Chemistry Reference
In-Depth Information
ionic liquid. As a consequence, the viscosity, which is sensitive to the liquid's
density, does not drop as much. The calculations predict that mixtures of ionic
liquids and nonpolar species, which would not cluster as easily as water, will
actually decrease the viscosity of the mixture more than does water.
The RNEMD method has also been used to compute the thermal con-
ductivity of 1-ethyl-3-methylimidazolium ethylsulfate ([C
2
mim] [EtSO
4
]) as
a function of water content
118
using the simple point charge (SPC) model
for water.
119
We are unaware of any experimental thermal conductivity
data for this system, and we have not seen any previous thermal conductivity
simulations for any other ionic liquids. Table 1 shows the computed values at
348 K. Note that the experimental thermal conductivity of pure water at this
temperature is 0.66W/(m K). Thus the SPC model overpredicts the thermal
conductivity of pure water by nearly 30%.
Only a few experimental measurements of ionic liquid thermal conductiv-
ity exist. For example, [C
2
mim][BF
4
] has a value of 0
:
193
0
:
006W/(m K) at
350K,
120
while [C
4
mim][PF
6
] has a thermal conductivity of 0
007W/
(m K) at 335K.
121
The computed values for [C
2
mim] [EtSO
4
] thus appear to be
reasonable. Note that the thermal conductivity of the pure ionic liquid is much
lower than that of water, and, it remains low even at a water mole fraction
of 0.75.
To summarize, less work has been done on modeling transport proper-
ties of ionic liquids than on modeling properties such as liquid structure and
density, but this area of research is undergoing rapid developments. We have
emphasized here that one must be very careful when computing transport
coefficients about the underlying assumptions being made. First, simulations
must be run for times longer than the relevant dynamical relaxation processes
in the liquid; at lower temperatures, these times can exceed 10 ns. Second,
we argued against using the Green-Kubo integration methods because of
the numerical inaccuracy associated with conducting lengthy integrations
of time correlation functions that fluctuate near zero. Either integrated
Einstein-like formulas or NEMD techniques should be used. Third, polariz-
ability in a force field tends to increase the dynamics of the system, but it
has not been shown that inclusion of polarizability is necessary to obtain
:
147
0
:
Table 1
Computed Thermal Conductivities for [C
2
mim][EtSO
4
]asa
Function of Water Content at 348 K
Mole Fraction Water
Thermal Conductivity (W/mK)
0.00
0
:
167
0
:
002
0.26
0
:
176
0
:
003
0.50
0
:
187
0
:
003
0.75
0
:
242
0
:
005
1.00
0
:
85
0
:
01
Source
: From Ref. 118.
