What is the function of surfactants?

Wetting Action: Wetting refers to the phenomenon where a gas adsorbed on a solid surface is replaced by a liquid. Substances that enhance this replacement ability are called wetting agents. Wetting is generally categorized into three types: contact wetting, immersion wetting, and spreading wetting. Among them, spreading is the highest standard of wetting, often measured by the spreading coefficient as an indicator of the wetting performance between systems.

2. Micelle and Solubilization: At low concentrations, surfactant molecules associate to form micelles. When their concentration exceeds the CMC (Critical Micelle Concentration), the surfactants arrange into spherical, rod-like, filamentous, lamellar/plate-like, and other structures.

3. Emulsification: The comprehensive affinity of hydrophilic and lipophilic groups within surfactant molecules for oil or water. A coarse dispersion system formed when one or more liquids, with droplets larger than 10^-7 meters in diameter, are dispersed within another immiscible liquid is called an emulsion. To maintain stability, an emulsifier is required. Depending on the structure of the emulsifier, water-in-oil (O/W) emulsions or oil-in-water (W/O) emulsions can be formed. Sometimes, to break down the emulsion, another surfactant called an anti-emulsifier is added to separate the dispersed phase and the dispersion medium. For example, an anti-emulsifier is needed to separate oil from water in crude oil.

4. Foaming and Defoaming Actions: Surfactants are also widely used in the medical industry. In formulations, many insoluble substances such as volatile oils, soluble cellulose, and steroidal hormones can be dissolved into transparent solutions and increased in concentration using the solubilizing action of surfactants. During the preparation process, they are essential as emulsifiers, wetting agents, suspending agents, foaming agents, and defoaming agents. "Bubbles" are formed by a liquid film enclosing gas. Some surfactants, when combined with water, can form a film of certain strength to enclose air and create foam, used in froth flotation, foam fire extinguishing, and cleaning. Such surfactants are known as foaming agents. Defoamers may also be needed; for instance, in sugar and starch production, excessive foam can be reduced by adding appropriate surfactants to weaken the film strength, eliminate bubbles, and prevent accidents.

5. Suspensory Aid: In the agricultural sector, wettable powders, emulsifiable concentrates, and thick emulsions all require a certain amount of surfactants. For instance, wettable powders commonly contain organic compounds that are hydrophobic. It is only under the presence of surfactants that the surface tension of water can be reduced, allowing the powder to be wetted by water and form a suspension.

6. Disinfection and Sterilization: Can be used as a disinfectant and sterilant in the industry. Their sterilizing and disinfecting effects are attributed to their strong interaction with bacterial biofilm proteins, causing denaturation or loss of function. These disinfectants have a relatively high solubility in water and, depending on the concentration, can be used for pre-operative skin disinfection, wound or mucosal disinfection, instrument sterilization, and environmental disinfection.

7. Detergent and Washing Function: Removing oil and dirt is a complex process, involving the wettability and foaming actions mentioned earlier. Detergents typically contain multiple auxiliary ingredients to enhance wettability, provide foaming, whitening, and prevent re-contamination of the cleaned surface. The cleaning process of the main ingredient, the surfactant, is as follows: Water has a high surface tension, poor wettability for oil stains, making it difficult to wash off the oil. The addition of surfactants positions the hydrophobic groups towards the fabric surface and the dirt, gradually releasing the dirt. The dirt is then removed from the surface by floating in the water or with the foam, and the clean surface is occupied by surfactant molecules.