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

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 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide5,7-triazabicyclo[4.4.0]dec-5-ene bis(trifluoromethylsulfonyl)imide ([MTBD][Tf 2 N]), and 7methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C 4 F 9 SO 3 ]). 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, [Tf 2 N] − shows stronger interactions with Pt(111) surfaces than [C 4 F 9 SO 3 ] − .

Importance of Anion–Anion Pairing for Capacitance of Carbon/Ionic Liquid Interfaces

“…In addition to tuning the compatibility of electrode nanostructures with ions, experimental characterizations have shown that diverse ion structure and capacitance of ILs at electrode interfaces can be achieved by selecting different chemical structures and compositions of cations ,,− and anions. ,,,− Of particular interest are ILs that possess both polar and nonpolar groups (e.g., alkyl chains on imidazolium cations and −CF 3 groups on certain anions). These ILs form distinctive ion ordering at electrode interfaces because they interact with the electrode surface, together with varying electrostatic and nonpolar interactions between molecular ions.…”

Structure of Deep Eutectic Solvents (DESs): What We Know, What We Want to Know, and Why We Need to Know It

“…Indirect measurements have been used primarily to elucidate structural heterogeneities in the bulk. Structural information, both at the interface and in the bulk, has been more commonly reported for direct measurements, including atomic force microscopy and sum frequency generation. − …”

Influence of surface nanostructure-induced innermost ion structuring on capacitance of carbon/ionic liquid double layers

“…5,[52][53][54] By selecting and designing the chemical structure and composition of cations and anions, the EDL structure of ILs has been thoughtfully tuned to optimize the IL performance for energy storage. 6,10,12,13,16,17,[21][22][23]32,[55][56][57][58][59] Of particular fundamental and practical interest is the EDL formed by ILs that possess both charged and nonpolar functional groups. These ILs are special in that their constituent ions interact with the electrode surface, intertwined with electrostatic attractions between charged groups and solvophobic (or nonpolar) interactions between nonpolar groups.…”