Superhydrophobic silica aerogels based on methyltrimethoxysilane precursor

The experimental results on the synthesis and physicochemical properties of superhydrophobic silica aerogels, with the highest ever obtained contact angle (-173degrees), using methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent and ammonium hydroxide (NH4OH) catalyst, are reported. The molar ratios of NH4OH/MTMS (N), H2O/ MTMS (H) and MeOH/MTMS (M) were varied from 4.25 x 10(-2) to 3.5 x 10(-1), 2 to 10 and 1.75 to 17, respectively. It has been found that the gelation time decreases with increase in N and H values and it increases with increase in M values. The bulk density of the aerogels was found to decrease with increase in N, H and M values. It has been observed that the volume shrinkage increases with decrease in N and H values and increases with M values. In the case of catalyst concentration variation, the contact angle (theta) increases slightly from 159 to 167 with increase in N values. On the other hand, in the case of H2O and MeOH variations, the theta first increases from 162 and 160 up to the values of 173 and 167 and then decreases to 160 and 159 with increase in H and M values, respectively. All the MTMS aerogels are opaque to the visible light. The aerogels retain their hydrophobicity up to a temperature of similar to480 degreesC. The thermal conductivity of the aerogels was found to be around 0.095 W/m K except for the aerogels with higher bulk density (>0.3 g/cm(3), at a lower H value of 2) whose thermal conductivity was around 0.109 W/m K. The aerogels have been characterized by Fourier transform infrared spectra (FTIR), thermogravimetric and differential thermal analyses (TGA-DTA) and scanning electron microscopy (SEM) techniques. The results have been discussed by taking into account the hydrolysis and condensation reactions and SEM observations. (C) 2003 Elsevier B.V. All rights reserved.