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

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.