What are surfactants and how do they work?

SURFACTANTS are the most versatile products of the chemical industry.

There are utilized inevery industrial area ranging from household detergents to drilling muds andfood items to pharmaceuticals.

The term surfactant comes from the word surface active agent. They areamphiphilic molecules and are thus absorbed in the air-water interface. At theinterface, they align themselves so that the hydrophobic part is in the air andthe hydrophilic part is in water. This will cause a decrease in surface orinterfacial tensions.

Surfactant basics

As said, surfactants are amphiphilic molecules that have hydrophobic andhydrophilic parts. The hydrophobic tail is a hydrocarbon, fluorocarbon orsiloxane. Surfactants are typically classified based on their polar head as thehydrophobic tails are often similar. If the head group has no charge, thesurfactant is called non-ionic. If the head group has negative or positivecharge, it is called anionic or cationic, respectively. If it contains bothpositive and negative groups, then the surfactant is called zwitterionic.

Anionic and nonionic surfactants are by far the most used surfactanttypes in the industry. Anionic surfactant finds use especially in cleaningproducts like laundry detergents and shampoos. Nonionic surfactants on theother hand are often used as wetting agents and in the food industry. Bothcationic and zwitterionic surfactants are more for special use as they are moreexpensive to produce.

Surfactants absorb at interfaces

Because of their amphiphilic nature, surfactants absorb at the air-wateror oil-water interface. At the interface, surfactants align themselves so thatthe hydrophobic part is in the air (or oil) and the hydrophilic part in water.

For simplicity, let’s consider only the air-water interface. Thecohesive forces between the water molecules are very strong making the surfacetension of water high.   As surfactants absorb they break theseinteractions. The intermolecular forces between surfactant and water moleculeare much lower than between two water molecules and thus surface tension willdecrease. When the surfactant concentration is high, they form micelles. Thepoint at which micelles are formed is called critical micelle concentration.

The main purpose of the surfactants is to decrease the surface andinterfacial tension and stabilize the interface. Without surfactants washinglaundry would be difficult and many of the food products like mayonnaise andice cream would not exist. Thus optimization of surfactants for differentapplications is highly important and surface and interfacial tensionmeasurements have a key role in it.