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

TL;DR

This study uses molecular dynamics simulations to explore how supercooling affects the thermodynamic and structural properties of dilute sodium chloride solutions in water, revealing shifts in density maxima and signs of a liquid-liquid critical point.

Contribution

It provides new insights into the supercooling behavior of saline solutions, highlighting differences from bulk water and identifying potential critical phenomena.

Findings

Spinodal line unaffected by ions at studied concentration

Temperature of maximum density shows mild shift and shape change

Evidence of a liquid-liquid critical point in the solution

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 analysing 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.