John Woodruff looks at the role of oils, fats and waxes in cosmetic formulation and suggests useful sources for quantifying their performance properties
Oils, fats and waxes are fundamental to cosmetic products. Without them there would be neither emulsions, lipsticks nor most other cosmetic product forms. So important are they to the cosmetics industry that in a publication about natural oils, fats, butters and waxes in cosmetics they are referred to as primary ingredients.[1] The authors classify the ingredients into animal-derived and plant-derived and according to carbon chain length and whether saturated or unsaturated.
Natural oils may also be characterised by their physical properties. They may be volatile and aromatic and used in perfumery and aromatherapy or non-volatile like the majority of vegetable seed oils. They may have a light or dry skin feel or be heavy and greasy to the touch. All these properties need to be taken into account when selecting oils for a particular purpose. Butters, oils and fats are all triglycerides and are defined by their re-solidification point. Thus oils remain liquid below 20°C, butters between 20°C and 41.5°C and melted fats solidify above 41.5°C. Waxes are defined as esters of long-chain fatty acids with long-chain fatty alcohols, and with the exception of jojoba oil, are solid at ambient temperature.
The 10th Edition of the INCI Dictionary lists in excess of 150 seed and kernel oils and with such a bewildering array to choose from selection may depend on whether it can yield a nice name to go on the label and if it has a history of cosmetic or therapeutic use in folklore. Skin feel and odour are important aspects affecting choice. Price and availability also play a part and the formulator will want to know its fatty acid profile and its chemical and physical properties.
Various publications and websites are available to assist in selecting oily components according to their sensorial properties.[2] Cognis, Degussa, Croda and other major suppliers publish guides to emollient selection, which include esters and silicone oils as well as triglycerides and hydrocarbon oils. Main headings are pick-up/rub-out/after-feel, with sub headings such as firmness/cohesiveness/greasiness/spreadability/gloss included. Degussa attempts to combine the properties of pour point, polarity, ease of spread and surface tension to the properties of the oils and esters described. Cognis meanwhile promotes its cascading theory of emollients whereby a sensorial feel is sought throughout the application of the product.
Floratech in the US undertook a quantitative comparison of 20 common emollients including vegetable oils, esters and jojoba oils and derivatives. The study measured selected attributes of each emollient and then used the numerical results to generate 20 graphs. Spread, slip, occlusive properties, viscosity, oxidative stability, fragrance fixative, moisturising and refractive index are graphed against penetration, refractive index, moisturising, fragrance fixative, oxidative stability, viscosity, occlusive properties and slip, with some very interesting results.
Oils in analysis
The following are the traditional analytical processes used to deter-mine the properties of natural oils; the term ‘value’ is interchangeable with the term ‘number’ and either may be used.
- Acid value is the milligrams of potassium hydroxide required to neutralise the free carboxyl groups in 1g of sample, usually expressed as mg/KOH per g. The acid value is determined in the cold and measures the free fatty acid content.
- Saponification value is the number of milligrams of potassium hydroxide required to react completely with all the reactive groups in lg of sample; also expressed as mg/KOH per g. Saponification is determined hot under reflux in an alcohol solution of potassium hydroxide (KOH). The triglycerides are broken down into glycerine and free fatty acids, which are neutralised with potassium hydroxide and the amount of KOH consumed per g of material is the saponification value. It is a particularly important figure when preparing soaps.
- Hydroxyl value is a measure of the degree of esterification. It is defined as the milligrams of potassium hydroxide required to neutralise the free hydroxyl groups in 1g of sample. The test is done by reacting the substance with a known amount of acetic anhydride in pyridine solution. Hydrolysis converts acetic anhydride to acetic acid and the excess of acetic acid after hydrolysis is determined. The number of mg of KOH used is the hydrolysis value expressed as mg/KOH per g. Hydroxyl groups increase the polarity of the oil and confer interesting properties.
