The equilibrium constants and thermodynamic parameters for complex ‎formation of 18-Crown-6 (18C6) with Tl+,Pb2+,Hg2+, and Zn2+ metal cations have ‎been determined by conductivity measurements inmethanol (MeOH)–water (H2O) ‎binary solutions. 18-Crown-6 forms 1 : 1 complexes with Hg2+and Zn2+cations,but ‎in the case of Tl+and Pb2+cations, in addition to 1 : 1 stoichiometry, 1 :2 ‎‎(ML2)complexes are formedin some binary solvents. The thermodynamic parameters ‎‎( and ), which were obtained from the temperature edependences of equilibrium ‎constants, show that in most cases the complexes are enthalpy destabilized but ‎entropy stabilized. Non-linear behavior is observed between the equilibrium constants ‎‎( ) of complexes and the composition of the mixed solvent. The selectivity of the ‎ligand for these metal cations is sensitive to the solvent composition, and, in some ‎cases, the selectivity order is reversed in certain compositions of the mixed solvent. ‎The results also show that the mechanism of complexation reactions and the ‎stoichiometry of complexes of some metal cations change with the nature and even ‎with the composition of the mixed solvent.‎

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