Molecular dynamics simulations of aqueous NaCl solutions at high pressures and temperatures

Molecular dynamics simulations have been performed on aqueous NaCl solutions over a range of concentrations, pressures and temperatures. At 300 K and 1 bar, simulated solution densities are in very good agreement with the experimental data at concentrations from 0.2 m to 5.2 m. Simulated densities at 1 and 5 kbars with a 1.74 m concentration solution are also in very good agreement with the experimental data. At 300 K and 1 bar the solution is a true strong electrolyte with Na+ and Cl− ions being completely separated. As temperature increases NaCl pairs tend to form as a direct response to the decreasing dielectric constant. At very low values of the dielectric constant, higher order NaCl complexes are formed. Two simulations at 1 kbar and at temperatures in the two phase region show evidence for phase separation as all the ions cluster together into a small volume of the simulation box. This phase separation vanishes when the pressure is increased to 5 kbars.