Crystallization of pristine cubic ice from liquid at ambient pressure
This study demonstrates that pristine cubic ice (Ic) can be reproducibly formed directly from the liquid state at atmospheric pressure by cooling a hydrate-forming aqueous solution confined within mesoporous silica, revealing that nanoconfinement and clathrate promoters enable a naturally accessible pathway for this previously elusive polymorph.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Water is one of the most familiar substances on Earth, yet it hides a secret that has puzzled scientists for decades. When water freezes, it usually turns into a crystal with a specific, orderly shape known as hexagonal ice. This is the ice we see in snowflakes, glaciers, and our freezers. However, water molecules can also arrange themselves into a different, rarer shape called cubic ice. For a long time, scientists debated whether this cubic form could ever appear naturally from liquid water or if it only existed as a fleeting, messy mixture of shapes that quickly turned into the common hexagonal kind. The question mattered because if pure cubic ice can form easily, it might be far more common in the atmosphere and in deep space than anyone realized, potentially changing how we understand weather patterns and the composition of comets.
A team of researchers has now found a way to make this elusive cubic ice appear directly from liquid water at normal air pressure, without needing extreme cold or high pressure. They did this by trapping a mixture of water and a specific organic chemical inside tiny, sponge-like holes in a material called silica. The silica acts like a container with millions of microscopic tunnels, each about 3.7 nanometers wide. Inside these tunnels, the researchers placed a liquid mixture of water and tetrahydrofuran, a chemical that is known to help form cage-like structures with water molecules. When they cooled this trapped mixture down to 150 Kelvin, the water froze.
The results were clear and reproducible. Using a powerful instrument that fires neutrons at the sample to see how the atoms are arranged, the researchers observed that the water had turned into pure, pristine cubic ice. This was not a messy mix of shapes; it was a single, uniform crystal structure. The experiment showed that this transformation happened reliably when the liquid filled between 0.34 and 0.60 cubic centimeters of space for every gram of silica. Crucially, the researchers proved that the cubic ice formed only because of the combination of the tiny tunnels and the presence of the tetrahydrofuran. When they ran control experiments using pure water or a different chemical that does not form cages, the water froze into the common hexagonal shape or a disordered mix, never the pure cubic form.
The study also revealed what happens to the parts of the mixture that do not freeze. While the water in the center of the tunnels turned into cubic ice, a thin layer of the liquid near the tunnel walls remained unfrozen, turning into a glassy, non-crystalline state. This glassy layer contains the tetrahydrofuran molecules, which seem to interact with the silica surface and prevent the ice from forming there. This separation suggests that the organic molecules play a direct role in guiding the water molecules into the cubic shape. The researchers measured the temperature at which this confined ice melts and found it to be 230 Kelvin, which is consistent with the behavior of ice in such small spaces.
This discovery suggests that cubic ice is not just a laboratory curiosity but a stable form of water that can exist under conditions found in nature. The setup used in the experiment mimics environments where water might be trapped on tiny dust particles in the atmosphere or in the icy grains of comets. In these places, organic molecules and mineral dust often coexist, creating the exact conditions the researchers used to grow the ice. The findings challenge the idea that ice in the upper atmosphere or in space is mostly a disordered mix of shapes. Instead, it suggests that pure cubic ice could be the primary form in these settings, formed directly from liquid water when the right ingredients are present. By showing that this process works at normal pressure and relatively high temperatures for ice, the study opens a new path for understanding how water behaves in the most extreme and distant corners of our universe.
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