Proton Spin Relaxation Study of Molecular Motions in the Lyotropic Liquid Crystalline System Potassium-Laurate Water

The paper presents and discusses measurements of the proton spin T₁ relaxation dispersion over a broad frequency range (100 Hz - 300 MHz) in the micellar, hexagonal, cubic and lamellar phases of lyotropic potassium-laurate water mixtures. The results clearly show that in the anisotropic phases (hexagonal, lamellar) T₁ at low Larmor frequencies is dominated by liquid crystalline order fluctuations, whereas this relaxation mechanism is negligible or absent in the isotropic phases (micellar, cubic). At medium and high Larmor frequencies the relaxation reflects rotational and translational molecular motions. Using theoretical models from the literature for the three kinds of reorientations the observed frequency and temperature dependences of T₁ can be described quantitatively. In contrast to the order fluctuation mechanism the rotational and translational reorientations of individual molecules depend only weakly on the mesogenic structure. The order fluctuation contribution is strongest for the lamellar phase.