We studied the diffusion properties of an hydrophilic molecular probe in a lyotropic lamellar phase which is a strongly anisotropic medium. Two behaviours are observed while varying the dilution of the system: a dilute regime in which molecules diffuse in the aqueous medium restricted by two walls, and a confined regime in which molecules diffuse as membrane probes. A simple model taking into account the membrane fluidity allows us to interpret our results and to illustrate the crucial role of the probe anisotropy. In the second part of this work, we studied the microscopic mechanisms of the smectic - cholesteric phase transition in lyotropic liquid crystals. We studied a mixture of two amphiphilic molecules with different spontaneous curvatures (DMPC/C12E5/H2O). We unveiled the apparition of dislocation loops in the smectic phase when approaching the phase transition. This validates experimentally theoretical predictions implying the unbinding of this kind of defects as the mechanism mediating the phase transition. This system thus appears as a good experimental tool to study this kind of transitions which can occur in several fields of soft condensed matter. Finally, the direct visualization of the defects, the characterization of their structure and of their organization allows us to propose the existence of new phases in the field of lyotropic liquid crystals (biaxial smectic, TGB).
