Preparation and characterization of hydrophobic silica aerogels

The present paper deals with preparation and characterization of hydrophobic silica aerogels. Silica sol was prepared by keeping the molar ratio of TMOS:MeOH:H2O constant at 1:12:4. respectively, while H2O was added as a dilute base (NH4OH) catalyst. In order to obtain the hydrophobic aerogels, tris(hydroxymethyl)aminomethane (TAM) was added in the sol. The TAM/TMOS molar ratio (A) was varied from 0 to 3.28×10−3 After gelation, silica alcogels were dried supercritically using a high temperature method. Before and after supercritical drying, the gels were flushed with N2 gas with a pressure of 5 bars in order to rarefy oxygen in the autoclave. Hydrophobicity of the aerogels was tested by measuring the percentage of water adsorbed by aerogels after putting them in water. It was found that medium A values between 0.41 × 10−3 and 0.99 × 10−3 resulted in hydrophobic aerogels, whereas lower (<0.41 × 10−3) and higher (>0.99 × 10−3) A values resulted in hydrophilic aerogels. Hydrophobicity at medium A values is due to the fact that OH groups are substituted by methyl groups. In order to study the thermal stability, the hydrophobic aerogels were heat treated in the temperature range from 25 to 500 °C. It has been found that below 223 °C the samples showed hydrophobic properties whereas above 223 °C the hydrophobicity vanished. The destruction of hydrophobicity above 223 °C is due to the weakening of C-H bonds. The results were supported by infrared (IR) characterization of the aerogel samples. The aerogels were also characterized by density and optical transmission measurements.