The effect of polyoxyethylene group number on the interfacial tension of short carbon chain extended surfactant and aromatic alkyl betaine compound system

As a new type of chemical flooding formula, the short carbon chain extended surfactant has attracted wide attention in recent years. However, existing research has primarily focused on the interfacial self-assembly and film construction of single-component systems, with relatively little attention paid to the interfacial properties of compound systems. In this paper, the effect of the number of polyoxyethylene (EO) groups on the oil-water interfacial tension (IFT) of a compound system consisting of short carbon chain extended surfactants (C₈P₁₅E_xC, x = 5, 10, 15) and aromatic alkyl sulfobetaine (XSB) was investigated using the rotating drop method. The experimental results demonstrate that the interfacial tension between different types of oil phase and water can be effectively reduced to ultra-low levels (< 10⁻² mN/m) by adjusting the surfactant ratio, and the oil phase properties play a critical role in the interfacial activity of the compound system. When the oil phase is n-alkanes, the IFT of the system is predominantly influenced by the short carbon chain extended surfactants, and it increases with the increase of EO groups, the ultralow IFT is attributed to the appropriate hydrophilic-lipophilic balance. When the oil phase is crude oil, the IFT of the system is dominated by betaine surfactant and decreases with the increase of EO, at this time, the ultralow IFT is due to the appropriate compounding ratio. The proposed mechanism and structure-activity relationship provide valuable insights and support for the application of short carbon chain extended surfactants in high temperature-high salinity reservoirs.