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 pi stacking, van der Waals, and hydrogen bonds, are described.

Automated summary

The aggregation behavior of asphaltene subfractions A1 and A2 in different solvents were studied using molecular dynamics simulations. The size and distribution of the aggregates were found to be correlated with the aromaticity of the molecules. In tetrahydrofuran (THF), large aggregates were observed to break down into smaller aggregates, which explains the bimodal size distribution often observed in gel permeation chromatography (GPC) analysis. The dipole moment of the aggregates in THF and toluene was found to decrease with an increase in the number of molecules, indicating a random structure of the aggregates. The contributions of different molecular interactions to the aggregation mechanism were discussed.