JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 16
porous silica its properties and use as support in column liquid chromatography
JOURNAL OF CHROMATOGRAPHY LIBRARY Volume 1 Chromatography of Antibiotics by G.H. Wagman and M.J. Weinstein Volume 2
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Volume 16
Porous Silica. Its Properties and Use as Support in Column Liquid Chromatography by K.K. Unger
Volume 17
75 Years of Chromatography - A Historical Dialogue edited by L.S. Ettre and A. Zlatkis
JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 16
porous silica its properties and use as support in column liquid chromatography K.K. Unger Professor of Chemistry, University of Maim
E LSEV I E R SC I ENTl FIC PUBLISH I NG COMPANY Amsterdam Oxford New York 1979
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ELSEVIER SCIENTIFIC PUBLISHING COMPANY 336 Jan van Galenstraat P.O. Box 211,1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIEWNORTH-HOLLAND INC. 52, Vanderbilt Avenue New York, N.Y. 10017
Library of Congress Cataloging in Publication Data
Unger, Klaus K 1936Porous s i l i c a . (Journal of chromatography librsry ; v. 16) Includes bibliographies end index. 1. Liquid chromatography--Equipent and supplies. 2. S i l i c a . I. T i t l e . 11. Series. ~ ~ 7 9 . C 4 5 4 ~ 5543' 3 .O8 79-12682 ISBN 0-444-41683-8
ISBN 0-444-41683-8 (Vol. 16) ISBN 0-444-41616-1 (Series) Elsevier Scientific Publishing Company, 1979 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, P.O. Box 330, 1000 AH Amsterdam, The Netherlands 0
Printed in The Netherlands
V Contents Preface..
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Chapter 1. General chemistry of silica
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XI 1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Classification of solid silica species . . . . . . . . . . . . . . 1 1.2 Bulk structure of silica . . . . . . . . . . . . . . . . . . . 4 1.2.1 Structure of crystalline silica modifications . . . . . . . . . . . 4 1.3 Surface structure . . . . . . . . . . . . . . . . . . . . . 6 1.3.1 Types of surface hydroxyl groups . . . . . . . . . . . . . . 6 1.3.2 Dehydration and dehydroxylation . . . . . . . . . . . . . . 8 1.3.3 Hydroxylation and hydration . . . . . . . . . . . . . . . 9 1.3.4 Infrared spectroscopy of surface silica species . . . . . . . . . . 9 1.4 Silica-water interactions. . . . . . . . . . . . . . . . . . . 11 1.4.1 Dissolution of silica . . . . . . . . . . . . . . . . . . . 12 1.5 References . . . . . . . . . . . . . . . . . . . . . . . 14 Chapter 2. Pore structure of silica
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2.1 Pore structure parameters . . . . . . . . . . . . . . . . . . 15 2.1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . 15 2.1.2 Fundamentals. . . . . . . . . . . . . . . . . . . . . 19 2.1.3 Experimental techniques . . . . . . . . . . . . . . . . . 23 2.1.4 Calculation procedures . . . . . . . . . . . . . . . . . . 27 2.2 Formation of pore structure . . . . . . . . . . . . . . . . . 40 2.2.1 Pore structure models . . . . . . . . . . . . . . . . . . 40 2.2.2 Origin of porosity in silica . . . . . . . . . . . . . . . . . 42 2.3 Controlled porosity silica packings. . . . . . . . . . . . . . . . 49 2.3.1 Modified sol-gel procedure followed by sintering . . . . . . . . . 50 2.3.2 Polyethoxysiloxane procedure . . . . . . . . . . . . . . . 50 2.3.3 Agglutination of finely dispersed non-porous silica particles. . . . . . 50 2.3.4 Controlled sintering . . . . . . . . . . . . . . . . . . . 51 2.4 Stability of porous silica . . . . . . . . . . . . . . . . . . . 52 2.4.1 Thermal stability . . . . . . . . . . . . . . . . . . . . 52 2.4.2 Chemical stability . . . . . . . . . . . . . . . . . . . 52 2.5 References . . . . . . . . . . . . . . . . . . . . . . . 53 Chapter 3. Surface chemistry of porous silica
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3.1 The surface structure of silica . . . . . . . . . . . . 3.1.1 Surface species . . . . . . . . . . . . . . . . 3.1.1.1 Surface hydroxyl groups and physically adsorbed water 3.1.1.2 Internal and surface hydroxyl groups . . . . . . 3.1.1.3 Types of surface hydroxyl groups. . . . . . . . 3.1.1.3.1 Free and bound hydroxyl groups . . . . . .
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57 58 58 61 62 62
VI 3.1.1.3.2 Paired and isolated hydroxyl groups . . . . . . . . . . 63 3.1.1.4 Determination of surface hydroxyl groups . . . . . . . . . 63 3.1.1.4.1 Chemical methods . . . . . . . . . . . . . . . . 64 3.1.1.4.1.1 Reaction between silica and diborane . . . . . . . . 64 3.1.1.4.1.2 Reaction between silica and dimethyldichlorosilane . . . 65 3.1.1.4.1.3 Reaction between silica and methyllithium . . . . . . 66 3.1.1.4.2 Physical methods . . . . . . . . . . . . . . . . 68 3.1.1.4.2.1 Infrared spectroscopy . . . . . . . . . . . . . 68 3.1.1.4.2.2 Isotopic exchange with DzO . . . . . . . . . . . 70 3.1.1.4.2.3 Isotopic exchange with HTO . . . . . . . . . . . 72 3.1.2 Reactivity ofsurface hydroxyl groups in adsorption . . . . . . . . 76 3.1.2.1 Fundamentals . . . . . . . . . . . . . . . . . . . 76 3.1.2.2 Silica-adsorbate interactions . . . . . . . . . . . . . . 78 3.2 Chemical modification of the silica surface . . . . . . . . . . . . . 83 84 3.2.1 Basic concepts . . . . . . . . . . . . . . . . . . . . 3.2.1.1 Types of bonds and functional groups . . . . . . . . . . . 84 3.2.1.2 Structure and stability of surface bonds . . . . . . . . . . . 85 3.2.1.3 Methods of forming surface bonds . . . . . . . . . . . . 88 3.2.1.3.1 Bulk modification . . . . . . . . . . . . . . . . 88 3.2.1.3.2 Surface modification . . . . . . . . . . . . . . . 91 3.2.1.3.3 Special topics in surface modification . . . . . . . . . . 96 3.2.1.3.3.1 Kinetics in surface reactions . . . . . . . . . . . 96 3.2.1.3.3.2 Reactors in surface modification . . . . . . . . . . 99 3.2.1.3.3.3 Conversion in surface reactions . . . . . . . . . . 99 3.2.1.3.3.4 Effect of surface modification on pore structure properties of silica . . . . . . . . . . . . . . . . . . 104 3.2.2 Synthesis and properties of chemically modified silica supports . . . . 108 3.2.2.1 Bulk modified products (organosilicon xerogels) . . . . . . . . 108 3.2.2.1.1 Xerogels made by condensation of organosilanetriols . . . . . 108 3.2.2.1.2 Xerogels made by co-condensation of sodium silicate and organosilanetriols . . . . . . . . . . . . . . . . 109 3.2.2.1.3 Xerogels made by co-hydrolysis and co-condensation of organotrialkoxysilanes and tetraethoxysilane or polyethoxysiloxane . . 1 10 3.2.2.2 Surface-modified products . . . . . . . . . . . . . . . 112 3.2.2.2.1 Si-X surface bonds (X = halogen, -NHz, -NRz, -R, -H) . . 112 3.2.2.2.2 Si-0-R surface bonds . . . . . . . . . . . . . . 116 3.2.2.2.3 Si-0-BX, surface bonds . . . . . . . . . . . . . 124 3.2.2.2.4 Polymerization . . . . . . . . . . . . . . . . . 128 3.2.2.2.5 Miscellaneous. . . . . . . . . . . . . . . . . . 129 3.3 Ion-exchange properties of silica . . . . . . . . . . . . . . . . 130 3.3.1 Surface sites of silica in aqueous solution and the origin of their acidity . . 130 3.3.2 Capacity and exchange ability as a function of pH . . . . . . . . . 133 3.3.3 Mechanism of cation exchange on silica and the theory of selectivity . . . 134 3.3.4 Isoelectric state and the possibility of anion exchange . . . . . . . 138 3.3.5 Measurement of ion-exchange selectivity . . . . . . . . . . . . 138
VII 3.3.6 Exclusion of electrolytes from the pores of silica 3.3.7 Kinetics of ion exchange on silica . . . . . 3.3.8 Applications . . . . . . . . . . . . . 3.4 References . . . . . . . . . . . . . . . Chapter 4 .Particle characteristics
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4.1 Particle size. shape and distribution: definitions . . . . . . . . . . . 147 4.1 .1 Particle size . . . . . . . . . . . . . . . . . . . . . 147 4.1.2 Particle shape . . . . . . . . . . . . . . . . . . . . . 148 4.1.3 Average particle diameter . . . . . . . . . . . . . . . . . 149 4.1.4 Presentation of size analysis data . . . . . . . . . . . . . . 151 4.2 Methods of particle size grading and size analysis . . . . . . . . . . . 153 153 4.2.1 Sieving . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Microscopy . . . . . . . . . . . . . . . . . . . . . 155 4.2.3 Sedimentation . . . . . . . . . . . . . . . . . . . . . 156 4.2.4 Fluid classification . . . . . . . . . . . . . . . . . . . 159 4.2.5 The Coulter Counter . . . . . . . . . . . . . . . . . . 161 4.3 Formation of silica particles . . . . . . . . . . . . . . . . . . 162 4.3.1 Irregularly shaped silica packings . . . . . . . . . . . . . . 162 4.3.2 Spherical silica packings . . . . . . . . . . . . . . . . . 162 163 4.4 Porous silica layers . . . . . . . . . . . . . . . . . . . . . 4.4.1 Preparation of PLBs with a porous silica layer . . . . . . . . . . 164 4.4.2 Variation of the pore structure and the thickness of the porous layer . . . 165 4.5 Acknowledgements . . . . . . . . . . . . . . . . . . . . 166 4.6 References . . . . . . . . . . . . . . . . . . . . . . . 166 packing procedures and performance characteristics . . 169 Chapter 5 . Silica columns . 169 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 5.2 Particle packing . . . . . . . . . . . . . . . . . . . . . 169 5.2.1 Geometrical analysis of column bed . . . . . . . . . . . . . 169 5.2.2 Factors influencing particle packing . . . . . . . . . . . . . 172 5.3 Packing procedures . . . . . . . . . . . . . . . . . . . . 175 5.3.1 Dry packing techniques . . . . . . . . . . . . . . . . . 175 5.3.2 Slurry packing techniques . . . . . . . . . . . . . . . . . 176 5.3.2.1 Pre-treatment of packing . . . . . . . . . . . . . . . 176 5.3.2.2 Slurry liquid . . . . . . . . . . . . . . . . . . . 177 5.3.2.3 Slurry preparation . . . . . . . . . . . . . . . . . 178 5.3.2.4 Apparatus . . . . . . . . . . . . . . . . . . . . 178 5.3.2.5 Filling procedure . . . . . . . . . . . . . . . . . . 179 5.4 Comparison of performances of silica columns . . . . . . . . . . . 179 5.4.1 Column permeability . . . . . . . . . . . . . . . . . . 180 5.4.2 Plate height-velocity dependences . . . . . . . . . . . . . . 181 5.4.3 Column stability . . . . . . . . . . . . . . . . . . . . 184 185 5.5 References . . . . . . . . . . . . . . . . . . . . . . .
VIII Chapter 6 . Silica and its chemically bonded derivatives as adsorbents in liquid-solid chromatography . . . . . . . . . . . . . . . . . . . . . . .
187
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 187 6.2 Silica as a polar packing in LSC . . . . . . . . . . . . . . . . 187 6.2.1 Retention mechanism . . . . . . . . . . . . . . . . . . 187 6.2.2 Characteristics of silica adsorbents in LSC . . . . . . . . . . . 193 6.2.2.1 Specific surface area . . . . . . . . . . . . . . . . . 194 6.2.2.2 Surface activity . . . . . . . . . . . . . . . . . . 195 6.2.2.3 Pore structure . . . . . . . . . . . . . . . . . . . 197 6.2.3 Support properties controlling retention . . . . . . . . . . . . 198 6.2.3.1 Specific surface area . . . . . . . . . . . . . . . . . 198 6.2.3.2 Degree of surface deactivation . . . . . . . . . . . . . . 198 6.2.3.3 Sample load and linear capacity . . . . . . . . . . . . . 202 6.2.4 Adsorbent selectivity of silica in LSC . . . . . . . . . . . . . 203 6.3 Reversed-phase silica packings in LSC . . . . . . . . . . . . . . 206 206 6.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . 6.3.2 Specificities in solute-solvent-reversed-phase adsorbent interactions . . 207 6.3.3 Characteristics of reversed-phase silica packings . . . . . . . . . . 209 6.3.4 Influence of reversed-phase packing properties on retention of solutes . . 213 6.3.4.1 Relationship between capacity factor and surface coverage . . . . 213 6.3.4.2 Relationship between capacity factor and chain length . . . . . . 216 6.3.4.3 Sample load and linear capacity . . . . . . . . . . . . . 216 6.3.5 Adsorbent selectivity ofreversed-phase packings . . . . . . . . . 216 6.4 Polar chemically bonded silica packings as selective adsorbents in LSC . . . . 217 6.4.1 Structure and properties of polar chemically bonded silica packings . . . 217 6.4.2 Relationship between structure of polar chemically bonded silica packings and retention of solutes . . . . . . . . . . . . . . . . . 219 6.4.3 Selectivity of polar chemically bonded silica packings . . . . . . . . 220 6.5 Adsorbent standardization . . . . . . . . . . . . . . . . . . 222 . . . . . . . . . . . . . . . . 222 6.5.1 Introduction . . . . 6.5.2 Physico-chemical standardization . . . . . . . . . . . . . . 223 . . . . . . . . . . . . 223 6.5.2.1 Morphology and size of particles 6.5.2.2 Specific surface area . . . . . . . . . . . . . . . . . 223 6.5.2.3 Specific pore volume . . . . . . . . . . . . . . . . . 225 6.5.2.4 Pore distribution . . . . . . . . . . . . . . . . . . 226 229 6.5.2.5 Stability . . . . . . . . . . . . . . . . . . . . . . 6.5.3 Chromatographic standardization . . . . . . . . . . . . . . 229 233 6.6 References . . . . . . . . . . . . . . . . . . . . . . . Chapter 7 . Silica as a support in liquid-liquid chromatography 7.1 General aspects . . . . . . . . . . . . . 7.2 Role ofthe support inliquid-liquidchromatography
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7.3 Preparation of columns inliquid-liquidchromatography . . . . . . 7.3.1 Solvent evaporation technique . . . . . . . . . . . . . 7.3.2 In situ coating technique . . . . . . . . . . . . . . . 7.3.3 Precipitation technique . . . . . . . . . . . . . . . 7.4 Effect of silica support properties on retention and column performance . 7.5 References . . . . . . . . . . . . . . . . . . . . .
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Chapter 8 . Chemically modified silica as packing in ion-exchange chromatography 8.1 Selectivity and kinetics of ion exchange . . . . . . . 8.2 Ion exchangers based on chemically bonded silica . . . 8.2.1 Synthesis . . . . . . . . . . . . . . . . 8.2.1.1 Preparation ofsurface-modified ion exchangers . 8.2.1.2 Preparation of bulk-modified ion exchangers . . 8.2.2 Characterization . . . . . . . . . . . . . . 8.2.2.1 Ion-exchange capacity . . . . . . . . . 8.2.2.2 Stability . . . . . . . . . . . . . . . 8.2.2.3 Selectivity . . . . . . . . . . . . . . 8.3 Selectivity and performance of silica-based ion exchangers 8.4 Acknowledgements . . . . . . . . . . . . . . 8.5 References . . . . . . . . . . . . . . . . . Chapter 9 . Silica as packing in size-exclusion chromatography
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9.1 Separation mechanism . . . . . . . . . . . . . . . . . . 9.2 Resolution in size-exclusion chromatography . . . . . . . . . . . 9.3 Optimization of silica support properties with respect to resolution and speed 9.4 Size separation on porous silica . . . . . . . . . . . . . . . 9.4.1 Introduction . . . . . . . . . . . . . . . . . . . . 9.4.2 Separation of synthetic polymers and oligomers . . . . . . . . 9.4.3 Separation of biopolymers . . . . . . . . . . . . . . . 9.4.4 Separation of oligomers by liquid-solid chromatography . . . . . 9.4.5 Characterization of colloidal dispersion . . . . . . . . . . . 9.5 References . . . . . . . . . . . . . . . . . . . . . . Appendix . Commercially available silica packings List of symbols and abbreviations . . . . . Subject index . . . . . . . . . . . . .
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XI
Preface Over the past decade porous silica has become the most important and widely used packing material in column liquid chromatography. In contrast to its widespread application, only a limited number of publications about silica, its properties and use as a support in column liquid chromatography are available. The most prominent contribution in this respect is L.R. Snyder’s book entitled The Principles of Adsorption Chromatography which deals with the role of silica in adsorption chromatography. In modern column liquid chromatography silica is utilized as porous microparticles having a controlled pore structure and a tailor-made surface composition. For a thorough understanding and prediction of retention of solutes, it is essential to deal with the properties of silica packings in relation to their chromatographic behaviour. This book attempts t o present a comprehensive treatment of both aspects. In the first part (Chapters 1 to 4) it provides a fundamental treatise on porous silica, starting with general silica chemistry and followed by a discussion of pore structure, surface chemistry and particle characteristics. Special emphasis is placed on the chemical modification of silica and on the characterization of modified products. In the second part (Chapters 5 to 9) the role of silica in the different types of column liquid chromatography (adsorption, partition, ion-exchange, size exclusion) is treated. A special chapter is reserved for column packing and column performance. The idea of writing this book arose in 1971 during a stay at Northeastern University, Boston, Mass. to give a course on the surface chemistry of silica. The basis of this project was my experience in this field over a long period. In view of the enormous literature COYering a diversity of subjects I tried to systemize and to assess critically the reported results. However, during writing, it became obvious that a series of experimental observations could not yet be explained sufficiently by the existing theories and a number of questions still remain open. Nevertheless, I hope that this book will provide a guideline and a basis for those who are interested in porous silica and its application in column liquid chromatography. My thanks go first to Prof. H.W. Kohlschuetter who gave me a thorough introduction to this subject over many years. I am indebted to several colleagues and friends for their helpful and stimulating discussions, in particular to Prof. I. Haldsz, Prof. J.F.K. Huber, Prof. B.L. Karger, Dr. J.J. Kirkland and Prof. J.H. Knox. I thank St. Doeller for writing the section on “The Ion Exchange Properties of Silica” in Chapter 3. In addition I would like to acknowledge the generous assistance of Dr. K.F. Krebs and Dr. W. Reich of E. Merck, Darmstadt. I am grateful to Dr. N. Becker, St. Doeller and R. Eksteen for proofreading. Finally, I am indebted to my family for their patience during the time required to write this book. Darmstadt. December 1978
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1
Chapter 1
General chemistry of silica 1.1 INTRODUCTION
As silica exists in various forms, our primary objective is to establish some criteria for its classification. Further, the term “porous silica” will be defined. Although silica means substances with the stoichiometric composition SiOz, this term also includes hydrated species with the composition SiOz *xH,O. This implies that water is chemically bound in a non-stoichiometric amount. 1.1.1 Classification of solid silica species Solid silica species can be classified on the basis of four main features: crystal structure, dispersity, surface composition and porosity.
1.1.1.1 Criterion of crystallinity Applying the first criterion, silica can be divided into crystalline and non-crystalline types. A series of natural occurring crystalline silica modifications with well defined structures such as quartz, tridymite, cristobalite, stishovite and coesite is known [ 1-31. Supercooled liquid silica, known as quartz glass, can be considered as an intermediate between the crystalline and amorphous forms [4]. Amorphous silica, found as opal, infusorial earth and diatomaceous earth in nature, has no regular structure and exhibits a higher degree of hydration than the crystalline forms [ 1-31. 1.I . I .2 Oiterion of dispersity
With respect to dispersity, silica is available in various forms such as soluble silica, silica sols, hydrogels, xerogels and aerogels and precipitated silicas. These forms are mostly amorphous and can generally be regarded as dispersed systems, in which solid silica is distributed in a liquid or gaseous dispersion medium. The various dispersed forms are discussed briefly below.
Soluble silica A molecular solution of silica is formed when amorphous or crystalline silica remains in contact with water. The solution contains mainly monosilicic acid in a low concentration. Silica sols Sols are usually made by adjusting the pH of soluble silicates t o 8-9 with acids. In this way polysilicic acids are formed by polycondensation and polymerization and grow into colloidal particles of size 1-100 nm. Special procedures must be followed in order to
2
stabilize the sol. The main characteristic of a silica sol is that it consists of discrete silica particles, which are spherical in shape, non-porous and amorphous.
Silica hydrogels Without stabilization, the dispersed silica particles tend t o aggregate, the term “aggregation” indicating all processes in which colloidal silica particles are linked together. Iler [5] differentiated four typical aggregation processes: gelling, coagulation, flocculation and COacervation. In gelling, the particles are linked together to form a three-dimensional packing of silica particles. This process leads to a gelatinous mass, called silica hydrogel, which fills the whole volume of the sol. The immobilized aqueous solution can be replaced with other solvents. In coagulation, the particles are linked together :o form relatively close-packed clumps, a coagulate being obtained that settles as a relatively dense precipitate. In flocculation, the particles are linked by bridges of the flocculation agent to form aggregates with a relatively open structure. In coacervation, the silica particles are surrounded by an adsorbed layer, which makes the particles less hydrophilic, but does not form bridges between them. The particles aggregate to a concentrated liquid phase which is immiscible with the aqueous phase. By coagulation and flocculation the colloid stage may be destroyed. According to Stauff, however, the hydrogel can still be regarded as a two-component colloidal system h71. Silica xerogels After washing the silica hydrogel, water is removed by heat treatment to yield a xerogel, which consists of hard porous grains. The dehydration process is simultaneously accompanied by shrinkage, which is caused by partial collapse of the globular structure. Further, the silica particles are cemented together by dissolution-deposition processes. Consequently, the xerogel differs widely from the hydrogel in its properties. Silica aerogels High-temperature hydrolysis of silica compounds such as silicon tetrachloride yields a voluminous powder, which consists of spherical amorphous silica particles of size 5-50 nm [8].This silica aerogel is commercially available as Aerosil or Cabosil. Hydrolysis of tetraalkoxysilanes followed by autoclaving is another means of preparing silica aerogels [9]. By grinding crystalline silica such as quartz to a particle size of 1 pm and smaller, silica aerogels can also be produced. Polymeric silica solutions It must be emphasized that silica also exists in the form of macromolecular solutions [lo]. For instance, by partial hydrolysis of alkoxysilanes, polyakoxysiloxanes are obtained. These polymers, built up of branched siloxane chains, are viscous liquids and are fairly soluble in organic liquids. The stoichiornetric composition of these products, however, deviates from SiOz *xHzObecause they contain carbon in the form of alkoxy group. When the remaining alkoxy groups are completely hydrolyzed the siloxane chains are threedimensionally linked to yield a gel or a precipitate [l 11.
3
1.1.1.3 Criterion of surface composition In the past 50 years, a large number of silica derivatives have been prepared that mostly contain organofunctional groups chemically bonded at the surface. The derivatives are made by means of surface reactions with an appropriate modifier. In some instances surface modification increases the weight by up to 30%. An additional criterion based on surface composition is needed for these species. The classification depends on the type of bond by which the functional groups are attached at the surface silicon atoms. Four types can be distinguished: (a) %%-OH and S i - 0 - S i z (b) ESi-O-CE (c) 3 i - C Z (d) S i - N = According to the functional groups bonded at the carbon and the nitrogen atom, the last three types can be further subdivided. 1.1.1.4 Criterion of porosity
Now, let us consider the term “porous silica”. It obviously implies a solid silica with a pore system. The pore system can be characterized by the width of the pores, their shapes and their distribution within the solid particles. The mean pore diameter may vary over a wide range covering several orders of magnitude, e.g., 1-104 nm. The porosity results in a high internal surface area. It should be noted that dispersed silica systems also exhibit a high surface area, which is due to their high degree of dispersity, but they are considered to be non-porous. Also, an assembly of smooth discrete silica particles is not regarded as a porous system, provided that the area of contact between the particles is small. This implies that porosity originates when dispersed silica particles are compacted or cemented together, the pore space being made up of interstices and voids between the particles. The compaction can be effected, for instance, by converting a hydrogel into a xerogel. Another possible route to pore formation is by means of hydrolytic polycondensation of polyethoxysiloxanes. A precipitate is obtained, the particles of which are porous after washing and dehydration. In contrast to xerogels, the pore space in these products is built up of a spongy-type threedimensionally linked network of siloxane chains. The terminology associated with porous silica needs comment. The terms silica, silica gel, silicic acid, etc., are often used with confused meanings. T o prevent this confusion, we shall reserve the term “silica gel” for silica hydrogel, and all porous silica species will be termed “porous silica”, irrespective of their origin. Porous silica species are mainly amorphous. In only a few instances, for example after prolonged high-temperature treatment, a small degree of crystallinity is observed. Porous particles that exhibit a pore system can be considered as a dispersed system. On the other hand, porous particles smaller than 1 p n can be distributed in a dispersion medium to yield an aerogel and colloidal silica. Porous silica also exists in a variety of surface-modified species. Special problems arise in the surface modification of porous silica, which are caused by the pore structure.
4
1.2 BULK STRUCTURE OF SILICA
As porous silica is an amorphous product, only limited information on its structure is available [12]. We are therefore dependent on our knowledge of the non-porous crystalline forms, which have the same bulk composition but a well known crystal structure, and which can be used as reference substances. The same is true of the surface structure of porous silica; theoretical calculations on its surface composition are always based on that of the crystalline forms. Unlike the crystalline forms, porous silica is a highly active system that is thermodynamically and kinetically unstable. In all of its reactions it has a tendency to achieve a more stable, lowenergy state. For example, in thermal or hydrothermal processes, in which porous silica is involved, non-porous crystalline silica is the final product. We shall therefore present a short fundamental review of crystalline silica chemistry, drawing conclusions relating to amorphous silica. 1.2.1 S t r u c t u ~of crystalline silica modifications All forms of silica contain the Si-0 bond, which is the most stable of all Si-X element bonds. The Si-0 bond length is about 0.162 nm, which is considerably smaller than the sum of the covalent radii of silicon and oxygen atoms (0.191 nm) [2]. The short bond length largely accounts for the partial ionic character of the single bond and is responsible for the relatively high stability of the siloxane bond. Each silicon atom is surrounded by four oxygen atoms, forming the tetrahedral unit [SiO,] "-, However, a six-fold octahedral coordination of the silicon atom has also been observed in the two minerals stishovite and coesite [2]. The arrangements of [SiO,] "- and [Si06]'- and the tendency of these units to form a three-dimensional framework structure are fundamental to silica crystal chemistry. Three enantiotropic forms of crystalline silica exist at room temperature and atmospheric pressure: quartz, tridymite and cristobalite (Fig. 1.1). The polymorphism is based on a different linkage of the tetrahedral [Si04]"- units. Quartz possesses the densest structure, tridymite and cristobalite having a much more open structure. All three forms exist in a- and 0-forms, which correspond to low- and high-temperature modifications, respectively. The a- and 0-modifications differ only slightly in the relative positions of the tetrahedral arrangements. This is evident from the fact that the conversion a $ 0 occurs at relatively low temperatures. Quartz is the most stable modification at room temperature, all other forms being metastable at this temperature. A peculiarity of quartz is its chirality, i.e., the possibility of separating two optically active forms. Many experimental data are available on the Si-0 bond length and the Si-0-Si bond angle [1,2]. Bond angles of 142"(aquartz), 150" (a-cristobalite) and 143" (quartz glass) have been found by means of diffraction measurements. Stretched Si-0-Si bonds have not yet been verified. Infrared transmission measurements have been made on powdered silica in the low frequency (700-1400 cm-') and high frequency (2800-4000 cm-I) regions [3]. Between 700 and 1400 cm-', three strong absorption bands at 800,1100 and 1250 cm-' were established, which are attributed to fundamental Si-0 vibrations. These characteristic frequencies do not differ greatly in the various silica modifications, whereas in the high-frequency region certain distinct differences are observed. Crystalline silica
5 liquified silica
I[ I[
1983 K
173-518K n-cristobalite a-cristobalite 17L3 K
O-tridyrnite
11
11L3 K
n-quartz
393433 K e a-tridymite
a16 K
e a-quartz
1
supercooled liquid vitreous silica, quartz glass
Fig. 1.1. Polymorphic forms of silica.
often contains impurities, particularly alkali and alkaline earth metal ions. In the presence of these ions, silicate structures are formed that may influence the chemical properties of these products. Quartz glass, which is a supercooled liquid, needs some comment. It is basically an amorphous product like porous silica, but the presence of some structural elements was established by electron and neutron diffraction measurements [3,4]. In contrast to quartz glass, porous glass is made from sodium borosilicate glass by thermal treatment and a subsequent leaching process [ 131. It is evident that there is an appreciable difference between the compositions and structures of non-porous quartz glass and porous glass. As a result of the structural differences, the silica forms vary in their densities, which are listed in Table 1.l. It is worth noting that cristobalite and tridymite have nearly the same TABLE 1.1 DENSITIES ( p ) OF CRYSTALLINE AND AMORPHOUS SILICAS [ 1-3 J Silica
P
Coesit a-Quartz p-QJartz pCristobalite p-Tridymite Quartz glass
3.01 2.65 2.5 3 2.21 2.26 2.20
Amorphous silica
(n/cm3) at 273 K
- 2.20
6
density as porous silica, which makes it likely that porous silica possesses a similar structure with a tetrahedral coordination of silicon atoms. However, its bulk structure is determined by a random packing of [SO4] 4- units, which results in a non-periodic structure.
1.3 SURFACE STRUCTURE A basic knowledge of the surface structure is of great help in understanding the adsorption behaviour and the chemical reactivity of silica in a variety of processes, particularly in chromatography. In the past, the surfaces of crystalline and amorphous silica have been extensively studied by means of various experimental techniques. In this section, our primary interest is confined to some general information about the silica surface species. We shall also consider some differences between the surface structures of amorphous and crystalline silica. The special features of the surface structure of porous silica are treated in detail in Chapter 3.
1.3.1 Types of surface hydroxyl groups The silicon atoms exposed at the surface will tend to maintain their tetrahedral coordination with oxygen. They complete their coordination at room temperature by attachment to monovalent hydroxyl groups, forming silanol groups. Theoretically, it is possible to use a pattern in which one surface silicon atom bears two or three hydroxyl groups, yielding silanediol and silanetriol groups, respectively (Fig. 1.2). Such types are indeed found in
\ SI -OH /
silanediol groups
’
\si /OH
(gerninal groups I
\OH
-si-
/OH
hydroxyl or silanol groups
OH
silanetriol groups
‘OH Fig. 1.2. Different types of silanol groups.
7
monomeric organosilicon compounds, such as diethylsilanediol, dimethylsilanediol and phenylsilanetriol [2]. Silanediol groups at the silica surface, termed geminal groups, were proposed by Pen and Hensley [ 141. Their existence, however, could not be established experimentally. Further, it seems improbable that silanetriol groups exist at a silica surface. Many attempts have been made to calculate the concentration of surface hydroxyl groups, WJH, starting with certain faces of crystalline silica and assuming that each surface silicon atom bears one hydroxyl group. Iler [15] found a value of %H close t o 8 hydroxyl groups per square nanometre based on the face of pcristobalite. De Boer and Vleeskens [16] found a value of aOH = 4.6 hydroxyl groups per square nanometre, choosing the octahedral face of 0-cristobalite and the basal and prism faces of 0-tridymite as standard surfaces. This value could be confirmed experimentally on fully hydroxylated amorphous silica species, after previous annealing at 673 K [ 171. In further discussions here we prefer to use the value of De Boer and Vleeskens. The reciprocal of aOH gives the mean molecular cross-sectional area,A,, of a hydroxyl group, w h c h is 0.217 nm2. From this result, the mean distance between two adjacent hydroxyl groups can be calculated to be about 0.5 nm. This implies, however, that these hydroxyl groups cannot interact via hydrogen bonding, as in these instances an 0-H.. .0 distance of less than 0.3 nm is observed [2,18]. With respect to the surface of crystalline silica, we can assume that all of the hydroxyl groups exist as free or isolated hydroxyl groups (Fig. 1.3). In contrast, the surface struc-
/H 0
0/ H
isolated or free hydroxyl groups
/T\
/H 0
0
t vicnal hydroxyl groups
ti
\
/H 0
hydroxyl groups bond to
Fig. 1.3. Arrangement of hydroxyl groups on a silica surface.
8
ture of amorphous silica is highly disordered and we cannot expect such a regular arrangement of hydroxyl groups. It can be assumed that some of them are adjacent to each other, and are possibly capable of interacting by hydrogen bonding. Such hydroxyl groups are termed vicinal [14]. Hence the surface of amorphous silica may be covered by isolated and vicinal hydroxyl groups. Irrespective of whether a surface contains both types or only isolated hydroxyl groups, complete surface coverage can be achieved; the surface obtained is termed fully or maximally hydroxylated. On exposing it to water vapour, it is further able to adsorb water physically by means of hydrogen bonding. In fully hydroxylated nonporous silica species, a multilayer of adsorbed water is built up by increasing the partial pressure. In fully hydroxylated porous silica species, additional capillary condensation takes place on the adsorbed multilayer. On increasing the partial pressure the pore volume is gradually filled with liquid water. The uptake of physically adsorbed water by means of adsorption and capillary condensation is termed hydration. The degree of hydration is directly proportional to the amount of adsorbed water at a given partial pressure. As is shown later, the hydration of the silica surface has an appreciable effect on its adsorption properties.
1.3.2 Dehydration and dehydroxylation We are now dealing with the simplest surface modification of silica, which can be obtained by increasing the temperature under vacuum. Physically adsorbed water should be removed at 393 K. In highly dispersed and porous silica, water is still held at higher temperatures. Thus, in order to remove physically adsorbed water, the drying temperature is vaned between 423 and 573 K in vacuo, depending on the type of silica [19,20]. At 473 K, dehydration of amorphous silica will be accompanied by dehydroxylation of vicinal hydroxyl groups according to the equation
The condensation proceeds with increasing temperature. At about 773 K, the vicinal hydroxyl groups are completely condensed. It should be noted that up to this temperature, the hydroxylation-dehydroxylation process is reversible. It is obvious that when an annealed silica is brought into contact with water vapour at room temperature, the siloxane groups are completely converted into hydroxyl groups. Above 873 K, the condensation of isolated hydroxyl groups is favoured by a lateral mobility of surface silicon atoms caused by the high temperature. The concentration of isolated hydroxyl groups then decreases gradually with increasing temperature. At 1473 K, the surface is nearly dehydroxylated and contains only siloxane groups. The resulting silica species has a pronounced hydrophobic character, evidence for which can be found by determining the differential heat of adsorption of polar basic vapours on fully hydroxylated and fully dehydroxylated silicas [2 11. On high-temperature treatment, some other processes may occur in addition to con-
9
densation. Using quartz, treatment up to 1473 K results in a phase transition to tridymite and cristobalite, as shown in Fig. 1.l.If the starting material is amorphous, crystallization takes place, depending on the temperature. Annealing of porous silica above 873 K may be accompanied by sintering, which generally causes destruction of the pore structure, resulting in an appreciable decrease in the specific surface area. 1.3.3 Hydroxylation and hydration As already indicated, the first step in the hydroxylation process of a dehydroxylated silica surface consists in adsorption of water molecules. In the second step, siloxane bonds are cleaved to form hydroxyl groups. Reversible hydroxylation behaviour is observed only after annealing of amorphous silica up to about 673 K; pre-treatment at higher temperatures leads to incomplete hydroxylation. Further, the rate of the reaction is very slow. A fully hydroxylated surface of such a sample, however, can be obtained by a treatment with water or water vapour at higher temperatures. This process, which is discussed in Section 2.2.2.5, is known as hydrothermal treatment.
1.3.4 lnfrafed spectroscopy of surface silica species A series of spectroscopic methods have been applied successfully in studies of surface structure and adsorption behaviour. Methods such as Raman, infrared, ultraviolet, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and Mossbauer spectroscopy are based on the adsorption of electromagnetic radiation b y matter. Some of them, which indicate the presence of certain properties and elements, have limitations as far as general application is concerned. Infrared spectroscopy, however, is a universal method and has been developed most comprehensively with respect to surface silica studies [21-231. Let us therefore discuss some results obtained in the high-frequency region between 2000 and 4000 cm-'. A temperature treatment under vacuum should be carried out first in order to investigate the original silica surface. This should be done preferably directly in the spectrophotometer cell and not by means of a separate procedure. Specially designed cells have been developed that permit work at elevated temperatures under high-vacuum conditions [24,25]. Such cells can also be employed for an in situ study of surface reactions. To reduce scattering of the infrared light, the silica should be employed as a fine powder with a particle size of about 1 pm. Further, a sufficiently high concentration of surface species should be placed in the path of the light beam. This can be achieved by using powdered samples and by increasing the thickness of the samples. For instance, quartz has to be ground t o small particles in order to achieve a measurable surface area. The sample thickness should not be too great, because it necessarily leads to an increase in the absorption bands of the bulk species. Generally, two techniques can be applied in the infrared spectroscopy of silica: the transmission technique, using pellets, powdered samples, mulls or crystals [21-231, and the internal reflection technique, using powdered samples [26]. A large amount of data has been published on infrared investigations on silica. For comparison purposes, only the results on Aerosil [27], a highly dispersed silica, and on a-quartz [25] are reported here (Fig. 1.4). Aerosil was used in the form of thin plates and
10
a-auartz
Aerosil
3800 cm-
> 3747*20 free or isolated hydroxyl groups
--
3700
3690 t 1 free hydroxyl groups
> 3660 t 90 hydroxyl groups involve( in hydrogen bonding, internal hydroxyl group'
=-
3600
> 3500
3LOO
>
3650*1 adsorbed water, isolated 3620 t 1 free hydroxyl groups
3520 * 2 0 0 hydroxyl groups hydrogt bonded t o adsorbed water
3 4 0 0 200 adsorbed liquid water, hydrogen bonded f
>
3400t 200 adsorbed liquid water, hydrogen bonded
Fig. 1.4. Band assignment of surface silica species in the high-frequency region (2000-4000 cm-'1.
a-quartz in the form of a powder. The appearance of absorption bands under vacuum is discussed below for pre-treatment temperatures of 298,773 and 1173 K. On Aerosil and Cabosil at 298 K , a sharp band at 3747 cm-' is observed, which merges into a broad asymmetric band ranging down to 3000 cm-' [27]. The band at 3747 cm-' is attributed to the fundamental stretching vibration of free or isolated hydroxyl groups. In comparison, the carbinol groups give an absorption band at 3614 cm-' [21]. The increase in frequency of the valence vibration of silanol groups compared with carbinol groups may be due to the higher acidity of silanol groups. Three bands can be distinguished on the low-frequency side. A broad band at 3400 cm-', which disappears completely after prolonged evacuation at room temperature, is assigned to physisorbed water. For the quantitative determination of physisorbed water Wining [28] and later Erkelens and Linsen [29] proposed the use of the combination band of water at 5265 cm-' (Section 3.1.1). Two broad bands at 3520 and 3660 cm-' still remain after evacuation at 298 K. The latter band may be due to perturbed hydroxyl groups which have previously been termed vicinal. These groups are located at a distance of less than
11
0.3 nm between them and are mostly capable of interaction by hydrogen bonding. However, the vibrations of hydroxyl groups inside the Aerosil particles lie in the same region, and these groups are termed internal hydroxyl groups [18]. Vicinal and internal hydroxyl groups can be distinguished by isotopic exchange with D20combined with mass spectrometric analysis [ 191. In contrast to internal hydroxyl groups, the vicinal hydroxyl groups at the surface very rapidly exchange deuterium from -OH to -OD. In this way, the amount of internal hydroxyl groups can be determined. The appearance of the band observed at 3520 cm-' has not yet been explained. Many workers have assumed that it is due to vibrations of hydroxyl groups hydrogen bonded to physisorbed water, while others correlated it with geminal hydroxyl groups. After degassing at 773 K, the two peaks at 3520 and 3660 cm-' disappear, which is in agreement with the previous statements. The vicinal hydroxyl groups condense to siloxane groups, and the narrow band at 3747 cm-' still exists. Annealing up to 1173 K does not affect the band at 3747 cm-' ,which indicates the high thermal stability of free hydroxyl groups. The infrared studies on aquartz were made with ground particles of size about 1 pm by means of the internal reflection technique [25]. At 323 K without degassing, three sharp symmetrical bands at 3690,3650 and 3620 cm-' and a broad band at 3410 cm-' arc observed. Absorption also occurs at 1635 cni-'. Evacuation of the sample at the same temperature eliminates the bands at 1635,3410 and 3650 cm-' and only two sharp bands remain at 3689 and 3620 cm-'. The bands at 1635,3410 and 3650 cm-' are attributed to physisorbed water. This could be established by leaving the sample in contact with humid air, as the three bands then appear again. After prolonged degassing at 523 K, the two sharp absorptions at 3690 and 3620 cm-' also disappear. The bands can be correlated with two types of free hydroxyl groups on the surface of quartz. It can be concluded that there are significant differences in the relative positions of the absorption bands in the OH stretching region in crystalline and amorphous silica. Free hydroxyl groups on amorphous silica exhibit a higher frequency in absorption than those on the quartz surface. The surface of quartz possesses two types of free hydroxyl groups, which is more like the surface of 7-alumina than that of amorphous silica.
1.4 SILICA-WATER INTERACTIONS
The surface chemistry of silica cannot be properly understood until the interactions between silica and water have been discussed in detail. These interactions will be discussed under three headings. The first and most important one deals with the dissolution of silica, the solute species and the parameters that influence the solubility. The second relates to ionexchange phenomena at the silica surface, which take place when silica is brought into contact with electrolytes. When porous silica is suspended in water, the pore structure may also be affected. The objective of this section is t o summarize the results with respect to the solubility mainly of amorphous silica. Ion-exchange phenomena are treated in Section 3.3. The stability of the pore structure towards aqueous solutions is discussed in Section 2.4.
12
The processes that occur between water and silica to yield soluble silica are very complex and difficult to interpret. On the one hand, the solubility is a function of a series of parameters such as pressure, temperature, structure of silica, particle size and pH of the aqueous solution. On the other hand, depending on the pH, different solute species exist, which are involved in consecutive hydration-dehydration reactions. The solubility behaviour of amorphous silica as monosilicic acid, its polycondensation to polysilicic acids and the formation of colloidal silica was discussed in detail by Iler [S]. Stober [30] considered the thermodynamic and kinetic aspects of hydrolysis and condensation reactions of silicic acids [30]. The silica-water system was reviewed from the standpoint of hydrothermal synthesis of silica minerals by Eitel [3]. Kolthoff and Elving [311 have intensively treated the problems of the analytical determination of silica. Let us first consider the basic reactions that determine the specific solubility behaviour of silica. As was pointed out in the previous section, the first reaction step between anhydrous silica and water or water vapour is the hydroxylation of the surface layer to form hydroxyl groups. Hydroxylation involves cleavage of siloxane bonds by water and can be generally termed hydrolysis. In the presence of water, the hydrolysis of siloxane bonds proceeds to give a water-soluble species, monosilicic acid: Si02 t 2 H20
@
Although monosilicic acid has never been isolated, diffusion measurements by Jander and Jahr [32] indicated the existence of a molecular species equivalent to Si(OH)+ The first dissociation constant, K1,of monosilicic acid has been calculated t o be cu. lo-'' [30]. In contrast to oligomeric and polymeric acids, Si(OH)4 is able to react rapidly with acidified ammonium molybdate solution to form yellow molybdosilicic acid. The determination of monosilicic acid as SiOz is carried out photometrically, after the light yellow solution has been converted into molybdenum blue by reduction. When we talk about soluble silica, we always mean monosilicic acid determined by the molybdate method. The photometric determination indicates that the amount of soluble silica is very low. The equilibrium solubility of amorphous silica in pure water at room temperature was measured t o be about 100 ppm [S,301. For quartz, however, the values obtained are sometimes considerably lower [ 151. There is no reason t o assume that the solubility of silica should depend on its structure, but it is evident that the rate of hydrolysis differs. Hydrolysis is very fast, for instance, with disilicic acid (HO)$i-O-Si(OH),
t H20
2 Si(OH)4
(1 -3)
because only one single siloxane bond per molecule has to be broken. At the surface of solid silica only silanol groups are present and thus several siloxane bonds have to be cleaved simultaneously in order to form monosilicic acid. Further, the activation energy of this rea2tion should be considerably higher for crystalline than for amorphous silica. As a result, the rate of dissolution of quartz is considerably slower than that of amorphous silica. The low concentration of soluble silica reported for quartz may be due to the fact that equilibrium has not been achieved [ 19,331. Further evidence for a high activation energy with respect to quartz was obtained by comparing the solubilities
13
of quartz and amorphous silica as a function of temperature [ 151. The solubility of amorphous silica increases linearly with temperature whereas that of quartz is constant at temperatures below 423 K and increases with temperature above 423 K. By means of a simple theoretical treatment, Stober [30] showed that the rate of hydrolysis is proportional t o the concentration of hydroxyl groups. Hydrolysis accelerates with increasing pH, and the rate of hydrolysis is also influenced by electrolytes and by impurities within the solid silica. Special conditions are observed in the presence of fluoride ions, because stable water-soluble complexes are formed. Despite the different rates of hydrolysis, the amount of soluble silica remains nearly constant in the pH range 1-9 "5,301. When the pH exceeds 9, a considerable increase in the solubility is observed, which is due to the formation of silicate ions in addition to monosilicic acid: SiOz t 2 HzO Si(OHk t OH-
t OH-
Si(OH)4 [Si(OH)sI-
(1 -5)
Above pH 10.7, silica dissolves mainly in the form of soluble silicates, and the concentration of monosilicic acid simultaneously decreases sharply. The silicate species are also measured as soluble silica. By acidifying the solution in the molybdate method they are converted into monosilicic acid. Up to now we have always assumed the presence of saturated or undersaturated solutions, but this has been verified in only a few instances. At higher concentrations, particularly in supersaturated solutions, condensation reactions take place, yielding polysilicic acids and water. Two condensation mechanisms have been proposed b y Iler [5], depending on the pH. At low pH (between 2 and 3), molecular linear and branchedchain silicic acids are formed by intermolecular condensations, such as 2 Si(OH)4
(HO),Si-0-Si(OH),
+ HzO
(1 -6)
They are temporarily stable in this pH range and react very slowly with the molybdate. In neutral and basic solutions, both intramolecular and intermolecular condensations take place, yielding polysilicic acids as colloidal particles. The interior of these particles consists mainly of linked siloxane groups, whereas their outer surface is covered with hydroxyl groups. Growth of the particles is favoured in the pH range 7-9. In a more theoretical treatment, Stober [30] showed that the rate of condensation also depends on the pH but not as a linear function. A maximum is predicted at pH 9, which is in fairly close agreement with the experimental findings above. Further, it should be noted that there is a strong analogy between the condensation reactions of monosilicic acid and the reactions that yield silica sols, discussed in Section 2.2. It has been established by Alexander et ul. [34] that in the molybdate method a certain amount of polysilicic acid is also measured as soluble silica, because hydrolysis of polysilicic acids takes place to yield monosilicic acid. Hydrolysis is particularly favoured at high pH. When a supersaturated solution of monosilicic acid is cooled or water is evaporated, silica is deposited at the surface. This deposition plays an important role in forming diatomaceous earth and silica minerals such as opal. Deposition of silica is also observed in the hydrothermal treatment producing macroporous silica (Section 2.2.2.5). As is generally known, solubility is a function of the particle size of the solid. Alexander [35] studied the effect of particle size on the solubility Q of amorphous silica using silica sols. He found that log S decreases proportionally with increasing particle size.
14
However, even at a constant particle size, the solubilities still vary, owing to differences in the preparation procedures. Another factor that may influence the solubility of silica is the content of inorganic impurities, such as metal ions, within the silica matrix. Systematic studies were carried out with a series of metal ions and it was found that aluminium adsorbed at the surface of colloidal silica particles drastically reduces its solubility [5]. From a practical point of view, one can draw the following conclusions. The solubility of amorphous silica in the pH range 1-8 at room temperature is about 100 ppm, while above pH 9 it increases exponentially and the bulk silica dissolves rapidly. The solubility increases linearly with temperature and increases exponentially with decreasing particle size. The solubility may be strongly affected by bulk impurities. 1.5 REFERENCES 1 E.G. Rochow, in J.C. Bailor, H.J. Erneleus and R. Nyholm (Editors), Comprehensive Inorganic Chemistry, Vol. 9, Pergamon Press, Oxford, 1973. 2 A.F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 3rd ed., 1962. 3 W. Eite1,Silicate Science, Vols. I-V, Academic Press, London, 1965. 4 H. Scholze, Glns, Vieweg Verlag, Braunschweig, 1965. 5 R.K. Iler, in E. Matijevic (Editor), Surface and Colloid Science, Vol. 6, Wiley-Interscience, London, 1973. 6 J. Stauff, Kolloidchemie, Springer Verlag, Gottingen, 1960. 7 J. Steinkopff, Konzepte der Kolloidchemie, D.D. Steinkopff Verlag, Darmstadt, 1975. 8 E. Wagner and H. Brunner, Angew. Chem., 72 (1960) 744. 9 G.E.E. Gardes, G. Nicolaon and SJ.Teichner, J. Colloid Interface Sci., in press. 10 W. Noll, Chemistry and Technology of Silicones, Academic Press, New York, 1968. 11 K. Unger, J. Schick-Kalb and B. Straube, J. Polym. Colloid Sci., 253 (1974) 658. 12 J.J. Fripiat, A. Leonhard and N. Barake, Bull. SOC. Chim Fr., (1963) 122. 13 H.P. Hood and M.E. Nordberg, USPat.,No. 2,215,039 (1940); No. 2,221,709 (1940); No. 2, 286,275 (1942). 14 J.B. Peri and A.L. Hensley, Jr.,J. Phys. Chem., 72 (1968) 2926. 15 R.K. Iler, The Colloid Chemistry of Silica and Silicates, Cornell University Press, New York, 1955. 16 J.H. de Boer and J.M. Vleeskens, Proc. K . Ned. Akad. Wet. Ser. B , 61 (1958) 2. 17 L.T. Zhuravlev and A.V. Kiselev, in D.H. Everett and R.H. Ottewill (Editors), hoceedings o f t h e International Symposium on Surface Area Determination, Butterworths, London, 1970, p. 155. 18 A.V. Kiselev and VJ. Lygin, Kolloidn. Zh., 21 (1959) 581. 19 V.Ya. Davydov, L.T. Zhuravlev and A.V. Kiselev,Russ. J. Phys. Chem., 38 (1964) 1108. 20 J.B. Peri,J. Phys. Chem., 69 (1965) 220. 21 A.V. Kiselev and V J . Lygin, Infrared Spectra of Surface Compounds, Wiley-Interscience, New York, 1975. 22 M.L. Hair, Infrared Spectroscopy in Surface Chemistry, Marcel Dekker, New York, 1967. 23 L.H. Little, Infrared Spectra of Adsorbed Species, Academic Press, London, 1966. 24 E. Gallei and E. Schadow, Rev. Sci. Instrum., 45 (1974) 1504. 25 E. Gallei,Ber. Bunsenges. Phys. Chem.. 77 (1973) 81. 26 N.J. Harrick, Internal Reflection Spectroscopy, Interscience, New York, 1967. 27 R.S. McDonald,J. Phys. Chem., 62 (1958) 1168. 28 G . Wining, Natunvissenschaften, 50 (1963) 466. 29 J. Erkelens and G.B. Linsen, J. Colloid Interface Sci., 29 (1969) 464. 30 W. Stober, Kolloid-Z., 147 (1956) 131. 31 I.M. Kolthoff and P.J. Elving (Editors), Treatise o n Analytical Chemistry, Part 11, Vol. 2, WileyInterscience, New York, 1971. 32 G. Jander and K.F. Jahr, Kolloid-Beih., 41 (1934) 48. 33 K.G. Schmidt and H.Luechtrath, Beitr. Silikose-Forschung, 37 (1955) 3. 34 G.B. Alexander, W.M. Heston and R.K. Iler,J. Phys. Chem., 58 (1954) 453. 35 GB. Alexander,J. Phys. Chem., 61 (1957) 1563.
Chapter 2
Pore structure of silica In chromatography, the rate of mass transfer of solute molecules into and out of the stationary zone is controlled mainly by their diffusion within the porous particles that constitute the column bed. In this way, the pore structure of the packing is important with respect t o column efficiency. Complete pore structure analysis is made possible by applying methods that are based on the physisorption of gases or vapours and on controlled penetration of fluids [ 1-71. The first section of this chapter is concerned with a study of these methods, their application, limitation and versatility. In Section 2.2, a thorough investigation is made of the formation of the pore system of silica and the factors that influence the corresponding pore structure parameters. As will be seen later, a knowledge of these quantities is especially important in high-performance liquid chromatography (HPLC), the different modes of which require packings with a tailormade pore structure. For this reason, procedures have been developed in the past for preparing porous silica supports with a controlled and reproducible porosity (Section 2.3). Another point that should be emphasized relates to the stability of the pore system. Workers in chromatography usually consider the framework of silica as an invariable geometric system. However, it can be shown by simple experiments that some of its properties are very sensitive to chemical treatment (Section 2.4).
2.1 PORE STRUCTURE PARAMETERS 2.1.1 Definitions Every porous system can be fully characterized by a limited set of parameters. The most important one is the mean pore diameter, D.Two other quantities are of fundamental interest, namely the specific surface area, S , expressed in square metres per gram of adsorbent, and the specific pore volume, V,, expressed in millilitres of liquid per gram of adsorbent. 2.1.1.1 Mean pore diameter, D
D may cover a range of several orders of magnitude, including pores in the molecular size range as well as macroscopic fissures and cracks. In 1971, a pore size classification was adopted by the IUPAC [ t i ] ,which was based mainly on the work of Dubinin [9]. Following his recommendation, pores with a width less than 2 nm are caIled rnicropores, those exceeding 50 nm macropores and those in the intermediate size range ( 2 < D < 50 nm) mesopores. The micropore range can be further subdivided into micropores and submicropores, the latter having a width less than 1 nm, but this borderline has yet to be defined. D is a mean value, which implies that one has to deal with the pore size distribution (PSD). When the PSD is homogeneous, this resembles a gaussian distribution (Fig. 2.la),
16
its standard deviation being a direct measure of the width of the dispersion. In most instances, heterogeneous distributions are obtained, the simplest one being called bimodal, exhibiting two distinct pore maxima (Fig. 2.lb). In practice, however, a still more complex variation of the pore sizes exists, as shown schematically in Fig. 2 . 1 ~When . a heterogeneous PSD is observed, it is difficult to interpret the results concerning the real distribution of pores within the particles. For instance, the larger pores may be accumulated at a thin surface layer, whereas the smaller ones may be located within the bulk of the particle, or vice versa. relative frequency N
I
-
mean pore diameter D
a
b
C
Fig. 2.1. Types of pore size distributions (differential curves): (a) homogeneous; (b) bimodal; (c) heterogeneous.
As every pore represents a distinct geometric space, its shape also has to be taken into consideration. Because of the irregularities in most porous solids, the real shape is known in only a few instances, and models have to be employed for an approximation (Fig. 2.2). The most common one is that of cylindrical pores open at one or both ends. Another relates to the so-called ink-bottle pores, which are described by two diameters: the width of the and the width of the wide body (Db).A third is the model of pores narrow neck (Dn) built up of parallel plates. Different pore geometries in relation to the shapes of sorption isotherms were discussed by De Boer [lo]. Pore analysis can also be performed without any model. In this approach, first proposed by Brunauer et al. [ 1 11, D is expressed as the ratio of volume to surface area and is termed the hydraulic pore diameter, Dh :
Dh
= 2vp/s
(2-1)
So far, our treatment has been restricted to a brief discussion of one parameter characterizing the pore itself. However, in practice, one has to consider the porous solid as a whole, i.e., the three-dimensional assembly of pores exhibiting a distinct size distribution [2,12]. The simplest case would be uniformly shaped pores with a regular array in the three dimensions of space, as is valid for synthetic zeolites. The crystal structure of the solid should be clear as a preliminary condition. Porous silica, however, is amorphous and the situation concerning the pore shape is well demonstrated by Fig. 2.3, showing an electron transmission photograph of a porous silica species [ 131. Especially in the description of porous silica, the lack of any symmetry (although some regions may have a relatively ordered structure) again requires the use of models, which will be discussed in Section 2.2.1.
17
2.1.1.2 Specific surface area, S The specific surface area of a porous solid is equal to the sum of its internal and external surface areas. The external surface area, Se, corresponds to the geometric surface of porous particles per gram. S, is an inverse function of the particle size. For spherical particles of equal size, Se = 61dp P
(2.2)
where d p is the diameter of the spherical particles and p the density of the porous solid. The internal surface area, Sj, originates from the pore walls. As by definition pores have to be open to the exterior of the particle, Si does not include the walls of closed pores. With respect to porous silica, Si is several orders of magnitude larger than Se. Let us assume we have a silica sample of particle size 10 pm,having a specific surface area of 300 rn2/g. According t o eqn. 2.2, S, = 0.3 m2/g. Thus, S represents predominantly an internal surface. a
a
l
I
b
b I
Fig. 2.2. Pore models: (a) cylindrical pores, clrcular in cross-section; (b) ink-bottle pores having a narrow neck and a wide body, D, = diameter of the narrow neck, Db = diameter of the wide body; ( c ) slitshaped pores with parakl plates.
18
Fig. 2.3. Electron transmission photograph of a highly porous silica sample [13].Dark areas = skeleton substance; light areas = pore space. Scale: 1 mm = 12.5 nm.
One should further bear in mind that there is generally an inverse relationship between the specific surface area and the average pore diameter of a porous solid: the larger is S, the smaller is D. A high specific surface area (S >500 m’/g) indicates the presence of very small pores, whereas a small value ( S < 10 m’/g) is characteristic of macroporous samples. This shows that, in contrast to macropores, micropores contribute a large amount to S. Normally, the specific surface area is obtained for all pores present within the porous particles. By applying special methods, the specific surface area of micropores and mesopores can be estimated separately. 2.1. I . 3 Specific pore volume, Vp
The specific pore volume, V p ,is the amount of liquid adsorbate that fills the total volume of pores per gram of adsorbent. To a first approximation, V p should be independent of the type of liquid, provided that the liquid wets the surface [2]. By analogy with the specific surface area, the total specific pore volume can be attributed to the volumes of micropores, mesopores and macropores. Once Vp is known, the particle porosity, f p , can be calculated by means of the equation
19
V, is the volume of pure solid per gram and corresponds to the reciprocal of the true density. Comparing the three parameters D , S and V p ,only the latter has real physical significance. It can be easily measured without any assumptions, whereas according to the previous sections the determination o f D and S must always be based on models. Hence, the question of how far these quantities can reflect reality is still left open. The highest degree of uncertainty is associated with the average pore diameter, as the pore models commonly used involve drastic simplifications. This is also valid for the pore size distribution, which in practice can be obtained only by plotting the distributions of Vp or S as functions of the mean pore diameter. Despite the restrictions outlined, it is very important to know these characteristic parameters of porous solids in order to permit an estimate t o be made of their sorptive properties, especially in chromatography. For an exact comparison of pore structure data, however, it is always necessary to describe the conditions and the methods used for their determination. 2.1.2 Fundamentals As all of the quantities mentioned above are evaluated by means of sorption and mercury penetration measurements, one has to consider the underlying phenomena and the basic equations derived therefrom.
2.1.2.1 Sorption isotherms In sorption, the amount of gas adsorbed, X,, on a given adsorbent is measured as a function of the equiiibrium partial pressure, p , of the adsorbate at constant temperature. X, may be expressed in millilitres (NTP) of gas adsorbed, in grams or in moles of adsorbate per gram of adsorbent. The equilibrium partial pressure, p . is preferably related to pol the saturation vapour pressure of the adsorptive. Measurements are made at a temperature at which the gas is in the liquid state. For nitrogen and argon 77 K and for methanol, water and benzene 298 K have been chosen. The sorption isotherms can be grouped into five types according to the Brunauer, Emmet and Teller (BET) classification [ 2 ] .We prefer a division that is based on the pore size of the adsorbent. Fig. 2.4 shows three nitrogen sorption isotherms that were obtained on a purely microporous, a purely mesoporous and a purely macroporous silica sample. As can be seen, there are distinct differences in the shapes of these isotherms. Their different slopes can be interpreted by means of three mechanisms that may occur during the adsorption and desorption runs. On the purely microporous silica, a Langmuir-type isotherm is obtained. Starting at a relatively low pressure, a sharp increase in adsorption is observed, which i s due to the gradual filling of the micropores with adsorbate. Adsorption proceeds until all pores are filled, which is indicated by the long flat branch that is nearly horizontal to the relative pressure axis. No hysteresis loop occurs. The courses of the adsorption and the desorption branch are identical. On the purely mesoporous silica, a multilayer of adsorbate is formed, increasing the relative pressure. According to the mean pore diameter at p / p o * 0.4, capillary condensation takes place on the multilayer, which results in a further increase in X,. The
20
horizontal branch at higher p / p o indicates that all mesopores are filled with liquid adsorbate. As is evident from Fig.2.4b, the desorption branch does not follow the adsorption branch, but gives a distinct hysteresis loop, which is reproducible. The hysteresis can be explained by a different filling mechanism of the mesopores by means of capillary condensation, depending on the pore shape [1,2]. On the purely macroporous silica monolayer, multilayer formation takes place in the adsorption run, whereas capillary condensation only occurs at a relative pressure of nearly unity. The desorption branch follows the same course as the adsorption branch. The sorption isotherm resembles that obtained on a non-porous silica such as finely divided quartz.
-
PIP,
Pig. 2.4. Nitrogen sorption isotherms at 77 K o n a purely (a) microporous, (b) mesoporous and (c) macroporous silica. Data obtained by the author.
Having briefly discussed the classification of sorption isotherms, the next step is to find equations that fit the experimental curves. As will be seen, these equations involve quantities that are directly proportional to pore structure parameters or are related to them. A variety of theories have been proposed to interpret the adsorption processes. The most useful with respect to surface area determination is the BET theory [1,2]. Based on an over-simplified model of adsorption, one obtains the following expression:
x, =-. x, c z (1-z)
1 - (n + 1) zn tnzn+l
1 t(c-I)z-cz"+l
(2.4)
21
where z
= relative pressure (p/po);
X , = amount adsorbed in moles per gram of adsorbent; X m = specific monolayer capacity in moles of adsorbate per gram of adsorbent; n = number of adsorbate layers; C = constant. Equation 2.4 is termed the three-parameter BET equation [ l ] because it contains three variable parameters, XU, p and n. Assuming n = wand z < 1 as one limiting case, the twoparameter BET equation is obtained: (2.5)
valid for the model of a plane surface at which an infinite number of adsorption layers can be built up. For n >4, the two-parameter BET equation will be a good approximation of eqn. 2.4 in the relative pressure range 0.05 < p / p o < 0.35. For n = 1 , eqn. 2.4 reduces to
which is equivalent to the equation of the Langmuir isotherm. In other words, the threeparameter BET equation gives results that agree, at n = 1 , with those obtained by means of the Langmuir equation. The objective of the BET theory is to evaluate X m , the monolayer capacity, from the multilayer region of the sorption isotherm. As eqns. 2.4-2.6 give linear plots, X m can be easily calculated from the slopes and the intercepts of the straight lines. The quantity n in eqn. 2.4 is obtained by a special approximation procedure. On the basis of the potential theory, Dubinin developed the concept of volume filling of micropores, leading to the following expression, which is valid over the range of relative pressure l*lO-'
(2.7)
where X , = amount adsorbed (moles per gram); W o = specific volume of micropores (millilitres of liquid adsorbate per gram); Vm = molar volume of liquid adsorbate (millilitres per mole); D =constant; p / p o = relative pressure. A plot of log Xa versus log(P0/p)* should give a straight line, with a slope D and an intercept1og(Wo/V,) at (logpo/p)2 = 0. As Vm is known, Wo,the specific micropore volume ( V y ) can be calculated. Eqn. 2.7 was modified by Kaganer [15], who replaced log (W,/Vm) with log X,, which corresponds to the case of a monolayer built up in the micropores.
22
By means of a thermodynamic treatment of capillary condensation, another equation can be derived [ 161 : 7 d S = ApdXa
(2.8)
where = surface tension of the adsorbate, assumed to be a liquid;
y
ds = decrease in surface area when the pores are filled with capillary condensed liquid; Ap = change in chemical potential; dX, = number of moles of adsorbate filling the pores. The change in chemical potential can be represented by
ar.C
= -RTlnb/po)
(2.9)
where T = absolute temperature; R = universal gas constant; p / p o = relative pressure. Inserting eqn. 2.9 into eqn. 2.8 and integrating, one obtains [17] (2.10) where S = surface area of the liquid adsorbate; Xa, = number of moles adsorbed at the beginning of the hysteresis loop; Xa, = number of moles adsorbed at the end of the hysteresis loop. Eqn. 2.10 can be transformed into the general form of the well-known Kelvin equation [2] (2.1 1) It should be mentioned that the derivation of eqn. 2.1 1 is based on the assumption of a zero contact angle between the liquid and the pore walls, which means complete wetting of the solid surface. d V/dS is the ratio of volume to surface area of pores in which capillary condensation takes place at a given relative pressure p / p o . Assuming pores that are circular in cross-section, one obtains
D dV --
_-
d s 4
(2.1 2)
Eqn. 2.1 1 then rearranges to (2.13) where DK is the average pore diameter according to the Kelvin equation. Eqn. 2.13 means that at a given relative pressure p / p o ,pores of size
23
2.1.2.2 Mercury penetration
In the previous section, the discussion of adsorption was correlated with the phenomenon of capillary condensation. Now we have to consider capillary depression, which is the basic phenomenon of mercury penetration as mercury is a non-wetting liquid. The minimum pressure, p , to force mercury into a cylindrically shaped capillary that is circular in cross-section is given by the Washburn equation [2] : 47 COS
p = - -
e
(2.14)
Dw
where Dw = mean pore diameter according to the Washburn equation; 7 = surface tension of mercury (0.480 N/m at 293 K)*; e = contact angle between mercury and the surface (140" at 293 K)*. Inserting the above values of 7 and 6 , one obtains 14,708
(2.15) Dw (nm) assuming that 7 and 0 are independent of the pressure applied and of the curvature of the pore walls. Eqn. 2.15 means that at p = 1 bar pores with D > 14,708 nm (1 5 pm) and at p = 1000 bar pores with D 2 15 nm are completely filled with mercury.
p(bar) =
2.1.3 Experimental techniques As already indicated, two experimental techniques are mainly applied in routine pore structure analysis: sorption measurements, using gases or vapours such as nitrogen, argon, benzene, methanol and water, and mercury intrusion measurements. Additionally, some special procedures have been developed t o evaluate V p and S of porous silica.
2. I . 3.1 Sorption techniques /2/ A preliminary condition for sorption measurements is the outgassing of the adsorbent, which involves the exposure of the adsorbent to a vacuum. For general purposes, a vacuum Pa is sufficient. Outgassing is often carried out at elevated of the order of loe2temperatures, in order to accelerate the removal of humidity and previously adsorbed gases. The temperature, however, may be a critical parameter concerning pore structure and surface composition. With respect to porous silica, an outgassing temperature of 473-573 K can be chosen, using nitrogen as an adsorptive. The sorption of benzene, and more especially of methanol and water, on silica is strongly affected by heating during pre-treatment owing to the diminution of the surface hydroxyl concentration when the temperature is increased above 473 K. For this reason, water isotherms on silica should be measured after outgassing the sample at about 298 K. *These values were mainly used for silica.
24
Sorption techniques can be divided into three modes: volumetric, gravimetric and dynamic. The most common mode is the first. Beginning at low pressures, a certain charge of the gas is admitted to the outgassed adsorbent. When equilibrium has been reached, the pressure in the dead space of the volumetric device is read on a manometer and the amount of unadsorbed gas is calculated by means of the gas law. The precision of the measurements strongly depends on the volume of the dead space, which should be determined as accurately as possible. The mass of gas adsorbed is given by the difference between the total amount admitted and the gas that remains unadsorbed. A review of the different designs of volumetric apparatus was given by Gregg and Sing [2]. In contrast to the volumetric technique, gravimetric methods permit the direct measurement of the amount adsorbed. The porous sample is suspended from a balance, a certain charge of gas is admitted and the increase in mass is read off. The balance can be either a beam balance or spring balance [ 181. A series of beam balances are available, which are based on the principle of centre point balancing. They differ in the type of primary fulcrum used and in the method of monitoring the mass changes. The spring balance consists of a helical spring on which a bucket containing the sample is suspended. The increase in mass causes an extension of the spring, which can be measured by means of a cathetometer. The balance is placed in a case and connected with hangdown tubes, which enclose the sample and the counter-weight suspensions. The case is linked with a vacuum system, with a reservoir of the adsorptive and a pressure-reading device. Pressure readings made at room temperature must be corrected for thermal transpiration when the sample is at a different temperature. It should be mentioned that in gravimetric measurements the effect of buoyancy has to be taken into account. Buoyancy is caused by the difference in volume and temperature between the sample and the counter weight and becomes significant only at relatively high pressures. Usually, the effect is eliminated by adjusting the volume of the counter weight. The balances differ in mass capacity and in sensitivity. The sensitivity required for sorption measurements will depend on the specific pore volume of the sample. For vacuum microbalances, the sensitivity is of the order of 0.1 pg per gram of load [2]. Dynamic methods are based on gas chromatographic measurements. The porous particles are packed in a column, which is placed in a gas chromatograph and conditioned with a stream of carrier gas at constant temperature. A certain amount of adsorptive vapour is injected and, after passing through the column, the concentration profile of the adsorbate is monitored by means of an appropriate detector. From the chromatogram, the retention time ( t R ) is measured and corrected with respect to pressure, temperature and pressure drop across the column. tR multiplied by the flow-rate (F)gives the retention volume UR of the adsorptive, which is then related to the mass of adsorbent present in the column, giving URm. It has been established by Kiselev and co-workers [19,20] that the URm value of the adsorptive is a direct measure of the specific surface area of macroporous silica samples. Cremer [21] and Huber [22] developed a method for the evaluation of the sorption isotherm by means of elution chromatography. The retention volumes are calculated from the chromatogram between the maximum and the end of the corresponding peaks. It is assumed that desorption begins at the maximum of a peak and is completed at the end of it. A plot of the ratio VR,/RT against the partial pressure, p, is equivalent t o the
25
first derivative of the isotherm. Thus, by means of graphical integration, the isotherm can be obtained, provided that the dispersion of the adsorbate in the mobile phase has a negligible effect on the width of the peak. Frontal analysis has also been applied for the evaluation of sorption isotherms 1231. A simple procedure for the determination of specific surface area has been developed by Nelson and Eggertsen [24], and a brief description of their so-called continuous flow method is given here. The sample, placed in a small-diameter glass tube, is outgassed in a helium atmosphere. After cooling, a stream of gas composed of nitrogen and helium passes through the tube and through the cell of a thermal conductivity detector. After a constant detector baseline has been attained the sample is cooled to the temperature of liquid nitrogen, adsorbing a certain amount of gas according to its partial pressure. When equilibrium has been established, the bath of liquid nitrogen is removed and the glass tube is thermostated at room temperature. The adsorbed nitrogen is desorbed, producing a symmetrical peak on the recorder. The peak area is calibrated by injecting known amounts of nitrogen into the helium stream. In this way, the volume of gas adsorbed per gram of adsorbent can be calculated as a function of the relative pressure. Sorption isotherms are usually measured in the range 0.05 < p / p o < 1.O. With the aid of these isotherms, the following pore structure parameters can be calculated: the specific pore volume, V p ,of the adsorbent; the specific pore volume, V p , as a function of the mean pore diameter, D ;the specific surface area, S , of the adsorbent; the specific surface area, S, as a function of the mean pore diameter, D ;and the mean pore diameter, which corresponds to the maximum value of the differential pore volume or surface area distribution curve. A computer program can be delivered by the author, in which the complete course of the isotherm is approximated from the values of the readings of the adsorption and desorption runs. The program also permits the calculation of the specific surface area, the specific pore volume and the pore volume distribution of mesoporous samples. 2.1.3.2 Mercury intrusion technique Mercury porosimetry is an effective means of pore structure analysis, particularly in the mesopore and macropore size ranges [ 2 , 3 , 2 5 ] .As mentioned previously, the method utilizes the phenomenon of capillary depression with mercury being a non-wetting liquid. The problem in this technique is to measure small changes in the mass of mercury when the external pressure is increased progressively. This can be effected by using a small glass vessel connected with a capillary. The apparatus is called a dilatometer or penetrometer. The penetrometer, containing a certain amount of adsorbent, is filled with mercury by means of a special filling device, followed by the immersion of the porous sample. The penetrometer is then placed in an autoclave, which is connected with a high-pressure system. By applying pressure, mercury is forced into the pores of the samples. The changes in the intruded volume of mercury at corresponding pressure readings can be measured by means of direct visual observation, using a resistance bridge or using a highly sensitive capacitance bridge. The volume readings have to be corrected with respect to the compressibility of mercury and the penetrometer. Pressure readings are made with precision dial gauges.
26
Commercially available porosimeters operate at pressures up to 2000 and 4000 bar, whereas special porosimeters permit operation at up t o 6000 bar [26]. By applying pressures from 1 to 4000 bar, the volume of macropores and mesopores down to an average pore diameter of 4 nm can be measured. The pore volume distribution may also be calculated from the V = f(p) curve. In Table 2.4 penetration data of a mesoporous silica are given.
2.1.3.3 Related techniques 2.1.3.3.1 Determination of the specific pore volume of silica according to Fisher and Mottlau /27] A certain amount of the sample (0.5-1.0 g) is weighed in a small erlenmeyer flask and titrated with water (or ethanol) from a burette, with stirring. By dropwise addition of liquid the pore volume of the sample is gradually filled, and the end-point of the titration is indicated by sticking of the porous particles. The volume titrated corresponds to the total specific pore volume of the sample. The precision of the method is about *20% and the reproducibility about *lo%. 2.1.3.3.2 Determination of the specific surface area of silica according to Sears [28] The specific surface area of fully hydroxylated porous or non-porous silica samples can be determined simply by the empirical method of Sears [28]. A suspension of the sample in sodium chloride solution is titrated with 0.1 N sodium hydroxide solution from pH 4.0 to 9.0. From the amount of alkali added, the specific surface area, S, can be calculated according to the equation S (m2/g) = 32 V - 25
(2.16)
where V is the volume of 0.1 N sodium hydroxide solution (millilitres) required to achieve pH 9.0 with 1.5 g of silica in 150 ml of 20% (w/w) sodium chloride solution. The linear relationship in eqn. 2.16 was obtained by plotting the specific surface areas of a series of fully hydroxylated silica samples determined by the BET method against the volume of base added.
2.1.3.3.3 Determination of the specific pore volume from the apparent densities with respect to helium and mercury f29/* The total specific pore volume of a porous material can be determined by measuring the apparent density with respect to helium and mercury by displacement. To a first approximation, p~~ is assumed to be proportional to the true density of a porous solid. Helium as a displacement fluid has the smallest molecular size and remains almost unadsorbed. is termed the lump or particle density, because mercury, a non-wetting liquid, does not penetrate into pores smaller than 15 pm at atmospheric pressure (eqn. 2.15). Helium and mercury pycnometers were described in detail by Spencer [29]. The total specific pore volume, V p ,is given by the difference in the reciprocal densities:
vp(cm’lg)
1
= PHg
1
-PHe
*Definitions of different densities arc given in ref. 98.
(2.17)
21
The particle porosity, ep, of the sample can be easily calculated according to
(2.18) 2.1.4 Calculation procedures
2.1.4.1 Specific surface area, S 2.1.4.1.1 S according to the BET method (SBET)
The most common method for determining the total surface area of a porous solid is the well known BET method, based on the two-parameter BET equation (eqn. 2.5). Nitrogen is usually employed as adsorptive and consequently sorption measurements were carried out at the temperature of liquid nitrogen (77 K). Data on the nitrogen isotherm between relative pressures (p/po) of about 0.05 and 0.35 were plotted in terms of the BET equation, which should result in a straight line. From the slope (s) and the intercept (i), the specific monolayer capacity ( X m )can be calculated. Finally,SBET is obtained by multiplying X m by the cross-sectional area of a nitrogen molecule, A m (Nz): SBET (mz/g) = XmAmN*10-'8
(2.19)
where N is Avogadro's constant (6.02 .loz3 molecules/mole) and Am(N2) is taken to be 0.162 nmZ/molecule [2]. The specific surface area obtained by means of the two-parameter BET equation is termed SBET(2). Instead of nitrogen, other adsorptives such as argon, krypton and water can be employed. The following A m values have been proposed [30] : Am(Ar) = 0.138 nmZ/atom Am(Kr) = 0.202 nmz/atom Am(HzO) = 0.125 nmz/molecule
( T = 77K) ( T = 77K) (T = 298 K)
Based on the mathematical nature of the two-parameter BET equation, reasonable results are obtained only when the constant C is greater than 10 [2]. In simple terms, the nitrogen isotherm should exhibit a distinct knee in the relative pressure range mentioned. C i s proportional to the heat of adsorption of the adsorbate in the multilayer region. Further, the BET plot should give a straight line with an intercept i 2 0 . The reproducibility ofSBET(2) iS about *5%. The two parameter BET equation can be applied successfully to calculations of the specific surface areas of mesoporous and macroporous silica. However, on evaluating the specific surface areas of microporous silicas, particularly those which also contain submicropores, we found that SBET(2) is always less than the values obtained by the threeparameter BET equation (eqn. 2.4) (see also Table 2.1). The n values calculated for these microporous samples vaned between 1 and 4. For n >4,SBET(3) agreed well w i t h S ~ ~ ~ ( 2 ) , so that the two-parameter equation is a good approximation to the three-parameter equation for n > 4. On the other hand, for n = l the three-parameter equation gives results that coincide with those obtained by means of the Langmuir equation (see Table 2.1).
TABLE 2.1 COMPARISON OF SPECIFIC SURFACE AREA DATA FOR POROUS SILICA SAMPLES Specific surface areas (9are given in mz/g.
No.
SBET(3)
n
SBET(2)
Sr
1 2 3 4 5
973 939 590 404 300
1.0 1.0 5.6 4.6 5.2
683 710 607 416 308
813 791 585 412 318
'
Scum(Pierce) (adsorption branch) -
465 -
S,rn(Pierce) (desorption branch)
SLangmuir
SK~S
597 484 436
954 928
407 35 4 308
-
-
-
2.1.4.1.2 S according to the Kiselev method (SKif)
This method, first proposed by Kiselev [17], is based on eqn. 2.10. Strictly speaking, SKiscorresponds to the surface area of the liquid adsorption film at the beginning of the hysteresis. This means that only the amount of capillary condensed liquid adsorbate is taken into account and the amount adsorbed on the pore walls is neglected in the surface ~ a measure only of the sum of the cores of mesopores, area determination. S Kis~therefore in which capillary condensation takes place. According to Brunauer et al. [16] ,the core is defined as that part of a pore which fills up by capillary condensation or remains empty after capillary evaporation. The procedure for calculating SKjs consists in the following steps. The data of the isotherm are converted into a plot of Xa versus log(po/p). From this plot, the integral in eqn. 2.10 is determined graphically for both the desorption and the adsorption branches within the given limits. The arithmetic average is multiplied by the factor 2.3 RTI7, giving S K ~ . Depending on the size of the mesopores, SKis is equal to or smaller than SBET(2), as shown in Table 2.1. The procedure provides an exact measurement of the isotherm in the hysteresis range. 2.1.4.1.3 S according to the Kaganer method ( S K ~ )
By means of the following equation, developed by Kaganer [15] (2.20) < p / p o < 0.2. The SKa values obtained on different adsorbents agree well with SBET(2), which were calculated from the isotherms in the relative pressure range between 0.05 and 0.3 [31]. Gottwald [32] showed, however, that the relatively close agreement between the BET and Kaganer monolayer capacities can be simply explained by the mathematical forms of the two equations. X m can be evaluated from the isotherm in the relative pressure range 1
2.1.4.1.4 S according to the t-method (S,)
After Lippens and co-workers [33,34] ,the amount of nitrogen adsorbed per unit surface area of an adsorbent is a unique function of the relative pressure. This could be established for a large variety of non-porous adsorbents. The amount of nitrogen adsorbed
29
per unit surface area of a non-porous adsorbent at a given relative pressure can be expressed as the statistical thickness, t, of the adsorption layer: t(nm) =
Vl ~
*
103
(2.21)
SBET(2)
where V1
= volume adsorbed in millilitres of liquid nitrogen per gram of adsorbent;
SBET(2) = BET surface area of the non-porous adsorbent (square metres per gram) having
the same chemical composition as the sample. Assuming that the plot of Vl versus t on a porous adsorbent is the same as on the nonporous reference solid, the slope of the function Vl = f(t) is a measure of the specific surface area of the porous sample. The calculation procedure begins with conversion of the measured amounts Xu into the corresponding Vl values. For each Vl at a given relative pressure, a t value is taken from the t curve of a non-porous reference solid. Then V1 is plotted against t for the porous adsorbent under investigation. The slope of the function through the origin gives St. If the right t-curve is chosen, the St value necessarily must coincide with SBET, as shown in Table 2.1. This indicates that the t-method depends on the BET evaluation of X,,, . The t-plot not only can be utilized for the determination of the surface area, but also permits a simple proof of the presence of submicropores, micropores or mesopores. Considering the experimental curves obtained with porous silica samples, one can distinguish the following three types (Fig. 2.5). (1) t-plot of a silica sample containing only micropores (Fig. 2.Sa). Curve (a) resembles a Langmuir isotherm. On the f i s t linear part, which can be extrapolated through the origin, a downward deviation from linearity follows, yielding a line parallel t o the abscissa at higher t values. This characteristic course is due to the presence of micropores.The surface area accessible decreases with the relative pressure as a result of micropores filing with liquid adsorbate. (2) t-plot of a silica sample containing micro- and submicropores (Fig. 2.Sb). Curve (b) represents a special case. It resembles (a) except that the first linear part cannot be extrapolated through the origin. This effect may be caused by the presence of submicropores with a mean pore diameter less than 1 .O nm. (3) t-plot of a silica sample containing only mesopores (Fig. 2 . 5 ~ )In . (c), after the linear part the curve deviates upwards owing to the onset of capillary condensation in the mesopores. The second straight part indicates that the filling of the mesopores is complete. The t-plot also provides the basis for the micropore analysis or MP method [16].
2. I .4.I .S S according to the Sing method (Ssing) Sing and co-workers [35,36] developed a procedure that is also based on the isotherm of a non-porous reference solid. Fransil is used as a standard for silica. The nitrogen isotherm of Fransil is rearranged in the form V, = f(as),where V, is the volume of nitrogen adsorbed in cubic centimetres (NTP) at a given relative pressure and asis the amount adsorbed at that relative pressure divided by Vu’,the amount adsorbed at p / p o = 0.4, as as = -
VU VU’
(2.22)
30
I
0.6
0.5
0.4
0.3
0.2
0.l
C
-----+ t ( n m )
Fig. 2.5. Characteristic types of t-plots. (a) Purely microporous silica;(b) microporous silica with submicropores;(c) purely mesoporous silica. Data obtained by the author.
The specific surface area of Fransil was determined to be 38.7 m2/g by means of the BET method. In order t o evaluate the specific surface area of a porous silica, the following procedure is applied. The data of the measured nitrogen isotherm are converted into a plot of V, versus a,, with a shape similar to that of the r-plots (Fig. 2.5) [37]. The initial part of the a, plot is extrapolated through the origin to calculate the slope V,/as.Multiplying this value by a factor, one obtains Ssing: ssing (m*/g) =
2.89 (V,/a&
(2.23)
The factor of 2.89 originates from calibration against SBETfor Fransil. Deviations from linearity in the upper section of the asplot are caused either by capillary condensation > 0.4) or by micropore fdliig (p/po < 0.4). Brunauer et al. [16] showed that the Ssing and SBET values are in fairly close agreement.
31
2. I . 4. I. 4 S according to the Sears method (Ssears)
The method of Sears (see page 26) was developed especially for the evaluation of the specific surface area of silica and standardized t o the BET method [28]. One must bear in mind that the calibration plot is valid only for fully hydroxylated species, otherwise an appropriate calibration graph has to be established with reference samples. This was shown for Aerosil by Meffert and Langenfeldt [38]. It should be further emphasized that the amount of alkali added to the silica suspension not only depends on the concentration of of surface hydroxyl groups (@OH) but is also a function of the mean pore diameter (0) the sample. Using fully hydroxylated silica of the same origin but differingin D, we found a marked deviation from linearity of the plot of SBETversus v (volume of 0.1 N sodium hydroxide solution), particularly with a high specific surface area. It therefore seems necessary t o investigate the linearity of the calibration plot for the samples under investigation.
2.1.4.1.7 S from pore distribution curves (Scum) A cumulative specific surface area can be obtained by summing the surface area contributions of all groups of pores corresponding to the distribution of S as a function of the mean pore diameter. The quantity of Scum depends on the choice of the sorption branch (desorption or adsorption), on the calculation method selected (Pierce [2] or Brunauer et al. [16]) and on the pore model. Hence, the S, values may be larger or smaller than or equal t o the SBETvalues (Table 2.1) [39]. 2.1.4.2 Specific pore volume, V p 2.1.4.2.1 V p obtained from density measurements [ vp(dm) As pointed out in Section 2.1.3, the total specific pore volume of porous silica can be . has the easily obtained by measuring the apparent densities, p~~ and p ~ As~helium smallest molecular size of the fluids available, the apparent density p~~ is in close agreement with the true density of the porous sample, provided that all pores are accessible. The reciprocal of p~~ corresponds to the specific volume of the solid bulk material. Instead of helium, other fluids such as benzene and carbon tetrachloride, which will penetrate all pores except those which are smaller than the diameter of the molecules, can be employed. As already indicated, the reciprocal of p~~ is proportional to the bulk volume of the ~ is a direct measure of the sample. According to eqn. 2.17, the difference 1 / p -~l/P& total specific pore volume. Calculated Vp(dm) values are in fairly good agreement with those obtained by other methods. 2.1.4.2.2 V p according to the Fisher and Mottlau method [vp(FM)] This method (see page 26) is suitable for the rapid evaluation of the total specific pore volume, particularly of porous silicas [27]. The reproducibility is +lo%. Some problems arise for highly porous silica samples containing macropores and for surfacemodified silica species containing non-polar functional groups. In the first instance the end-point of titration cannot be determined very accurately. For modified samples, ethanol should be used as a titrant instead of water, because ethanol wets the non-polar ) below), the coincidence is better than *lo%. surface. Compared with V p ( ~(see
32
2.1.4.2.3 Vp according to Dubinin and Radushkevich [Vp(DR)] The DR equation (eqn. 2.7) provides a means for determining the specific pore volume of microporous silica samples that exhibit a Langmuir-type isotherm (Fig. 2.4a). Depending on the molecular size of the adsorptive, molecular sieve effects can become ~ )differ for various adsorptives [2]. dominant, so that V p ( ~may 2.1.4.2.4 Vpfrom mercury intrusion [ vp(Hg)] Depending on the maximum pressure ( P m a ) that is attainable by the porosimeter, the specific pore volume of macropores and mesopores can be measured (eqn. 2.15). Errors may arise due to the compressibility of the specimen and damage of the pore walls under values agree satisfactorily with those obpressure. Despite the uncertainties, the Vp'p(~g) tained by the Gurvitsch rule [26]. 2.1.4.2.5 Vp according to the Gurvitsch rule [v p ( G ) / According to the Gurvitsch rule, the amount adsorbed at a relative pressure close to 0.98 represents complete filling of all pores with liquid adsorbate provided that the isotherm shows a course parallel to the relative pressure axis (Fig. 2.4b) [2]. Then Vp(p(c)is calculated as vp(G) ( d / g ) =
& 'vrn
(2.24)
where
X, = amount adsorbed (moles per gram); Vm = molar volume of liquid adsorbate (millilitres per mole) at the adsorption temperature. The rule implies that Vp(p(c)is independent of the type of adsorptive, provided that molecular sieve effects are absent [2 J.Indeed, close agreement was found in practice also in the case of different types of adsorbents [2].
2.1.4.2.6 Vp obtained from pore distribution [ Vp(,-um)/ The cumulative specific pore volume is obtained by summing the volume contributions corresponding to the distribution of the specific pore volume as a function of the mean ) silicas, pore diameter. It is apparent that V p ( a m ) corresponds to V p ( ~for~ microporous ) mesoporous silicas and to V p ( ~ gfor ) mesoporous and macroporous silicas. to V p ( ~for 2.1.4.3 Pore distribution 2.1.4.3.1 Micropore analysis ( M p method) Brunauer and co-workers [I 6,40,41] developed a procedure for analyzing micropores that is based on the t-method of Lippens and co-workers [33,34]. The approach is termed the micropore analysis or MP method. The calculation procedure will be demonstrated for a purely microporous silica. The measured amount of nitrogen is plotted as V1 versus t (Fig. 2.6). The slope, ml, of the straight line I through the first point and the origin will give S1, which is equal to the total surface area of the sample. The next new straight line I1 is drawn and from its slope, m 2 ,the surface area S2 is calculated. The difference S1 - Sz gives the surface
33
vI (rnllg 1
0.1
0.3
0.2
I
0.1
7 I I
0
L 1.5
1
0.5
t inmi
Fig. 2.6. Micropore analysis of a microporous sample (data from Table 2.2 were used).
area of the pore walls of the first group of pores, which are filled with liquid adsorbate. The slopes are calculated for all straight lines, yielding mi-i04 (m2/g) = ~i
(2.25)
Now, the AS values are calculated and the cumulative micropore surface area is obtained as
scum(m2/g)
=
Z ASj
(2.26)
If Mi is multiplied by the hydraulic radius, r h , a second parameter, the micropore volume, Vi, of the group of pores, results: Vi (ml/g) =
(Sj -Si+l) *(ti+ t j + l ) / 2
(2.27)
The average width of the group i of pores is expressed in terms of the hydraulic radius: rhi (nm) = 10-'
(ti
+ ti+1/2)
When the Vi values have been calculated, the cumulative micropore volume,
(2.28)
V3,gl, is
34
obtained: (2.29) On the basis of these results, the distribution of the specific micropore volume or the specific surface area of micropores as a function of the hydraulic radius can be constructed. Micropore analysis presupposes that Scumagrees with SBETand Vp(cum) with V,(G). For the sample in Table 2.2, the following results were obtained: Scum= 595 m2/g; SBET(2) = 610 m2/g; S B E T ( ~ )= 649 m2/g; = 0.352 ml/g; and ‘ V p ( ~=) 0.375 ml/g.
VZzrn)
2.1.4.3.2 Mesopore analysis from sorption data 2.1.4.3.2.1 Modelless and corrected modelless method Brunauer et al. [161 also suggested an approach for analysing mesopores based on capillary condensation and multilayer formation. The method will be applied to the hysteresis range of the nitrogen isotherm, which is typical of mesoporous samples. As already pointed out, in micropore analysis the mesopores are divided into groups. A given group i of pores will contribute a surface area, Sj, according to eqn. 2.10: (2.30)
where d V is the volume adsorbed in or desorbed from the ith group and Si is the uncorrected specific surface area of the ith group of cores. The core is defined as “that part of the pore which fills V, by capillary condensation or remains empty after capillary evaporation’, [ 161. For a small relative pressure difference, the integral in eqn. 2.30 can be approximated as follows: (2.31) 2 where K = R T / y .The last term on the right-hand side of eqn. 2.31 gives AVi, which corresponds to the uncorrected volume of the ith group of cores. So far, our treatment has considered only the core volume and surface area. Filling and emptying of the cores can be described by the well known Kelvin equation (eqn. 2.1 1). However, in addition to capillary condensation, multilayer adsorption also takes place in the still unfilled pores. Multilayer formation can be characterized by the t-plot of a nonporous reference solid provided that the same plot is valid for the mesoporous analogue. As a result, the volume and the surface area must be corrected with respect to the statistical thickness, t, of the adsorbed layer. The corrected volume, AVj, of the ith group of cores is given by i-1
(2.32) where Atj is the difference between the thicknesses of the adsorbed film at the lowest and highest values of relative pressures for the ith group.
35
TABLE 2.2 MICROPORE ANALYSIS DATA FOR A SILICA SAMPLE (SEE FIG. 2.5a AND 2.6) OBTAINED BY THE MP METHOD
I
596.00
I1
538.50
I€I
478.00
IV
406.00
V
348.00
VI
307.50
VII
157.40
VIII
104.40
IX
64.40
X
23.64
XI
13.84
XI1
5 .oo
XI11
0.75 Scum:
57.50
0.4090
0.0235
0.0235
60.50
0.4840
0.02529
0.04879
72.00
0.5 165
0.0374
0.08619
58.00
0.5515
0.032
0.11819
40.50
0.5900
0.0239
0.14209
150.10
0.6280
0.0943
0.23639
5 3.00
0.6670
0.035 37
0.27176
40.00
0.7115
0.02845
0.30030
40.76
0.7615
0.0311
0.33140
9.80
0.8215
0.008045
0.339445
8.84
0.8980
0.00794
0.347385
4.25
1.0805
0.00465 1
0.352036
Vcum:
0.352
595.25 ~~
The corrected surface area is calculated as (2.33) Now, a third pore structure parameter can be introduced that will be a measure of the pore width. Corresponding to eqn. 2.1, the hydraulic diameter, Dhj, of the ith group of cores is defined as
(2.34) In this way, the surface area or the volume of all groups of cores can be evaluated as a function of the hydraulic diameter. In Table 2.3 the data are listed for a purely mesoporous silica. The pore analysis is termed a modelless mode, because no assumptions are made about the pore shape. Considering the latter, the core properties must be transformed into pore properties. Choosing the model of cylindrically shaped pores, their diameter, D c , is calculated by the equation
36
D,
= Dh + I
(2.35)
where
Dh = hydraulic diameter (nanometres); t = statistical thickness of the adsorbed layer at a given relative pressure (nanometres). The surface area or the volume of the pores can be evaluated as a function of Dc (Table 2.3). This procedure is termed the corrected modelless mode, as corrections were made with respect to the pore shapes. Instead of regarding the pores as cylindrical, other models can be selected [ 161. All of the calculations are effected by a computer program U61.
2.1.4.3.2.2 Other methods of mesopore analysis A variety of other procedures have been proposed for evaluating the mesopore distribution from nitrogen isotherm data. One common method is that developed by Pierce and modified by Orr and Dalla Valle [2]. The calculation is based on the Kelvin equation,
TABLE 2.3 MESOPORE ANALYSIS DATA OF A SILICA SAMPLE (SEE FIG. 2.4b) USING NITROGEN ADSORPTION AT 77 K (DESORPTION BRANCH) AND THE METHOD OF BRUNAUER et al. Method/ Properties
Group of pores
Pressure range
(P/P,)
Core radius (nm)
Core volume
Core surface area
(ml/g)
(m' /g)
Group Modelless method, core properties
1 2 3 4 5 6 7 8 9 No.
0.95 0-0.925 0.925-0.900 0.900-0.875 0.875 -0.850 0.850-0.825 0.825 -0.800 0.800-0.775 0.775-0.750 0.750-0.725 Radius
(nm)
Volume (mild
7.36 5.19 3.98 3.21 2.68 2.29 1.99 1.75 1.56
Cumulative
0.0100 0.0100 0.0117 0.0217 0.0304 0.0521 0.1774 0.2295 0.3244 0.5539 0.1789 0.7327 0.0730 0.8058 0.0391 0.8448 0.0239 0.8687
Cumulative volume
1 2 3 4 5 6 7 8 9
8.25 5.98 4.69 3.86 3.27 2.84 2.50 2.24 2.02
0.0125 0.0155 0.0423 0.2554 0.48 36 0.2754 0.1156 0.0637 0.0400
0.0125 0.0280 0.0703 0.3257 0.8093 1.0847 1.2003 1,2640 1.3040
Cumulative
1.36 2.25 7.64 55.19 120.98 78.12 36.70 22.28 15.27
1.36 3.61 11.25 66.44 187.42 26554 302.24 324.52 339.79
Surface area (m2/d
Cumulative surface area (m2/g)
1.52 2.59 9.01 66.22 147.72 96.93 46.17 28.45 19.79
1.52 4.11 13.12 79.34 227.06 323.99 370.17 398.61 418.40
(mVd Corrected modelless method (cylindrical pores),pore properties
Group
31
assuming cylindrically shaped pores. Corrections were made with respect to the statistical thickness of the multilayer. Referring to the same sorption isotherm and assuming cylindrically shaped pores in both instances, the results of the Pierce method and that of the corrected modelless procedure agree fairly well. A computer program for calculating the pore volume distribution of mesoporous samples according to Pierce is available from the author.
