Synergism for lowering interfacial tensions between betaines and extended surfactants: The role of self-regulating molecular size

The synergism for reducing interfacial tensions (IFTs) between betaines and anionic extended surfactants with self-regulating ability was validated by experiments and molecular dynamics simulation. The IFTs of different betaines and different alkyl polypropylene ether sulfonates (APS) mixed solutions against decane and crude oil were investigated via the spinning drop method. The results demonstrate that the IFTs order of the three betaines is EDAS > ASB > XSB, which is consistent with their interfacial vacancies. Moreover, the interfacial activity of APS surfactants increases significantly and the IFTs decrease notably with the increase of propylene oxide (PO) numbers. For the betaines and APS surfactants mixed systems, the determining factor to reduce IFTs is not hydrophilic-lipophilic balance (HLB) but “size compatibility”. APS molecules can achieve a superior synergistic effect with different betaines by electrostatic attraction and size compatibility. The self-regulating characteristic of the flexible PO group enables the extended surfactants to exhibit certain synergistic effects with different betaines. What’s more, betaines and APS surfactants have a certain synergistic effect to reduce IFT values during a wide concentration ratio against crude oil. The findings of this study can help for better understanding of mixed surfactant solutions in enhanced oil recovery.