Simulation of Asphaltene Aggregation through Molecular Dynamics: Insights and Limitations

We report classical atomistic molecular dynamics simulations of four structurally diverse model asphaltenes, a model resin, and their respective mixtures in toluene or heptane under ambient conditions. Relatively large systems (similar to 50 000 atoms) and long time scales (>80 ns) are explored. Wherever possible, comparisons are made to available experimental observations asserting the validity of the models. When the asphaltenes are dissolved in toluene, a continuous distribution of cluster sizes is observed with average aggregation number ranging between 3.6 and 5.6, monomers and dimers being the predominant species. As expected for mixtures in heptane, the asphaltene molecules tend to aggregate to form a segregated phase. There is no evidence of the distinct formation of nanoaggregates, and the distribution of clusters is found to be continuous in character. Analysis of the shape of the clusters of asphaltenes suggests that they are generally spherical in character, with the archipelago models favoring longer prolate structures and the continental model tending toward oblate structures. The aggregates are seen to be diffuse in nature, containing at least 50% solvent on average, being denser in heptane than in toluene. Mixtures of asphaltenes with different architecture are found to have cluster properties that are intermediate between those of the individual components. The presence of resins in the mixture does not appear to alter the shape of the asphaltene aggregates or their size or density when toluene is the solvent; on the other hand, the resins lead to an increase in the density of the resulting aggregates in heptane. Quantification of these observations is made from histograms of the cluster distributions, potential of mean force calculations, and an analysis of the shape factors. We illustrate how the time scales for complete aggregation of molecules in heptane are longer than the longest of the simulations reported in the open literature and as an example report a long simulation (0.5 mu s) that fails to reach an equilibrium state, suggesting that acceleration techniques (e.g., using coarse-grained models) are needed to appropriately explore these phenomena.