Polarizable Ions at Interfaces

A nonperturbative theory is presented which allows us to calculate the solvation free energy of polarizable ions near water-vapor and water-oil interfaces. The theory predicts that larger halogen anions are adsorbed at the interface, while the alkali metal cations are repelled from it. The density profiles calculated theoretically are similar to those obtained using molecular dynamics simulations with polarizable force fields.

Figure 1

Ion of radius $a$ at an interface.

Figure 2

Density profiles ($\times {10}^5$) of KI, KBr, and KCl, from left to right. The radius of the water drop is $R=100\AA$ and it contains $N_{+}=N_{-}=380$ ions at physiological concentration of 150 mM. Solid curve is for cation (K) and the dashed ones are for anions. In all cases potassium is depleted from the interface, but there is a formation of a double layer, where the excess of anion directly at the interface results in a build up of potassium in its vicinity.