Interfacial Entropy Drives Crystallization of Covalent Framework Membranes for Precise Ionic Separation

ABSTRACT Achieving highly crystalline covalent organic framework (COF) membranes is essential for efficient mass transport but remains a longstanding challenge due to the inherent trade‐off between structural regularity and processability. Herein, we report an entropy‐regulated interfacial crystallization strategy that redirects membrane formation from kinetically trapped disorder to thermodynamically favored crystallization. By introducing ion‐dipole interactions at the interface, the configurational entropy of monomers is markedly reduced by 326.9 J mol −1 K −1 , enforcing ordered preorganization of monomers. Besides, solvent‐mediated diffusion induces framework growth beneath the nascent layer, giving rise to an asymmetric membrane structure composed of a dense, highly crystalline selective layer supported by a fibrous macroporous sublayer. The resulting membrane exhibits long‐range ordered channels, a high surface area up to 1721 m 2 g −1 , and enhanced mechanical robustness. Benefiting from these ordered channels, the membrane delivers a high Cs + permeation rate of 0.17 mol m −2 h −1 and an exceptional Cs + /La 3+ selectivity of 292 in mixed ion systems. This work establishes interfacial entropy regulation as a general and effective route for controlling crystallization in interfacial systems, offering new insights into the rational fabrication of framework‐based separation membranes.