Two-phase partition studies of alkali cation complexation by ionophores

Summary The partition of alkali cations and anions between an aqueous and an immiscible organic phase has been studied in the absence and presence of neutral and carboxylic ionophores of the valinomycin and nigericin types, respectively. Cation extraction into the organic phase was augmented considerably by the ionophores, and a cation specificity of K⁺Rb⁺>Cs⁺Na⁺ was found for all the neutral ionophores tested. Evidence is given that the actual values of ion specificity are a function of the solvent polarity, especially for valinomycin where an inversion of the K⁺/Rb⁺ specificity was observed. The ionophores examined have the following rank order of effectiveness for K⁺ extraction into a standard organic phase consisting of 70% toluene-30%n-butanol: valinomycin>18-crown-6trinactin>enniatin Bdinactin>monactin>nonactin. The ion affinity and selectivity data thus obtained have been compared with data previously reported.In a toluene-butanol solvent, extraction of cations in the absence of ionophores occurs as ion pairs. On the other hand, the neutral ionophores extract the cations by the mechanism of complexation, with the lipophilic anions coextracted as free gegenionic species at lower ionophore complex concentrations. When the concentration of extracted cations exceeds 1×10^–4 m, ion pairing between the ionophore complex and the anion occurs, and this tendency increases with increasing concentration and decreasing polarity of the organic phase. Anion pairing with the complexed cations is much less than for the free cations and this effect appears to be due to the larger distance of closest approach of the anion for the complexed cation.