Each has advantages and disadvantages
Last month’s article delved into the chemistry and properties of mineral oil, as well as some of its benefits and drawbacks when used as an ingredient in hair products or as a topical treatment. This month, I would love to expand on that topic a bit and dig a little deeper into the similarities and differences of petroleum-derived mineral oil and plant-derived oils, specifically coconut oil. Both types of oils are found in various hair care products, and coconut oil is quite often used in homemade treatments, as well. The superior properties of natural oils are frequently lauded, so it should be interesting to review a few scientific comparisons backed by data.
Water in - water out
Several published studies have summarized experiments done to evaluate and compare the emollient properties of mineral oil, coconut oil, and to a lesser extent, olive oil and safflower oil. In one paper, researchers reported using an analytical technique (dynamic vapor sorption, for those curious) to measure and determine the moisture diffusion coefficient for mineral oil, coconut oil, and other oils when applied to hair. They were interested in finding out was how much water vapor can penetrate into or diffuse out of hair that has been coated with oil.
The data obtained in the experiments revealed that both coconut oil and mineral oil form a protective barrier that effectively prevents the diffusion of moisture out of the hair in low-humidity environments, thereby improving moisture retention and minimizing dry, fly-away hair. All of the oil-treated hair samples showed this effect, whereas the untreated control remained unchanged.
It was noted that for coconut oil, the moisture-retention effects dissipated significantly over time. This is credible evidence that coconut oil absorbs into the hair shaft while mineral oil remains on the exterior surface. (Remember this—it will be important later).
All of the treated hair fibers showed a reduction in absorption of moisture vapor from the atmosphere in damp conditions. This is advantageous in an anti-humectant topical treatment, as it provides some protection from frizz that often occurs in high relative humidity. However, this effect was not total, and each sample was found to absorb significant amounts of water over time. Extremely hydrophobic mineral oil performed the best in terms of its ability to seal water out of hair, while the more polar fatty acids such as coconut oil allowed greater transport of moisture through the cuticle and into the hair shaft. This can certainly be an undesirable attribute if frizz and the tell-tale halo are not qualities you prefer in your hair.
Curl Formation and Clumping
Both coconut oil and mineral oil enhance clumping of adjacent hair strands. This mechanism aids in curl formation, definition of curl pattern, and curl retention. Capillary adhesion, the mechanism by which this is possible, occurs when oils form sufficiently thick films on the surfaces of hair strands and capillary forces between adjacent hairs attract them to one another, effectively binding them into clumps.
Researchers found that capillary adhesion between hair fibers remains constant with mineral oil, but is found to decrease over time with coconut oil, olive oil, and sunflower oil. The reason for this is that the very non-polar mineral oil molecules remain on the surface of the cuticle of the hair. In contrast, the saturated or mono-unsaturated fruit and vegetable oils in this study slowly penetrate into the cell membrane complex (CMC) and are transported into the hair shaft. As this diffusion occurs, the film thickness on the surface of the hair gradually decreases, which diminishes capillary forces. As a result the cuticle scale structure begins to dominate the behavior of the surface of the hair once more, and subsequent tangling and frizz can occur.
A Closer Look at the Penetration Behaviors of Mineral Oil and Coconut Oil
As you have probably ascertained from this article, many of the behaviors and performance of oils and moisturizers on the hair are affected by whether they remain on the surface or are absorbed into the hair. To get a more quantitative understanding of this, scientists performed direct study of the penetration behaviors of coconut oil and mineral oil on hair via spectrometry (secondary ion mass spec (SIMS) plus time-of-flight mass spec (TOF-MS). The results showed definitively that coconut oil does indeed penetrate the hair shaft, while mineral oil remains on the surface of the air.
Both mineral oil and coconut oil have pretty compact structures which should physically permit diffusion through the porous external layer of the hair shaft. So why does coconut oil do so, while mineral oil does not?
The answer lies in the atoms. While the chemical structure of the molecules present in mineral oil is purely carbon and hydrogen, rendering them very non-polar, triglycerides such as those found in plant-derived oils contain carboxylic acid groups, which lend a little polarity to the molecules. This polarity confers an affinity to these oils for other polar molecules, such as the various keratinous proteins of which hair is comprised. Thus, it is this inherent attraction to other polar molecules, coupled with the relatively simple structure of coconut oil that enables it to diffuse through the cell membrane cortex of the hair and penetrate into the central cortex. Mineral oil has no such affinity for proteins, and remains on the more hydrophobic exterior surface of the hair.
Coconut oil and improved resistance to wash-wear
The presence of coconut oil inside the cortex of hair provides multiple benefits. It acts as a plasticizer to soften the hair and provide more flexibility and toughness. Coconut oil also increases retention of keratin molecules within the hair shaft, which reduces protein erosion that normally occurs during wash cycles. Continuous loss of protein over time from routine washing damages hair and can result in color fading, split ends, and breakage, so anything that can moderate this phenomenon is beneficial.
An additional advantage to coconut oil inside the hair shaft is that it decreases the amount of swelling of the hair shaft that normally occurs when immersed in water. Normally, when hair is saturated with water during the washing process, it absorbs up to 30% or more of its weight in water. This causes each strand to swell considerably, which can lead to several undesirable effects. Increasing the diameter of the hair shaft causes the outer covering of cuticle scales to lift and separate, which increases tangling and breakage. But, perhaps more subtle, is the damage done over time from many cycles of expansion and contraction.
Hair is a highly complex biomaterial composed of layers of differing materials, ranging from varying types of keratin structures to pigment molecules to fatty acids. When it is saturated with water and swells and then subsequently dries via natural or thermal means, it undergoes what is known as differential drying and differential deformation (because each separate type of molecule within the overall structure dries and deforms at differing rates). This leads to moisture-induced stress on the hair, which can lead to delamination (cuticle layer stripping off), breakage, fiber fatigue, and rupture (split ends). This whole phenomenon is referred to as hygral fatigue. So, anything that reduces hygral fatigue is great for the health of your hair in the long term.
Which is the winner?
Well, both water insoluble oils have some distinct advantages for curly hair. By improving moisture retention within the hair shaft, they each can minimize drying and frizz which may occur in arid climates. Both enhance curl formation and clumping. However, in both of these things, mineral oil does the job better and for a longer time. On the other hand, coconut oil appears to have some real potential for improving the health and long-term vitality of hair, especially when it comes to wash-wear, whereas mineral oil is more of a topical treatment that is effective until it is washed away. Those with very porous hair may find that coconut oil penetrates too much into the interior of the hair, which can cause its own set of problems such as frizz, greasiness, and limp hair. So, adding either coconut oil or mineral oil to your hair care regimen may prove to be beneficial, but proceed with caution and see what works best for your own locks.