Molecular dynamics in lamellar lyotropic mesophases with ordered and disordered hydrocarbon chains

The molecular dynamics in the lyotropic liquid crystal system consisting of 70 per cent rubidium stearate/30 per cent water have been investigated using H2 nuclear magnetic relaxation time measurements. Data have been collected in the temperature range from -80 to 120°C for rubidium stearate selectively deuterated at carbon positions 2, 3, 11 and 18. Methyl group reorientation dominates the spin-lattice relaxation at C-18 over the whole temperature region studied. The activation enthalpy is 11·0 kJ mol-1 and the correlation time for the reorientation is less than 10-10 s even at -80°C. In the low temperature crystalline phase observed below -28°C, the central part of the hydrocarbon chain is rigid while there is some motion at the carboxyl end characterized by an activation enthalpy of about 26·8 kJ mol-1. The correlation time of this motion at the C-2 position is equal to the reciprocal of nuclear resonance frequency at -48°C. In the intermediate gel phase (L β) the relaxation time measurements for the methylene positions show that two types of motion dominate. Rotation of the lipid molecule about the long axis affects all positions equally and has an activation enthalpy of 53·5 kJ mol-1. Over the L β phase the correlation time for the rotation ranges from 3 × 10-9 to 5 × 10-12 s. A second molecular motion is evident from the results near the boundary with the L x phase. The relaxation data from the L x phase are the most difficult to interpret but there is some evidence to suggest that fluctuations in the normal to the lipid bilayer caused by undulation modes in the lamellae are the dominant relaxation mechanism.