Surfactant is a substance that can significantly reduce its surface tension and change the interface state of the system when a small amount is added to the solvent, resulting in wetting or dewetting, emulsification or demulsification, dispersion or aggregation, and foaming Or a series of functions such as defoaming and solubilization to meet the requirements of practical applications.

The function of surfactant
1. Wetting effect
When the solid is in contact with the liquid, the original solid/gas and liquid/gas interfaces disappear and a new solid/liquid interface is formed. This process is called wetting. For example, textile fiber is a porous material with a huge surface. When the solution spreads along the fiber, it will enter the gap between the fibers and drive out the air, turning the original air/fiber interface into a liquid/fiber interface. It is a typical wetting process; while the solution will enter the fiber at the same time, this process is called penetration. Surfactants that help wetting and penetration are called wetting agents and penetrants.

2. Emulsification
Emulsification refers to two immiscible liquids (such as oil and water), one of which is formed by evenly dispersing very small particles (with a particle size of 10-8~10-5m) into the other liquid The role of emulsion. Oil droplets dispersed in water are called oil-in-water emulsions (O/W), and water droplets dispersed in oils are called water-in-oil emulsions (W/O). Surfactants that can help emulsification are called emulsifiers. Surfactants used as emulsifiers have two functions: stabilization and protection.

3. Decontamination effect
Due to the emulsification effect of the surfactant, the grease and dirt particles detached from the solid surface can be stably emulsified and dispersed in the aqueous solution, and will no longer be deposited on the cleaned surface to form re-contamination.

4. Foaming effect
The state of gas dispersed in liquid is called bubble. If a certain liquid is easy to form a film and is not easy to break, the liquid will produce a lot of bubbles when it is stirred. After the foam is generated, the gas/liquid surface area in the system increases greatly, making the system unstable, so the foam is easy to burst. When the surfactant is added to the solution, the surfactant molecules are adsorbed on the gas/liquid interface, which not only reduces the surface tension between the gas/liquid phases, but also forms a monomolecular film with a certain mechanical strength to make the foam difficult to burst.

Application of surfactants
1. Cosmetics
Surfactants are widely used in various cosmetics as emulsifiers, penetrants, detergents, softeners, wetting agents, bactericides, dispersants, solubilizers, antistatic agents, hair dyes, etc. Non-ionic surfactants are most commonly used in cosmetics because they are not irritating and compatible with other components. Generally, they are fatty acid esters and polyethers.
2. Detergent
Surfactants have efficient cleaning and disinfection functions, and have long become the most important part of cleaning products. Surfactant is the main component of detergent. It interacts with dirt and between dirt and solid surface (such as wetting, permeating, emulsifying, solubilizing, dispersing, foaming, etc.) and taking advantage of Mechanical stirring obtains the washing effect. The most widely used and most widely used are anionic and nonionic surfactants. Cationic and amphoteric surfactants are only used in the production of certain special types and functions of detergents. The main varieties are LAS (referring to alkyl benzene sulfonate), AES (fatty alcohol polyoxyethylene ether sulfate), MES (α-sulfonic acid fatty acid salt), AOS (α-alkenyl sulfonate), Alkyl polyoxyethylene ether, alkylphenol polyoxyethylene ether, fatty acid diethanolamine, amino acid type, betaine type, etc.
3. The field of medicine
Surfactants have the functions of wetting, emulsifying, solubilizing, etc., so they are widely used as excipients for pharmaceutical preparations, especially in the pharmaceutical microemulsion technology that has been developed in recent years. In the synthesis of drugs, surfactants can be used as phase transfer catalysts, which can change the degree of solvation of ions, thereby increasing the reactivity of ions, making the reaction proceed in a heterogeneous system, and greatly improving the reaction efficiency. Surfactants are often used as solubilizers and sensitizers in analysis, especially in pharmaceutical fluorescence spectrometry. In terms of skin disinfection, wound or mucous membrane disinfection, instrument disinfection and environmental disinfection before surgery in the pharmaceutical industry, surfactants can strongly interact with bacterial biofilm proteins to denature or lose their function, and are used as bactericides and disinfectants being widely used.