Chemistry Reference
In-Depth Information
investigation. Many other examples can be given where seemingly small
changes in structure result in qualitative changes in physical properties. This
is because there is a subtle balance between van der Waals and electrostatic
forces among the ions and any dissolved species, and this balance can be dis-
rupted in unpredictable ways by small chemical or structural modifications.
The ability to make such changes is exciting to synthetic chemists because it
represents an opportunity to exploit the chemical diversity of ionic liquids
to prepare materials having just the right properties for a particular appli-
cation. It also presents them with a problem, however, because the very diver-
sity that offers such opportunities also means that there is no way all possible
ionic liquid compounds can be made and tested in the lab; there are simply too
many choices, even if combinatorial methods are used.
Because reliable methods for predicting how properties depend on struc-
ture and composition are lacking, the search for new ionic liquids relies on
chemical intuition or extrapolation of knowledge from related compounds.
This limits progress severely and, indeed, only a relatively small number of
ionic liquids have been made, characterized, and tested. The National
Institutes of Standards and Technologies maintains a database of physical
properties for ionic liquids,
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and while the number of entries is growing every
week, the actual number of different ionic liquids listed in the database is
small. A few companies have begun selling ionic liquids, and the list of differ-
ent ionic liquids in their catalogs is also growing. Still, the development of new
ionic liquids has followed the same time-honored tradition of most of chemical
research: Make a compound, test it, and use your instincts to tell you what to
make next to get the properties you want. Granted this is an oversimplifica-
tion, but not by much.
This is where atomistic simulation of ionic liquids enters the picture. Com-
putational methods and force fields have advanced over the years, as has been
well documented in the
Reviews in Computational Chemistry
series. This means
our ability to predict how the properties of a particular material depend on the
chemical constitution and structure of that material has increased substantially.
Because experimental studies of conventional organic substances have had
about a 100-year head start on computational studies, simulations have been
often used in a ''postpredictive'' method for organic compounds. Comparisons
are made between computed and known experimental properties, and some
insight is gained into why the properties are the way they are. Of course, not
all properties of all compounds are known experimentally at every state point,
and so simulations are important in this regard. However, because the field of
ionic liquids is so new, simulation methods are having a big impact on the
direction of the field. Computations on new compounds are being carried out
contemporaneously with experiments. Experimentalists are hungry for the
insights simulations can provide. Properties are being predicted before they
are measured, and indeed, simulations are helping drive the types of experiments
that are being carried out. This happy coincidence, that a brand new class of
