Chemistry Cleansing Agents
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Topics Covered :

● Cleansing Agents
● Soaps
● Types of Soaps
● Working of Soaps with Hard Water
● Synthetic Detergents

Cleansing Agents :

Two types of detergents are used as cleansing agents. These are soaps and synthetic detergents. These improve cleansing properties of water. These help in removal of fats which bind other materials to the fabric or skin.

Soaps :

`=>` Soaps are the detergents used since long.

`=>` Soaps used for cleaning purpose are sodium or potassium salts of long chain fatty acids, e.g., stearic, oleic and palmitic acids.

`=>` 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 `text(saponification)`.

`=>` In this reaction, esters of fatty acids are hydrolysed and the soap obtained remains in colloidal form.

● It is 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.

● Generally potassium soaps are soft to the skin than sodium soaps.

● These can be prepared by using potassium hydroxide solution in place of sodium hydroxide.

Types of soaps :

`=>` Basically all soaps are made by boiling fats or oils with suitable soluble hydroxide.

● Variations are made by using different raw materials.

● `color{green}("Toilet soaps")` 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.

● Soaps that float in water are made by beating tiny air bubbles before their hardening.

● Transparent soaps are made by dissolving the soap in ethanol and then evaporating the excess solvent.

● In medicated soaps, substances of medicinal value are added.

● In some soaps, deodorants are added.

● 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 contain fillers like sodium rosinate, sodium silicate, borax and sodium carbonate.

● 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 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. Builders make the soaps act more rapidly.

Why do soaps not 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.

`color{red}(undersettext(Soap)(2C_(17) H_(35) COONa)+CaCl_2 → 2NaCl + undersettext{Insoluble calcium strearate (Soap)}((C_(17)H_(35)COO)_2Ca))`

`=>` These insoluble soaps separate as scum in water and are useless as cleansing agent.

● In fact these are hinderance to good washing, because the precipitate adheres onto the fibre of the cloth as gummy mass.

● 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 :

`=>` 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.

`=>` Some of the detergents give foam even in ice cold water.

`=>` Synthetic detergents are mainly classified into three categories: (i) Anionic detergents (ii) Cationic detergents and (iii) Non-ionic detergents

(i) `color{green}("Anionic Detergents ")` : Anionic detergents are sodium salts of sulphonated long chain alcohols or hydrocarbons.

● Alkyl hydrogensulphates 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.

(ii) `color{green}("Cationic Detergents ")` : Cationic detergents are quarternary 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. 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.

`color{red}(undersettext(Cetyltrimethyl ammonium bromide)([CH_3(CH_2)_(15) - underset(underset(CH_3)(|)) overset( overset(CH_3)(|))N- CH_3]^(+) Br^(-)))`

(iii) `color{green}("Non-ionic Detergents ")` : Non-ionic detergents do not contain any ion in their constitution. One such detergent is formed when stearic acid reacts with polyethyleneglycol.

`color{red}(undersettext(stearic acid)(CH_3(CH_2)_(16) COOH) +undersettext(Polyethlenegycol)(HO (CH_2CH_2O)_n CH_2CH_2OH ) overset(-H_2O)→ CH_3(CH_2)_(16) COO(CH_2CH_2O)_n CH_2CH_2OH)`

● 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.

● Slow degradation of detergents leads to their accumulation.

● Effluents containing such detergents reach the rivers, ponds, etc.

● These persist in water even after sewage treatment and cause foaming in rivers, ponds and streams and their water gets polluted.

● 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.