Measurement of Critical Micelle Concentration (CMC)

The Critical Micelle Concentration (CMC) is the surfactant concentration above which the addition of a monomer leads to the formation of the first micelle. Above the CMC, monomers and micelles exist in dynamic equilibrium. Almost all physical properties of a surfactant solution exhibit a sharp concentration dependent discontinuity in the region of micellization. Below the CMC, the thermodynamic properties of surfactant solutions (osmotic pressure, conductivity, etc) follow ideal or regular laws of same type as those of solutions which contain a large molecular solute. Above the CMC, one can observe a clearly non-ideal behavior and a practically constant activity. The CMC of a surfactant is defined as the value at which the solution property of the molecule shows an abrupt change. The methods for measuring CMC can be classified into two types. The first are methods which detect abrupt changes in the monomolecular concentration, such as the surface tension and electric conductivity methods. The second type detects micelles and includes the dye adsorption method and the solubilization method.