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 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.
Highlights
Water is one of the most important and ubiquitous materials in nature[1,2,3]
Since the properties of water are critical for understanding aqueous electrolytes and the dynamics of water is significantly influenced by an existence of ions[14,16,17,26], it is necessary to explore the properties of water in specific conditions of electrolytes
The effects of ions on the structure and dynamics of water are of importance in a wide range of systems
Summary
Water is one of the most important and ubiquitous materials in nature[1,2,3]. It exhibits many distinct thermodynamic and dynamical properties compared to other liquids. When the hydrogen-bonding network of water is disrupted, one finds that water would exhibit special features different from bulk water. Restricted spatial freedom by nanoconfinement induces the distorted hydrogen-bonding network, so that nanoconfined water exhibits many different properties from bulk water[6,8,9]. One of the main issues in this field is how different ions would give perturbations on the HB network of bulk water, generating peculiar structural and dynamic properties of water in electrolyte solutions[12,13,14]. Recent studies have shown that ions can enhance or suppress the dynamics of water in the presence of different ions with the subtle change in the structure of water[16,17]. Motivated by the recent experiments[22,23,24], we study the dynamics of water in superconcentrated aqueous electrolytes for the first time
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