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Synthetic Detergents Chemistry Tutorial
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 (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.
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.
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No ads = no money for us = no free stuff for you! Classification of Synthetic Detergents
Detergents are classified as either:
Anionic: negatively charged head
Cationic: positively charged head
Non-ionic or neutral: uncharged head
The table below gives examples of each type of detergent, its properties and uses.
3(CH 2) 11OSO 3 -Na + sodium dodecyl sulfate Usually contain either
a sulfate (SO 4) head widely used due to cost and performance
-laundry detergents -dishwashing liquids -oven cleaners
3(CH 2) 11C 6H 4SO 3 -Na + sodium dodecylbenzenesulfonate or
a sulfonate (SO 3) head
3(CH 2) 11NH 3 +Cl - dodecylamine hydrochloride
3(CH 2) 15N(CH 3) 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
or neutral CH
3(CH 2) 14COOCH 2C(CH 2OH) 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
The table below lists some of the additives you will find in common detergents, and, the reason the additive is used.
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 C 12 is the best compromise for most anionic detergents.
Forms a protective hydrated adsorbed layer on cleaned fabric preventing redeposition of dirt.
builders (pyrophosphates, tripolyphosphates,
silicates eg, zeolites) Form soluble complexes with Ca
2+ and Mg 2+ in hard water and act as deflocculating agents preventing scum buildup.
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.
(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.
Enhance the odour of detergents.
The first detergents in use were highly branched alkylbenzenesulfonates,
3CH(CH 3)CH 2CH(CH 3)CH 2CH(CH 3)CH 2CH(CH 3)C
3 -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, CH
3(CH 2) 16CH 2OSO 3 -Na +, are also biodegradable.
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