Predicting Asphaltene Aggregate Structure from Molecular Dynamics Simulation: Comparison to Neutron Total Scattering Data

Molecular dynamics (MD) simulation is quickly growing in popularity as a technique for understanding asphaltene aggregate structure and dynamics. However, verification of the results of simulations against experimental data has, to date, been sparse. Here, we present total scattering data from Athabasca asphaltenes, as both a solid and dispersed at high concentrations in deuterated 1-methylnaphthalene. The advantage of total scattering is that the expected scattering can be calculated from knowledge of the atomic positions in the system of question, meaning that simulation and experiment can be directly compared. We find that the MD simulations for model monodisperse systems reproduce the general form of the scattering curves well, particularly for the slope and shape for the small-angle scattering curve of dispersed asphaltenes. However, we find a number of limitations in the MD techniques as commonly used in the literature; specifically, the size of the aggregates formed is considerably smaller than observed from the scattering data. We identify two main causes of this discrepancy, namely, the limited box size that can be reasonably simulated and the lack of molecular polydispersity.