PNNL

Abstract

A systematic study of anion partitioning, anion selectivity, and ion-pairing was performed for an extraction of individual cesium salts into 1,2-dichloroethane (1,2-DCE) using 4,5?-bis(tert-octylbenzo)dibenzo-24-crown-8 host. Equilibrium constants corresponding to the extraction of ion pairs and dissociated ions, formation of the 1:1 cesium/crown complex, and dissociation of the ion pairs in water-saturated 1,2-DCE at 25 °C were obtained from equilibrium modeling using the SXLSQI program. The standard Gibbs energy of partitioning between water and water-saturated 1,2-DCE was determined for picrate, permanganate, trifluoromethanesulfonate, methanesulfonate, trifluoroacetate, and acetate anions. The ion pairing behavior observed in the extraction experiments was shown to be consistent with the dissociation constant of the complex ion pair (Cs•4,4?- bis(tert-octylbenzo)dibenzo-24-crown-8)+NO3– determined independently by conductance measurements. The cesium/crown complex cation exhibited a weak tendency toward ion pairing and no discrimination among the tested anions, as attributed to the large effective radius of the complex cation, in agreement with the Fuoss treatment of ion pairing. These results provide insight into the inclusion properties of the clefts formed by opposing arene rings of the crown ether upon encapsulation of the Cs+ ion, whose lack of anion recognition likely reflects the preferential inclusion of 1,2-DCE molecules in the clefts. Observed anion selectivity, which was ascribed to solvent-induced bias selectivity in preference of large charge-diffuse anions, was nearly the same whether cesium salts were extracted as dissociated ions or ion pairs.