An efficient co-solvent tailoring interfacial polymerization for nanofiltration: enhanced selectivity and mechanism

The interfacial polymerization (IP) plays a fundamental role in the fabrication of polyamide (PA) nanofiltration (NF) membranes with high ion selectivity and water permeability, which has enormous value for the increasing permeate quality and production in desalination. Herein, this study adopted a weak-polar organic solvent of ethyl formate (EF) to effectively regulate the IP process between piperazine (PIP) and trimesoyl chloride (TMC). The engineered PA separate layer conferred a uniform and ultrafine pore size (∼0.195 nm), exhibited a precise solute separation, and maintained a desirable permeability (18.4 ± 0.9 L m−2 h−1 bar−1). Molecular dynamics (MD) simulation indicated that EF involvement would significantly reduce the interfacial tension to motivate a fast and abundant diffusion of amine monomers from the aqueous phase into organic phase during the IP reaction, contributing to engineering a PA film with higher crosslinking degree. The optimized PA NF membranes conferred a more efficient rejection for divalent anions and cations (e.g., 99.5% for Na2SO4 and 98.8% for MgCl2). Remarkably, the membrane endowed a fairly high mono/divalent ion selectivity at sub-angstrom size, such as 145 for Cl−/SO42− and 61 for Na+/Mg2+. The outstanding NF performance successfully broke the "trade-off" bound of perm-selectivity and was superior to the most "state-of-the-art" NF membranes. Overall, the co-solvent based IP strategy was beneficial to developing high-performance NF membranes without any significant changes to the existing fabrication process, and its facile and cost-effective merits were full of significance to the membrane manufacture.