# Atomic Force Spectroscopy on Ionic Liquids
## 2 Bulk Structure of Ionic Liquids
Hence, three distinguishable types of domains are present, namely the charged polar domain, the fluorophilic domain and the lipophilic hydrocarbon domain [33]. They establish a filamentary network throughout the bulk phase of the liquid, thus introducing complex order phenomena [34]. As the building blocks of the network, which have fundamentally different local properties, coexist on the nanoscale, triphilic ionic liquids promise the possibility of the precise local control of chemical reactions and transport properties and thus have become an emerging research topic [33]. A further step towards more structural complexity is the admixture of additives or cosolvents, e.g., by mixing different ionic liquids [35]. When two or more ionic liquids are mixed, the resulting properties cannot be simply understood as conventional \"double salt mixtures\" but as all constituting ions interacting with each other, thus forming an individual nanostructure that determines the properties of the new liquid [36]. In pure ionic liquids, mixture effects related to impurities must also be considered. Water in particular is a common impurity, as it can be easily absorbed from a humid atmosphere or purposely added to the ionic liquid to alter the mechanical or electrochemical properties [37; 38; 39; 40]. The uptake of water was found to depend specifically on the hydrophilicity of the anions, thus pointing to a close interaction between water and the ionic liquid nanostructure [41; 42; 43]. In general, the addition of water to an ionic liquid can be divided into different steps, as elaborated by [PERSON] et al. [44]. This scheme is illustrated in Figure 3. With small water contents, an embedment of water in the ionic liquid nanostructure can be established without clustering effects and thus this stage can be described as the solvent mixture (step II). With increasing water content, the ordered structure of the ionic liquid gets distorted and the dissociation and hydration occurs such that individual ion pairs can evolve (step III). Finally, at high water concentrations, a complete or partial dissociation and hydration takes place (step IV) and thus the mixture can be treated as a classical electrolyte. Depending on the individual properties of the ions, the solution of the ions in the water can also be related to the formation of micelles [45]. This has been studied in detail with ionic liquids containing the 1-alkyl-3-methylimidazolium family of cations and it has been found that the micelles formation can accelerate specific chemical reactions, thus showing potential for the practical application of ionic liquid-water mixtures in catalysis [46]. Figure 3: Schematic illustration of the mechanism of the intermixing between water and an ionic liquid. The ionic liquid is displayed in yellow (cations) and blue (anions) and the water in red (oxygen) and white (hydrogen). Adapted from [PERSON] et al. [44]. Considering the mechanisms of the formation of a heterogeneous nanostructure in the bulk of ionic liquids described so far, it must be expected that they will also influence the formation of the interface structure. As the presence of an interface breaks the symmetry of the system, a different nanostructure than that in the bulk evolves [47]. This can be readily seen in the simulation shown in Figure 2 where, at the surface of the liquid modelled as the interface to the vacuum, a preferential agglomeration of cationic tail groups occurs. When the ionic liquid is in contact with a solid, the interface structure is also affected by the topography, atomic arrangement, reactivity, local charge, and polarizability of the solid surface [11]. This interface structure becomes additionally distorted when electrical potential is applied to the solid, as is the case in electrochemical cells such as PEMFCs that employ ionic liquids as an electrolyte [48]. In consequence, a complex layered structure is formed in the ionic liquid that determines the kinetics of ion transport and reactions at the interface.