Properties of the different types of fatty acids
The protective outer cuticle layer of hair is not a solid surface, but is porous in order to allow transport of oils and water back and forth through the hair and into the cortex. The lipid-rich cell membrane complex layer just beneath the cuticle scales acts as a diffusion port, enabling fatty acids and moisture to travel into the interior of the hair strand.
Molecular size and shape determine the probability of a fatty acid to travel through the cuticle layer into the cortex of the hair. Saturated fatty acids such as stearic acid, lauric acid and palmitic acid diffuse easily through the pores of the cuticle layer and penetrate the cortex, where they provide flexibility and suppleness to hair strands. Spectroscopic studies demonstrate that despite their kinked structure due to the single double bond, monounsaturated fatty acids are also able to readily penetrate the interior of the hair via this route.
However, the more unwieldy structure of polyunsaturated fatty acids prohibits them from penetrating into the interior of the hair strand and they remain adsorbed onto the surface of the hair. Oils such as mongongo oil that are high in polyunsaturated fatty acids, form a protective and emollient film on the surface of the hair, where they act as a barrier preventing moisture from escaping the interior of the hair. These fatty acids can add gloss to hair and improve comb-ability by smoothing the surface of the hair. Due to the presence of the acid groups in their structure, these ingredients can also have a mild humectant effect.
Oils such as mongongo oil that are high in polyunsaturated fatty acids, form a protective and emollient film on the surface of the hair, where they act as a barrier preventing moisture from escaping the interior of the hair. These fatty acids can add gloss to hair and improve comb-ability by smoothing the surface of the hair.
The linoleic acid in mongongo oil is known for being emollient to the hair, and stearic acid, palmitic acid and oleic acid are good at penetrating to the cortex to supply elasticity and improved mechanical properties. Perhaps the most interesting properties of mongongo oil can be attributed to the presence of α-eleostearic acid, a conjugated trienoic fatty acid. This molecule has three double bonds in the middle of its structure that are conjugated, meaning they alternate (double bond-single bond-double bond-single bond…). These types of structures have unique chemical properties due to this conjugation, as they can delocalize certain of their electrons in response to various stimuli in a process known as resonance stabilization.
There are three reasons that this very specific feature of the organic structure of α-eleostearic acid is interesting to us in hair care applications. The first is that the conjugated diene structure enables this fatty acid to act as a mild sun protective agent via UV-absorption and subsequent resonance stabilization. The second reason is that the molecule is capable of undergoing a UV-initiated photopolymerization reaction, whereby the fatty acids molecules link together into a three-dimensional crosslinked network, forming a flexible film on the surface of the hair. This provides physical protection to the hair and also may impart style hold or curl retention. Thirdly, this polymerization mechanism (called curing) substantially reduces drying time for hair. Even once polymerized, the carboxylic acid groups on the molecule should be sufficient enough “hydrophilic handles” to permit removability in water, especially if conditioner and/or mild shampoo are used.
Since it is fun to experiment with our hair, it seems worthwhile to at least sample some of these new products containing this ingredient. Look for products that contain other quality ingredients and that feature mongongo oil sufficiently high up the ingredient list. Beware products that are comprised primarily of other oils and only include this as a trace ingredient as they may prove to not be a sound investment. Let us know your thoughts when you do try some of the new mongongo oil products.
- Dyer, J.M., et al, http://naldc.nal.usda.gov/download/22993/PDF, Differential Extraction of Eleostearic Acid-Rich Lipid–Protein Complexes in Tung Seeds, JAOCS, Vol. 75, no. 11 (1998)
- Yang et al. BMC Plant Biology 2010, 10:250, http://www.biomedcentral.com/1471-2229/10/250