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Colloidal suspensions of triglycerides are under investigation as potential drug carrier systems. The properties of the matrix lipids are altered in the nanoparticles compared to those of the bulk material. For instance, the metastable alpha-modification of the triglycerides usually transforms into the stable beta-polymorph quite rapidly in the colloidal particles. Recently, it was observed that the alpha-modification can be preserved for a considerable period of time in tristearin nanoparticles when the particles are stabilized with a blend of saturated long-chain phospholipids and the bile salt sodium glycocholate [Bunjes, H.; Koch, M. H. J. J. Controlled Release 2005, 107, 229-243]. As triglyceride nanoparticles in the alpha-modification may offer some advantages over those in the beta-form with regard to drug delivery applications, the structure of the corresponding dispersions was investigated in more detail with differential scanning calorimetry, X-ray diffraction, and electron microscopy. The electron microscopic investigations confirmed a platelet-like, layered structure for particles in the beta-modification and revealed a spheroidal shape with concentric layers for larger particles in the alpha-form. For the first time, not only was information on the internal structure of solid triglyceride nanoparticles obtained from freeze-fracture electron micrographs but also details were observed by cryoelectron microscopy.

The crystallization temperature and polymorphism of tripalmitin nanoparticles in colloidal dispersions prepared by melt-homogenization and stabilized with different pharmaceutical surfactants (sodium glycocholate, sodium oleate, tyloxapol, Solutol HS 15, Cremophor EL) and their combinations with soybean phospholipid (Lipoid S100) were investigated to establish the influence of the emulsifiers on these parameters. There were no major effects on the crystallization temperature but remarkable differences in the time-course of polymorphic transitions after crystallization of the triglyceride particles indicate interaction between the surfactant layer and the triglyceride matrix. The metastable alpha-modification was most stable in dispersions solely stabilized with glycocholate. Upon fast cooling from the melt, these dispersions form an uncommon type of alpha-modification that displays only a very weak small-angle reflection indicating poor ordering between triglyceride layers. Slow crystallization of these glycocholate-stabilized nanoparticles yields the usual alpha-form. Electron microscopic investigations reveal that, in both cases, the particles in the alpha-modification are less anisometric than those of the stable beta-form. These results indicate that major rearrangements still may take place in solid lipid nanoparticles after recrystallization.

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