Ion pairing

This review expounds the present (mid-2005) status of the ion-pairing concept and illustrate it with examples of well characterized ion pairs formed in electrolyte solutions in various solvents. The review is divided into several sections as follows: theoretical treatments of ion pairing, experimental methods for studying ion pairing, thermodynamic consequences of ion pairing, triple ions and further aggregation, solvation and ion pairing and ion pairing in polyelectrolytes. The section on theoretical treatments of ion pairing covers the Bjerrum approach, Fous treatment, multistep ion pairing, the pair correlation function treatment, the low concentration chemical model, the mean spherical approximation treatment and activity coefficients of charge-symmetrical ion pairs. The section on experimental methods for studying ion pairing includes conductometry, potentiometry, salt or solvent activity measurements, solubility measurements and relaxation methods. The section on thermodynamic consequences of ion pairing covers activity and osmotic coefficients, enthalpy and entropy and volume. The section on triple ions and further aggregation includes triple ion formation and quadruple ions and higher aggregates. Finally, the solvation and ion pairing section covers solvent and counterion competition, transfer of ion pairs between solvents, transfer of ion pairs between immiscible solvents and hydration vs ion pairing in molten salt hydrates.