Hair Coloring 148 (that the products and consumers interact with directly), the cuticles and the cortex (including the cell membrane complex). A change in each of these structures has different cosmetic consequences. For example, changes to the top surface layer will change deposition and feel, while changes to the cortex will affect the strength of the hair fiber. F-Layer Removal—Hair surface changesThe top surface of the hair (15 nm) consists of the F-Layer bound lipid. The F-Layer is the monolayer of covalently bound branched fatty acid (18-methyleicosanoic acid (18-MEA)) attached to the surface of the cuticle via thioester linkages. Treating hair with alkaline hydrogen peroxide attacks the thioester groups that bind 18-methyleicosanoic acid to the ultra-high sulfur protein backbone at the surface of the hair and in the upper beta layer of the cell membrane complex (CMC).228 This partially removes the hydrophobic surface barrier. The mechanism for 18-MEA removal is thought to be perhydrolysis of the thioester as shown in Figure 4. The removal of this lipid significantly changes the surface energy of the hair surface, making it more hydrophilic, and also increases the number of cysteic acid functional groups on the hair surface. Thus, multiple treatments with either Level 2 or Level 3 Permanent products or with bleach products will irreversibly remove, to some extent, the F-Layer from the hair surface. The removal of this lipid plays a very important role in how the hair feels and in adsorption of ingredients onto and into human hair.29 The surface properties are key to the frictional properties of hair,30 combing behavior,31 and to the handle or feel of hair.32 Figure 4 Mechanism of F-Layer Removal