Influence of Ions on Water Diffusion—A Neutron Scattering Study

Abstract

Using quasielastic neutron scattering spectroscopy, we measured the averaged translational diffusion of water in solutions of biologically relevant salts, NaCl, a kosmotrope, and KCl, a chaotrope. The analysis revealed the striking difference in the influence of these ions on water dynamics. While the averaged water diffusion slows down in the presence of the structure making (kosmotrope) Na(+) ion, the diffusion becomes faster in the presence of the structure breaking (chaotrope) K(+) ion. The latter means that, despite strong Coulombic interactions introduced by the K(+) ions, their disruption of the hydrogen-bonding network is so significant that it leads to faster diffusion of the water molecules.

Water and Lipid Bilayers

“…This aggregation of lipid molecules increases the ordering of the acyl chains, and results in thicker bilayers with reduced lipid in-plane mobility. Ions in solution have also been proposed to affect water structure and diffusional rates within the bilayer (Kausik and Han 2011), consistent with what has been observed in the bulk (Ishai et al 2013). Perhaps the most notable example of this is the role of the water-ion complex in the selectivity of ion channels (Doyle et al 1998).…”

The influence of polarizability and charge transfer on specific ion effects in the dynamics of aqueous salt solutions

“…Sodium diffuses slower than potassium and the water in a NaCl solution diffuses slower than in a KCl solution, as is known experimentally. 3,4 For the same ion, models which give slower ion diffusion constants also give slower water diffusion constants. Slower diffusing water is found not only inside but also outside the solvation shell (Fig.…”

“…The translational diffusion constant of water is changed by the addition of salt that strongly depends on the specific cation and only weakly on the anion. 3,4 Sodium salts sharply decrease while potassium, cesium, and rubidium salts do not change or slightly increase the water diffusion constant. The diffusion constant of the cations follows the same general trend, with the diffusion constant increasing going down the alkali group, reaching a maximum at cesium.…”

Membrane Hydration

“…This aggregation of lipid molecules increases the ordering of the acyl chains, and results in thicker bilayers with reduced lipid in-plane mobility. Ions in solution have also been proposed to affect water structure and diffusional rates within the bilayer (Kausik and Han 2011), consistent with what has been observed in the bulk (Ishai et al 2013). Perhaps the most notable example of this is the role of the water-ion complex in the selectivity of ion channels (Doyle et al 1998).…”