porous silica

JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 16

porous silica its properties and use as support in column liquid chromatography

JOURNAL OF CHROMATOGRAPHY LIBRARY

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Chromatography of Antibiotics by G.H. Wagman and M.J. Weinstein

Extraction Chromatography edited by T. Braun and G. Ghersini

Liquid Column Chromatography. A Survey of Modern Techniques and Applications edited by Z. Deyl, K. Macek and J. Jan&

DetecJors in Gas Chromatography by J . Sevdik

Instrumental Liquid Chromatography. A Practical Manual on High-Performance Liquid Chromatographic Methods by N.A. Parris

Isotachophoresis. Theory. Instrumentation and Applications by F.M. Everaerts, J.L. Beckers and Th.P.E.M. Verheggen

Chemical Derivatization in Liquid Chromatography by J.F. Lawrence and R.W. Frei

Chromatography of Steroids by E. Heftmann

HPTLC - High Performance Thin-Layer Chromatography edited by A. Zlatkis and R.E. Kaiser

Gas Chromatography of Polymers by V.G. Berezkin, V.R. Alishoyev and I.B. Nemirovskaya

Liquid Chromatography Detectors by R.P.W. Scott

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Instrumen tation for High-Performance Liquid Chromatography edited by J.F.K. Huber

Radiochromatography. The Chromatography and Electrophoresis of Radiolabelled Compounds by T.R. Roberts

Antibiotics. Isolation, Separation and Purification edited by M.J. Weinstein and G.H. Wagman

Porous Silica. Its Properties and Use as Support in Column Liquid Chromatography by K.K. Unger

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 F IC PUBLISH I NG COMPANY Amsterdam - Oxford - New York 1979

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 1936- Porous 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 ica . I. Ti t le . 11. Series. ~ ~ 7 9 . C 4 5 4 ~ 5 3 543' .O8 79-12682 ISBN 0-444-41683-8

ISBN 0-444-41683-8 (Vol. 16) ISBN 0-444-41616-1 (Series)

0 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, mechan- ical, 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

Printed in The Netherlands

V

Contents

P r e f a c e . . . . . . . . . . . . . . . . . . . . . . . . . . XI

Chapter 1. General chemistry of silica . . . . . . . . . . . . . . . . 1

1.1 Introduction . . . . . . . . . . . . . 1.1.1 Classification of solid silica species . . . .

1.2 Bulk structure of silica . . . . . . . . . 1.2.1 Structure of crystalline silica modifications .

1.3 Surface structure . . . . . . . . . . . 1.3.1 Types of surface hydroxyl groups . . . . 1.3.2 Dehydration and dehydroxylation . . . . 1.3.3 Hydroxylation and hydration . . . . . 1.3.4 Infrared spectroscopy of surface silica species

1.4 Silica-water interactions. . . . . . . . . 1.4.1 Dissolution of silica . . . . . . . . .

1.5 References . . . . . . . . . . . . .

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Chapter 2. Pore structure of silica . . . . . . . . . . . . . . . . . 15

2.1 Pore structure parameters . . . . . . . . . . . . . 2.1.1 Definitions . . . . . . . . . . . . . . . . . 2.1.2 Fundamentals. . . . . . . . . . . . . . . . 2.1.3 Experimental techniques . . . . . . . . . . . . 2.1.4 Calculation procedures . . . . . . . . . . . . .

. . . . . . . . . . . . 2.2.1 Pore structure models . . . . . . . . . . . . . 2.2.2 Origin of porosity in silica . . . . . . . . . . . .

2.3 Controlled porosity silica packings. . . . . . . . . . . 2.3.1 Modified sol-gel procedure followed by sintering . . . . 2.3.2 Polyethoxysiloxane procedure . . . . . . . . . . 2.3.3 Agglutination of finely dispersed non-porous silica particles. 2.3.4 Controlled sintering . . . . . . . . . . . . . .

2.4 Stability of porous silica . . . . . . . . . . . . . . 2.4.1 Thermal stability . . . . . . . . . . . . . . . 2.4.2 Chemical stability . . . . . . . . . . . . . .

