# A Presentation of Ionic Liquids as Lubricants: Some Critical Comments
## 1 Introduction  
Friction is a very general phenomenon of energy dissipation that appears between sliding pairs [1, 2]. For this reason, the reduction of friction plays a crucial role in many applications in which it is necessary to achieve the control of heat generation and of the constituents wearing out. The addition of a lubricant between two sliding solids may remarkably influence the friction even if it depends on the nature of the employed lubricant. A definition of \"good lubricant\" can be surely correlated to three prevalent factors:  
1. the achievement of the desired friction coefficient;
2. the lubricant has to be able to withstand the pressure without being squeezed out;
3. the lubrication performance should be as independent as possible of changes in the environment [3].  
A correlated aspect is that it should reduce the heat generated during surface interactions in order to prevent structure deformation during these processes.  
A typical lubricant is generally a mineral oil. Usually, it is added to additives, such as antioxidants, detergents, dispersants, friction modifiers, etc., to achieve other effects, such as reduction of wear, erosion, and corrosion of the contacting surfaces.  
The idea to employ ionic liquids (ILs) as synthetic lubricants was developed in 2001 [4], and several publications and patents on this topic were successively published to highlight their high-performing lubricant properties. ILs are organic salts with low melting temperatures possessing particular characteristics, such as low vapour pressure, low volatility, high thermal stability, high combustible temperature, and high miscibility with organic compounds. All these properties make ionic liquids performing lubricants or even additives to enhance the tribological properties of base oils.  
In this commentary, we summarize the research efforts in recent years to apply in ionic liquids in the lubrication field, making comments on the physicochemical factors determining their peculiar performance. We can already state that self-assembly and how it can change when approaching the surface will be considered. These considerations are very important to understand the molecular-level mechanisms of dissipation across ionic liquid films and, from this, to design ionic liquid lubricants with increased tribological performance for various applications.  
Even if there is much literature dealing with the lubricant properties of ionic liquids, we wish, instead, to furnish the reader with a critical view of this topic through a panoramic vision based on the physics of complex systems, avoiding detailed and sometimes case-specific descriptions, which are already available in the literature. Far from being a standard work, therefore, we try to furnish a simple but enriched vision of the topic. The present work constitutes an easy-to-read guide to young researchers to stimulate their imagination, hopefully stimulating new ideas and encouraging novel research.  
Our work is mainly organized in five parts:  
* Section 2 will give a general description of the properties of ionic liquids which allow their use as lubricants;
* Section 3 will show the mechanism of action of ILs as lubricants;
* Section 4 will give some hints on how to probe the IL structure;
* Section 5 will report some examples of the use of ILs as lubricants;
* Section 6, finally, will give some perspectives.