Ion Association in Nanoconfinement for Precision Separation
Selective ion transport in spatial confinement is central to environmental and energy sustainability, including desalination, energy conversion, and resource recovery. While biomimetic nanochannel membranes often rely on size exclusion or fixed ion-channel interactions, real-world separation systems involve multicomponent ionic mixtures where ion-ion interactions, such as dynamic pairing and competition, play a decisive yet underexplored role. Pan and coworkers address this gap by developing a functionalized membrane enabling dynamic manipulation of ion-ion interactions triggered by external ionic stimuli, achieving precise and reversible control over target ion transport. The membrane is constructed from aligned MXene nanosheets functionalized with γ-PGA through covalent and hydrogen bonding, establishing well-defined spatial confinements and adjacent amino and carboxyl groups that serve as anion-cation binding sites. This design enhances ion-pair formation, yielding ion selectivity and stimulus-responsive performance rivalling biological channels. Notably, when K+ and Mg2+ co-permeate, anions preferentially associate with K+ within the nanochannel, disrupting Mg2+ transport despite Mg2+'s higher affinity for channel walls. The MLM–γ-PGA membrane exhibits uniform channel architecture and remarkable aqueous stability. Ion transport characterization using a U-shaped diffusion cell with chloride salt solutions shows that with 0.2 M MgCl2 feed, Mg2+ permeation rate is 3.16 × 10−3 mol m−2 h−1. Introducing 0.2 M KCl suppresses Mg2+ permeation by two orders of magnitude to 3.2 × 10−5 mol m−2 h−1, with full reversibility over multiple cycles. This work highlights the potential of exploiting ion-ion interactions in nanoconfinement for precision separation.