model is in good agreement with the enthalphy value calculated by using the Woolley and Burchfield one-step mass action model (see Section 6.2). The value reported by Johnson and Olofson [132], AHM = 6.8kJ/mol at 5 ” C , for lithium perfluorononanoate is considerably lower than expected for the substitution of lithium for sodium and an increase of the chain length by one -CF2unit . The enthalpies and entropies of fluorinated surfactants are positive and higher than those of their hydrocarbon counterparts. The higher positive enthalpies and entropies have been attributed to the higher hydrophobicity of the fluorocarbon chain relative to the hydrocarbon chain. Micellization involves the transfer of the hydrophobic chain of the surfactant from an aqueous environment to the micelle. The positive entropy change indicates an increase in randomness which is unlikely to result from aggregation of surfactant monomers [103]. The increase in entropy is more likely to arouse from the destruction of “iceberg” structured water around the hydrophobic chain [ 13,24,127.129,133]. Micellization is endothermic [ 1341 and AH decreases with increasing temperature because at higher temperatures, there is less water structure to break [ 1291. Tadros attributed the difference between enthalpies and entropies of fluorinated surfactants and those of their hydrocarbon counterparts to hydrophobic interactions. The fluorocarbon chain has a higher water-structure-promoting ability than a hydrocarbon chain. Hence, the hydrogen-bonded water structure around fluorocarbon chains is more extensive and micellization requires breaking more water structure around a fluorinated surfactant chain than around the corresponding hydrocarbon chain, This condition leads to higher positive enthalpies and entropies of micellization. The involvement of water structure in the micellization process is also indicated by molar volume changes. The volume change associated with micellization is substantially greater for aperfluorinated surfactant than for a corresponding hydrocarbon surfactant [135,136]. A higher value of AV,,, has been attributed to a more pronounced volume change caused by structural perturbations of water. Thevolumechange upon micellizationreportedforlithiumperfluorononanoate by La Mesa and Sesta [ S I , AVhf = 18 -+ 2 mL/mol, corresponds to the AV, value of 20 mL/mol estimated by Sugihara and Mukerjee for sodium perfluorooctanoate [135]. However, the Vhfvalue of 14.2 mL/mol reported by Johnson and Olofsson [ 1321 for lithium perfluorononanoate is considerably lower than the value obtained by La Mesa and Sesta. The Vhl = 2 1.5 ? 1 mL/mol value estimated for perfluorooctanoic acid [88] is higher than that of its sodium salt and -

228

Chapter 6

5 mL/mol higher than expected for purely Coulombic interactions of H+ at the micelle surface. Shinoda and Soda [88] explained the apparent discrepancy by substantial covalent bonding between the perfluorooctanoate anions and H" ions at the micelle surface. Gonzilez-Martin et a1 [ 1371 determined AVhl for cesium perfluorooctanoate (CsPFO) from the apparent molal volume at a concentration higher than the cmc as well as from the partial volumes above and below cmc. The average AVh1 values determined by both methods at a given temperature were in good agreement. The AVMfor CsPFO, 14.9,14.0, and 13.9 mL/mol at 20°C, 30°C, and 40°C. respectively, was found to be concentration dependent, especially at high concentrations. The apparent molal volume of CsPFO is higher than that of sodium perfluorooctanoate (SPFO) [ 1381.

6.6 CRITICALMICELLECONCENTRATION The critical micelle concentration (cmc) denotes the surfactant concentration range at which surfactant molecules associate to form micelles. The term cmc has been defined as the surfactant concentration in a solution in which just half of the total surfactant present is in the monomeric form [ 1391. Because several physical properties of micellar solutions change abruptly at cmc, the value of cmc and its dependence on surfactant structure, composition of the solution, and physical conditions are of great practical importance. Wetting, detergency, solubilization, and other important processes depend on the cmcof the micellar solution. The critical micelle concentration of a surfactant solution can be determined by measuring surface tension, electric conductivity, osmotic pressure, light scattering, viscosity, dye solubilization, and other physical properties [lo71 (see Chapter 9). The cmc in aqueous solution can be determined also from a change in the chemical environment indicated by the "F-NMR chemical shift (6) [140), plotted as a function of inverse surfactant concentration (Fig. 6.14). The intersection in the lines indicates the cmc. The cmc values for fluorinated N-alkylpyridinium chlorides have been obtained from osmotic coefficients determined by a vapor pressure osmometer [ 1411. A critical discussion of the methods for the determination of cmc values has been published by Mukerjee and Mysels [ 1421.

Hydrophobe Structure The cmc of fluorinated or hydrocarbon surfactants decreases exponentially with increasingnumber of carbonatoms.accordingtotheempiricalequation [143-145) log cmc = A

-

Biz

(34)

where concentrations are expressed in moles per liter, 17 is the number of atoms in the hydrophobe carbon chain of the surfactant, and A and B are constants for apar-

I

Fluorinated Surfactants in Solution

229

-2.5

-2.0

-1.5

L o

a -1 .o -0.5

0.0

c

\

0.5 0

100

200

300

400

500

mol L" I LiFOS conc

FIG.6.14 Change in ''F-NMR chemical shift as a function of inverse concentration for LiFOS solutions at 30°C. Five peaks in the spectrum are numbered starting at the sulfonate group. The intersection in the lines indicate the cmc. (Reproduced with permission from Ref. 140. Copyright 0 1997 by Academic Press.)

ticular homologous series and temperature. The constant A varies with the nature and number of the hydrophilic group or substituents in the hydrophobic chain. B is reasonably constant (about 0.29 for alkane hydrocarbon chains having one ionic hydrophile) but varies with the number of hydrophiles [107]. B has a value of about 0.50 for nonionic hydrocarbon-type surfactants. In analogy to surfactants with a hydrocarbon chain, the critical micelle concentrations for fluorinated surfactants in an aqueous solution depend on the fluorocarbonchainlengthandthecounterion(Table 6.9)[58,59,75,77,96,139, 146-1541 and decrease with increasing hydrophobe chain length. A plot of log cmc versus the numbel of carbon atoms in the fluorocarbon chain of anionic fluorinated surfactants is linear (Fig. 6.15) [ 1481 in accord with Eq. (34). The same relationship also s e e m to hold for nonionic fluorinated surfactants [ 1551. Examples of cmc values for some nonionic fluorinated surfactants are given in Table 6.10. Anionic surfactants with a oligo(oxyhexafluoropropy1ene) hydrophobe obey the relationship expressed by Eq. (34) [ 1581. Caporiccio et al. [ 1581 prepared two series of surfactants. Monocarboxylic acids (Series A) were prepared by photooxidation of hexafluoropropylene. Dicarboxylic acids (Series B) were prepared by reductive cleavage of perfluoropolyperoxide resulting from tetrafluoroethylene photooxidation (see also Chapter 2). The log cmc values of the monocar-

Chapter 6

230 TABLE6.9

Compound

Critical Micelle Concentrations for Anionic Fluorinated Surfactants Refs.

cmc (mmol/L) 2600 2060 710,740,750 530 82,51 50 8.7, 9.0, 10.5, 9.1, 8.7 2.8, 5.6 0.89, 0.78, 0.85 0.48 98 10.6, 10.8 0.39 171 36,32 9.1 0.43 700 500 129,62 26.3, 27 9.1 0.9 0.34 28 110 33 6.7 0.48 6.5 6.1 0.54 4.5 2.7 8.5 32 30 =I .5 150 30,90 250, 110 38,28

58 152 58,147,150 152 58,151 152 58, 59, 71, 75, 147, 150 59, 152 59,147,150 152 96 77,96 96 96 36,75 96 96 153 58 96,153 58,89 153 58 96 96 147 59 96 96 96 96 96 77 77 59 59 59 59 146 146,147 146,147 146,147

Solution

in

Surfactants Fluorinated Continued

TABLE 6.9

cmc

Compound 9 187 164 51 24 220 88 27 17.5 6.3, 7.5 8.5 8.0 5.5 4.6 7.5 0.64 ~0.14

146 1 39 1 39 139 139 154 154 154 59 59,75 59 59 59 59 75 59 59

Note: Methods used for the determination of cmc: conductivity [58,75,77,96]; surface tension [58,59,177,150]; solubility versus temperature curve [59]; dye titration [146]; fluorine NMR [139]. Temperature: the temperatures at which cmc values weredetermined are given in the original references. The temperatures vary from 2.5"Cto 100°C.

boxylic acids, the dicarboxylic acids, and their ammonium salts correlated linearly with the number of oxyhexafluoropropylene units in the hydrophobic segment of the molecule (Figs. 6.16 and 6.17). Cationic surfactants exhibit a similar relationship. The cmc values and the enthalpy changes for micellization decrease with increasing alkyl chain length of N-alkylpyridinium chlorides, CF3(CF2),,CH~CH2N+C5HsCl-,I? = 3 , 5 , 7 [141]. Because Eq. (34) is empirical, attempts have been made to develop a theoretical basis for this relationship. Shinoda et al. [59,107] expressed the change in cmc as a function of hydrophobic chain length by the equation ln(cmc) = -

171 Lr)

(1

+ K,)kT

+K

where Kg and K are empirical constants, k is the Boltzmann constant, Tis the absolute temperature, n l is the number of carbon atoms in the fluorocarbon chain, and o is the free energy difference per -CF?group (or -CH2group) between micellar state and monomeric state.

Chapter 6

232 FLUOROCARBON CHAIN L E N G T H , n

0

1

2

3

4

5

6

7

8

9

1

FLUOROCARBONCHAINLENGTH,

0

1

1

n

FIG.6.15 Log cmc of fluorinated surfactants plotted against their number of CF2 groups in the hydrophobic chain. (From Ref. 148. Reproduced by permission of the American Chemical Society.)

Fluorinated Surfactants in Solution

233

TABLE 6.10 Critical Micelle Concentrations of Nonionic Fluorinated Surfactants in Water

cmc (mmol/L)

Compound

Temp. ("C)

0.35 0.048 20 0.61 0.1 6 0.55 0.012 -0.0003

Ref. 109 109

143 20 20 25 25 25

156 156 157 157 157

B is given by Eqs. (34) and (35): B=

0

(1

+ KJ(2.303kT)

The o value can be determined from the slope of the straight line obtained by plotting log cmc values versus the number of carbon atoms in a perfluoroalkane carbon chain (Fig. 6.18) [96].The slope, B, of the straight lines is the same for the Na and K salts. Substituting Kg = 0.52, obtained for n-C7FI5COOK

-1 0

5 -2 0

0,

0 J

-3 -4

1

2 n

3

1

2

3

n

FIG. 6.16 Log cmc versus number of oxyhexafluoropropylene units in the hydrophobic chain of monocarboxylic surfactants. (A-H) CF3(0C3F6),0CF2COOH, n = 1, 2, 3; (A-NH4)CF3(0C3F6),0CF2COONH 4, n = 1, 2, 3. (From Ref. 158. Reproduced by permission of Academic Press, Inc.)

Chapter 6

234

E-H -1 0

2 0

m

-2-

0

J

-

3I

I

1

1

2

3

P

FIG.6.17 Log cmc versus number of oxyhexafluoropropylene units in the hydrophobic chain of dicarboxylic surfactants, HOOCCF2[(0C2F4) - (0CF2)lpOCF,COOH. (B1-H), p = 1; (B2-H) p = 2; (B3-H) p = 3. (From Ref. 158. Reproduced by permission of Academic Press, Inc.)

[126], into Eq. (36), Kunieda and Shinoda [96] obtained w = 2.21rl.T. This value is about twice the w value (1.08rl.T)[41] found for the corresponding hydrocarbon surfactant. The critical micelle concentrations of fluorinated surfactants are compared with those of hydrocarbon surfactants in Table 6.1 1 [59,157]. The cmc values for C7 and Cs fluorinated surfactants are approximately equal to those for CI I and C12 hydrocarbon surfactants, respectively. The greater hydrophobicity of the fluorocarbon chain enhances the amphiphilic character of the surfactant and increases surface activity. reflected i n surface tensions below 20 dynkm and lower cmc values [ 1,1591. Perfluorination reduces cnlc values about four times per -CF2-group. The A value [or K value in Eq. (35)] does not change considerably with the type of the surfactant. Because the relation expressed by Eq. (34) is exponential, the cmc value of a fluorinated surfactant is approximately equal to that of a hydrocarbon surfactant with a hydrocarbon chain 1.5-1.7 times longer than the fluorocarbon chain [59,157,160]. At equal chain length, the critical micelle concentrations of perfluorinated surfactants are lower than those of their hydrocarbon analogs [ 1.75.142.161]. Potassium perfluorooctanoate has a 13-fold lower cmc than potassium octanoate. The difference between cmc values decreases with decreasing chain length and is only threefold for the, albeit poorly documented, hexanoates. The hydrophobicity of hydrocarbon and fluorocarbon chains of nonionic surfactants has been compared by Ravey and St6b6 [162]. I

I

Fluorinated Surfactants in Solution I

6

7

235

I

I

I

8

9

IO

Cham length of CnF,,+,COOMe

FIG.6.18 Log cmc plotted against the number of carbon atoms in the hydrophobic chain of perfluoroalkanoates. (From Ref. 96. Reproduced by permission of the American Chemical Society.)

TABLE 6.11 Critical Micelle Concentrations of Fluorinated Surfactants and Those of Ordinary Surfactants Compound C&17SO3Na CaF17S03K C7FI5COONa C7F15COOK F(CF2)&H2CON[(C2H40)&H3)]2

Source: Refs. 59 and 157.

cmc Compound (mmol/L) ("C) 8.5 80 36 27 25 0 55

Temp.

75 80 8 25.6

cmc (mmol/L) 81 C12F25S03Na C12F25S03K 81 C1H23COONa 25 26 25 25.5 C11HZsCOOK H(CH2)lI C O N [ ( C ~ H ~ O ) ~ C H25 ~)]~ 0.51

Temp ("C) 25 25

Chapter 6

236

Hydrophile Structure The cmc of a fluorinated surfactant also depends on the nature of the hydrophile but to a lesser effect than on the hydrophobe structure (Table 6.9). Thecarboxylates have higher cmc values than sulfonates (Table 6.1 l). This is in accord with the order of decreasing cmc values-carboxylates > sulfonates > sulfates-observed by Klevens [ 1451 for hydrocarbon-type surfactants. The critical micelle concentrations of nonionic fluorinated surfactants are much lower than those of their ionic analogs. The cmc values of nonionic fluorinated surfactants increase with increasing length of the oxyethylene chain log cmc

=A

+ Bn

(37)

where rz is the number of oxyethylene units in the hydrophile and A and B are constants [ 163,1641.The value of B calculated from the cmc values of fluorinated surfactants C6F13CH2CH2(OC2H4),,0H.with I? = 12 and 1 1.5, is 0.054 (or 0.125 when natural logarithms are used). The B value found is in good agreement with that of hydrocarbon surfactants (0.056), suggesting that the number of oxyethylene groups has the satne effect on critical micelle concentrations whether the hydrophobe is fluorinated or not [ 109.1561. Matos et al. [165] investigated the surface activity of nonionic fluorinated surfactants which contained both oxyethylene and thioethylene groups: (Series I) CF3(CF2),,zC2H~(SC2H4)(OC2H4)p(SC’IH-F)(0C2H3)~OH

(Series 11)

The critical concentrations CI (formation of micelles or mesophase particles) indicated that one inserted thioethylene group cancels out the effect of 1.6 oxyethylene groups. All of the data fit the same line (Fig. 6.19) when plotting log C1 versus the effective number of oxyethylene groups: 17

= p

+ q - 1.6

(38)

The slope of the line is the constant B in Eq. (37). Thevalue of B. 0.056, is in good agreement with the B value reported by Mathis et al. [ 1091. The foregoing examples indicate that oxyethylene groups increase the cmc of a fluorinated surfactant. However, oxyethylene groups inserted between a hydrophobe and a sulfate group have the opposite effect-the cmc value is reduced. Greiner et al. [ 1661 determined cmc values of ether-sulfate-type fluorinated surfactants derived from telomer alcohols H[CF~CF~],,CH~[OCH2CH2],,0S03NH4, where n = 2, 3, 4 and the average of In is 3. The cmc values for surfactants with the oxyethylene segment are lower than those for ammonium perfluoroalkyl sulfate without the oxyethylene segment (Fig. 6.20). The anomalous effect, attributed to a slight increase of hydrophobicity of the molecule, is similar to that observed with alkyl sulfates [ 167,1681.

Fluorinated Surfactants in Solution

237

o RXE, 0

- 5 -I 2

RXEp

\’Eq

n / P+4-1.6 e

4

6

FIG.6.19 Plot of log C,versus the effective number of oxyethylene units n (0)or n = p + q ( 0 ) .(From Ref. 165. Reproduced by permission of Academic Press, Inc.)

Counterion The cmc values of ionic fluorinated surfactants are affected by the nature of their counterion. The lines of the log cmc versus chain length plot (Fig. 6.18) have the same slope, B, but the cmc values for Na salts are higher than those for the corresponding K salts. Although the crnc values reported in the literature (Table 6.9) vary somewhat with the conditions and the method of their determination, i t is evident that cmc values decrease in the order of the counterions Li > Na > K. The cmc values decrease with increasing atomic radii, increasing ion binding, and decreasing hydration of the counterion. The cmc values of perfluoropolyether carboxylates decrease with decreasing counterion hydration Na+ > K+ > NH: [169]. The cmc value of tetraethylammonium perfluorooctaonoate is 4.2 times smaller than that of thesodium salt [ 1701, in accord with the hydrophobicity of the ( C ? H ~ ) ~ Nion. + Moroi et al. [171] determined the degrees of counterion binding for lithium I-perfluoroundecanoate over the temperature range from 5°C to 30°C by a method that combined electroconductivity and membrane potential measurements with the mass action model. The temperature dependence of the degree of counterion binding, the aggregation number, the enthalpy change of micellization, and the entropy change of micellization were found to be much greater than those for the corresponding hydrocarbon surfactant.

Chapter 6

238

0

IO”

\

CMC (mol/ L)

0

10-*

I 0-3

3

5

7

9

NUMBER OF C ATOMS FIG.6.20 Log cmc versus the number of carbon atoms in the hydrophobic chain of (-) H(CF2CF2),CH2(0CH2CH2),0S03NH4,n = 2,3, 4, m (average) = 3; (----) H(CF2CF2),CH20S03NH4, n = 2 and 3, m = 0. (From Ref. 166. Reproduced by permission of Dr. Alfred Huttig Verlag.)

Surfactants Fluorinated

239

in Solution

The effect of hydrophobicity of the counterion is also evident when an organic counterion of a hydrocarbon-type surfactant is fluorinated. Hoffmann et al. [78.80,172] studied cationic surfactants with perfluorinated counterions and observed that perfluorination of an organic counterion lowers cmc, in analogy to perfluorination of the hydrophobe. Perfluorination of the anion lowers the cmc as much as doubling the length of the hydrocarbon chain of the anion, as illustrated with the cmc values of dodecylpyridinium with C3F7COO- (cmc = 0.70 m M ) or C7HIsS03(cmc = 0.52) counterions [ 1721. The surface tension and cmc values decreased with increasing chain length of the perfluorinated anion in the order CF3COO- > C2F~COO-> C3F7COO-. The relation between the log of cmc and the number of -CF2- groups is linear. Kinetic measurements have indicated that the shorter-chain anions, CF3COO- and C2FsCOO-, are incorporated into micelles, but theanions with a longer chain, C3F7COO-, form acohesive film on the micelle surface and hinder a micellar interchange [78]. In accord with the results obtained by Hoffmann et al.. a study by Yoshida et al. [173] showed that the cmc values of rz-decyltrimethylammonium carboxylates decrease i n the order of Br > CF3COO- > C2F5COO-, whereas the aggregation numbers and the temperature dependence of the aggregation number increase in the sameorder.Thecmc of the n-decyltrimethylammonium perfluoropropionate,24.3 n M , ishigher than thecmcvaluefor 11-dodecyltrirnethylammonium bromide, 15.3 mM. However, the aggregation numbers, 51 and 55 (293 K), respectively. are about the same, although the surfactant anion of the bromide contains two more carbon atoms. Theeffect of the counteriononaqueousmicellarsolutions of 1(3,3,4,4,5.5.6,6-nonafluorohexyl)pyridiniumhalides has been examined by Fisicar0 et al. [174]. The cmc values and the values of rnicellization enthalpies of the surfactants decrease in the order C1- > Br- > I-. A comparison to hydrocarbon analogs suggests that fluorination of the alkyl group has restrained the strong effect the halides as counterions have on the micellar properties. The valence of the counterion has a considerable effect on crnc values. Surfactants with divalent counterions have a lower cmc than surfactants with monovalent counterions. In solutions of ionic surfactants, the activity of the ~.nicelle-formingspecies depends on the concentrations of both the surfactant ions and the counterions. The cmc values decrease with increasing counterion concentration (Fig. 6.21) [ 1271. If the change in the aggregation number is neglected or is very large. In crnc

=

-Kg In Cg+ const

(39)

where Kg is an experimentalconstant and Cg the counterionconcentration [127,175.176]. Yoshida et al. [ 1731 estimated the degree of counterion binding from the slope of this relationship (see Section 6.5). Fluorination of the counterion in-

Chapter 6

240

-I 4 I

-I 6

1

1

I

- 1.4

- 1.2

-1.0

log,,, C,+

(molality)

FIG.6.21 The effect of counterion concentration and temperature on cmc in aqueous solutions of potassium perfluorooctanoate. (From Ref. 127. Reproduced by permission of the American Chemical Society.)

creases the degree of counterion binding p to micelles of a hydrocarbon-type cationic surfactant, n-decyltrimethylammonium carboxylate.

Hydrophile-Hydrophobe Balance Micelle formation is apparently controlled by two opposing factors: the cohesive forces between hydrophobic surfactant tails and the affinity of the hydrophilic groups for water. At a given temperature, the balance between these factors determines the size, shape, and charge of the micelle. Lin and co-workers have shown that the effective chain length is more relevant than the actual chain length [148,149,177-1801. Branching of the hydrophobe chain increases cmc. Lin and Marszall [149] defined the hydrophobicity index, HI, as

where IZ is the nominal chain length expressed by the number of -CF2groups in the hydrophobe chain. The slope of the log cmc versus n plot (Fig. 6.15) yields neff values [ 148): log cmc

= A - Bneff= A -

I,? C p l

2.303kT(1

+ Kg)

where A and B are empirical constants, k is Boltzmann’s constant, and nq’ is the free-energy change associated with the transfer of the hydrophobic chain from the aqueous environment into the interior of the micelle. The use of the effective chain concept lead to a correlation between the

Fluorinated Surfactants in Solution

241

hydrophilic-hydrophobic (lipophilic) balance (HLB) values and cmc values. In a homologous series of surfactants, the chain length determines the HLB and cmc values. It is not surprising, therefore, that HLB values of fluorinated surfactants correlate with crnc values (Figs. 6.22 and 6.23), like those of hydrocarbon surfactants. The relationship between log cmc and HLB values for monocarboxylic and dicarboxylic perfluoropolyether surfactants with a poly(oxyhexafluoropropylene) hydrophobe is also linear [ 1581. HLB

20

21

22

23

24

25

26

21

28

HLB

FIG.6.22 Log cmc of fluorinated surfactants plotted against the corresponding HLB values ( 0 )C,F2,+1COOH; (A)C,F2,+1COOK. (From Ref. 148. Reproduced by permission of the American Chemical Society.)

Chapter 6

242

b-

FIG.6.23 The crnc and neffvalues plotted against corresponding HLB values for fluorinated surfactants. (From Ref. 149. Reproduced by permission of Carl Hanser Verlag.)

The general relationship between HLB and cmc values is given by [ 181) log cmc = CI

+ t?(HLB) = + 1HLB + Kg (1

~

where a and b are experimental constants and Kg is the ratio of counterions to surfactant ions in the micelle. Lin and Somasundaran [ 18I ] related HLB values to the free-energy change related to the transfer of hydrophobic chains from an aqueous environment into the micelle HLB

-

7

=

x(hydrophi1ic group numbers) -

L?cp' ~

2.303kT

The HLB group number for a CF? group was found to be 0.87 for long-

-

" .

"

"

Solution

in

Surfactants Fluorinated TABLE 6.12

243 HLB Group Numbers for Hydrophilic and Hydrophobic Groups ~~~

Hydrophilic group

Group number

-S04Na -COOK "COONa -S03Na NR3 (tertiary amine) Ester -COOH -OH -0-

+38.7 +21 .I +19.1 + I 1.o +9.4 +2.4 +2.1 + I .9 +0.5

F2

Lipophilic group

Group number

=CH-CHZ"CH"CH3 -C "cF3 Phenyl

-0.475 -0.475 -0.475 -0.475 -0.870 -0.870 - 1.662

Source: Refs. 149 and 182.

chain, unbranched polar molecules with terminal CF2 or CF3 groups [148]. The value for the -CF2-group is higher than the group number of 0.475 for the -CH2group (Table 6.12) [ 149,1821. The difference between the two group numbers is in accord with the higher hydrophobicity of the fluorinated alkane chain. Introducing the group number for the -CF2group, HLB becomes a linear function of the perfluorinated alkane chain length: HLB - 7

= x(hydrophi1ic

group numbers) - 0.8711,~~

( 44)

Lin [148] concluded that the relationship between cmc and HLB for fluorinated surfactants is analogous to that of hydrocarbon surfactants. The empirical HLB values relate to the free-energy change associated with the transfer of the hydrophobe from the aqueous environment into the micelle.

Partially Fluorinated Surfactants Partially fluorinated surfactants exhibit at least two apparent anomalies [1):

1. Their cmc values are higher than expected from the effect perfluorination usually has on cmc. Muller et al. [ 139,184-1881 found that perfluorination of the terminal CH3 group of a surfactant's alkyl chain approximately doubles the cmc instead of lowering it. 2. The w-trifluoromethyl group appears to be in a more aqueous environment than expected for its location in the micellar core. Both facts are in accord with the mutual phobicity of hydrocarbons and fluorocarbons, evidenced by the limited mutual solubility and the nonideality of the solutions [ 11. This mutual phobicity reduces micellization of partially fluorinated surfactants and explains the aqueous environment of the terminal w-trifluoromet h y1 group.

Chapter 6

244

If partially fluorinated surfactants were to behave ideally, fluorination of the hydrocarbon chain would decrease the cnlc value of the surfactant linearly with increasing number of hydrogens replaced by fluorine. Mukerjee and Mysels [I] found, instead, that substitution of a fluorine in the o-trifluoro group by a single hydrogen increases the cnlc value by a factor of 3 or 5 , bringing it closer to the cmc of the hydrocarbon surfactant. Replacement of three hydrogens in the terminal-CH3 group with fluorine increases cmc, instead of lowering it [l]. The anomalous cmc values suggest that theo-trifluoromethyl group has a “much more aqueous environment” than expected for the core average. The mutual phobicity of the fluorocarbon and hydrocarbon sections in the surfactant molecule counteract aggregation driven by the hydrophobicity of the hydrocarbon chain and the still stronger hydrophobicity of the fluorocarbon chain [ 11. Anomalies have also been observed with branched-chain anionic fluorinated surfactants. Kimura et al. [ 1891 investigated micellization of sulfopropylated N-alkylperfluorooctanan~ides,C7FIsCON(R)CH2CH2CH2SO3Na, where R is H or an alkyl group. The cmc values of the compounds depend on the chain length of the alkyl group. A short (<6 carbons) or a long (12 carbons) alkyl substituent increases the cmc of the surfactant. Alkyl groups of a length in the range of 6-1 0 carbon atoms give the lowest ctnc values (Table 6.13). The cmc minimum results from phobic interactions. The mutual phobicity between hydrocarbon and fluorocarbon groups in the surfactant molecule reduces micellization of the partially fluorinated surfactant. Surfactants consisting of an oligo(oxyhexafluoropropy1ene) hydrophobe and an aryl group exhibited an irregular cmc dependence on the hydrophobe chain length. Ishikawa and Sasabe [ 1901 prepared oil-soluble surfactants (HFPO),,-Ar, where Ar is an aryl group and (HFPO),, is an oligo(hexafluoropropy1ene oxide) TABLE6.13 CriticalMicelleConcentrations (cmc) andKrafft Points for Fluorinated Surfactants C7FI5CON(R)CH2CH2CH2S03Na R

cmc (mmol/L)

4.03 3.59 2.84 2.28 1.75 1.I5 2.44 3.27

Krafft point (“C) 1o/o

45 10
>IO0 >IO0 > I 00 > I 00

Source: Ref. 189. (Reproduced by permission of American Oil Chemists Society.)

Fluorinated Surfactantsin Solution

245

cmc x 103 (Molq/L)

\

0

1

2

3

4

5

6

Chain Length(n) FIG. 6.24 Log cmc values plotted against the number, n, of hexafluoropropylene oxide groups; 0 = surface tension, y; 0 = cmc. (Data from Ref. 190.)

group, n = 2-5. The logarithms of cmc values estimated for surfactants, (HFPO),,phenyl, from the bending point of surface tension curves in rn-xylene decreased linearly with 12 increasing from 3 to 6. However, the cmc value for the surfactant with n = 2 was much higher than predicted by the linear relationship (Fig. 6.24). The cmc values of the sulfonated surfactants, (HFPO),,-aryl’-S03Na, in water exhibited an irregular behavior with deep minima. In contrast, fluorinated surfactants with a predominantly fluorinated alkyl chain obey the cmc-chain length relationships expressed by Eqs. (34) and (37). Selve at al. [157] studied the surface activity of fluorinated nonionic surfactants with a two-chain polyoxyethylene hydrophilic head linked to the hydrophobe via an amide bond, F(CF2),(CH2),,,C(0)N[(C~H~O),,CH&. The cmcvalues of surfactants with a structure In = 1,rz = 3, and I = 6,8, and 10 are 0.55,0.012, and 0.0003 mmol/L, respectively. The ratio of the cmc values for the surfactants with I = 6 and 8 is 46, which corresponds to a 1.7-fold cmc decrease per C2F4unit. As expected, the cmc values of the fluorinated surfactant with six CF2 groups are very similar to the cmc values of a hydrocarbon surfactant with 10 CH2 groups (Table 6.10).

Chapter 6

246

f

J

r

I

1

I

I

0

20

40

60

80

TEMP

T

("C)

FIG.6.25 Effect of temperature on cmc: sodium decyl sulfate (O), sodium perflu(From Ref. 125. Reproduced orooctanoate (0),potassium perfluorooctanoate by permission of the American Chemical Society.)

(a).

Temperature The variation of the cmc with temperature is shown in Fig. 6.25 [125] and Fig. 6.21 [ 1261. The cmcof most ionic surfactants exhibits a minimum as the temperature is varied from about 0°C to 80°C. The cmc-temperature curves of nonionics do not usually exhibit minima, probably because their (fictitious) minima are at higher temperatures where the determination of cmc is limited by phase separation and the cloud point. The observed temperature dependence of cmc is a result of two opposing factors. An increase in temperature reduces hydration of the hydrophile and, consequently, favors micelle formation. The decrease of cmc with increasing temperature (the left-hand side of the curve in Fig. 6.25) is probably caused by a decrease of hydration of the monomer. However, an increase in temperature also lessens interactions between water molecules and reduces the hydrophobic-effect-related free-energy difference between a - C F 2 - in a monomeric and a micellar state. The cmc increase with temperature on the right-hand side of the curve in Fig. 6.25

Surfactants Fluorinated

in Solution

247

suggests that at higher temperatures, the disruption of water structures around the hydrophobe of the surfactant becomes the dominant factor. As an added complication, the aggregation number and the shape of micelles may vary with temperature. Finally, the experimental method employed to determine cmc values as a function of temperature may itself be affected by the changing temperature. For example, when cmc is determined by conductance measurements, a change in conductivity may be reflecting a change in fluidity of water [ 1251. Equation (20) implies a linear relationship between log cmc and 1/T. This assumption has been used to calculate the enthalpy of micellization. AH,,,, by a Clausius-Clapeyron type of equation [Eq. (21)]. Tadros [ 1291 attempted to calculate enthalpy values for Monflor fluorinated surfactants but found the dependence of log cmc on 1/T to be nonlinear. The Clausius-Clapeyron equation holds only when the surfactant concentration is constant [ 1361. Undoubtedly, the temperature dependence of cmc is a complex relationship affected by several factors, some having an opposite effect.

Pressure The cmc values are pressure dependent. Sugihara and Mukerjee [135] measured electrical conductivities of sodium perfluorooctanoate solutions as a function of pressure. The cmc values were found to increase with increasing pressure from 1 atm to 2000 kg/cm2 and then decrease slightly at higher pressures (Fig. 6.26)

1000

0

2000

Pressure ( k g l c r n2 1

FIG.6.26 Effect of pressure on the cmc of sodium perfluorooctanoate at 3OoC,using molality and molarity scales. (From Ref. 135. Reproduced by permission of the American Chemical Society.)

" " "

r."-

*-=-

"."

. . . - "=" " "

,

,

..

Chapter 6

240

47 46

45 44

43

42

t

41

1 0

I

1

I

1

20c

1000 Pressure ( kg / c m 2,

FIG.6.27 Effect of pressure on dWdc (a) above and (b) below cmc. (From Ref. 135. Reproduced by permission of the American Chemical Society.)

[ 1351. Below cmc, the differential conductivity, d ~ l d cwhere , c is the molar concentration, decreases linearly with pressure, in qualitative accord with increasing viscosity and decreasing mobility of single ions (Fig. 6.27) [ 1351. Above cmc, the differential conductivity increases linearly with pressure. The increase of d ~ l r l c with increasing pressure above cmc indicates that the degree of dissociation of micelles increases with increasing pressure. The volume change related to micelle formation has been calculated using the charged phase-separation model [ 191):

AV,,,

=

RT( 1

+ p)

8 I n (cmc)

where R is the molar gas constant, Tis the absolute temperature, P is pressure, and p is the degree of counterion binding. For the mass action model, the volume change AVS, is given by Eq. (46), assuming that 98% of the surfactant molecules are present as monomeric species at the experimentally determined cmc value [ 1351: AVZ, = RT(

1+p yz

-

1

)( 6 ln(cmc) 6P

)T

+ RT(%IT

ln[0.98(crnc)]

(46)

Surfactants Fluorinated

in Solution

249

When the aggregation number M is large, lln is small with respect to 1 + p and the first term of Eq. (46) is then approximately equal to the right-hand side of Eq. (45) derived for the charged phase-separation model. The estimated volume changes resulting from micellization are shown as a function of pressure in Fig. 6.28. At a low pressure, the volume changes calculated using the charged phase-separation model or the mass action model are similar, but at high pressures, the effect of (Spl8P)T is more significant and the curves diverge. Because the determination of p at high pressures is experimentally difficult, absolute values of (SplSP)Tmay not be exact, but the trend is probably real. A number of factors affect the net volume change on micellization. It is therefore difficult to relate AVP,, to molecular interactions. Ikawa et al. [ 1361 determined the critical solution temperature (Krafft point) and the critical solution pressure (Tanaka pressure) of sodium perfluorodecanoate in water. A phase diagram of sodium perfluorodecanoate versus pressure at 55°C is shown in Fig. 6.29. The curves of solubility versus pressure (aQb) and of cmc versus pressure (dQe) intersect at point Q, representing the Tanaka pressure. The phase diagram is divided into three regions: solution of monomolecular species (S), the tnicellar solution (M), and the hydrated solid (C). The rapid decrease of solubility with increasing pressure (curve aQ) was attributed to the transfer of surfactant from micelles to the hydrated solid phase, which is accompanied by a large decrease in partial molar volume.

0

1000

2000

Pressure ( kg / crn2 )

FIG. 6.28 Estimated volume changes on micelle formation: V;, using the charged phase-separation model [Eq. (45)]; V, using the mass action model [Eq. (46)]. (From Ref. 135. Reproduced by permission of the American Chemical Society.)

Chapter 6

250

0

C

Pressure ( o t m ) FIG.6.29 The phase diagram of sodium perfluorodecanoate concentration versus pressure at 55°C. M, S, and C denote the micellar, singly dispersed, and hydrated solid states, respectively; Q, a triple point; CSP, critical solution pressure. (From Ref. 136. Reproduced by permission of Plenum Publishing.)

The Krafft point increases with increasing Tanaka pressure (Fig. 6.30). Sodium perfluorodecanoate (SPFDe) has the highest Krafft point of the surfactants included in Fig. 6.29 at any pressure applied at a constant temperature. such as 50°C. Hence, the range where micelles can exist is narrower for sodium perfluorodecanoate than for the other surfactants shown.

Electrolytes and Additives The critical micelle concentration is affected by the presence of electrolytes and other components of the surfactant solution [ 142,168,192,1931.Electrolytes depress the cmc of ionic surfactants and increase the micellar size by decreasing the thickness of the ionic cloud around the ionic groups and reducing the electrostatic

Fluorinated Surfactantsin Solution

70 -

251

STS

I

2000

4000

Tanaka Pressure (atm) FIG. 6.30 Krafft temperature as a function of critical solution pressure for various surfactants. (From Ref. 136. Reproduced by permission of Plenum Publishing.)

repulsion between them. For example, the cmc of lithium perfluorooctanoate in water at 25°C is 0.0341M. but in 1.012MLiC1, the cmc value is 0.00277M [170]. Partial fluorination of the hydrocarbon tail of a surfactant does not alter the effect of the electrolyte concentration on cmc and micelle size of a cationic surfactant (Table 6.14) [ 1941. The effect of NaCl on the cmc of CF3(CH2)&OONa is given by the equation [ l 841. log cmc

= - 1.24 -

0.58 log(Na')

(47)

The linear relationship on logarithmic scales is based on the law ofmass action, applied to the formation of a single micelle which includes bound counterions [1841. The points for NaOH deviate from the straight line representing the points for NaCl (Fig. 6.3 1) [ 1841, indicating that at higher electrolyte concentrations the nature of the ions has a significant effect on cmc. For a cationic partially fluorinated surfactant DEFUMAC, having the structure CsFl7CH2CH(OH)CH2NCH3 (C2H40H)TCl-,Tamori et al. [195] found the

Chapter 6

252

TABLE6.14 Effect of Electrolyte on cmc Values and Aggregation Numbers of Cationic Surfactants DTAB and TDTAB

Surfactant

NaBr molality

Aggregation number

0.1 0 0.50 1.oo 1.50 0.1 0 0.50 1.oo 1.50

83 94 106 112 74 90 89 104

DTAB

TDTAB

cmc X 1O2 (mol/kg)

LS 0.46 0.1 6 0.14 0.09 0.90 0.40 0.22 0.1 7

DS 0.23

0.51

Abbreviations: DTAB, dodecyltrimethylammoniumbromide; TDTAB, 12, 12, 12-trifluorododecyltrimethylammoniumbromide; LS, light scattering; DS, dye solubilization. Source: Ref. 194. (Reproduced by permission of Academic Press.)

omlm I: : 00.5

00.1 0.1

.

.

.

0.2

0.3

0.5

1

Na Conc. (M)

FIG.6.31 The cmc values of CF3(CH2)&OONa versus the total concentration of sodium ions at the cmc, on logarithmic scales. Filled circles obtained using NaCI; empty circles obtained using NaOH. (From Ref. 184. Reproduced by permission of the American Chemical Society.)

Fluorinated Surfactants in Solution

253

following relationship between cmc values and the electrolyte concentration (Fig. 6.32): log cmc

=

-Kgs log C,.- log k,

(47a)

where Kg.is aconstant representing the degree of counterion binding of the spherical micelle (for this study of DFUMAC, Kg.rhad the value of 0.82) and k, is the equilibrium constant of the formation of spherical components in micelles. The addition of salt causes an elongation of the micelle, explained by a multiple equilibrium [ 195,1961. The cmc of a fluorinated surfactant depends on the solvent composition in a manner similar to that of its hydrocarbon counterpart [ 1851. The effect of an organic additive is complex, including changes in the dielectric constant and, consequently, in electrostatic forces, stabilization of the monomer by modifying the water structure, and hydrophobic interactions between the monomer and the additive. Additives may affect the composition and the aggregation number of mi-

- 2.4

- 2.0

log

C1/

- 1.6

-1 2

rn~l.drn-~

FIG. 6.32 Logarithmic plot of cmc (Co)versus the total free counterion concentration C,.The cmc is determined using electric conductivity (0)and pyrene-3-carboxaldehyde fluorescence method ( 0 ) .(Reproduced with permission from Ref. 195. Copyright 0 1992 by Steinkopff.)

Chapter 6 1

'

I

/

o DEFUMAC LIFOS

0

50 ethylene glycol /

100 VOIOO

i

o DEFUMAC

o ' b ' " " ' 10' ' " " ' (b)

20

ethanol / vol%

FIG.6.33 The cmc of LiFOS and DEFUMAC in (a) ethylene glycol-H,O and (b) ethanol-H20 mixtures. (Reproduced with permission from Ref. 198. Copyright 0 1995 by Elsevier Science B.V.)

Fluorinated Surfactants 255 in Solution

celles. Thus, alkanols and perfluoroalkanols decrease the cmc of LiFOS. The decrease in cmc is larger for longer-chain alkanols and for asurfactant-alcohol pair with similar alkyl chains [197]. Fluorine NMR has shown that additives, such as urea, glycine, glycerol, acetamide, methanol, ethanol, acetone, dioxane. and tetrahydrofuran, vary in their ability to penetrate into the micelle [ 1851. Even for the least penetrating additives, such as urea or glycine, the effect on crnc is not solely related to the variation of the dielectric constant. Esumi and Ogiro [ 1981 investigated micellization of an anionic and a cationic fluorinated surfactant by measuring surface tension and absorption spectra of a dye [2,6-diphenyl-4-(2,4,6-triphenyl1-pyridino)phenoxide]. The cmc values for lithium pex-fluorooctanesulfonate(LiFOS) and bis(2-hydroxyethyl)(2-hydroxy-3-perfluoroocty1propyl)methylammoniumchloride (DEFUMAC) were determined in ethanol-water and ethylene glycol-water mixtures, as well as in pure solvents. The cmc values for LiFOS decreased and then increased with increasing ethylene glycol or ethanol concentration. The cmc values for DEFUMAC varied only slightly with the ethanol concentration and increased with increasing ethylene glycol concentration (Fig. 6.33). The crnc values for both surfactants decreased with increasing dielectric constant or increased at dielectric constant values above 70 (Fig. 6.34). 60 o DEFUMAC 0 LIFOS

y

40

E -CY

E

E

..

20

I

II 0

30

40

50

60

70

80

dielectrlc constant FIG. 6.34 The cmc values of LiFOS and DEFUMAC as a function of dielectric constant of ethylene glycol-H,O (solid curve) and ethanol-H20 mixtures (dotted line). (Reproduced with permission from Ref. 198.Copyright 0 1995 by Elsevier Science B.V.)

Chapter 6

256

6.7

SOLUBILIZATION

Micelles in surfactant solutions can incorporate in or upon themselves molecules of substances which are otherwise insoluble in the solvent, usually water. The solutions are optically isotropic, clear, and thermodynamically stable. McBain [83,199] termed this process solubilization. Solubilization mechanisms are complex and, in addition to sorption in or on micelles, other mechanisms may be operative. It is sometimes difficult to differentiate between solubilization as viewed by McBain and other processes causing a solubility increase, such as hydrotropy and comicellization. Attwood and Florence [ 1921 have defined solubilization more broadly as “the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components.’’The definition includes dilute or concentrated solutions, hydrotropy, and comicellization. The definition does not assume a similar solubilization rnecl~anism, as long as the resulting solution is isotropic and thermodynamically stable. A definition given by Myers [ 1671 is even broader: the preparation of a thermodynamically stable, isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the addition of one or more amphiphilic compounds at or above their critical micelle concentration. This definition includes systems such as micellar emulsions and microemulsions. A molecule solubilized by a nonionic surfactant may be located in the core of the micelle, at the core-polyoxyethylene chain interface, in the polyoxyethylene layer, or on the surface of the micelle [98,167) (Fig. 6.35). A molecule solubilized by an ionic surfactant may be in the core of the micelle, between the hydrophobic chains, in the layer formed by the counterions, or on the surface of the micelle. Because the micelle is in a dynamic state, the loci of the solubilizate in a micelle cannot be sharply defined. Like surfactant molecules or ions in the micelle, molecules of the solubilizate are mobile and a rapid interchange between different sites may be possible. It is believed, however, that the preferred location of the solubilizate in a given micelle depends on the structure of the solubilizate as well. The amount of a solubilizate which can be solubilized depends on several factors. The dominant variables are the structures of the surfactant and the solubilizate. Both the structure of the hydrophobic chain and the nature of the counterion can affect solubilization. Although the relation between solubilization and surfactant structure is complex, it is clear that the interactions between a solubilizate molecule and the lipophobic hydrophobe of a fluorinated surfactant must be different from interactions between the solubilizate and the lipophilic hydrophobe of a hydrocarbon surfactant. Solubilization by fluorinated surfactants is therefore of great theoretical as well as practical interest. The published information on solubilization by fluorinated surfactants is, however, sparse.

Solubilization: (a) in the micellar core; (b) at the core- alisades interface; (c) in the palisades layer; (d) on the micelle surface. (From duced by permission of VCH Pu~lishers.)

Considering the nature of the fluorocarbon chain, the most likely compounds solubilizable by fluorinated surfactants must be nonpolar and have mainly dispersion forces as intermolecular forces. The mutual phobicity between hydrocarbons and ~ u o r o c ~ b o nsuggests s fluorocarbons or chloro~uoroc~bons as preferred solubilizates for fluorinated surfactants. Kunieda and Shinoda [96} solubilized CCl2FCCIF2 in aqueous solutions of pe~uoroalkanoateshaving ~ifferent counterions and fluorocarbon chain lengths. CC12CClF2,a nonpolar liquid, was chosen for thesolubilization study because its relatively small molal volume was expected to increase the amount solubilized. Various amounts of CC12FClF2 were added to a known volume of surfactant solution in an ampoule and theboundary between the two phases was measured. The authors assumed a linear dependence of solubili~ationon surfactant concen~ationand considered solubilization curves (Fig. 6.36) E961 based on only a few experimental points valid. Solubilization depended on the nature of the counterion. Because the chain length and the nature of the counterion affect the hydrophile-lipophi~ebalance (HLB) of the surfactant,

Chapter 6

258

molallfy of CnF,,+,COOM

x

10'

FIG.6.36 The effect of the counterion of perfluoroalkanoates on the solubilization of CCI2CCIF2at 45°C. (From Ref. 96. Reproduced by permission of the American Chemical Society.)

the authors concluded that a well-balanced HLB may be essential for effective solubilization. Li perfluorodecanoate (y = 20.5 mN/m) appeared to be too hydrophilic for solubilization, whereas the HLB of the ethanolammonium salt (y = 13.8) seemed to be favorable. Gerry and co-workers [ 1941 found that terminal perfluorination of dodecyltrimethylammonium bromide reduces solubilization of Orange OT by the surfactant (Fig. 6.37). Because terminal fluorination did not reduce the aggregation number of the micelles substantially, a decrease in micelle size did not appear to be a cause of reduced solubilization. The mutual phobicity between terminal CF3 groups and the hydrocarbon groups of the solubilizate, the dye molecule, was the more likely explanation. Accordingly, fluorinated surfactants are better solubilizers for decafluorobiphenyl than hydrocarbon surfactants [200]. Teddy and Wheeler [201] have described the three-component phase diagram for the ammonium perfluorooctanoate/octanol-water system. The phase structure was determined by optical microscopy. Octanol was found to be four times less soluble in ammonium perfluorooctanoate solutions than in sodium octanoate solutions. The low solubility of octanol was attributed to a low mutual solubility of fluorocarbon and hydrocarbon chains and not to different counterions, ammonium versus sodium. However, the ammonium counterion may have contributed to the high solubility of ammonium perfluorooctanoate in octanol.

Fluorinated Surfactants in Solution

1

2

3

259

4

5

6

7

8

S U R F A C T A N r M O L A L I T Y x 10’

FIG. 6.37 Solubilization of Orange OT by solutions of 12,12,12-trifluorododecyltrimethylammonium bromide (TDTAB) and dodecyltrimethylammonium bromide (DTAB) in 0.5M NaBr. (From Ref. 194. Reproduced by permission of Academic Press.)

Asakawa et al. [202] examined solubilization of octafluoronaphthalene (OFN) and pyrene by anionic surfactants with a fluorocarbon or hydrocarbon hydrophobe and their mixtures. The study confirmed the dependence of solubilization on mutual phobicity between the solubilizate and surfactant chains, as well as the micellar size. The replacement of the lithium counterion with the diethylamrnonium counterion increased solubilization of OFN by perfluorononoate and tetradecyl sulfate. The solubilization increase was attributed to an increase in micellar size and counterion binding. The positive effect of micellar size on solubilization was demonstrated by an adding salt (LiCI) or by mixing surfactants. Solubilization of benzene and alkylbenzenes by lithium 1-perfluoroundecanoate at low concentrations was studied by Takeuchi and Moroi [203]. The con-

Chapter 6

260

centration of thesolubilizatewasdetermined in aspecialglassapparatus [203,204] (Fig. 6.38). The solubilizate was placed into the middle of the apparatus in a thermostated environment. The surfactant solutions were agitated by a magnetically driven rotor. After equilibration, the concentration of the solubilizate in the surfactant solution was determined by ultraviolet spectroscopy. Spectral changes and thermodynamic parameters suggested that these solubilizates are solubilized on the surface of the micelles. The pseudophase model (see Section 6.2) for micellar solutions makes it possible to establish a partitioning coefficient for the partitioning of the solubilizate between the aqueous solution and the micellar pseudophase. Treiner et al. [206-2081 studied partitioning of alcohols and phenol in an aqueous solution of a fluorinated surfactant or of mixed anionic hydrocarbon and fluorocarbon surfactants. The fluorinated surfactants used in their studies were potassium or sodium perfluorooctanoate and lithium perfluorooctanesulfonate. Nakayama et al. [92] suggested that partitioning coefficients between water and a micellar pseudophase can be calculated from the depression of Krafft points by nonelectrolyte additives. Kaneshina et al. [209] showed that partitioning coefficients obtained by the Krafft point method were in excellent agreement with those

C

U

FIG.6.38 Solubilization apparatus for volatile solubilizates: (a) liquid solubilizate, (b) surfactant solution, (c) disk rotor, (d) magnetic stirrer. (Reproduced with permission from Ref. 204. Copyright 0 1993 by The American Chemical Society.)

Fluorinated Surfactants in Solution

261

determined directly by gas chromatography. Kaneshina used a Clausius-Clapeyron-type equation:

p = - [55.5 AHF (T - To)] IH,, RT; where AHF is the heat of fusion of the hydrated solid surfactant, T and To are respectively the Krafft point in the presence of a nonelectrolyte additive and in the absence of any additive, and m,, is the molality of the additive. Treiner and Chattopadhyay [206] used the method of Kaneshina et al. [209] but determined Krafft points and cmc values from conductance data. The Krafft point (To = 299.2 K) and the heat of fusion of the hydrated solid (AH, = 9.6 kcal/mol) for potassium perfluorooctanoate are known. Because the heat of fusion of sodium perfluorooctanoate was not known, an indirect method [210] was used to obtain the partition coefficient P. In a dilute micellar solution, log(cmco/cmc) = KMmN

(49)

where cmcOand crnc are the critical micelle concentrations in the absence and presence of a solute at molality InN. The micellization coefficient KM is given by

where k t is the Setchenov constant [210] of the solute in the premicellar region and P is the partition coefficient. Both and P are expressed on the mole fraction scale. The coefficient F expresses nonideal interactions between the solute and the micelle, in analogy to an activity coefficient. Figure 6.39 [206] shows a plot of log(cmc&mc) versus solute molality for sodium perfluorooctanoate. The semilogarithmic plot is linear, in accord with Eq. (48). The partition coefficients and cmc values, represented as the Khl coefficient, are given in Table 6.15. The partition coefficients yield the standard free energy of solubilization A G,”:

kt

AG,”= -RTln P

(51)

For n-alkanols solubilized by potassium perfluorooctanoate, AG,”= - 1.848 - 0.43812

(52)

and for 11-alkanolssolubilized by sodium perfluorooctanoate. AG,”= -2.081

-

0.434n

(53)

where IZ is the number of methylene groups. The slope, which is the same for both perfluorooctanoic acid salts, gives the contribution of methylene group to AGO,.

Chapter 6

262

1 Hex

OH

0.3

Bu OH

cw log 7

0.1

I

1

I 3

0.0050.01

0.010

1

5

t

m(mol/kg)

FIG.6.39 The effect of nonelectrolyte concentration on the cmc of sodium perfluorooctanoate at 298.15 K. cwiscmco; cis cmc in the presence of solute. (From Ref. 206. Reproduced by permission of Academic Press.)

The intercept for II = 0 represents the contribution of a methyl plus a hydroxyl group. The standard free energy of transfer of a methylene group to perfluorooctanoate micelles is about -440 cal/mol. The standard free energy of transfer of a methylene group to hydrocarbon-type surfactants is more negative: -600 cal/mol for sodium dodecyl sulfate micelles and -660 cal/mol for trimethyldodecylammoniurn bromide micelles. The difference, related to CH2/CF2phobicity, is, however, counterbalanced by the strong hydroxyl-carboxylate interaction. The electron-withdrawing CF2 groups increase the acidity of the carboxylic acid and. consequently, the ion-dipole interaction of the carboxylate head group with the hydroxyl group of the alkanol. Carlsfors and Stilbs [211] investigated solubilization of a series of alkanols, benzyl alcohol. and benzene in deuterium oxide containing sodium perfluorooc-

Fluorinated Surfactants in Solution

263

TABLE 6.15 Partition Coefficients P (Expressed as Mole Fractions) and Micellization Constants KM for n-Alkanols in Sodium or Potassium Perfluorooctanoate Solutions at 25.0"C

Alkanol

P

In potassium perfluorooctanoate solutions 1-Butanol 1-Pentanol 1-Hexanol 1-Heptanol In sodium perfluorooctanoate solutions 1-Butanol 1-Pentanol 1-Hexanol 1-Heptanol

KM

1 9 5 k 10 0.72 480 5 30 890 t 50 1860 k 50

I .02 2.24 5.02 10.6

320 620 1390 2900

1.72 +- 0.06 3.43 5 0.10 7.63 +- 0.20 16.1 2 0.3

Fa

0.72 0.70 0.69

F i s equivalent to an activity coefficient. It expresses nonideal interactions between the solute and the micelle [206]. Source: Ref. 206.

a

tanoate or a mixture of sodium perfluorooctanoate and sodium decanoate. The partition coefficients were obtained from multicomponent self-diffusion coefficients [212], determined by the Fourier transform NMR pulsed-gradient spin-echo (FT-PGSE) method [212,213]. The self-diffusion of fluorinated components was monitored by fluorine NMR. and the self-diffusion of hydrocarbon components by proton NMR. The observed diffusion coefficient, DObs,is a time average between free and solubilized molecules:

The quantity p indicates the amount of a compound solubilized. The partition coefficient,

p = -X A XB

(55)

where XA is the mole fraction of the solubilizate in the micellar pseudophase and X B is the mole fraction of the solubilizate in the aqueous pseudophase, was calculated from the p values and known solubilizate and surfactant concentrations. The fraction solubilized, by sodium perfluorooctanoate and the partition coefficient, increase with increasing chain length of the alcohol (Table 6.16) [211]. The increase in solubilization was attributed to the increase in hydrophobicity of the alcohols with increasing chain length. The partition ratio of benzene was found to approximate that of butanol. This unexpected result suggested that the solubilization mechanism, and possibly the site of benzene in the micelle, is different from that of alcohols [2111.

Chapter 6

264

TABLE 6.16 Partitioning in Micellar Solutions of Sodium Perfluorooctanoate (0.101n/r) and a Mixed Micellar Solution of Sodium Perfluorooctanoate (0.0997W and Sodium Decanoate (0.0404M)

Partitioning coefficient (ratio of mole fractions) Solubilizate (mmol/kg) Propanol (21) Butanol (24) Pentanol (32) Hexanol (27, 36) Heptanol (25) Benzene (13) Benzyl alcohol (13)

In sodium perfluorooctanoate

In sodium perfluorooctanoate + sodium decanoate

80 t 20 280 ? 30 480 ? 20 1000 t 100 4300 ? 500 2 3 0 t 10 280 20

81 ? 6 280 2 10 430 ? 20 810 t 40 3200 t 300 340 ? 10 190 t 50

*

Source: Ref. 2 11.

The partition coefficients in the sodium perfluorooctanoate-sodium decanoate system (Table 6.16) are similar to those of the pure perfluorooctanoate system. For benzene, however. the partition coefficient was higher i n the mixed system, suggesting again a different solubilization mechanism. The differences in thermodynamic parameters of solubilization between fluorinated surfactants and their hydrocarbon counterparts have been reviewed [ 124,2141. Fisicaro et al. [ 1341 pointed out the large discrepancies between partition coefficient values obtained by different techniques pointed out (compare the P values for alkanols in sodium perfluorooctanoate given in Tables 6.15 and 6.16). The limited reproducibility appeared to be related to the nature of the micellar systems studied: The equilibrium between different kinds of mixed micelles can easily be perturbed by the procedure used for the determination of partition coefficients and the equilibrium is slowly restored. Bongiovanni et a]. [2 151 solubilized perfluorocarbons in aqueous solutions of commercial-grade perfluorooctanoic acid (about 80% C8) and its sodium salt. The observed increase in solubility of perfluorooctanoic acid when neutralized with sodium hydroxide was related to lowering of the Uafft point. W/O and O N isotropic monophasic liquid systems were obtained by changing the pH of the system. Micelles in aqueous surfactant solutions provide nonpolar submicroscopic regions for sorption of gases [216-?18]. Solubilization of a gas in aqueous surfactant solutions depends on several variables, including the nature of gas, the presence of amphiphilic substances, the size of the hydrophobic group, the nature

Surfactants Fluorinated

in Solution

265

of the counterion of ionic surfactants, perfluorination of the hydrocarbon chain, salinity. temperature, pressure. and pH of an ionic surfactant solution. Prapaitrakul and King [219,220] studied solubilization of gases in aqueous solutions of sodium 1-heptanesulfonate, lithium perfluorooctanoate, sodium perfluorooctanoate, and lithium perfluorodecanoate. The solubility of each gas (oxygen, argon, methane, ethane, propane, and carbon tetrachloride) in lithium perfluorooctanoate and lithium perfluorodecanoate obeys Henry's law at all surfactant concentrations. The solubility of each gas in aqueous solutions of sodium 1-heptanesulfonate remained constant until the surfactant concentration reached cmc (Fig. 6.40) [219]. Above the cmc, the solubility of gas increases linearly with increasing surfactant concentration, evidencing micellar solubilization [Fig. 6.4 1 [219] and Fig. 6.42 [220]. Substituting sodium for lithium did not affect solubilization (Fig. 6.43) [220]. Lithium perfluorodecanoate is a more effective solubilizing agent than lithium or sodium perfluorooctanoate, in accord with evidence that a longer fluorocarbon chain increases solubilization. The constant-pressure (1 atm) solubilities of gases increase linearly when plotted logarithmically as a function of gas critical temperature. The solubilities

m

120 10 0

F

s X

F

C3H8

80 c 0

C2H6 CH4 0 Ar

-

(D

3 0

20

0

02

04

06

SURFACTANTCONCENTRATION

08

10

12

(moles kg-1)

FIG.6.40 Solubility of gas (moles of gas absorbed in 1000 g of water containing sodium I-heptanesulfonate) as a function of surfactant concentration at 25°C. (From Ref. 219. Reproduced by permission of Academic Press.)

Chapter 6

266

120

[I

100

C3H8 C2H6 A CH4

m

s

X ,A € c d

80

0 Ar V

l OI

CF4

Y

60 d

E

Y

1 40 m 2 2

0

v,

20

0 0

0 1

0-2 0 40 3

SURFACTANTCONCENTRATION

05

06

(moles kg-1)

FIG.6.41 Solubility of gas (moles of gas absorbed in 1000 g of water containing sodium perfluorooctanoate) as a function of surfactant concentration at 25°C. (From Ref. 219. Reproduced by permission of Academic Press.)

of C 0 2 and NO2 in sodium perfluorooctanoate solutions are higher than predicted by this linear relationship obtained for other gases [221]. The increase in solubility of C 0 2 and NO?. also observed for hydrocarbon-type surfactants, suggests highly specific adsorption phenomena at the surface regions of micelles. Parallel trends in solubility data for fluorocarbons and perfluorinated surfactants suggest that the micellar interior, where solubilization of the gas occurs. has solvent properties similar to those of a bulk perfluorocarbon solvent, such as perfluoroheptane [219]. The solubilities of gases in surfactant solutions are lower than the corresponding solubilities in perfluoroheptane, but the diminution in micellar solubility is afunction of micelle size alone. The discrepancy increases with decreasing length of the fluorocarbon chain, measured in t e r m of carbon number. The Laplace pressure model used to explain solubilization of gases in hydrocar-

Fluorinated Surfactants in Solution

267

C3H8

9 .o

C2H6

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6 .O

0

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07

1

I

0

0.1

I

0.2

I

I

1

0.3

SURFACTANTCONCENTRATION

1

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(moles kg-1)

FIG.6.42 Solubility of gas (moles of gas absorbed in 1000 g of water containing lithium perfluorodecanoate) as a function of surfactant concentration at 25°C. (From Ref. 220. Reproduced by permission of Academic Press.) bon surfactants [ 131,2221also applies to solubilization of gases in fluorinated surfactants [319,2331. Solubilization of dyes and luminescence probes (see Chapters 7 and 9) by surfxtants has an important role in the determination of micellar structure. Solubilization of a dye in mixed fluorinated and nonfluorinated surfactant systems is discussed in Section 7.2.

Adsolubilization Solubilization in adsorbed micelles has been termed adsolubilization [324-2281. A small amount of an anionic surfactant added to a dispersion of positively charged alumina causes flocculation, but a further addition of the same surfactant redisperses the flocculated alumina. The redispersion is attributed to the formation

Chapter 6

268 9.0

-

8.0

-

7.0

-

C7FlgCOOL~ (Shaded Symbols) C7FlgCOONa (Open S y m b o l s )

C3H8 C2H6

0 02

6.05.0 -

E'

4.0 -

3.0

-

2.0

-

-

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3

J

"L I

0

8-

/

I

0 1

I

I

0 2

i

I

0.3

1

1

0.4

SURFACTANT CONCENTRATION (moles kg-1)

FIG.6.43 Solubility of gas, expressed as moles of gas absorbed in 1000 g of water containing lithium perfluorooctanoate (shaded symbols) or sodium perfluorooctanoate (open symbols) and plotted as a function of surfactant concentration at 25°C. (From Ref. 220. Reproduced by permission of Academic Press.)

of a bilayer of the adsorbed surfactant. Esumi et al. [226] studied the adsolubilization of hexanol and 33,33,444-heptafluorobutanolinto the lithium dodecyl sulfate (LiDS) and the lithium perfluorooctanesulfonate (LiFOS) bilayers on cyalumina. The adsolubilized amounts of the alcohols increased with equilibrium concentration of the alcohols. The partition coefficients for hexanol were higher than those for heptafluorobutanol. The polarity of the bilayers was examined by the steady-state fluorescence of pyrene. The ratio of the first vibronic band to the third vibronic band of pyrene fluorescence indicated that adsolubilization of hexanol or heptafluorobutanol lowers the polarity in the LiDS bilayer, but adsolubilization of pentafluorobutanol barely affects the polarity of the LiFOS bilayer.

Fluorinated Surfactants in Solution

269

A two-site adsolubilization model proposed by Lee et al. [227] has been used todescribethesolubilization of 2,2,3,3,3-pentafluoropropanoland 2.2,3,3,4,4,4-heptafluorobutanolin micelles of sodium perfluoroheptanoate adsorbed on alumina [228]. One site for solubilization of the alcohol in the amount TCis in the palisade layer of the adsorbed micelles. The other site is the perimeter of the cylindrical micelles (radius R), where the amount of alcohol is TI,. 6.8 ASSOCIATIONWITH CYCLODEXTRINS Fluorinated surfactants can form inclusion compounds with cyclodextrins, macrocyclic oligosacharides obtained by the enzymatic conversion of starch. A cyclodextrin, abbreviated CD, may consist of six (a-CD), seven (p-CD), or eight (7CD) D-( +)-glucopyranose units [229-23 13. Cyclodextrins have a hydrophilic outside but a hydrophobic inside, shaped like a cone. The cone-shaped cavity can shelter hydrocarbon-based surfactants [232-2341 as well as fluorinated surfactants [235-2401. The inclusion of fluorinated surfactants in the CD cavity has been studied by I9F-NMR [236,238,240) (Chapter 9) and by measuring the velocity of sound [239]. The association constants between hydrocarbon surfactants and either a-CD or p-CD are of the same magnitude [238]. However, the replacement of hydrogen with fluorine reduces the association between the surfactant and aCD markedly because the fluorocarbon chain is too large to fit into the a-CD cavity [237]. However, fluorination enhances association between the surfactant and p-CD because of a favorable fit of the fluorocarbon chain in the p-CD cavity [236]. Complementary H (host) and I9F (guest) NMR chemical shift data have provided evidence that the hydrophobic effect dominates the binding affinities of p-CD and alkyl-substituted p-CDs with fluorinated surfactants [240]. Interactions between the hydrophilic group of the surfactant and the CD annulus are of secondary importance, although evident [240]. y-CD forms a 1 : 1 complex with fluorinated surfactants [238].

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Fluorinated Surfactants in Solution 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 2 10. 21 1. 212.

275

K. Shinoda, J. Phys. Chem. 59.432 ( 1 955). D. F. Evans and P. J. Whightman, J. Colloid Interf. Sci. 86. 515 (1982). I. J. Lin, J. P. Friend, and Y. Zimmels, J. Colloid Interf. Sci. 45, 164 (1973). I. J. Lin, B.M. Moudgil, andP. Somasundaran, Colloid Polym.Sci. 252,407 (1974). I. J. Lin. Israel J. Technol. 9, 621 (1971). I. J.Lin, Trans. AIME250, 225 ( I 97 1). I. J. Lin and P. Somasundaran,J. Colloid Interf. Sci. 37,73 1 (1 97 1). J. T.Davies, “Proceedings, 2nd International Congress of Surface Activity.” Vol. 1, p. 426, Butterworths, London (1 957). J. T. Davies and E. K. Rideal. “Interfacial Phenomena,” 2nd ed., Academic Press, New York ( 1 963). N.Muller and R. H. Birkhahn,J. Phys. Chem. 72.583 (1968). N.Muller and T. W. Johnson, J. Phys. Chem. 73,2042 (1969). N. Muller and F. E. Platko, J. Phys. Chem. 75, 547 (1971). N.Muller and H. Simsohn. J. Phys. Chem. 75,942 (1971). N.Muller, J. H. Pellerin. and W. W. Chen. J. Phys. Chem. 76. 3012 ( 1 972). C. Kimura, K. Kashiwaya.M. Kobayashi, and T. Nishiyama. J. Am. Oil Chem.SOC. 61, 105 (1984). N.Ishikawa and M. Sasabe. J. Fluorine Chem. 25.241 (1984). S. Kaneshina, M. Tanaka,T. Tomida, and R. Matuura, J. Colloid Interf. Sci. 48,450 ( 1974). D. Attwood and A. T. Florence, “Surfactant Systems: Their Chemistry, Pharmacy and Biology.’’ Chapman & Hall, London (1983). G. Perron, R. DeLisi, I. Davidson, S. Genereux, and J. E. Desnoyers, J. Colloid Interf. Sci. 79,432 (1981). H. E. Gerry, P. T. Jacobs, andE. W. Anacker. J. Colloid Interf. Sci. 62,556 (1977). K. Tamori, K. Kihara, K. Esumi, and K. Meguro, Colloid Polym. Sci. 270, 927 (1 992). K. Esumi, Colloids Surf. A 84,49 ( 1 994). Y. Muto. K. Yoda, N. Yoshida, K. Esumi, K. Meguro,W. Binana-Limbele, and R. Zana. J. Colloid Interf. Sci. 130, 165 (1989). K. Esumi and S. Ogiro, Colloids Surf. A 94, 107 (1995). J. W. McBain, “Colloid Science.” D.C. Heath, Boston(1950). T. Asakawa, T. Kitaguchi, and S. Miyagishi, J. Surfact. Deterg. 1, 195 (1988). G. J. T.Teddy and B. A. Wheeler,J. Colloid Interf. Sci. 47, 59 ( 1 974). T. Asakawa, J. Ikehara, and S. Miyagishi, J. Am. Oil Chem. SOC.73,21 (1996). M. Takeuchi and Y. Moroi, J. Colloid Interf. Sci. 197, 230 (1998). Y. Moroi and T. Morisue, J. Phys. Chem. 97, 12668 (1993). M. Takeuchi andY. Moroi, Langmuir 1 1,47 19 (1 995). C. Treiner and A. K. Chattopadhyay,J. Colloid Interf. Sci. 98,447 (1984). C. Treiner, J. F. Bocquet, and C. Pommier, J. Phys. Chem. 90, 3052 (1986). C. Treiner. A. A. Khodja, andM. Fromon. Langmuir 3,729 (1987). S. Kaneshina, H. Kamaya, and I. Ueda, J. Colloid Interf. Sci. 83.589 (1981). C. Treiner. J. Colloid Interf. Sci. 90.444 (1982). J. Carlfors and P. Stilbs, J. Colloid Interf. Sci. 103, 332 (1985). P. Stilbs. J. Colloid Interf. Sci. 87, 385 (1982).

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213. E. L. Hahn, Phys. Rev. 80,580 (1950). 214. S. Milioto and R. De Lisi, Langmuir 10, 1377 (1994). 215. R. Bongiovanni. E. Borgarello, F. M. Carlini. E. Fisicaro, and E.Pellizetti, Colloids Surf. 48,277 (1990). 216. J. W. McBain and J. J. O’Conner, J. Am. Chem. SOC. 62,2855(1940). 217. J. W. McBain and J. J. O’Conner, J. Am. Chem. SOC. 63,875 ( 1941). 218. J. W. McBain and A. M. Soldate, J. Am. Chem. SOC. 64,1556 (1942). 21 9. W. Prapaitrakul and A. D. King, J. Colloid Interf. Sci. 112. 387 (1986). 220. W. Prapaitrakul and A. D. King, J. Colloid Tnterf. Sci. 118, 334 (1987). 221. D. W. Ownby. W. Prapaitrakul, and A. D. King, J. Colloid Interf. Sci. 135, 526 (1988). 222. I. B. C. Matheson and A. D. King. J. Colloid Interf. Sci. 66,464 (1978). 223. J. M. Corkill and J. F. Goodman, Adv. Colloid Interf. Sci. 2,297 (1969). 224. Harwell, Ph.D. dissertation. The University of Texas at Austin (1983). 225. K. Esumi. Y. Sakamoto, T. Nagahama, and K. Meguro, Bull. Chem.SOC.Japan 62, 2502. (1989). 226. K. Esumi, M. Shibayama. and K. Meguro. Langmuir 6,826 (1990). 227. C. Lee, A. Yeskie, J. H. Harwell. and E. A. O’Rear, Langmuir 6. 1748 (1990). 328. C.-L. Lai, E. A. O’Rear, J. H. Harwell. and M. J. Hwa, Langmuir 13.4267 (1997). 229. W. Saenger. Angew. Chem. Int. Ed. Engl. 19, 344 (1980). 230. W. Saenger and A. Muller-Fahrnow. Angew. Chem. Int. Ed. Engl. 27,393 (1990). 231. S. Hamai. J. Phys. Chem. 80.2661 ( 1990). 232. T. Okubo, H. Kitano. and N. Ise, J. Phys. Chem. 80,2661 (1976). 233. J. Georges and S. Desmettre. J. Colloid Interf. Sci. 118. 192 (1987). 234. T. Okubo, Y. Maeda, and H. Kitano. J. Phys. Chem. 93,3721 (1989). 235. R. Palepu. J. M. Richardson, and V. C. Reinsborough, Langmuir 5, 218 (1989). 236. R. Palepu and V. C. Reinsborough, Can. J. Chem. 67, 1550 (1989). 237. E. Saint Aman and D. Serve,J. Colloid Interf. Sci. 138, 365 (1990). 238. W. Guo, B. M. Fung, and S. D. Christian, Langmuir 8.446 (1993). 239. E. Junquera, G. Tardajos, and E. Aicart. Langmuir 9, 1213 ( 1993). 240. L. D. Wilson and R. E. Verrall. Langmuir 14,4710 (1998).

t

Structure of Micelles and Mesophases

7.1

STRUCTURE OF MICELLES

Micelles are in a dynamic state. Their characteristics, size, and shape vary withthe structure of the surfactant and the solution conditions, such as the concentration, ionic strength, temperature, pressure, and the nature of additives. The aggregation number of the micelles is not independent of the method by which it is measured. The experimentally determined size and shape of micelles are therefore not unequivocally defined parameters of a surfactant solution but descriptive characteristics of micelles. The characteristics of micelles formed by hydrocarbon-type surfactants have been extensively studied. In dilute solutions, micelles are considered to be spherical or nearly spherical. The minimum space of the micelle interior has been calculated with the assumption that the radius of the interior region is approximately equal to the length of the fully extended hydrophobic chain and the empty space in the micelle interior is absent. The latter assumption is in accord with the liquidlike character of the micelle interior, supported by substantial evidence. However, micelles formed by hydrocarbon-type surfactants are larger than the space required for the hydrocarbon chain. Oblate or prolate micelle models have been proposed. At higher surfactant concentrations, a surfactant may form rodlike, cylindrical, disk-shaped, or lamellar micelles [ l ] or vesicles, which are lamellar micelles arranged in a spherical shape with water in between the lamellas (see Fig. 6.1). (See Section 7.4.) Micellar solutions of hydrocarbon-type surfactants have been characterized by several techniques, including light scattering, tensiometry, electric conductivity, solubilization, nuclear magnetic resonance (NMR), fluorescence quenching, 277

278

Chapter 7

small-angle neutron scattering (SANS), electric birefringence, ultrafiltration, sedimentation rates i n the ultracentrifuge, and x-ray scattering [2-61 (see Chapter 9). The size and shape of manyhydrocarbon-type surfactants are known as a function of a variety of conditions. The aggregation numbers of hydrocarbon type micelles in water range from 25 to over 10,000 [6]. The size of micelles increases with increasing length of the hydrophobic chain but decreases with increasing size of the hydrophilic group. A neutral electrolyte decreases the aggregation number of anionic surfactants. The effect of electrolytes on nonionic surfactant solutions is more complex. The aggregation numbers of ionic surfactants decrease with increasing temperature, but polyoxyethylated nonionic surfactants aggregate extensively at higher temperatures (cloud point, Section 6.3). In contrast to the extensive information on hydrocarbon-type surfactants, the knowledge of fluorinated surfactant micelles is sparse, probably because the fluorinated surfactant solutions are more difficult to investigate. Light scattering has been the most important tool for the study of hydrocarbon-type micelles. Unfortunately, the refractive indexes of fluorocarbons are close to that of water (see Chapter 3) and the determination of micellar shape and size of perfluorinated surfactants by conventional light-scattering techniques is difficult, if not impossible. Less sensitive or more complicated techniques have to be used for the study of perfluorinated surfactants. Micellar aggregation numbers of partially fluorinated surfactants have been determined by light scattering, however [7]. X-ray scntterilzg techniques have been used extensively in the investigation of micellar solutions [8-141. In solutions of hydrocarbon surfactants, three diffraction bands have been attributed to the surfactant: 1. The “M” or micelle thickness band 2. The “S” or short spacing band 3. The “I” or long spacing band, dependent on surfactant corlccntration The x-ray scattering pattern of fluorinated surfactants differs from that of hydrocarbon-type surfactants. M band spacings have been observed for potassium tetradecanoate but not for ammonium salts of C7 and C g perfluoroalkanoic acids (Fig. 7.1) [ 151. Arrington and Patterson [ 151 explained the lack of M spacing by a higher electron density of the fluorocarbon chain. Luminescence and electronic energy trmsfer have been used frequently tq determine aggregation numbers of hydrocarbon type surfactants [3]. In contrast, a few luminescence studies have been made for fluorinated surfactants [16-191. Turro and Lee [ 161 employed the luminescence method to determine the aggregation number for sodium perfluorooctanoate micelles. The aggregation number calculated from quenching of Ru(bpy)S+ phosphorescence was found to be seven. Although this numerical value may be inexact, the aggregation number of perfluorooctanoate micelles is undoubtedly much smaller than that of micelles of corresponding hydrocarbon surfactants. In spite of the small size, sodium perflu-

Structure of Micelles and Mesophases

279

0.50

E ' E! 4

0.25 4

1 --

0.50

COOH

COOH

WATER WATER

WATER

0.25

0

25

50

75

100

125

Distance, 7.1 Electron density: potassium tetradecanoate (upper); ammonium perfluoroalkanoate (lower). (From Ref. 15. Reproduced by permission of the American Chemical Society.) FIG.

Chapter 7

280 80

60 280 h

V a4

m

2 40

260

5

s

-“--e e

20

24 0

2 20 (

01

0302 04 Conc of LIFOS ( m o l / l )

05

FIG.7.2 Micelle aggregation number N (0),pyrene fluorescence lifetime T~ (A), and fluoresecnece intensity ratio /,/I3 ( 0 )as a function of lithium perfluorooctanesulfonate concentration at 25°C.(From Ref. 17. Reproduced by permission of Academic Press, Inc.)

orooctanoate micelles exhibit characteristics of a conventional micelle, such as solubilization of hydrophobic substances and enhanced quenching by quenchers solubilized in micelles containing energy donors. Muto et al.[ 171 determined the aggregation number, N , for Li perfluorooctanesulfonate by fluorescence decay of pyrene solubilized in micelles. The aggregation number was found to have a very low value at low concentrations, N = 6 at 2 X 10-’M. The aggregation number increased rapidly with increasing concentration up to 0 . M and slowly beyond this concentration (Fig. 7.2). The small micelle size is in accord with conductivity and kinetic data of Hoffmann and Tagesson [21].The degree of ionization was found to be large at the critical micelle concentration (cmc), indicating small micelles. Muto et al. [ 171 explained the small micellar size by two antagonistic effects. First, the cmc values of fluorinated surfactants are lower than those of hydrocarbon surfactants with an equal chain length, because of the strong hydrophobicity of a fluorocarbon chain. Second, the rigidity of the fluorocarbon chain may prevent the formation of spherical micelles right at the cmc, in contrast to hydrocarbon surfactants. Lai et a]. [20] investigated reversed micelles of perfluoroheptanoic acid (FHA) in aqueous 1,1,2-trichlorotrifluoroethane(TCFE) by fluorescence, NMR, infrared (IR), and ultraviolet (UV)-Vis spectroscopy. The spectra of three types of fluorescent probes were used: a hydrophobic probe (pyrene), ahydrophilic probe

Structure of Micelles and Mesophases

281

(NH4 or Na 8-anilino-l-naphthalenesulfonate,ANS), and a complex probe (pyrene in the presence of N,N-dimethylaniline). The Z1/Z3ratio (the ratio of the fluorescence intensity of the first and third vibronic peaks of pyrene), which indicates the polarity of the probe environment, increases with increasing FHA concentration and the formation of reversed micelles. The formation of reversed micelles is driven by interactions of TCFE, which are lyophobic with the carboxyl group of the surfactant and favorable with the fluorocarbon chain of the surfactant. The aggregation numbers and the amount of water solubilized in the reversed micelles increase with increasing FHA concentration. Chemical relaxation techniques[21-231 have been utilized to determine the aggregation numbers for salts of perfluorooctanesulfonic acid [2 1,221.The aggregation numbers have been calculated from the kinetic data, based on the assumption that the micellar distribution curve obeys a Gaussian distribution function. This procedure was validated with aggregation numbers for hydrocarbon-type surfactants, which were in agreement with those obtained from light-scattering data. The aggregation numbers for the tetraethylammonium salt were calculated from kinetic data with the assumptions that the incorporation of surfactant molecules in a micelle is diffusion controlled, the rate constant for the association of a surfactant molecule to the micelle k+ = lo9 L/mol s at 15"C, and the energy of activation is 8 kcal (Table 7.1). The aggregation numbers increased with decreasing temperature and increasing hydrophobicity of the counterion. The aggregation numbers were considerably larger than those expected for spherical micelles. The length of the carbon chain would have limited the aggregation number TABLE 7.1 Association and Dissociation of Some Perfluorinated Surfactants

Temp. ("c)

Surfactant C8F17S03N(C2H5)4

C7H15COOH C7FI5COO(H/Na = 1:l) C7FI5COONa (23) ~~

5 10 15 20 25 30 15 25 25

k+ (Umol . s)

4.8 X 6.0 X 7.3 X 8.2 X 1.0 X 1.2 X 7.3 X 1.0 X 1.0 X

108

I 08

lo8 108 109 109 108 109 109

k-

(s-l) 4.6 X 5.7 X 6.8 X 7.6 X 9.2 X 1.3 X I .o X 1.4 X 3.2 X

105 105 IO5 105 105 10' 107 107

lo7

n 210 204 183 155 140 11,000~ 10,ooom IO,OOO~ 12

U

26 28 29 30 32 48 43 50 <5

~

Note: Symbols: k+, rate constant for a micelle entering the micelle; k-, rate constant for a monomer exiting the micelle; n, average number of monomers in the micelle; 0; variance of the number of monomers in micelle; "', mesophases. Source: Ref. 22; reproduced by permission of Z. Phys. Chem. Neue Folge.

