Suspensions of platelet-like shaped tripalmitin nanocrystals stabilized by the pure lecithin DLPC and the lecithin blend S100, respectively, have been studied by small-angle x-ray scattering (SAXS) and optical observation of their birefringence at different tripalmitin (PPP) concentrations φPPP. It could be demonstrated that the platelets of these potential drug delivery systems start to form a liquid crystalline phase already at pharmaceutically relevant concentrations φPPP of less than 10 wt. %. The details of this liquid crystalline phase are described here for the first time. As in a previous study [A. Illing et al, Pharm. Res. 21, 592 (2004)] some platelets are found to self-assemble into lamellar stacks above a critical tripalmitin concentration

$\varphi _{PPP}^{st}$
φPPPst of 4  wt. %. In this study another critical concentration
$\varphi _{PPP}^{lc}\approx 7$
φPPPlc≈7 wt. % for DLPC and
$\varphi _{PPP}^{lc}\approx 9$
φPPPlc≈9  wt. % for S100 stabilized dispersions, respectively, has been observed.
$\varphi _{PPP}^{lc}$
φPPPlc describes the transition from a phase of randomly oriented stacked lamellae and remaining non-assembled individual platelets to a phase in which the stacks and non-assembled platelets exhibit an overall preferred orientation. A careful analysis of the experimental data indicates that for concentrations above
$\varphi _{PPP}^{lc}$
φPPPlc the stacked lamellae start to coalesce to rather small liquid crystalline domains of nematically ordered stacks. These liquid crystalline domains can be individually very differently oriented but possess an overall preferred orientation over macroscopic length scales which becomes successively more expressed when further increasing φPPP. The lower critical concentration for the formation of liquid crystalline domains of the DLPC-stabilized suspension compared to
$\varphi _{PPP}^{lc}$
φPPPlc of the S100-stabilized suspension can be explained by a larger aspect ratio of the corresponding tripalmitin platelets. A geometrical model based on the excluded volumes of individual platelets and stacked lamellae has been developed and successfully applied to reproduce the critical volume fractions for both, the onset of stack formation and the appearance of the liquid crystalline phase.

Topics
Sample handling, Liquid crystal phases, Birefringence, Nanocrystals, Nanoparticle, X-ray scattering, Drug delivery, Lipids, Lecithins, Fatty acids
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