Evaluation of the Characteristics of the Interface between Saline–Alkaline Solutions and Bitumen through Molecular Dynamics Simulations

Saline–alkaline solutions have a detrimental impact on the durability of bituminous pavements. Thus, it is important to understand the interaction between saline–alkaline environments and bitumen and investigate the corresponding interfacial characteristics. In this study, molecular dynamics simulations were carried out to evaluate the interfacial interaction behavior between different saline–alkaline solutions and bitumen. The interaction energy, mean square displacement, interfacial molecular arrangement, and ${\mathrm{Na}}^{+}$ distribution were analyzed for different bitumen-solution systems. The results revealed that the interaction between bitumen and alkaline salt solutions is more powerful than between bitumen and neutral salt solutions. The damaging impact of different solutions on bitumen can be written as: alkaline salt solution > neutral salt solution > deionized water. Oxidative aging can further strengthen the interaction between saline solutions and bitumen. Bitumen molecules diffusion is the fastest in an alkaline salt solution, as indicated by the high diffusion coefficient. Furthermore, adsorption occurs at the interface between a neutral salt solution and bitumen, leading to interspersing of the salt and bitumen molecules. The water molecules and ${\mathrm{Na}}^{+}$ ions in an alkaline salt solution can penetrate the bitumen surface, leading to a more chaotic molecular arrangement at the interface. The interfacial film formed in the bitumen-alkaline solution system has the highest thickness owing to the extremely strong interaction. Furthermore, ${\mathrm{Na}}^{+}$ ions aggregate at the bitumen surface, and aging accelerates the diffusion of ${\mathrm{Na}}^{+}$ ions toward the bitumen surface.