Langmuir Monolayer Diffraction Study
Grazing incidence X-ray Diffraction is used to reveal the structure of
Langmuir monolayers at a molecular level. Because the intensity of
diffraction from monolayers is very weak, strong synchrotron radiation
from a multi-pole wiggler is required.
The figure shows equal intensity contours of X-rays diffracted by an
arachidic monolayer on a 10-4M
cyaninedye aqueous solution at various
surface pressures. The diffraction peak at lower
2q corresponds to (1,-1)
reflection, and the other peak corresponds to overlapping (1,0) and (0,1)
reflections. It is found that arachidic acid molecules
on a 10-4M cyanine
dye solution at low surface pressure tilt toward their next nearest
neighbors in a two dimensional hexagonal lattice, and stand upright
at higher pressures. This is in contrast to the case on pure water
where the molecules tilt toward their nearest neighbors
at low surface pressure.
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