Mechanism of Permselectivity Enhancement in Polyelectrolyte-Dense Nanofiltration Membranes via Surfactant-Assembly Intercalation

Enhancing the water permeance while maintaining the solute rejection of a nanofiltration (NF) membrane can potentially result in significant cost-reduction for NF—a membrane process that excels in several unique environmental applications of growing interests. In this work, we demonstrate for the first time that intercalation of surfactant self-assemblies in the polyelectrolyte multilayer (PEM) can lead to significant performance enhancement of salt-rejecting dense NF membranes fabricated using layer-by-layer assembly of polyelectrolytes. Specifically, the intercalation of sodium dodecyl sulfate (SDS) bilayers in a PEM comprising poly(diallyldimethylammonium chloride) (PDADMAC) and poly (sodium 4-styrenesulfonate) (PSS) resulted in a decrease in PEM thickness, increase in pore size, and a smoother and more hydrophilic surface. The water permeance of the resulting PEM NF membrane increased by 100% without compromising the rejection of Na2SO4. Experiments with a quartz crystal microbalance also provide direct evidence that the intercalation of the surfactants substantially reduces the subsequent adsorption of the polyelectrolytes of a similar charge. Based on its mechanism of performance enhancement, surfactant intercalation may become a universally applicable and highly cost-effective approach for dramatically enhancing the performance of PEM NF membranes.