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

The molecular-level orientation and structure ofionic liquids (ILs) at liquid-solid interfaces are significantlydifferent than in the bulk. The interfacial ordering influencesboth IL properties, such as dielectric constants and viscosity, andtheir 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, asdetermined by atomic force microscopy. The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([emim][Tf2N]), 1-ethyl-3-methylimidazolium perfluorobutylsul-fonate ([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 ofthe ions. These ILs form a cation layer at the IL-solid interface, followed by a layer of anions. [Emim]+and [MTBD]+have similarorientations 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]-

Automated summary

This study investigates the layered structures of four ionic liquids (ILs) on liquid-solid interfaces and reveals the orientation and arrangement of ions on the surface are influenced by interfacial ordering.