chemistry of silica ralph iler faltando cap 6 e algumas pags

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Silica, the major component-Olthe
earth's solid surface and. the constitu-
ent of ordinary sand,'becomes involved
at some point in a great many phasesof .' .
modern technology and science. It is an
essential material in many, ·if not all,
. forms of life. Its role in human disease,
aging, and health is Just beginning to be
explored. Here is a comprehensive ac-
count of the basic chemistry invOWed in
a wide range of research and develop-
ment activities, as well as a wealth of in-
formation on production and produc-
tion control.
Beginning with the solubility of different.
. forms of silica and the factors that influ-
ence dissolution and deposition, the so-
lution chemistry of silica Is Introduced.
The author also compares and recom-
mends analytical methOds. The digest
of all currently available information
provides a solid background as to the
nature of soluble silicates and particu-
larly the mechanism of polymerization
of sHicic acid and formation of colloid.
:.'For the first time, the mechanism by
which silica sots, powders and gels-are
formed and their properties controlled
Is clearly described. Next, the many
types and uses of commercial concen-
trated sols, gels, and u'trafine powders
are examined, fotl~ by a discussion
of the biochemical properties and many
applications of the surface chemistry of
silica. The finat chapter draws together
all aspects of the occurrence and im-
portance of silica in different life forms.
Those engaged in research, develop-
ment, and production in the many di-
verse fields and Industries in which sili-
ca plays a vital role-such as chemis-
try, biology, medicine, agriculture, met-
allurgy, and mining-will find THE
CHEMISTRY OF SILICA an indispens-
able reference.
795
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THE CHltMISTRY
OF SILICA
Solubility, Polymerization, Colloid and
Surface Properties, and Biochemistry
RALPH K. ILER
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A Wiley-Interscience Publication
JOHN WILEY & SONS
New York • Chichester • Brisbane • Toronto
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To my wife, Mary, with gratitude for her never-ending
patience during the seemiaaly interminable
writing of this book
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Preface
This book was at first intended to be an updated second edition of my
earlier book, The Chemistry of Silica and Silicates (Cornell 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
RALPH K. ILER
Wilmington, De/aware
November /978
those in other fields who need specific information not otherwise easily
available.
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.ER
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./ Acknowledgments
I am indebted to the Cornell University Press for permission to include
some of my earlier book* with the following credit:
Reprinted from Ralph K. lIer: THE COLLOID CHEMISTRY OF
SILICA AND SILICATES. Copyright e 1955 by Cornell University.
Used by permission of the publisher. Cornell University Press.
I am also grateful to John Wiley and Sons for permission to include in
Chapter 5 portions of my monograph on "Colloidal Silica" in Col/Did and
Surface Science. VoL 6. 1973. edited by Egon Matijevic. .
This work would have been impossible without the generosity of E. I.
duPont de Nemours & Co. in making available to me. as a retiree. the
facilities of the Lavoisier Library at the duPont Experimental Station.
It is impossible to mention all those who have kindly reviewed drafts of
portions of the manuscript and given invaluable advice. My friend and
fellow scientist. Dr. Paul C. Yates has been very helpful with sound
technical counsel. .
To the late Mildred Syvertsen. who played an indispensible role in all my
earlier publications. I remain grateful for help in collecting references and
typing much of the present manuscript. The assistance of Patricia Cullen in
final typing. of Joseph A. Pankowski. Jr. in preparing illustrations and of
Jennifer J. Stiles in assembling indexes. is sincerely appreciated.
R.K.1.
• Now out of print.
ix
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Contents
Introduction
Previous Books and Reviews of Silica Chemistry
Selection of References
Terminology
References
.1 The Occurrence, Dissolution, and Deposition of Silica
The Silica-Water System
Thermodynamics of the System
Relating Particle Size and Composition
Energy Change with Changing Particle Size and Composition
Soluble Silica-Monosilicic Acid
Volatility in Steam
Soluble Silica in Nature
Phases of Silica
Anhydrous Crystalline Silicas
Relation between density and refractive index
. . . .
. Hydrated Crystalline Silicas
Amorphous Silicas
Microscopic sheet. ribbon, and fiberlike forms. Common
amorphous forms. Hydrated amorphous silica. Biogenic sili
The Solubility of Silica _ .
Solubility of Quartz at Ordinary Temperature
Cleaning the surface
Solubility of Quartz under Hydrothermal Conditions
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Contents
Solubility of Cristobalite and Tridymite 32
Solubility of Other Crystalline Forms ofSilica 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 Size 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
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Contents Contents xiii
32
3
34
40
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47
49
54
56
58
59
59
59
10
60
61
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 ofmonomeric silica, Silicification of biogenic
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 recom mended procedure,
Interfering substances, Molybdenum blue method, For
biological sample.
Methods of Concentrating Silica for Analysis 100
Depolym erizing 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
62
62
62
65
65
69
72
73
74
75
75
76
76
~ 76
2 Water-Soluble Silicates
Sodium and Potassium Silicates
Manufacture
Commercial Solutions
Soluble Crystalline Sodium and Potassium Silicates
Properties of Solutions
Fields of Use·
The Nature of Silicate Solutions
Theory
Physical Studies :
Effects of diluting silicate solutions. Effect of alkali metal
salts and other coagulants
Conversion to Suicic Acids
Reaction with molybdic acid. Conversion to esters of silicic
acids. Conversion to trirnethylsilyl derivatives of silicic acid
116
117
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119
120
120
121
123
126
130
137
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Silicates with Coordination Numbers Four and Six 142
~ .~ Solutions of Polysilicates 143
... : Sodium Polysilicate 144'-.'
i:! : .Potassium Polysilicate 145
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! Uses for Lithium Silicates and Polysilicates 149
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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
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Catechol Derivatives 156
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Humic Acids! . 157
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Other Organic Cornpounds : 157
Hydrated Crystalline Alkali Metal Polysilicates 158
Silicates Convertible to Crystalline Forms of (H2Si2Os),c 160
Precipitation of Insoluble Silicates 161
· i Soluble Silicate Glasses 163
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i· Peroxy Silicates 164;
· . References 165
3 Polymerization of Silica 172
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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 andboric acids, Sulfuric acid, Iron and uranium,
Chromium, Aluminum, Divalent cations
Characterization of Silicic Acids 195 \.......--
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 211
Polymerization: pH 2-7 213
Formation ofOligomers - 214
Oligomers as Particles 215
Nucleation Theory 218
Particle Growth in Acidic Solution 220
Depolyrnerization 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 227
Development of Microgel and Viscosity 231
Isolating "gel phase" or "rnicrogel", 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 Small Particles at Low pH 244
. Decrease in Viscosity on Conversion of Microgel to Sol 247
Thermal Effects 248
Energy of activation. Heat of polymerization
Summaries of Investigations 249
Investigations at Low pH 250
Iler; Alexander, Heston, and Iler: Schwarz and Knauff:
Bechtold; Goto; Okkerse: Audsley and Avcston: Weitz, Franck,
_...'::".1)'."0-.
Contents
142
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144
145
145
146
149
ISO
153
154
154
155
156
157
157
158
160
161
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164
165
172
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177
178
Is,
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on
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Contents xv
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xvi Contents
and Giller; Bechtold, Vest, and Plambeck; Acker: H~ebbel
. and Wieker
Investigations Through the Neutral pH Range 268
Merrill and Spencer; Ashley and Innes; Baumann; Coudurier,
Baudru, and Donnet; Marsh, Klein, and Vermeulen; Ginsberg
and Sheidina
Investigations Above pH 7 281
Greenberg and Sinclair; Greenberg; Goto; Tarutani; Iler: IIer
and Sears; Richardson and Waddams; Makrides et al.
Polysilicic Acids 287
Preparation of Polysilicic Acid 288
Hydrogen-Bonded Complexes with Polar Organic Compounds 288
Method of comparing hydrogen-bonding activity, Structure
versus activity, Liquid hydrogen bonded complexes-
coacervates, Complex of silicic acid with amine salt,
Interaction of silicic acid with phosphoric acid ester
Combinations with Organic Polymers 297
Prevention of hydrogen bonding by negative charge on silica,
Cationic organic compounds
Miscellaneous Interactions with Organic Materials 29~
Interaction with proteins-e-tanning. Esterification of
polysilicic acid
Activated Silica Sols-Water Treatment 301
Reaction of Polysilicic Acid with Metal Cations 303
References 304
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4 Colloidai Silica~Concentrated Sols 312
Definition of Colloidal Silica and Historical Development 312
Growth and Stabilization of Discrete Particles 313
Increasing Particle Size by Adding ..Active" Silica 313
Methods of Making Particles Under 10 nm in Size 317Stabilization Against Particle Growth 318
Stabilization Against Aggregation 323
.. , Stabilization by ionic charge, Addition of salt to lower
viscosity, Sterk stabilization .
