The Fat Side of the Cream II: Physical Properties and Impact on the Final Product

[Versione italiana]
[Back to the previous article: Emollients]

In the 2nd part of “The Fat Side of the Cream” we will see the following aspects of emollients:

  • Quick definitions
  • Melting point
  • Viscosity
  • Oxidation
  • Comedogenicity

Quick definitions

In the following paragraphs we will speak a lot about skin feel and spreadability. 

The skin feel includes everything that concerns the sensory properties of the cosmetic product, like spreadability, absorption, duration on the skin surface. In terms of skin feel, a cream can be defined as heavy, light, thick, thin, draggy, quick-absorbing, etc (7). 

The spreadability indicates how fast can the cosmetic product cover an area of the skin when applied to the skin (8). 

Melting point

The melting temperature is influenced by the chain length and by the degree of unsaturation of the fatty acids composing the triglycerides of the vegetable fats. The same goes for esters. 

Vegetable fats composed of mainly unsaturated fatty acids are typically liquid at room temperature. This happens because unsaturated fatty acids have at least one double bond, which constrains the hydrocarbon chain to assume a precise rigid conformation. As a result, the chain is more rigid, because it is “obliged” to rotate only in a certain way, and this does not allow a very good packing of the molecules. On the contrary, fats consisting of mostly saturated acids can pack very well and their molecules need more energy to get “dancing” and switch to the liquid state (9). 

Here are some emollients ranked from the highest to the lowest melting point (the detailed values I found are in the final table of this article below). For the ingredients that are liquid at room temperature, the melting temperature corresponds to the “freezing” point. 

Effect of the melting point on the final product

When we have mixtures of emollients, we cannot really speak of a melting point. We have rather a melting range. The mixture of emollients with different individual melting points will have a consequence on how thick and heavy the final mixture will be at room temperature. The higher the amount of “solids” (=emollients that have melting point > room temperature), the thicker and heavier the cream (7). 

  • If the amount of solid is too high, the cream will be very thick and difficult to apply. 
  • Increasing the amount of solids makes the skin feel heavier.
  • While it is true that solids should not be too high, it is also true that they help stabilising the emulsion and improving the skin feel. I would recommend adding a little amount also in creams that are meant to be “light”.
  • Among the “solids” you should also count the ingredient groups that we are not discussing in these articles: consistency factors (like cetyl alcohol) and waxes (like beeswax). They have similar effects on the consistency and texture of the creams, and they can have emollient activity. But typically they have give a dry skin feel, not greasy.

Viscosity

Remember rheology? You thought we wouldn’t meet it anymore, but you were wrong. Rheology is everywhere in our cosmetics and it has an important role also when we talk about emollients: they have their own viscosity, which has an influence on how a mixture of emollients will flow and spread on your skin. You can intuitively understand this if you imagine to rub some pure castor oil on your skin and compare it to pure jojoba oil. The two fluids have very different spreadability and skin feel. 

If you remember the rheology articles, you should know by now that viscosity is not the same concept as density. When we speak about the flow characteristics of fluids, we speak about viscosity (a parameter measured in Pa s), and not about density (measured in g/ml), like I read often in DIY cosmetics blog.

Here are the emollients discussed in this article ranked according to viscosity (from high to low viscosity). The precise values I could find are listed in the final table of this article below. I could not find them all: viscosity is a very nasty parameter and on top of that it also depends on the temperature. Take these values as a rough idea of how the relative viscosities are: comparing scientific data from different papers is a very delicate job, because even when the experimental parameters are the same (and they are never), the comparison could be not 100% reliable. The situation here is even more complicated when the viscosities are measured at different temperatures. 

Effect of viscosity on the final product

Now, if you understood the rheology stuff, you should know what I’m going to say now: an oil with high viscosity will not spread very easily on the skin. It takes a lot of “rubbing” to make it flow on the skin. The viscosity of the emollients is the main parameter affecting the spreadability of the mixture (the other one is the surface tension, but it kind of goes along with viscosity). 

But in our formulations we deal with mixtures of emollients, not with only one. The viscosity  usually reflects the composition of the mixture: it will be low if we mainly use low-viscosity emollients, high if the mixture consists of mainly high viscosity fluids. 

