Day four of my blog post series: Molecules in Cosmetics. Today we'll go over the structures of polyethylene glycol, polyols, quaternary ammonium compounds, sarcosinates, siloxanes and soap!
1. POLYETHYLENE GLYCOL (PEG)
Molecules derived from the reaction between ethylene glycol and ethylene oxide. PEG's are polymeric, so their chemistry is dependent on the amount of moles of ethylene oxide within the molecule. PEG's are often blends of multiple polymers. When you see a number next to PEG like "PEG 400," the 400 refers to the average molecular weight of the chemical in Daltons. In cosmetics, PEG's can be used as emollients or solvents. They can also be emulsifiers when combined with another molecule, like PEG 100 stearate.
Figure 1. General structure of polyethylene glycol, where n = number of ethylene oxide units
Figure 2. PEG-100 stearate
2. POLYOLS
An alcohol containing multiple OH groups. Glycerin and sorbitol are the most well-known polyoyls in cosmetics. These ingredients are used a humectants in a variety of different products.
Figure 3. Glycerin
3. QUATERNARY AMMONIUM COMPOUNDS
A nitrogen compound bonded to four groups, giving rise to its positive charge. Due to the negatively-charged nature of skin and hair, quaternary ammonium compounds are often used as conditioning agents since they are attracted to these negative charges and impart a smooth feel to the surface. For example, cetrimonium chloride is found in many hair conditioners. These compounds can also act as anti-static agents, hair setting & styling agents and sometimes anti-microbial agents.
Figure 4. Cetrimonium chloride
4. SARCOSINATES
An amino acid derivative, they are usually used as anionic surfactants in cleansers like sodium lauryl sarcosinate.
Figure 5. Sarcosinate structure
Figure 6. Sodium lauryl sarcosinate
5. SILOXANE
Molecules containing silicone (Si) and O bonds. Linear silanes are good opacifiers and lubricants (ex: triethoxycaprylylsilane, while cyclic silanes are more volatile, and are often used for their degreasing or dry feel. Cyclopentasiloxane is one of the most commonly used ingredients for these characteristics. I also like adding silicones to reduce foaming in my emulsions.
Figure 7. Siloxane structure
Figure 8. Triethoxycaprylylsilane
Figure 9. Cyclopentasiloxane
6. SOAP
There are two types of salts--water-soluble soap and water-insoluble soap. Water-soluble soaps are made up of salts of fatty acids like sodium stearate. These soaps are often used as emulsifiers in shaving products and shampoos. Sometimes they're used as gelling agents in hydro-alcoholic sticks like deodorant.
Water-insoluble soaps are derived from reactions between long-chain fatty amines and metallic or alkaline earth cations. Zinc stearate is a zinc soap that repels water and acts as a lubricant and emollient. Other water-insoluble soaps can be used as binders, fillers, emulsion stabilizers in w/o emulsions and thickeners for hydrocarbons.
Figure 10. Sodium stearate
Figure 11. Zinc stearate
7. SORBITAN
Derived from the polyol sorbitol, it's commonly used to synthesize surfactants. The most well known sorbitan-derived surfactants are the Spans.
Figure12. Sorbitan structure
Figure 13. Sorbitan monostearate (Span 60)
Phew, almost at the end! See you tomorrow!