Stability of hydroxyl and methoxy surface groups on silica aerogels

Silica aerogels were prepared by the hydrolysis of tetramethyl orthosilicate in methanol. Surface group stability was characterized by infrared spectroscopy. Results showed OH, H2O, and OCH3 surface groups present after supercritical drying. H2O groups are easily removed under a vacuum at ambient conditions; however, the OCH3 groups are strongly held and remained even at 600 degrees C. If air is admitted during heating, the OCH, groups disappear rapidly and surface OH groups are formed. The resultant surfaces were found to be quite hydrophilic, easily adsorbing water at room temperature. D2O vapor exposure showed an extremely active surface, with nearly 1/2 of the OH groups exchanged in 30 min at room temperature. Exposure to CCl4 vapor eliminated surface OH groups and the surface was found to be more hydrophobic after long term exposure to H2O.