Effect of solvent exchanging process on the preparation of the hydrophobic silica aerogels by ambient pressure drying method using sodium silicate precursor

Abstract

Experimental results obtained on the preparation of hydrophobic silica aerogels by ambient pressure drying method using the sodium silicate precursor with the variation of solvent exchanging process, are reported. The silica hydrogel was prepared by passing the 1.12 specific gravity sodium silicate through the Amberlite (TM) 120 Na⁺ resin and addition of 1 M ammonium hydroxide to silicic acid. The gel was kept in an oven for 3 h to strengthen the gel. Solvent exchange was carried out with ethanol and hexane for 36 h each followed by 24 h silylation using 20% hexamethyldisilazane (HMDZ) in hexane. Unreacted HMDZ was washed with hexane by keeping the gel in hexane for 24 h. Solvent was decanted and the gel was dried for 24 h by keeping the gel at 50 °C for 6 h, at 150 °C for 12 h and at 200 °C for 6 h. The low density (0.06 g/cm³), highly porous (96.9%), highly hydrophobic (contact angle of 160°), low thermal conductivity (0.07 W/m K) aerogels were obtained for the process of three times exchange with ethanol and three times exchange with hexane in 36 h each, followed by silylation with 20% HMDZ in hexane and two times washing with hexane in 24 h. FTIR studies showed the increase in the intensity of the Si–H and C–H bands of the aerogels with the increase of solvent exchanging times because of increase in silylation for more times of solvent exchange processes. It was found from the TG–DTA studies that the hydrophobicity of the aerogels retained up to the temperature of 325 °C. Water absorption studies show that the aerogels were remained hydrophobic up to 4 months when the aerogels were placed over the water as well as for up to 60 h in a 90% humid atmosphere. SEMs of the aerogels reveal that the pore sizes of the silica network increased, so the percentage of optical transparency decreased with the increase in exchange times with ethanol and hexane.