Composition, microstructure and rheology of aqueous two-phase cationic/anionic surfactant systems

Aqueous systems containing 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (12-3-12) and sodium dodecyl sulfonate (AS) can separate into a concentrated phase and a dilute phase at certain conditions. The composition analysis demonstrates that the 12-3-12/AS/water system must be treated as a quaternary with four independent components, the equality relations between 2m12−3−122+2m12−3−122+ and mBr−mBr−, mAS−mAS− and mNa+mNa+, are not held in general for the two separated phases. Tie lines are only possible to be described in a three-dimensional phase diagram. Microstructures of some concentrated phases in the phase-separated systems were determined by negative stained TEM method. For the concentrated bottom phase in aqueous two-phase system (ATPS) with water or 0.10 mol kg−1 NaF brine as solvent, vesicles are easy to form. Whereas in 0.10 mol kg−1 NaCl brine, irregular aggregates appear in the concentrated top phase of ATPS. The rheological behavior was examined indicating that the systems studied exhibit thixotropic, antithixotropic, or mixed thixotropic and antithixotropic depending on the compositions and microstructures. Effects of added sodium halide have also been studied.