Interfacial Tensions of Extended Surfactant Solutions: Effect of Alkane Chain Length on the Size of Propylene Oxide Groups

Abstract To explore the mechanism for lowering interfacial tension by extended surfactants, the interfacial tension values of extended surfactants (S–C13POnS and G-C16POnS) were measured using the spinning drop method. Additionally, molecular dynamics simulation was employed to validate the interfacial interaction mechanisms of these extended surfactants. The results indicate that both hydrophilic-lipophilic balance (HLB) and size compatibility determine the interfacial tension of linear extended surfactants (S–C13POnS). As NaCl concentration increases, the influence of oil chain length on size compatibility competes with its effect on HLB. When the PO chain is short, the oil chain length affects both HLB and the interfacial size of S–C13POnS. Consequently, the nmin values of S–C13POnS shift toward n-alkanes with lower carbon numbers as NaCl concentration increases. When the PO chain is long, PO chains are less affected by oil chain length, and HLB plays a dominant role in reducing interfacial tension. Therefore, the nmin values of S–C13POnS shift toward n-alkanes with higher carbon numbers as NaCl concentration increases. At low salt concentrations, size compatibility governs the interfacial tension of S–C13POnS, whereas at high salt concentrations, HLB becomes the determining factor. For extended surfactants with long branched chains (G-C16POnS), the size of the hydrophobic group is mainly determined by the branched alkyl chain rather than the PO group. Accordingly, the oil chain length has little effect on the interfacial size of G-C16POnS molecules. Consequently, the nmin values of G-C16POnS shift toward higher carbon numbers as NaCl concentration increases. The unique interfacial mechanism of extended surfactants provides a theoretical basis for designing high-performance extended surfactants for specific reservoir conditions.