Porous Particles 328.......:...
Contents
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Contents xvii
268
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297
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303
304
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317
318
323
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Elongated Particles 330
Particles with Non-Siliceous Cores 330
Methods of Making Sols 331
Neutralizing Soluble Silicates With Acids 33I
Electrodialysis 332
Ion Exchange 333
Peptizing Gels 334
Hydrolysis of Silicon Compounds 335
Dissolution of Elemental Silicon 335
Dispersion of Pyrogenic Silica 336
Purification. Concentration, Preservatives 337
Ion Exchange 337
Dialysis and Electrodialysis 338
Washing Procedures 338
Concentration 338
Evaporation of water, Centrifugation, Ultrafiltration,
Electrodecantation
Preservatives 343
Characterizing Sols 344
Chemical Analysis 344
Measuring pH. Electrolyte concentration
Particle Characteristics 345
Particle size. Specific surface area
Ionic Charge on Particles 355
Nature of ionic charge. Counterions and double layer
Viscosity 360
Aggregation of Particles 364
Definitions 364
Gelling 366
Effect of pH. Effect of particle size and concentration.
Electrolytes and organic liquids. Temperature. Theory of
strength of gels
Coagulation 372
Mechanism. Coagulation by electrolytes. Monovalent cations
as bridging agents. Coagulation by divalent metal ions.
Coagulation by polyvalent cations-basic metal salts. Effect
of silica concentration and other factors. Effect of
particle' size. Partly dehydrated surface
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Flocculation 384
Flocculation with cationic surfactants, Flocculation with
·
organic polymers
-: Coacervation 396~. ~
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~:: . ! . Aggregation into Ordered Structures-Precious Opal 398' . I 'I .
! ' Opal structure, Other ordered aggregates, Formation ofi ,
uniform inorganic particles, Synthesis of opal
.. i : Adsorption of Silica Particles on Surfaces 405
! .
Sols of Silica Particles with Modified Surfaces 407
~
· l Negatively Charged Surfaces 407
L Aluminosilicate ions, Other anions
~ .. Positively Charged Particles 410I·
-, : Polyvalent metal oxide coatings, Polyvalent organic cations ,
p- IOrganic Modified Surfaces-Organosols 412 -
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, : Organic ions, Esteri fication, Silylation
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L Commercial Colloidal Silicas 415
i Uses of Colloidal Silicas 415£
I Making Catalysts, Gels, Adsorbents 420IJ'( .
r~: . Inorganic Binder, Stiffener 420 ,-..
r Molded refractory bodies, Binders for fibers, Refractory
~ coatings, Molds for casting metalst,
r Frictionizing Effects 425+ . Fibers, Paper, Steel rails, Other surfaces. i
.
~ Antisoiling Surfaces 426
"j .
J Hydrophilizing Surfaces 427l _
j Modifying Adhesion 428
f' Increasing adhesion, Decreasing adhesioni :
i Coating Compositions 430
, .
Coatings on ships: tanks
Reinforcing organic polymers 432
t· .- Polishing Agent for Silicon Wafers 433~ ..
! Surfactant Effects 433,
.
Dispersing effects; Antifoaming effects
Modifying Viscosity-e-Gelling . 434
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Miscellaneous Optical Effects. Color, Photography 435
Use in Biological Research-.Density Gradient 436 <..
Contents Contents
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425 -.:
426
427
428
430
432
433
433
434
135
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384
396
398
405
407
407
410
412
415
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420
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Source of Chemically Reactive Silica 437
Soluble silicates, Silica bodies, GI3~\ compositions, Forming
solid silicates-cements, Other rea..' \ ions and uses
Colloidal Silicates' 439
References 439
5 Silica Gels and Powders 462
Definitions 462
Types of Gels 462
Types of Powders 463
Physical Characterization ?f Gels and P'lwders 464
Ultimate Particle Size 465
Electron micrographs, Specific sur(:II,;C area, Low angle X-ray
scattering
Aggregate Size-Powder Particles, Gd Granules 476
Pore Characterization 478
Particle size and packing, Loss of SUrface area by particle
packing
Characterizing Pores by Adsorption h'Jtherms 488
Pore volume, Pore size and size dislt'ibution, Miscellaneous
effects in micropores,
Nature of Silica Surface 505
Aggregate Strength-Interparticle B'IIIIJing 506
Electron micrographs, Partial diss,'lltlion method, Mechanical
strength of the aggregate . .
Silica pels 510
Sources of Silica Gel 511
From soluble silicates andminernb,From colloidal silica,
From hydrolysis of silicon com pou lids
Factors Controlling'Gel Characterist it:; 516
Size of pri~'~~ypa~iicles::'-pHefft.''', Wet gel strength.,
Particle size and packing density ill dried gels, Increased
porosity with 'removable fillers
Forming and Shaping Gel Particles 526
Wet Gel Treatments 528
Gel reinforcement
549
533
537
539
563
564
564
565
544
554
554
~b8
569
570
571
572
573
574
575
576 .
576
577
578
578
Contents
Drying and Shrinkage-Xerogels
Drying from low surface tension liquids
Drying without Shrinkage-Aerogels
Hydrothermal Treatments
Liquid or vapor phase
Heating in Air, Vacuum
Sintering uniform structures, Alkali metals, impurities,
crystallization
Special Gel Structures
Submicroporous gels-impervious silica, Porous glass,
Specific adsorbents
Precipitated Silica Powders
Silica Precipitated from Sodium Silicate Solution
Silica coagulated with sodium ions, Coagulation by adding
sodium or ammonium salts, Coagulation with calcium,
polyvalent metal ions, Coagulation with organic materials
Silica Precipitated from Fluoride Solution
Silica Precipitated from Organic Liquids
Silica Precipitated from Colloidal Silica Sols
Silica Precipitated from Vapor: Pyrogenic Silica
, "vaporized Si02, Oxidation of SiO vapor, Oxidation and
hydrolysis of SiCI. vapor, Oxidation and hydrolysis of silicon
esters vapors, Hydrolysis of SiF. vapor
Naturally Occurring Silica Powders
Microcrystalline Hydrated Silicas
Hydrophobic-Organophilic Silica Powders '
Adsorbed Organic Cations
Adsorbed Polyvalent Metal Cations with Organic Anions
Surface Esterification
Organosilicon Coatings
Organic Polymer Coatings '.
Silica Gels with Ion-Exchange Surfaces
Inorganic Ion-~xchang'e.Sites
Organic-Linked Ion-Exchange Sites
Commercial Silica Gels and Powders
Uses of Silica Gels and Powders
xx
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Contents Contents ui
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533 Reinforcing and Other Effects in Organic Solids 582
In rubber, In silicone elastomers, In various organic polymers
537 Reducing Adhesion 587
539 Increasing Adhesion- 587
Increasing Viscosity,·Thixotrophy 588
544 Mechanism, Lubricating grease, Paints, coatings, inks,
. Parmaceuticals and cosmetics, Miscellaneous compositions
Optical Effects-Flatting 593
549 Surfactant Effects 594
Stabilizing emulsions, Hydrophilic surface, Antifoam agent
Hydrophobing Effects 594
554
"Dry" water
554 Absorbent 595
Catalysts 596
Aerogels, Base for mitochondria, Spillover
I 597I Reactive Silica563 lI Cloud Seeding 597I564 ! Chromatographic Column Packings 598564
. References 599
J65
6 The Surface Chemistry of Silica 622
n
Reviews and Summaries 623
568 Nature of the Silica Surface 624
569 Structure of the Underlying Silica 624
570 Definition of Surface 625
571 The Hydroxylated Surface 625
572 . State of water at th-.7 hydroxylated surface, Electrical
573 . conductivity ofthe surface, Distinguishing adsorbed water
574 fromsilanol groups, Internal hydroxyl groups and trapped
water, Hydroxyl~groups per square nanometer, Theoretical
575
.concentration of surface hydroxyl groups
576 Dehydration and Rehydration 637
576 Surface Energies 645
577 Heat of Wetting Silica Surface 646
578 Physical Adsorption of Non-Ionic Low Molecular Weight
...:....,.
