## 3 Results and Discussion
### EPR Spectroscopy Investigation
#### 3.2.4 Saturation Curves  
Analysis of saturation curves of asphaltene samples shows a typical homogeneous saturation trend, as shown in Figure 7, characterized by the presence of a maximum followed by a decrease of the signal, which indicates the onset of spin saturation process. The homogeneous saturation trend is generally observed when a given set of spins is exposed to the same net magnetic field and has a uniform distribution in space. In fact, decrease of amplitude with increasing microwave power is characteristic for free radicals homogeneously located in the sample. Homogeneous saturation occurs when all free radical spins behave as a single spin system with the same relaxation behavior. In other words, the energy absorbed from the microwave field is distributed to all the spins and thermal equilibrium of the spin is maintained through resonance.  
The observed homogeneous saturation is in agreements with the molecular structure of asphaltenes, which can be visualized as polycondensates of a multicomponent system made up of individual molecules of aromatics, paraffins, naphthenics, macrocyclics, and heterocyclics characterized by extensive conjugation and wide electronic delocalization. As illustrated in Figure 7, all the saturation curves are affected neither by the presence of additives nor by the aging process.  
Analogous tests have been carried out on maltene samples isolated from both CTRL and bitumen modified with LCS, VC and RH additives, respectively. The correspondent power saturation curves, shown in Figure 8, manifest a definitely more complex trend.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Asphaltene**} & \multicolumn{4}{c}{**Maltene**} \\ \cline{2-9}
**Sample** & \multicolumn{2}{c}{\(H_{\text{PP}}\) (G)} & \multicolumn{2}{c}{\(R_{5}\)} & \multicolumn{2}{c}{\(H_{\text{PP}}\) (G)} & \multicolumn{2}{c}{\(R_{5}\)} \\ \cline{2-9}  & **Unaged** & **Aged** & **Unaged** & **Aged** & **Unaged** & **Aged** & **Unaged** & **Aged** \\ \hline CTRL & 6.2 & 5.8 & 1.4 & 1.5 & 5.4 & 4.9 & 1.5 & 1.5 \\ Bitumen + LCS & 5.9 & 5.6 & 1.4 & 1.5 & 5.2 & 5.3 & 1.5 & 1.3 \\ Bitumen + VC & 5.9 & 5.8 & 1.4 & 1.4 & 5.6 & 5.4 & 1.4 & 1.4 \\ Bitumen + RH & 5.7 & 5.7 & 1.5 & 1.4 & 5.7 & 5.6 & 1.4 & 1.4 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Peak-to-peak separation of the derivative peak (\(H_{\text{PP}}\)) and lineshape parameter (\(R_{5}\)) of EPR spectra of asphaltene and maltene fractions obtained both from unaged bituminous samples and after artificial aging through RTFOT.  
Figure 8: Power saturation profiles for maltene samples mixed with 2% \(w/w\) of antioxidants, as determined from the X-band EPR spectra registered at room temperature. CTRL indicates the control sample with no antioxidant added. Black lines, full circles: unaged samples; blue lines, full diamonds: samples artificially aged through the RTFOT method.  
Figure 7: Power saturation profiles for asphaltene samples mixed with 2% \(w/w\) of antioxidants, as determined from the X-band EPR spectra registered at room temperature. CTRL indicates the control sample with no antioxidant added. Black lines, full circles: unaged samples; blue lines, full diamonds: samples artificially aged through the RTFOT method.  
Maltene phase free of antioxidants shows a heterogeneous trend characterized by the absence of a maximum, which becomes homogeneous by following the simulated aging process in the laboratory, (see Figure 8). This circumstance may occur because aging of bitumen promotes a progressive increase of aromaticy accompanied by a consequent decrease in mass of saturated oils and resins [1]. The presence of antioxidant agents imparts an inversion of trend shown by the reference, i.e., from homogeneous to heterogeneous saturation upon RTFOT treatment. This aspect might be a consequence of a reduced oxidability of the organic matrix due to the antioxidant activity, the efficiency of which qualitatively increases along the series RH < VC < LCS.