The most abundant material on Earth exhibits some unusual
chemical properties when placed under extreme conditions.
Laboratory scientists have shown that water, in hot dense
environments, plays an unexpected role in catalyzing complex explosive
reactions. A catalyst is a compound that speeds chemical reactions without being
consumed. Platinum and enzymes are common catalysts. But water rarely, if ever,
acts as a chemical catalyst under ordinary conditions.
Detonations of high explosives made up of oxygen and hydrogen
produce water at thousands of degrees Kelvin and up to 100,000 atmospheres of
pressure, similar to conditions in the interiors of giant planets.
While the properties of pure water at high pressures and
temperatures have been studied for years, this extreme water in a reactive
environment has never been studied. Until now.
Using first-principle atomistic simulations of the detonation
of the high explosive PETN (pentaerythritol tetranitrate), the Livermore team
discovered that in water, when one hydrogen atom serves as a reducer and the
hydroxide (OH) serves as an oxidizer, the atoms act as a dynamic team that
transport oxygen between reaction centers.
“This was news to us,” said lead researcher Christine Wu.
“This suggests that water also may catalyze reactions in other explosives and in
planetary interiors.”
This finding is contrary to the current view that water is
simply a stable detonation product.
“Under extreme conditions, water is chemically peculiar
because of its frequent dissociations,” Wu said. “As you compress it to the
conditions you’d find in the interior of a planet, the hydrogen of a water
molecule starts to move around very fast.”
In molecular dynamic simulations using the Lab’s BlueGene L
supercomputer, Wu and colleagues Larry Fried, Lin Yang, Nir Goldman and Sorin
Bastea found that the hydrogen (H) atoms and hydroxide (OH) molecules in water
transport oxygen from nitrogen storage to carbon fuel under PETN detonation
conditions (temperatures between 3,000 and 4,200 Kelvin). Under both temperature
conditions, this “extreme water” served both as an end product and as a key
chemical catalyst.
For a molecular high explosive that is made up of carbon,
nitrogen, oxygen and hydrogen, such as PETN, the three major gaseous products
are water, carbon dioxide and molecular nitrogen.
But to date, the chemical processes leading to these stable
compounds are not well understood.
The team found that nitrogen loses its oxygen mostly to
hydrogen, not to carbon, even after the concentration of water reaches
equilibrium. They also found that carbon atoms capture oxygen mostly from
hydroxide, rather than directly from nitrogen monoxide (NO) or nitrogen dioxide
(NO2). Meanwhile, water disassociates and recombines with hydrogen
and hydroxide frequently.
“The water that comes out is part of the energy release
mechanism,” Wu said. “This catalytic mechanism is completely different from
previously proposed decomposition mechanisms for PETN or similar explosives, in
which water is just an end product. This new discovery could have implications
for scientists studying the interiors of Uranus and Neptune where water is in an
extreme form.”
The research appears in the premier issue (April 2009) of the
new journal, Nature Chemistry.
Source: https://newsline.llnl.gov/_rev02/articles/2009/mar/03.27.09-water.php
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