Aggregation and Partitioning of Model Asphaltenes at Toluene−Water Interfaces: Molecular Dynamics Simulations

Molecular dynamics (MD) simulations were used to study the nanoaggregation of model asphaltene molecules in binary mixtures of toluene and water. Four types of model asphaltenes were studied: continental (C), Violanthrone-79 (VO-79), anionic continental (AC), and thiophenic anionic continental (TAC). Of these, the first two were uncharged, whereas the AC and TAC contained a side chain with a carboxylic terminal group. In all simulations, the model asphaltenes partitioned completely into the toluene phase of the phase-separated solvent mixture. The asphaltene molecules containing charged terminal groups remained tethered to the toluene−water interface (interface-bound), whereas uncharged asphaltenes aggregated within the central region of the toluene layer (core-bound). The stacked polyaromatic rings of the asphaltene aggregates showed a distinct preference to incline almost perpendicular to the toluene−water interface. Analysis of the mean square displacements of the asphaltenes reveals that for both charged and uncharged model asphaltenes, the lateral diffusion parallel to the toluene−water interface is dominant, whereas the transverse diffusion (normal to the interface) becomes significantly suppressed. This observation suggests that in thin films of toluene trapped between two aqueous phases, both interface-bound and core-bound asphaltenes, although subjected to vastly different interaction scenarios, exhibit similar diffusion behavior.