Surfactants, also known as interfacial active agents, are substances that can significantly reduce the surface tension of the target solution, which can reduce the surface tension between two liquids or liquid solids. A wide variety. The most typical example is soap, which has the effect of decomposition and infiltration, and is widely used.

Surfactants are generally organic amphoteric molecules with hydrophilic and hydrophobic groups, usually amphiphilic organic compounds containing hydrophobic groups (" tail ") and hydrophilic groups (" head "). Therefore, they are soluble in both organic solvents and water.

Classification

--According to hydrophilic groups can be divided into:

Cationic surfactants: After dissociation in water, the head group has a positive charge, and quaternary ammonium salts are the most common. Because its aqueous solution has a strong bactericidal ability, it is often used for disinfection and sterilization. Cationic surfactants with two tail chains make fabrics supple and antistatic.

Anionic surfactant: After dissociation in water, the head base is negatively charged. These include bile acids containing carboxylic groups, soap-based surfactants, phospholipids partially containing phosphate groups, and sulfate and sulfonic groups.

Zwitterionic surfactant: Two head groups with opposite charges are covalently joined together. Such as egg yolk lecithin, betaine, amine oxide surfactants.

Nonionic surfactants: The head group is almost inseparable in water, and the water solubility depends on the hydrogen bond between the head group and water molecules. Such as oligoxyethylene, oligosaccharides.

--According to the hydrophobic tail chain, surfactants can be divided into :

Linear alkyl surfactants: have a linear hydrophobic tail chain, such as various alkyl salts, alkyl sulfates, etc.

Branched alkyl surfactants: Hydrophobic tail chain with branched chain structure, such as bis (2-ethylhexyl) succinate sodium sulfonate.

Unsaturated alkyl surfactants: Hydrophobic tail chains with carbon-carbon double bonds, such as sodium oleate.

Cyclic hydrophobic tail chain surfactants: Hydrophobic tail chains form annular structures, such as the cholesteric skeleton.

Fluorocarbon surfactant: hydrophobic tail chain formed by the replacement of hydrogen atoms on the hydrogen chain by fluorine atoms, with high surface activity, high heat stability and high chemical stability, both hydrophobic and oil-repellent. It is widely used in the preparation of hydrophobic materials.

Silicone surfactant: A mixture of aqueous solutions of fluorocarbon and hydrocarbon surfactants, with alternating silicon-oxygen bonds and methyl groups attached to silicon atoms as hydrophobic chains .

Polymeric surfactant: the polyoxypropylene chain obtained from the ring-opening polymerization of propylene oxide is used as the hydrophobic chain.

According to their molecular configuration, surfactants can be divided into :

Bola type surfactant: the two head groups are at both ends of the hydrophobic tail chain [5].

Gemini surfactant: Two monomers are covalently linked, double-headed and double-tailed.

Property

When dissolved in water, the surfactant can adsorb on the air/water surface to form a single layer film, and can enrich and reduce the interfacial tension at the oil/water interface, changing the structure and properties of the oil/water mask. In addition, they can also be adsorbed on solid surfaces, improving the wettability of solid substrates.

Application

Surfactants can reduce the surface tension of water so that water can more easily permeate and wet other materials.

Surfactants are widely used in detergents, with the ability to decontaminate, to dissolve oil in water, and form micelles or emulsions, so as to achieve cleaning effects.

Surfactants can coat the large size of the insoluble matter, dispersed in water, thereby increasing its stability. For example, for materials such as carbon nanotubes that are insoluble in water, surfactants can be coated on their surface to play a dispersing and stabilizing role.

Due to the amphiphilicity of the surfactant, it can be adsorbed at the liquid-liquid interface, thus forming an emulsion system, so that the originally insoluble liquids mix with each other. Emulsification has important applications in food industry and pharmaceutical preparations. When the insoluble gas is dispersed (wrapped) by the liquid, a stable foam system can be formed.

Some surfactants are widely used in biological systems. For example, the anionic surfactant sodium dodecyl sulfate (SDS) is used as a gel-assisted component to isolate proteins. In addition, surfactants can also play a role as drug carriers, biological adhesives, and the preparation of smart soft materials.