Chapter 7

282

to G20. Hoffmann and Tagesson [21] concluded, therefore, that the shape of the micelle must be nonspherical. The unusual relaxation behavior of magnesium and dimethylammonium salts of perfluorononanoic acid was attributed by Hoffmann et al.[24]toemulsion-droplet-likegiantmicellescontainingabout 1000 monomers. More recent studies by Hoffmann and co-workers have indicated, however. that the giant micelles are probably dispersions of liquid-crystalline mesophases [25]. Nuclear. rnagnefic resoncrnce is a very powerful tool for investigating micellar structures of fluorinated surfactants. NMR spectroscopy yields values of the free energy of micellization, AGS!, and the corresponding enthalpy and entropy changes, AH:?, and AS:,* [26-321. The effect of the environment on fluorine atoms in the micelle has been studied by NMR. Muller et al. [26,28,31] determined the chemical shifts of five structurally nonequivalent types of fluorine atom in salts of perfluorooctanoic acid to indicate the “degree of hydrocarbonlike character” of an atom in the micelle. They described the effect of the environment of CF3 numerically by the parameter 2, defined as

where S represents the surfactant ion and nz the aggregation number, S(S,,,) the chemical shift of the fluorine nuclei in the micellized surfactant, S(S) the chemical shift of the monomeric surfactant. and S(F) the chemical shift of the surfactant in a fluorocarbon medium. In the absence of water in the environment of the fluorocarbon groups S(S,,,) = SCF) and, consequently, Z = 1. If the surfactant is not soluble in a fluorocarbon, a correct value for S(F) is difficult to obtain. Empirically, Z=

S(S,,,) - SCS) 3.8 - S(S)

The Z values obtained by Muller et al. (Table 7.2) [33] suggested that the polar hydrophilic groups of the surfactant do not cover the micelle surface completely but leave some of the hydrophobic surface exposed. The environment is more organic for the o-carbon than for the P-carbon and the a-carbon, which is the least organic environment. The a-carbon isexposed to water because of its location vicinal to the hydrophilic group. The neighboring p-carbon is exposed to water to some extent. The 2 values suggested that even the o-carbon of an o-trifluoro group is slightly but significantly hydrated. The I9F spin-lattice relaxation times of heptafluorobutyric acid in water and D20 also have indicated that the fluorocarbon chains in the micelle are to some extent exposed to water [34]. However, the penetration of water into the interior

Mesophases Structure and of Micelles

TABLE 7.2

283

Degree of Organiclike Character Zvalues

C3F7COOH [29] C7F,,COONa [31] CF3(CH,),COONaa[26]

0.71 0.37

0.77 0.66

0.82 0.84 0.53

a n = 9,11,12. Source: Ref. 33; reproduced by permission of the American Chemical Society.

of the micelle has been disputed by others [34,35]. Ben. and Jones [35] found the model in which water does not penetrate into the fluorocarbon core to agree best with their SANS data. The NMR measurements of 13C spin relaxation [36.37] have provided useful information on micellar structure. Fur6 and Sitnikov [37] investigated the order parameter profile of the perfluorooctanoate chain in cesium perfluorooctanoate (CsPFO) micelle by I9F decoupled "C-NMR relaxation experiments at three different magnetic fields. The order parameter profile of the perfluorooctanoate chain differs both in shape and magnitude from that of the octanoate chain. The order parameter profile of the fluorinated chains is flat, indicating a chain packing that facilitates lamellarlike aggregation in oblate-shaped micelles of CsPFO. The micellar structures formed by LiFOS and TEAFOS (tetraethylammonium perfluorooctanesulfonate) have been examined by measuring NMR chemical shifts and self-diffusion coefficients of both TEA" and FOS- [38,39]. The micellar self-diffusion coefficient in the TEAFOS system is about an magnitude lower than that of the LiFOS system. The slower self-diffusion in the TEAFOS system is related to a strong binding of TEAf ions onto the micellar surface and a transformation fromspherical to threadlikemicelles,induced by TEA+ counterions. Partially fluorinated surfactants also have been investigated by NMR. Muller and Birkhahn [27] found the fluorine chemical shift for anionic terminally fluorinated swfrrctmzts, CF3(CH2)sCOONaand CF3(CH2),&OONa. to be essentially independent of ionic strength. An added electrolyte depressed cmc but did not affect the chemical shifts for the surfactant anions in the micelle. They explained their results by the formation of prolate-shaped spheroid micelles in the presence of sufficient electrolyte. An alternative explanation that electrolytes do not increase the micellar size was considered to be unlikely. Muller and Johnson [28] studied the effect of organic additives on sodium 12,12,12-trifluorododecylsulfate micelles. Mixed solvents used in the study included aqueous urea, glycine, glycerol, acetamide, methanol, ethanol, acetone,

284

Chapter 7

dioxane, and tetrahydrofuran. Changes in the shapes of the dilution-shift curves indicated a penetration of additive into micelles and a strong effect of theadditives on the micellar size. In some cases, the aggregation numbers were decreased to a value below 5. The demicellizing effectiveness of the additives was not related solely to a change in the dielectric constant. The effects of cosolvents, as well as temperature effects, were qualitatively similar to those of the corresponding nonfluorinated surfactants. In water, the tetramethylammonium salt of trifluorododecyl sulfate forms smaller micelles than the sodium or potassium salts. In 2.OM dioxane, the sodium or potassium salts form smaller micelles than in water, with aggregation numbers of about 10-20. The tetramethylammonium salt forms larger micelles in water containing dioxane than in water alone. In 2.OM tetrahydrofuran, the three salts gave similar results, but at the same concentration, tetrahydrofuran reduces the micellar size more than dioxane, to a very low aggregation number, possibly 3-4 [311. The NMR spectra of terminallv fluorinated nonionic sulfactantsresemble those of anionic surfactants with the same hydrophobe [30]. Thesurfactants 8 3 3 trifluorooctylhexaoxyethylene glycol monoether and 8,8,8-trifluorooctyl methyl sulfoxide yielded the same S(S) value, which is identical to that of corresponding anionic surfactants. Hence, the nearby environment of the CF3 group appears to be unaffected by the chemical constitution, size, or electric charge of the hydrophile. The aggregation number for 8,8,8-trifluorooctylhexaoxyethylene glycol monoether was found to be 16 at 27°C. At temperatures above 42”C, the micelles were larger. The addition of urea also caused an increase in the micelle size. The fluorine NMR chemical shifts of terminally fluorinated cationic s w factants 12,12,12-trifluorododecyltrimethylammoniumbromide and 10,10,10-trifluorodecyltrimetylammonium bromide are similar to those of the corresponding anionic and nonionic surfactants with a terminal trifluoromethyl group [32]. When the counterion is fluoride, instead of bromide, the chemical shift of the fluoride counterion is concentration dependent. The trifluoromethyl chemical shifts were interpreted utilizing a double-equilibrium model. The aggregation number was estimated to be 25 in dilute solutions from the curvature of the chemical shift concentration plots near the first cmc. Above the “second cmc,” the aggregation number was assumed to be 60, considered the spatial limit for 1Zcarbon surfactant chains in a spherical micelle. The micellar structures, lamellar phases, and vesicles formed by fluorinated surfactants and polymers have been investigated by electron spin resonance (ESR) [40-511 (see Section 9.8). Because the micellar systems do not have a net quantum mechanical angular momentum, a paramagnetic probe must be included in the sample (see Section 9.8 for structures of the probes). The use of spin probes is a useful technique for studying micellar systems provided that the probe does not perturb the aggregates being studied.

Structure of Micelles and Mesophases

285

Micellar systems of ammonium pentadecafluorooctanoate have been investigated by ESR of small and large cationic probes (TempTMA+ and CAT12) and long-chain neutral nitroxides (5-DXSA7 12-DXSA, and 16-DXSA) [42,43]. The information obtained by Ristori and Martini [42] using the small and positively charged TempTMA+ and the neutral long-chain doxy1 nitroxides (5- and 16DXSA) as ESR probes was comparable to that provided by other techniques. However, the charged long-chain probe CAT12 caused strong perburtations. Hence, the choice of the probe is critical. Micellar solutions of ammonium perfluoropolyethercarboxylate (PFFPENH4) have been studied by ESR using cationic nitroxide probes, TempTMA+ [40] and CAT12 [41], as well as using neutral spin probes (C12-Tenlpoand C16Tempo) [4 11. The cationic spin probe CAT12 caused strong perburtations near the cmc of the surfactant. Only when the ratio PFPENH4/CAT12 was above 20-25 did CAT 12 function as a true probe and provide useful information on micelle formation. At these dilutions of the probe, the CAT12 gave results similar to those obtained with TEMPTMA+. However, the comparison was complicated by the different location of the probes: CAT12 enters the micelle, whereas TEMPTMA+ is located on the surface of the micelles. The state of water in the interlamellar regions of perfluoropolyether ammonium carboxylates has been examined by using Cu(I1) and Mn(I1) ions as probes to avoid any possible perturbance caused by a hydrophobic chain of the probe [44,45]. The investigation of perfluorinated surfactants by light scattering is hampered by the low refractive index of fluorocarbons. Partially fluorinated surfactants are amenable, however, to the light-scattering technique. Gerry et al. [7] measured aggregation numbers of 12,12,12-trifluorododecy1trimethylammoniumbromide and dodecylammonium bromide in NaBr solutions by light scattering (Table 6.14 of Chapter 6). The aggregation numbers for the surfactant with the terminal CF3group were only slightly smaller than those for the corresponding unfluorinated surfactant, although fluorination had nearly doubled the cmc value. This result appeared to be puzzling at first, because fluorocarbons are more hydrophobic than hydrocarbons and fluorination of a surfactant should favor micellization. Surfactants with a completely fluorinated hydrophobic chain behave as expected. However, partially fluorinated surfactants are different. An o-trifluoromethyl group resists mixing with hydrocarbon chains and the resulting mutual phobicity causes the CF3- group to seek the water-micellar core interface, instead of the hydrocarbonlike core [33,52]. The micellar size constancy in the 0.5M to 1.OM NaBr concentration range suggests a geometrically defined upper size limit of the spherical micelle. The increase of the aggregation number at NaBr concentrations above 1.OM is related to a micellar shape transformation and polydispersity. Small-angle neutron scattering (SANS) is an excellent technique for the study of micellar solutions [35,53]. The large difference in scattering lengths be-

286

Chapter 7

tween 19Fand 'H provides a strong contrast between the aqueous ('H2O) solvent and the micelles of the perfluorinated surfactant [35]. Herbst et al. [54] observed that a sample of 30% (w/w) C8FI7COON(CH3)4in D 2 0 forms three lyotropic phases at different temperature ranges (see Section 7.4). The isotropic solution at 237°C close to the liquid-crystal boundary contained disklike micelles. The aggregation number of themicelles was about 150. The thickness of the micelles, including the counterions. was about 35 A. Hofftnann et al. [55] examined the size and shape of micelles of LiPFO (lithium perfluorooctanoate) and DEAFN (diethylammonium perfluorononanoate) by SANS. Mixtures of H20and D20 were used as the solvent to increase the accuracy of SANS data by varying the contrast of the solvent. Spherical micelles were found in LiPFO solutions and spherical vesicles of various size in DEAFN solutions. Berr and Jones [35] determined. by SANS, the charge and aggregation number for aqueous solutions of sodium perfluorooctanoate as a function of surfactant concentration above cmc. Their SANS data suggested a spherical micelle model with a water-free core of alkyl chains. The aggregation number increases linearly with the increasing square root of the surfactant concentration. The cmc of sodium perfluorooctanoate is an order of magnitude smaller than the cmc of sodium octanoate. The aggregation number of sodium perfluorooctanaoate is larger than that of the sodium octanoate, in accord with the greater hydrophobicity of the perfluoroalkyl chains and the smaller head-group charge of the fluorinated surfactant. The electronegative fluorine withdraws electron charge from the head group and the reduced Coulumbic repulsion between the head groups allows a closer packing in the micelle. Burkitt et al. [56] investigated the size and shape of tnicelles of ammonium salts of octanoic, decanoic, and perfluorooctanoic acids by SANS. Ammonium octanoate and ammonium decanoate formed spherical micelles having a micellar weight of 1640 and 12,576, respectively. Ammonium perfluorooctanoate formed cylindrical micelles with a mean micellar weight of 17,610. This corresponded to a mean association number of 43 at 0.12M. Burkitt at al. proposed a cylindrical micelle in which the head groups are hexagonally close-packed and the fluorocarbon chains form helical rows (Fig. 7.3). Berr and Jones [35] calculated the aggregation number for sodium perfluorooctanoate to be 28 at0.12M. The smaller aggregation number of sodium perfluorooctanoate allows the the micelle to remain spherical, whereas ammonium perfluorooctanoate with larger aggregation numbers [56] forms ellipsoidal micelles. Tamori et al. [ 191 studied the micellar properties of a cationic partially fluorinated surfactant CsF17CH2CH(OH)CH2NCH3(C2H40H)zCl(DEFUMAC) by electric conductivity, surface tension, fluorescence of pyrene-3-carboxaldehyde, dynamic light scattering, and viscosity measurements. An increase in electrolyte and surfactant concentration induced micellar elongation. The effect of salt

" "

" "

Structure of Micelles and Mesophases

287

A PF 0 Micelle FIG. 7.3 A cylindrical micelle model for ammonium perfluorooctanoate. (From Ref. 56. Reproduced by permission of Academic Press, Inc.)

on the shape of the micelles was attributed to a multiple equilibrium. The elongation resulting from an increase in surfactant concentration is rapid above a certain concentration, a second cmc. Asakawa et al. [57]used cyclic voltammetry with a suitable probe to measure the diffusion coefficients of fluorinated surfactant micelles. This method requires a complete solubilization of a small probe into micelles without altering the shape and size of the micelle, a theoretically unattainable condition. A cationic ferrocene derivative, (ferrocenylmethy1)trimethylammoniumbromide was introduced into an anionic surfactant (LiPFN, LiFOS, LiHFDeS) micelle by electrostatic interaction. The diffusion coefficient decreased with the addition of salt (LiCl) indicating a salt-induced micelle growth. Cryogenic transmission electron microscopy was used by Knoblich et al. [58]to examine the micellar structure of LiFOS and TEAFOS. TEA+ and Li+ as counterions not only screen the surface charge of the micelles but also modify the micellar surface. TEAFOS micelles are outwardly more hydrophobic than LIFOS the micelles. The hydrophobic nature facilitates aggregation of the spherical micelles to threadlike structures. As a result, the viscosity for TEAFOS solutions is significantly higher than that for LiFOS solutions [58,59]. The rheological behavior of TEAFOS threadlike micelles was investigated by Watanabe et al. [60].The threads are, like polymer chains, oriented under strain and exhibit viscoelastic stresses. However, unlike polymer chains, the threadlike micellar aggregates dissociate and reform reversibly. In the linear viscoelastic regime, the threads form a transient network and exhibit a single-mode terminal relaxation. Under shear flow, the TEAFOS threads exhibit thinning of the viscosity related to a distortion of the existing structural order. Shear thinning at high shear rates indicates a massive disruption of the threads. Cryogenic transmission electron microscopy was used also by Wang et al. [6 11 to study the aggregation behavior of cationic fluorinated surfactants 1,1,2,2-

Chapter 7

280

tetrahydroperfluorooctylpyridiniumchloride(HFOPC).1,1,2,2-tetrahydroperfluorodecylpyridinium chloride (HFDePC). 2-hydroxy- 1,1,2,3,3-~entahydroperfluorononyldiethylammonium chloride(I-C9). and 2-hydroxy-1,1,2,3,3pentahydroperfluoroundecyldiethylamrnoniumchloride (I-C 1 1) in aqueous solutions. The effects of surfactant and electrolyte concentration, counterion type, and perfluoroalkyl chain length were examined. The fluorinated surfactants like hydrocarbon surfactants assemble in solution into spherical micelles, threadlike micelles, vesicles, lamellar aggregates, and various lyotropic crystalline phases. However, fluorocarbon surfactants are more prone than hydrocarbon surfactants to fonn flatter structures, such as cylindrical micelles and bilayer structures. Even a small reduction of the repulsion between their head groups by an added electrolyte can cause a more rapid and extensive micellar elongation than observed for hydrocarbon surfactants. The different micellar behavior of fluorinated surfactants has been attributed to a more hydrophobic nature, a larger molecular volume, a larger cross-sectional area, and a higher stiffness of the fluorocarbon chain compared to the hydrocarbon chain.

7.2 THEORY FOR MIXED MICELLES The interest in mixed surfactant systems has theoretical as well as practical reasons. The realization that surfactants are used most often as mixtures has shifted research from pure surfactants to mixed systems. Industrial fluorinated surfactants are usually mixtures [62.63] because (1) they are synthesized as a mixture of isomers or telomers which are difficult to separate or (2) different surfactants are mixed to enhance the performance of the individual components by synergistic interactions. Some commercial fluorinated surfactants contain hydrocarbon-type surfactants which have been blended with the fluorinated surfactant to improve its performance. Mixed systems containing fluorinated surfactants and hydrocarbon surfactants are currently investigated for several theoretical and practical reasons: 1. The increased efficiency of surfactant mixtures allows one to reduce the amount of a fluorinated surfactant needed. 2. The fluorinatedsurfactant-hydrocarbonsurfactantmixtureshave unique properties. In two-phase systems of water and a hydrocarbon solvent. the fluorinated surfactant reduces surface tension and the hydrocarbon surfactant decreases the interfacial tension. For example, an aqueous foam of mixed surfactants spreads on a hydrocarbon solvent because the fluorinated surfactant adsorbs preferentially at the air-water interface, whereas the hydrocarbon surfactant adsorbs at the water-oil interface (see Chapter 8, Fire-fighting Foams).

Structure of Mesophases Micelles and

289

3. The fluorinated surfactant-hydrocarbon surfactant mixtures are of great theoretical interest for studying the formation and structure of micelles. Fluorocarbon groups are not only hydrophobic but are oleophobic as well. The phobic interactions between fluorinated and hydrocarbon chains in surfactant mixtures cause nonideal behavior and, in some systems, demixing. The formation of mixed micelles and the coexistence of two different kinds of micelles are being investigated to gain more insight into the behavior of micellar solutions. Because of the extensive practical and theoretical interest in mixed fluorocarbon-hydrocarbon surfactants, it is not surprising that the mixed-surfactant systems have been reviewed in two monographs [64,65], and numerous articles on fluorocarbon-hydrocarbon surfactant mixtures have appeared in print. A micellar solution containing a fluorinated surfactant and a selected hydrocarbon surfactant can exceed the surface activity predicted for an ideal solution in which the components do not interact. This synergism increases the effectiveness of the surfactant and, because of the lower cost of hydrocarbon-type surfactants, results in considerable savings. Because perfluorinated surfactants are not biodegradable (Chapter lo), the use of fluorinated surfactants at lower concentrations is also advantageous from the environmental point of view. The synergism observed in solutions of mixed dissimilar surfactants is governed by monomer-micelle equilibria and the interaction in micelles. The concentration of surfactant monomers below the cmc is so dilute that interactions between surfactant monomers are insignificant. The composition of mixed micelles has been determined from surface tension data, electric conductivity, NMR, selfdiffusion measurements, fluorescence quenching, solubilization, ultrafiltration, and gel filtration. Complete nonmixing of a fluorinated surfactant and a hydrocarbon surfactant, partial mixing, and the existence of two types of mixed micelles or one mixed micelle has been reported. In an ideal system of two similar surfactants of like charge, the total monomer concentration is between the cmc values of the two surfactants, if the total surfactant concentration is at or above the mixture cmc (Fig. 7.4) [64]. At equilibrium [64],

where C M is the monomer concentration, yA and ?'B are the mole fractions of monomers A and B, respectively, cmcA and cmcB are the cmc values of the monomers, respectively, and XA is the micellar mole fraction of component A.

290

Chapter 7

I

?

iz

E

.50

POSITIVE DEVIATION FRW IDEALITY: Sodium Tetradecyl Sulfate/Unspeclfied Fluorocarbon Surfactant In

V

r:

0

"-

0.05 M NaCl.

--

.20-,

-10-

-

/ " "

/ /

" "

/

/

NEGATIVE DEVIATION FROn IDEALITY: HexadecylPyridiniumChloride/Nonylphenol Polyethoxylatewith in 0.03 M NaC1.

10 EO groups

-. HW -&."4. 0

0.2

0.4

0.6

1

0.8

MONOMER MOLE FRACTIONOFSURFACTANT

A

FIG.7.4 Critical micelle concentrations for surfactant mixtures at 30°C (first surfactant listed is surfactant A). (From Ref. 64. Reproduced by permission of the American Chemical Society.)

Structure of Micelles and Mesophases

291

The ideal solution theory can predict the concentration of each surfactant in micelles or the monomeric state for mixed hydrocarbon surfactants of similar structure. However, hydrocarbon surfactant mixtures of dissimilar structures, such as ionic-nonionic or cationic-anionic surfactant mixtures, exhibit nonideal behavior. The deviation from the ideal relationship can be negative or positive (Fig. 7.4). Unlike mixed hydrocarbon-chain surfactants of similar molecular structure, mixtures of fluorinated surfactants and hydrocarbon-chain surfactants do not behave ideally, even when the surfactants have a similar hydrophilic group. Mixtures of anionic fluorinated surfactants with anionic hydrocarbon surfactants exhibit a positive deviation from the ideal relation (Fig. 7.4). In contrast, surfactant mixtures containing a nonionic surfactant or oppositely charged ionic surfactants exhibit a negative deviation from ideal predictions. The formation of mixed micelles is governed by hydrophobic interactions between hydrocarbon and fluorocarbon chains and electrostatic effects [66]. Introduction of nonionic surfactants into micelles of anionic fluorinated surfactants reduces electrostatic repulsion between the ionic head groups. Apparently, the resulting electrostatic effect overcomes the hydrophobic interaction between the fluorocarbon and hydrocarbon chains. If the positive deviation from ideal mixing in the micelle is severe, the mixed micelles are thermodynamically unstable and separate into two coexisting micelle types of different composition. Hence, surfactant mixtures containing anionic hydrocarbon and fluorocarbon surfactants form mixed micelles or two kinds of micelles, each rich in one of the components. The miscibility of hydrocarbon surfactants and fluorocarbon surfactants depends on the molecular structure of the surfactants, their concentrations, the composition of the solution, the temperature, and the pressure. Klevens and Raison [67] studied micellization of mixtures of sodium dodecyl sulfate and perfluorooctanoic acid. The phase diagram reveals mutual phobicity of hydrocarbon and perfluorocarbon chains of the surfactants (Fig. 7.5). The extent of the mutual phobia was examined by Mukerjee and Yang [68] with mixtures of sodium perfluorooctanoate (SPFO) and sodium decyl sulfate (SDeS) and with sodium laurate (SL). The cmc values of the mixed systems were calculated assuming (1) the extreme case of complete demixing or (2) ideal behavior. If the overall surfactant concentration is increased in a system of complete demixing, micelle formation begins when the concentration of one of the components exceeds its cmc value. If the surfactants are nonionic, the cmc versus mole fraction curve is given by cmc

cmcO X

= ___

(5)

where cmco is the cmc value for the pure surfactant and X is the mole fraction in

Chapter 7

292 FOA MICELLES

a W

0

I

1

1

1

1

1

I

05

C,,SO,Na

Molt fRACTIOH

1

1

1

pel

FIG.7.5 Equilibrium compositions of micelles and monomers in the sodium dodecyl sulfate and perfluorooctanoic acid system. Two types of micelles are present with limited mutual solubility. (From Ref. 33. Reproduced by permission of the American Chemical Society.)

a mixture. For ionic surfactants, the common ion effect of the counterion has to be taken in account: log(cmc)

=A -

B log[Na+]

where A and B are constants [69,70].

(6)

Structure of Micelles and Mesophases

293

The cmc value of a mixed system is given by cmc = (cmco) antilog

i'","")

The cmc values calculated for complete demixing and for ideal behavior are shown together with the observed cmc data in Fig. 7.6. The dashed lines show expected values for ideal mixing of micelles. assuming B = 0.645 for all three components. Curves 1, 2, 3, and 4 represent complete demixing of micelles [Eq. (7)]. Curve 1 was calculated for B = 0.53; for curves 2, 3, and 4, the value B = 0.645 was used. The experimental data are close to the curves for complete demixing, indicating severe nonideality of mixing. The cmc data of Fig. 7.6 suggest that two micellar species may coexist, one rich in the fluorinated surfactant and the other micellar species consisting mainly of the hydrocarbon surfactant. Mukerjee and Yang [68] measured differential conductance

50

-

Mole Fractlon of Hydrocarbon Surfactant FIG.7.6 Critical micelle concentrations for mixtures of sodium perfluorooctanoate with sodium decyl sulfate (A)and sodium laurate (0).Dashed lines represent expected values for ideal mixing, assuming B = 0.645 [Eq. (6)]. Curves 1, 2 , 3, and 4 show expected values for complete demixing of micelles [Eq. (7)]. Curve 1 was calculated for B = 0.53; curves 2 , 3, and 4 were calculated assuming B = 0.645. (From Ref. 68. Reproduced by permission of the American Chemical Society.)

294

Chapter 7

where K? and K~ are the specific conductance at two adjacent concentrations C? and C1.The differential conductance of mixtures of SPFO and sodium dodecyl sulfate (SDS) are plotted versus the mean concentration (C, + C1)/2in Fig. 7.7. Curves 1,5, and 6 represent single surfactants, curves 2 and 4 show data for mixtures containing a fluorinated surfactant (SPFO), and curve 3 represents a mixture of hydrocarbon-type surfactants (SDS and SDeS). Curves 2 and 4 show two inflection points before flattening at higher concentrations. Mukerjee and Yang [68] proposed that micelles composed of SDS are formed first. As the total concentration increases, the fraction of monomeric SPFO increases. At about the second inflection point, micelles form which consist mainly of SPFO. At higher concentrations, two kinds of micelles coexist. Advances in the theory of mixed-micelle formation have made it possible to calculate the composition of mixed micelles formed by two or more surfactants. A thermodynamic treatment of micellar solutions of mixed surfactants is usually based on the pseudophase separation tlzeoy [61,71-74]. The pseudophase models developed for binary surfactant solutions assume ideal mixing of the surfactants in the micelle.

I

FIG.7.7 Differential conductance, l o 3 ( K ~ K ~ (C2 ) - Cl) plotted against mean concentration (C, + C1)/2:curve 1, SDS; curve 2, SDS + SPFO (SDS mole fraction 0.5); curve 3, SDS + SDeS (SDS mole fraction 0.29); curve 4, SDS + SPFO (SDS mole fraction 0.2); curve 5, SPFO; curve 6, SDeS. (From Ref. 68. Reproduced by permission of the American Chemical Society.)

Structure of Micelles and Mesophases 295

The pseudophase separation model of micellar solutions considers a micelle to be a pseudophase in a liquid state. Because the micelles are assumed to have a liquidlike core, the mutual solubility of a fluorinated surfactant and a hydrocarbon surfactant in mixed micelles is, according to the pseudophase model, governed by the miscibility of the fluorocarbon and hydrocarbon chain. For example, heptane and perfluoroheptane are immiscible at 25°C but above 50°C these liquids are miscible in all proportions [75]. A terminal substitution of a hydrophilic group depresses the enthalpy of mixing and makes the components miscible at 25°C. The mutual solubility of the perfluoroalkanoic and alkanoic acids suggests that their salts should be miscible in micelles, at least partially. Depending on the chain length and the surfactant structure, the mixture may form a mixed micelle or two types of micelles: one rich in the fluorinated surfactant and the other rich in the hydrocarbon surfactant. Shinoda and Nomura [71] reported partial miscibility for the C8H17COONa-CI3H25S01Na system and miscibility in all proportionsforthe C7F1sCOONa-C~~H21S04Namixed micellesat25°C. NMR data indicated that C7FI5COOLimixes with C12H3-5S03Li in all proportions, but C8F17COOLi, CgF,&OOLi, and C10F31COOLido not mix completely with C12H2sS03Li.The mutual miscibility decreases with increasing chain length of the fluorocarbon and hydrocarbon surfactants [76]. The solubility and critical solution temperatures lead to the conclusion that a carbon chain longer than eight carbon atoms causes the liquid-liquid mixtures of alkanoic and perfluoroalkanoic acids to separate into two phases. For example, a 1 : 3 (v/v) mixture of C7FIsCOOHand C7H1sCOOHand an equimolar mixture of C8F17COOH and C1IH23COOH formasingleliquidphaseabove19°Cand 47OC, respectively. Below these temperatures, the respective perfluoroalkanoic acid precipitates.Studieshaveshown,however, that the interactionbetween hydrophilic groups can be more influential than the association of hydrophobic tails. The pseudophase separation model has been fairly adequate in describing the behavior of binary nonionic or anionic surfactant mixtures, especially when the surfactants have the same hydrophilic group. When the hydrophilic groups of the mixed surfactants are different. significant deviations have been observed. The interactions between the different groups have been taken into account in a pseudophase separation model, which assumes that the mixed micelle approximates a regular solutiorz [64,72,73,77,78]. For nonideal surfactant mixtures, equations derived from the regular solution theory [79] can give the interaction parameter, w, and the micelle composition at a given monomer composition:

Chapter 7

296

where R is the gas constant, Tis the absolute temperature, cmcA and cmcB are the cmc values for the individual components, respectively, YA and Y B are the monomer mole fractions of components A and B, respectively, and XA and X, are the micellar mole fractions of A and B, respectively. Scamehorn [64] has emphasized, however, that although these equations have accurately predicted mixture cmc values and monomer-micelle equilibrium. they do not validate the regular solution theory for nonideal surfactant mixtures. Ample evidence contradicts the validity of the regular solution theory in describing the nonideal surfactant mixtures. Rubingh [78], using the regular solution approximation, proposed an interaction parameter p for the treatment of the cmc of nonideal mixed surfactant solutions:

where C,, is the cmc of mixed surfactants, C1 is the crnc, and X1 the mole fraction of component 1, and a is mole fraction of surfactant 1 in the surfactant mixture present in the solution. Guo et al. [SO] have studied mixtures of sodium perfluorooctanoate (SPFO) with a nonionic, an amphoteric, and a cationic hydrocarbon-type surfactant by NMR and surface tension meaurements:

I

H N-triethoxylated heptanylamide(HEA8-3)

CH3

C"3(c"*)*-A*-C"2cH2C"2so; AH3

3-(decyldimethylammonio)-l-propanesulfonate(DEDIAP)

octyltrimethylammonium bromide(OCTAB)

Structure of Micelles and Mesophases

297

The anionic fluorinated surfactant (SPFO) forms with the nonionic or the amphoteric hydrocarbon surfactant mixed micelles containing both types of surfactants. Both systems exhibit a negative deviation from ideality. Changes of the I9F and 'H chemical shifts of the two surfactants upon mixing are consistent with pseudophase diagrams, calculated from the cmc dependence on the fluorinated surfactant mole fraction. The interpretation of data was based a modified regular solution theory and the phase-separation model. Osborne-Lee et al. [74] employed ultrafiltration to investigate binary mixtures of alkylbenzenesulfonates and oxyethylated nonylphenol. The regular solution approximation model was found to be inadequate for these mixtures. The model accurately represented cmc data but failed to predict correct micelle compositions. A new theory was proposed which included the effects of nonideal entropy variations resulting from nonrandom rearrangements in the mixed micelle and conformational entropy changes in the polyoxyethylene chain. The composition of mixed micelles was calculated using the regular solution theory and activity coefficients with a single empirical interaction coefficient. The solution of mixed micelles was treated as a classical multicomponent system. Treiner and co-workers [81] measured partitioning of 1-pentanol to elucidate the micellar composition of two aqueous mixed anionic surfactant solutions (sodium decyl sulfate + sodium perfluorooctanoate and sodium dodecyl sulfate sodium perfluorooctanoate). The partition coefficient exhibited a maximum in hydrocarbon-rich mixtures in both mixed micellar solutions. The synergistic effect was attributed to hydrocarbon-fluorocarbon repulsive interaction and found to be in accord with the regular solution model using interaction parameters determined for binary surfactant solutions. By applying the pseudophase model and considering the micelle as an ordinary solvent, the partition coefficient for the surfactant mixture can be expressed by

+

log PI,,= x log P, + (I - X ) log P?

+ PX(1 - X)

where P represents the partition coefficients of 1-pentanol in mixed (Pin) and pure surfactant solutions (Pl, P2) and X is the mole fraction of the surfactant in the mixed micelles. The p values proposed by Asakawa et al. [82] were used in the calculations: 2.1 for sodium decyl sulfate + sodium perfluorooctanoate and 1.8 for sodium dodecyl sulfate + sodium perfluorooctanoate. The agreement between the values predicted by Eq. (12) and the experimental values implies that the 1pentanol molecules are distributed throughout the micelles and not confined near the palisade layer as usually assumed for dilute single micellar solutions. The results suggested that the mixed micelles are dynamic structures in a highly disordered state. Treiner and co-workers [83] also determined the partition coefficient of 1pentanol between water and mixed micelles of lithium dodecyl sulfate and

298

Chapter 7

lithium perfluorooctanoate. The solubilization of 1-pentanol in the mixed-surfactant solution was calculated from the regular solution theory using the interaction parameter p. A positive deviation from ideal behavior was observed for the system as a consequence of the positive value of the interaction coefficient p. According to the regular solution theory, two types of mixed micelles should exist when the p value is above 2. The authors concluded. however, that the consequences of such a demixing effect on the solubilization profile depend on too many unknown parameters. The solubilization of n-alkanols (1-butanol. 1-pentanol, and 1-hexanol) and fluorinated alkanols (C,,H3F2rl-10H,IZ = 3. 4. or 5 ) in single and mixed aqueous solutions of fluorinated surfactants LIDS, LiFOS, LiPFO, and a nonionic hydrocarbon-type surfactant [hexaethyleneglycol dodecyl ether (6ED)I was investigated by Yoda et al. [84]. In single-surfactant solutions, partition coefficients between micelles and water were higher for solutions in which the alkyl chains of the surfactant and of the alcohol were similar, than for solutions containing a surfactant and an alcohol with dissimilar alkyl chains. In the LIDS-LiFOS system with a positive p value, the partition coefficients of alkanols or fluorinated alkanols were higher than for solutions of single surfactants. The LiFOS-LiPFO system exhibited an almost ideal behavior. The partition coefficients of the fluorinated alkanol were higher than those of alkanols. In contrast, the p values for the 6ED-LiFOS system were negative and the partition coefficients of the alcohols (alkanols or fluorinated alkanols) were lower. Kamrath and Franses [85] developed a single-micelle-size lnass action model for binary solutions of surfactants with the same hydrophilic group and counterion. The mass action model predicts micellar behavior more accurately than the pseudophase separation model [73] if the number of surfactant monomers in the mixed micelle is less than about 50. The single-parameter nonideal mixed-surfactant models are inadequate for treating the complex interactions between fluorinated surfactants and hydrocarbon surfactants. The possible involvement of water in micelle formation and electrostatic effects resulting from ionic interactions have to be taken into account. A more sophisticated model is therefore needed. Asakawa et al. [86] developed a group contribzction theory to characterize binary mixtures of surfactants. The more complex model is based on the concept that a solution is composed of many functional groups [87]. A group interaction parameter can be used to calculate mixed cmc values when the cmc values, the micelle counterion binding parameter, and molecular structure data are given for the pure components. The composition of micelles formed by mixed hydrocarbon and fluorocarbon surfactants is a controversial subject still being explored and debated. None of the existing theories is fully adequate to describe the micellar solutions of fluorocarbon-hydrocarbon surfactant mixtures. The interpretation of the interactions between the different surfactants depends on the specific micelle model used and

Structure of Mesophases Micelles and

299

the experimental conditions. The miscibility of fluorocarbon surfactants and hydrocarbon surfactants in binary solutions depends on the structure of the mixed surfactants, the composition of the solution, and the temperature. At least some of the apparent incongruities have been caused by differences in the composition of the systems studied and physical conditions. Undoubtedly, interactions of fluorocarbons and hydrocarbons are also profoundly nonideal in micellar systems. Research on the micelle structure and interactions of fluorinated surfactants is ongoing with the main focus on mixed-surfactant systems. Mixtures of fluorinated and nonfluorinated surfactants may consist of anionic, nonionic, or cationic components. Most of the systems investigated so far have contained a fluorinated anionic surfactant and an anionic hydrocarbon surfactant. Anionic fluorinated surfactant mixtures with nonionic or cationic hydrocarbon-type surfactants have been investigated as well. The nonionic fluorinated hydrocarbon surfactant mixtures and cationic fluorinated hydrocarbon surfactant mixtures have been the subject of only a few studies. Research on binary fluorinated surfactant-hydrocarbon surfactant mixtures is continuing with the main focus on the mutual solubility of mixed surfactants. It has been agreed [88] that an increase in the number of carbon atoms in either hydrophobe decreases the mutual solubility. An increase in the number of oxyethylene units in a nonionic hydrophile increases the mutual solubility of the mixed surfactants. An increase in temperature increases the mutual solubility. However, the mechanisms of fluorocarbon-hydrocarbon surfactant demixing and the formation of two types of coexisting micelles are still not completely understood. The gaps in knowledge are not caused by a lack of theories but are a result of experimental limitations. The development of more powerful analytical methods for the study of surfactants in micelles will advance the quantitative understanding of mixed-surfactant systems. Finally, most practical applications of surfactant mixtures involve adsorption at liquid-air, liquid-liquid, and solid-liquid interfaces. The theoretical understanding of the interactions in adsorbed surfactant mixtures is therefore of paramount importance (see Chapters 4 and 5).

7.3 SURFACTANT MIXTURES Mixtures of Anionic Surfactants. Perfluoroalkanoates The mutual, although limited, solubility of perfluoroalkanoic and alkanoic acids suggests that their salts should be miscible in micelles, at least partially [71]. The nature and the number of different types of mixed micelles are still being debated [89.90]. However, at least some of the apparent incongruities are caused by differences in surfactant structure, composition of the solution, and temperature. The miscibility and critical solution temperature (cst) of binary systems depend on the chain length, the surfactant structures, and the composition of the solution. Above

300

Chapter 7

cst, one kind of micelle exists in binary systems; below cst, two kinds of micelles can coexist. Mukerjee and Yang [68] have reported that in a solution containing sodium perfluorooctanoate and sodium decyl sulfate, two micellar species coexist: one rich in the fluorinated surfactant and the other micellar species consisting mainly of the hydrocarbon surfactant. On the other hand, studies made with pyrene as a fluorescence probe [2,17,91] found only one mixed micellar species. Kalyanasundaram[91]concluded that onlyonetype of mixed Li perfluorononanoate (LiPFN)-sodium lauryl sulfate (NaLS) micelle may exist over a wide range, in spite of severe nonideality of mixing. Burkitt et al. [92] studied mixtures of ammonium decanoate (AmDec) and ammonium perfluorooctanoate (APFO) by the contrast matching SANS technique [56]. The proportions of the two surfactants in an ammonium chloride-ammonium hydroxide buffer at pH 8.8 and an ionic strength of 0.1 were varied. The mixed micelles formed were cylindrical in shape at APFO/AmDec ratios of 2 : 1, 1 : 1, and 1 : 2. The micelle appeared to have a diameter of 20 A and a length of 150 A.The authors proposed that the original ammonium perfluorooctanoate micelle, which also had a diameter of 20 A,expanded along the lateral axis to accommodate the decanoate chains (Fig. 7.8). At a 1 : 9 APFO/AmDec ratio, the micelles were found to be cylindrical but larger than those formed by ammonium decanoate alone. Both the SANS results and surface tension measurements suggested that

FIG.7.8 Schematic mixed-micelle model. Fluorocarbon chains (hatched) form intersecting bands which wind gradually through the structure. The more flexible hydrocarbon chains (open circles) fill the channels. (From Ref. 72. Reproduced by permission of Dr. Dietrich Steinkopf Verlag.)

Structure of Micelles and Mesophases

301

perfluorooctanoate and decanoate chains can mix to form a micelle, but an incipient tendency toward phase separation exists and actual segregation may occur with longer hydrocarbon chains. The concept of a mixed micelle with intramicellar segregation between hydrocarbon and fluorocarbon chains is not consistent with the coexistence of two types of micelles. Carlfors and Stilbs [93], who based their conclusions on I9F-NMR self-diffusion measurements, proposed that alkanoate and perfluoroalkanoate mixtures form two kinds of micelles: one hydrocarbon rich and the other fluorocarbon rich. Burkitt et al. [92] explained the apparent contradiction by differences in the technique of measurement and suggested that the NMR technique measures temporal correlations, whereas SANS measures spatial correlations. The SANS external contrast-matching technique (see Section 9.10) was employed also by Caponetti et al. [89], who concluded that a mixture of sodium perfluorooctanoate and sodium decanoate form mixed micelles having the same composition and a very narrow size distribution. Clapperton et al. [94] examined binary mixtures of ammonium perflurooctanoate and ammonium decanoate by a combination of 'H- and I9F-NMR spectroscopy. The mixtures were buffered at pH 9.0 in NH40H-NH4CI solutions at an ionic strength of 0.1. The chemical shift data indicated nonideal, unfavored mixing of the hydrocarbon and fluorocarbon surfactants in the micelle. Most of the theoretical discussions of mixed-micelle formation have invoked the phase-separation model, although the model is inconsistent with the number of degrees of freedom given by the Gibbs phase rule (see Section 6.2). Moroi and Furuya [90] used the mass action model to examine micellization of a sodium perfluorodecanoate-sodium decanesulfonate mixture by the differential electric conductivity method [95]. The plots of specific conductivity against surfactant concentration revealed two distinct changes in slope, attributed to the first and second cmc. The first cmc belongs to sodium perfluorodecanoate, the second to sodium decanesulfonate. Sodium perfluorodecanoate forms independent micelles, whereas sodium decanesulfonate forms a mixed micelle including a small fraction of sodium perfluorodecanoate. Although more systematic studies of mixed micellization are needed, evidence obtained suggests that a mixture of a sodium alkanoate and a sodium perfluoroalkanoate forms under favorable conditions a mixed micelle or two kinds of mixed micelles. In contrast, potassium perfluorononanoylalaninate (PFNALA) and potassium acylalaninates are virtually immiscible [96]. Two types of micelles coexist, each micelle composed of only one surfactant. PFNALA is adsorbed on a water-air interface more strongly than potassium laurylalaninate.

Mixtures of Anionic Surfactants. Fluorinated Sulfonates Mixtures of fluorinated surfactants featuring a sulfonate hydrophilic group and hydrocarbon surfactants are either immiscible, form two types of micelles, or form

302

Chapter 7

a mixed micelle. The miscibility limits depend on the molecular structure of the surfactants, the composition of the solution, and physical conditions of the system. Micelles at equilibrium with surfactant monomers exhibit two relaxation times (see Chapter 9): one representing the exchange of monomers and the other the formation of new micelles. Bauernschmitt and Hoffmann [97] argued that if two different types of micelles exist in the micellar solution of mixed surfactants, the number of relaxation times should be four, instead of two. The observaand C8H17So3Lition that mixtures of C8F17S03N(CH3)3-C14H29S04N(CH3)4 C11H15S04Liexhibited only two relaxation times appeared to be evidence for comicellization. The cmc values of the surfactant mixtures lent more support for mixed micelles. The cmc values of the surfactant mixtures C8FI7S03N(CH3)~C14H29S04N(CH3)4 and C8H17S03Li-C12H25S04Li were found to be higher than the cmc values for the pure components. This result is consistent with mutual phobicity. However, the cmc values were lower than the cmc values expected for the mixture if no mixed micelles were formed. Hence, the cmc data are in accord with the formation of mixed micelles. Kinetic data indicated that mixed micelles of the C8F17S03NCH3)4-C14H'9S04N(CH3)~ mixture have lower aggregation numbers than the pure micelles. The residence time of the surfactant was found to be shorter i n mixed micelles than in micelles formed by a single surfactant. The solubility of the fluorinated surfactant in hydrocarbon surfactant micelles is higher than vice versa. The positive deviation of the lithium perfluorooctanesulfonate (LiFOS) and lithium dodecyl sulfate (LiDS) mixture from ideal predictions indicates that both molecules tend to exclude each other. Ueno et al. [98] varied the mole fraction of LiFOS and LiDS and examined the micellar properties of the surfactant mixtures by surface tension, electroconductivity, and the spectral changes of pinacyanol chloride in the surfactant solution. The surface tension values plotted against the logarithm of the total surfactant concentrations (Fig. 7.9) exhibit a minimum near the cmc. The total concentrations of mixed cmc values obtained from surface tension curves, shown in Fig. 7.9, and from conductivity measurements are plotted against the mole fraction of LiFOS in Fig. 7.10. For comparison, a curve is included which represents cmc values calculated for a hydrocarbon surfactant mixture by the Lange-Beck equation [77]. Both curves show a large positive deviation from ideal mixing and exhibit a maximum for an approximately equimolar mixture. The positive deviation of the cmc from ideal mixing was confirmed by spectral changes of pinacyanol chloride in single- and mixed-surfactant solutions. The coexistence of two kinds of mixed micelles has been confirmed by gel filtration and ultrafiltration [99,100]. These techniques have elucidated the monomer-micelle equilibria above the cmc of the mixture and micelle demixing. Gel filtration [ 101-1041 separates monomers from the micellar solution according to their molecular size. After the gel column has been equilibrated with a monomer solution. the sample solution is injected and eluted with the same

'i

4 a '+.\ \

1

LI FOS

Oa5

o-2

2 3 5 Total conc

10

20

50

mM

FIG.7.9 Surface tension versus total concentration of LiFOS and LiDS mixtures containing various mole fractions of LiFOS: (0)0.0: (H) 0.2; 0.33;(e)0.5; (0) 0.8; ( 0 )1.O. (From Ref. 98. Reproduced by permission of Academic Press.)

(a)

10

0

05 10 Mole fractlon of LIFOS

FIG.7.10 Mixed cmc values versus mole fraction of LiFOS (98): (a) obtained from surface tension curves, (b) obtained from conductivity data, and (c) calculated by Lange's equation. (From Ref. 77. Reproduced by permission of Academic Press.)

303

Chapter 7

304

n

3

E 0.7

U 1

v,

E

-

U

)r

.-

0.5

.->

+

u

3

U C

E l u t i o n Volume ( m I FIG.7.11 Gel filtration elution curves of the equimolar LiFOS-LiTS system at various total surfactant concentrations using monomer solutions as the eluents. A 1mLsample was introduced as a sandwichbetweenmonomer solutions. LiFOS-LiTS concentrations: (1) 50 mM, (2) 40 mM, (3) 30 mM, (4) 20 mM. Curve 5,30 mM, 0.5 LiDS-LiTS solution. (From Ref. 99. Reproduced by permission of the American Chemical Society.)

monomer solution (sandwich method). At concentrations of 30 nlM or above, Asakawa et al. [99] observed two peaks for the LiFOS-LiTS (lithium tetradecyl sulfate) system. but only one peak for the LIDS-LiTS system (Fig. 7.1 1).The first peak of the LiFOS-LiTS system corresponded to the LiTS-rich micelles and the second peak to the LiFOS-rich micelles. The LiFOS-rich peak diminished with decreasing total surfactant concentration and disappeared at low concentrations. The elution curves suggested that the LiTS-rich micelle exists above cmc, the second type of mixed micelle rich in LiFOS appears with increasing concentration, and the two types of micelles coexist at surfactant concentrations at or above 30 mM. The concentration needed to form the second type of mixed micelle was defined as the second cmc. The compositions of the mixed micelle remained constant in the demixing region, regardless of the mixing ratio. Ultrafiltration has been used to measure cmc, monomer composition, and micelle composition of binary surfactant mixtures [74,100,105]. Asakawa et al. [loo] used theultrafiltrationmethodtostudy SPFO-SDS systems and LiFOS-alkyl sulfate mixtures as a function of surfactant concentration. The alkyl sulfates had different chain lengths of 10 (LiDeS), 12 (LIDS), or 14 (LiTS) car-

Structure of Micelles and Mesophases

305

bons. The micelle demixing region increased with increasing alkyl chain length. The “azeotropic point,” where the micelle compositions are the same as the monomer surfactant composition, was found to be near the maximum of the mixture cmc curve for the LiFOS-LiTS and LiFOS-LIDS systems. The coexistence of two kinds of mixed micelles in LiFOS-LiDS and LiFOS-LiTS systems is also supported by fluorescence probe analysis and changes in ‘H-NMR and “C-NMR spectra [ 1061. Abrupt changes far above the mixture cmc were attributed to the formation of a second type of mixed micelle at the second cmc. The miscibility of LiTS and LiFOS has been examined by Matsuki et al. [ 1071 inaqueous solution and the adsorbed film. Surface tension measurements by the drop volume method indicated that the mixture of LiTS and LiFOS, like LiDS-LiFOS, forms a heterogeneous azeotrope (heteroazeotrope). The LiTSD and LiFOS molecules are partially immiscible in the micellar state and form two kinds of micelles which coexist in equilibrium only at the composition of the heteroazeotropic point. This situation is related to a strong repulsive interaction between the hydrocarbon and fluorocarbon chains in the micelle. A comparison of the results to those of the LiDS-LiFOS system suggested that the immiscible range of the composition of surfactants in the micelle increases with increasing chain length of the hydrocarbon surfactant. The hydrocarbon chain length also affects the miscibility of the surfactants in the adsorbed film at the water-air interface. A phase diagram for the LiFOS-LiDS-water system has been established by Tamori et al. [lo81 (see Section 7.4). Mysels [ 1091 introduced the concept of a critical demicellization concentration (cdc). Below cmc. essentially no micelles are present; above cmc, most of the surfactant has associated to form micelles (Fig. 7.12). When micelles in mixtures of surfactants have a limited solubility, a second cmc may exist [68]. As the surfactant concentration increases, the monomers of one surfactant attain their cmc and form micelles, whereas the other surfactant is still in its monomeric state. At a higher concentration, the second surfactant reaches its cmc and forms micelles. Above this cmc, two kinds of micelles are present. As the surfactant concentration increases further, one kind of micelle may disappear at a concentration denoted as cdc. The existence of cdc was demonstrated by Funasaki and Hada [ 1 101 Neos Ftergent (NF), with mixtures of an anionicfluorinatedsurfactant, [(CF3)2CF]2C=C(CF3)OC6H4S03Na [sodium y-(perfluoro-3-isopropyl-4methylpent-2-en-2-oxy)benzenesulfonate], and hydrocarbon surfactants. Funasaki and Hada [ 110-1 161 found mixtures of fluorinated and hydrocarbon surfactants to contain, under appropriate conditions, one type ofmixed micelle or two kinds of mixed micelles: one rich in one of the surfactants and the other rich in the other surfactant. The coexistence of two kinds of mixed micelles in a solution de-

Chapter 7

306

1

-

-

-

uI 2 0

"5

A

*

c 0 c -

A MICELLES

-

MONOMERS

0.5 4

A

I

I

I

!

MOLE FRACTION to

I

I

1

I

d

FIG.7.12 Hypothetical micellization diagram for a pair of nonionic Surfactants exhibiting a cdc. The parameters used are cA = 3; cB = 1, and proportionality constants are aA= 0.05 and aB= 0.07. (From Ref. i09. Reproduced by permission of Academic Press, Inc.)

pends on the nature of the surfactants, the molar ratio of the two surfactants, the total concentration of the surfactants, the temperature, and the concentration of added electrolyte [ 1 10-1 161. Lake [ 1 171 used kinetic dialysis to examine the existence of cdc. The cdc of the sodium decanoate-sodium perfluorooctanoate mixed-micelle system was found to occur at a specific surfactant concentration and mole fraction. Ben Ghoulam et al. [ 1 181 determined the demixing diagram of the Neos Ftergent (a branched alkylbenzenesulfonate by surface tensiometry) and measured second critical micelle concentrations. However, a critical demicellization concentration was not observed. The composition of mixed micelles in solutions containing NF and sodium tridecyl sulfate (STrS) above cmc was determined from the surface tension data

Structure of Micelles and Mesophases

307

[ 1 lo]. The solutions contained 0.05M NaCl to keep the surface tension value constant above the cmc. A typical plot of surface tension versus the total surfactant concentration is shown in Fig. 7.13. The mole fraction NF-STrS was kept cgnstant for each curve. The intersection of solid and dashed lines indicate the cmc. In Fig. 7.14, the surface tension is plotted against the mole fractions of STrS in coexisting monomers (curve e) and micelles (curve 0. The micellar composition was calculated assuming that (1) the surface tension of the pure surfactant remains constant above cmc and (2) the surface tension is a function of only the surfactant monomer concentrations regardless of coexisting micelles. In Fig. 7.14, the surface tension is constant at 18.73 mN/m between the mole fractions XA and XB. The compositions at both ends of this region, XA and X,, represent the mutual solubility in the mixed NF-STrS micelles. When the total concentration of the mixed surfactants of a fixed composition between XA and XB is increased, the number of mixed-micelle types increases from zero to one, then to two, and finally, decreases to one. The concentration at which the number of mixed-micelle types begins to decrease is the cdc. Comicellization of NF and STrS is shown in Fig. 7.15 as a function of the overall mole fraction of STrS. No micelles exist in region I. In region 11, micelles

FIG.7.13 Surface tension of the NF-STrS system versus the logarithm of total surfactant concentration in 50 m M sodium chloride solution at 30°C. The overall mole fraction XSTrS is kept constant in each curve: (a) XSTrS = 0.094; (b) XSTrS = 0.249; (c) XSTrS = 0.540; (d) XSTrS = 0.903. (From Ref. 110.Reproduced by permission of Academic Press, Inc.)

Chapter 7

308

I

0

A

E

02

0.4

,

0.6

mole fraction of STrS

E?

1

0.8

1

FIG.7.14 Surface tension of the NF-STrS system plotted against the mole fractions of STrS in coexisting monomers (curve e) and micelles (curve f). (From Ref. 110. Reproduced by permission of Academic Press, Inc.)

11

0

02

0.4 0.6 0.8 overall mole traction of STrS

1

FIG.7.15 Curves g and i show the cmc and cdc of the NF-STrSD system, respectively. Region I , no micelles; region II, NF-rich micelles; region Ill, two kinds of mixed micelles; region IV, STrSD-rich micelles. (From Ref. 110. Reproduced by permission of Academic Press, Inc.)

Structure of Micelles and Mesophases

309

are NF rich; in region IV, micelles are STrS rich; in region 111,two kinds of mixed micelles coexist. The limited solubility of mixed micelles has been attributed to the repulsion between hydrocarbon and fluorocarbon chains in the mixed micelle. To some extent, electrostatic interactions affect the miscibility of ionic fluorocarbon and hydrocarbon surfactants [ 111,112,1191. The fluorinated surfactant Neos Ftergent (NF) also forms two kinds of mixed micelles with sodium tetradecyl sulfate (STS) [ 1 1 1,1121. The plots of surface tension versus the logarithm of total surfactant concentration (Fig. 7.16), surface tension versus mole fraction of cmc (Fig. 7.17), and cmc versus the mole fraction of STS (Fig. 7.18) are similar to those of NF-STrS mixtures [110]. A comicellization diagram (Fig. 7.19) shows the relation between the total surfactant concentration and the overall mole fraction of STS. The area displayed is divided into four regions, I-IV, by solid lines. In region I, there are no micelles.

""

M

" 1

-3 0

FIG.7.16 (a) Surface tension plotted against the logarithm of total surfactant concentration at 30°C. The overall mole fraction, xsTs,of STS is kept constant: curve a, x s T s = 0;curve b, XSTS = 0.097; curve c, XSTS = 0.649. (b) Surface tension plotted against the logarithm of NF concentration at 30°C. The total STS concentration, CsTs (mol/L), is kept constant: curved d, CsTs = 2.01; curve e, CsTs = 5.75 x curve f, CsTs = 7.60 X (From Ref. 111. Reproduced by permission of the American Chemical Society.)

Chapter 7

310

L

0

.

'

"

0.2

"

"

'

'

0.4 0.6 mole fractlon of STS

08

1.o

FIG.7.17 Surface tension at the cmc (ycmc)plotted against the mole fraction of STS in adsorbed monolayers (curve a), in monomers (curve b), and in micelles (curve c). (From Ref. 111. Reproduced by permission of the American Chemical Society.)

The micelles in region I1 are NF rich; in region IV, the micelles are STS rich. In regions TI and IV, the compositions of coexisting monomers and micelles depend on the total concentration of the surfactants. In region 111, two kinds of micelles coexist with the monomers. The composition of the micelles is indicated by mole ratios XA and X B ; the monomer ratio is given by the point E. I

0

1

02

0.4

0.6

0.8

1

mole fraction of STS

FIG.7.18 Critical micelle concentration plotted against the mole fraction of STS in monomers (curve a) and in micelles (curve b). (From Ref. 111. Reproduced by permission of the American Chemical Society.)

c

Structure of Micelles and Mesophases

r

31 1

xB

,I ‘ !

-“Oh

04 06 08 overall mole fraction of STS

02

FIG.7.19 Comicellization shown as a function of total concentration and overall mole fraction of STS in 0.05M sodium chloride solution at 30°C: region I , no micelles; region ll, NF-rich micelles; region Ill, two kinds of mixed micelles; region IV, STS-rich micelles. The dashed lines represent the asymptotes at infinity total concentration. (From Ref. 111. Reproduced by permission of the American Chemical Society.)

The mutual miscibility of anionic fluorinated surfactants and hydrocarbon surfactants increases with increasing temperature,similar to the miscibility increase of fluorocarbon and hydrocarbon liquid mixtures [76,120]. In the LiFOS-LiTS system, the solubility of LiFOS increased substantially in the LiTS-rich micelle but only slightly in the LiFOS-rich micelle [99]. In comicellar systems, such as NF-STS [ 1121 and LiFOS-LIDS [76], the temperature dependence of mutual m i s cibility exhibits a critical solution temperature (cst) that corresponds to the transition from two types of micelle to one type of mixedmicelle. Above the cst, only one kind of mixed micelle exists; below thecritical solution temperature (cst), two types of micelles can coexist. The existence of two types of micelle depends also on the surfactant concentrations. As the total surfactant concentration is increased, one type of micelle disappears due to the larger solubilization capacity and growth of the other type of mixed micelle [76]. Above the critical demicellization temperature (cdc), only one kind of mixed micelle exists; below cdc, two micellar species can coexist [112]. The miscibility of an anionic fluorinated surfactant with an anionic hydrocarbon surfactant is affected by the electrolyte concer~trntionin the micellar solution [ 1211. Excess counterions increase the binding of the counterions to ionic head groups and reduce electrostatic interactions between the head groups. Furthermore, the cst and the composition of the surfactant mixture at cst depend on

312

Chapter 7

the concentration of added electrolyte (Fig. 7.20). The mole fraction of NF in micelles at 30°C and the cst increase when the NaCl concentration or the hydrocarbon chain length of sodium alkyl sulfate is increased [ 1 121. The nonideal behavior of mixed hydrocarbon and fluorinated surfactants is evidenced by a ~ ~ o Z z mchange e on comicellization [ 113,1141. According to the regular solution theory [79.122], the cst and the composition of a fluorocarbon and hydrocarbon surfactant mixture should depend only on the solubility parameter and the molecular volume of the solutes. For an ideal mixture of two surfactants 1 and 2 in micelles, the molar volume of the mixed micelles is given by [ 1141

where V,,,,is the molar volume of pure surfactant, i is the micelle, and .x-tTzt is the micellar molar fraction. For an ideal system, the volume change on mixing is zero:

AV= 0

(14)

The volume change of mixing micelles of two nonionic surfactants with hydrocarbon chains, DE5 and DE7, is zero indeed (Fig. 7.21, curve b) (DE5 is pentaoxyethylene glycol dodecyl ether, DE7 is heptaoxyethylene glycol dodecyl ether). However, the molal volumes of the fluorinated surfactant NF and the hydrocarbon Surfactant STS increase upon comicellization in the NF-STS system. A V is positive over the whole micellar composition range (curve a). Amixture of octaoxyethylene glycol dodecyl ether (DE8) and methyl p-hydroxybenzoate (MP) exhibited a negative molar volume change (curve c). The positive molar volume change of NTS and STS micelles is similar to a volunle increase when hydrocar-

h

Y

v

L

0

1

025 0 5

075

1

1

mole fraction of STS In mlcelles

FIG.7.20 The cst and the composition of mixed NF-STS micelles in 10 m M (0) and 50 m M ( 0 )sodium chloride solutions. (From Ref. 112. Reproduced by permission of Academic Press, Inc.)

Structure of Micelles and Mesophases

313

FIG.7.21 Volume changes on mixing of micelles as a function of micellar composition xrnSTs, XmDE7, or X,,MP: (0)NF-STS system; ( 0 )DE5-DE7 system; ( X ) DE8-MP system; curve calculated from A V = 0.0089Ge, for the DE8-MP system, taking o = - 1.58RT. (From Ref. 114. Reproduced by permission of the American Chemical Society.) bons and fluorocarbons are mixed [79]. The AV values for the NF-STS mixture appear to increase linearly with the mole fraction of STS in the range 0.065 < < 0.470, and decrease at higher STS mole fractions, x , , , ~ ~>s 0.470, indicating the coexistence of two kinds of mixed micelles.

Mixtures of Anionic Fluorinated Sutfactants A mixed system of two fluorinated surfactants was studied by Yoda et al. [123]. Mixtures of LiPFO and LiFOS behaved almost like ideal systems, with an interaction parameter [72] p = -0.48 (Fig. 7.22). In contrast, the LiFOS and LiDS mixtures exhibited a positive deviation from ideal predictions. The p value of 1.36 was attributed to repulsive interactions between the hydrocarbon and fluorocarbon groups (Fig. 7.23).

Anionic-Nonionic Surfactant Mixtures Binary hydrocarbon surfactant systems consisting of an anionic and a nonionic surfactant with almost equal cmc values generally exhibit a minimum cmc in the cmc versus composition curve. This negative deviation from the ideal cmc-com-

Chapter 7

314 I

20m I

s1 X

\

u

z

"

IO-

0

0.5 mole fraction of

1

LiFOS

FIG.7.22 Critical micelle concentrations for mixtures of LiPFO and LiFOS plotted against the mole fraction of LiFOS: the solid line represents values calculated for a nonideal mixed micelle with p = -0.48, and the dashed line represents cmc values calculated for ideal mixing. (From Ref. 123. Reproduced by permission of Academic Press, lnc.)

position relationship has been explained by electrostatic stabilization. Mixtures consisting of an anionic fluorinated surfactant and an anionic hydrocarbon surfactant exhibit a maximum in the cmc versus composition curve. The positive deviation from ideal predictions has been related to the mutual phobicity between fluorocarbon and hydrocarbon chains in the mixed micelle. In contrast, the deviation from ideal behavior is negative for mixtures consisting of an anionic fluorinated surfactant and a nonionic hydrocarbon surfactant. Apparently, in the micelle containing a fluorinated anionic surfactant and a hydrocarbon chain nonionic surfactant, the reduction of electrostatic repulsion between the hydrophilic groups overcomes the enthalpic destabilization resulting from the mixing of fluorocarbon and hydrocarbon chains [ 1121. A similar explanation was given by Yoda et al. [113] for mixtures of hexaoxyethylene dodecyl ether

1

I

Structure of Micelles and Mesophases

315

\ V

z

u

0

05

1

mole fraction of LiFOS FIG.7.23 Critical micelle concentrations for mixtures of LiDS and LiFOS plotted against the mole fraction of LiFOS: the solid line represents values calculated for a nonideal mixed micelle with p = + I .36,and the dashed line represents cmc values calculated for ideal mixing. (From Ref. 123. Reproduced by permission of Academic Press, Inc.)

(C12E6)and LiFOS, which deviated negatively from the ideal relationship with a /3 value of -4.8. The surface tensions of mixtures containing Neos Ftergent, an anionic fluorinated surfactant, and dodecyl-methyl sulfoxide (DMS) are plotted against the logarithm of total surfactant concentration Cr in Fig. 7.24 and against the mole fraction of DMS in Fig. 7.25. The mole fraction of curve c in Fig. 7.24 is equal to the composition of the intersection of curves a and b in Fig. 7.25. Funasaki and Hada [ 1121 calculated micellar compositions from surface tension data assuming that the surface tension of a pure surfactant remains constant above cmc. This condition was met in the presence of an excess electrolyte. A plot of the cmc values against the mole fraction of DMS has a minimum and, therefore, shows a negative deviation from ideal behavior. Abe et al. [124] have studied mixed-surfactant systems consisting of a nonionic hydrocarbon surfactant [C 16H330(C~H~0)20H] and an anionic fluorinated surfactant (ammonium perfluorooctanoate or ammonium perfluorodecanoate). Dynamic and static light-scattering and fluorescence probe measurements revealed mixed-micelle formation. Penetration of the anionic fluorinated

Chapter 7

316

t C

F l aJ

c

aJ V

r"3 20 ul

L

I

-40

-3.5

-3.0

Log Ct (Ct.mol/l)

FIG.7.24 Surface tension of the NF-DMS system versus the logarithm of total surfactant concentration in 50 m M sodium chloride solution at 30°C. The overall mole fraction xDMS is kept constant in each curve: (a) XDMS = 1.O;(b) XDMS = 0.961 ; (c) xDMS = 0.818; (d) XDMS = 0.572; (e) XDMS = 0. (From Ref. 112. Reproduced by permission of Academic Press, Inc.)

surfactant into the nonionic micelle causes intermicellar repulsion and, with increasing mole fraction of the fluorinated surfactant, the micelle size (aggregation number) decreases. Surface tension curves of mixtures of LiFOS and hexaethyleneglycol n-dodecylether (6ED) are consistent with mixed-micelle formation [ 1251. The surface

1

0

1

02

0.4 0.6 mole traction of DMS

08

I 1

FIG.7.25 Surface tension of the NF-DMS aqueous system plotted against the mole fractions of DMS in monomers (0)and in micelles (0).(From Ref. 112. Reproduced by permission of Academic Press, Inc.)

Structure of Micelles and Mesophases

70

317

-

h

E

\

2 60

W

z 50 v)

z w

IW

40

0

a

30

I

3 v)

Qn 1 I

I

I

LV

10-5

10-4

10-3

I

10-2

c, FIG.7.26 Surface tension of LiFOS in the presence and absence of 6ED versus the logarithm of LiFOS concentration, C1.Additive concentration of 6ED (mM): (O), 0 ; (0),0.005; (+), 0.01; (H),0.05;(El), 1.O. (From Ref. 125. Reproduced by permission of the American Oil Chemists Society.)

tension curve of single LiFOS exhibited only one break corresponding to the cmc (Fig. 7.26). The addition of small amounts of 6ED lowered the surface tension of LiFOS from 33.0 mN/m above the cmc to 25 mN/m. A minimum appeared near the cmc, suggesting the existence of a mixed micelle. The surface tension of 6ED and that of its solutions containing small amounts of LiFOS are shown in Fig. 7.27. Above the cmc, the surface tension of 6ED decreased with increasing amounts of LiFOS added. However, a minimum was not observed in the surface tension curves of 6ED and each curve exhibited a long plateau above cmc. Apparently, LiFOS molecules added to the 6ED solution can replace some 6ED molecules adsorbed at the surface of the solution and some adsorbed LiFOS molecules can be interchanged with 6ED tnolecules. At a higher concentration of LiFOS, the surface excess of 6ED molecules is higher than in the single system. An addition of 6ED lowers the sulface tension of LiFOS and a minimum in the curve (Fig. 7.26) suggests the formation of a mixed micelle. The effect of LiFOS on micellization of 6ED was investigated by keto-enol tautomerism of benzoylacetanilide (BZAA) [126]. BZAA exists in the enol form in 6ED solutions above the cmc. The enolization in the hydrocarbon chain surfactant solution is similar to that in organic solvents. In aqueous solutions of LiFOS, enolization was not observed. The enolic absorbance of BZAA at 3 15 nm above the cmc of 6ED decreases when LiFOS is added and the absorbance of the keto tautomer at about 250 nm increases. This result was explained by mixedmicelle formation and the existence of a limited number of sites in the micelle which BZAA or LiFOS could occupy.

Chapter 7

318

h

70 I

E f 60

- *.

W

z 0, 50

cn z w IW

0

a

-

*\

30 -

40

3

cn

3n I b"

1

10-6

I

10-5

I

10-4

I

10-3

c2 FIG.7.27 Surface tension of 6ED in the presence and absence of LiFOS versus the logarithm of 6ED concentration, C2.Additive concentration of LiFOS (mM): (O), 0; (m), 0.5; (0), 1.0; (O), 7.0. (From Ref. 125. Reproduced by permission of the American Oil Chemists Society.)

Meguro et al. [127] examined aqueous solutions of fluorocarbon and hydrocarbon surfactants by means of steady-state fluorescence of 8-anilino- 1-naphthalenesulfonic acid ammonium salt (ANS). The fluorescence of ANS increases in the hydrocarbon environment. The fluorescence intensity of ANS in a hydrocarbon surfactant solution below the cmc is constant. However, above the cmc, the fluorescence intensity of ANS in a solution of a hydrocarbon surfactant increases linearly with the surfactant concentration (Fig. 7.28). This indicates that ANS is solubilized by the hydrocarbon micelles. Unlike hydrocarbon surfactants, fluorinated surfactants do not solubilize ANS. These characteristics make ANS a useful probe for investigating mixed micellization. Three systems were studied: 6ED-LiFOS, 6ED-NFE7, and SDS-NFE7. The nonionic fluorinated surfactant NFE7 has the structure [(CF3)2CF]2C=C(CF3)0(CH2CH20)7CH3. (The structure of the surfactant coded NF in Ref. 127 is different from the structure of the anionic surfactant NFtergent coded NF in Ref. 110 and is apparently related to the nonionic surfactant coded NFE ill Ref. 116. The code NFE7 is used here to avoid possible confusion.) The fluorescence data revealed that 6ED and LiFOS form mixed micelles over a wide concentration range. At constant 6ED concentrations above the cmc of 6ED, the fluorescence intensity of ANS decreases with increasing LiFOS concentration (Fig. 7.29). suggesting that mixed micelles are formed in which ANS is less soluble.

319

Structure of Micelles and Mesophases

Ilydrocarbon Surfactant

-

Surfactant I

I 8

I 8

*I

CMC Conc of Surfactant FIG. 7.28 Fluorescence intensity versus surfactant concentration. (From Ref. 127. Reproduced by permission of the American Chemical Society.)

20

15

I

0

2 Cone

1

I

4 6 of LiFOS x

lo6

I

I

8

10

(mol/l)

FIG.7.29 The fluorescence of ANS as a function of LiFOS concentration in the 6ED-LiFOS mixed system. The numbers indicate fixed 6ED concentrations. (From Ref. 127. Reproduced by permission of the American Chemical Society.)

Chapter 7

320

Unlike the 6ED and LiFOS mixture, the surfactants SDS and NFE7 are immiscible over the entire concentration range studied and form separate micelles. Muto et al. [ 171 determined the aggregation number of lithium perfluorooctane sulfonate (LiF0S)-LiDS, hexaoxyethylene glycol dodecyl ether (C12E6)-LiFOS, octaoxyethylene glycol dodecyl ether (C12E8)-LiFOS, and C12E6-LiDS systems by fluorescence decay of micelle-solubilized pyrene in the presence of a micelle-solubilized quencher. The aggregation number increased considerably with the surfactant concentration. The measurements of the pyrene fluorescence lifetime and of the ratio of intensities of the first and third bands of the pyrene monomer fluorescence spectrum appeared to indicate that only one type of mixed micelle exists in the mixed system containing a fluorinated surfactant. Muto et al. [ 1281 studied interactions of anionic and nonionic surfactants by solubilization of a water-insoluble dye, Yellow OB. If the solubilizing power obeys the additivity rule, then the ideal solubilized amount, SA, can be calculated as SA =

SICMI

+ s2Ch37 + SW

(15)

where S is the solubilizing power of each surfactant, denoted with the subscripts 1 and 3,Ch1is the concentration of each surfactant in micelles, and Sw is the solubility of the solubilizate in water. The solubilization efficiency was indicated by the ratio SR: SR =

SA(observed) SA(ideal)

where SA (observed) and SA (ideal) are the solubilized amounts measured and calculated for the same molar fraction of the nonionic surfactant in the mixed micelle. When an anionic fluorocarbon surfactant (LiFOS) and a nonionic surfactant [hexaoxyethylene glycol dodecyl ether (C13E6) or octaoxyethylene glycol dodecy1 ether (CI2E8)]are mixed, the efficiency of solubilization decreases (Figs. 7.30 and 7.3 1). The incorporation of an anionic fluorinated surfactant into the nonionic micelle loosens the polyoxyethylene shell and causes desolubilization of the organic compound solubilized there [ 126,127,129,130]. The efficiency of solubilization decreases with increasing differences in the solubilization power of each individual component. The system containing LiFOS had a lower solubilization ratio than systems containing SDS or Aerosol OT (AOT). The micellar size and charge in the mixed-surfactant solutions of LiFOS-C ,?-E6and LiDS-C17E6 have been calculated using electric birefringence, interfacial tension, viscosity, dynamic light scattering. and electric conductivity data [ 1311. The component ratio at constant total surfactant concentration has a similar effect on the micellar shape and size for both systems (Fig. 7.32). The length of the mixed rodlike micelles in the LiFOS-C12E6 system increases with increasing mole fraction of the anionic component and decreases after exhibiting

Structure of Mesophases Micelles 321 and

0

05

1

0

0.5

1

X

X

FIG.7.30 The ideal (dashed line) and observed (solid line) amounts of solubilized Yellow OB (SA,) as a function of the molar fraction of (a) CI2E6 and (b) C12E8 in the mixed micelles. Total surfactant concentration 10 mmol/L. The symbols 0, 0, and A refer to solutions containing SDS, AOT, and LiFOS, respectively. (From Ref. 128. Reproduced by permission of Academic Press, Inc.)

1

1

&O 5

$0 5

r/)

0

05 X

1

0

05

I 1

X

FIG.7.31 The solubilized ratio (SR,) of Yellow OB as a function of the molar fraction of (a) C12E6 and (b) CI2E8 in the mixed micelles. Total surfactant concentration 10 mmol/L: 0, E!, A;20 mmol/L: 0 ,H, A.The symbols 0, 0 refer to solutions containing SDS; 0, to AOT; and A,A to LiFOS. (From Ref. 128. Reproduced by permission of Academic Press, Inc.)

Chapter 7

0.2

0.4 X

FIG.7.32 The dependence of the Kerr constant, K, on the mole fractionof LiFOS (0). in the mixture LiFOS-CI2EG (0)and the mole fraction of LiDS in LiDS-C& Total surfactant concentration0.1OOM, T = 25°C. (From Ref. 131. Reproduced by permission of Academic Press, Inc.)

a maximum value of 480 A at a LiFOS mole fraction of 0.07-0.08. For the hydrocarbon LiDS-C1&6 system, the rodlike micelles have a maximum value of 280 A at an LiDS mole fraction of 0.025. A rod-to-sphere transition point corresponds to a mole fraction of 0.4 for the LiFOS-C12E6 system and 0.2-0.3 for the LiDS-CI2E6 system. The mole fraction values determined by the electric birefringence method are in agreement with those obtained by viscosity and interfacial tension measurements. The LiFOS-C 12E6micelles seem to have greater internal order than the LiDS-CI2E6 micelles, probably because of the stiffer and shorter hydrophobe chain of the LiFOS surfactant than of the LiDS. Knoblich et al. [58] used cryogenic transmission electron microscopy to examinetheeffect of counterionsontheviscosityforsolutions of CloEOs [C10H7_1(OC2H4)s0H] mixed with LiFOS or FOSTEA (tetraethylammonium perfluorooctanesulfonate). Viscosity for CloE05solutions increased with increasing FOSTEA concentrations, indicating the formation of mixed micelles, threadlike micelles, and a looped structure. The mixtures of LiFOS and CloEOS formed spherical micelles, but at a 60 : 40 ratio of LiFOS and CloEOS,threadlike micelles without a looped structure were observed. Sugihara et al. [ 1321 investigated the effect of chain length on the pressure dependence of micellization in mixed-surfactant solutions of nonyl-M-methylglu-

Structure of Mesophases Micelles and

323

camine (MEGA9) and sodium perfluorooctanoate (SPFO). The cmc values of the mixed system are lower than the cmc values of the components (Fig. 7.33). The surface activity of the mixed system is higher than that of the components. The synergistic increase in surface activity leads to a decrease of the cmc [ 133,1341. The cmc curves (Fig. 7.33) indicate that the SPFO-MEGA9 system exhibits a negative deviation from ideal behavior, in contrast to the SPFO-SDeS system, which shows a positive deviation. The negative deviation implies that SPFO and MEGA9 are miscible in micelles and the repulsive forces between ionic head groups are reduced. The reduction in the repulsive forces probably results from the penetration of the nonionic surfactant into the space between the ionic head groups. The pressure dependence of this system is very small, probably because the volume change caused by micellization is small. The degree of counterion binding p, as well as the degree of head-group ionization a, vary much less with pressure than a and p of anionic-anionic surfactant mixtures.

SDeS-SPFO 1 atm

4 ! .

CMC X lo3 (mollkg)

\

U

3 O k \\

I

MEGA-9-SPFO

0 1 atm

\ \

0 2400 kg/cm2

\

0

0.5

1

MOLE FRACTION MEGA-9 OR SDeS

FIG.7.33 The first cmc plotted versus the mole fraction X, for the MEGAS-SPFO system at 30°C under 1 atm and 2400 kg/cm2 (lower curves.) The upper curve shows the first cmc values for the SDeS-SPFO mixed system. (From Ref. 132. Reproduced by permission of Plenum publishing.)

324

Chapter 7

Esumi et al. [135,136] studied the interactions between fluorocarbon- and hydrocarbon-type surfactants by their effect on monodispersed ferric hydrosols. The amounts of surfactants adsorbed showed that the formation of mixed bilayers of anionic fluorocarbon-nonionic hydrocarbon surfactants is more favorable than that of anionic fluorocarbon-anionic hydrocarbon surfactants (see Section 5.2). The interactions between LiFOS and octyl-P-D-glucoside (AG8) are dominated by the reduction of Coulombic repulsion between the head groups, and the mixtures deviate negatively from ideal behavior [ 1371. The aggregation number of the micelles and the mole fraction of AG8 in the micelles increase gradually when the mole fraction of AG8 increases from 0 to 0.8, and sharply when the mole fraction of AG8 exceeds 0.8. The change of the micellar radius is similar to that of the aggregation number. Hence, the micellar properties of AG8 are markedly affected by incorporation of LiFOS.

Nonionic Surfactant Mixtures The miscibility of anionic surfactant mixtures is affected by electrostatic effects which are absent in nonionic surfactant mixtures. This simplifies the interpretation oftheir solubility data. Furthermore. the miscibility of nonionic surfactants i n micelles can be compared to their mutual solubility in the liquid state. Funas'aki and Hada [ 1 161 examined the mutual solubility of a fluorinated nonionic surfactant [(CF3)2CF]2C=C(CF3)0(CH1CH20),,CH3(NFE. average rz = 18.4) and nonionic a hydrocarbon chain surfactant CH3(CH2)11O(CH2CH20),,,H (DEm,171 = 5,7.9, or 25). Curves of the surface tension plotted against the logarithm of total surfactant concentration for mixtures of NFE and DE7 are shown in Fig. 7.34. The constancy of surface tension beyond cmc (curve a) indicates that DE7 was highly pure, unlike NFE (curve f),which is difficult to purify and therefore contained impurities. The cmc values of NFE-DE7 mixtures exhibited a maximum at a mole fraction of 0.327 for DE7. Therelation between the surface tension and the mole fraction of monomeric DE7 in the mixture of NFE and DE7 monomers is shown in Fig. 7.35 with open circles. The filled circles show the surface tension at about 10-fold concentrations above cmc as a function of DE7 mole fraction in the whole system. A plateau region in the micellar composition curve indicates the coexistence of two kinds of mutually saturated mixed micelles. The surfactants NFE and DE7 mix partially in micelles and their mutual solubility increases with increasing temperature. The mutual solubility of NFE and DEm in micelles is shown in Fig. 7.36. Above the critical solution temperature (cst) only one kind of mixed micelle exists, whereas below cst, two kinds of micelles can coexist, depending on the total concentration and chemical composition of the surfactants. The cst and the critical mole fraction of DEmin the NFE-DEm system decrease with increasing number. nz, of oxyethylene units. The cst is

Structure of Micelles and Mesophases

325

L

36

3 FIG.7.34 Surface tension versus the log of the total surfactant concentration for mixtures of NFE and DE7 in water at 25°C. The overall mole fractions of DE7 are (a) 1.O,(b) 0.834, (c) 0.503, (d) 0.327, (e) 0.107, and (f) 0. (From Ref. 116. Reproduced by permission of the American Chemical Society.)

341

0

a5

Mole fractionof

DE7

FIG.7.35 Surface tension versus mole fraction of DE7 in the system ( 0 )and in monomers (0)for mixtures of NFE and DE7 in water at different temperatures: (a and b), 25°C; (c), 30°C; (d) 33°C. (From Ref. 116. Reproduced by permission of the American Chemical Society.)

Chapter 7

326

I A

0

0.5 Mole fraction of DEm in micelles

FIG.7.36 Solubility-temperature diagram for the mixed NFE-DEm micelle: (0) NFE-DE5; (A)NFE-DE7; ( 0 )NFE-DES. (From Ref. 116. Reproduced by permission of the American Chemical Society.)

higher for mixed NFE-DEm micelles than for liquid NFE-DEm mixtures, suggesting that the micellar core has a more extensively ordered structure than theliquid mixture. Steady-state fluorescence studies by Meguro et al. [ 1301 using ANS as the probeshowed that thefluorinatedsurfactant [(CH3)2CF]2C=C[CF3)0(CH2CH2)7CH3at concentrations below its cmc does not penetrate the micelles of the hydrocarbon surfactant C12H250(CH2CH20)6H (6ED). The surfactants form mixed micelles above the cmc of the fluorinated surfactant.

Oppositely Charged Surfactant Mixtures Binary mixtures of an anionic hydrocarbon surfactant and a cationic fluorocarbon surfactant have a much lower surface tension and interfacial tension than the individual components [ 1381 (Fig. 7.37). The same is true for mixtures of a cationic hydrocarbon surftlctant and an anionic fluorocarbon surfactant. Instead of normal micelles, the anionic-cationic mixture forms larger aggregates, probably containing ion pairs [80]. The mixtures investigated by Zhu and Zhao [138] included SPFO-OTAB, SPFO-DTAB, SPFO-HTAB, PFOA-OTAB, and FC3-SOS [OTAB, octyltrimethylammonium bromide; DTAB, dodecyltrimethylammonium bromide: HTAB, hexadecyltrimethylammonium bromide; SOS, sodium octylsulfate; PFOA, perfluorooctanoic acid: FC3, C3F70CF(CF3)CF20CF(CF3CONH(CH2Hs)2CH3Il.

Structure of Micelles and Mesophases

327

The surface tension values of the mixtures SPFO-OTAB and PFOA-OTAB were identical (Fig. 7.37), indicating that the anion and cation interact so strongly that the different counterions of the individual components have no effect. The strong electrostatic interaction between the surfactant anions and cations is conducive to the formation of mixed micelles. 'H-NMR spectra have indicated a penetration of SPFO into the interior of CTAB (cetyltrimethylammonium bromide) micelles [ 1391. The interaction between DEFUMAC [bis(2-hydroxyethy1)-(2-hydroxy-3perfluorooctylpropy1)methylammonium chloride] and sodium poly(oxyethy1ene) sulfate (SDE,,S, n = 3. 5 , 8) depends on the length of the oxyethylene chain [ 140.1411.The shorter the oxyethylene chain, the stronger the predominantly electrostatic interaction. In dilute solutions, the mixed cmc, the micellar composition, the interaction parameter, the surface tension at cmc, the apparent aggregation number, and the diffusion coefficient all change markedly with the length of the oxyethylene chain. In concentrated solutions, the DEFUMAC-SDE8S system does not form crystals at any molar fraction, whereas crystals are formed in the DEFUMAC-SDE3S and DEFUMAC-SDE5S systems. The mixed solutions of oppositely charged fluorocarbon and hydrocarbon surfactants have excellent spreading properties on oil. Hydrocarbon chains in the

2o

i

lo -

0 ,

5

4

3

2

1

0

Log C ( C mol d m - 3 )

FIG.7.37 Surface tension versus the log of the concentration curves for OTAB ( l ) , SPFO (2), PFOA (3), andtheir 1:l mixtures SPFO-OTAB (4, 0) and PFOA-OTAB (4, +) at 30°C. (From Ref. 138. Reproduced by permission of Dr. Dietrich Steinkopf Verlag.)

328

Chapter 7

adsorption layer at a water-oil interface reduce the mutual phobicity between the fluorocarbon chain of the surfactant and the hydrocarbon chain of the oil and decrease the water-oil interfacial tension. The mutual phobicity between hydrocarbon and fluorocarbon chains usually causes a positive deviation from ideal mixing with positive p values. In contrast, the strong interaction between anionic and cationic surfactants results in significant synergism with very large negative p values 111421. The interaction between oppositely charged ionic surfactants is much stronger than the interaction in mixtures of similarly charged ionic surfactants and nonionic surfactants. Iampietro and Kaler [ 1431 investigated aqueous mixtures of sodium perfluorohexanoate (SPFHX) and CTAB using SANS and tensiometry. Data analysis using the regular solution theory gave a large negative p value of - 19.4, indicating highly nonideal mixing. Below the cmc of SPFHX, the addition of small amounts of CTAB generates large structures, including vesicles. Above the cmc, SPFHX forms small globular micelles. The addition of CTAB induces micellar growth, evident by an increase in viscosity, and eventually causes a separation into two phases containing rodlike micelles. one enriched i n CTAB and the other in SPFHX. Perfluorodecanoic acid (PDA) forms solid 1 : 1 complexes with cationic copolymers, poly(dially1dimethylan~moniumchloride)-co-(N-methyl-N-vinylacetamide) [144]. The mesomorphic structure of the complexes consist of two-dimensional ordered columnar stacks of disklike aggregates. These long-chain complexes of low surface energy are of practical interest for the development of high-performance coatings.

Mixtures of Cationic Fluorinated Surfactantswith Nonionic and Cationic Surfactants Mixtures containing cationic fluorinated surfactants and nonionic or cationic hydrocarbon surfactants have not been investigated extensively. The interaction of cationic fluorocarbon and hydrocarbon surfactants was studied by Tamori et al. [145]. The mixed cmc of diethanolheptadecafluoro-2-2undecanolammonium chloride and dodecyltrimethylammonium chloride was determined by electric conductivity measurements. Partition coefficients of alcohols (methanol and C3F7H20H) and a fluorescent probe (pyrene-3-carboxaldehyde) between micelles and the bulk aqueous phase were determined. The data interpreted by a regular solution theory fitted an interaction parameter p = 1, indicating a much smaller repulsive interaction between the two cationic surfactants than that between an anionic fluorinated surfactant and an anionic hydrocarbon-type surfactant. The weak repulsion between the two cationic surfactants was explained by a large difference in their cmc values. Esumi [146] examined mixed micelles of DEFUMAC with other surfactants. such as dodecyltrimethylanlmoniunl chloride and octaoxytheylene glycol

.

"

Structure of Micelles and Mesophases

329

decyl ether (CloEg). Mixed cmc values obtained by the pyrene-3-carboxaldehyde fluorescence method are in close agreement with mixed cnlc values calculated from the regular solution theory (see Section 7.2) with the assumed interaction pahyrameter p = -0.5. Apparently, the unfavorable interaction between the drophobic segments is canceled out by a favorable interaction between the head groups. The mixed cmc values for mixtures of DEFUMAC with a cationic hydrocarbon surfactant DTAC were determined by plotting equivalent conductivity versus the square root of concentration. The cmc values agreed reasonably well with cmc values calculated from the regular solution theory assuming an interaction parameter value of p = l . The positive p value indicates a repulsive interaction between the two surfactants. Hence, the cationic-cationic surfactant mixture deviates more from the ideal regular solution theory than the cationic-nonionic system.

