Co-Ion and Ion Competition Effects: Ion Distributions Close to a Hydrophobic Solid Surface in Mixed Electrolyte Solutions

Abstract

We consider within a modified Poisson-Boltzmann theory an electrolyte, with different mixts. of NaCl and NaI, near uncharged and charged solid hydrophobic surfaces. The parametrized potentials of mean force acting on Na+, Cl-, and I- near an uncharged self-assembled monolayer were deduced from mol. simulations with polarizable force fields. We study what happens when the surface presents neg. charges. At moderately charged surfaces, we observe strong co-ion adsorption and clear specific ion effects at biol. concns. At high surface charge densities, the co-ions are pushed away from the interface. We predict that Cl- ions can also be excluded from the surface by increasing the concn. of NaI. This ion competition effect (I- vs. Cl-) may be relevant for ion-specific partitioning in multiphase systems where polarizable ions accumulate in phases with large surface areas.