Synthetic Detergents Chemistry Tutorial

Key Concepts

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 (London forces 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.

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

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Detergents are classified as either:

The table below gives examples of each type of detergent, its properties and uses.

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
Anionic	CH₃(CH₂)₁₁C₆H₄SO₃^-Na⁺ sodium dodecylbenzenesulfonate	or a sulfonate (SO₃) head

Cationic	CH₃(CH₂)₁₁NH₃⁺Cl^- dodecylamine hydrochloride CH₃(CH₂)₁₅N(CH₃)₃⁺Br^- 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

Non-ionic or neutral	CH₃(CH₂)₁₄COOCH₂C(CH₂OH)₃ 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 -cosmetics

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The table below lists some of the additives you will find in common detergents, and, the reason the additive is used.

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.

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The first detergents in use were highly branched alkylbenzenesulfonates,

CH₃CH(CH₃)CH₂CH(CH₃)CH₂CH(CH₃)CH₂CH(CH₃)C

SO₃^-Na⁺

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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