Aggregation of Asphaltene Subfractions A1 and A2 in Different Solvents from the Perspective of Molecular Dynamics Simulations

The aggregation behavior of model molecules of asphaltene
subfractions
A1 and A2 dissolved in heptane, toluene, and tetrahydrofuran (THF)
were studied using molecular dynamics simulations. The proposed asphaltene
molecular models are based on previously studied structures with two
new models, including a highly aromatic model with a prominent island-type
molecule and another molecule with a prominent archipelago-type architecture.
The aggregation mechanisms in toluene, THF, and heptane solvents were
studied. The results in heptane and toluene were consistent with the
solubility of asphaltenes and their subfractions in these solvents.
The size of the aggregates is well-correlated with aromaticity. When
considering THF, large aggregates are broken down into smaller aggregates.
This could lead to the mixture of high, medium, and low molecular
weight distribution bands usually observed when gel permeation chromatography
(GPC) analyses are conducted on asphaltene samples in THF. The size
distribution extracted from the simulations shows a bimodal distribution
with profiles similar to the size distribution profiles usually found
in GPC analysis for asphaltene samples in THF. The distributions of
dipole moments of the aggregates against the number of molecules in
aggregates were constructed in both THF and toluene and reveal that
the dipole moment of the aggregates vanishes when the number of molecules
increases as a result of a random structure of the aggregates. The
contributions of different molecular interactions to the aggregation
mechanism, such as π stacking, van der Waals, and hydrogen bonds,
are described.