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

Contents

Introduction 1

Previous Books and Reviews of Silica Chemistry 1 Selection of References 1

Terminology 2 References 2

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

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 NaC104 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 TV-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 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 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

Water-Soluble Silicates 116

Sodium and Potassium Silicates 117 Manufacture 117 Commercial Solutions 119

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 coagulants

Conversion to Silicic Acids 137 Reaction with molybdic acid, Conversion to esters of silicic acids, Conversion to trimethylsilyl derivatives of silicic acid

' 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 (Н281205)ж 160 Precipitation of Insoluble Silicates 161 Soluble Silicate Glasses 163 Peroxy Silicates 164 References 165

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 211

Polymerization: pH 2-7 213 Formation of Oligomers 214 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 227 Development of Microgel and Viscosity 231

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 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

Her; Alexander, Heston, and Пег; Schwarz and Knauff; Bechtold; Goto; Okkerse; Audsley and Aveston; Weitz, Franck,

XVI Contents

and Giller; Bechtold, Vest, and Plambeck; Acker; Hoebbel 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; Her; Her 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 299

Interaction with proteins—tanning, Esterification of polysilicic acid

Activated Silica Sols—Water Treatment 301 Reaction of Polysilicic Acid with Metal Cations 303

References 304

4 Colloidal 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 317

Stabilization Against Particle Growth 318 Stabilization Against Aggregation 323

Stabilization by ionic charge, Addition of salt to lower viscosity, Steric stabilization

Porous Particles 328

Contents

Elongated Particles Particles with Non-Siliceous Cores

Methods of Making Sols Neutralizing Soluble Silicates With Acids Electrodialysis Ion Exchange Peptizing Gels

Hydrolysis of Silicon Compounds Dissolution of Elemental Silicon Dispersion of Pyrogenic Silica

Purification, Concentration, Preservatives Ion Exchange

Dialysis and Electrodialysis Washing Procedures Concentration

Evaporation of water, Centrifugation, Ultrafiltration, Electrodecantation

Preservatives Characterizing Sols

Chemical Analysis Measuring pH, Electrolyte concentration

Particle Characteristics Particle size, Specific surface area

Ionic Charge on Particles Nature of ionic charge, Counterions and double layer

Viscosity Aggregation of Particles

Definitions Gelling

Effect of pH, Effect of particle size and concentration, Electrolytes and organic liquids, Temperature, Theory of strength of gels

Coagulation 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

xviu Contents

Flocculation 384 Flocculation with cationic surfactants, Flocculation with organic polymers

Coacervation 396 Silica spheres by coacervation

Aggregation into Ordered Structures—Precious Opal 398 Opal structure, Other ordered aggregates, Formation of uniform inorganic particles, Synthesis of opal

Adsorption of Silica Particles on Surfaces 405 Sols of Silica Particles with Modified Surfaces 407

Negatively Charged Surfaces 407 Aluminosilicate ions, Other anions

Positively Charged Particles 410 Polyvalent metal oxide coatings, Polyvalent organic cations

Organic Modified Surfaces—Organosols 412 Organic ions, Esterification, Silylation

Commercial Colloidal Silicas 415 Uses of Colloidal Silicas 415

Making Catalysts, Gels, Adsorbents 420 Inorganic Binder, Stiffener 420

Molded refractory bodies, Binders for fibers, Refractory coatings, Molds for casting metals

Frictionizing Effects 425 Fibers, Paper, Steel rails, Other surfaces

Antisoiling Surfaces 426 Hydrophilizing Surfaces 427 Modifying Adhesion 428

Increasing adhesion, Decreasing adhesion Coating Compositions 430

Coatings on ships, tanks Reinforcing organic polymers 432 Polishing Agent for Silicon Wafers 433 Surfactant Effects 433

Dispersing effects, Antifoaming effects Modifying Viscosity—Gelling 434 Miscellaneous Optical Effects, Color, Photography 435 Use in Biological Research—Density Gradient 436

Contents XIX

Source of Chemically Reactive Silica 437 Soluble silicates, Silica bodies, Glass compositions, Forming solid silicates—cements, Other reactions and uses

Colloidal Silicates 439

References 439

Silica Gels and Powders 462

Definitions 462 Types of Gels 462

Types of Powders 463 Physical Characterization of Gels and Powders 464

Ultimate Particle Size 465 Electron micrographs, Specific surface area, Low angle X-ray scattering

Aggregate Size—Powder Particles, Gel Granules 476 Pore Characterization 478

Particle size and packing, Loss of surface area by particle packing

Characterizing Pores by Adsorption Isotherms 488 Pore volume, Pore size and size distribution, Miscellaneous effects in micropores,

Nature of Silica Surface 505 Aggregate Strength—Interparticle Bonding 506

Electron micrographs, Partial dissolution method, Mechanical strength of the aggregate

