# Atomic Force Spectroscopy on Ionic Liquids
## 2 Bulk Structure of Ionic Liquids  
As ionic liquids are composed of ions, the main interaction in the liquid phase is the electrostatic force between the oppositely-charged molecules. In contrast to a classical molten salt however, these molecules, in particular the cations, show a significant molecular asymmetry that countervails the coulombic interaction. This effect is the main cause of the characteristic low melting temperature [21; 22]. Extensive theoretical and experimental research has been performed, especially on imidazolium-based ionic liquids, and it was found that they order in a self-assembled network, mediated by hydrogen bonding between the molecules in the solid as well as the liquid and even in the gas phase [23; 24]. Hence, they can be effectively described as a heterogeneous polymeric \" supramolecular fluid\" consisting of polar and nonpolar domains at the nanoscale [25]. In order to illustrate the bulk structure of ionic liquids, simulations related to molecular dynamics, together with experimental high-energy scattering methods, are frequently employed [26]. When simulating the forces between the molecules, particular care must be taken into account for steric effects, since the electronic landscape around the ions in the liquid reveals a distinct anisotropy [27]. These effects can be exemplified with the imidazolium cations 1-ethyl-3-methylimidazolium [EMIm]\({}^{+}\) and 1-butyl-3-methylimidazolium [BMIm]\({}^{+}\) (see Figure 1). They both consist of a methyl group and an imidazole ring, while the length of their alkyl side chain differs. As the positive charge of the cation is located in the methyl head group, the tail group can be regarded as being nonpolar. By performing molecular dynamics simulations of ionic liquids consisting of imidazolium cations and NO\({}_{3}\)\({}^{-}\) anions, [PERSON] et al. [28] could show that the nonpolar tail groups of the cations mainly aggregate due to van-der-Waals forces. In this way, they form domains beside a network consisting of the head groups of the cations and anions, which is kept together by electrostatic forces. In this way, a heterogeneous nanostructure of the ionic liquid is established. Figure 2 depicts a snapshot of such a simulation, making the aggregation phenomenon in the bulk visible. Comparing cations with different tail groups, [PERSON] et al. [28] concluded that the tendency for building heterogeneous bulk structures increases with the length of the alkyl chains, which can explain many macroscopic properties such as diffusivity and viscosity. Not only does this hold true for the cations, but it was also found that asymmetric anions with extended alkyl chains can be the cause of  
Figure 2: Molecular dynamics simulation of an ionic liquid consisting of imidazolium-based cations and NO\({}_{3}\)\({}^{-}\) sandwiched between two vacuum interfaces. The white spheres represent the nonpolar cationic tail groups, the gold spheres the charged cationic head groups, the red spheres the anions, and the blue lines the cathodic side chains connecting the head and tail groups. Adapted from [PERSON] et al. [28]. nanostructurization and domain formation. This opens up the opportunity to tailor ionic liquids with specific macroscopic properties by changing the configuration of their base ions [29; 30]. Depending on the specific geometry of the ions, a variety of complex bulk structures can be adopted, e.g., for the protic ionic liquid ethylammonium nitrate (EtNH\({}_{3}\)NO\({}_{3}\)), a defined structure composed of cationic layers with the anions distributed in between the layers as well as being intermixed with the cationic layers was proposed [31]. With the addition of further atomic species, the complexity increases. One example is triphilic ionic liquids, consisting of fluorinated, polar, and apolar constituents [32]. In these liquids, the cations typically contain the tails of alkyl chains while the anions have fluorous tails [33]. It was found that these fluorous tails agglomerate via self-assembly on the mesoscopic scale.