Structural characterization of the phospholipid stabilizer layer at the solid-liquid interface of dispersed triglyceride nanocrystals with small-angle x-ray and neutron scattering

Abstract

Dispersions of crystalline nanoparticles with at least one sufficiently large unit cell dimension can give rise to Bragg reflections in the small-angle scattering range. If the nanocrystals possess only a small number of unit cells along these particular crystallographic directions, the corresponding Bragg reflections will be broadened. In a previous study of phospholipid stabilized dispersions of β-tripalmitin platelets [Unruh, J. Appl. Crystallogr. 40, 1008 (2007)], the x-ray powder pattern simulation analysis (XPPSA) was developed. The XPPSA method facilitates the interpretation of the rather complicated small-angle x-ray scattering (SAXS) curves of such dispersions of nanocrystals. The XPPSA method yields the distribution function of the platelet thicknesses and facilitates a structural characterization of the phospholipid stabilizer layer at the solid-liquid interface between the nanocrystals and the dispersion medium from the shape of the broadened 001 Bragg reflection. In this contribution an improved and extended version of the XPPSA method is presented. The SAXS and small-angle neutron scattering patterns of dilute phospholipid stabilized tripalmitin dispersions can be reproduced on the basis of a consistent simulation model for the particles and their phospholipid stabilizer layer on an absolute scale. The results indicate a surprisingly flat arrangement of the phospholipid molecules in the stabilizer layer with a total thickness of only 12 Å. The stabilizer layer can be modeled by an inner shell for the fatty acid chains and an outer shell including the head groups and additional water. The experiments support a dense packing of the phospholipid molecules on the nanocrystal surfaces rather than isolated phospholipid domains.

Formation of liquid crystalline phases in aqueous suspensions of platelet-like tripalmitin nanoparticles

“…Figure 1) as the d 001 spacing of 4.02 nm for β-tripalmitin is fairly large. 17,18 Above a critical tripalmitin (PPP) concentration ϕ st P P P , interference maxima with a lamellar order arise in SAXS patterns (first three orders marked in Figure 1). They can be attributed to a self-assembly of individual platelets in stacked lamellae (named "stacks" hereafter).…”

“…The aqueous suspensions were prepared by highpressure melt homogenization according to the procedure described in Ref. 18. The native tripalmitin suspensions were prepared using 10% tripalmitin, 2.4% lecithin (DLPC or S100) and 0.6% NaGC in purified water.…”

Analysis of the structure of nanocomposites of triglyceride platelets and DNA

“…Their frequencies can be determined by a simultaneous fit of a linear combination of their macroscopic scattering cross sections (MSCSs) (dS/dO) to the experimental data. [21][22][23][24][25] Structures can be for example single nanocrystals 21 and stacks of such nanocrystals. 22 The nanocrystals can be covered by up to two shells.…”

“…21 For problem (1), the MSCSs (dS/dO) i sp were computed for five ensembles (i = 1,. . .,5), each consisting of 400 single platelets (sp).…”

“…In a previous study on the structure of the lecithin stabilizer layer of triglyceride nanocrystals, constraints between the EDs, NSLDs and thicknesses of the shells, based on a specific structural model of the lecithin head groups and acyl chains in the shells, were used in the fits. 21,22 However, for the samples studied here, this would be a challenging task because of the three different, interpenetrating stabilizer molecules (plus the DNA) in the shells.…”