Diffusion of surfactant micelles in fluid and crystal phases

Abstract

We have investigated diffusion of spherical surfactant micelles having a hard-core and weak repulsive potential in fluid, hexagonally close-packed (HCP), and body-centered cubic (BCC) phases by means of neutron spin echo (NSE) and dynamical light scattering (DLS) techniques. The intermediate scattering functions give short-time and long-time diffusion coefficients. In the fluid phase, the first-order virial coefficients of the normalized short-time self- and collective diffusion coefficients agreed with those of the hard spheres. The long-time collective diffusion coefficient exhibited the characteristic slowing down toward the fluid–HCP transition obeying a power law. At the fluid–HCP transition, the short-time diffusion coefficient increased abruptly, indicating increase of the effective free volume available for the micelles. Simultaneously, the long-time diffusion mode disappeared and a very slow mode characteristic of the crystal phase appeared, which might be related to the viscoelastic nature of the micelle crystals.