Dynamic features of water molecules in superconcentrated aqueous electrolytes

An existence of ions dissolved in water has significant effects on bulk properties of water. Superconcentrated conditions have been recently proposed to provide a new concept of lithium ion batteries in order to overcome limitations for practical applications. In those conditions, water would undergo significant changes in structure and dynamics compared to its bulk properties. However, little is known about water in superconcentrated aqueous electrolytes. Here we study the properties of water in aqueous electrolytes with various salt concentrations via molecular dynamics simulations. We find that new dynamic features of water arise in the limit of an extremely high salt concentration. In particular, we observe a decoupled temporal character of water molecules exhibiting a subdiffusive translation and a diffusive rotation in the superconcentrated condition. Furthermore, we find that the rotational dynamics for each principal axis of a water molecule differently responds to the salt concentration, resulting in an occurrence of anisotropy in the rotation as the salt concentration increases. The superconcentrated environments also invoke new features in the hydrogen-bonding characteristics of water such as an emergence of two time scales in the hydrogen bond dynamics of water with respect to the salt concentration.