Investigation of Multilayered Structures of Ionic Liquids on Graphite and Platinum Using Atomic Force Microscopy and Molecular Simulations

Zhichao Chen; Zixuan Li; Wei Zhao; Ray A Matsumoto; Matthew W Thompson; Oscar Morales-Collazo; Peter T Cummings; Filippo Mangolini; Joan F Brennecke

doi:10.1021/acs.langmuir.2c00024

Investigation of Multilayered Structures of Ionic Liquids on Graphite and Platinum Using Atomic Force Microscopy and Molecular Simulations

Langmuir. 2022 Apr 5;38(13):4036-4047. doi: 10.1021/acs.langmuir.2c00024. Epub 2022 Mar 21.

Authors

Zhichao Chen¹, Zixuan Li^{2

3}, Wei Zhao⁴, Ray A Matsumoto⁴, Matthew W Thompson⁴, Oscar Morales-Collazo¹, Peter T Cummings⁴, Filippo Mangolini^{2

5}, Joan F Brennecke¹

Affiliations

¹ McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States.
² Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States.
³ Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States.
⁴ Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States.
⁵ Walker Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States.

PMID: 35313730
DOI: 10.1021/acs.langmuir.2c00024

Abstract

The molecular-level orientation and structure of ionic liquids (ILs) at liquid-solid interfaces are significantly different than in the bulk. The interfacial ordering influences both IL properties, such as dielectric constants and viscosity, and their efficacy in devices, such as fuel cells and electrical capacitors. Here, we report the layered structures of four ILs on unbiased, highly ordered pyrolytic graphite (HOPG) and Pt(111) surfaces, as determined by atomic force microscopy. The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), 1-ethyl-3-methylimidazolium perfluorobutylsulfonate ([emim][C₄F₉SO₃]), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis(trifluoromethylsulfonyl)imide ([MTBD][Tf₂N]), and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C₄F₉SO₃]). Molecular dynamics simulations provide complementary information on the position and orientation of the ions. These ILs form a cation layer at the IL-solid interface, followed by a layer of anions. [Emim]⁺ and [MTBD]⁺ have similar orientations at the surface, but [MTBD]⁺ forms a thinner layer compared to [emim]⁺ on both HOPG and Pt(111). In addition, [Tf₂N]^- shows stronger interactions with Pt(111) surfaces than [C₄F₉SO₃]^-.