NaCl aggregation in water at elevated temperatures and pressures: Comparison of classical force fields

Abstract

The properties of water vary dramatically with temperature and density. This can be exploited to control its effectiveness as a solvent. Thus, supercritical water is of keen interest as solvent in many extraction processes. The low solubility of salts in lower density supercritical water has even been suggested as a means of desalination. The high temperatures and pressures required to reach supercritical conditions can present experimental challenges during collection of required physical property and phase equilibria data, especially in salt-containing systems. Molecular simulations have the potential to be a valuable tool for examining the behavior of solvated ions at these high temperatures and pressures. However, the accuracy of classical force fields under these conditions is unclear. We have, therefore, undertaken a parametric study of NaCl in water, comparing several salt and water models at 200 bar–600 bar and 450 K–750 K for a range of salt concentrations. We report a comparison of structural properties including ion aggregation, hydrogen bonding, density, and static dielectric constants. All of the force fields qualitatively reproduce the trends in the liquid phase density. An increase in ion aggregation with decreasing density holds true for all of the force fields. The propensity to aggregate is primarily determined by the salt force field rather than the water force field. This coincides with a decrease in the water static dielectric constant and reduced charge screening. While a decrease in the static dielectric constant with increasing NaCl concentration is consistent across all model combinations, the salt force fields that exhibit more ionic aggregation yield a slightly smaller dielectric decrement.

Selective recovery of critical materials in supercritical water desalination

“…In a low-dielectric medium, the formation of multiple ion aggregates enhances the mean dipole moment in a system [70]. Our recent MD simulations on high-temperature NaCl solutions modeled with several different molecular models also support the idea that the dielectric constant of high-temperature brine is increased as the brine concentration increases at simulated pressures of 20 MPa (approximately 30 MPa in reduced units) [71]. According to the MD simulations, a dielectric constant increment is observed when the temperature is above the critical temperature of pure water.…”

Ion association in hydrothermal aqueous NaCl solutions: implications for the microscopic structure of supercritical water

“…, water oxidation 23 . Numerous theoretical approaches have addressed the structural properties of the H 2 O-NaCl binary as function of temperature [24][25][26] , with the results of several studies pointing towards the formation of larger polyatomic NaCl clusters in the solution due to the decreasing dielectric constant of water with rising temperature 16,17,[27][28][29][30][31] . Several X-ray and neutron diffraction studies have been performed, on a variety of different salt solution and at a large range of different experimental conditions 12,[32][33][34] .…”

Structure and dynamics of aqueous NaCl solutions at high temperatures and pressures

“…15). The dependence of ion pairing on the force field chosen for the ions [76] has not been investigated for the state conditions covered in the present work.…”