Nanometric dispersions of particles and

Equation of State for Nanometric Dispersions of Particles and Polymers... 

In order to obtain good interfacial adhesion and mechanical properties, the hydrophilie elay needs to be modified prior to its introduction in most polymer matriees, which are organophilie. When nanometric dispersion of primary clay platelets is obtained, the aspect ratio of the filler particle is increased and the reinforcement effect is improved [75-25],... 

The exfoliated system is almost always quoted as the most desirable, since nanometric dispersion of day platelets maximizes the interfacial region between the filler and the polymer matrix, thus allowing to exploit the excellent mechanical properties of the individual day layers. Moreover, when exfoliation is attained, the number of reinfordng components is dramatically increased, since each day particle contains a very large number of day sheets. To further complicate matters, often a mixed dispersion is observed for day with different populations of tactoids or with partial exfoUation of single layers. Transmission electron microscopy (TEM) and WAXD are by far the most employed characterization techniques that assess the morphology of PLSN. 

A variety of industrial catalytic processes employ small metal-particle catalysts on porous inorganic supports. The particle sizes are increasingly in the nanometre size range which gives rise to nanocatalysts. As described in chapter 1, commonly used supports are ceramic oxides, like alumina and silica, or carbon. Metal (or metallic) catalysts in catalytic technologies contain a high dispersion of nanoscopic metal particles on ceramic oxide or carbon supports. This is to maximize the surface area with a minimum amount of metal for catalytic reactions. It is desirable to have all of the metal exposed to reactants. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

Polymer Colloids is a generic term encompassing all stable colloidal dispersions of polymers in aqueous or non-aqueous media for which the polymer particle size may be conveniently expressed in nanometres. For almost all synthetic and naturally-occurring polymer colloids the mean particle size falls in the 100-2000 nm range, but most commonly is 100-500 nm. 

Emulsions and suspensions are disperse systems that is, a liquid or solid phase is dispersed in an external liquid phase. While emulsions are sometimes formulated from oily drugs or nutrient oils their main function is to provide vehicles for drug delivery in which the drug is dissolved in the oil or water phase. Suspensions, on the other hand, are usually prepared from water-insoluble drugs for delivery orally or by injection, usually intramuscular injection. An increasing number of modern delivery systems are suspensions - of liposomes or of polymer or protein microspheres, nanospheres or dendrimers, hence the need to understand the formulation and stabilization of these systems. Pharmaceutical emulsions and suspensions are in the colloidal state, that is where the particles range from the nanometre size to visible (or coarse) dispersions of several micrometres. 

Figure 11.2. With the scanning tunnelling microscope, dispersed polyaniline (PAni) can be shown to consist of primary particles that are no larger than 10 nanometres (millionths of a millimetre). In (a) they can be seen as light, yellow-coloured patches. Once the volume concentration exceeds a critical threshold, these flocculate and—as can be seen in the scanning electron micrograph (b)—form network-like strucmres. Each of the particles behaves like a metal measuring a few nanometres, i.e. it possesses freely mobile electrons. These can tunnel between the particles and thereby conduct electricity. [Reproduced from ref. 17b with kind permission from Gordon and Breach publishers.]...

In this chapter, two new approaches for the synthesis of metal-polymer nanocomposite materials have been described. The first method allows the preparation of contact-free dispersions of passivated gold clusters in polystyrene, and it is based on a traditional technique for the colloidal gold synthesis—that is, the alcoholic reduction of tetrachloroauric acid in presence of poly(vinyl pyrrolidone) as polymeric stabilizer. The primary function of the stabilizer is to avoid cluster sintering, but it also allows us to isolate clusters by co-precipitation. It has been found that the obtained polymer-protected nanometric gold particles can be dissolved in alkane-thiol alcoholic solutions to yield thiol-derivatized gold clusters by thiol absorbtion on the metal surface. Differently from other approaches for thioaurite synthesis available in the literature, this method allows complete control over the passivated gold cluster structure since a number of thiol molecules can be equivalently used and the... 

The creation of 2D crystals of both micron sized and nanometre sized particles remains a somewhat empirical process due to the ill-defined role of the substrate or surface on which nucleation takes place. Perrin first observed diffusion and ordering of micron sized gamboge 2D crystals in 1909 under an optical microscope [32]. Several techniques have been proposed for the formation of 2D arrays at either solid-liquid surfaces or at the air-water interface. Pieranski [33], Murray and van Winkle [34] and later Micheletto et al. [14] have simply evaporated latex dispersions. Dimitrov and coworkers used a dip-coating procedure, which can produce continuous 2D arrays [35,36]. The method involves the adsorption of particles from the bulk solution at the tricontact phase line. Evaporation of the thin water film leads to an attractive surface capillary force which aids condensation into an ordered structure. By withdrawing the film at the same rate as deposition is occurring, a continuous film of monolayered particles is created. Since the rate of deposition is measured with a CCD camera, it is not possible to use nanometer sized particles with this method, unless a nonoptical monitor for the deposition process can be found. 

With the downsizing method, the synthesis of nanomaterials helps to control the size, morphology and compositional distribution at the nanometre level. The application of nanomaterials depends on the ability to disperse the particle in a fluid with compatible phase properties. 

Dispersions of nanometric particles are unusal because the potential barriers are low, since each particle carries a small number of effective charges. Moreover the range of repulsions is often rather short, because the dispersions have a very large surface area and this leads to high concentrations of ions in tiie aqueous medium. A consequence of these 2 features is that control of nanometric dispersions through electrostatic interactions is notoriously difficult. 

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