title: Specific ion effects in amphiphile hydration and interface stabilization.
abstract: Specific ion effects can influence many processes in aqueous solutions: protein folding, enzyme activity, self-assembly, and interface stabilization. Ionic amphiphiles are known to stabilize the oil/water interface, presumably by dipping their hydrophobic tails into the oil phase while sticking their hydrophilic head groups in water. However, we find that anionic and cationic amphiphiles adopt strikingly different structures at liquid hydrophobic/water interfaces, linked to the different specific interactions between water and the amphiphile head groups, both at the interface and in the bulk. Vibrational sum frequency scattering measurements show that dodecylsulfate (DS(-)) ions do not detectably perturb the oil phase while dodecyltrimethylammonium (DTA(+)) ions do. Raman solvation shell spectroscopy and second harmonic scattering (SHS) show that the respective hydration-shells and the interfacial water structure are also very different. Our work suggests that specific interactions with water play a key role in driving the anionic head group toward the water phase and the cationic head group toward the oil phase, thus also implying a quite different surface stabilization mechanism.
authors.affiliation: Laboratory for Fundamental BioPhotonics (LBP), Institute of Bio-Engineering (IBI), School of Engineering (STI), École Polytechnique Fédérale de Lausanne (EPFL) , Station 17, CH-1015 Lausanne, Switzerland.
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Scheu
authors.forename: Yixing
authors.initials: Y
authors.lastname: Chen
authors.forename: Hilton B
authors.initials: HB
authors.lastname: de Aguiar
authors.forename: Blake M
authors.initials: BM
authors.lastname: Rankin
authors.forename: Dor
authors.initials: D
authors.lastname: Ben-Amotz
authors.forename: Sylvie
authors.initials: S
authors.lastname: Roke
mesh_terms: D017738:Alkanesulfonic Acids / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D007477:Ions; D008958:Models, Molecular; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D013499:Surface Properties; D013501:Surface-Active Agents / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D017738:Alkanesulfonic Acids; D007477:Ions; D000644:Quaternary Ammonium Compounds; D013501:Surface-Active Agents; D014867:Water; C017273:dodecylsulfonic acid; C022806:dodecyltrimethylammonium