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 cesiu m pentadecafluorooctanoate-water system has been studied by means of t he H-2 pulsed gradient spin echo NMR technique. The principal componen ts of the macroscopic diffusion tensor were determined to high accurac y as functions of temperature and concentration. The orientational ord er 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 micros tructure in the nematic phase, our results are in qualitative disagree ment with previous interpretations of scattering and conductivity data as well as with theoretical predictions based on a hard-particle mode l. Our results thus indicate that the strong increase of orientational order with decreasing temperature is due to soft micelle-micelle inte ractions, presumably the anisotropic electrical double-layer repulsion , rather than to a growth-alignment coupling. On increasing the concen tration at fixed relative temperature, we find that the micelle size i ncreases while the orientational order decreases. The latter trend is rationalized in terms of the quadrupole-quadrupole component of the el ectrical double-layer interaction, which tends to destroy the orientat ional order.