UKPCS Science Chemistry: DETERGENTS:# 21

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Oracle IAS, the best coaching institute for UPSC/IAS/PCS preparation in Dehradun brings to you UKPCS Science Chemistry (paper 6). 


Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

A detergent is an effective cleaning product because it contains one or more surfactants. Because of their chemical makeup, the surfactants used in detergents can be engineered to perform well under a variety of conditions. Such surfactants are less sensitive than soap to the hardness minerals in water and most will not form a film.

Detergent surfactants were developed in response to a shortage of animal and vegetable fats and oils during World War I and World War II. In addition, a substance that was resistant to hard water was needed to make cleaning more effective. Today, detergent surfactants are made from a variety of petrochemicals (derived from petroleum) and/or oleochemicals (derived from fats and oils).

As in soap making, an alkali is used to make detergent surfactants. Sodium and potassium hydroxide are the most common alkalis.

Detergents are commonly available as powders or concentrated solutions. They are amphiphilic: partly hydrophilic (polar) and partly hydrophobic (non-polar). Their dual nature facilitates the mixture of hydrophobic compounds (like oil and grease) with water. Because air is not hydrophilic, detergents are also foaming agents to varying degrees.

Detergents are classified into three broad groupings, depending on the electrical charge of the surfactants : ANIONIC, CATIONIC AND NON-IONIC.



Three types of energy are needed for good cleaning results:

  1. chemical energy: provided by soap or detergent
  2. thermal energy: provided by warm or hot water
  3. mechanical energy: provided by machine or hands

These types of energy interact and should be in proper balance. Let’s look at how they work together.

Let’s assume we have oily, greasy soil on clothing. Water alone will not remove this soil. One important reason is that oil and grease present in soil repel the water molecules.

Now let’s add soap or detergent. The surfactant’s water-hating end is repelled by water but attracted to the oil in the soil. At the same time, the water-loving end is attracted to the water molecules.

These opposing forces loosen the soil and suspend it in the water. Warm or hot water helps dissolve grease and oil in soil. Washing machine agitation or hand rubbing helps pull the soil free.

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