[Explained] What are the Colloidal Properties of Soap?


Colloidal Properties of Soap

Introduction to Associated Colloids-Type of Colloid Formed By Soap and Synthetic Detergents

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 as known as micelles. Alternately they may be defined as the colloids that possess two distinct regions that is hydrophilic and hydrophobic portion within same molecule is called a micelle or an Associated Colloids.

 The associated colloids are usually formed by surface active agents like soap 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

Diagrammatic Representation of soap anion
Diagrammatic Representation of soap anion

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, a soap can be represented as RCOONa where R represents a long chain alkyl 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 in the figure above. The hydrocarbon  residue R is  hydrophobic i.e. water repelling whereas the COO- group is hydrophilic water loving. 

Therefore, RCOO- ion orients itself in such a way COO- ends dip in water and the group R stays away from the water. The COO- groups of different RCOO- ions tends to stay away from one another due to like charges, whereas the R-group try to approach each other to form a bunch. This leads to the formation of a micelle as shown in the figure below.


Aggregation of Rcoo ions to form a micelle aggregate particles
Aggregation of RCOO- ions to form a micelle

Thus a soap micelle is a negatively charged colloidal particle in which the negatively charged COO- groups are arranged in a spherical manner at the surface, while the hydrocarbon chains point towards the center. 

The COO- groups present at the surface of the micelle gets surrounded by Na+ ions which tend to drag the micelle in the bulk of the solution. A micelle may contain as many as 100 molecules or more.

The cleansing action of soaps

Soaps are used for cleaning dirty clothes. Clothes become dirty due to deposition of dust and oily or greasy substances. Water is not capable of wetting oily or greasy substances. However, the hydrocarbon residue R of the soap anion RCOO- can do so. 

When a dirty cloth is dipped into a soap solution, the hydrocarbon residue R of the soap anion RCOO- dissolves the oily or greasy dirt and encapsulates it to form a micelle. Thus, it emulsifies the dirt by micelle formation. When the cloth is rinsed with water, the micelles carrying the oily or greasy dirt gets washed away.

The cleansing action of detergents such as sodium lauryl sulfate or sulfonates of long chain hydrocarbons similar to that of soaps. In case of detergents, the polar groups are sulfate or sulfonate.

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