Aerogels are a class of materials with extraordinary properties like high porosity, low density, high thermal insulation value, ultra-low dieletric constant or low index of diffraction. That makes them suitable materials for different interesting applications: thermal insulation, drug encapsulation, desiccant, acoustic barrier, heat storage, supercapacitor, hydrogen storage, catalytic support, adsorption of toxic compounds from water among others.
Luis Miguel Sanz del Moral
On the other hand, silica aerogels are very brittle structures, and they are highly susceptible to fracture because of the collapse of their pore structure during the drying step of their production due to the surface tension of the liquid contained in the gel nanopores, that infers a capillary pressure gradient. Several techniques have been used in order to dry the aerogels, including atmospheric drying a
nd freeze drying, but supercritical drying, where the alcohol is solved in supercritical carbon dioxide, is one of the better established and more frequently used methods to dry aerogels overcoming the collapse of their structure.
Hydrophilic and hydrophobic silica aerogels have been produced following a sol–gel process. Tetramethylortosilicate or a mix of the first one with trimethylethoxysilane have been used as precursors. The hydrolysis and poly-condensation steps were followed by carbon dioxide supercritical drying (T = 45ºC; P = 105 bar). The resulting dry hydrophilic aerogels were subjected to a hydrophobic surface functionalization using supercritical carbon dioxide as the solvent for different
silane functionalization reactants: trimethylethoxisilane, octyltrimethoxisilane and clorotrimethylsilane. Effects of the working pressure were analyzed using FT-IR spectroscopy and exposing the treated aerogels to saturated moisture conditions in order to study the mass increment during the humidification. Nitrogen adsorption measurements show a considerable drop on the specific areas (13-17%) and on the pore volumes which were reduced by 50%. By the modification of the operating pressure and variation of the functionalization agent employed, the degree of functionalization could be gradually increased up to the values of the aerogels synthesized as hydrophobic in the sol-gel phase. That allows to work with the hydrophilic aerogel that can be advantageus in some cases and then provide protection against the humidity by functionalizing the surface.