Membrane transport systems. V. Coupled countertransport of alkaline earth cations and protons

Abstract

The extraction and transport of alkaline earth cations from a basic to an acidic solution across an artificial membrane by crown ether carboxylic acids are reported. Monocarboxylate carriers achieve the transport via 2:1 complexes or by ternary complexes in the presence of readily extractible anions, while dicarboxylate carriers transport cations as 1: 1 complexes. Both types of carrier exhibit two regimes of kinetic behaviour depending on the relative concentrations of the carrier and the metal ion: a zero order regime in which the rate depends only on the carrier concentration, and a reversible consecutive first order regime in which the rate depends only on the metal ion concentration. In the former, the rate of adsorption of the carrier on the interface is presumed to be rate limiting while in the latter, the diffusion of metal ion to the interface is rate limiting. Carrier structure exerts a general influence over both the rate and selectivity of transport but this influence varies with the kinetic regime considered.