Abstract
By considering the solution of the Mean Spherical Approximation (MSA) given by Lesser Blum, we addressed the problem of specificity in aqueous electrolyte solutions. The reference diameter of the cations is defined as the biggest possible diameter of the cation (obtained with the less associating anion). Then the specificity is taken into account by an association term similar to Bjerrum theory. The resulting activity coefficients can be used to describe pure electrolyte and electrolyte mixtures up to molar concentrations. Hydration appear to be relatively stable with small cations but it strongly depends on the anion for big cations. The validity of Zdanovskii-Stokes-Robinson mixing rule has also been tested and it appears to be valid with typically a 1% accuracy for molar concentrations.
Highlights
From a long time, thermodynamic properties of electrolyte solutions are described considering the solvent as a continuum characterized only by its dielectric constant
The calculated activity coefficient of the electrolyte has been converted from the McMillan-Mayer reference to the Lewis-Randall reference thanks to Eq (27)
It has long been recognized that the Mean Spherical Approximation (MSA) theory is powerful and able to evaluate the activity coefficients of simple aqueous electrolyte solutions [7,31]
Summary
Thermodynamic properties of electrolyte solutions are described considering the solvent as a continuum characterized only by its dielectric constant. The magnitude of the oscillating short-range potential between the ions is more than kBT for the first minima so that it is rigorously impossible to consider an equivalent hard-sphere model [24] and at least the contact ion pair has to be taken into account rigorously if the structure of the solution is to be described. If only the free energy of the solution is required, an effective diameter is possible, but there is a price to pay: non-additivity: the short-range potential strongly depends on the nature of the ions and specific attraction can occur between ions For example it is often stated [25] that small ions are dehydrated when they interact with small ions whereas they stay hydrated when they interact with bigger ions. The case of mixtures and the validity of mixtures rules are described in the last part
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