Distribution of Hydrophobic Ions and Their Counterions at an Aqueous Liquid−Liquid Interface:  A Molecular Dynamics Investigation

We report a molecular dynamics study on the distribution of spherical hydrophobic ions S + and S - (radius ≈ 5.5 Å) and hydrophilic counterions (halide X -; alkali M + ) at a water−“oil” interface, where “oil” is modeled by chloroform. The results reveal the surface activity of S + and S -, with marked counterion effects. The S + S - salt fully adsorbs at the interface, which is electrically neutral, while in the S + X - series, the anion concentration near the interface decreases in the Hofmeister order I - > Br - > Cl - > F -, thus increasing the change in interfacial electrostatic potential Δφ. A similar effect is observed with the S - M + salts, when Cs + is compared to Na + . We also investigate the effect of ion charge sign reversal, and find a larger Δφ for S + Na - than S - Na + salts, in relation with the higher hydration of the fictitious Na - anion compared to the isosteric Na + cation. The effect of the magnitude of the ion charge is studied with the divalent S 2+ vs S 2- ions and Na - vs Na + counterions. Despite their mutual repulsion, the S 2+ or S 2- like-charged species tend to self-aggregate at the interface and in water as a result of hydrophobic association and, again, differences in distributions are observed upon sign reversal. With regard to the treatment of electrostatics, the Ewald and Reaction Field methods qualitatively yield similar trends, but the latter underestimates the repulsion between like ions at the interface and thus exaggerates the calculated difference in interfacial potential Δφ. When compared to standard calculations, our results point to the importance of the treatment of cutoff boundaries on the distribution of hydrophilic counterions near the interface. Implications of these results concerning the mechanism of assisted ion transfer are discussed.