Solvation Structure and Energetics of Single Ions at the Aqueous Liquid-Vapor Interface.

Potentials of mean force for single, nonpolarizable monovalent halide anions and alkali cations are computed for transversing the water-air interface (modeling using polarizable TIP4P-FQ and TIP4P-QDP). Iodide and bromide in TIP4P-FQ show interfacial stability, whereas chloride, bromide, and iodide show interfacial stability in TIP4P-QDP. A monotonic decrease in coordination number and an increasingly anisotropic distribution of solvating water molecules is shown to accompany movement of the ions towards vapor conditions; these effects are most noticeable with increases in ion size/decreases in magnitude of hydration free energy.