Comparative studies on the surface chemical modification of silica aerogels based on various organosilane compounds of the type RnSiX4-n

We report the experimental results dealing with the surface chemical modification of silica aerogels using various precursors and co-precursors based on mono-, di-, tri- and tetrafunctional organosilane compounds of the type RnSiX4-n (where R = alkyl or aryl groups, X = Cl or alkoxy groups, n = 0-4). The precursors exhibit either tetrafunctional or trifunctional chemical functions while the co-precursors have a number of functional groups varying from 1 to 3. The organosilane based on the methyltrimethoxysilane (MTMS) can be used as a precursor as well as co-precursor. The chemically modified aerogels have been produced by (i) co-precursor, and (ii) derivatization methods. The co-precursor method results in aerogels with higher contact angle (theta approximate to 136degrees) but the aerogels are opaque, whereas transparent (>80% optical transmission in the visible range) aerogels with lower contact angle (theta approximate to 120degrees) are obtained using the derivatization method. The hydrophobicity of the phenyl-modified aerogels has been found to be thermally stable up to a temperature of 520degreesC. Using the MTMS precursor, aerogels with contact angles as high as 175 have been obtained, but the aerogels are opaque. The aerogels obtained using TEOS precursor along with the trimethylethoxysilane (TMES) co-precursor, show negligible volume shrinkage. Water intrusion into the MTMS-modified aerogels at pressures greater than the Laplace pressure exhibits hysteresis, as shown in the pressure-volume curves. Water droplets placed on surfaces coated with superhydrophobic aerogel powder with 8degrees of inclination showed velocities as high as 0.4ms(-1). The results are discussed with respect to the ratios of organic and inorganic components of the organosilane compounds. (C) 2004 Elsevier B.V. All rights reserved.