A study of asphaltene solubility and aggregation due to sulfur heteroatoms: molecular dynamics simulation

Context

This study describes the molecular dynamics (MD) simulations to investigate the effects of sulfur atom position on the solubility and aggregation properties of asphaltenes extracted from Zaghli crude oil (East Azerbaijan). Two different asphaltene models were studied, i.e., A1 with sulfur in the aromatic ring and A2 with sulfur in the aliphatic side chain. The radial distribution function (RDF) analysis revealed significant differences in aggregation trends. A1 exhibited rapid aggregation in both solvents used in the experiments, as evidenced by a significant decrease in the radius of gyration from 0.6 to 0.4 Å in octane and from 0.8 to 0.5 Å in xylene. In contrast, A2 showed increased solubility; especially in xylene with a marked increase in the radius of gyration from 0.5 to 2 Å. Furthermore, the energy analysis confirmed these results, i.e., A2 exhibited a higher total energy (451.16 kcal/mol) than A1 (221.28 kcal/mol); indicating a more energetically favorable and less aggregated state of A2. These computational results open up new possibilities for understanding the critical role of the sulfur atom position in the asphaltene structure on its aggregation propensity, which can be used to prevent asphaltene-related problems in the petroleum industry. The work is very useful in the oil industry for enhancing oil production by studying asphaltene solubility and aggregation.

Methods

MD simulations were performed using the COMPASS force field and Material Studio V.6 2017 software to evaluate the solubility of asphaltenes in octane and xylene solvents. Geometric optimization was carried out to address unstable interactions, with periodic boundary conditions applied. Simulations were conducted in the NVT ensemble at 298 K and 1 atm pressure, using a time step of 1 fs, a Nose thermostat for temperature control, and a Berendsen thermostat for pressure control. RDF analysis was utilized to examine the behavior of two distinct asphaltene models, which differed in the positioning of the sulfur atom, in the solvents. The total energy contributions, including van der Waals and electrostatic interactions, were also analyzed.