Shampoo and Conditioner Science 78 orient out of the surface plane and the chain-chain interactions cause the surfactant to behave as a two-dimensional solid. Irving Langmuir was awarded the 1932 Nobel Prize in Chemistry for measuring this effect and explaining it on a molecular basis.6 When a surfactant adsorbs to saturate an aqueous surface, the surface is largely composed of the surfactant’s hydrophobic groups, and this means that the surface essentially has low surface energy. As a consequence of the low surface energy, the surface area is easier to expand to a film. This means that the system is easier to foam, since aqueous foams really consist of water films with entrapped gas. If the foam surface is structured by the adsorbed surfactant, then foam stability can be achieved.7 Surfactant Micelles Relatively large aggregates form within solution just beyond the concentration at which the surface becomes saturated with surfactant.8 These aggregates are surfactant micelles in which the hydrophobes are segregated within the core of the aggregate and the hydrophilic groups are located on the surface where they interact strongly with water.9 For a given system, micelles initially form at the precise concentration at which the driving force for surface adsorption becomes equal to the driving force for aggregate formation. This driving force is the chemical potential of the surfactant species. The lowest concentration at which micelles form is named the critical micelle concentration (CMC). The aggregates are large for example, micelles of sodium dodecyl sulfate at the CMC contain about 100 molecules and the thickness of the head group layer is about 0.4 nm.10 Surfactant micelles have liquid centers. They effectively solubilize hydrophobic substances only when the temperature of the system is above the Krafft point. Krafft found this phenomenon in 1895, and 68 years later Shinoda explained that the Krafft point corresponds to the melting point of the hydrated solid surfactant.11 Micelles have different shapes. The simplest shape is the spherical micelle that was postulated by Hartley in 1936. The