NMR measurements of chain ordering in some liquid-crystalline lamellar phases

Measurements of orientational order parameters which express different aspects of relative order of the individual segments of the hydrocarbon chain are reported, some for the first time, in the lamellar $L_a$ region of the potassium palmitate-water system. The order parameters are time averages of spherical harmonic functions which are measured directly from deuterium spectral splittings. From these measurements it is found that the order of each hydrocarbon segment relative to that at the alpha position deviates strongly from cylindrical symmetry. This asymmetry is found to increase in a linear fashion proceeding from the head group toward the terminal end of the chain. The order of each segment relative to the alpha position reflects the noncylindrical time average conformation of the hydrocarbon chain. These measurements were made in the high temperature, phase I, of this lamellar system. In this phase the order of the molecular long axis was found to be anisotropic reflecting a biased rather than a free rotation about the molecular long axis as normally assumed in nuclear magnetic resonance studies of ordered lipids. A model is introduced for the other $L_a$ phases in this system which involves a change in axis about which the molecule rotates.