Mixtures of Cationic Surfactants with Perfluorocarboxylate Counterions Micellization of mixed cationic surfactants with different perfluorinated counterions has been investigated by Sugihara and co-workers [ 147-1491. The surfactants studied had a common dodecylammonium (DA) cation and different anions: perfluoroacetate (PA), perfluoropropionate (PP). perfluorobutyrate (PB), methanesulfonate (MS), and ethanesulfonate (ES). The cmc values were determined by electric conductance, and the effect of the hydrophobicity on solubility, cmc, and micellization (Krafft) temperature was examined. For binary mixtures DAPA-DAPB and DAPP-DAPB, the electric conductance change at the cmc was not sharp and the cmc values were determined by plotting the derivative of the , C, specific conductance, K , by total surfactant concentration (C,), d ~ l d C ,against [147] or The degree of counterion binding p was close to 1.0. The interaction between the perfluorinated anions in the mixed micellar state was investigated using a modified Rubingh's equation (see Section 72), which takes the counterion dissociation in account. The interaction between the counterions was found to be small and their mixing almost ideal. Mixing of fluorocarbon and hydrocarbon anions increased the stability of micelles containing only hydrocarbon anions.

e.

Mixtures Containing Amphoteric Fluorinated Surfactants Esumi and Ogawa [150] studied micellar solutions of an amphoteric fluorinated surfactant, [N-[3[[Tridecafluorooctyl)sulfonyl]an~ino]propyl]-N,N-dimethylammoniolacetate [C6F1 3C2H4S02NH(CH2)3N+(CH3)2COO-,FOSAB], and its mixtures with LiFOS and LiDS by surface tension, fluorescence probing, and viscosity measurements. At cmc and at 25OC, the surface tension value for FOSAB

Chapter 7

330

alone is 20 mN/m. LiFOS and LiDS lower the surface tension of FOSAB to 14.5 and 16.5 mN/m. This large reduction of surface tension results from electrostatic attraction between the cationic functionality of FOSAB and the anionic groups of LiFOS and LIDS. In the FOSAB-LIDS system, the mixed cmc values coincided almost with those predicted for an ideal system. Esumi and Ogawa [150] explained this ideal behavior by a mutual compensation of two effects: (1) an increase of the mixed cmc, caused by a weaker interaction between hydrocarbon and fluorocarbon chains in the mixed micelle than between individual hydrocarbon and fluorocarbon chains, and (2) favorable mixed-micelle formation because of electrostatic attraction between the cationic functionality of FOSAB and the anionic functionality of LiDS. At a 0.8 molar fraction of FOSAB, the FOSAB-LIDS system exhibited strong shear dependence, suggesting the presence of rodlike micelles. The cmc curves for the FOSAB-LiFOS system exhibited a large deviation from ideality, with an interaction parameter p = -8.3. 7.4

MESOPHASES AND LIQUID CRYSTALS

A discussion of surfactant solutions usually involves the solid surfactant phase,

dissolved surfactant monomers, and micelles in a solution above the cmc. Actually, a surfactant in a solvent can form several phases, depending on temperature and the surfactant concentration. A complete phase diagram of the surfactant-water system is essential for the understanding of the properties of the surfactant and its solutions [ 1511. The greater rigidity, volume, and hydrophobicity of fluorocarbon chains, relative to hydrocarbon chains, enhance the self-association of fluorinated surfactants into micelles, as well as into liquid crystals. mesophases, and vesicles. Liquid crystals are thermotropic or lyotropic, depending on the mode of their formation. In solid crystals, the ordering is three dimensional or, in some exceptional cases, two dimensional. When a solid is heated to its melting point, the crystal lattice of the solid collapses. When the melt is isotropic, molecules are randomly distributed and any orientational ordering is short range. In contrast, the melt of a thermotropic solid is anisotropic. The melt retains some its crystalline ordering, which is eliminated at a sufficiently higher temperature. At a second melting point, the anisotropic melt converts to an isotropic fluid. Lyotropic liquid crystals result from an interaction between a solid and a liquid. Hence, lyotropic liquid crystals are binary systems. When a solid surfactant is brought into contact with water, the predominantly crystalline solid disintegrates. Molecular and micellar solutions are isotropic. Interactions and ordering in molecular solutions are only short range. However, the dissolution of the surfactant in water does not proceed directly to a micellar solution but involves transitions to intermediate phases. These mesophases (meso = in between) have re-

Structure of Micelles and Mesophases

331

tained some of the crystalline character of the solid. This dualism is expressed in the term liquid crystal [ 1,152,1531. Mesophases also form when the concentration of a surfactant in its micellar solution is increased. When the concentration of the surfactant above cmc is increased, the number of micelles and their size increase, in accord with the mass action model. Dilute micellar solutions are isotropic, but at higher surfactant concentrations, intermicellar interactions produce mesophases which are anisotropic and have a one- or two-dimensional ordering. The structure of mesophases has been investigated by polarizing microscopy, light scattering. x-ray scattering. and SANS (see Chapter 9). The methods used in the past have been insufficient for the study of surfactant phases [ 1541. Recent studies have significantly benefited from NMR spectrography. The structures of mesophases have been described as rodlike hexagonal phases (HI and HII).a lamellar phase (Le), two sets of cubic phases (II. 111,and VI, VII), and nematic phases [ I , 153,155-1571. The liquid-crystalline mesophases feature one-dimensional (lamellar), two-dimensional (hexagonal), or three-dimensional(cubic)translationalperiodicity.Smecticphasescontainsurfactant molecules arranged in layers with the long molecular axes in a layer being parallel to one another and to the surfactant molecules of other layers, and perpendicular or slightly inclined to the plane of the layer. In nematic phases, the long molecular axes of surfactants in a layer are arranged in parallel lines to one another but not in layers. Nematic phases have only orientational order (Fig. 7.38). The hexagonal phases consist of rodlike micelles packed in anhexagonal assembly (Fig. 6.1). A lamellar phase consists of bilayer surfactant aggregates sepA

n 4

FIG.7.38 Nematic liquid-crystal structure. (From Ref. 152. Reproduced by permission of John Wiley & Sons.)

332

Chapter 7

arated by water layers. Two types of cubic phases are known to exist in hydrocarbon-type surfactant systems. The first type occurs between micellar solutions and hexagonal phases and the second type between lamellar and hexagonal phases. The structure of the first type of cubic phase is believed to be a cubic arrangement of small micelles. The second type has been described as a regular three-dimensional network of surfactant aggregates. The cubic phases are isotropic. In the hexagonal-lamellar phase transition region of hydrocarbon surfactants, either a cubic phase exists, as with nonionic surfactants and short-chain ionic surfactants, or birefringent intermediate phases form, as for long-chain anionic surfactants and nonionic surfactants. The term “intermediate phase” refers to birefringent phases formed between hexagonal and lamellar phases [ 153,1581. This term used by Tiddy [ 1531is restricted to birefringent phases and excludes cubic phases. Because both anionic and nonionic surfactants form intermediate phases, intermicellar repulsion is not essential for the formation of an intermicellar phase. Hall and Tiddy [ 1591 have postulated that the dominant factor which determines whether a cubic or an intermediate phase is formed is the balance between the type of the polar head and restrictions on alkyl chain conformational freedom and packing. The conformational restrictions increase with increasing chain length. Nematic phases are solutions of orientationally ordered discoid (phase ND) or columnar (Nc) micelles [ 1601. Nematic phases align in a magnetic field of adequate strength. The conditions for the existence of nematic phases are not completely understood [53,154]. Most of the binary surfactant systems at the transition from an isotropic solution to the liquid-crystalline phase do not give a netnatic phase but form a hexagonal or lamellar phase instead. One of the prerequisites for the formation of a nematic phase is the existence of anisotropic micelles in the isotropic phase. The magnitude of anisotropy and the size of micelles at the transition concentration must be within a narrow range favorable to the formation of the nematic phase. Like surfactants with hydrocarbon chains, fluorinated surfactants can form liquid crystals. Liquid crystals of fluorinated surfactants are important because of industrial applications and their use as biological membrane models (see Chapter 8). The mesomorphic phases formed by fluorinated surfactants have been the subject of numerous studies [25,52,53,108,155,161-1781. The phase behavior of fluorinated surfactants and hydrocarbon surfactants is remarkably similar. Tiddy and co-workers [ 161-1641 observed that anmoniutn perfluorooctanoate and lithium perfluorooctanoate, like hydrocarbon surfactants, form a hexagonal phase, a lamellar phase. and an intermediate phase. A reversed hexagonal structure originally postulated for the intermediate phase [ 163,1641 was found to be inconsistent with I9F-NMR observations, and an alternative lamellar structure was proposed [ 1651.

Structure of Micelles and Mesophases

333

The question of whether fluorinated surfactants form cubic phases has not been answered unequivocally. Tiddy [ 1631 has concluded that the preferential formation of intermediate phases instead of cubic phases depends on the balance between thepolar head and the conformational restrictions of thehydrophobic chain. The fluorocarbon chains are stiffer than hydrocarbon chains and, therefore, have less conformational freedom. The stiffness of fluorinated alkyl chains is caused by a large energy difference between the gauche and trans conformations [163]. Hence, fluorinated surfactants form intermediate phases rather than cubic phases. Although conformational restrictions decrease with decreasing chain length, even the shortest-chain fluorinated surfactant examined formed only intermediate phases [ 1591. Kekicheff and Tiddy [155] studied the structure of the intermediate phase using high-tensity, high-resolution x-rays from a synchrotron source. The intermediate phase was found to have a repeated layer structure closely related to the lamellar (La) phase. Kekicheff and Tiddy proposed that the intermediate phase is a lamellar phase where the layers have a regular array of holes through which water and ions can diffuse. Guo et al. [25] studied fluorinated surfactants (perfluoroheptanoic acid, its salts, and ethoxylated amide of perfluoroheptanoic acid). The characteristics of "F-NMR spectra and optical isotropy suggested that the aggregates of nonionic fluorinated surfactants (ethoxylated amides) may exist in a cubic phase. The formation of a cubic phase by a fluorinated surfactant has been reported also by Caboi et al. [ 1791. The effect of counterions on the phase behavior of perfluoropolyether carboxylates (Cl-PFPE-X, where X = Na, K, or NH4) of the general structure

CIC~F~O(CF~CFO)n(CFO),,,(CF~O),CF~COOX

I

CF3 CF3

I

where II 9 rn and q = 0, has been studied by Caboi et al. [ 1791 by NMR, optical microscopy, tensiometry, and specific conductivity measurements. The degree of binding p was obtained by the ratio between the slopes of the conductivity versus surfactant concentration in the micellar region (c > cmc) and in the premicellar region (c < cmc). (The Surfactants did not contain significant amounts of extraneous salts). The counterion binding p values of 0.23 for C1-PFPENa, 0.3 1 for C1PFPEK, and 0.40 for C1-PFPE-NH4were found to be inversely related to counterion hydration, NH4+ < K+ < Na+ (see also Sections 6.5 and 6.6). The surfactants with NH", K+. or Na+ counterions form lamellar phases in the moderate concentration range, but at high surfactant concentrations, different liquid crystal structures are observed. The 'H-NMR signal revealed a cubic phase for the sodium salt. A reverse hexagonal phase in equilibrium with a lamellar one was observed for the potassium salt and a second lamellar phase for the ammonium salt. The type of liquid-crystal phases is related to the packing parameter [ 1801.

Chapter 7

334

Methylation of the terminal hydroxyl in nonionic fluorinated surfactants has only a slight effect on the phases formed in water at low temperatures [181]. At temperatures above about 3O-4O0C,none of the isotropic and anisotropic phases can exist and no stable bilayer structures can be formed. Although the temperature range of the phases is reduced by capping of the terminal hydroxyl, the sequence of the phases does not change. Boden et al. [172] showed that a thermodynamically independent nematic mesophase exists in the cesium perfluorooctanoate (CsPF0)-water system between 37% and 87% (w/w) 'HzO and l 1-75°C. The nematic phase is intermediate to an isotropic micellar solution at higher temperatures and a smectic lamellar mesophase at lower temperatures. The isotropic phase consists of disk-shaped micelles. The lamellar phase has been described by a structure in which continuous lamellae of the surfactant are broken by irregular water-filled defects without interlayer correlations [ 183. In the nematic phase, the aggregates make the transition from discrete disks to continuous lamellae [ 160,1821.Both positional and oriental order increase when the temperature is lowered. The nematic phase of CsPFO-water is stable over a wide range of concentrations without needing a cosurfactant or salt as a stabilizer. The disk-shaped micelles of the nematic phase orient with their unique axis parallel to the direction of an applied magnetic field. The phase diagram of the CsPFO-water system is qualitatively similar to that for the CsPFO-D20 system. There are, however, quantitative differences which diminish with increasing temperature and eventually disappear [ 1831. Herbst et al. [54] have shown that a 30% (w/w) solution of tetramethylammonium perfluorononanoate in D 2 0 forms three lyotropic phases: an isotropic solution at 237"C, a nematic phase in the temperature range between 32°C and 37OC, and a lamellar phase in the 25-32°C temperature range. The isotropic solution near the liquid-crystal-phase boundary contains disklike micelles. The aggregation number of the micelles is about 150 and the thickness about 35 A.including head groups and counterions. Small-angle neutron scattering (SANS) indicated that the aligned nematic phase of tetramethylammonium perfluorononanoate maintains its alignment when it is cooled in a magnetic field to the temperature of the lamellar phase and the magnetic field is withdrawn. The lamellar phase of tetramethylammonium perfluorononanoate consists of double layers of disklike micelles alternating with layers of D20. The radius of the disks is probably about 59 The polydispersity of the disklike micelles is narrow. The layer does not have a two-ditnensional long-range order, but a longrange periodicity exists between the layers. In the nematic phase, the long-range periodicity perpendicular to the layers is missing. The lyotropic phases of the lithium and ammonium salts of perfluoropolyether (PFPE) carboxylic acids have been studied by optical microscopy [ 1841. surface tensiometry, 'H-, 14N-, and "C-NMR spectroscopy [179,184,185],

A.

Structure of Micelles and Mesophases 335

electron spin resonance (ESR) [40,44,45], SANS [186], and x-ray scattering (SAXS and SAXRD) [186,187] techniques (Chapter 9). The perfluoropolyether surfactants were synthesized from the intermediates of the photooxidation of hexafluoropropene. Their structure has been described as

RfO(CFCFZO),,(CFO),,(CF~O)tnCF~COO NHA

I

CF3

I

CF3

where 17 9 m , y ; n l

== p ==

0; Rf is the CI-C3 perfluoroalkyl group, [ 184,1851. and

CF3(0CF?CF),,OCF$2OOM

I

CF3 with 2 < n < 5 and M = NH4 or Li [44,186]. Gebel et al. [ 1861 identified three different regions in the phase diagram for the PFPE carboxylate-water system: (1) a liquid-crystal smectic phase, (2) a biphasic domain of smectic phase, and (3) at low concentrations an isotropic (micellar)phasecontainingflatparticles,probablylargevesicles.Lamellar (anisotropic) liquid-crystal phases are formed by the short-chain (average equivalent weight EW 450) and intermediate-chain (average EW 740) PFPE surfactants, whereas inverse hexagonal liquid crystals occur in the long-chain (average EW [ 1841. Small-angle x-ray diffraction 940) PFPE carboxylate-water systems (SAXRD) has indicated significant solvent (water or formamide) penetration in the lamellae [ 1871. Two water regions with different structural and dynamic properties have been identified in the interlamellar domains [44,188]. Vesicles are metastable aggregates which contain a certain amount of solvent and return gradually to the lamellar state from which they originated [ 1801. Vesicles have been observed in systems containing single-, double-, and triplechain surfactants, including cationic single-, double-, and triple-chain fluorinated surfactants [ 1891, hybrid surfactants containing a fluorocarbon and a hydrocarbon chain [ 1901, ionpairs of fluorinated surfactants [ 1911, mixtures of ammonium perfluoropolyethercarboxylate (PFPENH4) and n-dodecylbetaine [48-5 I]. nonionic surfactants derived from perfluorocarbon alcohols [ 1911, nonionic surfactants consisting of a fluorocarbon group separated from a polyoxyethylene chain by a methylene group [ 193,1941, nonionic surfactants consisting of a single fluorocarbon tail, a single- or double-chain hydrogenated tail. and a peptide link separating the hydrophobic tails from the hydrophilic head derived from natural disaccharides [ 1951 (see Section 10.4), double-chain fluorinated phosphatidylcholines, amphoteric single-chain perfluoroalkylated phosphocholine derivatives [ 1961, such as [2-(F-octyl)ethyl]phosphocholine [ 1971, and single-chain perfluoroalkylated dimorpholinophosphoramidates[ 1981. The formation of vesicles is facilitated by a multichain structure of amphiphiles. Single-chain amphiphiles need fa-

336

Chapter 7

vorable intermolecular interactions for their self-alignment and tight molecular packing. The stability of vesicles from fluoroalkylated single-chain amphiphiles depends on the chain length and is related to their truncated-cone geometry [ 1971. Vesicles are either unilamellar or multilamellar, determined by the geometry of the amphiphiles. For example, the structure of the aminoacid linkage in glycolipid based double-tailed surfactants [195] has a considerable effect on the structure of vesicles. Multilamellar vesicles are obtained with one glycine segment, whereas unilamellar vesicles are formed with a glycyl-glycine spacer. Bis(polyfluoroalkylated)bis(ammonium) compounds with a short spacer X, [RFC2H4XCOCH2N(CH3)2(CH2),IN(CH3)20CXC2H4Rf12+ 2Br-, form after sonication unilamellar vesicles in water [199]. Unilamellar vesicles are also formed by ammonium perfluoropolyethercarboxylate and n-dodecylbetaine, as evidenced by (dynamic and static) light scattering, NMR, and ESR data [49]. The formation of vesicles depends on the energy introduced into the system by mechanical agitation. Thus, brief sonication of systems containing perfluoroalkylated phosphocholines or dimorpholinophosphoramidates produces small vesicles, but prolonged sonication generates fibers and globules [ 1981. However, stable vesicles can form spontaneously, without mechanical agitation or a chemical treatment [48,49]. Ammonium perfluoropolyethercarboxylate and n-dodecylbetaine form spontaneously stable vesicles, in a narrow range of the total surfactant concentration and the betaine/PFPENH4 molar ratio [48]. Vesicles are of increasing interest for various practical applications, including drug and biomolecule delivery (see Chapter 10.5). The eflect of additives on mesophases of fluorinated surfactants has been studied by Tiddy and Wheeler [ 1 631 and Rosenblatt [ 1761. Tiddy and Wheeler described the effects of n-octanol on the ammonium perfluorooctanoate-water system with a three-component phase diagram (Fig. 7.39). The main differences between phase diagrams for this system and that for sodium octanoate- octanolwater were related to mutual phobicity between fluorocarbon and hydrocarbon chains. Octanol was found to be less soluble in the aqueous micellar phase of ammonium perfluorooctanoate than that of sodium octanoate. However, ammonium perfluorooctanoate is more soluble in octanol than sodium octanoate. This solubility difference is probably related to the effect of counterions, as ammonium salts are usually more soluble in octanol than sodium salts. Rosenblatt [ 1761 found that the addition of perfluorooctanol, a cosurfactant, to the oblate cesium perfluorooctanoate-water micellar system raised the nematic-isotropic transition temperature. The micelles grew substantially when the cosurfactant was added. Holmes et al. [182,200] have explored the effects of an electrolyte (CsC1) and a nonionic alcohol (lH, 1H-perfluroheptan-1-01)on the lamellar and nematic phases of the CsPFO-water system. Both additives facilitate a decrease in surface curvature and the growth of larger and flatter interfaces.

Structure of Micelles and Mesophases

337

'\ \

FIG.7.39 Phase diagram of APFO-octanol-water system at 298 K. (---) Boundaries not accurately determined. L,: aqueous solution; L2: octanol solution; D: lamellar phase consisting of lipid bylayers separated by water. (From Ref. 163. Reproduced by permission of Academic Press, Inc.)

Ishikawa et al. [201] established triangular phase diagrams of the LiDS-alcohol-water and LiFOS-alcohol-water systems,forfluorinatedalcohols (Cf1Fzn+ I CH20H, n = 1, 2, 3) and hydrogenated alcohols (C,lH2+I OH, IZ = 3, 4, 5, 6). When alcohol was added, the phase boundaries of hexagonal and lamellar liquid-crystal phases assumed an outline that is convex to the water side. The alcohol promoted the formation of these mesophases and this effect increased with their increasing carbon number of the alcohols. A phase separation between the hydrocarbon and fluorocarbon components did not occur. The phase behavior of a mixed-su$uctnnt system consisting of lithium dodecyl sulfate (LiDS), lithium perfluorooctanesulfonate (LiFOS), and water was investigated by Tanlori et al. [108]. The phase characteristics of the individual components, LiDS and LiFOS, are similar. The LiDS-water phase diagram, shown in Fig. 7.40, has three liquid-crystal regions: hexagonal (H), bicontinuous cubic (VI), and lamellar (Lam). The LiFOS-water phase diagram is similar (Fig. 7.41) and the same three liquid-crystalline phases exist. In the mixed LiDS-LiFOS system (Fig. 7.42), successive phase transitions, LI-H-VI-Lam-crystals, occur when the total surfactant concentration is increased. A phase region contain-

Chapter 7

338 100

50

Y-. I-

C Ice+ S

-5c

1

1

1

25

50

75

1

composttton/wto/oL~OS

FIG.7.40 The phase diagram of the LIDS-water system; L,: aqueous surfactant solution; H: hexagonal phase; V bicontinuous cubic phase; Lam: lamellar phase; S: indicates the presence of solid surfactant. (From Ref. 108. Reproduced by permission of Academic Press, Inc.) 100

LI

50

L1 *H-

Y \

+ 0

Ice+S

- 50

I

1

25

50

compost t ron/wt

'10

75 LiFOS

FIG.7.41 Phase diagram of LIFOS-water system. Symbols as for Fig. 40. (From Ref. 108. Reproduced by permission of Academic Press, Inc.)

I

Structure of Micelles and Mesophases

339

5 5'c

FIG.7.42 Triangular phase diagram for the LiFOS-LiDS-water system. Symbols as for Fig. 40. (From Ref. 108. Reproduced by permission of Academic Press, Inc.)

ing both Lam and H phases simultaneously has not been observed. This suggests that LiDS and LiFOS are mixed with each other, at least macroscopically. The phase region of S becomes narrower with increasing temperature, whereas other boundary lines between each LI, H, VI, and Lam remain constant. This situation is related to the much lower entropy of the hydrocarbon chain in the crystal than in the other phases. Tanlori et al. [lo81 investigated the structure of the hexagonal phase by xray diffraction. The diameter of the rodlike micelles was estimated from the longest Bragg spacing and the molar volume. The pure LiDS micelles in the hexagonal phase had a diameter of 35 A. The LiFOS micelle in the hexagonal phase had a diameter of 29 A.The radius, 14.5 A,is longer than the estimated length of the LiFOS molecule, 13 This is in accord with the observations that perfluorocarbons prefer the trans form to the gauche form [78,178]. The diameter of the mixed hexagonal phase decreases linearly with increasing LiFOS mole fraction. This observation suggests that the LiFOS molecule does not reach the center and the rodlike micelles have a core consisting of LiDS. In concentrated solutions, mixed micelles exist in all LIDS-LiFOS mole fraction ranges with a positive interaction parameter. The mixed micelle has a LiDS core, similar to the mixed hexagonal phase. Phase diagrams of the sodium perfluorodecanoate-sodium decyl sulfatewater system have been constructed from the dependence of solubility and cmc on temperature [202]. Two kinds of micelles were found: a fluorocarbon-rich mixed micelle and a hydrocarbon-rich mixed micelle. The phase behavior of oppositely charged fluorinated surfactant mixtures

A.

Chapter 7

340

has been studied with the DEFUMAC-LIDS and DEFUMAC-LiFOS systems [ 146,2031. The mixed cmc values obtained by the pyrene-3-carboxaldehyde (PAC) fluorescence measurements indicated a strong electrostatic attraction between the oppositely charged surfactants [146]. In the DEFUMAC-LIDS system, a dispersed or precipitated phase and vesicles are formed, confirmed by dynamic light scattering and transmission electron microscopy. Electrophoretic mobility measurements suggested that the outer layer of the vesicles consist predominantly of DEFUMAC in a DEFUMAC-rich mixture and of LiDS in a LiDS-rich mixture. Tn the DEFUMAC-LiFOS system, a lamellar-type phase was identified by polarization microscopy and x-ray diffraction. Dispersed disklike fragments of the lamellar phase were observed in dilute solutions when one surfactant was in excess [203]. 7.5

HYBRID SURFACTANTS

Mixtures of fluorocarbon and hydrocarbon surfactants have unusual interfacial properties. The fluorocarbon surfactant reduces the surface tension very effectively, whereas the hydrocarbon surfactant lowers interfacial tension. However, the use of hydrocarbon-fluorocarbon surfactant mixtures is complicated by the demixing of the micelles formed in solutions of the mixture. To avoid this problem, surfactants have been synthesized which contain both fluorocarbon and hydrocarbon chains in the same molecule [204-2071 (see Chapter 2). Guo et al. [206,208] synthesized hybrid surfactants with a fluorocarbon group (nz = 6-8) and a hydrocarbon group ( n = 1-9)

OS0,Na

CmF2m+1

-CI

In aqueous solutions, the surface tensions of these hybrid surfactants are about equal or slightly lower than that of the corresponding surfactant with a single hydrophobe. The cmc values are relatively low and are governed by the Kleven equation. The increase of the hydrocarbon-chain-length by a CH2 group decreases the cmc by about 35%. The increase of the fluorocarbon chain by each -CF?- group decreases the cmc by about 75%. Quantitative analysis of the "FNMR spectra has revealed that the residence time of the fluorocarbon chain in

Structure of Micelles and Mesophases

341

the micelle is longer than that of single-chain surfactants [206]. The residence time of the CF3- group increases monotonically with the increase in total surfactant concentration and is several times longer than that of the a-CF2-- group, The residence time of the latter does not depend on the surfactant concentration. The hybrid surfactants synthesized by Guo et al. [206] hydrolyze in moist air and have to be stored in a desiccator. Yoshino et al. [207] synthesized hybrid surfactants which contain an aromaticring C,zF2,1+ IC6H4COCH(S03Na)C,),H2,,z+ 1, where M = 4 and 6, nz = 2.4, and 6, and C6H4 = p-phenylene. These surfactants are stable in the presence of moisture and can emulsify a ternary system consisting of mutually immiscible components: hydrocarbon, water, and perfluoroether oil. The Krafft point, the area occupied by a surfactant molecule at the air-water and octane-water interfaces, and the aggregation number of micelles increase with an increase in fluorocarbon and/or hydrocarbon chain length of these hybrid surfactants [209]. The cmc, surface tension, and octane-water interfacial tension at the cmc decrease with an increase in fluorocarbon and/or hydrocarbon chain length. The surfactants lower the surface tension, as well as the hydrocarbon oil-water and the water-fluorocarbon oil interfacial tension. Their solutions in water can float on hydrocarbon liquids, such as benzene, cyclohexane, and decane. The 10% solution of the surfactant with nz = 6 and n = 4 chains exhibit rubberlike viscoelasticity [210]. Ito et al. [211] investigated the micellar aggregation of the hybrid surfactants (172 = 4,6; rz = 2,4,6) by the pyrene fluorescence method and Raman spectroscopy. The results suggested that the surfactants FC6-HCrz and FC4-HC6 form a loosely packed hydrated micelle first, but a dehydrated micelle coexists above the second cmc. Research on binary fluorinated surfactant-hydrocarbon surfactant mixtures is continuing with the main focus on the mutual solubility of mixed surfactants. It has been agreed [88] that an increase in the number of carbon atoms in either hydrophobe decreases the mutual solubility. An increase in the number of oxyethylene units in a nonionic hydrophile increases the mutual solubility of the mixed surfactants. An increase in temperature increases the mutual solubility. However, the mechanisms of thorocarbon-hydrocarbon surfactant demixing and the formation of two types of coexisting micelles are still not completely understood. The gaps in knowledge are not caused by a lack of theories but are a result of experimental limitations. The development of more powerful analytical methods for the study of surfactants in micelles will advance the quantitative understanding of mixed-surfactant systems. Finally, the advanced theoretical knowledge of micellar fluorinated surfactant systems will be applied to practical processes, such as adsorption, wetting, solubilization, emulsification, foaming, liquid-crystal formation, and others.

Chapter 7

342

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346 136. 137. 138. 139. 140. 141. 142.

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153.

154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166.

Chapter 7 K. Esumi, Y. Sakamoto, K. Yoshikawa. and K. Meguro,Colloids Surf. 36. 1 (1989). K. Esumi, T. Arai,and K. Tagasugi. ColloidsSurf. 11 1. 231 (1996). G-X. ZhaoGuo-Xi and B-Y. Zhu, Colloid Polym. Sci. 261, 89 (1983). J. Hao, H. Wang, and S. Dong, J. Dispers. Sci. Technol. 18. 379 ( 1 997). K. Esumi. N. Makamura, and K. Nagai, Langmuir 10,4388 ( 1994). K. Nagai and K. Esumi, Colloids Surf. A94,97 ( 1995). G-X. Zhao and B-Y. Zhu, in “Phenomena in Mixed Surfactant Systems.” J. F. Scamehorn. ed., ACS Symposium Series No. 3 1 1, p. 184, American Chemical Society. Washington. DC (1986). D. J. Tanlpietro and E. W. Kaler. Langmuir 15, 8590 (1999). A. F. Thiinemann and K. H. Lochhaas, Langmuir 15.6724 ( 1999). K. Tamori, A. Tshikawa, K. Kihara, Y. Ishii, and K. Esumi, Colloids Surf. 67, 1 ( 1992). K. Esumi, ColloidsSurf. A84,49 (1994). H. Furuya. Y. Mori, and G. Sugihara, Langmuir 1I . 774 ( I 995). G. Sugihara, F. 0. Nagao, T. Tanaka, and S. Lee. J. Colloid Interf. Sci. 171, 246 (1995). G. Sugihara, Y. Era, M. Funatsu, T. Kunitake, S. Lee. and Y. Sasaki, J. Colloid Interf. Sci. 187. 435 (1997). K. Esumi and M. Ogawa. Langmuir 9,358 (1993). R. G. Laughlin, in “Advances in Liquid Crystals,” G. H. Brown, ed.. Vol. 1. p. 41, Academic Press, New Ynrk (1975). D. B. DuPre‘. in “Kirk-Other, Chemical Encyclopedia.” Vol. 14, p. 395, John Wiley & Sons, New York ( I 98 1). G. J. Tiddy. in “Modern Trends of Colloid Sciencein Chemistry and Biology,”H. F. Eicke, ed., p. 148, Birkhauser, Base1 (1985). R. G. Laughlin,J. Am. Oil Chem. SOC.67,705 (1990). P. Kekicheff and G. J. T. Tiddy, J. Phys. Chem.93,2520 (1989). P. Ekwall. L. Mandell. and K. Fontell, in “Liquid Crystals,” G. H. Brown, ed., Vol. 2, p. 325, Gordon & Breach Science, London (1969). P. A. Winsor, Chem. Rev.68. 1 (1968). V. Luzzati. H. Mustacchi. A. Skoulios. and F. Husson, Acta Crystallogr. 13, 660 (1960). C. Hall and G. J. T. Tiddy, 6th International Symposium on Surfactants in Solution. New Delhi (August 1986). M. C. Holmes, D.J. Reynolds. and N. Boden,J. Phys. Chem. 91.5257 (1987). G. J. T. Tiddy, Symp.Faraday SOC. 5, 150 ( 1971). G. J. T. Tiddy. Trans.Faraday SOC.68,653 (1972). G. J. T. Tiddy and B. A. Wheeler, J. Colloid Interf. Sci. 47, 59 (1974). E. Everiss, G. J . T. Tiddy, and B. A. Wheeler, J.Chem. SOC.Faraday Trans. 72,1747 (1976). G. J. T. Tiddy, Trans.Faraday SOC.I, 73. 1731 (1977). P. G. Morris,P. Mansfield, and G. J.T. Tiddy, J.Chem. SOC.Faraday Symp. 13,37 (1 979).

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Structure of Micelles and Mesophases

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167. G. J. T. Tiddy. M. F. Walsh, and E. Wyn-Jones, J. Chem. SOC.Faraday Trans. I, 78. 389 (1982). 168. K. Fontell and B. Lindman. J. Phys. Chem. 87, 3289 (1983). 169. K. Fontell. in ”Surfactant Solutions.” K. Mittal and B. Lindman, eds.. Vol. I , Plenum Publishing, New York (1984). 170. H. Hoffnlann, G. Platz, H. Rehage, K. Reizlein, and W. Ulbricht, Makromol. Chem. 182,451 (1981). 171. K. Reizlein and H. Hoffmann, Prog. Colloid Polym.Sci. 69. 83 (1984). 172. N. Boden. P. H. Jackson, K. McMullen. and M. C. Holmes, Chem. Phys. Lett. 65, 476 ( 1 979). 173. R. M. Hedge, R. T.Thomas, M. Mortimer, and J. W.White, J. Chem. SOC.Faraday Trans. I. 76. 236 (1980). 174. M. C. Holmes, D. J. Reynolds, and N. Boden, J. Phys. Chem. 91, 5257 (1987). 175. (a) J. C. Ravey and M. J. Stebe, Comun. J. Esp. Deterg. 18, 271 (1 987). (b) J. C. Ravey and M. J. Stebe, Progr. Colloid Polym. Sci.82, 21 8 (1 990). 176. C. Rosenblatt. J. Phys. Chem. 91,3830 (1987). 177. R. Bongliovanni, E, Borgarello, C. Carniani. and C. Genova. Colloids Surf. 54, 75 (1991). 178. H. Hoffmann. Ber. Bunsenges. Phys. Chem. 88, 1078 (1984). P. Lazzari, and M. Monduzzi. Colloids Surf. A160, 37 179. F. Caboi. A. Chittofrati. ( 1999). 180. D. J. Mitchell and B. N. Ninham. J. Chem. SOC.. FaradayTrans. I1 77, 601 (1981 1. 181. S. Achilefu, C. Selve, M.-J. Stkbk, J.-C. Ravey, and J.-J. Delpuech, Langmuir 10, 2131 (1994). 182. M. C. Holmes, M. S. Leaver. and A. M. Smith, Langmuir 11,356 (1995). 183. N. Boden, K. W. Jolley, and M. H. Smith, J. Phys. Chem. 97, 7678 ( 1 993). 184. M. Monduzzi. A. Chittofrati, and V. Boselli, J. Phys. Chem. 98.7591 (1994). 185. F. Caboi. A. Chittofrati, M. Monduzzi, and C. Moriconi,Langmuir 12,6022 (1996). 186. G. Gebel, S. Ristori. B. Loppinet, and G. Martini, J. Phys. Chem. 97, 8664 (1993). 187. A. Chittofrati. V. Boselli, M. Visca, and S. E. Friberg, J. Dispers. Sci. Technol. 15, 7 1 1 (1994). 188. M. D’ Angelo, G. Martini.G. Onori, S. Ristori, and A. Santucci. J. Phys. Chem. 99, 1120 (1995). 189. (a) T. Kunitake, Y. Ohata, and S. Yasunami, J. Am. Chem. SOC.104. 5547 (1982). (b) T.Kunitake. Angew. Chem. Int. Ed. Engl. 3 I , 709 ( I 992). 190. T. Kunitake and N. Higashi, Makromol. Chem. 14(Suppl.), 81 (1985). 191. H. Fukuda, K. Kawata, and H. Okuda, J. Am. Chem. Soc. 112. 1635 (1990). 192. J. Wurtz and H. Hoffmann, J. Colloid Interf. Sci. 175. 304 ( 1995). 193. J. C. Ravey and M. J. StgbC, Colloids Surf. A84, 1I (1994). 194. J. C. Ravey. M. J. Stkbe‘, S. Sauvage, and C. Elmoujahid, Colloids Surf. A99, 221 ( 1995). 195. L. Zarif, T. Gulik-Krzywicki, J. G. Riess, B. Pucci, C. Guedj. and A. A. Pavia, Colloids Surf. A84, 107 ( I 993). 196. M.-P. Krafft, F. Giulieri, and J. G. Riess, Angew. Chem. Tnt. Ed. Engl. 32, 741 (1993).

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197. F. Giulieri and M. P. Krafft, Colloids Surf. A84. 121 ( 1 994). 198. M.-P. Krafft, F. Giulieri. and J. G. Riess. Colloids Surf. A84. 113 (1994). 199. M. Gaysinski, L. Joncheray, F. Guittard, A. Cambon, and P. Chang, J. Fluorine Chem. 74, 131 ( 1995). 200. M. C. Holmes, A. M. Smith, and M. S. Leaver, J. Phys. I1 3, 1357 (1993). 201. A. Ishikawa, K. Tamori, K. Esumi, and K. Meguro. J. Colloid Interf. Sci. 151, 370 (1992). 202. T. Akune, M. Abe, Y. Murata,T. Maki, Y. Moroi,H. Furuya and M. Tanaka, J. Colloid Interf. Sci. 18 1, 136 (1 996). 203. K. Tamori, K. Kihara, H. Sanda, K. Esumi, K. Meguro, C. Thunig, and H. Hoffmann, Colloid Polym. Sci. 270, 885 (1992). 204. E. G. Shafrin and W. A. Zisman, J. Phys. Chem. 66.740 (1962). 205. P. S. Abenin, F. Szonyi, and A. Cambon, J. Fluorine Chem. 55, 1 (1991). 206. W. Guo, Z. Li, B. M. Fung, E. A. O’Rear, and J. H. Harwell, J. Phys. Chem. 96,6738 (1992). 207. N.Yoshino, K. Hamano. and Y. Omiya, Langmuir 11,466 (1995). 208. W. Guo, B. M. Fung. and E. A. O’Rear, J. Phys. Chem. 96. 10,028 ( 1992). 209. A. Ito, H. Sakai, Y. Kondo. N. Yoshino, and M. Abe, Langmuir 12,5768 (1996). 2 10. M. Abe, K. Tobita, H. Sakai, Y. Kondo, N. Yoshino, Y. Kasahara, H. Matsuzawa, M. Iwahashi. N. Momozawa, and K. Nishiyama, Langmuir 13, 2932 (1997). 21 1. A. Ito, K. Kamogawa, H. Sakai, K. Hamano, Y. Kondo, N. Yoshino, and M. Abe, Langmuir 13,2935 ( 1997).

Applications

8.1

PERFORMANCECHARACTERISTICS OF FLUORINATED SURFACTANTS

The performance characteristics of fluorinated surfactants are related to the fluorocarbon segment in the surfactant molecule [l-91. The fluorocarbon segments are hydrophobic as well as oleophobic and the interaction between fluorocarbon chains is weak. Consequently, fluorinated surfactants can lower the surface tension of water more than hydrocarbon-based surfactants. Many fluorinated surfactants are powerful wetting agents. Fluorinated surfactants are needed for wetting surfaces that have a critical surface tension below 25 mN/m. For wetting surfaces of a higher critical surface tension, fluorinated surfactants can be useful because of their nonrewetting effect. Some fluorinated surfactants are strongly adsorbed on the surface with the fluorocarbon chains oriented toward the solution. As a result, the adsorbed fluorinated surfactant prevents or hinders rewetting of the surface by the solution. As an example, Zonyl FSP, an anionic surfactant, is a protective, anticorrosive agent for aluminum in acidic or alkaline media. Some fluorinated surfactants reduce the wettability of hydrocarbon surfaces, such as polyethylene. A unique characteristic of fluorinated surfactants is the chemical stability of the fluorocarbon chain to strong acids, oxidizing agents, and concentrated alkalis. Hence, fluorinated surfactants can be used in media where hydrocarbon-based surfactants would decompose. Fluorinated surfactants are useful emulsifiers and dispersants for systems in which either the continuous phase or the disperse phase is a fluorocarbon. Mixtures of fluorinated and hydrocarbon surfactants can be more effective and less expensive than each component alone. 349

350

Chapter 8

TABLE 8.1

Surface Tension of Fluorad@'

Fluorinated Surfactants Surface tension in water at 25°C (mN/m)

Fluorad Anionic FC-94 FC-I18 FC-120 FC-129 Cationic FC-135 Nonionic FC-17OC FC-171

Structure

0.001Yo

0.01Yo

0.1OYo

RfS03Li ( n - 8) RfCOONH4 ( n = 8) RfS03NH4 ( n - IO) R~SO~N(C~HS)CH~COOK ( n - 8)

63 60 20 56 49

54 53 26 23

39 44

RfS02NHC3HGN+(CH3)31- ( n - 8)

28

18

17

20 ( n -20 8) RfS02N(C2HS)(CH2CH20)xH RfS02N(C2Hs)(CH2CH2O)xCH3( n - 8)

22 39

22

20

17

Note: Rf = F(CF2),. Surfactant concentrations are given in percent active ingredient. Source: Ref. 8.

Leveling of paints andfloor polishes is also basedon the superior wetting power of fluorinated surfactants. Reductionof surface tension gradients in the paint film during dtying is essential for a uniform coating. Very low concentrations, 50-150 ppm, of a fluorinated surfactant can effectively overcome streaking and beading. The foaming properties of fluorinated surfactants vary widely (see Section 4.9). For example, amphoteric surfactants, Zonyl FSK and Zonyl FSC, are outstanding foaming agents. In contrast, the anionic fluorinated surfactant, Zonyl FSP, and the nonionic fluorinated surfactant, Zonyl FSN, are low foaming. In some systems. Zonyl FSP can function as an antifoarn agent. The structures and surface tensions of some Fluorad@tluorinated surfactants, produced by electrochemical fluorination. are shown in Table 8.1. 3M has discontinued the production of Flourad fluorinated surfactants based on perfluorooctanesulfonic and perfluorooctanesulfonic acid (see Chapter 10). The structures and surface tensions of Zonyl fluorinated surfactants, produced by telomerization. are shown in Table 4.4. 8.2

SELECTION OF FLUORINATEDSURFACTANTS

Before using a fluorinated surfactant in a product or system, the following questions should be answered [1,8]: 1. What is the desired effect of the fluorinated surfactant? Improved wetting, improved spreading. improved foam generation, reduced water

Applications

351

spotting, smaller gas bubbles, smaller droplets, enhanced liquid penetration, and stability to heat, acids, alkali, oxidation or reduction? 2. Are the physical and chemical properties of the fluorinated surfactant suitable for the system? 3. Will the fluorinated surfactant cause problems for the product or system? 4. Is the fluorinated surfactant cost effective and are its benefits significant? 5. Does the fluorinated surfactant have any adverse effects on the environment? The relatively high price of fluorinated surfactants limits their use to applications where hydrocarbon-based surfactants are inadequate. Usually, fluorinated surfactants are cost-effective because their high price is compensated by the low concentration needed. Sonletimes, as little as 50-150 ppm of the surfactant may be adequate. The fluorinated surfactants cost typically about 10 times more than silicones and 100 times more than hydrocarbon surfactant. However, fluorinated surfactants are about 10 times more effective than silicones and 50-100 times more effective than hydrocarbon surfactants. In some applications, a mixture of a fluorinated surfactant and a hydrocarbon-based surfactant is more cost-effective or performs better than either one alone. Fluorinated surfactants are available as liquids, pastes, or solids. Some are diluted with water or an organic solvent; some are sold in the 100% active ingredient form. If the system cannot tolerate water, an undiluted fluorinated surfactant or a surfactant formulated as an organic solution has to be used. Some nonionic fluorinated surfactants, (e.g., Fluorad FC-430, Zonyl FSN-100, and Zonyl FSO100) are soluble in several nonaqueous solvents. Generally, the surfactant must be soluble to be effective. Hence, solubility of the surfactant may limit its use in some systems. Other physical properties [e.g., the cloud point, critical micelle concentration (cmc), hydrophile-lipophile balance (HLB), pour point, and density] are important and have to be considered when selecting a fluorinated surfactant. The surfactant has to be compatible with other surfactants or components present in the system. Some systems can react with the surfactant. Although the fluorinated segment of the surfactant is resistant to chemical attack, the functional groups attached to it (e.g., the polyoxyethylene chain) may not withstand the intended chemical environment. Usually, a fluorinated surfactant does not have all the properties needed for a specific application. A fluorinated surfactant may dramatically improve wetting of a polyester or polyethylene sheet but not function as an effective emulsifier for oil inwater. When selecting a fluorinated surfactant, the desirable and undesirable side effects of its use have to be considered. The selection should not be made

352

Chapter 8

merely on the basis of the tabulated properties supplied by the manufacturer. For example, surface tension reduction depends not only on the structure of the surfactant but also on the nature of the interface. A fluorinated surfactant should, therefore, be tested in the system or product before it is used on a large scale. The surfactant should be tested at various concentrations and the surfxe-active properties and side effects should be observed. The effect of the fluorinated surfactant on the environment must be considered when planning an application of the surfactant. 8.3 SPECIFIC APPLICATIONS The use and application of fluorinated surfactants have been reviewed in several articles [2-71 and in trade literature [ 1,8.9]. This chapter is a more comprehensive review of various fluorinated surfactant applications, listed in alphabetical order.

Adhesives Fluorinated surfactants are used in solvent-based adhesives (e.g., Zonyl FSN-100 and FSO-100) or in water-based adhesives (e.g.. Zonyl FSA, FSP, and FSN). Fluorinated surfactants added to water-based adhesives facilitate wetting and penetration of the substrates being joined [l]. By improving leveling and spreading, fluorinated surfactants assure a cotnplete contact between the joining surfaces and retard foaming. Fluorinated surfactants should be evaluated at 0.001%. 0.01%, and 0.1% solids on weight of the formulated adhesive, as the effectiveness of fluorinated surfactants can vary significantly over this range. Anionic fluorinated surfactants (e.g., Zonyl FSA and FSP) should be evaluated in soft water. If hard water is used, a chelating agent should be added to reduce water hardness. Foamable hot-melt adhesives containing polyamides and fluorinated or silicone surfactants are useful on coarse surfaces [ 101. Fluorinated surfactants added to rubber (SKF 26) allows adhesiveless bonding to steel [ I 11.

Antifogging Fluorinated surfactants can form a durable antimist film on glass, metal, or plastic surfaces. Antimist formulations containing a fluorinated surfactant are very effective in preventing misting of glass surfaces exposed to humid atmospheres, such as mirrors in bathrooms, automobile windshields, and eyeglass lenses. Anionic, nonionic, or amphoteric fluorinated surfactants prevent fogging of glass [ 12-14] and plastic cover sheets used in agriculture. Potassium perfluorooc-

Applications

353

tanesulfonate and nonionic surfactants blended into transparent poly(viny1 chloride), polyethylene, or ethylene-vinyl acetate film reduce clouding caused by condensation of atmospheric moisture [ 15-1 91. A weather-resistant agricultural cover film, made by blending poly(viny1 chloride), phthalic ester plasticizers. nonionic surfactants, and a phthalate-insoluble fluorinated surfactant (Zonyl FSN), displayed antifogging even after 2 months use in the field [20]. Ueno et al. [21] studied the antifogging effect of surfactants on transparent cellulose ester films. Nonionic fluorinated surfactants, applied as a monolayer, were more effective antifogging agents than the corresponding nonionic hydrocarbon surfactants. The surfaces of the thinfilm placed in a closed chamber filled with water vapor at 60°C remained transparent for a long time. The antifogging effect of nonionic fluorinated surfactants was attributed to a double structure consisting of fluorocarbon and poly(ethy1ene oxide) chains in the n~onomolecular layer.

Antistatic Agents Antistats prevent the buildup of static electricity and dissipate the electric charge formed on the substrate. Brueck [22] studied the electrostatic properties of a triboelectric series of polymers having polyethyleneimine at the positive end and a hexafluoropropyIene-tetrafluoroethylene copolymer at the negative end. Various commercial polymers were compared to the polymers of the triboelectric series. The effects of different polar groups were in the order (from most positive to most negative) imino == oxy, carbonate = urethane = aliphatic carboxylate ester, carboxyamide, hydroxy, cyano, aliphatic hydrocarbon = aromatic hydrocarbon, arenecarboxylate == carboxylic acid, nitrate = chloro, fluoro. Surface treatments with fluorinated surfactants drastically altered the charging properties of polymers. Amphoteric surfactants, such as carboxymethyl-3-nonadecafluorodecaneamidopropylammonium hydroxide inner salt, dissolved in an organic solvent (e.g., propanol-2), function as antistatic agents for magnetic tapes and phonograph records [23]. Anionic fluorinated surfactants (e.g., C6F13S0&i) have been used in antistatic rubber compositions [24]. Mixtures of anionic surfactants having c4-16 perfluorocarbon chains ( e g , C6FI3So3Li) and nonionic surfactants having c3-16 perfluorocarbon and poly(oxyethy1ene) chains [e.g., C8FI7SO2N(C3H7) (CH2CH20)20H]have been claimed as antistatic agents for poly(viny1 chloride) [ 251. A nonionicfluorinatedsurfactant, C6F13SO?N[(CH2)5CH3]CH~CH2 (OCH2CH2),,0H. applied as a solution in isopropyl alcohol, reduced the surface charge of polyester film [26]. Nonionic fluorinated surfactants (e.g.. Monflor 5 1 and Monflor 52) are effective internal antistats for low-density polyethylene.

354

Chapter 8

Biomaterials The surface characteristics, such as surface composition, morphology, and wettability, affect the biological response of biomaterials. Grafting perfluorodecanaoic acid on polyurethane improved the compatibility with blood, related to the inertness, low surface energy, and morphology of the fluorinated surface [ 271. The low value, 6.9 mN/m, for critical surface tension, yc, of the highly hydrophobic surface has been related to the favorable orientation of -CF3 groups at the external surface. The surface of the perfluorodecanoic-acid-grafted polyurethane was analyzed by total reflectance Fourier transform infrared (FTIR), angular-dependent ESCA, SIMS, EDXA, x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and energy-dispersive x-ray analysis (EDXA) [28] (see Chapter 9).

Cement Additives Fluorinated surfactants reduce shrinkage of cement [ 291. Cement tiles pigmented with carbon-black dispersions containing fluorinated surfactants are more weather resistant than tiles made with ligninsulfonate dispersants [30]. Fluorinated surfactants improve primers used for coating cement mortar [3 I].

Cleaners for Hard Surfaces Small amounts [0.2% (w/w)] of fluorinated surfactants included in hard-surface cleaners formulated with conventional surfactants can substantially enhance the cleaning power [32]. Fluorinated surfactants facilitate wetting of hard surfaces and aid cleaning of low-energy surfaces, such as polyethylene. Theyalso promote a rapid runoff of rinse solutions. The fluorinated surfactant CloF21CONH(CH2)5COONH4 in a detergent reduces wiping stripes and reflection glitteringof cleaned glass[33]. Fluorinated surfactants (e.g., Zonyl FSN, Zonyl FSP, or Zonyl FSA) are used in cleaning formulations for removing calcium sulfate scale from reverse osmosis metnbranes [34]. The outstanding chemical stability allows fluorinated surfactants to be used in cleaners containing strong acids or alkali. A typical alkaline cleaner contains 5 1 0 % sodium hydroxide and 0.01% an anionic fluorinated surfactant (e.g., Fluorad FC-129). A cationic fluorinated surfactant (about 0.01%) facilitates wetting and the removal of oily soil on concrete [32] and cleaning concrete or masonry with a phosphoric acid-hydrochloric acid mixture, Fluorinated surfactants are also used to facilitate cleaning of metal surfaces (e.g., for cleaning the outside of airplanes) and degreasing of metals (see Metal Finishing). Fluorinated surfactants in automotive waxes aid spreading and improve the resistance of the polish to water and oil. Fluorinated surfactants are used in other car-care products as well: in cleaners for spray washing of automobiles [32] and shampoos for fabrics and vinyl surfaces.

Applications

355

Fluorinated surfactants [e.g., C8F17S02N(C3H7)CH2COOK]in nonaqueous cleaning agents aid the removal of adhesives [35] and in dry cleaning of textiles or metal surfaces [36,37]. Cured epoxy resins are removed from integrated circuit modules by cyclic alcohols containing small amounts of a surfactant [e.g.. C8F17S02N(C2Hs)CH,COOK] [38]. Machine parts, such as steel screws, are cleaned after nickel plating with trichlorotrifluoroethane containing a salt of perfluorooctanesulfonic acid [ 391.

Coatings Coatings have two interfaces: the coating-air interface and the coating-substrate interface. Fluorinated surfactants improve wetting and leveling of paints and control the surface tension during the application of the coating. as well as during the dynamic phase of drying and curing [1,8,9,4042]. Lowering of surface tension by fluorinated surfactants can overcome wetting and dewetting problems caused by contaminants on the surface, such as a film of hydrocarbon or silicone oil. Fluorinated surfactants are effective for the application of a second coat requiring a surface tension lower than that of the first coat. However, the effectiveness of a fluorinated surfactant in overcoating depends on the formulation of the coating. Linert and Chasman [ 431 tested various fluorosurfactant-containing coating formulations for recoatability. The effectiveness depended on the type of the coating. Some fluorinated surfactants improved recoating, others had no effect, and some even hindered recoating of thermal-cure and ultraviolet (UV)-cure systems. For example, Fluorad FC-430 improved recoatability of a thermal-cure epoxy-baked coating. but hindered recoating of a high-solids-polyester coating. Because the surfactant of choice depends on the formulation of the coating, a series of fluorinated surfactants should be tested to select the optimum surfactant for recoating. Fluorinated surfactants maintain a uniformly low surface tension during the application and drying phases. Leveling defects caused by brush marks and roller patterns are minimized by depressing surface tension uniformly throughout the drying and curing phase [8]. Fluorinated surfactants eliminate defects of the coatings related to surface tension gradients, such as orange peel, cratering, picture framing, edge crawling, and fish eyes [1.8,9,40-42]. Localized differences in surface tension can cause surface roughness resembling the skin of an orange. Craters are small bowl-shaped depressions caused by contaminants or particles at the surface. The resulting surface tension differences cause a migration of the resin away from the reduced surface tension area. A rapid evaporation of the solvent causes surface tension gradients and, consequently, a migration of the pigment particles and the resin. Fluorinated surfactants lower the surface tension of paint and reduce pigment flotation, which is quite common when strong tinting pigments are used. Fluorinated surfactants [ 1,8,9] also improve gloss, modify rheology, and control flow as well as foaming.

Chapter 8

356

Pigment dispersions containing a fluorinated surfactant (e.g., perfluoroalkyl phosphate) and an aluminum flake pigment are stable at high temperatures and can be used in automotive coatings applied by spraying and baking [44]. Fluorinated surfactants as paint additives improve the dust repellence of interior or exterior paints [45]. A dispersion containing acrylic and vinyl monomers. a fluorinated surfactant. and ammonium persulfate catalyst was mixed with Ti02 pigment and hexakis(methoxymethy1)melamine sprayed onto a metal surface and cured at 150°C to effect polymerization of the coating. In the absence of the fluorinated surfactant, similar coatings formed craters and cracks [46]. Fluorinated surfactants used in coatings are anionic or nonionic. The amount of the fluorinated surfactant used varies with the coating and the desired effect in the 0.05-0.5% range of the active surfactant, based on the resin solids [1,8]. The effectiveness of the surfactant depends on the coating system. For example, for epoxy coatings, the Fluorad FC-430 is highly effective, whereas the effectiveness of Fluorad FC-43 1 is low. However, Fluorad FC-43 1 is highly effective for cellulosic and acrylic systems, whereas the effectiveness of Fluorad FC-430 is only good [8]. (3M has discontinued the production of the Fluorads FC340 and FC-341, as well as the use of fluorinated surfactants in coatings and paints.)

i n Q) v1

Q)

-

60

0,

iii

C

6 ii

40

0

1

2

3

4

5

6

Surfactant concentration (wt%)

FIG. 8.1 Effect of fluorinated surfactants on the wettability of a poly(methy1 methacrylate) lacquer film: (i) polymerizable surfactant; (ii) nonreactive surfactant; (iii) fluorinated polymer. (Reproduced with permission from Ref. 47. Copyright 0 1998 by Wiley-Chichester.)

Applications

357

CZI,

I

F(CF,),

0

CH,

0

II

I

I1

- S - N -CH, -CH, - 0 - C - C = CH, I

II

0

CH,

CHI

F(CF,),

;F - -N S

I

0

It

CH,-CH,-

II

0 -C - C - CH,

H2

0

F(CF,), - S - N -CH, -CH,-

II

I1

0-C

...

111

0

FIG.8.1

(Continued.)

Surfactants in paint function as an emulsifier for the binder, a dispersant for the pigment. and a wetting agent. However, in the dried paint film, a surfactant can soften the film and impair the durability of the paint [47]. The problems created by a residual surfactant in coatings can be avoided by using [ l ] a destructible or hydrolyzable surfactant [48] or [3] a polymerizable surfactant [47,49]. The effect of polymerization has been demonstrated with two fluorinated surfactants, one polymerizable and one nonreactive, added to poly(methy1methacrylate) lacquer. After curing, the films were rinsed with a solvent. Contactangle measurements (Fig. 8.1) [47,49] showed that the film containing the polymerizable surfactant (i) had been permanently hydrophobized. whereas the nonreactive surfactant (ii) had been washed away. A preformed polymer (iii) also gave a permanently hydrophobic surface.

358

Chapter 8

Perfluorinated urethanes enhance the protective properties of anticorrosive paints [50].Fluorinated silicone surfactants also have been used as paint additives ~511. Fluorinated surfactants are used in floor finishes (see Polishes and Waxes).

Cosmetics Fluorinated surfactants are used in cosmetics as emulsifiers, lubricants, or oleophobic agents. Fluorinated surfactants in hair-conditioning formulations improve lubricity, facilitate wet cotnbing, and render hair oleophobic. Fluorinated surfactants are added to hair creams and rinses to keep the hair from becoming oily. Examples are (RfCH~CH~O),yPO(O-NH~),., x + v = 3(ZonylFSP),(RfCH2CH20)., PO(O-NHl), (OCH~CHZOH),.x + v + z = 3 (Zonyl FSE) [52,53]; perfluorooctanoic acid [54]; CF3(CF&(CH2),S(CH2),COOM, where x = 1-20. y = 1 4 , M = alkalimetalor ammonium' [55]; C8F,7(CH2CH20)sCOC,5H31[56]; C~F~~SO~N(C~H_S)CH~CH?_OP(O)(OH)~, C8F17SO?_N(C2H5)CH1COOK, ora cationic fluorinated surfactant C8F17S03_NH(CH2)3N+(CH~)~I[57]; C~0F2~SO~N(C2H5)CH~CH20P(O)(OH)~ [58], HOCH(CHZSCH2CH2C6F13). [59],and C8F17CH2CH2SCH2CH(OH)CH2CH(CIoH21)C12H2s [60]. Only a small amount (<0.05%) of a fluorinated surfactant (e.g., Lodyne S-106B, Lodyne S-l12B, Zonyl FSA, and Zonyl FSN) is needed to enhance the effectiveness of cationic hair-conditioning agents [6I]. Fluorinated surfactant oligomers have been patented [62] as hair-conditioning agents. Fluoroalkyl (metl1)acrylate polymers [63] also are useful in hair care formulations.

Crystal Growth Regulators The average size of Glauber salt (Na2S04 - 10H20)crystals formed in an aqueous solution in heat storage systems decreases when a fluorinated surfactant (potassium perfluorooctanesulfonate) is present [64].

Dispersions Ferromagnetic metal oxide particles (magnetite) have been dispersed with a fluorinated surfactant [C8F17S02N(C2HS)CH2COOK]in a fluorocarbon-type solvent to make magnetic fluids [65]. Poly(viny1 chloride) pipes manufactured from a polymer blend containing conductive carbon black dispersed with nonionic or cationic hydrocarbon-type or fluorinated surfactants have reduced surface electric resistance [66].

Applications

359

Fluorinated surfactants are used as dispersants in lubricating greases (see Greases and Lubricants).

Electroless Metallization Fluorinated surfactants improve the quality of electroless plating of copper [67,68] and stabilize the coating bath to deposit nickel-boron layers without a dentritic layer structure [69].

Electronics In the electronics industry, fluorinated surfactants are used in a variety of applications. Insulators for electric wires and cables are made from polyethylene containing a fluorinated alkyl ester [ 701 or a perfluoroalkanoic acid salt [ 7 11. Insulated wire with improved coiling propensity was prepared by coating the wire electrophoretically and treating the coated wire with a fluorinated surfactant solution before baking [72]. Insulating tapes have been prepared by impregnating glass-fiber-reinforced mica tapes with a mixture of a fluorinated surfactant with an epoxy resin and curing [ 73,741. A material for sealing electric circuits has been made by curing a mixture of polyamic acid, an epoxy compound derived from a silane, and a nonionic fluorinated surfactant in N , N-dimethylacetanilide (DMAC)

WI. Glass-fiber-reinforced epoxy resin boards treated with a fluorinated surfactant maintain their electric resistance when exposed to high humidity [ 761. In order to prevent hydrogen evolution and electrode corrosion in alkaline and zinc-carbon batteries, zinc has been amalgamated. The high overpotential of mercury decreases the rate of hydrogen evolution [ 77,781. However, mercury can no longer be used because of environmental restrictions. Fluorinated surfactants, such as Forafac 1110 [C6F13C2H4(OC2H4)120H] have been found to inhibit hydrogen evolution and serve as a substitute for mercury [77,78]. Fluorinated surfactants are added to the zinc battery electrolyte to prevent the formation of dendrites [79]. The nonionic fluorinated surfactant Forafac 1 110 changes a coarse-grained deposit on the zincate electrode into a fine-grained surface and inhibits the formation of entangled whiskers on the zinc counterelectrode [80]. A battery, having an improved charging-discharging cycle and increased service life, has a porous Zn anode, a nickel oxide cathode, and an electrolyte consisting of 0.001-0.1% of the nonionic fluorinated surfactant Zonyl FSN in 8N KOH saturated with ZnO [8 11. Alkaline manganese batteries have been made with Mn02 cathodes containing carbon black treated with a fluorinated surfactant (e.g., potassium perfluoroalkylcarboxylate) and a hydrocarbon nonionic surfactant [ 821. Nonionic fluorinated surfactants, CF~(CH?)~ICH~CH?O(CH?CH~O),,H, where m is 5-9 and rz is about 11, can make hydrophobic polymers, such as

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poly(tetrafluoroethylene) (PTFE), hydrophilic for use as microporous separators for electrolytic cells [83]. Fluorinated surfactants are also used as low-foaming noncorrosive wetting agents in solders for electronic parts and cleaning of electronic components [84,85]. A fluorinated surfactant in a polymerized rosin provides a dull finish flux useful for automatic soldering [86].

Electroplating The excellent chemical stability permits fluorinated surfactants to be used in electroplating baths where hydrocarbon-type surfactants would not survive [ 1.8,9,87-931. Fluorinated surfactants are remarkably stable in a solution of chromic oxide in sulfuric acid at 50°C [94,95]. (See Section 3.1.) Fluorinated surfactants, such as lithium or potassium perfluoroalakanesulfonates (Fluorads FC-94 and FC-93, prevent mist formed by gas bubbles evolving at electrodes during electroplating of chromium [ 1,5,8,96-991. Fluorinated surfactants reduce the size of gas bubbles by lowering the surface tension of the electrolyte solution. The fluorinated surfactant used for mist reduction has to meet several requirements. The surfactant should be readily soluble in the electrolyte, lower the surface tension rapidly, and have suitable foaming characteristics. Foam formed on the surface of the electrolyte solution acts like a barrier and retards the entrainment of the electrolyte. The formation of a layer of fine foam is advantageous but excessive foaming is undesirable. Copper has been deposited from an acid copper sulfate solution containing cationic and aniphoteric fluorinated surfactants, with the surfactant C6FI3So2N (CH2CHOHCH2S03Na)C3H6N+(CH3)2C2H40H]OHbeing the main component [92]. Fluorinated surfactants prevent haze of plated copper by regulating foam and improving the stability of plating baths. Brightness and adhesion are improved as well. Fluorinated surfactants are used as nonfoaming surfactants in nickel-plating baths to reduce the surface tension and increase the strength of nickel electroplate [ 1001 by eliminating pinholes, cracks, and peeling [ 100.10 I]. Nickel has been codeposited with graphite fluoride, dispersed in a plating bath containing a cationic fluorinated surfactant derived from octanesulfonarnide [ 1021. Fluorinated cationic. anionic, and nonionic surfactants were added to a suspension of graphite fluoride in a plating bath containing copper sulfate and sulfuric acid. The cationic surfactants were the most effective and the anionic surfactants were the least effective in increasing the deposition of graphite fluoride in the copper matrix [ 1031. A fluorinated surfactant added to a tin-plating bath [93] produces a plate of uniform thickness.

Applications

361

Cationic or amphoteric fluorinated surfactants can impart a positive charge to fluoropolymer particles and aid electroplating of the polymer (e.g., PTFE) onto steel for surface protection [ 1041. A nonionic fluorinated surfactant (Forafac F1110) is a leveling agent for zinc electrodeposition [ 1051. The electrochemical behavior of fluorinated surfactants at gold electrodes [106-1081, platinum [ 1091, and mercury [ 1091 electrodes has been studied and compared with that of hydrogenated surfactants. Electropolishing A fluorinated surfactant has been used for electropolishing gas turbine blades made from an Ni alloy [110]. Emulsions Fluorinated surfactants (e.g. ammonium perfluoroocatanaote and sodium perfluorooctanoate) are used to emulsify chlorocarbons and fluorocarbons in emulsion polymerization [ 111-1 141 (see Polymerization). Fluorinated surfactants are used as emulsifiers in medicine (see Fluorochemical Oxygen Carriers in Section 10.4) and in cosmetics. Fluorinated surfactants improve leveling of acrylic emulsions, floor polishes, and shoe brighteners (see Polishes and Waxes). Transportable aqueous crude petroleum emulsions have been prepared using a hydrocarbon surfactant and a fluorinated surfactant oligomer as emulsifiers [115]. Ammonium perfluoropolyether carboxylates are effective emulsifiers for perfluoropolyether oils [ 1161. Microemulsions of the oil-in-water type, consisting of perfluoropolyethers emulsified by fluorinated surfactants, are used as catholytes in electrochemical processes [ 117-1 191. Highly concentrated emulsions prepared from perfluorodecalin and a nonionic fluorinated surfactant, C6FI3C2H4(OCzH4)20H, can contain up to 98% water as the dispersed phase and behave like viscoelastic gels [120]. Certain fluorinated surfactants are used as emulsion breakers, for example. for breaking water-oil emulsions in oil wells [87]. Etching Flourinated surfactants are used as wetting agents in etch baths because of their low surface tension and stability in strong acids. Glass articles are polished and etched with solutions containing sulfuric acid, HF, and a fluorinated surfactant (e.g., tetraethylammonium perfluorooctanesulfonate or potassium perfluorooctanesulfonate). Fluorinated surfactants increase the speed of etching, acid polish-

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ing, or frosting of flat (sheet, plate) glass with hydrofluoric acid. PreferTed wetting agents are tetramethylammonium and tetraethylammonium perfluoroalkanesulfonates [87,121] or potassium perfluorooctanesulfonate [ 1221. Planar etching of fused silica [ 1231 is effected by a mixture of HF, H3P04. and a fluorinated surfactant. [CF3(CF2)14COO]2Ca. Thefluorinated surfactant lowers the surface tension to facilitate wetting and the removal of etching byproducts. Fluorinated surfactants, such as lithium perfluorooctanesulfonate. potassium perfluoroalkanesulfonate, and an amine perfluoroalkanesulfonate, are effective wetting agents in etching solutions for plastics. Polyacetals are etched in solutions containing phosphoric and sulfuric acid and a fluorinated surfactant [ 1241. Fluorinated surfactants increase the wettability and adhesion of isotactic poly(propy1ene) when etched with sulfuric and chromic acids [ 1251. Fluorinated surfactants used as wetting agents for manufacturing semiconductor devices [ 8,126-1391 are metal-free [e.g., ammonium perfluoroalkanesulfonate [8], C9F1&H2CH(OH)CH2NHCH(CH3)CHzCH3 [ 1261, H(CF&COOH [ 12711. The semiconductor devices consist of a metal (aluminum or copper) conductor, SiO? as the insulator, and a silicon semiconductor. The Si02 coating is etched to generate a fine pattern. Inadequate wetting during acid etching may cause an entrapment of small air bubbles which mask the area to be etched. A fluorinated surfactant in the etching bath facilitates the complete wetting of the entire area and produces a sharp detail of the pattern [8]. In printed circuit manufacturing, a metal film, typically copper, is coated with a photoresist film and etched with gaseous NO2 or aqueous nitric acid. A fluorinated surfactant (e.g.. Zonyl FSP) helps to remove copper from the circuit zone without underetching [ 1301. Fluorinated surfactants improve the efficient life of alkali baths used in etching of aluminum. In mild-steel etching, the effect of the fluorinated surfactant on the etching process depends on the structure of the surfactant. For example, Atsurf F-21 has given the best mist reduction and Atsurf F-35 the best surface. Fluorinated surfactants are also used for etching plastic preplates.

Fire-Fighting Foams and Powders Fire-fighting foams [ 1311 are formulated to float on flammable liquids and extinguish flames. The foams form a barrier for the vapors to escape and cool a hot surface to prevent reignition. Foams are arbitrarily divided into three groups [ 1321, based on the expansion ratio of the foam. The expansion ratios are 20 : 1 for lowexpansion foams, 30 : 1 to 100 : 1 for medium-expansion foams, and 200 : 1 to 1000 : 1 for high-expansion foams. High-expansion foams are used to fill large spaces with foam and extinguish flames without causing damage by water. The

F

Applications

363

high-expansion foam is light, however, and is easily blown away outdoors. Medium- and low-expansion foams are used more often than the high-expansion foams. Protein foam concentrates are produced by hydrolysis products of proteinaceous matter, such as hoof and horn meal, chicken feathers, or fish meal. Fluoroproteirz Concentrates are protein foam concentrates with fluorinated surfactants added. The fluorinated surfactants add to the cost of the protein-based foam agents but enhance fire-extinguishing efficiency by repelling hydrocarbon fuel when the foam is covered with fuel. Fluoroprotein foam introduced to a base of a burning fuel tank rises through the fuel and extinguishes the fire. The low surface tension allows fluoroprotein foam to move rapidly over a hydrocarbon fuel surface [133-1381. The nqzleozlsfilm-fo172zirlgfoaming (AFFF) agents contain synthetic chemicals instead of protein-based materials. Fluorinated surfactants used in AFFF agents lower the surface tension of water and form a film on the fuel surface. AFFF agents used in unaspirating equipment like water hoses are effective mainly because of the aqueous film, and not the foam [ 13I]. Alcohol-resistant AFFF agents contain a polymer for use on polar organic solvents. The polymer, usually a natural product, retards drainage and increases heat resistance of the foam [139,140]. An amine oxide containing a C4-20 alkyl group and two C,-14alkyl groups improves the flashback resistance of a fire-extinguishing agent containing an amphoteric fluorinated surfactant [ 1411. Fluorinated surfactants (e.g., C8F14{ OC6H4S02N[(CH,)20S03NH(CH2CH20H)]2}2), foam in seawater, and produce stable foams for fire extinguishers used in a seawater environment [ 1421. Schuierer et al. [ 1431 prepared fire-extinguishing foams from amphoteric fluorinated surfactants with and without phosphate ester groups. Fire-extinguishing agents containing a mixture of two amphoteric fluorinated surfactants [e.g., C7FIsCF=CH2N+Me2(CH2)20SOy and C7F1SCF=CH2NfMe2(C2H40H)Cl-], a nonfluorinated alkyl sulfate, butyl glycol, and ethylene glycol have also been patented [ 1441. Cordes and Achilles [ 1451 mixed amphoteric fluorinated surfactants, chlorofluorocarbons, and a propylene oxide-ethylene oxide copolymer to prepare an emulsion that foams on mixing with water. Mixtures of cationic and nonionicfluorinatedsurfactantshavebeen patented as foamable fire-extinguishing agents to minimize the evaporation of flammable solvents and protect the solvent (e.g., hydrocarbon) from ignition [146,147]. Mixtures of fluorinated surfactants and silicone surfactants have been patented [148,149]. Fire-extinguishing foams containing mixtures of a cationic, an amphoteric, and a nonionic fluorinated surfactant [ 1501 have been claimed to spread rapidly on hydrocarbon surfaces.

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An AFFF agent for extinguishing or preventing fires has been formulated with mixture a of an amphoteric fluorinated surfactant, N-[(dimethylamino)propyl]-2(or 3)-( 1,1,2,2-tetrahydroperfluoroalkylthio)succinami~ acid, an anionic fluorinated surfactant, perfluoroalkanoic acid, and an ionic and a nonionic nonfluorinated surfactant [ 1511. Foam fire extinguishers containing potassium perfluorooctanoate, calcium chloride, and aluminum lactate are claimed to be powerful fire-extinguishing agents for hydrocarbon and polar organic solvents [ 1521. Numerous other patents have issued for fluorinated surfactant-containing fire-extinguishing foams. Fluorinated surfactants are used in dry fire-extinguishing agents to make the powder nonwettable by hydrocarbons. The powder floats on the surface of the hydrocarbon and hinders the evaporation of the hydrocarbon. The hazard of reignition is thereby eliminated [ 153,1541. Environmental concerns have curbed the use of fluorinated surfactants in fire-fighting foams and powders.

Flotation of Minerals Fluorinated surfactants are used in ore flotation processes. Fluorinated surfactants are stable in concentrated acids and accelerate wetting of ore and the removal of the oxide layer. Perfluoroalkanoates, such as CF,(CF),,COOK (12 = 3,4, 6, 8) are more effective flotation agents than the corresponding hydrocarbon-type surfactants. The collecting power of the fluorinated surfactant in flotation of A1703increases with increasing 17 (chain length) [ 1551. Vanadium compounds, such as NH4V03, can be concentrated using a perfluorinatedsurfactant. At S 3 0 0 mg/Lvanadium, 100% sorptionhasbeen achieved [ 1561. Fluorinated surfactants are useful in nitrogen flotation to recover uranium. The fluorinated surfactant improves separation of uranium contained in sodium carbonate and/or sodium bicarbonate solutions. Tn one example, Zonyl FSP yielded 95.5% recovery, as compared with 23.6% using sodium dodecyl sulfate [l].

Foams and Defoaming Fire extinguishers-see Fire-Fighting Foams and Powders. Fluorinated surfactants usually differ widely in their foaming power (see Section 4.9). Cationic fluorinated surfactants and amphoteric fluorinated surfactants (e.g., Zonyl FSK) are foaming agents in aqueous media. On the other hand, nonionics (e.g., Zonyl FSN and Zonyl FSO) are low-foaming surfactants [ 11. Endcapping reduces foaming.

Applications

365

Fluorinated surfactants are used to aid foaming of plastics and polymers, such as polyolefins [ 1571, polyurethanes [158,159], poly(diethy1ene glycol diacrylate) [ 1601, siloxanes [ 16ll, and foamable adhesives [ 1621. Anionic, cationic, or amphoteric fluorinated surfactants with a hydroxyl group on the a-carbon improve foam stability [ 1631. Fluorinated surfactants are used to generate stable foam in chromium-plating baths and anodizing baths to prevent misting and spattering (see Electroplating). A nonionic fluorinated polymeric surfactant can produce stable foam in hydrocarbon solvents. Fluorinated surfactants of the phosphoric acid type can function as antifoaming agents. A 175-mm-high foam produced by 1 g/L of a secondary alkanesulfonate collapsed during 1 s when a 0.02-g/L mixture of heptadecafluorooctylphosphonic acid and bis(heptadecafluoroocty1)phosphinic acid was added [ 1641. As little as 0.004% of the fluorinated surfactant Atsurf F-12 (a phosphoricester-type anionic surfactant) lowered the initial foam of a typical nonionic surfactant from 80 to 38 mm and the 5-min reading of the Ross-Miles test from 38 to 5 mm [ 1651. An addition of 2.0% H(CF2)loCH20PO(OH)2to a detergent containing a coconut alcohol-ethylene oxide condensation product eliminated sudsing almost completely. A Dainippon Ink patent [ 1661 claims fluorinated surfactants containing a C3-?0 fluorinated aliphatic group and a >C (preferably C12-36) alkylgroup. The addition of thefluorinatedsurfactant C8F17S02NC18 H37(CH2CH20) reduces foaming ofNa dodecylbenzenesulfonate. The initial foam height of 230 mm and the height of 180 mm after 5 min decreased to a foam height of 10 mm initially and to zero foam height after 5 min. Fluorinated alcohols [e.g., CF3(CH2)s-I,C2H40H]reduce foaming of detergents used for mechanical washing [ 167,1681. Defoamers for fluorinated surfactants Fluorad FC- 129 (anionic), FC-135 (cationic), and FC-170C (nonionic) in aqueous media and Fluorad FC-430 (nonionic) in aqueous and organic solvent media were evaluated by defoamer manufacturers and the 3M Company. The list of effective defoamers compiled by 3M recommends different defoamers for each of the fluorinated surfactant tested. Because the effectiveness of the defoamer depends on the medium as well, a recommended defoamer must be tested for suitability in the coating system used [ 81. The theories of antifoam action are complex. Typical antifoam agents for silicone surfactants contain a silicone polymer and finely powdered silica [169]. A low surface tension of an antifoam agent, lower than that of the surfactant solution, is a necessary but insufficient condition. A siloxane surfactant solution cannot be defoamed by conventional siloxane antifoaming agents but is readily defoamed by an antifoam agent based on poly(methyl-3,3,4,4,5,5,6,6,6-nonafluorohexylsiloxane), [ { CH3[CF3(CF2)3(CH2)2]SiO},J, which has a low surface ten-

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sion. 19.2 mN/m [ 1701. The fluorosiloxane poly(n~ethyltrifluoropropylsiloxane) is not effective because of its insufficient fluorination and a higher surface tension, 24.4 mN/m [ 17 I]. However, electrostatic effects are as important as a low surface tension for defoaming ionic surfactants.

Graphic Imaging Printing inks are pigment dispersions or dye solutions which can be printed on a substrate and dried. Relief and lithographic inks are pigment dispersions in a viscous oil, and flexographic and gravure printing inks are liquids containing a resin in a volatile solvent. Ultraviolet and electron beam inks are cured and dried by a free-radical polymerization mechanism. The radiation printing process produces very little solvent vapor. Fluorinated surfactants reduce the surface tension and improve wetting properties of aqueous and oil-based inks [172]. The enhanced wetting is essential for printing on difficult-to-wet surfaces, such as plastics and metals. Fluorinated Surfactants also aid pigment dispersion and control probletns such as pigment flooding and flotation. The success of gravure printing depends on the ability of the ink to wet the surface being printed and to keep the roll clean and free of contaminants. Adding 0.5% Zonyl FSN by weight of press-ready ink can improve the cylinder life 25-40%, depending on the ink and cylinder, and itnprove the quality of print by reducing “snowflaking” and streaking of certain inks and maintaining ink viscosity. Fluorinated surfactants impart water resistance to water-based inks and improve the effective life of toner carrier materials. Fluorinated surfactants are added to inks for ballpoint pens [173-1761. tnarking pens [ 177-1 SO], anticlogging jet recording inks [181], and printing inks for plastics [ 182,1831, to improve leveling, wettability, a smooth flow, adhesion to the substrate, and water resistance of the print. Fluorinated surfactants in correction fluids repel ink and reduce bleeding of the print [ 1841. A cationic surfactant, a perfluoroalkyltrimethylammonium salt, in a developer for photosensitive lithographic plates facilitates the control of the development process [ 1851.

Greases and Lubricants Lubricating greases containing 15-40 wt% PTFE have been prepared by using a fluorinated surfactant as a dispersant [ 1861. Fluorinated surfactants { e.g. [(perfluoralkyl)alkoxy]alkylsulfonic acid salts [ 1871 and perfluropolyalkylethers} are useful as lubricants coated on the surface

Applications

367

of magnetic recording media such as magnetic tape, floppy disks, and disk drives [188,189]. Fluorinated organosilicones are useful lubricants for rubber surfaces [ 1901. Fluorosilicones with a favorable fluoroalkyl chain length and siloxane chainlength ratio are effective lubricants and reduce the friction coefficient of liquid paraffin [ 1911.

Herbicides and Insecticides The nonionic fluorinated surfactants tested by Sakakibara et al. [ 1921 exhibited hardly any herbicidal activity. Hence, selected fluorinated surfactants can be used safely as dispersants and adjuvants for agricultural chemicals. When compared to hydrocarbon surfactants, fluorinated surfactants are more powerful wetting agents for leaves (e.g., wheat leaves) [ 1931. Fluorinated surfactants are used in insecticide formulations to aid wetting and penetration of the insecticide into the insect. Insecticidal aerosols may contain an insecticide, solvent, and a fluorinated surfactant. An insecticide formulated with a fluorinated surfactant and dimethyl ether as the solvent is readily absorbed by insects [ 1941. Some fluorinated surfactants are insecticides in their own right, affecting the common housefly and the carmine mite [ 1921. The mechanism of insecticidal activity appears to be suffocation of the insect by the adsorbed fluorinated surfactant.

Leather Fluorinated surfactants are used in various leather manufacturing processes and repellent treatments of tanned leather. Fluorinated surfactants have been used in hydrating, bating, pickling, degreasing, and tanning processes. Fluorinated surfactants improve the efficiency of the process, reduce the processing time, and increase the quality of the product [ 195-1981. The use of fluorinated surfactants in leather tanning and dyeing processes has been investigated by Gratacos et al. [199]. In small amounts (0.025-0.05% on weight), an anionic fluorinated surfactant increased the exhaustion of the chrome tanning agent and dyes, but at higher concentrations, the fluorinated surfactant had the opposite effect. The distribution of Cr203 was more uniform when the skins were pretreated with a cationic fluorinated surfactant. The techniques suitable for applying fluorinated surfactants [195-1981 to leather after tanning are (1) tumbling in a drum, in which the leather sorbs the fluorinated surfactant from a emulsion, suspension. or solution, (2) spraying. and

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(3) cast coating. The water resistance of leather treated with fluorinated surfactants has been improved with cationic retention aids and by complexing with chromium and zirconium (see Chapter 12). The complexes provide oil repellency and are more water resistant than the parent fluorinated surfactants. The performance of monomeric fluorinated surfactants on leather has been substantially exceeded by designing polymeric fluorochemicals for repellent leather treatments. Fluorinated surfactants improve the leveling of acrylic brightener emulsions on leather.

Liquid Crystals Anionic fluorinated surfactants form liquid crystals with cationic fluorinated surfactants (see Chapter 7). Liquid crystals of fluorinated surfactants are used in various industrial applications and as biological membrane models. As an example, stirring C9F19CONH(CH2)3N+(CH2)31-, C9FlgC0ONH4, and H 2 0 at 60°C yields the CgF19CONH(CH2)3N+(CH2)3C9F19COOliquid crystal. The ion pair that forms the liquid crystal has a surface tension of 15.5 mN/m, compared to 18.3 mN/m for the cationic component and 2 1 .O for the anionic component alone [200,201]. Liquid-crystal cells are less likely to become turbid in the absence of image-producing ultrasonic vibration or electric voltage when the inside surfaces of their glass support plates are coated with a fluorinated surfactant [202].

Medical and Dental Uses Self-propelling aerosols of antiallergic, antibiotic, antitussive, or antianginal activities have been prepared using a fluorinated surfactant as the dispersant in Freon 114 and Freon 12 [203]. As anexample, epinephrine bitartrate has been dispersed with perfluoro-rz-octyl-N-ethylsulfonamidoethylphosphate. The use of fluorinated surfactants in intravascular oxygen carriers and blood substitutes is discussed in Chapter 10. A fluorinated surfactant in toothpastes containing potassium fluoride enhances fluoroapatite formation and inhibits caries [204]. A fluorinated surfactant formulation (1% Lodyne S-1 lo), which consists of an amphoteric fluoroalkylaminocarboxylic acid and a nonionic fluoroalkylamide synergist, in the toothpaste increases enamel-fluoride interactions. A fluorinated surfactant in pharmaceutical formulations and in toothpaste must be nontoxic for the intended purpose. Dispersions of cells are prepared in clinical laboratories to diagnose cell abnormalities. An anionic fluorinated surfactant facilitates the dispersion of cell aggregates from tissues in a saline solution [205].

