J. Chem. Phys. 107, 5531 (1997)

C60-propylamine adduct monolayers at the gas/water interface:
A Brewster angle microscopy and x-ray scattering study

Masafumi Fukuto, Konstantin Penanen, Ralf K. Heilmann, and Peter S. Pershan
Department of Physics and Division of Engineering and Applied Sciences,
Harvard University, Cambridge, Massachusetts 02138

David Vaknin
Ames Laboratory and Department of Physics and Astronomy,
Iowa State University, Ames, Iowa 50011


Brewster angle microscopy (BAM), x-ray specular reflectivity, and grazing- incidence x-ray diffraction (GID) studies of C60-propylamine adduct monolayers at the gas/water interface as a function of molecular area are reported. At large molecular areas (A > 150 Å2/molecule), BAM images reveal macroscopic heterogeneity in the film, consisting of the coexistence between regions covered with uniform solid-like monolayer and bare water surface. After compression to a limiting molecular area of 150 Å2/molecule, the film is observed to be homogeneous, with the uniform monolayer covering the entire available surface. Both the x-ray reflectivity results and the GID patterns are consistent with the formation of a uniform monolayer at A ~ 150 Å2/molecule, while the little dependence that the GID patterns have on the molecular area for A > 150 Å2/molecule is consistent with the heterogeneity in the film. Upon further compression to higher densities (A < 120 Å2/molecule), the x-ray reflectivity results suggest the formation of a partial layer either at the molecule/gas interface or at the molecule/water interface. In this high density regime, the shift in the observed GID pattern with the molecular area is much smaller than would be expected if the film were to remain a homogeneous monolayer, also consistent with the formation of an inhomogeneous partial layer. The analysis of the broad GID pattern observed from a uniform monolayer in terms of a model 2-D radial distribution function, implies a short range positional correlation, extending to only a few molecular distances. The average nearest neighbor distance (d ~ 13 Å), extracted from the GID analysis, is consistent with the limiting molecular area (A ~ 150 Å2/molecule) assuming local hexagonal packing. These results together with the sharp facets observed in the BAM images demonstrate that the monolayer when uniform is a two-dimensional amorphous solid.

Back to the trough page