Comparative studies of the physical and hydrophobic properties of TEOS based silica aerogels using different co-precursors

In the present paper, the experimental results on the physical and hydrophobic properties of tetraethoxysilane (TEOS) based silica aerogels using six different organosilane co-precursors (C.P) of the type RnSiX4-n as synthesis components, are reported and discussed. The aerogels have been produced by sol-gel processing followed by supercritical drying using methanol solvent extraction. The molar ratio of TEOS, ethanol (EtOH), water (0.001Moxalic acid (H2C2O4) catalyst) was kept constant at 1: 5: 7, respectively, and themolar ratio of C.P/TEOS(A) was varied from 0.1 to 0.6 and compared the aerogel properties. The hydrophobicity of the aerogels has been tested by the contact angle measurements. The contact angle (theta) has been found to be the highest (theta = 136 degrees)for the trimethylethoxysilane (TMES) co-precursor, while for the other co-precursors it is in between 120 degrees and 130 degrees. The surface chemical modification of the hydrophobic aerogels has been studied using Fourier Transform Infrared Spectroscopy (FTIR). As the C.P/TEOS molar ratio increased, the intensity of the C-H and Si-C peaks in the FTIR spectra increased, clearly indicating the organic modification of the aerogels. The aerogels based on mono-alkyl (CH3) trialkoxysilane co-precursor have shown higher optical transmission (approximate to 65%) compared to the phenyl, di or tri alkyl alkoxysilanes (5-50%). The trialkylmodified aerogels showed the lowest bulk density (118.3 kg/m(3)) and volumeshrinkage (<2%). The alkyl alkoxy/chloro-silanemodified aerogels have been found to be thermally stable up to a maximum temperature of 573 K, whereas the phenyl trialkoxysilane modified aerogels are stable up to a temperature as high as 823 K. The aerogels have been characterized by scanning electron microscopy, thermogravimetric and differential thermal analyses. (C) 2004 Elsevier Ltd. All rights reserved.