Spontaneously self-assembled fluidic bilayer of cholesteryl nonanoate at interfaces: Thermal stability and post collapse scenario

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.