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.
Keywords: femtosecond infrared; ion specificity; ionic solution; structure dynamics; water rotation.