2.1.4.3.3 Mesopore and macropore analysis from porosimetry By means of porosimetry, the intruded volume, V , of mercury is measured as a function of the equilibrium pressure, p (Table 2.4). The pressure at which pores with a width greater than or equal to D are filled is given by the Washburn equation (eqn. 2.14). In this way, the integral and differential pore volume distributions can be evaluated. Corrections are necessary on account of the compressibility of mercury and that of the penetrometer device. The most important assumption made in the derivation of eqn. 2.14 is that the pores are cylindrically shaped, with a circular cross-section. Further, both the surface tension of mercury and its angle of contact are assumed to be constant. The high-pressure treatment may also cause irreversible changes to the pore structure. Hence special care is required in interpreting data obtained by mercury porosimetry. For mesoporous silica [ 2 6 ] nearly the same pore volume distributions were found by means of porosimetry and nitrogen sorption for mean pore diameters between 4 and 40 nm. One should bear in mind that, in contrast to sorption data, porosimetry generally gives detailed information about the mesopore and macropore distribution. 2.1.4.4 Mean pore diameter, D By definition, the mean pore diameter, D , corresponds to the maximum value of the relative pore v h m e distribution curve. If the pore volume distribution is homogeneous and of gaussian shape, then D coincides with the 50% value of the cumulative distribution curve. In this instance, the standard deviation of the mean pore diameter, 0 0 , can be evaluated as a direct measure of the width of the distribution curve (Fig. 2.7). Pore distributions that cover a wide range of D should preferably be presented in logarithmic form, such as AV,/Alog D = f(1og D ) . According to Wheeler [42], a mean pore diameter, DWh, can be estimated that is proportional to the specific pore volume, V p ,divided by the specific surface area, SBET, of the adsorbent: 4 VP .lo3 (2.36) (nm) = SBET Again, it is assumed that the pores are cylindrically shaped. Fig. 2.8 shows a plot of DWh versus D , obtained from nitrogen sorption data applying the Pierce method. The samples were mesoporous silicas. It can be seen that &h is mostly greater than D and hence the Wheeler equation gives only rough information about the order of magnitude of D . When comparing pore distributions obtained by different methods, the AVp/AD and A S / A D values, respectively, should be equal for both distributions. However, such a comparison can be made only when the same pore models are chosen.
&h
38 TABLE 2.4 MESOPORE ANALYSIS DATA FOR A SlLlCA SAMPLE OBTAINED FROM MERCURY POROSIMETRY MEASUREMENTS A VIAL)
(mllg-nm)
1 .o
0.0
14,708
16.0
0.3948
919
21.0
0.3999
700
26.0
0.4038
566
35.8
0.4093
420
46.0
0.4 139
316
59.5
0.4184
247
74.3
0.423 1
198
89.3
0.4271
165
109
0.4328
135
139
0.4397
106
160
0.4447
92.0
193
0.4514
76.4
247
0.4648
59.6
304
0.476 1
48.4
388
0.4941
38.0
510
0.5228
28.8
612
0.5489
24.0
765
0~5911
19.2
1,014
0.6661
14.5
1,187
0.7138
12.4
1,387
0.7700
10.6
1,534
0.8147
9.6
3.948 *lo-’
13.789
7,814
2.863.10-5
5.149.10-s
219
810
2.352.10-5
3.894-10-’
135
633
2.891 .lo-’
5.449 -10-3
145
493
3.743.10-5
4.156.10-’
104
368
3.999
5.013.10-’
69.1
282
7.254-10-’
4.635 *lo-’
49.1
223
9.440*10-’
3.998*10-’
33.3
181
1.201
5.716*10-3
29.5
300
1.934.10-4
6.891 .lo-’
29.4
121
2.341
5.034 *lo-’
13.9
98.9
3.625
6.652 .lo-’
15.5
84.2
4.286.10-4
1.338 .lo-’
16.8
68.0
7.967 .LO-4
1.1 30.lo-’
11.2
54.0
1.008-lo-’
1.801 -lo-’
10.4
43.2
1.725 -lo-’
2.870*10”
9.1
33.4
3.150-10-’
2.614 .lo-’
4.8
26.4
5.436.
4.219.10-’
4.8
21.6
8.774*10-3
7.497.lo-’
4.7
16.9
1.588.10-’
4.773 .lo-’
2.1
13.5
2.252.lo-’
5.6 18.lo-’
1.8
11.5
3.146 *lo-’
4.476*10-’
1.0
10.1
4.408*10-’
1.116.10-’
0.86
9.2
1.259.10-’
39 TABLE 2.4 (continued)
1,685
0.9263
8.7
1,901
1.100
7.7
2,168
1.2044
6.8
2,539
1.2610
5.8
3,044
1.2864
4.8
3,386
1.2900
4.3
3,707
1.2905
4.0
4,05 3
1.2900
3.6
1.740*10-'
0.99
8.2
1.752*10-'
1.104.10"
0.95
7.3
1.092. lo-'
5.663.10-'
0.99
6.3
5.711.10-'
2.533.10-3
0.96
5.3
2.637-10-3
3.584
0.49
4.6
7.350 *
5.522
0.38
4.2
1.467. lo-'
-
-
-
-
-
-
percentage of cumulative pore volume f o r pores 2 D
Fig. 2.7. Cumulative pore volume distribution of a mesoporous silica (see Fig. 2.4b) obtained by the Pierce method presented as a probability plot.
40 DWh I nm I
3.0 /
/
/
/
/
/
/
/
/
2.0 a
'.
/
/
/
/ '
1.0
.r'
4.
0
I
1.0
"
"
1
"
"
I
2D
'
30
D Inml
Fig. 2.8. &+q, according to the Wheeler equation (eqn. 2.36) as a function of the mean pore diameter,
D ,derived from nitrogen desorption data of mesoporous silicas by means of the Pierce method.
2.2 FORMATION OF PORE STRUCTURE 2.2.1 Pore structure models
In the previous section we dealt with the parameters of pore structure, such as specific surface area, specific pore volume and mean pore diameter. These quantities can generally be used for a formal description of porous systems, irrespective of their chemical composition and their origin. For a more detailed study of the pore formation mechanism, the geometric aspects of pore structure are important. One simple model describes the porous solid as an assembly of non-intenecting cylindrical capillaries. This picture, however, oversimplifies the situation because it provides a pore uniformity that is far from reality. Thorough attempts have been made to achieve the mathematical evaluation of porous matter [43,44]. Karnaukhov [44] discussed the origin of porosity in various materials and concluded that there are two main types based on pore structure, namely corpuscular systems and spongy systems.
41
In corpuscular systems, the pores consist of the interstices between discrete particles of a solid material. In such a system the pore structure depends on the shape of the particles and on their mutual arrangement. The dimensions of the pores are controlled by the size of the particles and their packing density. Most of the amorphous xerogels and carbon blacks belong to this type [44]. In spongy systems, the pores consist of channels, hollows or cavities in the solid matter, interconnected by narrow passages. Representatives of this type are porous glasses and cellulose membranes [45,46]. In addition to these two main types, mixed systems can also be observed [44]. Most porous materials have been found to be corpuscular in structure. According to the shape and packing of the constituent particles, different models can be derived to describe corpuscular species. The most common is the globular model, in which the porous system is built up by a regular arrangement of a large number of packed spheres. The main parameters are the sphere diameter, D,,and the number of contacts (coordination number), n, with neighbouring spheres. The pore space consists of alternating wide bodies and narrow constrictions. Each wide body (pore cavity) is accessible by several constrictions (throats). Depending on the coordination number, polyhedrons can be constructed that differ widely in their porosity, e (Table 2.5). As n varies between 3 and 8 the porosity varies between 0.815 and 0.320. On the basis of these results, the model parameters diameter of the spheres (D,), coordination number (n), diameter of the pore throat ( d t ) and diameter of the pore cavity (d,) can be correlated with the specific pore volume ( Vp)and the specific surface area (S) [44,47].The specific pore volume and the particle porosity are independent of the globule diameter (D,)and will be a function only of n. S is given by the equation S(m2/g) = 6-103/D,p
(2.3 7a)
where p is the density of the porous solid and Dsis given in nanometres. The diameter of the pore throat, d t , is given by dt (nm) = 2 . 8 ~ 1 0 ~Vp/S . = 0.467 p VpDs
(2.3 7b)
Aristov et al. [48]put forward a theoretical approach, dealing with adsorption and capillary condensation in the interstices between regularly packed spheres. The results of TABLE 2.5 COORDINATION NUMBER (n)OF PACKED SPHERES AS A FUNCTION OF THE POROSITY ( E ) OF THE PACKING n
E
8 (cubic face centred)
0.320
6 (cubic simple)
0.476
4 (tetrahedral)
0.660
42
their calculations were confirmed experimentally using model adsorbents prepared by compacting Aerosil. Another model that may describe the corpuscular structure of a porous matter is the random sphere model (RSM) [49-531. It assumes a statistically random distribution of spheres without taking into consideration any correlation between the positions of the different entities. The pore space formed is made up by the interstitial volume and is specified by the void fraction, $. The statistical treatment of such a system yields a relationship between the volume fraction, the diameter of the solid spheres, Ds, and the specific surface area, S [49,53] :
6$
D, = --
S
-In $
(2.38)
The mean pore diameter, D, can be approximated by the equation 1491
D = (1.32 -$)*
4$ S
-
(2.39)
It has been found that D values calculated with this equation are in excellent agreement with those obtained by mercury porosimetry [49]. The globular structure of porous solids such as xerogels can be established by means of electron microscopy. Another effective means of characterizing porous media is by gas diffusion experiments. From these measurements the tortuosity factor, 7 , can be evaluated, which is an additional parameter describing the pore structure [54].
2.2.2 Origin of porosity in silica* Pore systems in inorganic oxides are fixed either in the course of their formation or by means of an after-treatment [55-571. Thus the pore structure parameters can be controlled by the type of the chemical reaction as well as by the experimental conditions. With respect to silica, the reactions are very complex (Fig. 29), particularly because the product remains amorphous over a wide range of temperatures. As starting materials, silicon compounds such as sodium silicate solution or water-glass, silicon tetrachloride and tetraakoxysilanes can be employed. By means of hydrolysis and polycondensation, these compounds are converted into a silica hydrogel. After ageing, the hydrogel is dehydrated to yield a xerogel, which can be further subjected to special treatments in order to change the pore structure. Instead of preparing the hydrogel in one step, a silica sol may be produced as an intermediate product, which is stabilized by protective agents (Fig. 2.9, solid arrows). Standardized stable silica sols are also commercially available [57]. The silica sol solidifies by gelling to the hydrogel, which can be further treated as described above. The use of well described silica sols offers the possibility of controlling the pore structure over a wide range. The xerogels obtained by the so-called sol-gel procedure exhibit a distinct globular structure. As shown by electron microscopy, the individual particles in xerogels are spherical and very similar in shape to those of the starting sol [58]. According to another method, partial hydrolysis and polycondensation of tetraethoxy*An extended treatment about the formation of porous silica is given in ref. 99.
43 stages
processes
sodium silicate alkoxy silanes silicon tetrachloride
I hydrolysis condensation
.
$ II
silica sol
I
II
condensation gelling
silica hydrogel condensation
Q
ageing dehydration
silica xerogel ( porous silica 1
Fig. 2.9. Schematic representation of porous silica formation (sol-gel procedure).
silane yields a polyethoxysiloxane (PES) as an intermediate product [59].In a subsequent heterogeneous reaction, hydrolysis and condensation of PES are completed with the formation of a hydrogel. The resulting xerogel exhibits a distinct spongy structure (Fig. 2.3), in contrast to species obtained by means of the sol-gel procedure. In most instances, macroporous and mesoporous silicas are produced by hydrolytic polycondensation reactions, macroporous samples being obtained exclusively by thermal or hydrothermal after-treatment of the hydrogel or xerogel.
2.2.2.1 Formation of silica sols Silica sols are generally prepared by adding a sodium silicate solution to an acidic solution [60,61]. Special procedures have been developed that utilize dialysis and ion exchange of the sol, peptization of the hydrogel, hydrolysis of silicon tetrachloride and tetraalkoxysilanes, dissolution of elemental silicon and dispersion of non-porous finely divided silica [60]. By acidifying sodium silicate solutions, soluble acids are formed, which can undergo further condensation to polysilicic acids. The polysilicic acids are the nuclei for the formation of colloidal silica particles, which, together with the polysilicic acids, are
44
additionally in a dissolution equilibrium with monosilicic acid (Section 1.3). The rate of the condensation reaction is influenced by various parameters, e.g., pH of solution, silica concentration, salt concentration and temperature [60-641. As indicated by Baumann [62], the rate of condensation of polysilicic acids is markedly affected by the pH. It is low between pH 2 and 4 and increases with increasing pH, owing to the catalytic effect of hydroxyl ions. As a result, the polysilicic acids formed at about pH 3 are temporarily stable. In alkaline solutions, the growth of polysilicic nuclei to colloidal particles is favoured, particularly at about pH 9. This growth can be enhanced by increasing the temperature. Electrophoresis measurements reveal, that the isoelectric point of colloidal silica lies at about pH 3 [60,64]. This is in close agreement with the observation that silica sols between pH 2 and 3 are fairly stable towards gelling whereas below pH 2 and above pH 3 the gelling time decreases [64]. Hence, a positive charge must appear at about pH 2 whereas at higher pH values adsorption of hydroxyl ions takes place. For this reason, the particles are surrounded by sodium or other cations, forming a diffuse double layer. This indicates that the stability of silica sols is also governed by the electrolyte concentration, an increase in which drastically reduces the thickness of the diffuse double layer surrounding the particles and may lead to gelling of the sol (see formation of silica hydrogel). As shown by Bolt [65],increasing the electrolyte concentration also increases the adsorption of hydroxyl ions at constant pH in the range 6-10. The dependence of the silica concentration in sols on the particle diameter has been thoroughly investigated by Iler [60]. As previously defined in Section 1.l,the silica sol consists of discrete silica particles, which are spherical in shape, non-porous and amorphous. Silica sols may be characterized by the content of silica, pH, type of stabilizer, mean particle diameter and specific surface area. Alkali-stabilized sols have been developed that contain up to 50% (w/w) of S O z [66]. The mean particle diameter, measured by means of electron microscopy, is 5-100 nm. Depending on the particle size, the specific surface area varies between 50 and 400 m2/g [601*
2.2.2.2 Formation of silica hydrogels In gelling, colloidal silica particles are linked to form a three-dimensional aggregate. Macroscopically, the silica sol becomes more and more viscous and finally solidifies to a gel mass (gel point). The gel then fills the whole volume of the sol and hence exhibits the same silica concentration. Gel formation is caused primarily by the collision of particles attracted by dispersion forces. Intraparticle bonding occurs by condensation of surface hydroxyl groups to form a significant number of siloxane bonds. In this way, the framework is strengthened. The rate of gelling is controlled by the following parameters: pH of solution, concentration of silica, concentration of electrolytes and temperature [60]. The effect of pH on gelling is two-fold. Firstly, as a consequence of the resulting higher negative surface charge, the rate of collisions between the particles decreases with increase in the pH of the solution above 6. Secondly, the extent of condensation between the silanol groups at the surface of neighbouring particles is proportional to the hydroxyl
45
group concentration in the solution. Thus, a pH above 3 favours gel formation. As a result of these two effects, a maximum rate of gelling is obtained at about pH 5 (Fig. 2.10). The effect of silica concentration on gelling is related with that caused by the mean particle diameter. As shown by Iler [60], sols that have the same ratio of silica concentration to mean particle diameter exhibit the same gelling rate. Gelling is markedly accelerated in the presence of electrolytes. As previously pointed out, the addition of electrolytes lowers the electric potential of the particles, which is due to the diminution of the thickness of the diffuse double layer. Consequently, the charge on the particles is reduced and the attraction forces between them overcome the electrostatic repulsion. Finally, it should be mentioned that the gelling rate increases with temperature, as the collision of particles depends on their mobility. Apart from the above effects, specific mechanisms of gelling have been proposed that take the nature of the silica surface into account [60]. When the hydrogel is allowed to remain in contact with the immobilized solution, ageing occurs, i.e., a strengthening of the gel framework, which is due to the formation of siloxane bonds between the particles. Further, re-condensation takes place, resulting in dissolution of silica. Dissolution and re-deposition processes lead to the growth of larger particles at the expense of smaller ones (Ostwald ripening). Ageing and ripening increase the size of silica particles as well as their packing density. Hence the specific surface area decreases and the specific pore volume increases [MI. Neimark and co-workers [67-731 studied extensively the effect of ageing of silica hydrogels in various media on the pore structure of the resulting silica species. Their results reveal that the type of acid or base and the pH have a strong influence on the structure of the xerogel. When the hydrogels are in contact with acids at pH 2-5 during ageing and washing, the dispersion will be stabilized. This may be caused by the development of a hydrate shell around the particles. Additionally, the condensation rate will be very slow in this pH range. Hydrogels prepared from sols in an acidic medium, washed with acids and dried carefully, exhibit an aggregation of very small silica particles. Hence, the xerogels have a high specific surface area [64]. Ageing and washing with solutions
log of gelling time
0
5
-
10
PH
Fig. 2.10. Gelling time as a function of the pH of the silica sol.
46
with pH above 5 accelerate the growth of silica particles and finally their aggregation, owing to the catalytic effect of hydroxyl ions on the rate 4condensation. Increasing the temperature ehances this effect.
2.2.2.3 Formation of silica xerogels After washing a hydrogel, water has t o be renowd by drying in an oven. As the water evaporates, a pronounced shrinkage of the relatively open silica framework is observed. Dehydration is associated with the formation of additional intraparticle bonds, which enhances the particle density. Water has a high surface tension and hence causes a high degree of shrinkage. When water is replaced with organic liquids before drying, the shrinkage is reduced. The xerogels obtained exhibit a smgn particle density and a high specific pore volume [74J. It must be emphasized that the condensation reaction proceeds further after drying, and continues as long as the porous silica is in contact with liquid water or water vapour (atmospheric moisture). The rate of condensation is markedly controlled by the pH of the aqueous solution. The condensation reaction expresses the tendency of hydroxyl groups to interact irreversibly to siloxane bonds.
2.2.2.4 Factors controlling the pore structure in the sol-gel procedure
As atready indicated, in the globuIar pore model the main parameters that determine the corpuscular pore structure are the particle dimensions and the packing density, expressed by the coordination number, n. As the structure of silica can be approximated fairly well by this model, one has to investigate the factors in the sol-gel pxocedure that significantly affect the dimensions of the silica paxticles and their packing in hydrogels and xerogels. These aspects were disoussed in detail by Sheinfain and Neimark [74], 2.2.2.4.1 Factors affecting the silica particle dimensions According to Sheinfain and Neimark [74], one has to distinguish between factors that inhibit and those which promote the growth of silica particles. Their dispersion is found to be stabilized by the following conditions: (1) attaining an acidic medium (pH 2-5) during the formation of the silica sol and the hydrogel; (2) washing the hydrogel with acidic solutions; (3) replacing the immobilized solution of the hydrogel with organic liquids. The growth of the particles (destabilization) is favoured by the following conditions: (1) increasing the pH from 5 to 8 during the formation of the silica sol and the hydrogel, as well as during ageing, ripening and washing; (2) increasing the working time and temperature; (3) treating the hydrogel with highly concentrated sulphuric acid or ammonia, the former accelerating dehydration, 2.2.2.4.2 Factors affecting the packing density of silica particles A high packing density will be obtained only by means of the dense compaction of
41
finely dispersed small silica particles. This can be achieved by preparing a hydrogel from acidic silica solutions and washing it with dilute acidic solutions [74,75]. The use of concentrated sulphuric acid, for instance, leads to a decrease in packing density, because the acid acts as a dehydrating agent, favouring the growth of the particles and strengthening the gel framework. A decrease in packing density is further obtained by replacing the intermicellar water with organic liquids that exhibit a lower surface tension than water. The effect can be explained by the role of capillary forces in the contraction of the silica skeleton during dehydration [75,76]. Numerous experimental data on the dependence of the pore structure parameters on the experimental conditions during the sol-gel procedure are available. The methods that influence and control the pore structure wd1 be summarized, following the outlines given above .
2.2.2.4.3 Variation of the p H in hydrogel formution Increasing the pH from 2 to 8 yields a family of xerogels, the specific surface areas of which vary between 800 (pH 2) and 200 m2/g (pH 8). The specific pore volume simdtaneously increases from 0.3 (pH 2) to 0.8 ml/g (pH 8) [64,74,78,79]. The changes in S and V,, correspond to a variation in pore structure from micropores to mesopores. 2.2.2.4.4 Variation of the duration of ripening and ageing of the hydrogel A prolonged time of ageing results in a diminution of the specific surface area and in an increase in the specifu: pore volume [64,74]. Fixing the globular structure during ageing by means of stabilizing agents makes it possible to obtain samples with an increased specific pore volume and a nearly constant specific surface area [74]. 2.2.2.4.5 Variation of the pH during washing of the hydrogel The pH of the washing water has the same influence as discussed with regard t o hydrogel formation. Some peculiarities in the changes of the pare structure parameters occur, depending on the type and concentration of the acid or base with which the hydrogel is treated. 2.2.2.4.6 Variation of the intermicellar liquid in the hydrogel Replacement of water with organic liquids leads to an increase in the specific pore volume [76,77]. The extent of the effect depends on the surface tension of the liquid as well as on its chemical nature. By combining the above four procedures, porous silica samples can be prepared with a wide variation in pore structure parameters in the micropore and mesopore size ranges. 2.2.2.5 Hydrothermal treatment of silica hydrogels and xerogels The aim of hydrothermal treatment is to increase the mean pore diameter, D , a f a given silica species. This increase will necessarily be associated with a decrease in the specific surface area of the sample. In this way, micropores may be converted into mesopores, smaller mesopores may be widened to larger ones or macropores may be created. It must be emphasized that by a hydrothermal treatment the surface of partially
P W
TABLE 2.6 INFLUENCE OF THE CONDITIONS OF HYDROTHERMAL TREATMENT ON THE PORE STRUCTURE OF SILICA [81,85] Sample No.
Treatment conditions Duration (h)
Temperature (K) ~~
Pressure (bar)
Specific surface area, S (m’ /g)
Specific pore volume, v p (ml/g)
D (nm)
Mean pore diameter,
210* 121* 39* 20* 1.4*
0.73 0.70 0.72 0.78 0.70
10.0*** 22.0*** 74.0*** 290.0** * 1420.0***
330* 63* 51* 48 * 38*
1.07 1.09 1.06 1.15 1.06
10.5*** 68.0*** 88.5* * * 88.O* ** 88.5***
498** 432** 395* * 356**
0.63 0.93 0.94 0.94
§ 5-1 7.2 §
~
~
~
Pore structure parameters
la lb lc Id le
383 45 3 523 573
4 4 4 4
2a 2b 2c 2d 2e
-
523 523 523 523
5
3a 3b 3c 3d
37 3 37 3 37 3
10 15 20 0.5
1.o 1.5
2 10 50
100 50 50 50 50 -
1 1 1
8.0 8.6 §
*Determined by adsorption of krypton (samples la-le, 2a-2e);Am(Kr) = 0.215 nm’/atom. **Determined by adsorption of nitrogen (samples 3a-3d);Am(N,) = 0.162 nm’/molecule. ***Determined by adsorption of benzene vapour and by mercury porosimetry (samples la-le, 2a-2e). §Determined by adsorption of nitrogen (samples 3a-3d) using the desorption isotherm and the method of Pierce.
49
dehydroxylated silica samples becomes completely hydroxylated. In the experimental procedure, silica is either suspended in water and heated in an autoclave or treated with a stream of steam at atmospheric pressure [80,81]. Heating silica in an aqueous solution accelerates the dissolution, particularly of smaller particles, that have a large surface curvature. Simultaneously, redeposition of silica takes place on the surface of large particles around their contact regions. As a result, the diameter of the pore throats increases [82]. Treatment at temperatures of 523 K and higher makes the large particles intergrow to vermicular particles, forming a macroporous spongy structure. This may be due to the highly supersaturated solution which causes redistribution of a large mass of amorphous silica and of crystalline quartz [83]. Kiselev and co-workers [82-871 have investigated the influence of the temperature and the duration of hydrothermal treatment on the pore structure of silica. As shown in Table 2.6, an increase in temperature from 373 to 573 K with a constant working time drastically reduces the specific surface area, S , whereas the specific pore volume, V p , remains nearly constant. The decrease in S is caused by the enlargement of silica particles. It is further evident from Table 2.6 that at constant temperature the specific surface area is considerably decreased during the first 5 h. Prolonging the hydrothermal treatment hardly affects the pore structure parameters, although it has been established by Kiselev eC al. [82], that it leads to a change of the globular into a vermicular structure. In contrast to the drastic autoclave conditions mentioned above, a slight change in the specific surface area can be obtained by heating the silica-water suspension at 373 K at atmospheric pressure (Table 2.6).
2.3 CONTROLLED POROSITY SILICA PACKINGS
According to the various modes of liquid chromatography (LC), silica packings are needed that exhibit specific pore structure properties. For instance, liquid-solid chromatography (LSC)requires a high specific surface area as well as mesopores. In liquid-liquid chromatography (LLC), the particles are utilized as supports for the liquid stationary phase and hence should have low specific surface areas and macropores. With respect to high column performance, microparticles must be employed as column packings. These can generally be obtained by grinding larger species followed by air sieving to provide the desired narrow size fractions. In contrast to the irregularly shaped particles, silica microbeads have the advantage of achieving more homogeneous and dense column packing. Efforts have therefore been made to modify the sol-gel procedure in order to obtain spherical particles and to control the particle size distribution. As silica is used in routine LC analysis, the pore structure properties should be reproducible from batch to batch. Hence silica LC packings require closer specifications than porous products serving as adsorbents in industrial processes. Therefore, in the past 10 years, special methods for the preparation of silica packings have been developed that differ widely from the sol-gel procedure previously discussed.
2.3.1 Modified sol-gel p
d
u followed ~ by sintering
According to a method developed by Le Page and co-workers [88,89], a silica sol is d o w e d to pass through a non-aqueous liquid, forming droplets that rapidly solidify to silica hydrogel beads. These beads are then dried and calcined at 673-1073 K. The alkali content of the sols (0.1 -lo%, w/w, Na20) and the calcination temperature strongly affect the pore structure parameters. An increasing alkali content of the sols makes the hydrogel beads more sensitive to the calcination treatment. As a result, the specific surface area and the specific pore volume decrease at a given temperature. Increasing the temperature from 673 to 1073 K at a constant alkali content also leads to a diminution o f S and V p . In this way, mesoporous and macroporous silica beads are produced that have a hornogeneow but fairly wide pore volume distribution.
2.3.2 Polyethoxysilaxane procedure [90] In this method, the formation of silica microbeads is performed in two steps. Firstly, a viscous liquid polyethoxysiloxane (PES) is prepared by partial hydrolysis of tetraethoxysilane. Secondly, the PES is emulsified with a mixture of ethanol and water with vigorous stirring, then a catalyst is added, which initiates the hydrolytic polycondensation of the droplets of PES to yield solid spheres of silica hydrogels as a precipitate. The hydrogel beads are washed and dehydrated 10 porous silica. Electron transmission studies of these products revealed that the pore structure can be considered to be a spongy type rather than a globular system (Fig. 2.3). According to this procedure, the pore size can be controlled in the micropore and mesopore size range. The main parameters that influence the pore structure are (1) the mean molecular weight of the PES, (2) the type of catalyst and its concentration, (3) the relative proportions of the ethanol-water mixture and (4)the reaction temperature [ 5 9 ] . As shown in Fig. 2.8, the pore volume distribution of the samples is fairly homogeneous and narrow. A slight modification of the above procedure yields silica microbeads with a wide pore volume distribution, extending from the mesopore to the low macropore size range [9]. Simultaneously, the porosity of the beads is increased to 90%. The materials are suitable as column packing in gel permeation chromatography [92],
2.3.3 Agglutination of fmely dispersed nomporous silica particles The principle consists in the agglutination of non-porous silica beads in the colloidal size range t o mechanically stable, large, porous particles, which are also spherical in shape (Fig. 2.1 1). Two procedures have recently been developed, the first utilizing well defined silica sols and the second Aerosil samples as starting materials.
51
2.3.3.1 Procedure 1 193-951 A silica sol with a narrow particle size distribution is mixed with a given proportion of melamine and formaldehyde. By adjusting the pH of the dispersion, co-condensation of the organic compounds starts, yielding microbeads with the non-porous colloidal silica particles distributed in the polymer matrix. By means of calcination at 773 K the organic constituent is completely burned out. After this treatment, the temperature is increased to 1273 K to produce slight sintering of the remaining silica species. In this way, the packing of the colloidal entities is strengthened and mechanically stable microbeads are obtained. The pore structure parameters are controlled by the mean diameter, D,,of the silica sol particles and by their packing density (eqns. 2.37a and b).
Fig. 2.1 1. Porous sfica beads made by means of agglutination of non-porous silica microparticles.
2.3.3.2 Procedure 2 [96] Aerosil prepared by high-temperature hydrolysis is suspended in water and the suspension is sprayed into an oven at 673 IC. In this way, spherical particles are obtained that consist of a n assembly of compacted non-porous Aerosil globules. In order to vary the pore structure, the products are subjected to hydrothermal treatment in an autoclave. According to the size of the Aerosil particles and the treatment conditions, mesoporous and macroporous silica beads can be prepared. The samples exhibit a high specific pore volume and a high thermal stability. 2.3.4 Controlled sintering [97]
Usually, sintering of porous silica at temperatures above 873 K leads to a gradual decrease in porosity. This effect can be largely prevented if the pore volume is filled with a high-melting salt such as sodium chloride. This can be done by suspending the sample in a salt solution followed by evaporation of the water and sintering at a temperature above the melting point of the salt. After this treatment, the salt is washed out and the products are dried. Applying this procedure, macroporous silica samples can be obtained from mesoporous
52
materials. Generally the enlargement of pores is controlled by the temperature, the type of the salt and the amount of loading. It should be emphasized that the specific pore volume decreases only slightly by this treatment (see Section 2.4.1). Both spherical and irregularly shaped silica particles yield the same results.
2.4 STABILITY OF POROUS SILICA 2.4.1 Thermal stability Generally, heat treatment of porous silica up to 673-773 K does not affect its pore structure parameters. Above 873 K sintering may take place, resulting in a gradual decrease in both the specific surface area and the specific pore volume. At temperatures higher than 1473 K, non-porous products are obtained. In the temperature range 473-673 K mainly vicinal or paired hydroxyl groups condense at the surface, forming strained siloxane bonds. Above 873 K, intraparticle condensation of free hydroxyl groups occurs, which is combined with a rearrangement of silica globules to produce a more stable configuration. At about 1073 K, the crystallization rate becomes significant, leading to further stabilization of the siloxane network. It has been found that the sintering characteristics sometimes differ from product to product. This can be explained on the basis of the origin of the samples. For instance, according to the conditions during the sol-gel formation, the resulting xerogels may vary in their content of internal hydroxyl groups, which are held within the non-porous silica globules. Further, porous samples made from Aerosil (Aerosil xerogels) exhibit higher thermal stability than the ordinary xerogels, because the Aerosil globules are made by a plasma process and therefore possess only a relatively low concentration of surface hydroxyl groups.
2.4.2 Chemical stability The pore structure of silica is very sensitive to treatment with aqueous solutions. Although some thorough investigations have been made, up to now the results could not be generalized. The reason is that a series of processes takes place when silica is soaked in aqueous solutions. Firstly, the condensation reaction of surface hydroxyl groups proceeds, leading to the growth of larger silica globules at the expense of smaller ones. The rate of condensation depends mainly on the pH, which also affects the dissolution and redeposition of silica, possibly changing the globule dimensions. With respect t o the rate of dissolution and condensation, one has to take into account that the silica surface bears weak acidic groups that are capable of ion-exchange interactions. The main factors that influence the pore structure are (1) the pH of the aqueous solution, (2) the electrolyte concentration and (3) the temperature. The most sensitive pore structure parameter has been found t o be the specific surface area, whereas the specific pore volume remains nearly constant. As a result, the mean pore diameter, D,increases. Treatment of porous silica samples in buffered aqueous solutions revealed that the specific surface area remains unaffected at about pH 2, but decreases markedly at higher
53
and lower pH values. At pH >9.0 the samples dissolve, forming silicates [74]. As stated by Okkerse [64], the electrolyte concentration has a smaller effect than pH on the variation of the specific surface area. Increasing the temperature increases the solubility and the rate of condensation. It has also been found that ionic impurities, such as metal ions, drastically influence the solubility and consequently the stability of the products. Thus such silica samples cannot be purified by boiling them with mineral acids. Summarizing the results, one can conclude that a prolonged treatment of silica with aqueous solutions decreases the specific surface area, the effect being more pronounced with samples that have a high specific surface area @BET >200 m2/g).
2.5 REFERENCES 1 E.A. Flood, The Solid-Gas Interface, Vols. 1 and 2, Marcel Dekker, New York, 1967. 2 SJ. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967. 3 H.Witzmann, in M.M. Dubinin (Editor), Methoden der Strukturuntersuchung an hochdispersen und porasen Stoffen (translated from Russian), Akademie Verlag, Berlin, 1961. 4 D.H. Everett and F.S. Stone (Editors), The Structure and Properties of Porous Materials, Proceedings of the Tenth Symposium of the Colston Research Society, Butterworths, London, 1958. 5 D.H. Everett and R.H. Ottewill (Editors), Proceedings of the IUPACInternationaISymposiumon Surface Arm Determinations, Butterworths, London, 1970. 6 B.G. Linsen, Physical and Chemical Aspects of Adsorbents and Catalysts,Academic Press, London, 1970. 7 S. Modry and M. Svata (Editors), Proceedings of the HilemlIUPAC International Symposium on Pore Structure and Properties of Materials, Vols. I-VI, Academia, Prague, 1974. 8 IUPAC, Manual of Symbols and Terminology, Appendix 2, Part I, Colloid and Surface Chemistry, Pure Appl. Chem., 31 (1972) 578. 9 M.M. Dubinin, J. Colloid Interface Sci., 23 (1967) 487. 10 J.H. De Boer, in D.H. Everett and F.S. Stone (Editors), The Structure and Properties o f Porous
11 12
13 14
15 16
17 18 19 20
Materials. Proceedings of the Tenth Symposium o f the Colston Research Society, Butterworths, London, 1958. S. Brunauer, RSh. Mikhail and E.E. Bodor, J. Colloid Interface Sci., 24 (1967) 45 1. D.H. Everett, in D.H. Everett and F.S. Stone (Editors), The Structure and Properties ofPorous Materials, Proceedings of the Tenth Symposium o f the Colston Research Society, Buttenvorths, London, 1958. B. Scharf, Thesis, Technical University, Darmstadt, G.F.R., 1976. M.M. Dubinin, Chem. Rev., 2 (1960) 235. M.G. Kaganer, Zh. Fiz. Khim., 33 (1959) 2202. S. Brunauer, J. Skalny and J. Odler, in S. Modry and M. Svata (Editors), Proceedings o f the Rilem/ IUPACInternatWnal Symposium on Pore Structure and Properties o f Materials, Vol. I , Academia, Prague, 1974, C-3. A.V. Kiselev, Usp. Khim., 14 (1945) 367. S.P. Wolsky and E J . Zdamek (Editors), Lntra Micro Weight Determination in Controlled Environments, Interscience, New York, 1969. L.D. Belyakova, A.V. Kiselev and N.V. Kovaleva, Anal Chem., 36 (1964) 1517. A.V. Kiselev, YuS. Nikitin, R.S. Petrova, K.D. Shcherbakova and Ya.J. Yashin, Anal. Chem., 36
(1964) 1526. 21 E. Cremer,Angew. Chem., 73 (1961) 461. 22 K. Huber, Thesis, University of Innsbruck, Austria, 1960. 23 S.J.Gregg and R. Stock, in D.H.Desty (Editor), Gas Chromatography,Butterworths, London, 1958.
54 24 F.M. Nelson and F.T. Eggertsen, Anal. Chern., 30 (1958) 1387. 25 J.H. M o l t e n , in R.L. Bond (Editor), Porous Carbon Solids, Academic Press, London, 1967, p. 225. 26 K. Unger, E. Schadow and H. Fischer, Z. Phys. Chem., N.F., 99 (1976) 245. 27 N.E. Fisher and A.Y. Mottlau, Anal. Chem., 34 (1962).714. 28 G.W. Sears, Jr.,Anal. Chem., 28 (1956) 1981. 29 D.H. Spencer, in R.L. Bond (Editor), Porous Carbon Solids,Academic Press, London, 1967, p. 86. 30 J.H. De Boer, in D.H. Everett and R.H. OttewiIl (Editors), Proceedings of the IUPAClnternatwnal Symposium on Surface Area Determination, Buttenvorthq London, 1970, p. 6. 31 R.A. Armstrong and J.P. Hobson,J. Phys. Chem., 67 (1963) 2000. 32 P.A. Gottwald, in D.H. Everett and R.H. Ottewill (Editors), Proceedings o f the IUPAClnternational Symposium on Surface Arm Determination, Buttenvorths, London, 1970, p. 59. 33 B.C. Lippens and J.H.De Boer,J. Catal., 4 (1965) 319. 34 J.H. De Boer, B.G. Lmsen and ThJ. Osinga,J. Catal., 4 (1965) 643. 35 M.R. Bambani, P.A. Cutting, K.S.W. Sing and D.H. Turk,J. Colloid Interface Sci., 38 (1972) 109. 36 K.S.W. Sing,in D.H. Everett and R.H. Ottewill (Editors), Proceedings of the IUPACInternational Symposium on Surface Area Determination, Butterworths, London, 1970, p. 25. 37 F.S. Baker and K.S.W. Sing, J. ColloidInterfaceSci., 55 (1976) 605. 38 A. Meffert and A. Langenfeldt,Z. Anal. Chem., 249 (1970) 231. 39 J.H. De Boer, in D.H. Everett and F.S. Stone (Editors), The Structure and Properties of Porous Materials, Buttenvorths, London, 1958, p. 68. 40 S. Brunauer, R.Sh. Mikhail and E.E. Bodor,J. Colloid Interfuce Sci., 24 (1967) 451. 41 R.Sh. Mikhail, S. Brunauer and E.E. Bodor,J. Colloid Interface Sci., 26 (1968) 45. 42 A. Wheeler, in P.H. Emmet (Editor), Gztalysis, Vol. 2, Reinhold, New York, 1955, p. 116. 43 A.E. Scheidegger, in S. Modry and M. Svata (Editors), Proceedings of the Rilern/lUPACInternational Symposium on Pore Structure and Properties of Materials, Vol. 111; Academia, Prague, 1974, A-3. 44 A.P. Karnaukhov, in S. Modry and M. Svata (Editors),Proceedings of the RilemlIUPAC International Symposium on Pore Structure and Properties of Materials, Vol. 1, Academia, Prague, 1974, A-3. 45 S.P. Zhdanov, Dokl. Akad. Nauk SSSR,82 (1952) 281. 46 K.H. Maier and E.A. Scheuermann, Kolloid Z., 171 (1960) 122. 47 B.G. Aristov, A.P. Karnaukhov and A.V. Kiselev, Russ. J. Phys. Chem., 36 (1962) 1159. 48 B.G. Aristov, V.Ya. Davydov, A.P. Karnaukhov and A.V. Kiselev, Russ. J. Phys. Chem., 36 (1962) 1497. 49 M.E. Van Kreveld and N. Van Den Hoed, J. Chromatogr., 8 3 (1973) 111. 50 H.L. Weissberg,J. Appl. Phys., 34 (1963) 2636. 5 1 W.C. Strieder and S. Prager, Phys. Fluids, 11 (1968) 2544. 52 W.C. Strieder,J. Chem. Phys., 54 (1971) 4050. 53 W. Haller,J. Chem. Phys., 42 (1965) 686. 54 P. Hugo, Ber. Bunsenges. Phys. Chem., 79 (1975) 149. 55 K.S.W. Sing, ColloquesInternationaux du CN.R.S., Termochimie, No. 201 (1972) 601. 56 K. Unger,Ber. Bunsenges. Phys. Chem., 79 (1975) 739. 57 K.S.W. Sing, in S. Modry and M. Svata (Editors),Proceedingsof the R ~ e m ~ i U P A C i ~ e r n a t w ~ l Symposium on Pore Structureand Properties of Materials, Vol. 111, Academia, Prague, 1974, B-5. 58 A.V. Kiselev, in D.H. Everett and F.S. Stone (Editors), The Structure and Properties o f Porous Materials, Buttenvorths, London, 1958, p. 195. 59 K. Unger, J. Schick-Kalb and B. Straube, J. Polym. CollokiSci., 253 (1974) 658. 60 R.K. Iler, in E. Matijkvic'(Editor), Surface and CoZloid Science, Vol. 6, Wiley, London, 1973. 61 P.C. Carman, Trans. Faraday SOC.,36 (1940) 964. 62 H. Baumann, Kolloid-Z., 162 (1959) 28. 63 G.B. Alexander, J. Amer. Chem. SOC.,76 (1954) 2094. 64 C. Okkerse, in B.G. Linsen (Editor), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, London, 1970, p. 213. 65 G.H. Bolt,J. Phys. Chem., 61 (1957) 1166.
55
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R J . Sippel, U.S.Pa?., No. 3,440,176 (1959). I.E. Neimark and R.Yu. Sheinfain, Kolloidn. Zh., 15 (1953) 145. I.E. Neimark, M.A. Piontkovskaya and J.B. Slinjakova, Kolloidn. Zh., 18 (1956) 61. I.E. Neimark and J.B. Slinjakova, Kolloidn. Zh., 18 (1956) 219. I.E. Neimark, R.Yu. Sheinfain, N.S. Kruglikova and O.P. Stas, Kolloidn. Zh., 26 (1964) 595. R.Yu. Sheinfain, B.A. Lipkind,O.P. Stas and I.E. Neimark, Kolloidn. Zh.,26 (1964) 734. S.A. Mitchell, Chem. Ind. (London), (1966) 924. O.P. Stas, R.Yu. Sheinfain and J.E. Neimark, Kolloidn. Zh., 32 (1970) 104. R.Yu. Sheinfain and I.E. Neimark, Kine?. Karal, 8 (1967) 433. R.Yu. Sheinfain, N.S. Kruglikova, O.P. Stas and I.E. Neimark, Kolloidn. Zh., 25 (1963) 732. 1.E. Neimark and R.Yu. Sheinfain, Kolloidn. Zh., 15 (1953) 145. F. Wolf and H. Beyer, Kolloid-Z., 165 (1959) 151. K.S.W. Sing and J.D. Madely,J. AppI. Chem., 3 (1953) 549. K.S.W. Sing and J.D. Madely,J. Appl. Chem., 4 (1954) 365. N.V. Akshinskaya, V.E. Beznogova, A.V. Kiselev and Yu.S. Nikitin, Russ. J. Phys. Chem., 36 (1962) 2277. N.V. Akshinskaya, A.V. Kiselev and Yu.S. Nikitin,Russ. J. Phys. Chew., 37 (1963) 927. A.V. Kiselev, YuS. Nikitin and E.B. Oganesyan, Kolloidn. Zh., 31 (1969) 525. A.V. Kiselev, V.M. Lukyanovich, YuS. Nikitin, E.B. Oganesyan and A.J. Sarakhov, Kolloidn. Zh., 31 (1969) 388. N.V. Akshinskaya, A.V. Kiselev and Yu.S. Nikitin,Russ. J. Phys. Chem., 37 (1963) 491. N.V. Akshinskaya, V.Ya. Davydov, L.T. Zhuravlev,G. Curthoys, A.V. Kiselev, B.V. Kuznetsov, YuS. Nikitin and V.V. Rybina, Kolloidn. Zh., 26 (1964) 529. V.M. Chertov, D.B. Dzhambaeva and I.E. Neimark, Kolloidn Zh., 27 (1965) 279. A.V. Kiselev, Yu.S. Nikitin, A J . Sarakhov and E.B. Oganesyan, Kolloidn. Zh., 30 (1968) 842. M. Le Page, R. Beau and J . Duchene, Fr. Pa?., No. 1,473,240 (1967). M. Le Page and A. d e Vries, Fr. Pa?., No. 1,475,929 (1967). K. Unger and J. Schick-Kalb, Ger. Pat., No. 2,155,281 (1971). K. Unger and B. Scharf,J. Colloid InterfaceSci., 55 (1976) 377. K. Unger and R. Kern,J. Chroma?ogr., 122 (1976) 345. JJ.Kirkiand,Ger. Pa?., No. 2,317,455 (1973). J.J. Kirkland, U.S.Pa?., No. 3,782,075 (1974). R.K. Iler and H.J. McQeston, US.Pa?., No. 3,855,172 (1974). A.V. Kiselev, G.L. Kustowa, B.A. Lipkind and J.S. Nikitin, Ger. Pa?., No. 2,225,452 (1972). K.F. Krebs and H. Heinz, Ger. Pat., No. 2,042,910 (1970). K.S.W. Sing, in G.D. Parfitt and K.S.W. Sing (Editors), Characterization o f Powder Surfaces, Academic Press, London, 1976, p. 3. D. Barry, in G.D. Parfitt and K.S.W. Sing (Editors), Characterization o f Powder Surfaces, Academic Press, London, 1976, pp. 353-425.
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51
Chapter 3
Surface chemistry of porous silica As the specific retention behaviour of solutes in adsorption chromatography at a given eluent composition is governed primarily by the chemical nature of the silica support, this chapter is devoted to the surface chemistry of porous silica. As an introduction, Section 3.1 deals with the surface structure of silica, summarizing the theoretical approaches and experimental techniques that allow one to identify and t o characterize the diverse surface species. It also includes a survey of the substantial features in adsorption interaction between the surface groups and various adsorbates. Section 3.2 provides a fundamental basis for understanding the surface reactions that are an effective means of preparing chemically bonded silica packings. Emphasis has been placed on different modes of chemical reactions at the silica surface from a chemical and physico-chemical point of view and on the synthesis and characteristic properties of surface-modified packings. Section 3.3 considers the acidity of the silica surface and ion-exchange phenomena, as porous silica alone is a weakly acidic cation exchanger and considerable efforts have been made to prepare silica-based ion exchangers by means of surface modification.
3.1 THE SURFACE STRUCTURE OF SILICA As already indicated in Chapter 1, the surface of hydrated silica is covered with hydroxyl groups that are attached in various ways to silicon atoms. The hydroxyl groups at the surface are involved in a temperature-dependent dehydroxylation-hydroxylation reaction written as 2 ESi-OH
+ ESi-O-Si=
t H20
(3.1)
Dehydroxylation usually starts at temperatures above 500 K, forming siloxane groups as surface sites which are less polar than hydroxyl groups. On the other hand, it must be emphasized that a hydrated silica surface, on being exposed to sufficiently high pressure of water vapour, is capable of taking up water by means of physical adsorption and capillary condensation. Thereby, the amount of physisorbed water is a function of temperature, water vapour pressure and specific surface area of the silica sample. The first fundamental study of the adsorption of water on quartz and fully hydroxylated porous silicas, the latter differing in their mean pore diameters, was made by Kiselev [ 11. The isotherms measured at room temperature were plotted in a reduced form, i.e., the amount adsorbed in moles of water per unit surface area of the sample versus the relative pressure, p / p o , of water. As the reduced isotherms of all samples follow nearly the same course in the low-pressure range 0.01 < p / p o < 0.2, Kiselev argued that (i) the samples under investigation exhibit the same adsorption behaviour towards water and (ii) the pore diameter does not affect to a great extent the amount adsorbed in the multilayer region (0.01 < p / p o < 0.2). At higher relative pressures, however, the shape of isotherm is highly dependent on the
58
mean pore diameter of the sample, as was previously indicated in Fig. 2.4 for nitrogen as adsorbate. In the presence of mesopores, for instance, a typical hysteresis is obtained between the adsorption and desorption branch of the water isotherm, which is reversible provided that the outgassing temperature of the sample does riot exceed 373 K [2,3]. When the sample is outgassed at temperatures above 673 K, it is observed that on desorption the course of isotherm does not approach the adsorption branch at small relative pressures and intersects the ordinate at a positive value at p/po = 0. This specific behaviour can be explained by dehydroxylation of hydroxyl groups taking place at the high outgassing temperature in the pre-treatment according to eqn. 3.1. The so-formed siloxane groups take up a certain amount of water vapour during the adsorption step to rehydroxylate. The amount of apparent adsorbed water at p / p = 0 then corresponds to the amount of chemisorbed water. On annealing the silica above 673 K, dehydroxylation of the silica surface increases. The increasing substitution of polar hydroxyl groups by less polar siloxane groups leads to a weakening in the strength of silica-water interactions, which is evidenced by a change of the course of the isotherm from a convex to a concave form [ 11. With respect to the chromatographic utility of porous silica as an adsorbent in column liquid chromatography, highly dehydroxylated samples are not suitable as packings because the siloxane groups present give rise only to non-specific interactions with the solutes, which results in a poor selectivity. Further, in the use of this type of packing water has to be totally excluded from the system in order to avoid rehydroxylation and to maintain reproducible conditions. In comparison with siloxane groups, the hydroxyl groups are considered to be the more decisive surface groups as they participate most effectively in adsorption by specific interactions. For instance, in non-polar eluents a fully hydroxylated silica adsorbent, being completely free from physically adsorbed water, exhibits its maximal surface activity, giving high retention values. By adding small amounts of water to the non-polar eluent, water is physically adsorbed to the surface hydroxyl groups by hydrogen bonding. As a consequence, the strength of interaction between hydroxyl groups and solutes decreases and the retention of solutes is reduced. Hence, by adjusting the water content of the eluent, the retention of solutes can be controlled over a wide range. The dependence of the capacity factor on the water content of the eluent is discussed in detail in Chapter 6. 3.1.1 Surface species
Considering the surface structure of silica, one can distinguish two main surface species [4], surface hydroxyl groups, which can be subdivided into different types according to their coordination to the silicon atoms (see Figs. 1.2 and 1.3), and siloxane groups, formed by dehydroxylation of hydroxyl groups. Additionally, one has to consider the amount of physically adsorbed water, which affects the strength of adsorption interaction between the hydroxyl groups and the adsorbate. 3.1.1.1 Surface hydroxyl groups and physically adsorbed water
With respect to water being physically adsorbed at the silica surface, the following two questions are of most interest:
59
(i) Is it possible to distinguish quantitatively between physically adsorbed water and surface hydroxyl groups? (ii) Are there defined conditions under which physically adsorbed water will be completely removed and the surface will bear only hydroxyl groups? To resolve the situation it is useful to investigate the behaviour of porous silica at annealing at higher temperatures. Thermogravimetry is often used for evaluating the water content. The weight loss monitored during annealing at a certain temperature is then assumed to be proportional to the amount of adsorbed water. This assumption, however, has to be carefully checked by means of a mass spectrometric analysis of the volatile product evolved. The total water content of silica, including physisorbed and chemisorbed water, then corresponds to the weight loss after subjecting the sample to a heat treatment at 1473 K under vacuum. By increasing the temperature successively, it can be expected that upon heating the sample above 373 K physisorbed water will be evolved first. At about 473 K hydroxyl groups may also start to condense to siloxane groups, yielding water. Condensation proceeds with increasing temperature until at 1473 K the concentration of hydroxyl groups will be vanishingly small. At a sufficiently high temperature hydroxyl groups that constitute the bulk phase of silica also begin to migrate and are finally dehydrated to siloxane groups, liberating water. Hence the total amount of water evolved on increasing the temperature from 373 to 1473 K will be the sum of three contributions: physically adsorbed water, surface structural water formed by condensation of surface hydroxyl groups and bulk water formed by condensation of internal hydroxyl groups. A clear distinction between physically adsorbed water and hydroxyl groups at a given temperature seems to be possible by applying infrared spectroscopy combined with thermogravimetry and other related techniques. The first thorough study in this direction was made by Fripiat and Uytterhoeven [5], utilizing infrared measurements at frequencies between 1000 and 4000 cm-' and thermogravimetry. Although they used Aerosil as a standard, the technique can be applied to finely divided porous species. According to Fripiat and Uytterhoeven [ 5 ] , the water content of the silica sample at a certain temperature is given by
N=X+2 Y
(3 .2)
where N i s the total amount of water expressed in moles of OH per gram of adsorbent, X the amount of hydroxyl groups in moles of OH per gram and Y the amount of physisorbed water in moles of H 2 0 per gram. N is found from thermogravimetric measurements and Y is calculated from the integrated intensities of the adsorption bands at 3400 and 1640 cm-', respectively. The band at 3400 cm-' is assigned to the vibration of physically adsorbed molecular water, whereas liquid water exhibits two vibrations, at 3445 and 3219 cm-' [6]. The band at 1640 cm-' corresponds to the bending vibration of adsorbed water. It has been found that both bands increase in intensity on addition of water to a porous silica sample and decrease in intensity on increasing the temperature [6,7]. The results of Fripiat and Uytterhoeven [5] indicate that the amount of physisorbed water estimated according to eqn. 3.2 decreases sharply on annealing the sample above 373 K (see Table 3.1). Above 5 13 K, the amount of physisorbed water becomes negligibly
60
small compared with that of hydroxyl groups. On the other hand, the content of hydroxyl groups has been shown to be scarcely affected by temperature in the range 293-5 13 K. The surface concentration of hydroxyl groups at 513 K was evaluated as 16.3 pmole/mZ. This relatively high value, however, accounts for both internal and surface hydroxyl groups. The amount of surface hydroxyl groups alone was calculated to be 6.7 pmole/mZ at 513 K
PI.
A second approach for evaluating the amount of physisorbed water utilizing infrared spectroscopy in the high-frequency range between 4000 and 10,000 cm-' was developed by Wirzing [8], making use of the combination band of water at 5265 cm-' . Employing a specially designed cell he was able to monitor spectroscopically the absence of physisorbed water and also different water contents of porous silica species. By varying the outgassing temperature, the plot of the integrated absorbance at 5265 cm-' of the sample investigated against its water content, w ,expressed in moles of HzOper mole of SiOz independently measured by the weight loss after annealing, gave a nearly straight line, intersecting the abscissa at a certain value w*.Wirzing concluded that this amount of water corresponds to chemically bonded water, i.e., hydroxyl groups. Later, Erkelens and Linsen [9] combined the method of ignition and infrared spectroscopy of the 5265 cm-' band for the evaluation of physisorbed water and hydroxyl groups. One of their conclusions was that pre-treatment of silica at 293 K under vacuum is sufficient to remove physisorbed water from the surface. The hydroxyl group concentrations of two samples at 293 K were estimated to be 13.0 and 35.0 pmole/mz, respectively. Again, these high values reflect the concentration of both internal and surface hydroxyl groups. A different method for estimating the amount of physisorbed water, based on the well known Karl Fischer titration procedure, was proposed by No11 et al. [lo]. By titrating monomeric organosilanols in the presence of known amounts of water, No11 et al. found that the reaction between water and Karl Fischer reagent (KFR) is much faster than that between the corresponding organosilanol and KFR. A similar behaviour was established in the titration with KFR of porous silica suspended in methanol. Whereas adsorbed water reacts rapidly with KFR, the surface hydroxyl groups undergo a very slow reaction. TABLE 3.1 AMOUNT OF PHYSISORBED WATER, Y,AND AMOUNT OF HYDROXYL GROUPS, X , ON AEROSIL (SBET = 180 m'/g> AS A FUNCTION OF THE PRE-TREATMENT TEMPERATURE (ACCORDING TO FRIPIAT AND UYTTERHOEVEN [S]) Pre-treatment temperature (K)
293 333 313 413 413 513
Y (mole H,O/g)
X (mole OH/g)
Under vacuum
At atmospheric pressure
Under vacuum
At atmospheric pressure
0.42 0.30 0.23 0.17 0.06 0.03
1.56 0.94 0.44 0.24 0.10
2.16 2.90 2.15 2.96 3.03 2.94
2.18 2.92 3.10 2.97 2.95
-
-
61
Considering the different kinetics, it becomes possible to correct the amount of KFR consumed in the titration due to physisorbed water. Additionally, the content of hydroxyl groups can be estimated by subtracting the amount of physisorbed water from the total amount of water that was derived from the weight loss on ignition of the sample at 1473 K. The results of No11 et al. [ 101 indicate that the concentration of adsorbed water decreases considerably above 473 K but at 573 K a small amount of physisorbed water could still be detected. In the titration of porous silica with KFR, a side-reaction may occur between the hydroxyl groups and methanol, which is one of the constituents of KFR, according to ESi-OH t CH30H + Si-OCH3 + HZO
(3.3)
In studying this reaction, Kellum and Smith [ 111 concluded that siloxane groups present in partially dehydroxylated silica samples are also able to react with methanol: ZSi-O-SiE
t CH@H ;= S - O H
+ CH30-Si=
(3.4)
The methoxylation of hydroxyl groups and the cleavage of siloxane groups can be drastically reduced by replacing methanol with a high-molecular-weight alcohol in KFR. Summarizing the results of the various studies, one can conclude that physisorbed water can be distinguished fairly well from hydroxyl groups. However, the temperature at which physisorbed water is completely removed and hydroxyl groups remain unreacted seems not to be well established and varies over a wide range between 373 and 623 K [5,8-10,12,13]. Obviously, this temperature cannot be expected to be constant for all porous and non-porous silica species, because it undoubtedly depends on the origin and history of the sample as well as on its mean pore diameter. For instance, physisorbed water will be held much more strongly within micropores than in meso- and macropores.
3.1.1.2 Internal and surface hydroxyl groups The next problem that arises is to distinguish between internal and surface hydroxyl groups. The existence of internal hydroxyl groups could be demonstrated by means of infrared spectroscopy combined with isotopic exchange using DzO vapour [ 14,151. According to Davydov and Kiselev [ 141, Aerosil in a specially designed I R cuvette was treated, after prolonged evacuation at 473 K, with D 2 0 vapour at room temperature. When deuteration of surface hydroxyl groups: -Si-OH
+ DzO =+S i - O D
t DOH
(3 5)
had occurred, the sample was again evacuated at 473 K. Infrared spectrograms were measured before and after deuteration. After deuteration, the band at 3750 cm-I virtually disappeared and a new narrow band at 2760 cm-' was formed, which may be assigned to the stretching vibration of free OD groups. The intensity of the broad band between 3000 and 3750 cm-' due to vicinal or bound hydroxyl groups diminished only after isotopic exchange. Simultaneously, a broad band due to bound OD groups appeared between 2200 and 2670 cm-'. After the reverse exchange according to Si-OD
+ HOH + S i - O H + DOH
(3.6)
the original spectrum was obtained again. These results confirm the assumption that a
62
proportion of the hydroxyl groups appears not to be accessible to deuteration. The hydroxyl groups termed internal must be located in the bulk of silica, i.e., in the interior of the non-porous silica globules of Aerosil. Their absorption band was assumed to lie at 3660 cm-' . Kiselev and co-workers [ 161 also devised a reliable technique for measuring quantitatively the proportion of surface to internal hydroxyl groups after heat treatment under vacuum. Three basic assumptions were made: (i) physically adsorbed water is completely removed at a temperature above 433 K under vacuum; (ii) only surface hydroxyl groups can undergo an isotopic exchange into OD groups; (iii) isotopic effects in the D20 exchange were neglected, Le., the isotopic equilibrium constant of reaction in eqn. 3.5 was assumed to be unity. The sample, subjected to prolonged annealing at a given temperature above 433 K under vacuum, was exposed to heavy water vapour at 433 K for a certain period necessary for reaching the isotopic exchange equilibrium. Then H20,HDO and D20 in the vapour phase were converted into H2, HD and D2, respectively, and the isotopic composition was determined mass spectrometrically. From the results the concentration of surface was determined. It should be noted that the reverse exchange hydroxyl groups, CIOH(~), gave identical results with those of the forward exchange (see eqns. 3.5 and 3.6). The total water content, a!OH(t), due to surface and internal hydroxyl groups was determined by the total loss of water on annealing from 473 to 1473 K. The difference q ) H ( t ) - CIOH(~) then corresponds to the amount of internal hydroxyl groups, &OH(i). Applying this technique, Zhuravlev and Kiselev [ 171 investigated a series of silica samples that differed widely in origin and porosity. From the results, they claimed that appears to be essentially independent the mean surface hydroxyl concentration, cuo~(~), of the specific surface area of the sample and varies between 7.0 and 9.5 pmole/m2 after prolonged evacuation at 473 K. A different behaviour was observed for the internal hydroxyl groups: aOH(i) decreased roughly exponentially with increasing SBETof the sample. This result can be explained theoretically by a model of silica as an assembly of non-porous globules having a fully hydroxylated surface and being internally saturated with hydroxyl groups. 3.1.1.3 S p e s of surface hydroxylgroups It is commonly accepted that several types of hydroxyl groups, differing in reactivity, exist at the surface of porous silica. Two basic approaches have emerged, derived from various experimental investigations. 3.1.1.3.1 Free and bound hydroxyl groups The Kiselev school [ 131 postulates two distinct types, namely free and bound hydroxyl groups. A major criterion of these two types is the relative distance between adjacent free and bound groups, respectively. Adjacent free hydroxyl groups are assumed to be separated by a distance considerably more than 0.31 nm between the oxygen atoms. Then, they are incapable of forming hydrogen bonds and give rise to a sharp absorption band at 3750 cm-' .Bound hydroxyl groups exhibit a do-0 distance less than 0.31 nm
63
and hence can interact via hydrogen bonding [ 181. As a result of the surface heterogeneity, one can expect the distance between bound hydroxyl groups to vary over a range between 0.24 and 0.31 nm. Consequently, this leads to a wide variation in the strength of O-H...O interactions and gives rise to a broad absorption band below 3750 cm-' . On the assumption that on vacuum treatment up to about 673 K only bound hydroxyl groups are removed from the surface, the concentration of the remaining free hydroxyl groups can be determined. Davydov et al. [ 131 reported a mean concentration of free hydroxyl groups of 4.3 pmole/m2 and of bound hydroxyl groups of 3.7 pmolelm' for a fully hydroxylated Aerosil sample. With these values, the average cross-sectional areas, A , , of free and bound hydroxyl groups can be estimated as 0.33 and 0.068 nmZ/OH, respectively, The mean distance between two free hydroxyl groups can then be calculated to be 0.65 nm and that between two bound hydroxyl groups to be 0.295 nm. Adsorption and reaction studies made by Kiselev and co-workers suggest that selective adsorption and reaction occur primarily upon free hydroxyl groups [ 13,151.
3.1.1.3.2 Paired and isolated hydroxyt groups In contrast to the above, Peri and Hensley [ 191 assumed that a fully hydroxylated silica contains predominantly paired hydroxyl groups (geminal or vicinal) in addition to isolated hydroxyl groups. Such a type of surface can be derived theoretically by a model of random partial dehydration of a (100)-face of P-cristobalite. The total concentration of hydroxyl groups, N , then corresponds to N=NgtNv+Ns
(3.7)
where Ng is the surface concentration of geminal hydroxyl groups, N , that of vicinal hydroxyl groups and N, that of isolated hydroxyl groups. At N = 7.6 pmole/m2, the concentration of geminal hydroxyl groups is calculated to be 2.0 pmole/m2. At N = 2.0 pmole/mz, achieved by annealing the sample at a temperature above 873 K, the vicinal hydroxyl groups completely disappear and only geminal and single hydroxyl groups exist at the surface. The idea that the surface hydroxyl groups exist mainly as pairs was established experimentally from the results of surface reactions between porous silica and a series of reagents such as SiCI4, AlC13, TiC14, BC13 and C12Si(CH3)2 [ 19211. In 1966, Snyder and Ward [21] suggested that the silica surface bears a large number of adjacent pairs of strongly hydrogen-bonded surface hydroxyl groups. The paired hydroxyl groups were considered to be more reactive than the isolated groups and to play the dominant role in adsorption and reaction processes of silica. 3.1.1.4 Determination o f surface hydroxyl groups Various experimental techniques have been developed for estimating the mean surface hydroxyl group concentration, c ~ o H ( ~of ) , porous silicas. They can be generally grouped into chemical and physical methods.
64
3.1.1.4.1 chemical methods Chemical methods are generally based on the reaction of surface hydroxyl groups with a selectively reacting compound to form a covalently bonded surface species of well known composition. As reactive compounds diborane [22-25], boron trichloride [20,26], diazomethane [27,28], organochlorosilanes [ 15,19,20,21,29-311 and organometallic compounds [S] have been employed. OH(^. is then derived from the amount of the chemisorbed species as well as the amount of volatile reaction products. In such an approach, however, the following are necessary: (i) the stoichiometry of the reaction is known, and also the composition of surface bonded species and volatile products; (ii) all surface hydroxyl groups are accessible to react and no steric exclusion of the reactant molecules takes place due to the pore structure of the support; (iii) the reactants and reaction products that are physisorbed at the surface must be completely removed before surface analysis. These conditions are fully realized in only a few surface reactions. Three instances will be given here that should illustrate the care that is necessary in order to obtain valid results for CXOH(~). 3.1.1.4.1.1 Reaction between silica and diborane Reaction between silica and diborane, BzH6, consists in hydrolysis of the diborane to form hydrogen and a surface species that contains a *i-O-B= bond. The conversion is expressed by the hydrolysis ratio, R , which is the ratio of hydrogen evolved to diborane consumed. According to the reaction mechanism, R is assumed to be different for physisorbed water and for surface hydroxyl groups. Naccache and Imelik [24] suggested the following reaction scheme for physically adsorbed water:
A distinction between physisorbed water and surface hydroxyl groups should be possible on the basis of the measured value of R . In contrast to the results of Naccache and Imelik [24], Shapiro and Weiss [22] proposed R = 6 for physisorbed water and 2
65
groups of the silica surface. The siloxane groups thereby act as lewis-base surface sites: BH3 t B2H6 + 2 Gsi-&siz
2 ESi-O-Sie
(3.10)
This hypothesis was supported by two findings: (i) the induction time was reduced considerably by increasing the outgassing temperature during pre-treatment of the silica, which necessarily leads to an increase in the concentration of siloxane groups; / H\, =B 8= or to (ii) no adsorption bands due to either bridged structures such as '\H/ =B-OH groups could be monitored by means of infrared spectroscopy. With BH3 adsorbed, the following reaction schemes were proposed [32] : H\
H o , BY I - 0 -st
I
/H
0 ,' '~
H\o
-o-s,
I
BH3 -0-
I
H \,.o/H
BH3 -0--5
-
o/ H' 1 BH3 1-0 I
0 I
I 0
I
-0-So-0-St-0-
I
HB
I I
+
4H2
13.11)
+
3 H*
13.12)
,0-BH2
I
0
I -0-sI-o-
I
13.13)
BH2
I
0
/
0
I BH3 I -0-Si-0-StI
I
H
-
yH2
BHz
0
0
I
-0-Si-0-St-
I
I
I I
+
H2
(
3.14)
On silica surfaces bearing physisorbed water, reactions 3.1 1 and 3.12 should occur, giving R > 2 , while on thoroughly dehydrated surfaces R should be less than 2. These postulates were confirmed by the experimental results. Additionally, the existence of BH2 and BH groups postulated above could be established by means of infrared spectroscopy. Further, using porous silica samples with different mean pore diameters, it could be shown that the reaction rate was not diffusion controlled. On the basis of their results, Fripiat and Van Tongelen [32] concluded that the diborane reaction is not suitable for distinguishing precisely between surface hydroxyl groups and adsorbed water. 3.1.1.4.1.2 Reaction between silica and dimethyldichlorosilane The reaction between silica and dimethyldichlorosilane can also be termed hydrolysis. Both physisorbed water and surface hydroxyl groups may react to form dimethylsilyl alone adsorbed water should be removed completely bonded species. To measure CYOH(~)
66
before reaction by means of a pre-treatment at 473 K under vacuum. Reaction of dimethyldichlorosilane may then take place with both free and adjacent surface hydroxyl groups:
-
=Si-OH
+
CI-S-CH3
+
ZSi-0-Si-CH,
I
I
CI
\ -Si -OH / 0
-\51-OH /
+
HCI
(3.75)
CI
\
+
’
-5-0
CI \ Si /CH3 CL/ \CH3
-
0
>s,-o’ /
\Sl/CH3
f
2HCl
(3.16)
‘CH3
Reaction according to eqn. 3.15 is termed monofunctional and that according to eqn. 3.1 6 bifunctional. It might be expected that the dimethyldichlorosilane would react preferentially with adjacent or paired surface hydroxyl groups provided that they are separated by a distance of less than 0.3 nm [ 131. The data reported in the literature on the mechanism proposed, however, gave no consistent picture. Whereas both Hair and Hertl [29] and Evans and White [30] concluded that both reaction mechanisms take place, Davydov et al. [I31 and Armistead and Hockey [31] postulated a monofunctional reaction of dimethyldichlorosilane. Armistead and Hockey [31] estimated the extent of conversion by determining the chlorine content of the solid product after removal of the excess of gaseous reactant by evacuation. Evans and White [30] used gravimetric measurements while Hair and Hertl [29] also determined the chlorine content of the reaction product. From the data reported, it is apparent that an estimation of CQH(~) on the basis of the surface reaction with dimethyldichlorosilane will be very imprecise.
3.1.1.4.1.3 Reaction between silica and methyllithium The most reIiable estimate of OH(^) can probably be obtained by reaction with methyllithium (CH3Li). Methyllithium dissolved in diisopentyl ether reacts with hydroxyl groups and with water as follows: ESi-OH
+ LiCH,
-+ESi-OLi
HzO + LiCH3 + LiOH + CH4
+ CH4
(3.17) (3.18)
According to eqn. 3.17, the amount of methane evolved is a direct measure of the content of surface hydroxyl groups, provided that physically adsorbed water is absent. Therefore, the sample must be subjected to prolonged evacuation at 473 K before an excess of dissolved methyllithium is added. The methane produced is measured either with a volumetric device [33] or, after trapping, by gas chromatography [5,34]. In this way the surface hydroxyl group concentration, CWOH(~), can be determined, assuming that the ) stoichiometry of reaction in eqn. 3.17 is valid. Fig. 3.1 shows a plot of c ~ o H ( ~obtained by reaction with methyllithium versus the pre-treatment temperature under vacuum for an Aerosil and two porous silica samples [35,36]. For comparison, this function is also plotted for a porous sample, the CYOH(~) values of which were determined by isotopic exchange with HTO [37].
67
10
'
'
r
A--k
9
0 4.5
7 6
5
3.0
4
3 1.5
2 ' 0
! , , , I , , , ! ~
1 I
273
D
473
673
873
1073
1273
4T ( K )
Fig. 3.1. Dependence of surface hydroxyl concentration, (YOH(~),of various silicas on the pre-treatment temperature under vacuum. Silica samples: 0,Aerosil200 (Degussa, Hanau, G.F.R.), SBET= 228 m*/g; A, macropororous silica (home-made preparation), SBET= 35 m2/g, D = 120 nm; X, mesoporous silica I (home-made preparation),SBET= 330 m2/g,D = 20 nm; 0,mesoporous silica 11, SBET= 475 mz/g, D = 7 nm. Method of determination of (YOH(~):0,A , X , reaction with methyllithium [ 361 ;0,isotopic exchange with HTO [ 371.
The course of the curves is in agreement with those published by other workers [5,15, 381. Whereas for the three porous silica species below 473 K the curves intersect the = 9.0 1.O pmole/mZ,Aerosil shows a considerably lower value. ordinate at about CIOH(~) This discrepancy is due to the fact that Aerosil is produced by means of a plasma process and hence its surface is not fully hydroxylated but bears a relatively large amount of siloxane groups. The results again confirm the statement of Zhuravlev and Kiselev [ 171 that the value of CIOH(~)= 9.0 pmole/m* on annealing at 473 K represents the saturation value of a fully hydroxylated silica surface [ 15,17,38]. As mentioned before, an important feature in surface reaction studies is the accessibility of surface hydroxyl groups in porous samples. In the methyllithium reaction, steric effects can be estimated by taking the size of solvated molecules into account. Methyllithium consists of a tetrameric unit in solution with a diameter of about 0.7 nm. The tetramer again forms a tetraetherate complex with diisopentyl ether used as the solvent. The diameter of this solvated species can be estimated to be about 2.5 nm. In comparison with the pore openings of the silica, however, this means that the dissolved methyllithium can penetrate by diffusion only into pores that have a mean pore diameter (0) twice as great as the diameter of the etherate complex, i e . , D should be larger than 5.0 nm [34]. Indeed, it was found experimentally that the '(YoH(~)values obtained by the methyllithium method on porous silica with micropores and small mesopores are considerably lower than those
*
68
obtained by the isotopic exchange method using tritium-labelled water, where no steric exclusion should take place. To summarize, the reaction between porous silica and organometallic compounds permits the most precise measurement of LIOH(~)because of its well known stoichiometry and easy detection. It has also been found that the LIOH(~)values obtained in this way agree well with those obtained by physical methods [ 17,381. 3.1.1.4.2 Physical methods Of physical methods, infrared spectroscopy in the frequency range between 2000 and 4000 cm-' is the most commonly applied technique for monitoring and controlling the surface hydroxylation of silica. In addition, heterogeneous isotopic exchange using heavy water (DzO)vapour or tritium-labelled water (HTO) has been developed as a suitable technique for the quantitative determination of surface hydroxyl groups. 3.1.1.4.2.1 Infrared spectroscopy [15,39,40] Most of the infrared spectroscopic studies have been carried out on Aerosil samples. The result shown in Fig. 1.4 can generally be transposed without restrictions to porous silica species. This is illustrated by the infrared spectra of a sample of mesoporous silica at different dehydration temperatures (see Fig. 3.2) [37]. The spectra were run with a PerkinElmer Model 325 double-beam instrument fitted with a measurement and a reference cell, which could be evacuated to pressures below lo-* Pa and heated to about 673 K. Pellets of the sample, with a mean particle diameter of less than 5 pm were prepared by pressing about 8 mg of SiOz powder under high pressure in a steel die. The pellet was placed in the sample holder of the measurement cell, evacuated for 20 h at less than lo-' Pa and then heated to the desired temperature (at a rate of 2-3 K/min) while maintaining the vacuum. The reference cell was also evacuated to less than lo-' Pa. For pre-treatment temperatures above 673 K, the pellet was annealed separately in an oven at the desired temperature, transposed to the IR cell under a dry nitrogen atmosphere and then evacuated at 673 K. The spectra were recorded using a spectral slit width of 1.8 cm-' at 3700 cm-' and a scanning speed of 0.5 crnlrnin. Inspection of the first spectra in Fig. 3.2 shows a broad absorption band made up of three bands located at 3747,3680 and 3535 cm-'. The sharp band at 3747 cm-' can be identified as the stretching vibration of free or isolated hydroxyl groups that do not interact together. This absorption band frequency can be described satisfactorily by the model of a free anharmonic oscillator composed of oxygen and hydrogen atoms [ 151. Evidence for this approach is the observation of a weak band at 3850 cm-' ,which can be interpreted as the P branch due to the rotational structure of the free hydroxyl groups. The corresponding R branch due to rotation will not be detectable because it is overlapped by the broad absorption band of hydrogen-bonded hydroxyl groups at the lower frequency side. It should be noted that it is believed that the band at 3747 cm-' is also produced by the vibration of geminal hydroxyl groups [41] while Camara er al. [421 suggested 37 I0 cm-' as the specific absorption band of geminal hydroxyl groups. The band at 3747 cm-' still exists after annealing the porous sample at 1100 K, indicating the high thermal stability of the free hydroxyl groups. After vacuum treatment of the sample at 473 K, a broad shoulder is observed next to
69
the band at the lower frequency side. This broad band between 3750 and 3000 cm-' is attributed to the vibration of hydrogen-bonded hydroxyl groups at the surface and in the bulk phase. Two distinct bands can be recognized at about 3680 cm-' and 3535 cm-' within this band. The band at 3680 cm-' is believed to be due to the stretching vibration of internal hydroxyl groups and that at 3535 cm-' to hydrogen-bonded hydroxyl groups. The broadness of the bands is caused by the fact that on the one hand the vibration of internal hydroxyl groups is strongly perturbed and on the other hand bond lengths and bond angles in hydrogen coupling of surface hydroxyl groups differ over a wide range owing to surface heterogeneity. On increasing the pre-treatment temperature, the broad flank on the low-frequency side decreases, indicating that hydrogen-bonded or vicinal hydroxyl groups which strongly interact will condense first. Above 873 K the band completely disappears. From this qualitative treatment, it is apparent that the quantitative determination of surface hydroxyl groups requires extensive sample preparation and carefully selected conditions. The basis for quantitative measurements in spectroscopy is given by the Lambert-Bouguer law: I = Zo exp -E*c-d
(3.19)
where I . is the incident intensity, I the transmitted intensity, E the extinction coefficient and d the path length. In practice, the spectrum is recorded in terms of the transmittance, T, which is given by T=I/Io=exp -e*c.d
(3.20)
The absorbance, A , is related to T by
A = In( 1 / T ) = ln(fo/Z)
(3.2 1)
A is directly p1,oportional to the concentration of the substance in the path of the radiation: A = ecd
(3.22)
which is the well known Lambert-Beer law, where E is the molar extinction coefficient (usually in litres per centimetre per mole) and c is the concentration (in moles per litre). For the integrated intensity Z of any considered band, one can write (3.23) Before quantitative evaluations of surface groups can be made, the value of E on the maximum of the adsorption band has to be known. Fripiat and Uytterhoeven [5] and later Borello et al. [43] postulated an approximation procedure for calculating E by combining the results of infrared and weight-loss measurements. It is noteworthy in this context that the extinction coefficient should be independent of the sample size but varies with the wavelength and the temperature. The absorbance, A , according to eqn. 3.23, is obtained by graphical integration of the peak area of the given band within the limits v l and u2. Again, the accuracy in the determination of A is rather poor because of background
70 a)
f
3530 -40 cm-’
3747 cm-’
ff t
‘I
3665 cm’
3675 c 6 ’
3747 cm-’
3747
Ern’
Fig. 3.2. Infrared spectra of a porous silica sample (SBET= 475 mz/g, D = 7 nm,d p < 5 pm) obtained after evacuation at different temperatures [37]. Conditions, see text. Temperature: (a) 473 K; (b) 673 K; ( c ) 773 K; (d) 873 K; (e) 973 K.
absorption and scattering effects. In order to minimize the radiation loss by scattering, particles in the 1-pm size range should be favourably employed [39]. Further, in order to enhance the resolution in transmission measurements, thin transparent discs of finely divided material without any dispersion medium should be utilized. from For the reasons outlined above, the reproducibility of the evaluation of CYOH(~) infrared measurements is not better than ?5-10% [43]. Further, such an evaluation makes sense only for samples ignited above 873 K, which gives rise to a sharp absorption band at 3747 cm-’. At lower temperatures the calculation is highly imprecise owing to the presence of the broad band between 3700 and 3000 cm-’. In order to prove the determination between infrared measurements and other correlation of the CYOH(~) techniques such as isotopic exchange in a more qualitative way, Fig. 3.3 shows a plot of CYOH(~) obtained from isotopic exchange using tritium-labelled water versus the total integrated intensity, I , of the absorption bands between 3750 and 3000 cm-’ of the porous silica taken from Fig. 3.2. Each point in Fig. 3.3 refers to a certain outgassing temperature of the sample. It can be seen that a fairly linear correlation results. 3.1.1.4.2.2 Isotopic exchange with D20 In deuteration, the protons of the surface hydroxyl groups of silica are replaced by deuterium atoms according to the equation
S - O H t DzO + 5%-OD t HDO
(3.24)
By subjecting the outgassed sample to an excess of D 2 0 vapour at 423 K, the exchange equilibrium is readily attained. D2O is preferably used rather than Dz because its exchange rate was found to be very fast while Dz reacts relatively slowly even at 523 K [41]. The isotopic composition of the vapour phase at equilibrium is determined mass spectrometri-
71
5
0
10
15
---- integrated intensity (cm2/m2S102)
Fig. 3.3. Dependence of CYOH(~) evaluated by means of the isotopic exchange method with HTO on the integrated intensity, I , of the adsorption bands between 3750 and 3000 cm-’ [ 371. I was measured by graphical integration using the tangent method from spectra in Fig. 3.2. Silica sample: SRET= 475 m2/g, D = 7 nm, d p < 5 urn. Pre-treatment temperature under vacuum: A, 473 K ; 0 , 5 2 3 K; X , 573 K; 0 , 6 7 3 K ; A, 773 K ; =, 873 K ; 0,973 K (the spectra at 523 and 573 K are not included in Fig. 3.2).
cally after converting the water species into D2, H2 and HD. The procedure was described in detail by Zhuravlev er al. [ 161. An interesting alternative was developed by Boehm and Sappok [44], who combined gravimetric sorption measurements using H 2 0 and D 2 0 with the isotopic exchange of these species. The concentration of surface hydroxyl groups being deuterated was determined from the weight change after isotopic exchange. In all papers published on deuteration, the calculation of (YOH(~)was based on the assumption that all hydroxyl groups were deuterated, i.e., the isotopic equilibrium constant, Ki,was assumed to be unity. Making use of the law of mass action, Kj for the isotopic exchange reaction is defined as
K 1. =
[Si-OD]
[=Si-OH]
[HDO] *
[D20]
(3.25)
where the brackets represent the molar concentration of the species concerned. Because of the large differences in mass between the isotopes H and D, however, the isotopic effect cannot be neglected. As shown in a survey of a series of homogeneous exchange reactions, involving D2, H2, D 2 0 and H 2 0 , the heavier isotope D has the tendency to be enriched mostly in the condensed phase, i.e., Kj is larger than unity [45,46]. A second critical comment which should be made refers to the observation of Zhuravlev er al. [16] that in D 2 0 exchange the H/D isotopic composition of the vapour phase, being in equilibrium with the deuterated surface, does not remain constant but has been found to be a function of the outgassing temperature of the sample, Surprisingly, H/D reaches a maximum at temperatures between 773 and 973 K. The drastic change in the H/D ratio in this temperature range is assumed to be due to the liberation of bulk water caused by the condensation of internal hydroxyl groups. In conclusion, the isotopic exchange with D 2 0 vapour for determining c ~ o H ( ~can ) only be considered to be a relative method. It should be added that isotopic exchange with D 2 0 can be usefully combined with
72
infrared spectroscopy. Owing to the large differences in mass between the isotopes H and D, the deuterated samples produce absorption bands in the region between 2000 and 2760 cm-I. For instance, the valence vibration of free hydroxyl groups at 3747 cm-' is replaced by the valence vibration of free OD groups at 2761 cm-' . A particular advantage of deuterated over protonated samples is that the transmission becomes greater at lower frequencies.
3.1.1.4.2.3 Isotopic exchange with HTO Tritium-labelled water is preferably employed instead of heavy water in isotopic exchange reaction studies on porous silica because the radioactive tracer provides a simple means of analysis. When the silica, outgassed at 473 K under vacuum, is exposed to tritiumlabelled water the following exchange reaction takes place at the surface: S i - O H t HTO + ZSi-OT + HzO
(3.26)
According to eqn. 3.26, highly diluted tritiated water is utilized, qssuming that a water molecule consists of one hydrogen and one tritium atom as in the formula HTO. In order to achieve a complete exchange of surface hydroxyl groups into OT groups, an excess of HTO has to be employed. With respect to a high exchange rate, it is also necessary to equilibrate at high temperatures. Another important point in the operation is to separate the reactants after equilibrium has been established and to measure the distribution of the label. The easiest way is to condense the total vapour phase in a cold trap, with separation from the silica. On the other hand, the tritiated silica sample, freed from physisorbed water, can be dissolved in sodium hydroxide solution and thus prepared for liquid scintillation counting too. The amount of tritium atoms in the vapour phase, U H T O ( ~ ) , or at the surface, U H T O ( ~ )under , equilibrium is given by
wro(v)= OHT TO *$/go
(3.27)
and
OHT TO*^"/^^
~ H T O (= ~)
(3.28)
where aoHTO is the initial amount of tritium-labelled water, expressed in moles of tritium atoms, g o its activity, 9' the activity of the condensed vapour HTO and HzO and3" the activity of the S C O T groups in the dissolved silica. It should be checked that the sum of the activities 9' and 9"is equal to%:
9'tg" = g o
(3.29)
otherwise the exchange does not take place exclusively at the silica surface. U H T O ( ~can ) also be determined by use of the relationship ~HTO(= ~)
OHT TO - OHTOW
(3.30)
The surface concentration, c ~ o H ( ~expressed ), in moles per square metre, is obtained by ) the sample weight and by the specific surface area of the sample. dividing U H T O ( ~by As already discussed, the larger the difference in mass between the isotopes the greater will be the equilibrium isotopic effect expressed by the quantity Kj. Consequently, the resulting value aHTO(s) has to be corrected by this factor. Unfortunately, there are n o
13
values of the isotopic equilibrium constant, Ki,available in the literature for the system HTO-Si02. Sorg [47] found that Ki values for the systems HTO-Ca(OH)2, HTO-CaSO, * 2 H 2 0and HTO-H3B03 ranged between 1.I9 and 1.32. In view of the lack of data for Ki for silica and in order to compare the results of HTO exchange with that of other methods, the following approach is suggested [37]. On an Aerosil sample with a specific surface area Of SBET = 228 m2/g and considered as a reference substance, the CVOH(~) value was measured by means of the methyllithium method to be 4.37 ?r 0.22 pmole/m2. On the same sample, the CYOH(~) value measured by means of HTO exchange according to the procedure described below, without any correction for the isotopic effect, was 3.45 f 0.17 prnolelm’. The value of 4.37 pmolelrn’ was then taken as a standard for the following reasons: (i) the surface of Aerosil resembles that of porous silica; (ii) most of the surface hydroxyl group determinations were carried out on Aerosil; values on Aerosil obtained by means of the methyllithium method and (iii) the CYOH(~) by means of the exchange with D 2 0 have been found t o be in close agreement [38]. Thus, the ratio of 4.37/3.45 (= 1.267) was defined as the correlation coefficient, ~ H T O , and was considered to be constant for all porous silica species. Clearly, k ~ is ~not othe true isotopic equilibrium constant, Ki,but represents a correction factor by which QHTO(~) is multiplied in order to obtain a value that is comparable t o that obtained by other methods. The detailed isotopic exchange on porous silica and Aerosil is carried out as follows [37] Between 0.5 and 1.O g of silica is weighed into the sample bulb at the left side-arm (a) of the glass apparatus shown in Fig. 3.4. A sealed ampoule containing about 100 pg of
L
e
If
Fig. 3.4. Diagram of the glass apparatus employed for isotopic exchange reaction between silica and HTO [37].(a) Sample bulb containing the silica; (b) holder containing a sealed ampoule fiied with 100 pg of tritium-labelled water; (c) constriction to facilitate the removal of the upper part during sealing the glass apparatus; (d) tap connected to a high-vacuum system; (e) oven for annealing the sample; (g) bath of liquid nitrogen for trapping the condensed phase after exchange.
14
titrium-labelled water is placed in the right side-arm (b). The activity of the starting HTO solution is about 10 pCi/ml. The glass cylinder is narrowed at position (c) by glass blowing, then the apparatus is connected by the tap (d) to a high-vacuum pumping system and evacuated to less than lo-' Pa for 15 h. Simultaneously, the sample bulb is annealed at 473 K by means of the oven (e) while the ampoule holder is maintained at room temperature. When the pre-treatment is finished, the tap (d) is closed and the glass apparatus is sealed under high vacuum at position (c). The exchange and the separation of reactants is performed in this closed system in the following way: (i) breaking the ampoule that contains the tritium-labelled water by carefully shaking the apparatus and admitting HTO into the reaction system; (ii) placing the whole apparatus in an air thermostat maintained at 353 f 5 K for 60 h in order to attain isotopic equilibrium; (iii) cooling the ampoule holder at 77 K in a bath of liquid nitrogen while the sample bulb is simultaneously heated at 473 K for 15 h. After these operations, the sealed end of the apparatus at position (c) is opened in a glove-box under a dry nitrogen atmosphere and the condensed water that has been trapped is washed out quantitatively into a 50-ml glass flask using reagent-grade dioxane (E. Merck, Darmstadt, G.F.R.) (solution A). For scintillation counting, a special solution (solution B) was prepared by dissolving 100 g of scintillation-grade naphthalene (Merck), 10 g of diphenyloxazole (Packard, (Packard) Frankfurt/M, G.F.R.) and 0.25 g of 1,4-bis-2-(4-methyl-5-phenyloxazolyl)benzene in 1 1 of reagent-grade dioxane. A 2-ml volume of solution A and 15 ml of solution B were mixed in a poly(viny1 chloride) flask especially supplied for scintillation counting (Packard), and shaken vigorously. After thermostating in a Tricarb Scintillation Counter (Packard) the activity, 9', was measured. In order to evaluate go,100 pg of tritium-labelled water were injected by means of a Hamilton syringe into 50 ml of reagent-grade dioxane (solution C). A 2-ml volume of solution C was mixed with 15 ml of solution B in a poly(viny1 chloride) flask and 9, was measured as described previously. The activity, 9 ",of the tritiated silica sample can also be monitored after dissolving it in 1.5 N sodium hydroxide solution and stabilizing the sodium silicate solution by use of a special scintillation solution which differs in composition from that of solution B. For details see reference [34]. In order to prevent an exchange between HTO and the hydroxyl groups at the surface of the glass apparatus and glass equipment, it is essential to hydrophobize all glass material prior to analysis. This operation is carried out by coating the inner surface of the burettes, pipettes, glass ampoules, glass apparatus, etc., with a thin film of silicone solution (Serva, Heidelberg, G.F.R.) and by subsequent heat treatment of the coated equipment at 523 K in an oven. For conducting calculations of IXOH(~)according to the procedure described, the data sheet in Table 3.2 provides more information. Values of IXOH(~)for a porous silica sample obtained at different pre-treatment temperatures in the range 473-1273 K are listed. Annealing of the sample was carried out separately in an oven and the materials annealed were stored before use in a vacuum desiccator over phosphorus pentoxide. Of all physical methods for the evaluation of IXOH(~),isotopic exchange with tritium-
TABLE 3.2 DATA SHEET FOR CXOH(~) DETERMINATION BY MEANS OF THE ISOTOPIC EXCHANGE METHOD WITH HTO [ 371 Outgassing temperature
SBET (mz/g)
(K)
Sample weight corrected due to weight loss
Activity,
Ratio
9'
g'/go
UHTO(~) (mmole T)
QHTO(~) (mmole T)
CXOH(~) (mmolelg)
IZOH(~) bmole/m')
IZOH(~) (car.) &rnole/m2)
8561 8204 8898 18238 19334 18615 18924 18142 18884 19747 21875 22062 22264
0.384 0.368 0.399 0.817 0.866 0.834 0.848 0.81 3 0.846 0.885 0.980 0.989 0.998
2.133 2.044 2.217 4.544 4.817 4.638 4.715 4.520 4.705 4.920 5.450 5.497 5.541
3.427 3.516 3.343 1.016 0.793 0.922 0.845 1.040 0.855 0.640 0.1098 0.0633 0.0129
3.218 2.856 2.195 1.281 1.090 1.043 0.918 0.830 0.706 0.623 0.110 0.0592 0.0140
7.363 6.804 5.27 3 3.106 2.657 2.570 2.325 2.242 2.093 1.964 1.607 1.214 1.048
9.329 8.620 6.681 3.935 3.366 3.256 2.945 2.840 2.65 1 2.488 2.036 1.538 1.327
k) 47 3 523 573 673 723 773 823 a73 923 973 1073 1173 1223
437.1 419.7 416.3 412.5 410.3 405.8 394.8 370.2 337.5 317.4 68.5 48.1 13.3
1.065 1.231 1.523 0.793 0.682 0.884 0.920 1.254 1.211 1.028 0.998 1.069 0.920
16
labelled water has the advantage of superior and easier detection over the D2O method. In comparison with infrared spectroscopy the procedure does not need any mechanical treatment such as pressing of discs [ 15,39,40]. In contrast to chemical methods such as reaction with methyllithium, no limitations exist owing to steric exclusion of the reactant because the diameter of a water molecule is about 0.3 nm and hence water is capable of penetrating ) for a porous silica sample obtained into small micropores [34], In Fig. 3.1, c ~ o H ( ~values by the HTO method are plotted against the pre-treatment temperature. The course of the curve is similar to that obtained on other porous samples by means of the methyllithium reaction [35,36]. A large decrease in c ~ o H ( ~is) observed in the temperature range between 523 and 723 K while at higher temperatures (YOH(~)decreases continuously. Even at 1223 K a relatively large amount of hydroxyl groups can be monitored, which is in close agreement with infrared spectroscopic measurements of other workers [39,40,43]. 3.1.2 Reactivity of surface hydroxyl groups in adsorption
Whereas the previous section considered the silica surface in terms of hydroxyl groups, here their reactivity will be discussed in relation to adsorption processes. Before specific systems are treated in detail, an attempt is made to understand the nature of intermolecular interactions between the silica surface and the various adsorbates. Then the adsorption behaviour is considered as a function of the electronic structure of the interacting molecules on the basis of adsorption studies and other techniques such as infrared spectroscopy and calorimetry. 3.1.2.1 Fundamentals
When an adsorbent and a vapour are brought into contact, it is found that at equilibrium the concentration of the vapour is greater at the interface than in its bulk phase. This phenomenon is called adsorption. The enrichment of the vapour in the interfacial layer is caused by attraction and repulsion forces. According to the type and strength of interactions, adsorption can be divided into physical adsorption (physisorption) and chemical adsorption (chemisorption) [48]. Physisorption takes place through short-range intermolecular attraction and repulsion forces [49] such as (a) London-type dispersion forces resulting from induced dipole-induced dipole and multi-pole attractions; (b) induction forces including interactions between induced or permant dipoles of molecules and the electric field present at the absorbent surface; (c) charge transfer between the adsorbed molecule and the adsorbent surface, resulting in a non-bonding resonance state. In contrast to these interactions, in chemisorption an electron transfer between the surface atoms of the adsorbent and the adsorbate occurs, resulting in a chemical bond between the species. Although a clear distinction between physisorption and chemisorption is not yet possible, some experimental criteria can be used in order to decide the type of adsorption involved [48]. The most significant criterion is the magnitude of the heat of adsorption. In chemisorption, the heat of adsorption usually exceeds 80 kJ/mole, whereas
the heats of physisorption vary between 8 and 40 kJ/mole, being comparable with the heat of liquefaction of the corresponding adsorbate. Further, chemisorption often requires an appreciable activation energy whereas physisorption is a non-activated process. Differences also arise in the thickness of the adsorbed layer: whereas chemisorption is restricted to the formation of a monolayer, in physisorption a multilayer is built up successively as the vapour pressure is increased. In the following discussion, we focus our attention on physisorption on silica, excluding chemisorption processes, which are treated in detail in Section 3.2. Kiselev and co-workers [50-521 made a thorough attempt to classify the short-range molecular interactions on the basis of the electronic structure of the surface functional groups of the absorbent and also the molecules that undergo adsorption at the solid-gas interface. The adsorbents can be divided in three groups: (I) those exhibiting no ions or active functional groups exposed at the surface; (11) those having positively charged surface sites such as Brbnsted and Lewis acid centres; (111) those carrying centres with a high electron density. The absorbate molecules can conveniently be classified into four groups: (a) those having a spherically symmetrical shell of electrons or a-bonds; (b) those exhibiting peripheral bonds or groups with a high electron density; (c) those having positively charged groups at the periphery; (d) those having both groups of electron density and those of positive charge concentrated on the periphery. The type I adsorbents, represented by graphitized carbon, boron nitride, etc., are thought to be capable only of non-specific interactions with all types of adsorbate molecules. The term “non-specific”, used merely for systematization rather than for a quantitative treatment, refers to London-type interactions. Porous silica, being fully hydroxylated, is a type I1 adsorbent because it carries localized positive charges at the surface. Although silica undergoes non-specific interactions with molecules of adsorbate groups (a)-(d), it gives rise to additional interactions with molecules of groups (b)-(d) such as ethers, nitriles, tertiary amines, alcohols and ketones. When the hydroxyl groups at the silica surface are totally removed by annealing at 1373 K, a dehydroxylated surface is produced carrying only siloxane groups, which are now unable to undergo specific interactions. To a first approximation, completely dehydroxylated silica is then a type I adsorbent. Thus, the difference of a measurable quantity such as the isosteric heat of adsorption on a hydroxylated and dehydroxylated silica sample for a certain adsorbate gives an estimate of the extent of specific interactions in the adsorption interaction. Additionally, the specific interactions can be manifested in infrared and electron spectra. Suitable quantities for the determination of the strength and specificity of short-range intermolecular interactions were derived from adsorption measurements. For a known weight of adsorbent, the volume of an adsorbate, v, at adsorption equilibrium is given by the pressure, p , of the adsorbate and the temperature, T (3.3 1)
At constant temperature, the volume is measured as a function of pressure and an adsorption isotherm is obtained, given 1 . ~ (3.32)
If the gas is below its critical temperature, eqn. 3.32 is plotted in the form
v = ~(P/PO)T= constant
(3.33)
where p o is the saturation vapour pressure of the adsorbate. If the volume is measured as a function of the temperature at fixed pressure, the adsorption isobar is obtained, given by (3.34) If the pressure is measured as a function of the temperature at a constant volume of adsorbate, the adsorption isoster is obtained, given by
P = f(T)v = constant
(3.35)
On the basis of the relationships 3.32-3.35, one can derive energetic parameters such as the isosteric heat of adsorption,.‘4 Other quantities such as the adiabatic or isothermal calorimetric heats and the integral heat of adsorption can be estimated from calorimetric measurements. The isosteric heat of adsorption, derived from the adsorption isoster, is defined as (3.36) In the derivation, it is assumed that the adsorbate behaves as an ideal gas and the molar volume of the adsorbed phase is negligibly small compared with that of the adsorbate [49] The relationship between the isosteric and differential heats of adsorption, qdiff,is given by the equation 4st
- RT = qdiff
(3.37)
Usually, q‘ or qdiff is plotted against the amount adsorbed, v, or the surface coverage, 8. Some uncertainty exists in choosing an appropriate value of 8 for comparing the various heats of adsorption. For convenience, 8 x 0.5 has been recommended [52]. Other thermodynamic quantities can be derived, such as the differential molar entropy of adsorption. For a detailed study, see refs. 49,53 and 54. 3.1.2.2 Silica-adsorbate interactions There is an enormous amount of published information on the adsorption interactions between porous silica and adsorbates with different electronic structures, which has been Reliable techniques for the study of reviewed by Kiselev [ 5 1 1 , Hair [39] and Little [a]. silica-adsorbate interactions are adsorption measurements combined mostly with infrared spectroscopy. In order to establish the specific role of hydroxyl groups in adsorption, both fully hydroxylated and completely dehydroxylated samples were investigated under the same conditions. Only a few results will be presented here to illustrate the terms “specific” and “non-specific interactions”. Following the recommendation of Kiselev and co-workers [50-521, the adsorption interactions will be discussed in the order of adsorbate molecules of groups (a) to (d). In the late 1950s McDonald [54,551 investigated spectroscopicaIly the perturbation of
79
surface hydroxyl groups that occurred when non-polar molecules were adsorbed at low temperatures on silica. Spectral shifts and intensities related to the OH stretching band at 3749 cm-‘ were reported for the adsorption of argon, krypton, xenon, methane, nitrogen and oxygen as a function of vapour pressure on Aerosil at 83 and 103 K, respectively (see Table 3.3). The first four adsorbate atoms possess a symmetrically spherical electron shell containing only o-bonds, whereas the latter two molecules additionally have n-bonds. The frequency shift observed for argon, krypton. and xenon was found to be proportional to their polarizibilities [39,55]. For oxygen and nitrogen, however, exhibiting nearly the same polarizibility, the spectral shifts were 12 and 24 cm-*, respectively, at the same surface coverage. This difference can be attributed to the different quadrupole moments of oxygen and nitrogen, depending on the orientation of the molecules at the surface [49,56]. The rare gases are assumed to be adsorbed by essentially non-specific interactions of London-type forces whereas the adsorption of oxygen and nitrogen is slightly specific owing to their quadrupole moments. In particular, the adsorption of argon and nitrogen on hydroxylated and dehydroxylated silicas has been studied by several workers with respect to their use as standards in surface area determinations by the BET method. Aristov and Kiselev [57] reported that the course of the argon isotherm in the monolayer region on a mesoporous silica sample at 78 K is independent of the degree of dehydroxylation of the silica surface, whereas the adsorption of nitrogen in the same range decreases sharply with increasing dehydroxylation of the surface owing to a reduction in the fieId gradient-quadrupole interaction. The specificity in the adsorption of nitrogen on non-porous, mesoporous and microporous hydroxylated and dehydroxylated silicas was also studied by Baker and Sing 1581. It was found that, in agreement with the results of Aristov and Kiselev [57], the influence of the specific field gradient-dipole interaction of nitrogen is mainly confined to the monolayer region of the isotherm, whereas the multilayer region is not much affected by dehydroxylation. The insensitivity in adsorption due to dehydroxylation could also be established for other nonpolar adsorbates such as n-alkanes [59,60] and carbon tetrachloride [61]. With regard to BET surface area determinations from argon isotherms, uncertainties exist in the value of the molecular cross-sectional area. Whereas Aristov and Kiselev [57] proposed an Am(Ar) vaIue of 0.137 nm2/atom, Payne etal. [62] adopted a value 0.182 nm’latom. Using the latter, the SBETvalues derived from the argon isotherm were found to coincide well with those of the nitrogen isotherm using Am(N2) = 0.162 nm2/molecule. Adsorbate molecules carrying localized centres of high electron densities such as extended s-electron systems and dipoles are expected to undergo specific interaction with a hydroxylated silica surface. In other words, the adsorption energy of the absorbates is very sensitive to the degree of hydroxylation of the silica surface. The change in the infrared spectra of surface hydroxyl groups during the adsorption of benzene, toluene, values was examined by p-xylene and mesitylene on Aerosil species with various OLOH(~) Galkin et ul. [63]. They concluded that mainly free hydroxyl groups participate in adsorption interactions and the shift of their adsorption band depends on the structure of the adsorbate molecules and the surface coverage. Changes in the spectra occur due to specific interactions between the large n-electron systems of aromatic molecules and the dipole of hydroxyl groups. A linear correlation could be established between the frequency shift, AVOH,of hydroxyl groups and the differential heat of adsorption, AQ, for a corre-
80
sponding adsorbate. AQ is the difference between the differential heats of adsorption on a fully hydroxylated and a completely dehydroxylated silica sample at a coverage of about half a monolayer of the given adsorbate: AQ = qdiff(hydroxylated) - qdiff (dehydroxylated)
(3.38)
0 = 0.5 was taken as a reference state because on the one hand at 0 << 0.5 the quantity qdiff is strongly affected by the heterogeneous distribution of the surface sites and on the other hand at 0 >> 0.5 adsorbate-adsorbate interactions will become noticeable and may influence qdiff. From the experimental data, it is concluded that AUOHincreases monotonically with increasing AQ [ 151. Another approach for determining the contribution of specific interactions between adsorbate molecules of groups (b)-(d) and a hydroxylated silica surface is based on choosing a reference molecule belonging to group (a) that is incapable of undergoing specific interactions [64,65]. The reference molecule should have the same molecular size and possess approximately the same polarizibility as those of molecules of other groups. For instance, argon can be used as reference for nitrogen and n-pentane for diethyl ether. It could be shown that the isosteric heats of adsorption of argon and nitrogen on adsorbents of type I (graphitized carbon black, dehydroxylated silica) are nearly the same whereas on adsorbents of type I1 (hydroxylated silica, alumina) the nitrogen quadrupole moment gives rise to an enhanced isosteric heat of adsorption [66]. The contribution of specific interaction in adsorption is then evaluated by the equation Qspecific
st
st
qb,d (hydroxylated) -48 (hydroxylated)
(3.39)
where 4:,d (hydroxylated)is the isosteric heat of adsorption of a molecule of group (b) and is the isosteric heat of adsorption (d) on the hydroxylated silica surface and 4~t(hydroxy~ted) of a reference molecule of group (a) on the hydroxylated silica surface. As shown by Ross and Olivier [49], q' can easily be determined by means of gas chromatographic measurements. The adsorbent under investigation is packed into a column and the corrected retention volume, VR, of the corresponding adsorbate is measured at various column temperatures, Tc. The plot of log VR versus 1/T, yields a straight line of slope qs'/2.303 R. On the basis of this technique, Elkington and Curthoys [67] made a comparative study of the relationship between 4& of different adsorbates and the shift of the hydroxyl group frequency, AVOH,in the infrared spectrum of a porous silica. In a broader sense, gas-adsorption chromatography provides a sensitive and simple means of deriving data that permit the determination of surface-adsorbate interaction as well as its specificity [69,70]. As the absolute retention volume of adsorbate molecules, Vh, expressed in millilitres per square metre of specific surface area of the adsorbent, depends on the isosteric heat of adsorption [49] according to the equation lim 4' = R[dlnVh/d(l/T)]
8+0
(3.40)
the ratio of the retention volumes, Vh (hydroxylated)/Vh (dehydroxylated), for a given adsorbate should correspond, to a first approximation, to the differences in the qs' of the adsorbate on a hydroxylated and a dehydroxylated surface. This was demonstrated by Kiselev et al. [71] for a series of adsorbates. Further, these workers not only reported a
81
linear relationship between Vk (hydroxylated)/Vh (dehydroxylated) and AQ at 0 0.5, but also between the ratio of absolute retention volumes and the frequency shift, AUOH. Gas chromatographic measurements are also an effective means of testing the surface polarity of novel adsorbents synthesized, for instance, by chemical modification of porous silica. In the silanization of the silica surface by treatment with trimethylchlorosilane or hexamethyldisilazane, an adsorbent of type I should be obtained. Then, the plot of the retention volume, V,, or the capacity factor, k’,of the solute against the volume polarizibility, a,of the adsorbate should yield a straight line [70]. As shown in Fig. 3.5, a linear relationship is obtained only for some n-alkanes and aromatic compounds, whereas small polar molecules such as acetone, diethyl ether and nitromethane have very large capacity factors compared with their low a-values and hence deviate considerably from linearity. These deviations are caused by the presence of residual hydroxyl groups at the modified surface which are accessible and capable of strong acid-base interactions with acetone, ethers, etc. [72]. In an extended treatment, it was claimed by Basila [68] and Galkin etal. [63] that AUOHdecreases linearly with increasing ionization potential, I , of the adsorbate. This indicates that the interaction between hydroxyl groups and adsorbates with electron-donor properties is of the charge-transfer type. The surface hydroxyl groups act as electron acceptors. In contrast, Kiselev and Lygin [ 151 stated that this linear relationship holds only for molecules with closely related structures, e.g., benzene derivates carrying alkyl substituents. For strongly polar molecules that exhibit centres of high electron density k’
a
01
0
5
--
10
15 O(
1 1 0 ~ ~ ~ ~ 1
Fig. 3.5. Dependence of k’ of various adsorbates obtained by gas chromatography on silanized silica supports on its polarizability, OL [ 721. Column dimensions, 1000 X 2 mm I.D.; column temperature, 373 K. Packing: (A) macroporous silica, SBET= 45 m’/g, d p = 100-200 pm, silanized with trimethylchlorosilane, (UTMS= 4.1 pmole/mz;( 0 ) macroporous silica, SBET= 45 rn’/g, d p = 100-200 jm, silanized with hexamethyldisilazane, C~TMS = 4.1 pmole/m2. Carrier gas, nitrogen; flame-ionization detector. Solutes: (1) dichloromethane, (2) trichloromethane, (3) n-pentane, (4) benzene, (5) cyclohexane, (6) n-hexane, (7) toluene, (8) n-heptane, (9) m-xylene, (10) n-octane, (1 1) dimethyl ether, (12) nitromethane, (13) acetone.
