MODELING OF AGGREGATION KINETICS OF COLLOIDAL SILICA PARTICLES

Solutions of tetraethoxysilane (TEOS), ethanol and water were prepared with a range of base catalyst (ammonia) concentrations. The size and morphology of the colloidal silica particles produced and the gelation/precipitation behaviour were systematically studied. Up to a critical value of the ammonia concentration the gelation/precipitation time decreased sharply. This could be explained by a corresponding increase in the ammonium counter-ion concentration in solution, which reduced the stability of the particles. For comparison the stability of the silica particles was calculated using a model based on DLVO theory, as a function of particle size and initial solution composition. The primary coagulation kinetics were also determined versus ammonia concentration. These calculations showed similarities with the experimental plots of gelation/precipitation times versus concentration and yielded a similar value of the critical coagulation concentration of catalyst. The results indicate that the rates of particle growth and of aggregation are strongly dependent on the stability of the colloidal silica particles. © 1990.