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.
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