Effect of reaction variables on the formation of silica particles by hydrolysis of tetraethyl orthosilicate using sodium hydroxide as a basic catalyst

Abstract

The formation of silica particles by hydrolysis of tetraethyl orthosilicate (TEOS) in ethanolic solutions was studied by multivariate analysis using a 2⁴⁻¹ reduced experimental design. The variables (factors) in the analysis were the starting concentrations of TEOS (0.1–0.5 M), NaOH (1–5 mM) and water (1–6 M) and the temperature (23–40 °C). The target parameters were the size and the polydispersity of the silica particles and the amount of silica formed. The size and the polydispersity of the particles were determined using photon correlation spectroscopy, while the amount of silica formed was determined gravimetrically after centrifugation. Analysis of the data by multiple regression analysis indicated that the variable of greatest importance for the amount of silica formed, the particle size and the polydispersity was the starting concentration of NaOH. In the concentration range studied, the conversion of TEOS to silica was low; however, the amount of silica formed increased in proportion to the concentration of NaOH. Our interpretation is that hydroxyl ions are consumed during the hydrolysis reaction, leading to the formation of charged intermediates. When most of the hydroxyl ions have been consumed the hydrolysis will stop. A higher concentration of NaOH resulted in the formation of smaller and more uniform silica particles and these were formed in greater number. In addition, both the particle size and the polydispersity index showed a strong negative correlation with the interaction term between water and NaOH, indicating that the ionisation of NaOH increases with increasing water content.