Osmotic pressure of aqueous electrolyte solutions via molecular simulations of chemical potentials: Application to NaCl

The osmotic pressure, Π, is an important thermodynamic property of aqueous electrolyte solutions, which is intimately related to the activity of the water solvent, and is sensitive to the details of the force field used in molecular simulations of such systems. Its calculation in the most important case of discrete water models has received scant attention in the literature; the only existing method involves a special-purpose molecular dynamics approach implementing virtual semi-permeable membranes separating solution and solvent phases. Here, we develop and demonstrate a new thermodynamically based approach utilizing simulation results for the salt chemical potential, μs, and for the solution specific volume, vm. The methodology may also be used in principle to calculate the activity of water and of the electrolyte from simulation data for Π and vm. We demonstrate our approach in the case of aqueous NaCl solutions at ambient conditions by calculating new results for both Π and the related osmotic coefficient property, ϕ, from simulation data for μNaCl. We compare with experimental data the predictions of two polarizable force fields (AH/BK3 and AH/SWM4-DP) and of a typical non-polarizable force field (JC). We find that AH/BK3 produces results in good agreement with experiment for both Π and ϕ over the entire experimentally accessible concentration range, and that the AH/SWM4-DP results are generally poor. The JC results are very good at concentrations below about 3 molal, but deteriorate rapidly at higher concentrations.