NMR study of the slow dynamics and local structure in lyotropic micellar liquid crystals

The first experimental evidence of director fluctuations in a micellar lyotropic liquid crystal, studied by ¹H spin-lattice relaxation rate, is reported. The system is constituted by the ternary mixture: potassium laurate/1-decanol/water. With the aim to test the slow dynamics, the experiment has been performed over a broad range of Larmor frequencies (2 × 10³− 6.6 × 10⁶ Hz), using fast field-cycling NMR relaxometry. The results evidence that in the nematic and isotropic mesophases and the poliphasyc region, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range (<10⁵ Hz). By increasing the intermicellar water content, a crossover was found between a quasi-isotropic 3D director fluctuations behavior to a 2D pseudo-lamellar undulation fluctuations. In order to explain the higher frequency range (>10⁵ 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. Received: 30 March 2000