title: Hofmeister effects in micromolar electrolyte solutions.
abstract: Ions induce both specific (Hofmeister) and non-specific (Coulomb) effects at aqueous interfaces. More than a century after their discovery, the origin of specific ion effects (SIE) still eludes explanation because the causal electrostatic and non-electrostatic interactions are neither local nor separable. Since direct Coulomb effects essentially vanish below ~10 μM (i.e., at >50 nm average ion separations in water), we decided to investigate whether SIE operate at, hitherto unexplored, lower concentrations. Herein, we report the detection of SIE above ~0.1 μM in experiments where relative iodide∕bromide populations, χ = I(-)/Br(-), were determined on the surface of aqueous (NaI + NaBr) jets by online electrospray mass spectrometry in the presence of variable XCl (X = H, Na, K, Cs, NH(4), and N(C(4)H(9))(4)) and NaY (Y = OH, Cl, NO(3), and ClO(4)) concentrations. We found that (1) all tested electrolytes begin to affect χ below ~1 μM and (2) I(-) and Br(-) are preferentially suppressed by co-ions closely matching their interfacial affinities. We infer that these phenomena, by falling outside the reach of even the longest ranged electrostatic interactions, are dynamical in nature.
authors.affiliation: The Hakubi Center, Kyoto University, Kyoto 606-8302, Japan. enami.shinichi.3r@kyoto-u.ac.jp
authors.forename: Shinichi
authors.initials: S
authors.lastname: Enami
authors.forename: Himanshu
authors.initials: H
authors.lastname: Mishra
authors.forename: Michael R
authors.initials: MR
authors.lastname: Hoffmann
authors.forename: Agustín J
authors.initials: AJ
authors.lastname: Colussi
mesh_terms: D000388:Air; D001965:Bromides / Q000737:chemistry*; D004573:Electrolytes / Q000737:chemistry*; D007454:Iodides / Q000737:chemistry*; D012996:Solutions; D013499:Surface Properties; D014867:Water / Q000737:chemistry
chemical_list: D001965:Bromides; D004573:Electrolytes; D007454:Iodides; D012996:Solutions; D014867:Water