Ambient pressure aerogels

The specific surface area of the surfactant- and polymer-templated gels, that is, the ambient pressure aerogels, is nearly the same as that of the aerogels prepared by the supercritical extraction of CO2. Also, the pore size and the pore volume, that is, the porosity, are larger (twice or more) than those of the xerogels, but smaller than those of the super-critically extracted aerogels, estimated to be ca. 55%. The pore size and pore volume depended on the properties of surfactants and polymers, molecular size, and micelle- or aggregate shape. Also, the concentration of polymers in the immersion solutions, as well as the kind of template materials, affected the pore size and pore volume (Table 7.2). 

Fig. 5. Schematic diagram of an extractive drying process that produces aerogels at ambient pressure. Reproduced from Ref. 49.

S. S. Prakash, C. J. Brinker, A. J. Hurd, and S. M. Rao, Silica aerogel films prepared at ambient pressure by using surface derivatization to induce reversible drying shrinkage. Nature (London) 374, 439-443 (1995). 

The equation above suggests that one approach would be lo use a pore liquid that has a low surface tension. In fact, with a pore liquid that has a sufficiently small surface tension, ambient pressure acid catalyzed aerogels with comparable pore volume and with bulk density to those prepared with supercritical drying (see Big. 1) have been produced. 

Fig. 2. Schematic diagram of an extractive drying process that produces aerogels at ambient pressure. Reproduced from D. M. Smith and co-workers. Mai. Res. Soe. Symp. Proc. 431, 291 (1996). Courtesy of the Materials Research Society...

Keywords fast gelation, hydrogensilsesquioxanes, aerogel, ambient pressure drying, highly porous hybrid... 

D. M. Smith, R. Desphande, and C. J. Drinker, Preparation of Low-Density Aerogels at Ambient Pressure, Better Ceramics through Chemistry V, 271 (M. J. Hampden-Smith, W. G. Klemperer, and C. J. Drinker, Eds.), Materials Research Society, Pittsburgh, pp. 567-72, 1992. 

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... 

Parvathy Rao A, Pajonk GM, Venkastewara Rao A (2005) Effect of preparation conditions on the physical and hydrophobic properties of two step processed ambient pressure dried sUica aerogels. J Mater Sci 40 3481-3489... 

Smith DM, Deshpande R, Brinker CJ (1992) Preparation of low-density aerogels at ambient pressure. Mat Res Soc Symp Proc Vol. 271 567-572... 

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