Precise Control of Charge-Structure Coupling in Polyamide Membranes for Mono- and Divalent Ion Separation

Selective separation of monovalent and divalent ions sharing the same charge sign is critical for treating complex aqueous streams such as natural brines and industrial wastewater. However, conventional nanofiltration (NF) membranes, which rely primarily on charge-based exclusion, often struggle to achieve efficient separation of counter-ions, limiting their applicability. Herein, we report a composition-regulated zwitterionic copolymer grafting strategy that enables precise tuning of membrane surface chemistry and pore structure of NF membranes. A zwitterionic copolymer, poly(2-methacryloyloxyethyl phosphorylcholine-co-2-aminoethyl methacrylate hydrochloride) (P(MPC-co-AEMA)), with tunable MPC/AEMA ratios, is grafted onto the polyamide (PA) selective layer via secondary interfacial polymerization. Rather than relying solely on the introduction of zwitterionic functionalities, this approach leverages the balance between reactive anchoring segments (AEMA) and zwitterionic segments (MPC) to regulate grafting density, surface charge, and structural evolution of the PA layer. Through systematic control of copolymer composition, the membrane properties can be finely tuned from strongly charged to near-neutral surfaces with concurrently reduced pore size, thereby suppressing charge-sign-dependent electrostatic interaction effects and promoting sterically governed ion transport. This work demonstrates that balancing reactive anchoring and zwitterionic segments offers a rational strategy for tuning membrane physicochemical properties and advancing NF membranes toward charge-sign-independent ion-selective separations.