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Abstract
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Molecular dynamics simulations are done to investigate the
structure and dynamics of a thin [Bmim][MeO 4 ] film in contact
with a hydroxylated silica surface on one side and with vacuum
on the other. An examination of the microscopic structure of
ionic liquid (IL) film shows that strong layered anionic/cationic
structures are formed at both interfaces. At the silica interface,
the imidazolium rings are closer to the silica surface (compared
to anions) and are coplanar with it. At the vacuum interface, the
charged imidazolium ring more concentrates in the interior of
the film, but the butyl side chain stretches out toward the
vacuum interface. While there exists an excess concentration of
the cations at the silica interface, at the vacuum interface an
excess concentration of anions (dissolved in the butyl chain) is
found. The influence of the interface on the dynamical proper-
ties is shown to depend on their time scales. A short-time
dynamical property, such as hydrogen bond formation is not noticeably perturbed at the interface. In contrary, long-time
properties such as ion-pair formation/rupture and translation of
ions across the film are largely decelerated at the silica interface
but are accelerate at the vacuum interface. Our findings
indicate that the structural relaxation time of ion-pairs, is
comparable to diffusion time scale in the IL film. Therefore, ion-
pairs are not stable species; the IL is composed of short-lived
ion-pairs and freely diffusing ions. However, the structural
relaxation times of ion-pairs is still long enough (comparable to
the time scale of diffusion) to conclude that correlated motions
of counterions influence the macroscopic properties of IL, such
as diffusion and ionic conductivity. In this respect, we have
shown that correcting the Nernst-Einstein equation for the joint
translation of ion-pairs considerably improves the accuracy of
calculated ionic conductivities.
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