Asphaltene, the heaviest, densest, and most polar component in petroleum, is classified as a complex mixture of heavy organic molecules that are insoluble in n-alkanes, including n-heptane and n-pentane soluble in aromatic solvents, such as toluene and benzene1–3. The viscosity of heavy oil is closely related to its asphaltene content, and the high concentration of asphaltene is the main reason for the high viscosity of heavy oil4, 5. In addition, the aggregation behavior of asphaltene molecules is responsible for the deposition, emulsification, and high viscosity of heavy oil, which significantly affect its utilization and value.
Asphaltenes are complex mixtures of polycyclic aromatic hydrocarbons substituted with alkyl side chains and heteroatoms, such as nitrogen, sulfur, and oxygen, which are typical substituents in conjugated cores6, 7 In general, asphaltene molecules have a polycyclic aromatic core that is composed of approximately 4–10 aromatic rings and fatty side chains with a length of 3–7 carbon atoms8, 9. In the past decade, various research groups have proposed and studied different asphaltene molecular models to study the mechanism behind asphaltene properties. Sjöblom10 reviewed the different types of proposed asphaltene models and summarized the composition and properties of asphaltenes and the research methods and results for different types of model asphaltene compounds.
During the past few decades, people have been exploring the mechanism of asphaltene aggregation. In 1990, Hunter and Saunders11 summarized three rules for porphyrins: (1) π-π repulsion dominates the face-to-face geometry; (2) π-σ attraction dominates the side geometry; and (3) σ-σ attraction dominates the offset π-stacking geometry. Three stacking modes are shown in Fig. 1. Pacheco12 employed classical molecular dynamics (MD) to simulate asphaltene aggregation under vacuum at different temperatures and concluded that the aggregation behavior of asphaltene molecular dimers could also follow these rules.
Takanohashi13 used MD simulation to study the stability of asphaltene aggregates of three model molecules at high temperatures and observed that aliphatic side chains and heteroatomic functional groups contribute to the stability of trimers. Rogel14 used the average structure model to study the interaction force of the binding process between asphaltene and resin. The results indicate that the stabilization energy obtained by asphaltene and resin is due mainly to the van der Waals forces between molecules. In contrast, the contribution of hydrogen bonds is low. In addition, some researchers have studied the relationship between asphaltene molecular structure and aggregation behavior 1, 15, 16.
Because of its many sources, asphaltene has a complex and diverse molecular structure, which has caused controversy for many years4, 17. Schuler18 utilized atomic force microscopy (AFM) and scanning tunneling microscopy (STM) to perform molecular orbital imaging for more than 100 asphaltene molecules and proposed that the molecular structure of asphaltene is an aromatic core, substituted with heteroatoms and a variable number of alkyl side chains. Sedghi1 employed MD simulations to investigate the relationship between the aggregation and molecular structure of asphaltene. The results indicate that the interaction between aromatic nuclei of asphaltene molecules is the main driving force of asphaltene aggregation. Furthermore, Jian16 carried out a series of MD simulations to study the effect of aliphatic side chain length on the aggregation behavior of asphaltenes. The degree of aggregation is not monotonically related to the side chain length. Asphaltene molecules with short or long side chains can form dense aggregates, while those with medium-length side chains cannot. Ekramipooy19 employed density functional theory and MD simulations to study the effect of heteroatoms on the aggregation of model asphaltenes. The results show that heteroatoms in the fatty side-chain more effectively increase the aggregation. The heteroatoms in the middle of the fat side chain strengthen the CH…C dispersion interaction through carbon polarization.
Although the studies on asphaltene viscosity and asphaltene aggregation behavior alone have been numerous and intensive, there are few studies on the relationship between the two, which is an important research direction needed to understand the microscopic mechanism of asphaltene molecules with different viscosities, so this is one of the focuses of this study. In addition, most of these studies14, 19 perform indirect characterization by calculating radial distribution functions and numerical distribution of interatomic distances/angles to study the asphaltene interactions. Yang20 proposed a visual method to study weak interactions as a direct characterization tool, referred to as the reduced density gradient (RDG) or noncovalent interaction (NCI) method. The so-called weak interaction refers to various forms of interaction whose strength is weaker than covalent chemical bonds, such as van der Waals interactions, π-π stacking, hydrogen bonds, halogen bonds, and dihydrogen bonds. To further identify the characteristics of the interactions between asphaltene molecules in a dimer, Wang2 carried out NCI visualization analysis, focusing on the intermolecular interactions in the asphaltene dimer, and screened the intramolecular interaction of asphaltene, concluding that π-π interactions are the main driving force of asphaltene aggregation. Ekramipooya19 adopted the RDG method to study the effect of heteroatoms on self-aggregation at different positions in a model of the asphaltene structure. By comparing the relationship between the RDG and sign (λ 2) ρ of different asphaltene dimer models, Ekramipooya concluded that the heteroatoms at the X3 position (in the middle of the fat side chain) strengthened the CH…C dispersion interaction through carbon polarization. Yang21 proposed the averaged RDG (aRDG) method (also known as the aNCI method), which is an extension of the original RDG method that can be used to analyze multi-frame structures, especially in combination with MD simulation techniques, to study weak interactions in equilibrium dynamic environments. Wu22 investigated the aggregation behavior of three reactive dyes in water by MD simulations. The dye-water molecular interactions were analyzed by the RDG and aRDG methods, and the position, intensity, and type of the interactions were visualized. The results show that the main interactions between the dye and anions are van der Waals and π-π stacking interactions.
The aRDG method can be employed to study average weak interactions between small molecules in kinetic processes and receptor-protein, small peptide-small peptide, and molecule-solid surface cases23–26. At present, although MD simulations have been widely employed to investigate the molecular structure and aggregation behavior of asphaltenes, few studies have used the RDG method, especially the aRDG method, to examine the weak interactions between asphaltene molecules. Schuler18 obtained the fundamental structures of some asphaltene molecules by using AFM. In this study, three representative structures of asphaltene molecules were selected and optimized appropriately.
MD simulations obtained the spatial aggregation morphology of three asphaltene molecules. Their shear viscosity was calculated by non-equilibrium molecular dynamics (NEMD) simulations to study the relationship among asphaltene molecular structure, viscosity, and aggregation behavior. Finally, the weak intermolecular interactions and their stability in the process of asphaltene aggregation were visualized by aRDG and TFI methods.