Highly selective cation transport through membranes containing lipophilic bis(monoaza-crown ether)s

Abstract

Ionophoric properties of lipophilic bis(monoaza-crown ether)s with 12-, 15-, and 18-membered rings, (1), (2) and (3)(n= 1,2,3), have been investigated in cation transport through supported liquid membranes. High ion selectivities were realized in facilitated cation transport through some membranes. Specifically, very high Na⁺ selectivity with respect to other alkali metal ions was found for the (1)(n= 1) membrane system. There are large differences in the transportability and the ion selectivity among the three bis(monoaza-12-crown-4) or bis(monoaza-15-crown-5) derivatives, while the bis(monoaza-18-crown-6) derivatives show a general selectivity for K⁺. The transportabilities and ion selectivities may be controlled by changing the pH in the aqueous phases, thus taking advantage of protonation by the bridgehead nitrogens. Proton-driven selective cation transport through the bis(monoaza-crown ether) membranes, based on the protonation–deprotonation cycle, is also demonstrated.