Abstract
Herein we employ molecular dynamics simulation technique to appreciate the microscopic structural and dynamic changes in the aqueous solution of LiCl when the electrolyte concentration is varied from dilute to high concentration. As a function of the LiCl concentration, we connect changes in the local order, entropy and transport properties and intend to relate them with experimentally determined phase diagram and the glass forming regime of the solution. We observe that a pronounced monotonic decrease in diffusivities of water, Li+ and Cl− is accompanied by an increase in the viscosity with increasing LiCl concentration that marks out a low-mobility regime in approximate agreement with the experimental glass-forming composition region. The fractional Stokes-Einstein relation is obeyed in the temperature regime studied here. The ionic conductivity shows a non-monotonic dependence on composition and pronounced negative deviations from the Nernst-Einstein relation are notable in the glass-forming regime. The breakdown of the hydrogen-bonded network of water as a function of the LiCl concentration along various isotherms is clearly indicated by the tetrahedral order parameter distributions, ensemble-averaged tetrahedral order (⟨qtet⟩) and triplet correlation functions. Further insights into the changes in the structure of the aqueous solution with increasing LiCl concentration is provided using radial distribution functions (RDFs) and corresponding coordination numbers. A useful connection between the structure and thermodynamics is provided by the pair entropy, computed from the atomic pair RDFs. We anticipate that these structure-thermodynamic-transport relations to be qualitatively the same for other alkali halide-water systems. Similarities and differences with binary ionic melts, e.g. LiF-BeF2, MgO-SiO2, with tunable tetrahedrality are also discussed.
Published Version
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