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

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 m² 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⁺/La³⁺ 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.