578 Compounds 648
Contentsxxii
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Adsorption of Vapors
Effect of dehydroxylation on adsorption
Adsorption from Solution-Nonionic
Nonaqueous solutions, Aqueous solutions, nonionic, hydrogen
bonding
Ionization and Surface Charge
The Hydroxylated Surface
The Dehydroxylated Surface
Nature of the Anionic Charge Sites
The "Site-Binding" theory
Forces Involved in Adsorption of Ions
Univalent cations: Metals and lower amines, Alkaline earth
metals and magnesium cations, Polyvalent metal cations
(Table 6.3)
Nonionic Reactions of the Silica Surface (Table 6.4)
Hydrophilic Coatings on Silica
Hydrophobic Silica Surface
Organic Cations and Bases
Hydrophobing effects, Cationic dyes, Aluminosilicate surfaces
Hydrocarbon Groups Attached through Polyvalent Metals
Surface Esters with Alcohols
Surface coverage, Reaction conditions, Methyl esterified silica,
Reaction of alcohols with dehydroxylated surface, Reaction of
hydrocarbons with dehydroxylated surface, Esterification in
micropores, Substituted alcohols, Hydrolysis of ester groups
Organic Groups attached by C-Si Bonds
Adsorption on Hydrophobic Surfaces
Adsorption ofwater, Adsorption of inert gases
Adsorption of Organic Polymers on the Silica Surface
From Aqueous Solution
Polyethylene oxide, Polyvinyl alcohol, Cationic polymers,
Proteins, Adsorption of polymers on dehydroxylated silicas,
Effect of salts
From Nonaqueous Solvents
Deposition of Multilayers of Charged Polyions and Particles
The Surface of Alumina-Silica
648
654
659
659
661
663
665
676
679
680
680
688
689
.695
699
702
704
709
710
710
'-.::..'
7 Silica in Biology 730
Introduction 730
Origin of Life 730
Earliest Life Forms 731
Biological Disintegration of Rocks 733
Association with Primitive Organisms 733
Viruses 734
Bacteria 734
Fungi and Lichens 734
Algae and Diatoms 734
Sponges 739
Gastropods, Sea Cucumbers, Limpets 739
Plants 740
Nature of Silica Deposits in Plants 741
Strengthening Plant Parts 742
Equisetum, Bamboo, Grasses, Spiny plants, Job's Tears, Palms,
Wood
Mechanism ofAbsorption, Movement, and Deposition of Silica 747
Relation of Soluble Silica to Soil Fertility 748
Beneficial and Protective Effects of Silica 750
Insects 752
Fish, Amphibians, Reptiles, Birds 753
Mammals: Man 753
Essential Role of Silica in Mammals 756
Toxicity of Silica 757
Cytotoxicity
Silica in Biochemical Combinations 761
Combination with polysaccharides, Combination with proteins.
Denaturation of proteins, coagulation of blood, Combination
with specific compounds, enzymes, Combination in .
phosphorus compounds: Nucleic acids, DNA, RNA,
Mutations, atherosclerosis, cancer
Contents
648
,,54
jen
659
659
661
663
665
676
679
i
680 r
680
:es
'88
689
rca,
of
s
695
699
702
704
709
710
710
........;...._-
Contents
Active Sites, Free Radicals, Active Oxygen, Ozone
References
xxiii
712
714
I
~
'.
,
~
~
..
i
-.
~
't
~
;:
J
~
~
.--;.----
Silicosis-Pneum oconiosis-Fibrinogenesis 769
Mechanism of silicosis, Amorphous versus crystalline silica
and particle size, Susceptibility to silicosis, An unusual
compound in silicotic tissues, Solubility theory, Silica
antagonists to prevent silicosis
Asbestosis-Microaciculosis 782
Beneficial Effects of Silica 783
Silicon Metabolism 783
Silica Gel as a Culture Medium 784
Organosilicon Compounds 785
Analytical Problems 786
Conclusion 787
References 787
xxiv Contents
Author Index 803
Subject Index 835
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Contents
769
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THE CHEMISTRY OF SILICA
..
~.-..
Introduction
PREVIOUS BOOKS AND REVIEWS OF SILICA CHEMISTRY
Numerous outlines or surveys of the chemistry of silica. especially the water-silica
system. have appeared over the past half-century. Properties of silica have been
described by Sosman (la) and more recently by BrUchner (Ib), The state of
knowledge of soluble silicic acid and colloidal silica was summarized in 1937 by
Fricke and HUttig (2). and the general aspects of "silicic science" were reviewed by
Hauser in 1955 (3). At the same time. Iler exhaustively covered the rapidly develop-
ing theory and practice of the colloid chemistry of silica and silicates. including new
surface chemistry and the role of silica in biology (4). Following an initial book on
the physical chemistry of silicates in 1954 (5). Eitel authored a monumental series of
six volumes on silicate science from 1964 to 1975 (6). The colloidal aspects of silica
are summarized in Volume I and brought up to date in Volume IV. A review of the
solubility of silica in water. the nature of soluble silica." and its equilibrium with
polymer species was published by Stober (7). Similar reviews. each with a particular
emphasis. were written by Greenberg (8). Wittman (9). Coyle (10). Maher (I I). and
Kolthoff and Elving (12).
j . Alexander has given an entertaining personal account of his researches in silica
chemistry (13). More recent summaries have been prepared by Kukolev (14) and
Hinz(!5).
SELECTION Of REfERENCES
! '
The major references used are those in which most variables have been deli ned. Thus
when pH has not been recorded in an aqueous system. or the source or characteris-
tics of the silica used in experiments are not well deli ned. less attention is given,
There has been enormous duplication of experimental work. particularly in coun-
tries where earlier work was not recognized because of lack of availability to the
literature or language difficulties. In such cases original work has been emphasized.
and secondary work is cited only as conlirmation.
Patents are cited when the technical information is not otherwise available in the
literature. Where possible. United Stales patents arc cited because of general
availability throughout the vor ld upon request. But where equivalent U.S. patents
could not be located. the patents in other countries arc cited.
2
TERMINOLOGY
Introduction
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'The term "silanol" is used for any OH group attached to silicon and will be under-
stood to include what has otherwise been referred to as "siloxanol" and "silicol"
groups. The term "polymerization," strictly speaking, means a linking together of
monomer units to form a polymer of the same composition, but, in the silica system
the monomer, Si(OH)., condenses to form polymer that ultimately has the composi-
tion (SiOz)II' However, the term "polymerization" has been so widely used for the
formation of condensation polymers that it will also be so used here.
"Silica" is used as a short convenient designation for "silicon dioxide" in all its
crystalline, amorphous, and hydrated or hydroxylated forms. This word does not
occur in many other languages, which generally use the term "silicon dioxide."
However, the latter implies only the composition SiOz• In analysis, the term "silica"
indicates only that the siliconcontent is given in terms of weight' of SiOz• regardless
of the form in which it is actually present.
REFERENCES
la. R. B. Sosrnan, The Phases of Silica, Rutgers University Press. New Brunswick, N.J .•
1965.
lb. R. Brilchner, "Properties and Structure of Vitreous Silica:' J. Non-Cryst, Solids. 5,
123, 177 (1911).
2. R. Fricke and G. F. HUttig. Handbuch der Allgemeine Chemle, Vol. 9. Hydroxides and
Oxyhydrates, Akademische Verlag. Leipzig, 1937, p. 146. '
3. E. A. Hauser, Silicic Science, Van Nostrand. Princeton. N.J., 1955.
4. R. K. Iler, The Colloid Chemistry of Silica and Silicates. Cornell University Press.
Ithaca, N.Y.• 1955.
5. W. Eitel. The Physical Chemistry of the Silicates. University of Chicago Press. Chicago.
III.. 1954.
6. W. E~tel. Silicates Science. Vols. I-VI. Academic. New York. 1964-1975.
7. W. Stober~ Kolloid Z.• 147, 131 (1956).
8. S: (Greenberg.J. Chem, Educ. 36, 218 (1959).
,9. A': Wittman. Oesterr. Chem, Z .• 62, 245. (1961).
10. T. D. Coyle. Kirk-Othmer Encyclopedia of Chemical Technology. 2nd ed.• Vol. 18.
Wiley. New York. 1969. p. 46.
1I. P. K. Maher. Kirk-Othrner Encyclopedia of Chemical Technology. 2nd ed.. Vol. 18.
Wiley. New York. 1969. p. 61.
12. I. M. Kolthoff, P. J. Elving, and E. B. Sandell. "Analytical Chemistry of Silicon."
Treatise on Analytical Chemistry. Part II. Vol. 2. Wiley-Interscience, New York. 1962.
, p. 120. .
...
13. G. B. Alexander. Silica and Me. Doubleday. Garden City. New York. 1967.
14. G. V. Kukolev, Chemistry ofSilicon and Physical Chemistry of the Silicates. Vols. 1-3.
translated from Russian by E. H. Murch. National Lending Library of Science and
Technology. Boston Spa. England. 19?1; reviewed inJ. Am. Ceram, Soc.• 55,126 (1972).
