A variety of shea butters.
Butters, oils, and waxes all come from fats that are derived from plants or animals, and have two basic components; fatty acids and alcohols. The difference between butters and oils is primarily whether or not they are solid at room and/or body temperature. Although they are both composed of groups of fatty acids, there are differences in the molecular composition and structure of butters and oils that are responsible for these differences in melting points.
Factors that determine melting point of lipids
- Molecular weight - lower-molecular-weight fatty acids have a lower melting point, so that they are liquid at room temperature or body temperature. Higher-molecular-weight fatty acids form crystalline structures that persist to higher melting points, and so they are usually solids at room temperature and higher.
- Saturated molecular structure — longer-chain fatty acids without any double bonds are straight chain molecules (like long snakes) that are able to closely pack next to one another This close-packing induces crystallization, which requires more energy to break apart than molecules not packed together into a crystalline or semi-crystalline structure. For this reason, the melting points of these types of fatty acids are much higher. This means the "oil" will exist in a solid state at room temperature or even body temperature.
- Unsaturated molecular structure — unsaturated molecules have at least one double bond somewhere in their structure. This creates a kink or branching effect in the geometry of the molecule. This prevents unsaturated fatty acids from getting too close to one another, thereby preventing crystallization. These molecules have lots of space between themselves, which allows for more mobility of the molecules and results in a lower melting temperature. These oils may be liquid at room temperature or melt upon contact with skin.
- Stearic acid, a saturated hydrocarbon molecule with 18 carbons (relatively long-chain fatty acid) has a melting point of 69.6°C (157.28°F). Oleic acid, a monounsaturated hydrocarbon molecule, has a double bond in it that creates a kink in its geometry, which makes it more difficult for adjacent molecules to pack tightly next to one another. It has a melting point of 10.5°C (50.9°F). Polyunsaturated acids, such as linoleic and linolenic, have multiple kinks in their chains and are liquid at very low temperatures (melt point = -5°C (23°F) for linoleic acid).
- Linolenic acid, polyunsaturated fatty acid.
- Linoleic acid, polyunsaturated, omega-6 fatty acid.
- Oleic acid, monounsaturated fatty acid.
- Stearic acid, saturated fatty acid.
Clearing up some misconceptions
Coconut butter, avocado, almond butter and peanut butter are not actually butters, in terms of the nomenclature we are discussing. In these products, the flesh of the nut or fruit is pressed and included with the oil, which provides the food source with proteins and water, as well as fatty acids. This is not a butter in the technical sense THEN WHAT IS IT? WHAT IS THE DEFINITION OF BUTTER?, but this is a common usage in food products. Another point to keep in mind is that in some cases, the unsaturated fatty acids in the oils of these fruits and nuts are hydrogenated to create a more solid texture. This can change the properties of the product significantly.
Another bit of confusion on this topic of oils, butters, and waxes is due to misleading terminology in the nomenclature system. It is not uncommon to read assertions that emulsifying is waxy or oily and prone to build-up. In fact, emulsifying wax is not a wax at all, nor is it an oil. It is a group of ingredients (derived from fatty acids ) used as a nonionic surfactant mixture that is highly effective at facilitating mixing of oils and waxes into aqueous solutions. Specifically, it is most often these ingredients: Cetearyl Alcohol, Polysorbate 60, PEG-150 Stearate & Steareth-20. All of those components are water soluble, with the exception of the fatty alcohol. It is possible that people who dislike products containing this ingredient are actually sensitive to the oil or butter being emulsified by emulsifying wax, or they are sensitive to buildup of fatty alcohols (cetearyl alcohol) on their hair.
Fatty acid composition of common vegetable butters and oils
|Vegetable||Butter or Oil||Fatty Acids|
Depending upon where it is grown and the environmental conditions of that year, as well as soil quality, shea butter can vary significantly in its ratio of stearic acid to oleic acid. This will affect its melting point, and thus, its softness. If there is a lot more oleic acid relative to stearic acid in a particular batch, it will be a much softer and oily product, and will behave somewhat differently on the hair. Processing (methods of extraction, filtering, use of heat, hydrogenation) can also drastically affect the unsaturated oil composition, so if purchasing shea butter itself, carefully read the label so you are aware of the quality of butter you are getting. A pure shea butter contains no emulsifiers or perfumes, but is purely the mixture of fatty acids that were extracted from the fruit.
