Classification of colloids

Colloids can be broadly classified as those that are lyophobic (solvent-hating) and those that are lyophilic and hydrophilic. Surfactant molecules, because of their dual affinity for water and oil and their consequent tendency to associate into micelles, form hydrophilic colloidal dispersions in water. Proteins and gums also form lyophilic colloidal systems. Hydrophilic systems are dealt with in Chapters 8 and 11. Water-insoluble drugs in fine dispersion or clays and oily phases will form lyophobic dispersions, the principal subject of this chapter. While lyophilic dispersions (such as phospholipid vesicles and micelles) are inherently stable, lyophobic colloidal dispersions have a tendency to coalesce because they are thermodynamically unstable as a result of their high surface energy. 

Pharmaceutical colloids such as emulsions and suspensions (Fig. 7.1) and aerosols are readily identified (Table 7.1). The disperse phase is the phase that is subdivided. The continuous phase is the phase in which the disperse phase is distributed. Many natural systems such as suspensions of microorganisms, blood, and isolated cells in culture, are also colloidal dispersions. Colloid science is interdisciplinary, for although dealing with complex systems it is nevertheless a unifying discipline as it bridges the physical and 

It is because of the subdivision of matter in colloidal systems that they have special properties. The large surface-to-volume ratio of the particles dispersed in a liquid medium results in a tendency for particles to associate to reduce their surface area, so reducing their contact with the medium. Emulsions and aerosols are thermodynamically unstable two-phase systems which only reach equilibrium when the globules have coalesced to form one macro-phase, for which the surface area is at a minimum. Many pharmaceutical problems revolve around the stabilisation of colloidal systems. 

Some biological phenomena can be understood in terms of the association of cells with other cells or with inanimate or other substrates. These, too, are kinds of colloidal 

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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. 

Colloids are introduced in the second half of the chapter. The various classifications of colloid types are discussed, together with ways of forming, sustaining and destroying colloids, i.e. colloid stability. Finally, association colloids ( micelles ) are discussed. 

CLASSIFICATION OF COLLOIDS BASED ON AFFINITY TO CARRIER FLUID... 

There are numerous ways in which viscosities are expressed in the literature. Some of the most common are defined Table 6.8. There is an entire lexicon of terms used to describe the different rheological classifications of colloidal dispersions [9-11,353,355]. 

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. 

It will be seen that the effect of Ostwald s classification of colloidal chemistry is to stimulate research for methods of determining the state of aggregation of the finely divided substances. The principles of this classification may be employed, however, as far as they are applicable. This is the case if we consider only three instead of Ostwald s nine classes. 

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 ... 

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... 

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