Soaps - preparation of soaps - Detergents - advantages and disadvantages

10th Chemistry : Soaps and Detergents [Carbon and Its Compound]

 SOAPS

Metallic salts prepared from natural fats are called soaps. Soap is a sodium salt or potasium salt of long chain fatty acids that has cleansing action in water. Sodium stearate, sodium oliate and sodium  palmitate are some examples of soaps.

Stearic acid is a long chain fatty acid. Its formula is C₁₇H₃₅COOH. This reacts with sodium hydroxide to give a compound called sodium stearate, which has properties of soap.

Similarly, sodium hydroxide reacts with oleic acid and palmitic acid forming sodium oleate   (C₁₇H₃₃COONa) and sodium palmitate (C₁₇H₃₁COONa) respectively.

Please Note!

Fatty acids are organic compounds. The molecules of these compounds contain a carboxyl  group                 {-COOH}. These molecules react with a trihydric alcohol called glycerol to form a class of compounds called esters.

Groundnut oil, coconut oil, palm oil, caster oil, gingely oil, animal fat etc are all esters of fatty acids. Fats and fatty oils are called glyceryl esters or tglycerides.

PREPARATION OF SOAP

Experiment : Take about 30 ml of a vegetable oil in a beaker. Add about 60ml of 20% sodium hydroxide solution to it. Heat slowly until the mixture boils. After 5-10 minutes of boiling, add about 5g of sodium chloride to separate soap from the solution. Cool the solution. Observe the creamy layer of soap floating on the  solution.

We need animal fat or vegetable oil, sodium hydroxide and sodium chloride for preparing soap. Fat or vegetable oil is a glyceride or a glyceryl ester. Soap is prepared by hydrolyzing fat or oil with bases such as sodium hydroxide or potassium hydroxide. This process of preparing soap is called saponification.

When a mixture of a oil/fat and a strong solution of sodium hydroxide is boild in an iron tank, a sodium salt of the fatty acid and glycerol are formed. The sodium salt of the fatty acid thus formed is nothing but soap.

Please Note!

A Swedish chemist, Carl Wilhelm Scheel, discovered accidentally the process of preparation of soap in 1783. He boiled olive oil with lead oxide and obtained a sweetish substance which was seat to taste. This substance is now known as glycerine.

Oil/fat + sodium hydroxide ----->  soap + glycerol

                                   Fig-01

Please Note! 

Glycerol

Glycerol is a sweet viscous liquid soluble in water. It is a byproduct of saponification process. It is commonly called glycerine. Its formula is CH₂OH.CHOH.CH₂OH. It is used in the manufacture of explosives, plastics, and pharmaceuticals. It is also used to prevent freezing.

Soap thus formed is separated with the help of sodium chloride. Addition of sodium chloride reduces the solubility of soap considerable. Soap, being lighter, floats like a cream on the solution. It is separated from the solution; suitable chemicals of suitable colour and odour are added to it and cast into moulds. When soap cools and solidifies, it is cut into desired shapes and packed. Glycerol exists in dissolved state in the solution. It is separated by distillation.

How does soap clean

Soap dissolves in water. This is due to the contrasting properties of the ends of the soap molecules.

A soap molecule has a long hydrocarbon end [E.g. CH₃(CH₂)₁₆] and a short ionic end containing COONa⁺.  The non-ionic hydrocarbon end has the property to repel water. The other end, which is ionic, has the property to attract water. This end attaches itself to water. The hydrocarbon end attaches to the dirt or grease to form structures called ‘micelles’.

                                                                           Figure-02

In micelles soap molecules are arranged radially. The end attached to water pulls out the dirt from the fabric. This can be washed off with water.

DETERGENTS

The term detergent is derived from a Latin word which means surface active. Synthetic detergents are commonly known as detergents. Detergents are also called soap less soaps.

Detergents do not form insoluble ‘scum’ with magnesium and calcium ions present in hard water. Therefore, detergents clean better than soap even in hard water.

Detergents are sodium salts of long chain benzene sulphonic acid or sodium salts of long chain alkyl hydrogen sulphate. A detergent molecule consists of a large hydrocarbon group that is non-ionic and a sulphonate (SO₃^- Na⁺) or a sulphate (SO₄^- Na⁺)  group that is ionic. Sodium n-dodecyl benzene sulphonate and Sodium dodecyl sulphate are two well-known detergents.

Note:

Long chain hydrocarbons obtained from petroleum are treated with concentrated sulphuric acid. The organic acids produced during this process are neutralized with sodium hydroxide. The sodium salt obtained is a detergent.

ADVANTAGES OF DETERGENTS OVER SOAPS

Detergents have more advantages than soap. Therefore, detergents are more widely used than soap. Let us now turn our focus towards the advantages of detergents.

! Detergents cleanse well even in hard water. Soap is not so good for washing when water is hard.

! Detergents are made from hydrocarbons obtained from petroleum. Soap is prepared from vegetable oil or animals fat that are usually edible. Detergents are not bio-degradable. Soaps are biodegradable.

! Detergents cleanse well even in acidic medium. Soap does not cleanse well in such a medium.

Detergents have their disadvanges.

! Detergents are not bio-degradable. They polutes water and soil.

Total Fatty Matter (TFM)

 Soaps are graded in terms of total fatty matter or TFM. Bureau of Indian Standards (BIS) has catogorised bath or toilet soaps as ‘normal’, ‘baby,  transparent, and antibacterial soaps. The last three are called specialty soaps  targeted to specific users. A toilet soap is a cosmetic by law and it must fulfil the  requirements of the relevant Indian standard.

 T. F. M or total fatty matter is a measure for identifying the amount of fatty matter  present in soaps. TFM of a sample of soap can be determined as follows. A known  weight of the soap is dissolved in water and the solution is treated with dilute  sulphuric acid. The soap decomposes to sodium suplphate and fatty acids. The fatty acids so formed can be estimated. From this TFM can be calculated. On the  basis TFM, toilet soaps can be classified into three grades.  

Grade	TFM	Moisture		Free salt (NaCl)
I	above 80	Max :	13.5	Max :	0.7
II	65 – 80		13- 15		0.8
III	55- 65		15-20		1.5

Q. Swapnesh, living in Ooty, was washing clothes in cold water. He found that  the clothes were not getting clean. Geeta , his niece, suggested that he wash  the clothes in warm water. Washing of clothes with soaps or detergents is  easier in Luke warm water than cold water.

(a)Why?

(b) What value do you derive from this?

Ans:

(a) Action of soaps is due to formation of micelles. But the formation of micelles takes place at a minimum temperature called ‘Kraft  temperature’. This temperature is reached in lukewarm water.

(b) Be humble to accept a scientific fact from a younger person