Effect of glycerol additive on physical properties of hydrophobic silica aerogels

Considering the need for obtaining good quality hydrophobic silica aerogels; in terms of monolithicity, low density and high optical transparency; the effect of glycerol (C3H5(OH)3, GLY) as a Drying Control Chemical Additive (DCCA) on physical properties of silica aerogels, is reported. The aerogels were produced by hydrolysis and condensation of methanol diluted tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMS) in presence of ammonia catalyst followed by supercritical drying. The molar ratio of GLY/TMOS (G) was varied from 0 to 0.2. It has been found that lower G (≈0.025) value resulted in low density (≈45 mg cm−3) and lesser shrunk (<6%) and transparent (≈93%) as well as monolithic aerogels, whereas the unmodified (G=0) aerogels were found to have higher density (≈70 mg cm−3), more shrinkage (≈20%), lower optical transmission (≈90%) and cracks. For higher G (≥0.075) values, monolithic but less transparent and more dense aerogels were obtained. Narrow and more uniform pore size distribution (PSD) was observed for the GLY-modified aerogels. The results have been discussed by taking into account the hydrolysis and condensation reactions and PSD.