Sensitivity of the aggregation behaviour of asphaltenes to molecular weight and structure using molecular dynamics

Asphaltenes are heavy crude oil compounds, defined as soluble in toluene and precipitating in alkanes. To understand the relation between asphaltene structure and aggregation, we perform equilibrium molecular dynamics with Large-scale Atomic and Molecular Massively Parallel Software (LAMMPS), using the atomistic force field PCFF+ in the MedeA (R) environment. The following three molecular models are considered: the continental model (1350g/mol) that has a large polyaromatic core and long alkyl chains, the island model (780g/mol) that has a smaller polyaromatic unit and shorter chains and the archipelago model (1350g/mol) that has three polyaromatic nuclei bridged with alkyl chains. The aggregation in a given solvent is monitored by visualising solvent-free configurations over 15ns trajectories at 350K. Nanoaggregates are characterised by stacked polyaromatic units separated by 0.33-0.4nm. Irreversible aggregation is found with the continental model in both solvents. Aggregation of the island model is significant in n-heptane and low in toluene. The archipelago model does not aggregate significantly. Our results confirm that the island model is a reasonable average model of asphaltenes [Headen TF, Boek ES, Skipper NT. Energy Fuels 2009;23:1220-1229]. The open structure of nanoaggregates and the limited number of stacked molecules are also in agreement with previous interpretations of experimental data [Fenistein D. et al. Langmuir 1998;14:1013-1020].