the formulas of Helmholtz free energy for fluid mixtures with charged ions and polar molecules are deduced. All kinds of interaction energies between solute-solvent
solvent-solvent and solute-solute are considered (including dispersion
repulsion
dipole-dipole
dipole-induced dipole
ion-ion and ion-dipole energies). The activity coefficients of solvents in LLE for three salt-containing mixed-solvent ternary systems [H2O-nC4H9OH-MX (MX = NaCl
KCl and KBr)] at 25℃ are calculated asd compared with experimental data.