These articles summarise what I believe to be the basics that one should know before making cosmetics at home. I do my best to link as many references and sources as possible to confirm what I’m writing, but you should never take anything as an absolute truth. Always check facts and information on more than one reliable source, and take these articles as starting point: keep studying and gathering chemistry knowledge before you actually start making cosmetics. If you notice errors, misleading information or you think I should cover further aspects of this theme, feel free to contact me!
You all know the story: if you mix oil and water in a glass, they will form a white-yellowish weird mixture as long as you keep mixing, but they will immediately separate as soon as you let them rest. We all know it: water and oil are immiscible.
However, creams – the ones used as cosmetics, as medications, but also some food recipes like mayonnaise – are made of water and oil. And they stay together. This happens because they are not simple mixtures: they are emulsions.
The concept: Colloids
Let’s start from far away. What happens when we mix two things? It depends on: the physical state of the things and on the miscibility of the things.
The easiest situation is when we have two miscible liquids, like two aqueous solutions, alcohol and water, or acetone and water. In this case, what we obtain is a solution and we won’t be able to distinguish the two original phases anymore.
In cosmetics, and especially in homemade cosmetics, the products that belong to this group are aqueous solutions like the skin toner or the scented spray, but also oil solutions like the cuticle oil.
The situation gets more complicated when you have two immiscible things. In this case, you will obtain different products (called colloids) according to the physical state of the things and also depending on who’s the external phase (meaning – in most cases – the phase that is present in highest amount) and who’s the inner phase.
If we have two liquids, for example oil and water, we will have emulsions. Specifically, we will get a:
- Water-in-oil (W/O) emulsion, if the inner phase is water and the outer phase is oil
- Oil-in-water (O/W) emulsion, if the inner phase is oil and the outer phase is water
This is what most of our cosmetic products are. Most creams are O/W emulsions, that can range from almost-oil-free creams to thick and heavy creams for dry skin. W/O emulsions are less present in cosmetics but still possible, and they are typically very fat creams, similar to ointments.
If we have a gas and a liquid, we will have a:
- Foam, if the inner phase is the gas and the outer phase is the liquid
- Aerosol, if the inner phase is the liquid and the outer phase is the gas
We can also have cases with a solid and a liquid, and we will get a:
- Suspension, if the inner phase is the solid and the outer phase is a liquid
- Solid emulsion, if the inner phase is the liquid and the outer phase is a solid
Finally, you can also combine gases with solids to get:
- Solid foams, where the gas is dispersed into the solid (an example is the pumice)
- Aerosol with solid dispersed in gas, which is the case of smoke
Ok, all this is not fundamental in our analysis of cosmetic creams, but it’s always interesting to know. See reference (1) for additional info.
Let’s go back to emulsions. As mentioned, emulsions are dispersions of one liquid in another liquid of which both are immiscible (2). Most of our creams are emulsions and especially O/W emulsions. This means that they consist of an inner oil phase that is dispersed in form of droplets in the water phase. This is what happens temporary while we mix water and oil in our glass but as we know, this conformation is not stable. To make it stable, we must add a third element to the system: the emulsifier.
Therefore, the basic components of an emulsion are:
- The inner phase: oil for O/W, water for W/O
- The outer phase: water for O/W, oil for W/O
- The emulsifier that makes them stay together
The emulsifier is a chemical compound characterised by a hydrophilic and a lipophilic part. The lipophilic portion is typically a long hydrocarbon chain, while the hydrophilic part consists of hydroxyl groups or ether moieties for non-ionic emulsifiers (3).
From the structural point of view, we can divide emulsifiers into 3 categories: anionic, cationic and non-ionic (4).
Anionic emulsifiers are not used in creams, but we encountered them in another chapter of our formulation life: they’re soaps and surfactants. They have a long lipophilic chain and an anionic head (a salt).
One must pay attention because they are sensitive to electrolytes (including some active ingredients).
Cationic emulsifiers are mostly used in haircare. We meet them every time we want to formulate a hair conditioner and sometimes when formulating a solid shampoo.
Non-ionic emulsifiers are the most applied in creams and lotions. Some examples are Ceteareth-20, glyceryl stearate, methyl glucose distearate, methyl glucose sesquistearate, but also lecithin-derivatives like Phospholipon 80H (hydrogenated lecithin).
For example, this is the emulsifier I use the most: methyl glucose sesquistearate.
The hydrophilic head is given by the methyl glucose molecule, whereas the lipophilic part is represented by the long hydrocarbon chain.
Emulsifiers can interact with both oil and water phase. They absorb at the interface of the two phases, thus lowering the interfacial tension and avoiding the coalescence of the inner phase droplets and consequent separation of the two phases (5).
In other words: in a o/w emulsion, the emulsifier will interact with the oil droplets with its lipohilic chain and with the surrounding water with the hydrophilic head and will form a sort of layer between the oil droplets and the water phase. This will stabilise the oil droplets and let them stay in water in form of tiny droplets, instead of looking for each other and coalesce into a big oil phase.
