Analysis of nanoscale microstructures in asphalts of different aging states

Asphalt surface features revealed in the tapping-mode atomic force microscope (AFM) have been extensively studied. Most of the existing studies focus on the so-called bee-shaped surface feature, believed to be made of paraffin wax crystals. In the research reported in this paper, five asphalts of widely different aging states and asphaltenes obtained from two of these asphalts were examined by using an AFM in tapping mode. Two types of surface features were identified in all the specimens, as follows: (1) disk-shaped feature rising from the specimen surface, and (2) elliptical (and sometimes irregular) feature slightly recessed from the surface. In addition, a Strategic Highway Research Program (SHRP) asphalt (AAD) was used to study the morphology of asphaltenes in wax-free asphalt. The analysis of the topography and phase images of the microstructures revealed by AFM suggests that the disk-shaped and elliptical microstructures may contain asphaltenes. The morphology of asphaltenes from the asphalt extracted from a 36-year-old pavement appears to match the hypothetical so-called island molecular structure.