TOWARD A DETERMINATION AT NEAR ATOMIC RESOLUTION OF 2-DIMENSIONAL CRYSTAL-STRUCTURES OF AMPHIPHILIC MOLECULES ON THE WATER-SURFACE - A STUDY BASED ON GRAZING-INCIDENCE SYNCHROTRON X-RAY-DIFFRACTION AND LATTICE ENERGY CALCULATIONS

A general approach is presented toward the determination at near atomic resolution of two-dimensional (2-D) crystalline structures of amphiphilic molecules at the air-water interface. It involves fitting of grazing incidence X-ray diffraction (GID) data from the 2-D crystal using molecular models, complemented by lattice energy calculations. In this way, one may eliminate ambiguities associated with assignment of the 2-D crystal symmetry, the molecular conformation, and the packing arrangement. The method was applied to the elucidation of the low temperature (5-degrees-C) structure of an uncompressed monolayer of triacontanoic acid C29H59CO2H. We deduced a packing of the molecules in a rectangular unit cell in plane group pg. The molecular chain axes are tilted from the vertical by 27-degrees along the short 5.53-angstrom axis of the unit cell and oriented so as to form the orthogonal O(perpendicular-to) packing type. The CO2H groups are not directly interlinked by O-H...O hydrogen bonds. A distinct improvement of the agreement between observed and calculated GID data was achieved by partial relaxation of the glide plane symmetry. The observed Bragg peaks suggested very anisotropic shapes for the crystallites. This anisotropy was correlated with the energy of attachment of a molecule to a specific crystallite facet.