Tuning Structure and Selectivity of Thin-Film Composite Membranes via <i>Ex Situ</i> Generated Nanobubbles

Nanobubbles (NBs) play a critical role in tailoring polyamide (PA) morphology and optimizing membrane separation performance. However, conventional in situ generated NBs during interfacial polymerization reactions suffer from uncontrolled size distribution and undesirable interference with heat and mass transfer, which complicates their application in precise membrane fabrication. To overcome these limitations, this study introduces ex situ generated NBs with controlled sizes into the IP process for the fabrication of thin-film composite (TFC) membranes used in reverse osmosis (RO) and nanofiltration (NF). By comprehensive characterization and analysis, we elucidate the distinct and competing mechanisms of NBs in affecting two types of membranes. In RO membranes, NB-induced diffusion inhibition leads to the formation of a thinner PA layer, thereby enhancing water permeability without compromising salt rejection. In NF membranes, where the IP reaction proceeds more slowly, NBs are more readily incorporated into the PA layer. Their collapse drives defect formation while also modifying surface characteristics of the NF membrane, leading to a notable decrease in membrane selectivity. This work not only demonstrates the versatility of NBs as a novel tool for modulating TFC membrane structure and performance but also highlights the critical interplay between NB stability and IP reaction dynamics. These findings provide new strategies for the rational design and optimization of advanced membranes for water treatment applications.