Relation between Surface Tension and Ion Adsorption at the Air-Water Interface: A Molecular Dynamics Simulation Study

Classical molecular dynamics simulations of aqueous solutions of sodium chloride and potassium fluoride at two different concentrations have been carried out using polarizable force fields and standard additive force fields (not including polarizability explicitly). We show that the presence of chloride ions at the air-solution interface, as predicted from the polarizable simulations of NaCl solutions, is reconcilable with the classical thermodynamics results of Gibbs absorption theory. We discuss the role of system size in the establishment of a bulklike region in which the ion densities have converged to the same value. We compare the results for NaCl solutions with those obtained for KF at two concentrations. We find that the computed surface tension and the Surface excess follow the experimental trend for each salt solution. We have characterized the extent of adsorption by calculating the fraction of the solution surface area that is occupied by each ion. The analysis reveals that, as expected, in the KF solution neither the cation nor the anion is present on the surface, regardless of whether or not a polarizable force field is employed. On the other hand, in the NaCl solutions, chloride anions occupy the surface to an extent that is roughly proportional to their bulk concentration, but only when a polarizable model is used.