Revealing the aging-induced chemical and microstructure evolution of asphalt via AFM-IR and quantum chemistry simulation

The evolution of nanoscale morphology plays a critical role in the degradation of the macroscopic properties of asphalt during oxidation. However, due to limitations in research scales and technical methods, it remains challenging to elucidate the mechanisms underlying nanoscale morphology changes and to establish the relationship between asphalt morphology and asphalt components. To address these challenges, this study utilized atomic force microscopy (AFM) and atomic force microscopy-infrared spectroscopy (AFM-IR) to investigate the nanoscale morphology and chemical characteristics of asphalt at various aging stages. The aggregation and oxidation characteristics of bee-structure phases and surrounding phases in asphalt were analyzed. Subsequently, quantum chemistry simulations were employed to simulate the oxidation susceptibility and aggregation behaviors of asphalt component molecules, establishing the relationship between asphalt phase distribution and asphalt components to explain the mechanism of morphology evolution during oxidation. The results indicate that the surface of asphalt exhibits distinct bee-structure phases and surrounding phases at the nanoscale. Before aging, there is no significant difference in the oxygen-containing functional groups between the bee-structure phase and surrounding phase. As oxidation progresses, the oxidation rate of bee-structure phase is higher than that of the surrounding phase, resulting in a higher content of oxygen-containing functional group in the bee-structure phase. Furthermore, a significant aggregation of bee-structure phase occurs as oxidation deepened. Asphaltenes exhibit the highest oxidation reactivity, followed by resins and aromatics, while saturates show no oxidation reactivity. The main components of the bee-structure phase are high-reactivity asphaltenes, whereas the surrounding phase is richer in low-reactivity aromatics and saturates. After oxidation, hydrogen bonding interactions appears in asphaltene dimers, and the electron cloud density in the PAH region increases, leading to a significant increase in the interaction energy of asphaltene molecules. The aggregation of bee-structure phases after oxidation is closely associated with the increased interaction energy of asphaltene molecules.