Molecular dynamics simulation of the heat-induced relaxation of asphaltene aggregates

The energy-minimum conformation calculated by molecular mechanics-molecular dynamics simulation for the asphaltene obtained from the vacuum residue of Khafji crude oils showed that structures aggregated through several noncovalent interactions are the most stable. Changes induced in aggregated structures by heating were investigated using molecular dynamics calculations. The simulation showed that the hydrogen bond between asphaltene molecules dissociated at 523 K, while aromatic-aromatic stacking interactions appeared to be stable. At 673 K, however, some stacking interactions could be disrupted, but some stable aggregates remained even at this high temperature where some decomposition reactions would be expected to occur. Simulations on two model compounds were carried out to investigate the effects of aliphatic chains and polar functional groups on the stability of asphaltene aggregates during heating. Aliphatic chains and polar functional groups contributed to the stability of aggregates; in simulations of imaginary structures in which the original structure was modified by removing the aliphatic side chains and then replacing heteroatoms with carbon, dissociation occurred at lower temperatures at to greater extents than for the original structure; van der Waals interactions between aliphatic chains acted cooperatively to stabilize the asphaltene aggregates.