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Cleansing Action of Soaps and Detergents

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  • Last Updated : 21 Sep, 2021
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In our day to day lives, we often use detergents and soaps to clean the dirt off our clothes. Have you ever wondered what soap molecule is made up of or how it cleanses the dirt? In this article, we shall look upon the structure of soap and its cleansing action but before that, we shall give a simple definition of what soap is.

What are Soaps?

Soap is produced by saponification reaction between sodium hydroxide or potassium hydroxide and vegetable oil or animal fats. Thus, soaps are potassium or sodium salts of a long chain of fatty acids. Soaps are water-soluble in nature.

Saponification reaction is the reaction between an ester and base to give alcohol and soap as the products. The general form of saponification reaction is shown below. 

Ester + Base → Alcohol + Soap

Properties of Soaps:

  • Hard in nature: Soaps are generally hard in nature i.e. they are in solid form.
  • Cleansing Action: Soaps are effective to clean away the dirt from a surface. Soaps have a hydrophobic tail and hydrophilic head which are equally important in the cleaning process.
  • Lather formation: Soaps form lather with soft water.
  • Conditioners: Soaps contain conditioners called emollients that are responsible to moisturize our skin once we use the soap.
  • Fragrant: Soaps generally have a unique fragrance added to them. These fragrances help calm us, soothe our minds and remove our body odours.

Structure of Soap

A soap molecule is a sodium or potassium salt of a long chain of fatty acids. Thus soap has two parts, one is the ionic part and the other is a long carbon chain. The two parts of the soap molecules possess very different properties. These two parts are known as:

  • Hydrophobic tail: This part of the soap is water repellent in nature and dissolves in oils. It is ionic in nature.
  • Hydrophilic head: This part of the soap molecule is water attractive or water-loving and dissolves in water. It is made up of a long chain of hydrocarbons.

As shown in the above image the soap molecule has a hydrophilic head which is generally the ionic part and a hydrophobic tail which is a long hydrocarbon chain and is generally represented using R. An example of soap is C17​H35​COONa+.

Structure of Soap.

Cleansing Action of Soap

The dirt is generally oily in nature and insoluble in water. The soap cleanses the dirt by the process of micelle formation. Before we study the cleansing action of soap in detail, we need to understand what micelle is. 

What is micelle?

A micelle is formed by the cluster of molecules where the molecules arrange themselves in a spherical shape with the hydrophobic end facing inwards and the hydrophilic end facing outwards. 

The soap molecules form micelles in water and perform cleansing action as follows:

  • As we know soap has a hydrophobic and hydrophilic part, so when the soaps are dissolved in water, the hydrophilic end is attracted by water and faces outwards while the hydrophobic tail is repelled by water and faces inwards. The below image shows soap molecules with the hydrophobic end facing away from water while the hydrophilic end facing towards water.

Micelle

  • Now, these molecules start aggregating around the dirt molecule with the hydrophilic head outwards and the hydrophobic tail towards the dirt molecule i.e. away from the water and form a spherical cluster of molecules that are known as a micelle. Micelle formation by soap molecule is shown below in the image.

Micelle Formation

  • The soap molecule in form of micelles can clean the dirt as the dirt is now trapped within the micelle.
  • The micelles formed to make a colloidal solution with the water.
  • When the cloth or surface is washed with soap and water, the hydrophilic end gets attracted by water and the dirt molecule that has been trapped inside the micelle gets washed away with the water.

Cleansing Action

  • This is how the cleansing action of soaps take place.

Advantages of soaps:

  • Soaps are readily available, cheap and convenient to use.
  • Soaps are very effective for cleaning in soft water
  • They are 100% biodegradable in nature and do not cause pollution as they are easily degradable by the bacteria in the sewerage.

Disadvantages of soap:

  • Soaps are not effective for cleaning in hard water as they form scum with hard water instead of lather formation. The reaction of soap with magnesium ions in hard water is shown below. Soaps react with magnesium and calcium ions from hard water to form calcium or magnesium salts of fatty acids which do not dissolve in water and form a white precipitate that is called scum.

2RCOONa+ + Mg2+ → (RCOO)2​Mg + 2Na+

  • Scum sticks to clothes and makes washing difficult
  • Highly branched soaps are not easily degradable and cause pollution.
  • As soaps are basic in nature, they cannot be used to clean woollen garments as they have acidic dyes.

Sample Questions

Question 1: Define soaps.

Answer:

Soaps are sodium or potassium salts of vegetable fats and are produced as a result of saponification reaction i.e. reaction between sodium or potassium hydroxide and vegetable oil or animal oil. Soaps are soluble in water.

Question 2: What are the two parts of soap molecules?

Answer:

The two parts of soap molecule are:

  • Hydrophobic tail: This part of soap is water repellent in nature and dissolves in oils.
  • Hydrophilic head: This part of soap molecule is water attractive or water loving and dissolves in water.

Question 3: State two properties of soaps?

Answer:

The 2 properties of soap molecules are as follows:

  • Cleansing Action: Soaps are effective to clean away the dirt from a surface.
  • Lather formation: Soaps form lather with soft water.

Question 4. What are the disadvantages of soaps?

Answer:

Disadvantages of soap are:

  • Soaps are not effective for cleaning in hard water as they form scum with hard water instead of lather formation
  • Scum sticks to clothes and makes washing difficult
  • Highly branched soaps are not easily degradable and cause pollution.

Question 5: Define micelle.

Answer:

A micelle is the aggregate cluster of molecules where the molecules arrange themselves in a spherical shape with the hydrophobic end facing inwards and the hydrophilic end facing outwards. 

Question 6: State the reaction of soap in hard water.

Answer:

The reaction of soap in hard water is as follows:

2RCOONa+ + Mg2+ → (RCOO)2​Mg + 2Na+

Here RCOONa++ is the soap molecule which reacts with Mg+2 ions from hard water to from scum i.e. (RCOO)2​Mg.

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