Density of deeply supercooled alkali chloride aqueous solutions: Experimental and simulation results

The density of supercooled aqueous solutions of lithium, sodium, and potassium chloride was experimentally determined at atmospheric pressure down to -60 °C using dilatometry. To avoid freezing of the solutions, they were dispersed in a hydrophobic matrix, forming an emulsion, which inhibits heterogeneous nucleation. The experiments were conducted for solutions with concentrations up to the solubility limit under ambient conditions (NaCl and KCl) or up to the eutectic (LiCl). The temperature of maximum density and the apparent molar volume were calculated from density data. Molecular dynamics simulations were carried out for these systems within the same temperature and concentration ranges, using the TIP4P/2005 and Madrid-2019 force fields. Simulation and experimental results are critically compared, evaluating the performance of the model to reproduce the experimental results. Good agreement is obtained, a fact that avails the use of the model to study the structure of these solutions. This is performed by analyzing a set of radial distribution functions and the angular distributions of water molecules with respect to ions. The structural comparison among the cations indicates that Na+ and K+ salts share similar solvation patterns, while Li+ shows a distinct configuration, characterized by a tetragonal arrangement of water molecules around the ion that resembles that observed in solid-state environments. This finding aligns with the experimental results, since the analyses of the temperature of maximum density and apparent molar volume reveal that LiCl deviates from the tendencies observed for NaCl and KCl.