The main reason for separation is "the coalescence of dispersed droplets", in other words, the oil clumps together. Because the oil has a lower specific gravity than the water, it floats to the top and separates from the lotion. Every oil and water lotion will fail eventually because it's not a natural state for oil and water to be mixed together like this, but that date should be far far into the future for a good emulsification system. (When I say far in the future, I mean you should be able to give a lotion to your currently infant daughter on her graduation day! I don't recommend this - no preservative is going to hold out THAT long - but that's the kind of time line I mean!)
A note in response to some lotion recipes I've seen on the 'net...Separation is NOT normal. If you're experiencing separation, you are doing something wrong. And you can't repair it by shaking the bottle before use.
When two or more droplets collide and combine with each other resulting in the formation of one larger droplet. This is an irreversible breach of interfacial film between the two phases and is not fixable. What causes it? Poor choice of emulsifier or too low a level of emulsifier, your pH level, too many salts, and temperature. What can we do about it?
Heating and holding is essential - getting both phases of your lotion up to 70C for 20 minutes can ensure you get all the ingredients to the right temperature. Solubility of ingredients generally increases when you have a higher temperature, and when you have two phases that really don't get along that well, you need every bit of help you can get to make them mix!
SEDIMENTATION OR CREAMING
I've seen this a few times - it's not pretty. Your oil phase floats to the top of the lotion, leaving a thin, milky coloured layer of water and other ingredients at the bottom. What caused this? Well, it's a natural thing for oil to float on top of water, so it could be a poor emulsification system, not enough emulsifier, failure to get the ingredients to the proper temperature, or failure to mix the lotion adequately. (In my case, I didn't get the two phases to the same temperature - I think. This was before I bought my thermometer!)
What can we do about this? Heating and holding, for one. You can increase the viscosity of the lotion with a gum or gel, although this really is a pain in the bum. You can ensure you have a good emulsification system. And remember to mix well.
I use a hand mixer for personal sized batches of lotions and a stand mixer (love my Kitchenaid!!!) for larger batches. You can also use a stick blender for smaller batchers, and a paint mixer attachment on a drill for huge ones. This is all about mechanical emulsification - again, we need to make the oil and water want to stay together, and mixing is a huge part of that. I mix quite a bit when we add the two phases together, I mix when the temperature drops a little, and I mix when I add the cool down ingredients (fragrance oil, preservative, and so on).
How to mix? LabRat recommended mixing until the lotion reaches 25C to 28C. I'm going to be honest, I don't do that when I'm using a hand mixer. It really is a lot of work and I don't have an hour to stand there holding a mixer! (I know, I know, I should but, I have so many things I want to make and so little time). If you have a stand mixer, put it on a low setting and let it run, checking the temperature periodically so you know when to add the cool down ingredients. If you're using a hand mixer, really it's your call. I find it works for me in the way I described above, but it isn't the best manufacturing practice you can do.
FLOCCULATION (my new favourite word! Try to use it in conversation!)
A process by which 2 or more droplets aggregate to form even larger drops (bigger than 2 mm!) This can promote sedimentation and creaming at a faster rate. The rate at which droplets aggregate is affected by the pH and ionic strength of the aqueous environment (meaning, it's about the water phase, not the oil phase). The floc (the joined droplets) float to the top and create the creaming effect.
This can also be caused by a too large oil phase coupled with a level of emulsifier that can't handle it. If you're using Polawax or e-wax, make sure your emulsifier level is 25% of your oil phase. Use a co-emulsifier or thickener like stearic acid or cetyl alcohol. If you continue to see this, use another emulsifier. I used to use general e-wax but switched to Polawax due to flocculation. I did try a number of things to get the e-wax to work, even increasing the amount to way more than I liked, resulting in a grippy lotion, and I finally decided to go back to Polawax despite the increased price.
When one droplet engulfs another, creating a larger droplet. This differs from the other forms of lotion failure because it's not so much about the collision of droplets as above. (I think of this one as one droplet eating another - a violent act - whereas the others are about droplets coming together and not wanting to be separated - an affectionate act.)
What can be done? Again, all the stuff mentioned above.
Lotions can go wrong for myriad reasons, but we can control most of them. Just remember lotions are formed through chemical, mechanical, and heat emulsification.
Chemical emulsification: Ensure you are using a good emulsification system at the right levels.
Mechanical emulsification: Mixing! Get a good mixer you feel comfortable holding for a while. Or treat yourself to a nice Kitchenaid mixer. Get a bowl and paddle for use exclusively for your bath and body products.
Heating: Always heat and hold at 70C for 20 minutes. Get your phases to the same temperature. Buy a nice candy thermometer (a bargain at $10 or less!) and check the temperatures regularly.
Measure your ingredients: Always go for weighted measurements instead of volume. Get a good scale - they're not expensive at $40 for a digital scale that weighs to 1.0 grams (and if you're really into making lotions, get one that goes to 0.1 grams. Good for mineral make-up as well!)
Know your ingredients: Knowing how to alter your emulsifier or which thickener to add will help you modify recipes properly.
A pH meter would be nice, but they're not cheap and are only good for one thing (whereas your scale is useful for so many things!) I know I have one on my Christmas list!
Join me tomorrow for fun with phase inversion!