Classification of surfactants
Surfactants can be categorized according to different perspectives, such as structure, function, and origin. In terms of structure, they can be categorized according to the types of hydrophilic and hydrophobic groups and the overall molecular configuration. In the following, the basic structure of surfactants, the types of common head groups and hydrophobic tail chains, and representative special structures are introduced in conjunction .
1. Classification according to hydrophilic groups
Since the hydrophobic groups of surfactants have been single for a long time, the classification according to the hydrophilic groups has become the main method of surfactant classification. According to this method, surfactants can be roughly divided into four categories: anionic surfactants, cationic surfactants, amphoteric surfactants and nonionic surfactants.
Anionic surfactants dissociate in water, making the head group negatively charged. It is the oldest surfactant, bile acids, soap-based surfactants, and some phospholipids are anionic surfactants. In addition to the carboxylic acids contained in bile acids and soap-based surfactants, and the phosphates contained in phospholipids, sulfates and sulfonates were later developed, further enriching the variety of anionic surfactants. In contrast, cationic surfactants (Fig. 1(b)) have positively charged head groups upon dissociation, and quaternary ammonium salts are the most common. Aqueous solutions of cationic surfactants have a strong bactericidal ability and are often used for sterilization. Since many solid surfaces, such as glass, are negatively charged, it is easier for cationic surfactants to be adsorbed, making the surface hydrophobic. Cationic surfactants with two tail chains are able to provide fabric softening and anti-static properties, and thus have
irreplaceable advantages in the textile industry. Anionic and cationic surfactants are sensitive to temperature. A decrease in temperature can reduce the dissociation ability of the polar groups and the flexibility of the hydrophobic tail chain of surfactant molecules, leading to a decrease in their performance. When the temperature reaches a critical point, the surface activity of surfactants decreases dramatically due to crystallization, and this temperature is called the Krafft point.
Amphoteric surfactants are formed when two oppositely charged head groups are covalently linked together (Fig. A representative of natural amphoteric surfactants is egg yolk lecithin, which contains both phosphate and quaternary ammonium cations, and plays an important role not only in the regulation of life activities, but also as an important additive in the food industry. As far as synthetic amphoteric surfactants are concerned, those containing both quaternary ammonium cations and carboxylic acids are called betaines, which are not only diverse but also have been realized in large-scale industrial production. In addition, there is also a class of amine oxide surfactants, whose amine oxide group can make its own charge unevenly distributed between nitrogen and oxygen, thus exhibiting amphoteric properties [6]. Most amphoteric surfactants are pH-sensitive, and under acidic conditions, the anion is easily protonated, and the whole molecule exhibits the characteristics of cationic surfactants.
Unlike ionic surfactants, the head group of nonionic surfactants hardly dissociates in water, and their water solubility mainly depends on the hydrogen bonding between the head group and water molecules. The common head group structures include oligooxyethylene, oligosaccharide, and so on. In terms of molecular size, there are not only small molecules of CnEm and alkyl glycosides, but also large-size PEO-PPO-PEO triblock copolymers (trade name Pluronic), etc. Due to the absence of electrostatic repulsion between the head groups, nonionic surfactants are more tightly arranged on the air/water surface, and have a stronger ability to lower the surface tension of water. Nonionic surfactants are highly stable, less disturbed by pH, heavy metal ions and electrolytes, and have excellent tolerance. Contrary to ionic surfactants, nonionic surfactants have better resistance to low temperatures, but at high temperatures due to the destruction of the hydrogen bonding structure from the water, the loss of surface activity, the entire appearance of the sample also becomes turbid. The temperature at which this transition occurs is called the cloud point. In order to ensure the effectiveness of surfactants, the temperature of ionic surfactants should be higher than the Krafft point, while the temperature of nonionic surfactants should be lower than the turbidity point.
The basic structure of surfactants, common head groups and hydrophobic tail chains, some special classes of surfactants. (a)~(d) Main types of surfactant head groups: anionic, cationic, amphoteric, nonionic. (g), (h) Two special structures of surfactants, Bola type, Gemini type. (e), (f) Common structures of hydrophobic tail chains of surfactants
