Dynamic features of water molecules in superconcentrated aqueous electrolytes

Abstract: 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 significat 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 concentreations via molecular dynamics simulations. We find that new dynamic features of water arise in the limit of an extremely high salt concentration. In the superconcentrated condition, particularly, we observe a decoupled temperal character of water molecules exhibiting a subdiffusive translation and a diffusive rotation. We also find that the rotational dynamics for each principle axis of water molecules differently responds to the salt concentration, resulting in an occurrence of anisotropy in the rotaion 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 short and long scales in the denpendence of the hydrogen bond dynamics on the salt concentration.