Molecular design-driven sub-nanometer cavity engineering in polyamide via programmable caprolactam modulation for high-performance SWRO membranes

The growing global freshwater demand drives the need for high-flux seawater reverse osmosis (SWRO) membranes with enhanced desalination performance. The aromatic polyamide (APA) layer of SWRO membranes is a two-dimensional nanofilm with a networked structure, formed through m-phenylenediamine (MPD) and trimesoyl chloride (TMC) condensation, followed by cross-linking, folding, and compaction processes. Therefore, precise regulation of the benzene ring spacing in APA layer, along with optimization of the size and distribution of free volume, holds great promise for significantly enhancing permeation flux while maintaining ultra-high rejection. This study employed a “chain linking - molecular detachment - cavity construction” strategy by introducing caprolactam (CPL) during interfacial polymerization. The CPL molecules reacted with acyl chloride groups and weakened hydrogen bonding and π-π interactions through steric hindrance effects, achieving sub-angstrom-level expansion of benzene ring spacing, thereby modifying the APA network. Subsequent water-induced CPL detachment from the APA matrix generated hydrophilic carboxylic groups while leaving molecular-scale cavities. This process simultaneously increased free volume dimensions and quantity while narrowing pore size distribution of APA. The optimized SWRO membranes achieved distinguished 59.85 L m⁻² h⁻¹ water flux with 99.70 % NaCl rejection. This work provides a novel approach for designing high-efficiency, low-energy SWRO membranes through molecular-level structural engineering.