The complexation processes of La3+, UO22+, Ag+ and NH4+ cations with macrocyclic ligand, dicyclohexyl-18-crown-6 (DCH18C6) was studied in acetonitrile-nitromethane binary solutions at different temperatures using the conductometric method. The conductance data show that with the exception of complexation of NH 4+ cation with this macrocyclic ligand, the stoichiometry of all the complexes which are formed in solutions, is 1:1 [M:L]. A different behaviour was observed in the case of complexation of NH4+ with DCH18C6 in which a 1:2 [M:L2] complex is formed in solutions. A non-linear behaviour was observed for changes of log Kf of (DCH18C6.La)3+, (DCH18C6.UO2)2+ and (DCH18C6.Ag)+ complexes versus the composition of the binary mixed solvents. This behaviour may be related to hetero selective solvation of the cations and the ligand and also solvent-solvent interactions in these binary mixed solvent systems. The values of thermodynamic quantities (ΔH°c, ΔS°c) for formation of the complexes were obtained from temperature dependence of their stability constants using the van t Hoff plots. The results show that in all solvent systems, positive values of ΔS°c characterize the formation of the complexes. The selectivity order of the ligand for the studied cations in pure acetonitrile and acetonitrile-nitromethane binary solutions (mol % AN = 50.0 and 75.0) was found to be La3+ > UO22+ > Ag+.

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