Self-Assembly of Crystalline Films of Interdigitated Long-Chain Cholesteryl Esters at the Air−Water Interface

Abstract

Cholesterol and some cholesterol-like derivatives such as stigma-sterol and cholesterol acetate self-assemble into semicrystalline monolayers, bilayers, or trilayers on the surface of water, as has been reported previously. Here we have extended our thin film studies toward cholesteryl esters that, in their bulk crystals, pack as interdigitated bilayers due to the mismatch between the cross-sectional area of the rigid cholesterol moiety (40 Å2) and the attached hydrocarbon chain (20 Å2). As shown by grazing incidence X-ray diffraction (GIXD), cholesteryl esters spontaneously self-assemble on the surface of water in order to form interdigitated bilayer films. The unit cell parameters of such bilayers at the water surface almost match those of their three-dimensional counterparts. Other experiments such as surface pressure vs area per molecule isotherms and ellipsometry measurements corroborate this result. To control the growth of the interdigitated films, we have used “tailor-made” additives (long-chain alcohols or acids) that fill the voids in the chain region of the cholesteryl ester layer caused by the structural mismatch, thereby inhibiting interdigitation. Using this strategy, we were able to form a mixed monolayer composed of the ester and the additive molecules in a 1:1 ratio, where the hydrocarbon chain of additive molecule intercalates between the ester chains and effectively inhibits the growth of the interdigitated bilayer.