Slow motion electron spin resonance line shapes of lyotropic liquid crystals in hexagonal phase

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In this paper the slow motion electron spin resonance (ESR) line shape theory is extended to hexagonal mesophases. The stochastic Liouville equation is applied to the dynamic description of both local molecular reorientation and azimuthal surface translational diffusion of the lipid around the cylinder axis. The established ESR line shape models allow a separation of contributions to the electron spin relaxation from the two motions, if macroscopically aligned hexagonal spectra are simulated. Such simulations can yield, not only the dynamic parameters such as local ordering parameter, local motional rate, and azimuthal surface diffusion coefficient, but also a disorder parameter accounting for residual cylinder disorder and the disorder effects due to finite cylinder length and curved surface along the cylinder axis. To test these models, they are applied to the analyses of X-band ESR spectra of the hexagonal phase of the sodium dodecyl sulphate (SDS)/decanol/water ternary system. The obtained surface diffusion coefficient and disorder parameter are in good agreement with previously reported values in a deuterium nuclear magnetic resonance (NMR) study. The local dynamic properties, which are not readily available by NMR method, are compared with those obtained for the micellar phase of the SDS/water binary system. Such a comparison reveals that while the local orderings are similar, the local dynamics is much slower in the hexagonal phase than in the micellar phase.