Orientational order and micelle size in the nematic phase of the cesium pentadecafluorooctanoate-water system from the anisotropic self-diffusion of water

The self-diffusion of water in the nematic micellar phase of the cesium pentadecafluorooctanoate-water system has been studied by means of the H-2 pulsed gradient spin echo NMR technique. The principal components of the macroscopic diffusion tensor were determined to high accuracy as functions of temperature and concentration. The orientational order parameter and the size of the discoidal micelles were derived from the data with the aid of a theoretical description of the obstruction and hydration effects, the accuracy of which was tested by stochastic simulations. While supporting the discrete-micelle model of the microstructure in the nematic phase, our results are in qualitative disagreement with previous interpretations of scattering and conductivity data as well as with theoretical predictions based on a hard-particle model. Our results thus indicate that the strong increase of orientational order with decreasing temperature is due to soft micelle-micelle interactions, presumably the anisotropic electrical double-layer repulsion, rather than to a growth-alignment coupling. On increasing the concentration at fixed relative temperature, we find that the micelle size increases while the orientational order decreases. The latter trend is rationalized in terms of the quadrupole-quadrupole component of the electrical double-layer interaction, which tends to destroy the orientational order.