Chemistry Reference
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dynamics of the system are overestimated significantly with the model, as is to
be expected.
Wang and Voth
129
studied the local structure of the [C
4
mim][NO
3
]
system using this coarse-grained model and found that the tail groups formed
relatively stable domains at a suitable temperature and when enough attrac-
tive interactions exist between the nonpolar groups on the cationic side chain.
These domains are depicted in Figure 23. At the same time, Canongia Lopes
and Padua
130
reported results using a fully atomistic model in which they also
observed ''nanodomains'' of order among polar and nonpolar parts of the
ionic liquid [C
n
mim][PF
6
], where
n
is a variable length alkyl chain. They col-
or-coded different regions of the ionic liquid (shown in gray tones in Fig-
ure 24) and observed segregation among the ''polar'' and ''nonpolar''
groups. In particular, they found that as the alkyl chain length increased,
the nonpolar regions percolated the entire simulation box. This finding
caused a great deal of excitement in the experimental ionic liquids commu-
nity, and since then many experimental
131
and theoretical
132
studies have
been carried out to confirm and explain that ordering. Coarse-grained models
can thus provide qualitative insight into the behavior of ionic liquids and can
stimulate and guide new experimental investigations while informing us of
the details of liquid-phase organization in ways that are difficult to see experi-
mentally.
Figure 23
One snapshot of a coarse-grained representation of [C
4
mim][NO
3
] with 1000
ion pairs at
T
700K with (a) all atoms, (b) tail groups only, (c) head groups only, and
(d) anions only. Notice the tail groups organize into domains. (From Ref. 129 and used
with permission.)
ΒΌ
