Thermodynamic behavior of complexation of 18-crown-6 with Tl+, Pb2+, Hg2+, and Zn2+ metal cations in methanol-water binary media

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 in methanol (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 formed in some binary solvents. The thermodynamic parameters (ΔHc0 and ΔSc0), which were obtained from the temperature dependences 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 (log Kf) 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.