Abstract
This PhD thesis deals with two different experimental studies based on original couplings between liquid crystals and surfactants. <br />The aim of the first study was to understand clearly the lyotropic organisation of mixture of didodecyldimethylammonium bromide (DDAB) and water in the isotropic 4-n-pentyl-4'-cyanobiphenyl liquid crystal (5CB). The formed microemulsion gets unstable when approaching the isotropic-to nematic 5CB or when slightly changing the composition by adding a co-surfactant. Our aim was to determine what the driving interactions were.<br />The structure of this water-in-oil micro-emulsion has been investigated by small-angle neutron and X-rays scatterings far above the nematic-isotropic (N-I) phase transition of the liquid crystal. From quantitative analyses, we obtain the shape and size of the aggregates. Next, in order to obtain new mesophases (for example a lamellar phase) in a liquid crystal, we have explored the effect of adding a co-surfactant. The study of the phase diagrams reveals a small extension of the micro-emulsion domain. Our experimental results are explained by a large attractive Van der Waals interaction between inverse micelles. Under cooling, we have also shown by neutron scattering experiments the presence of “paranematic fluctuations” interactions occuring close to the isotropic-nematic phase transition. We discuss these interactions in the light of the scattering data.<br />The goal of the second study was to formulate a direct emulsion of liquid crystal in water with new amphotropic surfactants that we have synthesized and characterized by NMR. These molecules were designed to be localized specifically to the water-5CB interface. A remarkable effect has been obtained with gemini molecules, which favour the formation of transient elongated cylindrical droplets composed by nematic 5CB. We have explored the static and dynamic properties of this new colloidal objects. The mechanisms controlling their formation have been discussed.