Types of colloidal systems

Characteristic property Suspension Colloidal solution True solution 

Visibility Particles are visible under a microscope or even with a naked eye. Particles are generally visible under ultramicroscope. Particles are not visible even under ultramicroscope. 

Diffusibility Do not diffuse Diffuse slowly Diffuse rapidly 

FHtrability Can be filtered even by an ordinary filter paper. Can be filtered through an animal membrane, through which the colloidal particles do not pass. Cannot be filtered 

Colour — Depends upon the shape and size of the particle Depends upon the nature of the ion. 

The term colloid usually refers to particles in the size range 50 A to 50 xm but this, of course, is somewhat arbitrary. For example, blood could be considered as a colloidal solution in which large blood cells are dispersed in water. Often we are interested in solid dispersions in aqueous solution but many other situations are also of interest and industrial importance. Some examples are given in Table 1.1. 

Dispersed phase Dispersion medium Name Examples 

Solid Liquid Sol or colloidal solution Paste at high concentration Au sol, Agl sol Toothpaste 

Liquid Solid Solid emulsion Opal, pearl 

Solid Solid Solid suspension Pigmented plastics 

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Figure 6.2. (a). Colloidal silica network on the surface of spores from Isoetes pantii (quill wort). Scale = 20 pm. (b). Polystyrene networks and foams produced as a biproduct of colloidal latex formation. Both types of colloidal system are typical of the diversity of patterns that can be derived from the interactions of minute particles. Scale (in (a)) = 50pm. 

One type of colloidal system has been chosen for discussion, a system in which the solid metal phase has been shrank in three dimensions to give small solid particles in Brownian motion in a solution. Such a colloidal suspension consisting of discrete, separate particles immersed in a continuous phase is known as a sol. One can also have a case where only two dimensions (e.g., the height z and breadth y of a cube) are shrank to colloidal dimensions. The result is long spaghettihke particles dispersed in solution—macromolecular solutions. 

Set up a circus of gels, sols, emulsions and foams, e.g. jelly , milk, pumice stone, polyurethane foam, bread, emulsion paint, cola, hair cream, aerosol dispenser, salad cream. A silica gel can be made from sodium silicate and hydrochloric acid. Classify the examples according to type of colloidal system. 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

Colloidal systems are widespread in their occurrence and have biological and technological significance. There are three types of colloidal systems ... 

Some of the more important types of colloidal systems outlined above are summarised in Table 1.1. For simplicity we shall limit ourselves in this book to a discussion of simple colloids, although the ideas developed can be extended and applied to more complex systems. 

The upsurge of interest in this type of colloidal system followed the development in the 1950s of the technique known as dispersion polymerization [3.54]. This process provides a means of preparing nonaqueous polymer dispersions in a controlled manner. A wide range of such dispersions have been made, mainly by free-radical addition polymerization. 

Figure 4.1 Particle size range of different types of colloidal systems.

A colloid consists of two distinct phases a continuous phase (referred to as the dispersion medium) and a fine, dispersed particulate phase (the disperse phase). In general, the two phases may be either solids, liquids, or gases, giving rise to various types of colloidal systems as listed in Table 4.1. The dispersed particles generally have dimensions ranging between 1 and 1000 nm, sometimes referred to as the colloidal size range. 

Table 4.1 Types of Colloidal Systems with Some Common Examples...

Since MEs have become commercially valuable for tertiary oil recovery, in the food and pharmaceutical industry, their characterization has become a pertinent feature of many publications dealing with this type of colloidal systems [4,7,38]. Knowing the microstructure of an ME is of great importance as the phase behavior will influence the ability to solubilize compounds and can determine drug release [39] or influence the manufacture of polymeric nanoparticles from these systems [40]. 

FIGURE 5.1 A foggy day on campus at the University of California, Merced. Fog is a type of colloidal system, a liquid aerosol of water droplets in air. 

We have not studied all types of colloidal systems in detail but limited ourselves to suspensions, siufac-tants, emulsions and foams. In terms of properties, the stability and associated concepts (double layer, van der Waals forces, steric effects) as well as the DLVO theory have been presented in detail, while kinetic and especially the optical properties have been discussed more briefly. 

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