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

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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.

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