(173g) Solvent Effects on the Structure of Petroleum Asphaltenes

The asphaltene nanoaggregate structure in liquid environments has been recently shown to differ from that of asphaltenes in the solid phase [1]. The correlations between asphaltene molecules can be described by their associated structure, which was shown to change in various solvents. In this study, neutron diffraction has been used to determine the influence of the surrounding liquid on the structure of asphaltene nano-aggregates in mixtures of solvents and anti-solvents (e.g., n-heptane). This talk will present first of their kind neutron diffraction results that were generated for samples containing >5 vol. % asphaltenes dissolved in various deuterated solvents and anti-solvents on the NOMAD instrument at the Spallation Neutron Source at Oak Ridge National Lab. Aromatic, polar and halogenated compounds were used as the solvents, while n-heptane served as the precipitant. Results were instrument, solvent background and inelasticity effect corrected to isolate the scattering signal of the structures formed by asphaltene molecules. This investigation focuses on the molecular spacing between asphaltene structures on the angstrom scale. The separation distance and intensity of each diffraction peak in the system were calculated and compared to previous wide-angle X-ray scattering results [1]. In the neutron diffraction results, the peaks corresponding to the organization of alkyl side chains (Ï? peak) and aromatic core stacking ([002] peak) were observed in agreement with previous X-ray results. The [002] peak is dominated by the correlation between asphaltene structures and represents the crystallinity of asphaltene nano-aggregates. The results showed the crystallinity is constant with changes in the concentrations of asphaltenes or precipitant. A new peak was identified at a separation distance located at a range between 1.7 to 2 angstroms that corresponds to the presence of intermolecular hydrogen bonding between asphaltene molecules. The properties of each diffraction peak vary with different kinds of solvents, which indicate the structure difference caused by solvent effects. The novel results generated with neutron diffraction provide the experimental data as the benchmark for the molecular simulation work and reinforce the need to consider the local solvent matrix on the structure and interactions of asphaltenes.

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