Aqueous silicates condensation

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. 

The structures of silicate polymers formed from alkoxides by the sol-gel process vary from weakly branched molecular networks to highly condensed particles that are similar to commercial aqueous silicates described by Iler (2). The sol-gel process combines this control of microstructure on molecular length scales with the ability to form specialized shapes such as fibers, films, monosized powders, and monoliths... 

Evidence for molecular siloxane networks is abundant in numerous Si NMR spectroscopy studies of alkoxides hydrolyzed under acidic conditions [35-40]. Figure 47.4 [41] shows a sequence of Si NMR spectra of TEOS hydrolyzed with 2 mol of water under acidic conditions in ethanol and for comparison a Si NMR spectrum of a commercial aqueous silicate (Ludox). With time the TEOS sol becomes more highly condensed, as evident from the disappearance of monomer (Q ) and the progressive formation of end groups and di-, tri-, and tetrasubstituted silicate species (Q -Q" species, respectively). However, even after 14 days, di- and trisubstituted species appear more prevalent than tetrasubstituted species. By comparison, Si NMR spectra of aqueous silicates (Figure 47.4d) and base-catalyzed alkoxides (Figure 47.5) are dominated by monomer and tetrasubstituted species. 

In aqueous silicate chemistry, the term polymerization is commonly used in its broadest sense to include reactions that result in an increase in molecular weight of silica. It includes the condensation of silanol groups... 

The most widely accepted condensation mechanism involves the attack of a nucleophilic deprotonated silanol on a neutral silicate species, as outlined earlier for the condensation in aqueous silicates [Eq. (5.4)]. This condensation mechanism pertains above the PZC (or lEP) of silica (pH > 2) because the surface silanols are deprotonated (i.e., they are negatively charged) and the mechanism changes with the charge on the silanol. Condensation between larger, more highly condensed species, which contain more acidic silanols, and smaller, less weakly branched species is favored. The condensation rate is maximized near neutral pH where significant concentrations of both protonated and deprotonated silanols exist. A minimum rate is observed near the PZC (or lEP). 

Euclidean objects (dense spherical particles) are most likely to form in systems (e.g., aqueous silicates) in which the particle is slightly soluble in the solvent. In this case, monomers can dissolve and reprecipitate until the equilibrium structure (having a minimum surface area) is obtained. In nonaqueous systems (e.g., silicon alkoxide-alcohol-water solutions), the solubility of the solid phase is so limited that condensation reactions are virtually irreversible. Bonds form at random and cannot convert to the equilibrium configuration, thereby leading to fractal polymeric clusters. 

In our laboratory, we have recently conducted gelation studies of silica nanopardcles in microgravity during the STS-95 space shuttle mission (28). Stable silica nanoparticle dispersions may be form either by polymerization of silicic acids in an aqueous system or through hydrolysis and condensation of silicon alkoxides (the sol-gel or Stober route). Comparison of small-angle x-ray scattering (SAXS) measurements of Ludox, a commercial aqueous silicate with acid- and base-catalyzed alkoxides shows that only aqueous silicate sols are uniform, whereas alkoxides generate fractal particles. As Brinker and Scherer point out (29), these results illustrate that sols derived from aqueous silicates are... 

Aqueous Silicates provides a concise review of hydrolysis and condensation of aqueous silicate systems in which Tier s views [1] are augmented by more recent studies that employ Si NMR. 

Because in aqueous silicate systems, gel times are observed to decrease below the isoelectric point of silica (Fig. 10b), it is generally believed that the acid-catalyzed condensation mechanism involves a protonated silanol species. Protonation of the silanol makes the silicon more electrophilic and thus more susceptible to nucleophilic attack. The most basic silanol species (silanols... 

Although the Si NMR spectra of aqueous silicate systems are quite complex, the speciation of polysilicates formed from tetraalkoxysilanes is more complicated, because hydrolysis and condensation occur concurrently. At the nearest functional-group level there are 15 distinguishable local chemical environments, which Kay and Assink have represented in matrix form in Fig. 22. The ordered triplet (X, Y, Z) represents the number of -OR, -OH, and -OSi functional groups attached to the central silicon [63,97]. 

Condensation rate versus pH for aqueous silicates. A and B correspond to regions of higher and lower sticking probability, respectively. From Brinker e al. [20]. 

MCM-41 mimicked the surfactant s liquid crystal mesophase in aqueous solutions. As a consequence, it was advanced that the siufactant forms a liquid crystal mesophase and the siUca species organize in the space around the surfactant micellar rods interacting with the polar heads of the surfactant molecules. The silicate condensation occurs in the second step once this organization has been set up and traps it. Alternatively, they have postulated that the liquid crystal phase does not pre-exist but forms upon the addition of the inorganic species. It was later evidenced that the MCM-41 synthesis takes place at concentrations lower than that necessary to form the liquid crystal phase, so the second pathway is more plausible when working in dilute media while the first one is true in more concentrated syntheses [11]. 

The most widely accepted mechanism for silica condensation reactions involves the attack of a nucleophilic deprotonated silanol Si—0 on a neutral silicate species as proposed by Her [43] to explain condensation in aqueous silicate systems. This pertains to reaction (Eq. (17.3)) above the isoelectric point of silica where surface silanols are deprotonated to a significant extent. Note that not all silanol groups are identical, as it depends on the electron density on the... 

The fate of silicic acid is of some interest. Silicic acid polymerizes, by condensation, and finally a silica gel is formed (Wilson Mesley, 1968). The insolubilization of silicic acid has been observed to parallel closely the precipitation of phosphate (Wilson Batchelor, 1967b) and is related to an increase of pH within the cement (Kent Wilson, 1969). A low concentration of silicic acid must remain in the matrix. All this is in accord with the known aqueous chemistry of silica. 

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