Structural changes in CTAB/H2O mixtures using a rheological approach

A complete, rheometrical analysis (steady, oscillatory and transient stress-relaxation experiments) may be performed to indirectly elucidate the inner structure of surfactant solutions over a wide range of physicochemical conditions. In this paper, structure and structural changes in cetyltrimethylammonium bromide (CTAB)/H2O mixtures have been studied by a set of rheological experiments as a function of composition (2-40 wt% CTAB) and temperature (20-70°C). This system was chosen for several reasons: firstly, it forms elongated micelles and exhibits a strong viscoelasticity at low concentrations (ca. 2 wt%); secondly, it has a low critical micellar concentration and a Krafft temperature close to the room temperature; third, a nematic phase (type-I, N) is formed in the concentration range 25-29 wt% with a clearing temperature of ca. 40°C. The nematic phase is intermediate to a micellar isotropic phase (L1), at low concentration, and to a normal hexagonal lyomesophase (H1) at high concentration of CTAB. The L 1 phase, consisting of a very large region, exhibited different rheological properties. At 30°C, viscoelastic data revealed a sphere-to-cylinder transition in shape of micelles in mixtures above 14 wt% CTAB. By increasing further the composition, a fast micellar growth was observed without reaching an entangled phase. The evolution of micellar morphology was also recorded with increasing temperature. The nematic and hexagonal phases, made by similar structural units (i.e. unconnected cylindrical aggregates), behaved like shear thinning materials and showed a comparable viscoelastic spectrum consisting of fast and slow relaxation processes. The slower relaxation times were interpreted as a relaxation of a shear-induced orientation that involved large lyotropic domains. Faster relaxation times (<1 s), on the contrary, could be related to dynamics or kinetics of stiff cylindrical aggregates.