The kinetic model of dissolution of silicon dioxide aerogels in an aqueous NaOH solution

Plausible explanations for previously obtained kinetic data on the dissolution of silicon dioxide aerogels in an aqueous 0.05 N NaOH solution in the temperature range 15-37 degrees C were considered. Because the coincidence of specific surface areas measured by the BET and small-angle X-ray scattering techniques suggests an absence of closed pores in the structure of investigated aerogels, the heterogeneity of their surfaces was considered to be the most probable reason for the variations in the shape of kinetic curves. A model of dissolution was proposed. The model is based on the assumption of two kinetically nonequivalent types of surface atoms (inactive and active) and takes into account finite sizes of these atoms. The model is a modification of the known Delmon model and uses a parameter E-s having a simple physical meaning of the initial number of active sites of dissolution on a primary aerogel particle. The activation energy of dissolution (80 +/- 6 kJ/mol) and the interfacial energy (0.09 and 0.24 J/m(2) for coarsely and finely dispersed samples, respectively) were obtained by approximating experimental data by calculated dissolution curves. It was shown that the number of active sites, E-s, increases linearly with the radius of primary particles.