Methyltriethoxysilane: New precursor for synthesizing silica aerogels

Traditionally, silica aerogels are synthesized using three silicon alkoxides, namely, tetraethoxysilane (TEOS), tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) for various applications in science as well as in technology. Among all these precursors, only MTMS-based silica aerogels are inherently superhydrophobic with so far reported, the highest contact angle of 173°. In the present paper, we reported a new precursor, namely, methyltriethoxysilane (MTES) for synthesizing the silica aerogels having the novel properties as that of MTMS-based aerogels. The aerogels have been prepared using the MTES by two-stage acid–base catalyzed sol–gel process followed by supercritical drying. The solvent and catalysts used for the synthesis were methanol (MeOH), oxalic acid (C2H2O4) and ammonium hydroxide (NH4OH), respectively. The aerogels of different densities were obtained by varying the molar ratio of MeOH/MTES (S) from 6.45 to 19.35. In order to get good quality aerogels in terms of low density, high contact angle and less volume shrinkage, the oxalic acid (A) and NH4OH (B) concentrations were varied from 0 to 1 and from 2 to 13.36 M, respectively. Monolithic aerogels have been obtained for the values of A = 0.01 M and B = 13.36 M. Simultaneously, the aerogels are superhydrophobic with contact angle as high as 163°. Furthermore, the effects of molar ratio of H2O/MTES (W1), i.e. acidic water and H2O/MTES (W2), i.e. basic water on the physical properties of the aerogels have also been studied. The molar ratio of MTES:MeOH:acidic water:basic water was optimized at 1:19.35:3.57:3.57, respectively. The aerogel thermal stability was studied by TGA–DTA while the hydrophobicity was quantified in terms of the contact angle measurements and FTIR studies. The as-prepared aerogels have been characterized by bulk density, porosity, volume shrinkage, thermal conductivity, contact angle measurements, transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The physical properties of the aerogels have been explained by taking into account of sol–gel reactions and the gel network formation.