The term surfactant comes from the English word 'surfactant'. It is actually a condensed word of the phrase 'surface active agent'. It also has a name called Tensor. Any substance that can significantly reduce the surface tension of a liquid by adding a small amount is collectively referred to as a surfactant. Their surface activity is specific to a particular liquid, usually referring to water. One end of the surfactant is a non-polar hydrocarbon chain (alkyl group) with extremely low affinity for water, often referred to as a hydrophobic group; On the other end, there are polar groups (such as - OH, - COOH, - NH ₂, - SO3H, etc.) that have a high affinity for water, hence they are called hydrophilic groups and collectively referred to as "amphiphilic molecules" (lipophilic hydrophilic molecules). To achieve stability, surfactants can be dissolved in water in two ways:
1. Form a monolayer on the liquid surface.
Leave hydrophilic groups in water and extend hydrophobic groups towards the air to reduce repulsion. The repulsive force between hydrophobic groups and water molecules is equivalent to exerting an outward push on the surface water molecules, offsetting the inward pulling force that the surface water molecules originally received, and even if the surface tension of water decreases. This is the basic principle of the foaming, emulsification, and wetting effects of surfactants. In oil-water systems, surfactant molecules are adsorbed at the interface between the oil and water phases, while polar groups are inserted into the water, and non-polar portions enter the oil and are oriented at the interface. This generates tension between the oil-water phase, reducing the interfacial tension between the oil and water. This property has a significant impact on the widespread application of surfactants.
2. Form 'micelles'.
Micelles can be spherical or layered, both of which try to hide hydrophobic groups inside the micelles and expose hydrophilic groups as much as possible. If polar groups are represented by spheres and hydrophobic non-polar groups are represented by columns, then monolayers and micelles can be obtained. If there are insoluble oils (a general term for organic liquids that are insoluble in water) in the solution, they can enter the center of spherical micelles and dissolve in the interlayer of layered micelles. This is called the solubilization effect of surfactants.
Surfactants can have various functions such as washing, emulsification, foaming, wetting, soaking, and dispersion, and the amount of surfactant used is small (usually a few percent to a few thousand percent). They are easy to operate, non-toxic, and non corrosive, making them ideal chemical products. Therefore, they have important applications in production and scientific research. When the concentration is the same, the non-polar components in the surfactant are larger and have stronger surface activity. Among homologues, those with more carbon atoms have higher surface activity. But when the carbon chain is too long, it has no practical value due to its low solubility in water.