Observation of elementary edge dislocations in phospholipid multilayers and of their annealing as a determination of the permeation coefficient

Abstract

We have observed the elementary edge dislocations in a phospholipid smectic liquid crystal lightly doped with a uniformly distributed, fluorescent, lipid analogue. Scanning the fluorescence intensity of a wedge crystal with 8 μm spatial resolution provided adequate photon statistics to detect changes of one bilayer in fifty. Fresh samples contain a dislocation density of about 1 × 107/cm2 which must anneal away before the bilayer edges in the wedge become clearly distinguishable. Annealing of dimyristoyl phosphatidylcholine (DMPC) requires 2-4 weeks at 35 °C in the Lα phase and does not occur during 3 months at 17 °C in the Pβ' phase. The annealing process requires the transfer of lipid molecules between neighbouring bilayers ; thus the annealing time depends upon the permeation coefficient λ p which appears in the viscoelastic equations for a smectic liquid crystal and provides a convenient determination of λp. We find λp is about 1 × 10-30 cm2/poise at 37 °C and is considerably smaller at 17 °C. Additional defects appear when the sample is cooled through the transition temperature; these anneal away within 2 days if the temperature is raised again within a short time, but require the full 2-4 weeks if the sample is in the Pβ' phase for more than a day