Nanofiltration membrane performance of layer-by-layer membranes with different polyelectrolyte concentrations

Nanofiltration membranes produced with polyelectrolytes via the layer-by-layer technique are frequently researched, but misunderstood parameter is the polyelectrolyte concentration. Higher polyelectrolyte (PE) concentrations are known to produce thicker PE layers, but its effect on the membrane performance has only been studied in a limited fashion, leading to premature conclusions. In this work, two well-known strong polyelectrolytes, PDADMAC and PSS were used to prepare membranes using coating solutions with polyelectrolyte concentrations of 0.01, 0.1, 1.0, 2.5 and 5.0 wt% and two different salt concentrations in the coating solution of 0.05 and 1 M, as higher salt concentrations lead to thicker PE layers. The membrane performance of the prepared membranes is researched in terms of pure water permeability (PWP), molecular weight cut-off (MWCO) and the retention of different salts. In the first bilayer, membranes coated with a 0.05 M salt solution showed lower PWPs and MWCOs and higher salt retentions by increasing the PE concentration. After a certain number of coated bilayers, the MWCO and salt retentions reach a plateau for all PE concentrations; but the plateau value was obtained earlier by coating with a higher PE concentration. The membranes coated with the 1 M salt concentration had lower or comparable retention rates, except for MgCl₂, than those coated with 0.05 M salt. The higher salt concentration resulted in more abundant PDADMAC in the membrane, which promotes the MgCl₂ retentions for all bilayers. In conclusion, we found that the polyelectrolyte concentration significantly alters the membrane performance, but after coating 7 bilayers, the same size exclusion plateaus are reached.