Getting Specific About Specific Ion Effects

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Science  29 Feb 2008:
Vol. 319, Issue 5867, pp. 1197-1198
DOI: 10.1126/science.1152799

Have you noticed that “lite” salt, which is a mixture of KCl and NaCl, tastes slightly different from ordinary table salt, which is essentially pure NaCl? If so, then you have experienced a specific ion effect. Such effects are ubiquitous in chemical and biochemical processes involving salt solutions and have traditionally been attributed to the influence of the salt ions on the structure of water. Yet, a surge of recent research has provided compelling evidence that we should instead think about these phenomena in terms of specific ion interactions with surfaces and influences on hydrophobic interactions (15).

In the 1880s, Hofmeister and co-workers investigated the relative ability of different salts to precipitate proteins from blood serum and egg whites (6). The work resulted in the following ranking for anions: SO42− >F > HPO42- > CH3COO > Cl > Br > NO3 > I > ClO4 > SCN. Ions on the left side of this Hofmeister series salt out (precipitate) solutes, whereas ions on the right salt in (dissolve or denature) solutes. An analogous series can be constructed for cations. Similar trends have been found in many solution properties (7, 8), including surface tensions, chromatographic selectivity, colloid stability, and protein denaturation temperatures. It is widely held that Hofmeister series reflect specific ion effects on the long-range structure of water: Ions on the left are structure makers, ions on the right structure breakers.

Two recent studies (1, 2) mount a strong case against this structure maker/breaker concept. Smith et al. (1) analyzed Raman spectra of water OH vibrations in potassium halide solutions. The position of the band centers and the line shapes of OH vibrational spectra are sensitive to details of the hydrogen-bonding network. Spectra of fluoride solutions are slightly blue-shifted compared with neat water, whereas solutions of the heavier halides are red-shifted. These effects were previously explained in terms of the structure-making and -breaking abilities of these ions. Using Monte Carlo simulations, Smith et al. show that the different halide ions do produce spectroscopically distinct changes to water hydrogen bonding, but these perturbations are largely confined to the first solvation shell. This result is consistent with an earlier spectroscopic study of the dynamics of halide ion solvation shells (9).

Mancinelli et al. found contradictions in the structure maker/breaker concept in a neutron diffraction study of NaCl and KCl solutions (2). According to the conventional Hofmeister series for cations, both Na+ and K+ are water-structure breakers. Yet, the analysis of the diffraction data suggested that, whereas water molecules are more orientationally disordered around the K+ ion, Na+ is more tightly solvated and more disruptive to water-water correlations.

Clues to more accurate explanations for specific ion effects are emerging from studies of the behavior of ions near interfaces. For example, molecular dynamics simulations have predicted that the propensity for anions to adsorb to the air-water interface follows an inverse Hofmeister series (SO42− < NO3 < I) (see the first figure) and is correlated with specific ion effects on surface tensions and surface potentials (10). These predictions have been confirmed by spectroscopic measurements (1114). The results suggest that specific ion effects could reflect differences in the hydration of ions near surfaces (for example, the air-water interface or biomolecular surfaces) compared to the bulk solution.

Specific ion effects at the air-water interface.

In snapshots from molecular dynamics simulations of air-solution interfaces of aqueous solutions (10, 11, 18), some ions accumulate near the interface, whereas others avoid the interface. Atom colors: Water O, blue; Na, green; S, magenta; sulfate O, orange; N, cyan; nitrate O, pink; I, purple.

This hypothesis was supported by recent studies. In a thermodynamic surface-bulk partitioning model, Pegram and Record (3) have found a good correlation between the propensity of ions to adsorb to the air-water interface and the effect of these ions on processes that involve protein surface hydration. Consistent with this analysis, Chen et al. have provided experimental evidence for changes in the hydration of ions when they interact with protein-like polymers at the air-water interface (4). In the case of cations, Pegram and Record did not find a similar correlation, probably because cations do not adsorb to the bare air-water interface (3). Inorganic cations are generally excluded from the air-water interface (10) and sit below the surface even in the presence of strongly adsorbing anions, such as I (see the first figure, bottom panel). However, recent x-ray photoemission experiments and molecular dynamics simulations have shown that surfactants containing polar groups can draw cations into the interfacial layer (15) (see the second figure).

Specific interactions between ions and surfactants (15).

The large enhancement of anions over cations at the interface in neat aqueous NaI (see the first figure, bottom panel) is suppressed when the surfactant butanol is present (this figure). Coloring as in the first figure, except butanol C, black; butanol O, red.

About 10 years ago, Baldwin argued that specific ion effects on protein stability could be described in terms of the ability of ions to salt in the polar peptide group and salt out the nonpolar side chains (16). At first glance, it seems plausible that salting-out ions strengthen hydrophobic interactions, whereas salting-in ions decrease them. Yet, a recent study suggests that specific ion effects on hydrophobic interactions are more complicated (5). In simulations of hydrophobic solutes in salt solutions, the propensity of the solutes to aggregate (salt out) increased with increasing ion charge density, as expected. However, ions of low charge density could induce salting out or salting in, depending on concentration. Low-charge-density ions can form complexes with the hydrophobic species, forming micelle-like aggregates at low ion concentration and soluble, surfactant-like complexes at high ion concentration. Thus, salt effects on hydrophobic interactions result from a subtle balance of ion-solute and ion-water interactions.

The existence and widespread applicability of Hofmeister series suggest an underlying simplicity. Yet, specific ion effects continue to defy all-encompassing theories. Fruitful directions for further research include the exploration of salt effects on hydrophobic interactions and the characterization of specific ion interactions with polar groups. For complex solutes such as proteins in salt solutions, hydrophobic and electroselective interactions appear to be intertwined (17). Techniques are available for interface-specific experiments that directly probe specific ion interactions, coupled with appropriate theory and simulations. It looks like scientists will remain immersed in salt water for some time.

References and Notes

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