Adsorption of nitrogen, water and certain organic vapours on fully and partially hydroxylated silica gels with different porosity characters

Abstract

The adsorption of several adsorbate molecules, namely nitrogen, water, carbon tetrachloride, benzene and methanol, on fully and partially hydroxylated silica samples was investigated. The samples tested had different porosity characters, i.e. they were microporous (Gasil 200), mesoporous (Gasil 35) and non-porous (TK800), in order to elucidate the effect of the porosity of the gel on specific and non-specific adsorbate-adsorbent interactions. Nitrogen adsorption, which is assumed to involve non-specific interactions, is influenced to a certain extent by the specific interaction between the nitrogen quadrupole and the surface hydroxyl. The smaller polar water molecules can penetrate into pores which are too narrow to accommodate larger adsorbate molecules. The marked reduction of the water uptake upon dehydroxylation of the silicas substantiates the important role of the specific interaction in the adsorption of water on silica surfaces. The carbon tetrachloride molecule appears to be a more sensitive probe than the nitrogen molecule for detecting microporosity, its only drawback being its larger size. Benzene adsorption depends mainly on the interaction between the mobile π bonding and surface hydroxyls. Finally, non-specific interactions appear to have a marked effect on the adsorption of polar methanol molecules on silica surfaces which involve essentially specific interactions.