Silica gelation, condensation

The insolubilization of cations and anions during the setting and hardening process is thus paralleled by that of silica. Under add conditions orthosilicic add condenses first to form polymeric silidc add and then silica gel (Her, 1979 Andersson, Dent Glasser Smith, 1982). These processes are discussed more fully in Section 6.5.4. Gelation of silica, like the formation of salt gels, is enhanced by a reduction in the acidity of solutions. 

After some time the colloidal particles and condensed silica species link together to become a three-dimensional network. The physical characteristics of the gel network depend greatly on the size of particles and extent of cross-linking prior to gelation. At gelation, the viscosity increases sharply, and a solid object results in the shape of the mold. 

Cellular structures are observed in poly(dimethylsiloxane) (PDMS)/silica gels when the sol-gel parameters (H2O, HC1 concentration and temperature) are adjusted to accelerate the gelation process167. PDMS, which contains direct Si—C bonds along its Si—O backbone, can co-condense with TEOS, as shown in equation 9. 

The variation of TPAOH amount during the synthesis of silica-aluminas has two macroscopic effects the change of the pH and of the relative amount of TPAOH to silico-aluminate oligomers. The textural properties of the final silica-aluminas may result from the different sequence of hydrolysis and condensation reactions (and the reverse reactions esterification and alcoholic or hydrolytic depolymerization) of the Si and A1 alkoxides. Indeed an increase in TPAOH content and therefore of the pH corresponds to an increase of OH" availability that can favor the hydrolysis and depolymerization reactions, giving rise to different gelation rate and to different network formation. 

Silica gels, such as those in Fig. 6.6, are prepared by mixing solutions of water glass (sodium silicate with SiOj/NajO 3.22) and HCl. This is an example of condensation and produces (H0)3Si-0-Si(0H)j sols of about 1.5 nm at a pH of 6. Gelation times of 10 min result in gels so stiff they may be cm into cubes. The hydrogel has a pore volume of 2.0cm g and contains about 60%-70% H O. 

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