The viscosity of a ternary system consisting of nonionic surfactant (Triton X-100), oil (dioctylphthalate), and water was measured over the shear rate range of 0.1-250 s-1 using a parallel plate geometry. From the viscosity behavior, the phase diagram was determined. When the surfactant forms spherical aggregates by association with oil and/or water, the solutions are optically isotropic and the flow is Newtonian. On the other hand, when the liquid crystalline phases are built up, the system becomes pseudoplastic and often shows a yield stress because of the anisotropic liquid crystalline structures. At low surfactant concentrations, the system forms conventional macroemulsions containing large drops with diameters of the order of 10 μm. These macroemulsions are turbid and thermodynamically unstable. Since the viscosity versus shear rate curve drastically changes in connection with phase transition, the phase boundaries can be rheologically detected. The rheological technique provides a useful complementary method of characterizing the phase equilibrium of surfactant-oil-water systems.
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