Molecular dynamics study of model molecules resembling asphaltene-like structures in aqueous organic solvent systems

Molecular dynamics (MD) simulations were used to study the aggregation of model molecules representing some structural features of asphaltenes in different solvents, namely, water, toluene, and heptane. Three different model molecules with similar molecular weights were developed as representatives of continental- and archipelago-type structures of asphaltenes. Two types of continental models were employed, one containing no charged entities and another containing one anionic terminal carboxylic group on an aliphatic chain. Analysis of the aggregate structures in single pure solvents indicate stacking of the polyaromatic rings as the dominant structural feature of the aggregates. Simulations were then conducted to study aggregation and partitioning of these model molecules in binary mixtures of water and toluene. A key observation from these simulations was that uncharged molecules did not congregate at the toluene-water interface, whereas charged terminal groups had a distinct affinity for the toluene-water interface. In the presence of charged entities, the resulting aggregate had a complex three-dimensional structure containing stacked polyaromatic rings tethered to the interface by the anionic terminal groups.