(601a) Molecular Dynamics Study of Asphalt-Water/Air Interface Systems

Asphalt is a complicated viscous liquid which is the tar from crude oil distillation. It is mainly applied on road pavement. One of the new developments in the asphalt technology is Warm Mix Asphalt (WMA), an environmentally benign and energy efficient technology compared with the state of practice with Hot Mix Asphalt (HMA). During WMA process, water was sprayed to facilitate the mixing of asphalt with aggregates. It is found that water and air can form interfaces with asphalt, which can influence the size and character of the foam, thus the mixing process. This study aimed to understand the mechanisms and effects of water and air have on asphalt mixtures by predicting important physical properties of asphalt interface systems. The original model asphalt mixture was represented by model asphaltene, alkane and saturate molecules with the chemical composition consistent with SHRP asphalt. All-atom molecular dynamics simulations using NAMD program and CHARMM forcefield were conducted, and the asphalt-air and asphalt-water interface systems were set up at different temperatures ranging from the room temperature to HMA temperature. Important physical properties such as the surface tension, diffusion coefficient of molecules on the interface and in the bulk of the system were calculated, and results were compared with experimental work. It was found that the surface tension of both kinds of interface systems decreased with the temperature increasing. The water interface systems have a higher surface tension than the corresponding air-asphalt interface systems at all temperatures, which agrees with experimental result well. Molecules on the bulk usually diffuse slower than on the interface because of the confinement effect, which sheds light on the mechanism of WMA in molecular level. This result from this work can supply insight on improving mixing process and efficiency of WMA during foaming process.