STUDIES OF THE KINETICS OF THE PRECIPITATION OF UNIFORM SILICA PARTICLES THROUGH THE HYDROLYSIS AND CONDENSATION OF SILICON ALKOXIDES

Reactions leading to the formation of uniform silica particles are investigated. Rates of silicon alkoxide reactions are reported and linked to particle growth rates. Seeded growth experiments indicate that reactions between soluble species occur at rates that are independent of the size or number density of seed particles. In addition, these experiments show that particles do not grow by a diffusion limited mechanism. Estimates of hydrolyzed alkoxide concentrations based on solution conductivities suggest that the soluble silica concentration is above that required for nucleation to occur until late in the precipitation reaction. These results are consistent with a particle growth mechanism involving reactions occurring in solution that proceed independently of the presence of particles and result in the formation of silica nuclei which aggregate to form particles. Uniformity in final particle size is produced through size dependent aggregation rates. This hypothesis is further checked by carrying out precipitations at different electrolyte concentrations. While rates of hydrolysis and condensation are not substantially affected by NaCl concentrations up to 10-2 M, final particle diameters increase dramatically. © 1991.