Why do we use detergent for washing clothes?

Detergent mixes with water to remove dirt and oil from clothes. Water can't remove oil and dirt from clothes. Detergent cleans by causing a chemical reaction with water to force dirt and debris out of clothing. Detergents work with water to loosen the dirt trapped in the material of clothing and clean them away. The detergents are alkaline in nature. They chemically react with the dirt or oil or stains. Once the chemical reaction takes place, the dirt/ stains and oil gets unseated from the fibres of the clothes. The detergent thereafter forms a layer around the molecules of the stain/ dirt/ oil and does not allow these to re-settle on the fabric.

Detergents are made up of a long chain of hydrogen and carbon atoms, as well as an ionic group that dissolves in water. Most detergents have an ionic group with a negative charge. These detergents are called anionic, and include surface-active agents. Some specialty detergents have a positive ionic charge. These detergents are called cationic and have germicidal properties. Cationic detergents are commonly used in hospitals.

Surface-active agents have hydrophobic or water-hating molecular chains and hydrophilic or water-loving components. These water-hating molecules are repelled by water, but are attracted to oil and grease.  Water-loving molecules will be attracted to water.  Thus the molecules that get attracted to oil and grease or stains wrap the molecules of these stains. The soapy solution then gets clear of the fabric.

Natural soaps tend to gel easily while Detergents lack this gelling problem

Detergents also react less strongly with hard water than soaps. Hard water is caused by dissolved minerals. So if the wash water is hard, it forms insoluble salts in contact with soap. The result is a fine white precipitate that settles on clothes, washing machines and any other surface.  This material is difficult to remove and quite sticky. Soap works well in areas with very soft water but causes problems when any minerals are present. Thus, detergents are best suited to wash clothes even if the water is hard.

So we use detergents for washing clothes.