Chemistry Reference
In-Depth Information
4
Spread Lipid Films
on Liquid Surfaces
(Langmuir-Blodgett
Films)
4.1 IntroductIon
Ancient Egyptians are known to have spread very small amounts of olive oil over
the water while their ships were sailing into harbors. Because this oil was known to
appreciably calm the waves, navigation into the harbors was made easier.
It was found centuries later that some lipid-like substances (almost insoluble in
water) formed
self-assembly monolayers
(SAMs) on the surface of water (Gaines,
1966; Adamson and Gast, 1997; Birdi, 1989, 1999, 2002; Chattoraj and Birdi, 1984).
A few decades ago, experiments showed that monomolecular films of lipids could be
studied by using rather simple experimental methods (Figure 4.1).
When a surface-active agent (such as surfactant or a soap) is dissolved in water
and adsorbs preferentially at the surface (surface excess, Γ), it shows that the concen-
tration of the surface-active agent may be as high as 1000 times more than in bulk.
The decrease in surface tension indicates this and also suggests that only a mono-
layer is present at the surface. For example, in a solution of sodium dodecyl sulfate
(SDS) of concentration 0.008 mol/L, the surface is completely covered with SDS
molecules. In this chapter, systems consisting of lipids presenting as monolayers on
water will be analyzed. In these, almost all the substance applied to the surface (in
the range of few micrograms) is supposed to be present at the interface. This means
that one knows quantitatively the magnitude of surface concentration (same as the
surface excess, Γ).
Thin organic films of thickness of 20 Å (greater than 2 nm) or more are now
found to be very useful structures. In 1774, Benjamin Franklin reported the effect of
very small amounts of oil on the surface of water. However, in 1920, Langmuir was
awarded the Nobel Prize for studying these monomolecular films in an apparatus
(Figure 4.1).
If one places a very small amount of a lipid on the surface of water, it may affect
surface tension in different ways. It may not show any effect (such as in the case
of cholesterol), or it may show a drastic decrease in surface tension (such as in the
case of stearic acid or tetra-decanol). An amphiphile molecule will adsorb at the
69
Search WWH ::
Custom Search