Molecular motion in the lyotropic liquid crystal system containing potassium palmitate: A study of proton spin-lattice relaxation times

Abstract

Proton magnetic resonance of mixtures of 1 mol potassium palmitate/6.3 mol D 2O have yielded information about the phase transitions as well as the frequencies and activation energies associated with the various molecular motions in the temperature range from 100 to 500 K. At low temperature in the coagel phase methyl group rotation occurs. At higher temperature in this same phase a small amplitude motion which could be the diffusion of water or the counterion is detected. At about 300 K there is a phase transition to the gel phase, where both rotation of the soap molecule about the long axis and translational diffusion are observed; diffusion being the slower process. The transition from the gel to liquid crystal phase occurs at about 323 K. The observed 1 (ω 0 ) 1 2 dependence of the spin-lattice relaxation demonstrates the importance of order fluctuations in the liquid crystal phase. A second relaxation mechanism is also observed which is probably caused by conformational changes within the fatty acid chain. Perdeuterated soap molecules and molecules with deuterated methyl groups were used to show that methyl group reorientation is an important relaxation mechanism only in the coagel phase and to probe the effect of the intermolecular nuclear dipole-dipole interaction.