15. W. Hinz, Silikate: Grundlagen der Sllikatwlssenschaft und Silikattechnlk, Vol. 2. Verlag ',-,
Bauwesen, East Berlin. 1971.
ntroduction
be under-
I "silicol"
agether of
ca system
composi-
ed for the
. in all its
does not
dioxide."
n "silica"
regardless
vick, N.J.•
Solids. S,
»xi and
sity Press•
• Chicago.
Vol. 18.
Vol. 18.
Silicon."
irk, 1962.
loIs. 1-3.
ence and
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CHAPTER 1
The Occurrence, Dissolution,
and Deposition of Silica
Silica is by far the major component of the earth's crust. yet much remains to be
learned of its chemistry and, in particular, its solubility behavior in water. The man-
ner of its deposition to form such curiosities as quartz crystals containing inclusions
of mineral oil, mercury, or liquid carbon dioxide remains' a mystery (I). Flint, which'
our remote ancestors recognized as the strongest and toughest stone available, was
apparently formed in some instances from the siliceous skeletons of ancient sponges
by a mysterious process of solution transport. Within some plants and marine
organisms, soluble silica is transported and deposited in characteristic intricate pat-
terns. Only recently has it been recognized that soluble silica, even in trace amounts.
plays a role in the development of mammals.
THE SILICA-WATER SYSTEM
.
As water is a unique liquid, so is amorphous silica a unique solid. They are much
alike, both consisting mainly of oxygen atoms with the smaller hydrogen or silicon
atoms in the interstices. As pointed out by Weyl and Marboe (2), "Some properties
of water and silica are so similar that the transition between hydrated silicic acids
and the aqueous mat-ri« is a gradual one." Washburn (3) noted that water and arnor-
. phous silica both have a temperature of minimum .volume. Ephraim (4) observed
another similarity between silica and water in that water is much less dense than
expected from close packing of the constituent atoms and from X-ray diffraction
studies. Bernal and Fowler (Sa) concluded that water molecules are arranged in a
rather open structure like quartz. and undcrcooled water has a still III are open struc-
ture,like tridymite. Another model has been proposed by Weres and Rice (5b).
These ideas lead to the suggestion that there is some relationship between the
density of water and the solubility of the various forms of silica. since both are
related to the close packing of oxygen atoms. Both silica and water consist. from the
stlWi4pQint of volume. largely of oxygen atoms. which are packed together with a
characteristic nnck inu density. The small hvdroucn and silicon atoms lit between the
4 The Occurrence. Dissolution. and Deposition of Silica
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oxygen atoms, contributing little to the volume. In pure orthosilicic acid, Si(OH). (if
it could be prepared). the small silicon and hydrogen atoms. lying in the interstices
between the large. oxygen atoms, would be more or less evenly distributed
throughout the mass. Polymerization of silicic acid to form solid silica and water
amounts to separation into two phases: in silica. the silicon atoms surround
themselves with oxygen atoms in a region of closer packing, and in water hydrogen
atoms surround themselves with oxygen in a region of more open packing. In amor-
phous SiO" there are 1.17 grams of oxygen per cubic centimeter; water of density
1.0, there is 0.89 gram of oxygen per cubic centimeter.
There is no evidence that silica is "soluble" to any appreciable degree in any
liquid other than water. However, that statement may depend on the definition of
"soluble." The dissolution of silica involves a chemical reaction or hydrolysis in an
excess of water:
Thus it is not a simple solution such as that of sugar in water, where the sugar
molecule exists intact in solution as in the crystal1ine state. Instead. it is analogous
to a hypothetical equilibrium of silica and ether in an excess of ether:
Since low condensation polymers such as [(HO)2SiO]. appear to be clear water-
miscible fluids resembling a polyhydroxy organic compound like glycerol (6). the
monomer, "soluble silica" or Si(OH)•. would probably be a clear liquid if it could be
isolated in anhydrous condition, In a pure state it might ~ven crystallize.
The unusual nature of the silica-water system has been noted by J. A. Kitchener
(7), who pointed out that the endless confusion in the literature concerning the
silica-water interface has arisen because the hydration and solubility characteristics
have not been understood. For example. there is the question as to why silica sols
are extraordinarily stable 'at pH 2 where the zeta potential is zero and become
increasingly sensitive to electrolytes at higher pH. where the potential is highest-in
contradiction to the generally accepted electrical double layer theory. Another
mystery is that crystalline quartz becomes coated with a film of amorphous silica
even though the solution is undersaturated with soluble silica with respect to a sur-
face of amorphous silica.
The dissolution and deposition of silica in water involves hydration and dehydra-
tion reactions catalyzed by OH - ions:
hydration(SiOz).. + 2 HzO t I (Si02)x _1 + Si(OH)•.
dehydration
For massive amorphous silica, the equilibrium concentration of Si(OH). at 25°C
corresponds to 70 ppm as SiO:. This is the "solubility" of anhydrous nonporous -.
amorphous Si02• However, except for fused StO, glass, the common forms of arnor- '--""
~--
1
c
t-
Supersaturated solutions of silicic acid in pure water are thermodynamically unsta-
ble because condensation polymerization through dehydration takes place. All
higher polymers of whatever size, molecular weight, or state of hydration can be
represented by a general formula containing n silicon atoms. The polymerization of
additional monomer 'molecules or the deposition of silica can be represented as
follows:
phous silica consist of extremely small particles of amorphous silica. orporous
aggregates, the surface of which is hydrated as SiOH groups. These exhibit a
somewhat higher solubility so that most powders and gels have a solubility of
100-130 ppm sto;
On the other hand, crystalline silica, such as quartz, almost universally present as
"sand," has a much lower solubility, of the order of 6 ppm SiOz.
Supersaturated solutions of monomeric Si{OH). are formed when silica is
dissolved in water at high temperature under pressure and then cooled, or when an
aqueous solution of soluble silicate is acidified:
5
..
The Silica-Water System
the sugar
analogous
ee in any
inition of
ysis in ~
m of Silica
,i(OH). (if
int ces
listriouted
and water
surround
hydrogen
In amor-
of density
[SiIlOZIl_C/UIZI(OH )lIz] + m Si(OH). -= .
[Sill+mOzlI-clIZlzl+zmlz-p,(OH)IlZHm-p,] + 2pm HzO
:ar -vater-
>1, the
t could be
Kitchener
rning the
acteristics
silica sols
j become
ghest-in
Another
ous silica
to a sur-
dehydra-
where n = number of silicon atoms in a polysilicic acid molecule or particle or
polymeric network
x = number of OH groups per silicon atom in the polymer, not exceeding 4
m = the number of monomeric silicic acid molecules added to the polymer
p = fraction of the hydroxyl groups per monomeric silicic acid molecule that
are converted to water during the polymerization reaction
Thus when p = 1, the monomer is converted to SiOz within the polymer molecule
without change in the number of OH groups in the polymer. There are, of course,
restrictions such as n and In having to be integers and the values of x and p being
limited by the possible structures of polymers and conditions of polymerization.
However, for the case where dense amorphous silica is being deposited on exten-
sive, massive silica surfaces from slightly supersaturated monomer solution. espe-
cially at high temperature and neutral or alkaline pH. x is very small. p is unity. and
n is large. Thus the deposited silica may be essentially dense and anhydrous:
• at 25°C
onnorous
o or-
. . .
Even vitreous or glassy silica contains some water. probably as SiOH groups. At a
given temperature and humidity there is an equilibrium "solubility" of water in vit-
reous silica, according to Hetherington and Jack (8). Flame-fused quartz contains
0.04 wt, % OH, whereas electrically fused material contains only 0.0003% as
detected byInfrared absorption at 2.73 micron wavelength. By extrapolation to'
-~"-
6 The: Occurrence. Dissolution. and Deposition of Silica
Thermodynamics of the System
Greenberg (13) calculated the following values for the thermodynamic functions:
Quartz
AF
(cal mole:")
- 1220 (~Fq - AFcs• 200°C)
- 550 (~Fvs - AFcs• 25°C)
Amorphous Silica
-210.260
-205.570
HfuaoK
(cal rncle ")
Si(c) + O2 (g) = SiC: (s)
for the overall equilibrium
30°C. Moulson and Roberts (9) concluded the equilibrium concentration of water in
silica glass may be as high as 0.22% H20 . probably present as internal SiOH . ps.
Let us return to the behavior of soluble silica in water. When the solution is hfghly
supersaturated and insufficient solid silica surface is available to permit rapid
deposition of soluble silica. new small nuclei particles are formed by intercondensa_
tion on monomer and low polymers. Silica is also deposited on these until supersajjj,
ration is relieved.
