Conductometric study of complexation process between dibenzo-18-crown-6 and K+, Na+, and La3+ cations in some binary mixed non-aqueous solvents

The complexation reactions between K+, Na+, and La3+ cations and the macrocyclic ligand dibenzo-18-crown-6 (DB18C6) were studied in acetionitrile (AN)-dimethylformamide (DMF) binary mixtures. Also, the complexation of K+ and Na+ in 50% ethanol (EtOH)-50% DMF and 50% EtOH-50% AN binary mixtures was studied. The conductivity data show that the stochiometry of all the complexes is 1: 1. A non-linear behavior was observed for the log Kf variation vs. composition of binary solvent, which was discussed in terms of heteroselective solvation and solvent-solvent interactions in binary solutions. It was found that the stability order of the complexes changes with composition of the mixed solvents. The stability sequence for AN-DMF (25 and 50 mol % DMF) solutions and pure AN at 25°C is [K(DB18C6)]+ > [Na(DB18C6)]+ > [La(DB18C6)]3+. However, at 75 mol % DMF it changes to [Na(DB18C6)]+ > [K(DB18C6)]+ > [La(DB18C6)]3+. The thermodynamical values (ΔHcpo, ΔScpo) for these complexation reactions were determined from the temperature dependence of the stability constants. The thermodynamics of the complexation reactions is affected by the nature and composition of the mixed solvents.