Organization of Asphaltenes in a Vacuum Residue: A Small-Angle X-ray Scattering (SAXS)-Viscosity Approach at High Temperatures

Temperature-dependent rheological behavior of heavy oils and bitumen is usually modeled with a colloidal approach, taking into account a temperature-dependent solvation effect (Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Rheological study of Ratawi vacuum residue in the 298-673 K temperature range. Energy Fuels 1995, 9,168-176). In addition to viscosity measurements for vacuum residue at various asphaltene contents, in the present study, we make use of small-angle X-ray scattering (SAXS) data on the 80-240 degrees C temperature range to propose an interpretation on asphaltene aggregation, consistent with both approaches. The radius of gyration R-g and molecular weight M-w of asphaltenes in a vacuum residue are measured and are of the same magnitude as asphaltenes in toluene. Dimensions and masses decrease with the temperature, while the small length scale remains unchanged, reinforcing the hierarchical aggregation scheme previously described in toluene. These findings enrich the viscosity data interpretation. A solvation factor has to be accounted for, as noticed in previous works. Its signification is made clear by SAXS data: lose asphaltene clusters in maltenes dissociate with temperature, decreasing their solvation.