Superhydrophobic silica aerogel powders with simultaneous surface modification, solvent exchange and sodium ion removal from hydrogels

We report a novel method for the rapid synthesis of superhydrophobic silica aerogel powders based on an inexpensive precursor such as sodium silicate via ambient pressure drying. The sodium silicate was directly polymerized without prior ion exchange by the addition of nitric acid and hexamethyldisilazane (HMDS) to yield silylated hydrogels. The indispensable steps such as surface modification, solvent exchange and sodium ion removal were simultaneously accomplished in one-step by immersing the silylated hydrogel in a non-polar solvent such as n-hexane. The organic modification in the aqueous phase, i.e. in the hydrogel essentially led to the displacement of pore water and simultaneous intrusion of n-hexane transformed the hydrogel into an organo-gel. During this process, sodium ions present in the hydrogel were removed with the displaced pore water. Using this process, it was possible to produce superhydrophobic silica aerogel powders with contact angle as high as 150 degrees, very low tapping density of 0.08 g/cm(3), high specific surface area (778 m(2)/g) and cumulative pore volume of 2.34 cm(3)/g. The total processing time of the aerogel powders could be restricted to 5 h. The textural properties of the aerogel powders have been investigated and discussed. (C) 2007 Elsevier Inc. All rights reserved.