Spread Lipid Films on Liquid Surfaces (Langmuir–Blodgett Films) - Surface and Colloid Chemistry: Principles and Applications - page 79

table 4.1
magnitudes of ao for different Film-Forming
molecules on the Surface of Water

compound ao (Å 2 )

Straight-chain acid 20.5

Straight-chain acid (on dilute HCl) 25.1

N -fatty alcohols 21.6

Cholesterol 40

Lecithins ca. 50

Proteins ca. 1 m 2 /mg

Diverse synthetic polymers (poly-amino acids, etc.) ca. 1 m 2 /mg

Source: Birdi, K. S., Lipid and Biopolymer Monolayers at Liquid
Interfaces , Plenum Press, New York, 1989; Gaines, 1966;
Adamson, A. W. and Gast, A. P., Physical Chemistry of
Surfaces , 6th ed., Wiley-Interscience, New York, 1997.

In general, the collapse pressure, Π col , is the highest surface pressure to which a
monolayer can be compressed without a detectable movement of the molecules in the
films to form a new phase (see Figure 4.7).
In other words, this pressure will be related to the nature of the substance and the
interaction between the subphase and the polar part of the lipid or the polymer mol-
ecule. However, some misinterpretation of the collapse phenomena exists, as found
in the older literature.
As described here, the monolayer of a lipid can be formed by different spreading
methods. The thermodynamics of the Π col analysis is given in the literature (Birdi,
1989). The monolayer collapse has been shown to provide much information also in
the case of protein monolayers.

4.3.1.2 reaction in monolayers at liquid Surfaces
The lipid monolayer is a well-defined structure with the polar part pointing toward
the water phase, and the apolar (hydrophobic) part pointing away from the water
phase. It is observed that monolayers are very sensitive model to investigate interac-
tions between lipids and other substances (such as ions in the water subphase). The
long-chain fatty acids exhibit gas-L ex -solid film states at high pH. However, the same
films, on substrates with Ca or Mg ions, exhibit only the solid state. Interestingly, this
method may also be used to investigate the kinetics of penetration by measuring the
change in Π with time.
ELLIPSOMETRY: The structure of liquid surfaces with monomolecular films
can be studied by measuring the light reflected from the surface. The range of thick-
ness that one generally considers to be measured varies from 100 to 1000 Å (10-100
nm). However, in monolayers in which the molecules are oriented and the thick-
ness involved is 5-50 Å, the methods have been not easily pursued. In a differential
method in which two beams of light from the same incandescent lamp were directed