The effect of pH on the physicochemical properties of silica aerogels prepared by an ambient pressure drying method

Hydrophobic silica aerogels were successfully synthesized by an ambient pressure drying method from various silicic acids with different pH values which were prepared from water glass. In this study, we selected xylene as the solvent and performed the surface modification in a TMCS (trimethylchlorosilane)/xylene solution, in order to improve the reproducibility of the aerogels. The densities of the aerogels were about 0.10–0.14 g/cm3 and the apparent porosities were in the range of 94.5–96%, depending on the processing conditions. Their specific surface area was in the range of 700–750 m2/g and their average pore size was around 20 nm. The transparency of the as-dried aerogels was enhanced as the pH value was increased. The transmittance of the heat-treated aerogels was slightly decreased with increasing heat-treatment temperature. On the other hand, an abrupt increase in the transmittance was observed when the temperature exceeded 400 °C and this phenomenon might be related to the change in the surface structure caused by the desorption of the methyl groups.