Evidence for Asphaltene Nanoaggregation in Toluene and Heptane from Molecular Dynamics Simulations

Molecular dynamics simulation techniques have been used to study the nanoaggregation of one resin and two asphaltene structures, generated by an updated quantitative molecular representation (QMR) technique (Boek, E. S.; Yakovlev, D. S.; Headen, T. F. Energy Fuels, manuscript submitted), in toluene and heptane. Analysis of the simulation trajectories, to yield the separation of asphaltene or resin pairs, over a 20 ns simulation has been used to investigate aggregation dynamics. The structure of aggregates has been investigated by the calculation of the asphaltene-asphaltene and resin-resin radial distribution functions, g(r), and the average angle between the polyaromatic planes as a function of separation. We calculate, for the first time, the asphaltene-asphaltene potential of mean force (PMF) from the g(r) and separately by a constraint force method. In general, it is observed that the asphaltenes form dimers and trimers in both toluene and heptane. Once formed, the dimers and trimers can separate and reform other aggregates with other asphaltene molecules. Aggregates persist for longer in heptane than in toluene. The resin molecule forms no aggregates in toluene, with some aggregation occurring in heptane. Significant peaks in the asphaltene-asphaltene g(r) are seen in both toluene and heptane between 0.5 and I nm separation. This is strong evidence for asphaltene nanoaggregation from molecular dynamics simulations. At the lowest separations, the angle between the aromatic planes is close to parallel. The calculated potential of the mean force gives similar results for both methods. For the asphaltene molecules in both toluene and heptane, free energy of dimer formation ranged from -6.6 to -12.1 kJ mol(-1).