A thermodynamic study of interaction of Na+ cation with benzo-15-crown-5 in binary mixed non-aqueous solvents

The complexation reaction between benzo-15-crown-5 (B15C5) with Na+ cation was studied in acetonitrile-methanol (AN-MeOH), tetrahydrofuran-methanol (THF-MeOH), acetonitrile-tetrahydrofuran (AN-THF) and nitromethane-dimethylformamide (NM-DMF) binary mixtures at different temperatures using conductometry method. The conductometric data show that in most cases, the stoichiometry of the complex formed between B15C5 and Na+ cation is 1:1 (ML), but in the cases of AN-THF (mol%THF = 65.7) binary mixture and in pure THF, a 1:2 (ML2) complex is formed in solutions, which shows that the stoichiometry of the complexes may change by the nature of the medium. The stability constant of the resulting 1:1 complex in pure non-aqueous solvents decreases in the order: AN > MeOH > DMF. The thermodynamic parameters (Delta HA degrees(c) and Delta SA degrees(c)) for formation of (B15C5.Na)(+) 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 both enthalpy and entropy stabilized. A non-linear behavior was observed for changes of log K-f of (B15C5.Na)(+) complex versus the composition of the binary mixed solvents, which was explained on the basis of changes occurring in the structure of the mixed solvents and also the preferential solvation of the cation, ligand and the resulting complex in solutions.