Chemistry Reference
In-Depth Information
Figure 2.2 Representation of formation of (a) micellar and (b) inverse micellar vesicular
assemblies from diblock and triblock copolymers, respectively.
example, the flexibility of the hydrophobic blocks is thought to be enhanced and thus
there is an entropic gain.
The propensity to form supramolecular assemblies by amphiphilic molecules is
estimated by their CAC, which is defined as the concentration below which only
single chains of polymer exist. Above the CAC, both single chains and aggregates
are observed. When studying a micelle, CAC is referred to as the critical micelle
concentration (CMC). The CMC is much lower in amphiphilic block copolymers
compared to small molecule amphiphiles. The most reliable method for determining
CMC involves surface tension measurements, although other techniques such as
absorption or fluorescence spectroscopy are also utilized (Alexandridis et al. 1994;
Holland et al. 1995; Lopes and Loh 1998). Several factors can influence the CAC
of an amphiphilic species, the most important component of which involves the
nature of the block that forms the core of the micelle. The nature of the hydrophilic
Figure 2.3 Representation of organized assemblies (a) bilayers, (b) cylindrical micelles, and
(c) lamellar structure.
