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swelling-deswelling behavior during the heating-cooling cycles [denoted as
breathing mode
(de Campo et al., 2004)]. This temperature-dependent behav-
ior indicated a reversible exchange of water into and out of the confi ned
internal particle structures during the cooling and heating cycles. For instance,
in oil-free samples, the internal structures follow the phase separation line in
Figure 5.1; the particles expelled water upon heating and reabsorbed water
upon cooling in a reversible way.
It should be pointed out that the slope of the separation line in the MLO-
water phase was negative. It was, therefore, similar to that observed in most
investigated monoglyceride-water systems. However, in certain cases, such as
in a monolaurin-water mixture, this line has a positive slope (Lutton, 1965),
thus suggesting that also a reverse breathing mode of dispersed monolaurin
particles could be feasible.
5.6
MODULATION OF THE INTERNAL NANOSTRUCTURES
This section describes, in two parts, the modulation of the internal self-
assembled nanostructure of the kinetically stabilized ISAsomes by varying the
lipid composition, as well as how to tune back the oil-loaded internal nano-
structure from the H
2
(hexosomes) or the W/O microemulsion system (EME)
to the V
2
phases (cubosomes). The underlying idea is based on a partial
replacement of the primary lipid MLO, which is favoring the formation of
nonlamellar phases, by a surfactant favoring the formation of the L
α
phases
(such as diglycerol monooleate, DGMO, or soybean phosphatidylcholine, PC)
(Yaghmur et al., 2006b).
5.6.1
Oil-Free MLO-DGMO- and MLO-PC-Based Systems
The impact of DGMO on the internal structure of the MLO-based aqueous
dispersions was investigated in the absence of oil (
0). Figure 5.11 shows
the SAXS curves at 25°C obtained from dispersions in which the
α
=
β
- ratio value,
defi ned as [[mass of DGMO (or mass of PC)]/(mass of MLO)
×
100], was in
the range of 0-25. At low DGMO concentrations (at
5 and 10), the scat-
tering curves still showed the characteristic peaks for a cubic Pn3m structure,
as well as a peak at low
q
values. The appearance of this additional peak sug-
gested the formation of two phases within the confi ned internal nanostructures
of our dispersed particles: the Pn3m cubic phase (diamond type
C
D
) coexisting
with the Im3m cubic phase (primitive type
C
P
). The Bonnet ratios for these
dispersed particles with the confi ned Pn3m-Im3m coexisting phases were in
the range of 1.3-1.34. These values corresponded well with the theoretical
value of 1.279 (Hyde, 1996). At a higher DGMO concentration, the three
observed peaks had ratios characteristic of a cubic phase with the symmetry
Im3m; a scattering curve for
β
=
25 is shown as an example in Figure 5.11. The
cryo-TEM micrographs obtained for this dispersion are shown in Figure 5.12.
β
=
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