Solvent activity in electrolyte solutions from molecular simulation of the osmotic pressure

Abstract

A method for determining the activity of the solvent in electrolyte solutions by molecular dynamics simulations is presented. The electrolyte solution is simulated in contact with the pure solvent. Between the two phases, there is a virtual membrane, which is permeable only for the solvent. In the simulation, this is realized by an external field which acts only on the solutes and confines them to a part of the simulation volume. The osmotic pressure, i.e., the pressure difference between both phases, is obtained with high accuracy from the force on the membrane, so that reliable data on the solvent activity can be determined. The acronym of the new method is therefore OPAS (osmotic pressure for activity of solvents). The OPAS method is verified using tests of varying complexity. This includes a comparison of results from the OPAS method for aqueous NaCl solutions to results from the literature which were obtained with other molecular simulation methods. Favorable agreement is observed not only for the solvent activity but also for the activity coefficient of NaCl, which is obtained by application of the Gibbs-Duhem equation.

Molecular simulation of binary phase diagrams from the osmotic equilibrium method: vapour pressure and activity in water–ethanol mixtures

“…From a simulation perspective, Gibbs Ensemble Monte Carlo (GEMC) is capable in predicting vapour-liquid equilibria of binary mixtures in good agreement with experiment [17][18][19][20]. Lísal et al [21] used the GEMC approach to simulate vapour-liquid equilibria of ethanol and water at 393.15 K. Other simulation methods for activities in solution, such as the molecular dynamics (MD) -based osmotic membrane method [22,23] or the McMillan-Mayer approach [24][25][26] only work for systems with charged particles. Zhang and Yang [27], Noskov et al [28] and more recently Ghoufi et al [29] studied the structural and physical properties of aqueous ethanol-water mixtures in the bulk liquid phase and at the liquid-vapour interface by the means of MD simulation.…”

Activity Coefficients of Concentrated Salt Solutions: A Monte Carlo Investigation

“…Interestingly, in a recent study [64], good agreement between the experimentally determined water activity in NaCl solutions and calculated by molecular dynamics simulations (MD) was also restricted to 1.5 mol·kg −1 . However, a good agreement between the mean ionic activity coefficients calculated by the MD simulations and experimental data was only found for concentrations less than 0.8 mol·L −1 .…”

“…However, a good agreement between the mean ionic activity coefficients calculated by the MD simulations and experimental data was only found for concentrations less than 0.8 mol·L −1 . This is explained by the model used in the MD simulations where ion size parameters were obtained by fitting to the density of aqueous solutions [64,65].…”

How clay delamination supports aseismic slip

“…The coupled pressure of the bulk phase represents the pore fluid pressure P f . In this study, we do not explicitly consider influence of solvated ions on the effective pore fluid pressure or osmotic pressure (Kohns et al 2016;Koop et al 2000). In principle, chemical potential of bulk phase simulated here is equal to that of the real pore fluid with the same osmotic pressure P f .…”