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

Abstract

The molecular dynamics have been investigated in the lyotropic liquid crystal system consisting of 70 per cent rubidium stearate/30 per cent water using 2H N.M.R. Spectra of 2H2O, perdeuterated rubidium stearate and rubidium stearate selectively deuterated at carbon positions 2, 3, 13 and 18 have been obtained over the temperature range from -80 to 120°C. In the L x phase above 58°C the molecular motions of the lipids are all fast on a time scale of 10-6 s and the observed deuteron quadrupole splittings are a direct measure of the C-D bond order parameter at each carbon position down the chain. The characteristic order parameter profile showing the expected decrease in order from the lipid-water interface to the centre of the bilayer is observed in the L α phase. The dominant molecular motion in the L β phase is rotation of the lipid molecule about its long axis. The correlation time for this motion is smaller than 10-7 s throughout the L β phase. Some trans-gauche isomerization does occur but with a high...