# A Presentation of Ionic Liquids as Lubricants: Some Critical Comments
## 4 Intermolecular Distances as Probes for Structure: How to Measure Them
### Scattering  
Scattering techniques are among the most employed experimental techniques for the investigation, in noninvasive way, of the structural properties of materials and complex self-assembly processes [75; 76; 77] in a large variety of material systems and different reactive environments [78], at least in the bulk phase.  
There are plenty of works dealing with such techniques, so the reader is redirected to the literature for details. Here, however, we just want to remind the reader that a peak in the scattering spectrum (reciprocal or q-space) equals a peak in the distances (real or R-spaces) according to a simple relation: R = 2\(\pi\)/q.  
Figure 7, reporting the comparison between the scattering spectra of a simple liquid (propylamine) and an IL, can shed light on this.  
In the IL scattering pattern, the broad band at wide angles (about 1.4 A\({}^{-1}\)) are due to the intermolecular short range distance taking place between first neighbouring molecules and due to intermolecular spacings of about 4.5 A, typical of the conventional liquid state [79]. This signal is generally found for any liquid. In the case of ILs, however, another  
Figure 7: Scattering patterns of a classical liquid (propylamine, with weak intermolecular interactions, lower plot) and an ionic liquid (a 1:1 mixture of propylamine and dibutylphosphate, upper plot).  
band occurs at lower q values (q \(\approx\) 0.5 A\({}^{-1}\)), revealing the occurrence of an intermolecular association in a pattern already known in the past for \(n\)-alcohols [80, 81] and liquid amphiphiles [82, 83, 84].  
The presence of associated species causes a new characteristic repetition distance to arise, in the present example, of about 13 A, which may be identified with the characteristic distance of polar/polar domains typical of ILs [31]. This is a quite universal signature in ionic liquids: generally, an X-ray peak below 1 A\({}^{-1}\) (and usually quite sharp) can be considered as a signature of the mesoscopic order [85] and is usually characteristic of such systems [86, 87]. In particular, as found for imidazolium-based ILs [28], but also for alcohols and their mixtures [30], the low-q contribution in the spectrum comes from the nanoscale structural heterogeneities in the bulk liquid state 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 [29]. It must be noted that the impact of temperature in such patterns is scarce [88].  
However, due to the liquid nature of the system, rapid molecular and charge association/fission processes take place, so the repetition distances we are discussing are to be meant as a time- and space-averaged distance with a characteristic polydispersion.  
It must be noted that a faint bump around 0.8 A\({}^{-1}\) is usually present and, although several structural features can contribute to this scattering signal, it is generally recognized as coming from charge-charge interactions, i.e., cation-anion characteristic distance repeating in the bulk [89]. This signal is generally quite weak (in Figure 7, for example, it is almost undetectable) and is generally associated with the sponge-like structure expected to be made by the charged groups [90].