## 3 Results and Discussion
### EPR Spectroscopy Investigation
#### 3.2.3 Linewidth and Lineshape  
The width of an EPR band is inversely proportional to the lifetime of the absorbing species in its excited state. It depends on the interaction between the unpaired electrons and their surroundings; the greater the interaction, the wider the band. Moreover, for complex and chemically heterogeneous samples, spectrum broadening could be the result of unresolved hyperfine structure; in bitumen samples, broadening of EPR signals due to unpaired electrons delocalized in aromatic \(\pi\) systems could arise from the hyperfine coupling with the adjacent aromatic H atoms [45]. The spectral linewidth is generally quantified by measuring, in gauss (G), the peak-to-peak distance (\(H_{\text{pp}}\)) of the first-derivative curve, which is the experimental output of the instrument. The (\(H_{\text{pp}}\)) values determined for the examined samples, collected in Table 3, show that, overall, the linewidth of the organic radical signal is only marginally affected by both aging and antioxidant addition. This indicates that both the molecular structure and the supramolecular organization of the sample do not change. Perusal of the table only reveals a slight narrowing for asphaltene in the presence of the LCS additive, thus suggesting reduced local interactions. Further information can be obtained from the analysis of the EPR signal lineshape. In general, it is affected by the unresolved hyperfine structure and the anisotropic effects and is classified as either Lorentzian or Gaussian. EPR lines have a trend toward a more Lorentzian character with increasing aromaticity [46]. However, radical species with different relaxation behaviors give rise to independent narrow absorptions, which yield a Gaussian-shaped envelope [46]. The accordance with one lineshape rather than another can be estimated by evaluating the ratio \(R_{n}\) of \(H_{n}\) to \(H_{\text{pp}}\) where \(H_{n}\) is the width at the position \(1/n\) of the peak-to-peak height of the first-derivative curve. For \(n=5\), the expected \(R_{5}\) values for a Lorentzian and Gaussian lineshape are 1.72 and 1.17, respectively [46]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{3}{c}{**Organic Radical Density \(N_{s}\) in 10\({}^{17}\) Spins-g\({}^{-1}\)**} \\ \cline{2-5}
**Sample** & \multicolumn{2}{c}{**Asphaltene**} & \multicolumn{2}{c}{**Maltene**} \\ \cline{2-5}  & **Unaged** & **Aged** & **Unaged** & **Aged** \\ \hline CTRL & 35.9 & 49.8 & 2.27 & 2.68 \\ Bitumen + LCS & 30.5 & 30.5 (39\%) & 1.92 & 1.27 (53\%) \\ Bitumen + VC & 35.2 & 32.0 (36\%) & 1.79 & 1.36 (49\%) \\ Bitumen + RH & 38.0 & 32.9 (34\%) & 1.96 & 2.23 (17\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Concentrations of paramagnetic centers \(N_{s}\) determined in asphaltene and maltene fractions obtained both from unaged bituminous samples and after artificial aging through RTFOT. Relative concentration differences with respect to the aged CTRL have been also indicated in brackets. The peak-to-peak separations of the asphaltene and maltene EPR derivative signals (\(H_{\text{PP}}\)) and the lineshape ratio \(R_{5}=H_{5}/H_{\text{PP}}\) are reported in Table 3. It is observed that for both the series of unaged and aged asphaltene samples, the reference parameter \(R_{5}\) remains almost constant and intermediate between Lorentzian and Gaussian lineshape, implying that the core size of the polyaromatic clusters remains essentially unchanged upon thermal treatment. An exception of this feature regards the unaged sample modified with RH additive, in correspondence of which a significant increment of \(R_{5}\) is recorded compared to the asphaltene samples of the same series. Considering the maltene fraction, calculated \(R_{5}\) data are slightly scattered compared to asphaltene, though manifesting the same intermediate character of the signal lineshape. Only for the sample supplemented with LCS a marked Gaussian tendency is observed in response to the aging treatment.