micelles

Micelles are aggregates of amphiphilic molecules, such as surfactants, that form in solution when their concentration exceeds the critical micelle concentration. In water, molecules orient with hydrophobic tails inward and hydrophilic heads outward, creating a hydrophobic core surrounded by a polar shell. This organization reduces the exposure of nonpolar regions to water and is stabilized by hydrophobic interactions and electrostatic or steric repulsion among head groups.

Formation is a dynamic self-assembly process; monomers continuously exchange with the bulk. The aggregation number—the average number of molecules per micelle—depends on surfactant type, temperature, and ionic strength; typical spherical micelles contain a few tens of molecules and have radii of about 2 to 10 nanometers.

Key factors include tail length and headgroup charge: longer hydrophobic tails decrease the critical micelle concentration, while adding salt screens headgroup repulsion and can promote micellization for ionic surfactants. Temperature can raise or lower the CMC depending on the system; some ionic surfactants exhibit a Krafft temperature below which micelles do not form.

Types and structures: In aqueous solutions, most common are spherical micelles. In nonpolar solvents, reverse micelles form with polar cores. At higher concentrations or with certain surfactants, rod-like or worm-like micelles may appear.

Functions and applications: Micelles solubilize hydrophobic compounds within their cores, aiding detergency, emulsification, and cleaning. They serve as carriers in drug delivery, cosmetics, and food products, enabling solubilization and controlled release. In research, micelles function as nanoreactors, templates for nanoparticle synthesis, or tools in spectroscopy.