Chemically Controlled Porosity in Silica Aeroglass

The relationship between the initial compositional make up and the ultimate pore structure has been probed by varying the initial amounts of methanol and water relative to the silica precursor. The gels were dried under conditions that were supercritical to the inter-pore liquid to yield aeroglasses. The structure of these materials has been investigated using nitrogen adsorption and mercury intrusion porosimetry. The primary compositional factor in determining the structure of sol-gel materials is the relative amount of water. A secondary but very important consideration is the relative dilution of the silane in the alcoholic solvent. Increasing this dilution has been shown to affect the shape of the isotherm, and the pore size distributions calculated from the adsorption and desorption branches. As the dilution increases the pore diameters show a general increase. This is related to the level of molecular separation between the condensing species. The relationship between the surface area and dilution is not as clear cut; the estimated BET area generally decreases with increasing dilution, with a possible maximum at a dilution of R(A) = 2 (where R(A) is the molar ratio of the alcohol with respect of the silane). It has been shown that there are intimate relationships between the initial sol-gel chemistry and the final pore structure of the aeroglasses. Manipulation of the various synthesis parameters thus allows control over the final morphology.