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Synthetic Detergents

Key Concepts

  • Synthetic detergents can be made from petrochemicals, fats and oils.

  • Synthetic detergent molecules, like soap molecules, generally consist of a hydrophilic head and a hydrophobic tail.

  • The hydrophobic, long hydrocarbon chain tail of the detergent molecule is attracted to particles of oil or grease by dispersion forces (Van der Waals or London or Weak Intermolecular Forces).

  • The hydrophilic, charged or polar head of the detergent molecule is attracted to water molecules.

  • Synthetic detergents are less sensitive to the effects of calcium and magnesium ions in hard water.

  • A number of additives are used to enhance the cleaning ability of detergents.

  • Branched-chain synthetic detergents are far less biodegradable than continuous-chain synthetic detergents.

Classification of Synthetic Detergents

Detergents are classified as either:

  • Anionic: negatively charged head

  • Cationic: positively charged head

  • Non-ionic or neutral: uncharged head

Class Example Chemical Properties Uses
Anionic CH3(CH2)11OSO3-Na+
sodium dodecyl sulfate
Usually contain either
a sulfate (SO4) head
widely used due to cost and performance
-laundry detergents
-dishwashing liquids
-oven cleaners
sodium dodecylbenzenesulfonate
a sulfonate (SO3) head

Cationic CH3(CH2)11NH3+Cl-
dodecylamine hydrochloride

hexadecyltrimethylammonium bromide

Usually contain a modified ammonium ion as active site which produces a germicidal action.
More expensive than anionic detergents.
-cleaning plastics
-hair shampoos
-nappy washes
-fabric softeners and conditioners

or neutral
pentaerythrityl palmitate
Contains polar parts, eg, OH groups, to provide water solubility.
No ionic groups so no reaction in hard water.
Low lathering prevents foam build up in dishwashers.
-car shampoos
-dishwasher detergents

Detergent Additives

additive Use
surfactants, wetting agents Small hydrocarbon chain length is a better wetting agent than a long one, but long hydrocarbon chain length is better for dirt removal and dispersion.
A chain length of about C12 is the best compromise for most anionic detergents.

carboxymethylcellulose Forms a protective hydrated adsorbed layer on cleaned fabric preventing redeposition of dirt.

builders (pyrophosphates, tripolyphosphates, silicates eg, zeolites) Form soluble complexes with Ca2+ and Mg2+ in hard water and act as deflocculating agents preventing scum buildup.

abrasives To scour.

foaming agents To promote and stabilize foam formation.

oxidisers, eg, perborates Bleaching

pH modifiers To promote the effectiveness of some ingredients.

enzymes To digest proteins, carbohydrates and fats.

optical whiteners
(fluorescent dyes)
Add brightness to fabrics by absorbing UV light and emitting blue light which masks any yellow tint which may develop in white fabrics.

fragrances Enhance the odour of detergents.

Environmental Concerns

The first detergents in use were highly branched alkylbenzenesulfonates,


Microorganisms in septic tanks and sewage-treatment plants cannot degrade branched chains.

To prevent buildup of detergents in rivers and lakes, modern-day detergents are designed to be biodegradable.

Alkylbenzenesulfonates with a continuous chain, rather than a branched chain are biodegradable.

Continuous chain alkysulfates, eg, CH3(CH2)16CH2OSO3-Na+, are also biodegradable.

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