New Insights into Asphaltene Structure and Aggregation by High-Resolution Microscopy

Transmission electron microscopy was used to probe the structural properties of asphaltenes and their aggregation behavior. Asphaltenes precipitated by different n-alkanes (n-C-5, n-C-7, and n-C-10) were considered in addition to the interfacial material (IM) extracted from n-C-7 asphaltenes. Measurements were first performed with powdered samples dispersed in methanol. Analysis of the amorphous layers observed in their segmented micrographs provided important structural parameters such as the edge-to-edge spacing between particles, the average area and shape factor of these particles, and the rugosity of the layer's boundary. Short-range order was observed in all samples due to the parallel stacking propensity of asphaltenes. Denser packing and lower particle circularity of the higher alkane asphaltenes resulted in the smallest interparticle spacing. On the other hand, disruption of stacking by the aliphatic side chains was more prevalent in the lower alkane asphaltenes as molecular polydispersity increased. The lower shape factor and average area of the n-C-7 IM indicated the presence of smaller particles than in the parent asphaltenes. Furthermore, the much smaller edge-to-edge and interlayer spacings suggested much tighter molecular packing due to the pi-pi stacking of the small aromatic cores and hydrogen bonding between the fatty acid chains. The higher crystallinity of the IM yielded thin nanosheets that were previously observed in liquid samples. Structure-function relations were also established by comparing micrographs of solid n-C-10 asphaltenes to those dispersed in heptol solutions. Crystalline structures seen in the solid samples appeared as small globular clusters in the liquid samples. Their shape and average size were consistent with the Yen-Mullins model.