NMR Diffusiometry Spectroscopy, a Novel Technique for Monitoring the Micro-Modifications in Bitumen Ageing  
[PERSON]  
1 Department of Chemistry and Chemical Technologies, University of Calabria, 87036 Arcavacata di Rende (CS), Italy; [EMAIL_ADDRESS] (P.C. ); [EMAIL_ADDRESS] (C.O.R.) [PERSON]  
2 Institute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Woodhouse Lane LS2 9 JT, UK; [PERSON]  
[PERSON]  
1 Department of Chemistry and Chemical Technologies, University of Calabria, 87036 Arcavacata di Rende (CS), Italy; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Chemistry and Chemical Technologies, University of Calabria, 87036 Arcavacata di Rende (CS), Italy; [EMAIL_ADDRESS] (P.C. ); [EMAIL_ADDRESS] (C.O.R.) [PERSON]  
4 Department of Physics and CNR-Nanotec, University of Calabria, via Bucci 31C, 87036 Arcavacata di Rende (CS), Italy; [EMAIL_ADDRESS]  
[PERSON]  
4 Correspondence: [EMAIL_ADDRESS] (M.P. ); [EMAIL_ADDRESS] (V.L. ); Tel./Fax: +39-09844920454  
[PERSON] defined the asphaltenes as insoluble in naphtha but soluble in carbon tetrachloride and also introduced the term \"carbenes\" for the fraction insoluble in carbon tetrachloride but soluble in carbon disulphide [4]. The word \"carboids\" for the part insoluble in carbon disulphide is also seldom used [5] (although not used by [PERSON] [4]). In all cases, these two additional fractions are present in very limited amounts in paving-grade bitumen [6] and are generally not mentioned in the road industry. Asphaltene aggregates remain in an oily apolar environment of lower molecular weight, known as maltenes, granting fluidity [7]. The apolar maltene phase, in turn, is composed of saturated paraffins, aromatic oils, and resins. Figure 1 shows a schematic splitting of bitumen into its main components. The proposed model of bitumen in the literature is that the asphaltene, in the form of polar nano-aggregates, remains dispersed as a peninsula or group of peninsulas within a more apolar continuous maltene phase [8]. In the conventional colloidal model, the resins-to-asphaltenes ratio characterizes the bitumen behavior to a specific path if the bitumen is in solution (sol). Sol bitumen is due to a high maltene concentration that leads to rapid relaxation modes (a liquid-like system), while the bitumen is gelatinous (gel) when the asphaltene concentration is dominant, resulting in slow relaxation modes [9]. In bitumen, resins are the dispersing agents for asphaltene molecules, combining them with aromatics and saturates; this produces the conditions for asphaltic flow. In fact, the chemistry of bitumen is the key element to defining its physical properties: following the analogy with reversed micelles in water-in-oil microemulsions [11], the stabilization of the polar domains is of pivotal importance for determining both the structure and the properties of the overall aggregates of organic-based materials [12; 13], even if, it must be pointed out, the stabilization mechanism is quite general, spanning from organic materials to inorganic complexes [14] and even nanoparticles [15]. Due to these strict relationships between intermolecular interactions, the aggregates' structure, and their dynamic properties, the rheology (ductility at a given temperature/frequency) and behavior of bitumen are dependent not only on its structure, but also on the maltene's glass transition temperature and the effective asphaltene content [10]. When it comes to the bitumen structure, the microstructural model offers a typical point of view. It describes the system as a complex solution of evenly distributed molecules, classifying them according to their molecular weight and polarity; these behaviors are the key to grasping whether  
Figure 1: Bitumen separation into its various constituents, highlighting the SARA (saturates, aromatics, resins, asphaltenes) distribution, reproduced from [10], Copyright Elsevier, 2020.  
the bitumen is modified.