Surface Penetration without Enrichment: Simulations Show Ion Surface Propensities Consistent with Both Elevated Surface Tension and Surface Sensitive Spectroscopy

Molecular dynamics was used to investigate ion surface propensities in NaCl, KBr, and CsI solutions with an MP2-based force field. Although NaCl is found to be strongly repelled from the liquid–vapor interface, softer ions, such as I^–, penetrate closely to the interface. Despite the surface penetration, the concentration of CsI near the interface is still lower than that in the bulk, thus leading to no surface enrichment. The salt concentration is found to affect relative surface propensities of the ions. More significant surface penetration is observed at higher salt concentrations. Softer ions at higher concentrations form a complex multilayer arrangement that can not be characterized as a simple surface bilayer. The simulated ion distributions explain the spectroscopic evidence of surface perturbation by soft ions with a negative surface excess consistent with an increased surface tension of salt solutions.