82
such as oxygen and nitrogen atoms in aniline, diethyl ether, etc., no correlation between &OH and I exists (see also Table 3.3). This means that charge transfer is not the only mechanism in specific interactions and other contributions caused by dipole orientation electrostatic forces should be included. The magnitude of the strength of interactions can be calculated from the AVOHvalues of various adsorbates as listed in Table 3.3. Whereas for group (a) molecules AUOHis relatively small (< 30 cm-') it increases to 100-200 cm-' for benzene, toluene, xylene, etc. Consideriq the adsorbate pair n-hexane-benzene, which have nearly identical polarizibilities, the increase in AVOH from n-hexane to benzene is due to the specific interaction between the n-electron system of benzene and the hydroxyl groups. The shifts produced by small polar molecules range between 300 and 500 crn-' .The greatest shifts are observed for polar molecules such as aniline and triethylamine that are additionally capable of acid-base interactions. The adsorption interactions between the surface hydroxyl groups and molecules that also contain hydroxyl groups such as alcohols and water are of great interest. However, in the study of interactions, peculiarities arise because (i) alcohols react chemically with both surface hydroxyl groups and with siloxane groups; (ii) water reacts with a dehydroxylated surface; and (iii) both water and alcohol form intermolecular hydrogen bonds. Alcohols such as methanol and ethanol are physically adsorbed at room temperature, whereas at higher temperatures chemisorption occurs through the formation of GSi-OR groups. TABLE 3.3 FREQUENCY SHIFT, AUOH,OF THE VALENCE VIBRATION OF FREE HYDROXYL GROUPS ON THE ADSORPTION OF A SERIES OF ADSORBATES AQ = heat of adsorption at d
Adsorbate
Argon Krypton Xenon Nitrogen Oxygen Methane n-Hexane Benzene Toluene p-Xylene Nitromethane Ethyl acetate Acetonitrile Acetone Tetrahydrofuran Diethyl ether Aniline Pyridine Triethylamine
= 0.5.
AUOH (cm-')
AQ (kJ/ mole)
8 16 19 24 12 32 36 125 146 154 160 280 300 330 470 500 600 775 990
Polarizability, 01 (nm3 10 3)
-
Ionization potential, IP (eV)
1.65 2.54 4.13 1.76 1.60 2.60 37.7 46.0 52.3 57.3 45.6 65.7 48.1 60.7 64.0 62.4 -
11.78 10.32 12.15 7.10 2.94 6.32 8.16 9.02 9.50
Reference
39 39 39 39 39 39 10.5 9.2 8.8 8.5 11.3 10.15 12.0 9.7 10.1 9.6 7.7 9.7 7.85
15,70 15,70 15,70 15,70 15,70 15,70 15,70 15,70 15,70 15,70 15,70 15,70 15,70
83
Physisorption studies of trimethylcarbinol on silica showed [73] that the interaction is based on hydrogen bonding with surface hydroxyl groups. It could also be concluded that mainly free hydroxyl groups take part in the interaction [73,74]. For water it could be established that both free and bound hydroxyl groups are involved in water adsorption. Lange [75] suggested two types of molecular adsorbed water. The first type requires an activation energy of about 29 kJ/mole for its removal and the second 42 kJ/mole. The characteristic behaviour of water in adsorption on nonporous, mesoporous and microporous silicas was recently studied by Baker and Sing [58].
3.2 CHEMICAL MODIFICATION OF THE SILICA SURFACE In silica chemistry, the term modification has a two-fold meaning. Procedures that permit the control and variation of pose structure parameters and the specific surface area of silica are exclusively termed geometric modification. In contrast, the term chemical modification is reserved for all processes that lead to a change in the chemical composition of the surface. In some instances, chemical modification may also occur during geometric modification, such as in hydrothermal treatment, calcination and sintering [76]. Hydroxylation, dehydroxylation and dissociation of silica surface species in aqueous solutions that are associated with these treatments, however, belong to the proper surface chemistry of the silica system. Chemical modification in its real sense means the covalent bonding of functional groups to the surface as a result of a chemical reaction between the surface species and an appropriate reactant. Difficulties sometimes arise in distinguishing whether a surface species being attached is chemisorbed, physisorbed or only mechanically held within the pores of the support. In the history of silica chemistry, chemical modification is a well known technique for improving the wettability behaviour of industrial silica products [77,78]. In gas chromatography it was first introduced as an approach for deactivating the heterogeneous surface sites of siliceous supports to prevent tailing and sample decomposition. Pioneering work in this field was carried out by Kiselev and Yashin [70]. Later, efforts were extended to develop chemically bonded silica supports for gas chromatography [76]. The potential advantage of chemical modification as an effective means for synthesizing specific adsorbents that differ widely in the type of functional group and hence in surface polarity was fully recognized some years later in the field of column liquid chromatography. Since then, a large number of chemically modified silica packings have been developed for liquid chromatography and these were surveyed by Grushka [79] and Rehak and Smolkova [go]. The following section deals with some basic studies in the synthesis and characterization of chemically bonded silica packings (CBSP). With respect to their chromatographic utility, the following features are of interest: to change significantly the adsorption behaviour of the parent support, functional groups that differ widely in their structure from hydroxyl groups should be introduced by bonding; the surface bond should offer high chemical and thermal stability; to minimize matrix interactions, a dense coverage of functional groups should be achieved; to ensure a sufficiently high rate of mass transfer due to diffusion and adsorption, the functional groups should be readily accessible to solutes.
84
3.2.1 Basic concepts
3.2.1.1 Types of bonds and functional groups With regard to a broad variation of functionality of bonded species, it has been found to be optimal to attach organofunctional groups, R, to the surface, where R may be nalkyl, aryl- or alkylaryl. Additionally, R may carry substituents of various functionality such as halogen, alcoholic or phenolic hydroxyl, amine, carbonyl and carboxyl. Further, carbon atoms in the chain or ring may be replaced with hetero atoms such as oxygen and nitrogen to give adsorbents with centres of electron-donating or electron-withdrawing properties. The functional group R can be linked to the surface silicon atoms in the following two ways: (i) R will be directly bonded to surface silicon atoms by means of a S i - C bond as S i - R . The formation of such a bond provides the elimination of hydroxyl groups that were originally bonded to the silicon atoms. This can be achieved by chlorination of the surface, yielding ZSi-Cl groups, followed by treatment of the chlorinated surface with organometallic compounds. (ii) R will be bonded via a bridge atom X to the surface silicon atoms as Si-X-R. The most common bridge structures are GSi-0-R
(a)
I ESi-0-Si-R
(c)
I
S i - 0 - R is known as an ester bond and is easily formed by the reaction between surface hydroxyl groups and an alcohol according to [4,76] Si-OH
+ HO-R
+Si-0-R
t H20
(3.41)
or by cleaving a siloxane bond [43] : Zji-O-SiE
+ CH30H + ZSi-OH
t =Si-OCH3
(3.42)
Structure (b) is obtained by treatment of a chlorinated silica surface containing S i - C l groups with an amine such as
sSi-Cl+ H2N-R
+ Si-N-R
H
t HCl
(3.43)
Structure (c) is formed through reaction of hydroxyl groups and organosilanes of type &Si&-n with reactive X groups such as halogen, ethoxy and methoxy [37,82] : S i - O H t X-Si-R
+ ESi-0-$i-R
+ HX
(3.44)
Evidently, the functional groups R being bonded as S i - R will possess the closest attachment to the original surface and also provide a denser coverage than that obtained by the
85
respective bridge structure. In the latter, the organic moiety is separated from the surface by fairly large X atoms differing in both bond length and bond angle. In the Si-0-Si-R bridge structure, for instance, the steric orientation of R is determined primarily by the tetrahedral arrangement of the siloxane bond.
3.2.1.2 Structure and stability of surface bonds For a better understanding of the stability of surface bonds, it is advantageous to consider their bond energies, bond lengths and bond angles. The S i - 0 - S i - e structure will be chosen as a representative of surface bonds because it combines the two main structural elements in silica chemistry, namely the siloxane bond, which is the basis of inorganic silica chemistry, and the silicon-carbon bond, which occurs in organosilicon compounds. As structural data for surface bonds are difficult to obtain experimentally, the crystalline silica modifications and monomeric and polymeric organosilicon compounds are considered as reference compounds. Bond energies give a rough estimation of the stability of a given bond. Thermodynamic bond energy terms are defined as the energy necessary to cleave the bond, assuming the compounds are in the ground state and the reaction occurs at 0 K in the ideal gas state. Comparison of bond energies, E(Si-X), of different bonds reveals that the Si-0 bond is the most stable compared with Si-C and Si-N (Table 3.4) [83].For a more precise treatment, one has to take into account that E(Si-X) depends markedly on the type of substituent at Si and X. For example, bond energy terms, E(Si-R), have been found to decrease in the sequence R = CH3, C2H5, C& (310,260,226 kJ/mole, respectively) [84]. Regarding the combination of the type S i - 0 - S i - E , bond additive parameters have to be introduced, corrected by structural interaction terms, in order to determine the bond energies [83]. Table 3.5 surveys the bond lengths in different Si-X bonds evaluated by means of electron diffraction, X-ray diffraction and microwave spectroscopy [85].The Si-0 distance is the smallest of the three bond lengths listed, which is in agreement with the highest bond energy of E(Si-0) compared with E(Si-C) and E(S1-N). The Si-0 distances in organosilicon compounds have been found to be slightly larger than that in crystalline silica modifications. The length of an Si-C bond is considerably larger than that of an Si-0 bond and also depends on the hybridization of carbon atoms linked to the silicon atom. The Si-N bond length is intermediate between those of Si-C and Si-0. It is interesting to compare the observed values with calculated distances derived by the TABLE 3.4 Si-X BOND ENERGIES (El [83] Type of bond
E (Si-X) (kJ/mole)
Reference compound
Si-0
469 444 306 318
SiO, [(CH,),Sil ,O (CH,),SiCl [(CH,),SiJ ,NH
Si-C Si-N
86
Shomaker-Stevenson relationship. This relationship accounts for the differences in electronegativity of atoms forming a heteronuclear and polar bond. The values calculated by this particular equation, however, are still larger than the observed distances. The additional shortening may be explained on the basis of either the formation of resonance structures or (d-p), bonding effects [ 8 6 ] , While the Si-0-Si bond angle in polymethylsiloxanes varies slightly around 130” [85], the angle is greater in crystalline silica modifications (cu. 140-1507. The C-Si-C bond angle coincides well with the tetrahedral angle of 109”.Transposing these results to a surface bond of the type FSi-0-Si-E a substantial variation of bond angles can be expected, owing to the heterogeneity of the silica surface. More precisely, the hydroxyl groups undergoing reaction to give siloxane bonds differ markedly in their coordination arrangement with oxygen atoms. On the other hand, steric effects may play a dominant role depending primarily on the size or volume of organofunctional groups giving rise to an additional deviation of the Si-0-Si bond angle. Both effects may lead to a weakening of the silicon-oxygen bond and hence to an increase in its reactivity. The situation is fairly similar to the so-called “strained” siloxane bonds which are believed to exist on Unfortunately, there are no experimental partially dehydrated silica surfaces [41,43]. data so far to confirm these assumptions but it is hoped that a more thorough examination of this effect will be undertaken. TABLE 3.5 Si-X BOND LENGTHS [ 851 Type of bond
Bond length (nm)
Reference compound
Si-0
0.1604 0.163 0.164 0.189 0.185 0.183 0.174
Average value of SiO, modifications (CH,),SiOSi(CH,), Si(OCH,), (CH,),Si SiH,CH=CH, SiH,C=CH (SiH,),N
Si-C
Si-N
As chemically modified silica supports are permanently in contact with organic or aqueous solvents in HPLC, it is worth examining their stability towards nucleophilic and electrophilic reagents. For clarity, we first treat the reactivity of the Si-0- and the -Si-C= bonds alone and then that of the S3i-O-Si-CZ bond [86]. It is well known that the siloxane bond in crystalline and amorphous silica is cleaved readily by nucleophilic reagents such as potassium hydroxide and sodium hydroxide according to
(3.45)
87
In aqueous solution, cleavage becomes noticeable at pH > 9.0. The siloxane bond, however, is also reactive to electrophilic agents such as Brensted and Lewis acids:
(3.46)
?;I The extent of electrophilic attack depends on the accessibility of the free electron pairs at the oxygen atom, which is controlled by electron-donating and -withdrawing substituents attached to the silicon atoms. With increasing strength of the electron-withdrawing substituents, the polarity of the siloxane bond decreases and becomes more stable against electrophilic attack. The reverse effect is observed on the introduction of electrondonating substituents. The S i - C E bond is relatively stable towards homolytic fission [86,87]. Thermal decomposition studies on tetraalkyl- and tetraarylsilanes have shown that decomposition is completed at about 1100 K. Primary breakdown of the S i - E bond, however, occurs at lower temperatures. One can expect that unsubstituted organofunctional groups, attached via a Si-O-Si-R bond at the silica surface, will exhibit a reasonable thermal stability up to 600-700 K. As a resuli of the weak ionic character of the Si-C bond, which is moderately polar in the sense Si-c, it can be cleaved by both nucleophilic and electrophilic agents according to:
(3.47)
(3.48) OH Y+
Unsubstituted alkyl groups attached to silicon are readily cleaved by nucleophilic reagents. For methylsiloxanes, the rate of cleavage has been found to decrease with the number of oxygen atoms linked to silicon in the sequence (Me3Si)20> (Me2SiO), > (MeSiOl.5), [87] Cleavage of the silicon-alkyl bond by electrophilic reagents such as Brbnsted and Lewis acids occurs very readily, as was shown for organosilanes of the type RSiRS [87]. It must be emphasized that the presence of polar hetero atoms substituted in the a-, 0-or yposition of the alkyl group facilitates the cleavage due to nucleophilic attack and decreases the ease of electrophilic cleavage. The effects due to hetero atoms linked at the carbon chain are expected to diminish with increasing distance from the silicon atom in the order a-, 0-and y-substituents. However, peculiarities are observed in a-,6- and y-substituted halogen compounds and a-nitrogen and 0-oxygen compounds [86,87]. When an aryl group is in an a-position with respect to an alkyl group attached to silicon, nucleophilic attack is markedly facilitated owing to the formation of a resonancestabilized carbanion. Electron-withdrawing substituents in the rn-position of the benzyl
88
group increase the nucleophilic attack. Si-benzyl groups are much less readily cleaved by electrophilic reagents than Si-alkyl groups as shown in acylation, nitration and sulphonation [87]. In contrast, Si-aryl bonds are easily attacked by electrophilic reagents, as is well known in electrophilic substitution of aromatics. Intensive investigations have been made on the influence of electron-donating and electron-withdrawing substituents on cleavage. In conclusion, electron-donating substituents markedly accelerate and electron-withdrawing substituents markedly retard the cleavage. Nucleophilic cleavage of the Si-aryl bond occurs more readily than that of the Si-alkyl bond. Cleavage seems to be facilitated by electron-withdrawing substituents in the aryl group. To a first approximation, one can expect a similar behaviour of organofunctional groups attached via Si-0-Si-C bonds at the silica surface, as is observed in monomeric organosilanes. However, this statement should be checked as far as possible in individual capes.
3.2.1.3 Methods of forming surface bonds The synthesis of chemically bonded silica supports can generally be performed in two ways. In the first method the characteristic feature is that the modification is carried out during the formation of the porous support. Commonly, organohalo- or alkoxysilanes are employed as starting materials, which are converted by hydrolysis, condensation and polymerization into rigid, insoluble and porous polyorganosiloxanes or organosilicon xerogels. In the final product, the organofunctional groups, R, bonded via an Si-C link to the silica framework are constituents of the bulk phase and are also distributed at the surface. Therefore, this mode is called bulk modification. In contrast, the second mode, termed surface modification, involves a surface reaction between a silica support of given porosity and an appropriate reactive organosilane. More specifically, reaction takes place between the hydroxyl groups and/or physisorbed water at the silica surface and the organosilane molecule to form a chemically bonded layer. In comparing the two modes, it seems obvious that bulk modification has the disadvantage of the need to control both the chemical composition and the pore structure whereas surface modification is simpler to perform and hence is more commonly used.
3.2.1.3.1 Bulk modification The starting compounds in bulk modification are mostly organosilanes of type b S i X 4 - (1 G n < 3 ) and also Si&, where X is halogen or an alkoxy group. According to the number of reactive Si-X groups, organosilanes are classified as mono-, diand trifunctional compounds. It is evident that every Si-X bond being hydrolysed is capable of forming a siloxane bond by means of condensation. Hence the functionality of the organosilane determines its crosslinking behaviour in hydrolytic polycondensation and polymerization to polyorganosiloxanes and finally the structure of the latter. Accordingly, polyorganosiloxanes are composed of mono-, di- and trifunctional structural units as shown in Table 3.6 [88]. Compounds of the type Six4 give pure silica consisting of quaternary units.
,
89
TABLE 3.6 STRUCTURAL UNITS IN SILICONE CHEMISTRY (ACCORDING TO NOLL [ 881) Structure of unit
Functionality
Composition
Source
R,Si-0-
Monofunctional = M
R,SiO, ,2
R,SiX
Difunctional = D
R2SiO*,2
R,SiX,
Trifunctional = T
R,SiO,,,
R,SiX
Quaternary = Q
sio,,,
Six,
R I
-0-Si-OI
R R I
-0-Si-OI
0 I I
0 I
-0-Si-OI
0 I
Monofunctional organosilanes such as R3SiX form only disiloxanes. In mixtures of RzSiX2, RSiX3 and Sixl they can also work as terminators to end a chain. Bifunctional compounds such as R2SiX2 alone form chains and also cyclic organosiloxanes. Threedimensionally linked products are obtained by hydrolysis of trifunctional compounds of the type RSiX3. Technical silicones, for instance, are manufactured by co-hydrolysis and co-condensation of a mixture of methyl- and phenylchlorosilanes containing mono-(M), di(D), tri-(T) and tetra-(Q) functional units. The products are liquids, oils and greases of various viscosities. Our main interest, however, is only in polyorganosiloxanes which exhibit a rigid pore structure. They can be obtained by total hydrolysis and condensation of trifunctional organosilanes: R-Six3
+ RSi(OH)3
+
(RSi0312)n
(3.49)
Although hydrolysis will be the first step, condensation follows rapidly so that both reactions proceed simultaneously at different rates. Usually reaction is initiated by addition of the organosilane to an excess of an acidic or basic solution [89]. As the reaction is exothermic, the heat liberated has to be removed by cooling. Acids or bases catalyse the hydrolytic cleavage of the Si-X bond. The Si-OH groups formed undergo spontaneous condensation to siloxane groups. In order to achieve complete condensation, a specific after-treatment is necessary. A large number of studies have been made in this field and most information is available on the reaction of methyl- and phenylsilanes [89]. The most important results are summarized below.
90
The rate of hydrolysis of an R-Si-X bond is known to be accelerated by increasing the polarity of the substituent X as well as by increasing the number of Si-X bonds per molecule. On the other hand, the bulkier the organo group R, the smaller is the rate of cleavage of the Si-X bond. This effect is due to steric hindrance to water molecules which attack the Si-X bond. For instance, hydrolysis of phenylchlorosilanes is much slower than that of the corresponding methylchlorosilanes. The type of solvent used has also been found to have considerable influence. Compounds such as toluene, xylene and diethyl ether which are immiscible or partially miscible in the aqueous phase, are known to be excellent solvents for both organosilane and polyorganosiloxanes. In this way, a high dilution of the organosilane can be achieved and the product formed readily dissolves in the organic phase before it undergoes further reaction. The choice of an appropriate solvent and the dilution of reactive organosilane is an effective means of controlling the intermolecular and intramolecular condensation of intermediately formed organosilanols. In contrast to inert solvents such as toluene, polar solvents such as alcohols form S i - O R groups by means of alcoholysis, which again influences the hydrolysis and condensation behaviour. Addition of bases instead of acids as catalysts leads to neutralization of the acid evolved by hydrolysis and may have a stabilizing effect on the intermediately formed organosilanols. The condensation of organosilanols is also catalysed by bases or acids. The mechanism of acid-catalysed condensation can be written as (3.50)
Whereas the first step occurs very rapidly, the second seems to be the rate-determining step in acidic condensation. The base-catalysed reaction mechanism may occur as follows: SSi-OH
+ OH- + GSiO- t HzO
=Si-O-
t GSi-OH
-+ ESi-O-Siz
(3.52)
+ OH-
(3.53)
Here, the nucleophilic attack of the siloxanyl anion is considered to be the rate-determining step. In highly crosslinked polyorganosiloxanes, the condensation of the residual hydroxyl groups proceeds very slowly owing to steric effects. Condensation may be accelerated and completed by treatment with organosilanes: ESi-OH t R-Sir
-+ =si-O-Sir
+ RH
(3.54)
This type of reaction is termed “silanolysis” [89]. Condensation may yield a homogeneously or heterogeneously crosslinked polyorganosiloxane. Heterogeneous crosslinking means that siloxane structural units are generated that differ widely in their arrangements of siloxane bonds. For instance, small cyclic units may be associated to quaternary units.
91
A comment should be made about the terms polycondensation and polymerization [89]. Polycondensation covers all processes in which organosilanols or organosiloxanols are involved. The term polymerization is reserved for the reactions of siioxanes free of hydroxyl groups. For instance, the conversion of low-molecular-weight cyclic siloxanes into highmolecular weight species by means of thermal treatment is of the polymerization type. Polymerization may also be base and acid catalysed. Also, copolymerization may occur. In the known cases it is extremely difficult to distinguish clearly between polycondensation and polymerization. So far we have discussed the formation of polyorganosiloxanes as a growth into a three-dimensionally linked network of siloxane bonds. In some processes, however, the formation is considered on the basis of the corpuscular theory previously described in Chapter 2. The organosiloxanes are viewed as particles with colloidal dimensions which grow and agglomerate to form a polyorganosiloxane gel or organosilicon gel. During dehydration the particles are cemented together to yield a porous xerogel [90]. The variety of products and product compositions can be considerably increased by co-hydrolysis, co-condensation and co-polymerization of organotrialkoxysilanes and tetraalkoxysilanes. This mode permits the control of the R:Si02 ratio in the final product by varying the amount of RSi(OR)3 relative to that of Si(OR)4. An increase in Si(OR), relative to RSi(OR)3 leads to a more mechanically stable product. Another feature that needs attention is that the different rates of hydrolysis and condensation of the two starting compounds may lead to a heterogeneously crosslinked framework. As an extreme instance, the formation of clusters having quaternary units incorporated in other structural units should be strictly avoided. Hydrolytic polycondensation of organosilanes carrying unsubstituted R groups yields products with a hydrophobic character. The hydrophobicity of the surface can be determined by means of sorption measurements using different polar gases or vapours. Using substituted organosilanes such as YR’SiX3, where Y is NH2 or S03H, hydrophilic silica supports can be synthesized by polycondensation of the silane alone or in combination with silanes of the type Six4. 3.2.1.3.2 Surface modification Surface modification simply means a heterogeneous chemical reaction which takes place at a solid-gas or a solid-liquid interface to yield an immobile chemically bonded surface layer. When the reaction is limited t o the formation of a monolayer, it can be viewed as chemisorption [ 9 1 ] ,Porosity of the support creates additional problems in surface modification owing to the diffusion of the reactant from the exterior of particles into the pores to the active surface sites. Diffusion can be activated or non-activated depending on the reaction system and on the conditions. Further, owing to the surface heterogeneity of silica some surfxe sites that are favourably positioned will react rapidly while others may require a considerable activation energy. For the reasons outlined above, a quantitative treatment of the surface modification of porous silica is extremely difficult and only limited evaluations can be made. In order to understand the type of reaction and its mechanism the different reactants should be considered.
92
An incompletely dehydrated silica surface is usually covered with a multilayer of physisorbed water. On addition of an organochlorosilane as a reactant the water present produces an organosilanol by means of hydrolysis, which rapidly undergoes condensation to an organosiloxanol and an organosiloxane. It is likely that at least the organosiloxanol will react with the surface hydroxyl groups. As an oversimplified picture, a chemisorbed layer is formed as a skin of crosslinked organosiloxane which is held to the surface by a few siloxane bonds. When the amount of physically adsorbed water becomes negligibly small, isolated, geminal and vicinal hydroxyl groups comprise the active surface sites. It seems reasonable that each of these types will possess a different reactivity depending on its structure and its accessibility. Annealing of silica at temperatures up to about 773 K causes partial surface dehydroxylation, yielding strained and hence highly reactive siloxane groups [41]. The role of such siloxane groups in addition to hydroxyl groups in surface interactions is not clear. When trace amounts of water are present in the reactant the siloxane groups will readily give hydroxyl groups. In the presence of alcohols, however, ester bonds are formed. In conclusion, the pre-treatment conditions of silica predominantly govern the type and reactivity of surface species that are involved in the reaction. In order to bind a desired organic group, R, to the silica surface, one has a choice between different homologous organosilanes of the type RnSiX4-, (1 < n < 3). Depending on the functionality of the modifier (mono-, di- or tri-), various reaction mechanisms may take place, yielding different kinds of surface species. With a monofunctional modifier, the following mechanism seems to be the only probable one, assuming that only hydroxyl groups are on the surface:
Si-OH t XSiR3 + Si-O-SiR3
t HX
(3.55)
i.e., one molecule reacts with one hydroxyl group. As a measure of the stoichiometry of
the reaction, a factor f may be introduced, defined as
f=
moles of hydroxyl groups reacted
(3.56)
moles of modifier reacted
In eqn. 3.55 f = 1. Using a bifunctional modifier of the type RzSiXz, two possible mechanisms exist: R
R S Si-OH
+
I
--c
X-SIR
I
I ss1-0-s~~+
HX
I
X
(3.57)
X
and \ -SI-OH
d\
-SI-OH /
X
+
\
X/
\ -5-0
R
Si
/ --C
\R
/ \S1/R ~S,-O/ R ‘
/
f 2HX
(
3.58)
93
Depending on the ratio of vicinal or geminal to isolated hydroxyl groups on the surface, fvaries in the range 1-2. The second mechanism (eqn. 3.58) will occur only with geminal and vicinal hydroxyl groups, the latter being separated from each other by not more than 0.3 nm [18]. m i x 3 may react according to the following mechanisms:
( 3.59)
-
\
-Si-OH
O\
\
+
/
\ -Si-OH
0 -St-OH \
SI
/
\
X
-St-OH /
/
X
X
+
R
’
x “st’
+
+
2nx
(3.60)
/
\ -5.1-0
X X \ / /SI\
X
\
-st-0
+
HO-SiG
R
’
0
-\S,-O’
O-SIE
’
\ SI
+
3HX
(3.61)
\R
/
/
While the first two mechanisms are probable, giving a stoichiometric factor off = 1 or 2, the third mechanism can be excluded for steric reasons. As a result of the different mechanisms, the following surface species will be produced: for R 3 S ( X :
sSI-O - SI R~
(a)
for R 2 S ~ X 2
=St-0-StRp
( b)
X and/or
X
and/or
x
-S-O / ,
O\ -Si-0
‘s,’
/
\
(e) R
/
In (a) and (c) bonded groups carry exclusively R groups. In (b) and (e), in addition to R groups, X groups are bonded in the same concentration. In (d), however, the number of new X groups being formed exceeds the number of R groups. Up to now the discussion has been highly general and the question arises of whether a certain stoichiometry can really be found in surface modification. This topic was con-
94
sidered in detail in a series of papers using different types of compounds. As an example, the results of the reaction between a given porous silica and the three homologues phenyltrichlorosilanc (PTCS), diphenyldichlorosilane (DPDCS) and triphenylchlorosilane (TPCS) will be discussed [92]. In order to establish the reaction mechanism, the following quantities were measured: carbon content of the product; chlorine content of the product before hydrolysis; increase in the silicon content of the product by means of the reaction; and the total weight increase of the product due to surface modification. Evaluating the ratio of the surface concentrations of different types of bonds in the three compounds, asi : ac : ac1, from the data in Table 3.7, one obtains the following: PTCS, 1.00:6.00:1.01;DPDCS, 1.00:11.60:1.01;and TPCS, 1.00:18.15:0. It can be concluded that in the reaction of PTCS, phenylmonochlorosilyl groups are formed, indicating that one molecule of PTCS reacts with two hydroxyl groups a n d f = 2. With DPDCS one molecule reacts with one hydroxyl group and diphenylchlorosilyl groups are formed. With TPCS, obviously triphenylsilyl groups are obtained. The weight increases for the reactions listed in the last column of Table 3.7 confirm the reaction mechanisms derived from the analytical data. The results are valid only for the phenylchlorosilanes and caution is required on generalization of these findings to all compounds of the type R,SiX4,. A bifunctional reaction mechanism could also be established for the surface reaction between porous (AEATS) as a modifier according silica and a 1-aminoethyl-3-aminopropyltrimethoxysilane to \
- Si -OH /
0
\
-Si-OH
CH,O
+
CH30’
’
OCH3
‘5,
‘CCY
___c
)3-NH-CH,-CHz-NHz
-\ 0 -0\5!/0cH3 \ / \ -51-0 (CH2)3-NH-CHz-CH2-NH2
+
2 CH30H
(3.62)
,’
The ratio of the surface concentrations expressed in microequivalents per square metre should be W : a N : a O C H , = 6.0:2.0:1.0
(3.63)
The ratio found was [93] : CVC:CKN:CYOCH, = 6.09:2.22:1.0
(3.64)
Surface modification is not restricted to the formation of a monolayer. Multilayers of desired thickness can be built up by repetitive reaction onto single layers, and chemically bonded polymer layers or polymer coatings can be deposited at the surface. The development of multilayers requires the preceding layer to have reactive surface sites. This can be achieved by the use of di- and trifunctional organochlorosilanes, yielding surface species such as
95 R
R
I =S~-O-SI-CI
I
or
=Si-O-s1-ccI
I
(3.65)
I
R
CI
After hydrolysis, 3 - C l groups are converted into S i - O H groups, the latter being capable of further reaction with organodi- or organotrichlorosilanes: R
R
CI
I GSN-O-SI-OH I
\
+
/
/SI,
Cl
R
-
R
I GSI-0-SI-0
's,/
1
R
CI
/
\
R
(3.66)
R
During reaction, crosslinking via siloxane links may take place between vicinal bonded groups. TABLE 3.7 ANALYTICAL RESULTS FOR SURFACE-MODIFIED PRODUCTS OBTAINED BY REACTION OF PHENYLTRICHLOROSILANE (PTCS), DIPHENYLDICHLOROSILANE (DPDCS)AND TRIPHENYLCHLOROSILANE (TPCS) AND A GIVEN SILICA Silica: SBET= 330 m*/g; V p = 1.65 ml/g;D = 20 nm [92]. -____.
Modifier
Temperature of reaction (K)
Content of elements (pequiv./m2)
c1
C
Si*
Total weight increase (mg/g)
PTCS
403 463
2.95 3.59
17.58 21.66
2.95 3.64
149 181
DPDCS
373 473
1.70 2.60
18.60 27.60
1.57 2.55
124 171
TPCS
413 573
-
27.60 29.70
1.48 1.66
133 140
*Increase of Si through surface reaction.
Instead of hydrolysing the remaining chlorine residues after reaction, the modified support can be treated directly with organodichloro- or organotrichlorosilanes in an atmosphere containing trace amounts of water or of water vapour, for example: R
I
=si-o-s1-x
R
I
R
ES, -O-S, "\ IR
/ X
SI
t 2H20
I
-0
I (3.67)
\ R
~s~-o-sI-o I
R
R
as,- 0 - S I
I
I
-x
96
It is likely that organosiloxanols will also be formed as precursors, which undergo further condensation with surface-bonded hydroxyl groups. Another type of silane reagent which offers a high variability with respect to substitution reactions was introduced by Parr and Novotny [94], viz., Y
'-
Y '- Si
I
(CH2lnO
C
H
2
X
Y*
where Y,X = C1 and Y',Y" = C1 or CH3. Substitution reactions are possible at the -C6H4-CH2Cl residue in the bonded layer. Furthermore, polymer layers can be deposited by treatment of the silica support with a mixture of selected trichloro-, dichloro- and monochlorosilanes. A similar procedure for producing polymer bonded layers starts with the preparation of polyorganoalkoxysiloxanes by partial hydrolysis of organoethoxysilanes under controlled conditions. The support is then coated with the polymer and reaction is completed by heat treatment in the range 273-523 K. The bonded polymer, of the type
ZSi-0-
-
[:In Si-0-
-
has a crosslinked structure of repetitive organofunctional groups. The thickness of the layer can be varied between 3 and 1000 nm [95].
3.2.1.3.3 Special topics in surface modification 3.2.1.3.3.1 Kinetics in surface reactions In surface reactions, the porous support is brought into equilibrium with the gaseous or liquid reactant. As the reaction takes place exclusively at the inner surface of the porous particles the kinetic aspects become important. From this point of view, the reaction can be divided into the following steps: (1) diffusion of the reactant to the active surface sites of the support; (2) adsorption of the reactant at the inner surface; (3) chemical reaction on the inner surface yielding surface-bonded species; (4) desorption of non-bonded products from the inner surface; and ( 5 ) diffusion of non-bonded products from the surface. Processes 1-5 occur consecutively and the slowest will determine the overall reaction rate. However, owing to their complexity it is not easy to separate the influence of the single steps. Pore diffusion is known to be different from ordinary molecular diffusion in gases and liquids because it is affected by the pore structure. As the free cross-sectional area available for diffusion is less than the same volume of bulk solution, the flux is correspondingly reduced. The proportionality factor will be the internal porosity, e P ,of the porous particles.
97
In addition, the effective diffusion coefficient, Deff(i),for a solute i is smaller in porous particles than in the bulk phase because of geometric factors. The most important of these is the tortuosity factor, 7 , which results in an enhanced diffusion path length. In conclusion, the relationship between the effective diffusivity, Deff(i),and the diffusivity in the bulk phase, Di, for a solute i is given by [96] (3.68)
Other equations were derived, involving an additional term that represents the slope of the adsorption isotherm [ 9 7 ] . Another pore structure parameter that affects the diffusion mechanism is the mean pore diameter, D, of the support and its ratio to the molecular diameter, di, of the reactant i. If D is several orders of magnitude larger than di, diffusion is similar to bulk diffusion. To a first approximation Deff(j)increases linearly with D [ 9 7 ] . If D is only a few times larger than di the effective diffusivity increases considerably with decreasing D . This is caused by an enhanced adsorption interaction between the solute i and the closely adjacent pore walls. When D becomes comparable to di, repulsion forces become noticeable and diffusion requires a high activation energy. The relationships discussed give only a rough estimation of the effect of D on kinetics because not only the pore size distribution of the support has to be considered but also the molecular diameter, dj, of a reactant has to be characterized by more than one linear dimension. If the pores are large enough (D > 100 nm), the gas is relatively dense or the pores are filled with liquid, the diffusion process follows that of bulk diffusion. At a very low gas pressure and if small pores are present, Kundsen diffusion takes place, resulting in a small K effective diffusivity Deff(+ (3.69) where DeKffci)is the Knudsen diffusion coefficient of solute i. Under these conditions the mean free path, &, of a gaseous solute i is larger than D (&/D>> 1 ) and the molecules collide with the pore walls much more frequently than with each other. At low gas pressures, adsorbed molecules may show a special transport phenomenon termed surface diffusion. In this particular case the diffusion of adsorbed species is much faster than for non-adsorbed species. In surface modification one has to deal mainly with a type of bulk diffusion, because the conditions for Knudsen and surface diffusion are not present. Further, one should bear in mind that the bulk diffusivities in liquids are of the order of lo4 times smaller than the diffusivities in gases, Le., very slow diffusion of solutes occurs in liquids. The range of diffusivities in gases and liquids is shown schematically in Fig. 3.6. Diffusion is also influenced by temperature. If the diffusion obeys Fick’s laws and is non-activated, its rate should be proportional to the square root of temperature. On the other hand, slow diffusions are activated and the rate is exponentially dependent on temperature [48].
98 2
D (ern Isecl
I
I
I
I I
Knudsen diffusion in m a l l pores ( A i / D 9 1 )
Fig. 3.6. Comparison of diffusivities of gases and liquids in bulk and porous media.
With respect to the overall rate of reaction, two cases can be distinguished: (i) if the rate of reaction (step 3) is small and a high rate of diffusion (step 1) exists, then the reaction rate will hardly be influenced by diffusion; (ii) however, if the reaction rate is relatively high and the diffusivity of reactant 1s small, then the reaction rate becomes diffusion controlled. Diffusion is followed by physical adsorption of reactants (step 2), which is usually nonactivated and exothermic [48]. In pure physical adsorption the amount adsorbed at constant pressure decreases with increasing temperature. When chemisorption takes place and it requires an activation energy, the amount adsorbed increases with temperature. Accordingly, differences may arise in the heat of physisorption and chemisorption processes. A thorough discussion of the various aspects was given by Hayward and Trapnell [48]. The chemisorption at the surface of porous silica can be considered as activated for the following reasons: the surface exposed t o the reactant is not clean in the strict physical sense but is covered with gases such as nitrogen, etc.; the active surface sites are heterogeneously distributed and hence possess a different reactivity; and steric effects may play a dominant role due to the pore structure. During reaction the formation of precursors and intermediates seems highly probable. For instance, in the reaction of porous silica and organochlorosilanes the hydrochloric acid evolved may be preferentially adsorbed at the unreacted surface sites, impeding further attack by the reactant. Measurements of activation energies were carried out on non-porous finely divided silica for the methylchlorosilanes as modifiers. By means of infrared spectroscopic studies Hair and Hertel [31] obtained values that are fairly close for all three compounds, suggesting a nearly identical reaction mechanism. In contrast, Evans and White [32] found from gravimetric adsorption measurements a higher value for trimethylchlorosilane than for the other t w o . Kinetic studies based on spectroscopic measurements [32] reveal that the monofunctional silane TMCS follows 1.@order kinetics with respect to the number of surface sites, which indicates that all surface groups have nearly the same reactivity and react independently of one another. For the di- and trifunctional methylchlorosilanes 1.5 0.2-order
*
99
kinetics were established, suggesting that 50% of the corresponding silane molecules react monofunctionally and 50%difunctionally. In a study of the reaction between porous silica and phenyltrichlorosilane it could also be confirmed that the final maximum conversion increased with the reaction temperature, again indicating that the reaction is activated [98].
3.2.1.3.3.2 Reactors in surface modification A variety of reactors have been proposed in surface modification. The design and scale is determined either by the amount of support to be modified or by the object of the study. If the main interest is the study of surface reactions alone by means of physical and physico-chemical methods, small amounts of sample are employed. For instance, in spectroscopic investigations the support material is pressed into small discs that are placed into specially designed cells [ 151. When conventional analytical investigations on surfacemodified products are to be carried out, appropriate glass apparatus is designed that permits outgassing, reaction and desorption consecutively without the need to transfer or move the sample [98]. With highly volatile reagents such as trimethylchlorosilane, silicon tetrachloride and thionyl chloride, and applying high reaction temperatures, the use of sealed glass ampoules is necessary [99]. Preparation on the laboratory scale (1-100 g of support) is preferably carried out in a round-bottomed glass flask, fitted with a stirrer, refluxer and a device for feeding in dry nitrogen [ 1001. After adding the pure reagent or its solution, reaction is initiated by increasing the temperature. The suspension is kept at constant temperature under stirring and refluxing for the period required to achieve maximum conversion. Then the suspension is filtered and the modified product is washed with different solvents to remove the excess of the modifier. As an alternative to washing outgassing under vacuum is also possible. An in situ reaction of silica support which was pre-packed in a column was proposed by Gilpin et al. [ 1011. After outgassing the support at a sufficiently high temperature in a gas stream, the silane solution was pumped continuously through the column for a given time. Soluble products were then washed out by rinsing the column with a range of solvents. For the production scale (1-10-kg charges), slurry reactors can be employed. The slurry is agitated by bubbling gas through the suspension or by mechanical means. Problems may arise in maintaining an overall constant temperature during the reaction because the support has a low heat conductivity and the reaction is exothermic. In this respect fluidized-bed reactors have advantages over slurry reactors because the wetted support particles are held in constant motion by means of a turbulent gas stream and a uniform temperature is maintained throughout the entire reactor [96]. 3.2,1.3.3.3 Conversion in surface reactions As already mentioned, reaction should preferably be carried out at high temperatures. The maximum temperature is limited by the decomposition behaviour of the reagent. The conversion at room temperature is usually low. For the methylchlorosilanes it has been found that on Aerosil that had been previously outgassed at 623 K only physisorption takes place at 293 K. Irreversible adsorption of methylchlorosilanes occurs at temperatures
100
above 573 K [32]. For the reaction of phenyltrichlorosilane with porous silica a sharp increase in the amount chemisorbed is observed on increasing the temperature above 323 K (see Fig. 3.7). However, an increase in reaction temperature above 473 K does not result in a higher conversion. This result may be evidence that a monomolecular layer of phenylchlorosilyl groups is formed under these conditions. On the other hand, instances are known in which a relatively high conversion was obtained at 300 K, e.g., for the reaction of a chlorinated silica sample with an ethereal solution of an organolithium compound [99] It is not easy to detect whether a surface-modified product contains only chemically bonded species. The most common way is to extract the product consecutively with a series of different solvents and to analyse the solvent aftei extraction. However, caution is required in this procedure; as shown by Aue [76], polymer layers physically bonded on supports withstand exposure to solvent extraction. In a mathematical evaluation of the maximum conversion in surface modification, it is usually assumed that the heterogeneous reaction is irreversible and its kinetics are simply first or second order. In the formation of a chemisorbed monolayer the conversion is limited by the maximum number of active surface sites that are accessible to the reactant and able to participate in the reaction under given conditions. Considering a fully hydroxylated silica surface after outgassing at 473 K, the total number of hydroxyl groups per unit surface area is about 4.8/nm2. This corresponds to a mean surface concentration of CYOH= 8.0 pmolelm’. From this quantity, the mean cross-sectionalarea occupied by one hydroxyl group is calculated to be &(OH) = 0.21 nm’. Consequently, the maximum
3.0
2.0
1.0
0 0
10
20
30
LO
reaction time ( h l Fig. 3.7. Surface concentration of phenylchlorosilyl groups expressed in microequivalentsof chlorine per square metre per gram of adsorbent as a function of the reaction time at different reaction temperatures [ 9 8 ] . Modifying reagent, phenyltrichlorosilane;sample, porous silica, SBET= 330 m’/g.
101
conversion in a 1 :1 reaction should never exceed 8 pmolelm’. The ratio of the number of hydroxyl groups that undergo reaction, nOH(react), to the total number of hydroxyl groups present before, nOH(total), can be termed the effectiveness factor, r): 9=
nOH(react) = “OH(react) nOH(tota1) aOH(total)
(3.70)
Using fully hydroxylated silica samples [aOH(total) = 8 pmole/m2], aOH(react) can be evaluated either by direct measurement after reaction or by indirect calculation from the carbon content provided that the reaction follows a certain stoichiometry, with r) < 1. However, 7) is not only determined by the number of hydroxyl groups that react under optimal conditions, but also by the mean cross-sectional area of the given bonded group. From this point of view the maximum conversion reaches a limiting value that corresponds to a dense two-dimensional arrangement of bonded groups at the surface. If the A m value of a bonded species exceeds that of the hydroxyl groups the effectiveness factor becomes smaller than unity and the maximum conversion is less than 8 pmole/m2. A m values of bonded species are not easy to derive because of uncertainties in bonding angles, bond lengths and configuration. The same problem arises in physisorption, in which A m values of adsorbed molecules are needed for the calculation of the specific surface area. To a first approximation A m for a chemisorbed species can be assumed to be equal to that for the same species in a physisorbed state [91]. Methods for evaluating A m values of adsorbed molecules were surveyed by McClellan and Harnsberger [ 1021. The most common determination is made from the density of adsorbed molecules, allowing for the shape and packing density. For spherical molecules in a plane hexagonal close packing arrangement the following equation results: A m (nm2/molccule) = 1.092
E)2 -
(
’3
10’
(3.71)
where 1.092 ii, a packing factor, considering the six nearest neighbour molecules in the plane, M the molecular weight, N Avogadro’s number and p the density of the adsorbate in the ordinary liquid or solid form [ 1031. For example, for nitrogen as the most commonly used adsorptive in surface area determinations a value of Am(N2) = 0.1 62 nm2/molecule can be calculated and is now fixed as an internationaI standard [ 1041. For the smallest hydrocarbon molecule, namely methane, which is spherical in shape, an A m value ranging between 0.1 5 and 0.20 mmz was proposed [ 1021. For aromatic hydrocarbons such as benzene the corresponding A m value will differ widely depending on the steric orientation of the adsorbed molecule relative to the surface [ 1021. The second approach for evaluating the A m value of an adsorbed molecule is based on its shadowed area obtained from molecular models and a shadow graph. Again, for molecules deviating from a spherical shape an over- or underestimation of A,,, is possible -r1021. Finally, A m can be assumed to be equal to the two-dimensional Van der Waals constant b derived from the critical constants of the bulk substance. According to [lo21
102
cy3
b (nm’) = 0.06354 -
(3.72)
where Tc is the critical temperature and Pc the critical pressure. According to the several possible definitions, the A , value derived should be accompanied by sufficient information. Using the A,,, value of a chemisorbed species a maximum conversion, expressed in moles per unit surface area, can be estimated as 10l8 amax(molelm’) = ___ AmN
(3.73)
where N is Avogadro’s number. On the other hand, the conversion obtained experimentally under optimal conditions can be calculated from analytical data, such as the carbon content: (3.74)
where w is the weight of chemisorbed species in grams per gram of adsorbent, M its the specific surface area of the starting silica corrected by molecular weight and SBET* the weight increase due t o modification. In this instance it is assumed that the bonded groups are homogeneously distributed a t the original surface and the mean pore diameter of the support is about one order of magnitude larger than the molecular diameter of the reactant . The ratio of both quantities gives the surface coverage, 8, as (3.75)
Summarizing the results, two aspects can be distinguished in surface modification. The first aspect considers the degree of conversion of hydroxyl groups that are present at the surface using the effectiveness factor, 77, where 9 < 1. The second aspect considers the degree of conversion on the basis of the mean molecular cross-sectional area of a bonded group. The surface concentration obtained experimentally, being a measure of the conversion, should never exceed the surface concentration derived theoretically from the A m value. In other words, the surface coverage should be 8 < 1.O. The two quantities 0 and 77 are each other’s complements. If 0 < 1 obviously 17 can never reach unity and should also be less than 1.0. If 8 = 1 the quantity 7 , however, needs not necessarily be unity, but can attain a smaller value depending on the A,,, value of the bonded group relative to the A m value of the hydroxyl groups. The validity of the expressions derived and their applicability will be demonstrated for some specific instances. The most intensive studies on surface modification have been made with trimethylchlorosilane (TMCS) as modifier. TMCS reacts with hydroxyl groups in a 1 : 1 ratio. The maximum surface concentration of trimethylsilyl groups, aeXp(TMS), is about 4.0 2 0.5 ymole/m’ on porous silicas of different origin and on ;on-porous finely dispersed silica such as Aerosil [ 1051. The molecular cross-sectional area of a trimethylsily1 group, A,(TMS), has been calculated to be 0.37 nm2 per group according t o eqn. 3.71 [ 106,107],
103
to 0.35 nm2 derived from Si-C and C-H bond lengths of 0.189 and 0.108 nm, respectively [ 131, and to 0.43 nm estimated from the Van der Waals radii of the group [ 1081. From these data an average value of 0.38 nm2 per TMS group results, giving a maximum surface concentration, a,a,(TMS), of 4.3 pmolelm’ . The agreement between a e x p ( m Sand ) amax(TMS)is fairly good, indicating a 100%surface coverage. While 6 = 1.O, the effectiveness factor, r ) , however, is 0.5 with fully hydroxylated porous silicas, i x . , only 50% of the total number of hydroxyl groups are able to take part in the reaction. Indeed, a surface concentration of &OH = 3.2 pmole/m2 could be measured with a TMCS-modified silica. This is due to unreacted hydroxyl groups and additionally supports the results derived from elemental analysis [35]. Adsorption studies on a trimethylsilylated Aerosil surface and calculation of adsorption potentials by Babkin and Kiselev [ 1091 reveal that the density of TMS groups is much less than those of oxygen atoms and hydroxyl groups at the original silica surface. Even with the closest packing of TMS groups the distances between two adjacent methyl groups correspond roughly to their Van der Waals radii of about 0.4 nm. Babkin and Kiselev [ 1091 stated that even with maximum coverage hydroxyl groups still remain between TMS groups, being less or more accessible to adsorptives. The occurrence of such coating defects in surface modification can generally be observed in the reaction of porous silica and organosilanes, particularly with reagents that possess bulky organic groups such as phenyl groups. For instance, the surface concentration, aeXp, was found to decrease in the order trimethylsilyl > dimethylphenylsilyl > methyldiphenylsilyl > triphenylsilyl groups under optimal conditions [ 1 101. On varying the chain length of the n-alkyl groups in n-alkyldimethylchlorosilanes, a similar effect can be observed [ 1 1 11. The results are given in Table 3.8. The maximum surface concentration, an-alkyl, ranges between 2 and 4 pmole/m2 and becomes smaller with increasing n-alkyl chain length. This is caused by the increasing average molecular cross-sectional area of modifiers in the order trimethylchlorosilane < n-butyldimethylchlorosilane < n-octyldimethylchlorosilane < n-dodecyldimethylchlorosilane < nhexadecyldimethylchlorosilane. As the respective monochlorosilane reacts according to a 1 :1 mechanism with the surface hydroxyl groups, the sum of an-alkyl + OH(^) (unreacted) should be equal to the CYOH(~) value of the native silica. As can be seen from the values in Table 3.8, this statement is supported by the experimental findings. On basis of the surface concentration, an-alkyl, the mean molecular cross-sectional area, A m , of the corresponding n-alkyldimethylsilyl group can be estimated and values are given in Table 3.8. Further, it is possible to calculate from A m the mean distance between two adjacent bonded groups. The mean distance between trimethylsilyl groups is 0.80 nm and that between n-hexadecylsilyl groups is 0.94 nm [ 11 11. A model of a surface modified silica with n-octyldimethylchlorosilane is presented in Fig. 3.8. The n-alkyl chains are bonded at one end via a siloxane link to the original surface and certainly have some rotational mobility. One case of surface modification was observed in which almost all hydroxyl groups can be brought into reaction [99]. According to S - O H + SOCl2
-+
=Si-Cl+ SO2 + HCl
(3.76)
the surface concentration of bonded Si-Cl groups evaluated from the chlorine content
104 TABLE 3.8 DATA FOR SILANIZED SUPPORTS Type of modifier
Trimethylchlorosilane n-But yldimethylchlorosilane n-Octyldimethylchlorosilane n-Dodecyldimethylchlorosilane
n-Hexadecyldimethylchlorosilane
Designation of product
C (%)
4.18 TMCSsilica 7.13 BDMCSsilica ODMcS-sili~i 10.43 DDMCS-silica 1 1.73 HDMCS-silica 15.67 Originalsilica
-
Surface concentration bmole/m') an-alkyl
aOH(s)
Mean molecular cross-sectional area (Arn)of bonded group (nm'/grouP)
3.37 2.97 2.71 2.20 2.36
2.98 3.68 3.73 3.81 4.24
0.49 0.56 0.61 0.76 0.70
-
7.63
-
H H-C'
I
H
I
\ HLC-H
H
/
H-CNH
AC-" H-C'
H-C'
/H
\
H\C-H
H-C'
/H \
H.P-,
Fig. 3.8. Schematic representation of a silica surface modified with n-octyldimethylsilyl groups.
was found to be (Yexp = 7.0 pmole/m2, choosing an optimal set of conditions. The starting material was fully hydroxylated OH = 8.0 pmolelm'). 3.2.1.3.3.4 Effect of surface modification on pore structure properties of silica It is sometimes neglected that surface modification not only changes the chemical nature of the parent silica but also affects its pore structure. The variation of pore structure parameters can easily be monitored by comparing the specific surface area, the specific pore volume and other properties of the native and the treated silica. The attachment of a chemisorbed layer, being of a monolayer or polymer layer type, is expected to
105
cause a decrease in the mean pore diameter by the thickness of the layer, t. Corresponding ly, the specific surface area and the specific pore volume also decrease. As the deposition of a polymer layer within a porous particle is less controllable with regard to a uniform thickness compared with a monolayer, we prefer to treat this subject exclusively on monolayer types of chemically bonded silica packings. The most striking feature that determines the extent of pore structure alteration is the ratio of the size of the modifier molecule to be bonded to the pore size of the silica. Depending on this ratio three cases can be distinguished: (i) The molecular diameter of the reagent, d,, is several orders of magnitude smaller than the pore diameter, D: (3.77)
The bonding of a monolayer on the walls of such large pores, i.e., macropores with D > 50 nm will have a neglible effect on the pore structure. (ii) The molecular diameter of the reagent is similar to the pore diameter: d,
=D
(3.78)
This situation may occur when using microporous silica supports with D < 3 nm and a bulky modifier. In this instance the diffusion of the modifier from the outer surface of the particle into the small pores is sterically hindered to a large extent and the molecules mainly react at the pore openings, blocking the remaining pore space. (iii) The molecular diameter of the reagent is one or two orders of magnitude smaller than the pore diameter: d, < D
(3.79)
This is the most common situation in surface modification and will occur when using mesoporous silicas with D = 5-50 nm and the usual silanizing reagents. This case will be discussed further in detail. Let us assume a mesoporous silica, the pore system of which can be represented as one pore of cylindrical shape of length I and radius r. The surface area of the pore before modification will be S = 2nd
(3.80)
and its volume will be V = nr21
(3.8 1)
After bonding a uniform layer of thickness t , the pore radius, r, is reduced by the thickness t , as shown in Fig. 3.9. The surface area of the modified pore then can be written as (3.82)
and the volume as (3.83)
Combining eqns. 3.80 and 3.82, one obtains
106
smod -
- 2dr-f)l
S
=
(7)
2nd
(3.84)
The same can be done with eqns. 3.81 and 3.83: (3.85)
Further, the following relationship should hold between eqns. 3.84 and 3.85: Smod
-
(
Vyd)
1'2
(3.86)
S
To prove the validity of the model [ 1121 one has to replace S and V by measurable quantities such as the specific surface area and the specific pore volume, respectively. To a first approximation, r is assumed to be equal to the most frequent pore radius of the relative pore volume distribution, rmax. From measurements both rmax of the untreated silica and rmax(mod) of the treated silica are available. rmax(mod) should obey the equation rmax(mod) = (rmax
-t )
(3.87)
where the thickness of the layer, t , can be estimated from scale models of the bonded moiety. This approach will be tested for a series of silanized silicas exhibiting a different chain length of the n-alkylsilyl group [ 1 1 1 1 . The data are given in Table 3.9. Firstly, in comparing the ratio rmax - t/rmax and the ratio r,x(mod)/rmax (Table 3.9), one can see a fairly good agreement, which indicates that the n-alkylsilyl groups are bonded perpendicularly at the surface as t is taken as the length of the respective n-alkylsilyl group. Comparison of the respective relative surface area ratio with the corresponding relative pore
1 I
I
I
I
Pig. 3.9. A cylindrical pore before and after bonding a uniform surface layer. r = pore radius before bonding; r* = pore radius after bonding; t = thickness of the bonded layer.
TABLE 3.9 VARIATION OF PORE STRUCTURE PARAMETERS BY MEANS OF SURFACE MODIFICATION SBET= specific surface area according to the BET method; V p = specific pore volume according to the Gurvitsch rule; t = thickness of the bonded layer; rmnY= most frequent pore radius of the relative pore volume distribution.
Originalsilica TMCSsilica BDMCSsilica ODMCSsilica DDMCSsilica HDMCSsilica
376 205 277 252 244 205
1.258 1.168 1.034 0.910 0.883 0.784
6.7 5.5 4.8 4.7
0.299 0.689 1.197 1.729 2.261
0.55 0.74 0.67 0.65 0.55
-
-
0.95 0.89 0.82 0.74 0.67
0.82 0.72 0.70
0.93 0.82 0.72 0.70 0.62
0.90 0.79 0.67 0.54 0.45
-
0.67 0.52 0.49
108
radius ratio (Table 3.9) shows that the surface area ratio is always smaller than the radius ratio. This is not surprizing as our model of a cylindrical pore is a rather poor description of the real pore system. The real pore structure provides intersecting voids and channels of varying cross-section, as discussed in Chapter 2. Therefore, it can be expected that the surface area ratio will be smaller than the radius ratio which is based on the cylinder pore model. In contrast, the relative pore volume ratios coincide fairly well with the squares of the relative pore radius ratios (Table 3.9). This example demonstrates that a considerable reduction in the specific surface area occurs on silanization (see also ref. 113). Strictly, the decrease in SBETon binding nhexadecyldimethylsilyl groups is about 45%of the starting silica and about 40%of the original pore space is filled with the organic moiety.
3.2.2 Synthesis and properties of chemically modified silica supports 3.2.2.I Bulk modified products (organosilicon xerogels) 3.2.2.1.I Xerogels made by condensation of organosilanetriols Organosilanetriols, RSi(OH)3, which are synthesized by controlled hydrolysis of organotrichlorosilanes, RSiC13, or metal organosilanolates, RSi(OH)2Me [ 1141, are the starting materials for producing polyorganosiloxanes (POS) of composition (RSi03,z)n. The mechanism of formation of POS resembles that of gel formation of silica from sodium silicate solutions, as discussed in Section 2.2.2. In the first stage intermolecular condensation takes place as follows: OH
I
R-Si-OH
I
OH
OH
OH
I
I
t HO-Si-R
I
OH
+
OH
R-Si-O-Si-R
I
OH
I
I
+ HzO
(3.88)
OH
Condensation proceeds rapidly by forming chain-branched, cyclic and crosslinked oligomeric and polymeric organosiloxanols, the latter growing to particles of colloidal dimensions. In addition to intermolecular condensation, intramolecular condensation may also occur, depending on the reaction conditions. While the rate of condensation of the monomeric organosilanetriols is very fast, it becomes noticeably slower with increasing molecular weight of the polyorganosiloxanols, owing to steric hindrances. In the further course of reaction primary particles grow and tend to aggregate by condensation of surface hydroxyl groups at the interface between adjacent particles. The solution becomes supersaturated and either solidifies as a gelatinous mass or a precipitate is obtained. After washing the coarse lumps or particles, an after-treatment is still necessary, involving heat treatment, in order to complete the condensation of residual hydroxyl groups yielding polyorganosiloxanes of composition (RSiO&. The pore space is then built up by the interstices between assembled or cemented primary particles. Based on the previous work of Hurd and Wintersberger [ 1151, Slinjakova and Neimark [ 1161 prepared a methylorganosiloxane xerogel by adding sodium methylsiliconate, CH3Si(OH)zNa, to hydrochloric acid. The gel obtained was washed and dried at 423 K to give a xerogel. The material consists of white, opaque, hydrophobic powder with a true
109
density of 1.35 g/cm3 at 293 K and a chemical composition corresponding to the formula (CH3Si03,z)n. Adsorption studies indicate a high adsorption capacity for hydrocarbons such as n-hexane and benzene and a negligibly small adsorption of water vapour. The hysteresis loop found between the adsorption and desorption branch of hydrocarbon isotherms indicates the presence of mesopores. It should be noted that the usual aqueous hydrolysis of methyltrichlorosilane carried with a out in spray towers gives a highly disperse powder of composition (CH3Si0, particle size below 1 pm [ 1171. Unger and Pohl [118] attempted to synthesize a corresponding phenylsiloxane xerogel following two routes: (i) thermal condensation of purified phenylsilanetriol, which is a stable white solid with a melting point between 398 and 400 K [ 1191, by successively increasing the temperature up to 673 K [product (a)] ; (ii) controlled hydrolysis and condensation of phenyltrichlorosilane in an acidic solution and thermal treatment of the gel obtained at 373 K under vacuum [product (b)]. While the elemental composition of product (a) was found to be very close to the theoretical value according to (C,HSSi031z)n the carbon content of product (b) was a few percent lower. Considerable differences in the properties, such as solubility and thermal stability, of the two products were observed. For instance, the weight loss during annealing under atmospheric pressure at 673 K over a period of 5 days of product (a) was less than 3%whereas product (b) lost 45% of its weight under the same conditions. The drastic decrease in weight of the latter product may be caused by the fact that lowmolecular-weight phenylsiloxanes are present, which become volatile on annealing. It could also be established that the condensation of product (b) was not complete because a hydroxyl group concentration, QOH,of 3.8 mmole/g was measured in the reaction with methyllithium. In contrast, product (a) had an CYOHvalue of 0.03 mmolelg. Neither material, however, is suitable as an adsorbent because they have a very small specific surface area of about 1 mZ/gand also a low porosity.
3.2.2.I .2 Xerogels made by co-condensation of sodium silicate and organosilanetriols After the synthesis of pure methylsiloxane xerogels, Slinjakova e t al. [ 1201 extended their studies on mixed gels prepared by adding a sulphuric acid to a mixture of sodium silicate (I) and sodium methylsiliconate (11). A series of mixed xerogels was synthesized by varying the ratio between I and I1 in the starting mixture. From adsorption measurements on these products it was concluded that the monolayer capacity for water, methanol and n-hexane vapour decreases significantly when the amount of I1 relative to I is increased. This change is thought to be due to the influence of methylsilicic acid on the gel formation process. The surface composition of these mixed gels with respect to their hydroxyl group distribution was later studied by Plachinda et al. [121] using the reaction with lithium aluminium hydride. A similar procedure based on the co-condensation technique was described by Runge and co-workers [ 122, 1231 for the synthesis of ion-exchange materials. Benzyltrichlorosilane was sulphonated to p-sulphobenzyltrichlorosilane, which was converted into the corresponding methoxy compound, the latter being hydrolysed to p-sulphobenzylsilanetriol, which could be stabilized in an acidic medium. This solution was then mixed
110
with a sodium silicate solution of given concentration, maintaining a pH of 3. On increasing the pH to 5-6 the solution rapidly gelled. After washing and drying a xerogel was obtained that exhibited cation-exchange properties. The product was particularly recommended for use as an acidic catalyst in catalysed reactions at temperatures up to 573 K.
32.2.1.3 Xerogels made by co-hydrolysisand co-condensationof orgmotrialkoxysilanes and tetraethoxysilane 01 pdyethoxysiloxane Analogous to the co-precipitation of gels made from sodium silicate and ocganosilanetriols, Unger et al. [ 1131 developed a procedure starting with tetraethoxysilane or polyethoxysiloxane as compounds that provide a sufficient stability of the final skeleton and an appropriate organotrialkoxysilane that carries the organofunctional group to he bonded. The reaction route permits great flexibility in varying the type of organofunctional group, the specific surface area and the pow structure paramters and also the size of the spherical particles. The synthesis can be divided into two stages, which are illustrated schematically in Fig. 3.10. In the first stage a mixture of tetraethoxysilane and oiganotriethoxysilane in a given ratio is partially hydrolysed in ethanol containing a certain amount of dilute hydrwhbric acid. The acid acts as a catalyst. Water is used in a stoichiometric deficiency After partial hydrolysis is complete, ethanol, water and unreacted monomers are completely removed by distiuation. The remaining polyorganoethoxysiloxaneis characterized by its mean molecular weight, M,,,and its kinematic viscosity, v. In the second step, a given amount of polyorganoethoxysiloxane, either pure or diluted with cyclohexane as an inert solvent, is emulsified by vigorous stirring in a wittw-ethanol mixture. Small liquid droplets are formed the size of which is controlled (i) by the stirring
1 V
(sloichiometric cteficiency)
( palyorganoelhoxy siloxane 1
I
1
+
H,O (OH7
( stoichiometric excess)
( polyorganosiloxane)
Pig. 3.10. Scheme of the preparation of polyorganosiloxane supports.
111
speed and (ii) by the viscosity of the polyorganoethoxysiloxane. The water-ethanol mixture contains an excess of water in order to ensure the complete hydrolysis of ethoxy groups to polyorganosiloxane. Hydrolysis followed by condensation of silanol groups is initiated by adding a basic catalyst, usually ammonia in a given concentration, to the stirred emulsion. The reactions mentioned above start at the surface of the droplets, which rapidly solidify to spherical gelatinous beads. Stirring is continued for about 1 h and then the particles are allowed to settle overnight. During settling, ageing of the gel occurs in the basic medium. After washing and drying at 473 K a xerogel is obtained. According to this procedure, a porous benzylsiloxane xerogel can be prepared by starting with a solution of tetraethoxysilane (I) and benzyltriethoxysilane (11). The molar ratio of I1 to I was varied between 0.10 and 0.45. Higher ratios should not be used because the final benzylsiloxane product then becomes mechanically unstable and also exhibits appreciable swelling in organic solvents. The mean molecular weight of benzylethoxysiloxane can be controlled in a range between 500 and 2000 by the amount of hydrochloric acid added to the starting solution. The amount of water also determines the viscosity of polybenzylethoxysiloxane. A typical composition of the charge is as follows [ 1241: 2.4 mole 619 ml of benzyltriethoxysilane, 9.6 mole 3 2138 ml of tetraethoxysilane, 210 ml of 0.01 M hydrochloric acid and 1500 ml of ethanol. The yield is 1300 ml of polybenzylethoxysiloxane with a mean molecular weight of M, = 740 and a kinematic viscosity of Y = 20 mmZ/sec.The chemical composition is [(C2H,0), .64(C6H5CHZ)~.z3Si0] 5.4, The process variables in the second stage permit the control of the particle size, porosity and specific surface area. The particle size is adjusted by both the viscosity of the polyorganoethoxysiloxane and the stirring speed during emulsification and solidification. The porosity is determined by the mean molecular weight of the polymer and can be increased considerably by adding an inert solvent such as cyclohexane to the polyorganoethoxysiloxane as a volume modifier [ 1251. The specific surface area of the final product can be varied by the type and the concentration of the catalyst in a range between 200 and 600 m2/g. As an example, polybenzylethoxysiloxane was converted into polybenzylsiloxane under the following conditions: 500 ml of polybenzylethoxysiloxane(Mn= 740, v = 20 mm2/s), 1400 ml of water-ethanol mixture (3 :1, v/v), 130 ml of cyclohexane, 200 ml of concentrated ammonia solution (25%, w/w), stirring speed 2500 rpm and reaction temperature 293 K. The yield was 270 g of dry polybenzylsiloxane beads with the following particle size distribution: >14 pm, 53.7%; 7-14 pm, 27.8%; and <7 pm, 18.5%. The carbon content was 24.45% (wlw). A simple calculation based on the carbon content shows that in fact every fifth silicon atom is attached to a benzyl group, as expected from the molar ratio of the initial two compounds. The density p~~ of the product was determined as 1.68 ml/g. The pore structure parameters obtained by nitrogen sorption measurements were: specific surface area, SBET= 652 mz/g; specific pore volume; Vp = 0.761 ml/g (according to the Gurvitsch rule); and most frequent pore diameter of the relative pore volume distribution, D = 3.2 nm. The procedure described can be modified in such a way that tetraethoxysilane is filst hydralysed and condensed alone to a pure polyethoxysiloxane (PES) of given mean molecular weight and viscosity. The modifier, the organotriethoxysilane, is then introduced in the second stage of the reaction as follows. A certain amount of the correspond-
112
ing triethoxysilane is dissolved in the PES. If necessary, cyclohexane can be added as a volume modifier. The solution is emulsified in a water-ethanol mixture by vigorous stirring and a certain amount of ammonia catalyst is added, which starts the hydrolysis of the ethoxy groups. Gelatinous beads are formed in the same way as mentioned previously. The main difference between the original and modified procedure is that in the first instance co-hydrolysis and co-condensation are performed in the first reaction step with the corresponding monomers, whereas in the second instance these reactions take place in the second reaction step with an oligomer and a monomer. In the modified procedure care is required in order to establish a homogeneous crosslinking between the organic modifier and the PES or, in other words, a statistical distribution of the organofunctional groups within the bulk phase as well as at the surface. According to the modified procedure microbeads can be prepared that carry bonded groups with an oxirane structure [ 1131.As an example, 2.0 mole (a45 ml) of tetraethoxysilane, 4.0 mole (e233 ml) of ethanol and 40 ml of 0.01N hydrochloric acid containing 2.2 mole of water gave 240 ml of polyethoxysiloxane with a mean molecular weight Mn = 800. As the modifier in the second reaction step 1,2-epoxy-3-propoxypropyltriethoxysilane (7-glycidoxypropyltriethoxysilane)was employed. The molar ratio of PES to the oxirane compound expressed in moles of SiOz per mole of SiOz in the starting solution was varied up to more than 2.A typical charge was as follows [ 1261 :180 ml of polyethoxysiloxane (G1.5 mole of SiOz),Mn = 800,180ml of y-glycidoxypropyltriethoxysilane(=0.65mole), 75 ml of cyclohexane, 135 ml of concentrated ammonia solution (25%, w/w), stirring speed 2100 rpm and reaction temperature 293 K. The yield was 200 g of poly(yglycidoxypropylsiloxane) with a particle size ranging between 5 and 15 pm and a carbon content of 17.8% (w/w). The specific surface area, SBET, was measured as 445 mz/g and the specific pore volume, V p ,was 2.05 ml/g. By means of the reaction conditions SBETcould be varied between 100 and 500 mz/g and Vp between 0.1 and 2.5 ml/g. In conclusion, the procedures offer a high potential in synthesizing bonded packings for HPLC with a great variety of functional groups.
3.2.2.2 Surface-modifiedproducts 3.2.2.2.1 Si-X surface bonds (X = halogen, -NHz, -NRz, -R, -H) Chlorination of silica surface. The reaction between silica (free of physisorbed water) and chlorinating reagents permits the direct replacement of hydroxyl groups with chlorine atoms, yielding highly reactive 31-Cl surface groups. The most common method for the preparation of Si-Cl groups is the treatment of silica with thionyl chloride: S i - O H t SOClz
+
Si-Cl t SOzt HCl
(3.89)
In order to achieve a maximum conversion, the reaction is preferably carried out at temperatures above the boiling point of SOClz. After reaction, the excess of SOClz and also the gaseous by-products, SOz and HCl, have to be removed completely by treating the
113
product at 473 K under vacuum. The chlorinated samples are useful as starting materials for preparing other surface-modified derivatives. For example, by reaction with Crignard reagents as phenyllithium, Si-aryl groups can be produced. Deuel et al. [ 1271 first studied the reaction of SOClz and silica as a means of synthesizing alkoxy derivatives by reaction of chlorinated silica with the corresponding alcohols. The maximum surface concentration, asjcl, was reported to vary from 3.3 to 5.3 pmole/ mz, depending on the reaction conditions. Based on these results, Boehm et al. [26,27] later utilized this reaction to evaluate the hydroxyl group concentration at the surface of Aerosil. After outgassing the Aerosil at 453 K, the concentration of Si-Cl groups was estimated to be between 3.48 and 5.16 pmole/mz. In contrast to Aerosil, the chlorination of porous silica yields much higher values of about asic1= 7.0 pmole/m2 [99]. This may be due to the fact that on fully hydroxylated porous silica the hydroxyl group population is higher than on the Aerosil surface, which was estimated to be about 8-9 I.tmole/m’. In other words, about 80%of the initial number of hydroxyl groups at the silica surface are accessible to reaction with S0C12. In a recent investigation, Janssen [34] studied the chlorination of the Aerosil surface using a variety of reagents such as S2ClZ,ClZCO,CC14, CH,COCl and S0Clz. Surprisingly, he found that with pure SOClz only a partial replacement of hydroxyl groups took place whereas dissolved SOClz gave a complete reaction with hydroxyl groups. Complete chlorination of surface hydroxyl groups can also be achieved by treatment of silica with Clz at 973-1223 K or CC14 at 623-873 K, whereas dry HC1 at 973 K did not give a noticeable reaction [ 1281.
Fluorination of silica surface. Complete fluorination of hydroxyl groups is reported to be possible by the reaction of silica with 30%ammonium fluoride solution at 973 K [ 1291. Partial replacement of surface hydroxyl groups by fluorine atoms gives a material with a high catalytic activity for the cracking of cumene [ 1301. Treatment of Aerosil with pure HF as well as in various solvents, carefully excluding water, leads to a relatively high fluorine content of the modified product, indicating that fluorination is not limited to the surface but also attacks the silica skeleton [34]. Only with the use of SF4 as a fluorinating reagent was complete replacement of hydroxyl groups without any side-reactions observed. Reaction of chlorinated surfaces with ammonia, amines and amine derivatives. Reaction between chlorinated samples containing Si-CI groups and ammonia, amines and amine derivatives can be utilized in order to form S - N = surface bonds. When chlorinated silica is exposed to ammonia vapour, NH4Cl is formed and can be monitored spectroscopically. Annealing of such a modified material at 673-873 K produces sublimation of NH,Cl, resulting in S i - N H z surface groups that can be identified by their absorption bands at 3445 and 3520 cm-’ [128]. Chlorinated silicas, prepared by reaction with SOC12, react rapidly with amine derivatives of formula HzN(CH2lnX, where n = 1-3 and X = CH3, NH2 or COOH, according to [81] S - C I + H2N(CH2),X
+
ESi-NH-(CH2),X
i- HCl
(3.90)
Maximum conversion was attained by refluxing the silica suspended in the pure or dissolved reagent at 333-373 K. In order t o prevent hydrolysis of Si-C1 groups water must be completely excluded. The excess of modifying reagent is removed by washing the product with diethyl ether and subsequently drying it under vacuum at 353 K. Under conditions that provide substantial stability of the S i - N = bond [81], further substitution reactions are possible at the terminal functional groups. In Table 3.10 silica species containing a variety of Si-N-based functional groups are listed. As can be seen from the experimental a values, the maximum surface concentration decreases considerably on increasing the chain length of the modifier. The a values permit an estimate of the average molecular cross-sectional area, A,,, , of the bonded species, which are listed in the last column in Table 3.10. The materials were especially developed for HPLC. The authors reported an adequate chemical stability within the pH range 3-8 Pll.
Reaction of chlorinated surfaces with organometallic compounds. Silicon-carbon bonds are most commonly produced by use of organometallic compounds, particularly
TABLE 3.10 SURFACE-MODIFIED SILICAS CONTAINING Si-N-C BONDS Data taken from ref. 81. Type of functional group
C(%)
N(%)
Surface concentration*, a (pmole/m’)
Mean crosssectional area of bonded group*, A m (nm’lgroup)
=Si-NH-CH, -CH,CH, =Si-N(CH, -CH,CH,), =Si-NH-C, ,H,, Si-NH-C,H,(CH,), =SSi-NH-(CH,), -NH, =Si-NH-CH, -CH, -NH, =Si-NH-CH,-CH, -SO,H =Si-NH-(CH,),-COOH Sj-NH-(CH,), ,-COOH =Si-N(CH,)-(CH,), -C=N
1.5 0.6 6.85 0.95 2.8 3.1 1.0 1.6 3.5 4.8 0.6 3.1 1.7 2.0 3.35 2.5 2.3 4.3 1.7 0.8 8.6
0.4 0.1 1.2 0.1 1.1 2.6 0.3 1.45 0.3 1.9 0.05 1.4 0.3 0.8 1.75 0.9 0.5 2.2 0.3 0.5 1.8
1.oo 2.16 1.66 0.22 1.08 2.99 0.86 1.82 0.66 2.33 0.22 1.oo 0.42 0.66 1.00 0.58 0.49 1.32 0.49 0.25 1.66
1.78 7.40 1.02 7.40 1.58 0.55 1.94 0.90 2.55 0.74 7.40 1.68 3.90 2.63 1.68 2.85 3.36 1.28 3.52 6.16
=Si-NH-C,H,-SO,H@)
=Si-NH-(CH,),-N=C(CH,)-CH, -CO-CH, =Si-NH-(CH,),-N=C(CH,)-CH, -CO-CH, =Si-NH-(CH,),-N=CH-C,H, =Si-NH-(CH,), -N=CH-C,H, -NO,@) =Si-NH-(CH,), -N=CH-C,H, -C=N@) Si-NH-(CH,), -N=CH-C, H,-COOH@) =Si-NH-(CH,),-N=CH-C,H,N Si-NH-(CH,), -NH-CH, -C=N Si-NH-(CH,), -NH-CH, -c, H, -c=N@) =Si-NH-(CH,), -NH-CH, -C,H,-NO, @) *Based on carbon content.
1.oo
11s
Grignard reagents, and silicon halides, especially chlorides: =Si-(J
+ Me-R
+ S i - R t Me(J
(3.9 1)
This reaction, which has been applied successfully in organosilicon chemistry 11311, can be transposed to chlorinated siIica surfaces. Reaction between silica and thionyl chloride, for instance, gives reactive monochlorosilyl groups. A variety of organometallic compounds have been utilized to attach both alkyl and aryl groups to surface silicon atoms. Pioneering work in this field was carried out by Dew1 et al. [ 1321 using silicate minerals such as montmorillonite as the basic material. Later, Ebert [60] reported a phenylsilica prepared by reaction of chlorinated silica and benzene with aluminium trichloride as a Friedel-Craft's catalyst. The surface properties of the modified product were characterized by means of infrared spectroscopy and by adsarption of ethylene and water. Grignard reactions with chlorinated Porasil C were carried out with benzyl-, naphthyl-, n-octyl-, n-octadecyl-, 9-anthracenyl- and phenanthrylmagnesium bromide in dry diethyl ether [ 1331. After bromination of naphthyl-Porasil the reaction with naphthylmagnesium bromide was repeated to produce a second layer of organic groups. Up to four layers were deposited by repetitive bromination and Grignard reaction. An n-butyl-Porasil was synthesized in a different procedure by reaction of chlorinated Porasil and n-butyl bromide in the presence of fine sodium slices according to Wurtz by Locke e t al. [133]. The benzyl-Porasil, prepared according to eqn. 3.91 was employed for further substitution reactions, e.g., to prepare ion exchangers. Organolithium compounds were preferred over Grignard reagents in a study by Unger et at. [99] because the first mentioned compounds react even more readily than Grignard reagents with silicon halides [131]. In addition to S0Cl2, SiC14 was employed as a chlorinating reagent giving -Si-O-SiCl3 and qSi-O)2SiC12 groups instead of the S i - C 1 groups obtained with SOClz. Chlorinated mesoporous and macroporous silicas were treated with ethereal solutions of phenyl-, naphthyl- and triphenylmethyllithium. For phenyl-silica a maximum surface concentration between 4.7 and 5.4 pmole/m2 was values reported, whereas for the bulkier naphthyl and triphenylmethyl groups the aarYl were 3.4 and 1.9 pmole/m2, respectively. The stoichiometry of the respective surface reactions was discussed and further substitution reactions were carried out at the modified silicas. Chlorinated silicas were treated with benzyl- and phenyllithium to produce benzyland phenyl-silica as starting materials for ion exchangers [ 1341. Surface reactions yielding S i - H bonds. In the reaction of organometallic compounds with monomeric silicon halides and alkoxides, the formation of silicon hydrides has sometimes been reported, particularly when the reaction was carried out at elevated temperatures [131]. One can assume that it is also possible that replacement of a S i - C l with a GSi-H group may occur in the reaction of chlorinated silica with organometallic compounds, but no evidence for a surface GSi-H bond was presented in any of the studies cited. However, the formation of GSi-H bonds was achieved in other ways. When methoxylated Aerosil was treated at temperatures above 873 K, the methoxy groups were cracked and a reactive surface remained carrying silanol and silane groups, the latter giving rise to an absorption band at 2300 cm-' [135,136]. Surface S i - H is probably
116
formed in the reaction between silica and trichlorosilane [ 137,1381:
The proposed mechanism was derived from the chlorine and hydrogen contents of the modified product. The mechanism according to eqn. 3.92 is valid only for a silica bearing a high concentration of surface hydroxyl groups. When the silica is annealed at temperatures above 873 K prior to reaction, the trichlorosilane is able to react only with one hydroxyl group giving SiC12H because of the large distance between adjacent free hydroxyl groups. The properties of the product are comparable to that obtained by hydrolysis of trichlorosilane: nHSiC13 t 1.5nH20
-+
(HSiOI.s)n t 3nHCl
(3.93)
termed a polysiloxane hydride xerogel. Such a bulk-modified silica was prepared and thoroughly characterized by adsorption measurements by Slinjakova et al. [ 1391. 3.2.2.2.2 Si-0-R surface bonds
Reaction with alcohols. The esterification of silica by reaction with alcohols is a well established procedure in silica technology for preparing hydrophobic and organophilic derivatives. Reaction of alcohols at the silica surface proceeds under anhydrous conditions as follows: S i - O H t R-OH
-+
Si-OR t H20
(3.94)
the hydroxyl groups being replaced by alkoxy groups. The reaction is preferably carried out at elevated temperatures in order to achieve a complete coverage. The state of art in esterification up to 1956 was reviewed by Iler [ 1401. So-called estersils were described, which are prepared by reaction of primary and secondary aliphatic alcohols with silica in the temperature range 463-548 K. Completion of surface coverage was controlled by adsorption of methyl red from benzene solution: completely esterified samples do not adsorb the red dye and remain colourless. For the butoxymodified silica a maximum surface concentration of (Yn-butoxy = 5.0 pmole/m2 was reported. After 1956, esterification of silica was thoroughly investigated by Ebert [60], Wartmann and Deuel [ 141,1421, Stoeber et al. [143], Bauer and Stoeber [ 1441 and Ballard et al. [ 1451 It is instructive to consider the reaction between silica and methanol in detail, as methanol is the simplest alcohol molecule. Fully hydroxylated surfaces undergo reaction with methanol as follows: ESi-OH
+ CH30H
-+
S-OCH3 t H20
(3.95)
Owing to the relatively large dimensions of methoxy groups bonded at the surface in comparison with those of hydroxyl groups, it seems highly improbable that all hydroxyl
117
groups can be substituted and hence a certain amount should remain unreacted. With increasing dehydroxylation of the silica surface, accomplished by a preliminary heat treatment at about 873 K, the distance between free hydroxyl groups increases so much that a complete substitution becomes possible [ 1461. During this treatment, however, siloxane groups are formed, which are also able to react with methanol: ~ S i - O - S i ~t CH30H + 3 i - O H
+ rSiOCH3
(3.96)
Chemisorption studies of methanol on Aerosil, outgassed between 673 and 973 K under vacuum, revealed that one methoxy group is formed for every two hydroxyl groups removed by dehydroxylation, supporting the mechanism postulated in eqn. 3.96 [43]. The methoxy groups formed offer a high thermal stability up to 700 K under vacuum. Above 873 K they slowly decompose, leaving surface silanol and silane groups [135,136]. Methoxylation of silica is also possible by reaction of silica with diazomethane in the absence of water [27,60,127,147] : 231-OH + CHzNz + Si,-OCH,
+ Nz
(3.97)
An alternative method for producing methoxylated silica consists in the treatment of chlorinated silica with methanol [127]:
rSi-Cl+ CH30H + Si-OCH3
+ HCl
(3.98)
Infrared spectroscopic studies of the reaction between porous glass and methanol were made by Folman and Yates [ 1481 and Sidorov [ 1491. During the reaction the intensity of the absorption band at 3750 cm-' decreases and two bands appear at 2965 and 2862 cm-' ,which belong to the valence vibrations of the C-H bond of methyl groups [ 1491. As mentioned before, n-butanol was preferably utilized in esterification studies. In accordance with the results of Iler [ 1401, Fripiat et al. [ 1501 also obtained a surface concentration of n-butoxy groups of 5.0 pmole/mz, which corresponds to an A m value of 0.33 nm2 per n-butoxy group. The activation energy for the reaction was estimated to be 90 kJ per mole of butoxy groups [ 1451. The esterification reaction is endothermic and requires 50 kJ/mole [ 1451. The attachment of n-butoxy groups gives rise to absorption bands at 2959,2933,2878,1471 and 1385 cm-' [151]; the bands at 2959 and 2878 cm-' refer to -CH3 groups, that at 2933 cm-' to =CH2 groups and those at 1471 and 1385 cm-' to the asymmetric and symmetric deformation modes of GC-CH3 groups. In a recent study, the mechanism of the chemisorption of branched alcohols such as on Aerosil was examined by means of ferr.-butanol and 2,2,4-trimethylpentan-3-01 gravimetric adsorption technique and infrared spectroscopy [41]. The utility of esterified silicas as modified adsorbents in gas chromatography (GC) was recognized by several workers. In 1960 Rossi et al. [ 1521 prepared GC packings by reaction of silica with benzyl and lauryl alcohol. Based on the work of Ballard et al. [ 1451, Kirkland [ 1531 used butoxy-modified silica as an adsorbent in GC. A few years later Halasz and co-workers [ 154-1 561 widely introduced silicas esterified with 3-hydroxypropionitrile, n-octanol, phenyl isothiocyanate and polyethylene glycols (Carbowaxes) as chemically bonded packings for GC and U:(cf: refs. 157 and 158).
118
Reaction with acetic anhydride. Surface hydroxyl groups are also able to react with acetic anhydride to form an acetyl derivative [ 1271, as follows: ZSi-OH
+ (CH3C0)20 + rSi-O-COCH3 + CH3COOH
(3.99)
Reaction with benzoyl chloride. Deuel et al. [ 1271 described the preparation of a benzoyl derivative according to the reaction ZSi-OH + C6H5COCl-+ SSi-O-COC6H5
+ HC1
(3.1 00)
Reaction with methylchlorosilanes. Reactions between methylchlorosilanes and silica have been extensively studied by means of infrared spectroscopy, isotopic exchange and adsorption techniques [ 15,39,40]. Of the three methylchlorosilanes of composition (( H3)nSiC14-n (1 < n < 3), trimethylchlorosilane is the most commonly used compound in silanization studies because of its simple stoichiometry in surface reactions with hydroxyl groups. Reactions between silica and trimethylchlorosilane were first studied by Kohlschuetter et al. [ 1591, Stober [ 1601 and Wartmann and Deuel [ 1611 and later with great intensity by the Kiselev school [ 151. The maximum possible number of trimethylsilyl (TMS) groups that can be grafted chemically at the silica surface was calculated to be 2.4 per nm' or 4.0 pmolelm'. From this value, the mean molecular cross-sectional area of TMS groups is estimated to be 0.42 nm', which is twice that of a hydroxyl group [162]. In the reaction between fully hydroxylated silica and TMCS vapour or dissolved TMCS, a surface concentration between 3.7 and 4.5 pmolelm' was achieved [ 1061. Although these values correspond to a surface coverage of about unity, 50% of the initial hydroxyl groups still remain unreacted. The existence of these hydroxyl groups was monitored by isotopic exchange combined with infrared spectroscopy [ 15,35,39,40], The role of these remaining hydroxyl groups in adsorption interactions is determined by their accessibility, i.e., by the degree of shielding by the trimethylsilyl groups. In this context, it is of interest to discuss the structure of the grafted trimethylsilyl layer [ 1091. While the silica surface represents a close-packed array of siloxane groups coordinatively saturated with protons, the trimethylsilylated silica surface, even at the densest coverage, is an extended layer of thickness 0.35 nm. The distances between adjacent methyl groups are mainly determined by their Van der Waals radii (ca. 0.4 nm). As a consequence, the dispersion interactions of an adsorbate with the TMS layer are relatively weak and caused mainly by (i) the saturated character of the methyl group and (ii) the large distance from the parent surface. The change in the interactions between an adsorbate and silica surfaces with increasing TMS coverage is demonstrated by the variation in the slope of the adsorption isotherms of benzene vapour on Aerosil at various coverages of TMS groups [I631 (see Fig. 3.1 1). It is beyond the scope of this book to discuss the enormous literature on spectroscopic and adsorption studies on trimethylsilylated silica surfaces. The spectroscopic aspects have been excellently treated in some monographs [ 15,39,40]. Only a few papers will be selected that describe the changes in the adsorption behaviour of TMS-modified silica compared with its hydroxylated form. Sorption isotherms of n-hexane, benzene, methanol and water were measured on two
119
-
N
'E
I
P/P.
