The molecular structure of surfactants is characterized by amphiphilic molecules

2021-03-24 17:07:06

Surfactant molecules are composed of two distinct parts: one is the lipophilic group (also known as hydrophobic group) which has affinity with oil, and the other is the hydrophilic group (also known as hydrophobic group) which has affinity with water. The structure of surfactant makes it soluble in water, hydrophilic group is attracted by water molecules, and lipophilic group is repelled by water molecules. In order to overcome this unstable state, we have to occupy the surface of the liquid surface, extend the lipophilic group to the atmosphere, and the hydrophilic group to the water.

Although the molecular structure of surfactants is characterized by amphiphilic molecules, not all amphiphilic molecules are surfactants. Only amphiphilic substances with sufficient length in the lipophilic part are surfactants.

For example, in the series of fatty acid sodium salts, the compounds with less carbon atoms (sodium formate, sodium acetate, sodium propionate, sodium butyrate, etc.) have lipophilic and hydrophilic groups and have surface activity, but they do not play the role of soap, so they can not be called surfactants. Only when the number of carbon atoms increased to a certain extent, the sodium fatty acid showed obvious surface activity and general soap properties. Most natural animal and vegetable oils are fatty acid esters containing C10 ~ C18. If these acids combine with a hydrophilic group, they will become lipophilic and hydrophilic surfactants with good solubility.

In the long-term production practice, it has been found that the solution of some substances can greatly change the surface properties of the solvent even when the concentration is very small, and make it suitable for some production requirements, such as reducing the surface tension or interfacial tension of the solvent, increasing the wettability, washing, emulsification and foaming properties, etc. In daily life, soap is one of these substances. A remarkable feature of substances such as soap is that adding a small amount to water can reduce the surface tension of water a lot.