Soaps & Detergents

To achieve effective cleaning, it is helpful to have a basic knowledge of soap and detergent chemistry.  Here we learn the composition of soaps and detergents and their cleaning action.

SOAPS

Soaps are sodium or potassium salts of long-chain fatty acids [carboxylic acids -Stearic acid, palmitic acid or oleic acid].

Saponification:  The process in which oil or fat is hydrolysed with Sodium hydroxide to get soap and glycerol is called saponification. The ionic-end of soap dissolves in water while the carbon chain dissolves in oil.

Types of Soap: Depending upon the nature of alkali used in the production of soap, they are classified into two types.

(a)  The sodium salt of long chain fatty acid is known as hard soap. It is difficult to dissolve in water. It is used as laundry soap.

(b)  The potassium salt of long chain fatty acid is known as soft soap, as it produces more lather. It is used as toilet soap and shaving soap.

In aqueous solution, soap ionises to form alkali ions. Since soaps have free alkali ions, they are alkaline in nature. Hence, the soap solutions are slippery to the touch.

Cleaning Action of Soap:  Ordinary water does not remove dirt from clothes or skin because the dirt present is oily or greasy in nature.

(a)  The soap molecule is generally represented as RCOONa. In solution, it ionizes to form RCOOand Na+. Each soap molecule has a polar head group (carboxylate ion, COO- group) and a long non-polar hydrocarbon tail (R group from long chain fatty acid). The polar head attracts the polar water molecule and is called hydrophilic end and the non-polar tail attracts the water-insoluble oily or greasy dirt particles.

soap molecule

(b)  When a dirty cloth is placed in a soap solution, the long non-polar hydrocarbon tail of soap molecules points towards the oily dirt particles and the polar heads point towards the water. This forms a spherical structure with polar parts of the molecule on the surface and non-polar parts in the centre. This spherical structure is called micelle.

(c )  This micelle is attracted towards water and carries the oily dirt particles along with it. This causes the dirt particles to detach from the fibres of the cloth. In this manner, clothes become free from dirt or dust.

CLASSIFICATION OF WATER

Water, along with soap, is used for washing purposes. On the basis of effective washing with soap, water has been classified as soft water and hard water.

(a)  Soft water: Water which produces good lather with soap is called soft water. When water falls as rain, it is naturally soft. Washing with soap is easy in soft water.

(b)  Hard water: Water which does not produce good lather with soap is called hard water. It is difficult to wash with soap in hard water. Water seeping through the ground becomes hard water. It is not useful for laundry and laboratory purposes.

DETERGENTS

Detergents. These are sodium salts of long chain sulphonic acids. The cleansing action of soaps and detergents are same. Rubbing of clothes with brush or agitation in a washing machine loosens the bond between the dirt particles and the fibres of clothes. This supports the cleansing action of soaps and detergents.

Cleaning Capacity of Soap with Hard and Soft Water

(a)  Although soap is a good cleaning agent, its cleaning capacity is reduced when used in hard water. Hardness of water is due to the presence of sulphates, chlorides or bicarbonate salts of Ca2+ or Mg2+ Soaps are sodium or potassium salts of long chain fatty acids. When soap is added to hard water, the Ca2+ and Mg2+ ions present in hard water react with soap. The sodium salts present in soaps are converted to their corresponding calcium and magnesium salts which are precipitated as scum. The insoluble scum sticks on the clothes and so the cleaning capacity of soap is reduced. The cleaning action of soap is very effective in soft water because it contains negligible calcium and magnesium ions.

(b)  Detergents are used in the case of hard water also because the calcium and magnesium salts of detergents are soluble in water. Detergents are more soluble than soaps and hence form more lather than soaps.

TYPES OF HARDNESS OF WATER

There are two types of water hardness.

(i) Temporary hardness:  It is a type of water hardness caused by the presence of dissolved bicarbonate salts of calcium or magnesium. Temporary hardness can be removed by boiling. When temporary hard water is boiled, the bicarbonates of calcium and magnesium undergo decomposition to form insoluble carbonates.  The insoluble carbonates of calcium and magnesium can be removed by filtration and the water thus obtained is free from calcium and magnesium ions and is soft.

(ii) Permanent hardness:  This type of water hardness is due to the presence of calcium or magnesium sulphates or chlorides. Permanent hardness cannot be removed by boiling. It can be removed by using water softeners.

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