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Interfacialpolymerization (IP) provides a versatile platform forfabricating defect-free functional nanofilms for various applications,including molecular separation, energy, electronics, and biomedicalmaterials. Unfortunately, coupled with complex natural instabilityphenomena, the IP mechanism and key parameters underlying the structuralevolution of nanofilms, especially in the presence of surfactantsas an interface regulator, remain puzzling. Here, we interfaciallyassembled polymer nanofilm membranes at the free water-oilinterface in the presence of differently charged surfactants and comprehensivelycharacterized their structure and properties. Combined with computationalsimulations, an in situ visualization of interfacial film formationdiscovered the critical role of Marangoni instability induced by thesurfactants via various mechanisms in structurally regulating thenanofilms. Despite their different instability-triggering mechanisms,the delicate control of the surfactants enabled the fabrication ofdefect-free, ultra-permselective nanofilm membranes. Our study identifiescritical IP parameters that allow us to rationally design nanofilms,coatings, and membranes for target applications.