Assembly‐Regulated Interfacial Polymerization for Janus Polyamide Membranes With Programmable Ion‐Transport Properties

Conventional interfacial polymerization is typically governed by rapid and disordered monomer diffusion, which limits precise regulation over through‐thickness structure and electrostatic distribution in polyamide nanofiltration membranes. Here, a reaction‐induced interfacial self‐assembly strategy is developed to regulate membrane growth dynamics and construct Janus polyamide membranes with directional ion‐transport behavior. By introducing 1,14‐dibromotetradecane into the piperazine/trimesoyl chloride interfacial polymerization system, amphiphilic quaternized intermediates are preferentially generated at the water/oil interface and reorganize the local reaction environment prior to continuous polyamide formation. This process transforms conventional diffusion‐dominated interfacial polymerization into an assembly‐regulated growth pathway, resulting in a selective layer with asymmetric through‐thickness chemistry, compact sub‐nanometer transport channels, and positively enriched bottom domains. The resulting membrane exhibits synergistic size‐sieving and localized Donnan exclusion effects for Mg 2+ /Li + separation, achieving a separation factor of 333.7 at a Mg 2+ /Li + mass ratio of 80 together with stable operation over 310 h. Molecular simulations further reveal that the asymmetric electrostatic architecture imposes substantially higher transmembrane free‐energy barriers for Mg 2+ than for Li + , thereby governing directional ion transport. This work demonstrates that transport functionality can be directly encoded during membrane formation through reaction‐field regulation, providing a feasible strategy for constructing asymmetric polyamide membranes with programmable ion‐transport properties.