ABSTRACT
Self-assembly has evolved as a powerful tool to fabricate various nanostructured materials for biomedical applications. Self-assembly involves non-covalent interaction among constituent molecules and hence self assembled structures are susceptible to diverse structural transformations by changing external conditions such as pH, ionic strength, and solvent. Some of the fundamental principles of the self-assembly process in amphiphilic molecules are discussed in this chapter. The applications of self-assembly process in developing new materials with tailored properties have also been addressed. Direct application of the assembled structures as vehicles for drug delivery and as scaffolds for the synthesis of other inorganic materials has demonstrated its potential. Some of the important challenges that need to be addressed for extending this approach over a wide range of applications are also discussed.
2.1 Phenomena: Principles of Self-AssemblySelf-assembly is a fundamental reversible process in which spontaneous organization of individual components into welldefined structures takes place under thermodynamic equilibrium conditions. This self-association of molecules forming hierarchical structures mostly involves a number of non-covalent interactions such as hydrogen bonds, electrostatic interaction, and van der Waals and hydrophobic forces. The self-assembly process can be classified as either static or dynamic. Static self-assembly involves the formation of ordered structures under equilibrium conditions without involving dissipation of energy. On the other hand, in the case of dynamic self-assemblies, the organization of disordered components requires dissipation of energy. These structures are described as “self-organized.” Self-assembly is involved in producing structural organization on all scales ranging from molecules to galaxies. If the constitutive elements of self-assembled structures are molecules, the process is named “molecular self-assembly.” Molecular self-assembly can be either intramolecular or intermolecular. In intramolecular self-assembly, the molecules assemble in such a way that from the random coil conformation, they change to well-defined stable structure. An example of intramolecular self-assembly is protein folding. In intermolecular self-assembly, the molecules associate themselves to form supramolecular assemblies, for example, formation of a micelle by amphiphilic molecules in solution [1-5]. Amphiphilic molecules, as the name suggests, contain both a hydrophilic and hydrophobic group, and to avoid the contact of their hydrophobic group with that of aqueous phase, they self-assemble at interfaces and in the bulk. Depending on the charge present at the head group of amphiphiles (surfactants), they have been classified into different categories: cationic, anionic, nonionic, and zwitterionic [6-8]. Figure 2.1 shows the schematic representation of a surfactant showing its hydrophilic and hydrophobic moieties and the examples for different categories of surfactants. The self-assembly process is very common in naturally occurring biological molecules such as lipids as well as in synthetic amphiphiles, including surfactants, block copolymers, etc. The self-association of amphiphilic molecules occurs above a narrow range of concentration, which is known as critical micelle concentration
