Colloidal Phenomena

J. D. Henry, Jr., and C. H. A. Kuo, paper presented at The Symposium on Recent Dere/opments in Colloidal Phenomena AIChE National Meeting, New Odeans, La., Nov. 1981. 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

Classical surface and colloid chemistry generally treats systems experimentally in a statistical fashion, with phenomenological theories that are applicable only to building simplified microstructural models. In recent years scientists have learned not only to observe individual atoms or molecules but also to manipulate them with subangstrom precision. The characterization of surfaces and interfaces on nanoscopic and mesoscopic length scales is important both for a basic understanding of colloidal phenomena and for the creation and mastery of a multitude of industrial applications. 

Hirtzel, C. s. Rajagopalam, R. "Colloidal Phenomena Advanced Topics," Noyes Pubs., New Jersey, 1985, pp 88 97. 

Lyklema, J. (1991) Fundamentals of Interface and Colloid Science, Volume I Fundamentals, Academic Press, London. (This book treats the most important interfacial and colloidal phenomena starting from basic principles of physics and chemistry.)... 

Fig. 2 Polymer brushes at various interfaces and their importance to surface science and colloidal phenomena [8]...

Thus, fundamentally the interest is in testing the limits and theory of polymer behavior in end-tethered systems, e.g., viscoelastic behavior, wetting and surface energies, adhesion, shear forces relevant to tribology, etc. It should be noted that relevant surfaces and interfaces can also refer to polymers adsorbed in liquid-liquid, liquid-gas, solid-gas, and solid-liquid interfaces, which makes these polymer systems also of prime importance in interfacial science and colloidal phenomena (Fig. 2). Correspondingly, a wide number of potential applications can be enumerated ranging from lubrication and microelectronics to bioimplant surfaces. 

TABLE 1.1 Some Examples of Disciplines and Topics for which Colloids and Colloidal Phenomena Are Important... 

Probstein, R. F. Physicochemical Hydrodynamics An Introduction, 2d ed., Wiley-Interscience, New York, 1994. (Primarily graduate level. Despite the confusing title, this is a textbook on topics in colloidal phenomena.)... 

While the Deborah number is often used to compare the time for deformation with the time of observation in experiments, it also inspires us to identify and formulate other dimensionless groups that compare the various characteristic times and forces relevant in colloidal phenomena. We discuss some of the important ones. 

A number of additional applications of the ideas of this chapter could be profitably considered if space permitted. Included among these are adhesives, lubricants, waterproofing, and the recovery of oil from the pores of rocks. Like detergency and flotation, these topics involve a variety of surface and colloid phenomena. The interested reader will find an introduction to these fields in some of the references listed at the end of this chapter, especially Adamson (1990), Davies and Rideal (1961), and Osipow (1962). 

What we have covered in this chapter barely scratches the surface of a vast area of applications of colloidal phenomena in chemical and materials processing industries and in environmental and other operations. There are many fundamental, as well as practical, problems in the above topics (especially ones involving polymers, polyelectrolytes, and polymer-colloid and polymer-surfactant mixtures) that are currently areas of active research in engineering, chemistry, physics, and biology. Some of the references cited at the end of this chapter contain good reviews of topics that are extensions of what we have covered in this chapter (see, e.g., Elimelech et al. 1995, Hirtzel and Rajagopalan 1985, Israelachvili 1991, Gregory 1989, and O Melia 1990). 

Hirtzel, C. S., and Rajagopalan, R., Colloidal Phenomena Advanced Topics, Noyes, Park Ridge, NJ, 1985. (Research monograph. A broad, qualitative review of colloidal phenomena. Discusses colloid stability as well as deposition phenomena and structural evolution in concentrated dispersions. Contains an extensive collection of references prior to 1985.)... 

Hirtzel, C.S. Rajagopalan, R. Colloidal Phenomena, Advanced Topics, Noyes Publ. Park Ridge, NJ, 1985. 

During that century, colloidal phenomena played a pivotal role in the genesis of physical chemistry by establishing a connection between descriptive chemistry and theoretical physics. For example, Einstein provided the relationship between Brownian motion and diffusion coefficient of colloidal particles. 

Flocculation is an entirely colloidal phenomena where the inter-ictlon Bitween protein molecules is determined by the balance between electrostatic repulsion due to the electric double layer and van der Waals attraction (2). 

The chains shown in Ffg. 3 are those of long pol yfions) deliberately added. However, there is some evidence to indicate that even in conventional emulsion polymerizations the particles formed may not be as smooth as those shown in Fig. I, and the charged groups may be heating a short distance in the medium as microhairs. In practice one should not be misled by the convenience of the smooth sphere model for theoretical modeling of colloidal phenomena. 

I n few, if any, industrial chemical processes are more ingredients used to produce " a single final product than in emulsion polymerization. Moreover, colloidal phenomena play a dominant role in these polymerization reactions. Both these features contribute to the complex nature of emulsion polymerization. 

The GC theory is used most often to estimate the potential drop across the diffuse layer. This quantity is important in colloid phenomena and electrode kinetics, for example. Noting that the inner boundary of the diffuse layer is located at distance Zd from the interface, the square of the electrical field at this location is given by... 

Double layers are also important in colloid chemistry. When a colloid particle is composed of an ionic crystal, it often preferentially adsorbs one of its component ions, thereby acquiring a charge. As a result the colloid particle is surrounded by a double layer. The interfacial properties are very important in determining a variety of colloidal properties, including electrophoresis and electroosmosis. It also plays a role in colloid stability and coagulation phenomena. The effects of the electrical properties of the interface are well known in colloid chemistry. The description of colloid phenomena is a well-developed area of physical chemistry which is often important in industrial processes. 

In a recent study, the necessity for understanding the effects that molecular probes (e.g., pyrene) can have on colloidal system has been clearly illustrated. " Mixtures of pyrene-labeled and unlabeled polyethylene oxide (PEO) were adsorbed on silica and the properties of the resulting suspension were monitored. Settling rate results (Eigure 7.29) clearly demonstrated that pyrene-labeled PEO has a marked effect on the flocculation of silica suspension. It has been shown that even relatively small amounts of labeled polymer, when mixed with unlabeled polymer, can dramatically affect the behavior of their mixtures. On the other hand, it has also been shown that when labeled polymer was used in sufficiently low amounts (<3%), the mixture behavior does not differ from that due to the unlabeled polymer. These findings once again underscore the fact that while certain measurement techniques are invaluable for investigating colloidal phenomena, potential effects caused by introduction of various molecular probes into the system should always be noted. 

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