The Membrane Surface Science group participates actively in the teaching of the Chemical Engineering curriculum, both in the BSc and the MSc phase. The following are the courses offered and their details:
- 193735000 - C.S. Membrane Technology
- 201200117 - Membranes for Gas Separation
- 201300049 – Advanced Molecular Separations
Many different BSc and MSc project are possible within the MSuS group. Projects can involve making or coating membranes (Material Science), but can also be more focused on characterizing membranes and optimizing their performance for a specific application (Process Technology). For a most up-to-date overview of possible projects please contact firstname.lastname@example.org.
Some examples of possible projects are given below:
Elif Nur Durmaz (1), Wouter Nielen(1), Joshua Willott (1), Wiebe de Vos(1)
(1)Membrane Surface Science (MSuS), Membrane Science and Technology Cluster
Membranes are used in a variety of processes including the production of clean drinking water, artificial kidneys, CO2 capture, and food processing. Due to their large application areas and many advantages of membranes over other separation technologies, the membrane market is continuously growing. However, a major concern regarding membrane production is the use of large quantities of aprotic organic solvents such as N-Methyl-2-pyrrolidone. Such solvents are harmful to humans and the environment. This project is focused on investigating a novel, environmentally friendly technique for the production of membranes without using organic solvents. Here, membrane formation relies on the phase separation of polyelectrolytes either through a pH switch or a complexation with an oppositely charged polyelectrolyte. The main aim is to understand which factors govern the morphology and separation performance of the membranes. Additionally, improving mechanical and chemical stability with cross-linking will be investigated.
Figure 1 - Porous symmetric and porous asymmetric structure of membranes obtained by tuning conditions of the aqueous phase separation
Polyelectrolytes are polymers that contain repeating charged groups. Due to these charges, they are soluble in polar solvents like water. Polyelectrolytes can be separated into two categories based on the charged groups. 1. Weak: when their charge depends on the pH of the medium or 2. Strong: when the charge is independent of pH. Weak polyelectrolytes can be used to precipitate a hydrophobic polyelectrolyte from water by shifting to a pH where the polyelectrolyte is uncharged. Alternatively, this can also be used to charge an uncharged hydrophilic weak polyelectrolyte. When this is mixed with an oppositely charged strong polyelectrolyte, they form a complex, which can be solid. In both cases, a phase separation occurs and under the right conditions, this can be used to form a porous, asymmetric membrane. By controlling the rate at which the polyelectrolyte is charged or uncharged the kinetics of the phase separation can be controlled which in turn gives control over the structure of the precipitate that is formed. This allows us to fabricate membranes with controllable properties. To fine-tune membrane performance e.g. membrane flux or retention, additives can be used. Additionally, the mechanical and chemical stability can be significantly improved with cross-linking. To assess the membrane performance several filtration techniques are used. In addition, the morphology is imaged using scanning electron microscopy. The membrane can then be optimized towards specific applications, such as gas-separation, ion removal or the treatment of oily wastewater.
This project is expected to lead to a new and sustainable approach to produce membranes. Moreover, the newly produced membranes are expected to have unique separation properties relevant to a number of applications.
Muhammad Irshad Baig(1), Wiebe M. de Vos(1)
(1)Membrane Surface Science, Membrane Science and Technology Cluster, MESA+ Institute for Nanotechnology, University of Twente, Faculty of Science and Technology, P.O. Box 217, 7500 AE, Enschede, The Netherlands.
A membrane is a barrier that regulates the flow of species in a selective way. The most commonly used membranes in water purification and gas separation applications are made from polymers that are only soluble in organic solvents. Ask anyone who works in the field of polymeric membranes about its fabrication procedure, and you will mostly hear one answer i.e. NIPS. It stands for Non-solvent Induced Phase Separation. Simply put, you take a polymer (Polysulfone, Polyimides for example) and dissolve it in an organic solvent such as N-methyl-2-pyrrolidone (NMP). When the solution is homogeneously mixed, it is then cast on a flat surface and put in a water bath. The solvent (NMP) is highly miscible in water and so it leaves the polymer solution and mixes with water. The polymer, at this point, becomes insoluble due to all the water that is around it, forming a solid porous membrane. The membranes prepared in this way are also known as ‘Loeb-Sourirajan Membranes (L-S membranes)’, named after the researchers who first made asymmetric cellulose acetate membranes for seawater desalination.
This simple procedure of making commercial polymer membranes has remained the same since the 1970s. NMP is by far the most widely used solvent to prepare membranes and the multi-billion dollars membrane industry uses this solvent in immense amounts.
