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Supra-additive slowdown of water rotation by outer-sphere ion pairs of densely charged ions

By: Ana Célia Vila Verde
From: Max Planck Institute of Colloids and Interfaces, Potsdam, Germany
At: Faculdade de Ciências, Ed. C8, 8.2.06
[2015-04-09] 11:00

Recent experimental reports indicate that densely charged ions such as magnesium and sulfate can supra-additively slow down the rotation of water molecules much beyond their first hydration shell [1]. These claims are not consistent with other experiments and simulations, which indicate that the effect of ions on water dynamics is limited to their first hydration shell and is mostly additive [2,3]. We address this controversy by investigating the effect of inorganic salts with high and low charge density on the rotational dynamics of water using molecular dynamics simulations and classical, atomistic, polarizable models. Magnesium sulfate and cesium chloride solutions are selected as representative of high and low charge density salts, respectively. The models are parameterized to reproduce the free energy of hydration of the single ions [4] and the activity derivative of solutions at 2.5m concentration, and are thus suitable for studies of water dynamics at high and low concentrations. Our results indicate that static contact or solvent-shared pairs of low charge density ions like cesium and chloride may already alter the rotational dynamics of water beyond what a simple additive model would predict. Because for CsCl these non-additive effects are small and the ion pairs have lifetimes comparable to the rotational dynamics of water, non-additive effects are not visible in the global water dynamics of CsCl solutions. In contrast, the supra-additive delay of the rotation of water molecules directly bridging solvent-shared ion pairs of magnesium and sulfate is intense. Because magnesium sulfate ion pairs are very long lived, the water rotational dynamics of concentrated MgSO₄ solutions can only be understood by explicitly including non-additive effects arising from ion pairs and, at larger concentrations, ion clusters. Our results highlight the connection between static properties of a solution - the concentration of solvent-shared ion pairs - and its dynamic properties, and give insight into the molecular scale mechanism behind it.

[1] Tielrooij et al., H. J., Science, 2010, 328, 1006-1009

[2] Funkner et al., JACS, 2012, 134, 1030-1035

[3] Stirnemann et al., JACS, 2013, 135, 11824-11831

[4] Vila Verde, A., Lipowsky, R. J. Phys. Chem. B, 2013, 117, 10556-10566