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

The lyotropic mesophases formed by the "amphitropic" surfactant triethylamm oniodecyloxycyanobiphenyl bromide in water have been studied using optical microscopy, X-ray diffraction, and NMR. Only a lamellar mesophase occurs, b ut this has a wide temperature and composition range. There is a marked dec rease 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 aris e from the occurrence of a defect-full lamellar phase with water-filled hol es rather than a conventional structure with continuous bilayers. The influ ence on the liquid crystals of added thermotropic mesogens (pentylcyanobiph enyl and heptyloxycyanobiphenyl) has been compared with that of a conventio nal alkane, hexane. The mesogenic additives stabilize the (defective) lamel lar 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 ani sotropic attractive interactions between the oxycyanobiphenyl moieties, whi ch are increased when the mesogenic additives are present. Some pointers fo r future directions in the search for true amphitropic mesophases are given .