Silica gels from aqueous silicate solutions:combined 29Si NMR and small-ange X-ray scattering spectroscopic study

\u3cp\u3eThe use of modern spectroscopic techniques in the study of the formation of aqueous silica gels is described. The oligomerization process of monomeric silicic acid was studied by silicon-29 NMR spectroscopy; at high pH values cyclic trimeric silicate species were favored compared to the linear structure. Aggregation of primary silica particles of molecular size (<1 nm) was studied by analysis of small-angle x-ray scattering patterns. All systems studies (pH 4.0) indicate reaction-limited aggregation. Polyvalent cations influence the rate of aggregate formation in a negative way: aluminum at low concentrations (1 mol%) significantly inhibits aggregation. A new model for the aging process that proposes that monomeric silicic acid is transported (via solution) from the periphery of the aggregate into the core of the solution is given.\u3c/p\u3