Classification of Surface Tension

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Chapter: Medicinal Chemistry : Surface Tension

Surface-active agents are classified as follows: Anionic surfactants, Cationic surfactants, Nonionic surfactants, Amphoteric surfactants


Surface-active agents are classified as follows:

  • Anionic surfactants: Ordinary soaps, salts of bile acids, salts of the sulphate or phosphate esters of alcohols, salts of sulphonic acids.

  • Cationic surfactants: High molecular weight aliphatic amines, quaternary ammonium derivatives.

  • Nonionic surfactants: Polyethylene ethers, glycol esters of fatty acids.

  • Amphoteric surfactants


The bactericidal activity of cationic quaternary ammonium compounds, such as benzalkonium chloride, cetrimide, cetyl pyridinium chloride, etc. is explained through their surface-active property. Many compounds, such as detergents, disinfectants, and antibiotics act through the surface phenomenon.

The anthelmintic activity of hexylresorcinols is reported to be increased by low concentrations of soap and decreased by high concentrations of soap. If the soap concentration is kept below critical micelle concentration (CMC), a 1:1 association of phenol and soap occurs, which facilitate the penetration of phenol through the surface of the worm. If the CMC is exceeded, the micelle competes favourably with the worms for phenol and there is decreased activity.

Compounds showing pronounced surface activity usually are unsuited for use in the animal body. Such compounds are lost through their adsorption by proteins, and they also have an undesirable feature of disorganizing the cell membrane and producing haemolysis of red blood cells. In general, highly surface-active agents are not used internally, but only topically, as skin disinfectants or sterilizers for sterilization of instruments. This is the case for ionic surfactants. Nonionic surfactants are largely employed in pharmaceutical preparations for oral (sometimes even parenteral) use as solubilizing agents of water insoluble or slightly soluble drugs.

Surface-active agents can be expected to have a pronounced effect on the permeability of a cell. Mildly surface-active agents may be adsorbed by cell membranes, and thereby interfere with the absorption of other compounds through this membrane or may alter membrane structure and function. Many central nervous system depressant drugs, such as sedative-hypnotic, anticonvulsant, and central relaxant agents possess the general structure of nonionic surface-active compounds.

The most commonly used surfactants are anionic and nonionic surfactants. Since the process of solubilization occurs due to the presence of micelles, generally, high concentrations of surfactants are needed to improve drug solubility significantly. One example of a surfactant based solution is Taxol (paclitaxel, Sigma-Aldrich, USA), an anticancer drug that is solubilized in 50% solution of Cremophor. Other examples include Valrubicin in 50% Cremophor and Cyclosporin in 65% Cremophor.

Surfactant preparations are used as replacement therapy for the treatment of premature infants suffering from neonatal respiratory distress syndrome (also known as hyaline membrane disease). A substantial deficiency in the endogenous lung surfactant is the principal factor contributing to the pathology of respiratory distress syndrome. The lung surfactant preparations are used in combination with supplemental oxygen and mechanical ventilation to facilitate gas exchange. The exogenous surfactants are either derived from animal or synthesized. For example, Beractant (modified bovine extract), Calfactant (extracted from the lungs of calves), and Poractant alfa (extract of porcine lung).

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