Study on π-π stacking between asphaltene molecules by DFT and MM methods

Key words:  asphaltene aggregates  π-π stacking  density functional theory(DFT)  molecular force field

Abstract:

To gain insight about the π-π stacking in asphaltene molecular aggregates, density functional theory (DFT) and molecular mechanics (MM) methods were adopted to study the potential energy surface (PES), the equilibrium center of mass distance (R_e) and the equilibrium interaction energy (ΔE_e). The results show that the typical PES are obtained to describe the S-type (sandwich), T-type (T-shaped) and PD-type (parallel-displaced) dimers of benzene or asphaltenes by using DFT-D, DFT/mGGA, MM, or DFT/GGA with DNP+ basis set, where the DFT/mGGA-M06L method has the highest accuracy. The ΔE_e and R_e calculated by mGGA-M06L are consistent with those reported in the literature for benzene dimers, and the errors are less than 4.60%. The R_e of the S-type asphaltene dimer is very close to that of benzene, both are 0.388 nm, and ΔE_e is more than ten times larger than that of benzene. For the T-type asphaltene dimers, the more atoms in the short-distance, the greater the ΔE_e, but there is only one or several atoms in the short-distance, resulting in the ΔE_e significantly smaller than that of the S-type dimer. The ΔE_e of the PD-type asphaltene dimers are greater than that of the S-type, and the ΔE_e is the largest when the vertical distance (R_v) is about 0.35 nm, which is close to twice that of the S-type. In this case, the interlamellar spacing of the dimer is significantly smaller than that of the S-type, resulting in the most stable structural conformation.