Studies of Surface Modification of Solids. III. Reaction of Silica with Trifluoroethanol

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The esterification of silica was performed by reactions with 2,2,2-trifluoroethanol vapor and ethanol vapor with and without basic catalysts, such as ammonia and pyridine. The esterification with ethanol was referred to as the control. The extents of the esterifications were observed by means of infrared spectroscopy. The reactivity of the silica with trifluoroethanol was higher than that with ethanol. This is presumably because trifluoroethanol is more acidic than ethanol. The esterification was accelerated by the addition of ammonia as a base catalyst. For example, in the reaction of the silica with trifluoroethanol of 10.7 Torr at 30 °C for 3 hr, 35% of the surface silanols were esterified without any catalyst, while with the ammonia (0.2 Torr) catalyst, 98% of them was esterified. This suggests that the extraction of the proton from the hydroxyl groups of alcohol by NH<SUB>3</SUB> contributes to the esterification. The addition of pyridine influenced the reaction rate as well, but the esterification itself was disturbed by the addition of excess pyridine, because pyridine was adsorbed strongly on the surface silanol and its heterocyclic ring prevented the alcohols from adsorption. It is supposed that the physical adsorption of trifluoroethanol on the surface silanol is necessary in the first stage of the esterification. The esterified silica became hydrophobic, and it was hydrolyzed in the presence of water.
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Studies of Surface Modification of Solids. III. Reaction of Silica with Trifluoroethanol

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