Ion specificity in the measured concentration depth profile of ions at the Vapor-Glycerol interface

Abstract

Hypothesis: Specific ion effects are manifest universally across many systems and solvents. Whilst broad understanding of these effects is emerging particularly for bulk effects, the perturbation introduced by the interfaces are generally not understood. We hypothesise that through a careful investigation of the distribution of ions at the glycerol-vapor interface we can better understand specific ion effects in this system and at interfaces.

Experiments: Neutral impact collision ion scattering spectroscopy (NICISS) is used to obtain and compare individual ion concentration depth profiles (CDP) for a range of monovalent inorganic anions and cations at 12 glycerol electrolyte solutions surfaces.

Findings: The distribution of ions at the vapor - glycerol interface is non-monotonic. Broadly, anions are concentrated at the outermost region of the interface and cations are depleted from the interface. The distribution of Cl^- and I^- is mostly independent of the counterion. However, for Br^- ions the distribution depends on the counterion where Cs⁺, K⁺, and Na⁺ ions lead to a desorption of Br^- ions from the interface. This is favoured by the large solvation energy of Br^- ions and consistent with the law of matching effective ion sizes.

Keywords: Anion-cation interactions; Concentration depth profiles; Neutral impact collision ion scattering spectroscopy; Specific ion effects; Surface tension increment; Vapour-Glycerol interface.