Lotions contain a water phase and an oil phase. Anyone who has made vinaigrette knows that oils do not dissolve in water no matter how long you shake them. As oil and water do not mix we add ingredients such as emulsifiers, stabilisers and use processing methods to help our lotion stay emulsified.
Creating an emulsion which is stable during the shelf life of the product is a challenge for most formulators. This article examines emulsions and provides guidance on how to create a stable product.
Why create an emulsion?
A common complaint of non-water containing products, such as body butters and balms are they are greasy, heavy and can be grainy. Emulsions allow oils and butters to be applied to the skin in an aesthetically pleasing fashion and offers the formulator a great degree of formulation flexibility and easy modification of characteristics such as viscosity, feel and appearance. Emulsions allow otherwise incompatible ingredients, such as oils, glycerin, actives, to be brought together in the same product.
What is an emulsion?
An emulsion can be defined as a dispersion of one material (usually the oil phase) inside another non-miscible (water) phase. However this is not a natural state – J W Gibbs, On the equilibrium of heterogeneous substances (1878), stated that “the only point in time where an emulsion is stable, is when it is completely separated”. As an emulsion will eventually separate over time, our aim is delay this separation for as long as possible.
Simple emulsions are either oil suspended in a water phase (o/w), or water suspended in oil (w/o). The emulsifier we choose determines whether the emulsion will be o/w or w/o. Most emulsions we make are o/w which are generally, more stable and this is the type of emulsion we will be discussing in this article.
What is an emulsifier?
Emulsifiers contain both a water-loving head group and an oil-loving tail. When added to an o/w emulsion, the emulsifier surrounds the oil droplet with it’s oil-loving tail extending into the oil, and it’s water-loving head facing the water. (In a w/o emulsion, the emulsifier’s orientation is reversed.) In this way, emulsifiers lower the interfacial tension between the oil and water phases, and form a protective layer around the oil phase keeping the oil droplets evenly dispersed and preventing them from flocculating.
How do we know our emulsion is sufficiently stable?
An emulsion could separate at any time. Tell-tale signs may appear during or soon after making the lotion or, these signs may appear months later, perhaps when a customer takes the product on vacation to a warmer climate:
- Creaming occurs when the oil phase migrates to form a layer at the top of the emulsion.
- Flocculation is when the oil phase droplets cluster together but they do not join into a larger droplet, giving the appearance of a “fluffy cloud”
- Coalescence is similar to flocculation but the droplets merge with each other to form a larger droplet
- Phase separation is the most obvious sign of emulsion instability and involves the collision and coalescence of the oil phase droplets ultimately resulting in the visible appearance of separate oil and water phases.
It is not realistic for a formulator to wait 1-2 years to check whether their emulsion is sufficiently stable. To speed this up, a 3 month stability test can be performed. As any changes in a lotion will take place faster at higher temperatures, samples are put at different environmental conditions which include a range of temperatures from 4°C (39°F) to 40-45°C (104°F-113°F). Typically the samples are checked at 2, 4, 8 and 12 week intervals. If the product has changed little in 8 weeks at elevated temperatures, it will likely last for a year at room temperature.
How can we create a sufficiently stable emulsion?
To prevent droplets from reuniting once they have been formed, energy barriers must be erected that either prevents the oil droplets from colliding or from fusing if they do collide. The barriers are erected by adding emulsifiers and can be maintained by adding stabilizers.
The steps below detail some of the key steps towards creating a sufficiently stable emulsion.
Step 1: Use an effective blend of at least two emulsifiers
A blend of at least two effective emulsifiers is usually required. Some emulsifiers, especially those marketed as “natural” can be tricky to work with and vary in effectiveness. The term “emulsifying wax”, (otherwise known as “ewax”), is a vague and general term as it can consist of different ingredients. Some ewaxes advertised as “self-emulsifying” or “complete” are not in fact self-emulsifying and another emulsifier needs to be added. Your supplier should reveal the INCI names which make up the emulsifier blend to enable you to check the ingredients. Examples of the INCI names of some emulsifier blends which are generally regarded as effective are:-
- polawax (a proprietary blend of emulsifiers)
- cetearyl alcohol and polysorbate 60
- glyceryl stearate or sorbitan stearate paired with ceteareth-20
Some formulators use the HLB system to tailor make their own emulsifier blend to fit their particular oil phase. Further information on this together with a free downloadable calculator can be accessed by clicking here.
Step 2: Use sufficient quantity of emulsifier
To attain stability, emulsifiers need to cover the surface of the droplets completely and remain firmly adsorbed. The amount of emulsifier needed for a given emulsion is highly dependent on the droplet size (ie, surface area), the emulsifier efficiency, and the composition of the two phases. When using the emulsifiers mentioned in Step 1, a general starting rule is to add the emulsifier blend at around 25% of the total of the oil soluble ingredients. For other emulsifiers, it is advisable to check the manufacturer’s data sheet.
Step 3: Strengthen the oil/water interface and slow down the rate of coalescence using thickeners and stabilizers
Thicken and stabilise your formula by adding 0.2-0.3% xanthan gum together with at least 1% cetyl alcohol.
Step 4: Use high shear mixing at the right temperature
For a stable emulsion, the oil droplets should be small and uniform as possible. The higher the mechanical energy used during the manufacturing process, the smaller and more even the droplet size. The high shear Silverson mixer is ideal for this application, however, a more economical option for the homecrafter is an ordinary stick blender. This high shear mixing usually commences when the water and oil phases are combined at around 167F/75C.
Undoubtedly, every formulator has experienced unstable emulsions. Emulsion stability is influenced by many factors including emulsifier type, concentration, thickeners/stabilizers and method of preparation.
Stability testing ensures that our product meets the intended physical, chemical quality standards as well as functionality and aesthetics when stored under appropriate conditions.
“Emulsions & Stability”, written by Jane Barber, was published in the January/February 2016 and March/April 2016 issues of Making Soap Cosmetics & Candles Magazine. It is reprinted here with full permission of the publisher, www.makingsoapmag.com