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.

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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.

Silica wet gels with the same silica content were prepared by the sonohydrolysis of tetraethoxysilane (TEOS) with additions of dimethylformamide (DMF). DMF plays a role in the overall hydrolysis/gelification/aging step of the sol-gel process, providing more consolidated wet gels with larger syneresis degrees and densities. The structure of the as-obtained wet gels can be interpreted as being built up of mass-fractal domains with fractal dimension D = 2.2 and radius of gyration decreasing from about 14 to 12 nm with increasing quantity of DMF. Monolithic hydrophobic aerogels were prepared after washing of the wet gels with isopropyl alcohol (IPA), silylation with trimethylchrorosilane (TMCS), and ambient-pressure drying (APD). The specific surface area of the APD aerogels was found to be about 900 m(2)/g, and the mean silica particle size was about 2.0 nm, approximately independent of the DMF quantity, whereas the porosity decreased slightly with increasing amount of DMF, fairly accompanying the behavior of the radius of gyration of the precursor wet gels. The mass-fractal characteristics were preserved in the APD aerogels, but the radius of gyration of the mass-fractal domains was reduced to values between 2.8 to 4.0 nm, with the values decreasing slightly with the DMF quantity, and the fractal domains developed a surface-mass-fractal structure with the overall washing/silylation/APD treatment. The structural characteristics of the APD aerogels as determined by SAXS were found to be in notable agreement with those inferred from nitrogen adsorption.

A kinetic study of the oxalic acid catalyzed and ultrasound-assisted hydrolysis of 3-glycidoxypropyltrimethoxysilane (GPTMS) was carried out using an ultrasound-adapted calorimetric method. During a relatively short time period (t p), the hydrolysis follows a heterogeneous pathway because of the initial immiscibility gap between GPTMS and water. Methanol produced in the early hydrolysis enhances the dissolution between GPTMS and water so that the hydrolysis crosses to a homogeneous pathway. A rate equation with a rate constant (k) of the order 3/4 with respect to GPTMS and first order with respect to water seems to describe well the experimental data in the homogeneous step of the reaction. k increased with increasing oxalic acid concentration up to a plateau value (∼3.2 × 10–3 M–3/4 s–1) at an oxalic acid concentration of about 0.083 M in the range studied (from 0.014 to 0.14 M). The rate constants k and 1/t p (proportional to k in the heterogeneous step) were found to follow approximately an Arrhenius equation with the hydrolysis temperature in the studied range (from 5 to 50 °C). The average activation energy for GPTMS sono-hydrolysis was found to be ΔE = (21.5 ± 1.9) kJ/mol. The structure of aged sonogels exhibited characteristics of a two-phase system (pore and hybrid solid phase) separated by a surface–fractal interface. The hybrid solid phase of the sonogels was constituted by polymeric linear chains, density fluctuation heterogeneities, and several intermolecular-interfering cagelike structures. The phase separation increased with increasing the hydrolysis temperature, apparently at the expense of the polymeric linear chains and the density fluctuation heterogeneities, and led to intensification of the intermolecular-interfering cagelike structures within the hybrid solid phase.