Loosely nanostructured polyamide membranes with rapid water transport for efficient molecule/ion separation

Highly permeable nanofiltration membranes comprising selective polyamide nanofilms hold significant promise for energy-efficient molecule/ion separations. However, current polyamide-based nanofiltration membranes, made through polymerization between highly reactive piperazine and triacyl chloride, exhibiting high retention of divalent salts, limiting their applicability for molecule/ion separations such as dye or antibiotics desalination. Herein, we report the fabrication of a loosely nanostructured poly(bipiperidine-amide) membrane via dorsal coating interfacial polymerization (DC-IP) using Kevlar hydrogel as porous support. The hydrogen-bonding and electrostatic interaction between bipiperidine and Kevlar hydrogel play a role in the formation of winkled ring-shaped nanostructures, which effectively enhance water transport area. By employing 4,4′-bipiperidine, a nonplanar monomer with a longer reaction size distance, the resulting membranes exhibited higher free volume and stronger pore connectivity compared to poly(piperazine-amide) counterparts, as evidenced by both experimental and simulation analyses. The impact of monomer concentration and solution pH on the DC-IP parameters influencing membrane separation performance was thoroughly investigated. Importantly, the optimized polyamide membranes demonstrated an exceptional water permeance of 70.1 L m⁻² h⁻¹ bar⁻¹, high dye removal efficiency (Congo red, 99.4 %), and low divalent salt rejection (Na₂SO₄, 38.6 %). Furthermore, the membranes exhibited high antifouling capability and long-term operational stability, rendering them highly promising for rapid and durable dye/salt separations. This study underscores the potential of utilizing moderately reactive bipiperidine to fabricate high-porosity polyamide membranes for fast molecule/ion separation.