Phase behavior and viscous properties of cetyltrimethylammonium bromide and sodium dodecyl sulfonate aqueous mixtures

Phase behavior of cetyltrimethylammonium bromide (CTAB)/sodium dodecyl sulfonate (AS)/water systems at 318.15 K with the total surfactant mole fraction xT ≤ 0.020 was investigated. In addition to the seven phase regions in the lower concentration range, two new phase regions, i.e. the hexagonal liquid crystalline phase region and its adjacent aqueous two-phase region were obtained in the higher concentration range. Shear viscosities of some isotropic single-phase systems, aqueous two-phase systems and lamellar liquid crystalline systems were measured. The experimental results indicate that at either isotropic single-phase region, under certain xT, there exists one shear viscosity maximum peak. As xT increases, the shear viscosity maximum peak shifts to the position far from the equimolar ratio of CTAB and AS, and meanwhile the value of the shear viscosity maximum peak increases. At each xT, the value of shear viscosity peak at the isotropic single-phase region with excess AS is less than that at the isotropic single-phase region with excess CTAB. The rheological behavior of the isotropic single-phase systems around the shear viscosity peak and that of lamellar liquid crystalline systems are complicated. Thixotropic, antithixotropic and complex thixotropic behaviors were all observed for these systems. Experimental results illustrate that both the isotropic single-phase systems and lamellar liquid crystalline systems located at position very close to the boundaries of the aqueous two-phase systems, as well as the coexisted phases in aqueous two-phase systems are not very viscous. When xT ≤ 0.005, their shear viscosities are usually several to dozens mPa s. In order to understand the rheological behaviors of the mixed systems, TEM micrographs of some isotropic single-phase systems and aqueous two-phase systems were investigated.