Soaps and Detergents
A detergent is generally a substance that improves the cleaning properties of water. A detergent functions by lowering the surface tension of water thus enabling the water to wet materials more effectively, while at the same time the detergent molecules break up and disperse greasy particles and oil stains.
The two main types of detergents are:
- Soapy detergents: They are generally called soaps. Soapy detergents are made from fats and oils.
- Soapless detergents: They are usually just called detergents. Soapless detergents are made from petroleum products.
Soaps
Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). They are manufactured by a process called saponification.
Definition: Saponification is the hydrolysis of glyceride esters of fatty acids found in fats and oils with an alkali to produce soap and glycerol.
Fat or Oil + Alkali → Soap + Glycerol
A typical sodium salt of fatty acid present in soap is sodium stearate (C₁₇H₃₅COONa), produced by saponification as follows:
Word equation:
Glyceryl stearate + sodium hydroxide → sodium stearate + glycerol
Chemical equation:
(C₁₇H₃₅COO)₃C₃H₅(glyceryl stearate) + 3NaOH → 3C₁₇H₃₅COONa(sodium stearate) + C₃H₅(OH)₃(glycerol)
Explanation of the reaction: When the glyceryl stearate in fat or oil is heated with caustic soda, it breaks down releasing fatty acid (C₁₇H₃₅COOH) and glycerol. The fatty acid is immediately neutralized by the caustic soda, forming a sodium salt of the fatty acid, i.e. sodium stearate or soap.
The Commercial Manufacture of Soap
Commercially, soap is manufactured by the saponification process. During this process, animal fats or vegetable oils such as palm oil, olive oil, groundnut oil etc. are boiled with sodium hydroxide solution. The mixture is stirred until saponification is completed. Concentrated sodium chloride solution is then added to make the soap less soluble and on cooling is precipitated as a hard cake on the surface. The addition of sodium chloride to precipitate the soap is called salting out, which separates the soap from glycerol.
The impure soap is purified with steam and water, and then dyes, perfumes and disinfectants are added where necessary, before the soap is pressed into bars or tablets.
Remark: Hard soaps for laundry are made from sodium hydroxide, while softer and milder toilet soaps (bathing soaps) are made from potassium hydroxide and animal fats.
Advantages of Soap
- It is an excellent cleanser in soft water.
- It is biodegradable, i.e. it can be easily decomposed by bacteria; hence it does not cause water pollution when washed into nearby streams and rivers.
Disadvantage of Soap
Soap forms a scum in hard water which can stain fabrics and causes wastage of the soap. The scum is an insoluble calcium or magnesium salt of fatty acids, formed when the Ca₂⁺ and Mg₂⁺ ions in hard water replace the Na⁺ and K⁺ ions in soap, e.g.
2C₁₇H₃₅COONa(sodium stearate, soap) + Ca₂⁺(from hard water) → (C₁₇H₃₅COO)₂Ca(calcium stearate, scum) + 2Na⁺
Soapless Detergents
Soapless detergents such as Omo, Pax, Surf, La Croix etc. are manufactured from complex alkenes (obtained by the cracking of petroleum products) and concentrated sulphuric acid.
To manufacture a soapless detergent, the complex alkene is first treated with concentrated sulphuric acid to obtain a sulphonated alkene. This is then treated with an alkali like sodium hydroxide to form the detergent, e.g.
C₁₈H₃₆(octadecene) + H₂SO₄(aq) → C₁₈H₃₄HSO₄(sulphonated alkene)
C₁₈H₃₄HSO₄(sulphonated alkene) + NaOH → C₁₈H₃₄SO₄Na(detergent) + H₂O
Advantages of Soapless Detergents
- They do not form scum in hard water.
- They are made from petroleum products and hence vegetable oils are saved for other uses.
- They wash brighter than soap.
Disadvantage of Soapless Detergents
Most soapless detergents are non-biodegradable, i.e. they cannot be broken down into harmless substances by micro-organisms. They usually persist in water causing water pollution. It is therefore unwise to disperse an oil slick with such detergents.
