Recent Developments in Lyotropic Liquid Crystals as Drug Delivery Vehicles - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 222

120

FI + water

100

H II + water

FI

80

Pn3m + water

60

la3d

Pn3m

40

L α

20

Lc

0 0

10 20 30

Composition (%w/w water)

40 50

Figure 8.6 Temperature-composition phase diagram of the monoolein-water system
(up to 50 wt % water). A cartoon representation of the various phase states is included
in which colored zones represent water. The mesophases are as follows: L c , crystalline
lamellar; L α , lamellar; Ia3d, gyroid inverted bicontinuous cubic; Pn3m, primitive inverted
bicontinuous cubic; H II , inverted hexagonal, and F1, reverse micelles fl uid phases
(Cherezov et al., 2006 ).

lipids should possess a small head-group area as compared to that of the tail
region. In contrast, direct mesophases are preferred when the surfactant head
cross section is larger than that of the tail, resulting in CPP 1. In the case of

lamellar mesophase these parameters are equal and hence result in CPP

<

1 .
The size of the polar head-group area is dictated by both molecular shape and
the degree of hydration.
Within the boundaries of a given mesophase the head-group area normally
increases with increasing hydration and decreases upon temperature rise. The
length and the volume of the lipophilic tails are also greatly affected by a
temperature increase. Stronger thermal motion of the tails eventually decreases
the length of the tails but increases their volume.
It was shown for monoglyceride-based LLC that H II mesophases are formed
from amphiphiles with CPP

=

∼ 1.7, cubic phases with CPP ∼ 1.3, and lamellar

structures with CPP 1 (Larsson, 1989). The lamellar phase is characterized

by zero curvature since the cross sections of the polar heads and the lipophilic
tails are similar. Upon increased hydration, cubic phases (Ia3d and Pn3m) with

=