Chemistry Reference
In-Depth Information
be used. This procedure is the same as the one used in the case of three-dimensional
gas systems.
4.3.1.1.2 Liquid Expanded and Condensed Films
The Π versus A data are found to provide much detailed information about the state
of monolayers at the liquid surface. In Figure 4.6, some typical states are shown. The
different states have been very extensively analyzed, and will therefore be described
in the following text.
In the case of simple amphiphiles (fatty acids, fatty alcohols, lecithins, etc.), in
several cases, transition phenomena have been observed between the gaseous and
coherent states of films, which show a very striking resemblance to the condensation
of vapors to liquids in three-dimensional systems. The liquid films shows various
states in the case of some amphiphiles, as shown in Figure 4.6 (schematic). In fact, if
the Π versus
A
data deviate from the ideal equation, then the following interactions
may be expected in the film:
Strong van der Waals forces
Charge-charge repulsions
Strong hydrogen bonding with subphase water
This means that the data deviations allow estimation of these interactions.
4.3.1.1.2.1 Liquid Expanded Films (L
exp
)
In general, there are two distinguish-
able types of liquid films. The first state is called the liquid expanded (L
exp
) (Gaines,
1966; Chattoraj and Birdi, 1984; Adamson and Gast, 1997). If the Π-
A
isotherm is
extrapolated to zero Π, the value of
A
obtained is much larger than that obtained for
close-packed films, shows that the distance between the molecules is much larger
than that in the solid film (to be discussed in later text). These films exhibit very
characteristic elasticity.
4.3.1.1.2.2 Liquid Condensed Films (L
co
)
As the area per molecule (or the
distance between molecules) is further decreased, a transition to a so-called liq-
uid condensed (L
co
) state is observed. These states have also been called “solid
expanded” films (Adam, 1941; Gaines, 1966; Birdi, 1989, 1999; Adamson and
Gast, 1997). The Π versus
A
isotherms of
n
-pentadecylic acid (amphiphile with a
single alkyl chain) have been studied, as a function of temperature (Birdi, 1989).
Π
‑A
isotherms for two-chain alkyl groups, such as lecithins, also showed similar
behavior.
4.3.1.1.3 Solid Films
As the film is compressed, a transition to a solid film is observed, which collapses
at higher surface pressure. The Π versus
A
isotherms, below the transition tem-
peratures, show the liquid to solid phase transition. These solid films have been also
called
condensed films.
. They are observed in such systems where the molecules
adhere to each other through van der Waals forces very strongly. The Π-
A
isotherm
shows generally no change in Π at high
A
, while at a rather low
A
value, a sudden
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