Structure of networks formed in concentrated solutions of nonionic surfactant studied by the pulsed-gradient spin-echo method

Surfactant self-diffusion coefficients (D) have been measured on concentrated micellar solutions (>10 wt %) of nonionic surfactant (C(16)E(7)) and liquid crystal (cubic and hexagonal) phases by using the pulsed-gradient spin-echo method. In the lower temperature range, the self-diffusion coefficient in the micellar phase is much smaller than that in the liquid crystal phases where the lateral diffusion is dominant. Also, the activation energy for the self-diffusion processes calculated from the temperature dependence of D is much larger than those in the liquid crystal phases and depends on concentration and temperature only slightly. These results confirm the validity of our diffusion model previously reported which takes into account intermicellar migration of surfactant molecules at the entanglement point. As the temperature is raised above about 45 degrees C (the lower critical solution temperature is 51 degrees C at about 1 wt %), however, the activation energy decreases rapidly toward the value for the liquid crystal phases. Above about 60 degrees C, the self-diffusion coefficient depends on concentration only slightly and its absolute value coincides with that expected from the temperature dependence of D in the cubic phase. From these results and the structure of the cubic phase, it is inferred that cross-links of wormlike micelles exist above about 45 degrees C and that the extent of cross linking increases with increasing temperature. Relations with phase behaviors are also discussed.