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Help, my emulsions aren’t stable!

April 1, 2012

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….

Stokes Law for Emulsions

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.

Amanda

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 🙂

 

10 Comments leave one →
  1. April 12, 2012 1:29 pm

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  2. November 15, 2013 10:54 am

    Great article. One point to add to your checklist – do heat and hold at 75c your 2 phases for 20 mins – this makes your phases more fluid which makes it easier to break the oil phase into very small droplets and also makes it easier for the emulsifier to migrate to the oil-water interface, which is where it needs to be for a stable emulsion.

    If you are making a lotion at home and don’t have a high shear mixer, you can use a stick blender instead. More free tips incl a lotion/moisturiser tutorial and recipe calculator here – http://www.makingskincare.com

    • RealizeBeautyEd permalink
      November 15, 2013 2:48 pm

      That might be the case but it isn’t necessarily practical, especially in scale up. I’d like to see some microscopic or stability based evidence of this producing better emulsions. I do see this working when you have to suspend iron oxides into the oil phase or where you are trying to combine different melting point waxes into a balm but no so much a standard emulsion.

    • RealizeBeautyEd permalink
      November 16, 2013 7:34 am

      Thanks, as I said! I’m sure it works best in some situations but I’d still like to see some experimental evidence as that would be really interesting. More importantly it would be effectual for large companies to have evidence before changing their manufacturing strategy as holding phases for 20or so minutes after melting would significantly slow down production times. That would be OK if the product turned out so much better but pointless if there were no benefits. Don’t get me wrong, I am very grateful for the info and will absolutely look into this because I’m all for process improvements, I just need to test the theory. I had a similar situation with Shea butter, the Internet repeated the mantra that we should ‘temper for 20 minutes’ to cure the gritty bits. I tried out various batches of Shea tempering for different times then leaving out to cool vs stirring to cool vs flash freezing. I then evaluated the samples under the microscope and found that there are many ways to reduce the grittiness and that the 20 minute tempering is only part of the answer. You can temper for 1 minute and it works if you do the next step properly. Thanks again, really appreciate the discussion 🙂

  3. November 16, 2013 2:22 pm

    First of all, you can’t use Stokes law for a non newtonian fluid like lotion. Stoke’s law is only valid for a newtonian fluid under laminar flow conditions. For non newtonian fluids, you need to use an exact form of the Navier Stokes equaiton which would require some form of numerical quadrature. Second, the equation you provided for Stoke’s Law is wrong. The equation is: Vs = 2/9 (del rho /mu) * g * R^2, where Vs is the settling velocity, del rho is the difference between the sphere and the fluid, g is 9.8 m/s^2, mu is the kinematic viscosity, and R is the radius of the spherical object. Your equation is incorrect as you left out the kinematic viscosity.

    • RealizeBeautyEd permalink
      November 16, 2013 3:21 pm

      Interesting. I’ll look at it again. Maybe starting with a ‘hello’ would have been a bit nicer though but anyway.

  4. November 17, 2013 4:32 am

    oh, sorry, hi! I found you via a link on facebook. I hope I didn’t come across as rude, I wrote this really late last night. I have a good bit of experience in fluid mechanics/rheology so would be happy to explain in more detail if you want to work through some problems. If you want to check out my site it is tikibarsoap.com.

    • RealizeBeautyEd permalink
      November 17, 2013 1:51 pm

      It’s no problem. I’ll talk to my peers in the cosmetic chemistry world and try out some more experiments.

  5. November 19, 2013 2:50 am

    Hi, your article is really interesting and gives helpful info for those of us making products without the scientific background. It’s good to have it explained in layman’s (or laywoman’s) terminology and I really am trying to grasp the science behind it.
    I’m interested in your method for tempering Shea butter (I do wonder if it’s partly to do with how the women actually produce it as they all have their own methods).
    I currently H&H for 20 mins and leave it to cool naturally which seems to work – 1 min H&H would be amazing! If you’ve written anything on it could you send me the link please.

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