CHARACTERIZATION OF FIELD-ORDERED AQUEOUS LIQUID-CRYSTALS BY NMR DIFFUSION MEASUREMENTS

Anisotropic water diffusion in three lyotropic liquid-crystal systems, cesium perfluorooctanoate (CsPFO)/D2O, dimyristoylphosphatidylcholine (DMPC)/CHAPSO/D2O, and DMPC/Triton X-100/D2O, has been measured using the NMR pulsed field gradient spin echo (PFGSE) technique. Field gradients were applied in directions parallel to the magnetic field and perpendicular to the field, in order to obtain very precise water diffusion coefficients along orthogonal directions. The results have been analyzed using a diffusion model in which water molecules move stochastically over fixed sites in a lattice. Micelles which makeup the liquid crystal are included as forbidden diskoidal regions of appropriate size and orientation in this lattice. The mean square displacement of a water molecule moving through the interstitial space of the obstacles is calculated as a function of time, and diffusion coefficients are extracted. From comparison of the simulated and the experimental results, the presence of diskoidal micelles of specific diameter and orientation is deduced, for two of the systems. In the CsPFO/D2O system where disk normals are parallel to the field, this observation agrees closely with published results obtained by different methods. In the case of DMPC/CHAPSO, where disk normals are perpendicular to the field, a useful characterization of a new liquid-crystalline system results. For the DMPC/Triton system, no adequate simulations of results could be obtained, suggesting that a more complex description of water interaction with micellar surfaces may be required.