Hofmeister effects in micromolar electrolyte solutions

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 similar to 10 mu M (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 similar to 0.1 mu M in experiments where relative iodide/bromide populations, chi = 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, NH4, and N(C4H9)(4)) and NaY (Y = OH, Cl, NO3, and ClO4) concentrations. We found that (1) all tested electrolytes begin to affect chi below similar to 1 mu M 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704752]