Monte Carlo Simulation of NaCl–H₂O Phase Equilibria: Comparison of Classical Force Fields

Abstract

Gibbs ensemble Monte Carlo simulations were performed to predict the pressure−composition phase diagrams of binary H 2 O−NaCl mixtures at 573 K and 633 K, pressures of 4500 to 19,000 kPa, and NaCl mole fractions of zero to 0.3. Understanding the high-temperature behavior of aqueous salt solutions is important for geochemical and industrial applications, including supercritical desalination. We employed both the SPC/E and TIP4P/2005 models for water, as well as the Joung-Cheatham (JC), Dang 1995 (D95), and Kirkwood-Buff (KBFF) force fields for NaCl. In addition, we performed simulations using the Madrid 2019 model for NaCl, which uses scaled charges and is parameterized only for use with TIP4P/2005. To compare the effect of scaling the charges, we also included a scaled charge version of the JC model. SPC/E with either the JC or KBFF NaCl force field results in the most accurate predictions for the pressure−composition diagrams, provided one removes the differences due to the vapor pressure of pure water. TIP4P/2005 with either JC or KBFF yields the best predictions for the liquid densities and partial molar heats of vaporization of water along the phase boundary. Scaling the charges from ±1 to ±0.85 reduces the mole fraction of NaCl in the liquid phase at a given pressure. We find a significant amount of ion pairing for all model combinations, with some notable differences in behavior. The D95 NaCl force field with either water model results in the highest Na + −Cl − coordination numbers, while the scaled charge force fields result in the least amount of ion pairing.