Energetics and structure of sol-gel silicas

High temperature calorimetry was used to measure the metastability, relative to fused silica glass, of an acid and a base catalyzed silica gel. This excess enthalpy can be separated into two parts, a 'fast' release enthalpy which occurs on heating the gel to 700-800 °C and a 'slow' release enthalpy detectable only when the Si{single bond}O{single bond}Si bridges are broken on dissolving the sample in molten lead borate. When the effects of H₂O, OH, and OC₂H₅ are subtracted by appropriate thermochemical cycles, the silica framework of the acid catalyzed gel is found to be metastable by 7.3 kJ/mol and that of the base catalyzed gel by 66.8 kJ/mol, with 'fast' release enthalpy accounting for most of the effect. The extremely large release of heat in the base catalyzed gel is proposed to be due mainly to the relaxation and rearrangement of metastable siloxane rings created from rapid condensation of monomers (due to high solubility of silica in basic solution) during drying. The much smaller metastability of the acid catalyzed gel may be associated with pore collapse as volatiles are removed. This proposal is consistent with chemical analyses, TGA/DSC and photoacoustic infrared spectroscopy of the materials.