title: How specific halide adsorption varies hydrophobic interactions.
abstract: Hydrophobic interactions (HI) are driven by the water structure around hydrophobes in aqueous electrolytes. How water structures at hydrophobic interfaces and how this influences the HI was subject to numerous studies. However, the effect of specific ion adsorption on HI and hydrophobic interfaces remains largely unexplored or controversial. Here, the authors utilized atomic force microscopy force spectroscopy at well-defined nanoscopic hydrophobic interfaces to experimentally address how specific ion adsorption of halide ions as well as NH4 (+), Cs(+), and Na(+) cations alters interaction forces across hydrophobic interfaces. Our data demonstrate that iodide adsorption at hydrophobic interfaces profoundly varies the hydrophobic interaction potential. A long-range and strong hydration repulsion at distances D > 3 nm, is followed by an instability which could be explained by a subsequent rapid ejection of adsorbed iodides from approaching hydrophobic interfaces. In addition, the authors find only a weakly pronounced influence of bromide, and as expected no influence of chloride. Also, all tested cations do not have any significant influence on HI. Complementary, x-ray photoelectron spectroscopy and quartz-crystal-microbalance with dissipation monitoring showed a clear adsorption of large halide ions (Br(-)/I(-)) onto hydrophobic self-assembled monolayers (SAMs). Interestingly, iodide can even lead to a full disintegration of SAMs due to specific and strong interactions of iodide with gold. Our data suggest that hydrophobic surfaces are not intrinsically charged negatively by hydroxide adsorption, as it was generally believed. Hydrophobic surfaces rather interact strongly with negatively charged large halide ions, leading to a surface charging and significant variation of interaction forces.
authors.affiliation: Department for Interface Chemistry and Surface Engineering, Max-Planck-Institut f. Eisenforschung GmbH, Max-Planck-Straße 1, D-40237 Düsseldorf, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Stock
authors.affiliation: Department for Interface Chemistry and Surface Engineering, Max-Planck-Institut f. Eisenforschung GmbH, Max-Planck-Straße 1, D-40237 Düsseldorf, Germany.
authors.forename: Melanie
authors.initials: M
authors.lastname: Müller
authors.affiliation: Department for Interface Chemistry and Surface Engineering, Max-Planck-Institut f. Eisenforschung GmbH, Max-Planck-Straße 1, D-40237 Düsseldorf, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Utzig
authors.affiliation: Department for Interface Chemistry and Surface Engineering, Max-Planck-Institut f. Eisenforschung GmbH, Max-Planck-Straße 1, D-40237 Düsseldorf, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Valtiner
mesh_terms: D000327:Adsorption; D064751:Ammonium Compounds / Q000737:chemistry; D002412:Cations / Q000737:chemistry*; D002586:Cesium / Q000737:chemistry; D006219:Halogens / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D018625:Microscopy, Atomic Force; D049329:Nanostructures / Q000737:chemistry; D058525:Quartz Crystal Microbalance Techniques; D012964:Sodium / Q000737:chemistry; D013057:Spectrum Analysis
chemical_list: D064751:Ammonium Compounds; D002412:Cations; D006219:Halogens; D002586:Cesium; D012964:Sodium