Surface tension is caused by a fluid’s intermolecular forces. Water molecules are polar and thus bond with each other electrically. These intermolecular Van der Waals forces are strong enough to produce tension on its surface. If you were to gently place a paper clip on the surface of water, the surface tension would prevent the paper clip from sinking. When a surface active agent is added, the surface tension reduces and the paper clip sinks.
Surface active agents, or surfactants, are chemicals that reduce a liquid’s surface tension. Each surfactant molecule has two main parts: a head and a tail. The head is polar and is thus attracted to the water molecules, making it hydrophilic. The tail is not polar and is not attracted to water molecules, making it hydrophobic. When a surfactant is added, its polar heads shift some of the electrical forces from between the molecules towards themselves. When millions of these molecules are added, a significant proportion of the intermolecular forces shifts towards neutralizing the polar heads and thus reduces the surface tension. Some common surfactants include soaps and emulsifiers.
Soaps and emulsifiers
Soaps and emulsifiers do the same thing. Emulsifiers prevent oil from separating from other liquids in an emulsion. Soaps make an emulsion by having dirt particles leave the dirty materials and mix with the water. With the dirt particles floating in the water, they can be rinsed out. Soaps and emulsifiers produce different results but do so using the same mechanism.
Oil and dirt particles don’t mix with water because they are nonpolar and are thus hydrophobic. The polar heads of surfactants are attracted to the water surrounding the particles, leaving the nonpolar heads to stick to the particles themselves. These form spheres called micelles, which are made up of the surfactant molecules and nonpolar particles. Micelles are able to float around in water and can be rinsed out, carrying dirt with them. This is how cleaning products remove dirt. Emulsifiers do the exact same thing, except the oil particles are meant to stay in the fluids. For example, lecithin and protein from egg yolk keep the oil from separating in mayonnaise.
Type of surfactants
There are many surfactants, but all of them can be placed into four different categories:
- Nonionic: These have no ions, but are still polar. Their polarity comes from one oxygen-rich end (negative polarity) and one end with a large organic molecule (induced positive polarity).
- Anionic: One end of the molecule is negatively charged.
- Cationic: One end of the molecule is postively charged.
- Amphoteric: Charge changes with pH. These can possess characteristics of any two of the three other types.