Shampoo and Conditioner Science 90 The Anatomy of a Shampoo Formulation Shampoos consist essentially of water, a primary surfactant, one or more co-surfactants and soluble salt. Other ingredients are added for fragrance, preservation, conditioning and styling attributes. Cleaning is achieved mainly by the primary surfactant which is often an anionic surfactant that would adopt a conical shape if it was present in water alone. The co-surfactant is usually a nonionic or zwitterionic surfactant with a relatively small head group surface area. This molecular shape allows the co-surfactant to serve two roles, namely, (i) it packs between the molecules of the primary surfactant to reduce the curvature and to promote the formation of worm-like micelles with their high viscosity and luxurious rheology and (ii) it packs between the primary surfactant in the lamellae of the foam to provide good lather which is easily removed on rinsing. Salt enhances the function of the co-surfactant by “damping down” the ionic repulsion between primary surfactant head groups and promoting the formation of wormlike micelles. If excess salt or co-surfactant is added, shampoo compositions can separate into phases that contain co-existing micelles and liquid crystals. These phase-separated compositions often exhibit thin viscosities and haziness. The Primary Surfactant The lauryl sulfates have been the primary surfactant workhorses of the shampoo industry for decades. The sulfate head groups bear an anionic charge when dissolved in water. The long chain alkyl tail group has an average length of 12 carbon atoms. It is important to understand that this is an average chain length commercial lauryl sulfates have a distribution of chain length from as short as 8 carbons to as long as 18 carbons. This chain length distribution changes from supplier to supplier and it also changes depending on the source of raw materials. Formulators should be aware that changes in the chain length distribution of the surfactants can lead to subtle changes in the properties of the shampoo product.