A synchrotron x-ray study of a solid-solid phase transition in a two-dimensional crystal

A measurement and interpretation on a molecular level of a phase transition in an ordered Langmuir monolayer is reported. The diagram of surface pressure (π) versus molecular area of a monolayer of chiral (S)-[CF ₃-(CF₂)₉-(CH₂)₂-OCO- CH₂-CH(NH₃⁺)CO₂^-] over water shows a change in slope at about π_s = 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at π_s. The diffraction pattern at low pressures reveals two diffraction peaks of equal intensities, with lattice spacings d of 5.11 and 5.00 angstroms; these coalesce for π ≥ π_s. Structural models that fit the diffraction data show that at π > π_s the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At π < π_s there is a molecular tilt of 16° ± 7°. Independent x-ray reflectivity data yield a tilt of 26° ± 7°. Concomitant with the tilt, the diffraction data indicate a transition from a hexagonal to a distorted-hexagonal lattice. The hexagonal arrangement is favored because the -(CF₂)₉CF₃ moiety adopts a helical conformation. Compression to 70 millinewtons per meter yields a unit cell with increased crystallinity and a coherence length exceeding 1000 angstroms.