Thermodynamic study of complex formation between 18-crown-6 and potassium ion in some binary non-aqueous solvents using a conductometric method.

The complexation reaction between a macrocyclic polyether, 18-crown-6 (18C6), and potassium ion was studied in methanol (MeOH)-acetonitrile (AN), dimethylformamide (DMF)-AN and propylecarbonate (PC)-DMF binary solvent systems at different temperatures using a conductometric method. It was found that the stability of the 1:1 complex formed between K(+) ion and this ligand increases with decreasing temperature. Standard enthalpies and standard entropies of the complex formation were obtained from the temperature dependence of the stability constant. In all cases negative DeltaH(o)(c) and DeltaS(o)(c) values characterize the formation of 18C6-K(+) complex. The results obtained show that the stability of the complex is governed by the solvent medium and the thermodynamic parameters DeltaH(o)(c), DeltaS(o)(c) and DeltaG(o)(c) are sensitive to the composition of the mixed solvents. In addition, it was found that the stability constant of the resulting 1:1 complex among various neat solvents used varies in the order PC > MeOH > AN > DMF.