Asphaltenes in asphalt: Direct observation and evaluation of their impacts on asphalt properties

Over the years, different hypotheses and speculations have been developed on asphaltenes, but their presence in asphalt has never been directly observed. In this study, asphaltenes in nine types of asphalt that vary in crude oil source and aging states are examined through scanning transmission electron microscopy (STEM) and atomic force microscopy (AFM). No solvent casting is used in sample preparation; hence, the natural state of the asphaltenes is preserved. Asphaltene particles are also separated and examined for their morphology and toughness. The impacts of asphaltene content and morphology on the zero shear viscosity (ZSV) and derived ductility of asphalt are investigated. STEM images reveal that asphaltenes in non-aged asphalt are generally well dispersed, with occasional formation of large agglomerates, and both the size of well-dispersed asphaltenes and the abundance of the agglomerates are dependent on asphalt source. Rod-shaped, crystal-like asphaltene particles can be found in both artificially-aged and field-aged asphalts. A probe into asphaltene particles using AFM suggests that the toughness of the particles is low. Rheological test results indicate that asphaltene content plays a predominant role in determining the relative ZSV (ratio between the system ZSV and the liquid phase ZSV), and asphaltene morphology likely plays a secondary role. Conversely, both the content and morphology of asphaltenes likely play major roles in determining the derived ductility of asphalt.