Hydrophobic silica aerogels supercritical drying

Yokogawa H., Yokoyama M. Hydrophobic silica aerogels. J. Non-Cryst. Solids 1995 186 23-29 Yokoyama M. Materials prepared by supercritically drying process. Dev. Appl. Porous Ceram. II 1991 295-309... 

In this article, production, properties and applications of hydrophobic silica aerogels have been described. The hydrophobic silica aerogels are at present especially important in the optical application, because they show highly valuable properties which cannot be attained optically by other available materials. So far, silica aerogel incurs high preparation costs due to the specialized process of supercritical drying, and the present consensus is that... 

Although many different varieties of aerogels can be made, the most common is silica aerogel. Without additional modification, silica aerogel is prone to the adsorption of water, which causes collapse of the structure. However, modification of the gel prior to the supercritical drying process with trimethylsilyl results in a hydrophobic silica aerogel [110]. This practice is very common in siUca aerogel production, especially in application as thermal insulation. 

El Rassy H, Buisson P, BouaU B, Perraid A, Pierre AC (2(X)3) Surface Characterization of Silica aerogels with Different Proportions of Hydrophobic Groups, dried by the CO2 Supercritical Method Langmuir, 19 358-363 Harreld JH, Ebina T, Tsubo N, Stucky G (2(X)2) Manipulation of pore size distributions in silica and ormosil gels dried under ambient pressure crmditions. J Non-Cryst Solids 298 241 251... 

El Rassy, H, Buisson, P, Bouali, B, Perrard, A, Pierre, A C (2003) Surface characterization of silica aerogels with different proportions of hydrophobic groups, dried by the CO2 supercritical method. Langmuir 19(2) ... 

Supercritical alcohol drying typically induces hydrophobic properties even in nonmodified (hydrophobized) silica gels. At the high process temperatures and pressures, free silanol groups react with ethanol, leaving behind mostly alkoxy-decorated gel. This surfece chemistry, however, is not hydrolytically stable under normal ambient conditions. A TEOS-based HTSCD aerogel dried in supercritical ethanol loses its hydrophobicity within a few hours when stored in ambient air. 

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