Monodisperse colloidal silica spheres from tetraalkoxysilanes: Particle formation and growth mechanism

The mechanisms behind the formation and growth of silica particles prepared from tetraalkoxysilanes in alcoholic solutions of water and ammonia were investigated. By analyzing the competitive growth of a dispersion of silica spheres with a bimodal distribution, it was established that the growth proceeds through a surface reaction-limited condensation of hydrolyzed monomers of small oligomers. By following the hydrolysis of tetraethoxysilane with 13C liquid NMR and the particle growth with time-resolved static light scattering, it was found that both processes were described by the same first-order rate constants. Therefore, despite the fact that the incorporation of hydrolyzed monomers proceeds through a reaction-limited process, the overall rate of the particle growth is limited by the first-order hydrolysis rate of the alkoxide. It was concluded that the particle formation (or particle nucleation) proceeds through an aggregation process of siloxane substructures that is influenced strongly by the surface potential of the silica particles and the ionic strength of the reaction medium. These conclusions were based on the dependence of the particle stability and final particle size on additions of LiNO3 to the reaction and dispersion medium and the independence of the growth rate on the same additions.