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

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.