Chemistry Reference
In-Depth Information
simulation, where a nearly planar water-ionic liquid interface forms after
about 40 ns. Sieffert and Wipfff computed the total amount of water that dis-
solves in the ionic liquid phase to find that the quantity varies depending on
whether the simulation starts from an initially homogeneous ''mixed'' state or
from a heterogeneous ''unmixed'' state. For example, on the order of 60 water
molecules are present in the ionic liquid phase when a homogeneous system
demixes, but only 25 water molecules infiltrate the ionic liquid phase from
an initially heterogeneous system. This is clear evidence that the simulations
have not reached thermodynamic equilibrium and are instead exhibiting ''hys-
teresis.'' The authors recognize this, stating that full convergence is not
obtained even with the long 20-40 ns simulations. This study demonstrates
nicely the problem with MD when it comes to computing phase equilibrium;
the time-scale limitations hinder significantly the ability of systems to come to
equilibrium, particularly when an interface is present, and this is why we
believe MC methods, like the CFC MC approach discussed above, are far
superior for computing phase equilibrium. Unfortunately, fewer choices exist
when it comes to general and easy-to-use MC codes than for MD codes, and
so, MC simulations have seen less use.
Molecular dynamics simulations focused on the solvation dynamics of
a hypothetical dimer probe molecule in [C
2
mim][PF
6
]and[C
2
mim][Cl]
have been done by Kim and co-workers.
91
The solvation dynamics were
characterized by the time correlation function of the vertical energy differ-
ence of two solute states relevant to a charge shift. The vertical energy
difference between an initial state
i
and a final state
f
,
E
i
!
f
,isassumed
to be comprised of only Coulombic terms. The time correlation function
is computed as
Þ¼
h
d
E
i
!
j
ð
t
Þ
d
E
i
!
j
ð
0
Þi
C
i
!
f
ð
t
½
11
2
hð
d
E
i
!
j
Þ
i
where the angle brackets refer to ensemble averages and
E
i
!
j
¼
E
i
!
j
h
E
i
!
j
i
½
12
These authors find that there is a very fast mode that causes the correlation
function to decay by about 70% in the first 0.2-0.3 ps at 400 K. Rapid oscilla-
tions of the correlation function exist in this regime, with a frequency of
roughly 26 ps
1
. This mode is attributed to the vibration of the anions in their
first solvation shell. A very slowly decaying multiexponential portion of the
correlation function also exists. The authors find that the short time dynamics
are dominated by the anion motion, which is mostly translational in nature.
