Oxidative aging causes major changes in asphalt binder’s physiochemical and rheological properties, giving rise to pavement distress and failure. Asphalt aging occurs due to two main processes: a loss of volatile components and a reduction of the malthene phase and oxidation of certain functional groups in asphalt, increasing the concentration of asphalt’s polar components. Performing SARA (saturate, aromatic, resin, and asphaltene) analysis and FTIR (Fourier transform infrared) spectroscopy on the crude oil shows that the ratio of polar components to nonpolar ones is higher in oxidized asphalt compared to virgin asphalt. However, when a biomodifier is introduced to virgin asphalt, the rate of carbonyl (as a polar functional) formation is reduced, which may indicate a delayed oxidation due to the presence of biomodifier molecules in the asphalt. To understand the fundamental origin of oxidative aging at the molecular level, this paper provides a comprehensive computational chemistry analysis conducted in conjunction with laboratory experiments. On the basis of the results of our analysis, the enhanced performance of biomodified asphalt binder stems from dual-protection mechanisms of biobinder components that defer asphalt aging: the less reactive molecular species found in biobinder that show little propensity toward oxidation and consequently are less affected by the new polar functionalities and the highly reactive components (such as α-tocopherol) that are the primary targets for oxidative attacks, acting as sacrificing elements to save key components of asphalt materials (such as asphaltenes) from oxidative agents. Polarizability calculations show that biobinder constituents are considerably less polarizable than asphalt molecules. Lower polarizability of biobinder indicates the lower tendency of these chemical species toward new polar functionalities arising from the presence of oxidative agents. In contrast, the high polarizability obtained for asphaltene molecules suggests that they are easily affected by the oxidative agents. Therefore, the presence of α-tocopherol in biobinder acting as a sacrificing element could delay asphaltene oxidation as evidenced by the lower carbonyl formation in asphalt samples containing biobinder.

g r a p h i c a l a b s t r a c t Micellar structure of asphaltene in a dispersing medium of lighter resins, aromatics, and saturates. a b s t r a c tIn the colloidal hypothesis of vacuum residua and asphalt, micelles are assumed to have an asphaltenic core with the greatest weight and aromaticity and an outer shell composed of lighter and less aromatic molecules. The true nature of the micelles, and the asphaltenic core in particular, has always been a debatable issue among asphalt researchers. One of the frequently asked questions is whether asphaltenes exist as stacked layers within the micelles or as single molecular units stabilized by resin molecules. In this paper, we investigate the correlation between the number of asphaltene sheets and the final stability of the system in a medium of asphaltene and resin components. Our evaluation is based on rigorous quantum mechanical calculations using a DFT-D approach. Our quantitative findings corroborate the view that the colloidal behavior of crude oil is better described by asphaltene-asphaltene associations. Moreover, our calculations show that if both asphaltene and resin are present, resin-asphaltene interactions are preferred over asphaltene-asphaltene interactions only if the number of resin molecules per micelle is greater than the number of asphaltene molecules per micelle.

The nature and origin of bee-like microstructures (bees) in asphalt binders and their impact on asphalt oxidation have been the subject of extensive discussions in recent years. While several studies refer to the bees as solely surface features, some others consider them to be bulk microcrystalline components that are formed due to co-precipitation of wax and asphaltene molecules. In this study, we use a rigorous theoretical and experimental approach to investigate the interplay of asphalt components (mainly asphaltene and wax) and their impact on bee formation. In the theoretical section, quantum-mechanical calculations using density functional theory (DFT) are used to evaluate the strength of interactions between asphaltene unit sheets in the presence and absence of a wax component, as well as the mutual interactions between asphaltene molecules (monomers and dimers) and paraffin wax. The results of this section reveal that paraffin waxes not only do not reinforce the interaction between the asphaltene unit sheets, they destabilize asphaltene assembly and dimerization. AIM (Atom in Molecules) analysis shows the destabilizing effect of wax on asphaltene assembly as a reduction in the number of cage and bond critical points between asphaltenes. This destabilization effect among interacting systems (asphaltene-asphaltene and wax-asphaltene) does not support the hypothesis that interaction between paraffin waxes and non-wax components, such as asphaltene, is responsible for their co-precipitation and bee formation. To further examine the effect of wax component on asphalt microstructure experimentally, we used atomic force microscopy (AFM) to study the surface morphology of an asphalt sample doped with 1% to 25% paraffin wax. In agreement with the conclusions drawn from the DFT approach, our experiments indicate that paraffin wax tends to crystallize separately and form lamellar paraffin wax crystal inclusions with 10 nm thickness. Moreover, the addition of 3% wax into asphalt results in a significant increase in surface roughness from 0.5 nm to 4.1 nm and an increase in bee wavelength from 651 nm to 1038 nm.