Micelles are aggregates of surfactant molecules that form in a liquid medium. These molecules have both hydrophilic (water-loving) and hydrophobic (water-hating) regions. In an aqueous solution, surfactant molecules arrange themselves in a specific manner to minimize their exposure to water. Micelles are formed when the concentration of surfactant molecules exceeds a certain value called the Critical Micelle Concentration (CMC).

When the concentration of surfactant molecules is less than the CMC, they exist as individual molecules dispersed in the solution. At this point, the surfactant molecules do not form micelles and do not exhibit colloidal behavior. They may still function as surface-active agents, reducing the surface tension of the liquid, but they do not form stable colloidal structures.

However, when the concentration of surfactant molecules exceeds the CMC, micelles start to form. The hydrophobic tails of the surfactant molecules cluster together in the center of the micelle, shielding themselves from the surrounding water molecules. The hydrophilic heads of the surfactant molecules face the aqueous environment, providing stability to the micelle. This arrangement allows the micelle to remain dispersed in the liquid medium.

At concentrations greater than the CMC, micelles behave as colloids. Colloids are a type of mixture where tiny particles are dispersed throughout a continuous medium. In the case of micelles, the surfactant molecules form the dispersed particles, and the liquid medium acts as the continuous phase. The micelles exhibit colloidal properties such as Brownian motion, Tyndall effect (scattering of light), and the ability to pass through filter paper.

In summary, micelles behave as colloids only when the concentration of surfactant molecules is greater than the Critical Micelle Concentration (CMC). At lower concentrations, they exist as individual molecules and do not exhibit colloidal behavior. The formation of micelles is a result of the self-assembly of surfactant molecules, driven by the need to minimize their exposure to water.