6. Red Brows: Chemical Explanation
Iron Oxide Reactions in the Skin
In the skin, iron oxides can interact with the protein ferritin, leading to the formation of ferric ions (Fe3+). These ions may undergo a series of redox reactions with oxygen and other elements, resulting in a shift in the iron's oxidation state. This alteration can cause the pigment to change its optical properties, manifesting as a shift from its original color to a reddish or rusty hue. The general simplified reaction is:
Fe2O3 + ferritin → Fe3+ + O2 + other products
This reaction alters the iron oxide's structure and bonding, leading to the "red brow" phenomenon.
Understanding Ferritin's Role
Ferritin is crucial in this context; it's like a biological magnet that attracts iron ions and facilitates chemical reactions leading to the degradation of iron oxide pigments. The longer a pigment stays in the skin, the more it's exposed to the effects of ferritin, increasing the likelihood of a color shift.
Ferritin's Complex Structure and Function
Ferritin's role goes beyond just attracting iron; it stores iron in a non-toxic form within a complex protein structure made of 24 subunits. It forms a nanocage holding iron ions, phosphate, and hydroxide, resembling ferrihydrite, and can store up to 4500 iron (Fe3+) ions. When ferritin aggregates, it transforms into a toxic form called hemosiderin.
The Shift in Iron Oxide Pigments
The interaction between iron oxides in semi-permanent pigments and bodily ferritin can trigger chemical reactions, altering the iron's oxidation state. This change affects how the pigment reflects light, leading to color changes to reddish, pinkish, or rusty hues.
Stabilizing Iron Oxide with Silica Coating
To combat this issue, chemists devised hybrid pigments by bonding mineral iron oxide molecules with organic polymetal silica, creating a protective barrier similar to an industrial metal coating. This silica coating acts like a shield, preventing the iron oxide from reacting with ferritin and staving off the red color transformation post-application.
Protective Coating Explained
This coating is achieved by merging iron oxide with organic polymetal silica. Originally developed for industrial use to protect metals from environmental damage, this technique uses silica-based coatings like tetraethoxysilane (TEOS) and mercaptopropyltrimethoxysilane (MPTMS) to modify the surface of iron oxide nanoparticles. These coatings have been proven to provide stability against environmental factors and high temperatures. Now adapted for semi-permanent makeup, the silica coating acts as a defensive layer, safeguarding the iron oxide from ferritin and mitigating the risk of pigment color change after application.