Anionic surfactants

Anionic surfactants
Krafft point
Krafft point: The temperature at which an ionic surfactant reaches its critical micelle concentration is called the Krafft point. The higher the Krafft point, the lower the critical micelle concentration.
According to the type of ion generated by hydrophilic group, surfactants can be divided into four major categories: anionic, cationic, amphoteric and nonionic.

①Soaps
Soaps are salts of fatty acids with the formula (RCOO)nM, and the fatty acid hydrocarbon R is generally a long chain of 11~17 carbons, and stearic acid, oleic acid, and lauric acid are common. 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, electrolyte can also make it 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 senior fatty alcohol sulfate esters. The aliphatic hydrocarbon chain R is between 12 and 18 carbons. The representative of sulfated oil is sulfated castor oil, commonly known as Turkish red oil. The advanced fatty alcohol sulfates are sodium dodecyl sulfate (SDS, sodium lauryl sulfate) and sodium aliphatic ethoxylate sulfate (AES).SDS is very emulsifying and stable, and is 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 membrane, so it is used as emulsifier of topical ointment, and it 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 better oil removal performance, and has a certain thickening effect.

③Sulfonate R-SO3-M
Aliphatic sulfonates, alkyl aryl sulfonates and alkyl naphthalene sulfonates belong to this category. Their water solubility and acid resistance to calcium and magnesium salts is slightly worse than sulfuric acid compounds, but not easy to hydrolyze in acidic solutions. Aliphatic sulfonates include: sodium sec-alkyl sulfonate (SAS-60), sodium fatty acid methyl ester ethoxylate sulfonate (FMES), sodium fatty acid methyl ester sulfonate (MES), and sodium dioctylsuccinic acid sulfonate (Alosol-OT) can be used as drug absorption enhancers for the non-oral route; sodium dodecylbenzene sulfonate, a class of alkyl aryl sulfonates, is a widely used detergent. Cholestatic acid salts such as sodium glycocholate and sodium taurocholate are often used as solubilizers of monofatty acid glycerides and emulsifiers of fats in the gastrointestinal tract.
Cationic Surfactants
Positively charged surfactants are called cationic surfactants. It is the cation that plays the role of surface activity, also known as cationic soap. The main part of its molecular structure is a pentavalent nitrogen atom, quaternary ammonium compounds, mainly benzalkonium chloride (Jelmai), benzalkonium bromide (new Jelmai), benzalkonium chloride, and so on. These surfactants have good water solubility, good surface activity, and are stable in acidic and alkaline solutions. Because of their strong bactericidal effect, they are mainly used for the disinfection of skin, mucous membranes, surgical instruments and so on. Certain varieties, such as benzalkonium chloride, can be used as bacteriostatic agents in ophthalmic solutions.