# A Presentation of Ionic Liquids as Lubricants: Some Critical Comments
## 2 Why Should ILs Be Used as Lubricants?
### Surface-Dependent IL Structure  
The structure of an IL is peculiar. By the term structure we mean the existence of preferential distances between constituents. In a simple liquid, the position of molecules can be influenced only by interactions taking place at short distances, so the preferential distance is generally that at which the first coordination shell forms, all higher distances being almost equally probable [32]. On the other hand, in ionic liquids, signatures of the mesoscopic order occur. In particular, it has been suggested (see the example given by imidazolium-based ionic liquid [33]) that nanoscale structural heterogeneities in the bulk liquid state are caused by the segregation of long enough apolar alkyl chains into domains which are embedded into a three-dimensional charged matrix generated by the positively and negatively charged heads [34]. This feature has also been found in alcohols and their mixtures [35] and also in liquid surfactants and their mixtures, whose structure is rationalized in terms of the coexistence of polar/apolar domains instead of positive/negative domains as in ionic liquids [36]. Generally speaking the tendency of intermolecular structure formation takes place when intermolecular interactions become strong enough to overcome thermal agitation. This can even give rise to self-segregation. It is clear that intermolecular interactions become increasingly stronger in the order of excluded volume, apolar interactions, polar interactions, H-bonds, and electrostatic interactions, so an increasingly higher structure with the increasing ionic liquid character is expected. However, it must be remembered that, in soft matter, parallel with the intensity of the interactions involved, a key factor is the number density of interactions. It can happen, in fact, that a high number of soft interactions can give rise to a significant structure, as in soft matter and bio-related materials, where a slight difference in temperature can induce striking changes in the structure. In our case, the interactions between IL and the contacting surface can be of the same strength (or even stronger) than the interactions present in pure IL  
Figure 2: Schematic collection of the IL properties for lubrication purposes.  
and characterizing the inner (bulk) part of the ionic liquids. This can cause a dramatic structural change when approaching the contacting surface. This is schematically depicted in Figure 3.  
The structure of an IL is therefore not invariant, since it can depend on the material that needs to be lubricated. Its function, therefore, must be addressed to the complex combination of IL properties and the potential interactions with the contacting material needing lubrication.  
In this context, it is interesting to notice that the recent developments of self-assembly processes in nanoscience are already providing innovative well-defined smart materials linking soft matter chemistry to hard matter sciences. This is the key point of this work: the ionic liquid structure (soft matter) is related to the solid surface (hard matter) when approaching the interface, which is a fundamental aspect in lubrication.