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

Vučemilović-Alagić, Nataša; Banhatti, Radha D.; Stepić, Robert; Wick, Christian R.; Berger, Daniel; Gaimann, Mario U.; Baer, Andreas; Harting, Jens; Smith, David M.; Smith, Ana-Sunčana (2019) Insights from molecular dynamics simulations on structural organization and diffusive dynamics of an ionic liquid at solid and vacuum interfaces. Journal of Colloid and Interface Science, 553 . pp. 350-363. ISSN 0021-9797

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Abstract

Hypothesis A reliable modelling approach is required for simultaneous characterisation of static and dynamic properties of bulk and interfacial ionic liquids (ILs). This is a prerequisite for a successful investigation of experimentally inaccessible, yet important properties, including those that change significantly with the distance from both vacuum and solid interfaces. Simulations We perform molecular dynamics simulations of bulk [C2Mim][NTf2], and thick IL films in contact with vacuum and hydroxylated sapphire surface, using the charge methods CHelpG, RESP-HF and RESP-B3LYP with charge scaling factors 1.0, 0.9 and 0.85. Findings By determining and employing appropriate system sizes and simulations lengths, and by benchmarking against self-diffusion coefficients, surface tension, X-ray reflectivity, and structural data, we identify RESP-HF/0.9 as the best non-polarizable force field for this IL. We use this optimal parametrisation to predict novel physical properties of confined IL films. First we fully characterise the internal configurations and orientations of IL molecules relative to, and as a function of the distance from the solid and vacuum interfaces. Second, we evaluate densities together with mobilities in-plane and normal to the interfaces and find that strong correlations between the IL’s stratification and diffusive transport in the interfacial layers persist for several nanometres deep into IL films.

Item Type:

Article

Additional Information:

We acknowledge funding by the German Research Council, which supports the Excellence Cluster "Engineering of Advanced Materi-als" at the FAU, support by the DAAD project Multiscale Modelling of Supported Ionic Liquid Phase Catalysis (2017-2018), and the NIC project 11311 at the Julich supercomputing facilities. R.D.B., C.R.W., A.-S.S. and D.M.S. gratefully acknowledge financial support from the Croatian Science Foundation project CompSoLS-MolFlex (IP-11-2013-8238). A-S.S and N.V.A. were supported by ERC Stg 33728 Membranes Act. We thank Zlatko Brkljaca (RBI) for assistance and helpful discussions in the early stages of the project.

Uncontrolled Keywords:

Ionic liquid ; Nano-scale film ; Sapphire substrate Interfaces ; Molecular dynamics simulation ; Non-polarizable force field ; Sampling ; Surface tension X-Ray reflectivity ; Structural order ; Lateral diffusion ; Residence time

Subjects:

NATURAL SCIENCES > Physics

Divisions:

Division of Physical Chemistry

Projects: