SCIENCE OBSERVER
Ice Entwined
Fenella Saunders
For a simple collection of hydrogen and oxygen atoms, water has an
astounding repertoire of contortionist tricks when frozen. It works
particularly well under pressure. In bulk quantities, there are 15
known forms of ice, only a few of which occur at normal atmospheric
pressure. "There are so many unique things about water,"
says Xiao Cheng Zeng, a chemist at the University of
Nebraska-Lincoln. Zeng decided to check out ice's act when it is
forced into microscopic containment—in this case, into carbon
nanotubes less than 2 nanometers in diameter. He and his colleagues
Jaeil Bai and Jun Wang have found that ice has a whole new
repertoire when it's pushed into a tight spot.
Zeng and his colleagues use supercomputers to model the molecular
dynamics of water molecules confined within carbon nanotubes. A few
years ago, the group created models for four new types of ice in
these close quarters at relatively low pressures—only a few
tens of times atmospheric pressure. Several groups have now
experimentally proven that these modeled forms of ice actually exist
when water is frozen in carbon nanotubes under such conditions. But
since bulk ice has so many more forms under high pressure, Zeng and
his colleagues wanted to see how ice would react if they put the
squeeze on the nanotubes as well.
In the December 26, 2006, edition of the Proceedings of the
National Academy of Sciences of the U.S.A., Zeng's group
reported that they had simulated subjecting the ice in the carbon
nanotubes to pressures of 10 to 40,000 atmospheres. Each
experiment's reaction time was no more than a few tens of
nanoseconds. The investigators expected that the ice would simply
roll up into a tubular structure—and for four of the six
resulting phases, this was the case. But for two sets of parameters,
the ice spontaneously transformed itself into nested helixes with
multiple strands.
In a 1.35-nanometer tube at a temperature of minus 9 degrees
Fahrenheit and 40,000 atmospheres of pressure, the ice became a
double helix that somewhat resembles DNA in structure, except the
outer sheath has eight strands and the inner layer has four. In
another simulation of a 1.9-nanometer tube at the same temperature,
when the pressure was suddenly raised from 10 atmospheres to 8,000,
the water abruptly froze into a helical formation with
triple-layered walls. In this case, the outermost wall had 18
strands and the two ice sheaths nested inside of it each had six.
"They are intertwined together like a braid of chains,"
explains Zeng. "At the highest pressures, we were surprised to
find that the helix is the most stable formation of ice."
High pressures force a large number of water molecules to pack
themselves into the carbon nanotube. Zeng suspects that the helix
may be an efficient way for the molecules to arrange themselves to
be as compact as possible at extreme densities. Hydrogen bonding is
also involved. Hydrogens from the inner chain match with those in
the outer wall, and each water molecule prefers to be bonded to four
others in a tetrahedral structure.
It took several years for experiments to confirm Zeng's previous
models of low-density ice in carbon nanotubes, and it's much more
challenging to create such high pressures in the lab than it is on a
computer, but Zeng is hopeful that the new high-density ice
structures will eventually be found in the laboratory. If so, that
will bring the total number of ice's known contortions to about 25
different structures.
Because the way that these ice structures assemble themselves
somewhat resembles the assembly of certain proteins, there is some
hope that the new study could be applied to understanding basic
biological processes, such as the protein misfoldings associated
with mad-cow disease and other illnesses. Also, because the water
molecules spontaneously assemble themselves into such a complex
arrangement, it might also be possible that Zeng's results could aid
in the fabrication of intricate materials at the nanoscale.
"This helix of ice is a cooperative arrangement of tens of
thousands of molecules together. They have to act in a cohesive way
to form this kind of unique structure. It's not like a polymer where
they are already connected by strong bonds," Zeng says.
But Zeng himself is most interested in knowing more about water for
its own sake: "Water and life go hand in hand, so that's the
most important reason why we study it."
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