Study of the interaction between surface hydroxyls and adsorbed water molecules on porous glasses by means of infrared spectroscopy

Abstract

Infrared absorption spectroscopy was employed to investigate the interaction between surface hydroxyl groups and adsorbed water molecules. Firstly, after a confirmation of the well-known frequencies of OH, H 2O and OD, D 2O vibration bands for the present system, the spectrum of adsorbed HDO molecules was studied. On this basis, the vibration of those surface hydroxyls which interact with adsorbed water was analyzed for the fundamental and first overtone OH stretching modes. These frequencies were used to estimate dissociation energies of interacting surface hydroxyls (118 kcal/mole) supposing that their potential curve can be described with sufficient accuracy by a Morse function as in the case of free OH groups. This high barrier for a formation of free protons is in contrast to the small activation energy (about 5 kcal/mole) for the proton life-time with respect to an exchange between surface hydroxyls and adsorbed water according to nmr data. This result rules out an exchange mechanism in which an classical H 3O + structure participates. Rather it points to a mechanism in which OH bands of the hydroxyl and the water molecule change places simultaneously, i.e. a synchronous reaction.