Nanotube Water Doesn't Freeze

Physicists in Argonne National Laboratories’ Intense Pulsed Neutron Source (IPNS) Division have discovered a new form of water. Called nanotube water, these molecules contain two hydrogen atoms and one oxygen atom but do not turn into ice — even at temperatures near absolute zero.

Research partners at MER Corp. (Tucson, AZ) supplied the nanotube samples made of nearly pure carbon only one atom thick. Each tube was 1.4-nm across and 10,000-nm long. At 8° K, four coordinated water molecules create an icy lining inside the naturally hydrophobic carbon nanotube. The lining free-floats inside the carbon nanotube with a 0.32-nm space all around it because that is as close as nature allows the water to the carbon.

Researchers attribute the peculiarities to the low "coordination numbers" of the molecules. In liquid water, an average of 3.8 (the coordination number) hydrogen bonds connect the molecule to its closest neighbors. In ice, four hydrogen bonds connect to its closest neighbors. In nanotube water, the number of hydrogen bonds for the chain water molecules is only 1.86.

To prepare for the experiment, the carbon nanotube sample was exposed to water vapor for several hours and dried to remove exterior water. Then researchers studied it with several neutron scattering techniques at the IPNS. Neutrons are uncharged particles found in nearly all matter. When the IPNS sends beams of neutrons through materials, they reveal a material's structural and dynamic properties.

First, researchers used the Small Angle Neutron Diffractometer to determine that water filled only the interior of the nanotube. If water were on the exterior, it would have skewed the neutron-scattering results. Other neutron diffraction techniques provided the atomic arrangement, and inelastic and quasielastic neutron scattering measurements revealed the water's molecular motions.

Next, a simulation was developed that illustrates how the new form of water behaves in the nanotube. The small scale of the materials was an advantage in creating the simulation, making it much faster in comparison to the simulation of, for instance, a biological structure thousands of times larger and more complex.

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Water behaves differently when confined inside a long, narrow nanotube. The copper-colored exterior rings represent the carbon nanotube 1.4 nm across. The red and white interior cylinder is an icy wall with permanent hydrogen bonds shown in red; white represents oxygen. The interior chain is in constant motion. Yellow represents the hydrogen in the chain. (Christian J. Burnham, University of Houston)