Molecular Dynamics Simulation Of Self-Aggregation Behavior Of Model Asphaltene Molecules With Polycyclic Aromatic Cores

In the past decades,asphaltene deposition has been affecting the extraction and transportation of crude oil and has brought cost increases in the petroleum industry.Studying self-aggregation of asphaltene molecules is of great significance for optimizing oil extraction,transportation and refining.Molecular modeling as a useful complement to experiments has been adopted by more and more researchers.In this work,polyaromatic asphaltene system is used to mimic the asphaltene molecules,and three research contents are includes by employthy molecular dynamics simulation:the first is the aggregation behavior of asphaltene molecules in the vacuum,the second is of the aggregation behavior of asphaltene aggregates in toluene solvent,and the third is the aggregation behavior of randomly distributed asphaltene molecules in toluene solvent.Firstly,the aggregation behaviors of nine model asphaltene molecules were investigated by atomistic molecular dynamic(MD)simulations at evaluated temperatures.The effects of aromatic core size,length and number of alkyl side chains on the assembly structures were probed.It is found that the ?-? interaction is a primary driving force that leads to association of polycyclic aromatic asphaltene molecules.Depending on the size of the aromatic nucleus,the asphaltene molecules formed different sized and shaped molecular aggregates.The larger the size of the polycyclic aromatic cores,the higher stability of ?-? stacking configuration.With the increase of the size of polycyclic rings,the formation of stable highly crystalline phase is found even at the evaluated temperature.Besides the alkyl side chains affect the aggregation behaviors of polycyclic asphaltene molecules significantly.In the present study,two kinds of 'phase-separated'stacking configurations are found.The model asphaltene molecules containing a polycyclic aromatic nucleus plus two evenly distributed alkyl side chains show a novel 'phase separated lamellar structure'(PSLS),in which the polycyclic nucleus formed stable monolayer stacking structure and the monolayers are separated by the alkyl side chains.For the model asphaltene molecules containing a central aromatic nucles and five evenly distributed alkyl side chains,the molecules aggregate into rod like structures.In each rod-like aggregate,long range ordered face-to-face ?-? stacking structure is clearly found.Seconedly,the influence of toluene solvents on asphaltene aggregates was discussed.After adding low concentrations of the toluene solution,the toluene molecules did not insert the layer formed by the asphaltene molecules and cannot destroy the ?-? interaction between them,but they moved rapidly at the boundary of aggregates,increasing the distance between the aggregates.Finally,the self-organization of five model side-chain decorated polyaromatic asphaltene molecules in toluene solvent was investigated by means of the atomistic molecular dynamic(MD)simulations.The PSLS and PSCS also maintain in the presence of small amount of toluene additive(30 wt%).However adding excess toluene molecules,the asphaltene molecules form highly dispersed nano-aggregates.The dynamic properties of ?-? stacking structures in the PSLS,PSCS as well as the nano-aggregates were probed.It was found that the number and size of alkyl side chains significanly impact the size and number of ?-? stacking structures in the aggregates.Through tracking the structure evolution of the nano-aggregates,a possible dissociation mechanism of nano-aggregate is suggested.