THERMODYNAMICS OF PROTON BINDING AT THE ALUMINA/AQUEOUS SOLUTION INTERFACE - A PHENOMENOLOGICAL APPROACH

The distribution function, f(log K), of apparent equilibrium constants for proton binding at the alumina/water interface was determined for various electrolyte concentrations and temperatures. The apparent distributions, obtained with electrostatic effects neglected, showed three well-defined peaks that correspond to protonation/deprotonation processes of distinct -O(H) and -OH(H) groups in various surface configurations. The peak positions were relatively invariant with respect to variations in ionic strength but showed a characteristic dependence on temperature. This phenomenological observation was used to determine, separately, the thermodynamic functions Delta H-i and Delta S-i for proton adsorption on distinct i surface sites. The neglect of electrostatic terms is not a drastic approximation because these terms appear as a minor perturbation when compared with much larger variations in Delta G(i)(intr) caused by the intrinsic structural terms for sites in various configurations. The results showed that adsorption of protons on structurally different sites at the alumina/water interface is both enthalpically and entropically driven. The effects of both temperature and electrolyte concentrations on the pH acquired by alumina suspensions in neutral electrolyte are discussed.