Importance of the Inclusion of Dispersion in the Modeling of Asphaltene Dimers

• Published in: Energy & fuels 2010-12, Vol.24 (12), p.6468-6475
• Main Authors: Mackie, Iain D, DiLabio, Gino A
• Format: Article
• Language: English
• Subjects: Materials
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/ef100797h

Modeling of asphaltenes presents many challenges, not least of which is their large size and the lack of definitive experimental structure information. However, a further fundamental issue of importance is the ability of the modeling methodology to accurately predict noncovalent interactions, particularly dispersion (London) forces. The self-aggregation properties of asphaltenes are primarily driven by such interactions. Therefore, for a modeling approach to be insightful, dispersion must be accounted for. This point is illustrated in this work by examining the effect of dispersion on the conformer distribution for a series of asphaltene model dimers, including perylene bisimide-type molecules and for a model of Maya asphaltene. Inclusion of dispersion using dispersion-correcting potentials on standard density-functionals is shown to be critical to both structure and conformer population for these noncovalently bound dimers. For the Maya asphaltene, a previously postulated “open” structure is shown to be ca. 9 kcal/mol less stable than a conformer that allows for greater π−π overlap within its central archipelago-type moiety. This finding is in line with recent NMR work, which indicates that “closed” structures dominate for asphaltenes. N,N′-dimethyl-perylene bisimide dimer has a binding energy (BE) of up to 29 kcal/mol, while the more complex N,N′-(1-hexylheptyl)-perylene bisimide model is slightly less strongly bound (BE = 25 kcal/mol) since the large alkyl substituents restrict the ability of the extensive π-regions to overlap. Such conclusions cannot be drawn when methods that do not incorporate dispersion, e.g., the B3LYP density-functional, are used.