Chemistry of Silica - Solubility, Polymerization, Colloid and Surface Properties and Biochemistry

980 Pages·1979·42.625 MB·English

by  Iler, Ralph K.

THE CHEMISTRY
OF SILICA
Solubility, Polymerization, Colloid and
Surface Properties, and Biochemistry
RALPH K. ILER
A Wiley-Interscience Publication
JOHN WILEY & SONS
New York Chichester Brisbane Toronto Singapore
Copyright @ 1979 by John Wiley & Sons, Inc
All rights reserved. Published simultaneously in Canada.
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Library of Congress Cataloging in Publication Data:
Iler, Ralph K.
The chemistry of silica.
"A Wiley-lnterscience publication."
Includes bibliographies and index.
I. Silica. 2. Silicates. 3. Colloids. 1. Title.
QD181.S6144 546l.683'2 78-23960
ISBN 0-471-02404-X
Printed in the United States of America
30 29 28 27 26 25 24 23 22 21
To my wife, Mary, with gratitude for her never-ending
patience during the seemingly interminable
writing of this book
Preface
This book was at first intended to be an updated second edition of my
earlier book, The Chemistry of Silica and Silicates (Cornel1 University
Press, 1955). It necessarily covers much of the same subject matter, but
with 2500 new references to consider, it had to be reorganized and expanded
to such an extent that it constitutes an almost entirely new work.
The purpose of the book is to present a complete and coherent account of
the chemistry of amorphous silica, including soluble silica and silicate
precursors of soluble silica, polymerization to polysilicic acids, colloidal sols
and gels, and the surface chemistry of silica. In discussing practical applica-
tions of sols and gels, emphasis is placed on the chemistry involved. The last
chapter on silica in living organisms is especially important in view of the
growing recognition that silica is present in many biological systems and
can function as an essential trace element.
Since publication of my earlier book in 1955, the literature on colloidal
metal silicates, including minerals, and on silicic esters has grown
enormously. Consequently these areas had to be omitted.
The title, The Chemistry of Silica, may be misleadingly broad but is
offset by a more definitive subtitle, “Solubility, Polymerization, Colloid and
Surface Properties, and Biochemistry.”
It is remarkable that silica, the major component of the earth’s solid sur-
face, has never become a separate branch of study or instruction. Science
students graudate with little or no knowledge of its properties or chemistry.
Yet sooner or later, in such diverse fields as industrial chemistry,
electronics, agriculture, mining, metallurgy, petroleum, power development,
and even biochemistry and medicine, problems arise involving this common
element oxide. This book is written not only for those already engaged in
these areas, who may find it a useful guide to the literature, but also for
vii
viii Preface
those in other fields who need specific information not otherwise easily
available.
RALPHK . ILER
Wilmingron, Delaware
Noveni ber I978
Contents
Introduction
Previous Books and Reviews of Silica Chemistry
Selection of References
Terminology
References
1 The Occurrence, Dissolution, and Deposition of Silica 3
The Silica-Water System 3
Thermodynamics of the System 6
Relating Particle Size and Composition 7
Energy Change with Changing Particle Size and Composition 9
Soluble Silica-Monosilicic Acid 10
Volatility in Steam 12
Soluble Silica in Nature 13
Phases of Silica 15
Anhydrous Crystalline Silicas 15
Relation between density and refractive index
Hydrated Crystalline Silicas 19
Amorphous Silicas 21
Microscopic sheet, ribbon, and fiberlike forms, Common
amorphous forms, Hydrated amorphous silica, Biogenic silica
The Solubility of Silica 30
Solubility of Quartz at Ordinary Temperature 30
Cleaning the surface
Solubility of Quartz under Hydrothermal Conditions 32
xi
xii Contents
Solubility of Cristobalite and Tridymite 32
Solubility of Other Crystalline Forms of Silica 33
Adsorbed Silica on Crystalline Silica 34
Solubility of Amorphous Silica 40
Establishment of solubility equilibrium, Effect of heating,
Solubility in water: pH 0-8, Possible solubility minimum at
pH 7, Solubility in nitric acid, Solubility in NaCIO, solutions,
Effect of electrolytes, Solubility under hydrothermal conditions
Solubility of Hydrated Amorphous Silica 46
Apparent High Solubility at High pH 47
Calculation of solubility and dissociation constant
Effect of Particle Site on Solubility in Water 49
Interfacial Energy 54
Effect of Impurities on Solubility 56
Effect of Organic Compounds on Solubility 58
Catechol and Related Compounds 59
Polyhydroxy Organic Compounds 59
N-Oxides 59
Organic Bases 60
Living Tissues 60
Solubility in Alcohols 61
Methanol, Higher alcohols
Solubility in Molten Salts 62
