Tuning Cationic Alkyl Chain Length in Zwitterionic Polysulfone Membranes for Enhanced Forward Osmosis Performance

Zwitterionic membranes are emerging as promising candidates for water treatment due to their highly hydrated surface layer. While common zwitterionic structures are well-established, there is a growing interest in incorporating varieties of cationic moieties without requiring an interfacial layer and while avoiding ester hydrolysis-prone bonds. However, few studies have investigated how the cationic moiety influences membrane performance. In this work, three polysulfone-based zwitterionic membranes were synthesized by varying the alkyl chain length, C4, C8, and C12 of the amine, while maintaining a constant sulfonate (SO3–) group. This set of membranes is analogous to that of polybetaines. Polymer modification was achieved via a surface ring-opening reaction between the 1,3-propane sultone (1,3 PS) and the amine-functionalized polysulfone (PSF-N). The fabricated membranes follow this labeling: PSF-zdiethylsulfonate (PSF-N+R4), PSF-zdibutylsulfonate (PSF-N+R8), and PSF-zdihexylsulfonate (PSF-N+R12). Structural characterization using 1H NMR and FTIR confirmed the successful completion of the zwitterion formation. Moreover, chain-length-dependent effects were observed in hydration behavior, surface charge, and morphology. All the modified membranes showed an enhancement in the surface hydrophilicity, with water contact angle values below 65°. In forward osmosis (FO), PSF-N+R4 had a 4-fold higher flux (57.2 ± 0.4) LMH over pristine PSF (9.3 ± 0.4) LMH. These water flux values are comparable to other works in the cited literature. Overall, these findings demonstrate the impact of cation design on the membrane performance and present a scalable approach for tuning zwitterionic materials. This work provides insight into the zwitterionic structure–function and another molecular view for the development of the next-generation FO membranes.