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However, transfer of the monolayers to solid substrates was inefficient because of the
small size of the hydrophilic anchor, which imparted a hydrophobic/hydrophilic
imbalance. Accordingly, increasing the size of the hydrophilic anchor facilitated
the formation of stable Langmuir films that could be transferred to solid substrates
with a transfer ratio of 1 (Nierengarten et al. 2001). Bayer's research group reported
a related approach in which the fullerene was modified with carboxylate-terminated
dendrons and alkyl chains (Maierhofer et al. 2000). This amphiphilic fullerene
derivative formed stable monolayers that could be compressed and expanded
without hysteresis (Fig. 11.46d; Maierhofer et al. 2000).
11.6. CONCLUSION
The localization of molecular components having similar polarity in water is a power-
ful, nondirectional driving force for the self-assembly and folding of amphiphilic
systems. The structure and self-assembly of the amphiphilic dendrimers discussed
here emerge from a complex interplay of topological factors with the need for
phase segregation. This interplay imparts amphiphilic dendrimers with properties
that differ significantly from those of traditional surfactants. Further, the ability to
readily adjust the dendrimer topology allows these physical properties to be adjusted
by minor structural changes. Many of the functional superstructures produced in
natural systems are constructed by a hierarchical self-assembly process that is
mediated by a synergistic array of directional and nondirectional intermolecular
forces. Although our understanding of how a particular noncovalent force contributes
to the assembly process is reasonably mature, the design of superstructures that
assemble via the action of multiple, interdependent forces is usually serendipitous.
Nevertheless, our capability to design molecular systems that assemble via multiple
interactions is rapidly increasing. Further progress toward the design of superstruc-
tures with a sophistication that rivals biological systems will require a better under-
standing of how directional interactions, such as hydrogen bonding, can be
synergized with amphiphilic phase separation.
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