Osmotic and activity coefficients from effective potentials for hydrated ions

Based on a method we previously suggested [Phys. Rev. E 52, 3730 (1995)], effective interaction potentials between ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{\mathrm{-}}$ ions have been derived from interionic radial distribution functions (RDF) in molecular dynamics (MD) simulations of aqueous NaCl solution. The effective interaction potentials between the hydrated ions, which reproduce the original ion-ion RDF curves, can be used further to construct a corresponding ionic solution in a much larger scale and to calculate any properties dependent on the structure of the electrolyte solution. In a subsequent Monte Carlo (MC) simulation, using the effective potentials, the osmotic and activity coefficients are calculated for the ions. Calculation of these properties directly from atomic MD or MC simulations is beyond the capacity of the present computers due to the very large number of molecules required in the simulations to obtain reliable results. A very good agreement with the experimental results is obtained. Effects of three-body interactions and concentration dependencies of the effective potentials are discussed.