Chemistry Reference
In-Depth Information
11.3.3. Hydrophilic-Hydrophobic Balance
The hydrophilic-hydrophobic size balance largely determines the size, shape, and
physical properties of the aggregate. For example, whereas natural phospholipids
have extremely low CMC values (,10
210
M), synthetic PEG-phospholipids
exhibit much higher CMCs because of the larger hydrophilic block. Tsuchida's
group (Takeoka et al. 2000) explored the impact of the hydrophilic-hydrophobic
balance by varying the dendron generation of related lipid-terminated lysine dendrons
with a focal PEG-OMe chain. The CMCs of the resulting conjugates generally
decreased as the number of terminal acyl groups increased at higher generations,
but they were still in the micromolar range (Takeoka et al. 2000). Recently,
Nguyen and Hammond (2006) observed that triblock copolymers of a poly(ethylene
oxide) (PEO) linear block terminated on both ends by PAMAM dendrons form
micellar aggregates, capable of encapsulating the antibiotic triclosan, with CMC
values ranging from 10
25
to 10
26
M (Fig. 11.20).
Amphiphilic peptide-dendritic diblock hybrids, composed of a dodecyl polyglu-
tamic acid hydrophobic linear segment appended to a hydrophilic PEO-terminated
polyester dendron, were recently reported by this group to form micellar aggregates
with remarkably low CMC values in the range of 10
28
M (Tian and Hammond
2006). The efficient self-assembly of these diblock systems was attributed to the
more efficient packing of a conelike morphology of the amphiphile, which
emerges from the globular shape of the dendron and the helical conformation of
the polyglutamate segment (Fig. 11.21).
11.3.4. Steric Impact of Dendritic Block
Israelachvilli (1991) predicted that small, high curvature aggregates are preferentially
formed by cone-shaped amphiphiles with polar, sterically demanding head groups
and relatively small hydrophobic blocks. Several studies reported variations in aggre-
gate morphology with dendron generation that generally confirm these predictions.
For example, Stupp's group constructed a series of surfactants composed of a
poly(
L
-lactic acid) chain functionalized on one end with an ammonium-
terminated lysine dendron and on the other end with cholesterol (Fig. 11.22;
Klok et al. 2002).
Figure 11.20 Structure of poly(amido amine) (PAMAM) (G2)-poly(propylene oxide)-
PAMAM (G2) triblock copolymers.
