Particle formation in the hydrolysis of tetraethyl orthosilicate in pH buffer solution

Particle formation in the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) was studied by varying pH (9.5-11) with the basic catalysts NH ₃, methylamine (MA), and dimethylamine (DMA) in the presence of 5 mol/m ³ CH ₃COOH, which was chosen to suppress time variations of pH and ionic strength during the reaction. Spherical particles were formed for MA and DMA at catalyst concentrations of 0.02-0.2 kmol/m ³ and for NH ₃ at catalyst concentrations of 0.1-1.5 kmol/m ³. In a common range of catalyst concentrations for spherical particle formation, average particle size was largest for DMA and smallest for NH ₃. Hydrolysis rate of TEOS could be quantified by the use of buffer systems as a function of TEOS and OH ^- concentrations. A specific relation was not found between the hydrolysis and the particle size. The zeta potential of silica particles measured in the reaction solvent was in the order DMA < MA < NH ₃, and ionic strength, estimated from pH in the reactions, was in the order DMA ≅ MA > NH ₃. This suggested that the particle sizes were controlled by electrostatic particle interactions.