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Copper-colored rings represent the inside walls of a carbon nanotube 1.4 nm in diameter. The red and white interior cylinder is an icy wall with permanent hydrogen bonds shown in red. White represents oxygen. The interior chain of hydrogen (yellow elements) is constantly moving. |
Researchers at Argonne National Laboratory were somewhat surprised when all the water they had carefully placed in nanotubes refused to freeze, despite being chilled to 8°K, the equivalent of 455° F. Some of it formed an icy lining on the inside of the naturally hydrophobic carbon walls. This lining was free floating with a 0.32-nm space between it and the wall because that's as close as nature lets water get to carbon. But inside the lining, a chain of liquid water remained and flowed through the tube and lining at temperatures near absolute zero.
The scientists attribute this behavior to the low coordination numbers of water molecules constrained in such a small space. (The coordination number is the average number of hydrogen bonds connecting a water molecule to its nearest neighbor.) In liquid water, the coordination number is usually 3.8 on average. In ice, it is usually 4. But inside the nanotube, the liquid portion had a coordination number of 1.86.
"Even though people have been modeling water for decades, we are only now appreciating the importance of quantum-level interaction of the motion of the hydrogen nuclei," says researcher Christian J. Burnham. This work could be useful to scientist studying how plants carry water in extremely small roots and how water travels through cell walls in plants and animals.