Self-assembly of crystalline films of interdigitated long-chain cholesteryl esters at the air-water interface

Cholesterol and some cholesterol-like derivatives such as stigma-sterol and cholesterol acetate self-assemble into semicrystalline monolayers, bilayer s, or trilayers on the surface of water, as has been reported previously. H ere 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 Angst rom (2) ) and the attached hydrocarbon chain (20 Angstrom (2)). As shown by grazing incidence X-ray diffraction (GIXD), cholesteryl esters spontaneous ly self-assemble on the surface of water in order to form interdigitated bi layer films. The unit cell parameters of such bilayers at the water surface almost match those of their three-dimensional counterparts. Other experime nts such as surface pressure vs area per molecule isotherms and ellipsometr y measurements corroborate this result. To control the growth of the interd igitated films, we have used "tailor-made" additives (long-chain alcohols o r acids) that fill the voids in the chain region of the cholesteryl ester l ayer caused by the structural mismatch, thereby inhibiting interdigitation. Using this strategy, we were able to form a mixed monolayer composed of th e ester and the additive molecules in a 1:1 ratio, where the hydrocarbon ch ain of additive molecule intercalates between the ester chains and effectiv ely inhibits the growth of the interdigitated bilayer.