Ions near the air/water interface: I. Compatibility of zeta potential and surface tension experiments

Abstract

A simple Structure Making/Structure Breaking (SM/SB) Modified Poisson Boltzmann approach is used to describe the distribution of ions in the vicinity of an air/water interface, which is assumed to be a layer possessing a thickness of a few Å. The interface is assumed to be depleted of SM ions, and the adsorption of SB ions on the interface is taken into account via Langmuir adsorption equations. Outside the interface, the ions are assumed to obey a modified Poisson–Boltzmann distribution, which includes the screened image forces. The adsorption constants of SB ions, determined by fitting the existing zeta potential data, are further used to determine the surface excesses of salt ions and the dependence of the surface tension on added salt. The number of sites available for the adsorption of OH− and H+ ions, considered the water molecules of the interface, is (by more than two orders of magnitude) smaller than the available sites in an ice-like layer. This result suggests a (much) lower density of water at the hydrophobic air/water interface than in the bulk. The model can qualitatively explain the dependence of the surface tension upon addition of salt determined experimentally: a large decrease at low concentrations, due to the formation of a double layer, a large increase at intermediate concentrations, where the screened image forces (being less screened than the double layer forces) dominate, and a slower increase at large concentrations, where both double layer and image forces are screened and the ion hydration forces dominate. However, for large surface potentials, the accumulation of ions in the double layer (either as adsorbed on the interface or in the ion clouds in the vicinity of the interface) leads to large surface excesses of ions and consequently to a surface tension that decreases upon addition of salt up to large concentrations, in contrast to experiment. According to the present model, only the zeta potentials experiments that predict low surface potentials and small adsorption of salt anions on the surface are compatible with the surface tension experiments.