Separating chemicals with a new type of nano-structured membranes

2020 Vidi laureates on their research plans

Physical chemist Martin F. Haase aims to use the Vidi grant he received last November to develop a new material for molecular separations. Haase is specialised in nano-structured membranes obtained from recently discovered so-called ‘bijels’. The material shows strong potentials to enhance the efficiency of processes such as obtaining drinking water from sea water.

Man-made problem

“Around the world, humans have created water scarcity where water had not been scarce before”, says the German soft materials researcher Martin Frodo Haase, of the Debye Institute for Nanomaterials Science. “Countries like the Netherlands are importing products such as strawberries, cocoa or cotton from arid regions around the world. In fact, most of the world’s drinking water is used for agriculture. I want to find a technical solution for this man-made problem.”

There is already a technical solution available: many areas facing a water shortage are located near the sea, and it is technically possible to desalinate the salt water via an osmotic process. Haase: “World-wide, there are around 20,000 desalinisation plants in operation, and together they produce one percent of the total volume of fresh water on earth. But the process is expensive, consumes a lot of energy and requires expensive equipment, such as powerful pumps and stainless-steel pipes that can resist corrosion. I want to improve the technology, to make fresh water more accessible.”

A bijel-membrane could potentially desalinate 400 times as much water as state-of-the-art membranes.

Martin F. Haase, assistant professor

Reverse osmosis

The most energy efficient way to separate salt from water is via what is known as ‘reverse osmosis’. Haase explains the principle of this method: “It uses a membrane that consists of a substrate with an ultra-thin film on its surface that is only 10 nanometres thick. The polymer chains in the film selectively allow water molecules to pass through, while holding back the salt molecules. But the process requires high pressure, and therefore a lot of energy. The system also must be able to handle the pressure, so the substrate of the membrane needs to be relatively thick, purely to provide the mechanical support.”

Graphical representation of membranes. Left: state-of-the-art hollow fiber membrane. Right: nano-structured membrane based on bijels.

Two years ago, Haase thought of a better way of designing the membrane: “Instead of using the substrate only for mechanical support, we can extend the thin film deep into the substrate. This would dramatically increase the surface area of the membrane and therefore the achievable freshwater production.” His idea to do that was to manufacture the membrane substrate as a sponge-like material, which allows water to flow inside where the film can separate water and salt. “This type of membrane could potentially desalinate 400 times as much water as state-of-the-art membranes.”

Hardening modelling clay

One condition for using a sponge-like membrane is that the salt molecules which remain behind in the structure, and will therefore “clog the membrane”, must be able to be led away from the membrane. Haase is an expert in research into a material that was only discovered in 2005: the bicontinous interfacially jammed emulsion, or ‘bijel’ for short. It is an emulsion of two fluids, with an ultra-thin layer of nanoparticles that hold the surface between the two fluids stable.

Left: in a bijel, two liquids are seperated by a thin film. Right: image of a bijel made with the use of a scanning electron microscope.

Haase: “This bijel normally has the consistency of modelling clay, but when you let it harden, it can be suitable for use as a scaffold for the type of membrane that I’d like to improve. While I was working in the United States, I conducted research that led to the discovery of a new method for producing bijels: STRIPS, or Solvent Transfer Induced Phase Separation.” STRIPS allows for the inexpensive mass production of nanostructured bijels with high internal surface areas and controllable shapes.  

The right reason for an approach

Before Assistant Professor Martin Haase came to Utrecht University, he had already held several positions in fundamental soft materials research. He earned his PhD at the Max Planck Institute of Colloids and Interfaces in Potsdam, then went on to do postdocs in New York and Philadelphia and an assistant professorship in New Jersey. “Fundamental science gives me the background I need to choose a specific approach for my research. Without that fundamental knowledge, you can’t arrive at an application.”

In his various positions, Haase has developed specific aspects of the knowledge he has been applying in Utrecht with funding from an ERC Starting Grant since 2019. And last November, he also received a Vidi grant. “I had already applied the principle of bijels to catalysis for my ERC grant, and now I’m using it to study membranes.”

In addition to desalinating water, bijels may be used to separate other substances in the future. “High surface-area membranes could potentially be used to remove micropollutants from wastewater, or to purify chemicals needed for the industrial production of pharmaceuticals, plastics, adhesives or cosmetics. The most energy-expensive part of these plants is the purification of chemicals.”