ε-Polylysine Regulated Nanofiltration Membrane with High Free Volume for Efficient Separation of Dye/Salt

Highly selective solute-solute separation remains a significant challenge for nanofiltration membranes, with the free-volume microstructure playing a critical role in determining perm-selectivity. This study presents a thin-film composite membrane fabricated through interfacial polymerization between bio-based ε -polylysine ( ε -PL) and triformyl chloride (TMC) for efficient dye/salt separation. ε -PL, with its unique molecular chain structure and steric hindrance, slows diffusion while maintaining adequate reactivity, promoting the formation of a selective layer with an optimal free-volume. In comparison to the small-molecule lysine used as an aqueous monomer, the polyamide membrane prepared with ε -PL exhibits a larger free-volume fraction (from 15.64% to 38.85%), enhanced pore connectivity, and a higher molecular weight cutoff. The optimized membrane demonstrates excellent performance, including high pure water flux (99.1 L·m -2 ·h -1 ), mixed solution permeate flux (58.1 L·m -2 ·h -1 ), and an impressive dye/salt separation factor (up to 597.8 for NaCl/Congo Red). The design strategy outlined here offers valuable insights for developing nanofiltration membranes with tunable pore structures, particularly for dye desalination applications.