Dynamic Sub-Nanoscale "Water Fingers" in Interfacial Polymerization

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.