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Basic theory of surfactant

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①Surface tension
The shrinkage force of any unit length of the liquid surface is called surface tension, and the unit is N • m-1.

②Surface active and surfactant
The property that can reduce the surface tension of the solvent is called surface activity, and the material with surface activity is called surface active material.
Surface active substances that can associate molecules in aqueous solution and form associations such as micelles and have high surface activity, as well as wetting, emulsifying, foaming, washing, etc. are called surfactants.

③ Molecular structure characteristics of surfactants
Surfactant is an organic compound with special structure and properties. They can obviously change the interfacial tension between two phases or the surface tension of liquid (generally water), and have the properties of wetting, foaming, emulsifying, washing and so on.
In terms of structure, surfactants have a common feature, that is, their molecules contain two groups with different properties, and one end is a long-chain non-polar group, which can be dissolved in oil but not in water. Hydrophobic groups or hydrophobic groups, such hydrophobic groups are generally long-chain hydrocarbons, sometimes also organic fluorine, silicone, organic phosphorus, organic tin chain, etc. At the other end is a water-soluble group, namely a hydrophilic group or a hydrophilic group. The hydrophilic group must be sufficiently hydrophilic to ensure that the entire surfactant is soluble in water and has the necessary solubility. Since the surfactant contains a hydrophilic group and a hydrophobic group, they can be dissolved in at least one phase in the liquid phase. This nature of surfactants that is both hydrophilic and lipophilic is called amphiphilic.

④Type of surfactant
Surfactant is an amphiphilic molecule with both hydrophobic and hydrophilic groups. The hydrophobic groups of surfactants are generally composed of long-chain hydrocarbons, such as straight-chain alkyl C8-C20, branched-chain alkyl C8-C20, alkylphenyl (alkyl carbon number 8-16), etc. . The difference of the hydrophobic group is mainly in the structural change of the hydrocarbon chain, the difference is small, and the type of the hydrophilic group is more, so the nature of the surfactant is mainly related to the size and shape of the hydrophobic group. It is also related to hydrophilic groups. The structural change of hydrophilic groups is larger than that of hydrophobic groups, so the classification of surfactants is generally based on the structure of hydrophilic groups. This classification is based on whether the hydrophilic group is ionic, and it is divided into anionic, cationic, nonionic, zwitterionic and other special types of surfactants.

⑤ Characteristics of surfactant aqueous solution
1. The adsorption of surfactant on the interface
Surfactant molecules have lipophilic and hydrophilic groups, and are amphiphilic molecules. Water is a strong polar liquid. When the surfactant is dissolved in water, according to the principle of similar polarity and polar exclusion, its hydrophilic group leads to water and is soluble in water, and its lipophilic group and water The phase repels and leaves the water. As a result, surfactant molecules (or ions) are adsorbed on the interface of the two phases, reducing the interfacial tension between the two phases. The more surfactant molecules (or ions) are adsorbed on the interface, the greater the decrease in interfacial tension.

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