Water on organosilane-treated silica surfaces

Abstract

Both a fully hydroxylated silica (HiSil 233) and a partially hydroxylated silica (Cab-o-Sil M-5) were treated with organosilanes and a silazane. Water adsorption isotherms were then monitored on the resulting products. Reflectance IR spectroscopic measurements were used to follow the reactions of the surface hydroxyls with the silanes and to examine the subsequent water adsorption. The main objective was to ascertain the water susceptibilities of the different functional groups and the availability to water of any residual hydroxyls. The silazane and silanes employed were: hexamethyldisilazane, vinyl-trichlorosilane, and the alkoxysilanes: γ-glycidoxypropyltrimethoxysilane, 3-mercaptopropyltrimethyoxysilane, and amino-propyltriethoxysilane. The latter three were reacted from the liquid phase, first hydrolyzing to the silanetriols, with additional complications from condensation reactions. The vinyl compound and the disilazane were reacted from the vapor phase. Sharp differences between the results for the fully hydroxylated and the partially (25%) hydroxylated silica were found. The silanetriols upon heat treatment formed a multilayer polymer network on the latter and only a two-dimensional network on the former apparently due to hydrogen bonding to residual hydroxyls. The reacted silanes formed umbrellas over reacted hydroxyls on the former which were nevertheless available to water molecules. With the latter, however, a much more hydrophobic surface was produced. A number of details were also provided by the reflectance IR measurements.