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 ~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.