Salt effect on the rheological properties of the aqueous mixed cationic and anionic surfactant systems

The effect of NaBr on the aqueous two-phase regions of 0.10 mol kg−1 cetyltrimethylammonium bromide (CTAB)/sodium dodecyl sulfonate (AS)/H2O mixed system at 318.15 K has been investigated. The aqueous two-phase region with excess AS or the aqueous two-phase region with excess CTAB all moves far from the equimolar line with the addition of NaBr. The effect of NaBr on the shear viscosities of the systems at the two isotropic single-phase regions has been also studied. At each isotropic single-phase region, one shear viscosity peak was observed for each mNaBr. The addition of NaBr to the mixed CTAB/AS/H2O systems shifts the viscosity peaks away from the equimolar line, has stronger effect on the viscosity peak at the isotropic single-phase region with excess CTAB, and weaker effect on that with excess AS. Seven 0.10 mol kg−1 CTAB/AS/H2O isotropic systems adjacent to the viscosity peaks were chosen to investigate the effect of NaBr on the shear viscosity. The rheological behaviors as well as the shear viscosity values of the seven chosen systems all change with the addition of NaBr. Salt-induced aqueous two-phase systems can be obtained with the addition of NaBr to the seven chosen systems. Just before the salt-induced aqueous two-phase separation was observed, the common tendency of the seven chosen systems is the decreases of the shear viscosities with the addition of NaBr, and those isotropic systems with NaBr aqueous solutions as solvents and very close to the phase boundaries are all thixotropic fluids. These results indicate that the salt-induced aqueous two-phase systems originate from thixotropic isotropic systems.