Deuterium NMR spectroscopy has been employed to characterize the concentrat
ion dependence of orientational order in DMPC/DHPC bicellar solutions with
molar ratios q = [DMPC]/[DHPC] = 3.3, 2.7, and 2.3. The stability of a disc
otic nematic phase can, in general, be predicted from a simple Onsager pict
ure involving the size and concentration of the mesogenic unit, but for the
bicellar solutions this model is not adequate. Specifically, macroscopic a
lignment is observed at total lipid concentrations well below that, 1-10% (
w/w) predicted by Onsager's model. Thus the discotic nematic phase is stabl
e to approximate to 3-5% (w/w) for q = 3.3-2.3, and the bicellar order is h
ighest just before phase separation occurs at the minimum total phospholipi
d concentration. This implies the presence of a DHPCbic reversible arrow DH
PCsol equilibrium in establishing bicellar size, thereby extending the rang
e of concentrations for which alignment occurs. Bicellar morphology has bee
n verified for a wide range of concentrations, temperatures, and q-values,
but as viscosity measurements demonstrate, major morphological changes take
place as the temperature is reduced below 30 degrees C. (C) 1998 Academic
Press.