Molecular mechanism of water reorientational slowing down in concentrated ionic solutions

Qiang Zhang; TianMin Wu; Chen Chen; Shaul Mukamel; Wei Zhuang

doi:10.1073/pnas.1707453114

New Research In

Contributed by Shaul Mukamel, August 10, 2017 (sent for review May 8, 2017; reviewed by Huib J. Bakker and Dongping Zhong)

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Water dynamics in concentrated electrolytes: Local ion effect on hydrogen-bond jumps rather than collective coupling to ion clusters - May 10, 2018

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Reply to Stirnemann et al.: Frame retardation is the key reason behind the general slowdown of water reorientation dynamics in concentrated electrolytes
- May 10, 2018

Significance

The dynamics of water molecules surrounding the hydrated ions affects many natural phenomena including protein processes and charge transfer in the aqueous rechargeable ion batteries. In the concentrated solutions, a long-standing puzzle is that all electrolytes retard water rotation regardless of whether they weaken or strengthen the water hydrogen-bonding network. We investigate this issue theoretically and find the deceleration to be largely due to the coupling of the slow, collective component of water rotation with the motion of sizable ion clusters in the concentrated solutions. This finding is at variance with the intuitive expectation that the deceleration is caused by the change in fast, single-molecular water hydrogen bond switching adjacent to the ions.

Abstract

Water dynamics in concentrated ionic solutions plays an important role in a number of material and energy conversion processes such as the charge transfer at the electrolyte–electrode interface in aqueous rechargeable ion batteries. One long-standing puzzle is that all electrolytes, regardless of their “structure-making/breaking” nature, make water rotate slower at high concentrations. To understand this effect, we present a theoretical simulation study of the reorientational motion of water molecules in different ionic solutions. Using an extended Ivanov model, water rotation is decomposed into contributions from large-amplitude angular jumps and a slower frame motion which was studied in a coarse-grained manner. Bearing a certain resemblance to water rotation near large biological molecules, the general deceleration is found to be largely due to the coupling of the slow, collective component of water rotation with the motion of large hydrated ion clusters ubiquitously existing in the concentrated ionic solutions. This finding is at variance with the intuitive expectation that the slowing down is caused by the change in fast, single-molecular water hydrogen bond switching adjacent to the ions.

Footnotes

↵¹To whom correspondence may be addressed. Email: wzhuang{at}fjirsm.ac.cn or smukamel{at}uci.edu.

Author contributions: W.Z. designed research; Q.Z., C.C., and W.Z. performed research; T.W., S.M., and W.Z. analyzed data; and W.Z. wrote the paper.
Reviewers: H.J.B., Fundamental Research on Matter Institute for Atomic and Molecular Physics; and D.Z., Ohio State University.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1707453114/-/DCSupplemental.

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