Loosening ultrathin polyamide nanofilms through alkali hydrolysis for high-permselective nanofiltration

Preparation of membranes with both ultrathin thickness and loose structure is often required for high separation performance, but remains a grand challenge. Herein, we report a two-step method to acquire ultrathin loose polyamide (ULPA) nanofilms. More specifically, in the first step, we prepare the ultrathin polyamide nanofilms by support-free interfacial polymerization and in the second step we loosen the ultrathin polyamide nanofilms by alkali hydrolysis. The uniform distribution and diffusion of piperazine at the aqueous-organic interface accelerates the self-sealing and self-healing of interfacial polymerization and generates ultrathin but dense polyamide nanofilms. Further, by hydrolyzing amide bonds with alkali solution, the pore size of polyamide nanofilms is dramatically enlarged. The composite membranes with ULPA nanofilm as separating layer display a water permeance of 29.3 L m−2 h−1 bar−1, 160% higher than that of the pristine polyamide membrane, with an ultrahigh Na2SO4 rejection of 99.1% at 1000 ppm concentration and an outstanding Cl−/SO42− selectivity of 91.8. This ultrathin loose structure may provide a new solution to design membranes with extraordinary permselectivity.