Choosing the Right Plant Oil for Hair and Skin Benefits

Civilizations for centuries have used plant oils to enhance their beauty. In ancient East Asia, seeds from the Camellia oleifera plant were crushed and pressed to release an oil that was a treasured commodity, especially in China. In Japan, geishas and their apprentices applied the camellia seed oil to maintain their skin’s perfection. Ancient Greeks and Egyptians both used olive oil for moisturizing skin and hair, and in India, the 5,000-year-old science of ayurveda used coconut oil in the ritual of hair oiling. All were harnessing the regenerative power of natural oils for hair, scalp and skin moisture and nourishment. Today, the same oils are used across the beauty industry. In 2022, one quarter of skin care launches and half of hair care launches in North America contained at least one plant oil or butter.¹ Figure 1 shows the top five oils in 2022 for both hair and skin launches in North America. 

This article details the natural sources of these oils, their wide variety of compositions and how these compositions impact the physical properties and stability of the oils. It also demonstrates how, with this understanding, product developers can choose the right oil to deliver the consumer’s desired benefit.

See related sidebar: Natural Oil Innovations

Definitions and Compositions

First, some definitions. A generic description of an oil is a nonpolar liquid composed primarily of hydrocarbons that is hydrophobic and lipophilic; i.e., mixes with other oils. A butter is similar but has a higher melting point (30-40°C), so is solid at room temperature but melts close to skin temperature. By contrast, a wax is a hydrophobic substance with a melting point above 40°C to more than 140°C. These materials are found in both animals and plants; this article will focus on the plant sources.

Oils and butters: Plant oils and butters are typically glycerol esters of fatty acids called triglycerides and most are low in saturated fats and high in saturated monounsaturated and polyunsaturated fats; i.e., each fatty acid chain has at least one or two double bonds (see Figure 2). The nomenclature used counts the fatty acid chain length number of carbons and the number of double bonds. For example, C12:0 is a fatty acid with 12 carbons and no double bonds and C18:1 is a fatty acid with 18 carbons and one double bond. The ω nomenclature notes the double bond position from the last carbon in the fatty acid chain.

These oils and butters are typically found in plant nuts or seeds and their purpose is to provide energy for the nut or seed to grow.² Each plant oil has a unique composition of triglycerides, with varying fatty acid chain lengths and degrees of saturation (see  Figure 2). Saturated fatty acids provide more energy to the growing plant than unsaturated fatty acids.³ Due to this energy difference, depending upon the location where the plant is grown, there will be variability in the types of triglycerides present in the oils.⁴

The triglycerides make up more than 95% of the oil composition but there may be a low level of di- and mono-glycerides (0-5%) as well as a low level of fatty acids (typically less than 1%) from triglyceride breakdown. As each plant is unique, so is the oil composition; and again, there is variation according to where the plant is grown. Plants that grow in colder temperatures have a higher proportion of unsaturated fatty acids, whereas plants that grow in warmer temperatures have a higher proportion of saturated fatty acids.⁵

Extraction methods: The typical method to extract oils and butters from nuts or seeds is via cold pressing, where the nuts and seeds are either crushed or ground and oil is forced out using pressure. The advantage of using low temperatures is that it protects the oil from oxidation. After extraction, the oils are often refined, which can involve the neutralization of free fatty acids, bleaching to remove low levels of pigments (e.g. chlorophyll, carotenoid) and deodorization to remove residual fatty acids and sterols.

Waxes: Plants also contain waxes that are found mainly on the plant leaves and in the outer layer of plant stems and fruit. These waxes are essential for barrier protection against environmental stress since they help protect the plant from water loss, UV penetration and attack from bacterial and fungal pathogens.⁶ There are a few plants that are harvested for their wax, including carnauba, candelilla, bayberry, sunflower and rice bran wax.⁷

The plant wax structures are generally more varied than the plant oils and butters. They can be saturated triglycerides or monoesters of long-chain acids (C12-C30) and alcohols (C20-30) and mixtures of these compounds.⁸ Due to the high melting point of these materials, they are not generally found in hair products but used for lipsticks, lip balms, lotion bars and some skin creams. An exception worth mentioning is jojoba oil produced in the seed of the Simmondsia chinensis (jojoba) plant, which contains monoesters similar to plant waxes but the C20-C22 fatty acids of the monoester are unsaturated, making jojoba oil a liquid. This liquid wax is commonly used in hair and skin products.

Formulation Consideration: Melting Point

With more than 100 plant oils and waxes available, how does the formulator know the best choice for the project at hand? The first point to consider is the oil melting point and thus whether the oil is a solid or liquid at room temperature. This property will be determined by the triglyceride composition. High levels of saturated triglycerides and fatty acid chain length are two important properties that increase melting point. This can be seen in Figure 3, which details the physical properties of pure triglycerides. Saturated fatty acids have higher melting points than unsaturated fatty acids because the molecules can align better to form stronger dispersion forces. Double bonds, especially cis- double bonds, cause the fatty acid chains to bend, making aligning the chains more difficult.

The majority of oils such as argan, almond, avocado, olive and sunflower oil contain mainly glycerol trioleate (C18:1) and glycerol trilinoleate (C18:2) and so, are liquid at room temperature. These oils would provide excellent lubrication and are often used in hair conditioning to provide smoothness and hair shine. Coconut oil has high levels of saturated short chain triglycerides (C8:0, C10:10, C12:O) and has a melting point of 24°C, so is a semi-solid at room temperature but will be a liquid at skin temperature. Cocoa butter and shea butter have higher levels of glycerol tripalmitate, so they melt in the 30-40°C range at around skin temperature. These butters are therefore often used where heavier moisturization benefits are needed; for example they are popular for moisturizing African hair.

