Monolayers and 3D films of cholesteryl derivatives at the air-water interface

Abstract

Cholesterol and its derivatives have drawn much attention because of their relevance to biology. They have also been studied at the air-water (A-W) interface. Here, we have systematically investigated the two-dimensional (2D) and three-dimensional (3D) phases of cholesteryl derivatives at the A-W interface employing surface manometry, epifluorescence, Brewster-angle and reflection microscopy techniques. We found that the short-chain ester cholesteryl acetate forms a stable monolayer. The higher homologues cholesteryl heptanoate and cholesteryl octanoate do not form monolayers; they yield 3D crystallites at very large area per molecule. Interestingly, we found cholesteryl nonanoate to be in the crossover regime in the homologous series of cholesteryl esters. It spontaneously forms a fluidlike bilayer at the A-W interface. The higher homologue cholesteryl laurate forms an unstable bilayer phase. The long-chain esters cholesteryl myristate, cholesteryl palmitate, and cholesteryl stearate exhibit crystalline bilayer and 3D structures. Cholesteryl benzoate, which contains a bulkier phenyl group instead of the flexible alkyl chain, forms a crystalline bilayer. We discuss the assembly of molecules in the different phases at the A-W interface for the cholesteryl derivatives with respect to their packing in the bulk and the onset of the smectic liquid-crystalline phase.