Chemistry Reference
In-Depth Information
was computed using thermodynamic integration. By comparing the signs and
magnitudes of the excess chemical potentials, the relative solubility of the
solutes in this ionic liquid could be determined. This was the first example
of a free energy calculation within an ionic liquid system, and it confirmed
the importance of hydrogen bonding and charge-charge interactions for the
solvation behavior of ionic liquids.
In that same
Green Chemistry
issue, Shah, Brennecke, and Maginn
39
reported the results of an isothermal-isobaric MC study on the ionic liquid
1-
n
-butyl-3-methylimidazolium hexafluorophosphate ([C
4
mim][PF
6
]). This
was the first (and still one of the few) MC studies of an ionic liquid. Nearly
all computational researchers study ionic liquids with MD, perhaps because
of the widespread availability of MD codes and the relative scarcity of similar
MC codes. As discussed later, this situation is changing and there are good rea-
sons for choosing to use MC methods to study these materials. Maginn's
group computed the molar volume, cohesive energy density (and thus enthalpy
of vaporization), isothermal compressibility, and volumetric expansion coeffi-
cient. They also computed the liquid structure of this system in the form of
radial distribution functions and found that the order was very long range—
much longer than what is observed in ordinary liquids, a phenomenon that
was explained as being due to the long-range nature of the Coulombic inter-
actions between the ions. Several new developments were introduced in that
work. First, [C
4
mim][PF
6
] was the most widely studied ionic liquid at the
time from an experimental standpoint, and a fair amount of experimental
property data was available against which to compare the results, thereby pro-
viding the first direct test of the ability of simulations to match experimental
properties. It was also the first simulation of a low-melting ''room tempera-
ture'' ionic liquid. Since then, we have learned that the PF
6
anion hydrolyzes
in the presence of water, giving off hydrofluoric acid, so this ionic liquid will
probably find limited practical use, although many experimental and
simulation studies are still being conducted on this compound. Second, this
study introduced a force field having the following functional form:
(
"
#
)
12
s
6
X
s
q
i
q
j
r
ij
ij
r
ij
ij
r
ij
¼
4
E
þ
þ
ð
f
Þ
½
3
U
tot
U
ij
i
;
j
<
i
where the repulsion-dispersion interactions were modeled with a 12-6 Mie (or
Lennard-Jones) potential. Because the cation contains a flexible butyl side
chain, a dihedral potential of the following form was used:
ð
f
Þ¼
n
0
þ
n
1
ð
f
Þ þ
n
2
f
Þ þ
n
3
U
2
½
1
þ
cos
2
½
1
cos
ð
2
2
½
1
þ
cos
ð
3
f
Þ
½
4
