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58.7 SOAPS AND SYNTHETIC DETERGENTS

The word ‘detergent’ means ‘cleansing agent’ and so the detergents are substances which remove dirt and have cleansing action in water. According to this definition of detergents, soap is also a detergent and has been used for more than two thousand years. There are two types of detergents:

  1. Soapy detergents or soaps
  2. Non-soapy detergents or soapless soaps.

 Soap

A soap is a sodium or potassium salt of some long chain carboxylic acids (fatty acid).

Sodium salts of fatty acids are known as hard soaps and potassium salts of fatty acids are known as soft soaps. Hard soaps are prepared from cheap oils and fats and sodium hydroxide. They contain free alkali and are used for washing purposes. Soft soaps are prepared from good oils and potassium hydroxide. They do not contain free alkali, produce more lather and are used as toilet soaps, shaving creams and shampoos.

A soap has a large non-ionic hydrocarbon group and an ionic COO-Na+ group. So for simplicity the structure of soap can be represented as

Some examples of soaps are: sodium stearate, C17H35COO-Na+, sodium palmitate, C15H31COO-Na+ and sodium oleate, C17H33COO-Na+.

Soaps of metals other than sodium and potassium are usually water-insoluble and do not find application as a cleansing agent. Therefore, hard water, which contains salts of magnesium and calcium, reacts with soap to form magnesium salt of fatty acid and calcium salt of fatty acid.

 

 

 

 

 

 

 

These calcium and magnesium salts of fatty acids are insoluble in water and separate as curdy white precipitate. Thus, a lot of soap is wasted if water is hard.

Activity 58.1 Study of Action of Soap in Hard Water

  • Take two test tubes and label them as A and B.
  • In test tube A, put 10 cm3 of distilled water (or rain water) and in test tube B take 10 cm” of hard water.
  • Add 5 drops of soap solution to both the tubes.
  • Shake the test tubes vigorously for an equal period of time.

What do you observe?

It is observed that more foam is formed in the test tube containing distilled water. In the test tube containing hard water less foam is formed and at the same time a curdy white precipitate is formed.

Note. If hard water is not available, it can be prepared by adding small amount of calcium chloride or magnesium sulphate to the ordinary water.

PREPARATION OF SOAP

Soap is prepared by heating oil or fat of vegetable or animal origin with calculated quantity of concentrated sodium hydroxide solution (caustic soda solution). Hydrolysis of fat takes place and a mixture of sodium salts of fatty acids and glycerol is formed.

 

 

 

 

 

 

Soap which is formed as a result of alkaline hydrolysis of oil or fat is separated from the solution by the addition of sodium chloride (common salt). Salt decreases the solubility of soap which is, therefore, released soap from the solution. The process is known as salting out. The crude soap that separates out is called grained soap. Now, soap being lighter than water floats on its surface from where it is removed. The lower aqueous layer is called lye.

After the removal of soap the solution which is left behind contains glycerol and sodium chloride. Glycerol is then recovered from this mixture as it is an important chemical
which finds use in drugs, paints, cosmetics and explosives.

TRADITIONAL SOAP MAKING

In Ghana and other parts of Western Africa, soap was prepared by traditional methods long before the modern methods were available. In the traditional methods, ashes from the burning of plantain peels, cocoa husk and wood are used in place of alkali. The ashes provide potassium hydroxide. Oils or fats are obtained from vegetable sources such as palm, coconut, cocoa, etc.

The oil is heated with dry ash or its aqueous solution, with constant stirring. In the traditional soap, glycerol is not recovered. The product thus obtained is usually coloured due to impurities and is not as good as the soap manufactured by modern methods.

ACTIVITY 58.2 Laboratory Preparation of Soap

  • Take about 25 cm3 of castor oil or vegetable oil in a beaker. To this add about 50 cm3 of 20 per cent sodium hydroxide solution slowly with constant shaking.
  • Heat the mixture slowly to boil and let it continue boiling for about ten minutes.

 

  • Remove the beaker from the burner and add about 5 g of common salt to the beaker.
  • Allow the mixture to cool.

What do you observe?

After sometime a solid crust of soap will be seen floating in the beaker.

DETERGENTS

These are also called synthetic detergents or syndets or soapless soaps or just detergents. So in our routine language when we say detergent, it means the synthetic detergent. These synthetic detergents are designed in such a way that while using them problems do not arise with the
hardness of water just as they arise with the use of soap. They do not form insoluble calcium and magnesium salts with hard water and can be used for washing even with hard water.

A synthetic detergent is the sodium salt of a long chain benzene sulphonic acid or the sodium salt of a long chain alkyl hydrogen sulphate.

Like soaps they contain an ionic group such as sulphonate group, SO3-Na+ or sulphate group, OSO3-Na+ and long chain hydrocarbon which is a non-ionic group.

PREPARATION OF SYNTHETIC DETERGENTS

Synthetic detergents are prepared by reacting hydrocarbons from petroleum with conc. tetraoxosulphate (VI) acid and converting the product into its sodium salt. Examples of synthetic detergents are sodium p-dodecyl benzenesulphonate and sodium lauryl sulphate.

Add To Your Knowledge

Washing powders available in the market contain about 15 to 30 per cent detergents by weight and the remaining part is other chemicals which are added to it. These are:

  • Tetraoxophosphate(V) compounds which prevent formation of insoluble compounds with Ca2+ ions and fatty acids
  • Bleaching agents to remove stains and to produce whiteness in clothes.
  • Perfumes
  • Fluorescents
  • Additives to prevent corrosion of metals, and to maintain the powdery texture of the detergent.

ADVANTAGES OF SYNTHETIC DETERGENTS OVER SOAPS

Synthetic detergents are widely used these days as cleansing agents, Synthetic detergents have the following advantages over soaps:

  1. Synthetic detergents can be used even in hard water whereas some of the soap gets wasted if water is hard.
  2. Synthetic detergents can be used even in acidic medium as they are the salts of strong acids and are not decomposed in acidic medium.
  3. Synthetic detergents have a stronger cleansing action than soaps.
  4. Synthetic detergents are more soluble in water than soaps.
  5. Synthetic detergents are prepared from the hydrocarbons obtained from petroleum. This saves vegetable oils which are otherwise used in preparation of soap.