When initially a small quantity of surfactant is added in water, the surfactant molecules initially form a monolayer at the water surface by orienting themselves almost in parallel fashion with the water-repelling nonpolar tails away from the water surface whereas the polar/ionic head groups interacting with the polar water molecules. If one attempts to dissolve more and more such amphipathic molecules, the temperature of the solvent should be more than the Kraft temperature. The solubility of surfactants in water depends on the temperature. The solubility of ionic surfactants rapidly increases above a particular temperature, called the Kraft temperature (Kraft point). The surfactant’s nonpolar chains “melt” at this temperature. Above the Kraft temperature, if one dissolves more and more of surfactants, above a particular concentration, called critical micelle concentration (CMC), tens to hundreds of these molecules cluster together forming colloid-sized aggregated structures that remain in dynamic equilibrium with monomers (free surfactant molecules) in solution. These aggregated structures which have nonpolar tails pointing inward away from the polar solvent and polar head groups pointing outward towards the polar solvent like water are called micelles. The aggregate structure is reversed in an nonpolar organic solvent and is known as reverse(d) micelles or inverted micelles. The CMC is about 10-4 to 10-3 mol/L. The number of surfactant molecules that form the micelle is called the aggregation number. The aggregation number can be 1000 or more for nonionic surfactant molecules but it is limited to 10 to 100 for ionic surfactants due to Coulomb repulsions between ionic head groups. For example, sodium dodecyl sulfate (SDS) or sodium lauryl sulfate (SLS) micelle in water has a CMC 8.1×10-3 mol/L and an aggregation number of ~80. Since the surfactant molecules self-assemble to form micelles having colloidal dimensions, micelles are known as association colloids. Micellar solutions scatter light that causes cloudy appearance in the soapy water. Micellar solutions are microheterogeneous media.

In water, the nonpolar hydrocarbon tails of the surfactant molecule being water repellent tend to orient away from water while the head groups being water loving interact with water. Initially when a small quantity of surfactant is added to water, the surfactant molecules form a monolayer at the water surface by orienting themselves almost in parallel fashion with the polar/ionic head groups interacting with the polar water molecules whereas the hydrophobic tails pointing away from the water surface. On increasing the quantity of the surfactants in water, above a critical concentration, the hydrophobic tail groups of tens to few hundred molecules cluster together. This results in the formation of an oily core with their hydrophilic head groups interacting with the polar water molecule (Figure 1). This kind of structure minimizes the repulsive interactions between the tail group and the solvent molecule and maximizes the attractive interactions between the head group and the solvent molecule. Thus, micelles form by a balance of forces like the water repelling hydrophobic forces associated with alkyl tail that promotes aggregation and the electrostatic repulsions of the ionic/polar head groups that inhibit aggregation. In the cases of an ionic surfactant, the charged head groups along with a fraction of the counter-ions constitute the Stern (or palisade) layer surrounding the core. The less tightly bound remainder of the counter-ions form another layer, the Gouy-Chapman layer that extends several hundred angstroms from the Stern layer into the solution. The shapes of the micelles depend on the surfactant concentration, molecular structures, etc. The shapes range from flattened spheres near CMC to cylindrical rods at higher concentrations. Micelles have both hydrophilic and hydrophobic regions. Micelles can therefore ‘‘solubilize’’ oil or nonpolar hydrophobic molecule to a limited extent in their oily interior. This property is used to disperse oil-soluble vitamins and similar oil-soluble solutes in water. On the other hand, small water-pools of the reverse micelles can also dissolve small quantity of water-soluble substances, such as proteins, in oil media. Transmission electron microscopy (TEM), light scattering, small angle neutron scattering, spectroscopy, surface tension measurements, etc. are used to characterize micelles.

Figure 1. Schematic representations of structures of (a) a surfactant molecule, SDS, and its stick diagram and (b) a partial cross-sectional view of an aqueous (normal) micelle.

Depending on the structure of the surfactant molecules, micelles can be classified as nonionic, cationic, anionic, or zwitterionic micelles, if the head groups of the micelle forming molecules are nonionic, cationic, anionic or zwitterionic, respectively. Triton X-100, CTAB (cetyltrimethylammonium bromide), and SDS are examples of nonionic, cationic, and anionic surfactants, respectively.

Cleansing action of soap and detergents. Detergency is an inherent property of amphiphiles. Surfactant molecules present in soaps and detergents are responsible for cleansing actions. Surfactant molecules used in soaps are sodium or potassium salts of various naturally occurring fatty acids. The properties of soaps depend on the length of the carbon chain of the fatty acid and the salt forming metal ions. For example, the potassium salts of the fatty acids produce a softer lather. Detergents are mostly synthetic ionic molecules. The core of the micellar structure is nonpolar in nature whereas the area near the head groups is polar in nature. Due to this dual nature, micelles can dissolve organic, nonpolar substances in its core as well as polar substances near the head groups. The cleansing action of soaps and detergents is based upon their ability to form micelles and dissolving oily, greasy materials into their cores.

Cleansing action occurs through the following steps. Dirt is often greasy or oily matters. Surfactant molecules reduce the surface tension of water that helps water to spread on the skin, fabrics, etc. Water cannot penetrate greasy or oily dirt matters due to their hydrophobic natures. The non-polar hydrocarbon tail of the surfactant molecules start interacting with the greasy or oily matters while the polar/ionic head groups sticking out of the oil droplets interact with water molecules. This pulls out the dirt matters into the water. Thus micelles formed from soap or detergent molecules trap dirt particles and get separated from skin or fabric and pass into washing solvent, water. The surface of each oily dirt particle becomes charged because of the head group charge of the surfactant molecules. This causes repulsions among the oily particles which help the particles to remain suspended in solution and to be washed away by the stream of water. Hard water that contains ions like calcium, magnesium, etc. does not form leather easily because these ions form insoluble scum by combining with the soap and detergent molecules.