Phase Diagram of Aqueous Solutions of LiCl: a Study of Concentration Effects on the Anomalies of Water

We perform molecular dynamics simulationsin order tostudy thermodynamicsand the structure of supercooled aqueous solutions of lithium chloride(LiCl) at concentrations c = 0.678 and 2.034 mol/kg.We model the solvent using the TIP4P/2005 potential and the ions usingthe Madrid-2019 force field, a force field particularly suited forstudying this solution. We find that, for c = 0.678mol/kg, the behavior of the equation of state, studied in the P-T plane, indicates the presenceof a liquid-liquid phase transition, similar to what was previouslyfound for bulk water. We estimate the position of the liquid-liquidcritical point to be at T (c) approximate to 174K, P (c) approximate to 1775 bar, and rho(c) approximate to 1.065 g/cm(3). When the concentration istripled to c = 2.034 mol/kg, no critical point isobserved, indicating its possible disappearance at this concentration.We also study the water-water and water-ions structurein the two solutions, and we find that at the concentrations examinedthe effect of ions on the water-water structure is not strong,and all the features found in bulk water are preserved. We also calculatethe hydration number of the Li and Cl ions, and in line with experiments,we find the value of 4 for Li+ and between 5.5 and 6 forCl(-), confirming the good performances of the Madrid-2019force field.

Automated summary

In this study, molecular dynamics simulations were used to investigate the thermodynamics and structure of supercooled aqueous solutions of lithium chloride (LiCl) at concentrations c = 0.678 and 2.034 mol/kg. The presence of a liquid-liquid phase transition was observed at c = 0.678 mol/kg, similar to bulk water. The estimated position of the liquid-liquid critical point was T (c) ≈ 174K, P (c) ≈ 1775 bar, and ρ(c) ≈ 1.065 g/cm(3). No critical point was observed when the concentration was tripled to c = 2.034 mol/kg, indicating a possible disappearance of the critical point at this concentration. The structure of water-water and water-ions in the solutions was also studied, and it was found that the effect of ions on the water-water structure was not significant, preserving the features of bulk water. The hydration number of Li and Cl ions was calculated, and the results matched experimental findings, confirming the good performance of the Madrid-2019 force field.