Engineering polyamide network topology via competitive interfacial polymerization for superior water permeance and ion selectivity

Nanofiltration (NF) plays a critical role in advanced water treatment; however, the performance of conventional polyamide (PA) membranes is fundamentally limited by the permeance–selectivity trade-off, a consequence of the densely cross-linked network formed via trimesoyl chloride (TMC) and piperazine (PIP) reaction. To address this issue, this study presents a molecular-level engineering approach that redesigns the PA network architecture by employing a competitive interfacial polymerization (IP) process using mixed acyl chloride monomers. Specifically, isophthaloyl chloride (IPC) is introduced as a crosslinking modulator that competitively restrains the over-crosslinking of TMC and acts as a chain extender to enhance the flexibility of the polymer matrix. Density functional theory (DFT) calculations revealed that the distinct electrostatic potential differences among monomers drive this precise regulation, resulting in a hybrid network with optimized fractional free volume. This rationally tailored structure led to a significant improvement in membrane performance, exhibiting a high pure water permeance of 18.9 L m⁻² h⁻¹ bar⁻¹ (2.5 times that of the pristine PA-TMC membrane) without compromising selectivity. Furthermore, the engineered membrane featured a highly electronegative and carboxyl-rich surface, which strengthens membrane–ion interactions via a synergistic surface complexation–charge shielding mechanism, thereby creating elevated ion transport barriers and enabling superior divalent ion separation (e.g., 82.9 % for MgCl₂ and >99.0 % for Na₂SO₄). Efficient removal of both natural organic matter and persistent negatively charged contaminants was achieved, with rejection rates exceeding 95 % for PFOA and reaching 99.5 % for large-molecule PFAS (e.g., PFODA). This study introduces a method to control polymer network formation using competitive reaction kinetics, yielding high-performance separation membranes while enhancing the understanding of the structure-property relationships in PA films for environmental applications.