THERMODYNAMIC PROPERTIES OF AQUEOUS SOLUTIONS AT HIGH PRESSURE FROM -20°C TO 400°C

The interactions between aqueous solutions and rocks from Earth’s surface to sub-crustal depths and in the interiors of outer solar system icy worlds provide important controls on the state and evolution of these planetary bodies. We provide a framework for the acquisition and description of high-pressure thermodynamic properties. This work improves determinations of phase equilibria and solubilities in multicomponent systems at high pressure. Although the thermodynamics of aqueous solutions are reasonably well characterized under near ambient conditions, data remain sparse in critical regimes of higher pressure (greater than 100 MPa) and both at low temperatures (deep icy world ocean conditions) and high temperatures (lithospheric conditions). We are extending measurements of sound speeds in solutions and determine Gibbs’ free energy as a function of pressure, temperature, and composition in new regimes of pressure and temperature. Below 1 GPa (1000 MPa), where pressure and temperature derivatives are large, we use conventional ultrasonic methods in a pressure vessel allowing precise control of both temperature (<0.1°C between -30°C and 200°C) and pressure (<0.2 MPa to 700 MPa). Sound speed measurements using impulsively stimulated light scattering are undertaken in both conventional pressure vessels with sapphire windows and (to much higher pressures) in an externally heated diamond-anvil cell. Results for pure water, MgSO_4(aq), Na₂SO_4(aq), and ammonia-water mixtures provide insights on their volumetric behavior. For the ionic solutions, where the partial molar volume at infinite dilution, V_o, is dominated by electrostriction at low pressure, the initial pressure derivative of V_o is large. At high pressure, where V_o is related to the “size” of the ions, it is only weakly pressure dependent. The non-ideal behavior of the sulfate solutions over an extended range of pressure and temperature is described using a standard three-term parameterization representing solvent (Debye-Huckel), solvent-ion, and ion-ion interactions. The last two terms show less dependence on pressure and temperature than V_o or the Debye-Huckel term. The non-ideal behavior in all solutions is suppressed at higher pressures.