There are many emulsifiers available for us DIY-formulators. The choice will first of all depend on the type of emulsion you want to make: o/w emulsifiers are different than w/o emulsifiers. In addition, some emulsifiers will yield creams with a richer touch than others or thicker than others, but it is always possible to modulate these aspects adjusting the rest of the formulation and keeping the same emulsifier.
The HLB concept
To establish whether a compound is a o/w or a w/o emulsifier, we take into account the HLB (hydrophilic-lipophilic balance) number. The HLB system is a very old fashioned way to distinguish and choose emulsifiers and nowadays no home-formulator uses it, since most suppliers offer us technical sheets for each emulsifier stating if it is a o/w or a w/o emulsifying agent. However, I believe it is always useful to know what is behind all this.
The HLB value is a function of the weight percentage of the hydrophilic portion of the molecule of a non-ionic emulsifier (6).
This value is proportional to the molecular mass of the hydrophilic portion, meaning that the higher the HLB for an emulsifier, the more it is hydrophilic (7) . A hydrophilic emulsifier will work better in o/w systems, where the prevalent phase is water; a lipophilic emulsifier (with a low HLB value) will prefer w/o emulsions.
Emulsifiers with HLB ranging from 4 to 6 are suitable for application as w/o emulsifiers, whereas emulsifiers with HLB 13-15 are used in o/w emulsions. Reference (8) provides a list of emulsifiers used in cosmetic formulation with their HLB value.
Every ingredient of our emulsion has a required HLB in order to be emulsified. For example, olive oil has a required HLB value of 7 (9).
So in theory, when you plan the formulation of a cream, you should carefully calculate the required HLB of the whole formulation and then decide which emulsifier to use and the amount you need.
Nowadays, nobody does it anymore. What is important to know is if the emulsifier you are about to use is a w/o or a o/w and what is the concentration range you need for it to work (10).
It is also important to know if the emulsifier can work well on its own or if it needs a co-emulsifying agent: for example, methyl glucose sesquistearate is always used as paired with cetyl alcohol, which helps stabilising the oil droplets in the fine dispersion.
The stability of an emulsion depends on many factors.
We already discussed the most important one: the presence of a suitable emulsifier. No emulsifier, no cream.
But there are other parameters that helps the cream to maintain a certain stability.
One of these is the presence of thickening agents in the water phase and of consistence factors in the oil phase.
There are hundreds of studies on the rheology of thickening agents and subsequent gel phases, but I will try to simplify the concept and tell you this: if you put a thickening agent in the water phase, the water phase will become jelly and firm. This will make it very difficult for the oil droplets to reach each other and coalesce in a big unique oil phase and separate from the water phase.
In this sense, the use of thickening agents enhances the stability of the emulsion.
In addition, thickening agents can act as moisturizers, as they typically retain water. They also increase the viscosity of the final emulsion, making it feel more like a… cream (11).
There is a vaste choice in terms of thickening agents and nowadays it is possible also for home-formulators to get high-performance thickening agents that give the creams the final touch that we are looking for. The same thickening agents are used to formulate hydrophilic gels and today we can choose between compounds (mainly based on polyacrylates) that yield wonderful transparent gels that flow on the skin like a soft cream.
With that being said, my personal choice always goes in the direction of green chemistry and having as less material as possible for as many applications as I can. Therefore, although I tested some polyacrylates in the past, right now my choice for creams is always the very basic and not-so-cool-final-touch xanthan gum. For hair conditioner, that require the use of cationic surfactants, I use guar gum (because xanthan gum is not compatible with cationic surfactants).
Another important factor that influences the stability of an emulsion is the size of the inner phase droplets.
Typically, the smaller the droplets, the more stable the emulsion. In fact, when the droplets are too large, they tend to result in phenomena like creaming, coagulation or coalescence and our cream will not look like a cream anymore (12).
To achieve micro- if not nanosized droplets, cosmetic labs have specific equipment to emulsify creams and lotions at high speed (which allow the formation of tiny droplets) and without incorporating air bubbles.
As DIY-formulator, our choice is limited by our budget, by the fact that we don’t produce tons of creams each time, and by the fact that we don’t have access to professional lab equipment. Therefore we use homogenisers or immersion mixers, that allow the emulsification of small quantities at a sufficient speed to form little inner phase droplets.
In summary, we have seen that a cosmetic cream (and a cream in general) is an emulsion, which is a colloidal system in which inner droplet of a liquid are dispersed in an outer phase made of another liquid, with the two liquids being immiscible. The whole system is stabilised by the presence of an emulsifier.
Specifically, in a cosmetic cream we can have oil droplets dispersed in water (o/w creams) or water droplets dispersed in oil (w/o creams).
The stability of a cream depends on several factors, that are not limited to the presence of the right emulsifier: the viscosity of the outer phase, modulated by the presence of thickening agents, has an important impact on the stability of the dispersed droplets; at the same time, the size of the dispersed droplets is very important, and the smaller the droplets, the more stable the emulsion.
Next article of the series: