Critical Micelle Concentration, CMC

(find instruments that measure CMC here)

Definition:

Critical micelle concentration (CMC) is the concentration of an amphiphilic component in solution at which the formation of aggregates (micelles, round rods, lamellar structures etc.) in the solution is initiated.

Summary/Description:

Amphiphilic molecules contain two distinct components, differing in their affinity for solutes. The hydrophilic part of the molecule has an affinity for polar solutes, such as water, and the hydrophobic part of the molecule has an affinity for non-polar solutes, such as hydrocarbons. Amphihilic molecules display distinct behavior when interacting with water. An amphiphilic molecule can arrange itself at the surface of the water such that the polar part interacts with the water and the non-polar part is held above the surface (either in the air or in a non-polar liquid). The presence of these molecules on the surface disrupts the cohesive energy at the surface and thus lowers the surface tension.

Another arrangement of amphiphilic molecules can allow each component to interact with its favored environment. Molecules can form aggregates in which the hydrophobic portions are oriented within the cluster and the hydrophilic portions are exposed to the solvent. Such aggregates can show a variety of conformations. The shapes of the aggregates depends largely of the properties of the amphiphilic molecules.

The proportion of molecules present at the surface or as aggregates in the bulk of the liquid depends on the concentration of the amphiphile. At low concentrations amphiphiles will favor arrangement on the surface. As the surface becomes crowded with amphiphiles more molecules will arrange into aggregates. At some concentration the surface becomes completely loaded with amphiphile and any further additions leads to arrangement into aggregates. This concentration is called the Critical Micelle Concentration(CMC). A graph of surface tension vs log of concentration may be used to determine the CMC point.