Investigation on the phase behaviors of aqueous surfactant two-phase systems in a mixture of N-dodecyl-N-methylpiperidinium bromide (C12MDB) and sodium dodecyl sulfate (SDS)

•The formation mechanism of ASTP is proposed.•The cationic ionic liquid C12MDB was synthesized and employed to form the ASTP.•The phase diagram for C12MDB/SDS aqueous solutions was mapped.•The driving forces to form vesicles were studied.

To study the phase behavior and formation mechanism of an aqueous surfactant two-phase system (ASTP), which was composed of a cationic surfactant N-dodecyl-N-methylpiperidinium bromide (C12MDB) and an anionic surfactant sodium dodecyl sulfate (SDS), an investigation was conducted. The phase diagram for C12MDB/SDS aqueous solutions was mapped, indicating that the ASTP forms between the homogeneous and heterogeneous phase regions in a narrow concentration range. By measurements of surface tension, transmission electron microscopy, dynamic light scattering, and zeta potential, the formation mechanism of the ASTP was proposed, which is attributed to the aggregation of vesicles. Different aggregation behaviors of vesicles lead to phase separation and the formation of the ASTP.

Graphical abstractThe phase diagram for C12MDB/SDS aqueous solutions was mapped, and the formation mechanism of ASTP is proposed. Firstly, vesicles are formed in the solution driven by the electrostatic attractions and hydrophobic interactions. Then, in the upper phase, vesicles aggregate together forming bigger vesicles, which distribute sparsely. While in the lower phase, vesicles are smaller and densely distributed. Different aggregation behaviors of vesicles lead to the formation of the C12MDB/SDS ASTP.Figure optionsDownload full-size imageDownload as PowerPoint slide