Transparent SiO2 aerogels prepared by ambient pressure drying with ternary azeotropes as components of pore fluid

Transparent SiO₂ aerogels were prepared by two-step sol–gel processing followed by ambient pressure drying at temperatures from 70 to 250 °C. The wet gels were synthesized via acid–base catalysis using tetraethyl orthosilicate as a silica precursor and isopropanol as a solvent. Isopropanol was exchanged with n-butanol, and the gel surface was modified using a trimethylchlorosilane solution in n-butanol. Next, the solvent was exchanged in several steps with saturated hydrocarbon in order to obtain pore fluids containing azeotropic mixtures of water, n-butanol and a corresponding hydrocarbon (hexane, heptane, octane, nonane). Ambient pressure drying was performed in two steps, at the boiling points of the ternary azeotropes and hydrocarbons, respectively. In this way, transparent, crack-free aerogels of different shapes, with a specific surface area of 1000 m²/g, average pore diameter of ～40–55 Å and density in the range 0.4–0.57 g/cm³ were obtained.