Nanotube water doesn't freeze — even at hundreds of degrees
below zero
ARGONNE, Ill. (May 13, 2005) — A new form of water has been discovered
by physicists in Argonne's Intense
Pulsed Neutron Source (IPNS) Division. 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.
Instead, inside a single wall tube of carbon atoms less than 2 nanometers,
or 2 billionths of a meter wide, the water forms an icy, inner wall of water
molecules with a chain of liquid-like water molecules flowing through the center.
This occurs at 8 Kelvins, which is minus 445 Fahrenheit. As
the temperature rises closer to room temperature, the nanotube water gradually
becomes liquid.
Researchers ranging from biologists to geologists and materials scientists
are interested in water's behavior in tightly confined spaces controlled by
hydrophobic – water repulsing – materials because this situation is found in
nature, for example when tiny roots carry water to plants. Some membrane proteins
also face this challenge, including aquaporin, which controls water flow through
cell walls.
This IPNS study is the first experiment with water in a nanotube. “I was surprised,” said
principal investigator Alexander Kolesnikov, “that no one has tried testing
water in nanotubes. There have been a large number of calculations, made even
more difficult because water is so difficult to model, but no experimental
work.”
“Even though people have been modeling water for decades,” said visiting scientist
Christian J. Burnham from the University of Houston, “we are only now just
beginning to appreciate the importance of including the correct quantum-level
description of the motion of the hydrogen nuclei and we are still working on
a more accurate mathematical description of the charge clouds enveloping each
water molecule.”
Researchers Kolesnikov, Chun Loong, Nicolas de Souza, Pappannan Thiyagarajan
and Jean-Marc Zanotti used the IPNS for the experiments. Instruments at the
IPNS study atomic arrangements and motions in liquids and solids. The IPNS
is open to researchers from industry, academia and other national and international
laboratories.
Research partners at MER
Corp., Tucson, Ariz., supplied the nanotube samples
made of nearly pure carbon only one atom thick. Each tube was 1.4 nanometers
across and 10,000 nanometers long; imagine a piece of dry, hollow spaghetti
200 meters long because the nanotube is more than 7,000 times longer than wide.
“With this one-dimensional confinement,” Kolesnikov said, “we expected something
new, but not the characteristics we observed. Something extraordinary appeared.”
What appeared was “totally different from bulk liquid or ice,” said Kolesnikov.
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 nanometer space all around it because that is as close as nature
allows the water to the carbon. “An interior chain is running inside the lining,
but compared to bulk water is much more mobile,” Kolesnikov said.
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.
“Because of the loose bonding, the water is very active and is always moving,” Kolesnikov
said. The icy lining is much more stable, but the mobile chain makes and breaks
bonds continuously between parts of the chain and sometimes with the icy wall.
A molecular divining rod
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, Burnham, an expert in modeling the molecular dynamics of water, developed
the simulation that shows 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.
Another advantage, according to Kolesnikov, is that scientists from other
disciplines will be able to isolate water's behavior in this one-dimensional
confinement. “In the inelastic neutron scattering experiment, the carbon is
almost invisible compared to the hydrogen atoms, so you only see the water.
Biologists can use our information to understand how the water behaves in their
much larger, complex models,” Kolesnikov said.
Funding for this research was supplied by the U.S. Department of Energy's
Office of Basic Energy Sciences.
Research continues. Burnham will expand his classical molecular dynamics research
to include quantum effects using parallel computing with funding from Argonne's Theory Institute. IPNS researchers plan to look at water in nanotubes with
smaller diameters close in size to membrane proteins that selectively transport
water. They also want to study the thermodynamic properties of nanotube water.
Kolesnikov said he has studyied water on and off during his career “because
it is so critical to everyday life – here on Earth and in the planetary system.”
This research was published in Physical
Review Letters, July 16, 2004. — Evelyn Brown
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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