Chemistry Reference
In-Depth Information
pressure. Nucleation growth has been studied by fluorescence (probe) and BAM
(without probe) methods. In the case of lipid monolayers with probes added, the effect
of probes on the monolayer structures may be questioned. Nucleation and growth
of three-dimensional crystallites in fatty acid monolayers at constant pressure was
investigated by BAM. The growth of domains could be followed by BAM, and vari-
ous shapes ranging from dots and triangles to thread-like strips were observed.
Grazing incidence x-ray diffraction (GID) measurements have indicated that
both precollapse and collapsed state monolayers at the air-water interface can be
crystalline (Birdi, 1989). A general procedure was delineated that could provide
near-atomic resolution of two-dimensional crystal structures of
n
-triacontanoic
acid (C
29
H
59
COOH). A monolayer composed of rod-like molecules would gen-
erally pack in such a way that each molecule has six nearest neighbors, that is,
hexagonal cell.
4.6.3 lb-d
e p o S I T e d
f
I l m
S
T r u c T u r e
No direct method exists by which monolayer film structures on water can be stud-
ied. Therefore, the LB method has been used to study molecular structures in past
decades. The most useful method for investigating the detailed LB-deposited film
structure is the well-known electron diffraction technique (or the scanning probe
microscope [Birdi, 2002a]). The molecular arrangements of deposited mono-
and multilayer films of fatty acids and their salts, using this technique, have been
reported. The analyses showed that the molecules were almost perpendicular to the
solid surface in the first monolayer. It was also reported that Ba-stearate molecules
have a more precise normal alignment compared to stearic-acid monolayers. In some
investigations, the thermal stability of these films has been found to be remarkably
stable up to 90°C.
Based on structural analyses obtained by the electron diffraction technique,
the deposited films are known to be monocrystalline in nature, and thus, can be
regarded as a special case of a layer-bilayer mechanical growth forming almost
“two-dimensional” crystals. However, there is evidence that Ba-behenate multilay-
ers do in fact show absence of crystallization, as demonstrated by electron micro-
graphic studies.
In many of the early reports, it has been shown that deposited films obtained by
the usual process of monolayer transfer invariably contained holes, cracks, or simi-
lar imperfections. These are not surprising because, at higher Π, the more compact
film will be removed. Nevertheless, it would be an oversimplification to regard
the film transferred at higher Π as perfectly uniform, coherent, and defect free.
Artifacts are indeed introduced if proper care has not been taken either during the
transfer process, and the subsequent thermal evaporation of metal electrodes over
the film may disturb the film structure. In some cases, radio autographic inves-
tigations have indicated that these deposited layers are generally uniform with
no apparent gross defects. It is thus found that recent modifications in deposition
technique have made it possible to obtain films largely free from gross defects or
imperfections. However, defects or pinholes are easily detected since resistance is
very low around them.
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