Schmiele Martin, Schindler Torben, Westermann Martin, Steiniger Frank, Radulescu Aurel, Kriele Armin, Gilles Ralph, Unruh Tobias
Friedrich-Alexander-Universität Erlangen-Nürnberg , Physik Department, Staudtstrasse 3, 91058 Erlangen, Germany.
J Phys Chem B. 2014 Jul 24;118(29):8808-18. doi: 10.1021/jp502580a. Epub 2014 Jul 2.
Aqueous suspensions of platelet-like shaped tripalmitin nanocrystals are studied here at high tripalmitin concentrations (10 wt % tripalmitin) for the first time by a combination of small-angle X-ray and neutron scattering (SAXS and SANS). The suspensions are stabilized by different lecithins, namely, DLPC, DOPC, and the lecithin blend S100. At such high concentrations the platelets start to self-assemble in stacks, which causes interference maxima at low Q-values in the SAXS and SANS patterns, respectively. It is found that the stack-related interference maxima are more pronounced for the suspension stabilized with DOPC and in particular DLPC, compared to suspensions stabilized by S100. By use of the X-ray and neutron powder pattern simulation analysis (XNPPSA), the SAXS and SANS patterns of the native tripalmitin suspensions could only be reproduced simultaneously when assuming the presence of both isolated nanocrystals and stacks of nanocrystals of different size in the simulation model of the dispersions. By a fit of the simulated SAXS and SANS patterns to the experimental data, a distribution of the stack sizes and their volume fractions is determined. The volume fraction of stacklike platelet assemblies is found to rise from 70% for S100-stabilized suspensions to almost 100% for the DLPC-stabilized suspensions. The distribution of the platelet thicknesses could be determined with molecular resolution from a combined analysis of the SAXS and SANS patterns of the corresponding diluted tripalmitin (3 wt %) suspensions. In accordance with microcalorimetric data, it could be concluded that the platelets in the suspensions stabilized with DOPC, and in particular DLPC, are significantly thinner than those stabilized with S100. The DLPC-stabilized suspensions exhibit a significantly narrower platelet thickness distribution compared to DOPC- and S100-stabilized suspensions. The smaller thicknesses for the DLPC- and DOPC-stabilized platelets explain their higher tendency to self-assemble in stacks. The finding that the nanoparticles of the suspension stabilized by the saturated lecithin DLPC crystallize in the stable β-tripalmitin modification with its characteristic platelet-like shape is surprising and can be explained by the fact that the main phase transformation temperature for DLPC is, as for unsaturated lecithins like DOPC and S100, well below the crystallization temperature of the supercooled tripalmitin emulsion droplets.
本文首次通过小角X射线散射和中子散射（SAXS和SANS）相结合的方法，研究了高浓度三棕榈酸甘油酯（10 wt%三棕榈酸甘油酯）下的血小板状三棕榈酸甘油酯纳米晶体水悬浮液。这些悬浮液由不同的卵磷脂稳定，即DLPC、DOPC和卵磷脂混合物S100。在如此高的浓度下，血小板开始自组装成堆叠结构，这分别在SAXS和SANS图谱的低Q值处导致干涉极大值。研究发现，与由S100稳定的悬浮液相比，由DOPC尤其是DLPC稳定的悬浮液中，与堆叠相关的干涉极大值更为明显。通过使用X射线和中子粉末图谱模拟分析（XNPPSA），只有在假设分散体的模拟模型中同时存在孤立的纳米晶体和不同尺寸的纳米晶体堆叠时，才能同时再现天然三棕榈酸甘油酯悬浮液的SAXS和SANS图谱。通过将模拟的SAXS和SANS图谱与实验数据拟合，确定了堆叠尺寸及其体积分数的分布。发现堆叠状血小板聚集体的体积分数从S100稳定的悬浮液中的70%上升到DLPC稳定的悬浮液中的几乎100%。通过对相应稀释的三棕榈酸甘油酯（3 wt%）悬浮液的SAXS和SANS图谱进行联合分析，可以以分子分辨率确定血小板厚度的分布。根据微量热数据，可以得出结论，由DOPC尤其是DLPC稳定的悬浮液中的血小板明显比由S100稳定的血小板薄。与DOPC和S100稳定的悬浮液相比，DLPC稳定的悬浮液表现出明显更窄的血小板厚度分布。DLPC和DOPC稳定的血小板较小的厚度解释了它们更高的自组装成堆叠的倾向。由饱和卵磷脂DLPC稳定的悬浮液中的纳米颗粒以其特征性的血小板状形状在稳定的β-三棕榈酸甘油酯变体中结晶，这一发现令人惊讶，并且可以通过以下事实来解释：DLPC的主要相变温度与DOPC和S100等不饱和卵磷脂一样，远低于过冷三棕榈酸甘油酯乳液滴的结晶温度。
