Chemistry Reference
In-Depth Information
solvated by the ionic liquids than is the uncharged ''aliphatic'' benzene,
although they were unable to compute absolute free energies and hence actual
solubilities. They found significant differences in the organization of the ions
about the two different benzene molecules. As shown in Figures 8(a) and 8(b),
the cations of the ionic liquid organize very strongly above and below the
plane of the charged benzene, while the anions organize around the equator
of benzene. This is driven by the charge distribution in benzene itself; it is
more positive around its equator and more negative above and below its plane.
In contrast, the cation and anion distribution is much more uniform about the
uncharged aliphatic benzene.
Hanke and Lynden-Bell
73
carried out another study of water dissolved in
hydrophilic [C
1
mim][Cl] and hydrophobic [C
1
mim][PF
6
] in which isochoric-
isothermal MD simulations were run at 127
C on mixtures containing water
mole fractions of 0.05, 25, 50, 75, and 99.5%. They computed excess
volumes, enthalpies, and internal energies of mixing and compared the results
between the two ionic liquids and found that the excess properties between the
two systems differed qualitatively, as might be expected. Unfortunately, no
experimental data was available for these systems against which to compare
their results. The authors pointed out, however, that the magnitude of com-
puted quantities such as excess molar volumes were consistent with those
for other ionic liquid systems for which data was available.
Cadena and co-workers
74
published a joint experimental and molecular
modeling study directed at explaining the high solubility of CO
2
in imiadzo-
lium-based ionic liquids. The simulations involved the liquid properties of
pure [C
4
mim][PF
6
] and 1-
n
-butyl-2,3-dimethylimidazolium hexafluoropho-
sphate ([C
4
mmim][PF
6
]). The latter material was studied to examine the
role played by the acidic proton at the C2 position of the cation; by blocking
this site with an additional methyl group, its effect on liquid structure and CO
2
solubility could be assessed. MD runs were done at 25, 50, and 70
C for the
pure ionic liquids and for mixtures containing 10mol% CO
2
. Simulated den-
sities are roughly 2% lower than experimental values for [C
4
mim][PF
6
] and 4-
6% higher than the experimental values for [C
4
mmim][PF
6
]. The simulations
found that the partial molar volume for CO
2
at the mixture concentration is
33 cm
3
/molin[C
4
mim][PF
6
] and 28 cm
3
/mol in [C
4
mmim][PF
6
]. The experi-
mental value for [C
4
mim][PF
6
] is about 29 cm
3
/mol but no experimental data
exist for [C
4
mmim][PF
6
]. The relatively small partial molar volumes reflect the
fact that the liquid volume expands very little upon dissolution of CO
2
into an
ionic liquid, indicating that the underlying liquid network is not greatly per-
turbed by the presence of CO
2
at these concentrations. The simulations show
that CO
2
does not associate to any great extent with the C2 position of the
cation, regardless of whether there is a methyl group present or not. This is
due to the fact that the anion associates preferentially with this part of the
cation, thereby ''blocking'' most direct interactions with CO
2
. It was found
that CO
2
associates preferentially with the [PF
6
] anion, and adopts a ''tangent''
