Thermodynamic potentials of NaCl(aq) between 0 - 0.7 GPa and 250 - 340 K.

Sodium and chloride are among the major ions expected in the aqueous chemistry of the outer solar system, and more specifically in the subsurface oceans of the large icy moons of Jupiter and Saturn. In that context, a representation of the thermodynamic potentials of NaCl bearing solutions is required to predict the dynamic liquid-ice equilibria taking place inside these moons. The properties of NaCl(aq), however, have only been coarsely explored in the 0 - 1.5 GPa pressure range (at the melting temperature of water ices) relevant to the hydrospheres of these bodies. As a second derivative of the Gibbs energy, the speed of sound offers a way to obtain the thermodynamic potentials of fluids. Using a high-pressure system previously developed at the University of Washington, we acquired new sound speeds in NaCl(aq) over the 0 - 0.7 GPa, 250 - 340 K, and 0 - 5 molal ranges. Previous experiments with pure water illustrated a precision of our measurements of 0.02%, with deviations to IAPWS within its claimed 0.3% accuracy at these pressures. Integration of the sound speeds gives precisions of 0.01% for the Gibbs energy, 0.02% for the density, and 0.5% for the heat capacity, allowing calculation of liquid-ice equilibria to within hundredths of a degree. We will present our new NaCl(aq) sound speed dataset and its corresponding integrated Gibbs energy surface (and derivatives) over the 0 - 0.7 GPa, 250 - 340 K, and 0 - 5 molal ranges. We will compare our results to available literature data and discuss the revision our data suggest to the current state of knowledge of NaCl(aq). These results will be used to illustrate the implications of our achieved accuracy for the calculation of liquid-ice equilibria under pressure.