Unraveling the role of physicochemical interactions between the layer-by-layer membranes and Pluronic® L-35 as a draw solution

While Layer-by-Layer (LbL) membranes have been extensively investigated for forward osmosis (FO) using conventional draw solutions (DSs) such as inorganic salts, the effect of LbL membrane properties on the physicochemical interactions with more complex compounds as DSs lacks understanding. Here, we investigate the influence of LbL multilayer properties on the physiochemical interactions with Pluronics®, a thermo-responsive non-ionic amphiphilic copolymer, as a recently proposed energy-efficient DS. The thickness and density of the polyelectrolyte (PE) multilayer build with poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrene sulfonate) (PSS) are tailored by varying the salt concentration in the coating solutions and the number of bilayers (BLs). The effect of these is studied by measuring Pluronic® L-35 retention and water flux during FO. The Pluronic® L-35 transport mechanism through the membrane below and above its critical micelle concentration is studied via in-situ optical reflectometry analysis. The Pluronic® retention improved with more BLs due to increased multilayer thickness and Pluronic® adsorption on the membrane surface. The best-performed BL number is further studied for the salt concentration effect in the coating solution. Low-salt solutions formed denser multilayers with higher Pluronic® retention due to intrinsic charge compensation, while high-salt solutions created more open multilayers with lower retention due to extrinsic charge compensation. Salt concentration had the most significant impact among LbL parameters on the Pluronic® transport due to its influence on the multilayer density. These findings on the physicochemical interaction mechanisms and membrane performance provide a foundation for future research on exploring LbL membranes in FO using Pluronic® as DSs.