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 at ambient conditions. Relatively large systems (~50,000 atoms) and long timescales (> 80 ns) are explored. Where ever possible , comparisons are made to available experimental observations asserting the validity of the models. W hen the asphaltenes are dissolved in toluene, a continuous distribut ion 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 a distinct formation of nanoaggregates, the distributions of clusters is found to be continuous in character. The analysis of the shape of the clusters of asphaltenes suggests that they are generally spherical in character, with the archipelago models favouring longer prolate structures and the continental model tending towards 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. M ixtures 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 a lter the shape of the asphaltene aggregate s , 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 the hi stograms of cluster distributions, the p otential 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 larger than the longest of the simulations reported in the open literature and as an example report a long simulation (0.5 μs) which fails to reach an equilibrium state , suggesting that acceleration techniques , for example , using coarse grained models , are needed to appropriately explore these phenomena.