Ion association and solvation of thiocyanates in water–dimethylformamide mixtures: an ultrasonic study

Ion Pairing

“…Further illustrations of the competition between ion solvation and ion association include the ultrasonic absorption study of lithium thiocyanate in mixtures of water and N , N -dimethylformamide (DMF). At x DMF > 0.6, ion pairing prevails over ion solvation, while the reverse is true at x DMF < 0.6 . Such a competition is enhanced as the concentration of the electrolyte increases and the ratio of solvent molecules to electrolyte decreases.…”

“…(iii) Are the activity coefficients of free ions calculated from theory beyond the validity of the limiting law (as distinct from the experimentally measured stoicheometric activity coefficient of the electrolyte) sufficiently reliable to differentiate between nonspecific ion-ion interactions and ion pairing? The widely used semiempirical Pitzer expressions for activity coefficients 58 do not recognize ion pairing explicitly. However, they include a coefficient β (2) that takes care of effects otherwise deemed to be due to ion pairing and can be shown to be closely correlated with K A values obtained by independent means (section 8.1).…”

“…At x DMF > 0.6, ion pairing prevails over ion solvation, while the reverse is true at x DMF < 0.6. 58 Such a competition is enhanced as the concentration of the electrolyte increases and the ratio of solvent molecules to electrolyte decreases. The entropy of mixing of free cations, free anions, ion pairs, and free solvent molecules depends on the amount of solvent bound to the ions and ion pairs.…”