A thermodynamic study between 18-crown-6 with Mg2+, Ca2+, Sr2+ and Ba2+ cations in water-methanol and water-ethanol binary mixtures using the conductometric method

The complexation reactions between Mg2+, Ca2+, Sr2+ and Ba2+ cations with the macrocyclic ligand, 18-Crown-6 (18C6) in water - methanol (MeOH) binary systems as well as the complexation reactions between Ca2+ and Sr2+ cations with 18C6 in water - ethanol ( EtOH) binary mixtures have been studied at different temperatures using conductometric method. The conductance data show that the stoichiometry of all the complexes is 1: 1. It was found that the stability of 18C6 complexes with Mg2+, Ca2+, Sr2+ and Ba2+ cations is sensitive to solvent composition and in all cases, a non-linear behaviour was observed for the variation of log K-f of the complexes versus the composition of the mixed solvents. In some cases, the stability order is changed with changing the composition of the mixed solvents. The selectivity order of 18C6 for the metal cations in pure methanol is: Ba2+ > Sr2+ > Ca2+> Mg2+. The values of thermodynamic parameters (Delta H-c(o) and Delta S-c(o)) for formation of 18C6 - Mg2+, 18C6 - Ca2+, 18C6 - Sr2+ and 18C6 - Ba2+ complexes were obtained from temperature dependence of the stability constants. The obtained results show that the values of Delta H-c(o) and Delta S-c(o) for formation of these complexes are quite sensitive to the nature and composition of the mixed solvent, but they do not vary monotonically with the solvent composition.