Study on the Interface Regulation Mechanism of Rejuvenators on Virgin and Aged Asphalt Based on Molecular Diffusion Theory

To address the issue of inefficient interfacial diffusion between virgin asphalt and the aged asphalt in Reclaimed Asphalt Pavement (RAP), this study investigates how a rejuvenator improves the interfacial blending behavior and restores the functional properties of aged asphalt. Molecular dynamics (MD) simulations were employed to construct aged asphalt–rejuvenator models with varying rejuvenator contents and to establish a bilayer dynamic model of the virgin-aged asphalt–rejuvenator diffusion system. The kinetic characteristics of the diffusion process were analyzed based on system density and relative concentration profiles, while the mean square displacement (MSD) and diffusion coefficients were calculated to elucidate the diffusion mechanism. The accuracy of the MD simulation results was validated using Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC), and the regulatory mechanism of the rejuvenator on the interfacial diffusion between virgin and aged asphalt was revealed at the microscopic scale. The results demonstrated that the addition of the rejuvenator effectively promotes the blending and diffusion at the virgin-aged asphalt interface. Specifically, a 6% rejuvenator significantly improved the diffusion efficiency at elevated temperatures, optimized system density toward virgin asphalt properties, and achieved the most uniform molecular distribution, thereby facilitating balanced intermolecular interactions. Meanwhile, the regenerant effectively restored the aromatic fraction content, reduced polar functional groups such as sulfoxide, and significantly lowered the glass transition temperature (Tg), thereby enhancing the low-temperature crack resistance and overall mechanical performance of RAP.