Nanostructure, electrochemistry and potential-dependent lubricity of the catanionic surface-active ionic liquid [P6,6,6,14] [AOT]

Yunxiao Zhang; Joshua B Marlow; Wade Millar; Zachary M Aman; Debbie S Silvester; Gregory G Warr; Rob Atkin; Hua Li

doi:10.1016/j.jcis.2021.10.120

Nanostructure, electrochemistry and potential-dependent lubricity of the catanionic surface-active ionic liquid [P_6,6,6,14] [AOT]

J Colloid Interface Sci. 2022 Feb 15;608(Pt 2):2120-2130. doi: 10.1016/j.jcis.2021.10.120. Epub 2021 Oct 23.

Authors

Yunxiao Zhang¹, Joshua B Marlow², Wade Millar³, Zachary M Aman⁴, Debbie S Silvester³, Gregory G Warr², Rob Atkin⁵, Hua Li⁶

Affiliations

¹ School of Molecular Sciences, The University of Western Australia, Perth, Western Australia, Australia.
² School of Chemistry and Sydney Nano Institute, The University of Sydney, Sydney, NSW 2006, Australia.
³ School of Molecular and Life Sciences, Curtin University, GPO Box U1987, Perth 6845, Western Australia, Australia.
⁴ Fluid Science and Resources Division, School of Engineering, The University of Western Australia, Perth, Western Australia, Australia.
⁵ School of Molecular Sciences, The University of Western Australia, Perth, Western Australia, Australia. Electronic address: Rob.Atkin@uwa.edu.au.
⁶ 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. Electronic address: Hua.Li@uwa.edu.au.

PMID: 34752982
DOI: 10.1016/j.jcis.2021.10.120

Abstract

Hypothesis: A catanionic surface-active ionic liquid (SAIL) trihexyltetradecylphosphonium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate ([P_6,6,6,14] [AOT]) is nanostructured in the bulk and at the interface. The interfacial nanostructure and lubricity may be changed by applying a potential.

Experiments: The bulk structure and viscosity have been investigated using small angle X-ray scattering (SAXS) and rheometry. The interfacial structure and lubricity as a function of potential have been investigated using atomic force microscopy (AFM). The electrochemistry has been investigated using cyclic voltammetry.

Findings: [P_6,6,6,14] [AOT] shows sponge-like bulk nanostructure with distinct interdigitation of cation-anion alkyl chains. Shear-thinning occurs at 293 K and below, but becomes less obvious on heating up to 313 K. Voltammetric analysis reveals that the electrochemical window of [P_6,6,6,14] [AOT] on a gold micro disk electrode exceeds the potential range of the AFM experiments and that negligible redox activity occurs in this range. The interfacial layered structure of [P_6,6,6,14] [AOT] is weaker than conventional ILs and SAILs, whereas lubricity is better, confirming the inverse correlation between the near-surface structure and lubricity. The adhesive forces of [P_6,6,6,14] [AOT] are lower at -1.0 V than at open circuit potential and +1.0 V, likely due to reduced electrostatic interactions caused by shielding of charge centres via long alkyl chains.

Keywords: Atomic force microscopy; Cyclic voltammetry; Friction; Nanostructure; Rheology; Surface-active ionic liquid.

MeSH terms

Electrochemistry
Ionic Liquids*
Nanostructures*
Scattering, Small Angle
X-Ray Diffraction

Substances

Ionic Liquids