Polyamide nanofiltration membranes with enhanced surface charge difference via large aqueous monomer

The charge properties of polyamide membranes play a crucial role in the selective ionic and molecular sieving processes. These polyamide membranes fabricated through interfacial polymerization normally exhibit a front surface enriched with carboxyl groups and a rear surface enriched with amine groups, rendering it with a Janus surface charge. However, this disparity in charge properties between the two surfaces has not been thoroughly investigated, despite its importance in understanding the membrane formation mechanism. In this study, polyethylenimine (PEI, Mw = 70000 Da) and piperazine (PIP, Mw = 86 Da), with distinct molecular size, were employed as aqueous monomers to react with trimesoyl chloride (TMC) at a support-free water hexane interface to synthesize polyamide nanofiltration membranes. The charge differences in the rear and front surface of PEI/TMC and PIP/TMC nanofiltration membrane was fully explored. The PEI/TMC membrane demonstrated a great charge disparity between the two surfaces with a negatively charged (isoelectric point = 5.62) front surface and a positively charged (isoelectric point = 9.59) rear surface. As a result, the rear surface exhibits a higher MgCl2 rejection (95.6 %) than the front surface (88.9 %). In contrast, the PIP/TMC membrane exhibited a similar negative charge and salt rejections on both surfaces. Moreover, the rear surface of PEI/TMC membrane exhibited a great anti-fouling performance to positively charged lysozyme (FDR = 51.1 %, FRR = 88.8 %). This work provides valuable insights into the mechanisms of an interfacial polymerization process