Hot Supercritical Drying Media Modulate Silica Aerogel Water Affinity

Using a custom-built, hot supercritical drying (SCD) system with automation and on-line monitoring capabilities, the drying process of silica aerogels was optimized to achieve negligible volumetric shrinkage, in addition to allowing control of the aerogel surface chemistry, from hydrophilic to “super-hydrophobic”. The procedure developed involved examining process parameters and exploring different drying media (solvents, mixtures and additives). We found that sample dissolution and bulk deformation could be minimized by introducing 2.5 vol.% H2O, in addition to ethanol or isopropanol, as the drying media. Similarly, versatility could be achieved with regard to the aerogel composition and affinity to water; mixing acetonitrile with up to 20 vol.% isopropanol can extend hydrophobic longevity from a few hours to well over a month of direct contact with water, while drying in pure isopropanol results in a “super-hydrophobic” aerogel. Alternatively, a few vol.% H2O added to acetonitrile results in a hydrophilic aerogel, with negligible carbon content.