title: The influence of concentration on the molecular surface structure of simple and mixed aqueous electrolytes.
abstract: We investigate various mechanisms contributing to the surface ion distributions in simple and mixed aqueous alkali-halide solutions depending on the total salt concentration, using a combination of photoelectron spectroscopy and molecular dynamics simulations. In simple solutions, the surface enhancement of large polarizable anions is reduced with increasing concentration. In the case of a NaBr/NaCl mixed aqueous solution, with bromide as the minority component, the situation is more complex. While the total anion/cation charge separation is similarly reduced with increasing salt content, this alone does not uniquely determine the ion distribution due to the co-existence of two different anions, Br(-) and Cl(-). We show that bromide is selectively surface enhanced at higher concentrations, despite the fact that the total anion surface enhancement is reduced. This phenomenon, which can be viewed as "salting out" of bromide by NaCl might have consequences for our understanding of the surface structure of mixed aqueous solutions subjected to concentration increase due to dehydration, such as seawater-born aerosols.
authors.affiliation: Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden. niklas.ottosson@fysik.uu.se
authors.forename: Niklas
authors.initials: N
authors.lastname: Ottosson
authors.forename: Jan
authors.initials: J
authors.lastname: Heyda
authors.forename: Erik
authors.initials: E
authors.lastname: Wernersson
authors.forename: Wandared
authors.initials: W
authors.lastname: Pokapanich
authors.forename: Svante
authors.initials: S
authors.lastname: Svensson
authors.forename: Bernd
authors.initials: B
authors.lastname: Winter
authors.forename: Gunnar
authors.initials: G
authors.lastname: Ohrwall
authors.forename: Pavel
authors.initials: P
authors.lastname: Jungwirth
authors.forename: Olle
authors.initials: O
authors.lastname: Björneholm
mesh_terms: D000468:Alkalies / Q000737:chemistry; D004573:Electrolytes / Q000737:chemistry*; D006219:Halogens / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D013499:Surface Properties; D014867:Water / Q000737:chemistry*
chemical_list: D000468:Alkalies; D004573:Electrolytes; D006219:Halogens; D014867:Water