Cleansing action of soap

We all know soap is used to remove dirt and grime from substances. Generally dirt and grime get stuck because they have an oily component which is difficult to remove by plain brushing or washing by water. A soap molecule has two parts: a long chain organic part and a functional group –COO^- Na⁺. There are a tail and a head. It has to be remembered that this is not an ion, the atoms are all covalently bonded, and electrical charges show how the charges get polarized in the group. A soap molecule has a tadpole like structure shown below.

The organic part is water insoluble but it is soluble in organic solvents or in oil or grease. The ionic part is soluble in water, as water is a polar solvent. When soap is added to water in which dirty clothes are soaked, the two parts of the soap molecule dissolves in two different mediums. The organic tail dissolves in the dirt, grime or grease and the ionic head dissolves in water. When the clothes are rinsed or agitated, the dirt gets pulled out of the clothes in the water by the soap molecule. In this way the soap does its cleaning work on dirty and grimy clothes or hands.

The soap molecules actually form a closed structure because of mutual repulsion of the positively charged heads. The structure is called a micelle. The micelle pulls out the dirt and grime more efficiently.

1.7 Synthetic detergents

Soaps have problems at use in hard water. Hard water contains calcium and magnesium ions, which does not form lather. Synthetic detergents overcome this problem. Synthetic detergents do not contain sodium salts of fatty acids. In spite of this detergents have all the properties of soap. Synthetic detergents have long chain molecules such as sodium n-dodecyl benzene sulphonate and sodium n- dodecyl sulphate. The chemical structure of these molecules is shown below.

We can see that a synthetic detergent molecule is similar to a soap molecule. It has a long hydrocarbon chain (a tail) and a short ionic head. These two parts are water repellent and water attracting respectively. Cleansing function of a synthetic detergent molecule is similar to action of a soap one.

Synthetic detergents are manufactured from long chain hydrocarbons obtained as a by-product of the petroleum industry. The hydrocarbons are treated with concentrated sulphuric acid and sodium hydroxide. Neutral sodium salt is obtained, which is synthetic detergent.

1.8 Advantages and disadvantages of synthetic detergents with respect to soaps

1. Soap cannot be used in hard water.	1. Synthetic detergents can be used in hard water.
2. Soap is made from vegetable/edible oils.	2. Synthetic detergents are made from by-products of the petroleum industry that allow to conserve valuable edible oils.
3. Soaps cannot be used in acid medium as in this case fatty acids will precipitate.	3. Synthetic detergents can be used in any medium including acid medium.
4. Soaps have a weak cleansing action.	4. Synthetic detergents have a strong cleansing action.
5. Soaps are not very soluble in water.	5. Synthetic detergents are high-soluble in water.
6. Soaps are biodegradable and do not cause any pollutions.	6. Some synthetic detergents are not biodegradable and cause water pollution. This phenomenon in some cases can become quite serious.