## 5 An Expensive Solution: Organic-Modification of Inorganic Nanoparticle Surface  
As the eager reader has probably already observed, some authors have reported problems in the chemical compatibility of the nanoparticles [51] and nanoparticles' tendency to agglomerate due to the high surface energy [59]. To solve this problem, an expensive but effective solution is represented by functionalization of the nanoparticle surface to change its chemical nature, making it more compatible with the apolar nature of the host matrix (bitumen, asphalt) thus avoiding instability problems.  
The role of aluminium oxide nanoparticles in the rheological properties of bitumen modified with acrylonitrile styrene acrylate (ASA) polymer was investigated by [PERSON] et al. [60]. An increase in viscosity, softening point temperatures and in storage stability and resistance against rutting were found. Nano-montmorillonite (size 1-2 nm), after organomodification to make it organophilic for a better dispersion in bitumen, was tested by [PERSON] et al. [61].  
The functionalization of silica nanoparticles with (3-aminopropyl) triethoxysilane (APTES) can favour their dispersion (probed by SEM) within the bituminous matrix, and consequently improve their performance (reduction of the chemical aging index--CAI--for carbonyl and sulfoxide groups, higher rutting parameter and lower cracking parameter) [62]. The same authors explored the possibility to combine different coupling agents to minimize nanoparticle agglomeration [63].  
Effectiveness of surface functionalization was confirmed by [PERSON] et al. [64] who found that a better dispersion of the nano-ZnO inside the bitumen is achieved if nanoparticles are silano-modified at the surface (FT-IR checks if the silano-modification is effective). Surface treatment of nano-SiO\({}_{2}\), nano-TiO\({}_{2}\) and nano-ZnO with \(\gamma\)-(2,3-epoxypropoxy) propyltrimethoxysilane as silane coupling agent to modify the nanoparticles' surfaces was performed by [PERSON] et al. [65]. Similar clues were found by [PERSON] et al. in 2015 [66].  
In general, it is now quite clear that the surface modification improves the dispersion. A worth of note is the work by [PERSON] et al. [67], who used modified nanoparticles (seven different nanoparticles: SiO\({}_{2}\); CaCO\({}_{3}\); montmorillonite; TiO\({}_{2}\); Fe\({}_{2}\)O\({}_{3}\); ZnO and bentonite with size in the range 15-30 nm functionalized with silane stearic acid or polyethylenimmine). They demonstrated how modification can give better performances (softening point and viscosity general increase, parallel to decrease in ductility) especially at high temperatures. It is important to note, in our opinion, how the authors are convinced about the importance of the surface modification to influence the final material properties. Probably for this reason, in fact, these authors improved the methodology for surface modification of nanoparticles. The previous procedure involved dosing the coupling agents on the amount of bitumen; instead, in their new procedure the percentage by weight of coupling agents is calculated with respect to the quantity of nano-SiO\({}_{2}\). In doing so, they discovered the optimal amount of coupling agent with consequent benefits procedure efficiency.