Interfacial surface charge and the selectivity of cation transport through a liquid membrane by a lipophilic crown ether carboxylic acid

Abstract

A kinetic model has been developed to describe the transport selectivities exhibited by a liquid membrane system incorporating a lipophilic crown ether carboxylic acid carrier. Under conditions of rate-limiting cation/carrier complex desorption from the interface, the rate an ion is extracted is related to the fraction of interfacial sites occupied by the carrier complex of that ion. The fractional site coverage is related to bulk aqueous concentrations and the surface charge by a modified Stern adsorption isotherm. Experimental values of constants of complexation and acidity at the interface were determined from single-ion transport studies and were utilized to correlate transport selectivities for binary K⁺/Na⁺ and Sr²⁺/K⁺ mixtures. The overall transport selectivity is shown to be a composite of both thermodynamic and kinetic factors. The relevance of the model to extraction and transport in related systems is discussed.