Ternary complex formation dramatically enhances metal-sorbing vesicle selectivity

Conventional wisdom derived from experimental and theoretical studies of metal ion transport in liquid membrane systems suggests that the selective behavior of closely-related metal-sorbing vesicles (MSVs) should depend on independent interaction of ions with the membrane-bound carrier and with the encapsulated water soluble chelator. From a theoretical perspective, however, interdependent interactions between carrier, chelator and metal ion in a ternary complex can be designed into MSVs to augment significantly their metal ion selectivity. In this paper, we compare and contrast two transport models so as to elucidate MSV selectivity based on initial metal ion uptake rates from single and multi-component metal ion solutions. Our findings show that metal ion transport mechanisms that allow for interdependent interactions between the carrier and chelator, namely formation of a ternary metal ion–carrier–chelator complex at the inner vesicle wall, can enhance the overall selectivity of MSVs in accordance with a multiplicative, rather than additive, function of equilibrium metal–ligand binding constants. Therefore, design of MSVs that rely on metal ion transport mechanisms involving ternary complex formation may provide for a more economic route to extremely selective systems that employ less extensively tailored and less expensive metal-binding ligands.