THERMODYNAMICS OF COMPLEXATION OF SODIUM, POTASSIUM, RUBIDIUM, CESIUMAND AMMONIUM-IONS WITH 18-CROWN-6 IN ACETONITRILE AND PROPYLENE CARBONATE

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 acetonitrile (MeCN) and propylene carbonate (PC) containing 0.05 mol dm(-3) (C2H5)(4)NCIO4 as a constant ionic medium at 25 degrees C. The formation of [M(18-crown-6)](+) (M = Na+, K+, Rb+, Cs+, NH4+) ions was revealed, and their formation cons tants, reaction enthalpies and entropies were determined. Consistent w ith the similar donor and acceptor properties of MeCN and PC, the stab ility of the 18-crown-6-cation complexes is similar in the two solvent s. However, the enthalpic and entropic behaviour are very different. T he formation enthalpy of the sodium complex is positive in MeCN, while negative in PC. The reactions for the potassium, rubidium, caesium an d ammonium ions are less exothermic in MeCN than in PC. On the other h and, the formation of the sodium complex in MeCN is accompanied by a m ore positive entropy change than that in PC. The reaction entropies fo r the potassium, rubidium, caesium and ammonium systems are positive i n MeCN, whereas they are negative in PC. Thus, enthalpy-entropy compen sation results in a similar stability of [M(18-crown-6)](+) (M = Na+, K+, Rb+, Cs+, NH4+) in MeCN and PC.