Anions of Alkali Halide Salts at Surfaces of Formamide Solutions: Concentration Depth Profiles and Surface Topography

Abstract

Previous computer simulations and surface-selective spectroscopic experiments indicated non-monotonic concentration depth profiles for soft inorganic ions (such as iodide) at the air/water interface while this surface effect was strongly quenched in methanol. Here, we investigate by means of neutral impact collision ion scattering spectroscopy and molecular dynamics simulations the surface propensity of anions of alkali halide iodide salts in formamide. By combination of experiments and calculations, we extract both the ion concentration depth profiles at the interface and the local solvent topography around the ions in or near the topmost layer of the liquid. We show that formamide, as a nonaqueous polar solvent containing no hydrophobic groups, exhibits an enhanced surface coverage of soft inorganic ions like iodide and that the ions are located in the outermost layer in valleylike structures. The non-monotonic interfacial concentration profile and surface propensity of iodide in formamide is quantified and compared to that in water.