(553e) Activity Coefficients of Salts in Aqueous and Nonaqueous Solvents and in Solvent Mixtures

In chemical technology, we frequently encounter nonaqueous and mixed-solvent electrolytes. For example, lithium-ion batteries and other electrochemical devices. The thermodynamic properties of these electrolytes influences device performance. Based on publications by Bernard and Blum,¹ Barthel et al.,² Simonin et al.,³ and others, this work presents and validates a molecular-thermodynamic model for salt activity coefficients in aqueous and non-aqueous single- and mixed-solvent systems.

The Binding Mean Spherical Approximation (BiMSA) gives electrolyte activity due to long-range electrostatic forces, short-range hard-sphere repulsion, and ion-pair formation. For molalities up to about 2 molar, this theory shows good agreement with measured salt activities in aqueous and non-aqueous solvents using the center-to-center distance of closest approach between oppositely charged ions as the single fitting parameter for each electrolyte system. For mixed-solvent electrolytes, the local solvation environment around the ions dictates short-range interactions. To account for preferential ion solvation in a mixed solvent, the center- to-center distance is obtained from Wang and coworkers’ Dipolar Self-Consistent-Field Theory.⁴ For a particular salt in a particular solvent mixture at fixed temperature, the model predicts salt activity coefficients using only the fitted single-solvent distance-of-closest approach.

References

1. Bernard, O.; Blum, L., Binding mean spherical approximation for pairing ions: An exponential approximation and thermodynamics. J. Chem. Phys. 1996, 104 (12), 4746-4754.
2. Barthel, J.; Krienke, H.; Holovko, M.; Kapko, V.; Protsykevich, I., The application of the associative mean spherical approximation in the theory of nonaqueous electrolyte solutions. Condens. Matter Phys 2000, 3 (3), 23.
3. Simonin, J.-P.; Bernard, O.; Blum, L., Ionic Solutions in the Binding Mean Spherical Approximation: Thermodynamic Properties of Mixtures of Associating Electrolytes. J. Phys. Chem. B 1999, 103 (4), 699-704.
4. Nakamura, I.; Shi, A.-C.; Wang, Z.-G., Ion Solvation in Liquid Mixtures: Effects of Solvent Reorganization. Phys. Rev. Lett. 2012, 109 (25), 257802.