The pH as a tool to tailor the performance of symmetric and asymmetric layer-by-layer nanofiltration membranes

Layer-by-layer assembly of polyelectrolyte layers is a versatile method to produce nanofiltration membranes. The membrane formation and subsequently performance is easily tailored with pH as it varies the polyelectrolyte charge density. Here, membranes are fabricated at different pH (4,8, and 9) with branched polyethyleneimine (PEI)/poly(sodium-4-styrenesulfonate) (PSS) layers (symmetric membranes) or a combination of poly(diallyldimethylammonium chloride) (PDADMAC)/PSS base layers terminated with PEI/PSS layers (asymmetric membranes). Overall, increasing the pH lowers the PEI charge density, which increases PEI adsorption, as measured by optical reflectometry and positive zeta potential. For symmetric systems, decreasing the charge density decreases the salt retention, because fewer intrinsic linkages are formed. Contrarily, asymmetric membranes, independent of charge density, show retentions >90% for MgSO4 and Na2SO4. Additionally, the benefit of asymmetric membrane formation is proven by comparing the best membrane performances. Asymmetric membranes prepared at pH = 4 form an open base layer and defect-free dense selective layer, resulting in much higher permeabilities compared to symmetric membranes (-13 and -9 L/ (m2hbar)), and significantly improving MgSO4 and Na2SO4 retentions (>95% compared to >90%). By combining two well-known polycations and tailoring the pH, versatile membranes are produced, without the need for synthesis or modification steps while obtaining improved water fluxes and salt retentions.

Automated summary

Layer-by-layer assembly of polyelectrolyte layers is used to fabricate versatile nanofiltration membranes, with pH as a determinant factor in membrane formation and performance. By varying the pH, the charge density of polyelectrolyte layers can be adjusted, affecting salt retention and adsorption. Symmetric membranes with different pH levels (4, 8, and 9) and asymmetric membranes with a combination of base layers terminated by PEI/PSS layers are compared. The results show that increasing the pH leads to lower PEI charge density, causing higher adsorption. However, reducing charge density in symmetric membranes decreases salt retention, while asymmetric membranes consistently achieve >90% retention for MgSO4 and Na2SO4. The advantage of asymmetric membrane formation is demonstrated by the improved performance compared to symmetric membranes, with higher permeabilities and salt retentions (>95% for MgSO4 and Na2SO4). This pH-tailored assembly method allows for the production of versatile membranes with enhanced performance in terms of water flux and salt retention, without additional synthesis or modification steps.