Chemistry Reference
In-Depth Information
results indicate that the model captures well the underlying liquid structure.
This latter result echoes the point made earlier in this chapter: Our under-
standing of the physical chemistry of ionic liquids is being advanced by the
simultaneous and synergistic application of modeling and experiment. This
is possible because molecular modeling has ''come of age'' just in time to
play a key role in understanding these new materials.
At the conclusion of 2002, four groups had published studies in which
the properties of different imidazolium-based ionic liquids had been computed
using five different force fields. Several things became clear as a result of these
studies. First,
relatively simple fixed charge force fields of the kind shown in
Eq. [5] do a remarkably good job of reproducing liquid densities and liquid
structure
. This statement has to be tempered somewhat by the fact that only
in a few of the cases were comparisons with experiment made, and then only
for a limited number of properties. Moreover, different force fields gave simi-
lar liquid densities, although explicit-atom models performed better than uni-
ted-atom models. The fact that different parameter sets give similar densities
means that density is not a particularly stringent test of a force field. Second,
information on liquid structure was being obtained, and it appeared that
ionic
liquids had longer range order than ordinary molecular liquids
. Third, simula-
tions of mixtures demonstrated the
importance of hydrogen bonding on solva-
tion
and suggested that solute polarity is important in determining its
solubility. Fourth, the simulations indicated that
ionic liquids should have
some detectable vapor pressure and enthalpy of vaporization
, even though
that aspect of ionic liquids had not yet been confirmed experimentally. Fifth,
and perhaps most important, these studies demonstrated that
carrying out
simulations of ionic liquids is only slightly more difficult than for ordinary
liquids
. In essence, these early studies paved the way for the broader research
community to use simulations to compute properties for ionic liquids. People
took notice, and the number of modeling studies on ionic liquids grew very
rapidly.
MORE SYSTEMS AND REFINED MODELS
Once it became apparent that standard modeling tools such as MD and
MC could be used with conventional force fields to simulate ionic liquids, a
number of other groups began making significant contributions to the field.
In fact, dozens of studies have appeared in the last few years on the topic.
In this section, we review some of the significant works and discuss some of
the different properties that have been computed while emphasizing
their successes and shortcomings. Because there exist so many works on this
topic in the literature, it is not possible to discuss every one; those reviewed
here represent a cross section of the kinds of systems being examined by scien-
tists and properties that are being computed.
