Dissipative particle dynamics to study asphaltenes and surfactants interactions at the oil–water interface

Abstract

Heavy oil is an important unconventional oil resource, but its high viscosity and complex composition make them difficult to transport. Heavy oil-in-water emulsion transportation is considered to be one of the most effective methods. Asphaltenes in crude oil is a natural surfactant that is responsible for the stability of the emulsions. It is necessary to better understand the influence of asphaltenes and surfactants on the stability of the oil–water interface during the transportation of heavy oil. Although several studies have been carried out on the properties of asphaltenes at the oil–water interface, the scientific understanding of asphaltenes in reducing the interfacial tension (IFT) is still limited in terms of simulation studies. In this paper, asphaltenes were extracted and characterized, and the molecular structure of asphaltenes was constructed. Dissipative particle dynamics (DPD) simulations are employed to study the distributions and interfacial behaviors of asphaltenes and surfactants in oil–water systems. The coarse-grained model of asphaltenes is constructed from basic structural fragments in the simulations, which is based on the inferred average molecular structure of asphaltenes. The results show that asphaltenes diffuse to the oil–water interface spontaneously and adsorb at the interface to form an interfacial film, which could reduce the IFT. However, the IFT is not controlled by the volume concentration of asphaltenes, but is directly related to the interfacial concentration. The IFT value of the oil–water system decreases by up to approximately 8 mN/m at an asphaltene concentration of 133 mM. In addition, the interfacial film formed by the synergistic adsorption of surfactants and asphaltenes has a tighter configuration, which further reduces the IFT and enhances the stability of emulsions. Moreover, the effect of surfactant head length on IFT of the oil–water interface was investigated in this study. The outcomes from this paper can help for a better understanding of asphaltenes and surfactants in stabilizing the oil–water interface and provide useful information for heavy oil transportation.