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Formation and Self-Assembly at the Nanoscale
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the individual components in these systems are able to move
They are “soft” in a sense that
with respect to one another.
the components may adjust their positions within the aggre-
gate, rather than being firmly stuck together or locked into place.
Thermodynamically, this moderates the extent of reduction in the
entropy term.
Hence, self-assembled biological structures are steady-state
structures at the dynamic balance point between order and disor-
der. For example, folded proteins are stabilised from the unfolded
state by only a small energy difference. These self-assembled
systems are thus sensitive towards perturbations exerted by the
external environment. A small fluctuation that alters the thermo-
dynamic variables, e.g. pH change, may lead to a marked change
in or even compromise the structure.
The weak cooperative
interactions are responsible for the flexibility of the structure and
allow for structural rearrangements in the direction determined
by ∆G. If fluctuations could bring the thermodynamic variables
back to the starting conditions, the structure is likely to revert to
its initial configuration. These structures are thus robust and even
self-healing.
Since self-assembled structures are made through minimis-
ing the Gibbs energy, they are always thermodynamically more
stable than the unassembled components. As a consequence, self-
assembled structures are also relatively free of defects. For exam-
ple, components which are not complementary or contain defects
are naturally excluded through the self-assembly process in bio-
logical systems.
7.3.2 Self-assembly of Molecules on Surfaces ch07
Our understanding of how Nature forms self-assembly structures
such as proteins and nuclei acids has inspired us to design new
and unique supramolecules (Section 4.4). The cell membrane is a
7
bimolecular layer of self-assembled amphiphilic molecules, and
has prompted our study of surfactants and micelles (Section 5.3).
The potential of self-assembly, however, extends beyond making
supramolecular complexes and micelles. Self-assembly is also
used as a strategy to produce ordered structures on surfaces.
7 Molecules that have both polar and non-polar segments, e.g. lipids, detergents,
surfactants (see Section 5.3).