Processing of low-density silica gel by critical point drying or ambient pressure drying

Various methods for the production of low-density silica from silica gel were studied. The silica gel was prepared by the `two-stepʹ sol–gel method. The `DSB processʹ (developed by Deshpande, Smith and Brinker), which takes the gel through solvent exchange, reaction with trimethylchlorosilane (TMCS) and ambient pressure drying (APD), was then applied. This processing provided a greater total pore volume, and more mesopores with diameters >50 Å, than critical point drying (CPD), the conventional method for producing an aerogel. The high porosity was found to be due primarily to the solvent exchange step; in fact, the reaction with TMCS reduced the porosity. Reaction of the gel with trimethylmethoxysilane (TMMS) in the mother liquor (ethanol/water) provided higher pore volumes than the DSB process. This extra porosity may be attributed to poor wetting of the polar solvent in contact with the surface made hydrophobic through reaction with the TMMS; thus, the capillary forces that cause the gel to shrink are reduced. Finally, it was discovered that some loss of porosity occurs when CPD is conducted with carbon dioxide; specifically, the pressure pulse that occurs when the CPD cell is initially flooded with this liquid can damage the structure of the silica network.