The size distributions of sulfur (S), vanadium (V), and nickel (Ni) compounds in four crude oils, two residues, and their saturate, aromatic, resin, and asphaltene (SARA) fractions were determined using gel permeation chromatography (GPC) coupled to inductively coupled plasma high-resolution mass spectrometry (ICP HR MS). The results show trimodal distributions of V, Ni, and S compounds in the crude oils and residues. V and Ni compounds are present in both resins and asphaltenes. Trimodal distributions are clearly apparent in the resins but not apparent in the asphaltenes. In the latter, the predominant compounds have a high molecular weight (HMW), even when the solution of asphaltenes is diluted by 40000-fold. In the resins, compounds with a medium molecular weight (MMW) were expected; however, HMW compounds were observed, indicating that nanoaggregates or large molecules exist in both the asphaltenes and resins. Low-molecular-weight (LMW) compounds are predominantly present in the resins and do not represent more than 22% of V and Ni present in crude oil. These compounds appear to have molecular weights similar to simple metalloporphyrins.

Asphaltenes are molecular structures that are composed of polyaromatic and polyheteroaromatic condensed nuclei, where benzene, pyridine, and thiophene rings are the smallest basic structural units. A deep understanding of the electronic features governing the interaction between these primary units is essential for subsequent rationalization of the nature of larger scale interand intramolecular interactions between asphaltenes, which could help to enrich the knowledge of why these compounds tend to aggregate and then to flocculate in oil operation processes. In this work, we study the intermolecular interaction potentials of benzeneÀbenzene, thiopheneÀthiophene, pyridineÀpyridine, benzeneÀpyridine, benzeneÀthiophene, and pyridineÀthiophene molecular complexes by using the self-consistent generalized gradient approximation density functional theory with the PerdewÀWang 91 functional (DFT/GGA PW91) in conjunction with the DNP double numerical basis set. In order to understand the dominant electronic interaction of these complexes in terms of the σÀπ and πÀπ electronic interactions, the three most important structural conformations (parallel, antiparallel, and T-shaped) were chosen. These calculations were performed with the DMol3Materials Studio 4.0 program. It was found that the results of the interaction energies calculated with DFT/GGA PW91 are consistent with those reported in the literature for benzene and thiophene dimers. To gain insight about the stability of the studied complexes, their molecular interaction polarizability was evaluated at the PW91PW91/6-31þG(d,p) level of theory. This property allowed us to explain the attraction and repulsion that occur in these dimer formation processes. These statements were also corroborated at the MP2 and MP4 levels of theory for benzene, pyridine, and thiophene homodimer parallel conformations. Additional CCSD/6-31þG(d,p) calculations were performed for the interaction energies of these dimers. To correlate these findings with the stability of crude oil, a real asphaltene and resin dimer complex structure was fully optimized at the PW91/DNP level. The results of interaction energy, which is in good agreement with the value predicted in the literature, and the large value of the interaction polarizability allowed us to explain the stability of these systems.

This paper describes the characterization of structural, energetic, and electric properties of the molecular complexes between Orinoco belt resins through the application of computational molecular mechanics (MM), semi-empirical parametrization (PM6), and density functional theory (DFT) (PW91 and HCTH) in conjunction with the double numeric and polarized (DNP) basis set. The resin sources for the studied molecules are Orinoco belt vacuum residues (Carabobo, Hamaca, Merey, Merey-Mesa, and Zuata). Molecular structures of these compounds were proposed from analysis of experimental characterization. The study of molecular interactions has shown that these resins are able to form stable van der Waals complexes, where their computed stability has a large dependence upon the applied theory level. A qualitative description of the formation of these complexes could be obtained with these methodologies. In particular, MM overestimates resin interaction energies when compared to PM6 and DFT (PW91 and HCTH) results. However, the HCTH/DNP approach leads to interaction energy values for resin−resin complexes that lie in the range from −2.39 to −7.09 kcal/mol, in better agreement with literature reports and chemical expectations than PW91/DNP interaction energy values. The stability of these complexes and the strength of the resin self-association can be rationalized considering their chemical nature and the induced electric properties (dipole moment and polarizabilities) by molecular interactions. Additionally, inclusion of dispersion in the DFT calculations of the resin molecular complexes improves the energetic pattern of the studied molecules significantly.

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