AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The use of quaternary ammonium salts as cationic surfactants.
Full specification wording
Nucleophilic properties (A-level only): The use of quaternary ammonium salts as cationic surfactants.
A quaternary ammonium ion has four groups bonded to the nitrogen and a permanent positive charge, because there is no lone pair left and no N-H bond to lose.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A quaternary ammonium ion has four groups bonded to the nitrogen and a permanent positive charge, because there is no lone pair left and no N-H bond to lose. Make one of those four groups a long hydrocarbon chain and you have a cationic surfactant: an ionic head that is attracted to water and a non-polar tail that is not. Unlike a soap, whose charge depends on the pH of the water, this charge is fixed. Wet hair and wet fabric fibres carry a slight negative charge on their surface, so the cations are attracted head-first onto the surface and their hydrocarbon tails stick outwards. That coating lubricates the fibres, so they slide over one another, and it carries away the charge that would otherwise build up as static.
What earns the marks
- The cation drawn with four groups on nitrogen, one of them a long hydrocarbon chain.
- The negative charge on wet hair or fabric attracting the positive head.
- The hydrocarbon tails pointing away from the surface.
- A stated effect, such as reduced friction between fibres or less static.
Common mistakes and how to put them right
Drawing nitrogen with three groups and a lone pair, which is an amine and not a quaternary salt.
Saying the hydrocarbon tail binds to the fibre and the charged head points outwards.
Describing soap's micelle mechanism instead of surface coating.
Claiming the charge on the head depends on pH, when a quaternary ion is permanently charged.
Exam tips
- Sketch the ion as a charged circle with one long zig-zag tail; that diagram earns the structure mark on its own.
- Name the surface you are coating. 'Hair' or 'fabric' with its negative charge is what turns a description into an explanation.
Try the lesson
The start of the Nucleophilic properties (A-level only): The use of quaternary ammonium salts as cationic surfactants. lesson, made from checked teaching notes for this specification point.
Have a go first
Which part of a quaternary ammonium salt molecule carries the positive charge?
Listen
A quaternary ammonium ion has four groups bonded to the nitrogen and a permanent positive charge, because there is no lone pair left and no N-H bond to lose. Make one of those four groups a long hydrocarbon chain and you have a cationic surfactant: an ionic head that is attracted to water and a non-polar tail that is not. Unlike a soap, whose charge depends on the pH of the water, this charge is fixed. Wet hair and wet fabric fibres carry a slight negative charge on their surface, so the cations are attracted head-first onto the surface and their hydrocarbon tails stick outwards. That coating lubricates the fibres, so they slide over one another, and it carries away the charge that would otherwise build up as static. A strong answer earns marks for these points. the cation drawn with four groups on nitrogen, one of them a long hydrocarbon chain. the negative charge on wet hair or fabric attracting the positive head. the hydrocarbon tails pointing away from the surface. a stated effect, such as reduced friction between fibres or less static. Exam tip: Sketch the ion as a charged circle with one long zig-zag tail; that diagram earns the structure mark on its own.
Key words
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permanent positive charge
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cationic surfactant
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hydrophobic tail
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attraction to negative surfaces
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See every definition
- permanent positive charge
- A quaternary ammonium ion has four groups bonded to the nitrogen and this charge, because there is no lone pair left and no N-H bond to lose.
- cationic surfactant
- An ionic head that is attracted to water and a non-polar tail that is not, made by giving one of the four groups a long hydrocarbon chain.
- hydrophobic tail
- The non-polar part of the surfactant that is not attracted to water and sticks outwards from the coated surface.
- attraction to negative surfaces
- Wet hair and wet fabric fibres carry a slight negative charge on their surface, so the cations are attracted head-first onto the surface.
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