| Abstract
| - We compare the packing characteristics of alkyl chainsin Langmuir monolayers of nonchiral and racemiccompounds as determined from available grazing incidence X-raydiffraction data. The analysisdemonstrates a gradual change of the projected unit cell dimensionsfrom those of a hexagonal packingof hydrocarbon chains, characteristic of high-temperature monolayerphases, to one of two more denserectangular packing modes with the projected unit cell dimensions 5.0× 7.5 Å2 and 4.4 × 8.7 Å2,characteristicof low-temperature phases. The 5.0 × 7.5 Å2 unitcell incorporates the well-known herringbone arrangement,with an ideally 90° dihedral angle between the planes of carbonbackbone chains. The 4.4 × 8.7 Å2cell,almost never observed in 3D structures, is characterized by a 40°dihedral angle. We characterize thepacking modes by lattice energy calculations. The distribution ofthe projected unit cell dimensions forthe various Langmuir monolayers reveals no discontinuity in the localmolecular order between crystallinephases and mesophases. The local symmetry of the molecularpacking, as determined from the X-ray data,is compared with the symmetry deduced from the Landau theory of phasetransitions. The symmetry ofthe local order in the mesophases is not maintained on the long-rangelength scale. We show that twophases of the same local molecular arrangement, a herringbone-orderedtwo-dimensional crystal and thecorresponding one-dimensional mesophase, possess mutually orthogonaldirections of glide symmetry.
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