Self-assembly and molecular packing in cholesteryl esters at interfaces

To understand the self-assembly and molecular packing in cholesteryl esters relevant to biological processes, we have studied them at the air-water and air-solid interfaces. Our phase and thickness studies employing imaging ellipsometry and atomic force microscopy along with surface manometry show that the molecular packing of cholesteryl esters at interfaces can be related to Craven's model of packing, given for bulk. At the air-water interface, following Craven's model, cholesteryl nonanoate and cholesteryl laurate exhibit a fluidic bilayer phase. Interestingly, we find the fluidic bilayer phase of cholesteryl laurate to be unstable and it switches to a crystalline bilayer phase. However, according to Craven, only cholesteryl esters with longer chain lengths starting from cholesteryl tridecanoate should show the crystalline bilayer phase. The thickness behavior of different phases was also studied by transferring the films onto a silicon substrate by using the Langmuir-Blodgett technique. Texture studies show that cholesterol, cholesteryl acetate, cholesteryl nonanoate, cholesteryl laurate, and cholesteryl myristate exhibit homogeneous films with large size domains, whereas cholesteryl palmitate and cholesteryl stearate exhibit less homogeneous films with smaller size domains. We suggest that such an assembly of molecules can be related to their molecular structures. Simulation studies may confirm such a relation.