Although water is a very simple compound from a chemist's point of view, its importance in nature cannot be overestimated. Suffice it to say that it constitutes as much as 60% of the human body weight, and its scarcity is one of the greatest problems affecting humanity. Despite the fact that water is one of the best-studied substances, its properties never cease to surprise.
Currently one of the intensively explored areas of research concerns substances capable of controlled and reversible absorption, storage and release of water. Such properties are particularly desirable in, for example, pharmaceutical formulations and desiccants. Most of the known materials of this type release water at higher temperatures, often well above the normal boiling point of water, making the dehydration process expensive and energy-intensive. To make water harvesting from the atmosphere cost-effective, materials are sought that can release water at temperatures only slightly above room temperature.
On April 12, 2023, the journal Nature published a paper entitled "Dehydration of a crystal hydrate at subglacial temperatures," in which scientists from the Adam Mickiewicz and Stellenbosch Universities demonstrated that crystals of a chiral macrocyclic compound, conventionally called trianglimine, have the ability to spontaneously absorb water and release it at sub-zero temperatures. More specifically, the lowest temperature at which this cyclic process can be observed is -70 °C. Below this temperature, the water exists in a glass-like state in the hydrate crystals, which is equivalent to losing its ability to flow.
The crystal structure of trianglimine resembles a so-called hollow brick, in which the channel diameter is 1 nm, and the walls are made of macrocycles. The process of water absorption and desorption is visualized as a change in the colour of the crystal from yellow to red and vice versa, which is a consequence of a change in the electron configuration within the macrocycle, associated with reversible proton transfer. Water uptake and release do not change the crystal structure; the process proceeds very fast and can be repeated many times without any signs of physical damage to the sample.
The presented research is an important step toward the rational design of materials capable of reversible water capture from the atmosphere, even at low humidity. This is particularly important in so-called arid regions of the world, such as steppes and deserts. Another area of application is humidity indicators for the food and pharmaceutical industries, including the transport and storage of frozen food and vaccines.
Open access to the article is available through the ID-UB program (link: https://rdcu.be/daMPe).