Surface Chemistry: Surfactant Wetting Properties
Surfaces are one of the most interesting and diverse parts of nature. Even though they’re made from the same materials as bulk solids, they are exposed to different forces, giving them unique (and sometimes problematic) properties.
Surfactants can modify these properties, making surfaces waterproof; easier to clean, paint, or process; or increase “wetting” when in contact with water.
Wetting: Contact Angles
When water touches a surface, the water molecules experience two forces: they are attracted to each other, but they can also be attracted to or repelled by the surface.
This leads to a range of possible contact angles (ϴ) depending on how these two forces compare.
Very hydrophobic surfaces (left) will have a high contact angle, and the surfaces will be nearly waterproof, because the water molecules are more attracted to each other than to the surface.
Hydrophilic surfaces will have a low contact angle (right) and the surfaces will be much wetter.
While the advantages of waterproofing are obvious, wetting can also be beneficial, especially in cleaning applications.
Oil & Water
Many stains, from food stains to industrial contaminants, are oily (non-polar) in nature, so they repel polar solvents like water.
This makes them difficult to clean with water alone, because the water can’t work its way between the stain and the surface, whether it’s a ceramic plate, a textile thread, or a piece of metal.
To improve wetting, a surfactant that can attract both polar and non-polar materials is needed.
Most of the time, this takes the form of a long-chain molecule that has a polar “head” and a non-polar hydrocarbon chain “tail.” These surfactants can coat the oily surface or be mixed in with the water.
The polar head can take many forms, from the anionic (negatively charged) molecules in soaps to the cationic (positively charged) molecules used in fabric softeners.
Anionic surfactants emulsify dirt very well but can form insoluble “scum” if the water is high in positively-charged minerals such as Ca2+ or Mg2+.
Cationic surfactants avoid this problem and can even improve the texture of fabrics that build up negative charges over time. Zwitterionic surfactants (a molecule that has a positive charge in one location and a negative charge in another) are also available.
Using this basic principle, surfactants can be designed to alter almost any property of a surface. They can repel water, attract oils, improve coating adhesion, or cause a surface to reject a substance completely.
They are found in almost everything from food emulsions to the foam in shampoo, and they quietly make life a little easier for manufacturers and consumers around the world.