# Atomic Force Spectroscopy on Ionic Liquids
## 5 Structure of the Interface Layer of Ionic Liquids Revealed by AFS
### The Electrified Interface
In the intermediate state, at zero charge, a checkerboard-like arrangement of the ions was assumed instead. Aside from the aforementioned investigations of the electrified Au surface, studies of the interface between ionic liquids and graphite have been conducted, as the understanding of carbon-based electrodes is of high significance for building fuel cells or electrochemical supercapacitors [100]. [PERSON] et al. [101] focused on the interface structure between 1-ethyl-3-methyl-imidazolium bis(trifluoromethylomenthanesulfonyl)imide ([EMIm][TFSI]) and highly oriented pyrolytic graphite (HOPG). The measured 2D force-separation histograms are presented in Figure 13 for the unbiased and positively and negatively polarized surface. In all cases, an ordered structure with up to four layers has been revealed. In order to gain a deeper insight into the ionic arrangement of the double layer, the measured curves were correlated with molecular dynamics simulations. The densities of anions and cations as a function of the separation from the surface were calculated from simulations performed at different  
Figure 12: Measurement of the [OMIM][PF6]/Au(111) interface using a modified tip. (**a**) Force–distance curves measured at a potential (E) above and below the potential of zero charge (PZC) with molecules having neutral, negative, and positive end groups attached to the tip (the inset tables display the force necessary to rupture the first, second, and third layer); (**b**) force necessary to rupture the third sublayer measured at different potentials applied to the surface; (**c**) model of arrangement of the ionic liquid at the interface depending on the potential. Adapted from [PERSON] et al. [85]. surface potentials, as shown in Figure 13 for +1 V and -1 V. In contrast to the uncharged surface, where different ion orientations are present, the electrostatic force induced by the application of the potential leads to a predetermined ion orientation in the first ion layer and a charge screening confined to approximately 1 nm. Further from the surface, a layered interface structure with absent preferential ion orientation was predicted. The conclusions derived from the simulation closely match the experimental results, showing the versatility of AFS for investigating the electric double layer in ionic liquids. [PERSON] et al. [102] extended the investigations of carbon-based materials to the graphene surface. They investigated the interface formed with [EMIm][TFSI] at zero charge as well as under applied potential, concluding that the strong interaction of the imidazolium ring with the graphene surface via \(\pi\)-\(\pi\) bonding results in a preferential orientation of the ions and to an overscreening, even on the uncharged surface. When a potential was applied, the behavior of the double layer changed from overscreening towards crowding, leading to a decrease in the force within the layered structure. While the investigation of model systems of atomically flat surfaces provides valuable insights into the fundamental nature of the double layer formation of ionic liquids, one must question if these results can be directly transferred to real applications where rough electrodes are present. In order to scrutinize this issue, [PERSON] et al. [103] conducted a study employing silica nanoparticles deposited on silica wafers. Additionally, they employed sharp tips as well as different silica colloids as AFM probes to modulate the roughness of the system confining the ionic liquid. Comparing the force curves measured in [HMIm][TFSI] shows that the roughness has a distinct influence on the layered structure. The presence of only a low density of nanoparticles modifies the measured double layer across the entire surface. Furthermore, the use of a rough probe contact was found to be related to a localized arrangement and agglomeration of the ionic liquid at \"multi-asperity\" contacts [103].