X-ray specular reflectivity at the liquid/gas interface
of dihexadecyl phosphate (DHDP) on pure H2O and
on
a series of pigment-containing aqueous solutions are reported along
with visible absorption spectra of
corresponding monomolecular Langmuir−Blodgett films on quartz
substrates. Molecular level interpretation
of the reflectivity from DHDP on pure water reveals that at large
surface pressure (>10 mN/m), the film is
closely packed with practically untilted hydrocarbon chains and
hydrated phosphate headgroups. On solutions
containing either water-soluble cationic tetraazaphthalocyanines or
tetrapyridylporphyrins, significant changes
in the organization of the lipid with respect to that on pure water are
found. Total film thicknesses are larger
and consistent with the adsorption of a single pigment layer contiguous
to the headgroup, whereas the
hydrocarbon tail region is shorter, suggestive of tilted
alkyl chains. In addition, film thicknesses for
phthalocyanine-containing films suggest formation of an iodide
counterion layer underneath the plane
containing
the pigments. This sharply contrasts the interfacial profile
obtained for porphyrin-containing films, in which
the iodide counterions appear to exist within the pigment
plane. Visible absorption spectra of all transferred
films indicate a closely packed single pigment layer, consistent with
the reflectivity results. The optical
spectra of the pigment are preserved (in relation to the aqueous
solution monomer spectra) in the transferred
film, indicating a suppression of pigment aggregation.
Reflectivity measurements at large molecular areas
on pure water indicate that DHDP forms an inhomogeneous film,
suggestive of phase segregation; on the
pigment-containing solutions, DHDP induces (through attractive Coulomb
interactions) the adsorption of a
homogeneous monopigment layer. The existence of a complete pigment
monolayer over the measured surface
pressure−molecular area (π−A) isotherms has been
evidenced by both X-ray reflectivity and visible optical
studies. Preservation of pigment functionality has been
demonstrated through the process of Coulomb
association of the chromophores with charged lipid monolayer headgroups
at the air/water interface. The
potential for applications as model photosynthetic antennae will be
discussed.
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