Ultrathin crown ether-based polyamide membrane for ion-ion separations

Membrane-based liquid separations that selectively extract valuable ionic species from water can enhance resource circularity across industries. However, commercial membranes lack the selectivity required to target specific ions. Here we design crown-ether-based polymeric membranes for ion–ion separations using principles inspired by biological ion channels. Ultrathin membranes (~6 nm) are fabricated via interfacial polymerization of crosslinked 18-crown-6 units. The membranes preferentially sorb and transport potassium, which forms the most stable complexes with the crown ether motifs. Sorption experiments show strong potassium preference in mixed-salt environments, where competitive interactions increase selectivity. Transport measurements demonstrate selective permeation of potassium over competing monovalent and divalent cations, with selectivities of ~4 over cesium and lithium. The combination of ultrathin architecture, high crosslinking degree, and high binding-site density enables this behavior. This work establishes interfacial polymerization as a strategy to incorporate macrocycles into membranes for precise ion separation.