Surfactant Micelles Dr. Aws Alshamsan Department of Pharmaceutics Office: AA87 Tel:

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 interface oil/water interface solid/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 PropertyEffect Surface tensionDecrease Interfacial tensionDecrease Equivalent conductivityDecrease Osmotic pressureIncrease Drug solubilityIncrease

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 SphericalCylindricalLamellar

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.