THERMODYNAMICS OF CESIUM CATION COMPLEXATION WITH DIBENZO-24-CROWN-8 IN MIXED NONAQUEOUS SOLVENTS

Thermodynamics of complexation of cesium cation by dibenzo-24-crown-8 was studied in three binary solvent mixtures: acetonitrile-nitromethan e (AN/NM), N,N-dimethylformamide-nitromethane (DMF/NM) and acetonitril e-propylene carbonate (AN/PC) using the Cs-133-NMR technique. In all c ases the variation of the formation constant, K-f, with the solvent co mposition was monotonic: K-f increased as the mole-% of the solvent of low donicity was increased. The temperature dependence of K-f indicat ed that the complexes are generally enthalpy stabilized, but entropy d estabilized. The enthalpy and entropy of complexation reactions are qu ite sensitive to the solvent composition. However, their variation wit h solvent composition was not monotonic but showed maxima or minima at the isosolvation points of the cation or the complexed cation. In all cases an enthalpy-entropy compensating effect was observed.