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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry