Theoretical insight into the effect of polar organic molecules on heptane-water interfacial properties using molecular dynamic simulation

Hydrocarbon-water interface plays an important role in many cases and industries. However, the effect of polar oil molecules on hydrocarbon-water interfacial properties is still unclear. In this work, 9 molecular scale heptane-water interface models containing different polar oil molecules were established by molecular dynamic simulation to study the effect of polar oil molecules on hydrocarbon-water interface properties. Interfacial tension (IFT), molecular configuration, molecular orientation and interaction energy between oil and water were calculated to quantitatively characterize the heptane-water interfacial properties containing different oil components. The IFT results show that IFT decreases as the molecular polarity increases. The molecular configuration results indicate that the stronger polar oil molecules tend to adsorb to the interface while the weaker polar oil molecules tend to stay in the oil bulk space. The molecular orientation shows that the polar oil molecules possess more orderly regularity at the interface while nonpolar oil molecules can only randomly adsorb at the interface. Specifically speaking, chain polar oil molecules tend to be perpendicular to the interface while the ring structure polar oil molecules tend to be parallel to the interface. The ring oil molecules containing branched functional groups should also tend to be perpendicular to the interface under the influence of the branches. The heptane-water interaction energy results indicate that the interaction energy increases as the molecular polarity increases. Furthermore, the electrostatic energy predominates over van der Waals energy when the oil contains polar oil molecules while van der Waals energy predominates over electrostatic energy when the oil contains nonpolar oil molecules. The conclusions in this work should provide some fundamental understanding of interfacial information and some industries involving hydrocarbon-water interface.