Chemistry Reference
In-Depth Information
4.2 monolayer ISothermS on Water SurFaceS
Lipids with a suitable hydrophilic-lipophilic balance (HLB) are known to spread on
the surface of water to form monolayer films. It is obvious that if the lipid-like mol-
ecule is highly soluble in water, it will disappear into the bulk phase (as observed for
SDS). Thus, the criteria for a monolayer formation are that it exhibits very low solu-
bility in water. The alkyl part of the lipid points away from the water surface. The
polar group is attracted to the water molecules and is inside this phase at the surface.
This means that the solid crystal, when placed on the surface of water, is in equilib-
rium with the film spread on the surface. A detailed analysis of this equilibrium has
been given in the literature (Gaines, 1966; Adamson and Gast, 1997; Birdi, 2009).
The thermodynamics allows one to obtain extensive physical data on this system. It
is thus apparent that, by studying only one monolayer of the substance, the effect of
temperature can be very evident.
4.3 SelF-aSSembly monolayer (Sam) FormatIon
The most fascinating characteristic some amphiphile molecules exhibit is that, when
mixed with water, they form
self-assembly structures
. This was already discussed
in Chapter 2 on micelle formation. Since most of the biological lipids also exhibit
self-assembly structure formation, this subject has been given much attention in the
literature (Birdi, 1999). Lipid monolayer studies thus provide a very useful method
to obtain information about SAM formation, both concerning technical systems and
cell bilayer structures.
4.3.1 S
T a T e S
of f
m
o n o l a y e r S
S
p r e a d
o n
W
a T e r
S
u r f a c e
It is found that, even a monolayer of lipid (on water), when compressed can undergo
various states. In the following text, the various states of monomolecular films will
be described as measured from the surface pressure, Π, versus area,
A
, isotherms, in
the case of simple amphiphile molecules. On the other hand, the Π-
A
isotherms of
biopolymers will be described separately since these have a different nature.
However, before presenting this analysis, it is necessary to consider some param-
eters of the two-dimensional states, which should be of interest in this context. We
need to start by considering the physical forces acting between the alkyl-alkyl
groups (parts) of the amphiphiles, as well as the interactions between the polar head
groups. In the process where two such amphiphile molecules are brought closer dur-
ing the Π-
A
measurement, the interaction forces would undergo certain changes that
would be related to the packing of the molecules in the two-dimensional plane at the
interface in contact with water (subphase).
This change in packing is thus analogous conceptually to the three-dimensional
P-V isotherms, as is well known in classical physical chemistry (Gaines, 1966;
Adamson and Gast, 1997; Birdi, 1989). We know that, as the pressure, P, is increased
on a gas in a container, when T < T
cr
, the molecules approach closer, and transition
to a liquid phase takes place. Further compression of the
liquid state
results in the
formation of a
solid phase
.
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