Phase behavior and physico-chemical properties of aqueous electrolyte solutions near the critical point via molecular dynamics simulation with gravity perturbation

In molecular dynamics (MD) simulation, large-scale density fluctuations bring about difficulties in evaluating the critical point from coexisting vapor-liquid densities. This study proposes a gravity perturbation method that improves the stability of vapor-liquid coexisting phases near the critical point by introducing gravity and boundary walls. Trial simulations were performed with a modified flexible SPC-TR (Toukan-Rahman) model and a critical point obtained for water (T_C=377.6°C, ρ_C=0.302gcm^-3) was in good agreement with the experimental data (T_C=374.0°C, ρ_C=0.322gcm^-3). The method was also applied to NaCl-water mixtures and it was found that simulations were greatly facilitated with the technique. The gravity perturbation method allows reliable determination of phase behavior in the vicinity of a critical point.