2.5 References . . . . . . . . . . . . . . . . . .

2.2 Formation of pore structure

. . . . . 15 15 19 23 27 40 40 42 49 50 50 50 51 52 52 52 53

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Chapter 3. Surface chemistry of porous silica . . . . . . . . . . . . . 57

3.1 The surface structure of silica . . . . . . . . . . . . . . . . . 57 3.1.1 Surface species . . . . . . . . . . . . . . . . . . . . 58

3.1.1.1 Surface hydroxyl groups and physically adsorbed water . . . . . 58 3.1.1.2 Internal and surface hydroxyl groups . . . . . . . . . . . 61 3.1.1.3 Types of surface hydroxyl groups. . . . . . . . . . . . . 62

3.1.1.3.1 Free and bound hydroxyl groups . . . . . . . . . . . 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

76 3.1.2.1 Fundamentals . . . . . . . . . . . . . . . . . . . 76 3.1.2.2 Silica-adsorbate interactions . . . . . . . . . . . . . . 78

3.2 Chemical modification of the silica surface . . . . . . . . . . . . . 83 3.2.1 Basic concepts . . . . . . . . . . . . . . . . . . . . 84

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 . . . . 108

3.2.2.1 Bulk modified products (organosilicon xerogels) . . . . . . . . 108 3.2.2.1.1 Xerogels made by condensation of organosilanetriols . . . . . 108

organosilanetriols . . . . . . . . . . . . . . . . 109

trialkoxysilanes 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

3.1.2 Reactivity ofsurface hydroxyl groups in adsorption . . . . . . . .

. . . . . . . . . . .

3.2.2 Synthesis and properties of chemically modified silica supports

3.2.2.1.2 Xerogels made by co-condensation of sodium silicate and

3.2.2.1.3 Xerogels made by co-hydrolysis and co-condensation of organo-

VII

3.3.6 Exclusion of electrolytes from the pores of silica . . . . . . . . . 138 3.3.7 Kinetics of ion exchange on silica . . . . . . . . . . . . . . 140 3.3.8 Applications . . . . . . . . . . . . . . . . . . . . . 140

3.4 References . . . . . . . . . . . . . . . . . . . . . . . 141

Chapter 4 . Particle characteristics . . . . . . . . . . . . . . . . . 147

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 4.2.1 Sieving . . . . . . . . . . . . . . . . . . . . . . . 153 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

4.4 Porous silica layers . . . . . . . . . . . . . . . . . . . . . 163 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

Chapter 5 . Silica columns . packing procedures and performance characteristics . . 169

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 169 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

5.4.1 Column permeability . . . . . . . . . . . . . . . . . . 180 5.4.2 Plate height-velocity dependences . . . . . . . . . . . . . . 181 5.4.3 Column stability . . . . . . . . . . . . . . . . . . . . 184

5.5 References . . . . . . . . . . . . . . . . . . . . . . . 185

5.4 Comparison of performances of silica columns . . . . . . . . . . . 179

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

6.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . 206 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 6.5.1 Introduction . . . . . . . . . . . . . . . . . . . . 222 6.5.2 Physico-chemical standardization . . . . . . . . . . . . . . 223

6.5.2.1 Morphology and size of particles . . . . . . . . . . . . 223 6.5.2.2 Specific surface area . . . . . . . . . . . . . . . . . 223

6.5.2.4 Pore distribution . . . . . . . . . . . . . . . . . . 226 6.5.2.3 Specific pore volume . . . . . . . . . . . . . . . . . 225

6.5.2.5 Stability . . . . . . . . . . . . . . . . . . . . . . 229 6.5.3 Chromatographic standardization . . . . . . . . . . . . . . 229

6.6 References . . . . . . . . . . . . . . . . . . . . . . . 233

Chapter 7 . Silica as a support in liquid-liquid chromatography . . . . . . . 237

7.1 General aspects . . . . . . . . . . . . . . . . . . . . . . 237 7.2 Role ofthe support inliquid-liquidchromatography . . . . . . . . . 238

