X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) measurements were made on asphaltenes and vacuum residua (denoted by the prefixes As and VR, respectively) isolated from three different crude oilssMaya (MY), Khafji (KF), and Iranian Light (IL)sto characterize the petroleum asphaltene aggregates present under various conditions. In the XRD experiments, the samples were loaded on a small plate sample holder that was kept horizontal while measurements were made at 30, 150, and 300 °C. The layer distances between aromatic sheets of asphaltenes were ∼3.6 Å, and the number of aromatic sheets in a stacked cluster decreased from eight to five as the temperature increased from 30 °C to 300 °C. The different crystallite parameters varied little between the three asphaltenes, although maltenes in the vacuum residua interacted with the asphaltenes and loosened their stacking by a small amount. In SAXS experiments, scattering patterns were obtained on the dry asphaltenes at room temperature in a flowing nitrogen atmosphere and the samples were then heated from 30 °C to 500 °C. The fractal aggregates of As-MY, As-KF, and As-IL broke down at 241, 179, and 243 °C, respectively. From these results, and earlier small-angle neutron scattering (SANS) data, a hypothetical hierarchical model of asphaltene aggregation is proposed.

The objective of this study is to examine changes in the structures of petroleum asphaltene aggregates in situ with small-angle neutron scattering (SANS). Asphaltenes were isolated from three different crude oils: Maya, Khafji, and Iranian Light. An aliquot of the 5 wt % asphaltene solution in deuterated Decalin, 1-methylnaphthalene, or quinoline was loaded in a special stainless steel cell for SANS measurements. SANS data measured at various temperatures from 25 to 350 °C showed various topological features different with asphaltene or solvent species. A fractal network was formed only with asphaltene of Maya in Decalin, and it remained even at 350 °C. In all of the solvents, asphaltenes aggregate in the form of a prolate ellipsoid with a high aspect ratio at 25 °C and got smaller with increasing temperature. That became a compact sphere with the size of around 25 Å in radius at 350 °C.

Mayan crude, residuum, and hydrocracked asphaltenes have been separated into two fractions by extended Soxhlet extraction in n-heptane. Although the solubility, composition, and molecular structures differ slightly, the greatest difference between the two asphaltene fractions is the degree to which they associate in solution. The vapor-phase osmometry molecular weight, molecular size by size-exclusion chromatography, and small-angle neutron scattering indicate that approximately 25% of Mayan asphaltene is not highly associated in aromatic solvents and, thus, is noncolloidal. By contrast, the remaining asphaltene forms large, rodlike colloidal particles in solution and has a higher apparent molecular weight. Although laser desorption mass spectrometry indicates that the molecular weight of the individual molecules in maltenes and asphaltenes is not very different, high-resolution mass spectrometry indicates that the size of the aromatic core of asphaltenes is significantly larger than those in maltenes. Furthermore, the tendency of the residuum fractions to form coke during thermal cracking is likely related to the size of the largest polyaromatic rings.