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have in fact prepared W/O emulsions without any external stabilizer (Kulkarni
et al., 2010a,b). In addition, it was possible to vary the amount of dispersed
water (from 50 to
90%) as well as the type of LC nanostructure (e.g.,
cubic—Pn3m or Fd3m and H
2
). The formation and characterization of these
W/O nanostructured emulsions (Kulkarni et al., 2010b) are discussed in the
following.
Water - in - oil nanostructured emulsions, hereafter nanostructured emul-
sions, were prepared using a custom-made shearing device based on the
Couette cell, details of which have been published elsewhere (Salentinig
et al., 2008). The oil phase (lipid or lipid
∼
+
oil) and aqueous phase (water or
water
hydrogelator) were injected simultaneously from the bottom into a
prechamber and mixed using a propeller-type device. The resultant raw emul-
sion was then pushed into the main Couette shear cell with vertical axis, where
the raw emulsion was converted into a nanostructured emulsion. The whole
ensemble was thermally controlled using a circulating water bath. The tem-
perature was set such that the corresponding LC phase was in the molten state
(L
2
phase). Shear rates were in the range of 31,000-78,500 s
− 1
; the higher values
were required for emulsions with high water content (e.g., 90% water)
(Kulkarni et al., 2010b).
These nanostructured emulsions also present a high level of structural
hierarchy and have been characterized using a wide range of techniques.
The type and dimensions of LC nanostructures were determined from
SAXS studies (data not shown). Polarizing optical microscopy and confocal
microscopy were utilized to examine microstructures involving various
sizes of water droplets (Fig. 6.16). Confocal microscopy was also capable
of showing the lipid network, which was stained by Nile Red dye (Sigma
Aldrich, St. Louis, MO).
There is a very strong link between the nano/microstructure and the proper-
ties of nanostructured emulsions, which facilitates their wider tunability. The
rheological properties of W/O-nanostructured emulsions can be modulated by
changing the composition of the components (
+
δ
and
ϕ
), temperature (
T
), and
the concentration of hydrogelling agent (
) (Fig. 6.17). The viscoelastic proper-
ties of these emulsions were investigated by using a strain-controlled rheom-
eter (Anton Paar Physics UDS 200, Graz, Austria).
It was also possible to form a hydrogel in the water reservoirs of these
nanostructured emulsions. This characteristic also allowed us to increase the
stability against water separation (particularly for the bicontinuous Pn3m
nanostructure) and also increase the viscosity of emulsions. In general, con-
centrated emulsions are stable if the volume fraction of an internal phase is
above the critical value
6
of 0.74; otherwise, they need an external stabilizer.
ε
6
The number 0.74 is a volume fraction of monodispersed spherical droplets or particles result-
ing into the most compact arrangement, which governs the stability. This number may vary for
deformed or polydisperse spheres. It is also called a critical volume fraction.
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