THERMODYNAMICS OF COMPLEXATION OF 18-CROWN-6 WITH SODIUM, POTASSIUM, RUBIDIUM, CESIUM AND AMMONIUM-IONS IN N,N-DIMETHYLFORMAMIDE

The complexation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) with sodium, potassium, rubidium, caesium and ammonium ions has been studied by titration calorimetry in N,N-dimethylformamide (DMF) contai ning 0.1 mol dm(-3) (C2H5)(4)NClO4 as a constant ionic medium at 25 de grees C, The calorimetric titration data were well explained in terms of the formation of the [M(18-crown-6)](+) (M = Na+, K+, Rb+, Cs+, NH4 +) and [Cs(18-cronw-6)(2)](+) complexes, and their formation constants , enthalpies and entropies were determined. The stability of the [M(18 -crown-6)](+) (M = Na+, K+, Rb+, Cs+, NH4+) complexes is much more pro nounced In DMF than In water. The formation entropies are similar for the sodium and potassium complexes' in DMF and water, but are smaller for the rubidium, caesium and ammonium complexes in DMF than for the c orresponding complexes in water. The stabilization of the [M(18-crown- 6)](+) complexes is thus due to the more exothermic complexation in DM F than in water. Because the solvation of the univalent cations is exp ected to be enhanced to a greater extent in DMF than in water owing to the relatively large electron-pair donating ability of DMF, the more exothermic complexation in DMF than in water is ascribed to the differ ent solvation of 18-crown-6 in the two solvents, i.e. 18-crown-6 is mu ch more stabilized in water by forming hydrogen bonds with water molec ules.