A tale of two charges: zwitterionic polyelectrolyte multilayer membranes

In this thesis, the development of selective membranes for water treatment facilities to cope with the aforementioned issues is covered. By using hollow fiber membranes, the water purification process can be simplified compared to using spiral wound membranes, a significant advantage for decentralized water treatment plants. The selectivity of ultrafiltration membranes is improved by coating a dense separation layer on the membrane. For this, the simple and versatile “layer-bylayer” (LbL) technique is used. By exposing a negative substrate to a polycation in an aqueous solution, a thin layer of that polycation is adsorbed on the surface. With the modified substrate, now positively charged, the same process can be done with an aqueous solution of a polyanion. This process can be repeated over and over again, steadily building a polyelectrolyte multilayer on top of the substrate. If the chosen substrate is a porous support (e.g., an ultrafiltration membrane), the formed polyelectrolyte multilayers can be used as a selective layer in water purification. The versatility of the LbL technique allows for an easy control over layer properties, such as thickness, density and charge, by varying the coating conditions, the type of polyelectrolytes and the amount of layers. This versatility makes the LbL system well suited for the design of dense filtration layers. A key property of all membranes for water treatment purposes is the life time of the membrane. In this thesis we show that an adequate membrane life time can indeed be obtained for polyelectrolyte multilayers modified membranes, if certain criteria are met (Chapter 2). First of all, the presence of ionic groups on the membrane support significantly enhances the adherence of the multilayer on the membrane when high shear forces and reversed flow are applied. Second, the capability to withstand chemical degradation by hypochlorite is superior when quaternary ammonium polycations are used in the layer. We show that when both criteria are met, backwashable hollow fiber nanofiltration membranes can be made with a life time that is comparable to commercial ultrafiltration membranes.