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.

Laser desorption mass spectrometry (LD-MS) was used to measure three asphaltenes, their sub-fractions separated using gel-permeation chromatography (GPC), and model compounds to ascertain their molecular weight distributions and averaged molecular weights. The asphaltenes were isolated from three crude oils:  Maya (MY), Khafji (KF), and Iranian Light (IL). To optimize the measurement conditions for LD-MS, the effects of instrument mode, matrix, and laser energy were determined. Linear mode detection gave larger ion counts and higher signal-to-noise ratios than did reflector mode. The use of a matrix to determine the asphaltene molecular weight distribution was not useful as it enhanced the ionization of only some of the asphaltene fractions. The chosen laser energy provided significant ionization of the high-molecular-weight fractions, while minimizing polymerization and fragmentation. Various model compounds were measured to clarify the dependence of the ionization capacity on molecular structure. Pure and small alkyl-substituted aromatics ionize easily, and tend to polymerize at relatively low laser energies. Bridged aromatics are easily disrupted at the alkyl bridge and do not polymerize. Aliphatic hydrocarbons are very difficult to ionize in the absence of an appropriate matrix. The higher the molecular weight of a compound, the more difficult it is to ionize. The characterization of the asphaltenes was aided by separating each into six sub-fractions using GPC. These fractions fell into two groups:  high- and low-aromaticity groups. In each group, the lower the molecular weight of a sub-fraction, the higher its aromaticity. Maya asphaltene (As-MY) had a larger molecular weight fraction than the other asphaltenes, but the distribution pattern of As-KF was similar to that of As-MY, except for this fraction. All the As-IL sub-fractions had lower molecular weights and higher aromaticity than the corresponding sub-fractions of the other asphaltenes. The averaged molecular weights of the asphaltenes were determined from the weighted average of the averaged molecular weight of each of the six sub-fractions. For As-MY, As-KF, and As-IL the values are 1657, 1628, and 1462 amu, respectively.

The energy-minimum conformation calculated by molecular mechanics−molecular dynamics simulation for the asphaltene obtained from the vacuum residue of Khafji crude oils showed that structures aggregated through several noncovalent interactions are the most stable. Changes induced in aggregated structures by heating were investigated using molecular dynamics calculations. The simulation showed that the hydrogen bond between asphaltene molecules dissociated at 523 K, while aromatic−aromatic stacking interactions appeared to be stable. At 673 K, however, some stacking interactions could be disrupted, but some stable aggregates remained even at this high temperature where some decomposition reactions would be expected to occur. Simulations on two model compounds were carried out to investigate the effects of aliphatic chains and polar functional groups on the stability of asphaltene aggregates during heating. Aliphatic chains and polar functional groups contributed to the stability of aggregates; in simulations of “imaginary” structures in which the original structure was modified by removing the aliphatic side chains and then replacing heteroatoms with carbon, dissociation occurred at lower temperatures at to greater extents than for the original structure; van der Waals interactions between aliphatic chains acted cooperatively to stabilize the asphaltene aggregates.

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The Hansen solubility parameters (HSPs) of asphaltenes extracted from oil sand bitumen samples produced at Athabasca in Canada and also from a vacuum residue fraction (VR) produced in the Middle East were determined by the Hansen solubility sphere method. For calculation of HSPs, the solubilities of asphaltenes were determined using a dynamic light scattering (DLS) method by dissolving or dispersing the asphaltenes in various solvents and measuring the particle size distributions thereof. The particle diameters of asphaltenes in good solvents were lower than its detection limit (<1 nm). It was demonstrated in the present study that asphaltenes differing in elemental composition had different HSP values corresponding to dispersion, dipole interaction, and hydrogen-bonding forces (δd, δp, and δh, respectively). Experimental results suggested that the differences in HSP values of the asphaltenes were influenced by the H/C ratio, oxygen content, and average asphaltene molecular weight.

Molecular weight (MW) is one of the most important properties of asphaltene. To determine the precise average MW using gel permeation chromatography (GPC), the MWs obtained for three different asphaltenes using GPC (MWps) and GPC-MSD (MWms) were compared. MWms proved independent of asphaltene type and had values similar to MWps for a MW of ∼900 but was significantly different in the lower-MW range, with a linear correlation between MWms and MWps. More-precise average MW values can be obtained using a revised calibration curve based on three new chemicals to convert MWps to MWms.

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