PNNL

Abstract

Surface segregation of iodide, but not of fluoride or cesium ions, is observed by a combination of metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS(HeI)) of amorphous solid water exposed to CsI or CsF vapor. The same surface ionic behavior is also derived from molecular dynamics (MD) simulations of the corresponding aqueous salt solutions. The MIES results provide first direct evidence for the suggested presence of heavier halides (iodide, bromide, and to a lesser extent chloride) at aqueous surfaces, and confirm the structural similarity between the amorphous solid and the corresponding liquid. In contrast, no appreciable surface segregation of ions is observed in methanol, neither in the experiment nor in the simulation, which points to the remarkable surface properties of water among polar solvents. The propensity of heavier halides for the air/solution interface has important implications for heterogeneous chemical processes, particularly on aqueous aerosols in the atmosphere. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy under contract DE-AC05-76RL01830.