Structure
of Alkylimidazolium-Based Ionic Liquids
at the Interface with Vacuum and WaterA Molecular Dynamics
Study
- Publication date
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Abstract
Ionic
liquid (IL) interfaces with vacuum and water are studied
by means of classical molecular dynamics simulations. Five ILs are
compared: [C<sub>2</sub>mim][TfO], [C<sub>12</sub>mim][TfO], [C<sub>2</sub>mim][NTf<sub>2</sub>], [C<sub>8</sub>mim][NTf<sub>2</sub>]
and [C<sub>12</sub>mim][NTf<sub>2</sub>], where [C<sub>2</sub>mim],
[C<sub>8</sub>mim] and [C<sub>12</sub>mim] stand for 1-ethyl-, 1-octyl-
and 1-dodecyl-3-methylimidazolium cation. Physical propertiesdensity,
thermal expansion coefficient, compressibility, surface tension, heat
of vaporization, self-diffusion coefficient, electric conductivity
and viscosityare calculated and validated against experimental
values. The structure of the interfaces is compared in terms of the
orientation of the molecules and segregation into layers. It is observed
that ILs with short alkyl chains orient at the surface; however, there
is no single preferred orientation. ILs with longer chains, on the
other hand, orient with alkyl chains protruding into the vacuum at
the IL/vacuum interface and into the bulk IL, at the IL/water interface.
Anions and water molecules tend to associate with polar imidazolium
groups