Applications

369

Metal Finishing Anionic, cationic, and nonionic fluorinated surfactants are used in various metal treatment processes. Metal surfaces are treated to prevent corrosion, reduce mechanical wear, or enhance the aesthetic appearance. Fluorinated surfactants are used in the phosphating process for aluminum and in bright dips for copper and brass. Some fluorinated surfactants (e.g., the anionic surfactants Zonyl FSA and Zonyl FSP) are strongly adsorbed on metals and provide water and solvent repellency [I]. An effective surface treatment requires a clean surface. Metal surfaces are cleaned with an alkaline, neutral, or weakly acidic cleaner, an organic solvent, or by pickling with molten-salt baths [5,87]. Fluorinated surfactants in a pickling and descaling bath disperse scum, speed runoff of acid when metal is removed from the bath, and increase bath life [206,207]. The fluorinated surfactant inhibits nascent hydrogen formation and, therefore, prevents embrittlement by hydrogen [208]. Some fluorinated surfactants function as corrosion inhibitors on steel [1,5,8,87,165,209-2151. For example, 0.01% Atsurf F-21 prevents corrosion of mild steel in 15% HCl for at least 20 days at ambient temperature [165]. Surface treatments with corrosion inhibitors containing fluorinated surfactants decrease the friction coefficient of magnetic audiotapes or videotapes [216-2181. Fluorinated surfactants promote the flow of metal coatings and prevent cracks in the coating during drying [219,220]. Some fluorinated surfactants are effective antiblocking agents for aluminum foil. For example. aluminum foil is coated with 0.025 g/m' (active ingredient) Monflor 9 1, applied as a 5% solution [43]. A nonionic fluorinated surfactant, Monflor 3 1. increases the penetration rate of penetrating oils by a factor of 3 [43]. Mild steel etching-see Etching.

Molding and Mold Release Fluorinated surfactants are effective mold-release agents because of their oleophobic and hydrophobic nature. Only small amounts of a fluorinated surfactant are needed, sometimes only one-fiftieth of the amount needed for hydrocarbon or silicone mold-release agents. Because the amounts of fluorinated surfactants used are very small, the molding can be painted, metallized, or adhered to another surface without removing the molding agent. Fluorinated surfactants are used as mold release agents [221-2251 for thermoplastics, polypropylene [221], epoxy resins [223], and polyurethane [224,225] elastomer foam moldings. Fluorinated surfactants reduce autoadhesion and blocking [226.227] and prevent orange peel on film casting and coating [228].

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Oil Containment Oil spills on water can be contained and prevented from spreading by injecting into water a chemical barrier containing a fluorinated surfactant and a maleic anhydride-derived polymer featuring carboxylic acid and ester groups [229]. The fluorinated surfactant CsFl7S02N(C3H7)CH2COOKprevents spreading of oils or gasoline on water [230]. Perlite or vermiculite, treated with a cationic fluorinated surfactant, is claimed to be hydrophobic and effective in cleaning oil spills [231].

Oil Wells Fluorinated surfactants are used in oil-well stimulation and for petroleum recovery by waterflooding [232] and in nonaqueous stimulation fluids for foaming hydrocarbon liquids (e.g., kerosine [233,234]). A foam, made by foaming a mixture of 1% F(CF2)8CH2CH20(CH2CH20)2H in methanol, stimulates underground formations in the production of petroleum [235]. Fluorinated surfactants lower the surface tension of stimulation fluids more than hydrocarbon surfactants and are stable in aqueous stimulation fluids at 100°C. Drainage of salt solution (2% KC1) from sand packs is inversely related to the surface tension. Fluorinated surfactants enhance drainage more than hydrocarbon surfactants. Flow rates of fluids through sand packs or low-permeability sandstone cores were unaffected by the fluorinated surfactant, indicating that formation blockage by a fluorinated surfactant is improbable. Aqueous fluorinated surfactant solutions do not form emulsions of kerosine. Gas wells blocked with a hydrostatic head of a condensate can be opened by using a fluorinated surfactant which allows the well gas to deliver the condensate and water to the surface as foam [8].

Paper Fluorinated surfactants function as oil and solvent repellents on paper and paperboard [ 195,236-2391. Monomeric fluorinated surfactants, their chromium or zirconium complexes, and polymeric fluorochelnicals are used for repellent treatments. Fluorinated surfactants can be added to the pulp sluuy. applied to the paper surface, or included in pigmented coatings. The surface treatment process is the most efficient mode of fluorinated surfactant application and easier to control than the internal application process. In pigmented coatings on the outside of boxes or bags, fluorinated surfactants are used to prevent soiling and maintain the appearance of the package. For internal application about 1.O-1.5% (based on the weight of dry fiber) of a fluoroalkyl phosphate is needed for good oil repellency [237]. Cationic retention aids are used withfluoroalkyl phosphates in internal application processes.

Applications

371

Materials treated with fluorinated surfactants include the following [237]: Liner board-for packaging machine parts, rope, twine, meat, etc. Folding cartons-for snack foods, carryout fast food, cake mixes, margarine, candy, bakery products, and pet foods. A repellent treatment with a fluorinated surfactant prevents fat and grease from seeping into the edges of stamped-out polyethylene- or polypropylene-lined cartons. Multiwall bags-snack foods, cake mixes, pet food. Flexible packaging-camyout fast food, candy wrap. Duplicator and reproduction paper-toluene holdout. Support cards-candy and bakery products. The treatment of paper or paperboard used for food and pet food packaging requires Food and Drug Administration (FDA) approval. This requirement excludes chromium and zirconium complexes of fluorinated surfactants. The monomeric fluorinated surfactants approved for the repellent treatment of paperboard and paper in directcontact with foodarefluoroalkylphosphates [ 195,336,237,2391 [e.g., a mixture consisting of a mono-(fluoroalkyl) phosphate ester (R&H2CH20)P(0)O$-[H2N+(CH2CH20H)& and a bis-(fluoroalkyl) phosphateester (RfCH2CH20),P(O)O-H2N+(CH2CH20H)2 (Zonyl RP) or (CsF17S02N(C2H5)CH~CH~O),,P(0)(ONH&-,z (Scotchban)]. Studies by Du Pont have revealed that very low amounts of fluorinated surfactant are extracted from paperboard into solvents simulating food. Highly sensitive analytical methods had to be developed for the determination of trace anlounts of organic fluorine in the extracts (see Section 9.2). Fluoroalkyl phosphates provide excellent oil and grease repellency. a moderate resistance to water penetration can be achieved with cationic retention aids and ketene dimer sizes or small amounts of alum. Excessive amounts of alum impede penetration of the fluorinated surfactant and reduce oil repellency. Optimum oil repellency with fluoroalkyl phosphates can be achieved by excluding alum from paper and paperboard or by using an alkaline sizing agent. Emulsions containing a fluorinated surfactant and waxes and/or paraffins are release agents for paper-coating compositions [240]. Cast-coated paper is produced by coating the paper with pigment- and adhesive-containing solutions, air drying, rewetting a polyethylene emulsion, and pressing the wet surfaces with a fluorinated surfactant-coated hot drum to give a paper with a high gloss [ 3411. Fluorinated surfactants are used in the manufacture of heat-sensitive recording paper [242-2451 and ink-jet printing paper [246,247].

Photography Fluorinated surfactants aid single-layer or multiple-layer, light-sensitive coating of photographic materials, such as films and papers, and function as wetting agents, emulsion additives, stabilizers, and antistats [9a,248-272]. Fluorinated

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surfactants impart antistatic and nontacky properties, prevent spot formation, and control-edge uniformity in multilayer coatings. In combination with a nonionic surfactant or in a hydrophilic protective layer, fluorinated surfactants prevent fogging and streaking caused by a static discharge. Fluorinated surfactants do not have undesirable effects on silver halide light-sensitive emulsions. In a diffusion-transfer photographic process, the photosensitive material and the image-accepting material are layered in a close contact to effect the diffusion transfer. When the photographic process is completed, the materials are peeled apart. Fluorinated surfactants in the timing layer of photographic diffusiontransfer materials provide a good contact when wet or dry, so that rupture or peeling of the emulsion layer is prevented [263-2771. A low-surface-tension processing solution containing a fluorinated surfactant eliminates air bubbles that can cause failures in image transfer [ 2781.

Plastics, Resins, and Films See also Antifogging, Antistats, Etching Mold Release, and Photography in this chapter. Fluorinated surfactants can reduce the surface tension of water and organic solvents and aid wetting of low-energy plastic surfaces. For example, a nonionic fluorinated surfactant, Fluorad FC-740, can lower the surface tension of some low-polarity solvents to 20-24 mN/m and facilitate wetting of plastic surfaces which might be contaminated with silicones, oil, or grease [SI. Antiblocking agents for vulcanized or unvulcanized rubbers have been formulated with a nonionic fluorinated surfactant [279]. Films of poly(viny1 alcohol) or saponified ethylene-vinyl acetate copolymers were treated with a nonionic fluorinated surfactant [C8FI7S02NRCH2CH20(CH2CH20)10H to reduce friction and blocking [ 2801. The water permeability of dialysis membranes containing fluorocarbon polymers can be increased by a surface treatment with a cationic fluorinated surfactant [281]. Partially fluorinated oligomers derived from hexafluoropropylene oxide { e.g., F[C(CF3)FCF20]&(CF3)FCOOCH2CH2(OCH2CH2)60CH3} lower the critical surface tension of polymers on which they are adsorbed [e.g., polystyrene, poly(methy1 methacrylate), and a vinyl chloride-vinylidene copolymer] [282]. An amphoteric fluorinated surfactant in silicone rubber sealants makes the seal soil resistant [283]. An anionic fluorinated surfactant prevents leakage of mineral oil around nitrile rubber seals [284]. Fluorinated surfactants facilitate coplating of polytetrafluoroethylene and metals onto a metal substrate [43]. A cationic fluorinated surfactant adsorbed onto the polymer particles imparts a positive charge and thus allows the polymer and metal to be electrolytically coplated.

Applications

373

Fluorinated surfactants improve wetting of fibers or fillers in composite resins and speed the escape of bubbles trapped in the viscous resin. A fluorinated surfactant in a poly(oxymethy1ene)diacetate polymer-nylon copolymer reduces the frictional coefficient of the polymer [285].

Polishes and Waxes Self-polishing liquid floor finishes depend on proper wetting and dry-down for complete coverage and shiny appearance. Some polymeric resin formulations do not wet floors completely and dry to a rough finish, especially on vinyl floors. As little as 50 ppm fluorinated surfactant added to the formulation can improve the appearance of the dried floor significantly by eliminating streaks and enhancing gloss. Fluorinated surfactants impart self-leveling properties to all types of polishes including styrene, acrylic, or wax-based floor polishes. Fluorinated surfactants are used as leveling agents in cleaner polishes forvinylfloors. The polishescontainusuallylow-molecular-weight a acrylic-methacrylic copolymer, a maleic anhydride-styrene copolymer, or an acrylic-styrene copolymer, a fluorinated surfactant, a hydrocarbon-type surfactant, a fugitive plasticizer, a dimethylpolysiloxane antifoaming agent, sodiunl bicarbonate, aqueous ammonia, a fragrance, and other ingredients [ 286-2931.

Polymerization Fluorinated surfactants used as an emulsifier in emulsion polymerization of fluoropolymers improve physical properties of the polymer and increase the rate of polymerization. Because the fluorinated surfactants are more effective in emulsion polymerization than hydrocarbon surfactants alone, the total surfactant concentration can be reduced. For example. in emulsion polymerization of vinyl chloride, 160 ppm Monflor 31 can reduce the required concentration of sodium dodeylbenzenesulfonate by about 40%. The emulsion-grade poly(tetrafluoroethy1ene) (PTFE) polymer is a dispersion of PTFE particles in an aqueous phase. PTFE is commercially produced by free-radical polymerization of tetrafluoroethylene in water containing a fluorinated surfactant [ 112-1 14,294-2991. The surfactants are usually alkali perfluoroalkanoates [e.g., ammonium perfluorooctanoate or lithium perfluoroalkanoates CF3(CF2),,COOLi,11 = 5-81. The surfactant solubilizes the monomer and stabilizes the PTFE particles formed. The polymerization rate, surfactant adsorption, and polymer particle morphology depend on the initial concentration of the surfactant [ 1 14,2951. Depending on the polymerization conditions, rod-shaped particles, spherical particles, small hexagons, or liquid-crystalline suspensions of PTFE whiskers are formed. A correlation exists between the association state of the surfactant during the initial stages of polymerization and the PTFE particle morphology [295]. The particle size and shape change near the cmc of the fluori-

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nated surfactant, suggesting that the particle nucleation is different in the two dominions. Below the cmc of the fluorinated surfactant, the nucleation step of PTFE particles is homogeneous, whereas in micellar systems, the nucleation step is heterogeneous. The micelles harbor the growing PTFE chains and as a tetrafluoroethylene reservoir provide a high concentration of the monomer. Copolymerization with a small amount of CF?=CFCF3 decreases the rod-shaped polymer formation [300]. Vinylidene fluoride has been polymerized in the presence of ammonium perfluorooctanoate [ 30 I], sodium perfluorooctanoate [ 3021, ammonium perfluoroisooctanoate [ 3041. or ammonium perfluorononanoate [ 3051. Ammonium u-hydroperfluorononanoate [3061, ammonium perfluoropelargonate [ 3071, and sodium perfluoroheptanoate [3081 function as emulsifiers for the polymerization of vinyl fluoride. Nonionic tluorinated surfactants are used as emulsifiers for homopolymerization or copolymerization of ethylene [ 3091. Kat0 et al. [ 3 101 studied emulsion polymerization of styrene in thepresence of sodium dodecyl sulfate and lithium perfluorooctanesulfonate. The fluorinated surfactant decreases the polymerization rate and the molecular weight of the polymer formed.

Repellency Fluorochemical repellents differ from silicone- or hydrocarbon-based repellents in several aspects, of which oil repellency is the most important [311]. Repellents with a fluorocarbon chain repel both water and oil, whereas repellents with silicone or hydrocarbon hydrophobes repel only water. The resistance of a porous substrate or a textile fabric to wetting and penetration of a liquid, such as water or oil, depends on the chemical nature, geometry, and roughness of thesurfacesandthecapillaryspacings i n thesubstrate [312-3 181. However, the initial repellency of a finish is not the only criterion for selecting a repellent. Durability to dry cleaning and laundering. resistance to abrasion and soiling, ease of application, and the cost of the repellent are important factors to be considered. Hydrocarbon repellents and their mixtures with fluorinated repellents. silicone and fluorosilicone repellents, and fluorinated repellents are described in Chapter 12. Fluorinated soil retardants are discussed in Chapter 13 and soil- and stain-resistant carpets in Chapter 14. The theory of repellency is discussed in Chapter 11. The relationship between repellency and the structure of the fluorinated repellent is in agreement with the critical surface tension concept developed by Zisman [ 3 191. Shafrin and Zisman [ 3201 determined the critical surface tensions of u-perfluroalkyl-substituted n-heptadecanoic acids and the wettabilities of their monolayers. The wettabilities

Applications

375

suggested that a terminal perfluoroalkyl chain of at least seven carbon atoms is sufficiently long to shield the nonfluorinated segment beneath the fluorinated segment. For fluorinated repellents on a textile fabric, about 10 perfluorinated carbon atoms are needed for maximum repellency. Mononleric as well as polymeric fluorinated surfactants are used to impart oil and fat repellency to paper or cardboard, and oil and water repellency to glass, leather, and metal surfaces. A water-soluble polymer (FC-759 by 3M), containing perfluoroalkyl, carboxylic, oxyethylated nonionic, and silanol [Si(OH)3]groups, has been applied to porous surfaces of concrete, grout. tile, granite, marble, terra cotta, and limestone. The polymer reacts with the multivalent ions in the surface, becomes water insoluble, and renders the porous surface water, oil, and stain resistant [8].

Surface Treatment of Glass See also Antifogging. Optical glass lenses for cameras and optical instruments are made hydrophobic and oleophobic by a surface treatment with a cationic fluorinated surfactant [321,322]. The treated glass surface is more resistant to fingerprint soiling than the untreated surface. Glass can also be made oil repellent by coating with a methanolic solution of a 70 : 30 mixture of anionic fluorinated surfactants, C6F13S03Kand C6FI3SO3NH4,and drying [323]. Fluorinated surfactants in windshield wiper fluids prevent icing of the windshield [324].

Textiles Fluorinated surfactants impart oil and water repellency to textiles and paper (see Repellency) and increase surface lubricity. As an example, a size for polyester yarn containing C 6-8 perfluoroalkanecarboxylic acid, poly(viny1 alcohol), and an acrylic polycarboxylate gave yarns easy to weave. When the fluorinated surfactant was replaced by potassium lauryl phosphate, the weavability was poor [ 3251. An anionicfluorinatedsurfactant, such assodium3-[3-perfluoromethylphenoxyll-1-propanesulfonate,has been claimed to increase the dye bath exhaustion of cationic dyes into acetate fibers [ 3261. In dry-cleaning formulations, fluorinated surfactants improve soil suspension in perchloroethylene and reduce redeposition.

Vapor Barrier, Evaporation Retarders Evaporation losses have been reduced by covering the liquid surface in petroleumproduct storage tanks with a floating layer of cereal grains (e.g., corn, wheat, or perlite treated with a fluorinated surfactant) [327].

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Evaporation of hydrocarbon fuel ( e g . gasoline) can be prevented with an aqueous layer containing anionic or amphoteric fluorinated surfactants [ 328,3291. Fluorinated surfactants are effective because of their low surface tension. Katrizky et al. [ 3301 examined the principles involved in vapor retardance. The postulated structural requirements for the optimum effect include a fluoroalkyl group, a rigid polar central section, and a lipophilic alkyl or aryl-alkyl substituent (see p. 149).

Wetting Agents See also Herbicides and Insecticides. Fluorinated surfactants are effective wetting agents in situations where conventional surfactants will fail ( e g . in strongly alkaline or acid media). Fluorinated surfactants are used as stable wetting agents during ore treatment with concentrated acids. More rapid wetting of the ore, quicker breaking of the protective oxide layer, and reduction of caking can be achieved. In soldering fluids, fluorinated, surfactants function as a low-foaming wetting agent and reduce foaming encountered with silicone surfactants. Fluorinated surfactants aid wetting of sheep skins during desalination [43]. The addition of 0.1% fluorinated surfactant can reduce desalination time by seven times. Mixtures of hydrocarbon surfactants and fluorinated surfactants are more effective wetting agents than either surfactant type used alone. A drop of water containing 0.1 % trimethyl[ 3-(perfluorooctylsulfonylamino)- 1-propyl]ammonium iodide and 1% sodium dodecyl sulfate spread on polyethylene and covered a circle with a 15-mm diameter. During the same time interval, drops containing 2% of either surfactant covered a circle having only a 6-mm diameter [3311. Tadros [ 1931 studied wetting of wheat leaves with aqueous droplets containing fluorinated surfactants, hydrocarbon surfactants, or their mixtures. The effect of surfactants on wetting was characterized by the spreading coefficient S and the retention factor F. The spreading coefficient S has been defined as

where ?LA is the liquid-air (vapor) interfacial tension and cos 8 the contact angle formed by the droplet on the leaf surface. The volume of liquid that will be retained on the sprayed surface is proportional to the retention factor F. Furmidge [332] derived the retention factor concept considering the sliding of drops on a tilted surface, the air-liquid surface tension, and the advancing and receding contact angles:

where Obf is the mean of the receding contact angle angle OA. and p is the density of the liquid.

eR and the advancing contact

Applications

377

0-10 -

-20-30-40 v)

-50

-

I

I

I

IO-4

10-3

10-2’

surfact ant concentrat

I on

FIG.8.2 Spreading coefficient S as a function of surfactant concentration, for Monflor 31 (C10F1g0C6H4S03Na), sodium dodecylbenzenesulfonate, and their mixtures. (From Ref. 193.)

-10-

-20-

-30v)

-40 -

-50 -60-701 1

10-5

I

IO-4

I

10-3

surfact ant concent ration FIG. 8.3 Spreadingcoefficient S as a function of surfactantconcentration, C~~H~~O(CH~CH~O)~H/CI~H~~O for Monflor 51, CloFlgO(CH2CH20),H, (CH2CH20),H (C16E17), and their mixtures. (From Ref. 193.)

Chapter 8

378

500

-

400

-

,300200 100I

I

IOsurfactant concentration

-2

10-

FIG.8.4 Retention factor F a s a function of surfactant concentration, for Monflor 31 (C10F190C6H4S03Na), sodium dodecyl benzenesulfonate, and their mixtures. (From Ref. 193.)

Tadros [ 1931 found the fluorinated surfactants to be more powerful wetting agents than hydrocarbon-type surfactants for wheat leaves. An order of magnitude lower concentration or even less of a fluorinated surfactant was needed to obtain the spreading coefficient exhibited by the corresponding hydrocarbon-type surfactant (Figs. 8.2 and 8.3). However, the retention coefficients for the fluorinated surfactants were lower than those for hydrocarbon surfactants (Figs. 8.4 and 8.5).

500 -

IO-5 10-4 10-3 surfactant concentration FIG. 8.5 Retention factor F as a function of surfactant concentration, for Monflor 51, CloF1gO(CH2CH20),H, C16H310(CH2CH20),HIC16H330(CH2CH20)nH (C16E17), and their mixtures. (From Ref. 193.)

Applications

379

To maintain satisfactory wetting and retention, Tadros proposed a mixture of fluorocarbon and hydrocarbon surfactants as a compromise. The use of mixed surfactants has a cost advantage and constitutes a lesser environmental burden as well.

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295. B. Luhmann and A. E. Feiring. Polymer 30, 1723 (1989). 296. D. Sianesi, G. Bernardi, G. Veroli (Mont Ed.), Ger. Offen. DE 1940293 (1970). 297. T. Yoshimura, K. Shimofukigoshi,T. Satokawa (Daikin KogyoCo.), Jpn. Patent JP 48/34836 (1973). 298. M. B. Mueller. P. P. Salatiello, and D. L. Sawhney (Allied). Ger. Offen. DE 2157170 (1972). 299. S. V. Gangal (Du Pont),U.S. Appl. 9 I6844(1978). 300. T. Shimizu. S. Yamaguchi, and H. Koizumi. Kobunshi Kako 30(lo), 473 (1981). 301. J. P. Stallings (Diamond Shamrock), Ger. Offen. DE221 3135 (1972). 302. Y.Toyoda and M. Shirai (Kureha). Jpn. PatentJP 47/51’233 (1972). 303. K. Kido, H. Wakamori, F. Suzuki, and G. Asai (Kureha), Jpn. Kokai Tokkyo Koho JP 54/85290 (1979). 304. J. E.Dohany (Pennwalt), U.S. Patent 4360652 (1 982). 305. S. S. Ivanchev. V. P. Budtov, A.I. Andreeva, G. A. Otradina, and Yu.A. Zaichenko, Vysokomol. Soedin. Ser. A 25( I 1). 2335 (1983). 306. M. Tatenloto and S. Sakata (Daikin), Jpn. Patent JP 49143386 (1974). 307. T. S. Sirlibaev. V. G. Kalyadin. I. Tirkashev, and Kh. U. Usmanov. Dokl. Akad. Nauk. USSR (S), 38 (1982). 308. M. Petruschke, K. H. Goebel, W. Jaeger, G. Reinisch. D. Prescher, H. Kaltwasser. I. Richter. and H. J. Wolf, Ger. (East) Patent DD 159079 (1983). 309. E. Jones and J. Walker (ICT), Ger. Offen. DE 2501239 (1975). 3 10. K. Kato, K. Esumi, and K. Meguro, Bull. Chem. SOC.Jpn. 59,249 (1988). 311. E. Kissa, Repellent Finishes, in “Handbook of Fiber Science and Technology,” M. Lewin and S. B. Sello, eds., Vol. TI, Part B, p. 143. Marcel Dekker, New York (1 984). 312. S. Baxter and A. €3.D. Cassie, J. Text. Inst. 36. T67 (1945). 313. A. B. D. Cassie and S. Baxter. Trans. Faraday SOC.40,546 (1944). 3 14. G. H. Segall, Textile Res. J. 22,736 (1952). 315. C. A. Davis. Am. Dyestuff Rep. 56, PS55 (1967). 3 16. M. Karrholm and G. Karrholm, Textile Res. J. 20. 215 (1950). 317. A. M. Sookne, F. W. Minor, J. E. Simpson, and M. Harris, Am. Dyestuff Rep. 35, 295 (1 946). 31 8. B. M. Lichstein. in ”Surface Characteristics of Fibers. Part11.” M. J. Schick, ed., p. 495, Marcel Dekker, New York(1977). 319. W. A. Zisman, in “Contact Angle. Wettability. and Adhesion.“ R.F. Good, ed.. Advances in Chemistry Vol. 43. p. 1, American Chemical Society, Washington, DC (1964). 320. E. G. Shafrin and W. A. Zisman, J. Phys. Chem. 66,740 (1962). 321. J. F. Padday and T. D. Blake (Kodak). Res. Discl. 180, 138 ( I 979). 322. J. F. Padday and T. D. Blake, Br. Patent GB1588962 ( 1 98 1): CA 95. 152499. 323. Dainippon Ink and Kawamura (Phys. Chem. Res. Institute), Jpn. Kokai Tokkyo Koho JP 58/213057 ( 1983). 324. 0. Wack and H. Schmid, Ger. Offen. DE 3208219 (1983). 325. M. Ueda. Jpn. KokaiTokkyo Koho JP 54 1 3 1094 (1979). 326. H. Moriga, Jpn. Patent JP 47043155 (1972). 327. P. R. Scott, W. D. Johnston, and J. L. Kyrish (Shell), U. S. Patent 4,035,149 (1977).

Applications

389

328. Dainippon Ink and Chemicals, Inc., Kawamura Rikagaku Kenkyoshu, Jpn. Kokai Tokkyo Koho JP 55/145780 ( I 980). 329. Dainippon Ink and Chemicals, Inc., Kawamura Rikagaku Kenkyoshu, Jpn. Kokai Tokkyo Koho JP 57/78473 ( 1982). 330. A. R. Katrizky. T. L. Davis, G. W. Rewcastle, G. 0. Rubel. and M. T. Pike. Langmuir 4,732 ( 1 988). 331. M. J. Owen and J. Thompson, Br. Patent GB 1337467 (1973). 332. C. G. L. Furmidge. J. Colloid Sci. 17, 309 ( 1962).

Analysis of Fluorinated Surfactants

9.1

DETERMINATIONANDCHARACTERIZATIONOF FLUORINATED SURFACTANTS

Analytical techniques are employed to determine the purity or the concentration of a fluorinated surfactant and to characterize a fluorinated surfactant and its solutions. Because most fluorinated surfactants are mixtures of homologs, the tern1 “purity” has to be redefined for each particular case. In most cases, the determination of purity begins with the analysis of intermediates used to synthesize the surfactant. Usually. the intermediates can be readily analyzed by chromatography and the homolog distribution determined. Gas chromatography has only a limited value for the analysis of fluorinated surfactants proper because most fluorinated surfactants are not sufficiently volatile for gas chromatography. In general, the concentration of a fluorinated surfactant in solution can be determined by conventional volumetric or spectroscopic methods used for hydrocarbon-type surfactants [l-51. In addition to the functional groups utilized for the analysis of hydrocarbon-type surfactants, the fluorine content is a unique feature useful for the determination of fluorinated surfactants. If the fluorinated surfactant is the only fluorine-containing species in a solution or a substrate, then the fluorine content indicates the concentration of the fluorinated surfactant. 9.2

ELEMENTALANALYSIS

Elemental analysis is more important for fluorinated surfactants than for hydrocarbon-type surfactants because the fluorine content can indicate the concentration of a fluorinated surfactant i n admixture with hydrocarbon-type surfactants or 390

Analysis 391 Surfactants of Fluorinated

nonfluorinated chemicals. Hence, the concentration of a fluorinated surfactant can be determined without having to resort to complicated separation schemes. Fluorine in anorganic substance can be determined by nondestructive methods or by destruction of the organic matter by combustion or fusion. Nondestructive methods include neutron activation [6] and x-ray fluorescence. Nondestructive methods for elemental fluorine analysis are rapid but require unusual equipment or are not adequately accurate, sensitive, or versatile. Fluorine in organic compounds is usually determined by converting organic fluorine to an inorganic fluoride. Various combustion methods are routinely used for this purpose. However, the carbon-fluorine bond is exceptionally strong and extremely vigorous conditions are needed for a quantitative mineralization. Conventional combustion conditions used for the determination of carbon and hydrogen in nonfluorinated organic compounds are not adequate for aquantitative analysis of fluorinated surfactants. The most vigorous analytical technique for the determination of fluorine in organic fluorochemicals is combustion in an oxyhydrogen flame. The original torch designed by Wickbold [7] used an oxygen-hydrocarbon gas mixture. The Wickboldtorch was modified in aDu Pont laboratory by Sweetser [8] (Fig. 9.1), who replaced the hydrocarbon with hydrogen. Dobratz [9], in Jackson Laboratory of Du Pont, provided the Sweetser apparatus with a bypass system to allow continuous operation during introductionof samples and collection of analyte without disrupting the operation of the oxyhydrogen flame. The temperature of the combustion chamberwas raised by cooling it with air instead of

0

FIG.9.1 The oxyhydrogen combustion apparatus: A, pyrolysis tube; 6,oxyhydrogen torch; C, flame chamber; D, absorber; E, spray trap; F, removable joint; G, sweep oxygen inlet; H, three-way stopcock; I, spiral condenser; J, joint; P, probe burner; x, oxygen inlet; y, hydrogen inlet. (From Ref. 8. Reproduced by permission of the American Chemical Chemical Society.)

392

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water. Some modifications of the apparatus were made by Kissa [ 101 and procedures for the determination of fluorine in biological samples and metalcontaining compounds were developed. The sample is placed into a sample boat and transferred to the quartz pyrolysis tube (A). For the combustion of volatile liquid samples. the boat is placed into the pyrolysis tube and the sample transferred to the boat with a syringe. The sample is gradually pyrolyzed or vaporized in the pyrolysis tube by a movable external burner. The vapors or pyrolysis products are swept into an oxygen-hydrogen flame burning at 2000°C in the quartz apparatus. Organic fluorine is converted to hydrofluoric acid, which is absorbed in water or in aqueous solutions of sodium carbonate or sodium hydroxide. The fluoride ion collected is determined with a fluoride ion-selective electrode [l 11. Metals in the satnple can retain fluorine by forming refractory fluorides. Acidification of the sample in the pyrolysis tube with sulfuric acid, immediately before closing the pyrolysis tube for the combustion, prevents fluoride retention. Two units are routinely used in Jackson Laboratory of Du Pont. and during their 30-year history, no serious incidents have occurred. Although the mixture of oxygen and hydrogen is potentially hazardous, elaborate safety devices, such asautomaticshutdown valves built intotheapparatus,assureasafe operation. Other versions of the oxyhydrogen torch have been developed [ 121. A cornbustion apparatus is commercially available from Heraeus [ 131. In an aspirating combustion apparatus, which draws a liquid sample directly into the oxyhydrogen flame, the sample may be swept through the flame without a complete combustion. A two-stage combustion sequence is essential for the complete conversion of organic fluorine to fluoride: (1) the pyrolysis and partial combustion of the sample in the pyrolysis tube and (2) the complete breakdown of the pyrolysis products in the oxyhydrogen flame. Combustion in an oxygen Parr bomb [ 14.151, although less vigorous than combustion in a oxyhydrogen flame. has given quantitative results for perfluorooctanoic acid and its salts. However, the method is not suitable for volatile organic fluorine compounds. Aqueous samples (e.g., blood) have to be dried and pelletized. Combustion in an oxygen flask [16,17], although convenient, tends to give low results. The combustion converts fluorine to the fluoride ion, which is determined titrimetrically [ 18-25] or with afluorideion-selectiveelectrode [ 1 1,26-291. Satisfactory results have been obtained for perfluorodecalin and perfluorotripropylamine [30]. However, the fluorine recovery of samples containing trifluorobenzoic acid andp-fluorobenzoic acid was found to be low (82-87%) by the oxygen flask combustion method [31]. Although the method is simple and easy to use, it is unsuitable for low fluorinated surfactant concentrations because the sample size islimited to 50 mg. Like the oxygen Parr botnb, the oxygen flask is not suitable for aqueous samples.

Analysis of Fluorinated Surfactants

393

Ashing [3 I] or fusion with metallic potassium [32] or with sodium biphenyl [33-351 may give low results caused by either losses of fluorine or incomplete mineralization. The limitations of various combustion and fusion methods leave the oxyhydrogen flame as the most powerful technique for quantitative mineralization of a fluorinated surfactant. 9.3 VOLUMETRIC METHODS AND ION-PAIR SPECTROSCOPY Volumetric methods used for hydrocarbon-type surfactants [l] are applicable to fluorinated surfactants, unless the solubility of the fluorinated surfactant imposes some limitations. Anionic fluorinated surfactants can be titrated potentiometrically with benzethonium chloride (Hyamine 1622), using a surfactant-selective [36] or a nitrate-selective electrode (Fig. 9.2). Cationic surfactants can be titrated

0 1 2 3 4 5 6 7 8 9

TITRANT ADDED (mL) FIG.9.2 Titration of Zonyl FSA with 0.05 N Hyamine 1622. Metrohm model 670 titrator, Orion model 9342BN surfactant-selective electrode, and model 90-02 double-junction reference electrode. (From Ref. 10.)

394

Chapter 9

with sodium dodecyl sulfate. Alternatively, turbidity of the titration medium can be used as an end-point indication [ 101. Two-phase titration methods [l] are less convenient and not applicable to a fluorinated surfactant, if a suitable water-immiscible solvent cannot be found for the ion pair formed by the fluorinated surfactant. Anionic hydrocarbon-type surfactants can be determined spectrophotometrically by forming an ion pair with a cationic dye [l].The ion pair is extracted into a water-immiscible solvent (e.g., chloroform) and determined spectrophotometrically. Jones [37] suggested the use of methylene blue for the determination of anionic surfactants, but his original method occasionally gave erroneous results. Numerous modifications of the method have been published [ 1,381. Shanna et al. [39] used a methylene blue method to determine perfluorinated carboxylic acids with a 7-10-carbon-long perfluorinated alkane chain. Mixtures of perfluorinated carboxylic acids and alkyl sulfates were analyzed by extracting the ion pairs from a citrate buffer and from 0.25N H2S04.In a medium of high pH, both surfactants, sodium dodecyl sulfate and sodium perfluorooctanoate, form a colored ion pair. At a low pH (sulfuric acid), the sodium salt of perfluorooctanoic acid is present as the free acid, which is extracted as a colorless species. Hence, the difference between the two absorbances represent the perfluorooctanoic acid concentration. 9.4

CHROMATOGRAPHY

Gas chromatogrcryhy (GC) [40-43] separates components of a mixture according to their volatility and interaction with a stationary phase or surfaces in the column. Gas chromatography is the main tool for analyzing intermediates for fluorinated surfactants. Most fluorinated surfactants are not sufficiently volatile for gas chromatography. Fluorinated surfactants suitable for gas chromatography include perfluoroalkanecarboxylic acids. perfluoropolyether surfactants [44], and semifluorinatedalkanes[45].Perfluorooctanoic acid has been derivatizedwith diazomethane and determined as its methyl ester by gas chromatography [46]. Inverse gas chromatography has been employed to determine the polarity of fluorine containing nonionic surfactants with oligooxyethylene groups [47]. The surfactants were placed into a GC column as a stationary phase on an inert support. The polarity of the surfactants was characterized by the difference between the retention indices of liquid standards, as suggested by Reynolds [48], as well as by the sum of these differences for the first five standards. Other polarity parameters examined were (1 ) the p coefficient (the ratio of the adjusted retention times of a polar standard, methanol or ethanol, to that of n-hexane. a nonpolar standard, (2) the polarity index defined by Huebner [49]. and (3) the partial molal Gibbs free energy of solution per methylene group. The polarity increased with increasing length of the oligooxyethylene chain and decreased with an increase in the fluorine content of the hydrophobe.

Analysis of Fluorinated Surfactants

395

The same inverse gas chromatography technique was used to determine the solubility parameters of nonionic surfactants with a oxyethylene chain and a partially fluorinated hydrophobe [50]. The solubility parameter, the corrected solubility parameter, and its polar components increased with increasing polarity of the surfactants. S~4percritictllfluid chronzntogi-qhy,with CO? as the mobile phase, can determine the telomer and homolog distribution in nonionic fluorinated surfactants (Fig. 9.3). Supercritical fluid chromatography with an octadecylsilane-bonded stationary phase has been utilized in studies of semifluorinated alkanes [511. High-pe$or7nnnce liquid clwonlntographv (HPLC), also termed high-pressure liquid chromatography, separates components of a mixture by adsorption on the stationary phase and partitioning between the stationary and mobile phases [52,53]. Mixtures of fluorinated surfactant and hydrocarbon-type surfactant monomers, separated from their micellar solutions by gel permeation [54] or ultrafiltration, have been analyzed by HPLC [ S I . Asakawa et al. [55]separated anionic surfactants on a Finepak SIL CI8S column with acetonitrile-water ( 5 : 4. v/v) as the mobile phase containing 10 mM tetrabutylammonium bromide. The nonionic fluorinated surfactant Fluowet OTN has been analyzed by reversedphase HPLC/MS/MS with a thermospray interface [56], using a 15-cm, 3.9-mminner diameter C column with 5-pn spheres. The methanol-water eluent gradient was from 80 : 20 to 20 : 80 in 10 min. After separation of the fluorinated surfactant, 0.10 mol/L ammonium acetate was injected on line.

22.00

23.75

25.50

29.00 27.25

30.75 ELUTION TIME (min)

32.50

34.25

36.00

FIG. 9.3 Supercritical fluid chromatography of a nonionic fluorinated surfactant RfCH2CH20(CH2CH20),H. A 50-pm-inner diameter capillary column, 10 m long, coated with a 0.25-pm dimethylpolysiloxane film. (Courtesy of J. J. Kozlowski, Du Pont.)

Chapter 9

396

Ion chrornntogrphy is a technique for the separation and determination of various ions. either anions or cations. The ion-exchange column is equilibrated with the eluent and the sample is introduced through the injection loop. The various ions compete with the eluent for exchange sites on the column and are eluted in accord with the strength of their interaction with the column. The ions are detected usually by spectroscopy or conductivity. The background conductivity is reduced by using a suppressor column or a membrane suppressor. Ion chromatography has been employed to determine fluorinated surfactants in acid batch used to etch semiconductor wafers. The fluorinated surfactant FC-93 was determined in a hydrofluoric acid (HF)-ammonium fluoride etch bath, whereas the fluorinated surfactant FC-95 was determined in an etch bath containing concentrated HF, HCI, and HN3 [57]. The analysis involved on-line elimination of the acid matrix, separation on a multiphase HPLC column, and detection by suppressed conductivity. Gel penneutiolz, also referred to as gel filtration, gel chromatography, steric exclusion, or size exclusion chromatography [58,59], separates particles based on their size or hydrodynamic volume. Gel permeation [60-631 is a chromatographic technique that can separate surfactant monomers from their micellar solution according to their molecular size. After the gel column has been equilibrated with a monomer solution, the sample solution is injected and eluted with the same 1nononler solution (sandwich method). Nakagawa and Jizomoto [61] developed a gel filtration method for binary surfactant solutions. Asakawa et al. [62] developed a new simulation technique based on the group contribution method [63] for the micelle-monomer equilibrium. The gel permeation column is considered to consist of a series of plates, each of which consists of a mobile phase and a stationary phase. The surfactants in the mobile phase move to the neighboring lower phase and the two phases are assumed to reach equilibrium immediately. The monomer exchange between the micelle and bulk phase is rapid compared to the elution rate. Hence, the kinetics of the monomer-micelle exchange or the micelle formation and dissociation are not taken in account. 9.5

ULTRAVIOLET AND INFRARED SPECTROSCOPY

Aliphatic carboxylic acids and their anions are known to absorb in the ultraviolet region as a result of n-n* and n-7~:~ transitions. Mukerjee et al. [64] found that long-chain perfluorocarboxylates, such as perfluorooctanoate ( E = 344 L/mol/cm at 35OC), have higher molar absorptivities in the 205-230-nm region than perfluoroacetate ( E = 57 L/mol/cm at 25°C). The absorptivity of perfluoroalkanoates is sufficient for aquantitative determination of thefluorinated surfactant down to the 10-5M concentration range using a IO-crn cell. Mukerjee et al. [64] observed that below the critical micelle concentration (cmc), perfluoroheptanoate and perfluorooctanoate solutions obeyed the Beer-Lambert law within 1%.A somewhat bet-

Analysis Surfactants of Fluorinated

397

ter linear relationship was obtained by relating absorbance data to the fluorinated surfactant concentration by

where A is absorbance, nl and bl are constants, and c is the fluorinated surfactant concentration. At the cmc. the absorptivity increases markedly. The increase (Fig. 9.4) is large enough to permit the determination of the cmc from ultraviolet (UV) absorption data. Anionic fluorinated surfactants of the structure RfCH2CH2SCH2CH2COOLi do not absorb in the UV, although they have a perfluoroalkyl chain and a carboxylate function. The application of UV spectroscopy to the analysis of nonionic hydrocarbon-type surfactants is limited to nonionics, which contain functional groups which absorb in the UVregion, such as aromatic nuclei [65]. The main functional group of nonionics, the oxyethylene ether linkage, does not absorb in the UV region. In spite of this limitation, UV spectroscopy can be useful for determining impurities in nonionic fluorinated surfactants. Infrared (IR) spectroscopy [66-681 is used mainly for identification and characterization of fluorinated surfactants. A beam of infrared radiation is passed through the sample and focused at a monochromator, which disperses the

A

C (mol I)

FIG.9.4 Plot of absorbance (A) at 230 nm versus perfluoroheptanoic acid concentration. (From Ref. 64. Reproduced by permission of the American Chemical Society.)

Chapter 9

398

radiation into a spectrum. An IR spectrum is recorded by plotting the percent transmission of the sample as a function of frequency or wavelength. IR spectra can be used to identify functional groups of the sample or to identify a compound by comparing its spectra to reference spectra of a known pure compound. Conventional dispersive IR spectroscopy is not very sensitive: the detection limit of a component in a mixture is above 0.2-1.0%. Quantitative analysis of surfactants by IR spectroscopy was less important until Fourier transform spectroscopy was developed [67]. In Fourier transform spectroscopy, the entire frequency range of interest is transmitted through an interferometer. The output signal is recorded as a function of interference, and the resulting interferogram is converted to a spectrum using a Fourier transform and a computer. Fourier transform infrared spectroscopy (FTIR) has increased the accuracy of absorption data by accumulating repetitive spectra and combining digitalized data electronically. The precision of IR spectroscopy is limited rnainly by the signal/noise ratio. Because noise israndom,cumulative collection of absorption data by Fourier transform spectroscopy increases the precision and sensitivity of quantitative IR spectroscopy. Fluorinated surfactants exhibit absorption bands arising from CF stretching and CF2 vibration modes (Fig. 9.5). Infrared spectroscopy is a sensitive tool for studying the structure of water in micelles of a fluorinated surfactant [69]. Rntmm syectroscopv [66,70,7 11 analyzes frequency changes in scattered monochromatic radiation. Light passing through a material medium is transmitted, absorbed, and/or scattered. When scattering involves only a direction change, .702 .56-

w

0

f

.42-

m U

5:m .28a

.14-

.oo 4000

I

I

3200 3200 2800 2dOO 2dOO

1200

1200

860

460

WAVE NUMBER FIG.9.5 Infrared spectra of a nonionic fluorinated surfactant consisting of homologs RfCH2CH20(CH2CH20),H. (Courtesy of J. T. Cronin, Du Pont.)

Analysis of Fluorinated Surfactants

399

the scattered light has the same frequency as the unscattered light. However, when light interacts with matter and various transitions are involved. the scattered light will have gained or lost energy. The resulting change in frequency is characteristic of the material studied. Because the intensity of Raman scattering is low, Raman spectroscopy was almost dormant until the development of the laser provided a high-density monochromatic light source. Ranlan spectroscopy has been very useful for conformation studies of hydrocarbon-type and partially fluorinated surfactants but has only a limited value for perfluorinated surfactants. Unlike a strong IR absorption band arising from the CF stretching mode, the intensity of Raman bands is low for these vibrations. In contrast to the characteristic CH stretching mode, the CF and CF2 modes are in a region where other molecular modes occur and complicate absorption patterns. Amorim da Costa and Santos [72] have nevertheless been able to show Raman spectroscopy to be useful for structure and conformation analysis of fluorinated surfactants (Fig. 9.6). Ito et al. [73] examined the micellization of fluorocarbon-hydrocarbon hybrid surfactants by Raman spectroscopy. The Spectrometer was equipped with a nlultireflection cell and an optical-fiber light-collecting device [74]. The Raman spectra were obtained at high surfactant concentrations above the cmc, because the Raman intensity below cmc was too weak to be measured. Developments such as Fourier Raman spectrometry and the charge-coupled device (CCD) detector enhance the usefulness of Raman spectroscopy for fluorinated surfactants. 9.6

MASSSPECTROMETRY

Mass spectrometry [75-831 involves four steps: (1) isolation of the component of interest, (2) ionization, (3) separation of the ions in a combination of electric and magnetic fields according to their masskharge ( d z ) ratio, and (4) detection. The molecular ions and ionic fragments are detected by an electrometer and their relative abundances are recorded in the mass spectra. The sensitivity of detection can be increased with an electron multiplier. The first step in mass spectrometry isolates a component of the sample by (1) vaporization using a direct insertion probe to heat the sample to about 200-300°C. (2) flash desorption at a very rapid heating rate to minimize thermal degradation, or (3) chromatography. Mass spectrometry is made more powerful by adding a chromatographic “front end” to separate the components of the Sample before they enter the mass spectrometer. Most fluorinated surfactants, even when derivatized, are not sufficiently volatile to be analyzed by gas chromatography-mass spectrometry (GC-MS), the most commonly used “hyphenated mass spectrometer.” HPLC is more useful for fluorinated surfactants. Several techniques have been developed for interfacing liquid chromatography (LC)

Chapter 9

400

727

. I doc

.

L

I400

. IO00

.

. 600

-I

b

Cm

FIG.9.6 The 200-1800-cm" Raman spectra of perfluorodecanoic acid (a) and its lithium (b), sodium (c), and ammonium (d) salts at 22°C. (From Ref. 72. Reproduced by permission of Academic Press, Inc.)

with a mass spectrometer [83]. Over a dozen LC-MS interfaces are commercially available, including a transport interface using a belt to transport the eluent through a desolvation chamber to the ionization source, direct liquid introduction into the ion source, particle beam, thermospray, electrospray, and others ~831.

I

I

"

Analysis of Fluorinated Surfactants

401

The thermospray technique [84-871 uses a heated vaporizer from which the HPLC eluent containing the dissolved electrolyte is sprayed as a jet into a heated chamber. A sampling orifice is positioned normal to the axis of the vaporizer probe. The ions and molecules are pumped through the sampling orifice into the mass spectrometer. Electron impact or collision-activated ionization, although optional, provides structural information. Schroder [88,89] analyzed fluorinated surfactants in water and wastewater using HPLC coupled by a thermospray interface to a tandem mass spectrometer (MS/MS). Alternatively, the chromatographic column was bypassed and the analyte was injected into the mass spectrometer (FIA, flow injection analysis). Supercritical fluid chromatography using CO? as the mobile phase eliminates the problems associated with the evaporation of a liquid eluent and is, therefore, more compatible than liquid chromatography with MS. The second step in mass spectrometry, ionization of the sample, is accomplished by one of several techniques, some of which include sampling as well. Usually, the sample is bombarded with a beam of electrons or energetic particles. Electron impact ionization employs electrons from a heated filament to ionize a gas-phase sample. The energy of the commonly used 70-eV electrons is in excess of the energy required for removing an electron from a molecule to produce a molecular ion. Hence, the electron impact ionization causes fragmentation of the sample and provides a fragmentation pattern which gives useful structural information. In field ionization a volatile sample passes through a strong electric field (107-108 V/cm) which generates molecular ions with little fragmentation. Both ionization techniques, electron impact ionization and field ionization, require a vaporizable sample, limiting their application to volatile surfactants or degradation products of nonvolatile surfactants. Modern "soft" ionization techniques have overcome the sample volatility requirement by combining the first two steps in mass spectrometry: sampling and ionization. The soft ionization techniques used for the analysis of surfactants include fast atom bombardment (FAB), field desorption (FD), desorption chemical ionization (DCI, also called direct chemical ionization), secondary-ion mass spectrometry (SIMS). and laser desorption methods. Fast atom bombardment directs a beam of energetic inert-gas atoms onto a sample in a viscous liquid matrix, usually in glycerol or triethanolamine. A flow of the sample in the solution replenishes the sample on the surface and limits degradation by the particle beam. The surfactant suppresses ionization of glycerol, and by providing a cleaner background, it increases the sensitivity of the FAB method [90,91]. The FAB techniques usually provide quasimolecular ions with only a few fragment ions. Electrolytes added to the sample solution facilitate the formation of quasimolecular ions, for example, adduct ions with alkali metal cations.

Chapter 9

402

:IA

477

80

c

70

60

W

I-

z W

5 a

50

40 30

20 ,911

10

1= '

0

400

200

600 1400 1200lo00 800

Mn

31

,1321

400

200

800

600

14001200lo00 Mi2

FIG. 9.7 FAB spectrum from Na 2-hydroperfluoroheptanesulfonate: (A) positive ions; (B)negative ions. (From Ref. 93. Reproduced by permission of the American Chemical Society.)

Analysis of Fluorinated Surfactants

403

Heller et al. [92] observed large cluster ions of perfluoroalkanesulfonates desorbed under FAB conditions. Cesium pertluorohexanesulfonate formed clusters containing as many as 29 anions and 30 cations. The abundance of these highmass ions produced by perfluoroalkanesulfonates was much higher than that of theionsformed by CsI. For example,theabundance of thecluster Cs(C6FI3SO3Cs)T9 at HI/: 10240.4 was eight times greater than the abundance of the Cs4011gat 10265.5. Discontinuities in surface tension of cesium perfluorohexanesulfonate and in the slope of cluster abundance occurred at the same surfactant concentration range in tetraglyme solutions. suggesting that the cluster formation and aggregation in solution are related. Lyon et al. [93] characterized fluroalkanesulfonates by FAB ionization combined with tandem MS/MS spectrometry. The samples were dissolved in glycerol or triethylamine, placed on the copper target of the FAB probe, and bombarded with 8-keV xenon atoms. The ions formed were accelerated into the analyzer of the mass spectrometer. Normal spectra were recorded by scanning the first spectrometer, MS-I, and leaving the second mass spectrometer, MS-11, fixed

M/Z

FIG.9.8 CAD spectrum of negative ions from mlz 431, the 2-hydroperfluoroheptanesulfonate anion. (From Ref. 93. Reproduced by permission of the American Chemical Society.)

Chapter 9

404

rI

C7F15S0383

28 1

21 9

I

i 1

rn 50

100

1 so

2d0

M/Z

FIG.9.9 CAD spectrum of negative ions from mlz 41 1, the perfluoroheptanesulfonate anion. (From Ref. 93. Reproduced by permission of the American Chemical Society.)

to pass all ions. Tandem mass spectrometry was used to enhance the FAB technique by collision-activated dissociation (CAD). An appropriate ion selected with MS-I was subjected to collisions with helium atoms in the collision cell and the CAD spectra were recorded by the MS-I1 unit. Examples of the FAB and CAD spectra are shown in Figs. 9.7,9.8. and 9.9 The fragmentation of perfluoroalkanesulfonates involved the cleavage of the C-C bond with the loss of a C12F3,1+-, followed by the detachment of tetrafluoroethylene:

CF3(CF2),l-CF~CF~CF2(CF~),~zSO~ + CF3(CF?),,*+ .CF2CF2CF2(CF?),,,SOST+ C F 2 4 F 2 + .CF2(CF2),,SOsT The fragmentation reaction sequence is analogous to the thermal decomposition mechanism of poly(tetrafluoroethy1ene). Substitution of hydrogen for oneof the terminal fluorine atoms changes the fragmentation mechanism. Formation of HF then becomes the main reaction in the fragmentation process.

Analysis of Fluorinated Surfactants

405

An amphoteric fluorinated surfactant, Du Pont's Zonyl FSB, has been used as a calibration standard for high-resolution FAB-MS measurements [94]. In FD, the sample is deposited directly onto carbon dendrites serving on the anode as activated emitters. For hydrocarbon-type anionic, cationic, and nonionic surfactants, FD usually produces molecular or quasimolecular ions free of fragmentation. For amphoteric nonfluorinated surfactants, molecular ions have been obtained together with fragment ions providing structural information [95-971, which showed that perfluoroalkanesulfonates are desorbed as high-mass clusters under FD conditions. Desorption chemical ionization (DCI) places the sample onto a direct-insertion probe located within the chemical ionization plasma [98-1001. Cationic surfactants produce molecular ions and decomposition ions useful for quantitative analysis [91,101a]. The DCI technique is less informative for anionic or nonionic surfactants. Batts and Paul [ 101bI used time-of-flight secondary-ion mass spectrometry (ToF-SIMS) to investigate the competitive adsorption of a cationic fluorinated surfactant (FC- 134) at the gelatin-air interface. ToF-SIMS is avery sensitive surface analysis technique. In the static mode, the sampling depth of ToF-SIMS is only one to two monolayers. However, the ToF-SIMS data are difficult to interpret in quantitative terms and experimental conditions must be carefully controlled. Batts and Paul used positive secondary-ion spectra only, although negative-ion spectra may have been used as well.

9.7

NUCLEARMAGNETICRESONANCE

Nuclear magnetic resonance (NMR) is a very powerful tool for investigating surfactant systems. The theory of NMR spectroscopy has been described in several books [102-1111 and will not be discussed here in detail. The applications of NMR to surfactant systems have been reviewed by Lindman et al. [112]. Nuclear magnetic resonance spectroscopy is based on the allowed orientation (Zeeman energy levels) of nuclei with nonzero angular momentum when the sample is placed into a magnetic field. The nuclei can be realigned by varying the external magnetic field or by radio-frequency irradiation. When the applied energy matches the energy required for the transition between Zeeman levels, resonance results. Nuclei do not all have the same resonance frequency because their chemical environment can vary the applied magnetic field. As a result of differences in shielding, nuclei in functional groups have characteristic resonance frequencies. The difference in the resonance frequencies of two chemically and/or magnetically unequal nuclei indicates the chemical shift, expressed in ppm. To calculate the chemical shift, the difference between the resonance frequency of the sample peak and the resonance frequency of the reference peak is divided by the reso-

Chapter 9

406

name frequency of the reference peak or by the "observed frequency'' given by the instrument manufacturer [ 1lo]. The chemical shifts observed by NMRdepend on the concentration of the species and the solvent [ 1 131. The solvent effect has been used to investigate the environment of atoms within the fluorinated surfactant micelle [ 114- 1171. Nuclear magnetic resonance spectroscopy yields structural information on surfactants and their micelles, values of the free energy of micellization, AGL, and the corresponding enthalpy and entropy changes, AH& and AS&. For the analyses of fluorinated surfactants, 'H-, 13C-,and 19F-NMR spectroscopies have been employed. 'H-NMR spectroscopy can provide information on the environment of the fluorinated surfactant in the micelle. The high sensitivity of the 'H nucleus is a definite advantage. Monduzzi et al. [ 1181 utilized the Fourier transform pulsedgradient spin-echo (FTPGSE) 'H-NMR technique to determine the self-diffusion coefficients of water in W/O (water-in-oil) microemulsions containing perfluoropolyether (PFPE) oils and an anionic surfactant with a PFPE hydrophobe. The self-diffusion data provided quantitative information on the amount of water in the composition range where continuous water coexists with water in droplets. Haoet al. [119] studied sodium perfluorooctanoate (SPFO) and cetyltrimethylammonium bromide (CTAB) mixed solutions by H-NMR. The results indicated a strong interaction between oppositely charged head groups and the penetration of SPFO molecules into the CTAB micelles. Monduzzi et al. [ 1201 identifiedlyotropiccrystallinephases of theammoniumsalts of perfluoropolyether carboxylic acids by 'H- and "N-NMR spectroscopy. "C-NMR spectroscopy, because of its high resolution and wide chemical shift range [121-124], can give qualitative information on molecular conformation of fluorinated surfactants in solution and quantitative information on cmc values. However, the 13C signal is relatively weak, for two reasons. The abundance of the "C isotope in carbon-containing substances is about 1.1% of carbon atoms. Furthermore, the I3C has a lower magnetogyric ratio (lower magnetic strength) than 'H and this reduces the sensitivity further to a total factor of about 1 : 5800. Several techniques have been developed to overcome the loss of sensitivity. Instead of sweeping the resonance frequencies successively, the Fourier transforln method uses a radio-frequency pulse to excite all resonance frequencies at once and the signal is enhanced by repeated pulses and signal averaging. For NMR studies of fluorinated surfactants, the most useful nucleus is 19F, in addition to 13C and 'H nuclei. Changes iu the 19F chemical shift at cmc are larger than changes in the proton chemical shifts and, therefore, provide more information on fluorinated surfactants and their micellar structures. I9F-NMR spectra have been recorded for structural characterization of perfluorononanoic acid [ 1251 and perfluoropolyether surfactants [ 1261. Micelle formation in solutions of

'

Analysis of Fluorinated Surfactants

407

o CAPRYLATE II PROPIONATE

-CF2 0.8

0.7 0.6 0-5

0.4 0.3

0.2 0.1

0

1

2

3

4

5

6

7

0

9

RECIPROCAL SURFACTANT CONCENTRATIONVALUES (M ')

FIG. 9.10 Plot of 'F chemical shift against the inverse concentration of sodium perfluorocaprylate and sodium perfluoropropionate: (a) CF,; (b) CF3. (From Ref. 127. Reproduced by permission of the American Chemical Society.)

fluorinated surfactants has been studied by measuring the I9F chemical shift [114-117,127-1331 (Fig. 9.10). Muller and co-workers studied the effect of the environment on fluorine atoms in a surfactant micelle by 19F-NMR [114-1171 (see Section 7.1 for the results and conclusions of their studies). Carlfors and Stilbs[134]usedtheFouriertransform NMR pulsed-gradientspin-echo (FTPGSE) method [ 135-1371 for the determination of multicomponent self-diffusion coefficients in micellar solutions of sodium perfluorooctanoate and sodium perfluorooctanoate-sodium decanoate. Partition coefficients were calculated from the self-diffusion data for a homologous series of 11-alkanols, benzene, and benzyl alcohol. Palepu and Rainsborough [138] measured the 19Fchemical shift changes for 1 : 1 mixtures of sodium perfluorooctanoate with a- and P-cyclodextrins. In a-cyclodextrin mixtures, the shifts for terminal fluorine atoms changed more than those for fluorine atoms in the middle of the chain. In P-cyclodextrin mixtures, the fluorine atoms in the middle of the chain were affected more than the

408

Chapter 9

terminal ones by cyclodextrin. Guo et al. [139] employed 19F-NMR to investigate the association of a-, p-, and 7-cyclodextrins with sodium perfluorobutanoate, sodium perfluoroheptanoate, sodium perfluorooctanoate, and sodium perfluorononanoate. Trifluoroacetic acid was used as the external reference, and the difference between the ‘9F chemical shift for the mixed system and that for the solution containing only the fluorinated surfactant was measured. The results of this systematic study showed a weak association of a-cyclodextrin with the fluorinated surfactants. Fluorinated surfactants with a short chain formed a 1 : 1 complex with p-cyclodextrin. Fluorinated surfactants having a longer chain form a 2 : 1 complex, especially at higher P-cyclodextrin concentrations. ?-Cyclodextrin forms a 1 : 1 complex with the fluorinated surfactants. The association complexes for the 1 : 1 complexes were calculated from the 19F chemical shifts measured for various cyclodextrin concentrations. Guo et al. [ 1391 explained the results by the cavity size of the host cyclodextrins. The cavity of CYcyclodextrin is apparently too small to accommodate fluorinated surfactants and form an inclusion complex. The combination of both I9F- and ‘H-NMR spectroscopy permits independent estimation of perfluorocarbon and hydrocarbon surfactants in their mixtures [140]. Bossev et al. [ 141,1421 studied the counterion effect on micellar systems formed by tetraethylammonium perfluorooctylsulfonate (TEAFOS) and lithium perfluorooctylsulfonate (LiFOS). ‘H- and I9F-NMR measurements of self-diffusion coefficients and chemical shifts showed that LiFOS, which forms spherical micelles, has a fast exchange rate. The TEA+counterions induce a transformation to threadlike structures. As a result, the self-diffusion coefficient for TEAFOS is by a magnitude lower than that of LiFOS. The dynamic parameters of fluorinated surfactant solutions have been studied by NMR relaxation methods. The theory of NMR relaxation has been discussed in detail by Henriksson and Odberg [ 1431 and reviewed by Lindman et al. [ 1121. Spin-lattice relaxation transfers energy from the higher energy level to the lattice as thermal energy. (The term “lattice” is used here to denote molecules other than the fluorinated surfactant in the sample.) Because the resulting temperature change is too small to be detected, the relaxation time of recovery of the absorption signal from saturation is measured. Fluorine relaxation times can be measured both in water and in deuterium oxide and the different magnetic properties of protons and deuterons can provide information about the environment of the fluorocarbon segments. Henriksson and Odberg [ 1431 determined I9F spin-lattice relaxation time for heptafluorobutyric acid by the 7~/2,7d2 pulse method and concluded that fluorocarbon chains in the heptafluorobutyric acid micelles are to some extent exposed to water. Ulmius and Lindman [144] measured I9F spin-lattice relaxation

Analysis of Fluorinated Surfactants

409

time for various perfluorinated or partially fluorinated carboxylic acids and concluded that the fluorocarbon chains come in contact with water only at the micellar surface. Because the deuterium relaxation time is affected by the state of water (bound or free), Burger-Guerisi et al. [145] studied phase transitions in fluorinated microemulsions by measuring 2H-NMR relaxation times. Serratrice et al. [ 1461 investigated the influence of water on I3C chemical shifts and relaxation times of nonionic fluorinated surfactants dissolved in a fluorocarbon. The observed chemical shift variations were attributed to the hydration of the hydrophilic chain. In another study [ 1471, relaxation data indicated similar flexibilities of the fluorinated chain in the various nonionic fluorinated surfactant molecules studied [147]. Tiddy [148] measured I9F spin-lattice and spin-spin relaxation times for the lamellar phase of the ammonium perfluorooctanoate-water system. The 19F relaxation rates were found to be qualitatively similar to the relaxation rates of protons in analogous hydrocarbon surfactant systems. The spin-lattice relaxation times indicated that the CF2 groups at 298 K rotate about the long axis of the hydrophobic chain more slowly than the CH2 groups in hydrocarbon systems. The activation energy of rotation is similar to or smaller than the activation energy for analogous hydrocarbon systems. Boden et al. [149] used ‘“Cs+-NMR spectroscopy for mapping phase diagrams of the cesium pentadecafluorooctanoate (CsPFO)/H20 systems. Fur6 and Sitnikov [ 1501 investigated cesium perfluorooctanoate micelles by ‘9F-decoupled “C-NMR relaxation rate measurements at three different magnetic fields. 9.8

ELECTRON SPIN RESONANCE

Electron spin resonance (ESR) spectroscopy [ 151-1 651 examines species having a net quantum angular momentum, usually arising from the spin of unpaired electrons. The resonant absorption of electromagnetic energy is measured in a magnetic field. Usually, the electromagnetic energy is provided by microwave radiation at a constant frequency and the magnetic field is varied. Because the sample may be paramagnetic without having unpaired electrons, the term “electron paramagnetic resonance” (EPR) has been used. The term ESR is more common and is used by Chemical Abstract Service as a generic term regardless of the origin of paramagnetism [ 1521. Because micellar systems do not have a net quantum mechanical angular momentum. a paramagnetic probe must be inserted into the sample. The use of spin probes is a useful technique for studying micellar systems provided that (1) the probe does not perturb the micelles and aggregates of the surfactant being studied, (2) the probe is stable at least for the duration of the ESR measurement, and (3) the probe is sensitive to the polarity, spatial restriction, and viscosity of its environment. The choice of the spin label is acritical step in the ESR study, as the

410

Chapter 9

interactions with the micellar aggregates and the location of the probe at or in the aggregate depend on the structure of the probe. Numerous nitroxide free radicals have been found to be sufficiently stable for the spin probe technique. In the ESR studies of fluorinated surfactants, Noxy1 derivatives of piperidine (I) or oxazolidine (11) have been used as the spin probes.

X X

= -N+(CH&

TempTMA+ 1 CH3 CAT12

X

= -OC(CH2),

.CH3

C1,-Tempo

X

= -OC(CH2)

,&H3 C

6-Ten1po

= -N+(CH3)2(CH?)I

112

+ I1 = 15

I1 121

12 10 7 5 1

3 5 8 10 14

5-DXSA 7-DXSA 10-DXSA 12-DXSA 16-DXSA

Micellar systems of ammonium pentadecafluorooctanoate and ammonium perfluropolyethercarboxylate have been investigated by ESR using a small cationic probe (TempTMA+) [ 155,1571, a large cationic probe (CAT12) [156], long-chain doxyl nitroxides (5-DXSA, 12-DXSA, and 16-DXSA) [157,158], as well as using neutral spin probes (Cl2-Tempo and CI6-Tempo)[ 1561. The results obtained by using the small and positively charged TempTMA+ or the neutral long-chain doxyl nitroxides ( 5 - and 16-DXSA) as ESR probes were comparable to those provided by other techniques. However, the positively charged longchain probe CAT12 caused considerable perburtations. The location of the probes was found to be different: CAT12 enters the micelle. whereas TenlpTMA+ is located on the surface of the micelles [ 1561. In order to avoid any possible perturbance caused by a hydrophobic chain of the probe, Ristori examined the state of water in the interlamellar regions of perfluoropolyether alnmonium carboxylates by using the corresponding Cu(I1) [ 159al and Mn(I1) [ 159bl salts as the spin probe. 9.9

CHEMICALRELAXATIONMETHODS

Chemical relaxation methods (CRM) observe a mixture of reactants and reaction products in thermodynamic equilibrium and perturb this equilibrium by generat-

Analysis of Fluorinated Surfactants

41 1

ing a rapid but very small change in one of the parameters affecting equilibrium, such as pressure or temperature [166]. As a result of the perturbance, the system shifts to another equilibrium, commensurate to the change in a parameter. The change of equilibrium is characterized by one or several time constants, the relaxation times, which are related to the rate constants of the chemical reactions studied. Chemical relaxation methods have been very useful in studies of Inicellization kinetics, based on the theory of Aniansson and Wall [167-1691, modified by Kahlweit and coworkers [ 170- 1741. Chemical relaxation techniques have been described in several articles and books [ 175-1 801 and reviewed by Lang and Zana [166]. The chemical relaxation methods usually employed are the temperaturejump, pressure-jump. shock tube, ultrasonic absorption, and stopped flow methods. The temperature-jump method utilizes rapid heating techniques, such as heating by a microwave pulse, discharge of a charged coaxial cable in the solution, discharge of a capacitor, and heating by a pulse of laser light. The pressure-jump method utilizes an autoclave with a thin metal diaphragm which bursts and allows the pressure of the autoclave to drop very rapidly to atmospheric pressure. A pressure-jump apparatus with conductivity detection and twin-cell arrangement is shown in Fig. 9.11. One cell contains the sample investigated and the other cell contains an electrolyte of similar conductivity but no relaxation. The shock tube technique is somewhat similar. The bursting of the diaphragm generates a pressure drop which propagates through a tube half-filled with water or ethanol. Reflections of the pressure jump at the bottom and the top of the tube cause addition and subtraction of the incident and reflected pressure waves. As a result, the equilibrium is shifted by a rectangular change in pressure. The ultrasonic absorption method shifts the equilibrium periodically by harmonic changes of pressure and temperature caused by the propagation of ultrasonic waves in fluids. The stopped flow method involves rapid mixing of two solutionsinless than amillisecond.Becausethemixtureobservedis not in equilibrium, the stopped flow method is not truly a chemical relaxation method. The stopped flow method is useful, nevertheless, for observing perturbations by composition jumps. Dilute micellar solutions of surfactants are characterized by two well-separated relaxation times. The molecular origin of the fast relaxation time has been related to a monomer-micelle exchange [ 181-1 841. It was realized later that the relaxation spectra of micellar solutions really exhibit two relaxation times. The theory of Aniansson and Wall [ 167,1851 assumes a stepwise aggregation of surfactant monomers to form micelles [186]. The fast relaxation time is attributed to the exchange of monomeric surfactants between the micelles and the intermicellar solution. The slow relaxation time is attributed to micelle formation and breakdown. The theory and its modifications by Kahlweit and co-workers [ 170-1741

412

Chapter 9

111

FIG.9.11 Pressure-jump apparatus: A, autoclave; C, and C2, conductivity cells; E, electrodes; M, elastic membrane; D, metal diaphragm; P, pressure pump; m, manometer; G, 40-kHz generator for the conductivity bridge; C3 and C4, tunable capacitors; R, and R2, helipot resistors; R3, potentiometer; Os, oscilloscope. (From Ref. 166.)

have been the basis for most interpretations of chemical relaxation times and provided valuable information on the kinetics of micellization. The pressure-jump method and a shock-wave method with conductivity detection have been used by Hoffmann and co-workers in their studies of micelles formed by perfluorinated surfactants [ 187-1931. The pressure-jump and shockwave techniques were utilized in micellization studies on cationic surfactants with pelfluorinated counterions as well [ 1911. The temperature-jump relaxation technique [ 1751 has been used by Hoffmann and Ulbricht [ 1881 with optical detection, utilizing a pH indicator (thymol blue) to observe relaxation processes of a 1 : 1 mixture of perfluorooctanoic acid and its sodium salt. Ultrasonic absorption has been measured to determine relaxation times in surfactant solutions [ 194-2051. The kmetics of micelle formation have been investigated using the ultrasonic relaxation method for alkali metal salts of perfluorooctanoic acid [206]. The

Analysis of Fluorinated Surfactants

413

periodic fluctuations in temperature and pressure caused by the acoustic wave are several magnitudes less than the temperature or pressure perturbations of jump techniques. Rassing et al. [206] suggested that the ultrasonic and jump methods measure different modes of micelle formation whose relaxation times differ by several orders of magnitude. Ultrasonic absorption techniques [204] have also been used to measure relaxation spectra of cesium perfluorooctanoate in water and in deuterium [205]. 9.10 SMALL-ANGLESCATTERINGMETHODS Small-angle scattering allows us to measure distances in the range 0.5-50 nm. Small-angle scattering methods differ in principle from imaging methods, such as microscopy. Imaging methods collect and focus radiation scattered by the objects being studied and reconstruct their image. In contrast, small-angle scattering methods produce an interference pattern of the radiation scattered by the objects under study. The interference pattern can be converted to reconstruct an average image and interpreted to obtain basic information on surfactant micelles. It is important to keep in mind that small-angle scattering techniques provide only an average image in the space of correlation functions. Averaging limits severely the amount of information obtainable because thermal agitation in surfactant solutions produces large fluctuations. However, this limitation has the advantage that all structures are described by several averaged parameters and, in that sense, averaging facilitates interpretation of data [207]. Small-angle and wide-angle x-ray diffraction techniques have been reviewed in several articles and books [207-2111. X-ray diffraction was used by Barton et al. [212] to study the monolayer structure of the acid CF3(CF2)9CH2COOHsupported on water. Shin et a]. [213] investigated the packing structures in monolayers of the partially fluorinated carboxylicacids CF3(CF3)&H2COOH, CF3(CF2)6CH7(CF?)3COoH,and CF3(CF2)6(CH2)4COOHon water. The molecular dynamics simulations indicated a breakup of the homogeneous ordered monolayer into an array of ordered islands when the area per molecule exceeds that of close packing. Zou and Barton [214] studied the packing and coverage of the surfactant CF3(CF2)7S02N(C2H6) (C2CH20)7.4CH3 at the liquid-vapor interface of a saturated solution in decane. The study showed x-ray reflection to be a valuable method for the determination of surface activity for fluorinated surfactants in organic solvents. X-ray reflection, as well as neutron reflection, can determine the absolute surface coverage, whereas surface tensiometry can only illustrate trends in surface activity as a function of surfactant concentration. Small-angle x-ray scattering (SAXS) has been used to investigate the structure of micelles and micellar phases [2 15-2171.

Chapter 9

414

Fontell and Lindman [318] investigated phase equilibria of two-component systems consisting of water and perfluorononanoic acid or its salts. SAXS showed liquid-crystalline regions in addition to regions of micellar solutions. The thickness of fluorocarbon layers in the liquid-crystalline region and the area per polar head group were estimated. Small-angle neutron scattering (SANS) methods have been described in the literature [207,219,220]. Hoffmann et al. [23 1,2321 measured SANS of lithium perfluorooctanoate. diethylammonium perfluorononanoate, and tetraethylammonium perfluorooctanesulfonate micelles in D 2 0 or in mixtures of D 2 0 and H.0. The radii. micelle concentrations, and aggregation numbers were calculated. Ravey and Stibe' [223,224] studied SANS of nonionic fluorinated surfactant gels. SANS spectra of systems containing a fluorocarbon. nonionic fluorinated surfactant, and large anlounts of water were interpreted in terms of water-in(water-in-oil microemulsions) emulsions. Mathis et al. [235] determined the aggregation number and droplet size of microemulsions prepared from a ternary mixture of water, a fluorocarbon, and a nonionic fluorinated perfluoroalkylpoly(oxyethy1ene) surfactant. The solutions were diluted with an H20-D20 mixture in the variable-contrast method that allows varying the scattering length [226] of the solvent. Scattering lengths, b,, where i = FC, S, A. B, or P, for the fluorocarbon (FC) and the surfactant (S) molecules, for the hydrophilic and hydrophobic moieties (A and B) of the surfactant. and for the whole particle (P) were computed from atomic coherent scattering lengths. Scattering densities were calculated from the scattering lengths, b,, and the corresponding molecular volumes

where N,, is the Avogadro number and V, are the molar volumes. The number of scattering particles per unit volume, Np. has been calculated assuming an average aggregation number ( N ) and neglecting the monomeric surfactant molecules (the cmc values were low):

where 2, and M sare the mass fraction and molecular weight of the surfactant and Vis the specific volume of the solution. Assuming that interparticle effects are negligible, the scattered intensity, I(q), for a sample containing monodisperse particles can be written as [227]

I(q) = KNILQJ,- Qll.)*V;Pdq) where K is a calibration constant,

(4)

is the scattering density of the H20-D20

Analysis of Fluorinated Surfactants

415

mixture, Qp is the scattering length density of particles, and PN(q)is the scattering form factor. Burkitt et al. [228,229] used SANS to examine the size and shape of micelles in solutions containing ammonium perfluorooctanoate or mixtures of ammonium perfluorooctanoate with ammonium decanoate. The SANS measurements were made by the external contrast variation technique using mixtures of water and D 2 0 as the solvent. By selecting appropriate H20-D20 ratios, it is possible to view hydrocarbon and fluorocarbon micelle species independently. At a match point, the scattering length density of the H20-D20 mixture is equal to that of the surfactant and the surfactant is at zero contrast. If the surfactants in a binary mixture form separate micelles, two match points are found. If mixed micelles are formed, scattering would occur at the contrast match points for each surfactant, but another match point is found as well. The basic scattering data were processed by a standard computer program to give the intensity of scattering, I(q), as a function of the scattering vector, Q, relative to water. The scattering vector Q for elastic scattering is defined by

where h is the wavelength of the radiation (neutrons) in the medium and 8 the scattering angle. Burkitt et a1 [228,229] suggested that perfluorooctanoate and decanoate chains can mix and form mixed micelles. However, their mixed micelle model allows for segregation between hydrocarbon and fluorocarbon chains within the micelle. Burkitt et al. [228,229] concluded that SANS is an excellent method for the determination of micellar weights of ammonium octanoate, ammonium decanoate, and ammonium perfluorooctanoate. The scattering data suggested that the ammonium perfluorooctanoate micelles are cylindrical. Small-angle neutron scattering coupled with the contrast variation technique was used by Caponetti et al. [230] to study solutions of sodium perfluorooctaonate, sodium dodecanoate, and their mixture. Their data indicated the existence of mixed micelles having the same composition and a narrow size distribution. SANS data obtained by Berr and Jones [231] indicated that sodium perfluorooctanoate forms in water spherical micelles in which the fluorocarbon chains reside in a water-free core. 9.1 1 LIGHTSCATTERING Light-scattering methods can be divided into two major categories: methods which measure time-averaged scattering (static methods) or methods which ob-

416

Chapter 9

serve the scattering fluctuation as a function of time (dynamic methods). Both methods can give useful information on the shape, size, polydispersity, and micellization of surfactant solutions and microemulsions. A large number of papers have been published and the theory of light scattering has been reviewed in several books [232-236]. The theoretical aspects of light scattering are in several ways similar to those of small-angle scattering. However, important differences exist in the status of experimental techniques. Dynamic light scattering is now in routine use, dynamic neutron scattering is a recently developed technique, but the practical feasibility of dynamic x-ray scattering is uncertain [235]. The use of light-scattering methods for studying micellar structures of fluorinated surfactants is limited mainly to partially fluorinated surfactants. The micelles of perfluorinated surfactants are very weak light scatterers. The refractive index-concentration slope, dnldc, is 10-100 times smaller than that for hydrocarbon chain surfactants [ 1921. However, partially fluorinated surfactant solutions are amenable to light-scattering measurements. Micellar aggregation numbers of fluorinated surfactants with a terminal "CF3 group (1 2,12,l'>-trifluorododecyltrimethylammoniun~bromide) have been determined by light scattering [237]. Dynamic light scattering has been used to determine the radius of aggregates in the mixed LiDS-DEFUMAC-water system (LiDS = lithium dodecyl sulfate; DEFUMAC = diethanolheptadecafluoro-2-undecanolmethylammonium chloride) [238] and in aqueous fluorinated nonionic surfactants [239,240]. Light-scattering measurements require optically clean samples free of extraneous matter, which can reduce the signal-to-noise ratio and distort the interpretation of data. The sensitivity of light scattering to contamination with dust is extremely critical for weak scatterers, such as perfluorinated surfactant systems. A continuous filtration technique [241] for removing dust has made it possible to measure dynamic light scattering even in a system containing a mixture of sodium perfluorooctanoate and tetrapropylamtnonium bromide. Lai et~21.[242] used dynamic light scattering to measure the size of nanometer-scale water droplets in reversed micelles of perfluoroheptanoic acid (PFHA) in 1,1,2-trichlorotrifluoroethane. 9.12 LUMINESCENCE PROBING METHODS

Luminescence probes are molecules or ions which, upon photoexcitation, emit light having characteristics sensitive to the immediate environment of the probe [243]. The characteristics of emitted light serve to characterize the environment of the luminescence probe. Luminescent probes can be divided into two groups: fluorescence probes and phosphorescence probes. Fluorescence is an emission of light associated with

Analysis of Fluorinated Surfactants

417

the transition from excited single states to the ground state. Phosphorescence is an emission of light associated with the transition from the lowest triplet state to the ground state (Fig. 9.12) [244]. Radiative lifetimes of fluorescence generally range from los to lo9 s-' and radiative lifetimes of phosphorescence range from 10" to lo3 s" [245]. Some molecules or ions can function as quenchers and inhibit luminescence. Quenching, caused by interactions between the luminescent probe and a quencher, may be reversible or irreversible. Excimer formation is a case of reversible quenching. Some luminescent probes react, in the excited state, with an identical molecule in the ground state and form an excimer:

where kE and k P E are the rate constants for excimer formation and dissociation [243]. A probe in the excited state may associate in an analogous manner with a dissimilar molecule in the ground state and form an exciplex. The fluorescence characteristics of excimers and exciplexes differ from those of the monomeric probe. The excimer or exciplex formation is sensitive to the viscosity of the environment around the probe and, therefore, provides useful information on the structure of surfactant solutions. Another reversible quenching technique used in micellar studies is energy transfer from the probe in the excited state to an energy-acceptor molecule.

I

I

Singlet excited states

A

\

crossing Absorption

Excited triplet state

Singlet ground state

FIG. 9.12 Molecular energy levels involved in photochemical processes. (From Ref. 244. Reproduced by permission of Prentice-Hall.)

418

Chapter 9

The luminescence methods involve solubilization of the probe in micelles and the determination of fluorescent spectra and fluorescence polarization. Various steady-state and transient-state fluorescence methods have been employed. Experimental details can be found in the literature [246-2501. A careful selection of the probe is essential for obtaining meaningful results. Luminescence methods are based on the assumption that the probe does not affect the fundamental nature of the solution and the micelles. This assumption must be validated for the system being studied. The fluorescence intensity of ammonium 1-anilinonaphthaline-8-sulfonate (ANS) in a solution of a hydrocarbon-type surfactant is constant below the cmc of the surfactant but increases linearly with increasing surfactant concentration above the cmc. The concentration dependence of fluorescence intensity indicates that the ANS probe is solubilized in the micelles of the hydrocarbon-type surfactant. In contrast, fluorinated surfactants do not solubilize ANS [251]. The ANS probe is therefore useful for investigating fluorinated surfactant and hydrocarbontype surfactant mixtures (see Section 7.3). Asakawa et al. [121] studied the micellar environment of mixed fluorinated surfactants and hydrocarbon-type surfactants by fluorescence intensities of ANS, auramine, and pyrene. The ANS fluorescence intensity is shown in Fig. 28 of Chapter 7 as a function of total surfactant concentration. The ANS fluorescence intensity increased with the increase in hydrocarbon-type surfactant (LiDS) concentration Because the ANS probe was not incorporated in LiFOS micelles, the fluorescence intensity increased very little with increasing fluorinated surfactant (LiFOS = lithium perfluorooctane sulfonate) concentration. Muto et al.[252] measured pyrene fluorescence lifetime roand the ratio Z1/13 of the intensities of the first vibronic and the third vibronic band of the monomelic pyrene. The pyrene fluorescence data revealed the existence of a single type of mixed nlicelle in solutions of LiDS-LiFOS, LiFOS-hexaoxyethylene glycol dodecyl ether, or LiFOS-octaoxyethylene glycol dodecyl ether mixtures. The lifetime and the intensity ratio of vibronic peaks have been usedto deternine the cmc of fluorinated surfactant micelles [253]. However, the solubility of pyrene in micelles of fluorinated surfactants is not adequate for determining the micelle aggregation number [253,254]. The 11/13ratio is very sensitive to the polarity of the medium sensed by the pyrene probe. Therefore, the pyrene fluorescence technique has been utilized for the characterization of adsorbed layers of hydrocarbon surfactants and fluorinated surfactants on alumina [255,256]. Asakawa et al. [257] used pyrene fluorescence to examine the coexistence of two types of micelles in solutions containing SPFO and DDS mixtures. Pyrene and a new quencher, 1,1,2,2-tetrahydroheptadecafluorodecylpyridini~mchloride (HFDePC), are separately solubilized into hydrocarbon-rich and fluorocarbonrich micelles, respectively. HFDePC quenches the fluorescence emission from

Analysis of Fluorinated Surfactants

419

pyrene in hydrocarbon-rich micelles but only barely in mixed micelles containing fluorocarbon and hydrocarbon surfactants. The pyrene fluorescence quenching method using a fluorocarbon quencher is an effective technique for investigating demicellization phenomena. Esumi et al. [258] studied adsolubilization of hexanol and heptafluorobutanol into the LiDS and LiFOS bilayers by measuring the steady-state emission of pyrene.

