The Chemistry Behind Cosmetics

Behind the beauty industry there is a world of chemistry to uncover. The structure of certain molecules commonly added into cosmetics affects their reactivity and thus changes their purpose when added to cosmetic products. Topics of emulsifiers, parabens, alcohols, and fragrances will be covered. It is important to learn the chemistry of what goes into the products that people apply to skin daily.

ACCM concepts covered:

• III. B. 2. Drawing Lewis structures is a common, simple model for describing atomic connectivity in molecules.
• III. F. Reactions of molecules can often be understood in terms of subsets of atoms, called functional groups.
• IV. A. Intermolecular forces are generally weaker, on an individual basis, than chemical bonds, but the presence of many such interactions may lead to overall strong interactions.
• IV. C. Intermolecular forces can be categorized based on the permanence and structural details of the dipoles involved.

Parabens:

Parabens are added to ingredients like deodorants, toothpastes, shampoos, conditioners, body lotions, and makeup. They are used to stop the growth of fungus, bacteria, and other microbes. A concern over parabens recently has been about how this branched structure has been shown to increase estrogenic activity. Parabens are a series of parahydroxybenzoates or esters of para hydroxybenzoic acid. In the structure listed below, different types of parabens vary in chains of carbons (4).

    

Figure \(\PageIndex{1}\): General Structure of paraben 

The longer the chain of the paraben, the stronger the estrogenic activity (7) Parabens that start with propyl- and butyl- are usually associated with said activity. Another issue related to the length of the side chain of parabens is skin irritation. The most commonly used types of parabens are methyl-, ethyl-, propyl-, isopropyl-, and butyl-.

The parabens are absorbed into the body through the skin which are then metabolized to then be excreted in urine and bile. According to the CDC propylparaben was designated as “generally recognized as safe” to be in food up to 0.1 percent (6). 

Table of Alkyl Groups and General Structure

  

Example 1:

1. Below a methyl paraben is shown, draw a butyl paraben. 

   

2. Will the butyl paraben be more or less likely to cause increased estrogenic activity? Explain. 

Solution

1. 

Molecules should have the same basic structure with a 4 carbon long chain instead of 1 like the methyl. The basis of the paraben is phenol with an ester bonded to it and at the other end of the ester is the carbon chain attached that can increase in number. The alkyl group butyl means that there are four carbons. 

2. The butyl paraben will be more likely to cause increased estrogenic activity due to its longer carbon chain length based on the research attached above. It has been recorded that the longer carbon chain is, the more linked to higher levels of estrogenic activity.

Emulsifiers:

Emulsions are composed of 2 or more immiscible materials like oil and water which many makeup products like foundations and moisturizers. The surface tension between oil and water is extremely high and they do not mix without an emulsifier. Some common emulsifiers are glycerin, propylene glycol, and ethanol which decrease the polarity of the water phase. While emulsifiers aid in reducing the surface tension of water, there are limits to emulsifiers. Cosmetic chemists try to minimize chemical reactions because of additions of ions that will cause products to become sticky and tacky when applied to the skin due to the reactivity (2).

The structure of an emulsifier is a hydrophilic tail made of long chain carbons and a hydrophilic head. The most broadly considered emulsions are oil-in-water and water-in-oil. In an oil-in-water emulsion, the hydrophobic tails surround the oil droplets, and in a water-in-oil emulsion the hydrophilic heads surround the water droplets (9). 

Figure \(\PageIndex{3}\): General Structure of Emulsifier

Figure \(\PageIndex{4}\): Anionic Emulsifier

In popular cosmetic brand Paula’s choice, emulsifiers like polysorbates, laureth-4, and potassium cetyl sulfate are used in their products. This brand is also known for its products with zinc, AHA and Salicylic acid which are held together through non-ionic emulsifiers since they are hard to stabilize (8). Non-ionic emulsifiers have the same structure except the hydrophilic head has no charge. Another emulsifier is a cationic emulsifier which tends to have longer hydrophobic tails that have the effect of reducing the charge density of the head of the emulsifier (2). This minimizes the possibility of irritation. The structure of a cationic emulsifier is the same as listed before except the hydrophilic head is positively charged. Another example of an emulsifier is liquid crystal emulsifiers. These emulsifiers form a lamella network in cream which closely mimics the skin barrier (2). The brand Paula’s Choice uses lecithin, a liquid crystal emulsifier, in their products. Lecithin specifically is used to effectively deliver active ingredients into the skin. Paula’s Choice is just one example of a brand that uses emulsifiers, however the brand is extremely honest about the ingredients and has definitions for the ingredients to educate their curious customers (10).

Example 2:

After looking at the structure of an emulsifier listed above, draw a diagram of how an oil-in-water emulsion would behave versus a water-in-oil emulsion. Label hydrophilic and hydrophobic parts of the emulsifier. 

