How Soap Works

Like water, soap is a daily essential. We depend on it for keeping our things, our surroundings, and our bodies hygienic. We witness its cleansing effect all the time—when we wash the laundry, do the dishes, wash our hands or take a bath. Yet very few of us can explain its action.

This is how soap works.

Perhaps we should begin with what it is. A bar of soap is actually a lump of fatty acid salts made from the interaction of fats and oils with alkali. The oil used may be either vegetable oil, such as palm, coconut and ground nut oil; or non-vegetable oil like lard and tallow.

How Soap Works By Reducing Surface Tension

Soap basically cleans in two ways. One of these is by reducing water’s surface tension, a term that refers to the cohesiveness of the molecules on the surface. Because of soap’s effect of lowering the attractive forces between the like molecules of liquid, water is better able to penetrate soiled areas of the skin and other objects.

How Soap Works Through Surfactants

Soap’s second way of cleaning, which it accomplishes by binding to dirtied oil, is more intensive. The surfactants in the soap are responsible for this action. A surfactant, also known as a surface-active agent, is a molecule with hydrophilic (water-loving) and hydrophobic (water-fearing) ends.

The hydrophilic portion allows the hydrophobic component, or fatty acids of the soap, to come into contact with other hydrophobic matter, mostly oil or fatty substances like grease, into which dirt has settled. Oil is a magnet for dirt, and in fact, most stubborn dirt are found in this form.

Oil/dirt matter attaches to the soap’s fatty acids, and is encapsulated in water droplets, or suspensions, which are then easily scrubbed off and washed away.

How Soap Works As an Emulsifier

Another, less technical way of describing this second cleansing mechanism is by saying that soap acts as an emulsifying agent. Emulsifiers have the ability to disperse a liquid that does not naturally mix with another, in this case, oil into water. Through this property, soap is able to catch oil-dirt matter in suspended form. This suspension can then be removed by washing.

The most active surfactants are found in the common bar soap, making this type of soap very effective in picking up grime from the skin for washing away. However, these surfactants are skin irritants that do not rinse easily. Soap makers solve this problem through superfatting, the addition of chemicals to prevent some of the oils or fats from being processed before production is completed. The result is soap with better moisturizing properties.

Terms That Help Us Understand Better How Soap Works

A few more terms relating to soap ingredients and we’re done with our discussion on how soap works. Familiarization with these terms will help us understand better the cleaning mechanisms behind soap as we go along.

We mentioned earlier that soaps are fatty acid salts. More precisely, they are sodium or potassium fatty acids salts, produced from the hydrolysis of fats in a chemical reaction called saponification. Simply put, hydrolysis is water reacting with another substance, while saponification is the production of soap as a result of oils of fats coming into contact with an alkali or base. In soap making that base is usually lye, a caustic alkaline solution made from wood ashes.