Fig. 3.1 1. Adsorption of benzene vapour on Aerosil, the surface of which has been progressively covered with TMS groups. I, untreated Aerosil; 11, OTMS = 60%; 111, OTMS = 80%; Iv,Iv A, OTMS = 90%;v, OTMS = 100%. (Taken from ref. 163.)
TMS-modified Aerosils differing in surface coverage (0 = 0.58 and 0.93) [164].The isotherms of modified Aerosil show a drastic reduction in the adsorption capacity of nhexane and benzene compared with the untreated Aerosil. Benzene is believed to be adsorbed exclusively onto the TMS layer whereas the interaction of methanol is restricted to the remaining hydroxyl groups that are accessible through the gaps in the TMS layer. The shape of the methanol adsorption isotherm changes from convex on the untreated Aerosil to a slightly concave form on the nearly completely covered surface. As water molecules are smaller than methanol molecules, the residual hydroxyl groups are more accessible and hence the amount adsorbed under comparable conditions is higher for water than for methanol. Babkin and Kiselev [ 109,1651 developed a model of a TMS surface on a skeleton of tridymite and calculated the adsorption energy of n-hexane and benzene to be 17.6 and 16.4 kJ/mole, respectively. In a later paper, Babkin ef d. [ 1661 discussed the adsorption mechanism of n-hexane and benzene on a TMS-modified surface in relation to the surface heterogeneity. The coating defects in the TMS layer of modified Aerosil can be monitored with small polar molecules that are able to interact by hydrogen bonding with residual hydroxyl groups in the gaps in the modified surface [ 1671. In order to estimate the effect of polar residuals on adsorption, the heats of adsorption of methanol and water as a function of the amount adsorbed on TMS-modified Aerosil were compared with those obtained on graphitized carbon. In a review paper, Kiselev and Shcherbakova [ 1681 demonstrated the various adaptabilities of TMS-modified silicas as selective packings in gas-adsorption chromatography. The most dense and uniform layer of TMS on silica was obtained by the treatment of silica in sealed glass ampoules at elevated temperatures. An additional treatment of TMSmodified silica with hexamethyldisilazane (HMDS)did not affect the sorption properties as detected by gas chromatographic measurements [ 1691. A mesoporous silica pre-treated at different temperatures from 573 to 1273 K was modified with TMCS at 427 K. On annealing up to 723 K the surface coverage of TMS was complete whereas at higher temperatures the formation of a dense layer was impossible, owing to the low concentration
120
of surface hydroxyl groups [ 1701. The effect of dehydroxylation and trimethylsilanization of surfaces of macroporous silicas on the adsorption interactions of various molecules was thoroughly examined by means of gas-adsorption chromatography [ 17 11. Other chlorosilanes for which surface reactions have been studied spectroscopically are [ 172,1731 C12Si(CH3)CH2COOCH3,C1;Si(H)CH=CH2, Cl2Si(CH3)CH2CH2C1, C12Si(CH3)CH2C1,ClgiH, CH&iHCl2 and (CH3),SiHC1. Reaction with long-chain alkylchlorosilanes. In contrast to the well investigated surface chemistry of methylsilylated silica surfaces only limited studies have been carried out with long-chain n-alkylchlorosilanes. This is surprising, considering that to-day most separations in HPLC are carried out on silicas modified with n-octyl- and n-octadecylchlorosilanes. The various procedures in treating porous silica with long-chain n-alkylchlorosilanes differ in (i) the choice of mono-, di- and trifunctional modifier and (ii) in the method of bonding the reagent to the silica surface (absence of water or presence of small amounts of water, utilizing the pure reagent or the dissolved reagent, reaction temperature). As previously discussed, the reactions between trifunctional organosilanes and silica in the presence even of trace amounts of water lead to the formation of polymeric or oligomeric organosiloxanes which may be, but need not be, chemically bonded to the surface. When water is carefully excluded from the silica surface, the n-alkyltrichlorosilane reacts bi- or monofunctionally with surface hydroxyl groups. So far there have been no detailed investigations on the reaction mechanism of long-chain n-alkyltrichlorosilanes.In order to overcome the difficulty in ascertaining the reaction mechanism, n-alkyldimethylchlorosilanes should be employed as modifiers, since they meet the requirement of a simple 1:1 stoichiometry in the reaction with surface hydroxyl groups and oligomerization is limited to the dimeric organosiloxanes. The previously heat-treated silica is either suspended in the pure liquid modifier or in a solution of the modifier using a dry high-boiling solvent such as toluene. The maximum reaction temperature is limited firstly by decomposition of the modifier, and never exceeds 473 K, and secondly by the boiling point of the solution. The reaction time should be sufficiently long to achieve maximum conversion otherwise a partially covered silica surface remains. As discussed before, even with complete coverage about 50% of the hydroxyl groups remain unreacted at the surface. In order to remove these hydroxyl groups, an aftertreatment with TMCS or HMDS is often recommended. Again, it can be assumed that only part of the residual hydroxyl groups will be accessible to react with TMCS or HMDS. The first studies in this direction were made by Abel et al. [174] and Stewart and Perry [ 1751,who treated Celite and diatomaceous earth with n-hexadecyl- and octadecyltrichlorosilane in order to obtain non-polar packings for GC. Considerable efforts to prepare non-polar bonded stationary phases for GC were also made by Aue et al. [76] , but these derivatives rather belong to the polymeric type than to the monolayer type and hence will be discussed in Section 3.2.2.2.4. Based on the work of Kirkland and De Stefan0 [ 1761, Majors and Hopper [ 1771 synthesized a series of non-polar surface-modified packings by reaction of silica with n-alkyl-
121
chlorosilanes under dry conditions. Bonding was confirmed by means of infrared spectroscopy. Kirkland [ 1781 described the preparation of an n-octadecylsilyl-modified silica by reaction with n-octadecyltrichlorosilane (ODTCS) dissolved in dry toluene according to a procedure introduced by Gilpin and Burke [ 1791. The excess of ODTCS was removed after washing by treating the product with tetrahydrofuran containing 10%of water. The final product was then treated with TMCS dissolved in dry toluene to remove the accessible hydroxyl groups that remained unreacted after the first step. The surface concentration of n-octadecylsilyl groups was reported to be 3.0 pmole/m2. Gilpin et al. [loll effected an in situ modification of silica packed in a column by pumping a solution of n-octadecylchlorosilane containing defined amounts of water. Kikta and Grushka [ 1801 prepared a series of n-nonylsilyl- and n-octadecylsilyl-modified packings with various nalkyl surface coverages by employing n-nonyltriethoxysilane and n-octadecyltrimethoxysilane in dry benzene as solvents. Hemetsberger and co-workers [ 100,181] prepared a series of n-alkyl-modified packings, namely n-alkyltrichloro and -dichloro derivatives with n = 7,10, 12, 14, 17 and 20, and also arylalkyltrichlorosilanes of composition C6HS(CH2),SiC13 with n = 0 , 2 , 4 , 6 . The respective reagent dissolved in benzene or toluene was added to the dried silica and the mixture was refluxed with stirring for a sufficient time. The product was washed with several solvents and dried at 385 K. Again, a treatment with TMCS was used in order to displace the accessible residual hydroxyl groups. n-Alkylmethyldichlorosilanesand n-alkyltrichlorosilanes with various chain lengths were utilized by Karch et al. [ 1821 to prepare non-polar packings for HPLC. The surface concentration obtained decreased from n-butylsilyl to n-octadecylsilyl from 4.9 to 2.5 pmole/m2. The decrease in Ctn-alkyl with increasing chain length of the n-alkyl modifier was also established by Roumeliotis and Unger [ 1 111 using n-alkyldimethylchlorosilanes of different chain length. In this study it was also shown that the reaction of monochlorosilanes with surface hydroxyl groups follows a 1 :1 mechanism; the sum of (Y,-alkyl and &OH measured for the respective modifier corresponded to the total concentration of surface hydroxyl groups on the silica prior to reaction. As the modifier becomes bulkier with longer chains, the choice of a support having sufficiently large pores becomes important in the preparation of reversed-phase packings. These aspects were considered in detail b by Eisenbeiss and Krebs [ 1831. Reaction with phenylchlorosilanes. In comparison with the extensive literature on the reaction of alkylchlorosilanes and silica, only a limited number of papers have dealt with surface modification with phenylchlorosilanes. The first attempt to bind Si-phenyl groups to the silica surface was made by Neimark and Chertov [184] using phenyltrichlorosilane and diphenyldichlorosilane. About 10 years later, Unger and co-workers [36, 921 examined in detail the reaction of the phenylchlorosilanes, i.e., phenyltrichlorosilane (PTS), diphenyldichlorosilane (DPDCS) and triphenylchlorosilane (TPCS), and porous silica. At a reaction temperature of 473 K maximum conversion was achieved for all three compounds. The maximum surface concentration of the respective bonded Si-aryl groups are given in Table 3.1 1 , and decreases in the sequence aphenylhydroxysilyl > (Ydiphenylhydroxysiiyl> atriphenylsfiyl. The decrease in (YSi-aryi is a necessary consequence of the increase in the molecular volume of the modifier by subsequent replacement of chlorine atoms with bulkier phenyl groups.
122 TABLE 3.1 1 CHARACTERISTIC DATA FOR POROUS SILICA MODIFIED WITH PHENYLCHLOROSILANES AND TRIMETHY LCHLOROSILANE Modifier
PTS DPDCS TPCS TMCS
Functional group bonded after hydrolysis
Surface concentration, a @mole/mz)
Mean molecular cross-sectional area, A,,, (nrn'lgroup)
Igosteric heat of adsorption, q"OSt' at 0 = 0.5 (kJ/mole) Benzene
Methanol
Phenylhydroxylsilyl Diphenylhydroxysilyl Triphenylsilyl Trimethylsilyl
3.6 2.6 1.9 4.5
0.46 0.64 0.87 0.37
18.9 18.0 5.3 6.6
38.1 29.5 6.2 9.0
Parent silica
8.0
0.21
28.3
49.6
On fully hydroxylated silicas, PTCS reacts bifunctionally whereas DPDCS and TPCS are able to undergo only a monofunctional reaction. The stoichiometry of the respective surface reaction was derived from the carbon and chlorine contents of the modified product, from the weight change during modification and from the change in OH between the native silica and the corresponding modified derivative. It is worth noting that PTCS also reacts monofunctionally on silicas that have been annealed at 873 K prior to reaction and have a concentration of surface hydroxyl groups of about 2.0 pmole/m2 [92]. On such a surface the distance between adjacent hydroxyl groups is so large that only one Si-Cl group in PTCS is able to react. The attachment of Si-aryl groups reduces the polar character of the surface as indicated by the isosteric heats of adsorption of benzene and methanol [ 1061. The qisost. values decrease from the native silica to the phenylhydroxylsilyl and diphenylhydroxysilyl derivatives. A sharp decrease is observed from the latter two to the triphenylsilyl-modified product whose isosteric heat of adsorption is comparable to that of a trimethylsilylated species.
Reaction with benzylchlorosilane. When fully hydroxylated porous silica is treated with pure benzyltriehlorosilane at 473 K, benzylchlorosilyl groups are bonded to the surface according to a bifunctional mechanism with a concentration of 4.1 prnolelm' [37]. In contact with water, hydrolysis takes place to yield benzylhydroxysilyl groups. Reaction with vinylchlorosilane. The surface reaction between silica and vinylmethyldichlorosilane was studied by Burushkina et al. [I851 by means of infrared spectroscopy. The reaction occurs very rapidly and the conversion increases with the reaction temperature and decreases with the concentration of surface hydroxyl groups. Wheals [ 1861 synthesized a series of vinyl-substituted modified silicas as packings for HPLC by treating silica with vinyltrichlorosilane in isooctane to give
I zSi-O-Si-CH=CH2. I This intermediate was utilized as starter to react with other vinyl monomers such as
123
acrylonitrile, acrylic acid, butyl methacrylate, 2-hydroxypropyl methacrylate and diethylaminoethyl acrylate, as follows:
I
Si-O-Si-CH=CH2
I
+ n(RCH=CHR’)
-+
I Z3i-O-Si(CH2)2-(CHR-CHR’)n
I
(3.101)
The product was Soxhlet extracted with acetonitrile and then acetone to remove the nonbonded material.
Reaction with special silanes. In the silanization of silica surfaces, TMCS is often replaced with hexamethyldisilazane, which reacts according to 2 S i - O H + (CH3)3SiNHSi(CH3)3 + 2 ESi-O-Si(CH3)3
+ NH3
(3.102)
liberating ammonia [ 161,187,1881. A comparative study of the efficiency of silanization with TMCS, HMDS and DMDCS was made by Kirkland [ 1891. Borisenko et al. [190] and Bossart [ 191] suggested the use of various substituted trialkoxysilanes as modifying reagents, such as N-(trimethoxysilylpropy1)ethylenediamine and N-bis(hydroxyethy1)aminopropyltriethoxysilane, to prepare selective supports for chromatography. Parr and Novotny [192] introduced a new family of silanes for solid-phase peptide synthesis and as modifying reagents in the preparation of bonded packings. These silanes have the formula Y Y’-
SI-
I
(CH,)n
Y“
where X = C1 or Br, Y = C1, Yf= Y” = C1 or CH3 and n = 0, 1 , 2 , 3 ..., and offer the possibility of preparing bonded monolayers as well as polymer layers with highly crosslinked structures. Two different approaches were made in the synthesis of these silanes: (i) A suitable substituted Grignard reagent is treated with silicon tetrachloride. For instance, p-tolylmagnesium bromide was treated with silicon tetrachloride and 4-bromomethylphenyltrichlorosilane. The intermediate was purified by distillation and identified by means of NMR spectroscopy. (ii) The respective halosilane is subjected to reaction with a corresponding olefin using tertiary amines or hexachloroplatinic acid as catalysts. According to this scheme, the following silanes were synthesized : trichloro- [3-(4-chloromethylphenyl)propyl] silane, methyldichloro-[4-(4-~hloromethylphenyl)butyl]silane and dimethylchloro- [4-(4chloromethylphenyl)butyl] silane. The silane chosen for modification was dissolved in dry tetrahydrofuran or benzene and added to the silica. The reaction conditions (temperature, duration) have to be optimized in every instance in order to achieve complete coverage. For synthesis of polymer layers, see Section 3.2.2.2.4. A highly selective packing for the separation of poiynuclear aromatics was synthesized in a two-step procedure as follows [ 1931 :
124
(i) 3-aminopropyltriethoxysilanewas subjected to reaction with silica to bond Si-(CH2)3-NH2 groups; yield(ii) the modified product was then treated with 2,4,5,7-tetranitro-9-fluorenone, ing a surface composition of
called the tetranitrofluorenimine phase. The reaction was accompanied by a colour change, the white aminated silica becoming black in the imine form. Recently, Engelhardt and Mathes [ 1941 reported a surface reaction between silica and substituted trialkoxysilanes such as CH3-CO-NH-(CH2)3-Si(OCzH5)3, CF3-CO-NH-(CH2)3-Si(OCzH5)3,CH3-S02-NH-(CH2)3-Si(OCzH5)3 and CH3-CO-NH-CH2-CO-NH-(CH2)3-Si(OC2H5)3.The dry silica to be modified was suspended in the reagent, which was dissolved in dry benzene. Reaction was carried out at elevated temperature and the ethanol formed was removed by azeotropic distillation. The intention of this approach was to attach already substituted silanes instead of binding the silane first and then introducing the functional groups by appropriate substitution reactions. Polypeptide-bonded silicas were synthesized by Grushka and Scott [ 1951 in a two-step procedure. Firstly, the silica was treated with l-trimethoxysilyl-2-(4-chloromethylphenyl)ethane dissolved in benzene to give Z3iO-$i-(CH2)2-C6H4-CH2Cl surface groups. The intermediate was then treated with a mixture of tert.-butyloxycarbonylglycineand triethylamine in dioxane at about 383 K.Seven additional glycine units were then attached to the surface. The separation of amino acid mixtures was studied on these modified packings. 3.2.2.2.3 Si-0-BX,, surface bonds B = B, Al, Ti, Si, P; X = halogen; n = 2 or 3, depending on the valency of B. Surface reactions between silica and halides such as BC13, AlC13, TiC14 and PC13 have mostly been examined on Aerosil and only a few have been carried out at porous silica. Further, the main object of these studies was less to prepare selective absorbents than to derive the coordination of hydroxyl groups on the surface from the stoichiometry of the surface reaction.
Reaction with boron trichloride. This reaction was investigated by Hambleton and Hockey [28] and Armistead et al. [20] on Aerosil, which was outgassed at different temperatures, various techniques being applied to monitor the surface composition of the modified product. Aerosil pre-treated under vacuum between 293 and 823 K was subjected
125
to reaction with BC13 at 473 K in sealed glass ampoules [26]. After removing the excess of BC13 by annealing at 473 K, the chlorine and boron contents of the product were determined. The C1:B ratio was found to increase from 0.99 at an outgassing temperature of 293 K to 1.81 at 823 K. According to this ratio, two possible surface species may be formed depending on whether BC13 reacts with isolated or with paired hydroxyl groups: ESI-OH
t BCI3
\
-S-OH
+ BCL,
,”
O\ -sI--oH /
-
=SI-O-BCl,
+
HCl
13.103)
CI 0 = 2 \
- 5-0 /
\ O -sr-o /
\ /
B-CI
+
2HCI
13.104)
CI:B =1
The results of Boehm et al. [26] can only be interpreted in such a way that at low temperatures mainly paired hydroxyl groups exist at the Aerosil surface and dehydroxylation at temperatures higher than 800 K creates solely single hydroxyl groups, giving =B-Cl groups in the reaction with BC13. An infrared spectroscopic investigation of a modified Aerosil treated with BC13 at room temperature and then treated with DtO to hydrolyse the =B-Cl surface bonds formed revealed that (i) a proportion of the surface hydroxyl groups are unable to react with BC13, presumably owing to steric hindrances; and (ii) BC13 reacts predominantly with paired hydroxyl groups giving \
-Si -0 /
0 \ -9 -0 /
\
after hydrolysis [28]. It was shown later that the first statement (i) was not tenable [ 1961. Again, studying the reaction by measuring the chlorine content of the modified product, Armistead et al [20] concluded that after outgassing the Aerosil at 453 K, reaction according to eqn. 3.104 occurs with paired hydroxyl groups whereas on annealing at 723 K mainly isolated hydroxyl groups remain at the surface that react according t o eqn. 3.103. Reaction with aluminium trichloride and tribromide. In the work of Peri and Hensley [ 191, various silicas subjected to a repeated hydration-dehydration procedure were treated with AICIJ vapour in a flow tube for a sufficient time to achieve a complete conversion of hydroxyl groups. The amount of HCl produced by the reaction was measured quantitatively. Finally, the modified product was subjected to hydrolysis by sweeping wet nitrogen through the tube reactor. The HCl liberated was trapped and again determined by titration. An AIC13 molecule is able to react with n surface hydroxyl groups where 1 < n < 3 leaving 3 - n chlorine atoms attached to the bonded aluminium atom. During the reaction n molecules of HC1 will be liberated and 3 - n additional molecules
126
of HC1 will be formed by complete hydrolysis of surface species. Let us define a ratio R given by
R=
HC1 on hydrolysis
(3.105)
HCl on initial reaction
The number of hydroxyl groups involved in the reaction is then
n =R+1
(3.106)
Assuming that a trifunctional reaction between OH and AC13 is highly unlikely, the percentage of hydroxyl groups that are bonded as pairs at the native silica surface, R ,is
R(%)=
200(n - 1)
(3.107)
The results obtained show that with silicas that have a surface concentration of CVOH between 6 and 9 pmolelm’ more than 90% react as pairs with AlC13 whereas at OH = 2.5 pmole/m’ R decreases to about 60%. The findings of Peri and Hensley [ 191 conflict with the results of Boehm et al. [ 2 6 ] , who studied the reaction between Aerosil and AlC13. At an outgassing temperature of 273 K, Boehm er al. established a C1:Al ratio of 1.72, which indicates a high tendency for a monofunctional reaction of AlC13 with hydroxyl groups. Kohlschuetter and Boegel [ 1971 later studied the reaction of AlBr3 and porous silicas differing in surface hydroxyl group concentration. Silica outgassed at 573 K was treated with AlBr3 at temperatures increasing stepwise from 393 to 573 K. The excess of AlBr3 was removed at 473 K under vacuum. At ad outgassing temperature of 573 K a Br:Al ratio of 1.77 was found, indicating the existence of surface species such as
Reaction with titanium tetrachloride. Infrared spectroscopic studies of the reaction of TiC14 and silica show that TiC14 reacts completely with hydroxyl groups on a fully hydroxylated surface [20]. From gravimetric measurements, a weight change of 10% was monitored after complete removal of the excess of TiC14. The mechanism is assumed t o involve both a mono- and a bifunctional reaction of TiC14: 51-OH
+
TiC14
-
SS-O-TICL,
+
HCI
(3.108)
(3.109)
121
which is in accordance with the observed weight change. From this result, Armistead et al. [20] concluded that a fully hydroxylated silica surface with CYOH= 7.66 pmole/m2 bears 2.33 pmolelm’ of isolated and 5.33 pmole/m’ of paired hydroxyl groups. It should be emphasized that these results do not confirm the model of Peri and Hensley [ 191, which assumed a totally hydroxylated silica surface consisting solely of pairs of hydroxyl groups. Reaction between TiCI4 and silica was also studied by Kohlschuetter and Boegel [ 1971. The silica was annealed at 573 K prior to reaction, reaction was carried out at 393 K and the excess of TiC14 was removed at 473 K. The C1:Ti ratio was measured as 2.2, which indicates a composition of surface species such as \
-s1--0
,
/
0
\
,T%
-s1-0 /
and hence a bifunctional reaction. It was further shown that this surface composition is not changed by varying the specific surface area of the silica with CQH held constant at 8 pmolelm’. In contrast to this finding, Kol’tsov and Aleskovskii [ 1981 found a Cl:Ti ratio of ca. 1 .O in studying the surface reaction between TiC14 and silica. Both outgassing and the reaction were performed at 453 K. This result indicates that three hydroxyl groups react with one TiC14 molecule giving a -Tic1 surface species. Reaction with silicon tetrachloride. In addition to AlC13, Peri and Hensley [ 191 also investigated the stoichiometry of the reaction between silica and SiC14 by measuring the amount of HCl on initial reaction and on hydrolysis. Again, with CYOHvalues of the starting silica between 5 and 9 pmole/m’ more than 88%of the hydroxyl groups react with SiCI4 as pairs according to a bifunctional mechanism. With CYOH< 2 pmole/m2 a monofunctional mechanism predominates. In a seperate study, Unger et al. [99] treated various porous silicas and Aerosil, outgassed between 473 and 673 K, with SiC14 at 473 K for a sufficient time to reach equilibrium. The excess of SiC14 was removed at 673 K under vacuum. With silicas annealed at 473 K prior to reaction an Q value of 13.1 pmole/m’ was found, which was independent of the specific surface area of the silica. With a purely monofunctional reaction, arc1 = 3 -8.0 = 24.0 @moleCl/m2 according to the surface composition =Si-0-Si-Cl3. For a solely bifunctional reaction, the paired hydroxyl groups should give a value of 1 -8.0 = 8.0 pmole Cllm’ according to the surface species / \
0
-sI-o
\ /sl-c12
/
The value of 13.1 pmole Cl/mz indicates that SiC14 reacts mainly bifunctionally with hydroxyl groups.
128
Reaction with phosphorus trichloride. Volkova et al. [ 1991 studied the reaction between silica outgassed at 393 K and PC13 at a reaction temperature of 393 K. The excess of PCIJ and HCl was removed at the same temperature by sweeping nitrogen through the reactor. Surprizingly, no chlorine could be found in the reaction product, which indicates that all three P-Cl bonds undergo reaction with surface hydroxyl groups. This reaction certainly should be investigated further very carefully. 3.2.2.2.4 Polymerization In discussing polymer layers that are deposited and grafted at the silica surface, a high degree of uncertainty exists in deriving valid and exact data about their structures and the stage at which they interact with the parent silica. In generating a polymer layer several modes can be distinguished: (i) The formation of layer upon layer by chemical reaction of monomeric silanes that are bi- or trifunctional, releasing reactive groups for further reaction. (ii) Bonding of a monomeric silane at the surface that carries unsaturated bonds, eg., vinyl groups. By adding a desired monomer and a catalyst polymerization can be performed. Another possibility is to maintain a certain water content on the surface and to add a trifunctional organosilane that hydrolyses and spontaneously undergoes polycondensation. (iii) The material to be bonded is pre-polymerized in an external solution, which is then added to the porous silica. By subsequent heat treatment or by removing volatile by-products a polymer layer is deposited on the surface. A necessary condition for forming bonded polymer layers is that the reactant (the monomer, intermediate or polymer) must contain reactive groups that are able to link with surface hydroxyl groups. There is no experimental criteria to prove whether or not a polymer layer is truly chemically bonded. Extraction studies with silicone polymers deposited on diatomaceous earths have shown that physically adsorbed materials behave as non-extractable [76]. This behaviour is similar to that of so-called snake-cage polymers that are polymerized within a porous support and held solely mechanically in the pore space. This was exemplified for styrene thermally polymerized in the pores of silica [200]. The synthesis and characterization of silicone polymers varying in structure and polarity were surveyed by Aue and Kapila [76] and Novotny [201]. Stewart and Perry [ 1751 adapted the procedure of Abel et al. [174] to synthesize an octadecylsilyl-bonded kieselguhr for liquid-liquid partition chromatography. The annealed support was immersed in a solution of n-octadecyltrichlorosilane in light petroleum. After removing the solvent a stream of warm air saturated with water was passed through the dry powder until liberation of HCl became negligible. Schmit et al. [202] introduced a pellicular-type material, namely Zipax coated and bonded with 1% (w/w) of a high-molecular-weightpolymer, called ODS Permaphase. Majors and Hopper [ 1771 studied in detail the reaction between silica and various polar and non-polar organochlorosilanes and -alkoxysilanes. Direct polymerization was achieved by treating the silica with a bi- or trifunctional organochlorosilane in the presence of water vapour. An alternative consists in a primary reaction between silica and a bifunctional organochlorosilane under anhydrous conditions and, after hydrolysis of the product, in a secondary reaction with the same reagent in the presence of water. The reactive hydroxyl groups in the final product were eliminated by
129
treatment with TMCS. Non-polar polymerized layers containing up to 30%(wfw) of carbon were obtained using vinylmethyldichlorosilane, vinylphenyldichlorosilane and allylphenyldichlorosilane.Kirkland and Yates [95,203]patented a procedure in which a pre-polymerized material representing an organofunctional siloxane was deposited on the silica support and subjected to further reaction. In this way a variety of polymer bonded packings were synthesized:
3-chlompropyl-
7 - gl ycidoxypropy l-
8 - cyonoethyl-
7 - m e t hacryloxypropyl-
The silanes introduced by Parr and Novotny [94]are also well suited to the synthesis of chemically bonded polymer films on silica surfaces. As in silicone chemistry, the degree of crosslinking, chain terminations, etc., of the bonded polymer are determined by the ratio of the trifunctional to difunctional and monofunctional silanes of formula X
X
in the starting mixture. For instance, trichloro- [3-(4chloromethylphenyl)propyl]silane and dichloromethyl- [4-(4chloromethylphenyl)butyl]silane were mixed in a 1:1 ratio to form a corresponding polymer layer. Further reactions of the chloromethyl groups are possible, leading to polymer layers with different functional groups. Treatment with a slightly alkaline solution gives a derivative carrying hydroxyl groups whereas the reaction with potassium cyanide yields a product with cyano groups.
3.2.2.2.5 Miscellaneous In the synthesis of selective adsorbents there is another method besides bulk and surface modification that seems to be a successful approach at first sight. This approach
130
consists in the incorporation of the intended adsorbate during the synthesis of the silica to tailor specifically its surface structure and to remove the tailoring substance by extraction after the xerogel has been formed. The status of tailored adsorbents up to 1968 was reviewed by Snyder [204]. Mostly organic adsorbates were employed as tailoring agents. Critical parameters in the preparation of a tailored silica are the rate of gelling and ageing and the washing and drying procedure [205]. In most instances reported in the literature [204] no relationship between the structure of the pre-treating agent and the selectivity of the adsorbent was evident.
3.3 ION-EXCHANGE PROPERTIES OF SILICA
The objective of the following sections is to discuss the most decisive features that govern the exchange processes of silica in electrolyte solutions. Readers who are especially interested in this field are also referred to several comprehensive reviews [206-2091. When silica is immersed in neutral de-salted water, the resulting suspension exhibits a pH of about 5 . This decrease in pH is due to the acidic properties of silica and may be partly explained by the so-called suspension effect [210-2121. The presence of weakly acidic surface hydroxyl groups was confirmed by infrared spectroscopy (see p. 68) and by isotopic exchange with DzO and HTO (see p. 70). As we shall see later, there are alternative points of view concerning the origin and nature of the acidic centres. For the sake of simplicity, it may suffice for the moment to consider the deprotonation of monosilanol groups: S - O H t H 2 0 + ESiO- t H 3 0 +
(3.1 10)
In electrolyte solutions the hydronium ions are exchangeable even in acidic media by cations and particularly by ions that readily coordinate with the oxygen ion to form silicate-metal complexes. With decreasing pH the equilibrium shifts to the right-hand side of eqn. 3.1 10. At a pH of about 2 the surface groups of silica are completely undissociated (isoelectric state), i.e., the surface is electrically neutral in aqueous solution. Consequently, at lower pH values silica probably should act as an anion exchanger (see Section 3.3.4). On the other hand, in strongly basic solution (pH > 9) the silanol groups become increasingly deprotonated and in the presence of metal cations silicates are formed. The pH markedly influences not only the dissociation state of the surface groups but also the hydrolysis of the species to be exchanged. Depending on pH, metal ions for example may exist in their non-hydrolysed or their hydrolysed form. As a consequence of the above considerations, one can expect a very complex ion-exchange behaviour of silica in electrolyte solutions. 3.3.1 Surface sites of silica in aqueous solution and the origin of their acidity
The acidic properties of silica depend strongly on the nature of the surrounding liquid. While water and aqueous solutions reduce the activity and acidity of the surface sites, these properties are considerably enhanced in anhydrous media. In the special case of silica catalysts used for various heterogeneous reactions such as hydration-dehydration
131
and isomerization, the acidity can be measured with the aid of Hammett and arylcarbinol indicators by titration with amines in organic solvents [213]. In this procedure not only the strength of the acidic surface sites but also their concentration under the given pretreatment conditions can be evaluated. Fig. 3.12 shows the distribution of the acidic surface sites as a function of the outgassing temperature for a mesoporous silica measured with arylcarbinol indicators [214]. It can be seen that in the “anhydrous state” silica exhibits a series of acidic groups characteristically differing in strength and surface concentration. Such studies may be of help in understanding the effect of surface sites on the activity and selectivity of silica-supported catalysts. A more detailed treatment of the conditions in non-aqueous media is beyond the scope of this chapter. Considering the behaviour of silica immersed in water or electrolyte solutions, one should first mention that in contact with water silica dissolves to a small extent, forming monosilicic acid, Si(OH)4. The pK, value of the first dissociation stage of Si(OH)4 is about 9.9 [215]. As mentioned earlier, silica is able to undergo cation exchange not only in neutral solutions but even in the pH range 2-6. Thus, the surface must bear more strongly acidic groups than the monomolecular acid. The possible nature of the acidic surface sites of silica in aqueous solutions has been treated in detail by Vysotskii and Strazhesko [216], who postulated two structural models. In the first model, it is assumed that the tendency for splitting off of a proton from a particular surface silanol group is markedly facilitated by its environment. According to modern concepts of the properties of siloxane bonds, the electron-accepting effect of a great number of undissociated hydroxyl groups can be transmitted by (d-p), interactions
concentration of acidlc groups (umolelg)
1
70
60 50 40
30 20 10
273
473
673 -
873 -->
1073
T ( K )
Fig. 3.12. Distribution of acidic functional groups on a mesoporous silica at various outgassing temperatures. The symbols refer to acidic groups of various strength characterized in terms of the acidity function HR [214]. HR:0,+0.82; A , +4.15; v, +5.61; 0,+6.90; a, +9.36.
132
along the chain of Si-0 bonds, resulting in a partially positive charge at the silicon atom of the silanol group:
(3.111)
s t r u c t u r e (0)
The effect of (d-p), conjugation in the siloxane chain was established in the formation of oligomers of silicic acids [2 171. It was found that the ionization constant increases by one order of magnitude on going from the dimer to the tetramer. Consequently, a further development of the polymerization process, finally resulting in the formation of silica, must lead to an even greater decrease in the pK, value. The second structural proposition is based on the idea that hydroxide ions (polarized water molecules) may be coordinated to unsaturated surface silicon atoms with vacant 3d orbitals according to ~
S-OH I
OH-,
H,O’
c
/OH
551
(3.112)
s t r u c t u r e (b)
In contrast to structure (a), the exchangeable counter ion H 3 0 + is now located at the coordinated hydroxide ion. A third proposition was made by Burwell et al. [218], who suggested that the increase in the acidity of silica compared with the first ionization constant of monosilicic acid is probably due to hydrogen bonding between adjacent surface silanol groups facilitated by the reaction with hydroxide ions:
A similar activating effect by hydrogen bonding, especially responsible for the adsorption of aromatic hydrocarbons on fine-pore silicas, was attributed by Snyder and Ward [21] to the so-called “reactive hydroxyls”:
It is probable that the three interactions proposed above together contribute to the observed acidity. By means of infrared measurements the pK, value of the acidic groups of silica was
133
estimated to be about 7.1 f 0.5 [219,220], which is in close agreement with the value obtained by titration procedures [217,221]. The pK, value of the silanol groups significantly depends on the degree of neutralization [217,221,222]. Obviously, the acidity of the remaining silanolic protons decreases as the reaction proceeds, owing to an increase in the negative surface charge density [221,222]. Effects of this kind are characteristic of polyelectrolytes.
3.3.2 Capacity and exchange ability as a function of pH
As the acidic silanol groups are responsible for the cation-exchange properties of silica, the theoretical specific capacity, Qo, in aqueous solution is equivalent to the concentration of surface hydroxyl groups, which is about 8 I.tmole/m2 for a totally hydroxylated silica. For example, a silica of this type with SBET= 500 m2/g provides Qo = 4 mequiv./g. Hence, the theoretical specific capacity depends on (i) the degree of hydroxylation of the silica surface, i.e., on its OH(^) value, and (ii) the specific surface area, SBET. The weight capacity, Q,, of a given silica can easily be determined by titration with an alkali metal hydroxide solution [217,221,223-2251. Owing to the weak acidic properties of the silanol groups, the total capacity is available only at high pH values. In other words, the capacity will be a strong function of the pH. For non-hydrolysed cations such as Li', Na' and K+ the capacity for a 600 mZ/gsilica is reported to be about 0.1-0.2 mequiv./g at pH 7, whereas at pH 9 Q, reaches about 1 mequiv./g [217]. A further increase in Q , is obtained on increasing the pH from 9 to 1 1. The maximum capacity attainable with various silicas varies from 1.2 to 1.5 mequiv./g [221,223,224], but it seems to be markedly dependent on the ionic strength of the solution. In the presence of 3 MNaC104, Schindler and Kamber [221] found an exchange capacity of about 3.6 mequiv./g. Moreover, the pretreatment of the silica under consideration may strongly influence its capacity. Ganguly [225], for example, observed a large capacity decrease when the sample was ignited at 1173 K before titration. This phenomenon can easily be interpreted as a result of the fusion of hydroxyl groups. For strongly hydrolysed multivalent cations, the sorption capacity enhances considerably on increasing the pH, as was shown by Vydra and co-workers [226-2281. The maximum capacity for soluble hydroxo complexes of divalent cations reaches about 0.0 1 mequiv./g at pH 8 [228] and about 0.03 mequiv./g for hydroxo complexes of tri- and tetravalent cations at pH 6 [226]. The sorption capacity for ethylenediamine, trisphenanthroline and ammine complexes of mono-, di- and trivalent metals was also found to be a function of the pH of the solution [227,229,230]. In contrast to the sorption capacity for hydroxo complexes, the maximum sorption in these instances reaches about 1 mequiv./g. The effect of pH on capacity is caused by its influence on the stability constant of the respective complex as well as on the number of negatively charged surface sites. Summarizing the above expIanations, it can be said that the weight capacity, even at high pH values, is only 10-20% of the theoretical specific capacity derived on the basis of the respective CIOH(~)value, i.e., only a small proportion of the total amount of surface hydroxyl groups is involved in ion exchange [229,230]. Because of its low capacity, silica is a suitable sorbent for the concentration of trace amounts of metal ions present in highly dilute solutions.
134
3.3.3 Mechanism of cation exchange on silica and the theory of selectivity
In an attempt to interpret their results, several workers applied the law of mass action to the simple ion-exchange reaction of non-hydrolysed ions: ZSi-OH t Men++ cSiOMe("- '1'
+ H+
(Me = metal ion)
(3.1 13)
In this case the thermodynamic equilibrium constant on a molar concentration basis is given by: (3.1 14) where quantities with bar refer to the exchanger phase. Rearranging and taking logarithms of this equation, Ahrland et at. 12231, by analogy with a procedure originally described by Kurbatov et al. [23 11, derived the expression (3.115) for the distribution coefficient KMe which is defined as KMe = [6n+]/[Me"+] (1 g-'). If one considers a constant low load of the metal ions, the quantities ?Me, 7~ and can be assumed to be fixed and the last term of eqn. 3.1 15 becomes constant. By plotting log KMeagainst the first term of the equation, a straight line of slope -1.6 was found for U 0 2 2 +(n = 2). This result clearly demonstrates that the law of mass action is only approximately valid in this instance. Using almost the same linearization procedure, Allen and co-workers [222,224] examined the validity of the law of mass action for the exchange of several metal ions on Ludox HS non-porous silica. Neglecting activity coefficients, they formulated the law of mass action according to the reaction of metal ions of charge n with rn silanol groups:
[n+]
(3.1 16) This equation can be transformed to [Me(" - m)+] = log K p + m log[n+] t mpH log [Me"']
(3.1 17)
Analogously to the foregoing approach for low coverages of metal ion, we have
z = log
KPt m log[Ii+]= constant
(3.118)
against pH should yield a straight line In this instance a plot of l~g[Me("-~)+]/[Me"+] of slope m and intercept Z . Unfortunately, a correlation between the slope and the ionic charge could only be found at pH < 5.5 while above this value all plots, although linear, exhibited a lower slope. Attempts to correct for inconstancy of [R'] at higher pH values were not successful. It is probable that the neglected variations of the activity coefficients could also have a significant influence. Three different models were proposed by the authors [222] to give a satisfactory explanation of their experimental results. In the first model, two types of silanol groups
135
with widely different reactivities were assumed to be involved in the ion-exchange process. In the second model the apparent equilibrium constant was assumed to vary as the reaction proceeds. As both approaches were found to be inadequate in describing the variation of the degree of exchange with pH, a third model was developed that combined the assumptions of the first two. In this last approach, two sorts of exchange sites were postulated with different exchange constants, each depending on the fraction of the respective site that is occupied by metal ions. Although the third model gave the most satisfactory results, its success may be mainly attributable to the number of adjustable parameters involved. Dugger el al. [232] also applied the law of mass action to the general case of the reaction of a non-hydrolysed cation, Me"', with m silanol groups. Based on the experimental results they assumed that in any case m = n. Further, they considered the ionic activities in the silica phase to be proportional to the mole fractions. Formulating an "apparent equilibrium constant" in terms of the equivalent ionic fraction XH,defined as the fraction of S O - groups in the hydrogen form, the following equation was obtained: (3.1 19) By extrapolation of log K r to [Men+] = 0 in a log KZe versus [Me"'] plot, it was possible to estimate the values of the thermodynamic equilibrium constant, With regard to the mechanism of the ion-exchange sorption of metal ions, Kohlschuetter and co-workers [233,234] considered the possibility of hydrolysis during ion exchange. They suggested a three-step mechanism, termed "hydrolytic sorption", as follows. Hydrolysis can be formulated as Men+ + H 2 0 + Me(OH)("-')'
+ H'
(3.120)
In the solution, the soluble hydroxo complex may undergo condensation, aggregation and polymerization to polynuclear hydrated oxy-hydroxides. It may also be adsorbed by the surface silanol groups according to the reaction S i - O H t Me(OH)("-
+ SiOH/(HO)Me(n-
')+
(3.1 21)
where the formulation 3iOH/(HO)Me(n-')t is general and is not based on any specific model representing the nature of the interaction between the metal ion and silica. Finally, a condensation process may take place: ESi-OH/(HO)Me(n- 'It
+ ESiOMe(n- '1' + H2 0
(3.1 22)
The addition of the three reaction steps of hydrolysis, adsorption and condensation leads to eqn. 3.1 13, i.e., to ion exchange as a gross reaction. As shown by Vydra and Stari [227] for soluble hydroxo complexes, the ion-exchange sorption of a metal complex may occur according to the mechanism m(Si-OH) + MeLn'
* [(~SiO)mMeL]('z-m)'+ mH+
(3.123)
where L = ligand. The validity of this mechanism was demonstrated for the ethylenediamine complexes of Ag+, Zn2+,Cu2 ', Co3+and Co2+ [229], for the trisphenanthroline complexes of Fez+,Co2+and ZnZt [230] and for the ammine complex of Co3' [227].
136
Ligand exchange is another possible feature in ion-exchange sorption of metal complexes. This was established for the sorption of ammine complexes of Cuz+,CdZ+,Zn2' and Ag+ [227]. The sorption of these species can be written as m(Si-OH) t Me(NH3):+
* [(SiO),Me(NH3)[,_
( m - n ) l ] (m-n)-
t (m- n
) ~ ~t n; ~ + (3.1 24)
In the sorption of soluble hydroxo complexes of the trivalent metal ions Fe3+and A13+ [229] and of the divalent metal ions Coz+,MnZ+,Cuz+, NiZ+and Zn2+ [228] on silica, the composition of the respective hydroxy complex adsorbed could be derived from the ratio of the amount of metal adsorbed to the amount of H+ released. The sorption of hydroxo complexes of the divalent metals can be described by the following equation [228] : S.3-OH t MeOH'
* SiOMeOH t H'
(3.125)
Burwell et ul. [218] also examined the sorption of metal complexes on porous silica. For labile coordination compounds such as nickel-amniine complexes, ammonia is readily interchanged by water as ligand according to the ammonia concentration in solution. Adsorbed inert complexes containing an aquo ligand react with surface silanol groups by binding silanol or S i O - groups in the coordination sphere. This is exemplified by the reaction of Co(en),Cl,+(aq.) (en = ethylenediamine). On base-treated wide-pore silica the cited complex is adsorbed according to the reaction [Co(en),Cl,] '(aq.) t ESi-OH green
+
[Co(en),Cl,]+ -0SiE t H30+
(3.1 26)
green
The adsorbed complex can be extracted with 0.1 M hydrochloric acid. On addition of a small amount of sodium hydroxide or ammonia solution to the water in contact with the gel, the green adsorbed complex is converted into a rose complex:
[ C~(en)~Cl,]+ - 0 S S t 3 0 - + [C0(en)~Cl(H,0)]~+2(-0Si=) t C1green
(3.127)
rose
The rose complex is rapidly and nearly completely removed if the silica is extracted with 0.1 M hydrochloric acid as [C0(en)~Cl(H~0)]~' (aq). The change in the coordination sphere of the complexes was monitored by their absorption spectra. The hydrolysis of the adsorbed complexes was also investigated in detail. Based on the structural model (a) presented in Section 3.3.1, Strazhesko etul. 12171 provided a reasonable explanation for the exchange interactions between metal ions and silica. First they considered the exchange on silica bearing acceptor compensating counter ions such as H+ or A13+,which tend to compensate for the electron-attracting effect of the adjacent surface hydroxyl groups. With regard to the possible intake of strongly basic alkali metal and alkaline earth metal cations, ix., elements of very low electronegativity, it is reasonable to expect that the ions with the lowest ionization potential and at the same time the highest basicity and greatest crystallographic radius will be sorbed preferably at pH <7. These
137
ions lead to a maximum increase in the negative charge on the oxygen atom of the few ESi0-M' groups present in acidic solution
( 3.128)
One should mention in this context the thermodynamic considerations of Dugger et al. [232], who pointed out that the free energy of the SiO-metal ion bond is mainly determined by the ionic charge density (defined by the ratio of the charge to the crystallographic radius) and that for a constant charge density the stability of the bond increases with an increase in the valency of the ion. If the silica bears basic counter ions such as Na+ or CaZ+,which are mainly bound by electrostatic forces, the strongest electron acceptors, i x . , the least basic ions, will be sorbed preferentially. These ions provide a maximum n interaction in the siloxane bonds adjacent to the exchange site.
(3.129)
The sorption series of multivalent acceptor ions, e.g., of the rareearth metals, is independent of the ionic form of silica as it is mainly determined by a strong 71 interaction in the O--Me:+ bonds which exceeds the influence of other counter ions located at neighbouring exchange sites. It should be mentioned that additional factors may also be involved, such as cation hydration and the nature of the anion. Most experimental results are in full accordance with the theory outlined above. At pH < 7, silica in its native form (H-form) exhibits the following order of preference for alkali and alkaline earth metal cations [217,232] : Li' < Na+ < K' < Rb' Be2+< Mgz+< Ca2+< SrZ+< BaZ+ At pH > 10, a selectivity reversal for thealkali metal ions could be detected [217], which may be explained by a mechanism similar to that presented in eqn. 3.129. On the calcium and sodium forms of silica, the above selectivity sequences for the alkali and alkaline earth metal ions exhibit the expected reverse order [217,235]. However, it should be mentioned that Tien [236], working with the sodium form of silica, surprisingly found contradictory results for the ion exchange of alkali metal ions. Independently of the ionic form of silica, the cations of the rare-earth metals gave the following sorption series [217] : La3+< Ce3+< Nd3+< Gd3+< Tb3+x Y3+< Er3+< Tm3+< Yb3+< Lu3+< Sc3+
138
3.3.4 Isoelectric state and the possibility of anion exchange Below its isoelectric state, i e . , in solutions of pH < 2, silica should exhibit a positive net charge due to the reaction ESi-OH t HX + SiOH,+ t X-
(3.130)
In a more general discussion about the acid-base properties of oxides of tetravalent metals Amphlett [237] in accordance with Ahrland et al. [223] excludes the possibility of anion exchange on silica considering lowering of the basicity of the silanolic oxygen atom due to the relatively high electron affinity of the adjacent tetravalent silicon ion. Though the behaviour of silica in the low pH region has so far been little investigated the validity of eqn. 3.130 now seems to be proved in contradiction to the above statement. Anion exchange could be observed with glass electrodes where the silanol groups were found to exhibit basic functions in strongly acidic solutions [238]. The possibility of the formation of a positive net charge on the silica surface could be established by direct electrophoretic measurements at pH 1.2 [216]. Moreover, Bannasch [239] studied the exchange interaction of silica with anions at pH values below its isoelectric state. Recently, Tschapek et al. [240] measured the enthalpy and the free energy of protonation of Aerosil by hydrochloric and nitric acid.
3.3.5 Measurement of ionexchange selectivity Two procedures are usually applied to the study of ion-exchange sorption on silica. The most common is the potentiometric titration technique, yielding the equilibrium pH when the exchange between Me”’ and H+ has been accomplished f224J.It is also possible t o carry out the familiar batch experiments, which are often performed by using the corresponding radioisotopes [223]. In these instances the adsorbed amounts are measured radiometrically. 3.3.6 Exclusion of electrolytes from the pores of silica In examining the sorption of a series of metal ions and metal complexes on silica with a high surface area, Dalton et al. [241] observed an exclusion of these species from part of the pore volume. To obtain unequivocal results with regard to this phenomenon, they either suppressed other possible effects, e.g., ion exchange, by choosing suitable experimental conditions, or calculated their magnitude in order to introduce a correction. The “percent availability”, A , defined as the actual fraction of the pore volume to which the solute has access, was calculated by means of the equation
A=
CjV-C’V-
WVp)
.-.1
100
(3.131)
W Cf VP where Ci and Cf mole/ml are the initial and final solution concentrations and V ml is the volume of solution mixed with W g of silica of specific pore volume Vp mllg. Eqn. 3.1 3 1 assumes equal densities of the external and the pore solution and the absence of significant adsorption exchange phenomena. By varying the concentration over a wide range and
139
extrapolating A for Cf= 0, the percentage availability at infinite dilution, A . (%), for the respective electrolyte was obtained. A . values were found to range between 24% for Al(N03)3 and 100%for CsN03. In order to understand the graduated availability of the pore volume, the hydration of the cations under investigation was considered. Large ions of low charge such as Cs’ can break up the water structure but form a relatively small hydrated shell (net-structure breakers). In contrast, small highly charged ions such as A13+and Mgz+not only break up the water structure but also form a new structure, i.e., large hydrates (net-structure formers). The net-structure formers were found to be excluded the most [241]. Consequently, the availability of the pore volume should decrease on increasing the radius of the respective cation hydrate. Indeed, it was found that A . correlates fairly well with the radii of the cation hydrates derived from diffusion and conductivity measurements. In later work McConnell et al. [242] concluded that the principal if not the only reason for ion exclusion is a purely geometric effect, which must be experimentally observable at any solid-solution interface, if the interface to volume ratio is hundreds of square metres per millilitre. As demonstrated in Fig. 3.1 3, this effect arises when there is a difference between the sizes of the solute and solvent species. According to McConnell et al. [242], the centre of the smaller species in a two-component solution has access to the volume between the planes AA’ and BB’ (perpendicular to the plane of the diagram and parallel t o the surface plane S S ‘ ) , while the centre of the larger species does not have access to this volume. Consequently, when the solution contacts the surface the larger species must be more dilute near the interface and at the same time enriched in the bulk. Making an assumption of the size of the “average” water cluster and considering the pore size distribution of the silica samples used, McConnell et al. [242] evaluated the S
A
B
A lI l‘
’A S‘
~
A’
I B‘
Fig. 3.1 3. Two-component solution contacting a plane surface [242].
140
radii of 42 solvated ions. The results were in close agreement with values found by other methods, which demonstrates the validity of the geometric interpretation. 3.3.7 Kinetics of ion exchange on silica
Only a few studies are known in this field. Ahrland et al. [223], who determined the rate of sorption of various metal ions on a Kebo silica of particle size 150-300 pm, found that non-hydrolysed ions are sorbed very fast whereas strongly hydrolysed ions forming polynuclear species are sorbed slowly. Almost the maximum sorption of Na+, Ca2+, Gd3+and UOZZ+could be attained within the first 5 min. In contrast to this result, the exchange of ions formed by Zr(IV), Nb, U(IV) and Pu(1V) was slow, even in acidic solutions. By comparison of the sorption rates for freshly prepared and aged solutions, a faster reaction of the freshly prepared solutions at the beginning of the sorption process was determined. It was concluded that the increased initial sorption rate would be due to lowmolecular-weight species still present as long as polynuclear hydrolysis had not yet finished. Consequently, in strongly acidic solutions where the ions are practically nonhydrolysed the rate of sorption was found to be of a similar nature to that for other nonhydrolysed ions. It is remarkable that another study showed a very fast sorption of the soluble hydrolysis products of Coz+,Mn2+and NiZ+[228]. Zaki and Abd-El-Moneim [243] studied the rate of the U(W) exchange on several pre-treated silica samples in hydrochloric acid and interpreted their kinetic measurements by a simple monomolecular reversible reaction. Both film and particle diffusion were believed to play a role in the case of colloidal silica. Pre-heated silica sorbed uranyl ions by a fast process followed by a slower one. The first process was assumed to be due to a direct exchange reaction at the suitably exposed silanol groups, whereas the second was attributed to particle diffusion and to a direct exchange reaction on newly formed or/and deeply oriented exchange sites. Previous soaking of the silica samples in hot 0.1 N hydrochloric acid increased the amount of suitably oriented silanol groups giving rise to a fast sorption process preceding a slow one. Prolonged soaking effected an increase in the sorption rate of both processes. An interesting investigation on the exchange of the cadmium(I1)-ammine complex was made by Bhaduri et al. [244]. They considered the exchange of Cd(NH,)? labelled with l 1'Cd on silica of particle size 10 pm, which had been pre-equilibrated with the unlabelled complex ion. As the fractional attainment of equilibrium at constant time increased with concentration, film diffusion at the particle surface was believed to be the predominant rate-controlling step. Because of the large size of the ions it was assumed that their diffusion through the silica particles would be infinitesimally slow. 3.3.8 Applications
The pronounced selectivity of silica towards simple metal cations and metal complexes, particularly at varying pH, permits the separation of cationic species by means of chromatography. Only a few characteristic studies will be mentioned here. Ahrland et al. [245] reported the separation of plutonium and fission products from uranium that had been irradiated with thermal neutrons. Kohlschuetter and co-workers [246,247] devised
14 1
procedures for separating divalent metal cations such as Ca2+,MgZ+,Ba2+,Ni2+,Zn2+ and Fez+ from trivalent metal cations such as Fe3+ and A13+.The separation of trivalent metal cations was studied by Vydra and Galba [248]. It is beyond the scope of this book to treat this subject in detail. A survey of ionexchange separations on silica was given by Bannasch and Stamm [207] and later by Markova and Vydra [208]. An up-to-date literature survey of this particular use of silica can be obtained from Doeller and Unger [249].
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Norden, Copenhagen, 1960. 152 C. Rossi, S. Munari, C. Cengarle and G.F. Tealdo, Chim. Ind. (Milan), 42 (1960) 724. 153 J.J. Kirkland, 4th International Symposium on Gas Chromatography, Academic Press, New York, 1963, p. 78. 154 I. Halasz and I. Sebestian, Angew. Chem., Int. Ed. Engl., 8 (1969) 453. 155 I. Halasz and I. Sebestian,J. Chromatogr. Sci., 12 (1974) 161. 156 1. Halasz and J. Asshauer, Justus Liebigs Ann. Chem., 758 (1972) 202. 157 J.N. Little, W.A. Dark, P.W. Farlinger and K.J. Bombaugh,J. Chrornatogr. Sci., 8 (1970) 647. 158 B.L. Karger and E. Sibley, Anal. Chem., 45 (1973) 741. 159 H.W. Kohlschuetter, P. Best and G. Wirzing,Z. Anorg. Allg. Chem., 285 (1956) 236. 160 W. Stober, Kol2oid.-Z., 149 (1956) 39. 161 J. Wartmann and H. Deuel, Helv. Chim. Acta, 42 (1959) 1166. 162 A.V. Kiselev, A.Ya. Korolev, R.S. Petrova and K.D. Shcherbakova, Kolloidn. Zh., 22 (1960) 671. 163 A.V. Kiselev, Quart. Rev., Chem. SOC., 15 (1961) 99. 164 I.Yu. Babkin, V.S. Vasil’eva, I.V. Drogaleva, A.V. Kiselev, A.Ya. Korolev and K.D. Shcherbakova, Dokl. Akad. NaukSSSR, 129 (1959) 131. 165 I.Yu. Babkin and A.V. Kiselev, Dokl. Akad. Nauk SSSR, 129 (1959) 357. 166 I.Yu. Babkin, A.V. Kiselev and A.Ya. Korolev, Dokl. Akad. NaukSSSR, 136 (1961) 373. 167 I.Yu. Babkin and A.V. Kiselev, Zh. Fiz. Khim., 36 (1962) 2448. 168 A.V. Kiselev and K.D. Shcherbakova, Abh. Dtsch. Akad. Wiss. Berlin, KI. Chem., Geol., Biol., 1 (1962) 207. 169 I.V. Borisenko, A.V. Kiselev, R.S. Petrova, V.K. Chuikina and K.D. Shcherbakova, Zh. Fiz. Khim., 39 (1965) 2685. 170 A.V. Kiselev, Yu.S. Nikitin, V.K. Chuikina and K.D. Shcherbakova, Zh. Fiz. Khim., 40 (1966) 140.
145 171 K.D. Shcherbakova and V.K. Chuikina, Abh. Dtsch. Akad. Wiss. Berlin, KI. Chem., Geol., Biol., 2 (1966) 117. 172 A.V. Kiselev, V.I. Lygin and I.N. Solomonova, Kolloidn. Zh., 26 (1964) 324. 173 A.A. Chuiko, V.A. Tertykh, V.A. Khranovskii, Yu.P. Egorov and L.M. Roev, Teor. Eksp. Khim., Akud. Nauk Ukr. SSR, 2 (1966) 247. 174 E.W. Abel, Z.H. Polard, P.C. Uden and G. Nickless, J. Chromatogr., 22 (1966) 23. 175 H.N.M. Stewart and S.G. Perry,J. Chromatogr., 37 (1968) 97. 176 J.J. Kirkland and J.J. de Stefano, J. Chromatogr. Sci., 8 (1970) 309. 177 R.E. Majors and H.J. Hopper, J. Chromatogr. Sci., 12 (1974) 767. 178 J.J. Kirkland, Chromatographia, 8 (1975) 661. 179 R.K. Gilpin and M.F. Burke, Anal. Chem., 45 (1973) 1383. 180 E.J. Kikta and E. Grushka, Anal. Chem., 48 (1976) 1098. 181 H. Hemetsberger, M. Kellermann and H. Ricken, Chromatographia, 10 (1977) 726. 182 K. Karch, I. Sebestian and I. Halasz, J. Chromatogr., 122 (1976) 3. 183 F. Eisenbeiss and K.F. Krebs, Pitsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, I 9 77, paper No. 3 11. 184 I.E. Neimark and V.M. Chertov, Dokl. Akad. Nauk SSSR,138 (1961) 977. 185 T.N. Burushkina, A.A. Chuiko, N.V. Khaber and L.V. Manchenko, Teor. Eksp. Khim., 4 (1968) 572. 186 B.B. Wheals, J. Chromatogr., 107 (1975) 402. 187 J. Bohemen, S.H. Langer, R.H. Perrett and J.H. Purnell,J. Chem. Soc., (1960) 2444. 188 R.A. Dewar and V.E. Maier,J. Chromatogr., 11 (1963) 295. 189 J.J. Kirkland, in L. Fowler (Editor), Gas Chromatography, Academic Press, New York, 1963, p. 77. 190 I.V. Borisenko, A.V. Kiselev, R.S. Petrova, V.K. Chuikina and K.D. Shcherbakova, Rum J. Phys. Chem., 39 (1965) 1436. 191 C.J. Bossart, US. Pat., 3,514,925 (1967). 192 W. Parr and M. Novotny, in E. Grushka (Editor), Bonded Stationary Phases in Chromatography, Ann Arbor Sci. Publ., Ann Arbor, Mich., 1974, p. 173. 193 C.H. Lochmueller and C.W. Amoss,J. Chromatogr., 108 (1975) 85. 194 H. Engelhardt and D. Mathes,J. Chromatogr., 142 (1977) 311. 195 E. Grushka and R.P.W. Scott, Anal. Chem., 45 (1973) 1626. 196 F.H. Hambleton, J.A. Hockey and A.J. Tylor,J. Catal., 13 (1969) 35. 197 H.W. Kohlschuetter and V. Boegel, Fortschrittsber. Kolloide Polym., 55 (1971) 29. 198 S.1. Kol’tsov and V.B. Aleskovskii, Zh. Prikl. Khim., 40 (1967) 907. 199 A.N. Volkova, S.1. Kol’tsov and V.B. Aleskovskii, Izv. Akad. Nauk SSSR, Neorg. Muter., 5 11969) 178. 200 K. Unger and W. Kamutzki, unpublished results. 201 M. Novotny, in E. Grushka (Editor), Bonded Stationary Phases in Chromatography, Ann Arbor Sci. Publ., Ann Arbor, Mich., 1974, p. 199. 202 J.A. Schmit, R.A. Henry, R.C. Williams and J.F. Dieckman,J. Chromatogr. Sci., 9 (1971) 645. 203 J.J. Kirkland and P.C. Yates, US. Pat., 3,722,181 (1973). 204 L.R. Snyder, Principles in Adsorption Chromatography, Marcel Dekker, New York, 1968, p. 171. 205 R.E. Majors and L.B. Rogers, Anal. Chem., 41 (1969) 1052 and 1058. 206 A.P. Dushina and V.B. Aleskovskii, Silica Gel: Inorganic Cation Exchanger, Goz. Nauchn-Tekhn. Izd. Khim. Lit., Leningrad, 1963. 207 W. Bannasch and H.H. Stamm, AEC Accession No. 15670, Rep. No. KFK-233, Avail. AEC, 54 pp., 1964. 208 V. Markova and F. Vydra, Chem. Listy, 60 (1966) 860. 209 D.N. Strazhesko, in B.P. Nikol’skii (Editor), Inorganic Ion-Exchange Materials, Vol. 1, Izd. Leningr. Univ., Leningrad, 1974, p. 192. 210 G. Wiegner, Kolloid-Z., 51 (1930) 49. 211 H. Pallmann, Kolloidchem. Beih., 30 (1930) 334. 212 Yu.M. Chernoberezhskii and K.I. Omarova, Vestn. Leningr. Univ., Fiz., Khim., 46 (1972) 106. 213 S.R. Morrison, The Chemical Physics of Surfaces, Plenum Press, New York, 1977.
146 214 U. Kittelmann, unpublished results. 215 S.A. Greenberg, J. Amer. Chem Soc., 8 0 (1958) 6508. 216 Z.Z. Vysatskii and D.N. Strazhesko, in D.N. Strazhesko (Editor), Adsorption and Adsorbents, Vol. 1, Wiley-Interscience, New York, 1973, p. 55. 217 D.N. Strazhesko, V.B. Strelko, V.N. Belyabov and S.C. Rubanik, J. Chromutog., 102 (1974) 191. 218 R.L. Burwell, Jr., R.G. Pearson, G.L. Haller, P.P. Tjok and S.P. Chock, Inorg. Chem., 4 (1955) 1123. 219 M.L. Hair and W. Hert1,J. Phys. Chem., 74 (1970) 91. 220 K. Marshl, G.L. Ridgewell, C.H. Rochester and J. Simpson, Chem Ind. (London), (1974) 775. 221 P. Schindler and H.R. Kamber, Helv. Chim Acta, 51 (1968) 1781. 222 L H . Allen, E. Matijevid and L. Meites, J. Inorg. Nucl. Chem., 33 (1971) 1293. 223 S. Ahrland, I. Grenthe and B. Noren, Acta Chem. Scand., 14 (1960) 1059. 224 L.H. Allen and E. Matijevid, J. Colloid Interface Sci., 33 (1970) 420. 225 A.K. Ganguly,J. Phys. Colloid Chem., 55 (1951) 1417. 226 F. Vydra and J. Galba, Collect. Czech. Chem. Commun., 32 (1967) 3530. 227 F. Vydra and V. Star& Collect. Czech. Chem. Commun., 33 (1968) 3883. 228 F. Vydra and J. Galba, Collect. Czech. Chem. Commun.,34 (1969) 3471. 229 F. Vydra and V. Markova, J. Inorg. Nucl. Chem., 26 (1964) 1319. 230 F. Vydra and V. Markova, Collect. Czech. Chem. Commun.,32 (1967) 1614. 231 M.H. Kurbatov, G.B. Wood and J.D. Kurbatov, J. Phys. Colloid Chem., 55 (1951) 1170. 232 D.L. Dugger, J.J. Stanton, B.N. Irby, B.L. McConnel, W.W. Cummings and R.W. Maatmann, J. Phys. Chem., 68 (1964) 757. 233 H.W. Kohlschuetter, H. Getrost, G. Hofmann and H.H. Stamm, Z. Anal. Chem., 166 (1959) 262. 234 H.W. Kohlschuetter, in K. Issleib (Editor), Anomalien bei Ionenaustnuschvorgangen (Ionenaustauscher in Einzeldarstellungen, Vol. I ) , Akademie Verlag, Berlin, 1962, p. 25. 235 M. Milone and G. Cetini, Atti Accad. Sci. Torino, 90 (1956) 3. 236 H.T. Tien,J. Phys. Chem., 69 (1965) 350. 237 C.B. Amphlett, Inorganic Ion Exchangers, Elsevier, Amsterdam, 1964. 238 N.A. Izmailov and A.G. Vasil'ev, Dokl. Akad. Nauk SSSR, 95 (1954) 579. 239 W. Bannasch, AEC Accession No. 41562, Rep. No. KFK-215, AEC, Karlsruhe, 1964,99 pp; Thesis, Technische Hochschule Karlsruhe, 1964. 240 M. Tschapek, S.G. De Bussetti and G. Pozzo Ardizzi, J. Electroanal. Chem., 52 (1974) 304. 241 R.W. Dalton, J.L. McClanahan and R.W. Maatman, J. Colloid Sci., 17 (1962) 207. 242 B.L. McConnelI, K.C. Williams, J.L. Daniel, J.H. Stanton, B.N. Irby, D.L. Dugger and R.W. Maatman, J. Phys. Chem., 68 (1964) 2941. 243 M.R. Zaki and I. Abd-El-Moneim, Z. Anorg. Allg. Chem., 365 (1969) 325. 244 A.K. Bhaduri, B. Bhushan, K.B. Pandeya and K.R. Kar, J. Radioanal. Chem., 33 (1976) 209. 245 S. Ahrland, I. Grenthe and B. Norin, Acta Chem. Scand., 14 (1960) 1077. 246 H.W. Kohlschuetter and H. Getrost, 2. Anal. Chem., 167 (1959) 264. 247 H.W. Kohlschuetter, S. Miedtank and H. Getrost, 2.Anal. Chem., 192 (1963) 381. 248 F. Vydra and J. Galba, 2. Anal. Chem., 235 (1968) 166. 249 St. Doeller and K. Unger, unpublished work.
147
Chapter 4
Particle characteristics It has been shown by a theoretical treatment that the column efficiency in liquid chromatography (LG) can be considerably improved by applying small particles [ I ] . As a result of a series of studies, 5-10pm particles have been found to be optimal as packings with respect to column efficiency and practical use [2-71. Silica packings are generally characterized by their average particle diameter, dp.However, a statement such as ‘the average diameter of the packing is 10pm’ may lead to confusion, because according to several possible definitions a series of average quantities can be derived and calculated by appropriate methods. Hence the reported averages should be accompanied by sufficient information, because otherwise column performance results cannot be accurately interpreted and compared. The objective of this chapter is first to define the quantities that are necessary for characterizing the particle size, shape and size distribution. Then some methods of size grading and size analysis are surveyed with respect to their application to silica packings. Finally, some detailed information is given about the preparation of irregularly shaped and spherical silica microparticles, and porous silica layers with a defined thickness and pore structure.
4.1 PARTICLE SIZE, SHAPE AND DISTRIBUTION: DEFINITIONS 4.1.1 Particle size [8-I 31
The size of a particle can be specified by a representative dimension, which is determined by its geometry. For spherical particles, the size corresponds to their diameter, d p . For other regularly shaped particles, dp can be defined by appropriate linear dimensions. The diameter of any given particle can then be expressed in terms of an equivalent spherical diameter as follows [ 131 : (a) volume diameter, dpv;dpv corresponds to a diameter of a sphere having the same volume as the particle; (b) surface diameter, dps ;dps corresponds to a diameter of a sphere having the same external surface as the particle; (c) surface volume diameter, dpsv;dpsv corresponds to a diameter of a sphere having the same external surface to volume ratio as the particle. Most of the silica packings used in LC are irregularly shaped. As their size can be described exactly only by an infinite number of linear dimensions, approximations are needed. One common approach is presented in Fig. 4.1, which gives the projected area diameter, dpa, of a particle; dpa corresponds to the diameter of a circle with the same area as that of the particle resting in a stable position. Other definitions are derived from particle sizing techniques. The sieve diameter, dpA, is equal t o the width of the minimum square aperture through which the particle will pass during the sieving process. The Stoke’s
148
diameter, dpSt, corresponds to the free-falling diameter of a particle in the laminar flow region at Reynolds number Re < 0.2 [ 131 .
n
W Fig. 4.1. Estimation of the projected area diameter, dpa, of an irregularly-shapedparticle.
4.1.2 Particle shape
A comprehensive description of particle properties also includes the particle shape. However, as a quantitative treatment should provide an exact definition of the shape, one must introduce further parameters, e.g., shape factors. For our purposes it is sufficient to deal with this subject more qualitatively. Regarding silica packing in LC, one can distinguish between spherical particles and irregular particles that lack any symmetry. According to the shape of these two types of particles, one can expect differences in the column bed characteristics. Firstly, the particle shape should influence the packing density of the column bed, usually expressed in terms of the interstitial porosity, eo. Secondly, the geometry of the interstitial voids between the particles should be a function of the particle shape. It is clear that spherical particles of equal size permit a more homogeneous and dense bed than irregular particles. The packing density should be equivalent to that of a statistically dense packing of spheres (e0 x 0.4). In contrast, irregularly shaped particles should yield a less dense packing, depending on their mutual orientation (e0 0.4). Differences should also arise with respect to the homogeneity of the interstitial voids. A packed bed of spheres creates more regular interstices between the particles and hence a more homogeneous flow profile than that of irregularly shaped particles. However, extreme caution is required in drawing conclusions about the retention of solutes and their dispersion behaviour in the mobile phase on the basis of such differences in packing properties. For instance, considering the outer surface of a spherical silica particle, one cannot observe a totally smooth surface. The surface rather shows a great number of pore openings exhibiting widths in the range from one tenth to one hundredth of the particle diameter, assuming dp = 10 pm. Further, some surface irregularities arise that may be due t o
149
agglomeration processes during the preparation of the particles. The question of whether these shape differences will really lead to some significant alteration in column performance is discussed in Section 5.1.
4.1.3 Average particle diameter Silica packings are always obtained with a particle size distribution. In order to evaluate an average quantity, the size analysis data are arranged in a frequency distribution. The particle size range under consideration is grouped into classes, each of which contains two levels (for the choice of an appropriate number and widths of classes, see textbooks on statistics [ 14,151). Every class is characterized by its average size. Generally, an average has to be defined in terms of a weighting factor, p , and the quantity, Q, being averaged [16] . Consequently, different averages can be derived that are named directly from the weighting factor. Assuming, for instance, that the diameter, dp, of spherical particles is the quantity being averaged, the following quantities can be defined : (a) number average, dPn;weighting factor, number of particles, n:
(b) surface average, dps;weighting factor, surface area of particles, s:
(c) weight average, dpw; weighting factor, weight of the particles, w:
Despite the different values of these averages, the following general relationship holds in any case [ 161 :
’
(4.4a) d ~ wd ~ > s dpn Additionally, a given average can be expressed by other weighting factors. Assuming that the weight of a sphere can be written as W i e Sidi
nidi3
(4.4b)
the weight average, dpw, of spherical particles in eq. 4.3 can be expressed in the following two forms:
150
(a) by s (weight-surface average): -
1
(4.5)
dpws-
I
(b) by n (weigh-number average):
-
i
(4.6)
dpwn-
I
The next step will be the calculation of the relative frequency, i.e., the percentage of the number of particles in each size interval. The relative frequency can then be convei ted into a cumulative frequency. Finally, from the frequency curve average values can be derived that represent the whole collection of data. The most common averages are the arithmetic mean, the median and the mode. The arithmetic mean, d p (arithm), is the sum of the diameters of the separate particles divided by the total number of particles. The mode corresponds to the value at the maximum of the relative frequency curve, whereas the term median is used to designate the dp value at 50% of the cumulative frequency and is therefore often indicated by dpso.It should be noted that for a symmetric distribution all three averages coincide, whereas for an asymmetric distribution one obtains the sequence dp (arithm) >median > mode. As an example, the results of a microscopic evaluation of the size distribution of a commercial product are given in Table 4.1. Within the size range 2-14 pm seven classes are chosen with the same interval of 2 pm. Column 3 represents the number of particles of the given size range in each class. The total amount of particles that are numbered is 536. Column 4 gives the relative and column 5 the cumulative percentage of the number distribution. Using the corresponding dp and n values, the weight of all particles belongTABLE 4.1 PARTICLE SIZE DISTRIBUTION OF LiCHROSPHER SI 100 (SUPPLIED BY DR. W. REICH, E. MERCK, DARMSTADT, G.F.R.) (1) Particle size (2) Average (3) Number Pcrcentage for a number
range (rm)
4,
d;*
0.0 2.0 4.0 6.0 8.0 10.0 12.0
2.0 4.0 6.0 8.0 10.0 12.0 14.0
size, cf: (rm)
Percentage for a weight frequency in distribution distribution * size range, n (4) Relative (5) Cumulative (6) Relative (7) Cumulative
1.0 3.0 5.0 7.0 9.0 11.0 13.0
11. 19. 55. 182. 203. 54. 12.
* Assuming wi * ni dj3.
2.1 3.5 10.3 34.0 37.9 10.1 2.2
2.1 5.6 15.9 49.8 87.7 97.8 100.0
0.003 0.162 2.175 19.752 46.824 22.741 8.342
0.003 0.166 2.34 1 22.093 68.917 91.658 100.000
151
ing to each class can be calculated according to the equation W i = ni di3.The total weight is obtained by summing the weight increments of the seven classes. Then the relative and cumulative frequency based on a number-weight distribution can be calculated (columns 6 and 7). From the frequency distribution, the following averages can be evaluated: (a) number average, d&: dp: (arithmetic) = 7.8 pm;
d p i (median) = dps0 = 8.0pm; (b) number-weight average, dpiW :
d p i w (arithmetic) = 9.3 pm; d p i w (median) = dps0 = 9.4 pm. From Fig. 4.2, the standard deviation (a) is calculated to be k1.9 pm.
4.1.4 Presentation of size analysis data [13] As discussed previously, the tabulated particle size data can be presented as a relative or a cumulative frequency distribution. Usually one plots the cumulative distribution in such a manner that a straight line results. One approach is to fit the given distribution curve to either a normal (gaussian) distribution or a log-normal distribution function. The gaussian distribution is given by the equation dp - dr, (arithm) n = - z n -exp (4.7)
4
1- ;[
U
121
where n is the number of particles of diameter d p , Cn is the total number of particles, dp (arithm) corresponds to the arithmetic mean of the particle diameter and u is its standard deviation. The log-normal distribution is given by the equation ti
=
Cn ~
log u f i
exp
1-
1 log d p - log d p (geom) log u
[
where dp (geom) is the geometric mean of the particle diameter. In the graphs the cumulative distribution is plotted on normal probability or log-probability paper. The l o g probability graph is particularly useful in instances when the particle size covers a wide range. For a gaussian distribution, a straight line results, provided that (1) a large number of measurements are made and (2) the class intervals are negligibly small. In Fig. 4.2 the data in Table 4.1 are plotted on normal probability paper. I n this instance, small deviations from a straight line occur at the lower and upper ends of the distribution. The distribution can then be characterized by the median, which corresponds to the particle size at SO% cumulative frequency, and by the standard deviation, which is half the difference between the 84.1% and the 16.9% sizes. As they can be obtained from a plot such as that in Fig. 4.2, the dps0value and the standard deviation will give insufficient information about the fine-particles that may exist in a given distribution. I n this
152 percentage of cumulative undersize
9 9.8 99
95 90
80 70 60 50 LO
30 20
10
5 1
0.2 0.05
10
5
I
Ipm)
mean particle diameter dp: lo], dp:w
[A]
Fig. 4.2. Particle size distribution plotted on normal probability paper (data taken from Table 4.1).
instance the Rosin-Rammler distribution is very helpful [8, 131. This is an empirical distribution that can be applied successfully to materials that have been milled and size graded by sieving. The function is written as
n (4.9)
where R is the cumulative percentage weight oversize retained on a sieve of a given aperture, dp the particle diameter, dp’ the particle diameter at 36.8% cumulative oversize and n a constant of the material being sieved (n FS: 1.O). Eqn. 4.9 reduces to log log - = n logdp
(
+ log(1oge)- n l o g d i
.
.. .
(4.10)
. . . . . . . .. , l. loo\ AS the last two terms on me right-nand slae are constant, a plot or log log - versus log dp gives a straight line. The Rosin-Rammler distribution isparticularly suitable for controlling milled materials that exist with a large size range and also for those which show a skewed distribution. Further, deviations from the normal size distribution at the lower and upper ends can easily be recognized. ,
C .
I €4
153
4.2 METHODS OF PARTICLE SIZE GRADING AND SIZE ANALYSIS [9,12,13] There are a variety of size grading methods, based on different phenomena and techniques. As it is beyond the scope of this book to survey all of the methods available, the aim of this section is to discuss some selected techniques that are suitable for the separation of silica particles in the size range 1-100 pm. 4.2.1 Sieving
Sieving is the most common and easiest method of separating particles larger than 100 pm. Although it is generally applicable to powders finer than 100 pm, one must bear in mind that (1) very expensive micromesh sieves have to be employed, (2) only small amounts of material can be sieved and (3) the time to reach the end-point of the sieving operation is considerably greater than for large particles. Therefore, sieving of silica supports in the size range 10-100pm is preferably applied for evaluating the size distribution rather than preparing narrow cuts of large amounts. In sieving, the particle diameter, dP*, is determined by the sieve size and corresponds to the minimum square aperture through which the particles can pass. Different types of sieves are currently in use, namely the German Standard DIN 4188, the ASTM Standard E 11-61, the American Tyler Series and the British Standard BSS 410. Sieve specifications for the aperture range up to 125 pm are listed in Table 4.2. Silica particles can be dry sieved down to about 5 pm without any precautions with regard to particle agglomeration. In order to decrease the sieving time, several screening devices are employed. Two instances are given below. The Allen-Bradley sonic sifter uses a vertical oscillating column of air and a repetitive mechanical pulse to move the particles through a sieve or a set of sieves. In this way, the particles are held in a periodic vertical motion that reduces blinding of the sieves and particle aggregation. In the Alpine air-jet sifter (see Fig. 4.3) the material is fluidized by blowing air through a rotating slit that is mounted under the enclosed sieve [ 171. While the powder is kept in motion the fines are drawn through the bottom of the sieve by vacuum collection of the undersize particles on the suction side. The sieving action is very gentle and the process can be performed within a few minutes. The end-point of the sieving operation can be determined either by Kaye’s method or by microscopic examination [ 131. A typical result of a sieving operation may be presented in the following form:
% (w/w) Sieving fraction >25pm ...... 3 20-25pm . . . . . . 16 14-20pm . . . . . , 3 9 12-14 pm . . . . . . 3 7 <12pm ...... 5
154 TABLE 4.2 SIEVE SPECIFICATIONS Aperture pm 20 22.4 25 28 32 33 36 40 45 50 53 56 63 71 75 80 96 100 104 112 120 125
DIN4188 (mm)
in.
ASTM E 11-61 (mesh/in.)
Tyler (meshlin.)
BSS4lO (mesh/in.)
0.020 0.0224 0.025 0.028 0.032 0.0015 0.0017
0.036 0.040 0.045 0.050
0.0021 0.0025
0.056 0.063 0.071
0.0030 0.0035
0.080 0.090 0.100
0.0041
400
400
325
325
350
270
270
300
230
250
240
200
200
200
170
170
170
140
150
150
120
115
120
0.112 0.0049
0.125
Fig. 4.3. Diagrammatic section of Alpine Air-Jet Sifter.
155
4.2.2 Microscopy Microscopy is a simple and valuable tool for particle size analysis, particularly when applied to particles in the size range from 1 to 50 pm. Owing to their Brownian motion, smaller particles do not stay in a stable position during observation, and magnifications up to 5OOX provided by the best microscopes are therefore sufficient for the investigations. Microscopy has the potential advantage of clearness, as no assumptions concerning the particle shape are required because the images are viewed directly. There are no restrictions with respect to the type of sample that can be used. Qualitative information is rapidly available for estimating the size range of a given product and comparisons can be made using standards with well-known size distributions. In this way, the efficiency of a size grading process can also be easily controlled. Moreover, it is possible with the aid of number counting to obtain quantitative information such as the cumulative fie-quency distribution.
4.2.2.1 Sample preparation As the images of individual particles are viewed and measured, particle agglomeration should be avoided. For this reason, the sample is dispersed uniformly in an appropriate agent. For silica particles immersion oil or water containing wetting agents are usually employed. Dispersion can be carried out in different ways, a common technique being to place the sample on a microscope slide and to add a few drops of the dispersing agent, ensuring that the fluid is homogeneously distributed.
4.2.2.2 Measuring device An optical microscope, fitted with binocular eyepieces and equipped with a set of oculars of graduated magnification, is used. The sample is either viewed directly or by projection using the transmission technique. For particle size evaluation, an ocular with a calibrated scale is needed, The slide placed on the microscope stage is then moved and the particle image passes over the scale while it is sized. Instead of using oculars with a linear scale, several graticules have been developed that exhibit opaque or transparent circles. These graticules have the advantage of providing a better estimation of the projected area diameter, dpa [13].
4.2.2.3 Methods of size evaluation As the counting of particles and size estimation are based on a statistical evaluation, the sample viewed should be representative of the whole material. Hence a considerable number of particles should be counted, preferably 200-500. To make the evaluation more reliable, in practice several slides have to be prepared. In order to prevent a large systematic error, (1) always the same operator should carry out the counting and ( 2 ) a second operator may assist the first by writing down the results on a data sheet.
156
4.2.3 Sedimentation
4.2.3.1 Introduction Sedimentation of particles in an appropriate fluid is widely used for size fractionation as well as for the evaluation of the size distribution. The motion of a small particle settling in a fluid under the influence of gravitational forces can be described by the well known Stokes law. If the motion is steady, a constant velocity, the so-called terminal velocity, uSt, is reached. The relationship between the diameter, dpSt, of a spherical particle and the terminal velocity is given by (4.1 1) where p1 is the density of the particle, p z the density of the fluid used, dpst the Stokes diameter of the particle,g the gravitational constant and 77 the viscosity of the fluid. Substituting uSt by (4.12)
uSt = h / t
where h is the falling depth of a spherical particle at a given time t and rearranging eqn. 4.1 1, one obtains dpst
=[
18qh (P1
- Pz)gt
]
(4.13)
It is necessary to discuss briefly the assumptions made in the derivation of eqn. 4.1 1. Firstly, the relationship is valid only in the laminar flow region with a Reynolds number Re < 0.2. At higher Reynolds numbers, corrections have to be applied. Secondly, the settling velocity is strongly influenced by the concentration of the particles dispersed in the fluid, particularly if they are irregularly shaped. In relatively dense suspensions the interactions between the particles affect the flow pattern and result in a change of uSt compared with the value predicted by eqn. 4.1 1. It was found that at concentrations lower than 1% (v/v) this effect is negligibly small. At higher concentrations the particles tend to settle en masse. Thirdly, if the particles are about 1 pm in size, Brownian motion becomes noticeable in addition to gravitational settling. Finally, it should be emphasized again that the Stokes diameter, dpSt, is related to spherical particles. Thus, for an irregularly shaped particle, dpSt corresponds to the diameter of a sphere of the same material with the terminal velocity of the particle under consideration. Most of the sedimentation procedures are carried out in liquids. The liquid should wet the particles and produce a homogeneous and stable dispersion. In some instances wetting agents are added in a minimum concentration. For silica particles water is mostly employed as dispersing medium and sodium hexametaphosphate (Calgon) or sodium pyrophosphate as dispersing agent [ 131 .
157 TABLE 4.3 DENSITIES @He) AT 293 K OF COMMERCIAL SILICA PACKINGS DETERMINED WITH A BECKMAN MODEL 930 PYCNOMETER USING HELIUM Type of packing
Batch No.
PHe (dm3)
Merckosorb Si 60 Merckosorb Si 100 LiChrospher Si 100 LiChrospher Si 500 LiChrospher Si 1000
YN 13 YE 210 YE 187 YE 422 YE 244
2.30 2 2% 2.29 2.30 2.46 2.49
In order to apply eqn. 4.1 1 for size analysis, it is further necessary to know the tiue density of the particles. As an example the densities of some commercial silica packings determined by means of a helium pycnometer are listed in Table 4.3. The average value, ;He, is calculated as 2.36 0.05g/cm3 at 293 K. For spherical silica particles suspended in water of density p 2 = 0.998 d c m 3 at 293 K and of viscosity q = N/sm2 at 293 K, the times t to reach a falling depth of 20 cm are calculated as follows: dpst = 5 pm, c = 179.3 min; dpSt = 10 pm, t = 44.8 min; dpSt = 20 pm, t = 11.2 min; dpSt = 40 pm, t = 2.8 min. It can easily be shown from eqn. 4.13 that the size distribution can be measured by determining the time at constant h or vice versa in terms of the amount of settled material.
*
4.2.3.2 Methods of sedimentation size analysis
There are two fundamental techniques for the preparation of a sedimenting suspension. In the two-layer technique, a thin layer of the suspension is placed on top of the pure liquid that is used as the dispersing medium. In the so-called homogeneous suspension technique, the particles are uniformly dispersed in the whole liquid. Sedimentation is usually carried out in tall glass cylinders. In size evaluation, one must distinguish between incremental and cumulative methods. In incremental methods one determines the amount of particles either at a given time at different falling depths or at a given falling depth at different times. The f u e d depth method is preferably applied. In cumulative methods, the amount of particles that settle out of the suspension in a given time at the bottom of the cylinder is measured. Here we deal only with the incremental methods based on a fixed depth. 4.2.3.2.1 Pipette method In the pipette method, aliquots of the settling suspension are withdrawn at different times at a fixed level of the cylinder. The mass of the particles in these aliquots can be determined simply by weighing after drying. The most common pipette used is that first introduced by Andreasen. The Andreasen pipette consists of a graduated glass cylinder (0-20 cm) that contains about 500 ml of liquid when it is filled up to the 20-cm mark. The stem of the pipette is centrally positioned, its end corresponding to the zero mark. The socket in which the pipette is fused
158
holds a two-way tap and a 10-ml container. The two-way tap can be turned to empty the container into a beaker. A suspension with a volume concentration of about 0.2%(v/v) at p1 = 2.5 glcm’ is prepared. Dispersion may be assisted by ultra-sonic treatment. The pipette is then filled to the 20-cm level with the suspension and the glass cylinder is vigorously shaken by hand for about 1 min. After a further 1 min, when agitation ceases, the first sample is taken by transferring under suction 10 ml of suspension into the container. Finally, the vessel is emptied by m a n s of the two-way tap, cleaned with pure liquid and prepared for the next measurement. The masses m l , m2 ... taken at times r l , t 2 ... are determined by weighing. Then the percentage of undersize, P,is calculated according to the equation
P (%)
- -mt -
Vt
.-Vamo ’
100
(4.14)
where m t is the mass of dried particles in a pipetted volume vt and mo is the mass of dried particles in a pipetted volume vo at the start. The largest size of particles remaining in the corresponding sample suspension after the time t is then calculated by means of eqn. 4.13.
4.2.3.2.2 The pho tosedimentom eter In the photosedimentation technique the concentration of settled particles in a suspension is determined directly by means of a photoelectric measurement instead of withdrawing an aliquot of the suspension, drying and weighing the powder. A commonly used measuring device working on this principle is the wide-angle scanning photosedimentometer (WASP).Inside this apparatus a n a r o w horizontal beam of parallel light is split into a sample beam and a reference beam, the former passing through the sedimentation cell containing the suspension. As the apparatus is equipped with separate photocells, the light flux can be adjusted to zero before measurements. A recorder is connected to the electrical output of the photocells, which measure the change in the optical density, D, of the suspension while the sedimentation cell is scanned downwards at a rate of 1 cm/min. The sedimentation cell consists of a rectangular glass cylinder with a volume of 600 ml. The concentration of particles in the light beam is assumed to be the same as that in the suspension. The particles passing the light beam at a given time t have a diameter smaller than dpSt. The latter can be evaluated according to eqn. 4.13 as all other quantities (pl ,p 2 , q) are known and only the falling depth, h, and the time, t , are monitored. The absorbances, A , measured at a given time, t, and height, h , can be expressed a5 a percentage undersize. In this way the cumulative percentage frequency distribution can be calculated. An example IS given in Table 4.4. 4.2.3.3 Size fractionation by sedimentation Fractionation by sedimentation can be generally applied in sizing silica packings. Small amounts of sample are dispersed in water or water-ethanol mixtures in a glass cylinder. The particles are allowed to settle in an appropriate period that depends on the smallest
159
TABLE 4.4 PARTICLE SIZE DISTRIBUTION OF LICHROSORB SI 100 (SUPPLIED BY DR. W. REICH, E. MERCK, DARMSTADT) DETERMINED BY THE WASP METHOD Conditions (for symbols see eqn. 4.13): p1 = 2.24 g/cm3;pz = 1.00 &m3; T = 293 K ; 77 = 9.36 * lo4 Ns/m2 ;g= 9.81 m2/sec; concentration of the suspension, c = 0.04%(v/v). thin)
h (cm)
Absorbance, A
+st
10.000 17.000 19.000 21.000 22.000 23.000 23.4500 24.0000 24.4500 24.7000 24.8500
17.7000 10.7000 8.7000 6.7000 5.7000 4.7000 4.2500 3.7000 3.2500 3.0000 2.8500
0.1960 0.1960 0.1920 0.1860 0.1650 0.1210 0.0900 0.0500 0.0200 0.0050 0.0001
31.2088 18.6105 15.8735 13.2501 11.9403 10.6041 9.9865 9.2106 8.5525 8.1753 7.9442
Q.UII)
Cumulative frequency surface distribution
100.0000 100.0000 97.9592 94.8979 84.1836 61.7341 45.9184 25.5102 10.204 1 2.5510 0.0510
size desired. For a known height of a cylinder, the time f can be calculated according to eqn. 4.13. After this time, the supernatant solution or suspension is decanted carefully, and the procedure is repeated until a sufficient separation is obtained. The efficiency of fractionation can be controlled by using a microscope. It can easily be shown by a simple evaluation that the procedure for obtaining narrow cuts in the 5-10-pm size range is very time consuming. Sometimes it is necessary to remove the fines from commercial silica packings that have not been well sized. In this instance fractional decantation is the method of choice. On the other hand, it also provides a simple proof of the presence of fines in a given product.
4.2.4 Fluid classification Fluid classification covers all processes of separation in which particles suspended in a fluid are moved under the influence of field forces such as gravitational, centrifugal, Coriolis and other forces. Air is usually used as the fluid. Thus, in elutriation an air flow moves up or down or circulates, carrying all particles with a settling velocity by gravitational forces that is less than the fluid velocity. In contrast to sedimentation, in elutriation the Stokes law cannot be used to choose the appropriate conditions of size separation because ( 1) the Reynolds number often exceeds 0.2 and ( 2 ) the flow profile is very complex. In centrifugal elutriators, the particles are separated under the influence of centrifugal forces. The separators can be divided into counter-flow and transverse-flow classifiers. Fluid classification was reviewed by Allen [13].
160
4.2.4.1 The Gone11 elutriator This apparatus is an air elutriator of the up-blast type. A cylindrical brass tube of width 3.5, 7.0 or 14.0 cm is mounted on a stand. The upper end of the tube, connected with a cone, is housed in a glass bell while the lower end is fitted with a funnel-like glass container in which the air is compressed by means of a blower. The flow-rate can be controlled with a valve and measured with a rotameter. The material to be sieved is placed in the lower glass container, an appropriate flow is set and the brass tube is tapped mechanically. During separation, the fines are collected in the glass bell at the top of the tube, whereas the coarse material is left in the lower glass cylinder. In order to separate particles in a given size range, an appropriate brass tube and a corresponding flow-rate have to be chosen. 4.2.4.2 The Alpine Multiplex Zig-ZagiClassi'er [ I 81 (see Fig. 4.4) The Alpine Zig-Zag Classifier belongs to the centrifugal type of elutriators and was developed for analytical and preparative purposes. The process consists in four steps: (1) closing, (2) dispersing, (3) classifying and (4) separating the fines from the coarse material. The material to be classified is filled into a wide-mouthed bottle, which is attached to the feeding device of the classifier with a bolt spring lever, thus making it airtight. The feeding device is attached to the frame by means of a bolt, and consists of a screw, the screw housing and a stirrer system, which is driven by the screw. The purpose of this system is to ensure an even flow of material between the mouth of the bottle and the transition piece in the dosing screw. The dosing screw is driven by hand or via a control-gear motor by means of a flexible shaft. Then dispersion of the material in an air
Fig. 4.4 Section of the Multiplex Laboratory Zig-Zag Classifier (schematic). a = Drive shaft; b = dosing screw; c = feed material; d = classifier wheel; e = classifying air; f = coarse grains; g = fine grains.
161
stream is started. Fig. 4.4 shows the admission of air both in front and at the back of the classifier wheel. The air admitted at the back conveys the material to the periphery of the classifier chamber. On its way the material passes a number of cams on the periphery of the zig-zag wheel, which increase the dispersion effect. The core of the classifier is the rotating wheel, which is 100 mm in diameter and has 32 zig-zag channels. A particle in the classifier wheel is radially affected by the centrifugal force towards the outside and by the frictional force of the air towards the inside. The material is separated into two fractions on the periphery of the classifier chamber or in the external components of the zig-zag wheel. The induced resistance that occurs in the stationary zig-zag classifier is thus created in each component. By means of this induced resistance, classification is gradually effected in the systematic counter-flow of fine and coarse grains, the latter being carried out of the classifier wheel by centrifugal force. The internal components maintain the fines free from spatter grains. The coarse grains revolving on the periphery of the classifier chamber are separated through an opening of special design and are then fed to the coarse-grain chamber and subsequently into a glass bottle. The fine grains are retained in the air stream leaving the zigzag wheel. The major portion of the fines is separated in the following cyclone. As the separation ratio depends strongly on the particle size distribution, it is impossible to quote any numerical data. The cyclone itself consists of two parts; the top section (inlet spiral) is removed by loosening two bolts. The fines collecting bottle serves as a separating chamber; it is the same kind of wide-mouthed bottle that is used for the feed material and the coarse grains and is also attached by means of bolt spring levers. 4.2.5 The Coulter Counter [lo, 131
The Coulter Counter principle is different from all of the methods discussed above. Originally the technique was developed for counting blood cells, but it can also be utilized for the determination of the number and size of solid porous particles such as silica packings. The material under investigation is suspended in an electrolyte solution, preferably sodium chloride solution of low concentration. By means of a controlled vacuum the charged particles are forced to flow through a small orifice with two electrodes mounted on either side. As the particles pass through the orifice, the resistance of the electrolyte changes, generating a voltage pulse the amplitude of which is assumed to be proportional to the volume of the particles. The pulses are counted and sized according to the magnitude of the amplitude. Calibration is needed for a given electrolyte and tube, using a standard with a well known particle size distribution. As a result a number-volume average diameter, dpnv, is obtained. Errors due to the relationship between the response and the particle volume may arise. Another problem is the end-point determination counting at the lowest level, and several extrapolation techniques have been proposed to overcome this difficulty. Nevertheless, the Coulter Counter is a valuable tool in determining particle size distributions at levels down to 1 pm. To summarize, we can say that sieving and elutriation are the most effective means of sizing silica packings, whereas size analysis is preferably performed by using microscopy,
€62
sedimentation or the Coulter Counter. According to the technique used, different average diameters are obtained that do not necessarily coincide. For a comparison of particle size distribution data from various methods, see also references 10, 17 and 36.
4.3 FORMATION OF SILICA PARTICLES Porous silica is an industrial product with wide applications as an adsorbent for the purification of gases, vapours and liquids, as a carrier for catalysts, etc. As its application to LC as a support requires certain specifications concerning mechancial stability, purity, pore structure and particle size, one must pay particular attention to the preparation procedures. The most frequently used industrial process in manufacturing silica is the sol-gel procedure previously described in Section 2.2.2. A silica hydrogel is first prepared by adding water-glass to an acidic solution. After ageing, the cake is subdivided into suitably sized portions, washed and dehydrated. A substantial shrinkage of hydrogel lumps occurs upon dehydration, yielding hard, porous grains of silica xerogel that are irregular in shape. When spherical particles are desired, the sol-gel procedure has to be modified in such a way that the liquid silica sol droplets being generated in a heterogeneous phase system are converted into hydrogel beads by gelling. In addition to this mode, a series of special procedures for preparing spherical silica packings have been developed. Their potential advantage is the possibility of obtaining a variation of both the pore structure parameters and the size and size distribution of the microbeads. In the following sections, some procedures for the preparation of spherical and other silica microparticles with well defined properties are considered. 4.3.1 Irregularly shaped silica packings Milling of silica xerogels is the most convenient means of producing irregularly shaped microparticle packings. The main objective is to obtain a high yield in the 5 - 1 Q m size range with a minimum weight loss due to dust or fines. Theoretically, this can be realized by a continuous shifting of the maximum of the relative distribution curve from large to small diameters. In practice, the grains of 1-6 mm in size pass a series of different breakers, mills and sieves and the yield of microparticles may range between 20 and 70% depending on the mass of starting material. The yield depends on (1) the mechanical behaviour of the silica to be milled, (2) the efficiency of the operations and (3) the adjustment of the single units in the plant.