Solution

In an oil-in-water emulsion the hydrophobic tails surround the oil droplet inward and the hydrophobic heads border the droplet. In the water-in-oil emulsion the hydrophilic heads face inward and surround the water droplet while the hydrophobic heads face outward in the oil. This is because of the structure of the molecule and whatever the surrounding liquid is. With oil being non polar, the oil loving carbon chain will face inward towards the oil and the hydrophobic heads line the outside of the oil droplet. The opposite happens in a water-in-oil solution since the oil loving tails will face outwards in the oil, while the hydrophilic, polar head of the molecule surrounds the inside of the water droplet.

Alcohols:

       

Figure \(\PageIndex{5}\): General Structure of Alcohols

Alcohols are compounds containing an hydroxyl (-OH) group attached to a carbon atom in the alkyl group. Alcohols are separated into primary, secondary, and tertiary alcohols depending on which atom the hydroxyl group is attached to, and there may also be multiple hydroxyl groups (15).

Alcohols are often added into cosmetics for their high volatility, drying and antimicrobial properties, and solubility (16). However, there is controversy around the use of alcohol in cosmetics, with many manufacturers labeling their products as "alcohol free." However, this label usually refers to short-chain alcohols, such as ethanol, and isopropyl alcohol, which should be differentiated from fatty alcohols (14). These short-chain alcohols are used as solvents for ingredients that are insoluble in water. The hydroxyl group in alcohol is hydrophilic and is able to form hydrogen bonds with water, while the alkyl group is hydrophobic. However, due to their small size, they remain soluble. Furthermore, they may also be used to leave a cool feeling on the skin due to how fast they evaporate. 

 

Figure \(\PageIndex{6}\): Different 3D Representations of Ethanol

Ethanol is used to improve the penetration of other ingredients (15). The exact mechanism by which ethanol enhances permeation is unknown, but it is known that it reduces the amount of lipids in the skin bilayer. This makes it more permeable for ingredients, but it also causes dryness due to the reduced amount of lipids (17). Denatured alcohol, which contains ethanol, is commonly used in acne treatment. Products containing denatured alcohol dry faster, which creates a cooling effect and de-greases the skin. However, the skin can overcompensate for the lack of oil by producing more oil, which can lead to increased acne production (15). 

 

Figure \(\PageIndex{7}\): Avocadyne long fatty alcohol with three hydroxyl groups 

Not all alcohols in cosmetics have negative effects. Long chain aliphatic alcohols, also known as fatty alcohols, are usually between 12 and 18 carbon atoms long and are often used in lotions and creams (13). Due to their long alkyl chain, they’re hydrophobic, which gives them a different use from smaller sized alcohols. Since they’re hydrophobic, they don’t penetrate the skin and instead help to prevent water loss (17). 

Example 3: Isopropanol

1. Isopropanol is a common alcohol found in many cosmetics and cleansing products. The chemical formula for isopropanol is \(C_3H_8O\), and it's a secondary alcohol. Draw the Lewis dot diagram. 

2. Identify the hybridization of the oxygen and carbon atoms in your Lewis dot diagram. 

3. Using your drawn Lewis dot diagram, predict and explain the polarity of the molecule. Use polarity to explain why isopropanol helps to improve the absorption of the products when used in cosmetics.

Solution

1. Isopropanol has 26 valence electrons. According to its chemical formula, its a three carbon chain, and since it's a secondary alcohol, the hydroxyl is attached to the middle carbon. The remaining seven hydrogen atoms are attached to the carbon atoms to complete their octet. This gives us a total of 22 valence electrons, and the last two lone pairs are bonded to the oxygen atom to complete its octet. This gives the following Lewis structure. 

2. To determine an atom's hybridization, look at its steric number. All the carbon atoms and oxygen atom have a steric number of 4, which gives it a \(sp^3\) hybridization.

3. Isopropanol is a polar molecule because there is a hydroxyl group attached to the center carbon. The oxygen is more electronegative than carbon and hydrogen, so there is a slight negative charge on the oxygen atom. 

Fragrances: 

Fragrances are volatile compounds that vaporize into the air, which is what allows us to smell them. They’re added to cosmetics to give a scent or to mask another scent. These compounds need to be small and lipophilic, meaning they’re hydrophobic and can dissolve in lipids (18). Their volatility allows them to easily evaporate and leave their source, and their hydrophobicity allows them to pass through the mucus to react with olfactory receptors (20). 

It’s difficult to arrange smells into distinct classes, but the functional groups are linked to certain odors. Below is a chart with a few functional groups and their associated smells (19). 

Table of Functional Groups and their Associated Odors

Functional Groups	Related Odor
Alcohol (-OH)	Floral, mint, sweet or pungent
Acid	pungent
ester	Glue, Fruity
Pyrazine	Earth, Spice, Green pepper, Popcorn
Phenols	Wood smoke, Tarry

Bright light and air can damage perfume due to chemical reactions. Energy from light can break the bonds in molecules, and fragrant molecules may react with chemicals in the air (21). Small changes in the chemical structure can also lead to different smells. Below is a table of possible changes to the structure and examples of how each change affects the odor. 

