A thermodynamic study of complex formation between 18-crown-6 with T1+, Hg2+ and Ag+ metal cations in some binary mixed non-aqueous solvents using the conductometric method

The complexation reactions between T1+, Hg2+ and Ag+ metal cations with 18-Crown-6 (18C6) were studied in acetonitrile (AN)-methanol (MeOH) and benzonitrile (BN)-methanol (MeOH) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complexes in most cases is 1:1 (ML), but in the case of the Tl+ cation, in addition to a 1:1 complex, a 1:2 (ML2) complex is formed in solutions. A non-linear behaviour was observed for the variation of log K-f of the complexes vs the composition of the binary mixed solvents. The stability of 18C6 complexes with T1+, Hg2+ and Ag+ cations is sensitive to solvent composition and in some cases, the stability order is changed with changing the composition of the mixed solvents. The values of the thermodynamic parameters (DeltaHdegrees(c), DeltaSdegrees(c)) for formation of 18C6-T1(+), 18C6-Hg+2 and the 18C6- Ag+ complexes were obtained from the temperature dependence of the stability constants and the results show that the thermodynamics of the complexation reactions is affected by the nature and composition of the mixed solvents and in most cases, the complexes are enthalpy destabilized but entropy stabilized.