Molecular dynamics simulation of the influence of oxidized functional groups on asphalt-aggregate bonding properties

To elucidate the mechanism by which oxidized functional groups influence asphalt-aggregate adhesion behavior at the microscopic level, molecular dynamics (MD) simulations were employed based on the Derek four-component model-a model well-matched with real asphalt's density and thermal expansion coefficient. Asphalt molecular models containing 12 characteristic molecules were constructed, with two stable oxidized functional groups (carbonyl, C=O; sulfoxyl, S=O) introduced to represent asphalt aging. Three concentration gradients (0%, 50%, 100%) were designed by adjusting the number of functional groups. Meanwhile, silica (SiO₂) aggregate models were cleaved along the (0 0 1) crystal plane, transformed into an orthorhombic structure, and passivated with hydrogen atoms to simulate real aggregate surface properties. An asphalt-aggregate interface system with a "SiO₂ layer-10 Å vacuum layer-asphalt layer-50 Å vacuum layer" sandwich structure was established. The model rationality was verified by three key indicators: density (1.011~1.079 g/cm³), solubility parameter (18.604~19.237 (J cm^-3)^0.5), and glass transition temperature (237.97~247.61 K), ensuring consistency with published experimental and simulation data. The effects of functional group type and concentration on asphalt-aggregate adhesion were analyzed by evaluating interfacial energies (interaction energy, van der Waals energy, electrostatic energy). As the concentration of carbonyl (C=O) groups increased, the interaction energy, van der Waals energy, and electrostatic energy in the asphalt-aggregate system all showed an upward trend. The increase at 100% concentration was greater than that at 50% concentration, with electrostatic energy exhibiting the largest overall increase. As the sulfoxyl group (S=O) content increased, the interaction energy and van der Waals energy in the asphalt-aggregate system decreased, while the electrostatic energy increased. When both carbonyl (C=O) and sulfoxyl (S=O) groups coexist, the variation patterns of various interfacial energies resemble those observed under sulfoxide influence alone. This indicates that, under identical conditions, sulfoxyl (S=O) exerts a greater impact on interfacial energies. Oxidized functional groups significantly influence the adhesion properties between asphalt and aggregates.

To investigate the effects of oxygen-containing functional groups with different concentrations and types on the asphalt-aggregate adhesion behavior, asphalt molecular models were first constructed via the molecular assembly method using the Amorphous Cell module in Materials Studio 2023 software, referencing TLC-FID data of asphalt four components to determine the molecular quantity of each component. A total of 15 asphalt models were established, covering three functional group types (C=O, S=O, C=O + S=O) and three concentration gradients. SiO₂ aggregate models were built by cleaving SiO₂ crystals along the (0 0 1) direction, inserting a 10 Å vacuum layer to form periodic unit cells, and passivated with hydrogen atoms to simulate real aggregate surface properties. Subsequently, molecular dynamics simulations were performed on the asphalt-aggregate models via the Forcite module, where the COMPASS III force field was adopted to describe intermolecular bonding and non-bonding interactions. The simulation process included three key steps: (1) geometric optimization (medium precision, 50,000 iterations) to eliminate unreasonable atomic configurations, (2) five annealing cycles (298~498 K, 200 ps total) under the NVT ensemble (controlled by a Nose thermostat) to stabilize the system, (3) equilibrium simulations (500 ps NVT + 1000 ps NPT) at 298 K and 1.0 × 10^-4 GPa (controlled by a Berendsen barostat).