Fabrication of thin-film composite membranes for organic solvent nanofiltration by mixed monomeric polymerization on ionic liquid/water interfaces

In this study, we developed a novel ionic liquid (IL) assisted method to fabricate polyamide/poly (m-phenylene isophthalamide) (PMIA) thin-film composite (TFC) membranes for organic solvent nanofiltration (OSN). The porous PMIA membrane substrates with a pore size range of 7–11 nm were cast from dope solutions prepared by an environmentally benign IL [EMIm][OAc]. While the selective layer was synthesized on the PMIA surface via interfacial polymerization (IP) between amine monomers (i.e., piperazine and polyethyleneimine) in the aqueous phase and aromatic acyl chlorides (i.e., phthaloyl chloride and 1,3,5-benzenetricarboxylic chloride) in the IL [BMIm][Tf2N] phase. The effects of the concentrations of each monomer on the separation performance of the TFC membranes were investigated. Results showed that a suitable selection of mixed monomers in IP dramatically boosted the total permeance (0.6 vs. 2.15 L m−2h−1bar−1) without sacrificing a high rejection to congo red (99.4% vs. 99.9%). Importantly, the TFC membrane prepared with ILs showed nearly 3 times improvement of the permeance than that prepared with n-hexane as the organic phase solvent and maintained high rejections (>99%) for both congo red and vitamin B12. This improvement is probably due to the high viscosity of the IL reduced the diffusion rate of the monomers in the organic phase and thus slowed down the interfacial reaction, making the selective layer much thinner and less cross-linked. The optimal membranes also exhibited strong tolerance to various organic solvents with good stability during the 54-h OSN tests. Positron annihilation spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy were also used to shed light on the enhanced separation performance due to ILs. Furthermore, the TFC membranes prepared by using recycled IL phase still exhibited a good separation performance. Accordingly, our results manifested the great potential and sustainability of the IL-assisted method to fabricate TFC membranes for practical OSN application.