(169c) Ultra-Small-Angle Scattering Investigation of the Structure and Self-Assembly Mechanism of Asphaltenes in Solvent Mixtures

It is known that molecules of petroleum asphaltenes can self-assemble in crude oil and solvents into fractal clusters with Rg<40 nm and fractal dimensions ranging between 1.6 and 2.1 [1]. With the addition of an antisolvent (e.g., heptane), the stable clusters of asphaltenes can be destabilized and further aggregate into insoluble clusters with sizes in the micrometer length scale and a more compact internal structure (i.e., higher fractal dimension) [1]. However, it is still unclear what structural changes occur at the nano and micrometer length scales for both soluble and insoluble asphaltenes during the destabilization and precipitation process. In this talk, ultra-small-angle X-ray scattering (USAXS) results generated on beamline 9-ID-C at the Advanced Photon Source at Argonne National Lab will be presented to explore the mechanism of asphaltene destabilization, self-assembly and precipitation. Because of the ability of USAXS to be able to probe a wider range of scattering vectors (q), both soluble and precipitating asphaltenes with larger length scales can be detected simultaneously. The results show the capability of USAXS to track time-resolved changes in structural properties of asphaltenes as they transit from the nano to the micrometer length scale. The precipitation process was monitored as a function of time after adding an antisolvent by measuring the structural changes of both soluble and insoluble/precipitating clusters for long mixing times (>24 hours) and large length scales (>1000 nm). It was observed that the compactness and size of insoluble clusters increase as a function of time, while the structural properties of soluble asphaltenes remain the same. Moreover, it is revealed that the precipitating asphaltene clusters have surface fractal geometry or rough surfaces, but with a compact core. Trace quantities of inorganic solids (0.1nm<R<1 micron) were detected and likely contributed to the precipitation mechanism by acting as nucleation sites. Inorganic solids are present in real-world oil production and processing environments, and this result is of direct scientific and industrial importance. Overall, this study provided an improved mechanistic understanding of asphaltene assembly during precipitation and showed the powerful ability of USAXS for the investigations of asphaltene-related problems.

 

[1] Hoepfner, M. P.; Vilas Boas Favero, C.; Haji-Akbari, N.; Fogler, H. S. The Fractal Aggregation of Asphaltenes. Langmuir 2013, 29, 8799–8808.