What are the classifications of surfactants?

According to the type of ions generated by the hydrophilic group, surfactants can be divided into four categories: anionic, cationic, zwitterionic and nonionic.

Anionic surfactants

① Soaps

It is a salt of higher fatty acids, with the general formula: (RCOO)nM. The fatty acid hydrocarbon R is generally a long chain of 11 to 17 carbons, and stearic acid, oleic acid and lauric acid are common. According to the different substances represented by M, it can be divided into alkali metal soaps, alkaline earth metal soaps and organic amine soaps. They all have good emulsification properties and the ability to disperse oil. But they are easily destroyed. Alkali metal soaps can also be destroyed by calcium and magnesium salts, and electrolytes can also cause salting out.

Alkali metal soaps: O/W

Alkaline earth metal soaps: W/O

Organic amine soaps: triethanolamine soaps

② Sulfates RO-SO3-M

Mainly sulfated oils and higher fatty alcohol sulfates. The fatty hydrocarbon chain R is between 12 and 18 carbons. The representative of sulfated oil is sulfated castor oil, commonly known as Turkish red oil. Advanced fatty alcohol sulfates include sodium dodecyl sulfate (SDS, sodium lauryl sulfate) and sodium fatty alcohol polyoxyethylene ether sulfate (AES). SDS has strong emulsification, is relatively stable, and is more resistant to acid, calcium, and magnesium salts. In pharmacy, it can produce precipitation with some high molecular cationic drugs, has a certain irritation to the mucous membrane, and is used as an emulsifier for external ointments, and is also used for wetting or solubilizing solid preparations such as tablets. Sodium fatty alcohol polyoxyethylene ether sulfate (AES) has the ability to resist hard water, has good oil removal performance, and has a certain thickening effect.

③ Sulfonates R-SO3-M

This category includes aliphatic sulfonates, alkyl aryl sulfonates, and alkyl naphthalene sulfonates. Their water solubility and acid and calcium and magnesium salt resistance are slightly worse than sulfates, but they are not easily hydrolyzed in acidic solutions. Aliphatic sulfonates include: sodium secondary alkyl sulfonate (SAS-60), sodium fatty acid methyl ester ethoxylate sulfonate (FMES), sodium fatty acid methyl ester sulfonate (MES), sodium dioctyl succinate sulfonate (Alosol-OT), etc., which can be used as drug absorption enhancers for non-oral routes; sodium dodecylbenzene sulfonate of alkyl aryl sulfonates is a widely used detergent. Cholelithium salts such as sodium glycocholate and sodium taurocholate are often used as solubilizers for monoglycerides and emulsifiers for fat in the gastrointestinal tract.

Cationic surfactants

Surfactants with positive charges are called cationic surfactants. The cation, also known as positive soap, plays a surfactant role. The main part of its molecular structure is a pentavalent nitrogen atom, which is a quaternary ammonium compound, mainly benzalkonium chloride (Chlorhexidine), benzalkonium bromide (Chlorhexidine), benzalkonium chloride, etc. This type of surfactant has good water solubility, good surface activity, and is relatively stable in both acidic and alkaline solutions. Because of its strong bactericidal effect, it is mainly used for disinfection of skin, mucous membranes, surgical instruments, etc. Some varieties, such as benzalkonium chloride, can be used as antibacterial agents in ophthalmic solutions.

Amphoteric surfactants

This type of surfactant has both positive and negative charge groups in its molecular structure, and can exhibit the properties of cationic or anionic surfactants in media with different pH values.

① Lecithin

Lecithin is a natural zwitterionic surfactant, mainly derived from soybeans and egg yolks. The composition of lecithin is very complex and is a mixture of multiple compounds. Due to its different sources and preparation processes, the proportions of each component will also be different, and thus the performance will also be different. Lecithin is very sensitive to heat, easily hydrolyzed under the action of acid, alkalinity and esterase, insoluble in water, soluble in organic solvents such as chloroform, ether, and petroleum ether, and is the main excipient for the preparation of injectable emulsions and lipid microparticles.

②Amino acid type and betaine type

Amino acid and betaine are synthetic amphoteric surfactants, whose anion part is mainly carboxylate, and whose cationic part is amine salt, which is amino acid type (R-NH2+-CH2CH2COO-), and quaternary ammonium salt, which is betaine type: R-N+(CH3)2-COO-. Its characteristics are: in alkaline aqueous solution, it has the properties of anionic surfactants, with good foaming and decontamination effects; in acidic solution, it has the properties of cationic surfactants, with strong bactericidal ability, strong bactericidal effect and less toxicity than cationic surfactants.

Nonionic surfactants

Fatty acid glycerides

Mainly fatty acid monoglycerides and fatty acid diglycerides, such as monostearate glyceryl. Insoluble in water, easily hydrolyzed into glycerol and fatty acids, not very active, HLB value of 3 to 4, often used as W/O type auxiliary emulsifier.

Sucrose fatty acid ester

Sucrose ester for short, belongs to polyol type nonionic surfactant, is a class of compounds formed by the reaction of sucrose and fatty acids, including monoester, diester, triester, and polyester. It can be decomposed into sucrose and fatty acids in the body and used. HLB value is 5-13, often used as O/W emulsifier and dispersant, and is also a commonly used food additive.

Sorbitan fatty acid

It is a mixture of ester compounds obtained by the reaction of sorbitan and its anhydride with fatty acids, and its trade name is Span. Due to its strong lipophilicity, it is often used as a W/O emulsifier, with an HLB value of 1.8-3.8, and is mostly used in lotions and ointments. However, Span 20 and Span 40 are often used as O/W mixed emulsifiers in combination with Tween.

Polysorbate

It is a polyoxyethylene sorbitan fatty acid ester. On the remaining -OH of Span, polyoxyethylene is combined to obtain an ether compound, and its trade name is Tween. This type of surfactant has greatly increased its hydrophilicity due to the addition of hydrophilic polyoxyethylene, becoming a water-soluble surfactant. The HLB value is 9.6-16.7, and it is often used as a solubilizer and O/W emulsifier.

Polyoxyethylene fatty acid ester

It is an ester generated by the condensation of polyethylene glycol and long-chain fatty acids. The trade name Myrij is one of them. This type is water-soluble and has strong emulsification properties. It is often used as an O/W emulsifier and solubilizer.

Polyoxyethylene fatty alcohol ether

It is an ether generated by the condensation of polyethylene glycol and fatty acids. The trade name Brij is one of them. It is often used as an O/W emulsifier and solubilizer.

Polyoxyethylene-polyoxypropylene polymer

It is formed by the polymerization of polyoxyethylene and polyoxypropylene, also known as Poloxamer, and the trade name is Pluronic.