Chemistry Reference
In-Depth Information
In biological systems, the lung fluid exhibits surface pressure characteristics that
are related to its lipid composition. The ratio between two different lipids has been
shown to be critical for lung function in newborn babies. Recent studies of mixed
monolayers have been made using AFM (Birdi, 2002a).
4.5 monolayerS oF macromoleculeS
at the Water SurFace
Many macromolecules (such as proteins) form stable monolayers at the water surface
if the hydrophilic-lipohilic balance (HLB) is of the right quantity. Especially, almost
all proteins (hemoglobin, ovalbumin, bovine serum albumin, lactoglobulin, etc.) are
reported to form stable monolayers at the water surface.
4.6 lanGmuIr-blodGett (lb) FIlmS (tranSFer
oF lIPId monolayerS on SolIdS)
Some decades ago (Langmuir, 1920), it was reported that, when a clean glass plate
was dipped into water covered by a monolayer of oleic acid, an area of the monolayer
equal to the area of the plate dipped was deposited on withdrawing the plate. Later,
it was found that any number of layers could be deposited successively by repeated
drippings, later called the Langmuir-Blodgett (LB) method (Petty, 1996; Roberts,
1990; Birdi, 1989).
In another context, the electrical properties of thin films obtained by different
procedures, for example, thermal evaporation in vacuum, have been investigated in
much detail. However, the films deposited by the LB technique have only recently
been used in electrical applications. Thickness in LB films can be varied from only
one monomolecular layer (ca. 25Å = 25 10
−10
m), while this is not possible by evapo-
ration procedures.
Monomolecular layers (LB films) of lipids are of interest in a variety of appli-
cations, including the preparation of very thin controlled films for interfaces in
solid-state electronic devices (Gaines, 1966; Birdi, 1989, 1999). Langmuir (1920)
investigated the process of transferring the spread monolayer film to a solid surface
by raising the solid surface through the interface. The process of transfer is depicted
(schematic) in Figure 4.11. It is seen that, if the monolayer is in a closely packed
state, then it is transferred to the solid surface, most likely without any change in
the packing density. Detailed investigations have, however, shown that the process
of transfer is not as simple as shown in Figure 4.11. The monolayer may or may not
be stable on a solid surface, and defects may be present. However, by using modern
AFM techniques, one can determine the molecular orientation and packing of such
LB films (Birdi, 1989, 1999, 2002, 2002a; Gaines, 1966).
Scientists are currently using
LB film assemblies as solutions to problems in
diverse areas such as microlithography, solid-state polymerization, light guiding,
electron tunneling, and photovoltaic effects. In the case of such films as Mg stear-
ate, if a clean glass slide is dipped through the film, a monolayer is adsorbed on the
downstroke. Another layer is adsorbed on the upstroke. Under careful conditions,
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