Coarse-Grained Molecular Simulations to Investigate Asphaltenes at the Oil-Water Interface

摘要

In the present work, we investigate, by means of theoretical simulation, the preferred orientation of a model asphaltene molecule at the oil-water interface (monomer). The coarse-grained model molecules at the mesoscale level, using dissipative particle dynamics (DPD), are adopted. The central polycyclic aromatic hydrocarbon (PAH) core and the peripheral alkanes in the asphaltene are considered. The asphaltene model construction by coarse grain mapping is proposed and analyzed, as well as the effect of using different solubility parameters in the construction of the potential interaction of the beads in the coarse grained asphaltene model. Also, the effect of surface coverage for a structure where steric effects dominate is presented as well as the effect of asphaltene coarse-grain nanoaggregates at the oilwater region. Finally, the orientation at the oilwater interface of an asphaltene with peripheral oxygen moieties is studied. Toluene is used as a model of oil. Three different orientations of the asphaltene model are used as starting configurations: horizontal to the oil-water interface, perpendicular to the oilwater interface, and tilted 45 degrees with respect to the oil-water interface. In all cases, it is found that the asphaltene molecule stays at the oil-water interface with the preferred orientation where the aromatic region lays in the plane of the oil-water interface while the aliphatic chains are perpendicular to the oilwater interface and in the oil region. This molecular orientation remains in the case of higher asphaltene surface coverage. Due to steric hindrance, some of the asphaltenes migrate to the bulk and some remain at the oil-water interface. Asphaltene molecules in the bulk oil are found to interact by pp stacking interaction of the aromatic cores. For the case of the nanoaggregate, where the aromatic core is surrounded by alkyl chains, it is observed that the aggregate migrates to the bulk of the oil region; thus these nanoaggregates do not load onto the oil-water interface. For the case of the coarse grain asphaltene with peripheral oxygen moieties, it is found that the oxygen moieties orient in plane at the oil-water interface while the PAH orients out of plane and into the toluene region. These many findings are consistent with extensive experimental results as discussed. The combination of coarse grain and DPD dynamics, while maintaining the major structural characteristics in the coarse grain mapping of asphaltene species, represents a powerful tool that will help to answer questions about oil-water emulsions.