A surfactant (surface active agent) is a molecule with a water-loving (hydrophilic) part and a water-hating (hydrophobic) part, so it collects at surfaces. A cationic surfactant is one in which the hydrophilic head is positively charged.Quaternary ammonium salts with at least one long hydrocarbon chain are cationic surfactants. An example is hexadecyltrimethylammonium bromide:CH3(CH2)15N+(CH3)3 Br− The ion has two parts:
• a positively charged head, the N+ with its small methyl groups, which is attracted to water and to negatively charged surfaces
• a long non-polar tail, the C16 chain, which is hydrophobic and attracted to other non-polar substances such as greaseMany surfaces, including wet hair and fabric fibres such as cotton, carry a small negative charge. The positive heads are attracted to these surfaces and the hydrocarbon tails point outwards, forming a thin coating. This reduces static and makes the surface feel smooth and soft. That is why cationic surfactants are used in fabric softeners and hair conditioners.
The head is a quaternary nitrogen, which has no N-H bond to lose, so it stays positively charged whatever the pH. The surfactant therefore works in both acidic and alkaline water. Some cationic surfactants are also used as antiseptics and disinfectants.Worked example. Which part of CH3(CH2)11N+(CH3)3 Cl− sticks to a fabric fibre? The positive N+(CH3)3 head is attracted to the negative charge on the fibre; the C12 tail points away from it.
• a positively charged head, the N+ with its small methyl groups, which is attracted to water and to negatively charged surfaces
• a long non-polar tail, the C16 chain, which is hydrophobic and attracted to other non-polar substances such as greaseMany surfaces, including wet hair and fabric fibres such as cotton, carry a small negative charge. The positive heads are attracted to these surfaces and the hydrocarbon tails point outwards, forming a thin coating. This reduces static and makes the surface feel smooth and soft. That is why cationic surfactants are used in fabric softeners and hair conditioners.
The head is a quaternary nitrogen, which has no N-H bond to lose, so it stays positively charged whatever the pH. The surfactant therefore works in both acidic and alkaline water. Some cationic surfactants are also used as antiseptics and disinfectants.Worked example. Which part of CH3(CH2)11N+(CH3)3 Cl− sticks to a fabric fibre? The positive N+(CH3)3 head is attracted to the negative charge on the fibre; the C12 tail points away from it.