7.3 Preparation of columns inliquid-liquidchromatography . . . . . . . . 241 7.3.1 Solvent evaporation technique . . . . . . . . . . . . . . . 241 7.3.2 In situ coating technique . . . . . . . . . . . . . . . . . 241 7.3.3 Precipitation technique . . . . . . . . . . . . . . . . . 242

7.4 Effect of silica support properties on retention and column performance . . . 243 7.5 References . . . . . . . . . . . . . . . . . . . . . . . 247

Chapter 8 . Chemically modified silica as packing in ion-exchange chromatography . 249

8.1 Selectivity and kinetics of ion exchange . . . . . . . . . . . . . . 249 8.2 Ion exchangers based on chemically bonded silica . . . . . . . . . . 252

8.2.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . 253 8.2.1.1 Preparation ofsurface-modified ion exchangers . . . . . . . . 255 8.2.1.2 Preparation of bulk-modified ion exchangers . . . . . . . . . 258

8.2.2 Characterization . . . . . . . . . . . . . . . . . . . . 259 8.2.2.1 Ion-exchange capacity . . . . . . . . . . . . . . . . 259 8.2.2.2 Stability . . . . . . . . . . . . . . . . . . . . . 261 8.2.2.3 Selectivity . . . . . . . . . . . . . . . . . . . . 262

8.3 Selectivity and performance of silica-based ion exchangers . . . . . . . 264 8.4 Acknowledgements . . . . . . . . . . . . . . . . . . . . 269 8.5 References . . . . . . . . . . . . . . . . . . . . . . . 269

Chapter 9 . Silica as packing in size-exclusion chromatography . . . . . . . . 271

9.1 Separation mechanism . . . . . . . . . . . . . . . . . . 9.2 Resolution in size-exclusion chromatography

9.4 Size separation on porous silica

. . . . . . . . . . . 9.3 Optimization of silica support properties with respect to resolution and speed

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 . . . . . . . . . . . . . . . . . . . . . .

. 271

. 274

. 278

. 280

. 280

. 281

. 282

. 285

. 287

. 287

Appendix . Commercially available silica packings . . . . . . . . . . . . 291 List of symbols and abbreviations . . . . . . . . . . . . . . . . . 303 Subject index . . . . . . . . . . . . . . . . . . . . . . . . 315

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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. Snyders 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 pre- diction of retention of solutes, it is essential to deal with the properties of silica packings in relation to their chromatographic behaviour. This book attempts to 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 em- phasis 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 COY- ering 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 proof- reading. 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. Super- cooled 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

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

Sols are usually made by adjusting the pH of soluble silicates t o 8-9 with acids. In this

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 to aggregate, the term aggrega-

tion indicating all processes in which colloidal silica particles are linked together. Iler [5] differentiated four typical aggregation processes: gelling, coagulation, flocculation and CO- acervation.

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.

a coagulate being obtained that settles as a relatively dense precipitate.

aggregates with a relatively open structure.

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

In coagulation, the particles are linked together :o form relatively close-packed clumps,

In flocculation, the particles are linked by bridges of the flocculation agent to form

In coacervation, the silica particles are surrounded by an adsorbed layer, which makes

h 7 1 .

Silica xerogels

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. Conse- quently, the xerogel differs widely from the hydrogel in its properties.

After washing the silica hydrogel, water is removed by heat treatment to yield a xerogel,

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 tetra- alkoxysilanes 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 *xHzO because 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 iz

(c) 3 i - C Z (d) S i - N =

(b) ESi-O-CE

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. To 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 pn 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 atmo- spheric 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

1983 K I[ 173-518K

n-cristobalite a- cristobalite

17L3 K I[ 393433 K

O-tridyrnite e a-tridymite

11L3 K 11 a16 K

n-quartz 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 sub- sequent 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 (n/cm3) at 273 K

Coesit 3.01 a-Quartz 2.65 p-QJartz 2.5 3 pCristobalite 2.21 p-Tridymite 2.26 Quartz glass 2.20

Amorphous silica - 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 hydroxyl or silanol groups

\si /OH silanediol groups

\OH (gerninal groups I

/OH

OH -si- OH silanetriol groups

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 to 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, whch 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 /H 0 - isolated or free hydroxyl

groups /T\

/ H 0 0 t vicnal hydroxyl groups

hydroxyl groups bond to \ / H

ti

0

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 non- porous 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 1 1.