Formulation Consideration: Viscosity

For liquid oils, the compositions also influence viscosity, which is a second important consideration. Oil viscosity will influence how the oil spreads and how heavy it feels;jojoba oil has one of the lowest viscosities (~40 cps at 1/s^- 1). Many of the C18:1 and C18:2 oils (e.g., argan, almond, camellia, avocado) have viscosities in the 60-70 cps range at 1/s^-1, which also makes them easy to spread and light weight.

A more viscous triglyceride oil is castor oil (~760 1/s^-1), which contains high levels of triglyceride ricinoleic acid fatty chains. This fatty acid has a hydroxyl group that increases intermolecular chain interactions and thus viscosity, making it more suitable for heavier moisturization. For skin, this viscosity will translate into ease of spreading and a moisturized skin feel.

The oil also acts to reduce moisture loss via an occlusion mechanism, reducing the appearance and feel of dry skin. For hair, this will translate into lower combing forces and a softer, smoother feel. How effective they are at lubrication will depend upon the level used, e.g., higher levels will provide better surface coverage, and how well they spread.

Formulation Consideration: Penetration

Besides the ability to spread over a surface, another important property of these oils is their ability to penetrate the substrate of interest. Coconut oil has been used for centuries by Indian women to deliver soft and healthy hair, and at least some of this benefit is driven by penetration into hair. This penetration was visualized by Kamath, et al., using Time of Flight (TOF) SIMS with cross-sections of hair treated with coconut oil⁹ (see Figure 4). The authors’ previous work has also shown that hydrophobic oils such as fatty acids and fatty alcohols will partition into the lipid-rich cell membrane complex (CMC)¹⁰ and it is assumed that plant oils would do the same.

Kamath, et al., also quantified coconut oil penetration into hair using radiolabeled (tritiated) oil and hexane to extract surface oil, which showed between 15% and 25% oil penetration depending on the hair type.¹¹ Longer chain length triglycerides including sunflower and olive oil have also been shown to penetrate into hair but to a lower level than coconut oil.¹²

The beneficial impact of these penetrating oils has been studied by several authors and has been recently shown to reduce porosity, reduce break stress and improve color retention.¹³ It is known that cell membrane lipids in hair are lost over time, with the fastest loss from unsaturated fatty acids such as oleic acid,¹⁴ and it is likely that these benefits are linked to replacement of these lost lipids in the CMC. The penetration of plant oils into skin has only been shown to reach the very upper layers¹⁵ and their moisturization benefits are mainly driven by occlusion properties to prevent water loss.

Formulation Consideration: Oxidative Stability

A critical property to consider based on the oil composition is oxidative stability. Double bonds in the fatty acid chains are susceptible to oxidation, forming lipid peroxides, as shown in Figure 5, which then decompose to form volatile and non-volatile aldehydes. As oxidation occurs, the oil can darken and smell rancid.

Antioxidants such as tocopherols are either found naturally in the oil or added by manufacturers to protect against this oxidation. Tocopherol acts as a radical scavenger by halting the radical chain reaction by donating a proton to the lipid radical and forming a stabilized radical. Oxidation rates significantly increase with more double bonds and also with temperature.

It is difficult to predict the oxidative stability of plant oils because antioxidant levels will vary according to the oil source and how it has been processed. However, in general, oils with mainly saturated fatty acid triglycerides such as coconut oil will be more than stable than oils with unsaturated fatty acid triglycerides. Higher levels of C18:2 and C18:3 fatty acid chains will have even higher susceptibility to oxidation. All oils have low levels of other plant components (typically less than 1% total), along with their triglycerides such as tocopherols, sterols and sterol esters and phospholipids that can act as antioxidants and protect the oil from oxidation. Additional antioxidants can also be added to ensure stability and typically most of the plant oils can be stabilized in cosmetic formulations as long as high temperatures are avoided.

Skin and Hair Benefits

Along with the main benefits of moisturization and protection against water loss, plant oils have been shown to have antimicrobial and anti-inflammatory benefits for skin.¹⁶ Jojoba oil, for example, has a history of use to treat scalp and skin disorders, and shea butter has been used to treat cuts and scrapes. The exact chemistry delivering these benefits is not always well understood but one mechanism is the ability of the oils to restore skin barrier function. This maintains moisture, which in turn prevents the entry of irritants, allergens and microbes.

However, compounds present in low concentrations in the oil can also be important. For example, shea butter has between 7% and 10% of unsaponifiables comprising triterpene esters that are thought to contribute to a reduction in the release of the intracellular proinflammatory mediator IL-1α cytokine.¹⁷ Coconut oil contains monolaurin, a monoester formed from glycerol and lauric acid, which has demonstrated antimicrobial properties,¹⁸ and oat oil has anti-inflammatory and anti-itch properties, possibly due to the minor phytochemicals avenanthramides.¹⁹

Conclusion

Plant oils have been used in cosmetic products for centuries for very good reasons. They deliver a wide range of benefits and by understanding their properties, the formulator can choose the right ones for the desired skin and hair benefits.

References

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