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.