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 by 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 to 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

Aerosil a-auartz

3800 cm-

3700

3600

3500

3LOO

> 3747*20 free or isolated hydroxyl groups

> 3660 t 90 hydroxyl groups involve( in hydrogen bonding, internal hydroxyl group'

> 3520 * 200 hydroxyl groups hydrogt bonded to adsorbed water

> 3 4 0 0 f 200 adsorbed liquid water, hydrogen bonded

-- 3690 t 1 free hydroxyl groups

3650*1 adsorbed water, isolated

=- 3620 t 1 free hydroxyl groups

> 3 4 0 0 t 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.

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.

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

On Aerosil and Cabosil at 298 K , a sharp band at 3747 cm-' is observed, which merges

Three bands can be distinguished on the low-frequency side. A broad band at 3400 cm-',

11

0.3 nm between them and are mostly capable of interaction by hydrogen bonding. How- ever, 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 D20 combined 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.

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.

It can be concluded that there are significant differences in the relative positions of the

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 to 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 [3 11 have intensively treated the problems of the analytical determination of silica.

of silica. As was pointed out in the previous section, the first reaction step between an- hydrous 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:

Let us first consider the basic reactions that determine the specific solubility behaviour

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 to 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 photo- metric 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 to be about 100 ppm [S, 301. For quartz, however, the values obtained are sometimes considerably lower [ 151. There is no reason to 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 H 2 0 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 t OH- - - Si(OH)4

Si(OHk t OH- [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 by 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).

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.

As is generally known, solubility is a function of the particle size of the solid.

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].

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.

From a practical point of view, one can draw the following conclusions. The solubility

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 the

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 VJ . 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 on Analytical Chemistry, Part 11, Vol. 2, Wiley-

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.

Interscience, New York, 1971.

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 tailor- made 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 197 1, 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 submicro- pores, 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),

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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 narrow neck (Dn) and the width of the wide body (Db). A third is the model of pores 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 char-

acterizing 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.

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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 to eqn. 2.2, S, = 0.3 m2/g. Thus, S represents predominantly an internal surface.

a

l a

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 ) slit- shaped pores with parakl plates.

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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, Vp, is the amount of liquid adsorbate that fills the total volume of pores per gram of adsorbent. To a first approximation, Vp 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, fp, can be calculated by means of the equation

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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 signif- icance. 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 to 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

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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 on 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, cz 1 - (n + 1) zn tnzn+l (1-z) 1 t(c-I)z-cz"+l

x, =-. (2.4)

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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 two- parameter 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 three- parameter 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.

of micropores, leading to the following expression, which is valid over the range of relative pressure l*lO-'

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By means of a thermodynamic treatment of capillary condensation, another equation can be derived [ 161 :

7 d S = ApdXa (2.8)

where y = surface tension of the adsorbate, assumed to be a liquid; 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 Xa, = number of moles adsorbed at the beginning of the hysteresis loop; Xa, = number of moles adsorbed at the end of the hysteresis loop.

= surface area of the liquid adsorbate;

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 /po . Assuming pores that are circular in cross-section, one obtains

dV D d s 4

Eqn. 2.1 1 then rearranges to

- _ - - (2.1 2)

(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

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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 e Dw

p = - -

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

p(bar) = 14,708

Dw (nm)

(2.14)

(2.15)

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.

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 to 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 of the order of loe2 - Pa is sufficient. Outgassing is often carried out at elevated temperatures, 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.

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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].

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 to the

In contrast to the volumetric technique, gravimetric methods permit the direct

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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, Vp, 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.

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Commercially available porosimeters operate at pressures up to 2000 and 4000 bar, whereas special porosimeters permit operation at up to 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

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%.

Mottlau /27]

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 relations

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