Osmolytic Co-Solute Perturbing the Surface Enhancement of Halide Ions

Abstract

We have investigated the variation in the surface binding free energy with the choice of halide ion, F^–, Cl^–, Br^–, and I^–, in water–glycerol binary mixtures with varying glycerol concentrations using umbrella sampling with a polarizable force field. We have found that halide surface adsorption is significantly perturbed by glycerol. At no or low glycerol concentration, the surface preference follows the Hofmeister series (I^– > Br^– > Cl^– > F^–). However, at the highest concentration, Br^– is preferentially stabilized. Decomposition of the free energy indicates that the halide surface adsorption is dominated by enthalpy and, specifically, by the solvent–solvent polarization interaction. The differences in this interaction between the chaotropic halides are reduced by glycerol addition, which is in line with a recent measurement of the solvent excess enthalpy for the same systems studied here. Moreover, our calculations indicate that the effect of an osmolyte (glycerol) on surface ion concentrations parallels the known effect of osmolytes on protein folding