Investigation of the relation between surfactant head group size and formation of different types of lyotropic nematic phases in binary surfactant/dye systems

In the frame of this thesis, properties of some lyotropic binary surfactant mixtures, composed of surfactant, Sunset Yellow, dodecanol, and water, were studied at constant total surfactant concentrations. Dodecyldimethylammonium bromide (DDMABr), dodecyldimethylethylammonium bromide (DDMEABr), and dodecyltriethylammonium bromide (DTEABr) were chosen as surfactant molecules. The binary surfactant mixtures consisted of DDEMABr/DDMABr and DDMEABr/DTEABr. By changing the relative concentrations of each surfactant in the mixtures, lyotropic mixtures presenting lamellar, nematic, and hexagonal phases were obtained. The textures of the lyotropic phases were analyzed using a polarized optical microscope. The uniaxial to biaxial nematic phase transitions were obtained from the temperature dependences of the birefringences of the nematic phases via laser conoscopy technique. Small-angle X-ray Scattering was used to determine some micellar parameters. The results indicated that the formation of different lyotropic structures, especially biaxial nematic phases, is related to the average area surfactant head group. Those results were interpreted from the interactions between ionic species, considering their kosmotropic and chaotropic properties.