Solute-solvent interactions in supercritical water: density-, and temperature-dependent effects on the Krichevskii parameter of different monovalent salts

Reliable models describing chemical equilibria of aqueous fluids at high temperatures (T) and pressures (P) are crucial to several areas of the geoscience, ranging from problems in ore deposit geology to the use of geothermal energy systems.

Despite this importance, to date there is no model that can accurately predict the thermodynamic properties of supercritical aqueous fluids at and near the critical point (CP) of water.

This is due to the strongly divergent nature of certain thermodynamic properties of solutes, such as partial molar volumes (PMV), whose derivatives approach infinity at the critical point and are therefore difficult to constrain experimentally. Therefore, it is important to find ways to avoid direct treatment of anomalously behaving solution properties under CP conditions.

In this context, the Krichevskii parameter AKr is considered a promising key variable, which is closely related to the PMV of a solute and is known to describe a smooth function even in the vicinity of the CP of water. In an effort to better understand the behavior of AKr of different monovalent salts, we perform classical molecular dynamics (MD) simulations of supercritical aqueous solutions in a wide range of temperatures and densities. We describe the effect of solute type, T and water density on the behavior of AKr and draw connections to the molecular structure of water under the respective conditions. Our observations contribute to a deeper understanding of solute-solvent interactions under supercritical conditions and serve as part of a molecular-level guide for the development of future thermodynamic models.