Molecular packing of amphiphiles with crown polar heads at the air-water interface

An amphiphilic compound containing a benzyl-15-crown-5 focal point, azobenzene spacer, and a dodecyl tail as a peripheral group has been investigated at the air-water interface. X-ray reflectivity and grazing incident diffraction (XGID) were performed on the Langmuir monolayers to elucidate molecular packing and orientation of molecular fragments for the compound with mismatch between cross-sectional areas of hydrophobic and hydrophilic segments. At high surface pressure, we observed intralayer packing of the alkyl tails with doubling parameters of the conventional orthorhombic unit cell (supercell) and long-range positional ordering. High tilt of the alkyl tails of 58degrees from the surface normal was a signature of molecular packing caused by a large mismatch between the cross-sectional areas of the polar head (45 Angstrom(2)) and the alkyl tail (20 Angstrom(2)).