Fluorine-induced hydrophobicity in silica aerogels

The use of the R-substituted organosiloxanes in hydrolysis-condensation reactions with tetraalkoxysilanes is a flexible way to incorporate functional groups into silica gels and aerogels. When R = CH2CH2CF3, chemically and physically stable gels and aerogels that are hydrophobic can be readily made. These types of aerogels have been shown to be adsorbents for organics compounds from aqueous phase. In order to understand the role of the fluorinated group in making these materials hydrophobic, various formulations of CF3CH2CH2-modified aerogels have been synthesized and characterized. These materials were formed by the reaction of CF3CH2CH2Si(OCH3)3 and Si(OCH3)4 in CH3OH with NH4OH as the catalyst followed by supercritical drying in CO2. A 0.7 mole ratio of the reactants CF3CH2CH2Si(OCH3)3/Si(OCH3)4 is necessary to make hydrophobic aerogels that meet the 2.5% water adsorption limit. Lower reactant mole ratios yield materials that adsorb more water, reaching a saturation value in roughly 1 month, where the amount adsorbed is inversely proportional to the amount of CF3CH2CH2 group incorporated.