Unformatted text preview: is called micellar solubilization. The inverted micelles formed by oilsoluble surfactant which dissolves in a hydrocarbon solvent can solubilize watersoluble compound which is located in the center of the micelle, out of contact with the solvent. 26 Micelles of nonionic surfactants consist of an outer shell containing their polyethylene glycol moieties mixed with water and an inner core formed by their hydrocarbon moieties. Some compounds like phenols and benzoic acid form complexes with polyethylene glycols by hydrogen bonding and/or are more soluble in liquids of intermediate polarity like ethanol or ethyl ether than in liquids of low polarity like aliphatic hydrocarbons. These compounds locate in the aqueous polyethylene glycol outer shell of nonionic micelles on solubilization. Drugs which are soluble in oils and lipids can be solubilized by micellar solubilization. 27 As Micellar solubilization depends on the existence of micelles; it does not take place below the CMC. So dissolution begins at the CMC. Above the CMC, the amount solubilized is directly proportional to the surfactant concentration because all surfactant added to the solution in excess of the CMC exists in micellar form, and as the number of micelles increases the extent of solubilization increases . Compounds that are extensively solubilized increase the size of micelles in two ways: o The micelles swell because their core volume is augmented by the volume of the solubilizate.
o The number of surfactant molecules per micelle increases. 28 Foaming and Anti Foaming agents Foams are dispersion of a gas in a liquid (liquid foams as that formed by soaps and detergents ) or in a solid (solid foams as sponges ). Foaming agents
Many Surfactants solutions promote the formation of foams and stabilize them, in pharmacy they are useful in toothpastes compositions. Anti Foaming agents
They break foams and reduce frothing that may cause problems as in foaming of solubilized liquid preparations. in pharmacy they are useful in aerobic fermentations, steam boilers. 29 Detergents Detergents are surfactants used for removal of dirt. Detergency involves: •Initial wetting of the dirt and the surface to be cleaned. •Deflocculation and suspension, emulsification or solubilisation of the dirt particles •Finally washing away the dirt. 30 Structural Classification A single surfactant molecule contains one or more hydrophobic portions and one or more hydrophilic groups. According to the presence of ions in the surfactant molecule they may be classified into: Ionic surfactants
o Anionic surfactants: the surface active part is anion (negative ion ) e.g. soaps, sodium lauryl sulfate
o Cationic surfactants: the surface active part is cation (positive ion) e.g. quaternary ammonium salts o Ampholytic surfactants: contain both positive and negative ions e.g. dodecylBalanine. 31 Ionic surfactants
They are the metal salts of long chain fatty acids as lauric acid.
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- Spring '14
- Surface tension, CMC, surfactants, interfacial tension