Why Are Some Oils Solid and Some Oils Liquid

May 22, 2019

Why Are Some Oils Solid and Some Oils Liquid?

Solid Oils Vs Liquid Oils

When deciding on a new skincare formula, it is essential to consider how your product will be used. That will determine your packaging since you’ll need to choose between different containers. Also, consider how the skin care solution should be applied and in what amounts. An essential consideration is the desired density of your product.

Using oils in cosmetics

The application of oils as an emollient and for other cosmetic purposes started with vegetable oil and animal fats. They were used to smooth and sooth skin and hair. Fish oils were avoided due to the odor. With the development of the cosmetic industry came modifications of natural fats and oils for use in beauty products. The largest unmodified application of an oil today is the use of castor oil as a base for lipsticks.

You are most likely going to use a plant-based oil in your skincare product and will need to determine which oil it will be. Since oils have different consistencies, the density of your skin care formula will change. Oils are either in liquid or solid form at room temperature, and you might need to change out a favored oil or update your skin care formula with a different ingredient than you thought you’d use originally.

The molecular make-up of oils

The different consistencies of oils at set temperatures have to do with their molecular make-up. Time for a bit of organic chemistry. The study of carbon-containing compounds, such as oils, and their structure, properties, composition, and reactions.

The main constituent of oils are triglycerides, which are long strings of carbon, hydrogen, and oxygen with three fatty acids attached. All oils are a combination of saturated, monounsaturated and polyunsaturated fatty acids. This is what determines the density of the oil.

The more unsaturated fats in an oil, the more liquid it appears to be. The double bonds cause “kinks” or bends in the unsaturated fats preventing the molecules from stacking close together.  Therefore, an oil with mostly unsaturated fats such as olive oil with a whopping 73% is in liquid form at room temperature. Coconut oil, on the other hand, is solid at room temperature. From this, we can correctly deduce that there are more saturated fatty acids in coconut oil. Coconut oil is made up of 92% saturated fat.

Hydrogenated oils

The process of hydrogenation has enabled us to create more stable oils, which are very useful in cosmetics. Hydrogenated oils are unsaturated fatty acids to which hydrogen is added, turning a liquid fat into a solid. In culinary application, these are often referred to as trans fats and are used to increase the shelf life of products. As you might have heard, consuming large amounts of trans fats is very unhealthy and could lead to several health issues. However, in the cosmetic application hydrogenated oils offer a wider range of properties and does not carry the same risks.

The popularity of natural oils in skin care formulation

Recently, the use of natural or “raw” oils has been growing in popularity. As mentioned before, the most common use of an unmodified or natural oil is that of castor oil as a base for lipstick. There are also other examples such as the growing popularity of Argan oil, which is sometimes described as having a “gel meets oil” texture.

Without modification, oils can become rancid. It is therefore important to pay close attention to the iodine and peroxide value in raw materials. Raw materials have a Certificate of Analysis (COA) which ensures that the product adheres to certain specifications, your assurance that the product has not degraded. Iodine value indicates the amount of unsaturation in oils (or the number of double bonds). As we’ve seen above, the higher the number of double bonds (or iodine) the more liquid the oil will be. The peroxide value has to do with the amount of oxidation that occurs during the heating and storage of the oil. Too much oxidation can cause bad odors and decomposition.

It is essential to pay close attention to the COA when selecting the ingredients for your skin care formulation. It will also help to determine the shelf life of your product. Knowing a bit more about oils will empower you to make an educated decision as to what should be included in your skin care formula.

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