Discussion on Complexation Reactions between Dicyclohexyl-18-Crown-6(DCH18C6)with Na~+,K~+,Cs~+,Rb~+,and Tl~+Metal Cations in Acetonitrile-Water Binary Mixtures

摘要

The complex formation between Na~+,K~+,Cs~+,Rb~+ and Tl~+ metal cations with macrocyclic ligand,dicyclohexyl-18-crown-6(DCH18C6)was studied in acetonitrile-water(AN-H2O)binary systems at different temperatures using conductometric method.DCH18C6 forms 1:1 complexes with these metal cations.The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program Genplot.The results show that the selectivity order of DCH18C6 for the metal cations in acetonitrile-water mixtures(AN=25.3 and 50.4 mol %)is:Tl~+>K~+>Rb~+>Cs~+>Na+.A non-linear behaviour was observed between the log K_f of the complexes versus the composition of the mixed solvent which it related to changes of acidity,basicity,polarity and also polarizability of AN-H2O mixtures with the composition of this binary solution.The values of standard enthalpy changes(DELTA S_s~0)for complexation reactions were obtained from the slope of the van't Hoff plots and the changes in the standard entropy(DELTA S_s~0)were calculated from the relationship:DELTA G_(s,298.15)~0=DELTA H_s~0- 298.15 DELTA H_s~0.The obtained results show that in most cases,the complexes are enthalpy stabilized but entropy destabilized.