Extremely slow dynamics of ionic liquid self-assembled nanostructures near a solid surface

Hua Li; Jianan Wang; Gregory G Warr; Rob Atkin

doi:10.1016/j.jcis.2022.10.123

Extremely slow dynamics of ionic liquid self-assembled nanostructures near a solid surface

J Colloid Interface Sci. 2023 Jan 15;630(Pt B):658-665. doi: 10.1016/j.jcis.2022.10.123. Epub 2022 Oct 28.

Authors

Hua Li¹, Jianan Wang², Gregory G Warr³, Rob Atkin⁴

Affiliations

¹ School of Molecular Sciences, The University of Western Australia, Perth, Western Australia, Australia; Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Western Australia, Australia.
² School of Molecular Sciences, The University of Western Australia, Perth, Western Australia, Australia.
³ School of Chemistry and Sydney Nano Institute, The University of Sydney, Sydney, New South Wales, Australia.
⁴ School of Molecular Sciences, The University of Western Australia, Perth, Western Australia, Australia. Electronic address: Rob.Atkin@uwa.edu.au.

PMID: 36343557
DOI: 10.1016/j.jcis.2022.10.123

Abstract

Hypothesis: The dynamics of the self-assembled liquid nanostructure of the ionic liquids (ILs) near a mica surface can be determined from video-rate atomic force microscopy (AFM) data.

Experiments: Video-rate AFM has been used to record the nanostructure dynamics of two most widely studied ILs, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM TFSI) and ethylammonium nitrate (EAN), as well as EAN-water mixtures, above a model anode, mica. Diffusion coefficients were extracted from the AFM videos using dynamic differential microscopy and direct tracking.

Findings: Video rate AFM is able to record the movement of the IL nanostructure. This is the first time that any liquid has been directly visualized at a scale of 10 nm × 10 nm in real-time. Diffusion coefficients determined from AFM videos reveal IL nanostructures near surfaces diffuse orders of magnitude more slowly than individual ions in the bulk. Thus, rather than free-flowing liquid, the near-surface nanostructure is better conceptualized as self-assembled aggregates of IL ions diffusing slowly over the cation-rich Stern layer, akin to adsorbed surfactant micelles in aqueous systems. This new and surprising insight affects wide-ranging processes involving the interfacial dynamics of concentrated electrolytes.

Keywords: Dynamics; Electrode; Electrolyte; Ionic liquids; Nanostructure.