Interfacial Protonation Augments Piperazine Terminals in Polyamide for Intensified Membrane Positive Charge Effects

Nanofiltration (NF) has been widely adopted to accelerate lithium extraction from salar brines, addressing a pressing need in the lithium-ion battery industry. This highlights the current urgency to devise NF membranes with high Li⁺/Mg²⁺ selectivity while ensuring ease of industrial production. Herein, with acids blended into the organic phase before initiating the conventional interfacial polymerization (IP) of classic monomers, piperazine (PIP) and trimesoyl chloride, interfacial protonation of PIP during IP was implemented. Mechanistic insights, probed through acid-PIP interactions in an organic milieu, unveiled interfacial binding, enabled by strong acids, which preferentially preserves amine terminals. With the unexpected and remarkable enhancement of membrane surface positivity, resulting membranes exhibited substantially higher rejection of MgCl₂ compared to Na₂SO₄ and an exceptional Li⁺/Mg²⁺ selectivity (>30), outperforming most NF membranes fabricated via one-step IP. Also, they served as the best qualified candidate to distinguish the steric/charge effects in Li⁺/Mg²⁺ separation. At the early yet faltering exploration of developing positively charged membranes, interfacial protonation, not only providing one innovative thinking of augmenting amine groups, empowered by the diverse range of available acids, is poised to represent an IP regulation paradigm shift to ignite extensive research.