Highly Selective Polyelectrolyte Multilayer Membranes Through Hydrophobic Interactions

Polyelectrolyte multilayer (PEM) membranes are highly promising for the removal of organic micropollutants (OMPs) from wastewater. However, for the removal of small OMPs, dense membranes with a low molecular weight cutoff (MWCO) are required. It has been recently demonstrated that MWCO correlates with PEM swelling by water. We therefore propose that dense membranes could be fabricated by enhancing hydrophobic interactions to decrease swelling. Controlled synthesis of hydrophobic polycations was achieved by quaternization of poly(4-vinylpiridine) (P4VP) with alkyl chains of varying length, which will also enhance its chemical stability. Optical reflectometry shows that multilayers can be successfully grown with the quaternized P4VPs (QP4VPs) and poly(styrenesulfonate) (PSS). Permeability and MWCO tests demonstrate that the membrane density can indeed be increased by increasing the length of the alkyl chain. Polycation terminated membranes are denser than polyanion membranes, likely due to the higher ratio of hydrophobic polycation. Propyl-QP4VP/PSS membranes have a MWCO as low as 230 Da. In line with the MWCO, OMP retention increases with increasing alkyl chain length. The QP4VP membranes studied here outperform commonly used poly(diallyldimethylammonium chloride) PDADMAC/PSS membranes in selectivity and are made with more chemically stable polyelectrolytes than other dense PEM membranes. The presented research confirms that PEM density correlates with hydrophobicity, and that PEM membranes can be densified through the addition of alkyl chains.