Neutron Scattering as a Tool to Characterize Bulk and Interfacial Asphaltene Supramolecular Self-Assembly(ABSTRACT)

摘要

Because of their role in water-in-crude oil emulsion stabilization, in forming deposits during flow, and in causing challenges in the refinery, asphaltene characterization is of great relevance and importance to the petroleum industry. What we will describe here are highlights from our investigations using small angle neutron scattering (SANS) and ultra small angle neutron scattering (USANS) to illuminate the nature of the self-assembled structures formed by asphaltenes in bulk fluids as well as at the oil water interface.We have repeatedly used SANS to investigate the nanoscale aggregate structures of asphaltenes. A careful analysis of several form factor models reveals that asphaltene nanoscale aggregates are best characterized by a discoidal shape. Furthermore, application of this form factor facilitates evaluation of some telling features of aggregates in various solvents, including interaggregate association and thermodynamic interactions (osmotic 2~(nd) Virial coefficient), the structural variation among asphaltene subtractions of unique elemental chemistry, and solvent entrainment within interstitial cavities of aggregates. In fact, our observations suggest that asphaltenic aggregates are quite rich in solvent, often 30-50% by volume, and that the degree to which solvent entrains, as well as the composition of the entrained solvent, significantly depends on the nature of the solvent-asphaltene interactions. Through contrast variation in SANS experiments of asphaltenes dissolved in mixtures of toluene with n-heptane, decalin, or 1-methylnaphthalene, we detect the preferential entrainment of the "better" solvent within the aggregate for each solvent pair. SANS is also a useful tool in probing the physics and chemistry of water-in-oil emulsions, particularly the average thickness and composition of asphaltenic interfacial films of these emulsions. A popular criticism of SANS in polydisperse systems is the ambiguity that accompanies the ability to fit data with multiple form factor models. In the emulsions we describe here, we control the composition of the water-phase and replace exhaustively the continuous phase with fresh deuterated solvent. This enables us to fix the scattering length densities of bulk and continuous phases in emulsions and to eliminate scattering due to asphaltenic aggregates in the bulk. Optical microscopy provides the droplet size distribution and supports the use of a core/shell form factor model. We observe that the films coating our rinsed emulsions are often 100-120 A thick, with no direct correlation to emulsion stability determined by centrifugation. It is clear, however, that the density of asphaltene molecules in these films is what dictates the overall emulsion stability. In samples we also ran on USANS, we saw good agreement for film properties determined by SANS alone and those obtained by simultaneous fitting of SANS and USANS data. We conclude our overview of the application of neutron scattering techniques to problems in asphaltene chemistry and characterization by relating asphaltene structure and its interaction with solvent to emulsion stabilization.