Proton relaxation study of paraffinic chain motions in a lyotropic liquid crystal

Abstract

In the phases of lyotropic liquid crystals, such as soap‐water systems, a large scale periodic organization of aqueous and paraffinic media coexists with a short range disorder of both media. We present here a nuclear magnetic resonance study of the paraffinic protons in the mesophases of the potassium laurate‐D₂O system. Special aspects of the NMR properties of the proton spin system in such phases are discussed. Taking benefit of the geometrical features of the lamellar, cubic, and hexagonal phases we can distinguish several motions: rapid diffusion of the molecules with their polar heads remaining on the soap‐water interface; configurational changes which can be due to flexion, torsion, or isomeric rotations. Owing to the existence of the interface and to the presence of neighboring molecules the local deformation motions are highly anisotropic. The diffusion coefficient, measured in the cubic phase, with the field gradient method, has a value of ${\text{2 × 10}}^{\text{−6}}{\mathrm{cm}}^2{\sec }^{\text{−1}}$ at 90°C its activation energy is 0.25 eV. The characteristic times of the chain deformations studied by relaxation measurements ${\mathrm{(T}}_1{\mathrm{,T}}_{\text{1 ρ}}{\mathrm{,T}}_D)$ , are distributed from ${\text{3 × 10}}^{\text{−6}}\sec$ to about 10⁻⁹sec at 90°C. Possible correlations between diffusion and deformations are discussed.