It is in this manner that colloidal particles of silica are formed. These. in turn.
may be aggregated to form silica gel or may be laid down as opal. both of which are
highly porous with an extensive internal surface covered with SiOH groups. Thus
"hydrated" silicas are formed. Very slow deposition may produce quartz. .
Alpha quartz (q)
Colloidal silica (cs)
Vitreous silica (vs)
The heat of formation of silica by the reaction
was reported (10. II): ~H~ISOK ... - 217.5 == 0.5 kcal mole':' i for alpha quartz. and
-215.9 ± 0.3 for amorphous silica.
Greenberg and Price (12) give somewhat different estimated values:
I'.
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AH (kcal mole-I)
~F~9S0K (kcal mole ' ')
.;lS~9"oK (cal deg" ' mole:")
+2.65 == 0.28
+3.98 == 0.04
-2.82 == 0.50
+7.34 ± 0.37
+5.20 ± 0.04
+4.53 ± 0.71
!
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~ ..
According to these data. the heat of formation of quartz from amorphous solid silica
is ~H = -4.69 kcal mole:", which is areatcr than the value -1.78 found bv W~'et
..... .. /'
1. and Deposition of Silica r~'Silica-Wat.r System 7
mcenrration of water in
ir 11 SiOH groups.
:n the solution is highly
lable to permit rapid
med by intercondensa-
1 these until supersatu-
irmed, These. in turn,
ipal, both of which are
h SiOH groups. Thus
rce quartz.
..
a1. (14). The latter is closer to the value 0.54 :I: 0.2 more recently calculated by
Cochran and Foster (15).
Other reported values for the above hydration reaction of amorphous silica were
given by Morey, Fournier, and Rowe (16), who found
Kitahara (17a) measured the solubility of amorphous silica between 9 and 100°C
and calculated AHa.oK .. 3.2 kcal mole:".
Walther and Helgeson (17b) calculated the thermodynamic properties of aqueous
silica and the solubility of quartz and its polymorphs over a wide range of tempera-
tures and pressures. The thermodynamic constants derived from all available data
were evaluated as follows:
'or alpha quartz, and
Constant
Entropy, So (cal deg " mole-I)
Volume, VO (em! rnole ")
Gibbs free energy, AG ° (cal mole:")
Enthalpy, AH (cal mole:")
Alpha Quartz
9.88
22.69
-204.65
-217.65
Amorphous Silica
14.34
29.0
-202.89
. -214.57
alues:
. AFcs• 200°C)
. AFc•• 25°C)
/narnic functions:
Quartz
+ 7.34 :I: 0.37
+5.20 :I: 0.04
+4.53 :I: 0.71
.rphous solid silica
I f(\""d by Wise et
Coefficients were also given with equations for calculating the values over a wide
range of temperatures and pressures. .
Relating Particle Size and Composition
In most sols that consist of discrete spherical particles of amorphous silica, the
interior of the particles consists of anhydrous Si02 with a density of 2.2 gem -'. The
silicon atoms located at the surface bear OH groups which are not lost when the
silica is dried to remove free water."
The relation of particle composition to particle size can be calculated purely from
geometry and densities of the components. Let
. . .
n, = total number of silicon atoms in a particle
n, = number of silicon atoms at the particle surface
d . = diameter of particle on anhydrous basis (nm)
d" ~ diameter of hydroxylated particle (nm)
x = ratio of SiOH groups to total Si atoms
= ns/nt assuming one OH per surface silicon
w ... weight of one anhydrous Si02 particle (grams)
WIt = weight of one surface hydroxylatcd particle
p = average number of silicon atoms across the diameter of a particle
SOLUBLE SILICA-MONOSILICIC ACID
surface had decreased and the particles have grown to a certain size. Further spon-
taneous changes are unlikely to occur.
One point that has not been considered in the foregoing discussion is that the
energy values have been generally determined on types of silicas that have already
reached a relatively stabilized state of particle growth. On the other hand. for much
finer silica, for example, with a specific surface of more than 600 m Z g-l, the radius
of curvature of the surface is then less than 25 A, and the silanol groups must be
spread apart so that less hydrogen bonding can occur between neighboring hydroxyl
groups. In turn. it might be expected that this would increase the heat of "wetting,
decrease the heat of dehydration, and decrease particle density and surface energy.
Under these conditions, it is certain that particle growth occurs with decrease in the
radius of curvature, but energy data onsuch materials have not been obtained,
particularly in regard to surface energy of the silanol-water interface.
I
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The Occurrence. Dissolution. and Deposition of Silica
·.I
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The soluble form of silica is monomeric, containing only one silicon atom and
generally formulated as Si(OH)•. This is often called monosilicic acid or orthosilicic
acid. The state of hydration is not known. although at high pressure there is some
indication that one water molecule is linked to each OH group, probably by
hydrogen bonding, so the hydrated molecule is represented by Willey (20) as
Si(OH: OHz).'
The structure of monosilicic acid is assumed to involve silicon coordinated with
four oxygen atoms as in amorphous vitreous silica and in crystalline quartz.
Although there are rare minerals such as the stishovite form of SiOz (21) or thau-
masite (22), in which silicon is coordinated with six oxygen atoms, silicon in most
oxides and silicates is surrounded by only four oxygen atoms. If the monomer had
the structure HzSi(OH)., one would expect it to be a strong acid like the analogous
HzSiF., but in fact it is a very weak acid.
It is essentially non ionic in neutral and weakly acidic solution and is not
transported by electric current unless ionized in alkaline solution. .lt is not salted out
of water nor can it be extracted by neutral organic solvents.
It remains in the monomeric state for long periods in water at 25°C, as long as
the concentration is less than about 2 x 10-3 M. but polymerizes. usually rapidly. at
higher concentrations. initially forming polysilicic acids of low molecular weight and
then larger polymeric species recognizable as colloidal particles.
The question often arises as to whether the term "soluble silica" should include
the low polymers such as tetrarner or decarner, which are classed as "oligorners." It
becomes a matter of definition. "Soluble" materials have been recognized as those
that pass through a dialysis membrane. whereas colloids do not: but even though
membranes can now be made with pores sufficiently small to separate dextrose from
sucrose. we think of sucrose as being "soluble," On the other hand. sucrose is cer-
tainly not colloidal.
For the purpose of this book. the following terminology is used:
Deposition of Silica Soluble Silica-Monosilicic Acid
~--...;;.-.-.-
11
Soluble silica (or monosilicic acid). Si(OH)4'
Polysilicic acid (oligomers). Polymers with molecular weights (as Si02) up to
about 100,000, whether consisting of highly hydrated "active" silica or dense
spherical particles less than about 50 A in diameter.
Colloidal silica. More highly polymerized species or particles larger than about 50
A, although sometimes down to 10-20 A.
Silica sol. May refer broadly either to polysilicic acid or colloidal silica.
The arbitrary borderline of 50 A or mol. wt. 100,000 is based on the general
observation that below this point the polymer species are generally unstable, having
only a transient existence owing to gelling or particle growth.
Also, as has already been shown, it is below this size range that less than half of
all the silicon atoms are present as Si02 that is, as "silica," whereas more than half
are each associated with at least one hydroxyl group. The term "silicic" acid is thus
justifiable.
The preparation and reactions. for example. polymerization. of dilute solutions of
monosilicic acid are further described in .Chapter 3. Meanwhile, some of its charac-
teristics are noted, as follows, prior to discussing solubility:
1. It is characterized by its rapid rate of reaction with molybdic acid to form the
yellow silicomolybdic acid.
2. It is generally inert in neutral solution if the concentration is below the saturation
level with respect to amorphous silica. Thus it is almost universally present at a
concentration of a few parts per million in most natural waters and in living
organisms.
3. It combines with metal ions to an increasing degree with increasing pH, thus
reducing the concentration of free monosilicic acid. (Ferric and uranyl ions react
at a pH as low as 2, whereas most other metal ions combine only at higher pH.)
4. Above pH 9 it is ionized first to (HO)3SiO - or at still higher pH to (HO)2Si022-.
The first equilibrium constant (13.23) is approximately (25°C)
size. Further spon-
cus.. • is that the
that have already
ier hand, for much
m2g r ', the radius
01 groups must be
ghboring hydroxyl
ie heat of wetting,
nd surface energy.
lith decrease in the
lot been obtained,
Ice.
silicon atom and
acid or orthosilicic
.sure there is some
'oup, probably by
Jy Willey (20) as
1 c. dinated with
crystalline quartz.
