Magnetically alignable phase of phospholipid "Bicelle" mixtures is a chiral nematic made up of wormlike micelles

We have studied the phase behavior of binary mixtures of long- and short-chain lipids, namely, dimyristoyl phosphatidylcholine (DMPC) and dihexanoyl phosphatidylcholine (DHPC), using optical microscopy and small-angle neutron scattering. Samples with a total lipid content of 25 wt %, corresponding to ratios Q ([DMPC]/[DHPC]) of 5, 3.2, and 2, are found to exhibit an isotropic (I) → chiral nematic (N*) →?? lamellar phase sequence on increasing temperature. The I-N* transition coincides with the chain melting transition of DMPC at Q = 5 and 3.2, but the N* phase forms at a higher temperature for Q = 2. All three samples form multilamellar vesicles in the lamellar phase. Our results show that disklike "bicellar" aggregates occur only in the lower temperature isotropic phase and not in the higher temperature magnetically alignable N* phase, where they were previously believed to exist. The N* phase is found to consist of long, flexible wormlike micelles, their entanglement resulting in the very high viscosity of this phase.