Characteristics of Colloidal Systems

Although the classification of colloids covers an extremely diverse array of materials, in general colloidal systems can be identified by the following characteristics  

Droplets or particles of one phase are dispersed in a continuous phase of another material. 

The particles in the dispersed phase have a size between about 10 nm and about 10 pm. 

The dispersed material is characterized by a very large surface area-to-volume ratio, resulting in a large interfacial area between the two components. 

The energy of interparticle interactions between particles is close to the thermal energy k T. 

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Accordingly, in the earlier exercises, 23 in number, attention is centered on methods of manipulation for overcoming the usual varieties of difficulties and in gaining experience in the use of preparative processes. Later exercises are arranged with reference to a number of types of compounds and the reactions available for their preparation. A final group of exercises is provided to illustrate the chief characteristics of colloidal systems. 

In the colloidal realm, given the large surface-to-volume ratio and the relatively small range of force that can sway the disposition of a colloidal particle, it is easy to appreciate the importance of controlling surface properties. Research literature abounds with the characteristics of colloid systems and model systems that mimic colloid surfaces. Applications permeate the fields of materials processing, adhesion, coatings, food science, and medicine. 

From the middle of the nineteenth century on, understanding of the phenomenology of interfaces became better at the molecular level, although the nature of the forces involved remained uncertain until the advent of quantum mechanical theory in the 1930s. The study of colloidal phenomena followed a similar track in that certain characteristics of colloidal systems were recognized and studied in the last century (and before), but a good quantitative understanding of the principles and processes involved remained elusive. 

One of the most obvious properties of a disperse system is the vast interfacial area that exists between the dispersed phase and the dispersion medium [48-50]. When considering the surface and interfacial properties of the dispersed particles, two factors must be taken into account the first relates to an increase in the surface free energy as the particle size is reduced and the specific surface increased the second deals with the presence of an electrical charge on the particle surface. This section covers the basic theoretical concepts related to interfacial phenomena and the characteristics of colloids that are fundamental to an understanding of the behavior of any disperse systems having larger dispersed phases. 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

In the past few decades, a specific kind of colloidal system based on monodis-perse size has been developed for various industrial applications. A variety of metal oxides and hydroxides and polymer lattices have been produced. Monodisperse systems are obviously preferred since their properties can be easily predicted. On the other hand, polydisperse systems will exhibit varying characteristics, depending on the degree of polydispersity. 

COLLOID SYSTEMS. Colloids are usually defined as disperse systems with at least one characteristic dimension in the range 10 7 lo ll> centimeter. Examples include sals (dispersions or solid in liquid) emulsions (dispersion of liquids in liquids) aerosols (dispersions of liquids or solids in gases) /inum (dispersion of gases in liquids or solids) and gels (system, such as common jelly, in which one component provides a sufficient structural framework for rigidity and other components fill the space between the structural units or spaces). All forms of colloid systems are encountered in nature. Products of a colloidal nature arc commonly found in industry and are notably extensive in the food field. Foams, widely used in industrial products, but also the causes of processing problems are described in entries on Foam and Foamed Plastics. 

As illustrated by the results presented in Figure 2 and in Table 2 at high ionic strength and high Ca2 + for favorable particle-particle interactions (e.g., in the deposition of non-Brownian particles, F = F%Taviiy + Fdrag +FlVDW Fchem = 0), transport models based on physical and hydrodynamic characteristics of a system can predict the initial kinetics of aggregation and deposition processes in aquatic systems quantitatively. In the presence of repulsive chemical interactions, however, quantitative theoretical predictions of such kinetics are very inaccurate and even many qualitative predictions are not observed. The determination of Fchem in aquatic systems merits study and development,- it is necessary for the quantitative prediction of the kinetics of colloid chemical processes in these systems. 

A comparison of the desorption rates at pH 7, shown in Figure 7 for the plutonium sorbed from fresh and aged solutions, indicates that the total desorption curve may be interpreted in terms of two different sorbed species. This is expressed in Equations 2, 3, and 4 as two first order processes. For both the fresh and aged systems, the relative quantities of the Ao(d or loosely-held species were almost identical, as were their desorption rate constants. It is likely that the A0<2 or tightly-held species were colloidal in size, since irreversibility is a widely known characteristic of colloid sorption. This was found to apply, for example, in the case of the sorption of colloidal americium on quartz (27). 

Aggregative stability, the most characteristic for colloidal systems. Colloidally stable means that the particles do not aggregate at a significant rate (6). As explained earlier, aggregate is used to describe the structure formed by the cohesion of colloidal particles. 

In later chapters we shall discuss the properties of a variety of colloidal systems. However, as a preparation for our consideration of the general properties and stability of dispersions, it will be useful to use one simple example to outline some of their more important characteristics. It so happens that one of the first colloidal dispersions to have been examined systematically will suit our purpose admirably. 

Cationic surfactants, in contrast to anionic surfactants, usually reduce both the number of particles involved in the polymerization and the rate of polymerization. The nature of the stabilizing emulsifier has a marked effect on the polymerization kinetics. For example, addition of a non-ionic stabilizer [e.g., poly(vinyl alcohol), a block copolymer of carbowax 6000 and vinyl acetate, or ethylene oxide-alkyl phenol condensates] to a seed polymer stabilized by an anionic surfactant decreased the rate of polymerization to 25% of the original rate. The effect was as if the nonionic stabilizer (or protective colloid) acted as a barrier around the seed particles to alter the over-all kinetics. It may be that the viscosity of the medium in the neighborhood of the nonionic surfactant coating of the polymer particle is sufficiently different from that of an anionic layer to interfere with the diffusion of monomer or free radicals. There may also be a change in the chain-transfer characteristics of the system [156]. 

It is possible therefore to subdivide the complex systems according to the latter principle of classification into various cases but it does not strike the essential point which is characteristic of complex systems and indeed produces no rational order in the mutually very varied behaviour of the complex colloid systems. 

It is characteristic of tricomplex systems that here the specific charge elements of the colloid ions and micro ions taking part play a part to a very large extent. Before we go into this more fully a general consideration may first be put forward in which we take the above given simple working hypothesis as our starting point. 

In practical situations, the stipulation of constant surface charge is often found to be invalid, especially in concentrated colloidal systems where the distance between interacting surfaces is relatively small. In those cases, a number of events can occur that will result in changes in the net surface charge, and therefore the overall electrical characteristics of the system. The most important of these processes is the specific absorption of ions at the interface. 

In Chapter 17, we discuss rheological properties, in particular viscosity and elasticity, of colloidal systems. These properties are at the basis of quality characteristics such as strength, pliancy, fluidity, texture, and other mechanical properties of various materials and products. In addition to bulk rheology, rheological features of interfaces are discussed. Interfacial rheological behavior is crucial for the existence of deformable dispersed particles in emulsions and foams. Emulsions and foams, notably their formation and stabilization, are considered in more detail in Chapter 18. 

Fig. 8. Characterization of colloidal systems by characteristic length scales.

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