The kinetic model of dissolution of silicon dioxide aerogels in an aqueousNaOH solution

Plausible explanations for previously obtained kinetic data on the dissolut ion of silicon dioxide aerogels in an aqueous 0.05 N NaOH solution in the t emperature range 15-37 degrees C were considered. Because the coincidence o f specific surface areas measured by the BET and small-angle X-ray scatteri ng techniques suggests an absence of closed pores in the structure of inves tigated 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 activ e) and takes into account finite sizes of these atoms. The model is a modif ication 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 fin ely dispersed samples, respectively) were obtained by approximating experim ental data by calculated dissolution curves. It was shown that the number o f active sites, E-s, increases linearly with the radius of primary particle s.