Investigating molecular conformation and packing of oxidized asphaltene molecules in presence of paraffin wax

This study investigates the effect of paraffinic wax (as a base component in many commercial rejuvenators) on the rheological and intermolecular properties of aged asphalt binder. Samples of oxidized asphalt binder doped with (1%, 3%, 5%, and 10%) paraffin wax were characterized using the rotational viscometer (RV), dynamic shear rheometer (DSR), bending beam rheometer (BBR) and direct tension test (DTT). The RV results showed an improvement trend in workability of aged binder as wax content increased. The DSR results (for a temperature range of 10–76 °C) showed lower complex modules for wax-doped specimen than control specimen. The BBR test results performed at sub-zero temperature showed that an increase of wax dosage led to higher creep stiffness modulus indicating that the asphalt binder became generally stiffer in presence of wax at low temperature. However, fracture energy measured through DTT test showed a significant reduction in presence of wax. The latter can be attributed to plausible weak secondary bonds between wax and asphaltene molecules as well as crystallization of wax molecules at low temperature within the asphalt matrix. This in turn can lead to wax crystals playing as stress localization point giving rise to crack nucleation at the wax-asphalt interface reducing overall fracture energy; it was further observed that as the wax content increased, the asphalt binder became more brittle. To further investigate the effect of wax on the molecular conformation and packing in aged asphalt binder, a molecular simulation was performed on a system of wax and oxidized asphaltene. The results of molecular simulations showed a reduction in formation of oxidized asphaltene nano-aggregates as the amount of wax increased in the wax-doped oxidized asphaltene matrix at room temperature, which was also confirmed by size exclusion chromatography. Furthermore, the radial distribution function results showed a less packed structure of oxidized asphaltene molecules in presence of wax molecules. Increasing the wax content also increased the diffusion coefficient of wax into the oxidized asphaltene matrix within a solvent medium. It was also showed that interaction energy of a dimer of oxidized asphaltene is at a lower energy state in presence of wax molecules, which suggests that wax molecules can promote dimerization of oxidized asphaltene molecules while suppressing nano-aggregates formation.