Crown ethers are macrocyclic polyethers which form stable selective complexes with suitable cations. The ability of crown ethers to form stable complexes with cations has spurred interest in these compounds [1]. Various investigations have extensively studied thermodynamics of solution of macrocyclic polyether complexes during the past three decades [2] and Izatt and coworkers have compiled kinetic data[3]. The equilibrium constants and the thermodynamic parameters for complex formation of benzo-15-crown-5 (B15C5) and phenylaza-15- crown-5 (PhA15C5) with La+3 cation have been determined by conductivity measurements in acetonitrile-dimethylformamide (AN/DMF), acetonitrile-dimethylsulfoxide (AN/DMSO), acetonitrile-1,2dichloroethane (AN/1,2DCE) and acetonitrile-chloroform (AN/CHCl3) binary solutions. Phenylaza-15-crown-5 forms a 1:1 complex [M:L] with La3+ cation, but in the case of benzo-15-crown-5 ligand, a 1:2 [M:L2] complex is formed in some binary solutions. The stability of the complexes is sensitive to the solvent composition and a non-linear behavior was observed for variation of logKf of the 1:1 complexes versus the composition of the binary mixed solvents. The thermodynamic parameters (ΔH˚c , ΔS˚c) for formation of (B15C5.La)3+ and (PhA15C5.La)3+ complexes were obtained from temperature dependence of the stability constant using the Van‘t Hoff plots. The results show that in most cases, the complexes are both enthalpy and entropy stabilized. A non-linear behavior was observed for changes of logKf of the1:1 complexes versus the composition of the binary mixed solvents which was explained in terms of solvent–solvent interactions and also the heteroselective solvation of the species involved in the complexation reactions.

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