4.3.2 Spherical silica packings The basic principles of all processes using the sol-gel procedure are (1) emulsification of a silica sol in an immiscible non-polar liquid by stirring, dropping, etc., and (2) converting the droplets being formed in this way into gelatinous beads of silica hydrogel. The size of the liquid droplets and hence the particle size can be controlled by the viscosity of the sol. Gelling of silica sol droplets should occur very rapidly and has t o be completed during their residence time in the organic phase. The difficulty that arises is the careful
163
adjustment of the conditions and factors that govern the gelling time of the silica sol. The factors that have the most significant effect on gelling are the pH, the concentration and the temperature of the silica sol [ 191 . A series of processes that are based on the emulsification of a silica sol in an immiscible organic liquid while the droplets being generated harden to gelatinous beads have been patented [20]. An alternative approach to emulsification is to spray silica sols, made from water-glass or by dispersing finely divided silica, into a column or an oven, preferably at high temperatures. These spraying techniques, however, produce less or more uniform particles with sizes generally larger than 20 pm [21,22]. In order to obtain uniformly-sized silica microbeads, Kirkland patented a process that consists in the following steps [23, 241 : (1) preparing a silica sol consisting of silica particles of defined size dispersed in a polar liquid; (2) adding a solution of a polymerizable organic material such as formaldehyde and urea or melamine; (3) initiating copolymerization of the organic consituent yielding a coacervate of the organic polymer with the colloidal silica particles in the form of microbeads with a diameter of about 0.5-20 pm; (4) washing and drying the microparticles; (5) burning out the organic material by means of heat treatment; (6) sintering of the microbeads at 1173 K in order to strengthen the framework of interconnected silica particles. In 1971, Unger and Schick-Kalb [25] developed a procedure starting with poly(ethoxysiloxane) (PES) instead of silica sol. The PES is emulsified by vigorous stirring in ethanolwater, forming liquid droplets. Addition of a basic catalyst initiates the hydrolytic polycondensation, converting the droplets into gelatinous beads of silica hydrogel. The bead size and the size distribution can be easily controlled by the viscosity of PES and by the stirring speed (see Fig. 4.5). On the basis of these results, organosilica beads, termed bulkmodified silica, can also be prepared by using either poly(organoethoxysi1oxane) or a solution of PES and organotriethoxysilane instead of pure PES for the emulsification 1261.
4.4 POROUS SILICA LAYERS Although porous-layer beads (PLB) were firstly introduced as supports in gas chromatography to make possible high-speed separations, they became widely used as packing materials when high-performance liquid chromatography was developed [27-291. Since then, PLBs have decreased in importance, being replaced by totally porous microparticles, but they are still commercially available. The PLBs consist of a non-porous glass core coated with a thin porous layer. The diameter of the glass beads may vary between 30 and 50 pm and the thickness of the porous layer is about 1 pm.
164 percentage of cumulative undersize
____P
mean particle diameter dp&,
(lm)
Fig. 4.5. Cumulative frequency distribution of silica microbeads obtained by the PES procedure as a function of the stirring speed during emulsification. Preparation conditions: 1000 ml of PES (mean molecular weight 730); 3500 ml of ethanol-water mixture (2 : 3, v/v); 100 m1,of 25% ammonia solution.
4.4.1 Reparation of PLBS with a porous silica layer
In 1962, Halasz and Horvfith [29,30], described a procedure for the preparation of PLBs that were suitable as supports in gas chromatography. The glass beads were carefully cleaned, then shaken with a suspension of a finely divided adsorbent such as Aerosil. After evaporation of the solvent, coated beads with a thin skin of aggregated silica particles remained. By means of subsequent heat treatment, the particles were cemented together to form a rigid pore structure in the layer. Later, Kirkland [31] devised another procedure in which cleaned glass beads were coated alternately with a layer of colloidal silica particles and a layer of organic microparticles [31] . The organic interlayer provides an oppositely charged surface that is necessary for the attraction and retention of the colloidal silica. In the coating process, a silica sol containing particles of 200 nm in size and an organic sol such as poly(diethylaminoethylmethy1methacrylate) acetate or poly(methylmethacry1ic acid) are employed. Coating is repeated several times followed by intermediate drying until a layer of thickness 0.5-1 .Opm is achieved. The product is then subjected to heat treatment to burn out the organic polymer. In order to improve the mechanical strength of the PLBs, they were finally heated to about 1000 K. The material commercially available as Zipax has a specific surface area of SBET = 0.5 mz/g and a mean pore diameter of D = 38 nm [32]. On the basis of the PES procedure described on page 50, a process for the formation of a porous silica layer was developed [33,34]. The procedure involves the following steps:
165
(1) cleaning the glass beads to be coated; (2) preparing a solution of a certain amount of PES in a low-boiling solvent; (3) suspending the glass beads in the PES solution; (4) evaporating the solvent carefully under vacuum in order to obtain virtually dry PES-coated beads; (5) suspending the coated beads in water-ethanol; ( 6 ) adding a portion of ammonia solution with vigorous shaking in order to convert the liquid layer rapidly into a hydrogel layer; (7) ageing, washing and drying.
4.4.2 Variation of the pore structure and the thickness of the porous layer The PES coating procedure permits a systematic variation of both the pore structure parameters and the thickness of the porous layer. The latter, designated by ds,is determined by the amount of PES used per grain of glass beads and can be evaluated according to the equation (4.15) where dp is the average diameter of the glass beads, the relative amount of PES in grams per 100 g of glass beads, pb the density of glass beads and PPES the density of PES. It proved possible to vary ds reproducibly between 0.1 and 1.0 pm by using graduated amounts of PES, all other conditions (mean molecular weight of PES, ratio of ethanol to water and amount of ammonia added as a catalyst) being held constant. An increase ;n ds simultaneously causes an increase in the specific surface area; the maximum value attained was 40 m2/g at d p = 23 pm. By varying the amount of ammonia while the other factors were held constant, the mean pore diameter of the porous layer could be controlled between 2 and 20 nm, because the catalyst affects the rate of the TABLE 4.5 PORE STRUCTURE DATA OF A SERIES OF PLBs WITH GRADUATED MEAN PORE DIAMETER Preparation conditions: d of glass beads = 35 &./T;I mean molecular weight of PES = 1300; kinematic viscosity of PES = 6.10- m’/s, mass of coated PES = 7% (w/w); amount of ethanol-water mixture
r
(1 : 2, v/v) = 30 ml; amount of concentrated ammonia solution (reagent grade, 25%) = 1, 3 , 5 and 7 ml. No.
Amountof ammonia solution (ml)
1
2 3 4
1 3 5
7
* SBET: using nitrogen
vp **
SBBT* im 18)
(ml/g)
D*** inm)
10.2 8.9 7 .a 6.8
0.022 0.023 0.024 0.026
6.6 7.6 9.0 1 .o
withAm(N2) = 0.162 nm2/molecule.
** V p : millilitres of liquid nitrogen per gram at p/po = 0.95. ***D: calculated from the nitrogen desorption branch using the method of Pierce.
166
hydrolytic polycondensation of PES to the hydrogel and hence the crosslinking of the siloxane network, which finally determines the pore width [35].Table 4.5 gives pore structure data for a series of PLBs whose mean pore diameter was varied in about 1.0-nm intervals. These studies were part of a comprehensive investigation on the structure and properties of PLBs with the aim of optimizing the structure of the porous layer with respect to mass transfer of solutes in column liquid chromatography [33] .
4.5 ACKNOWLEDGEMENTS Valuable comments and help from Dr. K.F. Krebs and Dr. W. Reich, E. Merck, Darmstadt, G.F.R., concerning this chapter are greatly appreciated. 4.6 REFERENCES 1 J.F.K. Huber, Ber. Bunsenges. Phys. Chem., 77 (1973) 179. 2 J.F.K. Huber, Chimia, Suppl. (1970) 24. 3 J.J. Kirkland, J. Chromatogr. Sci., 10 (1972) 593. 4 R.E. Majors, Anal. Chem., 44 (1972) 1722. 5 J.J. Kirkland, in S.G. Perry (Editor), Gas Chromatography 1972, Appl. Science, Barking, 1973, p. 39. 6 R.E. Majors, J. Chromatogr. Sci., 11 (1973) 88. 7 J.J. Kirkland, J. Chromatogr.. 83 (1973) 149. 8 C. Orr, Jr., and J.M. Dalla Valle, Fine ParticleMeasuremenf Macmillan, New York, 1959. 9 G. Herdan, Small Particle Statistics, Butterworths, London, 1960. I 0 R.R. Irani and C.F. Callis, Particle Size: Measurement, Interpretation and Applicatioa, Wiley, New York, 1963. 11 R.D. Cadle, Particle Size: Theory and Industrial Apptication, Reinhold, New York, 1966. 12 C. Orr, Jr., Particulate Technology, MacMillan, New York, 1966. 13 T. Allen, Particle Size Measurement, Chapman and Hall, London, 1974. 14 A. Linder, Statistische Methoden, Birkhauser Verlag, Bade, 1964. Roscoe, Fundamental Research Statistics for the Behavioral Sciences, Holt, Rinehart and 15 J.T. Winston, New York, 1975. 16 J.T. Bailey, W.H. Beattie and C. Booth, J. Chem. Educ., 39 (1962) 196. 17 0. Lauer, Feinheitsrnessungen an Technischen Stauben, Alpine AG, Augsburg, 1963. 18 0. Lauer, Sonderdruck, No. 44, Alpine AG, Augsburg. 19 W. Foerst (Editor), Ullmanns Enzyklopadie der Technischen Chemie, Vol. 15, Verlag Urban und Schwarzenberg, Berlin, 1951, p. 723. 20 M. Le Page, R. Beau and J. DuchBne, Fr.Pat., No. 1,473,240 (1967); M.Le Page and A. de Vries, Fr. Pat., No. 1,475,929 (1967); I. Sebestian and I. HalBsz, Ger. Rat., No. 2,155,045 (1971); J.H. Knox, Ger. Pat., No. 2,364,159 (1973). 21 H.E. Bergna and F.A. Simko, Jr., U.S. Pat., No. 3,301,635 (1965). 22 A.V. Kiselev, G.L.Kustova, B.A. Lipkind and J.S. Nikitin, Ger. Pat., No. 2,225,452 (1972). 23 J.J. Kirkland, US.Pat., No. 3,782,075 (1974). 24 R.K.Iler and H.J.McQueston, US.Pat., No. 3,855,172 (1974). 25 K. Unger and J. Schick-Kalb, Ger. Pat., No. 2,155,281 (1971). 26 K. Unger and J. Schick-Kalb, Ger. Pat., No. 2,357,184 (1973). 27 M.J. Golay, in R.P.W. Scott (Editor), Gas Chromatography, 1960, Butterworths, London, 1960, p. 139.
167 28 J . Bohemen and J.H. Purnell,J. Chem. Soc., (1961) 360. 29 I. Hal& and C. Horvith,Anul. Chem., 36 (1964) 1178 and 2226. 30 I. Halls2 andC. Horvith, Ger. Put., No. 1,183,715 (1962) and 1,282,323 (1964). 31 J.J. Kirkland, US.Put., No. 3,505,785 (1970). 32 K. Unger, P. Ringe, J . Schick-Kalband B. Straube, 2. Anal. ehem., 264 (1973) 267. 33 J. Schick-Kalb, Thesis, Technische Hochschule, Darmstadt, 1975. 34 H.W. Kohlschiitter, K. Unger and J. Schick-Kalb, Ger. Put., No. 2,225,973 (1973). 35 K. Unger and B. Scharf,J. Colloid InterfaceSci, 55 (1976) 377. 36 W. Reich, Kontukte (E. Merck), 3 (1977) 26.
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Chapter 5
Silica columns - packing procedures and performance characteristics
5.1 INTRODUCTION The column is the core of any chromatographic system. In the high-performance mode of column liquid chromatography (HPLC), the primary objective is to attain a column bed as a uniform, dense and stable array of microparticles in order to minimize band broadening and to obtain a reasonable pressure drop. Two types of silica packings are in use: (1) superficially porous packings, otherwise termed pellicular or porous layer beads, which consist of a non-porous glass core coated with a thin layer of porous silica; whereas the diameter of the coated beads ranges between 30 and 50 pm, the porous shell is only 1-2 pm thick; (2) totally porous packings in the 5-10-pm size range, spherical or angular in shape. The most important parameter of silica packings that determines the method of packing to be used is the mean particle diameter, d,. When d , is greater than 20 pm,the silica is dry packed into the column, preferably by means of the tap-fill method as described by Snyder and Kirkland [ I ] . At first in HPLC diatomaceous earth particles were also packed by a dry-tamping technique, but this procedure is tedious and requires an experienced operator [2]. Silica supports with d p less than 20 pm were found to be packed satisfactorily by means of the slurry technique, in which the suspended particles are forced into the column under pressure at a high flow-rate. Pioneering work in this field was done by Sie and Van den Hoed [3], Majors [4], Strubert [5] and others. Scott and Lee [ 6 ] first applied this technique of dynamic column packing in the preparation of ion-exchange columns filled with organic resin microparticles. Since 1973, the number of slurry procedures proposed has increased greatly, differing in the type of slurry liquid, the slurry concentration, the equipment, the pressure drop needed, etc. Up to now no generally valid and conclusive judgement can be made about these various techniques with regard to optimal packing and high column performance. This chapter is an attempt to clear up the situation, mainly from the point of view of the properties of silica.
5.2 PARTICLE PACKING 5.2.1 Geometrical analysis of column bed
For a better understanding of column performance, it may help to consider the column bed dimensions. Let us assume a tube of length 100 mm and I.D. 5 mm being uniformly packed with 5-pm silica beads. The packing is assumed to have a specific surface area of 320 m2/g, a specific pore volume of 1.10 ml/g and a helium density of 2.30
170
g/ml. The volume of the column, V,, can be written as the sum of three volume contributions: Vc =
Vi,
f
Vin + V ,
(5.1)
where Vi, is the interstitial volume due to the interstices and voids between the particles, Vin the internal volume originating from the pore space of the particles and Vsp the volume of the purely solid packing. The volume of the empty column for this particular instance is calculated t a be 1963 p1 and the volume of a single microbead is 6.5 lo-’ pl. To compare the volume data of columns independent of length and diameter, the volumes in eqn. 5.1 are converted into dimensionsless quantities; by dividing Vjs, Vi, and Vsp by V,, one obtains the following three porosities: Vis fis p = -
(interstitial porosity)
VC
‘sp =-vsp
VC
(internal porosity)
(5.3)
(fraction of space fdled up by the solid packing)
(5.4)
In a regularly packed bed, the interstitial porosity is about 0.4. In the instance being considered, 1963 -0.4 = 785 pl accounts for the interstitial volume. The total porosity, er, of the column, which is the fraction of space filled with the eluent, is given by et = fisp + einp
(5 3) The total column porosity can easily be calculated from experimental data by use of the equation
where f v is the volume flow-rate of the eluent, to the elution time of an unretained solute, L the column length and dc the column diameter [7]. The total porosity of the column taken as an example was calculated in ref. [8] to be 0.83, utilizing eqn. 5.6. The interstitial porosity, initially assumed to be 0.40,can also be measured experimentally as follows. A totally excluded polymer is injected and its elution volume, VE,is measured accurately; VE corresponds to the interstitial volume of the column provided that the volume contribution due to connections, etc., are negligibly small. VE divided by Vc gives Eisp
-
The internal porosity made up by the pores inside the particles can easily be derived at a known specific pore volume of the packing as follows. The specific pore volume is related t o the particle porosity, e P ,which is the volume of pores relative to the total volume of a particle:
171
where Vs is the specific volume of the purely solid packing, which is the reciprocal of the true density. In the instance being considered, one obtains Ep =
1.1
= 0.72
1.1 +- (1/2.30)
(5
The particle porosity should not be confused with the internal column porosity. The following relationship between eP and Einp is valid: (5 *9)
Einp = epP(1- ~ i s p )
Assuming eisp = 0.4, the internal column porosity is 0.43. The total column porosity is then 0.4 t 0.43 = 0.83, which is in excellent agreement with the value derived from eqn. 5.6 171. It should be noted that a clear distinction between internal and interstitial porosity is possible only if there is a signifEant difference between the mean pore diameter attributable to the particle pore structure and to the interstitial pore space. The two pore systems, often termed primary and secondary pore systems, can be shown by means of mercury porosimetry. This is illustrated in Fig. 5.1 for the silica sample under consideration. On forcing mercury into the assembly of 5-pm particles, the large interstitial voids
$
rnl intruded mercury , p e r g silica
1 1
~ n l e r s t i t i o lvolume
1
between oggregoted particles
~
,
,
.
, ,
10
,
. , , ,
100
,
,
,
, @ J ,, , , , 00 0
,,,,'
1000 -1
-_
D (nrnl
Fig. 5.1. Cumulative pore volume as a function of mean pore diameter for a silica packing with a mean pore diameter of 12 nm and a particle size of 5 pm measured by mercury porosimetry.
172
(= secondary pore system) between 1 and 5 pm were first filled at low pressures. The volume of intruded mercury corresponds to the interstitial volume of the agglomerated dry particles. Obviously, this volume may be larger than that of the densest packing of particles in the column obtained by the pressure filtration technique. The next increase in intruded volume is observed at high pressures at which the mesopores were successively filled with mercury. On converting this cumulative curve into a relative pore volume distribution, two distinct maxima occur at 2000 and 12 nm, the difference between these two values being about two orders of magnitude. Assuming an interstitial porosity of 0.40, the volume attributable to the porous packing is 1963 -0.6 = 1 178 pl. The number of particles, N , being present in this volume is
N=
1178
6.5 * lo-'
= 1.8.10'o
(5.10)
The weight of the packing in this column is about 1.O g. The total external surface area of the particles, Se, is Se = 1.4 mZ/g
is 320 m2/g. whereas the total internal surface area of the particles, SBET, To obtain an impression about the size relationships between the different linear dimensions of the column and packing, the following data should be compared: d, = 5 mm = 5*106nm diameter of the column: d p = 5 p m = 5 1 0 3 nm mean particle diameter of packing: lo3*dp = d, particle diameters across the column: most frequent pore diameter: D = 12 nm dm ~ 0 . nm 5 molecular diameter of solute: 5.2.2 Factors influencing particle packing
The column bed consists of compacted solid silica particles. For a high column performance the bed properties should satisfy the following requirements: (i) the packing should be as dense as possible in order to provide high stability; (ii) the interstitial voids between the compacted particles should be of uniform dimensions in order to give a homogeneous flow profile; (iii) local inequalities of packing density across and along the column axis should be strictly avoided. Particle compaction can be treated theoretically as a coordination of particles of uniform size and shape to give a three-dimensionally linked structure. Most of the approaches are based either on a regular packing of hard spheres or on randomly packed packings [9]. Smalley [101 postulated nine different types of packed beds of spheres in which the interstitial voids are described in terms of polyhedrons. In a more practical way, JSrischer [ 1 1 1 modelled several regular packings by compacting stainless-steel balls. By means of permeability studies, he calculated the interstitial porosities of the packings, which were compared with the theoretically predicted porosity. The results given in Table 5.1 show that the experimental value is always slightly higher than the theoretical value for a given type of packing structure. Depending on the coordination number of the
173
spheres in a particular structure, the interstitial porosity covers a wide range between 0.3 and 0.5. Although this attempt is far removed from chromatographic reality, it shows clearly that the number of contacts and the contact area between particles in a compact assembly determine the volume fraction available and the geometry of the interstitial voids. In an attempt to evaluate the structure characteristics of random beds packed with spheres and irregularly shaped particles, Rumpf and Debbas [ 121 derived an expression that enables one to calculate the variance of the interstitial porosity in the cross-section within a random packed bed. TABLE 5.1 THEORETICALLY PREDICTED AND EXPERIMENTALLY MEASURED INTERSTITIAL POROSITIES, q S p ,OF VARIOUS REGULAR PACKING STRUCTURES OF STAINLESS STEEL BEADS (2 mm IN DIAMETER) [ 111 Type of packing structure
Eisp (theoretical)
Eisp (experimental)
Cubic Rhombedric Octaedric Tetraedric Disordered
0.476 0.396 0.258 0.258 -
0.480-0.485 0.404-0.428 0.321 -0.330 0.294-0.302 0.37 -0.38
Taking into consideration the above results, one can make some qualitative predictions on packing structure in liquid chromatography (LC). The packing density and strength will be influenced by both the shape and the size distribution of particles. Spherical particles are expected to give a more regular packing than angular particles. Angular particles may be packed to various densities and a very slight, uneven compaction of the bed may result in local size differences in interstitial voids and in non-uniformity. Uniform interstitial voids can be obtained only with narrow-sized packings. A special case in which particles with a bimodal size distribution were packed into columns was recently studied by Bristow [ 131. When the maxima of the two distributions differ by about one order of magnitude, the smaller particles are able to occupy the interstitial space due to the larger particles, resulting in a low permeability of the column. In studies of the compaction of powders [9] that have comparable sizes to silica packings, it was shown that interparticle forces depending on the particle size, content of moisture and degree of pore filling with a capillary condensed liquid have a dominant effect on the particle packing structure. With the use of 5-10-pm silica particles that have an external specific surface area of about 1 m2/g, adhesion forces between adjacent particles become noticeable and aggregates and clusters are formed. For packing columns with silica microparticles, the slurry technique was introduced in which the particles are suspended homogeneously in an appropriate liquid in order to avoid particle-to-particle.bridging and sedimentation. Depending on the type of liquid and the surface composition of the silica used, it is often observed that the suspension does not remain stable but flocculation occurs. Flocculation, i e . , aggregation of particles, is caused by dispersion attraction forces between the particles. in colloidal silica chemistry
174
[ 141, typical flocculation agents are cationic surfactants, basic metal salts, cationic polymers, alcohols, amides and ethers, providing a certain pH value. The basic mechanism in applying these agents is that either the negative charge of colloidal silica particles is compensated for by attracting positive ions or positively charged sites of species or hydrogen bridges are formed between pait
0.90, which corresponds to a specific pore volume of VP > 3.0 ml/g, break up under a pressure of 300 bar during column packing. Messer [19] investigated the packing stability of silica beads of graduated pore diameter. A tube of length 200 mm and I.D. 4.2 mm was packed with silica at 300 bar by the slurry technique. Then the pressure was increased stepwise up to 1000 bar while measwing the flow-rate. On reaching a certain pressure which was typical of the particular silica under investigation a sharp decrease in the flow-rate was observed. With a further increase in pressure the flow-rate again decreased drastically and the column became impermeable. The limiting pressures, P h i t . , for a series of silicas are given in Table 5.2. It seems obvious that pfi,,,it. is a function of the mean pore diameter, D, of the packing. Whereas mesoporous silica with D = 10 nm is able to withstand pressures greater than 1000 bar and a flow-rate more than 40 ml/min, wide-pore silica with a pore diameter of 400 nm fractionates at 530 bar and 35 mllmin. It should be noted that the particle porosities of an samples in Table 5.2 are nearly identical at about 0.65, so that the porosity may not be the determining factor. It is not certain that the breakage of the particle structure in wide-pore silica is due only to the thin pore walls. Another possible explanation may be that the wide-pore silica, which shows a highly fissured outer surface with broad pore openings (see Fig. 7.1), is sensitive to shear forces at high flow-rates, and will be partially broken up into small
175 TABLE 5.2 LIMIT OF PACKING STABILITY OF SPHERICAL SILICA PACKINGS WITH DIFFERENT MEAN PORE DIAMETERS [ 191 Column, length 200 mm, I.D. 4.2 mm; eluent, n-heptane; pump, MS 80/8(Orlita, Giessen, G.F.R.). Mean particle diameter, dp:, (rm)
Specific surface area, SBET(m’/g)
Mean pore diameter, D (nm)
Limiting pressure, Plimit. (bar)
9.8 10.3 10.4 10.7
236 45 19 4.7
9.4 48.8 97.4 385.0
>loo0 925 f 50 750 f 40 530 f 20
pieces. In contrast to wide-pore silica, mesopore silica exhibits a relatively smooth particle surface, as can be observed by examining electron micrographs. For a deeper understanding of the observed behaviour, however, more careful experiments are required.
5.3 PACKING PROCEDURES 5.3.1 Dry packing techniques
The basic procedure consists in a continuous or batch-wise addition of small amounts of packing, by hand or with an appropriate mechanical device, into a vertically mounted column, which is bounced at a frequency of about 100 times per minute or subjected to vibration in a vertical direction. During the filling, the outside of the tube is tapped gently or vibrated at the level of the top of the packing in order to facilitate settling of the particles to give a dense packing and to prevent size segregation near the tube walls. A variety of procedures have been proposed that differ in the use of rotation, vibration, bouncing, etc., and in the amplitude of vibration, the vibration intensity, the method of tapping, etc. The different features in dry packing and the tap-fill method as the most recommended one are described by Snyder and Kirkland [ l ] and Bristow [22]. Dry packing is the optimal method for packing large particles with dp > 20 pm but smaller particles, in the 10-14-pm size range, have also been dry packed successfully [23]. Comparison of the column performance data between dry- and slurry-packed columns filled with the same material show that the column resistance factor 4 [22] is twice as great with the former. Porous layer beads such as Corasil, Zipax and Perisorb are preferably dry packed. Owing to their spherical shape and high bulk density of about 2.5 glml, a dense and regular packing is obtained. Dry packing of micro-sized silica particles on a large scale is performed in the production of large diameter glass columns, marketed as Lobar columns by E. Merck (Darmstadt, G.F.R.). As already indicated by Bristow [22], the packing density in these large-diameter columns can more easily be assessed by accurate measurements of the column volume and weight of paclung than it can in small-diameter columns. The packing density of Lobar columns of sizes A, B and C filled with LiChrosorb Si 60 is reported to be 0.55 g/ml [24].
176
Scott and Kucera [25] measured the bulk densities of a series of dry-packed commercial silica types, which ranged between 0.38 and 0.56 g/ml. 5.3.2 Slurry packing techniques
Slurry packing is now the most widely used method for packing satisfactorily columns with microparticles in the 5-10-pm size range. The technique was originally developed by Sie and Van den Hoed [3] to improve the packing quality of LC columns using large particles and by Scott and Lee [6] in packing ion-exchange columns with small particles. The purpose of the traditional slurry technique was [4,5] to prepare a homogeneous distribution of isolated particles in the slurry medium by subjecting the suspension to ultrasonic treatment, to prevent sedimentation of particles by adjusting the density of the slurry to that of the packing (balanced-density slurry) and to force the slurry into the column at high pressure and high flow-rate by displacement of the slurry by a second immiscible liquid to achieve a dense and uniform bed. Both the improved qualities of silica packings and the development of new types, such as chemically bonded packings, have distinctly changed the criteria that were valid for packing technology at the beginning of 1970. Firstly, the silica packings that are marketed now possess a much narrower size distribution than before. The standard deviation of the mean particle diameter, d p , for instance, is reported t o be about +lo-20% for 5-10-pm particles. Secondly, chemically bonded silica packings, particularly the reversed-phase type, have gained increasing importance over pure silica. The hydrophobic nature of this packing created new problems in packing highefficiency columns. Thirdly, in the last 2 years relatively short microparticle columns of length 100-200 mm have come into use. To minimize peak broadening of such highefficiency columns, particular attention has to be paid to the particle packing on the top of the bed, along the tube walls and at the column end, in addition to taking care of extra-column contributions. The purpose of this section is less to discuss the advantages and disadvantages of the various slurry techniques with regard to column performance but more particularly to find out the controlling process variables and parameters in the packing procedures and to discuss their effect on packing quality. 5.3.2.I Pre-treatment of packing The question of the pre-treatment of a purchased silica support before its use is often asked. Most commercially available silicas are made by the sol-gel procedure, sometimes followed by an after-treatment such as hydrothermal treatment. The surface of these products is fully hydroxylated, exhibiting a surface concentration of hydroxyl groups, OH, of 8-9 pmoie/m2 after annealing at 473 K. In many papers it has been suggested that the silica should be treated with dilute or even concentrated boiling acid in order to purify the material and/or to hydroxylate the surface fully. However, it is well known in the surface chemistry of silica (see Chapter 3) that an after-treatment with boiling acid changes the specific surface area and the pore structure considerably. Further, it is a long procedure subsequently to wash out the acids completely. If it is not done, the surface
177
acidity and its activity will be drastically changed. In order to attain reproducible and comparable chromatographic results, one should avoid a chemical pre-treatment of the packing. Another feature which may be brought into the discussion of pre-treatment is the removal of fines that may be present in packings. As outlined in Chapter 4, sizing of particles is probably the most critical and also the most expensive step in the production of packings. The particular problem in sizing silica is to cut the particle size distribution at the lower end. Depending on the sizing technology and efficiency, a few tenths of 1% (wlw) of fines will still remain in the product. This small proportion, however, corresponds to a relatively large number of fine particles that may block the column frit and cause other difficulties. In order to remove the fines, the material to be used is subjected to a sedimentation as follows: prepare a dilute and homogeneous slurry of silica in water by shaking or ultrasonic treatment; with reversed-phase packings, use dioxane or benzene instead of water as the slurry medium; allow the slurry to sediment for a sufficient time in a beaker or glass cylinder; remove the upper part of the suspension that contains the fines when the major amount of particles has settled at the bottom; control the particle size of the fines and that of the sediment by viewing under a microscope; and separate the solid particles from the solution by filtering the suspension through an appropriate filter device and dry the packing at 473 K.
5.3.2.2 Slurry liquid The original idea in employing a slurry was to disperse the particles in an appropriate liquid in such a way that their settling and aggregation are prevented. From this point of view, a balanced-density slurry was made by mixing liquids of high viscosity such as tetrabromoethane and tetrachloroethylene with liquids of lower density, eg., dioxane. The composition of the mixture was adjusted to have a density equal to the true density of silica (cu. 2.3 g/ml). In the further development of slurry packing, other liquids were introduced, without balancing the density, e.g., acetone [26], methanol [ 171, methyl iodide [24], trichloromethane [27], tetrachloromethane [28] and dilute ammonia solution [29]. Columns were found to be packed satisfactorily by means of these slurry liquids. A possible reason for the good results may be that the packings used were narrower sized than that in the early days of HPLC and sedimentation and segregation d o not occur to an appreciable extent under these non-balanced conditions. A basic requirement for utilizing a liquid as a slurry medium is that it should wet the surface of the particles. Pure silica is well wetted by both polar and non-polar solvents. Reversed-phase packings having a hydrophobic surface are not wetted by pure water but they are by polar organic solvents or their mixtures with water. Dispersion of silica supports in non-wetting or partially wetting liquids causes air bubbles to remain, which may give rise to inhomogeneities or uneven compaction of the bed. Bristow [ 221 proposed to discriminate between flocculating and deflocculating slurry liquids. In flocculation, silica particles become linked together to form aggregates with a relatively open structure (see Chapter I), whereas in deflocculating liquids, the silica particles settle at a velocity given by Stoke’s law, the flocculated particles settling more rapidly “en masse” to leave a clear supernatant solution. Table 5.3 lists a series of liquids,
178
TABLE 5.3 FLOCCULATING AND DEFLOCCULATING LIQUIDS IN SLURRY PACKING FOR SILICA AND REVERSEDPHASE SILICA PACKINGS ACCORDING TO BRISTOW [ 221 Packing type
Liquid Flocculating
Deflocculating
SitiCa
Halocarbons, n-hexane, diethyl ether
Acetone, water, methanol
Reversed-phase silica
Water, methanol
Acetone
divided into flocculating and deflocculating. At first sight, one might expect that deflocculating liquids would give superior packing qualities, but the performance data do not permit a clear decision to be made. Another important property of slurry liquids that has a considerable effect on the speed of packing is its viscosity. According to the Stoke’s equation (eqn. 4.1 l), the settling velocity, ust, of a spherical particle is inversely proportional to the viscosity, 7 , of the liquid in which it sediments. In other words, high-viscosity liquids reduce the settling velocity compared with low-viscosity liquids. This was the background to the introduction of highly viscous liquids such as polyethylene glycol, cyclohexanol, etc., as slurry liquids [30]. With highly viscous slurries, however, appreciably longer times are required for forcing the slurry into the column than with lowwiscosity slurries at a constant set of conditions. It is worth mentioning that tetrabromoethane and tetrachloroethylene are extremely toxic. Additionally, on prolonged storage these compounds start to decompose, giving reactive bromine and chlorine, respectively. Therefore, it is recommended that they be purified before use by slowly passing them through a column containing an activated coarse silica. The halocarbons are also able to attack the surface bonds of chemically bonded silica packings, as discussed in Chapter 3.
5.3.2.3 Slurry preparation In preparing the slurry, a known amount of dry silica is weighed into a given volume or mass of slurry liquid and then subjected to ultrasonic treatment for a few minutes. A major point of discussion in slurry preparation is the concentration of silica in the slurry. The concentrations reported vary over a wide range, between 1 and 30% (wlw) [ 17,19,27]. Bristow et al. [17] found superior results with dilute (1-5%, wlv) compared with thick slurries using methanol and applying an upward packing technique. Messer [19] recommended 10%(wlw) as the optimal concentration for tetrachloroethylene.
5.3.2.4 Apparatus The apparatus for slurry packing consists of a solvent reservoir, pump, pressure gauge, drain valve, slurry reservoir, extension, column and beaker. Large reservoirs are needed for dilute slurries. When high pressures, say between 300 and 400 bar, are applied the wall thickness of the slurry reservoir has to be satisfactorily thick for safety reasons. Narrow-
179
bore constrictions between the reservoir and the extension should be avoided because the flow of particles is then disturbed, which results in uneven compaction. For the same reason, the diameter of the reservoir should not be Less than that of extension and column. 5.3.2.5 Filling procedure
The f a n g procedures reported differ in various ways. Theoretically, one can distinguish two cases, concerning flow-rate and related pressure. The first case is to hold the flow-rate constant during the whole filling procedure. This operation requires a constantflow pump furnished with a relief valve or an electrical motor cut-off. In practice, this mode can be realized only with a prolonged packing, i.e., with viscous slurries. When using dilute and low viscosity slurries, the filling operation is completed within a few minutes. In the second case, the pressure is held constant during filling and hence the flow-rate varies. In most studies the constant-pressure mode has been applied. The pressure required for slurry packing is also a matter of wide speculation. Low pressures (ca. 10 bar) have been recommended, but pressures up to 1000 bar have also been employed [ 18,191. The pressure needed may depend on the density of the slurry and on the state of the particles, e.g., whether they are flocculated or not. Bristow et ~ l [ 171 . suggested that a pressure of CQ. 100 bar is satisfactory for dilute slurries in methanol. Messer [19], however, concluded that the optimal pressure range is between 400 and 500 bar for tetrachloroethylene as a slurry medium at a 10%(w/w) concentration. He also studied the influence of filling pressure between 400 and 800 bar on the chromatographic permeability and column efficiency using spherical a~ndangular silicas of similar size. Above 500 bar he established an increase in the plate height at a given velocity, associated with a reduction in chromatographic permeability, Kto. High-pressure treatment was also recommended by Kirkland [3 11 in packing Zorbax columns. A repetitive high-pressure pulse of the packing by rapidly opening the isolation valve to produce a type of shock wave has also been suggested to give a dense packing [24,31]. A special modification of the slurry technique was introduced by Godbille and Devaux [32,33], in which the column was used as its own reservoir, the slurry being compressed into the bed by a piston. The procedure was developed mainly for large-diameter columns. The technique proposed by Linder ef al. [34] “involves a combination of stirring action to keep the particles floating and high initial pressure action to transport the particles rapidly from the mixing vessel via high-pressure tubing into the column”. The last step in column packing is to displace the slurry liquid with an appropriate solvent and to condition the column with the eluent to be ready for use. In some instances when larger differences in polarity between the slurry liquid and the eluent exist one or two solvents of intermediate polarity should be employed as displacement liquids to ensure the complete removal of the slurry liquid.
5.4 COMPARISON OF PERFORMANCES OF SILICA COLUMNS A major problem in HPLC that is still unsatisfactorily resolved consists in deriving representative quantities from experimental data that fully characterize column perfor-
180
mance and that can be utilized in comparing packing materials. The chromatographic process concerning band spreading of a solute can be treated by theoretical and semitheoretical approaches. An excellent review of the current state of band spreading theories was given by Grushka et al. [35]. In discussing performance in LC, Knox [36] divided the properties of a column into thermodynamic, kinetic and hydrodynamic. The thermodynamic properties, determined mainly by the type of solute and the phase system, are considered to be almost independent of the kinetic and hydrodynamic properties, both being affected by the particle size of the packing, the flow-rate, etc. In following the concept of Knox and co-workers [35,36], a reasonable measure of the kinetic properties of the column will be the column permeability and the plate height-velocity dependence. 5.4.1 Column permeability
The permeability of the column determines the pressure drop at a particular flow-rate and given column dimensions and hence controls the analysis time. The basic aspects in defining column permeability and its meaning for column liquid chromatography were thoroughly discussed by Deininger [37]. The permeability of an open tube through which a liquid is flowing is defined as (5.1 1)
where fv is the flow-rate, 7 the viscosity of the eluent, L the column length, A p the pressure drop and d, the inner diameter of the column [7,8]. In a regularly packed bed of non-porous spheres, the permeability is calculated according to the Karman-Cozeny equation [37] : dP2
KKc
’
eo
=a (1 -
(5.12)
where d p is the mean particle diameter of the spheres, 6 a shape factor and e0 the interstitial porosity. As a measure of the flow resistance of a bed packed with porous particles, the chromatographic permeability, Kto,was introduced:
K =-UVL to Ap
(5.13)
By inserting u = L/to into eqn. 5.13, one obtains
(5.14)
Kto and K F are related by the total porosity of the column, e t : K F = etKt,,
(5.15)
KF and Kt, have the dimensions of area, preferably expressed in square micrometres. Bristow and Knox [7] suggested the use of a dimensionless quantity, namely the column resistance factor, $, which is defined as
181
(5.16) The value of 4 depends on the particle shape, the particle size distribution, the porosity and pore structure of the support. For columns packed with impervious glass beads, the value of # varies between 500 for 15-pm glass beads and 800 for 100-pm beads, as shown in Table 5.4 [38]. For columns packed with pellicular supports with a negligibly small porosity, the column resistance factor is reported to be 500-700 [39,40]. For porous particles, 4 is expected to be larger as a result of the increase in total porosity. For porous microbeads, Kirkland [39] obtained 4 values of the order of 500-700, which agreed with the results of Knox and Pryde [41] for spherical silica and its chemically modified derivatives. A comparative study of column resistance factors between spherical and angular silicas was made by Unger et al. [42]. For all angular silica supports, the resistance factor is greater by roughly a factor of two than for spherical silica particles. This difference cannot be discussed only in terms of particle shape because both types of packings also differ in porosity and mean pore diameter. Conversely, the relationship between the square of the mean particle diameter, d p 2 , and the chromatographic permeability, Kto,can be utilized to estimate the effective mean particle diameter: (5.17)
4 is then assumed to have a constant value of 500 [43] or 1000 [8]. 5.4.2 Plate height-velocity dependences
The plate height, Hi,of a solute i is given by
(z) 2
Hi = L
(5.18)
where uyi is the standard deviation of the eluted peak in time units, tRi the retention time TABLE 5.4 CHROMATOGRAPHIC PERMEABILITIES, K t , , AND COLUMN RESISTANCE FACTORS, 0,OF COLUMNS (LENGTH 500 mm, I.D. 2 mrn) PACKED WITH GLASS BEADS OF VARIOUS SIZES [ 381 Mean particle diameter, dp:, (Pm)
Kto
15.3 23.2 31.5 48.0 75.8 104.6
0.45 0.96 1.65 3.25 6.45 13.50
@
bm’) ~
520 560 601
I08 890 810
~
~-
182
of solute i and L the column length. Usually, the plate height measured for an unretained solute and retained solutes is plotted as a function of the linear velocity, u, of the eluent as shown in Fig. 5.2 for a column packed with Spherisorb S5W (dp = 5 pm). Instead of using absolute values, Giddings [44] introduced reduced parameters such as the reduced plate height, hj, given by h . =-Hi I dP
(5.19)
and the reduced velocity, v:
v=- UdP (5.20) Dim where Dim is the diffusion coefficient of the solute calculated according to the WilkeChang equation [7]. The concept of reduced parameters as a measure of performance has been widely advocated by Knox and co-workers [7,35,36,40]. The major advantage of using reduced parameters is the ease of comparison of column data relating to different particle sizes. The h versus Y function is preferably plotted on a log-log scale as shown in Fig. 5.3 for the same column as in Fig. 5.2. In practice, columns are operated at the minimum of the curve at 1 < v < 10. The shape of the h versus v curve can be adequately approximated by
I
I
1
2
3
4
5
'
6
7
-
8
9
I
I
1
'
0
u (rnmlsec)
Fig. 5.2. Plate height, H , as a function of linear velocity, u, for Spherisorb S5W. Column length, 100 mm; I.D., 4.5 mm;column temperature, 298 K; eluent, n-heptane (33%relative humidity); detector, UV (254 nm). Solutes: benzene (k' = 0.26), diphenyl (k' = 0.70), rn-terphenyl (k' = 1.41) and mquaterphenyl (k' = 2.63). t o marker: tetrachloromethane.
183
the equation h = B/u t
(5.21)
t0
The B term reflects the influence of axial diffusion of the solute, which becomes noticeable at low reduced velocities. A is the contribution from the flow in the intraparticle space and is a measure of the goodness of packing. C reflects the contribution caused by the rate of mass transfer of solute between the mobile and stationary phase and is dominant at high reduced velocities. Examination of reduced plots obtained on a variety of columns permits the derivation of the following optimal values of the constants: A = 1 , B = 2 and C = 0.1. Hence, eqn. 5.21 reduces to h = 2/u t
t0 . 1 ~
(5.22)
This relationship is suggested for use in further optimization studies in H P K , including particle size and flowrate. Reduced plots for various packing materials show that ion exchangers have poor performance compared with adsorbents and liquid-liquid partition systems [35].Extended studies on the performance of Zipax of various particles sizes were carried out by Done and b o x [40]. The h versus u plots (see Fig. 5.4) show minima at 2 < u < 10. The corresponding reduced plate height was h x 2.0 for an unretained solute. For retained solutes, the h values are 1.5-2.0 times larger than those of unretained solutes. Comparative plate height studies on various types of porous silica packings were made by Kirkland [45,46], Majors [47], Laird et ul. [23], Unger et ul. [42] and others. From this comparison, Laird et al. [23] stated that “little if any advantage arises from the use of spherical
--
v
Fig. 5.3. Dependence of reduced plate height, h , on reduced velocity, u, for Spherisorb S5W. Conditions as in Fig. 5.2.
184
particles rather than broken chips”. This is in agreement with recent results obtained by Unger et al. [42]. Well packed silica columns offer a reduced plate height of h % 2 at the minimum of the h versus v curve for unretained solutes. This means that absolute plate height, H, is equal to twice the particle diameter of the packing. For retained solutes, H is about 2-5 times greater than d p . Reduced velocities, v, in the minimum range of 1-10 correspond to a linear velocity, u , between 1 and 10 mm/s, depending on the particle size and the diffusion coefficient of the solute.
0
log v Fig. 5.4. Dependence of h upon Y on Zipax, d p = 39 pm, loaded with 1%of 3,3’-oxydipropionitrile [40] for various solutes. (Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, Inc.)
The analysis of experimental data on the basis of the plate height-velocity dependences provides a helpful means of assessing performance and packing properties, but some questions still remain open. As it is defined, the plate height, H, refers to a certain solute of given capacity factor. Consequently, this concept does not permit the calculation of H values for other types of solutes unless their capacity factors and h versus v dependences are measured experimentally. Further, the sub-division of the plate height-velocity dependence into three independent and additive contributions (see eqn. 5.21) for solutes with varying capacity factors is not yet well understood. For instance, usually the h uersus v plot of an unretained solute is taken to indicate the order of packing effects on plate height. On the other hand, one can argue that an unretained solute with k’ % 0 not only passes through the interstitial voids but also penetrates the pore space as a totally permeating solute in terms of SEC. Therefore, H not only reflects the quality of the packing but is also affected by the pore structure of the packing, Lastly, in making high-efficiency columns the contributions of extra-column effects to the total plate height become significant. At present there is no experimental method available that is able to distinguish clearly between these two contributions. 5.4.3 Column stability
A final comment should be made about column stability which is a very important feature in practical work. It is desirable that the good column performance is maintained over a long period of time. Long-term stabilities of more than a few months have
185
been reported in its use in industrial laboratories [48]. Long-term stability, however, does not depend only on a stable column packing but is also influenced by the purity of the eluent, the sample mixture to be introduced, etc.
5.5 REFERENCES 1 L.R. Snyder and J.J. Kirkland (Editors), Introduction ro Modern Liquid Chromatography, Wiley, New York, 1974. 2 J.F.K. Huber, Chimin, Suppl. (1970) 24. 3 S.T. Sie and N. van den Hoed, J. Chromatogr. Sci., 7 (1969) 257. 4 R.E. Majors,Anal. Chem., 40 (1972) 1722. 5 W. Strubert, Chromatographia, 6 (1973) 50. 6 C.D. Scott and N.E. Lee, J. Chromatogr., 42 (1969) 263. 7 P.A. Bristow and J.H. b o x , Chromatogmphia, 10 (1976) 279. 8 R. Endele, 1. Halgsz and K. Unger, J. Chromatogr., 99 (1974) 377. 9 H.M. Sutton, in G.D. Parfitt and K.S.W. Sing (Editors), Characterizarion of Powder Surfaces, Academic Press, New York, 1976, p- 107. 10 LJ. Smalley, Powder Technol., 4 (1970) 97. 11 0. Krischer, Die wissenschaftlichen Grundlagen der Trocknungstechnik, Springer Verlag, Gottingen, 1963, pp. 211-213. 12 H. Rumpf and S . Debbas, Chem. Eng. Sci., 21 (1966) 583. 13 P.A. Bristow, J. Chromatogr., 149 (1978) 13. 14 RK. Iler, in E. Matijkvic' (Editor), Surface and Colloid Science, Vol. 6, Wiley-Interscience, New York, 1973, pp. 1-100. 15 S.A. Greenberg, R. Jaruwtowski and T.N. Chang, J. ColZoid Sci., 20 (1965) 20. 16 P.N. Mishra, D.E. Severson and T.C. Owens, Chem. Eng. Sci., 25 (1970) 653. 17 P.A. Bristow, P.N. Brittain, C.M.Riley and B.F. Williamson, J. Chromatogr., 131 (1977) 57. 18 J.C. Kraak, H. Poppe and F. Smedes, J. Chromatogr., 122 (1976) 147. 19 W. Messer, Thesis, Technische Hochschule, Darmstadt, 1977. 20 H. Hauck, Thesis, Technische Hochschule, Darmstadt, 1976. 21 R. Kern, Thesis, Technische Hochschule, Darmstadt, 1976. 22 P.A. Bristow, Liquid Chromatography in Practice, hetp, Handforth, Cheshire, 1976. 23 G.R. Laird, J. Jurand and J.H. Knox, Proc. SOC.Anal. Chem., 11 (1974) 31 1. 24 F. Eisenbeiss, Research Department, E. Merck, Darmstadt, personal communication. 25 R.P.W. Scott and P. Kucera,J. Chromatogr., 125 (1976) 251. 26 G.B. Cox, C.R. Loscombe, M.J. Slucutt, K. Sudgen and J.A. Upfield, J. Chromatogr., 117 (1976) 269. 27 T.J.N. Webber and E.H. McKerrel, J. Chromatogr., 122 (1976) 243. 28 R.M. Cassidy, D.S. Le Gay and R.W. Frei, Anal. Chem., 46 (1974) 340. 29 J.J. Kirkland, J. Chromatogr. Sci., 10 (1972) 593. 30 3. Asshauer and I. Halisz, J. c)"rromatogr.Sci., 12 (1974) 340. 31 J.J. Kirkland, Chromatographia, 8 (1975) 661. 32 E. Godbille and P. Devaux, J. Chromatogr. Sci., 12 (1974) 564. 33 E. Godbille and P. Devaux, J. Chromarogr. Sci., 122 (1976) 317. 34 H.R. Linder, H.P. Keller and R.W. Frei, J. Chromatogr. Sci., 14 (1976) 234. 35 E. Grushka, L.R. Snyder and J.H. Knox, J. Chromatogr, Sci., 13 (1975) 25. 36 J.H. Knox, Annu. Rev. Phys. Chem., 24 (1973) 29. 37 G. Deininger, Ber. Bunsenges. Phys. Chem., 77 (1973) 145. 38 J. Schick-Kalb, 7'hesis, Technische Hochschule, Darmstadt, 1974. 39 J.J. Kirkland, J. Chromatogr. Sci., 10 (1972) 129. 40 J.N. Done and J.H. Knox, J. Chromatogr. Sci., 10 (1972) 606. 41 J.H. Knox and A. Pryde, J. Chromatogr. Sci., 112 (1975) 171.
186 42 K.K. Unger, W. Messer and K.F. Krebs, J. Chromatogr., 149 (1978) 1. 43 J.H. Knox, Chem. Ind. (London), (1975) 29. 44 J.C. Giddings (Editor), Dynamics of Chromatography,Part I, Principles and meory, Marcel Dekker, New York, 1965. 45 J.J. Kirkland, J. Chromatogr. Sci., 10 (1972) 593. 46 J.J. Kirkland, J. Chromatogr., 83 (1973) 149. 47 R.E. Majors, J. Chromatogr. Sci., 11 (1972) 88. 48 A. Wehrli, Sandoz, Basle, personal communication.
187
Chapter 6
S i k a and its chemicaIIy bonded derivatives as adsorbents in Iiquid-solid chromatography 6.1 INTRODUCTION Adsorption chromatography, introduced by Tswett [ 11 and Day [2] in 1903, was the first chromatographic separation technique. In its classical form, liquid-solid chromatography (LSC) uses wide-bore columns packed with large particles and separation is achieved by selective adsorption of the solutes on the surface sites of the adsorbent. Silica and alumina are the most commonly used packings. The development of microparticulate packings and appropriate column technology in modern LSC led to a considerable improvement in the efficiency and speed of separation. Special techniques, such as gradient elution and column coupling, have further expanded the applicability of this method. The introduction of chemically bonded silica packings opened a new dimension in LSC with respect to selectivity and convenience. Although the surface composition of these modified packings differs from that of parent silica and some specific solute-solventadsorbent interactions arise, the retention can be basically understood as adsorption. Most chemically bonded packings offer a high specific surface area @BET > 100 m2/g) and hence possess a large solid-liquid interface accessible to solute-solvent interactions. Moreover, the bonded layer, which is mostly of the monolayer type, is 1-2 nm thick and hence deviates considerably in its properties from a bulk liquid of the same composition. While the basic retention mechanism on silica can be understood in terms of a competitive adsorption process [3], the retention behaviour of solutes on chemically bonded silica packings, including reversed-phase materials, is not yet completely clear in detail. Irrespective of all discussions, it is important to realise that nowadays most problems in HPLC can be solved on reversed-phase packings. Other chemically bonded packings are of less importance, i.e., their selectivity is restricted to special groups of compounds. State-of-the-art reviews on adsorption chromatography, including reversed-phase chromatography, were given by Saunders [4], Engelhardt [ 5 ] , Cox [6] and Horvath and Melander [ 7 ] . The main objective of this chapter is to discuss the basic relationships between the support properties and the retention characteristics.
6.2 SILICA AS A POLAR PACKING IN LSC 6.2.1 Retention mechanism When a packed silica column is conditioned with an eluent, solvent molecules, S, are adsorbed on the surface forming a monolayer. After introducing the sample into the
188
eluent, the solute band migrates through the column bed and solute molecules, X , diffuse into the pores of silica. When they approach the adsorbed layer competitive adsorption by attractive forces takes place between X and S and the active surface sites:
x t Sad * Xad t s
(6.1)
where the subscript ad refers to the adsorbed species. The adsorption-desorption process is assumed to be reversible and its kinetics to be SO fast that thermodynamic equilibrium is readily attained. The adsorption of the solute X is then determined by (i) the relative volumes of the adsorbed and non-adsorbed phases; and (ii) the net adsorption energy of X . To a first approximation, the volume of the adsorbed phase is the product of the specific surface area, SBET,of the silica and the thickness, t, of the adsorbed monolayer:
v, = SBET
(6.2)
The adsorption of X originates from attractive forces such as London-type dispersion forces, induction forces and charge-transfer interactions (see p. 76). The last two effects, termed specific interactions, are most important with respect to selective adsorption and retention. The amount adsorbed, expressed in molar fraction of X in the adsorbed phase, NX,ad, at constant temperature and pressure is a function of the amount of solute in the nonadsorbed phase, expressed in molar fraction of X, N x . In most liquid-solid adsorption processes, the course of the sorption isotherm resembles that described by Langmuir. In the linear part of the isotherm the total moles of solute in the adsorbed (nx,ad) and non-adsorbed (nx) phases will be small in comparison with the total moles of solvent in the adsorbed (ns,ad) and non-adsorbed (ns) phases. The molar fractions of solute X are then given by nX
Nx =-
nS
and nX,ad NX,ad =%,ad
(6-4)
The thermodynamic equilibrium constant, K f i , can be written as Kth =-
nXadnS n X nS,ad
When one uses molar concentrations instead of molar fractions, a distribution coefficient, KO, can be defined as
189
where CX,ad and cx are the molar concentrations of the solute in both phases. Ku, and KO are related by the equation
where V, is the volume of an adsorbed solvent monolayer per unit weight of adsorbent [ 8 ] . Assuming that the net energy of adsorption is greater than the energy of solution, Snyder [3] postulated an equation that relates KO to the properties of the adsorbent, the solute and the solvent:
where V, is the surface volume as given in eqn. 6.7, (Y the relative activity of the adsorbent referred t o a standard adsorbent with a: = 1.OO,So the relative adsorption energy of the solute interacting with the surface of the standard adsorbent, A s the molecular surface area required for an adsorbed solute molecule and eo the relative adsorption energy of the eluent per unit surface area of the standard adsorbent. On replacing KO with the capacity factor, k', one obtains
log k' = log
(y)
+ a(SO- A s e O )
where W is the mass of adsorbent in the column and V o the dead volum_e of the column. According to eqn. 6.9, the retention of a solute is a function of the properties of the adsorbent (V, and a:), the structure and composition of the solute (So and As) and the type of solvent (E'). The quantities So and A S can be calculated as shown in detail by Snyder [3]. e0 represents the effect of solvent type on retention for a given adsorbent and solute and is therefore termed the solvent strength. Values are tabulated for single solvents as a so-called eluotropic series in Table 6.1. The solvent strength can also be estimated for binary solvent mixtures. As indicated in Fig. 6.1, eo is not a linear function of the composition of the binary mixture. Small amounts of polar solvents in less polar solvents considerably enhance E O . Further, it can be seen that a nominal solvent strength can be achieved by different solvent mixtures (see dashed line in Fig. 6.1 at e0 = 0.30). In order to cover the whole range of polarity, several solvents must be used. The aspects in choosing the right mixture for LSC and related topics were discussed in detail by Saunders [4]. The parameters of the adsorbent that control retention are the surface volume of the adsorbed solvent, V, and the relative activity, ci, both termed activity parameters. V, represents a fundamental property of the adsorbent and is independent of the type of stationary phase. For a silica that is completely free of physisorbed water, V, attains a maximum value of V,m=
= 0.00035 SBET
(6.10)
The factor 0.00035 corresponds to the average thickness of a monolayer of adsorbed solvent molecules. V, can be decreased by adding small amounts of polar compounds such as water and alcohols to the non-polar eluent because they are preferentially adsorbed on the surface and replace the adsorbed solvent molecules. Assuming that a volume
190 TABLE 6.1 ELUOTROPIC SERIES OF SOLVENTS ON SILICA [ 3,4] Solvent
Solvent strength c 0
Solvent
n-Pentane 2,2,4-Trimethyl pentane Tetrachloromethane 2-Chloropropane Benzene Trichloromethane Dichloromethane Tetrahydrofuran Diethyl ether
0.00 0.01 0.11 0.22 0.25 0.26 0.32 0.35 0.38
Ethyl acetate Acetone Dioxane Acetonitrile 2-Propanol Methanol Water Acetic acid
I
Solvent strength,
I
E
0.38 0.47 0.49
0.50 0.63 0.73 >O. 7 3 >0.73
I
I
I
'1. MeOH in I P r C l
Fig. 6.1. Solvent strength of mixed solvents o n silica [4]. Hx = n-hexane; IhCl = isopropyl chloride (2-chloropropane); MC = methylene chloride (dichloromethane); Et,O = diethyl ether; ACN = acetonitrile; MeOH = methanol. (Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, Inc.)
191
of water replaces an equal volume of adsorbed solvent, the following relationship is obtained:
V, = 0.00035S~~~ - (ml water added per gram of silica)
(6.1 1)
The linear relationship between V, and the percentage of water surface coverage has been established in various ways [9]. The second activity parameter, the dimensionless quantity a, is a measure of the surface activity, which depends strongly on the types of hydroxyl groups at the silica surface and on their reactivity, which is also considerably affected by the specific surface area and the pore structure. The relative activity reaches its maximum value only with a completely hydroxylated surface, whereas it is considerably decreased by the adsorption of water, which occurs preferentially at reactive surface hydroxyl groups. For a given silica at a constant water content, a can be derived from the slope of the plot of k' versus So - A s e' for a series of compounds [ 10,111. As an example, for silica with SBET = 400 m2/g the values of log V, and (Y vary with the amount of adsorbed water in the following way [ 121:
Wateradsorbed Log V, (%, wjw,
a
2.0 2.7 3.8 6.7 11.8
0.72 0.70 0.69 0.69 0.69
-0.92 -0.95 -0.99 - 1.14 -1.66
Eqn. 6.8 was derived under the assumption that the interactions of solute and solvent molecules with the solvent bulk phase are so small that they can be neglected in the adsorption equilibrium. However, with increasing polarity of the solvent, solute-solvent interactions become noticeable. Polar solvents are also capable of displacing adsorbed water from the silica surface. In these instances, the activity parameters a and V, lose their original meaning and eqn. 6.8 no longer describes accurately the retention behaviour of solutes. Moreover, deviations from the predicted dependences are to be expected if the adsorption interaction between the solute molecule and the surface is influenced by several different surface sites instead of a single site. Surface geometry factors may also become important in determining the relative orientation of the adsorbed molecules. Another aspect that has to be considered relates to the pore structure of the adsorbent. In some instances microporous silicas were employed, the fine pores of which may cause partial exclusion of large solvent molecules, whereas water, owing to its small size, is preferentially adsorbed. In comparison with larger pores, such small pores are preferentially filled with water on deactivation. This may also lead to a false estimate of KO. Eqn. 6.8 can be corrected for these so-called secondary adsorbent activity effects by an additive term, Aeas, on the right-hand side. Nevertheless, as demonstrated in numerous papers (see ref. 13), eqn. 6.8 predicts very well the retention behaviour of solutes on silicas in solvents of low and medium strength.
192
Scott and Kucera [14,15] developed a dynamic equation for the distribution coefficient: K , of a solute in LSC that is based on the various kinds of solute-adsorbent interactions. K is defined by
K=
forces between solute and stationary phase X probability of interaction forces between solute and mobile phase X probability of interaction
(6.12)
The forces between the interacting species and phases are dependent on the temperature, the nature of the solute and the nature of the mobile and stationary phases. The probability of interaction will be a function of the concentration of the interacting species. If ionic forces are absent and only dispersion and polar forces are operative, the effective distribution coefficient becomes (6.13) where FL and FA are the polar and dispersion forces between the solute and the stationary phase, F p and Fd the respective forces between the solute and the mobile phase and PL, PA,Pp and Pd are the probabilities of the solute molecule interacting with polar and dispersive moieties of both phases. The probability of interaction of a solute is assumed to be a function of the absolute temperature and to be proportional to the concentration of the interacting moieties. Eqn. 6.13 then gives (6.14) where T is the absolute temperature and CL, C; and Cp, c d are the concentrations of polar and dispersive moieties in both phases. The concentration c d can be substituted according to cd =Ap
(6.15)
where A is a constant and p is the density of the dispersing medium. Now, the distribution coefficient K can be replaced: (6.16) where V' is the corrected retention volume and & the volume of the stationary phase Vs). Inserting eqn. 6.16 into eqn. 6.14 and resolving to lfV', one obtains
(E
(6.17) The term FAf2(OCi can be neglected when a polar or semi-polar mobile phase is used and the dispersive factor for the stationary phase can be eliminated. For practical convenience, eqn. 6.17 can be simplified in the following way: (i) when using different concentrations of more than 3% (wlv) of a polar or semi-polar
193
solvent in a mobile phase such as n-heptane the surface activity of silica is assumed to be constant and eqn. 6.17 gives 1 -=A V'
t BCp
(6.18)
where B is a constant and Cp the concentration of the polar moderator; (ii) when employing a constant concentration, Cp,of the polar moderator in a series of dispersion solvents and again maintaining Cp > 3% (wlv), the polar interactions in the mobile phase will be constant and eqn. 6.17 gives 1 -=AtBp V'
(6.19)
where p is the density of the dispersing solvent. The validity of eqns. 6.18 and 6.19 was verified experimentally for a large number of polar moderators and dispersion solvents. Fig. 6.2 is a graph of l/V' versus the concentration of various polar moderators in n-heptane for phenyl methyl carbinol as solute.
b'"""
lld TETRAHYDROFURAN
0 4
A
10
MOBILE PHASE C o Y m T m
15 XIVr OF POLAR
20 SOLVENT IN n-HEPTANE
Fig. 6.2. Graphs relating the reciprocal of the corrected retention volume of phenyl methyl carbinol to the composition of the mobile phase containing different polar solvents in n-heptane [ 141. Column, 25 X 4.6 mm I.D., packed with Partisil 10.
6.2.2 Characteristics of silica adsorbents in LSC
The most decisive parameters of silica that control retention and selectivity in LSC are the specific surface area and the surface activity. The pore structure properties, such as the average pore diameter and the specific pore volume, are linked with the specific surface area as previously discussed. For a given silica chosen as adsorbent the specific
194
surface area and the surface activity can be gradually diminished in two different ways: (i) by thermal treatment at T > 473 K which results, through dehydroxylation, in a ) at T > 873 K, which results, through sintering, in a decrease in decrease of c ~ o H ( ~and SBET ; (ii) by addition of increasing amounts of water to the adsorbent which will be adsorbed and will lower the surface activity and the effective specific surface area. It is not possible to state that only one silica of standardized specific surface area and surface activity is best suited for separations in LSC. As the requirements for both quantities depend on the structure and composition of the samples to be separated, the trend in LSC is t o use a series of silica packings with a wide range Of SBET values. The commercial products and their properties are surveyed in Appendix A. A comprehensive survey of packings was also recently published by Majors [ 161.
6.2.2.1 Specific surface area The general objective is to have a sufficiently high specific surface area available for solute-surface interactions which is accessible to the solutes, Le., relatively large pores and a high particle porosity should be present. As we know, however, the higher is SBET the smaller are the average pore diameter and the porosity of the packings. Hence, a fairly good compromise between a high specific surface area and the average pore diameter is achieved by employing mesoporous silicas with SBETin the range 200-400 m’/g, which corresponds t o an average pore diameter of 6-10 nm. Inspection of the data in Appendix A shows that most of the commercial silicas fall in this range. A few exceptions exist. Low-surface-area silicas are Vydac TP (SBET= 100 m’lg) and LiChrospher 500, 1000 and 4000 (SBET= 5-50 m’lg). Some high-surface-area products are also marketed, such as Spherosil XO 600,800 and 1000. The last two are microporous adsorbents with an average pore diameter of less than 3 nm. It is worth mentioning that with such microporous silicas the specific surface area loses its physical meaning and can be considered only as an arithmetic quantity. Usually the specific surface area is quoted in square metres per gram, whereas from the chromatographic point of view the specific surface area per column volume is of greater interest. The latter can be estimated by dividing SBETby the bulk density of the silica. The bulk density is commonly determined by measuring the weight of silica when dry packed to a minimal bed volume, for instance in a cylinder. This bulk density, however, particularly with porous silica microparticles, deviates from that obtained in a slurrypacked column. Hence, an easy means of deriving the specific surface area per column volume is t o empty the packed column and to weigh the packing. SBET(m’lml) is then given by (6.20) where W is the weight of packing and V, the column volume. An important aspect of the use of silica in LSC is the batch-to-batch reproducibility. By careful control of the process parameters a reproducibility of +lo%for SBETcan be achieved for mesoporous silicas, whereas the reproducibility is poorer for products with
195
SBET> 500 m2/g. For silicas with SBET< 100 m2/g the reproducibility is better than +lo%. 6.2.2.2 Surface activity Surface activity depends on the types of surface hydroxyl groups and their distribution and reactivity. The maximal surface activity of a given silica is attained in its completely hydroxylated state after heat treatment at 473 K under vacuum. Adsorption of water or other polar compounds, even in trace amounts, lowers the surface activity. Many efforts have been made t o differentiate the surface hydroxyl groups into free, bound, paired, reactive, etc., to estimate their concentrations and to correlate these data with chromatographic measurements, i e . , with the activity parameter, a.Up to now no generally valid and conclusive method has been developed for the quantitative determination of single types of hydroxyl groups at the silica surface. However, one can draw some qualitative conclusions. Large-pore silicas with relatively low surface areas are expected to have a fairly homogeneous distribution of surface hydroxyl groups, because in their manufacture by hydrothermal treatment or sintering the amorphous surface starts to be converted into a crystalline state. Going to mesoporous and in particular to microporous silicas the surface structure becomes increasingly disordered and highly heterogeneous. Consequently, the surface activity will be higher for small- than for large-pore silicas. An additional effect that gives rise to enhanced surface activity is the curvature of the pore walls and their distance from each other. With decreasing pore size the curvature of the surface and the relative strength of active surface sites increase. In micropores the force field of oppositely positioned surface sites will be superimposed and a force cage is formed instead of a force field at a plane surface. As already shown by Snyder [ 171, the activity parameter, a,for small-pore silicas in their activated state is higher than that for large-pore silicas under comparable conditions. Significant differences are also observed in the deactivation behaviour. Deactivation, i.e., a decrease in surface activity, can be accomplished either through dehydroxylation above 473 K or through partial or complete coverage of the surface with polar adsorbates such as water or alcohols. Water is the most commonly studied moderator in LSC. Heat treatment above 473 K causes condensation of surface hydroxyl groups and lowers OH(^). Some representative cyOH(s)-temperature dependences are given in Fig. 3.1. Above 873 K, dehydroxylation is accompanied by sintering, which leads to a reduction in SBET. Snyder [17] has shown that the treatment temperature is correlated with the surface activity: a decreases during heating from 473 to 673 K. The uptake of water on silica is possible by (i) exposing the outgassed sample to water vapour of known pressure, or (ii) immersing the outgassed sample in a water containing solvent. The amount of water adsorbed per gram of silica can be measured gravimetrically or indirectly from the decrease in the water content in the solution at equilibrium. Both methods were applied by El Rassi et af. [ 181 to a series of commercial small-particle silica packings. The results, taken from Figs. 2 and 8 in ref. 18, are illustrated in Figs. 6.3 and 6.4. In Fig. 6.3 the amount of adsorbed water, in number of water molecules per A', is plotted vs. the relative humidity of the vapour phase, whereas in Fig. 6.4 the amount adsorbed in g water per g adsorbent is plotted vs. the water content of the solvent dichloromethane.
196
Fig. 6.3 indicates that all curves follow nearly the same course up to a relative humidity of 50%. Above 50%, a steep increase of the amount adsorbed is observed, which is caused by capillary condensation of water vapour in the pores of the packings. The uptake of water increases until the whole pore volume is filled with water. At about 100%relative
Fig. 6.3. Water adsorption isotherms on commercial silicas from the vapour phase at 293 K [ 181. (1) Spherosil XOA 1000, SBET= 1096 mz/g; (2) Spherosil XOA 600, SBET= 600 m*/g; (3) Spherosil XOA 400, SBET= 465 m'/g; (4) Spherosil XOA 200, SBET= 170 mz/g; ( 5 ) LiChrosorb Si 60, S B= 500 ~ m'/g; (6) Partisil20, SBET= 400 m'/g.
197
0
xor
1000
X O A 600 .a20
A
XOA
* *
LICHROSORB SI 60
400
A x O A 200
-0.19
.O>l
PARTlSlL
5
LICHROSORB SI 100
.
0.15
.a73 .a11
.0.08
.M7
ppm
200
400
600
800
1000
1200
.,
1400
n20 tn
1600
mobile Phase
la00
2000
Fig. 6.4. Water adsorption isotherms on commercial silicas from solution (solvent dichloromethane) [ 181. 0,Spherosil XOA 1000, SBET= 1096 mZ/g; Spherosil XOA 600, SBET= 600 rn2/g; A, Spherosil XOA 400, SBET= 465 m2/g;A , Spherosil XOA 200, SBET= 170 rn2/g;*, LiChrosorb Si 60, SBET= 500 rn2/g;0 , LiChrosorb Si 100, SBET= 400 rn2/g;*, Partisil 5, SBET= 400 rn'/g.
humidity, condensation of the vapour additionally occurs within the interstitial voids of the small particles. In contrast, the absolute amount of adsorbed water taken up from the water-dichloromethane solution is considerably less than that from the vapour phase but the conditions are also different. Fig. 6.4 reveals that the uptake of water increases approximately linearly with its concentration in the solution. Moreover, the difference in the absolute amount of water adsorbed between the various packings seems not to be significant.
6.2.2.3 Pore structure As indicated before, the average pore diameter of silicas spans several orders of magnitude. Microporous silicas such as Spherosil XO 800 and 1000 are most active owing to their high specific surface area and to the presence of a large number of fine pores. At low loadings with water a drastic decrease in surface activity compared with mesoporous silicas is observed with increasing deactivation of Spherosil XOA 1000 [ 181. At 40%relative humidity (see Fig. 6.3) or at 2000 ppm of water in dichloromethane (see Fig. 6.4) almost half of the total pore volume of Spherosil XO 1000 is filled with liquid water. At this stage it is no longer justified to consider the interactions as pure adsorption phenomena. In addition to the high surface activity of silicas containing micropores and small mesopores, special effects in retention can be observed that are caused by steric hindrance
198
during adsorption owing to the size and shape of the sample molecules. These effects often permit a selective separation of isomers. Most of the packings are mesoporous, exhibiting an average pore diameter between 6 and 10 nm and a specific pore volume of about 1.O ml/g. Adsorption data for water from solution on mesoporous silicas indicate that, up to 2000 ppm of water in dichloromethane, the amount adsorbed corresponds to a multilayer and concentrations considerably higher than 2000 ppm are needed in order to fill the major part of the pore volume. 6.2.3 Support properties controlling retention 6.2.3. I Specific surface area The optimal capacity factor range of k’ = 1-10 for solutes to be separated is commonly attained by adjusting the solvent strength of the eluent. Another potential means of controlling retention that is often not fully appreciated is to choose a series of silica packings with graduated specific surface areas at constant eluent composition. According to the equation (6.21)
where VJVm is the phase ratio and ‘Ki0 the distribution coefficient of solute i at infinite dilution, k/ depends linearly on the volume of the stationary phase, V,, and hence on the specific surface area, SBET(see also eqn. 6.1). For this reason, a variation in SBETat a given eluent composition enables the capacity factor of a solute to be roughly changed by about two orders of magnitude. The above relationship is strictly valid only at a constant average pore diameter, D, of the silica or, in other words, it is assumed that ‘Ki0 remains unaffected by D when SBETis altered. There is one procedure of adsorbent preparation known by which the requirement of constant D and variable SBETcould be fulfilled: batches of glass beads of d p = 3 1.5 pm are coated with a porous silica layer of increasing thickness so that the SBETof the resulting porous layer beads is in the range 5-35 m2/g. As all parameters except the mass of deposited silica are kept constant, all batches exhibit an average pore diameter of 6 nm (nitrogen desorption measurements). Fig. 6.5 shows the graph of k’versus the specific surface area of this pellicular type of packing obtained at constant eluent composition, column dimensions, temperature, etc. [ 191. According to eqn. 6.21, a straight line results for every solute, which can be extrapolated to the origin. In a recent study on a series of mesoporous and macroporous silica adsorbents, Huber and Eisenbeiss [20] observed a linear correlation between k’ and SBETat constant eluent composition. This result indicates that the dependence of k’ on SBETobeys eqn. 6.21 for meso- and macroporous silicas even with a variable average pore diameter.
6.2.3.2 Degree of surface deactivation In the previous section, silica adsorbents with different specific surface areas were compared, assuming a constant level of surface deactivation. Here we consider a given silica
199
Fig. 6.5. Dependence of capacity factor on specific surface area on pellicular-type materials of varying SBETand constant average pore diameter [ 191. Columns: length 500 mm, I.D. 4.0 mm. Packings: porous layer beads (homemade preparation);dp = 31.5 ctm;d, = 0.1-1.0 ctm; SBET= 4-34 m’/g; D = 6 nm. Eluent: n-heptane (30%relative water content). Detector: UV (254 nm). Samples: ( 0 ) mterphenyl; (A) m-quinquephenyl.
adsorbent and vary its surface activity continuously from the highest possible activated state to increasing deactivation. Several activation procedures can be applied: (i) heating in an oven at 473 K in air or under vacuum for about 8 h; (ii) storage over desiccants such as phosphorus pentoxide or concentrated sulphuric acid; this procedure requires longer times to achieve the activated state than the previous method; (iii) “in-column activation” by washing the column with polar solvents such as diethyl ether or dichloromethane, followed by a dry non-polar solvent such as n-hexane or nheptane. Deactivation is accomplished through: (i) heating the packing in an oven above 473 K in air or under vacuum to achieve partial dehydroxylation of the surface. One particular case is known [21] in which the whole packed column was heat treated in an oven while flushing dry nitrogen through the packing. After cooling, the column was connected to the chromatograph, conditioned with the eluent and measurements were performed. By increasing the pre-treatment temperature from 493 to 978 K a steady increase in capacity factors was observed at constant
200
eluent composition. This result is unexpected and conflicts with all data reported in the literature ; (ii) equilibrating the silica with the water vapour phase over sulphuric acid or salt solutions of fixed concentration; (iii) flushing a non-polar eluent of controlled water content through the packed column until equilibrium is achieved. Extended thermal deactivation studies on a series of commercial silicas were made by El Rassi et al. [ 181. For aromatic hydrocarbons as solutes and dry 2,2,Ctrimethylpentane as eluent the capacity factors were observed to decrease continuously with increase in the pre-treatment temperature above 573 K. Between 573 and 873 K the decline in k‘ is caused mainly by the successive reduction in the surface hydroxyl concentration as a result of dehydroxylation. Above 873 K, the dominant effect on k’ is the decrease in SBETthrough sintering. When using thermally deactivated silicas and water-modified eluents, rehydroxylation of the surface during conditioning of the column takes place to a certain extent. The degree of rehydroxylation is a function of the pre-treatment temperature, If, for example, LiChrosorb Si 60 is heated to 623 K complete rehydroxylation takes place on running the column with the moist eluent for a sufficient time [ 181. Above 673 K, the water uptake from the moist eluent falls drastically, indicating partial dehydroxylation which is irreversible. In practice, separate equilibration of silica by storing it in a controlled water atmosphere is less convenient than the “in-column’’ deactivation as, after the equilibration procedure, the adsorbent has to be packed into the column by the slurry method. The slurry liquids used should be free of water because otherwise the water content, Le., the degree of deactivation, changes. Traditionally, deactivation is performed with water but other polar liquids such as ethylene glycol, glycerine, aliphatic alcohols, tetrahydrofuran, acetonitrile, esters and dichloromethane are also often employed [5]. Most of the solvents used as eluents in LSC, however, a priori contain trace amounts of water in a concentration of about 50-2000 ppm. Such small water contents are detected by means of either gas chromatography or Karl Fischer titration. The detection limit of the Karl Fischer titration is about 5-10 ppm of water and requires large sample volumes [5]. For comparison, the maximal solubility of water at ambient temperature is reported to be 100 ppm for nheptane and 2000 ppm for dichloromethane [ 5 ] . In order to obtain reproducible retention data when employing silica adsorbents, the water content of the eluent has to be adjusted to a certain level and has to be maintained constant during the chromatographic measurements, i.e., column operation demands the total exclusion of moisture from the atmosphere. In routine work, the determination of the water content by Karl Fischer titration is a very tedious and time-consuming procedure. A more practical method consists in blending a water-free and a water-saturated non-polar solvent in the desired proportions. Solvents are preferably dried by storing them over activated molecular sieve 3A or 4A, which adsorb water very rapidly. Other adsorbents, such as porous silica and alumina, have also been recommended [4,5]. Water saturation of solvents is accomplished by shaking the solvent with an excess of water at ambient temperature. The composition of such blended solvents is commonly described by the term relative water content. A 0%(v/v) relative water content refers to the dry state and a 100%relative water content corresponds to the water-saturated solvent. In working with
201
silica columns it is usual in LSC to maintain an intermediate relative water content between 20 and 5%. Various techniques are recommended for holding the water content constant during chromatography. A common procedure is to insert between the eluent reservoir and the injection system a large-bore column packed with coarse silica particles of high capacity. A special device, called the moisture control system (MCS), which is inserted before the pump, was recently introduced by Engelhardt and Boehme [22]. It consists of a thermostated funnel filled with coarse-grained silica which is loaded with a known amount of water. The eluent is introduced through this funnel and recycled. Depending on the water content of the silica in the MCS system, the eluent is dried or moistened to a desired equilibrium water content. The effect of the water content of the eluent on the capacity factor of the solute is exemplified in Fig. 6.6 for a series of commercial silica packings [ 181. As expected, the capacity factor falls continuously with increasing water content, Le., increasing deactivation of the silica. At low water levels a sharp decrease in the dependence of k' on water content is observed. This decrease is greater for silicas of high than of low surface area. In the intermediate range of water contents, k' decreases only slightly because the most active surface sites of silica are already blocked at low water concentration. A further increase in the water level has only a minor effect on deactivation. Above 1500 ppm, the adsorbed water fills a considerable proportion of the pore volume and starts to act as a partitioning medium. Inspection of the family of curves in Fig. 6.6 confirms that at a constant water
Fig. 6.6. Dependence of capacity factor on water content of eluent [ 181 for various silicas. Columns: (a) length 500 mrn, LD. 4.6 mm; (b) length 100 mm, LD. 4.6 mm. Packing: 0, Spherosil XOA 1000, d p < 4 0 pm (a); =, Spherosil XOA 600, d p < 40 pm (a); *, Spherosil XOA 400, dp < 40 pm (a); A , Spherosil XOA 200, d p < 40 pm (a); 0 , LiChrosorb Si 60, dp.= 5 pm (b); *, Partisil5, dp = 5 pm (b); *, LiChrosorb Si 60, dp = 5 pm (b). Eluent: dichloromethane w t h varying water content (200-2000 PPd.
202
level the capacity factor is proportional to the specific surface area of the sample. Dependences similar to those shown in Fig. 6.6 were obtained by using polar moderators other than water [23]. The addition of small amounts of polar moderators to non-polar or moderately polar eluents not only leads to deactivation and to a decrease in k’ but also gives rise to some other favourable effects, as discussed by Saunders [4] : (i) reduction of peak tailing by increasing the linear sample capacity; (ii) enhanced column efficiency; and (iii) diminished retention drift. The concentration range of moderators used in LSC is shown in Fig. 6.7. As suggested by Engelhardt [ 5 ] , the term moderator should be restricted to a concentration lower than 1% because at higher concentrations solvent mixtures are present. ___-_ - --- - --Water in n-Heptane
- - - - - - - - - - - Water in Dichloromcthane
4
Ethanol in n-Heptone
(Lcetonitrile in HcKOnC
I
4
popropanol in Oichloromethane Methanol Oichloromethanc in H e m e
I
Iroorooanol in n-Heotane Ethylacetate in n-Hcplanc, ?IF in n-Hepfane Solvent-Miiturer --.
Gradient-Elution
I
1 ppm
10 PPm
100 ppm
lOOOppm O.l*/.
I
1-1.
107.
100%
Moderator Concentration
Fig. 6.7. Concentration range of moderators used in LSC [S].(Reproduced from the Journal of ChromatographicScience by permission of Preston Publications, Inc.)
6.2.3.3 Sample load and linear capacity In the so-called linear elution adsorption chromatography, separations are carried out at low sample concentrations for which the adsorption isotherm becomes linear and can be described by a distribution coefficient, ‘Kjo, which is independent of the sample size. At higher sample concentrations the shape of the isotherm deviates from linearity and shows either a downward or an upward inflection. In using polar packings in LSC, one mostly observes that the adsorption isotherm is of the Langmuir type. This means that at higher sample concentrations the slope of the curve decreases and ‘Ki becomes smaller than ‘Kio. The linear capacity, of an adsorbent is defined as the sample size in grams per gram of adsorbent which causes a 10%relative change in the value of ‘Kio on the linear part of the isotherm. As ‘Kjo is proportional to the capacity factor, 80.1 corresponds to a 10%change in the k‘ value obtained at low sample concentration (Fig. 6.8). The linear sample capacity of silica in LSC is reported to be about 10-3-104 g/g [17]. Bo.l depends on the structure of the sample, the composition of the solvent, the temperature and the adsorbent properties. The various influences that affect the linearity of
203
-
sample size
g/g1
Fig. 6.8. Definition of linear sample capacity.
the isotherm were discussed in detail by Snyder [24]. For a given solute and solvent, f?o.l is primarily dependent on the specific surface area of the silica and its surface activity. In their highest activated state, large-pore silicas show a higher linear capacity than those of mesoporous and microporous samples, because of their more uniform and homogeneous surface [24]. As mentioned before, deactivation of silica with water decreases the surface heterogeneity. The linear capacity with large-pore silicas reaches a maximum at moderate deactivation and then decreases at higher deactivation. At the maximum, most of the heterogeneous surface sites are covered with water, whereas at higher loadings uniform surface sites are successively covered. In contrast, the Bo.l value of narrow-pore silicas at low deactivation exceeds that of large-pore samples because the addition of water deactivates a large number of heterogeneous surface sites. As a general rule, the linear capacity of silica will usually increase by one order of magnitude on going from the activated to the optimally deactivated state. It was shown by Saunders 1251 that deactivation with acetonitrile gives a superior linear sample capacity than that with water.
6.2.4 Adsorbent selectivity of silica in LSC
LSC on silica columns is known to separate non-ionic compounds of varying polarity in the molecular weight range from 200 to 2000 provided that the sample is sufficiently soluble in the organic eluent. According to Karger et al. [26], selectivity can be divided into various types, termed molecular weight or size selectivity, functional group selectivity, shape selectivity, isomer selectivity, optical selectivity and biochemical selectivity. As exemplified in Figs. 6.9-6.1 1 for LiChrosorb Si 60, LSC offers a good potential for functional group and isomer selectivity [27].
204
Selectivity in LSC as measured by the selectivity coefficient, rji, for two components can be discussed as being originated either by the solvent or by the adsorbent. Consequently, solvent selectivity means that for a given silica adsorbent of known surface activity the selectivity coefficients are considered to be a function of the eluent composition at constant solvent strength [28]. Most separation problems in LSC are solved in this way. The adsorbent selectivity refers to the effect of the nature and properties of silicas on the selectivity coefficient of solutes at constant eluent composition and solvent strength. Theoretical and experimental examinations of adsorbent selectivity are carried out to a lesser extent than those on solvent selectivity [29-331. Numerous studies in this context were performed by the Waksmundzki school [34] ,but mainly in the field of thin-layer chromatography. Three parameters seem to influence adsorbent selectivity: (i) the specific surface area; (ii) the mean pore diameter; and (iii) the surface activity. Applying the Snyder equation for two solutes at constant eluent composition, the selectivity coefficient increases with the activity (a)of the silica. With weakly adsorbing compounds, the effect of a on the distribution coefficient (K)and hence on the selectivity coefficient is small, so that K will increase mainly due to the quantity V,. As Va is proportional to SBET, silicas of high surface area are more selective than those of low surface area in this particular case [35]. This was exemplified in a study by Vermont et al. [36] on Spheror The effect of the mean pore diameter of silica on isomer selectivity was recently shown by Eisenbeiss [37] in separating DDT and its degradation products on LiChrosorb Si 40, Si 60 and Si 100 (Fig. 6.12).
0--.--
S
10
1s
20
t[min]
-
Fig. 6.9. Separation of anthraquinones [27]. Column: length 330 mm, LD. 2.7 rnm. Packing: LiChrosorb Si 60, d p = 10 Nm. Eluent: n-heptane-dioxane (89: 11, v/v). Temperature: 328 K. Pressure: 58 bar. Flow-rate: 1.1 ml/min. Detector: W (254 nm). Sequence of elution: (1) anthraquinone; (2) 2-nitroanthraquinone;(3) 1-aminoanthraquinone;(4) 1-nitroanthraquinone;(5) 1,s-diamino(8) 1-nitro-&aminoanthraquinone;(6) 1,8-diaminoanthraquinone;(7) 1,7-dinitroanthraquinone; anthraquinone;(9) 1,s-dinitroanthraquinone;(1 0) 1,8-dinitroanthraquinone.
205 1
:iL. I
I
0
5
10
$5
-
tlrnin]
Fig. 6.10. Separation of aniline derivatives and aromatic alcohols [27]. Column: length 250 mm, I.D. 3.2 mm. Packing: LiChrosorb Si 60, d p = 5 pm. Eluent: isooctane-dichloromethane-isopropanol (900:90:10,v/v). Pressure: 158 bar. Flow-rate: 1.5 ml/min. Detector: W (254 nm). Sequence of elution: (1) 2,6-diethylaniline; (2) 2-methyl-6-ethylaniline; (3) o-isopropylaniline; (4) o-ethylaniline; (5) 2-phenyl-2-propanol; (6) armethylbenzyl alcohol; (7) benzyl alcohol; (8) cinnamyl alcohol. 3
I
OH
c-0
0
Fig. 6.11. Separation of carbamates [27]. Column: length 300 + 200 mm, I.D. 3 mm. Packing: LiChrosorb Si 60. Eluent: dichloromethane-ethanol-water (960: 24 :16, v/v). Pressure: 260 bar. Flow-rate: 1.8 ml/min. Detector: UV (250 nm). Sequence of elution: (1) promacyl; (2) promecarb; (3) 3-isopropyl-5-methylphenol.
206 3
3
1
1
Si 60
Si 40
Si 100
i L 0
30
60
d
90
0
30
60
40
0
30
60
f
[sec]
Pi. 6.12. Separation of isomers on LiChrosorb Si 40,60 and 100 d e r identical conditions [ 371. Column: length 200 mm, LD. 3 mm. Packings: LiChromb Si 40,60 and 100. Eluent: n-heptane, adjusted to a constant relative water content. Flow-rate: 2 ml/min. Samples: 1 = p,p'-DDE;2 = 4,4'-DDM; 3 = OJJ'-DDT;4 =p,p'-DDT.
6.3 REVERSED-PHASE SILICA PACKINGS IN LSC 6.3.1 Introduction
Among chemically bonded silicas, reversed-phase (RP) gadtings have gained the greatest importance in HPLC and even exceed that of untreated silica. RP packings exhibit a hydrophobic surface as they possess n-alkyl group; bonded via a Si-0-Si linkage to the silica matrix. The most popular n-alkyl groups arm-octyl and n-octadecyl but short-chain groups such as methyl and n-butyl are also utilized to some extent. The corresponding
207
eluents in reversed-phase chromatography ( R K ) consist of water mixed with a polar water-miscible organic solvent or buffered solutions. Owing to the hydrophobic nature of the surface and the polar eluent, the elution sequence of solutes is the reverse of that in normal-phase chromatography: the solutes are eluted in order of decreasing polarity. The popularity of RPC is due to the fact that solutes of a wide range of polarity, including ionic species, can be separated. Additionally, the chemical nature of the surface can be modified systematically by adding small amounts of organic amines, acids, salts oi ionic detergents to the eluent. These derivative forms of RPC, termed ion suppression, ion-pair partition, soap chromatography, etc., further enhance the versatility and applicability of Rp packings. A practical advantage in operating RP columns is that no precautions are necessary for controlling the surface activity, contrary to the situation with the use of bare silica. However, a serious limitationexists owing to the solubiIity of RP packings when working in buffered eluents of pH > 9. 6.3.2 Specificities in solute-salvent-reversed-phase adsorbent interactions
In contrast to the retention on conventional polar silica packings, which occurs on the basis of competitive adsorption, the retention mechanism in Rpc is still a matter of discussion. In earlier studies [38,39], RPC was considered as a liquid-liquid partitioning process. Partitioning of the solute was assumed to occur between the stationary liquid within the swollen network of the bonded phase and the moving bulk liquid. The retention order was found to be controlled mainly by the solubility of the solute in the given eluent [38,40421. Secondary effects caused by the adsorption of solutes at the non-polar surface sites have also been discussed [41]. The partitioning mechanism may be justified in the case of polymeric-type RP packings which possess a high loading and hence an extended framework, but is less probable with the monomeric RP packings that are now the most comrnonIy used. A simple adsorption as the dominant retention mechanism can also be excluded because the interactions of solutes with a non-polar surface are mainly of the dispersion type and hence are less specific. On the other hand, it is evident that the surface of a RP packing is not purely hydrophobic like the crystalline surface of graphite, which consists of closely packed carbon atoms. The n-alkyl chains, bound at one end to the heterogeneous silica surface, extend to about 2 nm in thickness and form a relatively open surface layer. Irrespective of their structure, the surface sites are accessible through a pore system with narrowings and openings that may additionally affect the degree of interaction. The various arguments for the partition or adsorption mechanism on RP packings have recently been discussed by Colin and Guiochon [43]. Intensive efforts to understand the retention mechanism in RPC on monolayer-type packings were undertaken by Horvath el al. [44,45], who utilized the theory of solvophobic interactions developed by Sinanoglu [46] and Sinanoglu and Abdulnur [47]. The solvophobic interaction between the solute and the surface of RP packings is considered as a reversible association of solute molecules, S, with the so-called hydrocarbonaceous ligand, L, resulting in a complex LS:
S+L*LS
(6.22)
208
Three important parameters govern the strength of association between L and S and hence the retention of solute S: (i) the total hydrocarbonaceous surface area of the solute, TSAs; (ii) the total hydrocarbonaceous surface area of the ligand L (bonded n-alkyl group), mAL; (iii) the surface tension, y, of the eluent. In dissolving a hydrophobic compound in water, the original water structure has to be broken and a cavity is formed for the solute surrounded by water molecules [48]. The energy of cavity formation is proportional to the surface tension of water and to the molecular surface area of the hydrocarbon solute. The process is similar when the hydrocarbonaceous ligand is wetted with water or a polar eluent. The driving force for the dispersion interactions between a hydrophobic compound and the hydrocarbonaceous ligand is the tendency of water to minimize the hydrophobic surface created around the solute and the ligand. The net energy of interaction which finally controls the retention of the solute is largely determined by both the total hydrocarbonaceous surface area of the solute and the total hydrocarbonaceous surface area of the ligand. It was demonstrated experimentally [45,49] that for a given type of RP packing (constant TSAL)the logarithm of the capacity factor is a linear function of TSAs at constant eluent composition and temperature. It could also be established that for a given solute the capacity factor increases linearly with TSAL at constant eluent composition and temperature, as shown in Fig. 6.13. Another important parameter in determining retention is the surface tension of the eluent. The higher is 7 , the more energy is liberated during the association between L and
100
I0
1
0
1
3
2 ----D
4
5
TSA ( nrn*/group )
Fig. 6.13. Plot of k' of selected solutes normalized to the specific surface area, SBET,of the RP packing uersus the TSAL of n-alkyldimethyl groups with different chain lengths [49]. Column: length 25 cm, I.D. 4.2 mm. Eluent: water-methanol (65:35, v/v). Column temperature: 323 K. A, Phenol; 0,bromobenzene; +, nitrobenzene; 0,benzaldehyde; q2-chloroaniline; v ,rn-xylene.
209
S and the stronger will be the retention of S. Hence, the capacity factor of a solute decreases on increasing the amount of polar solvent in water as y diminishes in the same direction. On the other hand, the capacity factor increases on adding an electrolyte to water as the eluent, as the surface tension increases with increasing salt concentration. The effect of the ionic strength and pH of the eluent on retention was studied intensively by Horvath et ~ l [45] . for a variety of compounds. Summarizing the results, one can conclude that the retention increases with the number of hydrophobic groups in the solute molecule, the length of the n-alkyl chain of the RP packing, and the surface tension of the eluent.
