We report our investigations on the thermal stability of the intermediate homologue of cholesteryl ester, cholesteryl nonanoate (ChN) at the air–water (A–W) and air–solid (A–S) interfaces. Surface manometry studies on ChN at the A–W interface reveal that the limiting area and the collapse pressure of the film decrease with an increase in the temperature. Brewster angle microscopy studies show the co-existing gas (G) and the homogeneous phase (bilayer) that, with compression, transforms to a bilayer phase followed by a collapse to circular domains. These collapsed circular domains (CCDs) coarsen and nucleate to form 3D structures, and their evolution and growth are further tracked at different temperatures using the reflection mode of the microscope. For temperatures between 288 K and 293 K, we find that the dendritic growth is favored. From 298 K to 303 K, the CCD transforms to a fractal domain with its branches changed from the left-handed to right-handed sense via an intermediate state. Based on these observations, a morphological phase diagram ChN in the collapsed state with different temperatures is constructed. Topography images of the ChN film using an atomic force microscope yield a thickness of about 3.5 nm, which is larger than its molecular length (2.7 nm). We attribute this to the partial vertical segregation of ChN molecules at the interfaces, which is consistent with the m-ii packing model proposed by Guerina and Craven (J. Chem. Soc., Perkin Trans. 2 1979, 1414). We have also investigated the thermal stability of the bilayer at the A–S interface through imaging ellipsometry. It highlights that the thermal dewetting of the confined bilayer proceeds via the random nucleation and growth of voids and the transition temperature is estimated to be 396.3 ± 1.2 K with a width of 7.6 ± 0.8 K.

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.

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