Thermodynamic behavior and structural properties of an aqueous sodium chloride solution upon supercooling

Abstract

We present the results of a molecular dynamics simulation study of thermodynamic and structural properties upon supercooling of a low concentration sodium chloride solution in TIP4P water and the comparison with the corresponding bulk quantities. We study the isotherms and the isochores for both the aqueous solution and bulk water. The comparison of the phase diagrams shows that thermodynamic properties of the solution are not merely shifted with respect to the bulk. Moreover, from the analysis of the thermodynamic curves, both the spinodal line and the temperatures of maximum density curve can be calculated. The spinodal line appears not to be influenced by the presence of ions at the chosen concentration, while the temperatures of maximum density curve displays both a mild shift in temperature and a shape modification with respect to bulk. Signatures of the presence of a liquid-liquid critical point are found in the aqueous solution. By analyzing the water-ion radial distribution functions of the aqueous solution, we observe that upon changing density, structural modifications appear close to the spinodal. For low temperatures, additional modifications appear also for densities close to that corresponding to a low density configurational energy minimum.