Chemistry Reference
In-Depth Information
There is a huge disparity between the cation and anion dynamics, a feature
first observed for [C
4
mim][PF
6
] by Morrow and Maginn.
49
To assess the polarity of the ionic liquids, Kim's group
91
computed
the average energy difference between a nonpolar probe solute and an
ion-pair solute in which formal charges of
1 were placed on
the dimer atom centers. This energy difference is expected to give qualita-
tively similar results as those obtained using spectroscopic probes of
polarity. The calculations result in large blue shifts that are qualitatively
similar to experimental observations and are consistent with the known sol-
vation power of ionic liquids. The reorganization free energy for transi-
tions between a nonpolar and an ion-pair solute were computed to be
45-40 kcal/mol,
รพ
1and
indicating that considerable solvent
reorganization is
required for these systems.
At about the same time as this work was published, Znamenskiy and
Kobrak
92
simulated the absorption spectrum of betaine-30, a commonly
used solvatochromic probe molecule, in [C
4
mim][PF
6
]. They investigated the
interactions responsible for the solvatochromic shift. Because this shift is used
experimentally to assess solvent polarity, the calculations can thus provide a
direct window into the nature of polarity in ionic liquid systems. To conduct
the study, a single molecule of betaine-30 was immersed in a liquid containing
200 ion pairs. Twelve independent 1-ns runs were then carried out, and from
that the absorption spectrum was computed. They observe two distinct time
scales: one on subpicosecond time scales and one that is on the order of
100 ps. This result is consistent with previous simulation studies as well as
time-resolved fluorescence spectroscopy experiments.
93
Although the actual
absorption spectra computed do not agree quantitatively with experimental
results, the qualitative features do.
IMPLICATIONS OF SLOW DYNAMICS WHEN
COMPUTING TRANSPORT PROPERTIES
As mentioned earlier when discussing some of the work by Margulis and
Berne,
47
Del Popolo and Voth,
62
and Morrow and Maginn,
49
the dynamical
behavior of ionic liquid systems is complex and its behavior is characterized as
akin to that of a supercooled liquid.
47
A large number of experimental and
simulation studies have been carried out recently that investigated the
dynamics of ionic liquid systems. It is not an exaggeration to say that an entire
review article could be written on this topic alone, so a comprehensive review
of that literature is not possible. Instead, we summarize briefly a few of those
results and point out some of the things to be aware of when computing
dynamic properties of ionic liquids, while referring the interested reader to
the original studies for more details.