Si02 (21) or thau-
ns, silicon in most
the monomer had
like the analogous
lution and is not
.lt is not salted out
t 25°C. as long as
usually rapidly. at
ilecular weight and . or
[(HO)3SiO-]
[OH-] [Si(OH)4] 1.5 X 10
4
ca" should include
as "oligorners." It
ecognized as those
t: but even though
.rate dextrose from
LOd. sucrose is cer-
.,
.l.
[(HO)3SiO-] [H -]
[Si(OH)4]
Even though the silica solution is neutral. if it is passed through a bed of strong-
base cation-exchange resin in the free-base form. the soluble silica in contact with
the resin is ionized and is then held as silicate ions. In a mixture of Si(OH)4 in
equilibrium with colloidal silica particles at pH 7-8, the particles bear a negative
charge. According to Goto, Okura, and Kayarnu (24) electrophoresis and trans-
12
.... ~ ... -
--The Occurrence. Dissolution. and Deposition of Silica
;I'
, ,
til:. '
l: :
~ . :.
!;.,.. ~"
n ~ .
t!r..
"1( :'
I
•
port studies show that the colloid. not the "molecularly soluble" Si(OH)., is the
charge carrier. When the mixture is passed through a mixture of strong-bas'
anion- and cation-exchange resin, monosilicic acid is removed. but not the
colloidal particles. [After a time the particles dissolve surticiently to reestablish
the equilibrium concentration of Si(OH)•.]
5. It is converted to H2SiF. by reaction with HF in aqueous solution:
Si(OH). + 6 HF == 2 H+ + SiFi- + 4 H20
6. It is converted to a complex anion by reaction with o-dihydroxy aromatic com-
pounds, such as catechol, in neutral solution:
Si(OH). + 3 o-C2H.(OH)2 + 2 NH.+ + 20H-
(o-C2H.02)3Sj2- + 2 NH. + + 8 H20
Volatility in Steam
Although Si(OH). is nonvolatile at ordinary temperature and polymerizes quickly
when heated, nevertheless at elevated temperature and pressure in 'water its solubility
is greatly increased and it can exist in equilibrium as the vapor phase in the steam,
as shown by Kennedy (25). This is of importance in very high pressure boilers in
power plants where deposits build up on turbine blades unless all silica is remover
from the feed water. Brady (26) supposes the volatile species is Si(OH). 0,.
(HO)3SiOSi(OHh. Astrand (27) found that volatility increased with decreasing
alkalinity in experiments conducted up to 350°C and 300 atm. This. of course. sug-
gests that Si(OH). is more volatile than the silicate ion. Wendlandt and Glemser (28)
reviewed evidence from earlier workers and calculated the equilibrium constants
involved whence the species in the vapor were related to the density of the water
vapor:
. ,
.
: Quartz
Si02 + 2 H20 == Si(OH).
2 Si02 + 3 H20 == (HO)3SiOSi(OH)3
Si02 + H20 == OSi(OH)2
Gas Density Range (g em-3)
Up to 0.05
Up to 0.45
Above 0.65
:
I
I
.. I.
I
Similarly. Martynova, Fursenko, and Popov (29) found that in a solution satu-
rated with soluble silica at 263-364°C about a third of the silica in the vapor was
present as disilicic acid, whereas in the range lSI-223°C it was all monomeric.
Heitmann (30) concluded that deposition in turbines was minimal if the silica
concentrationwas less than 0.01 ppm. iron concentration less than 0.005 ppm. and
the conductivity less than 0.1 micromho cm -I. According to Heitmann's measure-
ments (3 I). the silica concentration in the vapor phase ranges from O. I rng kg -I r
400°C to 5 mg kg:" at 600°C at a pressure of 0.3 kg em -2, but increases to mor.. _
than 100mg kg -I under an applied pressure of 300 kg ern -2.
Silica is constantly dissolving and precipitating over a large part of the earth's sur-
face. The sedimentary cycles have been described in complex detail by Siever (32).
Soluble silica is mainly derived from the weathering of minerals which, in some
cases, results in amorphous silica residues that then dissolve. Very little can come
from the "sands of seashore," or quartz, which is soluble to only a few parts per
million; furthermore, the rate of dissolution is extremely slow. River waters range
from S to 3S ppm Si02, a few up to 7S ppm, and by the time they reach the sea may
range from 5 to 15 ppm. Seawater varies widely but the silica content may range
from 2 to 14 ppm (33). However, Lisitsyn and Bogdanov (34) report that surface
waters in the Pacific Ocean contain only 0.0001-0.3 ppm Si02• Plankton convert
6 X 10' tons Si02 from soluble to suspended form each year, but this is only 0.16%
of the available silica.
In addition to the silica carried into the sea by fresh water, additional soluble
silica comes from the suspended colloidal clays and related minerals. Tests show
that common colloidal silicates like clay will dissolve in seawater sufficiently to give
a silica concentration of 10 ppm (35).
Concentrations of silica of around 2 ppm were reached in dilute salt solution with
mica and kaolin and up to 15 ppm with montmorillonite (36). When seawater was
enriched with soluble silica to 25 ppm Si02, it remained at that level for a year in the
absence of these minerals, but when the latter we~e then added, the silica was
removed from solution down to the 2-15 ppm level that was reached when the
minerals alone were added. Since many ocean waters contain 2-10 ppm Si02, it is
possible that this value is reached as the equilibrium solubility of colloidal alumino-
silicate in suspension. The above experiment is consistent with the fact that in pure
water, pure amorphous silica dissolves to give a concentration of monosilicic acid of
100-110 ppm, but in' the presence of polyvalent metal cations such as iron, alu-
minum, and other metals, colloidal silicates are formed with a much lower solubility
with respect to monosilicic acid. lIer (37) has shown that soluble aluminum reduces
the solubility of amorphous silica from about 110 to less than 10 ppm.
Willey (38, 39) has studied the natural interaction of soluble alumina and soluble
silica in 0.6 N sodium chloride solution. Addition of aluminum ion to 200 ppm
Si(OH). retards polymerization. Probably there is formed a colloidal complex which
reacts as monomer when put into the strongly acidic molybdate reagent. Avery low
concentration of soluble silica also causes the precipitation of alumina.
Soluble silica as determined by the molybdate: method is not necessarily present as
Si(OH)•. Bogdanova (40) reported that in natural waters that contained only about 5
ppm total silica, 4-9% of the silica was polymeric but was converted to monomer by
acid. It is most likely that the "polymeric" silica was actually very small colloidal
particles of aluminum silicate that liberated monomer when acidified..
Silica is continuously removed from seawater by biochemical processes. Diatoms
and sponges as well as plants remove silica which is stored within the organism.
Although Calvert (41) believes that the concentration of silica in the sea is mainly
controlled by biological activity, Harder (42) reports that amorphous hydroxides of
AI, Fe, Mn, or Mg can react with and precipitate soluble silica, thus reducing the
concentration to as low as 3 ppm. Both processes are no doubt operative.
13
"'';--'..
Soluble Silica in Nature
Soluble Silica-Monosilicic Acid
~ -.
"
:'~.
~-
'..-.,
r,
i
I
romatic com-
: (g cm -3)
solution satu-
.he vapor was
meric.
I if the silica
~05 ppm, and
nn's measure-
.1 rn~. kg-I at
ea. 0 more
erizes quickly
r its solubility
in the steam,
ure boilers in
:a ie removed
H). or
th decreasing
,f course, sug-
. Glemser (28)
urn constants
of the water
isition of Silica
,i(OH)., is the
.r ~··"'ng-base
b, lot the
to reestablish
______t;;...' _
14
-'.;..... -
-The Occurrence. Dissolution. and Deposition of Silica
Dissolved Silica Estimated Maximum Measured Temperature
.. (ppm) Temperature (OC) in Drill Hole (0C)
660 . 246-252 250 ~-....
425 .' . 215-220 220
245 178-180 170
,
:> ,-
.
l
j .
,
~ .
~
-'
.'
I .
I
I
Amorphous silica is probably more soluble in seawater at great depths owing to
the higher pressure. Willey (20) and Jones and Pytkowicz (43) found that at about "
O°C, the solubility increased with pressure as follows:
Willey (O°C) Jones (2°C)
Ib in. -2 ppm Si02 Ib in.- 2 ppm Si02
15 64 15 56
4,000 74 7,500 628,000 80
12,000 85 15,000 7018,000 94
The salt at this concentration no doubt promoted more rapid establishment of equi-
librium but would have little effect on solubility.
Hot springs in some areas produce a supersaturated solution of silica. Knowing
the solubility of quartz, Fournier and Rowe (44) have shown that the total silica
content of the water permits an estimate of the subterranean temperature at which
the water has become saturated with respect to quartz, which is the major phase that
usually determines the solubility. Typical results are as follows:
The method is, of course, not dependable if the water encounters previously
deposited amorphous silica at any point. Also, the loss of water as steam must either
be prevented or taken into account.
