Study on complex formation of dicyclohexyl-18-crown-6 with Mg2+, Ca2+ and Sr2+ in acetonitrile-water binary mixtures by conductometry

 The complexation reactions between Mg2+, Ca2+ and Sr2+ cations and dicyclohexyl-18-crown-6 (DCH 18C6) have been studied in acetonitrile–water binary mixtures at different temperatures by conductometry. The formation constants of the resulting 1:1 (M:L) complexes for all the three cations were determined from computer fitting of the molar conductance versus mole ratio data. The results show that the selectivity order of DCH 18C6 for the metal cations in the acetonitrile-water binary solvent at different temperatures (25, 35 and 45 ºC) was Sr2+ > Ca2+ > Mg2+ based on size fit concept. A linear relationship was observed between the formation constants (log Kf) of these complexes and the composition of acetonitrile-water binary mixtures (70:30, 80:20, 90:10, mol%). The values of thermodynamic parameters (∆Ho and ∆So) for complex formation in binary systems were obtained from temperature dependence of formation constants of complexes using the van’t Hoff plots. The complex formation reactions were spontaneous and enthalpy stabilized.