title: Oppositely Charged Ions at Water-Air and Water-Oil Interfaces: Contrasting the Molecular Picture with Thermodynamics.
abstract: The surface-active ions tetraphenylarsonium (Ph4As(+)) and tetraphenylboron (Ph4B(-)) have a similar structure but opposite charge. At the solution-air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph4As(+) compared with Ph4B(-), in addition to opposite alignment of interfacial water molecules. At the water-oil interface, the interfacial tension is 7 mN/m lower for Ph4As(+) than for Ph4B(-) salts, but this can be fully accounted for by the different bulk solubility of these ions in the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measurements.
authors.affiliation: van der Waals-Zeeman Institute, Institute of Physics, University of Amsterdam , Science Park 904, 1098 XH Amsterdam, The Netherlands.
authors.forename: Odile
authors.initials: O
authors.lastname: Carrier
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Ellen H G
authors.initials: EH
authors.lastname: Backus
authors.affiliation: van der Waals-Zeeman Institute, Institute of Physics, University of Amsterdam , Science Park 904, 1098 XH Amsterdam, The Netherlands.
authors.forename: Noushine
authors.initials: N
authors.lastname: Shahidzadeh
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Franz
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Manfred
authors.initials: M
authors.lastname: Wagner
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Nagata
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Mischa
authors.initials: M
authors.lastname: Bonn
authors.affiliation: van der Waals-Zeeman Institute, Institute of Physics, University of Amsterdam , Science Park 904, 1098 XH Amsterdam, The Netherlands.
authors.forename: Daniel
authors.initials: D
authors.lastname: Bonn