Gelation aqueous silicates

Figure 18-13a. Typical SAXS-curve of an aqueous silicate solution after gelation has occurred, b SAXS-curve of a silica gel prepared at pH = 3.9 from potassium waterglass. The gel has been aged for one year at room temperature. The dotted line presents a nonlinear least squares fit of the fractal region in the curve, yielding D = 2.0. The dashed line presents the Porod-law.

Despite the relatively long time scales and large length scales involved in gelation of aqueous silicate precursors, several groups have used molecular dynamics simulations and atomistic potentials to study the sol-gel process and sol-gel derived silica materials. 

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... 

Gelation is initiated in aqueous silicate systems by pH changes, and in alkox-ide precursor systems by addition of water (hydrolysis reactions to generate Si-OH groups). 

The sol-gel technique uses mild conditions to prepare a cross-linked, robust metal oxide gel. Gelation of silanes, such as tetraethoxysilane (TEOS) or sodium silicate, involves the use of aqueous solutions, sometimes containing alcohol, with a slight adjustment of pH to catalyse the condensation polymerisation. The cross-link density, material porosity and homogeneity of functionalised sites, are dependent on the solution pH, extent of ageing, method of solvent evaporation, water content in solution and type of catalyst, all of which provide a number of handles to modify the material of interest. 

For commercial silica gels, water glass Na2Si03 is used as a cheap silica source then, before the gelation step, sodium is ion-exchanged from the aqueous sodium silicate solution to get a silicic acid (Schwertfeger et al., 1998). 

Silica gel is a macroporous polymer that precipitates from water or aqueous alcohol as it is formed from silicic acid, Si(OH)4, or tetraethyl orthosilicate, Si(OEt)4. The network structure is completed by removal of water and ethanol after gelation. The dried silica is more highly cross-linked than PS-DVB, because each silicon atom is tetrafunctional. It swells even less than poly-DVB resins, and commonly has 300-700 m g of internal surface. 

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