Micelle size and order in lyotropic nematic phases from nuclear spin relaxation

Abstract

Nuclear magnetic resonance (NMR) relaxation of quadrupolar nuclei is introduced as a new method for determining micelle size and nematic order in lyotropic nematic mesophases from the dependence of the spin relaxation rates on molecular diffusion over the curved micelle surface. The approach is illustrated by an experimental study of two uniaxial nematic phases (the calamitic N_C and discotic N_D phases of the sodium dodecyl sulphate/decanol/water system) using two nuclei: ²H in the α‐deuterated surfactant and ²³Na in the counterions. The two nuclei yield similar results: an apparently temperature independent axial ratio of 3–4 in both phases and a nematic order parameter which decreases from ca. 0.9 (0.75) at the lowest temperature to ca. 0.6 (0.5) at the highest temperature in the N_C (N_D) phase. As compared to the predictions of the Maier–Saupe theory, the nematic order parameter in the N_C phase is considerably larger and decreases more strongly as the nematic–isotropic transition is approached.