Influence of charge density and ionic strength on diallyldimethylammonium chloride (DADMAC)-based polyelectrolyte multilayer membrane formation

Although the layer-by-layer technique to produce nanofiltration membranes has been studied frequently, the influence of ionic strength of the coating solution in relation to the charge density lacks understanding. Here, we investigate the effect of both parameters systematically by comparing two strong polyelectrolyte sets: poly(acrylamide-co-diallyldimethylammoniumchloride) (P(AM-co-DADMAC))/ poly(sodium-4-styrenesulfonate) (PSS) and polydiallyldimethylammoniumchloride (PDADMAC)/PSS, with a polycation charge density of 32 and 100%, respectively. The influence of the charge density and the ionic strength during layer formation (5∙10−5 M – 1 M) has been researched in terms of polyelectrolyte adsorbance, membrane surface charge, and filtration performance. A low charge density is a limiting parameter for polyelectrolyte adsorption and retention, due to limited uptake of two bilayers. Consequently, it is impossible to form NF membranes with solely P(AM-co-DADMAC)/PSS layers. For high charge density polyelectrolytes, increasing the ionic strength is crucial to increase PE adsorption and MgSO4 retention. However, there is an opposite effect of ionic strength and charge density on membrane surface charge and pure water permeability. An increasing ionic strength results in a decreasing surface charge and increasing permeability for P(AM-co-DADMAC)-based membranes, and vice versa for PDADMAC. These opposite trends undoubtedly show the importance of simultaneously taking into account the charge density and ionic strength.