Surface chemical modification of silica aerogels using various alkyl-alkoxy/chloro silanes

The experimental results on the surface chemical modification of silica aerogels using various alkyl-alkoxy/chloro silane (organosilane) compounds, are reported. Silica alcogels, prepared by keeping the molar ratio of tetramethoxysilane (TMOS), organosilane compound, methanol (MeOH), water (H2O) and ammonia (NH4OH) constant at 1:0.5:14:4:3.7 x 10(-3) respectively, were dried supercritically to obtain the aerogels. In all, 10 organosilane compounds having zero to three functional groups were used. Large variations were observed in the hydrophobic and physical properties of the aerogels depending on the type of the organosilane incorporated in the gel. The contact angle (theta) increased from similar to95degrees for a monoalkyl organosilane compound such as methyltrimethoxysilane (MTMS) to similar to135degrees for trialkyl compound such as hexamethyldisilazane (HMDZ). Tetramethylsilane (TMS) modification resulted in hydrophilic acrogels. While all the surface modified aerogels were found to be thermally stable up to a temperature of 275 degreesC, the HMDZ and other trialkylorganosilane modified aerogels showed higher thermal stability (similar to300 degreesC). However, the long term water intake is slightly more for the trialkylorganosilane incorporated aerogels than the monoalkylsilane modified aerogels, with the exception of HMDZ. The alkylchlorosilane modified aerogels have higher volume shrinkage and density and lower optical transmission than the alkylalkoxysilane modified aerogels. The alkylchlorosilane resulted in cracked aerogels and alkylalkoxysilane gave rise to monolithic aerogels. (C) 2002 Elsevier Science B.V. All rights reserved.