Ion specificity at solvent surfaces: concentration depth profiles of monovalent inorganic ions

Specific ion effects that are commonly discussed in terms of the Hofmeister series in aqueous solutions are perturbed in number of ways, including the solvent identity, more so at interfaces. We hypothesise that through a careful investigation and comparison of the distribution of ions at the vapour-solvent interface for a range of solvents, our conceptual understanding of the adsorption of ions at surfaces can be improved.

In this study, the relative concentration of monovalent inorganic ions as function of the depth depth from the vapour-solvent interface in four nonaqueous solvents, propylene carbonate (PC), benzyl alcohol (BA), glycerol and formamide (FA) are investigated. Neutral impact collision ion scattering spectroscopy (NICISS) is used to directly measure these concentration depth profiles (CDPs) of monovalent inorganic ions (Cl^-, Br^-, I^-, Na⁺, K⁺, and Cs⁺) in solution.

The distribution of inorganic ions at the vapour-solvent interface is strongly solvent dependent. Concepts often used for explaining specific ion effects such as solvated ion size, ion polarisability, desolvation energy, the law of matching affinity, electrostatic and dispersion interactions are not able to describe in isolation the observed phenomena presented here. The results are described by a multistage approach in which the surface tension of the solvent is the dominant factor.