Heterogeneity of proton binding sites at the oxide/solution interface

Abstract

Proton adsorption isotherms were used to calculate the affinity distribution of proton binding sites for [gamma] alumina and F-modified alumina samples. The isotherms were obtained by potentiometric titration over as broad a pH interval as allowed by the samples being studied. The affinity distribution were calculated using the local solution of the adsorption integral equation. A smoothing spline procedure was employed, and a criterion for optimal smoothing was proposed. The method was tested by model calculations and verified on simple chemical compounds with known proton affinity characteristics. The apparent distribution spectra for oxides, with electrostatic effects neglected, showed well-defined peaks that were assigned to protonation/deprotonation processes of distinct -O(H) or -OH(H) groups in various surface configurations. A correlation was obtained between the evaluated log K values for proton association equilibria and values predicted by structural models. These results confirm that proton adsorption isotherms are sensitive to the heterogeneity existing at oxide surfaces. We find that ionic strength effects on apparent distribution were minor when compared with the effect of surface structure and chemical composition. 56 refs., 18 figs., 1 tab.