Aug 30 2008
Certain explosives may soon get a little greener and a little more precise.
Fluoride ionic liquid as a novel super-efficient solvent can lead to high-quality single crystals of technologically important materials. The molecules in red, white, blue and gray are the explosive, TATB. The green balls (fluoride anions) and the gray and blue sticks (cations), act as the solvent. The rocks in the background are TATB crystals.
LLNL researchers added unique
green solvents (ionic liquids) to an explosive called TATB (1,3,5-triamino-2,4,6-trinitrobenzene)
and improved the crystal quality and chemical purity of the material.
This work, supported under the Transformational Materials Initiative (TMI)
Laboratory Research and Development project, appears on the cover of the Sept.
1 issue of the journal Physical Chemistry Chemical Physics.
“Improving crystal quality and purity leads to explosive materials that
are safer (less likely to react violently) when subjected to mechanical impact
or heat,” said Larry Fried, the project’s principal investigator
and a co-author of the paper.
Most explosives belong to a general class of materials called molecular crystals,
which have become important building blocks in a number of other applications
ranging from drugs, pigments, agrochemicals, dyes and optoelectronics. Many
of these materials, including TATB, are bound together by a strong network of
hydrogen-bonds. This extended network often makes these materials nearly insoluble
in common organic solvents, leading to poor quality and limited size crystals,
which in turn hinders progress in many technological applications.
So the TMI team looked for a suitable alternative, which happened to be ionic
liquids – a special type of molten salt that becomes liquid under the
boiling point of water (100 degrees Celsius). Chemists recently became interested
in ionic liquids because they are solvents with almost no vapor pressure, and
do not evaporate, even under high temperature conditions. They also provide
researchers an endless number of choices due to the large combinations of positive
and negative ions involved.
To narrow the choices down, lead author Amitesh Maiti used state-of-the-art
quantum mechanical simulations to identify a special class of ionic liquids
containing fluoride anions that are highly effective in dissolving hydrogen-bonded
materials such as TATB. (An anion is an atom with a net negative charge, i.e.,
more electrons than protons.)
“The design of custom solvents through first principles modeling opens
up new possibilities for the dissolution of materials that are hard to dissolve,”
Maiti said.
The next step involved an experimental team, led by Phil Pagoria, who was successful
not only in dissolving TATB in such solvents, but also in growing large defect-free
crystallites (more than 97 percent pure TATB), which will lead to a better formulated
material for explosive applications.
The solvents and the dissolution process developed by the TMI team have applications
in other fields as well, such as the production of polymers (plastics) or molecular
solids (pharmaceuticals, paints, propellants, explosives). For instance, the
team found that fluoride ionic liquids are highly effective in dissolving cellulose
(plant fiber), a versatile bio-renewable polymeric material with many applications.
However, the immediate goal is to find a cost-effective way to improve the
quality of low purity TATB. TATB is an extremely safe explosive that is used
by the Department of Energy, the Department of Defense and the mining industry.
Other Livermore researchers include: Alex Gash, Yong Han, Christine Orme and
Richard Gee.