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Published April 21, 2012 | Published
Journal Article Open

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_4H_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.

Additional Information

© 2012 American Institute of Physics. Received 30 December 2011; accepted 3 April 2012; published online 19 April 2012. This work was financially supported by the Japan Science and Technology Agency (JST) PRESTO program. We are grateful to Dr. Drew Parsons and Professor Barry Ninham for a helpful discussion. S.E. is grateful to Professor K. Takahashi, Professor M. Shiotani, and Ms. A. Hatano of Kyoto University for assisting the startup of a new laboratory. Valuable discussions with Professor William Goddard and Dr. Tod Pascal of California Institute of Technology are also acknowledged.

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