Elucidating the Roles of Polyamide Layer Structural Properties in the Permeability-Selectivity Tradeoff Governing Aqueous Separations

A tradeoff relationship between permeability and selectivity of reverse osmosis and nanofiltration aqueous separation membranes is increasingly being documented. However, there is currently no comprehensive mechanistic framework to describe the roles of membrane structural properties in the transport tradeoff. This study investigates two key structural properties of the widely used thin-film composite polyamide (TFC-PA) membranes, namely, the free volume element (FVE) size and the effective transport pathway, and examines their influence on the tradeoff behavior. Permeability and selectivity performance were characterized by challenging chemically modified TFC-PA membranes with two neutral organic tracers. Positron annihilation lifetime spectroscopy (PALS) determined that the FVE diameters slightly increased for more permeable membranes, but the marginal size enlargement cannot fully account for the permeability trend. Instead, analysis using the hindered transport model showed that shortening of the effective transport pathway is identified as having a more significant effect on raising the water permeability. On the other hand, membrane selectivity is found to be dominated by FVE size and is essentially independent of the transport pathway. Lastly, a framework reconciling experimental evidence with transport theory is proposed to relate the influence of membrane structural properties on the permeability-selectivity tradeoff. Findings of this study provide fundamental insights for understanding the transport phenomena in aqueous separation membranes.