(352d) Solvent Effects on the Structure of Petroleum Asphaltenes

The asphaltene nanoaggregate structure in liquid environments has been recently shown to differ from that of solid asphaltenes [1]. The nature of asphaltene-asphaltene interactions is manifested in their associated structure, which was shown to change in various solvents, and prompts further study. Neutron diffraction provides a quantitative experimental description of the influence of the surrounding liquid matrix on structure of asphaltene nanoaggregates in mixtures of solvents and anti-solvents. In addition, neutron diffraction allows for isotopic substitution of the solvent and investigation of the interaction of hydrogen atoms, which are virtually transparent to X-rays. 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 anti-solvent. Results were instrument and solvent background corrected to isolate the scattering of the asphaltene molecules and their aggregated structures. This investigation differs from previous small-angle scattering studies by examining the molecular spacing between asphaltenes on the angstrom scale as opposed to viewing larger (e.g., nanometer) associations with small-angle scattering. The separation distance of each diffraction peak was calculated and compared to previous X-ray scattering results [1]. In the neutron diffraction results, the peaks corresponding to the organization of alkyl side chains and aromatic stacking were observed in agreement with previous X-ray results. A new correlation peak was identified at a separation distance of approximately 2 angstroms that corresponds to the presence of hydrogen bonding. The novel results generated with neutron diffraction reinforce the need to consider the local solvent matrix on the structure and interactions of asphaltenes. 

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