Transfer of alkali-metal and hydrogen ions across liquid/liquid interfaces mediated by monensin. A voltammetric study at the interface of two immiscible electrolyte solutions

Abstract

Under certain conditions the interface between an aqueous phase and an organic phase (ITIES, interface of two immiscible electrolyte solutions) has certain properties analogous to a metal/electrolyte-solution interface. By polarization of the ITIES using potential-sweep voltammetry reproducible voltammograms corresponding to ion transfer across the ITIES are obtained. In the presence of cation-complexing ionophores in the organic phase cation transfer is facilitated. The acidic form HX of carboxylic ionophores, monensin A and B, acts in the nitrobenzene/water system as a sodium carrier while the complex of its anion with sodium or lithium cation (M⁺) is a proton carrier. The equilibrium constants of the reactions M⁺+ HX ⇌ MHX⁺ and MX + H⁺⇌ MHX⁺ have been determined from the voltammograms. The stabilities of the complexes correspond to the series Li⁺ > Na⁺≫ K⁺. The substitution of methyl (monensin B) for ethyl (monensin A) on ring C increases the stability of the sodium complex by ca. 1 kJ mol^–1 and has no effect on the acidity of the carboxy group.