Effect of double chain anionic surfactant on the dynamic interfacial tensions of betaine solutions

Abstract

To clarify the effect of the anionic surfactant with double alkyl chains on the interfacial tension (IFT) of betaine solutions, the IFT values of the sodium dialkyl sulfosuccinates (AOT) mixed with different betaines (alkyl sulphobetaine (ASB) and benzyl substituted alkyl sulphobetaine (BSB)) against n-alkanes and crude oil were detected by the spinning drop method. Additionally, molecular dynamics (MD) simulation was employed to verify the interaction mechanism between AOT and different betaine molecules. The results demonstrate that AOT molecules reach hydrophilic-lipophilic balance and form a relatively compacted interfacial film at the dodecane-water interface. Owing to the flexible double alkyl chains, AOT molecules possess a robust self-regulating ability at the oil–water interface. It is the self-regulating ability of AOT molecules and the electrostatic attraction between betaines and AOT molecules that achieve superior synergistic effects between AOT and betaine molecules with different molecular sizes. Besides, mixed adsorption and hydrophilic-lipophilic balance (HLB) reduce the IFT values of mixed systems containing AOT and different betaines effectively. The equilibrium IFT of the AOT-ASB system and the AOT-BSB system can be reduced to ultralow vales of 5.7 × 10⁻³ mN/m and 8.2 × 10⁻³ mN/m, respectively. At the crude oil–water interface, active components in crude oil compete with AOT molecules, which results in a significant increase in IFT values for AOT solution alone. Despite the different molecular sizes of ASB and BSB molecules, the flexible double alkyl chains of AOT molecules enable AOT to have synergistic effects with both ASB and BSB molecules at crude oil–water interface. Ultralow IFTs against crude oil can be obtained at optimized ratio of AOT to betaines, which is meaningful for the application of mixed solutions containing betaines and anionic surfactants in enhanced oil recovery.