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Surfactant Micelles
Dr. Aws AlshamsanDepartment of Pharmaceutics
Office: AA87Tel: 4677363
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
Surfactants
• Water attracts the polar head and repels the hydrocarbon tail
Surfactant
• Surfactants tend to adsorb at:air/water interfaceoil/water interfacesolid/water interface
Surface tension
• Surface tension decreases as the surfactant concentration increases up to the critical micelle concentration (CMC)
Surfactant micelles
• Above CMC, excess surfactant molecules will associate into small aggregates called micelles
Changes occurring after the CMC
Property Effect
Surface tension Decrease
Interfacial tension Decrease
Equivalent conductivity Decrease
Osmotic pressure Increase
Drug solubility Increase
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
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
Micellar shapes
Reverse Micelle
Spherical Cylindrical Lamellar
Spherical micelles
• The polar heads orient toward the outer shell, while hydrocarbon tails form a spherical core
Cylindrical micelles
• Larger micelles are more asymmetric where spherical shape converts to cylindrical shape
• Cylinders tend to arrange in parallel hexagonal contour
Lamellar micelles
• Surfactant molecules arrange in parallel bimolecular sheets with a tail-to-tail orientation
• Water is stratified between the sheets
Reverse Cylindrical
Micelle
Cylindrical Micelle
Lamellar Micelle
How to reduce the CMC?
• Propose two methods to reduce the critical micelle concentration (CMC) of ionic surfactants.