Application of the Y-Rule Aromaticity and Structural–Topological Analysis to Understand the Effect of an External Magnetic Field on the Aggregation of Oil Asphaltenes

Asphaltenes play a major role in oil–water emulsion stability, aggregation, deposition, viscosity, and change of wettability to oil-wet; thus, a deep understanding of the interaction of an external magnetic field and asphaltenes is of utter importance to developing technologies friendly with the environment based on the application of magnetic fields to manage asphaltene-related issues in the oil industry. In spite of the studies reported in the literature of the effects of the magnetic field on crude oil and asphaltenes, the mechanisms in which the magnetic field interacts with asphaltenes are not completely understood. In the present contribution, the effect of an external magnetic field on the aggregation of three different asphaltenes (α-, β-, and γ-asphaltenes) extracted from light, medium, and heavy crude oils is studied experimentally and theoretically. The average chemical structures of the three asphaltenes and their structural isomers, which compose the polydispersity, are obtained through experimental analysis. Solutions in toluene (aromatic medium) of the three asphaltenes are placed under the effect of a constant external magnetic field of 0.1 T strength at a constant temperature of 30 °C, and the effect of the magnetic field on the aggregation of the asphaltene solutions is followed with fluorescence emission measurement at different times of magnetization for a total of 24 h. All of the fluorescence emission measurements under the magnetic field were performed in triplicate, and the error obtained is 1%. The magnetic field uniformity and stability were checked in each fluorescence emission measurement using a high-precision magnetic field meter. The analysis of the effect of the magnetic field on the aggregation is carried out by the rationalization of the average electronic density distribution in the polycyclic aromatic hydrocarbon (PAH) core of the asphaltenes and their structural topology using the Y-rule aromaticity, the percentage of electrons in π-aromatic sextets, Y-rule mapping, and topological–structural parameters in the different asphaltenes. All the PAH average asphaltene structures were analyzed by their HOMO–LUMO energy gap, which falls within the reported experimental gap; the most predominant number of fused aromatic rings (nFAR); the π-electronic distribution in resonant sextets (NR) obtained with the Y-rule aromaticity; the percentage of aromatic sextet (%NR); and Y-rule mapping (%NR/nFAR). It is found that the magnetic field strongly propitiates the aggregation of asphaltenes that contain a higher percentage of aromatic sextets, %NR = (total aromatic π-sextet electron/total π-electrons) × 100, in the distribution of the electronic density and a higher HOMO–LUMO energy gap. Density functional theory calculations of the magnetic susceptibility or magnetizability of the asphaltenes show that the physical origin of the magnetizability is the induced currents within the asphaltene molecules and that, under the effect of the external magnetic field, asphaltene molecules reorient with the smallest magnetizability component aligned antiparallel along the direction of the external magnetic field. The γ-asphaltenes present the highest magnetic susceptibility (χiso) and magnetic anisotropy (Δχan), but due to their large size (11FAR), it is more difficult for the γ-asphaltenes’ magnetic dipole to reorient. The reorientation of the asphaltenes position them face-to-face or displaced face-to-face; thus, facilitating their aggregation. The aggregation quenches the λ0–0 band in the fluorescence emission spectrum. This effect is more pronounced for the α-asphaltenes, which are less bulky and less heavy (9FAR). The β-asphaltenes are the least magnetizable, the smallest (7FAR), and the ones with the smallest %NR. The heteroatoms in asphaltenes influence their behavior under magnetic fields by introducing lone pairs and partial localized charges that contribute to the magnetic susceptibility.