Nanobubbles-induced self-assembly of amphiphilic molecules for structural design and application to polyamide reverse osmosis membranes

Abstract

During the interfacial polymerization (IP) in liquid-liquid two-phase systems, nanobubbles introduce additional gas-liquid interfaces, enabling the self-assembly of amphiphilic molecules either in the bulk phase or at the interface. In this work, a strategy involving nanobubble-induced self-assembly of anionic and cationic surfactants was employed to create nanovoids with rapid water transport properties within polyamide (PA) reverse osmosis (RO) membranes, thereby enhancing the membranes' water permeance. Building on this concept, the formation of nanobubble@artificial water channel (AWC) nanoparticles—termed armored nanobubbles—was promoted through nanobubble-induced self-assembly of amphiphilic HCx (Synthesized by histamine (H) and isocyanate (Cx)) compounds. During the IP process, these stable armored nanobubbles facilitated the formation of abundant nanovoids within PA layers and anchored HCx AWC nanoparticles around the nanovoids via their unique carrier effect, achieving the directional loading of nanoparticles within PA layers. The synergistic effect of nanovoid water channels and localized sub-nano AWC nanoparticles significantly enhanced both water permeance and water/salt permselectivity of RO membranes. By varying the alkane chain length of HCx (x = 6, 7, 8), the PA-HC6 membrane exhibited superior water permeance (4.37 L m⁻² h⁻¹ bar⁻¹) and salt rejection (98.6 %) in brackish water desalination. This "one stone, three birds" strategy (nanobubble-induced self-assembly of amphiphilic molecules, nanovoid templates, and carrier effect) involving nanobubbles proposed in this study brings a new wave of technological innovation in combining amphiphilic molecules with IP engineering, providing a pathway for designing high-performance PA thin-film composite membranes.