Chemists discovered the opposite effect: How dilution with water makes solutions hard
Graphical representation of phase transitions. Credit: Koen Pieterse
In ScienceTU/e researchers have published their study of new phase transitions of solutions and gels in water, which seem to contradict the basic principles of chemistry, and which they discovered by accident.
In chemistry, hydrogels turn into liquids by diluting them with water. For the reverse transition, you increase the hydrogel concentration. However, TU/e researchers led by Bert Meijer accidentally discovered that their liquid solution turned into a hydrogel when diluted. This phenomenon has never been studied or described before and can have consequences in many fields in chemistry and biology.
This study focuses on the formation of certain hydrogels. This means that it starts with an aqueous solution of, in this case, two substances (a surfactant and a monomer). Research shows that gels form at a certain ratio of these two substances in water. This gel is formed by a long supramolecular network consisting of both substances. The amount of these substances in water (concentration) also determines where the gelling phase transitions are located. When lowering the concentration without changing the ratio between the two components, the gel dissolves and becomes a liquid. So far, this is familiar territory.
What is remarkable, however, is that if the solution is diluted further, a gel is once again formed. Another supramolecular structure is now formed and becomes a hydrogel again. And if it is then diluted further, it becomes liquid again. This paper carefully examines what the correct proportion of the active substance should be and at which concentration the phase transition occurs. This transition is also completely reversible. If the concentration is increased, the transition from liquid to gel to liquid to gel occurs at the same point. This phenomenon is supposed to be present in other fields, such as biology, but has never been studied and documented before.
Found by accident
This so-called dilution-induced self-assembly has been the subject of research in Bert Meijer’s research group for about ten years. However, it is difficult to achieve this transition to form a solid and make it work in aqueous solution. But Jesús Mosquera and Cyprien Muller accidentally discovered in October 2019 that their two-component liquid mixture became a hydrogel when diluted.
“It’s valuable to point out that what you learn in your first chemistry lesson doesn’t always apply. When you dilute, not all gels and solutions are liquid by definition,” says Meijer.
Lu Su, lead author of Science paper, immediately attracted by the results. “It was a special and accidental discovery. And I immediately saw more possibilities. What if we could show a double transition? So from gel to liquid, back to gel and back to liquid just by adding more water.”
“I have to be honest, when Jesús and Cyprien started their research, I wasn’t immediately convinced that we had anything special on our hands,” says Meijer. “But as a group leader, you have to support and trust young researchers when they’re starting something they’re passionate about and you haven’t seen for yourself. Since this is a new area for us, my intuition tells me to be positive. And that’s the funny thing about research. Often, what you design beforehand doesn’t work out, but you can come across the unexpected by accident. That’s why I’ve supported them from the start in preparing and developing their research.”
Lockdown
The timing of the discovery is important to mention because not long after, the whole world was faced with a lockdown and the TU/e lab was also closed. That gave Su plenty of time to think about how he could cope with this with his colleagues after Jesús and Cyprien left the Netherlands to pursue careers abroad. “During a Teams meeting, we brainstorm with the group on how we can set up experiments. Together, we make good preparations for additional experiments.”
So when the lab reopened in the summer of 2020, they immediately started exploring their ideas. “And it turned out to be true!” said Su. “Within a month, we’ve done the basics of our experiment and demonstrated the hydrogel-solution-hydrogel (gel-sol-gel-sol) transition.”
In the fall, the researchers co-authored the first draft of their article, but the work wasn’t done yet. “When you find something special, you want to make sure your interpretation is correct. That’s why I encourage them to invest in ensuring reproducibility and broadening their scope,” says Meijer. A new series of experiments was prepared by Su. Unfortunately, due to her pregnancy, she was unable to do it herself in the chemistry lab, but her colleagues were able to take over from her. The experiment was repeated several times to get more reliable results.
According to Meijer, it is difficult to say how widespread the effect of their discovery will be, but it is certain that it will have a major impact on chemistry and biology. “Such hydrogels might be a good solution to existing challenges, such as for stem cell culture. In a gel, cells can safely divide into three dimensions and, once sufficient, you dilute the solution and the cells can be used properly. That is, for example, research what we’re doing together with Patricia Dankers’ group. The trick, of course, is to find the right substances that exhibit this behavior but don’t react to cell walls or stick to them. . In this way, our research leads to other studies.”
Evidence found for the existence of two forms of liquid water
Lu Su et al, The dilution-induced gel-sol-gel-sol transition by a competitive supramolecular pathway in water, Science (2022). DOI: 10.1126/science.abn3438. www.science.org/doi/10.1126/science.abn3438
Matthew J. Webber, Less is more when gelling by dilution, Science (2022). DOI: 10.1126/science.abo7656, www.science.org/doi/10.1126/science.abo7656
Provided by Eindhoven University of Technology
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