# Atomic Force Spectroscopy on Ionic Liquids
## 5 Structure of the Interface Layer of Ionic Liquids Revealed by AFS
### Influence of Water  
When hydrophilic ionic liquids are employed for applications under environmental conditions, water is the major impurity that significantly influences the nanoscale structure of the double layer [106; 107]. In particular, in ionic liquids used as electrolytes in fuel cells, water is generated as the product of the reaction between hydrogen and oxygen and can thus alter the reactions at the electrode surfaces [41]. Even small amounts of water can modify the interface structure of an ionic liquid under applied electric potential [108]. On the interface between butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide ([BMP][TFSA], also referred to as [Py\({}_{1A}\)][TFSI]) and Au(111), [PERSON] et al. [109], using AFS, observed that upon a water increase from 30 ppm to 90 ppm, the stiffness of the first interface layers decreased significantly while the thickness of the layers increased. This indicates that at this low concentration, water absorbs on the surface and distorts the defined orientation of ions in the layer. A comparable result was obtained by [PERSON] et al. [110] on different 1-alkyl-3-methylimidazolium bis(trifluoromethylenesulfonyl) amide ([C\({}_{n}\)Mim][TFSI]) ionic liquids. Water was found to distort the interfacial structure formed with mica, which also impacts the wetting behavior of the liquid [111]. The dissolution of the structured interface layer upon water uptake is also a challenge when using ionic liquids as lubricants due to higher friction [112]. In contrast to many reports showing that even small amounts of water lead to a distortion of the structured interface layer, [PERSON] et al. [105] note that the presence of water is necessary for the evolution of a layered structure at the interface. They also investigated [RMIm][TFSI] ionic liquids in contact with mica and found that the force-separation curves were not indicative for a layered interface structure when using carefully dried ionic liquids, as is shown in Figure 14a,b. The force curve was found to be continuous with a repulsive force detected from approximately 12 A away from the surface, which reveals that only an unstructured interface layer exists. Only after exposing the ionic liquid to humid ambient conditions could the characteristic instabilities in the force curve related to layering be observed (see Figure 14c,d). It was concluded that water or other small molecules are necessary for a charging of the mica surface via the dissolution of K\({}^{+}\) ions, inducing a structuring of the ionic liquid at the interface. In additionally wetted ionic liquids, layering was also present, but the jumps between the innermost layer and surface were larger, suggesting that water forms an adsorption layer on the mica surface, altering the interface structure [105]. Figure 14: Investigation of the interface between mica and [C\({}_{2}\)Mim][TFSI] (**a**,**c**,**e**) and [C\({}_{5}\)Mim][TFSI] (**b**,**d**,**f**) using force-separation histograms at different water contents. Adapted from [PERSON] et al. [105]. The effect of higher amounts of water on the interface structure was examined by [PERSON] et al. [114] using 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm][TfO]) in contact with Au (111). They found that the number of layers detected by the force-separation curves decreased with increasing water content. For the pure ionic liquid, five layers were seen, whereas after the addition of 50 vol% of water, only one layer was resolved under cathodic polarization. Assisted by complementary experiments using vibrational spectroscopy, [PERSON] et al. [114] concluded that a \"water in IL\" to \"IL in water\" transition takes place at 20-30 vol%. As a result, the water molecules weaken the interaction between the ions and the multilayer structure at the interface is diluted to a classical Helmholtz-like double layer. Depending on the molecular configuration of the ionic liquids, this transition can also be related to the formation of micelles. Reverting the roles of ionic liquid and water, [PERSON] et al.