Molecular dynamics simulation study on interfacial behaviors of betaines and extended surfactants

The mixed system between different surfactants has characteristic and synergy to reduce the oil/water interfacial tension (IFT). As the reservoir conditions become more and more severe, it is necessary to explore new surfactants suitable for complex geological environments. In this work, we designed the mixed systems between betaine and extended surfactants, and studied the interfacial properties at the decane/water interface by molecular dynamics (MD) simulations and interfacial tension measurements. The effects of betaine structure and oxypropylene (PO) chain length of extended surfactant on the interfacial behaviors were analyzed in detail from the density profile, tilt angle, average rising height of PO unit and radial distribution function. The results showed that the polar part of betaine extended into the water phase and the hydrophobic chain was more upright in the mixed system due to the electrostatic interaction. For the mixed system, the rise height change of the PO unit indicated that the interfacial behavior of short PO chain was controlled by betaine structure, while that of long PO chain was independent of betaine structure because of the poor deformation ability of it. IFT experiments showed that xylyl substituted alkyl sulfobetaine (XSB) mixing with the short PO chain extended surfactant system had a significant synergistic effect to produce ultra-low IFT but had a negative effect with the long PO chain extended surfactant. This work demonstrated that the steric hindrance of betaine and the PO chain length of extended surfactant synergistically affected the interfacial arrangement and IFT of surfactants, which was of significance for improving and designing more effective surfactant flooding systems to enhance oil recovery.