Layer-by-layer coatings on ion exchange membranes: Effect of multilayer charge and hydration on monovalent ion selectivities

Electrodialysis (ED) is an important process to desalinate brackish water, which contains scale-forming divalent ions. Hence, there is an interest in separating mono- and divalent ions. In recent years, layer-by-layer (LbL) polyelectrolyte coatings on ion exchange membranes (IEMs) have provided high monovalent ion selectivities. However, there is a lack of understanding on the structure-property relationships for polyelectrolyte multilayers for membrane applications and lack of evaluation under practical conditions. In this work, we evaluate polyelectrolyte multilayer properties by optical techniques, connect them to monovalent-selectivity using resistance measurements and upscale this to practical monovalent-selectivities for an ED unit. Excess positive poly(allyl amine) (PAH) was observed in the PAH/PSS (polystyrene sulfonate) multilayer, resulting in a positively charged multilayer. Coated multilayers with low hydrations (0.2) are able to achieve high monovalent-selectivities (of 5.7 up to 7.8) on CEMs comparable to the commercial monovalent-selective CSO (with 6.9). This enhanced selectivity was only observed for cations with LbL-coated CEMs and not for anions with LbL-coated AEMs, which we argue is due to the excess positive charge of the PAH/PSS multilayers. Finally, LbL-coatings show an improved monovalent-selectivity in ED stack experiments with artificial brackish water of 1.7 for LbL-coated CEMs compared to uncoated CEMs (with a monovalent-selectivity of 0.5).