Connectivity of the hexagonal, cubic, and isotropic phases of the C12EO6/H2O lyotropic mixture investigated by tracer diffusion and X-ray scattering

The connectivity of the hydrophobic medium in the nonionic binary system C1 2EO6/H2O is Studied by monitoring the diffusion constants of tracer molecul es at the transition between the hexagonal mesophase and the fluid isotropi c phase. The increase in the transverse diffusion coefficient on approachin g the isotropic phase reveals the proliferation of bridgelike defects conne cting the surfactant cylinders. This suggests that the isotropic phase has a highly connected structure. Indeed, we find similar diffusion coefficient s in the isotropic and cubic bicontinuous phases. The temperature dependenc e of the lattice parameter in the hexagonal phase confirms the change in co nnectivity close to the hexagonal-isotropic transition. Finally, an X-ray i nvestigation of the isotropic phase shows that its structure is locally sim ilar to that of the hexagonal phase.