Hydrophobic and physical properties of the two step processed ambient pressure dried silica aerogels with various exchanging solvents

The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient pressure dried hydrophobic silica aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and condensation with NH4OH of ethanol diluted tetraethylorthosilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution and contact angle (0) of the silica aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the silica aerogels strongly depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (approximate to 90% optical transmission at 700 nm for 1 cm thick sample), low density (approximate to 0.060 g/cm(3)), low thermal conductive (0.070 W/m-K), high % of porosity (97%), high surface area (750 m(2)/g), uniform porosity and hydrophobic (theta approximate to 160 degrees) aerogels compared to other solvents. On the otherhand, xylene resulted in aerogels with higher hydrophobicity (theta approximate to 172 degrees) among other solvents.