Molecular Dynamics Simulation to Investigate the Effect of the Molecular Structure of Warm-mix Additive on Its Adsorption Properties

Abstract

In this study, the effect of the molecular structure of an asphalt warm-mix additive (WMA) on its adsorption characteristics was determined by a molecular dynamics simulation. Calculation models were established of the adsorption-film forming of an imidazoline WMA on the surfaces of the typical minerals (CaCO₃) and quartz (SiO₂). The effect of the change in the length of the WMA's hydrophobic carbon chain on system adsorption and lubrication was further investigated. The simulation results show that the HOMO and LUMO orbital distributions of the long-chain aliphatic imidazoline WMA are mainly delocalized on the imidazoline ring and hydrophilic group. As the length of the carbon chain at the hydrophobic end of the WMA increases, the E_HOMO of the WMA gradually increases, the E_LUMO gradually decreases, and the energy difference between HOMO and LUMO decreases. Also, as that length increases, the adsorption energy of the WMA on SiO₂ (0 0 1) and calcite (1 0 4) surfaces, as well as the film's cohesive energy, all increase continuously. In addition, the adsorption energy of the WMA on the SiO₂ (0 0 1) surface and the film's cohesive energy exceed those on the calcite (1 0 4) surface. In the adsorption films formed on the SiO₂ (0 0 1) and calcite (1 0 4) surfaces, the MSD of the WMA in each system increases with time. Meanwhile, as the hydrophobic-end alkyl chain length of the WMA increases, the MSD and the self-diffusion coefficient gradually increase, and the molecular diffusivity is enhanced. Additionally, the MSD and self-diffusion coefficient of the WMA on the SiO₂ (0 0 1) surface exceed those on the calcite (1 0 4) surface. In conclusion, the study researched the adsorption-film-forming behavior of the surfactant WMA on the aggregate surface, providing a theoretical basis for the mechanism of warm-mixing and the development of a new WMA.