Annealing of Polyelectrolyte Multilayers for Control over Ion Permeation

Abstract

AbstractPolyelectrolyte multilayer based membranes are highly promising systems to create stable and versatile nanofiltration membranes. One very popular and well‐studied polyelectrolyte pair, is the polycation poly(diallyldimethylammonium chloride) (PDADMAC) and polyanion poly(sodium 4‐styrenesulfonate) (PSS), due to its excellent separation properties and high chemical and physical stability. Membrane charge can be easily controlled by simply terminating the multilayer by either PDADMAC or PSS. Unfortunately, a phenomenon that occurs during multilayer coating, is overcompensation by PDADMAC. In this study, it is shown that overcompensation of PDADMAC results in a positive surface charge even when the multilayer is PSS‐terminated. In addition, it is shown that this leads to poorer membrane separation properties with sulfate retention decreasing from 94 to 39%. At the same time, it is demonstrated that a so‐called annealing cycle with a high salt concentration leads to recovery of the negative surface charge, increasing the sulfate retention from 39 to 95%. Even for multilayers at which no irreversible positive surface charge is measured, separation properties improved substantially (increasing sulfate retention from 94 to 97%, at a higher membrane permeability) after salt‐annealing. It is concluded that post‐treatment by salt‐annealing results in an improved membrane performance and allows an additional degree of control over the membrane separation properties.