As exciting as it is to browse through the hundreds of different vegetable oils that are available to today’s cosmetic chemists, most of them, for the most part, are chemically quite similar. What I’m referring to here is the fact that the majority of vegetable oils are comprised of triglycerides dominant in Oleic, Linoleic and Linolenic fatty acids. While there is absolutely nothing wrong with that, there are occasions when it’s nice to work with something different, something that behaves a little more interestingly, and that stands out from all the rest in some way. That’s where Castor oil steps in.
If you prefer to listen rather than read, this article is available as a podcast and can be found here.
If you’ve ever wondered why castor oil performs better than your typical veggie oil at solubilising pigments, why it is often used in skin cleansing formulations or why it’s popular in hair and eyelash treatments, it is due to its unique and interesting chemistry.
Castor oil conforms to the usual oil structure of being dominant in triglycerides but its fatty acid of choice is Ricinoleic rather than oleic, linoleic or linolenic acid.
Ricinoleic is a C18 fatty acid and just like Oleic it also contains a single double bond. However, Ricinoleic also contains something else – an extra hydroxyl group on its fatty tail – another -OH or, in chemistry speak Oh My God That Changes Everything!!!
This extra -OH group on a C18 fatty acid that typically makes up between 85-90% of the oil makes Castor a whole lot more polar and surface-active than your average oily triglyceride. That explains why Castor oil is much more interested in getting up close and personal with other polar chemicals in a similar way to that of a surfactant/ emulsifier. While it would be misleading to hint that this oil can be used as an emulsifier – it’s not polar enough for that – the extra -OH’s does give Castor oil the ability to modify the surface tension of a formula in a way that non-polar oils simply cannot and that feature can be very, very useful!
Surface tension is a repellant force that exists at the interface between an oil and water phase. This may be within a product (say, at the oil: water boundary of an emulsion) or during product use (when you try and spread a product through the hair or across the skin). The bigger the difference in polarity between the phases, the greater the interfacial force and potentially the greater the resistance to flow (less spreadable product).
While some surface tension between phases is desirable and required to maintain the integrity of an emulsion or multi-phase product, it’s often possible and preferable to create formulations that are a little less highly strung and more relaxed. Adding more polar ingredients to an oil phase of an emulsion or at the emulsion interface (eg caprylyl glycol) can improve spreadability. In formulations that contain a water phase, another surface-tension modification option is to decrease the polarity of the water phase and this can be achieved by adding glycerin, glycols or alcohol. Often multiple strategies are used to balance out the benefits of reduced surface tension with the requirements for creating a stable product. So it could quite easily be the case that polar oils are used to increase the polarity of the oil phase while glycols are used to reduce the polarity of the water phase.
This surface-tension modification can be used by formulators to increase the spreadability and flow of a formula, and so the fact Castor oil can help with this more so than most other oils is what makes it such an interesting and useful ingredient.
Castor oil is quite safe to use and has a low potential for triggering allergies but it can turn rancid quite quickly thanks to this very feature that makes the oil so interesting – the joys of double bonds and oxygen groups! As such, it’s definitely best to avoid storing this in warm environments, in UV-exposed locations or where moisture can get into it – I’d definitely recommend limiting the oils head space but would not advocate storing this in the fridge, a room temperature cupboard should suffice.
In terms of identifying when castor oil is going off, that’s not too difficult actually. This oil becomes much darker in colour when it oxidises going from a light yellow to a darker yellow and onto a slight orange tone. Ironically the oils propensity for this is what got it kicked out of most lipstick formulations back in the day as people got fed up with their lipsticks getting progressively more orange after a few months of being used.
In spite of its susceptibility to oxidation, this is a good oil. It’s cheap, widely available and has some really interesting chemistry that delivers a multitude of benefits in a formula.
I love it when a simple, low-key ingredient that many people take for granted is uncovered as having chemistry superpowers like these. It's stories like this that make my chemistry world go around!
Enjoy.
Amanda x
PS: for those who are interested, here is a link to one of the articles I reviewed when checking up on the composition of Castor Seed Oil.
Mutlu, H., & Meier, M. (2010). Castor oil as a renewable resource for the chemical industry. European Journal of Lipid Science and Technology, 112(1), 10-30.
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