Point of interest: I've amended this post like silly to reflect the studies I've been reading over the last few days. The information might seem a bit disjointed because I wanted to amend it as soon as I can. I'll edit for readability later this week.
It seems to me that a lot of the information on the 'net about hygral fatigue and oils good for our hair is based on one study - this one - that has been referenced by many many sites I've read. I think the references you see on most sites are references to a blog post by the Beauty Brains rather than the actual study. The Beauty Brains wrote a post on the study, then added an amendment to note that they were discussing oils based on another study, not this one. This study only looked at mineral oil and coconut oil. The Beauty Brains mention meadowfoam seed, olive, sunflower, avocado, and jojoba oil, but amend their post to note these other oils are from a different study, not the one linked above. I'm also basing this belief of this study being the basis for these blog posts because there are two things in it I've never seen before - the idea of coconut oil being polar and the mention of the term "hygral fatigue".
There's no doubt that water in the hair shaft might not be our friend - swelling causes friction and friction causes damage - and there's no doubt that our hair can stretch up to 30% when wet, so we must be very careful when brushing or combing in that state, but I have never seen the term "hygral fatigue" used in a textbook, paper, or study or by a cosmetic chemist. In all the years I've been studying hair care stuff, I saw it the first time this week by a commenter writing about the Curly Girl technique.
Correction: I found it in this study, the only place I've seen this term.
This is one of the reasons those of us with frizzy hair concerns use cyclomethicone and dimethicone - they keep the water out and keep our hair smooth. It's also the reason they're used as heat protecting products - they keep the water we have in our hair from escaping!
OILS AREN'T POLAR
Coconut oil is not a polar oil. No natural oils are polar. (I will be writing more about this later this week...) Mineral oil isn't polar. To be polar, a molecule has one end that is more negatively charged and one end that is more positively charged. This is a result of the electronegativity of the atoms involved in the bonding. The larger the difference between the electronegativities of the atoms, the more polar the molecule. If the polarization gets too much, it becomes an ionic bond. Coconut oil and other oils, are considered non-polar.
Edited to add: I know that study says coconut oil is polar, but I'm confused by this. Oils are generally held as being non-polar, so calling this oil polar is very strange. I realize Yahoo answers isn't a scholarly resource, but here's a description why coconut oil isn't polar, which I quote to demonstrate that the non-polarity of coconut oil is an accepted thing. Here's a site from Elmhurst University about the non-polarity of oils. But then I found this study that said, "Coconut oil has shorter alkyl side chains than SBO and therefore has a higher polarity and lower molar volume." But this is a comparison with soy bean oil, so it's more polar than soy bean oil...but what does that mean? The thing is that polarity is a continuum, not an either/or situation. So it can be more polar than soy bean oil, but not be considered polar. Then I found this review about coconut oil that stated that, "Medium chain triglycerides are widely used in the flavor industries as they are more polar and therefore more hydrophilic and can dissolve a variety of polar substances that are insoluble in conventional fats and oils." But this is about the triglycerides, not coconut oil. I'm investigating this further...
NOT ALL OILS PENETRATE THE HAIR STRAND
Edited: I really recommend you check out this post for the studies I've found about oils and hair penetration.
It appears that coconut oil can penetrate the hair strand into the outer cortex! See the studies and the conclusions in this post for more information. Its fatty acids seem to have an affinity for the protein on our strands. This study is the one everyone quotes, which notes...
coconut oil was the only oil found to reduce the protein loss remarkably for both undamaged and damaged hair when used as a pre-wash and post-wash grooming product. Both sunflower and mineral oils do not help at all in reducing the protein loss from hair. This difference in results could arise from the composition of each of these oils. Coconut oil, being a triglyceride of lauric acid (principal fatty acid), has a high affinity for hair proteins and, because of its low molecular weight and straight linear chain, is able to penetrate inside the hair shaft. Mineral oil, being a hydrocarbon, has no affinity for proteins and therefore is not able to penetrate and yield better results. In the case of sunflower oil, although it is a triglyceride of linoleic acid, because of its bulky structure due to the presence of double bonds, it does not penetrate the fiber, consequently resulting in no favorable impact on protein loss.
(Summary can be found here...)
Mineral oil was found to not penetrate the hair strand in quite a few studies. Having said that, we don't need an oil to penetrate our hair strand to be awesome. A coating of oil - or silicone, or other hydrophobic thing - can reduce water retention and friction.
Weekend Wonderings: Oils good for our hair?
Coconut oil in hair products (with leave in conditioner recipe)
Conditioners: Adding oils - coconut oil (with various recipes)
Coconut oil? Coconut oil!
Question: What oils are good for your hair?
If you want something to penetrate your hair strand, oils aren't necessarily the right ingredient, especially if you want moisturizing. (Coconut oil, yes. Unconfirmed for the rest.) You'd want to turn to ingredients like hydrolyzed proteins - silk or wheat to penetrate the hair shaft, oat to coat it - or humectants. If you want to coat your hair strand, any oil or butter you might like to use will work, as will silicones and some esters.
As an aside, there is a difference between moisturizing and hydrating. Moisturizing is about preventing water loss, which means you want to use emollients, like oils, butters, silicones, and esters. Hydrating is about maintaining an adequate level of water in our skin and hair, so you want to use humectants. Conditioning is about preventing friction damage, so you want to use conditioners for that.
- Hygral fatigue isn't a cosmetic chemistry thing - I've only seen it the once in that oft quoted study.
- Coconut, mineral, and avocado oils aren't polar.
- Coconut oil can penetrate your hair strand, but mineral oil can't. I haven't found anything confirming that avocado or olive oil can penetrate your hair strand, although it seems like the monounsaturated fatty acids, like oleic acid, might.
- Every oil will form a coating on your hair, which will keep water out.
- Water penetrating your hair strand isn't always a bad thing, depending upon hair type.
I found a great trove of information at Ktani's Hair Sense on the topic. I really recommend you wander over there and take a look! And check out this post she wrote on Hair Protein Loss and Coconut Oil, Hygral Fatigue and Mineral Oil. Bookmark this page! I love her use of references in her posts and her willingness to question everything! Awesome awesome blog. (Why don't I have this on my blog roll? Well, I do now!)
I need to point out that I am no expert on African hair - everything I know I have learned from my textbooks - so I do not write this to act as if I am. (Although this topic of hygral fatigue isn't just about African hair types, most of the resources I found were for this hair type.) I know there are issues with breakage for the African hair type - click here for a very interesting read - and I, unfortunately, don't have any solutions. I'm also not an expert on curly hair, another hair type that I've seen associated with hygral fatigue. I write this post because it was interesting to me!
And thanks to the people who shared their thoughts and resources with me. I'm always about the learning, and I've learned a lot the last few days. As you can see, I'm still waiting for more information to come in!
The chemistry of our hair: Overview
The chemistry of our hair: Medulla and cortex
The chemistry of our hair: Cuticle
Definition of good condition
Quick summary about damage
African hair types
Some interesting things I found about African hair types
Chemistry of our hair: Straight, curly, and frizzy hair!