Researchers Reveal The Dance Of Water
August 13, 2009
Researchers Reveal The Dance Of Water
Water is
familiar to everyone — it shapes our bodies and our planet. But despite
this abundance, the molecular structure of water has remained a mystery,
with the substance exhibiting many strange properties that are still
poorly understood. Recent work at the Department of Energy's SLAC
National Accelerator Laboratory and several universities in Sweden and
Japan, however, is shedding new light on water's molecular
idiosyncrasies, offering insight into its strange bulk properties.
In all, water exhibits 66 known anomalies, including a strangely varying
density, large heat capacity and high surface tension. Contrary to other
"normal" liquids, which become denser as they get colder, water reaches
its maximum density at about 4 degrees Celsius. Above and below this
temperature, water is less dense; this is why, for example, lakes freeze
from the surface down. Water also has an unusually large capacity to
store heat, which stabilizes the temperature of the oceans, and a high
surface tension, which allows insects to walk on water, droplets to form
and trees to transport water to great heights.
"Understanding these anomalies is very important because water is the
ultimate basis for our existence: no water, no life," said SLAC
scientist Anders Nilsson, who is leading the experimental efforts. "Our
work helps explain these anomalies on the molecular level at
temperatures which are relevant to life."
How the molecules arrange themselves in water's solid form, ice, was
long ago established: the molecules form a tight "tetrahedral" lattice,
with each molecule binding to four others. Discovering the molecular
arrangement in liquid water, however, is proving to be much more
complex. For over 100 years, this structure has been the subject of
intense debate. The current textbook model holds that, since ice is made
up of tetrahedral structures, liquid water should be similar, but less
structured since heat creates disorder and breaks bonds. As ice melts,
the story goes, the tetrahedral structures loosen their grip, breaking
apart as the temperature rises, but all still striving to remain as
tetrahedral as possible, resulting in a smooth distribution around
distorted, partially broken tetrahedral structures.
Recently, Nilsson and colleagues directed powerful X-rays generated by
the Stanford Synchrotron Radiation Lightsource at SLAC and the SPring-8
synchrotron facility in Japan at samples of liquid water. These
experiments suggested that the textbook model of water at ambient
conditions was incorrect and that, unexpectedly, two distinct
structures, either very disordered or very tetrahedral, exist no matter
the temperature.
In a paper published yesterday in the Proceedings of the National
Academy of Sciences, the researchers revealed the additional discovery
that the two types of structure are spatially separated, with the
tetrahedral structures existing in "clumps" made of up to about 100
molecules surrounded by disordered regions; the liquid is a fluctuating
mix of the two structures at temperatures ranging from ambient to all
the way up near the boiling point. As the temperature of water
increases, fewer and fewer of these clumps exist; but they are always
there to some degree, in clumps of a similar size. The researchers also
discovered that the disordered regions themselves become more disordered
as the temperature rises.
"One can visualize this as a crowded dance restaurant, with some people
sitting at large tables, taking up quite a bit of room — like the
tetrahedral component in water — and other people on the dance floor,
standing close together and moving slower or faster depending on the
mood or 'temperature' of the restaurant — like the molecules in the
disordered regions can be excited by heat, the dancers can be excited
and move faster with the music," Nilsson said. "There's an exchange when
people sitting decide to get up to dance and other dancers sit down to
rest. When the dance floor really gets busy, tables can also be moved
out of the way to allow for more dancers, and when things cool back off,
more tables can be brought in."
This more detailed understanding of the molecular structure and dynamics
of liquid water at ambient temperatures mirrors theoretical work on "supercooled"
water: an unusual state in which water has not turned into ice even
though it is far below the freezing point. In this state, theorists
postulate, the liquid is made up of a continuously fluctuating mix of
tetrahedral and more disordered structures, with the ratio of the two
depending on temperature — just as Nilsson and his colleagues have found
to be the case with water at the ambient temperatures important for
life.
"Previously, hardly anyone thought that such fluctuations leading to
distinct local structures existed at ambient temperatures," Nilsson
said. "But that's precisely what we found."
This new work explains, in part, the liquid's strange properties.
Water's density maximum at 4 degrees Celsius can be explained by the
fact that the tetrahedral structures are of lower density, which does
not vary significantly with temperature, while the more disordered
regions — which are of higher density — become more disordered and so
less dense with increasing temperature. Likewise, as water heats, the
percentage of molecules in the more disordered state increases, allowing
this excitable structure to absorb significant amounts of heat, which
leads to water's high heat capacity. Water's tendency to form strong
hydrogen bonds explains the high surface tension that insects take
advantage of when walking across water.
Connecting the molecular structure of water with its bulk properties in
this way is tremendously important for fields ranging from medicine and
biology to climate and energy research.
"If we don't understand this basic life material, how can we study the
more complex life materials — like proteins — that are immersed in
water?" asked Postdoctoral Researcher Congcong Huang, who conducted the
X-ray scattering experiments. "We must understand the simple before we
can understand the complex."
This research was conducted by scientists from SLAC, Stockholm
University, Spring-8, University of Tokyo, Hiroshima University, and
Linkoping University. The work was supported by the National Science
Foundation, the Swedish Foundation for Strategic Research, the Swedish
Research Council, the Swedish National Supercomputer Center and the
Japanese Ministry of Education, Science, Sports and Culture through a
Grant-in-Aid for Scientific Research.
SLAC National Accelerator Laboratory is a multi-program laboratory
exploring frontier questions in photon science, astrophysics, particle
physics and accelerator research. Located in Menlo Park, California,
SLAC is operated by Stanford University for the U.S. Department of
Energy Office of Science. SLAC's Stanford Synchrotron Radiation
Lightsource is a national user facility which provides synchrotron
radiation for research in chemistry, biology, physics and materials
science to over two thousand users each year.
SOURCE: SLAC
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