6.3.3 Characteristics of reversed-phase silica packings The most relevant quantities that fully describe an RP material are the type of bonded functional groups, their surface concentration, a,and surface coverage, 8,and the effectiveness factor, 77, of the surface reaction. The hydrocarbonaceous ligand may be an alkyl, an aryl or an alkylaryl group. The ligand is anchored through a Sil-0-Si2-C linkage to the silica matrix, where Sil represents a surface silicon atom originally present at the surface and Siz a silicon atom being introduced by the organosilane modifier. It is worth considering the way in which the remaining valencies of Si2 are saturated. The following four possibilities can be distinguished:
I 0
I
-Sil-0-Siz-R
I 0
I
(a)
I 0
I I -Sil -0-Si2-R I I
OH
(b)
I
R'
I
I
R'
I
-Sil -0-Si2-R
-Sil-O-Siz-R
OH
R'
I
I
(c)
I
I
(dl
where R and R ' are organic radicals. Type (a) represents a sort of polymerized layer with lateral siloxane bonds. Type (b) can be obtained by incomplete polymerization whereas structure (c) is formed through the reaction of a bi- or trifunctional modifier and structure (d) is accomplished through reaction with a monofunctional compound. Except for the last case, it is difficult to decide which type of anchor group really forms the bridge between the hydrocarbonaceous ligand (L) and the silica matrix represented by Sil . When the desired reaction is carried out and the surface composition is known, the surface concentration, CYexp., can be evaluated according to eqn. 3.74 from the carbon content and the specific surface area of the parent silica. For a given organosilane and silica the surface concentration reaches a maximum value under optimal conditions that correspond to the closest packing of bonded functional groups. This value should coincide with amax. derived theoretically on the basis of the molecular cross-sectional area, A m , of the modifier molecule (see eqns. 3.71 and 3.73). The ratio Cie,p./ama. indicates the surface coverage, 8, which should not exceed unity. Therefore, the designation of the carbon content alone provides incomplete information on a RP packing. This will be demonstrated by an example. A carbon content of 10%(wlw) can be obtained by moderate conversion of a high-surface-area silica as well as by maximal conversion of a
210
sample with a lower specific surface area. Assuming that both products exhibit the same surface composition, their surface concentrations differ significantly and hence will lead to different retention characteristics of solutes at constant eluent composition and temperature. Far this reason, aMP.is a necessary quantity for comparing RP packings. The values of am=. and 8 provide a rough measure of the degree of monolayer formation. It is much more difficuft to indicate the effectiveness factor, 8, of the surface reaction or, as it is alternatively called, the residual polarity of the RP packing. As it is the ratio of cvOHdreaet) to dOH(total), this quantity provides a measure of the surface hydroxyl concentration before and after the reaction by means of heterogeneous isotopic exchange, which was described in detail in Chapter 3. As this procedure is very tedious, special tests were developed to detect qualitatively the presence of residual hydroxyl groups. A common approach is to use methyl red adsorption [50,51], which is carried out as follows. A small sample of the packing, which has previously been dehydrated, is placed on a slide and a few drops of a solution of methyl red in benzene are added. The untreated silica is coloured red whereas RP packings remain colourless after addition of the indicator. Methyl red is an acid-base indicator which changes colour from red (acid form) to yellow (base form) when the pH of an aqueous solution is altered from pH 4.4 to 6.2. The pK, of the indicator is 4.80 [ 5 2 ] . In the methyl red test, one applies a dilute solution of the indicator in benzene to indicate a solid acid, namely the hydroxyl groups. For this reason, according to the results cited above it is assumed that hydroxyl groups are virtually absent from RP samples. The methyl red method is commonly used in catalyst characterization in order to determine the surface acidity of heterogeneous catalysts by means of titration with amines [52-54]. Careful inspection of the literature on the determination of surface acidity on carriers and catalysts indicates the necessary precautions aad the limitations of this procedure. Firstly, the silica under investigation should be completely in its dehydrated state because adsorbed water acts as a poison and gives false results [55]. Secondly, adsorbed bases or acids should be completely removed from the surface in order to avoid their detection instead of the hydroxyl groups [56]. Thirdly, the applicability of the method is dependent on the particle size of the support and its pore size, i.e., the accessibility of acidic groups, on the concentration of the indicator, etc. [57,58]. For these reasons, the methyl red test in its common form cannot be considered as a representative test for the identification of hydroxyl groups. Another test for indicating the residual polarity of an RP material is to use it as a polar adsorbent with dry n-hexane or n-heptane as the eluent. Polar molecules such as nitrobenzene [51], acetone [59,60], diethyl ether [59], methanol [59,60], amines [60], 2,6-dinitrotoluene [6 13 and azobenzene [62] are recommended to be chromatographed as these species are able to interact with the hydroxyl groups of a RP packing specifically via hydrogen bonding. Provided that the hydroxyl groups are virtually inaccessible or absent, the capacity factor of polar molecules is assumed to be nearly zero. Again, this procedure can be considered only as a crude test because of the problems of maintaining an absolutely dry eluent. A procedure for masking the remaining hydroxyl groups on a RP packing is the reaction of the modified product with short-chain organosilanes such as trimethylchlorosilane or hexamethyldisilazane [63,64]. If some hydroxyl groups are accessible, the carbon content
21 1
increases after this end-capping modification. Again, one can argue that this procedure will be only partially successful because only the accessible hydroxyl groups will react with TMCS and HMDS. Further, one should remember that the molecular cross-sectional area of a TMS group is twice that of a hydroxyl group and therefore, irrespective of the question of the way in which these hydroxyf groups contribute to retention in RPC,this subject needs more investigation. In discussing and comparing RP packings the nature and properties of the starting silicas are sometimes neglected. The specific surface area controls the amount of bonded material in the packing and hence determines the retention of the solute. The mean pore diameter may also play a decisive role, particulady when it is less than 6 nm, and bulky longchain modifiers are employed in modification. Under these conditions, either the pores of the packing are nearly completely filled with the bonded moiety or their entrances can be blocked. Both effects can be monitored by a drastic decrease in the SBETof the modified product compared with that of native silica. The choice of an appropriate silica with an optimal mean pore diameter with regard to the type of modifier was discussed by Eisenbeiss and Krebs [ 6 5 ] ,who recommended a mesoporous silica withD between 10 and 20 nm for long-chain alkylchlorosilanes. In order to achieve a maximal conversion of a given silica, not only suitable conditions should be chosen but also the starting sample should be completely hydroxylated, i.e., it should exhibit a hydroxyl concentration Of a0H = 8-9 pmol&n2, obtained by previous heat treatment at 473 K under vacuum. Temperatures above 600 K cause a pronounced dehydroxylation of the surface and hence yield a lower surface concentration of bonded groups than in the case of complete hydroxylation. When a monolayer type of packing is required, it is further important to carry out the reaction under totally anhydrous conditions (anhydrous in this sense means that the starting silica is free of physisorbed water and a dry reactant and solvent are employed). Although this subject is discussed in Chapters 3 and 8, a comment should be made here with regard to the chemical and thermal stability of RP packings. The latter is of minor importance as thermal degradation of bonded n-alkyl groups in the atmosphere usually starts above 600 K and RP columns are seldom run at temperatures above 330 K. The chemical stability of RP packings, however, determines the lifetime of the column. Considering the Si-0-Si-C link of a bonded group, the attack of a reagent occurs at the siloxane bond, at the silicon-carbon bond or at both positions. The cleavage of the respective bonds through nucleophilic and electrophilic attack is discussed in Chapter 3. Serious limitations to the use of RP packings exist in aqueous solutions, particularly at pH > 9. This is due to the cleavage of the siloxane bond by means of nucleophilic attack of hydroxyl ions forming soluble silicates. In a recent study, Wehrli et al. 1661 have shown that primary, secondary and tertiary alkylamines inhibit the dissolution process to a certain extent compared with sodium hydroxide and quaternary ammonium hydroxides. By adjusting the buffer sdution to pH 12 with triethylamine, selective separations were performed that could be reproduced over a period of a few days. An explanation for the observed behaviour of basic compounds towards Rp materials cannot yet be given. In using electrolyte solutions as eluents, a cation exchange takes place with the remaining weakly acidic hydroxyl groups of the RP packing. For monovalent non-hydrolysable cations such as Na*, K+and Li' the exchange becomes noticeable at ptf 7 and then in-
212
creases considerably with increasing pH. From this point of view, it is obvious that residual hydroxyl groups also affect the stability of RP packings and finally the retention behaviour of solutes. However, careful measurements are still necessary in order to clarify this subject. Following this discussion, we shall now substantiate the characteristic properties of RP packings by data from the literature. Inspection of the commercial RP packings listed in Appendix C shows that the most popular hydrocarbonaceous ligands are n-octyl and noctadecyl. A few packings are supplied with bonded dimethylsilyl, phenylsilyl and phenylalkylsilyl groups. The first major study aimed at binding non-polar ligands of widely varying composition and structure on to silica was made by Majors and Hopper [62]. Later, Gilpin et al. [67], Knox and Pryde [68], Kirkland [63], Kingston and Gerhart [69], Kikta and Grushka [70], Karch et al. [51], Hemetsberger et al. [64], Unger et al. [59], Hemetsberger etal. [71] and Roumeliotis and Unger [49] dealt with the synthesis of RP packings. The results of the silanization of porous silica species are collected in Table 6.2. It is significant that most silanes employed in surface reactions are trichloroorganosilanes and only limited studies have been made on di- and monochloro compounds. With a few exceptions which relate to the synthesis of polymer layers, anhydrous conditions were chosen. An excess of the silane is dissolved in a dry, inert solvent such as toluene or benzene and the solution is refluxed at about 380-390 K for a sufficient time to complete conversion. Basic catalysts such as pyridine are sometimes added in order to bind the liberated hydrochloric acid. The pure silane without a solvent can also be employed [40, 711. This procedure permits higher reaction temperatures but it is less economic in view of the high price of modifier in large-scale production. In one instance in situ bonding was performed by pumping the dissolved silane through the packed column [67]. The surface concentrations according to eqn. 3.74 are also given in Table 6.2. For trimethylchlorosilane the maximal value reported is (IITMCS = 4.5 ymole/m2, which is in excellent agreement with amax. derived from the molecular cross-sectional area of a trimethylsilyl group [59]. Corresponding to this small molecular size, this surface concentration is the highest to be obtained for a silanizing reagent. Only in one instance [51] is the surface concentration of a dimethyldichlorosilane modified silica claimed to be as high as 8.7 ymole/m2, which is unreasonably high because it indicates a 100% conversion of hydroxyl groups. The replacement of methyl groups with bulky phenyl groups drastically reduces the surface concentration [59] while replacement of a terminal methyl group of an n-alkyl chain with a phenyl group has only a slight effect on a. These results are not surprising if one takes into account that a phenyl group bonded to the same silicon atom as the reactive chlorine atom impedes the attachment of the reactant molecule at a surface hydroxyl group for steric reasons. Comparing the surface concentration of different silicas modified with the same n-alkyltrichlorosilane, one finds a wide variation in a. For instance, for n-octadecyltrichlorosilane a ranges from 1.4 to 4.0 pmolelm’, as shown in Table 6.2. This phenomenon possibly originates from (i) the differences in the nature of the parent silicas and in their state of hydroxylation; (ii) the differences in the reaction and after-treatment conditions and (iii) the use of imprecise specific surface areas. An interesting feature is the influence of the chain length of the n-alkylsilane on the surface concentration. It is obvious that this problem can be examined only by using the
213
same batch of silica, the same pre-treatment reaction and after-treatment conditions and the same series of n-alkylsilanes of identical functionality. The results of two independent studies made by Hemetsberger e t al. [64] and Unger et al. [59] reveal that 01 decreases only slightly on lengthening the n-alkyl chain up to 16 and 21 carbon atoms, respectively. This result, however, implies a mainly perpendicular and regular orientation of bonded n-alkyl groups to the original surface. Obviously, an increase in the chain length leads only to an extension of the thickness of the layer. On the other hand, the monolayer formed seems not to be so closely packed and impermeable to solutes because otherwise the logarithm of the capacity factor should not increase linearly with the total hydrocarbonaceous surface area of the bonded group, which is theoretically derived on the basis of the Van der Waals radii (see Fig. 6.13). In other words, the monolayer must have a relatively open structure. There is only one examination known in which the hydroxyl group concentration was measured on both the native silica and the modified products [49]. The results (Table 3.8) show that at the maximal surface concentration of the n-alkyl group, nearly half of the hydroxyl groups originally present undergo a reaction, i.e., the effectiveness factor is about 0.5. It was also found, with n-octylchlorosilanes, that the residual hydroxyl group concentration increases considerably when a bi- or tri-functional silane is used instead of a monofunctional modifier. 6.3.4 Influence of reversed-phase packing properties on retention of solutes Comparative studies of the retention characteristics of solutes on commercial RP packings grafted with the same n-alkyl group sometimes lead to inconsistent results [72], whereas in other instances only small deviations are observed [6]. It is unquestionable that the nature of the basic silicas will have some influence, even when their specific surface areas and pore structures are the same and the reaction conditions are kept constant. This is due to the fact that they are produced according to a variety of procedures, which finally result in a specific distribution of the surface hydroxyl groups in the product. It is possible to determine the average surface hydroxyl group concentration but a further discrimination into free and isolated, vicinal, paired and geminal hydroxyl groups is highly speculative. As the production of one standardized silica is impracticable, standardization should preferably be performed by chromatographic means, i.e., by adjusting the eluent composition to achieve a defined sequence of elution. This comment seems to be necessary before discussing the effect of packing properties on retention.
6.3.4.1 Relationship between capacity factor and surface coverage It seems obvious that an RP packing with a maximal surface concentration of bonded n-alkyl groups will give the highest capacity factors of solutes eluted at a constant solvent composition. On the other hand, with decreasing surface concentration, the density of non-polar groups decreases and the lipophilic character of the RP packing is reduced. As a consequence, the capacity factors of solutes decrease in the same order. This behaviour was established experimentally by Kikta and Grushka [70] on n-nonyl-modified Corasil 11.
!P 2
TABLE 6.2 PREPARATION CONDITIONS AND CHARACTERISTICDATA FOR TOTALLY POROUS REVERSED-PHASE PACKINGS Abbreviations: a = anhydrous conditions; h = hydrous conditions; s = with solvent; ws = without solvent. Basic silica
Porasil C Porasil C Porasil C Porasil C LiChrosorb Si 60 Porasil C LiChrosorb Si 60 Porasil C Porasil C LiChrosorb Si 60 P o dc Porasil C Homemade Homemade Spherosil XOA 400 Spherosil XOA 400 Spherosil XOA 400 PSM*SO PSM 50 PSM 50 PSM 500 PSM 500 PSM 500 Porasil B LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100
SBET W/g) 50
50 50 50
366 50
366 50 50
366 50 50 200 200 313 373 373 262 262 26 2 52 52 52 200 360 360 360 360
D (nm)
Modifying reagent
Reaction conditions
Reaction temperature, T(K)
Surface concentration, aexp. bmole/m*)
-
Benzyldimethylchlorosilane Phenyldimethylchlorosilane Trimethylchlorosilane Dimethyldichlorosilane n-Hexyltrichlorosilane n-Dodecyltrichlorosilane n-Dodecyltrichlorosilane n-Octadecyltrichlorosilane n-Octadecyltrichlorosilane n-octadecyltrichlorosilane Vinylmethyldichlorosilane Allylphenyldichlorosilane n-octadecyltrichlorosilane Trimethylchlorosilane Trimethylchlorosilane Dimethylbenzylchlorosilane n-octadecyltrichlorosilane Trimethylchlorosilane Dimethylbenzy lchlorosilane n-octadecyltrichlorosilane Trimethylchlorosilane Dimethylbenzylchlorosilane n-Octadecyltrichlorosilane n-Octyltrichlorosilane Dimethyldichlorosilane Di-n-butyldichlorosilane n-Decylmethyldichlorosilane n-Octadecyltrichlorosilane
a, s a, s a, s a, s a, s a, s a, s h, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s
388 388 388 388 388 388 388 388 388 388 388 388 383 383 383 383 383 383 383 383 383 383 -
-
-
60 60 60 -
6 6 6 7.5 7.5 7.5 29 29 29 10 10 10 10
Reference
3.45 1.40 1.06 Polymer layer 0.90 1.44 Polymer layer Polymer layer 2.6 5.3 2.51 2.50 3.13 2.49 2.36 3.10 3.25 3.08 4.00 3.03 8.7
2.6
62
1
68
63
69
1
J
51
360 282 282 282 282 282 282 282 282 282 282 211t(301)* 211(301) 211(301) 211(301) 211(301) 211(301) 211(301) 282 282 282 282 376 376 376 376 376 376 376
*PSM = porous silica microspheres. ?Batch 1. *Batch 2.
LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 Homemade Homemade Homemade Homemade Homemade Homemade Homemade LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 LiChrosorb Si 100 Homemade Homemade Homemade Homemade Homemade Homemade Homemade
10 10 10 10 6.7 6.7 6.7 6.7 6.7 6.7 6.7
-
-
-
-
-
-
10 10 10 10 10 10 10 10 10 10 10 n-Octadecylnleth yldichlorosilane Phen yltrichlorosilane Phenylethyltrichlorosilane Phenylbutyltrichlorosilane Phenylhexyltrichlorosilane n-Octyltrichlorodane n-Undecyltrichlorosilane n-Tridecyltrichlorosilane n-Pentadecyltrichlorosilane n-Octadecyltrichlorosilane n-Heneicosyltrichlorosilane Trimethylchlorosilane Dimeth ylphenylchlorosilane Triphenylchlorosilane n-Bu tyldimeth ylchlorosilane n-Butyldiphenylchlorosilane n-Octyldimeth ylchlorosilane n-Octadecyldimeth ylchlorosilane n-Octylmethyldichlorosilane n-Undecylmethyldichlorosilane n-Pentadecylmethyldichlorosilane n-Octadecylme thyldichlorosilane Trimethylchlorosilane n-Butyldimeth ylchlorosilane n-Octyldimeth ylchlorosilane n-Octyltrichlorosilane n-Octylmeth yldichlorosilane n-Dodecyldimethylchlorosilane n-Hexadecyldimethylchlorosilane a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, s a, ws a, ws a, ws a, ws a, ws a, ws a, ws a, s a, s a, s a, s a, ws a, ws a, ws a, ws a, ws a, ws a, ws ~
388 388 388 388 388 388 388 388 388 388 473 473 473 473 473 473 473 388 388 388 388 473 473 473 473 473 473 473
2.6 3.78 3.70 3.78 4.13 3.74 3.52 3.59 3.47 3.44 3.35 4.5 (4.1) 2.6 1.5 3.6 (3.7) 1.8 (1.7) 3.8 (3.4) 3.4 (3.0) 3.84 3.37 3.04 3.24 3.37 2.97 2.71 (2.35) 2.35 2.40 2.20 2.36 49
71
59
64
51
216
6.3.4.2 Relationship between capacity factor and chain length
As mentioned earlier, a given type of n-alkyl group can be grafted by employing the mono-, bi- or trifunctional organosilanes. With RP silicas thoroughly treated with n-octyldimethylmonochlorosilane (ODMCS), n-octylmethyldichlorosilane (OMDCS) and n-octyltrichlorosilane (OTCS), the capacity factor decreases in the sequence ODMCS-silica > OMDCS-silica > OTCS-silica at a constant eluent composition (methanol-water, 70:30). This result is consistent with the slight change in the hydrophobic character of the packings in the same direction [49]. The effect of chain length on the capacity factor has been investigated in numerous studies [51,59,62,64,70]. In all instances a fairly good linear relationship between k' and the number of carbon atoms in the n-alkyl chain was established, i.e., the retention power increases with increasing chain length at constant eluent composition. In order to compare the retention data of RP packings of different specific surface areas, it was suggested that the capacity factor be normalized to SBETof the modified product [59]. The quantity ~'/SBET was found to be independent Of SBETfor a given bonded group and solute. The question arises here of whether the specific surface area obtained by means of nitrogen adsorption is a real measure of the surface which is responsible for the solvophobic interactions [7]. Including the TMS-modified silica in the discussion of the dependence of k' on chain length, Roumeliotis and Unger [49] found an exceptional retention behaviour in comparison with longer n-alkyl chains: the k' values obtained on TMS-silica are remarkably low and do not fit the straight line which approximately describes the results obtained with long-chain silicas (see Fig. 6.13). The specific behaviour already noticed by Knox and Pryde [68], who compared retentions on TMS- and n-octadecyl-silica,may be caused by the small thickness of the TMS layer giving rise to interactions with the underlying polar siloxane matrix. 6.3.4.3 Sample load and linear capacity Karch et aL [511 established that the linear capacity, Bo.l, of RP packings increases with increasing n-alkyl chain length and exceeds that of the parent silica by one order of magnitude. This was also confirmed in a study by Done [61]. 6.3.5 Absorbent selectivity of reversed-phase packings
As the selectivity of FU' packings has been broadly discussed in some recent review articles [6,7,43], only brief comments will be given here. Considering n-alkyl bonded packings, n-octadecyl-silica offers a better selectivity for aromatic hydrocarbons than n-octyl silica, as exemplified in several studies [63,73,74]. In separating polar substituted aromatic hydrocarbons, such as chloro compounds, phenols, esters and acids, both n-octyland n-octadecyl-silica were found to be well suited [64,75-791. For some particular applications, e.g., in resolving tetracyclines [80] and long-chain fatty acids [81], methylsilica was employed owing to its higher selectivity in comparison with long-chain silica. A tremendous increase in the use of n-octyl- and n-octadecyl-silica has been observed
217
in the field of biochemistry for the separation of catechol compounds [82], biogenic amines [83], urinary acids [84], nucleic acids [85], peptides [86,87], etc. It is easy to predict that this trend will continue in the future. An example of such a separation is shown in Fig. 6.14. Although n-octadecyl-silica,owing to its long chain, gives rise to a greater retention than n-octyl-silica, no final judgement is possible about the preferential selectivity of one of them. This must be checked in every instance. 0.29
€ 1
205nm
0.15
0.10
8 Qo5
0 L
0
I
5
10
I
15
MIN
Fig. 6.14. Separation of amino acids and oligopeptides on a reversed-phase packing [ 881. Column: length 250 mm, I.D. 4.6 mm. Packing: LiChrosorb RP-18, dp = 5 um. Eluent: 0.1 M phosphate buffer (pH 2.1). Temperature: 323 K. Pressure: 260 bar. Detector: W (205 nm). Sequence of elution: 1, Cly; 2, Ala; 3, (Ala),;4,(Ala),;5, unknown;6,(Ah),; 7, Ala-Val; 8, (Ala),; 9, (Val),; 10, (Ala),.
6.4 POLAR CHEMICALLY BONDED SILICA PACKINGS AS SELECTIVE ADSORBENTS IN LSC 6.4.1 Structure and properties of polar chemically bonded silica packings
Inspection of the chemically bonded silica packings other than RP listed in Appendix C shows that they are of polar and moderately polar types. The commonest functional groups are cyano, nitro, amino and diol. The trade-name of each packing is derived from the bonded functional group, such as cyano phase, amino phase, etc. This procedure may lead to an incomplete nomenclature considering only the respective polar groups. In fact, the bonded moiety consists of three parts linked together in the following sequence: (i) the anchor group, Si-0-Si-C, which binds the organic moiety to the underlying parent silica surface; the different possibilities in the composition of the anchor group are discussed on p. 209;
218
(ii) the spacer group, consisting of a n-alkyl group, ether group, etc., which is thought t o provide a sufficiently large distance between the original surface and the proper functional group in order to prevent interference from possible matrix effects; (iii) the terminal functional group, which consists of a n-alkyl or aryl group carrying the polar substituent. Thus, considering the solute-adsorbent interactions on a molecular basis, the net energy of interaction and hence the retention is a result of various contributions according to the different constituents linked in the bonded chain. The strength of interaction further appears to be dependent on the solvent strength of the eluent used. For instance, in non-polar eluents the terminal polar groups will determine retention whereas in polar eluents the reversed-phase character caused by a non-polar spacer group may have a decisive influence. Additionally, the steric orientation of bonded moieties or parts of them will affect the retention of solutes for the following reason. Of all the terminal functional groups, the polar substituent will be most readily accessible for solutes because of its favourable position, which is due to the large extension in the solvent filled pore space. The Si-0-Si-C anchor group is less accessible because the solute molecule has to migrate along the whole chain of the moiety before an interaction can take place. Depending on the reaction conditions and on the special requirements of the surface modifier, the surface concentration of the organic moiety determined fiom the carbon content and evaluated according to eqn. 3.71 usually varies between 2 and 4 pmole/m2. Typical instances are listed in Section 3.2. In most instances the chemically bonded packings are prepared in two steps. The first step consists in the reaction between silica and an appropriate modifier, resulting in the fixation of the desired n-alkylsilyl, arylsilyl or arylalkylsilyl group at the surface. In the second step, the polar functional group is introduced by means of the corresponding substitution reaction. Owing to steric hindrances of the reactants, the degree of substitution is less than that achieved in homogeneous solution. Therefore, provided that a monosubstitution takes place, the surface concentration of polar groups and of groups such as amino and nitro is usually less than or equal to that of the organic group to which they are bonded. Calculating the two surface concentrations based on the carbon and nitrogen content, respectively, and comparing the data enables an estimate of the degree of substitution to be made. Owing to the variety of different groups linked together in an organic moiety such as I -Si-O-St-CH2-CH2-NH-CH2-CH2-NH2 (see eqn. 3.62), the surface exhibits a type of zwitter character when exposed to organic solvents. In non-polar solvents, polar groups such as NH2 and NH govern the interaction with the solute, whereas in polar solvents, the non-polar constituents become dominant. At a distinct solvent composition both influences balance out to a certain extent. When working in aqueous solutions, the amino group may undergo protonation at pH < 7 while deprotonation of residual silanol groups starts at pH > 7. Compared with silica-based ion exchangers on the one hand and RP silica packings on the other, the chemically bonded packings discussed above have medium surface polarity. There is still another favourable property which is provided by the described surface modifications, namely that the most heterogeneous surface sites carrying the hydroxyl groups of highest reactivity undergo reaction and hence disappear.
219
6.4.2 Relationship between structure of polar chemically bonded silica packings and retention of solutes Only a few systematic studies have been carried out on the dependence of the retention of solutes on the structure of chemically bonded silicas. To demonstrate the effects, a silica packing carrying diol groups such as Si-(CH2)3-O-CH2-CH(OH)-CH2(0H) will be considered. In order to study the influence of the eluent composition on retention, the capacity factors of solutes were measured in various binary solvents on untreated and diol-modified silica. On the basis of eqn. 6.8, developed by Snyder, the solvent strength, eo, was calculated in relation to that of n-heptane, which is zero on untreated and modified silica, and arranged in an eluotropic series (see Table 6.3). Comparison of the eo values on native and modified silica for the same eluent composition shows that the magnitude of the solvent strength on bare silica is considerably larger than on did-modified silica. With weakly polar solvents such as dichloromethane a significant change in eo on diol-modified silica compared with the unmodified material is observed (see last column in Table 6.3). It is possible that in this range the hydrophobic parts of the spacer group of the diol sample contribute to the retention. With increasing polarity of the solvent mixture, the ratio &ica/&l-elica remains nearly constant. A specific effect of the diol modification is also observed on deactivating the surface with water as a polar moderator in n-heptane [89], which was already noticed in an earlier study [90]. Fig. 6.15 shows a graph of log k' of selected solutes versus the relative water content of n-heptane as eluent for bare and diolmodified-silica. On the unmodified packing the logarithm of the capacity factor decreases linearly with increasing relative water content, which is consistent with the results in Fig. 6.6. In contrast, log k' on diol-modified silica at first increases slighly at low relative water contents and then levels out to a constant value. The course of the curves indicates that highly active surface sites are removed by surface modification and that the surface exhibits a rather homogeneous character.
TABLE 6.3 ELUOTROPIC SERIES OF BINARY SOLVENTS ON UNTREATED AND DIOL-MODIFIED SILICA is91 Eluent composition (v/v)
chca
Ciiol-silica
n-Heptane n-Heptane-dichloromethane (90: 10) n-Heptane-dichloromethane (25 :75) Dichloromethane Dichloromethane-acetonitrile (95 5 ) Dichloromethane-acetonitrile (20:80) Acetonitrile-methanol ( 9 5 5 ) Acetonitrile-methanol (30:70)
0 0.13 0.30
0
-
0.014 0.052 0.077 0.097 0.125 0.175 0.194
9.3 5.8 4.6 3.9 3.8 3.7 3.8
0.35 0.38 0.48 0.56 0.75
&ica/Eliol-silica
220 k'
k'
f
f 10
1
0.1
0
20
40
60
80
100
0
20
4
40
60
80
100
rel. water content (%)
Fig. 6.15. Dependence of the capacity factor of a solute on the relative water content on (a) untreated and (b) diol-modified silica [ 8 9 ] .Column: length 200 mm, I.D. 4 mm. Packings: LiChrosorb Si 100, d p = 10 pm,and LiChrosorb Si 100 modified with 1,2-epoxy-3-propoxypropyltriethoxysilane (diolsilica). Eluents: n-heptane with controlled relative water content in the range 0-100% (v/v), corresponding to 10-500 ppm of water. Flow-rate: 1.0 ml/min. Detector: UV (254 nm). Samples: X , benzene; v ,diphenyl; 0,m-terphenyl; A, m-quaterphenyl; 0,rn-quinquephenyl.
6.4.3 Selectivity of polar chemically bonded silica packings
Selective retention on chemically bonded silica packings in non-polar or moderately polar eluents is based on specific interactions such as dipole-orientation, hydrogen bonding or charge transfer between the solute molecules and the polar terminal groups. This is demonstrated in Table 6.4 for some polar aromatic solutes. Capacity factors are measured on untreated silica and on a series of bulk modified derivatives, all packings having nearly the same specific surface area. The three bulk-modified silicas differ in the composition o i their terminal groups as follows:
I
-Si-(CH2)3-O-CH2-CH(OH)-CH20H I (a)
I
-Si-(CH2)3-O-CH2-CH(OH)-CH2-NH2 I
(molar ratio OH/hWz= 6)
(b)
I
-Si-(CH2)3-O-CH2-CH(OH)-CH2-NH~ I (c)
(molar ratio OH/NH2 = 1)
221
For all solutes investigated, the capacity factors increase from packing (a) to (c), which is in agreement with the increasing substitution of hydroxyl groups by amino groups in the diol structure of the packing. Maximal retention is obtained with the hydroxylaminemodified packing, which also exceeds that obtained on bare silica at the same conditions. The variation in capacity factors results in a significant improvement in the selectivity coefficient of the solute pairs considered. TABLE 6.4 CAPACITY AND SELECTIVITY FACTORS OF POLAR SOLUTES ON DIFFERENT BULKMODIFIED SILICA PACKING AT CONSTANT ELUENT COMPOSITION (DICHLOROMETHANEACETONITRILE, 95:5, v/v) [59] Column length, 200 mm. Column 1: SBET= 400 m'/g; original silica. Column 2: SBET= 394 m'/g; functional group, Si(CH,),-0-CH,-CH(0H)-CH,OH. Column 3: SBET= 401 m'/g; functional group, Si(CH,),-0-CH, -CH(OH)-CH,OH(NH,) (molar ratio OH:NH, = 6). Column 4: SBET= 430 m'/g; functional group, Si(CH,),-0-CH,-CH(0H)-CH,NH, (molar ratio OH:NH, = 1). Sample
Column 1
k' 2-Chlorophenol
Column 2 rji
k'
3.75 0.80
4-Chlorophenol
0.69
CNitrophenol Aniline
0.85 1.49
1.54 1.75
1.83 2.26 2.26
1.00
1.6 1.14
1.16
1.48
1.3
5.18 9.08 4.08
2.0 4.6
rji
0.72
1.23
1.18
k'
2.3
0.94
1.30
rji
1.06
0.80
-
Column 4
0.46
0.85
1.23
large
k'
3.4
0.86
cu-Picoline
rji
0.25
0.21
Phenol
Column 3
2.25
10.8
Another example to demonstrate the selectivity of bonded packings on the basis of specific interactions is the retention behaviour of polynuclear aromatics (PNAs) on silica carrying 3-(2,4,5,7-tetranitrofluorenimino)propylsilylgroups [9 11. The tetranitrofluorenimino (TNF) group is able to form charge transfer complexes with PNAs. Retention studies on this packing in acetonitrile-saturated isooctane show a pronounced selectivity for methyl-substituted PNAs which is explained in terms of n complex formation between the respective solute molecule and the TNF group. In practice, some chemically bonded silica packings were found to exhibit an excellent selectivity for particular groups of substances. An example is shown in Fig. 6.16, which demonstrates the separation of monosaccharides on an amino-silica in acetonitrile-water (921. Unfortunately, the selectivity of a chemically bonded packing cannot be predicted but has to be determined experimentally.
Fig. 6.16. Separation of monosaccharides [92]. Column: length 250 mm, I.D. 3 mm. Packing: LiChrosorb NH,, d p = 10 rm. Eluent: acetonitrile-water (80:20, v/v). Pressure: 78 bar. Flow-rate: 2 ml/min. Detector: R1. Sequence of elution: 1, xylose; 2, arabinose; 3, fructose;4, glucose; 5, sucrose (saccharose); 6, lactose.
6.5 ADSORBENT STANDARDIZATION 6.5.1 Introduction
Adsorbent standardization is a fundamental problem in LSC that was recognized several years ago by Snyder [93] and other workers [94,95].It provides the basis for comparisons of packings, columns and chromatographic data and may be valuable in predicting the retentions of solutes of known composition. Standardization implies (i) the definition of a standard state of the adsorbent or packing and (ii) the measurement of representative physico-chemical and chromatographic properties of the adsorbent and column under defined and recognized test conditions making use of reference adsorbents. Most work in this field has been carried out on native silica, resulting in a singleparameter adsorbent activity scale that is related to the water content or activity [93] of the adsorbent. The Snyder equation (eqn. 6.8) then provides the relationship between the distribution coefficient of the solute and the standardized properties of the adsorbent, eluent and sample. A similar approach was developed by Horvath el d. I961 for reversedphase silica packings.
223
The purpose of this section is not to discuss the various theoretical aspects in defining standard states of adsorbents, but to examine critically the phy;ico-chemical and chromatographic properties and related test procedures that are suitable for standardization purposes. 6.5.2 Physico-chemical standardization This term implies three classes of adsorbent properties: (i) data that characterize the morphology and size of the adsorbent particles; (ii) data that describe the specific surface area and the pore structure of adsorbents; (iii) data that characterize the mechanical, thermal and chemical stability of adsorbents.
6.5.2.I Morphology and size of particles The term morphology simply refers to the particle shape, which is either spherical or angular. Further characterization of angular particles by introducing shape factors [97] is possible but makes the subject more complicated. In defining particle size, the most exact and representative dimension is the diameter of a spherical particle. According to Sing [ 9 8 ] , the term sphericity is defined as “the ratio of the surface area of a sphere having the same volume as a given particle to the actual surface area of that particle”. Consequently, spherical packings are recommended as reference materials for the assessment of size distribution data and the size of angular particles should be quoted as an equivalent diameter of a sphere. Considering the methods of size analysis, microscopic determination gives the most precise estimate because direct viewing of particles is involved. All other methods, such as sedimentation and the Coulter Counter method, are not able to discriminate exactly between individual particles and particle assemblies. As suggested in Chapter 4 (p. 150), about 500-700 particles of a given batch should be investigated by microscopic viewing. The range of particle diameters considered should be divided into about 10 classes. By using appropriate equations, the number-average distribution obtained can be converted into a weight- or volume-average distribution. The results are preferably plotted as a cumulative frequency distribution, the 50%value of which should be defined as the mean particle diameter, d p . The particle size, however, is only fully characterized by the whole size distribution. In this respect, it is desirable to illustrate the course of the distribution at its lower end by a sufficient number of experimental data. It is very helpful for chromatographic purposes to designate separately the content of fines with a particle size smaller than 2 pm. Spherical particles that have a well defined particle size distribution should be employed as reference materials to calibrate other size analysis techniques such as sedimentation and the Coulter Counter method. It is worth mentioning that considerable efforts have been made by the Community Bureau of Reference (BCR) [99] (see also ref. 100) to supply certificated reference materials for particle size analysis.
6.5.2.2 Specific surface area An extended study on surface area standards sponsored by the IUPAC was carried out by Everett e f dl: [ l o l l . The method chosen was the conventional nitrogen BET surface
224
area determination taking A,(N2) to be 0.1 62 nm2/molecule. The materials investigated were carbon blacks, a finely divided silica and two micronized porous silicas. Based on the experience of the author, the following test conditions are recommended for determining S B of ~silica and its chemically bonded packings: ( i ) Initial experimental data Weight of sample
Particle size of sample Outgassing temperature Outgassing time Outgassing pressure Adsorbate Chemical purity of N2 Temperature of measurement Apparatus Type of silica reference material for calibration
Depending on the value of SBETand the type of equipment 473 K 12 h 99.99% 77.6 K Gravimetric or volumetric device
(ii)Measurements Equilibrium pressure of adsorbate Saturation pressure of adsorbate at 77.6 K Amount adsorbent of N2 in g or ml “TP) (iii)parameter obtained fiom data given in (i) and (ii)
SBET(m’/g)
A,(N2)N* lo-’*
(6.23)
where N is Avogadro’s constant (6.02 molecules/mole), Am(N,) the molecular crosssectional area of a nitrogen molecule, taken as 0.162 nm2, and X , the specific monolayer capacity in moles per gram of adsorbent (see also eqn. 2.19). X, is derived from the plot of Xa versus p/po using the equation (6.24) where Xa is the amount of nitrogen adsorbed in moles per gram of adsorbent, p / p o the relative pressure and C a constant (see also eqn. 2.5). The applicability of the two-parameter BET equation has to be checked carefully according to the following criteria [ 1021 : (i) the BET isotherm should be a straight line; (ii) the intercept of the straight line according to eqn. 6.24 should be zero or positive; (iii) C should be larger than 10.
225
6.5.2.3 Specificpore volume In contrast to other parameters such as specific surface area and pore size, the estimation of the specific pore volume, Vp,does not provide any models or assumptions other than that Vp corresponds to the volume of a given liquid that completely fills the open pores of the porous particles.
6.5.2.3.1 Vp of purely mesoporous silicas with D in the range 2.0-50.0 nm Assuming that the adsorbate condensed in the pores behaves as an ordinary liquid, the specific pore volume can be determined from the finite uptake of nitrogen at p/po = 0.97, provided that the sorption isotherm exhibits a course parallel to the relative pressure axis. ( I ) Initial experimental data Nitrogen sorption measurements at 77.6 K (see pp. 23-25). (ii)Measurements Amount of nitrogen adsorbed as a function of relative pressure. (iii)Parameter obtained from data given in (i)and (ii) (6.25) V~(G) (ml/g) = x u Vm where Xu is the amount of adsorbed nitrogen at p / p o = 0.97 and V , the molar volume
of liquid nitrogen at 77.6 K, taken as 34.6 ml/mole. 6.5.2.3.2 Vp of mesoporous and macroporous silicas with D in the range 3-14,708 nm In this instance the specific pore volume is derived from the volume of intruded mercury at atmospheric pressure up to pmax = 5000 bar. The volume of mercury has to be corrected for its compressibility. The relationship between V p of pores filled with mercury and the equilibrium pressure,p, is given by the Washburn equation (see eqns. 2.14 and 2.15). (i)Initial experimental data Filling the penetrometer Type of apparatus Dimensions of penetrometer (length, inner diameter) Weight of sample Particle size of sample Outgassing temperature Outgassing pressure Outgassing time Purity of mercury used Apparatus for measuring the volume of intruded mercury as a function of pressure Type of gauges Method of detecting the changes of volume of mercury Temperature (ii) Measurements Pressure P (bar) Volume of intruded mercury corrected for its compressibility V p (ml)
226
(iii)Parameter obtained from data given in (i) and (ii) V p ( ~ g ) = Volume of intruded mercury at atmospheric pressure up to pmaxof the porosimeter device. The smallest pore diameter to be filled with mercury depends on the maximum pressure of the porosimeter (see eqn. 2.15). For instance, in order to fill pores with D = 3.0 nm a pressure of 5000 bar has to be applied. When microparticulate silicas are measured, Vp(Hg) has to be corrected for the volume of mercury that fills the void volume between the particles. 6.5.2.3.3 V, fiom the apparent particle densities due to helium and mercury In principle, the specific pore vdume can also be determined by measurement of the , to mercury, PHg. The reciprocal apparent particle densities with respect to helium, P H ~and of pHe is equal to the specific volume of the pure solid, including closed pores and excluding open pores, whereas the reciprocal of p~~ corresponds to the specific volume of the pure solid, including closed pores and excluding open pores with D 2 15 m. (i) Initial experimental data for both helium and mercury dens@ measurements Type of pycnometer Weight of sample Particle size of sample Pre-treatment conditions (outgassing temperature, pressure, time) Temperature of measurement Purity of helium and mercury (ii)Measurement Volume of purely solid packing including the volume Apparent helium density of closed pores (ml) Apparent mercury density Volume of intruded mercury at atmospheric pressure (ml) (iii)Parameters obtained from data given in ( i ) and (ii) Apparent particle density with respect to helium, PHe (g/ml) Apparent particle density with respect to merucry, PHg (g/ml)
vp =-
1
PHg
-
1
-
(6.26)
PHe
Difficulties arise in the measurement of PHg of microparticles; owing to the incomplete filling of the small interstices between the microparticles at atmospheric pressure the value of pHg becomes too small.
6.5.2.4 Pore distn’bulion It should be mentioned that either the distribution of the specific pore volume or the distribution of the specific surface area as a function of the mean pore diameter can be estimated, but not the true frequency distribution of pore sizes. The calculation of the pore volume distribution of micro- and mesoporous silica is derived from the full nitrogen isotherm on an adsorbent at 77.6 K, whereas the pore volume distribution of meso- and
221
macropores is obtained from mercury penetration measurements. In the assessment of mesopore distribution from nitrogen sorption measurements, a number of methods have been proposed that differ in the following respects: (i) choosing the branch (desorption and/or adsorption) of the nitrogen isotherm suitable for calculation; (ii) the assumptions regarding the pore shape; (iii) the type of equation that relates the amount adsorbed at a given relative pressure to the mean pore diameter; (iv) the mode of correction with respect to the adsorbed multilayer of nitrogen. A comparative study on pore measurements on a mesoporous silica was performed by Havard and Wilson [103], which typically reflects the actual situation in this field. Three methods will be presented here that permit the calculation of the micropore, mesopore and macropore distribution.
6.5.2.4.I Micropore analysis [1041 (i) Initial experimental data Sorption measurements with nitrogen at 77.6 K (see pp. 23-25) (ii) Measurements Amount of nitrogen adsorbed as a function of relative pressure. (iii) Parameters obtained from data given in ( i ) and (ii)
-
Si (m'/g) = mi lo4
(6.27)
where Si is the specific surface of the pore group i and m i the corresponding slope of the graph Vl versus t (see Fig. 2.6). Scum (m2/g) = ZAsi
(6.28)
where Scumis the specific cumulative micropore surface area. (6.29) where Vj is the specific micropore volume of pore group i and t the thickness of the adsorbed layer. micro J'p(cum) (ml/g) = vi micro . where Vp(c"m) is the specific cumulative micropore volume.
(6.30)
(6.3 1) where rhj is the hydraulic radius of pore group i. The results are presented, for instance, as a relative distribution: (6.32)
228
where
6.5.2.4.2 Mesopore analysis 6.5.2.4.2.1 Method according to Pierce modified by Orr and Dalla Valle /lo51 (i)Initial experimental data Nitrogen sorption measurements at 77.6 K (see pp. 23-25) (ii)Measurements Amount of nitrogen adsorbed as a function of relative pressure (iii)Parameters obtained from data given in (i)and (ii) D=DK + t (6.33) where D is the actual pore diameter, DK the Kelvin diameter assuming cylindrical pores and t the thickness of the adsorbed layer.
D (nm) =
Di + @ + I 2
(6.34) (6.35) (6.36) (6.37)
where A Vco,, is the volume decrement of capillary-condensed liquid
(6.38)
A vp(cum) ( d d = L:AVp AS(cum)(m2/g) = ZAS = L: 62.4 A VP D 0 -
(6.39)
The results can be presented, for instance, as a relative distribution, A Vp/AD= f(@.
6.5.2.4.2.2 Corrected modelless method [lo61 (i) Initial experimental data Nitrogen sorption measurements at 77.6 K (see pp. 23-25) (ii)Measurements Amount of nitrogen adsorbed as a function of relative pressure (iii)Parameters obtained from data given in (i)and (ii) Dhi (nm) =
2 Vi(corr)
Si(corr) where Dhi is the hydraulic pore diameter of the core group i , and Vi'ifcom)and Si(com) are the specific pore volume and specific surface area, respectively.
(6.40)
229
(6.41) (6.42) (6.43)
6.5.2.4.3 Meso- and macropore analysis by mercury penetration (1071 ( i )Initial experimental data Mercury porosimetry measurements (see pp. 25,26) (ii)Measurement Volume of mercury intruded as a function of pressure. (iii) Parameters obtained from data given in (i) and (ii)
D (nm) =
Di+Di+i 2
(6.44)
6.5.2.5 Stability Mechanical stability becomes important when the adsorbent is packed into a column at high pressures (300-500 bar) at high flow-rates. A procedure for indicating the so-called packing stability of an adsorbent was developed by Messt [ 1081. The column was slurrypacked with the material under investigation and n-I. itane was pumped through at increasing pressure while increasing the flow-rate. Fo?acking, a point on the curve of pressure ver: 4s flow-rate exists above which the funct deviates from linearity and shows an upwa,d inflection, indicating the beginning of rUiticle or packing destruction. As can be seen From the results in Table 5.2, the stability of a packing is dependent on average pore diameter and has to be carefully considered when large-pore silicas are ..icked. Thermal stability can be characterized by means of thermal gravimetric measurements and is of interest with chemically bonded silica packings, particularly with ion exchangers. I n these instances, thermal gravimetric measurements should be accomplished by means of elemental analyses for carbon, sulphur, nitrogen, etc., to indicate thermal cleavage of bonded functional groups. Chemical stability is the most important property of bonded packings because it limits the lifetime of the column. It can be monitored either by extraction experiments with a given solvent or solvent mixture, or by chromatographic measurements of the retention times of certain test solutes. 6.5.3 Chromatographic standardization
The comparison of packings, column packing methods and columns requires test procedures that enable calculation of representative test parameters. We shall follow here the recommendations of Bristow and Knox [ 1091, who considered this subject in detail.
230
In testing a column, the following data should be recorded: (i) operating conditions: temperature of column, designation of packing, method of packing, composition of eluent, composition of test sample, detection method, injection method; (ii) properties of eluent and solute(s): viscosity of eluent, diffusion coefficients of solute(s) in eluent; (iii) geometrical parameters: column bed length, column diameter, particle size; (iv) chromatographic parameters: volume of sample injected, elution distance on recorder chart of unretained and retained solute(s), peak width of solute(s) at half-height as distance on recorder chart, recorder chart speed, pressure drop across column, volume flow-rate of eluent. A scheme is outlined in Formats 1 and 2 taken from ref. 109:
Format 1 Initial experimental data Conditions of test Packing material Method of packing Temperature of column (K) Method of injection Detector Eluent composition Sample solute Properties of eluent and solute(s) Viscosity of eluent (N s m-') Diffusion coefficient of solute (mZ sec-') Geometrical parameters Length of bed, L (mm) Diameter of bed, dc (mm) Particle size d p (pm) Range of d p (cun) Format 2 Chromatographic measurements Elution distance of unretained solute peak measured as a distance on recorder chart, ro (mm) Elution distance of retained solute peak measured as a distance on recorder chart, rR Peak width at half-height for retained solute peak measured as a distance on recorder chart, w, (mm) Chart speed, s (mm sec-') Pressure drop, Ap (bar) Volume flow-rate of eluent, fv (dmin-') On the basis of the data in Formats 1 and 2, a series of chromatographic properties can be calculated:
-
231
The elution time or retention time: t o = ro/s
tR = rR /s
(6.47)
The peak width in time units: Wt
(6.48)
= WJS
The number of theoretical plates to which the column is equivalent, or the plate number:
N = (tR/Wr)’ 5.54
(6.49)
a
The height equivalent to a theoretical plate, or plate height:
(6.50) The reduced plate height:
(6.51) The mean linear velocity of the eluent: u = L/to
(6.52)
The reduced velocity of the eluent: - UdP - LdP v=-
D,
(6.53)
toDm
The reduced column length: 1 = L/dp
(6.54)
The column capacity ratio:
k’ = ( t R
-
(6.55)
to)/to
The chromatographic permeability:
uqL K=-=AP
qL2
(6.56)
AP~O
The dimensionless flow resistance parameter: 2
(6.57) K
7)
f 4
The performance index: IT=- -
N2 - 5.54’ t~~ -
RAP
APWP
(6.58)
232
The separation impedance: (6.59)
(6.60)
= h 2 4 = H2/K
The box-Parcher ratio: (6.6 1)
I = d;/Ldp The total column porosity:
(6.62) According to their importance, the above quantities are divided into two categories: (a) major quantities: H, KO,u and E or h, 4, v and E ; (b) subsidiary quantities: k', I, €tot. According to Bristow and Knox [ 1091, for a good column h should be about 3 at a reduced velocity of 5 and should not exceed 20 at a reduced velocity of about 100. The column resistance factor should not exceed 1000. The test solutes to be selected should be readily available, stable and of low toxicity in solution. We suggest the use of polar solutes of low molecular weight exhibiting an adequate extinction coefficient at 254 nm. A mixture of solutes should contain an unretained and three retained solutes with capacity factors of 0.2, 1.Oand 5.0, respectively. To measure the capacity factors and selectivity coefficients, the following test conditions are recommended:
For silica Eluent Solutes
Injection volumes Linear velocity
n Rexane-me thanol(99.5 :0.5, v/v) Dissolved in the eluent (1) n-pentane (unretained) (2) toluene (3) nitrobenzene (4) acetophenone (5) 2,6-dinitrotoluene (6) 1,3,5-trinitrobenzene 5 pl (syringe injection) 20 pl (valve system) 3 mmlsec for d p = 5 pm 1.5 mmlsec for d p = 10 pm
233
For reversed-phase silica Eluent Methanol-water (60:40, vlv) Solutes Dissolved in eluent containing a slight excess of methanol (1) methanol-water, RI disturbance (2) acetone (3) phenol (4) p-cresol (5) 2,5-xylenol (6) anisole (7) phenetole Injection volume See above Linear velocity 1 mmlsec for dp = 5 p.m 0.5 mmlsec for d p = 1 0 pm A computer program is given in ref. 109 for computation of data.
6.6 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
M. Tswett, Trav. SOC.Nut. Warsowie, 14 (1903) 6. D.T. Day, Science, 17 (1903) 1007. L.R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker, New York, 1968. D.L. Saunders, J. Chromatogr. Sci., 15 (1977) 372. H. Engelhardt, J. Gromatogr. Sci., 15 (1977) 380. G.B. Cox,J. Chromatogr. Sci., 15 (1977) 385. C. Horvath and W. Melander, J. Chromatogr. Sci., 15 (1977) 393. L.R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker, New York, 1968, pp. 130-131. L.R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker, New York, 1968, p. 132. F. Geiss (Editor), Die Parameter der Dunnschichtchromatographie. F. Vieweg, Braunschweig, 1972, pp. 92-105. L.R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker, New York, 1968, p. 146. L.R. Snyder, Principles ofAdsorption Chromatography, Marcel Dekker, New York, 1968, p. 145. D.L. Saunders, Anal. Chem., 46 (1974) 470. R.P.W. Scott and P. Kucera,J. Chromatogr., 112 (1975) 425. R.P.W. Scott, J. Chromatogr., 122 (1976) 35. R.E. Majors, J. Chromatogr. Sci., 15 (1977) 334. L.R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker, New York, 1968, pp. 160-162. 2. El Rassi, C. Gonnet and J.L. Rocca, J. Chromatogr., 125 (1976) 179. K. Unger, Chimia, 28 (1974) 679. J.F.K. Huber and F. Eisenbeiss, J. Chromatogr., 149 (1978) 127. J.M. Bather and R.A.C. Gray,J. Chromatogr., 122 (1976) 159. W. Boehme and H. Engelhardt, J. Chromafogr., 133 (1977) 67. J.J. Kirkland, J. Chromatogr., 83 (1972) 149. L.R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker, New York, 1968, pp. 75-97. D.L. Saunders, J. Chromatogr., 125 (1976) 163. B.L. Karger, L.R. Snyder and C. Horvath (Editors), A n Introduction to Separation Science, Wiley, New York, 1973, pp. 174-178.
234 27 High Performance Liquid Chromatography, Catalogue, E. Merck, Darmstadt, 1976. 28 L.R Snyder and J.J. Kirkland, Introduction to Modern Liquid Chromatography, Wiley, New York, 1974, pp. 261-264. 29 E. Soczewinski, Anal. Chem., 41 (1969) 179. 30 E. Socmwinski and W. Golkiewicz, Chromatographia, 5 (1972) 431. 31 W. Golkiewicz and E. Soczewinski, Chromatographia, 5 (1972) 594. 32 E. Soczewinski and W. Golkiewicz, Chromatographia, 6 (1973) 269. 33 R.P.W. Scott and P. Kucera, AnaL Chem., 45 (1973) 749. 34 A. Waksmundzki and J. Rosylo, Chem Anal., 14 (1969) 1217. 35 L.R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker, New York, 1968, p. 139. 36 J. Vermont, M. Deleuil, A.J. de Vries and C.L. Cuillemin, Anal. Chem., 47 (1975) 1329. 37 F. Eisenbeiss, Kontakte (E. Merck, Darmstadt), 1 (1976) 19 and 2 (1976) 17. 38 D.C. Locke, J. Chromatogr. Sci., 12 (1974) 433. 39 A. Pryde, J. Chromatogr. Sci., 12 (1974) 486. 40 R.B. Sleight, J. Chromatogr., 83 (1973) 31. 41 J.N. Seiber, J. Chromatogr., 94 (1974) 151. 42 S. Omura, Y. Susuki, A. Nakagawa and J. Hata, J. Antibiot., 26 (1973) 794. 43 H. Colin and G. Guiochon, J. Chromatogr., 141 (1977) 289. 44 C. Horvath, W. Melander and I. Molnar, J. Chromatogr., 125 (1976) 129. 45 C. Horvath, W. Melander and I. Molnar, AnaL Chem., 49 (1977) 143. 46 0. Sinanoglu, in B. Pullmann (Editor), Molecular Associations in Biology, Academic Press, New York, 1968, p. 427. 47 0. Sinanoglu and S . Abdulnur, Fed Proc., Fed. Amer. SOC.Exp. Biol., 24 (1965) 12. 48 C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes, WileyInterscience, New York, 1973. 49 P. Roumeliotis and K.K. Unger, J. Chromatog., 149 (1978) 211. 50 J. Shapiro and J.M. Kolthoff, J. Amer. Chem. SOC.,72 (1950) 776. 51 K. Karch, I. Sebestian and I. Halgsz, J. Chromafogr., 122 (1976) 3. 52 L. Forni, Catal. Rev., 8 (1973) 65. 53 K. Tanabe, Solid Acids and Bases, Academic Press, New York, 1970. 54 S.R. Morrison, The Chemical Physics of Surfaces, Plenum Press, New York, 1977. 55 L.D. Sharma and R.P. Mehrotra, Indian J. Chem., 14A (1976) 352. 56 H.A. Benesi, J. Amer. Chem. Soc., 78 (1956) 5490. 57 I. Matsuzaki, Y. Fukuda, I. Kobayashi, K. Kubo and K. Tanabe, Shokubai, 11 (1969) 210. 58 0. Johnson, J. Phys. Chem., 59 (1955) 827. 59 K.K. Unger, N. Becker and P. Roumeliotis, J. Chromatogr., 125 (1976) 115. 60 F. Eisenbeiss (E. Merck, Darmstadt), personal communication. 61 J.N. Done, J. Chromatogr., 125 (1976) 43. 62 RE. Majors and H.J. Hopper, J. Chromatogr. Sci., 12 (1974) 767. 63 J.J. Kirkland, Chromatographia, 8 (1975) 661. 64 H. Hemetsberger, W. Maasfeld and H. Ricken, Chromatographia, 9 (1976) 303. 65 F. Eisenbeiss and K.F. Krebs, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, 1977, Paper No. 311. 66 A. Wehrli, J.C. Hildenbrand, H.P. Keller, R. Stampfli and R.W. Frei, J. Chromatogr., 149 (1978) 199. 67 R.K. Gilpin, J.A. Korpi and C.A. Janicki, Anal. Chem,, 46 (1974) 1314. 68 J.H. Knox and A. Pryde, J. Chromatop., 112 (1975) 171. 69 D.G.I. Kingston and B.B. Gerhart, J. Chromatogr., 116 (1976) 182. 70 E.J. Kikta and E. Grushka, Anal. Chem., 48 (1976) 1098. 71 H. Hemetsberger, M. Kellermann and H. Ricken, Chromatographia, 10 (1977) 726. 72 R.P.W. Scott and P. Kucera,J. Chromatogr., 142 (1977) 213. 73 J.A. Schmit, R.A. Henry, R.C. Williams and J.R. Dieckmann,J. Chromatogr. Sci., 9 (1971) 645. 74 R.B. Sleight, J. Chromatogr., 83 (1973) 31. 75 RS.H. Yang, F. Coulston and L. Goldberg, Anal. Chem., 58 (1975) 1167.
235 76 77 78 79 80 81 82 83
U.A.Th. Brinkman, A. de Kok, H.G.M. Reymer and G. de Vries, J. Chromatogr., 129 (1976) 193. K. Callmer, L.E. Edholm and B.E.F. Smith, J. Chromatogr., 136 (1977) 45. A. Otsuki, J. Chromatogr., 133 (1977) 402. W. Winkle, Chromatographia, 10 (1977) 13. J.H.Knox and J. Jurand, J. Chromatogr., 110 (1975) 103. F. Eisenbeiss (E. Merck, Darmstadt), personal communication. I. Molnar and C. Horvath, Ciin. Chem., 22 (1976) 1497. A.P. Graffeo and B.L. Karger, Clin Chem., 22 (1976) 184. 84 I. Molnar and C. Horvath, J. Chromatogr., 143 (1977) 391. 85 R.A. Hartwick and P.R. Brown, J. Chromatogr., 126 (1976) 679. 86 J.J. Hansen, T. Greibokk, B.L. Currie, K.N.-G. Johannson and K. Polkers, J. Chromatogr., 135 (1977) 155. 87 K. Krummen and R.W. Frei, J. Chromatogr., 132 (1977) 27. 88 I. Molnar, Separation of Biologically Important Compounds on Reverse Phase Packing, Dr. H. Knauer, Berlin, 1977. 89 N. Becker, Thesis, Technische Hochschule, Darmstadt, 1977. 90 K. Unger, N. Becker and E. Kraemer, Chromatographia, 8 (1975) 283. 91 C.H. Lochmueller and C.W. Amoss, J. Chromatogr,, 108 (1975) 85. 92 Application of LiChrosorb N H , , 76-4, E. Merck, Darmstadt, 1976. 93 L.R. Snyder, Principles o f Adsorption Chromatography, Marcel Dekker, New York, 1968, pp. 143-153. 94 H. Halpaap, J. Chromatogr., 78 (1973) 63. 95 H. Halpaap, J. Chromatogr., 78 (1973) 77. 96 C. Horvath, W. Melander and 1. Molnar, J. Chromatogr., 125 (1976) 129. 97 T . Allen (Editor), Particle SizeMeasurement, Chapman & Hall, London, 1974, pp. 76-85. 98 K.S.W. Sing, in G.D. Parfitt and K.S.W. Sing (Editors), Characterization o f Powder Surfaces, Academic Press, London, 1976, p. 2. 99 Community Bureau of ReferenceBCR, Directorate General XII, Commission of the European Communities, 2000, rue de la Loi, E l 0 4 9 BNSS~~S. 100 E. Robens, Sprechsaal, 110 (1977) 716. 101 H. Everett, G.D. Parfitt, K.S.W. Sing and R. Wilson, J. Appl. Chem Biotechnol., 24 (1974) 199. 102 S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, pp. 35-54. 103 D.C. Havard and R. Wilson, J. Colloid Interface Sci., 57 (1976) 276. 104 S. Brunauer, J. Skalny and J. Odler, in S. Modry and M. Svata (Editors),Proceedings of the RilemlIUPAC International Symposium on Pore Structure and Properties o f Materials, Vol. I, Academia, Prague, 1974, C-3. 105 S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, pp. 162-172. 106 S. Brunauer, R.Sh Mikhail and E.E. Bodor, J. Colloid Interface Sci., 24 (1967) 451. 107 S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, pp. 182-184. 108 W. Messer, Thesis, Technische Hochschule, Darmstadt, 1977. 109 P.A. Bristow and J.H. Knox, Chromatographia, 10 (1977) 279.
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237
Chapter 7
Silica as a support in liquid-liquid chromatography 7.1 GENERAL ASPECTS Partition chromatography, introduced by Martin and Synge [ l ] in 1941 as an alternative to counter-current extraction, has been developed to a powerful separation technique in column liquid chromatography. In liquid-liquid chromatography (LLC) the phase system consists of two liquids, one being mobile the other being held stationary on a solid support. The solutes, when introduced into the mobile phase, are partitioned between the two phases and selective retention takes place according to the different distribution coefficients. As the distribution process in LLC is equivalent to simple extraction, the distribution coefficient of a solute i, Ki, in a given liquid-liquid system can be written as
where ‘Ki is the distribution coefficient of solute i, x i a and x i p are the molar fractions of i in phases (Y and p [2]. Partitioning in linear chromatography is assumed to occur at infinite dilution of solute i. Thus, “Ki becomes
where xKio is defined as the partition coefficient of solute i at infinite dilution and xioa and Xiop are the molar fractions of i at infinite dilution in phases (Y and 0. In chromatography, it is more convenient to use concentrations instead of molar fractions, which gives a distribution coefficient, ‘Kio, defined as (7.3)
where Ciop and Cioa are the concentrations of i at infinite dilution in the two phases a and 0. The two distribution coefficients, ’KiO and ‘Kj0, are related in the following way:
where Va and V p are the molar volumes of the compounds (Y and 0 that constitute the phase system. The particular advantage of LLC is that it is possible to determine ‘Kjo values by simple batch experiments and to predict the capacity factors in the given phase system by use of the equation
238
k! =-.vs cK io
’
(7.5)
vm
where V, and Vm are the volume of the stationary and mobile phase, respectively. By measuring the static distribution coefficients, ‘KjoStat, of selected solutes in a limited number of phase systems and applying an approximation procedure, the capacity factors of a large number of solutes can be predicted according to eqn. 7.2 and hence it is possible to choose an optimal liquid-liquid system. This approach has been widely advocated by Huber and co-workers [3,4], In order to achieve differences in the distribution coefficients, the two phases should not be totally immiscible otherwise the ‘Ki0 value of the solute tends to either zero or infinity. In practice, binary or ternary liquid-liquid systems are preferably employed as phase systems [3]. For instance, by mixing water, ethanol and 2,2,44rimethylpentane in certain proprotions a polar or a non-polar phase is obtained. In column operation there is the choice of holding the polar phase stationary and using the non-polar phase as the mobile phase or vice versa. Provided that the separation mechanism is purely partitioning, the distribution coefficient, ‘Ki0, of a given solute i in phase system I (polar stationary phase/non-polar mobile phase) is replaced with its reciprocal when changing to phase system I1 (non-polar stationary phase/polar mobile phase) at the same phase composition: (‘Kio)r = (~/‘K~o)II
(7 4
Phase system I1 is then the reversed-phase system relative to phase system I.
7.2 ROLE OF THE SUPPORT IN LIQUID-LIQUID CHROMATOGRAPHY
In LLC, the porous packing material is used to support the stationary liquid phase. The support surface should be homogeneously coated with a liquid layer of defined thickness. In this way a maximum liquid-liquid interface is produced, permitting a large number of distribution stages in the column comparable to that of a multistage extraction cascade. A necessary condition for depositing a liquid on a porous support is the wetting of the surface by the liquid, ie., the contact angle, 8,between the liquid and the surface should be less than 90”. The deposited liquid is then held by adsorption and capillary forces. Depending on the polarity of the stationary phase, either a polar or a non-polar support is required. In practice, the homogeneous dispersion of a liquid on the inner surface of a porous support creates some problems. Owing to the geometric heterogeneity of the surface and to the irregularity of the pore shape, the thickness of the deposited layer cannot be considered as constant but varies over a wide range. This occurs particularly with supports that have small pores and/or pores of the ink-bottle type, i.e., narrow openings and wide bodies. As a result of this uneven distribution, (i) adsorption interaction may contribute to the retention and (ii) the rate of mass transfer of the solute between the stationary phase and the non-moving liquid held in the pore space may become slow so that the column performance deteriorates.
239
Taking the above points into consideration, the porous support should offer a low specific surface area with large pores. Suitable supports of this type are macroporous silica and diatomaceous earths, the latter being a natural product which is subjected to an after-treatment. Although the pore structure parameter of diatomaceous earths [5] satisfies the support requirements, the softness of the particles limits their use as packings in HPLC. As listed in the Appendix, macroporous silica packings are commercially available with a wide range of graduated mean pore diameters. Nevertheless, one can find a large number of separations in LLC that are performed on mesoporous silicas that exhibit specific surface areas of a few hundred square metres per gram. In operating liquid-liquid systems on these supports, the solutes are retained by a mixed retention mechanism due to adsorption plus partition, which may sometimes lead to a specific selectivity different from that in pure partition. However, it has been found that the stability and reproducibility of such phase systems are much worse than those of liquid-liquid systems using low-surfacearea supports. As there is still some uncertainty and confusion about the pore structure parameters of silica supports used in LLC, this subject will be treated here in more detail in a qualitative way. Some quantitative correlations are given in Section 7.4. The specific pore volume, V p , of the support limits the maximal load of a support with a liquid stationary phase. The load can be expressed as (i) grams of liquid stationary phase per gram of unloaded support, or (ii) grams of liquid stationary phase per gram of loaded support. The latter quantity should preferably be used, particularly when the load is expressed in weight per cent. Large-pore silicas usually exhibit a Vp ranging between 0.5 and 1.O ml/g, and sometimes exceeding 2.0 ml/g [6]. There is also a lower limit of liquid loading, which is achieved when the mobile phase saturated with the stationary phase is pumped through the column fop a sufficient time. Under steady-state conditions, a certain amount of stationary liquid covers the surface as an adsorbed multilayer. The load can be gradually varied between the minimum and maximum. In order to maintain a constant load and constant distribution coefficients (capacity factors), the column should be thermostated. Using diatomaceous earths of low surface area as supports, Huber er ul. I41 have shown that the distribution coefficients of steroids in ternary liquid-liquid systems derived from static measurements are in excellent agreement with those calculated from chromatographic measurements using the equation tRi = to(1
+ qcKjo)
(7.7)
where q is the phase ratio. This finding indicates that pure partition is possible and the contribution of adsorption to retention is negligibly small. By using high-surface-area supports, adsorption effects may become increasingly important towards retention, which is indicated by (i) a non-linear relationship between k' and the liquid load of the given system; and (ii) a positive intercept on the ordinate on extrapalating to zero loading in the plot of k ' versus liquid load. It should be mentioned that the plots of retention time and retention volume versus liquid loading have positive intercepts at zero loading that are due to ro and Vo, respectively (see also eqn. 7.7). Although there have been only a limited number of studies in LLC on the influence of
240
the specific surface area on the retention characteristics, one can generally conclude that a support with SBET< 5 m2/g manifests a pure partitioning mechanism. The mean pore diameter, D, is usually correlated to the specific surface area in the following way, as shown for the following series of silicas: SBET (m*/g): D (nm):
300 12.7
45 44
15 123
2 400
It can be seen that low-surface-area supports have large pores. Concerning the use of supports in LLC, there is a limiting maximal value of the mean pore diameter related to the mean particle diameter of the support. If the mean pore diameter, D, is of the same order of magnitude as the mean particle diameter, d p, then the eluent flow may partially penetrate these large pores, which have nearly the same dimensions as the interstitial voids between the particles. Under these conditions the liquid stationary phase held in the large pores is stripped out by means of erosion. As a consequence, in order to ensure a stable system, a significant difference in D between the two separate pore systems should be established. In practice, the mean pore diameter of the packing should not exceed one tenth of the mean particle diameter. For instance, at d p = 10 pm, D should be smaller than 1000 nm. Fig. 7.1 shows an electron scanning micrograph of a large-pore silica microparticle which gives a fairly good illustration of the relationship between the mean pore diameter and the mean particle diameter.
Fig. 7.1. Electron scanning micrograph of a large-pore silica (LiChrospher Si 4000, dp = 10 pm, D = 400 nm). Scale: 1 mm 4 100 nm.
24 1
Another important feature in the use of silica supports in LLC is the chemical nature of the surface. In order to provide a reasonable attraction of the liquid stationary phase, the surface should be polar with a polar stationary phase and non-polar with a non-polar stationary phase. In utilizing non-polar stationary liquids such as squalane, the surface of the silica support has to be silanized by treatment with trimethylchlorosilane, dimethyldichlorosilane, hexamethyldisilazane or long-chain n-alkyltrichlorosilanes. Although an apparent dense coverage may be achieved, coating defects on the modified surface still exist so that the surface is not completely hydrophobic. As a result, the wetting of nonpolar stationary liquid phase will be incomplete, which leads to a diminution of the stability of the column.
7.3 PREPARATION OF COLUMNS IN LIQUID-LIQUID CHROMATOGRAPHY There are different techniques in use for the preparation of columns that contain a liquid-coated packing [7] ,and these are considered below. 7.3.1 Solvent evaporation technique This procedure is common in the preparation of columns in gas-liquid chromatography. A certain amount of the liquid to be used as a stationary is dissolved in an excess of a volatile solvent. For instance, with 3,3 '-oxydipropionitrile (ODPN)as a stationary phase, dichloromethane is employed. The solution is added to a known weight of dry support, then the solvent is slowly removed by rotating the suspension using a rotary evaporator until a dry and free-flowing powder is obtained. The technique described can be used only for supports with d p > 20 pm which are dry packed after loading in the column. The procedure is not applicable to microparticles in the S-lO-pm size range which are packed by means of the slurry technique because in most instances the stationary liquid dissolves in the slurry liquid. For such type of packings the following two techniques are recom mended. 7.3.2 In situ coating technique According to this technique, the loading occurs on the pre-packed column. The precipitation of the liquid in the pores of the support can be performed in various ways. One method is to pump the liquid mobile phase saturated with the stationary liquid phase through the column until a constant load is obtained [8]. The achievement of a steady state can be controlled by measuring the capacity factor of a test solute on repeated injections under constant conditions: at the steady state, the capacity factor should approach a constant value. There is no general agreement in the literature about the question of whether the minimal load obtained by this technique really represents an equilibrium [7], but it is apparent that this load corresponds to the minimal value at which the column can be operated and, provided that the capacity factors remain constant, reproducible separations can be performed. Higher loadings than the minimum can be gradually achieved by injecting small portions of stationary liquid into the flow ci mobile phase saturated with
24 2
the stationary phase and passing it through the column [4]. The maximal load is attained by pumping the pure liquid stationary phase through the dry, pre-packed column for a sufficient time, followed by pumping the saturated mobile phase. In this way the stationary liquid phase held in the interstitial voids of the column is displaced and a fully loaded pore space remains. 7.3.3 Precipitation technique This procedure is based on precipitation of the stationary liquid within the pore system. The stationary liquid is dissolved in a good solvent at a relatively high concentration and the solution is pumped through the pre-packed column. Then the eluent is displaced by a solvent that is immiscible with the stationary liquid, which results in precipitation of the latter in the pores of the support. Again graduated loadings can be obtained by varying the concentration of the stationary liquid in the first eluent. The percentage loading may range within a few per cent for pellicular type of supports [9] and may achieve more than 50% for porous packings, as shown in Table 7.1. The maximal liquid load in terms of volume units, I/s(max), equals the total specific pore volume of the packing in the column. For an accurate measurement of V,(ma,), a known amount of support is titrated with the stationary liquid according to the procedure of Fisher and Mottlau [lo]. The volume of the stationary liquid phase at the minimal load, Vg(min), can be calculated by using the following equation
where Vm(mh) and L'm(mm) are the volumes of the mobile phase at the minimal and maximal load, respectively. The volume of mobile phase at any given load is given by
v,
(7 -9)
=f"fo
where f v is the flow-rate af the eluent and to is the retention time of an unretained solute under the given conditions;f, and toshould be measured at various flow-rates in order to TABLE 7.1 MINIMAL AND MAXIMAL LIQUID LOADS OF 3,3'-OXYDIPROPIONITRlLE ON A SERIES OF SILICAS [ 6 ] Type of silica
HK 1 HK 2 HK 3 HK 4 LiChrospher Si 100 LiChrospher Si 500 LiChrospher Si 1000
dp Cm)
-
g ODPN/g unloaded silica (g ODPN/g loaded silica) 100 (%)
Minimum
Maximum
Minimum
Maximum
1.31 1.99 3.03 3.48
28.0 30.5
6.5
0.39 0.44 0.37 0.38
56.8 66.5 15.2 71.1
10.0 10.0 10.0
0.35 0.17 0.13
1.79 0.88 0.68
6.5 6.5 6.5
27.0 27.6 25.8 14.6
11.1
64.1 46.1 40.3
24 3
derive an average value for V , . The quantity VJV,, termed the phase ratio, q , can be determined by a procedure proposed by Huber et al. [4] as follows. For a series of solutes in the given system, the retention times measured are plotted against the distribution coefficient of solutes, ‘Kj0, derived fmm static measurements. Provided that retention is based on a pure partition mechanism, the function according to eqn. 7.7 gives a straight line whose slope s corresponds to the phase ratio q : (7.10) and the positive intercept i equals t o as to =
~
R - ~~
‘K2o
-
‘Ki o
R~
~K
~
~K
~
~
(7.1 1)
An important feature in the operation of a liquid-liquid system is the stability of the column. Bleeding may occur by the loss of stationary liquid either due to dissolution in the unsaturated mobile phase or due to mechanical stripping from large pores of the support at high flow-rates. Thus, necessary conditions for reproducible and accurate work are (i) saturation of the mobile phase with the stationary liquid phase, or the use of a precolumn exhibiting a high capacity with respect to liquid load; (ii) thermostating of the system, including the column and the eluent reservoir. Emphasis should be also directed to the temperature gradient between the top and the end of column at high flowrates, i.e., at high pressure drops. This temperature difference may affect the reproducibility, because the capacity factor is temperature dependent. By careful adjustment of the conditions a column in LLC may remain stable for several months [ l l ] .
7.4 EFFECT OF SILICA SUPPORT PROPERTIES ON RETENTION AND COLUMN PERFORMANCE Assuming pure partition as the retention mechanism, the capacity factor, k ’ , of a solute in a given liquid-liquid system should increase linearly with an increase in the amount of liquid stationary phase. This linear dependence was established experimentally only with low-surface-area supports. When high-surface-area silicas are employed as supports, adsorption interactions additionally contribute to retention and the above dependence then deviates from linearity, as is exemplified lor the system 3,3 ’-oxydipropionitrile-n-heptane on LiChrospher Si 100 (dp = 20 p )as the support [6] (see Table 7.2). Whereas for the last three solutes in Table 7.2 the capacity factor increases with increasing load, a specific course is observed for the homologous series of m-oligophenylenes. These findings were also confirmed by Engelhardt and Weigand [ 121 for a similar liquid-liquid system (ODPN-n-hexane) but with different solutes. Various models have been developed for establishing the influence of adsorption effects on retention in gas-liquid and liquid-liquid systems [13-161. The approach introduced by Conder et al. [ 161 was used by Asshauer and Halasz [ 171 for a quantitive
244 TABLE 7.2 CAPACITY FACTORS OF SOLUTES AT VARIOUS LIQUID LOADINGS ON A SILICA COLUMN [ 61 Liquid stationary phase, ODPN; support, LiChrospher Si 100, d p = 20 gm; eluent, n-heptane, saturated with ODPN; temperature, 298 K; column, length 250 mm, I.D. 4.2 mm. Solute
Liquid load [(g ODPN/g loaded support) -1001 (%)
28.57 Tetrachloromethane Benzene Diphenyl m-Terphenyl mQuaterpheny1 m-Quinquephenyl m-Hexaphenyl Acetophenone Nitrobenzene Dimethyl phthalate
0.29 f lo%* 0.50 0.83 1.45 2.58 4.50 8.03 3.80 3.43 17.97
37.50
44.45
50.00
0.38 0.63 0.91 1.31 2.09 3.42 5.30
0.36 0.73 1.01 1.40 2.05 3.08 4.96
0.46 0.90 1.13 1.39 1.85 2.42 3.42
3.99 4.18 20.14
6.06 6.79 29.09
7.50 8.27 35.85
*Standard deviation.
study of adsorption effects on retention in gas-liquid chromatography. Asshauer and Halasz [ 171 found that even at high loadings of ODPN on a silica support (Porasil A, SBET= 450 m2/g), adsorption interactions contribute to a large extent to retention. This concept was also applied to LLC to evaluate the magnitude of the adsorption effect when using high-surface-area supports [6,12,18]. Taking only liquid-solid interactions into consideration and neglecting the liquid-liquid interface adsorption term, one obtains for a given solute, i, the relationship (7.12)
where KAs(j) ml/cm2 is the adsorption coefficient of solute i, V,“ ml/ml the volume of stationary liquid normalized to unit volume of the column, ‘Kjodyn the distribution coefficient derived from the chromatographic retention: (7.13) ‘KioStat the distribution coefficient at infinite dilution of solute i estimated from batch
experiments and A: cm2/ml the specific surface area of the support normalized to unit volume of the column. For the system ODPN-n-heptane, the ‘KjodYn and ‘KjoStat values are listed in Table 7.3 for a silica support at different loadings [6]. By comparing the data in Table 7.3 with respect to the differences between ‘Kjodyn and ‘Kjostat, it can be seen that at the minimal liquid load adsorption is dominant over partition in retention, With increasing load the ‘KiodYn values of a given solute decrease considerably but with a 50% load, which is lower than the maximum (64.1 %, w/w), t‘i; dynamic distribution coeffi’ cients are of the same order as the static values or sligh:’ 1able 7.4 gives the KA# values calculated according to eqn. 7.12 for the same sol , a5 111 Table 7.3. ’
245
TABLE 7.3 STATIC AND DYNAMIC DISTRIBUTION COEFFICIENTS OF SOLUTES AT VARIOUS LOADINGS Conditions as in Table 7.2. Solute
CK.dyn
CK.
lo
stat
10
Load [(g ODPN/g loaded silica) .100] (%) 28.51
37.50
44.45
50.00
Tetrachloromethane Benzene Diphenyl m-Terphenyl m-Quaterphenyl m-Quinquephenyi m-Hexaphenyl
1.10 1.89 3.15 5.50 9.81 17.11 30.51
0.79 1.29 1.87 2.69 4.29 7.04 10.90
0.51 1-05 1.45 2.00 2.92 4.39 7.09
0.42 0.81 1.02 1.26 1.67 2.18 3.09
Ace tophenone Nitrobenzene Dimethyl phthalate
14.44 13.04 68.28
8.21 8.59 41.42
8.65 9.70 41.52
6.76 1.46 32.33
0.20 0.60 0.90 1.11 1.40 1.65 1.79 4.38 4.49 30.13
TABLE 7.4 ADSORPTION COEFFICIENTS ( K A ~ml/crn’) , OF SOLUTES AT VARIOUS LOADINGS [ 61 Conditions as in Table 7.2. Solute
Load [ (g ODPN/g loaded silica) 1001 (%) 28.51
37.50
44.45
50.00
Tetrachloromethane Benzene Diphenyl m-Terphenyl mQuaterpheny1 m-Quinquephenyl m-Hexaphenyl
7.85 3.78 4.36 6.94 10.50 16.42 28.03
7.59 3.02 2.79 3.72 5.35 8.50 13.21
5.36 2.57 2.08 2.77 3.10 5.70 10.10
4.82 1.58 0.58 0.61 0.85 1.45 3.24
Acetophenone Nitrobenzene Dimethyl phthalate
4.01 3.33 2.21
3.87 3.28 1.97
3.32 3.07 1.29
2.42
2.36 0.33
In the homologous series of m-oligophenylenes, the K A values ~ are increased considerably on increasing the number of n-systems per molecule from benzene to m-hexaphenyl at a given load. A plausible explanation for this result is that (i) the mystems are capable of undergoing specific adsorption interactions with the surface of the silica support, and (ii) the solubilities of the solutes in the stationary liquid decrease with increasing molecular weight, the difference between the homologues always being 77 molecular weight units. With gradual filling of the pore space the stationary liquid more and more acquires bulk ~ to approximately zero. properties and K A tends
24 6
An extended study of the dependence of ‘Kjodyn measured chromatographically on the specific surface area of the support was made by Rossler and Halasz [ 181. The liquidliquid system was ODPN-n-heptane and the support was Porasil with various SBETvalues. The supports were coated to the maximal load. The results supported the previous statements that the capacity factors of solutes become independent of adsorption interactions when SBET< 10 m2/g, and then correspond to that obtained on silanized Chromosorb W.The dependence of k’ on SBETfor the supports was found to be more pronounced for compounds that are capable of specific interactions with surface hydroxyl groups than for those which interact solely by dispersion forces. When using low-surface-area supports at maximum load, caution is required in applying high flow-rates because the stationary liquid is stripped out of the pores. At high flow-rates in the range 5-10 cmlsec the specific surface area should be not less than 50 mz/g, which corresponds approximately to a mean pore diameter of 50 nm. In accordance with the load, columns packed with heavily loaded support exhibit a high linear capacity, doel;Bo.l was estimated to be about 2-100 mg of sample per gram of stationary liquid and hence considerably exceeds the capacity of common columns. To summarize the results, one can distinguish two cases in the selection of a support, depending on its surface area: (1) high-surface-area silica supports loaded to the maximum can be run at high flowrates, offering a high linear capacity; retention and selectivity are then influenced by support effects, which may or may not be an advantage; (2) low-surface-area supports (SBET << 10 mz/g) can be moderately loaded with liquids with respect to high-flow-rate operation; In this instance partition is the dominant retention mechanism. Most of the studies dealing with highly efficient and highly selective separations using liquid-liquid systems utilize high-surface-area silica supports [ 19-21]. It should be clearly stated that Zipax, being a pellicular material, also belongs in this category [ 191. Its specific surface area is about 1 mZ/gwhen related to the total mass, but this value is a factor of 100 larger when it is related t o the mass of the thin porous layer. An interesting but difficult problem is to evaluate the dependence of the column efficiency, Le., the plate height (H), on the amount of liquid load. At maximum load the liquid stationary phase fills the whole pore space of the support, whereas at lower loadings a fraction is filled by the non-moving eluent. One would expect that a variation of the liquid load between the maximum and minimum would significantly affect the rate of mass transfer of solutes between the phases and hence the slope of the plot of Huersus u , particularly at high velocities. Theoretical treatments of the mass transfer of solutes lead to equations that involve the thickness of the liquid layer, the effective diffusivity of the solute and its capacity factor. These quantities vary with the liquid load, and it is difficult to establish the correlation of any one of these parameters with experimental results such as the slope of the H versus u curve. Snyder and Kirkland [7] found no difference in plate heights of solutes that have the same capacity factors on Zipax at different loadings. Other results do not confirm this result [ 6 ] .The data presented in Table 7.5 for the C terms and capacity factors of various solutes at the two loadings reveal that a simple interpretation and correlation is not possible. If one recognizes that the plate height being measured
247 TABLE 7.5 C TERMS AND CAPACITY FACTORS OF SOLUTES AT MINIMAL AND MAXIMAL LOADS [6]
Stationary liquid, ODPN; support, HK 2 porous silica, SBET= 363 m*/g, V p = 2.07 ml/g, d p = 6.5 Hm; temperature, 298 K; eluent, n-heptane, saturated with ODPN; column, length 250 mm, I.D. 4.2 mm. The C term corresponds to the slope of the H versus u curve between u = 1.0 and 5.0 mm/sec. Solute
Minimal load, 30.5% (w/w) C(rnsec)
Maximal load,
66.5% (w/w)
k'
C(msec)
k'
-
Tetrachloromethane Benzene Diphenyl m-Terphenyl m-Quaterphenyl m-Quinquephen yl m-Hexaphenyl
26 12 17 25 30 35 38
0.07 0.13 0.27 0.48 0.85 1.47 2.58
26 37 52 95 97 115 144
0.19 0.77 1.00 1.20 1.50 1.92 2.52
Acetophenone Nitrobenzene Dimethyl phthalate
38 10 9
1.99 1.07 8.94
14 8 12
7.24 9.04 36.40
~
reflects not only the contribution of the rate of mass transfer but also other effects, an estimation of or statement about the optimal load concerning column efficiency remains highly speculative.
7.5 REFERENCES 1 A.J.P. Martin and R.L.M. Synge, Biochem. J., 35 (1941) 194. 2 B.L. Karger, L.R. Snyder and C. Horvath (Editors), A n Introduction to Separation Science, WileyInterscience, London, 1973. 3 J.F.K. Huber,J. Chromatogr. Sci., 9 (1971)72. 4 J.P.K. Huber, C.A.M. Meijersand J.A.R.J. Hulsman,Anal. Chern.,44 (1972)111. 5 H. R k l e r , in E. Stahl (Editor), Diinnschichtchromatographie, Springer-Verhg, Berlin, 1967, pp. 27-29. 6 H. Hauck, Thesis, Technische Hochschule, Darmstadt, 1976. I L.R. Snyder and J.J. Kirkland, Introduction to Modem Liquid Chromatography, Wiley-Interscience, New York, 1974,pp. 208-214. 8 B.L. Karger, H. Engelhardt and K. Conroe, in B. Stock and S.G. Perry (Editors), Gas Chromatography, Institute of Petroleum, London, 1971,p. 124. 9 J.J. Kirkland, Modern Practice of Liquid Chromatography, Wiley-Interscience, New York, 1971. 10 N.E. Fisher and A.Y. Mottlau, Anal. Chem., 34 (1962)714. 1 1 R.E. Leitch,J. Chromatogr. Sci.,9 (1971)531. 12 H. Engelhardt and N. Weigand, Anal. Chem., 45 (1973)1149. 13 R.L. Martin, Anal. Chem., 35 (1963)116.
248 14 15 16 17 18 19 20 21
V.G. Berezkin and V.M.Fateeva,J. Chromatogr., 58 (1971) 73. B.L. Karger, R.C. Castells, P.A. Sewell and A. Hartkopf, J. Phys. Chem., 75 (1971) 3870. J.R. Conder, D.C. Locke and J.H. Purnell, J. Phys. Chem., 73 (1969) 700 and 708. J. Asshauer and I. Halasz,Anal. Chem., 45 (1973) 1142. G. RSssler and I. Halasz,J. Chromatogr., 92 (1974) 33. J.J. Kirkland, J. Chromatogr. Sci., 10 (1972) 593. C. Gonnet and J.L. Rocca, J. Chromatogr., 120 (1976) 419. E.P. Horwitz, C.A.A. Bloomquist and W.H. Delphin,J. Chromatogr. Sci., 15 (1977) 41.
249
Chapter 8
Chemically modified silica as packing in ion-exchange chromatography Ion-exchange chromatography, combining the phenomena of adsorption and ionic exchange interaction, is a very promising field in high-performance liquid chromatography (HPLC) especially for separations in biochemistry. Considerable progress has been made in recent years in the development of novel ion-exchange packings. Highly cross-linked and pressure-stable microbeads in the 5-10 pm size range have been synthesized by copolymerization of styrene and divinylbenzene and are now commercially available. As an alternative pellicular ion exchangers were introduced that consist of an impenetrable nonporous core and a thin polymer or silica layer containing ionic fixed sites [ I ] . Though with these materials the separations were rapid the sample capacity was relatively low. For this reason, non-swelling, totally porous and surface-modified silica ion-exchange microparticles seem to provide a reasonable compromise between the pellicular and the organic types of resin. This chapter deals exclusively with the current state of silica-based ion exchangers and discusses their preparation, characteristic properties and selectivity in both batch and column operation.
8.1 SELECTIVITY AND KINETICS OF ION EXCHANGE
To become familiar with the complex separation mechanism in ion exchange, it is advisable to start with some basic considerations. An ion-exchange particle, as shown schematically in Fig. 8.1, consists of a crosslinked or porous matrix that carries ionic functional sites X, which are negatively charged in the case of a cation exchanger. In column operation the charge of X-is compensated for the most part by mobile counter ions El+ provided by the eluent and to a lesser degree by counter ions A' and B+, the charged species to be separated. Y-designates the co-ions maintaining electroneutrality in solution. Let us consider the competition of the two monovalent counter ions A' and B' with El' for the fixed ionic sites X-. In analogy to ordinary chemical reactions we may describe the situation as IzAI(X- + El')
+ AzA + (IzAIX- + AZA) + IzAIE~'
(8.1)
where Z A and ZB are the charges of ions A+ and B+. For simplicity Z A = Z B and Z E ~= 1 Applying the law of mass action to eqns. 8.1,8.2, the respective thermodynamic equilibrium constants are given by the following activity quotients
250
Fig. 8.1 Schematic representation of a cation-exchange particle. Dark Line, cross-liiked matrix; X-, negatively charged functional group chemically bonded to the matrix; El+, eluent ion (counter ion); A*, B+, counter ions to be separated; Y', co-ion.
lZBl
KO$ = aEl El
liEI'ZB
-
aB
' ag
(8.4)
where quantities with a bar refer to the exchanger phase. By use of the two constants, KZ: and KZF the thermodynamic separation factor is given by:
and in this case (yo is identical to the selectivity constant. Since chromatographic theory preferably makes use of classical, concentration-based equilibrium terms, we may express the separation factor by the two distribution coefficients:
and
where [A] and [B] represent the molar concentrations of the counter-ions in the solution A is termed and [A]and [GI their corresponding concentrations in the exchanger phase. KB the selectivity coefficient. The distribution coefficients, KA and KB, can be expressed by the classical constants KA and K[l, as
25 1
and (8.10)
As the eluent ion, El*, occupies practically all accessible charged sites of the exchanger phase, the numerators of the last terms on the right hand sides of eqns. 8.9 and 8.10, as well as K A and K i can be considered as constant. Taking logarithms we get the two equations : logKA = - l z ~ I log [El] +constant
(8.1 1)
logKB =-lzgI log [El] +constant'
(8.12)
yielding straight lines with slopes of - Iz I if plotted as log K against log [El] [2]. In other words, both quantities KAand K B depend on the eluent concentration, which also influences the separation factor, a.With weak bases and acids that are involved in a pHdependent dissociation reaction, the pH controls the separation to a large extent. Complexing may also be a powerful means of influencing the selectivity, provided that the ions to be separated can form complexes. Of course there are additional factors underlying ion-exchange selectivity [3], the most important being: the nature of the counter ions and fixed ions; the surface concentration and distribution of the fixed ions within the exchanger matrix; the pK, and pKb values of all ionic functional groups participating in the exchange reaction; the type of matrix, i e . , its chemical composition; the pore structure and swelling behaviour of the ion exchanger; and the eluent composition, i.e. its polarity . To summarize as a selectivity rule, the ion exchanger tends to prefer: the ion of higher valence (electroselectivity); the ion with the smaller solvated equivalent volume; the ion with the greater polarizability; the ion which interacts more strongly with the fixed ionic groups or with the exchanger matrix; the ion which has the weakest interaction with other constituents of the eluent. It has been found in most cases that selectivity increases with increasing capacity and crosslinking of the exchanger and decicases with increased temperature and concentration. Most of the selectivity studies reported refer to organic ion exchangers and inorganic ions, and result in the well known selectivity sequences [4]. Only a few investigations have dealt with the ion-exchange selectivity of charged organic species [S]. In this particular instance hydrophobic bonding between the non-polar part of organic molecules occurs, changing the water structure and forming hydrogenbonded clusters. In addition to this effect, which tends to force the organic ions from the solution into the non-polar exchanger phase, the hydrophobic parts of organic ions interact with the non-polar groups of the exchanger by dispersion forces. This may lead to a
25 2
specific selectivity which sometimes can become dominant over that caused by electrostatic forces. Steric exclusion effects may also take place when the solvated organic ions are larger than the pore diameter of the exchanger. As selectivity in ion-exchange is affected by a variety of factors and is difficult to predict, it is advisable to measure it in simple batch experiments and to draw conclusions on chromatographic selectivities from these data. This method was mainly used by Inczidy and co-workers [6-91 in separating organic bases and acids. The kinetics of this reaction are a very important aspect of ion exchange. According to the experimental results of numerous investigators ion exchange, in practically all cases, is mainly a diffusion-controlled process. Considering the migration of counter ions from a stirred solution or from a solute band travelling through a column into the ion-exchange particles, two diffusion processes can be distinguished: (1) diffusion through a liquid layer of given thickness adhering to the particle surface (film diffusion); (2) diffusion within the ion exchanger particles (particle diffusion). As both processes are sequential, the slower one is the rate-controlling step. There are criteria developed by theoretical treatments that permit a prediction of the rate controlling step by taking into account the capacity, the solution concentration, the particle size, the frlm thickness and the diffusion coefficients [ 101. It is also possible to distinguish experimentally between film and particle diffusion. The criteria mentioned above can be expressed in a more qualitative way:
Conditionsfor filmdiffusion
Conditionsfor particle diffitsion
High concentration of ionic functional groups Low solution concentration (C < 0.01 M> Low degree of crosslinking Small particles Moderate stirring or decreased flow-rate
Low concentration of ionic functional groups High solution concentration (C > 0.1 M) High degree of crosslinking Large particles Vigorous stirring or high flow-rate
As in HPLC ion-exchange microparticles and low concentrations are employed, film diffusion is believed to be the rate-determining step. The effect of particle size distribution on the kinetics of ion exchange was treated thoroughly in a recent paper [ 111.
8.2 ION EXCHANGERS BASED ON CHEMICALLY BONDED SILICA
Commercially available silica-based ion exchangers differ widely in their pore structure and in the type of attachment of their ionic functional groups. Considering the porosity, the exchangers can be divided into totally porous microparticles, of spherical or irregular shape, and pellicular particles bearing a thin porous layer deposited on a non-porous core. Whereas totally porous microparticles offer porosities of about 50%and more, pellicular materials have a porosity of less than 10% relative to the total volume of the coated cores.
25 3
Ion exchangers made by means of surface modification of a silica of given porosity are nonswelling and hence exhibit a so-called permanent porosity. Other materials synthesized by means of bulk modification as treated in Section 3.2 may swell to a considerable extent and give rise to porosity due to swelling in addition to the permanent porosity. The latter type of exchanger is comparable in its behaviour to macroreticular (macroporous) organic resins [ 121. Special attention should be given to the distributions of porosity and pore volume as a function of the mean pore diameter. For kinetic and other reasons a pore volume distribution in the mesopore range of 3 < D < 50 nm is preferred. The presence of micropores is undesirable as the diffusion rate of charged solutes into and out of these small pores is drastically diminished and micropores also give rise to specific steric exclusion effects. On the other hand it is advantageous to produce some macropores that form the entrances to the mesopores and hence facilitate the mass transport. There is still another reason for the preference of the mesoporous type: as it exhibits a sufficiently large specific surface area, surface modification yields a relatively high content of ionic functional sites and a high capacity compared with low-surface-area supports. For these reasons most totally porous and pellicular exchangers that are now marketed are of the mesopore type (see Appendix). 8.2.1 Synthesis
There are different possibilities for attaching an ionic functional group via an anchor group to the siloxane matrix. As previously discussed in Section 3.2, chemical modification can be performed either by bulk modification or by surface modification. Bulk-modified products are made by co-hydrolysis, co-condensation and co-polymerization of an appropriate trifunctional organosilane and a tetrafunctional tetraalkoxy- or tetrachlorosilane. The final products are crosslinked polyorganosiloxanes built up from the following structural units:
I
O
R
I I -0-Si-O-Si-OI I 0 0 I I Another possible way of bulk modification is the co-polymerization of vinyl monomers such as styrene, divinylbenzene or vinylpyridine, and organosilanes, yielding totally porous particles or layers grafted onto non-porous beads. As with bulk modification, surface modification can be realized in various ways. One route consists of a consecutive two-step reaction. Firstly, a carbonaceous back-bone is or Si-CE link by the reaction of a selective organosilane bonded via a Si-0-Si-CE with silica: *i-OH
I + X-Si-R
+
I ESi-0-Si-R
+ HX
(8.13)
254
and a i - X t R-Me
-+
S i - R t MeX
(8.14)
where X is a halogen. Depending on the functionality of the modifier and on the presence or absence of water during the reaction, a monolayer or a polymer-layer type of support is obtained. The organic radical may be one of the following:
phenyl
a
naphthyl
trlphenylmethyl
-c
(Q),
- ( C H2In-C
n-alkyl
H3
alkylphenyl
The acidic or basic functional groups are then introduced by subsequent reactions such as sulphonation, chloramethylation followed by amination, etc. Another possible route consists of the reaction between silica and an organosilane of type X-&-R’-Y
I
- (CH2)3-
where R‘-Y is a group such as:
NH2
-(CH21,-NH-CH2-CH2-NH,
By appropriate substitution of the organic radical (route 1) or by suitable choice of a selective reagent (route 2) a wide variety of ionogenic functional groups are available (see Table 8.1). They differ in their acid and base strength according to their pK, and pKb values. Even in -the case of a complete surface coverage by the organofunctional groups a relatively large number of surface hydroxyl groups still remain unreacted and operate as
255
TABLE 8.1 TYPES OF IONIC FUNCTIONAL GROUPS ATTACHED VIA A CARBONACEOUS BACKBONE TO THE SILICA SURFACE Acidic groups
Basic groups
Amphot eric groups
-S03H -COOH -CH2 -COOH
-CH,N(CHs),CI -CH,N(C, H,),CI -CH2 N(CH ,)2 C,H40H C1
-CH-CH,-CH,-NH,
-CH-CH2
-CH,NH(CH,),CI -CH2NH(C2H,)2CI -CH,-NH,
I
I
OH OH
I
COOH -(CH2),0-CH2 -CH-CH, 1
1
OH NH,
weak acidic sites in ion-exchange separations. Due to this specificity in their surface structure, silica-based ion exchangers are considered as bifunctional. Of course these structural properties are responsible for some peculiarities concerning the selectivity behaviour. In the following sections the preparation procedures are treated in detail.
8.2.I. 1 Preparation of surface-modified ion exchangers Runge and Zimmermann [ 131 described a procedure in which a porous dehydrated silica is treated with benzyltrichlorosilane. The benzylsilica is then sulphonated, yielding a strong cation exchanger. In a comprehensive investigation, Neimark and co-workers [ 14161 obtained a series of surface-modified silica ion exchangers that were characterized by means of infrared spectroscopy, elemental analysis and adsorption methods. A phenyItrichlorosilane-treated silica was converted into a cation exchanger by reaction with oleum or chlorosulphonic acid [ 171. Chlorosulphonic acid was found to be preferable to oleum because it reduces the heterolytic cleavage of the moderately polar Si-C bond. The formation of cation exchangers was also investigated on silica modified with diphenyldichlorosilane and triphenylchlorosilane [ 18,191. According to eqn. 8.14, both cation and anion exchangers were prepared by the Grignard reaction of a halogenated silica with naphthylmagnesium bromide and subsequent substitution reactions [20]. In the same way, chlorinated silicas were reacted with various organolithium compounds such as phenyllithium, naphthyllithium and triphenylmethyllithium [21]. It was established that about half of the Si-phenyl and Si-naphthyl bonds were cleaved during sulphonation, whereas the Si-C bonds in the triphenylmethylsilyl groups proved to be relatively stable. Differences in the degree of sulphonation between the three products were also observed. As a suitable packing for the separation of proteins a weak anion exchanger called Silochrom was prepared by reaction of y-aminopropyltriethoxysilane with macroporous silica [22]. A comprehensive study of the preparative aspects of the formation of sulphonic acid cation exchangers was made by Weigand et al. [ 2 3 ] ,who used n-alkylchlorosilanes, their w-bromo derivatives, and phenylalkylchlorosilanes as backbone silanizing reagents.
256
Sulphonation of unsubstituted n-alkyl and alkylphenyl groups was performed with oleum, chlorosulphonic acid and sulphonyl dichloride, whereas o-bromo-n-alkyl bonded silicas were sulphonated by reaction with sodium sulphite and sodium hydrogen sulphite, respectively. The degree of Si-C cleavage by sulphonation was reported to vary between 25 and 65% relative to the amount of the initial organosilyl groups. The Si-C bonds of the benzylsilyl and phenylpropylsilyl groups proved to be the most stable. Because of its superior chemical stability, benzyl silica has aroused particular interest with regard to the preparation of silica based ion exchangers [24-271. In agreement with earlier findings [171 chlorosulphonic acid as sulphonating agent was found to give negligible bond cleavage. In chloromethylation with chloromethyl methyl ether, the use of zinc chloride as the catalyst gave a much higher conversion than tin(1V) chloride. About 80%of the total chloromethyl groups could be reacted in the sequential amination reaction. Another possibility for introducing a benzyl moiety as a carrier of ionic functional groups is by means of the reaction of chlorinated silica with benzyllithium [28,29]. Meiller [30] reported the synthesis of an anion exchanger by the reaction of ychloropropyltrichlorosilane with silica in xylene as solvent and treatment of the modified product with dibutylamine. Weak anion exchangers, namely a yaminopropylsilyl bonded silica and a 2-(4-pyridyl)ethylsilyl bonded alumina, were also synthesized by Knox and Pryde [3 I]. A strong anion exchanger was prepared by treatment of silica with chlorodimethyl-[4-(4-~hloromethylphenyl)butyl]silane [32,33]. The chloromethylated product obtained was subjected to further reaction with trimethylamine in methanol:
By conversion of the intermediate chloromethylated product into its nitrile derivative with potassium cyanide and subsequent reaction with sulphonic acid-acetic acid, a weak cation exchanger of composition
was obtained [33]. Another novel silane, chlorodimethyl-(4-phenylbutyl)silane, was utilized as a modifier by the same workers [33] to synthesize a back-bone for a strong cation exchanger with the following composition
A series of ion exchangers were prepared by Cox et al. [34]. A strong cation exchanger
was obtained by chlorosulphonation of 2-phenylethyltrichlorosilane-treatedsilica whereas
25 I
treatment of 3-chloropropyltrichlorosilane-reactedsilica with benzyldimethylamine led to strong anion exchangers. In order to produce a weak cation exchanger the authors used 0-alanine instead of benzyldimethylamine in the foregoing reaction. Gareil et ul. [35] reported a high-capacity anion exchanger prepared by reaction of diethylaminopropylsilane with silica. y-Glycidoxypropylsilane is a widely used modifier with numerous possibilities for introduction of ionic functional groups by subsequent coupling reactions [36,37]. Depending on the type of reagent that cleaves the oxirane ring, a weakly acidic diol group such as Si-(CH&-O-CHz
-CH-CHz
I
I
OH OH or an amphoteric hydroxylamine group such as Si-(CH&O-CHz-CH-CH2 I I OH NHz can be synthesized, as shown recently by Becker [37]. On the other hand, yglycidoxypropylsilyl-modified products permit a variety of selective reactions at the oxirane ring yielding anion and cation exchangers with the following ionic functional groups [36] : -N(CHZCH3)2 -OCH2CH2N(CH3)z -OCHzCHzN(C2H5)2 -(NHCHZCHZ)~NHz -0CHzCOOH -S03H
diethylamine (DEA) dimethylaminoethanol (DMAE) diethylaminoethanol (DEAE) polyethyleneimine (PEI) carboxymethyl (CM) sulphonic acid (SA)
Most of the bonded ion exchangers mentioned above belong to the monolayer type of surface-modified packings. There have also been attempts to deposit thick polymer layers with ionic functional groups within the pore space of silica. Kirkland and Yates [38] patented a procedure in which an organotrialkoxysilane dissolved in a non-aqueous solvent is pre-polymerized with an excess of water at elevated temperatures using an acidic catalyst. The reaction of the pre-polymerized silane with porous silica is carried out between 373 and 423 K in different ways: (i) The silica is stirred into the solution that contains the polymerized silane. After careful removal of the solvent, toluene or xylene is added to the wet powder and the water-solvent azeotrope is continuously distilled until the reaction is complete. (ii) The pore space of silica is loaded with the appropriate organotrialkoxysilane using the solvent-evaporation technique. The coated silica is then treated with moist air or steam to effect polymerization and reaction in one step. (iii) The silica is soaked in a solution containing organotrialkoxysilane being hydrolyzed in an acidic medium. The excess of solution is filtered off and the coated support is heated to 373 K for polymerization and reaction with the surface.
