Polyamide–based membranes with nanoscale homogeneity and asymmetric structure for ultrafast ion separation

Polymeric membranes are the most practical separation technology for extracting target ionic species necessary for water purification and clean energy, but are limited by the multi-scale heterogeneity, and transient and dynamic free volume pores. Modulated interfacial polymerization techniques are emergent concepts for preparing polyamide composite membranes with reduced thickness and homogeneous pore structures to satisfy the requirements for efficient ion separation applications. Herein, N–Cyclic cation-assisted kinetically regulated interfacial polymerization strategy to generate polyamide nanofilm composite membranes with tuned pore network and asymmetric structures for water purification is reported. Experimental characterization techniques and molecular dynamic simulations revealed that the N-Cyclic cations control the kinetic diffusion behaviour of the amine monomers to the aqueous/organic interface conducing to the formation of nanofilm membranes with fine-tuned polyamide network structures. Meanwhile, the N–Cyclic cation–regulated IP process not only affords to decrease the thickness of the formed polyamide nanofilm selective layers but also effectively narrows the effective pore size of the membranes. These membranes with thinner, homogeneous and asymmetric polyamide network structures exhibit enhanced water permeance of 55.7–60.42 L m−2 h−1 bar−1, and showed Na2SO4 rejection of 99.59–99.66% and MgCl2 rejection of 98.94–98.96% in nanofiltration separations, thereby outperforming the permeance–selectivity performance of the state-of-the-art polyamide-based membranes. The membranes also exhibit improved forward osmosis performances. We envision that this research will provide insightful foundations for the molecular tuning and construction of the next-generation structurally modulated polyamide nanofilms for various applications in molecular separation and water purification.