Synthetic Detergents

Synthetic Detergents: Three Structural Categories That Work in Hard Water Too

Synthetic detergents share all the practical cleansing properties of soap, yet contain no true soap in their formulation at all. Their key chemical advantage over soap is that they clean effectively in both soft and hard water alike (some synthetic detergent formulations will even foam in ice-cold water), because - unlike soap's calcium and magnesium salts - the corresponding calcium and magnesium salts of synthetic detergents remain soluble rather than precipitating out as scum.

Synthetic detergents fall into three structural categories, distinguished by the chemical nature of their hydrophilic (water-attracting) head group:

• Anionic detergents are sodium salts of sulphonated long-chain alcohols or hydrocarbons - for example, sodium laurylsulphate, made by treating a long-chain alcohol with sulphuric acid and then neutralising the product with alkali, or sodium alkylbenzenesulphonates, made by neutralising the corresponding alkylbenzenesulphonic acids. In both cases it is the negatively charged (anionic) part of the molecule that performs the actual cleansing action. Anionic detergents are mostly used for household cleaning work and are also incorporated into toothpaste.
• Cationic detergents are quaternary ammonium salts of amines, paired with acetate, chloride or bromide counter-ions, and carry both a long hydrocarbon chain and a positively charged nitrogen centre - cetyltrimethylammonium bromide is a popular example, used in hair conditioners. Cationic detergents possess germicidal properties in addition to their cleansing action, but they are comparatively expensive, which limits their use to more specialised applications. …