The effect of the H2O/TEOS ratio on the structure of gels derived by the acid catalysed hydrolysis of tetraethoxysilane

Silica gels were produced by the acid catalysed hydrolysis of tetraethoxysilane (TEOS) using H₂O/TEOS ratios from 2 to 50. After heat treatment the structure of the gels was studied using nitrogen adsorption, small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and bulk density measurements.All the gels possessed microporosity in the region of 30 Å or less. For H₂O/TEOS = 25 and 50 the matrix density was found to be uniform, but for gels from solutions with H₂O/TEOS = 2,4 and 10, density fluctuations in the matrix were detected from Porod analysis of the SAXS data. These results indicate that in higher water content solutions, rearrangement of the polymeric chains leads to small densified particles, but for lower water content solutions, gelation results from the entanglement of linear chains leaving free volume on a molecular scale between the chains.