Fig. 3: The mechanisms of lignin alkali (LA) in fabrication of thin film composite (TFC) membranes.

a The snapshot of the water-hexane interface of piperazine (PIP) solution with LA after 10 ns molecular dynamics (MD) simulation (Supplementary Fig. 4). b The free energy (ΔG) of PIP versus the z coordinates perpendicular to the water-hexane interface (right image is the schematic illustration of pulling the PIP from the aqueous phase to the organic phase). c The partition coefficient of PIP in organic and aqueous phases and PIP partitioned amount in hexane (the partition coefficient of PIP was calculated by Eq. (8), and PIP partitioned amount was determined by UV spectrophotometer characterization). d The normalized concentration of trimesoyl chloride (TMC) in hexane over reaction time during the interfacial polymerization (IP) process (the initial concentration of TMC is 0.05 wt%). e The diffusion rate of PIP in aqueous phase bulk solution by MD simulation (the insert represents the simulation box). f Surface element compositions and optical photos of polysulfone (PSF) substrates with PIP only and with both PIP and LA adsorption. g Schematic diagrams of formation of control-TFC (C-TFC) and LA-TFC membranes with different morphological polyamide (PA) layer. h and i Free energy variations for the reactions between acyl chloride group of TMC and amine group of PIP/phenolic hydroxyl group of LA, respectively (the alkyl chain in LA molecule was deleted in order to let structure convergence during the calculation process). Error bars represent standard deviation of three independent measurements.