Chemistry Reference
In-Depth Information
9.1 LIPID SELF-ASSEMBLY AND TRANSITIONS BETWEEN
SELF-ASSEMBLED STRUCTURES
9.1.1
Lipid Self-Assembly
Amphiphilic lipids and surfactants self-assemble in aqueous environments to
form a variety of possible structures, depending on the solubility and effective
geometry of the assembling amphiphiles. These structures form spontaneously
and in the absence of physical or chemical changes to the system are thermo-
dynamically stable. The spectrum of frequently encountered structures is
represented in Figure 9.1, which shows the usual progression of these struc-
tures as water content is decreased from left to right. Not all amphiphile
systems show all of the structures represented.
Micelles are the simplest geometry, and in their “normal” confi guration they
are denoted as an L
1
phase and typically occur at relatively high hydration.
The addition of water to relatively soluble surfactants and lipids usually results
in the observation of a range of the structures in Figure 9.1 from right to left,
with an eventual transition to the L
1
micellar phase. However, oftentimes the
addition of water to surfactant does not induce a transition through to the
normal L
1
micellar phase. If the matrix has a fi nite capacity to incorporate
additional water, the extra water coexists as a separate phase. Such systems
form the basis of bilayer structures that coexist in excess water, as in the case
of cellular membranes and liposomes. Such systems are typically formed by
lipids such as phospholipids, which have an inherent geometric shape suitable
for the formation of bilayer structures.
The addition of excess water to poorly water-soluble amphiphilic lipids can
also result in the formation of fi nite swelling inverse nonlamellar geometries,
C
U
B
I
C
C
U
B
I
C
C
U
B
I
C
C
U
B
I
C
L
2
L
1
I
1
H
1
V
1
L
2
V
2
H
2
I
2
(Hexagonal)
(Lamellar)
(Micelles)
(Reversed
hexagonal)
(Reversed
micelles)
Increasing amphipnile concentration
Figure 9.1
Spectrum of commonly encountered amphiphile self-assembly structures
in water. [Reproduced from Kaasgaard and Drummond (2006) with permission of the
PCCP Owner Societies.]
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