Molecular perspectives on interaction and replacement of surfactant to asphaltene at oil–water interface

The efficiency of reservoir development, oil–water separation, mixed transportation, as well as leaked oil treatment hinges on the ability to regulate oil–water interfaces. Most research has overlooked the impact of active substances (especially asphaltenes) in oil that have already occupied the oil–water interface. In this study, the influence of surfactant type on the replace adsorption for asphaltene is first analyzed through molecular dynamics simulation. Then, the effects of temperature, surfactant structure, and concentration on replace adsorption are studied. Finally, the replace adsorption mechanisms of surfactants for the case of asphaltene are analyzed. It is found that anionic surfactants (sodium dodecyl sulfate, SDS; fatty alcohol primary alcohol ethoxylate ether, AS) exhibit stronger replace adsorption ability than cationic surfactants (dodecyl trimethyl ammonium bromide). Interfacial tension (IFT) decreased by 62.95% and interfacial thickness increased by 28.65% when the concentration of SDS increased from 5 to 20 wt. %. This phenomenon occurs because surfactants at high concentrations have a stronger tendency to arrange themselves in an orderly manner at interface. This orderly arrangement also aids in moving asphaltene into oil phase, resulting in the creation of a dynamic recombination layer at interface. AS stands out because it contains ethylene oxide (EO), which gives its molecules a distinctive and unique structural shape. The number of hydrogen bonds increases by 30.7% and the interfacial thickness is increased by 14.5% when the number of EO is increased from 3 to 8. Moreover, temperature exhibits a dual regulatory effect on interfacial behavior. Interfacial thickness increases by 38.4% and IFT decreases by 41.54% when temperature increases from 313 to 373 K. This is attributed to the disorder conformation of surfactant due to intensified thermal motion and enhanced π–π stacking of asphaltene. The dispersion of the surfactant to a π–π stacking structure can be divided into two stages. The surfactant first encapsulates the π–π stacking structure to achieve separation from interface. Then, the stacking structures are disrupted through hydrogen bond between the surfactant and heteroatoms of asphaltene to realize dispersion.