Surfactants or Surface Active Agents

Also known as surfactants, surface-active agents are basic cleaning agents in soaps and detergents. These agents are added to wash water to lower its surface tension, thereby to increase the wetting and spreading properties of water. Surfactants are usually organic compounds, which are amphiphilic, meaning they are soluble in both organic solvents and water. Surface active agents have two parts, one is hydrophilic (water loving) and another is hydrophobic (water repellent). Surface-active molecules concentrate at the areas of contact or interfaces, between oil and water. One end of the molecule seeks oil, while the other end seeks water. At the interface of water and oil, surface-active agents emulsify oil and mix it into the liquid in the same way fat is mixed in milk. At the interface of water, these agents trap air molecules to produce foam.

By reducing the surface tension water, surfactants improve the cleaning performance by enabling the solution to wet a surface (for example, dishes, clothes, and countertops) quickly and effectively, and hence the soil can be readily loosened and removed. Surface active agents also emulsify oily soils and keep them suspended and dispersed so they do not settle back on the surface. To achieve superior cleaning performance, most of the cleaning products contain two or more surfactants.

Types of Surface Active Agents

Surfactants are generally classified on the basis of their ionic properties (electric charge) in water. On the basis of ionic properties, surfactants can be classified into four types -

• Anionic Surfactants
• Non-ionic Surfactants
• Cationic Surfactants
• Amphoteric Surfactants

Properties of Surfactants

The molecular structure of surface-active agents means that they have unusual characteristics, leading to their uses in widespread and highly specialized applications. The properties of these agents can be categorized into two types -

• Adsorption
• Self Assembly

Structures of Common Surfactants Used in Biochemistry

Adsorption

Adsorption is the tendency of the molecule of a surfactant to collect as an interface. It is the taking up of a liquid or gas at the surface of substance, generally a solid (for example, activated charcoal adsorbs gases). The process involves molecular attraction at the surface.

The adsorption property of surfactants mean that their molecules are generally found at the interface between a water phase and an oil phase an air phase and a water phase. This molecular property results in the macroscopic properties of wetting, detergency, foaming and emulsion formation. The molecules of a surface-active agent tend to adsorb to the surface of oil droplets. While the hydrophilic heads stick out into the water phase, the hydrophobic tails stick into the oil phase.

Self Assembly

Self-assembly is the inclination of surfactant molecules to organize and coordinate themselves into the extended structures in water. The process includes the formation of micelles, liquid crystals and bilayers that are formed when the hydrophobic tails of surfactant molecules cluster together to produce small aggregates, like micelles, or large layer structures like bilayers that are similar to a cell wall. These characteristics of surfactants make them an interesting study and an area of research.

Surfactants can also organize to form micelles, which allows the hydrophobic tails to get out of the water, however it still allows the hydrophilic heads to stay in the water. There is typically between a few dozen to a couple of hundred surfactant molecules in a micelle.

Surfactants also aggregate to develop and produce extended structures in water like that of surfactant bilayers.

The concentration at which surfactants start forming micelles is known as the critical micelle concentration or CMC. When micelles are formed in water, their tails form a core, which can encapsulate an oil droplet, and their (ionic/polar) heads produce an outer shell, which maintains a favorable contact with water. When the surfactants assemble in oil, their aggregate is referred to as reverse micelle. In a reverse micelle, the heads remain in the core and the tails maintain a favorable contact with the oil.

Thermodynamics of surface-active agents is of great importance theoretically as well as practically. This is due to the fact that these agents represent systems between the ordered and disordered states of matter.

Applications of Surface Active Agents

The surfactants have found variety of uses and applications in the detergent industry, in emulsification, lubrication, catalysis, tertiary oil recovery, and in drug delivery. Some of the important applications of surfactants include -

• Used in the investigation of the denaturation of bacteriorhodopsin and in thermal stability experiments of rhodopsin
• In superior performance liquid chromatography, some common techniques, like ion-exchange HPLC, reversed-phase HPLC and sizeexclusion-HPLC require surfactants to solubilize membrane proteins. Ion pair HPLC requires surfactants as reagents so as to achieve the separation conditions
• The operations of removal and exchange of surfactants bound to membrane proteins are important and have been successfully applied to a variety of these proteins
• Integral membrane proteins can be separated from hydrophilic proteins and can be described as such in crude surfactant extracts of membrane or cells
• Surfactants are also used to encourage and push the dissociation of proteins from nucleic acids on extraction from biological material
• Affinity surfactants have found uses as reversibly bound ligands in high performance affinity chromatography
• Some other examples of surfactants in biochemistry include the solubilization of enzymes in apolar solvents via reversed micelles and the isolation of hydrophobic proteins
• Crystallization of membrane proteins can be achieved using short chain surfactants that are believed to shield the hydrophobic inter membrane part of the molecule