One layer at a time: the fabrication and performance of layer-by-layer membranes
On September 26, 2023, Daniëlle Scheepers successfully defended her PhD thesis 'One layer at a time: the fabrication and performance of layer-by-layer membranes'
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Nanofiltration membranes are vital in several industrial applications, such as (partial) water desalination. Unfortunately, these membranes generally show a trade-off between a high water permeability and a good salt retention. This trade-off either limits the quality or the quantity of the purified stream. Nanofiltration membranes produced with the layer-by-layer (LbL) technique can positively shift this trade-off. However, it is still essential to understand the role of the different parameters in producing new LbL-based membranes, to further improve this trade-off, to make water more accessible and shortages less likely. Therefore, this work focused on understanding the influence of frequently used coating parameters in the LbL process.
The LbL technique uses polyelectrolytes (plastic with a charge) to continuously build layers because the positively charged and negatively charged polyelectrolytes bind to each other. How these layers are built is highly dependent on the conditions used. The build-up can be visualized as different layers of chicken wire, where every layer results in more overlapping of the wire with its underlying layers. The water stream and the pollutants go through these layers, where a higher number of layers increases the resistance, resulting in more difficulty for the water to go through these layers. This can be seen by the water permeability, which is generally lower with more layers, which is unwanted. Contrarily, the resistance by adding layers is one reason why pollutants and salt are retained, resulting in the purified water stream.
During this work, it was realized that there are different methods to control the thickness of the polyelectrolyte layers, hence controlling how much water can permeate relative to how much pollutants you retain. The thickness of the layers can be controlled by adding salt to your polyelectrolyte, adding more polyelectrolyte, or using different polyelectrolytes that carry less charge. The use of a combination of several polyelectrolytes in a membrane with different properties has also been investigated, which gives even more latitude to increase the water permeability. Moreover, by varying these parameters, you can also vary the overall charge your membrane has, e.g. negative or positive. This is a great advantage, as the composition of water streams can be very different depending on their origin. Many pollutants are also charged and are easier removed when the membrane carries the same charge. Hence a membrane can now be made tailored to the composition of the feed stream and the demands of the purified stream.
Daniëlle Scheepers defended her thesis 'One layer at a time: the fabrication of layer-by-layer membranes' on Tuesday, September 26, 2023. She was supervised by prof.dr.ir. Kitty Nijmeijer and Dr. Zandrie Borneman