Dissolution kinetics of SiO2 aerogels in aqueous NaOH solutions

The dissolution kinetics of SiO2 aerogels was investigated by three complementary experimental techniques - the kinetic method, static light scattering, and small-angle X-ray scattering. Four aerogel samples synthesized by the catalytic hydrolysis of alkoxides with the use of one- and two-stage procedures were investigated. The aerogels differed in their specific surface areas, densities, and optical properties. Regularities of the dissolution of the aerogels in 0.05-0.4 M NaOH aqueous solutions were studied in the kinetic- and diffusion-controlled regimes over the temperature range 15-56°C. In spite of the different conditions of aerogel synthesis, the activation energies of their dissolution measured in the kinetic regime were identical and equal to 82 ± 6 kJ/mol. Once the critical degree of dissolution for the aerogels was reached, the threshold disintegration of their secondary structure occurred, and silica clusters (SiO2)cl consisting of primary particles appeared in the solution. In situ registration of scattering showed that the disintegration of the aerogels resulted in 10-300 nm fragments. A correlation between the shape of peaks in light-scattering curves of the aerogel dispersions and their optical properties was found.