Thermodynamic properties of charge asymmetric mixed electrolytes in the primitive model

Abstract

Theoretical results for the structure and thermodynamics of charge asymmetric but equal size electrolyte mixtures obtained by the hypernetted-chain (HNC) integral equation are presented. The electrolytes are modeled as charged hard spheres in a dielectric continuum. The results for the excess energies, osmotic and activity coefficients for mixtures of +2:−1 and +1:−1 electrolytes with a common ion were calculated as a function of composition in the range of ionic strength from I=10 −4 to 1.2 M. From these data, the first two mixing coefficients for the excess internal energy ( q i ) and excess osmotic pressure ( w i ) were determined. The hypernetted-chain results were used to analyze experimental data for mixtures of NaCl and MgCl 2 in water. The zeroth mixing coefficient w 0 for the excess osmotic pressure was compared with the experimental data for this quantity, obtained from osmotic pressure measurements of MgCl 2 and NaCl mixtures in water. In addition, the Harned coefficients for the activity coefficients were calculated and compared with the experimental values. The agreement between theory and experiment for the w 0 results and Harned coefficients α ij is fair. In addition, the hypernetted-chain calculations were compared with existing and new Monte Carlo (MC) data for the excess energies and osmotic coefficients as functions of composition of the model mixture.