Development of thermodynamic properties of electrolyte solutions with the help of RISM-calculations at the Born–Oppenheimer level

Abstract

We derived a formula of the chemical excess potential μ^exc_i of electrolytes in solutions (NaCl, NaNO₃, Me₄NCl) within the framework of the RISM model of liquid systems as a partial derivative of the Helmholtz energy with respect to the amount n_i of the compound i. Since it is possible to perform RISM-calculations at very small electrolyte concentrations we compared the concentration dependence of the chemical excess potential with the Debye–Hückel limiting law. In general the concentration dependence of the chemical excess potential of the electrolytes follows the square-root law of Debye–Hückel. The slopes of these functions, however, need a very individual discussion. The HNC-closure relation yields a much too large slope due to the overwhelming electrostatic interaction, whereas the closure relation of Kovalenko and Hirata gives very promising results for atomic ions. According to the deficiencies of the RISM model of the SSOZ-theory the slope furnishes only a dielectric constant of water according to this model. Molecular ions, however, show larger deviations from the postulated concentration dependence.