INPLANE X-RAY-DIFFRACTION FROM MONOLAYERS OF PERFLUORINATED FATTY-ACIDS - EVIDENCE FOR AZIMUTHAL ORDERING IN THE CONDENSED-PHASE

We report the results of new grazing incidence X-ray diffraction studi es of Langmuir monolayers of F(CF2)11COOH and F(CF2)10CH2COOH supporte d on water; in each case the locations and intensities of the first th ree diffraction peaks ((1, 0), (1, 1) and (2, 0)) have been measured. These new data are consistent with earlier ones in that both monolayer s have simple hexagonal packing of molecules with their long axes near ly perpendicular to the water surface at maximum surface densities. By comparing the ratios of the intensities of the several diffraction pe aks observed in each case, one can conclude that at least one of the m olecules is not in a free rotator phase. These ratios are used, togeth er with a fit to the Debye-Waller factor, to discriminate between diff erent models of the azimuthal ordering in the monolayer. A model of cl ose packed monolayer with freely rotating molecules is inconsistent wi th the integrated diffraction intensities for F(CF2)11COOH. Much bette r agreement is obtained when an orientational ordering of the long axe s of the amphiphilic molecules is assumed in the monolayer. On the oth er hand, in the case of F(CF2)10CH2COOH, either of the two models, fre e rotating or azimuthal ordering, appears to be equally consistent wit h the diffraction data.