Researchers discover how water can affect its own filtration

Membranes with microscopic pores are suitable for water filtration. The effect of pore size on water filtration is well known, as is the role of ions, charged atoms, that interact with the membrane. For the first time, researchers have successfully described the influence of water molecules on other water molecules and ions as part of the filtration mechanism. The researchers describe a feedback system between water molecules that opens up new design possibilities for highly selective membranes. Applications can contain virus filters.

Synthetic chemistry is a field of research that is concerned with creating and researching new substances and materials that do not exist in nature. Sometimes a certain property or behavior of a material is required for an application such as pharmaceutical or high-tech manufacturing. Synthetic chemistry can help to find, manufacture or refine suitable materials. For example, so-called synthetic liquid crystal membranes could be used for water filtration.

When filtering water or other liquids, the goal is to separate chemical components such as ions from your target liquid. Using a porous membrane can be the primary method for this. It is intuitively obvious that holes in a surface prevent anything larger than the hole from going through. However, advanced membranes such as synthetic liquid crystal membranes can have pores barely a few nanometers, billionths of a meter in diameter. At these scales, membrane functionality offers more than just the size of a pore.

“Chemistry plays a big role in what happens on these small scales,” said Professor Takashi Kato of the Institute of Chemistry and Biotechnology at the University of Tokyo. “In the case of water filtration, the pores are dimensioned in such a way that nothing larger than water is let through. However, there are also electrostatic forces between ions and pores. If the material is constructed correctly, these forces act as an additional barrier against ions if they are smaller than the pores. This is pretty well understood. But there is another important substance that can affect water filtration, and that is actually the water molecule itself. ”

Professor Yoshihisa Harada of the UTokyo Institute of Solid State Physics and his team set out to fully describe what has long been suspected but never been explained: how water molecules at the site of a pore interact with surrounding water molecules and ions. This is actually very important on this tiny scale where even minor forces can affect the overall performance of the filtration membrane. It is also extremely difficult to extract this type of information from the physical systems.

“In theory, we could use computer simulations to model exactly how water behaves and interacts during filtration. However, such simulations would require enormous amounts of supercomputing, ”said Harada. “At least initially, we turned to a physical method to investigate these mechanisms, high-resolution synchrotron-based soft X-ray emission spectroscopy. This itself was an extremely complex challenge. ”

In this process, X-ray emissions are taken from a synchrotron, a particle accelerator, and directed at the sample to be analyzed. The sample, in this case the membrane and water molecules, changes some properties of the X-ray beam before it is detected and recorded by a high-resolution sensor. The changes imposed on the X-ray beam tell researchers with great accuracy what was going on in the sample.

“It’s not easy,” said Harada. “Due to the thinness of the membranes, the signals that we expected from the target water molecules in the pores are difficult to distinguish from the background signals due to the mass of other water molecules. So we had to subtract the background signals to generate them. ”Our target signals are more visible. But now I am pleased that we can present the first description of water that functions as part of its host material. By doing this type of basic research, we hope it provides tools that others can build on. ”

The team’s new models describe how the interactions of water molecules are modulated by charged particles in the immediate vicinity. In membrane pores, water molecules modulated in a certain way preferentially connect with other modulated water molecules in the volume. A dynamic system like this, in which one change in one property causes another change in the same property, is called a feedback loop. Although they can seem mathematically complicated, these models can help engineers develop new and effective filtration methods.

“Liquid crystal membranes already have perfectly sized pores, while earlier types of membranes were more diverse,” said Kato. “In combination with our new knowledge, we want to create membranes that are even more selective about what they let through than anything that has happened before. These could do more than just purify water; they could be useful in building lithium, for example. Ion batteries, as electrolytes that transport lithium ions between electrodes, and even as virus filters. Because these membranes are so highly selective, they can be set to only block very specific things, which means they can also be used for long periods of time before they become saturated. ”

There are several areas that Harada, Kato, and their colleagues wish to explore further. These first physical experiments will affect computer models, so advanced computer simulations are such an area. But you also want to study cell membranes that naturally mediate the passage of ions such as potassium and sodium. Studying these membranes could also help improve artificial membranes.

“What’s exciting here is combining chemistry, physics, and biology to solve such seemingly complex things,” said Harada.

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More information:
Ryusuke Watanabe et al., Ion Selectivity of Water Molecules in Subnanoporous Liquid Crystalline Water Purification Membranes: A Structural Study of Hydrogen Binding, Angewandte Chemie International Edition (2020). DOI: 10.1002 / anie.202008148

Provided by the University of Tokyo