Dynamic Sub‐Nanoscale “Water Fingers” in Interfacial Polymerization

Abstract Interfacial polymerization (IP) is widely used to fabricate high‐performance membranes, yet the molecular‐level dynamics that govern monomer transport across liquid–liquid interfaces remain poorly understood. Here it is reported that sub‐nanoscale “water fingers”—transient chains of water molecules—modulate the interfacial behavior of amine monomers during IP, dictating the structure and performance of the resulting polyamide films. Using molecular dynamics simulations of archetypal membrane‐forming systems ( m ‐phenylenediamine (MPD)–trimesoyl chloride (TMC) for reverse osmosis and piperazine (PIP)–TMC for nanofiltration), it is revealed that water fingers differentially stabilize monomer transport across the aqueous‐organic interface, correlating with experimentally observed disparities in film density and permeability. These findings offer a new physical picture of interfacial reactivity, establishing water fingers as critical, tunable elements of monomer transport. This work provides mechanistic insights into a century‐old reaction and opens new design strategies for ultrathin films and interfacial materials.