# Atomic Force Spectroscopy on Ionic Liquids
## 5 Structure of the Interface Layer of Ionic Liquids Revealed by AFS  
### The Electrified Interface  
The history of investigating the ionic liquid/solid interface layer by means of AFS goes back to the year 2006 when [PERSON] and [PERSON] performed the first measurements on the atomically flat model substrates mica, silica, and graphite [88]. They investigated the three different ionic liquids 1-ethyl-3-methylimidazolium acetate (C2 MImAc), propylammonium nitrate (PAN), and ethylammonium nitrate (EAN). As is exemplarily shown in Figure 8, the force-separation curves exhibit oscillations with frequencies corresponding to the dimension of the ion pairs of the respective ionic liquids. A strong correlation between the surface charge, surface roughness, and molecular arrangement with the observed layered structure was found. In particular, in the case of graphene, a pronounced layering was identified with up to seven layers for C\({}_{2}\)MImAc. Hence, it was concluded that steric effects such as the local interaction of the alkyl chains of the ionic liquid molecules with the carbon atoms of the substrate are decisive for the formation of the interfacial structure. Comparing the ionic liquids under investigation, it has been determined that an increased internal flexibility (e.g., PAN compared to EAN) results in a weakening of the layer structure and an increase in the compressibility. Figure 8: Force–separation curves measured on the silica surface (left) and the graphite surface (right) using three different ionic liquids, EAN (A), PAN (B) and C\({}_{2}\)MImAc (C). Adapted with permission from [PERSON] and Warr [88]. Copyright 2007, American Chemical Society. As the mechanical measurement of layered interfaces is prone to fluctuations, it is useful to not measure only one but a set of several tens of force curves (as drawback information about dynamic effects, which are often investigated in biological AFS, gets lost). Subsequently, their average can be calculated and presented using 2D histograms, as is shown for the [EMIm][TFSI]/mica interface in Figure 9. Four series of measurements obtained using different set points of maximum force are presented, illustrating a major challenge when performing AFS in liquids. Due to the calculation of the separation via the cantilever deflection (Equation (3)) and the increasing hardness of layers when approaching the surface, it is demanding to determine the point of zero separation, which denotes the point when the tip is in direct contact with the surface [20]. In all measurements, the layered interface structure of the ionic liquid can be clearly observed, but when choosing too low of a set point, the real surface cannot be reached. This also constitutes a requirement on the cantilever selection. Using a soft cantilever with a low spring constant has the advantage of high sensitivity at the expense of a limit on the maximum force. Furthermore, the large deflection of soft cantilevers can result in a distortion of the force curves due to the nonlinearities of the photodetector [20]. Hence, one should select a cantilever that can provide sufficient force to reach the surface. In the example of Figure 9, a set point of 14 nN was needed to puncture all liquid interface layers. This was concluded by the fact that a further increase of the force set point to 18 nN did not reveal further layers. An additional indication for hitting the surface can be derived by regarding the retraction curves, often showing a distinct change in adhesion after direct tip-surface contact [20]. Using a tip attached to a cantilever as a mechanical probe, one should also keep in mind that the probe itself can have an impact on the measurement. The radius of commercial AFM tips is typically in the range of a few nanometers, which is much larger than the distance between ionic liquid interface layers that are supposed to be measured.