Chemistry Reference
In-Depth Information
higher curvature and consequently higher CPP are preferred. The effective
CPP theory can supply a reasonable explanation for the temperature-induced
structural shifts from lamellar through cubic to reverse hexagonal phases,
requiring higher curvature than in the lamellar phase. Increasing the thermal
motion of both the hydrocarbon chains and the water molecules would increase
the CPP values by way of expanding the volume of the lipophilic moiety, but
decreasing the chain length and the head-group area. This leads to an increase
in curvature and therefore induces the formation of cubic and hexagonal
mesophases.
It should be noticed that the CPP concept may only be used as a rule of
thumb to predict transitions between various forms of LLC. For example, it
was shown that it fails to predict the appearance of the inverse bicontinuous
phases and an intermediate phase (Shearman et al., 2006; Shah and Paradkar,
2005). This is the reason a thermodynamic approach considering the total free
energy of the LLC systems was applied by Seddon and co-workers (Seddon
et al., 1996, 2000; Shearman et al., 2006); however, this is beyond the scope of
this chapter.
8.3 H
II
MESOPHASE COMPOSED OF GMO-TRIGLYCERIDE-WATER
AS DRUG DELIVERY SYSTEMS
It was shown that in the monoolein-based system, the cubic phase is trans-
formed into an H
II
mesophase upon heating at ca. 85°C (Drummond and Fong,
2000). CPP theory can supply a reasonable explanation to the temperature-
induced structural shifts from lamellar through cubic to reverse hexagonal
phases, requiring greater curvature than in the lamellar phase. Increasing the
thermal motion of both the hydrocarbon chains and the water molecules
would increase the CPP values via expanding the volume of the lipophilic
moiety but decrease the chain length and the head-group area. This leads to
an increase in curvature and therefore induces the formation of cubic and
hexagonal mesophases. (Gin et al., 2008; Israelachvili et al., 1976).
Bearing in mind the thermodynamic and structural considerations noted
above, systematic research was conducted to decrease the cubic to hexagonal
temperature transition and stabilize the GMO-based H
II
mesophase at room
temperature (Amar-Yuli and Garti, 2005). To achieve this goal, experiments
relative to incorporation of triglycerides (TAGs) with various chain lengths to
the binary GMO-water system were conducted. Amar-Yuli and Garti sur-
mised that immobilization of a TAG between GMO tails would lead to a
change in the geometry of monoolein molecules from cylindrical to wedge-
shaped and thereby an increase in the CPP value of the system (Amar- Yuli
and Garti, 2005). This should encourage transition from lamellar or cubic
phases to hexagonal structures. In addition, the immobilization of TAG in the
system was expected to reduce the packing frustration, stabilizing the hexago-
nal LLC at room temperature. These experimental results showed that a
Search WWH ::
Custom Search