Acid acceptor modulated interfacial polymerization with piperazine-2-carboxylic acid as diamine monomer for ultrathin polyamide nanofiltration membrane

When introducing acid groups in the diamine monomer for the fabrication of polyamide (PA) membranes through interfacial polymerization (IP), the acid groups not only reduce the diffusion flux of diamine monomer for the polymerization to prepare ultrathin membrane but also provide the negative charges on the membrane surface. However, the introduction of acid groups often weakens the reactivity of diamine monomers, resulting in the formation of thick and loose PA membranes with low rejection of ions. This study proposes a strategy of modulating the IP process of piperazine-2-carboxylic acid (CPIP) by employing an acid acceptor (NaOH) to prepare thin nanofiltration membranes. The acid acceptor can neutralize the HCl to prevent CPIP from being protonated during the IP process, thereby facilitating the cross-linking reaction rate. The thickness of the resulting membrane decreases from 180 nm to 30 nm, accompanied by the fortification of the negative charge, an increase in cross-linking density, and a reduction in pore size. The resulting PA-AA/CPIP_1.0 membrane exhibits a water permeance of 44.6 L m⁻² h⁻¹ bar⁻¹, with a significantly increased Na₂SO₄ rejection of 98.4 % compared with 50.4 % of the PA-AA/CPIP_0 membrane fabricated without acid acceptor. This work may open a new avenue to fabricating high-performance PA membranes for nanofiltration.