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The aggregation kinetics of solutions of vinyltriethoxysilane‐derived organic/silica hybrid species were studied by small‐angle X‐ray scattering (SAXS) in a strongly basic medium. The SAXS intensity was analysed by a modified Sharp–Bloomfield (SB) global function and its evolution was found to be compatible with the growth, coiling and branching of the polymeric macromolecules in solution. A form factor valid for randomly and nonrandomly branched polycondensates and for polydisperse coils of linear chains was used in the modified SB model, instead of the Debye function valid for monodisperse coils of linear chains. The aggregation kinetics are accelerated with increasing base concentration in the studied range, but all the kinetics curves can be matched to a unique curve using an appropriate time scaling factor. The aggregation kinetics suggest that physical forces (hydrothermal forces) associated with phase coarsening could be active in the aggregation process, together with diffusion mechanisms.

A comparative study using small-angle x-ray scattering ͑SAXS͒ and nitrogen adsorption has been carried out in the structural characterization of silica xerogels and aerogels, obtained from tetraethoxysilane sonohydrolysis. The specific surface and the mean pore size as measured by both the techniques were found to be in notable agreement in all cases for aerogels and xerogels. According to the SAXS data, aerogels at 500°C exhibit a mass fractal structure with fractal dimension Dϳ2.4 in the range between the correlation length ϳ5.3 nm and aϳ0.75 nm. An experimental method to probe the mass fractal structure of aerogels from exclusively nitrogen adsorption isotherms has been presented. For aerogels at 500°C, we have found Dϳ2.4 in the range between the pore width 2r ϳ33 nm and 2r a ϳ4.5 nm, which is in notable agreement with the SAXS results (D ϳ2.4, ϳ5.3 nm, aϳ0.75 nm) if we assign the pore width 2r probed by the Kelvin equation in the adsorption method to the Bragg distance 2/q associated to the correlation length 1/q probed by SAXS.

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