Fouling-resistant membranes with zwitterion-containing ultra-thin hydrogel selective layers

Zwitterions are neutrally charged molecules with an equal number of cationic and anionic groups. Their exceptional fouling resistance makes them excellent membrane materials. This study focuses on thin-film composite (TFC) ultrafiltration membranes where the selective layers consist of zwitterion-containing hydrogels. To synthesize these membranes with ultra-thin hydrogel selective layers we used a novel method, Interfacially Initiated Free Radical Polymerization (IIFRP). While previous studies focused on poly(ethylene glycol) (PEG)-based selective layers, we aimed to determine the effect of incorporating two zwitterionic monomers, sulfobetaine methacrylate (SBMA) and 2-methacryloyloxyethyl phosphorylcholine (MPC), on permeability, selectivity, and fouling resistance. Compared to a PEG-based hydrogel selective layer, all zwitterion-containing hydrogel selective layers showed increased water permeance (12.2 ± 1.8 L/m2 h bar for SBMA, 7.3 ± 1.8 L/m2 h bar for MPC). Interestingly, this permeance increase was not accompanied by a major decrease in protein rejection despite the zwitterion-containing hydrogels having lower chemical cross-link densities, with myoglobin rejections staying >90%. This was likely due to physical cross-links between zwitterionic moieties. For the same total monomer concentration, higher zwitterion concentrations led to a permeance increase of almost an order of magnitude along with only minimal decrease in protein rejection. Moreover, all hydrogel selective layer TFC membranes tested exhibited excellent oil fouling resistance.