The complexation reaction of macrocyclic ligand, dibenzo-18-crown-6 (DB18C6) with UO 2 2? cation was studied in ethylacetate-1,2-dichloroethane (EtOAc/ DCE), acetonitrile-1,2-dichloroethane (AN/DCE), methanol-1,2-dichloroethane (MeOH/DCE) and ethanol-1,2-dichloroethane (EtOH/DCE) binary solutions at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complex formed between DB18C6 and UO 2 2? cation is affected by the nature of the solvent systems. A non-linear behaviour was observed for changes of log K f of (DB18C6.UO 2 ) ?2 complex versus the composition of the binary mixed solvents. The values of thermodynamic quantities (DS°c, DH°c) for formation of (DB18C6.UO 2 ) ?2 complex were obtained from temperature dependence of the stability constant using the van't Hoff plots. The results show that in most cases, the complex is enthalpy stabilized and in all cases entropy stabilized and both parameters are affected by the nature and composition of the mixed solvents. In addition, the complex formation between dicyclohexyl-18-crown-6 (DCH18C6) and UO 2 2? cation was studied in pure AN and the results were compared with those of the (DB18C6.UO 2 ) ?2 complex.

In the present work, the complexation process between UO 2 2? cation and the macrocyclic ligand, dicyclohexyl-18-crown-6 (DCH18C6) was studied in ethyl acetate/ 1,2-dichloroethane (EtOAc/DCE), acetonitrile/1,2-dichloroethane (AN/DCE), methanol/1,2-dichloroethane (MeOH/ DCE) and ethanol/1,2-dichloroethane (EtOH/DCE) binary solutions at different temperatures using the conductometric method. The conductance data show that in most cases, the stoichiometry of the complex formed between DCH18C6 and UO 2 2? cation is 1:1 [M:L], but in some solvent systems also a 1:2 [M:L 2 ] complex is formed in solutions. The values of stability constant of (DCH18C6ÁUO 2 ) 2? complex which were obtained from conductometric data, show that the stability of the complex is affected by the nature and also the composition of the solvent system and in all cases, a non-linear behavior is observed for the variation of (log K f ) of the (DCH 18C6ÁUO 2 ) 2? complex versus the composition of the binary mixed solvents. The values of thermodynamic quantities DH c and DS c for formation of (DCH18C6ÁUO 2 ) 2? complex were obtained from temperature dependence of the stability constant using the van't Hoff plots. The experimental results show that depending on the nature and composition of the solvent systems, the complex is enthalpy stabilized or destabilized, but in most cases, it is stabilized from entropy view point and both thermodynamic parameters are affected by the nature and composition of the binary mixed solutions.

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