Chemical Reactions in Cosmetic Science

In college, most chemistry classes were focused on creating chemical reactions. We were constantly challenged to figure out what reaction would happen when you mix chemicals together.

Cosmetic science is not usually reactive

So, you might find it surprising that cosmetic scientists usually do the opposite. We mix chemicals together and hope that nothing happens. In a cosmetic formula, chemical reactions are bad. They are a sign of instability and we do everything we can to prevent them.

This drive to make nonreactive systems might lead you to wonder whether all those chemical reactions you memorized in Organic Chemistry were a waste of time. And if you work as a cosmetic formulator, you might wonder, “Are there any chemical reactions in cosmetic science?”

Yes, there is!

Reactive cosmetic products

While most of the chemical reactions in our industry occur at the raw material suppliers labs, there are some cosmetic products specifically designed to chemically react.  Here’s a list of the most common.

Permanent Waves

These products are designed to permanently change the shape of hair. People with straight hair often use permanent waves to get a little curl in their hair. A permanent wave formula has a reducing agent like thioglycolic acid that reacts with the di-sulfur bonds in the cystine amino acids breaking down the hair structure. Hair is first shaped into curlers, then the product is put on hair. It begins reducing hair and is rinsed with water to stop the reaction. A neutralizing chemical like hydrogen peroxide, is added which reforms the di-sulfur bonds into the new configuration.

Hair Relaxers

These products do the opposite of permanent waves. They make curly hair permanently straight. The method is similar you chemically break down hair, reshape it, then reform the protein bonds in the new configuration. Sometimes ammonium thioglycolate is used but most often it is sodium hydroxide or lithium hydroxide. The compounds break down the di-sulfur bonds in hair and the neutralizing step stops the reaction.

This is the most damaging chemical treatment for hair.

Hair Bleach

Hair bleaching is a process used to turn brunettes into blonds. You didn’t really think that there were that many blonds in the world did you? Hair bleach is a chemical reaction between melanin (the material in hair that gives it color) and hydrogen peroxide.

Hair Colors

Hair coloring is a slightly more complicated version of hair bleach. It uses hydrogen peroxide to break down hair’s natural color, then the peroxide also oxidizes a polymeric reaction with dye monomers. When the dye polymerizes inside the hair, it creates a color molecule that is too big to easily come back out.

Skin Darkening

These products are designed to give fair-skinned people a tanned look. They work by using an ingredient called dihydroxyacetone or DHA. It reacts with the proteins in the stratum corneum via the Maillard reaction to produce the brown (although sometimes orange) color. All the steps haven’t been worked out, but basically when DHA is exposed to skin protein, it is converted to pyruvaldehyde, which then reacts with arginine, lysine, and histidine amino acids in skin to form brown/yellow pigments called melanoidins.

Be sure to see Kelly’s article about DHA in Cosmetics & Toiletries magazine.

Depilatories

These are products designed to help people to remove unwanted hair. The primary active in these types of cosmetics is some version of thioglycolic acid. The acid reacts with the cystine amino acids in hair and breaks down the S-S linkages. The hair is reduced to a jelly like mass that can then be wiped away. Note this is the same reaction as in permanent waves.

2SH-CH2-COOH(thioglycolic acid) +R-S-S-R(cystine)—--> 2R-SH + COOH CH2 SS CH2 COOH (dithiodiglycolic acid)

Chemical reactions are not the primary focus of most cosmetics, but there are a few reactions so don’t fret. Memorizing those reactions in Organic Chemistry wasn’t a complete waste of time.

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