Molecular Dynamics Simulations of Asphaltenes at the Oil-Water Interface: From Nanoaggregation to Thin-Film Formation

We have investigated the interfacial behavior of asphaltene molecules at the oil water interface using molecular dynamics simulations. Oil precipitants and solvents are represented by heptane and toluene, respectively. It was found that asphaltenes are preferably distributed in the oil phase in the case of pure toluene, whereas they accumulate at the oil-water interface for pure heptane. Interestingly, the interfacial tension (IFT) of the interfacial system containing a small amount of asphaltene molecules is close to that of a pure heptane-water system, while the IFT of the system containing a large amount of asphaltene molecules is much reduced, similar to 12 mN/m. Further, it was shown that the reduced IFT results from a complete asphaltene film formed at the oil-water interface when asphaltenes are abundant. In addition, it was found that a small amount of asphaltene molecules stacked their aromatic planes and formed a nanoscale aggregate, which exhibited an exotic molecular oscillation behavior of asphaltene molecules at the oil-water interface.