Nanotechnology 3: Self organizing Micelle nanostructures
(K. G. Furton and A. Norelus in J. Chem. Educ. 70, 254-257 (1993))
Surfactants are amphiphilic substances consisting of a long hydrophobic “tail” and a polar
“head” group. Sodium dodecyl sulfate (SDS) for example has a hydrophobic hydrocarbon chain
and a negatively charged sulfate head group.
In low concentrations, surfactants exist as isolated molecules in hydrophilic environment such as
water. Above a certain concentration the molecules arrange themselves in nanosized structures
called micelles. The hydrophobic part is typically inside the micelles shielded from the solvent
while the hydrophilic part is in contact with water. The concentration at which the molecules
form spontaneously micellar structures is called the critical micelle concentration (CMC).
Cross section of micelles
Problem: Micellar structures are just formed above the CMC. The CMC is dependent on the
nature of the surfactant molecule and the conditions of the environment (solvent, temperature,
pH, pressure, additives such as salt or other detergents).
Aim: In this project you should determine the CMCs for different surfactants and document the
influence of pH and salt on the CMC for these surfactants.
Lab work: A water-insoluble hydrophobic dye (1-(2-pyridylazo)-2-naphthol) is used to visualize
the micelle formation. First the dye will be placed on a filter paper. The filter will be dried and
placed in water. The dye cannot dissolve in water. Addition of surfactants to the water will result
in micelle formation at a certain concentration of surfactants. The CMC of this surfactant can
then be estimated by measuring absorption of the dye at 470 nm.
Examples: Surfactants can be found in many products such as motor oil, pharmaceuticals, and
cleaning products such as detergents (washing powder) or soap. They can be used as delivery
systems in medicine and as emulsifier for different organic products such as paint.