Complex membrane formation using surfactant-regulated Marangoni instability at immiscible liquid-liquid interface

Interfacial polymerization (IP) is a method to fabricate uniform and defect-free polymer nanofilms for various academic and industrial applications, including sensors, electronics, batteries, catalysts, drug carriers, and separation membranes. The IP process occurs at an immiscible liquid-liquid interface through a type of step-growth polymerization. Therefore, understanding a mass transport mechanism is crucial to control the IP process. Specifically, surfactants have been introduced for the IP process, which can change the membrane’s characteristics, for instance the complexity of the interface shape and the filtration efficiency of polymer membranes. So far, various studies attempted to understand how nano-materials form at the interface, and several hypotheses have been introduced to explain the instability-triggering mechanism. However, the interfacial instability with surfactants is still unclear due to the lack of direct experimental observation. Thus, we decided to observe an interfacial flow field near the immiscible liquid-liquid interface. Here, the instability-driven mechanism underlying the surfactant-regulated interfacial formation of PA nanofilms was clarified by in situ visualization via particle image velocimetry (PIV). To focus on interfacial phenomena at the immiscible liquid-liquid interface, support-free interfacial polymerization (SFIP) was used to assemble a freestanding PA nanofilm at the interface between MPD (m-phenylenediamine) aqueous and TMC (trimesoyl chloride) organic solutions. A series of representative anionic (sodium dodecyl sulfate, SDS), cationic (dodecyl trimethyl ammonium bromide, DTAB), and neutral (Triton X-100, TX-100) surfactants by varying concentrations were added to the MPD solution to characterize the effect of different surfactants to the formation of PA nanofilms. Finally, we concluded that the critical role of Marangoni instability induced by the surfactants via various mechanisms in structurally regulating the nanofilms by observing the interfacial flow patterns.