Elucidating permselectivity mechanisms in polyamide membranes through controlled chlorination and hydrolysis

Polyamide membranes are widely used in freshwater production because of their high water permeance and water–salt selectivity, but emerging applications require tunable permselectivity. We investigate the structure–property–performance relationships governing membrane behavior by modifying permselectivity through sequential chlorination and alkaline treatment. Chemical, structural, and transport properties are examined using X-ray characterization, free-energy barrier analysis, and molecular dynamics simulations. We find chlorine–alkaline modification more than doubles water permeance through hydrolysis, which enhances swelling, increases water–membrane interactions, and promotes water transport. Despite the resulting looser structure, the hydrolyzed membrane maintains water–salt selectivity because of entropic barriers to salt transport, likely caused by the presence of bulky chlorine groups on the aromatic rings. These findings identify distinct mechanisms by which chemical modification alters water and salt transport, challenge the prevailing view that swelling is detrimental to selectivity, and provide new design insights for tuning polyamide membrane performance.