R. E. Mesmer

Simulation of supercritical water and of supercritical aqueous solutions

Molecular dynamics (MD) calculations have been performed to determine equilibrium structure and properties of systems modeling supercritical (SC) water and SC aqueous solutions at two states near the critical point using the simple point charge (SPC) potential model of Berendsen et al. for water. Both thermodynamic and dielectric properties from the simulations for pure water are accurate in comparison with experimental results even though the SPC model parameters were fitted to properties of ambient water. Details of the near‐critical clustering in SC water have been predicted which have not been measured to date. MD studies have also been undertaken of systems that model sodium and chloride ions and neutral argon in SC water at the same states. The first solvation shell in SC water is observed to be similar to that in ambient water, and long‐range solvation structures in SC water are similar to those observed for simple SC solvents. An excess of water molecules is observed clustering around ionic solute...

Na+–Cl− ion pair association in supercritical water

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 dynamics simulation of the limiting conductance of NaCl in supercritical water

Abstract We report molecular dynamics calculations of the ionic mobility and limiting conductance of NaCl in supercritical water as a function of density along an isotherm 5% above the critical temperature. The number of hydration water molecules around ions is found to dominate the behavior of the limiting conductance in the higher-density region while the interaction between the ions and hydration water molecules is found to dominate in the lower-density region. The different effects in the lower- and higher-density regimes lead to different slopes for the limiting conductance as a function of density in the two regimes.

Temperature and density effects on the high temperature ionic speciation in dilute Na+/Cl− aqueous solutions

Molecular simulation of dilute NaCl aqueous solutions is performed to study the Na+/Cl− ion‐pair association and the constant of equilibrium between the solvent separated (shared) and contact ion pairs at high temperature. Using the simple point charge, the Pettitt–Rossky, and the Fumi–Tosi models for the water–water, the ion–water, and the ion–ion interactions, we determine the density dependence of the constants along the Tr=T/Tc=1.05 isotherm, and the temperature dependence along the ρr=ρ/ρc=1.5 isochore. The simulation results for the association constant are then compared with the predictions of two recent correlations based on conductance measurements at high temperature. The outcome of the comparison is interpreted in terms of the microstructural behavior of the solvent around the ions and the realism of its dielectric behavior.

Solvation in high-temperature electrolyte solutions. II. Some formal results

Our molecular-based formalism for infinitely dilute supercritical nonelectrolyte solutions is extended to electrolyte solutions by establishing rigorous connections between the microscopic behavior of the solvent around individual ionic species and their macroscopic solvation behavior. The formalism relies on the unambiguous splitting of the mixture’s properties into short-ranged (finite) and long-ranged (diverging) contributions, associated with the corresponding solvation and compressibility-driven phenomena, respectively. The salt (solute) and solvent’s residual chemical potentials are linked to the change of the local solvent’s environment around the infinitely dilute anion and cation, and the salt partial molar properties are interpreted in terms of the individual ion partial molar counterparts without introducing any extra-thermodynamic assumption. This is achieved with the use of Kusalik and Patey’s version of the Kirkwood–Buff fluctuation theory of mixtures. Moreover, the salt-and the individual i...

SOLVATION IN HIGH-TEMPERATURE ELECTROLYTE SOLUTIONS. I. HYDRATION SHELL BEHAVIOR FROM MOLECULAR SIMULATION

The behavior of the first hydration shell of species in solution and its relevant thermophysical properties are studied by molecular dynamics of infinitely dilute NaCl aqueous solutions at high temperature. The ion-induced effects on the water local properties are assessed in terms of the corresponding radial profiles for the local density, the local pressure, the local electric field, the local dielectric constant, and two alternative types of coordination numbers, along the near-critical reduced isotherm Tr=1.05 and the supercritical reduced isochore ρr=1.5. Simulation results are discussed in the context of their usefulness in enhancing the understanding and the modeling of supercritical aqueous electrolytes.

MOLECULAR SIMULATION STUDY OF SPECIATION IN SUPERCRITICAL AQUEOUS NACL SOLUTIONS

Abstract Molecular simulation of dilute NaCl aqueous solutions are performed to study the thermodynamics and kinetics of Na+/Cl− speciation at high temperature. Using the SPC, the Pettitt-Rossky, and the Fumi-Tosi models for the water-water, the ion-water, and the ion-ion interactions respectively, we determine the enthalpy and volume of association at the reduced conditions of Tr =1.05 and ϱr = 1.5, and compare with the corresponding predictions of a recent correlation based on conductance measurements at high temperature. The forward and backward (transition state theory) kinetic rate constants for the interconversion between contact and solvent-shared ion pair configurations along the reduced isotherm of Tr = 1.05, and the transmission coefficient at Tr = 1.05 and ϱr = 1.0 are assessed by simulation and compared with the corresponding quantities at ambient conditions.

Molecular approach to high-temperature solvation. Formal, integral equation and experimental results

The solvation of infinitely dilute CsBr in high-temperature aqueous solutions is analysed by integral equation calculations, according to the recently proposed molecular-based formalism which connects the solvent environment around individual ionic species and their macroscopic solvation behaviour. Recent experimental data for infinitely dilute CsBr aqueous solutions are interpreted via the same formalism and compared with their analogous integral equation calculations. Finally, some relevant theoretical implications regarding the modelling of high-temperature aqueous electrolyte solutions are discussed and illustrated by integral equation results.

Thermodynamics and kinetics of ion speciation in supercritical aqueous solutions : a molecular-based study

Molecular simulation of infinitely dilute NaCl aqueous solutions are performed to study the Na{sup +}/Cl{sup -} ion pairing in a polarizable and a nonpolarizable solvent at supercritical conditions. The Simple Point Charge, Pettitt-Rossky, and Fumi-Tosi models for the water-water, ion-water, and ion-ion interactions are used in determining the degree of dissociation, its temperature and density dependence, and the kinetics of the interconversion between ion-pair configurations in a nonpolarizable medium. To assess the effect of the solvent polarizability on the stability of the ion-pair configurations, we replace the Simple Point Charge by the Polarizable Point Charge water model and determine the anion-cation potential of mean force at T{sub r}=1.20 and {rho}{sub r}=1.5.