Revealing self-aggregation mechanism of asphaltenes during oxidative aging using quantum mechanical calculations

The density functional theory (DFT-D3) was used to reveal the molecular mechanisms responsible for self-aggregation of asphaltene. The results showed that electrostatic potential on the molecular vdW surface, atomic charge distribution and transfer, highest occupied, also lowest unoccupied orbitals all appeared at the aromatic rings and heteroatoms that in the aromatic core or aliphatic side chains of asphaltene molecules. The polar functional groups introduced by oxidation changed the electrostatic potential and charge distribution on these regions, which resulted in that electron over carbonyl groups (CO) around the aromatic core decreased the spatial extent of the π-electron cloud and the electron density, thus reduced some favorable contribution of π-π stacking effect in this region. The loss of interactions was partially compensated for by new forms of OH··N hydrogen bond and θ-θ interaction between the side chains in another part of the molecules. Compared to unoxidized dimer, the contribution of electrostatic interaction of oxidized asphaltenes enhanced from 2.42 % to 8.08 % and contribution of dispersion attraction slightly reduced from 63.41 % to 53.98 %. Bond critical points (BCPs) were distributed at aromatic rings and heteroatoms and the number of that were increase. The contributions of atom pair (δGpair (%)) around the strong OH··N bond (0.97 %) in oxidized dimer was significantly raised than that of unoxidized dimer (0.61 %). These changes were consistent with the electronic characteristics of asphaltene molecules after oxidation.