Nature of Asphaltene Aggregates

The size and fractal dimension of asphaltene aggregates were investigated for two Western Canadian bitumens diluted with n-heptane at concentrations above the onset of precipitation. Asphaltene aggregate size distributions were measured over time using focused beam reflectance and micrographic methods in a series of batch experiments at different n-heptane contents and shear rates. The fractal dimensions of the aggregates were determined from the volume of the settled aggregates. The asphaltenes formed approximately log-normal size distributions with volume mean diameters of tens to hundreds of micrometers, depending mainly upon the n-heptane content. The distributions were established in less time than the first measurement could be obtained (about 30 s) and changed relatively little afterward. The average aggregate size increased with an increasing n-heptane content but reached a plateau value at 70-80 wt % n-heptane. The fractal dimension reached a maximum at a similar n-heptane content. Micrographic images and fractal dimensions indicated that, near the onset of precipitation, compact linear and planar aggregate structures dominated. At higher n-heptane contents up to approximately 75 wt % n-heptane, the aggregates remained compact but became more three-dimensional. At higher n-heptane contents, larger, looser structures were formed that could be broken under sufficient shear but did not reaggregate. The results were consistent with simultaneous nucleation, growth, and flocculation processes, where the precipitating material was initially sticky but lost its stickiness over time. The loss in stickiness was confirmed with surface force adhesion measurements.