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 (ΔH s0 ) for complexation reactions were obtained from the slope of the van\\\\\\\'t Hoff plots and the changes in the standard entropy (ΔS s0 ) were calculated from the relationship: ΔG s,298.150 = ΔH s0 - 298.15ΔS s0 . The obtained results show that in most cases, the complexes are enthalpy stabilized but entropy destabilized.

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