Molecular Dynamics Simulation of Electrolyte Solutions in Ambient and Supercritical Water. 1. Ion Solvation

Free energies of solvation for Cl-, OH-, Na+, HCl, and H2O are calculated 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 trend also applies to the magnitude of the solvation free energy, although the changes for Na+ are very small, from ambient conditions all the way to 673 K and 0.087 g/cm3. Bivalent cations exhibit a well-defined second shell that remains at supercritical conditions. Modest discrepancies are observed between simulation and experiment both for the local density of water about Sr2+ and for the free energy of solvation of NaCl, illustrating the challenges to both theory and experiment.