Determination of the thermodynamic parameters, stoichiometry, and stability constants for complex formation of dicyclohexyl-18-crown-6 and 15-crown-5 with Y3+, La3+, and Hg2+ cations in binary solutions

The complexation of Y3+, La3+, and nd Hg2+ cations with macrocyclic ligands, dicyclohexyl-18-crown-6 (DCH18C6) and 15-crown-5 (15C5) have been studied in acetonitrile (AN)-N,N-dimethylformamide (DMF) binary solutions at different temperatures using conductometric method. The conductance data revealed 1: 1 [ML] stoichiometry for most complexes in pure DMF and AN-DMF binary solutions, except for the (DCH18C6-Y3+) complex in pure AN (1: 2, [ML2]). The stability constants of DCH18C6-La3+ and 15C5-La3+ in pure AN were higher than in pure DMF at all temperatures. Nonlinear behavior was observed for the stability constants of complexes against the composition of AN-DMF binary solutions at all temperatures. The minimum log Kf value for the 15C5-La3+ complex in AN-DMF binary solutions was obtained at χAN = 0.5, which may be due to negative excess viscosities ηE of AN-DMF mixtures over the whole composition range with a minimum value of χAN = 0.5. Moreover, the selectivity order of DCH18C6 and 15C5 for Y3+, La3+, and Hg2+ cations 25°C depended on the AN-DMF ratio. The thermodynamic parameters (ΔHC0) for complex formation were obtained from the temperature dependences of the stability constants of the complexes using the van’t Hoff plots, and the standard entropy (ΔSC0) was calculated from the relationship: ΔGC, 298.150 = ΔHC0 − 298.15ΔSC0.