Chemistry Reference
In-Depth Information
parameters used for the anion. Enthalpies of vaporization were computed and
found to range from 50 to 185 kJ/mol, depending on the compound and tem-
perature. The lower value is small for ionic liquids and suggests significant
volatility; it would be interesting to see if experimental vapor pressure
measurements confirm this. Finally, by analyzing distribution functions, it
was found that the amino group on the cation formed hydrogen bonds with
all five anions studied.
The same group then proposed a force field for 11 different cyclic gua-
nidinium cations paired with the nitrate anion.
71
Their approach to force field
development followed the same methods used before. Production runs were
carried out for 1 ns at temperatures between 320 and 450 K. Because most
of the compounds studied have glass transition temperatures or melting points
above room temperature, only a small number of experimental densities exist
for comparison. In general, it was found that the simulations systematically
underpredict the liquid density by about 5%, which the authors speculate is
due to problems in the parameterization of the Lennard-Jones parameters in
the force field. Self-diffusion coefficients were computed from the Einstein
relation over the trajectories from 200 to 600 ps. As discussed below, the
authors were careful to test that their systems were in the diffusive regime.
It is surprising that true diffusive motion was observed, however, given that
a total mean-squared displacement of only about 0.06 nm
2
was observed
over the time in which the self-diffusivity was computed. This length is
much shorter than the axis of either ion. Nevertheless, estimates of the self-
diffusivity were on the order of 1
10
11
m
2
/s, and from this and the Stokes-
Einstein model, estimates of the viscosity and conductivity were made.
SOLUTES IN IONIC LIQUIDS
Although pure ionic liquid properties are important, many applications
envision the use of ionic liquids as solvents or separation agents, where proper-
ties of ionic liquids mixed with other species are paramount. Relatively few
simulations have been carried out to study the properties of ionic liquids
with dissolved solutes, but the number of studies on this topic has increased
recently. Here we review several of these studies.
As mentioned earlier, Hanke, Atamas and Lynden-Bell
37
conducted the
first simulation study of solvation in ionic liquids by investigating the behavior
of water, methanol, dimethyl ether, and propane in [C
1
mim][Cl]. Using ther-
modynamic integration,
38
they computed the difference in excess chemical
potential of those same solutes in [C
1
mim][Cl] and ranked the solubilities
on the basis of relative excess chemical potentials. In 2003, they examined
72
benzene and a hypothetical uncharged ''benzene'' in [C
1
mim][Cl] and
[C
1
mim][PF
6
] by using the same thermodynamic integration technique
and showed that the charged ''aromatic'' benzene is much more favorably
