# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 5 Perspectives
### New/Novel Rejuvenators  
The overall characteristics of a bitumen are, as a matter of fact, the consequence of its complex structure. The term \"complex\" does not refer to \"complicated\" or \"hard to describe\" but, instead, the term is used on a physical basis. The peculiar aspect involved in this topic will be better clarified in the next paragraph. Aging perturbs and changes the complex organization of a bitumen through various mechanisms whose consequence is to change the overall complex organization of the material and not, strictly speaking, one single specific aspect of the molecular organization of the bitumen. That is why, if only one specific structural feature is looked at, it may not correlate the dynamic behavior. In this respect, novel rejuvenators can be thought of. A typical class of molecule usually related to complex behavior is that of surfactant, and more generally that of amphiphiles. Amphiphiles, simultaneously possessing polar and apolar moieties within their molecular architecture, can give a wide scenario of possible intermolecular interactions: polar-polar, polar-apolar, apolar-apolar interactions, eventual directional H-bonds, steric hindrance, etc. For this reason, some of them are a surfactant, i.e., surface-active agents, when dissolved in water: they expose to the air their apolar part while binding water through their polar head, thus, decreasing the surface tension [78; 79]. The same principle holds when trying to mix polar and apolar substances. Bypassing their natural tendency to remain separated, they actually can be effectively mixed if an amphiphile is present. Thanks to its capability to simultaneously linking both the polar and the apolar phases, the amphiphile act as a bridging molecule between the two. The two phases can be, therefore, homogenized to such an extent that the system can become homogeneous at the macro-scale although heterogeneous at the nano-scale, with the formation of local domain of one phase stabilized by one or more layers of opportunely oriented amphiphilic molecules and dispersed in the other phase [80]. Micelles, vesicles, bicontinuous structures and liquid crystals are only examples of stabilization of the systems through the formation of local intermolecular assemblies. This principle can be used, in our opinion, to the bitumen cases also. Let us consider that, in the micellar model, the bitumen is constituted by polar aggregates stabilized by polar resins and dispersed in a more apolar matrix. In this case, an amphiphilic molecule can bind on a side the asphaltene cluster, and on the other side, the apolar maltene phase. The overall outcome of these simultaneous interactions would be to disperse the asphaltene clusters contrasting aging better, or even, drawing back to rejuvenation. On the other hand, it must be admitted that the general mechanism of action shown by amphiphiles is already well-known and used for several issues in various fields: amphiphiles have been proved to be effective in stabilizing organic molecule clusters within an apolar solvent [81; 82], as well as metal clusters [83], nanoparticles [84], and ionic clusters [85], so in our opinion their direct application to bitumen represents an obvious and immediate step. This idea has been recently tested in preliminary works where the surfactants have been successfully used to prepare warm mix asphalt binders. The results showed that the use of the surfactant-based additive reduces surface free energy. It increases after short-term (Rolling Thin Film Oven) and reduces after long-term (Pressure Aging Vessel) aging [86]. Moreover, the addition of DBSA (Dodecylbenzosulfonic acid) based surfactant enhanced viscoelastic response of bitumen and reduced glass transition temperatures since it promotes the association of asphaltene molecules/aggregates into larger clusters in bitumen [87].