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 509 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 100,000 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 Catherine
Foster (630/252-5580 or cfoster@anl.gov )
at Argonne.
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