II+IW model for the activity of electrolytes

We proposed a simple, but successful model for the non-monotonic concentration dependence of the activity coefficient of aqueous electrolytes. It is well known that the mean activity coefficient  of many electrolytes shows a nonmonotonic behavior as a function of concentration: increasing the concentration from zero the infinite dilution limit the activity coefficient decreases from one with a slope obeying the Debye–Hückel (DH) limiting law, reaches a minimum at a large concentration, then increases again often above unity as the concentration approaches saturation.

In the framework of the implicit water model of electrolytes (also known as the Primitive Model) it was reproduced by giving unrealistically large diameter to the ions. It was considered as a solvated diameter of ions representing ions together with their solvation shells. In this picture, the non-monotonic behavior is a result of the balance between the (repulsive) hard sphere exclusion and the (attractive) Coulomb interaction.

We argued against this view and proposed that the non-monotonic behavior is a result of balance between ion-ion (II) and ion-water (IW) interactions [1]. The II part can be computed in the framework of an extended DH [6], the Mean Spherical Approximation [1], or Grand Canonical Monte Carlo simulations [1-7]. The IW part is approximated from the Born theory using a concentration dependent dielectric constant that is a crucial quantity in our theory linking the two terms that are otherwise computed independently.

  abstract2

[7] M. Valiskó, D. Boda. Activity coefficients of individual ions in LaCl3 from the II+IW theory. Mol. Phys. 115(9-12):1245-1252, 2017.

[6] M. Valiskó, D. Boda. Comment on ``The Role of Concentration Dependent Static Permittivity of Electrolyte Solutions in the Debye−Hückel Theory. J. Phys. Chem. B,  119(44):14332-14336, 2015.

[5] M. Valiskó, D. Boda. Unraveling the behavior of the individual ionic activity coefficients on the basis of the balance of ion-ion and ion-water interactions J. Phys. Chem. B, 119(4):1546-1557, 2015.

[4] M. Valiskó, D. Boda. The effect of concentration- and temperature-dependent dielectric constant on the activity coefficient of NaCl electrolyte solutions J. Chem. Phys., 140(23):234508, 2014.

[3] D. Boda. Monte Carlo simulation of electrolyte solutions in biology: in and out of equilibrium. Ann. Rep. Comp. Chem., Editor R. A. Wheeler, volume 10, pages 127–163. Elsevier, 2014.

[2] J. Vincze, M. Valiskó, and D. Boda. Response to "Comment on 'The nonmonotonic concentration dependence of the mean activity coefficient of electrolytes is a result of a balance between solvation and ion-ion correlations' " [J. Chem. Phys. 134, 157101 (2011)]". J. Chem. Phys., 134(15):157102, 2011.

[1] J. Vincze, M. Valiskó, and D. Boda. The nonmonotonic concentration dependence of the mean activity coeffcient of electrolytes is a result of a balance between solvation and ion-ion correlations. J. Chem. Phys., 133(15):154507, 2010.