Direct observation of the asphaltene structure in paving-grade bitumen using confocal laser-scanning microscopy

The structure of the asphaltene phase in the bitumen is believed to have a significant effect on its rheological properties. It has traditionally been difficult to observe the asphaltene phase in unaltered samples of bitumen. The maltenes are thought to form a continuous phase in which the asphaltenes are ‘dispersed’. In this study, confocal laser‐scanning microscopy (CLSM) operating in fluorescence mode was used to examine the structure of paving‐grade Safaniya and San Joaquin bitumen. The asphaltene fraction fluoresces in the 515–545 nm wavelength range when irradiated with light with a wavelength of 488 nm. The major advantages of CLSM are that the bitumen sample requires little pretreatment or preparation that may affect the original dispersion of asphaltenes and the bitumen is observed at ambient temperature and pressure. This reduces the possibility of producing images that are not representative of the original material. CLSM was able to show the distribution of maltene and asphaltene components in bitumen. The asphaltene aggregates in the bitumen were observed to be 2–7 µm in size and formed a dispersed ‘sol’ structure in the continuous maltene matrix rather than a network ‘gel’ structure. Surprisingly, the structure and fluorescence of the asphaltene phase does not appear to alter radically upon oxidative ageing. The structure of the asphaltene phase of an AR4000 San Joaquin bitumen was found to be more homogeneous than that of Safaniya bitumen, illustrating the range of structures that can be observed in bitumens by this method.