Silica Gels 510 Sources of Silica Gel 511

From soluble silicates and minerals, From colloidal silica, From hydrolysis of silicon compounds

Factors Controlling Gel Characteristics 516 Size of primary particles—pH effect, Wet gel strength, Particle size and packing density in dried gels, Increased porosity with removable fillers

Forming and Shaping Gel Particles 526 Wet Gel Treatments

Gel reinforcement

XX Contents

Drying and Shrinkage—Xerogels 533 Drying from low surface tension liquids

Drying without Shrinkage—Aerogels 537 Hydrothermal Treatments 539

Liquid or vapor phase Heating in Air, Vacuum 544

Sintering uniform structures, Alkali metals, impurities, crystallization

Special Gel Structures 549 Submicroporous gels—impervious silica, Porous glass, Specific adsorbents

Precipitated Silica Powders 554 Silica Precipitated from Sodium Silicate Solution 554

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 563 Silica Precipitated from Organic Liquids 564 Silica Precipitated from Colloidal Silica Sols 564 Silica Precipitated from Vapor: Pyrogenic Silica 565

Vaporized Si02, Oxidation of SiO vapor, Oxidation and hydrolysis of SiCl4 vapor, Oxidation and hydrolysis of silicon esters vapors, Hydrolysis of SiF4 vapor

Naturally Occurring Silica Powders 568 Microcrystalline Hydrated Silicas 569 Hydrophobic-Organophilic Silica Powders 570

Adsorbed Organic Cations 571 Adsorbed Polyvalent Metal Cations with Organic Anions 572 Surface Esterification 573 Organosilicon Coatings 574 Organic Polymer Coatings 575

Silica Gels with Ion-Exchange Surfaces 576 Inorganic Ion-Exchange Sites 576 Organic-Linked Ion-Exchange Sites 577

Commercial Silica Gels and Powders 578 Uses of Silica Gels and Powders 578

Contents XX i

Reinforcing and Other Effects in Organic Solids 582

In rubber, In silicone elastomers, In various organic polymers Reducing Adhesion 587 Increasing Adhesion 587 Increasing Viscosity, Thixotrophy 588

Mechanism, Lubricating grease, Paints, coatings, inks, Parmaceuticals and cosmetics, Miscellaneous compositions

Optical Effects—Flatting 593 Surfactant Effects 594

Stabilizing emulsions, Hydrophilic surface, Antifoam agent Hydrophobing Effects 594

"Dry" water Absorbent

Catalysts Aerogels, Base for mitochondria, Spillover

Reactive Silica Cloud Seeding Chromatographic Column Packings

References

1

595 596

597 597 598 599

622

623 624

624

625 625

The Surface Chemistry of Silica

Reviews and Summaries Nature of the Silica Surface

Structure of the Underlying Silica Definition of Surface The Hydroxylated Surface

State of water at the hydroxylated surface, Electrical conductivity of the surface, Distinguishing adsorbed water from silanol groups, Internal hydroxyl groups and trapped water, Hydroxyl groups per square nanometer, Theoretical concentration of surface hydroxyl groups

Dehydration and Rehydration 637

Surface Energies 645

Heat of Wetting Silica Surface 646 Physical Adsorption of Non-Ionic Low Molecular Weight Compounds 648

XXII Contents

Adsorption of Vapors 648 Effect of dehydroxylation on adsorption

Adsorption from Solution—Nonionic 654 Nonaqueous solutions, Aqueous solutions, nonionic, hydrogen bonding

Ionization and Surface Charge 659 The Hydroxylated Surface 659 The Dehydroxylated Surface 661 Nature of the Anionic Charge Sites 663

The "Site-Binding" theory Forces Involved in Adsorption of Ions 665

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) 676 Hydrophilic Coatings on Silica 679 Hydrophobic Silica Surface 680

Organic Cations and Bases 680 Hydrophobing effects, Cationic dyes, Aluminosilicate surfaces

Hydrocarbon Groups Attached through Polyvalent Metals 688 Surface Esters with Alcohols 689

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 695 Adsorption on Hydrophobic Surfaces 699

Adsorption of water, Adsorption of inert gases Adsorption of Organic Polymers on the Silica Surface 702

From Aqueous Solution 704 Polyethylene oxide, Polyvinyl alcohol, Cationic polymers, Proteins, Adsorption of polymers on dehydroxylated silicas, Effect of salts

From Nonaqueous Solvents 709 Deposition of Multilayers of Charged Polyions and Particles 710 The Surface of Alumina-Silica 710

Contents XXIII

Active Sites, Free Radicals, Active Oxygen, Ozone 712 References 714

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 of Absorption, 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

XXIV Contents

Silicosis-Pneumoconiosis-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-M icroaciculosis Beneficial Effects of Silica

Silicon Metabolism

Silica Gel as a Culture Medium Organosilicon Compounds Analytical Problems Conclusion

References

horIndex

»iect Index

782

783 783 784

785 786

787 787

803

835

;