surfactant micelles dr. aws alshamsan department of pharmaceutics office: aa87 tel: 4677363...
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Surfactant Micelles
Dr. Aws AlshamsanDepartment of Pharmaceutics
Office: AA87Tel: 4677363
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Association colloids
• Refers to colloids that are formed by self-assembling small molecules enough to produce aggregates in the colloidal size range
• Surfactant micelles• Microemulstions• Liposomes• Polymeric micelles
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Surfactants
• Water attracts the polar head and repels the hydrocarbon tail
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Surfactant
• Surfactants tend to adsorb at:air/water interfaceoil/water interfacesolid/water interface
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Surface tension
• Surface tension decreases as the surfactant concentration increases up to the critical micelle concentration (CMC)
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Surfactant micelles
• Above CMC, excess surfactant molecules will associate into small aggregates called micelles
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Changes occurring after the CMC
Property Effect
Surface tension Decrease
Interfacial tension Decrease
Equivalent conductivity Decrease
Osmotic pressure Increase
Drug solubility Increase
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Surfactant micelle
• The concentration of the micellar surfactant equals to the total surfactant concentration minus the CMC
• Diluting the solution below the CMC causes the micelles to break up into single or non-associate surfactant monomers
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Micellar dynamics
• Micelles are not static: they dissociate, regroup, and re-associate rapidly (fraction of second)
• Surfactant molecules (free, adsorbed, and micellar) are in dynamic equilibrium
Micelle
Head
Tail
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Micellar shapes
Reverse Micelle
Spherical Cylindrical Lamellar
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Spherical micelles
• The polar heads orient toward the outer shell, while hydrocarbon tails form a spherical core
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Cylindrical micelles
• Larger micelles are more asymmetric where spherical shape converts to cylindrical shape
• Cylinders tend to arrange in parallel hexagonal contour
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Lamellar micelles
• Surfactant molecules arrange in parallel bimolecular sheets with a tail-to-tail orientation
• Water is stratified between the sheets
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Reverse Cylindrical
Micelle
Cylindrical Micelle
Lamellar Micelle
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How to reduce the CMC?
• Propose two methods to reduce the critical micelle concentration (CMC) of ionic surfactants.