Help, my emulsions aren’t stable!
For those of you that have tried making your own creams, lotions, sunscreens or liquid foundation (to name but a few) you will empathise with me when I say that emulsions can be scary things. Armed with a bullet proof knowledge that oil and water don’t mix without help we ready ourselves with an emulsifier or two (the help) a good mixer and a strong cup of tea (essential) and prepare to beat our proposed dispersed phase into a neat and orderly submission. Only our cunning plans don’t always work. Do they?
There are many ways for an emulsion to leave you crying and unfortunately your choice of emulsifier/ emulsifier combo is only the beginning. I’m talking about Stokes Law….
Yes it does look rather scary at first but once you become familiar with all the V’s, d’s, g’s and triangles the world becomes a much brighter place!
Now, before we go any further we should re-introduce ourselves to a few features of an emulsion. As we are just learning we will pretend that emulsions are simple things with only two possible options – oil in water or water in oil. Us science people like to think of an emulsion as two phases – the continuous phase (the bulky bit that the other drops sit inside of) and the dispersed phase (the small bubbles that float around inside the continuous phase). When you have oil as the continuous phase you have a water-in-oil emulsion and when you have water as the continuous phase you have oil-in-water. The most common is oil-in-water as these are cheaper, feel lighter and are generally suitable for most applications.
So, V = velocity. This is the speed at which your tricky little dispersed phase moves. The quicker it moves, the shorter the time that the product will be stable so the slower the dispersed phase moves, the longer the product will be stable for. Pretty much all emulsions will break at some point but in the world of cosmetics we like to aim for a minimum of 1 year shelf life if not 2 or even 3. 6 months is a painful minimum (painful as manufacturing this way is a massive headache).
d = the size of the dispersed phase. Small, evenly sized dispersed particles are the best option in pretty much all cases and that is why it is important to thoroughly mix your emulsions. Homogenisers are also useful bits of kit as these not only bash up the dispersed phase, they put them through the mill to make them all the same size and this further improves stability.
The triangle of love – This relates to the density (weight) of the two phases. Sometimes you do everything right but the dispersed phase still floats or sinks. This can be because it is either too light or too heavy for the continuous phase. You can play around with density by adding other materials to the lighter phase. To make water denser add humectants and/or salts. Adding gums can sometimes make them even lighter which makes the problem worse. To make oil phases denser add waxes or butters.
g= gravity. We can slow down the effects of gravity by adding an invisible fishing net effect to our continuous phase. If we have made an oil in water emulsion we need to net-up the water phase so something like a bentonite clay or a veegum may work electrostatically by repelling the dispersed phase or you could use natural gums, polymeric thickeners such as carbopol, acrylates, cellulose derivatives or guar. Some of these thicken the continuous phase as well as structuring it so make sure you chose one that works with your product concept – a milk is not a milk if it is creamy thick 🙂
There are ingredients that you can get to structure up the oil phase too if that’s an issue. Sometimes natural waxes and butters are enough but if these aren’t giving you what you need try polyglyceryl esters, wax esters, fatty acids or synthetic polymers such as silicone elastomers.
So, the golden checklist for emulsion stability is as follows:
- Understand your phases – What is in each, what are the properties of each, how much will they ‘like’ each other?
- Understand your emulsifier/ emulsifier system – what type of emulsion are you going to make and will your emulsifiers give you that?
- Even out the densities of your two phases if you need to.
- Add a ‘safety net’ to your continuous phase so that no little dispersed phase particles can slip through.
- Manufacturing method – make sure it is going to help you to get what you want – if making a water in oil be sure to add the water into the oil rather than pouring the oil into the water!
- Mix it like you mean it. Give it some welly.
- Homogenise if you can – not all emulsions need homogenising but many can benefit from this. A couple of minutes is all it takes usually but sunscreens and foundations may take longer.
- If all else fails, try to make it thicker as it will take more energy for your dispersed phase to travel through thick stuff than thin.
Good luck with it all and remember that seeing is believing so get out there, give it a go and most importantly try to get into the mind of your emulsion to really understand what is going on.
PS: If your emulsion of choice is a zinc only sunscreen you are in for a world of pain. There are many more things to consider with zinc that I won’t even start on here. Patience is important but so is an understanding of the science so don’t delay, get educated today 🙂