Table of Structural Changes that Affect Odor and Examples

Change	Example
Elongation of a carbon chain	G-nonalactone smells like coconut, but adding two methyl groups gives the smell of peaches.
Exchange of functional groups	1-butanol, an alcohol, smells pungent, but carboxylic acid smells like rancid butter.
Position of functional group	1-propanol, which has a hydroxyl on the first carbon, smells like ethanol, whereas 2-propanol, which has a hydroxyl on the second carbon smells sweet

Example 4: Citronellol

(-)-citronellol is used in perfumes. Its structure is shown below. 

1. Using the tables from this section and the representations of citronellol, predict what it would smell like.

2. Label each of the carbons with their geometric shape. 

3. When a carbon is bonded to four different groups, it’s called a chiral center. At a chiral center, there are two different ways to arrange the four groups. Draw an arrow pointing to the chiral center and explain the two different ways to arrange the four molecules in 3D space. 

Solution

1. Floral, mint, sweet or pungent are all acceptable answers. Citronellol contains a hydroxyl group, and according to figure 8, hydroxyl groups are associated with floral, mint, sweet or pungent smells. 

2. To identify a carbon's geometric shape, look at its steric number. All the carbon atoms, except for the two in the carbon-carbon double bond, have a steric number fo 4, which gives them a tetrahedral shape. The two carbon atoms in the carbon-carbon double bond have a steric number of 3, which give them a trigonal planar shape.

3. The carbon at position 3 is the chiral center. The methyl group at can point towards us while the hydrogen points away, or the positions can be switched so that the hydrogen points towards us and the methyl group points away. 

References

1. Anionic emulsifier structure https://rahabitumen.com/bitumen-emul...ier-structure/ 

2. Selecting and Evaluating Emulsifiers for Cosmetics https://cosmetics.specialchem.com/se...-for-cosmetics

3. Wikipedia Contributors. Paraben https://en.wikipedia.org/wiki/Paraben 

4. Should People Be Concerned about Parabens in Beauty Products? https://www.scientificamerican.com/a...20by%20binding 

5. Moore, J.; Zhou, J.; Garand, E. 6.15: Basics of Organic Nomenclature https://chem.libretexts.org/Bookshel...c_Nomenclature 

6. Stoiber, T. What Are Parabens, and Why Don’t They Belong in Cosmetics? | Environmental Working Group https://www.ewg.org/what-are-parabens 

7. Blair, R. M. The Estrogen Receptor Relative Binding Affinities of 188 Natural and Xenochemicals: Structural Diversity of Ligands. Toxicological Sciences 2000, 54 (1), 138–153. https://doi.org/10.1093/toxsci/54.1.138.

8. Emulsifier | Paula’s Choice https://www.paulaschoice.com/ingredi...mulsifier.html 

9. GCSE CHEMISTRY - What is an Emulsifier? - How does an Emulsifier work? - What are Hydrophilic and Hydrophobic Molecules? - GCSE SCIENCE. https://www.gcsescience.com/o77.htm 

10. Lecithin | Paula’s Choice https://www.paulaschoice.com/ingredi...-lecithin.html

11. Wong, S. F.; Lim, J. S.; Dol, S. S. Crude Oil Emulsion: A Review on Formation, Classification and Stability of Water-In-Oil Emulsions. Journal of Petroleum Science and Engineering 2015, 135, 498–504. https://doi.org/10.1016/j.petrol.2015.10.006.

12. Wong, S. F.; Lim, J. S.; Dol, S. S. Crude Oil Emulsion: A Review on Formation, Classification and Stability of Water-In-Oil Emulsions. Journal of Petroleum Science and Engineering 2015, 135, 498–504. https://doi.org/10.1016/j.petrol.2015.10.006

13. Alcohols in cosmetic formulations https://www.virtualbeauty.co.nz/reso...-formulations/.

14. Nutrition, C. for F. S. and A. “Alcohol Free.” FDA 2020.

15. Are all alcohols in cosmetics necessarly bad? https://cosmedoc.si/en/2019/06/13/ar...ecessarly-bad/.

16. Alcohol in cosmetics| Inside Our Products - L’Oréal https://inside-our-products.loreal.c...ohol-cosmetics.

17. Ingólfsson, Helgi I.; Andersen, Olaf S. Alcohol’s Effects on Lipid Bilayer Properties. Biophysical Journal 2011, 101 (4), 847–855. https://doi.org/10.1016/j.bpj.2011.07.013.

18. Börsch, A. Small molecules make scents. https://www.scienceinschool.org/article/2007/scents/.

19. Genva, M.; Kenne Kemene, T.; Deleu, M.; Lins, L.; Fauconnier, M.-L. Is It Possible to Predict the Odor of a Molecule on the Basis of Its Structure? International Journal of Molecular Sciences 2019, 20 (12). https://doi.org/10.3390/ijms20123018.

20. Barakat, A. What Makes a Molecule Smell? https://monell.org/what-makes-a-molecule-smell/ ‌

21. How Perfume Works https://science.howstuffworks.com/perfume2.htm

22. Hydrocarbons | Chemistry for Majors https://courses.lumenlearning.com/ch.../hydrocarbons/.