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.