# Atomic Force Spectroscopy on Ionic Liquids
## 1 Introduction  
In general, ionic liquids are defined as liquids that are composed entirely of ions [1]. This definition includes melts of crystalline ionic materials, which are commonly referred to as \"molten salts\", as well as mixtures of any type of ions in the liquid state. In a narrower sense, \"room-temperature ionic liquids (RTIL)\" are understood as salts, which often contain organic ions, with melting points below room temperature [2; 3]. Starting from their first discovery in 1914, they have become of central interest in fundamental and applied chemistry in recent decades, mainly because they are considered as promising alternative environmentally-beneficient solvents [4]. As their properties can be as systematically tailored as both their constituents, anion and cation, can be varied, they have been labelled \"designer solvents\" [5]. In general, ionic liquids provide a variety of desirable characteristics for chemical engineering, such as a high chemical and thermal stability, a high ionic conductivity, a negligible vapor pressure, and low flammability [6]. Potential fields of application comprise not only all techniques of chemical synthesis and processing where conventional molecular solvents are employed, but also mechanical engineering where ionic liquids may serve as lubricants and coatings [7]. Moreover, they have attracted enormous attention in electrochemistry, as they can be used as stable electrolytes for sustainable energy storage and conversion by supercapacitors, batteries, dye-sensitized solar cells, or polymer electrolyte membrane fuel cells (PEMFCs) [8]. As an illustration of the composition of ionic liquids, selected cations and anions are presented in Figure 1.  
The analysis and description of ionic liquids opens a new field of science, not only because of the tremendous number of potential ionic liquids (up to \(10^{6}\) binary and \(10^{18}\) ternary ionic liquidshave been predicted [9]), but rather because ionic liquids behave fundamentally differently than conventional molecular solvents. Classically, liquids are understood as matter without inner structure and long-range ordering [10]. As experiments have shown that this does not hold true for ionic liquids, a paradigm shift in the understanding of their structure has occurred. As [PERSON] et al. [11] put it, molecular solvents can be described as \"homogeneous, coherent, and essentially irregular\", but ionic liquids must be regarded as \"nanoheterogeneous, coherent, and essentially regular\". This regular ordering not only involves the bulk of ionic liquids but also surfaces and interfaces, which is of particular importance when using ionic liquids in electrochemical applications. In molecular solvents, the interface between a solid electrode and a liquid solvent can be described as the formation of a double-layer, as elaborated by [PERSON], [PERSON], [PERSON], and [PERSON] [12], but in ionic liquids, the internal ordering also influences the electrochemical processes at the interface [13]. Hence, a multitude of experimental approaches have been applied in order to elucidate the structure of ionic liquids at electrified interfaces. Among them, at first, electrochemical impedance spectroscopy (EIS) must be mentioned as a well-established method for estimating the average double layer thickness and the potential of zero charge (PZC) [14]. This method has been complemented by optical methods of interface analysis such as sum frequency generation vibrational spectroscopy (SFG), or infrared (IR) and Raman spectroscopy [15; 16]. High-energy reflection and scattering using X-rays and neutrons have also been employed, revealing that ionic liquids form regular layers at the interface [17; 18]. Direct mechanical access to the interface structure has been provided by using a surface force apparatus (SFA) to measure the thickness of anion and cation layers at the interface with utmost precision down to the Angstrom scale but averaging over a large contact area in the square micrometer range [19].