hygral fatigue and coconut oil, I stated that oils can't penetrate our hair shaft. I was wrong! Some oils can penetrate our hair shaft - it's all about the size and type of molecules in the oil.
When I was thinking about oils penetrating the hair shaft, for some reason I thought about it penetrating all the way to the medulla, which they do not, instead penetrating through the cuticle and outer cortex of the hair strand. I opined that I thought smaller molecules and things other than fatty acids in the oil might penetrate, which was on the way to correct, but not far enough! What's totally embarassing is that I quoted not one but two studies on the original post that demonstrated coconut oil could penetrate our hair shaft, and for some reason, I blanked on it!
This study shows that coconut oil can penetrate the hair shaft, but mineral oil and sunflower oil can't. (I encourage you to read page 11 to learn more about how our hair is damaged.) They concluded that coconut oil is better as a pre-wash than a post-wash oil. Interestingly, the study noted that post-wash application of coconut oil was less effective at reducing water retention in the hair shaft because the oil only formed a coating without penetrating. (Point of interest: The Swift quoted in this study isn't me. I wish it were!) But they also noted that a coating on the hair strand can prevent water retention.
Check out this study that showed that tripalmitin, found in hydrogenated palm oil, could penetrate the cuticle and outer cortex of damaged hair!
This study showed that hydrogenated palm oil was an effective conditioning agent for repair of damaged hair because it inhibits water-induced swelling of damaged hair. To understand the functional mechanism of hydrogenated palm oil, we conducted TOF-SIMS analysis of the penetration of this oil into damaged hair. TOF-SIMS analysis revealed that tripalmitin, a constituent of hydrogenated palm oil, penetrated into the cuticle and the outer cortex of damaged hair. This is considered as the mechanism responsible for inhibition of hair swelling by hydrogenated palm oil.
Reference: Kojima, T., Tsuji, S., Niwa, M., Saito, K., Matsushita, Y., & Fukushima, K. (2012). Distribution Analysis of Triglyceride Having Repair Effect on Damaged Human Hair by TOF-SIMS. International Journal Of Polymer Analysis & Characterization, 17(1), 21-28. doi:10.1080/1023666X.2012.638435
In this study below, it's noted that there seems to be an inverse correlation between the amount of oil found on the surface of the hair and its ability to penetrate the hair shaft. Mineral oil leaves more of a film while coconut oil doesn't. I was interested in this study as they mention olive oil, but they don't mention whether it was absorbed or not - they imply it, but don't state it completely.
In this work we have explored capillary adhesion between hair fibers treated with different types of oils. With coconut, olive, and sunflower oils the capillary adhesion was found to decrease with time, but not with mineral oil. Application of heat reduced the capillary adhesion further for coconut and sunflower oils. Again, this was not observed with mineral oil. Based on an earlier study, where coconut oil was found to penetrate hair while mineral oil was unable to do so, it was hypothesized that the reduction in capillary adhesion resulted from the penetration of oil into the fiber, leaving a thinner oil film on the surface....As the oil is absorbed into the hair, the film thins with time and application of heat, and the scale structure reappears...The agreement between the two methods supports the concept that the reduction in capillary adhesion between hair fibers is most likely because of thinning of oil films by absorption of oil into the hair.
Investigation of penetration abilities of various oils into human hair fibers (K. Keis*, D. Persaud*, Y. K. Kamath* and A. S. Rele)
I really enjoyed this next summary of a study done to investigate why some oils penetrate and others don't. In general, it's all about the type of fatty acids.
