Structural Characterization of Lecithin-Stabilized Tetracosane Lipid Nanoparticles. Part II: Suspensions

Using
photon correlation spectroscopy, transmission electron microscopy,
microcalorimetry, wide-angle X-ray scattering (WAXS), and small-angle
X-ray and neutron scattering (SAXS, SANS), the structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-stabilized colloidal
tetracosane suspensions was studied from the molecular level to the
microscopic scale as a function of the temperature. The platelike
nanocrystals exhibit for tetracosane an unusual orthorhombic low-temperature
crystal structure. The corresponding
WAXS pattern can be reproduced with a predicted orthorhombic unit
cell (space group Pca21), which usually
occurs only for much longer even-numbered n-alkanes.
Special emphasis was placed on the structure of the DMPC stabilizer
layer covering the nanocrystals. Their structure was investigated
by SAXS and SANS, using suspensions with different neutron scattering
contrasts. As for the emulsions in Part I,
the crystallized nanoparticles are covered by a DMPC monolayer. Their
significant smaller thickness of 10.5 Å (for the emulsions in Part I: 16 Å) could be related to a more tilted
orientation of the DMPC molecules to cover the expanded surface of
the crystallized nanoparticles.