Cationic Surfactants

Like anionic surfactants, it is fairly easy to recognise the cationic surfactants - it has a positive charge. Fatty amine salts (or ammonium salts) were developed as the first cationic surfactants.

Fatty amine salts were the first style of cationic surfactant synthesised.

Unfortunately, ammonium salts can also be sensitive to pH. If we take these cationic surfactants up to high pH (10 or 11) then it is possible to deprotonate the amine, thus leaving us once again with an uncharged molecule. In the same way as the fatty acid salts tended to precipitate out once they were protonated, these fatty amine salts will precipitate out once deprotonated.

In response to this shortcoming, the alkyl pyridinium and quaternary ammonium salts were developed (quaternary means that there are four substituents on the nitrogen atom). These surfactants are incredibly stable so that they do not lose their charge in high pH conditions.

The development of alkyl pyridinium and quaternary ammonium salts provided excellent surfactants that could be used over a vast range of conditions.

Uses of Cationic Surfactants

Cationic surfactants are typically used in things like hair-conditioner and fabric softeners. The fatty amine salts proved quite useful in blends with nonionic surfactants, giving good stability over a range of pH levels. Cationic surfactants are generally rated as being more irritating to the skin than anionic surfactants (although this is probably a gross over-generalisation...).

You might be interested to know that having a "bad hair day" is usually due to residual surfactant on your hair. The cationic surfactants used in shampoos and hair-conditioners can stick to your hair even when you rinse it under water. This results in slightly positively charged hairs, which repel each other, giving you that "bad hair day" look.