(234t) Solvent Effects on the Crystallinity of Petroleum Asphaltenes

Authors: 
Yang, Y., University of Utah
Hoepfner, M. P., The University of Utah
Headen, T., Science and Technology Facilities Council, Rutherford Appleton Laboratory
Petroleum asphaltenes are the most aromatic and heaviest fraction of crude oil and they can destabilize and precipitate on surfaces with changes in composition, temperature, and pressure [1]. The molecular structure of asphaltenes is that of a large aromatic core along with alkyl stabilizing side chains. Asphaltenes have been shown in primarily exist as stable semi-crystalline aggregates, called nanoaggregates (~2nm diameter), made of ~3-8 molecules in crude oils and good solvents. Previous research indicated that the aromatic areas of asphaltenes’ molecules stack at a similar separation distance to graphite in the solid phase. However, it is reported that the correlations between asphaltene molecules can be influenced by the liquid surroundings. In this study, neutron diffraction has been used to determine the influence of the surrounding liquid on the crystallinity of asphaltene nano-aggregates (stacked graphite-like structure) in mixtures organic solvents (e.g., toluene).

This talk will present first of their kind neutron diffraction results that were generated for samples containing 5 vol. % asphaltenes dissolved in deuterated solvents and anti-solvents on the NOMAD instrument at the Spallation Neutron Source at Oak Ridge National Lab. Moreover, the crystallinity of different subfractions of asphaltenes (extracted from crude oil using different fraction of the precipitant) were studied using neutron diffraction technique on the NIMROD instrument at the Rutherford Appleton Laboratory. Results were instrument, solvent background and inelasticity effect corrected to isolate the scattering signal of the structures formed by asphaltene molecules.

In the neutron diffraction results, the peak corresponding to aromatic core stacking ([002] graphite peak) was observed in agreement with previous X-ray results. The [002] graphite peak is dominated by the correlation between asphaltene structures and represents the crystallinity of asphaltene nano-aggregates. The results showed the crystallinity is approximately constant with changes in the concentrations of asphaltenes or precipitant, indicating a highly stable aggregated state. The characteristics of other diffraction peaks varied with different kinds of solvents, which indicate different solvation mechanisms. The results at different temperatures will also be presented (25 – 240℃). The novel results generated with neutron diffraction provide necessary experimental data as a benchmark for molecular simulations and reinforce the need to consider the local solvent matrix on the structure and interactions of asphaltenes and other self-assembling hydrocarbon systems. This work also demonstrates the capability of neutron diffraction technique to probe assembly structures at Angstrom length scale of dispersed semi-crystalline materials.

[1] Hoepfner, M. P. and Fogler, H. S., “Multiscale Scattering Investigations of Asphaltene Cluster Breakup, Nanoaggregate Dissociation, and Molecular Ordering,” Langmuir, 29 (49), 2013.