ION ASSOCIATION AND LIQUID STRUCTURE IN SUPERCRITICAL WATER SOLUTIONS OF SODIUM-CHLORIDE - A MICROSCOPIC VIEW FROM MOLECULAR-DYNAMICS SIMULATIONS

Salt solvation and precipitation are important problems in the supercritical water oxidation process. This paper examines the properties of pure water and sodium chloride in supercritical water at infinite dilution using the results of molecular dynamics simulations. The equation of state and the dielectric properties of the SPC water are calculated at 700-1000 K and densities (similar to 0.1 g/cm(3)) corresponding to 250 bar. Our results show that while the SPC model of water describes the equation of state adequately, the calculated dielectric constant is about 10% larger than the experimental one. This slight discrepancy is due to the larger dipole moment of the SPC water in comparison with the actual dipole moment of an isolated water molecule. We calculated the potential of mean force between sodium-chloride, sodium-sodium, and chloride-chloride ion pairs. This calculation shows that sodium chloride stays as the bound ion pair in supercritical water. The potential of mean force calculations also show that supercritical water exhibits strong solvation effects upon the ions and the bound ion pairs at the conditions of interest. The dissociation constant predicted from this potential of mean force is about 1-2 orders of magnitude smaller than that which would be obtained from the extrapolation of experimental measurements at higher densities. We also investigate the solvation of the ions using the pair correlation function of the solutions. These correlation functions provide valuable insight into the ion hydration models used in studying NaCl solution properties.