credential_live_c4016700-a6b5-4104-8e07-a6a4b97e5cb9 Understanding Polarity in Choosing Cosmetic Lipids/Oils
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Understanding Polarity in Choosing Cosmetic Lipids/Oils

The lipid category of ingredients includes oils, butters, waxes, silicones, fatty acids, solvents, and other related substances. Lipids are essential components in formulations due to their wide range of beneficial properties. They protect the skin barrier, prevent water loss, provide hydration and moisturization, and improve skin conditioning. These substances also offer sensory advantages by enhancing product texture, reducing greasiness and tackiness, improving spreadability, and enhancing the overall feel and appearance. Additionally, some lipids serve as texture modifiers, stabilizers, and absorption enhancers, while others provide antioxidant benefits.

 

Selecting which lipids to use in a formulation is not a random process. Careful consideration is necessary because the lipid composition can greatly influence the physical and chemical stability of formulations. Considerations such as temperature, pH, emulsifiers, and lipid compatibility are important. Incompatible lipids can result in phase separation and unstable formulations. A common cause of lipid incompatibility is polarity; mixing oils with opposing polarities can cause them to separate (Genesis Formulab).


Lipid Polarity


Polarity refers to the tendency of compounds or substances to be attracted to or repelled by other compounds. Lipid polarity is understood based on the electronegativity of bonds between atoms in lipid molecules. When bonds have different levels of electronegativity, molecules have what is called a partial charge. Polar compounds are attracted to other compounds to achieve balance. When the relative electronegativity between atoms is similar, the molecules have no charge and are classified as non-polar. A third category of lipid, called amphiphilic, exhibits both polar and non-polar characteristics.

 

Lipids exhibit varying degrees of polarity and are defined on a spectrum ranging from polar to non-polar. The degree of lipid polarity is given by an index. Two common measures of polarity are the dielectric constant, which classifies lipids according to their polarity, and the polarity index (PI), which is a measure of interfacial tension.


Dielectric Constant

The dielectric constant measures the permittivity of oils at a reference temperature, usually room temperature. It is calculated by determining the differences in polarization within molecules. A higher dielectric constant indicates higher polarity, while zero indicates non-polarity. Table 1 shows the dielectric constants of different lipids.



Interfacial Tension Based Polarity Index

The other measure, the polarity index, indicates the interfacial tension between oil and water phases. Measured in milli-Newtons per meter, it quantifies the energy required to combine water and oil phase components. The interfacial tension-based PI signifies the difficulty in emulsifying an oil; a higher PI indicates a greater difficulty in emulsification. According to the Rahm Group, certain polar oils may exhibit high interfacial tension. Table 2 , below lists the PI values for different commonly used oils, as found in the literature.



Polar Oils

Polar lipids are lipids whose molecules have atoms exhibiting different degrees of electronegativity. This category of lipids includes glycols, glycerol, esters, solvents, and some refined oils. Although it is common to categorize lipids as either polar or non-polar, lipids exist in varying degrees of polarity. Lipids that exhibit medium to high polarity are often classified as polar. Polar lipids contribute to skin hydration, moisturization, and can have anti-aging properties. For instance, polar vegetable oils are often loaded with skin-beneficial compounds, including antioxidants, in addition to their emollient properties. They also have the added benefit of deriving from renewable resources. However, they are prone to oxidation and rancidity. Advances in technology have developed multifunctional esters that have properties similar to natural vegetable oils but are temperature and heat stable, and have a long shelf life. Caprylic/Capric Triglyceride is one such ester; it is multifunctional and has the added benefit of sensory enhancement.


Amphiphilic Lipids

Amphiphilic lipids have both polar and non-polar components, allowing them to mix with both water and oil. These lipids include natural oils such as lecithin and lanolin, as well as petrol-chemical derived Polyethylene Glycols (PEGs).

 

In addition to their emollient properties, lecithin functions as skin conditioning, wetting, and adsorption enhancing agents. Lanolin, derived from wool-bearing sheep, is an eco-friendly and non-toxic emollient with water-binding and skin-soothing properties. While lanolin is widely used in personal care products, it can cause skin irritation.

 

Polyethylene Glycol (PEG) or ethoxylated lipids are another example of amphiphilic lipids. Ethoxylated lipids are lipids that have undergone a chemical process to make them more water-soluble.  Common examples include ethoxylated Lanolin and PEG-40 Hydrogenated Castor Oil, PEGs are multifunctional, highly stable, biodegradable, and non-toxic. They are thermal and pH stable and compatible with most ingredients. PEGs are non-comedogenic emollients with humectant, emulsifying, and solubilizing properties. They also enhance the effectiveness of active ingredients.


Non-Polar Lipids


In non-polar lipids, the bonds between atoms have similar levels of electronegativity. Unlike polar oils, they have a balanced charge and are considered neutral. Non-polar lipids include petroleum-based hydrocarbons, silicones, waxes, esters, and most oils derived from natural sources. This category of lipids includes both natural and synthetic substances with a wide range of skin-beneficial properties. Among their common benefits is their ability to keep the skin moisturized and hydrated by preventing trans-dermal water loss.


