Membranes capable of selectively separating ions with the same charge and valency are of great interest for the recovery of critical minerals, such as lithium and sodium from continental brines, and nickel, cobalt, and manganese from battery recycling wastes.1 Crown ethers (CEs) are promising candidates for selective ion recognition and separation due to their ability to form complexes with specific cations through coordinative metal-oxygen interactions.2 However, integrating CEs into membrane structures remains challenging, as their coordination behavior becomes less predictable within dense polymer matrices. Factors such as cavity size, ring strain, chain spacing, and metal ion properties (e.g., valency, hydration energy, and the presence of ligand-field effects) can significantly influence their interactions.3 Thus, prior studies have reported conflicting results regarding CE-metal ion interactions in polymer membranes functionalized with CEs. For instance, while 12-crown-4 ether is known to be ideal for Li+ binding due to its cavity size, it has been observed to exhibit stronger binding toward Na+ when embedded in certain polymer matrices.4 In other cases, larger crown ether-based moieties, such as 15-crown-5 or 18-crown-6 ethers, have been explored instead to enhance Li+ selectivity, highlighting the need for a deeper understanding of membranes with crown ether functionalities.5,6
To address this knowledge gap, particularly in the context of interfacially polymerized nanofiltration membranes, we incorporated diaza-crown ether moieties with varying cavity sizes and ether-to-amine ratios into the polymer backbone during interfacial polymerization. Chemical and physical characterization techniques, including FTIR spectroscopy and X-ray diffraction, were combined with ion transport experiments using a broad library of cations (alkali metals, alkaline earth metals, and transition metals) to establish structure–property–transport relationships. In addition, data-driven supervised learning models were employed to predict ion–ligand interaction strengths and correlate them with transport behavior. We found that incorporating crown ether moieties into the polymer backbone may restrict the conformational flexibility necessary for specific ion recognition. Instead, ion selectivity appeared to be governed by the modulation of ion diffusivity and ion–oxygen interactions, both of which correlate with the dehydration energy of the ions. Additionally, we found that strong ion–oxygen interactions can lead to increased ion rejection despite enhanced partitioning, suggesting a more complex transport mechanism. These findings prompt a reevaluation of the working principles traditionally attributed to nanofiltration membranes. Overall, this study evaluates the feasibility of incorporating diaza-crown ether into the backbone of interfacially polymerized nanofiltration membranes and provides insights into the rational design of crown ether-based membrane materials for selective separation of cations from aqueous sources.
References
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