Terminating layer effect on forward osmosis performance of layer-by-layer membranes with Pluronic® L-35 draw solution

While the application of layer-by-layer (LbL) assembly to fabricate nanoscale selective layers on the membrane support to be used in the forward osmosis (FO) process is well established, the role of terminating layer composition in governing physicochemical interactions with non-conventional draw solutions (DSs) remains unexplored. This work focuses on the physicochemical interactions between Pluronic® L-35 as an alternative DS, a thermoresponsive amphiphilic copolymer, and a polyacrylonitrile (PAN) ultrafiltration membrane coated with polyallylamine hydrochloride (PAH) and poly(sodium styrene sulfonate) (PSS). To facilitate LbL, the native PAN membrane was hydrolyzed in an alkaline medium to introduce carboxylate groups, thereby imparting a negative surface charge. The LbL-coated membranes were characterized by membrane surface charge, hydrophilicity, and in-situ Pluronic® adsorption. PAH-terminated membranes exhibited a positive surface charge and reduced hydrophilicity, while PSS-terminated membranes displayed a negative surface charge and increased hydrophilicity. PAH-terminated multilayers showed a higher swelling degree than PSS-terminated ones upon Pluronic® solution exposure, resulting in a more open multilayer structure. The pure water permeability of LbL membranes increased with successive coating steps due to enhanced membrane exposure time to acidic coating solutions, which promotes membrane deswelling, thereby enhancing the membrane pore size. The overall FO performance of the LbL membranes was governed by multiple independent parameters: Pluronic® absorption within multilayer’s water channels was critical for lower bilayer numbers, resulting in lower water flux; coating solution exposure time became the predominant factor for intermediate bilayer numbers, increasing water flux; multilayer density emerged as the limiting parameter for higher bilayers, declining the water flux. • Effect of terminating layer in LbL FO membranes with Pluronic® L-35 investigated • PAH-terminated membranes swell more than PSS-terminated ones upon Pluronic® exposure • Pluronic® absorption in the multilayers lowers J W for membranes with fewer bilayers • Membrane deswelling due to charge screening in acidic conditions enhances J W • Multilayer thickness and density decrease JW overruling Pluronic® absorption effect