Definition and classification of surfactants
Surfactant (surfactant) is a substance with fixed hydrophilic and lipophilic groups, which can be arranged directionally on the surface of a solution and can make the surface tension drop significantly. The molecular structure of surfactant is amphiphilic: hydrophilic group at one end, hydrophobic group at the other end; hydrophilic groups are often polar groups, such as carboxylic acid, sulfonic acid, sulfuric acid, amino or amine groups and their salts, but also hydroxyl, amide, ether bond, etc.; and hydrophobic groups are often nonpolar hydrocarbon chains, such as more than 8 carbon atoms hydrocarbon chain. Surfactants are divided into ionic surfactants and nonionic surfactants.

In fact, there are many ways to classify surfactants. According to the structure of hydrophobic group, they are classified as straight chain, branched chain, aromatic chain, fluorine-containing long chain, etc. According to the classification of hydrophilic group, they are classified as carboxylate, sulfate, quaternary ammonium salt, PEO derivatives, lactone and so on; some researchers classify them into ionic and nonionic according to their ionic molecular composition, and there are also various classification methods according to their water solubility, chemical structure characteristics, raw material source, etc. WSI understands that surfactants are classified into ionic and nonionic surfactants. WSI understands that, in fact, many classification methods have their limitations, and it is difficult to position surfactants appropriately without overlapping conceptual connotations.
It is generally recognized that it is more appropriate to classify surfactants according to their chemical structure. That is, when the surfactant is dissolved in water, it is divided into ionic surfactant and nonionic surfactant according to whether it generates ions and its electrical properties.
Classification according to the dissociation nature of polar groups
1、Anionic surfactants: stearic acid, sodium dodecylbenzene sulfonate
2、Cationic surfactants: quaternary ammonium compounds
3、Amphoteric surfactants: lecithin, amino acid type, betaine type
4、Nonionic surfactants: fatty acid glycerides, dehydrated sorbitan fatty acid esters (spironolactone), polyoxyethylene dehydrated sorbitan fatty acid esters (tween)
Anionic active agent

1、Soap type
Is a salt of high-level fatty acids, general formula: (RCOOˉ) n M. Fatty acid hydrocarbons R is generally a long chain of 11 to 17 carbons. Commonly, there are stearic acid, oleic acid, and lauric acid.
Depending on the substance represented by M, they can be further divided into alkali metal soaps, alkaline earth metal soaps and organic amine soaps. They all have good emulsifying properties and the ability to disperse oil.
But easy to be destroyed, alkali metal soap can also be calcium, magnesium salt destruction, electrolytes can also make the salt precipitation.
Alkali metal soap: O/W
Alkaline earth metal soap: W/O
Organic amine soap: triethanolamine soap

2、Sulfate RO-SO3-M
Mainly sulfated oils and advanced fatty alcohol sulfate esters. Aliphatic hydrocarbon chain R between 12 ~ 18 carbon.
The representative of sulfated oil is sulfated castor oil, commonly known as Turkish red oil.
Advanced fatty alcohol sulfates are sodium dodecyl sulfate (SDS, sodium lauryl sulfate)
Very emulsifying and stable, more resistant to acids and calcium and magnesium salts. In pharmacy, it can produce precipitation with some polymer cationic drugs, and it has certain irritation to mucous membranes, used as emulsifier for topical ointment, and also used for wetting or solubilization of solid preparations such as tablets.
3、Sulfonate R-SO3-M
There are aliphatic sulfonates, alkyl aryl sulfonates and alkyl naphthalene sulfonates belonging to this category. Their water solubility and acid resistance to calcium and magnesium salts than sulfuric acid slightly worse, but not easy to hydrolyze in acidic solutions.
Commonly used varieties are: sodium dioctylsuccinate sulfonate, sodium dodecylbenzene sulfonate, sodium glycocholate.
Cationic active agent
This type of surfactant is called cationic soap because it is cationic in nature. The main part of their molecular structure is a pentavalent nitrogen atom, so they are also called quaternary ammonium compounds. It is characterized by large water solubility, more stable in acidic and alkaline solutions, with good surface activity and bactericidal effect. Commonly used varieties include benzalkonium chloride and benzalkonium bromide.
Amphoteric ionic active agent
This kind of surfactant has both positive and negative charge groups in its molecular structure, which can show the properties of cationic or anionic surfactant in different pH media.
1、Lecithin: It is the main excipient for the preparation of emulsion and lipid particle preparation for injection.
2、Amino acid type and betaine type:
Amino acid type: R-NH+2-CH2CH2COO-
Betaine type: R-N+(CH3)2-COO-
It is an anionic surfactant in alkaline aqueous solution, with good foaming and decontamination effects; and it is a cationic surfactant in acidic solution, with strong bactericidal ability.
Nonionic surfactant
HLB is 3~4, mainly used as auxiliary emulsifier for W/O type emulsion.
Sucrose ester: HLB (5~13) O/W emulsifier, dispersant
Sorbitan fatty acid ester (Span): W/O emulsifier
Polyoxyethylene sorbitan fatty acid ester (Tween): O/W