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 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 ∼0.1 M in experiments where relative iodidebromide populations, 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 ₄H ₉) ₄) and NaY (Y OH, Cl, NO ₃, and ClO ₄) concentrations. We found that (1) all tested electrolytes begin to affect below ∼1 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.