25 8
A corresponding anion exchanger can be prepared by hydrolysis and pre-polymerization of y-aminopropyltriethoxysilane:
(8.16)
373-423 K
The amino group can be quaternized by appropria e organic reactions ain a strongly basic anion exchanger. Similar polymerization reactions effected on the silica surface using y-glycidoxypropyltriethoxysilanewere reported by Chang et.al. [36].
8.2.1.2 Preparation of bulk-modified ion exchangers Only a few systematic studies have been made on bulk-modified ion exchangers based on polyorganosiloxanes with ionic functional groups. Runge et al. [39] reported on the synthesis of polyvinylsiloxanes by co-hydrolysis of tetrachlorosilane and vinyltrichlorosilane. By appropriate graft co-polymerization, acidic or basic groups were introduced. According to Wolf el al. 1401 cation exchangers were produced by co-precipitation of the sulphonic acid of benzylsiloxane and sodium silicate. The principle was utilized to synthesize anion exchangers by co-hydrolysis of tetrachlorosilane and amino-functional organosilanes [41]. In the patent literature one can find some poorly described procedures for the synthesis of polyorganosiloxanes with acidic and basic functional groups [42-451. A refinement of this approach was effected by Unger et al. [46]. Benzyltriethoxysilane and tetraethoxysilane were partially hydrolysed to a viscous liquid of oligomeric benzylethoxysiloxane with a mean molecular weight of about IOOO. The purified polybenzylethoxysiloxane was then emulsified in a water-ethanol mixture with vigorous stirring to form liquid droplets. Addition of a basic catalyst initiated the hydrolysis of the remaining ethoxy groups to form silanol groups which condensed to gelatinous beads of polybenzylsiloxane. After washing and drying, the hydrophobic porous beads were subjected to appropriate reactions to produce the respective anion or cation exchanger.
25 9
8.2.2 Characterization 8.2.2. I Ion-exchange capacity
According to the IUPAC recommendations on ion-exchange nomenclature, the capacity can be expressed by three quantities [47] : (1) theoretical specific capacity, Qo = milliequivalents of ionogenic groups per gram of dry ion exchanger in the H + or C1- form calculated by elemental (sulphur, nitrogen) analysis; (2) weight capacity, Q, = milliequivalents of ionogenic groups per gram of dry ion exchanger in the H+ or C1- form; (3) break-through capacity, QB = milliequivalents taken up per gram of dry ion exchanger or per cubic centimetre of bed volume. QB corresponds to the effective capacity of a bed of ion exchanger measured under specified experimental conditions as the amount of species that has been taken up when the species is first detected in the effluent or when the concentration in the effluent reaches some arbitrarily defined value. The most common method for determining Q, of cation exchangers in the H+ form is the direct titration procedure with 0.1 N sodium hydroxide solution. The capacity of anion exchangers in the C1- form is usually determined by titrating the displaced C1- ions with 0.1 N silver nitrate solution. A typical titration curve of a strong silica cation exchanger is presented in Fig. 8.2. The curve obtained is not symmetrical, but shows a deviation in the upper part at pH > 6. The additional consumption of sodium hydroxide in this range may be due to the neutralization of the weakly acidic hydroxyl groups that remain after surface modification. In consistency with this observation, the weight capacity, Q, , should exceed the theoretical specific capacity derived from the sulphur content. This was confirmed for sulphonic benzylsilica [25]. Thus an evaluation of Q , on the basis of the titration curve is rather imprecise. A much better procedure is as follows [48]: Treat about 0.5 g of cation exchanger with a large excess of 1 N hydrochloric acid to convert it to the H + form. Place the exchanger in a small column fitted with a frit at the outlet, and wash the exchanger with de-salted water to neutrality to remove the excess of acid. Elute with 50 ml of 2 M sodium chloride solution at a flow-rate of about 1 ml/min to convert the exchanger completely into the Na+ form. Collect the eluate and titrate an aliquot potentiometrically with 0.01 N sodium hydroxide solution. It was further observed that the weight capacity of these materials does not remain constant but is reduced to about 50% during the first recyclings [27,49]. With totally porous exchangers the capacity levels-out to a constant value after about 10 recycling operations. By extraction of the cation exchanger and by UV spectroscopic studies of the solution, benzyl species could be detected [48]. This observation seems to be consistent with our hypothesis that soluble oligomeric sulphonated benzylsiloxanes that are not strongly bonded within the pores, slowly diffuse into the extracting solution and hence cause a
260 PH
f
9'0 8.0
7.0
6.0
5.0
4.0
3.0
4
0.01 N NaOH(ml)
Fig. 8.2. Determination of the weight capacity Qw of a strong sulphonic acid benzylsilicaexchanger by titration with 0.01 N sodium hydroxide (481.
capacity decrease. As a consequence, silica cation exchangers must be carefully regenerated several times before use until a constant capacity is achieved. The extraction effect may be responsible for the large discrepancies in Q, often cited in the literature. It should be emphasized that the decrease in capacity is exclusively observed with surface-modified ion exchangers. Bulk-modified exchangers based on polybenzylsiloxane retain their capacity on repeated loading and regeneration [SO]. The maximum attainable weight capacity of sulphonic benzylsilica can be easily evaluated using a simple calculation. The maximum surface concentration in the reaction between silica and benzyltrichlorosilane has been reported as about 4.0 pmole/mz [25]. Provided that there is complete sulphonation, the surface concentration of sulphonic acid groups should achieve a similar value. For silica with a specific surface area of 300 m'/g the capacity is calculated to be about 1.2 mequiv./g, whereas weight capacities reported in the literature vary between 0.2 and 1.O mequiv./g for certain types of porous silica exchangers. Corresponding to their thin porous layer, the weight capacity of pellicular cation exchangers is about 10-100 pequiv./g [Sl]. During the preparation of anion exchangers which is frequently carried out in two consecutive steps, amination rarely achieves a 100% conversion of chloromethyl groups. Therefore, the weight capacity of an anion exchanger is lower than that of the corresponding cation exchanger made from the same carbonaceous backbone [27].
26 1
8.2.2.2 Stability Stability has to be considered in terms of mechanical, thermal and chemical stability. Ion-exchange microparticles made by surface modification are pressure stable and can be packed into columns by the slurry technique described in Chapter 5. A comment should be made in this context about the drying procedure for ion exchangers. It is advisable to use air-dried instead of oven-dried products for the following reasons: by suspending a completely dry ion exchanger in an aqueous solution, considerable osmotic forces will become operative which may lead to bursting of the particles. This behaviour can be easily seen under a microscope. Even when a few particles burst, some fines will be formed that block the frit of the column and/or give rise to a low column permeability. For this reason there is by no means an unequivocal relation between the permeability of a column and the very small swelling effects of its contents, which may in practice be neglected. With bulk-modified ion exchangers, a considerable volume swelling is observed when the particles are brought into contact with aqueous solutions. The extent of swelling is a function of the carbon content of the packing, the type of ionic functional group, the type of eluent ion, the eluent concentration, etc. [47].In applying pressure to an ionexchange column, the swollen microbeads undergo a reversible elastic deformation, i. e. the column bed is compressed to a certain extent at high pressures and expands when the pressure is lowered. This property places a serious limitation on the utility of bulk-modified exchangers in HPLC. Silica-basedion exchangers are expected to be stable towards buffer solutions at pH 0-8. Above pH 9 the silica matrix dissolves readily. However, even in the pH range given above, the stability of exchangers can be limited as a result of a chemical attack on surface bonds by appropriate reagents. Considering a patch of the surface on a molecular scale as illustrated schematically in Fig. 8.3, three surface bonds are chemically attackable by reagents. Firstly, depending on the surface coverage, unreacted silanol groups on the parent silica surface are accessible to the aqueous solution. At these patches, silica dissolves forming monosilicic acid in the solution until the equilibrium solubility is attained. At pH > 7 the silanol groups are converted to siloxanyl ions by deprotonation. This process may be accomplished by the formation of soluble silicates. In the presence of appropriate reagents,
I
~
. _.I
-
I --
Fig. 8.3. Schematic representationof possible attacks of reagents on surface bonds of a chemicallybonded silica cation exchanger.
262
nucleophilic or electrophilic cleavage of the ESi-O-SiG and the 3 i - E bonds may occur as discussed in Chapter 3. The nature and mechanism of the surface reactions taking place in buffered solutions seems rather complicated. The only feasible way to obtain information about these phenomena is by carefully analysing all soluble or extractable products such as soluble silica, organosiloxanes and organosilanes. Provided that precautions in the storage of ion exchangers are followed carefully, columns can be run at temperatures up to 600 K [27]. The thermal stability of exchangers on annealing the particles in an oven at higher temperatures has also been investigated [25]. Surface-modified sulphonic benzylsilica was found to be stable up to about 600 K depending upon the carbon and sulphur content. However, this thermal stability has no practical advantage in HPLC. Stability tests should preferably be carried out in columns operating with buffered solutions over a wide pH range. By repeated injections of sensitive test solutes, the capacity factor, plate height and peak shape are measured with a given eluent [37]. Another possible method is to treat the support for a certain period with a buffered solution at elevated temperature. After washing and filtering the product, the carbon content is measured and compared with the original. 8.2.2.3 Selectivity
Selectivity can be discussed in two different respects. Selectivity, expressed in terms of the separation factor, a,is derived from isotherms measured under static or dynamic conditions. On the other hand, selectivity can be expressed in terms of the selectivity coefficient, rii, which relates t o chromatographic measurements, Le., retention times. The following section deals with the selectivity based on isotherms obtained from batch experiments, whereas the chromatographic selectivity is considered in Section 8.3. In batch experiments, a known volume of solution containing the counter-ion A to be adsorbed is added to a known amount of ion exchanger carrying the counter-ion El. Let the cation exchanger* be in the H form, then El = H. After shaking the suspension for sufficient time to attain equilibrium, the loaded exchanger is filtered from the supernatant solution and completely rinsed with desalted water. If there is no super-equivalent sorption [4], the difference between the initial and the equilibrium concentration of A in the solution gives the true amount taken-up by ion exchange. Consequently if the analytical determination is confined to only one counterion species, it is advisable to detect the amount of the ion given up (El) by the exchanger, because for an equivalent exchange reaction it naturally equals the numbers of exchanger sites occupied by ions A. In this way data are obtained that can be used to construct the isotherm. As illustrated in Fig. 8.4, isotherms are usually plotted in three different ways. In the first case, shown in Fig. 8.4a, the concentration of A in the exchanger phase at , in mequiv./g is plotted against its equilibrium concentration equilibrium, q ~expressed CA in the solution, expressed in mequiv./ml. A more common approach is to use equivalent fractions, X , instead of concentrations. FAcorresponds to the equivalent fraction of A in the exchanger phase and is defined as *The positive charge is omitted for simplicity.
263
(8.17) where FA is the number of sorbed moles of A at equilibrium and the denominator is the summation of all numbers of moles being in the exchanger phase*. The equivalent fraction of A in the solution at equilibrium, X A , is given by (8.18) where C i s the total equivalent concentration of the outer solution. In the graph of Fig. 8.4b TAis plotted vs, X A . Another form of the isotherm is given in Fig. 8 . 4 ~ where the logarithm of the selectivity coefficient, K ; (see eqn. 8.8), is plotted us. X A . It is necessary to discuss briefly the dependence Of XA upon X A since selectivity is evaluated according to the different forms of the isotherms. For an interpretation of selectivity, three characteristic isotherms are pictured in Fig. 8.4b. Selectivity is always given by the separation factor, a,which in terms of equivalent fractions becomes (8.19)
In other words, the selectivity of an ion exchanger for different ions can be compared directly if either the composition of the solution (XA) or of the exchanger phase ( f ~is ) held constant. When the isotherm follows the diagonal, the exchanger exhibits no selectivity, i.e. a! = 1 over the whole range of X A . In the case of an upward deviation from the diagonal, with the curve showing a convex form relative to the abscissa, the separation factor a > 1 and the exchanger prefers the counter ion A to El. With a downward deviation of the isotherm a
b
-
C
'A
i = 'A .
Fig. 8.4 Graphic presentation of ion-exchange isotherms. (a) q A equilibrium concentration of ion A in the exchanger phase (mequiv./g), C A equilibrium concentration of ion A in the solution (mequiv./ml); (b) X A equival nt fraction of ion A in the exchanger phase, X A equivalent fraction of ion A in the solution; (c) K g selectivity coefficient of ion A according to eqn. 8.8, XA equivalent fraction of ion A in the solution.
si
*If all exchanger sites are equally accessible to the counter-ionsunder consideration, C z i may be identified with the weight capacity.
,
264
from the diagonal, Le. a concave form relative to the abcissa, a < 1 and the exchanger prefers El to A. In the special case of an S-shaped isotherm there is a selectivity reversal at the inflection point. As is apparent from the isotherms presented, the separation factor is a function of the equivalent fraction of counter ion A in the solution. Since, in ionexchange elution Chromatography, very low concentrations of the counter ion A.are applied, one operates under conditions that correspond to X A + 0 and the isotherm can be considered as linear with constant a. Sorption isotherms on sulphonic benzylsilica exchangers were measured for a series of metal cations [25,50]. The sequence of their separation factors at X = 0.5 was found to follow the same order as observed with organic ion exchangers bearing sulphonic groups [3,4]. Much more interest is attached to isotherms of charged organic substances. See, for example, the curves of Fig. 8.5 representing the exchange of H+for benzylammonium chloride (B) and phenethylamrnonium chloride (P), respectively. The ion exchanger used is a sulphonic benzylsilica with a weight capacity of 0.5 mequiv./g. The total concentration of the ions in solution was 0.01 mol/l. 1*
1.0
4 0 5 H 0 0 0.5 1.o ----D
'A
Fig. 8.5. Ion-exchange isotherms for the exchange of H+ for benzylammonium chloride ( 0 ) and phenethylammoniumchloride (X) on a sulphonic benzylsilica exchanger [48]. Conditions: H-form of cation exchanger;Q, = 0.5 mequiv./g; T = 293 K; C = l.O-lU-' mol/l (total concentration).
It can be seen from the figure that the exchanger tends to prefer both B and P to H. Further, it shows a higher selectivity for P than for B at low concentrations (X< 0.3) whereas in the range 0.4 < X < 0.9, both isotherms follow the same course. At X > 0.9, a selectivity reversal occurs which may be explained by the presence of the silanol groups, Le. the bifunctional character of the exchanger. On the basis of the selectivity data obtained by batch experiments, it seems possible to develop a strategy for prediction of the selectivity coefficients of charged organic species in ion-exchange chromatography.
8.3 SELECTIVITY AND PERFORMANCE OF SILICA-BASED ION EXCHANGERS
In conditioning an ion-exchange column when passing the eluent through it, the exchanger is equilibrated with the eluent ions, El, as counter ions. On introducing the
265
sample mixture containing A and B into the mobile phase a competitive sorption takes place between A, B and El. The extent of this competitive interaction controls the retention of both species A and B. As a result, the solutes are eluted in order of their increasing strength of interaction with the exchanger. The selectivity rule (see page 251) allows one to explain and to a certain extent, to predict qualitatively the possible interactions of solutes in terms of charge, polarizability, etc. However, the question of the weights which the single interactions contribute to the net interaction remains open. With monovalent inorganic ions the determination of relative retention if fairly easy. For large organic ions which possess an extended hydrophobic part, possibly carrying several different ionic groups, it may be difficult to predict or to explain their retention behaviour. Recently Semmens [ 5 2 ] gave an excellent review of the factors that influence the selectivity of ion-exchange resins for organic ions. Before a survey is given of the chromatographic selectivity of silica-based exchangers towards organic compounds the factors that generally control retention of solutes in ion-exchange chromatography will be considered. Of primary importance in adjusting retention will be the choice of the appropriate counter ion, El. One usually employs ions such as H+, Na+, K+, CaZ+,etc. for cation and NO3-, S042-, P043-, etc., for anion exchangers. The selectivity sequences for the respective exchangers give a rough estimate of the relative strength of interaction of the eluent ion and the oppositely charged fixed ion. This may be the first way of controlling the retention of solutes to be separated. For instance, by substituting the eluent ion of an the relative strength between El- and anion exchanger in the sequence C1-, SO4’-, the fured ion increases and the retention of a given anion A- decreases. The concentration of ions El, or more exactly the ionic strength of the eluent, has a remarkable effect on retention, as previously indicated in eqns. 8.1 1 and 8.12: the logarithm of the distribution coefficient of solute A, K decreases with increasing eluent concentration. Knox and Pryde [31] showed that for a weak anion exchanger, the capacity factor of an acidic solute is inversely proportional to the ionic strength of the eluent. Therefore, in ion-exchange chromatography a gradient in eluent concentration is a powerful technique for regulating retention. Another significant quantity in retention control is the pH of the eluent, because it governs the degree of ionization at weakly acidic and weakly basic fixed ionic sites on the exchanger as well as the ionization of weakly acidic and basic solutes. The corresponding conditions can best be understood by reference to Fig. 8.6, which qualitatively represents the influence of solution pH on anion-exchange with the participation of weakly ionized groups [52]. When the solutes in the mixture span a broad range of pK values, a pH gradient in elution will be an effective means of achieving separation. In the pH range far below the pK, of an acidic solute or far above the pKb of a basic solute, retention is believed to occur only by adsorption interactions which may cause shorter residence times within a chromatographic column. As stated by Knox and Pryde [31], “the effects of variation of ionic strength and pH still provide the best tests for the mechanism of retention on a supposed ion exchanger”. A comment should be made about the retention of non-ionized species on silica exchangers. The decisive factor in retention will be the solubility of the solute in the eluent, which can be influenced by adding an organic solvent to the aqueous solution or
*,
266 b
a
resin exchange
sorption of organic
0 1 2 3 4 5 6 7 8 --o
PH
-
1 2 3 4 5 6 7 8 PH
Fig. 8.6. (a) Influence of solution pH on the exchange of a weakly acidic organic ion with a strongly basic resin (taken from ref. 52); (b) influence of solution pH on the exchange of a weakly acidic organic ion with a weakly basic resin (taken from ref. 52).
by changing its salt concentration. By increasing the concentration of methanol in the eluent the retention of neutral compounds is diminished, as shown by Twitchett et al. [53] and Asmus et al. [33]. The reverse effect on retention is observed on increasing the ionic strength: the solubility of neutral species in the eluent decreases and the strength of hydrophobic interactions between the solute and the ion exchanger is enhanced. As in adsorption chromatography, retention is diminished by increasing the column temperature, as was exemplified by Unger and Nyamah [27], Saunders et al. [29], and Cox ef al. [34]. The general guidelines in the control of retention and selectivity outlined above can now be substantiated by actual separations carried out on silica-based ion exchangers. Most of the applications refer to separations of organic compounds in biochemical systems. Historically, the first studies in this field were made with superficially bonded ion exchangers [54-561 which provided fast separations but suffered from their low capacities. Horvath [ l ] and Brown [57] presented up-to-date surveys of pellicular-type silica ion exchangers. For this reason, the main objective here is more related to totally porous, microparticulate, chemically-bonded silica exchangers. The commercially available types are listed in the Appendix. Isomeric pyridine carbonitriles and amino acids were firstly separated on sulphobenzylsilica microparticles [27]. A weak anion exchanger exhibiting a monolayer of bonded y aminopropylsilyl groups was utilized for the separation of DNA nucleotide monophosphates, penicillin derivatives and various carboxylic acids [31]. It was shown that at pH 3.0 the capacity factors of o-phthalic acid (pK, = 2.6) and p-toluenesulphonic acid (pK, < 1) followed a straight line when plotted against the inverse of the ionic strength, whereas for nearly non-ionized acids k' is independent of the ionic strength. Peculiar
267
behaviour was observed in examining the dependence of log k' of these acids upon the pH of the eluent (0.1 M NazHP04 + H3P04): as the pH was increased, the plot of log k' US pH for each particular acid passed through a maximum and then fell rapidly. The observed decline in k' on increasing the pH above 4.5 is believed to occur due to a charge compensation of the y-aminopropyl cations by adjacent siloxanyl anions of the silica matrix. The performance of the 7-pm anion exchanger considered was reported to be worse than that of the parent silica (see Fig. 3 of ref. 31). Excellent baseiine separations of complex mixtures of 5 'mono-, di- and triphosphate nucleotides of adenine, guanine, hypoxanthine, xanthine, cytosine, uracil and thymine were achieved on a microparticulate, surface-modified strong anion exchanger by applying a salt gradient [58]. The separation mechanism with silica based ion exchangers was investigated by Asmus el al. [32,33] using a variety of ionic compounds and different conditions of pH, ionic strength and eluent composition. The various exchanger types exhibited the following functional groups: strong anion e x c h a n g e r .
weak catlon e x c h a n g e r
strong cation e x c h a n g e r
A variety of acids were chromatographed in a buffered solution at pH 3.0. From the capacity factors measured it was concluded that the dominant retention mechanism was due to ionic interactions, i.e. the acids under investigation were eluted in the sequence of increasing pKa. Matrix effects caused by residual hydroxyl groups or hydrophobic interactions between the carbonaceous part of the respective solutes and the fixed ionic functional groups seemed to play a minor role in retention. On the other hand, it was shown that these matrix interactions can b t utilized in the separation of nucleic acid bases and nucleosides. The dependence of the capacity factors of the acids on the pH of the eluent between 3 and 6.0 was also studied. In agreement with their pKa values a maximum retention of the aromatic acids was observed at pH 3-5 whereas at pH > 5 retention decreases. The results coincide fairly well with those obtained by Knox and Pryde [31]. It is worth noting that the retention of aromatic bases such as pyridine, quinoline and nicotine increased with increasing pH. Comparison of their retention data on the ion exchanger, the parent silica and its unsubstituted phenyl derivative under the same conditions revealed that there is only a small contribution from the remaining hydroxyl groups of the strong anion exchanger to the retention of these compounds. The effect of changing the mobile phase
268
composition (by adding methanol) on retention was also examined. Whereas with nucleosides retention decreased with increasing amounts of ethanol, consistent with the solubility, for nucleotides and acids a competition between solubility effects and an ionexchange mechanism was observed resulting in a special course of the plot of k‘ against the ethanol concentration of the eluent. In testing the weak and strong cation exchangers by the retention of basic model compounds such as adenine, cytosine, uracil, thymine, xanthine, guanine and hypoxanthine, selectivity differences could be established which coincided with the findings of other workers [59,60].The dependence of k’upon the pH of the eluent showed a drastic reduction in retention with increasing pH, which is plausible since the solutes became less cationic. Several biogenic amines included in the investigation were separated at pH 3.0 using a NH4H2P04buffer. In contrast to the compounds mentioned above the capacity factors of biogenic amines remained unaffected on variation of the pH from 3-6 owing to their high pK values. At a constant pH of 6 the capacity factors of biogenic amines dropped sharply with increasing concentration of NH4HzP04, which again indicates an ion-exchange mechanism. In a systematic study of drug separation Twitchett et al. [53] tested a microparticular strong cation exchanger (Partisil SCX, Reeve-Angel, Maidstone, Great Britain) as a selective packing. About 30 representative compounds were chosen, covering acids, neutral compounds and bases, and the effects of pH, ionic strength and methanol content of the buffered eluent upon retention were investigated. Using an eluent containing NH4HzP04 and 40%(vlv) of methanol the acidic drugs were eluted rapidly while the more basic drugs were strongly retained. For basic solutes, the capacity factors were found t o be an inverse function of the ionic strength. For acidic drugs the increase in k’ was attributed to salting-out effects. An increase in the pH at a given eluent concentration from pH 3 to 7 enhanced retention of basic drugs, while for acidic and neutral compounds k’ decreased. This behaviour cannot be explained on the basis of a pure ion-exchange mechanism. On increasing the methanol content of the eluent, a considerable decrease in retention was observed, indicating a type of reversed-phase interaction of the solutes with the exchanger phase. Although the column showed a relatively high performance, variations in the plate height with pH, ionic strength and methanol content occurred which could not be explained fully. Two types of silica-based ion exchangers, namely a strong cation exchanger carrying functional groups like:
and an anion exchanger with groups: S&-(CH2)3-N(CH3)2 C1
I
CH2-C 6H 5 were employed for the separation of pharmaceuticals [ 6 0 ] .On the strong cation exchanger the retentions of various nucleosides were examined as functions of pH, ethanol concentration and temperature. On the anion exchanger the separations of benzoic acid, toluic acid and aspirin at pH 2.1 and of caffeine, aspirin and salicylamide at pH 9.2 were studied.
269
The columns showed an excellent performance of about 5700 theoretical plates with a column length of 15 cm. Recently, Houdeau et al. [6 11 described a strongly basic trimethylammonium exchanger suitable for separation of monochloroacetic acid (pK = 2.85), dichloroacetic acid (pK = 1.40) and trichloroacetic acid (pK = 0.65) in water-acetonitrile eluent at pH 5 containing 0.1 M sodium nitrate.
8.4 ACKNOWLEDGEMENTS I very much appreciated the help of St. Doeller, H. Kramer and W.Jost in preparing this chapter.
8.5 REFERENCES 1 C. Horvath, in E. Grushka (Editor), Bonded Stationary Phases in Chromatography, Ann Arbor Sci. Publ., Ann Arbor, Mich., 1974, p. 59. 2 H.L. Rothbart, in B.L. Karger, L.R. Snyder and C. Horvath (Editors), Introduction t o Separation Science, Wiley, New York, 1973, p. 337. 3 D. Reichenberg, in J.A. Marinsky (Editor), Ion Exchange, Vol. 1, Marcel Dekker, New York, 1966, p. 227. 4 F. Helfferich, Ionenaustauscher, Vol. I, Verlag Chemie, Weinheim/Bergstr., 1959. 5 J. Feitelson, in J.A. Marinsky (Editor), Ion Exchange, Vol. 2, Marcel Dekker, New York, 1969, p. 135. 6 J. Inczidy, Acta Chim. Acad Sci. Hung., 69 (1971) 265. 7 J. Gail and J. InczLdy, Acta Chim Acad. Sci. Hung., 76 (1973) 113. 8 J. Inczidy, J. Chromatogr., 102 (1974) 41. 9 A. Marton and J. Inczay, J. Chromatogr., 102 (1974) 165. 10 F. Helfferich, in J.A. Marinsky (Editor), Ion Exchange, Vol. 1, Marcel Dekker, New York, 1969, p. 65. 11 K. Bunzl, Anal. Chem., 50 (1978) 258. 12 J. Seidl, J. Malinsky, K. Dusek and W . Heitz, Advan. Polym. Sci., 5 (1967) 113. 13 F. Runge and W . Zimmermann, DDR Pat., No. 8560, Dec. 29, 1953. 14 I.E. Neimark and V.M. Chertov, Dokl. A h d Nauk SSSR, 138 (1961) 877. 15 I.E. Neimark, Neftekhimiya, 3 (1963) 149. 16 A.A. Chuiko, V.A. Tertykh, G.E. Plavnik and I.E. Neimark, Kofloidn. Zh., 27 (1965) 903. 17 K. Unger and K.H. Berg, 2. Naturforsch. B, 24 (1969) 454. 18 K. Unger, K. Berg, E. Gallei and G. Erdel, Fortschn'ttsber. Kofloide Polym., 55 (1971) 34. 19 K. Berg and K. Unger, Kolloid-Z.Z. Polym., 246 (1971) 682. 20 D.C. Locke, J.T. Schmermund and B. Banner, Anal. Chem., 44 (1971) 90. 2 1 K. Unger, W. Thomas and P. Adrian, Kolloid-Z.Z. Polym., 251 (1973) 45. 22 Yu.A. Eltekov, A.V. Kiselev, T.D. Khokhlova and Yu.S. Nikitin, Chromatogmphia, 6 (1973) 187. 23 N. Weigand, I. Sebestian and I. Halasz, J. Chromatogr., 102 (1974) 325. 24 D. Nyamah, Thesis, Technische Hochschule, Darmstadt, 1974. 25 K. Unger, K. Berg, D. Nyamah and Th. Lothe, Colloid Polym Sci., 252 (1974) 317. 26 K. Unger and K. Berg, Ger. Pat., No. 2,225,904, Dec. 13, 1974. 27 K. Unger and D. Nyamah, Chromatographia, 7 (1974) 63. 28 R.A. Barford, L.T. Olszewski, D.H. Saunders, P. Magidman and H.L. Rothbart, J. Chromatogr. Sci., 12 (1974) 555.
270 29 D.H. Saunders, R.A. Barford, P. Magidman, L.T. Olszewski and H.L. Rothbart, Anal. Chem., 4 6 (1974) 834. 30 F. Meiller, Ger. Pat., No. 2,433,409, Feb. 6, 1975. 31 J.H. Knox and A. Pryde, J. Chromatogr., 112 (1975) 171. 32 P.A. Asmus, Ch. Low and M. Novotny, J. Chromatog., 119 (1976) 25. 33 P.A. Asmus, Ch. Low and M. Novotny, J. Chromatogr., 123 (1976) 109. 34 G.B. Cox, C.R. Loscombe, M.J. Slucutt, K. Sugden and J.A. Upfield, J. Chromatogr., 117 (1976) 269. 35 P. Gareil, A. Heritier, M. Caude and R. Rosset, Amlusis, 4 (1976) 71. 36 Sh.H. Chang, K.M. Gooding and F.E. Regnier,J. Chromatogr., 120 (1976) 321. 37 N . Becker, Thesis, Technische Hochschule, Darmstadt, 1977. 38 J.J. Kirklandand P.C.Yates, U.S. Put., No. 3,795,313, March 5, 1974. 39 F. Runge, H. Ehrhardt and G. Penndorf, Mukromol. Chem., (1964) 68. 40 F. Wolf,H. Beyer and U. Hadicke, J. Prakt. Chem., 24 (1964) 154. 41 F. Wolf, H. Beyer and U. Hadicke, L Prakt. Chem., 24 (1964) 158. 42 Imperial Chemical Industries Ltd., Fr. Pat., No. 1,390,072, Feb. 19, 1965. 43 Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences, Ukrainian SSR, USSR Par., No. 182,719, June 9, 1966. 44 Z.I. Belinskaya, D.N. Vaskevich, G.G. Petukhov, P.G. Konovalov and A.1. Subbotina, USSR Put., No. 209,748, Jan. 26, 1968. 45 I.B. Slinjakova, M.G. Voronkov and E.Ya. Lukevits, USSR Pat., No. 245,364, June 4, 1969. 46 K. Unger, H. Kramer and M. Engler, 19th Annual Conference on Analytical Chemistry, Denver, Cola, 1977. 47 IUPAC Commission on Analytical Nomenclature, Atre Appl. Chem., 29 (1972) 619. 48 St. Doeller, unpublished results. 49 N. Weigand, Thesis, University of Saarland, Saarbrucken, 1973. 50 H. Kramer, unpublished results. 51 R.E. Majors, Int. Lab., Nov./Dec. (1975) 11. 52 M.J. Semmens, Chemical Engineering Progress (AICHE Symposium Series), 7 1 (1975) 214. 53 P.J. Twitchett, A.E.P. Gorvin and A.C. Moffat, J. Chromatogr., 120 (1976) 360. 54 J.J. Kirkland,J. Chromatogr. Sci., 7 (1969) 361. 55 J.J. Kirkland, J. Chromatogr. Sci., 8 (1970) 277. 56 R.A. Henry, J.A. Schmit and R.C. Williams,J. Chromatogr. Sci., 11 (1973) 358. 57 Ph.H. Brown, Aduan Chromatogr., 12 (1975) I. 58 R.A. Hartwick and P. Brown, J. Chrometogr., 112 (1975) 651. 59 C. Horvath and S.R. Lipsky, Anal. Chem., 44 (1969) 1227. 60 C.F. Crampton, F.R. Frankel, A.M. Benson and A. Wade, Anal. Biochem., 1 (1960) 249. 61 M. Houdeau, M. Thibert and M. Caude, Analusis, 5 (1977) 286.
27 1
Chapter 9
Silica as packing in sizeexclusion chromatography 9.1 SEPARATION MECHANISM
Size-exclusion chromatography (SEC) is a mode of column liquid chromatography used to separate solutes by their size and shape. Provided that the samples are readily soluble in an aqueous or nonaqueous medium, separations can be performed in a molecular weight (MW)range extending from a few hundred to several million MW units. The phase system required consists of a liquid able to dissolve the sample and a porous packing of appropriate pore size. The solutes passing through the column participate in a distribution process between the moving mobile phase and the solvent held stationary in the pore space of the packing. At equilibrium, the distribution of a solute can be defined in terms of a distribution coefficient, KSEC,as
where C, and Cm are the concentrations of the solute per unit volume of stationary and mobile phase, respectively. The quantity being measured chromatographically is the elution volume, V,y, which is related to KSW as follows: VE = Vo t KsW' V s
(9.2)
where V, Vj, is the void or interstitial volume of the column and V s is the volume of solvent held stationary in the pores of the packing (see eqn. 5.1). In contrast to other modes of liquid chromatography, the distribution coefficients of solutes in SEC as defined in eqn. 9.1 are relatively small and vary between zero and unity. When the solute molecule is too big to enter the pores of the packing it is totally excluded from the pore space and will be eluted first;KSEC then becomes zero and V,y = Y O . A monomeric solute with a very small size compared with an excluded solute polymer is able to penetrate the whole pore space and hence will be eluted last. Provided that no adsorption interactions contribute to retention, KSECin this instance becomes unity and V,y = Vo t V s . Between these two molecular size limits, a selective permeation takes place. The solutes of intermediate size are eluted in order of decreasing size, Le., decreasing MW, with elution volumes between VE = VOand VE = VO + V s . As the mobile and stationary phases are of the same chemical nature, the differences in the elution volumes are expected to be due primarily to steric effects, which apparently depend on the ratio of molecular size to pore size. Several theories have been proposed to explain this particular separation mechanism [ 1I. According to Bly [2] they can be divided into three categories, steric exclusion, restricted diffusion and thermodynamic, as follows. (1)Steric exclusion In this mechanism, first introduced by Flodin [3], a diffusional equilibrium is attained, i.e., the diffusion rate of solute into and out of the porous particles is higher than the interstitial velocity of the eluent moving around the particles.
272
The distribution coefficient of a solute that selectively permeates the pore space is given by
where Vs(acc.) is the fraction of total pore volume of packing accessible to the solute. Rearrangement of eqn. 9.3 gives eqn. 9.2. The validity of this steric exclusion model was demonstrated for a large number of polymers on various types of packings [2]. (2) Restricted diffusion. In this mechanism, proposed by Ackers [4] and later developed by Yau and co-workers [S-71, restricted diffusion of solutes in the stationary phase is thought to cause the size separation. By comparing the distribution coefficients of dissolved polystyrenes derived from static and dynamic, i. e., chromatographic measurements, Yau et al. [ 6] established that the steric exclusion mechanism is dominant over restricted diffusion. The latter has to be considered only at high flow-rates of the eluent and for high-molecular-weightspecies. (3) Thermodynamic theories. An extended approach based on random flight statistics to describe the molecular conformation of polymer chains was made by Casassa [8,9]. The distribution of the polymer between the mobile and stationary phase is caused by the loss of conformational entropy that occurs on transfer of the chain from the mobile phase to the stationary phase. Four assumptions were made in this concept [lo]: (a) the solution of dissolved polymer is dilute; (b) the polymer-solvent system is at the Flory temperature; (c) no adsorption of polymer takes place on void surfaces; (d) the polymer-solvent interactions are not altered in the neighbourhood of this surface. The general problem in all theoretical models is to approximate the molecular size and the pore space by appropriate quantities. For rigid molecules the “mean external length”, L,which is a “mean length of projection of the molecule along an infinite number of axes”, is a most useful parameter for characterizing the size [ 1 1 1 . For polymers the hydrodynamic volume, Vh , can be considered as the most decisive size parameter. As discussed by Billmeyer and Altgelt [ 11 1 , the hydrodynamic volume calculated from the mean molecular weight must take into account “the coiling of the polymer, its flexibility and its interaction with the solvent”. Other valuable quantities that describe the size of a random coiled polymer are the root mean square distance between its end, the radius of gyration, the root mean square distance of the elements of the chain from its centre of gravity and the mean square end-to-end distance. The size and shape of the pores of the packing are approximated by models of simple geometry such as spheres, slabs and cylinders [ 10,121 or by a random array of packed spheres [ 131. Theoretical analysis produces an equation that relates the ratio of the size of the polymer to the pore diameter to the fraction of the accessible pore volume of the packing. In Fig. 9.1 such a relationship is shown graphically [ 141. The ratio of the root mean square radius of gyration of a solute polymer, E, to the equivalent hydraulic radius of a pore, rh , is plotted on a logarithmic scale against the distribution coefficients, K S E C of , solute polymers. The two solid lines refer to two different pore models.
213
1
1.0
0.5
0.2
0.1
0
02
04
0.6 0.8 4
10
KSEC
Fig. 9.1. Ratio of-molecular size to pore size as a function of distribution coefficient, KSEC (taken from ref. [ 141). R = root mean square radius of gyration of a solute polymer; rh = equivalent hydraulic radius of a pore.
A more common presentation is the dependence of the mean molecular weight of solute polymer on elution volume on a log-linear scale. Again, calculated theoretical curves can be compared with experimental curves to prove the validity of the model. Such an experimental plot measured for dissolved polystyrenes on silica packings of different pore size is shown in Fig. 9.2 [ 151. As predicted in Fig. 9.1, the single curves in Fig. 9.2 have nearly the same slopes in the log-linear range. The calibration curves of log MW versus VE hold only for a given type of polymer, solvent and packing and are not generally valid for all solute polymers. From the log MW versus VE dependence the following information can be obtained: (i) exclusion limit of the packing, which refers to the mean molecular weight, M *, below which the solutes are able to penetrate the packing and will be selectively retarded; (ii) working range of the packing, which corresponds to the mean molecular weight range, AMW, of the linear part of the curve, at which selective permeation takes place and separation can be expected; (iii) molecular weight selectivity, which is expressed by the reciprocal of the slope of the calibration curve in the linear, i.e., working, range, as
The smaller the slope the higher is S and the better is the selectivity to size differences.
214 MW
f
lo6
105
104
103
102
20
25
30 __a
35 retention volurnc(ml)
Fig. 9.2. Log-linear calibration curve for standard polystyrenes dissolved in tetrahydrofuran on silica packings of graduated pore sizes (taken from ref. [ 151). The nominal pore diameters are indicated on the curves.
9.2 RESOLUTION IN SIZE-EXCLUSION CHROMATOGRAPHY
In SEC one observes a unique situation in the elution behaviour of solutes compared to that of other modes of liquid chromatography: all solutes are eluted within a narrow volume range between VE = VOand VE= V ot Y s . In practice, it corresponds to about two column volumes. The smallest elution volume, Vo, belongs to the totally excluded solute and the largest, Vo + Vs,to the monomeric solute which totally permeates the pore space. When no adsorption interactions occur, the totally permeating solute can be considered to be unretained in the sense of adsorption chromatography, exhibiting a retention time to and hence a capacity factor k’ = 0. Applying this concept to SEC, solutes that are eluted prior to the monomer will have negative capacity factors. In order to characterize retention in SEC, Knox [16] suggested the use of a capacity factor, k”, defined as
where the unretained solute is the totally excluded polymer with k” = 0. Applying this concept, the capacity factors, k”, of solutes in SEC vary between zero
215
and about unity [ 171. When one formally uses the common resolution equation 11 81
where Rji is defined as
where a v(i) is the standard deviation of the peak of solute i in volume units, one can readily derive the following conclusions: (i) as the capacity factors, k", in SEC fall in the range between 0-1 .O, the retardation term kf'/(l + k;') in eqn. 9.6 Iargely influences resolution; (ii) as the selectivity coefficient, rji, in its original definition is very small in SEC, say between 1.O and 1.5, the column efficiency, i.e., the total number of theoretical plates, Ni, should be as high as possible to achieve a high resolution. A more realistic resolution equation is that suggested by Yau ef al. [ 141, which involves the molecular weight:
where Rs in this instance is defined by
D2 is a constant related to the dope of the linear part of the calibration curve (= 11s) and a v(i)and u vv) are the volume standard deviations of solute peaks i and j , respectively. Eqn. 9.8 can be simplified and normalized to units of column length, giving a specific resolution equation: (9.10) where L is the column length. In order to achieve a high resolution, the product OD*in the denominator of eqn. 9.10 should he as small as possible. As the slope D2 of the calibration curve is limited by the pore size distribution of the packing, u has to be minimized. Recently, Scott and Kucera [19] introduced a similar equation that relates to the minimal difference in MW, AM", between two solutes being completely resolved to the plate number and the elution volume: (9.1 1) where Vi and yi are the interstitial volume of the column and the pore volume accessible to the solute, respectively. This equation again illustrates that extremely high column
276
efficiencies are needed in order to achieve a reasonable molecular weight selectivity. Scott and Kucera utilized very long, narrow-bore columns that produced up to 250,000 theoretical plates. On these columns, homologous n-alkylbenzenes that differed by two methylene groups or 32 MW units could be resolved. In discussing peak broadening with SEC columns, one has to recognize that the total variance of an eluted peak, u v 2 is a sum of three independent contributions:
'v (total) = 2
2
0
v (extra-column)
+
0
v
2
2
(column)
+
0
v WWD)
(9.12)
where the terms on the right-hand side have the following meanings: 2
u
(extra-column)
= variance of the peak caused by extra-column effects (injection
volume, volume of connections, volume of detector cell, mixing phenomena in these volumes); 2
= variance of the peak caused by mixing phenomena in the column
' V (column)
(longitudinal diffusion, eddy diffusion, mass transfer): 2
= variance of the peak due to the molecular weight distribution of
V (MWD)
the polymer solute. 2
The variance u (MWD)of the polymer sample depends on its polydispersity, P,defined as (9.13) where MWw is the weight-average and MW, the number-average molecular weight. b o x by the following and McLennan [20] developed an approach for estimating equation: 2
UV
( W D ) = S 2 ( p - 1)(1 +(Y)
(9.14)
where S is the reciprocal of the gradient of the linear portion of the calibration curve (see eqn. 9.4) and (Y is a correction factor. By use of eqn. 9.14, it is possible to evaluate the true plate height, H , of a solute polymer with P < 1.1 from its apparent plate height, Ifapp. Happis calculated from the experimental peak width and the retention time or volume. At present, however, no experimental data are available on the peak broadening of a polymer of fixed mean molecular weight but with varying polydispersity that can be used to prove the validity of this approach. Irrespective of the polydispersity of the polymer sample, one expects a dependence of 2 u (column) on the MW of sample due to the variation in the diffusion coefficients that affect the mass transfer. As shown in a theoretical treatment on zone broadening in SEC [21], the diffusion coefficient of solute polymer, Dm , is in the denominator of the mass-transfer term. As D , decreases with increasing MW, the plate height, H , should also increase, particularly at high flow-rates at which the mass-transfer term is dominant over the other terms that contribute to the total plate height. The dependence of H on the linear velocity on a silica column for various standard polystyrenes is shown in Fig. 9.3 [22], Evidently, at a constant linear velocity the mea-
217
sured plate height increases with the MW of polymer solutes that are selectively retarded. Further, the gradient of the H versus u curve in the linear part increases with the MW of the solute. In deriving the number of theoretical plates, N, instead of plate height, it was found that the logarithm of MW decreases linearly with the square root of N for a given polymer and a fNed set of conditions [ 17,231. Considering a particular value of N,it is dependent on two quantities being measured, namely the retention volume, V E ,and the standard deviation, u y , of the eluted peak: (9.15)
VE decreases with increasing MW of the polymer. Hence the question of what happens to the function u y = f(MW) at constant linear velocity remains open. As analysed by Kirkland [24], the dependence of the logarithm of MW upon the standard deviation, u v, of a solute polymer was essentially linear only for one particular column, while for other columns the observed values diverged considerably from linearity. For this particular column, u v increased only slightly with increasing logarithm of the mean MW.This finding could also be confirmed by our results obtained on LiChrospher Si 100 (dp = 10 pm) for the same type of polymer [22]. A few comments will be made on the effect of column temperature (Tc) on resolution in SEC. As silica possesses a non-swelling rigid structure, the column temperature can be
3.0
PS 98000 PS 51 000 I
2.0
Ps 19000
1.0
PS 10 000
PS 5000
- ethylbenzene PS 600 1
0.2
0.4
0.6 4
0.8 u (cm/sec)
Fig. 9.3. H versus u curve for standard polystyrenes dissolved in tetrahydrofuran on a column length of 250 mm and I.D.4 mm packed with LiChrospher Si 500, d p = 10 rm [21]. Column temperature: 298 K.
218
increased considerably without deformation of the particles and the particle packing. The maximal column temperature is limited by the boiling point of the solvent used as the eluent. However, by utilizing a sufficiently high back-pressure on the end of the column, vaporization at high temperatures can be avoided. In this way, the column temperature for tetrahydrofuran (THF) could be increased from 293 to 410 K [25]. Although the hydrodynamic volume of the solute polymers is a function of temperature in a given solvent, the calibration curve for standard polystyrenes in THF on a silica column remained unchanged following this substantial increase in Tc.The increase in Tc:however, resulted in an improvement in column efficiency, which was more pronounced fur the high- than for the low-MW species. The number of theoretical plates increased by a factor of ca. 2-3 on increasing Tc from 293 to 410 K. This result may be due to a simultaneous increase in the diffusion coefficients of polymer solutes. Increasing the column temperature has a significant effect on the speed of analysis. The analysis time in SEC is given by the retention time of the totally permeating solute, t o , which can be expressed by the following equation 1261 : (9.16) where 17 is the viscosity of the eluent, L the column length, $J the column resistance factor, d p the mean particle diameter of the packing and A p the pressure drop across the column. Holding L, d p and p constant, an increase in T, results in a decrease in 17 and hence in to. On keeping t o , L and dp constant, an increase in T, reduces the column pressure. In the particular instance discussed above, the pressure decreased by a factor of 2 when T, was increased from 273 to 410 K [ 2 5 ] . 9.3 OPTIMIZATION OF SILICA SUPPORT PROPERTIES WITH RESPECT TO
RESOLUTION AND SPEED Resolution in SEC can be required in either a narrow or a broad MW range spanning several decades. In order to resolve polymers with a narrow MW distribution, a packing is needed with uniform pores of equal size in the desired range. This requirement can never be met for practical reasons. Porous packings such as silica always have a pore size distribution owing to the processes of pore formation and to the amorphous nature of silica. The minimal standard deviation of the relative pore volume distribution of porous silica is as shown by Holdoway [27] in a comparative study of the pore strucabout _+10-15%, ture of commercial silica packings. Using silica with such a narrow pore distribution the linear range of the calibration curve spans more than one MW decade. Theoretical calculation by Yau el al. [ 1.51 have shown that, assuming a packing of a single pore size, a substantial fractionation range results, between 1.5 and 2.0 decades. In this respect, SEC in a narrow MW range is considerably limited. The only way of overcoming this problem is to apply adsorption chromatography in which the solutes are eluted in order of increasing W . &&-resolution separations, however, are possible only in the low-MW range (-3000). The other case to be considered requires a long log-linear calibration curve over the whole MW range. In the past, this was realized by utilizing a set of columns coupled
219
in the sequence of decreasing exclusion limit. In Fig. 9.4, it is clearly shown that coupled columns, each packed with a certain type of silica, are not necessarily needed. High-resolution separations over a broad MW range can also be performed on one column packed with a mixture of silica packings of graduated pore sizes. In order to achieve an equal pore volume contribution of the packings in the column, appropriate amounts should be mixed, depending on the specific pore volume of the silica.
0
5
10
min
Fig. 9.4. Separationof standard polystyrenes on a silica column [ 221. Column dimensions, length 500 mm, I.D.4 mm; packings, LiChrospher Si 100, d p = 10 pm (1.2 g), LiChrospher Si 500, dp = 10 pm (1.4 g), LiChrospher Si 1000, d p = 10 fim (1.5 g); column temperature, 417 K ; eluent, tetrahydrofuran; flow-rate, 0.5 ml/min; pressure, 40 bar; detector, W (254 nm). Sequence of elution: PS 2,145,000; 867,000; 41 1,000; 173,000; 98,000; 51,000; 19,000; 5000; 2000; ethylbenzene.
Recently, Yau et al. [ 151 postulated that a silica packing that exhibits a bimodal pore size distribution would optimally fit the requirement of a long log-linear calibration range. The specific pore volumes due to both pore sizes should be nearly the same and the maxima of the relative pore volume distribution should differ by one order of magnitude. The use of a bimodal pore size was predicted by theoretical calculations and could be verified experimentally by utilizing either two types of silica with different pore sizes or one type of silica whose particles offer a bimodal distribution. For instance, on employing a 4.7-nm plus a 120-nm silica support, a MW range of five decades could be spanned. Another property that can be optimized with respect to resolution and capacity is the specific pore volume of the packing. As the elution volume is limited by the pore volume of the packing, V p should be made as large as possible. Strictly, not only is V, a decisive parameter, but even more so the ratio V,s/Vo.In regularly packed columns the interstitial volume expressed in terms of interstitial porosity, e i s p , reaches about 0.4. Hence a porosity of 0.6 remains for the internal volume plus the volume of the purely solid packing. Replacing the ratio Vs/Vo by the corresponding porosities one obtains (9.17)
The variable that should be maximized is the particle porosity or the specific pore volume. Fig. 9.5 shows the dependence of ep(l - eisp)/eiSpon the specific pore volume for porous
280
1.5
1.0
0.5
Fig. 9.5. Phase ratio of silica columns in SEC as a function of the specific pore volume of the packing at three different interstitialcolumn porosities [ 251.
silicas at three different ' i s p values. It is apparent that the ratio increases considerably at high V p values but then approximates to a constant value at high V p .In other words, a specific pore volume greater than 2.0 ml/g does not contribute significantly to Vs/Vo. Vp = 2.0 ml/g corresponds to a particle porosity of 82%. Porous silica packings usually exhibit a specific pore volume between 0.3 and 1.1 ml/g [ 19,241. Highly porous silica packings have been synthesized that offer a maximal specific pore volume of 3.0 ml/g [22,25]. Experiments have shown that such a type of silica can be satisfactorily slurry-packed into columns up to a specific pore volume of about 2.0-2.5 ml/ With respect to a high column performance, silica microparticles 5 pm in size should be employed. The columns to be packed should be 100-500 mm in length in order to improve the poor selectivity in SEC.An unanswered question up to now in the use of tailor-made silica packings for SEC with a particular pore size and pore structure is the way in which the arrangement of pores within microparticles affects the peak broadening of polymers. Most of the commercially available silica packings (see Appendix) are mesoporous and exhibit a mean pore diameter between 3.0 and 10.0 nm. When these packings are utilized in SEC,an MW fractionation range between 10' and 5*104can be spanned [ 151. For separations in a higher MW range, macroporous silica(s) with a mean pore diameter up to 1000 nm are needed. The only macroporous packings available are LiChrospher Si 500, 1000 and 4000.
9.4 SIZE SEPARATIONS ON POROUS SILICA
9.4.1 Introduction
Selectivity in SEC can be considered in two different respects. The primary objective is to resolve solute species that exhibit a small MW difference, e.g., about 10-100 MW units. This demand, however, is impossible to realize as the packing always exhibits a pore size
281
distribution and even in the case of uniform pore size of the packing an inclined calibration curve is theoretically predicted [ 151. In the separation of polymers on silica columns it is possible to resolve two solutes that differ by a factor of 2 in their MW, as illustrated in Fig. 9.4. In the oligomer range silica columns are able to discriminate between two methylene goups or one phenyl group [ 191. In its most common sense, SEC is utilized as a separation technique for identifying the MW distribution of polymer species over a broad MW range spanning several decades. This can be performed in a traditional way by coupling columns of decreasing exclusion limits [28] or in a more modern way by using a mixture of two packings in one column [ 151. The utility of porous silica as a packing for size-exclusion separations was fully recognized in the period between 1965 and 1970. De Vries [29] and independently Unger [30] stated that silica-packed columns are suitable for eluting dissolved polystyrenes in order of decreasing MW by means of a distribution process between the mobile phase and the quasi-stationary phase held in the pores. Initial fractionations and separations were carried out on low-efficiency classical columns packed with coarse spherical and angular particles of graduated pore size [31-341. The silica beads prepared by Le Page and co-workers [35,36] by a special procedure then became available under the trade-names Spherosil and Porasil. Most investigations in SEC carried out during this period were performed with this type of packing [37-391. In these examinations, the basic separation features concerning the relationship between the pore size of the packing and the molecular size of the solute were established, and also the dependences of flow-rate, particle size, etc., on peak broadening. Silanization of the silica surface was shown to have a considerable effect on the amount of dissolved polystyrene adsorbed in static experiments: on a fully trimethylsilylated silica surface the adsorption was negligibly small compared with that of the native silica [40]. A renaissance in SEC on porous silica packings has occurred in the last 3 years. Small silica microparticles became available that could be packed efficiently and reproducibly into columns. Such silica columns, exhibiting a high performance and offering a high speed of analysis, were tested for the separation of different types of polymer. However, the short analysis time produced by a high flow-rate may lead to errors in the calculation of MW distribution attributable to flow-rate variations [41]. As a result of its high mechanical and thermal stability, a potential advantage of silica is that one can chromatograph at high column temperatures with no loss of performance or selectivity. This is particularly useful for less soluble polymers such as polyethylene and polypropylene. Although silanization of the silica surface reduces adsorption interaction considerably, mainly nonpolar and moderately polar polymers can be separated. 9.4.2 Separation of synthetic polymers and oligomers
Whereas polystyrene solutes on native silica and its TMS derivative show no distinct differences in the calibration curve and in column performance [24], relatively polar polymers such as poly(methy1 methacrylates) (PMMA) are exclusively separated on long chain n-alkylsilicas [42]. It should be noted that such n-octyl- and n-octadecyl-modified packings suffer a decrease in internal column volume due to the decrease in the pore diameter of the chemisorbed layer. Long-chain n-alkylsilicas are also the most suitable
282
supports for separating fatty acid esters and polyethylene glycols [43]. A recent survey of the applications of SEC in polymer chemistry was published by Eisenbeiss et al. [43]. High-performance SEC of oligomers on porous silica microparticles was especially studied by Kirkland and Antle [44]. Columns packed with silanized and untreated 4.0- and 5 .O-nm pore-size silicas were employed, with n-alkanes and biphenyls as solutes, calibration curves being obtained with acetone, dimethylformamide and tetrahydrofuran as eluents. Sizeexclusion separations were found to take place on the basis of differences in the hydrodynamic volume of the solutes. Separations of dissolved epoxy resins, polyethylene terephthalate oligomers, etc., were achieved. The separation of hydrophilic polymers such as polydextrans and polyethylene glycols in water and polar organic eluents on chemically bonded hydrophilic silica packings was reported by Unger e t al. [45] and recently by Engelhardt and Mathes [46]. The former workers utilized a bulk-modified silica carrying Si(CHz)30CHz-CH(OH)-CH2(0H) groups as the packing, its synthesis later being described in detail [47].Elution of polyethylene glycols in pure water could be performed with no irreversible adsorption on the modified surface. According to Engelhardt and Mathes [46], surface-modified silicas were synthesized by means of a reaction between silica and o-substituted organotriethoxysilanes, e.g., N-(3-triethoxysilylpropyl)silane, giving bonded supports with the following hydrophilic organic moieties: Si-(CH2)3 -NH2 ES~-(CH~)~-NH-CO-CH~ Si-(CH2)3-NH-S02-CH3
=Si-(CH2)3-NH-CO-CH2-NH-CO-CH3 ES~-(CH~)~-OCH~ -CH(OH)-CH2(OH) The packings showed a specific behaviour with respect t o their elution sequence towards polar polymers such as polyethylene glycols (PEGs) and dextrans. On some packings dextrans were separated according to a size-exclusion mechanism whereas PEGs were often strongly retarded, indicating that an adsorption mechanism partially takes place. 9.4.3 Separation of biopolymers
Particular interest is attached to size-exclusion separations of biological macrmolecules such as proteins, nucleic acids, enzymes and viruses on inorganic supports. The porous inorganic supports have the advantages of mechanical stability and permanent porosity over swelling carbohydrate gels such as crosslinked dextrans. The use of inorganic supports, however, is seriously limited by the fact that they either irreversibly adsorb or denature the sensitive biological substances. To overcome these disadvantages, the surface has to be chemically modified, taking into consideration the following objectives: (i) the organic moiety to be bonded to the surface should be similar in its chemical structure to that of biological macromolecules; (ii) the reaction should provide a dense surface coverage of organofunctional groups in order to minimize adsorption interactions of the solute with the native surface; (iii) the bonded support should offer a sufficient chemical stability towards buffered solutions in the pH range 3-9.
28 3
It is obvious from the foregoing discussions about the chemical modification of silica (Section 3.2) that these requirements cannot be completely satisfied in the preparation and modification of supports. Pioneering work on the size separation of biopolymers on inorganic supports was carried out by Haller and co-workers [48,49] and Hiatt e t d . [SO] using controlledporosity glass. Adsorption and denaturation were eliminated to a great extent either by adding polar compounds such as alcohols, polyethelene glycols and poly(ethy1ene oxide) to the eluent in a low concentration or by treatment of the glass surface with y-aminopropyltriethoxysilane (APTES). In 1973, Eltekov et al. [5 11 investigated the adsorption = 80 m2/g, D = 50 nm) modified behaviour of proteins on a macroporous silica (SBET with APTES. The surface concentration of amino groups was reported to be 1.9 pmole/m2. The bonded surface layer was of a polymeric type because small portions of water were added in the course of the modification. As shown for lysozyme and albumin in static experiments, the amount of adsorbent on APTES-modified silica was negligibly small compared with the native silica. The results of column separations in buffered solutions with graduated salinity and pH, however, revealed that the order of elution of the proteins used did not follow the size exclusion order, Le., V, increased with decreasing MW of the solute. It seems that the selectivity is still specifically influenced by adsorption interactions. Three years later, Regnier and Noel [52] developed a suitable support by reaction of controlled-porosity glass and 1,2epoxy-3-propoxypropyltrimethoxysilane (EPPTMS) as modifier, which was termed a glycophase-bonded support. Bonding was carried out in acidic solution at pH 3.5 at 363 K by refluxing. Under these conditions, the silane was attached to the surface as a polymeric layer. Simultaneously, the oxirane ring of the modifier was opened to a glycol structure: -
=Si-(CH2)3-O-CH2-CH-CH2
I
1
OH OH
From the carbon content and the specific surface area, a surface concentration between 1.O and 1.5 pmole/m2 was obtained. The thickness of the bonded layer was estimated to be about 1.85 nm. Recovery of enzymatic activity from solutions of enzymes brought into contact with the glycophase support was studied under static conditions. Additionally, recovery studies on proteins under dynamic conditions on columns were performed. In both instances the recovery of a series of proteins and enzymes was much better than on the original glass, sometimes reaching about 100%. On this glycophase support, sizeexclusion separations of proteins were carried out in buffered solution under isocratic conditions [53,54]. Derivatives of glycophase supports such as weakly acidic and basic ion exchangers are commercially available from Pierce (Rockford, Ill., U.S.A.) [ 5 5 ] .Becker [561 and Unger and Becker [57] also employed EPPTMS and additionally 1-aminoethyl-3aminopropyltrimethoxysilane(AAPTMS) as selective reagents in the surface modification of silica. Reaction was carried out at 463 K, water was carefully excluded and no solvent was used, so that a dense monolayer coverage could be achieved. The EPPTMS-modified silica was after-treated either with sulphuric acid to cleave the oxirane ring and to give
284
Z~~(CHZ)~-O-CH~-CH-CH2 I I OH OH
1,2-dihydroxy-3-propoxypropylsilyl or with ammonia solution to give S ~ ( C H Z ) ~ - O - C H-CH-CH2 ~
I
I
OH NH2 1-amino-2-hydroxy-3-propoxypropylsilyl
The surface concentration of both moieties was found to range between 2.0 and 2.5 pmole/m2, whereas the theoretical value is 2.65 pmole/m2 [ 5 6 ] . The AAPTMS-modified silica contains the following functional group: S ~ ~ ( C H ~ ) ~ - N H - C-CH2 H Z -NHz 1-aminoethyl-3-aminopropylsilyl
The surface concentration varied between 2.7 and 3.3 pmole/mz, which is in fairly close 015.)
1, 1
0.E
0
= aldolase (rabbit)
MW I 158 000 2 = chymotrypsinogen A M W = 25000 3 = lysozymdhuman) MW = 14 300 4 glutathione(red.) MW I 307
OJO
10 min
1 = albumin ( hen) M W = 45000 2 = chymotrypsinogenA M W = 25 COO 3 lysozyme (human) MW = 14 300
0.01
1 2 3 4 min
Fig. 9.6. Separation of proteins and enzymes on a silica (dp = 10 pm) carrying sSi(CH,),OCH,CH(OH)CH,(OH) groups (a= 2.4 #mole/m') [ 5 6 ] . (a) Column, length 300 mm, I.D. 4 mm; eluent. 0.05 M phosphate buffer, 0.1 M NaCl, pH = 7.5; pressure, 30 bar; b e a r velocity, 2.9 mmlsec; detector, UV (210 nm). (b) Column, length 300 mm, I.D. 4 mm; eluent, 0.05 M phosphate buffer, 0.1 M NaCl, pH = 7.5; pressure, 90 bar; linear velocity, 7.0 mmlsec; detector, UV (210 nm).
28 5 TABLE 9.1 RECOVERY OF ENZYMES ON EPPTMS-MODIFIED SILICA SUPPORT HAVING A DIOL STRUCTURE AND RETENTION DATA OF ENZYMES ON A COLUMN PACKED WITH THE SAME SUPPORT (561 Enzyme
MW
PI
Relative recovery (%) Static (incubation)*
Dynamic (column)**
I Glutathione (reduced) Cytochrome C (horse heart) Lysozyme (human) Haemoglobin (human) Try psin Chymotrypsinogen A (beef) Pepsin (hog) p-Lactoglobulin (cow) Albumin (hen) Albumin (ox) Aldolase (rabbit) Catalase (beef)
307 12,500 14,300 16,000 23,300 25,000 33,000 36,000 45,000 67,000 158,000 240,000
9.2 10.5 7.0 8.5 9.2 2.9 5.2 4.6 5.1 9.5 8.0
98.5 98.7 94.6 97.1 89.6 98.9 84.4 96.7 -
Retention volume (ml)***
I1
87.3 94.7 97.9 98.5 93.7 95.2 96.3 93.3
98.1 99.3 99.1 99.3 98.9 99.4 97.6 96.6 100 100 98.1 98.6
3.76 3.42 3.33 3.03 3.03 2.40 2.28 2.28 2.10 2.04 1.86 1.86
*Test conditions: support, 100 mg; incubation time 1 h; sample, protein (enzyme), loA6M in 0.07 M phosphate buffer + 0.1 M NaCI, pH 6.8. **Test conditions: (I) column, 300 X 4 mm I.D.; temperature, room temperature; detector, UV (210 M or lower in 0.07 M phosphate buffer, pH 6.8; linear velocity, 2.85 mm/sec; nm); sample, (11) as for (I) but linear velocity 0.95 mm/sec and 0.05 M phosphate buffer + 0.1 M NaCI, pH 7.5. ***Test conditions: as above for (11).
agreement with the theoretical value of 3.04 pmole/m*. The pH stability of supports towards buffered solutions was tested in columns over a broad pH range between 5 and 9 by controlling the retention time, the plate height and the peak shape of thiamine and ascorbic acid as representative solutes. Small variations in these quantities were observed between pH 6 and 9. Recovery studies on a series of enzymes and proteins were made under static and dynamic conditions. The results (Table 9.1) indicate that negligibly small irreversible adsorption and/or denaturation occurs. Chromatographic measurements were carried out on columns packed with the modified silica supports under isocratic conditions. The results (see the last column in Table 9.1) reveal that the elution volume increases with decreasing h4W of the biopolymer, indicating a size-exclusion mechanism. In Fig. 9.6 two separations are shown as examples that demonstrate the potential advantage of these packings. 9.4.4 Separation of oligorners and polymers by iiquid-solid chromatography
Recently, Klein and Treichel IS81 presented a theoretical treatment of the superposition of separation based on both size-exclusion and adsorption mechanisms. In pure SEC, solutes
N W o\
Fig. 9.7. Separation of a polyethylene glycol (PEG 1000) into its homologues on a pre-packed silica column (Hibar LiChrosorb RP-8; Merck) [ 5 9 ] . Column, length 250 mm, I.D. 4 mm; packing, LiChrosorb RP-8, d p = 5 pm; eluent, methanol-water (42: 58, v/v); flow-
rate, 1.0 ml/min.
287
are eluted with decreasing MW whereas in U C the reverse elution order is obtained. In a mixed mechanism, adsorption can contribute to net retention only when the surface within the pores is accessible to the solute. As a result, the total retention in this instance cannot be considered to be proportional to the sum of K s E c t KLSC.Eisenbeiss and EhIerding [59], in a study on oligomer separations by LSC, stated that in general a solute should have an MW about one-tenth of that of totally excluded polymers to be separated exclusively by adsorption on porous silica. They demonstrated the potential of separation selectivity that could be achieved by LSC on reversed-phase columns for polyethylene glycols (see Fig. 9.7) under isocratic conditions. The versatility of the method can be considerably enhanced by utilizing the column switching technique. In this way it becomes possible to resolve complex oligomeric mixtures. 9.4.5 Characterization of colloidal dispersion
It is not widely known that SEC can also be successfully applied to characterize colloidal dispersions. Only a few attempts have been made to separate stabilized polystyrene and poly(methylmethacry1ate) latices on macroporous silicas [60,61]. As the latices consist of dispersed particles of discrete size, the separation mechanism can be considered to be similar to that in SEC using dissolved polymers. Particle size analysis can be performed separately by means of light scattering or electron microscopy. Under constant conditions, the elution volumes, V E ,of standard dispersions with known and graduated mean particle sizes are measured. By plotting the mean particle size against the elution volume, a calibration curve is obtained. As the latex particles cover a broad range of diameters up to a few microns, largepore silica packings have to be employed. It has been shown that the elution volume of defined latices is a function of the flow-rate. Further, to prevent adsorption the dispersion has to be stabilized by adding surfactants or salts. Detection also creates some problems, because differential refractometers do not show a linear response to concentration. Heitz, however, presented a calculation procedure for adjusting for non-linearity [62]. Regardless of these serious limitations, the method opens a new field for characterizing finely divided particulate matter.
9.5 REFERENCES 1 K.H. Altgelt, Adv. Chromatogr., 7 (1978) 3. 2 D.D. Bly, in B. Caroll (Editor), Physical Methods in Macromo2ecular Chemistry, Vol. 2, Marcel Dekker, New York, 1972, pp. 1-89. 3 P. Flodin, Thesis, University of Uppsala, Sweden, 1962. 4 G.K. Ackers, Biochemistry, 3 (1964) 723. 5 W.W. Yau and C.P. Malone, J. Polym Sci., Part B, 5 (1967) 663. 6 W.W. Yau, C.P. Malone and S.W. Fleming, J. Polym Sci., Part B, 6 (1968) 803. 7 W.W. Yau, J. Polym Sci., Part A-2, 7 (1969) 483. 8 E.F. Casassa, J. Phys. Chem., 75 (1971) 3929. 9 E.P. Casassa, J. Polym Sci, Part B, 5 (1976) 773. 10 E.P. Casassa, Polym. Lett., 5 (1967) 773.
288 11 W. Billmeyer, Jr., and K.H. Altgelt, in K.H. Altgelt and L. Segal (Editors), GelPermeation Chromatography, Marcel Dekker, New York, 1971, pp. 3-12. 12 J. Giddings, Anal. Chem., 40 (1968) 2143. 13 M.E. van Kreveld and N. van den Hoed, J. Chromatogr., 83 (1973) 111. 14 W.W. Yau, J.J. Kirkland, D.D. Bly and H.J. Stoklosa,J. Chromatogr., 125 (1976) 219. 15 W.W. Yau, C.R. Ginnard and J.J. Kirkland, J. Chromatogr., 149 (1978) 465. 16 J.H. Knox, IIIrd International Symposium on Column Liquid Chromatography, Salzburg, 1977. 17 K. Unger, R. Kern, M.C. Nmou and K.F. Krebs, J. Chromatogr., 99 (1974) 435. 18 J.F.K. Huber,Z. Anal. Chem., 277 (1975) 341. 19 R.P.W. Scott and P. Kucera, J. Chromatogr., 125 (1976) 251. 20 J.H. Knox and F. McLennan, Chromatographia, 10 (1977) 75. 21 J.C. Giddings and K.L. Mallik, Anal. Chem., 38 (1966) 997. 22 R. Kern, Thesis, Technische Hochschule, Darmstadt, 1976. 23 D.D. Bly, J. Polym Sci, Part A - I , 6 (1968) 2085. 24 J.J. Kirkland,J. Chromatogr., 125 (1976) 231. 25 K. Unger and R. Kern, J. Chromatogr., 122 (1976) 345. 26 P.A. Bristow and J.H. Knox, Chromatographia, 10 (1977) 279. 27 M.J. Holdoway, Report No. M2749, Atomic Energy Research Establishment, Harwell, U.K., 1976. 28 D.J. Harmon, in K.H. Altgelt and L. Segal (Editors), Gel Permeation Chromatography, Marcel Dekker, New York, 1971, pp. 13-23. 29 A.J. de Vries, IUPAC Internal Symposium on Macromolecular Chemistry, Prague, 1965, Preprint 139. 30 K. Unger, Thesis, Technische Hochschule, Darmstadt, 1965. 31 H.W. Kohlschutter, K. Unger and K. Vogel, Makromol. Chcm., 93 (1966) 1. 32 K. Unger, K. Vogel and H.W. Kohlschutter, Z . Naturforsch., 22B (1967) 8. 33 A.J. de Vries, M. Le Page, R. Beau and C.L. Guilliemin, Anal. Chem., 39 (1967) 935. 34 W. Heitz, K. Klatyk, F. Kraffczyk, K. Pfitzner and D. Randau, Proceedings of the Seventh InternationaI Seminar on GPC, Monaco, 1969, Waters Associates, Milford, Mass., 1969. 35 M. Le Page, R. Beau and J. Duchene, Fr. Pat., 1,473,240, 1967. 36 M. Le Page and A. de Vries, Fr. Pat., 1,475,929, 1967. 37 K.J. Bombaugh, W.A. Dark and R.F. Levangie, Proceedings of the Fifth International Seminar on GPC, London, 1968, Waters Associates, Milford, Mass., 1968. 38 K.A. Boni and F.A. Sliemers, Proceedings of the Sixth International Seminar on GPC, Miami Beach, 1968, Waters Associates, Milford, Mass., 1968. 39 Z. Grubisic and H. Benoit, Proceedings of the Seventh International Seminar on GPC, Monaco, 1969, Waters Associates, Milford, Mass., 1969. 40 K. Unger and P. Ringe, J. Chromatogr. Sci., 9 (1971) 463. 41 D.D. Bly, H.J. Stoklosa, J.J. Kirkland and W.W. Yau, Anal. Chem., 47 (1975) 1810. 4 2 J.J. Kirkland, J. Chromatogr., 125 (1976) 231. 43 F. Eisenbeiss, E. Dumont and H. Henke, Angew. Makromol. Chem., in press. 44 J.J. Kirkland and P.E. Antle,J. Chromatogr. Sci., 15 (1977) 137. 45 K. Unger, N. Becker and E. Kramer, Chromatographia, 8 (1975) 283. 46 H. Engelhardt and D. Mathes,J. Chromafogr., 142 (1977) 311. 47 K.K. Unger, N. Becker and P. Roumeliotis, J. Chromatogr., 125 (1976) 115. 48 W . Haller, Nature (London), 206 (1965) 693. 49 W. Haller, K. Tympner and K. Hannig, Anal. Biochem., 35 (1970) 23. 50 C.W. Hiatt, A. Shekokov, E.J. Rosenthal and J.M. Galimore, J. Chromatogr., 56 (1971) 362. 51 Yu.A. Eltekov, A.V. Kiselev, T.D. Khokhlova and Yu.S. Nikitin, Chromatographin, 6 (1973) 187. 52 P.E. Regnier and R. Noel, J. Chromatogr. Sci., 14 (1976) 316. 53 S.H. Chang, K.M. Gooding and RE. Regnier, J. Chromatogr., 125 (1976) 103. 54 T.D. Schlabach, D.C. Tseng and P.E. Regnier, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, 1977, paper No. 185. 55 Catalogue, Pierce, Rockford, Ill., 1976.
289 56 N. Becker, Thesis, Technische Hochschule, Darmstadt, 1977. 57 K. Unger and N. Becker, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, 1977, paper No. 171. 58 J. Kiein and K. Treichel, Chromatographio, 10 (1977) 604. 59 F. Eisenbeiss and S. Ehlerding, Kontakte (Merck), 1 (1978) 22. 60 F. Eisenbeiss, Kontakte (Merck), 3 (1973) 35. 61 H. Coll, G.R. Fague and K.A. Robitlard, 49th National Colloid Symposium, Potsdam, N. Y., June
1 9 75. 6 2 W. Heitz, Kontakte (Merck), 2 (1977) 29.
This Page Intentionally Left Blank
29 1
Appendix
Commercially available silica packing [ 1,2]
LIST OF SUPPLIERS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Alltech Associates Altex Scientific Applied Science Laboratories Chromatec E.I. Du Pont de Nemours E. Merck (U.S.A.: MCIB Manufacturing Chemists) Macherey Nagel & Co Perkin-Elmer Phase Separation Ltd. Reeve Angel (Whatman) Rhone-Progil (France) Shandon Varian Associates Waters Associates Woelm Pharma GmbH & Co
ADDITIONAL ABBREVIATIONS = angular particles = spherical particles p. c. = available as prepacked columns b.p. = bulk packing SBET= specific surface are according to BET Vp = specific pore volume D = most frequent pore diameter ds = thickness of the porous layer at pellicular packings SCX = strong cation exchanger WCX = weak cation exchanger SAX = strong anion exchanger WAX = weak anion exchanger = weight capacity of ion exchanger in mequiv./g Q, MW* = exclusion limit for polystyrenes in tetrahydrofuran at 293 K AMW = log-linear calibration range for polystyrenes in tetrahydrofuran at 293 K a s
APPENDIX A TOTALLY POROUS SILICA PACKINGS FOR LIQUID-SOLID AND LIQUID-LIQUID CHROMATOGRAPHY Trade name Biosil A Chroma Sep SL Hi Eff Micro Part Hi-Flosil
Supplier
Particle shape
Particle size b m )
Trade form
Description
b.p. p.c. p.c. b.p.
SBET= 400 m'/g identical with LiChrosorb Si 60 SBET= 250 m'/g silica and magnesia as constituents
b.p.
SBET= 200 m2/g D =lO+3nm SBET= 675 m2/g; Vp = 0.65 ml/g D =4nm SBET= 500 m'lg; Vp = 0.82 ml/g D =6nm
2 4 3 3
a a a a
12
S
LiChroprep Si 40
6
a
40-63
b.p.
LiChroprep Si 60
6
a
5-20; 15-25; 25-40; 40-63
LiChroprep Si 100
6
a
40-63
b.p. p.c. (40-63 only) b.p.
LiChrospher Si 100
6
S
5; 10; 20
LiChrospher Si 300
6
S
5; 10
b.p. p.c. b.p.
LiChrospher Si 500
6
S
10
b.p.
LiChrospher Si 1000
6
S
10
b.p.
LiChrospher Si 4000
6
S
10
b.p.
LiChrosorb Si 60
6
a
5; 7; 10; 30
LiChrosorb Si 100
6
a
5; 7; 10; 30
13
a
5; 10
p. c. b.p. b.p. p.c. p.c.
Hypersil
Micropak Si
2-10 5; 10; 20 5; 10 4 0 ; 50-75; 75-250 5-7
SBET= 300 m'/g; V p = 1.05 ml/g D =1Onm SBET= 256 m'/g; V p = 1.20 ml/g D =12nm[3] SBET= 250 m'/g; Vp = 2.00 ml/g D = 3 0 n m [3] SBET= 45 m'/g; V p = 0.88 ml/g D = 5 0 n m [3] SBET= 19 m'/g; V p = 0.72 rd/g D = l 0 0 n m [3] SBET= 4.7 m2/g; V p = 0.78 ml/g D = 3 8 5 n m [3] SBET= 475 m'/g; Vp = 0.76 ml/g D =6nm[3] SBET= 278 m'/g; V p = 1.02 ml/g D = 1 0 n m [3] identical with LiChrosorb Si 60
Nucleosil 50
I
S
5; 1.5; 10
b.p.
Nucleosil 100
I
S
5; 1.5; 10
b.p.
Nucleosil 100 V
I
S
5; 7.5; 10
b.p.
1, 10
a
5; 10; 20
p
Partisil
p
Porasil
14
a
8-12
b.p. p.c. p.c.
Porasil A
14
S
31-15: 15-125
b.p.
Porasil B
14
S
31-15; 15-125
b.p.
Porasil C
14
S
31-15; 15-125
b.p.
Porasil D
14
S
31-15; 15-125
b.p.
Porasil E
14
S
31-15; 15-125
b.p.
Porasil F
14
S
31-15: 15-125
b.p.
Porasil T
14
S
15-25; 25-31
b.p.
Sil60
I
a
5; 10; 15; 20
b.p.
sil-x-I
8
a
13 5 5
b.p.
Silica A
8
a
1355
b.p.
Silica Woelm
15
a
b.p.
Spherisorb S
9
S
3-6; 1-12; 10-18; 18-32 5 ; 10; 20
11 11
S
5; 10; 20 5; 10
b.p. b.p.
b.p.
P. Spherosil XOA 600 Spherosil XOA 800
S
SBET= 500 m'/g; V p = 0.80 ml/g D =5nm SBET= 300 m'/g; V p = 1.0 ml/g D =10nm SBET= 430 m'/g; V p = 1.5 ml/g D =10nm SBET= 400 m'/g D =4-5nm SBET= 400 m'/g; V p = 1.0 ml/g D =10nm SBET= 350-500 m'/g; V p = 1.05 ml/g D -10nm SBET= 125-250 m'/g; V p = 0.90 ml/g D -15nm SBET= 50-100 m'/g; V p = 0.10 ml/g D -30nm SBET= 25-45 m'/g; V p = 0.60 ml/g D -60nm SBET= 10-20 m'/g; Vp = 0.40 ml/g D -100nm SBET= 2-6 m'/g; Vp = 0.25 ml/g D >150nm SBET= 300 m'/g D =15nm SBET= 500 m'/g; V p = 0.75 ml/g D -6nm SBET= 400 m'/g D -10nm SBET= 400 m'/g D -10nm SBET= 500-600 m'/g; V p = 0.80 ml/g D -6nm SBET= 183 m2/g; Vp = 0.52 ml/g D = 10.9 nm (31 SBET= 600 m'/g SBET= 830 mZ/g D -3nm
(Continued on p. 294)
h)
W
W
h)
APPENDIX A (continued)
W P
Trade name
Supplier
Particle shape
Particle size (pm)
Trade form
Description
Spherosil XOA 400 (identical with Porasil A) Spherosil XOA 200 (identical with Porasil B) Spherosil XOB 075 (identical with Porasil C) Spherosil XOB 030 (identical with Porasil D) Spherosil XOB 015 (identical with Porasil E) Spherosil XOB 005 (identical with Porasil F) Zorbax Sil
11
s
10
b.p.
11
s
10
b.p.
11
S
10
b.p.
11
s
10
b.p.
11
s
10
b.p.
11
S
10
b.p.
5
s
5-7
P. c
SBET= 350-500 m2/g; Vp = 1.05 ml/g D <10nm SBET=125-250 m2/g; V p = 0.90 ml/g D -15nm SBET= 50-100 m’/g; V p = 0.70 ml/g D -30nm SBET= 25-45 m2/g; V p = 0.60 ml/g D -60nm SBET= 10-20 m’/g; V p = 0.40 mllg D -100nm SBET= 2-6 m2/g; Vp = 0.25 ml/g D >150nm SBET= 300 m’/g D = 7.5 nm
3, 7
S
10
p.c.
Vydac TP Silica
APPENDIX B SUPERFICIALLY POROUS OR PELLICULAR SILICA PACKINGS FOR LIQUID-SOLID AND LIQUID-LIQUID CHROMATOGRAPHY Trade name
Supplier
Particle shape
Particle size(pm)
Trade form
Description
Corasil I
14
S
37-50
b.p.
Corasil I1
14
S
37-50
b.p.
Pellosil HS Pellosil HC Perisorb A
10 10 6
S
37-44 37-44 30-40
b.p. b.p. b.p.
SBET= 15 m'/g; V p = 0.040 ml/g D = 7 n m [4] SBET= 27 m'/g; V p = 0.049 ml/g D =5nm[4] the HC type has a thicker porous layer than the HS type
S S
SBET= 14 m'/g; V p = 0.024 ml/g =6nm[4] SBET= 12 m2/g SBET= 12 m'/g D = 6 nm;ds = 1 rm SBET= 0.9 m'/g; Vp = 0.023 ml/g D = 3 8 n m [4]
D sil-x-I1 Vydac 101 Si
8 7
S S
35 5 15 30-44
b.p. b.p.
Zipax
5
S
25-37
b.p.
t4
W
ul
APPENDIX C
h)
CHEMICALLY BONDED SILICA PACKINGS (EXCLUDING BONDED SILICA ION EXCHANGERS)
OI
Trade name
Supplier
Particle shape
Particle size (rm)
Trade form
Description
8
a
13 * 5
allylphenyl groups
14
a
31-75
14
a
8-12
Bondapak phenyl (Porasil D) Hi Eff Micro Part C 18
14
a
31-15
b.p. p.c. b.p. p.c. b.p. p-c. b.p.
3
a
10
octadecylsilyl groups
Hypersil SAS Hypersil ODS LiChrosorb RP 2
12
S
12 6
S
a
5 -1 5-1 5; I; 10
LiChrosorb RP 8
6
a
5; 7; 10
LiChrosorb RP 18 LiChroprep RP-2 LiChroprep RP-8
6 6 6
a a a
5;I; 10
LiChroprep RP-18 Micropak CH
6 13
a a
25-40 5-20; 25-40; 40-63 25-40 10
7
S
5; 7.5; 10
b.p. p. c. b.p. b.p. b.p. p.c. b.p. p.c. b.p. b.p. b.p. (40-63 only) b.p. b.p. p.c. b.p.
Reversed phase packings (totally porous} Allylphenyl Sil-X-I Bondapak C 18 (Porasil B) p Bondapak C 18
Nucleosil C 8 Nucleosil C 18
I
S
5; 7.5; 10
ODS S1-X-I
8
a
13+5
Partisil ODS
10
a
10
P.c
b.p. p.c. b.p. p.c. b.p. P.C
octadecylsilyl groups octadecylsilyl groups phenylsilyl groups
dimethylsilyl groups octadecylsilyl groups dimethylsilyl groups octylsilyl groups octadecylsilyl groups dimethylsilyl groups octylsilyl groups octadecylsilyl groups octadecylsilyl groups octylsilyl groups octadecylsilyl groups octadecylsilyl groups octadecylsilyl groups
W
phenylsilyl groups
5-1
b.p. p.c. b.p. p.c. b.p. p.c. p.c.
Phenyl Sil-X-I
8
a
13*5
Spherisorb ODs
9
S
10
Vydac TP Reverse Phase
1
a
10
Zorbax ODS
5
S
octadecylsilyl groups octadecylsilyl groups octadecylsilyl groups
Reversed phase packings (superficially porous or pellicular) Bondapak C 18 Corasil Bondapak Phenyl Corasil CO : PELL ODS ODS Sil-X-I1 Perisorb RP 2 Perisorb RP 8 Perisorb RP 18 Phermaphase ODS Vydax 201 RP Zipax HCP
14
S
37-50
b.p.
octadecylsilyl groups
14
S
31-50
b.p.
diphenylsilyl groups
1,lO 8 6
S S S
6 6
S
5 7 5
S S S
44-53 35 * 15 30-40 30-40 30-40 25-31 30-44 21-35
b.p. b.p. b.p. 5.p. b.p. b.p. b.p. b.p.
octadecylsilyl groups octadecylsilyl groups dimethylsilyl groups octylsilyl groups octadecylsilyl groups octadecylsilyl groups octadecylsilyl groups Zipax coated with an unpolar saturated hydrocarbon polymer layer
S
Polar and moderately polar chemically bonded silica packings (totally porous) (s. also silica ion exchangers) Bondapak Carbohydrate p Bondapak CN p
cyano sil-x-I Durapak Carbowax (Porasil C) Durapak OPN (Porasil) FE Sil-X-I
14
a
8-12
14
a
8-12
8
a
13*5
14
a
31-75
b.p. p.c. b.p. p.c. b.p. p.c. b.p.
14
a
31-75
b.p.
8
a
13t5
b.p. p.c.
cyano groups cyano groups Carbowax 400 chemically bonded oxydipropionitrile groups bonded by a Si-0-C bond fluoroether groups N
W
(Continued on p. 298)
4
APPENDIX C (continued] Trade name
Particle shape
Particle size (gm)
Trade form
Description
6
a
5; 10
diol groups
Micropak CN
13
a
10
Nucleosil CN
7
S
5 ; 10
Nucleosil NO,
7
S
5; 10
Partisil PAC
9
a
10
Sil6WN
7 7 7
a a a
5; 10 5; 10 10
b.p. p.c. b.p. p.c. b.p. p.c. b.p. p. c. b.p. p.c. b.p. b.p. b.p.
Lichrosorb DIOL
Sil 6O-NO2 Vydac TP polar
Supplier
alkylnitrile groups cyano groups nitro goups alkylnitrile groups cyano groups nitro groups
alkylnitrile groups
Polar and moderately polar chemically bonded silica pckings (superficinlly porous or pellicuhr) CO : PELL PAC Durapak Carbowax 400 Corasil Permaphase ETH Perisorb PA 6 vydac 501 PP Zipax ANH Zipax PAM
1 14
S
5 6 7 5
S
5
37-53 37-53
b.p. b.p.
nitrile groups Corasil coated with Carbowax 400
S
25-37 30-40 30-44 25-37
b.p. b.p. b.p. b.p.
S
25-37
b.p.
ether groups Perisorb coated with a polyamide propionitrile groups Zipax coated with a cyanoethylsiliwne polymer layer nylon coated Zipax
S
S S
APPENDIX D TOTALLY POROUS CHEMICALLY BONDED SILICA ION EXCHANGERS Trade name
Supplier
Particle shape
Particle size (urn)
Trade form
Description
b.p. b.p. b.p. b.p. b.p.
SCX; sulphonic acid groups SCX; sulphonic acid groups SCX; sulphonic acid groups SCX; sulphonic acid groups SCX; sulphonic acid groups
p.c. b.p. p.c. b.p. b.p. b.p. p.c. b.p. p.c. b.p. p-c. b.p. P*C b.p. p. c. b.p. b.p. b.p. b.p. b.p.
WAX; alkyl amino groups WAX; alkyl amino groups
ation exchangers
LiChrosorb KAT Nucleosil SA Partisil SCX Vydac TP Cation Zorbax SCX
6
a
10
I 10 I 5
S
5 ; 10
a a S
10 10 5-1
8
a a
1325 10
Anion exchangers p
Amino Sil-X-I Bondapak-NH,
14
Hypersil APS LiChrosorb AN LiChrosorb NH,
12 6 6
S
a a
5-1 10 5; 10
Micropak NH,
13
a
10
Nucleosil-N(CHJ
I
S
5; 10
Nucleosil-NH(CH,),
I
S
5; 10
Nucleosil-NH,
I
S
5; 10
Partisil SAX Sil-60 NH, Sil-60 N(CH,), Vydac TP Anion Zorbax SAX
10
I I 2 5
10 5; 10 5; 10 10 5-1
WAX; y-aminopropyl groups SAX; dimethylaminoethanol groups WAX; alkyl amine groups WAX; alkyl amine groups SAX; quaternary ammonium groups WAX; dimethylamino groups WAX; alkyl amine groups SAX; quaternary ammonium groups WAX; alkyl amine groups WAX; dimethyl amine groups SAX; quaternary ammonium groups SAX; quaternary ammonium groups h)
W W
W
APPENDIX E
0
0
SUPERFICIALLY POROUS AND PELLICULAR CHEMICALLY BONDED SILICA ION EXCHANGERS Trade name
Supplier
Particle shape
Particle size (pm)
Trade form
Description
14
S
31-50
b.p.
SCX; sulphonic acid groups
6
S
I
S
5
s
30-40 30-44 . 25-31
b.p. b.p. b.p.
SCX; sulphonic acid groups SCX; sulphonic acid groups SCX; sulphonic acid groups
14
S
31-50
b.p.
SAX; quaternary ammonium groups
6 5
S
b.p. b.p. b.p. b.p. b.p.
SAX; dimethylaminoethanol groups SAX; quaternary ammonium groups SAX; quaternary ammonium groups SAX; polymer layer on Zipax, strong basic functional groups WAX; polymer layer on Zipax, weak basic functional groups
Cation exchangers
Bondapak CX Corasil Perisorb KAT Vydac 401 SA Zipax SCX
Anion exchangers Bondapak AX Corasil Perisorb A N Permaphase AAX Vydac 301 SB Zipax SAX Zipax WAX
I
s
5
S
30-40 25-31 30-44 25-31
5
S
25-31
s
APPENDIX F SILICA PACKINGS FOR SIZE EXCLUSION CHROMATOGRAPHY
This list collects only packings with graduated pore sizes Trade name
Supplier
Particle shape
Particle size (pm)
Trade form
Description
40-63 63-125 40-63 63-125
b.p.
SBET= 150 m’/g; V p = 0.65 ml/g D =20nm SBET= 50 m2/g; V p = 0.65 ml/g D =50nm
Fractosil200
6
a
Fractosil500
6
a
b.p.
b.p.
a
40-63 63-125 63-125
6
a
63-125
b.p.
Fractosil 10000
6
a
63-125
b.p.
Fractosil25000
6
a
63-125
b.p.
LiChrospher Si 100
6
S
5; 10; 20
for pore structure data see .4ppendix A aMw= 3*1U3-5*1U4 [ 5 ]
S
5; 10 10 10 10
b.p. p.c. b.p. b.p. b.p. b.p.
37-75 75-125 31-75 75-125 37-75 75-125 31-75 75-125 37-75 75-125 37-75 75-125
b.p.
for pore structure data see Appendix A
7 7 7 7 7 7
b.p. b.p. b.p. b.p. b.p. b. p.
Fractosil 1000
6
a
Fractosil2500
6
Fractosil5000
LiChrospher LiChrospher LiChrospher LiChrospher
Si 300 Si 500 Si 1000 Si 4000
6 6 6 6
S
S
S
Porasil A
14
S
Porasil B
14
S
Porasil C
14
S
Porasil D
14
S
Porasil E
14
S
Porasil F
14
S
Spherosil XOA 400 Spherosil XOA 200 Spherosil XOB 075 Spherosil XOB 030 Spherosii XOB 015 Spherosil XOC 005
11 11 11 11 11 11
S S S
S S S
b.p.
SBET= ZU m’/g; Y p = U.bU ml/g
D = 100nm SBET= 10 m2/g; VP = 0.60 ml/g D =250nm SBET= 3 m’/g; V p = 0.50 mug D =500nm SBE;T = 2 m’/g; V p = 0.50 ml/g D =lOOOnm SBET= 0.8 m’/g; V p = 0.40 ml/g D =2500nm
-
AMW= 1.5e1u4-1.5~105 [51 a M W = 3*1U4-2*1U6 [ 5 ] aMw= 1O5->7*1O6 [ 5 ]
b.p. b.p. b.p. b.p. b.p.
for pore structure data see Appendix A
W
s
302
REFERENCES 1 R.E. Majors,Inrerrz. Lob.. Nov./Dec. (1975) 71. 2 ti. EngeJhardt, HochdncckflilssigchrorPlotogmphie,Springer-Verlag, New York, 1977. 3 Data measured by the author for a given batch. 4 K. Unger, P. Ringe, J. Schick-Kalb and B. Straube, Z. Anal. Chem, 264 (1973) 267. 5 JJ. Kirkland,J. Ommutogr., 125 (1976) 231.
Addendum
A recent survey about commercial packing is given by R.E. Majors, J. Chromutogr. Sci., 15 (1977) 334.
