A variety of experimental techniques were applied to a single source asphaltene sample at the same experimental conditions in order to reveal the possible size distributions of asphaltene monomers and aggregates. The asphaltene sample was divided into solubility cuts by selective precipitation in solutions of heptane and toluene. Asphaltene self-association was assessed through a combination of density, vapor pressure osmometry (VPO), elemental analysis, Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry, and time-resolved fluorescence emission spectra measurements performed on each cut. The physical dimensions of the asphaltenes were assessed using SAXS, DLS, membrane diffusion, Rayleigh scattering, and nanofiltration measurements. Molecular and nanoaggregate dimensions were also investigated through a combination of interfacial tension, interfacial adsorption, and surface force measurements. All of the measurements indicated that approximately 90 wt % of the asphaltenes self-associated. Ultrahigh resolution spectrometry suggests that the nonassociated asphaltenes are smaller and more aromatic than bulk asphaltenes indicating that the associating species are larger and less aromatic. On the basis of VPO, the average monomer molecular weight was approximately 850 g/mol, while the molecular weight of the nanoaggregates spanned a range of at least 30000 g/mol with an average on the order of 10000 to 20000 g/mol. SAXS and DLS gave molecular weights 10 times larger. The physical dimensions of the nanoaggregates were less than 20 nm based on nanofiltration and with average diameters of 5 to 9 nm based on diffusion and Rayleigh scattering. SAXS and DLS gave average diameters of 14 nm and indicated that the nanoaggregates had loose structures. Film studies were consistent with the lower molecular weights and dimensions and also demonstrated that asphaltene monolayers swell by a factor of 4 in the presence of a solvent. The most consistent interpretation of the data is that asphaltenes form a highly polydisperse distribution of loosely structured (porous or low fractal dimension) nanoaggregates. However, the discrepancy between VPO and SAXS molecular weights remains unresolved.

Physical separation of heavy oils and bitumen is of particular interest because it improves the description of the chemical and structural organization in these industrial and challenging fluids (Zhao, B.; Shaw, J. M. Composition and size distribution of coherent nanostructures in Athabasca bitumen and Maya crude oil. Energy Fuels 2007Fuels , 21, 2795Fuels −2804. In this study, permeates and retentates, differing in aggregate concentrations and sizes, were obtained from nanofiltration of a vacuum residue at 200 °C with membranes of varying pore size. Elemental composition and density extrapolations show that aggregates are best represented as n-pentane asphaltenes, while the dispersing phase corresponds to n-pentane maltenes. Small-angle X-ray scattering (SAXS) measurements are processed, on this basis, to calculate the size and mass of the aggregates. Aggregates in the vacuum residue are similar in size and mass to asphaltenes in toluene, and temperature elevation decreases the size of the aggregates. Wide-angle X-ray scattering (WAXS) highlights a coherent domain observed for fluids containing aggregates, corresponding to aromatic stacking described for dry asphaltenes. The scattered signal in this region, not observed in maltenes, grows as aggregate content increases, and the signal persists up to 300 °C. A generic behavior of aggregation in the vacuum residue is depicted, from nanoaggregates to large fractal clusters with high aggregation numbers, that is similar to the organization in toluene.

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Complex viscosity and phase-angle measurements for Athabasca bitumen and Maya crude oil were performed with a rotational rheometer using parallel plates and a double gap cylinder in the oscillatory mode over the temperature range of (200 to 410) K. A large range of shearing conditions were applied (frequency of oscillations, shear strain, or stress), and up to three orders of magnitude of variations in measured viscosity values for individual samples at a fixed temperature were obtained. Athabasca bitumen and Maya crude oil were found to be solid-like materials up to (260 to 280) K and (230 to 240) K, respectively. Athabasca bitumen is a non-Newtonian shear-thinning fluid up to (310 to 315) K, whereas Maya crude is a shearthinning fluid up to (280 to 285) K. Both are Newtonian at higher temperatures. Maya crude oil was also found to possess thixotropic behavior. Athabasca bitumen reveals the thermal irreversibility of complex viscosity, if it is heated above 360 K. These rheological behaviors are attributed to the multiphase behavior of these materials over the temperature range of interest, and these results can be used to reconcile the large differences in reported viscosity values for bitumen and heavy oil obtained with diverse viscometers where shear rate and other variables are not controlled. Additional artifacts introduced during measurements are also addressed. Sample variation due to geographical location, depth of formation, and production and postproduction processing can also result in up to three orders of magnitude of differences between the measured "viscosity" of bitumen when the measurement method and temperature are fixed. The flow properties of bitumen and heavy oil are expected to be strong functions of sample source and the hydrodynamics prevailing in situ or in processes at temperatures where non-Newtonian behaviors prevail.