MICELLE SIZE AND ORIENTATIONAL ORDER ACROSS THE NEMATIC-ISOTROPIC TRANSITION - A FIELD-DEPENDENT NUCLEAR-SPIN-RELAXATION STUDY

Magnetic field-dependent counterion Na23 spin relaxation and quadrupole splitting measurements are reported from the discotic nematic (ND) and isotropic phases of the system sodium dodecylsulphate-decanol-water. Carefully designed relaxation experiments were used to determine the individual field-dependent motional spectral densities. A quantitative analysis of the splitting and spectral density data allowed a simultaneous determination of the nematic order parameter S and the axial ratio of the disklike micelles as functions of temperature throughout the extension of the ND phase and into the isotropic phase. The axial ratio varies only weakly with temperature in the ND phase, from 6 at 10°C below the nematic-isotropic transition temperature TNI to 4 just below TNI, but then increases to 8 in the isotropic phase just above TNI. This behavior is contrary to the existing theoretical predictions. The nematic order parameter decreases from 0.9 at 10°C below TNI to 0.4 just below TNI. While the latter value is close to that predicted by Maier-Saupe theory, the temperature dependence of S is much stronger than that predicted theoretically. © 1995 The American Physical Society.