By Sunil Bhardwaj


The surfactant is a substance which gets preferentially adsorbed at the airwater, oil-water and solidwater interfaces. It forms an oriented monolayer where in hydrophilic group points towards the aqueous phase and the hydrocarbon chain points towards the air or oil phase. The surfactant can be cationic, anionic or non-ionogenic. Sodium salts of higher fatty acids such as sodium palmitate, sodium sterate, sodium oleate are anionic surfactants. Those which dissociate in water to yield positively charged ions are cationic surfactants. e.g. Octadecyl ammonium chloride, cetyl triethyl ammonium chloride and cetyl pyridinium chloride. Non-ionogenic surfactants are those whose molecules cannot undergo dissociation. When an alcohol having high molar mass reacts with several molecules of ethylene oxide, a non-ionogenic surfactant is produced.

Willard Gibbs showed that surface activity is due to an unequal distribution of solute between the surface and the bulk of the solution. He derived an equation, on the basis of thermodynamics, which is given by, $$ S\quad =\quad \frac { C }{ RT } \frac { d\gamma }{ dC } $$ where, S = excess concentration of solute per square cm. of surface as compared to that in the bulk of the solution.
C = equilibrium concentration of the solute in the given volume of the solution.
\(\frac { d\gamma }{ dC } \) = rate of change of surface tension with concentration.

Substances that increase the surface tension of a solvent, when dissolved are called surface inactive. This class includes substances dissociating into ions, and substances more polar than water. In this case, the value of S will be -ve. Hence, the concentration of the solute will be lower in the surface than in the bulk of the solution.

Detergents: Surfactants are used as cleaners and detergents to improve wetting of the cloth fiber and spreading of the detergent. Detergents are mixtures of low and high HLB, which remove oil, fat and grease, the higher HLB detergent helps solubilize the less water soluble, low HLB detergent into an aqueous system. Broadly we can classify the Detergents into 4 groups viz. Anionic, Cationic, Nonionic and Amphoteric.

Food industry: The surfactants in food industry are generally called as food emulsifiers and they have various effects on the production process of food and improve the quality of food. They are used to improve the palatability and shelf storage properties. They also function as waterinoil emulsifiers to promote air retention and other physical properties of food where the water content is low. They can function in a similar manner as an oilinwater emulsifier for products with high water content.

Pesticides formulations: Surfactants facilitate and enhance the absorbing, emulsifying, dispersing, spreading, sticking, wetting or penetrating properties of pesticides. Surfactants when used with herbicides help a pesticide to spread over and penetrate the waxy cuticle (outer layer) of a leaf or to penetrate through the small hairs present on a leaf surface. Because of the high surface tension of water, spray mixture droplets can maintain their roundness and sit on the leaf hairs or waxy surface without much of the herbicide actually contacting the leaf. The pesticide mixture then becomes susceptible to degradation by sunlight or by gravity that can cause herbicide runoff from the leaf surface. This can be avoided by use of suitable surfactant, which will reduce the surface tension of the water and increase the spreading and sticking property of pesticides.