Small-angle X-ray scattering study of oil- and deposit-asphaltene solutions

Multiscale scattering investigations of asphaltene cluster breakup, nanoaggregate dissociation, and molecular ordering

Small-angle X-ray and neutron scattering (SAXS/SANS) by asphaltenes in various solvents (toluene, tetrahydrofuran, and 1-methylnaphthalene) at dilute concentrations of asphaltenes are presented and discussed. As asphaltenes are diluted, it was found that the cluster size decreases and follows a fractal scaling law. This observation reveals that asphaltene clusters persist to dilute concentrations and maintain fractal characteristics, regardless of concentration. For the first time, the fraction of asphaltenes that exist in nanoaggregates compared to those molecularly dispersed was estimated from the scattering intensity. Significant dissociation was detected at concentrations similar to the previously reported critical nanoaggregate concentration (CNAC); however, the dissociation was observed to occur gradually as the asphaltene concentration was lowered. Complete dissociation was not detected, and aggregates persisted down to asphaltene concentrations as low as 15 mg/L (0.00125 vol. %). A simplified thermodynamic aggregation model was applied to the measurements, and the free energy change of association per asphaltene-asphaltene interaction was calculated to be approximately -31 kJ/mol. Finally, novel solvent-corrected WAXS results of asphaltene in a liquid environment are presented and reveal three distinct separation distances, in contrast to the two separation distances observed in diffraction studies of solid phase asphaltenes. Significant differences in the WAXS peak positions and shapes between aromatic and nonaromatic solvents suggests that there may be large differences between the solvation shell or conformation of the asphaltene alkyl shell depending on the surrounding liquid environment.

Asphaltenic aggregates are polydisperse oblate cylinders

Small-angle neutron scattering (SANS) has proven to be very useful for deducing the sizes and morphologies of asphaltenic aggregates in solution. A wide variety of intra-particle structure factors have previously been applied to SANS scattering spectra, but the studies often provided limited information concerning the quality of the fits and the Q range over which the models were applied. Selection of an appropriate form factor that closely approximates the structure of asphaltenic aggregates is important for determining the properties of asphaltenic aggregates, such as the radius of gyration (R(G)), molar mass, and apparent fractal dimension. This study evaluates various mono- and polydisperse intra-particle structure factor models as applied to four asphaltene scattering spectra. Agreement of the model fit parameters (I(0) and R(G)) with those obtained from Guinier analyses suggests that such a form factor model is physically reasonable. Reduced chi2 values for each non-linear least squares fit indicates how well a given model fits to the entire Q range studied for the scattering intensity distribution. In the polydispersity analyses, an analytical function is introduced to model the scattering behavior of oblate cylinders with a Schultz distribution of radii. Results indicate that the polydisperse radius oblate cylinder model best approximates the shape of asphaltenic aggregates.