Influence of stabilizer systems on the properties and phase behavior of supercooled smectic nanoparticles.

Colloidal dispersions of cholesterol esters in the supercooled smectic state (supercooled smectic nanoparticles) are potential novel carrier systems for poorly water soluble drugs. As the supercooled smectic state is metastable, evaluation of its stability and of parameters influencing it is essential. In the present study, the effect of different emulsifiers on the stability of the supercooled smectic state of cholesteryl myristate (CM) nanoparticles and their crystallization was investigated. Nanoparticles were prepared by high-pressure melt homogenization and characterized by dynamic light scattering (DLS), laser diffraction combined with polarization intensity differential scattering (LD-PIDS), synchrotron radiation small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM, negative staining and cryo-preparation). The various stabilizers resulted in clear differences in the crystallization behavior of the nanoparticles: stabilizers containing a fatty acid chain in their molecule (e.g. phospholipids, sodium oleate and sucrose monolaurate) induced a multiple crystallization event accompanied by a comparatively high recrystallization tendency. In contrast, the recrystallization tendency of nanoparticles stabilized with polymers (e.g. gelatin polysuccinate, poloxamer, poloxamine, polyvinyl alcohol) and sodium glycocholate was much lower and a single crystallization event was observed. The high stability against recrystallization during storage of smectic nanoparticles stabilized with polysorbate 80 in spite of the presence of a fatty acyl group in the molecule suggests that the polar head group (e.g. polyethylene glycol chains) of the emulsifier may also play a significant role.