Adsorption of water vapour by silica and the effect of surface methylation

Abstract

A comparative study of nitrogen (78 K) and water vapour (273–298 K) adsorption isotherms has been carried out with precipitated silica, a flame hydrolysis product (Cabosil) and with surface modified Cabosil obtained by treatment with trimethylchlorosilane. The latter produces a hydrophobic surface by methylating the surface hydroxyl groups. Infrared examination of pressed discs of the samples has been used to characterise the hydroxyl groups on the surface and to establish the effect of outgassing conditions.

Following relatively mild outgassing conditions, 373–423 K for example, Cabosil and precipitated silica gave water isotherms of B.E.T. type II. The isosteric enthalpies of adsorption were consistent with a physisorption mechanism with hydrogen bonding to surface hydroxyls. Following completion of an isotherm, evacuation at 298 K invariably left some water on the sample. The precipitated silica was more hydrophilic than the Cabosil as measured by their respective surface area ratios, A(H₂O)/A(N₂), of 1.09 and 0.38. Most of the hydrogen bonded hydroxyls were removed from the silicas by evacuation at 773 K, whilst most of the free hydroxyl species were removed from the Cabosil at 1173 K. The water isotherm on a Cabosil sample so treated was B.E.T. type III, typical of a hydrophobic surface. Vapour phase rehydroxylation occurred during the determination of the desorption branch of the isotherm, although complete rehydroxylation was not achieved. The extent of rehydroxylation was reduced by prolonging the evacuation at 1173 K.

By treating Cabosil with liquid (CH₃)₃SiCl (by soaking and refluxing), the free hydroxyl groups were replaced with OSi(CH₃)₃ groups. This reduced the hydrophilicity of the material appreciably giving an are ratio of only 0.08. The sample obtained by soaking still gave a type II water isotherm however, whereas that obtained by refluxing gave an isotherm which was generally more of type IV in shape with significant hysteresis; the hysteresis loop failed to close as the pressure approached zero. With these surface-modified Cabosil samples there was no evidence for the multilayer step-wise adsorption, reported previously for similarly treated surfaces.

With the surface-modified Cabosil, the hydrogen bonded hydroxyl groups were progressively removed at temperatures above 400 K. The –OSi(CH₃)₃ groups were hardly affected at temperatures below 900 K; an appreciable effect was only apparent at 973 K. Complete dehydroxylation of the surface was achieved at 1073 K. The completely dehydroxylated surface was shown to be rehydroxylated by exposure to water vapour, although there was no evidence of free hydroxyl groups being formed at this stage.