title: Surface behavior of hydrated guanidinium and ammonium ions: a comparative study by photoelectron spectroscopy and molecular dynamics.
abstract: Through the combination of surface sensitive photoelectron spectroscopy and molecular dynamics simulation, the relative surface propensities of guanidinium and ammonium ions in aqueous solution are characterized. The fact that the N 1s binding energies differ between these two species was exploited to monitor their relative surface concentration through their respective photoemission intensities. Aqueous solutions of ammonium and guanidinium chloride, and mixtures of these salts, have been studied in a wide concentration range, and it is found that the guanidinium ion has a greater propensity to reside at the aqueous surface than the ammonium ion. A large portion of the relative excess of guanidinium ions in the surface region of the mixed solutions can be explained by replacement of ammonium ions by guanidinium ions in the surface region in combination with a strong salting-out effect of guanidinium by ammonium ions at increased concentrations. This interpretation is supported by molecular dynamics simulations, which reproduce the experimental trends very well. The simulations suggest that the relatively higher surface propensity of guanidinium compared with ammonium ions is due to the ease of dehydration of the faces of the almost planar guanidinium ion, which allows it to approach the water-vapor interface oriented parallel to it.
authors.affiliation: Department of Physics and Astronomy, Uppsala University , P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: Josephina
authors.initials: J
authors.lastname: Werner
authors.forename: Erik
authors.initials: E
authors.lastname: Wernersson
authors.forename: Victor
authors.initials: V
authors.lastname: Ekholm
authors.forename: Niklas
authors.initials: N
authors.lastname: Ottosson
authors.forename: Gunnar
authors.initials: G
authors.lastname: Ohrwall
authors.forename: Jan
authors.initials: J
authors.lastname: Heyda
authors.forename: Ingmar
authors.initials: I
authors.lastname: Persson
authors.forename: Johan
authors.initials: J
authors.lastname: Söderström
authors.forename: Pavel
authors.initials: P
authors.lastname: Jungwirth
authors.forename: Olle
authors.initials: O
authors.lastname: Björneholm
mesh_terms: D064751:Ammonium Compounds / Q000737:chemistry*; D005740:Gases / Q000737:chemistry; D019791:Guanidine / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D056951:Photoelectron Spectroscopy; D012492:Salts / Q000737:chemistry; D013499:Surface Properties; D014867:Water / Q000737:chemistry
chemical_list: D064751:Ammonium Compounds; D005740:Gases; D007477:Ions; D012492:Salts; D014867:Water; D019791:Guanidine