Improvement in optical transmission of the ambient pressure dried hydrophobic nanostructured silica aerogels with mixed silylating agents

The experimental results on the preparation of transparent and hydrophobic silica aerogels based on ion exchanged sodium silicate (Na2SiO3) using mixed silylating agents of trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDZ) are reported. Hydrogels were prepared with ion exchanged Na2Si03 of 1.10 specific gravity with 1 N ammonium hydroxide (NH4OH). The resulted gels were exchanged with methanol solvent followed by silylation using HMDZ + HMDSO or TMCS + HMDZ or TMCS + HMDSO mixture in methanol and hexane and dried the gels at room temperature for 24 h, at 50 and 200 °C for 1 h each. It has been observed that the percentage of the silylating agent in the mixture, time interval of addition of one agent followed by another and volume of silylating mixture have an effect on density, % of optical transmission, % of porosity, porevolume, thermalconductivity and refractive index of the aerogels. Hydrophobicity of the aerogels was studied by contact angle measurements. The TMCS + HMDSO aerogels have been found more hydrophobic (contact angle > 150°) than the other aerogels. Further, aerogels have been characterized by pore size distribution, Fourier Transform Infra Red Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and thermogravimetric analysis and differential thermal analysis (TGA–DTA) techniques. It has been found that the weight increase is the highest (325%) for HMDSO + HMDZ aerogels and lowest (1.2%) for HMDSO + TMCS aerogels by keeping the aerogels in the atmosphere over a period of 18 months. Low density (0.042 g/cc) transparent (85%), low thermal conductive (0.047 W/m K), low refractive index (1.0088) and hydrophobic (152°) silica aerogels were obtained with equivolume TMCS + HMDSO mixture of 10 ml, time interval of addition of 12 h between the two reagents prior addition of TMCS followed by HMDSO for 24 h of silylation period. It was found from the TGA–DTA of the aerogels that hydrophobicity of the aerogels remained up to the temperature of 425 °C.