On the dissociation degree of ionic solutions considering solvation effects

In this work the impact of solvation effects on the dissociation degree of strong electrolytes and salts is discussed. The investigation is based on a thermodynamic mixture theory which explicitly accounts for the solvation effect. Based on a remarkable relationship between differential capacity maxima and partial molar volume of ions in solution the solvation number of specific ions in solution is determined. A subsequent investigation of the electrolytic space charge layer shows that a saturated solution of 1 mol L-1 solvated ions forms near the electrode, and we point out some fundamental similarities of this state to a saturated bulk solution. This finding challenges the assumption of complete dissociation, even for moderate electrolyte concentrations, whereby we introduce an undissociated ion-pair in solution. We re-derive the equilibrium conditions for a two-step dissociation reaction, including solvation effects, which leads to a new relation to determine the dissociation degree. A comparison to Ostwald's dilution law clearly shows the shortcomings when solvation effects are neglected and we emphasize that complete dissociation is questionable beyond 0.5 mol L-1 for aqueous, monovalent electrolytes.