Microstructure of amphiphilic monodendrons at the air-water interface

Grazing incident X-ray diffraction and X-ray reflectivity have been performed on Langmuir monolayers of low generation monodendrons containing a crown-ether polar group, azobenzene spacer, and varying number of peripheral alkyl chains of 1, 2, 4, and 8. We observe that the cross-sectional mismatch between the bulky polar head and the alkyl tails has a profound effect on the local ordering of the alkyl tails. It is found that the alkyl chains in a single-tail molecule are significantly tilted away from the surface normal. The tilt is eliminated in molecules with two or more alkyl chains where the cross-sectional mismatch is in favor of the peripheral tails. The molecule with one tail possesses a supercell orthorhombic packing caused by structural nonequivalency on the neighboring tails. The two- and four-tail molecules form a mixed structure best described by a quasi-hexagonal unit cell, and the eight-tail molecule forms a more stable hexagonal unit cell. Peripheral tails for these molecules are in standing-off orientation. We suggest that the steric constraints cause lower correlations and a staggered packing structure of monolayers from the eight-tail molecule. We suggest that branching alkyl tails off the same phenyl ring and the presence of the phenyl rings in the vicinity of the branching are limiting factors on the chain packing at the air-water interface in monodendrons with multiple peripheral tails. We conclude that a significant portion of the molecules is submerged in the water subphase and possesses a "kink" shape.