Probing the Effect of Amine N-Oxide Zwitterionic Polymer Additives in Polysulfone Forward Osmosis Membranes

Forward osmosis (FO) is a passive liquid separation process where a hypertonic solution drives water across a semipermeable membrane. Compared to other membrane-based water purification processes, FO has the potential of producing clean water with minimal energy consumption. To achieve its potential, membranes with enhanced properties, such as high fouling resistance and solute rejection, high hydrophilicity and water permeability, and a wide pH and temperature tolerance, must be developed. In this work, we studied the effect of amine N-oxide zwitterionic polymer additives in the properties and morphology of polysulfone (PSf) based membranes. These additives were synthesized via a three-step reaction in which analogues of trimethylamine oxide (TMAO) were covalently incorporated into PSf. TMAO is a well-known organic osmolyte and its antifouling properties offer an interesting avenue to explore its potential use in water purification applications such as FO. Moreover, we studied three analogues which varied in their alkyl chain length: diethyl, dibutyl, and dihexyl. Our interest in these three alkyl groups was to systematically increase the carbon chain and study their effect in the overall membrane formation and performance. The successful synthesis of the amine N-oxide zwitterionic polymer additives was confirmed by FT-IR, NMR, and XPS. Then, we fabricated the membranes through the nonsolvent induced phase separation (NIPS) process. The morphology of the membranes was evaluated with SEM, while the hydrophilicity was monitored with contact angle measurements. Our results showed that the addition of these TMAO analogues significantly affects the PSf membrane formation and properties. Moreover, the PSf–DEAO membrane with the shortest carbon chain improve the membrane surface hydrophilicity and enhanced the water flux by a 4-fold increase. Altogether, our results suggest that longer alkyl groups could compromise the antifouling properties of the amine oxide.