- Iodine value indicates the number of double bonds present and therefore the degree of unsaturation. The higher the iodine value the more double bonds in the molecule and the lower its viscosity and solidification point. High iodine values generally mean that the oil may be prone to rancidity.
- Peroxide value is a measure of the oxidation processes that have occurred in an oil, fat or wax and is an indication of rancidity. Rancidity or peroxidiation is to be avoided as it usually results in malodours and unwanted by-products. The addition of BHT, BHA or tocopheryl and other antioxidants helps prevent this problem. Many oils have a natural tocopherol content that renders them stable against oxidation.
- Anisidine value is a colorimetric reaction that detects substances such as aldehydes and ketones, resulting from further oxidation of the oil.
- Viscosity is related to the carbon chain length and therefore the molecular weight of the oil and generally the longer the carbon chain the more viscous and oily the oil will feel.
- Solidification point is also known as titre and is determined by melting butters, fats and waxes and then recording the temperature at which they solidify. It is more accurate than melting point for the complex mixtures that comprise most of these natural materials.
Peroxide value and acid value are the most useful primary indicators as to the quality of an oil in terms of refinement and freshness since both will give an early indication of the level of reactive fatty acids and breakdown products. Thereafter the Anisidine value, which measures secondary oxidation compounds, gives a better relative measure of an oil’s oxidative state. It can help determine how old an oil is, how well or badly it has been stored and processed and how the shelf-life may be affected.
Other characteristics like refractive index, flash points etc may also be determined and modern analysis relies on gas chromatography (GC), high performance liquid chromatography (HPLC), and spectroscopic determinations. These are of particular relevance when purchasing essential oils and some of the other more expensive fixed oils, which may be adulterated by unscrupulous traders. For more information about the analysis of oils, fats and waxes see the IFSCC publication on the subject.[3]
Natural oils may be from certified organic sources, in which case they will also be cold-pressed and have undergone the minimum of processing. However, these are expensive and are also likely to be more coloured and of a stronger odour than refined oils, which will also be less likely to cloud in cold conditions. Further treatment may be undertaken to optimise the inherent properties of the oil in order to obtain greater functionality and stability and some suppliers are now blending oils to enhance certain properties.
Formulation favourites
Cocoa butter has a long-standing tradition as an appreciated emollient and moisturiser for cosmetics and personal care. Due to its inherent complex crystallisation properties, however, its applications have mainly been limited to low contents in emulsions and anhydrous systems. Problems include grainy emulsion structures or greyish formations and bloom on the surface of finished products. AarhusKarlshamn (AAK), Sweden, claims that Lipex Cocoasoft overcomes these problems. It is cocoa butter that has undergone a rapid crystallisation process resulting in a material with an extended melting point. It is claimed that the stable crystal form facilitates the formation of homogenous and stable formulations. The good temperature stability offers robust systems less affected by temperature fluctuations. At room temperature, Lipex Cocoasoft adds structure and stable consistency to the formulation. When applied on the skin it melts smoothly and results in a nice soft after-feel.
A similar problem exists when incorporating Butyrospermum parkii (shea butter) in formulations and Lipex Shea is designed to overcome them. The crystallisation stability of Lipex Shea is superior to that of traditional shea butter, the latter having a triglyceride composition that requires tempering to achieve a stable crystal form and thus also a cosmetic product with a long and stable shelf-life. Lipex Shea is a white, odourless and solid lipid made from a vegetable fat derived from the fruit of the shea tree (Butyrosperum parkii). It is rich in unsaponifiable lipids that help protect the skin, is solid at room temperature and has a creamy texture and superior heat stability.
AAK also modifies natural oils to optimise their properties. The uniqueness of shea butter comes from the high content of unsaponifiable lipids, especially the dominating triterpenes. By means of patented technology the typical triterpenes, amyrins, lupeol and butyrospermol known for anti-inflammatory activities have been enriched in Lipex Shea-U.
