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 dist ributions, obtained with electrostatic effects neglected, showed three well-defined peaks that correspond to protonation/deprotonation proce sses of distinct -O(H) and -OH(H) groups in various surface configurat ions. The peak positions were relatively invariant with respect to var iations in ionic strength but showed a characteristic dependence on te mperature. This phenomenological observation was used to determine, se parately, the thermodynamic functions Delta H-i and Delta S-i for prot on adsorption on distinct i surface sites. The neglect of electrostati c 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 vari ous configurations. The results showed that adsorption of protons on s tructurally different sites at the alumina/water interface is both ent halpically and entropically driven. The effects of both temperature an d electrolyte concentrations on the pH acquired by alumina suspensions in neutral electrolyte are discussed.