In this communication, penetration of vegetable oils into hair fibers has been investigated by the TOF-SIMS (Time-Of-Flight Secondary Ion Mass Spectrometry) method. In earlier work , the method was found suitable to study the penetration of coconut and mineral oils into human hair. Therefore, the study has been extended to a group of vegetable oils with different types of unsaturation in the fatty acid components. Different patterns of penetration have been observed for oils of different molecular structure. The general pattern which emerges from this study is that polyunsaturated oils do not penetrate at all, or do so only sparingly into the structure of hair. Most of these molecules seem to penetrate only into the cuticular region of the hair fiber. Oils with polyunsaturated fatty acids seem to have difficulty in penetrating hair. It is possible that these molecules do not fit into the fiber’s cell membrane complexes, which are known to be the diffusion pathways in the keratin fiber. On the other hand, monounsaturated oils, such as olive oil, with more compact molecular structure seem to penet- rate readily into the hair fiber.
Reference: Mapping penetration of cosmetic compounds into hair fibers using time-of-flight secondary ion mass spectrometry (TOF-SIMS) S. B. Hornby*, Y. Appa*, S. Ruetsch and Y. Kamath
Here's a summary of a study that everyone has quoted all over the 'net (J Cosmet Sci. 2001 May-Jun;52(3):169-84) As a note, if you do a search for this summary, you'll see it quoted as being about coconut oil, mineral oil, olive oil, and avocado oil. The study is about coconut oil and mineral oil only. I think the confusion comes out of this post on the Beauty Brains where they acknowledge they were extrapolating to help their readers understand other oils that will penetrate hair. I also have to point out they use the term hygric fatigue, which is the only place I have ever seen it! (Click here for the actual study.) And they also say that coconut oil is polar, which is the first and only time I have seen this stated. But this explains why people are saying coconut oil is polar - everyone is quoting this study! As a note, it isn't just about the size of the molecules - coconut oil may penetrate due to the affinity of it for the proteins.
An attempt has been made to show the difference in the penetrability of coconut oil and mineral oil in human hair....The results show that coconut oil penetrates the hair shaft while mineral oil does not. The difference may be due to the polarity of the coconut oil compared to the nonpolar nature of the mineral oil. The affinity of the penetrant to the protein seems to be the cause for this difference in their behavior. This study also indicates that the swelling of hair is limited by the presence oil. Since the process of swelling and deswelling of hair is one of the causes of hair damage by hygral fatigue, coconut oil, which is a better penetrant than mineral oil, may provide better protection from damage by hygral fatigue.
Monounsaturated oils are those with at least one double bond, like oleic acid. Polyunsaturated fatty acids are those with at least two double bonds, like linoleic, linolenic, gamma-linolenic acid, and so on. You can see from the picture, that some fatty acids lie straight - like palmitic (C16) and stearic (C18) - while the more unsaturated fatty acids have kinks in them. This is likely due to the trans or cis configuration of the fatty acid.
WHAT HAVE WE LEARNED?
We've learned that some oils can penetrate the hair strand, depending upon the fatty acid size and saturation. The smaller the fatty acids, the more likely it is to penetrate the cuticle and outer cortex of our hair. Polyunsaturated fatty acids seem to have trouble penetrating the hair. If we extrapolate this, it means that we want to use oils with shorter fatty acids and high levels of saturation. This means oils like coconut oil, palm oil, or fractionated coconut oil. But coconut oil might also be able to penetrate the hair strand thanks to its affinity for our hair's protein.
Oils that contain a lot of longer fatty acids and lower levels of saturation like sunflower or soy bean oil will still coat your hair shaft, but they won't penetrate the strand. (This isn't a bad thing as having a coating on your hair will still reduce water retention in your hair strand.) I haven't been able to confirm anything about olive oil, avocado oil, or jojoba oil, although I have read in a few places that jojoba oil's fatty acids are just too big. (The study above that notes, "On the other hand, monounsaturated oils, such as olive oil, with more compact molecular structure seem to penetrate readily into the hair fiber" says "seems".)
We've learned we should be using penetrating oils works best as a pre-wash instead of a post-wash for maximum water retention prevention.
Want to know more about oils? Click the links above!
And this is why I love science! I learn something new every day! And thanks to my readers for sending along studies and other pieces of great information to me so I could do more research. (What to do if you think I'm wrong? Check out this post!)