Chemistry Reference
In-Depth Information
1
3
1
1
<
Vl(AI)
<
Vl(AI)
<
Vl(AI)
>
1
Vl(AI)
≥
1
Vl(AI)
=
1
2
3
L
II
OR H
II
L
a
Cubic
Hex
Micelles
CCP
Water-in-oil
Oil-in-water
1
Figure 1.2
Relationship between the critical packing parameter (CCP) and the
expected morphology in lyotropic liquid crystals.
forming a convex interface against water. On the other hand, for
v
/(
Al
)
1, a
phase inversion occurs and “water-in-oil” morphologies are found, with
concave lipid head surfaces against the water. The fl at interfaces, correspond-
ing to the L
α
lamellar phase are found for
v
/(
Al
)
>
1. More in details, inverted
micelles/inverted hexagonal, inverted cubic phases, lamellar, hexagonal phases,
and direct micelles are expected when the CPP has a value of
v
/(
Al
)
=
>
1 ,
v
/
1 ;
3
<
vAl
/(
)
<
1
2
, and
vAl
/(
)
<
1
3
, respectively (Israelachvili
(
Al
)
≥
1 ,
v
/(
Al
)
≈
et al., 1991; Jonsson et al., 2001).
The CPP has been widely employed to predict and rationalize differences
observed among liquid crystalline phases and can capture some of the physical
changes. For example, changes occurring on CCP with temperature and com-
position can be understood to some extent. Increases in temperature leads to
partial breaking of hydrogen bonds, with the number of water molecules
hydrating the polar heads of the lipids, and this leads to an increase of the CPP
because it decreases the effective head area. This can well explain the cubic-
to-hexagonal transition, for example (Qiu and Caffrey, 2000). Similar argu-
ments can be used to explain the L
α
to cubic transition induced by temperature
raises. However, other transitional changes, such as the concentration-induced
lamellar
cubic transition remains unexplained by the application of the CCP
concepts. The concept of the CPP is limited to a qualitative interpretation of
the phase diagrams and cannot be used to bring insight into the structural
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