However, NMP is not a friendly chemical by any means. It is partly flammable, and most importantly, it has been proven to be repro-toxic to humans and harmful for the environment.1 Therefore, it has to be recycled and/or removed from waste streams plus it has to be removed from the membranes before they can be utilized for water production. The EU has introduced a law that restricts the use of such chemicals throughout the Union.2
Having discussed the nature of aprotic, polar organic solvents and their harmful effects, we have an alternative solution which uses only water as a solvent. It is extremely difficult and almost impossible to dissolve conventional polymers in water. This leaves us with a special class of polymers called ‘Polyelectrolytes’ which can be easily dissolved in water. These polyelectrolytes are charged polymers with a unique property that their charge can be manipulated by a pH switch. Some polyelectrolytes are only charged below a particular pH. They are called weak polyelectrolytes. On the other hand, certain polyelectrolytes remain charged in almost all the pH range. These are called strong polyelectrolytes. When the two oppositely charged polyelectrolytes come across each other, they instantly form a ‘Polyelectrolyte Complex’. So if we make a homogeneous solution of these two polyelectrolytes such that they are uncharged in the solution and then cast it in a water bath with a pH where both become charged, we get a polyelectrolyte complex membrane. This novel approach to making membranes is termed ‘Aqueous Phase Separation’ (APS).
Project Details and Outcome
In this project, Poly(sodium 4-styrenesulfonate) and Poly(allylamine hydrochloride) will be used as strong and weak polyelectrolytes, respectively. The aim of this study would be to investigate the effect of the monomer molecular weight on the final membrane properties. During the course of the project, the effect of pH and salt concentration of coagulation bath would also be investigated in detail. Monomer molecular weight is a very important factor that determines the final structure and properties of the membrane. A low molecular weight polyelectrolyte usually results in an open structure (porous) membrane with water fluxes that are in the range of Microfiltration and Ultrafiltration. However, this is not the only control parameter. Coagulation bath pH and the salt concentration in the bath are one of the most crucial parameters as well. In this project, all these effects will be studied to have a better understanding of APS.
Figure 1. Schematic illustration of polyelectrolyte complexation induced by a pH switch.
The membranes prepared in this study will be used for water treatment and purification. The applications are as diverse as the new APS field itself. From oil/water separation (Microfiltration) to seawater desalination (nanofiltration and reverse osmosis), the outcomes of this study are diverse.
It is important to mention here that one of the inventors of L-S membranes, Loeb Sidney, was an MSc. student at UCLA when he got the breakthrough in membrane science and technology. Similarly, this project, part of a larger research goal, will play an important role in shaping the next generation of polyelectrolyte membranes.
- D. Prat, J. Hayler and A. Wells, Green Chem., 2014, 16, 4546–4551.
- L. Bergkamp and N. Herbatschek, Rev. Eur. Comp. Int. Environ. Law, 2014, 23, 221–245.
Dennis Reurink(1), Wiebe M. de Vos (1)
(1)Membrane Surface Science (MSuS)
An emerging separation and desalination technology that shows great potential is forward osmosis (FO). FO uses osmotic pressure gradients to transport water across a semipermeable membrane. These membranes are usually based on reverse osmosis (RO) membranes, which lack, however, suitability for FO applications . A way to create new membranes truly optimized for FO, is by incorporating aquaporin containing vesicles (AQPVs) in the separating layer . These vesicles have the aquaporin protein incorporating in the vesicle wall as shown in Figure 1. This aquaporin protein acts as a water channel that allows solely water and block all other solutes . In this way the permeation and rejection properties are enhanced.
Another versatile technique is the self-assembly of oppositely charged polyelectrolytes (PEs) on the surface of a porous support membrane. In this so-called Layer-by-Layer (LbL) assembly, the support membrane is alternatively exposed to polycations and polyanions . Such a PEM coating is easily applied on all geometries. In this study, the focus will be on making a PEM coating with the incorporation of AQPVs. Since the interaction between the vesicles and the multilayer is important, the distribution of the AQPVs is first studied. Subsequently, a layer will be produced and tested on its FO and RO performance.
In order to create a more permeable and selective layer, aquaporin containing vesicles (AQPVs) can be used. These vesicles are incorporated in the separating layer of the membrane.
In order to control the distribution of the aquaporin vesicle, various polyelectrolytes can be used which have different interaction with the AQPVs. Also different coating environments, like salt concentration, pH, and solvent can be used to control the distribution of the vesicles.
In this MSc project, the distribution of the AQPVs will be studied on model surfaces. When the distribution is controlled and understood a separating layer will be made and incorporated on a membrane.
PEMs can be made by dip-coating silicon wafers or membranes in a solution containing a certain polyelectrolyte. AQPVs can be also be coated by means of dip-coating.
The distribution of AQPVs can be seen with a high resolution scanning electron microscope. Adsorption of both the PEs and AQPVs are measured with reflectometry and further characterization can be done with ellipsometry, zeta potential, and contact angle measurements. The knowledge obtained from model surfaces (silicon wafers) will be translated into hollow fiber membranes. The membranes will be coated under the same conditions as the model surfaces and will be tested on their performances in forward and reverse osmosis operating conditions.
Figure 1: The water-selective aquaporin protein incorporated in the vesicle wall.
