Molecular dynamics insight into the adsorption and distribution of bitumen subfractions on Na-montmorillonite surface

• The Combined ClayFF-CVFF is developed to characterize bitumen-clay interaction. • Asphaltenes have preference for parallel adsorption near bitumen-clay interface. • The higher asphaltene and resins content near bitumen-clay interface (1.0～1.5 nm). • Thermal effect reverse the distribution by inconsistently increasing the diffusion. How bitumen works with clay minerals is an essential matter for the application of nano-clay minerals (e. g., montmorillonite, vermiculite) in bitumen modification. In this paper, the combination of ClayFF and consistent-valence force field (CVFF) is developed to characterize the bitumen-clay interaction. The adsorption configuration, distribution pattern of bitumen subfractions on the Na-montmorillonite surface and thermal effect are systematacially investigated by examining the evolution of bitumen model at different temperatures based on the molecular dynamics simulation. For comparision, 12 isolated bitumen monomers and bitumen SARA fractions (saturates, aromatics, resins, asphaltenes) are attached to Na-montmorillonite surface respectively. The results show that asphaltenes have preference for parallel adsorption near bitumen-clay interface, but the tightly adsorbed configuration can be disrupted by their self-aggregation and light fractions. The distribution pattern is highly influenced by the adsorption strength and molecular diffusion, despite non-negligible spatial effect. Herein, the interaction energy of isolated monomers with clay platelet shows the adsorption strength of SARA fractions in the order of asphaltenes > resins > aromatics > saturates, while the diffusion can be ranked as: Saturates > Resins > Aromatics > Asphaltenes. The distribution pattern of bitumen inferred from the synergy of interaction strength and diffusion suggests that the bitumen exhibits stratified property on clay surface, the asphaltenes and resins content is likely to be higher in the range (1.0～1.5 nm) close to the surface. As for the thermal effect, it is observed that the asphaltenes and resins desorb from the clay surface as the temperature rises as aromatics dominates the adsorption zone, also supported by the finding that asphaltenes dimers are formed and the interaction energy of bitumen with clay platelet decreases to 72.98% at elevated temperature, which implies the thermal effect can reverse the distribution by inconsistently increasing the diffusion of bitumen subfractions. Overall, these findings are helpful for understanding bitumen-clay interaction, optimizing processing temperature and screening surfactant to improve compatibility of nano-clay modifiers with bitumen.