Multiscale Evaluation of Synergistic and Antagonistic Interactions between Bitumen Modifiers

Introduction of various organic and inorganic modifiers to bitumen has been commonly practiced mainly to upgrade the bitumen performance and its sustainable properties; polyphosphoric acid (PPA) and nanosilica are among such modifiers. Here, we study synergy between silica nanoparticles and PPA to increase effectiveness of PPA to modify bitumen. We further examine plausible antagonistic effects of bitumen’s wax content on the synergistic features of silica–PPA. Our density functional (DFT) results and energy decomposition analysis (EDA) provide evidence on the ability of the silica surface to set up an efficient interaction with PPA through noncovalent interactions, giving rise to PPA–bitumen interactions. In parallel, our laboratory investigations show significant reduction in crossover modulus and an increase in complex viscosity of low-wax bitumen containing silica-supported PPA. However, the above synergistic role diminishes in the high-wax bitumen to a point that introduction of silica-supported PPA could not improve the complex viscosity of high-wax bitumen as much as it did in the case of low-wax bitumen. The latter was made evident in the significant contribution of the dispersion energy to the total interaction energy between wax and silica-supported PPA indicating the formation of a series of induced dipole–dipole interactions between wax and PPA. This is further supported by the molecular dynamic (MD) results indicating blocking the active sites of PPA by wax, preventing PPA from effective interaction with bitumen components. The results of the paper highlight the importance of accounting for such antagonistic or synergistic interactions among various modifiers/additives to enhance the efficiency of modifiers used in sustainable construction.