Realizing Precise Molecular Differentiation in Heterogeneous Polyamide Nanofilms Formed via Multi‐Stage Interfacial Polymerization

ABSTRACT High‐flux nanofiltration membranes capable of precisely separating similar‐sized molecules in solvents are essential for sustainable separations. Realizing such molecular‐level differentiation requires stringent control over mean pore size, pore uniformity, and the nature of the surface charge of the membrane. However, this remains a bottleneck in conventional thin‐film composite membranes due to their structural limitations, as a outcome of the interfacial polymerization at a single liquid‐liquid interface. Here, a surfactant‐mediated multi‐stage interfacial polymerization forming a tri‐layer polyamide nanofilm with structural heterogeneity along its thickness in the form of a composite membrane is reported. This tri‐layer structure is formed as a mould of the liquid‐liquid interface defined by three distinct reaction zones with two sub‐interfaces. The multi‐stage interfacial polymerization enables precise control over the thickness, degree of crosslinking, and the mean pore size of the nanofilms. The resulting nanofilm composite membranes show high methanol permeance (8.3 Lm −2 h −1 bar −1 ), efficient methanol‐rich mixed solvent separation, and precise molecular differentiation between two similar‐sized molecules (azobenzene and neutral red) with an extremely high selectivity of >480. This selectivity surpasses that of state‐of‐the‐art organic‐solvent‐nanofiltration membranes (OSN). The combination of outstanding separation performance and scalable fabrication establishes a strategy for the rational design of application‐specific OSN membranes.