Investigations on the physical state of lipid nanoparticles by synchrotron radiation X-ray diffraction

Submicron-sized parenteral drug carrier systems based on lipids solid at room temperature in the bulk phase were prepared by high pressure homogenization of an emulsion of molten lipids in an aqueous phase. Phospholipids, bile salts and block copolymers of polyoxyethylene and polyoxypropylene were used as emulsifiers. The mean diameters obtained under standardized homogenization conditions determined by photon correlation spectroscopy lie between 70 and 300 nm, and depend on the amount and type of emulsifier. The present study describes the results of an investigation on the physical state, the melting behaviour and the recrystallization of the lipids in these colloidal carrier systems using X-ray diffraction. Debye-Scherrer diffraction patterns were first obtained on a conventional X-ray source, but the long exposure times (8–24 h) precluded a systematic investigation. Therefore, simultaneous small and wide angle diffraction measurements were made using synchrotron radiation with exposure times around 3 min. All investigated carrier systems were in the β-crystalline modification at room temperature. At body temperature the carriers were either β-crystalline or amorphous liquids depending on the matrix constituent. Incorporation of coenzyme Q10 into the lipid particles did not significantly alter the recrystallization tendency of the nanoparticles. Time-resolved X-ray diffraction measurements during temperature scans demonstrated that the recrystallization of the lipid nanodispersions is different from that of bulk material. After heating above the bulk melting temperature and cooling down under controlled conditions the dispersed lipids recrystallize in the α-form whereas the bulk lipids recrystallize in the β′-modification and transform rapidly into the β-form