Chemistry Reference
In-Depth Information
of the solid slide across the interface. On the other hand, in X-type films, transfer
takes place only when the slide is dipped, and in the Z-type, only when the slide is
withdrawn. The orientation is termed
exotropic
when the methyl groups touch the
solid surface and the polar groups (carboxyl) remain away from it in the first layer.
If the orientation of these two groups is reversed, it is termed
endotropic
. Thus, an
X-type film is made up of a series of exotropic layers, a
Y
-type a series of alternating
exotropic and endotropic layers, and an
Z
-type a series of endotropic layers.
Z
-type
films are, rather uncommon.
There are two crucial factors for making satisfactory electrical measurements on
the LB films uniform packing and thickness. These two requirements are generally
fulfilled to a greater extent when one uses LB films. Metrical thicknesses of several
fatty acid monolayers have been measured using the best-known optical methods. A
high quality of fringes and, accordingly, high-precision measurements were obtained
employing a protective colloid ion layer.
As regards uniformity in the packing of LB films, the selection and purity of
the substrate needs to be carried out very carefully. In fact, to achieve the required
uniformity in the packing of LB films, the deposition of the first monolayer has been
found to be very critical since any voids or imperfections in it generally leads to major
disruption of the subsequent monolayers. Therefore, it has been suggested that the
solid slide emerging from the liquid surface must be completely dried before being
reimmersed. If the wet slide is reimmersed, generally no multilayers are adsorbed
(for lecithin, fatty acids, and protein monolayers).
Ba- or Cu-stearate films could be built up to as many as 3000 layers simply by
repeated dipping and withdrawal processes (Birdi, 1989, 1999). On the other hand,
fatty-acid films or other sources have cracking and fogging tendencies. It was sug-
gested that addition of the Cu ion [10
−6
M] solved these problems (Birdi, 1999).
4.6.2 o
p T I c a l
a n d
r
e l a T e d
m
e T h o d S
of f
a
b S o r p T I o n
Optical reflection from a monomolecular film must be measured from the interface
with a very small amount of material present. Therefore, in these methods, repeated
interaction of the light beam with one or more identical films is generally employed.
The simplest way to observe a light beam that has passed through several identical
monolayers is to transfer portions of the layer to suitable transfer end plates, which
are then stacked and examined in a conventional spectrophotometer. This method
was used where monolayers of chlorophyll were deposited on glass slides by the LB
method. Ferrodoxin and chlorophyll monolayers were investigated by measuring the
spectra (550-750 nm) of these films at the interface.
To examine monolayers on liquid surfaces in situ, multiple interactions by mount-
ing mirrors above and below the surface have been used. X-ray reflectivity measure-
ments have been used to study the counterion overlayers at the interface between
electrolyte solutions and monolayers of carboxylic acids terminated alkanethiols
self-assemblies on Au (Birdi, 1999).
The Brewster angle microscope (BAM) has been used to investigate the mono-
layers of polyamic acid at the air-water interface in situ and without the addition
of probes. The morphology of the monolayer was observed as a function of surface
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