Lyotropic phase behavior of triethylammoniodecyloxycyanobiphenyl bromide and the influence of added thermotropic mesogens

The lyotropic mesophases formed by the "amphitropic" surfactant triethylammoniodecyloxycyanobiphenyl bromide in water have been studied using optical microscopy, X-ray diffraction, and NMR. Only a lamellar mesophase occurs, but this has a wide temperature and composition range. There is a marked decrease in the estimated bilayer thickness (decrease in alkyl chain "ordering", increase of micelle surface curvature) with an increase of temperature, particularly at the lower surfactant concentrations. This is likely to arise from the occurrence of a defect-full lamellar phase with water-filled holes rather than a conventional structure with continuous bilayers. The influence on the liquid crystals of added thermotropic mesogens (pentylcyanobiphenyl and heptyloxycyanobiphenyl) has been compared with that of a conventional alkane, hexane. The mesogenic additives stabilize the (defective) lamellar phase, while hexane destabilizes it. The phase behavior and the NMR and the X-ray data all suggest that in the mesophase there are significant anisotropic attractive interactions between the oxycyanobiphenyl moieties, which are increased when the mesogenic additives are present. Some pointers for future directions in the search for true amphitropic mesophases are given.