Effect of solvent on the stability constant of complex formation and the thermodynamic parameters between dicyclohexyl-18-crown-6 with Eu3+, La3+, Er3+ and Y3+ cations

The complexation reaction between Eu3+, La3+, Er3+ and Y3+ cations with the dicyclohexyl-18-crown-6 (DCH18C6) in acetonitrile (AN)–dimethylformamide (DMF) and AN–methanol (MeOH) binary systems have been studied at different temperatures using conductometric method. The conductometric data show that the stoichiometry of the complexes is 1:1 [ML]. The results show that the stability constant of complexes in various solvents is: AN > MeOH > DMF. In the some cases, the minimum of logKf for (DCH18C6–Eu3+), (DCH18C6–La3+), (DCH18C6–Er3+) and (DCH18C6–Y3+) complexes in AN–MeOH binary systems obtain at χMeOH ~ 0.75, and also, the logKf of (DCH18C6–Er3+) complex in AN–DMF binary systems show a minimum at χAN ~ 0.75. Non-linear behavior was observed for the stability constant of complexes versus the composition of the solvent systems. The experimental data show that the selectivity order of DCH18C6 for these cations in AN–MeOH binary systems (mol% AN = 50, 75) at 25 °C is: Y3+ > Er3+ > Eu3+ > La3+. The values of thermodynamic parameters (∆H˚C) for formation of complexes were obtained from temperature dependence of stability constants of complexes using the van′t Hoff plots and the standard entropy (∆S˚C) were calculated from the relationship: ∆G˚C, 298.15 = ∆H˚C −298.15∆S˚C. The results show that the values of these thermodynamic parameters are influenced by the nature and the composition of the binary systems.