AQA A Level Chemistry复习笔记7.5.5 Quaternary Ammonium Salts

• Quaternary ammonium salts are chemical analogues to ammonium chloride, NH4Cl, in which the hydrogens have been replaced by four alkyl groups
• They are very useful in the manufacture of cleaning products such as conditioners for washing hair and in fabric softeners

The structure of a quaternary ammonium salt. The alkyl groups can be the same or different.

• In this role they act as cationic surfactants
• Surfactants are chemicals that lower the surface tension between immiscible liquids and allow wetting to take place
• They are cationic due to the positive charge on the nitrogen atom
• The hydrocarbon groups are hydrophobic and the amine group is hydrophilic
• This allows the molecules to cluster on the surface of water with their hydrophilic charged ends in the water and their hydrophobic tails on the surface, hence the term surfactant

Cationic surfactants

• When hair or clothing is wet it can pick up negative charges that facilitate the build up of static electricity when dry
• The conditioner or softener is attracted to the wet surfaces due to the positive charge on the surfactant
• This forms a coating on the hair or clothing, giving a smooth coating and preventing the build up of static