We have already seen that the colloidal particles present in a colloidal system have size lying in the range 1nm-100nm. Depending upon how different substances forming colloidal solution acquire the size of particles in this range, colloidal solutions may be classified into the following three categories.
(i) Multimolecular colloids
(ii)Macromolecular colloids
(iii) Associated colloids
1. Multimolecular Colloids: When a large number of atoms or small molecules (having diameters of less than 1nm) of a substance combine together in a dispersion medium to form aggregates having size in the colloidal range, the colloidal solutions thus formed are called multimolecular colloids. The species (atoms or molecules) constituting the dispersed particles in multimolecular colloids are held together by Vander Waals’ forces.
The gold sol, sulphur sol etc. are some examples of multimolecular colloids. A gold sol may contain particles of various size composed of several atoms of gold. Similarly, sulphur sol consists of particles containing about a thousand of S8molecules.
2. Macromolecular Colloids: Certain substances form large molecules whose dimensions are comparable to those of colloidal particles. Such molecules have very high molecular masses and are termed as macromolecules. When such substances are dispersed in suitable dispersion medium, the resulting colloidal solutions are known as macromolecular colloids. Thus, in macromolecular colloids, the dispersed particles are themselves large molecules having very high molecular masses.
Most of the lyophilic sols are macromolecular colloids. For example colloidal dispersion of naturally occurring macromolecules such as starch, proteins, gelatin, cellulose, nucleic acids etc. are macromolecular colloids. Synthetic polymers such as polyethylene, polypropylene, synthetic rubber etc. also form macromolecular colloids when dispersed in suitable solvents.
3. Associated Colloids (Micelles): Associated colloids are those colloids which behave as normal strong electrolytes at low concentrations but exhibit colloidal properties at higher concentrations due to the formation of aggregated particles. The aggregated particles thus formed are called micelles.
The associated colloids are usually formed by surfactants (surface active agents) like soaps and synthetic detergents. These agents form micelles when present in solution at a concentration greater than critical micellization concentration (CMC). The formation of a micelle can be understood by taking the example of soap solution as described below.
Micelle formation in soap solution: The commonly used soaps are sodium or potassium salts of higher fatty acids such as palmitic acid (C15H31COOH), stearic acid (C17H35COOH) etc. The most commonly used washing soap is sodium stearate, C17H35COONa. In general, soap can be represented as RCOONa, where R represents a long chain alky1 group. When dissolved in water, soap ionizes to give RCOO– and Na+ ions. The RCOO– ion consists of two parts – long hydrocarbon chain R and the polar group -COO– as shown diagrammatically. The hydrocarbon residue R is hydrophobic, i.e. water repelling, whereas the -COO-group is hydrophilic, i.e. water loving. Therefore, RCOO– ion orients itself in such a way that -COO- end dips in water and the group R stays away from water. The -COO- groups of different RCOO-ions tend to stay away from one another due to like charges,whereas the R-groups try to approach each other to form other to form a bunch. This leads to the formation of a micelle as shown in the figure below.
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