It is the moment of truth for a moisturizer. You scoop a small amount out of the pot and rub it into your skin. How does it feel? Is it greasy or smooth? Does it spread well? Do you smell the fragrance?
If you love the experience you may have just met an oleogel.
Oleogels are absolutely fascinating materials. Exquisitely designed microstructure transforms a liquid oil into a gel which can stay solid for years until the moment when it is rubbed onto your skin. They provide a convenient way to apply liquid oils to the skin or hair – oils that absorb easily. They avoid the need for the emulsifying agents used in many cream products. And they have a wonderful texture and feel.
Over the last 20 years research groups from around the world have studied these materials. They have carefully deciphered the links between the chemical structures of the ingredients, the way that they are combined, and the stability of the resulting gels. Oleogels have been the subject of 1000’s of academic papers.
But when a formulator in industry is creating a new commercial product they don’t start with specific molecules. They start with the complex mixtures of materials that make up natural oils such as olive, apricot, camelina, and rapeseed oils. Which is why studies such as the one recently published by scientists from L’Oreal are important.
What is an oleogel and how does it work?
The key thing about an oleogel is that liquid oil is physically immobilized. The oil is still oil, it just can’t flow. It is trapped in a cage – in a maze of micron-scale crystals.
The cage can be created from mixtures of saturated fatty alcohols and fatty acids – the detached arms of the triglyceride octopuses from my last post. The fully saturated fatty acids can stack and they crystalize into plate-like structures as they cool. If there are enough of these waxy plates the whole thing becomes a solid.
This is definitely a situation where a picture is worth 1000 words. Here is a good one from the European Journal of Lipid Science and Technology
Since formulators want the plate-like crystals to form as easily as possible they use fatty acids and alcohols that are long. In this study the authors used docosanoic acid and docosanoic alcohol with 22 carbons.
Interesting note: These molecules may be listed on cosmetics labels as behenic acid and behenic alcohol. They were originally extracted from the seeds of Moringa trees found in Iran and are named from the Persian month Bahman.
So what did the L’Oreal researchers discover?
Previous research had revealed that there was an optimum ratio of docosanoic alcohol and docosanoic acid for creating strong, durable, oleogels. The best recipes had a ratio of 7:3 alcohols to acid. The oil mixes with these materials at 85°C. As the mixture cools they self-assemble to form the plate-like structures which trap the oil.
When the L’Oreal researchers used the natural oils they found that for sunflower, apricot, and rapeseed oils 7:3 was indeed the optimal ratio. But olive and camelina oils required an 8:2 ratio.
Why do you need to change the cage materials for different oils?
The minor components found in natural oils (the stuff that isn’t triglycerides) might be making the difference. Olive oil can contain significant amounts of polyphenols. These are directly related to the health benefits of olive oil. According the EU Health Claim Labeling Regulations manufacturers can make the claim “olive oil polyphenols contribute to the protection of blood lipids from oxidative stress”. But these hydrophilic (water loving) molecules could easily be responsible for changing the stability of the oleogel.
A natural oil contains a huge range of different triglycerides with different length arms. Camelina seed oil contains 35%–40% of α-linolenic acid – an essential omega-3 fatty acid which is kinked in three places. It also contains fully saturated, easily straightened chains.
This diversity of structure in natural materials can provide multiple benefits to products. But it presents a challenge to formulators creating the next generation of personal care products.
Next time you apply a moisturizer to your skin take a moment and think about the micron-scale structures that are creating this experience for you!
Read the paper: “The Effect of Vegetable Oil Composition on the Structural Properties of Oleogels Based on Behenyl Alcohol/Behenic Acid Oleogelator System,” by Marion Callau, Nina Jenkins, Koudedii Sow-Kebe, Clement Levivier, And Anne-Laure Fameau, L’Oréal Research and Innovation.
Here is a fun student video (YouTube) showing oleogel formation in the laboratory. In this case Aya Haj Eissa is using ethylcellulose rather than mixtures of fatty acids and alcohols.