- The word detergent means “cleansing agent”. Detergent is derived from Latin word “detergere” means “to wipe off”.
- Cleansing agents are the substances which remove dirt and have cleansing action in water.
- Detergents can be classified into 2 types”
- Soapy detergents or soaps.
- Synthetic detergents
- Soaps are sodium or potassium salts of long chain fatty acids (containing 15-18 carbon atoms), e.g., stearic, oleic and palmitic acids.
- Sodium salts of fatty acids are known as hard soaps and potassium salts of any fatty acids are known as soft soaps.
Preparation of Soap
- Soaps containing sodium salts are formed by heating fat (i.e., glyceryl ester of fatty acid) with aqueous sodium hydroxide solution. This reaction is known as saponification.
- In saponification esters of fatty acids are hydrolysed and the soap is obtained remains in colloidal form.
- It is then precipitated from the solution by adding sodium chloride.
- The solution left after removing the soap contains glycerol, which can be recovered by fractional distillation.
- Only sodium and potassium soaps are soluble in water and are used for cleaning purposes.
- As compared to sodium, potassium soaps are soft to the skin. These can be prepared by using potassium hydroxide solution in place of sodium hydroxide.
Types of Soaps
- By using different types of raw materials different kinds of soaps are made.
- Toilet Soaps:-
- They are prepared by using better grades of fats and oils and care is taken to remove excess alkali.
- Colour and perfumes are added to make these more attractive.
- Floating Soaps:-
- Soaps that float in water are made by beating tiny air bubbles before their hardening.
- Transparent Soaps:-
- They are prepared by dissolving the soap in ethanol and then evaporating the excess solvent.
- Medicated Soaps:-
- In this type of soaps substances of medicinal value are added.
- Shaving Soaps:-
- Shaving soaps contain glycerol to prevent rapid drying. A gum called, rosin is added while making them. It forms sodium rosinate which lathers well.
- Laundry Soaps:-
- They contain fillers like sodium rosinate, sodium silicate, borax and sodium carbonate.
- Soap Chips:-
- Soap chips are made by running a thin sheet of melted soap onto a cool cylinder and scraping off the soaps in small broken pieces.
- Soap Granules:-
- They are dried miniature soap bubbles.
- Soap powders and scouring:-
- Soaps contain some soap, a scouring agent (abrasive) such as powdered pumice or finely divided sand, and builders like sodium carbonate and trisodium phosphate.
Why Soaps don’t work in hard water
- Hard water contains calcium and magnesium ions. These ions form insoluble calcium and magnesium soaps respectively when sodium or potassium soaps are dissolved in hard water.
C17H35COONa + CaCl2 à 2NaCl + (C17H35COO) 2 Ca
Soap Insoluble calcium stearate (Soap)
- These insoluble soaps separate as scum in water and are useless as cleansing agent. In fact these are hindrance to good washing, because the precipitate adheres onto the fibre of the cloth as gummy mass.
- Soaps cannot be used in acidic solutions since acids precipitate the insoluble free fatty acids which stick to the fabrics and thus reduce the ability of the soaps to remove oil and grease from fabrics.
RCOONa + H+ à RCOOH + Na+
Soap Free fatty
acid precipitate out
- Hair washed with hard water looks dull because of this sticky precipitate. Dye does not absorb evenly on cloth washed with soap using hard water, because of this gummy mass.
- Synthetic detergents are cleansing agents which have all the properties of soaps, but which actually do not contain any soap.
- These can be used both in soft and hard water as they give foam even in hard water. Because calcium and magnesium salts are also detergents like their sodium salts are also soluble in water.
- Some of the detergents give foam even in ice cold water.
- Straight chain alkyl group contains detergents which are biodegradable whereas branched alkyl group containing detergents are non-biodegradable.
- Synthetic detergents are mainly classified into three categories:
- (i) Anionic detergents (ii) Cationic detergents and (iii) Non-ionic detergents
- Anionic detergents:-
- Anionic detergents are sodium salts of sulphonated long chain alcohols or hydrocarbons.
- Alkyl hydrogen sulphates formed by treating long chain alcohols with concentrated sulphuric acid are neutralised with alkali to form anionic detergents. Similarly alkyl benzene sulphonates are obtained by neutralising alkyl benzene sulphonic acids with alkali.
- In anionic detergents, the anionic part of the molecule is involved in the cleansing action.
- Sodium salts of alkylbenzenesulphonates are an important class of anionic detergents.
- They are mostly used for household work. Anionic detergents are also used in toothpastes.
- Cationic Detergents:
- They are quaternary ammonium salts of amines with acetates, chlorides or bromides as anions.
- Cationic part possesses a long hydrocarbon chain and a positive charge on nitrogen atom.
- Hence, these are called cationic detergents. Cetyltrimethylammonium bromide is a popular cationic detergent and is used in hair conditioners.
- Cationic detergents have germicidal properties and are expensive, therefore, these are of limited use.
- Non-ionic Detergents:
- They do not contain any ion in their constitution.
- One such detergent is formed when stearic acid reacts with polyethylene glycol.
- Liquid dishwashing detergents are non-ionic type. Mechanism of cleansing action of this type of detergents is the same as that of soaps.
- These also remove grease and oil by micelle formation. Main problem that appears in the use of detergents is that if their hydrocarbon chain is highly branched, then bacteria cannot degrade this easily.
- These days the branching of the hydrocarbon chain is controlled and kept to the minimum. Unbranched chains can be biodegraded more easily and hence pollution is prevented.
Advantages of Synthetic detergents over Soaps
- They can be used with hard water which soap cannot do.
- They can be used in acidic medium unlike soaps.
- They are more soluble in water so form better latter than soaps.
- They have stronger cleaning action than soaps.