STRUCTURAL TRANSITIONS IN A MONOLAYER OF FLUORINATED AMPHIPHILE MOLECULES

We present results of an extensive x-ray diffraction study of a monolayer of C10F21CH2COOH spread on water (pH2) at 19.1-degrees-C. Lever-rule analysis of the in-plane scattering is used to show that there is a coexistence region between ordered condensed islands and a dilute disordered phase. The coexistence region is found to be bounded by molecular areas of 29 angstrom 2 and 2000 +/- 600 angstrom 2, in agreement with the pressure-area isotherm. The molecular tilt of the ordered phase remains unchanged from closet packing (near collapse of the monolayer) throughout the coexistence region, and has a value of 2 +/- 3 degrees with respect to the normal to the liquid surface. These results are contrasted with those for hydrocarbon monolayers in which the onset of order in the coexistence region is close to the condensed phase boundary, and the ordered phase is compressible with a continuously variable tilt angle ranging from 30 to 0 deg at closet packing. The differences are attributed to the enhanced chain stiffness of the fluorinated chain. To illustrate this interpretation, preliminary scattering results are presented for a monolayer of C8F17(CH2)4COOH on water, which has some of the features of the aliphatic lipid monolayers. Recent molecular dynamics simulations have been found to reproduce all the important qualitative features of these systems [S. Shin, N. Collazo, and S. A. Rice, J. Chem. Phys. 96, 1352 (1992)].