Study on the polarity, solubility, and stacking characteristics of asphaltenes

Abstract

The structure and transformation of fused aromatic ring system in asphaltenes play an important role in the character of asphaltenes, and in step affect the properties of heavy oils. Polarity, solubility and structural characteristics of asphaltenes derived from Tahe atmospheric residue (THAR) and Tuo-826 heavy crude oil (Tuo-826) were analyzed for study of their internal relationship. A fractionation method was used to separate the asphaltenes into four sub-fractions, based on their solubility in the mixed solvent, for the study of different structural and physical–chemical properties, such as polarity, solubility, morphology, stacking characteristics, and mean structural parameters. Transmission electron microscope (TEM) observation can present the intuitive morphology of asphaltene molecules, and shows that the structure of asphaltenes is in local order as well as long range disorder. The analysis results showed that n-heptane asphaltenes of THAR and Tuo-826 had larger dipole moment values, larger fused aromatic ring systems, larger mean number of stacking layers, and less interlayer spacing between stacking layers than the corresponding n-pentane asphaltenes. The sub-fractions that were inclined to precipitate from the mixture of n-heptane and tetrahydrofuran had larger polarity and less solubility. From the first sub-fraction to the fourth sub-fraction, polarity, mean stacking numbers, and average layer size from the TEM images follow a gradual decrease. The structural parameters derived from TEM images could reflect the largest fused aromatic ring system in asphaltene molecule, yet the parameters derived from 1H NMR data reflected the mean message of poly-aromatic ring systems. The structural parameters derived from TEM images were more consistent with the polarity variation of sub-fractions than those derived from 1H NMR data, which indicates that the largest fused aromatic ring system will play a more important role in the stacking characteristics of asphaltene molecules than the mean parameters of poly-aromatic ring systems.