1. Shaffer, D.L., et al., Forward osmosis: Where are we now?, Desalination, 2015. 356.
2. Tang, C., et al., Biomimetic aquaporin membranes coming of age, Desalination, 2015. 368.
3. Murata, K., et al., Structural determinants of water permeation through aquaporin-1, Nature, 2000. 407(6804).
4. Decher, G., Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites, Science, 1997. 277(5330).
Dennis Reurink (1), Wiebe M. de Vos (1)
(1)Membrane Surface Science (MSuS)
Nowadays most membranes for nanofiltration (NF) and reverse osmosis (RO) are based on thin film composite (TFC) membranes. On a TFC membrane, a thin film is coated on a porous support with interfacial polymerization. A major disadvantage is that defect free hollow fiber membranes based on the TFC principle is merely impossible . A new versatile technique for membrane the self-assembly of oppositely charged polyelectrolytes (PEs) on the surface of a porous ultrafiltration support membrane. In this so-called Layer-by-Layer (LbL) assembly, the support membrane is alternatively exposed to polycations and polyanions . Such a polyelectrolyte multilayer (PEM) coating is easily applied on all geometries. In this study, the focus will be to create a PEM based membrane suitable for RO applications. In order to achieve RO performance, PEMs should be made as dense as possible by means of crosslinking and the use of different kinds of PEs. Subsequently, the layer will be characterized and tested on its RO performance.
Creating more intrinsic bonds will increase the rejection properties of an active membrane layer. Crosslinking is a manner to create more intrinsic bonds and can easily be done by heat or a catalyst. Many moieties of PEs are amines or carboxylic acids which can easily be crosslinked, as shown in Figure 1.
Crosslinking of PEMs has already shown to be a promising way to create an active layer capable of retaining salts . For FO purposes, the layer has to be as dense as possible in order to cope with highly concentrated saline streams. In order to control the layer density, different materials can be used that influence the intermolecular distance.
In this MSc project, different types of crosslinking techniques and materials will be evaluated. Materials will vary from aliphatic, branched, to aromatic structures while controlling and monitoring the performance of the membrane.
PEMs can be made by dip-coating silicon wafers or membranes in a solution containing a certain polyelectrolyte. The growth and properties of these multilayers can be monitored by using techniques like reflectometry, ellipsometry, contact angle, and zeta potential measurements.
Crosslinking can be done catalyzed or non-catalyzed. Non-catalyzed reactions take place under the influence of heat. To see if crosslinking has taken place, the layer will be characterized by, e.g., FTIR measurements.
The knowledge obtained from model surfaces (silicon wafers) will be translated into hollow fiber membranes. The membranes will be coated and crosslinked under the same conditions as the model surfaces and will be tested on their performances in forward osmosis operating conditions.
Figure 1: Crosslinking of polyelectrolyte multilayers .
1. Lau, W.J., et al., A recent progress in thin film composite membrane: A review, Desalination, 2012. 287.
2. Decher, G., Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites, Science, 1997. 277(5330).
3. Park, J., et al., Desalination membranes from pH-controlled and thermally-crosslinked layer-by-layer assembled multilayers, Journal of Materials Chemistry, 2010. 20(11).
4. Sullivan, D.M. and M.L. Bruening, Ultrathin, cross-linked polyimide pervaporation membranes prepared from polyelectrolyte multilayers, Journal of Membrane Science, 2005. 248(1-2).
Joshua Willott (1), Wiebe M. de Vos (1)
(1) Membrane Surface Science (MSuS)
Solid surfaces coated with water-soluble stimuli-responsive polymers (specifically end-tethered brushes) exhibit many diverse macroscopic behaviors such as tuneable wettability and lubrication. For these promising coatings to be applied in ‘real world’ scenarios their nanoscale properties (solvation, charge state, thickness) and how they response to stimuli (pH, temperature, salt) and stresses (confinement, added foulants like surfactants, particles) must be very well understood.
The scope of this project encompasses designing specific polymer architectures to control the responsive nature of end-tethered polymer layers. Recently, it has been shown that polydispersity within such layers can be used to control its thickness response. However, this is difficult to achieve synthetically, and something that can be easier to control is polymer architecture. The polymeric layers will be designed and then studied using numerical self-consistent field theory (Willott et al. 2018, Macromolecules 51, 1198-1206 and Langmuir, 2019, 35, 2709-2718) with comparisons made to experimental observations.
In this project, you will work to investigate how the architecture (molecular structure) of end-tethered polymer chains (e.g. linear, star, comb, dendrimer) changes their solution behaviour. The polymer response to stimuli including temperature, pH, salt concentration and salt type will also be studied by changing the type of monomers (or segments) and the solvent. Preliminary work shows that architecture is a control parameter for brush behaviour as shown in Figure 1 where linear and comb polymers are compared. Figure 1 also shows that behaviour depends on which sections within the brush contain the responsive monomers.
Figure 1. Thickness response of the polymer layer as a function of chi parameter (proportional to temperature) for linear vs. comb polymer architectures.