9.13 X-RAYPHOTOELECTRONSPECTROSCOPY X-ray photoelectron spectroscopy (XPS, commonly termed ESCA as an abbreviation for electron spectroscopy for chemical analysis) is eminently suited to the study of surfactant adsorption. The XPS method is highly sensitive to the surface composition and can characterize adsorbed surfactant layers without elaborate sample preparation. The theory and praxis of XPS have been reviewed in several monographs and journal articles [259-2651. The sample is placed into a chamber and positioned for analysis. The chamber is evacuated to a high vacuum of < IO” torr and the sample is irradiated with soft x-rays, usually from a MgK, (1253.6 eV) or AlK, (1486.6 eV) source. The x-ray irradiation generates photoelectrons which are emitted with kinetic energy, EK, governed by the energy of the exciting radiation, hu, and the binding energy, EB,of the electron: The work function 4 depends on the sample and the spectrometer used for measuring photoelectron emission. The binding energies of the electrons are characteristic of the element and the environment of the atom in the molecule. Hence, XPS can characterize the composition and the chemical state of the near-surface region. The XPS spectra are strongly affected by the orientation of the sample, the source, and the spectrometer. Almost all (about 95%) of the signal emerges from the distance 3h within the solid, where h is the inelastic mean free path of the electron, also called the attenuation length of the emerging electron. The sampling depth, d. of the subsurface analyzed by XPS is given by d = 3h sin cy

(8)

where cy is the exit angle of the emitted electron, relative to the sample surface. The mean free path depends on the kinetic energy of the photoelectron, which, in turn, is affected by the energy of the radiation source. The sampling depth has a maximum when cy = 90 and is usually below 50 A. Although the method is considered to be nondestructive, sample damage and evaporative losses have been of concern [266]. The fluorine-to-carbon pho-

Chapter 9

420

toelectron peak intensity ratios, F(ls)/C( 1s). have been found to decrease during an x-ray exposure of several minutes, depending on experimental conditions [2671. X-ray photoelectron spectroscopy can yield qualitative and quantitative information on adsorbed surfactant layers. The overlayer on the substrate decreases the intensity, Ze, of a photoelectron peak, originating from a component in the substrate, by a factor

where 8 is the thickness of the overlayer sampled, I, is the intensity of the photoelectron peak originating from the covered substrate, A, is the inelastic mean free path (IMFP) of the electron in the overlayer, and a is the electron takeoff angle relative to the sample surface [268] IMFP, the average distance a photoelectron travels before an inelastic collision, depends on the binding energy of the photoelectron and the composition of the sample. Angle-dependent XPS (variable takeoff angle) can confirm the surfactant overlayer thickness and determine the continuity of the surfactant overlayer [268] (Fig. 9.13). The effective sampling depth of XPS analysis is a function of the electron IMFP and the takeoff angle. By decreasing the takeoff angle, the signal intensity contributions to the photoelectron spectrum from the top surface region can be selectively enhanced. This relationship serves as the basis for angle-dependent depth profiling. The angle-dependent ratio of overlayer to

e'

I 4 t FIG.9.13 The principle of angle-dependent XPS, where A , is the IMFP of the electron being analyzed and 8 is the angle between the sample surface and the emitted electrons. (From Ref. 268. Reproduced by permission of the American Chemical Society.)

Analysis of Fluorinated Surfactants

421

substrate, R. can be calculated using a simplified expression given by Fadley [269]: Overlayer

( Substrate )

=

.[

(&)

-

'1

where K is a function of atom density, instrument response, the kinetic energies of the substrate and overlayer atoms within the measured levels, and the effective cross sections of the atoms. The effective overlayer thickness, r, is given by

where 6 is the actual overlayer thickness. The orientation of an adsorbed surfactant can be determined by measuring the intensity of the peak for an atom on one end of the surfactant molecule relative to the intensity of a peak for an atom on the opposite end. Fluorine bound to carbon in CF3(CFZ)tl- groups induces a chemical shift to a higher electron binding energy. The resulting peak is readily distinguished in the C( 1s) spectrum from the peak for the carbon in the nonfluorinated portion of the molecule. If the surfactant molecule prefers a certain orientation, one peak is enhanced relative to the other for a given takeoff angle [268]. Gerenser et al. [268,270] studied adsorption of Zonyl FSC, a cationic fluorinated surfactant, on Si02 and poly(ethy1ene terephthalate). Samples of adsorbed Zonyl FSC were irradiated with monochromatic AlK, x-rays. Use of a monochromatic x-ray source minimized radiation damage of the sample. The pressure in the spectrometer was typically 5 X torr. Gerenser et al. [268] found that the thickness of the adsorbed surfactant layer calculated by the angle-dependent method [Eq. (lo)] is always larger than the thickness value calculated by the substrate attenuation method [Eq. (1l)]. The discrepancy was explained by orientation of the surfactant. which affects only the angle-dependent method and has no effect on the attenuation method. The results of their study are discussed in Section 5.1. Batts [267] used XPS to study the surface chemistry of dried gelatin layers which contained a cationic fluorinated surfactant FC-134. The samples were irradiated by x-rays from a magnesium target (MgK,, 1253.6 eV). Conditions were chosen to attain a theoretical resolution of 1 eV and to minimize sample exposure. Errors caused by sample damage were minimized by recording the spectra for each takeoff angle and surfactant concentration on fresh surfaces. Contact-angle and XPS data indicated that the progressive adsorption of a fluorinated surfactant at the aqueous gelatin-air interface can be monitored by analyzing the corresponding dried layers. The surface chemical composition determined by XPS was in accord with wettability data (see Section 5.1).

Chapter 9

422

Claesson et al. [27 I] studied the adsorbed monolayers of a cationic, doublechained fluorinated surfactant on mica. The XPS spectrometer was equipped with an AlK, x-ray source. The known number of exchangeable potassium and sodium ions on the mica basal plane served as the internal standard for the quantitative determination of adsorbed surfactant. The surfactant oriented preferentially with both nitrogen atoms or only the quaternary ammonium group toward the surface, depending on the deposition method. Mitsuya [272] examined chemisorption of 11 -H-eicosafluoroundecanoic acid from hexane onto fluorine-terminated silicon wafers by XPS. The fluorinated acid was chosen as an adsorbate to distinguish the adsorbate from hydrocarbon contaminants and to minimize x-ray-induced sample damage by the chemical stability of the C-F bond.

9.14

ELECTROCHEMICALMETHODS

Electric Conductivity Electric conductivity provides highly useful information on the association of surfactants in solution. The conductivity is measured in a thermostated cell calibrated with a standard KC1 solution. Polarization is avoided by using alternating current or applying short pulses of opposing polarity. The conductance data are related to the surfactant concentration by one of the following graphic presentations: 1. The specific conductivity is plotted against the surfactant concentration [273-2761 (Fig. 9.14). or against the square root of the surfactant concentration [277]. 2. The first derivative of specific conductivity with respect to surfactant concentration is plotted against the surfactant concentration [275] (Fig. 9.15) or against its square root [276]. 3. The equivalent conductivity is plotted against the square root of surfactant concentration [272,278]. If an ionic surfactant is completely dissociated, the specific conductivity increases below the cnlc linearly with increasing surfactant concentration. The slope of the linear function is the sum of the individual ionic conductivities. Above the cnx, in anideal case the concentration of surfactant monomers and. consequently, the conductivity are constant. In a real system, the micelles are ionic and contribute to conductivity. Hence, the conductivity increases with increasing surfactant concentration but with a lower slope than below the cmc. The break in the conductivity curve indicates the cmc [279]. At concentrations well below the cmc, a decrease in equivalent conductance with increasing surfactant concentration indicates a formation of premicellar aggregates (dimers, trimers, etc.) [278].

Analysis of Fluorinated Surfactants

423

500-

DAPA

Surfactantconcentration

/ mmoi kg"

FIG. 9.14 Change of specific conductance with surfactant concentration (DAPA = dodecylammonium perfluoroacetate). (Reproduced with permission from Ref. 275. Copyright 0 by the American Chemical Society.) 100

A 80-

-

60-

%

3 4020-

0

2

4

6

8

10

Surfactant concentration / mmol kg" FIG.9.15 Change of AAlAC with total surfactant concentration. DAPP-DAPB mixture (DAPP-dodecylammonium perfluoropropionate, DAPB = dodecylammonium perfluorobutyrate). (Reproduced with permission from Ref. 275. Copyright 0 by the American Chemical Society.)

Chapter 9

424

Wurtz and Hoffmann [240] measured the conductivity of nonionic surfactants (ethoxylated perfluoroalkanols) in 20 mM sodium chloride. The conductivity decreased linearly with increasing concentration of surfactant. The decrease in conductivity was explained by the incorporation of ions in the vesicles formed by the surfactant. The ions in the vesicles can no longer contribute to the conductivity. The temperature dependence of electric conductivity can be used to determine the Krafft point (see Section 6.3).

Transient Electric Birefringence Electric birefringence can give useful information on the shape and size of micelles and fluorinated surfactant aggregates, especially when complemented by other physical methods [243.280-2901. Electric birefringence has been successfully used to determine the shape and size of polymers, polyelectrolytes, and surfactant micelles. Colloidal particles or molecular aggregates, which have a permanent dipole moment or are polarized anisotropically, orient in an electric field. The colloidal solution becomes optically anisotropic and exhibits electric birefringence, termed the Kerr effect [246,291,2921. An apparatus for electric birefringence measurements is shown schematically in Fig. 9.16 [288]. Rectangular high-voltage pulses of short duration are applied to the solution, and the buildup and decay of electric birefringence are measured. The beam of an He-Ne laser is polarized by a Glan prism set at 45" with

I "

L

P

A

HPlOOO

HV FIG.9.16 Apparatus for electric birefringence measurements. L, He-Ne laser; P, polarizer; KC, Kerr cell; A, analyzer; D, photodiode detector; V, amplifier; TR, transient recorder; 0, oscilloscope; HP, HPlOOO computer; HV, high-voltage pulse generator; TI, trigger impulse to start the recording system. (From Ref. 288. Reproduced by permission of the American Chemical Society.)

Analysis of Fluorinated Surfactants

425

respect to the electric field applied across the Kerr cell. The polarized light traverses the Kerr cell containing the sample and passes through the analyzer to the photodiode detector. The analyzer and the polarizer are in a crossed position. The signal of the detector is digitized by a recorder interfaced with a computer. The electric birefringence, An = ~ z l l- nL. is the difference in refractive indexes parallel and perpendicular to the direction of the applied electric field. Electric birefringence is related to the optical retardation or phase shift, 8, by the equation (12) where 1 is the path length of the Kerr cell and h is the vacuum wavelength [283]. The Kerr constant, B, is given by B=lim

E+O

A?? -

hE2

where E is the field strength. The Kerr constant depends on the temperature and the surfactant concentration (Fig. 9.17) [283]. Below the cmc, the surfactant does not exhibit birefringence and only the solvent birefringence is observed. The Kerr constant of ionic surfactants can assume positive or negative values, depending on temperature and the counterion [283]. The rise and decay of the electric birefringence of a fluorinated surfactant are shown in Fig. 9.18 [283]. The birefringence relaxation time, rB,is related to the rotational diffusion constant, DR: TB =

(6DR)-

'

(14)

The rotational diffusion constant, DR,is proportional to LP3,where L is the length of a rodlike micelle or the diameter of a disklike spheroid. Hence, rBvalues cannot distinguish between disks or rods, unless complemented with Kerr constant values or data obtained by other methods. Tamori et al. [290] used electric birefringence to estimate the micellar size and shape in mixed-surfactant solutions containing hexaoxyethylene glycol dodecy1 ether and lithium perfluorooctanesulfonate or lithium dodecyl sulfate. Wurtz and Hoffmann [240] estimated the radius of vesicles formed by ethoxylated perfluoroalkanols by measuring the decay of the electrical birefringence signal. The time dependence of birefringence is affected by intermicellar interaction, electrolytes, and polydispersity. If the aggregates are polydisperse, the time dependence of birefringence deviates from a single exponential relationship. The size distribution function must be known, because the deviation depends on the width of the distribution function [243]. In spite of these limitations, Shorr and

Chapter 9

426

0

1

2

3

L

S

6

7

8

9

1

0

FIG. 9.17 Kerr constant B of tetraethylammonium perfluorooctanesulfonate (FOSET) and tetramethylammonium perfluorooctanesulfonate (FOSMe) as a function of total concentration co for T = 20°C. (From Ref. 283. Reproduced by permission of the American Chemical Society.)

Hoffmann [283,288]conclude that electric birefringence measurements are useful for the determination of the dimensions of anisotropic surfactant aggregates. Although electric birefringence data alone are not sufficient to define the type of surfactant aggregates, the electric birefringence method complements other physical methods. 9.1 5

ULTRAFILTRATION

Ultrafiltrationtechniques [293-2951 have been used to separatesurfactant monomers from their micelles. Asakawa et al. [296] used the ultrafiltration

Analysis of Fluorinated Surfactants ~~

~

>

FOSMe c, =10mM

v)

E= G.510%’;

A

L z w Iz W

r:

t-

I

E

427

m

!-21

0 “-4.3 0

I

lo

I

2 0 3 0

1

G O 5 0

FIG. 9.18 The rise and decay of the electric birefringence of FOSMe. The duration of the applied electric field pulse is indicated with the arrow. (From Ref. 283. Reproduced by permission of the American Chemical Society.)

method to study fluorinated surfactant and hydrocarbon-type surfactant mixtures. The method is based on the assumption that surfactant monomers pass through an ultrafiltration membrane which has pores sufficiently small to prevent the passage of micelles. The membrane used (YC-05, Amicon Corp.) can exclude molecules having a molecular weight larger than 500. The surfactant solution was forced through the membrane by applying pressurized nitrogen gas. The concentrations of the filtrand and the filtrate were determined by HPLC or isotachophoresis. a high-resolution electrophoretic method [297]. Ultrafiltration experiments were conducted as a function of initial surfactant concentration. Below the cmc, the surfactant concentrations in the filtrand and filtrate were equal. Above the cmc, the surfactant concentration in the filtrate became constant and was equal to the cmc. in accord with the postulate that the filtrate contains monomeric surfactant molecules in equilibrium with micelles which did not pass the filter. 9.16 SURFACE TENSION It is perhaps needless to state that surface tension is the most important physical property of a surfactant to be determined. Methods for surface and interfacial tension measurement have been the subject of numerous papers and review articles [298-3 141. In spite of the apparent simplicity of surface tension measurement, correct and reproducible values are not always readily obtainable. In addition to the specific limitations of each technique, the time dependence of surface tension of surfactant solutions can be a major complication. Surface tension depends on the

428

Chapter 9

adsorption and orientation of molecules at the liquid-air interface. In pure liquids, only microseconds are needed for equilibrium orientation, whereas in surfactant solutions, hours or even days may be needed to attain equilibrium surfactantadsorption at afreshly created surface. Adsorption and orientation kinetics are especially critical when measuring the surface tension of surfactant mixtures. Surface tension methods measure either static or dynamic surface tension. Static methods measure surface tension at equilibrium, if sufficient time is allowed for the measurement, and characterize the system. Dynamic surface tension methods provide information on adsorption kinetics of surfactants at the air-liquid interface or at a liquid-liquid interface. Dynamic surface tension can be measured in a timescale ranging from a few milliseconds to several minutes [315]. However, a demarkation line between static and dynamic methods is not very sharp because surfactant adsorption kinetics can also affect the results obtained by static methods. It has been argued [316] that in many industrial processes, sufficient time is not available for the surfactant molecules to attain equilibrium. In such situations, dynamic surface tension, dependent on the rate of interface formation, is more meaningful than the equilibrium surface tension. For example, peaked alcohol ethoxylates, because they are more water soluble, do not lower surface tension under static conditions as much as the conventional alcohol ethoxylates. Under dynamic conditions, however, peaked ethoxylates are equally or more effective than conventional ethoxylates in lowering surface tension [3 171. Most techniques stretch the liquid-air surface at the moment of measurement. For example, the drop weight method [3 181 and the ring method [319-3221 stretch the surface during detachment. However, instruments are now available which measure surface tension without detaching the ring from the liquid (e.g.. the Quss Tensiometer K12). The surface tension methods measure a force, pressure, or drop size (volume, weight. or dimensions). Examples of methods which measure a force are the ringmethod [319-3221 andtheplatemethod[323,324].Capillaryheight [325-328,3361 and the maximum-bubble-pressure method [329-3361 measure pressure.Pendantdrop [328,337-3391 sessiledrop[328.340],dropvolume [341-3431, dropweight [3 18,336,344-3471. andspinningdropmethods [348-3501 measure the size or the dimensions of a drop. Special techniques [3 151 for measuring dynamic surface tension include the oscillating jet [35 I], dynamic drop volume [315.352], inclined plate [353], strip [354]. free falling [355], pulsed drop [356], dynamic maximum-bubble-pressure [3 16,331-334,357-3591, and dynamic capillary [3151 methods. Modern tensiometers are interfaced with computers to increase the accuracy of the measurement and obtain dynamic surface tension readings within short but accurately measured time intervals.

Analysis of Fluorinated Surfactants

429

The ring rnethod [3 19-3221 is one of the most frequently used techniques for surface tension measurement. A platinum ring, attached to a vertical wire, is placed horizontally into the liquid (Fig. 9.19). The force, P, needed to pull the ring through the interface is measured. If one assumes that the ring supports a cylinder of liquid, P

= 4ryR

(15)

where R is the radius of the ring. Actually, the liquid column lifted by the ring is not a cylinder (Fig. 9.19) and a correction factor [319-322,2601 is needed. The accuracy of surface tension values obtained by the ring method is limited by the accuracy of the correction factor. Lunkenheinler [361] considers Huh and Mason correction factors to be sufficiently accurate for a ring with a two-point suspension and suggests that the correction factors of Harkins and Jordan should not be used. To simplify calculations, modern tensiometers are calibrated to make the correction factor for water at 20°C equal to 1. Consequently, the deviation of the obtained value from the actual value is reduced. The number of papers being published on the ring method indicates, however, that this technique still has problems [322,361-3651. The ring method gives reproducible and accurate values only if certain precautions are taken [361,366]. The surface of the liquid should be sucked to remove impurities in the surface layer. Complete wetting of the ring by the liquid is a prerequisite. The ring should perpendicular to the suspension and planar. The ring should not be detached from the liquid during the measurement. Sufficient time should be allowed for the surfactant to attain equilibrium. The vessel should provide a liquid surface much larger than the area covered by the ring.

FIG.9.19 The ring method. (Reproduced by permission of Kruss USA.)

Chapter 9

430

It has been argued that the ring method is suitable only for measuring the surface tension of pure liquids. The applicability of the method for the measurement of surface tension of surfactants has been debated [328,367-3691. The wire loop method [370,371] is similar to the ring method. The Wilhelmy plate method [323,324], the sessile drop method [328.340]. and the capillary height method [325-3281 measure equilibrium surface tension, if sufficient time is allowed for the adsorption of surfactant molecules at the surface to attain the state of equilibrium. The Wilhelmy plate method measures the force exerted on a vertical plate partially immersed in the liquid (Fig. 9.20).If wetting of the plate is complete, the force, F , is proportional to the surface tension, y, and the circumference. L. of the plate: F

=

(16)

yL

The Wilhelmy plate method has the advantages of measuring strictly static surface tension and being less sensitive to vibrations of the vessel or a slight deformation of the plate. The prerequisite is complete wetting of the plate. indicated by a zero contact angle. Significant contact angles in the wetting of the plate by some liquid systems have been observed [372]. Wetting is facilitated by using a roughened plate or a platinized platinum plate from which the liquid does not recede. Tadros [373] used a glass plate for measuring the surface tension of fluorinated surfactants by the Wilhelmy method. Hirt et al. [316] used a

The sample contamer will be raised against the plateedge

When the plate has touched the liquid lifted the surface tension will position again. pull the plate into the sample

The plate will be up to zero-

FIG.9.20 The Wilhelmy plate method. (Reproduced by permission of Kruss USA.)

Analysis of Fluorinated Surfactants

431

FIG. 9.21 The pendant drop method. (Reproduced by permission of Kruss USA.)

platinum wire instead of a plate for surface tension measurements of fluorinated surfactants. The principle of the Wilhelmy method has also been enlployed for the measurement of wettability of single fibers [374]. The pendant drop method has been described by Andreas et al. [375] and others [328,337-3391. The apparatus is simple, but the technique requires skill for forming the drop and maintaining its size and shape during the measurement of the diameters d l and d2 (Fig. 9.31 j. The measurement is complicated by optical effects (diffraction and dispersion). The detrimental effects of vibration and evaporation have to be considered. A correction table has been compiled by Fordham [376]. The measurements of the pendant drop are usually determined from a photograph of the drop [339]. The laborious calculations have been greatly facilitated by using video equipment interfaced with a conlputer [339]. The drop volunw method [341-3431 requires only a buret or a syringe (Fig. 9.22). Either the volume required to form the drop, V. is measured or the number of drops formed by a measured volume of liquid is counted:

FIG. 9.22 The drop volume method. (Reproduced by permission of Kruss USA.)

Chapter 9

432

The dripping radius, I-, has to be equal to the capillary radius or be known. The method is not very precise and demands a careful manipulative skill. Semiautomatic [377] and automatic drop volume methods [378] have been developed. The calculation of interfacial tensions corrected for transport processes inside the growing drop is simplified by interfacing the tensiometer with a computer. The drop weight method measures the weight of a drop (or several drops) emerging from a capillary of known dimensions [318,336,344-3471. Slight vacuum is applied to the apparatus through a tubing until the drop, forming at the outlet of the capillary, assumes almost its full size. The drop is then allowed to detach itself from the capillary. Surface tension is calculated from the equation

.=pi. where 177 is the mass of the drop, g the acceleration due to gravity, Y the capillary radius, and F a factor dependent on the drop volume and I- [379]. The drop volume can be calculated from the drop weight and the density of the liquid. With proper corrections [380], the method is quite accurate [381,382]. ~ ~ ~ i ~ 7 l u ~ ~ l - b u b b l e - p Ymethod e s s l ~ I[3 - e13,316,329-3361 measures pressure in a bubble formed at the end of a capillary when a gas (e.g., air) is blown through the capillary into the liquid. The pressure increases when the bubble grows and attains its maximum value when the bubble has obtained the shape of a hemisphere (Fig. 9.23). The pressure decreases when the bubble grows further and finally bursts. Maximum-bubble-pressure methods have been compared [383] and equipment for automated surface tension determination by maximum-bubble-pressure measurement has been developed [384-3871.

2r

other positable forms of bubbles

Maximum-bubble-pressure method. (Reproduced by permission of Kruss USA.)

FIG. 9.23

Analysis of Fluorinated Surfactants

433

w 1I

measuring-microscope

outlet

w i n y

t

7il-Bath inlet

1

capilla tube

-

syringe

heavy Phase

\

Septum

Lighting FIG.9.24 Spinning drop tensiometer. (From Ref. 301. Reproduced by permission of Kruss USA.)

Dynamic surface tension measurements by Hirt et al. [316], based on the maximum-bubble-pressure method, revealed large differences between equilibrium and dynamic surface tension values of fluorinated surfactants (see Section 4.4). The surface tension transition from equilibrium values to dynamic diffusionlimited values depended on the surfactant type, concentration, and bubble generation rate. The spirznirzg drop method [348-3501 is used to determine interfacial tensions between two liquids. A capillary tube is mounted in a chamber leaving the ends open (Fig. 9.24). The chamber and the tube are filled with the heavier of the two liquids and the capillary is rotated at a high speed (about 2000 rpm). A drop of the other liquid having a lower density is placed into the capillary. The drop moves into the center of the capillary tube and usually assumes the shape of a cylinder with curved edges. The radius of the drop is measured using a camera or a microscope: y = fr3Ap o3

(19)

where r is the radius of the drop, Ap the density difference, and o the angular velocity. The accuracy of the method depends on the technique used for measuring the radius of the drop. A camera interfaced with a computer allows the image to be frozen for an accurate measurement [388,389].

Chapter 9

434 TABLE9.1 Examples of Surface TensionMethodsUsed Surfactants

Method Ring

Wire loop Plate

Pendant drop Drop volume Drop weight

Maximum bubble pressure

for Fluorinated

Author Bernett and Zisman (1959) Caporiccio et al. (1984) La Mesa and Sesta (1987) Glockner et al. (1989) Thoai (1977) Funasaki and Hada (1979) Tadros (1980) Jost et al. (1988) Jarvis and Zisman (1959) Motomura et al. (1989) Zhao Guo-Xi and Zhu Bu-Yao (1983) Shinoda and Nakayama (1963) Mukerjee and Handa (1981) Nishikido et al. (1989) Scholberg et al. (1953) Hirt et al. (1990)

Reference 390 126 39 1 392a 371 393 373 394 395 396,397 398,399 38 1 318,400 401 402 316

Examples of methods used to measure surface tensions of fluorinated surfactants are shown in Table 9.1.

9.17 FLUORINATEDSURFACTANTS IN BIOLOGICAL SYSTEMS The discovery of two types of fluorine, organic and inorganic, in human blood [403407] intensified the interest in the absorption and retention of fluorochemicals in biological systems. The conjecture that organic fluorine in blood originated from exposure to fluorinated surfactants prompted analyses of blood and other biological samples for fluorinated surfactants. A study of the exposure to fluorinated surfactants [408] found fluorine concentrations ranging from 1 to 71 ppnl in the blood of workers handling ammonium perfluorooctane. The methods used for the determination of fluorinated surfactants in biological samples can be divided into two groups: (1) the determination of organic fluorine, which represents the concentration of a fluorinated surfactant if other fluorochemicals are absent, and (2) a specific method for the fluorinated surfactant of interest. The determination of organic fluorine in biological samples, such as whole blood, serum, and plasma, involves destruction of organic matter by combustion

Analysis of Fluorinated Surfactants

435

or ashing to convert organic fluorine to inorganic fluoride and the determination of fluoride in the sample. The organic fluorine is calculated as the difference between fluoride found in the combusted sample minus inorganic fluoride present in the uncombusted sample. Ashing in the presence of magnesium carbonate [405], magnesium oxide [407], or calcium phosphate [409,410,4 1 11 leads to low results and has been superseded by combustion in a closed system with oxygen. Venkateswarlu [411] and Belisle and Hagen [15] have employed an oxygen Parr bomb for the determination of fluorine in whole blood or senlm. Because liquid blood cannot be combusted in an oxygen Parr bomb, Belisle and Hagen [15] removed water from blood i n I ? ~ C U Oat ambient temperature. The solids were pelletized after mixing with a preweighed amount of benzoic acid. The pellet was combusted in an oxygen Parr bomb which contained 0.025NNaOH to absorb fluoride. The reaction mixture was acidified with perchloric acid and fluoride extracted into In-xylene containing triethylsilanol. The fluoride, converted to triethylfluorosilane, was determined by gas chromatography. The method gives accurate results for perfluorooctanoic acid in blood, but volatile fluorochemicals in the sample evaporate together with water when the blood sample is dried. Kissa [lo] combusted biological samples, including whole blood, serum, and various organic tissues, in an oxyhydrogen flame. A 0.5-1.O-g sample of whole blood, serum, or plasma was introduced into the combustion apparatus. Hydrofluoric acid formed during the combustion is collected in water and determined with a fluoride ion-selective electrode [ 111. To calculate the organic fluorine concentration, the inorganic fluoride concentration of the sample was determined with ananalyte addition method using a fluoride ion-selective electrode [412] and deducted from the total fluorine concentration. The method has the advantage of being applicable to liquid samples without conversion of the liquid sample to a solid. The oxyhydrogen combustion method can determine volatile fluorochemicals which would belost if a liquid sample had to be dried. A large nutnber of samples of blood and organic tissues have been analyzed routinely by the oxyhydrogen combustion method in a DLIPont laboratory [ 101. The direct determination of fluorinated surfactants is possible if the fluorinated surfactant is amenable to chromatography or spectroscopy. Belisle and Hagen [46] determined perfluorooctanoic acid in blood, urine, and liver tissue. Perfluorooctanoic acid was extracted from blood or other biological samples with hexane in the presence of hydrochloric acid and converted to its methyl ester with diazomethane. The recovery of known amounts of perfluorooctanoic acid added to human plasma was essentially quantitative. The precision of the method was inferior to that of the determination of perfluorooctanoic acid by elemental fluorine analysis but could probably be improved by using a capillary chromatographic column instead of the packed column used by the authors.

Chapter 9

436

9.18

FLUORINATED SURFACTANTS IN THE ENVIRONMENT

The presence of fluorinated surfactants in the environment is of concern in air and in water or wastewater. Fluorinated surfactants can enter air as vapor if volatile or as a liquid or solid aerosol. To protect the health of workers, fluorinated surfactants have to be monitored in air at industrial sites where fluorinated surfactants are produced or used. The determination of fluorochemicals in air usually involves two steps: collection of an air sample and determination of the fluorochemical in the sample collected. Sampling techniques used have included grab sampling, concentration by cryogenic techniques, trapping in a solvent, and adsorption on a solid adsorbent. The latter method is convenient for collecting a sample. However, the common adsorbents hold fluorinated surfactants firmly and desorption of the collected species for its determination is difficult and frequently not quantitative. Kissa [413] therefore introduced the concept of the total organic fluorine concentration in air. The fluorinated surfactant in air is collected on a solid adsorbent, such as activated carbon, graphitized carbon, silica, or Tenax. The adsorbent is combusted in an oxyhydrogen torch. Hydrogen fluoride, formed by combustion of organic fluorine on the adsorbent, is collected in water and determined with a fluoride ionselective electrode [l 11. When only one fluorine-containing species is present in air. the fluorine content represents its concentration. When several fluorochemicals are present. the fluorine concentration indicates the maximum concentration a fluorinated surfactant may possibly have in the mixture of fluorochemical air contaminants. If this maximum concentration value indicated by the total fluorine concentration is below the acceptable limit, a specific analytical method is not needed. Fluorinated surfactants present in air as solid aerosols, such as dust, are collected on filters made of mixed cellulose esters and combusted together with the filter in the oxyhydrogen torch [413]. Volatile fluorochemicals can be collected on a solid adsorbent in an adsorption tube connected to the outlet of the filter. The deterinination of fluorinated surfactants in water and wastewater is essential for (1) the detection of pollution by fluorinated surfactants, (2) study of biodegradation. and (3) determining the effect of fluorinated surfactants on aquatic life. If a specific method is not needed, the oxyhydrogen combustion method is the most effective [lo]. By introducing a 10-mL water sample into the oxyhydrogen torch in several portions, as little as 20-40 ppb fluorinated surfactant can be detected without the need to concentrate the sample before combustion. When other fluorochemicals are present, the fluorinated surfactant has to be separated and determined by a specific analytical method. Some of the conventional methods for the analysis of hydrocarbon-type surfactants [ 11 are also

.

"

..

"

Analysis of Fluorinated Surfactants

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applicable for the determination of fluorinated surfactants (see Sections 9.3 and 9.4). However, when the fluorinated surfactant has to be identified and structural informationisneeded, mass spectrometry[414]isthemethod of choice. Schroder [56,88,89] employed a tandem mass spectrometer (MSMS) to analyze fluorinated surfactants in water and wastewater. HPLC was coupled by a thermospray interface to a MS/MS. A nonionic fluorinated surfactant (Fluowet OTN) was separated by reversed-phase HPLC using a 15-cn1, 3.9-mm inner diameter CI8column with 5-pm spheres. The gradient of the methanol-water eluent was from 80 : 20 to 20 : 80 in 10 min. Ammonium acetate (0.10 mol/L) was injected on line after separation. Alternatively, the chromatographic separation was bypassed and the analyte injected on line by the flow injection analysis method. An anionic fluorinated surfactant (Fluowet PL 80) and a cationic fluorinated surfactant (Fluowet L 3658-1) were quantitatively analyzed in water. However, analyses of these fluorinated surfactants in wastewater containing sludge were complicated by strong adsorption of the surfactants on the sludge. Extraction of the surfactants with acidified methanol was incomplete. The determination of the anionic surfactant (Fluowet PL 80) by combustion in an oxyhydrogen flame gave a quantitative result, although only 41% of the fluorine in the nonionic surfactant (Fluowet OTN) was recovered.

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10 Toxicology and Environmental Aspects

10.1 TOXICOLOGY

The toxicology of organic fluorine compounds is unusual in the sense that most fluorine compounds are harmless, whereas some are among the most toxic nonproteins known. The structural differences between a toxic and a nontoxic compound are usually not extensive [ 11. Therefore, organic fluorine compounds have to be treated with caution unless proven to be nontoxic by toxicological testing. The toxicity of organic fluorine compounds depends on their biochemical stability and the toxicity of metabolites. The carbon-fluorine bond is highly resistant to biochemical degradation and, as a consequence, perfluorocarbons are physiologically inactive. However, if a fluorochemical is metabolized, the toxicity of the metabolized fluorocompound depends on the structure of the metcfbolites. Monofluorinated alkanoic acids are toxic when the P-oxidation mechanism can produce monofluoroacetic acid [2]. Steric hindrance and branching of the alkane chain reduce toxicity (Table 10.1). Monofluoroacetic acid is one of the most toxic conlpounds known. Its toxicity is related to blocking the citric acid cycle by hindering the enzyme acotinase. Compounds which can readily be hydrolyzed or oxidized to fluoroacetic acids (e.g., esters of fluoroacetic acid and fluoroethanol) are also highly toxic. In addition to metabolism, chemical reactions, such as pyrolysis, can convert a nontoxic fluorine compounds to a toxic one. Pyrolysis of some organic fluorine compounds can produce perfluoroisobutylene. (CF3)2C=CF?, probably one of the most highly toxic fluorocompounds known [3]. The toxicity of intermediates used for synthesizing fluorinated surfactants also has to be considered. The toxicity of perfluoroalkylethyl iodides depends on 451

Chapter 10

452 TABLE 10.1 Toxicity of Monofluorocarboxylic Acids Compound F(CH2CH2)3CH2COOH CH3

I F(CH2CH2)3CHCOOH CH3CH2

0.64

1.1

I F(CH2CH2)3CHCOOH CH3CH2CH2

5.5

I F(CH&H2)3CHCOOH CHsCHCH3

75

I F(CH2CH2)3CHCOOH

100

Source: Ref. 1.

the length of the perfluoroalkyl group. The inhalation toxicity LC50 value of C2F5CH2CH21is 400-500 ppm, that of C I F ~ C H ~ C H is ~4000 I ppm [4]. The biochemical efects of fluorinated surfactants are not completely understood. Perfluorocarbons are used as oxygen carriers in blood without toxic effects [5,6].The toxicity of some fluorinated surfactants is so low that they have been tested in vivo as emulsifiers in blood substitutes and biomedical oxygen carriers (see Section 10.4). However, some fluorinated surfactants are considerably toxic although they are not metabolized or, if metabolized, produce presumably a nontoxic fluorocompound. A discussion of the toxicity of fluorinated surfactants has to distinguish between the toxicity of (1) a pure surfactant, ( 3 )a surfactant containing impurities, and (3) a commercial product consisting of a fluorinated surfactant and a solvent. The exposure to the fluorinated surfactant may be acute or chronic to small amounts for along period of time. Both systemic effects and local effects have to be considered. Systemic effects result from absorption of a fluorinated surfactants by oral intake, inhalation, or skin penetration. Local effects are observed at the location of contact with a surfactant. Irritation of skin, eye, or nasal mucous membranes and sensitization of skin are typical local effects. The intrinsic toxicity of some fluorinated surfactants has been related to their exceptionally high szrlfxe activio. The acute oral toxicity of tetraethylammonium perfluorooctanesulfonate has been reported to be considerable [LDSo= 190 mg/kg (Wistar rat)] although the surfactant does not irritate mucous membranes and is not a bactericide. At low concentrations, the surfactant is not toxic

Aspects Environmental and Toxicology

453

to fish. However, when the concentration of the surfactant i n water is increased, toxic effects appear at a concentration where the surface tension starts to drop markedly (see Aquatic Toxicity in Section 10.2) [7]. Surfactants are adsorbed on interfaces and interact with biological membranes, proteins, and enzymes [8]. Anionic surfactants form ionic adsorption complexes with proteins, whereas the interaction of nonionic surfactants is weak. This is in accord with the observed toxicity, which is much higher for anionic than for nonionic fluorinated surfactants. The lipophobicity of fluorinated surfactants also may contribute to their toxicity. It is plausible that the lipophobicity of fluorinated surfactants in conjunction with a high surface activity and consequent strong adsorption may interfere with normal functioning of cells. The interaction of fluorinated surfactants with cells is evidenced by the effect of fluorination on hemolytic activity of the surfactant. Fluorinated surfactants containing both fluorocarbon and hydrocarbon segments are less hemolytic than their hydrocarbon analogs. The hemolytic activity decreases with increasing fluorocarbonhydrocarbon ratio (see Section 10.4). Theundoubtedly conlplex mechanism of hemolysis is not fully understood [9]. It is usually believed that a hydrocarbon surfactant is adsorbed onto the erythrocyte membrane before it penetrates the cell and causes its disintegration. The weaker hemolytic activity of fluorinated surfactants has been explained by a low affinity of fluorocarbon chains for nonfluorinated materials and weak adhesion to the cells [9]. In contrast to this generally accepted view, it may be argued that the decrease in hemolytic activity is related not to weakadsorption but to strong adsorption. The fluorocarbon tails of the adsorbed fluorinated surfactant may render the blood cell lipophobic as well as hydrophobic and hinder penetration of the cell. Although the lipophobicity of the surfactant appears to be beneficial for reducing hemolytic activity, lipophobicity of fluorinated surfactants may be detrimental to normal diffusion processes in cell. Because most fluorinated surfactants are commercial products containing several components, the toxicity of impurities in fluorinated surfactants has to be considered. Commercial fluorinated surfactants are usually sold as solutions in an aqueous solvent [ 101. In some cases, the solvent may cause more systemic or local toxic effects than the surfactant itself. The solvent and volatile impurities may dominate the toxic effects produced by inhalation. Nonionic surfactants with a poly(oxyethy1ene) hydrophilic chain may contain 1.4-dioxane, which has shown carcinogenic activity in some animal tests. 1,4-Dioxane is a by-product found in nonionic surfactants, regardless of whether the surfactants are fluorinated. However, the concentration of 1,4-dioxane in nonionic surfactants is carefully controlled and is usually very low (about 0.1 % or less). Air monitoring has indicated that at a workplace where there are nonionic fluorinated surfactants containing about 0.1 9% dioxane. the 1,4-dioxane concentration in air would be below 1 ppm,

Chapter 10

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well below the acceptable concentration limit of 25 ppm, the ACGIH threshold limit value (TLV) [ 113. The acute toxicities of some commer-cia1fluorirlnted su?$xtnnts marketed as solutions in an aqueous solvent are given in Table 2 [ 101. A 10-dose oral subacute test in rats has been conducted for Zonyl TBS [ 1 11. Zonyl TBS was administered by intragastric intubation to three groups of male rats, 10 rats per group, 5 times a week for 2 weeks in repeated dose levels of either 10. 100, or 1000 mg/kg. Half ofthe rats were sacrificed after the last dose, the other half after a 14-day recovery period. For the 10- and 1OO-mg/kg doses, the mortality was 0/10. For the 1000-mg/kg dose, the mortality was 9/10; only one rat survived the recovery period. The organic fluorine content of blood was analyzed by combustion in the oxyhydrogen flame. Kissa [12] found the organofluorine content of blood to increase linearly with the square root of the dose level (Fig. 1 ). A deviation from linearity at the highest dose level may have been related to the high mortality rate. Substantial amounts of organofluorine remained in the blood after the 14-day recovery period, indicating a relatively slow elimination rate, typical of anionic fluorinated surfactants. The sorption and elimination kinetics of fluorine in blood for a subchronic inhalation study of Zonyl TBS are discussed in Section 10.3. Zonyl TBS has a low acute toxicity by inhalation. Groups of six male rats were exposed to dust atmospheres of Zonyl TBS in the free acid form for a single, 4-h period. All rats survived the 14-day postexposure period after the exposure to up to and including 2100 mg/m3 [ 1 11.

TABLE10.2 Toxicity of Zonyl Fluorinated Surfactants

Name Zonyl FSA Zonyl TBS Zonyl FSE Zonyl FSP Zonyl FSC Zonyl FSN Zonyl FSO Zonyl FSK

Structurea

Solvent

Acute oral (rat)

Anionic (25)(38) Anionic (33)(3) Anionic (14)(24) Anionic (35)(20) Cationic (50)(25) Nonionic (40)(30) Nonionic (50)(25) Amphoteric (47)(53)

Propanol-2 Acetic acid Ethylene glycol Propanol-2 Propanol-2 Propanol-2 Ethylene glycol Acetic acid

4.7 1.9 4.0 4.7 6.1 13.8 >I7 3.3

Skin (rabbit) 12.9 2.1 >22 12.9 22 12.9 >22 1.I

a The first number in parentheses indicates the percent solids; the second number indicates the weight percent of solvent in the product.The difference between the sumof the two numbers and 100 is the percent water in the product. Source: Ref. 10.

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Organic F in Blood (ppm F)

0

-

2

4

6

8

10

12

FIG.10.1 Concentration of organic fluorine in rat blood as a function of the dose in a 10-dose oral subacute test. (0)4-h recovery; (0)14-day recovery. (From Ref. 12.)

Zonyl TBS is a moderate skin irritant. In contrast, the anionic fluorinated surfactant Zonyl FSA with a -CH2CH2COOLi group is neither a skin irritant nor an eye irritant and is not a skin sensitizer. However, it is a good practice to avoid skin or eye contact with commercial-grade fluorinated surfactants and flush the contaminated skin area with water, should a contact occur. From a practical viewpoint, the important question is, how does the toxicity of fluorinated surfactants compare to that oftheir hydrocarbon analogs? A general statement cannot be made because the toxicity of fluorinated surfactants varies greatly with their structure. Some fluorinated surfactants derived from natural products, such as carbohydrates or lipids, are sufficiently biocompatible to be considered for intravascular use (see Section 10.4). Clearly, a fluorinated surfactant per se is not necessarily more toxic than a nonfluorinated surfactant. The toxicities of surfactants with corresponding structures have to be compared. Most fluorinated surfactants have been tested as commercial products containing a solvent. Toxicity data for pure, well-characterized fluorinated surfactants are scarce. The acute oral toxicities of solvent-free fluorinated surfactants are listed with those of hydrocarbon surfactants in Table 10.3. The acute oral toxicity of the nonionic fluorinated surfactants listed is low and comparable to that of their hydrocarbon analogs. However, the data suggest that anionic surfactants when ingested more toxic than their hydrocarbon counterparts. The tox-

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456

TABLE 10.3 Acute Oral Toxicity of Surfactants with a Fluorocarbon or Hydrocarbon Hydrophobe

Surfactant Nonionic 13.8 R~CH~CH~O(CH~CHPO)"H 17.0 RfCH2CH20(CH2CH20),H Alkyl polyoxyethylene ethers 0.87->25 Anionic 0.47, 0.54 Ammonium perfluorooctanoate C7F15COONH4 Sodium myristate >IO C13H2,COONa Tetraethylammonium perfluorooctanesulfonate 0.1 9 c8F17S03(C2H5)4 Sodium octanesulfonate 2.0 (mouse) C8H17SO3Na

11 11 13a,b

11, 13c,d 15 7

15

icity of sodium carboxylates (e.g., sodium myristate) is relatively low. In contrast, ammonium perfluorooctanoate is moderately toxic when ingested but toxic when inhaled, with a 4-h ALC (approximate lethal concentration) in the rat of 800 mg/m' [ 111. The local toxicity of anionic fluorinated surfactants depends on their structure, as can be expected. The surfactant FT-248, tetraethylammonium perfluorooctanesulfonate, is not a skin irritant or a eye irritant [ 7b]. Sodium perfluorooctanoate is a moderate skin and eye irritant [11,13c,d]. It is important to recognize that fluorinated surfactants are used only in small quantities, usually 10-100 times smaller than the amounts of hydrocarbontype surfactants. Most fluorinated surfactant applications are in industry, where the handling and disposal of fluorinated surfactants can be controlled. Hence, a consumer is exposed much less to fluorinated surfactants than to hydrocarbon surfactants. It is reasonable to conclude that the toxicity of most fluorinated surfactants should not prohibit their proper use. Under usual application conditions, an exposure to fluorinated surfactants can be kept well within acceptable limits. 10.2

ENVIRONMENTALASPECTS

Release to the Environment Fluorinated surfactants can affect the environment by their occurrence in air or water. Some fluorinated surfactants are sufficiently volatile to be present in air;

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some are introduced into the air as dust particles. It is believed that the presence of organic fluorine in blood (see Section 10.3) originates from the inhaled vapors or dust of fluorinated surfactants. Fluorinated surfactants are used in small quantities and, for this reason, do not constitute a heavy burden on theenvironment. The largest concentrations of fluorinated surfactantscan be found in the industrial environment. where they can be more readily contained. Fluorinatedsurfactants used infire-fightingfoams (see Chapter 8) are of recent environmental concern[ 141. The population at large is not exposed to fluorinated surfactants in the same way as to hydrocarbon-type surfactants. Unlike hydrocarbon-type surfactants, fluorinated surfactants are not used in household detergents. Hence, fluorinated surfactantsdo not come in frequent contact with the skin and are not inevitablyingested like small amounts of hydrocarbon-type Surfactants.

Aquatic Toxicity The aquatic toxicity of a chemical depends not only on its intrinsic toxicity but also on its biodegration rate. Chemicals which are toxic but are degraded at a rapid rate may not affect aquatic life. However, most fluorinated surfactants are biochemically stable and their aquatic toxicity is not reduced by degradation. The aquatic toxicity of fluorinated surfactants has been studied by Knaack and Walther [ 161. The biochemical oxygen demand of three fluorinated surfactants was measured by the Warburg method, using a mixed bacterial culture. The fluorinated surfactants tested were the following: (A) RfCF=CFS03Na, (B) R&F=CF(OCH2CH&OH.

where Rf = C6F13 to C9FI9,average CgF17 where Rf = C6F13 to CgFlg.average C8FI7;

(C) RfCON(CHzCH20H)2,

where Rf

11

=

=7

C7FI5

Fluorinated surfactant A in concentrations below 100 mg/L did not affect the oxygen consumption of the bacterial culture significantly. Above the concentration of 100 mg/L, the biochemical oxygen demand decreased drastically (Fig. 10.2). Fluorinated surfactants B and C were less toxic to the bacteria. The oxygen consumption was not inhibited until complete inhibition occurred at about 7500 mg/L (Fig. 10.3). Themedian inhibitory concentration values (ICs0) for bacterial cultures of the three fluorinated surfactants tested decreased in the order of increasing toxicity: C (6750 mg/L) < B (5650 mg/L) < A ( I 10 mg/L). The anionic fluorinated surfactant, like anionic hydrocarbon-type surfactants, exhibited higher antibacterial activity than the nonionic surfactants tested. The high ICso values of the two nonionic fluorinated surfactants indicate that these surfactants in normal use should not affect aquatic bacterial flora. Prescher et al. [17] studied the aquatic toxicities of four fluorinated surfactants to guppy (Poecilin reticulntn) and green algae (Monornphidium grifithii).

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g

100

a

E z

60

w

(3

> x 0

20

o 200 100 (mg/L) SURFACTANT CONCENTRATION FIG. 10.2 Biochemical oxygen demand of the anionic fluorinated surfactant A. (From Ref. 16.)

The structuresof the four fluorinated surfactantswere as follows:

(I) (IT) (111) (IV)

CF3(CF2),,CF=CF(OCH2CH2)70H. CF3(CF2),,CF=CFCF2S03Na. CF3(CF2),,CFHCOONa, CF3(CF2),,COONa.

where 11 where 17 where 17 where rz

=4

to 7 6 to 7 = 4 to 10 = 7 to 10 =

The median lethal concentration(LC50/96h) values for Poecilia reticulcrtcr were 88, 11, 9, and 4 mg/L, respectively. The IC50 values for Morzorcphidiunz

-

%

n

-

180

z

2

140

W

n w

100

(3

5t

60

20 0

1

2

3

4

5 i 6 i 7

8

9

CONCENTRATION (g/L) FIG.10.3 Biochemical oxygen demand of the nonionic fluorinated surfactants B and C. (From Ref. 16.)

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459

griffithii were 41 8, 42. 15, and 30 mg/L, respectively. The suggested maximum permissible concentrations are S O . 1,0.2, s0.05, and 0.05 mg/L, respectively. With the exception of fluorinated surfactant 111, the fluorinated surfactants tested are less toxic to algae than to fish. Fluorinated surfactant I is less toxic to algae and to fish than the con-esponding hydrocarbon-type nonionic surfactants (oxyethylated Iz-tetradecanol, with 10-15 EO (ethylene oxide) units). The noeffect concentrations and the IC50 values for fluorinated surfactant I1 and for its nonfluorinated counterpart (sodium pentadecanesulfonate) are about the same, but the LC5ovalue for the hydrocarbon-type surfactant is about 10 times lower. The aquatic toxicity of fluorinated surfactants to fish has been related to their high surface tension. The very sensitive orfes (Leuciscus idus r~~elcrmotus) have tolerated as much as 20 mg/L of tetraethylammonium perfluorooctanesulfonate for 70 h without noticeable effects. However, when the concentration of the surfactant was increased to a level where the surface tension started to drop markedly, toxic effects were immediately observed [7].

Nonaquatic Biological Activity The biological activity of nonionic fluorinated surfactants as herbicides, fungicides, and insecticides was studied by Sakakibara et al. [ 181. The nonionic fluorinated surfactants were derived from alcohols of the structure F(CF2CF2)3CH20H orH(CF2CF2)2-4CH20H by adding 1-19 oxyethyleneunits. The nonionic ethylenediamine derivatives had the structure CF3(CF2)7_,,6CONHCH2CH2NH?-o(CH2CH2OH)o.-?. The 18 nonionic fluorinated surfactants studied exhibited hardly any significant herbicidal activity. Foliar spray application had only weak effects on plant life and the herbicidal activity was even weaker for fluorinated surfactants with a longer oxyethylene chain. None of the surfactants tested were strong fungicides. Based on these results, the authors concluded that the fluorinated surfactants tested can be used safely as adjuvants, such as emulsifiers and dispersants, for agricultural chemicals. Some of the fluorinated surfactants tested were found to be insecticides, affecting especially the common housefly (Musca domesticn) and carmine mite (Tetrmzychzls cinrznbnrius). The mechanism of the insecticidal activity appeared to be suffocation of the insect, attributable to adhesion of the surfactant to the cuticle of the insect.

Biodegradation The oxygen demand curves obtained by Knaack and Walther [ 161 (Figs. 10.2 and 10.3) suggested that surfactants B and C were at least partially biodegraded. The

460

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oxygen demand for these surfactants exceeded loo%, suggesting that the nonfluorinated hydrophile of these surfactants was degraded. unless the oxygen consumption was caused by impurities. Because surfactant A consisted of a perfluorinated group and a sulfonate hydrophile, the surfactant can be expected to be nonbiodegrable. However, Knaack and Walther could not verify this experimentally. The high antibacterial activity of surfactant A permitted biodegradation tests only at low surfactant concentrations, where accuracy is somewhat uncertain. Prescher et al. [ 191 found no evidence of significant biodegradation. Their study included one nonionic and three anionic fluorinated surfactants (I, 11,111, and IV; see structures above). The conclusion was based on biological oxygen demand (BOD), KMn04 consumption, ignition loss. and dehydrogenase activity. Prescher et al. [19] classified the fluorinated surfactants as biochemically not degradable or difficult-to-degrade substances. Schroder [20] studied the biodegradation of an anionic, a cationic, and a nonionic surfactant. The anionic fluorinated surfactant, Fluowet PL 80, was found to be a mixture of a phosphonic acid, C12F3,2+lPO(OH)2, and a phosphinic acid, (C,IF3,2+ I)2PO(OH). Thephosphinic component was adsorbed on activated sludge, whereas the phosphonic component of the fluorinated surfactant remained in the aqueous phase. Biodegradation of the perfluoroalkane chain did not occur, and no metabolites were detected [20a]. The cationicsurfactantFluowet L 3658hasthestructure C12F2,2+l CHFCF2N+(CH2CH20H)3X-. In wastewater containing activated sludge, the cationic surfactant was adsorbed on the sludge. Deactivation of the sludge with sodium azide had no effect on the removal of the cationic surfactant from the aqueous phase, indicating that the sorption of the surfactant on the sludge did not involve biodegradation. In the absence of sludge, the cationic surfactant remained in the aqueous phase, but biodegradation was not evident [20a]. Schroder [20b] used high-performance liquid chromatography (HPLC)/ mass spectrometric (MS)/MS with a thermospray interface [~OC] to detect, identify, and quantify metabolites of Fluowet OTN, a nonionic fluorinated surfactant with the structure R,2F2,2+ CH2CH2(0CH2CH2),,,0H. The biodegradation was limited to the poly(oxyethy1ene) hydrophile [20b]. The absence of fluoride ions indicated that the perfluorocarbon chain was not degraded. The biodegradation of FC-17 1, a nonionic fluorinated surfactant, was measured by the modified I S 0 Standard Aerobic Shake Flask Test [21a]. The test uses nutrient salt in a brine (seawater) solution in which the fluorinated surfactant is the only organic solute. No significant biodegradation was observed after 9 days, but after 57 days, dissolved organic carbon (DOC) was reduced from 14.5 to 6.0 mgL. Because the perfluoro chain was probably not degraded, the results indicate a 90% degradation of the oxyethylated part of the surfactant molecule. A study by Keyet al. [ 2 1b,21 c] concluded that perfluorooctanesulfonic acid is resistant to biodegradation. However, the partially fluorinated octanesulfonic

Toxicology andAspects Environmental

461

acid, C6F13C2H3S03H,was partially degraded by a Pseudomonad under aerobic and sulfur-limiting conditions, yielding volatile fluorinated compounds.

Removal Becausefluorinatedsurfactantsarenotbiodegradable,theirremovalfrom wastewater is an important practical problem. The results obtained by Schroder [20a] indicate that ionic fluorinated surfactants are strongly adsorbed on sludges and sediments and their concentrations in wastewater are probably low. Prescher et al. [ 221 investigated the removal of four fluorinated surfactants by chemical and physical means. The structures of the fluorinated surfactants were the same as in their previous study [ 171. The study showed that the nonionic surfactant can be removed by flocculation with aluminum oxychloride, by chlorination, or by adsorption on activated carbon. For the anionic fluorinated surfactant, adsorption on activated carbon or on a resin, such as Wofatit EA60. is the most effective removal method. A treatment of synthetic sewage containing 6.0 mg/L FC-17 1, a nonionic fluorinated surfactant, with activated sludge reduced the toxicity of the surfactant to the water flea (Daylmin magna). The 48-h LCso was increased from 0.26 to 2.4 mg/L (14 days of continuous operation before sampling). Staude et al. [23] evaluated the removal of tetraethylammonium perfluorooctanesulfonate from rinse solutions of the electrochemical plating industry by hyperfiltration. Fluorinated surfactants have unique properties and are therefore indispensable. A potential effect on the environment can bereduced by (1 ) using synergism with hydrocarbon-type surfactants to minimize the concentration of fluorinated surfactant where feasible and (2) removing fluorinated surfactants from wastewater atindustrialsites by adsorptionorconvertingthesurfactant by partial biodegradation to physiologically inert substances.

10.3 PHYSIOLOGY:SORPTION, METABOLISM,AND EXCRETION Taves et al. [24,25] observed that human blood serum contains organic fluorine, in addition to inorganic fluoride. The presence of organic fluorine was related to fluorinated surfactants which are significantly volatile and absorbed in the body by inhalation. Perfluorooctanoic acid is known to be adsorbed on protein in blood [26,27]. The blood of workers handling ammonium perfluorooctanoate was found to contain from 1 to 71 ppm organic fluorine [28]. Thelong retention times of fluorinated surfactants in the body have inspired efforts to minimize exposure to fluorinated surfactants by reducing their volatility. However, the elimination rate of fluorinated surfactants from the body by expiration is also related to volatility.

462

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Hence, the main benefit of using less volatile fluorinated surfactants may be a decrease in sorption but not necessarily a shorter retention time of organic fluorine. It should be kept in mind that fluorinated surfactants are not the only fluorine-containing substances which may be present in air. The sorption and elimination kinetics of a fluorinated surfactant in blood has been investigated by Kissa and Kinney [ 291. An inhalation subchronic study on Zonyl TBS, a partially neutralized ammoniunl salt of telomer sulfonic acids, used rats as the test animals. Airborne Zonyl TBS was formed using two-stage dust generators. Zonyl TBS powder was agitated in the reservoirs of the generator and swept through the elutriators into the rat exposure chambers. The total fluorine content of rat blood was determined by combustion in an oxyhydrogen torch [30-321 and analysis of the combusted analyte by the fluoride ion-selective electrode [33]. Inorganic fluoride in blood was determined by an analyte addition method using a fluoride ion-selective electrode [34].

PPm F in Blood

500L 200

100 -

50 20

-

10 -

5-

21

2

5

10

20

50 100 200

500 1,000

c, (mg/m3) FIG.10.4 Organofluorine concentration in rat blood as a logarithmic function of fluorinated surfactant (Zonyl TBS) concentration in air inhaled during exposure; 0, 2, and 6 weeks of postexposure time. (From Ref. 29. Reproduced by permission of Carl Hanser Verlag.)

463

Toxicology and Aspects Environmental

The concentration of organic fluorine in blood, Cb. increased exponentially with increasing concentration, C,, of the fluorinated surfactant in the air inhaled (Fig. 10.4):

where K, is the sorption coefficient. The value of 11 was found to bein the 0.5-0.6 range. A plot of the residual organic fluorine concentration in blood versus the concentration of the fluorinated surfactant in air is reasonably linear (Fig. 10.5). The elimination of fluorocompounds from the body is usually assumed to be a first-order kinetic process. The half-life of a fluorocompound in the blood or body is used to characterize retention [5,35,36]. However, the first-order elimination plot for Zonyl TBS exhibited curvature, suggesting a departure from uncomplicated kinetics (Fig. 10.6). The elimination of fluorinated surfactants from blood can be viewed as a first-order process with a postexposure time, t, dependent rate constant, K,: log (G) LO = K,t , -

\

PPm F in Blood 150

.A

/

100

50

15

10 0

5

20

(mg/m3)0-5

FIG.10.5 Organofluorine concentration in rat blood versus the square root of fluorinated surfactant (Zonyl TBS) concentration in air inhaled during exposure; 0, 2, and 6 weeks of postexposure time. (From Ref. 29. Reproduced by permission of Carl Hanser Verlag.)

Chapter 10

464

PPm F

in Blood

(Log Scale)

0.1 I 0

I

I

2

4

I

1

6 128 Time (Weeks)

I

I

10

FIG.10.6 A first-order plot of fluorinated surfactant (Zonyl TBS) elimination from blood. (From Ref. 29. Reproduced by permission of Carl Hanser Verlag.)

where

and K, is the elimination rate constant for the first-order process (Fig. 7). Kissa and Kinney explained the deviation from simple first-order kinetics by adsorptive site-dependent variation of adsorptive bonding, heterogeneity of the telomer-derived fluorinated surfactant, and involvement of other organs. Perfluorooctanoic acid adsorbs on protein in blood [26]. It is is reasonable

Toxicology and Environmental Aspects

465

ppm F in Blood 200

100

50

20

10

5

2

I

I

I

I

I

1

0.5

1.0

1.5

2.0

2.5

3.0

Ji-

(Week 0 . 5 )

FIG.10.7 A plot of the logarithm of organofluorine concentration in rat blood versus the square root of postexposure time. (From Ref. 29. Reproduced by permission of Carl Hanser Verlag.)

to assume that Zonyl TBS, a partially neutralized fluorinated alkanesulfonic acid, may also be adsorbed on protein in the blood. The strength of the adsorptive bond may depend on the particular adsorption, site and the less strongly held fluorinated surfactant molecules may be eliminated faster than the more strongly held ones. The elimination kinetics may be complicated by the heterogeneous composition of the fluorinated surfactant. Zonyl TBS is a mixture of fluorinated surfactants differing in their chain length and, conceivably, in their diffusion rate. The elimination kinetics of fluorinated surfactants from blood are also complicated by

Chapter 10

466

the involvement of other organs, mainly the liver and the spleen, in the storage and transport of the fluorinated surfactant. Although the persistent adsorption of fluorinated surfactants had been known for over 20 years without any observed adverse health effects, the Minnesota Mining and Manufacturing Company (3M) has phased out Scotchgard products and Zonyl surfactants derived from perfluorooctanesulfonate (PFOS). This decision was influenced by an animal test using PFOS. Female rats were dosed with a relatively high dose (1.6 mg/kg/day) of PFOS during their pregnancy. More than one-third of the pups died within 4 days of birth. No mortality was observed at a lower level of dosage. It is uncertain that these results are relevant to human beings. Nevertheless, the 3M Company decided to withdraw products based on PFOS [14b]. MacNicol and Robertson [37] have cautioned that perfluorochemicals cannot be assumed to be inert in biological systems. Kissa and Kinney [29] found, however, that the presence of fluorinated surfactant-related organic fluorine in blood does not affect the inorganic fluoride concentration significantly (Table 10.4). The increase in inorganic fluoride concentration in blood was barely significant, even when the organic fluorine concentration in blood exceeded 2000 mg/L after an exposure to very high fluorinated surfactant concentrations in air. The very slightly elevated inorganic fluoride concentration in blood returned to its initial level shortly after the exposure. Biological inertness and biocompatibility are essential when considering the use of fluorinated surfactants in biomedical oxygen-caving emulsions (Section 10.4).

TABLE10.4

Inorganic (Ionic) Fluoride in Rat Blood

Exposure: Zonyl TBSA concentration in air (mg/m3) 0 0 0 0 480 480 480 480

Postexposure time (PPm) (weeks)

Organofluorine concentration in blood (PPm)

Inorganic F concentration in blood

1.4 0.7 0.7 1.I 112 140 168 41

0.016 0.007 0.025 0.019 0.034 0.055 0.063 0.020

Source: Ref. 29. Reproduced by permission of Carl Hanser Verlag.

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467

10.4 FLUOROCHEMICALEMULSIONSFORBIOMEDICAL OXYGEN TRANSPORT

Blood Substitutes and Biomedical OxygenCarriers Transfusion with whole blood has several limitations and problems [38]. The storage stability of donated whole blood is limited. Blood must be stored at 4°C or frozen, but even under favorable conditions, the lifetime of refrigerated blood is only 5-6 weeks. Therefore, a blood reserve must be replenished continuously. It is not practical to build up a large blood reserve for an unforeseeable natural disaster or war because most of the stored blood would have to be discarded from time to time. The transfusion with whole donated blood is complicated by the intergroup incompatibility of red blood cells. About 20 antigenic systems have been identified and are considered for blood-type matching. Cross-matching of blood types requires time, but hypovolemia must be treated as rapidly as possibly because the time a patient remains hypovolemic is critical for recovery. The incompatibility of blood groups complicates the supply and storage of donated blood as well. In a large-scale emergency situation, such as a natural disaster or war, large volumes of blood may be needed and shortages of particular donor blood types may develop. A significant number of patients has so many antibodies that they cannot receive foreign red cells [ 381. An additional problem with transfusions of donated blood is the potential risk of transmitting bacterial or viral diseases, such as hepatitis or the human immunodeficiency (HIV) virus. Blood donors have to be carefully screened for their past and present diseases to reduce the risk of disease transmittal. Blood substitutes are needed for patients who refuse blood transfusion for religious reasons, in spite of their critical condition. Because of the limitations and complications with transfusions of whole blood, a biocompatible synthetic blood substitute is an important research objective. The blood substitutes, used to supplement blood, can be divided into two classes: (1) plasma substitutes used to restore the blood volume and (2) oxygen carriers which not only can correct blood volume deficits but transport respiratory gases as well. Although hypovolemia is usually a life-threatening condition caused by a massive hemorrhage, a supply of oxygen is essential in case of continuing hemorrhage. The administration of a colloidal blood plasma substitute can restore the normal blood volume but is insufficient when oxygen supply has to be restored as well. Oxygen-carrying resuscitation fluids have therefore been developed. Hemoglobin solutions [38] and emulsions of fluorocarbons [5,6,38-44] have been investigated. There are differences and similarities between the two categories of blood substitutes. Hemoglobin or synthetic metal chelates combine oxygen chemically.

Chapter 10

468

I I

NORMAL BLOOD

; I

W

10

20

30

40

50 60 70 PO, (kPa)

80

90 100

FIG.10.8 The oxygen content of normal human blood and Fluosol-DA 20% as a function of oxygen partial pressure. (From Ref. 61. Reproduced by permission of Ellis Horwood Ltd.)

whereasfluorocarbonsdissolveoxygenwithoutachemicalreaction.Both hemoglobin and perfluorocarbons can transport oxygen to the tissues. but blood substitutes derived from hemoglobin or modified hemoglobin have oxygen sorption and desorption characteristics of whole blood [28]. However, blood substitutes based on modified hemoglobins are mainly of interest for acute blood volume restoration, because of their relatively short residence times in blood [45]. Fluorochemical blood substitutes differ from blood in several important aspects. Oxygen uptake in fluorochemical emulsions increases linearly with the partial oxygen pressure p02, unlike the S-shaped oxyhemoglobin dissociation curve (Fig. 10.8) [42]. Desorption of oxygen is rapid, because oxygen is not chemically bonded but is dissolved in fluorochemical emulsions. Fluorochemical particles in properly prepared emulsions have a diameter smaller than 0.3 pm, about 70 times smaller than red blood cells. The fluorochemical particles can pass through fine capillaries and emboli [46-491, which are impermeable for the much larger red blood cells. Because of the small particle size. the surface area of the fluorochemical particles in the emulsion is large. The rapid desorption of oxygen from the emulsion and the large surface area of the fluorochemical particles facilitate the delivery of oxygen to tissues [50]. Therefore, fluorochemical emulsions are of considerable interest for cytotoxic treatments and diagnostic procedures. Development of more concentrated injectable perfluorochemical emulsions has extended their diagnostic and therapeutic applications in medicine. Fluoro-

469

Aspects Environmental and Toxicology

chemical oxygen carriers have a considerable potential for treating myocardial and cerebral ischemia. Fluorochemical emulsions enhance the effectiveness of radiation therapy of cancer by delivering oxygen to ischemic areas of tumors [5 11. Fluorochemical oxygen carriers containing bromine are radiopaque. 1-Bromoperfluorooctane has been tested as a contrast agent in x-raytomography and for magnetic resonance imaging (MRI) [52-561. A very promising potential application of fluorochemical emulsions is in the perfusion and preservation of donated organs [ 3937-611.

Fluorochemical OxygenCarriers Fluorochemicals (Table 10.5) are attractive as biomedical oxygen carriers for several reasons [5,6,39,42,43]. Perfluorocarbons are inert, available in large quantities, and pathogen free. The solubility of oxygen and carbon dioxide in fluorochemicals is related to weak intermolecular forces. Within a homolog series, the solubilities of oxygen and carbon dioxide decrease with increasing molecular weight and molal volume of the perfluorocarbon (Fig. 10.9) [42,62]. For a given molecular weight, the linear fluorocarbons dissolve more oxygen than fluorocar-

TABLE10.5 Examples of Fluorocarbons Used in Biomedical Oxygen-Carrying Emulsions

N(C4FQ13 Mol. wt. 462 F-decalln (FDC)I PPSI

N(C3F713

Mol.wt. 521 F-tripropylamine (mPA)

Mol. wt 671 F-tributylamine (FTW I FC-43

FluOSOl-DA : FDCIFI'PA70130

mN\ CH3

Mol. wt. 495 F-N-methylisoquinoline (FMIQI

C8F17Br Mol. wt. 409

F-octylbromide (PFOB)

@(3"CH3

Mol. wt. 595 F-N-(4-methylcyclohexyl)pipelidlne

C~FQCHPCHC~F~

CeFlsCH=CHCeF1:,

Mol. wt. 464 bls-(F-buty1)ethene(F-44E)

Mol. wt. 664 bis(F-hcqW,hene (F-66E)

Note: Examples given as molecular weight (code name) ]trade name[. Source: Ref. 42. Reproduced by permission of Ellis Horwood Ltd.

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470

60

MW

RELEVANT

37°C

0Itnear (6F- nn’E)

-e

.jQ

50

40

MW

FIG.10.9 Solubility of oxygen (a) and carbon dioxide (b) in various fluorocarbons tested for biomedical use. (From Ref. 42. Reproduced by permission of Ellis Horwood Ltd.)

Toxicology andAspects Environmental

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bons with a cyclic structure. This is contrary to earlier beliefs that cyclic fluorocarbons are to be preferred. The solubility of oxygen in dilute emulsions parallels the solubility of oxygen in neat fluorocarbons [63]. Perfluorocarbons dissolve oxygen, about 30-50 mL 02/100 mL at 760 torr and 37°C. However, solubility of oxygen in the commercial perfluorocarbon emulsions is only about 0.8-8 mL 02/1 00 mL [64,65]. The concentration of oxygen in a perfluorocarbon emulsion is much lower than the oxygen concentration in saturated arterial blood [66]. The saturation concentration of oxygen in blood with a normal hemoglobin content is about 20 n L 02/100 niL at about 120 torr. At this pressure, Fluosol-DA, fluorocarbon-water emulsion, can dissolve about 1.2 mL O&OO mL. Although Fluosol-DA can transport less oxygen than whole blood, the solubility of oxygen in Fluosol-DA is about three times higher than that in plasma [61]. The delivery of oxygen to tissues depends, in addition to the solubility of oxygen in the carrier, on the release rate of oxygen from the carrier. Perfluorocarbon emulsions release oxygen more readily than blood because fluorocarbon emulsions, unlike blood, dissolve oxygen without a chemical reaction. Hence, perfluorocarbon emulsions can deliver substantial amounts of oxygen to tissues if the patient inhales supplementary oxygen [67]. In order to be suitable for in vivo application, fluorochemical oxygen carriers have to meet several stringent specifications. The solubility of oxygen, chemical and biochemical inertness, consistent availability in high purity, nontoxicity, a sufficient retention time in the circulating blood, a reasonably fast excretion from the body, and a structure favorable for emulsion stability are the most important and desirable properties. The excretion of the fluorochemical from the body occurs mainly by expiration through the lung [68,69]. The retention time of the perfluorocarbon in blood depends on the in vivo stability of the perfluorocarbop in blood depends on the in vivo stability of the perfluorocarbon particles in circulation [70]. Tsuda etal. [7 1] have proposed that particles of a perfluorocarbon emulsion in the vascular system are deposited in tissues of the reticuloendothelial system, such as in liver and spleen, where they are stripped of their surfactant and moved through the cell membranes to blood vessels and adipose tissues. The diffusion rate of the perfluorocarbon across cell metnbranes is determined by the lipophilicity of the perfluorocarbon [71] and the stability of the emulsion [70]. Lipoproteins in blood transport the perfluorocarbons to the lung, where they are excreted into the expired air. The volatility of the fluorochemical is therefore important. The logarithm of the rate constant for expiratory elimination is inversely related to the boiling point of the fluorocarbon [35] (Fig. 10) and increases with increasing vapor pressure of the fluorocarbon (Fig. 1 1) [42,72]. Because the volatility of a compound depends not only on the size of the molecule but also on the shape, it was originally believed that cyclic perfluorochemicals would be mostsuitable for blood substitutes. Riess and Le Blanc [42] have argued.

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0

1. FC43

2.FMD 3. FDEA 4. FTC 5.FBA 6. FDC

100

120 140 160 180 BOILING POINT "C

200

FIG.10.10 The logarithm of rate constant of the expiratory elimination rate versus the boiling point of the fluorocarbon in rats given 4 g/kg of fluorocarbon intravenously. (From Ref. 72.)

however, that the molecular weight, not the molecular shape, is the dominant factor in determining the retention time of the fluorochemical in the body. Unfortunately, the retention time of the fluorochemical oxygen carrier in the body and the stability of its emulsion are conversely related [42]. Both the excretion rate and emulsion stability depend on the vapor pressure of the fluorocarbon. Excretion by exhalation through the lungs is facilitated by the volatility of the fluorocarbon, but a higher vapor pressure enhances molecular diffusion and increases emulsion instability. Volatility and excretion require a low molecular weight, but for emulsion stability, a larger molecule is needed [42]. Increasing the molecular weight decreases the volatility of the fluorochemical but increases emulsion stability. The effect of a fluorochemical on the stability of its emulsion is believed to depend largely on its molecular weight [42]. The volatility and stability requirements limit the molecular weight of fluorochemicals suitable for intravascular use to a range between 460 and 520 [42]. Fluorochemicals of molecular weight 460 are too volatile and may cause embolism, whereas fluorochemicals of molecular weight

Toxicology and Environmental Aspects

0.1’ 0

1

I

5

10

473

15

VAPOR PRESSURE AT 37°C (mm Hg)

FIG.10.11 The logarithm of rate constant of the expiratory elimination rate versus the vapor pressure of the fluorocarbon. Symbols are the same as in Fig. 10.10. (From Ref. 72.)

above 520 have too long retention times in organs. Examples of fluorochemicals used for preparing oxygen-carrying emulsions are shown in Table 10.5. Perfluorodecalin (FDC), used in the first generation of fluorochemical blood substitutes, has a half-retention time in organs of only 6 days, but the stability of its emulsions is insufficient [5].Perfluorotripropylamine is added to perfluorodecalin in Fluosol-DA to increase emulsion stability. However, the retentiontime of perfluorotripropylamine in the body is longer than that of perfluorodecalin. Fluosol-DA is stored frozen and diluted with two aqueous solutions containing electrolytes and additives. The emulsions of perfluorotributylamine (Fluosol 43) are more stable and can be stored refrigerated at 1-10°C [42,43], but the retention time in organs is very long [69,73]. Perfluorotributylamine is eliminated from blood, probably by storage in tissues. At a dose of 4 g k g in rats, the half-life of perfluorotri-rz-butylamine in the body has been estimated to be 900 days. 1-BromoperfIuorooctane (perfluorooctyl bromide, PFOB) has several advantages over other oxygen carriers in a blood substitute [56,62]: (1) The residence time in tissues is relatively short [74,75]. The residence time of l-bromoperfluorooctane is shorter than expected from the relationship between retention times and molecular weights [42,76]. (2) The bromine substitution increases the solubility of oxygen in a perfluoroalkane [62,77]. (3) The bromine atom permits a convenient determination of 1-bromoperfluorooctane in tissues by neutron acti-

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vation. (4) 1-Bromoperfluorooctane is readily available in pure form via l-iodoperfluorooctane by telomerization of tetrafluoroethylene, or as a mixture of normal and isoperfluorooctyl bromide by electrochemical fluorination. ( 5 ) 1-Bromoperfluorooctane forms stable highly concentrated emulsions [ 781. (6) 1-Bromoperfluorooctane is radiopaque [52-561 and is useful for diagnostic applications [55.56,78-841. 1-Bromoperfluorooctane emulsified with egg-yolk phospholipids as a surfactant did not exhibit any toxic effects in the liver, spleen, and bone marrow [ S I . Bis(perfluor0-n-buty1)ethene (F-44E) can be obtained from pure products, n-butyl iodide and ethylene, by a two-step process [6,42,86]. The double bond of the molecule has raised the question of chemical stability, but in biological conditions, no evidence of chemical reactions has been found. Animal tests have indicated satisfactory biocompatibility. Bis(perfluorohexy1)ethane (F-66E), however, has caused some toxic effects [85]. The intravascular use of fluorochemicals demands the absence of toxicity and carcinogenic, mutagenic. or teratogenic effects. The toxicity of fluorocarbons is largely affected by their purity. Most fluorocarbons are toxic, unless carefully purified. Tissue culture assays are needed to test for toxicity before a fluorochemical can be considered for biomedical application. Perfluorodecalin and perfluorotripropylamine have been used clinically [35,41,87-891.

Fluorochemical Emulsions Perfluorochemical oxygen carriers are not soluble in water. Therefore, perfluorinated chemicals cannot be administered in the pure form but have to be converted to an aqueous emulsion. A surfactant, selected for its effectiveness and biochemical compatibility, serves as an emulsifier. Osmolarity and oncotic pressures are adjusted by adding electrolytes and oncotic agents, such as hydroxyethyl starch. Nutrients, thrombolytic agents. therapeutic agents, and other additives may be included in the e,mulsion, depending on the particular clinical application of the emulsion. The first-generation oxygen carriers Fluosol-DA and Fluosol43 are fluorochemical emulsions manufactured by the Green Cross Corporation in Japan, Ftorosan has been made in the Soviet Union (replaced by Perftoran). and Emulsion No. 2 in China. Fluosol-DA 2096, the first of commercial perfluorochenlical blood substitutes, contains (TOw/v) perfluorodecalin (14.0), perfluorotripropylamine (6.0), P h o n i c F-68 (2.7), yolk phospholipids (lecithin) (0.4), potassium oleate (0.032), glycerol (0.8), hydroxyethylstarch (3.0), NaCl (0.60), KC1 (0.034),MgCl? (0.020). CaC12 (0.028), NaHC03 (0.210). and glucose (0.180) [41]. Fluosol-DA has been tested clinically as an oxygen-carrying blood substitute [89] and has been approved by the Food and Drug Administration for supplying the myocardium with oxygen during percutaneous transluminal coronary angioplasty.

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Fluosol 43 [73] contains 20% (wh) perfluoro-tri-12-butylamine as the oxygen carrier, and 2.56% Pluronic F-68 as the emulsifier; the other additives have the same composition as in Fluosol-DA. Riess and Le Blanc [90] have advised against clinical use of Fluosol 43 because of the long retention time of perfluorotri-n-butylamine in the body. An improved oxygen carrier, the F-44E emulsion (Therox) [91,92]. contains 24% ( v h ) [42% (w/v)] F44-E, egg-yolk lecithin as the emulsifier, and an isotonic buffer. The half-life of the emulsion in blood is about 2.5 h and the dwell time of F-44E in tissues is about 7 days [90], compared to a blood half-life of 12 h forFluosol-DA. A second-generation fluorochemical emulsion, Oxygento, contains 60% (wh) perfluorooctyl bromide stabilized with egg-yolk lecithin [93]. This emulsion can be stored without freezing and the organ half-time is 4 days. The energy needed for emulsification of the perfluorochemical oxygen carrier can be provided by high-pressure homogenization or by sonication. The first step in the high-pressure emulsification process is adispersion, prepared by highspeed agitation of the perfluorochemical in a surfactant solution. The coarse dispersion is passed through a high-pressure homogenizer until the particle size is reduced to the desired range. about 0.1-0.2 pm. Oxidative degradation of some components, such as lecithin and nonionic hydrocarbon-type surfactants, is prevented by blanketing with nitrogen. Emulsification by sonication needs a low temperature, close to 0°C. The emulsion is blanketed with carbon dioxide to prevent oxidation and fluoride formation. Sonication permits working on a smaller scale than high-pressure homogenization, but the particle size is usually coarser and the size distribution wider [5]. Sterilization of the emulsion by heat can affect the stability of the emulsion. Sterilization of the prepared emulsion by autoclaving can cause coarsening of the emulsion and fluoride formation. Coarsening of the emulsion [94-1001 during sterilization can be avoided by sterilizing all components prior to emulsification, but this is a less practical process. The fluorochemical content of the emulsion can be determined by the density of the emulsion or by analyzing the emulsion for fluorine. The quantitative analysis of fluorochemicals in organs is usually done by gas Chromatography [loll. The organs or tissues are homogenized in water. Ethanol is added to break the emulsion. The mixture is centrifuged, the upper layer is removed, and the lower layer extracted with F- 113. The extract is dried, an internal standard added, and a sample injected into a gas chromatograph. The determination of the fluorochemical oxygen carrier in the emulsion or organs by gas chromatography assumes that the extraction of the fluorochemical is quantitative. Alternatively, the fluorochemical content of the emulsion can be determined by combustion in an oxygen Parr bomb or, preferably, in a Wickbold

476

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oxyhydrogen torch [20,23]. A combustion method can detennine fluorinated surfactants, which are not sufficiently volatile for gas chromatography. A oxyhydrogen flame combustion method [32] has been used successfully for the analysis of fluorochemical emulsions and fluorochemicals in blood and organs. The intravascular persistence of perfluorotributylamine in blood has been measured by 19F-NMR (nuclear magnetic resonance as well [73]. The detection limit of can be reached with I9F-NMR Fourier transform spectrometry.

Surfactants Clinical tests of perfluorochemical oxygen carriers revealed three major problems: (1) insufficient stability evidenced by coarsening of the emulsion, (2) toxicity, and (3) unsatisfactory retention time of the fluorochemical in blood and in organs. The toxicity and stability are to some extent related. The biocompatibility of perfluorochemical emulsions is impaired by an increase in particle size [35]. The coarsening of fluorochemical emulsions has been attributed the progressive increase in particle size to Ostwald ripening [ 94-99]. The diameter profiles for the emulsified perfluorocarbon droplets in Fluosol-DA and Fluosol43 have been determined by sedimentation field-flow fractionation [96]. A strategy for solving these problems is to prepare a stable emulsion by using an effective biocompatible emulsifier. The retention time can behopefully adjusted by fine-tuning of the surfactant structure. Tsuda et al. [ 701 have suggested that the efficacy of a perfhorochemical and its retention time in blood depend on the in vivo stability of the emulsion in circulating blood. The properties of a perfluorochemical emulsion depend critically on the surfactant used for emulsification. A surfactant used as an emulsifier in fluorochemical blood substitutes has to meet several criteria: (1) provide a fine stable emulsion; (2) be nontoxic, nonmutagenic, and nonhemolytic; (3) be compatible with blood and endothelial cells; (4) be pharmacologically, physiologically, or biochemically inactive; and ( 5 )either be excreted unchanged or in the form of harmless metabolites [41]. The surfactants used in the first-generation fluorochemical blood substitutes were ordinary soaps (potassium oleate), egg-yolk phospholipids (lecithin), or Pluronic F-6s. These emulsifiers were used individually or as their mixtures. Pluronic F-68, a nonionic block polymer, consists of a polyoxypropylene segment located between polyoxyethylene segments. Pluronic F-68 lowers the interfacial tension between perfluorodecalin and water only to 31 mN/m. The stability of a perfluorodecalin-Phonic F-68 emulsion is therefore not based on a low interfacial tension but probably on a steric stabilization mechanism. The coarsening of perfluorocarbon emulsions, apparently by the Ostwald ripening mechanism [94-991, has been inhibited by the addition of high-boiling polycyclic perfluorocarbons [ 102,1031. The third component is believed to form a retaining

Toxicology andAspects Environmental

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film at the fluorocarbon-water interface and hinder the transmission of the fluorocarbon into water [104-1061. The biocompatibility of Pluronic F-68 is questionable, if not inadequate [67,107,108]. Although intravenously infused Fluosol-DA has been reported to be innocuous in rats [ 1091, adverse effects of Fluosol-DA in vivo have been observed and related to Pluronic F-68 or its impurities [67,89,110,11 I]. Transitory anaphylactoid reactions observed in clinical studies have indicated that more biocompatible emulsifiers are needed [89,112]. Lecithins, found in egg yolk, are natural products biocompatible with blood. The storage stability of emulsions lecithin has formed with some fluorochemicals is considerable [78,113]. Lecithin-PFOB emulsions, once sterilized, can be stored at room temperature for several months [53,78]. It has been speculated that the bromine atom of PFOB contributes to emulsion stabilization by lecithin. Mukherji and Sloviter [ 1 141 prepared stable emulsion of perfluorodecalin with egg-yolk lecithin by sonication in Tyrode's buffer (pH 7.4). The viscosity of the emulsion did not change when stored for 60 weeks at 5°C. At 21 "C, viscosity increased after 20 weeks of storage. The viscosity increase was related to slow oxidation of lecithin on storage of the emulsion at either 5°C or 21°C. Lecithins are sensitive to light and oxygen. Some emulsions made with lecithin were not sufficiently stable to be sterilized [42]. Mukherji and Sloviter [ 1141 measured oxidation of lecithin by analyzing the emulsion for malondialdehyde, a lipid oxidation product. The formation of malondialdehyde during storage and, consequently, the oxidation of lecithin, at either 5°C or 21°C was markedly reduced by the antioxidant tocopherol. The egg-yolk lecithins are used in secondgeneration fluorochemical emulsions (e.g., Therox and Oxygent). However, their shortcomings indicate that synthetic surfactants, specifically designed for the emulsification of fluorochemical oxygen carriers, are needed. Fluorinated surflctunts can lower the interfacial tension between a fluorocarbon, such as per perfluorodecalin, and water to as low as 1 mN/m. This suggests that fluorinated surfactants should be effective emulsifiers for biomedical fluorochemicals [42]. The first fluorinated surfactants [ 112-1211 used in fluorochemical emulsions included potassium oleate, a perfluorinated amine oxide [ 115,1161, a nonionic fluorinated surfactant derived from perfluoroalcohol [ 1171, a fluorinated surfactant with two perfluoroalkyl end groups [118]. and perfluoroalkylated polyols [119]. However, the toxicity of these fluorinated surfactants has hindered their intravascular use. The search for stable perfluorochemical emulsions for biomedical applications has included microemulsions formed by fluorinated surfactants (see Section 4.8). Microemulsions form spontaneously, are thermodynamically stable, and have a small particle size. However, microemulsions have shared with coarser emulsions the toxicity problems associated with fluorinated surfactants. The early microemulsionsmade with fluorinatedsurfactantsweretoxicandviscous.

