Magnetic field-dependent counterion ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ spin relaxation and quadrupole splitting measurements are reported from the discotic nematic (${\mathit{N}}_{\mathit{D}}$) 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 ${\mathit{N}}_{\mathit{D}}$ phase and into the isotropic phase. The axial ratio varies only weakly with temperature in the ${\mathit{N}}_{\mathit{D}}$ phase, from 6 at 10 °C below the nematic-isotropic transition temperature ${\mathit{T}}_{\mathit{N}\mathit{I}}$ to 4 just below ${\mathit{T}}_{\mathit{N}\mathit{I}}$, but then increases to 8 in the isotropic phase just above ${\mathit{T}}_{\mathit{N}\mathit{I}}$. This behavior is contrary to the existing theoretical predictions. The nematic order parameter decreases from 0.9 at 10 °C below ${\mathit{T}}_{\mathit{N}\mathit{I}}$ to 0.4 just below ${\mathit{T}}_{\mathit{N}\mathit{I}}$. 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.

• Received 17 June 1994

DOI:https://doi.org/10.1103/PhysRevE.51.466