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predictions on free water, H-bonded lipid, and H-bond free lipids, as a function
of temperature and composition, respectively.
As expected intuitively, the amount of free water increases progressively
with increasing temperature or total water content. Particularly interesting is
the case of dependence of free water on the total amount of added water (Fig.
1.8b): More than 50% of the water is bound at any composition. These trends
have been already assessed experimentally by Garti and colleagues by measur-
ing the subzero (°C) water behavior in nonionic surfactants via differential
scanning calorimetry (Garti et al., 1996). These authors found that free water
can only be measured at high water volume fractions, but that at typical com-
positions used for microemulsions and lyotropic liquid crystals a high amount
of water (
50%) is typically bound to the lipid polar heads.
These fi ndings have not only fundamental relevance but might also assist
the design of lyotropic liquid crystalline phases for real applications. For
example, by varying the amount of free water contained within the water
channels, one can also tune the water activity, and ultimately also the state
of molecules encapsulated within the hydrophilic regions of the mesophases.
One clear example would be the control of activity of enzymes encapsulated
within the mesophases by varying their activity via the amount of free and
bound water.
Another very successful achievement of SCFT implementing H-bond fea-
tures has been the prediction of the exact size of the water channels in the
inverted lyotropic liquid crystals, as a function of physical parameters such as
concentration and temperature. Figure 1.9 shows the density profi les predicted
by SCFT for the hexagonal, Ia3d, and Pn3m phases (Lee et al., 2008). The
water channels are compared directly to the size (black circles) predicted by
either simple geometrical consideration, such as in the case of the hexagonal,
or by triply periodical minimal surfaces (TPMS).
Again, this has a direct practical relevance in view of lyotropic liquid crys-
tals as encapsulating agents, as it is know that the capacity of these mesophases
to encapsulate macromolecular compounds depends on the ratio between the
drug diameter and the size of the hosting water channels (Mezzenga et al.,
2005c ).
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1.4 DYNAMICS OF ORDER-ORDER TRANSITIONS AND
ATOMISTIC SIMULATIONS
All the approaches discussed above only describe the status of lipids and water
at the fi nal thermodynamic equilibrium. Of particular interest, however, is also
the dynamic of structural transitions from an ordered state to another ordered
one, since this has direct relevance also in the fi eld of biology in which lipid
membrane fusion has a vital role in phenomena such as viral infections, cell
adhesion, penetration, and the like. This is a complex phenomenon that
involves several intermediate steps occurring within very short times and, as
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