Thermodynamic properties of aqueous MgSO4 to 800 MPa at temperatures from −20 to 100 °C and concentrations to 2.5 mol kg−1 from sound speeds, with applications to icy world oceans

Thermodynamic properties of aqueous magnesium sulfate solutions (MgSO4 concentrations to 2.5 mol kg−1) are reported in a previously unexplored regime of pressure and temperature. Solution densities (to a few hundred parts-per-million), specifics heats (to a few percent), and volumetric mixing parameters were determined to 800 MPa in the temperature range from −20 to 100 °C from sound speeds measurements. Equilibrium data extrapolate smoothly to temperatures below which liquids are stable, providing a basis for equilibrium freezing calculations. In more compressed water at high pressure, where electrostrictive effects are smaller, the partial molal volume at infinite dilution is positive and changes less with pressure, while the non-ideal contribution to the apparent molal volume is reduced. Ion–solvent and ion-ion contributions are small under all conditions, while solvent contributions to non-ideality show greater variation with pressure and temperature. In application to Ganymede, Callisto, and Titan, the current results suggest that concentrations of aqueous MgSO4 would be exist that would be denser than overlying ice and thus buoyantly stable at the ice VI-rock interface, or between overlying layers of ice VI–V or V–III. More generally, the current data and analysis provide a comprehensive framework that can guide investigations of other single and multi-component aqueous systems.