Kinetic and thermodynamic studies of silica nanoparticle dissolution

Small-angle X-ray scattering (SAXS) and microcalorimetry were used to study the dissolution of silica nanoparticles that serve as precursors in the synthesis of the pure-silica zeolite, silicalite-1. Temporal changes in nanoparticle size were monitored by SAXS to obtain radial dissolution rates on the order of 1 x 10(-2) nm/min, 10 times greater than those of silicalite-1. Nanoparticle dissolution rates are independent of solution alkalinity (above pH 11) and particle surface area, although contributions from the latter account for more than 60% of the nanoparticle enthalpy of dissolution (13.5 +/- 0.1 kJ/mol SiO2 relative to silicalite-1). We show that dissolution enthalpies and rates correlate to the molecular structure of silicates. Comparisons among amorphous silica, silicalite-1, and silica nanoparticles suggest that the latter are amorphous and therefore not simply small fragments of a crystalline zeolite. Nevertheless, they do possess a degree of ordering greater than that in dense amorphous silica. Dissolution experiments were also performed on heat-treated nanoparticles grown via Ostwald ripening. With increasing time of heat treatment, the nanoparticle dissolution rates and enthalpies decrease in magnitude toward those of silicalite-1, suggesting a structural reorganization of silica within the particles. The results offer insight on silicalite-1 nucleation as well as relevant time scales and rate-determining steps involved in zeolite crystallization.