Ions near the air/water interface. II: Is the water/air interface acidic or basic? Predictions of a simple model

摘要

A simple modified Poisson-Boltzmann model, compatible with zeta potential experiments, is used to calculate the concentrations of ions on the interface. It is shown that, for most pH values and salt concentrations, both H ~+ and OH ~- accumulate on the interface, making their interfacial concentrations larger than in the bulk. However, at extreme pH values, the interfacial concentrations of both H ~+ and OH ~- have been found to be lower than in bulk, which might explain some contradictory results from literature. The ratio between H ~+ and OH ~- concentrations is smaller at the interface than in bulk for most (but not all) pH values and salt concentrations. Recent experimental results, which show that H ~+ is present on the interface only at very low pH values, are explained on the basis of this model. It is shown that, when the adsorption of Cl ~- on the interface is sufficiently large compared to that of OH ~-, the number of H ~+ ions adsorbed on the interface passes through a maximum with salt concentration, also in agreement with experiment. The concentration at which the maximum occurs is of the order of the adsorption equilibrium constant of Cl ~- on the interface.