Ice Nanotubes Inside Carbon Nanotubes


Hideki Tanaka Department of Chemistry, Okayama University

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The carbon nanotube provides a well-defined quasi one-dimensional (1-D) space where one can prepare a quasi 1-D material and explore its new properties that have not been found for the corresponding bulk material. Water is one of the substances that may actually be confined to the carbon nanotube, but its properties in the quasi 1-D space had been as little studied as, or even less studied than, those of other quasi 1-D materials despite the fact that water itself has been more studied than any other substance. However, recent theoretical studies and the following experimental studies of water confined in the carbon nanotube showed that the confined water freezes into crystalline structures that were never found in the bulk water and exhibits the phase behavior of melting and freezing that no bulk system has ever shown. This article is a review of these theoretical studies of the confined water. There are other theoretical or experimental studies on water in the carbon nanotube, focusing on the dynamic properties of water, and more studies of confined water in general.

Computer simulation studies show that the structure of solid water in the zigzag (ℓ, 0), single-walled, carbon nanotube (SWCN) (with ℓ = 13, 14, …, or 17) is quite different from the bulk ice structure; it is a 1-D array of n-gonal “rings” consisting of n molecules. As in ordinary ice—in fact, as in any one of the 12 bulk ice polymorphs excluding ice X—every H2O molecule in the solid water in the carbon nanotube is hydrogen bonded to its four neighboring molecules. Because it has a hollow-tube structure and its width is of the order of 1 nm ( = 10− 9 m), this quasi 1-D ice is called the ice nanotube. Simulations show that liquid water confined in carbon nanotubes freezes into a square, pentagonal, hexagonal, and heptagonal form of the ice nanotube, each corresponding to n = 4, 5, 6, and 7, respectively. Which structure is selected on freezing is dependent on the diameter of the carbon nanotube and the external conditions such as pressure and temperature. It is also found from the simulations and free-energy calculations that the phase behavior of confined water is qualitatively different from the bulk counterpart. Melting and freezing in any bulk system occur as an infinitely sharp change of the state of matter, i.e., as the first-order phase transition; however, freezing into and melting from the ice nanotube are either continuous with no well-defined melting point or discontinuous as in any bulk system.

In 2002, it was reported that the structure of water inside the carbon nanotube at low temperatures determined by X-ray diffraction is consistent with that of the ice nanotube found in the computer simulation.