How Functional Groups Influence Asphaltene Aggregation: Molecular Simulations and Small-Angle Neutron Scattering

Asphaltenes are polycyclic aromatic molecules found in high-molecular-weight fractions of crude oil. They dissolve in aromatic solvents like toluene but not in aliphatic solvents like heptane. Low solubility leads to aggregate formation and many problems in transport applications. Natural asphaltene fractions contain complex mixtures of molecules that vary widely between sources of crude oil, and so the chemical and structural properties are difficult to characterize. Herein, three synthetic asphaltenes containing different oxidation states of sulfur in binary mixtures of toluene and heptane are studied. Aggregate formation is investigated using a combination of small-angle neutron scattering (SANS) and molecular dynamics (MD) simulations. The extent of aggregation is found to depend strongly on both the composition of the solvent and the functionality of the sulfur atom. A benzothiophene-functionalized asphaltene, without any oxygen, forms nanoaggregates in toluene that do not change significantly on addition of heptane. In contrast, asphaltenes containing the sulfoxide or sulfone analogs form nanoaggregates in toluene and much larger clusters above 40% by volume heptane content, with the initial nanoaggregates of the sulfone being slightly larger. Such behavior is apparent in both measured and simulated scattering profiles, and while these techniques probe different length scales, the results are consistent. The microscopic structures of the simulated aggregates are detailed. In systems with low heptane content, the asphaltenes form small clusters of 2-4 molecules, depending on the functionality. In systems with greater heptane content, the sulfoxide and sulfone form larger clusters. The variations in clustering behavior between functional groups and solvents are attributed mainly to the electrostatic interactions between the polar sulfur-containing functional groups, which stabilize “head-to-tail” configurations in the sulfoxide aggregates and more complex branched structures in the sulfone aggregates.