Chemistry Reference
In-Depth Information
exchanged during the swap moves, the momentum flux is known. It is a simple
matter to combine this with the measured shear rate to compute the shear
viscosity via Eq. [23]. The method requires no modification to boundary
conditions or the Ewald sum.
We have confirmed that the RNEMD method gives identical results to
equilibrium MD and SLLOD calculations for the Lennard-Jones fluid, molten
NaCl, water, alcohols, and alkanes
116
and have applied it to compute the visc-
osity of [C
2
mim][Tf
2
N] as a function of temperature and water content.
117
Figure 21 shows the computed viscosity as a function of temperature. The
solid line is a Vogel-Fulcher-Tamman fit to a large number of experimental
data points. The agreement with experiment is excellent. Note that these
results were obtained with a fixed-charge model in which the cation and anion
were forced to have a formal charge of unity. The viscosity was also calculated
for mixtures of [C
2
mim][Tf
2
N] and water; it is known that water tends to low-
er the viscosity of ionic liquids dramatically, and the simulations captured this
trend well (although the drop in calculated viscosity was not as great as what is
observed experimentally). Most interesting is that the drop in viscosity
observed experimentally and in the simulations is
less
than what would be pre-
dicted from simple empirical correlations. The conventional wisdom that
small amounts of water dissolved in ionic liquids causes some inordinately
large drop in the viscosity is not true; water does not decrease the viscosity
as much as would be expected from ''ideal'' mixture viscosity models. The cal-
culations show that water forms hydrogen-bonded clusters in the ionic liquid.
As a result, the molar volume of the mixture remains somewhat lower than
what would occur if the water had been mixed ideally (uniformly) with the
80
Simulations
Fit to Experimental Data
60
40
20
0
250
300
350
Temperature (K)
400
450
500
Figure 21
Computed viscosity as a function of temperature (symbols) compared to a
correlation of a large number of data points (solid line) for [C
2
mim][Tf
2
N]. (Results are
from Ref. 117.)
