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 grad ients were applied in directions parallel to the magnetic field and pe rpendicular to the field, in order to obtain very precise water diffus ion coefficients along orthogonal directions. The results have been an alyzed using a diffusion model in which water molecules move stochasti cally over fixed sites in a lattice. Micelles which makeup the liquid crystal are included as forbidden diskoidal regions of appropriate siz e and orientation in this lattice. The mean square displacement of a w ater molecule moving through the interstitial space of the obstacles i s calculated as a function of time, and diffusion coefficients are ext racted. From comparison of the simulated and the experimental results, the presence of diskoidal micelles of specific diameter and orientati on is deduced, for two of the systems. In the CsPFO/D2O system where d isk 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 usef ul characterization of a new liquid-crystalline system results. For th e DMPC/Triton system, no adequate simulations of results could be obta ined, suggesting that a more complex description of water interaction with micellar surfaces may be required.