  The connectivity of the hydrophobic medium in the nonionic binary system
C$_{12}$EO$_6$/H$_2$O is studied by monitoring the diffusion constants of
tracer molecules at the transition between the hexagonal mesophase and the
fluid isotropic phase. The increase in the transverse diffusion coefficient on
approaching the isotropic phase reveals the proliferation of bridgelike defects
connecting the surfactant cylinders. This suggests that the isotropic phase has
a highly connected structure. Indeed, we find similar diffusion coefficients in
the isotropic and cubic bicontinuous phases. The temperature dependence of the
lattice parameter in the hexagonal phase confirms the change in connectivity
close to the hexagonal-isotropic transition. Finally, an X-ray investigation of
the isotropic phase shows that its structure is locally similar to that of the
hexagonal phase.
