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Krzysztof Szalewicz, UD professor
of physics and astronomy, led the development of the first
computer model that can accurately predict both the properties of
a pair of water molecules and of liquid water.
Foto by Kathy F. Atkinson,
University of Delaware. |
"Water as a liquid is not simple at all and
has several properties different from most other liquids," Szalewicz
said. "For example, a well-known anomaly of water is that its density
is highest at four degrees Celsius above the freezing point. Thus, ice
floats on water, whereas the solid state of other compounds would sink
in their liquids."
Among its many properties, water also can absorb
large amounts of heat before it begins to get hot, and it releases
heat slowly during cooling. Otherwise, pools of water, from puddles to
oceans, might boil during the day or freeze solid at night, regardless
of the season.
Water's unique characteristics are directly related
to its molecular structure and the ability of water molecules to form
hydrogen bonds with other water molecules.
The hydrogen side of the water molecule has a
slight positive charge, while a slight negative charge exists on the
opposite side of the molecule.
"For a long time, most researchers agreed that, in
its liquid state, each water molecule coordinates on average with four
other water molecules by forming hydrogen bonds," Szalewicz said. "However,
a 2004 paper in Science claimed that this coordination takes place
with only two molecules, a discovery that, if correct, would turn over
the whole water paradigm."
The experimental claim was not dismissed right away,
Szalewicz said, because existing theoretical models of liquid water
were "parameterized" or coordinated to a specific class of experiments.
"However, the ambiguities about the structure of
liquid water may be resolved if the structure is predicted directly
from the laws of physics," Szalewicz said.
Through the use of quantum mechanics, the
application of the laws of physics at the microscopic level, the
scientists were able to generate a new theoretical framework for
describing the structure and behavior of the water molecule atom by
atom.
"This became possible recently when fast
multiprocessor computers enabled very accurate solutions of the
equations of quantum mechanics describing the forces that water
molecules exert on each other," Szalewicz said. "Once these forces are
known, one can find motions in an ensemble of water molecules and
predict all the properties of liquid water."
The UD researchers used clusters of Linux computers
to perform the large-scale computer calculations required for the
research. The study took several months to complete.
The result is a new model - the first that can
accurately predict both the properties of a pair of water molecules
and of liquid water.
Among its many applications, the research should
help scientists better understand water in not only its liquid form,
but in other states as well, such as crystalline forms of ice, and
water in extreme conditions, including highly reactive "supercritical"
water, which is used to remove pollutants in wastewater and recover
waste plastic in chemical recycling, Szalewicz said. |
