Impact of the\nHydrophobic Phase on the Interfacial\nDilational Rheology of Alkoxy Carboxylate/Cetyltrimethyl Ammonium\nChloride Mixtures

Despite extensive investigations on the interfacial activities\nof mixed anionic and cationic surfactants (S<sub>a/c</sub>), the influence\nof the hydrophobic phase on their interfacial assembly and dilational\nrheology remains unaddressed. In this study, the interfacial dilational\nrheology of alkoxy carboxylate (anionic)/cetyltrimethylammonium chloride\n(cationic) surfactant mixtures was studied at various interfaces.\nThe dilational modulus of S<sub>a/c</sub> increases linearly with\ninterfacial pressure at the interfaces of air, <i>n</i>-hexane/<i>n</i>-octane/<i>n</i>-hexadecane, and toluene. The\nlimit elasticity (ε<sub>0</sub>) is similar at air and alkane\ninterfaces but significantly decreases at the toluene interface. To\nexplain these phenomena, all-atom molecular simulation was carried\nout to investigate the microscopic features of surfactants at the\ninterface. The findings emphasize the crucial role of anionic/cationic\nsurfactant bound pairs in regulating interfacial rheology. S<sub>a/c</sub> tend to form large aggregates at the air/water surface. When mixed\nwith alkanes like octane, most S<sub>a/c</sub> remain as ion pairs.\nHowever, when toluene is employed, the coordination number between\nanionic and cationic surfactants sharply decreases due to π–π\ninteractions between the toluene molecules and the phenyl groups in\nthe anionic surfactant. This leads to a much lower interfacial modulus\nbecause interactions between oil molecules and surfactants cannot\ncompensate for weakened interactions among anionic/cationic surfactants.\nThese results suggest that S<sub>a/c</sub> in this study tolerate\nalkanes but are not resistant to aromatics, which helps explain why\nS<sub>a/c</sub> demonstrate excellent performance for the chemical\nenhanced oil recovery of a high-wax reservoir and further provides\nfundamental knowledge of their potential applications, such as gas\nwell deliquification using foamers in the presence of condensate oil,\ntextiles, etc.