Molecular dynamics simulation on ion-pair association of NaCl from ambient to supercritical water

Molecular dynamics (MD) simulations for aqueous NaCl solution were performed from ambient to supercritical conditions (25 °C, 1.0 g cm⁻³; 250–350 °C, 0.67–0.8 g cm⁻³; 380 °C, 0.2–0.8 g cm⁻³; and 400–600 °C, 0.4 and 0.7 g cm⁻³) in the canonical ensemble to examine how the hydration structure relates with the thermodynamics of the ion-pair association. Hydration structure and the potential of mean force (PMF) of Na⁺Cl⁻ ion-pair were calculated. Ion-pair association constants were also calculated from the PMFs. Energies and entropies of the ion-pair at arbitrary inter-ionic distances from 2.0 to 8.0 Å were evaluated from the temperature derivative of the PMFs. From the calculation of energies and entropies, Na⁺–Cl⁻ pair association was found to be endothermic and promoted by the entropy gain. PMFs had minimums and a slight maximum corresponding to CIP, SShIP, and the transition state between CIP and SShIP, and similar minimums and maximum were only observed for the energy term and not clearly observed for the entropy term. This result indicates that the shape of the PMF and stability of SShIP are determined by the energy of the system. Relationship between the hydration structure and the energy of the system was examined and it was confirmed that the hydration structure in the first hydration shell of the ion-pair was one of the important factor, which made the minimums and maximum in the energy terms and PMFs, and stabilized the SShIP structure.