Chemistry Reference
In-Depth Information
There are several parameters that affect on what type of LB film is produced.
These are the nature of the spread film, subphase composition and temperature, sur-
face pressure during deposition and deposition speed, the type and nature of the solid
substrate, and the time the solid substrate is stored in air or in the subphase between
deposition cycles.
The quantity and quality of the deposited monolayer on a solid support is mea-
sured by a so-called transfer ratio,
t
r
. This is defined as the ratio between the decrease
in monolayer area during a deposition stroke, Al, and the area of the substrate, As.
For ideal transfer, the magnitude of
t
r
is equal to 1. Depending on the behavior of
the molecule, the solid substrate can be dipped through the film until the desired
thickness of the film is achieved. Different kinds of LB multilayers can be produced
and/or obtained by successive deposition of monolayers on the same substrate (see
Figure 4.11). The most common one is the Y-type multilayer, which is produced when
the monolayer deposits on the solid substrate in both up and down directions. When
the monolayer deposits only in the up or down direction, the multilayer structure is
called either
Z
-type or
X
-type. Intermediate structures are sometimes observed for
some LB multilayers, and they are often referred to be
XY
-type multilayers.
The production of the so-called alternating layers that consist of two different
kind of amphiphiles is also possible by using suitable instruments. In such an instru-
ment, there is a trough with two separate compartments, both possessing a float-
ing monolayer of a different amphiphile. These monolayers can then be alternately
deposited on one solid substrate.
The monolayer can also be held at a constant surface pressure, which is enabled
by a computer-controlled feedback system between the electrobalance and the motor
responsible for the movements of the compressing barrier. This is useful when pro-
ducing LB films, that is, when the monolayer is deposited on a solid substrate.
The most simple procedure generally used is one in which a clean and smooth
solid surface (of suitable surface area) is dipped through the interface with the mono-
layer. Alternatively, one can also place the solid sample in the water before a mono-
layer is spread and then drawn up through the interface to obtain the film transfer.
It is obvious that such processes involving monomolecular film transfers will eas-
ily be disturbed by defects arising from various sources. As will be shown in the fol-
lowing text, these defects are in most cases easily detected. The structural analysis of
the molecular ordering within a single LB monolayer is important both to understand
how the environment in the immediate vicinity of the surface (i.e., solid) affects the
structure of the molecular monolayer and to ascertain how the structure of one layer
forms a template for subsequent layers in a multilayer formation.
Studies of the order within surfactant monolayers have been reported for many
decades. Multilayer assemblies have been studied by electron as well as infrared
absorption. Motivated by an older model proposed for the orientation of molecules
(Langmuir, 1933; Epstein, 1950), and by recent theoretical calculations, these two
potential models for tilt disorder in the monolayer have been examined. Both models
arise because the monolayer structure tries to compensate for the difference between
the equilibrium head-head and chain-chain distances that each piece of the mol-
ecule would want to attain if it were independent. In one model, the magnitude of
the tilt is fixed, but the tilt direction wanders slowly through the lattice. In the second
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