# Atomic Force Spectroscopy on Ionic Liquids
## 5 Structure of the Interface Layer of Ionic Liquids Revealed by AFS
### The Electrified Interface
Additionally, the presence of the tip leads to the formation of a second solid-liquid interface where, depending on the material and charge of the tip, a layered interface structure can also evolve. Hence, one interface structure is probed with another. [PERSON] et al. [20] performed tests employing two different tips on the same [EMIm][TFSI]/mica interface. They compared a SiN tip with a Au-coated SiN tip. As is shown in Figure 10, the layered interface structure can be seen in both measurements. The forces needed to puncture the layers are significantly different and mainly relate to the tip radius, which was up to three times higher for the Au-coated tip than for the uncoated one. Despite this, the measured thicknesses of the layers are the same for both tips, as becomes obvious from the separation histogram on the right of Figure 10.  
Figure 9: Force–separation curves measured during approach of the [EMIm][TFSI]/mica interface using different force set points. The 2D histograms were calculated using 50 curves. Adapted from [PERSON] et al. [20]. In order to gain information on the electric double layer as a function of the electrical polarization of the solid surface, miniaturized electrochemical cells have been constructed and introduced in AFM setups. [PERSON] et al. [89] investigated the Au(111) surface in contact with the ionic liquids 1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate ([EMIM][FAP]) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py\({}_{1,4}\)][FAP]) under different electrical polarizations. Figure 11 shows the resulting force-separation curves for the [Py\({}_{1,4}\)][FAP]/Au(111) interface at open circuit potential (OCP) and positive and negative voltage. The oscillatory behavior, which was already present at OCP, became modified upon polarization, leading to an increase in the force necessary to punctuate the innermost layers and an increase in the number of detected layers ([PERSON] et al. [90] even measured up to 12 ion pair layers at -2 V). It has been observed that this effect is more pronounced at negative potentials, suggesting that the [Py\({}_{1,4}\)]\({}^{+}\) cations absorbed on the surface are a more effective template for establishing an ordered interface structure. Figure 11: Force–distance measurements on the [Py\({}_{1,4}\)] FAP/Au(111) interface at different surface potentials. Adapted with permission from [PERSON] et al. [89]. Copyright 2011, American Chemical Society. These findings by [PERSON] et al. [89] finally provided direct experimental evidence that the electric double layer in ionic liquids cannot be traditionally described using a Gouy-Chapman-Stern approach, and an oscillatory capacitive double layer is present, as is illustrated in Figure 5. Hence, in recent years, AFS investigations under applied potential have been extended to a variety of different combinations of ionic liquids and conducting surfaces relevant for electrochemical applications [91; 92]. For example, [PERSON] et al. [93] performed detailed measurements on the Au(111) surface in contact with 1-\(n\)-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]). They were able to resolve three to four layers using AFS at the interface. By recording force-separation curves at potentials between \(-0.9\) V and \(+0.5\) V, they concluded that the electric double layer is confined to a few charged interior layers, but that additionally neutral exterior layers are also present. The complexity of the interface structure under electrical polarization has been further elucidated by combining AFS investigations with STM mapping. For both investigated ionic liquids [Py1,4][FAP] and [EMIm][FAP], the innermost ions interact strongly with the electrode surface depending on the potential [94]. However, significant differences between the two liquids are present, considering surface reconstruction. While the [Py1,4][FAP]/Au interface undergoes herringbone reconstruction in the cathodic regime [95], no reconstruction has been observed for [EMIm][FAP]/Au [94].