Asthenumberofsurfactantmoleculesadsorbedatthewaterair

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Unformatted text preview: yl sulfate or Sodium Lauryl Sulfate is used in toothpaste and ointments Triethanolamine dodecyl sulfate is used in shampoos and other cosmetic preparations. Sodium dodecyl benzene sulfonate is a detergent and . has germicidal properties Sodium dialkvlsulfosuccinates are good wetting agents. 32 Cationic surfactants Cationic surfactants These are chiefly quaternary ammonium compounds. They have bacteriostatic activity probably because they combine with the carboxyl groups in the cell walls and of microorganisms by cation exchange, causing lysis. Among the most popular antiseptics in this category are benzalkonium chloride, cetylpyridinium chloride and cetyltrimethylammonium bromide, Ampholytic Surfactants These are the least common, e.g. dodecyl­β­alanine 33 Non­ionic surfactants Widely used in pharmaceutical formulations e.g. Tweens, Spans, Brij and Myrj. They are polyethylene oxide products. Surfactants based on sorbitan are of pharmaceutical importance. Esterification of the primary hydroxyl group with lauric, palmitic, stearic or oleic acid forms sorbitan monolaurate, monopalmitate, monostearate or monooleate These are water­insoluble surfactants called Span 20, 40, 60 or 80, respectively. Addition of about 20 ethylene oxide molecules produces the water­soluble surfactants called polysorbate or Tween 20, 40. 60 or 80. 34 As a Surface active substance contains a hydrophilic and a hydrophobic portions, it is adsorbed as a monolayer at the interfaces. At water­air interface At water­air interface Surface­active molecules will be adsorbed at water­air interfaces and oriented so that the hydrocarbon chains of are pushed out of the water and rest on the surface, while the polar groups are inside the water. Perhaps the polar groups pull the hydrocarbon chains partly into the water. At oil­water interface Surface­active molecules will be oriented so that the hydrophobic portion is inside the oil phase and the hydrophilic portion inside the water phase. 35 At low surfactant concentrations: The hydrocarbon chains of surfactant molecules adsorbed in the interface lie nearly flat oh the water surface. At higher concentrations: They stand upright because this permits more surfactant molecules to pack into the interfacial monolayer. As the number of surfactant molecules adsorbed at the water­air interface increased, they tend to cover the water with a layer of hydrocarbon chains. Thus, the water­air interface is gradually transformed into an non polar­air interface. This results in a decrease in the surface tension of water. 36 Micelle Formation Micelle Formation When the surfactant molecules adsorbed as a monolayer in the water­air interface have become so closely packed that additional molecules cannot be accommodated with ease, the polar groups pull the hydrocarbon chains partly into the water. At certain concentration the interface and the bulk phase become saturated with monomers. Excess surfactants add will begin to agglomerate in the bulk of the solution forming aggregates called .Micelles and the free energy of the system is reduced The lowest concentration at which micelles first appear is called the critical concentration for micelle formation [CMC ] 37...
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This document was uploaded on 02/05/2014.

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