Chemistry of Self-Assembling Bilayers and Related Molecular Layers

The two-dimensional packing of organic molecules (small and large) produces ultimately thin organic films. Several possibilities exist for achieving this result from linear polymers, taking advantage of surfaces and interfaces as spatial templates. Monomolecular polymer layers can be prepared on the surface of water by polymerization of monolayers. They can be transferred onto solid supports by the Langmuir-Blodgett-Kuhn technique. Polyion complexation at the air-water interface provides a second method of two-dimensional arrangements of linear polymers. More recent approaches include polymerization-induced epitaxy (PIE) and alternate polyion adsorption. In the case of PIE, growing polymer chains in solution are adsorbed onto immersed graphite plates, most probably as single polymer layers (Figure la). Chain alignment is commensurate with the graphite lattice, as confirmed by scanning tunneling microscopy (STM). This epitaxial adsorption is independent of polymerization mechanisms. On the other hand, the alternate adsorption consists of sequential dipping of charged solid supports in aqueous solutions of oppositely charged linear polyions (Figure 1b). The regularity of the layer thickness is remarkable and, under appropriate conditions, the deposition is repeatable without limit. This method is applicable not only to pairs of linear polyions but also to combinations of linear polyions with water-soluble proteins or inorganic nanoparticles.