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Science 2 December 1988:
Vol. 242. no. 4883, pp. 1286 - 1290
DOI: 10.1126/science.242.4883.1286
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Articles

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

SHARON GRAYER WOLF 1, EHUD M. LANDAU 1, MEIR LAHAV 1, LESLIE LEISEROWITZ 1, MOSHE DEUTSCH 2, KRISTIAN KJAER 3, and JENS ALS-NIELSEN 3

1 Structural Chemistry Department, Weizmann Institute of Science, Rehovot 76100, Israel.
2 Physics Department, Bar-Ilan University, Ramat Gan 52100, Israel.
3 Physics Department, Risø National Laboratory, DK4000 Roskilde, Denmark.

A measurement and interpretation on a molecular level of a phase transition in an ordered Langmuir monolayer is reported. The diagram of surface pressure (pgr) 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 pgrs= 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at pgrs. 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 pgr gepgrs. Structural models that fit the diffraction data show that at pgr> pgrs the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At pgr < pgrs 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.

Submitted on June 1, 1988
Accepted on September 7, 1988


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Molecular Recognition at Crystal Interfaces.
I. WEISSBUCH, L. ADDADI, and L. LEISEROWITZ (1991)
Science 253, 637-645
   Abstract »    PDF »
Crystallinity of the Double Layer of Cadmium Arachidate Films at the Water Surface.
F. LEVEILLER, D. JACQUEMAIN, M. LAHAV, L. LEISEROWITZ, M. DEUTSCH, K. KJAER, and J. ALS-NIELSEN (1991)
Science 252, 1532-1536
   Abstract »    PDF »


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