Surfactant self-diffusion and networks of wormlike micelles in concentrated solutions of nonionic surfactants

Abstract

Phase behavior, surfactant self-diffusion coefficients (D) and small-angle x-ray scattering (SAXS) have been measured on concentrated solutions of a nonionic surfactant (C₁₆E₇) where entanglement of wormlike micelles has been suggested in our previous light scattering study. In the lower temperature range, the self-diffusion coefficient/activation energy for self-diffusion processes (E _D) are much smaller/larger than those in the liquid crystal (LC) phases where the lateral diffusion is dominant, which confirms the validity of our diffusion model previously reported where intermicellar migration of surfactant molecules at the entanglement point is considered. As the temperature is raised above about 45 °C, however, E _D increases rapidly towards the value for the LC phases. Above about 60°C, the D 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 crosslinking of wormlike micelles occurs above about 45°C and that its extent increases with increasing temperature. SAXS results are consistent with such a change in the network structures. Relations with phase behaviors are also discussed.