# Atomic Force Spectroscopy on Ionic Liquids
## 5 Structure of the Interface Layer of Ionic Liquids Revealed by AFS
### Addition of Metallic Solvates  
As ionic liquids are promising electrolytes for applications containing metallic components such as batteries or electrodeposition, many AFS studies have been performed that focus on the formation of the interface structure in the presence of the dissolved metals. [PERSON] et al. [115] performed a study of [Py1,4][FAP] on the Au(111) surface, revealing that the addition of LiCl changes the interface drastically. While the AFM tip experienced a repulsive force when approaching the surface as a pure ionic liquid, an attractive force was present in the Li-containing mixture. The addition of Li only influenced the innermost layer of the interface structure, leaving layers further away from the interface unchanged. Using N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imid ([Py1,4][FSI]), which intrinsically shows a stronger interaction with the Au(111) surface than [Py1,4][FAP], comparable experiments, with the addition of Na, were performed. [PERSON] et al. [116] reported that the interfacial multilayer structure is not significantly affected after adding small amounts of Na. However, when adding more than 0.25 M of Na, the innermost layers changed, indicating the local formation of [Na(FSI)3]2-. The impact of a layered electric double layer on the electrochemical performance has also been illustrated by [PERSON] et al. [117], addressing the mechanisms in rechargeable zinc batteries. Zinc dicyanamide salt (Zn(dca)2) has been admixed to two ionic liquids, N-butyl-N-methylpyrrolidinium dicyanamide ([C4 propyl][dca]) and 1-ethyl-3-methyl-imidazolium dicyanamide ([C2 mim][dca]), while force-separation curves were recorded as a function of the water content and applied potential. Layering at the interface was readily present in the pure ionic liquids in contact with the HOPG. The admixture of the zinc salts did not lead to a significant dissolution of the multilayer structure; it even increased the number of layers for [C4 propyl][dca]. These results demonstrate that interface ordering can be challenging when an effective mass transport to the surface is needed and that for chemically similar ionic liquids, electrochemical performance can significantly differ. The subtle interplay between metallic precursors and ionic liquids was elucidated by [PERSON] et al. [118], looking at the electrodeposition of Ta and Ga from halides in [Py1,4][TFSA], as well as [Py1,4][FAP] on the Au(111) surface. Using AFS, they concluded that in mixtures of [Py1,4][TFSA] and TaF5, the electric double layer becomes enriched in Ta-containing molecules, thus allowing for their further reduction to Ta in metallic form directly at the interface. When they tried to reproduce this experiment using [Py1,4][FAP], it was not possible to induce the electrodeposition process, as the Ta species was pushed away from the interface. By changing the precursor to GaCl3, however, the electrodeposition of Ga could be successfully started in both ionic liquids. In order to simulate the interaction between solutes and ionic liquids, [PERSON] et al. [119] developed a mathematical semi-continuum model based on AFS investigations of the interface between Au(111) and in [Py1,4][TFSA] with varying admixtures of Ag salt. As presented in Figure 16, a clear correlation between Ag concentration and the detected layers at the interface was observable. With increasing Ag concentrations, the widths of the layers at first increase, suggesting the presence of larger AgTFSA complexes within the double layer. Eventually, at concentrations above 500 \(\upmu\)M AgTFSA, the ordered structure collapses into a simple double layer. The developed thermodynamic continuum model, which treats the ionic liquid as being composed of hard spheres, could be successfully applied to describe the interfacial structure under the influence of Ag additives and to simulate the force-separation curves [119].