The stability constants of 1:1 (M:L) complexes of benzo-15-crown-5 (B15C5) with Li+, Na+, K+ and NH4+ cations, the Gibbs standard free energies (DGºc), the standard enthalpy changes (DHºc) and standard entropy changes (DSºc) for formation of these complexes in acetonitrile-methanol (AN-MeOH) binary mixtures have been determined conductometrically. The conductance data show that the stoichiometry of the complexes formed between the macrocyclic ligand and the studied cations is 1:1 (M:L). In most cases, addition of B15C5 to solutions of these cations, causes a continuous increase in the molar conductivities which indicates that the mobility of complexed cations is more than the uncomplexed ones. The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program, GENPLOT. The results show that the selectivity order of B15C5 for the metal cations changes with the nature and composition of the binary mixed solvent. The values of standard enthalpy changes (DHºc) for complexation reactions were obtained from the slope of the van’t Hoff plots and the changes in standard entropy (DSºc) were calculated from the relationship DG◦c,298.15 =DH◦c - 298.15DS◦c. A non-linear behavior was observed between the stability constants (log Kf) of the complexes and the composition of the acetonitrile-methanol (AN-MeOH) binary solution. The results obtained in this study, show that in most cases, the complexes formed between B15C5 and Li+, Na+, K+ and NH4+ cations are both enthalpy and entropy stabilized and values of these thermodynamic quantities changes with the composition of the binary solution.

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