# Atomic Force Spectroscopy on Ionic Liquids
## 5 Structure of the Interface Layer of Ionic Liquids Revealed by AFS
### The Electrified Interface
Similar effects have been observed comparing 1-ethyl-3-methylimidazolium bis[trifluoromethylomethyl]-sulfonyl]amide ([EMIm][TFSA]) and 1-butyl-1-methylpyrrolidinium bis[(trifluoromethylomethyl)-sulfonyl]amide ([Py1,4][TFSA]). Unlike the [EMIm]\({}^{+}\) cations, the [Py1,4]\({}^{+}\) cations were found to be strongly bound to the surface, leading to the evolution of a wormlike reconstruction pattern under cathodic polarization [96]. This illustrates that even small changes in the molecular configuration of the cations can drastically change the electrochemical properties of the ionic liquid. Moreover, the configuration of the anions can also have an impact on the interfacial structure as was addressed, e.g., by [PERSON] et al. [97]. In their work, ionic liquids having the same [EMIm]\({}^{+}\) cation but four different anions bis(trifluoromethylsulfonyl)imide [TFSI]\({}^{-}\), trifluoromethylsulfonate [To]\({}^{-}\), methylsulfonate [Oms]\({}^{-}\), and acetate [OAc]\({}^{-}\) were investigated in contact with the Au (111) surface. AFS measurements under cathodic polarization revealed that the number of layers and force necessary for their punctuation increases systematically with anion size, and it was concluded that this is also the cause for differences in macroscopic electrowetting behavior [97]. In order to extract further information from the force-separation curves, approaches to controlled tip modification have been developed. One method to enlarge and accurately define the size of the contact area is to use colloidal probes instead of conventional pyramidal AFM tips [98]. Using a spherical silica probe with a diameter of 5 \(\upmu\)m, [PERSON] et al. [99] were able to elucidate the interface structure between Au and imidazolium-based ionic liquids with cations having different alkyl chain lengths, such as [EMIm]\({}^{+}\), [BMIm]\({}^{+}\), and [HMIM]\({}^{+}\). They found that under cathodic polarization, [BMIm]\({}^{+}\) was relatively weakly bound to the surface, while [EMIm]\({}^{+}\) as well as [HMIM]\({}^{+}\) showed stronger interactions. For [EMIm]\({}^{+}\), this was attributed to the stronger interaction of the imidazolium ring with the surface. For [HMIM]\({}^{+}\), steric effects related to the increased length of the alkyl chain, leading to a stronger solvophobic force, have been made responsible. A second approach of tip modification was followed by [PERSON] et al. [85]. They attached alkylthiols with different end groups, such as -NH3\({}^{+}\), -COO\({}^{-}\), and -CH3 to the tip, thus creating charge-sensitive probes, as illustrated in Figure 12a. They investigated the interface between 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIM][PF6]) and Au (111). Regarding the force-distance curves in Figure 12a measured under anodic and cathodic substrate polarization, distinct differences between the neutral, positive, and negative tips can be seen. Positively charged tips are more sensitive for layer detection at positive substrate potential and vice versa. This becomes obvious with regard to the force, which is necessary to punctuate the third sublayer as a function of the potential (Figure 12b). A \"camel-shaped\" dependency can be observed for all tips, indicating that the structural organization of the electric double layer increases with the potential at both polarities, but at positive potential, the positively-modified tips show the highest force, while at negative potential, the force for the negatively modified tips is the highest. Based on these findings, [PERSON] et al. [85] elaborated a molecular model of the interface, as shown in Figure 12c. At negative substrate potential, the cations are modeled to be directly attached to the surface via their imidazolium ring where the positive charge is located. At positive bias, the smaller anions form the first interface layer with the cationic imidazolium ring attached to it while the cationic tail groups are pointed away from the surface.