Definition
detergent: A substance added towater to improve its cleaning properties.Although water is a powerfulsolvent for many compounds, it willnot dissolve grease and natural oils.Detergents are compounds that causesuch nonpolar substances to go intosolution in water. Soap is the originalexample, owing its action tothe presence of ions formed fromlong-chain fatty acids (e.g. theoctadecanoate (stearate) ion,CH
3(CH
2)
16COO
-). These have twoparts: a nonpolar part (the hydrocarbonchain), which attaches to thegrease; and a polar part (the –COO–group), which is attracted to thewater. A disadvantage of soap is thatit forms a scum with hard water (seehardness of water) and is relativelyexpensive to make. Various synthetic(‘soapless’) detergents have been developedfrom petrochemicals. Thecommonest, used in washing powders,is sodium dodecylbenzenesulphonate,which containsCH
3(CH
2)
11C
6H
4SO
2O
- ions. This, likesoap, is an example of an anionic detergent,i.e. one in which the activepart is a negative ion. Cationic detergentshave a long hydrocarbonchain connected to a positive ion.Usually they are amine salts, as inCH
3(CH
2)
15N(CH
3)
3+Br
-, in which thepolar part is the N(CH
3)
3+ group.Nonionic detergents have nonionicpolar groups of the type–C2H4–O–C2H4–OH, which formhydrogen bonds with the water.Synthetic detergents are also used aswetting agents, emulsifiers, and stabilizersfor foam.