Chapter 10

478

Chabert et al. [122] used an oxyethylated perfluoroalkanol as the fluorinated surfactant. Yiv has claimed [123] that the toxicity of this fluorinated surfactant is greatly reduced, although not eliminated, if a tertiary carbon atom is inserted between the perfluorinated alkyl group and the oxyethylene hydrophile. Serratrice et al. [ 1241 prepared microemulsions using nonionic fluorinated surfactants of the structures C6F13C2H4SC3H4(OC2H4),,0H and C6F13C2H4SC3H4(0C2H4),SC,H4 (OC?H&,OH. Viscosity and conductivity measurements showed that in order to form microemulsions of perfluorodecalin or CgH I 7CH=CH2, the hydrophilelipophile balance (HLB) value of the surfactant must be about 7.5, corresponding to a total number of five to six units of -0C2H4--. Therefore, the use of fluorinated surfactants in biomedical applications hinges on the question, Are all fluorinated surfactants intrinsically toxic or is it possible to synthesize fluorinated surfactants which are not toxic? Meussdoerffer and Niderpriim [ 7a] have suggested that the toxicity of fluorinated surfactants is related to their extremely low surface tension and high surface activity. However, the toxicity of nonionic fluorinated surfactants derived from a telomer mixture and tris(hydroxymethy1) aminomethane by Pavia et al. [121],

I c-0 I

HNC(CH?OH)3

is low, in spite of their high surface activity. Riess and co-workers have reported that perfluoroalkylated xylitol derivatives are biocompatible, in spite of their strong surface activity [ 1251. Riess and Le Blanc [42] has argued that there is no reason to believe that fluorinated surfactants should necessarily be toxic. Most commercial fluorinated surfactants are mixtures of undefined purity and their toxicity in intravascular use is, therefore, not surprising. Unfortunately, attempts to reduce their toxicity by various purification methods to a biomedically acceptable level have been unsuccessful. Hence, other approaches to achieve emulsion stability coupled with biocompatibility have been explored: 1. Reduce toxicity by lowering the surfactant/fluorocarbon ratio [ 126-1 301 2. Attach a perfluoroalkyl group to a biocompatible molecule [131-1551. 3. Useatwo-componentemulsifier,consisting of asemifluorinated alkane (see Section 1.8) and ahydrocarbonsurfactant [ 104.105, 129,130,1561. Fundamentally, the stability of fluorochemical emulsions can be increased by one of the two approaches: by making either the surfactant more "fluorophilic"

Toxicology and Environmental Aspects

479

[ 125,1271 or the oxygen carrier less "fluorophilic" [ 1281. In accord with the latter approach, attempts have been made to prepare stable fluorochemical emulsions using a conventional nonfluorinated surfactant, such as Pluronic F-68 or lecithin, but replacing the perfluorocarbon with a partially fluorinated oxygen carrier [42,128]. Cecutti et al. [128] synthesized a microemulsion consisting of a partially fluorinated alkane, C8F17CH2CH==CHC4H9,and a biocompatible hydrocarbontype surfactant, Montanox 80. The solubility of oxygen in the partially fluorinated hydrocarbon was similar to that in blood but lower than the solubility of oxygen in perfluorodecalin. Tests in the rat suggested that these microemulsions may be less toxic. On the other hand, the interfacial tension between the fluorochemical and water can be lowered and the stability of emulsions can be increased by using a fluorinated surfactant as a single emulsifier or as a cosurfactant with a hydrocarbon-type surfactant [ 104,105,129,130]. The search for nontoxic fluorinated surfactants is shifting from fully synthetic surfactants to perfluoroalkylated natural products. Fluorinated surfactants havebeenpreparedfromnontoxicbiocompatibleintermediates[42,117, 131-1 331. Riess and his co-workers [ 13I] at the University of Nice, France, have made perfluoroalkylated surfactants with a modular structure:

Hydrophile -

-Lipophilic-Fluorocarbon spacer(s) tail(s)

Poly01 Monosaccharide Disaccharide Amino acid Amine oxide Phospholipid Glycophospholipid Perfluoroalkyl groups of various lengths are combined with hydrocarbon segments, which, in turn, are attached to a hydrophilic polar group. To minimize toxic effects, the hydrophilic head groups were derived from atoxic natural products,suchaspolyols [43,125,157-1591, sugarsandsugarderivatives [42,43,125,127,134-145,157,158,160], amines and amino acids [42,43, 146-149,1571, amine oxides [ 148,1611, phospholipids, including phosphatidylcholines [150-154.1571, and glycophopholipids [157,162]. The lengths of the perfluoroalkyl and the hydrocarbon segments affect the solubility and the hemolytic activity of the surfactant (9,145) and have to be carefully selected. Perfluoroalkylated phosphotidylcholines [ 150-1 531 are more effective emulsifiers for fluorocarbons than the natural egg-yolk phospholipids. Concentrated [50% (w/v)] perfluorodecalin emulsions containing only 1% surfactant

480

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were stable at 50°C for over 1 month. However, the in vivo tests using mice as test animals gave LD50values too low for intravascular use. The polyhydroxyperfluoroalkylated surfactants are derived from natural products such as monosaccharides and disaccharides, linear pentitols and hexitols, and so forth, which are not toxic [42,125,127.134-1451. Examples of fluorinated surfactants derived from xylitol or maltose are shown in Fig. 10.12 [127]. Their solubility in water depends, as expected, on the length of the perfluorocarbon chain and the number of hydroxyl groups. The maltose derivative (surfactant 2) is the most soluble and the xylitol derivative (surfactant 1) the least soluble in water. These semisynthetic fluorinated surfactants can lower the surface tension of water to 18-23 mN/m and the water-perfluorodecalin interfacial tension to 1-5 mN/m, a large improvement over the interfacial tension of 3 1 mN/m obtained with Pluronic F-68. Small amounts of such fluorinated surfactants increase the stability of fluorocarbon emulsion prepared with Pluronic F-68 significantly. Riess et al. [ 125,127,138,1551 have shown that perfluoroalkylated xylitol and maltose derivatives exhibit a synergistic stabilizing effect in perfluorocarbon emulsions made with Pluronic F-68. The synergistic effect resulted from an interfacial tension reduction by the fluoroalkyl group and hydrogen bonds formed between the hydroxyl groups of the poly01 and the ether groups of Pluronic F-68. The average droplet size measured as a function of time indicated that neither Pluronic F-68 nor the fluorinated surfactants 1, 2, or 3 alone can produce a stable perfluorodecalin emulsion. The synergism between the two surfactants, Pluronic F-68 and a fluorinated surfactant, is essential for achieving storage stability. The hemolytic activity of perfluoroalkylated carbohydrates decreases with increasing length of the perfluoroalkyl group, decreases with the increasing length of the hydrocarbon connective link, and decreases with increasing size of the hydrophile [9]. This observation suggests that hemolytic activity increases with increasing lipophilicity of the surfactant. Because Pluronic F-68 has caused adverse reactions in some patients, fluorinated surfactants are being developed to replace Pluronic F-68 as the sole emulsifier. Attempts to use perfluoroalkylated surfactants derived from galactose, glucose,maltose,andrelatedpolyolsassinglesurfactantswereunsuccessful [ 125,127,139,143,155]. However, a study of perfluoroalkylated fatty acid monoesters of trehalose and sucrose (Fig. 13) [144,145] showed that 6-0-[3’-(perfluorooctyl) propanoy1)-a,a-trehalose can emulsify perfluorodecalin and form stable emulsions in the absence of Pluronic F-68. The biological compatibility of perfluoroalkylated fatty acid monoesters derived from trehalose or sucrose was evaluated by an in vitro cell culture test, a hemolytic activity test, and in vivo tests performed in mice. In the cell culture test, the growth and viability of Namalva lymphoblastoid cells in the presence of the surfactant tested were compared to those of control cells. At a concentration of 0.1

X

0

481

Chapter 10

HO

OH

OH

HO

FIG.10.13 Structures of perfluoroalky1a;ed fatty acid esters of trehalose (1) and sucrose (2) and the structure of maltose (5). (From Ref. 145. Reproduced by permission of the American Oil Chemists Society.)

g/L, none of the fluorinated surfactants affected the cell growth and viability significantly, in spite of their high surface activity. The hemolytic activity of the perfluoroalkylated fatty acid monoesters derived from trehalose or sucrose decreased with increasing length of the perfluoroalkyl chain and decreased with increasing the hydrocarbon segment. Abouhilale et al. [145] concluded that the most surface-active compounds are the least hemolytic. However, their data appear to suggest also that hemolytic activity is related to lipophilicity, which is reduced by fluorination. Fluorinated surfactants derived by Riess et al. [ 144,1451from natural products appear to be essentially nontoxic in intravascular use, but more testing is needed to remove any doubts about their biocompatibility. The purification of fluorinated natural products is difficult and of a practical concern. Other emulsification systems are therefore being developed.

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The fundamental difficulty with the emulsification of fluorochemical oxygen carriers in water is the interface between the fluorochemical and water. The use of ordinary surfactants as emulsifiers results in a fluorophilic-lipophilic interface between the droplet and the surfactant and, consequently, in a high interfacial tension. The stability of emulsions prepared with lipophilic-hydrophilic-type surfactants is therefore only marginal. Therefore, emulsifier systems consisting of two components have been proposed [ 102-106,130]. Davis et al. [102,156] were concerned mostly with the coarsening of fluorochenlical emulsions by the Ostwald ripening mechanism [94-991. To the emulsion they added a component which had a lower vapor pressure than the fluorochemical oxygen carrier. The stabilizing effect of these perfluorinated polycyclic oils, such as perfluoroperhydrophenanthrene, was related to the boiling point and, consequently, to the molecular weight of the additive [ 102,1631. Meinert et al. [ 104-1061 measured the average particle size and the interfacial tensions of perfluorodecalin-water emulsions containing various fluorinated dimorpholines. Their data indicated that the additive was enriched at the perfluorodecalin-water interface and retarded Ostwald ripening by hindering the transmission of perfluorodecalin from the emulsified droplet into water. The stabilizing effect increased with the decreasing time the additive needs to form the retaining film. F-dimorpholinopropane and F-dimorpholinobutane were most effective of the F-dimorpholinoalkanes tested. Meinert et al. [104-1061 explained the stabilizing effect of perfluorodimorpholinoalkanes and semifluorinated alkanes by the enrichment at the interface between the fluorochemical oxygen carrier and a hydrocarbon-type nonionic surfactant, such as Pluronic F-68. Meinert et al. designated such partially fluorinated compounds "interfacial active compounds.'' The additives have a lower vapor pressure and a higher molecular weight than the fluorochemical oxygen carrier. Riess et al. [ 1301 suggested separating the surfactant interactions at the fluorochemical-water interface into two contributions: a fluorophilic-lipophilic interface and a lipophilic-hydrophilic interface. In accord with this concept, two amphiphiles, a fluorophilic-lipophilic amphiphile (see Section 1.8) and a conventional lipophilic-hydrophilic amphiphile, are used to bridge the fluorocarbon and water surfaces. A similar theory was developed by Meinert et al. [104-1061. A partially fluorinated alkane [ 104-1061 or alkene [ 1301 adsorbs on the fluorochemical oxygen carrier with its fluorophilic segment in the surface of the droplet. Its hydrocarbon group oriented away from the droplet surface forms a lipophilic interface. The nonfluorinatedsurfactantadsorbsonthisinterface with its lipophilic segment. The hydrophilic groups of the surfactant are oriented toward water and form an hydrophilic outer shell (Fig. 10.14) [105]. The partially fluorinated alkane or alkene is surface active in hydrocarbon media (see Section 1.8) and can be visualized to function as a cosurfactant in the emulsion.

Chapter 10

484

. s

0

0

e

\

’ -* ’

HYDROPHILIC SEGMENTS OLEOPHILIC SEGMENT OLEOPHILIC CHAIN FLUOROCARBON CHAIN

t

t

PLURONIC F68 SEMIFLUORINATED ALKANE

PFC-DROPLET FIG.10.14 Concept of RFRHforming a hydrocarbon sphere around the fluorocarbon droplet. (From Ref. 105.)

Meinert et al. [104-1061 found that already small quantities [ l or 2% (w/v)] of partially fluorinated alkanes, C,,lF2r?l+ 1C,IH7_,,+ 1, stabilize perfluorodecalinPluronic F68 emulsions. Riess et al. [ 1301 described a binary emulsifier system. consisting of a nonfluorinated surfactant in conjunction with a partially fluorinated alkene. Riess et al. [130] named the fluorinated amphiphile a “dowel,” suggesting that its fluorophilic end adsorbs in the fluorocarbon surface and its lipophilic end penetrates the lipophilic part of the egg-yolk phospholipid. The “dowel,” CsFI7CH=CHCsHl7, increased the stability of a perfluorooctyl bromide (PF0B)-egg-yolk lipid emulsion stability considerably. The droplet size (0.25 pm) remained constant over 9 months, even at 40°C. In the absence of the “dowel,” the droplet size more than doubled at 25°C to 0.49 pm. The theories of Meinert et al. [104-1061 and Riess et al. [130] are similar, except for the interaction between the two amphiphiles forming the surfactant system. Meinert et al. [104-1061 visualized the attraction between the two surfactant components as a result of a decrease in interfacial tension. Riess et al. [ 1301 have suggested that the lipophilic group of the partially fluorinated hydrocarbon penetrates the lipophilic segments of the surfactant and is located preferentially at the fluorocarbon-water interface-hence the term “dowel” [ 164.1651. This interfacial structuring by the “dowel” is more effective when the fluorocarbon is linear rather than cyclic.

Future The history of fluorochemical blood substitutes started with the dramatic demonstration that a mouse can stayalive while submerged in a fluorochemical saturated with oxygen [ 1631. Since the early days of great optimism, the limitations of fluorochemical enlulsions as blood substitutes have been recognized [5]. Difficulties with the preparation of stable emulsions, a low oxygen content at atmospheric pressures, and in vivo accumulation of fluorochemicals in tissues have restricted their intravascular use as blood substitutes [38]. The main emphasis is now on diagnostic applications and oxygen transport into ischemic tissues.

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Because the molecular weight range of fluorochemicals suitable for intravascular oxygen carriers is limited [42], further progress hinges on the development of more effective, biocompatible surfactants [42,104-106,129,130,166]. The fluorochemical oxygen carrier is lipophobic and repels lipophilic groups of a conventional surfactants. This phobicity results in a high interfacial tension between the fluorochemical and the surfactant and a low emulsion stability. A fluorinated surfactant is the logical choice, if nontoxic biocompatible fluorinated surfactants can be found. Fluorinated surfactants derived from natural products, such as carbohydrates, appear to be effective, although the toxicity of the least toxic surfactants is marginally acceptable at best. Binary surfactant systems consisting of a fluorophilic-lipophilic fluorinated amphiphile and a lipophilic-hydrophilic surfactant appear promising if adequate biocompatibility can be achieved. The intravascular persistence of fluorocarbon emulsions needs to be improved further. The dose-dependent half-life of recent fluorocarbon emulsions is typically 4-1 2 h [ 1611. This is sufficient only for surgical procedures, but it is inadequate for cases of trauma and much too short for chronic anemia [ 1611. The fear of HIV has heightened the demand for safe blood transfusion procedures and blood substitutes. The safest procedure, autologous blood transfusion, uses blood the patient has donated before the surgery. However, emergency and trauma cases require homologous blood transfusions. The search for safe blood substitutes has therefore intensified. Two avenues are now being explored using either modified hemoglobin or a fluorochemical as the oxygen carrier. The development of blood substitutes based on hemoglobin has been frustrated by the toxicity of hemoglobin deprived from its red blood cell membrane. Free hemoglobin molecule breaks down into two nonfunctional dimers which are filtered by the kidney and cause renal toxicity. The toxicity problem of hemoglobin has lead to modifications of hemoglobin, including cross-linking, genetic engineering, and attachment of a polyoxyethylene chain to the hemoglobin molecule. In spite of earlierdifficultieswithsideeffects,bloodsubstitutesbased on modified hemoglobin have entered the stage of human trials. A second-generation fluorocarbon emulsions have been developed and submitted to clinicaltrials [ 78,167,1681. First-generation fluorochemical oxygen carriers had a low concentration of the fluorochemical and a marginal storage stability, requiring storage in a freezer or refrigerator. One of thefirst fluorochemical oxygen carriers, FluosoPf,has been approvedby the FDA for a limited use in humans, for oxygenation of myocardium during percutaneous transluminal coronary angioplasty [169]. The inadequate storage stability requiring frozen storage and an arduous reconstitution procedure frustrated the commercial acceptance. The secondgeneration emulsions have a higher concentration of fluorochemicals [169,1701. These emulsions are made with egg-yolk lecithins as theemulsifier and do not need to be frozen for storage. A heat-sterilized, ready-for-use, stable 60% (wh) perfluorooctyl bromide emulsion, OxygentTM, has been tested clinically. Further progress in stabilizing fluorochemical emulsions and minimizing side effects will allow the

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use of fluorochemical oxygen carriers as temporary red blood cell substitutes and for a variety of biomedical uses, such as treatment of cerebral ischenlia, organ preservation, diagnostic procedures. drug delivery, and others [ 161,169,170]. 10.5

DRUG DELIVERY AND OTHER PHARMACEUTICAL APPLICATIONS

A fluorinated chain in an amphiphile enhances its propensity to form vesicles and other assemblies (see Section 7.4). Various neutral and cationic fluorinated surfactantshave been found to formstablebilayermembranesandvesicles [ 17 1,1721.Riess and co-workers [ 161.1681 have obtained stable fluorinated vesicles from amphoteric fluoroalkylated phosphocholines, phospholipids, glycolipids, or anionic sugar phosphates. Vesicles can enclose substances used for biological and pharmaceutical applications in the limiting membrane or in the inner cores and can carry drugs, prodrugs, immunoactive materials, genetic material, contrast agents, vaccines, and so forth [ 1611. The unique properties make fluorinated surfactants attractive for drug delivery and drug release systems [ 168,1731. Fluorinated surfactants can be employed as covalently bonded drug carriers, as a form of prodrugs facilitating the incorporation of the drug into an appropriate delivery system, as dispersants, and in the form of fluorinated vesicles. Riess and co-workers [ 161,1681 have synthesized a large number of fluoroalkylated amphiphiles. Their versatile modular design allowed a stepwise modification of the surfactant size and charge, as well as the hydrophilic, lipophilic, and fluorophilic character. The nature of the head, the number of tails (identical or different), the spacers, the connecting units, and the sites were altered in order to manipulate the physical and biological characteristics of emulsions, vesicles, and other colloidal systems. The permeability of the vesicle membrane can be reduced by incorporating an impermeablefluorinatedsheetinsidethelipidicfilm of themembrane [168,174]. The permeability can be fine-tuned by varying the length of the fluorinated segment in the surfactant. modifying the fluorocarbon-hydrocarbon ratio, and having fluorocarbon and hydrocarbon tails present in the surfactant molecule. Ristori et a1 [175-1771 have studied mixed fluorocarbon-hydrocarbon surfactant vesicles of interest as drug carriers for biological and pharmaceutical substances. Ammonium perfluoropolyether carboxylate and n-dodecylbetaine, when mixed in an appropriate molecular ratio, form spontaneously stable vesicles. These mixed vesicles were investigated as carriers of model biomolecules [I761 and metalloproteins [ 1771. The high solubility of respiratory gases allows the use of fluorocarbons for liquid ventilation and drug delivery by the pulmonary route [178]. For administration of drugs via the pulmonary route, reverse water in fluorocarbon emulsions

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have been prepared by Riess and co-workers [178]. The emulsions containing water in perfluorooctyl bromide (PFOB) or perfluorooctylethane (PFOE) were stabilized with a perfluoroalkyl(alky1) dimorpholinophosphate (FnCmDMP) [ 1791. Various drugs, including antibiotics, vasodilators, and anticancer drugs not soluble in the fluorocarbon phase, were incorporated in the aqueous phase. In an in vitro study, the release of 5,6-carboxyfluorescein encapsulated in the internal water phase in a reverse water-in-PFOB emulsion was slower than the release in the reverse water-in-PFOE emulsion, and much slower than the release from a reverse water-in-n-octylbromide emulsion. Apparently, the high hydrophobicity of fluorocarbons hinders as a physical barrier the diffusion of the species from the water phase. The release rate and, consequently, the delivery of drugs may be regulated by including semifluorinated alkanes (see Chapter 1) in the emulsion. Fluorinated surfactants and polymers may have a therapeutic effect of their own. Sawada et al. [ 180-1821 have prepared fluoroalkylated anionic [ 180,1811 and cationic polysoaps [ 1821. The fluoroalkylated acrylic acid co-oligomers containing dimethylsilicone segments [ 1801 and fluoroalkylated 2-(methacry1oxy)ethanesulfonic acid oligomers [ 1811 were found to function as inhibitors of HIV- 1 virus replication. A fluoroalkylated 4-vinylpyridinium chloride-acrylic acid cooligomer exhibited both virus-inhibiting and antibacterial activities [ 1821.

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124. G. Serratrice. L. Matos. J. J. Delpuech, and A. Cambon. J. Chim. Phys. 87. 1969 (1990). 125. L. Zarif. J. Greiner, S. Pace. and J. G. Riess. J. Med. Chem. 33. 1262 (1990). 126. J. G. Riess. Curr. Surg. 45,365 (1988). 127. L. Zarif, A. Manfredi, C. Varescon, M. Le Blanc, and J. G. Riess, J. Am. Oil Chem. SOC.66, 1515 (1989). 128. C. Cecutti, A. Novelli. I. Rico. and A. Lattes, J. Dispers. Sci. Technol. 11, 115 (1990). 129. J. G. Riess. Artif. Organs 15,408 (1991). 130. J. G. Riess. L. Sole-Violan, and M. Postel. J. Dispers. Sci. Technol. 13, 349 (1991). 131. A. Faradji, M. Giunta, Y. Dayan. L. Foulletier, and F. Oberling, Rev. Fr. Transf. Tmmunohenlatol. 22, 1 19 (1979). 132. G. Mathis and J. J. Delpuech. Br. Eur. Pat. No. 0.051.526 (1981). 133. G. Mathis. P. Leempoel. J.-C. Ravey, C. Selve. and J.-J. Delpuech. J. Am. Chem. SOC.106,6162 (1985). 134. (a) A. Milius, J. Greiner. and J. G. Riess, New J. Chem. 15, 337 (1991). 135. A. Milius. J. Greiner, and J. G. Riess, Colloids Surf. 63,28 I ( 1992). 136. A. Milius. J. Greiner, and J. G. Riess, New J. Chem. 16,771 (1992). 137. J. G. Riess, J. Greiner. S. Abouhilale, and A. Milius. Progr. Colloid Polym. Sci. 88. 123 ( 1992). 138. J. G. Riess. C. Arlen, J. Greiner. M. Le Blanc, A. Manfredi. S. Pace, C. Varescon, and L. Zarif. Biomater. Artif. Cells Artif. Organs 16, 421 (1988). 139. J. Greiner, A. Manfredi, and J. G. Riess, New J. Chem. 13, 247 (1989). 140. A. Manfredi, S. Abouhilale, J. Greiner, and J. G. Riess. Bull. SOC.Chim. Fr. 872 (1989). 141. A. Milius. F. Guillod. E. Myrtil. and J. G. Riess. J. Fluorine Chem. 58. 205 (1992). 142. L. Zarif, J. Greiner. and J. G. Riess, J. Fluorine Chem. 44,73 (1989). 143. C. Varescon, A. Manfredi. M. Le Blanc, and J. G. Riess, J. Colloid Interf. Sci. 137, 373 (1990). 144. S. J. Abouhilale. J. Greiner, and J. G. Riess. Carbohydr. Res. 312, 55 (1991). 145. S. Abouhilale, J. Greiner, and J. G. Riess, J. Am. Oil Chem. SOC.69, 1 (1992). 146. C. Blaignon, M. Le Blanc, and J. G. Riess, Patent Appl. France FR 244 803 (1988). 147. C. Blaignon, M. Le Blanc, and J. G. Riess, Proceed. 2nd World Surfactant Congress (Paris, May 1988), Vol. 2, pp. 137-142 (1988). 148. J. B. Nivet. M. Le Blanc, and J. G. Riess, J. Dispers. Sci. and Technol. 13. 627 (1992). 149. J. B. Nivet, M. Le Blanc, and J. G. Riess. Eur. J. Med. Chem. 26,953 (1991). 150. M. P. Krafft, J. P. Rolland, P. Vierling, and J. G. Riess, New J. Chem. 14, 869 (1990). 151. C. Santaella, P. Vierling, and J. G. Riess, J. Colloid Interf. Sci. 148, 288 (1991). 152. C. Santaella, P. Vierling, and J. G. Riess. New J. Chem. 15, 685 ( 1 991). 153. C. Santaella, P. Vierling, and J. G. Riess, Angew. Chem. Int. Ed. Engl. 30, 567 (199 1). 154. J. G.Riess, F. Jeanneaux,M. P. Krafft, and P. Vierling, U.S. Patent Appl. 071542,227 (1990).

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155. J. G. Riess, “Proc. 2nd World Surfactant Congress,” Vol. 4, p. 256. A.S.P.A., Paris ( 1 988). 156. S. S. Davis, H. P. Round. and T. S. Purewal. J. Colloid Interf. Sci. 80, 508 (198 1 ). 157. J. Greiner, J. G. Riess. and P. Vierling, in “Organofluorine Compounds in Medicinal Chemistry and Biomedical Applications,” R. Filler, Y. Kobayashi, and L. M. Yagupolskii, eds., pp. 339-380, Elsevier, Amsterdam (1993). 158. B. Charpiot, J. Greiner. M. Le Blanc, A. Manfredi, J. G, Riess, and L. Zarif, US Patent 4,985.550 (1991). 159. A. A. Pavia, B. Pucci, J. G. Riess, and L. Zarif, FR Patent 2.665,705 ( 1993); U.S. Patent 5,527,962 (1996). 160. E. Myrtil, L. Zarif, J. Greiner. J. G. Riess, B. Pucci, and A. A. Pavia, J. Fluorine Chem. 71. 101 (1995). 161. J. G. Riess, and M. P. Krafft, Biomaterials 19, 1529 (1998). 162. J. G. Riess, J. Greiner, A. Milius, P. Vierling, F. Guillot. and S. Gaentzler, Fr. Patent 2,677,360 (1995); U.S. Patent 5.679.459 (1997). 163. L. C. Clark and F. Gollan, Science 152, 1755 (1966). 164. C. Cornelius, M.-P. Krafft, and J. G. Riess, J. Colloid Interf. Sci. 163, 391 (1994). 165. L. Trevino, M. Postel, and J. G. Riess, J. Colloid Interf. Sci. 166.414 (1994). 166. J. Greiner, J. G. Riess, and P. Vierling, in “Organofluorine Compounds in Medicinal Chemistry and Biomedical Applications,’’ R. Filler, Y. Kobayashi, and L. M. Yagupolski, eds., Elsevier, Amsterdam (1993). 167. J. G. Riess, J. L. Dalfors, G. K. Hanna, D. H. Klein, M. P. Krafft. T. J. Pelura, and E. G. Schutt, Biomater. Artif. Cells Immob. Biotechnol. 20, 839 (1992). 168. J. G. Riess, Colloids Surf. A 84, 33 (1994). 169. J. G. Riess, Vox Sang 61,225 (1991). 170. J. G. Riess, Biomater. Artif. Cells Immob. Biotechnol. 20, I83 ( 1 992). 171. T. Kunitake. Angew. Chem. Int. Ed. Engl. 31,709 (1992). 172. H. Ringsdorf, B. Schlarb, and J. Venzmer, Angew. Chem. Int. Ed. Engl. 30, 113 (1992). 173. R. H. Muller, “Colloidal Carriers for Controlled Drug Delivery and Targeting: Modification, Characterization and in Vivo Distribution.” CRC Press. Boca Raton, FL (1991). 174. F. Frezard, C. Santaella, P. Vierling. and J. G. Riess. Biomater. Artif. Cells Immob. Biotechnol. 22 ( I 993). 175. S. Ristori, C. Maggiulli, J. Appell, G. Marchionni, and G. Martini, J. Phys. Chem. B101,4155 (1997). 176. S. Ristori, S. Rossi, G. Ricciardi, and G. Martini. J. Phys. Chem. BIOI, 8507 (1997). 177. G. Martini, S. Ristori, and S. Rossi, J. Phys. Chem. A102, 5476 (1998). 178. V. M. Sadtler, M.-P. Krafft, and J. G. Riess. Colloids Surf. A 147. 309 (1999). 179. M. P. Krafft, J. G. Riess, and P. Vierling, Eur. J. Med. Chem. 26,545 (1991). 180. H. Sawada, A. Ohashi. M. Oue, M. Baba, M. Abe, M. Mitani, and H. Nakajima, J. Fluorine Chem. 75, 121 (1995). 181. H. Sawada, A. Ohashi, M. Baba, T. Kawase, and Y. Hayakawa, J. Fluorine Chem. 79, 149 (1996). 182. H. Sawada, A. Wake, T. Maekawa, T. Kawase. Y. Hayakawa, T. Tomita, and M. Baba, J. Fluorine Chem. 83, 125 (1997).

11 Theory of Repellency

11.I

DEFINITIONS

Repellency is a condition of limited wettability. Stain repellency of a treated fabric is the ability of the fabric to withstand penetration by liquid soils under static conditions involving no other forces than capillary forces and the weight of the drop [l]. In accord with this definition, oil repellency is tested by placing a drop of oil on the fabric and observing the resistance of the fabric to sorption of oil by the fabric. A series of hydrocarbon homologs, aligned in decreasing order of their surface tensions, yLv (subscripts L and V are liquid and vapor phases of the liquid, respectively), is used to rate oil repellency. The hydrocarbon with the lowest yLvto remain above the fabric during the duration of the test is used to indicate oil repellency. Water repellency is more difficult to define, because various static and dynamic tests are used to measure water repellency. In general, water repellency can be defined as the ability of the fabric to withstand wetting or penetration by water under the test conditions. It is important to distinguish between the terms “water repellent” and “waterproof.” A fabric is made water repellent by depositing a hydrophobic material on the fibers. Water-repellent fabrics have open pores and are permeable to air and water vapor. Waterproofing involves filling the pores in the fabric with a substance impermeable to water and usually to air as well. Coating fabric with rubber is an example of a waterproofing. The characteristics of waterresistant fabrics are listed in Table 11.1. The main difference between water-repellent and waterproof fabrics is the greater permeability of the former to water under hydrostatic pressure and to water vapor [2]. Water-repellent fabrics, but not waterproof fabrics, permit passage 494

Theory of Repellency

495

TABLE11.1 Water-Resistant Fabrics Waterproof Pores Water-vapor permeability Air permeability Resistance to water penetration

Water repellent

Filled None to very small

Open Small to large

None to small Highly resistant even under external hydrostatic pressure

Usually large Resistant to wetting by raindrops and spreading and wicking of water; permits water passage under external hydrostatic pressure

Source: Ref. 2.

of water once hydrostatic pressure is sufficiently high. Because the use of the term “waterproof’ isbeing discouraged as an overstatement, the more descriptive term “impermeable to water” may be used instead.

11.2 WETTING Because repellency is a condition of low wettability, a discussion of repellency is not possible without reviewing the principles of wetting. Wetting is a displacement of a solid-air (vapor) interface with a solid-liquid interface. In a broader sense, the term “wetting” has been used to describe the replacement of a solid-liquid or liquid-air interface with a liquid-liquid interface. Wetting is adynamic process. Spontaneous wettingis a migration of a liquid over a solid surface toward thermodynamic equilibrium. Forced wetting, on the other hand, involves external hydrodynamic or mechanical forces to increase the solid-liquid interface beyond the static equilibrium. Wetting of fibers is a displacement of a fiber-air (vapor) interface with a fiber-liquid interface. Wetting of a fibrous assembly, such as a fabric, is a complex process. Various wetting mechanisms. such as spreading, immersion, adhesion, and capillary penetration, may operate simultaneously. Undoubtedly, wetting is one of the most important phenomena in the processing and use of textiles. Detergency, repellency, absorbency, and other performance characteristics of textiles are affected by the wettability of the fabric. Wetting is a complex process complicated further by the fibrous structure of the textile fabric. In his classic thermodynamic treatise, Gibbs [3] related wetting to a decrease of free energy. Spontaneous wetting occurs when the sum of interfacial en-

Chapter 11

496

ergies, F , decreases as the result of the liquid-solid contact: F

=A

s~s+ v ALYLV+ ASLYSL= CAY

(1)

where A denotes areas, y is surface tension (surface energy per unit area), and the subscripts S, L, and V are solid, liquid, and vapor of the liquid, respectively. Wetting is spontaneous when the change in free energy, AF. is negative:

AF

= F? -

FI

=

C(Ay)2

-

C(Ay)l

(2)

Wetting of textiles involves several primary processes: immersion, capillary sorption, adhesion, and spreading. During immersion (Fig. 1 1.1a) or capillary sorption (Fig. 1 1.lb) a solid-vapor interface disappears and a solid-liquid interface appears. By convention, the work of immersion, Wr.or the work of penetration, W p , performed during capillary sorption are defined as the free-energy change when the contacting solid and liquid are separated (reversal of wetting). For spontaneous penetration (e.g., a positive capillary rise), the work of penetration has to be positive. This is the case when the interfacial energy of the solid in contact with vapor exceeds the interfacial tension between the solid and the liquid. For interfaces of unit area,

Adhesion is attraction between two surfaces in contact (Fig. 1 1.2). When the contacting surfaces are those of a solid and a liquid, the work of adhesion, WA,is equal to the change of surface free energy of the system when the contacting liquid and the solid are separated. The separation results in the loss of their interface with interfacial tension, ysL, and the formation of two new surfaces with surface tensions ySl7 and yLv. The work of adhesion is given by the Dupri equation (4) per unit area of interfaces [4]:

WA

'YSV

+ 'YLV - Y S L

(4)

Theory of Repellency

497

ysL=

FIG.11.2 Adhesion between a liquid and a solid. Application of Eq. (4) to a liquid yields the work of cohesion, Wc, which is the reversible work to pull apart a liquid column, creating two liquid surfaces, with each having an interfacial tension yLv:

wc = 2 Y L V

( 5)

Spreading is the flow of liquid at least two molecular layers thick over a solid. During spreading (Fig. 11.3), the solid-liquid and liquid-vapor interfaces increase, whereas the solid-vapor interface decreases. Again by definition. the work of spreading, Ws, is the reversible work equal to the free-energy change that occurs when the solid and liquid are separated (reversal of spreading). Per unit area,

ws = Ysv -

YLV

-

YSL

(6)

For spreading to be spontaneous, the work of spreading, Ws, has to be positive. The work of spreading has also been called the spreading coefficient. A drop placed on a solid flattens when it spreads. As a result, the liquid-vapor interface per unit area of the solid beneath the liquid decreases with the decreasing curvature of the drop. The decrease in the liquid-vapor interface is relaYLV " " " " " "

W s = YSV-YSL-Y LV

FIG.11.3 Spreading of a liquid on a solid.

Chapter 11

498

tively small and is usually neglected in Eq. (6). Figure 11.3 shows a simplified model with a liquid film of constant thickness and shape. Equations (3)-(5) are valid only for ideal, smooth, homogeneous, impermeable, and nondefornlable surfaces. Because textile fibers do not have such ideal surfaces, their wetting phenomena are more complicated. In addition, the prediction of wetting phenomena (e.g., spreading) from wetting energetics is difficult because a direct method for determining ysv, a term found in Eqs. (3). (4), and (6), is not available. It is more convenient to use the forces in balance at a three-phase (solid, liquid, vapor) boundary as an indication of wettability. 11.3 THE EQUILIBRIUM CONTACT ANGLE

When a drop of liquid placed on a solid surface does not spread, the drop assumes a shape that appears to be constant and exhibits an angle, 8 (Fig. 1 1.4). The angle 8 is called the contact angle and is considered to be characteristic of the particular liquid-solid interaction. Therefore, the equilibrium contact angle serves as an indication of the wettability of the solid by the liquid. The emphasis here is on equilibrium, because valid conclusions can be drawn from the value of the contact angle only when equilibrium is assured. Many years ago, Young [ 5 ]proposed that a liquid drop on a plane solid surface (Fig. 11.4) is subject to the following equilibrium forces: YSV = Y S L

+ YLV cos8

(7)

where 8 is the contact angle in the liquid at the solid-liquid-vapor boundary. The validity of Young’s equation has been questioned [6], but thermodynamic derivations [ 7-91 have shown it to be correct for ideal systems in equilibrium. The surfaces in such systems have to be smooth, homogeneous, impermeable, and nondeformable. Equations (7) and (4) can be combined, and if the adsorption of vapor

SOLID FIG.11.4

Equilibrium contact angle.

Theory of Repellency

499

on the solid surface is included,

wA= y L V ( l + case)

(8)

Equation (8) is more useful than Eq. (4), because it includes measurable quantities. Equation (8) relates adhesion to cohesion of the liquid, because yLv = Wc. Equation (8) also appears to suggest that when the contact angle 8 is 0, adhesion is equal to the cohesion of the liquid. 2yLv, and the spreading coefficient, Ws, is equal to zero. Such a conclusion would not be correct, however, because Eq. (8) applies to an equilibrium condition, which spreading is not. It is better to visualize that the wettability of a solid is higher by liquids that exhibit a smaller contact angle when placed on the solid; when the contact angle approaches zero, the wettability has its maximum limit.

3

11.4

CONTACT ANGLES IN REAL SYSTEMS

The contact-angle concept has been very useful, but nevertheless complex and problematic, if not controversial, mainly because the equilibrium contact angle in an ideal system has been confused with an apparent contact angle measured in real nonequilibrium systems. Furthermore, the term “contact angle’’ has several meanings. The contact angle is the angle between the tangent to the liquid-vapor (air) interface and the solid-liquid interface. The contact angle is formed at the contact line. This is the region where three interfaces (solid-vapor, solid-liquid, and liquid-vapor) intersect. The intrinsic contact angle or the true contact angle is the angle at a very short (molecular) distance from the contact line on the solid [lo]. The equilibriurn contact angle is the single-valued intrinsic contact angle described by the Young-Dupri equation for an ideal system. However, a real solid-liquid system may exhibit several stable contact angles. An experimentally observed contact angle is an apparent contact angle,measured on a macroscopic scale, for example, through a low-power microscope [ 101. The measurement of the true intrinsic contact angle at the contact line is very difficult [ 111. On rough surfaces, the difference between the apparent and intrinsic contact angles can be considerable [ 101. Shuttleworth and Bailey [ 81 have defined the apparent contact angle as the sum of the intrinsic contact angle and the slope angle of thesurface at the point of contact. The slope angle can be positive or negative. Because the Young-Dupri equation applies to the interfacial tensions and the intrinsic contact angle at the contact line, the substitution of an apparent contact angle into the equation can give only an approximate result [ 101. Because the surfaces of textile fibers are not ideal, wetting is complicated by surface roughness, heterogeneity, and adsorption of liquids or surfactants with a consequent change of surface energy. Whereas Eqs. ( 7 ) and (8) deal with ideal systems, on nonideal surfaces the measured (apparent) contact angle is not single valued. The contact angle displayed after the liquid front has advanced is usually

500

Chapter 11

larger than the contact angle after the liquid has receded from a previously wet surface. The difference between the advancing and the receding contact angles is contact angle hysteresis [ 12-17], which has been attributed to adsorption of the liquid on the solid with a consequent change of the surface energy of the solid [12], surface heterogenity [ 131, or roughness of the solid surface. Real solid surfaces are not absolutely smooth but appear rough on microscopic examination. Wenzel [18] expressed the effect of roughness on the contact angle by the roughness factor, given by

where A , is the observed (microscopic) surface area, A ,., is the real surface including surface rugosities, 0’ is the measured contact angle, and 8, is the true contact angle on a smooth surface. Equation (9) indicates an increase in the contact angle with increasing surface roughness when 0 > 90” and a decrease in contact angle with increasing surface roughness when the contact angle is <90°. Johnson and Dettre [ 14,151analyzed the theories of Wenzel [ 181, Cassie and Baxter [ 191, Shuttleworth and Bailey [8], and Good [20] in terms of an idealized sinusoidal rough surface. Contact-angle hysteresis was attributed to energy barriers between metastable states of a drop, as first suggested by Shuttleworth and Bailey [8]. Because of the existence of these metastable states, a contact angle measured on a rough surface will usually not be given by Eq. (9) but will depend on the direction of movement of the three-phase boundary prior to measurement of the angle. Inhomogeneity of the surface or contamination of the solid surface or the liquid can also give rise to contact-angle hysteresis [ 131. The surfaces of fibers treated with a repellent are not homogeneous but are covered with patches or islands deposited during chemical finishing. Therefore, the effect of surface heterogeneity on repellency is of practical importance. Pease [21] explained the effect of surface heterogeneity on contact-angle hysteresis by associating the advancing contact angle with regions of high intrinsic contact angle and the receding contact angle with regions of low intrinsic contact angle. A more quantitative study was made by Johnson and Dettre [ 22,231 for aspecific model system. They postulated in analogy with their roughness theory that the contact angle that the drop assumes onaheterogeneoussurfacealsodependsonenergybarriersbetweenthe metastable states. The experimentally observed hysteresis was similar to that expected from conclusions based on the model surface. Usually, the advancing contact angle is employed in discussions of wetting. However, it should be kept in mind as well that real wetting and wicking processes are dynamic. The capillary flow is not determined by a constant advancing con-

501

Theory of Repellency

tact angle OA, as frequently assumed, but depends on a dynamic contact angle corresponding to the instantaneous velocity of the moving meniscus. The contact angle of a moving liquid front, the dynamic contact angle, can be quite different from the contact angle formed by a static liquid [24,25]. The term “dynamic contact angle” has two meanings, however. The dynamic contact angle can be dependent on the velocity of the moving contact line, on time, or both. 11.5 CRITICAL SURFACE TENSION AND SURFACE ENERGY

Equations ( 3 ) , (4). (6), and (7) indicate that wetting depends on the surface free energies of the solid and the liquid. This suggests that the wettability of a solid by a liquid could be predicted if the surface free energy of the solid were known. Zisman and co-workers [26] measured advancing contact angles of homologous series of liquids on low-energy surfaces (fluorocarbons, hydrocarbons) and plotted the cos 8 values against the surface tensions of the liquids (Fig. 11.5). The lines representing homologous series of liquids extrapolated approximately to the same yLv value at cos 8 = 1. Zisman named this surface tension value the critical surface tension of the solid yc and proposed that only liquids having surface tensions below this value spread on the solid. Liquids with a surface tension above yc form a finite contact angle on the solid. When the plot of cos 8 against yLv resulted in a rectilinear band (Fig. 11.6), Zisman chose the intercept of the lower limb of the band at cos 8 = 1 as the critical surface ten-

FIG.11.5 Wettability of polytetrafluoroethylene by n-alkanes. (From Ref. 26.)

502

Chapter 11

503

Theory of Repellency TABLE 11.2

Critical Surface Tensions and Surface Free Energies of Polymers ~~

Poly(tetrafluoroethy1ene) Poly(trifluoroethy1ene) Poly(viny1idene fluoride) Poly(viny1 fluoride) Polyethylene Poly(chlorotrifluoroethy1ene) Polystyrene Poly(viny1 alcohol) Poly(viny1 chloride) Poly(methy1 methacrylate) Poly(viny1idene chloride) Poly(ethy1ene terephthalate) Poly(hexamethy1eneadipamide)

Zisma [26]

Owens [28]

WU [29]

Yc

YS

[Yc,+Irnax

18 22 25 28 31 31 33 37 39 39 40 43 46

19.1 23.9 30.3 36.7 33.1 29.5 42.0

22.6 29.5 36.5 37.5 35.9 32.1 43

41.5 40.2 45.0 41.3 47.0

43.8 42.5 45.2 43.8

sion of the solid. Critical surface tensions of smooth, plasticizer-free polymeric solids are given in Table 112. Although the critical surface tension concept has been criticized for its empirical nature [27], critical surface tension data have been very useful in developing water- and oil-repellent finishes. One reason for this practical impact may be the similarity between the systems studied by Zisman and the surfaces of waterand oil-repellent finishes. All polymers listed in Table 11.2 can be considered hydrophobic, because their critical surface tensions are well below the surface tension of water (72.8 dydcm at 20°C). Zisman and co-workers used the critical surface tension principle to correlate wettability with the constitution of low-energy surfaces. Condensed monolayers were prepared on polished platinum or glass by adsorption from solution and isolation by the retraction method. Data obtained by these methods [26] are tabulated in Table 11.3. The surface comprised of closest packed -CF3 groups has the lowest yL‘ and the lowest surface energy if yc correlates with the surface free energy. Substitution of one of the fluorine atoms by a hydrogen atom more than doubles ?(.. The effect of fluorine atoms on wettability of the surface is evident also by progressive replacement of hydrogen atoms in polyethylene by fluorine. The critical surface tension decreases linearly with the increasing fluorine substitution. Replacement of hydrogen atoms by chlorine increases yc. Zisman concluded that the critical surface tension and the wettability were determined by the nature and packing of the exposed surface atoms of the solid

Chapter 11

504 TABLE11.3 Surfaces

CriticalSurfaceTensions

Surface constitution Fluorocarbon surfaces -C F3 "CF2H -CF3 and -CF2-CF2-CH2-CF3 "CF2-CFH"CF2-CH2-CFH-CH2Hydrocarbon surfaces -CH3 (crystal) -CH3 (monolayer) -CH2-CH2and .=CH= .=CH= (phenyl ring) Chlorocarbon surfaces -CClH-CH2-CCl2-CHz=CCI;!

of Low-Energy yc, (dyns/cm at 20°C) 6 15 17 18 20 22 25 28

22 24 31 33 35

39 40

43

Source: Reprinted with permission from Ref. 26. 0 1964, American Chemical Society.

and are otherwise independent of the nature and arrangement of the underlying atoms [26]. This concept has predicted correctly the wettability of repellent surfaces in the absence of internal forces. Zisman was one of the first to recognize that the critical surface tension concept is strictly empirical and to suggest that -yc needs to be replaced by parameters having a thermodynamic or statistical mechanical basis [26]. Fox and Zisman [ 301 have cautioned that 'yc varies between liquid types and that it is not a measure of the surface energy of the solid -yso. The critical surface tension method for determining wettability also has some practical limitations. Several contact-angle measurements involving a series of liquids are necessary to determine 'yc of a single solid. Another limitation is the curvature of the cos 8 versus -yLv plot. Although the plot should yield a straight line, the relation is curvilinear even for a homologous series of liquids. This can cause considerable error when liquids with very low contact angles are not available and extensive extrapolation is necessary.

Theory of Repellency

505

Because of the deficiencies and limitations of the critical surface tension concept, the need to determine the surface free energy of solids has remained. Because the surface free energy of solids is difficult to measure, attempts have been made to estimate the surface free energy from interaction with liquids. Girifalco and Good [31-331 proposed that the interfacial free energy, yAB, between liquids A and B is given by YAB

=

YA

+ YB -24(~A~B)o.'

(10)

where YA and yBare the surface free energies of the components A and B and 4 an interaction parameter. The value of 4 depends on the interaction between two phases. When the interaction is limited to dispersion forces between immisible phases, 4 is approximately 1. Combining Eq. (10) with the Young equation (7). Good and Girifalco related the surface free energy of a solid, yso, to the contact angle 8

Good and Girifalco obtained a reasonably straight line when plotting cos 8 versus [ll(yLvloSand estimated the surface free energy, yso, of tetrafluoroethylene to be about 28 erg/cm2. The critical surface tension concept assumes that yLv = yC,when 8 = 0. It follows from Eq. (1 1) that when 8 = 0 and cos 8 = 1.O, Yc

Yso = 2

4

Thus, yc = yso if 4 = 1.O. Wu [ 291 proposed an equation of state by replacing yso in Eq. (1 1)with yC,+,to emphasize that 'yc is dependent on 4. The maximum of a curve obtained by plotting yC,+versus yLvwas considered to be equal to yso. Surface tension values of various solids obtained by this method are included in Table 1 1.2. Fowkes [28,34] suggested that the surface tension of a liquid or solid or the interfacial tension is the sum of the contributions made by intermolecular forces at the surface. The surface tension of a protic solvent such as water is therefore the sum of dispersion forces $ and hydrogen-bonding forces 7;. Fowkes assumed that only similar forces contribute to the interfacial tension and postulated a geometric mean rule for interaction. If only dispersion forces are operable across the interface, YSL

= Ysv

+

YLV

- 2 ( Y t Yfv)o.s

(13)

Chapter 11

506

By combining Eq. (13) with the Young equation (7), Fowkes derived the relationship

For interfaces where both dispersion forces and hydrogen-bonding forces operate YSL

= ysv

+ YLV - 2(y$y9°.5 - 2( y $ yi)O

(15)

Owens and Wendt [ 351 wrote Eq. (15) in a more general form: p)O.s

1 + c o s H = 2 ( cy$YLV Yi9°.s ) + 2 [

YLV

)

By measuring 8 of two different liquids against a solid and solving the simultaneous equations obtained for y$ and y;. the contributions of various forces to the surface free energy can be estimated and the sum of these components, the total solid surface free energy ys can be calculated [ 351. By using this method, Owens and Wendt found that in general the agreement between yc and ys was reasonable (Table l 1.2). 11.6 KINETICS OF WETTING The interaction of liquids with textiles may involve one or several fundamental physical phenomena: (1) wetting of the fiber surface, (2) transport of the liquid into an assembly of fibers, (3) adsorption on the fiber surface, and (4) diffusion of the liquid into the interior of the fibers [36]. Thetransport of a liquid into a fibrous assembly, such as yarn or fabric, may be caused by external forces or by capillary forces only. A spontaneous transport of a liquid driven into a porous system by capillary forces is termed “wicking.” Because capillary forces are caused by wetting, wicking is a result of spontaneous wetting in a capillary system. On basis of the mode of the liquid-fabric contact and the relative amount of liquid involved, the wicking processes can be divided into two groups: (1) wicking from an infinite liquid reservoir: immersion, transplanar wicking. and longitudinal wicking; (2) Wicking from a finite (limited) liquid reservoir (a single drop wicking into a fabric). The wicking rate from an infinite liquid reservoir depends on the capillary dimensions of the substrate and the viscosity of the liquid [37,38]. For a theoretical treatment of capillary flow in fabrics, the fibrous assemblies are usually considered to consist of a number of parallel capillaries [39]. The advancement of the liquid front in a capillary occurs in small jumps. The advancing wetting line in a single capillary stretches the meniscus of the liquid until the elasticity of the

Theory of Repellency

507

meniscus and the inertia of flow are exceeded. The meniscus contracts and pulls more liquid into the capillary to restore the equilibrium state of the meniscus. The movement of the liquid may be discontinuous for another reason as well. The wetting front advances into the capillary system in small jumps if the capillary spaces are irregular with various dimensions. The volume rate dVldt of flow in a tube is given by Poiseuille’s equation dV dt

-

m4P

” -

871

where Y is the radius of the tube, 1 is the distance covered by the liquid front during time t, and P is the pressure drop across the distance I. For linear flow dlldt,

Poiseulle’s equation states that the flow rate in a tube is inversely related to the distance of the liquid movement. Based on Poiseulle’s equation, Lucas [40] and Washburn [41] developed an equation for flow rates in capillaries:

The limitations of the Washburn-Lucas equation are frequently overlooked. The equation assumes incorrectly a constant advancing contact angle OA for the moving meniscus r42.431. The Washburn-Lucas equation (19) does not take in account the inertia of the flow [ 251, and implies that at time t = 0 and 1 = 0, the flow rate is infinite. In spite of these limitations, a variety of liquids have obeyed the Washburn-Lucas wicking kinetics [44]. Other forms of the Washburn-Lucas equation have been suggested [45” 11. Wicking is affected by the morphology of the fiber surface and may be affected by the shape of the fibers as well. The common belief that fiber shape does not affect wetting is valid only for the wetting of single fibers. The shape of fibers in an assembly such as yarn or fabric affects the size and geometry of the capillary spaces between fibers and, consequently, the rate of wetting. The flow in a capillary may stop when geometrical irregularities allow the meniscus to reach an edge and flatten [ lo]. The kinetics of wicking from a finite (limited) liquid reservoir (a single drop wicking into a fabric) are more complicated than those of wicking from a liquid pool of essentially constant volume. Wicking of a drop can be divided into two phases of different kinetics [52-551. At first, the drop spreads on the substrate and penetrates the porous substrate underneath. As long as most of the liquid remains on the outer surface of the substrate, the capillary penetration is kinetically similar to that from an infinite reservoir [56]. During the second phase of the capillary

Chapter 11

508

penetration process, all of the liquid is contained within the substrate and spreads radially under the influence of capillary forces. Gillespie [52] measured the radius of a the spreading liquid in paper and developed an equation based on d’Arcy’s law :

where

and where R denotes the radius of the circle covered by the spreading liquid at time t, Ro is the radius of the wet spot at time zero, V is the volume of liquid, y is the surface tension, 7 is the viscosity of the liquid, b is a constant descriptive of the substrate, q is the permeability of the substrate, eA is the advancing contact angle, h is the thickness of the substrate, and c is the saturation concentration of the liquid in the substrate. Unlike paper, most textile fabrics are not isotropic and the spreading liquid does not usually form a circle with a measurable well-defined radius. Therefore, Kissa [53-551 measured the area covered by the spreading liquid by a photometric technique and wrote the equation

where A denotes the area of the circle covered by the s reading liquid at time t, A. is the area of the wet spot at time zero, and K , = If the initial area covered by the drop resting on the surface is relatively small and can be neglected, the area A covered at time t by the liquid spreading in a fabric is given approximately by the equation [ 53,541

d

where Vis the initial volume of the drop, the superscripts e, f , and g are constants, and K is a coefficient [53-551. Usually, e and g have the value 0.33, and f is 0.67. The coefficient K depends on the dynamic contact angle, the permeability, and the thickness of the substrate. Equation (23) holds for capillary penetration of a drop in assemblies of fibersimpermeable to theliquid,suchasspreading of hydrocarbonsin a polyester-cotton fabric [53,54]. Marmur [57] compared the kinetics of penetration from a infinite and finite reservoir using a simplified model for radial penetration [58]. Under the conditions simulating a limited (finite) reservoir, the slope of the logarithmic area-time plot was found to be 0.32, in excellent agreement

Theory of Repellency

509

*t II

I

I

0.I

0.5

I

I

I

I

5 IO 2 TIME (rnin 1

I

I

20

50

FIG. 11.7 Spreading of n-alkanes (drop size 0.10 mL) in 65 : 35 poly(ethylene terephthalate)-cotton blend fabric: Clo-n-decane; C14-n-tetradecane. (Reproduced with permission from Ref. 55. Copyright 0 1981 by Academic Press.)

with the slope 0.31-0.33 that Kissa had found for the penetration of a drop (Fig. 11.7). The area covered by a hydrocarbon liquid within a certain time period is correlated with the oil repellency of the fabric (Fig. 11.8). Usually, oil repellency is tested inder static conditions by using a series of hydrocarbon liquids of different surface tensions (see Section 12.5). A measured drop of the liquid is placed on the fabric. After 30 s, the drop is removed and the wicking of the liquid into the fabric is observed. If the drop has not left a significant wetted spot on the fabric, the next higher numbered liquid of lower surface tension is placed on the fabric, until noticeable wetting occurs. Instead of using different hydrocarbon liquids, the area covered by the liquid of the lowest surface tension in the series can be measured to determine oil repellency. The wicking process is kinetically quite different when capillary penetration is accompanied by diffusion of the liquid into the fibers into a finish on fibers. Sorption within fibers decreases the volume of the liquid flowing in the capillary spaces and reduces the interfiber spaces available for capillary penetration because of swelling of the fibers. As a consequence of these complications, the exponent g in Eq. (23) is no longer constant but depends on the drop volume [53,54]. The area covered by the liquid spreading within the fabric does not correlate with the drop absorbency time. When the drop absorbency time is used to evaluate fabric absorbency, an inadequate drop volume can lead to misleading results if cap-

Chapter 11

510

30

- 20

(v

E

Y

U

w

a U

IO

:\ I

I

I

0

I

I

3 4 2 OIL REPELLENCY

5

I

I

FIG.11.8 Area covered by the spreading liquid versus oil repellency (areas covered in 60 s by 0.10 mL n-heptane spreading on a 65 : 35 poly(ethy1ene terephthalate)-cotton blend fabric finished with various fluorinated repellents). (From Ref. 59.)

illary penetration is accompanied by absorption of the liquid within a finish or the interior of fibers [60,61]. When the capillary spaces in a fabric are not uniform, the liquid may not spread as a continuous front but penetrate some capillaries before others and form a “fingering” pattern. A nonuniform wicking patterns complicates the measurement of the wetted area, even when using imaging techniques. However, the pattern itself provides useful information about the wetting characteristics of the fabric. 11.7

REPELLENCY OF FABRICS

The resistance of a fabric to wetting and penetration of a liquid, such as water or oil, depends on the chemical nature, geometry, and roughness of the fiber surfaces and the capillary spacings in the fabric. The hydrostatic pressure, AP, required to force a liquid through a fabric is given by the Laplace equation

Theory of Repellency

511

where R and R2 are the principal radii of curvature of the liquid surface. If the liquid surface is part of a sphere, R1 and R 3 are equal, so that

The pressure required for the penetration of fabrics through spaces between the yarns is given by [61]

where n is the radius of yam, 2b is the distance between yarns (2b + 2a is the distance between the centers of adjacent yarns), and OA the apparent advancing contact angle on the yarn (Fig. 11.9). According to Eq. (26), the pressure needed for penetration increases with the decreasing spacing of the yarns if the apparent contact angle is above 90". Equation (26) suggests that a tight weave and high bulk density are beneficial for repellency by reducing the spaces available for penetration. On the other hand, the contact angle is also affected by the bulk density. Cassie and Baxter [ 191 showed that an apparent advancing contact angle OD of a liquid drop resting on an ideal porous surface is related to the advancing contact angle for the nonporous surface by cos eo = f lCOS eA - f 2

(27)

The apparent receding contact angle Ow is related to the receding contact angle for

iu FIG.11.9 Contact angle formed by a liquid on a repellent fabric.

Chapter 11

512

a nonporous surface, OR, by

where

+

and r is the radius of fibers with their axes 2(r d ) apart. Baxter and Cassie [62] verified Eq. (27) and (28) experimentally by measuring contact angles of water on model porous surfaces made of parallel strands of fine copper wire. The apparent contact angles calculated for a wide range of bulk density values were found to increase with the decreasing bulk density of the yarn. The large values of the apparent advancing and receding contact angles calculated for water on ideal porous yarns suggested that raindrops falling on a fabric made of such yarns should withdraw from a pore after the initial impact and roll offthe fabric surface without wetting it. This was based on the assumption that the kinetic energy of the drops falling onto the fabric is not so large that the drops can penetrate the fabric on impact. The apparent-contact-angle values for water on wool fibers are high (120"). partly because of the rough and scaly fiber surface. Because the bulk density of the wool yarn is usually low (0.5 g/cm2), the contactangle values for water on wool yarn are even higher (>145"), and a wool fabric should provide sufficient resistance against rain. This is, indeed, the case with the first raindrops falling on the fabric. After continued exposure, the drops no longer re-form but wet the fabric. Baxter and Cassie [62] explained this by postulating that formation of water films between the fibers causes the receding contact angle to decrease to a low value. These authors suggested that to avoid film formation, the fibers should be uniformly separated and held in the fabric so that the surface tension of water cannot pull them together. The duck's feather, a perfect example of a water-repellent structure, has barbs equipped with fine fibers called barbules. The barbules on one side of the barb carry hooks and those on the other side are notched. The two types of barbules link together and provide for an open structure that cannot be drawn together by water [ 191. Segall [63] pointed out that tightly woven cotton fabrics are more resistant to rain than the best woolen fabrics. According to the Baxter and Cassie theory, tight weaving of fine yarns should have facilitated interfiber film formation and increased wetting. Segall suggested that optimum water resistance is obtained when thehydrostatic pressure resistance of the fabric is equal to the interfiber penetration pressure of the yarn surface forming the walls of the fabric pores. Segall's concept is not entirely in conflict with the views of Baxter and Cassie, who also favor a uniform fiber spacing. Baxter and Cassie were working with wool fabrics,

Theory of Repellency

513

whereas Segall was concerned with water resistance of cotton fabrics. The swelling of cotton wetted by water affects the pore size and interfiber spacings. and by proper construction, self-sealing fabrics can be made. A cotton fire hose is an example. Because modern rainwear fabrics are made water repellent by chemical finishing, the main concern is to select the fabric structure best suited for finishing with repellent chemicals. Davis [64] suggested that the fabric should be very tightly woven to eliminate shifting of fibers during wear. A careful preparation of the fabric before finishing is essential for the absorbency of the tightly woven fabric during finishing. Wetting agents should be avoided because they may reduce the water repellency of the finished fabric. The presence of impurities and residual chemicals can lower the repellency considerably [65]. The nature of the support under the repellent fabric and its compressibility are important [66,67]. Penetration is more likely to occur on a rigid support than on a compressible surface. The wettability of the liner under the fabric can also affect the repellency of the garment. An absorbent liner can increase wetting, whereas a repellent liner can function as a second line of defense against penetration by water. The resistance of a fabric to penetration and wetting by a liquid (e.g., water or oil) depends on the chemical composition, geometry, and roughness of fiber surfaces and the capillary spacings between the fibers [59]. The repellent finishes differ in their ability to lower the surface energy of the fibers and increase the contact angles. The critical surface energy concept developed by Zisman [26] is quite useful in predicting the chemical nature of the repellent finish needed to resist wetting by a liquid. Water has a high surface tension (72.0 d y n h n at 25°C). Therefore, repellent finishes featuring a hydrocarbon hydrophobe with a yc value of about 30 or polysiloxane finishes with a yc value of about 24 dyne/cm are sufficient for water repellency. To repel hydrocarbon oils with surface tensions ranging from 20 to 3 1 dynkm, a fluorocarbon finish is needed to lower the critical surface tension of the fibers to below 15 dyn/cm. However, the initial repellency of a finish is not the only criterion for selecting a repellent. Durability to dry cleaning and laundering, resistance to abrasion and soiling, ease of application, and the cost of the repellent are important factors to be considered.

REFERENCES 1. R. E. Read and G. C. Culling, Am. Dyestuff Rep. 56, P881 (1967). 2. J. W. Rowen and D. Gagliardi, J. Res. Natl. Bur. Stand. 38, 103 (1947). 3. J. W. Gibbs, Trans. Connecticut Acad. 3 (1876-1878); Collected Works, vol. I. Longmans, Green, New York (1928). 4. A. DuprC,“ThCorieMCchanique de la Chaleur,” p. 369Gauthier-Villars.Paris ( I 869). 5. T. Young, Phil. Trans. Roy. SOC. (London) 95,255 (1858).

514

Chapter 11

6. J. J. Bikerman, Kolloid Z. 218,52 (1967). 7. C. G. Sumner, “Symposium on Detergency.” p. 15, Chemical Publ. Co.,New York (1937). 8. H. Shuttleworth and G. L. Bailey, Disc.Faraday SOC.3, 16 (1948). 9. R. E. Johnson,J. Phys. Chem. 63,1655 (1959). IO. A. Marmur, in “Modern Approaches toWettability,” M. E. Schrader and G. I. Loeb, eds., Plenum Press, New York(1992). 11. A. W. Adamson.“Physical Chemistry of Surfaces,” 5thed., John Wiley& Sons, New York (1990). 12. N. K. Adam, in “Contact Angle, Wettability and Adhesion.” R. F. Gould. ed. Advances in Chemistry Series Vol. 43, p. 52, American Chemical Society. Washington, DC (1964). 13. F. M. Fowkes and W. D. Harkins. J. Am. Chem. SOC. 62.3377 (1940). 14. R. E. Johnson and R. H. Dettre,in “Contact Angle, Wettability and Adhesion,” R. F. Gould, ed.. Advances in Chemistry Series Vol. 43, p. 112. AmericanChemical Society, Washington, DC (1969). 15. R.H. Dettre and R. E. Johnson,in “Contact Angle, Wettability, and Adhesion,” R. F. Gould, ed., Advances in Chemistry Series Vol. 43. p. 136, American ChemicalSociety, Washington, DC (1964). 16. J. F. Oliver. C. Huh, and C. G. Mason, Colloids Surf. 1.79 (1980). 17. M. Tagawa. K. Gotoh. A. Yasukawa, and M. Ikuta. Colloid Polym. Sci. 268, 589 (1990). 18. R.N. Wenzel, Ind. Eng. Chem. 28.988 (1936). 19. A. B. D. Cassie and S. Baxter, Trans.Faraday SOC. 40, 546 (1944). 20. R. J. Good, J. Am. Chem. SOC.74,504 (1952). 21. D. C. Pease,J. Phys. Chem. 49,107 (1945). 22. R. E. Johnson and R. H. Dettre. J. Phys. Chem. 68, 1744 (1964). 23. R. H. Dettre andR. E. Johnson. J. Phys. Chem. 69, 1507 (1965j. 24. (a) L. Grader, Colloid Polym. Sci. 264, 719 (1986); (b) A. Siebold, M. Nardin, J. Schultz, A. Walliser. and M. Opplinger, ColloidsSurf. A161, 81 (2000). 25. A. A. Jeje, J. Colloid Interf. Sci. 69.420 (1979). 26. W. A. Zisman. in “Contact Angle, Wettability, and Adhesion,” R. F. Good, ed., Advances in Chemistry Series Vol.43, p. 1, American Chemical Society, Washington, DC (1964). 27. B. Miller, in “Surface Characteristicsof Fibers and Textiles,” M. J. Schick, ed., p. 417. Marcel Dekker, New York(1977). 28. (a) F. M.Fowkes, J. Phys. Chem. 66,382 (1962); (b) J. Phys. Chem. 67,2538 (1963). 29. S. Wu, J. ColloidInterf. Sci. 71,605 (1979). 30. H. W. Fox and W. A. Zisman, J. Colloid Sci. 7, 109 (1956). 31. L. A. Girifalcoand F. J. Good, J. Phys. Chem. 61.904 ( 1957). 32. R. J. Good, L. A. Girifalko, and G. Kraus. J. Phys. Chem. 62, 1418 (1958). 33. R. J. Good and L. A. Girifalko, J. Phys. Chem. 64,56 1 (1960). 34. F. M. Fowkes, Ind. Eng. Chem.56,40 (1 964). 35. D. K. Owens and R. C. Wendt, J. Appl. Polym. Sci. 13, 1741 (1969). 36. E. Kissa, Textile Res.J. 66, 660 (1996). 37. S. Chwastiak, J. Colloid Interf. Sci. 42,298 (1973).

Theory of Repellency 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59.

60. 61. 62. 63. 64. 65. 66. 67.

515

T. Gillespie and T. Johnson. J. Colloid Interf. Sci. 36,282 (1971). P. R. Lord, Textile Res. J. 44,5 16 (1974). R. Lucas. Kolloid Z. 23, 15 (1918). E. W. Washburn, Phys. Rev. 27,273 (1923). L. R. Fisher and P. D. Lark, J. Colloid Interf. Sci. 69,496 (1979). P. Joos, P. Van Remoortere, and M. Bracke. J. Colloid Tnterf. Sci. 136, 189 (1990). K. T. Hodgson and J. C. Berg, J. Colloid Tnterf. Sci. 121,22 (1988). J. Szekely, W. Neumann, and Y. K. Chuang, J. Colloid Interf. Sci. 35,273 (1971). M. Bracke, F. De Voeght, and P. Joos. Prog. Colloid Polym.Sci. 79, 142 (1989). E. Chibowski, Langmuir 9, 330 (1 993). S. Chwastiak, J. Colloid Tnterf. Sci. 42, 298 (1973). R. J. Good, J. Colloid Interf. Sci. 42.473 (1973). R. J. Good and N.-J. Lin, J. Colloid Interf. Sci. 54, 52 ( I 976). P.Joos, P. Van Remoortere, andM. Bracke. J. Colloid Interf. Sci. 136. 189 ( 1990). T. Gillespie, J. Colloid Interf. Sci. 13. 32 (1958). E. Kissa, J. Colloid Interf. Sci. 83,265 (1981). E. Kissa, Pure Appl. Chem. 53,2255 (1981). E. Kissa, in “Detergency. Theory and Technology,”W. G. Cutler and E. Kissa. eds., pp. 209-212, Marcel Dekker, New York, (1987). T. Kawase, S. Sekoguchi, T. Fujii, and M. Minagawa, Textile Res.J. 56,409 (1986). A. Marmur, Adv. Colloid Interf. Sci. 39, 13 (1992). A. Marmur, J. Colloid Interf. Sci. 124. 301 (1988). E. Kissa, in “Functional Finishes, Part B, Handbook of Fiber Science and Technology.” M. Lewin andS. B. Sello,eds., Vol. 11, pp. 144-210, Marcel Dekker,New York (1984). C. Heinrichs, S. Dugal, G. Heidemann, and E. Schollmeyer, Textil-Praxis 37, 515 ( 1982). C. Heinrichs, S. Dugal, G. Heidemann, and E. Schollmeyer, Melliand Textilber. 67. 807 (1986). S. Baxter and A. B. D. Cassie, J. Textile Tnst. 36, T67 (1945). G. H. Segall, Textile Res.J. 22.736 (1952). C. A. Davis, Am. Dyestuff Rep. 56, P555 (1967). R. L. Wayland, Jr., et al. Am. Dyest. Rep. 52, P1059 (1963). M. Karrholm and G. Karrholm, Textile Res. J. 20,215 (1950). A. M. Sookne, F. W. Minor, J. E. Simpson. and M. Harris, Am. Dyestuff Rep. 35,295 (1946).

Fluorinated Repellents

12.1

REPELLENTSWITHHYDROCARBONHYDROPHOBES

Waxes and Waxlike Substances Repellents have hydrocarbon, polysiloxane. and/or fluorocarbon hydrophobes. Only fluorinated repellents are oleophobic and repel both water and oil. The oldest and most economical way to make a fabric water repellent is to coat it with a hydrophobic substance, such as paraffin. Wax or waxlike substances have been applied as solids to the fabric followed by heating them to a molten state, or as solutions in organic solvents or aqueous emulsions. The application of wax or paraffin emulsions containing aluminum acetate or formate was at one time the most important water-repellent treatment of cotton [l-31. At first, the aluminum acetate and the wax were applied separately in a two-step process. Later, stable wax emulsions were developed and applied from a single bath that also contained aluminum acetate or formate. Proteins, such as glue or gelatin, were used as protective colloids to increase the bath stability [2,4]. The efficacy of the water-repellent treatment was increased further by incorporating aluminum soaps together with aluminum acetate and a wax emulsion in the same bath [2]. The durability of the wax-containing finishes to laundering and dry cleaning was increased substantially by replacing aluminum salts with zirconium acetate, ammonium dizirconyl carbonate, or zirconium oxychloride [ 1-3,5]. The stability of wax dispersions and the durability of wax-containing repellent finishes have been increased by formulating wax dispersions with polymers, such as poly(viny1 alcohol) [6], polyethylene [ 71, acrylic polymers, for example, poly(buty1 acrylate) [8], copolymers of stearyl acrylate or stearyl methacry516

Repellents

517

Fluorinated

late-dodecenylsuccinic acid-acrylic or methacrylic acid [9], ethylenemethacrylic acid copolymers [ 101, or ethylene-methacrylic acid-vinyl acetate copolymers [ 1 11. Cross-linking reactants are coapplied to increase the durability of the finish and impart dimensional stability and wrinkle resistance to cellulosic fabrics. The water repellents formulated with waxes and polymers are used as extenders for polymeric fluorochemical repellents.

Aluminum and Zirconium Soaps One of the oldest water repellents is aluminum acetate [ 1.41 hydrolyzed on fibers to basic aluminum acetates and hydroxides of undefined structure. The poor adhesion and dusting of the repellent was a major shortcoming of the finish. An improvement was made by applying a water-soluble soap to the fibers and precipitating it with an aluminum salt, such as the acetate, formate, or sulfate:

+ )3HCOONa ~A~ 3C17H3sCOONa+ (HC00)3A1-+( C I ~ H ~ ~ C O O Instead of the two-bath treatment, aluminum soaps have been applied as solutions in an organic solvent or as aqueous dispersions. Aluminum soaps, such as aluminum palmitate or aluminum stearate, are insoluble in water but dissolve in an alkaline detergent solution. Therefore, the washfastness of the water-repellent finishes made with aluminum soaps is poor. Because zirconium soaps are more hydrophobic and resistant to alkali than aluminum soaps, the durability of repellency was improved by replacing the aluminum with zirconium in the form of the acetate [4] or the oxychloride; the latter was buffered with sodium acetate to protect the fabric against HC1 formed by hydrolysis of ZrOC13. Copper soaps have been used to produce water-repellent finishes that also function as fungicides and protect the fabric against rot [ I].

Metal Complexes In 1950, Du Pont marketed Werner-type chrome complexes under the trademark Quilon [ 12,131with the structure

H

Quilon chrome complexes are sold as a solution in isopropyl alcohol. Be-

Chapter 12

518

cause the solutions contain a small amount of water, the complexes ionize as shown in the following: 2+

717"35

CI-Cr

H

2CI

Dilution with water, raising the pH of the solution, or heating causes the complex to hydrolyze. With further heating or on prolonged standing, the complex polymerizes forming -Cr-0-Crlinkages. During application, the polymerization of the chrome complexes in the treating baths is not allowed to proceed so far as to cause the precipitation of the polymer. When the fabric treated with the chrome complex is cured by heating at 15O-17O0C, further polymerization of the complex occurs. The complex can also form covalent bonds with -OH, "COOH, "CONH,, or " S 0 3 H groups in the surface of the fibers. Because the inorganic part of the complex is bonded to the surface, the organic hydrophobic part is oriented perpendicularly away from the surface, providing water repellency to the fabric. 2+ 717H35

0

i

I

Ho-c~o~cr-oH H

The above structures show Quilon S, the stearic acid complex. Quilon M is the chrome complex of myristic acid and Quilon C is a modified M- and S-type chrome complex. Quilon solutions are acidic. When treating cellulosic or other acid-sensitive materials, the pH has to be adjusted with hexatetramethylenetetratnine or NaOH to 4.5 (Quilon C) or 3.5 (Quilon S or M). Quilon chrome complexes produce semidurable water-repellent finishes on natural and synthetic fabrics. The treated fabrics have very good initial water repellency and a soft handle. Quilon complexes are used where their blue-green coloration is not objectionable, such as for tents, awnings, boat covers, and so forth.

519

Fluorinated Repellents

To eliminate the color of the metal complex, Du Pont developed an aluminum complex of myristic acid. However, it was less effective on cellulosic fibers than the chrome complex.

Pyridinium Compounds The interesting history of the pyridinium-type water repellents has been reviewed by Harding [4]. In 1931, Deutsche Hydrierwerke [ 141 patented quarternary ammonium compounds prepared by reacting stearyl alcohol with formaldehyde and dry HCl. CIgH370H + HCHO

+ HCl + CIgH370CHaCl + H20

and reacting the chloromethyl ether produced with a tertiary amine. In one of the patent examples, the tertiary amine was pyridine C18%70CH2CI

+ @u -

-ci,CH20C18H37

A later patent [ 151 suggested the use of these quaternary ammonium compounds as dyeing auxiliaries and stated that they react with cellulose to form an acetal. Water-repellent properties of these compounds were also mentioned. Reynolds, et al. (ICI) [ 161 discovered that pyridinium compounds featuring a long alkyl chain reacted with cellulose when heated and produced a durable water-repellent finish on cotton. The reaction with cellulose was thought to involve the chloroalkyl ether as an intermediate,

+CI~H~$T~CI CELLULOSE CELLmOCH2OC18H37

Stearyloxymethylpyridiniunl chloride was laterreplaced by stearamidomethyl pyridinium chloride, produced by reacting stearamide with pyridine hydrochloride, pyridine, and paraformaldehyde [ 171:

The product was marketed under the trademark Velan PF by ICI. Davis [ 181 confirmed that stearamidomethylpyridinium chloride reacts with

Chapter 12

520

cellulose but suggested that the reaction involves methylolstearamide as the reactive intermediate: C + J ~ C O N H C H ~ ~ ~ H20C I - C17H35CONHCH20H

I

+

0

HCI

1

HEAT CELLULOSE C,~H~SCONHCH~O-CELL

Davis concluded that up to 2% owf (of the weight of the fabric) stearamidomethylpyridinium chloride can, via the methylol intermediate. react with cellulose. An excess forms only decomposition products not bonded to the fabric. Schuyten and co-workers [ 191 reinvestigated the reaction and confirmed that stearamidomethylpyridir~iumchloride reacts with cellulose to the extent of 1-2% of the weight of the fabric. This indicated that the reaction is restricted to the surface of the fibers. However, methylolstearamide did not react with cellulose, in contrast to the conclusions by Davis. Schuyten and co-workers concluded, therefore, that the quaternary stearamidomethylpyridinium salt itself is the reactive species. Distearamidomethane formed during curing is deposited on fibers and also contributes to nonpermanent repellency. Stearamidomethylpyridinium chloride is applied to cellulosic fabrics by a pad-dry-cure process. A buffer, usually sodium acetate. is added to prevent tendering of the fabric by the HCl formed. Pyridine liberated during the reaction has an unpleasant odor, and the fabric has to be scoured after the cure. De Marco and Dias [ 201 observed a synergistic effect with pyridinium-type water repellents coapplied with fluorochemical repellents, resulting in a long-lasting protection against rain and a good durability to laundering. This finish was named Quarpel by its inventors. The production of pyridinium-type water repellents has been recently curtailed because of toxicological considerations.

Methylol Compounds N-Methylol chemistry, utilized for cross-linking of cellulose, has also been successfully exploited as a means for producing durable water repellents. N-Methylo1 compounds are prepared by reacting an amine or amide with formaldehyde: RNH? + HCHO -+RNHCH2OH RCONHz

+ HCHO + RCONHCHZOH

In the presence of acidic catalysts, N-methylol groups can react at elevated temperatures with hydroxyl groups of cellulose: -NHCH20H

+ HO-Cell -+-NHCHZO-Cell

llents

Fluorinated

521

Willard and co-workers [21] have shown that the preferred reaction site on cellulose is the primary (C,) hydroxyl. N-Methylol compounds can also react with other N-methylol compounds and form resins by self-condensation. The reaction with cellulose is, therefore, accompanied by the formation of resinous products in varying amounts. Because N-methylol compounds can react with compounds that have an active hydrogen atom ( e g , alcohols, amines, and carboxylic acids), they are very useful for introducing hydrophobes into the repellent molecule. Because acids catalyze the self-condensation to resins, N-methylo1 compounds are converted with an alcohol, usually methyl alcohol, to ethers, which are more stable and can be reacted with a long-chain fatty acid to form the repellent molecule. The simplest water repellent based on methylol chemistry is derived from stearamide [ 22,231 C 17H 35CONH2 + HCHO + C 17H 35CONHCHZOH or stearylurea C17H35CONHCONH2 + HCHO + C17H35CONHCONHCHZOH The reaction with formaldehyde has also been carried out i n situ by applying stearamide or stearylurea first, and then treating the fabric with formaldehyde. Urea can react readily with 2 mol of formaldehyde:

NH? CONH.,

NH~CONHCH~OHHCHO. HOCHZNHCONHCH20H

A third mole of formaldehyde reacts only under vigorous conditions. However, melamine can react with 1-6 mol of formaldehyde because each of its three amino groups is capable of reacting with 2 mol of formaldehyde. This produces a molecule with up to six functional groups that can bond hydrophobes, react with cellulose, or condense to resinous products: y42

NHCH20H I

Chapter 12

522

A variety of water repellents have been prepared by reacting methylolmelamines or methoxymethylmelamines with a hydrophobic compound having an active hydrogen atom (e.g., an acid, alcohol, amine). A large number of patents have been issued since the beginning of this activity in the 1930s. The early patents described formation of the water repellent irz sitzr by heating the fabric impregnated with the reactants. For example, octadecylamine acetate was reacted on fibers with melamine and formaldehyde [ 241. Methoxymethylmelamine was reacted with a long-chain alcohol [25]. Later processes were developed to prepare a repellent product and apply it to the fabric as an emulsion or dispersion [26,27]. The water-repellent melamine derivative is usually coapplied with a wax to boost repellency. For example, in one of the patents to CIBA [28] methylolmelamine is converted with methyl alcohol to its methyl ether, which is reacted with stearic acid. The resulting ester is mixed with molten paraffin, a nonionic surfactant, dimethylethanolamine, poly(viny1 alcohol) or hydroxyethylcellulose, and water. The emulsion obtained is acidified with HCl to pH 5.0, aluminum sulfate is added, and the emulsion is diluted with water to make a pad-bath. The padded fabric is heated 5 min at 150°C to cure the finish.

Other Fiber-Reactive Water Repellents Chemical reactions other than those of pyridinium and tnethylol compounds have been explored in an effort to bond the hydrophobic species covalently to the fiber and produce a durable water-repellent finish. Long-chain isocyanates have been patented [29] as water repellents. The reaction of octadecyl isocyanate with cellulose was studied by Hamalainen and coworkers [30], who concluded that only a part of octadecyl isocyanate reacted with cellulose: C sH 37NC0 + Cell-OH

-+Cell-OCONHC

8H37

The rest of the octadecyl isocyanate reacted with water, producing mainly dioctadecylurea, which was deposited on the fibers and contributed to the repellency durable only to mild laundering conditions. Because cellulosic fibers contain water, the side reactions of isocyanates with water cannot be avoided by solvent application [29,311 of the repellent. Although the direct application of isocyanates proved to be impractical, their reaction products have been in commercial use as water repellents. Octadecy1 isocyanate was reacted with ethylenimine to yield an aziridinyl compound [ 32-34]: C18H3,NC0 + HN/TH1 \ +

Fluorinated Repellents

523

which, in turn, is reacted with cellulose:

The reaction with cellulose is accompanied by polymerization of the aziridinyl compound. The durability of water repellency to laundering was improved by coapplying a di(aziriny1) compound, such as

The dispersion containing both the mono(aziriny1) and the di(aziridiny1) compound-was applied to the fabric by padding, followed by drying and curing at 150°C. The reaction of epoxides with cellulose in the presence of alkali or acid [35-371 offers another route to durable water repellency [38]. /O\ C14H&H-CH2

+ HO-Cell

-+ C14H29CH(OH)CH~O-Cell

The repellent was applied from a volatile solvent to suppress side reactions with water. Cotton fabric has been made water repellent by acylation with isopropenyl stearate [39,40]:

ii

'iH3 C17H&-O-C=CH2

0

II + HO-Cell + C17 H3s C-0-Cell

+ CH3COCH3

The fabric is padded with a solution of the reactant and an acid catalyst (e.g., p toluenesulfonic acid) in benzene and heated 4-7 at 180-210°C. Although benzene, considered to be a carcinogen, could be replaced by another solvent such as toluene, the practicality of a finishing process using flammable solvents is limited. The chemistry of fiber-reactive dyeing of cellulose has also been considered for producing durable water repellency. Monochlorotriazines and dichloro-

Chapter 12

524

triazines with a hydrophobe bonded via a -NH[411.

linkage have been proposed

+

co2

In analogy to vinyl sulfone fiber-reactive dyes, water repellents with the vinyl sulfone function have been patented [42]. The P-hydroxyethylsulfate group, which, in the presence of alkali forms the reactive vinyl sulfone intermediate, functions as a water-solubilizing group and gives the water repellent an amphiphilic character until reacted with alkali. C18H37S0ZCH2CH20SO3Na

1..

ClsHyS02CH

I

= CH2

HO -Cell

C18H37S02CH2CH20-Cell Water repellents with a chlorotriazine or vinyl sulfone functional group react with cellulose in the presence of alkali. Therefore, they are not compatible with cross-linking reactants requiring acid catalysis for the reaction with cellulose. This limitation, in addition to the cost, is one of several reasons why fiber-reactive chemistry developed for dyes has not been successfully adaptable to repellent finishing.

