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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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Silica wet gels were prepared from hydrolysis of tetraethoxysilane (TEOS) with additions of sodium dodecyl sulfate (SDS). The surfactant was removed after gelation. Wet gels exhibited mass-fractal structure with mass-fractal dimension D (typically around 2.25) in a length scale extending from a characteristic size ξ (typically about 10 nm) of the mass-fractal domains to a characteristic size a0 (typically between 0.3 and 0.4 nm) of the primary particles building up the fractal domains. ξ increased while D and a0 diminished slightly as the SDS quantity increased. Aerogels with typical specific surface of 1000 m(2)/g and density of 0.20 g/cm(3) were obtained by supercritical drying of the wet gels after washing with ethanol and n-hexane. The pore volume and the mean pore size increased with the increase of the SDS quantity. The aerogels presented most of the mass-fractal characteristics of the original wet gels at large length scales and exhibited at a higher resolution level at about 0.7 nm a crossover to a mass-surface fractal structure, with apparent mass-fractal dimension Dm ∼ 2.4 and surface-fractal dimension Ds ∼ 2.6, as inferred from small-angle X-ray scattering (SAXS) and nitrogen adsorption data.

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The formation of calcium silicate hydrates (C−S−H) during the hydration of tricalcium silicate (C3S) in pure water and in water solutions containing 1% CaCl2 (accelerator) and 0.01% saccharose (retarder) was studied by small-angle X-ray scattering (SAXS). SAXS measurements were performed under isothermal conditions within the temperature range 25 °C < T < 52 °C. The experimental results indicate that the time variation of the mass fraction of the C−S−H product phase, α(t), can be fitted, under all conditions of paste setting, by Avrami equation, α(t) = 1 − exp(−(kt) n ), k being a rate parameter and n an exponent depending on the characteristics of the transformation. The parameter n is approximately equal to 2 for hydration of C3S in pure water. Depending on temperature, n varies from 2 to 2.65 for hydration in the presence of CaCl2 and saccharose. The value n = 2 is theoretically expected for lateral growth of thin C−S−H plates of constant thickness. The time dependence of SAXS intensity indicates that the transformed phase (C−S−H) consists of colloidal particles in early stages of hydration, evolving by two-dimensional growth toward a disordered lamellar structure composed of very thin plates. The activation energy ΔE for the growth of C−S−H phase was determined from the time dependence of X-ray scattering intensity. These data were obtained by “in situ” measurements at different temperatures of hydration. The values of ΔE are 37.7, 49.4, and 44.3 kJ/mol for hydration in pure water and in water solutions containing CaCl2 and saccharose, respectively.