Analytical insights on ion behaviour at interfaces

We report new analytical results for the behaviour of the free energy of an ion between two media that are characterized by different dielectric permitivities including the ion polarizability as a phenomenological parameter. The differences in the free energy induced by the new terms related to the polarizability are on the order of a few kBT (∼25 meV), kB being the Boltzmann constant and T the temperature of the system not far from the room temperature, and may explain the enhanced propensity of halogen anions for the water–air or water–lipid interfaces with increasing ionic polarizability.