# Functionalization and Modification of Bitumen by Silica Nanoparticles
## 1 Introduction  
One of the methods for improving the characteristics of bitumen binders is the modification brought about by the use of nanomaterials [1]. The addition of nanomaterials indeed changes the rheological properties of bitumen and can also lead to changes in intermolecular forces in the structure of bitumen binders. This is due to the fact that as the dimensions of the materials reach the nanoscale, interactions at the phase boundaries become much more intense [2].  
In this context, silica nanoparticles (SNPs) are among the nanomaterials that are proposed to be used as additives in asphalt mixtures [3]. It was recently shown that the addition of nanosilica leads to an increase in rigidity, tensile strength, elastic modulus, fatigue life, and resistance to permanent deformation and damage from moisture in asphalt concrete mixtures [4]. In particular, evaluation of the characteristics of bitumen incorporating SNPs showed a gain in high fatigue resistance, alsoimproving oxidative aging and rutting properties [5; 6; 7]. Furthermore, a study of the effect of SNPs on the thermal properties of bitumen binders showed that nanosilica could improve the temperature sensitivity of asphalt mixtures [8].  
Bitumen is an organic binder formed by high molecular weight hydrocarbons and their derivatives. The contrast between an organic based matrix and an inorganic additive might result in a poor compatibility resulting in a scarce dispersibility of the nanoparticles within bitumen [9]. To improve the dispersion of nanosilica in organic matrices and favor interaction with the medium, functionalization of the surface of nanosilica can be used. In this way, the number of silanol structures are reduced changing the structure of surface functional groups thus mitigating the mismatch between nanosilica particles and the bitumen matrix [10]. Thus, surface properties such as the physicochemical adsorption of nanosilica and the surface free energy of the nanoparticles can be adequately altered, and the agglomeration phenomenon between the particles inside the matrix decreases.  
A recent study reported that NH\({}_{2}\)@SNP: silica nanoparticles functionalized with APTES (3-aminopropyltriethoxysilane), can effectively improve bitumen resistance to oxidation and mechanical stresses, while reducing the agglomeration phenomena observed with pristine SNPs. Up to 4 wt% of modified SNPs were dispersed in bitumen without observing any agglomeration process [11].  
The present study intends therefore to use the free amine moieties of APTES-modified SNPs as functional groups for further alkylation derivatization and check the eventual improvements of such modifications onto the thermorheological properties of bitumen. To this end, alkylated SNPs (C\({}_{14}\)N@SNP) were synthesized with the aim of increasing the hydrophobicity of the nanoparticles, thus improving their dispersion in the bitumen binder, as well as enhancing its thermorheological properties.  
However, before testing our newly functionalized SNPs, a curiosity arose: does the size of the SNPs have any influence on the rheological properties of bitumen? To answer this question, a series of SNPs were first prepared by varying the experimental conditions in order to finely control the diameter of the SNPs produced. Indeed, in the sol-gel method the final size of SNPs is greatly affected by different parameters, such as the chemical nature of the alcoholic solvent, the alcohol/water ratio, the silica precursor, the catalyst, the surfactant concentration and the reaction temperature [12; 13]. In this study, SNPs of different size were obtained by varying two parameters in turn: the ethanol/water ratio and the reaction temperature. Finally, the comparison between surface-functionalized and pristine SNPs on the rheological properties of bitumen was conducted.