  • Very low viscosity mixture: the skin feel will be thin and light. The product spreads fast and doesn’t leave a residue film on the skin.
  • Very high viscosity mixture: the final product will be perceived as greasy, difficult to rub in. It will leave a greasy film on the skin. 

This doesn’t mean that we should avoid high viscosity oils: they typically help stabilise the emulsion and they are of course useful in lotions and butters that are meant to be very emollient and slow absorbing. Also in this case I would recommend to use them anyway, even in low percentage, to give some body to the cream.

The equilibrium of high, medium, and low viscosity emollients yields products that spread well and give a thin film on the skin that will not live eternally on the skin but will disappear after a while.

Rancidity processes: peroxide and iodine value

This parameter doesn’t have much to do with the skin feel of the cosmetic product, but it has a substantial impact on another important aspect of our DIY cosmetics: their shelf life. 

Oxidation is a process that occur in the emollient molecule when exposed to oxygen. It results in breakdown molecules that can change appearance and smell of the emollient, and that can trigger irritation and inflammatory responses in the skin (7). 

Emollients rich in saturated carbon chains are less prone to oxidation, because they are not very reactive. 

On the contrary, unsaturated carbon chains and in particular polyunsaturated chains are susceptible of oxidation, because the carbon atoms close to the double bonds can react. 

Oxidation reactions also result in the production of hydrogen peroxide. The measurement of the hydrogen peroxide amount in the emollient sample gives us an idea of how advanced is the oxidation status of the emollient (that’s the peroxide value). 

This value doesn’t tell us if the emollient is generally more prone to rancidity and oxidation or not: it just tells us about its current status of oxidation. 

The parameter that can give us an idea of how easily an emollient is oxidised is the iodine value. The iodine value is the grams of iodine consumed by 100 g of substance. The double bonds react with iodine compounds, giving a higher iodine value. Thus, a higher iodine value tells us that the emollient has a higher unsaturation level and, consequently, is more prone to oxidation.  

Effect of oxidation susceptibility on the final product

The susceptibility to oxidation process does not directly have an impact on the physical characteristics of the final product. But you should take it into account when designing the emollient blend: if your mixture is rich in polyunsaturated fatty acid-rich oils (like linseed oil), you should consider adding antioxidants to the mixture. Tocopherol and tocopheryl acetate in 0,5-2% should do the job. Keep in mind that tocopherol and tocopheryl acetate are also high viscosity emollients. 

Comedogenicity

The comedogenicity of an ingredient is the tendency to cause clodged pores. 

It is not a trivial parameter to measure in an objective way, as all skins are different and react differently to the same ingredients. 

The most common lists concerning comedogenicity of oils are based on a scale that goes from 0 (= believed to be non-comedogenic) to 5 (= high probability that it will be comedogenic in most cases). 

Exhaustive explanations and lists are found in References 10 (a-e). Please note that these are not scientific papers or handbooks. The lists and information are well made, but take these values again as rough idea of how it could be, not as a law. 

Indeed, it’s a bit difficult to find references with scientific background about these ratings, because these ratings are mainly based on consumer experiences with products. However, in the references cited here there are only minor differences in the ratings and the big picture is always the same concerning which oils are definitely comedogenic and which ones are not.

I have to mention that there is a bit of confusion concerning an oil that is not mentioned in the list: Rosehip oil (Rosa moschata or Rosa rubiginosa, Rosa mosqueta in Italian). In the Italian community of natural cosmetic enthusiasts it has been banished from face products because it is believed to be the most comedogenic oil of the world. In the references I find it rated as 1, so with a low comedogenic ranking. Hm.

Effect of comedogenicity on the final product

The choice of one or another oil in terms of comedogenicity mainly depends on you and your need. Again, consider that the rankings I mentioned are not based on objective and scientific data. Things could work differently for you. Maybe if you have acne-prone skin you could consider avoiding coconut oil, wheat germ oil or cocoa butter in your face cream, just to be sure. But don’t take these rankings as absolute laws.

References

[Next article: Approaches to emollient blending]

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