Polyelectrolyte multilayer membranes: An experimental review

Within just two decades, polyelectrolyte multilayer (PEM) membranes have moved from invention to their world wide application. One key advantage of these membranes is their versatility, allowing easy optimization through various tuning parameters. But it is exactly this versatility that has made it difficult to fairly compare PEM membranes. Previously, only Krasemann and Tieke (2000) have compared a large set polyelectrolyte pairs for their membrane performance, concluding that the polyelectrolyte charge density was a key tuning parameter. However, in these early days of PEM membrane research, characterization was rather limited. In this work, we have performed a thorough experimental review, studying 9 common polyelectrolyte pairs for their layer properties on model surfaces and their separation properties as pressure driven membranes. All systems were prepared under identical conditions to allow for a fair comparison, and membranes were characterized for their water permeability, molecular weight cut off, and ion retention. We find that the porous support is closed by a PEM when its thickness is in the order of the membrane support pore size, which was expected but not demonstrated before. Of all the PEM parameters, the degree of swelling stands out as a parameter that plays a key role and can be easily experimentally determined. Interestingly, we find that swelling increases with polyelectrolyte charge density, which is opposite to what Krasemann and Tieke proposed. We attribute this to the higher hydrophilicity of polyelectrolytes with a higher charge density. In turn, we find correlations between swelling and the water permeability as well as size exclusion, dielectric exclusion and Donnan exclusion. Still, the connection between polyelectrolyte properties and membrane properties is more complex and cannot be solely understood by swelling. Further understanding of this complexity will provide the tools to develop even better polyelectrolyte multilayer membranes.