Mixtures of Hydrocarbon and Fluorocarbon Repellents Hydrocarbon-based repellents are relatively inexpensive but they repel only water. They do not impart oil and soil repellency to textiles. In contrast, fluorinated repellents provide oil and soil repellency as well as water repellency. However, the higher cost of fluorinated repellents limits their applications. To lower the cost of repellent finishing, fluoropolymers are usually coapplied with hydrocarbontype repellents. With a hydrocarbon repellent as an extender, fluorinated repellents can be usedat a lower concentration without an adverse effect on repellency [43-571. The numerous examples of extenders described in the patent literature include maleic anhydride-octadecene copolymers [55],carbodiimide extenders prepared by reacting isocyanates with isostearyl alcohol [53], and stearyl acrylate-2hydroxyethyl methacrylate-N-rnethylolacrylarnidecopolymers [54].

llents

Fluorinated

525

With some hydrocarbon-type repellents, a synergistic effect has been observed. De Marco et al. [58,59] found that a pyridinium-type repellant, stearoyloxymethylpyridinium chloride, has a synergistic effect when coapplied with an acrylic fluorinated copolymer and a poly(octy1 methacrylate). The effect of nonfluorinated repellents on the performance of fluorinated repellents is complicated by other adjuvants in the finish, such as cross-linking agents for cotton, softeners, and sewing aids. The interactions between the various components and their effect on repellency and physical properties of the fabric must be considered when formulating a repellent finish. An extender effect can be achieved as well by including a long-chain alkyl group in the fluorinated repellent polymer (e.g., by copolymerizing a fluorinated acrylate or methacrylate with stearyl acrylate) [60-621. 12.2 SILICONES (POLYSILOXANES)

Chemical Reactionsof Silanes The hydrophobicity of silicones was discovered at General Electric by Patnode [63], who observed that paper treated with chloromethylsilanes became water repellent when exposed to moist air. Chloromethylsilanes are hydrolyzed by water to silanols, which condense spontaneously to siloxanes. Chlorotrimethylsilane yields hexamethyldisiloxane:

+

+ HOSi(CH3)3 2HOSi(CH3)3 + (CH3)3 SiOSi(CH3)3 + H20

C1Si(CH3)3 H 2 0 + HCl

Dichlorodimethylsilane yields, depending on reaction conditions, 20-50% cyclic siloxanes and 80-50% linear polydimethylsiloxanes: H20/[(CH313Si0]

I CH3

n>3

OSi-OH I CH3 X

Trichloromethylsilane yields cross-linked polymethylsiloxanes. Condensation reactions can occur between "SiOH and "SiH groups, if present, and between two "SiOH groups. In the presence of peroxides or upon irradiation, two -SiCH3 groups can also undergo a condensation reaction. The hydrolysis and

Chapter 12

526

condensation reactions of chloromethylsilanes or chlorohydrogensilanes are, therefore, more complex than those shown above. Chloromethylsilanes are corrosive and difficult to handle. Processes for the treatment of textiles with chloromethylsilanes [63] that used ammonia to absorb the hydrochloric acid formed were not successful. The replacement of chlorine, with acetoxy [64], alkoxy [65,66], amino [67], or isocyanato [68] groups has been patented. None of these conversions has resulted in a practical process. Therefore, silanes are not usedas such for textile finishing but are converted to polysiloxanes, which can be applied to textiles as solutions in organic solvents or as aqueous emulsions [69].

Polysiloxanes The chemistry and technology of polysiloxanes have been thoroughly described in an excellent monograph by No11 [70]. The silicones used as water repellents for textiles are polymers with a -0-Si-0backbone. According to proper chemical nomenclature, these polymers are polysiloxanes: H

H

I

I I R

I

I -n

R

The substituent R can be a hydrogen, hydroxyl, alkyl, aryl, or alkoxy1 group. The substituents in the polymer chain can be all of the same kind or can be different. For textile applications, R is usually either methyl or hydrogen. Polydimethylsiloxanes form a flexible film on fibers and produce a finish with a soft handle. The cure temperatures required for efficient water repellency are, however, too high for textiles unless lowered with a catalyst. A process using irradiation for graft polymerization of polydimethylsiloxane has been patented [711. The reactivity of polydimethylsiloxanes can be increased by mixing with polymethylhydrogensiloxanes. The Si-H bond is hydrolyzed to -Si-OH, which can condense with another Si-OH group or a -Si-H group and forms cross-links. However, hydrolysis produces hydrogen, which may create a firehazard and a storage problem. The Si-H bond hydrolyzes rapidly in an alkaline or strongly acidic medium but can be stabilized with certain organic additives in an aqueous medium buffered at pH 3-4. Oxidation of-Si-H groups by atmospheric oxygen or oxidizing agents can produce -Si-OH groups, which can also contribute to eventual cross-linking of the finish on the fabric. Polymethylhydrogensiloxanes produce a hard brittle film on fibers and the

lents

Fluorinated

527

finish has a harsh handle. They are, therefore, not used alone, but in admixture with polydimethylsiloxanes [72], which act as plasticizers and improve the handle of the finished fabric. However, Bullock and Welch [73] have reported that polymethylhydrogensiloxanes can produce a highly water-repellent finish with a soft handle when cross-linked on cotton in the presence of organic peroxides. The water repellency of the fabrics finished with “silicones” has been attributed to a polysiloxane sheath around the fiber [ 741 that is oriented with the oxygen atoms toward the fiber surface and the methyl groups away from it (Fig. 12.1). When this film is broken or cracked during laundering (e.g., by swelling of the fibers), repellency may be lost without losing the polysiloxane polymer. The proper orientation of the polymer film on the fibers is essential for repellency. This necessitates the use of adjuvants that improve bonding of the polysiloxanes to the fiber, ensure proper orientation on the fiber, and accelerate cross-linking. These adjuvants are known in the trade as catalysts, although their action is not truly catalytic but can involve several mechanisms dependent on the chemical structure of the adjuvant. Tetrabutyl zirconate or tetrabutyl titanate, used mostly with solvent-

H ,

/

H3 H

, 7

3 H\

7

3 H\

/”””

,H

7

3

“‘0 /s’\o/s\o/si\o/s\o/si\o,~~

POLY(HYDROGENMETHYLSIL0XANE)

POLY (DIMETHYLSILOXANE)

FIG. 12.1 Orientation of polysiloxanes on a fiber surface coated with an additive to improve adhesion and alignment of the polysiloxane. (From Ref. 75.)

520

Chapter 12

based nonreactive polydimethylsiloxanes, hydrolyze producing an insoluble oxide layer on the fiber. The Zr or Ti atoms coordinate with the oxygen atoms of polysiloxanes and contribute to the sorption and orientation of the water repellent. Zirconium oxychloride [ 761 or zirconium acetate used in aqueous emulsions of polysiloxanes function in a similar manner. Divalent carboxylates [77], such as stannous octoate, zinc octoate, and zinc naphtenate, may function as phase transfer agents. Organic peroxides catalyze cross-linking probably by a free-radical mechanism [ 731. Organic resins, such as polycondensed epoxyamides and epoxyamines, have also been used as adjuvants for water-repellent finishing with polysiloxanes r781.

Water-RepellentFinishing with Polysiloxanes Repellent finishing with polysiloxanes has been reviewed by Madaras [ 791, Glenz [ 70b], and Medico [ 781. The silicones used as water repellents for textiles are supplied in the neat form or as aqueous emulsions. Polysiloxanes are soluble and stable in hydrocarbon solvents (e.g., xylene) or in chlorinated solvents, such as trichloroethylene or tetrachloroethylene, but special equipment is needed for the solvent-finishing operation. Although polysiloxanes used as water repellents for textiles are hydrolytically unstable, sufficiently stable aqueous emulsions are now available. The water repellency achieved with aqueous emulsions is usually somewhat inferior to that obtained with solutions of polysiloxanes because residual emulsifiers can impair repellency. The polysiloxanes are applied by padding followed by drying and heating for afew minutes at 120-150°C to cure the finish, or by exhaust procedures [70b]. Because most textile fabrics have a negative surface potential in water, cationic surfactants are used for emulsifying polysiloxanes for exhaust application. The padding or exhaust application should deposit about 1-2% polysiloxane onto the fabric; the amount can be smaller in coapplication with a zirconium or titanium “catalyst.” Because the cure conditions of the polysiloxane water repellents are similar to durable press cross-linking treatments with methylol compounds, polysiloxanes can be coapplied with a durable press finish [80]. The durable press resins enhance the durability of the “silicone” water repellent [ 781. The water repellency of “silicones” on synthetic fabrics, especially those made of filament fibers, is fairly resistant to laundering and dry cleaning with pure solvents. The loss of water repellency during dry cleaning is caused mainly by adsorption of hydrophilic substances, such as detergents, and to a lesser extent by dissolution of the silicone finish in the solvent [ 701. The durability of silicone finishes on cellulosic fabrics is impaired by swelling of cotton fibers during launder-

ellents

Fluorinated

529

ing. The expansion of the fibers ruptures the silicone film essential for water repellency [74,75]. Because the repellent polysiloxane film does not melt and flow, the cracks in the ruptured film cannot be sealed and the initial repellency restored by heating. Attempts have been made to form a covalent bond between the polysiloxane and cellulose fibers [SO], but the -Si-0-Cell bond is not stable to hydrolysis. The outstanding feature of fabrics treated with polysiloxanes, especially polydimethylsiloxanes, is their soft handle. The polysiloxanes have a cost advantage over fluoropolymers, which are, however, more durable to laundering and dry cleaning.

Fluorinated Polysiloxanes The outstanding feature of polysiloxane repellents is the soft handle they impart to the treated fabric. However, unlike fluorinated repellents, polysiloxanes do not provide oil repellency and soil resistance. Hence, a combination of polysiloxanes with fluorinated repellents promises to offer the advantages of both systems [Sl]. Numerous repellent compositions containing a polysiloxane and a fluorinated polymer have been described in the patent literature. For example, a patent to 3M claims a blend of succinic anhydride-terminated dimethylsiloxane and a poly(fluoroalky1)methacrylate [82]. Repellents containing both fluorine and silicone atoms in the same molecule have been prepared by Holbrook and Steward [83,84] and by many others. Pittman et al. [85] prepared fluoroalkylsiloxanes using hexafluoroacetone as the starting material and obtained good oil and water repellency on wool. Since then, fluorinated silicone-containing repellents have been described in numerous patents and patent applications. Some examples of the repellents are shown in Section 12.3. Fluorinated siloxy pyromellitates have been patented by Oxenrider and Long [86]. Amixture of rn- and y-pyromellitic diesters with CF3(CF),CH2CH2( n = 5 , 7 , 9 , 11) ester groups was converted to the diester chloride and reacted with mercaptopropyltriethoxysilane to yield a siloxy pyromellitate which waterproofed a nylon fabric. A water- and oil-repellent copolymer prepared from a perfluoroethyl acrylate, styrene, and siloxane CH2=C(CH3)COO(CH2)3Si(OSi(CH3)3)3 has been claimed by Abe et al. [87]. Luedemann et al. [88,89] reacted perfluoroalkyl group (avg. CI0F2,)containing polyurethane with a, w-dicarbinoldimethylpolysiloxane to obtain an suspension for producing water- and oil-repellent fabric finishes. Trimethylsilyl-terminated siloxanes containing C4-20fluoroaliphatic groups have been blended with acrylic fluoropolymers to form oilproofing and waterproofing emulsions for textiles [90,91]. Compositions containing silanes or siloxanes having perfluoroalkyl groups, (n = 3-7), and organosiloxane ladsuch as CF3CF2(CF2CF2),,CH2CH2Si(OCH3)3 der polymers have shown to impart water and oil repellency and durable soil re-

530

Chapter 12

sistance to textiles [92]. Fluorinated acrylic or methacrylic copolymers have been condensed with organosilsesquioxane ladder polymers [93]. (Fluoroalky1thio)alkyl siloxanes have been prepared by the addition reaction of vinyl siloxanes with thiols containing a perfluoroalkyl end group [94]. A mixture of OH-terminated methyl(perfluoroocty1)ethylsiloxane, (perfluoroocty1)ethylhydrogen siloxane, and a curing catalyst was used to treat textiles, plastics, metals, or ceramics for water and oil repellency [95,96]. A reaction product of trimethylsilyloxy-terminated methylhydrogensiloxane and N-allylperfluoro- 1 -octanesulfonamide was reacted with 2-methyl-3propenyl and 2-propenylethanol and the product obtained was used with dimethyloldihydroxyethyleneurea to treat a cotton/polyester fabric for oil repellency [97]. Dimethylsiloxane terminated with succinic acid anhydride groups was reand then acted with N-methyl-N-(2-hydroxyethyl)-perfluorooctanesulfonamide with ammonia for awater- and oil-repellent treatment of leather and polyester cotton fabric with a soft handle [98]. Acidic hydrolysis of heptadecafluorodecyltrimethoxysilane in aqueous isopropyl alcohol gave a water repellent for automobile windows [99]. Nylon fabric was made oil- and water-repellent by treating with a solution of F3C(CF2)7(CH2)2Si(NC0)3 in C2Cl3F3[ 1001. Judging by the continuing patent activity, this field is still being explored. 12.3 FLUOROCHEMICALREPELLENTS

Structure and Repellency Fluorochemical repellents differ from silicone or hydrocarbon-based repellents in several aspects, of which oil repellency is the most important. Fluorocarbons repel both water and oil, whereas repellents with silicone or hydrocarbon hydrophobes repel only water. The ability of fluorocarbons to repel oil is related to their low surface energy. The repellency of fluorocarbon finishes depends on the structures of the fluorocarbon segment, the nonfluorinated segment of the molecule, the orientation of the fluorocarbon tail, the distribution and the amount of the fluorocarbon moiety on fibers, and the composition and geometry of the fabric [ 1011. The relationship between repellency and the structure of the fluorocarbon segment is in accord with the critical surface tension concept developed by Zisman and co-workers (see Chapter 11). Shafrin and Zisman [lo21 determined the wettabilities and critical surface tensions of w-perfluoroalkyl substituted n-heptadecanoic acids synthesized by Brace [103]. Once the seven outmost carbon atoms are fully fluorinated (x = 7), the wettability of monolayers of the acids F(CF2)x(CH2)1&OOHapproaches that of the perfluorocarboxylic acid F(CF2),COOH (Fig. 12.2). This suggests that a terminal perfluoroalkyl chain of seven carbon atoms is sufficiently

Fluorinated Repellents

531

0 F(CF2),COOH

( f r o m solution)

X = NUMBER OF FLUORINATED CARBON ATOMS PER MOLECULE

FIG.12.2 The effect of fluorination of the adsorbed acid monolayer on the critical surface tension of wetting by n-alkanes. (From Ref. 102.)

long to shield the nonfluorinated segment beneath the fluorinated segment. For fluorocarbon repellents on a fabric, approximately 10 fully fluorinated carbon atoms are needed in a normal alkane chain to achieve maximum repellency. Data by Grajeck and Petersen [1041 suggest that oil repellency of a cotton fabric treated with poly(RfCH20-acrylates) increases with the increasing chain length of the perfluoroalkyl group Rfuntil the chain is about 12 carbon atoms long (Fig. 12.3). The coverage of fiber surfaces by the fluorochemical repellent is avery important variable affecting repellency. Hence, the amount of a fluorochemical needed for maximum repellency depends on the fabric construction (Fig. 12.4) and the structure of the fluorochemical repellent. At an equal number of carbon atoms, normal perfluoroalkane chains are more effective than branched chains. Under otherwise identical conditions, a surface comprised of closely packed -CF3 groups has the lowest surface energy and the highest repellency. Pittman and co-workers [ 1051concluded that polyacrylates containing a perfluoroisopropyl group have a lower critical surface tension than those containing a n-perfluoropropyl group. However, the comparison was made

Chapter 12

532 OILREPELLENCY ( 3 M CO. T E S T )

15C

IO(

5(

I

1

I

I

I

I

2 3 4 5 6 7 NUMBEROFFLUORINATED

I

8

1

I

9 1 0 1 CARBON ATOMS

I

1

1

1

J 2

FIG. 12.3 The effect of the fluorination of a hydrocarbon chain on oil repellency. (Data from Table IV in Ref. 104.)

between poly( lH, 1H-heptafluorobutyl acrylate) and poly(heptafluoroisopropy1 acrylate) or poly(heptafluoroisopropy1 methacrylate). Hence, structural features other than branching of the perfluoroalkane chain were also involved in the comparison.

Synthesis of Fluorochemical Repellents The patent literature on fluorocarbon repellents is voluminous. However, most patients disclose variations of nonfluorinated structural features of the repellent molecule. Structural variations of the fluorinated segment are limited by the small number of practical chemical routes to fluorinated intermediates. The most important commercial processes for producing fluorinated intermediates are based on either electrochemical fluorination or telomerization of tetrafluoroethylene (see Chapter 2).

Fluorinated Repellents

533

120 110 I

"-8

100 (3

zl-

90

1;

80

a X

z W -I

-I W

70

a w

0 Cotton fabric A

60

=!

X Cotton fabric B

0

50

0 Cotton fabrlc C

0

0

1.o

0.5

1.5

2.0

% FC 206 SOLIDS ON FABRIC

FIG.12.4 Oil repellency as a function of fabric construction and the amount of fluoropolymer applied. (From Ref. 104.)

Electrochemical fluorination of carboxylic acids in anhydrous hydrofluoric acid was invented by Simons [ 106,1071. A process developed by the 3M Co. [108,109] yields perfluorocarboxylic acid fluoride, which can be converted to esters, amides, or other intermediates for the production of repellents: C,2H2n+l COOH

+ (211 + 2)HF + C,2F2,2+ COF+ By-products

The yield of the perfluorinated product is higher when fluorinating a carboxylic acid chloride or fluoride, instead of the carboxylic acid itself or its anhydride [110]: CnH2,2+1 COCl

+ (211 + 2)HF + C,2F212+COF + HC1 + By-products

Electrochemical fluorination of alkanesulfonyl chlorides or fluorides yields the corresponding perfluorosulfonyl chloride or fluoride in an even higher yield (12-79%, depending on the alkane chain length), together with shorter-chain sulfonyl chlorides or fluorides [ 111,1121: C,tH2rt+l S02Cl+ (211 + 2)HF + C,2F2,2+1 SO2F + HCl

+ By-products

Hence, it is not surprising to find the perfluoroalkanesulfonyl moiety in commercial repellents (for toxicity concerns see Sectionl0.3).

Chapter 12

534

Telomerization of tetrafluoroethylene with iodopentafluoroethane, analogous to that with iodotrifluoromethane [ 1131, was developed by the Du Pont Company [114-1161:

The reaction produces a mixture of telomers differing in the length of their carbon chain. The perfluoroalkyl iodides do not react with nucleophiles (e.g.. OH-, NH3) and cannot be converted directly to common intermediates of fluorocarbon repellents. Therefore, the perfluoroalkyl iodides are reacted with ethylene:

The alkyl iodides, with two methylene -CHZgroups isolating the iodine atom from the perfluoroalkyl segment, can be readily converted to the corresponding alcohols or amines and used for building repellent molecules. Pittman and co-workers [ 105,1171 reacted hexafluoroacetone with nucleophilic agents such as alkoxide, cyanide, and an alkali-metal fluoride to prepare fluoroalkyl acrylates and methacrylates, which were polymerized to repellent polymers. The reaction scheme produces esters with a perfluoroisopropyl group; for example [ 1 181

+ KF -+ (CF3)zCFO-K+ (CF3)2CFO-K+ + CHz=CHCOCl+ CHZ=CHCOOCF(CF3)2 + KC1 (CF3)zC-0

A process developed by IC1 [ 1191 is based on anionic polymerization of tetrafluoroethylene pioneered by Graham [120]. The branched oligomers produced can be reacted with phenol and then with chlorosulfonic acid to form sulfonyl chlorides useful as intermediates:

BAS E

-

T"\

\

OC6H4S02CI

Fluorinated Repellents

535

Monomeric Fluorocarbon Repellents Fluorochemicals used in water- and oil-repellent finishes have been monomeric or polymeric.Thefirstfluorochemicalrepellents used commerciallywere monomeric. In analogy to repellents with a hydrocarbon hydrophobe (see Section 12.1),perfluoroalkanoicacids,andphosphateestershavebeenappliedas chromiumcomplexes [121-1231, zirconiumortitaniumalkoxidesorsalts [ 124-1261 or have been converted to quaternary amines [ 127,1281:

Because the repellent finishes produced with perfluorocarboxylic acid complexes were not durable to laundering [ 1251, attempts were made to bond the repellent covalently to fibers. Benerito et al. [129] reacted cellulose with either perfluoroctanoyl chloride or perfluorobutanoic chloride in dimethylformamide (DMF) in the presence of a tertiary amine as the acid acceptor. The partial esters of cellulose thus produced exhibited good oil and water repellency but were hydrolyzed by alkaline detergents. Repellent finishes made by reacting glycidyl ethers of l, l-dihydrofluoroalkanols with cellulose [130] were more durable but were not commercially successful. Triazines with a reactive chlorine or methylolamino groups have been utilized to bond the fluorinated repellent to cotton fibers. Thus, a cotton fabric was reacted with 2,4-bis(N-ethylperfluorooctylsulfonamido-6-chloro-striazine in the presence of Na2C03by drying and steaming [ 1 3 11. Methylol groups of triazine or arylguanamine have been reacted with cellulose or cross-linked to improve wash fastness and stabilize the fabric [132]. The monomeric fluorochemical repellents were superseded by fluoropolymers. However, the interest in monomeric fluorochemical repellents is increasing again. Modification of the fiber surface by (1) copolymerization with a fluorinated monomer (e.g., acrylonitrile copolymerized with lH, lH,2H,2H--heptadecafluorodecyl methacrylate [133], (2) adding fluorochemicals to the polymer melt [134-1401, or (3) by applying fluorochemicals to preformed fibers [141,142] is being explored and exploited commercially.

Fluoropolytper /3qellents Most polymeric fluorine-containing repellents in commercial use are vinyl polymers of the acrylic or methacrylic type, although polymers of vinyl esters. vinyl ethers, allyl esters, and thiomethacrylates also have been patented (Table 12.1). The vinyl polymers can be visualized as consisting of the perfluoroalkyl group, the polymer backbone, and the nonfluorinated linkage between the two. Many

536

+

7

<

Y 0

0 0

I

0

II

0

I" v)

0

T

d

5

a

( D o 3 7

m

@ 3

m

@ 3

7

L O L O

m LO

@ 3

c

0

+

a 3 n

m m LO? 0

Chapter 12

z

c3

t: 0

4-J

3

a

a

oc

v) (D

Fluorinated Repellents

0, . r

3 L i

(D (D

cn

+

7

Uj

cj

m

03 (D

z

537

TABLE 12.1

UI 0

Continued

Patent

Date issued

Ger. Offen. 1,918,079 U.S. 3,462,296

1969 1969

U.S. 3,424,785

1969

U.S. 3,529,995

1970

cx,

Inventor E. Kleiner, M. Knell S. Raynolds, T. K. Tandy

A. G. Pittman, W. L. Wasley S. Smith

Assignee

Structure of monomers

Geigy

Fluorinated esters of fumaric acid.

Du Pont

-75% (I)CH2=C-COOCH2CH2C,F2,+1

U. S. Dept. of Agric. 3M

I

CH3 (n = 6, 8, 10) -25% (11) 2-ethylhexyl methacrylate 0.25% (111) 2-hydroxyethyl methacrylate 0.25% (IV) A/-methylolacrylamide Polymer mixed with terpolymer of II, IV, and ethylene dimethacrylate FC(CF3)20(CH2)50COCH=CH2 Fluorinated acrylate of methacrylate 1 hydroxyalkyl acrylate or methacrylate,e.g., (C8F17)2CC=CH2 HOCSH602CC=CH2

I

I

b

7

m

0

Fluorinated Repellents

c

+"

0 a. 3 n

zz

C D b

030

4 3

ntb

0

-

I

a a cn

539

540

Chapter 12

patents claiming fluorinated vinyl polymers differ mainly in the linkage between the perfluoroalkyl group and the backbone. The structure of the perfluoroalkyl group depends on the source of the fluorocarbon and is, in some instances, dictated also by the patent situation. Repellent polymers are also made by copolymerizing one or more fluorinated monomers with one or more monomers not containing fluorine, including vinyl acetate, vinyl chloride, 2-vinyl-2-oxazoline, vinylidene chloride, maleic anhydride, dioctyl maleate, (meth)acrylic acid, acetoacetoxyethyl methacrylate, cyclohexyl methacrylate, N-methylolacrylamide, 2ethylhexyl methacrylate, hydroxyethyl (meth)acrylate, 3-chloro-2-hydroxypropyl (meth)acrylate,polyethyleneglycol(meth)acrylate,polypropyleneglycol (meth)acrylate,glycerolmono(meth)acrylate,glycidylmethacrylate,butyl methacrylate, stearyl (meth)acrylate, and cationic (meth)acrylic monomers, such as CH2=C(CH3)COOCH2CH2N+(CH3)3Cl-. The composition and ratio of the comonomers in such polymers is another variable. The structure of nonfluorinated monomers and their ratio to the fluorinated monomer affect repellency as well as other properties of the polymer such as melt flow and hardness. Comonomers with a cross-linking function, such as a hydroxyl, epoxy, or vinyl group, are used to increase the durability of the repellent polymer. “Hybrid’ fluoropolymers, consisting of hydrophobic and hydrophilic segments, are discussed in Chapter 13. To lower the cost of fluoropolymers or to improve their performance, fluoropolymers are mixed with a nonfluorinated extender polymer, usually of acrylic or methacrylic type. Table 12.1 contains some examples of extended polymers. (see also Section 12.1). The durability of a fluorinated repellent to washing and abrasion can be increased by incorporating the perfluoroalkyl group in a urethane. For example, perfluoroalkyl-terminated urethane oligomers are obtained by reacting an alcohol CnF2?*+ CH2CH20H with a hexamethylene diisocyanate oligomer, 2,4-toluenediisocyanate and polyethylene glycol [143]. Melt-spun nylon 6 filaments were treated with a lubricant mixture of a perfluoroalkyl containing aliphatic esters and a urethane 2,4-CH3CsH3[NHCOO(CH2)4N(CH3)S02C8F17]2 [144]. Fluoropolymers are also used as mixtures with nonfluorinated water repellents [145]. De Marco et al. [ 1461 found that the combination of a fluoropolymer and a pyridinium-type water repellent on cotton has a synergistic effect, resulting in long-lasting resistance to rain and durability to laundering. A patent to the U.S. Department of the Army [ 201 claims repellent composjtions comprising > 1% of a pyridinium compound, for example, stearoyloxymethylpyridinium chloride, and 20.5% of a mixed fluorocarbon polymer, a copolymer of IH, 1H72H,2H-perfluoroalkane methacrylates and poly(octy1 methacrylate). In recent years, fluorochemical repellents have been coapplied mainly with wax dispersions made durable with cross-linking adjuvants. Although a variety of water repellents with hydrocarbon-type hydrophobes enhance the repellency and durability of fluoropolymer repellents, silicones may reduce their oil repellency

ellents

Fluorinated

541

[104]. However, the patent literature contains numerous examples of compositions containing both fluorine and silicone (see Section 12.2) as water and oil repellents for cotton, polyamide, polyester, and acrylics, as well as glass and other inorganic material. Repellents containing both fluorine and silicone atoms in the same molecule have been prepared. A large number of patents have been issued claiming compounds containing both fluorine and silicone as water and oil repellents for cotton, polyamide, and polyester. Judging by the continuing patent activity, this field is still being explored. 12.4 REPELLENTFINISHINGWITHFLUOROPOLYMERS

Fluoropolymers are used for repellent finishing of fabrics made of synthetic and natural fibers. Formulations for cellulosic fabrics include cross-linking reactants to increase durability of fluoropolymer finishes, to impart wrinkle resistance, wash and wear, and durable press properties. Cross-linking reactants of the melamine, triazine or modified triazine, carbamate, or glyoxal type are used. Because fluoropolymers, unlike silicones, do not soften the fabric, softeners may be needed. Coapplication with cross-linking reactants may also require lubricants, such as nonionic polyethylene dispersions, to assure satisfactory sewing properties of the fabric. A typical formulation for polyester-cotton rainwear and outerwear is shown in Table 12.2 [148]. Fluoropolymer repellent finishes are applied by padding, spraying, or exhaust. Generally, the fabric is padded, dried at 120-1 80"C, and cured for 1-3 min at 150-182°C. The coapplied adjuvants and resins may require the fabric to be process washed and dried at 150-175°C. Fluorochemicals (e.g., CH2=CHCOO(CH2)&FI7) [149] have been applied to fabrics by plasma coating as well. Producers of fluoropolymer repellents require that textiles meet minimum oil- and water-repellency specifications to bear the producer's label. The durability of repellency to laundering and dry cleaning is also specified (Table 12.3) [148].

TABLE12.2 Formulation for Polyester-Cotton Rainwear/Outerwear Ingredient Fluorinated repellent Extender Thermosetting resin Catalyst Softener Bath stabilizer Acetic acid, glacial

Concentration (% owf) 2.0-3.0 2.0-3.0 2.0-7.0 0.4-1.4 0.5-2.0 0.03-0.05 0.05-0.1

ul

P

N

TABLE12.3 Specifications for Fabric Protected with a Fluorinated Repellent Specification ratings Water repellency (AATCC Meth. 22-1 996)

All fabrics, fibers and blends of fibers for all uses except flat woven rainwear and all upholstery fabrics Flat woven rainwear fabrics All upholstery fabrics except cut pile Cut pile upholstery fabrics

Oil repellency (AATCC Meth. 118-1 997)

Initially

After 10 home launderings and tumble dryings

After 2 tumble jar dry cleanings

Initially

After laundering

After dry cleaning

90

80

80

5

4

4

100

80

80

5

4

4

80

-

-

5

-

-

80

-

-

4

-

-

Note: A tolerance of 10 is allowed on Spray Test Ratings. A tolerance of 1 is allowed in Oleophobic Test Ratings.

3 P)

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llents

Fluorinated

543

Fabrics that cannot be finished with aqueous fluoropolymer dispersions can be finished by spray application of a fluoropolymer dispersed in a volatile solvent, such as stabilized trichloroethylene [150] or tetrachloroethylene [ 15 I]. After spraying, fabrics pass directly from the spray booth into a dryer heated at 38-66°C. The finish is cured for 60 s at 150°C or at a lower temperature if the fabric cannot tolerate heating at 150°C [ 1501. Equipment has been built for continuous pad-dry-heat application of fluoropolymers from a volatile solvent, which is recovered almost completely and recycled [ 1511. The use of volatile solvents has caused environmental concerns. 12.5 REPELLENCYTESTS

Water-Repellency Tests A variety of water-repellency tests have been developed and used over the years. The limitation of space permits description of only the most widely used tests. The reader is referred to reviews by Norris [ 1521 and Lewedag [ 1531 and papers published by Slowinske [154-1561 for more information on the testing of water-repellent finishes. The test methods can be divided into three main classes [ 157,1581: Class I. Spray tests to simulate exposure to rain CZass II. Hydrostatic pressure tests to measure the penetration of water as a function of pressure exerted by water standing on the fabric Class III. Sorption of water by the fabric immersed in water

The class I spray and sprinkling tests most frequently used are listed in Table 12.4. The AATCC 22-1996 spray test (Fig. 12.5) developed by the Du Pont Co. [ 1541 is the simplest and most widely used. In addition to the simplicity of the apparatus, it has the advantage that fabric and garments can be tested without cutting out samples. However, the test does not discriminate among fabrics with good water repellency, and other tests may be needed to supplement it. The fabric to betested is mounted on a 6-in. (15.2-cm) embroidery hoop and placed on a stand at an angle of 45". A 250-m volume of water is poured through a funnel with a nozzle 6 in. (15.2 cm) above the center of the fabric mounted on the hoop. The edge of the hoop is tapped against a solid object. The hoop is then rotated and the opposite edge istapped. The wet or spotted pattern of the fabric is compared with photographic standards and the repellency rated. Complete wetting of upper and lower surfaces is rated 0; 50 is given for complete wetting of the upper surface and 100 for not wetting; ratings 70, 80, and 90 are given for partial wetting of the upper surface. The AATCC 42-1994 impact penetration test [l55] uses an apparatus similar to that of the spray test, but the fabric is clamped to the inclined stand with a

544

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c. c.

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Chapter 12

545

Fluorinated Repellents

or Glass Laboratory Funnel

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- Tubing Dia. Rubber 2" Long - Aluminum Spray Nozzle .035" Dia. $6"

I

19 Holes

I

I

- Fabric Sample (7" x 7")

-

Metal Embroidery

.+

Hoop (6'* Dia.)

Metal Holder and Stand

p'FIG.12.5 AATCC

8"

spray tester.

I

Chapter 12

546

paper blotter beneath the specimen (Fig. 12.6). The water volume and the height of the nozzle are larger than in the spray test. After the fabric has been sprayed, the blotter underneath it is removed and weighed. The weight increase indicates penetration of water. The AATCC 35- 1994 rain test developed by Slowinske and Pope [ 1561 simulates exposure to rain of variable intensity (Fig. 12.7). A sample of the fabric backed by a weighed paper blotter is sprayed with water for 5 min. The blotter is weighed to determine the amount of water that has leaked through the fabric. The hydrostatic pressure of water and, consequently, the intensity of the impact of its drops on the fabric is varied in increments of 0.3 m (1 ft) from 0.6 m to 2.4 m (2 ft to 4 ft). Therain test has ranked fabrics in the same order as exposure to a 3-in. rain, but some discrepancies became evident in the correlation with 1-in./h rain.

\,

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6" GLASS LABORATORY FUNNEL

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FUNNEL LABORATORY BURETTE

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I

1 1

I

I

t

I

3/81' RUBBER TUBING CONNECTING

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6" SPRING CLIP

7

h c\1

I

L /

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6" SPRING CLIP (WEIGHT OF CLIP AND PLATE EQUALS 1 LBI

FIG.12.6 AATCC impact penetration tester.

10"

Fluorinated Repellents

h

547

/OVERFLOW

TEST SPECIMEN

TEST SPECIMEN 8" x 8"

rm< w OETAl LOF TEST SPECIMEN HOLDER

6

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" x 6"

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PLASTIC OR MASON ITE BOARD A

FIG. 12.7 AATCC rain tester.

Nevertheless, Slowinske and Pope concluded that the rain test correlated with the exposure to rain (simulated in a rain room) better than any other test. The Bundesmann test [152,153a,159-1671 is most widely used in Germany (DIN 53 333) and England. The test was designed to simulate the conditions of a garment worn inrain. The apparatus consists of a shower and a cup assembly (Fig, 12.8). The fabric sample is mounted over a cup that collects the water penetrating

Chapter 12

548 T

WAT FE WFR EI A TDH BFRI ILCT E R

OVER FI

WIPER /ARMS . BEZELS -EST CUPS

I

J,

FIG.12.8

I

I

1

I

I

Bundesmann apparatus for water repellency testing.

Fluorinated Repellents

549

the fabric. Each cup is equipped with a wiper with four arms, which rotates during the test, rubbing against the underside of the fabric. The cups are tilted outward at an angle of 10-1 5" and are mounted on a spindle that rotates under the shower at 5 rpm for 10min. The fabric samples are removed from the cup assembly. The surplus water on the fabric is removed by shaking by hand, or by a mechanical shaker [ 161~1,or by centrifugation [ 164,1651. The amount of water sorbed by the sample is then determined by weighing. The amount of water that has penetrated the fabric and collected in the cup isalso determined. However, the sorption of water by the fabric has been found to be a more precise measurement [ 1671. The variables of the Bundesmann test have been studied in Germany [ 163-1671 and in Great Britain [ 160,1611. Interlaboratory trials in the United Kingdom [ 1611 revealed that Bundesmann apparatuses built by different manufacturers gave different results. It was concluded, however, that the Bundesmann test is well established and suitable for testing within a laboratory. The drop penetration test was developed by Sookne et al. [168]. The apparatus consists of a drop-forming device with capillaries mounted at the bottom of a container, a tube to shield the spray from draft, and a sample holder designed to collect the water penetrating the fabric. The time needed to collect 10 mL of water is measured. The hydrostatic pressure tests [ 1541 measure water penetration under pressure, but the test results do not correlate with resistance to penetration by rain. The AATCC Test Method 127- 1998 [ 1691 uses a Suter apparatus, which subjects the sample to water pressure that is increased at a constant rate by increasing the hydrostatic head (1 cm/s). The lowest hydrostatic head needed to cause water penetration at three different places of the fabric is recorded. The test is suitable for testing tent cloths, tarpaulins, and fabrics coated with an impermeable film (e.g., rubber). Absorption tests measure the weight increase of the sample that has been immersed in water, placed between paper blotters, and passed through a wringer to remove excess water. The now discontinued static absorption test [170] (AATCC Test Method 21-1983) immersed a weighed sample in water at an average hydrostatic head of 8.9 cm (3.5 in.) for 20 min. The dynamic absorption test [ 17 1,1721(AATCC Test Method 70- 1997) tumbles preweighed samples in water in a jar for 20 min. The weight increase is reported as percent water absorbed. The test measures hydrophobicity of the fibers and yarns in the fabric independently of the fabric construction. Before testing water repellency, the fabric should be conditioned in an atmosphere of 65 ? 2% relative humidity at 21 ? 1"C (70°F) for at least 4 h(United States) or at 20 -+ 2°C for at least 24 h (Great Britain and Germany). The AATCC test methods specify the temperature of water used in the test to be kept at 27 ? 1°C. In the latest revision of the Bundesmann test, the water temperature is kept

Chapter 12

550

at 20 ? 0.1 "C [ 1671. The purity of water also can affect test results; usually, distilled water is used. In conjunction with repellency tests, the air permeability and the water-vapor permeability (ASTM D 737-96; DIN 53 887) [ 153bl of a fabric are sometimes determined. Water-vapor transmission properties of a fabric are important variables affecting wearing comfort of repellent garments [173].

Oil-Repellency Tests The purpose of an oil-repellency test is to evaluate the resistance of a fabric or carpet to wetting by hydrocarbons and, more important, to detect the presence and quality of a fluorochemical finish. The AATCC Test Method 118-1997 [1741 uses eight hydrocarbon liquids in a series of decreasing surface tensions (Table 12.5). Beginning with the lowest-numbered test liquid, a 0.05-mL drop is placed carefully on the fabric. If no penetration or wetting occurs in 30 s, a drop of the nexthigher-numbered test liquid is placed on the fabric. The procedure is continued until a test liquid wets the fabric under or around the drop within 30 s. The repellency rating is the highest-numbered test liquid which does not wet the fabric in 30 s. The AATCC test is similar to Du Pont's Oleophobic Test. The 3M (Minnesota Mining and Manufacturing Co) test uses mixtures of Nujol oil and heptane in various proportions numbered from 50 (100% Nujol) to 150 (100% n-heptane) [ 1071. Oil repellency is determined under static conditions, and the rating depends entirely on the contact angle of the oil (hydrocarbon) on the fibers. The rating does not indicate the resistance of the fabric to the spreading of an oil that can wet the TABLE 12.5 Standard Test Liquids AATCC oil repellency rating number 31.2

a

1 2

Data by R. H. Dettre.

Composition Nujol oil 65 : 35 Nujol : n-hexadecane by volume at 70°F (21"C) n-Hexadecane n-Tetradecane n-Dodecane n-Decane n-Octane n-Heptane

Surface tension at 25°C (dyn/cm)" 28.7 27.1 26.1 25.1 23.5 21.3 19.8

Fluorinated Repellents

551

fibers. The spreading of the liquid in the fabric depends on the viscosity of the liquid, the geometry of the fabric, and other factors. Kissa has shown that with a polyester-cotton fabric finished with six different fluorochemical repellents the area covered in 1 min by a 0.10-mL drop of heptane spreading on these fabrics correlated with the AATCC oil-repellency ratings however (Fig. 11.8).

12.6 FUTURE Although water-repellent finishing is a mature technology, research is continuing to improve the repellents and application processes and to ease the inflationary pressure on their cost. Low add-on application, to reduce the energy needed to dry the fabric and minimize migration of the dispersed repellent, is one of the recent developments on a commercial scale. Fluorochemicals will remain the most effective, durable, and expensive repellents. The use of nonfluorinated extenders, where appropriate, will continue to improve the cost-effectiveness of fluorochemical repellents. Silicones and repellents with hydrocarbon-type hydrophobes will share the market where the performancekost relationship is in their favor. The dry soil-resistant finishing with fluorochemicals has been extended to carpet protection, which is now a rapidly growing field. The development of repellent finishing is benefiting from recent progress in surface analyses by electron spectroscopy for chemical analyses, scanning electron microscopy, and the attenuated total reflection technique of infrared spectroscopy.

REFERENCES 1. E. B. Higgins, in “Waterproofing and Water-repellency,” J. L. Moilliet, ed., p. 188, Elsevier, Amsterdam (1963). 2. J. L. Moilliet, in “Waterproofing and Water-repellency,” J. L. Moilliet, ed., p. 52, Elsevier, Amsterdam (1 963). 3. E. B. Higgins, Textile Inst. Ind. 4,255 (1966). 4. T. R. Harding, J. Textile Inst. 43, P691 (1951). 5. W. B. Blumenthal, Ind. Eng. Chem. 42,640 (1950). 6. N. J. Glade (American Cyanamid), U.S. Patent 2,971.930 (1961). 7. Badische Anilin-und Soda-Fabrik A. G., Fr. Patent 1,557,348 (1969); CA 71, 82,598. 8. M. A. Thomas (to Deering Miliken Res. Corp.), Ger. Offen. 2,156,908 ( I 972). 9. Y. Sasaki, A. Fulkunishi, and M. Goto (to Sanyo Chem. Ind.), Jpn. Patent 74,10,315 (1974). 10. J. W. McDonald and R. E. Stahl (to DuPont), Ger. Offen. 2,216,161 (1972). 11. R. E. Stahl (to Du Pont). Ger. Offen. 2,420,971 (1974). 12. R. J. Pavlin, TAPPI 36, 107A (1953).

552

Chapter 12

13. 14. 15. 16. 17. 18. 19.

R. K. Iler, Ind. Eng. Chem. 46,766 (1954). Deutsche Hydrierwerke. Br. Patent 394.196 (1933). Deutsche Hydrierwerke. Br. Patent 426,482 (1935). R. J. W. Reynolds, E, E. Walker, and C. S. Woolvin, Br. Patent 466,817 (1 937). A. W. Baldwin and E. E. Walker (to ICI), Br. Patent 475,170 (1937). F. V. Davis, J. SOC.Dyers Colour. 63, 260 (1947). H. A. Schuyten, J. W. Weaver. J. G. Frick, Jr., and J. David Reid, Textile Res. J. 22. 424 (1962). C. G. De Marco and G. M. Dias (to U.S. Dept. of the Army), U.S. Patent 3,347,812 ( I 967). J. J. Willard, R. Turner, and R. F. Schwenker, Textile Res. J. 35.564 (1965). I. G. Farbenindustrie, A.G., Br. Patent 463,300 (1937). H. L. Sanders (to General Aniline), Br. Patent 679.8 1 1(1 952). I. G. Farbenindustrie, A.G., Br. Patent 467,166 (1937). J. M. Grim. (to American Cyanamid Co.), U.S. Patent 2,426.770 (1947). CIBA, Ltd., Swiss Patent 301,453 (1954); Swiss Patent 303.917 to 303,926; CA 50. 15,130. M. Hoffmann. J. Backmann, and H. Becker (to VEB Fettchemie), Ger. Offen. 2,625,776 (1 977). A. Hiestand (to CIBA). Swiss Patent 458,278 (1968); CA 70. 5209. I. G. Farbenindustrie A.G., Br. Patent 461,179 (1937); CA 31,5600. C. Hamalainen. J. D. Reid. and W. N. Berard, Am. Dyestuff Rep. 43,453 (1954). Farberei A.G.. Br. Patent 474,403 (1937); CA 32, 3637. H. Bestian and G. v. Finck (to I.G. Farbenindustrie A.G.), Ger. Patent 731,667 (1943); CA 38, 650. Farbwerke Hoechst A.G.. Br. Patent 795,380 (1958); CA 53,221. W. Baird, in “Waterproofing and Water-repellency,” J. L. Moilliet, ed.. p. 62, Elsevier, Amsterdam, ( 1963). C. W. Schroeder and F. E. Condo, Textile Res. J. 27, 135 (1957). J. B. McKelvey, B. G. Webre, and E. Klein, Textile Res. J. 29,918 (1959). J. B. McKelvey. B. G. Webre, and E. Klein, Am. Dyestuff Rep. 49, 804 (1960). DEHYDAG, Ger. Patent 874,289 ( 1 953); CA 5 1 , 10083. L. S. Silbert, S. Serota. G. Maerker, W. E. Palm, and J. G. Phillips, Textile Res. J. 48,422 (1978). L. S. Silbert. S. Serota. and G. Maerker, U.S. Patent 4,152.115 (1979). A. J. Hall, “Textile Finishing,” 3rd ed., p. 334, Heywood, London. (1966); Br. Pat. 8 18.492; Br. Patent 8 18,778. J. Heyna and W. Schumacher (to Farbwerke Hoechst) U.S. Patent 2,670265; CA 52.7914. E. N. Alexander et al., Am. Dyestuff Rep. 49(10), 818 (1960). C. G. De Marco, A. J. McQuade, and S. J. Kennedy, Mod. Textile Mag. 41. 50 (1960). J. H. B. Goldstein, Textile Res. J. 31, 377 (1961). E. J. Grajceck and W. H. Petersen, Textile Res. J. 32. 320 (1962). L. I. Heffner et al.. Am. Dyestuff Rep. 52(2) 82 (1963). E. B. Hoggins. Textile Inst. Ind. 4, 255 (1966).

20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48.

lents

Fluorinated

553

49. T. F. Cooke and W. F. Baitinger. Encycl. Polym.Sci. Technol. 13,740 (1970). 50. R. E.Read and G. C. Culling,Am. Dyestuff Rep. 56( l), 1 P881 ( 1 967). 51. R. J. Dams, J. E. De Witte, and C. C. Maes (to Minnesota Mining Mfg.), PCT Int. Appl. WO 92,17,636 (1992). 52. D. M. Coppens (to Minnesota Mining Mfg.), Eur.Patent Appl. EP 648,887 (1995). 53. F. A. Audenaert and H. Lens (to Minnesota Mining Mfg.), Eur. Patent Appl. EP 7 13.863 (1996). 54. Y. Suzuki and T. Higuchi (to Dainippon Ink Chem.), Jpn. Kokai Tokkyo Koho JP 09 31,852 (1997). 55. D. M. Coppens (to Minnesota Mining Mfg.). Eur. PatentAppl. EP 648,887 (1995). 56. F. A. Audenaert and H. Lens (to Minnesota Mining Mfg.), Eur. Patent Appl. EP 713.863 (1996). 57. Y. Suzuki and T. Higuchi (to Dainippon Ink Chem.), Jpn. Kokai Tokkyo Koho JP 09 3 1,852 (1997). 58. C. G. De Marco, A. J. McQuade, and S. J. Kennedy, Mod. Textile Mag. 41, 50 ( 1960). 59. C. G.De Marco andG. M. Dias (to U.S. Dept. of the Army), U.S. Patent 3,347,812 ( 1967). 60. Y. Amimoto, A. Senda, and S. Okamoto (to Daikin Ind.), Jpn. Kokai Tokkyo Koho JP 61 264,081 (1986). 61. K. Ito, T. Kamata, and K. Kaneko (to Asahi Glass), Jpn.Kokai Tokkyo Koho JP 06 240,239 (1994). DE 19,516,907 (1996). 62. K.-G. Berndt and W. Knaup (Hoechst Trevira), Ger. Offen. 63a. W. I. Patnode, U.S. Patent 2.306,222 (to General Electric) (1942);CA 37,3272. 63b. W. I. Patnode and D. F. Wilcock, J. Am. Chem. SOC.68,358 ( 1 946). 64. A. J. Barry (to Dow Chemical), U.S. Patent 2,405,988 (1946);CA 41,477. 65. Coming Glass Works, Br. Patent 588,762 (1944). 66. M. E. Thayer (to Dow Corning), U.S. Patent 2,474,704 (1949);CA 43,6759. 67. British Thomson-Houston,Br. Patent 593,727 (1944). 68. D. X. Klein (to DuPont), U.S. Pat. 2,532,559 (1950); CA 45, 3409. 69. F. L. Dennett (to Dow Corning), U.S. Patent 2,588,365 (1952);CA 46, 5332. 70a. W. Noll, “Chemistry and Technology of Silicones,’’ Academic Press, New York (1968). 70b. 0. Glenz. in “Chemistry and Technology of Silicones.” p. 585. Academic Press, New York ( 1968). 71. E. E. Magat and D. Tanner (to Du Pont), U.S. Patent 3,284.156 (1966); CA 66, 19733. 72. F. L. Dennett (to Dow Corning). U.S. Patent 2.588,366 (1952); CA 46,5333. 73. J. B. Bullock and C. M. Welch, Textile Res. J. 35,459 (1965). 74. F. Fortess, Ind. Eng. Chem. 46.2325 (1954). 75. E. Kurz, Textilveredlung 4,773 (1969). 76. J. A. C. Watt, J. Textile Inst. 48, T175 (1957). 77. J. A. C. Watt, J. Textile Inst. 5 1, TI (1960). 78. A. Medico, Ciba-Geigy Rev. 1972(3), 32 (1972). 79. G. W. Madaras, J. SOC.Dyers Colour. 74, 835 (1958). 80. J. W. Gilkey, Textile Res. J. 33. 129 (1963).

554

Chapter 12

81. H. Sawada. Y. Minoshima. T. Matsumoto. and M. Nakayama, J. Fluorine Chem. 59. 275 ( 1992). 82. D. M. Coppens and K. E. A. Allewaert, Eur. Patent Appl. EP 648.890 (1995). 83. G. W, Holbrook and 0. Steward (to Dow Corning), U.S. Patent 3,012,006 (1961). 84. G. W. Holbrook and 0. Steward (to Dow Corning), U.S. Patent 3,015.585 (1962). 85. A. G. Pittman and W. L. Wasley. Am. Dyestuff Rep. 56, 808 (1967). 86. B. C. Oxenrider and D. J. Long (to Allied Corp.), U.S. Patent 4,447,629 (1984). 87. A. Abe, Y. Hara. andH. Ohashi (to Shin-Etsu Chem.), Jpn. Kokai Tokkyo Koho JP 60,190.408 (1985). 88. S. Luedemann, M. Bernheim. B. Sandner. E. Roessler. and H. B. Vogel (to Chem. Fabrik Pfersee), Eur. Patent Appl. EP 325,918 (1989). 89. S. Luedemann. M. Bernheim, and B. Sandner (to Chem. Fabrik Pfersee), Ger. Offenb. DE 3,802,633 (1989). 90. T. Enokida. S. Kutnatnoto, and T. Mizuno (to Nippon Mectron), Jpn. Kokai Tokkyo Koho JP 01 36,674 (1989). 91. T. Enokida, S. Kumamoto, and T. Mizuno (to Nippon Mectron), Jpn. Kokai Tokkyo Koho JP 01 36.677 (1989). 92. S. Takubo and Y. Chiba (to Daikin Ind.), Jpn. Kokai Tokkyo Koho JP 01 95.181 (1989). 93. M. Shinjo. S. Okamoto, Y. Katakura. and S. Takubo (to Daikin Ind.), Eur. Patent Appl. EP 271.054 (1988). 94. F. Mosch (to Chem. Fabrik Pfersee), Eur. Patent Appl. EP 380,972 (1990). 95. M. Ozaki and I. Ona (to Dow Corning Toray Silicone), Eur. Patent Appl. EP 472,2 15 ( 1992). 96. M. Ozaki and I. Ona (to Dow Corning Toray Silicone), U.S. Patent 5,300,239 ( 1994). 97. R. J. Gambale and B. C. J. Barbera (to OS1 Specialites), U.S. Patent 5,348,769 (1994). 98. Minnesota Mining and Mfg.. Ger. Offen. DE 4,240.274 (1994). 99. R. Kamimura, I. Nakamura, and S. Sugawara (to Nissan Motor), Jpn. Kokai Tokkyo Koho JP 07 179,850 (1995). 100. M. Yokoyama. T. Shimada. and T. Yoneda (to Asahi Glass), Jpn. Kokai Tokkyo Koho JP 10 1 10.160 (1998). 101. E. Kissa. in “Functional Finishes,” Part B. “Handbook of Fiber Science,’’M. Lewin and S. B. Sello, eds., Vol. TI, pp. 144-210, Marcel Dekker, New York (1984). 102. E. G. Shafrin and W. A. Zisman, J. Phys. Chem. 66, 740 (1962). 103. N. 0. Brace. J. Org. Chem. 27,4491 ( 1962). 104. E. J. Grajeck and W. H. Petersen, Textile Res. J. 32, 320 (1962). 105. A. G. Pittman, D. L. Sharp. and B. A. Ludwig, J. Polym. Sci. Part A-1. 6, 1729 (1968). 106. J. H. Simons, “Fluorine Chemistry,’’ Vol. I, p. 414, Academic Press, New York (1950); vol. 11, p. 340 (1954). 107. J. H. Simons, 3M Co. U.S. Patent 2,519,983 (1950); CA 45, 51. 108. E. A. Kauck and A. R. Diesslin, Ind. Eng. Chem. 43,2332 (1951). 109. A. R. Diesslin, E. A. Kauck, and J. H. Simons (to 3M Co.), U.S. Patent 2,567.01 1 (1951); CA 46, 1375. U.S. Patent 2,593,737 (1952): CA 46, 6016.

lents

Fluorinated 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141.

555

H. M. Scholberg and H. G. Bryce (to 3M Co.), U.S. Patent 2,717,871 (1955). T. J. Brice andP. W. Trott(to 3M Co.), U.S. Patent2,732,398 (1956); CA 50, 13982. T. Gramstad and R. N. Haszeldine, J. Chem. SOC.1956, 173. R. N. Haszeldine, J. Chem. SOC.1949, 2856. R. E. Parsons (to Du Pont), U.S. Patent 3,132,185 (1964);CA 61. 1755. A. Blanchard and J. C. Rhode (to Du Pont), U.S. Patent 3,226.449 (1965); CA 64, 803 1. R. E. Parsons (to Du Pont). U.S. Patent 3,234,294 (1962). A. G. Pittman, D. L. Sharp, and R. E. Lundin, J. Polym. Sci., Part A-1, 4, 2637 (1966). A. G. Pittman and D.L. Sharp, Textile Res. J. 35, 190 (1965). W. R. Deem (to ICI). Br. Patent 1,302,350 (1973); CA 78, 1 1 1926. D. P. Graham, J. Org. Chem. 3 1,955 ( 1966). T. S. Reid (to 3M Co.), U.S. Patent 2,662,835 (1953). F. J. Philips, L. Segal, and L. Loeb, Textile Res.J. 27. 369 ( I 957). K. Hara. Y. Itami, T. Masutani,N. Nose, T. Enomoto, and A. Ueda(to Daikin Ind.). PCT Tnt. Appl. WO 93 15,254 (1993). V. W. Tripp, R. L. Clayton, and B. R. Porter, Textile Res.J. 27, 340 ( I 957). L. Segal, F. J. Philips, L. Loeb. and R. L. Clayton, Jr., Textile Res. J. 28,233(1958). E. Nishida andU. Maeda (to Nippon Soda), Jpn. Kokai Tokkyo Koho JP 06340,870 (1 994). R. J. Koshar and H. A. Brown (to 3M Co.) U.S. Patent 3,359.131 (1967); CA 68, 50896. D. D. Gagliardi (to Colgate-Palmolive Co.). U.S. Patent 3,350, 21 8 (1967);CA 68, 115,657. R. R. Benerito, R.J. Berni, and T. F. Fagley. Textile Res. J. 30, 393 (1960). R. J. Berni, R. R. Benerito. andF. J. Philips, Textile Res. J. 30, 576 (1960). C. Willis (to Secretary of State for Defense, UK), Br. Patent Appl. GB 2,291,439 ( 1996). T. Oishi (to Mitsui Toatsu Chem.), Jpn. Kokai TokkyoKoho JP 03 145,475 (1991). N. Oosuga, H. Ito, J. Fukui, and S. Hagura (to Mitsubishi Rayon), Jpn. Kokai Tokkyo Koho JP 05 71,010 (1993). F. Mares and B.C. Oxenrider. Textile Res. J. 47, 55 1 (1977). R. A. Lofquits, G. C. Weedon, and C. J. Cole (to Allied), U.S. Patent 4,59 1,473 (1986). 0. Shinonome, T. Kitahara, and S. Murakami (to Unitikia), Jpn. Kokai Tokkyo Koho JP 62 206,019 (1987). A. Ueda, S. Takubo, and Y. Amimoto (to Daikin Ind.), PCT Int. Appl. WO 93 22,483 (1993). R. S. Buckanin (to Minnesota Mining Manuf,), Eur. Patent Appl. EP 599,023 (1 994). P. H. Fitzgerald, K. G. Raiford, and E. J. Greenwood (to Du Pont), PCT Int. Appl. WO 97 22,660 (1997). T, A. L i s , K. G. Raiford. andE. J. Greenwood (to Du Pont), PCT Int. Appl. WO 97 22,582 (1997). F. Mares and B.C. Oxenrider. Textile Res. J. 48,218 (1978).

556

Chapter 12

142. N. Fujimaru, Y. Shimizu, Y. Sakane. S. Kai, and S. Ishikawa (to Unitika), Jpn. Kokai Toklcyo KohoJP 63 06,114(1 988). 143. M. Matsuo and M. Tamura (to Asahi Glass), Eur.Patent Appl. EP 231.927 (1987). 144. T. Kitahara, S. Murakami, Y. Sakane. and N. Tsuchiya (to Unitika), Jpn. Kokai Tokkyo Koho JP 62 238,869 (1987). 145. H. B. Goldstein, TextileRes. J. 3 1, 377 (196 1). 146. C. G. DeMarco, A. J. McQuade, and S. J. Kennedy, Mod. Textile Mag. 41, 50 (1960). 147. N. C. Shane and H. G. Weiland, U.S. Patent 3,336,157 (1967),CA 67,74.434. 148. Zepel B and Zepel DR Fabric Fluoridizers Specifications forFabric Protected with Zepel Fabric Fluoridizer, Industrial Chemicals Information Bulletin, E. I. du Pontde Nemours & Co. Wilmington. DE. 149. J. P. Singh, S. R. Coulson, C.R. Willis. and S. A. Brewer (to The Secretaryof State for Defense,UK), PCT Int. Appl. WO 98 58,117 (1 997). 150. Zepel S Fabric Fluoridizer for Solvent-Spray Application, Zepel SC Fabric Fluoridizer for Solvent Application, Industrial Chemicals Information Bulletin,I.E. du Pont de Nemours & Co., Wilmington. DE. 151. W. Bernheim and H. Ruile, Textilveredlung2,463 (1967). 152. C. A. Norris, in “Waterproofing and Water-repellency.” J. L. Moilliet, ed.. p. 265, Elsevier, Amsterdam (1963). 153. D. Lewedag, Textil-Praxis(a) 24,401(1969): (b) 24.459 (1969). 154. G. A. Slowinske. Am. Dyestuff Rep. 30, P6 (1941). 155. G. A. Slowinske, Am. Dyestuff Rep.32, P85 (1943). 156. G. A. Slowinske and A. G. Pope. Am. Dyestuff Rep. 36, P108 (1947). 157. J. W. Rowen and D. Gagliardi,Am. Dyestuff Rep. 36,533 (1947) 158. J. D. Reid and W. J. Connick, in “Kirk-Othmer, Encyclopediaof Chemical Technology,” 2nd ed., Vol. 22, p. 135, Interscience,New York (1970). 159. H. Bundesmann, Melliand Textilber. 16, 128 (1935). 160. Tentative TextileStandard No. 7, 1947, J. Textile Inst. 38. S1 ( 1947). 161. (a) Tentative Textile StandardNo. 8. 1955, J. Textile Inst. 46, S51 (1955);(b) 46, S 57 (1946),(c) J. Lord, J. Textile Inst. 46, S83 (1955):(d) 49. P255 (1958). 162. H. Bundesmann,Melliand Textilber. 41, 217 (1960). 163. H. Machemerand L. Hintz. Melliand Textilber. 41, 86 (1960). 164. H. Bundesmann and I. Matuschke. Melliand Textilber. 42. 561 (1961). 165. H. Bundesmann, Melliand Textilber. 45, 802 (1964). 166. H. Mendrzyk and H. Beyer. Z. Ges. Textilind.69,779 (1967). 167. H. Beyer, Melliand Textilber. 56,558 (1975). 168. A. M. Sookne, F. W. Minor. J. E. Simpson, and M. Harris, Am. Dyestuff Rep. 35, 295 (1946). 169. AATCC Test Method 127-1998, AATCC Technical Manual 75,211 (2000). 170. AATCC Test Method 21-1978, AATCC Technical Manual 55,266 (1978). 171. AATCC Test Method 70- 1997,AATCC Technical Manual 75,97 (2000). 172. J. E. Simpson and R. M. Howorth, Textile Res. J. 17,497 (1947). 173. G. E. Martin, H. S. Sell, and B. W. Habeck, Textile Res. J. 20, 123 (1950). 174. AATCC Test Method 118- 1997,AATCC Technical Manual 75, 19I , (2000).

13 Fluorinated Soil Retardants

13.1 SOILS Textiles come in contact with a variety of soils, which are usually complicated mixtures of components differing in their chemical and physical properties. Soils can be defined as unwanted substances that make a textile appear to be uncleana wrong substance in a wrong place. In general, soils are colored or colorless. The broad definition of soils includes stains, which because of their intensive color are discussed separately in Chapter 14. Colorless soils (e.g., colorless oil or fat) can enhance soiling by making particulate soil on the fabric more visible and facilitating adsorption of particulate soil on fibers. Soils are liquid, solid, or a mixture of liquid and solid substances. Liquid soils may be water insoluble, such as oils, or water soluble, such as aqueous stains and blood. Textiles in use encounter also a variety of particulate soils: airborne soil particles (e.g., dust and soot, carried by air currents) or particulate soil on the ground, and solid particles on soiled surfaces, usually mixed with fatty or oily soil. Particulate soils may be organic soils, such as skin cells mixed with sebum, or inorganic particles, usually mixed with organic soil, such as humus or oil. Water-insoluble liquid soils are commonly known as oily soils. Naturally occurring oily soils include hydrocarbons, saturated or unsaturated fatty acids, esters of fatty acids, and alcohols. Natural oily soils found on textiles are mixtures of oily components. Frequently, oil soils contain dispersed solid particulate matter (e.g., used motor oil). The most important properties of oilysoils are their viscosity [ 1,2], polarity [3,4], and solubility in detergent solutions or dry-cleaning solvents. The removal of oily soilby detergency is facilitated by a low viscosity at the wash temperature. The polarity of soil affects adhesion of the soil on fibers, interaction with 557

558

Chapter 13

the detergent, and the water-soil interfacial energy. Hence, the effect of polarity on detergency depends on the substrate and the detergent. Some polar soils, namely free fatty acids, react with alkali and form soaps dispersible in water. Very common natural soil are mixtures of sebum and skin cells containing organic pigments. These fatty soils are transferred from skin to a fabric i n contact with skin. The major constituents of atmospheric particulate soil, dispersed as an aerosol, are carbon soot, organic matter, and inorganic carbonates [ 5 ] .The composition of dust collected from air filters is variable; it may even vary withthe season of the year [6]. Sanders and Lambert [7] analyzed particulate soil commonly found on streets and found the composition to be fairly constant in 10 cities of the United States. Based on their analyses, Florio and Mersereau [8] suggested an artificial particulate soil consisting of seven components: carbon black (1.75%), iron oxide (0.50%),peat moss (38%0, cement (17%). silica (17%), red iron oxide (3%), carbon black (gas soot; 2%), and mineral (paraffin oil; 5%).

13.2 SOILING MECHANISMS Soiling is a natural process, because it increases overall randomness and, consequently, the entropy of the system. Therefore, an effort is needed to prevent soiling and maintain the cleanliness of a textile. This effort has been made easier by developing textile finishes that retard soiling and facilitate the removal of soil by detergency. A soiling process consists of two steps: (1) the transport of soil to the substrate and (2) entrapment of soil in the substrate and/or adhesion of the soil to substrate. Either step can determine the rate of soiling. The transport of soil onto a textile can occur by one of three mechanisms: (1) direct soiling (e.g., a drop of grease falling on a tablecloth or particulate soil deposited on a textile by air currents) (2) transfer of soil from a soiled surface to a cleaner one (e.g., soiling of armrests of chairs touched by hands), and (3) electrostatic soiling, caused by attraction of airborne soil by electrostatically charged textile surfaces (e.g., curtains). The mechanisms and kinetics of soiling with liquids differ from those of particulate soil. Soiling with liquid soils is dominated by wetting and wicking processes discussed in Chapter 11. The resistance of fabrics to soiling with a liquid is greatly enhanced by fluorochemical repellents (Section 12.7.) Soiling of textiles with particulate soil has been attributed to mechanical entrapment [9-131, adsorption of soil particles on an oil film covering the fiber surface [9,10,12-161, electrostatic attraction [9,10,12,13,15], and adhesion of particulate soil to the fiber surface [ 12,13,15,17- 191. The amount of soil that adheres to a fabric depends on the accessible fiber surface area, the topography of the fiber

Fluorinated Soil Retardants

559

surface, and the cross section and twist of the yarns in the fabric. Soiling with particulate soil is undoubtedly a complicated process governed by several properties of the soil and the substrate and mechanical action to which soil and the substrate are subjected. Kinetics of soiling with particulate soil has been studied by Kissa [19-231. using a dynamic method for precise soil application under carefully controlled but reproducibly variable conditions. Soil is deposited uniformly by an accelerotor. (The accelerotor has been designed for abrasion tests, but theabrasive collar is not used when performing soiling tests.) A rapidly rotating impeller agitates soil and fabric swatches in a closed chamber in a random fashion [ 19-22]. Excess loose soil is removed by suction. Quantitative analysis of soiled fabrics indicated that soiling depends on the nature of the soil and the fabric. The amount of soil deposited on the fabric increases with the 2/3 power of the amount of soil applied to the fabric and is exponentially related to the duration of the mechanical action and the kinetic energy of the soil particles. A soil particle impinging on a fabric is captured or bounces off the fabric surface. The capture of the particle can occur by adhesion or entrapment mechanisms. Scanning electron micrographs indicate that soil particles on a fabric that has been vacuum cleaned adhere to the fiber surface and are distributed over the fiber surface. Kling and Mahl [17], Powe [18], and Kissa [ 191 have shown that the main cause of soiling is adhesion, not entrapment of soil. Kissa [19] has demonstrated that particulate soil can adhere to initially smooth surfaces, such as polyester or polyethylene film. The impingement of soil particles can, however, deform the film surface and increase the contact area. The adhesion of soil to the fiber surface is caused mainly by van der Waals forces that are effective only over very short distances. Therefore, the soil particle must be brought into close contact with the fiber surface to adhere. Because the surface of the fiber and the soil particle are not microscopically planar, but are usually curved and irregular, the area of contact is small. The contact area increases when the fiber surface or the particle is deformed on impact and conforms to the shape of the interface (Fig. 13.1). Hence, pressure, which causes plastic deformation or indentation of the fiber surface, increases soiling (Fig. 13.2).

sol L

FIBER FIG.13.1

Plastic deformation of the fiber surface by a soil particle.

560

Chapter 13

FIG.13.2 Effect of pressure on soiling of polyester ( 0 )and cotton poplin (A)fabrics with iron oxide. (From Ref. 19.)

A liquid film on the fiber surface may increase the contact area and promote adhesion. It is well known that an oily film on fibers increases soiling. Kissa [ 19.231 found that liquids belonging to different classes of chemical compounds (hydrocarbon, fatty acid, alcohol, and ester) increased soiling of a hydrophilic fabric (cotton) and a hydrophobic fabric (polyester) with different types of particulate soil. Attempts to correlate soiling with the refractive index, viscosity, surface tension, dipole moment, and dielectric constant showed that soiling increases with the ratio of the viscosity to the dielectric constant of the liquid. Because hydrocarbons are nonpolar and have a low dielectric constant, a viscous hydrocarbon film is especially prone to capture particulate soil. A repellent finish cannot prevent oil from being forced into the fabric or carpet. An oilrepellent can, however, prevent the oil from spreading and wicking into the interior (e.g., the pile of a carpet). This localizes the oil and, consequently, the particulate soil to the outer area of the carpet where it can be removed more rapidly. 13.3 THEORY OF SOIL RETARDATION

Soil retardants are used to reduce soiling of textiles that cannot be laundered, such as upholstery fabrics and carpets. The soils encountered are usually composite soils, containing particulate matter and various amounts of liquid components, such as water or oil. The soils can therefore be fluids or particulate solids.

Fluorinated Soil Retardants

561

Soil retardants designed for upholstery of furniture or automobiles are water and oil repellents that retard sorption or waterborne or oily stains by the fabric. A liquid soil on a repellent fabric can be wiped off immediately after the spills. When the soil is rubbed into the fabric with sufficient force to overcome repellency, a fluorocarbon finish facilitates spot cleaning [24]. Soil retardants for carpets reduce accumulation of soil, mainly particulate soil, transferred onto the carpet from shoe soles. Because the carpet is subjected to mechanical forces, such as pressure, rubbing, and impingement, the retardant has to be resistant to abrasion. According to older theories [10,11], soiling with particulate soil is caused primarily by a fatty film on the fibers (oil binding) or entrapment (micro-occlusion) of soil particles by the crevices in the fiber surface and in the intrayarn and interyarn spaces (macro-occlusion). This led to the idea to retard soiling by depositing colorless particles on the fibers. These particles, it was thought [8,25], would occupy sites on the fiber that would otherwise be accessible to the soil particles. The carpet saturated with the “presoil” was supposed to stay visibly clean in subsequent soiling because the soil particles would not find sites for their attachment. Colloidal silica, alumina and silica, titanium dioxide, and other colorless compounds have been suggested as soil retardants [ 8,25,26]. Inorganic oxides have been coapplied with film-forming organic macromolecules. The efficiency of these soil retardants is not quite adequate. however, and the inorganic oxides imparted a “grainy” handle to the carpet. Therefore, attempts were made to cover the rugosities of yarns by film-forming organic polymers without inorganic oxides. Antistats have been included in these finishes. Newer soiling studies have established the adhesion of soil particles to the fiber surface as the dominant soiling mechanism. Kling and Mahl [ 171 showed that soil particles were evenly distributed on surfaces of wool and cotton fibers. Powe [ 181 found that soil particles on cotton fibers were larger than the rugosities in the fibers and concluded that sorptive bonding is the main mechanism of soiling. Kissa 1191 confirmed the adhesion mechanism with synthetic fibers and showed that soil particles could adhere even to initially smooth polymer films. Because soiling with particulate soil is caused mainly by adhesion, resistance to soiling is antiadhesion. The latest generation of soil retardants for carpets are repellent compounds that can lower the adhesion of soil particles to fibers. The attraction between nonpolar soil and the fiber surface is probably caused by London dispersion forces arising from fluctuations of electron clouds. Hence, energy of the interaction between the soil and the fiber surface should depend on electron by the polarizability,which, in turn,isrelated to therefractiveindex Lorentz-Lorenz equation: p E =--n 2 - 1 M n2+2 d

Chapter 13

562 TABLE 13.1

Refractive Indices and Surface Tensions of Benzene Derivatives

X

Atomic refractive constant of X

Refractive index (nD 1

F H CI Br I

0.95 1.IO 5.97 8.87 13.90

1.47 1.50 1.52 1.56 1.57

Surface tension

(Y) (vapor)20°C 27.3 28.9 33.6 35.8 39.3

Source: Ref. 27.

where P Eis polarizability, n is the refractive index, M is the molecular weight, and d is the density. The molecular refraction is an additive function of atomic refractions. Fluorine has the lowest atomic refraction (Table 13.1) of the atoms used to build compounds or polymers for coating fiber surfaces. Consequently, the refractive index and polarizability of fluorocarbons are lower than those of the corresponding hydrocarbons and chlorocarbons. Because dispersion forces constitute the major part of their cohesive forces, their surface energy is low. However, a low-surface energy is not the sole criterion for an effective soil retardant. The dispersion forces are effective only over short distances. A finish deformable by soil particles provides a larger area in close contact with soil than a finish that does not flow on impact. Hence, hardness of finish is also important. Soiling can also be affected by a liquid film on fibers, such as oil or fat, which can fill the voids between the surfaces of soil and fiber and act as an adhesive. Adhesion of soil on a film of oil or fat has been recognized for some time as one of the important soiling mechanisms [ 10.111. An oil-repellent finish can limit wicking of an oily substance into the yarn bundle. An oil-repellent finish also hinders residual oil, left at the bottom of the carpet pile after shampooing, from wicking back up to the top of the pile and collecting particulate soil. The oil-repellent soil retardants, therefore, reduce the reappearance of soiled spots after cleaning. However, oil repellency is not a dominant factor in determining the resistance of soiling with particulate soil. Berni et al. [28] reported that a fluorine-containing acrylic polymer, Poly FBA, offered only little resistance to dry soiling, although the oil repellency of the finish was excellent. Using his accelerotor method, Kissa [20] soiled polyester-cotton and automotive nylon fabrics finished with four different fluoropolymers. The Florio-Mersereau soil [8] that was used contained 8.75% (w/w) mineral oil (Nujol). Differences in soil retardance imparted to the woven polyester-cotton fabrics by the four fluoropolymers were clearly discernible (Fig. 13.3). The soil resistance of automotive nylon tricot fabrics treated

Fluorinated Soil Retardants

563

D

0.7 0

0.6

0.5

> W

3

0

>

0.4

0.2 Solled In Accelerotor with 10.0%0.w.f.

Synthetic So11for 30 sec a t 1500 rpm

0.1

I

I

I

1

0.1

0.2

0.3

I

0.4 % 0.w.f. Fluoropolymer Concn

FIG.13.3 Soiling of 65/35 polyester-cotton fabric finished with fluoropolymers A, B, C, and D. (Reproduced with permission from Ref. 20. Copyright 0 1971 by Textile Research Journal.)

with the same fluoropolymers is shown in Fig. 13.4. Although the polyester-cotton and nylon fabrics differ in their construction and chemical composition, the efficacy of the fluoropolymers with particulate soil in retarding soiling is the same order, A > B > C > D. The soiling values of Fig. 13.4 plotted against oil repellencies of the fabrics indicate that the resistance of the fabrics finished with fluoropolymers to soiling with oily particulate soil is not a function of oil repellency (Fig. 13.5). A decrease in soiling with increasing oil repellency of fabrics treated with the same fluoropolymer is related to the increasing amounts of fluoropolymer applied, as shown in Figs. 13.3 and 13.4. The fluoropolymer finishes, A, B, C, and D, differed in their hardness and resistance to abrasion. Fluoropolymer D, inefficient as a soil retardant, was more deformable than the others. The most efficient soil retardant, fluoropolymer A,

Chapter 13

564

22

20

> I6 -3 m > cn E I 6 0

VI

14

12 Solled In Accelerotor wlth 5.0% 0.w.f Synthetlc So11for 30 sec at 1500 rpm

I

.o 1

01

I 0.2

I

1

0 3

0 4

O/O

0.w.f

.

Fluoropolymer Concn

FIG.13.4 Soiling of Nylon 66 tricot finished with fluoropolymers A,

B, C, and D. (Reproduced with permission from Ref. 20. Copyright 0 1971 by Textile Research Journal.)

was also most resistant to abrasion. However, a harder finish is not always the more durable. A hard but brittle finish may break and flake off when the fibers are flexed. The resistance to plastic deformation has to be balanced with sufficient pliability. The temperature dependence of the physical properties and the transition temperatures are important for soil-retardant finishes cured at elevated temperatures. A soil retardant that melts below thecure temperature can spread on the fibers during cure. Quantitative data like those shown in Figs. 13.3-13.5 have not been published for soil retardants on carpets. Although laboratory soiling methods for carpets have been developed, in-service traffic tests have been considered to be more realistic and are being used instead. Bierbrauer et al. [29] have concluded from their inservice tests that although fluorochemicals with a low surface energy reduce soiling, low surface energy is not the sole explanation for soil retardance. Their data show that the efficacy of fluorochemicals in reducing soiling is not

Fluorinated Soil Retardants

>

2.0

-

1.8

-

.'

- O"

W

3 J

3

1.6

-

1.4

-

1.2

-

565

o

c

0

f 1

1

I I

I

I

I

I

2

3

5

6

1 4 OIL REPELLENCY

FIG.13.5 Soil resistance versus oil repellency of Nylon 66 tricot treated with fluoropolymers A, B, C, and D. (Reproduced with permission from Ref. 20. Copyright 0 1971 by Textile Research Journal.)

solely dependent on oil repellency of the treated carpet. This is in accord with the conclusions published earlier by Kissa [20] for repellent fabrics (Fig. 13.5).

13.4

FLUORINATEDSOILRETARDANTS

Most fluorinated soil retardants are vinyl polymers, urethanes, or oligomers derived from pyromellitates. Numerous fluorinated soil retardants have been disclosed in the patent literature, but only some examples can be given here (Table 13.2). Usually a vinyl polymer consists of (1) a fluoroalkyl group containing monomer [e.g., 2-(perfluoronony1)ethyl acrylate, the methacrylate CH=C(CH3) COOCH2CH2C,,F2,,+1, or CH"C(CH3) COOCH2CH?(CH3) SOZ(CF~)~F], (2) a nonfluorinated alkyl acrylate or methacrylate (e.g., butyl or stearyl acrylate or methacrylate), and (3) a cross-linkable monomer for durability (e.g. N-methylo-

TABLE 13.2 Examples of Fluorinated Soil Retardants for Carpets

Patent

Date issued ~~

Inventor ~~

___

Assignee -

Ger. Offen. 2,424, 447 (CA 82,126, 562)

1974

A. P. Downing, R. L. Powell

ICI

US. 3,896,fl35 (CA 83, isTn, '048) U.S. 3,897,227 (CA 86,18323)

1975

W. J. Schultz, S. Smith

3M

U.S. 3,916;053 (CA 84,S52,1'50) U.S.4,043,964 (CA 87, 153,375

1975

P. 0. Sherman, S. Smith

3M

U.S. 3,923,715

1975

R. H. Dettre, E. J. Greenwood

Du Pont

u.s.4;029335

1977

R. H. Dettre, E. J. Greenwood

cn

Du Pont

~

Composition _

_

_

~

A bifunctional fluorocompound [e.g., CloFl OC6H4S02N(CH2CH20H)2]reacted with a polyanhydride [e.g., pyromellitic dianhydride] and pH adjusted with NH3to 8-9 Urethane adduct not containing fluorine + urethane adduct of a fluorocompound (e.g., Nethylperfluorooctanesulfonamidoethanol); both adducts are water insoluble and have major transition temperature >45"C and melt flow temperature ~ 2 0 0 ° C Component A: a1 phase of a water-insoluble polymer (e.g., 3 : 7 : 90 itaconic acid-methyl acrylate-vinylidene chloride polymer) Component B: a 1 phase of a fluoropolymer [e.g., 90 : 10 C8F17S02N(CH3)CH2CH202 C-CH=CH2-butyl acrylate copolymer] At least one of the phases is continuous; components A and B must have at least one major transition point (glass, transition, melting temperature, etc.) above 45°C 0-95% nonfluorinated polymer having a Vickers hardness of 10-20; 5 4 100% perfluoroalkyl ester of a carboxylic acid of 3-20 carbon atoms; ester volatile at about 200-300°C Citric acid esters of C,F2,+,CH2CH20H, where n = 6-1 4, are polymerizedwith 1-methyl-2,4diisocyanatobenzene

9

5g

U

.-a, 3

H

3

n

0 0 0

Fluorinated Soil Retardants

U

.-a, 3

m

co b

0

03 r-

2

b 0

n

T-

5

U L ! ? 0

(u

0

cu

0 X

567

568

Chapter 13

lacrylamide) [30-321. Vinyl chloride or vinylidene chloride has been used as one of the monomers [33]. Nylon carpet yarn has been treated with a CF3(CF?1)5-1 ,CH2CH202CCH=CH2 and acrylic acid copolymer [34]. A soil-resist composition has been formed by reacting a maleic anhydride-styrene copolymer with allylamine and perfluoroalkyl iodide [35]. Fluorinated soil retardants based on polyurethanes are tough but resilient and can withstand foot traffic on the carpet [36-381. Urethanes are carbamic acid esters [e.g., -(CH2),,-NH-C(=O)-OCH2CH~(CF2),CF~] formed by a reaction of an isocyanate with an alcohol. The isocyanates used include hexamethylenediisocyanate. polymethlenepoly(phenyleneisocyanate), isophorone diisocyanate, trimethylolpropenediphenylmethylenediisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, and others. Blocked isocyanates, obtained by the reaction of an isocyanate with an oxime such as methylethylketoxime, dissociate when heated. The regenerated isocyanates react with hydroxyl or amino groups available and form covalent bonds and cross-links [3943]. Wehowsky et al. [44] reacted epichlorohydrin with C6-12F13-250H.The product obtained was reacted with toluene diisocyanate and oligoepichlorohydrine to yield an urethane useful as a soil retardant. Chang et al. [45] reacted 1.5 mol methylene bis(4-phenyleneisocyanate) with 1 mol N-ethyl(perf1uorooctane)sulfonamidoethyl alcohol. The remaining isocyanate groups were converted to carbodiimide groups and reacted with dibutylamine to yield a guanidine derivative. Smith[46]reactedatri-isocyanateorhigherorderisocyanatewith C8Fl7SO?N(CH3)C2H4OH,then reacted 5-50% of the isocyanate groups in the reaction product with water and, subsequently, reacted the remaining isocyanate groups with poly(ethy1ene glycol) (Carbowax 600). Soil retardants can be applied to fibers, yarns, fabrics, or carpets by spraying, padding, kiss-roll, or foam application techniques. Some soil retardants are applicable also by exhaust methods. Spraying is the most popular method for applying soil retardants to carpets. The required amount of a soil-retardant product is typically 0.5-1.6% of the weight of dry face fiber or about 200 ppm as fluorine. Usually, the soil retardant as applied as the last step before the carpet is dried. The presence of a fluorinated finish on the carpet can be confirmed by an oil-repellency test, based on the AATCC 118-1997 test (see Chapter 12), or a water-repellency test. Fuorier transform infrared and x-ray photoelectron spectroscopy (ESCA) (Chapter 9) provide semiquantitative information on the fluorinated soilretardant concentration on the fibers. 13.5 SOIL-RESISTANCE TESTS Soiling with particulate soil or fatty soils containing particulate matter occurs by transfer from a soiled surface onto a clean one. For example, upholstery fabrics

s

Soil

Fluorinated

569

are soiled mainly by the transfer of fatty soil from skin to fabric. To test the soil resistance of repellent fabrics, the transfer soiling has been simulated in a laboratory. Berch et al. [47] applied soil to felt cubes and transferred soil to the fabric by tumbling the fabric samples and the soiled cubes in a jar. TheFIRA test [48] transfers soil from felt cubes to fabric samples mounted at both ends of a tumbler. These procedures are time-consuming and soiling is not always uniform and reproducible, because the cubes are used repeatedly [49]. The transfer method developed by Dave and Kissa [49,50] combines the advantages of the dynamic soiling method [19,20,49] with those of the transfer method by Berch et al. [47]. A measured amount of soil is placed onto four polyurethane foam cubes and rotated in the closed chamber of the accelerotor for 1 or 2 min at 2000 rpm. The chamber is opened to introduce two 10 X 1O-cm (4 X 4-in.) fabric samples which are rotated with the soiled cubes for 1-3 min at a constant speed in the 1500-3000-rpm range, usually for 2 min at 2000 rpm. Soiling is very uniform, reproducible, and can be varied from barely visible to heavy soiling [50]. Soiling of carpets with particulate soil and mixed soils occurs mainly by transfer of soil from shoe soles to carpet fibers. Special techniques have been developed to simulate floor soiling by an accelerated test in a laboratory. The older test procedures involved tumbling carpet samples and soil in a cylinder [ 121 or sprinkling soil on a carpet mounted to the inside wall of a rotating cylinder [8,25]. Mechanical compression of the carpet being walked on was simulated by steel balls in the cylinder. Florio and Mersereau [8] designed a ball-mill soiling apparatus with a center perforated tube for dispensing soil (Fig. 13.6). A similar device

A F F LE

DR I VE

MECHAN ISM

I

( CENTRAL

SAMPLE MOUNTING STEEL BALLS

PERFORATED TUBE FOR

DISPENSING SOIL

FIG. 13.6 Diagrammatic sketch of ball-mill soiling apparatus. (Reproduced with permission from Ref. 8. Copyright 0 1955 by Textile Research Journal.)

was used by Salsbury and co-workers [25], who used a perforated capsule to dispense soil. The accelerated AATCC soiling test [51] tumbles specimens of carpet together with the soilin a laboratory ball mill for a predetemined time. The samples (18 X 9 cm) are placed in a porcelain ball-mill jar with the back against the inside wall of the jar. Soil (13 g) and 60 steel balls (12.6 rnin diameter) [52] or 50 flintpebbles are added to the jar, and the closed jar is rotated at 250-300 rpm. Comparison of laboratory soiling with actual traffic soiling has shown that soiling in the ball mill does not represent the mechanical action that a carpet encounters on thefloor. Therefore, various devices have beenconstructed to simulate grinding of the soil into the carpet by walking. The Tetrapod Walker [53-551, designed by ~ o u ~ a u l Engineering. ds Ltd. (England), consists of rotating drum and a plastic-footed tetrapod (Fig. 13.7).The drum is lined with carpet smples, pile face inward, the tetrapodis placed into the drum, and thedrum isrotated at 50rpm. The tetrapod rotates freely in the drum, and its feet, located at the apexes of a tetrahedron, make several random contacts with thecarpet during each revolution. The soil is distributed from a perforated egg-shaped container placed into the drum. This soiling technique was adopted in the British Standard for carpet shampoos [56]. However, East and Ferguson [57]found it to bedeficient in reprod~cibilityand uniformity. They adapted a transfer soiling method developed by Berch and co-workers E471 to carpet soiling. Benisek [58] also used the adaptedtransfer method.

