Study on oil-water interface self-assembly and ultra-low tension mechanism of betaine and internal olefin sulfonate mixed system

As a novel temperature-resistant and salt-tolerant surfactant, internal olefin sulfonate has attracted extensive attention in recent years. Nevertheless, to date, most studies on its oil-water interfacial properties have focused on the impact of reservoir environments, while research on the interfacial self-assembly of its mixed systems remains relatively few. In this paper, the interfacial self-assembly behavior of linear alkyl betaine (ASB) and internal olefin sulfonates (IOS-C₁₈, IOS-C₂₄) with total carbon number of branched alkyl of 18 and 24 was investigated using the spinning drop method, and the influence of oil phase properties was further investigated. The experimental results indicate that the branched structure of the hydrophobic group in internal olefin sulfonate exhibits a certain self-regulating capacity, which not only has a certain interfacial activity, but also can make up for the adsorption void of betaine molecules on the oil-water interface. Under optimal hydrophilic-lipophilic balance (HLB), the composite system can reduce the n-decane-water interfacial tension (IFT) to an ultra-low level. Meanwhile, the properties of the oil phase represent a key factor affecting the composition and structure of mixed adsorption film. The fractions in Shengli crude oil compete with surfactant molecules for adsorption; consequently, the oil-water IFT is high. In contrast, the small-molecule active components in Changqing crude oil can be mixed with the system, exhibiting low branching degree, and the IFT is low. The research on the structure and properties of mixed adsorption films provides a reliable theoretical basis for the field application of internal olefin sulfonate and its mixed system in high-temperature and high-salinity reservoirs.