The dissolution kinetics of SiO2 aerogels in aqueous NaOH solutions

The dissolution kinetics of SiO2;? aerogels was investigated by three compl ementary experimental techniques-the kinetic method, static light scatterin g, and small-angle X-ray scattering. Four aerogel samples synthesized by th e catalytic hydrolysis of alkoxides with the use of one- and two-stage proc edures were investigated. The aerogels differed in their specific surface a reas, densities, and optical properties. Regularities of the dissolution of the aerogels in 0.05-0.4 M NaOH aqueous solutions were studied in the kine tic- and diffusion-controlled regimes over the temperature range 15-56 degr ees C. In spite of the different conditions of aerogel synthesis, the activ ation energies of their dissolution measured in the kinetic regime were ide ntical and equal to 82 +/- 6 kJ/mol. Once the critical degree of dissolutio n for the aerogels was reached, the threshold disintegration of their secon dary structure occurred, and silica clusters (SiO2),(ct) consisting of prim ary particles appeared in the solution. In situ registration of scattering showed that the disintegration of the aerogels resulted in 10-300 nm fragme nts. A correlation between the shape of peaks in light-scattering curves of the aerogel dispersions and their optical properties was found.