  Room temperature ionic liquids (ILs) at solid surfaces have been recognized
for their significant interfacial properties in electrochemical and electronic
devices. To ascertain the interface effects, we investigate dynamical and
structural properties of two ILs in nanoscale confinement at various
temperatures. Specifically, we perform all-atom molecular dynamics simulations
for ILs composed of 1-butyl-3-methylimidazolium cations and hexafluorophosphate
([Bmim][PF6]) or tetrafluoroborate ([Bmim][BF4]) anions sandwiched between
amorphous silica slabs. Density profiles of the ionic species across the slit
reveal that [PF6] and [BF4] anions tend to stay closer to the slab wall than
[Bmim] cations resulting in a bi-layered arrangement in the interfacial region.
For the cations, we observe a preferred orientation at the surface with the
methyl groups pointing towards the wall and the butyl tails projected inwards.
Mean square displacements and incoherent scattering function reveal slowed and
heterogeneous dynamics of all ionic species in the slit pore. In particular,
spatially resolved analyses show that the structural relaxation times increase
by about two orders of magnitude when approaching the silica surfaces, an
effect to be considered when designing applications. The altered structural and
dynamical features of the confined ILs can be related to an existence of
preferred sites for the anions on the amorphous silica surfaces. Detailed
analyses of relations between the broadly distributed site and surface
properties show that particularly stable anion sites result when triangular
arrangements of silanol groups enable multiple hydrogen bonds with the various
fluorine atoms of a given anion, elucidating an important trapping mechanism at
the silica surface.
