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 |
CH3(CH2)11C6H4SO3-Na+
sodium dodecylbenzenesulfonate |
or
a sulfonate (SO3) head |
|
| Cationic |
CH3(CH2)11NH3+Cl-
dodecylamine hydrochlorideCH3(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 |
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,
| 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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