Solvent Effects on the Structure of Petroleum Asphaltenes

Neutron total scattering was employed to study the impact of a solvent on the behavior of asphaltenes by comparing structural interpretations of solid phase samples to those dispersed in 1-methylnaphthalene (1-MN) as a function of temperature and asphaltene solubility. This powerful technique enables structural interpretation on length scales ranging from local molecular order to the formation of nanoaggregates and/or fractal clusters in a single experiment. We observed that more unstable asphaltenes exhibit greater aromatic stacking on the local molecular length scale and also produce more frequent nanometer-sized associations. However, when the temperature was elevated, the breakup of larger asphaltene clusters was correlated to a reduction in the extent of asphaltene side-chain interactions. Additionally, dispersed asphaltenes demonstrated an increase in aromatic stacking as the temperature was increased, suggesting an entropic driving force for assembly driven by solvent depletion interactions. This observation is novel in asphaltene science and serves as a pathway to assess the impact of local solvent composition on both enthalpic and entropic assembly pathways. The results suggest that while more unstable asphaltenes exhibit greater aromatic stacking, further association may be assisted by an increase in the disordered side-chain interactions. The experimental results also serve as a useful benchmark to validate simulation predictions of asphaltene structure.