Effect of nearest-neighbor interactions on surface titrations

Abstract

By considering the discreteness of surface groups and using either the random-mixing or quasichemical approximations for mixing ionized and un-ionized groups, analytical expressions are presented for the apparent dissociation constant in terms of nearest-neighbor interaction energies. These expressions are exact at zero and one degree of dissociation. A smeared surface charge model or a model calculating Δp K using only the electrostatic potential at a given charge due to the electrostatic potential of all the other charges is insufficient to explain the difference observed in the p K at a surface and an analogous nonaggregating molecule in bulk solution for titrations of dodecylamineoxide micelles and phosphatidic acid vesicles in the literature. In particular for a neutralized surface or when the electrostatic potentials cancel, there is still a discrepancy in the p K's even if image forces are considered. For dodecylamineoxide, assuming only a charge-charge interaction in the nearest-neighbor model is still inconsistent with the data, while an additional attractive interaction such as hydrogen bonding is consistent.