Expulsion of ions from hydrophobic hydration shells.

Raman spectroscopy is combined with multivariate curve resolution to quantify interactions between ions and molecular hydrophobic groups in water. The molecular solutes in this study all have similar structures, with a trimethyl hydrophobic domain and a polar or charged headgroup. Our results imply that aqueous sodium and fluoride ions are strongly expelled from the first hydration shells of the hydrophobic (methyl) groups, while iodide ions are found to enter the hydrophobic hydration shell, to an extent that depends on the methyl group partial charge. However, our quantitative estimates of the corresponding ion binding equilibrium constants indicate that the iodide concentration in the first hydrophobic hydration shell is generally lower than that in the surrounding bulk water, and so an iodide ion cannot be viewed as having a true affinity for the molecular hydrophobic interface, but rather is less strongly expelled from such an interface than fluoride.