Effect of protic solvents on the physical properties of the ambient pressure dried hydrophobic silica aerogels using sodium silicate precursor

Abstract

Considering the importance of the highly porous, low density, transparent and nanostructured hydrophobic silica aerogels in the scientific and industrial applications, the experiments have been carried out to prepare the low density silica aerogels using the 1.12 specific gravity water glass (sodium silicate, Na2SiO3) precursor, ammonium hydroxide (NH4OH) catalyst, trimethylchlorosilane (TMCS) silylating agent, various first exchanging protic solvents and hexane as a second exchanging aprotic solvent. The first exchanging solvents used were: methanol, ethanol, propanol, isopropanol, butanol, isobutanol and hexanol. The molar ratio of the Na2SiO3:H2O:NH4OH:TMCS was kept constant at 1:56:0.02:0.4 respectively. The ambient pressure dried method was used for the preparation of hydrophobic silica aerogels. The effect of the exchanging protic solvents on the physical properties of the aerogels such as density, % of volume shrinkage, % of porosity, % of optical transmission, thermal conductivity, thermal stability and contact angle of the aerogels with water, were studied. FTIR studies were carried out to confirm the silylation of the aerogel samples. It was found that the exchanging protic solvents have profound effect on the physical and hydrophobic properties of the aerogels. Low density (0.07 g/cm3), high porosity (96.6 %), low thermal conductivity (0.091 W/mK), high contact angle (166°) silica aerogels could be prepared by using the isopropanol first exchanging solvent followed by the hexane as the second exchanging solvent along with the TMCS silylating agent with sodium silicate precursor.