Na+–Cl− ion pair association in supercritical water

Abstract

Molecular dynamics simulations of supercritical electrolyte solutions with three different ion–water models are performed to study the anion–cation potential of mean force of an infinitely dilute aqueous NaCl solution in the vicinity of the solvent’s critical point. The association constant for the ion pair Na+/Cl− and the constant of equilibrium between the solvent-separated and the contact ion pairs are determined for three models at the solvent critical density and 5% above its critical temperature. The realism of the aqueous electrolyte models is assessed by comparing the association constants obtained by simulation with those based on high temperature conductance measurements. Some remarks are given concerning the calculation of the mean-force potential from simulation and the impact of the assumptions involved.

Molecular Simulations of Dense Hydrothermal NaCl−H2O Solutions from Subcritical to Supercritical Conditions

“…This decrease in ion hydration can be attributed to screening effects caused by high salt concentrations and the possible formation of ionpair clusters that would disrupt the hydration structure observed in infinite-dilution ion pair studies. 16,15 Ion association in the supercritical phase is illustrated in Figure 11. In the ion-ion correlation function, g Na-Cl , the magnitude of the first peak increases dramatically above 452°C , reflecting ion pairs that only associate as pairs or clusters of pairs.…”

“…14 Chialvo and co-workers expanded the study of Na-Cl ion pairs to examine the effect of intermolecular potential models and to calculate the association constant and compare it to electrical conductance measurements. 15 Harris and Cui used molecular dynamics (MD) and the rigid SPC model to estimate the solubility and activity coefficients of NaCl in supercritical water and calculate the potential of mean force for the Na-Cl ion pair. 16 Expanding the investigation to finite concentrations, Cummings and co-workers examined the structure of SPC water solutions at supercritical conditions and established a baseline of qualitative results for supercritical solution structure.…”

Systematic comparison of force fields for microscopic simulations of NaCl in aqueous solutions: Diffusion, free energy of hydration, and structural properties

“…The water models used for the aqueous solution were SPC, SPC/E, TIP3P and TIP4P. In previous studies some static properties such as radial distribution functions have been studied, but in each case for one particular choice of force field only [7,8,9,10,11,12,16]. In addition, the effects of temperature [17] and salt concentration [18] have recently been studied.…”

Thermodynamically Consistent Force Field for Molecular Dynamics Simulations of Alkaline-Earth Carbonates and Their Aqueous Speciation

“…A similar approach was used by Chialvo et al70 In ref 69 the potential of mean force was obtained analytically by assuming only a screened electrostatic interaction between the ions and the minimum cation-anion distance was regarded as a fitting parameter. However, from our metadynamics calculations we have access to the all-atom pairing free energy profiles and we therefore calculate the dissociation constant by using them instead of the analytic potential of mean force obtained from a…”