Surfactant Micelles

Dr. Aws AlshamsanDepartment of Pharmaceutics

Office: AA87Tel: 4677363

aalshamsan@ksu.edu.sa

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.