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The Role of Molecular Packing in Dictating the Miscibility of Some Cholesteryl n-Alkanoates at Interfaces
Langmuir ( IF 3.9 ) Pub Date : 2021-09-15 , DOI: 10.1021/acs.langmuir.1c01025
Pinchu Xavier 1, 2 , Brindhu Malani S 1, 3 , Padmanabhan Viswanath 1
Affiliation  

Cholesteryl n-alkanoates of saturated fatty acids and their mixtures are widely studied in different physical states and also due to their significance in biology. Here, we address the miscibility of some homologues of cholesteryl n-alkanoates at interfaces, which are known to exhibit different (cholesteryl octanoate, ChC8, and cholesteryl stearate, ChC18) or the same (cholesteryl nonanoate, ChC9, and cholesteryl laurate, ChC12) molecular packing in bulk. Surface manometry and Brewster angle microscopy studies on ChC8 (cholesteryl–cholesteryl interaction, referred to as m-i packing)/ChC9 (cholesteryl–chain interaction, referred to as m-ii packing) and also on ChC18 (chain–chain interactions, referred to as the crystalline bilayer)/ChC9 mixtures reveal phase separation at the air–water (A–W) interface plausibly due to the difference in the molecular packing. In contrast, ChC12/ChC9 (both m-ii packing) mixtures form a homogeneous phase and exhibit a higher collapse pressure (almost twice) than that of ChC9 indicating higher stability. At the air–solid (A–S) interface, the height profiles extracted from the surface topography images using an atomic force microscope yielded thicknesses of 3.6 ± 0.1 and 5.6 ± 0.1 nm for ChC18/ChC9 mixtures (at 0.66 and 0.5 mole fractions (MF)) corresponding to individual assembly, whereas a uniform thickness of 3.5 ± 0.2 nm is obtained for the case of ChC12/ChC9 mixtures (at 0.2, 0.5, and 0.8 MF) corresponding to m-ii packing. Ellipsometry studies reveal that the desorption temperature increases with the mole fraction of ChC9 and attains a maximum at 406.8 ± 4.8 K for 0.4 MF of ChC9, beyond which it decreases. Raman spectroscopy studies are carried out for ChC12/ChC9 mixtures in the homogeneous phase and in the collapsed state. Here, the dependency of peak positions on different physical states was assessed. Our studies offer new insights into the compatibility of molecular packing influencing the phase behavior and may be of relevance to tear film studies and on the formation of crystals in atherosclerosis.

中文翻译:

分子堆积在决定某些胆固醇正链烷酸酯在界面处的混溶性中的作用

饱和脂肪酸的胆固醇链烷酸酯及其混合物在不同的物理状态下被广泛研究,也因为它们在生物学中的重要性。在这里,我们解决了一些界面处胆固醇链烷酸酯同系物的混溶性,已知它们表现出不同(辛酸胆固醇,ChC8 和硬脂酸胆固醇,ChC18)或相同(壬酸胆固醇,ChC9 和月桂酸胆固醇,ChC12)分子包装散装。ChC8(胆固醇-胆固醇相互作用,称为mi堆积)/ChC9(胆固醇-链相互作用,称为m-ii)的表面测压和布鲁斯特角显微镜研究堆积)以及 ChC18(链-链相互作用,称为结晶双层)/ChC9 混合物显示,由于分子堆积的差异,空气-水 (A-W) 界面处的相分离似乎是合理的。相比之下,ChC12/ChC9(均为m-ii填料)混合物形成均相并表现出比 ChC9 更高的坍塌压力(几乎两倍),表明更高的稳定性。在空气-固体 (A-S) 界面处,使用原子力显微镜从表面形貌图像中提取的高度剖面对 ChC18/ChC9 混合物(摩尔分数为 0.66 和 0.5 MF)) 对应于单个组件,而对于 ChCl2/ChC9 混合物(在 0.2、0.5 和 0.8 MF)的情况下,获得了 3.5 ± 0.2 nm 的均匀厚度,对应于m-ii包装。椭圆光度法研究表明,解吸温度随着 ChC9 的摩尔分数的增加而增加,对于 0.4 MF 的 ChC9,解吸温度在 406.8 ± 4.8 K 处达到最大值,超过此温度会降低。对均相和塌陷状态的 ChCl2/ChC9 混合物进行拉曼光谱研究。在这里,评估了峰值位置对不同物理状态的依赖性。我们的研究为影响相行为的分子堆积的兼容性提供了新的见解,并且可能与泪膜研究和动脉粥样硬化中晶体的形成有关。
更新日期:2021-09-28
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