Self-Assembly in Food – A New Way to Make Nutritious Products - Food Colloids: Self-Assembly and Material Science - page 27

mesophases, however, is a complex task. They show only a limited stability in
water, and they quickly phase separate after dispersion. Intrinsic to the
behaviour of these systems is the problem of lipophilicity of the surfaces
formed after fragmentation of the mesophases. Since the inner self-assembled
structure of the particle is bent towards the water phase (i.e., it has a negative
natural curvature), the lipophilic domains become exposed to the continuous
water phase, which is energetically unfavourable. Therefore, an external stabi-
lizer has to be used to protect these lipophilic particles against aggregation. 21,22
Most work relevant to food has been done on dispersions of reversed bicon-
tinuous cubic phases, made of glyceryl monoolein (GMO). Landh 70 and
Gustafsson et al. 21,22 have reported that amphiphilic block copolymers provide
an exceptional stabilization capability for reversed bicontinuous cubic phase
dispersions, most probably through steric stabilization. Poloxamer 407 (from
BASF, also denoted as Pluronics F-127) is by far the most studied block
copolymer in this context. The dispersed bicontinuous cubic particles are called
'cubosomes' and the dispersed reversed hexagonal particles 'hexosomes'. 21,22
Using the method of Gustafsson et al., 22 it has recently been demonstrated 24
that it is indeed possible to disperse W/O microemulsions, cubic phases and
hexagonal phases without losing the structural features of the bulk phase inside
the particles after dispersion.
We have shown 71 that the use of cryo-TEM in combination with fast Fourier
transform analysis and tilting experiments is an effective alternative to small-
angle X-ray scattering for obtaining information on the crystallographic
structure and space group of particles having reversed bicontinuous cubic
and hexagonal structures. A major advantage of cryo-TEM is the possibility to
analyse single particles. This allows identification of particles present at very
low concentrations and the coexistence of particles having different internal
self-assembly structures. Using this technique one can visualize a multitude of
different self-assembly particles as indicated in Figure 3. Further details on
structural and thermodynamic properties of emulsified mesophases can be
found elsewhere. 72
2.7 Addition of Guest Molecules to Mesophases
The addition of guest molecules to monoglyceride mesophases can significantly
influence the phase behaviour of the binary monoglyceride + water system.
After solubilization of a certain 'critical' amount of additive, a phase transition is
induced. Each additive affects the interfacial curvature of the lipid bilayer in a
slightly different way. Lipophilic additives such as oleic acid, 73,74 triglycerides 22,75
or n-tetradecane 24 induce a transition from a reversed bicontinuous cubic phase
to a reversed hexagonal phase, resulting in a more negative mean interfacial
curvature. More hydrophilic additives such as diglycerol monooleate 76,77 induce
the transition from reversed bicontinuous cubic to lamellar, and the mean
interfacial curvature changes towards zero. However, addition of hydrophilic
molecules, such as polysaccharides, can also induce a transition towards more