Onlythecounterionsofnonassociated

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Unformatted text preview: At a given concentration, temperature, and salt content, all micelles of a given surfactant usually contain the same number of molecules, i.e. they are usually monodisperse. For different surfactants in dilute aqueous solutions, this number ranges approximately from 25 to 100 molecules. The diameters of micelles are approximately between 30 and 80 Ao. Because of their ability to form aggregates of colloidal size, surfactants are also called association colloids. Micelles are not permanent aggregates. They form and disperse continually. 38 Please wait Please wait 39 Surfactant shapes in colloidal solution a­ Cone­shaped surfactant resulting in b­normal micelles c­ Hampagne cork shaped surfactant resulting in d­reverse micelles with control of their size by the water content e­ Interconnected cylinders. f­ Planar lamellar phase. g­ Onion­like lamellar phase. 40 Normal spherical micelles In dilute aqueous solutions micelles are approximately spherical. The polar groups of the surfactants are in the periphery and the hydrocarbon chains are oriented toward the center, forming the core of the micelles Inverted spherical micelles In solvents of low polarity or oils micelles are inverted. The polar groups face inward to form the core of the micelle while the hydrocarbon chains are oriented outward Cylindrical and lamellar micelles In more concentrated solutions of surfactants, micelles change from spherical either to cylindrical or lamellar phase. 41 Changes occurred at the CMC Changes occurred at the CMC Properties of surfactant Solutions as functions of concentration: I­ A continuous decrease in Surface and interfacial tensions with surfactants concentration until CMC the Surface and interfacial tensions level become constant owing to crowding of surfactant molecules adsorbed at surfaces and interfaces. A, surface tension B, interfacial tension C, osmotic pressure; D, equivalent conductivity; E, solubility of compounds with low or zero solubility in water 42 Changes occurred at the CMC Changes occurred at the CMC II­ The osmotic pressure (and all other colligative properties, lowering of the vapor pressure and of the freezing point), rises much more slowly with increasing surfactant concentration above than it does below the CMC because it depends on the number of dissolved particles. A, surface tension B, interfacial tension C, osmotic pressure D, equivalent conductivity E, solubility of compounds with low or zero solubility in water 43 Changes occurred at the CMC Changes occurred at the CMC III­ For ionic surfactants, the equivalent conductivity drops sharply above the CMC. Only the counterions of non­associated surfactant molecules can carry current. A, surface tension B, interfacial tension C, osmotic pressure D, equivalent conductivity E, solubility of compounds with low or zero solubility in water 44 Changes occurred at the CMC Changes occurred at the CMC IV­Solubility of many drugs are increased after CMC. Other solution properties changing at the CMC: intrinsic viscosity and turbidity increase, while diffusion coefficient decreases All these properties can be used to determine the CMC. A, surface tension B, interfacial tension C, osmotic pressure D, equivalent conductivity E, solubility of compounds with low or zero solubility in water 45 Factors af...
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This document was uploaded on 02/05/2014.

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