Classification of colloidal systems

We have already pointed to the difference between hydrophobic and hydrophilic colloids. Closely related to this distinction is the division into reversible and irreversible colloids. 

Reversible colloids are thermodynamically stable, which means that they are, at constant temperature and pressure, in their state of minimum Gibbs energy. It implies that the dispersion is formed spontaneously upon mixing the components. Solutions of macromolecules (hydrophilic colloids) and association colloids belong to the reversible colloids. 

Irreversible colloids are thermodynamically unstable. The material of which the particles are composed is not soluble in the surrounding medium. Hence, irreversible colloids are not spontaneously formed upon mixing special tricks are involved to prepare (and maintain) them. The colloidal state of irreversible colloids is only metastable it requires an activation energy to transfer the system to the thermodynamically stable state, which is the state where the particles are segregated to minimize 

Because irreversible colloids are a fine dispersion of one phase (the dispersed phase) in another, continuous, phase (the medium), this group of colloids may be classified on the basis of its constituting phases, as is done in Table 1.1. 

Some examples of reversible and irreversible colloids are summarized in Table 1.2 but many more could be given. 

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Nomenclature. Colloidal systems necessarily consist of at least two phases, the coUoid and the continuous medium or environment in which it resides, and their properties gready depend on the composition and stmcture of each phase. Therefore, it is useful to classify coUoids according to their states of subdivision and agglomeration, and with respect to the dispersing medium. The possible classifications of colloidal systems are given in Table 2. The variety of systems represented in this table underscores the idea that the problems associated with coUoids are usuaUy interdisciplinary in nature and that a broad scientific base is required to understand them completely. 

As mentioned earlier, colloidal systems are composed of two phase of matter. The dispersed phase, also called the discontinuous or internal phase is made up of colloidal particles while the dispersion medium, also called the continuous or external phase is made up of the solvent in which the dispersion takes place. Numerous types of combinations of these two phases are possible (Table 3.1) and it is difficult to classify colloids strictly. However, various attempts have been made and the classification of colloidal systems as two distinct types - the lyophillc colloids and the lyophobic colloids, based upon the Interaction of phases has found wide acceptance. 

Classification of Colloidal Systems.— Unlike true solutes, the various components of a colloidal system can be any one or more of the three states of matter. 

CLASSIFICATION OF COLLOID SYSTEMS ACCORDING TO THE MATERIAL OF THE PARTICLES... 

Classification of colloid systems according to the material of the particles... 

Definition and Classification of Emulsions. Colloidal droplets (or particles or bubbles), as they are usually defined, have at least one dimension between about 1 and 1000 nm. Emulsions are a special kind of colloidal dispersion one in which a liquid is dispersed in a continuous liquid phase of different composition. The dispersed phase is sometimes referred to as the internal (disperse) phase, and the continuous phase as the external phase. Emulsions also form a rather special kind of colloidal system in that the droplets often exceed the size limit of 1000 nm. In petroleum emulsions one of the liquids is aqueous, and the other is hydrocarbon and referred to as oil. Two types of emulsion are now readily distinguished in principle, depending upon which kind of liquid forms the continuous phase (Figure 2) ... 

Along with the classification of disperse systems based on the phase state of the dispersed phase and the dispersion medium, and their classification as coarse dispersed or colloidal, structured or unstructured, dilute or concentrated, one can also subdivide disperse systems into lyophilic or lyophobic types. Systems belonging to these principally different classes differ in the nature of colloid stability and in the intensity of interfacial intermolecular interactions. High degree of similarity between the dispersed phase and the dispersion medium, and, consequently, compensation of the... 

In Chapter 1 the importance of the various classes of colloidal systems to modern science and technology was indicated in a general way. Because of the wide variety of colloidal systems one encounters, each having certain unique features that distinguish it from the others, it is convenient to discuss each major classification separately. For that reason, chapters have been devoted to specific systems such as solid dispersions, aerosols, emulsions, foams, lyophilic colloids (i.e., polymer solutions), and association colloids. There is a great deal of overlap in many aspects of the formation, stabilization, and destruction of those systems, and an effort will be made not to repeat more than is necessary. However, for purposes of clarity, some repetition is unavoidable. 

A particular focus of this chapter is colloidal dispersions of solid particles in a liquid. These are both industrially important but also scientifically interesting since model systems can be prepared with which we can probe the intermolecular interactions responsible for colloidal aggregation. As indicated in Table 3.1, such systems are termed sols. Sometimes they are also known as lyophobic solids. This reflects a now-outmoded classification of colloids into those that are solvent hating (lyophobic) and those that are solvent loving (lyophilic). Some examples of sols are described in Section 3.9, whilst the aggregation of model sols is discussed in Section 3.15. Other examples of commonly encountered colloids are described in Sections 3.10 to 3.14. 

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

This section presents an overview of the great variety of soft particles encountered both in fundamental science and in applications. We propose a classification based on composition and architecture, distinguishing colloidal-like particles, network particles, polymer-colloid systems, and surfactant particles, as illustrated in Fig. 1 and discussed below. 

C. H. Giles, T. H. MacEwan, S. N. Nakhwa, and D. Smith, Studies in adsorption. Part XI A system of classification of solution adsorption isotherms and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids, J. Chem. Soc., London, 3973 (1960). C. H. Giles, D. Smith, and A. Huitson, A general treatment and classification of the solute adsorption isotherm. I Theoretical, /. Colloid Interface Sci. 47 755 (1974). C. H. Giles, A. P. D Silva, and I. A. Easton, A general treatment and classification of the solute adsorption isotherm. Part II Experimental interpretation, /. Colloid Interface Sci. 47 766 (1974). 

Colloidal systems and dispersions are of great importance in many areas of human activity such as oil recovery, coating, food and beverage industry, cosmetics, medicine, pharmacy, environmental protection etc. They represent multi-component and multiphase (heterogeneous) systems, in which at least one of the phases exists in the form of small (Brownian) or large (non-Brownian) particles (Hetsroni 1982, Russel et al. 1989, Hunter 1993). One possible classification of the colloids is with respect to the type of the continuous phase (dispersions with solid continuous phase like metal alloys, rocks, porous materials, etc. will not be consider). 

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