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, NH4, and N(C4H9)4) and NaY (Y = OH, Cl, NO3, and ClO4) 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.
- Publication:
-
Journal of Chemical Physics
- Pub Date:
- April 2012
- DOI:
- 10.1063/1.4704752
- Bibcode:
- 2012JChPh.136o4707E
- Keywords:
-
- electrolytes;
- jets;
- mass spectroscopy;
- separation;
- sodium compounds;
- surface chemistry;
- 82.45.Gj;
- 82.80.Ms;
- Electrolytes;
- Mass spectrometry