Hair Chemistry

The Chemical Foundation of Hair

Hair is primarily composed of a protein called keratin, a complex fibrous protein also found in skin, nails, and animal horns. This keratin is arranged in a specific helical structure, and its unique chemical bonds determine hair's elasticity, strength, and overall behavior.

Keratin Structure

Keratin itself is made up of amino acids linked together in long chains. These chains then twist into an alpha-helix, which is further organized into larger bundles. The integrity and arrangement of these structures are crucial for hair health.

Key Chemical Bonds

Several types of bonds contribute to hair's structure:

• Peptide Bonds: These covalent bonds link amino acids together to form the polypeptide chains of keratin. They are strong and not easily broken by water or heat.
• Hydrogen Bonds: These are weak, temporary bonds that are easily broken by water (e.g., when hair gets wet) and heat. They are responsible for hair's ability to be styled and reshaped, as they reform when hair dries or cools. Hydrogen bonds account for approximately one-third of hair's strength.
• Salt Bonds (Ionic Bonds): Also weak and temporary, salt bonds are formed between positively and negatively charged amino acid side chains. Like hydrogen bonds, they are easily broken by changes in pH and reform when the pH is normalized. They also contribute to about one-third of hair's strength.
• Disulfide Bonds (Cystine Bonds): These are strong covalent bonds formed between two sulfur atoms of cysteine amino acids. Disulfide bonds are critical for hair's strength, elasticity, and shape. They are not broken by water or heat but can be broken and reformed chemically, for example, during perming, relaxing, or permanent straightening processes. These bonds provide approximately one-third of hair's strength.

pH and Hair

pH (potential of hydrogen) is a measure of the acidity or alkalinity of a substance, expressed on a scale from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline (or basic). Hair and skin have a natural pH range of approximately 4.5 to 5.5, which is slightly acidic. This acidic mantle helps to keep the cuticle closed, preventing moisture loss and protecting against bacterial and fungal growth.

Impact of pH on Hair

• Acidic Products (pH < 7): Products with a low pH (e.g., acid rinses, some conditioners) help to close the hair cuticle, making hair smoother, shinier, and less prone to tangling. This is beneficial for maintaining hair integrity.
• Alkaline Products (pH > 7): Products with a high pH (e.g., many shampoos, permanent waves, hair relaxers, hair dyes) cause the hair cuticle to swell and open. This allows chemicals to penetrate the hair shaft for cleansing or chemical processing. Prolonged exposure to high pH can lead to cuticle damage, increased porosity, and reduced strength.

Chemical Services and Their Effects

Hair chemistry is fundamental to understanding how various chemical services alter hair.

Permanent Waving (Perms)

Perms involve chemically altering the disulfide bonds to change the hair