Molecular Cartography of A1 and A2 Asphaltene Subfractions from Classical Molecular Dynamics Simulations

The physical chemistry of the heavyweight fraction of crude oil is still a subject of vivid discussions due to the complications which arise from the tendency of this fraction to aggregate and finally hinder the refining processes. Asphaltene molecules, that compose the largest part of this fraction, can be separated into two subfractions, A1 and A2, through a treatment with p-nitrophenol. In this paper, starting from the molecule models suggested by Acevedo et al. (Acevedo et al. Energy Fuels 2018, 32, 6669-6677.), we screened the chemical structure and composition of the two subtractions and investigated their aggregation mechanism using classical molecular dynamics simulations. The results show that oxygen atoms, present as hydroxyl or carboxylic groups, are a key factor in the formation of large aggregate. From the analysis of the simulations, we estimated the size of the aggregates and showed how the flexibility of the molecules may affect the size of the aggregates. Finally we showed that in addition to the structural differences (such as the H/C ratio and DBE) that distinguish the A1 and A2 subtractions, the solubility of the subfractions is also strongly dependent on the ability of the molecules to bind through hydrogen bonds.