Activity Coefficients of Aqueous Sodium, Calcium, and Europium Nitrate Solutions from Osmotic Equilibrium MD Simulations.

Osmotic and activity coefficients of three aqueous electrolyte solutions with cations of similar ionic radius, but different charges, are described by molecular dynamics with the help of the osmotic equilibrium method using polarizable force fields up to high concentration. Simulations of vapor-liquid interfaces of aqueous solutions of NaNO, Ca(NO), and Eu(NO) at different concentrations and at 298.15 K provide time-averaged number density profiles and consequently the quantity of solvent molecules in the vapor phase. These three cations of similar ionic radii exhibit an increasing amount of water in their first coordination sphere due to their increasing charge. The solvent activity is directly determined by the vapor phase density at different salt concentrations with respect to the vapor phase density of the pure solvent. The obtained densities of the liquid bulk and the osmotic and activity coefficients for the three different nitrate salts are in good agreement with the experimental results. Time-averaged concentration profiles and the interpretation of radial distribution functions are used to explain the role of coordination on the thermodynamic properties of aqueous electrolyte solutions.