Comment on “The Role of Concentration Dependent Static Permittivity of Electrolyte Solutions in the Debye–Hückel Theory”

Shilov and Lyashchenko generalized the Debye-Hückel theory for the case of a concentration dependent dielectric constant, a concept that was also the central idea of our papers Vincze et al. J. Chem. Phys. 2010, 133, 154507 and Valiskó and Boda J. Phys. Chem. B 2015, 119, 1546-1557. This assumption leads to breaking the activity coefficient into two parts associated with ion-ion (II) and ion-water (IW) interactions. These two terms appear in the paper of Shilov and Lyashchenko too (denoted by subscripts 1 and 2, respectively). Shilov and Lyashchenko state that the IW term (corresponding to solvation) developed by them is different from the "plain" Born-like expression proposed by us. In this comment, we prove that the two formulas are equivalent. We show that the experimentally motivated Born radius should be used in the equation instead of the Pauling radius. We also show that the theory of Shilov and Lyashchenko can also predict individual ionic activity coefficients even for 2:1 electrolytes.