Molecular mechanism of water reorientational slowing down in concentrated ionic solutions

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.

Effect of Ions on Water Dynamics in Dilute and Concentrated Aqueous Salt Solutions

“…These experimental results therefore strongly differ from the conclusions of a previous study on sodium and potassium thiocyanate. 12 It was suggested that the increase in viscosity was the main reason for retarded dynamics at high concentrations, a conclusion that was meant to be general for all ion solutions. 12 In contrast, the above-described experimental results support the conclusions of our prior study, 7 where we suggested that the increased viscosity becomes important at high concentration for some specific salts such as sodium perchlorate, but that this does not apply to many other salts where viscosity has a minor effect on water dynamics.…”

“…M), which are likely due to limitations of this simplified description, and do not necessarily affect the applicability of the jump model, in contrast to what was implied in ref 12. …”

“…The frame retardation ρ frame should thus scale with ρ η , as clearly shown in previous studies. 7,12 We had previously suggested that the ion effects on viscosity could actually be understood within the simple microscopic model developed by Eyring, 51 where viscosity scales as…”

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

“…This same argument, however, can be used to reason that the general deceleration does not entirely come from the retarded jump either, since the decelerations in many concentrated electrolytes are larger than Our point in ref. 5, instead, is that at higher concentrations the ions can have nonnegligible effects on both jump and frame components. However, the effect on the jump component can be either acceleration or deceleration (e.g., figure 4 in ref.…”

“…Stirnemann et al (1) claim that the high-concentration regime (>5 M) studied in ref. 5 is not relevant for aqueous batteries or for biological processes. A recent paper (7), however, reported using the aqueous electrolytes up to 21 M in lithium-ion batteries, which enables high-voltage aqueous lithium-ion chemistries.…”