Sythetic detergents have advantage over usual soaps as far as cleansing power is concerned. But use of synthetic detergents over a long time creates environmental pollution. How can the pollution caused by synthetic detergents be minimised? Classify the detergents according to their chemical nature.
Sythetic detergents have advantage over usual soaps as far as cleansing power is concerned. But use of synthetic detergents over a long time creates environmental pollution. How can the pollution caused by synthetic detergents be minimised? Classify the detergents according to their chemical nature.
Synthetic detergents are cleansing agents which have all the properties of soaps but which actually do not contain any soap. These can be used in soft as well as in hard water.
They are mainly classified into three categories
(1) Anionic Detergents
Anionic detergents are sodium salts of sulphonated long chain alcohols or hydrocarbons. Alkyl hydrogen sulphates formed by treating long chain alcohols with conc. H_2SO_4 are neutralised with alkali to form anionic detergents. Similarly alkyl benzene sulphonates are obtained by neutralising alkyl benzene sulphonic acids with alkali.
CH3(CH2)10Lauryl alcoholH2SO4−−−−→CH3(CH2)10CH2OSO3HLaurylhydrogen sulphateNaOH(aq)−−−−−−→ CH3−(CH2)10CH2OOS−O3Na+Sodium Lauryl sulphate (Anionic detergent)
In these detergents. the anionic part of the molecule is involved in the cleansing action. They are mostly used for household work. They are also used in toothpaste.
(2)
Cationic detergents
They are quaternary ammonium salts of amines with acetates, chlorides or bromides as anions. Cationic part possess a long hydrocarbon chain and a positive charge on nitrogen atom. Cetyltrimethylammonium bromide is a popular cationic detergent and is used in hair conditioners.
⎡⎢
⎢⎣H3C−(CH2)15−CH3|n|CH3−CH3⎤⎥
⎥⎦+Br−
Cationic detergents have germicidal properties and are expensive, therefore, these are of limited use.
(3) Non -ionic Detergents
Non-ionic detergents do not contain any ion in their constitution. One such detergent is formed when stearic acid reacts with polyethylene glycol.
H3C−(CH2)16Stearic acidCOOH+HO(CH2CH2O)n−CH2−CH2OHpolyethylene glycol−H2O−−−−→ CH3−(CH2)16COO(CH2CH2O)nCH2CH2OH
Liquid dishwashing detergents are non-ionic type
Advantages of Synthetic Detergents over Soaps
(i) Synthetic detergents can be used In hard water without any wastage while some of the soaps gets wasted.
(ii) Synthetic detergents can be used in acidic medium while soaps get precipitated.
(iii) Synthetic detergents are more soluble in water and hence produce lather more easily than soaps. Some synthetic detergents produce lather even in ice cold water.
(iv) Synthetic detergents decrease the surface tension of water to greater extent and hence have a stronger cleansing action than soap.
Synthetic detergents have advantages over usual soaps but the use of synthetic detergents over a long time creates environmental pollution because some detergents have highly branched hydrocarbon chains.
These branches or side chains stop bacteria from attacking and breaking the chains. This result in slow degradation of detergent molecule leading to their accumulation, Effluents containing these detergents reach the rivers, ponds etc. These persist in water even after sewage treatment and thus water get polluted.
Since, unbranched (i.e., straight) chains are more prone to attack by bacteria, therefore, in most of the detergents used these days, the branching is kept to a minimum so that the detergents become easily biodegradable and hence pollution is prevented.
Synthetic detergents are cleansing agents which have all the properties of soaps but which actually do not contain any soap. These can be used in soft as well as in hard water.
They are mainly classified into three categories
(1) Anionic Detergents
Anionic detergents are sodium salts of sulphonated long chain alcohols or hydrocarbons. Alkyl hydrogen sulphates formed by treating long chain alcohols with conc. H_2SO_4 are neutralised with alkali to form anionic detergents. Similarly alkyl benzene sulphonates are obtained by neutralising alkyl benzene sulphonic acids with alkali.
CH3(CH2)10Lauryl alcoholH2SO4−−−−→CH3(CH2)10CH2OSO3HLaurylhydrogen sulphateNaOH(aq)−−−−−−→ CH3−(CH2)10CH2OOS−O3Na+Sodium Lauryl sulphate (Anionic detergent)
In these detergents. the anionic part of the molecule is involved in the cleansing action. They are mostly used for household work. They are also used in toothpaste.
(2)
Cationic detergents
They are quaternary ammonium salts of amines with acetates, chlorides or bromides as anions. Cationic part possess a long hydrocarbon chain and a positive charge on nitrogen atom. Cetyltrimethylammonium bromide is a popular cationic detergent and is used in hair conditioners.
⎡⎢ ⎢⎣H3C−(CH2)15−CH3|n|CH3−CH3⎤⎥ ⎥⎦+Br−
Cationic detergents have germicidal properties and are expensive, therefore, these are of limited use.
(3) Non -ionic Detergents
Non-ionic detergents do not contain any ion in their constitution. One such detergent is formed when stearic acid reacts with polyethylene glycol.
H3C−(CH2)16Stearic acidCOOH+HO(CH2CH2O)n−CH2−CH2OHpolyethylene glycol−H2O−−−−→ CH3−(CH2)16COO(CH2CH2O)nCH2CH2OH
Liquid dishwashing detergents are non-ionic type
Advantages of Synthetic Detergents over Soaps
(i) Synthetic detergents can be used In hard water without any wastage while some of the soaps gets wasted.
(ii) Synthetic detergents can be used in acidic medium while soaps get precipitated.
(iii) Synthetic detergents are more soluble in water and hence produce lather more easily than soaps. Some synthetic detergents produce lather even in ice cold water.
(iv) Synthetic detergents decrease the surface tension of water to greater extent and hence have a stronger cleansing action than soap.
Synthetic detergents have advantages over usual soaps but the use of synthetic detergents over a long time creates environmental pollution because some detergents have highly branched hydrocarbon chains.
These branches or side chains stop bacteria from attacking and breaking the chains. This result in slow degradation of detergent molecule leading to their accumulation, Effluents containing these detergents reach the rivers, ponds etc. These persist in water even after sewage treatment and thus water get polluted.
Since, unbranched (i.e., straight) chains are more prone to attack by bacteria, therefore, in most of the detergents used these days, the branching is kept to a minimum so that the detergents become easily biodegradable and hence pollution is prevented.
