A Synchrotron X-ray Study of a Solid-Solid Phase Transition in a Two-Dimensional Crystal

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Science  02 Dec 1988:
Vol. 242, Issue 4883, pp. 1286-1290
DOI: 10.1126/science.242.4883.1286


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)-[CF3-(CF2)9-(CH2)2-OCO-CH2-CH (NH3+)CO2-] over water shows a change in slope at about πs= 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at πs. The diffraction pattren 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 ≥πs. Structural models that fit the diffraction data show that at π> πs the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At π < πs 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.