Infrared study of the interactions between the surface of silica and methyl fluorosulphate vapour

Abstract

Infrared studies of the interactions between silica and methyl fluorosulphate vapour have established that isolated surface silanol groups are hydrogen bonded to the oxygen atoms of sulphuryl groups in adsorbed methyl fluorosulphate molecules. Subsequent slow chemisorptive reactions occur and generate the surface species SiOMe, SiF, SiOSO₂OMe and SiOSO₂OH. The presence of hydrophobic SiOMe groups decreased the ability of the oxide to adsorb water. However replacement of SiOMe by SiF groups increased the ability of the oxide to adsorb water and in some situations gave a surface which adsorbed more water at high vapour pressures than untreated silica. Complete dehydroxylation of silica by treatment with methyl fluorosulphate at 306 K followed by evacuation at ∼853 K gave a hydrophobic surface. Surface silanol groups are considered to be the primary adsorption sites for water on partially fluorinated silica. The presence of SiF groups enhances multilayer adsorption of water on the primary adsorption sites.