A substance added to water to improve its cleaning properties. Although water is a powerful solvent for many compounds, it will not dissolve grease and natural oils. Detergents are compounds that cause such nonpolar substances to go into solution in water. Soap is the original example, owing its action to the presence of ions formed from long-chain fatty acids (e.g. the octadecanoate (stearate) ion, CH3(CH2)16COO−). These have two parts: a nonpolar part (the hydrocarbon chain), which attaches to the grease; and a polar part (the –COO− group), which is attracted to the water. A disadvantage of soap is that it forms a scum with hard water (see hardness of water) and is relatively expensive to make. Various synthetic (’soapless’) detergents have been developed from petrochemicals. The commonest, used in washing powders, is sodium dodecylbenzenesulphonate, which contains CH3(CH2)11C6H4SO2O− ions. This, like soap, is an example of an anionic detergent, i.e. one in which the active part is a negative ion. Cationic detergents have a long hydrocarbon chain connected to a positive ion. Usually they are amine salts, as in CH3(CH2)15N(CH3)3+Br−, in which the polar part is the –N(CH3)3+ group. Nonionic detergents have nonionic polar groups of the type –C2H4–O-C2H4–OH, which form hydrogen bonds with the water. Synthetic detergents are also used as wetting agents, emulsifiers, and stabilizers for foam.