. Major studies of silica in geothermal waters have been made in 1956 by White.
Brannock: and Murata (45) and in 1970 by Fournier (46). all of the united States
Geological Survey. Detailed analyses of waters from Yellowstone National Park,
Wyoming, have been published by Rowe, Fournier, and Morey (47).
Soluble silica is found in essentially all plants and animals. For example. human
blood contains I ppm. Ingested monosilicic acid as an undersaturated solution
rapidly penetrates all tissues and body fluids and is excreted apparently without any
effect (48). Plants, especially grasses. including the grains and rice. take up silica and
deposit it in the tissues as characteristic microscopic amorphous opaline particles.
which are later found in the soil and in the intestinal tracts of grazing animals (49).
The widespread occurrence and possible role of silica in living systems are more fully
discussed in Chapter 7.
In regard to the weathering of soils. it is noted that aluminosilicates (clays)
. undergo weathering in the tropical regions with dissolution of silica, leaving a .-'
residue high in alumina (bauxite) whereas in colder regions alumina seems to be
removed preferentially, leaving more highly siliceous residues (50).
PHASES OF SILICA
Since different phases of silica exhibit different solubility behavior, they are briefly
described. By far the commonest crystalline form is quartz, the main constituent of
common sand. However, under certain conditions in nature and in the laboratory,
other forms are produced. These forms in turn may be divided into the following
classes:
A possible explanation is that in the tropics the decomposing vegetation produces
tannins and other catechol-like materials that are known to dissolve silica in neutral
solution. In colder regions, .less organic matter is likely to be present and the pH
may be lower because more dissolved carbon dioxide is present, so that alumina is
preferentially dissolved.
15Phases of Silica
ths owingto
hat '1bout
sition of Silica
nent of equi-
:a. Knowing
e total silica
ure at which
or phase that
Temperature
(0C)
1. Anhydrous crystalline Si02•
2. Hydrated crystalline Si02·xH20.
3. Anhydrous amorphous silica of microporous anisotropic form such as fibers or
sheets.
4. Anhydrous and hydrous amorphous silica of colloidally subdivided or
microporous isotropic form such as sols, gels, and fine powders.
S. Massive dense amorphous silica glass.
Of these, 2, 3, and 4 exhibit extensive external or internal surfaces and are thus
pertinent to the present study.
Anhydrous Crystalline Silicas
Sosman (51) classified the more common phases as follows:
s previously
1 must either _
;6 by White,
mited States
tional Park,
nple, human
ted solution
without any
up silica and
ne particles.
mirnals (49).
-e more fully
cates (clays)
a, l-uving a
se, to be
Thermodynamically
Stable at Atmospheric
Pressure
Quartz low
Quartz high
Tridymite S-1
Tridyrnite S-II
Tridyrnite S-lIl
Tridymite S-IV
Tridymitc S-V
Tridyrnite S- VI
Tridyrnite 1\1:1
Tridymitc M-I r
Tridymitc M-1I1
Cristobalite low
Cristobalitc high
Thermodynamic
Stability Range (OC)
To 573
573-867 - tridyrnite
To'64
64-117
117-163
163-210
210-475
475-I·HO - cristobalite
To 117
117-163
Above 163
To ~72
272-1723
_______---..J:. " _
16 The Occurrence. Dissolution. and Deposition of Silica
The different forms of quartz. tridyrnite, and cristobalite are transformed spon-
taneously with temperature so that from the standpoint of solubility there are only
the three phases to be considered.
The next group of three phases are those formed only under conditions of high
temperature and pressure. '
Thermodynamically Stable Range
Therrnodynam ically
Stable at High Pressure Temperature (OC) Pressure (kilobars)
Surveys of these phases and their properties have been published by Fronde! (52).
Sosman (53). and Fldrke (54). Wells relates the structure of the different forms of
silica to various crystalline silicates (55).
Quartz, the commonest phase found in nature. ranges from huge crystals. to amor-
phous-looking powders a few microns in size. to shapeless masses of chalcedony agate
or flint consisting of densely packed. interlocked microscopic crystals. The
transformations between the three common forms and vitreous silica is as follows:
~'. -
\.
vitreous
0.8-1.3
15
40
160
147QOC
~ cristobalite
400-500
From 300
To 1700
1200-1400
tridymitequartz
Keatite
Coesite
Stishovite
The transformation to tridymite apparently requires traces of certain impurities or
mineralizers.
The three phases metastable at ordinary pressure were recognized only recently.
Keatite was discovered by Paul Keat (56) in 1954. and its formation via cristobalite
and transformation to quartz were studied by Carr and Fyfe (57). Hoover (58), in a
patent filed in 1954. described the preparation of a very similar if not identical
material from "silicic acid". that is. hydrated amorphous silica powder. by heating it
in water at about 3000 atm pressure and 500-625°C in the presence of about 1%
alkali based on silica.
Coesite was discovered by Coes, in 1953 (59). It is made from amorphous silica in
the same temperature range as for keatite, but at 10 times the pressure and with
weakly acidic catalysts such as boric acid or ammonium chloride (59). It was found
in nature in 1960 at Meteor Crater, Arizona. apparently formed under the high
temperature and pressure conditions of the im pact.
Similarly. stishovite was first made in the laboratory in 1961 by Stishov and
Popova (60) and discovered in Meteor Crater by Chao. Shoemaker. and Madsen. in
1962 (61). A most interesting story of the isolation of substantial amounts of coesite
and stishovite from the crater is told by Bohn and Stober (62.63).
There arc also some unusual anhydrous crystalline forms. as follows (64).
i
~ . .
I
,
, ,
fj .;
i '
~
•e
"
:! .
't· .
rI '( ,
r
L
E
i
I
L
rI'
t',
.!~
j'
.
•
.'j
1
I
J
1
i
f
;; .";
L.i .
.,
.t ~
~
Silica W is a fibrous crystalline silica with a density of 1.97 g em -, described by
Weiss and Weiss (65), formed in the gas phase by oxidizing silicon monoxide vapor
at 1200-1400°C and deposited as paperlike films. It is unstable above about 1400°C.
It is fairly stable in dry air, but is converted by moisture to amorphous hydrated
silica, still retaining a swollen fibrous form. In this transformation. only about 0.08
mole of H20 is taken up per mole of Si02, forming SiOH groups. Silica W can have
no true equilibrium solubility in water. Instead, it must decompose rapidly in water
to give monosilicic acid. When the powder is suspended in water and within 2 min
centrifuged to obtain a clear solution. then titrated with NaOH solution at pH
10.2-10.5 (thymolphthalein). 2 equivalents of base are required per mole of Si02 in
solution (66). After the solution has been aged for 1 hr, only 0.1 equivalent is
required .. Initially. therefore, the solution must have been supersaturated with
Si(OH)4, which when titrated with base, requires 2 equivalents of alkali. but after the
monomer has polymerized. much less alkali is required to neutralize the surface
acidity of the colloid. If the solution is mixed with silver salt an orange. light-sensi-
tive AgSiO, is precipitated. In absolute methanol the fibers swell and form a
polymeric methyl ester containing one methoxy group per silicon atom which, when
heated in vacuum at 300-500°C, yields cyclic methyl esters [(CH,OhSiO]II.'.u,
_When hydration of the fiber by water is followed by suitable technique under the
microscope it can be seen that the reaction starts at the end of the fiber and proceeds
rapidly along its length as the crystal swells and is converted to hydrated amorphous
silica:
17
~-_.
Phases of Silica
1.3
5
o
o
npurities or
vitreous -
'ondel (52),
nt forms of
Is. to arnor-
edony agate
/stals. The
'ollows:
kilobars)
mge
ons of high
°med spon-
re ft --: only
:ion of Silica
Iy recently.
cristobalite
er (58). in a
ot identical
'y heating it
f about 1%
ous silica in ...
re and with
t was found
er the high
Stishov and
Madsen. in
lS of cocsitc .
4).
Anhydrous fibrous silicas formed in connection with high temperature metallur-
gical operations were noted as long ago as 1852 by Schnabel and 1859 by Rose.
Soft. silky fibers of more than 98% Si02 were classed as aphanitic (invisible) silica.
.' and also known as lussatite. Around 1910. in the mouths of electric furnaces making
silicon carbide. a sort spongy gray deposit called "elephant's ear" was identified as
microfibrous amorphous silica (67). It is likely that all of these were silica W.
