Molecular view of the asphaltene aggregation behavior in asphaltene-resin mixtures

The aggregation behavior of asphaltenes in asphaltene-resin mixtures within different host media is studied on the basis of the asphaltene/resin molecular structures and considering the embedding medium as a uniform background field. The approach employs models of molecular structures for both asphaltenes and resins (as derived from characterization data), and uses Molecular Mechanics (MM)/Molecular Dynamics (MD) calculations to predict the effect of the intermolecular interactions on the aggregation process as a function of composition and embedding medium. The peptizing behavior of resins is analyzed as a function of the ratio of resin to asphaltene molecules in each host medium by constructing the corresponding MD-generated radial distribution functions as well as the associated potentials of mean force (PMF). In all cases, the PMF presents repulsive barriers characteristic of aggregate systems showing a strong aggregation effect in a highly precipitant medium such as heptane. For an intermediate precipitant, as toluene, the results indicate the formation of stable asphaltene cores peptized by resins with no further association. Finally, it is shown that for a highly dispersive medium-such as pyridine-the system behaves as a typical solution. All calculations have been performed at T = 273 K. The results of this study point to the usefulness of MM/MD simulations as a complementary tool to understand the behavior of asphaltene-resin mixtures embedded in different host media.