Rate of Dissolution of Silica 62
Mechanism 62
Effect of pH on Rate 65
Relation Between Rate of Dissolution and Particle Size 65
Rate of Dissolution Of Very Small Particles 69
Rate of solution as particle dissolves
Rate of Dissolution of Particles of Different Sizes 72
Dissolution of Crushed Powders 73
Neutral Solutions-Effect of Salts 74
Retardants of Dissolution 75
Rate of Dissolution in Presence.of Catechol 75
Rate of Dissolution in Aqueous HF 76
Comparative Rates of Dissolution 76
Removal and Deposition of Silica from Water 76
Contents xiii
Removal of Silica from Water 78
Precipitation mechanisms, Nucleation of quartz, Adsorption and
precipitation by hydrous oxides, Removal by ion exchange
Deposition of Silica from Water 83
Rate of deposition of monomeric silica, Silicification of biog- enic
materials, Rate of deposition of colloidal silica
Methods of Analysis 94
Atomic Absorption 94
Chemical Methods 95
Methods Involving Silicomolybdic Acid 95
The beta silicomolybdate method, A recommended procedure,
Interfering substances, Molybdenum blue method, For
biological sample
Methods of Concentrating Silica for Analysis 100
Depolymerizing Colloidal Silica before Analysis 101
Standard Silica Solutions 101
Miscellaneous Colorimetric Methods 101
Detection of Colloidal Silica on Surfaces 102
Rapid Titration of Total Silica as Fluosilicate 102
Titration as the Silicomolybdic Acid 103
References 104
2 Water-Soluble Silicates 116
Sodium and Potassium Silicates 117
Manufacture 117
Commercial Solutions 1 I9
Soluble Crystalline Sodium and Potassium Silicates 120
Properties of Solutions 120
Fields of Use 121
The Nature of Silicate Solutions 123
Theory 126
Physical Studies 130
Effects of diluting silicate solutions, Effect of alkali metal
salts and other ccagulants
Conversion to Silicic Acids 137
Reaction with molybdic acid, Conversion to esters of silicic
acids, Conversion to trimethylsilyl derivatives of silicic acid
xiv Contents
Silicates with Coordination Numbers Four and Six 142
Solutions of Polysilicates 143
Sodium Polysilicate 144
Potassium Polysilicate 145
Lithium Silicates 145
Lithium Polysilicates 146
Uses for Lithium Silicates and Polysilicates 149
Organic Base Silicates 150
Mixed Organic Base-Alkali Metal Base Silicates 153
Other Organic Base Silicates 154
Complex Metal Ion Silicates 154
Organic Chelates of Silicon 155
Catechol Derivatives 156
Humic Acids 157
Other Organic Compounds 157
Hydrated Crystalline Alkali Metal Polysilicates 158
Silicates Convertible to Crystalline Forms of (H,Si,O,), 160
Precipitation of Insoluble Silicates 161
Soluble Silicate Glasses 163
Peroxy Silicates 164
References 165
3 Polymerization of Silica 172
General Theory of Polymerization 174
Overall Effect of pH on Gelling 177
Monosilicic Acid 177
Preparation 178
Dissolving silica, Hydrolysis of monomeric silicon compounds,
Dissolving monomeric silicates in acid
Characteristics of Silicic Acid 180
Diffusion constant, Ionization constants, Increase in ionization
constant with polymerization, Isoelectric point, Point of zero
charge, Stability of monomeric silica
Reactions of Monosilicic Acid 189
Phosphoric and boric acids, Sulfuric acid, Iron and uranium,
Chromium, Aluminum, Divalent cations
Characterization of Silicic Acids 195
Contents xv
Reaction with Molybdic Acid 195
Alpha and beta silicic acids, Measurement of reaction rates,
Reaction rate constants, Composition of molybdic acid
reagents, Other observations
Separation of Silicic Acids 202
Particle Size and Surface Area by Titration 203
Correction for soluble silica
Coagulation with Gelatin-Salt 206
Mechanism of Condensation and Hydrolysis 209
Catalytic Effect of HF 21 1
Polymerization: pH 2-7 213
Formation of Oligomers 2 14
Oligomers as Particles 215
Nucleation Theory 218
Particle Growth in Acidic Solution 220
Depolymerization in Acidic Solution 220
Polymerization by Aggregation-Gel Formation 222
Molecular versus Particle Chains 222
Mechanism of Interparticle Bonding 223
Formation of Chains of Particles and Networks 225
Partial Coalescence of Particles in Chains 221
Development of Microgel and Viscosity 23 1
Isolating “gel phase” or “microgel”, Effect of electrolytes and
coagulants, Gel density and structure, Increase in viscosity
Formation of Larger Particles by Coacervation 239
Polymerization above pH 7 239
Spontaneous Growth of Particles 239
Final Size of Particles versus Temperature 242
Viscosity of Sols before Aggregation Begins 244
Viscosity of Sols of Very Sinal1 Particles at Low pH 244
Decrease in Viscosity on Conversion of Microgel to Sol 241
Thermal Effects 248
Energy of activation, Heat of polymerization
Summaries of Investigations 249
Investigations at Low pH 2 50
Iler; Alexander, Heston, and Iler; Schwarz and Knauff;
Bechtold; Goto; Okkerse; Audsley and Aveston; Weitz, Franck,