Chemistry Reference
In-Depth Information
Figure 7
Experimental conductivities (open squares) and viscosities (filled circles)
plotted against the calculated number of hydrogen bonds among anions in [P(C
4
)
4
]
amino acid ionic liquids.
69
The mobility of the liquid decreases as hydrogen bonding
among anions increases. (Used with permission.)
computed in this manner, they may have agreed better with experiment. It
would be interesting to investigate this issue further.
A hydrogen-bonding analysis was also performed by Zhou and co-
workers
69
in which the number of hydrogen bonds between an oxygen and
a hydroxyl hydrogen on an anion were counted. Figure 7 shows a plot of
the experimental viscosity and conductivity versus the number of hydrogen
bonds calculated for that particular ionic liquid. The correlation is clear—
the greater the number of hydrogen bonds, the slower the dynamics (i.e., high-
er the viscosity and lower the thermal conductivity). The authors conclude that
the relatively high viscosity of this class of material is due to the ease with
which these materials form hydrogen bonds. If this trend can be shown to
hold for other ionic liquids, it might be an effective screening method in mole-
cular design for identifying ionic liquids with high and low viscosities.
Liu et al. developed force field parameters for
N
,
N
,
N
0
,
N
0
-tetramethyl-
guanidinium (TMG) ionic liquids paired with formate, lactate, perchlorate,
trifluoroacetate, and trifluoromethylsulfonate anions.
70
The force field has
the typical AMBER form, and parameterization was done in much the same
way that other force fields have been developed, with harmonic bond and
angle terms fit to quantum-derived vibrational frequencies, and where dihedral
angle parameters were fit to ab initio energy profiles computed for gas-phase
ions. Liquid densities were computed from 100-ps MD runs at temperatures
ranging from 298 to 373 K. The computed densities agree well with experi-
ment for all but the system containing the trifluoromethylsulfonate anion, in
which case the density was overpredicted by a fair amount. The authors attri-
bute this to possible problems with the charge distribution and Lennard-Jones
