Solvent Nuances Modulate the Decreasing Effect of a Model Asphaltene on Interfacial Tension

摘要

In this work, a series of molecular dynamics simulations was performed to investigate the role of organic solvents in modulating the decreasing effect of a model asphaltene on interfacial tension (IFT). Pentane, pentol with a volume ratio of 50:50 (pentane/toluene), heptane, and heptol with a volume ratio of 56:44 (heptane/toluene) were adopted as the organic solvents; violanthrone-79 (VO-79), a widely tested compound, was chosen as the representative model for asphaltene. Our simulations revealed that, while VO-79 can reduce solvent/water IFT, the reduction magnitude of pentane/water IFT is smaller than that of heptane/water IFT. That is, despite the fact that the pentane/water IFT is lower than that of the heptane/water interface without VO-79, the presence of VO-79 reverses this trend, resulting in higher pentane/water IFT values compared to those of the heptane/water interfaces. Similar observations hold for the comparison between pentol/water and heptol/water IFT values. Detailed analysis on the interfacial and bulk properties of VO-79 suggests that, depending on the nature of the solvents, distinct mechanisms exist that are responsible for these observations. Pentane and heptane are poor solvents for VO-79, and compared to heptane, pentane can introduce even more unfavored solvation. That is, a stronger self-aggregation of VO-79 is observed at pentane/water interfaces, which could decrease their mobility and thus entropy, and ultimately reduce the ability to decrease solvent/water IFT. Contrarily, due to the presence of toluene in pentol and heptol, the migration of VO-79 into an organic solvent phase also affects its ability to decrease solvent/water IFT. Our results here demonstrate that solvent nuances can modulate the decreasing effect of model asphaltenes on IFT and suggest that precaution should be taken when using IFT reduction as an indicator for asphaltene adsorption extent.