Incorporation of shea butter into a conditioning product involves melting it and dissolving it into an emulsifier and then mixing that into the product. Although the butter is melted and mixed into a liquid, its mixture of fatty acids should remain intact (unless high heat was used, which is not typical). Therefore, it is still the same "butter", simply because the term butter is not actually very meaningful. The inherent molecular structure is unchanged.
It is also interesting to note that while coconut oil is comprised almost entirely of saturated fatty acids, it is still referred to as an oil, rather than a butter. This is due to the lower molecular weight of the major fatty acids in coconut oil, which give it a lower melting point; typically right around room temperature. This is another clue that the terms butter and oil are not always very precise or meaningful. For this reason, it is a good idea to look at the fatty acid content of a particular butter or oil you would like to try and to understand what sort of performance you might expect based upon its chemistry, rather than what it is called.
Which is best for our hair?
So, we have established that all of these butters and oils are made up of different mixtures fatty acid molecules. What specifically accounts for the varying preferences expressed by consumers? Some curly-haired people extol the virtues of butters, while others adamantly proclaim oils as their holy grail ingredient.
The major components of butters and coconut oil are one or more saturated fatty acids, while the major components of most oils are a mixture of mono- and polyunsaturated fatty acids. One must assume that the differences in performance when used as emollients for the hair are a direct result of these differences in molecular structure. This is exactly the reason, and the science is fairly straight forward.
You will recall recent discussions we have had regarding the nature of the cuticle layer of the hair. Pores in the cuticle layer (whether from damage or from its being slightly open due to being wet) allow passage of some molecules into the cell membrane complex layer that is just beneath the cuticle scales. The fatty acids in this lipid layer act as a diffusion port that allows some fatty acids to penetrate the hair shaft. However, due to molecular structure and geometry, not all fatty acids are created equal in their ability to diffuse into the hair.
Generally, molecules with a straight chain geometry (saturated fatty acids), such as stearic acid, lauric acid, and palmitic acid can easily fit through the pores of the cuticle layer and slither through the CMC and into the interior of the cortex. Recent spectroscopic studies have allowed scientists to confirm that monounsaturated fatty acids are also able to readily penetrate the interior of the hair via this route. However, polyunsaturated fatty acids seem unable to penetrate into the interior of the hair at all, and remain either adsorbed onto the exterior surface of the hair or may get wedged into the cuticle layer.
Fatty acids in the interior of the hair can provide brittle hair with much-needed suppleness and elasticity. However, porosity is a very important factor to consider when using easily absorbed oils and butters. If one has very porous hair, it can absorb excessive quantities of these oils, which can lead to a host of problems. Among these could be greasy feel, dull appearance, limp hair, a swollen and open cuticle, frizz, and tangling. It can be very difficult to remove excess absorbed oils, in my experience, requiring the use of harsh surfactants, which strip the hair of its own lipids.
Oils high in polyunsaturated fatty acids may provide ease of wet combing and prevent static build up and fly-away hair. In addition, they form a barrier film on the surface of the hair, preventing moisture from escaping the interior of the hair. However, they might contribute to hair that feels greasy or sticky to the touch. Oils on the surface of the hair can also attract dirt to your hair. Another potential concern is that if these types of oils are indeed wedged into the cuticle layer at all, hair becomes vulnerable to the dangers of having a raised or rough cuticle.
My recommendation is to try small quantities of different oils and butters and carefully observe your results and preferences. Then, when you find things you really like or dislike, look into the fatty acid composition of the things you tried. This will help you determine if your hair needs saturated, monounsaturated, or polyunsaturated fatty acids, and you can then choose your products accordingly, armed with your knowledge of the underlying chemistry!