micelle

Micelles are aggregates of amphiphilic molecules that form spontaneously in certain liquids, most commonly aqueous solutions. Surfactant molecules have a hydrophobic tail and a hydrophilic head, arranging themselves with tails inward and heads facing the solvent to create a compact, colloidal-scale particle. In water, micelles typically range from about 2 to 20 nanometers in diameter and act to shield hydrophobic regions from contact with the solvent.

Micelle formation occurs when the concentration of amphiphiles exceeds the critical micelle concentration (CMC). Below the CMC, molecules exist mainly as monomers; above it, they assemble into micelles. The shape and size depend on molecular geometry and conditions such as temperature and ionic strength. While spherical micelles are common, cylindrical, disk-like, and wormlike micelles can also form. The packing parameter, p = v/(a0l), helps predict shape: p below ~1/3 favors spheres, around 1/3–1/2 favors cylinders, and larger values can lead to bilayers or vesicles. Ionic head groups introduce electrostatic repulsion and generally raise the CMC compared with nonionic surfactants.

Reverse micelles can form in nonpolar solvents, with the polar core oriented inward and the hydrophobic tails outward, enabling solubilization of water and polar compounds in nonpolar media.

Applications of micelles are broad: detergency and cleaning, solubilization of hydrophobic substances, drug delivery systems, cosmetics, and food products. In biology, bile salts form micelles that solubilize fats for digestion. Techniques such as surface tension measurements, dynamic light scattering, and small-angle scattering are used to determine CMC, size, and shape.