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Home » Featured Research » That is hot ice and wondrous strange snow.

That is hot ice and wondrous strange snow.

Posted on : August 28, 2019 | Modified on : August 29, 2019

---> Theoretical Chemistry Laboratory

How can I change the freezing temperature of water (freezing point)? If you don't mix anything, the water will turn into ice at 0°C when you cool it, but you can easily lower the freezing temperature by mixing it with salt or alcohol. One idea is to use the fact that salt is ejected when salt water freezes to desalinate seawater by freezing the water. However, since the temperature is generally high in regions that want to make fresh water, the energy required to cool the water to below 0°C is not negligible and has not been put into practical use.

How do you make the freezing temperature of ice higher than 0°C? For example, if you can freeze water at 15 degrees, you can make an ice pillow that cools down confortably and lasts longer. If you make fresh water and freeze it at a higher temperature, you can save a lot of energy for freezing. But raising the freezing temperature is much more difficult than lowering it.

To increase the freezing temperature of water, it is necessary to freeze the water into a crystal structure different from that of normal ice. For example, if 7,000 atm or more is added, the crystal structure of ice changes and the melting point becomes higher than 0°C. When methane and water are frozen together under pressure, methane hydrate with a complex crystal structure is formed and its freezing temperature is slightly higher than 0°C (depending on the pressure,). The freezing point of water mixed with tetrahydrofuran (THF), an organic solvent, at atmospheric pressure also rises to about 4°C, and mixing water with a substance called tetrabutylammonium bromide raises the freezing point to above 10°C. In either case, the crystal structure is different from normal ice.

However, extra energy is required for pressurization, and when toxic tetrahydrofuran or tetrabutylammonium bromide is used, it cannot be used as fresh water without removing them. Is there any way to raise the freezing point without mixing water?

In our laboratory, we focused on the fact that the crystal structure of ice is similar to that of inorganic crystals called zeolites, and have been searching for ways to make ice with various new crystal structures by computer simulation. We have found that ice dtc, which has cylindrical cavities, is relatively stable. To make it more stable, we need to put some other molecule into the cavity. A computer simulation predicted that when water is poured into a nano brush made of very thin wiry-shaped molecules called carbon nanotubes arranged at regular intervals, the water freezes into the structure of ice dtc, and its freezing point can reach as high as 50°C!

Beyond the freezing point, the water between the nanotubes melts, but since the brushes themselves are solid, they do not dissolve in water like tetrahydrofuran or tetrabutylammonium bromide, and fresh water can be easily obtained. You can also adjust the diameter and spacing of the nanotubes to control the freezing temperature of the water.

 In this study, we demonstrated for the first time that a solid surface can be used to increase the freezing point of water. At present, there is no technology to precisely form the nano-brush, so it cannot be put into practical use immediately. In the future, however, it will be useful for ice packs with freely adjustable temperatures and desalination of seawater in tropical regions.

Figure 1 Structure of ice dtc. 

 

Figure 2 Ice dtc frozen in the nanbrush.

Takuma Yagasaki, Masaru Yamasaki, Masakazu Matsumoto, and Hideki Tanaka
Formation of Hot Ice Caused by Carbon Nanobrushes
J. Chem. Phys. 151, 064702 (2019).

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