Polymerization of silica

Freundlich appeared to recognize these alternatives when he wrote  

Whether it is rather a matter of polysilicic acids, which give larger micellae, being formed from simple silicic acid, or whether the crystalloid particles originally present already consist of polysilicic acids, but are exceedingly fine amicrons which continually increase in size—cannot yet be said with certainty. 

In his terminology, a micella is a colloidal particle in which foreign substances (ions, water) are present in its structure, that is, a porous aggregate, whereas the amicron is a discrete particle too small to be seen with the ultramicroscope. He recognized that such particles in a colloidal solution could consist of one very large molecule, in other words, a single unit, not an aggregate. 

It was Carmen (4c) who first clearly stated that silicic acid polymerizes to discrete particles which then aggregate into chains and networks. 

Linking of particles together into branched chains, then networks, finally extending throughout the liquid medium, thickening it to a gel. 

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The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

Her, R. K. (1979). The polymerization of silica. The Chemistry of Silica. New York Wiley-Interscience Chapters 3 and 6. 

Cha, J.N., Chimizu, K., Zhou, Y., Christiansen, S.C., Chmelka, B.F., Stucky, G.D. and Morse, D.E. (1999) Silicatein filaments and subunits from a marine sponge direct the polymerization of silica and silicones... 

These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of free bromide anion increasing at pH around 11 when the polymerization of silica starts. 

Silica materials have been studied extensively because of the structural flexibility of silica (through Si04 tetrahedral connections), easy control of hydrolysis and polymerization of silica species, high thermal stability of silica framework, easy modification of the silica surface, and well known silica and zeolite chemistry. Amorphous silica is also the main inorganic component for certain natural materials obtained from bioassembly, such as diatoms. Various mesoporous silica materials have been reported, which are very important for both fundamental research and applications. 

With a silica-loaded exchanger, a unique reaction of polymerization of silica inside and its migration into the interior of the exchanger also takes place that can... 

The general theory of nucleation and polymerization in aqueous systems, in which silica shows some solubility, is discussed in detail in Iler s book (3). However, very little was known at the time the book was published (1979) about the polymerization of silica when Si(OH)4 is formed in nonaqueous systems. Progress made up to 1990 in the understanding of the hydrolysis and condensation of silicon alkoxides that leads to silica gels or to silica sols of large particle diameter are lucidly discussed by Brinker and Scherer (8). Brinker s chapter in this book (Chapter 18) includes a clear explanation of the difference between hydrolysis and condensation of aqueous silicates and silicon alkoxides. 

Fig. 5. Chimie douce inorganic polymerization of silica precursors.

Silica Silica is the main constituent of the majority of glasses. Successful determination of silica is essential for analysis of glass. This is usually achieved by fusion with sodium carbonate or a mixture of sodium carbonate and borax (2 1) with subsequent dissolution of the cooled melt in 0.1 N hydrochloric acid to avoid polymerization of silica. 

The reaction of silica with catechol, pyrocatechol, and 2,3-naphthalenediol has been studied by several investigators (158-162), but Bartels and Erlenmeyer appear to have been the only ones to use this reaction to characterize the rate of depolymerization of silica (163a). For example, monomeric silicic acid from ethyl silicate in a standard solution of catechol in 0.8 N HCl reacted rapidly and could be titrated to a constant pH of 8.5 with an equivalent amount of standard NaOH solution in a few minutes. An equivalent amount of silica gel required 2.5 hr, but ignited gel reacted only slightly in 5 hr. The rate of reaction, followed by a constant pH titration, provides a way to estimate the relative degree of polymerization of silica or possibly the specific surface area. 

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