Effects of Silica Surfaces on the Structure and Dynamics of Room Temperature Ionic Liquids: A Molecular Dynamics Simulation Study

摘要

Room temperature ionic liquids (ILs) at solid surfaces have been recognizedfor their significant interfacial properties in electrochemical and electronicdevices. To ascertain the interface effects, we investigate dynamical andstructural properties of two ILs in nanoscale confinement at varioustemperatures. Specifically, we perform all-atom molecular dynamics simulationsfor ILs composed of 1-butyl-3-methylimidazolium cations and hexafluorophosphate([Bmim][PF6]) or tetrafluoroborate ([Bmim][BF4]) anions sandwiched betweenamorphous silica slabs. Density profiles of the ionic species across the slitreveal 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 themethyl groups pointing towards the wall and the butyl tails projected inwards.Mean square displacements and incoherent scattering function reveal slowed andheterogeneous dynamics of all ionic species in the slit pore. In particular,spatially resolved analyses show that the structural relaxation times increaseby about two orders of magnitude when approaching the silica surfaces, aneffect to be considered when designing applications. The altered structural anddynamical features of the confined ILs can be related to an existence ofpreferred sites for the anions on the amorphous silica surfaces. Detailedanalyses of relations between the broadly distributed site and surfaceproperties show that particularly stable anion sites result when triangulararrangements of silanol groups enable multiple hydrogen bonds with the variousfluorine atoms of a given anion, elucidating an important trapping mechanism atthe silica surface.