Chemical reaction kinetics leading to the first Stober silica nanoparticles – NMR and SAXS investigation

Abstract 29 Si-NMR and 13 C-NMR were used in methanol and ethanol to monitor the intermediates or hydrolyzed monomers that lead to the formation of the first primary particles as detected by small angle X-ray scattering. This identification was facilitated by using initial NH 3 and H 2 O levels at the lower end of those experienced in Stober synthesis to slow the reaction kinetics. We found that [NH 3 ] and [H 2 O] control the balance between hydrolysis of tetraethylorthosilicate (TEOS) and the condensation of its hydrolyzed monomers. Transesterification between methanol and TEOS did occur; however, it was negligible compared to the production of hydrolyzed intermediates. The first nanostructures appear at a hydrolyzed monomer concentration around 0.1 M, indicating that formation of the primary structures is thermodynamically controlled by a supersaturation of the intermediate species. Differences in particle size between methanol and ethanol are attributed to thermodynamic interactions between the solvent and the hydrolyzed intermediates.