Charge exclusion as a strategy to control retention of small proteins in polyelectrolyte-modified ultrafiltration membranes

While retention in commercial ultrafiltration membranes is commonly governed by size exclusion, addition of charged polyelectrolytes to the membrane surface has been proposed as a facile and inexpensive method to modulate retention during filtration of charged compounds, such as proteins. This study demonstrates that selected combinations of common polyelectrolytes can be efficiently used to control the retention of proteins. We show how the retention of positively charged lysozyme increased from ~7% to ~50% when a negatively charged polyelectrolyte was deposited on a commercial polysulfone membrane, most probably due to a combination of associated pore narrowing effect and protein adsorption onto the membrane. By contrast, the retention decreased again (from ~50% to ~30%) upon further addition of a positively charged polyelectrolyte onto the membrane. The electrostatic repulsion between the positively charged surface and proteins prevented protein adsorption onto the membrane, while pore size was still large enough that size exclusion was limited. The retention decreased further to ~22% when increasing amounts of polyelectrolytes of the same charge as the protein were deposited on the membrane. In addition, variations in pH revealed that the negative effect of polyelectrolytes of the same charge as the protein on lysozyme retention was reversed when the charge density of lysozyme reached a certain level beyond which the positively charged electrolyte promoted dramatic increases of rejection. The results of this study suggest that use of polyelectrolyte-modified ultrafiltration membranes with a much larger pore size than the charged solute to be filtered could be a strategy to control retention of such solutes by minimizing the effect of size exclusion and fouling.