Specific ion effects can be described by the Hofmeister series in aqueous solutions. These effects, however, are significantly at surfaces due to the discontinuity at this interfaces. We show 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 has to be reconsidered.
In this study, the concentration of monovalent inorganic ions as function of the depth, i.e. concentration depth profiles (CDPs), from the vapour-solvent interface in five solvents including the 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 the CDPs of monovalent inorganic ions (Cl-, Br-, I-, Na+, K+, and Cs+) in solutions of all five solvents.
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 the observed phenomena in isolation. The results are described by a multistage approach in which the surface tension of the solvent is the dominant factor.