Spread Lipid Films on Liquid Surfaces (Langmuir–Blodgett Films) - Surface and Colloid Chemistry: Principles and Applications - page 74

Solid Monolayer

Water

FIGure 4.3 Orientation of solid films of aliphatic alcohols or acids as films.

Solid Liquid Gas

FIGure 4.4 Lipid monolayer phases.

structure, which is the SAM phenomena. It has been suggested that, during evolu-
tion, life was first conceived by a SAM process.
The most convincing results were those obtained with normal fatty alcohols and
acids. Their monomolecular films are stable and exhibit very high surface pressures
(Birdi, 1989, 1999). A steep rise in Π is observed around 20.5 Å 2 regardless of the
number of carbon atoms in the chains. The volume of a [-CH 2 -] group is 29.4 Å 3 ,
which gives the length of each [-CH 2 -] group perpendicular to the surface, or the
vertical height of each group, as ca. 1.42 Å. It compares very satisfactorily with the
x-ray data, which has a value of 1.5 Å. This means that such straight-chain lipids are
oriented in this compressed as vertical (Figure 4.3).
As high pressures lead to transition from gas to liquid or to solid phases in the
three-dimensional system, a similar state of affairs would be expected in the two-
dimensional film compression Π versus A isotherms (Figure 4.4), as described in the
following text.

4.3.1.1.1 Gaseous Films
The most simple type of amphiphile film or a polymer film would be a “gaseous”
state. This film would consist of molecules that are at a sufficient distance apart from
each other such that lateral adhesion (van der Waals forces) are negligible. However,
there is sufficient interaction between the polar group and the subphase that the film-
forming molecules cannot be easily lost into the gas phase, and the amphiphiles are
almost insoluble in water (subphase).
When the area available for each molecule is many times larger than molecular
dimension, the gaseous-type film [state 1] would be present. As the area available
per molecule is reduced, the other states, for example, liquid-expanded [L ex ], liquid-
condensed [L co ], and, finally, the solid-like [S or solid-condensed] states would be
present.
The molecules will have an average kinetic energy, that is, 1/2 k B T, for each
degree of freedom, where k is the Boltzmann constant (=1.372 10 -16 erg/T), and T is
the temperature. The surface pressure measured would thus be equal to the collisions