Scientific Concepts
Anatomy, chemistry, and hair analysis
Skin has three layers: the epidermis, whose outer stratum corneum is the barrier; the dermis, holding nerves, glands, and collagen; and the subcutaneous layer of fat. The acid mantle sits near pH 4.5 to 5.5.
Hair grows from a follicle with a bulb and papilla, fed by a sebaceous gland, with an arrector pili muscle. The shaft has three layers: cuticle, cortex, and medulla. Color and strength live in the cortex; the cuticle controls how much anything can enter. Growth cycles are anagen (active, several years, most hairs), catagen (transition), and telogen (rest and shedding). Losing 50 to 100 hairs daily is normal.
Bonds drive every chemical service. Hydrogen bonds break with water and reset on drying or heat. Salt bonds break with pH change and reset when pH returns. Disulfide bonds break with thioglycolate or hydroxide and reset by oxidation. Peptide bonds break only with damage; they do not re-form.
pH is logarithmic: each whole unit is a tenfold change in hydrogen ion concentration, so pH 3 is one hundred times more acidic than pH 5. Hair and skin sit near pH 5; shampoos are slightly acidic; relaxers sit at pH 12 to 13.
Trap to expect: candidates confuse salt and disulfide bonds, or assume hydrogen bonds are permanent.
Worked example: a client wants a relaxer and a color service in one visit. Because both break bonds in the cortex, you schedule them on separate days and analyze elasticity first.