Scientists identify new benchmark for freezing point of water at -70 C

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Photomicrographs of an initially red single crystal showing how it changes to yellow upon dehydration at -20°C. Credit: Nature (2023). DOI: 10.1038/s41586-023-05749-7

Scientists have discovered another amazing aspect of the weird and wonderful behavior of water, this time when subjected to nanoscale confinement at sub-zero temperatures.

The discovery that a crystalline substance can easily drop out of water at temperatures as low as -70°C, published in the journal Nature on April 12, has major implications for the development of materials to extract water from the atmosphere.

A team of supramolecular chemists from the University of Stellenbosch (SU), consisting of Dr Alan Eaby, Professor Catharine Esterhuysen and Professor Len Barbour, made this discovery while trying to understand the particular behavior of a type of crystal that has first aroused their interest for ten years ago.

“Scientists are currently adept at designing materials capable of absorbing water,” says Barbour. “However, it is much more difficult to get these materials (we call them ‘hydrates’) to then release the water without having to supply energy in the form of heat. As we all know, energy is expensive and rarely completely “green.”

The chemical compound in question was originally synthesized by Professor Marcin Kwit, a specialist in organic stereochemistry at Adam Mickiewicz University in Poland. It was then crystallized and brought to Barbour’s lab for further study by postdoctoral researcher Dr. Agnieszka Janiak. This was mainly due to Barbour’s interest in ring-shaped molecules and the way they form channels when packed together in crystals.

Perspective view VT-SCXRD along [210] showing the evolution of differential electron density in T1-R upon cooling from -30 to -125 °C. Host molecules in the foreground have been omitted for clarity. Credit: Nature (2023). DOI: 10.1038/s41586-023-05749-7

Janiak noticed that the crystals were yellow on some days and red on others. It didn’t take him long to figure out that the crystals would only turn red on days when the humidity was above 55%. When humidity levels fell below this level, the crystals would turn yellow again.

“Not only was this behavior rather unusual,” Barbour explains, “it also happened very quickly. It appears that the crystals absorbed water just as quickly at high humidity as they lost it again at low humidity. materials designed to absorb water, it is highly unusual for a material that easily absorbs water to lose it so easily.”

Why do these crystals have such special properties? This question launched a nearly decade-long investigation, which initially focused on explaining the mechanism behind the color change. Theoretical modeling by Esterhuysen and master’s student Dirkie Myburgh showed that water absorption causes slight changes in the electronic properties of crystals, causing them to turn red. With such remarkable properties, Barbour was convinced that the crystals would also have other interesting properties.

It was then that the Ph.D. student Alan Eaby started playing with the material. Initially, he focused on room temperature studies for his master’s research, but later turned his attention to measuring properties at lower temperatures when he undertook his doctorate. three years ago. He wanted to know how the crystals would behave when subjected to different levels of temperature and humidity: “I was intrigued by the color change and wanted to explore what was happening on the atomic scale,” explains- he.

Having learned to develop instruments and methods from Barbour, he embarked on the use of non-standard techniques to understand the mechanisms of absorption and release of water in the material.

One day he observed something strange happening at temperatures below zero degrees Celsius. “I noticed that the crystal still changed color at subzero temperatures. Initially I thought there was something wrong with the experimental setup or the temperature controller, because the Crystal hydrates are not expected to release water at such low temperatures,” he explains.

After many conversations and coffee breaks with Barbour and Esterhuysen, and after modifying the experimental setup several times, they realized that Alan’s observations could be explained by the narrowness of the channels in the material. The channels in the crystal are only one nanometer wide, or one thousandth the diameter of a human hair.

We already knew that at the nanometric scale, water can remain mobile in the channels at temperatures below 0°C. However, this study showed for the first time that such channels can also allow uptake And release of water at temperatures well below its normal freezing point.

To understand this process, Eaby undertook a large systematic series of X-ray diffraction studies of red and yellow crystals at different temperatures and humidities. This allowed him to construct a computer-generated “movie”, with atomic-scale resolution, of what happens to the channels when cooling or heating, and in the presence or absence of water. These animations indicated that the water molecules in the nanochannels move freely until they are cooled to -70°C, after which they undergo a “reversible patterning event” to resemble a glassy state. This “glass transition” ends up trapping water in the material at temperatures below -70°C.

If it weren’t for the color changing behavior of the crystals in the first place, they wouldn’t have become aware of the very low temperature water loss capability. “Who knows,” says Barbour, “there may be many other materials that can absorb and release water at very low temperatures, such as metal-organic structures and covalent organic structures.

“We just don’t know because we haven’t been able to visualize it. Now that we know such behavior is possible, it opens up a whole new field of research and potential applications. Researchers can use these new information to identify other materials with similar properties, and also use the principles we have developed to refine the release of water at low temperatures.This could lead to dramatic reductions in the energy costs of atmospheric water collection , with implications for society and the environment,” he concludes.

More information:
Alan C. Eaby et al, Dehydration of a hydrated crystal at subglacial temperatures, Nature (2023). DOI: 10.1038/s41586-023-05749-7

Journal information:

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