303
List of symbols and abbreviations activity of ion A in the solution (see eqn. 8.3) activity of ion A in the exchanger phase (see eqn. 8.3) amount of tritium atoms at the surface (moles of tritium atoms) (see eqn. 3.28) amount of tritium atoms in the vapour phase (moles of tritium atoms) (see eqn. 3.27) initial amount of tritium atoms in HTO (moles of tritium atoms) (see eqns. 3.27 and 3.28) absorbance (optical density) (see eqn. 3.21) constant in eqns. 6.15,6.18 and 6.19 charged species to be separated in ion-exchange chromatography (see p- 249) percent availability (actual fraction of the pore volume accessible to the solute) (%) (see eqn. 3.131) percentage availability at infrnite dilution (%)(see eqn. 3.13 1) mean molecular cross-sectional area of physisorbed and chemisorbed molecule or group, respectively (nm2/molecule) (see eqn. 3.7 1) molecular surface area of an adsorbed solute molecule (nm2/molecule) (see eqns. 6.8 and 6.9) specific surface area of support normalized to unit volume of the column (cm’/ml) (see eqn. 7.12) Van der Waals constant (nmp (see eqn. 3.72) constant in eqns 6.18 and 6.19 charged species to be separated in ion-exchange chromatography (see p. 249) concentration of solute i at infinite dilution in phase (Y and phase 0, respectively (molell) (see eqn. 7.3) molar concentration of solute X in the non-adsorbed phase (mole/l) (see eqn. 6.6) molar concentration of solute X in the adsorbed phase (molell) constant in the BET equation (see eqns. 2.4 and 2.5) concentration (molell) total equivalent concentration of ions in the solution (mequiv./l) (see eqn. 8.18) equilibrium concentration of ion A in the solution (mequiv./l) (see eqn. 8.18) concentration of the dispersive moieties in the mobile phase (molell) (see eqn. 6.14) concentration of the dispersive moieties in the stationary phase (molell) (see eqn. 6.14) final solution concentration (molelml) (see eqn. 3.13 1) initial solution concentration (mole/ml) (see eqn. 3.13 1) concentratinn of solute nnlvmer oer unit volume of mobile phase
304
concentration of the polar moieties in the mobile phase (mole/l) (see eqn. 6.14) concentration of the polar moieties in the stationary phase (molell) (see eqn. 6.14) concentration of solute polymer per unit volume of stationary phase (glml) (see eqn. 9.1) path length (cm) (see eqn. 3.19) diameter of a column bed (mm) diameter of a pore cavity (nm) molecular diameter of reactant i (nm) (see p. 97) particle diameter (pm) particle diameter at 36.8%cumulative oversize (pm) (see eqns. 4.9 and 4.10) projected area diameter of a particle (see p. 147) sieve diameter of a particle (pm)(see p. 147) arithmetic mean of the particle diameter (pm) (see eqn. 4.7) effective mean particle diameter derived from column permeability K m (p) (see eqn. 5.17) geometric mean of the particle diameter (pm) (see eqn. 4.8) mean particle diameter at 50% of the cumulative frequency distribution (pm) (see p. 150) mean particle diameter at the maximum of the relative frequency distribution (pm) (see p. 150) number average of particle diameter (pm)(see eqn. 4.1) number-weight average particle diameter (pm) (see Table 4.1) surface average particle diameter (pm) (see eqn. 4.2) surface diameter (pm) (see p. 147) surface volume diameter (pm) (see p. 147) Stoke’s diameter of a particle (pm)(see pp. 147-148) volume diameter ( p ) (see p. 147) weight average particle diameter (pm) (see eqn. 4.3) weight-number average particle diameter (pm) (see eqn. 4.6) weight-surface average particle diameter (pm) (see eqn. 4.5) = dp (median) (pm) molecular diameter of reactant molecule (di) (see p. 105) thickness of a porous layer (pm) (see eqn. 4.1 5) diameter of a pore throat (pm) (see eqn. 2.37b) mean pore diameter (nm) constant in the Dubinin-Radushkevich equation (see eqn. 2.7) width of a wide body (nm) (see p, 17) pore diameter of cylindrically shaped pores (nm) (see eqn. 2.35) effective diffusion coefficient for solute i(m2/s) (see eqn. 3.68) effective diffusivity of solute i at Knudsen diffusion (m2/s) (see eqn. 3.69)
om)
dp (arithm) dPeff dp (geom) dp (median) dp (mode)
305
D Wh
D2 E El (El+) R (Si-X)
.f” F Fd
hydraulic pore diameter (nm) (see eqn. 2.1) diffusivity of solute i in the bulk phase (m2/s) (see eqn. 3.68) diffusion coefficient of a solute i in the mobile phase m (m2/s) mean pore diameter according to the Kelvin equation (nm) (see eqn. 2.13) width of a narrow neck (nm) (see p. 17) diffusion coefficient of a solute polymer (m2/s) (see p. 276) mean particle diameter of a solid sphere (nm) (see eqn. 2.38) mean pore diameter according to the Washburn equation (nm) (see eqn. 2.14) mean pore diameter according to Wheeler (nm) (see eqn. 2.36) constant being proportional to the reciprocal of the slope of the log-linear calibration curve in SEC (see eqn. 9.8) separation impedance (bar S m2/Ns) (see eqn. 6.59) eluent ion (= mobile counter ion provided by the eluent) (see p. 249) bond energy of a silicon-element (X) bond (kJ/mol) (see Table 3.4) moles of hydroxyl groups reacted divided by moles of modifier reacted (see eqn. 3.56) volume flow rate of the eluent (ml/min) (see eqn. 5.1 1) flow rate (ml/min) dispersion forces between solute and mobile phase (see eqns. 6.12 and 6.13) dispersion forces between solute and stationary phase (see eqns. 6.12 and 6.13) polar forces between solute and mobile phase (see eqns. 6.12 and 6.13) polar forces between solute and stationary phase (see eqns. 6.12 and 6.13) gravitational constant
HPLC i I I I 10 90
height, falling depth of a spherical particle (cm) (see eqn. 4.12) reduced plate height (see eqns. 5.19 and 6.51) apparent plate height (mm, pm) (see p. 276) plate height or height equivalent to a theoretical plate (mm, pm) (see eqns. 5.18 and 6.50) high-performance liquid chromatography intercept of a straight line transmitted intensity (see eqn. 3.19) ionization potential of adsorbate (eV) (see Table 3.3) Knox-Parcher ratio (see eqn. 6.61) incident intensity (see eqn. 3.19) initial activity of tritium labelled water (pCi/ml) (see p. 72)
306
9' 9l' I EC
activity of the condensed vapour HTO (pCi/ml) (see p. 72) activity of Si-OT groups in the dissolved silica (pCi/ml) ionexchange chromatography
~HTO
correlation coefficient for HTO exchange (see p. 73) capacity factor of solute capacity factor to characterize retention in size exclusion chromatography (see eqn. 9.5) (see eqn. 2.3 1) distribution coefficient of a solute in LSC (see eqns. 6.12 and 6.16) distribution coefficient of solute at adsorption (see eqn. 6.6) separation factor of ion A (see eqn. 8.6) separation factor of ion B (see eqn. 8.7) selectivity coefficient of ion A according to eqn. 8.8 adsorption coefficient of solute i (ml/cm2) (see eqn. 7.12) stoichiometric equilibrium constant of A and B, respectively (see p. 250) thermodynamic equilibrium constant of A (see eqn. 8.3) thermodynamic equilibrium constant of B (see eqn. 8.4) permeability of an open tube (cm2) (see eqn. 5.1 1) stoichiometric equilibrium constant for the ion-exchange reaction between a cation exchanger in the H form and a metal ion (see eqn. 3.1 16) thermodynamic equilibrium constant for the ion-exchange reaction between a cation exchanger in the H form and a metal ion (see eqn. 3.1 14) isotopic equilibrium constant (see eqn. 3.25) distribution coeficient of solute i at infinite dilution (see eqns. 6.21 and 7.3) static distribution (partition) coefficient of solute i at infinite dilution (see eqn. 7.1 2) dynamic distribution (partition) coefficient of solute i at infinite dilution (see eqns. 7.12 and 7.13) distribution (partition) coefficient of solute i (see eqn. 7.1) distribution (partition) coefficient of solute i at infinite dilution (see eqn. 7.2) permeability according to the Karman-Cozeny equation (cml) (see eqn. 5.12) distribution coefficient in size exclusion chromatography (see eqn. 9.1) chromatographic column permeability (cm2) (see eqn. 5.13) thermodynamic equilibrium constant of solute at adsorption (see eqn. 6.5) distribution coefficient of metal ion in ion-exchange equilibrium (see eqn. 3.1 15)
k' k" K = RT/-/ K
KO KA KB KJ:
KF
KHM"
Ki cKiO
KKC
KM"
307
LC LLC
Ls LSC m m
M
n
n n = 3/(R +I) nA nA OH(react) nOH(total)
nS ,ad nX nX,ad
N N
length of a cylindrical pore (nm) (see eqn. 3.80) reduced column length (see eqn. 6.54) column length (mm, cm) hydrocarbonaceous ligand in RPC (see eqn. 6.22) mean length of projection of the molecule along an infinite number of axes (nm) (see p. 272) liquid chromatography liquid-liquid (partition) chromatography complex between the hydrocarbonaceous ligand, L, and the solute, S, in RPC (see eqn. 6.22) liquid-solid (adsorption) chromatography mass 63) slope of the straight line in the evaluation of the micro pore distribution (m*/g) (see pp. 32,33) molecular weight (glmol) molarity (mol/l) minimal difference in mean molecular weight between two solutes being completely resolved in SEC (see eqn. 9.1 I) mean molecular weight in SEC above which the polymer solutes will be totally excluded (exclusion limit) (dmol) metal number average molecular weight (g/mol) weight average molecular weight (glmol) molecular weight distribution number of adsorbed layers in the three-parameter BET equation (see eqn. 2.4) number of contacts (coordination number) in an assembly of spheres (see Table 2.5, p. 41) number of hydroxyl groups involved in surface reaction (see eqn. 3.106) number of moles of ion A in the solution (see eqn. 8.18) number of moles of ion A in the exchanger phase (see eqn. 8.17) number of hydroxyl groups that undergo reaction (see eqn. 3.70) total number of hydroxyl groups present before reaction (see eqn. 3.70) total moles of solvent molecules in the non-adsorbed phase (see eqn. 6.3) total moles of solvent molecules in the adsorbed phase (see eqn. 6.4) total moles of solute molecules in the non-adsorbed phase (see eqn. 6.3) total moles of solute molecules in the adsorbed phase (see eqn. 6.4) Avogadro's number (molecules/mol) number of theoretical plates (see eqn. 6.49)
normality (mequiv./l) total amount of water (mol OH/g adsorbent) (see eqn. 3.2) surface concentration of geminal hydroxyl groups (pmol/m2) (see eqn. 3.7) surface concentration of isolated hydroxyl groups (pmol/m2) (see eqn. 3.7) surface concentration of vicinal hydroxyl groups (pmol/mz) (see eqn. 3.7) molar fraction of X (solute) in the non-adsorbed phase (see eqn. 6.3) molar fraction of X (solute) in the adsorbed phase (see eqn. 6.4) pressure (bar, Pa) weighting factor (see p. 149) isoelectric point of enzyme and protein, respectively (isoelectric pH) (see Table 9.1) negative logarithm of the dissociation constant of an acid vapour saturation pressure (bar, Pa) percentage of undersize (%) (see eqn. 4.14) critical pressure (bar) (see eqn. 3.72) probability of the solute molecule interacting with the dispersive moieties of the mobile phase (see eqns. 6.12 and 6.13) probability of the solute molecule interacting with the dispersive moieties of the stationary phase (see eqns. 6.12 and 6.13) probability of the solute molecule interacting with the polar moieties of the mobile phase (see eqns. 6.12 and 6.13) probability of the solute molecule interacting with the polar moieties of the stationary phase (see eqns. 6.12 and 6.13) polydispersity of a polymer sample (see eqn. 9.13) phase ratio in LLC (see eqn. 7.10) equilibrium concentration of ion A in the exchanger phase (mequiv./g) (see Fig. 8.4) differential heat of adsorption (kJ/mol) (see eqn. 3.37) isosteric heat of adsorption (kJ/mol) (see eqn. 3.36) st qa(hydroxylated) isosteric heat of adsorption of a reference molecule of group (a) on the hydroxylated silica surface (kJ/mol) (see eqn. 3.39) st qb,d(hydroxylated) isosteric heat of adsorption of a molecule of group (b) and (d) on the hydroxylated silica surface (kJ/mol) (see eqn. 3.39) AQ difference between the differential heat of adsorption on a fully hydroxylated and a completely dehydroxylated silica at a coverage of about half a monolayer of the given adsorbate (see eqn. 3.38) break-through capacity (mequiv./g) (see p. 259) QB Qspecific QW
Qo
st st =qb,d(hydroxylated) -qa(hydroxylated) (see eqn. 3.39)
wcight capacity (mequiv./g) (see p. 259) theoretical specific capacity (mequiv./g) (sec p. 259)
309
r rh rji
rmax rmax(mod)
R R
R R
- 200(n-1) n
Re Rji
RP RS
S
S
so SBET SBET(2)
radius of a cylindrical pore (see eqn. 3.80) hydraulic radius (nm) (see eqn. 2.28) selectivity coefficient (see eqn. 9.6) most frequent pore radius of the relative pore volume distribution of untreated silica (nm) (see eqn. 3.87) most frequent pore radius of relative pore volume distribution of treated silica (nm) (see eqn. 3.87) revolutions per minute (min-') elution distance of a retained solute peak measured as a distance on recorder chart (mm) (see p. 230) elution distance of an unretained solute peak measured as a distance on recorder chart (mm) (see p. 230) universal gas constant (J/mol K) hydrolysis ratio (ratio of HCl on hydrolysis to HCl on initial reaction) (see eqn. 3.105) cumulative percentage weight oversize retained on a sieve of a given aperture (%) (see eqn. 4.9) hydrolysis ratio (ratio of hydrogen evolved to diborane consumed) (see p. 64) percentage of hydroxyl groups bonded as pairs at the silica surface (%) (see eqn. 3.107) root mean square radius of gyration of a solute polymer (nm) (see p. 272) Reynolds number resolution related to solute j and i, respectively, in column liquid chromatography (see eqns. 9.6 and 9.7) reversed phase resolution in SEC related to molecular weight difference (see eqns. 9.8 and 9.9) resolution in SEC normalized to units of column length (see eqn. 9.10) chart speed of a recorder (mm/s) (see p. 230) slope of a staight line surface area of a cylindrical pore ( m') (see eqn. 3.80) solvent molecule (see eqn. 6.1) molecular weight selectivity in SEC (see eqn. 9.4) reciprocal of the gradient of the linear portion of the log-linear calibration curve in SEC (see eqn. 9.14) solubility (g/g;mol/g) (see p. 13) specific surface area (m'/g; m'/ml) relative adsorption energy of the solute interacting with the surface of the standard adsorbent (see eqns. 6.8 and 6.9) specific surface area according to BET (m'/g) (see eqn. 2.19) specific surface area according to the two-parameter BET equation (m'/g) (see eqns. 2.4 and 2.19)
310
SBETQ)
SBET*
SKk
TSAs
U
USt
vt
V V V
vo
specific surface area according to the three-parameter BET equation (m'/g) (see eqns. 2.5 and 2.19) specific surface area of support corrected by the weight increase due to surface modification (m'/g) (see eqn. 3.74) cumulative surface area (m'/g) (see p. 3 1) external surface area (m2/g) (see eqn. 2.2) size exclusion chromatography internal surface area (m2/g) surface area of group i of cores (m'lg) (see eqns. 2.25 and 2.30) specific surface area according to the Kaganer method (m'/g) (see eqn. 2.20) specific surface area according to the Kiselev method (m'/g) (see eqn. 2.10 and p. 28) specific surface area according to the Langmuir equation (m'/g) (see eqns. 2.6 and 2.19) surface area of a cylindrical pore after modification (m') (see eqn. 3.82) specific surface area according to the Sears method (m2/g) (see p. 3 1) specific surface area according to the Sing method (m'lg) (see eqn. 2.23) specific surface area according to the t method (m2/g) (see eqn. 2.21) time (s, min, h) statistical thickness of an adsorbed layer (nm) (see eqn. 2.21) thickness of a chemisorbed layer (nm) (see pp. 105,106) elution time of an unretained solute (s) (see eqn. 6.47) absolute temperature (K) transmittance (see eqn. 3.21) column temperature (K) critical temperature (K) (see eqn. 3.72) total hydrocarbonaceous surface area of ligand L (bonded n-alkyl group) (nm'lgroup) (see p. 208) total hydrocarbonaceous surface area of a solute, S (nm'lgroup) (see p. 208) linear velocity (mmls) (see eqn. 6.52) terminal velocity of a sphere during settling (cm/s) (see eqn. 4.1 1) pipette volume at time to(start) (ml) (see eqn. 4.14) retention volume (ml) (see p. 24) retention volume related to unit mass of adsorbent in the column (ml/g) (see P. 24) pipette volume at a given time t (ml) (see eqn. 4.14) volume (ml, 1) volume of a cylindrical pore (ml) (see eqn. 3.81) corrected retention volume (ml) (see eqn. 6.16) dead volume of column (ml) (see eqn. 6.9)
311
VO
va
Vi
vm
vi
Vdacc.)
void volume of column in SEC (Vm) (ml) (see eqn. 9.2) volume of adsorbed solvent monolayer per unit mass of adsorbent = specific surface area x thickness of adsorbed monolayer of solvent (ml/g) (see eqns. 6.2 and 6.7) volume of the column (ml) volume decrement of capillary condensed liquid (see eqn. 6.37) elution volume of solute in SEC (ml) (see eqn. 9.2) hydrodynamic volume of a dissolved polymer ( n m 3 interstitial volume of column and pore volume accessible t o solute i (ml) (see eqn. 9.1 1) specific micropore volume (ml/g) (see p. 33) internal volume originating from the pore space of the packing (ml) (see eqn. 5.1) interstitial volume due to interstices and voids between particles (ml) (see eqn. 5.1) interstitial volume of column and pore volume accessible to solute j (ml) (see eqn. 9.1 1) molar volume (ml/mol) volume of mobile phase (ml) (see eqn. 7.5) volume of mobile phase at maximum load (ml) (see eqn. 7.8) volume of mobile phase at minimum load (ml) (see eqn. 7.8) volume of a cylindrical pore after modification (ml) (see eqn. 3.83) specific pore volume (ml/g) cumulative specific pore volume (ml/g) (see p. 32) cumulative pore volume of micropores (ml/g) specific pore volume obtained from density measurements (ml/g) (see p. 3 1) specific pore volume according to Dubinir-Radushkevich (ml/g) (see eqn. 2.7) specific pore volume according to Fisher and Mottlau (ml/g) (see p. 3 1) specific pore volume according t o the Gurvitsch rule (ml/g) (see eqn. 2.24) specific pore volume from porosimetry measurements (ml/g) (see p. 32) specific pore volume of macropores (ml/g) specific pore volume of mesopores (ml/g) specific pore volume of micropores (ml/g) corrected retention volume of an adsorbate in GC (ml/g) (see p. 80) absolute retention volume (ml/m2) (see p. 80) volume of stationary phase (ml) (see eqn. 7.5) volume of solvent held stationary in the pores of packing in SEC (ml) (see eqn. 9.2) specific volume of pure solid (ml/g) volume of stationary liquid normalized to unit volume of the column (ml/ml) (see eqn. 7.12) pore volume of packing accessible to the solute in SEC (ml) (see eqn. 9.3)
312
vp
W W W*
Wr W/W
W W WO
volume of stationary liquid at maximum load (ml) (see eqn. 7.8) volume of stationary liquid at minimum load (ml) (see eqn. 7.8) volume of the purely solid packing (ml) (see eqn. 5.1) molar volume of compound a! that constitutes a phase (ml/mol) (see eqn. 7.4) molar volume of compound @ that constitutes a phase (ml/mol) (see eqn. 7.4) water content of adsorbent (mol HzO/mol SiOJ (see p. 60) mass of chemisorbed species (g/g adsorbent) (see eqn. 3.74) chemically bonded water of adsorbent (mol HzO/mol SiO,) (see p. 60) peak width at half-height for retained solute peak measured as a distance on recorder chart (mm) (see p. 230) weight percent (%) mass of silica in the solution (g) (see eqn. 3.131) mass of adsorbent in the column (9) (see eqn. 6.9) specific volume of micropores (ml/g) molar fraction of solute i in phase a! and 0, respectively (see eqn. 7.1) molar fraction of solute i at infinite dilution in phase (Y and 0, respectively (see eqn. 7.2) ionic functional site of an ion exchanger (see p. 249) amount of hydroxyl groups (mol OH/g adsorbent) (see eqn. 3.2) solute molecule (see eqn. 6.1) amount adsorbed at a given relative pressure (g/g, mol/g, ml NTP/g) equivalent fraction of ion A in the solution (see eqn. 8.18) equivalent fraction of ion A in the exchanger phase (see eqn. 8.17) equivalent ionic fraction (see eqn. 3.1 19) specific monolayer capacity (mollg)
Y(Y -,Y+) Y z = PIP0 ZA
ZB a! a!
a!
a! a! (Yo
mobile counter ion maintaining electroneutrality in ion-exchange separation (see p. 249) amount of physisorbed water (mol HzO/g adsorbent) (see eqn. 3.2) relative pressure charge of ion A (see p. 249) charge of ion B (see p. 249) surface concentration of physisorbed or chemisorbed groups hmol/mz) relative activity of an adsorbent referred to a standard adsorbent with a! = 1.OO (see eqn. 6.8) separation factor in terms of equivalent fraction ratios (see eqn. 8.19) correction factor (see eqn. 9.14) volume polarizability (nm3) thermodynamic separation factor (see eqn. 8.5)
313 ffexp
&OH (i) &OH($ “OH(t) ffS
E f
E0
9 9
6
e e 8 0.1
surface concentration of chemisorbed species under given conditions (pmol/mz) (see eqn. 3.74) maximum surface concentration of chemisorbed species (pmol/m2) (see eqn. 3.73) concentration of inner hydroxyl groups (pmol/mz) (see p. 62) concentration of surface hydroxyl groups (pmol/m2) (see p. 62) total concentration of hydroxyl groups (pmol/mz) (see p. 62) amount adsorbed at a given relative pressure divided by the amount adsorbed at p / p o = 0.4 (see eqn. 2.22) surface tension (N/m) additive term in the expanded Snyder equation corresponding to secondary adsorbent activity effects (see p. 191) molar extinction coefficient (l/mol cm) (see eqn. 3.19) porosity relative adsorption energy of the eluent per unit surface area of the standard adsorbent (= solvent strength) (see eqns. 6.8 and 6.9) interstitial porosity of a packing internal porosity of a column (see eqn. 5.3) interstitial porosity of a column (see eqn. 5.2) particle porosity fraction of column volume filled up by the purely solid packing (see eqn. 5.4) total porosity of the column = fraction of column volume filled with the eluent (see eqns. 5.5 and 5.6) dynamic viscosity (Ns/m2) effectiveness factor (see eqn. 3.70) shape factor in the Karman-Cozeny equation (see eqn. 5.12) surface coverage (see eqn. 3.75) contact angle between a liquid and a solid surface (”) linear sample capacity of an adsorbent (= sample size in gram per gram adsorbent which causes a 10%relative change in the “Ki,value on the linear part of the isotherm) (g/g) (see p. 202) mean free path of a solute i (nm) (see p. 97) chemical potential kinematic viscosity (m2/s) reduced velocity (see eqn. 5.20) wavelength (cm-’) frequency shift of hydroxyl groups in IR spectroscopy (cm-’) performance index (see eqn. 6.58) density (glml) density of glass beads (g/ml) (see eqn. 4.1 5) apparent density due to helium (g/ml) apparent densitj due to mercury (g/ml) density of polyethoxysiloxane (g/ml) (see eqn. 4.1 5 ) standard deviation
314
uv 2 (column)
av2 @
V(i)
(MWD) U P (total) 7
@ 4s J/
variance of the peak caused by mixing phenomena in the column (m1') variance of the peak caused by extra-column effects (ml') (see eqn. 9.12) standard deviation in volume units of an eluted peak of solute i (ml) (see eqn. 9.9) variance of the peak due to molecular weight distribution (m12) (see eqn. 9.12) total variance of eluted peak in SEC (m1') (see eqn. 9.1 2) tort uosi t y factor column resistance factor volume of stationary phase (EVS)(ml) (see eqn. 6.16) void fraction
315
Subject Index A Absorbance 69 Absorption bands, of hydroxyl groups 59 Acetic acid, solvent strength on silica 190 Acetic anhydride, reaction with silica 118 Acetone - retention on silanized silica 81 - solvent strength on silica 190 Acetonitrile-methanol mixtures, solvent strength on diol-modified silica 219 Acidic surface sites at silica, structural models 131,132 Acidity of silica surface 130-133 Activation energy of methylchlorosilanes at reaction on silica 98,99 Activation procedures of silica in LSC 199 Adsorbates, classification according to Kiselev 77 Adsorbent-adsorbate interactions 76 Adsorbents, classification according to Kiselev 77 Adsorbent standardization 222-233 - chromatographic standardization 229-233 capacity factor 231 chromatographic permeability 23 1 elution time 231 flow resistance parameter 231 Knox-Parcher ratio 232 mean linear velocity 231 number of theoretical plates 231 performance index 23 1 plate height 231 reduced column length 23 1 reduced plate height 231 reduced velocity 231 separation impedance 23 1 test conditions 230-233 test solutes 232,233 total column porosity 232 - physico-chemical standardization 223-229 morphology and size of particles 223-229 pore distribution 226-229 specific pore volume 225-226 specific surface area 223-224 stability 229 Adsorption 76-78 - of alcohols on silica 82, 83 of n-alkanes on silica 79 - of argon on silica 79 - of benzene on silica 79 - of carbon tetrachloride on silica 79 - of krypton on silica 79 - of mesitylene on silica 79 - of methane on silica 79
- of oxygene on silica 79 - of toluene on silica 79 - of water on silica 82, 83 - of xenon on silica 79 - of p-xylene on silica 79 Adsorption, equilibrium 77,78 Adsorption, heat of adsorption, definitions 78 Adsorption, in LSC - adsorption process 188 - distribution coefficient 188 - opcrating forces in solute-adsorbent interactions 188,192 ’ - retention mechanism 187-193 - Snyder equation 189-191 - thermodynamic equilibrium constant 188 Adsorption isobar, definition 78 Adsorption isoster, definition 78 Adsorption isotherm, definition 77 Aerosil - adsorption of argon, benzene, krypton, mesitylene, methane, nitrogen, oxygen, toluene and p-xylene 79 - concentration of surface hydroxyl groups 67 - fluorination 113 - formation of =Si-H surface bonds 115, 116 - IR absorption bands 9-11 - IR spectroscopy on deuterated samples 61, 62 - 1R studies 59,60 - isotopic exchange with HTO 73 - reaction with alcohols 117 - reaction with trimethylchlorosilane 102, 103 - reaction with thionylchloride 113 Ageing of silica hydrogel 45 Agglutination of silica particles 50,51 Aggregation, definition 2 Albumin, separation on chemically modified silica 285 Alcohols - adsorption on silica 82,83 - reaction with silica 116, 117 Aldolase (rabbit), separation on chemically modified silica 285 n-Alkanes - adsorption on silica 79 - separation by means of SEC 282 Alkylamines in buffer solution, used in RPC 211 n-Alk ylchlorosilanes - in synthesis of reversed-phase packings 212, 214,215 - reaction with silica 120,121 surface concentration 121 Allen-Bradley sonic sifter 153 Allylphenyldichlorosilane as modifier in RPC 214
316
Alpine air jet sifter 153, 154 Alpine Multiplex Zig-Zag Classifier 160, 161 Alumina, modified with 2-(4-pyridyl)ethylsilyl groups 256 Aluminium tribromide, reaction with silica 125, 126
Aluminium trichloride, reaction with silica 125,
BET - theory 20 - three-parameter equation 20, 21 - two-parameter equation 21 Biopolymers - separation by means of SEC 282-285 - size exclusion on chemically modified silica 285
126
Amination of chloromethylated benzylsilica
Bond energy terms for silicon-element bonds 85
256
Amines, biogenic, separation on chemically modified silica ion exhangers 268 Amino acids, separation on reversed phase silica packmgs 2 17 l-Aminoethyl-3-aminopropyltrimethoxysilane, reaction with silica 94 - stoichiometry 94 7-Aminopropyltriethox ysilane, reaction with silica 258 Analysis time in SEC 281 Aniline derivatives, separation on LiChrosorb 205
Anion exchanger see ion exchanger 259 Anthrachinones, separation on LiChrosorb 204 Argon - adsorption on silica 79 - molecular cross-sectional area on adsorption 79
Aromatic alcohols, separation on LiChrosorb 205
B Balanceddensity slurry technique 176 Benzene - adsorption on silica 79 - solvent strength on silica 190 Benzoyl chloride, reaction with silica 118 Benzylammoniumchloride, sorption isotherm on sulphonic benzylsilica exchanger 264 Benzylchlorosilane, reaction with silica 123 Benzyldimethylchlorosilane as modifier in RPC 214
Benzyllithium, reaction with chlorinated silica 256
Benzylsiloxane, sulphonated, co-precipitation with sodium silicate 258 Benzylsiioxane xerogel, formation 111 Benzyltrichlorosilane as reagent in synthesizing surface modified ion exchanger 255 Benzyltriethoxysilane - co-hydrolysis and co-condensation with tetraethoxysilane to benzylsiloxane xerogel 111 - hydrolysis and condensation to benzylethoxysiloxane 258
Boron trichloride, reaction with silica 64,124, 125
Bridge structures, types of bonds in chemical modification of silica 84 w-Bromo-n-alkylchlorosilanes,reaction with silica 255,256 Bulk modification of silica 88-91 - synthesis of ion exchangers 258 n-Butanol, reaction with silica 117 n-Butyldimethylchlorosilane as modifier in RPC 215 n-Butyldiphenylchlorosilane as modifier in RPC 215
C Cabosil see Aerosil Calibration curve in SEC 272-274 - molecular weight range 278,279 Capacity factor - dependence on hydrocarbonaceous surface area of solute and ligand, resp. 208 - dependence on relative water content in LSC 201 - dependence on specific surface area of packing in LSC 198,199 - in adsorbent standardization 231 - of solutes in SEC 274,275 - vs. chain length of n-alkyl group in RPC 216 - vs. surface coverage of n-alkyl group in RPC 213
Capacity of ion exchanger - break-through capacity 259 - determination 259 - theoretical specific capacity 259 - weight capacity 259 Capacity of silica in ion exchange 133 Capillary condensation 22 Carbamates, separation o n LiChrosorb 205 Carbinol groups, absorption band in 1R spectroscopy 10 Carbon tetrachloride, adsorption on silica 79 Carboxylic acids, separation on chemically modified silica ion exchanger 266 Catalase (beef), separation on chemically modified silica 285
317
Cation exchanger see ion exchanger 259 Chain length of n-alkylsilyl bonded silica packings, effect on surface concentration 213 Charge transfer between an adsorbed molecule and the adsorbent surface 76 Chemically bonded silica packings - as packings in LSC 206-235 - bulk modification 88-91 - functionality of modifier 89 - methods of forming surface bonds 88-108 - polar chemically bonded silica packings in LSC 217-222 - product requirements 83 - stability of silicon-carbon bond towards nucleophilic and electrophilic reagents 87, 88 - stability of siloxane bond towards nucleophilic and electrophilic reagents 86, 87 - structure and stability of surface bonds 85-88 - synthesis 108-130 - types of bonds and functional groups 84, 85 Chemical modification of silica - basic concepts 84-108 - bulk modification 88-91 - definition83 - functionality of modifier 89 - mcthods of forming surface bonds 88-108 - structure and stability of surface bonds 85-88 - types of bonds and functional groups 84,85 Chemisorption - in surface reaction a t silica 98 - vs. physisorption 76,77 Chlorinating reagents 1 13 Chlorination of silica surface 112, 113 Chloroacetic acids, separation on chemically modified silica ion exchanger 269 Chlorodimethyl-[4-(4-chloromethylphenyl)butyllsilane, reaction with silica 256 Chlorodimethyl-(4-phenylbutyl)silane,reaction with silica 256 Chlorome thylation - catalysts 256 - with chloromethyl methyl ether 256 2Chloropropane, solvent strength on silica 190 3Chloropropyltrichlorosilane, reaction with silica 256,257 Chromatographic permeability in adsorbent standardization 231 Chymotrypsinogen A (beef), separation on chemically modified silica 283 Coacervation, definition 2 Coagulation, definition 2 Cocondensation of organotrialkoxysilanes 9 1
Coesit - density5 - structure4 Co-hydrolysis of organotrialkoxysilanes 9 1 Colloidal dispersion, characterisation by means of SEC 287 Column - column porosity 170, 171 - comparison of performances 179-186 - factors influencing particle packing 172-1 75 - interstitial volume 271 - packing density and strength 173 - packing procedures and performance characteristics 169- 186 - pore systems of column bed 171, 172 - size relationships between d p d,, D, etc. 172 Column bed - geometrical analysis 169-172 - internal volume 170 - interstitial porosity, effect of column capacity in SEC 279 - interstitial volume 170 - packing stability, mechanical 174, 175 - volume of purely solid packing 170 Column coupling in SEC 279,281 Column length in SEC 280 Column packing see packings Column packing procedures 175-179 - dry packing technique 169, 175, 176 - packing stability (mechanical) 174, 175 - slurry packing technique 169, 176-179 general consideration 173-175 - tap-fii method 169 Column performance - column Permeability 180,181 - column stability 184,185 - plate height-velocity dependence 181-189 - properties (thermodynamic, kinetic, hydrodynamic) 180 Column permeability 180,18 1 - chromatographic permeability 180 - chromatographic permeability vs. effective mean particle diameter of packing 181 - column resistance factor 180,181 - of an open tube 180 - of a packed bed 180 Column resistance factor 180, 181 Column temperature - in SEC, effect on analysis time 278 - in SEC, effect on resolution 277,278 Condensation, intermolecular and intramolecular, of organosilanols 90 Condensation reaction of polysilicic acids, effect of pH and electrolyte concentration on rate of condensation 44
318
Controlled porosity glass - reaction with y-aminopropyltriethoxysilane 283 - reaction with 1,2epoxy-3-propoxypropyltrimethoxysilane 283 - separation of biopolymers on 283 Controlled porosity silica packings 49-52 - agglutination of finely dispersed non-porous silica particles 50,51 - controlled sintering 5 1,52 - modified sol-gel procedure followed by sintering 50 - polyethoxysiloxane procedure 50 Conversion, in surface reaction 99-104 - as function of temperature 100,102 - effectiveness factor 101,102 - maximum conversion 100, 102 Coordination number, in particle packing 172, 173 Co-polymerization of organotrialkoxysilanes 9 1 Corpuscular theory, in the formation of polyorganosiloxanes 9 1 Correlation coefficient in isotopic exchange reaction between silica and HTO 73 Coulter Counter 161 Cristobalite - density 5 - random partial dehydration 63 - structure4 Cross-sectional area - ofargon 27 - of hydroxyl groups 63 of krypton 27 - of nitrogen 27 - ofwater 27 Crystallinity of silica 1 Cytochrome C (horse heart), separation on chemically modified silica 285 ~
D DDT derivatives, separation on LiChrosorb 206 Deactivation procedures of silica in LSC 199, 200 n-Decylmethylchlorosilane as modifier in RPC 214 n-Decyltrichlorosilane as modifier in RPC 214 Dehydration of silica 8 Dehydroxylation of silica 8,57,58 Density - apparent, due to helium 26 of silica packings 157 apparent, due to mercury 26 Deuteration of surface hydroxyl groups 61,62, 70-72 Diborane, reaction with silica 64, 65 ~
Di-n-butyldichlorosilane as modifier in RPC 2 14 Dichloromethane-acetonitrile mixtures, solvent strength on diol-modified silica 219 Diethyl ether - retention on silanized silica 81 - solvent strength on silica 190 Diffusion - bulk diffusion vs. diffusion in porous media 98 - film diffusion in ion exchange 252 - particle diffusion in ion exchange 252 Diffusion coefficient - effective diffusion coefficient 97 - Knudsen diffusion coefficient 97 - related to the reduced velocity 182 Diffusion mechanism, in surface reaction 96-99 Dimethyldichlorosilane - as modifier in RPC 214 - reaction with silica 65,66 Diol-silica, bulk modified, capacity factors and selectivity coefficients 221 Diol-silica, k' us. relative water content of solvent 220 Dioxan, solvent strcngth on silica 190 Diphenyldichlorosilane, reaction with silica 255 Dispersions, characterisation by means of SEC 287 Dispersity of silica 1 Distribution coefficient, in ion exchange on silica 134 Distribution coefficient, in LSC 188, 192,193 Distribution coefficient, in SEC 271,272 Distribution coefficient, of solute in liquidliquid systems 237, 238 DNA nucleotide monophosphates, separation on chemically modified silica ion exchanger 266 n-Dodecyltrichlorosilane as modifier in RPC 214 Drugs, separation on chemically modified silica ion exchanger 268 Dry packing techniques 175,176
E Effectiveness factor in surface modification 101,102 Eluotropic series of binary solvents on untreated and diol-modified silica 219 Eluotropic series of solvents on silica 189, 190 Elution volume in SEC 271 Elutriation, in particle sizing 159-161 Enzymes - isoelectric point 285
319 recovery on chemically modified controlled porosity glass 283 recovery on 1,2dihydroxy-3-propoxypropylsilyl modified silicas 285 separation on chemically modified silica carrying 1,2dihydroxy-3-propoxypropylsilyl groups 284 Epoxy resins, separation by means of SEC 282 Ester bonds at the silica surface 116-124 Estersils 116 Ethyl acetate, solvent strength on silica 190 Exclusion limit in SEC 273 Exclusion of electrolytes from the pores of silica in cation exchange 139,140 Extinction coefficient 69
F Flocculating agents 174 Flocculation, definition 2 Flocculation of silica particles 173, 174 Flow resistance of a packed bed see column permeability Flow resistance parameter in adsorbent standardization 232 Fluid classification of particles 159-16 1 Fluorination of silica surface 113 Functionality of modifier, in surface modification of silica 92 G
Gas chromatography - absolute retention volume of adsorbate 80 - detection of surface polarity of chemically modified silica packings 81 - estimate of isostetic heat of adsorption 80 - retention behaviour of solutes on silanized silica 81 Gel chromatography see size exclusion chromatography Gel formation in the synthesis of silica 44 Gelling, definition 2 Gelling rate of silica hydrogels, controlling factor 44,45 Gel permeation chromatography see size exclusion chromatography Glass beads - coating with a thin porous layer 163-166 - permeability of columns packed with 181 Globular structure of silica 42-49 Glutathione, separation on chemically modified silica 285 yGlycidoxypropylsilane, reaction with silica 257
y-Glycidoxypropyltriethoxysilane (= 1,2epoxy-3-propox ypropyltriethox ysilane), co-hydrolysis and cocondensation with polyethoxysiloxane 112 Glycophase see porous glass treated with 1,2epoxy-3-propoxypropyltrimethoxysilane Gonell elutriator 160 Grignard reagents, reaction with chlorinated silica 115 H Haemoglobin (human), separation on chemically modified silica 285 Heat of adsorption - difference between the differential heat of adsorption on a fully hydroxylated and completely dehydroxylated silica 80 - differential 78 - differential vs. frequency shift of hydroxyl groups 79,80,82 - isosteric 78 estimate by means of GC 80 - specific heat of adsorption 80 Heat treatment of silica 57,59 n-Heneicosyltrichlorosilane as modifier in RPC 215 n-Heptane-dichloromethane mixtures, solvent strength on diol-modified silica 219 n-Hexyltrichlorosilane as modifier in RPC 214 Hydration of silica 57,59 Hydraulic diameter (radius) 272 Hydrocarbonaceous ligand - in RPC 207,208 - linkage to silica surface 209 Hydrocarbonaceous surface area of solute and ligand, resp., in RPC 208 Hydrodynamic volume in SEC 278 Hydrodynamic volume of polymers 272 Hydrogel see silica hydrogel Hydrolysis of R-Si-X bond, rate of 90 Hydrolysis ratio in the reaction of silica with diborane 6 4 , 6 5 Hydrolytic sorption in cation exchange on silica 135 Hydrothermal treatment of silica, effect of temperature, duration and water vapour pressure 48 Hydrothermal treatment of silica hydrogels and xerogels 47-49 Hydroxo complexes, sorption in cation exchange on silica 135, 136 Hydroxylamine silica, bulk modified, capacity factors and selectivity coefficients 221 Hydroxylation of silica 9 , 5 7 , 5 8
320 Hydroxyl groups 58-76 - absorption bands 59 - absorption bands in the frequency range between 2000 and 4000 cm-l 68-70 - bonded to water by hydrogen bonding 7 - bound hydroxyl groups 62,63 - completely hydroxylated silica surface 62 - deprotonation, dependence on pH 130 - determination by means of HTO exchange 72-76 - determination, correlation between U O H ( ~ ) and integrated intensity 70, 71 - deuteration of surface hydroxyl groups 61, 62 - distinction between hydroxyl groups and physisorbed water 58-61 - free hydroxyl groups 7 , 6 2 , 6 3 - geminal hydroxyl groups 63 - interaction with alcohol 82 - interaction with water 82 - internal vs. surface hydroxyl groups 6 1 , 6 2 - isolated hydroxyl groups 6 , 6 3 - isolated (free) hydroxyl groups, absorption bands in IR spectroscopy 10 - isotopic exchange with deuterated water 70-72 - isotopic exchange with HTO, procedure 73-75 - mean cross-sectional area 7,63 - paired hydroxyl groups 63 - ratio of paired to isolated hydroxyl groups 63 - reaction with diborane 64,65 - reaction with dimethyldichlorosilane 65,66 - reaction with methyllithium 66,67 - reactivity of surface hydroxyl groups in adsorption 76--83 - residual, in surface modification 103, 104 - role in adsorption 78-83 - spectral shifts at the adsorption of argon, krypton, methane, nitrogen, oxygen, xenon 79,82 - surface concentration 7 - surface concentration of hydroxyl groups vs. pretreatment temperature 67.75 - surface concentration, total 62 of internal hydroxyl groups 62 of surface hydroxyl groups 62 - surface hydroxyl groups, basic concepts 62, 63 - surface hydroxyl groups, determination 63-76 - surface hydroxyl groups, types 62,63 - types 6 - vicinal hydroxyl groups 7 , 6 3
I
“In column” deactivation of silica in LSC 200 Induction forces 76 Infrared spectroscopy of silica 9-1 1 - absorption bands in the frequency range between 2000 and 4000 cm-’ 68,69 - procedure 68 - sample preparation 9 - spectral shift of hydroxyl groups a t adsorption us. ionization potential of adsorbate 81, 82 Intensity - incident 69 - integrated 69 - integrated vs. surface concentration 70,7 1 - transmitted 69 Interactions, molecular - non-specific 7 - short range 76,77 - specific 17 ion exchange see also ionexchange chromatography W C ) - capacity, definition 259 - capacity, determination 259 - capacity of pellicular types 260 - capacity of totally porous types 260 - distribution coefficients 250, 251 - isotherm 203 - isotherms on sulphonic benzylsilica exchanger 264 - kinetics 252 fdm diffusion 252 particle diffusion 252 - regeneration 259 - selectivity 249-252 - selectivity coefficient 250 - selectivity constant 250 - selectivity, general 262-264 - selectivity of charged organic species 251 - selectivity rules 25 1 - separation factor 250 - stability, chemical 261 - stability, mechanical 261 - stability, test 261 - stability, thermal 261 - thermodynamic equilibrium constant 249 Ion-exchange chromatography (IEC) 249-270 - chemically bonded silica ion exchangers, commercial products 299, 300 - effect of eluent concentration 251 - evaluation of selectivity from batch experiments 252 - factors affecting selectivity 251 - kinetics, film diffusion 252
321 - kinetics of ion exchange 252 - kinetics, particle diffusion 252 - performance of 7-aminopropylsilica 267 - retention, effect of temperature 266 - retention, in alcohol/water solution as eluent 26 8 - retention of acids and bases 265, 266 - retention of non-ionized species 265,266 - selectivity 249-252 adsorption effects 268 effect of counter ion 265, 267 effect of ionic strength of eluent 265, 267 effect of pH of eluent 265,267 factors that influence selectivity 265, 266 - separation mechanism 249-252 strategy in evaluation of selectivity 264 Ion exchange properties of silica 130-141 - acidic surface sites, pK, value 132, 133 - acidic surface sites, structural models 131, 132 - applications 140 - capacity and exchange ability as a function of pH 133 - exclusion of electrolytes from the pores of silica 139,140 - isoelectric state and possibility of anion exchange 138 - kinetics 140 - mechanism of cation exchange on silica and the theory of selectivity 134-138 - selectivity of metal cations 137 - surface sites and origin of acidity 130-133 Ion exchanger types, porous and pellicular 252 Ionization potential of adsorbates, correlation with spectral shift of hydroxyl groups 81, 82 Isoelectric state of silica 138 Isotherms of water on silica 57, 58 Isotopic composition at deuteration of silica 71 Isotopic effect 62,72, 73 Isotopic equilibrium constant 71 Isotopic exchange of silica with deuterated water 61,62, 70-72 Isotopic exchange of silica with tritium-labelled water 72-76
K Karl Kscher titration 60, 6 I Karman-Cozeny equation 180 Kelvin equation 22 Kinetics in surface reactions 96-99 - chemisorption 98 - physisorption 98 - pore diffusion mechanisms 96-98
reaction order during reaction of methyl chlorosilanes at silica 98,99 Knox-Parcher ratio in adsorbent standardization 232 Knudsen diffusion 97 Krypton, adsorption on silica 79 -
L p-Lactoglobulin (COW),separation on chemically modified silica 26 Lambert-Beer law 69 Lambert-Bouguer law 69 Langmuir isotherm, equation 21 Latices, separation by means of SEC 287 LiChrosorb - dependence of k' on relative water content of eluent 201 - selectivity in LSC 204 - water sorption isotherms 196, 197 LiChrosorb NH,, separation of monosaccharides 22 1,222 LiChrosorb RP-8, separation of polyethylene glycols 286 LiChrospher, as packing in SEC 279,280 Lifetime, of column 184, 185 Linear sample capacity o f packings in RPC 216 Linear sample capacity of silicas in LSC 202, 203 Linear velocity of eluent in adsorbent standardization 231 Liquid-liquid chromatography (LLC) 237-248 - adsorption coefficient 244,245 - adsorption effects 243-246 - basic aspects 237,238 - chromatographic us. static distribution coefficients 238,243-246 - column bleeding 243 - column preparation 241-243 column preparation, in situ coating technique 241 - column preparation, precipitation technique 242 - column preparation, solvent evaporation technique 241 - commercial silica packings superficially porous 295 totally porous 292-294 - distribution coefficient 237 - distribution process 237 - effect of amount of liquid load on plate height 246,247 - effect of support properties on retention 243-246 - k' vs. liquid load 239, 243,244 - load of support 239,242
322
phase ratio 238, 243 phase systems 237,238 - role of support 238-241 adsorption effects 239 effect of pore size 240 effect of specific pore volume 239 effect of specific surface area 239, 240 requirements 238 wetting behaviour 241 - sample size 246 silica supports 292-295 - support selection according to its specific surface area 246 Liquid-solid chromatography (LSC) - activation and deactivation procedures 199, 200 - activity parameters of adsorbent ( Va and (I) 189-191 - adjustment and control of relative water content of eluent 200,201 - adsorbent selectivity 203,204 - adsorption process 188 - capacity factor YS. specific surface area of adsorbent 198,199 - characteristics of silica adsorbents 193-198 - commercial silica packings superficially porous 295 totally porous 292-294 - dependence of k’ on relative water content of eluent 201 - distribution coefficient 188,192, 193 - effect of concentration of polar moderator on retention 193 - effect of water adsorbed on surface activity 191 - historical review 187 - linear sample capacity 202,203 - molecular surface area, AS, required for an adsorbed solute molecule 189 - operating forces in solute-adsorbent interaction 188,192 - pore structure of silica packings 197,198 - relative activity 01 of adsorbent referred to a standard adsorbent with a = 1.OO 189 - relative adsorption energy of eluent, 2 per unit surface area of standard adsorbent 189 - relative adsorption energy of solute So, interacting with the surface of standard adsorbent 189 - retention mechanism on silica 187-193 concept of Scott and Kucera 192,193 concept of Snyder 188-192 - sample load 202,203 - secondary adsorbent activity effects 191 - silica packings, watcr content 196,197 - Snyder equation 189-191 - solvent effects 191 --
-
~
- specific surface area of silica packings 194, 195 - superposition of adsorption and size exclusion mechanism 285,287 support properties controlling retention 198-203 degree of surface deactivation 198-202 specific surface area 198 surface activity of silica packings 195-197 surface volume Va (= volume of an adsorbed solvent monolayer per unit weight of adsorbent) 189 Liquid-solid chromatography on polar chemically bonded silica packings 217-222 - commercial products 296-298 - eluotropic series of binary solvents 219 - k’ YS. relative water content of solvent 219 - retention mechanism on polar chemically bonded silica packings 219 - retention on bulk modified diol- and hydroxylamine-silica 22 1 - selectivity 220-222 - structure and properties of polar chemically bonded silica packings 217-220 - structure of polar chemically bonded silica packings vs. retention 219-220 - synthesis of polar chemically bonded silica packings 21 8 Lobar columns, packing density 175 London-type of dispersion forces 76 Lysozyme (human), separation on chemically modified silica 285
M Macropores, definition 15 Macroporous silica - synthesis by means of controlled sintering 51,52 - synthesis by means of hydrothermal treatment 47-49 Masking, of remaining hydroxyl groups on reversed phase silica packings 210, 211 Mean external length of solute polymer 272 Mechanism, of cation exchange on silica 134-138 Mercury penetration 23 - meso- and macropore analysis in adsorbent standardization 229 - technique 25,26 Mesitylene, adsorption on silica 79 Mesopore analysis in adsorbent standardization 228,229 Mesopores, definition 15 Mesoporous silica - synthesis by means of the sol-gel procedurc 42-49
323
synthesis by means of other procedures 50, 51 Methane, adsorption on silica 79 Methanol - reaction with silica 117 - solvent strength on silica 190 Methylchlorosilanes - activation energies in surface reaction at silica 98,99 factors controlling the rate of hydrolysis 90 - hydrolysis and condensation 89,90 - rcaction with silica 118-120 Methyllithium, reaction with silica 66,67 Methyl red adsorption 210 Methylsiloxane xerogel 108,109 Microbeads see particles, silica packings Microparticles see particles or silica packings Micropore analysis in adsorbent standardization 227,228 Micropores - definition 15 - theory of volume filling 21 Microporous silica, synthesis by means of the sol-gel procedure 4 2 - 4 9 Microscopy, size determination of particles 155 - measuring device 155 - sample preparation 155 - size evaluation 155 Moderators, in LSC 193,202 concentration range 202 Modifier in surface modification of silica, functionality 92 Modifier in the synthesis of reversed phase silica packings 2 12 -2 15 Moisture control system in LSC 201 Molecular cross-sectional area of argon, krypton, nitrogen and water 27 Molecular cross-sectional area of bonded species, methods of evaluation 101, 102 Molecular cross-sectional area of n-alkyl chlorosilanes in surface reaction of silica 103,104 Molecular cross-sectional area of trimethylchlorosilane in surface reaction of silica 102, 103 Molecular size of polymers 272 Monolayer capacity 21 - assessment by means of the Kaganer equation 21 - calculation by means of the BET method 21 Monolayer formation in surface modification of silica 91-108 Monosaccharides, separation on LiChrosorb NH, 221,222 Multilayer formation in surface modification of silica 94-96 -
~
N Naphthyllithium - reaction with chlorinated silica 115,255 - reaction with halogenated silica 255 Nitrogen, adsorption on silica 79 Nitromethane, retention on silanized silica 8 1 Nucleic acid bases, separation on chemically modified silica ion exchanger 267,268 Nucleosides, separation on chemically modified silica ion exchanger 267 Number of theoretical plates in SEC 277
0 n-Octadecyldimethylchlorosilane as modifier in RPC 215 n-Octadecylmethyldichlorosilaneas modifier in RPC 215 n-Octadecyltrichlorosilane as modifier in RPC 214 n-Octylchlorosilanes as modifier in RPC 216 n-Octyldimethylchlorosilane as modifier in RPC 215 n-Octylmethyldichlorosilane as modifier in RPC 215 Oligomers - separation on silica by means of LSC 285-287 - separation by means of SEC 280, 281 Oligopeptides, separation on reversed phase silica packings 2 17 Organolithium compounds, reaction with chlorinated silica 115 Organosilanes - functionality 89 - hydrolytic polycondensation and polymerization to polyorganosiloxanes 88,89 Organosilanetriol in the synthesis of organosilicon xerogels 108- 1 10 Organosilanols - estimation of hydroxyl group content by means of Karl Fischer titration 60 - mechanisms of acid and base catalyzed condensation 90 Organosiloxanes, structural units 89 Organosiloxanols in polycondensation of polyorganosiloxanes 9 1 Organosiloxanols in the synthesis of organosilicon xerogels 108 Organosilicon gel see polyorganosiloxane gel Organo trialkoxysilanes - co-condensation 9 1 - co-condensation with tetraethoxysilane and polyethoxysiloxane to organosilicon xerogel 110-112
324
co-hydrolysis 9 1 - co-polymerization 9 1 - reaction with silica 257 Oxygen, adsorption on silica 79 -
P Packing procedures 169-186 - dry packing techniques, modes and process parameters 175 - dry tamping technique 169 - performance of dry packed vs. slurry packed columns 175 - slurry packing technique filling device 178, 179 fiiling procedure 179 filling procedure, effec of pressure and flow rate 179 general considerations 73-185 historical review 176 pretreatment of packing 176-179 slurry concentration 178 slurry liquids 177, 178 slurry preparation 178 - tap-fii method 169 Packings - angular, production 162 - chemically bonded for RPC superficially porous or pellicular 296-298 totally porous 296-298 - chemically bonded ion exchanger superficially porous 295 totally porous 299 - density of silica particles in silica hydrogel and xerogel46,47 - for size exclusion chromatography 300, 301 - internal vs. external surface 172 - packing structure 172, 173 - polar and moderately polar chemically bonded silica packings superficially porous 296-298 totally porous 296-298 - spherical, formation 162, 163 - superficially porous and pellicular for LSC andLLC 295 - superficially porous, formation 164-166 - totally porous for LSC and LLC 292-294 - types 169 Packing stability - dependence on pore diameter of packing 174,175 - evaluation 174 Particle - angular, formation 162 - diameter, average 149-151
- diameter, average, arithmetic 150, 151 - diameter, average, geometric 151 - diameter, average, median 150, 151 - diameter, average, mode 150, 151 - diameter, definitions 147-151 - diameter, dpsovalue 151 - diameter, projected area diameter 147, 148, 155 - diameter, related to the width of the minimum square aperture 147,153,154 - diameter, Stokes' diameter 147, 148, 156-158 - formation of silica particles 162-166 - packing 169-175 factors influencing particle packing 172-175 interstitial porosity 172, 173 packing structures 172, 173 - porosity, effect on column capacity in SEC 279,280 - shape 148,149 - shape, spherical vs. angular 148, 149 - size 147-152 - size analysis, comparison of methods 161,162 - size analysis, Coulter Counter 161 - size analysis, frequency distribution 150-152 - size analysis, gaussian distribution 151 - size analysis, log-normal distribution 151 - size analysis, methods 153-162 - size analysis, microscopy 155 - size analysis, presentation of data 151,152 - size analysis, Rosin-Rammler distribution 151,152 - size analysis, sedimentation 156 - size analysis, sedimentation, methods of analysis 157,158 - size analysis, sieving 153, 154 - size grading, elutriation 159-161 - size grading, fluid classification 159-161 - size grading, fractionation by sedimentation 158,159 - size grading, methods 153-162 - size grading, sieving 153,154 - spherical, formation 162, 163 Partisil - dependence of k' on relative water content of eluent 201 - water sorption isotherms 196, 197 Partition chromatography see liquid-liquid chromatography (LLC) Peak broadening see plate height rz-Pentadecyltrichlorosilaneas modifier in RPC 215 n-Pentane, solvent strength on silica 190
325 Pepsin (hog), separation on chemically modified silica 285 Performance characteristics of columns 169 - 186 Pcrformancc index in adsorbcnt standardization 231 Performance of columns 179 - 186 Permeability see column permeability Pharmaceuticals, separation on chemically modified silica ion exchanger 268 Phase ratio in SEC 279 Phenethylammonium chloride, sorption isotherm on sulphonic benzylsilica exchanger 264 Phenylalkylchlorosilanes, reaction with silica 255,256 Phenylbutyltrichlorosilane as modifier in RPC 215 Phenylchlorosilanes - factors controlling the rate of hydrolysis 9 0 - hydrolysis and condensation 8 9 , 9 0 - reaction with silica 121, 122 isosteric heat of adsorption 122 mean cross-sectional area 122 surface concentration 121 - stoichiometry in surface reaction with silica 94,95 Phenyldimethylchlorosilane as modifier in RPC 214,215 Phenylethyltrichlorosilane as modifier in RPC 215 2-Phenylethyltrichlorosilane, reaction with silica 256-259 Phenylhexyltrichlorosilane as modifier in RPC 215 Phenyllithium, reaction with chlorinated silica 115,255 Phenylsilanetriol in the formation of phenylsiloxane xerogels 109 Phenylsiloxane xcrogel 109 Phenyltrichlorosilane - as modifier in RPC 2 15 - as reagent in synthesizing surface modified ion exchanger 255 - formation of phenylsiloxane xerogel 109 Phosphorus trichloride, rcaction with silica 128 Photosedimcntometer in evaluating particle size distribution 158 Physisorbed water, determination by means of Karl Fischer titration 6 0 , 6 1 Physisorbed water, on silica - absorption bands 5 9 , 6 0 - content vs. pretreatment temperature 6 0 Physisorption us. chemisorption 76, 77 Pipette method, in evaluating particle size distribution 157, 158
Plate height - as function of capacity factor 182-184 - definition 181 dependence on velocity 181 184 - in adsorbent standardization 231 - inSEC apparent 276 as a function of column temperature 277, 278 as a function of linear velocity of eluent 276,277 as a function of mean molecular weight of solute 276,277 as a function of polydispersity of polymers sample 276 Polarizability us. capacity factors of solute in GC 81 Polybenzylethoxysiloxane, formation of benzylsiloxane xerogel 11 1 Polybenzylsiloxane as ion exchange resin 258 Polydextrans, separation by means of SEC 282 Polydispersity of polymer sample 276 Polye thoxysiioxane - as starting material for porous layer bead production 165, 166 in synthesis of silica microbeads 50 Polyethylene, separation by means of SEC 281, 282 Polyethylene glycols separation by means of SEC 282,286 - separation on reversed phase silica 286 Poly(yglycidoxypropylsiloxane), formation by means of co-hydrolysis and co-condensation of yglycidox yprop yltriethoxysilane and polyethoxysiloxane 112 Polymerization in surface modification of silica 128,129 Polymerization of polyorganosiloxanes 9 1 Polymer layers, formation in surface modification of silica 96 Polymers, separation by means of LSC 285-287 Polymers, separation by means of SEC 280-287 Polymers, synthetic, separation by means of SEC 280,281 Poly(methylmethacrylate), latices, separation by means of SEC on macroporous silica 287 Poly(methy1 methacrylates), separation by means of SEC 281 Polymethylsiloxanes, siloxane bond angle 86 Polynuclear aromatics, separation on 3-(2,4,5,7te tranitrot1uorenimino)propyl-silica 221 Polyorganoethoxysilowane, formation of polyorganosiloxane 110-1 12 Polyoganosiloxanes as basic materials for ion exchangers 253 -
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326 concentration of residual hydroxyl groups 90 - formation of bonded po!ymer layers at silica 96 - formation of structure 9 1 - modes of formation 108-1 12 - polycondensation us. polymerization 91 Polypropylene, separation by means of SEC 281 Polysilicic acids - in the formation of silica sols 43,44 - rate of condensation reaction effect of electrolyte concentration 44 effect of pH 44 Polysiloxane hydride xerogel 116 Polystyrene, latices, separation by means of SEC on macroporous silica 287 Polyvinylsiloxanes, synthesis by cohydrolysis of tetrachlorosilane and vinyltrichlorosilane 258 Porasil as packing in SEC 281 Pore diameter according to Wheeler 39,40 Pore diameter (radius), hydraulic 16 Pore diffusion, in kinetics of surface reactions 96,97 Pore distribution 32-38 - in adsorbent standardization 226-229 - mesopore analysis from sorption data 34-37 - mesopore and macropore analysis from porosimetry 37, 38 - micropore analysis 32-34 Pore models 16, 17 Pore size - classification 15 - distribution, types of 15, 16 Pore structure - assembly of pores 16 - mean pore diameter 39,40 - mean pore diameter, classification according to its size I S - mesopore analysis from sorption data 34-37 - mesopore and macropore analysis from porosimetry 37,38 - micropore analysis 32-34 - models 40-42 - parameters, definitions 15 -19 - pore diameter according to the Kelvin equation 22 - pore distribution 32 -38 - pore models 15,16 - specific pore volume 3 1, 3 2 - t-plots 29, 30 Pore structure models, corpuscular and spongy systems 40-42 Pore structure of silica, effect of surface modification on 104-108 -
Pore systems, primary and secondary pore systems in a packed column 171, 172 Pore volume distribution, bimodal 279 Porosimetry see mercury penetration 23 Porosity - column porosity internal porosity 170, 171 interstitial porosity 170, 171 - controlled, in processing of silica packings 49-52 - definition 3 - of column bed in SEC 279 - of packed spheres 4 1 , 4 2 - of silica, origin of 42-49 - particle porosity 18,170, 171 - particle porosity, effect on column capacity in SEC 279,280 - permanent porosity of ion exchanger 253 swelling porosity of ion exchanger 253 Porous glass see controlled porosity glass Porous layer beads - formation 163-165 - pore structure 164,165 Porous silica layers 163-165 preparation procedures 164,165 variation of pore structure 165, 166 2-Propano1, solvent strength on silica 190 Proteins - adsorption on silica, chemically modified with y-aminopropyltriethoxysilane 283 - isoclectric point 285 - recovery from chemically modified controlled porous glass 283 - recovery from 1,2dihydroxy-3-propoxypropylsilyl modified silica 285 - separation on chemically modified silica carrying 1,2dihydroxy-3-propoxypropylsilyl groups 284 - separation on weak anion exchanger 255 P yridine carbonitriles, separation on chemically modified silica ion exchanger 266 -
-
Q Quadrupole moment - of nitrogen 79 - ofoxygen79 Quartz density 5 - IR absorption bands 9-1 1 - solubility 12, 13 - structure4 Quartz glass, density 5
R Radius of gyration 272 Reaction mechanism of organosilanes in surface modification of silica 92, 93
321 Reactors in surface modification of silica 99 Reduced column length in adsorbent standardization 231 Reduced linear velocity - definition 182 - in adsorbent standardization 231 Reduced parameters, concept 182, 183 Reduced plate height, definition 182 %educed plate height in adsorbent standardization 23 1 Reduced plate height-reduced velocity dependences - effect of particle shape 184, 185 - effect of particle size 183, 184 - effect of type of packing 183, 184 Relative water content of eluent in LSC 200, 201 Resolution - in SEC 274-278 - in SEC, related to mean molecular weight 215 - in SEC, specific resolution 275 - selectivity coefficient in SEC 275 Retention mechanism - in adsorption chromatography using silica 187-193 - in RPC 207-209 Retention of solutes in SEC 274 Retention volume, absolute, of adsorbate in GC
80 Retention volume, corrected, dependence on concentration of polar moderator in LSC 192,193 Reversed phase chromatography (RPC) 206-235 - characteristics of reversed phase silica packings 209-213 - control of retention 208,209 - dependence of k' on hydrocarbonaceous surface area of solute and ligand, resp. 208 influence of reverscd phase packing propcrties on solute retention 213,216 capacity factor vs. chain length 216 capacity factor us. surface coverage 213 - retention behaviour on packings modified with homologues n-octylchlorosilanes 2 16 retention mechanism 207-209 - retention mechanism, concept of Horvath 207-209 - sample load and linear capacity 216 selectivity of reversed phase packings 216, 217 - silica reversed phase packings, commercial products 296-298 - stability of reversed phase silica packings 211,212 surface concentration of bonded groups on reversed phase silica packing 212-215 ~
~
~
surface coverage on reversed phase packings 209,210 synthesis of reversed phase silica packings 212,214,215 test on residual polarity of reversed phase silica packings 2 10 - types of bonded hydrocarbonaceous ~ o u p (ligand) 212,214,215 Reversed phase silica packings see silica, chemically modified as packing in RPC -
~
~
S Sample load of silica, in LSC 202, 203 Scintillation measurements 74,75 Sedimentation - of particles 156-159 - size analysis photosedimentation 158, 159 pipette method 157, 158 - size fractionation 158, 159 Selective adsorbents see polar chemically bonded silica packings 217-222 Selectivity coefficient, rjj 262 - in ion exchange 250 - inSEC275 Selectivity constant in ion exchange 250 Selectivity in ion exchange, graphical presentation 263 Selectivity in ion exchange on silica 134-138 Selectivity, molecular weight 273,281 Separation factor 01 250, 262 Separation factor, thermodynamic, a" 250 Separation impedance, in adsorbent standardization 232 Separation o f oligomers on silica 280-287 Separation of polymers on silica 280-287 Shrinkage of silica hydrogel 46 Sieving of particles 153, 154 - end point 153 sieve specifications 153, 154 Silanediol groups see also geminal groups - a t the silica surface 6 - in monomeric silanes 7 Silanetriol groups 6 Silanization of porous silica with hexamethyldisilazane and trimethylchlorosilane 81 - of silica packings in SEC 281 - proof of surface polarity by means of GC 81 Silanol groups see hydroxyl groups Silanolysis 40 Silica - acidic properties 130-133 - adsorbate interactions adsorption of alcohof and water 82, 83 adsorption of n-alkanes 79 ~
~
328
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adsorption of argon 79 adsorption of benzene 79 adsorption of carbon tetrachloride 79 adsorption of krypton 79 adsorption of methane 79 adsorption of nitrogen 79 adsorption of oxygen 79 adsorption of toluene 79 adsorption of xenon 79 adsorption of p-xylene 79 estimate of specific interactions 80 specific heat of adsorption 80 aerogels 2 amorphous bulk structure 4-6 density 5 carrying 1 -aminoethyl-3-aminopropylsilyl groups 284 carrying l-amino-2-hydroxy-3-propoxypropylsilyl groups 284 carrying 1,2dihydroxy-3-propoxypropylsilyl groups 284 criteria for its classification 1-3 crystalline modifications 4 -6 dehydration 8 dehydroxylation 8,57,58 dehydroxylation by thermal treatment 8 dissolution 11-14 drying 8 heat treatment 57-59 hydration 9 hydrogel 2 hydroxylation 9,57,58 hydroxyl groups arrangement at the surface 7 bond length between them via hydrogen bonding 7 bond to water by hydrogen bonding 7 free 7 geminal6 isolated 7 isolated, adsorption bands in IR spectroscopy 10 surface concentration 7 types 6 vicinal7 infrared absorption bands, assignment 10 infrared spectroscopy 9-1 1 interaction between silica and adsorbate 76-83 interactions with water 11-14 mean pore diameter 39,40 classification according to its size 15 mesopore analysis from sorption data 34 -3 7 mesopore and macropore analysis from porosimetry 37, 38 micropore analysis 32-34
- physisorbed water 57,58 absorption band in IR spectroscopy 10 - polymeric solutions 2 - polymorphism 4 - pore distribution 32-38 - pore structure 15-55 formation 40-49 - pore structure parameters, dcfinition 15-19 - porosity 3 origin of 42-49 - reaction of chlorinated silica with ammonia, amines and amine derivatives 113, 114 - reaction of chlorinated silica with benzyllithium 256 - reaction of chlorinated silica with naphthylmagnesiumbromide 255 - reaction of chlorinated silica with organolithium compounds 255 - reaction of chlorinated silica with organometallic compounds 114,115 - reaction with acetic anhydride 118 - reaction with alcohols 116, 117 - reaction with n-alkylchlorosilanes 120, 121, 212,214,215,255,256 - reaction with allylphenyldichlorosilane 2 14 - reaction with aluminium tribromide 125, 126 - reaction with aluminium trichloride 125, 126 - reaction with 1-aminoethyl-3-aminopropyltrimethoxysilane 283 - reaction with y-aminopropyltrichlorosilanc 256 - reaction with y-aminopropyltriethoxysilane 258 - reaction with ammonium fluoride 113 - reaction with benzoyl chloride 118 - reaction with benzylchlorosilane 122 - reaction with benzyldimethylchlorosilane 214 - reaction with benzyltrichlorosilane 255 - reaction with boron trichloride 124, 125 - reaction with w-bromo-n-alkylchlorosilanes 255,256 - reaction with n-butyldimethylchlorosilane 215 - reaction with n-butyldiphenylchlorosilane 21 5 - reaction with chlorodimethyl-[4-(4-chloromethylpheny1)butylJsilane 256 - reaction with chlorodimethyb(4-phenylbuty1)silane 256 - reaction with y-chloropropyltrichlorosilane 256,257 - reaction with n-decylmethyldichlorosilane 214 - reaction with ndecyltrichlorosilane 214 - reaction with diborane 64,65 - reaction with di-n-butyldichlorosilane 214 - reaction with dimethyldichlorosilane 65,66, 214 - reaction with diphenyldichlorosilane 255 - reaction with ndodecyltrichlorosilane 214 - reaction with y-glycidox ypropylsilane
329
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(= 1,2-epoxy-3-propoxypropyltrimethoxy silane) 257,283 reaction with n-heneicosyltrichlorosilane 21 5 reaction with n-hexyltrichlorosilane 214 reaction with methanol 117 reaction with methylchlorosilanes 118-120 reaction with methyllithium 66,67 reaction with n-octadecyldimethylchlorosilane 215 reaction with n-octadecylmethyldichlorosilane 215 reaction with n-octadecyltrichlorosilane 214 reaction with n-octyldimethylchlorosilane 2 15 reaction with n-octylmethyldichlorosilane
215 - reaction with n-octyltrichlorosilane 215 - reaction with organotrialkoxysilane 257 - reaction with n-pentadecylmethyldichlorosilane 215 - reaction with n-pentadecyltrichlorosilane 215 - reaction with phenylalkylchlorosilanes 255,
256 reaction with phenylbutyltrichlorosilane 21 5 - reaction with phenylchlorosilanes 121, 122 stoichiometry 94,95 - reaction with phenyldimethylchlorosilane
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214,215 - reaction with phenylethyltrichlorosilane 21 5 - reaction with 2-phenylethyltrichlorosilane
256,257
- solubility dependence on particle size 13, 14 effect of impurities 14 factors affecting the rate of dissolution 13 in aqueous solutions 11-14 pH dependence 12,13 temperature dependence 12,13 - soluble 1 - sorption isotherms 19-22 - sorption isotherms of nitrogen 20 - specific pore volume 3 1,32 according to Fisher and Mottlau 26 assessment from apparent densities due to helium and mercury 26 - specific surface area 27-31 - structure of crystalline modifications 4-6 - surface chemistry 57-146 absorption bands of hydroxyl groups 59 activity of hydroxyl groups YS. siloxane groups 58 bound hydroxyl groups 62,63 free hydroxyl groups 62,63 fully hydroxylated surface 62 geminal hydroxyl groups 63 HTO exchange 72-76 HTO exchange procedure 73-75 hydroxylated and dehydroxylated surface, retention volume of adsorbates on 80 internal vs. surface hydroxyl groups 6 1,62 IR absorption bands of hydroxyl groups
- reaction with phenylhexyltrichlorosilane 21 5 - reaction with phenyltrichlorosilane 215, 255 - reaction with phosphorus trichloride 128 - reaction with silicon tetrachloride 127 - reaction with special silanes 123 - reaction with specific compounds to evaluate the surface hydroxyl concentration
68-70 isolated hydroxyl groups 63 isotopic exchange of surface hydroxyl groups by means of D,O 61,62,70-72 paired hydroxyl groups 63 physically adsorbed water 58-61 reactivity of surface hydroxyl groups in adsorption 76-83 silica-adsorbate interactions 78-83 surface activity 58 surface concentration of hydroxyl groups 62 surface concentration of hydroxyl groups vs. pretreatment temperature 67,75 surface hydroxyl groups, determination
64-6 8 - reaction with wsubstituted organotriethoxysilanes 282 - reaction with sulphur tetrafluoride 113 - reaction with thionylchloride 103,104,
112,113 - reaction with titanium tetrachloride 126, 127 - reaction with n-tridecyltrichlorosilane 215 - reaction with -N-(3-triethoxysilylpropyI)silane 282 - reaction with trimethylchlorosilane 2 12,
63-76
214,215 molecular cross-sectional area 102,103 - reaction with triphenylchlorosilane 215, 255 - reaction with n-undecylmethyldichlorosilane 2 15 - reaction with n-undecyltrichlorosilane 215 - reaction with vinylchlorosilane 122 - reaction with vinylmethyldichlorosilane 214 - sintering9 - sols 1
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surface hydroxyl groups us. physically adsorbed water 58-61 types of surface hydroxyl groups 62,63 vicinal hydroxyl groups 63 surface composition 3 surface species 58-76 surface structure 6-11,57-83 types of bonded groups 3 water adsorption isotherms 196, 197 water content, total 59 water, physically adsorbed 8 water, physically adsorbed, removal 8 water sorption isotherms 57,58
330 - xerogel2 Silica, as adsorbent in LSC 187-206 - activation and deactivation procedures 199,
200 commercial products superficially porous 295 totally porous 292-294 - linear sample capacity 202,203 - retention mechanisms 187-193 concept of Scott and Kucera 192, 193 concept of Snyder 188-192 - selectivity 203, 204 - support properties controlling retention -
198-203 degree of surface deactivation 198-200 specific surface area 198 Silica, as packing in SEC 27 1-289 - advantages, disadvantages in the separation of biopolymers 282 - advantages over organic gels 281 - analysis time 281 - applications 280-287 - calibration curve 272-274 molecular weight range 278, 279 - column coupling 279 - effect of column temperature on resolution -
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277,278 effect of specific pore volume on column capacity 279,280 exclusion limit of a column 273 historical review 281 interstitial porosity of column, effect on capacity 279 molecular weight selectivity 273, 281 number of theoretical plates 277 peak broadening 276 phase ratio 279 plate height, apparent 276 plate height, as a function of column temperature 277,278 plate height, as a function of linear velocity of eluent 275-277 plate height, as a function of mean molecular weight of solute 276,277 pore volume distribution 278 pore volume distribution, bimodal 279 resolution 274-278 separation mechanism 271-274 restricted diffusion 272 steric exclusion 271 -272 thermodynamic theories 272 separation of biopolymers 282-285 separation of synthetic oligomers and polymers 28 1 , 2 8 2 silanization 281 specific pore volume 279,280 standard deviation of eluted peak 276, 277 support properties, optimization 278-280
- working range of a column 273 Silica, as support in LLC 237-248 - commercial products superficially porous 295 totally porous 292-294 Silica beads, processing 5 0 , s 1 Silica, chemically modified, as ion exchanger
252-264 -
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capacity definition 259 deter mination 25 9 capacity, of porous and pellicular ion exchanger 260 cohydrolysis of tetrachlorosilane and vinyltrichlorosilane 258 commercial products 299, 300 extraction 259,260 mean pore diameter 253 monolayer vs. polymer layer type 257 performance 264-269 porosity 252,253 recycling 259 regeneration 259 selectivity 264-269 separation mechanism 267 stability chemical 26 I mechanical 26 1 thermal 26 1 stability tests 262 sulphobenzyl silica 264,266 swelling behaviour 26 1 synthesis 253-258 synthesis, by means of bulk modification
258 synthesis, by means of hydrolysis and condensation of benzyltriethoxysilane 258 - synthesis, by means of reaction between sulphonated benzylsilane and sodium silicate 258 - synthesis, by means of surface modification -
255-258 - types of ionic functional groups 255 Silica, chemically modified, as packing in RPC 206-235 - characteristics 209-213 - commercial products 296-298 - control of retention 208 - effect of native silica 21 1 - influence of reversed phase packing properties on retention of solutes 213, 216 capacity factor us. chain length 216 capacity factor 1’s. surface coverage 213 - retention mechanism 207-209 - selectivity 216,217 - stability of reversed phase packings 21 1, 212 - surface concentration of bonded groups on reversed phase silica packings 212-215
331 surface coverage 209, 210 synthesis of reversed phase silica packings 212,214, 215 - test on residual polarity 210 types of bonded unpolar group 212-215 Silica, chemically modified, as packing in SEC - adsorption of proteins on silica modified with yaminopropyltriethoxysilane 283 - applications 281 -287 - n*ctyl and n-octadecyl modified silica 281 - separation of enzymes and proteins on silica carrying 1,2dihydroxy-3-propoxypropylsilylgroups 284,285 - separation on silanized silica 282 - separation on silica carrying -Si(CH,),-NH, 282 -Si(CH,),-NH-CO-CH, 282 =Si(CH,),-NH-SO,-CH, 282 ~~
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-=Si(CH,),-NH-CO-CH,-NH-CO-CH, 282
-Si(CH,),OCH,-CH(OH)-CH,(OH) 282 Silica, chemically modified, as polar packing in LSC 217-222 - commercial products 296-298 - k’ vs. relative water content of solvent 219, 220 - retention mechanism 2 18 - retention on bulk modified did-silica 221 - retention on bulk modified hydroxylaminesilica 221 - selectivity 220-222 - structure and properties of packings 217-220 - structure of bonded packing YS. retention 219,220 - synthesis 218 Silica, chemical modification 83-130 - definition 8 3 - basic concepts 84-108 - bulk modification 88-91 - functionality of modifier 89 - methods of forming surface bonds 88-108 - structure and stability of surface bonds 85-88 - surface modification 9 1- 108 - types of bonds and functional groups 81 -85 Silica columns - column porosity 170, 171 - comparison of performances 179-186 - factors influencing particle packing 172-175 - packing density and strength 173 - packing procedures, general 169 - performance column pcrmcability 180, 181 column stability 184, 185 plate hcight-vclocity depcndcncc 181-184
pore systems of column bed 171,172 size relationships d p d,, D etc. 172 Silica columns, packing procedures 175- 179 - dry packing techniques 175, 176 - packing stability (mechanical) 174, 175 - slurry packing techniques 176-179 - slurry technique, general consideration 173-175 Silica, controlled porosity packings 49-52 agglutination of finely dispersed non-porous silica particles 50, 5 1 - controlled sintering 51, 52 - modified sol-gel procedure followed by sintering 50 - polyethoxysiloxane procedure 50 Silica gel see silica hydrogel Silica, geometric modification 83 see also hydrothermal treatment, sintering, calcination Silica hydrogel - ageing45 factors controlling ageing 45 - effect of electrolyte concentration on gelling 45 - effect of pH on rate of gelling 44,45 - effect of silica concentration on rate of gelling 45 - effect of temperature on rate of gelling 45 - factors controlling the rate of gelling 44,45 - formation 44-46 - gelling44 Silica, hydrothermal treatment 47-49 - effect of temperature, duration and watcr vapour pressure 48 Silica, ion-exchange properties 130-141 - acidic surface sites, pK, value 132, 133 - acidic surface sites, structural models 131,132 - applications 140 - capacity and exchange ability as a function of pH 133 - exclusion of electrolytes from the pores of silica 139, 140 - isoelcctric state and possibility of anion exchange 138 - kinetics 140 - mechanism of cation exchange on silica and the theory of selectivity 134-138 - selectivity of metal cations 137 - surface sites and origin of acidity 130-133 Silica, macroporous - as packing for separation of latices by means of SBC 287 - synthesis by means of controlled sintering 51,52 - synthcsis by means of hydrothermal treatment 47-49 Silica, mcsoporous - synthesis by means of the sol-gel procedure 42-49 -
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332 - other procedures 50,51 Silica, microporous, synthesis by means of the sol-gel procedure 4 2 - 4 9 Silica, packing characteristics in LSC 193-198 - pore structure 197, 198 - specific surface area 194, 195 - surface activity 195-197 Silica packings - formation 162-166 - formation of angular particles 162 - formation of spherical particles 162, 163 - internal vs. external surface 172 - packing procedures dry packing techniques 175,176 slurry packing techniques 176-1 79 - particle porosity YS. internal porosity of column bed 170,171 - pre-treatment before column packing in slurry packing technique 176, 177 - removal of fines 177 - spherical, formation by means of an emulsification procedure 163 - spherical, formation by means of polyethoxysiloxane procedure 163 - spherical, formation by means of spraying technique 163 - types 169 Silica particles - compaction of 172-175 Silica, porosity, origin of 42-49 silica, porous - chemical stability 52,53 - globar us. vermicular structure 49 - pore volume distribution bimodal 279 standard deviation 278 - surface chemistry 57-146 - thermal stability 52 Silica sol - factors affecting the rate of condensation of polysilicic acids 44 - formation 43,44 - stabilization 44 Silica, surface modification - basic 91-108 - conversion 99-104 - conversion, maximum 100,102 - conversion vs. temperature 100 - effectiveness factor 101,102 - effect on pore structure properties of silica 104-108 - in presence of water 95,96 - in situ reaction 99 - kinetics in surface reaction 96-99 - molecular cross-sectional area of bonded species, evaluation 101,102 - multilayer formation 94-96 - reaction mechanism 92,93
- reaction with n-alkylchlorosilanes 103, 104, 107 - reaction with trimethylchlorosilane 102-104,107 - reactors 99 - residual hydroxyl groups at silanized silica 103,104 - role of water 92 - stoichiometric factor 92 - stoichiometry of reaction 92,93 - surface concentration as function of chain length of modifier 103, 104 - surface concentration as function of volume of modifier 103 - surface coverage, definition 102 - surface coverage vs. effectiveness factor 102 - surface species produced in the reaction of o rganosilanes and silica 93 - thickness of bonded layer 105-107 Silica, synthesis by means of sol-gel procedure 4 2-49 - duration of ripening and ageing of the hydrogel 47 - factors controlling the pore structure 46-49 - hydrothermal treatment 47-49 - packing density of silica particles 46 - pH during washing 47 - pH in hydrogel formation 47 - silica particle dimensions 46 - substitution of intermicellar liquid in the hydrogel 47 Silica, synthesis of bulk modified products 108-112 - co-condensation of sodium silicate and organosilanetriols 109, 110 - co-hydrolysis and cocondensation of organotrialkoxysilanes and tetraethoxysilane 110-112 - condensation of organosilanetriols 108,109 Silica, synthesis of chemically modified products 108-130 Silica, synthesis of surface modified products 112- 130 - chlorinating reagents 112,113 - chlorination 112,113 - fluorination 113 - formation of silica with =Si-H bonds 115 - polymerization 112-130 - reaction of chlorinated silica with ammonia, amines and amine derivatives 113,114 - reaction of chlorinated silica with organometallic compounds 114,115 - reaction with alcohols 116, 117 - reaction with n-alkylchlorosilanes 120, 121 - reaction with aluminium tribromide 125,126 - reaction with aluminium trichloride 125,126 - reaction with benzylchlorosilane 122 - reaction with boron trichloride 124, 125
333 - reaction with methylchlorosilanes 118-120 - reaction with phosphorus trichloride 128 - reaction with silicon tetrachloride 127 - reaction with titanium tetrachloride 126, 127 - reaction with vinylchlorosilanes 122 - Si-0-BX, surface bonds 124-128 - Si-0-R surface bonds 116-124 - Si-X surface bonds (X = halogen, -NH,, -NR,, -R, -H) 112-116 Silica, synthesis, starting materials 42 Silica, water content in LSC 195-197 Silica xerogel, formation 46 Silicates, soluble 1 3 Silicic acid - condensation mechanism 13 - disilicic acid 12 - intermolecular condensation 13 - intramolecular condensation 13 - monosilicic acid 12 determination 1 2 dissociation constant 12 pK, value 1 31 Silicon atoms, coordination in silica 4 Silicon-carbon bond - bond angle 86 - bond energy 85 bond length 86 - cleavage 261 - in benzylsilica, stability 256 - in phenylpropylsica, stability 256 - stability towards nucleophilic and electrophilic reagents 87,88 - thermal decomposition 87 Silicon-element bonds, energies 85 Silicone polymers 128 Silicon-halogen surface bonds, formation 112,113 Silicon-hydrogen surface bonds 115,116 Silicon-nitrogen bond - bond energy 85 - bond length 86 Silicon-nitrogen-carbon surface bonds, formation 114 Silicon-nitrogen surface bonds, formation 113,114 Silicon tetrachloride, reaction with silica 127 Siloxanebond - absorption bands in IR spectroscopy 4 - bond angle 4 , 8 6 - bond energy 85 - bond length 4 , 8 6 - cleavage 261 - ( d - p ) , conjugation 1 31,132 - hydrolysis 12 - stability towards nucleophilic and electrophilic reagents 86,87 - strained siloxane bonds 86 ~
Siloxane groups 58-76 - reaction with methanol 6 1 Sizeexclusion chromatography (SEC) 271 -289 - advantages of silica packings over organic polymer gels 281 - analysis time 278,281 - calibration curve 272-274 molecular weight range 278-279 - capacity factor 274, 275 - column coupling 279,281 - column length 280 - commercial silica packings 300, 301 - distribution coefficient, definition 271 - effect of column temperature on analysis time 278 - effect of column temperature on resolution 277,278 - effect of interstitial porosity of column on capacity 279,280 - effect of specific pore volume of packing on column capacity 279, 280 - elution volume of solute 271 - exclusion limit of a column 273 - hydrodynamic volume of solute polymers 278 - molecular weight selectivity 273, 281 - number of theoretical plates 277 - on silica, historical review 281 - peak broadening 276 - phase ratio 279 - plate height apparent 276 as a function of column temperature 277, 278 as a function of linear velocity of eluent 276,277 as a function of mean molecular weight of solute 276,277 as a function of polydispersity of polymer sample 276 - pore volume distribution of packing, bimodal 279 - resolution 274-278 related to mean molecular weight 275 - resolution, specific 275 - retention in SEC vs. retention in LSC 274 - separation mechanism 271 -274 restricted diffusion 272 steric exclusion 271, 272 thermodynamic theories 272 - separation on chemically modified silica packings 281-287 - separation on silanized silicas 282 - separation on untreated silica 280,281 - separation of n-alkanes 282 - separation of biopolymers 282-285 - separation of biopolymers on controlled porosity glass 283
334 separation of colloid dispersions 287 separation of enzymes and proteins on porous glass chemically modified with 1,2epoxy-3-propoxypropyltrimethoxysilane 283 - scparation of polyethylene 281 - separation of polypropylene 281 - separation of proteins and enzymes on chemically modified silica carrying 1,2dihydrox y-3-propoxyprop ylsilyl groups 283 - separation range 27 1 - silanization of packing 281 ~- silica support, pore volume distribution 278 - silica us. carbohydrate gels 282 - size-exclusion mechanism of biopolymers -
Solvent mixtures, solvent strength on silica 189,
190
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285 - specific pore volume of packings 279,280 - superposition of sizeexclusion and adsorption mechanism 285,287 - support properties, optimization 278-280 - standard deviation of eluted peak 276,277 - working range of a column 273 Slurries - high-viscosity 178 - low-viscosity 178 Slurry liquids - acetone 177 - ammonia, dilute 177 - dioxane 177 - flocculating us. deflocculating 177,178 - methanol 177 - methyliodide 177 - tetrabromoethane 177 - tetrabromoethane/tetrachloromethane, dioxane 177 - tetrachloroethylene 177 - trichloromethane 177 Slurry packing technique 176-1 79 - apparatus 178,179 - filling procedure 179 - pre-treatment of packings 176, 177 - slurry concentration 178 - slurry liquid 177, 178 - slurry preparation 178 Snyder equation 189-191 Sol see silica sol Sol-gel procedure, in the synthesis of silica
42-49 factors controlling the pore structure 46-49 - duration of ripening and ageing of the hydrogel 47 hydrothermal treatment 47-49 pH during washing 47 pH in hydrogel formation 47 substitution of intermicellar liquid in thc hydrogel 47 Solvent effects, in LSC 191 -
Solvent strcngth, of singlc solvcnts and mixed solvents on silica 189,190 Solvophobic interactions, in RPC 207,208 Sorption isotherms 19-22 - classification according to BET 19 - classification according to the pore size of adsorbent 19,20 - of watcr on silica 196,197 - techniques 23-25 Specific pore volume 18,19 - according to Dubinin and Radushkevich 32 - according to Fisher and Mottlau 26,31 - according to the Gurvitsch rule 32 - assessment from the apparent densities due to helium and mercury 26 - availability for cations in ionexchange of silica 139 - calculation procedures 31,32 - from density measurements 31 from mercury intrusion 32 - from pore distribution ( Vp(cum)) 32 - in adsorbent standardization 225,226 - in surface modification of silica 105-107 - of packings in SEC 279,280 Specific surface area 17, 18 according to the BET method 27,28 - according to the Kaganer method 28 - according to the Kiselev method 28 - according to the Sears method 26,31 - according to the Sing method 29 - according to the t method 28 - calculation proccdures 27-31 - comparison of data 27,28 - external 17 - from pore distribution curves (Scum) 31 - in adsorbent standardization 223,224 - internal 17 - molecular cross-sectional areas of adsorbates -
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27 of silica in surface modification 105-107 - relationship to the average pore diameter 18 - t plot 29 Spherisorb S5W - h VS. u plot 183 - H US. u plot 182 Spherosil - as packing in LSC 194-197 - as packing in SEC 281 - dependence of k o n relative water content of eluent 201 - selectivity in LSC 204 - water isotherms 196,197 Stability of columns 184,185 Stability of ion-exchanger - chemical 261 -
335 mechanical 261 stability tests 262 - thermal 261 Stability of porous silica - chemical stability 52, 53 - thermal stability 52 Stability of reversed phase silica packings 21 1, 212 Standard deviation of eluted peak in SEC 276, 277 Standardization of adsorbents 222-233 see also adsorbent standardization Steric exclusion mechanism in SEC 271, 272 Stishovite, structure 4 Stoicliiometry of surface reaction at silica - 1-aminoethyl-3-aminopropyltrimethoxysilane 94 - phenylchlorosilanes 94, 95 Stokes’ equation 156 Sulphonation - of n-alkylphenylsilica 256 - of n-alkylsilica 256 - of benzylsilica 255 - of w-bromo-n-alkylsilica 256 - of naphthylsilica 255 - of 2-phenylethylsilica 256, 257 - of phenylsilica 255 - of triphenylmethylsilica 255 - with chlorosulphonic acid 255, 256 - with oleum 255 - with sodium hydrogen sulphite 256 - with sodium sulphite 256 - with sulphonyl dichloride 256 Sulphonic benzylsilica exchanger - isotherms 264 - selectivity 269 Supplier, for silica packings in HPLC 291 Support properties, optimization in SEC 278-280 Surface activity of silica 58 - in LSC, control of 194 Surface chemistry of silica 57-146 Surface composition of silica 3 Surface concentration of bonded groups at silica - as function of chain length of modifier 103, 104 - as function of volume of modifier 103 Surface coverage in surface reaction, definition 102 Surface diffusion 97 Surface modification of silica - basic concepts 91 -108 - conversion 99-104 - conversion, maximum 100,102 - conversion 11s. tempcrature 100 - effectiveness factor 101, 102 -
-
- effect on pore structure properties on silica 104- 108 - in presence of water 95,96 - in situ reaction 99 - kinetics in surface reaction 96-99 - molecular cross-sectional area of bonded species, evaluation 101,102 - multilayer formation 94-96 - reaction mechanism 9 2 , 9 3 - reaction with n-alkylchlorosilancs 103,104, 107 - reaction with phenylchlorosilanes 9 4 ,9 5 - reaction with trimethylchlorosilane 102-104,107 - reactors99 - residual hydroxyl groups at silanized silica 103,104 - role of water 92 - stoichiometric factor 92 - stoichiometry of reaction 92,93 - surface concentration as function of chain length of modifier 103, 104 - surface concentration as function of volume of modifier 103 - surface coverage 102 - surface coverage vs. effectiveness factor 102 - surface species produced in the reaction of organosilanes with silica 93 - synthesis of ion exchanger general routes 253,254 introduction of ionic functional groups 254, 255 types of organic radicals 254 - thickness of bonded layer 105-107 Surface species of silica 58-76 Surface species, produced in the reaction of organosilanes and silica 9 3 Surface structure of silica 57-83 Surface tension of eluents in RPC 208, 209 Suspensions in column packing techniques 173, 174 Synthesis of bulk modified silica products 108-112 - cocondensation of sodium silicate and organosilanetriols 109, 110 - co-hydrolysis and co-condensation of organotrialkoxysjlanes and tetraefhoxysilane 110-112 - condensation of organosilanetriols 108, 109 Synthesis of chemically modified silica supports 108-130 Synthesis of surface modified silica products 112-130 - chlorinating reagents 1 12, 113 - chlorination 112, 113 - fluorination 11 3 - formation of silica with =Si-H bonds 115
336 - polymerization 112-130 - reaction of chlorinated silica with ammonia, amines and amine derivatives 113,114 - reaction of chlorinated silica with organcmetallic compounds 114,115 - reaction with alcohols 116,117 - reaction with n-alkylchlorosilanes 120, 121 - reaction with aluminium tribromide 125,126 - reaction with aluminium trichloride 125,126 - reaction with benzylchlorosilane .reaction with boron trichloride 124, 125 - reaction with methylchlorosilanes 118-120 - reaction with phosphorus trichloride 128 - reaction with silicon tetrachloride 127 - reaction with titanium tetrachloride 126,127 - reaction with vinylchlorosilane 122 - Si-OBX, surface bonds 124-126 - Si-0-R surface bonds 116-124 - Si-X surface bonds (X = halogen, -NH,, -NR,, -R, -H) 112-116
T Tap-fill method 175 Test conditions, in adsorbent standardization 2 30-233 Test solutes, in adsorbent standardization 232,233 Tetrachloromethane, solvent strength on silica 190 Tetraethoxysilane - cocondensation with organotrialkoxysilane to polyorganosiloxane 110-112 - formation of polyethoxysiloxane 50 Tetrahydrofuran, solvent strength on silica 190 Thermogravimetry, evaluation of water content of silica 59,60 Thickness of bonded layer in surface reaction of silica 105-107 Thionylchloride, reaction with silica 103,104, 112,113 Titanium tetrachloride, reaction with silica 126, 127 Toluene, adsorption on silica 79 Tortuosity factor, in pore diffusion 97 Total column porosity in adsorbent standardization 232 t plots, characteristic types 29, 30 Transmittance 69 Trichloromethane, solvent strength on silica 190 n-Tridecyltrichlorosilaneas modifier in RPC 215 Tridy mite - density 5 - structure4
Trime th ylchlorosilane - as modifier in RPC 212,214,215 - reaction with silica 118-120 model of a trimethysilyl modified silica surface 119 molecular cross-sectional area 118 sorption isotherms on trimethylsilyl modified silica 118-120 2,2,4-Trimethylpentane, solvent strength on silica 190 Triphenylchlorosilane - as modifier in RPC 215 - reaction with silica 255 Triphenylmethyllithium, reaction with chlorinated silica 115 Tritium-labelled water in the determination of surface hydroxyl groups 72-76 Trypsin, separation on chemically modified silica 285 U
n-Undecylmethyldichlorosilaneas modifier in RPC 215 n-Undecyltrichlorosilane as modifier in RPC 213 V Vinylchlorosilane, reaction with silica 122 Vinylmethyldichlorosilane as modifier in RPC 214 Vinyltrichlorosilane, co-hydrolysis with tetrachlorosilane 258
W Washburn equation 23 Water - adsorption isotherms on silica from solution 197 from vapour phase 196 - adsorption on silica 82,83 - content, in solvent, determination 200 - content, of silica 59 - solvent strength on silica 190 - surface coverage on silica in LSC 191
X Xenon, adsorption on silica 79 Xerogel see silica xerogel p-Xylene, adsorption on silica 79 2 Zipax, h vs. u plot 184
WoeIm
f o r Chromatography 0
active Slllcas actlve Alumlnas for chromatography with elevated pressure for dry-column chromatography for preparative and classical column chromatography for thin-layer chromatography for TLC on glass and aluminium backing SiliTech and AluTech for plant and pilot plant use
TLC=Appllcator for accurate line- and spot-application-of samples on chromatographic layers
I GmbH8Co. D-3440 Eschwege West-Germany 402
PART S L Coumn Media for HPLC A complete line of famous Partisil and Partisil-based media for HPLC is now available worldwide.
PARTISIL-10 PAC: Cyano-amino groups (Si-0-Si-C) bonded to Partisil 10 for normal-phase partition HPLC.
PARTISIL: Structured-irregular, totally porous, high purity silica gel; 400+ m2ig surface area; 70-80A pore diameter; extremely narrow particle size distribution.
PARTISIL-10 SAX: - h R 3 groups (Si-0-Si-C) bonded to Partisil 10 for strong anion exchange HPLC.
PARTlSlL 5: 5 p m dp Partisil silica gel for adsorption HPLC. PARTlSlL 10: 10 p m dp Partisil silica gel for adsorption HPLC. PARTlSlL 20: 20 p n dp Partisil silica gel for adsorption HPLC. PARTISIL-10 ODS: CIS groups (Si-0Si-C) bonded to Partisil 10 for R-P HPLC; carbon load, 5%; coverage, 50%. PARTISIL-10 ODS-2: C I S groups (Si0 - S i - c ) bonded to Partisill0 for R-P HPLC; carbon load, 15%; coverage, 75%.
PARTISIL-10 SCX: Benzene-sulfonic acid groups (Si-0-Si-C)bonded to Partisil 10 for strong cation exchange HPLC.
All Partisil media are fully quality assured, have excellent batch to batch uniformity, pack well into uniform, high density beds for excellent, high efficiency column performance. Prepacked analytical (4.6 mm ID x 25 cm) and preparative (9.4 mm ID x 50 cm) columns with all Partisil media are also available.
Whatman Inc. rn 9 Bridewell Place, Clifton, New Jersey 07014 Telex 133426 Tel. (201) 777-4825 Whatman Ltd. Springfield Mill, Maidstone, Kent ME14 2LE, England Tel. (0622) 61681 rn Telex 961 13 Whatman S.A. m Zone Industrielle. BP N. 12, 45210 Ferrieres, France Tel. 95 74 15 Telex 780229
I
“Howdo you want your peak performers3 I
”
1
‘’...by the bottle?” k y
Pack your own columns with Shandon Hyperspheressupplied in bulk The uniform distribution of pore sizes of optimum dimension gives more efficient separations than any other packing material - peak performanceevery time
Shandon Hyperspheres are now available in pre-packedcolumns for guaranteed performanceand convenience Based on the infinite diameter principle developed by Professor J H Knox, Director of the Wolfson Chromatography Unit, in the Chemistry Department of Edinburgh University,the columns are compatible with any HPLC instrument
“Ora complete system?” Like all families, Shandon Hyperspheres,columns and injectors work best together. In fact, because it’s a complete system, purpose built without compromise for total compatibility-the Shandon family is the ultlmate in peak performance And for those little family problems the Shandon HPLC applications laboratory ISalways availablefor advice. However you want your peak performers-find out more.
515 Broad Street Sewickley PA 15143 Tel (412) 741 8400 Telex 86 6200
MERCK Silica
The bigger the selection, the harder the choice. We have made your choice easier by settling for
standardized types To get down to basics, we manufacture silica gels of defined pore structure, 0 and special surface modified structures, 0 with particle sizes specially attuned to the intended chromatographic technique, 0 for preparation by the user or in ready-to-use form. Each silica product is designed specifically for one of the various liquid and gas chromatographic techniques.
irregular types
spherical types
silica gel (CC, TLC, HPTLC) Fractosil@(GPC) LiChrosorbB (HPLC analytical) LiChroprep@(HPLC preparative)
LiChrosphere (HPLC) Perisorbe (HPLC) VolaspheP (GC)
"silanol-active" silica gels types 40, 60, loo* Fractosil@200,500,1000 2500,5000,lO 000,25 OOO* Si 60, Si loo* Perisorb@A Si 100,300,500,1000,4000* (LiChrospherm)
and
chemically derivatized silanized silica gel 60' (dichlorodimethylsilane) RP-2, RP-8, RP-18 NH2, DIOL, KAT, AN 100 CH-8,500 CH-8, 1000 CH-8,4000 CH-8* (LiChrospheP) Volaspher@A 2
* Mean pore diameter of the basic silica in nm can be derived by dividing numerical suffixes by 10.
Our silica types and silica derivatives form a system which permits a corresponding type to be selected when changing from one particular chromatographic technique to another, i. e. one type for TLC or HPTLC is matched by a corresponding type for conventional column chromatography (CC) or HPLC. Scale-up is a simple matter when one knows that TLC plates have an equivalent in the form of PLC plates or that LiChrosorb@is matched by LiChroprep@ for preparative scale work.
Please ask for our special brochures. 405
E. Merck, Darmstadt, Germany
