Insights from molecular dynamics simulations on structural organization and diffusive dynamics of an ionic liquid at solid and vacuum interfaces

Abstract

HypothesisA prototypical modelling approach is required for a full characterisation of the static and equilibrium dynamical properties of confined ionic liquids (ILs), in order to gain predictive power of properties that are difficult to extract from experiments. Such a protocol needs to be constructed by benchmarking molecular dynamics simulations against available experiments. SimulationsWe perform an in-depth study of [C 2 Mim] + [NTf 2 ]in bulk, at the vacuum and at hydroxylated alumina surface. Using the charge methods CHelpG, RESP-HF and RESP-B3LYP with charge scaling factors 1.0, 0.9 and 0.85, we search for an optimum non-polarizable force field by benchmarking against self-diffusion coefficients, surface tension, X-ray reflectivity data, and structural data. FindingsBenchmarking, which relies on establishing the significance of an appropriate size of the model systems and the length of the simulations, yields RESP-HF/0.9 as the best suited force field for this IL overall. A complete and accurate characterisation of the spatially-dependent internal configurational space and orientation of IL molecules relative to the solid and vacuum interfaces is obtained. Furthermore, the density and mobility of IL ions in the plane parallel and normal to the interfaces is evaluated and the correlation between the stratification and dynamics in the interfacial layers is detectable deep into the films.

Structural characterization of an ionic liquid in bulk and in nano-confined environment using data from MD simulations

“…S1 of Supporting Information to Ref. [1]), demonstrates a good reproduction of the average structural features. In Fig.…”

“…10. Both of these imply a checkerboard arrangement rather than the usually expected bilayer arrangement of the ions of the IL [1]. INND profiles presented in Fig.…”

“…5 the rdfs of the H9 of cation (cf. Scheme 1 of [1]) with nitrogen, sulphurs and oxygens of the anion, obtained for all three charge methods are compared for the scaling factor of 0.9. For details see Ref.…”

“…For details see Ref. [1] Fig. 6 shows results of our QM calculations using Gaussian09 software [8] that the preferred conformation of the anion in the gas phase is trans .…”

“…Detailed discussion of structural organization at the two interfaces and its implications for dynamics are discussed in detail in Ref. [1].

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