Toward an Understanding of the Specific Ion Effect Using Density Functional Theory

Although it is now accepted that some anions adsorb at the air water interface following a reverse Hofmeister series [Kunz, W.; Hen le, J.; Ninham, B. W. Curr. Opinion Colloid Interface Sci. 2004, 9, 19-37; Tobias, D. J.; Hemminger,. C. Science 2008, 319, 1197-1198], the nature of the microscopic interactions driving ions to interfaces is currently the subject of active research. We use extensive density functional theory (DFT)-based interaction potentials to study the free energy of transfer of iodide from the interior to the surface in both a cluster and the extended air-water interface. Our research supports a picture that empirical polarizable interaction potentials may overestimate surface adsorption for iodide. These results, in conjunction with previous theoretical and experimental studies on iodide solvation, have implications toward the necessary interactions that give rise to the specific ion effect at the air water interface.