Effects of Hydration Ability of Cations on Water Reorientation Dynamics

Water structures and dynamics in ionic solutions play an important role in a plethora of chemical, biochemical, and environmental processes, and they are greatly affected by the hydration degree of ions. Unfortunately, the effects of cations with different degrees of hydration on water properties have hitherto been poorly understood. Here, we investigate how the structures and dynamics of water molecules are affected by typical cations with strong Mg²⁺ and week Cs⁺ hydration abilities using molecular dynamics simulations. We analyze the simulated data by the extended jump model that decomposes the reorientation of water molecules into two different processes (jump reorientation and frame reorientation). The results indicate that although both cations retard the two types of reorientations of water molecules, Mg²⁺ shows much stronger influences than Cs⁺. We elucidate the underlying mechanisms by our newly developed theoretical models and provides an insightful molecular picture of water structures and dynamics surrounding cations.