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.