An ab initio approach to understanding the specific ion effect

Abstract

Although there is a consensus that large, polarizable anions will adsorb to the air–water interface, the precise interactions that give rise to surface enhancement are still being debated. Previously, we have demonstrated that there is a significant dependence on the choice of molecular interaction potential for I⁻ adsorption at the air–water interface. Specifically, density functional theory (DFT) based interaction potentials lead to significantly less adsorption than empirical interaction potentials that include polarization. We have also demonstrated that DFT based interaction potentials can accurately capture the structure of the first solvation shell of both simple and polyatomic anions as compared to multi-edge X-ray absorption fine structure (XAFS) experiments. We utilize DFT to examine the local hydration structure of SCN⁻ and IO⁻₃ that exhibit both chaotropic and kosmotropic characteristics. We compare and contrast the solvation structure of the polyatomic anions with the series of halide anions. A picture emerges where we can correlate local solvation structure to the ions’ position in the Hofmeister series.