The importance of cation-ligand complexes due to non-covalent interactions became a focous of attention to crown ethers. Due to their ability to bind strongly and selectively to metal cations and substrate molecules, macrocyclic polyethers and their derivatives may find use in many diverse separation processes.Crown ethers have found applications in many of areas such as chemical sensors[1], bonded to the stationary phase in chromatographic techniques[2], phase transfer catalysts[3] and design of ion-selective electrodes[4].The complexation reactions between K+,Ag+,NH4+ and Hg2+ cations with the macrocyclic ligand dibenzo-18-crown-6 (DB18C6) were studied in ethylacetate(EtOAc)-dimethylformamide (DMF) binary mixtures at different temperature using conductometric method.The conductance data show that the stochiometry of all complexes is 1:1. A non-linear behaviour was observed for the variation of logKf of the complexes versus the composition of binary mixed solvents. It was found that the stability order of the complexes is changed with changing the composition of the mixed solvents which was discussed in terms of preferential solvation and solvent-solvent interactions in binary solutions. The sequence of stabilities for the K+,Ag+,NH4+ and Hg2+ complexes with DB18C6 in EtOAc-DMF binary solutions (mol % DMF= 25.0) and (mol % DMF = 50.0 ) at 25˚C is : (DB18C6.Ag)+ > (DB18C6.K)+ >(DB18C6.Hg)2+> (DB18C6.NH4)+, but in the case pure DMF is: (DB18C6.Hg)2+>(DB18C6.Ag)+≈ (DB18C6.NH4)+>(DB18C6.K)+. The values of thermodynamic quantities (∆H˚c , ∆S˚c) for these complexation reactions have been determined from temperature dependence of the stability constants and the results show that the thermodynamics of complexation reactions is affected by nature and composition of the mixed solvents and in all cases, positive values of ∆S˚c characterize the formation of these complexes.In addition , the experimental results show that the values of entropies for complexation reactions between K+.Ag+,NH4+ and Hg2+ cations with DB18C6 in EtOAc-DMF binary solutions do not change monotically with the solvent composition.

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