J. M. Simonson

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.

The structure of water from 25∞C to 457∞C: comparison between neutron scattering and molecular simulation

The microstructure of water as predicted by four rather diAerent water intermolecular potential models, two flexible (BJH and SPC-mTR) and two polarizable (TIP4P-FQ and PPC), is compared against the corresponding newest neutron scattering data in a wide range of state conditions. The purpose of this study is to assess the reliability of current, more complex potential models for the simulation of water at high temperature, and consequently, to gain some insight into the impact of flexibility and polarizability on the thermodynamic and structural properties as a function of state conditions. The comparison clearly indicates the superiority of polarizable over (simply) flexible water models to properly describe structure and polarization (enhancement of the molecular dipole over that of the monomer). ” 2000 Elsevier Science B.V. All rights reserved.

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...

Aqueous Na+Cl- pair association from liquidlike to steamlike densities along near-critical isotherms

An extensive molecular-based study of ion-pair formation in near-critical dilute aqueous NaCl solutions is performed along three near- (super- and sub-) critical isotherms and from liquidlike to steamlike densities. The study encompasses the determination of the ion-pair association constant via potential of mean force calculations. The main goal is to find answers to some relevant questions regarding the thermodynamic and corresponding microscopic behavior of the ion-pair formation at steamlike densities, where experimental data are extremely difficult to obtain accurately. A direct comparison is made between simulation, theoretical developments, and experiment to aid the interpretation of experimental data and their macroscopic modeling.

H3O+/Cl− ion-pair formation in high-temperature aqueous solutions

The radial profiles of the mean force and corresponding potential of mean force for the Cl−⋯H3O+ pair are determined by constraint molecular dynamics of an infinitely dilute near-critical aqueous solution, as described by the SPC/E water model and either the Gertner–Hynes or the Kusaka et al. hydronium model. These profiles are used to test the prediction of a continuum primitive model, and to predict the ion-pair association constant. The reliability of these intermolecular potential models is assessed by comparing the predicted association constants with those determined experimentally by conductance and solubility measurements. This comparison suggests that the most accurate experimental data available for the association constant of HCl fall between the predictions of the two models, and tends to support the superiority of the Gertner–Hynes over the Kusaka et al. hydronium model. Moreover, the simulation results allow a quick test of the reliability of the simple continuum dielectric model to represen...

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.