.7 Tetrapod soiling apparatus with the soiling capsule. (From Ref. 55.)

Fluorinated Soil Retardants

""" ""

571

"""_

FIG.13.8 Lisson soiling apparatus. (Reproduced with permission from Ref. 61.) The Lisson Walking Wheel [59,60] soils the carpet in a planar position and subjects the carpet to compression, simulating foot traffic (Fig. 13.8). However, its mechanical compression was found to be unrealistically high. Borjesson [61] reduced the weight of the wheel from 25 to 10 kg and added sand to simulate abrasion in service soiling. Another soiling apparatus described by Brinkmann et al. [62] of the German Carpet Research Institute consists of a soil-dispensing cylinder and two grooved rollers for working the soil into the carpet. The soil applicator moves reciprocally over the carpet sample. A control sample, mounted parallel to the sample being soiled, does not receive soil but is subjected to the same mechanical action as the soiled sample. Within a limited range of fiber characteristics, both of the above mechanical devices are capable of producing results which correlate with those obtained by traffic soiling of carpets on the floor (Fig. 13.9). A carpet soiling test developed by Lamb [63] uses two drums in contact with each other. The lower drum is coated with a rubber sneaker-sole material and the carpet specimen are mounted on the upper drum (Fig. 13.10). The lower drum is driven by a motor at alow speed (5 rpm). The pressure on the carpet is controlled by a weight at the end of the lever arm. Soil is supplied by a conveyor belt (Fig. 13.11). A blade that rubs against the carpet helps to distribute the soil evenly before the soil reaches the compression zone. Soil resistance is evaluated using a Hunter ColorQuest or asimilar chromameter.

Chapter 13

572

0 24

5 ’ =

0.93

0-

18

e5

‘L:”

e9

5

10

15

FIG.13.9 A comparison of traffic spoiling (abscissa) and soiling in the Lisson apparatus (ordinate) (Hunter AEvalues). (Reproduced with permission from Ref. 61 .)

Although the test is reproducible, it does not present the carpet to soil in a normal position. The carpet on the drum is not flat, like a carpet on the floor, but is bent on the drum and the carpet pile is held in a downward position while being soiled. Hence, the carpet pile is more open than during actual use on the floor. This may make a difference when the concentration of the soil-resistant fluoropolymer is higher at the top of the pile than at the bottom. The existing laboratory soiling tests, in spite of their increasing mechanical sophistication, do not predict soil resistance of the full range of commercial car-

Fluorinated Soil Retardants

573

Carpet r

I

I o

01

0

FIG. 13.10 The Textile Research Institute Carpet Walker. (Reproduced with permission from Ref. 63. Copyright 0 1992 by Textile Research Journal.)

Smearing blade I

+

Belt

‘Oil

n....... ..::.:.:

Carpet

Sneaker sole

FIG. 13.1 1 Adaption of the Textile Research Institute Carpet Walker for soiling tests. (Reproduced with permission from Ref. 63. Copyright 0 1992 by Textile Research Journal.)

574

Chapter 13

pets. At present, exposing carpet samples to foot traffic is still considered the most meaningful test procedure [64,65]. Although soiling of the carpet on a floor by foot traffic is a realistic test, the uneven distribution and variability of soil make visual rating of soil resistance difficult and exclude an instrumental measurement of soiling. The development of laboratory test procedures to simulate soiling of carpets in service is therefore continuing. 13.6 FLUORINATEDSOIL-RELEASEAGENTS

Although soil retardants reduce soiling, the deposition of soil onto a textile cannot be entirely prevented. If the textile can be washed, soil-release finishes can facilitate the removal of soil considerably. The term "soil release" suggests a separation of soil from a fabric immersed in water, but such a spontaneous separation is possible only with liquid oily soils. Solid soils cannot separate spontaneously and require mechanical action for their removal. The mechanisms of soil release have been reviewed by Patterson and Grindstaff [66] and by Kissa [23,67]. The adhesive forces between soil and the fiber surfaces can be reduced by coating the fibers with a polymer of low surface energy. However, the removal of soil by aqueous detergency requires a hydrophilic fiber surface of high surface energy. The requirement to lower the surface energy of fibers is in apparent conflict with the requirement to increase hydrophilicity, which is a high-surface-energy condition. The opposing effects have been colligated by designing hybrid block copolymers with fluorinated segments of low surface energy alternating with hydrophilic poly(oxyethy1ene) segments [68,69] (Fig. 13.12). Conventional fluorocarbon polymers repel oil in the dry state, but are wetted under water by oil in preference to water. Therefore, the oily soils forced in the fabric are very difficult to wash out. The hybrid fluoropolymers are oleophobic in air and in water and hydrophilic in water. The dual function is based on different orientation of the oleophobic and hydrophilic segments in air and in water. In air, the poly(oxyethy1ene) segment is coiled and the repellent fluorocarbon segments dominate the interface. Under water, the oxyethylene chains are hydrated and expand, driven by the interfacial tension imparting hydrophilicity to the surface. During drying, the hydrophilic segment is dehydrated and the fluorocarbon segments reassume their dominant position. Pittman et al. [70] suggested that the dual action of oleophobicity and hydrophilicity could be considered in terms of separate contributions to wetting or adhesion. They argued that fluorinated polymers repel nonpolar liquids by dispersion-force interactions. However, in an environment of water, attractive forces of hydrogen-bonding are stronger than dispersion force interactions. The resultant force of both dispersion and hydrogen-bonding interactions should therefore be sufficient to cause spreading of water on the polymer surface.

Fluorinated Soil Retardants

575

F-

li-

00

V

v)

Nlr)

o x

2-0 N

N

I

X 0

0

(u

I 0

0

0

n co f C H 2

FIG.13.12 Average structure of a fluorinated soil release block copolymer. (From Ref. 68.)

Smith [70] has pointed out that if this mechanism is valid, water should instantaneously assume a low contact angle on fluoropolymers with hydrophilic segments. This conflicts with the observed slow decrease of the water-contact angle on such surfaces, which supports the orientation mechanism. It should be noted that other plausible mechanisms can explain the "dual action" and the slow decrease of the water-contact angle. The additional advantages of the hybrid soil-release polymers are increased dry soil resistance and good soil-release performance when coapplied with selected cross-linking formulations on polyester-cotton blends. In contrast, the soilrelease performance of nonfluorinated acrylic polymers deteriorates when coapplied with a durable press finish. Disadvantages of the hybrid soil-release polymers are the higher cost of the fluorochemical moiety and a slower water sorption in cross-linking finishes. Examples of fluorine-containing soil-release polymers are shown in Table 13.3. The fluorinated soil-release polymer [3M Protective Chemical FC-248 [7 111 has been applied by padding, usually as a component in a durable press finish formulation: Polyester/cotton or polyestedrayon (50% wet pickup) 0-200 g/L glyoxal crosslinking resin 0-50 g/L catalyst 10-20 g/L polyethylene sewing lubricant 20-30 g/L FC-248

.~

576

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Chapter 13

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cn

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Chapter 13

Fluorinated Soil Retardants

579

The amount of FC-248 applied was usually 1.35% product on fabric, but some fabrics needed a larger amount. The drying and curing of the finish was accomplished by either a one- or two-step process. The curing temperature of the one-step process was about 150-1 80°C [ 7 I]. FC-248 imparts soil release, oil repellency, and limited resistance to water. Like other 3M products derived from perfluorooctanesulfonate, FC-248 has been phased out.

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.

( 1 963). W. C. Powe, H. Am. Oil. Chem. SOC. 40,290 E. Kissa. Textile Res. J. 41,760 (1971). R. P. Harker. J. Textile Inst. 50, T189 (1959). J. C. Stewartand C. S. Whewell, Textile Res. J. 30,912 (1960). G. T. Wolff and R. L. Klimisch. eds., “Particulate Carbon:Atmospheric Life Cycle.’’ Plenum, New York (1982). ed., Vol. I,p. 47. AcademicPress, New York M. Corn, in “Air Pollution,’’A. C. Stern, (1968). H. L. Sanders and J. M. Lambert, J. Am. Oil Chem. SOC.27, 153 (1950). P. A. Florio and E. P. Mersereau, TextileRes. J. 25,65 1(1955). A. M. Schwartz, J. Am. Oil Chem. SOC.26,2 12 (1949). F. D. Snell, C. T. Snell, and I. Reich, J. Am. Oil Chem. SOC. 27,62(1950). J. Compton and W. J. Hart, Ind. Eng. Chem. 43, 1564 (1951). E. W. K. Schwartz, A. Leonard, K. Barnard, J. F. Hagen, E. Hansen, G. Slowinske, R. Smith, and E. I. Valko, Am. Dyestuff Rep. 41, 322. (1952). N. F. Getchell, TextileRes. J. 25, 150 (1955). F. Fortess and C. E. Kip, Am. Dyest. Rep. 42,349 (1953). W. H. Rees,J. TextileRes. J. 45,612 (1 954). T. Fort, Jr.. H. R. Billica, and C. K. Sloan, TextileRes. J. 36,7 (1966). W. Kling and H. Mahl, Melliand Textilber. 35,640 (1954). W. C. Powe, Textile Res. J. 29. 879 (1959). E, Kissa, Textile Res. J. 43, 86 (1973). E. Kissa, Textile Res. J. 41, 621 (1971). E. Kissa. in “Detergency. Theory and Technology,’’G. Cutler and E. Kissa, eds., pp. 193-225, Marcel Dekker. New York (1987). E. Kissa, in “Detergency. Theory and Technology,” G. Cutler and E. Kissa,eds., pp. 30-40, Marcel Dekker, New York( 1 987). E. IQssa, in “Handbookof Fiber Scienceand Technology,” M. Lewin andS. B. Sello, eds., Vol. IIB, pp. 21 1-289, Marcel Dekker, New York (1984). -W. Bernheim and H. Ruile, Textilveredlung2,463 (1967). H. Enders and K. H. Wiest, Melliand Textilber. 41. 1135 (1960). J. M, Salsbury, T.F. Cooke, E. S. Pierce, and P. B.Roth, Am. DyestuffRep. 45, P190 (1956). (a) E. I. Cogovan and E. D. Friderici (to Mohawk Carpet Mills), U.S. Patent 2.622,307 (1952); (b) E. P. Frieser, SVF Fachorgan 16, 382 (1961). “Lange’s Handbook of Chemistry,” 14ed.. J. A. Dean, ed.. McGraw-Hill Book Co.. New York ( 1992).

580

Chapter 13

28. R. J. Berni, R. R. Benerito, and F. J. Philips, Textile Res. J. 30, 576 (1960). 29. C. I. Bierbrauer, K. D. Goebel, and D. P. Landucci, Am. Dyestuff Rep. 69 (6), 19 (1979). 30. S. K. K. Obayashi, Jpn. Kokai Tokkyo Koho JP 59, 160,415 (1984). 31. G. Michels, H. A. Ehlert, and R. V. Meyer (to Bayer), Ger. Offen. DE 4,113,893 ( 1992). 32. G. Michels, H. A. Ehlert, and U. Zweering(to Bayer). Eur. Patent Appl. EP 713,939 (1996). 33. M. Yamana, I. Yamamoto. M. Usugaya, andT. Sano(to Daikin Ind.), PCT Int. Appl. WO9743,481(1997). 34. G. Olive and S. Olive (to Monsanto), Eur. Patent Appl. EP 161,382 (1985). 35. D. D. May (to Du Pont), U.S. Patent 5,408.010 (1995). 36. J. R. Kirchner (to Du Pont), Eur. Patent Appl.EP 435 641 (1991). 37. K. Itoh, G. Enna, and S. Otoshi (to Asahi Glass), Eur. Patent Appl. EP 414 155 (1991). 38. J. R. Kirchner (to Du Pont), PCT Int. Appl. WO 93 17.165 (1993). JP 06240.239 39. K. Ito, T. Kamata, and K. Kaneko(to Asahi Glass), Jpn. Kokai Tokkyo (1994). 40. T. Hashimoto, M. Shinada, and T. Ichikawa (to Enu 00 Kee). Jpn. Kokai Tokkyo Koho JP 05 179,573(1993). 41. F. A. Audenaert, R.J. Dams, and R.F. Kamrath (to Minnesota Mining andMfg.) U.S. Patent 5,466.770 (1995). 42. F. A. Audenaert, K. E. M. L. A. Allewaert. G. Hooftman, M. Nagase, and H.R. Lens (to Minnesota Mining and Mfg.). PCTInt. Appl. WO 97 44,375 (1997). 43. F. A. Audenaert. K. E. M. L. A. Allewaert, G. Hooftman, M. Nagase, and H.R. Lens (to Minnesota Mining and Mfg.). PCTInt. Appl. WO 97 44,508 (1997). 44. F. Wehowsky, R. Kleber, and L. Jaeckel (to Hoechst), Ger. Offen. DE 3 540 147 (1987). 45. J. Chang, R. D. Howells, and K. L. Williams (to Minnesota Mining Manuf.), Eur. Patent Appl. EP 108.512 (1984). 46. R. S. Smith (to Minnesota Mining Manuf.). PCT Int. Appl. WO 97 14,842 (1997). 47. J. Berch, H. Peper, J. Ross, and G. L. Cranke, Am. Dyestuff Rep. 56, 167 (1967). 48. T. M. Brown and D. J. Morley, Textile Inst. Ind. 9, 344 (1971). 49. A. M. Dave and E. Kissa, Textile Chem. Color. 12,255 (1980). 50. A. M. Dave andE. Kissa. Textile Res. J. 5I, 650 (1981). 51. AATCC Test Method 123-1989. Carpet Soiling: Accelerated Soiling Method, AATCC Technical Manual 70,208 (1995). 52. B. M. Reagan, S. Dusaj. D. G. Johnson. and D. M. Hodges, Am. Assoc. Textile Chem. Color. 22(4), 16 (1 990). 53. E. A. Ainsworth and G. E. Cusick. Textile Res. J. 37. 608 (1967). 54. K. C. Laughlin and G. E. Cusick, Textile Res. J. 37. 608 (1967). 55. H. Ninow and G. L. A. Burgers, Melliand Textilber. 59,288 and 383 (1978). 56. Specification for Carpet Shampoos, British Standards4088, Appendix G (1988). 57. G. C. East and J. P. Ferguson, J. Textile Inst. 60,400 (1969). 58. L. Benisek. Textile Res. J. 42,490 (1972). 59. G. Satlow and R. Liining, Z. Ges. Textilind. 71. 523 (1969).

Fluorinated Soil Retardants 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

581

G. Satlow and R. Liining, Z. Ges. Textilind. 71, 853( 1969). A. Borjesson. Melliand Textilber. 62. 854 (1981 ). K. Brinkmann, A. Lehnen, and G. Satlow. Chemiefasern 23,202 (1973). G. E. R. Lamb, Textile Res. J. 62,325 (1992). AATCC Test Method 122-1989, Carpet Soiling: Service Soiling Method, AATCC Technical Manual 70.206 (1995). H. Klingenberger and F. D. Diinnwald, Chemiefaserflextilind. 34(86), 198-200, 202-204 ( 1 984). H. T. Patterson and T. H. Grindstaff, in “Surface Characteristicsof Fibers and Textiles, M. J. Schick, ed., Part 11.” p. 447, Marcel Dekker, New York (1977). E. Kissa. in “Detergency; Theory and Technology,’‘W. G. Cutlerand E. Kissa. eds., pp. 333-369. Marcel Dekker, New York (1987). P. 0. Sherman, S. Smith, and B. Johannessen, TextileRes. J. 39.449 (1969). P. 0. Sherman, S. Smith, and B. Johannessen, TextileRes. J. 39.441 (1969). A. G. Pittman,J. N. Roitman, and D. Sharp, Textile Chem. Color.3, 175 (1971). “Protective Chemical FC-248.” 3M Product Bulletin, 1999.

14 Stain-Resistant Carpets

14.1SOILINGANDSTAINING Carpets on the floor are subjected to soiling under severe mechanical conditions. Particulate soil is transferred from shoe soles onto the carpet while the carpet pile is being compressed, rubbed, and twisted by the mechanical action of the foot traffic. Thesoil particles left on fibers are ground into the fiber surface by the continuous traffic. The mechanisms of soiling with particulate soil and the deterrence of soiling by fluorinated soil retardants have been described in Chapter 13. Soiling of a carpet by foot traffic is an unavoidable process which continuously affects the appearance of the carpet. In contrast, soiling with food and liquids, mostly beverages, is of accidental nature and avoidable in theory but quite common in real life. Therefore, the emphasis has shifted from the prevention of spills by careful behavior to the stain resistance of the carpet. The liquid is usually spilled from a considerable height and impacts the carpet with a significant force. Because most liquids coming in contact with a carpet contain colored substances, the stain resistance of carpets has become an important practical concern. The mechanisms governing the capillary sorption of liquids into textiles are described in Chapter 11; the consequential staining processes and protective stain-resist systems are discussed in this chapter.

14.2CARPETFIBERS Most carpets are made of nylon, wool, polypropylene, polyester, or cotton fibers. Silk isused occasionally in expensive carpets and acrylic fibers do not have a significant market share any more. Nylon is the most important carpet fiber because 582 i

Stain-Resistant Carpets

583

of its resilience, abrasion resistance, dyeability, and aesthetics. Both nylon 66 and nylon 6 polyamide fibers are used in carpets: Nylon 6

H

O

H O

H

Nylon 6,6

"N"(CH~)~"N"C-(CH~)~-C"N-CH;!)~-

I

H

I II

H

O

II I

O H

Nylon 6 is a more open fiber than nylon 6.6 [ 1,2] and, therefore, a more difficult fiber to protect against staining. Nylon fibers contain amine end groups which function as dye sites for acid dyes. The number of amine groups in nylon 6,6 can be increased by using an excess of hexamethylenediamine or by a varying the molecular weight of the polymer. The number of amine groups per lo6 g of fiber is 15-20 in "light dyeable," 3 5 4 5 in "medium dyeable," and 60-70 in "deep-dyeable" nylon 6,6 [3]. Nylon is especially vulnerable to staining by synthetic food dyes which have acid groups similar to the acid dyes used for dyeing nylon. Nylon 6,6 can be made dyeable with cationic (basic) dyes as well by incorporating -S03H or -COOH groups in the polymer. Cationic dyeable nylon is used mainly for styling purposes and has not been a major item in the stain-resist technology. Virtually all residential nylon carpet sold in the United States today is protected against traffic soiling with a fluorinated soil retardant. The repellency provided by the fluorinated soil retardant protects the carpet against staining by a liquid as well, if the liquid is blotted off immediately. However, the fluorochemical does not form an impervious shield around the fibers and a liquid can migrate or be forced into the carpet and stain the fibers. To protect the carpet against staining with food colors, stain-resist technology has been developed and stain-resistant carpets have been introduced by Allied Chemical (Ansco V), Du Pont (Stainmaster), Monsanto (StainBlocker), and BASF (Zeftron). A stain-resistant carpet is a system including three essential components: 1. Nylon fiber designed to reduce soil visibility. The carpet fiber produced by Du Pont has a three-lobal cross section. A carpet fiber made by IC1 contained bubbles to hide soil optically. 2. A fluorinated soil repellent (see Chapter 13) repels liquids, aqueous as well as oily, and protects the carpet against traffic soiling. Teflon MF, manufactured by Du Pont, is a fluorinated urethane derived from

Chapter 14

584

Telomer B alcohol [4]. Scotchgard FC-1395, a fluorinated polymer, was made by 3M [5]. The fluorinated soil retardant is undoubtedly the most important component of the soil and stain-resist finish on the carpet because soiling by foot traffic is a daily occurrence, whereas staining with liquids is an accidental event. 3. A stain-resist agent contains phenolic, sulfonic, or carboxylic groups needed for attachment to nylon fibers and stain prevention. The stain resistance of nylon carpets depends on several variables [6]: Type of fibers and heat-setting history Stain-resist agent and its chemical and physical properties Application process for the stain-resist agent Fluorinated soil retarder Wool fibers contain amine groups like nylon and the staining characteristics and stain-resistant treatments are similar to these of nylon [7,8]. Polypropylene and polyester fibers are less vulnerable to staining by synthetic food dyes than nylon but need fluorochemical protection against traffic soiling. 14.3 STAINS

Stains are intensively colored substances which, in small amounts, can affect the color and appearance of a textile. The term "stain" also denotes the discoloration caused by these intensely colored substances. From a physical-chemical point of view, it is useful to distinguish between stains and soils. A soil on a fabric constitutes a separate phase: a liquid, a semisolid, or a solid. In contrast, a stain is usually caused by molecules of colored substances deposited on or in fibers. The term "stain" usually implies a degree of permanency not necessarily associated with soil [9]. Common stains are as follows [ 10-131: Natural colors Synthetic food colors Substances of biological origin (e.g., blood, proteins, and pigments) Oxidation products of residual soil or antioxidants Common stains are coffee, tea, chocolate, wine, mustard, grass, rust, blood, lipstick, ink, natural organic colorants in general, and food dyes. Tea and coffee are most frequently consumed beverages and their stains are, therefore, common. Teacontainscoloredflavonoidpolymers,suchastheaflavines,and polyphenols [ 141 which have a considerable affinity to nylon. In the United States, tea stain on carpets has been a lesser problem than coffee stains. The color of tea

Stain-Resistant Carpets

585

0

(3

H + o OH

Lawsone

HO

Cyanidin

&Carotene C 02H

Crocetin FIG.14.1 Examples of natural colors. (Reproduced with permission from Ref. 13. Copyright 0 1991 by American Oil Chemists Society.)

is usually not as dark as coffee and tea is less popular in the United States, especially in offices and conference rooms. However, some herbal teas contain synthetic food colors which can stain nylon fibers severely. Most colored organic substances found in the nature are nonionic and do not dissociate to ions in water. However, some natural colors are ionic and can form in water negatively or positively charged colored ions (Fig. 14.1). Almost all synthetic colorants used in foods are anionic dyes with sulfonic or carboxylic acid groups (Fig. 14.2) and behave like acid dyes when staining fibers. The intensity of stain depends on the nature of the fibers, the physical state and location of stain, and the chemical composition of the staining substance. The interaction of stains with textile fibers is similar to dyeing with textile dyes. The intensity of the resulting stain depends, in analogy to dyeing, on the affinity of the staining substance to fibers. The affinity of a dyecan be defined as the difference between the chemical potential of the dye in its standard state in fiber and the chemical potential of the dye in the liquid medium surrounding the fibers at equilibrium. The affinity of a stain can be described by an analogous definition. The affinity of a stain is the difference between the chemical potential of

Chapter 14

586 I

I

I

FD&C Yellow N0.5 Tartrazine ,OCH3 tda03S-@-N=N

cti3

CY""""

FD&C Red N0.3 Erythrosine

HO

\

8

FD&C Red No. 40 Allura Red

FD&C B!ue No. 1 Brilliant Blue FCF

S 03NH,

FIG.14.2 Examples of synthetic food colors. (Reproduced with permission from Ref. 13. Copyright 0 1991 by American Oil Chemists Society.)

the stainer in its standard state in fiber and the chemical potential of the stainer in the liquid medium surrounding the fibers at equilibrium. The affinity of a stain depends on the structure of the staining colorant and the nature of the fibers. Cotton is stained mainly by cationic and nonionic colorants. Anionic food colors have a low affinity to cotton in a neutral medium but acidity enhances staining markedly. Synthetic food colors and other ionic substances have no affinity to polyester fibers. Nonionic colorants do not stain polyester fabrics readily at ambient temperature but diffuse into the polyester fiber at a higher temperature and are then difficult to remove. Nylon is stained by ionic as well as nonionic colorants. Synthetic food colors behave like anionic (acid) dyes are sorbed on nylon readily. An acid, such as citric acid present in beverages, enhances the staining of nylon. The resulting stain is difficult to remove with detergents and can create a problem for nylon carpets. Nonionic colors dissolved in an oil or fat can stain polyester, nylon, and cotton. However, the stain is located in the fatty or oily film on fibers and can be removed together with the fat or oil, unless driven into fibers by heat. Staining can occur by one of the two mechanisms: (1) transfer of the stain from a soiled substrate to the initially clean substrate or (2) a direct deposition of

Stain-Resistant Carpets

sa7

70 60

73 50 I

c/)

40

E 30

*8

20 10

0 0

05

15

k (fi)

2

25

FIG.14.3 Sorption of F&D Red 40 on nylon 6,6 Type 1150 yarn at 25°C as a function of the square root of time. (Reproduced with permission from Ref. 13. COPYright 0 1991 by American Oil Chemists Society.)

the staining substance to the fabric [ 12,151. A liquid dropped onto a fabric spreads on the fabric by capillary sorption [ 12-1 71. The rate of spreading depends on the wettability of fibers by the staining liquid. A colored substance can diffuse into the fibers, if it has affinity to the fibers. This is illustrated in Fig. 14.3 for a synthetic food dye sorbed on nylon. The amount of the food dye sorbed by fibers increases with the increasing square root of time, in accordance with Crank's diffusion law [ 181. The rate of diffusion increases with increasing temperature, but the diffusion rate of food dyes into nylon fiber is already considerable at ambient temperature. 14.4 COFFEE STAINS ON NYLON Although the resistance to staining by synthetic food colorants has been the main thrust of the commercial development of stain-resistant carpets, coffee has remained the worst offender [ 191. Coffee is a very popular beverage and coffee stains on textiles are common, especially on tablecloths and carpets. The interaction between coffee and fibers depends on the nature of the fibers. The affinity of coffee stain to fibers, indicated by resistance to detergency, increases in the order polyester < cotton < nylon [20]. The coffee stain has very little affinity to polyester fibers but adheres to cotton and most firmly to nylon. The high affinity of coffee to nylon is a formidable challenge to the stain-resist finishing. Contrary to a common belief, coffee stain is not caused mainly by colored

Chapter 14

588

pigmentlike particles suspended in coffee [201. The particles in coffee have a negative zeta potential [20] and are probably not attracted to textiles having a negative surface potential in water. Filtration of coffee through a very fine filter with a 0.2-pm pore size removes most of the particles but does not affect staining significantly [20]. Staining of nylon fibers by coffee is caused predominantly by water-soluble acidic colorants or colloidally dispersed polymeric substances. Although coffee has been consumed for centuries, the chemical composition of coffee is not completely known. The composition of coffee is complex, over 700 components have been detected so far [21-241. Roasted coffee contains a substantial amount of colored matter: brown to black pigments, polymers, and water-soluble colored substances. The composition of the colored species is largely unknown. Coffee is an acidic beverage [ 201. The pH of percolated coffee is about 4.9-5.0. Coffee is known to contain acids [21,22,25], mainly chlorogenic acids [23-241 (Fig. 14.4), and smaller amounts of nicotinic and citric acids. Linoleic, oleic, palmitic, and stearic acids are present as glycerides. These acids are colorless and cannot stain fibers. However, they can contribute indirectly to the formation of colored species when coffee beans are roasted. The colored species in roasted coffee are formed by thermal degradation and polymerization of monosacharides and sucrose to caramel [21]. The brown caramel can react with chlorogenic acids to form brown-black humic acids [21,26,27]. Another reaction mechanism being considered is the formation of melanoidins from amino acids and sugars. Most colored compounds in coffee are formed during roasting by thermal degradation and condensation reactions from carbohydrates. The resulting higher-molecular-weight species feature hydroxyl groups in addition to carboxylic groups. The hydroxyl groups can form hydrogen bonds with nylon fibers and create several attachment points to the fibers. Al-

3-Feruloylquinic Acid CH=CH-COOH

-R = O H -R = OCH, -R = H

Caffeic Ferulic P-Coumaric

R

H

Chlorogenic Acids

FIG.14.4 Acids in coffee. (Reproduced with permission from Ref. 20. Copyright 0 1995 by American Oil Chemists Society.)

Stain-Resistant Carpets

589

A€

I

35 30 -

25 20 15 10-

\

2

4

6

8

1

0

2

PH

FIG.14.5 The pH dependence of staining nylon 6,6 with coffee. (Reproduced with permission from Ref. 20. Copyright 0 1995 by American Oil Chemists Society.)

though a hydrogen bond is weaker than an ionic bond between an amine end group and an acid, the multitude of hydrogen bonds can increase the affinity to nylon fibers considerably. Staining of nylon by coffee depends on the acidity of coffee and decreases when the pH ofcoffee is increased (Fig. 14.5).The pH dependence of staining and the strong affinity to nylon indicate that ionic interactions between the carboxyl groups of the staining substances with amine end groups in nylon dominate staining of nylon fibers with coffee, in analogy to dyeing with acid dyes. In the absence of a stain-resist finish, adsorption of coffee stain on nylon fibers is rapid, followed by a slow diffusion of some of the colored species into fibers. The sorption of coffee by nylon fibers is accelerated by heat (Fig. 14.6). Because coffee is usually consumed hot, spilled coffee is a real challenge to a stainresistant finish. Coffee stain has remained a formidable challenge. A protective fluorinated finish retards the diffusion of the stain into fibers. However, when coffee penetrates the fluorinated repellent finish, stain-resist agents in the fibers must hinder staining by coffee. 14.5 THEORIES FOR STAINRESISTANCE

Discoloration of a textile by a stain involves the same physical-chemical interactions which govern the sorption of a textile dye by fibers. Hence, staining can be described as an unwanted dyeing process. Consequently, the objective of a stainresist agent is to prevent dyeing of fibers by any colored species which may con-

Chapter 14

590

AE

20

30

50 60 70 Temperature ("C)

40

80

90

FIG.14.6 Effect of coffee temperature on staining of nylon 6,6 carpet (5-min immersions). (Reproduced with permission from Ref. 20. Copyright 0 1995 by American Oil Chemists Society.)

tact fibers. Because the principle of stain resistance is antidyeing, the mechanisms of dyeing have to be considered in order to develop a theoretical basis for stain resistance. Dyeing of fibers occurs in two steps: (1) adsorption of the dye on fibers and (2) diffusion of the dye into fibers. Hence, the staining of carpet can be prevented by (1) hindering adsorption of the staining colorant on fibers and (2) retarding its diffusion in fibers. Obviously, if adsorption on fibers can be prevented, a diffusion into the interior of fibers cannot occur. The first defense line against staining is the fluorinated soil retardant (see Chapter 13) which reduces soiling with particulate soil and hinders wetting of fibers. When a liquid is spilled on the carpet, the fluorinated repellent provides time for the removal of the liquid by blotting before the liquid wicks into the carpet. However, if the liquid is allowed to wick into the carpet, a stain-resist agent in the fiber surface or inside the fibers must prevent staining. Because the synthetic dyes used in foods and beverages are bright colors, leaving a visible discoloration on a textile, the stain-resist drive focused first on synthetic food colors. A red dye, FD&C Red 40 found in beverages, became the standard test substance. Most synthetic food dyes contain sulfonic acid groups (see Section 14.3). like the acid (anionic) dyes used to dye nylon. It is well known that acid dyes are sorbed onto nylon in an acid medium. The sorption of the dye decreases with increasing pH and drops abruptly at a neutral pH [28]. In an acid medium, the amine groups in nylon are protonated and form a cationic site for the acid dye [29]. The

Stain-Resistant Carpets

591

sorption of synthetic food dyes on nylon also depends on the pH of the medium, indicating similar ionic interactions with protonated dye sites in nylon fibers. Therefore, it was conjectured that staining can be prevented by occupying all accessible dye sites with a colorless species. The dye-site-blocking model relates stain resistance to reduced dye adsorption on fibers. The adsorption of an acid dye on amine dye sites governs the diffusion rate of the staining dye in the fibers. Blocking of the accessible dye sites with a stain-resist agent which has a higher affinity to fibers than the staining dye prevents adsorption of the staining dye. The diffusion coefficient of the staining colorant may be reduced by a physical space limitation, but this is a secondary effect. The notion that a complete saturation of dye sites impedes staining with a typical synthetic food dye can be demonstrated by acylating the amine and groups (e.g., by reacting nylon with acetic anhydride). Acylation of nylon reduces staining with anacid (anionic) food dye. The amount of dye sorbed drops considerably from complete staining for the untreated fiber, although acylation does not prevent staining completely. The effect of dye-site blocking on staining and the importance of affinity can be demonstrated also with dyes. Kissa [ 301 selected two acid dyes, one of low affinity (C.I. Acid Yellow 29) and the other with a dye of high affinity (C.I. Acid Blue 113). Nylon yarn was dyed to the saturation level with one of the dyes and then immersed in a test solution containing the food color FD&C Red 40. The food color stained nylon fibers dyed with the low-affinity dye (C.I. Acid Yellow 29), but nylon dyed with the high-affinity dye (C.I. Acid Blue 1 13) was quite resistant to staining with the food color. The results can be explained by a displacement of the dye in the nylon fibers. The food dye did replace some of the lowaffinity dye but did not have the affinity to exceed that of the high-affinity dye. The rationale that the dye occupying a dyesite must have a higher affinity than the attacking species applies to dye-site blocking by a colorless “dye” as well. The dye-blocking mechanism predicts, therefore, that the stain-resist agent should be a “colorless acid dye” of high affinity and a slow diffusion rate in fibers. The surface of the fibers is the most effective location for the stain-resist agent [31]. However, the fiber surface is not a smooth and homogenous ideal plane but has a third dimension formed by the three-dimensional an-angement of polymer chains and their segmental mobility. This third dimension can be defined as the fiber surface accessible for the adsorption of dye molecules. To resist staining, the stain-resist agent has to occupy dye sites in the subsurface deeper than the depth accessible for the staining species. If the amount of stain-resist agent applied is justadequate for occupying the dye sites in the surface of the fibers, a diffusion of the stain-resist agent into the fibers will reduce stain resistance. On the other hand, excessive amounts of a stain-resist agent may affect the physical properties of the carpet, lower the lightfastness, and cause a partial displacement of the carpet dye.

592

Chapter 14

The dye-site-blocking model has some significant limitations. First, dye-site blocking does not prevent staining by nonionic stains. Second, the resistance to staining with food colors is compromised by the “overdyeing” phenomenon. If the dye concentration exceeds the saturation concentration for the amine groups functioning as dye sites, some of the excess dye may be held on fibers by hydrogen bonds and dispersion forces. Hence, food colors may stain fibers by this “overdyeing” mechanism, even when all of the dye sites are occupied. The limitations of the model for dye-site blocking has made it necessary to invoke a barrier model as well. A fluorinated soil retarder functions as a repellent nonionic barrier on the fiber surface. The fluorinated polymer does not form a continuous film around the fibers and cannot completely prevent the stain from diffusing into fibers. An additional barrier is needed in the subsurface of fibers as well to hinder the diffusion of the stain. Barrier models have been postulated to explain resistance to both ionic and nonionic staining. A nonionic barrier prevents the diffusion of the offending colored species into the fibers by physically hindering their adsorption onto the surface of fibers. In contrast, an ionic barrier model [32-341 attempts to relate stain resistance to electrostatic repulsion. An ionic barrier model envisions an anionic stain-resist agent on the surface or in the subsurface of the fiber that repels anionic substances, such as acid food dyes, with its electronegative potential. According to the ionic barrier model, a stain-resist polymer has functional groups that provide the coating on the fiber with an electronegative potential and repel negatively charged staining colorants. The size of the polymer prevents the polymer from significantly penetrating the fibers and provides extensive hydrogen-bonding needed to keep the barrier on the fiber surface. The effect of an ionic polymer on the diffusion of an acid dye has been explained by an electrostatic repulsion [35].However, a measurement of diffusion coefficients has shown that the retardation of dye diffusion into fibers is caused mainly by the prevention of initial dye adsorption rather than by electrostatic repulsion by a barrier [36]. The barrier model does not relate stain resistance to an interaction with dye sites in fibers. However, a barrier must be durable and adhere to fibers. The adhesion of a barrier to fiber surfaces relies mainly on ionic and/or hydrogen bonds formed with amine groups that function as dyes sites. Hence, the barrier model cannot exclude interaction with dye sites and consequently dye site blocking as well. On the other hand, most stain-resist agents featuring sulfonic acid groups are oligomers or polymers and their effectiveness exceeds that of the stain blocker model. Actually both stain-resist mechanisms, dye site blocking, and barrier action, are operable. 14.6 STAIN-RESIST AGENTS When it was recognized that fluorinated soil retardants alone cannot provide a complete protection against staining with liquids, the fibers were treated with stain-resist agents.

Stain-Resistant Carpets

593

Theoretical considerations predict that a stain-resist agent must have a strong affinity toward fibers, interact with the dye sites, and hinder adsorption of unwanted colorants on the fibers. In analogy to theories of dyeing, affinity of a stain-resist agent can be defined as the difference between the chemical potential of the stain-resist agent in its standard state in fiber and its chemical potential in the liquid medium surrounding the fiber at equilibrium. In simple terms, affinity indicates the strength of the interaction with fibers and, consequently, the strength of thebond between the stain-resist agent and the fiber. Strong affinity is required for exhaustion from a solution to fibers, adequate washfastness, and resistance to replacement by an offending stain. The affinity of the stain-resist agent to nylon has to be considerably higher than that of the offending species. The relative affinity test developed by Kissa [30] measures competitive adsorption of the satin-resist agent and a food dye on nylon fibers. The stain-resist agent and a food dye, the FD&C Red 40, used as the reference are applied to fibers from the same solution at an appropriate temperature. The species having a higher affinity is adsorbed preferentially on amine dye sites of nylon fibers. This simple test has accurately predicted the effectiveness of the stain-resist agents at a given pH. The structural functions essential for affinity to nylon fibers are ionic bonds (sulfonic acid groups and carboxylic groups), hydrogen bonds, and dispersion forces (aromatic nuclei). Hence, the stain-resist activity and the attraction to fibers require sulfonic, carboxylic, or phenolic functional groups. The interaction with amine groups in fibers is strongest with sulfonic groups; carboxylic groups require a low pH to be effective. The attraction of the stain-resist agent by ionic interactions with amine groups is augmented by hydrogen bonds formed between the phenolic groups of the stain-resist agent and the amine and amide groups of nylon. Blocking of the phenolic groups by propoxylation has been shown to lower stain resistance [ 371. The molecular size of the stain-resist agent is critical. For a strong adhesion to the fibers, a larger molecular size of the stain-resist molecule is advantageous. A large molecule cannot diffuse into fibers, which to some extent is desirable. However, the solubility of polymers decreases with increasing molecular weight as well and the solubility of the stain-resist polymer can be a problem, especially in an acid medium. Stiffness and brittleness are potential problems. The stain-resist agent should not interfere with the soil retardation by the fluorinated polymer or increase particulate soiling. A stain-resist agent must be applicable to nylon fibers which has been heat-set by various techniques and differs in their crystallinity. The twist of nylon yarn is heat-set by dry heat at 195-205°C (Suessen) or by steaming at 130-140°C (Superba). Consequently, to design of the stain-resist agent is a complicated task necessitating several compromises and trade-offs. Because larger molecules remain near the fiber surface, a larger molecular size is needed for fibers with an open structure [31]. Usually, stain-resist agents are mixtures of components with different molecular sizes.

Chapter 14

594

OH

FIG.14.7 Sulfonated condensates of phenols. (Reproduced with permission from Ref. 43. Copyright 0 1998 by Textilveredlung.)

The first-generation stain-resist agents were syntans, colorless compounds for blocking dye sites in nylon [37]. Synthetic tannic agents [ 381, referred to as syntans as an abbreviation, have been used as washfastness improvers for dyes in nylon 6 [39-41]. Their function, to keep the dye from leaving nylon, prevents a dye from moving into nylon as well [33,36,41,42]. Hence, the first-generation commercial stain-resist agents were syntan related, including Cibatex PA, FB, and RN (Ciba-Geigy). Dyapol SB40 (ICI), Erional PA, Intratex N (Crompton and Knowles), Matexil FA-SN (ICI), Mesitol NBS (Mobay), Nylofixan P (Sandoz), Unional SN (Tejin), and FX-369 (3M). Stain-resist agents related to syntans are condensation products of formaldehyde with aromatic compounds, including phenols, 4,4'-diphenyl sulfone, p-phenolsulfonic acid, sulfonated naphthalene, and sulfonated bisphenol (Fig. 14.7). The condensates contain sulfonic acid groups or are sulfonated after the condensation reaction [44-541. A polycondensation of a phenolsulfonic acid with formaldehyde may involve desulfonation and consequent cross-linking [55].Because the condensation reactions are complex, the formaldehyde condensates used as stain-resist agents are mixtures of several components [56]. Stain-resist agents containing a formaldehyde condensate of mainly bishydroxyphenyl sulfate and the remainder of cresol, methyl p-hydroxybenzoate, ory-

595

Stain-Resistant Carpets

phenolsulfonic acid and a mercaptocarboxylic acid have been disclosed in a patent 1571. Plischke and Snooks [58] patented fluorinated oligomeric formaldehyde condensation products derived from perfluoroalkyl groups containing sulfonates, such as sulfonated 2,2-bis-(4-hydroxyphenyl)hexafluoropropane. The fluorinated product obtained was claimed to be a more effective stain blocker than the nonfluorinated HCHO-4,4'-dihydroxyphenyl sulfone copolymer. The syntan-related formaldehyde condensation products of phenols, phenolsulfonic acids, and sulfonated dihydroxyphenylsulfones are indeed remarkably effective stain-resist agents. However, the lightfastness of these phenolic stain-resist agents is inadequate and yellowing of carpets became a serious problem [34]. Photodegradation of the stain-resist agent oxidizes phenolic groups, reduces the hydrogen-bonding with fibers, and lowers stain resistance [37]. The addition of thiocyanates, preferably ammonium thiocyanate, to reduce yellowing of phenolic stain-resist agents, has been patented [59]. However, this and other measures to prevent yellowing were not adequate. The yellowing problem was finally solved by stain-resist agents based on polycarboxylates, such as hydrolyzed maleic acid copolymers with styrene [60] (Fig. 14.8), with butadiene [61] and an allyl or vinyl ether [62,63]. The low affinity of polycarboxylate copolymers to nylon in neutral and alkaline medium re-

coo

4 coo

x=5

x=5

FIG. 14.8 Stain-resistant polycarboxylates. (Reproduced with permission from Ref. 43. Copyright 0 1998 by Textilveredlung.)

Chapter 14

596

quires a low pH, about 2.5. for their application and lowers the durability to shampooing. The solubility of polycarboxylates in acid medium decreases while the wash fastness of the stain-resist finish increases with molecular size. Because polycarboxylates are mixtures of components of different chain lengths and stereoconfigurations, the molecular size distribution of polycarboxylates is a critical factor for maintaining adequate solubility during application and durability of the stain-resist finish to shampooing. The inadequate shampoo fastness of polycarboxylates was improved by blending with sulfonated phenolic condensates [64-691. Thus, syntan-related sulfonated phenolic condensates have been blended with hydrolyzed maleic anhydride-styrene copolymers [66-681, with maleic anhydride-butadiene copolymers [69] or with methacrylic acid copolymers [62]. Improved durability to shampooing by adding an epoxy resin to the stain-resist agent has been claimed in a patent [70]. Methacrylic acid has been esterified with a sulfonated phenolic condensate [71]. A mathacrylic polymer or copolymer has been coapplied with a sulfonated and phosphated resol resin [72]. Pechhold [73] patented a condensation product of bis(hydroxypheny1)sulfone, applied with marcaptoacetic acid and a Mg salt in the presence of ammonium or sodium hydroxide. A new generation of stain-resist agents in commercial production does not contain sulfonate, carboxylate, or phosphate groups. A resol, patented by Buck and co-workers [ 74,751 is obtained by condensing bis(hydroxypheny1)sulfone with formaldehyde (Fig. 14.9). The resol has a high affinity to nylon although it does not contain sulfonic or carboxylic acid groups. The resol is applied at higher pH values (between 5 and 9) than the syntan-related stain-resist agents or polycarboxylates and is, therefore, less corrosive. Yellowing has been minimized as OH

OH

OH

OH

OH

OH

FIG.14.9 Formaldehyde condensate of 4,4'-bis(hydroxypheny1) sulfone. (Reproduced with permission from Ref. 43. Copyright 0 1998 by Textilveredlung.)

Stain-Resistant Carpets

597

1 Stain Resist Application

_ I ,

Steam

Fuation

”--+

Wash & Extract

Dry

FIG.14.10 Continuous dyeing and finishing of carpets. (Reproduced with permission from Ref. 43. Copyright 0 1998 by Textilveredlung.)

well. Because the phenol or phenoxide groups of resol have a high affinity to nylon in a neutral and slightly alkaline medium, the durability to shampooing is excellent. The Du Pont Antron@Teflon@protection system [43,761 utilizes the resol in combination with a fluorinated urethane as the soil retardant. The novel stainprotection system has been stated to be effective against all common household stains, including coffee, tea, and red wine [43]. Theoretically, the stain-resist agent can be (1) incorporated in the fibers [77,78], (2) applied to the fiber as a spin finish [79,80], (3) to yam [81], (4) coapplied together with the dye to the carpet [45,82], or ( 5 ) applied to the dyed carpet [64,83,84]. Although the stain-resist function has been incorporated into nylon fibers on a commercial scale, the application of the stain-resist agent to the carpet has become the dominant practice. The stain-resist agent can be applied before dyeing, coapplied with the dye, or applied to the already dyed carpet. Although all of the three options have been practiced commercially, the stain-resist treatment of the dyed carpet has proven to be the most practical (Fig. 14.10). Coapplication of the stain-resist agent with a dye is difficult at best. Stain-resist agents hinder the sorption of dyes, especially acid dyes, by nylon fibers; coapplication with acid dyes is not practical. A process of coapplying a syntan with disperse dye has been demonstrated in a laboratory [85] but has not been established commercially. Usually, the stain-resist agent is applied to the dyed carpet, either by exhaust application in a dye beck or mainly by a continuous process, by using the Kiister Flex-Nip equipment, by spraying, or as foam. The stain-resist agent is sorbed onto the fibers by a steaming or heating. A 3M stain-resist agent (FC-661. FX-668F, or FX-657) was coapplied with the Scotchgard FC-1395 fluorinated soil retardant in the presence of magnesium sulfate at pH 2.0, either by saturation application (Flex-Nip) or as foam. The carpet was steamed for a minimum of 30 s at steam temperatures of 98°C or higher [5a]. The coapplication technology has been patented by 3M [86]. The fluorinated soil retardant Scotchgard FX-l367F was applied alone or coapplied with its com-

-

598

Chapter 14

panion stain-resist agent FX-668F by a foam application machine. The carpet was heated at 2 1 10-12 1“C for 1-3 min after the carpet was completely dry [5b]. The production of these Scotchgard soil retardants has been discontinued by 3M (see Chapter 10). The concentration of the stain-resist agent on fibers is determined by dissolving nylon fibers in concentrated sulfuric acid [30] or in formic acid and measuring the absorbance at 260 nm in the ultraviolet region. Because a stain-resist agent is a mixture of various components, some of which are sorbed on fibers preferentially. only an approximate average value can be obtained. 14.7 STAIN-RESISTANCETESTS Stain-resistance tests have two different objectives: (1) to control the quality of the soil-resist treatment, to assure that specifications are met and control the proper operation of the application process, and (2) to evaluate the effectiveness of developmental stain-resist systems. Whereas quality control requires mainly a pass or no-pass result, quantitative data are needed for research and development. Small differences in stain resistance reveal trends related to variation of the stainresist agent structure, active concentration, and application conditions. Hence, the objectives of stain-resist tests can be quite different. The design of a stain-resistance test has to deal with two problems: how to (1) apply the test stain to the carpet in a realistic and reproducible manner and (2) to evaluate or measure the resulting coloration of the carpet. Stain resistance of carpets is usually evaluated by the AATCC test [ 871. A known volume (20 mL) of a dye solution (0.100 g/L FD&C Red 40, adjusted to pH 2.8 with citric acid) is poured into a ring on a 15 X 15-cm carpet sample (Fig. 14.11). The dye solution is forced into the carpet pile by moving a cup, like a plunger, five times up and down in the ring. After allowing the stained carpet to stand for 24h, the excess dye isremoved by rinsing with water. The excess water is removed by centrifugation or suction. The carpet is dried and the intensity of stain is rated visually using a stain scale [87]. The conventional AATCC Red Stain Scale [87] for stain-resistance rating has 10 transparent squares, each dyed with a different amount of a red dye having a color similar to the FD&C Red 40 used in beverages. The color intensities of the squares correspond to the stain-resistance ratings from 1 to 10, where 10 iscolorless (“no staining”) and 1 is “severely stained.’’ The red stain on the carpet specimen is visually rated by placing the clear area of the stain scale over the stained area of the specimen. The color of the stained area is compared to that of the colored squares. The stain-resistance rating is the number of the square that matches the color of the stain. The AATCC test simulates practical staining conditions without expensive instrumentation. The staining procedure attempts to simulate practical conditions, but the application of the test liquid is operator dependent and not highly repro-

Stain-Resistant Carpets

599

1

I '

I

CUP L

1

-70 ML-6OML-50 ML-40 ML-30 ML-2OML-1OML-

-

1

t

MOVE UP AND DOWN 5 TIMES

I

PRESS FIRMLY

FIG.14.11 AATCC stain-resistance test. (Reproduced with permission from Ref 87. Copyright 0 2000 by AATCC.)

ducible. The standard deviation is 0.5 units for within-lab testing and 1 unit for interlaboratory testing [87]. This corresponds to 95% confidence limits of 1 and 2 units, respectively. As useful as the AATCC test may be for the trade, the test is inadequate for creating quantitative data needed for the development of stain-resist agents and investigation of staining and stain resistance. The precision of the stain-resist test can be improved by (1) improving the stain-application procedure and (2) colorimetric measurement of the stain on the carpet. The instrumental measurement of stain on carpets is complicated by the geometry of the pile and orientation effects on the reflectance [88,89]. Laughlin and Lynch [88] have shown that compaction of the carpet pile affects the observable and measurable coloration. Therefore, Kissa [90] developed a colorimetric method that positions the carpet sample face-down onto a sample holder on top of

Chapter 14

600

AE

2o

1 0.01

0.02

0.05

0.10

0.20

0.50

1.0

Dye Conc in Fibers (mg/g) FIG.14.12 Visible staining, indicated by CIELAB A,!? values, plotted on logarithmic scales versus the FD&C Red 40 content of the initially blue cut pile carpet (BCP), blue level loop carpet (BLL), and white level loop carpet (WLL). White standard plate as the reference (target). (Reproduced with permission from Ref. 90. Copyright 0 1995 by AATCC).

the Minolta chromameter. This technique minimizes pressure and orientation effects on the reflectance and color values. The CIELAB colorimetric AE*and Aa* values correlate on logarithmic scales with the stain content of the carpet (Fig. 14.12):

AE"

= EoC

(1)

where Eo is the AE* value for the unstained carpet, C is the stain (dye) concentration in the carpet, and n is acoefficient dependent on the distribution of the stain (dye) in the carpet [Sl] and the carpet structure. A colorimetric stain evaluation has some limitations, however. First, a multicolor pattern frustrates instrumental measurement. Second, if only the roots of the carpet pile are stained, the carpet may appear to a visual observer and to a colorimeter to be stain resistant until the pile is shifted. Therefore, Kissa [30] developed a test method that differs from the conventional AATCC test in two aspects: (1) The carpet specimen is immersed in a me-

Stain-Resistant Carpets

601

chanically agitated dye solution and (2) staining of the fibers is assessed by measuring spectrophotometrically the depletion of the dye in the solution. The sorption measurement may be augmented by a colorimetric measurement of the stain on the specimen. A total immersion of the specimen in a mechanically agitated dye solution has two significant advantages: (1) Mechanical agitation forces the liquid into the carpet pile in a reproducible manner and produces a uniformly stained carpet, suitable for colorimetric evaluation, and (2) stain resistance can be evaluated by measuring dye sorption by the depletion technique. A sufficient amount of the dye solution in the jar is filtered through the syringe filter to remove suspended debris and its absorbance measured. If the weight of the specimens is kept essentially constant, stain resistance can be reported as percentage dye sorbed. Otherwise, stain resistance can be expressed as milligrams of dye sorbed per gram of carpet. A “blank” specimen immersed in a solution not containing a dye (citric acid and surfactant only) serves as the reference for the rating. The carpet sample is then removed from the dye solution and rinsed with water without squeezing the carpet pile. The color of the dried carpet is measured colorimetrically to complement the dye sorption values. The amount of dye sorbed as a function of stain resistance is shown in (Fig. 14.13). The precision of the stain resistance test measuring dye sorption by the depletion technique was determined using eight replicate samples of a white levelloop carpet, treated with a small amount of a stain resist agent. The average dye sorption was 60.9% (2.40 mg/g) and the standard deviation was found to be

Dye Sorbed %

\

I

0

I

WLL

”.

0:l

0:2

013

0:4

015

0.6

0.7

Stain Resist Agent(% w/w) FIG.14.13 Sorption of FD&C Red 40 stain as a function of stain resistance. WLL = white level loop; BCP = blue cut pile. (From Ref. 30.)

602

Chapter 14

0.45%, or 0.74% of the average value. Clearly, the precision of the dye depletion test is greatly superior to that of the AATCC test. In principle, the AATCC test and the dye depletion technique provide different information. The AATCC test rates the apparent resistance to staining with a red food dye. In contrast, the depletion of dye indicates the overall resistance to sorption of a stain regardless of the color of the staining dye and construction of the carpet. If the distribution of the stain-resistant finish on the yarn is not uniform, an average value for dyesorption is obtained. Furthermore, colored patterns and extensively sculptured texture exclude instrumental measurements and complicate visual estimation of stain but do not interfere with the dye sorption measurement. However, the evaluation of stain resistance by dye depletion is limited by its sensitivity to very small amounts of stain. Minute amounts of stain located on the top of the carpet pile may be visible but too small to be reproducibly and accurately measured by dye depletion. If a dye concentration measurement indicates that less than 3% of the dye initially present has been sorbed by the carpet, the depletion technique must be augmented with colorimetric measurements. Because dye sorption increases linearly with the square root of time (Fig. 14.3), stain resistance can be evaluated by measuring the rate of dye sorption, related to the slope of the line in Fig. 14.3. The time required for the test can be reduced by measuring dye sorption by the depletion technique at a higher temperature (e.g., at 60°C). Instead of a carpet sample, yarn sheared or pulled from a finished carpet can be used for testing [30]. Stain-resist finishes must be tested for their resistance to nonionic stains as well, including natural-colored substances found in food (Fig. 14.1). However, the resistance to traffic soiling is the most important factor governing the appearance of the stain-resistant carpet, with the added requirement that the stain-resist finish must withstand foot traffic. Soil-resistance tests are described in Chapter 13 and, in addition, stain resistance is tested before and after subjecting the carpet to floor traffic. The durability of the stain-resist finish to shampooing is another important requirement. Soil and stain-resist technology has been accepted by the consumer, and virtually all residential nylon carpets sold in the United States have been treated with soil and stain retardants. However, the development of the stain-resist technology is continuing and further improvement as well as theoretical insights can be expected.

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603

4. Du Pont, Teflon@ Carpet Protector Bulletin. 5. Minnesota Mining & Manufacturing,(a) Protective Chemical FC-1395, Product Bulletin (1 999); (b) ScotchgardTM Carpet ProtectorFX- 1367F,Product Bulletin, St. Paul, MN, (1994). 6. D. J. Jose,B. F. Lewis,J. M. Materniak, E. Rivet, R. M. Shellenbarger, Y.Vinod, and E. D. Williams,Can. TextileJ. 105(1I), 34 (1 988). 7. S. M. Burkinshaw and N. Nikolaides, DyesPigments 15,225 (1990). 8. S. M. Burkinshaw and N. Nicolaides, Dyes Pigments 16,299 (1991). 9. C. P. McClain. in “Detergency. Theory and Test Methods,” W. G. Cutler and R. C. Davis. eds. Vol. 5, Part 11, p. 519, Marcel Dekker, New York (1975). 10. M. Wentz, A. C. Lloyd, and A. Watt IV, Textile Chem. Color. 7(10), 30 (1975). 11. A. M. Sarmadi, M. L. Tate, and R. A. Young, (AATCC Midwest Section), Textile Chem. Color.20(2), 23 (1988). 12. E. Kissa, in “Detergency.Theory and Technology.” W. G. Cutler and E. Kissa, eds., p. 1. Marcel Dekker, New York( I 987). 13. E. Kissa, J. M. Dohner, W. R. Gibson, and D. Strickman, J.Am. Oil Chem. SOC. 68, 532 (1991). 14. D. A. Balentine, in “Kirk-Othmer Encyclopedia of Chemical Technology,’’ 4th ed.. M. Howe-Grant, ed., Vol.23, p. 746, John Wiley& Sons. New York ( I 997). 15. E. Kissa, in “Functional Finishes. Part B, Handbook of Fiber Science and Technology,” Vol. TI, Marcel Dekker, New York(1984). 16. E. Kissa, J. Colloid Interf. Sci. 83,265 (1981). 17. E. Kissa, Textile Res.J. 66. 660 (1996). 18. J. Crank, “TheMathematics of Diffusion.’’ Oxford University Press, London(1 956). 19. H. Klingenberger, ChemiefaserrdTextilindustrie 38/90, 660 (1988). 20. E. Kissa, J. Am. Oil Chem. SOC.72,793 (1995). 21. R. Viani, “Ullmann‘s Encyclopedia of Industrial Chemistry.” 5thed., Vol. A7,p. 3 15. VCH. Weinheim (1986). 22. G. Wasserman, H. D. Stahl. W. Rehman, and P. Whitman, “Kirk-Othmer Encyclopedia of Chemical Technology.” 4th ed., M. Howe-Grant, ed., Vol. 6, p. 793, John Wiley & Sons, New York. 1993. 23. M. N. Clifford and J. J. Wright, J. Sci. Food Agric. 27.73 (1976). 24. L. C. Trugoand R. Macrae,Analyst 109,263 (1984). 25. J. Wurziger, “Ullmann’sEncyclopedia of Industrial Chemistry,”4th ed., Vol. 13, p. 429, Verlag Chemie. Weinheim (1977). 26. R. Ikan, T. Dorsey, and I. R. KapIan, Anal. Chim.Acta 232, 11 (1990). 27. M. A. G. T. Van den Hoop, H. P. Van Leeuwen. andR. F. M. J. Cleven, Anal. Chim. Acta 232, 141 (1990). 28. T. Vickerstaff, “The Physical Chemistry of Dyeing,’’ 2nd ed., Oliver & Boyd, London (1954). 29. C. L. Bird and W. S. Boston, eds., “The Theory of Coloration of Textiles,” TheDyers Company Publication Trust, West Yorkshire, U.K. (1 975). 30. E. Kissa, to be published. 31. P.W. Harris and D.A. Hangey, Textile Chem. Color.21 (1 1) 25 (1989). 32. C. C. Cook and C. G. Herbert,J. Appl. Chem. Biotechnol. 28, 105 ( 1978).

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68. P. H. Fitzgerald, N. S. Rao, Y. V. Vinod, and J. R. Alender (to Du Pont), Eur. Patent EP 328 822 (1989). 69. E. Pechhold (to Du Pont), PCT Int. Appl. WO 93 19,238 (1993); U.S. Patent 5,460,887 (1995). 70. E. Pechhold (to Du Pont), PCT Appl. WO 94 25,662 (1994). 71. R. R. Sargent and M. S. Williams (to Peach State Labs.), PCT Int. Appl. WO 92 18.332 (1992). 72. Y. M. Yassin (to Trichromatic Carpet), U.S. Patent 5,736,468 (1996). 73. E. Pechhold (to Du Pont), PCT Int. Appl. WO 93 21,375 (1993). 74. R. C.Buck, E. Pechhold. and D.D. May (to Du Pont), U.S. Patent5.447.755 (1995). 75. R. C. Buck, E. Pechhold,and D. D. May,U.S. Patent 5,460,891 (1995). 76. R. C. Buck, Tinctoria 96(3). 44 (1999). 77. P. A. Ucci, U.S. Patent 4,579,762(1 985). 78. M. B. Hoyt, A. Coons, and D. N. Dickson (to BASF), Eur. Patent Appl. EP 517.203 ( 1992). 79. R. C. Blyth and P. A. Ucci, U.S. Patent 4,680,212 (1987). 80. R. C. Blyth and P. A. Ucci (to Monsanto). Jpn. Kokai Tokkyo Koho JP 62 257,465 (1 987). 81. L. Yeh (to BASF), Eur. Patent Appl. EP 579.976 (1994). 82. R. C. Blyth,P. A. Ucci, and G. R. McLellan. U.S. Patent 4,619,853 (1986). 83. K. Tajiri and A. Teruhiko (to Teijin), Jpn. Patent2,216,275 (1990). 84. R. C. Blyth and P. A. Ucci (to Monsanto). Jpn. Kokai Tokkyo Koho JP 62 257.467 ( 1987). 85. X. X. Huang, H.-D. Weigman, and L. Rebenfeld, Textile Asia 24(December). 54 ( 1993). 86. J. C. Clark, J. C. Newland, R. F. Kamrath, M. B. Burleigh, and K. R. Schaffer, PCT Int. Appl. WO 98 50,619 (1998); U.S. Patent 4,875,901; U.S. Patent 4382,373. 87. AATCC Test Method 175-1998, AATCC Technical Manual 75,319,2000. 88. K. C. Laughlin and S. Lynch, Textile Chem. Color. 2(6), 37 (1970). 89. R. F. Hoban, Textile Chem. Color.13(5). 16 (1981). 90. E. Kissa. Textile Chem. Color. 37(lo), 29 (1995). 91. E. Kissa, Textile Res.J. 54,497 (1984).

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Index

Acid strength, 87-90 Adhesion. 496 Adhesives, 352 Adsolubilization, 167-268 Adsorption at solid-liquid interface, 175-1 89 characterizationof adsorbed surfactant, 185 chemisorption. 175 effect of substrate, 175 effect of surfactant structure. 175 electrode-solution boundary,189 ionic, 175 isotherms, 177 measurement, 176 of mixed surfactants, 190-194 on dyes, 188 physical, 175 simultaneous, 189. 194 unequal, 183 Adsorption of surfactant, at liquid-vapor boundary, 103-1 39 competitive adsorption, 124 cross-sectionalarea, 110 kinetics, 133-139 phase transitions, 120

Adsorption on solids, 175-197 Aggregation number, 204,22 1, 252 Alkane sulfonyl chlorides, 34 Amine oxides, 8 synthesis, 58 Amphoteric fluorinated surfactants, 3, 8, 11 synthesis, 59-63 Analysis of fluorinated surfactants, 390-450 Anionic fluorinated surfactants, 3.4,44 Bunte salts, 54 carboxylates, 45-49 chemical stability, 141-142 mixtures, 299-3 13 phosphates, 55, 56 sulfates, 54 sulfonates, 49-52 sulfosuccinates, 53 surface tension in acids, 141. 143 surface tension in alkali, 142, 144 synthesis, 44-56 Anionic-nonionic surfactant mixtures, 3 13-324 critical micelle concentration. 3 13 Antifogging, 352

607

608 Antistatic agents, 353 Aquatic toxicity, 457-459 Area occupied by surfactant molecule, 110 Association with cyclodextrins, 269 Atsurf surfactants, 168. 362, 365. 369 Benzoylacetanilide (BZAA),3 17 Biodegradation,4 5 9 4 6 I Biological systems, 461-466 Biomaterials, 354 Biomedical oxygen carriers, 467-486 fluorinated surfactants. 477-485 fluorochemical,468486 Bis(perfluorobuty1)ethene (F-44E). 475 Blood, 461486 inorganic fluoride, 466 sorption and excretionof fluorinated surfactants, 46 1-466 Blood substitutes, 467468 fluorochemical, 468 hemoglobin based, 467 Boiling points,94-99 1 -Bromoperfluorooctane(PFOB), 473 Bunte salts, synthesis, 54 Capillary sorption, 496 Carbon-fluorine bond, 82 Carboxybetaines, synthesis,59-6 1 Carboxylic acidsand salts, 5 Carboxylic acid anhydrides. 33 Carboxylic acid fluorides, 33 Carpet fibers, 582-584 Cationic fluorinatedsurfactants, 3, 7, 9 synthesis, 56-58 Cement additives, 354 Cesium perfluorooctanoate,mesophases, 334,336 Chemical properties,80-90 Chemical relaxation, 206-210, 28 1, 410413 Chemical stability, 80-82, 86-87 Chromatography, 394-396 Cleaners forhard surfaces, 354 Cloud point, 2 17-220 dehydration theory,217 effect of surfactant structure, 219

Index Coatings, 355-358 Coffee stains, 587-589 Condensed monolayers, 176 Contact angle, 498-501 advancing. 500 apparent, 499 at equilibrium, 498-499 dynamic, 501 hysteresis. 500 in real systems, 499-501 intrinsic, 499 receding, 500 roughness factor, 500 Cosmetics, 358 Crank’s diffusionlaw. 587 Critical demicellization concentration (cdc), 305-306 Critical micelle concentration (cmc). 106, 202,228-255 anionic fluorinated surfactants, 230,23 1 cationic fluorinatedsurfactants, 23 1 counterion, 237 determination, 228 effect of chain length, 228,23 1 , 232 electrolytes and additives, 250-255 HLB ,240-243 hydrophile structure, 236 hydrophobe structure, 228-235 mixed surfactants, 293 nonionic fluorinated surfactants, 233, 236 partially fluorinated surfactants. 243-245 pressure, 247-250 temperature, 246-247 Critical solution pressure (Tanaka pressure), 239 Critical solution temperature (cst), 299 Critical surface tension,181, 185,501-504 Cryogenic transmission electron microscopy. 287. 322 Crystal growth regulations, 358 Cyclic voltammetry. 287 Decomposition of perfluoroalkanoates,82 Defoaming, 365

Index Degree of counterion binding, 222,237, 239 Density, 99-100 Determination of fluorinated surfactants, 390-450 by chromatography, 394-396 by ion-pair spectroscopy, 394 by ultraviolet and infrared spectroscopy, 396-398 in biological systems, 434-435 in the environment, 436-437 Raman spectroscopy,398-399 volumetric methods, 393 Dicarboxylic acid fluorides, 33 Direct fluorination, 29 Disjoining pressure, 166 Dispersions, 358 Drug delivery, 486-487 Du Pont telomerization process, 36 DuprC equation, 496

609

Fluorinated acids, strength, 87-90 Fluorinated alkylsulfates, 7 synthesis, 54 Fluorinated amphoteric surfactants, 3, 8-10, 11.59 heterocyclic, 61 synthesis, 59-63 Fluorinated carboxylic acidsand salts, 5, 45 density, 99-100 melting points, 91-94 Fluorinated cationicsurfactants, 7-8, 9 synthesis. 56-58 Fluorinated counterions, 4, 210 Fluorinated epoxides, 43,48 Fluorinated nonionic surfactants, 10, 12. synthesis, 64-70 Fluorinated phosphates, 8 synthesis, 55-56 Fluorinated phosphatidylcholines. 479 Fluorinated polyethers. 16-1 9 Electric birefringence,320,424-426 Fluorinated repellents, 530-541 Electric conductivity, 422-424 application, 541-543 Electrochemical fluorination, 29, 31-36 monomeric, 535 mechanism, 32 polymeric, 535-541 Electroless metallization, 359 structure. 530 Electrolytic cell, 3 1, 32 synthesis, 532-535 Fluorinated soil release agents. 574-579 Electron spin resonance (ESR). 284-285. 409-4 10 Fluorinated soil retardants, 565-568 Fluorinated sugars and sugarderivatives, Electronegativityof fluorine, 80 479-482 Electronic applications,359 Fluorinated sulfates. synthesis, 54 Electroplating, 360 Fluorinated sulfonic acids and salts, 6 Electropolishing.361 synthesis, 49-52 Elemental analysis, 390-393 Fluorinated sulfosuccinates, synthesis, 53 Elemental fluorine, 29, 80 Fluorinated surfactants Emulsions, 160-165, 361 chemical stability, 145 Environmental effects, 456 in biomedical oxygencarriers, 477-484 removal of fluorinated surfactants from in solution, 196-276 wastewater, 461 physical and chemical properties, Esters of fluorinated acids, 147 80- 102 Etching. 361 synthesis, 29-78 Fire-fighting foamsand powders, 362-364 Fluorinated surfactantswithout a Flotation of minerals, 364 hydrophile, 14 Fluorad surfactants, surface tension. 350 Fluorinated vinyl polymers, 19 Fluorescence. 280, 3 18-320, 329,4 16,418 Fluorination with elemental fluorine, 29

610 Fluorine, 80 electronegativity, 31 Fluorine NMR. 255,301. 332, 340, 406409,476 Fluorine, organic,434,461 determination in blood andbiological samples, 435 Fluorosilicone copolymers,11-14 Fluorosiloxanes, 13 Fluosol-DA, 473 F I u o s o I - ~473 ~, Fluowet surfactants, 437,460 Foam elasticity, 166 Foams, 166-169,364 Fourier transform infrared spectroscopy (FTIR), 398 Fourier transformNMR, 263,406,407 Fusion method, 393 Gas chromatography, 394 Gel chromatography (filtration). 302, 396 Gibbs equation, 105, 1 10 Graphic imaging, 366 Greases and lubricants, 366 Group contribution theory,298 Harkins equation, 145 Hemolytic activity, 453,480.482 Heptafluoroisopropyl iodide, 30 Herbicides and insecticides, 367 Hexafluoroacetone, 44 Hexafluoropropylene, 30 Hexafluoropropylene oxide(HFPO), 17 High performance (pressure) liquid chromatography (HPLC), 395 HLB, 160 Hybrid surfactants. 53, 340-341 synthesis, 53-54 Hydrofluoric acid, 29 Hydrogen fluoride, 29 Hydrophile-lipophile balance (HLB),10, 160,240-243 Hydrophobe, 3 partially fluorinated, 4 Hydrophobicity index, 240

Index Ideal solution theory, 291 Immersion, 496 Infrared spectroscopy, 397-398 Interfacial boundary, 103 Interfacial tension, liquid-liquid, 155 Inverse gas chromatography, 394 Iodine pentafluoride,37 Ion chromatography, 396 Ionomers, 16 Iron(IT1) oxide hydratesols, 190-193, 324 Kinetics of wetting, 506-510 Krafft point, 108, 210-216 critical solution pressure, 215, 249 effect of carbon chain length, 212 effect of counterion, 21 5 of nonionic surfactants, 215 surfactant mixtures, 215,218 Langmuir film balance, 115, 145 surface area-pressure curves,115 Langmuir-Blodgettfilms, 187 Laplace's equation, 166 Leather, 367 Lecithines, 477 Light scattering, 285,415-416 Liquid crystals, 368 Liquid-liquid interface, 155-1 60 Liquid-vapor and liquid-liquid boundaries, 103-1 74 Lithium dodecylsulfate (LiDS), 302, 305 Lithium perfluorononanoate, 300 enthalpy of micellization, 227 Lithium perfluorooctanesulfonate (LiFOS) adsorption on iron(II1)oxidehydrate, 192-194 aggregation number. 280 critical micelle concentration, 255 in surfactant mixtures, 302-306, 3 16-322 thermal stability, 85 Lithium perfluorooctanoate adsorption on graphite. 188 critical micelle concentration,25 1 thermal stability, 83 Lodyne surfactants, 358,368 Luminescence, 278-280.41 6-419

Index Marangoni effect, I67 Mass-action theory of micellization, 203, 212,220,248,298 Mass spectrometry, 399-405 Medical and dental uses. 368 Melting points, 90-94 Mesophases, 330-340 cubic, 333 effect of additives, 336 hexagonal, 331 lamellar, 331 mixed surfactant systems, 337, 339, 340 nematic, 332 Metabolism of fluorinated surfactants, 45 1 Metal finishing, 369 Micelle structure, 203, 277-288 Micelles, 202 aggregation number, 277 characterization,277 monomer residence time, 2 10 size and shape, 203,283,284 Micellization (micelle formationand dissociation), 202-210 charged phase separation, 205, 248 enthalpy, 224, 227 entropy, 223, 224, 227 free energy, 222 heat, 221 kinetics, 205 mass action theory, 203, 212,220, 248 molar volume change, 227.248 pseudophase (phase separation)theory, 205,211,220,248,260,294-295 relaxation times, 206-210,281, 302 stepwise association,203, 206 thermodynamics, 220-228 Microemulsion. 161 Miscibility of fluorinated and hydrocarbon surfactants, 299-330 effect of electrolytes, 304 effect of temperature, 304 Mixed micelles, 288-330 composition, 289-330 critical micelle concentration, 290 effect of temperature. 3 11,324 model, 300

611 [Mixed micelles] pressure dependence,322,323 theory, 288-299 Mixtures containing amphoteric surfactants, 329-330 Mixtures of anionic and nonionic surfactants, 3 13-324 Mixtures of anionic fluorinated surfactants, 3 13 Mixtures of anionic surfactants, 299-3 13 fluorinated sulfonates,301-3 13 perfluoroalkanoates,299-301 Mixtures containing cationicsurfactants, 328-329 Mixtures of nonionic surfactants. 324-326 Mixtures of oppositely charged surfactants, 326-328 Mixtures of surfactants. 120-124, 299-330 monolayer, 121-124 occupied area, 120 Molding and mold release, 369 Monflor surfactants, area covered, 112-1 13,225 as wetting agents, 377, 378 thermodynamic parameters. 224, 225. 226 Monofluoroaceticacid, 45 1 Monolayers, 176, 177 condensed, 176 force-area curve,114, 121 preparation, 176. 177 Neos Ftergent, in surfactant mixtures, 306-310,315 Neutron activation, 39 1 Neutron reflection, 108, 109, 111 Nonionic fluorinated surfactants,3, 10. 12 chemical stability, 10, 144 monodisperse, synthesis, 68-69 polyhydroxy hydrophile, synthesis, 69-70 surface tension in acid and alkali, 144 synthesis. 64-70 thioethylene groups, 67, 117 Nonionic surfactant mixtures,324-326

612 Nuclear magnetic resonance (NMR), 282-284.405-409 relaxation methods, 282. 283.408-409 Oil containment, 370 Oil repellency, 494 Oil repellency tests, 550-55 1 Oil wells. 370 Oleophobic-oleophilicsurfactants, 14-15 synthesis, 70 Oligomerization of hexafluoropropene oxide. 43,44 Oligomerization of tetrafluoroethylene, 40-43 Ostwald ripening mechanism, 483 Oxyethylation, 64 Oxygen difluoride, 31, 34 Oxygen flask, 392 Oxygen Parr bomb, 392 Oxyhydrogen combustion apparatus, 39 1 Ozonization, 49 Paper. 370 Partially fluorinated surfactants, 2, 243-245,25 1,283 critical micelle concentration, 243-245, 283 interfacial tension, I56 surface tension,130-1 33 Partitioning, 297 Pentafluoroethyl iodide,30 Perfluorinated surfactants, 2 Perfluoroalkanesulfonic acidsand salts, 6. 34,49 boiling points, 98-99 chemical stability, 86 Krafft point, 212,214 solubility, 199 synthesis, 49 thermal stability, 84-86 Perfluoroalkanesulfonyl chloride,34,49 Perfluoroalkanesulfonylfluoride, 34,49 Perfluoroalkanoates in mixtures, 299-301 Krafft points, 212, 214 solubility, 199 thermal stability, 82, 83

Index Perfluoroalkanoicacid fluoride, 33 Perfluoroalkanoic acids boiling points, 95-97 density, 99 Krafft points, 212,214 melting points, 93-94 monolayers, 178 solubility, 198 surface tension, 126 synthesis, 45 thermal stability, 83 Perfluoroalkyl iodides. 45 Perfluoroalkyl-2-ethanethiols, 40 Perfluoroalkyl-2-ethano1,38,45 Perfluoroalkylethyl iodide,38-40 hydrolysis, 38,40 Perfluoroalkylethyl iodides, 37.38,39 Perfluoroalkylethane-2-thiols, 40 Perfluorocarboxylicacid fluorides, 33 Perfluorocarboxylicacids, synthesis, 33, 45 Perfluorodecalin, 473 Perfluoroheptanoicacid (FHA), aggregation number, 28 1 Perfluorooctanesulfonates aggregation number. 28 1 cross-sectional area, 1 11 in mixtures. 301-3 13 kinetics of micellization, 207 Perfluorooctanoates, 111 degree of binding, 223 melting points, 92,93 micelle structure, 282 solubility, 217 surface tension, 129 Perfluorooctanoicacid, 45, 184 aqueous solutions, 184 in blood, 461 Krafft point, 2 15 mixtures, 121, 132 Perfluoropolyethersurfactants, 16, 88 boiling points, 98, 99 critical micelle concentration, 233, 234 cross-sectional areas, 117- 119 density, 98 mesophases, 335 microemulsions. 161, 165

Index [Perfluoropolyether surfactants] refractive index, 98 surface tension, 115, 1 16 Perfluorosulfonyl fluoride, 35, 36 Perfluorotributyl amine, 473 Performance characteristics, 349-350 Phosphatobetaines, synthesis,62-63 Phosphorescence, 416 Photoelectron spectroscopy (XPS), 4 19-422 Photography, 37 1 Photooxidation, 48 Physical and chemicalproperties, 80-1 02 Physiology of fluorinated surfactants. 46 1-466 Plastics, resins, and films, 372 Pluronic F68.476 Poiseuille’s equation, 507 Polishes and waxes, 373 Polymeric fluorinatedsurfactants, 15 structure, 15 Polymerizable fluorinatedsurfactants, 20 Polymerization, 373 Poly(oxyethy1ene)ethers withfluorinated end groups, 18 Polysiloxanes. 14,525-530 fluorinated, 529-530 Potassium perfluorooctanoate,234. 235 Potassium perfluoro-3-oxaalkane sulfonate, 51 Propiolactone, 59 Pseudophase separationtheory, 205.21 1. 220,248,260,297 Purity. 390 Radioisotope tracermethods, 108 Raman spectroscopy, 398-399 Refractive index, 100-101 Regular solution theory, 295, 312 Relaxation methods, 4 10-4 13 Relaxation times. 206-2 10, 28 1,302 Repellency, 374,494,5 10-5 13 Repellency tests. 543-55 1 oil repellency, 550-55 1 water repellency, 543-550

613 Repellents, 516-541 aluminum and zirconiumsoaps, 5 17 fiber-reactive, 522 fluorinated, 530-541 fluorinated polysiloxanes,529-530 hydrocarbon and fluorocarbon mixtures, 524 metal complexes, 517 n~ethylol compounds,520-522 silicones, 525-530 waxes, 5 16 Selection of fluorinated surfactants. 350-352 Semiflourinated alkanes,14-15 synthesis, 70 Si-F bond, 11 Silanes, 525 Silicon containing fluorinatedsurfactants, 11-14 Siloxanes, 14 Small-angle neutron scattering, 285-286, 414-415 Small-angle scatteringmethods, 413-41 5 Sodium dodecyl sulfate (SDS), 190, 191, 193,292,294,303,320 Sodium perfluorodecanoate (SPFDe), 250 Sodium perfluorononanoate (SPFN), 2 13 Sodium perfluorooctanesulfonate, surface tension, 134, 135 Sodium perfluorooctanoate (SPFO), 207. 222 aggregation number, 286 enthalpy of micellization. 227 interfacial tension, 155. 158 microemulsions, 161 surface tension, 125 surfactant mixtures, 291,296-297 Soiling and staining, 582 Soiling mechanisms. 558-560 Soil resistance tests. 568-574 Soil retardation. 560-568 theory. 560-565 Soils, 557-558 Solid-liquid boundary, 175-197

614 Solubility, 149-202 effect of hydrophile, 198 effect of hydrophobe, 198 Solubility of surfactant mixtures, 131 electrolyte concentration, 131 temperature, 31 1 Solubilization, 256-269, 320 gases, 264-266 HLB. 258 in adsorbed micelles,267-268 standard free energy,261 surfactant mixtures, 263. 320 Solutions of fluorinated surfactants, 198-276 Spectrophotometric determinationas an ion pair, 294 Spin probes, 4 10 Spreading, 496,497 Spreading coefficient, 145 Stability. chemical, 82. 86, 87. 145 Stability, thermal, 82-86 Stain-resist agents, 592-598 Stain resistance, theory,589-592 Stain resistancetests, 598-602 Stain resistant carpets, 582-605 Stains, 584-587 Starting materials, 29-3 1 Structure of fluorinated surfactants, 1 Stuart molecular models, estimationof cross-sectional areas, 116 Sulfatobetaines, synthesis, 62 Sulfobetaines, synthesis, 62 Sultone. 52 Supercritical fluid chromatography, 395 Surface excess concentration,105, 108, 125 effect of fluorination, 1 10 effect of hydrophilic group,108 effect of hydrophobic group, 108 Surface free energy, 505-506 Surface tension. 1,2, 103-155,427434 drop volume method, 43 1 drop weight method, 432 dynamic methods, 138,428 effect of fluorination, 110, 125 effect of hydrophile, 109,127-1 30

Index [Surface tension] effect of hydrophobe, 109, 125-127 effect of surfactant structure, 124-1 33 in acids and alkali, 139-145 in organic liquids, 145-155 maximum bubble pressuremethod. 432 measurement, 427-434 minimum value, 125-126 of fluorinated surfactants, 11 1 of partially fluorinated surfactants, 130-133 pendant drop method, 43 1 ring method, 429 spinning dropmethod, 433 static methods, 428 surfactant structure, 124-133 synergism, 139 theory, 103-1 08 Wilhelmy plate method, 430 Surface treatment of glass, 375 Surfactant effectiveness,108 Surfactant efficiency,107 Surfactant mixtures, 299-330 surface tension, 303,306-3 10.3 15-3 18, 327 volume change on mixing, 3 12-3 13 Surflon surfactants, 167 Surfmers, 20 Synthesis of fluorinated surfactants, 29-79 Tanaka pressure,249 Taxogen, 36 Telogen, 36 Telomer, 36 Telomer chlorides,47 Telomerization, 30,36-40 catalysts, 37, 38 mechanism, 37 Terminally fluorinated surfactants, aggregation number, 284 Tetrafluoroethylene.30, 37 Tetrafluoroethylene oligomers, 40 Tetramethylammonium perfluorononanoate,mesophases, 334

Index Textiles, 375 Thermal stability, 82-86 Titration of fluorinated surfactants, 393 Toxicity, 45 1-459 aquatic, 457 biological activity, 459 inhalation, 454 local, 456 oral, 455 Toxicology, 45 1 4 5 6 aquatic, 457-459 nonaquatic, 459 Traube’s rule, 126- 127 Trifluoromethyl iodide,37 Ultrafiltration, 297. 304,426-427 Ultraviolet spectroscopy, 396-397 Vapor barrier, evaporation retarders, 365 Vesicles, 335-336 Washburn-Lucas equation, 507 Water, structure, 227 Water repellency, 494 Water repellency tests, 543-550 absorption tests, 549 drop penetration, 549 hydrostatic pressure, 549 impact penetration, 543,546

615 [Water repellency tests] rain test, 544,546-549 spray test, 543,545 Waterproof, 494 Wetting, 178.495-498 forced, 495 kinetics, 506-5 10 spontaneous, 495 Wetting agents, 376 Wicking, 506 X-ray diffraction, 41 3 X-ray photoelectron spectroscopy (XPS, ESCA), I86,4 19-422 X-ray reflectivity, 186 X-ray scattering, 278, 413 Young’s equation, 181,498 Zeta potential, 190-193 Zonyl surfactant mixtures,139 Zonyl surfactants, 124, 139, 140, 153, 186. 187, 188,349’352-354,358,359, 362,364,366.37 1,421,454,455, 462-466 adsorption on coal, 187 foaming action, 167, 168 solubility, 200-202 surface tension in acids and alkali, 140 surface tension in water, 124