RESUMEN
Over the past decade polyelectrolyte multilayer (PEM)-based membranes have gained a lot of interest in the field of nanofiltration (NF) as an alternative to conventional polyamide-based thin film composite membranes. With great variety in fabrication conditions, these membranes can achieve superior properties such as high chemical resistance and excellent filtration performance. Some of the most common polyelectrolytes used to prepare NF membranes are weak, meaning that their charge density depends on pH within the normal window of operation relevant for potential applications (pH 0-14). This might cause a dependency of membrane properties on the pH of filtered solutions, as indicated by other applications of PEMs. In this work, the susceptibility of membrane structure (swelling and surface charge) and performance (permeability, molecular weight cutoff, and salt retention) toward the pH of the filtration solution was studied for four fundamentally different PEM systems: poly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium-4-styrenesulfonate) (PSS) (strong/strong), poly(allylamine hydrochloric acid) (PAH)/poly(acrylic acid) (PAA) (weak/weak), and PAH/PSS (weak/strong) and PAH/PSS+PAH/PAA (asymmetric). Slight variations in structure and performance of the PDADMAC/PSS-based membranes were observed. On the contrary, structure and performance of PAH/PAA-based membranes are very susceptible to feed solution pH. A continuous change in charge density with variation in pH significantly affects salt retention. An increased swelling at pH 9 translates to variation in permeability and molecular weight cutoff of the membrane. The susceptibility of PAH/PSS-based membranes to pH is less pronounced compared to the PAH/PAA-based membranes since only one of the polyelectrolytes involved is weak. No structural changes were observed, indicating additional specific interactions between the polyelectrolytes other than electrostatic forces that stabilize film structure. A combination of the PAH/PSS and PAH/PAA system (8 + 2 bilayers) also displays a clear dependency of both membrane structure and performance on solution pH, where PAH/PSS is dominating due to a higher bilayer number.
RESUMEN
Polyelectrolyte multilayers (PEMs) are highly promising selective layers for membrane applications, especially because of their versatility. By careful choice of the types of polyelectrolyte and the coating conditions, the PEM material properties can be controlled to achieve desired separations. Less understood, however, is how the molecular weight (Mw) of the chosen polyelectrolytes (PEs) will impact layer build-up and thus separation properties. In this work, we investigate the influence of Mw on the performance of two types of PEM-based membranes. PEM membranes have been fabricated from low (15-20 kDa) and high (150-250 kDa) Mw poly(allylamine hydrochloride) (PAH), poly(sodium-4-styrenesulfonate)(PSS), and poly(acrylic acid) (PAA) to obtain PAH/PSS- and PAH/PAA-based nanofiltration membranes. For the linear growing PSS/PAH system, with low PE mobility, the Mw is found to influence the pore closure of the support membrane during coating but not its subsequent performance. In contrast, for the exponentially growing PAH/PAA system with a high PE mobility, much stronger effects of Mw are observed. For low-Mw PAH/PAA PEM membranes, separation properties are found that would be expected of a negatively charged separation layer, while for high-Mw PAH/PAA PEMs a positive separation layer is found. Moreover, molecular weight cutoff (MWCO) measurements show that the low-Mw PAH/PAA multilayers are much denser than their high-Mw counterparts. Here the higher mobility of the small PE chains is expected to lead to more optimal binding between the oppositely charged PEs, explaining the denser structure. Lastly, we find that PEM pH stability is lowest for low-Mw PAH/PAA multilayers which can again be attributed to their higher mobility. Clearly, the Mw can significantly influence the separation performance of PEM-based membranes, especially for more mobile PEM systems such as PAA/PAH.