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.

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Asphaltenes self-associate, and the molecular weight and density distributions are a factor in asphaltene precipitiation. To determine these distributions, heptane-extracted asphaltenes from four crude oils were fractionated into solubility cuts. The asphaltenes were dissolved in toluene and then partially precipitated at specified ratios of heptane/toluene to generate sets of light (soluble) and heavy (insoluble) cuts. The molecular weight and density were measured for each cut. The asphaltenes were found to include both associating and non-associating asphaltenes. The content of non-associating components was up to 15 wt % of the asphaltenes. The density distributions were determined directly from the data. The molecular weight data were fitted with a self-association model to predict the distributions at any given concentration. Then, a guideline was developed to represent the molecular weight distribution of non-associated and associated asphaltenes with a Γ distribution function. Finally, the density of asphaltene cuts was correlated to their molecular weight. This correlation fit the data with an average absolute deviation of 11 kg/m 3 .

Densities and refractive indices were measured for native crude oils, distillation cuts, and thermo-and hydrocracked oils at 20°C and for some samples up to 60°C, all at atmospheric pressure. These data as well as pure component and crude oil data from the literature were used to develop correlations between: the refractive index function (FRI) and density; the thermal and FRI expansion coefficients, and; the binary interaction parameters used in mixing rules for the density and FRI of mixtures. The density (or FRI) of the components (or liquids treated as a single component) were both correlated to within 2%. The density (or FRI) of a mixture was fitted also to within 2% using mixing rules with only the known component density (or FRI) at 20°C as the input. The mixture density (or FRI) can be predicted to within 4% using only the correlated component density (or FRI) at 20°C. The correlations have yet to be tested at temperatures above 60°C and pressures above atmospheric.

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