. Melanophlogite, a long known but strange and little understood mineral. is found
in volcanic sulfur deposits in Sicily. Skinner and Appleman reviewed its history and
showed that it was a new cubic ·polymorph of silica (68). It has a cubic. very open
structure. containing 92.4% SiO; and about 5.7t:(. SO, (2.2SC;c as sulfur). 1.2%
carbon. and 0.81% hydrogen. The density is 2.052 ± 0.013 g COl -'. The initial
refractive index is 1.467. but when these volatile materials are driven off by heating.
the crystalline silica residue has a refractive index of 1.425 ± 0.002 and a density of
1.99 g COl -'. which arc substantially lower than those of amorphous or glassy silica.
The silica crystal is stable up to about 900°C. above which it changes into cristo-
. balite. However. ,vhen crystals arc subjected to grinding in a mortar at ordinary
temperalure.· the open structure collapses to fine-grained quartz. Its solubility h~IS
not been measured. and it is doubtful if il can ha vc a true equilibrium solubility in
,
18
-~
- - The Occurrence. Dissolution. and Deposition of SilicaI,
I'I.
I,·. ,
. '; .
I
,
J.
r
water. The heat-purified material will probably react with water rapidly to give a
highly supersaturated solution of Si(OH)4' similar to the behavior of silica W.
The nature of the hydrocarbon and sulfur content is still not clear. However. cal-
culations based on density data would seem to support earlier suggestions that the
sulfur must be present as S03 or H2S04 within the silica lattice. The optical charac-
teristics of the mineral show that the organic matter occurs in films between the
faces of the crystals. On the other hand, calculations based on the difference in
densities of the original mineral and the pyrolyzed silica crystals show that the sulfur
compounds at least must be within the crystal lattice. Kamb offers evidence (69) that
the silica structure is a clathrate with S02. H20. and CH4 in the lattice analogous to
the known 12 A gas hydrates of water, 6X .46H20, where X is CH4, H 2S, CO2, S02.
C12• etc., and in fact the structure is the complete analogue of 6C1 2 • H20.
Silica 0 crystallizes from lithium silicate glasses during devitrification at low
temperature. It has a crystal lattice similar to quartz and may simply be "high
quartz" stabilized below 573°C. the normal transition temperature, to low quartz
(53. 70) by inclusions of metal ion impurities. The only way pure material can be
obtained is by neutron bombardment of quartz.
Silica X is a microcrystalline form obtained as spherical aggregates of radial
fibers up to 12 microns in diameter by heating pure amorphous hydrated silica
("silicic acid") with 2% KOH solution in sealed tubes at 150°C for a few weeks. The
refractive index of 1.484 ± 0.004 is close to that of cristobalite. It is anhydrous.
maintaining its structure up to about 600°C. above which it is converted to cristo-
balite (71a. 71b).
Silicalite. a very unusual new form of anhydrous crystalline silica hornogeneousl..
permeated by uniform pores 6 A in diameter and having a density of only 1.76 g
ern -3, has been described by FJanigen et al. (71c) and patented by Grose and
Flanigen (71d). The pores constitute 33% of the volume of the crystal. A most
remarkable feature is that this silica is hydrophobic; the pores are lined with oxygen
atoms that are highly hydrophobic and organophilic or oleophilic. Thus the crystals
preferentially absorb hexane in the presence of water. which does not enter the pores
even at saturation pressure.
This type of silica is made first as a crystalline quaternary ammonium silicate. for
example, tetrapropylammonium silicate: (TPA).O .48Si02· H20. It is then heated to
red heat to remove the organic matter and water. leaving uniform cylindrical chan-
nels throughout the three-dimensional crystalline framework of silica.
A similar but even more hydrophobic. anhydrous. microporous crystalline silica
was obtained by Flanigen and Patton (7le) by conducting the hydrothermal synthe-
sis in the presence of some ammonium fluoride. which facilitated the formation of
crystals 2-15 microns in size at only 100°C rather than at the higher temperature
and pressure required for making silicalitc. After crystallization from solution a
typical composition was about 88% by weight of silica. 11.0% tetrapropylarnmonium
oxide. and 0.9% fluorine, but after calcination at 600°C the porous crystalline
product was essentially pure Si02 (containing less than 0.1 % fluorine) with a mean'
refractive index of 1.39 ± 0.01 and a specific gravity of" 1.70 ± 0.05. These value"
arc the same as those of silicalite and fall on the same curve with other forms 0
silica in Figure 1.1. "'~
Hydrated Crystalline Silicas
Until the advent of X-ray diffraction, it was not clear whether solids containing only
silica and water were definite compounds. that is, had a definite stoichiometry or
structure. In 1905, Tschermak (73) believed that he had obtained definite hydrates
based on ratios of silica to water corresponding to Si02 : 2 H:O, 2 Si02 : 2 H20, 3
Si02 :2 H20 , etc.• by carefully leaching the metals out of certain crystalline silicates
and drying in air. Then Van Bemmelen (74) and Theile (75) gave evidence that no
definite silicic acids were thus produced.
Nevertheless, since then· numerous instances have been found where definite
crystalline materials. having characteristic X-ray diffraction patterns and crystal
structures. have been made by extracting the cations from certain crystalline silicates
withacid.' .
19
4
COESITE
QUARTZ
KEATITE
'------ CRIST06:'UTE
TRIDYMiTE
AMORPHOUS
'------ MELANOPHLOGITE
2 3
DENSITY G -CM-)
Density versus refractive index of various forms of silica.
to
2.0
Figure 1.1.
x
w
o
~
w
>
i=
~ 1.5
0::
l.l.
W
0::
Z
<l:
W
::E
-..,.
Phases of Silica
Relation Between Density and Refractive Index
The anhydrous crystalline phases were arranged in order of increasing density and
refractive index, and found to have a linear relationship, by Skinner and Appleman
(68). Stishovite falls on the s~me line (72) (Figure 1.1).
It will be noted that the line for silica polymorphs has been extrapolated to meet
that of two forms of water. It seems odd that neither of these lines extrapolates to a
refractive index of 1.0 (for a vacuum) at zero density.
The analogous structures of Si02 and H20 have been compared by Kamb (69),
who points out that the ratios of the densities of the various phases or polymorphs of
Si02 and water to those of the corresponding forms of Ice I and cristobalite are very
similar, and for each type of silica there is an ice counterpart with the same type of
crystal structure.
on of Silica
; of radial
ated silica
veeks. The
anhydrous,
I to cristo-
silicate, for
1 heated to
rical chan-
on at low
, be "high
.ow quartz
-ial can be
illine silica
nal synthe-
rrnation of
emperature
solution .1
irnmoniurn
crystalline
ith a mean
11:S" values
:r . s of
og, usly
mly 1.76 g
Grose and
J. A most
·ith oxygen
he crystals
r the pores
to give a
W
veve. cal-
lS that the
.al charac-
etween the
Terence in
: the sulfur
e (69) that
alogous to
CO2, S0 2.
20
~ -:-- -.-
Th~ Occurrence. Dissolution. and Deposition of Silica
I
fj
,
I ~
-,
f iI
1 "
:
< 1
• .<....! :,;
· -1:
F
1
\
Jj
I
·1
t
I
1
(
)
>
"
'..~
There is no instance of a hydrated silica being crystallized directly from a solution
of silica and water. Yet there is the peculiarity that certain hydrates. once formed,
exhibit what seems to be a characteristic solubility. implying that since an equilib-
rium is reached, silica must pass from solution to the solid phase as well as the
reverse. However, the data are conflicting.
Sodium disilicate, Na2Si20s, may be converted to hydrated silica. Thus !?y treating
the silicate with concentrated cold acid, washing out salts with water and water with
acetone, and drying in vacuum at 40°C, a crystalline "disilicic acid" is obtained (76,
77). Such layerlike structures are termed "lepidoidal" (scalelike) or phylloidal (Ieaf-
like). Liebau distinguished two types of layer structures. They are internally
hydrogen bonded and exhibit only weak acidity (78).
Crystalline Na:Si03·4H20 gives another form of silica. Here treatment with dry
HCI gas at -25°C gave definite silica hydrates containing 2.0. 1.5, and 1.0 moles of
H20 per mole of Si02, as the product was dried under vacuum at progressively
increasing temperatures. The silica was anhydrous at 90°C (79).
From gillispite, BaFeSi4010, Pabst (80) obtained flakes of crystalline hydrated sil-
ica consisting of Si40 l o4 - ions linked into sheets with the composition 4 H4Si40 l o +
2 H20; density calculated 2: 15, found 2.05; refractive index 1.45 ± 0.0 I.
From sheetlike KHSi20S' Le Behan, Kalt, and Wey (81) obtained by treatment
with a dilute acid a still different leaflike disilicic acid, H 2Si20s.