Membrane Surface Science
For many membranes and membrane processes, it is the outer few nanometers of a membrane surface that determine critical performance parameters such as selectivity and fouling. Within MSuS, we aim to understand and control the interactions at the membrane surface for improved performance, but also to create additional functionalities. The focus is on polymeric membranes and the application of polymeric coatings for various functional enhancements, including anti-fouling, enhanced separations, easy-to-clean membranes and virus in-activation. Typical coatings include polymer brushes, polyelectrolyte multilayers and thin layers of self-assembled diblock-copolymer. Moreover, a strong driver for the group is the desire to make membrane technology more sustainable. Membrane materials are prepared under completely aqueous conditions, without utilizing the unsustainable and toxic organic solvents that are usually required.
The group is built on the expertise and vision of Associate Professor Wiebe M. de Vos and combines the more fundamental fields of physical chemistry, surface science and polymer science with the more applied field of membranes. The research within the group is split between three distinct but strongly related research lines: 1. Interactions at the membrane surface, 2. Membrane coatings and 3. Advanced membrane materials. Here we provide a short overview of research within these research lines.
Interactions at the membrane interface
Fig. 1 Schematic picture of well stabilized (left) and poorly stabilized (right) oil droplets at the interface of an ultrafiltration membrane.
In complex feed mixtures such as surface waters, biological systems, and industrial waste waters it can be very difficult to understand and predict membrane performance. A key example is produced water, that contains oil droplets, surfactants, salts, inorganic particles and dissolved organics (e.g. benzene, toluene). Membrane Technology is a very promising technique for the treatment (cleaning) of such produced water, but fouling and a lack of oil rejection are common problems. The complexity of such systems can be overwhelming, but by using well characterized feed streams and careful experiments it does become possible to understand the major effects playing a role. For example, well stabilized oil droplets will from an open cake layer (figure 1, left side) while poorly stabilized droplets will from an oil layer blocking the porous structure (figure 1, right side). Clearly the oil droplets are the main fouling agent, but the chemistry of the water (surfactants, salts, pH) determines the fouling severity (Dickhout et al. 2017, JCIS, 487, 523-534).
Figure 2. A polyelectrolyte multilayer on the inside of a hollow fiber UF membrane, imbues it with NF/RO type separation properties to remove small organic molecules.
A coating can imbue a membrane with new separation properties, and can allow an ultrafiltration membrane to become a nanofiltration membrane (Grooth et al. 2015, JMS, 475, 311-319), or allow a microfiltration membrane to remove viruses (https://www.utoday.nl/science/64543/safe-drinking-water-for-developing-countries). But within MSuS, we always strive to create coatings that are multifunctional, for example coatings that provide the desired separation properties while at the same time leading to anti-fouling, responsive and/or easy to clean membranes (Ilyas et al. JCIS, 446, 386-39). Another important driver for the group is the development of novel NF/RO type membranes specifically designed for the removal of so-called micro-pollutants (figure 2). These are small organic molecules (100-1000 Dalton) that stem from industrial, medicinal and agricultural waste, and that can damage the environment and human health already at very low concentrations. For these coatings we apply self-assembly of oppositely charged polyelectrolytes at the interface of a porous support membrane. In this so-called Layer-by-Layer assembly, the support membrane is alternatively exposed to polycations and polyanions, to build polyelectrolyte multilayer’s (PEMs) of controllable thickness. A large advantage of this approach is that the properties of the PEM layer, responsible for the separation properties of the membrane, can be tuned by choice of polymer and by the employed coating conditions such as pH and ionic strength. This method thus allows the design of a membrane truly optimized towards micro-pollutant removal, while, for example, still allowing small ions to permeate (Ilyas et al. 2016, JMS, 537, 220-228)
Advanced membrane materials
Under the right conditions, membrane formation and membrane functionalization can be attained in a single step. For example, the use of diblock-copolymer self-assembly for membrane fabrication not only leads to very uniform pore sizes (figure 3a), but membranes can be made responsive to pH and temperature by choice of the right block-copolymer. Control over the exact composition of the solvent during membrane casting, provides further control over the membrane structure (Vriezekolk et al. JMS, 2016, 504, 230-239). But controlled precipitation of responsive polymers is also a highly promising method to create novel membranes completely from water. In figure 3b, we show an example of a porous membrane prepared completely under aqueous condition. The membrane retains a responsive nature. Clearly organic solvents are not always required to obtain promising membrane structures.