Spread Lipid Films on Liquid Surfaces (Langmuir–Blodgett Films) - Surface and Colloid Chemistry: Principles and Applications - page 78

increase in Π is observed, as shown in Figure 4.5. In the case of straight-chain mol-
ecules, such as stearyl alcohol, the sudden increase in Π is found to take place at
A = 20 − 22 Å 2 , at room temperature (that is, much lower than the phase transition
temperature).
From these descriptions, it is seen that the films may, under given experimental
conditions, show three first-order transition states, such as (i) transition from the
gaseous film to the liquid-expanded (L ex ), (ii) transition from the liquid-expanded
(L ex ) to the liquid-condensed (L co ), and (iii) from L ex or L co to the solid state if the
temperature is below the transition temperature. The temperature above which no
expanded state is observed has been found to be related to the melting point of the
lipid monolayer.

4.3.1.1.4 Collapse States
The measurements of Π versus A isotherms generally exhibit, when compressed, a
sharp break in the isotherms that has been connected to the collapse of the mono-
layer under given experimental conditions. The monolayer of some lipids, such as
cholesterol, is found to exhibit an unusual isotherm (Figure 4.7). The magnitude of Π
increases very little as compression takes place. In fact, the collapse state or point is
the most useful molecular information from such studies. It has been found that this
is the only method that can provide information about the structure and orientation
of amphiphile molecules at the surface of water (Birdi, 1989).
However, a steep rise in Π is observed and a distinct break in the isotherm is
found at the collapse. This occurs at Π = 40 mN/m and A = 40 Å 2 .
This value of A co corresponds to the cholesterol molecule oriented with the
hydroxyl group pointing toward the water phase. Atomic force microscope (AFM)
studies of cholesterol in Langmuir-Blodgett (LB) films has shown that there exist
domain structures (see Chapter X). This has been found for different collapse lipid
monolayers (Birdi, 2003). Different data have provided much information about the
orientation of lipid on water (Table 4.1).
It is worth mentioning that monolayer studies are the only procedure that allows
estimation of the area per molecule of any molecule as situated at the water surface.

45

Cholesterol

Collapse

0

40
Area/Molecule

FIGure 4.7 Π versus A isotherm of cholesterol monolayer on water.