Nuclear magnetic resonance proton-spin relaxation study of the local director fluctuations in the lyotropic liquid crystal:Potassoium Iaurate/1-decanol/water

摘要

Field cycling nuclear magnetic resonance relaxometry was used to study the slow molecular dynamics in the nematics and isotropic phases and polyphasic region of the phase diagram in the thernary mixture: potassium laurate/l-decanOl/water. The experiment has been performed over a broad range of Larmor frequencies (2 X — 6.6X 106 Hz). The first experimental evidence of director fluctuations in a micellar lyotropic nematic liquid crystal, studied by ‘H spin-lattice relaxation rate, is reported. The results evidence that in the nematic mesophases, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range (~ l0~ Hz). By increasing the intermicellar water content, a crossover was found between a quasi-isotropic three-dimensional (3D) director fluctuation behavior and a two-dimensional (2D) pseudo-lamellar undulation fluctuation. In spite of the fact that no menatic phases of this micellar complex fluid are isotropic with respect to light scattering, they present spin-lattice relaxation profiles as driven by local director fluctuations. The polyphasic region, at lower temperatures, shows a quasinematic 3D director fluctuation behavior; meanwhile, the isotropic phase, at higher temperatures, presents 2D pseudo-lamellar undulation fluctuation modes. We conclude that the micelles, in the isotropic phase, preserve the pseudo-lamellar structure, already found in the nematic phases but forming nematic domains with the directors randomly oriented. In order to explain the higher frequency range (2z l0~ Hz), two relaxation mechanisms are assigned: (i) molecular reorientation by translational diffusion on the micellar surface and (ii) molecular exchange between the micelle and the bulk.