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Figure 3

Figure 3. Transmission electron micrographs (TEM) of (a) zinc sulfide, (b) manganese(II) phosphate, (c) and (d) hematite particles.

Figure 4
Figure 4. Scanning electron micrograph (SEM) of colloidal particles of (a) Ca-naproxenate, and (b) Ba-naproxenate. Naproxen is a common anti-inflammatory drug.

Monodispersed Colloids

It is then easily comprehended, that developing a quantitative understanding of the relationship between the particle size and shape of a given material to its various properties requires the availability of uniformly subdivided matter, usually designated as "monodispersed". Over many years, Matijevic''s group has developed several techniques that yield such dispersions of a large number of particles, of internally simple or composite natures, coated, or hollow, with modal sizes ranging from nanometers to micrometers, and in a variety of shapes. The electron micrographs in Figure 3 illustrate some of these dispersions of inorganic particles of different chemical compositions or shapes. A more recent development in these studies involves the preparation of organic materials, especially of drugs, an area where very little work has been done before. Again, examples of such powders are displayed in Figure 4.

Monodispersed solids produced by Matijevic and his collaborators have already found many uses, best testified by a number of patents dealing with pigments for the use in printing inks or as paper whiteners, catalysts for electroplating, semiconducting particles for the analysis of blood cell populations, and perovskites for multilayer capacitors, to mention a few.

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While much success can be cited in the area of the preparation of uniform particles, there are still numerous practical and theoretical problems that need careful attention, some of which will be addressed below.

The most common and versatile methods for the synthesis of monodispersed particles are based on the precipitation in homogeneous (mostly aqueous) solutions of the reactants. From the applications point of view, there is a need for scale-up processes, that would efficiently yield such dispersions in large quantities. One important aspect, which must be considered in this task, is the sensitivity of precipitation of most solids to the experiential parameters. Thus, in many instances a change in temperature, pH, or reactant concentrations may either yield a solid of different chemical composition, or irregular or polydispersed particles.

From the theoretical point of view, much work is required to develop a better understanding of the mechanisms of the formation of monodispersed systems. The precipitation in a solution proceeds through several stages, all of which can affect the properties of the final product. Indeed, it has been documented that uniform particles can be achieved by two main, but quite different processes. In the first, a short burst of nuclei is followed by their growth, due to the incorporation of constituent reactants (ions, molecules), through diffusion toward the existing initial solids. While this mechanism is rather appealing, because of its simplicity in concept, it operates during the initial phase of precipitation, but it leads less frequently directly to final larger uniform (colloid) particles.

Indeed, it has been shown that in many instances the nuclei grow to nanoparticles, which then aggregate. Under certain conditions, the latter process can yield monodispersed colloids. Obviously, it is more difficult to derive a theoretical interpretation of such a complex mechanism. However, in collaboration with his colleagues, Professors Privman and Goia, models have been developed that show how the aggregation of primary (nanosized) precursors can lead to size selection, i.e. uniformity of the secondary particles.