Page 73 - Nanotechnology an introduction

Figure 6.11 A polyion approaching a surface already covered with its congeners (see text).

Figure 6.12 Overcharging resulting from adsorbed polyion tails (see text).
(Monovalent) counterions screen the polyions in the usual Debye–Hückel fashion, diminishing the charging energy of the polyion more than its
correlation energy, enhancing the charge inversion. (If the monovalent counterion concentration is very high the correlation disappears and APED is
no longer possible.) Multivalent counterions are more difficult to treat theoretically and APED in their presence would appear to be a fruitful area of
investigation. The hydrogen ion may play a special role; for example, it has been found that the porosity of built layers can be reversibly controlled
by varying ion concentration (e.g., pH).
Instead of polymeric polyions, nanoparticles composed of materials with ionizable surface groups can be used. In this case, although the
electrostatic charge of the surface of the coating is always reversed, typically not all the inner charges are compensated because of steric
hindrances, and hence electrostatic charges build up, and the long range electrostatic force ultimately prevents further particles from being
deposited [116]. This is an example of a two-dimensional supersphere (Section 8.2.9).

If polymeric polyions are used as the polyelectrolyte of one sign, and ionizable particles as the polyelectrolyte of the opposite sign, the particles act
as stress concentrators, thus greatly increasing the toughness of the built material. Large aspect ratio nanoparticles are very useful for diminishing
the deleterious effects of defects (pinholes) in multilayer films. In this way sophisticated coatings can be built up. It has been proposed that the
shells of many marine organisms, such as the abalone, are assembled using this principle, producing materials that are both robust (cf. equation
2.25) and beautiful: anisotropic nanoparticles are dispersed in a biopolymer matrix, which only occupies a few volume percent of the total mass.
Natural biopolymers, which are nearly all heteropolymers, primarily based on amino acids as monomers, but also possibly incorporating
polysaccharides and nucleic acids, can incorporate enormous functional variety in ways that we can only dream about at present in synthetic
systems. Nevertheless, developments are moving in this direction, as exemplified by the APED variant known as the surface sol-gel process [99].
6.6. Summary
The terminological framework for nanomaterials is given. The two main divisions are nano-objects and nanostructured materials. The various kinds
of nano-objects (particles, fibers and plates) are described, along with their methods of manufacture. The most important nanostructured materials
are currently nanocomposites. Fabrication and applications are discussed. It must be emphasized that there is a vast body of detailed knowledge
concerning this area, part of which is published but unpublished work undertaken in commercial confidence probably preponderates. Wetting of the
embedded nano-objects by their matrix is of crucial importance, regarding which present synthetic capabilities reveal themselves as far inferior to
what nature can do. Composites with enhanced mechanical properties require careful consideration of how load is transferred between matrix and
particle or fiber, and how many cycles of repeated loading and unloading fatigue the material.
6.7 Further Reading
Swalen, J.D.; et al., Molecular monolayers and films, Langmuir 3 (1987) 932–950.