Acid-base interactions on surfaces of wet and dry inorganic oxides

The acid-base chemistry of the surfaces of inorganic solids, both in their wet and (semi) dry state, is reviewed. The results of different experimental methods to probe acidity, namely the Proton Affinity Distribution (PAD) approach and FTIR spectroscopy, are compared. These two methods provide information on the types and acid strength of surface hydroxyl groups on the wet and (semi) dry surface of oxides, respectively. Several examples show that a relationship can be established between the acid-base properties of the wet and (semi) dry surface of the same oxide. This is the result of a common structure of the outermost surface layers. In particular, for oxides, it is proposed that at both the wet and (semi) dry surfaces the proton binding / proton donor sites are surface hydroxyl groups. These groups terminate the oxide layer and may bind a surface layer of water molecules, which are in contact with either an adjacent liquid phase or a gaseous environment. Following an analysis of the factors that determine the acid-base properties of oxide surfaces, several examples are given which illustrate the various principles and practical procedures for the control of the acidity or basicity of several classes of oxide materials.