Applications of Colloid Systems

The stability of colloid suspensions is an important criteria in the manufacture of a large number of industrial products where these are the basic building blocks (food colloids, pollution control, emulsions, wastewater treatment). 

An example of a food colloid is mayonnaise (a mixture of vegetable oil plus egg yolk and vinegar which is an emulsion of oil in water). 

The electrostatic forces in many systems play a dominant role, such as the separation process (filtration) in wastewater treatment. 

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In drug studies, of main interest is the application of colloidal systems, which show specific and unspecific interaction with mainly lipophilic substances. An obvious application is the study of highly lipophilic and poorly absorbable drugs that are administered orally or transdermally (2). Such interactions with surface-active agents may either cause a diminution of the... 

These phenomena play nn important part in many technical applications of colloidal systems, especially of suspensions, in the properties of the soil, etc. 

Major problems inherent in general applications of RO systems have to do with (1) the presence of particulate and colloidal matter in feed water, (2) precipitation of soluble salts, and (3) physical and chemical makeup of the feed water. All RO membranes can become clogged, some more readily than others. This problem is most severe for spiral-wound and hollow-fiber modules, especially when submicron and colloidal particles enter the unit (larger particulate matter can be easily removed by standard filtration methods). A similar problem is the occurrence of concentration-polarization, previously discussed for ED processes. Concentration-polarization is caused by an accumulation of solute on or near the membrane surface and results in lower flux and reduced salt rejection. 

This chapter describes the basic principles involved in the development of disperse systems. Emphasis is laid on systems that are of particular pharmaceutical interest, namely, suspensions, emulsions, and colloids. Theoretical concepts, preparation techniques, and methods used to characterize and stabilize disperse systems are presented. The term particle is used in its broadest sense, including gases, liquids, solids, molecules, and aggregates. The reader may find it useful to read this chapter in conjuction with Chapters 8, 12, and 14, since they include some of the most important applications of disperse systems as pharmaceutical dosage forms [1]. 

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. 

One area related to gold clusters is the field of nanoparticles and nanoclusters. These materials have actually been known since ancient times when they were used for their esthetic appeal and also for their therapeutic properties in the form of colloidal gold. In recent decades, the field of nanoparticle research has emerged and in-depth research has dealt with the properties and potential applications of these systems. 

Finally, a discussion of surfactant self-assembly will not be complete without a mention of surfactant assemblies in biological systems. Although they are outside the scope of our book, we have already drawn attention to such biological applications of colloid science in Chapters 1 and 7 and above in this chapter. Some additional discussion is provided in the last section of this chapter (Section 8.11). 

Similar incorporation of ligand-based targeting motifs is applicable to colloidal systems. In the final analysis, the effectiveness of ligand-based targeting drug delivery systems depends on the density of the target receptor on the cell surface, the rate in which the systems are internalized into the cell, and recycling and reexpression of the receptor on the cell surface after internalization. 

The application of light scattering to the characterisation of colloidal systems has advanced rapidly over the last few decades. This has been made possible by the development of (a) lasers as intense, coherent and well-collimated light sources, (b) sophisticated electronic devices for recording data, and (c) computers for the complex data processing that is involved. 

One widespread application, which has been much researched in recent years, is the use of colloidal systems for controlled uptake and release purposes. 

Fluorocarbons and fluorinated amphiphiles have found a variety of applications in materials science and medicine [1-5]. As many of these applications involve colloidal systems stabilized by a monolayer of fluorinated amphiphiles, it is essential to understand the structure and properties of these interfacial films. Such knowledge can provide improved control over the engineering and properties of highly fluorinated colloids and interfaces [6]. 

The applications of colloid solutions are not restricted to paints and clay. They are also to be found in inks, mineral suspensions, pulp and paper making, pharmaceuticals, cosmetic preparations, photographic films, foams, soaps, micelles, polymer solutions and in many biological systems, for example within the cell. Many food products can be considered colloidal systems. For example, milk is an interesting mixture containing over 100 proteins, mainly large casein and whey proteins [6,7]. 

This review is a discussion of the kinetic modelling of the photoelectrochemistry of colloidal semiconductor systems. This area is currently attracting significant attention from the scientific community due to the applications of colloidal semiconductors within two rapidly advancing research fronts heterogeneous photocatalysis and nanocrystalline particle technology. 

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. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

Applications of colloid stability theory to other systems 265... 

Some progress toward an understanding of these systems is also possible by considering the influence of the presence of water within the oil drops on the interaction between the oil drops and by consideration of the influence of the size of the internal water droplets on their internal stability and on the possibility of coalescence with the external aqueous phase. It is premature to consider all this in detail as the application of colloid stability theory to simpler emulsions has not been particularly successful (37). For type A w/o/w emulsions, the approach of Void (38) may perhaps be used if the oil layer is thought of as the homogeneous adsorbed layer. 

So far, we have shown that redox-modulated recognition, a prevalent feature in biological systems, can be employed in the design of functional devices. As mentioned earher, one challenge to the efficient application of such systems is the ability to immobilize, order and thus individually address them. One way to provide the desired anisotropy is through the use of colloids functionalized with self-assembled monolayers (SAMs). " In a recent model smdy, a diacyl diaminopyri-dine-functionahzed gold colloid (DAP-Au), capable of binding flavin, has been... 

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