MOLECULAR-DYNAMICS SIMULATION OF ELECTROLYTE-SOLUTIONS IN AMBIENT ANDSUPERCRITICAL WATER .1. ION SOLVATION

Free energies of solvation for Cl-, OH-, Na+, HCl, and H2O are calcula ted at ambient conditions and several supercritical water (SCW) states using molecular dynamics-free energy perturbation computer simulation . From ambient to SCW, the magnitude of reductions in the coordination number, number of hydrogen bonds, and other specific interactions are in the opposite order of the solute-water interaction energies, which are, from strongest to weakest, Na+ > OH- > Cl- > H2O > HCl. This tre nd also applies to the magnitude of the solvation free energy, althoug h the changes for Na+ are very small, from ambient conditions all the way to 673 K and 0.087 g/cm(3). Bivalent cations exhibit a well-define d second shell that remains at supercritical conditions. Modest discre pancies are observed between simulation and experiment both for the lo cal density of water about Sr2+ and for the free energy of solvation o f NaCl, illustrating the challenges to both theory and experiment.