Hydrocarbons


Hydrocarbons do not contain oxygen atoms. Lipids in this category are sourced from natural origins like petroleum, plants, or marine life. Oils derived from petroleum, such as mineral oils, petrolatum, and liquid paraffin, are excellent occlusives and emollients. They create a barrier on the skin that prevents moisture loss and keeps the skin well-hydrated. These oils do not enter the skin, so they do not cause any physiological changes. Additionally, they are non-reactive (do not trigger allergic reactions), non-comedogenic, and moisturizing. Structurally, they are very stable, resistant to oxidation, and have a long shelf life. Compared to other oils, petroleum-based lipids are more cost-effective to produce. They are generally safe as long as the manufacturing process meets international standards. However, because they are primarily derived from crude oil, a non-renewable resource, they may not be the top choice for environmentally conscious formulators.


Silicones

Silicones are a class of synthetic polymers, with over 600 varieties commonly used in skincare. The most prevalent type is Dimethicones, which are classified as non-polar. Dimethicones possess a unique blend of physical and sensory properties that can improve formulations. They help create products that feel silky, non-greasy, and easy to spread, providing a luxurious sensation. Although they are not moisturizers themselves, Dimethicones give the skin a moisturized appearance. Additionally, they form a protective barrier on the skin and exhibit excellent water and sebum repellent properties.


Waxes

Waxes are a sub-category of hydrocarbons also derived from renewable natural sources, making them an eco-friendly option for product formulations. Unlike petroleum-based hydrocarbons, waxes are rich in nutrients such as antioxidants that are easily absorbed by the skin. They serve various functions in formulations, including as emollients, occlusives, stabilizers, and texture enhancers. When combined with borax, waxes can also emulsify products.

 

Waxes are commonly used to alter, thicken, or harden the consistency of formulations. For example, in lipsticks, waxes provide luster and durability, improve color dispersion, and enhance spreadability (Nong, 2023).  They are generally non-comedogenic and compatible with most ingredients. Due to their high melting points, waxes are not suitable for cold-processable formulations.


Esters


Innovation in skincare technologies has led to the development of non-polar esters that are multifunctional, eco-friendly, stable, and compatible with a wide range of ingredients. These esters include fatty alcohols, acids, and other solvents. Unlike petroleum-based lipids and vegetable oils, esters serve as stabilizers, absorption enhancers, degreasers, foam boosters, solvents, and sensory enhancers. Non-polar esters encompass both synthetic and nature-identical lipids. A commonly used synthetic alcohol, Isopropyl Myristate (IPM), reduces the greasiness of formulations with high oil content and enhances the effectiveness of active ingredients. High-performing esters like Dicaprulyl Carbonate and Dipentaerythrityl hexacaprylate/hexacaprate improve the sensory properties of products by creating luxurious, velvety textures while reducing greasiness. Additionally, Dicapryllyl Carbonate aids in dissolving hard-to-solubilize substances. Common fatty acids such as Cetyl alcohol act as foam boosters, thickeners, and stabilizers.


Natural Plant-derived and Seed Oils


Some vegetable oils are non-polar due to the presence of long-chain hydrocarbons or triglycerides. They are derived from natural and renewable sources such as plant and fruit seeds. These emollients contain skin-beneficial antioxidants and other skin conditioning compounds. Common non-polar vegetable oils include coconut oil, known for its hydrating and moisturizing properties, Argan oil, rich in skin-identical fats and great for hair care, Jojoba butter, which creates luxurious formulations, and sesame oil, packed with antioxidants and providing gloss and spreadability to products. Unlike petroleum-based lipids, they can have physiological effects on the skin by penetrating it. However, they are susceptible to oxidation, rancidity, and have a shorter shelf life.



References

Alvarez A and  Rodríguez ML (2000). Lipids in pharmaceutical and cosmetic preparations. Grasas Aceites 51:74–96. https://doi.org/10.3989/gya.2000.v51.i1-2.409

 

Behrens, S., ; Bleckmann, A.,  Clausen, A; Uta, M,  & Nielsen, J.  (2007) . Emulsion for treating e.g. skin aging comprises a lipid phase of polar and/or moderately polar, having specified interfacial tension toward water; and at least one 1,2-alkanediol having specified 1-octanol/water distribution coefficient. DE 10 2005 051 862 A1

 

Endress and Hauser. Dielectric Constant (DC value) Compendium. https://portal.endress.com/wa001/dla/5000894/4733/000/00/CP01076F00EN0114.pdf

 

 

Genesis Formu-lab.  Advanced Formulation with Lipids. https://genesisformulab.com/wp-content/uploads/2023/10/Lipids-Emollients.pdf


Mendrock-Edinger, C. (2020). Cosmetic composition comprising specific hyperbranched copolymers and oils . https://patents.google.com/patent/WO2020109262A1/en

 

Nong, Y et al. (2023). A review of the use of beeswax in skincare. Journal of Cosmetic Dermatology.

 

Traversier , M  et. Al (2018). Polar lipids in cosmetics: recent trends in extraction, separation, analysis and main applications. Springer Nature.

 

 





 
 
 
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