Surfactants can be classified from different perspectives such as structure, function, and source. In terms of structural classification, it can also be classified according to the types of hydrophilic and hydrophobic groups and the overall molecular configuration. Below, in conjunction with Figure 1, we will introduce the basic structure of surfactants, the types of common head groups and hydrophobic tail chains, and representative special structures.
1. Divided by hydrophilic groups
Due to the fact that the types of hydrophobic groups in surfactants have been relatively single for a long time, classification based on the types of hydrophilic groups has become the main method for classifying the types of surfactants. According to this method, surfactants can be roughly divided into four categories: anionic surfactants, cationic surfactants, zwitterionic surfactants, and non ionic surfactants.
Anionic surfactants (Figure 1 (a)) undergo dissociation in water, resulting in a negative charge on the base band of the head. It is the oldest surfactant, with bile acids, soap based surfactants, and some phospholipids all belonging to the category of anionic surfactants. In addition to the carboxylate groups contained in bile acids and soap based surfactants, as well as the phosphate groups contained in phospholipids, people later developed sulfate and sulfonic groups, further enriching the variety of anionic surfactants. On the contrary, the head group of cationic surfactants (Figure 1 (b)) dissociates and carries a positive charge, with quaternary ammonium salts being the most common. The aqueous solution of cationic surfactants has strong bactericidal ability, so it is often used for disinfection and sterilization. Due to the negative charge on many solid surfaces such as glass, it is easier for cationic surfactants to adsorb, making the surface hydrophobic. Cationic surfactants with two tail chains have irreplaceable advantages in the textile industry due to their ability to provide flexibility and antistatic properties to fabrics. Anionic and cationic surfactants are more sensitive to temperature. A decrease in temperature can reduce the dissociation ability of polar groups and the flexibility of hydrophobic tail chains of surfactant molecules, leading to a decrease in their effectiveness. When the temperature reaches a critical point, the surface activity of the surfactant decreases sharply due to crystallization and precipitation, and this temperature is called the Krafft point.
When two head groups with opposite charges are covalently connected together, a zwitterionic surfactant is formed (Figure 1 (c)). The representative of natural amphoteric surfactants is egg yolk lecithin, which contains both phosphate and quaternary ammonium cations. It not only plays an important role in regulating life activities, but also is an important additive in the food industry. As for artificially synthesized zwitterionic surfactants, those containing both quaternary ammonium cations and carboxylic acid radicals are called betaine. These surfactants not only have a wide variety of types but also have achieved large-scale industrial production. In addition, there is a type of amine oxidizing surfactant, whose amine oxidizing group can cause uneven distribution of its own charge between nitrogen and oxygen, thereby exhibiting the characteristics of zwitterions [6]. Most zwitterionic surfactants are pH sensitive, and under acidic conditions, anions are easily protonated, and the entire molecule exhibits the characteristics of a cationic surfactant.
