title: Long-range Hofmeister effects of anionic and cationic amphiphiles.
abstract: Specific ion effects at aqueous interfaces play key roles in many important phenomena. We recently reported that ions interact specifically over unexpectedly long distances on the surface of sub-micromolar electrolyte solutions (Enami et al. J. Chem. Phys. 2012, 136, 154707). Whether the anionic and cationic headgroups of the organic amphiphiles present at most water/hydrophobe interfaces act similarly or display new behaviors, however, is not known. Here we report the results of experiments in which we apply online electrospray ionization mass spectrometry (ESI-MS) to investigate how carboxylate, RCOO(-) (R = CH3, C5H11, C7H15), and alkylammonium, R'(CH3)3N(+) (R' = CH3, C14H29), ions affect the ratio χ = I(-)/Br(-) at the aerial interface of 1 μM (NaI + NaBr) aqueous solutions. We found that χ is systematically but selectively depressed by these ionic amphiphiles and minimally affected by the neutral surfactant 1-octanol. The depressing effects induced by cationic headgroups are stronger than those caused by anionic surfactants and always increase with the length of the alkyl chains.
authors.affiliation: The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302, Japan. enami.shinichi.3r@kyoto-u.ac.jp
authors.forename: Shinichi
authors.initials: S
authors.lastname: Enami
authors.forename: Agustín J
authors.initials: AJ
authors.lastname: Colussi
mesh_terms: D001966:Bromine / Q000737:chemistry; D002264:Carboxylic Acids / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D007455:Iodine / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D002264:Carboxylic Acids; D007455:Iodine; D001966:Bromine