Textron is a supplier that provides a number of virgin and organic oils, a more extensive range of refined oils and a further list under the heading of Evoil Basics, which have been developed to replicate the classical fatty acid composition of each specific plant or fruit oil without sacrificing any of the intrinsic beneficial qualities of the oil. Textron also supplies combinations of various oils and oil components designed for specific purposes under the heading of Evoil Actives. There are suggestions for face treatment products, anti-wrinkle creams, body care and baby care as well as for hair care and post-natal creams.
Replicating natural materials using other natural materials is a technique used by Carrubba Inc to provide natural fragrances that have the odour of essential oils but which are stable and less likely to disappear in a few months than would the original oil. They are made exclusively from natural aromatics as defined by IFRA and all the ingredients are physically obtained from plants without altering their chemical structure. France’s Laboratoires Expanscience offers Prunus domestica seed oil, cold pressed from the kernels of Entes plum almonds. The oil is said to be wonderfully perfumed with a top note of bitter almonds over a complex fruity fragrance. The oil is readily absorbed and has a dry, silky skin feel and its natural tocopherol content ensures good oxidative stability.
Blending for improved properties is also undertaken by Bertin SA of France, which seeks to combine the skin conditioning properties of one oil with the oxidative stability of another. For example, macadamia nut oil is blended with kiwi seed oil and apricot kernel oil is blended with echium seed oil. The blends result in oils with a high linolenic and linoleic acid content that are also resistant to oxidation. Bertin also supplies virgin oils, refined oils and macerates of leaves and flowers in virgin oils and provides essential waxes from flowers that are claimed to be waterproof and to extend the shelf-life of active ingredients in cosmetics.
Many oils can be grouped together according to their fatty acid profile and generally members of each group will all function in a similar way, with very little difference in efficacy. However, some plant lipids do have unique and interesting profiles and may be very rich in a particular fatty acid or may have more active fatty acids such as alpha-linoleic acid and stearidonic acid. Omega-3 fatty acids are a family of polyunsaturated fatty acids which have in common a carbon-carbon double bond in the omega-3 position. Three major omega-3 fatty acids are ingested through foods and used by the body: alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The body converts ALA to omega-3s including EPA, and to a smaller extent DHA. However, the conversion of ALA to EPA and then to DHA is very inefficient, and only a small percentage of ALA consumed is made into the beneficial omega-3s. An omega-3 fatty acid called stearidonic acid (SDA) is more efficiently converted by the body to EPA because it bypasses a step in the conversion.
Building on its knowledge of nutritional oils, the UK’s Northstar Lipids has continued to develop its product line of oils rich in omega-3 and omega-6 fatty acids for cosmetic use. It has recently launched two new fruit seed oils. Actinidia deliciosa (Kiwi) seed oil is a rich source of ALA and studies have shown that it affords excellent emolliency and skin after-feel combined with relatively quick absorption. Citrus aurantifolia (lime) seed oil has a broader, more balanced profile and combines the functions of a basic emollient with the properties desirable from a more protective skin conditioner and is ideal for body butters and dry skin creams. The Northstar website (www.northstarlipids.co.uk) includes much useful information about natural oils and their uses in nutrition and skin care. Northstar maintains that the skin is one of the largest parts of the body for absorbing nutrients so the application of oils to the skin is another way of providing the body with some of the key elements of essential nutrition.
Research carried out by Northstar shows that oils rich in omega-9 oleic acid are absorbed through the skin barrier more quickly than the more unsaturated oils, making them good natural emollients. In addition they are very useful carriers or base oils as they aid the transfer of other actives and skin nutrients into the lower skin layers. Oils rich in omega-3 and 6 fatty acids are absorbed more slowly and can be considered to be more protective. More crucially they have a key role in the tissue regeneration process and in the control of the natural inflammatory response. Northstar has created a blend of oils, NorOmega PC369, described as a 100% naturally derived lipid formulation containing all the essential nutrient fatty acids in a ratio that favours good skin nutrition and absorbance whilst maintaining good product stability and odour.
